Surface Species Formed after NO Adsorption and NO + O2

1290−1220, and 1042−1004 cm-1) develop and enhance the Lewis acidity of ... of Photocatalytic NO Oxidation on SrO-Clusters@Amorphous Carbon Ni...
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Langmuir 2002, 18, 1619-1625

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Surface Species Formed after NO Adsorption and NO + O2 Coadsorption on ZrO2 and Sulfated ZrO2: An FTIR Spectroscopic Study Konstantin Hadjiivanov,* Valentina Avreyska, Dimitar Klissurski, and Tsvetana Marinova Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria Received July 16, 2001. In Final Form: October 29, 2001 Adsorption of NO and its coadsorption with oxygen on ZrO2 and sulfated ZrO2 have been studied by FTIR spectroscopy. It has been found that NO adsorption on ZrO2 results in formation of N2O (2282, 2244, and 1233 cm-1) and small amounts of nitro species (1443 and 1423 cm-1), nitrates (1558, 1189, and 1057 cm-1), and nitric acid (1621 and 1142 cm-1). With time, the nitrate bands (1640-1560, 1290-1220, and 10421004 cm-1) develop and enhance the Lewis acidity of nearby Zr4+ cations. As a result, coordinated NO (1906 cm-1) is formed. On sulfated zirconia, nitrosyls (1908 cm-1) are produced immediately after NO adsorption evidencing an increased acidity of the Zr4+ cations as a result of the sulfate inductive effect. In addition, nitrates and nitric acid are produced similarly to the case of the sulfate-free sample. Addition of oxygen to the NO-ZrO2 system results in formation of nitrites (1180 cm-1, disappearing in excess oxygen) and in a strong increase in concentration of the nitrates. Besides, NO+ (2206 cm-1) is found. The nitrate species manifest combination modes in the 2700-2400 cm-1 region, which facilitates their spectral identification. Coadsorption of NO and O2 on the sulfated sample results in a similar picture, but the nitrates formed are less stable.

1. Introduction The increasing ecological requirements rouse an immense interest in selective catalytic reduction (SCR) of nitrogen oxides. This process is commercially applied with immobile NOx sources using ammonia as a reducing agent.1,2 The most efficient catalysts are vanadia or vanadia-tungsta supported on TiO2.1,2 However, efforts have been made to replace TiO2 as a support by ZrO2 because of the higher thermal stability of the latter.3,4 At present, many studies are devoted to SCR of NOx by hydrocarbons (HC-SCR). This reaction has initially been reported to proceed over metal-exchanged zeolites,5-8 but after that transition metal cations well dispersed on oxide supports have also been found to be efficient catalysts.9,10 In particular, transition metal cations dispersed on ZrO2 are reported to be promising HC-SCR catalysts.11-15 More recently, a preliminary sulfatation of zirconia was found (1) Janssen, F. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Weitkamph, I., Eds.; Willy-VCH: Weinheim, Germany, 1997; p 1633. (2) Busca, G.; Lietti, L.; Ramis, G.; Berti, F. Appl. Catal., B 1998, 18, 1. (3) Indovina, V.; Occhiuzzi, M.; Ciambelli, P.; Sannino, D.; Ghiotti, G.; Prinetto, F. In Proceedings of the 11th International Congress on Catalysis - 40th Anniversary; Hightower, J. W., Delgass, W. N., Iglesia, E., Bell, A. T., Eds.; Elsevier: Amsterdam, 1996; p 691. (4) Makedonski, L.; Nikolov, N.; Klissurski, D.; Slavov, S.; Stefanov, P.; React. Kinet. Catal. Lett. 1995, 54, 247. (5) Iwamoto, M.; Yahiro, H.; Yu-u, Y.; Shundo, S.; Mizuno, N. Shokubai 1990, 32, 430. (6) Held, W.; Ko¨nig, A.; Richter, T.; Puppe, L. Soc. Automot. Eng. Trans., Section 4, No. 900 496, 1990, 209. (7) Li, Y.; Armor, J. N. Appl. Catal., B 1992, 1, L31. (8) Traa, Y.; Burger, B.; Weitkamp. J. Microporous Mesoporous Mater. 1999, 30, 3. (9) Inaba, M.; Kintaichi, Y.; Haneda, M.; Hamada, H. Catal. Lett. 1996, 39, 269. (10) Hamada, H.; Kintaichi, Y.; Inaba, M.; Tabata, M.; Yoshinari, T.; Tsuchida, H. Catal. Today 1996, 29, 53. (11) Haneda, M.; Kintaichi, Y.; Inaba, M.; Hamada, H. Catal. Today 1998, 42, 127. (12) Bethke, K. A.; Kung, M. C.; Yang, B.; Shah, M.; Alt, D.; Li, C.; Kung, H. H. Catal. Today 1995, 26, 169.

