Chen Zhao-Xu, Lim Kok Hwa, Neyman Konstantin M, Rösch Notker
Department Chemie, Technische Universität München, 85747 Garching, Germany.
J Phys Chem B. 2005 Mar 17;109(10):4568-74. doi: 10.1021/jp044843e.
The decomposition of methoxide (CH(3)O) on a PdZn alloy is considered to be the rate-limiting step of steam re-forming of methanol over a Pd/ZnO catalyst. Our previous density functional (DF) studies (Langmuir 2004, 20, 8068; Phys. Chem. Chem. Phys. 2004, 6, 4499) revealed only a very low propensity of defect-free flat (111) and (100) PdZn surfaces to promote C-H or C-O bond breaking of CH(3)O. Thus, we applied the same DF periodic slab-model approach to investigate these two routes of CH(3)O decomposition on PdZn(221) surfaces that expose Pd, (221)(Pd), and Zn, (221)(Zn), steps. C-H bond cleavage of CH(3)O is greatly facilitated on (221)(Pd): the calculated activation energy is dramatically reduced, to approximately 50 kJ mol(-1) from approximately 90 kJ mol(-1) on flat PdZn surfaces, increasing the rate constant by a factor of 10(8). The lower barrier is mainly due to a weaker interaction of the reactant CH(3)O and an enhanced interaction of the product CH(2)O with the substrate. The activation energy for C-O bond scission did not decrease on the (221)(Pd) step. On the (221)(Zn) step, the calculated reaction barriers of both decomposition routes are even higher than on flat surfaces, because of the stronger adsorption of CH(3)O. Steps (and other defects) appear to be crucial for methanol steam re-forming on Pd/ZnO catalyst; the stepped surface PdZn(221)(Pd) is a realistic model for studying the reactivity of this catalyst.
在钯锌合金上甲醇盐（CH(3)O）的分解被认为是甲醇在钯/氧化锌催化剂上进行蒸汽重整反应的速率限制步骤。我们之前的密度泛函（DF）研究（《朗缪尔》2004年，20卷，8068页；《物理化学化学物理》2004年，6卷，4499页）表明，无缺陷的平整（111）和（100）钯锌表面促进CH(3)O的C-H或C-O键断裂的倾向非常低。因此，我们采用相同的DF周期性平板模型方法来研究CH(3)O在暴露有钯（(221)(Pd)）和锌（(221)(Zn)）台阶的PdZn(221)表面上分解的这两条途径。在(221)(Pd)表面上，CH(3)O的C-H键断裂得到极大促进：计算得到的活化能大幅降低，从平整钯锌表面上的约90 kJ mol(-1)降至约50 kJ mol(-1)，速率常数增加了10(8)倍。较低的势垒主要归因于反应物CH(3)O与底物之间较弱的相互作用以及产物CH(2)O与底物之间增强的相互作用。在(221)(Pd)台阶上，C-O键断裂的活化能并未降低。在(221)(Zn)台阶上，由于CH(3)O的吸附更强，两条分解途径的计算反应势垒甚至高于平整表面。台阶（以及其他缺陷）对于甲醇在钯/氧化锌催化剂上的蒸汽重整似乎至关重要；台阶表面PdZn(221)(Pd)是研究该催化剂反应活性的一个现实模型。
