Fourier Transform Infrared Spectroscopy Study of the Nature and

Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria. Langmuir , 2003, 19 (8), pp 3323–3332. DOI: 10.10...
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Langmuir 2003, 19, 3323-3332

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Fourier Transform Infrared Spectroscopy Study of the Nature and Reactivity of NOx Compounds Formed after Coadsorption of NO and O2 on Cu/ZrO2 Tzvetomir Venkov, Konstantin Hadjiivanov,* Atanas Milushev, and Dimitar Klissurski Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria Received October 4, 2002. In Final Form: January 29, 2003 Adsorption of CO on Cu/ZrO2 leads to the formation of Cu+-CO (2126 cm-1) and Zr4+-CO (2192 cm-1) carbonyls and of carbonato-hydrogen-carbonate structures (bands in the 1650-1000 cm-1 region). Adsorption of NO on the same sample produces bands characterizing N2O (2246 cm-1), NO+ (2122 cm-1), mononitrosyls coordinated to Cu+ (1759 cm-1) and Cu2+ (1872 cm-1) ions, respectively, and nitro (1527, 1327 cm-1) and nitrito compounds (1185 cm-1). Introduction of oxygen into the system results in an initial increase in concentration of NO2- compounds followed by oxidation of these complexes to nitrates (1630, 1580, 1554, 1287, 1225, 1031, and 1004 cm-1). The nitrates thus obtained are stable up to 573 K. They enhance the Lewis acidity of the neighboring Zr4+ sites on the support as a result of which mononitrosyls of the Zr4+(NO3)-NO type (1932 cm-1) are formed. The copper nitrates formed on the surface begin to react with C2H4 at 523 K, yielding an organic nitro-containing deposit which decomposes to reactive cyanides (2142 cm-1). The mechanism of selective catalytic reduction of nitrogen oxides with hydrocarbons on Cu/ZrO2 and the oxidation state of the copper ions during the process are discussed.

1. Introduction Selective catalytic reduction of nitrogen oxides with hydrocarbons (HC-SCR) is of great interest because it provides the principal possibility of controlling the emissions of nitrogen oxides from diesel exhaust gases.1-4 A series of cation-exchanged zeolites1-15 and supported oxides16-34 have shown a high activity in HC-SCR. The * Corresponding author. E-mail: [email protected]. (1) Shelef, M. Chem. Rev. 1995, 95, 209. (2) Paˆrvulescu, V. I.; Grange, P.; Delmon, B. Catal. Today 1998, 46, 233. (3) Iwamoto, M.; Yahiro, H.; Yu-u, Y.; Shundo, S.; Mizuno, N. Shokubai 1990, 32, 430. (4) Held, W.; Koenig, A.; Richter T.; Puppe, L. Society of Automotive Engineering Paper 900496; Society of Automotive Engineering: Warrendale, PA, 1990. (5) Iwamoto, M.; Hamada, H. Catal. Today 1991, 10, 57. (6) Valyon, J.; Hall, W. K. Stud. Surf. Sci. Catal. 1993, 74, 1339. (7) Petunchi, J.; Hall, W. K. Appl. Catal., B 1993, 2, L17. (8) Aylor, A.; Larsen, S.; Reimer, J.; Bell, A. J. Catal. 1995, 157, 592. (9) Matsumoto, S. Catal. Today 1996, 29, 43. (10) Burch, R.; Scire, S. Appl. Catal., B 1994, 3, 295. (11) Bennett, C. J.; Bennett, P. S.; Golunski, S. E.; Hayes, J. W.; Walker, A. P. Appl. Catal., A 1992, 86, L1. (12) Liu, D.; Robota, H. Appl. Catal., B 1994, 4, 155. (13) Ma´rquez-Alvarez, C.; Rodrı´guez-Ramos, I.; Guerrero-Ruiz, A.; Haller, G. L.; Ferna´ndez-Garcı´a, M. J. Am. Chem. Soc. 1997, 119, 2905. (14) Li, Y.; Armor, J. N. Appl. Catal., B 1992, 1, L12. (15) Kikuchi, E.; Ogura, M.; Terasaki, I.; Goto, Y. J. Catal. 1996, 161, 465. (16) Hamada, H.; Kintaichi, Y.; Inaba, M.; Tabata, M.; Yoshinari, T.; Tsuchida, H. Catal. Today 1996, 29, 53. (17) Inaba, M.; Kintaichi, Y.; Haneda, M.; Hamada, H. Catal. Lett. 1996, 39, 269. (18) Feeley, J. S.; Deeba, M.; Farrauto, R.; Beri, G.; Haynes, A. Appl. Catal., B 1995, 6, 79. (19) Burch, R.; Millington, P. Catal. Today 1996, 29, 37. (20) Shimitzu, K.; Satsuma, A.; Hattori, T. Catal. Surv. Jpn. 2000, 4, 115. (21) Hamada, H.; Kintaichi, Y.; Sasaki, M.; Ito, T.; Tabata, M. Appl. Catal. 1991, 75, L1. (22) Otsuka, K.; Zhang, Q.; Yamanaka, I.; Tono, H.; Hatano, M.; Kinoshita, H. Bull. Chem. Soc. Jpn. 1996, 69, 3367. (23) Tanaka, T.; Okuhara, T. Appl. Catal., B 1994, 4, L1. (24) Shimizu, K.; Maeshima, H.; Satsuma, A.; Hattori, T. Appl. Catal., B 1998, 18, 163.

catalytic activity of zeolites containing transition metal ions and above all the activity of Cu-ZSM-53-13 in HCSCR is well studied. However, zeolites also have some disadvantages, the most important among them being their low hydrothermal stability, which leads to catalyst deactivation.1,9,18 This imposes the necessity of developing some metal oxides as supports for SCR catalysts as they have a higher stability.16-19,28 Zirconia is characterized by a high thermal stability, mechanical strength, and chemical resistance, which makes it a suitable support of new catalyst systems for selective catalytic reduction.34 It has been reported that Cu/ZrO2 is active in various reactions of NOx conversion such as HC-SCR25-34 and N2O decomposition.35-37 Delahay et al.27 have shown that Cu/ZrO2 is selective toward NO (25) Bethke, K.; Alt, D.; Kung, M. Catal. Lett. 1994, 25, 37. (26) Bethke, K.; Li, C.; Kung, M.; Yang, B.; Kung, H. Catal. Lett. 1995, 26, 169. (27) Delahay, G.; Coq, B.; Ensuque, E.; Figueras, F. Catal. Lett. 1996, 39, 105. (28) Bethke, K. A.; Kung, M. C.; Yang, B.; Shah, M.; Alt, D.; Li, C.; Kung, H. H. Catal. Today 1995, 26, 169. (29) Morales, J.; Caballero, A.; Holgado, J. P.; Espino´s, J. P.; Gonza´lezElipe, A. R. J. Phys. Chem. B 2002, 106, 10185. (30) Sadykov, V.; Bunina, R.; Alikina, G.; Ivanova, A.; Kochubei, D.; Novgorodov, B.; Paukshtis, E.; Fenelonov, V.; Zaikovskii, V.; Kuznetsova, T.; Beloshapkin, S.; Kolomiichuk, V.; Moroz, E.; Matyshak, V.; Konin, V.; Rozovskii, A.; Ross, J.; Breen, J. J. Catal. 2001, 200, 117. (31) Pietrogiacomi, D.; Sannino, D.; Tuti, S.; Ciambelli, P.; Indovina, V.; Occhiuzzi, M.; Pepe, F. Appl. Catal., B 1999, 21, 141. (32) Delahay, G.; Ensuque, E.; Coq, B.; Figueras, F. J. Catal. 1998, 175, 7. (33) Pietrogiacomi, D.; Sannino, D.; Magliano, A.; Ciambelli, P.; Tuti, S.; Indovina, V. Appl. Catal., B 2002, 36, 217. (34) Indovina, V.; Occhiuzzi, M.; Ciambelli, P.; Sannino, D.; Ghiotti, G.; Prinetto, F. In Proceedings of the 11th International Congress on Catalysiss40th Anniversary; Hightower, J. W., Delgass, W. N., Iglesia, E., Bell, A. T., Eds.; Elsevier: Amsterdam, 1996; p 691. (35) Centi, G.; Gerrato, G.; D’Angelo, S.; Finardi, U.; Giamello, E.; Morterra, C.; Perathoner, S. Catal. Today 1996, 27, 265. (36) Tuti, S.; Pepe, F.; Pietrogiacomi, D.; Indovina, V. React. Kinet. Catal. Lett. 2001, 1, 35. (37) Morterra, C.; Giamello, E.; Cerrato, G.; Centi, G.; Perathoner, S. J. Catal. 1998, 179, 111.

