Dr Cyril Thomas
email: cthomas@ccr.jussieu.fr

Olivier Gorce

Céline Fontaine

Jean-Marc Krafft

Françoise Villain

Gérald Djéga-Mariadassou

On the promotional effect of Pd on the propene-assisted decomposition of NO on chlorinated Ce 0.68 Zr 0.32 O 2

Keywords: CO-FTIR, XANES, Lean deNO x, Mechanism, C 3 H 6, Pd catalysts, Ceria-Zirconia, TPD of NO x

The Selective Catalytic Reduction (SCR) of NO x assisted by propene is investigated on Pd/Ce 0.68 Zr 0.32 O 2 catalysts (Pd/CZ), and is compared, under identical experimental conditions, with that found on a Pd/SiO 2 reference catalyst. Physico-chemical characterisation of the studied catalysts along with their catalytic properties indicate that Pd is not fully reduced to metallic Pd for the Pd/CZ catalysts. This study shows that the incorporation of Pd to CZ greatly promotes the reduction of NO in the presence of C 3 H 6 . These catalysts display very stable deNO x activity even in the presence of 1.7% water, the addition of which induces a reversible deactivation of about 10%. The much higher N 2 selectivity obtained on Pd/CZ suggests that the lean deNO x mechanism occurring on these catalysts is different from that occurring on Pd 0 /SiO 2 . A detailed mechanism is proposed for which CZ achieves both NO oxidation to NO 2 and NO decomposition to N 2 , whereas PdO x activates C 3 H 6 via ad-NO 2 species, intermediately producing R-NO x compounds that further decompose to NO and C x H y O z . The role of the latter oxygenates is to reduce CZ to provide the catalytic sites responsible for NO decomposition. The proposed C 3 H 6 -assisted NO decomposition mechanism stresses the key role of NO 2 , R-NO x and C x H y O z as intermediates of the SCR of NO x by hydrocarbons.

Introduction

A tremendous number of investigations have been made within the past decades to develop catalysts capable of decreasing the emissions of air pollutants such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NO x ) from automotive gas exhausts [START_REF] Taylor | Catalysis Science and Technology[END_REF][START_REF] Shelef | [END_REF][3][4][5][6]. The development of improved catalysts to meet the ever more stringent emissions standards is, however, still of key importance. To achieve this goal, the most promising means would obviously be to find a catalytic system that could directly decompose NO over a wide range of temperatures as pointed out by Pârvulescu et al. in a rather recent review [4].

Although a large number of catalysts have already been tested, no such system has as yet been found.

Three-Way Catalysts (TWC) were among the first possibilities investigated for lean NO x abatement in car exhausts. These catalysts, which typically operate with an air-to-fuel ratio (A/F) close to the stoichiometry [START_REF] Taylor | Catalysis Science and Technology[END_REF][5][6][7], are generally made up of a quite complicated combination of noble metals supported on an oxide carrier. These noble metals have shown to dissociate readily NO when present in their fully reduced state [START_REF] Taylor | Catalysis Science and Technology[END_REF][START_REF] Shelef | [END_REF]4,[8][9][10].

In addition to the ever more restricting limitations on carbon monoxide and nitrogen oxides, new regulations have appeared concerning the emission of carbon dioxide which is well known to be one of the greenhouse gases responsible for global warming. From this point of view, diesel engines, which operate typically under a lean mixture (air-to-fuel ratio (A/F) greater than unity), is among the solutions that must be considered to meet the CO 2 emission requirements. Indeed, such burning conditions lead to a lowering of fuel consumption and consequently to a decrease of the CO 2 emissions. Although these lean conditions have solved essentially the carbon monoxide and unburned hydrocarbon emission problems, they have made the classic TWC ineffective in removing nitrogen oxides from lean automotive gas exhausts.

The catalytic solution using a traditional TWC associated with a storage component (barium carbonate: BaCO 3 ) [11] which has been adopted for lean gasoline engines has still to be adapted for diesel cars which, as yet, have no suitable catalytic solution. A more attractive alternative would be to develop a specific catalyst capable of reducing nitrogen oxide emissions under lean conditions. Among all the different formulations evaluated, to date, zeolite-based catalysts are probably those which have been studied most [3,4]. This might be due to the promising findings of Iwamoto et al. [12,13] and Lee and Armor [14] who showed that Cu-exchanged ZSM5 is able to decompose nitrogen monoxide at RT. This decomposition process is, however, self-inhibiting since as soon as nitrogen monoxide is decomposed on the catalytic sites (copper ions) they are poisoned by the oxygen left from nitrogen monoxide decomposition and, thus, the reaction stops. It is important to notice the striking similarity with model TWC reported in the case of stoichiometric mixture by Djéga-Mariadassou and co-workers [7,15,16]. These authors demonstrated the competition between the CO oxidation reaction and the assisted decomposition of NO by CO. In lean exhausts, carbon monoxide cannot be further used to assist the decomposition of nitrogen monoxide [16]. Consequently, other reductants such as hydrocarbons must be used to assist the decomposition of nitrogen monoxide. In the case of model exhausts, it has been shown by Matsumoto et al. [17] that alkenes, and more particularly propene, are the most efficient reductants to assist the nitrogen monoxide reduction. Alkenes are, however, also known as coke precursors for acidic catalysts which accounts for the dramatic deactivation of zeolite-based catalysts when propene is present in the lean mixture [18]. In addition to this coking phenomenon, zeolite-based catalysts are rather sensitive to water, and deactivation was also reported due to their low hydrothermal resistance [4]. These drawbacks have led researchers to focus on oxidessupported Platinum Group Metals (PGMs) catalysts. Due to their intrinsic higher activity [19][20][21], supported Pt catalysts have been the subject of the greatest number of studies [9,[22][23][24][25][26][27][28][29][30][31][32][33].

These are followed by supported Rh [10,[19][20][21]33] and Pd [19][20][21]33,34] catalysts. Although these catalysts exhibit significant lean deNO x activity with C 3 H 6 as reductant and quite reasonable stability, they have a rather narrow temperature operating window [19] and fairly poor selectivity to N 2 [9,30,34,35]. N 2 O, which is well known as a component of the greenhouse effect [START_REF] Degobert | Automobiles and Pollution[END_REF], is, indeed, formed in substantial amounts. High selectivity to N 2 has, however, been reported for Rh/Al 2 O 3 catalysts [10,19,33]. Alumina-supported Rh catalysts usually operate at much higher temperatures than either supported Pt or Pd catalysts. This peculiarity led Obuchi et al. to the conclusion that this high selectivity to N 2 was attributable to an additional catalytic effect of the alumina support [33]. The influence of the nature of the support was also investigated [19,20,30]. These studies, however, did not report on PGMs supported on ceria-zirconia (CZ) until recently for Rh/CZ [START_REF] Orlik | [END_REF] or Pt catalysts supported on CZ-Al 2 O 3 [31] and pure CZ [32]. Finally, CZ showed only very moderate lean deNO x activity in the presence of C 3 H 6 [38].

