Josefine Schnee
email: josefine.schnee@upmc.fr

Laurent Delannoy

Guylène Costentin

Cyril Thomas
email: cyril.thomas@upmc.fr.

Unraveling the Direct Decomposition of NO x over Keggin Heteropolyacids and Their Deactivation Using a Combination of Gas-IR/MS and In Situ DRIFT Spectroscopy

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Heteropolyacids (HPAs) are metal-oxygen clusters which have a discrete ionic structure consisting of mobile elementary units, namely heteropolyanions and protons. This mobility is the source of very interesting catalytic properties. [START_REF] Kozhevnikov | Sustainable heterogeneous acid catalysis by heteropoly acids[END_REF][START_REF] Kozhevnikov | Catalysis by heteropoly acids and multicomponent polyoxometalates in liquid-phase reactions[END_REF] HPAs exhibit unique redox properties which can be tuned by varying their chemical composition. [START_REF] Kozhevnikov | Sustainable heterogeneous acid catalysis by heteropoly acids[END_REF][START_REF] Ganapathy | Location of protons in anhydrous Keggin heteropolyacids H3PMo12O40 and H3PW12O40 by 1H{31P}/31P{1H} REDOR NMR and DFT quantum chemical calculations[END_REF] They also exhibit a very strong Brönsted acidity, being much stronger than that of conventional inorganic acids, and approaching the super-acid region. [START_REF] Kozhevnikov | Sustainable heterogeneous acid catalysis by heteropoly acids[END_REF][START_REF] Li | New progress of Keggin and Wells-Dawson type polyoxometalates catalyze acid and oxidative reactions[END_REF][START_REF] Bielański | Polyoxometalates as the catalysts for tertiary ethers MTBE and ETBE synthesis[END_REF] Among a large diversity of structures, Keggin HPAs are the most studied. Indeed, they are the most stable and the most easily available ones. [START_REF] Kozhevnikov | Catalysis by heteropoly acids and multicomponent polyoxometalates in liquid-phase reactions[END_REF][START_REF] Janik | A computational and experimental study of anhydrous phosphotungstic acid and its interaction with water molecules[END_REF] The Keggin unit is composed of a heteropolyanion having the formula [XM12O40] n-(typically, X = P or Si and M = W or Mo) and being stabilized by n acidic protons. [START_REF] Janik | A computational and experimental study of anhydrous phosphotungstic acid and its interaction with water molecules[END_REF] The heteropolyanion is made of a central XO4 tetrahedron, surrounded by 12 MO6 octahedra. [START_REF] Zhang | Formation, location, and photocatalytic reactivity of methoxy species on Keggin 12-H3PW12O40: A joint solid-state NMR spectroscopy and DFT calculation study[END_REF] It contains 3 types of oxygen atoms: central (Oa), bridging, and terminal (Od) ones. The bridging ones are either corner-sharing (Ob) or edge-sharing (Oc). In the hydrated solid, Keggin units are coordinated with finally further enabled the bulk of the HPA to become accessible also to methanol from the reaction feed at 150 °C. This low-temperature methanol pre-exposure led to a 5 times higher methanol-to-DME conversion than observed over classically activated HPA. On this basis, in the present work, the NOx decomposition reaction will be investigated without or with pre-exposure of the hydrated HPA sample to the NO-O2 feed at RT. Through a combination of essentially gas-infrared analysis, mass spectrometry and in situ diffuse reflectance infrared Fourier-transform spectroscopy under various reaction conditions, the present work sheds light on the activity of H3PW12O40 in the aforementioned NOx decomposition process, the nature of its active species in that process, and the sources of its deactivation.

Thereby it unprecedentedly places HPAs into the category of materials to be considered on the way towards efficient formulations for direct NOx decomposition.

Experimental section

2.1. Sample preparation H3PW12O40 (hereafter HPW12) was purchased from Sigma-Aldrich in the form of H3PW12O40.xH2O (reagent grade). The powder was placed overnight under vacuum (< 5000 Pa) at room temperature (RT) in order to evacuate as much physisorbed water as possible. As revealed by a subsequent thermogravimetric analysis, the evacuated HPA contained 6 molecules of structural water per HPW12 unit (x = 6, thermogram not shown). Before experiments, the HPW12.6H2O powder was sieved below 125 µm.

Direct NOx decomposition experiments

Direct NOx decomposition experiments were carried out in a U-shape quartz reactor (15 mm i.d.) at atmospheric pressure. The samples (2.5 g, < 125 µm) were held on plugs of quartz wool, and the temperature was controlled by a temperature controller (Eurotherm 2408) using a K type thermocouple.

The reaction feed consisted of NO (~ 2000 ppm) in Ar(13%)/He, with or without O2 (5%, if present), depending on the steps of the experiments (ppm and % referring to volume). The constitutive gases were fed from independent gas cylinders (Air Liquide), through independent mass flow controllers (Brooks 5850TR). The total flow rate was 50 mLNTP/min. The reactor inlet and outlet gas flows were analyzed with a gas-infrared (IR) analyzer (MKS MultiGas 2030), which allowed simultaneously detecting H2O, NO, NO2 and N2O (with 1 record every sec), as well as with a Pfeiffer Vacuum OmniStar mass spectrometer (MS) to qualitatively monitor the formation of N2 using Ar as an internal standard (with 1 record every 6-6.5 sec), and occasionally with an Agilent CP490 micro-gas chromatograph (µ-GC) to quantify N2 (with 1 injection every 3.5 min). As the concentration of N2O remained below 0.6 ppm in all experiments, it is not reported in the results and discussion section.

In a first experiment, the sample -hereafter referred to as "N-HPW12" sample (N standing for non-NOx-pre-saturated) -was heated from RT to 380 °C, the reaction temperature of interest in this work, at 10 °C/min under He (50 mLNTP/min). Once H2O was no longer detected by the gas-IR analyzer, i.e. once all of the structural water had been released (Figure S1a in the Supplementary Information), the sample was exposed to the O2-free reaction feed (~ 2000 ppm NO -13% Ar/He) for 60 minutes. Then, the sample was kept under static NO-Ar/He for 2 hours. Meanwhile, O2 was introduced into the NO-Ar/He feed. 2 hours was the time needed to achieve stable IR and MS traces. Finally, the sample was exposed overnight to the O2-containing reaction feed (~ 2000 ppm NO -5% O2 -13% Ar/He).

In a second experiment, the sample was first pre-exposed to the O2-containing reaction feed at RT for 4.5 days until the NOx (NO + NO2) inlet readout was equivalent to the NOx outlet readout. The resulting "P-HPW12" sample (P standing for NOx-pre-saturated) was then heated from RT to 380 °C at 3 °C/min under Ar(8%)/He (50 mLNTP/min). The temperature ramp was slower than that used for N-HPW12 to avoid decomposing the NOx species, that might have been retained by the sample at RT, into N2 before reaching 380 °C. [START_REF] Yang | A new approach to decomposition of nitric oxide using sorbent/catalyst without reducing gas: Use of heteropoly compounds[END_REF] At 380 °C, once H2O was no longer detected by the gas-IR analyzer, i.e. once no structural water was released anymore (Figure S1b), the sample was first exposed overnight to the O2-free reaction feed (~ 2000 ppm NO -13% Ar/He), before being flushed under He (50 mLNTP/min) for 10 hours, and then being exposed once again to the same O2-free reaction feed for 3 hours. Finally, after having been kept under static NO-Ar/He for 2 hours (for the same reasons as N-HPW12 in the first experiment), the sample was exposed overnight to the O2-containing reaction feed (~ 2000 ppm NO -5% O2 -13% Ar/He). As explained in the introduction section, pre-exposing the hydrated HPW12 sample to the O2-containing NO feed at RT before dehydration (upon heating to 380 °C) was likely to enable the protons located in the bulk of its crystals to participate in the direct decomposition of NOx from the feed at 380 °C.

The exposure of the samples to the O2-containing reaction feed at 380 °C in the above described experiments was repeated over fresh N-HPW12 and P-HPW12 samples (not having been beforehand exposed to the O2-free reaction feed at 380 °C). As the results were essentially the same as those obtained in the above described experiments, they are not reported in the results and discussion section.

As references for these experiments, two blank experiments were also carried out, under the same reaction conditions as those aforementioned, but with only quartz wool in the reactor. The first reference experiment was performed with the O2-free reaction feed, whereas the second one was performed with the O2(5%)-containing feed.

In situ diffuse reflectance Fourier-transform infrared (DRIFT) spectroscopy

In order to gain further insight into the behavior of N-HPW12 and P-HPW12 under the O2-free and O2(5%)-containing reaction feeds, in situ diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy was carried out on similarly pre-treated samples under reaction conditions very close to those described in section 2.2, using a Bruker TENSOR II spectrometer. The reaction feed consisted of NO (~ 2000 ppm) in Ar(40%)/He (50 mLNTP/min total flow rate), with or without O2 (5%, if present), depending on the steps of the experiments. It was prepared using the gas distribution setup described in section 2.2, and then conducted through a Harrick heatable variable atmosphere cell (HVAC) equipped with two ZnSe windows and one quartz window, in which the sample holder was filled with powder of the HPW12 samples (< 125 µm). The content of Ar in the feed was higher than that used in the experiments described in section 2.2, for thermal conductivity reasons. The samples, referred to as N-HPW12 and P-HPW12, were both heated from RT to 380 °C at 3 °C/min under Ar(40%)/He (50 mLNTP/min). The pre-exposure of P-HPW12 to NOx at RT was carried out in the IR cell, under the O2containing reaction feed. At 380 °C, the first exposure to NOx was performed once the IR absorption band at 1740 cm -1 , characteristic of the structural water within HPW12, [START_REF] Herring | In Situ infrared study of the absorption of nitric oxide by 12tungstophosphoric acid[END_REF] had disappeared from the DRIFT spectrum (Figure S2, spectrum c vs. spectrum a of N-HPW12 at 380 °C vs. RT, and spectrum d vs. spectrum b of P-HPW12 at 380 °C vs. RT). [START_REF] Herring | In Situ infrared study of the absorption of nitric oxide by 12tungstophosphoric acid[END_REF] In situ DRIFT spectra of the samples were recorded using 128 scans and a resolution of 4 cm -1 . A reference spectrum was recorded with KBr (Fluka, purity > 99.5%) under the same operating (Ar(40%)/He (50 mLNTP/min)) and temperature conditions. The absolute spectra are reported in log 1/R with R = Isample / IKBr. The difference DRIFT spectra are reported in log 1/R ' with the relative reflectance R ' = Isample / I(reference sample). [START_REF] Sirita | Quantitative analysis of adsorbate concentrations by diffuse reflectance FT-IR[END_REF] Spectra processing was done using the Thermo Scientific OMNIC 9 software.

