The effect of pre-adsorbed atoms on the reactivity of methanol- d4 on Ru(0 0 1): Comparison between hydrogen and oxygen

Abstract

The influence of pre-adsorbed H and O on the adsorption and decomposition of methanol- d 4 on Ru(0 0 1) surfaces is analysed by RAIRS. It is shown that the reactivity of CD 3OD at 90 K is not determined by the nature of the modifying atom nor by the structure of the pre-adsorbed layer: a low dose of CD 3OD (0.1 L) undergoes O–D bond breaking, yielding CD 3O–, both on Ru(0 0 1)–H (0.5 ⩽ θ H < 1 ML) and on Ru(0 0 1)–O (0.25 ⩽ θ O ⩽ 0.6 ML) surfaces. At 90 K, methoxide- d 3 acquires a tilted configuration on all these surfaces, despite the fact that oxygen forms ordered phases whereas hydrogen (adsorbed at this temperature) does not. A fraction of the methoxide- d 3 undergoes C–D bond breaking at 110 K on all the modified surfaces, in a lower extent than on clean Ru(0 0 1). The stabilizing effect is more pronounced on the O modified layers, and is coverage dependent. The chemical nature of the pre-adsorbed atom is determinant of the unreacted methoxide geometry, as only oxygen is capable of inducing a reorientation of this species towards C 3v local symmetry. Confirmation of the adsorption geometries, both at 90 and 110 K, was obtained from the RAIR spectra of the selectively labelled CHD 2OH, adsorbed on the same surfaces. The long-range repulsive interactions between the pre-adsorbed atom and the final decomposition product at 130 K (carbon monoxide) are more evident on the denser O layer (0.6 ML), since this species does not remain on the surface. No partially hydrogenated intermediates were detected on the H modified surfaces, suggesting that, in case exchange reactions occur, they yield only gaseous products. On the contrary, direct evidence for the participation of pre-adsorbed O was supplied by the detection of deuterated formate (DCOO) for θ O = 0.6 ML.