Spectroscopic studies of surface–gas interactions and catalyst restructuring at ambientpressure: mind the gap!

Abstract

Recent progress in the application of surface vibrational spectroscopy at ambientpressure allows us to monitor surface–gas interactions and heterogeneous catalyticreactions under conditions approaching those of technical catalysis. The surfacespecificity of photon-based methods such as polarization modulation infraredreflection absorption spectroscopy (PM-IRAS) and sum frequency generation(SFG) spectroscopy is utilized to monitor catalytically active surfaces while theyfunction at high pressure and high temperature. Together with complementaryinformation from high-pressure x-ray photoelectron spectroscopy (HP-XPS) andhigh-resolution transmission electron microscopy (HRTEM), reaction mechanisms canbe deduced on a molecular level. Well defined model catalysts, prepared underultrahigh vacuum (UHV), are typically employed in such studies, including smoothand stepped single crystals, thin oxide films, and oxide-supported nanoparticles.A number of studies on unsupported and supported noble metal (Pd, Rh) catalysts arepresented, focusing on the transformation of the catalysts from the ‘as-prepared’ to the‘active state’. This often involves pronounced alterations in catalyst structure andcomposition, for example the creation of surface carbon phases, surface oxides orsurface alloys, as well as nanoparticle restructuring. The reactivity studies includeCH3OH,CH4 and CO oxidation with gas phase analysis by gas chromatography and mass spectrometry.Differing results between studies under ultrahigh vacuum and ambient pressure,and between studies on single crystals and supported nanoparticles, demonstratethe importance of ‘minding the gap’ between idealized and realistic conditions.