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

摘要

Photoemission spectroscopy was used to study Ce/Ta(110) and Ce/W(110) interfaces, and the catalytic oxidation effect on both surfaces. 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 Ta[subscript]2O[subscript]5. Ce/W(110) adsorption patterns were determined by low-energy electron diffraction. For [theta] ≤ 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 [gamma]-Ce to 3% smaller than that for [alpha]-Ce. Correspondingly the Ce 4f photoelectron spectrum evolves to resemble that of [alpha]-Ce. W 4f core levels are also correlated to the adsorption structure. A significantly widened W SCS is found at low Ce coverages. After the formation of the hexagonal patterns, the Ce-W registration is lost, and the interaction within the Ce layer increases. The SCS for the top layer of W is partially restored. Electronic charge polarization from Ce toward the W (or Ta) substrate is believed to be the key mechanism for the Ce-induced SCS. The interface charge polarization affects deeper layers in the substrate. The W 4f \u22bulk\u22 component broadens slightly toward lower binding energies, despite no major interface disruption. Oxidation of Ce/W(110) results in the rapid formation of 1 ML of W surface monoxide. No other W oxides were observed. A comparative experiment on O[subscript]2/Ce/W(111) shows the growth of WO[subscript]3 instead of WO. Therefore 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. Ce/Ta and Ce/W interface reactions are excluded as the general cause of the catalytic oxidation. An earlier suggestion is reconfirmed that changes in the Ce oxide states convert O[subscript]2 to oxygen ions and thus promote the oxidation of the substrate.