Langmuir 1999, 15, 5733-5741
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Surface Heterogeneity of Zirconia-Supported V2O5 Catalysts. The Link between Structure and Catalytic Properties in Oxidative Dehydrogenation of Propane† A. Adamski, Z. Sojka, and K. Dyrek*,‡ Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Cracow, Poland
M. Che§ Laboratoire de Re´ activite´ de Surface, UMR-7609 CNRS, Universite´ Pierre et Marie Curie, 4 place Jussieu, Tour 54-55, 75252 Paris Cedex 05, France
G. Wendt and S. Albrecht Universita¨ t Leipzig, Fakulta¨ t fu¨ r Chemie und Mineralogie, Institut fu¨ r Technische Chemie, Linne´ strasse 3, 04103 Leipzig, Germany Received October 13, 1998. In Final Form: March 18, 1999 Undoped and alkali-doped zirconia-supported vanadia catalysts for the oxidative dehydrogenation of propane were prepared by impregnation and characterized by various techniques. The chemical state of surface and bulk vanadium was investigated as a function of the calcination temperature, V2O5 loading, and the nature and content of alkali-metal additives. It is found that on the surface vanadium is present in the form of isolated vanadyl species or oligomeric vanadates, or as nanocrystalline V2O5 and that V5+ and V4+ ions coexist in octahedral and tetrahedral coordination, while within the bulk of zirconia matrix, V4+ ions are stabilized in a VxZr1-xO2 solid solution. Presence of the alkali-metal additives and water changes the dispersion of surface vanadium species favoring, in both cases, formation of mononuclear vanadyl surface complexes. Surface heterogeneity plays a vital role for the catalytic performance of V2O5/ ZrO2 catalysts in oxidative dehydrogenation of propane. Catalytic properties are related to the nature of VOx surface species and correlates well with their reducibility. The maximum of catalytic activity was observed for catalysts with vanadia content between 3 and 5 mol %, for which octahedral polyvanadate surface species are dominant. It is proposed that the catalytic activity is affected by the nucleophilicity of bridging oxygen in V-O-V entities, modified by the adjacent alkali cations.
Introduction Vanadia-based catalysts are widely used in many industrial oxidation processes.1-5 In particular, selective oxidative dehydrogenation of propane (ODP) and other light alkanes to the corresponding alkenes is an attractive subject because of its importance as a source of cheap feedstocks for polymerization or production of acrylates.6 Supported vanadia-containing catalysts are complex oxide systems showing interesting surface and catalytic † Presented at the Third International Symposium on the Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland, August 9-16, 1998. * To whom correspondence should be addressed. Phone: (48-12) 633 63 77, ext. 2244. Fax: (48-12) 634 05 15. E-mail: dyrek@ trurl.ch.uj.edu.pl. ‡ Regional Laboratory of Physicochemical Analyses and Structural Research, Ingardena 3, 30-060 Cracow, Poland. § Institut Universitaire de France.
(1) Spencer, N. D.; Pereira, C. J. J. Catal. 1989, 116, 399. (2) Parmaliana, A.; Arena, F.; Frusteri, F.; Miceli, D.; Sokolovskii, V. Catal. Today 1995, 24, 231. (3) Fierro, J. L. G.; Arrua, L. A.; Lo´pez-Nieto, J. M.; Kremenic, G. Appl. Catal. 1988, 37, 323. (4) Grabowski, R.; Grzybowska, B.; Samson, K.; Słoczyn´ski, J.; Stoch, J.; Wcisło, K. Appl. Catal. A 1995, 125, 129. (5) Corma, A.; Lo´pez-Nieto, J. M.; Paredes, N.; Dejoz, A.; Vazquez, I. Stud. Surf. Sci. Catal. 1994, 82, 113. (6) Grabowski, R.; Grzybowska, B.; Wcisło K. Pol. J. Chem. 1994, 68, 1803. (7) Corma, A..; Lo´pez-Nieto, J. M.; Paredes N. J. Catal. 1993, 144, 425.
properties that depend on a variety of factors. The most important include preparation method, thermal treatment,7-9 V2O5 loading,3,10 presence of alkali-metal additives,11-13 the nature of support, and its interaction with the deposited vanadia.14-17 On the other hand, catalytic reaction products such as water can considerably affect the properties of supported vanadia catalysts.18,19 All these factors play an essential role in the formation and stability of the heterogeneous VOx surface species and are pivotal for the catalytic activity. A series of undoped and Li-, Na-, and K-doped zirconiasupported V2O5 catalysts was prepared by a wet impreg(8) Saleh, R. Y.; Wachs, I. E.; Chan, S. S.; Chersich, C. C. J. Catal. 1986, 98, 102. (9) Rohan, D.; Hodnett, B. K. Appl. Catal. A 1997, 151, 409. (10) Went, G. T.; Leu, L.-J.; Rosin, R. R.; Bell, A. T. J. Catal. 1992, 134, 492. (11) Tanaka, T.; Nishimura, Y.; Kawasaki, S.-I.; Ooe, M.; Funabiki, T.; Yoshida, S. J. Catal. 1989, 118, 327. (12) Malet, P.; Mun˜oz-Pa´ez, A.; Martı´n, C.; Rives, V. J. Catal. 1992, 134, 47. (13) Kung, M. C.; Kung, H. H. J. Catal. 1992, 134, 668. (14) Scharf, U.; Schraml-Marth, M.; Wokaun, A.; Baiker A. J. Chem. Soc., Faraday Trans. 1991, 87 (19), 3299. (15) Deo, G.; Wachs, I. E. J. Catal. 1994, 146, 323. (16) Akimoto, M.; Usami, M.; Echigoya, E. Bull. Chem. Soc. Jpn. 1978, 51, 2195. (17) Deo, G.; Wachs, I. E.; Haber, J. Crit. Rev. Surf. Chem. 1994, 4, 141. (18) Schraml-Marth, M.; Wokaun, A.; Pohl, M.; Krauss, H.-L. J. Chem. Soc., Faraday Trans. 1991, 87, 2635. (19) Topsøe, N.-Y.; Slabiak, T.; Clausen, B. S.; Srnak, T. Z.; Dumesic, J. A. J. Catal. 1992, 134, 742.
