Oxidation of Small Organic Molecules on Noble Metals and Pseudomorphic Layers of Noble Metals

Abstract

Ethanol, due to its low toxicity and relatively high energy density, is a promising candidate to be used in fuel cells. It is mainly obtained from biomass, thus its oxidation does not influence the total CO2 amount in the biosphere. However incomplete and slow electrooxidation of ethanol impedes the commercialization of low-temperature ethanol fuel cells. Electrocatalytic oxidation of ethanol is a complex, multi-step process with not completely understood mechanism. Factors influencing the catalytic activity of many systems towards ethanol electrooxidation are still unclear, and effective catalyst for ethanol electrooxidation has not been discovered yet. Most systems used for electrocatalytic oxidation of ethanol contain platinum-group metals in the form of nanoparticles, but even for those systems high overpotentials and incomplete oxidation of ethanol is observed. Another factor influencing the catalytic activity towards ethanol oxidation is poisoning of catalysts’ surface by products of incomplete oxidation. In particular for Pt, a quick decrease in surface activity during ethanol electrooxidation has been observed, which is related to surface poisoning by strongly adsorbed CO, a by-product of ethanol electrooxidation reaction. Adsorption of CO inhibits further electrooxidation of ethanol, and as a result high electrode potential is required to oxidize ethanol, as the surface CO must be oxidized first. To overcome the problem of surface poisoning by CO, bi-metallic surfaces, such as Pt-Ru and Pt-Sn were proposed, due to the fact, that the second metal provides adsorbed OH groups, required to oxidize the adsorbed CO, at lower potential than Pt. This so-called “bifunctional mechanism” was firstly described for methanol electrooxidation on Pt-Ru by Watanabe and Motoo in 1975. However in the case of ethanol electrooxidation the presence of Ru or Sn on Pt surface, despite yielding higher current density, leads to even further decrease in CO2 yield, increase the amount of acetic acid produced. This shift in product distribution significantly decrease the maximum amount of energy, which can be obtained from ethanol electrooxidation in conditions relevant to fuel cells on Pt-Sn or Pt-Ru, as it is impossible to oxidize acetic acid in those conditions, making oxidation of ethanol effectively a 4-electron process. It is thus imperative to develop new catalysts more active towards C-C bond scission, which is impossible without good understanding of the factors influencing catalytic activity in general. It is postulated, that electronic properties are in part responsible for catalytic activity of a given material. Theoretical DFT calculations suggest that chemisorption energy, activation barrier and energy of dissociation of small molecules on metal surface can be correlated to the d-band center of gravity of that metal (so called d-band center theory). This holds true for many systems and reactions it is possible that for more complex system other factors, not included in the d-band center theory, can play a significant role. For instance for more complex reactions, such as oxidation of simple alcohols, the relation between electronic and electrocatalytic properties are much more complicated, because those reactions are multi-step processes and the change of electronic properties may influence the elementary reactions to a different degree. One of the model systems allowing for studying the relation between catalytic and electronic properties of transition metals are pseudomorphic layers. Such systems consist of one metal (substrate) on which another metal is deposited forming a thin layer, usually a monolayer. It has been discovered, that in those systems the top atom layer mimics the crystallographic structure and lattice parameter of the substrate. Possible examples of such systems are monocrystal substrate with a monolayer of other metal or core-shell nanoparticles. Pseudomorphic layers allow to introduce and study changes in electronic properties due to modification of lattice parameter of the topmost layer and changes resulting from possible partial charge transfer between topmost layer and substrate material. At the same time, if pseudomorphic layer provides full coverage of the substrate’s surface, non-electronic effects, such as above mentioned bi-functional mechanism, can be eliminated. We focused on correspondence between electronic properties of noble metals (especially Pt and Pd) and their pseudomorphic layers observed using X-ray Photoelectron Spectroscopy (XPS) and UV Photoelectron Spectroscopy (UPS) and catalytic activity towards CO, formic acid and ethanol electrooxidation reactions. Catalytic activity was investigated using electrochemical methods. Oxidation of formic acid was selected as a model electrode reaction while oxidation of CO is a crucial step in electrooxidation of ethanol on noble metals. This project was funded from Polish National Science Centre budget based on decision number DEC-2013/09/B/ST4/00099