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reduction with n-C10H22 at 573 K. In investigations on Cu/SO42--ZrO2 samples, the same authors32 have established that the catalyst activity depends not only on the dispersion of the copper ions but also on the presence of surface sulfate ions which have a promoting effect. In the literature,37 the formation of large CuO aggregates on the zirconia surface is assumed to reduce its activity to a large extent. This is in agreement with a series of investigations where a high catalyst activity has been found with catalysts characterized by a highly dispersed active phase.16,17,20 According to Sadykov et al.,30 however, the formation of small copper oxide clusters on the zirconia surface enhances the catalyst activity. In the literature, there are different opinions concerning the mechanism of SCR of nitrogen oxides with hydrocarbons. Some authors11,22,24,29 assume that in the presence of oxygen the hydrocarbons are partially oxidized, forming HxCyOz complexes which reduce the nitrogen oxides. Another group of authors7,21,38-42 suppose that the initial stage of the process is oxidation of nitrogen oxide. This leads to formation of NO27,21 or nitro and nitrato complexes.38-42 These surface NOx species react with the hydrocarbons from the gas phase, and the so-called organic deposit is formed.40-42 During the decomposition of the latter, CN- and NCO- compounds appear as intermediates.40-43 Li et al.43 have proved their presence on a Cu/ZrO2 catalyst using IR spectroscopy in a study of SCR of nitrogen oxides with C2H6 and C2H5OH. Analysis of literature data on the mechanism of selective catalytic reduction of nitrogen oxides with hydrocarbons on Cu/ZrO2 reveals a certain ambiguity with respect to some intermediate steps of the process, for example, the stability and reactivity of the surface nitrate formed during the process. The nature of the intermediate compounds resulting from interaction between the hydrocarbons and the NOx compounds on the surface has not yet been established. The purpose of the present work was to study, by means of IR spectroscopy, the nature of surface compounds formed on the Cu/ZrO2 surface during adsorption of nitrogen oxide and its coadsorption with oxygen as well as the interaction of these compounds with ethene. To achieve better characterization of the surface active sites which participate in the processes, we investigated additionally CO adsorption and CO + NO coadsorption on the sample under consideration as well as the thermal stability of the surface compounds. On the basis of the summarized results, some new data were obtained on the mechanism of selective catalytic reduction of NOx with ethene on Cu/ZrO2. 2. Experimental Section 2.1. Reagents and Materials. Commercially available zirconia (Degussa) with a specific surface area of 53 m2 g-1 was used for the experiments. The Cu/ZrO2 sample was prepared by impregnating zirconia with a 0.05 M solution of Cu2+ obtained by dissolving Cu(NO3)2‚ 3H2O in distilled water followed by addition of concentrated NH3 until a pH value of 9 was reached. After the synthesis, the sample was calcined in air for 1 h at 673 K. Since the Cu2+ ions are (38) Hadjiivanov, K.; Klissurski, D.; Ramis, G.; Busca, G. Appl. Catal., B 1996, 7, 251. (39) Beutel, T.; Adelman, B.; Sachtler, W. M. H. Appl. Catal., B 1996, 9, L1. (40) Hadjiivanov, K.; Kno¨zinger, H.; Tsyntsarski, B.; Dimitrov, L. Catal. Lett. 1999, 62, 35. (41) Djonev, B.; Tsyntsarski, B.; Klissurski, D.; Hadjiivanov, K. J. Chem. Soc., Faraday Trans. 1997, 93, 4055. (42) Chen, H. Y.; Voskoboinikov, T.; Sachtler, W. M. H. J. Catal. 1998, 180, 171. (43) Li, C.; Bethke, K. A.; Kung, H. H.; Kung, M. C. J. Chem. Soc., Chem. Commun. 1995, 813.

Venkov et al. adsorbed strongly on zirconia from alkaline solutions,44 the synthesis method was expected to lead to a highly dispersed supported phase. This is in line with the blue color of the sample, which indicated no formation of a separate CuO phase. The nominal copper concentration in the sample was 0.75 wt %. 2.2. Instrumental Methods. The IR measurements were carried out using a Nicolet Avatar 320 spectrophotometer with a resolution of 2 cm-1, accumulating 128 scans. Self-supporting pellets of the sample under investigation were prepared by pressing a powdery sample under a pressure of 104 kPa. The sample was subjected to direct treatment in the IR cell which was connected to a vacuum apparatus with a residual pressure of about 10-3 Pa. Prior to the adsorption measurements, the sample was activated by heating for 1 h at 673 K under oxygen and evacuation for 1 h at the same temperature. The XPS measurements were performed by an ESCALAB Mk II (VG Scientific) apparatus with an aluminum anode (hν ) 1486.6 eV). The binding energy values were corrected using the C 1s level (285 eV) of the carbon contaminants on the surface.

3. Results 3.1. Sample Characterization. The IR spectrum of the activated sample contains, in the ν(O-H) region, two bands with maxima at 3774 and 3672 cm-1. This spectrum is practically identical with the spectrum of an activated zirconia sample which leads to the conclusion that the above bands are due to hydroxyl groups bound to Zr4+ ions. According to literature data,45-47 the band at 3774 cm-1 characterizes the O-H stretchings of hydroxyl groups of the Zr4+-OH type, whereas the band at 3672 cm-1 corresponds to bridged hydroxyl groups coordinated to two Zr4+ ions. The XPS spectrum of the sample displays a Cu 2p3/2 signal with a maximum at 933.2 eV. A careful analysis of the spectrum also shows the existence of a peak of a very low intensity at ca. 937 eV. According to the literature data,29,48 the signals of the Cu2+ and Cu+ cations almost coincide. However, a criterion for distinguishing between Cu2+ and Cu+ cations is the fact that the higher-energy satellite is much more intense with Cu2+. Thus, the results suggest predominant existence of Cu+ cations on the sample surface. Unfortunately, due to the low intensity of the peaks, we were not able to make a more detailed analysis. It was calculated that the atomic ratio between the detected Cu and Zr cations was 1:39. This ratio is higher than the Cu/Zr atomic ratio in the sample (1:68), which is in agreement with the surface localization of copper. 3.2. Adsorption of CO. Adsorption of CO (2 kPa equilibrium pressure) on the sample leads to the appearance of an intense band with a maximum at 2126 cm-1 and of a lower intensity band at 2192 cm-1 (Figure 1, spectrum a). After evacuation at room temperature, the band at 2192 cm-1 disappears, which indicates that it is due to compounds weakly bound to the surface (Figure 1, spectrum b). Simultaneously, the band at 2126 cm-1 is shifted to 2120 cm-1. With rising temperature of evacuation up to 373 K, the band at 2120 cm-1 decreases in intensity and its maximum is shifted to 2112 cm-1 (Figure 1, spectrum c). The band disappears after evacuation at 423 K (Figure 1, spectrum d). On the basis of summarized literature data,49 we assign the band at 2126 cm-1 to the (44) Vassileva, E.; Furuta, N. Fresenius’ J. Anal. Chem. 2001, 370, 52. (45) Tsyganenko, A.; Filimonov, V. Uspekhi Photoniki 1974, 4, 51. (46) Hadjiivanov, K.; Lavalley, J. Catal. Commun. 2001, 2, 129. (47) Bensitel, M.; Moravek, V.; Lamotte, J.; Saur, O.; Lavalley, J.-C. Spectrochim. Acta 1987, 43A, 1487. (48) Gru¨nert, W.; Hayes, N. W.; Joyner, R. W.; Shpiro, E. S.; Siddiqui, M. R. H.; Baeva, G. N. J. Phys. Chem. 1994, 98, 10832. (49) Hadjiivanov, K.; Vayssilov, G. Adv. Catal. 2002, 47, 307.

