Raman and infrared spectroscopies as in situ probes of catalytic adsorbate chemistry at electrochemical and related metal–gas interfaces: some perspectives and prospects

Abstract

Some recent progress in the utilization of infrared and especially Raman spectroscopies for the in situ vibrational characterization of adsorbates at electrochemical interfaces having relevance to catalytic chemistry is briefly outlined, and illustrated by means of examples culled chiefly from our laboratory. The primary factors responsible for the differences as well as similarities in the experimental strategies pursued for metal–solution interfaces as compared with metal surfaces in gas‐phase and ultrahigh vacuum (UHV) environments are discussed, and the distinct virtues of surface‐enhanced Raman scattering (SERS) and infrared reflection‐absorption spectroscopy (IRAS) for scrutinizing the first interfacial type are assessed. The detailed influences of the electrochemical double layer on chemisorbate vibrational properties at ordered metal–solution interfaces as gleaned by in situ IRAS data in comparison with spectra for analogous “model electrochemical” interfaces in UHV are described, and briefly illustrated for carbon monoxide on Pt(111) and Ir(111). The significance of the surface potential φ in controlling chemisorbate properties on metal surfaces in gaseous and UHV as well as electrochemical environments is pointed out. Evidence for the occurrence of “redox pinning” of φ by gaseous species in ambient‐pressure systems is outlined, along with possible catalytic implications. The burgeoning prospects for utilizing SERS as a versatile as well as uniquely sensitive vibrational probe of catalytically significant, especially transition‐metal, interfaces in both electrochemical and gas‐phase environments are delineated. Emphasis is placed on the typically richer vibrational spectra attainable for SERS compared to IRAS, arising from differing surface selection rules along with the greater sensitivity and wider wavenumber ranges accessible to the former method, and exemplified by benzene adsorption on rhodium and palladium electrodes. The anticipated power of SERS for assessing the reactivity as well as identity of adsorbed intermediates in ambient catalytic systems by means of transient in situ spectral measurements is noted, and illustrated briefly for ambient‐pressure methanol oxidation on rhodium.