10.1021/la981431m CCC: $18.00 © 1999 American Chemical Society Published on Web 05/07/1999
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Figure 1. Effect of the alkali-metal dopant on propane conversion over 3V/Zr catalysts. Light-gray bars correspond to alkali metal-to-vanadium ratio A/V ) 0.05 and dark-gray bars to A/V ) 0.5; UD stands for undoped sample (white bar).
nation method and examined by XRD (X-ray diffraction), TPR (temperature-programmed reduction), RS (Raman spectroscopy), EPR (electron paramagnetic resonance), and UV/vis techniques. The samples were tested in an ODP reaction and the complete results were published earlier.20,21 Solid-state properties of V2O5/ZrO2 catalysts were the subject of our previous XRD and EPR studies that focused on the phase transitions in a ZrO2 matrix occurring during calcination and their influence on the incorporation of the V4+ ions into the bulk of zirconia.22 Although the role of surface heterogeneity of supported transition metals in catalytic oxidation of various hydrocarbons was addressed in several papers,8,17,23-25 the influence of size and structure of the oxometallic species on the partial oxidation of propane still remains an open question. This paper is devoted to catalytic, TPR, and spectroscopic (mainly EPR and UV/vis) studies aiming to rationalize the intimate relationship between the surface structure of undoped and alkali-metal-doped V2O5/ZrO2 catalysts and their reactivity in ODP. In particular, the impact of temperature, loading, and alkali-metal doping on the molecular architecture of surface vanadium species is investigated. Experimental Section Materials. ZrO2 support was prepared by precipitation from aqueous solution of ZrO(NO3)2 (Aldrich 99.99%) with ammonia at room temperature. V2O5/ZrO2 catalysts containing 1, 3, 5, 10, 20, and 40 mol % V2O5 were prepared by the impregnation of ZrO2 with an aqueous solution of NH4VO3 (Merck) and by further firing in air at 500-700 °C for 6 h. The 3 mol % V2O5/ZrO2 samples containing Li-, Na-, and K-dopants with an atomic ratio of A/V (A ) alkali metal) equal to 0.05, 0.1, 0.3, and 0.5 for each alkali metal were prepared in an analogous way. In this case, along with NH4VO3, appropriate alkali nitrates were added in the mixture. In model experiments where the migration of V4+ into ZrO2 was studied, a sample of precursor containing 1 mol % V2O5 was calcined stepwise up to 1200 °C, and every 100 °C the EPR spectrum was recorded. The samples are labeled as follows: the value of the atomic ratio A/V is given in parentheses and precedes the alkali-metal symbol; next, the value of V2O5 loading and the V/Zr description are added e.g., (0.3)K-3V/Zr. Further details of the preparation are given elsewhere.20,21
Techniques. Temperature-programmed reduction (TPR) measurements were performed in a flow system using a TCD detector and a gas mixture containing 8 vol % of hydrogen in argon. The heating rate was equal to 10 °C/min and the flow rate adjusted to 3 dm3/h. For calibration CuO was used as a reference. Catalytic tests were performed under ambient pressure in a fixed-bed flow reactor in the temperature range from 400 to 500 °C. The reaction mixture contained propane in air and nitrogen with a volume ratio of 1:9:10. A gas space velocity from 4 to 20 dm3/h was used. The principal products detected by on-line gas chromatographic analysis (Hewlett-Packard HP 5890) were C3H6 and COx. The typical weight of the sample was about 0.05 g. More details on the TPR and catalytic measurements are given in previous papers.20,21 EPR (X- and Q-band) spectra were recorded at liquid nitrogen temperature (77 K) on a Bruker ESP 330 spectrometer with 100 kHz field modulation. EPR parameters were determined by computer simulation using the SIM14A program.26 For quantitative EPR measurements VOSO4 dispersed in K2SO4 was used as a standard. It shows a single EPR line with giso ) 1.977, which is typical for VO2+. UV/Vis reflectance spectra were recorded at room temperature in the range of 200-800 nm (12500-50000 cm-1) on a CARY 5E Varian spectrophotometer (equipped with a deuterium and a tungsten radiation source operating in the 190-310 and 310800 nm range, respectively). The samples were exposed to air during the measurements. Decomposition of the spectra was performed using the Spectra Calc program.
Results and Discussion Catalytic Test Studies. In the presence of oxygen, propane is selectively oxidized to propene (target product) and to COx over bare and alkali-metal-doped V2O5/ZrO2 catalysts. All samples showed distinct catalytic activity that was clearly dependent on their composition, while (20) Albrecht, S.; Wendt, G.; Lippold, G.; Adamski, A.; Dyrek, K. Solid State Ionics 1997, 101-103, 909. (21) Albrecht, S. Ph.D. Thesis, Universita¨t Leipzig, Leipzig, Germany, 1998. (22) Adamski, A.; Sojka, Z.; Dyrek, K.; Che, M. Solid State Ionics 1999, 117, 113. (23) Lischke, G.; Hanke, W.; Jerschkewitz, H.-G.; O ¨ hlmann, G. J. Catal. 1985, 91, 54. (24) Baiker, A.; Monti, D. J. Catal. 1985, 91, 369. (25) Hanke, W.; Heise, K.; Jerschkewitz, H.-G.; Lischke, G.; O ¨ hlmann, G.; Parlitz, B. Z. Anorg. Allg. Chem. 1978, 438, 176. (26) Lozos, G. P.; Hoffman, B. M.; Franz, Ch. G. Sim14A Program; Chemistry Department, Northwestern University: Illinois; QCPE No. 265.