The aim of this work is to investigate the influence of Pd addition to a chlorinated ceria-zirconia support on lean deNO x with C 3 H 6 as reductant. To our knowledge, such catalysts have not been investigated for this particular reaction. They have, however, been widely studied in the fields of TWC [39][40][41][42][43], CH 4 total oxidation [44,45] and hydrocarbon reforming [46]. For comparison, the performances of a Pd/SiO 2 catalyst are also reported.

Pd/CZ catalysts show enhanced lean deNO x activity and much higher N 2 selectivity compared to Pd/SiO 2 . This suggests the involvement of distinct lean deNO x reaction mechanisms within these two catalytic systems. Based on transient and steady-state experiments, a mechanism of the lean deNO x is proposed for the Pd/CZ catalysts. The CZ solid solution was provided by Rhodia. The CZ was added to chloridric acid solution (pH 1.9) prepared by adding HCl to distilled water. After ageing for 2 hours under vigorous stirring, CZ was filtered and washed with distilled water before being dried in air at 120°C for 3 hours.

Experimental

Pd/CZ

Two ceria-zirconia-supported Pd catalysts (0.54 and 0.89 wt% Pd) were prepared by incipient wetness impregnation of the chlorinated support by an aqueous solution of PdCl 2 , 3H 2 O (Johnson Matthey). After impregnation, the catalysts were aged for 2 hours and dried in air at 120°C for three hours.

Pd/SiO 2

For comparison with Pd/CZ catalysts, a silica (Degussa, Aerosil 50)-supported palladium catalyst (0.93 wt% Pd) was prepared by incipient wetness impregnation of the support by an aqueous solution of PdCl 2 , 3 H 2 O (Johnson Matthey). After impregnation, the catalyst was aged for 2 hours and dried in air at 120°C for three hours.

Catalyst characterization

Metal and chlorine contents were determined by chemical analyses (CNRS -Vernaison). The chlorine content was about 1 wt% for the CZ-based catalysts before and after reaction.

The specific surface areas were determined by physisorption of N 2 at 77K using a Quantasorb Jr. dynamic system equipped with a thermal conductivity detector (TCD). The specific surface areas were calculated using the BET method. For CZ-supported catalysts, the specific surface areas were about 100 m 2 g -1 before and after testing, whereas that of Pd/SiO 2 was about 50 m 2 g -1 .

Determination of the percentage of exposed zero-valent Pd atom (PEM 0 )

With the well-known reducibility of the CZ support [START_REF] Overbury | Ceria Surfaces and Films for Model Catalytic Studies Using Surface Analysis Techniques[END_REF], the determination of the percentage of exposed zero-valent Pd atom was done by means of propene hydrogenation [START_REF] Salin | [END_REF].

Prior to propene hydrogenation, the catalyst sample (30-60 mg deposited on sintered glass of a pyrex reactor) was heated in flowing H 2 (100 mL NTP min -1 ) at atmospheric pressure with a heating rate of 3°C min -1 up to 300°C and was kept at this temperature for 2h. After cooling to -78°C under H 2 , the reaction was started. The partial pressure of propene was 51.8 torr, and the total flow rate was 107 mL NTP min -1 with H 2 as balance.

The composition of the effluents was analyzed by means of an on-line gas chromatograph (Hewlett Packard 5890, FID) equipped with a CP-Al 2 O 3 /KCl (Chrompack, 50 m long, 0.32 mm inner diameter, 5 µm film thickness) capillary column. The only detected product was propane.

From the initial reaction rates obtained, the numbers of exposed zero-valent Pd atoms were calculated according to a turnover rate of 0.40 s -1 for the propene hydrogenation reaction at -78°C [START_REF] Salin | [END_REF].

Temperature-Programmed Reduction

Temperature-Programmed Reduction (TPR) experiments were performed in a conventional system equipped with a thermal conductivity detector. After calcination at 500°C for 2h, the reduction was carried out in a flow of H 2 (5%) in Ar (25 mL min -1 ) using a heating rate of 5°C min -1 . Typically, the reduction was carried out up to 700°C and then the sample (0.02-0.05 g) was kept at this temperature for 15 min. H 2 O evolving in the course of the TPR experiments was trapped by a 5A molecular sieve. The amount of H 2 uptake in the TPR was estimated from integrated peak areas.

X-Ray diffraction

X-Ray diffraction (XRD) patterns of CZ, SiO 2 , Pd(0.93)/SiO 2 and Pd/CZ were obtained on a SIEMENS D500 diffractometer with a Cu K  monochromatized radiation.

X-ray absorption near edge spectroscopy (XANES)

X-Ray absorption measurements were carried out using synchrotron radiation of the XAS13 station at LURE (Orsay, France) on line D42 ( May 3-4, 2001). Fluorescence yield spectra of Pd(0.54)/CZ were recorded at RT using a Ge solid detector (Eurisys, 7 elements) combined with a multichannel analyser to select the Pd K fluorescence, whereas those of the reference compounds (Pd foils, 20 µm thick, PdO (> 99% pure, Lancaster)) were collected in the transmission mode. The incident beam was monochromatized using a Ge (400) double monochromator. XANES spectra were collected at the Pd K edge with a sampling step of 2.0 eV/point from 24300 to 24500 eV and an integration time of 10s and 2s in fluorescence and transmission modes, respectively. The energy was calibrated using the Pd foil. XANES spectra were obtained from Pd(0.54)/CZ either after oxidation or reduction treatments under flowing air or H 2 (100 mL NTP min -1 ) for 2h at 500°C. To avoid the exposure of the reduced sample to O 2 , the catalyst was then transferred into a UV cell (Suprasil quartz, 5 cm long, 1cm wide and 1 cm high) that was finally sealed under vacuum (5x10 -5 torr). In the case of the oxidized sample, we checked that the high purity silica glass did not interfere with incident and fluorescent beams. Superimposed spectra were, indeed, obtained with this sample located either into the UV cell or between Kapton tapes. The cell was set to 45° with respect to the incident RX beam and the fluorescence detector. The extraction of the data was done with Michalowicz's software package [START_REF] Michalowicz | Logiciels pour la chimie[END_REF][START_REF] Michalowicz | [END_REF].

Fourrier transform infra-red spectroscopy

Fourrier transform infra-red (FTIR) spectra of adsorbed CO on CZ and Pd(0.89)/CZ were collected on a Bruker Vector 22 FTIR spectrometer equipped with a liquid N 2 -cooled MCT detector and a data acquisition station. 256 scans were averaged with a spectral resolution of 2 cm -1 .

The samples were pressed into self-supporting wafers of 6-11 mg cm -2 . The wafers were loaded in a moveable glass sample holder, equipped on top with an iron magnet, inserted in a conventional pyrex-glass cell (CaF 2 windows) connected to a vacuum system. The iron magnet allowed the transfer of the catalyst sample from the oven-heated region to the infrared light beam.

