用UBI-QEP方法分析钴系Fischer-Tropsch合成催化反应机理

采用unity bond index-quadratic exponential potential(UBI-QEP)方法, 以Co(0001) 单晶为模型催化剂, 对Fischer-Tropsch(F-T)合成的三种可能反应机理(表面碳化物机理、烯醇机理和CO插入机理)进行了全面的能学分析. 计算结果表明, 通过表面碳化物机理生成烃类产物从能学角度看较为合理, 其中CO_ads表面解离和C_ads加氢具有较高的活化能垒, 可能是整个F-T反应序列中的慢步骤; 通过CH_2,ads插入金属-烷基键实现链增长的活化能垒最低, 是能量上有利的链增长方式; 在Co(0001)晶面上烷基经β-H消除生成烯烃的活化能垒低于加氢生成烷烃, 而通过CO插入机理生成的有机含氧化合物的二次反应能垒较低, 从而导致其在Co催化剂上的低选择性. 此外, 与Fe/W(110)相比, Co金属上的CH_x,ads加氢以及CH_2,ads插入的活化能垒较低, 从而解释了Co催化剂上甲烷选择性较高和倾向于生成重质烃类产物的特性.

UBI-QEP Analysis for the Mechanism of Fischer-Tropsch Synthesis

The unity bond index-quadratic exponential potential (UBI-QEP) model is used to evaluate the heat of chemisorption of adsorbed species and activation barriers for the elementary reactions in the mechanisms for Fischer-Tropsch synthesis (FTS) reaction over a model catalyst Co(0001), including carbide mechanism, hydroxycarbene mechanism, and CO insertion mechanism. It is demonstrated that the reaction pathway for the formation of hydrocarbons proposed in the carbide mechanism is energetically favorable. The insertion of CO is the pathway for the formation of oxygenations. Dissociation of adsorbed CO and hydrogenation of C_ads have higher activation barriers than other elementary steps in the reaction pathway. The energetically preferred pathway to initiate the carbon chain growth is via insertion of CH_{2, ads} intermediate into the carbon-metal bond of CH_{3, ads} or CH_{2, ads} group. The activation barrier for termination of carbon chain propagation by β-H elimination is lower than hydrogenation. The olefins and oxygenates in the primary products during FTS are apt to conduct the secondary reaction via readsorption on the catalyst surface because of its low active energies. Compared with Fe/W(110), the activation barriers for CH_{x, ads} hydrogenations and CH_{2, ads} insertion are lower on Co(0001), which results in higher selectivity of methane and more yield of heavy hydrocarbons.