Adsorption and Desorption of NOx on Commercial Ceria-Zirconia

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Adsorption and Desorption of NOx on Commercial Ceria-Zirconia (CexZr1-xO2) Mixed Oxides: A Combined TGA, TPD-MS, and DRIFTS study B. Azambre,* L. Zenboury, A. Koch, and J.V. Weber UniVersite´ Paul Verlaine de Metz, Laboratoire de Chimie et Me´thodologies pour l’EnVironnement (LCME) EA 4164, Rue Victor Demange, IUT Chimie, F-57500 Saint-AVold, France ReceiVed: January 29, 2009; ReVised Manuscript ReceiVed: June 17, 2009

The adsorption/desorption of NOx on commercial CexZr1-xO2 solid solutions (x ) 1, 0.85, 0.69, 0.5, 0.21, 0.1, and 0) is reported in this paper. In the first part, the characterization of the ceria-zirconia is carried out by N2 adsorption at 77 K, TEM, Raman, XRD, and DRIFTS in order to establish relationships between their bulk/surface properties and their NO and NO2 adsorption/desorption behavior. As deduced from thermogravimetric experiments, the adsorption of NO2 is promoted in comparison to that of NO with typical surface densities spreading between 3.1-4.6 NO2/nm2 and 1.5-4.5 NO/nm2 among the CexZr1-xO2 series. DRIFTS experiments reveal that NOx adsorption proceeds mainly via electron transfer and/or participation of surface oxygens and OH groups, inducing the formation of negatively charged surface species, such as hyponitrites, nitroxyl anions, nitrites, and nitrates. The mechanistic pathways promoting the formation of the different kinds of ad-NOx species are discussed in function of the catalyst pretreatment, the temperature, the nature of the reactive gas, and the ceria-zirconia composition. 1. Introduction The use of ceria and related materials in environmental catalysis has attracted much attention these recent years.1 Common applications include the removal of organics from wastewaters and their use as additives in combustion processes or fluid catalytic cracking (FCC).1 However, most of the research effort devoted to ceria-based materials in the field of environmental cleanup has been focused on their use as key component in three-way catalysts (TWC), NOx traps, or in soot abatement from diesel engine exhaust.1-4 In automotive exhaust gas converters, the role of ceria is to assist supported noble metals, such as Pd, Pt, or Rh, by providing a rapid cycling of oxygen between the gas phase and the catalyst, and reversely, under lean (storage) and rich (release) conditions.3,4 The strong redox properties of ceria are related to the facile oxidation/reduction of the cerium component with the concomitant creation/filling of oxygen vacancies. Among all ceria-related materials, ceriazirconia mixed oxides of composition CexZr1-xO2 have known a particular interest due to their good thermal resistance to sintering and superior oxygen storage capacities (OSC).1 Redox properties of ceria-zirconia solid solutions strongly depend on their crystalline structures, the Ce/Zr ratio, the specific surface area, and the presence of a promoter, and accordingly, many nanostructural characterization studies have been made.1,5-14 Noteworthy, it has been shown that CexZr1-xO2 solid solutions still include Ce3+ species and O vacancies after extensive oxidation in oxygen gas.15 Recently, some attention has been paid to the use of ceria-zirconia as support or catalyst in the selective catalytic reduction (SCR) of NOx by hydrocarbons from stationary sources.16,17 Despite the nature of the interaction between NOx and ceria-containing catalysts is crucial for mobile and stationary applications, there are only few studies devoted to the adsorption/desorption behavior of NO and NO2 on ceria and CexZr1-xO2.15,18-21 Moreover, the results obtained using * To whom correspondence should be addressed. E-mail: bazambre@ univ-metz.fr. Phone: +33387939106. Fax: +33387939101.

model or powdered catalysts are sometimes contradictory, maybe because of the different conditions or methodologies employed and the complexity of the CeO2-ZrO2 phase diagram.1 From a general viewpoint, the interaction of NOx with metal oxide surfaces at the molecular level is only partly understood and may depend both on the surface acid-base and redox properties.22,23 NOx (x ) 1,2) differs from most adsorbates in that they are odd-electron molecules. Because of their relatively low ionization potentials in the gas phase and high electron affinity, NOx molecules can undergo either one-electron oxidation or reduction reactions forming Lewis acid (NOx+) or Lewis base (NOx-) species, respectively.23 If the surface redox chemistry was the only mechanism for the formation of these species, one would expect that only defective surfaces with high density of steps, kinks, and vacancies would exhibit the formation of NOx- species in addition to decomposition products. However, the observation of these negatively charged species on a MgO defect-free surface and subsequent theoretical calculations have shown that the chemisorption of NOx species could be described on this material using a cooperative model involving both acid-base and oxidation-reduction processes.24,25 First, the formation of cationic and anionic species is generated from an electron transfer between two NOx adsorbates. These species can then chemisorb through acid-base interactions on the metal oxide surface. Finally, charged adsorbed NOx species pair on the surface forming locally neutral and strongly bound adsorbates.24,25 Whether simple electron transfer or cooperative effects are preponderant depends on the nature of the metal oxide and its preparation.22,23 The highly reducible character of ceria-based materials is thought to promote the formation of NOx anionic species via electron transfer from reduced cations to NO or NO2 adsorbates.18,20-23 In addition to side-reactions occurring with coadsorbed molecules, e.g., water, the study of NOx adsorption processes is complicated by the simultaneous existence of many possible reactions, such as chemisorption on cationic or anionic

10.1021/jp9008674 CCC: $40.75  2009 American Chemical Society Published on Web 07/01/2009

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sites, surface reoxidation, and NOx decomposition.18-23 In a combined FTIR and EPR study, Martinez-Arias et al. have shown that the adsorption of NO at 25 °C on CeO2 powder leads to the formation of nitrites (NO2-) by reaction of the NO radical with a surface oxygen and hyponitrites (N2O22-) by reaction with two neighboring Ce3+-0 sites (0 denotes an oxygen vacancy).20 Upon heating, nitrites further react with surface O2-, yielding nitrates, whereas hyponitrites decompose to N2O.20 In another EPR-FTIR study performed on different compositions of CexZr1-xO2 powders, Adamski et al. have found that NO adsorption induces the formation of mononitrosyls complexes, dimers, and adsorbed N2O.19 Upon interaction of NO2 with the same catalysts, it was found that disproportionation into NO3- and NO+ is promoted at low Ce content, whereas dimerization is preferred for the rich-CeO2 samples.19 In this study, we aimed to assess, for a series of wellcharacterized ceria-zirconia commercial catalysts, the parameters governing both the type/structure/thermostability of adsorbed NOx species and their mechanisms of formation/transformation. First, the effect of ceria-zirconia composition/structure on NOx adsorption/desorption processes is investigated using temperature-programmed techniques. In the second part, the conditions of formation of the different ad-NOx species and the most relevant adsorption mechanisms are determined, namely near room temperature, according to the type of NOx molecule (NO or NO2) and the nature of the pretreatment by considering the peculiar example of the Ce0.69Zr0.31O2 composition, which is well representative of the best ceria-zirconia catalysts in terms of OSC and redox activity. 2. Experimental Section Nanometric ceria-zirconia mixed oxides of composition CexZr1-xO2 (with x ) 0, 0.1, 0.21, 0.5, 0.69, 0.85, 1) were supplied by Rhodia-France (La Rochelle) and obtained from nitrate precursors. Prior to any characterization, they were thermally treated at 500 °C under air (unless otherwise stated). Raman scattering experiments have been performed onto a Jobin-Yvon FT-Raman spectrometer with a double monochromator Spex and the 632.8 nm line of a He-Ne laser (P ) 9 mW). Spectra were acquired using 100 scans at a resolution of about 2 cm-1 in the range between 130 (detector cutoff) and 4200 cm-1. Thermogravimetric experiments of NO (500 ppm/Ar, Air Products) and NO2 (1000 ppm/Ar, Air Products) adsorption were achieved in a Mettler-Toledo SDTA851 thermobalance (accuracy 10-6 g) using 75 mg samples and a flow rate of 60 mL/ min. Before NOx adsorption at 25 °C, the catalysts were pretreated in situ at 500 °C under argon. After 6 h of exposition to NOx, the reaction gas was switched to pure argon and the temperature was increased from 25 to 500 °C using a heating rate of 10 °C/min. TEM analyses were carried out on a JEOL JEM 2011 UHR microscope (200 kV). Prior to analysis, the samples were dispersed in ethanol and deposited on a carbon film embedded on a copper grid. TPD-MS analyses were performed on a Pyrolyzer coupled to a HP 5973 mass detector working in electron impact (EI) mode at an energy of 70 eV. Individual m/z profiles were recorded over the mass range 2-100 amu at a heating rate of 10 °C/min from 25 °C up to 800 °C using purified He as the carrier gas. Diffuse reflectance (DRIFTS) spectra were recorded in the 4000-700 cm-1 range (resolution 4 cm-1, 100 scans) on a Biorad FTS 185 spectrometer equipped with a MCT detector