Surface Heterogeneity of V2O5/ZrO2 Catalysts
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Figure 2. TPR profiles of massive vanadia and undoped V/Zr catalysts of various V2O5 loading (40, 20, 10, 5, 3, and 1 mol %).
the parent V2O5 and ZrO2 oxides were almost inactive. In the case of the alkali-free samples, the highest activity (14-16% of propane conversion) was found for V2O5 loading between 3 and 5 mol %. Moreover, with the increasing calcination temperature of the samples the catalytic activity decreases.21 On the basis of our earlier EPR and XRD investigations this effect was explained to be mainly due to thermally induced migration of V4+ ions toward the bulk of zirconia with the formation of VxZr1-xO2 solid solution. This leads to depletion of the active vanadium species from the catalyst surface.22 As shown in Figure 1, for alkali-doped 3V/Zr samples propane conversion decreases in the following sequence: undoped (UD) > Li > Na > K. A similar effect was observed previously for titania- and silica-supported V2O5.4,5,27 The sequence remains the same whatever the alkali-metal concentration in the catalysts may be, but the higher the A/V atomic ratio, the lower the catalytic activity observed. For example, in the case of the potassium-doped sample (K/V ) 0.5) the activity is 25 times lower in comparison to that found for the alkali-free catalyst. TPR Results and Reducibility. Reduction of the V2O5/ ZrO2 catalysts is a multistep process and obviously depends on the vanadia loading. As shown in Figure 2, in the temperature range of 650-860 °C, the TPR profile of the parent V2O5 exhibits four peaks. The first two correspond to the following consecutive reduction steps V2O5 f 1/3V3O16 f 2VO2, whereas the last ones all together account for a 2VO2 f V2O3 transformation in which V6O11 is a possible intermediate.28,29 Some of the peaks (at 660, 686, and 854 °C) appear also for the samples with the vanadia loading above 5 mol %, but at the positions shifted to the lower temperatures. Zirconia-supported samples, (27) Milisavlevich, B. S.; Ivanov, A. A.; Polyakova, G. M.; Serzhantova, V. V. Kinet. Katal. 1975, 16, 103. (28) Bosch, H.; Kip, B. J.; van Ommen, J. G.; Gellings, P. J. J. Chem. Soc., Faraday Trans. 1 1984, 80, 2479. (29) Bosch, H.; Sinot, P. J. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1425.
Figure 3. TPR profiles for undoped (UD) and alkali metal containing 3V/Zr samples with an A/V ratio equal to 0.05 and 0.5 (A ) Li, Na, or K).
containing more than 1 mol % V2O5, are characterized by a new peak appearing already at 460-490 °C. This peak occurs at the lowest temperature for a 3V/Zr sample, in which, as will be described later, vanadium exists mainly in the form of surface oligospecies with the octahedral coordination. In the case of 1V/Zr the dominant peak is shifted to higher temperatures and occurs at 526 °C. Thus, zirconia interacting with the supported vanadia modifies its structure significantly and leads to the formation of surface species more easily reducible than the parent V2O5. Furthermore, vanadium species corresponding to the lowest coverage are harder to reduce than those at the intermediate V2O5 loadings. For all samples containing less than 10 mol % V2O5 the calculated reduction degree (defined as a change of the average oxidation number from V5+ to V3+) was in the range of 0.7-0.9. The results of the temperature-programmed reduction of undoped 3V/Zr were similar to those obtained for (0.05)A-3V/Zr catalysts (Figure 3). Apparently, small amounts of alkali additives do not affect the reducibility of vanadium surface species to a considerable extent. However, the situation changes dramatically with the increasing A/V ratio, since a systematic shift of the reduction peak is observed for (0.5)A-3V/Zr samples
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Figure 4. UV/vis diffuse reflectance spectra and the corresponding decomposition of the bands for V/Zr catalysts of various V2O5 loadings (1, 3, 5, and 10 mol %). Absorption is expressed in arbitrary units.
(Figure 3). In this case, when compared to the undoped samples, the vanadium surface species are more hardly reducible and this process clearly depends on the nature of the alkali dopant. The reduction temperatures for the (0.5)A-V/Zr catalyst increase from 460 to 527 °C in the following sequence: undoped (UD) < Li- < Na- < K-doped samples. Since the catalytic activity in propane conversion (Figure 1) also decreases in the same order, it suggests that there is a correlation between the reducibility of surface vanadium species and their catalytic properties. This remains in agreement with earlier results of Deo et al.15,17 who concluded that the reducibility parallels the reactivity in methanol oxidation over supported vanadium oxide catalysts. It is worth noting that the main TPR peak for alkalicontaining (0.5)K-3V/Zr catalysts (Figure 3) occurs at practically the same temperature (527 °C) as that for the undoped 1V/Zr catalyst (Figure 2), suggesting a similar structure of vanadium species in both cases, despite the loadings being different. This point will be discussed in more detail later on. Although TPR is not as sensitive as spectroscopic methods used for the characterization of supported vanadium oxide catalysts, it can provide some crude information about the surface species. As can be inferred from the experimental results, large agglomerates are reduced faster and at lower temperatures. The sequence VdO < V-O(-V)2 < V-O-V of increasing reducibility proposed by Deo et al.30 is fully confirmed also in our case. The TPR peak shifts from 526 °C for isolated vanadyl species (1V/Zr) to 460-490 °C for higher vanadia loading (3-5V/Zr), where oligomers exhibiting V-O-V entities prevail. Identification of Oxovanadium Surface Species by UV/Vis and EPR Spectroscopy. UV/Vis Results. The UV/Vis diffuse reflectance spectra recorded for catalysts of various V2O5 loadings exhibit strong absorp(30) Deo, G.; Wachs, I. E. J. Catal. 1991, 129, 307.
