Sara Navarro-Jaén

Luis F Bobadilla

Francisca Romero-Sarria

Oscar H Laguna ▽ , #

Nicolas Bion

José A Odriozola

Evaluation of the Oxygen Mobility in CePO 4 -Supported Catalysts: Mechanistic Implications on the WGS Reaction

The hexagonal and monoclinic phases of CePO4 have demonstrated to be excellent catalytic supports for Ptbased WGS catalysts. Consequently, the elucidation of the WGS reaction mechanism in these materials constitutes a fundamental aspect in order to explain their catalytic behavior. Since the observed WGS reaction path is closely related to the absence or presence of oxygen vacancies in the support, the study of the oxygen mobility in these solids constitutes a key factor for the understanding of the materials structure and its influence on the reaction mechanism. In this study, the oxygen mobility in the CePO4 supports and the corresponding Pt catalysts has been evaluated by means of isotopic exchange experiments using 18 O2 and C 18 O2 as probe molecules. Results demonstrate that the evaluated solids present a low exchange activity when 18 O2 is used, indicating the absence of oxygen vacancies in these solids and thus, suggesting a poor influence of the WGS redox mechanism. On the contrary, a high oxygen exchange activity is observed using C 18 O2, demonstrating that the exchange in these materials takes place through the formation of carbonate-like intermediates, and thus suggesting the associative mechanism of the WGS reaction as the preferred path in these solids. Operando DRIFTS experiments under WGS reaction conditions confirm these results, proving that the WGS reaction in the studied materials takes place through a formate-mediated associative mechanism.

INTRODUCTION

The use of hydrogen as energy carrier combined with the development of fuel cells have gained increasing interest in the last years. Particularly, polymer electrolyte membrane fuel cells (PEMFCs) constitute the most promising option for portable applications. However, the purification of the hydrogen used as feed in these systems results mandatory [START_REF] Song | Fuel processing for low-temperature and hightemperature fuel cells: Challenges and opportunities for sustainable development in the 21st century[END_REF] . Among the fuel processing reactions used with this objective, the water-gas shift (WGS) reaction is the most relevant step, since it removes the highest CO content while increasing the H2 content [START_REF] Ilieva | Nano-gold catalysts on Fe-modified ceria for pure hydrogen production via WGS and PROX: Effect of preparation method and Fe-doping on the structural and catalytic properties[END_REF] .

In this context, oxide-supported noble metal catalysts have been extensively studied and have demonstrated to be excellent WGS catalysts for fuel cell applications [START_REF] Gonzalez Castaño | Pt vs. Au in water-gas shift reaction[END_REF][START_REF] Vecchietti | Understanding the Role of Oxygen Vacancies in the Water Gas Shift Reaction on Ceria-Supported Platinum Catalysts[END_REF] . However, in the last few years some studies have been devoted to the use of phosphate-type solids as WGS catalytic supports [START_REF] Miao | Water-gas shift reaction over platinum/strontium apatite catalysts[END_REF][START_REF] Miao | Platinum/Apatite Water-Gas Shift Catalysts[END_REF][START_REF] Navarro-Jaén | Phosphate-type supports for the design of WGS catalysts[END_REF] .

In a previous work [START_REF] Navarro-Jaén | Pt/CePO4 catalysts for the WGS reaction: influence of the water-supplier role of the support on the catalytic performance[END_REF] , we reported the use of Pt/CePO4 catalysts as highly active and selective catalytic systems for the WGS reaction. CePO4 can present two different crystal struc-tures: the hexagonal (rhabdophane-type) phase, which contains structural channels able to hold water molecules and the monoclinic (monazite-type) phase, in which the vanishing of the structural channels takes place. We studied the influence of the support structure on the WGS catalytic activity, concluding that the rhabdophane-type phase (CeP400) enhanced the WGS catalytic performance with respect to the monazite-type phase (CeP600) as a consequence of an enhanced water adsorption capacity of the support, which allowed a continuous water supplying during the reaction.

Since phosphate-type solids constitute a promising option as WGS catalytic supports [START_REF] Miao | Water-gas shift reaction over platinum/strontium apatite catalysts[END_REF][START_REF] Miao | Platinum/Apatite Water-Gas Shift Catalysts[END_REF][START_REF] Navarro-Jaén | Phosphate-type supports for the design of WGS catalysts[END_REF] , the understanding of the WGS reaction mechanism in these solids results of great interest.

Although WGS reaction mechanism has been thoroughly studied [START_REF] Vecchietti | Understanding the Role of Oxygen Vacancies in the Water Gas Shift Reaction on Ceria-Supported Platinum Catalysts[END_REF][START_REF] Kalamaras | Mechanistic Studies of the Water-Gas Shift Reaction over Pt/CexZr1-xO2 Catalysts: The Effect of Pt Particle Size and Zr Dopant[END_REF][START_REF] Gokhale | On the mechanism of low temperature water gas shift reaction on copper[END_REF][START_REF] Choi | Catalytic behavior of metal catalysts in hightemperature RWGS reaction: In-situ FT-IR experiments and firstprinciples calculations[END_REF][START_REF] Fu | Active Nonmetallic Au and Pt Species on Ceria-Based Water-Gas Shift Catalysts[END_REF] , the reaction path is still a matter of debate. However, two mechanisms have been generally proposed: the redox and the associative mechanism, which imply both the participation of the metal phase and the support [START_REF] Ratnasamy | Water Gas Shift Catalysis[END_REF][START_REF] Olympiou | Mechanistic aspects of the water-gas shift reaction on alumina-supported noble metal catalysts: in situ DRIFTS and SSITKA-mass spectrometry studies[END_REF][START_REF] Byron | A Review of the Water Gas Shift Reaction Kinetics[END_REF] .

In the redox mechanism, the oxidation-reduction cycles occurring in the support are responsible for the WGS reaction. CO adsorbs on metal sites and reacts with the oxygen of the support to yield CO2, thus generating an oxygen vacancy. Afterwards, the generated oxygen vacancy is re-oxidized by H2O. The interaction of water with the vacancies of the solid generates OH and eventually, H groups. This mechanism has been typically observed on partially reduced oxides containing oxygen vacancies, such as TiO2, CeO2 or CeO2/ZrO2 [START_REF] Ratnasamy | Water Gas Shift Catalysis[END_REF] .

On the contrary, the associative mechanism is an adsorption-desorption model where the adsorbed species interact to form an adsorbed intermediate which decomposes into the reaction products. Thus, it implies the adsorption of CO and H2O on the catalyst surface to form surface intermediates such as formate, carbonate or carbonyl species, that decompose into the reaction products, CO2 and H2 [START_REF] Vecchietti | Understanding the Role of Oxygen Vacancies in the Water Gas Shift Reaction on Ceria-Supported Platinum Catalysts[END_REF][START_REF] Ratnasamy | Water Gas Shift Catalysis[END_REF][START_REF] Reddy | Water Gas Shift Reaction: Research Developments and Applications[END_REF] .