Prior to adsorption of CO, the catalysts were submitted to a dynamic (50 cm 3 min -1 ) reducing pretreatment (5% H 2 in Ar, Air Liquide, 99.999%) at 500°C for 2 h at atmospheric pressure. The catalyst samples were then evacuated (10 -6 mbar) at 500°C for 60 min before the temperature was decreased to RT under dynamic vacuum. CO (Air Liquide, 99.999%), trapped at 77 K, was adsorbed at RT at equilibrated pressures of 0.9 and 18 torr.

The spectrum of the pretreated catalyst was used as reference and subtracted from the spectra of the catalyst exposed to CO.

Catalytic runs

Prior to catalytic runs, the samples were calcined in situ in dry air at 500°C (4°C.min -1 ) for 2 hours with a flow rate of 500 mL NTP min -1 per gram of catalyst.

Steady-state, Temperature-Programmed Desorption (TPD) and Temperature-Programmed Surface Reaction (TPSR) experiments and were carried out. After being contacted with the appropriate gas mixture at RT, temperature transient experiments were carried out from RT to 500°C with a heating rate of 10°C min -1 . Before the TPD experiments, the catalyst samples were flushed in N 2 to remove RT physisorbed species from the adsorption mixture.

The experiments were carried out in a U-type quartz reactor. The sample (0.2 g unless otherwise specified) was held between plugs of quartz wool, and the temperature was controlled through a WEST 4000 temperature controller using a K type thermocouple.

Reactant gases were fed from independent mass flow controllers (Brooks 5850E). The total flow was 250 mL NTP min -1 to which corresponds a HSV (Hour Space Velocity) of 112,500 h -1 .

Catalytic experiments were carried out with C 3 H 6 as reductant. Typically, the composition of the C 3 H 6 -NO-O 2 reacting mixture was: 1900 ppm C 3 H 6 , 340 ppm NO and 8% O 2 in N 2 . The reactants, diluted in N 2 , were fed from independent gas cylinders (Air Liquide).

The reactor outflow was continuously analysed using the combination of four different detectors. An Eco Physics CLD 700 AL chemiluminescence NO x analyser (for NO and total NO x (i.e. NO + NO 2 )) allowed the simultaneous detection of both NO and NO x . Two Ultramat 6 IR analysers were used to monitor N 2 O, CO and CO 2 . A FID detector (Fidamat 5A) was used to follow the concentration of hydrocarbons. We checked that, under our experimental conditions, CO and CO 2 had a negligible response on the N 2 O IR analyser, whereas that of C 3 H 6 was significant. C 3 H 6 contribution to the N 2 O signal was taken into account to calculate the "true" N 2 O concentration ([N 2 O]) as follows (Eq. 1):

inlet 6 3 meas. 6 3 0 2 meas. 2 2 ] H [C ] H [C x O] [N O] [N O] [N   (1)
where concentrations measured in the course of the reaction, respectively. As N 2 was used as balance, the conversion of NO x to N 2 was calculated assuming 100% nitrogen mass balance (Eq. 2):

[N 2 O] 0 , [C 3 H 6 ] inlet , [C 3 H 6 ] meas.
inlet x 2 outlet x inlet x 2 x ] [NO O]) [N 2 ] [NO ( ] [NO (%) N to NO of Conversion x    (2)
where [NO x ] inlet and [NO x ] outlet are the concentrations of NO x at the inlet and at the outlet of the reactor, respectively.

In one experiment, over Ce 0.68 Zr 0.32 O 2 , propene was substituted by 1-propanol (Merck, spectroscopic grade) with a concentration of 2200 ppm by means of a saturator.

Results

Catalyst characterization

Determination of the percentage of exposed zero-valent metal atoms (PEM 0 )

Table 1 lists the percentage of exposed zero-valent palladium atoms of the CZsupported Pd catalysts and Pd(0.93)/SiO 2 . The percentage of exposed zero-valent Pd atoms is about 17 and 22% for Pd/CZ and Pd(0.93)/SiO 2 , respectively.

TPR results

Fig. 1 shows the TPR profiles of the calcined catalysts. For Pd(0.93)/SiO 2 , the trace of which is magnified by a factor of 4 for the sake of clarity, a reduction peak is observed at about 50°C.

For the CZ sample, the trace of which is magnified by a factor of 3, a broad reduction peak starting around 400°C and ranging to a maximum at 530°C is observed. This reduction temperature region corresponds well with that reported for various ceria-zirconia materials [51][52][53][54][55].

The presence of noble metal on CZ modifies substantially the features of the TPD profiles. Indeed, the reduction peak revealed over the CZ sample vanishes, while a strong signal appears at temperatures below 200°C depending on the catalyst sample. It is observed that the reduction peak shifts to lower temperature with increasing Pd content. Luo and Zheng [56] reported a similar feature despite the fact that their TPR traces were substantially different from ours.

As it is well established that the incorporation of noble metals increases the reducibility of the ceria-related materials [START_REF] Overbury | Ceria Surfaces and Films for Model Catalytic Studies Using Surface Analysis Techniques[END_REF]51,52,56,57], quantitative analysis of the TPR traces is not discussed. Overall, the amount of H 2 consumed in the course of the TPR was close to 0.5 10 -3 mol g -1 for all samples. This value agrees well with those listed by Luo and

Zheng for Ce 0.5 Zr 0.5 O 2 -supported Pd catalysts [56].

Finally, the comparison of the TPR traces of Pd(0.93)/SiO 2 and Pd(0.89)/CZ clearly shows that the Pd species interact to a greater extent with CZ than with SiO 2 . The reduction peak of these Pd species supported on CZ is, indeed, shifted to higher temperature compared with that of Pd(0.93)/SiO 2 .

XRD

XRD patterns of the calcined SiO 2 , Pd(0.93)/SiO 2 and CZ samples are shown in Fig. 2.

After calcination of Pd(0.93)/SiO 2 , a diffraction peak is observed at about 33.8° which corresponds to PdO (101 plane). Two very weak peaks at about 41.9 and 54.8° can also be assigned to PdO. As shown in Fig. 2, the diffraction pattern of CZ exhibits 4 peaks at about 29, 33.8, 48.3 and 57.2°. Owing to the weak intensity of the PdO peaks and the overlapping with CZ support, the presence of PdO on the CZ-supported catalysts could not be established through XRD.

XANES measurements

XANES spectra of the Pd(0.54) catalyst after calcination or reduction are shown in Fig. 3. The shape of the XANES spectra of the Pd(0.54)/CZ sample either calcined (----) or reduced ( ___ ) corresponds fairly well to that of the PdO ( ) and Pd metal () references (Fig. 3), respectively. This result markedly differs from that of the reduced Rh/CZ catalyst, which showed a XANES spectrum different from both Rh foil and Rh 2 O 3 references [58].

