ZrO2 for the selective catalytic reduction of NO

J.W. Hightower, W.N. Delgass, E. Iglesia and A.T. Bell (Eds.) 11th International Congress on Catalysis - 40th Anniversary

Studies in Surface Science and Catalysis, Vol. 101 9 1996 Elsevier Science B.V. All rights reserved.

691

The activity of VOx/ZrO2 for the selective catalytic reduction of NO V. Indovinaa, M. Occhiuzzi a, P. Ciambelli b, D. Sannino b, G. Ghiotti c and F. Prinettoc, aChem. Dept., "La Sapienza" University, Roma, P.le A. Moro 5, 00185, Roma, Italy bChem and Food Engineering Dept., Salemo University cChem. Inorg., Chem. Phys. and Materials Dept., Torino University Abstract

Samples VOx/ZrO2, prepared by i) adsorption from aqueous NH4VO3 solutions at pH=l-4, ii) dry impregnation with the same solution, or iii) adsorption from vanadyl acetylacetonate solutions in toluene, were characterized by means of ESR, XPS and IR spectroscopies. In the selective catalytic reduction of NO with NH3 in the presence of 02 (SCR), VOx/ZrO2 catalysts were active and stable. In the NO+NH3 reaction, they had much lower catalytic activity. Their activity depended only on the vanadium content, not on the method used for preparing the catalysts. Catalytic activity (molecules nm -2 s -1) markedly increased with the vanadium concentration up to 3 atoms nm -2 and changed little thereafter, paralleling the increased concentration of specific polyoxovanadates, detected by IR. The surface concentration of NH~ also paralleled the SCR activity. The results suggest a possible role in SCR for NH; ions and adjacent chelating nitrates, also identified by IR.

1. INTRODUCTION The selective catalytic reduction of NO with NH3 in the presence of 02 (SCR) has been extensively studied mainly on VOx supported on TiO2 [1-4]. The commercial catalysts for the SCR of flue gases from stationary sources are V205-TiO2 and V205(-WO3)/TiO2. Many studies have investigated the dispersion, the nuclearity and the oxidation state of vanadium supported on TiO2 [5-14]. All these properties might depend on the support and it was therefore of interest to extend the study to other supports and particularly ZrO2. Szakacs et al. [15] have studied the SCR activity of VOx/ZrO2 catalysts prepared by adsorption on ZrO2 of VO(acetylacetonate)2 from toluene solutions. The SCR mechanism has been investigated by isotopic tracers [16-18], and the surface species on VOx/TiO2 by spectroscopy [19-22]. Takag! et al. [23] proposed a mechanism involving the reaction of adsorbed NO2 with NH~. Although Tops~e et al. [22] evidenced the participation of NH~ species, they are in favour of a mechanism involving reaction with gas phase NO or weakly adsorbed NO. Several investigators have proposed a redox mechanism involving vIv/v v species [1, 3, 6-8, 24] and have pointed out the need for two adjacent V sites [2, 4, 9, 15]. In this paper we report (i) the catalytic activity for SCR of VOx/ZrO2 samples prepared by various methods (adsorption from aqueous metavanadate solutions at different pH values, dry impregnation, and adsorption from VO(acetylacetonate)2 in toluene), (ii) sample characterization (nuclearity, dispersion and oxidation state) by means of XPS, ESR and FTIR and (iii) the nature and reactivity of the surface species observed in the presence of the reactant mixture. Catal .ytic results are here reported in full. Characterization data relevant to the discussion of the catalytic activity will be given, whereas details on the catalysts preparation and

692

characterization will be reported elsewhere.

2. E X P E R I M E N T A L 2.1. Sample preparation

The zirconia support was prepared by hydrolysis of zirconium oxychloride with ammonia, as already described [25]. Before its use as support, the material was calcined in air at 823 K. VOx/ZrO2 samples were prepared by three methods: (i) adsorption from a solution of ammonium metavanadate (AV) at pH values from 1 to 4, adjusted by nitric acid, (ii) dry impregnation with AV solutions and (iii) adsorption from a solution of VO(acetylacetonate)2 in toluene. VOx/ZrO2 catalysts were designated as ZVx(y)pHz, where x gives the analytical vanadium content (weight percent), y specifies the preparation method (a, adsorption, i, impregnation or acac, acetylacetonate) and z the AV solution pH. The V-content was determined by atomic absorption (Varian Spectra AA-30) after the sample had been dissolved in a concentrated (40%) HF solution. BET surface areas (SNm2g -1) were measured by N2 adsorption at 77 K. The SA of ZrO2 was 49 m2g -1. The SA of some ZV samples was determined after the various treatments. All these samples had SA values ranging from 45 to 49 m2g -1, slightly smaller than those of zirconia.

