Temperature programmed desorption studies of OD coadsorbed with H2 on Pt(111)

A molecular beam source of pure hydroxyl radicals has been developed and used to explore the water reaction catalyzed over Pt(111). An electrostatic hexapole selectively focused OD radicals from a supersonic corona discharge source onto a Pt target at a surface temperature of T-S=143 K. Subsequent D2O temperature programmed desorption (TPD) spectra revealed two major features, one near T(S)similar to 170 K from desorption of molecular water overlayer and a second near T(S)similar to 210 K from the decomposition of an adsorbed OD intermediate. The latter feature was isolated and analysis of TPD spectra revealed that the D2O production reaction was approximately half-order in total oxygen coverage with a pre-exponential factor ranging from v(d)=4 +/- 1x10(16) to 5 +/- 2x10(18) molecules(1/2) cm(-1) s(-1) and activation energy E-a=9.7 +/- 0.1 to 11.5 +/- 0.1 kcal mol(-1) for initial coverage ranging from theta(0)=0.04 to 0.25 ML. Coadsorption studies of OD and H-2 revealed that H atoms drive reactions with adsorbed OD at T(S)similar to 180 K to form all three water isotopes: D2O, HDO, and H2O. Oxygen (O-2) TPD spectra contained three desorption features (T-S=700 K, 735 K, and 790 K). The relative abundance of O-2 from these three features was virtually the same in all low temperature (T-S=143 K) TPD experiments. At elevated dosing temperatures (T-S=223 K) the two features at T-S=700 K and 790 K could be selectively titrated from the surface by hydrogen. The presence of hydrogen prior to OD exposure at this elevated temperature prevented the accumulation of oxygen on the surface. The implications of these observations on our mechanistic understanding of the low temperature (T-S< 210 K) water reaction are discussed. (C) 2000 American Institute of Physics. [S0021-9606(00)70214-5].