Electronic Structure Evolution with Composition Alteration of RhxCuy Alloy Nanoparticles

Natalia Palina,Hiroshi Kitagawa,Hirokazu Kobayashi,Katsutoshi Nagaoka,Tokutaro Komatsu,Osami Sakata,Chulho Song,Katsutoshi Sato,Kohei Kusada,L S R Kumara

doi:10.1038/srep41264

Abstract

The change in electronic structure of extremely small RhxCuy alloy nanoparticles (NPs) with composition variation was investigated by core-level (CL) and valence-band (VB) hard X-ray photoelectron spectroscopy. A combination of CL and VB spectra analyses confirmed that intermetallic charge transfer occurs between Rh and Cu. This is an important compensation mechanism that helps to explain the relationship between the catalytic activity and composition of RhxCuy alloy NPs. For monometallic Rh and Rh-rich alloy (Rh0.77Cu0.23) NPs, the formation of Rh surface oxide with a non-integer oxidation state (Rh(3−δ)+) resulted in high catalytic activity. Conversely, for alloy NPs with comparable Rh:Cu ratio (Rh0.53Cu0.47 and Rh0.50Cu0.50), the decreased fraction of catalytically active Rh(3−δ)+ oxide is compensated by charge transfer from Cu to Rh. As a result, ensuring negligible change in the catalytic activities of the NPs with comparable Rh:Cu ratio to those of Rh-rich and monometallic Rh NPs.

Highlights

Interest in alloy catalysts is driven by initial industry demand for bimetallic catalysts[1,2,3] and quantum theory of alloys; in particular, theories predicting the surface composition of alloys[4,5]
To identify alloy compositions suitable for use as three-way catalysts (TWCs), here we study the evolution of electronic structure as a function of compositional alteration utilizing hard X-ray photoelectron spectroscopy (HAXPES)
Information that can be obtained from core level (CL) and valence band (VB) data will help determine the optimal composition of alloys to achieve the most stable and catalytically effective NPs

Summary

Introduction

Interest in alloy catalysts is driven by initial industry demand for bimetallic catalysts[1,2,3] and quantum theory of alloys; in particular, theories predicting the surface composition of alloys[4,5]. With the rapid growth of the number of automobiles in the modern world, effective three-way catalysts (TWCs) to purify harmful exhaust gases including nitrogen oxides (NOx), carbon monoxide (CO) and hydrocarbons have increasingly been viewed as a necessary technology to prevent serious atmospheric pollution[12,13,14,15]. Use of hard X-rays can effectively overcome the above-stated limitation of laboratory XPS instruments and provide reliable data about the electronic states of the metals in the alloy NPs. Information that can be obtained from core level (CL) and valence band (VB) data will help determine the optimal composition of alloys to achieve the most stable and catalytically effective NPs. Note that the RhxCuy NPs investigated in this work exhibit comparable catalytic activity (Supplementary Figure SI 2). Obtained electronic structure data provide essential inputs for theoretical calculation of realistic alloy NP models, including orbital projected DOS. The ultimate goals are to control and predict the physicochemical properties of new alloy NPs

Methods

Results

Conclusion