Insights into the WOx Coverage-Dependent Location and Oxidation

Article pubs.acs.org/JPCC

Insights into the WOx Coverage-Dependent Location and Oxidation State of Noble Metals Supported on Tungstated Oxides: The Case of Rh/WOx−Ce0.62Zr0.38O2

Thomas Bonnotte,†,‡,# Rachel P. Doherty,†,‡ Céline Sayag,†,‡ Jean-Marc Krafft,†,‡ Christophe Méthivier,†,‡ Mickael̈ Sicard,§ Frédéric Ser,§ and Cyril Thomas*,†,‡ †

Sorbonne Universités, UPMC Univ Paris 06, UMR 7197, Laboratoire de Réactivité de Surface, 4 Place Jussieu, case 178, F-75005, Paris, France ‡ CNRS, UMR 7197, Laboratoire de Réactivité de Surface, 4 Place Jussieu, case 178, F-75005, Paris, France § ONERA, The French Aerospace Lab, Centre de Palaiseau, BP80100, Palaiseau F-91123, France ABSTRACT: Noble metals-promoted tungstated oxides have been shown to be profitable in a wide variety of catalytic reactions of environmental interest but to be detrimental in the hydrogenation of aromatics. The origin of the deleterious effect of tungstates on the hydrogenation performance of noble metals is still being debated. To provide further insights into this, the location and the oxidation state of Rh were investigated as a function of the W surface density (0−10 W/nm2) of Rh/WOx− Ce0.62Zr0.38O2 (Rh/W−CZ) catalysts after high-temperature reduction. For that purpose, a thorough characterization of the oxide phases was performed through N2sorption, X-ray diffraction, Raman spectroscopy, NOx temperature-programmed desorption, and X-ray photoelectron spectroscopy (XPS), whereas the metallic phases were characterized by low-temperature H2 chemisorption, XPS, N2 Fourier transform infrared spectroscopy and benzene hydrogenation. It was found that Rh deposited on both tungstates and CZ, and did not sinter with increasing W surface densities. The observed linear decrease in the Rh hydrogenation performance of the WOx-promoted Ce0.62Zr0.38O2 below pseudo monolayer coverage of CZ (4.8 W/nm2CZ) was assigned to a strong metal support interaction effect between the Rh particles and the nonreducible underlying WOx phase, resulting in the formation of electron-deficient Rh species (Rhδ+). and in the catalytic hydro/dehydrogenation properties8,19,22 of the WOx-promoted NM catalysts. The origin of the deleterious effect of adding tungstates to supporting oxides on the metallic function has been attributed to the appearance of a strong metal support interaction (SMSI) between tungstates and the noble metals. The SMSI effect was originally reported by Tauster in the case of Pt/TiO2 when exposed to a reductive atmosphere at temperatures greater than 400 °C.26 Regalbuto et al. initially proposed that the SMSI effect was due to decoration of the Pt metal particles by tungstates on the basis of CO chemisorption, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) investigations.23,27 The evidence of this decoration phenomenon, which would result in a decrease in the number of accessible Pt0 sites,23 has been provided on the basis of TEM micrographs of model catalysts.27 Later, it was proposed that decoration of the NM particles by WOx species occurred for calcination temperatures greater than 500 °C.11,28 Some authors also suggested that the strong interaction between NM and tungstates may result in the formation of

1. INTRODUCTION Since the original discovery of the acidic properties of tungstated zirconias by Hino and Arata,1 tungstated oxides have been the subject of many studies (ref 2 and references therein). The combination of a noble metal (NM) and a tungstated support has been shown to be profitable in a wide variety of catalytic reactions such as CO−NO on Pt/WO3− SiO2 and Pt/WO3/Ce0.65Zr0.35O2,3,4 toluene decomposition in the presence of H2 on Pt/WOx−ZrO2,5 the isomerization of alkanes on Pt/WOx−ZrO2,6−13 the selective catalytic reduction of nitrogen oxides by hydrocarbons on Pd/WOx−ZrO214−17 or by H2 on Pt/WOx−ZrO2,18 the selective ring-opening of naphthenic molecules on Ir/WOx−ZrO2,19 and, more recently, in the oxidations of soot on Pt/WOx−Al2O320 and propane on Pd/WOx−TiO2.21 Contreras and Fuentes also reported that W promotion of a Pt/γ−Al2O3 material led to more stable catalysts toward deactivation in the hydrogenation of benzene, although these Pt/WO3−Al2O3 catalysts were initially less active than Pt/γ−Al2O3.22 Overall, the promotion of the bare oxides by tungstates (WOx) has been shown to be detrimental to the metallic function of the NM/WOx-oxides materials. This resulted in a decrease in the H2 and CO chemisorption capacities7,8,19,22−25 © 2014 American Chemical Society

Received: November 4, 2013 Revised: February 6, 2014 Published: March 13, 2014 7386

dx.doi.org/10.1021/jp410848f | J. Phys. Chem. C 2014, 118, 7386−7397

The Journal of Physical Chemistry C

Article

oxidized NM species (NMδ+),11,20,25,29,30 as also anticipated by Sen and Vannice for Pt/TiO2 reduced at high temperatures.31 Only a few studies provided evidence of the presence of NMδ+ species on the basis of CO Fourier transform infrared (FTIR)25 spectroscopy, and extended X-ray absorption fine structure (EXAFS)32 measurements. It was also concluded that these species were not present in extensive quantities.25,32 Other authors attributed the decrease in the metallic function capacity of NM/WOx-oxides to the sintering of the NM phase and a corresponding increase in the size of the NM metal particles.4,11,19 Pt sintering on WOx−ZrO2 has been shown to occur for calcination temperatures greater than 550 °C.11 Yet Duchet and co-workers reported that the Pt dispersion of WOx−ZrO2-supported samples calcined at 480 °C, estimated after removing the fraction of Pt embedded in ZrO2,33 did not change on W addition.12 To our knowledge, a systematic and comprehensive study of the metallic function of NM/WOx-oxides has not been reported to date over a wide range of W surface densities, i.e., from 0 to 10 W/nm2. In addition, WOx−CexZr1‑xO2 oxides have been the subject of a very limited number of investigations,4,34−37 and Rh has not been deposited over such tungstated supports. This work thus aims at providing further insights into the location and oxidation state of Rh when deposited on a WOx−Ce0.62Zr0.38O2 support over a broad range of W surface densities. For that purpose, a thorough characterization of the oxide phases was performed through N2sorption, X-ray diffraction (XRD), Raman spectroscopy, NOx temperature-programmed desorption (TPD), and XPS, whereas the metallic phases were characterized by H2-chemisorption, XPS, N2-FTIR, and benzene hydrogenation.