tion between 17000 and 50000 cm-1 (Figure 4). The highest energy bands above 40000 cm-1 are due to the zirconia matrix, while the bands below this range can be attributed to the O2-(2p) f V5+(3d) charge-transfer transitions (CT).31 The charge-transfer energy is strongly influenced by the coordination of vanadium and the cluster size.18,23,31 An apparent absence of the absorption bands due to d-d transitions in the range of 12500-17000 cm-1 (Figure 4 insert) points out that the amount of V4+ in the investigated samples is too small32,33 to be revealed by UV/vis spectroscopy. This observation is well-supported by quantitative EPR measurements, which indicated that the amount of EPR-visible V4+ ions is smaller than 10%. Thus, both techniques, UV/vis and EPR, are complementary for these studies, while the former can probe only pentavalent vanadium species present on the surface of the catalysts; the latter reveals the presence of V4+ ions exclusively. Since the coordination of vanadium depends inter alia on its valence state and may change upon reduction,34 coordinations deduced from EPR and UV/vis may not be necessarily the same as they refer to vanadium in different oxidation states. Analysis of the absorption bands presented in Figure 4 shows that a correlation between the vanadia loading and the band-edge position exists. The band edge is redshifted with the increasing V2O5 content, which can be related to the enlarged size of vanadium clusters. This effect was earlier observed for supported molybdenum oxides and was explained by the relationship between the cluster size and the band gap energy.35 Indeed, as in the case of a particle-in-the-box model where the separation between energy levels decreases with increasing box size, (31) Rasch, G.; Bo¨gel, H.; Rein, C. Z. Phys. Chem. (Leipzig) 1978, 259, 955. (32) Inomata, M.; Mori, K.; Miyamoto, A.; Ui, T.; Murakami, Y. J. Phys. Chem. 1983, 87, 754. (33) Mukhlenov, I. P.; Pak, V. N.; Shvedova, I. V.; Dobkina, E. I. Kinet. Katal. 1978, 19, 259. (34) Iwasawa, Y. Adv. Catal. 1987, 35, 187.
Surface Heterogeneity of V2O5/ZrO2 Catalysts
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Table 1. Average Positions of the UV/Vis Bands Obtained from Decomposition of the Spectra of Undoped V/Zr Catalysts and Their Assignment to the Corresponding Surface Vanadium Species sample νj/cm-1 νj/cm-1 νj/cm-1 νj/cm-1 νj/cm-1 νj/cm-1
1V/Zr
30300 ( 1145 38000 ( 390
3V/Zr
5V/Zr
23000 ( 250
23000 ( 435
36000 ( 330
35200 ( 265
the band gap energy decreases as the cluster size increases.35 As shown in Figure 4 the UV/vis spectra are composed of 3-4 bands overlapped in the region of 17000-40000 cm-1. Their average positions are given in Table 1 and the assignments proposed in the last column are based on the scrutinized literature about surface vanadia species and the table published by Baiker et al.14 Because of the broad and overlapping character of the UV/vis lines, there is a certain degree of uncertainty in the band positions corresponding to particular species. This should not, however, affect the correctness of the proposed attribution in a significant way, as their assignment is corroborated by parallel RS studies.36 The small differences in the observed band positions when compared to those from earlier studies on the same system reflect in our opinion specific deviations that naturally occur within the particular structure of surface vanadium at the same loading, and most probably reflect various preparation conditions. The spectra of 3V/Zr and 5V/Zr are quite similar and both are composed of two broad bands centered around 23000 and 35000 cm-1, ascribed to vanadium in octa- and tetrahedral coordinations, respectively.37 Octahedral vanadium exists in the form of oligomeric surface species similar to the decavanadate ion (V10O286-) that exhibits a CT band at 24000 cm-1.11,14 Vanadium in tetrahedral coordination can be found in small islandlike clusters containing VO43- units, as was earlier observed for titania- and silica-supported vanadia.14,38 These observations confirm the dominance of the structures containing V-O-V entities when the vanadia content is higher than 3 mol %. The shift of the average peak position from 36000 to 34000 cm-1 with the increasing V2O5 loading can then be explained by a growth of the cluster size. In the case of lower V2O5 contents, pentavalent vanadium is tetrahedrally coordinated forming metavanadate-like chains (band around 30300 cm-1) or small oxovanadium clusters, occurring between decavanadate and completely isolated species (typical for vanadia loading lower than 1 wt %). They represent intermediate states of vanadium agglomeration.14,23,33 At 10 mol % of the V2O5 content, pentacoordinated species appear, giving rise to strong absorption around 26300 cm-1. This band corresponds to vanadium in square pyramidal coordination present in dehydrated V2O5 layers. Depending on the lateral extent, these structures form patches, ribbons, or two-dimensional layers.14,33,38 Such species coexist with small clusters absorbing at 34000 cm-1 (mentioned above) and V2O5 nanocrystals for which the low-energy band about 21500 cm-1 is characteristic.14,33 EPR Results. The EPR technique provides detailed information about paramagnetic surface and bulk V4+(3d1) ions and Zr3+(4d1) matrix defects. It can also be a (35) Weber, R. S. J. Catal. 1995, 151, 470. (36) Adamski, A. Ph.D. Thesis, Jagiellonian University, Cracow, Poland, 1998. (37) Busca, G.; Centi, G.; Marchetti, L.; Trifiro´, F. Langmuir 1986, 2, 568. (38) Hanke, W.; Bienert, R.; Jerschkewitz, H.-G. Z. Anorg. Allg. Chem. 1975, 414, 109.
