In situ spectroscopy-guided engineering of rhodium single-atom catalysts for CO oxidation

Max J Hülsey,Tsunehiro Tanaka,Peng Zhang,Wei Chen,Hiroyuki Asakura,Ning Yan,David A Do,Zhirui Ma,Zili Wu,Bin Zhang

doi:10.1038/s41467-019-09188-9

Abstract

Single-atom catalysts have recently been applied in many applications such as CO oxidation. Experimental in situ investigations into this reaction, however, are limited. Hereby, we present a suite of operando/in situ spectroscopic experiments for structurally well-defined atomically dispersed Rh on phosphotungstic acid during CO oxidation. The identification of several key intermediates and the steady-state catalyst structure indicate that the reactions follow an unconventional Mars-van Krevelen mechanism and that the activation of O2 is rate-limiting. In situ XPS confirms the contribution of the heteropoly acid support while in situ DRIFT spectroscopy consolidates the oxidation state and CO adsorption of Rh. As such, direct observation of three key components, i.e., metal center, support and substrate, is achieved, providing a clearer picture on CO oxidation on atomically dispersed Rh sites. The obtained information are used to engineer structurally similar catalysts that exhibit T20 values up to 130 °C below the previously reported Rh1/NPTA.

Highlights

Single-atom catalysts have recently been applied in many applications such as CO oxidation
No Rh–Rh scattering for the Rh1/NPTA catalyst is observed and the white line intensity is similar to the Rh2O3 reference sample in the Rh K-edge Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectra (Supplementary Figures 6a and 6b)
The W LIII-edge XANES and EXAFS spectra reveal the stability of the phosphotungstic acid after adsorption of rhodium (Supplementary Figures 7a and 7b)

Summary

Introduction

Single-atom catalysts have recently been applied in many applications such as CO oxidation. A combination of both L–H and E–R mechanisms is proposed where the first molecule of CO is oxidized in an L–H manner via the formation of a peroxide-like intermediate followed by the oxidation of another CO molecule with the remaining oxygen on the single atom[48]. The controversial mechanisms proposed in the literature highlight the importance of performing in situ spectroscopic investigation on structurally simple and well-defined SACs to understand the roles of metal, support, and the fate of CO during a full catalytic cycle. In situ spectroscopic experiments have proven success in elucidating the structure and active sites of SACs under reaction conditions[50,51], for CO oxidation it has so far been primarily limited to IR42,45 and XPS46 spectroscopy, tools that only directly or indirectly provide information on the oxidation state of the catalyst. The rational design of SACs with improved catalytic performance based on mechanistic understanding is lacking so far

Methods

Results

Conclusion