mW; resolution, 4 cm−1; accumulation time, 10 s; 30 scans per spectrum). A microscope with an X50 long working distance (W.D. = 8.0 mm) lens was used. The NOx-temperature-programmed desorption (NOx-TPD) experiments were carried out in a U-shape quartz reactor (15 mm i.d.). The samples were held on plugs of quartz wool, and the temperature was controlled by a Eurotherm 2408 temperature controller using a K type thermocouple. Reactant gases, used as received, were fed from independent gas cylinders (Air Liquide) by means of mass flow controllers (Brooks 5850TR) with a total flow rate of 230 mLNTP/min. Prior to the NOx-TPD experiments, the samples (0.3 g) were calcined in situ in O2(20%)/He at 500 °C (3 °C/min) for 2 h with a flow rate of 100 mLNTP/min. Typically, the composition of the adsorption mixture consisted of 400 ppm NOx (∼ 96% NO + 4% NO2) and 8% O2 in He. The reactor outlet was continuously monitored by a chemiluminescence NOx analyzer (Thermo Environmental Instruments 42C-HT) that allowed the simultaneous detection of both NO and NO2. The samples were exposed to the adsorption mixture at RT until the outlet NOx readout was equivalent to the inlet NOx. This latter parameter was set to ensure that saturation coverage was reached for all the samples investigated, as the time after which no change was observed in the gas phase NOx concentrations was strongly dependent on the W surface density of the samples. It has been clearly demonstrated that the nature of formed ad-NOx species on such materials is affected to a significant extent by the presence of O2 in the NO-containing mixture.39 As the presence of parts per million levels of O2 cannot be excluded, NOx adsorption and desorption were thus carried out in the presence of a large excess of O2 (8%). Before the NOx-TPD experiments, the samples were flushed in O2(8%)/He at RT to remove weakly chemisorbed species until the NO and NO2 concentrations detected at the outlet were negligible. NOx-TPD experiments were carried out from RT to 550 °C at a heating rate of 3 °C/min under a mixture of 8% O2 in He. As has already been reported,40 it should be noted that NOx chemisorption does not occur on WOx species. Rhodium accessibility was evaluated via the irreversible H2 chemisorption method. These measurements were performed in a static mode using a conventional volumetric apparatus (Belsorp max, Bel Japan) and at −196 °C to minimize the spillover of hydrogen to the CZ support.41 Typically, about 0.3 g of catalyst was used. Before the H2 chemisorption measurements, the catalyst was reduced in H2 (50 mL/min) at 600 °C for 2 h (3 °C/min heating rate) with subsequent evacuation at 600 °C for an additional 2 h. Then, the sample was cooled under vacuum to −196 °C. Two H2 adsorption isotherms were obtained. After the first isotherm, the catalyst was evacuated again for 2 h at −196 °C. The amounts of total and reversible H2 uptakes were then estimated by extrapolating the quasi-linear portions of the isotherm to zero pressure. The difference between these two values gave the amount of irreversible H2 uptake. The low-temperature H2 chemisorption methodology was validated on a Rh(0.68 wt %)/CeO2 sample also characterized by EXAFS under H2 at 600 °C. For this particular sample, the mean Rh particle size was found to be in excellent agreement between low-temperature H2 chemisorption (2.3 nm) and EXAFS (2.5 nm). XP spectra of the calcined and reduced series of samples, stored in air under ambient conditions following their pretreatment, were collected on a SPECS (Phoibos 5MCD 100) X-ray photoelectron spectrometer using a Mg Kα (hν =

2. EXPERIMENTAL SECTION 2.1. Catalyst Synthesis. The Ce0.62Zr0.38O2 support (CZ, Rhodia) was first calcined in air at 750 °C (2 °C/min) for 4 h in a muffle furnace. W was then deposited on CZ by incipient wetness impregnation with aqueous solutions of (NH4)6H2W12O40 (Aldrich) leading to various W loadings up to 17.3 wt %, which corresponds at maximum to a W surface density38 of 10 W/nm2. After aging at RT for 6 h and drying at 100 °C overnight, the tungstated materials were calcined in air at 500 °C (2 °C/min) for 2 h in a muffle furnace. Rh (∼1.2 wt %) was then introduced on the tungstated materials by incipient wetness impregnation with an aqueous solution of Rh(NO3)3 (Aldrich). After aging for 6 h at RT, the samples were dried overnight at 100 °C and finally calcined in air at 400 °C (2 °C/min) for 2 h in a muffle furnace. The samples will be denoted as W(W/nm2)−CZ and Rh/W(W/nm2)−CZ. 2.2. Catalyst Characterization. Chemical analysis of the supported catalysts was performed by inductively coupled plasma atom emission spectroscopy (ICP/AES) at the CNRS Centre of Chemical Analysis (Solaize, France). N2-sorption measurements were carried out on a Belsorp max instrument (Bel Japan) at 77 K after evacuation of the samples at 300 °C for 3 h. XRD measurements were carried out using a θ-θ D8 Advance (Bruker) powder diffractometer with Cu Kα radiation (0.1548 nm) operated at 30 kV and 30 mA and equipped with a 1D LynxEye detector. The LynxEye detector was set to a 3° opening, and the scanning range was 5−60° by step of 0.01°. Raman spectra of the samples were collected from a KAISER (RXN1) Optical system equipped with a charge-coupled detector (CCD) and a laser with λ = 785 nm (power, 0.6 7387



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Table 1. Chemical Elemental Analysis, Specific Surface Area, and Apparent H/Rh Values Estimated by Low-Temperature H2 Chemisorption of the Studied Samples sample

Rh (wt %)

W (wt %)

BET surface area (m2/g)

surface area (m2/gCZ)

W surface densitya (W/nm2)

apparent H/ Rhb

Rh-free W(W/nm2)− CZ

W(0.0)−CZ

−

0.00

66.0

66.0

0.00

−

− − − − − −

1.39 2.50 5.06 6.73 7.32 11.01 13.53 17.32

65.8 64.0 60.2 57.9 54.9 51.8 49.3 44.5 64.6 64.0 63.3 58.3 56.7 54.9 52.3 49.0 44.0

67.0 66.1 64.3 63.3 60.5 60.2 59.4 56.9

0.68 1.24 2.58 3.49 3.97 6.00 7.46 9.97

Rh/W(W/nm2)−CZ

W(0.7)−CZ W(1.2)−CZ W(2.6)−CZ W(3.5)−CZ W(4.0)−CZ W(6.0)−CZ W(7.5)−CZ W(10.0)−CZ Rh/W(0.0)−CZ Rh/W(0.7)−CZ Rh/W(1.2)−CZ Rh/W(2.6)−CZ Rh/W(3.5)−CZ Rh/W(4.0)−CZ Rh/W(6.0)−CZ Rh/W(7.5)−CZ Rh/W(10.0)−CZ

