(Invited) Electronic-Level Insight on CO2 Reduction Reaction

Abstract

There is general agreement that catalytic activity is correlated to electronic properties of the catalyst’s surface. For selected molecules and simple surfaces it was shown by Norskov and co-workers that the strength of adsorption of such molecule and the kinetics of the respective catalytic reaction can be correlated to theoretically predicted electronic properties of the surface, i.e. catalyst d-band center (in relation to Fermi edge). However the calculated energy of the d-band center cannot be always correlated to catalytic activity, and the calculated electronic structure (DOS) is not always representative of the experimentally determined electronic structure, especially for non-uniform systems. This is the major drawback of the theoretical approaches currently being used to predict or explain the observed catalytic activity. Thus for assessing the factors responsive for catalytic activity, experimental determination of surface electronic structure is imperative. X-ray Photoelectron Spectroscopy (XPS) and UV Photoelectron Spectroscopy (UPS) experiments are often used to study the electronic properties of materials. In particular core-level electron’s binding energy and valence band (VB) electron’s density of state can be determined using XPS and UPS. Due to interactions of ejected photoelectrons with matter those experiments are in general limited to the Ultra-High Vacuum (UHV) conditions (ca. 10-9 mbar). In the recent years however the Near Ambient Pressure X-ray Photoelectron Spectroscopy/UV Photoelectron Spectroscopy techniques (NAP XPS/UPS) were developed, allowing for local (at the sample surface) increase in pressure up to tenths of millibars, bridging the gap between UHV adsorption studies and real-life systems. Using of the NAP methods presents unique opportunity to study the electronic properties (and changes of electronic properties) of surface during the adsorption from gas phase. This is of paramount importance as insight can be gained on electron-level interactions between the surface and adsorbing molecule, bond breaking/bond forming and similar phenomena, which was not possible before. Investigating the electronic properties of catalysts, in particular valence band DOS, in both – Ultra-High Vacuum (UHV) and Near Ambient Pressure (NAP) conditions allows for experimental determination of electronic-level properties of surface in absence and presence of CO2 molecule, the electronic-level changes induced by CO2 adsorption and experimental verification of CO2 adsorption theories. It can be expected that utilizing NAP XPS will results in better understanding of CO2 adsorption processes, fine-tuning the theoretical models and advance the science and technology of catalytic CO2 reduction.