Oxidation of dilute H2 and H2/O2 mixtures by hydrogenases and Pt

Hydrogenase enzymes that allow micro-organisms to gain energy from oxidation of H2 undergo efficient electrocatalysis of H2 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 H2 and continue oxidising H2 in the presence of O2 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 H2 and mixtures of H2 and O2. We show that Pt is a poor catalyst for oxidation of sub-atmospheric levels of H2 compared to the MBH from Ralstonia eutropha H16, and that at a platinised electrode, H2 oxidation competes less favourably with reduction of O2 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 H2.