(734a) Trends in Formic Acid Electro-Oxidation on Pt and Pd Monolayers on Transition Metal Surfaces: A Theoretical and Experimental Study

Authors: 
Elnabawy, A., University of Wisconsin-Madison
Liang, Z., Brookhaven National Laboratory
Adzic, R. R., Brookhaven National Laboratory
Mavrikakis, M., University of Wisconsin-Madison
Formic Acid (FA) is a viable carbon-neutral fuel that can be produced from biomass1 or via the reduction of CO22. Formic acid electro-oxidation (FAO) in low-temperature direct formic acid fuel cells (DFAFCs) has attracted considerable interest due to its numerous advantages compared to other similar fuel cells3. DFAFCs show an equilibrium cell voltage (1.40 V) that is higher than that shown by hydrogen (1.23 V) and methanol (1.21 V) fuel cells among others4. Additionally, FA is liquid (unlike hydrogen) and, compared to methanol, is less toxic4,5 and shows less crossover through Nafion® membranes in fuel cells6. Platinum (Pt) and Palladium (Pd) are the best monometallic catalysts for FAO, however, they suffer from stability issues in acidic media, are poisoned by CO during the reaction, and are too expensive to make DFAFCs economically viable 3. One way to mitigate these issues is to deposit pseudomorphic monolayers of Pt or Pd atop other less expensive metals 7. Such structures would reduce the amount of Pt or Pd in the catalyst (and thereby the cost), and could improve the stability and activity of Pt and Pd8-10. To this end, we present a first-principles, self-consistent periodic density functional theory (PW91-GGA) study of FAO on model (111) and (100) facets of Pt and Pd monolayers atop six fcc metals (Au, Ag, Pt, Pd, Ir, and Rh), and (0001) facets of Pt and Pd monolayers atop three hcp metals (Os, Ru, and Re). On all surfaces, we calculate the free energies of the adsorbed intermediates: carbon monoxide (CO), hydroxyl (OH), carboxyl (COOH), and formate (HCOO). Using these data, we develop thermochemical free energy diagrams and, thereby, calculate onset potentials of three mechanisms: direct oxidation via carboxyl, direct oxidation via formate, and the indirect mechanism leading to the formation of CO. Comparing these onset potentials on the surfaces studied reveals trends in the preference to either direct mechanisms, as well as trends governing their propensity towards CO poisoning. For a subset of the close-packed surfaces, we compare the calculated theoretical trends to experimental trends, as determined by electrochemical measurements. Finally, having studied both close-packed and open facets of fcc systems, we discuss the structure sensitivity of FAO on Pt and Pd monolayers on transition metals.

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