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Abstract:
Supported vanadia catalysts are important catalysts for oxidation reactions. Depending on the kind of support and on the doping degree, vanadia is either spread over the sup-ports surface forming thin layers of vanadates or it agglomerates at higher loadings yielding V2O5 crystallites. Commonly, vanadates are proposed to consist of VO4 units, in which mono- and polyvanadates are distinguished (e.g. [1-3]). In this concept, in the oxidized state all V atoms are assumed to be in the oxidation state +5 in vanadate as well as in crystalline phases. However, for vanadate catalysts, often in TPR H/V ratios of less than 2 are observed (e.g. [4-6]) and, consistently, in other papers (e.g. [7-9]) by XPS, UV-VIS or ESR significant fractions of V(IV) species were measured.
In order to clarify this discrepancy, a systematic TPR- and XPS-based study was performed for catalysts with loadings of 1.6-15.7 % V using γ-Al2O3 as support and VO(acac)2 or NH4VO3 as vanadia precursors. After impregnation all catalysts were calcined in air at 650 °C for 8 h. The investigations were focused not only on fresh samples but also include catalysts after exposure to reduction-reoxidation sequences with diluted H2 or O2 gas mixtures and after "time on stream" in the oxidative dehydrogenation of ethane. Furthermore, BET, XRD, DRIFTS and TPD of pyridine were applied for catalyst characterization. Finally, information on the present V species was derived from structure-activity analysis in ethane oxidation.
From this data pool and based on TEM micrographs of selected samples, for the fresh catalysts the formation of an amorphous vanadate phase with "isolated" species at low loadings (< 3 % V) and of a well-arranged polymerized vanadate phase at moderately higher loadings were concluded. At loadings > 9 % V V2O5 crystallization accompanied by pore clogging was observed. Herein, the vanadate phases demonstrate identical reducibility in TPR, but can be distinguished via their different Brønstedt acidity [10,11]. The reducibility of the V2O5 crystalline phase is significantly lower.
The phase composition can change after exposure to "time on stream". So, in the H2/O2 redox sequences the formation of a new even easier reducible vanadia species was observed from the amorphous vanadates. In contrast, the high loaded samples demonstrate an increase of V2O5 agglomerates. A different behavior was found after "time on stream" in ethane oxidation: Here the two vanadate phases remain unchanged but the V2O5 crystallites are transformed into a vanadate-like multilayer phase with improved reducibility similar to vanadate catalysts.
XPS spectra of the fresh samples demonstrate oxidation states of 4.3 for all vanadate catalysts below monolayer coverage and of 4.8 for those catalysts containing crystalline V2O5 species, and the absence of V(III). In TPR, H/V ratios of ~1.3 and ~1.8 were measured, respectively. This implies that the present VOx is completely reduced to V(III) by hydrogen. Despite on changes in phase composition, no changes were observed regarding oxidation states as far as the catalysts are not exposed to reducing conditions. The correlation between XPS and TPR data indicates that only 1/3 of V in the vanadate phases but 80 % of V in the crystalline V2O5 phase and the multilayer phase formed from the latter can really reach oxidation state +5. The residual fraction of V cations remains at oxidation state +4 even after exposure to oxidizing environment (air) at elevated temperatures. This conclusion is supported by structure-activity analysis of data from ethane oxidation, where for the initial ethylene formation in agreement with the Mars-van-Krevelen mechanism a significant correlation of turnover rate constants to the present amounts of accessible V(V) species was found for oxygen-lean and oxygen-excess conditions as well. Similar correlations can be established also for silica-supported samples.
The presence of 2/3 of the vanadium as V(IV) cations in the vanadate catalysts which cannot be oxidized to V(V) under moderately oxidizing conditions contradicts the established postulate that the vanadate phases consist in the oxidized state exclusively of V(V) cations. Consequently, this finding implies the existence of two different kinds of V cations within the vanadate structures. However, vanadate structures still have to include M-O-V(V) bonds as active sites in hydrocarbon oxidation and V=O terminal bonds to reach consistence with the DRIFT spectra and with the observed support influence on reducibility, acidity and catalyst performance.
The smallest unit fulfilling all requirements is [V3O8]3- with three bonds to the support (see Fig. 1a). We suggest that this structure is present mainly at low V loadings and represents the "isolated" vanadates, which in our opinion should replace the commonly assumed (VO4)3- monovanadate structure. For polyvanadates, chains with the sequence (-V(IV)-O-V(V)-O-V(IV)-O-)n may be postulated (Fig. 1b). The proposed structures do not contradict previous spectroscopic findings, and are consistent with the observed redox and activity behavior.
Acknowledgements
Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged (DFG research unit 447, "Membrane supported reaction engineering"). Further, we thank R. Wagner for assistance with the DRIFTS measurements, and D. S. Su, N. Pfänder and G. Weinberg for TEM images.
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J. Catal., in press (2007)
