Coupling Ambient Pressure X-ray Photoelectron Spectroscopy with Density Functional Theory to Study Complex Surface Chemistry and Catalysis

Abstract

Ambient pressure X-ray photoelectron spectroscopy (APXPS) experiments narrow the pressure and materials gaps between UHV surface science experiments and applications. Upon closing these gaps, ambiguity can enter the analysis of the spectra due to overlapping peaks from different elements or functional groups of both the sample and the gas phase. Additionally, reaction intermediates and mechanisms are often inaccessible from interpretation of APXPS data alone. In many cases, these issues can be overcome with the aid of density functional theory (DFT) calculations. Here, we outline our process of combining DFT calculations with APXPS experiments by describing our recent investigations of the adsorption of dimethyl methylphosphonate (DMMP) on MoO3 and CuO. We begin by showing the importance of the characterization of the isolated gas phase molecule before adsorption onto a surface. In particular, strong agreement between theory and experiment helps identify plausible decomposition pathways of the isolated molecule and provides a baseline for interpretation of spectra showing evidence of DMMP interaction with surfaces. The intact adsorption of DMMP on MoO3 offers an illustration of how moving beyond pristine single crystalline surfaces in calculations can enable better modeling of experimental trends that result from surface defects. Studies of DMMP adsorption on CuO exemplify the powerful synergy of APXPS combined with DFT for elucidation of complex reaction mechanisms on surfaces.