The adsorption, desorption, and exchange reactions of oxygen, hydrogen, and water on platinum surfaces. III. Water adsorption and exchange with oxygen and hydrogen

Abstract

Ultra-high vacuum thermal desorption experiments have been carried out on the adsorption of water, and the D 2 O/H and D 2 16 O/ 18 O exchange reactions on platinum previously characterized by oxygen and hydrogen adsorption studies. Water is weakly adsorbed and only at T < 150 K, an observation confirmed by using H 2 adsorption as a chemical probe for the presence of adsorbed water. The thickness of the adsorbed D 2 O layers was determined by a novel method involving monitoring the power output of the temperature programmer. Exchange occurs between adsorbed H and a D 2 O overlayer and the extent of exchange increases with the thickness of the overlayer. Isotope distribution studies show that the surface is heterogeneous and the exchange reaction does not occur at uniform rate. Exchange occurs between adsorbed 18 O and a D 2 16 O overlayer. The extent of exchange is constant up to T ∼ 300 K showing that D 2 O is more strongly bound on O-covered Pt presumably as a result of H bonding. Heterogeneity also is apparent in the O-exchange reaction. The apparent activation energies for exchange are 1.8 and 2.9 kcal mol −1 for the half- and fully-covered surfaces, respectively.