Electrooxidation of Ethanol and Formic Acid on Model Systems Containing Noble Metals - Combined Spectroscopic and Electrochemical Studies

In electrooxidation of small organic molecules, such as ethanol and formic acid, noble metals exhibit exceptional catalytic activity. However even for the most active catalysts known, both above mentioned reactions proceed with high overpotentials at room temperature. The overall low activity of known catalysts towards electrooxidation of organic molecules render the low temperature fuel cells fed with liquid organic fuels, such as direct ethanol fuel cells, far from being economically viable.

Better understanding of the factors influencing catalytic activity is crucial to design new, more active catalytic systems. It is widely accepted that catalytic and electronic properties of catalyst‘s surface are strongly related. Electronic properties of catalyst can be purposely modified, for instance by modifying its lattice parameter. In general, changes of the lattice parameter of metals will induce changes in their electronic properties as degree of overlapping of band forming orbitals change . When metal lattice is contracted, the conduction band becomes broader, and when metal lattice is expanded the conduction band (d-band) shrinks. Depending of the degree of filling of the d-band, the changes in d-band width causes the shift of the d-band center in opposite directions: if the conduction band is filled in more than half the broadening of the conduction band leads to shift of the d-band center towards Fermi level; when it is filled in less than half, the d-band center shifts in the opposite direction [1]. The changes in d-band center position relative to Fermi level can be correlated to adsorption properties of the surface [1]. We used the above mechanism to purposely induce the changes in the electronic properties and to investigate how the changed electronic properties influence the catalytic activity.

To study the correlation between electronic and electrocatalytic properties of catalysts we used two distinct types of model systems: i) epitaxial layers of catalytically active noble metals (platinum, palladium) on another noble metals (in form of core-shell nanoparticles) and ii) nanoalloys, where catalytically active metal was alloyed with metal inactive towards the studied reaction, such as Pt-Pd which we used for electrooxidation of ethanol in acidic conditions. In those conditions palladium is catalytically inactive, and also does not participate in so called “bi-functional mechanism”, thus it can be assumed, that the overall catalytic activity can be to a large extend attributed to changes in surface geometry and  to changes of electronic properties of platinum.

To confirm changes in the electronic properties of our catalysts we used X-ray Photoelectron Spectroscopy (XPS) and UV Photoelectron Spectroscopy (UPS). Valence band and core-level spectra allows for determination of the electronic properties of the material, and together with electrochemical results the relation between electronic and electrocatalytic properties can be studied.

This project was funded from Polish National Science Centre budget based on decision number DEC-2013/09/B/ST4/00099

[1] A. Ruban, B. Hammer, P. Stoltze, H.L. Skriver, J.K. Norskov, J. Mol. Catal. A, 1997, 115, 421-429.