Oxidation and Reduction of Ir(100) Studied by High-Energy Surface X-ray Diffraction

Stefano Albertin,Edvin Lundgren,Ann-Christin Dippel,Lindsay R Merte,Uta Hejral,Rachel Martin,Olof Gutowski,Jason F Weaver

doi:10.1021/acs.jpcc.1c10250

Abstract

The oxidation and reduction of an Ir(100) surface using 2.5, 5, and 10 mbar O2 partial pressure and a sample temperature of 775 K have been studied by using high-energy surface X-ray diffraction (HESXRD) which allowed to record large volumes of reciprocal space in short time periods. The complex 3D diffraction patterns could be disentangled in a stepwise procedure. For the 2.5 mbar experiment the measurements indicate the formation of an Ir(100)-O c(2 × 2) oxygen superstructure along with the onset of epitaxial IrO2(110) bulk oxide formation. For the 5 and 10 mbar O2 partial pressures the formation of additional IrO2 bulk oxide epitaxies with (100) and (101) orientations as well as of polycrystalline IrO2 was observed. Upon CO reduction, we found the IrO2 islands to be reduced into epitaxial and metallic Ir(111) and (221) oriented islands.

Highlights

The interaction of oxygen with the Ir(100) surface has been investigated in great detail during the past few decades due to the importance of Ir as an oxidation catalyst.[1−5] These earlier investigations have been limited to chemisorbed oxygen formed under UHV-like conditions
A recent timelapsed ambient pressure X-ray photoelectron spectroscopy study concluded a rapid and autocatalytic growth mechanism that, only occurs after a long induction period with constant oxygen coverage.[16]. These findings make a detailed investigation of the oxidation dynamics of the Ir(100) surface necessary, with the goal to identify conditions that stabilize the formation of desired IrO2 orientations
Our experiment shows that multiple IrO2 phases and epitaxies form, in line with previous studies,[12] and that the number of differently oriented oxides make the Ir(100) difficult to use as a model system to produce well-ordered IrO2(110) surfaces, in particular when using a flow reactor with added complexity in determining the exact pressures and temperatures

Summary

Introduction

The interaction of oxygen with the Ir(100) surface has been investigated in great detail during the past few decades due to the importance of Ir as an oxidation catalyst.[1−5] These earlier investigations have been limited to chemisorbed oxygen formed under UHV-like conditions. It was observed that an epitaxial rutile IrO2(110) film may form on the Ir(100) surface at significantly higher oxygen exposures and that this surface has the ability to dissociate and activate methane (CH4) at surprisingly low temperatures.[6] Because alkane C−H bonds are among the least reactive known, no process for direct conversion of CH4 into methanol has so far been developed; the facile CH4 dissociation occurring even at liquid nitrogen temperatures on the IrO2(110) surface could have importance for the development of novel and efficient methane to methanol conversion catalysts. A recent timelapsed ambient pressure X-ray photoelectron spectroscopy study concluded a rapid and autocatalytic growth mechanism that, only occurs after a long induction period with constant oxygen coverage.[16] These findings make a detailed investigation of the oxidation dynamics of the Ir(100) surface necessary, with the goal to identify conditions that stabilize the formation of desired IrO2 orientations

Results

Discussion

Conclusion