Results and Discussion

In the two blank experiments -one under the O2-free reaction feed, the other one under the O2(5%)containing one -the MS-recorded mass to charge (m/z) ratio 28 associated with N2 normalized by the m/z 40 one associated with Ar, as well as the sum of the NO and NO2 concentrations recorded by the gas-IR analyzer, remained essentially unchanged after flowing the reaction feed through the reactor at 380 °C. This indicates that there was no NOx decomposition to N2 occurring in the absence of HPW12 sample, and that there was no unwanted source of N2 formation associated with the reactor. As an example, the N2/Ar trace and the NO+NO2 concentrations recorded in the presence of O2 in the feed are shown in Figure S3. Due to the presence of O2 in the feed, the NO and NO2 concentrations were impacted by the non-catalytic conversion of NO to NO2, to a larger extent after the feed had flown through the reactor at 380 °C. This needs to be kept in mind for discussing the results obtained over the HPW12 samples in the following subsections.

Activity of non-NOx-pre-saturated H3PW12O40 (N-HPW12) in the direct decomposition of NOx

In the absence of O2 in the reaction feed

Figure 1 shows the NO and NO2 concentrations recorded by the gas-IR analyzer (panel a) as well as the MS-recorded mass to charge (m/z) ratios 28 and 32 associated with N2 and O2, respectively, normalized by the m/z 40 one associated with Ar (panels b and c, respectively) as a function of time on stream in the NOx decomposition experiment carried out with the N-HPW12 sample at 380 °C, in the absence of O2 in the reaction feed. The sample was previously heated from RT to 380 °C at 10 °C/min under He (50 mLNTP/min). The NOx decomposition experiment was launched at 380 °C once no structural water was released from the sample anymore, as monitored with the gas-IR analyzer (Figure S1a). The reaction feed consisted of NO (~ 2000 ppm) in Ar(13%)/He (50 mLNTP/min total flow rate).

In period (i) of Figure 1, the reaction feed was analyzed for 5 min (time -5 to 0 min) without being contacted with the sample (upon flowing exclusively through lines at RT). In period (ii) of Figure 1, the reaction feed was continuously contacted with the sample at 380 °C, and the outlet gas flow was analyzed over time on stream. -c) as a function of time on stream in the NOx decomposition experiment with N-HPW12 (2.5 g, < 125 µm, N standing for non-NOxpre-saturated) at 380 °C and a reaction feed consisting of NO (~ 2000 ppm) in Ar(13%)/He (50 mLNTP/min). In (i), the reaction feed was analyzed without being contacted with the sample (upon flowing exclusively through lines at RT). In (ii), the reaction feed was continuously contacted with the sample at 380 °C, and the outlet gas flow was analyzed over time on stream. N-HPW12 was previously heated from RT to 380 °C at 10 °C/min under He (50 mLNTP/min).

As measured by the gas-IR analyzer, the reaction feed contained 1921 ppm of NO and 8 ppm of NO2 (Figure 1a, period (i)). Upon starting period (ii) (time = 0 min), the concentration of NO in the outlet gas flow (Figure 1a) dropped from 1921 to 1520 ppm within the first 2 min on stream, and then immediately increased until reaching 1874 ppm at 6 min on stream. This initial peak of apparent NO consumption is attributed to the purge of the dead volume of the reactor, filled with He in period (i), upon introduction of the reaction feed. Afterwards, the recorded NO concentration evolved as a function of adsorption/reaction phenomena over the N-HPW12 sample. The concentration of NO first decreased from 1874 ppm at 6 min on stream to 1859 ppm at 13 min on stream. Then, it slowly increased until reaching 1891 ppm at the end of period (ii). The concentration of NO2 remained between 6.5 and 7.5 ppm over the whole period (ii). The fact that it was lower in period (ii) than in period (i) suggests that NO2 was partly adsorbed and/or converted on N-HPW12. The possible impact of the latter process on the interaction of N-HPW12 with NO will be discussed in the next sections (3.1.2, and 3.2 to 3.4).

The evolution of the N2/Ar trace recorded by the MS (Figure 1b) inversely follows that of the NO concentration (Figure 1a). In period (ii), following its initial peak attributed to the evacuation of traces of air from the dead volume of the reactor, the N2/Ar trace first increased from 5 to 13 min on stream, and then slowly decreased until the end of the experiment, reaching a final signal being however above that measured in period (i). According to a calibration based on µ-GC measurements (the error associated to which is estimated to be about 1.6%), the difference between the final N2/Ar signals in periods (ii) and (i) corresponds to about 18 ppm of N2 produced, thus close to half the concentration of NO (30 ppm) missing in the outlet gas flow as compared to the feed (Figure 1a, end of period (ii) vs. period (i)). These results demonstrate, to our knowledge for the first time, that direct decomposition of NO into N2 (2 NO  N2 + O2) occurs over HPW12 under continuous NO feeding at 380 °C (with a rate of 5.9 x 10 -16 µmol/gHPW12/s after 60 min on stream, with 2.5 g of HPW12 and a reaction feed consisting of NO (~ 2000 ppm) in Ar(13%)/He (50 mLNTP/min), i.e. a WHSV of 3.2 x 10 -3 g/gHPW12/h). The evolution of this NO decomposition activity with time on stream is further discussed in section 3.4.

In period (ii) of Figure 1, the initial peak observed for the N2/Ar trace (Figure 1b), which was attributed earlier to the evacuation of traces of air from the dead volume of the reactor, was also observed for the O2/Ar trace (Figure 1c). Afterwards, however, while the N2/Ar trace increased from 5 to 13 min on stream, and then slowly decreased until the end of the experiment, reaching a final signal being however above that measured in period (i) (Figure 1b), the O2/Ar trace first decreased until 8 min on stream, and then increased until the end of the experiment (rapidly from 8 to 15 min on stream, then more slowly from 15 min on stream to the end), reaching a final signal being slightly below that measured in period (i) (Figure 1c). This comparison suggests that, while N2 was produced over N-HPW12 by NO decomposition, traces of O2 contained in the NO feed and/or O2 produced together with N2 over N-HPW12 by NO decomposition were getting adsorbed/not getting desorbed onto/from the Keggin units.

The link between the latter phenomenon and the decreasing activity of N-HPW12 in Figure 1a-b will be made in section 3.4.2. Non-catalytic NO oxidation into NO2 with traces of O2 contained in the NO feed or O2 produced from the NO decomposition process over N-HPW12 must be negligible, as the concentration of NO converted (30 ppm at the end of the experiment, Figure 1a) was close to that expected from the concentration of N2 produced (18 ppm at the end of the experiment, Figure 1b).

In the presence of O2 in the reaction feed

Figure 2 shows the NO and NO2 concentrations recorded by the gas-IR analyzer in the NOx decomposition experiment over N-HPW12 at 380 °C in the presence of O2 in the reaction feed (with panels a and b showing the same data over two different time scales, for the sake of clarity). In period (i), the reaction feed consisting of NO (~ 2000 ppm) in O2(5%)-Ar(13%)/He (50 mLNTP/min) was analyzed for 5 min without being contacted with the sample (upon flowing exclusively through lines at RT). It was found to contain 1770 ppm of NO and 194 ppm of NO2 (Figure 2a). Meanwhile, the sample was kept under static NO-Ar/He (from the O2-free reaction feed used in the preceding step of the experiment, Figure 1). In period (ii), the reaction feed was continuously contacted with the sample at 380 °C, and the outlet gas flow was analyzed over time on stream. The total NOx concentration in the outlet gas flow in period (ii) remained lower than that in the feed in period (i) for about 300 min on stream. Given that N2/Ar trace remained essentially unchanged after contacting the reaction feed with the sample (not shown), as also reported for the blank experiment over quartz wool (Figure S3b-c), NOx species did not decompose into N2, but were retained by the N-HPW12 sample in these first 300 min on stream (Figure 2). The NOx species retained by the sample consisted mostly of NO2, as indicated by the comparison of the concentrations of NO and NO2 in the first 300 min on stream with those after 300 min on stream (Figure 2b), once the total NOx concentration had finally reached the same level as in the feed (no NOx species were retained anymore by the N-HPW12 sample).

In the latter period, after 300 min on stream, the final NO/(NO+NO2) and NO2/(NO+NO2) concentration ratios (1718/1964 = 0.875 and 246/1964 = 0.125, respectively, as measured in several repeat experiments) were slightly higher and lower, respectively, than the reference ratios observed over quartz wool at 380 °C (1675/1947 = 0.86 and 272/1947 = 0.14, respectively, Figure S3a), with the reference ratios being dictated by the non-catalytic oxidation of NO to NO2. This suggests that N-HPW12 catalyzed the conversion of some NO2 back to NO (whereas at RT, N-HPW12 was found to catalyze the oxidation of NO, Figure 3). Indeed, at 380 °C, even if their Keggin structure is maintained, [START_REF] Kozhevnikov | Sustainable heterogeneous acid catalysis by heteropoly acids[END_REF] the heteropolyanions may be deficient in structural oxygen atoms, and therefore may act as reducing agents towards NO2. Actually, the filling of their O vacancies along with the reduction of NO2 might explain the inactivity of N-HPW12 in the direct decomposition of NOx in the present experiment, whereas this sample was found to be active in this reaction in the absence of O2 in the feed (Figure 1). In collaboration with the protons known to activate NO and NO2 to form NOH + and HNO2 + species, respectively, [START_REF] Chen | Activation of nitric oxide by heteropoly compounds: Structure of nitricoxide linkages in tungstophosphoric acid with Keggin units[END_REF][START_REF] Yang | A new approach to decomposition of nitric oxide using sorbent/catalyst without reducing gas: Use of heteropoly compounds[END_REF] the lacunary heteropolyanions might be the reductants required to finally decompose NO into N2. A direct filling of the O vacancies by O2 might then also contribute to the deactivation of N-HPW12, together with a possible poisoning of the protons by NO2 to form stable HNO2 + species preventing NO from being activated. The deactivation of N-HPW12 after exposure to the O2-containing reaction feed at 380 °C was found to be irreversible as no NOx decomposition could be recorded when removing O2 from the reaction feed (not shown).