According to the aforementioned aspects, the structure and more specifically, the presence or not of oxygen vacancies has demonstrated to play a substantial role regarding the WGS reaction mechanism in oxide-supported catalysts 9, 17-18 . Consequently, studying the mobility of the oxygen atoms contained in the solids become of great interest in order to justify the reaction path followed by the catalysts under study. In this context, isotopic exchange techniques using molecules containing [START_REF] Maciel | Hydrogen production and purification from the water-gas shift reaction on CuO/CeO2-TiO2 catalysts[END_REF] O provide answers to characterize the catalysts behavior regarding the mobility and reactivity of the oxygen atoms constituting the solid, as well as the elucidation of reaction mechanisms [START_REF] Maupin | Direct evidence of the role of dispersed ceria on the activation of oxygen in NaX zeolite by coupling the 17 O/ 16 O isotopic exchange and 17 O solid-state NMR[END_REF][START_REF] Yang | Design of nanocrystalline mixed oxides with improved oxygen mobility: a simple non-aqueous route to nano-LaFeO3 and the consequences on the catalytic oxidation performances[END_REF] . Generally, the exchange reaction mechanism includes three main steps [START_REF] Duprez | Isotopes in Heterogeneous Catalysis[END_REF] :

1. Adsorption and subsequent dissociation of the labelled molecule (*X2) to form atoms or ions adsorbed on the surface (*Xads).

2. Exchange of such species (*Xads) with the atoms or ions naturally present in the surface or the bulk of the solid (Xs).

3. Desorption of the exchanged elements in their molecular form (*X2, X2 or *XX) in the gas phase.

Additionally, diffusion phenomena can take place if the adsorbed labelled species and those present in the solid are not in the same region of the catalyst after the adsorption of the labelled molecule.

The change in the gas phase composition during the exchange reaction allows to calculate characteristic parameters of the isotopic exchange as the atomic fraction of [START_REF] Maciel | Hydrogen production and purification from the water-gas shift reaction on CuO/CeO2-TiO2 catalysts[END_REF] O in the gas phase, the rate of exchange, the diffusion coefficients on the surface and in the bulk and the number of exchanged and exchangeable oxygen atoms in the catalyst, which are useful parameters for the knowledge of the catalytic properties of the solids [START_REF] Duprez | Isotopes in Heterogeneous Catalysis[END_REF] .

Particularly, isotopic exchange using 18 O2/ 16 O2 and C 18 O2/C 16 O2 is a powerful technique for the study of the WGS reaction mechanism. Differences in the oxygen rate of exchange depending on the labelled molecule are a consequence of a difference in the exchange mechanism: in the case of oxygen, a dissociative mechanism in which the rate limiting step is the dissociation of the oxygen molecule, whereas in the case of CO2 an associative mechanism takes place, via formation and decomposition of intermediate species such as carbonate, hydrogen carbonate or formate species [START_REF] Ojala | Isotopic Oxygen Exchange over Pd/Al2O3 Catalyst: Study on C 18 O2 and 18 O2 Exchange[END_REF] .

Consequently, the isotopic exchange experiments using 18 O2 result of great interest for evaluating the absence or presence of oxygen vacancies in the supports, which play an important role in the WGS redox mechanism. On the other hand, experiments using C 18 O2 allow the evaluation of the formation of carbonate-like intermediate species, implied in the WGS associative mechanism. Thus, the use of these labelled molecules could provide information about the preferred mechanism and/or the competence between them, which are crucial aspects during the WGS reaction.

According to the aforementioned considerations, this work presents the study of the oxygen mobility in two types of CePO4 supports with different structure and the corresponding WGS Pt catalysts by means of isotopic exchange techniques. Operando Diffuse Reflectance Infrared Spectroscopy (DRIFTS) was employed to obtain mechanistic insights, in order to correlate the inherent oxygen mobility of the solids to the observed WGS reaction mechanism.

EXPERIMENTAL SECTION

CePO4 supports were prepared by a hydrothermal method previously described [START_REF] Navarro-Jaén | Pt/CePO4 catalysts for the WGS reaction: influence of the water-supplier role of the support on the catalytic performance[END_REF] . The obtained materials were calcined in air at two different temperatures: 400 °C, to obtain the rhabdophane-type phase of CePO4 (CeP400), and 600 °C to obtain the monazite-type phase (CeP600). Pt catalysts with a nominal content of 2 wt.% were prepared by wet impregnation and calcined at 350 °C.

For the X*/X (X= O2, CO2) isotopic exchange experiments of the evaluated samples, 20 mg of the solids were introduced in a quartz U reactor placed in a closed recirculation system, connected to a mass spectrometer in order to follow continuously the evolution of the gaseous species as a function of the time, and to a vacuum pump. A recirculation pump eliminated the limitations due to the gas phase diffusion. A scheme of the reaction system has been presented elsewhere [START_REF] Duprez | Isotopes in Heterogeneous Catalysis[END_REF] .

The studied supports were first activated under 50 ml•min -1 of 16 O2 at the calcination temperature and atmospheric pressure. Regarding the catalysts, the activation was carried out at 350 °C under 50 ml•min -1 of H2 in order to ensure the reduction of the metallic species. Subsequently, the temperature was decreased and the activation gas was evacuated to introduce the isotopic molecule in the reaction system. For the heteroexchange experiments, 65 mbar of 18 O2 (Sigma-Aldrich, 99.9%) or C 18 O2 (Sigma-Aldrich, 97%) were introduced in the system. Two different experiments were performed:

Temperature programmed isotopic exchange (TPIE), where the temperature was progressively increased along the experiment at a rate of 2 °C•min -1 .

Isothermal isotopic exchange (IIE), where the temperature was kept constant along the experiment.

During the 18 O2/ 16 O2 isotopic exchange experiments, the mass-to-charge ratio (m/z) 32 ( 16 O2), 34 ( 16 O 18 O) and 36 ( 18 O2) were recorded, whereas for the C 18 O2/C 16 O2 isotopic exchange experiments, the mass-to-charge ratio (m/z) 44 (C 16 O2), 46 (C 16 O 18 O) and 48 (C 18 O2) were evaluated, with the purpose of following the isotopomer distribution during the isotopic exchange reaction. The absence of air during the experiment was verified by mass spectrometry in both cases.

The [START_REF] Maciel | Hydrogen production and purification from the water-gas shift reaction on CuO/CeO2-TiO2 catalysts[END_REF] O atomic fractions in the gas phase (αg,t) were calculated according to Equation (1a) for [START_REF] Maciel | Hydrogen production and purification from the water-gas shift reaction on CuO/CeO2-TiO2 catalysts[END_REF] Where Px,t corresponds to the partial pressure of the gaseous species of mass/charge x at time t.