to favor the dispersion of supported metal cations which would facilitate good SCR performance.14 Another advantage of sulfated zirconia as a support could be the enhanced surface acidity which might promote the activation of hydrocarbons during SCR. That is why detailed knowledge of the surface species formed on ZrO2 and sulfated ZrO2 under SCR conditions is of great importance when studying the SCR mechanism on zirconia-supported catalysts. This would allow unambiguous discrimination between the species formed on the support and the compounds produced with the participation of the active phase on different zirconia-supported catalysts. A peculiarity of SCR is that it proceeds in excess of oxygen.1,2,16 As a rule, the activity of the catalysts under anaerobic conditions is rather low. That is why it is important to study the surface species formed in the presence of oxygen. The aim of this paper is to establish the nature of the surface species produced on ZrO2 and sulfated ZrO2 after NO adsorption and NO + O2 coadsorption and to characterize them. We have also made a detailed analysis of the spectra in the regions of the overtones and combination modes since, as proposed recently,17 this analysis can provide valuable information on the nature and structure of the surface nitrogen-oxo species. 2. Experimental Section 2.1. Samples and Reagents. The ZrO2 used for the experiments was a commercial Degussa sample with a monoclinic (13) Figueras, F.; Coq, B.; Ensuque, E.; Tachon, D.; Delahay, G. Catal. Today 1998, 42, 117. (14) Pietrogiacomi, D.; Sannino, D.; Tuti, S.; Ciambelli, P.; Indovina, V.; Occhiuzzi, M.; Pepe, F. Appl. Catal., B 1999, 21, 141. (15) 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, G.; Rozovskii, A.; Ross, J. R. H.; Breen, J. P. J. Catal. 2001, 200, 117. (16) Shelef, M. Chem. Rev. 1995, 95, 209. (17) Hadjiivanov, K. Catal. Lett. 2000, 68, 157.

10.1021/la0110895 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/01/2002

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structure having a specific surface area of 53 m2 g-1. Sulfated zirconia was prepared by suspending ZrO2 powder in 0.5 M H2SO4. After 15 h of stirring, the precipitate was filtered, washed with 0.05 M H2SO4, dried at 383 K, and calcined for 1 h at 773 K. The specific surface area of the sample thus obtained was 44 m2 g-1. Nitrogen monoxide (>99.5) and carbon monoxide (>99.99) were supplied by Merck. 2.2. Methods. IR spectroscopy studies were carried out with a Nicolet Avatar 320 apparatus at a spectral resolution of 2 cm-1 accumulating 128 scans. Self-supporting wafers (ca. 10 mg cm-2) were prepared by pressing the sample powders at 104 kPa. The samples were heated directly in an IR cell equipped with KBr windows. The latter was connected to a vacuum/adsorption system with a residual pressure of less than 10-5 Torr. Prior to the adsorption measurements, the sample was activated by calcination at 723 K for 1 h and evacuation for 1 h at the same temperature.

3. Results 3.1. IR Characterization of the Initial Samples. The spectrum of the activated zirconia sample coincides with those described in the literature.18,19 In particular, two bands at 3760 and 3680 cm-1, due to two kinds of surface OH groups, have been detected. It is believed that the groups characterized by the band at 3760 cm-1 are I type (Zr-OH), whereas the band at 3680 cm-1 is due to the existence of II type hydroxyls, namely, (Zr)2OH.18 However, an alternative interpretation19 proposes that the observation of two kinds of surface hydroxyls is related to the existence of two kinds of oxygen, tri- and tetragonal, in the monoclinic zirconia. The spectrum of the activated sample of sulfated zirconia displays a strong band at 1380 cm-1. In agreement with many data from the literature,20,21 this band is being assigned to SdO stretching modes of surface sulfates. A weak band at 1460 cm-1 is also visible. At present, its assignment is not evident, but it obviously arises from the sulfatation of zirconia since it is not present on the pure oxide. In the ν(OH) region, two bands with maxima at 3765 and 3660 cm-1 are detected. These two bands can be assigned to surface zirconia hydroxyls, the position of the latter band being slightly affected by the presence of sulfates. The so-called “cutoff”, that is, the lowest frequency at which bands of surface species can be detected, is for ZrO2 at ca. 950 cm-1. For sulfated zirconia, the cutoff value is at ca. 1100 cm-1 and the noise in the higher-frequency region is more significant, which hinders detailed analysis of the spectra in the O-H stretching region and in the regions of overtones and combination modes. 3.2. Adsorption of NO on ZrO2. Introduction of NO (4 kPa equilibrium pressure) to the activated zirconia sample results in the appearance of several bands with very low intensities. Their maxima are at 2466, 2282, 2244, 1906, 1621, 1558, 1443, 1423, 1321, 1233, 1189, 1142, 1057, and 982 cm-1 (Figure 1, spectrum a). Simultaneously, the Zr-OH band at 3760 cm-1 slightly decreases in intensity and a weak and broad absorbance around 3600 cm-1 develops instead. A weak band at 3445 cm-1 is also discernible. With time, the bands at 1558, 1443, 1423, 1321, 1189, and 1057 cm-1 slightly rise in intensity whereas the intensities of the bands at 2486, 2224, and 1233 cm-1 are reduced (Figure 1, spectrum b). (18) Bensitel, M.; Moravek, V.; Lamotte, J.; Saur, O.; Lavalley, J. C. Spectrochim. Acta 1987, 43A, 1487. (19) Jacob, K.; Kno¨zinger, E.; Benies, S. J. Mater. Chem. 1993, 3, 65. (20) Hadjiivanov, K.; Davydov, A. Kinet. Katal. 1988, 29, 460. (21) Yamaguchi, T.; Jin, T.; Ishida, T.; Tanabe, K. Mater. Chem. Phys. 1987, 17, 3.

