Döbler Jens, Pritzsche Marc, Sauer Joachim
Institut für Chemie der Humboldt Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany.
J Am Chem Soc. 2005 Aug 10;127(31):10861-8. doi: 10.1021/ja051720e.
The oxidation of methanol to formaldehyde on silica supported vanadium oxide is studied by density functional theory. For isolated vanadium oxide species silsesquioxane-type models are adopted. The first step is dissociative adsorption of methanol yielding CH3O(O=)V(O-)2 surface complexes. This makes the O=V(OCH3)3 molecule a suited model system. The rate-limiting oxidation step involves hydrogen transfer from the methoxy group to the vanadyl oxygen atom. The transition state is biradicaloid and needs to be treated by the broken-symmetry approach. The activation energies for O=V(OCH3)3 and the silsesquioxane surface model are 147 and 154 kJ/mol. In addition, the (O=V(OCH3)3)(2) dimer (a model for polymeric vanadium oxide species) and the O=V(OCH3)3(*+) radical cation are studied. For the latter the barrier is only 80 kJ/mol, indicating a strong effect of the charge on the energy profile of the reaction and questioning the significance of gas-phase cluster studies for understanding the activity of supported oxide catalysts.
采用密度泛函理论研究了甲醇在二氧化硅负载的氧化钒上氧化生成甲醛的反应。对于孤立的氧化钒物种，采用倍半硅氧烷型模型。第一步是甲醇的解离吸附，生成CH3O(O=)V(O-)2表面络合物。这使得O=V(OCH3)3分子成为一个合适的模型体系。限速氧化步骤涉及氢从甲氧基转移到钒氧基原子。过渡态是双自由基型的，需要用破缺对称性方法处理。O=V(OCH3)3和倍半硅氧烷表面模型的活化能分别为147和154 kJ/mol。此外，还研究了(O=V(OCH3)3)2二聚体（一种聚合氧化钒物种的模型）和O=V(OCH3)3(*+)自由基阳离子。对于后者，势垒仅为80 kJ/mol，这表明电荷对反应能量分布有强烈影响，并对气相团簇研究对于理解负载型氧化物催化剂活性的意义提出了质疑。
