Reactivity of methanol on clean Ru(0 0 1) studied by RAIRS: effect of deuterium substitution

The effect of deuteration on the reactivity of methanol on clean Ru(0 0 1) has been studied by comparing the adsorption and decomposition of methanol (CH3OH), methanol-d3 (CD3OH) and methanol-d4 (CD3OD) on that surface, using reflection–absorption infrared spectroscopy (RAIRS). At 90 K and low exposure, the three methanol isotopomers adsorb dissociatively as methoxide (CH3O– or CD3O–), in a tilted geometry. The isotope effect is revealed by a lower coverage of CD3O– for methanol-d4, reflecting the higher O–D versus O–H bond enthalpy. The deuterated alcohols undergo C–D bond scission at 110 K, leaving mostly CO(ads), whereas CH3OH does not undergo C–H bond cleavage bellow 120 K. This higher reactivity of the deuterated alcohols was interpreted in terms of a surface assisted mechanism. No isotope effect is observed upon adsorption above 120 K. At 190 K, all the methanol isotopomers decompose into stable η2-formaldehyde (H2CO or D2CO, respectively) on passivated Ru(0 0 1) surfaces, obtained by sequential dosing.