Self-activated surface dynamics in gold catalysts under reaction environments

Naoto Kamiuchi,Masakazu Tane,Keju Sun,Ryotaro Aso,Seiji Takeda,Takehiro Tamaoka,Hideto Yoshida

doi:10.1038/s41467-018-04412-4

Abstract

Nanoporous gold (NPG) with sponge-like structures has been studied by atomic-scale and microsecond-resolution environmental transmission electron microscopy (ETEM) combined with ab initio energy calculations. Peculiar surface dynamics were found in the reaction environment for the oxidation of CO at room temperature, involving residual silver in the NPG leaves as well as gold and oxygen atoms, especially on {110} facets. The NPG is thus classified as a novel self-activating catalyst. The essential structure unit for catalytic activity was identified as Au–AgO surface clusters, implying that the NPG is regarded as a nano-structured silver oxide catalyst supported on the matrix of NPG, or an inverse catalyst of a supported gold nanoparticulate (AuNP) catalyst. Hence, the catalytically active structure in the gold catalysts (supported AuNP and NPG catalysts) can now be experimentally unified in low-temperature CO oxidation, a step forward towards elucidating the fascinating catalysis mechanism of gold.

Highlights

Nanoporous gold (NPG) with sponge-like structures has been studied by atomic-scale and microsecond-resolution environmental transmission electron microscopy (ETEM) combined with ab initio energy calculations
The amount of residual Ag was estimated below 1.0 at.% by quantitative analyses of TEM-energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) measurements (Supplementary Tables 1 and 2)
During and/or after a single catalytic cycle of a heterogeneous catalyst such as the Au catalysts, it is likely that the catalytic structure reorganized at the atomic scale and an intermediate atomic structure is formed with a relaxation time that is commensurate with the time scale of the ETEM observation, for instance, the cluster of Au–AgO and that of Au–Ag without O

Summary

Introduction

Nanoporous gold (NPG) with sponge-like structures has been studied by atomic-scale and microsecond-resolution environmental transmission electron microscopy (ETEM) combined with ab initio energy calculations. Deronzier et al.[18] already attempted to fabricate a pure NPG by dissolving a Au0.5Zr0.5 alloy in hydrofluoric acid This NPG, having significantly less residual Zr (0.03%), exhibited poor activity toward CO oxidation even at an elevated temperature of 140 °C, and the authors concluded that the residual impurity is indispensable for catalyzing CO oxidation reactions[18]. Zugic et al.[20] suggested that the surface restructuring of NPG, caused by ozone pretreatment followed by the reduction process in CO and/or CH3OH, resulted in metal oxide particles that covered the NPG surface up to a surface coverage of ~11% They concluded that the oxygen in the specific state can catalyze the selective oxidation of CH3OH. The result unifies experimentally the catalytically active structure of extended gold catalysts such as NPG and AuNP catalysts into heterogeneous nanostructures of gold and metal oxides

Methods

Results

Conclusion