− − − − − − − − 0.33 0.39 0.32 0.16 0.11 0.08 0.03 − 0.01

1.23 1.30 1.21 1.21 1.21 1.22 1.16 1.17 1.17

a Tungsten surface density calculated on the basis of the content of W and the BET surface areas corrected for the amounts of W as WO3 (m2/ gCZ).38 bApparent H/Rh ratio estimated from the amount of H2 irreversibly chemisorbed at low temperature.

samples were pressed into self-supporting wafers of about 15 mg/cm2 (∼31 mg for wafers of 1.6 cm diameter). The wafers were loaded in a moveable glass sample holder, equipped on top with an iron magnet, and inserted in a conventional quartzglass cell (CaF2 windows) connected to a vacuum system. The iron magnet allowed for the transfer of the catalyst sample from the oven-heated region to the infrared light beam. Before N2 adsorption, the catalysts were submitted to a dynamic (50 cm3/ min) reducing pretreatment (5% H2 in Ar, Air Liquide, 99.999%) at 600 °C for 2 h at atmospheric pressure. The samples were then evacuated (7.5 × 10−7 Torr) at 600 °C for 1 h. Finally, the temperature was decreased to −173 °C under dynamic vacuum. The samples were then exposed to 10 Torr of N2 (Air Liquide, 99.999%) further purified by passing through a liquid nitrogen trap. The spectrum at −173 °C of the pretreated sample was used as a reference and subtracted from the spectrum of the sample exposed to N2. 2.3. Benzene Hydrogenation. Before benzene hydrogenation, the catalyst sample (0.050 g deposited on a plug of quartz wool inserted inside a quartz reactor) was heated in flowing H2 (100 mLNTP/min) at atmospheric pressure with a heating rate of 3 °C/min up to 600 °C and held at this temperature for 2 h. After cooling to 50 °C under H2, the reaction was started. The partial pressure of benzene (C6D6, Aldrich) was 51.8 Torr (1 Torr = 133 Pa) and the total flow rate was 107 mLNTP/min with H2 as balance. The composition of the effluent was analyzed using an online gas chromatograph (Hewlett-Packard 5890, FID) equipped with a PONA (paraffins−olefins−naphthenes−aromatics; HP, 50 m long, 0.20 mm i.d., 0.5 μm film thickness) capillary column. Cyclohexane was the only product detected.

1253.6 eV) radiation source having a 300 W electron beam power. The emission of photoelectrons from the sample was analyzed at a takeoff angle of 90° under ultrahigh vacuum conditions (1 × 10−8 Pa). For selected samples, XP spectra were also collected after H2-reduction without exposure to air. In this case, the samples were reduced in a quartz reactor equipped with shut-off valves. After being reduced in H2 (100 mL/min) at 600 °C for 2 h, the samples were cooled to RT under H2 and the shut-off valves of the reactor were closed, hence maintaining the samples under H2. The reactor was then transferred into a glovebox that was continuously flushed with high purity N2 (>99.999%). The reactor was then opened and the samples were deposited onto the XPS sample holders before being introduced into the XPS facility. XP spectra of these reduced samples were collected on an Omicron SHERA X-ray photoelectron spectrometer, using a monochromatic Al Kα (hν = 1486.6 eV) radiation source having a 300 W electron beam power. The emission of photoelectrons from the sample was analyzed at a takeoff angle of 45° under ultrahigh vacuum conditions (1 × 10−8 Pa). XP spectra were collected at pass energy of 10 or 20 eV for C 1s, O 1s, W 4f, Ce 3d, Ce 4d, Zr 3d, and Rh 3d core XPS levels. After data collection, because of the known difficulties in referencing the binding energies to the C1s line,42 the binding energies were calibrated with respect to the binding energy of the O 1s peak at 529 eV.35,36 All binding energies reported in this work were measured within an accuracy of ±0.2 eV. The peak areas were determined after subtraction of a Shirley background. The atomic ratio calculations were performed after normalization using Scofield factors.43 Spectrum processing was carried out using the Casa XPS software package and Origin 7.1 (Origin Lab Corporation). N2-FTIR spectra of adsorbed N2 on Rh/W(W/nm2)−CZ samples were collected in transmission mode on a Bruker Vertex 70 FTIR spectrometer equipped with a liquid N2-cooled MCT detector and a data acquisition station. A total of 128 scans were averaged with a spectral resolution of 2 cm−1. The

3. RESULTS The composition of the catalysts and their specific surface areas are listed in Table 1. The amount of W in the synthesized materials increased up to 17.3 wt %, and that of Rh was about 7388



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1.2 wt %. The addition of Rh had no influence on the specific surface area of the samples. The specific surface area of CZ (square meters per gram) decreased by about 30% with the addition of 17.3 wt % of W. The average surface area corrected for the amounts of W as WO3 was 62 ± 5 m2/gCZ (Table 1). As already shown for tungstated zirconias,19,38 this indicates that the tungstates did not provide additional surface area. The W surface density thus varied from 0 to 10 W/nm2 in the series of catalysts (Table 1). 3.1. Characterization of Oxide Phases. 3.1.1. XRD. The XRD patterns obtained on the calcined Rh/W(W/nm2)−CZ samples are shown in Figure 1. The XRD patterns of the

Figure 1. XRD patterns of the calcined Rh/W(W/nm2)−CZ samples.

W(W/nm2)−CZ samples were similar to those displayed in Figure 1 and therefore are not shown. The diffraction peaks observed at 2θ of 29.0, 35.6, and 48.2° on all samples are attributed to the reflections of the 111, 200, and 220 planes of the cubic fluorite phase of ceria-zirconia.44,45 These diffraction peaks were symmetrical, which indicates the presence of a ceriazirconia solid solution. A broad contribution of very weak intensity was also observed from 23 to 25° for W surface densities greater than or equal to 6.0 W/nm2 (insert of Figure 1). This broad contribution is assigned to the presence of WO3 crystallites of very small sizes, as the three reflections expected from the monoclinic phase of WO3 (JCPDS file 32-1395) in this range of 2θ were not resolved. This shows that the tungstate phase was very well-dispersed on the CZ support. The Rh/W(W/nm2)−CZ samples did not show any diffraction peaks related to Rh species most probably because of its elevated dispersion on the W(W/nm2)−CZ supports. 3.1.2. Raman Spectroscopy. Figure 2a shows the Raman spectra of the calcined Rh/W(W/nm2)−CZ samples. The band at 472 cm−1 is assigned to the F2g vibration of the fluorite type lattice46 (symmetric breathing vibrational mode of the O anions around Ce cations47). The bands at 714, 806, and 980 cm−1 are due to the tungstate species.35,36,48 The contribution at 980 cm−1, observed as soon as W was added to the CZ support (Figure 2b), is assigned to the symmetric vibration mode of the WOt of the tungstate species.35,36,48 In agreement with earlier studies,35,36 its intensity increased as the W surface density increased up to 6.0 W/nm2 in the Rh/W(W/nm2)−CZ series and remained essentially constant for greater W surface densities. The bands at 714 and 806 cm−1 are assigned to W− O−W stretching modes (Eg and A1g) of distorted WO3 crystallites.36 It can be seen that these contributions were detected for W surface densities greater than or equal to 4.0 W/ nm2 and increased in intensity as the W surface density increased. The W surface densities for which WO3 crystallites