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Table 1. Assignment of the Bands Observed after Adsorption on NO and CO and after Coadsorption on NO + O2 and NO + O2 + C2H4 on Cu/ZrO2 band, cm-1

species

assignment

site

note

2192 2126-2120 1677 1625 1558 1405 1320 1270 1220 1058

Zr4+-CO Cu+-CO bidentate carbonates hydrogen-carbonates carbonates hydrogen-carbonates carbonates bidentate carbonates hydrogen-carbonates carbonates

Species Produced after Adsorption of CO low stability ν(C-O) Zr4+ decreases in intensity in the presence of NO ν(C-O) Cu+ νas(CdO) decomposed at 423 K Cu+ νas(CdO) decomposed at 423 K Cu+ + ν(CdO) Cu νs(CdO) decomposed at 423 K Cu+ νas(OCO) Cu+ νs(CdO) decomposed at 423 K Cu+ Cu+ decomposed at 423 K νs(OCO) Cu+

2246 2122 1872 1759 1527 1327 1185

N2O NO+ Cu2+-NO Cu+-NO nitro compounds nitro compounds bridging nitrito species

Species Produced after Adsorption of NO low intensity ν(N-N) Cu2+ or Zr4+ ν(N-O) O2disappears in the presence of small amounts of O2 blue shifted in the presence of small amounts of O2 ν(N-O) Cu2+ disappears in the presence of NO ν(N-O) Cu+ νas(NO2) disappears at excess of O2 Cu2+ νs(NO2) disappears at excess of O2 Cu2+ νs(NO2) disappears at excess of O2 Zr4+

1932

Zr4+(NO3-)-NO

1892 1630 1580 1554 1287 1225 1031 1004

Species Produced after Coadsorption of NO and O2 increases in intensity in the presence of small amounts ν(N-O) Zr4+ of O2 N2O3 ν(NdO) Cu2+ or Zr4+ disappears after evacuation bridging nitrates ν3′ decomposed at T ) 573 K Cu2+ or Zr4+ bidentate nitrates ν3′ observed also with ZrO2 Cu2+ or Zr4+ monodentate nitrates + N2O3 ν3; νas(NO2) observed also with ZrO2 Cu2+ or Zr4+ 2+ 4+ monodentate nitrates + N2O3 ν3; νs(NO2) observed also with ZrO2 Cu or Zr bridging nitrates ν3′′ observed also with ZrO2 Cu2+ or Zr4+ monodentate nitrates ν1 Cu2+ or Zr4+ observed also with ZrO2 bridging nitrates ν1 Cu2+ or Zr4+ observed also with ZrO2

2142

CN-

1700-1600 1552 1450 1430 1340 1258

C-H-N-O deposit carboxylates carboxylates carboxylates carbonaceous deposit carbonaceous deposit

Species Produced after Coadsorption of NO, O2, and C2H4 ν(C-N) Cun+ appeared after decomposition of the C-H-N-O deposit at 623 K Cun+ decomposed to nitriles + νas(COO ) high stability Cu νs(COO-) high stability Cu+ νs(COO-) high stability Zr4+ νas(NO2) tentative assignment Cu2+ νs(NO2) tentative assignment Cu2+

Figure 1. IR spectra of CO adsorbed on Cu/ZrO2: equilibrium pressure of 2 kPa (a), after evacuation at room temperature (b), at 373 (c), and at 423 K (d).

ν(C-O) stretching vibrations of Cu+-CO monocarbonyls. The high stability of these species is due to the synergism between the σ and π components of the bond between the Cu+ ions and CO.50 In contrast, the band at 2192 cm-1 disappears after evacuation already at room temperature. At this frequency, carbonyls of both the Zr4+ sites of the support46 and Cu2+ ions could absorb.49,51,52 Under the conditions of our experiment, however, formation of Cu2+(50) Davydov, A. IR Spectroscopy Applied to Surface Chemistry of Oxides; Nauka: Novosibirsk, 1984.

CO carbonyls is hardly probable because they are observed mainly at low temperatures.52 On this basis and according to literature data,46,49 we have assigned the band at 2192 cm-1 to the ν(C-O) modes of surface Zr4+-CO carbonyls. After CO adsorption, a series of bands also appear in the 1800-1000 cm-1 region. Their maxima are at 1625, 1558, 1405, 1320, 1220, and 1058 cm-1 (Figure 1, spectrum a). The bands show a negligible decrease in intensity after evacuation at room temperature, and a band at 1677 cm-1 becomes clearly visible (Figure 1, spectrum b). Evacuation at 373 K leads to an intensity drop of the bands at 1625, 1558, and 1058 cm-1, and the band at 1220 cm-1 almost disappears (Figure 1, spectrum c). With rising temperature of evacuation up to 423 K, the bands at 1677, 1625, and 1058 cm-1 disappear and a broad band remains in the range of 1600-1300 cm-1 (Figure 1, spectrum d). The appearance of the bands at 1800-1000 cm-1 is associated with the formation of (hydrogen)carbonate species on the surface.53 In particular, the bands with maxima at 1625, 1405, and 1220 cm-1 are due to HCO3- compounds while those at 1558, 1320, and 1058 cm-1 are produced by CO32complexes (see Table 1).37,36,50,53 The formation of such surface compounds indicates reduction of Cu2+ ions from the surface to Cu+. 3.3. Adsorption of NO. Adsorption of NO (2 kPa equilibrium pressure) on the Cu/ZrO2 sample leads to (51) Hadjiivanov, K.; Kno¨zinger, H. Phys. Chem. Chem. Phys. 2001, 3, 1132. (52) Fu, Y.; Tian, Y.; Lin, P. J. Catal. 1991, 132, 85. (53) Busca, G.; Lorenzelli, V. Mater. Chem. 1982, 7, 89.

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Figure 2. IR spectra of NO (2 kPa equilibrium pressure) adsorbed on Cu/ZrO2 (a) and time evolution of the spectra (b,c).