2.2. Procedure and characterization techniques Specimens were placed in a silica reactor that was equipped with two side tubes for XPS and ESR measurements and connected to a circulation apparatus, described elsewhere [25, 26]. The catalysts, dried at 383 K, were characterized as prepared (a.p.), after heating in dry oxygen at 773 K (s.o.), or after reduction with CO. In some experiments, as specified, samples were exposed to NO, NH3, or various mixtures NO-O2-NH3. Electrons per V atom (eN) were determined from the CO consumed. The average oxidation number of vanadium was calculated as 5 - eN. FT-IR spectra were recorded at RT on a Perkin-Elmer 1760-X spectrophotometer equipped with a cryodetector, at a resolution of 2 cm -1 (number of scans -100). In the 1070-960 cm -1 region, band integration and c u r v e fitting were carried out by "Curve fit, in Spectra Calc." (Galactic Industries Co.). Powdered materials were pelleted in self-supporting discs of 25-50 mg cm -z and 0.1-0.2 mm thick, placed in an IR cell allowing thermal treatments in vacuo or in a controlled atmosphere. The ESR measurements were made at RT or 77 K on a Varian E-9 spectrometer (X-band), equipped with an on-line computer for data analysis. Spin-Hamiltonian parameters (g and A values) were obtained from calculated spectra using the program SIM14 A [26]. The absolute concentration of the paramagnetic species was determined from the integrated area of the spectra. Values of g were determined using as reference the sharp peak at g = 2.0008 of the E'I center (marked with an asterisk in Fig. 3); the center was formed by UV irradiation of the silica dewar used as sample holder. XPS measurements were obtained with a Leybold Heraeus LHS 10 spectrometer operating in FAT mode and interfaced to a 2113 HP computer. Mgk(~ (1253.6 eV) radiation (12 kV and 20 mA) was used. The a.p. sample was pressed onto a golddecorated tantalum plate attached to the sample holder. After the various treatments (s.o., or reduction with CO), the specimen was transferred into the above mentioned XPS tube without exposure to the atmosphere. The spectra were collected by the computer in a sequential manner (figure in parenthesis gives the kinetic energy): Ols (719.0 eV), V2pl/2 (724.5 eV), V2P3/2 (732.0 eV), Zr3d3/2 (1062.5 eV) and Zr3d5/2

693

(1066.0 eV). The binding energy (BE) of Ols (530.0 eV) was taken as reference. The spectrum analysis involved (i) satellite subtraction of Mgk(z components; (ii) inelastic background removal by a linear integral profile; (iii) curve-fitting by a least-squares method, using a mixed Gaussian-Lorentzian function and (iv) determination of the peak area by integration. Satellite subtraction of ec3 and (~4 oxygen components allowed V2pl/2 to be partially resolved. 2.3. Catalytic experiments Catalytic experiments were done in an apparatus consisting of a flow measuring and control system (mass flow controllers, Hitech), fixed-bed flow microreactor, electrically heated and equipped with a temperature programmer-controller (Ascon), two on-line IR analyzers, one for NO (Radas 1G, Hartmann & Braun) and the other for NH3 (Siemens, Ultramat 5E), and an on-line gas chromatograph (Dani 86.10 HT), equipped with a 2 m length column (AIItech CTR) for the analysis of 02, N2 and N20. Typical experiments were conducted in the temperature range 473-723 K, feeding a gas mixture containing 700 ppm NO, 700 ppm NH3 and 3.6 % 02 in helium. The effect of 02 partial pressure was also tested. The flow rate of the reactant gas was 60 L/h (W/F = 5x10 -6 g h cm-3). NH3 was oxidyzed by feeding a gas mixture containing 700 ppm NH3 and 3.6 % 02 in helium. The nitrogen mass balance was better than 90%. Catalytic data were expressed as NO or NH3 conversion percent, or calculated as apparent kinetic constants (k/NO molecules nm -2 s-l), assuming the occurrence of a single reaction (4NO + 4NH3 +02 = 4N2 + 6H20), first order with respect to NO and zero order with respect to NH3.

