Coordination-number-determined activity of copper catalyst in water-gas shift reaction

Abstract

The water–gas shift reaction (WGSR) is of great significance in industrial hydrogen production. In this paper, Cun/TiO2(n = 1,2,4,8) and Cu (111) crystal planes were studied to explore the effect of size effect on the activity of WGSR. Density functional theory (DFT) calculations showed that the adsorption energy of H2O molecule was significantly enhanced on the surface of Cu1 and Cu2 clusters compared with Cu4 and Cu8 clusters, while CO adsorption strength first increased and then decreased. Among many copper clusters of different sizes, the redox pathway cannot proceed due to the lack of active sites on Cu1 and Cu2 clusters. According to the analysis of the microkinetic model, the order of WGSR activity on the surface of copper clusters of different sizes is: Cu4 > Cu2 ≈ Cu8 > Cu1 > Cu (111). The active center structure of the copper cluster is composed of 3 copper atoms in which the Cu-Cu coordination number is ca. 2 ∼ 3, and the active center is partially oxidized copper. Essentially, the low-coordination copper atoms determined by the geometry structure promote the WGSR activity of the copper clusters. This work not only reveals the copper cluster size effect on WGSR, but also provides the guidance for the size design and control of other supported metal catalysts.