Gold Atomic Layers and Isolated Atoms on MoC for the Low-Temperature Water Gas Shift Reaction

Abstract

Au 2–4 atomic layers, monolayers, and isolated atoms were generated by redispersing a Au particle (8 nm) during MoO3 carburization to α-MoC with a CH4/H2 mixture at 600–750 °C. The spherical Au particle initially wetted on the developing molybdenum carbides, reshaped into large thin films, then decomposed to smaller islands, and finally cracked into 2–4 atomic layers at 600 °C. These layered Au species further split into isolated atoms when carburized at 750 °C, which could be reversed into monolayers and 1–2 atomic layers by H2-treatment at 630–650 °C, followed by activation with a CH4/H2 mixture at 590 °C. When tested for the low-temperature water gas shift reaction, the 2–4 atomic layers possessed a much higher activity than the monolayers and isolated atoms. Interestingly, the specific reaction rate showed a volcano-type pattern with respect to the coordination number of Au–Au in the size-specified gold species, where the Au 2–4 atomic layers having a Au–Au coordination number of 7.7 were intrinsically more active. Detailed structural analysis, by STEM and EXAFS, identified that the interfacial gold atom coordinated simultaneously with the Mo atom in MoC and Au atoms in the layers.