Photoemission study of Ce/Ta(110) and Ce/W(110) interfaces and the catalytic oxidation effect

A weak Ce/Ta interface reaction is illustrated by the reduced Ta 4f surface core-level shift (SCS) upon Ce adsorption. No Ce/Ta interdiffusion was found. The Ce layer may greatly enhance the oxidation of Ta(110). Oxidation of Ce/Ta(110) at 300 K yields one monolayer (ML) of Ta suboxide (∼TaO), followed by the rapid formation of Ta2O5. Ce/W(110) adsorption patterns were determined by low-energy electron diffraction. For Θ ≤ 0.5 ML, the adsorption structure is characterized by one-dimensional commensuration along the [110] direction. It changes abruptly to a hexagonal pattern after 0.5 ML. The interatomic spacing shrinks continuously from 9% larger than that for γ-Ce to 3% smaller than that for α-Ce. Correspondingly, the Ce 4f photoelectron spectrum evolves to resemble that of α-Ce. W 4f core levels are also correlated to the adsorption structure. The surface monoxide formation found on Ce/Ta(110) and Ce/W(110) is not an inherently necessary step in the catalytic oxidation, but rather an interface product on the most densely-packed bcc (110) surfaces