Stability of formate species on β-Ga2O3
Stability of formate species on β-Ga2O3
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DOI：
10.1039/b800519b
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发表时间：
2009-01-01
影响因子：
3.3
通讯作者：
Bonivardi, A. L.
中科院分区：
文献类型：
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作者：
Calatayud, M.;Collins, S. E.;Bonivardi, A. L.
在涉及甲醇、CO和H-2的多种催化反应中，氧化镓已被证明能提高金属催化剂的性能。甲酸盐作为这些反应的关键中间体的存在已经被报道，尽管它们的作用仍然是一个有争议的问题。在这项工作中，结合了理论和实验的方法，以表征这种甲酸物质在氧化镓表面的形成。H-2（或D-2）预处理的β - ga2o3吸附CO的红外光谱实验显示了几种甲酸物质的形成。利用周期性DFT计算方法对β - ga2o3（100）表面进行了建模。讨论了所述物种的稳定性及其振动模式分配，并讨论了甲酸互转化势垒。基于实验和理论结果，提出了一种可能的机理：CO首先插入表面（单坐标）羟基，形成单坐标甲酸；这个物种可能进化成热力学上最稳定的二配位甲酸盐，或者可能将氢转移到表面氧化成二氧化碳，产生氧空位和氢化物基团。计算得出，第一步CO插入的势垒明显高于单坐标甲酸转换步骤。因此，单配位甲酸盐是CO氧化反应中起关键作用的短寿命中间体，而双齿甲酸盐主要是旁观者。
Gallia(gallium oxide) has been proved to enhance the performance of metal catalysts in a variety of catalytic reactions involving methanol, CO and H-2. The presence of formate species as key intermediates in some of these reactions has been reported, although their role is still a matter of debate. In this work, a combined theoretical and experimental approach has been carried out in order to characterize the formation of such formate species over the gallium oxide surface. Infrared spectroscopy experiments of CO adsorption over H-2 (or D-2) pretreated beta-Ga2O3 revealed the formation of several formate species. The beta-Ga2O3 (100) surface was modelled by means of periodic DFT calculations. The stability of said species and their vibrational mode assignments are discussed together with the formate interconversion barriers. A possible mechanism is proposed based on the experimental and theoretical results:first CO inserts into surface (monocoordinate) hydroxyl groups leading to monocoordinate formate; this species might evolve to the thermodynamically most stable dicoordinate formate, or might transfer hydrogen to the surface oxidizing to CO2 creating an oxygen vacancy and a hydride group. The barrier for the first step, CO insertion, is calculated to be significantly higher than that of the monocoordinate formate conversion steps. Monocoordinate formates are thus short-lived intermediates playing a key role in the CO oxidation reaction, while bidentate formates are mainly spectators.