Figure 1. Raman spectra of the commercial ceria-zirconia powders in the 100-900 cm-1 range with their corresponding normalization factor: (a) m-ZrO2; (b) Ce0.1Zr0.9O2; (c) Ce0.21Zr0.79O2; (d) Ce0.5Zr0.5O2; (e) Ce0.69Zr0.31O2; (f) Ce0.85Zr0.15O2; (g) CeO2.

and the “Graseby Specac” optical accessory. In situ experiments were performed on pure samples into a Spectra-Tech environmental cell designed to work under flowing gases and temperatures up to 500 °C. The nature and thermostability of NOx ad-species generated upon reactive adsorption of NO (500 ppm/ Ar) or NO2 (1000 ppm/Ar) at 30 °C was investigated for some CexZr1-xO2 compositions (x ) 0, 0.21, 0.69, 1). Before admission of the reactive gas, the samples were pretreated in situ inside the DRIFTS cell under synthetic air or Ar at 500 °C. Time-course difference spectra were calculated using as reference the single-beam (SB) spectrum of the ceria-zirconia in Ar just prior to NO or NO2 adsorption. TPD-DRIFTS spectra reflecting the thermostability of the ad-NOx species from 30 to 500 °C under Ar were referenced to the SB spectrum of the sample outgassed under Ar at the same temperature. 3. Results and Discussion 3.1. Characterization of CexZr1-xO2 Catalysts. 3.1.1. Bulk Structural Properties. It is well-known that the crystallites size, specific surface area, and structures of CexZr1-xO2 polymorphs are strongly dependent on the preparation method.1 Structural analyses of our commercial ceria-zirconia powders have been previously performed by XRD.17 However, these data are now compared with those obtained now by FT-Raman spectroscopy (Figure 1) due to the better sensitivity of this technique for the detection of impurities or small crystalline domains and also to its ability in distinguishing the cubic and tetragonal forms of zirconia.26,27 The Raman spectrum obtained for pure ceria (Figure 1g) displays only one peak at 459 cm-1, as expected for the cubic

NOx on (CexZr1-xO2) Mixed Oxides

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TABLE 1: Structural and Porous Characteristics of the Commercial Ceria-Zirconia Powders ceria-zirconia sample

specific surface area SBET (m2/g)a

CeO2 (Ce0.85Zr0.15O2 Ce0.69Zr0.31O2 Ce0.50Zr0.50O2 Ce0.21Zr0.79O2 Ce0.10Zr0.90O2 ZrO2

284 185 131 87 184 73 22

pore volume (cm3/g)a

average pore diameter (Å)a

0.184 0.183 0.315 0.213 0.339

25.1 39.5 102.1 98.4 71.0

phase assignation (Raman/XRDa) c c c + t″ t′or t″ (and c, m) t (and m) m m

a Textural characteristics were calculated using the same samples and adapted from ref 17; c ) cubic; t′, t′′ ) tetragonal (metastable); t ) tetragonal; m ) monoclinic (in italic ) minor phases).

fluorite structure.1,12,26,27 This peak corresponds to the fundamental with F2g symmetry, which can be regarded as a symmetric O-Ce-O stretching.27 For CexZr1-xO2 solid solutions with x g 0.7 (Figure 1e,f), Raman spectra are still dominated by the F2g vibration of the cubic fluorite lattice. However, both the broadening of the F2g peak, its marked decrease in intensity and shift toward higher frequencies (from 459 to 470 and 475 cm-1 for Ce0.85Zr0.15O2 and Ce0.69Zr0.31O2, respectively) have to be connected with the increase of disorder in the fluorite structure and cell contraction due to partial substitution of Ce by Zr and/or existence of oxygen vacancies.28 The appearance of weak bands ca. 304 and 625 cm-1 for samples Ce0.69Zr0.31O2 and Ce0.5Zr0.5O2 (Figure 1d,e) is assigned to a tetragonal displacement of oxygen atoms from their ideal fluorite lattice positions leading to phases t′′ or t′.1,12 For Ce0.5Zr0.5O2, the strong asymmetry of the band at 466 cm-1 and the appearance of the diagnostic twin bands at 179 and 191 cm-1 of m-ZrO2 suggest the coexistence of a tetragonal phase together with CeO2 and m-ZrO2 impurities in some minor parts of the sample. The spectrum of Ce0.21Zr0.79O2 (Figure 1c) displays only weak bands at 179, 262, 320, 377, 472, and 620 cm-1, which are mostly characteristic of small or highly defective crystalline domains of the tetragonal t phase.1,12 By contrast, the Raman spectrum of Ce0.1Zr0.9O2 (Figure 1b) reveals only peaks similar to those of pure zirconia with monoclinic structure (Figure 1a). However, both the marked decrease in the intensities of these peaks and the shift of the main band to 462 cm-1 for Ce0.1Zr0.9O2 (Figure 1b) indicate that an expansion occurs in the monoclinic cell due to the replacement of some Zr4+ by the bigger Ce4+ cations. The structural properties associated with each CexZr1-xO2 composition are summarized in Table 1. 3.1.2. Textural Properties. As stated from N2 adsorption measurements at 77 K (Table 1), the porosity of ceria-zirconia samples is due mainly to the existence of mesopores, consistent with literature data.12,28 The monotonic decrease observed in the specific surface area (SBET values in Table 1) of catalysts from x ) 1 to x ) 0.5 has to be related to the incorporation of zirconium ions in the cubic fluorite lattice of ceria. This process, according to XRD data obtained on the same samples,17 promotes the growth of crystallites. It is worth noting that the reverse trend was obtained for CexZr1-xO2 compositions adopting predominantly the monoclinic structure (x ) 0 and 0.1), i.e., SBET values increase when cerium is inserted in the zirconia lattice (Table 1). The tetragonal Ce0.21Zr0.79O2 phase, which is characterized by a rather important specific surface area (184 m2/g) and an average particle size of 7 µm, was studied by HRTEM and SEM imaging (not shown here). Particles in this sample consisted in irregular crystallites of 2-5 nm in size having no preferential orientation and connected to each other by joint boundaries to form larger assemblies. Both the surface roughness and the existence of nonstructured voids in some parts