IR

The CO-FTIR technique was used to probe the nature of the metal species [59] supported on CZ.

The FTIR spectra of adsorbed CO under equilibrated pressures of 0.9 and 18 torr of CO are shown in Fig. 4 for CZ and Pd(0.89)/CZ. It is worthwhile noting the fairly low intensity of the CO absorption bands. This is why the CO-FTIR of the Pd(0.54)/CZ catalyst was not investigated.

Over CZ (Fig. 4a), two CO absorption bands are observed at 1575 and 2167 cm -1 . The band at 1575 cm -1 can be assigned to the formation of carbonates [60]. This band appears as soon as the equilibrated pressure of CO is 0.9 torr, whereas the band at 2167 cm -1 is observed only at a pressure of 18 torr. This latter band has been attributed to the weak interaction of CO with Ce surface sites [61][62][63][64].

The FTIR spectra of adsorbed CO on Pd(0.89)/CZ (Fig. 4b) differ markedly from those of CZ. The formation of carbonate is not observed, and a rather broad absorption band from 1850 to 2090 cm -1 is revealed, with two maxima at 1950 and 2083 cm -1 . The band at 1950 cm -1 , also found in a previous work of Badri et al. on a chlorinated Pd/CeO 2 sample [61], can be assigned to bridged CO species bonded to zero-valent Pd atoms (Pd 0 2 CO) [43,65,66]. In contrast, the band with the maximum at 2083 cm -1 corresponds to CO species singly bonded to Pd 0 atoms [43,59,61,63,65]. CO coordinated to Ce z+ centers, acting as Lewis acid sites, gives rise to an absorption band at 2175 cm -1 , as already reported by Badri et al.

[61].

NO oxidation

As reported earlier [38 and references therein], the oxidation of NO to NO 2 is suspected to be an essential step of the deNO x process. This reaction was, thus, investigated over CZ and Pd(0.54)/CZ under steady-state conditions. As shown in Fig. 5, NO oxidation is the greatest at 400°C. At higher temperatures, the conversion of NO decreases due to thermodynamic limitations. It is important to note that the addition of Pd does not promote NO oxidation, and this reaction is only catalysed by the CZ support.

Adsorption of NO-O 2 -TPD in N 2

Fig. 6 shows the NO x TPD profiles in N 2 of samples contacted with NO-O 2 (340 ppm-8%). The addition of Pd to CZ does not affect the TPD profiles to a large extent. In both cases, two NO peaks are observed at about 100 and 420°C, and a broad NO 2 desorption profile is seen from 50 to 450°C. This NO 2 feature exhibits two maxima at low and high temperatures. The high temperature maximum is close to 350°C and is not affected by the introduction of Pd. In contrast, the low temperature maximum shifts from 116 to 136°C when Pd is added to CZ (Fig. 6c). For both samples, the quantities of adsorbed and desorbed NO x closely match (Table 2). Table 2 shows that the amount of adsorbed NO x fluctuates to some extent. This was related to slightly different times of exposure and not to differences in the starting conditions of the samples. In agreement with previous work [67], PdO/SiO 2 did not chemisorb NO x . Consequently, the TPD in C 3 H 6 -O 2 after NO x chemisorption was not performed.

Adsorption of NO-O 2 -TPD in C 3 H 6 -O 2

After exposure of the samples to a NO-O 2 mixture, the TPD profiles obtained in C 3 H 6 -O 2 are shown in Fig. 7 for CZ and Pd(0.89)/CZ. Before TPD, the samples were flushed by N 2 to remove physisorbed NO x and then contacted with the desorption gas mixture (C 3 H 6 -O 2 ) for a few minutes at RT. For both samples, a C 3 H 6 desorption profile is observed at low temperature. It is worth mentioning that in the case of the oxidation of C 3 H 6 by O 2 without pre-adsorption of NO x (not shown), a similar desorption feature was observed. This low temperature desorption feature is, thus, attributed to propene species chemisorbed on catalytic sites different from those occupied by adsorbed NO x species. For both catalysts, HC consumption starts at 80°C and a more drastic HC consumption is observed at about 110°C. It is worthwhile mentioning that the HC consumption profile is slightly different for Pd(0.89)/CZ compared with that of CZ. At temperatures higher than 150°C, the HC concentration reaches its input concentration for CZ (Fig. 7a). This phenomenon, however, is not observed for Pd(0.89)/CZ (Fig. 7b). The first column of Table 3 shows that the light-off temperature of C 3 H 6 in the course of the TPD in C 3 H 6 -O 2 decreases drastically with the addition of Pd.

Considering desorption of the N t O x (NO, NO 2 and N 2 O) species, rather complicated profiles are observed (Fig. 7). A detailed description of the traces seen for CZ is reported in a recent article [38]. The amounts of adsorbed and desorbed N t O x species are listed in Table 2.

For both catalysts, a comparison of the desorbed quantities (Table 2) and profiles of N t O x species of the two TPD experiments either in N 2 (Fig. 6) or in C 3 H 6 -O 2 (Fig. 7) after exposure of the samples to NO-O 2 shows striking differences. First, NO becomes the major N t O x species of the TPD in C 3 H 6 -O 2 . In contrast, NO 2 is the major species of the TPD in N 2 .

Second, the global amount of desorbed N t O x species, taking into account that N 2 O originates from the recombination of two NO x molecules, is always lower than that adsorbed for the

TPD in C 3 H 6 -O 2 .
On the other hand, adsorbed and desorbed NO x quantities agree well for the TPD in N 2 . These results suggest that part of the adsorbed NO x is reduced to N 2 in the presence of C 3 H 6 in the desorption mixture. Finally, the N t O x TPD profiles (Fig. 7) suggest that part of the adsorbed NO x species reacted with C 3 H 6 in the low temperature region (50-115°C). For N t O x species, the NO x desorption feature is clearly truncated in this low temperature region compared to that of the TPD in N 2 (Fig. 6). As already discussed [38],

these concomitant consumptions suggest the interaction of adsorbed NO x species (ad-NO x )

with propene and the formation of mixed compounds (R-NO x ) stored on the catalytic material. For Pd(0.89)/CZ, it is worth adding that a stabilised HC consumption occurs up to higher temperatures, ca. 150°C, compared with that observed on CZ (Fig. 7). Overall, the Fig. 8 shows that the C 3 H 6 oxidation profiles are obviously different depending on the nature of the catalyst support.