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Figure 1. FTIR spectra of NO (equilibrium pressure of 4 kPa) adsorbed on ZrO2: spectra taken immediately after NO admission (a) and after 30 min (b). The spectra are background corrected.

There are only a few studies of NO adsorption on ZrO2, and the results obtained are controversial. Pozdnjakov and Filimonov22 have reported formation of nitrates (1520 and 1040 cm-1) and nitrites (1280 and 1190 cm-1). Miyata et al.23 have proposed development of nitrates (band at 1630 cm-1), nitrites (bands at 1186 and 1105 cm-1), and nitro/nitrato compounds (1325 cm-1). According to Indovina et al.,24 the species formed are monodentate nitrates (1505 and 1423 cm-1), monodentate nitrites (1394 and 1274 cm-1), and chelate nitrites (1329, 1290, 1177, and 1140 cm-1). A set of bands in the 1650-1400 cm-1 region has been registered and attributed to nitro/nitrato species by Delahay et al.25 Despite some coincidences, the spectra described in this study differ from the reported ones. These results as well as the different observations by various authors suggest that the species arising after NO adsorption of zirconia are strongly affected by the sample structure, morphology, and/or hydroxylation degree. Some difference in the spectra could also arise from small oxygen contaminants in NO produced during its disproportionation. Since a large number of different IR bands have been described in this study, for easier understanding their assignments are presented in Tables 1 (ZrO2) and 2 (sulfated ZrO2). The bands at 2244 and 1233 cm-1 are assigned to the N-N and N-O stretching modes, respectively, of adsorbed N2O,26 whereas the weak band at 2486 cm-1 is attributed to the first overtone of the N-O stretches at 1233 cm-1. Usually, the overtones are observed at frequencies slightly lower than 2ν, where ν is the fundamental vibration. The high frequency of the overtone observed in this study could be explained by the big noise level in the overtone region which does not allow an accurate determination of the peak maxima. The surface concentration of N2O decreases with time probably as a result of the progressive blocking of the adsorption sites by other compounds. The intensity of this band is by about 1 order of magnitude lower than the intensity of the principal band, which is normal for overtones. Similar results have been reported for N2O adsorption on ZrO2,22 where the fundamental bands are registered at 2242 and 1237 cm-1. Examination of the presented spectra shows (22) Pozdnjakov, D.; Filimonov, V. Kinet. Katal. 1973, 14, 760. (23) Miyata, H.; Konishi, S.; Ohno, T.; Hatayama, F. J. Chem. Soc., Faraday Trans. 1995, 91, 1557. (24) Indovina, V.; Cordischi, D.; de Rossi, S.; Ferraris, G.; Ghiotti, G.; Chiorino, A. J. Mol. Catal. 1991, 68, 53. (25) Delahay, G.; Coq, B.; Ensuque, E.; Figueras, F.; Saussey, J.; Poignant, F. Langmuir 1997, 13, 5588. (26) Hadjiivanov, K. Catal. Rev.sSci. Eng. 2000, 42, 71.