of the grain are expected to be responsible of the mesoporosity in these ceria-zirconia nanopowders. 3.1.3. Surface Properties. In the literature,10 it is reported that a pretreatment of ceria at temperatures as low as 200 °C is able to create surface defects either via the thermal elimination of adsorbed impurities or via removal of O capping atoms (in that case, a higher temperature is needed). In the present study, all the ceria-zirconia catalysts were pretreated at a temperature of 500 °C, i.e., at a temperature sufficient to remove most of adsorbed impurities. Hence, some reduced surface sites are expected to exist on all the ceria-zirconia surfaces investigated. Although the processes leading to surface reduction are usually not well-known, one may suspect that the thermal removal (by oxidation during the pretreatment) of some hydrocarbons adsorbed, for instance, during sample storage, may contribute to generate defects (O vacancies and reduced Ce sites) on the surface of ceria-zirconia. Also, the diffusion of O vacancies from the bulk to the surface during pretreatment may also be another source for the creation of these species. The existence of reduced sites will be addressed with more details in the next parts relevant to NOx adsorption processes. OH groups present on the surface of metal oxides were often found to play a role in NOx adsorption [ref 23 and references therein]. Here, their coordination and relative amounts on the surface was studied by in situ DRIFTS for some CexZr1-xO2 compositions (Figure 2). The presence of surface water at low/ moderate temperatures usually precludes the accurate determination of OH group positions due to H-bonding interactions. Therefore, the spectra presented in Figure 2 were obtained after pretreatment at 500 °C under inert atmosphere (peak temperature during catalyst pretreatment before NOx adsorption), i.e., in a dehydrated (and partially dehydroxylated) state. Hydroxyl group species on the surface of metal oxides are commonly assigned based on the number of coordinating cations; the higher frequency species (generally above 3700 cm-1) represent terminal groups (type I) and the lower frequency species represent either bi- or tribridging groups (types II and III, respectively).30 In that respect, type I OH groups have the most basic character, and the ease of removal of hydroxyl groups parallels their basicity.30 The DRIFTS spectrum of CeO2 (Figure 2) displays three main bands ca. 3690, 3644, and 3500 cm-1. The OH group at 3690 cm-1 (and the shoulder at 3712 cm-1) is assigned to Ce-OH terminal hydroxyls.10,32 The 3500 cm-1 band is assigned to cerium oxyhydroxide impurities within the ceria pores.31 The unresolved and intense band centered at 3644 cm-1 is assigned to Ce2(OH) doubly bridging groups on different facets of ceria crystallites and/or in different local environments.10,32 In agreement with previous assignments,27,31 the bands observed in the spectrum of m-ZrO2 (Figure 2) are connected with the existence of terminal hydroxyls (3758 cm-1) and bridged Zr2(OH) groups

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Figure 2. DRIFT spectra in the region of hydroxyl groups for some CexZr1-xO2 solid solutions after in situ pretreatment under Ar at 500 °C.

in defective (3725 cm-1) or regular (3661 cm-1) positions. Zr3(OH) groups are found around 3400 cm-1. By comparison with the single oxides, CexZr1-xO2 solid solutions are characterized by rather similar features, the most intense bands at 500 °C being still of the bridged M2(OH) type. Most peculiarly, terminal and bridged OH groups of Ce0.21Zr0.79O2 are found at IR frequencies rather close to zirconia, which is consistent with the composition of this mixed oxide. By contrast, hydroxyls on Ce0.69Zr0.31O2 are rather different from those of ZrO2 but also from those of CeO2. This material has the lowest amount of terminal hydroxyls among the CexZr1-xO2 materials investigated, as shown by the weak band ca. 3735 cm-1 (assigned to some Zr-OH species). Additionally, some bridged CeZr(OH) species are also expected to contribute to the absorptions observed in the 3700-3600 cm-1 range, but they can not be easily distinguished from bridged OH existing on ceria and zirconia. 3.2. Adsorption/Desorption Behavior of NO and NO2 on CexZr1-xO2. 3.2.1. ThermograWimetry of NO and NO2 Adsorption. Thermogravimetric curves displaying the evolution of the mass uptake during NO and NO2 adsorption at 30 °C on the series of ceria-zirconia are plotted in Figure 3. Although the amounts adsorbed near saturation drastically depend on the composition and specific surface area of the mixed oxides (Table 1), thermogravimetric curves were found to have rather similar shapes. The quasi-steady state of the mass uptake observed above the inflection point on TG curves (after 200-250 min in most cases) indicates that saturation of the surface by adsorbed species has already occurred. Above this point, the adsorption rate is mainly controlled by surface to bulk diffusion processes. Whatever the composition of the CexZr1-xO2 oxide, the adsorbed mass was not found to decrease after prolonged pretreatment under inert atmosphere. This confirms that NO and NO2 were irreversibly chemisorbed on ceria-zirconia. For each CexZr1-xO2 composition, the adsorption capacities extrapolated at saturation were first calculated by fitting the TG

Figure 3. Mass uptakes obtained by thermogravimetry during the adsorption of NO (500 ppm/Ar) and NO2 (1000 ppm/Ar) at 30 °C on CexZr1-xO2.

curves with the Freundlich equation and then the amount of sites per surface unit were obtained by subsequent normalization


Figure 4. Evolution of the surface densities (SNO and SNO2) computed from TG curves in function of the cerium content (x) within the on CexZr1-xO2 series.

of the data by the specific surface area. These surface densities of NO and NO2 (labeled SNO and SNO2 respectively, thereafter) on CexZr1-xO2 are plotted against the cerium content (x) in Figure 4. Remarkably, the distribution of SNO2 values among the CexZr1-xO2 series spreads between 3.1 and 4.6 NO2 equiv/ nm2, whereas the corresponding SNO values are generally well below (1.5-4.5 NO equiv/nm2). As SNO2 is always superior to SNO for a given composition (average SNO2:SNO ratio ≈ 1.8), this evidences the preferential affinity of ceria-zirconia surfaces for NO2. This result is in line with the general knowledge on the reactivity of NOx and the rather basic character of the mixed oxides.1 Within all the series of CexZr1-xO2 mixed oxides, SNO2 and SNO were not found to follow a regular trend, the maximal surface densities being observed for x ) 0.5 (NO2 and NO) and the minimal ones for x ) 0.2 (NO2) or x ) 1 (NO). Although the total number of adsorption sites is mainly influenced by the specific surface area of the mixed oxides (Figure 4 and Table 1), the variations of S within the CexZr1-xO2 series point out the existence of additional effects on adsorption. Indeed, the surface structure peculiar to each oxide, which is itself dependent both on the bulk structure and the pretreatment, will influence the redox processes involved in adsorption and therefore the nature of ad-NOx species. These aspects will be discussed with more details in a later section. 3.2.2. ThermograWimetry of NO and NO2 Desorption. Thermogravimetric (TG) curves obtained during the desorption at 500 °C under Ar of the NO and NO2 ad-species are displayed for the Ce0.69Zr0.31O2 sample in Figure 5. At 500 °C, all the NOx ad-species generated by the adsorption of NO or NO2 have almost desorbed. The mass loss due to the removal of surface species left by NO adsorption occurs predominantly below 300 °C, whereas for NO2 adsorption, the reverse trend is found (crossing point on Figure 5). This behavior is even more evident on the first derivatives (DTG) of thermogravimetric curves, which are displayed for the series of CexZr1-xO2 oxides in Figure 6 (desorption after NO2 and NO adsorption). The shape of DTG curves and peak temperatures revealed to be rather similar among the series of CexZr1-xO2 oxides, especially for those obtained following desorption of adsorbed NO2. On NO and NO2 DTG curves (Figure 6), two broad contributions can be easily separated, the first (80-150 °C) corresponding to desorption from weakly bonded sites and the second (300-500 °C) to strongly adsorbed species, i.e., nitrates.21 By comparing


Figure 5. TG curves obtained during the desorption under inert atmosphere (Argon) of ad-NOx species generated either by NO or NO2 adsorption on Ce0.69Zr0.31O2 at 30 °C. V ) 10 °C/min from 30 to 500 °C.