Figure 2. Raman spectra of (a,b) the calcined Rh/W(W/nm2)−CZ samples and (c) the calcined and reduced (post C6D6 hydrogenation) Rh/W(10.0)−CZ sample.

were detected through Raman spectroscopy (Figure 2a) were much lower than those at which the onset of detection of WO3 crystallites was observed by XRD (Figure 1). This is consistent with the much greater sensitivity of Raman spectroscopy to smaller WO3 crystallites compared to that of XRD, as reported earlier for tungstated zirconias.36,49 The comparison of the Raman spectrum measured on Rh/ W(10.0)−CZ after benzene hydrogenation, hence after hightemperature reduction pretreatment, with that obtained on the calcined catalyst is shown in Figure 2c. In addition to a significant decrease in the intensity of the CZ-related band at 472 cm−1 because of the reduction of the support, and also because of changes in the optical properties of the reduced sample (dark) compared to the calcined sample (brown), the bands related to the presence of WO3 crystallites at 714 and 806 cm−1 of the calcined sample vanished after reduction. In contrast, the contribution at 980 cm−1 due to the tungstate species (WOx) was still observable on the reduced sample. This indicates that the WO3 crystallites were reduced at 600 °C under H2. From the Raman spectra shown in Figure 2c, it is difficult to conclude about the extent of reduction of the WO3 crystallites. Our Raman results are consistent with earlier works 7389



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obtained on the Rh-free series of samples. As has already been observed on ZrO2,40 the W-free sample (W(0.0)−CZ, i.e., CZ) showed low- and high-temperature (LT and HT) NOx desorption peaks, with the intensity of the LT peak being much lower than that of the HT peak. Such a NOx-TPD profile is consistent with those reported recently by Azambre et al.50 on comparable CZ materials, although the experimental conditions under which the adsorption−desorption of NOx was carried out were different from those used in the present work. The distribution of the NOx products (NO and NO2, not shown) was similar to that found on ZrO2.40 As was also the case for tungstated zirconias,40 (i) the LT peak of tungstated CZs was of comparable intensity (up to 4.0 W/nm2) and was positioned as those found for CZ and (ii) the HT peaks of the W(W/nm2)−CZ samples were shifted to slightly lower temperatures and decreased in intensity with increasing amounts of W up to W surface densities lower than or equal to 4.0 W/nm2 (Figure 3a). As was concluded for tungstated zirconias,40 the tungstate species prevented the formation of the most stable adsorbed NOx species, i.e., the nitrates.50 For W surface densities greater than or equal to 6.0 W/nm2, the intensity of the low LT peak decreased and stabilized, whereas the HT peak vanished. From a qualitative point of view the NOx desorption profiles were strongly affected by the introduction of Rh (Figure 3b) on the W(W/nm2)−CZ series (Figure 3a). The LT and HT peaks were much less resolved in the presence of Rh with NOx desorption at intermediate temperatures (120−250 °C). As was the case for the Rh-free series (Figure 3a), the amounts of NOx stored on Rh/W(W/nm2)−CZ series decreased as the amount of added W increased (Figure 3b). Figure 3c shows the NOx uptakes, normalized per unit surface area of CZ, determined on the W(W/nm2)−CZ (open symbols) and Rh/W(W/nm2)−CZ (solid symbols) samples. The NOx surface density obtained on the W-free samples (W(0.0)−CZ and Rh/W(0.0)−CZ, Figure 3c) of about 5.3 μmol/m2 (3.2 NOx molecules/nm2) is in agreement with the values reported by Azambre et al.50 (3.1−4.5 NOx molecules/ nm2 for CZ samples contacted with NO2). With a nominal loading of 1.2 wt % in Rh, the addition of Rh did not influence the NOx uptakes obtained on the W(W/nm2)−CZ series to a significant extent. Consequently, the NOx species desorbing in the 120−250 °C range of temperatures on the Rh-promoted samples (Figure 3b) cannot be attributed to species chemisorbed on Rh oxide. The fact that Rh2O3 supported on SiO2 did not chemisorb NOx either (not shown) supports this conclusion. The main effect of Rh would therefore be to help desorb the most stable adsorbed NOx species to lower temperatures and/or prevent their formation.40 The reason behind the significant alteration of the NOx-TPD profiles with the introduction of Rh remains unclear at present. As was also observed for tungstated zirconias,38,40 the amount of NOx released on the W(W/nm2)−CZ and Rh/ W(W/nm2)−CZ series decreased linearly, with a very good correlation coefficient (0.988), as the W surface density increased and then stabilized for W surface densities greater than or equal to 6.0 W/nm2 (Figure 3c). Wu et al. also reported a decrease in the amount of NOx released with the introduction of 10 wt % WO3 to a Pt/Al2O3 sample (W surface density of 1.7 W/nm2).20 From Figure 3c, the pseudo monolayer coverage of CZ by tungstates was estimated to be 4.8 W/nm2. This value is greater than that determined for tungstated zirconias38 (4.1 W/nm2) and in excellent agreement with the values reported

in which it has been shown that WO3 was much more easily reducible than WOx for tungstated zirconias.7,24 3.1.3. NOx-TPD. Recently, we have shown that the extent of coverage of zirconia by tungstates could be determined accurately with the help of a newly developed NOx-TPD method.38,40 Figure 3a shows the NOx desorption profiles

Figure 3. NOx (NO + NO2) TPD profiles on (a) W(W/nm2)−CZ and (b) Rh/W(W/nm2)−CZ (W(0.0)−CZ and Rh/W(0.0)−CZ, gray; W(1.2)−CZ and Rh/W(1.2)−CZ, red; W(2.6)−CZ and Rh/ W(2.6)−CZ, blue; Rh/W(3.5)−CZ, purple; W(4.0)−CZ and Rh/ W(4.0)−CZ, green; W(6.0)−CZ, W(7.5)−CZ, Rh/W(6.0)−CZ, and Rh/W(7.5)−CZ, black), (c) NOx uptakes, and (d) fraction of CZ covered by tungstates of the W(W/nm2)−CZ (open symbols) and the Rh/W(W/nm2)−CZ (solid symbols) series of samples. 7390



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Table 2. Carbon contents (Atom Percent) and Atomic Ratios Deduced from the XPS Data of the Rh/W(W/nm2)−CZ Samples after (a) Calcination at 400 °C, (b) Reduction at 600 °C Followed by C6D6 Hydrogenation Reaction at 50 °C and Storage in Air, and (c) Reduction at 600 °C without Exposure to Air C (atom %)

Ce/Zr (bulk = 1.63)

Rh/(W + Ce + Zr) (bulk = 0.019)