Adsorption of NOx on ZrO2 and Sulfated ZrO2

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Table 1. Assignment of Bands Due to NOx Surface Species on Zirconia band, cm-1

formation conditions

species

assignment

concomitant bands, cm-1

3445 2609-2605 2584-2580 2484 2466 2244 2210 1926 1906

NO NO + O2 NO + O2 NO + O2 NO NO NO + O2 NO + O2 NO;

(HNO2) bridging nitrates bidentate nitrates nitrites N2O N2O NO+ Zr4+(NO3-)-NO Zr4+(NO3-)-NO

ν(O-H) ν3′ + ν1 ν3′ + ν1 νs + νas 2ν(N-O) ν(N-N) ν(N-O) ν(N-O) ν(N-O)

1621, 1142, 982 1640, 1233, 1003 1588, 1280, 1040 1294, 1180 2244, 1233 1233, 2466 -

tentative assignment low intensity low intensity low intensity low intensity

1878 1753 1640-1621 1621 1588 1558 1443 1423 1321 1308 1240 1233 1224 1189 1180 1142 1057 1042-1040 1003 982

NO + O2 NO + O2 NO + O2 NO + O2 NO NO + O2 NO NO + O2 NO + O2 NO + O2 NO + O2 NO + O2 NO NO + O2 NO NO + O2 NO NO NO + O2 NO + O2 NO

N2O3 N2O4 bridging nitrates (HNO2) bidentate nitrates bidentate nitrates nitro species nitro species nitro species nitro species bidentate nitrates N2O bridging nitrates bidentate nitrates nitrites (HNO2) bidentate nitrates bidentate nitrates bridging nitrates (HNO2)

ν(N-O) νas(N-O) ν3′ ν(NdO) ν3′ ν3′ νs νas νs νas ν3′′ ν(N-O) ν3′′ ν3′′ νas δ(N-O-H) ν1 ν1 ν1 ν(N-O)

(1550, 1295) 2610, 1233, 1004 3445, 1142, 982 2584, 1280, 1040 1189, 1057 1423 1443 1308 1321 2584, 1588, 1040 2466, 2244 2609, 1640, 1003 1558, 1057 2484, 1294 3445, 1621, 982 1558, 1189 2580, 1588, 1240 2609, 1640, 1233 3445, 1621, 1142

disappears after evacuation disappears after evacuation decomposed at T > 673 K tentative assignment decomposed at T > 673 K

note

disappears in excess of O2 increases in intensity in the presence of small amounts of O2

disappears in excess of O2 disappears in excess of O2 easily disappears in the presence of O2 easily disappears in the presence of O2 decomposed at T > 673 K decomposed at T > 673 K disappears in excess of O2 tentative assignment decomposed at T > 673 K decomposed at T > 673 K tentative assignment

Table 2. Assignment of Bands Due to NOx Surface Species on Sulfated Zirconia band, cm-1

formation conditions

species

2610-2600 2580 2505 2490 2450 2243 2224 1942 1908 1900 1750 1628-1617 1618 1585-1561 1554 ≈1500 ≈1300 1240 1233 1226 1173 1180 1144 1113

NO + O2 NO + O2 NO + O2 NO + O2 NO NO NO + O2 NO NO NO + O2 NO + O2 NO + O2 NO NO + O2 NO NO + O2 NO + O2 NO + O2 NO + O2 NO NO NO + O2 NO NO + O2

bridging nitrates bidentate nitrates monodentate nitrates nitrites N2O N2O NO+ Zr4+(NO3-)-NO Zr4+(SO42-)-NO N2O3 N2O4 bridging nitrates (HNO2) bidentate nitrates bidentate nitrates monodentate nitrates monodentate nitrates bidentate nitrates bridging nitrates N2O bidentate nitrates nitrites (HNO2) nitrates

concomitant bands, cm-1

assignment ν3′ + ν1 ν3′ + ν1 ν3′ + ν1 νs + νas 2ν(N-O) ν(N-N) ν(N-O) ν(N-O) ν(N-O) ν(N-O) νas(N-O) ν3′ ν(NdO) ν3′ ν3′ ν3′ ν3′′ ν3′′ ν3′′ ν(N-O) ν3′′ νas δ(N-O-H) ν1

that in this case again, a 2ν(NO) overtone has been recorded. The weak band at 1906 cm-1 is assigned to some kinds of Zr4+-NO complexes. Since this band is negligible but rises in intensity in the presence of oxygen (when nitrates are developed, see below), we believe that the Zr4+ cations on the bare zirconia surface are not acidic enough to form nitrosyl complexes. Formation of nitrates bound to the same site enhances the Lewis acidity, and as a result nitrosyls are formed. Thus, it appears that the band at 1906 cm-1 characterizes species of the Zr4+(NOx-)-NO type.