On Pd(0.89)/CZ, C 3 H 6 oxidation is rather similar to that observed after adsorption of NO-O 2 (Fig. 7b) with comparable light-off temperature (Table 3). On such a catalyst, however, the consumption of C 3 H 6 in the C 3 H 6 -O 2 reaction starts at a higher temperature and is more pronounced before 250°C than after adsorption of NO-O 2 . In contrast, C 3 H 6 oxidation proceeds much more steeply in a much narrower range of temperature (from 195 to 260°C) on Pd(0.93)/SiO 2 than on Pd(0.89)/CZ (Fig. 8). In addition, C 3 H 6 does not chemisorb at RT on Pd(0.93)/SiO 2 , as is the case of Pd(0.89)/CZ that shows a low temperature C 3 H 6 desorption feature. For Pd(0.93)/SiO 2 , it is worthwhile noting that the experiment reported on Fig. 8 was carried out after a previous C 3 H 6 -NO-O 2 Temperature-Programmed Surface Reaction (TPSR).

It will be shown later (please refer to part 3.1.10.) that for Pd(0.93)/SiO 2 such an experimental sequence does significantly influence the light-off temperature of C 3 H 6 , whereas it has no impact on CZ-supported catalysts. Compared with the TPSR profiles found on CZ (Fig. 12 of [38]), the desorption peak of NO reported in this experiment at about 300°C is no longer observed in the course of the TPSR on Pd(0.89)/CZ, as a NO x deficit occurs in this temperature range (Fig. 9).

The TPSR profiles of Pd(0.93)/SiO 2 (not shown) are less complicated than those of Pd(0.89)/CZ. The most interesting peculiarity is that if the temperature is decreased from 500°C to RT (reverse TPSR) after the first TPSR experiment, the C 3 H 6 light-off temperature and the selectivity to N 2 are modified significantly. The C 3 H 6 light-off temperature and the N 2 selectivity of the TPSR and the reverse TPSR decrease from 298 to 237°C (Table 3) and from 70 to 30%, respectively. On the other hand, the conversion of NO x to N 2 remains almost constant and close to 9% at maximum NO x conversion. Such phenomena were not observed for the CZ-based catalysts for which the C 3 H 6 and NO x profiles of the TPSR and the reverse TPSR were comparable.

As in the case of the C 3 H 6 -O 2 reaction, Table 3 shows that the C 3 H 6 light-off temperature for the TPSR is lowest for Pd(0.93)/SiO 2 and highest for CZ. Pd also promotes significantly C 3 H 6 oxidation for the CZ-based catalysts.

Steady-state NO-C 3 H 6 -O 2 reaction

Prior to steady-state measurements, the catalysts were submitted to a TPSR under the complete reacting mixture from RT to 500°C with a heating rate of 10°C min -1 .

Fig. 10 shows the performances of the catalysts for the reduction of NO x under steadystate conditions with stepwise increase of the temperature. The temperature of maximum of N 2 formation, the corresponding NO x and C 3 H 6 conversions are listed in Table 4.

It is obvious that the introduction of Pd leads to a significant increase of NO x reduction to N 2 as well as a drastic decrease of the C 3 H 6 light-off temperature. Table 3 lists the temperature of light-off of C 3 H 6 over various Pd-containing catalysts with respect to reaction conditions. From this table it can also be concluded that the presence of NO does not influence this light-off temperature, as was the case for CZ ( [38] and first line of Table 3).

Conversion of NO x to N 2 higher than 20% is obtained on the Pd/CZ catalysts. On the other hand, Pd(0.93)/SiO 2 shows a rather low conversion of NO x to N 2 close to that found for CZ, despite the fact that the light-off temperature of propene is similar to that of the most active CZ-based catalyst (Pd(0.89)/CZ). One also may note that propene oxidation is steeper over Pd(0.93)/SiO 2 than over Pd/CZ catalysts. It is worthwhile noting that N 2 selectivity is higher than 80% on CZ-based catalysts, whereas that of Pd(0.93)/SiO 2 is of 28% (Table 4).

Finally, Fig. 10e shows that NO oxidation to NO 2 is almost eliminated in the presence of C 3 H 6 before 400°C, the temperature of which corresponds to the almost complete conversion of C 3 H 6 (Fig. 10c).

To illustrate the high stability of the CZ-supported catalysts, Table 5 reports on the influence of time on stream on the reduction of NO x on Pd(0.89)/CZ. This catalyst exhibits NO x conversion to N 2 by about 30% and does not deactivate in the absence of water for 1 h.

After about 1 hour of run, 1.7% of water was introduced in the reaction mixture. This addition decreases the conversion of NO x by about 10%. More interesting is that the catalyst does not deactivate over a period of 19 hours on stream in the presence of water. After 19 hours on stream, the initial activity is restored when water is removed from the feed. This suggests that H 2 O competes with the active sites responsible for N 2 formation. C 3 H 6 conversion remains almost unaffected and close to 60%. Chemical analysis of this sample after reaction reveals a carbon content of only 0.4 wt%.

Steady-state NO-C 3 H 6 -O 2 reaction over mechanical mixtures

The results obtained over two mechanical mixtures of Pd(0.89)/CZ + SiO 2 and Pd(0.93)/ SiO 2 + CZ are shown in Fig. 11. It is obvious that the mechanical mixture of Pd(0.89)/CZ + SiO 2 (Fig. 11a) is more active than its counterpart (Fig. 11b) over a wide range of temperature. Table 6 compares the performances of the mechanical mixtures at about 250°C. Given that SiO 2 exhibits no deNO x , the activity reported for the mechanical mixture of Pd(0.89)/CZ and SiO 2 is only due to Pd(0.89)/CZ. It is quite unusual to note that the HSV does not influence the performance of the Pd(0.89)/CZ catalyst as the experiments carried out with 0.10 or 0.20 g of this sample give comparable deNO x activities (Table 4 and Table 6, Fig. 10d and Fig. 11a). This unexpected result suggests that the geometry of the reactor used in this study was not the best to achieve optimised deNO x conversions. The deNO x activity reported for the mechanical mixture made up of Pd(0.93)/SiO 2 and CZ roughly corresponds to the sum of the activities of both catalytic systems. These conclusions are corroborated by the selectivity to N 2 that is: (i) close to that reported for Pd(0.89)/CZ (Table 4) for the mechanical mixture made up of Pd(0.89)/CZ and SiO 2 (Table 6), and (ii) intermediate to those of Pd(0.93)/SiO 2 and CZ (Table 4) for the corresponding mechanical mixture (Table 6). as reductant (Fig. 10b), CZ exhibits a higher lean deNO x activity in the presence of 1-C 3 H 7 OH over a broader range of temperatures than with C 3 H 6 . In comparison, the highest activity, 19% NO x to N 2 at 283°C when 1-C 3 H 7 OH is used as reductant, is more than twice that listed at 361°C with C 3 H 6 (Table 4). The selectivity to N 2 in the presence of C 3 H 7 OH (76%) is comparable to that found in the presence of C 3 H 6 (81%).

Discussion

State of Pd in the supported Pd catalysts

The XRD pattern of the calcined Pd(0.93)/SiO 2 sample (Fig. 2) indicates, as expected, the presence of PdO. The corresponding reduced sample exhibits propene hydrogenation activity (Table 1).