10V/Zr
structure and type of species
21500 ( 280
V2O5-like multilayers octahedral oligomers square pyramidal species tetrahedral metavanadate-like chains tetrahedral small clusters tetrahedral small clusters
26300 ( 390 34000 ( 220
sensible tool to characterize catalytically active mixedvalence centers. The origin of V4+ in oxide-supported V2O5 systems still remains an open question. It is well-known that during thermal treatment polycrystalline vanadia partially dissociates, loosing oxygen. This leads to the formation of V4+ defects responsible inter alia for semiconductive properties of V2O5.39-41 Some authors suggest that upon calcination at elevated temperatures a specific reaction between dispersed V5+ ions and the matrix occurs, resulting in the formation of V4+, even in the presence of air.42,43 However, following other papers, nearly all vanadium deposited onto the support is fully oxidized and V4+ ions appear during the catalytic reaction only.44 In the Q-band EPR spectrum of 1V/Zr recorded at 77 K, two well-separated signals can be distinguished. A broader one with hyperfine structure (hfs) due to the interaction of the unpaired electron with the 51V nucleus (I ) 7/2, 99.76% of natural abundance) was attributed to V4+ species while a weaker narrow signal around gav ) 1.97 without any resolved hfs was assigned to Zr3+ ions.22 For the vanadia content 3-10 mol %, the EPR spectra are similar and a typical spectrum is shown in Figure 5. All of them are complex in nature and can be interpreted as a superposition of several signals attributed to paramagnetic centers shown schematically in Figure 6. Thus, V4+ can be present in the form of isolated vanadyl groups not interacting with other paramagnetic species or they can be associated with V-O-V bridges of surface vanadates or nanocrystalline V2O5. In our case no EPR signal due to V4+-O-V4+ species with magnetically interacting V4+ ions was observed. Taking into account mere abundance of V4+ in the samples, this is consistent with the existence of mixed-valence V4+-O-V5+ groups at the higher V2O5 loadings, since the probability of having two adjacent V4+ ions is very low. We have shown that during calcination above 735 °C the V4+ ions migrate toward the ZrO2 matrix and become magnetically isolated by forming a VxZr1-xO2 solid solution.22 The invoked centers add to the shape and intensity of the EPR spectra of the particular samples and their specific contributions depend on the catalyst composition (vanadium loading, alkali-metal doping) and on the calcination temperature. While the composition is responsible mainly for the surface heterogeneity, the calcination temperature determines the ratio between surface and bulk paramagnetic vanadium. Four generic components, A, B, C, and D, can be distinguished by computer simulation of the spectra recorded for samples with various V2O5 and alkali(39) Serwicka, E. M. Z. Phys. Chem. Neue Folge 1990, 167, 87. (40) Dyrek, K. Bull. Acad. Pol. Sci. Ser. Sci. Chim. 1974, 22, 605. (41) Dyrek, K.; Serwicka, E. Bull. Acad. Pol. Sci. Ser. Sci. Chim. 1976, 24, 639. (42) Centi, G.; Giamello, E.; Pinelli, D.; Trifiro´, F. J. Catal. 1991, 130, 220. (43) Balikdjian, J. P.; Davidson, A.; Tatiboue¨t, J. M.; Che, M.; Delamar, M. Submitted for publication. (44) Wachs, I. E.; Saleh, R.; Chan, S.; Cherich, C. Appl. Catal. 1985, 15, 339.
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Figure 5. X-band EPR spectrum of 10V/Zr recorded at 77 K and its computer simulation. The signal consists of four components (Figure 7), the average parameters of which are given in Table 2.
Figure 6. Schematic picture of generic surface and bulk paramagnetic V4+ species revealed in zirconia-supported V2O5.
the very close spectroscopic properties, those ligands cannot be distinguished in normal CW-EPR investigations.48 Tetragonally distorted octahedral coordination is also a plausible structure, which is consistent with the observed parameters. However, as the oxygen ligand trans to the vanadyl bond is weakly coordinated, only slightly modifying the unpaired electron distribution, such an octahedron can be considered as a perturbation of the square pyramidal structure.49 The distortion of the symmetry can be characterized by parameter B ) ∆g||/∆g⊥ (where ∆g|| ) g|| - 2.0023 and ∆g⊥ ) g⊥ - 2.0023) proposed by Sharma et al.46 In our case the value of B = 3.0 indicates considerable tetragonal distortion due to the shortening of the VdO bond, which is accompanied by the corresponding release of the metal-ligand distance in the equatorial plane. For comparison, in the case of V2O5 single crystals where the sixth oxygen ligand is at a distance 2.78 Å from the vanadium center, B ) 4.9 (as calculated from the parameters given in ref 45). The relatively high values of A|| observed for the investigated samples are characteristic of high ionicity of the V-O bonds. Assuming that the principal components of the 51V hfs tensor can be expressed in terms of the approximate equations50 3 A| ) P[- 4/7β/2 2 - Keff + ∆g| + /7∆g⊥]
Figure 7. Principal components of the EPR spectra obtained by computer simulation attributed to (A) isolated V4+ in the surface vanadyl species, (B) V4+ in surface oligomers or in V2O5 microcrystalls, (C) V4+ isolated in the bulk VxZr1-xO2 solid solution, and (D) Zr3+ defects in bulk zirconia.
metal contents (Figure 7). Their average EPR parameters are given in Table 2. An axial signal A (g|| ) 1.923; g⊥ ) 1.976) with the wellresolved eight-line hfs is characteristic of the isolated vanadyl ion in a square pyramidal C4v symmetry (g⊥ > g|| and A|| > A⊥) and the ground state of the unpaired electron described mainly by a 3dxy orbital.45,46 The ligands constituting the inner sphere of the V4+ ions can be either surface oxygens of the ZrO2 matrix, terminal hydroxyl groups, or coordinated water molecules.45,47 Because of (45) Davidson, A.; Che, M. J. Phys. Chem. 1992, 96, 9909. (46) Sharma, V. K.; Wokaun, A.; Baiker, A. J. Phys. Chem. 1986, 90, 2715. (47) Bahranowski, K.; Łabanowska, M.; Serwicka, E. M. Appl. Magn. Res. 1996, 10, 477.