W/(Ce + Zr)

W surface density (W/nm2)

(a)

(b)

(a)

(b)

(c)

bulk

(a)

(b)

(c)

(a)

(b)

(c)

0.0 1.2 2.6 3.5 4.0 6.0 10.0

14.6 11.9 12.8 − 8.4 7.5 6.0

12.6 10.9 8.7 − 12.7 14.0 13.1

1.65 1.77 1.85 − 1.94 1.90 1.96

1.73 1.80 1.80 − 1.79 1.70 1.71

1.66 1.71 − 1.76 − 2.08 −

0.000 0.019 0.040 − 0.060 0.096 0.166

0.000 0.046 0.089 − 0.117 0.156 0.171

0.000 0.039 0.082 − 0.124 0.171 0.205

0.000 0.040 − 0.100 − 0.145 −

0.026 0.026 0.027 − 0.031 0.038 0.041

0.022 0.024 0.027 − 0.030 0.034 0.037

0.019 0.021 − 0.027 − 0.031 −

by Chen and co-workers on WOx−CeO234 and WOx− Ce0.50Zr0.50O2 samples.35 This further supports the validity and the sensitivity of the NOx-TPD method.38,40 The fraction of CZ covered by the tungstate species estimated from the NOx-TPD results (Figure 3c) is shown in Figure 3d. It can be seen that this fraction reached 0.90 at pseudo monolayer coverage (Figure 3d), which is comparable to the value found for tungstated zirconias (0.85).40 3.1.4. XPS. XPS analyses were carried out on selected samples (W surface densities of 0.0, 1.2, 2.6, 4.0, 6.0, and 10.0 W/nm2) of the Rh/W(W/nm2)−CZ series after calcination at 400 °C or reduction at 600 °C followed by benzene hydrogenation reaction. The samples submitted to the benzene hydrogenation experiments were removed from the reactor and stored in air before XPS analyses. From this experimental methodology, reoxidation of the samples to various extents is likely. Consequently, the oxidation states of Ce and Rh will not be discussed in detail over these two series of samples. The binding energies (BE) of the Ce 3d5/2, Ce 4d5/2, Zr 3d5/2, and W 4f7/2 were 881.8, 108.0, 181.5, and 34.6 eV, respectively. These values are in good agreement with those reported earlier for the BE of Ce 3d5/2 in CeO2 related materials34,36,51−61 (881.8−883.1 eV), Ce 4d5/2 for WOx− CeO256 (108.6 eV), Zr 3d5/2 for Ce0.68Zr0.32O255 (181.7 eV), and W 4f7/2 in WO356,62,63 and WOx−CeO234,36,56 (35.3−35.8 eV). Regarding the BE of W 4f7/2, it can be concluded that W was in an oxidation state close to +6 (WO3-like species) even after high-temperature reduction prior to benzene hydrogenation. As the XPS measurements were performed before the Raman investigations, in which it was shown that the WO3 crystallites were reduced after high-temperature reduction (Section 3.1.2, Figure 2c), the +6 oxidation state of W found by XPS suggests that the amounts of WO3 crystallites were very low even in the sample including the highest content of W (W(10.0)−CZ). Table 2 shows that the amounts of carbon in the samples were in the range of 6.0−14.6 atom % for both series irrespective of the pretreatment of the samples. This indicates that exposure of the samples to benzene at 50 °C in the course of the hydrogenation reaction did not lead to appreciable C deposition in comparison to the oxidized parent samples. The Ce/Zr atomic ratio was estimated to be 1.65 on the calcined W-free sample (W surface density of 0.0 W/nm2, Table 2), which is in good agreement with that expected from the bulk composition of CZ (Ce0.68Zr0.32O2) ascertained by chemical analysis (Ce/Zr = 1.63). This ratio increased with the addition of W up to 4.0 W/nm2 and then stabilized (1.93 ± 0.03). For W surface densities lower than or equal to 4.0 W/

nm2, the observed increase in the Ce/Zr ratio may tentatively be attributed to the preferential interaction of the tungstates with the Zr surface atoms rather than with the Ce ones. In such a case the amount of signal from the Zr 3d core level screened by a WOx overlayer would be more important than that of the Ce 4d core level. An opposite conclusion was reached by Chen and co-workers35 on a Ce0.50Zr0.50O2 support but for samples also including 0.4 mmol CuO/100 m2 CZ (2.4 Cu/nm2). The Ce/Zr atomic ratio remained essentially constant on the series of samples which were reduced at high temperature before being evaluated in the benzene hydrogenation reaction (Ce/Zr = 1.76 ± 0.04, Table 2). The origin of this peculiarity remains unclear. Figure 4 shows the W/(Ce + Zr) atomic ratios, deduced from the XPS data, plotted as a function of the W surface

Figure 4. XPS W/(Ce + Zr) atomic ratios as a function of the W surface density of Rh/W(W/nm2)−CZ samples after (■) calcination at 400 °C or (□) reduction at 600 °C followed by benzene hydrogenation reaction at 50 °C.

densities of the Rh/W(W/nm2)−CZ samples calcined at 400 °C (■) or reduced at 600 °C followed by evaluation in the benzene hydrogenation reaction (□). A clear discontinuity is observed for W surface densities greater than or equal to 6.0 W/nm2 for the calcined samples. This indicates that pseudo monolayer coverage of CZ by the tungstates has been reached for a value of W surface density between 4.0 and 6.0 W/nm2. The value of the W surface density at which pseudo monolayer coverage was reached, which corresponds to the intercept of the two straight lines,34,64 was estimated to be 4.9 W/nm2. This value is in excellent agreement with the value provided by the NOx-TPD method (4.8 W/nm2, Section 3.1.3) and reported in earlier studies as well.34,35 In the reduced series of samples, the W/(Ce + Zr) ratios were found to be in better agreement with those of the calcined series below pseudo monolayer coverage than with those at higher coverages (Figure 4 and Table 2). 7391