As mentioned in the results (part 3.1.3.), due to the overlapping of the diffraction peaks of PdO with those of CZ, XRD characterisation could not be successfully used to investigate the nature of the Pd species of the Pd/CZ catalysts. XANES measurements of oxidised and reduced Pd(0.54)/CZ samples show that they closely resemble those of PdO and the Pd foil, respectively (Fig. 3). As in the case of Pd/SiO 2 , these XANES measurements suggest that Pd is present as PdO for the calcined sample (Fig. 3a). Pd/CZ catalysts also catalyse propene hydrogenation (Table 1), which agrees fairly well with the XANES measurements of the reduced Pd(0.54)/CZ sample (Fig. 3), indicating the presence of reduced Pd clusters. Holles and Davis have reported, however, that Pd(4.0)/CeO x /Al 2 O 3 could not be fully reduced even after exposure to CO at 400°C [68]. Comparable conclusions were also drawn by Matsumara and coworkers for Pd(3.0)/CeO 2 and Pd(2.0)/ZrO 2 samples reduced under H 2 at 300 and 400°C [69,70], respectively. As X-ray absorption spectroscopy is known to provide an average picture of the targeted metal species [START_REF] Vaarkamp | Handbook of Heterogeneous Catalysis[END_REF], CO-FTIR measurements were also performed on Pd(0.89)/CZ (Fig. 4b). In agreement with XANES measurements, CO-FTIR spectra recorded after H 2 reduction at 500°C do not reveal the presence of Pd oxidised species, as is the case with a CZ-supported Rh catalyst [58].

The characterisation of these catalysts after oxidising or reducing pretreatments were motivated due to the initial calcination of the samples and the C mixture, Pd was not reduced to Pd 0 for Pd(1.0)/Ce 0.5 Zr 0.5 O 2 up to reaction temperature as high as 400°C, but remained in an oxidised state due to contact between PdO clusters and ceriazirconia [START_REF] Fernàndez-Garcia | [END_REF]. It is, thus, very likely that the same conclusion also applies to the Pd/CZ catalysts of the present work. This assumption is supported by: (i) the TPR traces revealing greater interaction of the Pd species with CZ than with SiO 2 (Fig. 1), and (ii) the steady-state catalytic results (Table 4 and Table 6) that show markedly different N 2 selectivity for Pd/CZ and Pd/SiO 2 catalysts. The N 2 selectivity found over the Pd/CZ catalysts is, indeed, more than twice that observed over Pd/SiO 2 . The N 2 selectivity listed for Pd/SiO 2 agrees well with those already reported over a comparable catalytic system [19], over Pd/Al 2 O 3 [34] or over Pt 0 clusters [19]. This means that PdO is reduced to Pd 0 when being supported on SiO 2 even under a lean atmosphere. Such a conclusion has also been reported by Fernàndez-Garcia et al.

for alumina-supported Pd catalysts [START_REF] Fernàndez-Garcia | [END_REF]. As already mentioned above, however, these authors have demonstrated that this was not the case for a Pd(1.0)/Ce 0.5 Zr 0.5 O 2 catalyst. The obvious higher N 2 selectivity for the Pd/CZ catalysts might also suggest that the deNO x mechanism occurring over these materials is different from the dissociation mechanism reported over zero-valent PGMs atoms [6,9] and, thus, that the nature of the Pd species of the Pd/CZ catalysts is different from that of Pd 0 . These arguments are further supported by the comparison of the light-off of C 3 H 6 of the TPSR and reverse TPSR (see part 3.1.10.) that decreases to a significant extent on Pd(0.93)/SiO 2 , and it is hardly affected at all on Pd/CZ catalysts. Under the experimental conditions of the present study, it is, therefore, very likely that Pd is present as PdO x species, with x close to unity [73], rather than Pd 0 .

Lean deNO x mechanism proposal over PdO x /CZ catalysts

The results of the present work show that the incorporation of Pd to CZ promotes significantly lean deNO x activity (Fig. 10). In addition, CZ-supported catalysts exhibit a much higher N 2 selectivity compared with that of Pd 0 (0.93)/SiO 2 (Table 4). Centi et al. [31] and Liotta et al. [32] have also recently reported elevated N 2 selectivities over Pt/Ce 0.60 Zr 0.40 O 2 -Al 2 O 3 and Pt/Ce 0.60 Zr 0.40 O 2 catalysts. These authors, however, did not comment about this peculiarity. As already mentioned in the preceding characterisation section, this obvious difference in N 2 selectivity between Pd 0 /SiO 2 and PdO x /CZ catalysts suggests that the lean deNO x mechanism occurring over PdO x /CZ catalysts is different from that occurring over Pd 0 /SiO 2 . The fairly low N 2 selectivity found for Pd 0 /SiO 2 (Table 4) indicates that lean deNO x most likely proceeds through the NO dissociation mechanism [9] on zero-valent Pd clusters, the oxygen left from NO dissociation on the reduced surface being "cleaned" by the C 3 H 6 oxidation reaction.

In a recent article [38], we suggested that reaction intermediates such as NO 2 , organic nitrogen-containing compounds (RNO x ) and oxygenates (C x H y O z ) might be involved in the lean deNO x catalysis on CZ. In the proposed mechanism, the production of NO 2 and C x H y O z , the formation of the latter resulting from the decomposition of R-NO x intermediates produced by the reaction of NO 2 and C 3 H 6 , has been assumed to be critical for lean deNO x activity, as also reported by other authors [34]. The present study also shows that NO oxidation to NO 2 is achieved on the bare support, as the introduction of Pd does not promote such a reaction (Fig. 5). This conclusion is consistent with the work of Krishna et al. in which the authors reported that CeO 2 is a fairly efficient catalyst in the oxidation of NO [74].

The activation of C 3 H 6 proceeds unambiguously on the PdO x catalytic function, as demonstrated by the C 3 H 6 light-off temperature that decreases significantly over supported Pd catalysts (Fig. 10 and Table 3). In addition, it is obvious that this low temperature activation of C 3 H 6 is responsible for the lean deNO x activity of PdO x /CZ catalysts, as both C 3 H 6 activation and deNO x occur concomitantly (Fig. 9 and Fig. 10c,d).

One might, however, wonder about the activation process of C 3 H 6 in the course of the deNO x reaction. On that point, the TPD in C 3 H 6 -O 2 after adsorption of NO-O 2 is very informative. Fig. 7b shows that C 3 H 6 is activated through ad-NO 2 species, as already proposed on CZ [38], the low temperature (130-180°C) consumption of C 3 H 6 being different from the C 3 H 6 profiles observed in the C 3 H 6 -O 2 reaction on Pd(0.89)/CZ (Fig. 8) and in the TPD in C 3 H 6 -O 2 after adsorption of NO-O 2 on CZ (Fig. 7a). In this temperature range, it is worthwhile noting that the NO 2 concentration of the TPD in N 2 after adsorption of NO-O 2 on Pd(0.89)/CZ is greater than that of CZ (Fig. 6c, 130-180°C region). These observations strongly suggest that NO 2 species adsorbed on the PdO x catalytic function account for the activation of C 3 H 6 . Such a conclusion is also supported by the fact that NO is mainly desorbed in the TPD in C 3 H 6 -O 2 after adsorption of NO-O 2 (Table 2), whereas NO 2 would have been expected to be the most abundant desorbed compound from the TPD in N 2 (Table 2 and Fig. 6b). As discussed previously [38], the activation of C 3 H 6 by ad-NO 2 and the concomitant release of NO might be attributed to the intermediate formation of R-NO x compounds, the decomposition of which produces NO and C x H y O z .

