A Density-Functional Theory Study of the Water−Gas Shift Mechanism on Pt/Ceria(111)

Density-functional theory has been used to model the interactions between ceria(111) and CO adsorbed on platinum in order to provide insights into the mechanism behind the water−gas shift reaction on this material. Morphological studies on ceria show the presence of various defect sites, both cerium and oxygen vacancies, which are believed to be the reason for the catalytic activity of the substrate. Of these defect sites, the reported calculations clearly show the preference for platinum to bind in cerium vacancies, as opposed to a defect-free surface or an oxygen deficiency site. Currently there are two main pathways proposed for this mechanism: a direct redox pathway and a formate-intermediate pathway. In both scenarios the initial step is the same: the oxygen storage capacity of ceria provides the oxygen necessary to oxidize CO into CO2; water then adsorbs and is dissociated in these oxygen vacancies helping reform the ceria surface. The key difference between the two mechanisms is either the desorption of CO2 and the formation of H2 when a second water molecule adsorbs at a site where water had been previously dissociated (catalyzed by platinum), or the formation of a formate species from the interaction of the adsorbed CO2 with adjacent water molecules. Energy pathways are mapped for both processes, demonstrating the preference toward the initial direct redox pathway. However, the resulting formation of adsorbed hydroxide species in the oxygen vacancies adjacent to the platinum hinders this pathway. A bifunctional mechanism is then presented as a means to remove these species and thereby form formate. The activation of these adsorbed hydroxides is also studied.

本研究采用密度泛函理论（Density-functional theory）对二氧化铈(111)晶面与吸附于铂表面的一氧化碳（CO）之间的相互作用进行建模，以期为该体系上水煤气变换反应的反应机理提供理论见解。针对二氧化铈的形貌学研究表明，其表面存在多种缺陷位点，包括铈空位与氧空位，这类缺陷被认为是该衬底具备催化活性的核心原因。针对上述缺陷位点的已有计算结果清晰表明，相较于无缺陷表面或氧缺陷位点，铂更倾向于结合于铈空位处。目前针对该反应机理已提出两条主流路径：直接氧化还原路径与甲酸盐中间体路径。两种路径的初始步骤完全一致：二氧化铈的储氧能力可为将一氧化碳氧化为二氧化碳提供所需氧原子；随后水分子吸附于上述氧空位并发生解离，助力二氧化铈表面的结构复原。两种机理的核心差异分为两类情况：其一，当第二个水分子吸附于先前已由铂催化发生解离的水分子位点时，会伴随二氧化碳脱附并生成氢气；其二，吸附态二氧化碳与邻近水分子相互作用生成甲酸盐物种。研究人员对两条路径的能量走势进行了描图分析，结果表明初始的直接氧化还原路径更具优势。然而，在铂邻近的氧空位中生成的吸附态氢氧根物种会抑制该路径的进行。随后本研究提出双功能机理，用以移除上述氢氧根物种并生成甲酸盐，同时对该类吸附态氢氧根的活化过程展开了系统研究。