Beyond surface redox and oxygen mobility at pd-polar ceria (100) interface: Underlying principle for strong metal-support interactions in green catalysis

Abdul Hanif Mahadi,Lin Ye,Simon M Fairclough,Jin Qu,Simson Wu,Wei Chen,Evangelos I Papaioannou,Brian Ray,Timothy J Pennycook,Sarah J Haigh,Neil P Young,Karaked Tedsree,Ian S Metcalfe,S.C Edman Tsang

doi:10.1016/j.apcatb.2020.118843

Abstract

When ceria is used as a support for many redox catalysis involved in green catalysis, it is well-known that the overlying noble metal can gain access to a significant quantity of oxygen atoms with high mobility and fast reduction and oxidation properties under mild conditions. However, it is as yet unclear what the underlying principle and the nature of the ceria surface involved are. By using two tailored morphologies of ceria nanocrystals, namely cubes and rods, it is demonstrated from Scanning Transmission Electron Microscopy with Electron Energy Loss Spectroscopy (STEM-EELS) mapping and Pulse Isotopic Exchange (PIE) that ceria nanocubes terminated with a polar surface (100) can give access to more than the top most layer of surface oxygen atoms. Also, they give higher oxygen mobility than ceria nanorods with a non-polar facet of (110). A new insight for the possible additional role of polar ceria surface plays in the oxygen mobility is obtained from Density Functional Theory (DFT) calculations which suggest that the (100) surface sites that has more than half-filled O on same plane can drive oxygen atoms to oxidise adsorbate(s) on Pd due to the strong electrostatic repulsion.

Highlights

Platinum group metals (PGMs) on a ceria support system have been shown to exhibit strong metal support interactions or so called SMSI [1,2,3]
The corresponding Fast Fourier Transform (FFT) of the HRTEM matches with the expected model describing the ceria cubes
The redox properties of the Pd deposited on ceria cubes with polar (100) surface and rods with non-polar (110) surface were investigated with Scanning Transmission Electron Microscopy with Electron Energy Loss Spectroscopy (STEM-EELS), Pulse Isotopic Exchange (PIE) and Density Functional Theory

Summary

Introduction

Platinum group metals (PGMs) on a ceria support system have been shown to exhibit strong metal support interactions or so called SMSI [1,2,3]. In contrast to traditional supports such as carbon and titania, PGM metals on ceria supports have been shown to exhibit enhanced catalytic activity. Both theoretical and experimental studies since the 1980s have explored the effects of using ceria support [4], the Applied Catalysis B: Environmental 270 (2020) 118843 properties of which have been thought to be due to the reducible nature of the ceria. PGM/ceria has been applied in three-way catalysts to remove hydrocarbons/CO/NOx from exhaust gas [5,6,7], CO oxidation to CO2 [8,9,10], water gas shift reaction to produce H2 [11,12], oxygen sensors [13,14,15] and solid oxide fuel cells for clean energy [16,17,18]. The ability of ceria to shift between the oxidised and reduced state (Ce3+ ↔ Ce4+) under operating conditions is suspected to be of importance for all of these processes [6,7,19,20,21]

Objectives

Results

Conclusion