Pre-saturation of H3PW12O40 (HPW12) with NOx species to yield the P-HPW12 sample

In the second NOx decomposition experiment performed in the present work, HPW12.6H2O was first exposed to NO (~ 2000 ppm) in O2(5%)-Ar(13%)/He (50 mLNTP/min) at RT before being heated under Ar(8%)/He (50 mLNTP/min) to the reaction temperature 380 °C (3 °C/min). The sample obtained at the end of the pre-exposure step at RT is labelled as "P-HPW12" (P standing for NOx-pre-saturated). The NO and NO2 concentrations recorded by the gas-IR analyzer during the NOx-pre-exposure step are shown in Figure 3 (with panels a and b showing the same data over two different time scales, for the sake of clarity). In period (i), the NO-O2 feed was analyzed without being contacted with the sample. It was found to contain 1732 ppm of NO and 177 ppm of NO2 (Figure 3a), due to the non-catalytic conversion of NO to NO2 in the stainless-steel feeding lines. In period (ii), the feed was contacted with the sample, and the outlet gas flow was analyzed over time on stream. The sum of the NO and NO2 concentrations remained below its initial level of period (i) for 4.5 days on stream (Figure 3b). The missing NOx (NO+NO2) concentration in the outlet gas flow was integrated over the whole 4.5 days on stream, by roughly interpolating the NOx concentration in the periods of missing data (i.e. night and weekend periods during which the cryo-detector of the analyzer could not be filled with liquid N2). This yielded a value of 0.0020 mol of NOx retained by the sample, that corresponds to 81% of the total amount of protons (0.0024 mol) contained in the sample (2.5 g). Given that surface protons represent only 4% of the total number of protons of HPW12, [START_REF] Schnee | Lifetime of the H3PW12O40 heteropolyacid in the methanol-to-DME process: A question of pre-treatment[END_REF] the amount of NOx stored in the present experiment as NOH + or HNO2 + species [START_REF] Herring | In Situ infrared study of the absorption of nitric oxide by 12tungstophosphoric acid[END_REF][START_REF] Weng | DRIFT studies on promotion mechanism of H3PW12O40 in selective catalytic reduction of NO with NH3[END_REF] (81% of the protons) demonstrates that NOx had largely entered the bulk of the P-HPW12 crystals, which confirms the great potential of Keggin HPAs as NOx absorber materials, as pointed out in the introduction section. [START_REF] Chen | Activation of nitric oxide by heteropoly compounds: Structure of nitricoxide linkages in tungstophosphoric acid with Keggin units[END_REF] Thereby, the structural water was progressively evacuated from the P-HPW12 crystals during the pre-exposure to NOx at RT (Figure S4). This explains why, upon heating the P-HPW12 sample afterwards from RT to 380 °C (3 °C/min) under Ar(8%)/He (50 mLNTP/min), only 25% of the total amount of structural water theoretically contained in HPW12.6H2O (2.5 g) was released (Figure S1b). It can therefore be concluded that the pre-exposure of the HPW12 sample to NOx at RT had already evacuated 75% of the structural water.

At the very end of period (ii), when the total NOx concentration in the outlet gas flow was equal to that in the feed (1909 ppm in period (i)), indicating that NOx absorption by P-HPW12 was completed, the concentrations of NO and NO2 were significantly lower and higher, respectively, than those in the feed (1174 vs. 1732 ppm and 736 vs. 177 ppm, respectively). This is attributed to the catalytic oxidation of NO into NO2 at the surface of P-HPW12. [START_REF] Herring | In Situ infrared study of the absorption of nitric oxide by 12tungstophosphoric acid[END_REF] The total amounts of NO and NO2 retained by the sample can be assumed to be similar, as upon heating the P-HPW12 sample afterwards from RT to 380 °C (3 °C/min) under Ar(8%)/He (50 mLNTP/min), nearly the same amounts of NO and NO2 were released (7.42 x 10 -5 mol of NO vs. 7.61 x 10 -5 mol of NO2, Figure S5a). Notice that the total amount of NOx released (1.503 x 10 -4 mol) upon heating the P-HPW12 sample from RT to 380 °C corresponds to only 7.5% of the 0.0020 mol of NOx initially retained at RT. Some N2 was also detected upon heating the P-HPW12 sample from RT to 380 °C (Figure S5b), but as the temperature ramp used was as slow as 3 °C/min, according to earlier literature reports, the corresponding N2 amount remained limited to about 5% of the pre-absorbed NOx species. [START_REF] Chen | Activation of nitric oxide by heteropoly compounds: Structure of nitricoxide linkages in tungstophosphoric acid with Keggin units[END_REF][START_REF] Yang | A new approach to decomposition of nitric oxide using sorbent/catalyst without reducing gas: Use of heteropoly compounds[END_REF] Once the temperature was stabilized at 380 °C, no N2 was formed anymore, showing that the pre-absorbed NOx species were unable to decompose at a constant temperature of 380 °C under inert atmosphere. Overall, it can be concluded that the sample still contained the majority of its pre-absorbed NOx species at the start of the experiment performed with continuously fed NOx at 380 °C.

Activity of NOx-pre-saturated H3PW12O40 (P-HPW12) in the direct decomposition of NOx

In the absence of O2 in the reaction feed -Elucidating the deactivating impact of NO2 on the activity of HPW12

Figure 4a shows the NO and NO2 concentrations recorded by the gas-IR analyzer as a function of time on stream in the NOx decomposition experiment over the P-HPW12 sample at 380 °C, in the absence of O2 in the reaction feed. In period (i), the reaction feed -NO (~ 2000 ppm) in Ar(13%)/Hewas analyzed for 5 min without being contacted with the sample (upon flowing exclusively through lines at RT). It was found to contain 1919 ppm of NO and 7 ppm of NO2. Meanwhile, the sample was kept under static Ar/He, the atmosphere under which the sample was previously heated from RT to 380 °C. In period (ii), the reaction feed was continuously contacted with the sample at 380 °C, and the outlet gas flow was analyzed over time on stream. Figure S6 shows the same concentrations over a longer time scale (600 min). Within the first 3 min of period (ii), the concentrations of NO and NO2 in the outlet gas flow rose up quickly to 2280 and 565 ppm, respectively, whereas they were found to drop in the equivalent experiment over N-HPW12 (Figure 1). This indicates that, during the preceding period (i), the static Ar/He atmosphere had been progressively enriched in NO and NO2 being released from the sample, until it was evacuated along with the introduction of the reaction feed. Then, following this initial NOx peak in period (ii), both the NO and NO2 concentrations decreased until reaching, at about 35 min on stream, stable levels of 1969 and 57 ppm, respectively. These concentrations were both 50 ppm higher than those in the feed recorded in period (i), indicating that the sample was continuously releasing 50 ppm of both NO and NO2, namely 4.4 times less NO and NO2 than released under Ar(8%)/He right before switching to period (i) (Figure S5a, 219 ppm of both NO and NO2 released at 184 min on stream). Thus, under reaction conditions, the NO feed essentially prevented the sample from releasing its preabsorbed NOx species. However, unlike in the experiment carried out over N-HPW12 in the absence of O2 in the feed (Figure 1), NO from the feed was not decomposed to N2, as the N2/Ar MS trace remained essentially unchanged from period (i) to period (ii) (Figure 4b). The absence of NOx decomposition activity of P-HPW12, whereas N-HPW12 was active under the same O2-free conditions, can only be attributed to the presence and/or the release of NO2 in the/out of the bulk of its crystals, which is the only significant difference between the experiments carried out on these two samples in the absence of O2 in the feed. As in the experiment performed over N-HPW12 in the presence of O2 in the feed, NO2 might have filled the O vacancies of the heteropolyanions, preventing them from playing their possible role as NO reductants, or might have poisoned the protons to form unreactive HNO2 + species, eventually preventing NO from being activated.

Following this first NOx decomposition experiment over P-HPW12 at 380 °C in the absence of O2 in the feed (Figure 4), the sample was purged under He (50 mLNTP/min) at 380 °C for 10 hours, before being exposed a second time to the same O2-free reaction feed for 3 hours (Figure S7). During this second exposure to the O2-free reaction feed, the concentrations of NO and NO2 continuously released from the sample were lower than in the first reaction step (Figure S7 vs. Figure 4). From about 50 min on stream, once the concentrations had stabilized, only 20 ppm of both NO and NO2 were released (whereas 50 ppm were released in the first experiment, Figure 4). This shows that the amount of NOx species released in the feed in the course of the experiment depends on the amount of NOx species stored in the sample, the latter amount being lower after the purge step during which a part of the pre-absorbed NOx species was released (not shown). As in the first experiment, the NOx (fed and pre-absorbed) were not found to decompose into N2 (not shown).

In the presence of O2 in the reaction feed -Elucidating the positive impact of O2 on the activity of HPW12

The P-HPW12 sample already used in the two latter experiments (section 3. 5c. In period (i), the reaction feed consisting of NO (~ 2000 ppm) in O2(5%)-Ar(13%)/He (50 mLNTP/min) was analyzed for 5 min without being contacted with the sample (upon flowing exclusively through lines at RT). It was found to contain 1736 ppm of NO and 181 ppm of NO2. Meanwhile, the sample was kept under static NO-Ar/He (from the O2-free reaction feed used in the preceding step of the experiment, Figure 4). In period (ii), the reaction feed was continuously contacted with the sample at 380 °C, and the outlet gas flow was analyzed over time on stream.

In period (ii) of Figure 5a-b, the total NOx concentration in the outlet gas flow first decreased, and then immediately increased to slightly above its level in the feed. Then, it decreased again up to about 20 min on stream, before remaining below its level in the feed until the end of the experiment. Thus, unlike in the absence of O2 in the feed (Figure 4a), NOx from the feed were retained by the P-HPW12 sample in the presence of O2 in the feed. The fact that the NO, NO2 and total NOx concentrations remained essentially constant from about 20 to 100 min on stream (Figure 5a-b) indicates that NOx were retained not only at the surface but also in the bulk of P-HPW12. Indeed, in the pre-saturation of P-HPW12 with NOx at RT, the NOx concentration was also constant for a significant period of time (Figure 3b), and this was demonstrated in section 3.2 to arise from bulk absorption, probably due to diffusion limitations inbetween the constitutive HPW12 units. [START_REF] Schnee | Lifetime of the H3PW12O40 heteropolyacid in the methanol-to-DME process: A question of pre-treatment[END_REF] In the present experiment, during the last 200 min on stream (Figure 5b), the total NOx concentration was relatively close to that in the feed (only 14 ppm lower, whereas it was 71 ppm lower at 100 min on stream), which means that the sample was still retaining In (b), for the sake of clarity, the same data as in (a) is shown over a longer time scale. In (i), the reaction feed was analyzed without being contacted with the sample (upon flowing exclusively through lines at RT). In (ii), the reaction feed was continuously contacted with the sample at 380 °C, and the outlet gas flow was analyzed over time on stream. The N2/Ar peak at around 5 min on stream has no analytical interest here. It is attributed to the release of air trapped in the valve that served for contacting the reaction feed with the sample. Its upper intensity was cut from the graph, as indicated by the rising arrow. This experiment with P-HPW12 was the third one over the same sample. The two first ones were carried out in the absence of O2 in the feed (Figure 4 and Figure S7, respectively). The P-HPW12 sample was previously heated from RT to 380 °C at 3 °C/min under Ar(8%)/He (50 mLNTP/min).

The fact that the presence of O2 in the feed induced a diffusion of NO2 into the bulk of P-HPW12 is attributed to the relatively high concentration of NO2 in the feed under these conditions (181 ppm in Figure 5a-b vs. 7 ppm in Figure 4a, periods (i)). The fact that, at the same time, it also enabled the diffusion of NO into the bulk may be assigned to two possible origins. Firstly, NO might have been brought up into the bulk of HPW12 concomitantly with NO2 via the formation of N2O3. [START_REF] Belanger | A comparative study of the adsorption and reaction of nitrogen oxides on 12-tungstophosphoric, 12-tungstosilicic, and 12-molybdophosphoric acids[END_REF] Secondly, O2 might have reacted with NO at the surface of the crystals to form intermediate NOx y-adspecies being easily protonated by the protons located in the near-surface bulk of HPW12 (thereby regenerating O2) and consequently leading to the absorption of NO into the bulk of HPW12. [START_REF] Herring | In Situ infrared study of the absorption of nitric oxide by 12tungstophosphoric acid[END_REF] Overall, the presence of O2 in the feed appears to be required for NO to form a species that is able to diffuse into the bulk of HPW12.