The rate of exchange (Re), Equation ( 2), is generally calculated by the initial rate method, that is taking into account the rate at the beginning of the exchange reaction.

𝑅 𝑒 = -𝑁 𝑔 𝑑𝛼 𝑔 𝑑𝑡 (𝟐)
Where Ng is the number of oxygen atoms in the gas phase at the beginning of the reaction, which can be calculated according to Equation (3).

𝑁 𝑔 = 2𝑁 𝐴 • 𝑃 𝑇 𝑅 ( 𝑉 𝑟 𝑇 𝑟 + 𝑉 𝑐 𝑇 𝑐 ) (𝟑)
Where NA is the Avogadro number, PT corresponds to the total pressure, R is the ideal gas constant, Vr and Vc are the volumes and Tr and Tc are the temperatures of the heated and non-heated parts of the reaction system respectively.

Finally, the number of oxygen exchanged atoms (Ne) was calculated according to Equation (4).

𝑁 𝑒 = 𝑁 𝑔 (1 -𝛼 𝑔 ) (𝟒)
If the diffusion rate in the support network becomes significant, the kinetics of exchange can be explained using the model developed by Kakioka et al. [START_REF] Duprez | Isotopes in Heterogeneous Catalysis[END_REF][START_REF] Kakioka | Échange isotopique de 18 O du gaz carbonique avec les ions oxygène de l'hémipentoxyde de vanadium[END_REF][START_REF] Kakioka | II. Échange isotopique de 18 O du gaz carbonique avec les ion oxygène de l'hémipentoxyde de vanadium dopé par l'ion molybdène Mo 6+[END_REF] , based in the following premises:

-Surface exchange is fast, thus the isotopic equilibrium between the gas phase and the surface atoms is rapidly reached -Bulk diffusion takes place in spherical particles In such case, the kinetics of diffusion in the solid follows Equation (5).

-𝐿𝑛 𝛼 𝑔 𝛼 * = 2 𝜋 𝜌𝐴 𝑁 𝑔 √ 𝐷, 𝑡 (5)
Where α* is the value of αg once the surface exchange is completed and bulk diffusion starts. Consequently, the coefficient of bulk diffusion can be calculated from the slope "a" of Lnαg vs. t 1/2 for t > tA. This equation is valid if α* and αg >> αs 0 .

Operando diffuse reflectance infrared spectroscopy (DRIFTS) measurements were performed in a high-temperature reaction chamber supported on a DRIFTS optical system (Praying Mantis, Harricks) with ZnSe windows. Spectra were collected by a Thermo Nicolet iS50 FT-IR spectrometer equipped with a liquid-nitrogen cooled MCT detector at a resolution of 4 cm -1 and an average of 64 scans. 40 mg of the finely ground solids were loaded in the catalytic cell and the reaction flow containing 4.5% CO (Linde, ≥99.5%) and 10% H2O in Ar (Air Liquide, ≥99.999%) with a total flow rate of 50 ml•min -1 was introduced using a series of AALBORG mass-flow controllers and a KNAUER Smartline 1050 HPLC pump for the introduction of water in the system. The system was heated at 100 °C to avoid water condensation during the reaction. Experiments were performed every 50 °C from 150 to 350 °C, previous activation of the samples at 350 °C under 50 ml•min -1 of 10% H2 (Linde, ≥99.999%)/Ar (Air Liquide, ≥99.999%) flow.

CO2 concentration at the exit of the reaction chamber was measured using a VAISALA GMT220 series detector.

Acquisition and analysis of DRIFTS spectra were carried out using OMNIC 8 (Thermo Scientific ®) software. For IR band heights normalization, band intensities were re-scaled to the unit interval, using a second order polynomial for baseline correction.

RESULTS AND DISCUSSION

3.1. 18 O2/ 16 O2 isotopic exchange experiments 3.1.1. Isothermal 18 O2/ 16 O2 isotopic exchange In order to study the presence of oxygen vacancies, the oxygen mobility in the CePO4-based solids was firstly evaluated using [START_REF] Maciel | Hydrogen production and purification from the water-gas shift reaction on CuO/CeO2-TiO2 catalysts[END_REF] O2 in isothermal conditions. The selected temperature to carry out these experiments was 300 °C, considering the high WGS activity of the CePO4-based catalysts at this temperature 8 (See Supporting Information, Figure S1).

The evolution of the O2 isotopic distribution as a function of the time during the exchange experiment at 300 °C on the CeP400 support is presented in Figure 1. At this temperature, the concentration of 18 O2 does not decrease, whereas no formation of [START_REF] Reddy | Water Gas Shift Reaction: Research Developments and Applications[END_REF] O 18 O and/or 16 O2 is detected, indicating that the oxygen exchange is not significant in this support. Similar results were obtained for the CeP600 support and the corresponding Pt catalysts (not presented). In both cases, the oxygen exchange is not significant in the evaluated temperature range for the WGS reaction, that is from 200 to 350 °C. The CeP400 support does not exchange oxygen in all the temperature range evaluated, which indicates no mobility of the oxygen atoms of the support. This result is consistent with the description of the rhabdophane-type phase of CePO4, where the oxygen atoms are stabilized forming the structural channels typical of this phase [START_REF] Mooney | X-ray Diffraction Study of Cerous Phosphate and Related Crystals. I. Hexagonal modification[END_REF] . However, the solid CeP600 exchanges oxygen from 400 °C, reaching a number of exchanged atoms (Ne) equal to 1.5 x 10 21 atoms per gram of support at 600 °C, which corresponds to a 15% of the total number of atoms present in the solid. Considering that in the monazite-type phase of CePO4 the oxygen atoms are not stabilized forming channels, the mobility of oxygen is expected to increase with respect to the rhabdophane-type phase, in good agreement with the obtained results.

Nevertheless, these results point out the low oxygen mobility in both CePO4 supports, in contrast to the results previously reported for CeO2 [START_REF] Madier | Oxygen Mobility in CeO2 and CexZr(1-x)O2 Compounds: Study by CO Transient Oxidation and 18 O/ 16 O Isotopic Exchange[END_REF][START_REF] Larese | Effects of the CePO4 on the oxygen storage and release properties of CeO2 and Ce0.8Zr0.2O2 solid solution[END_REF] , in which the totality of the surface oxygen atoms are exchanged at 400°C. The presence of oxygen vacancies associated to the CeO2 redox properties, that is the presence of the Ce 3+ /Ce 4+ redox pair, has demonstrated to play a substantial role regarding the oxygen mobility in this solid, which influences the behavior and reaction mechanism of CeO2-based catalysts during the WGS reaction 4, 28-29 . In our case, the low oxygen mobility is explained by the great stability of the Ce 3+ ion in both CePO4 structures [START_REF] Larese | Effects of the CePO4 on the oxygen storage and release properties of CeO2 and Ce0.8Zr0.2O2 solid solution[END_REF] previously demonstrated by XPS [START_REF] Navarro-Jaén | Pt/CePO4 catalysts for the WGS reaction: influence of the water-supplier role of the support on the catalytic performance[END_REF][START_REF] Pemba-Mabiala | XPS Study of Mixed Cerium-Terbium Ortophosphate Catalysts[END_REF] , thus excluding the redox role in these supports.