11 /14∆g⊥] A⊥ ) P[2/7β/2 2 - Keff +
where Keff is the Fermi contact term and P ) 184.5 G46 for the free V4+ ion, the value of the coefficient β*2 2 can be assessed. It reflects the fraction of the unpaired electron on the dxy orbital of the V4+ ion and provides a direct measure for the in-plane V-O bond ionicity:46 7 5 β/2 2 ) /6∆g| - /12∆g⊥ -
7 A | - A⊥ 6 P
Calculated values of β*2 2 range from 0.64 for 10V/Zr to 0.75 for (0.05)K-3V/Zr and show increasing ionicity of the V-O bond in the presence of alkali-metal ions located close to the V4+ species. (48) Dyrek, K.; Che, M. Chem. Rev. 1997, 97, 305. (49) Boucher, L. J.; Tynan, E. C.; Yen, T. F. In Electron Spin Resonance of Metal Complexes; Yen, T. F., Ed.; Plenum Press: New York, 1969; p 111. (50) Kivelson, D.; Lee, S. K. J. Chem. Phys. 1964, 41, 1896.
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Table 2. EPR Parameters of the A, B, C, and D Signals (Presented in Figure 7) and Their Assignment to the Corresponding Paramagnetic Species Shown in Figure 6 signal
g|
g⊥
gav
A|/G
A⊥/G
Aav/G
type of species
A B D
1.923 ( 0.002 1.947 ( 0.003 1.980 ( 0.002
1.976 ( 0.001 1.974 ( 0.002 1.969 ( 0.003
1.958 ( 0.002 1.965 ( 0.003 1.973 ( 0.002
185 ( 3.0 hfs not resolved hfs not observed
64 ( 1.0
105 ( 1.0
isolated surface V4+ interacting V4+ Zr3+ ions
signal
g1
g2
g3
gav
A1/G
A2/G
A3/G
Aav/G
type of species
C
1.889 ( 0.013
1.977 ( 0.004
1.942 ( 0.011
1.936 ( 0.009
159 ( 10.0
67 ( 4.0
15 ( 1.0
80 ( 5.0
isolated bulk V4+
At high vanadium contents, when VOx oligomers or V2O5 nanocrystals prevail on the surface, magnetic interactions lead to pronounced line broadening and the hfs practically disappears.43,51 This gives rise to the appearance of the broad, structureless signal B (∆Hpp ) 100-500 G), which is dominant (∼65%) in all calcined samples containing more than 3 mol % V2O5. This signal, centered at gav ) 1.96, is slightly anisotropic and very similar to that observed for partially reduced vanadia in polycrystalline39,52 and single-crystal form.52 A well-resolved hyperfine structure characteristic of isolated V4+ ions appears also in the case of signal C. However, its shape and parameters are very different from that found for signal A (Table 2). The values gav ) 1.936 and Aav ) 80.5 G are consistent with the V4+ ion in a distorted octahedral symmetry without vanadyl bonding.45 This suggests that such ions are located within the lattice of the ZrO2. As already mentioned, during the calcination a considerable amount of V4+ ions migrate toward the zirconia matrix and substitute for Zr4+ in the cationic sublattice to form the VxZr1-xO2 solid solution. Similar behavior was observed previously for vanadia deposited on TiO2, SnO2, and GeO2.53-55 XRD measurements of the 3V/Zr sample revealed that practically all vanadium is incorporated into the matrix as V4+ after heating in air at 1000 °C for 0.5 h.22 Contribution of signal C to the overall EPR spectrum of the catalysts studied varies from ∼3% for 3V/Zr to ∼25% for 10V/Zr. As mentioned above, Zr3+ ions are responsible for the weak, narrow signal labeled D (Figure 7)56,57 The intensity of this signal is generally low in all samples calcined below the threshold temperatures of vanadium migration, where the broad signal B dominates. The EPR results indicate that at higher V2O5 contents when the magnetically interacting species are preponderant, V4+ ions occur mainly within the mixed-valence V4+-O-V5+ bridges, whereas at low vanadium loadings, highly dispersed monomeric V4+ species dominate. Furthermore, this technique evaluates also the role of thermal treatment in determining the ratio between surface and bulk V4+.22 Summarizing, the UV/vis and EPR results confirm the general trend (e.g., refs 10 and 58) that with increasing the V2O5 content progressive agglomeration of the monomeric vanadium surface species occurs, leading, at intermediate loading, to the formation of larger VOx clusters and above the monolayer coverage to the appearance of nanocrystalline V2O5. The agglomeration (51) Gatteschi, D.; Tsukerblatt, B.; Barra, A. L.; Brunel, L. C.; Mu¨ller, A.; Do¨ring, J. Inorg. Chem. 1993, 32, 2114. (52) Kera, Y.; Matsukaze, Y. J. Phys. Chem. 1986, 90, 5752. (53) Davidson, A.; Morin, B.; Che, M. Colloids Surf. A 1993, 72, 245. (54) Gallay, R.; van der Klink, J. J.; Moser, J. Phys. Rev. B 1986, 34, 3060. (55) Michel-Calendini, F. M.; Fichelle, G. Phys. Status Solidi B 1975, 69, 607. (56) Torralvo, M. J.; Alario, M. A.; Soria, J. J. Catal. 1984, 86, 473. (57) Morterra, C.; Giamello, E.; Orio, L.; Volante, M. J. Phys. Chem. 1990, 94, 3111. (58) Topsøe, N.-Y.; Topsøe, H.; Dumesic, J. A. J. Catal. 1995, 151, 226.