One might wonder whether lean deNO x also occurs on the PdO x catalytic function.

Given that lean deNO x occurs to a significant extent on CZ when C 3 H 6 is substituted by 1-C 3 H 7 OH (Fig. 12), it seems more likely that NO reduction takes place via a lacunar mechanism on reduced CZ sites, as proposed by several groups [75][76][77][78]. These authors, indeed, provided evidence for the decomposition of NO to N 2 on prereduced CeO 2 surfaces. It is worth reporting that this decomposition pathway has also been considered in Three-Way Catalysis in which CO is used as reductant [15]. In the course of the lean deNO x reaction, the creation of the sites responsible for the decomposition of NO would, thus, be achieved by reduction of CZ by the oxygenates (C x H y O z ) produced by the decomposition of R-NO x formed on the PdO x catalytic function.

The corresponding detailed mechanism of the C 3 H 6 -assisted decomposition of NO on PdO x /CZ catalysts is described in Fig. 13. This mechanism stresses the key role of NO to give NO back and the competition between the NO oxidation and decomposition reactions on the CZ catalytic sites also account for the limited formation of NO 2 in the course of the lean deNO x process (Fig. 7b, Fig. 9 and Fig. 10e). At elevated temperatures, complete oxidation of C 3 H 6 by molecular O 2 becomes predominant [80] on PdO x , and the deNO x process cannot proceed further.

Conclusion

This study shows that the incorporation of Pd to CZ greatly promotes the reduction of NO in the presence of C 3 H 6 . These catalysts display very stable deNO x activity even in the presence of 1.7% water, the addition of which induces a reversible deactivation of about 10%.

Ex situ characterisation of the catalysts through propene hydrogenation reaction, XRD, XANES, CO absorption followed by FTIR along with the catalytic results indicate that PdO is reduced to Pd 0 clusters on Pd/SiO 2 , this catalyst having been studied as a reference catalyst.

This complete reduction process, however, does not occur for PdO x /CZ catalysts under our experimental conditions.

The PdO x /CZ catalysts also exhibit much higher selectivity to N 2 than that of Pd 0 /SiO 2 , the selectivity of which is consistent with those already reported in literature data over supported zero-valent noble metal catalysts. The much higher N 2 selectivity obtained on PdO x /CZ suggests that the lean deNO x mechanism occurring on these catalysts is different from that occurring on Pd 0 /SiO 2 which consists of the dissociation of NO on reduced palladium sites, followed by the regeneration of the active Pd 0 sites by C 3 H 6 . The results of the catalytic experiments led us to propose a detailed mechanism (Fig.

  and [N 2 O] meas. are: the concentrations of N 2 O due to the contribution of the inlet concentration of C 3 H 6, the inlet concentration of C 3 H 6 , C 3 H 6 and N 2 O

Figure captions Fig. 1 :

 captions1 Figure captions

Fig. 2 :Fig. 3 :

 23 Fig. 2: XRD patterns of SiO 2 , Pd(0.93)/SiO 2 and Ce 0.68 Zr 0.32 O 2 .

Fig. 4 :

 4 Fig. 4: FT infrared spectra of (a) Ce 0.68 Zr 0.32 O 2 (CZ) and (b) Pd(0.89)/CZ, reduced under

Fig. 5 :

 5 Fig. 5: Steady-state catalytic conversion of NO oxidation to NO 2 in the course of the NO-O 2

Fig. 6 :

 6 Fig. 6: NO x Temperature-Programmed Desorption profiles in N 2 after exposure of the

Fig. 10 :

 10 Fig. 10: Steady-state catalytic conversions of C 3 H 6 and NO x to N 2 in the course of the C 3 H 6 -

Fig. 11 :Fig. 12 :

 1112 Fig. 11: Steady-state catalytic conversions of C 3 H 6 and NO x to N 2 in the course of the C 3 H 6 -

Fig. 13 :Fig. 13

 1313 Fig. 13: (a) Schematic representation of the catalytic functions, (b) Mechanism of the lean

  300°C) with the incorporation of Pd. In addition, the amount of NO 2 desorbed at temperatures higher than 200°C is much lower over Pd(0.89)/CZ than over CZ. That of N 2 O is, however, greater.

	broad N t O x desorption features observed at low temperatures (100-200°C) over CZ become thinner over Pd(0.89)/CZ, whereas those at high temperatures (220-400°C) are clearly shifted to lower temperatures (200-3.1.9. C 3 H 6 oxidation by O 2

Table 3

 3 shows that the C 3 H 6 light-off temperature for the C 3 H 6 -O 2 reaction is the lowest for Pd(0.93)/SiO 2 and the highest for CZ. Finally, Pd promotes significantly C 3 H 6 oxidation for the CZ-based catalysts. The features observed are comparable to those seen for the TPD in C 3 H 6 -O 2 after adsorption of NO-O 2 (Fig.7b). NO desorption is the major N t O x species with a two peak profile (100 and 220°C). The formation of NO 2 and N 2 O are, however, more limited than those obtained for the TPD experiment in C 3 H 6 -O 2 (Fig.7). C 3 H 6 and NO x consumptions detected in the low temperature region in the case of the TPD experiment in C 3 H 6 -O 2 are no longer observed. C 3 H 6 is moderately consumed from 100 to 200°C and at higher temperatures the oxidation of C 3 H 6 accelerates. More interesting is that the NO x profile shows a decrease of the NO x concentration at 297°C corresponding to 50% of the NO x inlet concentration. At this temperature, the conversion of C 3 H 6 is 79%. At higher temperatures, the NO x concentration reaches back its inlet concentration, and NO is oxidized to NO 2 when full conversion of C 3 H 6 is achieved at about 400°C.

	3.1.10. C 3 H 6 -NO-O 2 TPSR experiment

TPSR of the complete reacting mixture was performed after exposure of Pd(0.89)/CZ to the reacting mixture at RT (Fig.

9

).