The same experiment was performed on the corresponding Pt catalysts, in order to determine if the presence of the metal induces changes in the exchange capacity of the solids. Results are shown in Figure 3. Results obtained for the Pt/CeP400 catalyst do not show any difference with respect to the support. In the temperature range evaluated, no exchange occurs, indicating that the presence of Pt does not influence the exchange capacity of the CeP400 support. Although Pt is expected to activate and dissociate the oxygen molecule, which could be reflected in the oxygen isotopic exchange results, previous works [START_REF] Holmgrem | A Model of Oxygen Transport in Pt/Ceria catalysts from Isotope Exchange[END_REF][START_REF] Holmgrem | Oxygen Storage Dynamics in Pt/CeO2/Al2O3 Catalysts[END_REF] have proved that highly dispersed Pt particles present lower oxygen activation and dissociation capacity than Pt particles with bigger sizes, which could explain the obtained results, considering the dispersion values reported for the Pt/CeP400 catalyst 8 (Supporting information, Table S1).

Regarding the Pt/CeP600 catalyst, the exchange starts at the same temperature than in the corresponding support, that is 400 °C, and the number of oxygen exchanged atoms at 600 °C corresponds to a 13% of the total oxygen atoms present in the support, thus being similar to that value observed for the support. However, the major species observed during the experiment is [START_REF] Reddy | Water Gas Shift Reaction: Research Developments and Applications[END_REF] O [START_REF] Maciel | Hydrogen production and purification from the water-gas shift reaction on CuO/CeO2-TiO2 catalysts[END_REF] O being the production of [START_REF] Reddy | Water Gas Shift Reaction: Research Developments and Applications[END_REF] O2 almost insignificant. As previously mentioned, the presence of Pt is expected to increase the oxygen dissociation capacity. Thus, if [START_REF] Maciel | Hydrogen production and purification from the water-gas shift reaction on CuO/CeO2-TiO2 catalysts[END_REF] O2 dissociates into two [START_REF] Maciel | Hydrogen production and purification from the water-gas shift reaction on CuO/CeO2-TiO2 catalysts[END_REF] O adsorbed atoms on a catalyst containing principally [START_REF] Reddy | Water Gas Shift Reaction: Research Developments and Applications[END_REF] O, the formation of [START_REF] Reddy | Water Gas Shift Reaction: Research Developments and Applications[END_REF] O [START_REF] Maciel | Hydrogen production and purification from the water-gas shift reaction on CuO/CeO2-TiO2 catalysts[END_REF] O is expected [START_REF] Holmgrem | A Model of Oxygen Transport in Pt/Ceria catalysts from Isotope Exchange[END_REF] in good agreement with the obtained results. This indicates that contrary to the results obtained for Pt/CeP400, the exchange of oxygen species in the CeP600 support is influenced by the presence of Pt, presumably located in the nearby vicinity of the lattice oxygen ions and OH groups [START_REF] Ducarme | Study of Surface Atom Behaviour on Pt/SiO2 and Pt/Al2O3 Catalysts by Isotopic Exchange of Oxygen from CO2[END_REF] . The oxygen dissociation capacity of the Pt particles in this solid is in good agreement with its lower Pt dispersion value compared to the Pt/CeP400 catalyst [START_REF] Navarro-Jaén | Pt/CePO4 catalysts for the WGS reaction: influence of the water-supplier role of the support on the catalytic performance[END_REF] .

These results allow to confirm the lack of influence of oxygen vacancies in the WGS temperature range where the solids were tested, in good agreement with the lack of the Ce 3+ /Ce 4+ redox pair. Since Pt does not allow to improve the exchange activity, it can be concluded that the rate limiting step is the incorporation or the mobility of oxygen atoms in the CePO4 lattice. Consequently, the redox mechanism proposed for the WGS reaction, which implies the existence of oxidation-reduction cycles and the participation of the support oxygen atoms [START_REF] Vecchietti | Understanding the Role of Oxygen Vacancies in the Water Gas Shift Reaction on Ceria-Supported Platinum Catalysts[END_REF][START_REF] González-Castaño | Deep insight into Zr/Fe combination for successful Pt/CeO2/Al2O3 WGS catalyst doping[END_REF][START_REF] García-Moncada | New concept for old reaction: Novel WGS catalyst design[END_REF] can be discarded or have poor influence in this type of solids.

Isothermal C 18 O2/C 16 O2 Isotopic Exchange

Contrary to the results obtained using 18 O2, CePO4 supports and Pt catalysts demonstrated to be highly active oxygen exchange materials when using C 18 O2 as labelled molecule. Figure 4 shows the evolution of the CO2 isotopic distribution at 300 °C for the CePO4 supports. Similar results were obtained for the Pt catalysts, as shown in Figure 5. The exchange mechanism was the same than that observed in the supports, and by the same token the exchange demonstrated to be very fast at the beginning of the exchange reaction, taking long times to reach the equilibrium. The little effect of Pt addition regarding oxygen exchange suggests that C 18 O2 exchange takes place mainly on the supports through a direct process [START_REF] Ojala | Isotopic Oxygen Exchange over Pd/Al2O3 Catalyst: Study on C 18 O2 and 18 O2 Exchange[END_REF] . In order to compare the oxygen mobility in every solid, the number of oxygen exchanged atoms (Ne) as a function of the time is presented in Figure 6. Additionally, Ne data after two hours of reaction and Re are summarized in Table 1. Nevertheless, in the presence of Pt Ne values become similar for both solids, although the Pt/CeP400 catalyst presents a slightly superior exchange capacity, particularly at the beginning of the reaction. According to the data after two hours of reaction (Table 1), both Ne and Re values for the Pt/CeP400 catalyst are lower than those of the CeP400 support. Additionally, the number of oxygen exchanged atoms in such catalyst corresponds to a 21% of the total atoms present in the solid, very close to the 22% exchanged in the Pt/CeP600 catalyst. Consequently, the presence of Pt induces a negative effect on the oxygen exchange capacity of the CeP400 support, especially marked during the exchange reaction with the bulk oxygen atoms.

