Research Article pubs.acs.org/acscatalysis
Probing the Structure of a Water-Oxidizing Anodic Iridium Oxide Catalyst using Raman Spectroscopy Zoran Pavlovic, Chinmoy Ranjan,* Qiang Gao, Maurice van Gastel, and Robert Schlögl Department of Heterogeneous Catalysis, Max Planck Institute for Chemical Energy Conversion, 45470 Mülheim an der Ruhr, Germany S Supporting Information *
ABSTRACT: Iridium oxide is one of the most important catalysts for water oxidation. The atomic structure of this catalyst remains unknown. We have studied anodically grown iridium oxide catalyst films using Raman spectroscopy. In addition to deuteration and 18O substitution experiments, theoretical models were also constructed using density functional theory to interpret the experimental data. The material was characterized over a large potential range which included that for the oxygen evolution reaction (0.0−1.8 V). The material was found to be composed of [IrO6]n edge-sharing polyhedra (with n ≥ 3). Ir centers are connected to each other via μ-O type oxygen linkages that allow for the Ir centers to electronically couple to each other. The most intense peaks in Raman spectra were characterized by stretching movement of Ir−μ-O bonds in the basal plane of the octahedra coupled to OH bending movements of hydroxyl groups bound to the Ir centers. Oxidation of Ir3+ to Ir4+ at 0.7−1.2 V within a μO linked polymeric geometry results in a blue coloration of the material at high potentials. Theoretical calculations indicate that the optical transition responsible for the color is essentially an Ir to Ir charge transfer transition. The active compound that carries out oxygen evolution is resistant to further structure-directing influence of oxidation. In the course of oxidation, it was observed that IrO2 with a rutile structure could form at potentials greater than 1.2 V as a side product of the reaction. KEYWORDS: iridium oxide, iridium hydrous oxide, oxygen evolution reaction, water electrolysis, electrocatalysis, Raman spectroscopy, electrode characterization
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INTRODUCTION Water oxidation remains one of the central challenges toward solving the problem of renewable hydrogen.1,2 Iridium-based oxides (IrOx) remain the best water oxidation catalysts (WOC) under acidic conditions.3−6 Dimensionally stable anodes (DSA) have traditionally used IrO2 either as a catalyst layer or as nanoparticles dispersed on a support such as TiO2.7,8 However, a DSA material needs to undergo elaborate activation protocols involving potential cycling before it can be used as a catalyst.9 IrOx materials can be roughly divided into two types: those that are thermally synthesized7,8,10 and those that are electrochemically synthesized.11 A large degree of variation exists in the synthetic methods used in the creation of thermal IrOx. Reier et al. showed thermally prepared IrOx to have both crystalline and amorphous components, with the latter having higher intrinsic activity.10 Electrochemically, IrOx can be grown on Ir foils by potential cycling, as reported by Buckley and Burke, or by anodic deposition of IrOx from solution using electroprecipitation, as reported by Zhao et al.11−15 These anodic iridium oxide films (AIROFs) are highly hydrous and electrochromic. AIROFs are different from thermally formed films (e.g., DSA) which are not hydrated, are much higher in density, and do not show electrochromism. In addition, AIROFs show high oxygen evolution activity from the outset without requiring any pretreatment. Although they have been © 2016 American Chemical Society
extensively studied, the detailed atomic structure of the IrOx catalysts remains unknown. The design of improved IrOx catalysts awaits an understanding of the atomic structure of the active site. For the first time, we have characterized the atomic structure of an anodic IrOx-WOC using Raman spectroscopy, isotopic substitution experiments, and theoretical calculations. Previously, this material has been studied using Xray absorption spectroscopy (XANES, EXAFS)16−22 and X-ray photoelectron spectroscopy (XPS and UPS).23−26 A singular study by Mo et al. on the IrOx/Au system used Raman spectroscopy as a minor component, but the paper essentially focused on XANES to elucidate the electronic structure of Ir.18
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RESULTS AND DISCUSSION Catalysis is a transformative process for both the catalyst and the substrate. Raman spectroscopy probes all allowed vibrations from all of the materials present in the zone of inquiry. Since catalysis is a local phenomenon, Raman spectroscopy has significant advantages over other techniques such as XRD and EXAFS (which show an averaged picture) as a structural probe for the catalyst site. Over the potential range of 0−1.5 V, Received: August 16, 2016 Revised: October 15, 2016 Published: October 21, 2016 8098
DOI: 10.1021/acscatal.6b02343 ACS Catal. 2016, 6, 8098−8105
Research Article
ACS Catalysis
oxidation state of Ir changes from ∼+3 (at
