An infrared spectroelectrochemical approach for understanding electrocatalysis at supported metal nanoparticles
Keywords: In situ infrared spectroscopy
Carbon monoxide stripping
Nanoparticle
Dipole-dipole coupling
Formic acid oxidation
Attenuated total reflectance
Surface selection rule

This thesis describes the development and application of a new in situ infrared (IR) approach to studying electrocatalysis at supported nanoparticle catalysts that are used in proton exchange membrane (PEM) fuel cells. Such fuel cells running on small organic molecules are an attractive technology for use in transport applications and portable electronic devices, however one major challenge facing this technology is the slow oxidation of the organic molecules at the electrode surface. Furthermore, the mechanisms by which these organic molecules are oxidised are still not clear, hampering the design of new electrode materials. In situ IR spectroscopy has been used extensively to investigate the mechanism of reactions on model catalysts, however extension of these techniques to real fuel cell catalysts is challenging and much less advanced.
A new approach to in situ IR spectroscopy of supported electrocatalysts is therefore developed, inspired by the geometry of PEM fuel cell electrodes. The approach overcomes many of the limitations of previous approaches, allowing solution flow over the catalyst layer, cyclic voltammetry up to scan rates of 1 V s-1 and spectroscopic detection of surface adsorbed species with time resolution of 0.5 s. The utility of this approach is demonstrated through a study of the mechanism of two model reactions, carbon monoxide (CO) and formic acid (FA) oxidation, on a commercial fuel cell catalyst.
In situ IR measurements made during CO stripping experiments on a commercial carbon-supported Pt catalyst reveal two strongly bipolar IR peaks in the CO stretching region. An empirical model for the bipolar peak shape is developed and used to extract peak parameters. Electrochemical measurement of the CO coverage then enables calibration of the IR peak intensity with coverage. This quantitative relationship enables features such as dipole-dipole coupling in the CO adlayer to be discussed. In situ IR spectra recorded during the stripping voltammogram reveal the presence of two linear CO peaks, assigned to different sites on the catalyst. The potential dependence of the two peak intensities is used to discuss the mechanism of CO oxidation on the catalyst.
The in situ approach is extended to the study of FA oxidation on the commercial Pt catalyst. As well as adsorbed CO, two potential-dependent peaks are assigned to adsorbed formate - the first time formate has been observed on a nanoparticle catalyst during electrooxidation. Furthermore, one of the peaks is assigned to an IR surface selection rule-prohibited mode, providing evidence for the previously proposed size-dependence of the selection rule. The effects of concentration, pH, isotope and supporting electrolyte on the adsorbed species are examined and related to the current in order to understand different aspects of the mechanism on nanoparticle catalysts. The results are discussed in the context of previous work on macroscopic electrodes.
Overall an approach to in situ IR spectroscopy of nanoparticle electrocatalysts is presented and is used to probe the mechanisms of CO and FA oxidation under conditions relevant to fuel cells.