Sample

Ne (10 21 at•g -1 ) Re (10 18 at•g - Considering the long times required to reach the equilibrium in both the supports and catalysts, the calculation of the diffusion coefficient using Kakioka's model results of great interest [START_REF] Kakioka | Échange isotopique de 18 O du gaz carbonique avec les ions oxygène de l'hémipentoxyde de vanadium[END_REF][START_REF] Kakioka | II. Échange isotopique de 18 O du gaz carbonique avec les ion oxygène de l'hémipentoxyde de vanadium dopé par l'ion molybdène Mo 6+[END_REF] . In order to calculate the oxygen diffusion coefficient, it is necessary to distinguish (1) the exchange with the surface oxygen atoms, which takes place at a high rate with the oxygen atoms of CO2 and (2) the exchange with the bulk oxygen, determined by the diffusion rate in the inside of the solid. Thus, such model has to be applied once the fast oxygen exchange with the surface atoms has taken place. Linear fitting of the results and diffusion coefficient values are displayed in Figure 7 and Table 2, respectively.

Diffusion coefficient of the CeP400 support is smaller than that of CeP600, which could be related to a hindered oxygen diffusion in the former due to the presence of channels in its structure. In the presence of Pt, no significant differences are observed between CeP600 and Pt/CeP600, thus denoting no influence of the metal particles during the diffusion processes. On the contrary, the diffusion coefficient of Pt/CeP400 decreases significantly with respect to CeP400. In our previous work [START_REF] Navarro-Jaén | Pt/CePO4 catalysts for the WGS reaction: influence of the water-supplier role of the support on the catalytic performance[END_REF] , TEM micrographs suggested the arrangement of some Pt particles inside the channels of CeP400, thus the presence of Pt both the surface and in such cavities could hinder the diffusion of oxygen in the bulk of the CeP400 support (Supporting information, Figure S2). The obtained results demonstrate a high oxygen mobility in the solids using C 18 O2, which implies the formation and decomposition of carbonate-like intermediates and thus, suggesting that the WGS associative mechanism could be the preferred path in these solids.

Operando DRIFTS study of the WGS reaction mechanism

18 O2 isotopic exchange results have demonstrated that both the CePO4 supports and Pt catalysts present no activity towards oxygen exchange in the WGS temperature range evaluated in our samples (200-350 °C), which discards the participation of oxygen vacancies in these solids and thus suggests a poor influence of the so-called WGS redox mechanism. On the contrary, the great oxygen exchange activity using C 18 O2 suggests a preference for the WGS associative mechanism in these solids.

In order to elucidate the WGS reaction mechanism taking place in these solids, operando DRIFTS experiments were performed under WGS reaction atmosphere. Difference spectra during the reaction at each temperature were obtained by subtraction of the spectra of the samples before the introduction of the reaction mixture.

Difference spectra of the Pt/CeP400 catalyst when subjected to the reaction mixture (4.5% CO, 10% H2O, Ar balance) are shown in Figure 8. Results reveal the presence of adsorbed water even at relatively high temperature as observed in previous experiments [START_REF] Navarro-Jaén | Pt/CePO4 catalysts for the WGS reaction: influence of the water-supplier role of the support on the catalytic performance[END_REF] , in agreement with the appearance of bands at 1625 cm -1 , corresponding to the H-O-H bending mode of hydrogen-bonded water, and those in the 3100-3500 cm -1 range, attributed to surface-bonded water [START_REF] Romero-Sarria | CO oxidation at low temperature on Au/CePO4: Mechanistic aspects[END_REF] . Bands in the 5100-5300 cm -1 range, ascribed to the overtones of the water stretching vibrations, allow to confirm the presence of molecular water [START_REF] Frost | Near-infrared spectroscopic study of selected hydrated hydroxylated phosphates[END_REF] , Figure 8(a).

Additionally, bands corresponding to linear-bonded CO, generally found between 2000-2100 cm -1 , appear at 2055, 2073 and 2080 cm -1 , as shown in Figure 8(b). The presence of these contributions exposes different environments of the Pt particles in the Pt/CeP400 solid. The shifts of the CO stretching frequency could be explained by several reasons, as changes in the Pt exposed faces or changes in the electron density of the metal particles induced by the support, which affects the number of available electrons for back-bonding to the CO π* orbitals [START_REF] Kappers | Correlation between CO frequency and Pt coordination number. A DRIFT study on supported Pt catalysts[END_REF][START_REF] Lane | Infrared Spectroscopy of Adsorbed Carbon Monoxide on Platinum/Nonacidic Zeolite Catalysts[END_REF] . In our case, these bands have been assigned to CO linearly adsorbed on different adsorption sites of platinum crystallites possessing surface atoms with different coordination. Greenler et al. [START_REF] Greenler | Stepped single-crystal surfaces as models for small catalyst particles[END_REF] obtained similar features during CO adsorption on Pt/SiO2 catalysts. They reported three bands at 2063, 2070 and 2081 cm -1 , ascribed to CO linearly adsorbed on corner, edge and face atoms of Pt crystallites, respectively. Furthermore, a shoulder at about 2010 cm -1 is observed in our spectra, which has been ascribed to CO linearly adsorbed on weakly coordinated Pt atom. According to Bazin and co-workers [START_REF] Bazin | FT-IR study of CO adsorption on Pt/CeO2: characterisation and structural rearrangement of small Pt particles[END_REF] , the origin of this Pt site could be a Pt coordination site located onto steps or kinks of small particles or correspond to CO linearly adsorbed on very small Pt particles. An additional band at very low frequency (1965 cm -1 ) is also observed, which have been tentatively attributed to CO linearly adsorbed on the Pt species occluded in the channels of the rhabdophanetype phase. The CeP400 channels are constituted by oxygen atoms which present local negative charge, being able to donate electron density to the adjacent Pt atoms. Thus, the increased Pt electron density could provoke an increased bonding to the CO π* orbitals, decreasing the CO stretching frequency. This result is in accordance with previous works, where a contribution at ca. 1970 cm -1 has been observed in the spectra of Pt catalysts supported on zeolites, being attributed to CO linearly adsorbed on Pt located on the zeolite cages [START_REF] Kappers | Correlation between CO frequency and Pt coordination number. A DRIFT study on supported Pt catalysts[END_REF][START_REF] Lane | Infrared Spectroscopy of Adsorbed Carbon Monoxide on Platinum/Nonacidic Zeolite Catalysts[END_REF] .

