Stabilizing mechanism of single-atom catalysts on a defective carbon surface

Lianping Wu,Shengping Shen,Teng Li,Wenshan Yu,Shuling Hu

doi:10.1038/s41524-020-0292-y

Abstract

Single-atom (SA) catalysts represent the ultimate limit of atom use efficiency for catalysis. Promising experimental progress in synthesizing SA catalysts aside, the atomic-scale transformation mechanism from metal nanoparticles (NPs) to metal SAs and the stabilization mechanism of SA catalysts at high temperature remain elusive. Through systematic molecular dynamics simulations, for the first time, we reveal the atomic-scale mechanisms associated with the transformation of a metal NP into an array of stable SAs on a defective carbon surface at a high temperature, using Au as a model material. Simulations reveal the pivotal role of defects in the carbon surface in trapping and stabilizing the Au-SAs at high temperatures, which well explain previous experimental observations. Furthermore, reactive simulations demonstrate that the thermally stable Au-SAs exhibit much better catalyst activity than Au-NPs for the methane oxidation at high temperatures, in which the substantially reduced energy barriers for oxidation reaction steps are the key. Findings in this study offer mechanistic and quantitative guidance for material selection and optimal synthesis conditions to stabilize metal SA catalysts at high temperatures.

Highlights

Single-atom (SA) catalysts contain isolated single-metal atoms dispersed on a support surface, and represent the ultimate limit of atom use efficiency for catalysis[1,2]
We find that thermodynamically stable metal SA formation is governed by the interplay between the density/type of defects on the carbon surface, the cohesion energy between metal atoms, and the metal–carbon-binding energy
We carry out molecular dynamics (MD) simulations to reveal the effect of defects in a carbon surface on the transformation of a Au-NP into Au-SAs at high temperature

Summary

INTRODUCTION

Single-atom (SA) catalysts contain isolated single-metal atoms dispersed on a support surface, and represent the ultimate limit of atom use efficiency for catalysis[1,2]. Recent studies have sought to improve the thermal stability of SAs by forming strong metal-support interactions at 800–1200 °C on different supports (e.g., TiO210,20, CeO27, FeOx21, nitrogen-doped carbon[22], and carbon nanofibers[23]). In some of these experiments, a counterintuitive transformation from nanocluster to SAs is observed at a high temperature[22]. Encouraging experimental evidence of high- temperature-assisted conversion of metal nanoparticles (NPs) into SAs aside, the underlying SA formation mechanisms remain elusive To this end, it is desirable to decipher such underlying mechanisms through comprehensive atomistic simulations of the NP-to-SA process to capture key atomicscale events that occur at a short period of time. The mechanistic findings from this study offer fundamental insight into facile and robust approaches to synthesize thermodynamically stable metal SA catalysts

RESULTS AND DISCUSSION

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METHODS