The diffusion of NOx into the bulk of P-HPW12 led to the decomposition of NOx from the feed and/or of NOx pre-absorbed in the bulk into N2. Indeed, in period (ii) of Figure 5c, following its initial peak centered at around 5 min on stream, attributed to the evacuation of traces of air trapped in the 4-way valve used for contacting the reaction feed with the sample, the N2/Ar trace stabilized at a higher level than that measured in period (i). As shown in Figure S8, the production of N2 remained stable for about 200 min on stream, before decreasing to the level measured in period (i) within the next 300 min.

According to µ-GC measurements (the error associated to which is estimated to be about 1.6%), about 5 ppm of N2 were produced per unit of time in the first 200 min on stream, which corresponds to 10 ppm of NOx decomposed per unit of time. From 200 to 500 min on stream, while the N2/Ar signal was decreasing (Figure S8), the NO, NO2, and total NOx concentrations were increasing (Figure 5b). This shows that the deactivation of P-HPW12 was directly related to the decreasing capacity of its bulk to further incorporate NOx species. To explain this finding, two non-exclusive hypotheses may be proposed. Firstly, once the NOx from the feed were no longer able to react over the deactivated bulk HPW12 units, they accumulated inside the bulk until saturating it. In this hypothesis, the HPW12 units may have been deactivated following the two non-exclusive mechanisms proposed earlier in the present work: 1) the O vacancies in the heteropolyanions getting filled by O2 and/or NO2, and/or 2) the protons associated with the heteropolyanions getting poisoned by NO2 as HNO2 + species. Secondly, once the NOx species pre-absorbed in the bulk had all been consumed by the decomposition reaction with NOx from the feed, the bulk was essentially emptied, and therefore no longer accessible. [START_REF] Schnee | Elucidating and exploiting the chemistry of Keggin heteropolyacids in the methanol-to-DME conversion: enabling the bulk reaction thanks to operando Raman[END_REF]

Comparison of the NOx decomposition activities of N-HPW12 and P-HPW12

Table 1 qualitatively summarizes the activities observed over the N-HPW12 and P-HPW12 samples in the direct decomposition of continuously fed NOx at 380 °C in the absence and in the presence of O2 in the reaction feed. In section 3.4.1, the role of pre-absorbed NOx species in the activity of P-HPW12 in the decomposition of NOx from the O2-containing reaction feed is discussed (by comparing the activities of N-HPW12 and P-HPW12 shown in Figure 2 vs. Figures 5 andS8, respectively). In section 3.4.2, the activity of N-HPW12 in the absence of O2 in the reaction feed (Figure 1) is compared to that of P-HPW12 in the presence of O2 in the reaction feed (Figures 5 andS8) in terms of NO conversion and stability over time on stream.

Table 1. Qualitative summary of the activities observed over the N-HPW12 and P-HPW12 samples (2.5 g, < 125 µm) in the direct decomposition of continuously fed NOx at 380 °C in the absence and presence of O2 (5%) in the reaction feed (Figures 12, 4-5 and S6 to S8). The reaction feed contained NO (~ 2000 ppm) in Ar(13%)/He (50 mLNTP/min total flow rate). The N-HPW12 and P-HPW12 samples were previously heated from RT to 380 °C at 10 °C/min under He and at 3°C/min under Ar(8%)/He, respectively (50 mLNTP/min). Unlike the N-HPW12 sample, the P-HPW12 sample was pre-saturated with NOx at RT (N and P standing for non-NOx-pre-saturated and NOx-pre-saturated, respectively). "Min" are min of reaction, not including the 5 min in which the reaction feed was analyzed without being contacted with the sample, namely period (i) in Figures 12, 4-5 and S6 to S8.

The positive impact of the pre-saturation of HPW12 with NOx species on its NOx decomposition activity in the presence of O2 in the reaction feed, and the balance between positive and deactivating impacts of O2

The second hypothesis proposed at the end of section 3.3.2 to account for the observed deactivation of P-HPW12, namely that a NOx-emptied HPW12 bulk is no longer accessible to NOx from the feed, arises from the fact that NOx were essentially unable to enter the bulk of the N-HPW12 sample at 380 °C despite the presence of O2 in the feed. This assumption is supported by the absence of plateau in the NO and NO2 concentration traces when the total NOx concentration was significantly below that in the feed (when NOx were retained by the sample), i.e. in the first 100 min on stream (Figure 2). Within this period, NOx were essentially adsorbed onto the surface of N-HPW12, with a higher rate than that observed in the case of P-HPW12 (Figure 2 vs. Figure 5), as the surface of the latter had been already partly covered with NOx species during the pre-saturation step at RT (Figure 2 vs. Figure 5). Thus, the presence of pre-absorbed NOx appears to be necessary for NO and NO2 to enter the bulk of HPW12 crystals at 380 °C, unlike at RT (Figure 3). This suggests that, rather than depending on the temperature as a source of energy to overcome the activation barrier of bulk absorption, the ability of NOx to enter the bulk depends on the temperature as a modulator of the balance between the strength of adsorption at the surface of the crystals and the activation energy required to enter the bulk. At 380 °C, the adsorption of NOx at the surface of N-HPW12 must be weaker than at RT, whereas the activation energy to enter its bulk is likely higher, due to the absence of structural water which requires the NOx to open the way in-between the bulk HPW12 units. This makes the diffusion of NOx into the bulk more difficult, unless the presence of pre-absorbed NOx species has already made the diffusion pathway in-between the bulk HPW12 units accessible, as in the case of P-HPW12.

In summary, the different NOx decomposition activities observed for the N-HPW12 and P-HPW12 samples at 380 °C in the presence of O2 in the feed (Figures 2 and5, respectively) resulted from a balance between the negative impact of O2 and/or NO2 deactivating the heteropolyanions and/or their associated protons, and the positive impact of O2 and/or NO2 initiating the diffusion of NO into the bulk of the HPW12 crystals. The latter positive impact took place to a noticeable extent only in the case of P-HPW12 (Figure 5), thanks to NOx species pre-absorbed in the bulk of HPW12 having made the diffusion pathway in-between the HPW12 units accessible to the NOx from the reaction feed. This explains that P-HPW12 exhibited a NOx decomposition activity in the presence of O2 in the reaction feed, whereas N-HPW12 was found to be inactive in this reaction under the same conditions.

Comparison of the NOx decomposition activity of N-HPW12 in the absence of O2 in the reaction feed to that of P-HPW12 in the presence of O2 in the reaction feed

The balance between the negative and positive impacts of O2 and/or NO2 described in section 3.4.1 was, however, still not sufficiently favorable for P-HPW12 to reach a NOx decomposition activity in the presence of O2 in the reaction feed (Figure 5) as high as that recorded on N-HPW12 in the absence of O2 in the reaction feed (Figure 1). At 60 min of reaction, 10 ppm of NOx were decomposed over P-HPW12 in the presence of O2 in the feed (Figures 5c andS8), whereas 30 ppm of NOx were decomposed over N-HPW12 in the absence of O2 (Figure 1). Nevertheless, P-HPW12 exhibited a stable NOx decomposition activity for as long as 200 min, thanks to the increased fraction of accessible HPW12 units whose deactivation was likely limited by NOx diffusion inside the bulk, [START_REF] Schnee | Lifetime of the H3PW12O40 heteropolyacid in the methanol-to-DME process: A question of pre-treatment[END_REF] whereas N-HPW12

showed a continuously decreasing activity in the considered 60 min of reaction. Indeed, even in the absence of O2 intentionally added into the feed, the latter still contained a few ppm of deactivating NO2 and traces of O2, as well as O2 that was produced along with the decomposition of NO over N-HPW12, which might have also led to the progressive filling of the O vacancies in the HPW12 units and thus to the deactivation of the sample. Such a progressive filling of the O vacancies might explain the progressive increase of the O2/Ar signal from 8 min on stream in Figure 1c (the Keggin units exhibiting less and less O vacancies to be filled as the decomposition reaction proceeded).

Identifying the active sites of HPW12 in the direct decomposition of NOx, and possible origins of their deactivation

Figure 6 shows the in situ DRIFT difference spectra at steady state of N-HPW12 at 380 °C under a 50 mLNTP/min feed of NO (~ 2000 ppm) in Ar(40%)/He which first contained no O2 (spectrum 1, NO) and subsequently contained 5% of O2 (spectrum 2, NO+O2). For the sake of clarity, panels a and b show the same spectra in two different spectral regions. Both difference spectra were obtained by subtraction of the last spectrum recorded at 380 °C under Ar(40%)/He before NO was added into the feed. N-HPW12 was previously heated from RT to 380 °C at 3 °C/min under Ar(40%)/He (50 mLNTP/min).

Both difference spectra show a positive band at 2261 cm -1 , and a broad negative band centered at 3230 cm -1 . In the presence of O2 in the feed (spectrum 2 in Figure 6), the area of these two bands is 4.8 and 1.3 times higher, respectively, than in the absence of O2 in the feed (spectrum 1 in Figure 6). In earlier studies about the absorption of NO and/or NO2 into the bulk of HPW12 at temperatures around 150 °C, the band at 2261 cm -1 was assigned to ν(N=O) stretches within both NOH + and HNO2 + species. [START_REF] Herring | In Situ infrared study of the absorption of nitric oxide by 12tungstophosphoric acid[END_REF][START_REF] Weng | DRIFT studies on promotion mechanism of H3PW12O40 in selective catalytic reduction of NO with NH3[END_REF] The negative band at 3230 cm -1 was neither shown nor commented in earlier literature studies. It is attributed here to the consumption of HPA protons upon NOH + and/or HNO2 + formation. The higher areas of the bands at 2261 and 3230 cm -1 in the presence of O2 in the feed can be attributed to two non-exclusive origins. Firstly, it may result from the 26 times higher concentration of NO2 in the O2-containing feed (181 ppm vs. 7 ppm in Figures 5a-b and4a, respectively), which led to an increase of the concentration of HNO2 + adspecies, that may have reached the near-surface bulk of the HPW12 crystals to a little extent. [START_REF] Belanger | A comparative study of the adsorption and reaction of nitrogen oxides on 12-tungstophosphoric, 12-tungstosilicic, and 12-molybdophosphoric acids[END_REF] Secondly, it may result from the formation of NOH + species in the near-surface bulk of HPW12, thanks to either NO2 as a carrier or O2 having reacted with NO at the surface to form intermediate NOx y- adspecies that are potentially required to initiate bulk absorption. 19 A similar study was performed on a HPW12 sample which was beforehand saturated with NO/NO2 at RT to provide a P-HPW12 sample, and then heated from RT to 380 °C at 3 °C/min under Ar(40%)/He (50 mLNTP/min). At 380 °C, the P-HPW12 sample was exposed to an O2-free feed of NO (~ 2000 ppm)

in Ar(40%)/He (50 mLNTP/min) (in situ DRIFT difference spectra shown in Figure S9, panel 1). The P-HPW12 sample was then purged under Ar(40%)/He (50 mLNTP/min) before exposure to a feed of NO (~ 2000 ppm) in O2(5%)-Ar(40%)/He (50 mLNTP/min). Figure 7 shows the in situ DRIFT difference spectra of P-HPW12 at 380 °C as a function of time of exposure to the O2-containing NO feed, in the spectral region of N=O stretches within NOH + and HNO2 + species. During the exposures to the two NO feeds at 380 °C, the difference spectra were obtained by subtraction of the last spectrum recorded at 380 °C under Ar(40%)/He before the respective NO feed was contacted with the P-HPW12 sample.