formation of a series of bands with maxima at 2246, 2122, 1872, 1759, 1527, 1327, and 1185 cm-1 (Figure 2, spectrum a). The intensity of the bands at 2122, 1872, 1527, 1327, and 1185 cm-1 increases with time, whereas that of the bands at 2122 and 1872 cm-1 reaches saturation more rapidly. Simultaneously, the intensity of the band at 1759 cm-1 decreases while the band at 2246 cm-1 remains unchanged (Figure 2, spectra b and c). The bands at 1872 and 1759 cm-1 can be attributed to ν(N-O) stretching modes of Cu2+-NO and Cu+-NO complexes, respectively.6,8,33,50-52,54-58 Until recently, in the literature there were data on nitrosyls of Cu+ ions detected at room temperature mainly with zeolite systems.6,8,54,55,59 However, more recent data report the formation of Cu+-NO compounds at room temperature on sulfated Cu/ZrO2 (band at 1810 cm-1).33 The higher frequency of the ν(N-O) modes of Cu+-NO nitrosyls formed on Cu/SO42--ZrO2, as compared to the band at 1759 cm-1 registered by us, can be explained by the presence of SO42- ions. This effect will be discussed below. The fact that the band at 1759 cm-1, which is due to Cu+NO nitrosyls, decreases in intensity with time whereas that of the band at 1872 cm-1 (characterizing Cu2+-NO) increases evidences that in a NO atmosphere the Cu+ ions are oxidized to Cu2+. The bands at 1527, 1327, and 1185 cm-1 can, in general, be ascribed to surface nitro/nitrito compounds.59 The band at 2246 cm-1 characterizes the ν(N-N) stretching modes of adsorbed N2O,59 and that at 2122 cm-1 is due to ν(N-O) vibrations of NO+ species localized on coordinatively unsaturated O2- anions.59,60 3.4. Coadsorption of NO and O2. The introduction of small amounts of oxygen (133 Pa initial equilibrium pressure) to the IR cell already containing 2 kPa NO leads to the appearance of a broad low-intensity band with a maximum at 1554 cm-1 (Figure 3, spectrum b). Simultaneously, the band at 1185 cm-1 becomes more intense and is slightly shifted to lower frequencies (1177 cm-1). After a longer contact of the sample with the gas mixture, three bands at 1630, 1580, and 1554 cm-1 appear in the 1650-1000 cm-1 region (Figure 3, spectra c and d). In (54) Spoto, G.; Bordiga, S.; Scarano, D.; Zechina, A. Catal. Lett. 1992, 13, 39. (55) Szanyi, J.; Paffett, M. J. Catal. 1996, 164, 232. (56) Wagif, M.; Lakhdar, V.; Saur, O.; Lavalley, J. C. J. Chem. Soc., Faraday Trans. 1994, 90, 2815. (57) Padley, M.; Rochester, H. C.; Hutchings, G.; King, F. J. Chem. Soc., Faraday Trans. 1995, 91, 141. (58) Delahay, G.; Coq, B.; Ensuque, E.; Figueras, F.; Saussey, J.; Poignant, F. Langmuir 1997, 13, 5588. (59) Hadjiivanov, K. Catal. Rev.sSci. Eng. 2000, 42, 71. (60) Hadjiivanov, K.; Bushev, V.; Kantcheva, M.; Klissurski, D. Langmuir 1994, 10, 464.

Venkov et al.

Figure 3. IR spectra of NO (2 kPa equilibrium pressure) adsorbed on Cu/ZrO2 (a), after addition of O2 (133 Pa initial equilibrium pressure) (b) and time evolution of the spectra (c,d), and after addition of more O2: 530 (e) and 4700 Pa (f) initial equilibrium pressure.

addition, the intensity of the band at 1177 cm-1 drops, and a broad band with a maximum at 1225 cm-1 and a shoulder at 1285 cm-1 appears along with two lowintensity bands at 1031 and 1004 cm-1 (Figure 3, spectrum c). These facts indicate oxidation of NO2- species (1177 cm-1) to species in which the formal oxidation state of nitrogen is higher than +3. The intensity of the band at 1225 cm-1 rises with time (Figure 3, spectra c and d). The increase in amount of the introduced oxygen (up to 4.7 kPa) is accompanied by an intensity increase of the 1225 cm-1 band up to a saturation degree (Figure 3, spectrum e). Simultaneously, the intensity of the band at 1287 cm-1 displays a strong increase while the bands at 1580 and 1554 cm-1 overlap, which results in a broad band with a maximum at 1554 cm-1 (Figure 3, spectra e and f). Under the conditions used, formation of surface nitrato complexes is expected. On this basis and in agreement with literature data,59 we assign the bands with maxima at 1630 and 1554 cm-1 to ν3′ and those at 1287 and 1225 cm-1 to ν3′′ modes of surface nitrates. The bands with maxima at 1031 and 1004 cm-1 are produced by the corresponding ν1 vibrations.59 Obviously, the ν3′ mode is most sensitive to the nature of active sites, as a result of which at lower coverages this band splits. The kind of surface nitrates formed will be discussed below. In the 1950-1800 cm-1 region, the introduction of oxygen causes an intensity increase and a shift to higher frequencies (1884 cm-1) of the Cu2+-NO band at 1872 cm-1 (Figure 3, spectra b and c). At the same time, another low-intensity band becomes visible at 1932 cm-1 (Figure 3, spectrum c). With increasing oxygen amount, the band at 1932 cm-1 decreases and vanishes whereas that at 1884 cm-1 is shifted to higher frequencies (1892 cm-1) and loses part of its intensity (Figure 3, spectra d-f). The bands in the region of 1950-1850 cm-1 are due to N-O stretching modes. However, both nitrosyl complexes and adsorbed N2O3 can appear in this region.59 An interpretation of these bands will be presented in the Discussion. At 3800-3000 cm-1, the Zr4+-OH band at 3774 cm-1 drops in intensity after nitrogen oxide adsorption while the intensity of the band at 3672 cm-1 shows no change (Figure 4, spectrum a). Immediately after the introduction of oxygen (133 Pa initial equilibrium pressure), the band at 3672 cm-1 also begins to decrease in intensity, which is accompanied by formation of a broad low-intensity band with a maximum at about 3510 cm-1 (Figure 4, spectrum b). The bands at 3774 and 3672 cm-1 show an additional erosion with time, while the band at 3510 cm-1 becomes more pronounced (Figure 4, spectra c and d). The increase

NOx Species on Cu/ZrO2

Figure 4. Changes of the IR spectra in the O-H stretching region after adsorption of NO (2 kPa equilibrium pressure) on Cu/ZrO2 (a), after addition of O2 (133 Pa initial equilibrium pressure) (b) and time evolution of the spectra (c,d), and after addition of more O2: 530 (e) and 4700 Pa (f) initial equilibrium pressure.

Figure 5. Stability of the nitrates on Cu/ZrO2: coadsorption of NO (2 kPa equilibrium pressure) and O2 (2 kPa initial equilibrium pressure) (a), after 10 min evacuation at room temperature (b), and at 373 (c), 423 (d), 473 (e), and 573 K (f).

in oxygen amount to 530 Pa and 4.7 kPa, respectively, is accompanied by a continuous intensity decrease and disappearance of the bands at 3774 and 3672 cm-1 while the 3510 cm-1 band grows (Figure 4, spectra e and f). The band at 3510 cm-1 characterizes hydrogen-bonded hydroxyl groups and can be due either to the interactions of Zr4+-OH groups with adsorbed compounds or to water being evolved during the processes. 3.5. Stability of the NOx Compounds Obtained. To investigate the thermal stability of the NOx compounds formed, we carried out coadsorption of NO (equilibrium pressure 2 kPa) and O2 (equilibrium pressure 2 kPa) on Cu/ZrO2 and then evacuated the sample at room temperature. The spectrum showed bands at 1630, 1560, 1285, 1226, 1031, and 1004 cm-1 (Figure 5, spectrum b) which were already assigned to surface nitrates. After evacuation at 373 K, the intensity of the above bands dropped (Figure 5, spectrum c). With rising temperature of evacuation up to 423 K, the intensities of the bands at 1560, 1285, and 1031 cm-1 displayed an additional decrease whereas the intensity of the 1630 and 1226 cm-1 bands negligibly increased (Figure 5, spectrum d). This shows that the bands at 1630, 1226, and 1004 cm-1 characterize nitrates with the same type of symmetry while the bands at 1560, 1287, and 1031 cm-1 correspond to another type. When the sample is evacuated at 473 K, the intensities of the bands at 1630 and 1226 cm-1 sharply drop (Figure 5, spectrum e). In addition, the band at 1560 cm-1 exhibits a slight intensity decrease while the maximum is slightly

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Figure 6. IR spectra of CO, NO, and O2 coadsorbed on Cu/ ZrO2: adsorption of CO (2 kPa equilibrium pressure) (a), evacuation at ambient temperature (b), addition of NO (2 kPa equilibrium pressure) (c), addition of O2 (133 Pa initial equilibrium pressure) (d), and addition of more O2: 530 (e) and 2000 Pa (f) initial equilibrium pressure.