Figure 6. Effect of ceria-zirconia composition on DTG curves corresponding to the desorption of ad-NOx species generated by: (A) NO2 and (B) NO adsorptions performed at 30 °C. Heating rate ) 10 °C/min.

the DTG curves obtained with each of the reactive gases, it can be seen that NO2 promotes the formation of adsorbed species rather relevant to the second contribution while the opposite is observed for NO. A closer look at NO2 DTG curves (Figure 6A) allows establishing that the second contribution consists in two unresolved features, whose positions on the temperature scale depend on the composition of CexZr1-xO2 mixed oxides.

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Figure 7. Corrected MS data corresponding to the desorption of adNOx species generated from NO2 adsorption at 30 °C on Ce0.69Zr0.31O2. Heating rate ) 10 °C/min.

For the low Ce contents (x ) 0.1 and 0.21), the temperatures corresponding to the maximal rate of nitrates desorption are ca. 405 and 460 °C, whereas intermediate compositions (x ) 0.5, 0.69, 0.85) present two peaks at slightly lower temperatures (375 and 430 °C). When x ) 1 (CeO2), only one broad unresolved feature is obtained with a peak ca. 430 °C. These results show that the thermostability of nitrates adsorbed on tetragonal phases is weakened compared with the monoclinic phase and to a lesser extent also with the cubic fluorite phase. Depending on the intrinsic redox activity of sorption sites and the temperature, the thermal release of ad-NOx species can be either a simple desorption process or involves complex decomposition/reduction mechanisms. In the literature, it has been reported that under conditions with low or zero partial pressures of oxygen, the adsorption equilibrium of nitrate species could be dramatically reduced, this process creating a driving force for nitrate decomposition.33 Among the CexZr1-xO2 series, the most active mixed oxides in terms of redox activity are usually found between x ) 0.5 and x ) 1,1 i.e., the Ce-rich compositions with tetragonal or fluorite structures. Therefore, it is believed that the CexZr1-xO2 catalysts with x ) 0.5-1 are also the most active for NO2 dissociation in absence of O2, as is the case for the present TG experiments. Moreover, it can be added that CeO2 presents an additional contribution ca. 200 °C that is not or only hardly observed for CexZr1-xO2 mixed oxides (Figure 6A). DTG curves corresponding to desorption of NO (Figure 6B) show that the first desorption peak is monotonically shifted from 133 (CeO2) to 104 °C (Ce0.1Zr0.9O2) as the Zr content in CexZr1-xO2 increases. This trend is accompanied by the increase of another weak desorption peak at 250 °C, which is absent for CeO2. Therefore, it can be concluded that some NO ad-species in the vicinity of Ce sites desorb mostly around 130 °C, whereas for those connected with Zr sites, the desorption temperatures are ca. 100 and 250 °C. 3.2.3. Analyses of Desorbed NOx Species by TPD-MS. More information on the desorption behavior of NOx on CexZr1-xO2 can be extracted from TPD-MS analyses. TPD signals relevant to the decomposition of ad-NOx species generated by NO2 adsorption are displayed for the Ce0.69Zr0.31O2 composition on Figure 7. First, it is interesting to note that the whole TPD profile has a shape rather similar to the corresponding DTG curve displayed in Figure 6A. Between 100 and 300 °C (lowtemperature contribution on NO2 and NO DTG curves), the

Azambre et al. decomposition of the less stable adsorbed species proceeds via the evolution of NO, NO2, and to a lesser extent N2O and N2. The presence of the latter species indicates the existence of redox processes during desorption. The intense MS signals observed above 300 °C can be readily correlated with the main desorption peak present in the 300-500 °C range on NO2-DTG curves (Figure 6A). This corresponds mainly to the decomposition of nitrates as NO + O2 (main peak ca. 405 °C) and NO2 (small peak at 350 °C). Here, the detection of gaseous oxygen is thought to originate partly from the decomposition of the desorbed NO2 in the TPD equipment due to the displacement of the thermodynamic equilibrium NO2 T NO + 1/2O2 to the right as the temperature increases. However, the decomposition of nitrate species via surface peroxide O22- or superoxide O2intermediates may also contribute to the production of O2.29 In the literature, superoxide Ce4+-O2- species were detected upon O2 adsorption by EPR and FTIR and are formed via electron transfer from reduced cerium Ce3+-0 centers (0 represents an anionic vacancy).19 Considering the latter possibility, the decomposition of the most stable form of nitrates, e.g., bridged/ bidentate,23 is thought to involve the reduction of some metal centers, which were initially oxidized for a NOx-covered ceriazirconia surface. This may be represented here by the following reaction: M4+ ) O2(NO)- f M3+0 + O2(g) + NO(g)(M ) Ce, Zr)

(1) In the present conditions, it is however difficult to determine whether most of the NO + O2 evolved is due to thermodynamics or direct decomposition of the nitrates. The NO2-TPD experiment was repeated for the Ce0.21Zr0.79O2 composition (not shown here) and yielded results very similar to Ce0.69Zr0.31O2, despite the (NO + O2) peak was delayed by about 25 °C, which is again consistent with the results obtained by TGA. Finally, when NO and O2 were coadsorbed at 30 °C, the corresponding TPD profiles (not shown here) were also close to that of NO2-TPD in the 300-500 °C range. This confirms that the oxidation of NO to NO2 occurs to a significant extent already at rather low temperatures on ceria-zirconia with high specific area. 3.2.4. DRIFTS Study of NO2 Adsorption on Ce0.69Zr0.31O2. In this section, we aimed to describe the general chemical processes relevant to the reactive adsorption of NOx on ceriazirconia. For this, the adsorption of NO2 at 30 °C on a Ce0.69Zr0.31O2 catalyst pretreated in situ under air at 500 °C is considered first. As described in the introduction, the reaction of NOx with ceria-based materials is expected to occur mainly via oxygen or electron transfer. Considering the strong oxidative character of NO2, one of the primary steps in its adsorption on Ce0.69Zr0.31O2 should involve the reoxidation of surface defects. Owing to the pretreatment of the catalyst under air, the amount of reduced sites prior to adsorption is supposed to be fairly low, but not nil, as we will see. In that respect, it was shown in the literature14,15 that the stability of oxygen vacancy defects on the Ce0.8Zr0.2O2 (111) surface was higher than that for the CeO2 (111) surface and that reduced sites were preserved even after prolonged oxidation in oxygen gas. In the present study, information on the state of the surface was obtained indirectly by DRIFTS. First, an overall decrease of the spectral baseline was observed in the 3000-1700 cm-1 region (with a minimum around 2060 cm-1) during adsorption. It has been evidenced in the literature10-12 that the IR spectrum of CeO2 in a reduced