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3.2. Characterization of the Rh Phases. 3.2.1. Rh Accessibility: Low-Temperature H2 Chemisorption. The use of TEM on such tungstated CZs to estimate the Rh dispersion is limited because of the poor contrast between the NM particles and the tungstates11,27,28 and between the Rh particles and the ceria-related supports.41 As also reported by many different groups, the estimation of the metal dispersion on tungstated oxides is not trivial. H2 and CO chemisorptions should be used with the greatest care, as hydrogen spillover has been reported to occur on such materials7,24 together with SMSI effects24 and the corresponding loss in chemisorption capacity of the metallic function.7,8,19,22−25 Hydrogen spillover and the limitations of CO chemisorption have also been highlighted on ceria-related materials.41 Yet, low-temperature H2 chemisorption measurements have been shown to be an interesting alternative to the conventional measurements performed at RT on ceria-related materials.41 The apparent H/Rh values deduced from low-temperature H2 chemisorption measurements performed on the Rh/W(W/ nm2)−CZ catalysts are listed in Table 1. Apart from the value obtained on the Rh/W(0.7)−CZ sample, the H/Rh values were found to decrease with increasing W surface densities. Such a behavior may be ascribed to a decrease in the dispersion of Rh, a decoration of the Rh particles by the tungstates, and/or changes in its electronic properties on the interaction with tungstates. As a blank test, the low-temperature H 2 chemisorption performed on a W(10.0)−CZ bare support did not lead to any irreversible hydrogen uptake. 3.2.2. XPS. The BE of the Rh 3d5/2 were 308.6 and 308.2 eV in the calcined and reduced series of Rh/W(W/nm2)−CZ samples stored under ambient air before being analyzed, respectively. These BE indicate that Rh was mainly in an oxidized form on both series.42,57−59,65−67 The shift to slightly lower BE in the reduced series suggests the presence of Rh0 atoms resistant to reoxidation upon exposure of the reduced samples to air under ambient conditions. Although the dispersion of Rh cannot be determined formally by XPS, this powerful technique may provide useful information on the relative dispersion of a metal in a series of samples by studying the NM/support ratio.23 The Rh/(W + Ce + Zr) ratios of the calcined and reduced samples were found to be greater than that expected from the bulk composition (0.019, Table 2). In the case of well-dispersed NM particles, this is not surprising as the proportion of Rh atoms analyzed by XPS is expected to be greater than the proportion of Ce and Zr atoms analyzed from support particles of sizes bigger (13 nm from the specific surface area of CZ) than those of the Rh particles.41 More interesting is that this ratio remained essentially constant (0.027 ± 0.03) on the introduction of W to the CZ support for W surface densities lower than or equal to 4.0 W/ nm2 (Table 2). This suggests that the size of Rh particles did not increase to a significant extent below pseudo monolayer coverage of CZ by WOx. Above pseudo monolayer coverage (W surface density > 4.8 W/nm2), the higher experimental values of the Rh/(W + Ce + Zr) atomic ratio are to be considered with the greatest care because of the Ce and Zr screening by WO3 overlayers, as illustrated by the discontinuity observed in the W/(Ce + Zr) ratio (Figure 4). To gain further insights into the influence of WOx on the oxidation state of Rh after high-temperature reduction, selected samples were reduced under H2 at 600 °C and transferred into the XPS facility without intermediate exposure to air (Section

2.2). Figure 5 shows that the investigated samples exhibited two Rh 3d5/2 contributions at about 307 and 308 eV. In agreement

Figure 5. Rh 3d photoelectron spectra of Rh/W(0.0)−CZ, Rh/ W(1.2)−CZ, Rh/W(3.5)−CZ, and Rh/W(6.0)−CZ after reduction at 600 °C for 2 h under H2 and transfer into the XPS facility without intermediate exposure to air. The numbers reported in the black squares indicate the percentage of Rh0 (Rh 3d5/2 ∼ 307 eV) in the samples.

with earlier studies,65−67 these contributions are assigned to Rh0 and Rhδ+ species, respectively. It is worth reporting that both the binding energies (306.7 and 308.2 eV, Figure 5) and the percentage of Rh0 (74%, Figure 5) found for the W-free sample (Rh/W(0.0)−CZ) are in excellent agreement with those reported earlier by Eriksson et al.66 (307.3 and 308.2 eV, and 68% Rh0) on a Rh/Ce−ZrO2 catalyst of comparable Rh surface density (0.7 Rh/nm2 compared to 1.1 Rh/nm2 in the present study), although the composition of the ceria-related material and the reduction temperature were somewhat different. Figure 5 also shows that the percentage of Rh0 decreased and the Rh0 and Rhδ+ contributions shifted to higher binding energies with the addition WOx. This clearly indicates an enhanced electron transfer from Rh to the support with the addition of WOx. Finally, a decrease in the Rh/(W + Ce + Zr) ratio below pseudo monolayer coverage of CZ by WOx would have indicated the occurrence of an encapsulation phenomenon68 of the Rh particles by WOx. However, because this ratio did not decrease with the addition of WOx on CZ (Table 2), the possibility of a decoration of the Rh particles by WOx can be ruled out. 3.2.3. N2-FTIR. The oxidation state of Rh after hightemperature reduction was probed by monitoring the adsorption of N2 by FTIR (Figure 6). This probe molecule was preferred to CO, although it is intrinsically less sensitive (because of an extinction coefficient much lower than that of CO69) and technically more demanding (low temperatures or high N2 pressures are required to chemisorb N2), because the position of the N2 stretching vibration is not dipole−dipole coupling-dependent70−72 as that of CO is.69 Moreover, CO may alter the catalytic sites onto which the adsorption occurs, by reducing oxidized NM species69,73 and/or oxidizing zerovalent Rh species according to the well-established disruption phenomenon of Rh0 particles.74−76 These reduction−oxidation processes will be definitely avoided with the use of N2 as a probe molecule. The absorption band observed on all samples at about 2335 cm−1 (Figure 6) is due to the N2 stretching vibration of N2 interacting with the OH groups of the support.70,71,77−79 As 7392



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(Table 1) considering a H/Rh stoichiometry of 1,84 were found to remain essentially constant (0.16 ± 0.04 s−1) for W surface densities lower than or equal to 4 W/nm2 (Figure 6), whereas at higher W surface densities a drastic increase in the C6D6 HYD TOR was observed. Below 4 W/nm2, the C6D6 HYD TOR agree with those reported earlier on SiO2- and Al2O3supported Rh catalysts.83 The much higher C6D6 HYD TOR observed at W surface densities higher than 4 W/nm2 are more surprising. The benzene hydrogenation rates were also measured on the W(4.0)−CZ and W(10.0)−CZ bare supporting oxides. The benzene hydrogenation rate on W(4.0)−CZ (0.03 μmol C6D6/(s gcat)) was found to be negligible compared to that on Rh/W(4.0)−CZ (2.01 μmol C6D6/(s gcat)). In contrast, the contribution of the benzene hydrogenation rate measured on W(10.0)−CZ (0.24 μmol C6D6/(s gcat)) was found to be as high as 21% of that on Rh/ W(4.0)−CZ (1.12 μmol C6D6/(s gcat)). These experiments suggest that WO3 was reduced to W metal, which is known to catalyze the benzene hydrogenation reaction.85 Hence, the C6D6 HYD TOR estimated for W surface densities higher than 4 W/nm2, for which the presence of WO3 crystallites was confirmed by XRD (Figure 1) and Raman spectroscopy (Figure 2a), cannot be taken as representative of the activity of Rh only.