3. RESULTS AND DISCUSSION 3.1. Vanadium uptake For samples ZV(a)pH1, up to about 2.5 mmol L -1, the vanadium uptake (atoms nrn -2) was proportional to the AV concentration, but as the concentration increased further, the uptake tended to level off (Fig. 1, a). o,I

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Figure 1. Section a V-uptake (atoms nm -2) vs. AV concentration (retool L-l) at (z~) pH=l, (El) pH=2, (O) pH=3 and (o) pH=4. Section b: V-uptake (atoms nm -2) vs. the vanadium available in the AV solution (referred to the ZrO2 surface area) at (z~) pH=l, (El) pH=2, (O) pH=3 and (o) pH=4. For samples ZV(a)pH2-4, up to about 2.5 mmol L-1 the uptake increased linearly

604 and thereafter remained about constant up to 6.5 mmol L-1 . In contrast to ZV(a)pH1, in ZV(a)pH2-4 samples, at AV concentration > 6.5 mmol L -1, the vanadium uptake increased sharply. The increase depended on the precipitation of a vanadium phase on the zirconia support. Accordingly, in these samples, the X-ray analysis showed the presence of a segregated phase, which transformed into V205 after calcination in air at 773 K or s.o. treatment. Because V-uptake is a surface phenomenon, we plotted (Fig. 1, b) the V adsorbed per unit area of zirconia_(atoms nm -z) as a function of V atoms available in the AV solution (V atoms nm-z, referred to the ZrO2 surface area). For ZV(a)pH2+4 samples, up to 2.5 atoms nm -z, all the available V was adsorbed. As the available V increased further, uptake reached an extended plateau, corresponding to about 3 V atoms nm -2. By contrast, for ZV(a)pH1 samples, V-uptake progressively increased throughout the region of the available V. The maximum V-uptake was about 2.4 atoms nm -2. 3.2. XP$ characterization For all samples, both a.p. and s.o., irrespective of the preparation method, the experimental intensity ratios, V2p/Zr3d, increased proportionally to the V-content up to 3 atoms nm -2 (Fig. 2). The ratio approaches those calculated with the "spherical model" proposed recently by Cimino et al. [27] (full line in Fig. 2). For ZV samples with V-content _< 3 atoms nm -2, this finding shows that vanadium species are uniformly spread on the ZrO2 surface. On ZV catalysts with a larger V content (not shown in Fig. 2), the intensity ratios were markedly larger than the corresponding values yielded by the spherical model. The results obtained on samples with V-content > 3 atoms nm -z point therefore to a V surface enrichment.

Figure 2. Intensity ratio, V2p/Zr3d, vs. V-content. Samples: ZV(a)pH1 (o) a.p. and (l) s.o.; ZV(a)pH2 (z~) a.p. and (&) s.o.; ZV(a)pH3 (El) a.p. and (11) s.o.; ZV(a)pH4 (<>) a.p. and (O) s.o.; ZV(i) ( v ) a.p. and ( v ) s.o.; ZV(acac) (+) a.p. and (x) s.o.

Figure 3. ESR spectra at RT of reduced VOx/ZrO2 samples (CO at 623 K). Samples: (a)ZV0.05(a)pH1; (b) ZV0.33(a)pH4; (c) ZV0.58(a)pH1; (d) ZV1.09(a)pH4.

Because of the intensity of V2ppeaks, which is much lower than that of the nearby Ols peak, the vanadium oxidation state could be reliably ascertained only for ZV(a) and ZV(i) specimens with a V loading > 1% (2.5 atom nm-2). The binding energy value of the V2p3/2 component, obtained by curve fitting of the region Ols-V2p,