The difference spectra (d-spectra hereafter) recorded within the first 10 min under the O2-free NO feed (Figure S9, panel 1) do not show the appearance of any IR absorption contribution. The d-spectrum recorded after 30 min on stream shows the appearance of a slight negative band at 2261 cm -1 , which was found to remain essentially unchanged at longer exposures to the O2-free NO feed (d-spectra at 60 and 80 min in Figure S9, panel 1). In contrast, on the d-spectra recorded under the O2-containing NO feed (Figure 7), a sharp negative band was quickly observed at 2261 cm -1 (d-spectrum at 2 min). The area of this band then continuously decreased until the end of the experiment (d-spectra from 4 to 900 min, Figure 7). From 2 to 34 min on stream, it remained higher than that on the last d-spectrum recorded under the O2-free NO feed (Figure S9, panel 2 vs. panel 1). At longer exposures to the O2-containing NO feed (d-spectra from 60 min in Figure 7), the area of the band at 2261 cm -1 became lower than that on the last d-spectrum recorded under the O2-free NO feed (Figure S9, panel 1), without however reaching zero on the final spectrum at 900 min on stream.

The negative band at 2261 cm -1 indicates the disappearance of NOH + and/or HNO2 + species which had been stored in the bulk of the sample during its pre-exposure to NOx at RT, due to their desorption and/or reaction under the NO-O2 containing feed. The limited disappearance of pre-absorbed NOH + and/or HNO2 + species under the O2-free NO feed (Figure S9, panel 1) can be attributed to the slight desorption of such species in agreement with the release of NOx species recorded in Figure 4a and the absence of N2 formation (Figure 4b). In contrast, the sharp disappearance of pre-absorbed NOH + and/or HNO2 + species under the O2-containing feed (Figure 7) is rather attributed to their reaction as supported by the presence of N2 in the effluent of the corresponding NO decomposition experiment described in section 3.2 (Figure 5c). The latter experiment showed that the presence of O2 in the reaction feed allowed NOx from the latter feed to enter into the bulk of the sample, which led to NOx decomposition into N2. The present DRIFT results show that this decomposition process relied on both the NOx from the feed and the NOx species pre-absorbed in the bulk. The NOx from the feed reacted over the HPW12 units with the involvement of pre-absorbed NOH + species (NOgas + NOH + = N2 + O2 + H + , or NOgas + H + = NOH + followed by NOH + + NOH + = N2 + O2 + 2 H + ). The involvement of pre-absorbed HNO2 + species seems unlikely in view of the deactivating impact of NO2 demonstrated in section 3.3.2. Hence, the present data suggest that NOH + species are important intermediates in the direct decomposition of NOx over HPW12 at 380 °C, which thus relies on the protons of HPW12.

The fact that the band at 2261 cm -1 became less and less negative with increasing time on stream under the O2-containing NO feed (Figure 7) indicates that unreactive NOH + and/or HNO2 + species were formed/adsorbed on the HPW12 units along with the reaction of pre-absorbed NOH + species with NOx from the feed. This accumulation of unreactive NOH + and/or HNO2 + species may have occurred according to two non-exclusive mechanisms. Firstly, along with the consumption of NOH + species through their reaction with NOx from the feed, the regenerated protons might have been poisoned with NO2 by forming unreactive HNO2 + species. Secondly, considering that the lacunary heteropolyanions might act as reductants in the direct decomposition of NOx over HPW12, as proposed earlier in section

Conclusions

For the first time, the activity of H3PW12O40 (HPW12) in the direct decomposition of NOx was investigated under continuous NOx feeding, at 380 °C. From the recorded NO decomposition and in situ DRIFT data, the observed NOx decomposition activity of HPW12 was deduced to occur either through the reaction of two neighbouring NOH + adspecies, or through the reaction of one NOH + adspecies with gas phase NO. The presence of NO2 -either in the reaction feed due to the presence of O2 or in the bulk of the sample -was found to deactivate HPW12, likely because NO2 poisoned the protons by forming unreactive HNO2 + species. Nevertheless, the presence of O2 -in the feed and/or generated by the decomposition process -was also found to have a positive impact, namely to initiate the diffusion of NOx into the bulk of the HPW12 crystals. However, this diffusion process effectively took place only when the bulk of HPW12 already contained NOx beforehand pre-absorbed at RT, i.e. when the HPW12 crystals were still hydrated. Indeed, at 380 °C, the pre-absorbed NOx species preserved the diffusion pathway in-between the bulk HPW12 units and therefore allowed the diffusion of the NOx from the feed and their reaction with NOH + species. In the presence of O2 in the feed, the bulk-type reaction allowed the otherwise inactive HPW12 material to remain active for 8 hours. By providing the key parameters involved in the NOx decomposition process over HPW12 at 380 °C, the present work paves the way for future investigations in order to further improve the lifetime and activity of HPW12. Future investigations may focus, for instance, on tuning the chemical composition of the surface and/or bulk heteropolyanions in order to make the HPA more resistant to poisoning by O2 and/or NO2.

Supporting information

Structural water desorption profiles upon heating the two HPW12 samples from RT to the reaction temperature 380 °C under inert atmosphere; Full in situ DRIFT absorbance spectra of the two HPW12

Figure 1 .

 1 Figure 1. NO and NO2 concentrations recorded by the MKS MultiGas 2030 IR analyzer (a) and mass to charge (m/z) ratios 28 and 32 associated with N2 and O2, respectively, normalized by the m/z 40 one associated with Ar recorded by the MS (b-c) as a function of time on stream in the NOx decomposition experiment with N-HPW12 (2.5 g, < 125 µm, N standing for non-NOxpre-saturated) at 380 °C and a reaction feed consisting of NO (~ 2000 ppm) in Ar(13%)/He (50 mLNTP/min). In (i), the reaction feed was analyzed without being contacted with the sample (upon flowing exclusively through lines at RT). In (ii), the reaction feed was continuously contacted with the sample at 380 °C, and the outlet gas flow was analyzed over time on stream. N-HPW12 was previously heated from RT to 380 °C at 10 °C/min under He (50 mLNTP/min).

Figure 2 .

 2 Figure 2. NO and NO2 concentrations recorded by the MKS MultiGas 2030 IR analyzer as a function of time on stream in the NOx decomposition experiment with N-HPW12 (2.5 g, < 125 µm, N standing for non-NOx-pre-saturated) at 380 °C and a reaction feed consisting of NO (~ 2000 ppm) in O2(5%)-Ar(13%)/He (50 mLNTP/min). In (b), for the sake of clarity, the same data as in (a) is shown over a longer time scale. In (i), the reaction feed was analyzed without being contacted with the sample (upon flowing exclusively through lines at RT). In (ii), the reaction feed was continuously contacted with the sample at 380 °C, and the outlet gas flow was analyzed over time on stream. N-HPW12 was previously heated from RT to 380 °C at 10 °C/min under He (50 mLNTP/min).

Figure 3 .

 3 Figure 3. NO and NO2 concentrations recorded by the MKS MultiGas 2030 IR analyzer as a function of time on stream in the pre-exposure of HPW12 (2.5 g, < 125 µm) to a 50 mLNTP/min feed of NO (~ 2000 ppm) in O2(5%)-Ar(13%)/He at RT, to yield the P-HPW12 sample. In (i), the feed was analyzed without being contacted with the sample. In (ii), the feed was contacted with the sample, and the outlet gas flow was analyzed over time on stream. (b) shows the same traces as (a), but over a longer time scale. The periods marked with crosses (xxx) correspond to night and/or weekend periods in which the cryo-detector of the IR analyzer could not be filled with liquid N2.

Figure 4 .

 4 Figure 4. NO and NO2 concentrations recorded by the MKS MultiGas 2030 IR analyzer (a) and mass to charge (m/z) ratio 28 associated with N2 normalized by the m/z 40 one associated with Ar recorded by the MS (b) vs. time on stream in the NOx decomposition experiment with P-HPW12 (2.5 g, < 125 µm, P standing for NOx-pre-saturated) at 380 °C and a reaction feed consisting of NO (~ 2000 ppm) in Ar(13%)/He (50 mLNTP/min).In (i), the reaction feed was analyzed without being contacted with the sample (upon flowing exclusively through lines at RT). In (ii), the reaction feed was continuously contacted with the sample at 380 °C, and the outlet gas flow was analyzed over time on stream. The N2/Ar peak at around 5 min on stream has no analytical interest here. It is attributed to the release of air trapped in the valve that served for contacting the reaction feed with the sample. Its upper intensity was cut from the graph, as indicated by the rising arrow. The P-HPW12 sample was previously heated from RT to 380 °C at 3 °C/min under Ar(8%)/He (50 mLNTP/min).

  3.1) was finally submitted to a NOx decomposition experiment at 380 °C in the presence of O2 in the reaction feed. The NO and NO2 concentrations recorded by the gas-IR analyzer as a function of time on stream are shown in Figure 5a-b (with panels a and b showing the same data over two different time scales, for the sake of clarity), whereas the N2/Ar trace recorded by the MS is shown in Figure

  only a limited amount of NOx from the feed. By comparing the individual NO and NO2 concentrations in that final period with those in the period from 20 to 100 min on stream (1670 vs. 1643 ppm of NO, and 233 vs. 207 ppm of NO2), it can be concluded that, in the period from 20 to 100 min on stream, both NO and NO2 were absorbed by the sample in similar proportions (27 ppm of NO absorbed per unit of time vs. 26 ppm of NO2).

Figure 5 .

 5 Figure 5. NO and NO2 concentrations recorded by the MKS MultiGas 2030 IR analyzer (a-b) and mass to charge (m/z) ratio 28 associated with N2 normalized by the m/z 40 one associated with Ar recorded by the MS (c) as a function of time on stream in the NOx decomposition experiment with the P-HPW12 sample (2.5 g, < 125 µm, P standing for NOx-pre-saturated) at 380 °C and a reaction feed consisting of NO (~ 2000 ppm) in O2(5%)-Ar(13%)/He (50 mLNTP/min).In (b), for the sake of clarity, the same data as in (a) is shown over a longer time scale. In (i), the reaction feed was analyzed without being contacted with the sample (upon flowing exclusively through lines at RT). In (ii), the reaction feed was continuously contacted with the sample at 380 °C, and the outlet gas flow was analyzed over time on stream. The N2/Ar peak at around 5 min on stream has no analytical interest here. It is attributed to the release of air trapped in the valve that served for contacting the reaction feed with the sample. Its upper intensity was cut from the graph, as indicated by the rising arrow. This experiment with P-HPW12 was the third one over the same sample. The two first ones were carried out in the absence of O2 in the feed (Figure4and FigureS7, respectively). The P-HPW12 sample was previously heated from RT to 380 °C at 3 °C/min under Ar(8%)/He (50 mLNTP/min).

