THE CHEMISORPTION AND REACTIONS OF FORMIC-ACID ON CU FILMS ON ZNO(000(1)OVER-BAR)-O

The adsorption and reactions of formic acid (HCOOD:HCOOH = 3:1) on the oxygen-terminated ZnO(0001BAR)-O surface and on thin Cu films deposit ed on the ZnO(0001BAR)-O surface have been studied with temperature pr ogrammed desorption (TPD) and XPS. Small amounts of formic acid dissoc iate at defect sites on clean ZnO(0001BAR)-O to yield surface formate (HCOO). The acid D(H) from this dissociation does not reappear in TPD, and is lost to the ZnO bulk, as confirmed by nuclear reaction analysi s. The surface HCOO decomposes to yield nearly simultaneous CO2 (37%), CO (63%) and H-2 TPD peaks at 560 K. Substantial amounts of D (approx imately 20%) are incorporated in this hydrogen TPD peak resulting from formate decomposition at ZnO defects, indicating that bulk D is readi ly accessible. Submonolayer and multilayer Cu films that are deposited at 130 K and partially cover the ZnO surface as 2D and 3D islands ads orb formic acid and decompose it into formate and hydrogen much like t he Cu(110) surface. The surface formate from the Cu film decomposes at 470-500 K to give primarily CO2 and H-2, also much like Cu(110), alth ough atom-thin Cu islands also give approximately 40% CO. Annealed Cu films give formate decomposition peaks at 25-50 K lower in temperature , attributed to thickening and ordering of the Cu islands to form Cu(1 11)-like sites. The acid D(H) atom from the formic acid is partially l ost by hydrogen spillover from the Cu islands into the ZnO substrate, especially for thin Cu films. This effect partially desorbs and is enh anced upon preannealing the Cu layers, due to increased H diffusion ra tes across the annealed Cu islands, and/or the decrease in island size . Bulk D(H) is slowly removed as D2, HD and H-2 above 400 K in diffusi on-limited desorption, catalyzed by Cu.