state exhibits a very broad absorption ca. 2120 cm-1 due to the 2 F5/2 f 2F7/2 forbidden electronic transition of Ce3+. Therefore, the decrease of the overall absorption between 3000 and 1700 cm-1 probably has to be related to the reoxidation of the catalyst during NO2 adsorption. For Ce0.69Zr0.31O2, this transition may be expected at a different wavenumber (around 2060 cm-1 here) because of differences in band gaps with CeO2. The second kind of information on the presence of reduced sites on the surface was brought by the nature of adsorbed/ gaseous species in the initial stages of adsorption. Indeed, preliminary experiments carried out using a flow reactor have revealed that some NO was released due to NO2 reduction, this process presumably involving either the disproportionation of N2O4 (as discussed later) or the reoxidation of some Ce3+-0 reduced sites by NO2. Time-course DRIFT spectra recorded during the exposure of Ce0.69Zr0.31O2 to NO2 (1000 ppm/Ar) at 30 °C are given for the 1800-700 cm-1 region in Figure 8A. Up to 15-20 min of adsorption, the IR spectra are dominated by a band ca. 1187 cm-1. Meanwhile, weaker bands develop at 1000, 1070, 1277, 1375, 1390, 1481, 1563, 1592, and 1610 cm-1. After 20 min of adsorption, the intensity of the 1187 cm-1 band decreases progressively at the expense of very strong bands, whose new maxima after 42 min adsorption are 1009-1025, 1242-1275, 1520-1565-1585, and 1612 cm-1 (Figure 8A). The growth of the latter bands is accompanied by the increase of absorptions ca. 1740, 1932, 2009, 2243, 2290, 2499, 2563, and 2607 cm-1 (Figure 8C). Finally, all the changes observed were accompanied by the continuous decrease of OH groups at 3688 cm-1 and the increase of positive absorptions in the 3650-3100 cm-1 region (Figure 8B). Interpretation of these spectral data is based on common IR assignments of NOx ad-species and the known reactivity of zirconia, ceria, and the mixed oxides.1,18-27 An accurate assignment of the different ad-NOx species identified in this study is given in Table 2. First, it should be outlined that none of the absorptions detected in the initial stages of adsorption could be assigned to Mx+-(NO)x neutral nitrosyls. By contrast, the strong absorption at 1187 cm-1 (Figure 8A) can be ascribed to the ν(N-O) vibration of anionic NOx surface species such as the nitroxyl anion NO- and/or surface nitrites. Therefore, this confirms that redox mechanisms predominate in the present conditions. Rather accurate information on the structure of the nitrites can be obtained by IR spectroscopy. The free nitrite anion NO2- has a C2V symmetry and manifests νas(NO2) at 1260 cm-1 and νs(NO2) at 1330 cm-1. However, coordination of a nitrite anion to one or more metal cations is accompanied by drastic changes in its IR spectrum, which have been described in detail by Hadjiivanov.23 The low intensities observed in the 1520-1375 cm-1 region compared with those of the 1187 cm-1 band indicate that these surface nitrites have in most cases no corresponding ν(NdO) modes (Figure 8A). Hence, these surface nitrites are thought to preferentially adopt the bridged bidentate (M2(O2)dN-) and/or chelating bidentate (M(O2)dN-) configurations. One possible way for the formation of bidentate nitrite species is via electron transfer from a reduced Lewis center (Ce3+ or Zrx+) to a NO2 molecule with a simultaneous reoxidation of an oxygen vacancy, which can be depicted for instance as:

Ce3+0 + NO2 f Ce4+(NO2)-

(2)

Alternatively, isolated absorptions around 1200-1150 cm-1 have also been assigned to the nitroxyl anion NO- because it

Figure 8. DRIFTS spectra corresponding to the adsorption of NO2 (1000 ppm/Ar) at 30 °C on Ce0.69Zr0.31O2 pretreated at 500 °C under air. (A): 1800-700 cm-1 region; (B) 3800-3000 cm-1 region; (C) 2700-1800 cm-1 region. (a) 30 min; (b) 42 min; (c) 60 min; (d) 82 min; (e) 91 min; (f) 120 min.

is not spectroscopically distinguishable from bidentate nitrites.23 By analogy with eq 2, NO- species can be formed via electron transfer to a NO molecule, released by the dissociation of NO2:

Ce3+0 + NO f Ce4+(NO)-

(3)

These species can dimerize to yield hyponitrites, as suggested by the study of Martinez-Arias et al.20 performed on ceria, and

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TABLE 2: IR Assignments of ad-NOx Species on CexZr1-xO2 Surfaces surface species x+

2-

Ce -cis-N2O2

HNO2 (ads) interacting with H2O (ads) H2O (ads) NO2- (bidentate nitrites) and/or NO- on CexZr1-xO2 NO2- (monodentate nitrites) on ZrO2 NO3- (bridged bidentate)**b NO3- (chelate bidentate I)*,**a,b Ce-rich compositions

NO3- (chelate bidentate II)*,**a,b Ce-rich compositions

NO3- (chelate bidentate III)**b Ce-rich compositions NO3- (monodentate)**b Ce-rich compositions ON-Cex+-(NO3-) N2O (ads) N 2 O4 NO+ a

band positions (cm-1)

vibration modes

1360-1320 1070-1040 3585-3570 1700-1620 1550-1300 around 3400 1625-1610 1200-1185 1462 1160 1610 and 1220 around 1000 2607 1520 and 1227 1030 3018, 2430 2540 and 2249 2052 1544-1536 and 1242 1012 3050, 2440 2540 and 2240 2010 1586-1565 and 1235 1006 2568-2525 and 2275 2004 1480 and 1295-1280 1005 2480 and 2294 1940-1930 2245-2230 1255 1760-1740 2300-2200

ν(N-N) νs(N-O) ν(O-H) ν(NdO) δ(HON) ν(O-H) δ(HOH) ν(N-O) ν(NdO) ν(N-O) νas(NO3-) split νs(NO3-) νas(NO3-) + νs(NO3-) νas(NO3-) split νs(NO3-) 2νas(NO3-) split νas(NO3-) + νs(NO3-) 2νs(NO3-) νas(NO3-) split νs(NO3-) 2νas(NO3-) split νas(NO3-) + νs(NO3-) 2νs(NO3-) νas(NO3-) split νs(NO3-) νas(NO3-) + νs(NO3-) 2νs(NO3-) νas(NO3-) split νs(NO3-) νas(NO3-) + νs(NO3-) ν(NdO) ν(N ) N) ν(N-O) ν(NdO) ν(NdO)

split

split

split

split

split

* Nitrates formed following the oxidation/thermal decomposition of bidentate nitrites. b ** Nitrates formed following reaction with NO2

the presence of characteristic bands at 1070 cm-1 and 1375 cm-1, assigned to cis-N2O22- species (Figure 8A and Table 2):

2Ce3+-0 + 2NO f 2Ce4+(NO-)2 f 2Ce4+(N2O22 ) (4) According to ref 20, cis-hyponitrites can easily decompose at low temperatures to yield N2O. These species were also postulated as potential candidates for the formation of N2 in SCR reactions.23 In our case, the evolution of N2O and N2 during the He-TPD, as shown on Figure 7, may perhaps be linked to the decomposition of hyponitrites. In the IR spectra, it is also possible that part of the absorptions around 2240 cm-1 (Figure 8B) and 1255 cm-1 (Figure 8A) could be due to adsorbed N2O. However, this assignment is not obvious due the overlapping of IR bands in this region, as shown in Table 2. NO- species generated from eq 3 can be oxidized by an activated O* surface species (the nature of O* species will be discussed afterward) to increase the amount of nitrites:

NO- + O* f NO¯ 2 + e

(5)