Figure 8. The X-band EPR spectra of non-calcined 3V/Zr catalysts with alkali-metal loading A/V ) 0.3, showing the influence of dopants on the resolution of the hyperfine structure of isolated V4+ ions. A better resolution of the hyperfine structure indicates better dispersion of surface vanadium.
process of vanadium is the main cause of the enhanced heterogeneity of the surface oxospecies. On the basis of the EPR data, V4+-containing species are penta- or octa-coordinated, while UV/vis spectra imply that tetrahedral coordination can be also ascribed to V5+ centers; thus, depending on the oxidation state, different coordinations can exist on the ZrO2 surface. Influence of Alkali-Metal Dopants on the State of Surface Vanadium Species. In Figure 8 the EPR spectrum of undoped 3V/Zr is compared to the spectra of (0.3)A-3V/Zr samples (A ) Li, Na, or K). Computer simulation revealed that for the undoped catalyst signal D, because of the Zr3+ ions, excels the spectrum, while the signal of the V4+ ions is hardly observed. Contrary to this, for all alkali-metal-containing samples, the signal with the resolved hyperfine structure, characteristic of isolated V4+ species (signal A), was present. Development of the hfs is apparently related to the kind of alkali dopant. The efficiency of potassium as a vanadium-dispersing agent is the highest, as it can be inferred from the appearance of a superb hyperfine structure, in the case of the (0.3)K-3V/Zr catalyst. Since the samples were not calcined, the tetravalent vanadium ions are then located on the catalyst surface. Thus, alkali-metal additives play an important role in improving the dispersion of surface
5740 Langmuir, Vol. 15, No. 18, 1999
Adamski et al.
Table 3. Average Positions of the UV/Vis Bands Obtained from Decomposition of the Spectra of Alkali-Doped A-V/Zr Catalysts A/V ) 0.05
A/V ) 0.5
sample
Li-3V/Zr
Na-3V/Zr
K-3V/Zr
Li-3V/Zr
Na-3V/Zr
K-3V/Zr
νj/cm-1 νj/cm-1 νj/cm-1 νj/cm-1
23000 ( 640
24000 ( 650 (30400 ( 400) 35500 ( 265
23500 ( 625 31000 ( 590
24000 ( 755 29800 ( 380
22500 ( 390 29000 ( 425
23000 ( 515 31000 ( 425
36500 ( 365
38000 ( 410
37000 ( 600
38000 ( 450
34500 ( 280
Table 4. Comparison of EPR Parameters for Spectra Recorded at 77 K for Calcined (0.3)Na-3V/Zr and 1V/Zr Samples after Adsorption of Water at 261 HPa (at 66 °C) (0.3)Na-3V/Zr g| 1.924 1.946 1.977
g⊥ 1.980 1.976 1.967
A|/G 186
1V/Zr after H2O adsorption A⊥/G 64
center V4+
isolated surface V4+ surface oligomers Zr3+ ions
vanadium oxide. This effect strongly depends on the size of the alkali metal and indicates a geometric influence of the dopants on the state of V2O5/ZrO2 catalysts. The sequence of the increasing dispersion of vanadium (undoped < Li- < Na- < K-doped) correlates with the sequence of decreasing propane conversion (Figure 1). In the presence of potassium or sodium surface vanadium species are strongly dispersed and their catalytic activities are distinctly lower. Formation of the isolated vanadium species at low coordinations is confirmed by UV/vis investigations. The results obtained from decomposition of the spectra of the catalysts with low (A/V ) 0.05) and high (A/V ) 0.5) alkalimetal content are summarized in Table 3. The band positions observed for (0.05)Li-3V/Zr remain basically unaltered in respect to the unpromoted 3V/Zr catalysts. In the case of a sodium-doped catalyst a broadening at ∼30400 cm-1 can be attributed to the formation of a tetrahedral quasi-isolated metavanadate species. This band is clearly observed in the (0.05)K-3V/Zr catalyst. At high alkali-metal loading the differences between the samples doped with various alkali metals disappear. In all cases tetrahedrally coordinated surface vanadates coexist with oligomers of the octahedrally coordinated vanadium. It means that alkali-metal dopants interact with a part of the clustered VOx species, dispersing them into isolated vanadium, whereas the rest of VOx remains unaffected by the dopants. Through the formation of V-O-A bonds, alkali cations can act as structure-terminating agents. This prevents the formation of the V-O-V bridges constituting oligomeric species, which gives rise to the improved dispersion of the supported vanadium.1,6 As a result, the broad EPR signal (type B) transforms into a well-structured and intense signal A characteristic of the isolated V4+ ions. Thermal treatment leads to an oxolation process involving the formation of the V-O-V bridges as schematically shown in Figure 9. Alkali-metal dopants interfere in this reaction in two ways, depending on their content. At low concentrations they protect isolated vanadium species from condensation by blocking the centers of H2O elimination, since this reaction principally conditions that process. At high concentrations an alkalimetal dopant can also attack the V-O-Zr bridges, changing the number of vanadium links with the surface. This weakens the interaction of deposited vanadia with the ZrO2 support. Elimination of water during oxolation is a reversible process and after adsorption of H2O refragmentation of the polymeric surface species occurs. This can be clearly seen in Figure 10 where the EPR spectra recorded for a
g|
g⊥
A|/G
A⊥/G
1.926 1.944 1.978
1.975 1.978 1.966
186
63
Figure 9. Schematic illustration of the influence of alkali dopants and water on condensation processes of surface vanadium species during calcination of the V/Zr system. For the sake of simplicity vanadium species, regardless the actual structure, are reduced to their essential features important for condensation.
