Reaction paths in the hydrogenolysis of acetic acid to ethanol over Pd(111), Re(0001), and PdRe alloys
Reaction paths in the hydrogenolysis of acetic acid to ethanol over Pd(111), Re(0001), and PdRe alloys
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DOI：
10.1006/jcat.2002.3585
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发表时间：
2002-07-25
影响因子：
7.3
通讯作者：
Neurock, M
中科院分区：
文献类型：
--
作者：
Pallassana, V;Neurock, M
非局部密度泛函理论（DFT）计算用于检查替代机制醋酸氢解乙醇在Pd。在低表面覆盖率的情况下，计算了乙酸在Pd（111）上转化为乙醇的许多不同可能途径的总表面反应能。本文还报道了沿这些路径形成的各种含氧C-2中间体的结合能。利用总反应能提出了醋酸氢解的合理机理。在假定的机理中，乙酸解离形成乙酰基表面中间体。乙酰基中间体随后通过乙醛表面中间体的形成氢化成乙醇。详细的反应坐标计算用于分离过渡态，并计算了Pd（111）上乙酸解离成乙酰基（DeltaE(act) = + 142 kJ/mol）和乙酰氢化成乙醛（DeltaE(act) = +66 kJ/mol）的激活势垒。实验观察和DFT计算表明，这两个步骤很可能是醋酸氢解速率决定。在Re（0001）和Pd在Re上的伪晶覆盖层（Pd- ml /Re(0001)）上也计算了这些相同步骤的势垒和总反应能。结果表明，C-OH键离解反应比Re（0001）更有利，因为它具有更开放的d带。然而，与PdML/Re（0001）相比，诸如乙酰氢化之类的键结合反应更受青睐，PdML/Re（0001）具有类似于贵金属的电子d带结构。最佳平衡可能需要Pd/Re合金。在Pd0.66Re0.33合金上进行的计算证明了C-OH键断裂和C-H键形成的标称屏障。这可能是理想的醋酸氢解乙醇。然而，应避免使用铼系，因为它们会导致醋酸脱羧。(C) 2002 Elsevier Science （USA）。
Nonlocal density functional theory (DFT) calculations are used to examine alternative mechanisms for the hydrogenolysis of acetic acid to ethanol over Pd. The overall surface reaction energies, at low surface coverage, are computed for a number of different possible paths by which acetic acid may be converted to ethanol over Pd(111). Binding energies of the various oxygenated C-2 intermediates formed along these paths are also reported. The overall reaction energies were used to propose a plausible mechanism for acetic acid hydrogenolysis. In the postulated mechanism, acetic acid dissociates to form an acetyl surface intermediate. The acetyl intermediate is then subsequently hydrogenated to ethanol via the formation of an acetaldehyde surface intermediate. Detailed reaction coordinate calculations were used to isolate the transition states and calculate activation barriers for acetic acid dissociation to acetyl (DeltaE(act) = + 142 kJ/mol) and acetyl hydrogenation to acetaldehyde (DeltaE(act) = +66 kJ/mol) over Pd(111). Experimental observations and DFT calculations suggest that these two steps are likely to be rate determining in acetic acid hydrogenolysis. The barriers and overall reaction energies for these same steps are also computed on Re(0001) and pseudomorphic overlayers of Pd on Re (Pd-ML/Re(0001)) as well. The results suggest that the C-OH bond-dissociation reaction is more favored over Re(0001) since it has a more open d band. However, bond-association reactions such as acetyl hydrogenation are more favored over PdML/Re(0001), which has an electronic d-band structure similar to that of a noble metal. The optimal balance may require a Pd/Re alloy. Calculations performed over a Pd0.66Re0.33 alloy demonstrate a nominal barrier for both C-OH bond breaking and C-H bond formation. This may be ideal for acetic acid hydrogenolysis to ethanol. Rhenium ensembles, however, should be avoided as they lead to acetic, acid decarboxylation. (C) 2002 Elsevier Science (USA).