Figure 6 .

 6 Figure 6. In situ DRIFT difference spectra of N-HPW12 (N standing for non-NOx-pre-saturated) at 380 °C under a feed of NO (~ 2000 ppm) in Ar(40%)/He (50 mLNTP/min) 1) since 12 h and 2) 8 h after adding O2 (5%) into the feed, in the spectral regions of (a) structural O-H + stretches and (b) N=O stretches within NOH + and HNO2 + species. N-HPW12 was heated from RT to 380 °C at 3 °C/min under Ar(40%)/He (50 mLNTP/min). Both difference spectra were obtained by subtraction of the last spectrum recorded at 380 °C under Ar(40%)/He before NO was added into the feed. Log (1/R') is expressed in arbitrary units, with the relative reflectance R ' = Isample / I(reference sample).

3. 1 . 2 ,

 12 NO2 and/or O2 might have oxidized the heteropolyanions, thereby making them non-reductive and thus inactive in the direct decomposition of NOx from the feed, and leading to the accumulation of NOH + species which were no longer able to react. Through the first and/or the second mechanism, this accumulation of unreactive species finally contributed to the deactivation of P-HPW12 observed in section 3.3.2 (Figures5a-b and S8).

Figure 7 .

 7 Figure 7. In situ DRIFT difference spectra of P-HPW12 (P standing for NOx-pre-saturated) at 380 °C vs. time of exposure to a feed of NO (~ 2000 ppm) in O2(5%)-Ar(40%)/He (50 mLNTP/min), in the spectral region of N=O stretches within NOH + and HNO2 + species. P-HPW12 was heated from RT to 380 °C at 3 °C/min under Ar(40%)/He (50 mLNTP/min). All difference spectra were obtained by subtraction of the last spectrum at 380 °C under Ar(40%)/He measured before NO and O2 were added into the feed. Log (1/R') is expressed in arbitrary units, with the relative reflectance R ' = Isample / I(reference sample).

  samples under various conditions; NO and NO2 concentrations and N2/Ar trace recorded vs. time on stream in the blank NO decomposition experiments with only quartz wool in the reactor; H2O concentration recorded vs. time on stream during the pre-saturation of P-HPW12 with NOx at RT; NO and NO2 concentrations and N2/Ar trace recorded vs. time on stream upon heating P-HPW12 from RT to the reaction temperature 380 °C under inert atmosphere; Comparison of the NO and NO2 concentrations recorded vs. time on stream in the two successive NO decomposition experiments over P-HPW12 at 380 °C in the absence of O2 in the feed; N2/Ar trace recorded vs. time on stream in the NO decomposition experiment over P-HPW12 at 380 °C in the presence of O2 in the feed, over a longer time scale (600 min) than shown in the manuscript (30 min); Detailed comparison of the in situ DRIFT difference spectra in the region of N=O stretches within NOH + and HNO2 + species measured in the experiments over P-HPW12 at 380 °C in the absence vs. presence of O2 in the feed.

Figure S2 .

 S2 Figure S2. In situ DRIFT spectra of (a,c) N-HPW12 and (b,d) P-HPW12, at (a-b) RT and (c-d) 380 °C, under 50 mLNTP/min of (a,c-d) Ar(40%)/He and (b) NO (~ 2000 ppm) in O2(5%)-Ar(40%)/He (since 48 h, i.e. at the end of the pre-treatment yielding the P-HPW12 sample). Spectra (c) and (d) were measured after heating the samples from RT to 380 °C at 3 °C/min under Ar(40%)/He. "Abs." means absorbance, and it is expressed in arbitrary units. "O-Hw" means O-H within water. The band marked with a star (*) on spectra (a), (c) and (d) is not attributed in the literature, to our knowledge. Within the fingerprint of the Keggin structure, due to a saturation effect, the bands are shifted to higher wavenumbers as compared to their equivalents on the spectra of HPW12 diluted (20 wt.%) in diamond powder measured in the same conditions (not shown). In agreement with the literature (Janik, M. J.; Campbell, K. A.; Bardin, B. B.; Davis, R. J.; Neurock, M., Applied Catalysis A: General 2003, 256 (1-2), 51-68), the latter bands are positioned at 1080, 980, 885 and 790 cm -1 , respectively. Log (1/R) is expressed in arbitrary units, with the reflectance R = Isample / IKBr. The weak bands at 1300 and 1405 cm -1 on spectra (b) and (c) are attributed to bidentate nitrate and nitrite species, respectively (Weng, X.; Dai, X.; Zeng, Q.; Liu, Y.; Wu, Z., Journal of Colloid and Interface Science 2016, 461, 9-14).

Figure S3 .

 S3 Figure S3. NO and NO2 concentrations recorded by the MKS MultiGas 2030 IR analyzer (a) and mass to charge (m/z) ratio 28 associated with N2 normalized by the m/z 40 one associated with Ar recorded by the MS (b-c) as a function of time on stream in the blank NOx decomposition experiment with only quartz wool in the reactor (no catalyst) at 380 °C and a reaction feed of NO (~ 2000 ppm) in O2(5%)-Ar(13%)/He (50 mLNTP/min). In (b) and (c), the same N2/Ar trace is shown with two different y axis scales. In (i), the reaction feed was analyzed without flowing through the reactor (upon flowing exclusively through lines at RT). In (ii), the reaction feed was flowing through the reactor at 380 °C, and the outlet gas flow was analyzed over time-on-stream. In (a), the negative peaks centered at 10 min are attributed to the purge of the dead volume of the reactor upon introduction of the reaction feed. In (b) and (c), the N2/Ar peak centered at 8 min is attributed to the release of air from the dead volume of the reactor, and/or to the introduction of air trapped in the valve that was turned for contacting the reaction feed with the sample. In (c), the upper intensity of this peak was cut from the graph, as indicated by the rising arrow.

Figure S5 .

 S5 Figure S5. (a) NO and NO2 concentrations and temperature trace recorded by the MKS MultiGas 2030 IR analyzer as a function of time on stream upon heating P-HPW12 (prepared by the treatment monitored in Figures 2 and S4) from RT to 380 °C at 3 °C/min under Ar(8%)/He (50 mLNTP/min). The areas under the NO and NO2 traces correspond to 7.42 x 10 -5 mol of NO and 7.61 x 10 -5 mol of NO2, respectively. (b) In the same experiment, temperature trace recorded by the MKS MultiGas 2030 IR analyzer and mass to charge (m/z) ratio 28 associated with N2 normalized by the m/z 40 one associated with Ar recorded by the MS.

Figure S6 .

 S6 Figure S6. Same traces as those in Figure 4, but over a longer time scale -NO and NO2 concentrations recorded by the MKS MultiGas 2030 IR analyzer vs. time on stream in the NOx decomposition experiment with P-HPW12 (2.5 g, < 125 µm) at 380 °C and a reaction feed consisting of NO (~ 2000 ppm) in Ar(13%)/He (50 mLNTP/min).In (i), the reaction feed was analyzed without being contacted with the sample (upon flowing exclusively through lines at RT). In (ii), the reaction feed was continuously contacted with the sample at 380 °C, and the outlet gas flow was analyzed over time on stream. P-HPW12 was heated from RT to 380 °C at 3 °C/min under Ar(8%)/He (50 mLNTP/min).

Figure S7 .

 S7 Figure S7. NO and NO2 concentrations recorded by the MKS MultiGas 2030 IR analyzer vs. time on stream in the second NOx decomposition experiment with P-HPW12 (2.5 g, < 125 µm) at 380 °C and a reaction feed consisting of NO (~ 2000 ppm) inAr(13%)/He (50 mLNTP/min). In (i), the reaction feed was analyzed without being contacted with the sample (upon flowing exclusively through lines at RT). In (ii), the reaction feed was continuously contacted with the sample at 380 °C, and the outlet gas flow was analyzed over time on stream. This experiment was launched after P-HPW12 had been purged for 10 h under He at 380 °C (50 mLNTP/min) following the first NOx decomposition experiment monitored in FigureS6.

Acknowledgements

This work has benefited from a funding managed by the French National Research Agency (DECOMPNOx project: ANR-18-CE07-0002). The authors acknowledge Jean-Marc Krafft (Sorbonne Université, CNRS, Laboratoire de Réactivité de Surface) for the technical support in in situ DRIFT experiments.

Author information

Corresponding authors *E-mails: josefine.schnee@upmc.fr, josischnee@hotmail.com, cyril.thomas@upmc.fr.

ORCID numbers

Josefine Schnee: 0000-0001-8521-9922 Laurent Delannoy: 0000-0002-3263-5065 Guylène Costentin: 0000-0003-1559-6890 Cyril Thomas: 0000-0003-4224-6095

Competing interests

The authors declare no competing interests. In (i), the reaction feed was analyzed without being contacted with the sample (upon flowing exclusively through lines at RT). In (ii), the reaction feed was continuously contacted with the sample at 380 °C, and the outlet gas flow was analyzed over time-on-stream. P-HPW12 was heated from RT to 380 °C at 3 °C/min under Ar(8%)/He (50 mLNTP/min). Except the first one attributed to the release of air trapped in the valve that served for contacting the reaction feed with the sample, the peaks marked with arrows are artifacts related to the simultaneously ongoing µ-GC analysis.

Supplementary information

Figure S9.

In situ DRIFT difference spectra of P-HPW12 at 380 °C vs. time of exposure to a feed of 1) NO (~ 2000 ppm) in Ar(40%)/He (50 mLNTP/min) and subsequently 2) NO (~ 2000 ppm) in O2(5%)-Ar(40%)/He (50 mLNTP/min), in the spectral region of N=O stretches within NOH + and HNO2 + species. P-HPW12 was heated from RT to 380 °C at 3 °C/min under Ar(40%)/He (50 mLNTP/min). The difference spectra were obtained by subtraction of the last spectrum at 380 °C under Ar(40%)/He measured before the reaction feed -NO without O2 in step 1, vs. NO with O2 in step 2 -was contacted with the P-HPW12 sample. The low signal to noise ratio of the spectra recorded in the absence of O2 in the feed is attributed to the presence of water in the atmosphere of the spectrometer due to a reduced purge. Log (1/R') is expressed in arbitrary units, with the relative reflectance R ' = Isample / I(reference sample).


M Christie

H.-Y Wu

T.-Y Li

Editors V Gandhi

Qiu-Jie Wu

Chih-Hsuan Kuo

Hui-Yin Wu

Tsai-Yen Li

Designing an Adaptive Assisting Interface for Learning Virtual Filmmaking

Keywords: CCS Concepts, Human-centered computing → Interaction design process and methods, User studies, • Computing methodologies → Virtual reality, • Applied computing → Media arts

In this paper, we present an adaptive assisting interface for learning virtual filmmaking. The design of the system is based on the scaffolding theory, to provide timely guidance to the user in the form of visual and audio messages that are adapted to each person's skill level and performance. The system was developed on an existing virtual filmmaking setup. We conducted a study with 24 participants, who were asked to operate the film set with or without our adaptive assisting interface. Results suggest that our system can provide users with a better learning experience and positive knowledge harvest.