Figure 6. N2-FTIR spectra of the (a) Rh/W(0.0)−CZ, (b) Rh/ W(0.7)−CZ, (c) Rh/W(1.2)−CZ, (d) Rh/W(2.6)−CZ, (e) Rh/ W(3.5)−CZ, (f) Rh/W(4.0)−CZ, (g) Rh/W(6.0)−CZ, (h) Rh/ W(7.5)−CZ, and (i) W(4.0)−CZ samples after reduction at 600 °C for 2 h, evacuation (7.5 × 10−7 Torr) at 600 °C for 1 h, and exposure to 10 Torr of N2 at −173 °C.

indicated earlier, the contribution at about 2150−2100 cm−1 (not shown) is not attributed to N2 chemisorbed species but to the reduction of the CZ support.72,80 A broad absorption band with a maximum at 2190 cm−1 was observed on the W-free sample (Figure 6, spectrum a, Rh/W(0.0)−CZ). This band was attributed to the stretching vibration of N2 chemisorbed on Rh0 sites supported on CZ.72,80 This band decreased in intensity to a significant extent and shifted to higher wavenumbers (2240 cm−1) with the introduction of increasing amounts of W (Figure 6, spectra b−f). This was attributed to the appearance of electron-deficient Rh species (Rhδ+)70−72,77,79,80 on W addition. Because of changes in the optical properties of reduced Rh/W(10.0)−CZ, it was not possible to investigate such a sample by N2-FTIR. 3.2.4. Benzene Hydrogenation. Figure 7 shows the benzene hydrogenation rates normalized per gram of Rh measured on

4. DISCUSSION In agreement with earlier studies performed on Pt/WOx− ZrO28,22 and Ir/WOx−ZrO2,19 the introduction of Rh to W(W/nm2)−CZ supports with increasing W surface densities led to a drastic decrease in the benzene hydrogenation rate and thus to a significant alteration of the Rh metallic function (Figure 7). In the present work the benzene hydrogenation rates were found to decrease linearly below pseudo monolayer coverage of CZ by WOx (4.8−4.9 W/nm2, Sections 3.1.3. and 3.1.4, and refs 34 and 35) and then to remain essentially constant up to 10.0 W/nm2 (Figure 7). Although not emphasized by Lecarpentier et al., the toluene hydrogenation performance of Ir/W(W/nm2)−ZrO2 samples decreased almost linearly with increasing W surface densities19 below pseudo monolayer coverage of ZrO2 by WOx38 (4.1 W/nm2). These authors attributed the decrease in the hydrogenation performance to a decrease in the Ir dispersion with increasing W coverage.19 Such a proposal would be consistent with the decrease in the H/Rh values, i.e., to an increase in the size of the Rh particles, observed on the Rh/W(W/nm2)−CZ catalysts (Table 1). An increase in the size of the Rh particles with increasing W surface densities does, however, conflict with the XPS results which suggested that the size of Rh particles did not increase to a significant extent below pseudo monolayer coverage; the XPS Rh/(Ce + Zr + W) ratios remained essentially constant for W surface densities lower than or equal to 4 W/nm2 (Table 2). A more likely explanation to account for the observed decrease in the benzene hydrogenation rates below pseudo monolayer coverage of CZ by WOx (Figure 7) lies in a decrease in the fraction of the Rh particles interacting with CZ and a concomitant increase in the fraction of those interacting with the tungstates with increasing W surface densities. In this model, the Rh particles interacting with the tungstate species are less active for the benzene hydrogenation reaction than those interacting with CZ, as the W-free catalyst (Rh/W(0.0)− CZ, i.e. Rh/CZ) exhibited the highest rate of hydrogenation (Figure 7). The lower hydrogenation performance of the Rh particles interacting with the tungstates could be attributed to a

Figure 7. C6D6 hydrogenation rates (●) and turnover rates (TOR, ▲) of the reduced (H2 600 °C, 2 h) Rh/W(W/nm2)−CZ samples.

the Rh/W(W/nm2)−CZ series. It has been reported that such a reaction is structure insensitive81−83 and can be used as a model reaction to probe the efficiency of the metallic Rh function. It can be noted that the rate of benzene hydrogenation decreased linearly up to a W surface density lower than or equal to 4.0 W/nm2 and then remained essentially constant at higher W surface densities. The benzene hydrogenation turnover rates (C6D6 HYD TOR), estimated from the benzene hydrogenation rates shown in Figure 6 and the low-temperature H2 chemisorption data 7393



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s−1 for Rh0/CZ (Figure 7), this leads to an estimated C6D6 HYD TOR of 2.8 s−1 for Rhδ+/WOx. Figure 8 shows that the

SMSI effect induced by the tungstate species on the NM particles5,8,9,20,24,28,73 resulting in a decoration3,6,7,11,21,23,25,27 and/or an electronic modification11,18,22,25,29,30 of the NM particles by WOx. The decrease in the intensity of the IR contribution assigned to the vibration of N2 chemisorbed on Rh0 sites of particles interacting with the CZ support (2190 cm−1) together with the appearance of absorption contributions at higher wavenumbers as the W surface density increased (Figure 6) provide support for the formation of electrondeficient Rh sites (Rhδ+80) in interaction with tungstates at the expense of those interacting with CZ. A shift in the absorption band to higher wavenumbers is indeed indicative of a strengthening of the N2 bond due to decreased back-electron transfer from the metal into the N2 π* antibonding orbital. This finding is in agreement with a limited number of earlier works in which the presence of Ptδ+ supported on WOx−SiO2 and WOx−ZrO2 could be revealed by FTIR using CO as a probe molecule.3,18 To our knowledge, the present work provides the first spectroscopic evidence of the presence of electrondeficient Rh species because of their interaction with tungstates. In the case of supported Rh catalysts, Sachtler and co-workers also claimed the presence of oxidized Rh species for Rh interacting with Mn and Mo oxides supported on SiO2 by means of the extraction of the Rh(I) ions by acetylacetone.86 Finally, it was found that the intensity of the N2−Rhδ+ contributions (νN2 > 2190 cm−1) did not compensate for the loss of intensity of the N2−Rh0 contribution (νN2 = 2190 cm−1) (Figure 6). As the charge transfer from Rh to the supporting oxides was found to increase with increasing W content (Figure 5) and the decoration of the Rh particles could be excluded on the basis of the XPS results below pseudo monolayer coverage (Section 3.2.2.), this can be attributed to the lower molar extinction coefficient of N2−Rhδ+ compared to that of N2−Rh0. We have indeed previously shown that the molar extinction coefficient of the CO-Rhδ+ was much lower than that of CORh0.72 Below pseudo monolayer coverage of CZ by WOx (4.8 W/ nm2), the benzene hydrogenation rates can be predicted on the basis of a model in which the hydrogenation rates measured experimentally (rC6D6 HYD exp., μmol/(s gRh)) correspond to the sum of the rates of the Rh0 (rC6D6 HYD Rh0, μmol/(s gRh0)) and Rhδ+ (rC6D6 HYD Rhδ+, μmol/(s gRhδ+)) sites (eq 1): 0

δ+

rC6D6 HYD exp. = (1 − x)rC6D6 HYDRh + xrC6D6 HYDRh

Figure 8. Comparison between the experimental (●) and predicted (--○--) C6D6 hydrogenation rates of the Rh/W(W/nm2)−CZ samples.

predicted hydrogenation rates on the Rh/W(W/nm2)−CZ series (open symbols) are in excellent agreement with those measured experimentally (solid symbols) below pseudo monolayer coverage (W surface density < 4.8 W/nm2). At higher W surface densities, the model does not apply because of the contribution of W−CZ supports to the hydrogenation reaction (Section 3.2.4). The location and the oxidation state of Rh are schematically represented in Figure 9 below CZ pseudo monolayer coverage

Figure 9. Model description of the location and the oxidation state of Rh with increasing W surface density for Rh/W(W/nm2)−CZ catalysts below pseudo monolayer coverage of CZ by tungstates (WOx).

