The Decomposition Pathways of Methanol on Clean Ru(0001), Studied by Reflection−Absorption Infrared Spectroscopy (RAIRS)

Abstract

The adsorption and decomposition of methanol on clean Ru(0001) were investigated by reflection−absorption infrared spectroscopy (RAIRS). At low temperature (90 K) and coverage (0.1 L), it was confirmed that methanol adsorbs dissociatively as methoxide (CH3O−). No experimental evidence was obtained of an alternative decomposition for high coverage. Different bonding sites and geometries, depending on temperature and coverage, were proposed for methoxide and correlated with the corresponding CO stretching wavenumbers. Methoxide may either undergo complete dehydrogenation into CO(ads) and H(ads), if annealed in small temperature steps (in the range between 110 and 320 K), or partial dehydrogenation into very stable η2-formaldehyde, by a one-step thermal activation (from 130 K to at least 190 K), in the presence of previously formed products (CO and atomic species). At high temperatures (≥190 K), methanol undergoes O−H, C−H, and C−O bond scission, leaving surface fragments undetectable by RAIRS. However, in a sequential dosing, the fragments from the first methanol molecules that hit the surface seem to have a passivating effect on Ru(0001). Subsequent doses undergo only partial dehydrogenation, yielding η2-formaldehyde, which was isolated on the surface in two bonding configurations: bridging [μ2-η2(C,O)-H2CO] and chelating [μ1-η2(C,O)-H2CO], characterized by the νCO mode at 1262 and 1277 cm-1, respectively. This assignment was confirmed by adsorbing CD3OH. The bridging form is favored at lower coverage. Formaldehyde prepared by sequential dosing is stable on the surface up to at least 290 K, although some dehydrogenates to CO(ads) above 190 K.

Ruthenium–Platinum Catalysts and Direct Methanol Fuel Cells (DMFC): A Review of Theoretical and Experimental Breakthroughs

“…In this subsection, we analyze relevant experimental published results in Ru catalysts regarding DMFC or methanol electrooxidation context [44,45]. They explore several aspects of the reaction and catalyst role, in particular those relating with thermal evolution.…”

“…Barros et al experiments discriminate temperature effects per surface exposure. At 90 K (considered a low temperature), experimental data indicate a fast dehydrogenation of methanol to methoxide (CH 3 O) in the solo low coverage crystal Ru(0001) surface, while at the same temperature and catalyst metal surface, a high coverage presented no evidence of detection of scission of the C-O bond [44]. Gadzincki et al found isolated methanol molecules in solo Ru(0001) surfaces, when both temperature (≈80 K) and coverage was low [45].…”

“…Formation of methanol multilayers was also detected and investigated. Though capable of forming at low temperature (below 110 K), they desorb, become after annealing into methanol clusters only at temperatures of 120 K or with increasing surface coverage [44,45]. In fact, when the process of surface annealing occurs in small steps, at high temperatures, the formed methoxide decomposes, yielding adsorbed carbon monoxide and adsorbed hydrogen [44].…”

“…Formaldehyde formation occurs when combining CO and CH 3 [44]. Formed formaldehyde through reactivity reduction regarding cleavage of C-H and C-O bonds, partially decomposes to CO ads above 190 K, and otherwise remaining stable up to 290 K [44].…”

“…In fact, after methanol adsorption and decomposition through high temperature annealing (600 K), there is a complete lack of methane desorption as well as no detection of oxygen residues [45]. Interestingly, when imposing methanol adsorption at 190 K, there were no detectable products from this direct methanol adsorption through reflection-absorption infrared system (RAIRS), either for high or low coverage [44].…”