Introduction

Film cinematography has the function of eliciting emotions from the audience, which influences not only how scripts are written, but also how the director chooses to direct the movie. The techniques applied by the cinematographer can lead to a wide variety of interpretations of the same story. All of these show the role of cinematography in telling the story of the film. It is important for the users studying film production to be able to experiment with different ways of filming and observe the effects on the audience.

To improve one's skills, we believe that practice makes perfect. However, typically, building a film set with real props and actors is costly and not readily available to most novice filmmakers. Recently, virtual reality has been deployed to develop a virtual film set [LLGC18] that allows users to take on the role of the cinematographer, director, or editor, thus allowing trainees and students to experience the process of filmmaking at a lower cost, and with minimal professional equipment and personnel. Unfortunately, virtual reality technologies introduce a high technical barrier for those with little to no experience in 3D virtual environments. In this work, we design an innovative adapting assisting interface to help the user learn to operate a virtual film set. Our primary goal is to assist the users in familiarizing themselves with the virtual set without the need for in-person mentoring. A secondary goal is to offer nonexpert users the opportunities to make a head start on cinematography.

To this end, we refer to the scaffolding theory, introduced in the late 1950s by Jerome Bruner. The term "scaffolding" represents the helpful interactions that enable the students to do something beyond his or her independent efforts [START_REF] Slotta | The cambridge handbook of the learning sciences (r. keith sawyer[END_REF]. Based on this concept, we designed an adaptive assisting learning interface that builds on an existing virtual film set, which tutors the users through visual and audio messages. The user is assigned an initial skill level at the beginning of the tutorial. As they advance through the hands-on tutorial, the system automatically adjusts the level of help according to the user's performance. We carried out a user study to evaluate the assisting interface and scaffolding mechanism that we have designed, the results of which are reported in this paper.

The paper is organized as follows. First, the state of the art on the scaffolding theory and virtual camera control is presented. We then show how the adaptive assistant is constructed, and our system architecture. This is followed by the description and results of a study in which we invited users to evaluate our system. Finally, we summarise the findings and limitations and offer perspectives on future work.

Related Work

In this section, we provide an overview of research on the scaffolding theory and the camera control methods for 3D environments that concern the design and operation of the virtual film set.

Intelligent tutoring systems and virtual training

For many years, virtual reality (VR) has been expected to bring on the next evolution in learning technologies, introducing sev-eral advantages, such as low cost, high comfort, and unlimited distance [PIB * 16, RBB * 00, You98]. In a recent work on adaptive technologies in virtual reality training [VGD16], data from trainee assessment was used as an input to an autonomous system for customized training and automated difficulty level adjustment to meet individual needs.

Scaffolding Theory

Scaffolding theory was first introduced in the late 1950s by Jerome Bruner, a cognitive psychologist. Scaffolding represents the helpful interactions that enable students to do something beyond his or her independent efforts [START_REF] Slotta | The cambridge handbook of the learning sciences (r. keith sawyer[END_REF]. Instructional scaffolding is the support given to a student by an instructor throughout the learning process. This support is specifically tailored to each student such that the "scaffolds" are incrementally removed when the students acquire the knowledge or skills under the learning target. This instructional approach allows students to experience studentcentered learning, which tends to facilitate more efficient learning than teacher-centered learning.

In the learning process, the educators may question a student's approach to a difficult problem and provide constructive feedback. The type and amount of support needed are dependent on the needs of the students during the time of instruction [VL14]. Scaffolds are developed to assist students with a difficult task [SB02]. The key is that the assistance needs to be planned.

Scaffolding is also used to support problem-based learning (PBL), an instructional approach in which students are presented with an ill-structured problem [START_REF] Ali | Problem based learning: A student-centered approach[END_REF]. After being presented with the problem, defining it, and generating learning issues, students proceed to address their learning issues, and then develop a potential solution, backed with evidence. There is some evidence that PBL can be effective when students work individually, leading to strong learning outcomes. Many educators incorporate PBL in their classrooms to engage students and help them become better problem solvers. However, educators must first identify the content that needs scaffolding support, the appropriate time to implement the support, and determine when the scaffold can be removed [Laj05].

The design of our interface was also inspired by game-based learning, which can help learners acquire real-world knowledge through a game-like environment. A study by Barzilai and Blau [BB14] suggests that presenting scaffolds may have "problematized" learners' understandings of the game by connecting them to disciplinary knowledge. Implications for the design of scaffolds for game-based learning are discussed. Zook et al. [DZO * 13] also demonstrate how a rule-based intelligent system can reduce the frequency of errors that novices make by providing information about rule violations without prescribing solutions, and discuss the role of error reduction in creativity support tools.

Camera Control Methods

In computer graphics and animations, autonomous camera control has been a challenge to find from infinite possibilities to film a set, solutions that fulfill geometrical, storytelling, and navigation For virtual cinematography [HCS96], Jhala and Young created the Darshak system that generates shot sequences to fulfill goals of showing specific actions and events in a 3D environment [JY11]. O'Neill et al. [START_REF] O'neill B | Towards intelligent authoring tools for machinima creation[END_REF] proposed a system of blockings that would choreograph the relative positions of actors and cameras in an environment. In these systems, context -the placement of characters and cameras in accordance with the story and the environmentis crucial, making camera control in 3D environments a complex problem.

The adoption of cinematic idioms for virtual camera control was first demonstrated through rule-and constraint-based systems [CAH * 96, BGL98], and through the representation of state transitions between various shot types [HCS96]. Ronfard et al. then proposed the Prose Storyboard language (PSL) [RGB13] as a formal representation for describing shots visually. PSL illustrates the spatial and temporal structure of movie frames with arbitrarily complex shots. For instance, each shot in a movie consists of a variety of sizes and angles, and the visual composition of every frame is defined as relative position and orientation by applying the prose storyboard language. The Director's Lens [LCRB11] was the first to introduce a mixed-initiative creative interface (MICI) with viewpoint suggestions and a tangible camera device to control static and dynamic cameras. Further along, optimization approaches [GRLC15] and film editing patterns [WPRC18] were proposed to ensure spatial and temporal continuity, and to apply common film idioms over long sequences of shots respectively.

System Framework

Virtual Film Set

In this section, we describe the design of our system. To understand our approach, we first briefly introduce the virtual film set that has been developed in previous work [LLGC18]. We then describe the design of the adaptive assisting interface for learning that is built on the virtual film set.

The architecture of our virtual film set, depicted in Figure 1, is composed of three main parts: (1) a navigation component to move around cameras and the monitor, (2) shot management components including a database, recommendation and editing functions, (3) the workspace allowing users to take on various filmmaking roles, and export the output to a video. The system was developed using Unity 3D and runs on HTC VIVE. While operating the virtual film set, a user simply puts on the headset and interacts with the set using the two hand controllers.

In this virtual film set, a user can take on the role of either the director, photographer, or editor. Each of these represents important roles in the filmmaking process. As a director, the user can decide the number of cameras that should be placed by observing the whole film set from the god's perspective and the storyboard; as a photographer, the user can determine where to place the cameras to have the desirable framing; as an editor, the user is expected to arrange the story by composing a sequence of shots from the various camera streams. In this work, the capabilities of the photographer are enhanced through the design of a recommendation component for static camera shots. The recommendation is based on a shot database built from the experience of filmmaking experts. We invited eight professional photographers to shoot a sequence on the reconstructed animation from a clip in Back to the Future, and analyzed their shot compositions on the keyframes. Finally, we added the results to the shot database to improve the quality of the shot recommendation function provided by our system.

Assisting Learning Interface

Although the goal of design the proposed virtual film set system is to help users learn or experience filmmaking through virtual reality, the user has to learn how to use the system first. Therefore, we have designed a user interface that can assist users in learning such a system. The assisting interface has three main modules: the user model, scaffolding tools, and tasks as shown in Figure 2. The system divides users into three levels: novice, skilled, and master, based on the user's self-evaluation and their understanding of the virtual film set. Different instructional tools are adopted in different tasks according to the context and difficulty of the task. We have designed several different types of tasks to illustrate how assisting learning interface can be designed to fit the need of the tasks. In our current system, three main tasks are available to the users: still photography, dynamic photography, and the use of shot recommendation as shown in Figure 3. Each task may be further divided into several subtasks (Table 1), which are smaller learning units that are easier for the users to master through practice.

When the user starts a task, the assisting learning interface will start timing. In the whole learning process, we have three important time points (denoted by t 1 , t 2 , t 3 in the following text).

If the time reaches t 1 , and the user has not started to do anything, the assisting learning interface will give the user a text hint, which will introduce the user to the task in detail again and instructions on how to do it.

After t 2 seconds, if the user still has not completed the task, the assisting learning interface will give the user a pointer hint to show which button to press next, the position of the button, and the function of the button.

If the user still has not completed the task by t 3 , the assisting learning interface will give the user an audio hint. Through a voice, the assisting learning interface will guide the user to complete the task as a teacher may do besides the student.

In our design, some hints appear in all tasks. According to the learning context and difficulty of the task, the assisting function is either enabled or optional. Currently, three hint functions (text, pointer, audio) are the basic ones for all tasks while the other hints are used to provide further scaffolds when needed. For example, the video hint is provided when the operations used in completing a task are more complex and cannot be described clearly with text. In this case, showing a demonstration video may help the user understand the specific operations that need to be carried out.

Besides, when the system wants to provide feedback to the user upon completion of an operation or task, the vibration function can be used. Further specific information about the use of hints in different tasks are given in Table 1 .

When a user enters our system to learn a specific task in the virtual film set, the system will give hints (scaffolds) to the user according to the task as well as the proficiency of the user. The hints appear progressively. For example, usually, a text hint appears first after t 1 seconds of delay. When the task consists of several operations, the pointer hint will then appear t 2 seconds after the text hint. If the user still cannot accomplish the task, the audio hint will follow after t 3 seconds of delay. For different tasks and different levels of users, these delay parameters will be given different values. For example, for users of the novice level, t 1 is equal to 0 seconds, which means that a text prompt will be given immediately when they start the task. For skilled and master users, t 1 is set to 4 and 6 seconds, respectively. In other words, the hints will be prompted at a later time for more proficient users as they may not need the scaffold to finish the task. Similarly, the ranges of t 2 and t 3 are 2 to 6 seconds and 4 to 8 seconds, respectively. These parameters are set by the designer according to their empirical experience and may vary slightly for different tasks.

Adaptive Mechanism

According to the principle of instructional scaffolding, the assisting mechanism provided to the users should be customized and adjusted dynamically as their proficiency improves. In our system, we have designed an adaptive mechanism to adjust the level of our assisting learning interface adaptively.

In the adaptive mechanism, the user can provide active feedback to the frequency of the hints such that the level of user proficiency can be adjusted immediately. In the assisting learning interface, we have designed a 'help' button and an 'annoying' button for the user to provide their feedback on the frequency of the hints. When the user encounters difficulties in using the system, he can press the 'help' button, and the system will give him a hint. When the user feels that there are too many prompts and feels 'irritated', he can press the 'annoying' button to reduce the frequency of prompts.