(1)

In this model, it was assumed that the number of active sites remained essentially constant, as supported by the XPS measurements (Section 3.2.2), and that the fraction of Rhδ+ sites x corresponded to the fraction of CZ covered by the tungstates, as estimated by the NOx-TPD experiments (Section 3.1.3, Figure 3d). The hydrogenation rate measured experimentally on Rh/W(1.2)−CZ (371 μmol C6D6/(s gRh)) was used to estimate the hydrogenation rate on Rhδ+ sites with the help of eq 1 and considering (i) the hydrogenation rate on Rh0 sites measured experimentally on the catalyst free of tungstates (451 μmol C6D6/(s gRh0) on Rh/W(0.0)−CZ) and (ii) that the fraction of CZ covered by the tungstates on that particular sample was 0.22 (Figure 3d). This resulted in a hydrogenation rate of 89 μmol C6D6/(s gRhδ+). It is thus deduced that the Rh0 species interacting with CZ are 5.1 times more active in the benzene hydrogenation than the Rhδ+ species interacting with amorphous tungstates. Considering a C6D6 HYD TOR of 14.2

for a series of Rh/W(W/nm2)−CZ catalysts reduced at 600 °C. When introduced by the incipient wetness impregnation technique, Rh deposited both on CZ and tungstates. More importantly, the size of the Rh particles did not increase to a significant extent with increasing W surface densities, as indicated by XPS (Section 3.2.2). With increasing W surface densities and below pseudo monolayer coverage of CZ by WOx (W surface densities < 4.8 W/nm2), it is proposed that the fraction of Rh particles interacting with the tungstates increased at the expense of those interacting with CZ. The Rh particles interacting with CZ were found to be in a zerovalent oxidation state (Rh0) after reduction, whereas those located on the amorphous tungstates (WOx) were shown to be electrondeficient (Rhδ+, Section 3.2.3) and less active for benzene hydrogenation than Rh0/CZ because of a SMSI effect of the 7394



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tungstates. This accounts for the linear decrease observed in the benzene hydrogenation rates with increasing coverage of CZ by the tungstates up to 75% (W surface density ≤ 4.0 W/nm2, Figure 7). At tungstate coverages greater than or equal to 75% (W surface density ≥ 4.0 W/nm2), WO3 crystallites began to form in the calcined samples and were reduced after hightemperature exposure to H2, as shown by Raman spectroscopy (Section 3.1.2).

Article

AUTHOR INFORMATION

Corresponding Author

*UPMC, UMR 7197, Laboratoire de Réactivité de Surface, 4 Place Jussieu, Case 178, F-75005, Paris, France. E-mail: cyril. [email protected]. Tel: + 33 1 44 27 36 30. Fax: + 33 1 44 27 60 33. Present Address #

(T.B.) Univ. Lille Nord de France, F-59000, Lille, France. CNRS UMR 8181, Unité de Catalyse et Chimie du Solide UCCS, 59655, Villeneuve d’Ascq, France.

5. CONCLUSIONS This work reports on the influence of the W loading of Rh/ WOx−CZ catalysts, in other words, of the coverage of CZ by tungstate species (WOx and/or WOx + WO3), on the location and the oxidation state of Rh after high-temperature reduction over a wide range of coverages, i.e., from 0 to slightly more than twice WOx pseudo monolayer coverage. To achieve this goal, the Rh/WOx−CZ system was thoroughly characterized through N2-sorption, XRD, Raman spectroscopy, NOx-TPD, and XPS for the oxide phases and by low-temperature H2 chemisorption, XPS, N2-FTIR, and benzene hydrogenation for the metallic phases. The pseudo monolayer coverage of CZ by tungstate species was determined via the NOx-TPD method reported recently for tungstated zirconias.38,40 A tungstate pseudo monolayer was found to occur at a W surface density of 4.8 W/nm2. This value is corroborated by the XPS results obtained in the present work and in excellent agreement with a limited number of earlier investigations performed on tungstated ceria-related materials.34,35 The present work therefore provided further support for the reliability and the accuracy with which pseudo monolayer coverage can be estimated by the NOx-TPD method on tungstated oxides and highlights that this method can be an interesting alternative to the commonly used physicochemical techniques (XRD, Raman, and XP spectroscopies). In agreement with earlier literature reports on noble metals (NM) supported on tungstated oxides, the incorporation of W to the Rh/CZ system was found to be detrimental to the hydrogenation performance. As a consequence, the benzene hydrogenation rates of the W-promoted Rh/CZ catalysts were always lower than that found for the W-free catalyst. It was shown for the first time that the benzene hydrogenation rates decreased linearly up to W surface densities lower than or equal to 4.0 W/nm2, thus below tungstate pseudo monolayer coverage (4.8 W/nm2). As indicated by XPS, this decrease in the number of the Rh0 sites from particles located on tungstatefree CZ domains was not attributed to a decrease in the dispersion of Rh, i.e., to an increase in the size of the Rh0 particles, but to a SMSI phenomenon between the Rh particles interacting with the underlying WOx species. The SMSI phenomenon was assigned to the electronic perturbation of the Rh0 particles by the amorphous WOx species onto which the Rh particles are located, as indicated by XPS and N2-FTIR with the appearance of electron-deficient Rh species (Rhδ+). The Rhδ+ species were formed at the expense of the Rh0 ones. Below pseudo monolayer coverage of CZ by amorphous WOx, the hydrogenation rates measured experimentally could be fitted satisfactorily with a model in which the turnover rate of the Rh0 sites was about 5.1 times higher than that of the Rhδ+ sites supported on amorphous WOx. Above pseudo monolayer coverage, the contribution of the WOx−CZ support to the hydrogenation reaction prevented the use of the model developed below pseudo monolayer coverage.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The ONERA (Office National d’Etudes et de Recherches Aérospatiales: The French Aerospace Lab) is acknowledged for funding this work. Dr. J.T. Miller is acknowledged for the EXAFS characterization of the Rh(0.68 wt %)/CeO2 sample.

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