shifted to lower frequencies, and the band at 1287 cm-1 also rises. All bands in the 1700-1000 cm-1 region vanish after evacuation at 573 K (Figure 5, spectrum f). In the region between 3800 and 3000 cm-1, the rise of the evacuation temperature is accompanied by an intensity drop of the band at 3510 cm-1 (see inset in Figure 5). 3.6. Interaction of Surface NOx Compounds with NO. Distinguishing between NO2- and NO3- complexes by IR spectroscopy is a difficult task. One of the possible methods in this respect is interaction of these complexes with NO.41,59 Surface nitrates could be reduced by nitrogen oxide, which should result in an intensity decrease of their characteristic bands. In contrast, surface compounds with a +3 oxidation degree of nitrogen (nitro and nitrito compounds) do not react with nitrogen oxide.41,59 However, the same can also happen with nitrates of low reactivity. The sample was covered with nitrates by treating with a NO + O2 mixture (equilibrium pressures of 2 and 4.7 kPa, respectively) followed by evacuation. Adsorption of NO (equilibrium pressure 1 kPa) on the sample thus treated led to no substantial changes in spectrum at the first moment (the spectra are not shown). However, the bands at 1632, 1560, 1285, 1226, 1031, and 1004 cm-1 showed a slight intensity decrease with time, which was accompanied by formation of a low-intensity band with a maximum at 1881 cm-1. The appearance of this band can be attributed to the formation of N2O3 on the surface as a result of the interaction between the surface nitrates and nitrogen oxide from the gas phase:

2NO3- + 4NO f 3N2O3 + O2The results obtained are an indication that the 1630, 1554, 1287, 1225, 1031, and 1004 cm-1 bands are due to surface nitrates. 3.7. Coadsorption of CO, NO, and O2. Coadsorption of CO and NO on Cun+-containing oxides and zeolites permits selective determination of the oxidation degrees of copper ions accessible to adsorption.51,52 According to Fu et al.,52 CO is selectively adsorbed on Cu+ sites, forming surface Cu+-CO complexes, whereas NO is bonded preferentially to Cu2+ ions, producing Cu2+-NO species. Carbon monoxide adsorption (equilibrium pressure 2 kPa) on the Cu/ZrO2 sample already used for the above experiments produces a high-intensity band with a maximum at 2120 cm-1 and a lower intensity band at 2190 cm-1 (Figure 6, spectrum a). The two bands have been attributed to carbonyls of Cu+ and Zr4+, respectively.

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The band of Cu+-CO carbonyls is observed at a lower frequency (2120 cm-1) and is less intense than the band registered after CO adsorption on a freshly activated sample (2126 cm-1, Figure 1). At 1800-1000 cm-1, there are bands characterizing surface carbonates and hydrogencarbonates. These bands also have a lower intensity than the corresponding bands discussed in section 3.1. This indicates that CO desorption (section 3.1) has resulted in surface changes of the sample investigated. After evacuation at room temperature, the intensity of the band at 2120 cm-1 slightly decreases while the band at 2192 cm-1 completely disappears (Figure 6, spectrum b). Subsequent adsorption of nitrogen oxide (1 kPa equilibrium pressure) results in a low-intensity band with a maximum at 1872 cm-1, assigned to Cu2+-NO species (Figure 6, spectrum c). No Cu+-NO band (expected at about 1760 cm-1) is visible, which is in agreement with the blocking of the Cu+ ions by CO. After introduction of small amounts of oxygen (133 and 530 Pa) to the gas mixture, the band at 2120 cm-1 produced by Cu+-CO carbonyls drops in intensity while the opposite is observed with the band at 1872 cm-1 attributed to Cu2+-NO complexes (Figure 6, spectra d and e). This leads to the conclusion that addition of oxygen to the NO + CO mixture leads to oxidation of the Cu+ ions to Cu2+. With increasing oxygen pressure up to 2.1 kPa, the band at 2120 cm-1 vanishes and that at 1872 cm-1 is slightly shifted to higher frequencies (1880 cm-1) (Figure 6, spectrum f). At higher oxygen pressures, in the ν(N-O) region there is a single band at 1880 cm-1. In the 1800-1000 cm-1 region, addition of NO and small amounts of oxygen provokes the appearance of bands at 1334 and 1186 cm-1 (Figure 6, spectra d and e). These bands have been observed after nitrogen oxide adsorption on a freshly activated sample and are assigned to surface nitrito compounds.59 After the introduction of a higher amount of oxygen (2.1 kPa initial pressure), these bands disappear and a new series of bands appear at 1618, 1589, 1524, 1297, 1239, and 1047 cm-1, which is associated with the formation of surface nitrates (Figure 6, spectrum f). The initial hydrogen(carbonate) bands due to CO reactive adsorption gradually decrease and almost disappear from the spectrum after NO + O2 coadsorption. This suggests displacement of (hydrogen)carbonates by nitrates. 3.8. Adsorption of Ethene. Introduction of ethene (7.33 kPa equilibrium pressure, followed by evacuation) to the activated sample does not lead to the formation of adsorption species. However, heating the sample in an ethene atmosphere at 423 K results in the appearance of two bands with maxima at 1554 and 1450 cm-1, the latter having a pronounced shoulder at 1443 cm-1 (Figure 7, spectrum a). These bands are assigned to carboxylate species,53 which suggests oxidation of ethene by the Cu2+ cations. Increase of the interaction temperature up to 573 K (Figure 7, spectra b-d) provokes an increase in intensity of the bands at 1554, 1450, and 1443 cm-1 and the appearance of two weak bands at 1352 and 1257 cm-1. The latter are assigned to carbonaceous deposits. All of the bands are not affected by evacuation. No bands are observed in the C-H stretching region, which implies that no C-H-containing species exist on the surface. 3.9. Interaction of Surface NOx Compounds with Ethene. Although it is generally considered that the SCR mechanism involves interaction of surface nitrates with hydrocarbons, we have also studied the interaction of preadsorbed ethene with NOx. The spectrum registered after interaction between ethene and the sample at 573 K is already discussed (see Figure 7, spectrum d). Subsequent introduction of a NO + O2 mixture (2 and 3.3

Venkov et al.

Figure 7. Interaction of C2H4 with Cu/ZrO2: heating of the sample for 15 min in an atmosphere of 7.3 kPa ethene at 423 (a), 473 (b), 523 (c), and 573 K (d). The gas-phase spectrum is subtracted.

kPa, respectively) results in development of nitrate bands as already described for the pure surface. The carboxylate band at 1450 cm-1 disappears after heating the sample in the gas mixture at 573 K, which is explained by their displacement by nitrates. The behavior of the other bands produced after ethene adsorption at 573 K cannot be followed accurately since they are masked by the strong nitrate bands. Evacuation of the sample at elevated temperature up to 673 K leads to a decrease in intensity/ disappearance from the spectrum of all bands due to adspecies. No other compounds were detected. Since the spectra of the surface nitrates formed on the pure support61 are similar to the spectra of the nitrates registered in the present work, it is difficult to determine, on the basis of the frequencies of separate bands alone, which of them are formed with the participation of copper cations. For that reason, we studied the interaction between surface nitrates and ethene from the gas phase on a zirconia sample, after which the reactivity of the nitrates on Cu/ZrO2 was examined under the same conditions. During coadsorption of NO (equilibrium pressure 2 kPa) and O2 (equilibrium pressure 2.7 kPa) on zirconia followed by evacuation at room temperature, a series of bands with maxima at 1636, 1591, 1561, 1287, 1226, 1047, and 1004 cm-1 appeared due to the formation of surface nitrates on Zr4+ cations of zirconia.61-63 After ethene adsorption, the above bands showed practically no change (Figure 8, spectrum b). After that, the sample was heated at rising temperature under ethene. With a temperature increase up to 573 K, the intensity of bands in the 1650-2000 cm-1 region showed almost no change (Figure 8, spectra b-f). Only at 623 K did the bands at 1636 and 1591 cm-1 display an intensity drop while those at 1287 and 1226 cm-1 disappeared. At the same time, a broad band with a maximum at about 1450 cm-1 (Figure 8, spectrum g) appeared in the spectrum. The latter was assigned to carbonaceous deposits.61 To elucidate the nature of the intermediate surface compounds appearing on the Cu/ZrO2 surface during selective catalytic reduction of NOx with ethene, we at first covered this sample with nitrates (as described above for zirconia). As a result of this procedure, a series of bands with maxima at 1890, 1630, 1573, 1283, 1215, 1030, and 1004 cm-1 appeared in the spectrum (Figure 9, spectrum (61) Hadjiivanov, K.; Avreyska, V.; Klissurski, D.; Marinova, T. Langmuir 2002, 18, 1619. (62) Pozdnjakov, D.; Filimonov, V. Kinet. Katal. 1973, 14, 760. (63) Kantcheva, M.; Ciftlikli, E. J. Phys. Chem. B 2002, 106, 3941.