However, there is another pathway leading to nitrites (from NO), which is evidenced in IR spectra by the decrease of the 3688 cm-1 band (Figure 8B) and the concomitant development of a broad absorption in the 3600-3000 cm-1 range. It involves

the reaction of hydroxyl groups with NO and the release of a nitrous acid intermediate and water:

2NO + OH- f HNO2+ NO-(+ OH-) f NO-+ NO2 + H2O (6) By contrast, the remaining intense bands in the 1700-1000 cm-1 range (Figure 8A) and the weaker ones in the 2700-2000 cm-1 region (Figure 8B,C), which further grow after 20 min adsorption, are unambiguously ascribed to surface nitrates.21,23 In ref 23, it is proposed that the possible configurations of nitrates species on the surface may be further distinguished by the extent of splitting between their νas(NO3-) modes (two bands between 1620 and 1200 cm-1) and the positions of the combination modes and overtones in the 3100-2000 cm-1 region. Hence, the intense bands ca. 1585-1565, 1530-1515, and 1275-1240 cm-1 and 1610 and 1220-1200 cm-1 are ascribed to various forms of bidentate and bridged nitrates, respectively, whereas the bands around 1500-1450 and 1300-1270 cm-1 better correspond to monodentate species.21,23 Additionally, all these surface nitrates also present a characteristic νs(NO3-) vibration mode in the 1050-990 cm-1 range,23 which is clearly visible on ceria-zirconia due to the weak absorption of lattice vibration modes in this spectral region. When possible, the combination modes and overtones of the different kinds of nitrates in this study were identified. They are given in Table 2.



As for the nitrites, there are several ways to form nitrates, which can be evidenced through a detailed analysis of IR data. Because of the concomitant decrease of the nitrite band at 1187 cm-1 after 20 min adsorption, one major pathway leading to their formation on ceria-zirconia is the reaction of nitrite species with an activated O* species present following the pretreatment or a surface oxygen left by the dissociation of another NO2 molecule: NO2 + O* f NO3

(7)

This surface oxidation process has already been reported for ceria and alumina-based catalysts and was found to be promoted both by gaseous oxygen and the temperature.20,21,33 Here, the fact that reaction 7 occurs significantly at room temperature is in line with the well-established redox properties of ceriazirconia.1 However, the question now arises on the nature of these O* activated species. The forms of O2 activation on Cecontaining surfaces following dioxygen adsorption have been studied both by EPR,19 Raman,34 and FTIR35 spectroscopies. These studies have shown that O2 can be adsorbed following electron transfer by one-electron or two-electron surface defects as superoxide O2- or peroxide O22-, respectively. Among these species, the surface superoxides are the most thermally unstable/ reactive and convert first into peroxides as the temperature increases and then to lattice oxygen O2- when the thermal activation is high enough to provide enough electron density to break the O-O bond. Duprez et al.35 have reported that the concentration of superoxide O2- species resulting from O2 adsorption at room temperature reaches a maximum for x ) 0.6-0.8, e.g., for samples exhibiting the highest OSC among the CexZr1-xO2 series. In our previous study,21 we have reported the influence of the type of pretreatment (500 °C under air or inert atmosphere) on the adsorption of NO and NO2 on Ce0.69Zr0.31O2 at room temperature. Whatever the type of NOx molecule adsorbed (NO or NO2), nitrates were present only when the pretreatment was carried out to 500 °C in air and not when performed under inert atmosphere. Hence, it is deduced that only the pretreatment in air is able to bring the O* species needed for the production of nitrates near room temperature. Considering the known thermostability/reactivity of the different possible O* species34,35 and the rather oxidized state of the surface, it is therefore proposed that the O* species needed for nitrite oxidation may be mainly of superoxide O2- type and/or consist as O2- surface oxygens20 in special locations, e.g., the side-terminations of the crystallites. A second possibility for the formation of nitrates involves the participation of basic/neutral hydroxyls:

2OH- + 3NO2 f 2NO3 + H2O + NO

(8)

The validity of this mechanism is proven by the continuous decrease of the 3688 cm-1 OH band all along the adsorption process, i.e., after all the nitrites have been converted to nitrates but also by the synchronous growth of absorptions near 3600-3000 cm-1 due to H-bonded water molecules. Finally, a third pathway for the formation of nitrates via selfionization of N2O4 and cooperative effects has previously been suggested in the literature, as described in the introduction.24-26 It has been reported to occur on the terrace sites of defect-free oxides and implies the participation of acid-base pairs on the surface. From this process, both nitrates and cationic NOx

species, such as NO+ and NO2+, are expected. Because of their charge, these cationic species were reported to absorb at higher wavenumbers than gas-phase NO in the 2300-2000 cm-1 range.23 In the present study, new absorptions at 2280, 2240, and 2010 cm-1 (Figure 8B were found to grow simultaneously to nitrates after 20-30 min adsorption. However, the presence of these bands can not be considered as direct evidence for the existence of cationic NOx species. In fact, all these absorptions can alternatively be assigned to some combination modes and overtones of nitrates (Table 2), excepted the 1932 cm-1 band, which is tentatively assigned to ON-Ce4+-(NO3-) species. On the basis of this assignment, it is possible that NO2 disproportionation to NO+ and NO3- species also take place on ceriazirconia, but the reaction is immediately accompanied by a single electron transfer reaction leading to a ON-Ce4+-(NO3-) species or to NO in the gas phase. To sum up, the adsorption of NO2 at 30 °C on ceria-zirconia pretreated in situ under air at 500 °C has led to the formation of nitrates after an initial period of reoxidation of the surface and the evolution of hyponitrites and nitrites species. Though several mechanisms may apply, the most probable pathway leading to nitrates, on the basis of IR data, seems to be the surface oxidation of nitrites. When the Ce0.69Zr0.31O2 sample was pretreated under inert atmosphere (500 °C under argon) instead of air, nitrites were still the dominant ad-NOx species on the surface following exposure to NO2 at 30 °C over the same time period (120 min). Because of the nature of the pretreatment, a lower concentration of the most reactive O* species, i.e. a higher concentration of reduced sites, is expected on ceria-zirconia.15,21 Therefore, the surface reoxidation and transformation of nitrites to nitrates are both expected to be much slower, which explains the results obtained. Moreover, it can be added that the NO2 adsorption following an in situ pretreatment at 500 °C under H2 (5% in Ar) yielded IR spectra somewhat similar to those obtained under pure Ar. This means that the H2 pretreatment in our conditions has not led to a strong reduction of the ceriazirconia. In that respect, Adamowska et al. have shown by TPR onto same ceria-zirconia powders that the reduction becomes significant only above 500 °C, which is in line with the present data.17 However, when discussing the possible formation pathways leading to nitrites/nitrates, one should bear in mind that the preponderance of one mechanism over another may depend not only on the intrinsic reactivity of the sample but also on the adsorption conditions (temperature, presence of water, etc). 3.2.5. DRIFTS Study of NO Adsorption on Ce0.69Zr0.31O2. Following a pretreatment at 500 °C under air, the exposure of Ce0.69Zr0.31O2 to NO at 30 °C led initially to the same ad-NOx species as those observed in the case of NO2 adsorption in Figure 8 (spectra obtained at 5, 7, 15, and 20 min). Hence, the spectra are not given here. However, after 20 min on stream, the evolution of nitrates was much slower in the case of NO adsorption than it was for NO2 (Figure 8). These results showed that in the case of an “oxidized” Ce0.69Zr0.31O2 surface, a significant part of the NO is oxidized to NO2 even at room temperature, and therefore the generated ad-NOx species are those corresponding to NO2 adsorption. After this initial period, the NO oxidation rate quickly decreases because the reservoir of active O* species undergoes a rapid depletion in absence of gaseous oxygen. The surface conversion of nitrites to nitrates, which indeed requires the participation of these reactive oxygen species, does not occur at a rate similar to what was observed during NO2 adsorption. Thermogravimetry was used in order to give a more quantitative picture of the NO and NO2