695

showed Vv only (517.1 eV), in a.p. and s.o. samples, and complete reduction to V Iv (516.6 eV), after reduction with CO at 500 K. On the same sample the redox cycle showed eN=l, corresponding to an average vanadium oxidation state of 4. 3.3. E SR characterization In a. p. and s.o. ZV(a) and ZV(i) samples, no ESR signals were detected. In a.p. ZV(acac), a weak ESR signal of vanadyl species was detected (5% of total V), absent after the s.o. treatment. The spectra of samples reduced with CO at 400 to 623 K consisted of a signal showing a resolved hyperfine structure (Vh), overlapping a broad (,~Hpp = 300 Gauss) and nearly-isotropic band (Vb, giso = 1.97) (Fig. 3). When recorded at 77 K, both Vh and Vb maintained the same shape as at RT, and their intensity as a function of temperature followed the Curie law. The spectroscopic features of Vh (gll=1.913, gL=1.983, and All=204 Gauss, Aj.= 76 Gauss, line-width, dependence on recording temperature, and number of lines), allow the signal to be assigned to mononuclear isolated V TM in a square pyramidal configuration (vanadyl species). The absence of a hyperfine structure and the large value of the line-width of Vb, both features arising from dipolar and exchange interactions among paramagnetic species, suggest its assignment to magnetical!y interacting V IV, formed by the reduction of polyoxoanions anchored to the zlrcon=a surface. The sequence of spectra, referring to ZV samples with increasing V-content, shows that in the low-loading ZV samples up to 0.2 atoms nm -2 isolated mononuclear V Iv species prevailed, whereas with increasing V-loading interacting VIV became prevalent (Fig. 3). Exposure of s.o. samples to NH3-NO at 623 K caused the formation of a weak Vh signal. A subsequent treatment with NO-NH3-O2 mixtures, containing increasing amounts of 02, caused a progressive decrease in Vh and its disappearance with 1-2% 02. Exposure to NO-NH3 of reduced samples (CO at 623 K), therefore containing Vh and Vb, caused a decrease in the ESR detected V IV by 65%. After this treatment Vb species were nearly absent. Exposure of reduced samples to NO-NH3-O2 caused the complete oxidation of V Iv species. 3.4. FTIR characterization in a.p. ZV(a) and ZV(i) samples, broad bands arising from hydrated vanadates were detected in the 800-1100 cm -1 region. Metavanadate-like species (band at 920 cm -1) prevailed on ZV samples with V-content < 1.5 atoms nm -z and decavanadates (bands at 850-880 cm -1 and 960-990 cm -1) in the range 1.5-3 atoms nm -2. A.p. ZV(acac) samples showed bands from CH3 and C=O, suggesting the adsorption of VO(acac)2 as such (spectra not reported). Spectra of s.o. samples differed markedly from those of a.p. samples and were unaffected by a subsequent evacuation up to 673 K (Fig. 4, a). Spectra consisted of a composite envelope of heavily overlapping bands at 980-1070 cm -~, with two weak bands at 874 and 894 cm -1. Irrespective of the preparation method, the integrated area (cm -1) of the composite band at 980-1070 cm "1 was proportional to the V-content up to 3 atoms nm -z. An analysis of spectra by the curve-fitting procedure showed the presence of several V=O modes. The relative intensity of the various peaks contributing to the composite band depended only on the V-content and did not depend on the method used for preparing the catalysts. Samples with V > 3 atoms nm -~" had IR-spectra features similar to those of pure V205 (spectrum 8 in Fig. 4, a). According to the dependence of the intensity of the various peaks on the V-content, we distinguished vanadates with different nuclearities (roughly three types). The first, corresponding to peak 3 and prevailing in most dilute samples (spectra 2 and 3 Fig. 4, b), is a low nuclearity species possibly mononuclear (type-I). Type-II vanadates had an increasing concentration in the vanadium range 0.4-0.8

696

atoms nm "2, peaks 1 2 and 4 (Fig. 4, b and r Type-Ill vanadates had a markedly increasing concentration in the vanadium range 1.5-3 atoms nm -2 and increased little thereafter, peaks 5, 6 and 7 (Fig. 4, b and r

Figure 4. IR spectra of s.o. samples. Section a: ZrO2, curve 1; Z V0.18(a)pH4, curve 2; ZV0.30(acac), curve 3; ZV0.58(a)pH4, curve 4; ZV0.83(a)pH4, curve 5; ZVl.05(i), curve 6; ZV1.21 (a)pH4, curve 7; ZV4.65(a)pH4, curve 8. Section b: curve fitting of the band at 980-1070 cm-1; 990-1000 cm -1, peak 1, 1007-1008 cm -1, peak 2, 1017-1020 cm -1, peak 3, 1025-1029 cm -1, peak 4, 1034-1038 cm -1, peak 5, 1042-1045 cm -1, peak 6 and 1050-1052 cm -1, peak 7. Sample s.o. ZV0.58(a)pH4. Section c: as in section b. Sample s.o. ZV1.21(a)pH4.