NOx Species on Cu/ZrO2

Figure 8. Interaction of C2H4 with the nitrates formed on ZrO2: coadsorption of NO (2 kPa equilibrium pressure) and O2 (3 kPa initial equilibrium pressure) followed by evacuation at room temperature (a), addition of C2H4 (6.7 kPa equilibrium pressure) (b), and heating the sample for 15 min in the presence of C2H4 at 423 (c), 473 (d), 523 (e), 573 (f), and 623 K (g).

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1340, and 1258 cm-1 vanish while that at 1443 cm-1 is slightly shifted to higher frequencies (Figure 9, spectrum h). Thus, the only bands that remain in the spectrum after evacuation at 623 K are those at 1552 and 1450 cm-1, which shows that they are due to compounds strongly bound to the surface. On the basis of the difference between the spectra registered after the reaction of ethene on Cu/ZrO2 and pure zirconia, the following conclusions may be drawn: (i) The broad feature at 1700-1600 cm-1 which characterizes a C-H-N-O deposit and the C-N band at 2142 cm-1 correspond to intermediate compounds adsorbed on copper ions on the Cu/ZrO2 surface. This is evidenced by the fact that the bands are registered on Cu/ZrO2 alone and are absent from the spectra recorded on pure zirconia. (ii) Nitrates formed on the zirconia surface are inert toward ethene even at high temperatures. This is proved by the fact that in the spectra obtained after C2H4 adsorption on pure zirconia the intensity of nitrate bands practically does not change with increasing temperature up to 573 K. In addition, the spectra show no bands of intermediate compounds formed by interaction between surface nitrates and ethene. Hence, nitrates adsorbed on Cu2+ surface sites play the key role during reduction of nitrogen oxides with hydrocarbons on Cu/ZrO2. 4. Discussion

Figure 9. Interaction of C2H4 with the nitrates formed on Cu/ZrO2: coadsorption of NO (2 kPa equilibrium pressure) and O2 (3 kPa initial equilibrium pressure) followed by evacuation at room temperature (a), addition of C2H4 (6.7 kPa equilibrium pressure) (b), and heating the sample for 15 min in the presence of C2H4 at 423 (c), 473 (d), 523 (e), 573 (f), and 623 K (g) and after evacuation at 623 K (h).

a). The bands in the region of 1650-1000 cm-1 were already ascribed to surface nitrates, and the low-intensity band at 1890 cm-1, to N2O3. Introduction at room temperature of C2H4 (6.7 kPa) caused no substantial spectrum changes (Figure 9, spectrum b). With rising temperature of interaction up to 423 and 473 K, respectively, the band intensity in the 1630-1000 cm-1 region decreased while the band of N2O3 (1890 cm-1) disappeared (Figure 9, spectra c and d). However, at 523 K the bands at 1630, 1283, 1215, 1030, and 1004 cm-1 vanished and a new band of low intensity with a maximum at 1443 cm-1 became visible (Figure 9, spectrum e). With rising temperature of interaction up to 573 K, the band at 1573 cm-1 produced by nitrates disappeared, and three new low-intensity bands at 1552, 1340, and 1258 cm-1 appeared (Figure 9, spectrum f). A broad feature in the 1700-1600 cm-1 region also emerges. The bands at 1552, 1450, 1340, and 1258 cm-1 were already observed when ethene reacted with the pure sample. The new feature around 1650 cm-1 most probably indicated a C-H-N-O deposit. This suggests that at the above temperature the surface nitrates react with ethene from the gaseous phase. At 623 K, a new band with a maximum at 2142 cm-1 is seen in the 2200-2100 cm-1 region (see inset in Figure 9, spectrum g) and is attributed to CN- compounds. After evacuation of the sample at the same temperature, the bands at 2142,

Since a high number of bands were registered during the experiments, for the sake of convenience their interpretation was summarized in Table 1. 4.1. Localization of Copper Ions on the Cu/ZrO2 Surface. The similarity of the IR spectra of ZrO2 and Cu/ZrO2 in the region of O-H stretching modes is an indication that supported copper ions are localized in the part of the support which is not characterized by the presence of surface hydroxyl groups. A similar distribution is typical of a series of cations adsorbed on titania. In this case, the cations are localized on coordinatively unsaturated O2- anions.64 The neutral charge in the system is preserved by adsorption of hydroxyl groups on the titanium cations, which is followed by recombination and water evolution:

As a result, the coordinatively unsaturated titanium cations are blocked. Obviously, a similar mechanism is also valid for zirconia. This is confirmed by the low intensity of the band for Zr4+-CO carbonyls (lower than that of the pure support), which evidences blocking of part of the zirconium cations on the catalyst surface after the deposition of copper cations. This mechanism is also supported by the X-ray photoelectron spectroscopy (XPS) data indicating a Cu/Zr ratio on the surface that is higher than the ratio according to the chemical composition of the sample. The Cu+-CO carbonyls are usually characterized by a high stability due to the synergism between the σ and π components of the Cu+-CO bond. Indeed, we have found that these species decompose after evacuation at 423 K (Figure 1). On the contrary, the bond between Zr4+ ions and CO has mainly an electrostatic and/or σ character. This arises from the fact that the Zr4+ ions have no (64) Hadjiivanov, K.; Klissurski, D.; Kantcheva, M.; Davydov, A. J. Chem. Soc., Faraday Trans. 1991, 87, 907.

