Oxidation of dilute H 2 and H 2/O 2 mixtures by hydrogenases and Pt

Abstract

Hydrogenase enzymes that allow micro-organisms to gain energy from oxidation of H 2 undergo efficient electrocatalysis of H 2 oxidation or production when adsorbed on a graphite rotating disk electrode [K.A. Vincent, A. Parkin, F.A. Armstrong, Chem. Rev. 107 (2007) 4366]. Combining potential sweeps or steps with precisely controlled gas exchanges is enabling us to build up a detailed understanding of the many factors that control the chemistry of nickel–iron membrane-bound hydrogenase (MBH) enzymes. The observation that the MBH enzymes from Ralstonia strains have extremely high affinity for H 2 and continue oxidising H 2 in the presence of O 2 and CO has relevance for selective fuel cell catalysis [K.A. Vincent, J.A. Cracknell, J.R. Clark, M. Ludwig, O. Lenz, B. Friedrich, F.A. Armstrong, Chem. Commun. (2006) 5033; K.A. Vincent, J.A. Cracknell, O. Lenz, I. Zebger, B. Friedrich, F.A. Armstrong, Proc. Natl. Acad. Sci. U.S.A. 102 (2005) 16951], and this has led us to compare the ability of hydrogenases and platinum to oxidise low levels of H 2 and mixtures of H 2 and O 2. We show that Pt is a poor catalyst for oxidation of sub-atmospheric levels of H 2 compared to the MBH from Ralstonia eutropha H16, and that at a platinised electrode, H 2 oxidation competes less favourably with reduction of O 2 compared to the situation at hydrogenase-modified graphite. This should have implications for development of future selective energy catalysts able to concentrate the energy available from dilute H 2.