1628-1617, 1224 1585, 1240 ≈1500, ≈1300 1180 2243, 1226 2450, 1226

(1550, 1295) 2610, 1233, 1116 1144 2580, 1260 1173 cm-1 2505, ≈1300 2505, ≈1500 2580, 1588-1568 2610, 1640, 1116 2450, 2243 1554 cm-1 2490 1618 cm-1 2610, 1628, 1233

note low intensity low intensity low intensity low intensity low intensity relatively stable disappears in excess of O2 disappears in excess of O2 disappears after evacuation disappears after evacuation decomposed at T < 673 K tentative assignment decomposed at T < 673 K unstable unstable decomposed at T < 673 K decomposed at T < 673 K disappears in oxygen tentative assignment

It is difficult to propose, on the basis of the presented results, unambiguous assignments of all the other bands produced by NO adsorption on ZrO2. However, the set of bands at 3445, 1621, 1142, and 982 cm-1 do not change with time and could be assigned to adsorbed HNO2.28 The bands at 1558, 1189, and 1057 cm-1 change synchronously and are most probably due to bidentate nitrates.22,26 The bands at 1443, 1423, and 1321 cm-1 (the latter having a pronounced shoulder at 1308 cm-1) seem to develop in (27) Miller, T. M.; Grassian, V. H. Catal. Lett. 1997, 46, 213. (28) Laane, J.; Ohlsen, J. R. Prog. Inorg. Chem. 1986, 28, 465.

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Figure 2. FTIR spectra of NO (equilibrium pressure of 4 kPa) adsorbed on sulfated ZrO2: spectra taken immediately after NO admission (a), time evolution of the spectra (b,c), and after evacuation (d). The spectra are background corrected.

parallel and are due either to one species or to different species formed as a result of one surface reaction. Additional experiments have evidenced that the bands at 1443 and 1423, on one hand, and those at 1321 and 1308 cm-1, on the other, change synchronously. We tentatively assign these bands to two kinds of nitro compounds. An important conclusion is that all of the species observed during NO adsorption on ZrO2, except for N2O, do not manifest bands due to overtones or combination modes with a measurable intensity. 3.3. Adsorption of NO on Sulfated ZrO2. Introduction of NO (4 kPa equilibrium pressure) to the activated sulfated zirconia leads, similarly to the case of ZrO2, to the appearance of a strong band at 2243 cm-1 and a series of low-intensity bands. Their maxima are located at 2450, 1942, 1908, 1618, 1554, 1352, 1333, 1300, 1226, 1174, 1144, and 1113 cm-1 (Figure 2, spectrum a). Simultaneously, the Zr-OH band at 3766 cm-1 slightly decreases in intensity and a weak and broad absorbance around 3600 cm-1 develops instead. A negative band at 1390 cm-1 is also detected. This evidences that at least some of the surface species produced are located in the vicinity of the sulfate anions. With time, all bands develop and the band at 1908 cm-1 only slightly decreases in intensity. A decrease in the equilibrium pressure leads to a gradual decrease in intensity of the band at 1908 cm-1 which disappears completely after evacuation. The bands at 2450, 2243, and 1226 cm-1 also decrease in intensity but are not fully removed by evacuation. Comparing the results with those obtained with pure zirconia, we assign the bands at 2243 and 1226 to the ν(N-N) and ν(N-O) modes, respectively, of adsorbed N2O. The respective 2ν(N-O) overtone is found at 2450 cm-1. The ν(N-O) and 2ν(N-O) modes are observed at somewhat lower frequencies than on pure ZrO2, which is evidently due to the increased surface acidity of zirconia after sulfatation and suggests that N2O can be used for testing surface acidity. The surprisingly high stability of N2O in this case is in line with the above hypothesis. The band at 1908 cm-1 has been observed immediately after NO adsorption, and its intensity practically does not depend on the development of the other surface species. The band is definitely more intense than the band at 1906 cm-1 observed with ZrO2 and is assigned to Zr4+(SO42-)NO species. Indeed, it is well-known that metal cations on sulfated samples are characterized by an enhanced

Hadjiivanov et al.

Figure 3. FTIR spectra of NO and O2 coadsorbed on ZrO2: adsorption of NO, 4 kPa equilibrium pressure (spectrum b from Figure 1) (a) and admission of 0.1 kPa O2 (b), time evolution of the spectrum (c), admission of 4.0 kPa O2 (d), and after evacuation (f). The spectra are background corrected.

