The adsorption of H 2O on clean and oxygen-covered pd(100): Formation and reaction of OH groups

Abstract

The adsorption of H 2O on clean and oxygen-covered Pd(100) was studied with temperature programmed reaction spectroscopy (TPRS), high resolution electron energy loss spectroscopy (EELS), and low energy electron diffraction (LEED). H 2O adsorbs molecularly on the clean surface at 100 K and desorbs in two states at 170 and 180 K. The first state is due to multilayer desorption with a zero order desorption energy of 10.3 kcal mol . The 180 K state is due to the desorption of H 2O bound directly to the surface; this state saturates at a coverage of 0.25 monolayer. H 2O adsorbed at 100 K reacts with preadsorbed oxygen as the sample is heated above 175 K to form stable hydroxyl groups (OH). LEED showed that OH groups are adsorbed in a p(2 × 1) structure, and EELS evidence indicates that the adsorption site is not a four-fold hollow. The coverage of OH groups is dependent on the oxygen precoverage, and a maximum in OH coverage of 0.22 is obtained at an oxygen precoverage of 0.16 monolayer. Oxygen precoverages less than about 0.16 react quantitatively with H 2O adsorbed at 100 K to form OH groups. Larger oxygen precoverages (up to 0.31) gradually decrease the amount of H 2O that could initially be adsorbed at 100 K and thus reduce the OH coverage. The OH groups recombined directly (which was verified by H/D exchange measurements) to give atomic oxygen and H 2O desorption at 250 K for oxygen precoverages less than 0.16. This OH disproportionation occurs in a different state at 230 K for oxygen precoverages greater than 0.16. The minimum Pd-O bond energy for OH groups is estimated to be 51 kcal mol .