As a consequence of the interaction between the reactants, surface carbonaceous species are formed, according to the contributions emerging at 1370, 1381, 1589 and 2850 cm -1 (Figure 8(a) and (c)). These features seem to indicate the formation of formate (HCOO -) species. The bands at 1589 and 1370 cm -1 have been assigned to the asymmetric and symmetric CO stretching vibrations (νas CO and νs CO), respectively, whereas the bands at 2850 and 1381 cm - 1 have been attributed to the CH stretching (νCH) and CH bending (δCH) vibrations, respectively. In order to identify the type of coordination of the formate species, a general procedure consists on the evaluation of the splitting between the CO asymmetric and symmetric stretching modes, Δνas-s [START_REF] Nakamoto | Infrared and Raman Spectra of Inorganic and Coordination Compounds Part B: Applications in Coordination, Organometalic, and Bioinorganic Chemistry[END_REF][START_REF] Oomens | Free Carboxylate Stretching Modes[END_REF][START_REF] Busca | Infrared spectroscopic identification of species arising from reactive adsorption of carbon oxides on metal oxide surfaces[END_REF] . In this case, the splitting is not representative of the typical coordination structures described for surface formate species. This could be explained by the formation of hydrogen bonds, since fields due to water molecules can affect the splitting value [START_REF] Busca | Infrared spectroscopic identification of species arising from reactive adsorption of carbon oxides on metal oxide surfaces[END_REF][START_REF] Ramis | Low-temperature CO2 adsorption on metal oxides: spectroscopic characterization of some weakly adsorbed species[END_REF] . Miao et al. obtained similar bands in Pt/apatite catalysts 6 , which were attributed to formate ions in aqueous solution [START_REF] Ito | The vibrational spectra of the formate, acetate, and oxalate ions[END_REF] and icy matter [START_REF] Schutte | Weak ice absorption features at 7.24 and 7.41 mu m in the spectrum of the obscured young stellar object W 33A[END_REF] . Furthermore, the broad band at about 1770 cm -1 , also present in our spectra, agrees with the value reported for the CO stretching mode (νCO) of gaseous formic acid [START_REF] Maçôas | Vibrational spectroscopy of cis-and trans-formic acid in solid argon[END_REF] , which is produced by means of the reaction between CO and H2O. Canepa et al. [START_REF] Canepa | Affinity of hydroxyapatite (001) and (010) surfaces to formic and alendronic acids: a quantum-mechanical and infrared study[END_REF] carried out an ab initio study on the interaction of formic acid with hydroxyapatite surfaces, which explained properly the WGS intermediates observed by Miao et al. [START_REF] Miao | Platinum/Apatite Water-Gas Shift Catalysts[END_REF] in Pt/apatite catalysts. According to their results, CO and H2O react to produce formic acid, which can adsorb molecularly on the (001) surface. However, the acid dissociates spontaneously in the water-reconstructed (010) surface to pro-duce formate species, which are the WGS reaction intermediates. Thus, in our Pt/CeP400 catalyst, the adsorption of formic acid could take place in those surfaces presenting less affinity by water, whereas the formate species appear in the water-enriched surface of CeP400.

On the other hand, Lustemberg et al. [START_REF] Lustemberg | Insights into the Nature of Formate Species in the Decomposition and Reaction of Methanol over Cerium Oxide Surfaces: A Combined Infrared Sepctroscopy and Density Functional Theory Study[END_REF] studied several coordination modes of formate species, considering both the ∆ν values and the position of the vibration bands. According to these authors, the CO asymmetric and symmetric stretching vibrations of bidentate formate species appear at 1550 and 1371 cm -1 , respectively (∆ν=179 cm -1 ). In our case, these bands are found at 1589 and 1370 cm -1 (∆ν=218 cm - 1 ), suggesting the presence of bidentate formate species in the surface of the Pt/CeP400 catalyst. Difference spectra of the Pt/CeP600 catalyst are shown in Figure 9. As for the Pt/CeP400 catalyst, bands ascribed to adsorbed water at 1622 cm -1 and in the 3100-3500 cm -1 region appear (Figure 9(a)), although their intensity are lower than those of the Pt/CeP400 catalysts, in accordance with the lowest water adsorption capacity of the support CeP600 8 . Furthermore, the band in the 5100-5300 cm -1 range observed for the Pt/CeP400 catalyst does not appear in the spectra of this solid, indicating the absence of a water layer in which the formate species could be dissolved.

Bands attributed to linear Pt 0 -CO are observed in the 2000-2100 cm -1 range (Figure 9(b)). Differently from the Pt/CeP400 solid, only two bands at 2057 and 2076 cm -1 are observed, pointing out the lower number of coordination environments of the Pt particles in Pt/CeP600. Such bands correspond to CO linearly adsorbed on corner and edge atoms of Pt crystallites [START_REF] Greenler | Stepped single-crystal surfaces as models for small catalyst particles[END_REF] , respectively. As for the spectra of the Pt/CeP400 catalyst, a shoulder at about 2010 cm -1 is present, attributed to CO linearly adsorbed on a weakly coordinated Pt atom [START_REF] Bazin | FT-IR study of CO adsorption on Pt/CeO2: characterisation and structural rearrangement of small Pt particles[END_REF] . Furthermore, the band at 1965 cm -1 present in the Pt/CeP400 spectra is not observed in this catalyst, reinforcing the assignment of this contribution to Pt particles located in the rhabdophane-type phase structural channels.

As a result of the interaction between reactants, bands corresponding to bidentate formate groups are present at 1370, 1381, 1588 and 2843 cm -1 (Figure 9(a) and (c)), similarly to the results obtained for Pt/CeP400. The contributions at 1588 and 1370 cm -1 have been attributed to the asymmetric and symmetric stretching vibrations of the CO group (νas CO and νs CO), whereas the bands at 2843 and 1381 cm -1 have been ascribed to the CH stretching (νCH) and CH bending (δCH) vibrations. However, a second type of formate species is present, according to the bands at 1337, 1362, 1564 and 2976 cm -1 . As reported by Lustemberg et al. [START_REF] Lustemberg | Insights into the Nature of Formate Species in the Decomposition and Reaction of Methanol over Cerium Oxide Surfaces: A Combined Infrared Sepctroscopy and Density Functional Theory Study[END_REF] , (CO) asymmetric and symmetric modes of monodentate formates are observed at 1580 and 1335 cm -1 (=245 cm -1 ). In our case,  is 227 cm -1 . Therefore, we can conclude that in this solid, the lower amount of water favours the stabilization of monodentate formate species.

In addition, a weak band at 1478 cm -1 points out the simultaneous formation of carbonate species.

Whatever the catalyst, no new -OH groups resulting from the interaction of the water molecule with the oxygen vacancies of the supports appear during the WGS reaction, which is in good agreement with the absence of oxygen exchange in these solids demonstrated by [START_REF] Maciel | Hydrogen production and purification from the water-gas shift reaction on CuO/CeO2-TiO2 catalysts[END_REF] O2 isotopic exchange experiments. Consequently, the WGS redox mechanism can be discarded in this type of solids. According to the previously mentioned results, only bidentate formate species are formed on the surface of the Pt/CeP400 catalyst, whereas bidentate and monodentate species are observed in Pt/CeP600. Jacobs et al. [START_REF] Jacobs | Low-Temperature Water-Gas Shift: In-Situ DRIFTS-Reaction Study of a Pt/CeO2 Catalyst for Fuel Cell Reformer Applications[END_REF] studied the WGS reaction mechanism on a Pt/CeO2 catalyst after a pre-treatment under H2. They demonstrated that the partial reduction of ceria leads to the formation of geminal OH groups, which react with CO to form bidentate formate species, the active intermediate species during the WGS reaction. However, some authors claim that formate species act as spectators during the WGS reaction, being carboxyl species the true intermediates [START_REF] Grabow | Mechanism of the Water Gas Shift Reaction on Pt: First Principles, Experiments, and Microkinetic Modeling[END_REF] .

