Keller Daphne E, Airaksinen Sanna M K, Krause A Outi, Weckhuysen Bert M, Koningsberger Diederik C
Inorganic Chemistry and Catalysis, Department of Chemistry, Utrecht University, P.O. Box 80083, 3508 TB Utrecht, The Netherlands.
J Am Chem Soc. 2007 Mar 21;129(11):3189-97. doi: 10.1021/ja0667007. Epub 2007 Feb 27.
The potential of atomic XAFS (AXAFS) to directly probe the catalytic performances of a set of supported metal oxide catalysts has been explored for the first time. For this purpose, a series of 1 wt % supported vanadium oxide catalysts have been prepared differing in their oxidic support material (SiO2, Al2O3, Nb2O5, and ZrO2). Previous characterization results have shown that these catalysts contain the same molecular structure on all supports, i.e., a monomeric VO4 species. It was found that the catalytic activity for the selective oxidation of methanol to formaldehyde and the oxidative dehydrogenation of propane to propene increases in the order SiO2 < Al2O3 < Nb2O5 < ZrO2. The opposite trend was observed for the dehydrogenation of propane to propene in the absence of oxygen. Interestingly, the intensity of the Fourier transform AXAFS peak decreases in the same order. This can be interpreted by an increase in the binding energy of the vanadium valence orbitals when the ionicity of the support (increasing electron charge on the support oxygen atoms) increases. Moreover, detailed EXAFS analysis shows a systematic decrease of the V-Ob(-M(support)) and an increase of a the V-O(H) bond length, when going from SiO2 to ZrO2. This implies a more reactive OH group for ZrO2, in line with the catalytic data. These results show that the electronic structure and consequently the catalytic behavior of the VO4 cluster depend on the ionicity of the support oxide. These results demonstrate that AXAFS spectroscopy can be used to understand and predict the catalytic performances of supported metal oxide catalysts. Furthermore, it enables the user to gather quantitative insight in metal oxide support interactions.
首次探索了原子X射线吸收精细结构（AXAFS）直接探测一组负载型金属氧化物催化剂催化性能的潜力。为此，制备了一系列负载量为1 wt%的负载型氧化钒催化剂，其氧化载体材料不同（SiO₂、Al₂O₃、Nb₂O₅和ZrO₂）。先前的表征结果表明，这些催化剂在所有载体上都具有相同的分子结构，即单体VO₄物种。研究发现，甲醇选择性氧化为甲醛以及丙烷氧化脱氢为丙烯的催化活性按SiO₂ < Al₂O₃ < Nb₂O₅ < ZrO₂的顺序增加。在无氧条件下丙烷脱氢为丙烯时观察到相反的趋势。有趣的是，傅里叶变换AXAFS峰的强度也按相同顺序降低。这可以通过载体离子性增加（载体氧原子上电子电荷增加）时钒价轨道结合能的增加来解释。此外，详细的扩展X射线吸收精细结构（EXAFS）分析表明，从SiO₂到ZrO₂，V-Ob(-M(载体))系统地减少，V-O(H)键长增加。这意味着ZrO₂的OH基团更具反应性，与催化数据一致。这些结果表明，VO₄簇的电子结构以及催化行为取决于载体氧化物的离子性。这些结果表明，AXAFS光谱可用于理解和预测负载型金属氧化物催化剂的催化性能。此外，它还能让使用者对金属氧化物与载体的相互作用有定量的认识。