surface acidity.20,29 This is consistent with the above proposed assignment of the band at 1906 cm-1 observed on pure zirconia to Zr4+(NO3-)-NO species. The band at 1942 cm-1 is also attributed to nitrosyls of Zr4+ cations. However, its intensity rises with the development of the other species detected in the lower-frequency region. That is why this band is ascribed to Zr4+(NOx-)-NO nitrosyls also affected by sulfate anions. Our results are in agreement with the observations of Delahay et al.25 These authors have reported that no nitrosyls are produced on pure zirconia whereas NO adsorption on a sulfated sample gives rise to Zr4+-NO species (broad band at ca. 1960 cm-1). By analogy with pure ZrO2, the set of bands at 1618 and 1144 cm-1 can be assigned to adsorbed HNO2.28 In this case, the contaminant bands at ca. 3445 and 982 cm-1 have not been observed because of the high light scattering and the high position of the sample cutoff. The bands at 1554 and 1173 cm-1 are probably due to bidentate nitrates.22,26 At least some bands around 1300 cm-1 are due to sulfates: it is well-known that the 1380 cm-1 sulfate band shifts to lower frequencies after adsorption of different gases.20 However, the probability for one or more bands in this region to be due to surface nitrogen-oxo species cannot be ruled out. It is to be noted that no nitro species are formed on sulfated zirconia. 3.4. Coadsorption of NO and O2 on ZrO2. Introduction of a small amount of oxygen to the ZrO2-NO system leads to drastic changes in the IR spectrum (Figure 3, spectrum b). First of all, the bands due to adsorbed N2O (2466, 2244, and 1233 cm-1) disappear. Difference spectra also suggest disappearance of the bands at 1321 and 1308 cm-1. Simultaneously, bands generally much more intense than those produced after NO adsorption are developed at 1926, 1906, 1621, 1588, 1294, 1180, 1040, and 1003 cm-1. Note that the 1180 cm-1 band has two pronounced high-frequency shoulders. Two weak bands at 2605 and 2584 cm-1 and a broad band at 2484 cm-1 appear in the 2700-2400 cm-1 region. In the ν(OH) region, the bands due to Zr-OH groups almost disappear and an intense broad band around 3510 cm-1 emerges instead (see the inset in Figure 3). With time (Figure 3, spectrum c), the bands at 2605, 2584, 1621, 1588, 1287, 1233, 1040, and 1003 cm-1 develop, whereas the bands at 2484 and 1180 cm-1 vanish. When the amount of oxygen added to the system increases (29) Jin, T.; Machida, M.; Yamaguchi, T.; Tanabe, K. Inorg. Chem. 1984, 23, 4396.

Adsorption of NOx on ZrO2 and Sulfated ZrO2

(Figure 3, spectra d), those bands additionally increase in intensity and a broad band at 1530 cm-1 is clearly distinguished. A weak and broad band at 2210 cm-1, assigned to NO+,30 is also visible. The bands at 1926 and 1906 cm-1 almost disappear, and a band at 1878 cm-1 emerges in the region. The latter can be assigned to adsorbed N2O3,26 the other bands typical of this compound (expected at 1550 and 1295 cm-1) being masked by the strong nitrate bands. Another band at 1752 cm-1 is unambiguously associated with adsorbed N2O4.26 The band at 2605 cm-1 also rises in intensity and is shifted to 2609 cm-1. Both bands at 1443 and 1423 cm-1 disappear. In the ν(OH) region, the broad band due to H-bonded hydroxyls also develops. After evacuation, all bands in the 2300-1700 cm-1 region disappear. The difference spectra show that bands at 1650, 1208, and 1001 cm-1 develop, whereas bands at 1606, 1588, 1535 (broad), 1297, and 1260 cm-1 decrease in intensity. In the 2700-2400 cm-1 region, the band at 2584 cm-1 is slightly reduced in intensity, whereas the 2610 cm-1 band slightly rises. A weak and broad feature at 2426 cm-1 is also visible. Most of the bands produced after contact of the sample with the NO + O2 mixture are easily assignable. The bands in the 1700-900 cm-1 region, which are resistant toward evacuation, are generally assigned to different anionic NOx- species.26 The band at 1180 cm-1 (Figure 3, spectrum b) is observed in the presence of small amounts of oxygen only and disappears when the amount of oxygen increases. That is why this band has to be assigned to species where the nitrogen oxidation state is 3+, that is, to nitro or nitrito compounds. A similar band with a similar behavior has been observed recently with Al2O3 and assigned to nitrites.31 Our results indicate that this band is associated with a broad band at 2484 cm-1 in the region of overtones and combination modes. This suggests that the band at 1180 cm-1 has a contaminant mode around 1300 cm-1, which is most probably the band at 1294 cm-1. Thus, both bands can be attributed to the N-O stretching modes of surface nitrito species. The intense band at 1644-1621 cm-1 is formed in the presence of oxygen and is stable in excess oxygen and toward evacuation. This band changes in parallel with the bands at 1233 and 1004 cm-1. In the region of overtones and combination modes, a band at 2609 cm-1 follows the changes of the 1644-1621 cm-1 band. All this allows, in agreement with data from the literature,22,26 assignment of the set of bands at 1644-1621, 1233, and 1003 cm-1 to bridging nitrates. Here, the ν3 split mode gives rise to the bands at 1644 and 1233 cm-1 (ν3′ and ν3′′, respectively) and ν1 is observed at 1004 cm-1. Thus, the band at 2609 cm-1 appears to be a combination mode of the ν3′ and ν1 vibrations. Usually, the combination modes are observed at frequencies somewhat lower than the sum of both vibrations. Note that the intensity of the combination band is ca. 150 times lower than the intensity of the fundamental band at 1640 cm-1. However, this ratio is perhaps somewhat overestimated, since a water band at 1620 cm-1 probably participates in the absorption in this region. Indeed, it is well-known that formation of nitrates on oxide surfaces results in replacement of surface OH groups by NO3- with simultaneous formation of water.32 (30) Hadjiivanov, K.; Saussey, J.; Freysz, J.-L.; Lavalley, J.-C. Catal. Lett. 1998, 52, 103. (31) Bo¨rensen, C.; Kirchner, U.; Scheer, V.; Zelner, R. J. Phys. Chem. A 2000, 104, 5036. (32) Kantcheva, M.; Bushev, V.; Hadjiivanov, K. J. Chem. Soc., Faraday Trans. 1992, 88, 3087.