In order to clarify the role of the bidentate formates in our catalysts, the evolution of the ~2850 cm -1 band intensity was compared to the CO2 production rate as a function of the temperature. In the case of the Pt/CeP600 catalyst, the intensity of the bands at 2976 and 1478 cm -1 attributed to monodentate formate and carbonate species, respectively, are also evaluated. Results are shown in Figure 10. Results demonstrate a greater catalytic performance of Pt/CeP400 with respect to Pt/CeP600, in good agreement with the catalytic activity tests (Supporting Information, Figure S1).

For both catalysts, an initial increase of the bidentate formate band intensity takes place between 150 and 200 °C, at low CO conversion values. As the temperature increases, CO2 production increases and the bidentate formate band intensity decreases to the same extent, suggesting that bidentate formates are the intermediate species in both catalysts. This has been confirmed by comparing the CO2 production registered during the experiment and the ~2850 cm -1 band intensity as a function of the time, which shows that both the increase in the CO2 production and the decrease in the band intensity present the same slope with opposite signs in all the temperature range evaluated, thus confirming that CO2 formation is the result of the decomposition of the bidentate formate species.

Regarding Pt/CeP600, the formation of surface carbonate species could take place through two possible mechanisms 53 : 1) the interaction of the produced CO2 with the oxygen atoms of the support, in which case the drop of the formates band intensity must initiate before the formation of carbonates, or 2) the oxidation of the formate species by the available surface oxygen to yield carbonate. In this case, carbonates formation should start simultaneously to formates consumption. Thus, according to our results, the formation of surface carbonates results from the interaction of CO2 with the CeP600 support. According to Hilaire and co-workers [START_REF] Hilaire | A comparative study of water-gas-shift reaction over ceria supported metallic catalysts[END_REF] , carbonate is stable in reduced CeO2, where cerium is present as Ce 3+ similarly to the CePO4 supports. However, carbonate species are easily decomposed by oxidation with H2O. Consequently, the stabilization of carbonates on the Pt/CeP600 surface could be explained by the lower water retention capacity of CeP600 with respect to CeP400. Since water is available on the surface of the latter even at 350 °C, the decomposition of the carbonate species could take place, increasing reaction rates [START_REF] Hilaire | A comparative study of water-gas-shift reaction over ceria supported metallic catalysts[END_REF] . The trend followed by the carbonates and monodentate formates bands in Pt/CeP600 indicates that the concentration of both species increases along the WGS reaction. Hence, both carbonate and monodentate formate species may have a spectator or catalytic poisoning role, contributing to the blocking of the active sites and thus, explaining its lower catalytic performance with respect to Pt/CeP400.

Consequently, results suggest that the WGS reaction in both catalysts proceed via an associative mechanism through the generation of formate species, according to the following scheme: CO(g) + H2O(g) ↔ CO* + OH* + H* ↔ HCOO* + H* ↔ CO2* + 2H* ↔ CO2(g) + H2(g)

Where the asterisk (*) implies that the corresponding species is adsorbed.

Considering this reaction scheme, which implies the dissociation of the water molecule into OH and H species and taking into account that the CePO4 supports do not dissociate the water molecule 8 , it could be concluded that the OH (2) In the case of Pt, the activation energy barrier for OH dissociation is larger than that for the abstraction of the first H atom in the H2O molecule. Accordingly, in WGS conditions, atomic H is the most abundant surface intermediate, followed by OH and finally, O. These results are in agreement with the reaction mechanism observed in Pt/CeP400. However, the observation of the Pt-H vibration is difficult, since it appears in the same region than the Pt-CO one [START_REF] Palecek | Characterization of the Platinum-Hydrogen Bond by Surface-Sensitive Time-Resolved Infrared Spectroscopy[END_REF] . Likewise, the observation of the Pt-OH vibration results difficult in the presence of H2O, since it appears in the 3600-3400 cm -1 range, where the broad contributions ascribed to OH groups of bonded water are also present, thus obscuring this region [START_REF] Morrow | Infrared Spectra of Hydroxyl Groups on the Surface of Platinum[END_REF] .

According to DRIFTS results, the WGS reaction in these solids occurs through a bidentate formate-mediated mechanism. Since CePO4 supports do not present the capacity of dissociating the water molecule, the water dissociation step is carried out in the Pt particles, as well as the adsorption of CO. Consequently, the formation of bidentate formate species seems to place on the phase. Furthermore, Pt has demonstrated to promote H-transfer by assisting the hydrogen abstraction from the formate intermediate and a neighboring OH group, allowing H2 recombination and desorption from the catalyst surface, whereas the presence of H2O lowers the activation energy of formate decomposition and changes the selectivity to favor formate dehydrogenation over dehydration [START_REF] Jacobs | Surface interfaces in low temperature water-gas shift: The metal oxide synergy, the assistance of co-adsorbed water, and alkali doping[END_REF][START_REF] Shido | Reactant-Promoted Reaction Mechanism for Water-Gas Shift Reaction on Rh-Doped CeO2[END_REF] . Therefore, the metal-support interface could have an important function regarding the decomposition of the formate species. Whereas the role of the CePO4 supports is to supply H2O in the vicinity of Pt during the WGS reaction, Pt particles could as well assist the decomposition of the formate species to generate the reaction products. The higher H2O content in the CeP400 surface could facilitate formates decomposition to a greater extent, thus explaining the higher WGS catalytic performance of the Pt/CeP400 catalyst.

4.

CONCLUSIONS Oxygen mobility in the rhabdophane (CeP400) and monazite-type (CeP600) phases of CePO4 and the corresponding Pt catalysts has been evaluated by [START_REF] Maciel | Hydrogen production and purification from the water-gas shift reaction on CuO/CeO2-TiO2 catalysts[END_REF] O2/ 16 O2 and C 18 O2/C 16 O2 isotopic exchange experiments. Every solid present poor oxygen exchange capacity using 18 O2 as labelled molecule, pointing out the absence of oxygen vacancies able to participate in the WGS reaction and thus, suggesting a poor influence of the WGS redox mechanism. On the contrary, the evaluated solids present high oxygen exchange capacity using C 18 O2, indicating that the formation and decomposition of carbonate-like intermediates can occur in the solids surface and, consequently, suggesting the WGS associative mechanism as the main path in these catalytic systems. Operando DRIFTS results confirm that the WGS reaction occurs through an associative mechanism via bidentate formate intermediate species, whereas no formation of new -OH groups is observed, ruling out the redox mechanism, in good agreement with the isotopic exchange results.

