Experimentally Determined Correlation Between Electronic and Electrocatalytic Properties in Oxidation of Small Organic Molecules for Systems Consisting of Noble Metal

Abstract

It is widely accepted that catalytic and electronic properties of catalyst‘s surface are strongly related. It has been postulated that the catalyst’s center of the d-band (εd) can be used to predict the adsorption strength and dissociation energy of simple molecules adsorbed on it. However there are not many studies correlating electronic properties of catalysts determined experimentally to the catalytic activity, because usually the electronic properties of catalysts surface are calculated using DFT. In this presentation we will focus on experimental verification of the role of electronic properties on the electrocatalytic activity of selected systems containing Pt, Pd and Rh, with purposefully modified electronic properties, in electrooxidation of small organic molecules, such as ethanol and formic acid. Model systems with purposely modified electronic properties were obtained by alloying and by forming of pseudomorfic layers of one metal on the surface of other metal. As a result in the investigated catalysts the lattice parameter was modified which, together with the possible partial charge transfer, induced the changes in their electronic properties. Modification of the lattice parameter changes the degree of overlapping of band forming orbitals and, according to the d-band center theory, it can be expected that when metal lattice is contracted, the valence band becomes broader, and when metal lattice is expanded the valence band (d-band) shrinks. The changes in d-band width causes the shift of the d-band: the broadening of the valence band leads to shift of the d-band center away from Fermi level and when the d-band shrinks, the d-band center shifts in the opposite direction. The changes in electronic properties can be observed using X-ray Photoelectron Spectroscopy (XPS) and UV Photoelectron Spectroscopy (UPS), and for the same material the catalytic properties can be monitored using electrochemical methods. We used this approach to investigate how the changed electronic properties influence the catalytic activity. We focused on CO, formic acid and ethanol electrooxidation reaction. Electrooxidation of small organic molecules, such as ethanol and formic acid, on noble metals is of particular interest due to its potential application in low temperature direct ethanol/formic acid fuel cells (DEFCs/DFAFCs), and for better understanding the mechanisms of electrode reactions. In particular CO and formic acid oxidation are considered model electrode reaction, where oxidation of ethanol, albeit more complicated, is of particular interest due to its importance in DEFCs. Despite the significant work already devoted to find a good low temperature catalyst for electrooxidation of ethanol, there is no known ethanol electrooxidation reaction (EOR) catalyst, allowing for complete electrooxidation of ethanol to CO2. Even for the most active systems, incomplete and slow EOR renders the DEFCs not economically viable. Better understanding of the factors influencing the catalytic activity should allow for development of novel, more active and selective electrocatalysts, and for further development of direct, liquid fuel feed, low temperature fuel cells. This project was funded from Polish National Science Centre budget based on decision number DEC-2013/09/B/ST4/00099