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Figure 4. FTIR spectra of NO and O2 coadsorbed on sulfated ZrO2: adsorption of NO, 2.4 kPa equilibrium pressure (a) and admission of 270 Pa O2 (b), time evolution of the spectrum (c), admission of 1.2 (d) and 2.4 kPa O2 (e), and after evacuation (f). The spectra are background corrected.

Another set of bands, at 1588, ca. 1280, and 1040 cm-1, also change in synchrony. These bands are assigned to bidentate nitrates. The respective (ν3′ + ν1) combination mode for these species is observed at 2584 cm-1. During evacuation, bands around 1530 and 1285 cm-1 disappear from the spectrum. This is evidently due to the removal of N2O3 from the sample surface. However, the probability for some nitro species to be characterized by bands around 1530 and 1285 cm-1 cannot be totally ruled out. The disappearance of the bands due to isolated surface OH groups indicates that surface NOx species are attached to the same Zr4+ cations where the OH groups have been (or are) bonded. The broad band at 3450 cm-1 is evidently due to H-bonded hydroxyls. However, there are two possibilities to explain the appearance of H-bonded OH groups. First, they could be affected by nearby species. Second, water could be formed as a result of a surface reaction as proposed for titania.32 We support the second possibility since some experiments suggest that part of the band around 1600 cm-1 arises from adsorbed water (see below). In contrast to the results obtained after NO adsorption, our results on NO + O2 coadsorption are in general agreement with the reported literature data concerning not only ZrO23 but ZrO2-TiO233 and other oxides:26 it is found that the most stable species produced are different kinds of surface nitrates. 3.5. Coadsorption of NO and O2 on Sulfated ZrO2. Here again, the changes in the IR spectrum of the sample are much more pronounced after NO + O2 coadsorption than after NO adsorption (Figure 4). Introduction of a small amount of oxygen (270 Pa initial equilibrium pressure) to the system NO-sulfated zirconia leads to gradual disappearance of the N2O bands and appearance of a strong band at 1925 cm-1. A set of bands at 1617, 1561, 1340, 1325, 1180, and 1116 cm-1 also appear and rise with time. Simultaneously, the sulfate band at 1380 cm-1 additionally decreases in intensity. In the region of overtones and combination modes, the N2O band also disappears and a broad band at 2490 cm-1 develops. A very weak band at 2600 cm-1 is also visible. In the OH stretching region, the OH bands at 3765 and 3665 cm-1 vanish and a broad band at ca. 3600 cm-1 develops instead. An increase in amount of the added oxygen (1.2 kPa initial equilibrium pressure) leads mainly to an increase (33) Kintaichi, Y.; Haneda, M.; Inaba, M.; Hamada, H. Catal. Lett. 1997, 48, 121.

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Figure 5. Stability of the surface NOx species on ZrO2: coadsorption of NO (4 kPa initial equilibrium pressure) and O2 (4 kPa initial equilibrium pressure), followed by 15 min evacuations at 293 (a), 373 (b), 473 (c), 573 (d), and 673 K (e). The spectra are background corrected.

Figure 6. Stability of the surface NOx species on sulfated ZrO2: coadsorption of NO (2.4 kPa initial equilibrium pressure) and O2 (2.4 kPa initial equilibrium pressure), followed by 15 min evacuations at 293 (a), 373 (b), 473 (c), 573 (d), and 673 K (e). The spectra are background corrected.