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d-electrons, which could participate in the formation of a back π-donation. That is why Zr4+-CO carbonyls are unstable and decompose after evacuation at room temperature (see Figure 1). It has been established that with decreasing CO coverage the band at 2126 cm-1, characterizing Cu+-CO carbonyls, is shifted to lower frequencies. This cannot be due to the effect of surface (hydrogen)carbonates because their bands show almost no change after evacuation (Figure 1, spectra a and b). For that reason, we are of the opinion that the shift of the Cu+-CO band at high coverages is due to the dipole interaction between the CO molecules adsorbed on the surface, as a result of which the so-called dynamic shift to higher frequencies is observed with the band characterizing the C-O stretching modes. Increasing CO coverage on oxide systems is usually accompanied by a shift of the carbonyl bands to lower frequencies as a result of the prevailing static effect.49 In our case, however, the coverage increase is accompanied by a blue shift of the Cu+-CO band maximum, which indicates a predominating dynamic effect. As was already mentioned, we do not assume formation of a separate CuO phase on the surface of our catalyst because even after thermal treatment the sample color remains blue. The coverage dependence of the Cu+-CO band is, however, an indication that the copper ions on the surface of our Cu/ZrO2 sample are not isolated. These results can be explained by the formation of one- or twodimensional CuO-like phases on the surface, similarly to the case of Cu/TiO2.64 4.2. Redox Properties of Copper Ions on a Cu/ZrO2 Surface. Our results show that Cu2+ and Cu+ cations coexist on the surface of the activated sample. Thus, after adsorption of nitrogen oxide on the surface of Cu/ZrO2, mononitrosyls coordinated to Cu+ (1759 cm-1) and Cu2+ (1872 cm-1) sites are formed. With time, the Cu+ sites are oxidized under NO and small amounts of surface nitro/ nitrito compounds (1527, 1327, and 1185 cm-1) and N2O (2246 cm-1) and NO+ (2122 cm-1) appear. The coexistence of Cu2+ and Cu+ ions on the sample is also consistent with our XPS results. The Cu+-NO complexes are usually unstable and are observed mainly during adsorption at low temperatures. A band of Cu+-NO nitrosyls obtained after NO adsorption at room temperature has been observed for the first time on Cu-ZSM-5.54 In this case, the Cu+-NO nitrosyls are converted to Cu+(NO)2 dinitrosyls in the presence of NO.6,8,54,55 This can be explained by the low coordination number of the Cu+ ions in the zeolite matrix, which permits coordination of a second NO molecule. Studying the adsorption of nitrogen oxide on Cu/Al2O3 at room temperature, some authors have observed a band in the region of 1780-1740 cm-1 and attributed it to the ν(N-O) stretching modes of Cu+-NO complexes.56,57 No bands of Cu+-NO nitrosyls have been observed with Cu/ZrO2 so far. However, at the initial moment of nitrogen oxide adsorption on our Cu/ZrO2 sample we have observed formation of small amounts of Cu+-NO species (Figure 2). The band characterizing them shows an intensity drop with time, whereas the band due to Cu2+-NO species becomes more intense. On this basis, it may be concluded that in a nitrogen oxide atmosphere the Cu+ ions on the sample surface are oxidized to Cu2+.51 A similar effect has also been observed on Cu-ZSM-5.54 Our experiments have shown that oxidation of Cu+ ions to Cu2+ also occurs during coadsorption of CO and NO (Figure 6). After introduction of nitrogen oxide to the CO-Cu/ZrO2 system, the band for monocarbonyls of Cu+ ions becomes weaker while that due to Cu2+-NO increases in intensity.

Venkov et al.

Adsorption of CO on the sample was accompanied by formation of a series of carbonato-hydrogen-carbonate structures which indicates that during CO adsorption, reduction of Cu2+ ions to Cu+ occurs. The results obtained show that the Cu2+ ions on the Cu/ZrO2 surface are reduced relatively easily and, vice versa, the Cu+ ions formed are easily oxidized to Cu2+. Since SCR proceeds as a rule under strongly oxidative conditions, it might be concluded that the main part of the copper ions under the reaction conditions are in oxidation state +2. Similar conclusions have been made by Ma´rquez-Alvarez et al.13 with a CuZSM-5 sample. However, more recently it has been shown12,29 that in the presence of hydrocarbons in the gas phase and at the SCR reaction temperatures a significant part of the copper is in the form of Cu+ cations. For instance, the cyanide species registered in the present work are probably coordinated to Cu+ ions due to the high stability of the Cu+-CN complexes. It is also beyond any doubt that the carboxylates produced during interaction of ethene with Cu/ZrO2 are associated with Cu+ cations. It is evident that the Cu/ZrO2 sample also catalyzes the disproportionation and/or decomposition of NO because under NO bands which characterize NO2- and NO+ appear with time. The bands observed at 1600-1000 cm-1 during adsorption of nitrogen oxide were attributed to surface nitro or nitrito compounds. Studying the adsorption of nitrogen oxide on zirconia, some authors61,62,65,66 found a band at 1190-1177 cm-1 and ascribed it to nitrito compounds. On this basis, we assign the band at 1185 cm-1 to surface nitrito compounds coordinated to Zr4+ sites. The bands with maxima at 1527 and 1327 cm-1 probably characterize the formation of nitro compounds on the surface.59 Due to the higher intensity of the band at 1185 cm-1 as compared to the bands at 1527 and 1327 cm-1, one can assume that the concentration of nitrito compounds is higher than that of nitro compounds formed on Cu/ZrO2. 4.3. Formation of Surface Nitrates. Introduction of small amounts of oxygen into the Cu/ZrO2-NO system causes at first an increase in concentration of the surface nitro/nitrito compounds, that is, the bands at 1527, 1327, and 1185 cm-1 increase in intensity (Figure 3, spectra a and b). When the oxygen amount increases, the NO2compounds are oxidized to nitrates which are identified by bands at 1630, 1580, 1554, 1287, 1225, 1031, and 1004 cm-1 (Figure 3). From the behavior of the bands during the study of the thermal stability of surface NO3complexes, it has become clear that the bands at 1630, 1225, and 1004 cm-1 correspond to nitrates of the same type of symmetry whereas those at 1554, 1287, and 1031 cm-1 could be identified as nitrates of another symmetry (Figure 5). According to literature data,59 the bands at 1630, 1225, and 1004 cm-1 characterize the corresponding ν3′, ν3′′, and ν1 vibrations of bridged nitrates while the bands at 1554, 1287, and 1031 cm-1 can be attributed to monodentate nitrates. The band at 1580 cm-1 is probably produced by bidentate nitrates. The band characterizing the ν3′′ vibration of these nitrates most probably overlaps with the bands at 1287 and 1225 cm-1. In addition to the oxidation of surface NO2- species, there are three possibilities of formation of surface nitrates during coadsorption of NO and O2: (i) oxidation of a surface cation; (ii) disproportionation of N2O4; and (iii) displacement of another anion, usually a hydroxyl group. Our results permit rejecting the first possibility since even (65) Miyata, H.; Konishi, S.; Ohno, T.; Hatayama, F. J. Chem. Soc., Faraday Trans. 1995, 91, 1557. (66) Indovina, V.; Gordischi, D.; Rossi, S.; Ferraris, G.; Ghiotti, G.; Chiorino, A. J. Mol. Catal. 1991, 68, 53.

NOx Species on Cu/ZrO2

under NO, the Cu+ cations are oxidized to Cu2+. It is evident that part of the nitrates are formed by the second mechanism, which is also supported by the formation of a certain amount of NO+. Analysis of the spectra in the regions of O-H stretching modes permits the assumption that a large part of the nitrates are formed upon interaction with surface hydroxyl groups, for example,

2OH- + N2O3 + O2 f 2NO3- + H2O The water formed produces a wide band in the region of the O-H stretching modes and a δ(H2O) band overlapping with ν3′ of the nitrates. 4.4. Effect of the Surface Nitrates on the Lewis Acidity. Admixture anions (such as sulfates, nitrates, carbonates, and phosphates) modify the oxide surfaces.58,61,63,67-72 Due to their negative induction effect, the anions can enhance the Lewis acidity of the neighboring metal ions on the oxide surface. This effect was observed for the first time during coadsorption of CO2 and CO on ZnO: the carbonates formed by CO2 enhanced the electrophilicity of the Zn2+ ions.67 As a result, the σ-bond between the cations on the surface and the adsorbed CO molecules became stronger, which led to a higher order of the C-O bond and a shift of the ν(C-O) stretching modes to higher frequencies. A similar phenomenon was also observed after adsorption of CO68,69 on sulfated oxides. Moreover, this effect was noticed when testing the surface acidity with NO.58 Similar results are published on coadsorption of NO and O2 on TiO2,70 ZrO2,61,63 and Al2O3.71 The data were considered as evidence that the nitrates formed on the surface increased the acidity of neighboring cations. It should be noted that not all of the anion modifiers enhance the Lewis acidity. Thus, it has been established that surface phosphates block the cation sites.72 At high concentrations, the anionic nitrogen oxocompounds can also block the cationic sites. Indeed, as a result of such blocking the ν(N-O) bands corresponding to Mn+(NO3-)-NO species usually disappear at high coverages with nitrates.61,63,70,71 If the anionic admixtures enhance the electrophilicity of cations possessing d-electrons in their last electron layer, the shift of the ν(N-O) vibration to higher frequencies can occur at the expense of both (i) strengthening of the σ-bond and (ii) hindering the back π-donation of electrons due to enhanced electrophilicity of the cation.58 The same is valid for the shift of the C-O stretching modes. A criterion determining which of the two effects is prevailing is the stability of the appearing Mn+(X)-NO or Mn+(X)CO complexes (X ) SO42-, NO3-, CO32-). If the complexes thus formed are more stable than species formed on an unmodified surface, it may be assumed that the shift of the ν(N-O) or ν(C-O) modes appears at the expense of strengthening of the σ-bond. However, if the reason for the band shift to higher frequencies is the weakening of the π-bond, these complexes will decompose more easily than the complexes formed on a clean surface. On the basis of the foregoing, one could also explain the effect of nitrates on the behavior of the nitrosyl bands. Let (67) Saussey, J.; Lavalley, C. J.; Bovet, C. J. Chem. Soc., Faraday Trans. 1982, 78, 1457. (68) Hadjiivanov, K.; Davydov, A. Kinet. Katal. 1988, 29, 460. (69) Lange, F.; Hadjiivanov, K.; Schmelz, H.; Kno¨zinger, H. Catal. Lett. 1992, 16, 97. (70) Hadjiivanov, K.; Kno¨zinger, H. Phys. Chem. Chem. Phys. 2000, 2, 2803. (71) Venkov, T.; Hadjiivanov, K.; Klissurski, D. Phys. Chem. Chem. Phys. 2002, 4, 2443. (72) Hadjiivanov, K.; Klissurski, D.; Davydov, A. J. Catal. 1989, 116, 498.