The greater catalytic activity of the Pt/CeP400 catalyst is explained by the formation of only bidentate formate species in the catalyst surface, which are proposed as the active species during the WGS reaction. On the contrary, the lower surface water content in the Pt/CeP600 catalyst gives rise to the additional formation of monodentate formate and carbonate species, which could block the active sites and provoke a decrease on the catalytic performance.

Figure 1 .

 1 Figure 1. O2 isotopic distribution (%) as a function of the time during isotopic exchange reaction at 300 °C over CeP400

Figure 2 .

 2 Figure 2. O2 isotopic distribution (%) as a function of the temperature during temperature-programmed isotopic exchange reaction over (a) CeP400, (b) CeP600

Figure 3 .

 3 Figure 3. O2 isotopic distribution (%) as a function of the temperature during temperature-programmed isotopic exchange reaction over (a) Pt/CeP400, (b) Pt/CeP600

Figure 4 .

 4 Figure 4. CO2 isotopic distribution (%) as a function of the time during isotopic exchange reaction at 300 °C over (a) CeP400, (b) CeP600The oxygen exchange using C 18 O2 occurs at a high rate, giving rise firstly to the formation of C 16 O 18 O, followed by the generation of C 16 O2. Consequently, the mechanism of the C 18 O2 exchange reaction in both supports proceeds through two consecutive simple exchange mechanisms via transfer of one oxygen atom in every stage, as expressed by equation (5):C 18 O2(g) +[START_REF] Reddy | Water Gas Shift Reaction: Research Developments and Applications[END_REF] O(s) ↔ C[START_REF] Reddy | Water Gas Shift Reaction: Research Developments and Applications[END_REF] O 18 O(g) +[START_REF] Maciel | Hydrogen production and purification from the water-gas shift reaction on CuO/CeO2-TiO2 catalysts[END_REF] O(s) (5a) C[START_REF] Reddy | Water Gas Shift Reaction: Research Developments and Applications[END_REF] O 18 O(g) +[START_REF] Reddy | Water Gas Shift Reaction: Research Developments and Applications[END_REF] O(s) ↔ C 16 O2(g) +[START_REF] Maciel | Hydrogen production and purification from the water-gas shift reaction on CuO/CeO2-TiO2 catalysts[END_REF] O(s) (5b) The second step involving the formation of C 16 O2 takes place almost instantly to the first one, which demonstrates a fast activation of CO2 on the supports. Nevertheless, although the initial rate of exchange (corresponding to the exchange of oxygen surface atoms) is high, long times are required to reach the equilibrium, pointing out that the exchange with the bulk oxygen atoms is hindered.Similar results were obtained for the Pt catalysts, as shown in Figure5. The exchange mechanism was the same than that observed in the supports, and by the same token the exchange demonstrated to be very fast at the beginning of the exchange reaction, taking long times to reach the equilibrium. The little effect of Pt addition regarding oxygen exchange suggests that C 18 O2 exchange takes place mainly on the supports through a direct process[START_REF] Ojala | Isotopic Oxygen Exchange over Pd/Al2O3 Catalyst: Study on C 18 O2 and 18 O2 Exchange[END_REF] .

Figure 5 .

 5 Figure 5. CO2 isotopic distribution (%) as a function of the time during isotopic exchange reaction at 300 °C over (a) Pt/CeP400, (b) Pt/CeP600

Figure 6 . 1 .

 61 Figure 6. Ne evolution as a function of the time for the CePO4 supports and Pt catalystsTable 1. Number of oxygen exchanged atoms (Ne) and C 18 O2 rates of exchange (Re) of the CePO4 supports and Pt catalysts at 300 °C

Figure 7 .Table 2 .

 72 Figure 7. Linear fitting using Kakioka's model from C 18 O2 isotopic exchange results at 300 °C over (a) CePO4 supports, (b) Pt catalystsTable 2. Diffusion coefficient (D) of the CePO4 supports and Pt catalysts calculated by Kakioka's model Sample D (10 -22 m 2 •s -1 ) CeP400 1.47 CeP600 4.41 Pt/CeP400 0.86 Pt/CeP600 4.93

Figure 8 .

 8 Figure 8. (a) Difference spectra (general) of the Pt/CeP400 catalyst under WGS reaction flow, and enlargement of (b) 2200-1900 cm -1 and (c) 1900-1340 cm -1 range, from 150 (top) to 350 °C (bottom).

Figure 9 .

 9 Figure 9. (a) Difference spectra (general) of the Pt/CeP600 catalyst under WGS reaction flow, and enlargement of (b) 2200-1900 cm -1 and (c) 1900-1300 cm -1 range, from 150 (top) to 350 °C (bottom)

Figure 10 .

 10 Figure 10. Comparison between the CO2 production during operando DRIFTS experiments (continuous line) and the evolution of the main IR band for the detected carbonaceous species (dashed lines) for (a) Pt/CeP400 and (b) Pt/CeP600 catalysts.

  )

	and H species participating in the formate generation are
	those produced by the dissociation of the water molecule on
	the Pt particles. Phatak et al. 55 developed an interesting DFT
	investigation on the adsorption and dissociation steps of
	water on different metals, relating them to the WGS reaction
	mechanism. In every case, water dissociation occurs by
	means of the following 2 steps:
	H2O* + * → OH* + H*	(1)
	OH* + * → O* + H*
	(a)
			Bidentate formates	Normalized IR intensity (a. u.)
	(b)	3
	)
	0 1 2 CO 2 production rate (10 -9 mol•g -1 -1 •s	Carbonates Bidentate formates	Monodentate formates	Normalized IR intensity (a. u.)
		150	200		250	300	350
						Temperature (ºC)

ASSOCIATED CONTENT

Supporting Information WGS catalytic performance of the Pt catalysts, mean Pt particle size and metal dispersion values, TEM micrographs. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author

* Sara Navarro-Jaén

Present Addresses † Sara Navarro-Jaén. Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181, UCCS, Unité de Catalyse et Chimie du Solide, F-59000, Lille, France. Current e-mail address: sara.navarro-jaen@univ-lille.fr # Oscar H. Laguna. Departamento de Ingeniería Química, Medioambiental y de los Materiales. Campus Científico Tecnológico de Linares-Universidad de Jaén. Av. De la Universidad, s/n, 23700 Linares (Jaén), Spain

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no conflict of interest.

ACKNOWLEDGMENT

The work presented in this manuscript is reproduced in part from: Navarro-Jaén, S., Phosphate-based catalysts for the WGS reaction: synthesis, reactivity and mechanistic considerations. Ph.D. Thesis, University of Seville, January 2019. The authors acknowledge the financial support of the Spanish Ministry of Economy and Competitiveness through the Project ENE2015-66975-C3-2-R. S. Navarro-Jaén is thankful for the financial support provided by the Erasmus+ Mobility Agreement between the University of Seville and the University of Poitiers.