1V/Zr sample after the adsorption of water at different partial pressures are compared to that of a sodiumcontaining (0.3)Na-3V/Zr catalyst. Similarly, as in the case of alkali-metal dopants, the appearance of the hfs due to 51V is taken as an indication for better spreading of vanadium, caused by adsorbed water. Indeed, the intensity of the EPR signal of isolated vanadium (Figure 10, insert) increases substantially. The contribution of isolated species (34%) is slightly higher in the sample of 1V/Zr saturated with water than in the case of (0.3)Na3V/Zr (23%). A close resemblance of the EPR spectra after alkalimetal doping and upon water adsorption shows that water can act comparatively to alkali dopants as a decoupling agent. However, the mechanism in both cases is opposite, while water molecules tend to erode the V-O-V bridges, the addition of alkali-metal cations prevents their formation. Possible Implications of Surface Heterogeneity for the ODP Process. Catalytic activity of the investigated A-V/Zr samples in ODP is related to the distribution of surface vanadium between the monomeric and polymeric species and the maximum corresponds to the loading of 3-5 mol %. A similar dependence between the concentration of the species with V-O-V groups and the activity in ODP over V2O5 deposited on various supports was reported earlier.10,30,59 At low vanadium loadings, where isolated vanadyl species are abundant, catalytic activity decreases, indicating that those species are clearly less efficient in an ODP reaction than the oligomeric VOx.
Surface Heterogeneity of V2O5/ZrO2 Catalysts
Langmuir, Vol. 15, No. 18, 1999 5741
presence of very basic oxygen in V-O-V entities is responsible for too strong adsorption of the hydrocarbon intermediates, so they can be easily oxidized to COx. Therefore, such centers are not very selective in comparison to those with oxygens of lower nucleophilicity,6,30,58 Catalytic properties can thus be modified by the addition of alkali-metal dopants. Irreversible replacement of protons in terminal OH groups of the vanadium species leads to a location of alkali cations close to the active sites (Figure 9). Owing to their high electropositivity, they modify the nucleophilic character of adjacent oxygen anions, which is beneficial for the selectivity. On the other hand, by enhancing the dispersion of surface vanadium, alkali promotors are responsible for the formation of less active isolated centers. The intensity of these effects is related to the size and the chemical nature of the alkali cations. Summarizing, efficient zirconia-supported V2O5 catalysts have to contain about 3 mol % of V2O5 (because of the highest concentration of V-O-V active species) and A/V = 0.3, to optimize the basicity of bridging oxygen and simultaneously to not disperse the vanadium significantly. Sodium or lithium seems to be the most appropriate alkali metals to achieve such requirements. The optimal temperature of catalyst calcination should not exceed 500 °C to avoid thermally induced diffusion of surface vanadium into a zirconia matrix.
Figure 10. Influence of adsorbed water on the shape of the EPR spectra of V/Zr catalysts. In the insert a plot of the associated changes in the intensity of the EPR signal with water partial pressure is shown. Beneath, for the sake of comparison, the spectrum of (0.3)Na-3V/Zr sample is added (dotted line). The close resemblance of the latter spectrum to that obtained after the adsorption of water at 261 hPa indicates a similar dispersion of vanadium in both cases.
In accordance with the literature data, the proposed active site is a mixed-valence center involving a V4+ ion. This type of catalytically active sites was proposed by several authors for selective oxidation processes over supported vanadia,42,60 molybdena,61 or chromia62 catalysts. To avoid a deep oxidation of propene, immediate desorption of C3H6 from the active center is necessary. This process can be controlled by an appropriate tailoring of the size of oxovanadium clusters and the acid-base properties of the bridging oxygen atoms.4,23,30,63 The (59) Corma, A.; Lo´pez-Nieto, J. M.; Paredes, N.; Pe´rez, M.; Shen, Y.; Cao, H.; Suib, S. L. Stud. Surf. Sci. Catal. 1992, 72, 213. (60) Andersson, A. J. Catal. 1982, 76, 144. (61) Mendolovici, L.; Lunsford, J. H. J. Catal. 1985, 94, 37. (62) Lugo, H. J.; Lunsford, J. H. J. Catal. 1985, 91, 155. (63) Ramis, G.; Busca, G.; Bregani, F. Catal. Lett. 1993, 18, 299.
Conclusions Preparation conditions and the chemical composition of V2O5/ZrO2 catalysts are directly responsible for the presence of various forms of V5+ and V4+ on the surface of zirconia, including isolated vanadyl species, oligomers containing V-O-V entities, and V2O5 nanocrystals. Surface heterogeneity is of essential importance for the propane oxidative dehydrogenation. The most efficient sites in this reaction are those with V-O-V bridges that are also the easiest to reduce. Doping with alkali metals or the presence of water favor the formation of less active isolated vanadyl species. The nucleophilicity of the bridging V-O-V oxygen can be effectively modified by the presence of alkali dopants and is supposed to be responsible for facilitated propene desorption. Acknowledgment. This work was financially supported by the Polish Committee for Scientific Research (KBN) (Grant No. T09A 104 15) and by Deutsche Forschungsgemeinschaft, Graduiertenkolleg der Physikalischen Chemie der Grenzfla¨chen, Universita¨t Leipzig. The students from E Ä cole Nationale Supe´rieure de Chimie de Clermont-Ferrand (France), A. Lucas, C. Bettarel, O. Haillant, Y. Bunel, and S. Schaeffer, who worked with A. Ad. during their training sessions on EPR and UV/vis results, are gratefully acknowledged. The authors sincerely thank Mrs. Ewa Bidzin˜ska (Regional Laboratory of Physicochemical Analyses and Structural Research, Cracow) for her help in the quantitative EPR measurements. LA981431M
