THE COADSORPTION OF H AND O ON THE W(110) PLANE

The coadsorption of O and H on a W(110) plane has been studied by ther mal desorption, work function and XPS measurements. Attainable coverag es depend on adsorption order. For H preadsorption, O coverage decreas es linearly from O/W = 0.56 at H/W = 0 to O at H/W = 0.95. For O preco verage H decreases nonlinearly, reaching 0 at O/W = 0.35. Whether pre- or post-adsorbed, O decreases the peak desorption temperatures of H-2 to 400 K from their values of 560 to 450 K (depending on coverage) fo r H/W(110). This provides a rough estimate of Delta H for desorption o f H-2, namely 23 kcal/mol at high O coverage and 28 kcal/mol of H-2 at low O coverage. These values should be compared with Delta H = 34 kca l/mol of H at low PI coverage and 30 kcal/mol at high coverage on clea n W(110). For O post-adsorption work function increases are similar to those on clean W(110). However, for H post-adsorption complex behavio r is seen and differs for O/W(110) layers prepared and kept at 90 K an d those annealed at 600 K, suggesting differences in H binding. The im portance of these differences disappears at H-2 desorption temperature s, since desorption behavior is independent of adsorption order or sur face pretreatment. No evidence for OH or H2O formation under any condi tions was found. Sticking coefficients of O on H precovered W(110) dec rease linearly with O coverage, instead of showing a plateau as on cle an W(110), suggesting that I-I decreases the lifetime of O-2 precursor s. (C) 1998 Elsevier Science B.V. All rights reserved.