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Figure 9. Kinetic rate constants corresponding to the gravimetric adsorption of NO and NO2 at 30 °C on Ce0.69Zr0.31O2 (calcined at 500 °C in air) in IGA apparatus.

adsorption process. In that respect, the “apparent” rate constants (kNO and kNO2) corresponding to the adsorption of NO and NO2, respectively, at 30 °C (following an in situ pretreatment at 450 °C under air) on Ce0.69Zr0.31O2 were determined from the mass uptakes recorded at increasing exposure times and concentrations of the reactive gas. In Figure 9, one point corresponds to the rate constant determined over a period of 300 min at a fixed concentration of the reactive gas. Although the experimental setups in the DRIFTS reactor and the IGA are different, the same conclusions were obtained. In the initial stages of adsorption (first points on Figure 9), kNO and kNO2 are of the same order due to the oxidation of NO in NO2. According to DRIFTS, this period corresponds mainly to the build-up of surface nitrites (and hyponitrites). Then, kNO2 increases drastically, whereas kNO slightly decreases, which means that the mass uptake due to the formation of supplementary ad-NOx species is rapid for NO2 adsorption and rather unfavored in the case of NO (Figure 9). This has to be connected with the production of nitrates, which is strongly promoted only in the case of NO2, as mentioned above. The effect of ceria-zirconia composition on the nature and thermostability of ad-NOx species generated by NO adsorption for argon-pretreated surfaces (x ) 1, 0.69, 0.21, and 0) was investigated next. Time-course DRIFT spectra recorded during the exposure of Ce0.69Zr0.31O2 to NO (500 ppm/Ar) at 30 °C are given in Figure 10. During the first 15 min of adsorption, only weak bands at ca. 1640, 1545, 1327, and 1063 cm-1 are visible in the 1700-1000 cm-1 region. According to previous assignments (Table 2) and the literature,21,23 the band at 1063 cm-1 has to be assigned to the hyponitrites anion N2O22-. As mentioned before, the pretreatment of the Ce0.69Zr0.31O2 catalyst under Ar is thought to create a higher amount of thermally induced oxygen vacancies and reduced metal centers than the pretreatment under air. On these defective sites, NO can undergo partial or total decomposition, leading to the formation of reduced species such as isolated N and O atoms (not detectable by IR) or even N2O (small band at 2240 cm-1). Increasing the time of exposure to NO led to the shift and growth of almost all the bands in the 1700-1000 cm-1 region but also to the formation of a new intense band at 1200-1190 cm-1 characteristic of bridged/bidentate nitrites NO2- and/or NO- anionic species (Figure 10).21,23 Here also, the absence of neutral Mx+NO nitrosyls indicates that ceria-zirconia surfaces rather promote the stabilization of ad-NOx species by electron or oxygen transfer. The evolution of the spectral features observed in the

Figure 10. DRIFTS spectra corresponding to the adsorption of NO (500 ppm/Ar) at 30 °C on Ce0.69Zr0.31O2 pretreated at 500 °C under Ar. (A): 1800-700 cm-1 region; (B) 3900-3000 cm-1 region.

1700-1000 cm-1 region (Figure 10) is, as for NO2 adsorption, accompanied by complex changes in the hydroxyl region. All along the NO adsorption process, a consumption of type II OH groups is observed at 3696, 3686 (shoulder), and 3666 cm-1 and broad absorptions develop around 3400 and 3210 cm-1 due to the production of water. Synchronously, new OH groups with increased Bro¨nsted acidity appear as a broad and unresolved band near 3570 cm-1. It is worth noting that the 3735 cm-1 band previously assigned to some residual terminal Zr-OH groups (Figure 2) do not apparently interact with NO. Upon switching the reactive gas to argon, a strong decrease of the overall absorptions in the 3650-3000 and 1650-1250 cm-1 regions is observed, whereas the NO2- and/or NO- band at 1194 cm-1 further grow with time. Moreover, the 3696 cm-1 band becomes more negative and some of the new OH in the 3650-3550 cm-1 still maintain. These data strongly suggest the reaction of OH groups with NO to give adsorbed water and HNO2 such as described in eq 6. First and upon exposure to NO, an adsorbed layer constituted by H-bonded HNO2 and H2O adducts is formed. In the literature,23,36 the trans- and cis- forms of HNO2 were reported to absorb at 3600-3400 cm-1 (νOH), 1700-1600 cm-1 (νNdO), and 1400-1200 cm-1 (δHON), whereas the presence of adsorbed water on oxides usually yields broad absorption features around 3400 and 1620 cm-1. More-



Figure 11. Semiquantitative DRIFTS data corresponding to the evolution of nitrite and hyponitrite species during the adsorption of NO (500 ppm/Ar) at 30 °C on CexZr1-xO2 (x ) 1, 0.69, 0.21, 0).

over, the association of water and nitrous acid through Hbonding interactions was found to induce significant red-shifts for the νOH band of HNO2 and to a lesser extent for the νNdO band in comparison with the positions of these modes for the isolated nitrous acid molecule, the extent of the shift for each mode being dependent on the HNO2:H2O ratio.34 Hence, the absorptions observed in the course of NO adsorption in the 3650-3000 and 1650-1250 cm-1 regions of IR spectra are representative of the adsorption of HNO2 on a ceria-zirconia surface with different degrees of hydration. Upon purging the cell with argon, some adsorbed water molecules are lost, as shown by the overall decrease of the absorptions at 3600-3000 and 1640-1620 cm-1. Simultaneously, the absorptions ca. 1545 and 1327 cm-1 due to hydrated NO2 moieties also decrease and shift, whereas the 1194 cm-1 band increases (Figure 10). Therefore, the HNO2-H2O adducts formed during adsorption are progressively replaced by bidentate nitrites and/or Mx+(OH)ONO- species, the latter kind of OH groups having an increased Bro¨nsted acidity due to presence of the NO2 group in their vicinity. 3.2.6. DRIFTS Study of NO Adsorption/Desorption: Effect of CexZr1-x1O2 Composition. Although slight changes in bands positions and intensities were observed due to adsorption on different kinds of sites, it is worth noting that rather similar conclusions can be drawn for the other CexZr1-xO2 compositions (x ) 1, 0.21, 0) investigated, therefore the corresponding IR spectra are not given here. The main adsorption pathway still involves the reaction of NO with type II OH groups to produce adsorbed H2O-HNO2 complexes and their decomposition products, namely NO2-, NO-, and/or HNO- species. Briefly, the IR spectra of NO adsorbed on the mixed oxides resemble more to those of their closer parent single oxide, i.e., the IR spectra obtained for Ce0.69Zr0.21O2 are more close to those obtained for CeO2 than those of ZrO2. A semiquantitative of the evolution of the main ad-species was attempted next. The intensities of the bidentate nitrite band ca. 1200-1190 cm-1 and that of the 1065-1040 cm-1 band corresponding to the hyponitrite anion are plotted for the different CexZr1-xO2 compositions investigated in Figure 11. Taking into account the differences existing in terms of specific surface area (see Table 1), it can be deduced that the amount of hyponitrites on Ce0.69Zr0.21O2 is almost twice that present on ceria (Figure 11),