Figure 5. IR spectra of s.o. samples after various treatments. Section a: after adsorption of NH3 (1 mbar) at RT: ZrO2 (curve 1), ZV0.58(a)pH4 (curve 2), ZV1.21(a)pH4 (curve 3). Section b: ZV0.58(a)pH4 sample after adsorption of NO+O2 at 623 K (curve 1), after subsequent adsorption of NH3 (1 mbar) at RT (curve 2) and after subsequent heating at 623 K (curve 3). Bands assigned to bridged bidentate nitrates (*) and to chelating nitrates (**). Section c: the same treatments as in section b on s.o. ZV 1.21 (a)pH4. At RT, NH3 adsorbed on Lewis acid sites, Zr Iv and V v. Accordingly, the intensity of bands from NH3 decreased little with the V-content, by 15% at most, as expected on account of the similar Lewis acid strengths of Zr Iv and Vv. The symmetric bending

697

of NH3 was 1157 cm -1 on pure ZrO2, and shifted to higher frequency on ZV. In particular, on most dilute ZV the frequency band was 1195 cm -1 and with increasing V-content it progressively increased up to 1208 cm -1 (Fig. 5, a). NH~ did not form on ZrO2 and ZV samples with V-content < 1.5 atoms nm -2, whereas it did form on more concentrated samples and markedly increased with V-content up to 3 V/atoms nm -2 (Fig. 5, a). At RT, NO adsorption on s.o. ZV samples gave weak bands from N20 and nitrites. The same species formed on pure ZrO2. Adsorption of NO+O2 gave strong bands from bridged bidentate-nitrates (spectra 1 in Fig. 5, b and r On both ZV0.58(a)pH4 and ZV1.21(a)pH4 after evacuation at RT, the subsequent addition of NH3 at RT gave more intense NH~ bands than those on s.o. samples and caused the concomitant transformation of bridged nitrates into chelating nitrates (spectra 2 in Fig. 5, b and r NH~ species were much more intense in ZV1.21(a)pH4 than in ZV0.58(a)pH4. A subsequent heating at 623 K caused the disappearance of chelating nitrates in ZV1.21(a)pH4 (spectrum 3 in Fig. 5, r and their decrease (to 50%) in ZV0.58(a)pH4 (spectrum 3 in Fig. 5, b), while surface-OH formed and H20 and N2 were detected by analysis of the gas phase. Exposure of s.o. samples to NH3-NO at 623 K, caused a slight reduction of V v to V iv, whereas exposure to NO-NH3-O2, did not affect the vanadium oxidation state. Exposure of reduced samples (CO at 623 K) to NH3-NO caused slight oxidation, whereas exposure to NO-NH3-O2 oxidized all V IV. Catalytic activity On all catalysts, the activity for SCR was stable as a function of the time on stream and the ratio NO/NH3 remained very close to unity. 3.5.

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Figure 6. NO conversion (%) vs. temperature. Section a: (n) ZV0.18(a)pH4, (<>) ZV0.34(a)pH4, (m) ZV0.58(a)pH1, ( v ) ZV0.60(a)pH4, (~) ZV0.83(a)pH4, (o) ZV1.17(a)pH4, (zx) ZV4.79(a)pH4. Section b: (<>)ZV0.17(i), (v)ZV0.32(i), (~)ZV0.64(i), (m)ZVl.05(i), (z~)ZV0.30(acac), (o)ZV0.96(acac), (D)ZV1.36(acac)and (x)ZrO2. In samples ZV(a) (Fig. 6, a), ZV(i) and ZV(acac) (Fig. 6, b), NO conversion increased with V-loading at all temperatures. In the whole temperature range, the selectivity to N2 was ve~ high. A small amounts of N20 (< 3%) were detected only above 573 K. Pure zirconla showed some SCR activity at T > 573 K, comparable with

698

that of ZV0.18(a)pH4. The best performance was obtained with the catalyst ZV1.17(a)pH4, containing 2.8 V atoms nm -2, namely a V-content close to that of the adsorption plateau in ZV(a) samples (3 atoms nm-Z). On the sample ZV4.79(a)pH4 (12.6 V atoms nm-2), containing segregated V205, NO conversion reached a maximum at 623 K and decreased thereafter. Higher reaction temperatures resulted in the formation of very large amounts of N20 (>10 %) arising from the oxidation of NH3; NH3 conversion still monotonically increased with the temperature. In all ZV catalysts, the apparent activation energy (Ea/kJ mol "1) was nearly independent of the V-content (42 + 4 kJ mol-1). Therefore, the dependence of catalytic activity on the V-content can be conveniently inspected through k ~ values, the pre-exponential factor of the Arrhenius equation. The finding that k ~ values, irrespective of the method used for preparing the catalysts, stay on the same curve, shows that the SCR activity is mainly controlled by the vanadium content (Fig. 7, a). The marked and non-linear increase of k ~ with the V-content clarifies that the concentration of the active vanadium is not proportional to the V-loading. Namely, only specific configurations are active. To identify the active vanadium configuration, we divided k ~ values by the intensity of i) the composite band at 980-1070 cm -1, ii) peak corresponding to type-I vanadates, iii) peaks of type-II polyvanadates, and iv) peaks of type-Ill polyvanadates. Normalized k ~ values monotonically and markedly increase by a factor 8 to 11, but normalized k ~ values for type-Ill polyoxovanadates remain about constant, well within a factor of two (Fig. 7, b).

