The energies of formation and mobilities of Cu surface species on Cu and ZnO in methanol and water gas shift atmospheres studied by DFT

Abstract

Catalysts based on copper, such as the Cu/ZnO/Al2O3 system are widely used for industrial scale methanol synthesis and the low temperature water gas shift reaction. A common characteristic of these catalysts is that they deactivate quite rapidly during operation and therefore understanding their deactivation by sintering is highly relevant. In this work, we study the nature of the species that are responsible for transport of the Cu metal in this catalyst type using density functional theory calculations within a chemical potential formalism. The stability and mobility of Cu–X (Cu, OH, CO, CH3O, HCOO) species are investigated in relevant synthesis gas compositions. The CuCO and Cu2HCOO species are identified to be predominant for metal transport on Cu particles, which may contribute to sintering of Cu by particle migration and coalescence. Furthermore, transport of Cu on ZnO is found mostly to occur through CuCO species, which indicates that CuCO is an important species for Ostwald ripening in a Cu/ZnO catalyst. These results provide atomistic perspective on the diffusion of the species that may contribute to catalyst sintering, therefore lending a valuable foundation for future investigations of the stability of Cu catalysts.