  3.1.13. Steady-state NO-C 3 H 6 OH-O 2 reaction over Ce 0.68 Zr 0.32 O 2 Over CZ, C 3 H 6 was substituted by 1-C 3 H 7 OH as reductant. The results of this steadystate experiment are shown in Fig. 12. In contrast to the steady-state measurements with C 3 H 6

  3 H 6 -NO-O 2 TPSR experiment that was carried out before steady-state measurements, the TPSR experiment leading to the exposure of the calcined sample to the C 3 H 6 reductant. The characterisation of the Pd/CZ catalysts indicates that Pd is present as PdO in the calcined samples. Nevertheless, one might wonder about the nature of the Pd species in the course of the lean NO x reaction. Fernàndez-Garcia et al. have recently shown that even under a stoichiometric C 3 H 6 +CO+NO+O 2

Table 1 :

 1 13) for which CZ achieves both NO oxidation to NO 2 and NO decomposition to N 2 , whereas PdO x activates C 3 H 6 via ad-NO 2 species, intermediately producing R-NO x compounds that further decompose to NO and C x H y O z . The role of the latter oxygenates is to reduce CZ to provide the catalytic sites responsible for NO decomposition. Finally, the proposed C 3 H 6 -assisted NO decomposition mechanism stresses the key role of NO 2 , R-NO x and C x H y O z as intermediates of the Selective Catalytic Reduction (SCR) of NO x by hydrocarbons. Determination of the percentage of exposed zero-valent Pd atoms (PEM 0 ) over Ce 0.68 Zr 0.32 O 2 (CZ)-and silica-supported catalysts by means of propene hydrogenation reactions[START_REF] Salin | [END_REF].

	Catalysts Pd(0.54)/CZ Pd(0.89)/CZ	Pd(0.93)/SiO 2
	Pd 0 (%)	17	16	22

Table 2 :

 2 Amounts of adsorbed or desorbed NO x (NO+ NO 2 ) and desorbed N 2 O in the course of the TPD experiments after exposure of the catalysts to NO-O 2 (340 ppm -8%) in N 2

			Amounts of adsorbed (ads.) or desorbed (des.) species (10 -5 mol g -1 )
	Catalysts	experiment	NO x ads.	NO des. NO 2 des. NO x des. N 2 O des. N balance*
	Ce 0.68 Zr 0.32 O 2 (CZ) TPD in N 2	54.8	8.1	46.5	54.6	0.0	0.1
		TPD in C 3 H 6 -O 2	48.2	19.0	7.8	26.8	5.8	4.9
	Pd(0.89)/CZ	TPD in N 2	47.6	6.2	36.4	42.7	0.0	2.5
		TPD in C 3 H 6 -O 2	51.8	20.3	5.1	25.4	5.4	7.8
	* N balance calculated with respect to N 2 equivalent

Table 3 :

 3 C 3 H 6 light-off temperatures (°C) of Ce 0.68 Zr 0.32 O 2 (CZ)-and silica-supported catalysts; 340ppm NO, 8% O 2 , 1900 ppm C 3 H 6 and N 2 balance

		Ads NO-O 2	Transient	Transient	Steady-state
		TPD in O 2 -C 3 H 6	O 2 -C 3 H 6	NO-O 2 -C 3 H 6	NO-O 2 -C 3 H 6
	CZ	450	473	387	380
	Pd(0.54)/CZ	-	277	276	303
	Pd(0.89)/CZ	276	251	252	243
	Pd(0.93)/SiO 2	-	-	298*	-
			236	237	237
	* Light-off temperature measured in the course of the first NO-O 2 -C 3 H 6 TPSR

Table 4 :

 4 Temperature of maximum of N 2 formation and percentage of NO x converted to N 2 at this temperature over Ce 0.68 Zr 0.32 O 2 (CZ)-and silica-supported catalysts for the steady-state NO-O 2 -C 3 H 6 reaction (340 ppm -8 % -1900 ppm, N 2 balance)

	Catalysts	CZ	Pd(0.54)/CZ Pd(0.89)/CZ Pd(0.93)/SiO 2
	Sample Weight (g)	0.20	0.20	0.20	0.07
	T max N 2 (°C)	361	263	243	243
	Conversion of NO x to N 2 (%)	8	21	29	7
	Selectivity to N 2 (%)*	81	83	87	28
	C 3 H 6 conversion (%)	34	38	50	58
	* Selectivity to N 2 referred to as (N 2 /(N 2 + N 2 O) x 100)

Table 5 :

 5 Influence of the addition of 1.7% water on the conversions of C 3 H 6 and NO x to N 2 at 278°C in the course of the C 3 H 6 -NO-O 2 reaction (1900 ppm-340 ppm -8 %, balance N 2 ) versus time on stream on Pd(0.89)/Ce 0.68 Zr 0.32 O 2 .

			Time on stream (h)
		0	1	20	20.5
	H 2 O (%)	0	1.7	1.7	0
	Conversion of NO x to N 2 (%)	27	24	25	29
	Selectivity to N 2 (%)*	75	76	76	77
	C 3 H 6 conversion (%)	65	62	62	60
	* Selectivity to N 2 referred to as (N 2 /(N 2 + N 2 O) x 100)

Table 6 :

 6 Percentage of NO x converted to N 2 over Ce 0.68 Zr 0.32 O 2 (CZ)-and silica-supported catalysts for the steady-state NO-O 2 -C 3 H 6 reaction (340 ppm-8 % -1900 ppm, N 2 balance)

	Catalysts	Pd(0.89)/CZ + SiO 2 Pd(0.93)/SiO 2 + CZ
	Sample Weight (g)	0.10 + 0.10	0.10 + 0.10
	T (°C)	250	255
	Conversion of NO x to N 2 (%)	29	12
	Selectivity to N 2 (%)*	86	51
	C 3 H 6 conversion (%)	47	50
	* Selectivity to N 2 referred to as (N 2 /(N 2 + N 2 O) x 100)

Table 2

 2 ). Temperature-Programmed Desorption profiles in C 3 H 6 -O 2 (1900 ppm -8%O 2 , balance N 2 ) after exposure of the catalysts to NO-O 2 (340 ppm -8%, balance N 2 ), at RT: (a) Ce 0.68 Zr 0.32 O 2 (CZ), (b) Pd(0.89)/CZ. Temperature-Programmed Surface Reaction profiles of C 3 H 6 -NO-O 2 (1900 ppm -340 ppm-8%) after exposure of Pd(0.89)/Ce 0.68 Zr 0.32 O 2 to C 3 H 6 -NO-O 2 (1900 ppm -340 ppm -8%, balance N 2 ) at RT.

	Fig. 7: Fig. 8: Transient C 3 H 6 -O 2 (1900 ppm -8% O 2 , balance N 2 ) reaction over Pd(0.93)/SiO 2 and
	Pd(0.89)/Ce 0.68 Zr 0.32 O 2 .
	Fig. 9:

Acknowledgments

Rhodia contributed to part of the financial support for this work; the Ministère de l'Enseignement Supérieur et de la Recherche organization supported the work of Dr. Gorce (Grant 98-4-10713) and Ms Fontaine (Grant 8449-2003). We also thank G. Blanchard for his interest in this work and P. Lavaud for his invaluable help in technical support.