Effects of titania on the coadsorption of H 2 and CO on nickel surfaces: Consequences for understanding methanation over titania-supported nickel catalysts

Abstract

The effects of titania adspecies on the coadsorption of CO and hydrogen on polycrystalline nickel were studied under ultrahigh vacuum conditions using temperature-programmed desorption. Titania blocks CO adsorption at strongly bound sites (desorption energies ~130 kJ · mol −1) and enhances CO adsorption at more weakly bound sites (desorption energies ~90 kJ · mol −1). In addition, the presence of titania on nickel increases the strength of hydrogen adsorption. This net weakening of CO adsorption and strengthening of hydrogen adsorption allows hydrogen to compete more effectively for nickel adsorption sites. Model calculations of this coadsorption behavior at temperatures and pressures representative of experimental methanation conditions (450–700 K and 0.1–10 kPa, respectively) showed that the measured changes in CO and hydrogen heats of adsorption on nickel should lead to a one to two order of magnitude increase in the H-adatom concentration for titania-containing surfaces. Assuming a mechanism wherein CO dissociation and carbon hydrogenation are balanced at steady state (neither step is rate limiting), this higher H-adatom coverage leads directly to a one to two order of magnitude higher methanation activity. Thus, the higher methanation activities of titania-supported nickel catalysts which exhibit so-called strong metal-support interactions can be understood in terms of more competitive hydrogen adsorption on the nickel surface.