possibly because of a more important of reduced sites on the former. However, these species were barely detected for zirconia and only as a small shoulder near 1040 cm-1 on Ce0.21Zr0.79O2, despite its rather high specific surface area (180 m2/g). Therefore, this confirms that the formation of hyponitrites is promoted by the presence of Ce3+ ions interacting with oxygen vacancies.20 On the other hand, it can also be observed from Figure 11 that nitrites are formed in each case after an induction period, corresponding to surface reoxidation. Also, the formation of these species does not seem specific to a peculiar domain of CexZr1-xO2 compositions. DRIFT spectra recorded under Ar from 30 to 500 °C during the TPD of NO adsorbed on Ce0.69Zr0.31O2 are shown in Figure 12. The absorptions corresponding to hyponitrites species at 1327 and 1067 cm-1, which already started to decrease upon evacuation, completely vanish in the 30-100 °C range, probably due to their transformation into gaseous N2O or N2, as suggested by TPD-MS (Figure 12 and Figure 7). Within the same temperature range, the removal of adsorbed water induces a further decrease in the absorptions related to HNO2-H2O complexes and a slight increase of the band corresponding to metal nitrites. Therefore, the first DTG peak observed during the desorption of NO ad-species (Figures 5 and 6) has to be related mainly to the loss of water, hyponitrites species, and part of the nitrites. Second, the nitrite band ca. 1190 cm-1 decreases above 150-200 °C and transforms progressively into new sharp and narrow bands at 1522, 1223, and 1030 cm-1, which become obvious above 250-300 °C and are ascribed to bidentate nitrates (Figure 12). The growth of these bands is accompanied by the synchronous increase of the prominent ν1 + ν3 combination mode at 2543 cm-1 and the corresponding overtones of the 1522, 1223, and 1030 cm-1 bands at ca. 3020, 2435, and 2050 cm-1, respectively. It is worth noting that all these bands were found at the same positions for CeO2 (but not for Zr-rich compositions). Therefore, they are assigned to Cex+d(O2)NO- species. A further increase of the temperature led to the increase of another kind of bidentate nitrate species characterized by bands at 1568-1557, 1245, and 1011 cm-1. These results show that the oxidation of nitrites to nitrates, which is kinetically limited at room temperature in the absence of “reactive” oxygen, can occur more significantly at higher temperatures because the oxygen mobility in the material has

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Figure 12. TPD-DRIFTS spectra obtained during the desorption of ad-NO species on Ce0.69Zr0.31O2 from 25 to 500 °C under argon (V ) 10 °C/min).

Figure 13. Reaction network corresponding to NOx adsorption/desorption on nanometric ceria-zirconia powders. Plain arrows represent mechanistic pathways confirmed by the present study and dashed arrows represent other possible pathways.

increased. Apart from this direct surface oxidation mechanism, another possibility to form nitrates arises from the desorption of nitrites and readsorption of the desorbing NOx on the surface. In the 300-500 °C temperature range, adsorbed N2O and Mx+(NO3-)NO species were also detected at 2230 and 1940 cm-1, respectively. The desorption of all these species, i.e., nitrates, adsorbed N2O and NO can be connected with the occurrence of the second DTG peak observed by TGA during the desorption of ad-NO species (Figure 5 and 6). Finally, it could be added that nitrates were also formed in the TPD of NO adsorbed on Ce0.21Zr079O2 and ZrO2 but only in very

small amounts in comparison with CeO2 and Ce0.69Zr0.21O2. This seems in line with the weaker redox activity of the Zr-rich compositions in comparison with the Ce-rich ones.1 Taking into account all the results obtained, an overall mechanism representative of the interaction of NOx with CexZr1-xO2 is given on Figure 13. The proposed pathways can be divided into two groups whether they were formally observed by IR spectroscopy in the present study (plain arrows) or not (dashed arrows). Hence, the formation of nitrites and nitrates via surface oxidation pathways and reactions with OH groups has been formally evidenced by IR spectroscopy and are important pathways for NOx adsorption on ceria-based

NOx on (CexZr1-xO2) Mixed Oxides materials. By contrast, the reactions involving NO- as an intermediate are only postulated (or transient) pathways here because the existence of such species can not be simply revealed spectroscopically in the used conditions. Moreover, it can be added that NO- species evidenced on ceria20 and ceria-zirconia19 by EPR at subambient temperatures may have a short lifetime above room temperature because of an increased reactivity of the system. Taking it overall, we can neither rule out nor confirm the participation of NO- to the adsorption/desorption processes of NOx on ceriabased materials. 4. Conclusions In this study, the NOx adsorption/desorption properties of well-characterized commercial ceria-zirconia powders was investigated in function of the CexZr1-xO2 composition/structure. The correlation between thermogravimetric and spectroscopic data has given consistent and complementary results: (i) The adsorption capacities of NO and NO2 estimated by TGA at 30 °C were mainly influenced by the specific surface area of CexZr1-xO2 rather than by the peculiar phase composition or structure. Typical surface densities spread between 3.1-4.6 NO2/ nm2 and 1.5-4.5 NO/nm2 among the ceria-zirconia series. No regular trend could be identified regarding the phase diagram of CexZr1-xO2. (ii) DRIFTS experiments allowed identification of the main ad-NOx species both from their fundamental vibrations, combination modes, and overtones. The type and amount of adsorbed species was found to be dependent both on the reactive gas (NO or NO2), the CexZr1-xO2 composition, the temperature, and the pretreatment/presence of O2. On Ar-pretreated surfaces, the adsorption of NOx at room temperature led mainly to the formation of bidentate nitrites after an initial period of surface reoxidation. Hyponitrites were also identified for the Ce-rich compositions (x ) 1 and 0.69) in the first steps of adsorption. Although several mechanisms are operative, one of the main pathways for the formation of nitrites from NO involved the reaction of the latter with hydroxyl groups to form HNO2-H2O complexes. On Ce0.69Zr0.31O2, the formation of bidentate and bridged nitrates from NO2 adsorption at 30 °C was promoted only when a pretreatment was carried out in air at 500 °C and/ or in the presence of gaseous O2; although the concentration of reduced surface sites was not nil following the pretreatment under air, the same pretreatment performed under Ar led to a more important formation of surface defects, i.e., oxygen vacancies and reduced metal centers. On Ar-pretreated surfaces, the lack of the most reactive oxygens hinders partially the oxidation of nitrites to nitrates at room temperature, but this process is promoted at higher temperatures due to the enhancement of the oxygen mobility in the materials. (iii) The thermodesorption of ad-NOx species under inert atmosphere is characterized by well-differentiated TG-DTG profiles depending on if these species were generated from NO or NO2 adsorption. ad-NO species, i.e., mostly nitrites and hyponitrites, decomposed in general through simple desorption or redox processes below 250 °C as NO, NO2, N2, and N2O. By contrast, the thermostability of NO2-adsorbed species, mainly nitrates, was much higher. TPD-MS experiments reveal that nitrate species decompose mainly between 300 and 550 °C as NO2 and/or NO + O2. In general, the thermostability of the different ad-NOx species was found to be rather similar among the CexZr1-xO2 series. In the range x ) 0.1-1, the most stable forms of nitrates were found for the CexZr1-xO2 solid solutions with predominantly monoclinic structure (x ) 0.1, 0.21) and the less stable ones for tetragonal (t′ and t′′) and fluorite structures (x ) 0.5, 0.69, 0.85) due to their superior redox activity.

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JP9008674

Adsorption and Desorption of NOx on Commercial Ceria-Zirconia

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