After a task is completed, the level of user proficiency will be adjusted if needed according to two criteria: (1) the time that the user took to complete the last task, and (2) the number of times the user provides feedback by pressing the 'help' and the 'annoying' buttons. We also studied the quality of the accomplished task of users and found that most of the user tasks are completed very well, so we decided to use their completion time as a more important ba-sis for judging their proficiency. The system records the time Tc that a user takes to complete a task and uses it to determine if the level needs to be adjusted. The decision is based on the standard completion times Ts for each level of users obtained according to experiments conducted in a pilot study. If Tc is closer to the Ts of a level than others, we will adjust the user's proficiency to this level accordingly. Different tasks for users of different levels of proficiency will have different values of Ts as the difficulty of a task can vary greatly, which affects the completion time.

In addition to the user's completion time, the times that the users press the 'help' and the 'annoying' buttons will also affect the adjustment of level. If the number of times that the 'annoying' button is pressed is over a threshold, the proficiency level will be raised by one level. On the other hand, if the help button is pressed too often, the level will be lowered. Among these two types of criteria, the active feedback provided by the user takes precedence over the completion time since we think it is more subjective and should take priority.

Experiments

To validate our system design and receive feedback on the effectiveness of our system, we designed an experiment in which recruited users were asked to carry out several tasks on the virtual film set under different conditions of our assisting learning interface. In this experiment, twenty-four participants (16 females and 8 males) with diverse backgrounds were recruited online. Their ages ranged from 18 to 32, and 9 of them had never experienced virtual reality before. We will detail the procedure and task design, and the experimental results in this section.

Procedure

Figure 5: Here we depict the main process of our experiment, which is composed of three tasks: (1) the basic controls task such as moving around and grabbing objects on the set, (2) still photography task of manipulating a static cameras, and (3) dynamic photography task of playing animations and tracking moving actors.

First of all, a pre-experiment survey of the experiences of the participants in cinematography and using VR related systems was carried out. At the beginning of the experiment, the participant would select a level that best described how familiar he or she was with the virtual film set. During the experiment, the participant was asked to finish three main tasks which would teach them how to film a sequence in the virtual film set. The main tasks were composed of knowledge about the basic controls, still photography, and dynamic photography, and each of which may consist of multiple sub-tasks.

Figure 6: The relationship between prior-knowledge and experiment time shows that prior-knowledge did not seem to affect the time it took to complete tasks.

As shown in Figure 5, the first task is for the participant to practice the basic controls in the virtual film set, including moving around, selecting items, and grabbing objects by using VIVE's controllers in the virtual environment. The next task is to learn how to use the still photography functions, such as adding, deleting, and moving a camera. Also, the participant is asked to try out the new shot recommendation function that is first introduced in this work. The last task is to instruct the participants on how to use dynamic photography functions, including playing the actors' animation and tracking a moving actor with a camera in the virtual film set. We adopted a between-subject design in our experiment and divided the participants into three groups. The first group was the control group that was only offered the step-by-step tutorial outside the virtual environment, which means that the participant would not see any instructions inside the virtual film set. The second group was the first experimental group that was provided assisting learning interface with a fixed setting. This means that the users in this group did not have access to the adaptive functions and their level of proficiency would remain at the initial setting and would not change throughout the experiment. The third group was the second experimental group that was provided with both the assisting learning interface as well as the adaptive mechanism described in the previous section.

Experimental Results

Below we analyze the results of the experiment on our system concerning the user's prior knowledge, the performance for each group, and finally the user feedback via a post-experiment survey.

Prior knowledge

In Table 2, we show the statistics of the three groups of participants on their prior knowledge (K), perceived difficulty levels (D), and the time they used in the experiment (T ). We used questionnaires to acquire prior knowledge and perceived difficulty. The questions about the prior knowledge include the level of understanding of VR, and basic knowledge of camera shooting.

We use a 10-point scale for the participants to rate, with 1 meaning the user has no prior knowledge and 9 meaning expert. The perceived difficulty of the tasks is based on the subjective evaluation of the difficulty of completing all three tasks on a scale of 1 to 20 points from easiest to most difficult. The null t-test is applied to the prior knowledge of the three groups of users and the result shows that there is no significant difference among them despite that their averages differ. We have also found that, for the three groups, the correlation coefficients for K vs. D are all below 0.4, which means that there is no strong relationship between the prior knowledge and perceived task difficulty.

The prior knowledge (K) and the experimental time (T ) for each participant are plotted in Figure 6. As one can observe, there is no direct relationship between prior knowledge and experimental time. The correlation coefficients between K and T for all three groups are below 0.4. Therefore, we believe that users' evaluation of tasks was not affected by their prior knowledge and should be credible.

Learning Assisting Function

As shown in Table 1, different tasks may adopt different types of hints. To see the effects of different hints in the assisting learning interface, we have asked the participants in the experimental groups (groups 2 and 3) to provide feedback on these hints with a questionnaire after the experiment is finished.

Table 3 shows the second and third groups of users' evaluations of the five types of hints provided by the assisting learning interface. The evaluation was composed of a list of questions ranked on the 5-point Likert scale with 1-5 meaning mostly disagree to mostly agree. According to Table 3, the average score of the five types of hints is 3.5, which indicates that the assisting learning interface is effective in general.

Among these types of hints, the participants seem to be most satisfied with the pointer hint and the vibration hint, and they think that these are effective for learning virtual film sets. The average score for the video hint is 3 points, which is lower than expected, which means that video hint is not very effective. According to the user's feedback after filling the questionnaire, the sound of the video seems to be too low, and the speed is too fast to clearly understand the content. This problem will be corrected in our future work.

We also found that the pointer hint received the highest score. The participants considered that this hint helped them effectively, especially in complex multi-step operations.

User Feedback

We asked the participants in the two experimental groups to rank their overall experience with the system on a Likert scale of 1 to 5 with 1 for most disagree and 5 for most agree on three statements:

(1) With the assisting learning interface, I can easily understand the operations of the virtual film set, (2) I would like to learn the virtual film set through this assisting learning interface, (3) I think the virtual tutoring functions can appear when I need them. Table 4 shows the average and standard deviation of the user responses. It is clear from the results that the third group responded more positively towards the statements. Also, the standard deviation of each question is lower than that of the second group. We think this indicates that the adaptive mechanism is an effective way of learning the operations of the virtual film set.

An additional question about the participants' opinion on the frequency that the hints appear was also asked at the end of the survey. The scale of 1 means too infrequent, and 5 means too often. As shown in Table 4, the average score for the second group is 3.5 points while it is 3.0 for the third group. The third group of participants found the frequency of hints to be most appropriate which shows that the adaptive mechanism is indeed effective.

Discussion

The main limitation of this work, which is also an ongoing challenge in the area of assisted creativity and smart learning tools, is to correctly identify the user's skill level and barriers, and provide guidance and tips that are personalized to their learning goals. Though similar systems have been proposed such as for computer programming [PZD * 18], there is very little work on smart tutoring systems for domains such as filmmaking that have both higher technical barriers and also artistic freedom, and evaluation of such systems is a great challenge.

Another aspect worth noting is the type of hints that are suitable for such an assisting interface. In our experiment, we have observed that pointer hints which highlight the next step in the task, and vibration hints for task completion have been best-received by the users, whereas video hints are very dependent on the quality of the content in the videos. This seems to indicate that the most useful hints are not necessarily the ones containing the most information, but rather those that give the user small indications and encouragements while leaving them with more time to develop their hands-on solutions to the task.

Conclusion

We have proposed an adaptive assisting interface for learning virtual filmmaking based on the scaffolding theory. Our system guides the user in operating a virtual film set, by providing text and vocal messages that are adapted to each person's skill level and performance. Through a user evaluation with 24 participants, we see that our system has strong potentials for training on virtual film sets, and can provide users with a better learning experience and fruitful learning outcome.

We hope these first steps explored in this work can open new avenues of research in using virtual reality for smart filmmaking environments, that would also have strong impacts towards assistive creativity in other professional and artistic domains.

Acknowledgement

This research was supported by the Ministry of Science and Technology, Taiwan, R.O.C., under contract no. MOST107-2221-E-004-008-and MOST108-2221-E-004-007-MY3.

Figure 1 :

 1 Figure 1: The architecture of our virtual film set is composed of three main parts: (1) a navigation component to move around cameras and the monitor, (2) shot management components (right of the figure) including a database, recommendation, and editing functions, (3) the workspace allowing users to take on various filmmkaing roles, and export the output as a video.

Figure 2 :

 2 Figure 2: Our assisting learning interface has three modules: the user model, scaffolding tools, and tasks

Figure 3 :

 3 Figure 3: Three tasks have been designed to illustrate the assisting learning interface as well as to help the users learn the use of virtual film set: still photography, dynamic photography, and the use of shot recommendation

Figure 4 :

 4 Figure 4: Our system provides five kinds of hints to the user: text, pointer, audio, video, and vibration. This figure shows examples of the text, video, and pointer hints.

Figure 7 :

 7 Figure 7: Evaluation on different types of hints in the assisting learning interface

Table 1 :

 1 Hints for each subtask

	Tools / Tasks	Basic Control			Still Photography		Dynamic Photography
		Movement	Teleport	Grab	Add Camera	Delete Camera	Move Camera	Shot Recommendation	Play Media	Track Actor
	Text
	Step
	Audio
	Video
	Vibration

Table 2 :

 2 Comparison of prior knowledge, perceived difficulty, and experimental time (AVG (STD))

		Prior Knowledge Level (K)	Perceived Difficulty Level (D)	Experimental Time (min)(T)
	1st Group	5.13 (1.36)	16.88 (2.03)	11.50 (3.38)
	2nd Group	4.00 (1.6)	14.88 (2.36)	13.63 (1.85)
	3rd Group	3.63 (1.92)	14.87 (2.85)	12.88 (3.4)
	Average	4.25	15.54	12.67

Table 3 :

 3 The user reported scores for the five types of hints provided by our assisting learning interface show that the pointer and vibration hints were generally most well received by both groups.

		Average score of	Average score of	Average score of
		the 2nd Group	the 3rd Group	both groups
	Text	3.25	3.38	3.31
	Pointer	4.25	4.28	4.26
	Audio	3.38	3.45	3.41
	Video	2.63	2.88	2.75
	Vibration	4.00	4.23	4.11
	Average	3.56	3.71

Table 4 :

 4 User Feedback on 4 questions in the post-experiment survey between the 2nd and 3rd user groups show that users found the assistive functions useful in familiarizing with the virtual film set.

			With the assisting learning	I would like to learn the	I think the virtual tutoring	I consider the frequency
			interface, I can more easily	virtual film set through this	functions can appear when I	that the virtual tutoring
			understand the operations	assisting learning interface	need them (1=disagree,	functions appear to be
			of the virtual film set	again (1=disagree,	5=agree)	(1=too infrequent, 5=too
			(1=disagree, 5=agree)	5=agree)		often)
	2nd Group	Average Std. Dev.	4.00 (0.93)	4.25 (0.71)	4.00 (0.93)	3.50 (0.53)
	3rd Group	Average Std. Dev.	4.25 (0.71)	4.25 (0.46)	4.13 (0.64)	3.00 (0.53)

© 2020 The Author(s) Eurographics Proceedings © 2020 The Eurographics Association.