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Figure 7. The dependence of catalytic activity on the V-content (a) and its correlation with type-Ill polyoxovanadates (b). Section a: 10-3 k ~ vs. V-content on ZrO2 (0), ZV(a) (El), ZV(i) (z~), and ZV(acac) (O). Section b: 10.2 k ~ divided by the total integrated area (cm -'1) of bands in the region 980-1070 cm -1 (El), and 10-~k ~ divided by the sum of components 5, 6 and 7 (0) areas (cm-1). In the absence of 02, NO reduction continued, however at a rate about ten times lower than that in the presence of 02. During 20 h experiments NO conversion remained constant. On 02 addition, the catalytic activity increased with 02 content in the mixture up to about 1000 ppm, and changed little thereafter. We noticed that increasing the 02 concentration caused NO conversion to become lower than that of NH3, probably due to changes in the stoichiometry of the overall reaction (the NO/NH3 ratio passed from 1.5 to 1). Catalytic tests of NH3 oxidation with 02 yielded high selectivity to N2 (66-90%), which decreased with the higher loading catalysts. in special experiments we tested the activity of ZV samples prereduced in an NH3

699

flow (700 ppm in He) for 1 h at 623 K. In the SCR and NO + NH3 reaction the prereduced ZV1.04(a)pH4 sample showed the same activity as the s.o. sample. 4. C O N C L U S I O N S The heating in 02 at 773 K of VOx/ZrO2 stabilizes surface vanadates of various nuclearities. Provided that the V-content in the samples is < 3 atoms nm -2, XPS shows effective spreading of vanadates on the ZrO2 surface. As the V-content increases, the concentration of monomers increases little, whereas that of polyoxovanadates increases markedly, particularly that of type-Ill polyoxovanadates. The relative abundance of the various vanadates depends only on the V-content, not on the method used for catalyst preparation (impregnation, or adsorption from both aqueous and toluene solutions) and pH of the solution used for adsorption (pH =1-4 for AV solutions). This finding strengthens our earlier proposal [28, 29], based on the results obtained on the related MoOx/ZrO2 system, that the ZrO2 surface has a buffer effect. When samples are heated, the buffer effect causes the condensationdecondensation of vanadium species, therefore making the surface composition independent of the nature of the precursor-adsorbate. In agreement with the results from the characterization, the SCR activity of VOx/ZrO2 also depends only on the V-content, not on the method used for catalyst preparation. The marked increase in SCR activity with the V-content shows that only specific vanadium configurations are active. Although we assess the V=O modes associated with these active configurations, IR analysis did not specify the structure of active polyoxoanions. The presence of V Iv on the surface before catalysis is unessential for ca.tai~ic activity. We cannot however rule out an SCR redox mechanism involving Vv-v Iv. ESR and IR results show that the oxidation state of surface vanadium at the reaction temperature is controlled mainly by the composition of the reactant mixture.

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Figure 8. The catalyt!c activity for SCR (10 -3 k~ vs. concentration of NH~ (from the IR area of <5asym/Cmq). (n) ZrO2, (o)ZV(a), and (A) ZV(acac) samples. For the abatement of NO with NH3 in the absence of 02, ZV catalysts give low, but stable activity. The activity is strongly enhanced in the presence of 02. This is a common feature to all SCR catalysts, including the ZSM-5 based system [30]. In the presence of 02, our results show the formation of bidentate nitrates and, on NH3 addition, their transformation into chelating nitrates. Our results also show that the

700 concentration of NH~ ions correlates with the SCR activity of VOx/ZrO2 catalysts (Fig. 8), as already observed by Topsoe et al. [22] for VOx/TiO2 catalysts. The concomitant formation at RT of NH~ species and chelating nitrates and their reactivity at higher temperature suggests that they participate in the SCR reaction.

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