Hydrogen oxidation on oxygen-rich IrO2(110)

Tao Li,Aravind Asthagiri,Jason F Weaver,Minkyu Kim,Zhu Liang

doi:10.1080/2055074x.2018.1565002

Abstract

We investigated the adsorption and oxidation of H2 on O-rich IrO2(110) using temperature programmed reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. Our results show that H2 dissociation occurs efficiently on O-rich IrO2(110) at low temperature and initiates from an adsorbed H2 σ-complex on the coordinatively-unsaturated Ir atoms (Ircus). We find that on-top oxygen atoms (Oot), adsorbed on the Ircus sites, promote the desorption-limited evolution of H2O during subsequent oxidation of the adsorbed hydrogen on IrO2(110) while suppressing reaction-limited production of H2O via the recombination of bridging HO groups (HObr) (~500 to 750 K) during TPRS. The desorption-limited TPRS peak of H2O shifts from ~490 to 550 K with increasing Oot coverage, demonstrating that Oot atoms stabilize adsorbed OH and H2O species. DFT predicts that molecularly-adsorbed H2 dissociates on O-rich IrO2(110) at low temperature and that the resulting H-atoms redistribute to produce a mixture of HObr and HOot groups, with equilibrium favouring HOot groups. Our calculations further predict that subsequent H2O evolution occurs through the recombination of HObr/HOot and HOot/HOot pairs, and that these reactions represent desorption-limited pathways because the dissociative chemisorption of H2O is favoured over molecular adsorption on IrO2(110). The higher stability of HOot groups and their preferred formation causes the higher-barrier HOot/HOot recombination reaction to become the dominant pathway for H2O formation with increasing Oot coverage, consistent with the experimentally-observed upshift in the H2O TPRS peak temperature.

Highlights

Understanding the interactions of hydrogen with IrO2 surfaces is important for improving applications of electrocatalytic water splitting and developing IrO2based catalysts that can efficiently convert light alkanes to value-added products
We find that the dissociative chemisorption of hydrogen occurs efficiently on O-rich IrO2(110) at low temperature and that on-top oxygen atoms (Oot) atoms facilitate the subsequent oxidation of adsorbed hydrogen
Our results show that Oot atoms promote the desorption-limited evolution of H2O during H2 oxidation on IrO2(110) while suppressing reaction-limited H2O production arising from recombination of HObr groups

Summary

Introduction

Understanding the interactions of hydrogen with IrO2 surfaces is important for improving applications of electrocatalytic water splitting and developing IrO2based catalysts that can efficiently convert light alkanes to value-added products. Prior studies show that O2 dissociation occurs efficiently on RuO2(110) at room temperature, and can generate variable coverages of Oot atoms that coexist with Rucus and Obr atoms [8,9,10]. We exposed the film to ~23 L of O2 while cycling the surface temperature between 300 and 650 K to fill oxygen vacancies that may have been created during sample transfer from the reaction cell to the analysis chamber. This procedure produces a high-quality IrO2(110) surface that has a stoichiometric surface termination and consists of ~10 layers of IrO2(110), corresponding to a thickness of 3.2 nm. All reported binding energies and energy barriers are corrected for zero-point vibrational energy

Experimental results

H O desorption yield 2

Summary