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us discuss first the band with a maximum at 1932 cm-1 (Figure 3, spectrum c). Probably part of the nitrates are bound to Zr4+ ions from the bare zirconia surface. This leads to an increase in Lewis acidity of the Zr4+ ions, as a result of which formation of mononitrosyls of the type Zr4+(NO3-)-NO is possible.61,63 This band was not found during adsorption of nitrogen oxide on the sample. The formation of such nitrosyls characterized by a band at 1926 cm-1 (with moderate nitrate coverages) was also observed after coadsorption of nitrogen oxide and oxygen on pure zirconia.61 With increasing nitrate concentration in the presence of larger oxygen amounts, the Zr4+ sites are blocked and the band at 1932 cm-1 vanishes. A similar effect has been observed when studying the coadsorption of nitrogen oxide and oxygen on zirconia.61,63 Simultaneously with the formation of nitrates on zirconia, a band with a maximum at 1926 cm-1 appears and has been assigned to Zr4+(NO3-)-NO. The surface nitrates exercise an analogous effect on the surface of Al2O3, as a result of which complexes of the type of Al3+(NO3-)-NO are formed.71 The formation of nitrates probably leads to an enhancement of the Lewis acidity of Cu2+ sites to which they are bonded. As a result, the order of the ν(N-O) vibration of Cu2+-NO increases, due to which the band characterizing these mononitrosyls is shifted to higher frequencies. A similar effect of the anion modification has been observed by Delahay et al.58 in a study on the adsorption of nitrogen oxide on sulfated Cu/ZrO2. During adsorption of nitrogen oxide on pure Cu/ZrO2 and Cu/SO42--ZrO2, the authors have observed a weak shift to higher frequencies of the band assigned by them to Cu2+-NO species. They have attributed this phenomenon to the effect of surface sulfate ions which, owing to their negative induction effect, decrease the influence of the back π-bond between the Cu2+ ions on the surface and the molecule of the nitrogen oxide. Careful analysis of our spectra, however, shows that the behavior of the band in the 1890-1875 cm-1 region (see Figure 3) cannot be explained on the basis of this effect only. Obviously, with the introduction of oxygen to the system, N2O3 is formed on the surface and is adsorbed on Cu2+ and Zr4+ sites. The frequency of the band characterizing the ν(NdO) vibration of N2O3 is close to that of the band attributed to Cu2+-NO. The introduction of larger oxygen amounts (initial equilibrium pressure of 4.7 kPa), however, results in disappearance of the bands for Cu2+-NO and Zr4+(NO3-)-NO, and only a band with a maximum at 1892 cm-1, belonging to N2O3,59 remains. The bands characterizing the νas(NO2-) and νs(NO2-) modes of N2O3 (at 1550 and 1295 cm-1, respectively) overlap with the corresponding bands of the nitrates. 4.5. Interaction of Surface Nitrates with Ethene. The results on ethene adsorption on the pure sample indicate that no C-H-containing species are produced in this case. Subsequent NOx adsorption only leads to destruction of the carboxylates formed and appearance of surface nitrates. No intermediate species, such as nitriles or isocyanates, are isolated in this case. These results suggest that the formation of a C-H-N-O deposit (which is decomposed to isocyanates and/or nitriles) requires interaction of surface nitrates with hydrocarbons. Analysis of the spectra registered after interaction of ethene with the nitrates formed on pure zirconia and Cu/ ZrO2 has shown that only nitrates bound to Cu2+ surface ions react with ethene from the gas phase. In contrast to them, nitrates coordinated to Zr4+ sites are highly inert. An organic C-H-N-O deposit characterized by a broad feature at 1700-1600 cm-1 is formed as a result of

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interaction between copper nitrates and ethene at 573 K. With the rise of temperature of interaction up to 623 K, one more low-intensity band appears at 2142 cm-1. All these bands are absent from the spectra registered after ethene adsorption on nitrate-precovered zirconia, which is an indication that they belong to compounds coordinated to copper ions from the active phase. Studying the mechanism of SCR of nitrogen oxides with C2H6 and C2H5OH over Cu/ZrO2, Li et al.43 have established the appearance of two bands with maxima at 2190 and 2140 cm-1. On the basis of experiments involving adsorption of isotope-labeled molecules, the authors have proved these bands to be due to NCO- and CN- species, respectively. Indeed, it has been reported in the literature that isocyanates are characterized by higher frequencies than the CN- species.73 On this basis, we ascribe the band at 2142 cm-1 to CN- compounds. Evacuation at 623 K leads to disappearance of the bands at 2142 cm-1, and only bands at 1552 and 1450 cm-1 are visible. These bands probably characterize νas(COO-) and νs(COO-) vibrations, respectively, of surface carboxylates50 appearing as a product of oxidation of hydrocarbons by nitrogen oxides. The same species were formed during interaction of ethene on a pure sample. Finally, our results confirm the general scenario for the SCR mechanism currently accepted by many authors: that is, (i) oxidation of NO to surface nitrates, (ii) interaction of the nitrates with hydrocarbons to a C-H-N-O deposit, (iii) decomposition of this deposit to CN- and/or NCOspecies, and (iv) interaction of the latter with NOx to form (73) Bion, N.; Saussey, J.; Seguelong, T.; Daturi, M. Phys. Chem. Chem. Phys. 2001, 3, 4811.

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nitrogen. Cu/ZrO2 appears to be a promising catalyst for HC-SCR because of the high hydrothermal stability of the support. 5. Conclusions In the presence of CO, Cu2+ ions from the Cu/ZrO2 surface are reduced to Cu+, and carbonate-hydrogencarbonate structures are simultaneously formed. On the contrary, in an NO atmosphere the Cu+ ions are oxidized to Cu2+. Adsorption of NO on Cu/ZrO2 leads to formation of small amounts of surface NO2- species which are oxidized to nitrates in the presence of oxygen. The nitrates enhance the Lewis acidity of Cu2+ and Zr4+ on the surface, as a result of which Mn+(NO3-)-NO species are formed. The surface nitrates coordinated to Cu2+ ions from the surface of Cu/ZrO2 are characterized by a much higher reactivity with respect to ethene than is the case for nitrates bonded to Zr4+ sites. Organic nitro compounds and cyanides are intermediate compounds during the reduction of nitrogen oxides with ethene on the surface of Cu/ZrO2. Carboxylates are produced as a result of ethene oxidation. Cu/ZrO2 is a promising SCR catalyst. An important advantage of it is the high hydrothermal stability of the support. Acknowledgment. This work was supported by the Alexander von Humboldt Foundation to which the authors are indebted. LA026655Q