Cu_2O(111)催化水煤气变换反应机理的理论研究

采用密度泛函理论结合周期平板模型方法系统地研究了水煤气变换反应在Cu_2O(111)表面上的反应机理,包括氧化还原机理、羧基机理和甲酸根机理.结果表明,在Cu_2O(111)表面,羧基机理和甲酸根机理均可行,且甲酸根机理更为有利,其最佳反应途径为H_2O~*→H~*+OH~*;CO(g)+H~*+OH~*→trans-HCOOH~*(1)→cis-HCOOH~*→CO_2~*+H_2(g).其中trans-HCOOH~*(1)→cis-HCOOH~*为其决速步,该基元反应的能垒仅为59 kJ·mol~(-1).羧基机理的最优反应路径同样是以H_2O的解离反应开始,随后CO(g)+OH~*→cis-COOH~*→trans-COOH~*→CO_2(g)+H~*,最后产生的两个吸附的H原子先迁移再结合生成H_2,整个反应的控速步骤为H原子的迁移,迁移能垒为96 kJ·mol~(-1).氧化还原机理则由于OH解离需要越过一个很高的能垒(254 vs.187 kJ·mol~(-1))而不可行.

Reaction Mechanisms of WGSR Catalyzed by Cu2O(111): A Theoretical Study

The detailed mechanisms of water-gas shift reaction (WGSR) on Cu2O (111) have been investigated by periodic densityfunctional theory (DFT).Three typical WGSR mechanisms,i.e.,the redox,carboxyl and formate ones,are explored.Our transition state calculations reveal that the carboxyl and formate mechanisms are both viable for WGSR on Cu2O (111);but formate mechanism preferentially operates.For the formate mechanism,the most feasible reaction pathway is H2O*→H*+OH*;CO(g) +H*+OH*→trans-HCOOH*(1)→cis-HCOOH*→CO2*+ H2(g).The rate-limiting step is trans-HCOOH*(1)→cis-HCOOH*,in which only a moderate energy barrier (59 kJ · mol-1) needs to be overcome.When WGSR takes place along the carboxyl mechanism,the most feasible pathway is also started from the dissociation of H2 O.But next,CO (g) + OH*→cis-COOH* →trans-COOH* →CO2 (g) +H*.Finally,the formed H atoms occur migration and then react to form H2.The migration of H atom is the rate-limiting step.The barrier involved in this step is 96 kJ · mol-1.The redox mechanism is unfeasible due to the high energy barrier of OH dissociation (254 vs.187 kJ · mol-1).