in intensity of the bands at 1617, 1561, and 1235 cm-1, whereas the band at 1180 cm-1 disappears. Simultaneously, the band at 2660 cm-1 becomes clearly visible. Additional increase of the amount of oxygen in the system (2.4 kPa initial equilibrium pressure) results in a drop in intensity of the band at 1925 cm-1 and its slight red shift. Bands at 2224 cm-1 (assigned to NO+) and at 1750 cm-1 (due to adsorbed N2O4) are also registered. In addition, broad features at ca. 1500 and 1300 cm-1 appear. The shifted OH band has an enhanced intensity and is now centered at 3450 cm-1. Evacuation results in the disappearance of all bands in the 2300-1700 cm-1 region. The broad features at 1500 and 1300 cm-1 are strongly reduced in intensity. The 1116 cm-1 band also becomes weaker. A small fraction of the OH groups at 3665 cm-1 is restored. By analogy with pure zirconia, we assign the bands at 1628 and 1585 cm-1 to nitrato species and the band at 1180 cm-1 to nitrites. The bands at 1628 and 1585 cm-1 characterize bridging and bidentate nitrates, respectively. The respective combination modes for the nitrates are detected at 2610 and 2600 cm-1. The band at 2490 cm-1 seems to be a combination mode of the nitrites. The set of bands at ca. 1500 and 1300 cm-1 change together with the band at 2505 cm-1. That is why we assign these bands to monodentate nitrates. These species are more typical of the sulfated sample and are probably favored by the existence of a sulfate anion coordinated to the same Zr4+ site. Their removal during evacuation can be explained by a surface reaction with NO+:

1530 and 1288 cm-1 almost disappear and the bands at 2612, 1642, and 1220 cm-1 rise in intensity. Evacuation at 573 K results in full disappearance of the bands at 1530 and 1288 cm-1. All the other bands decrease in intensity, whereas the band at 1566 cm-1 is the only one that has gained some intensity. Note that even at this evacuation temperature the bands at 1513 and 1280 cm-1 have kept a measurable intensity. At higher temperatures, only intensity decrease of the bands is observed and no bands are left in the region after 773 K evacuation. Upon 673 K evacuation, the spectrum of the Zr-OH groups at 3680 cm-1 is almost restored, whereas the hydroxyls characterized by a band at 3770 cm-1 reappear after 673 K evacuation. Note that generally the combination modes follow the changes of the nitrate bands in the 1650-1500 cm-1 region. A careful inspection of spectra c and d in Figure 5 shows that in this case the intensity of the combination mode does not follow the intensity of the band at ca. 1620 cm-1. This may be explained by evolution of water during the evacuation at 573 K. 3.7. Stability of the Surface Species on Sulfated ZrO2. Evacuation at 373 K leads to a slight alteration of the spectrum of nitrate-precovered sulfated zirconia (Figure 6, spectrum b). Mainly, two shoulders at ca. 1500 and 1300 cm-1 are removed from the spectrum. The intensity of the nitrate bands slightly decreases after evacuation up to 573 K, and they disappear from the spectrum after 673 K evacuation. Evacuations in the temperature interval of 373-573 K lead to gradual restoration of the 3662 cm-1 OH band. The band at 3765 cm-1 is not observed after 573 K evacuation but is almost completely restored after evacuation at 673 K. Thus, the results demonstrate that the species produced after NO + O2 coadsorption on ZrO2 and sulfated ZrO2 are similar. However, the sulfatation has decreased the stability of the surface nitrates.

NO3- + NO+ f N2O4 More detailed analysis of the lower-frequency region is hindered because of the superimposition of nitrate and shifted sulfate bands. 3.6. Stability of the Surface Species on ZrO2. It is of interest to determine the thermal stability of the surface nitrates and to establish how it is affected by the presence of surface sulfates. The spectra recorded after evacuation of the ZrO2 sample (preliminary covered with nitrates) at different temperatures are presented in Figure 5. Evacuation at 373 K leads to a very slight alteration of the spectrum (Figure 5, spectrum b). Only a negligible increase in intensity of the set of bands at 1530 and 1288 cm-1 and a negligible erosion of the bands at 1640, 1588, and 1567 cm-1 are detected. After evacuation at 473 K, the set of bands at

4. Conclusions At room temperature, NO forms no nitrosyl complexes with Zr4+ sites on a pure zirconia surface. However, the acidity of the Zr4+ ions is enhanced by the presence of nearby situated anions (nitrates, sulfates), and as a result Zr4+(A)-NO species (A ) sulfates, nitrates) are formed in the presence of NO. Coadsorption of NO and O2 on ZrO2 and sulfated ZrO2 results in formation of different anionic species: nitrites (that are easily oxidized to nitrates), nitro species (not produced on the sulfated sample), different kinds of

Adsorption of NOx on ZrO2 and Sulfated ZrO2

nitrates, NO+, N2O3, and N2O4 (the latter two are easily removed by evacuation). Analysis of the IR spectra in the region of the overtones and combination modes can be helpful with the assignment of the different bands due to anionic nitrogen-oxo species. The most stable species produced after NO and O2 coadsorption on ZrO2 and sulfated ZrO2 are different

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nitrates. These species are of a lower stability on the sulfated sample. Acknowledgment. This work was supported by EC (Project ENVA A-CT97-0633). K.H. is indebted to the Alexander-von-Humboldt Foundation. LA0110895