Surface-Bound Intermediates in Low-Temperature Methanol Synthesis on Copper: Participants and Spectators

The reactivity of surface-adsorbed species present on copper catalysts during methanol synthesis at low temperatures was studied by simultaneous infrared spectroscopy (IR) and mass spectroscopy (MS) measurements during "titration" (transient surface reaction) experiments with isotopic tracing. The results show that adsorbed formate is a major bystander species present on the surface under steady-state methanol synthesis reaction conditions, but it cannot be converted to methanol by reaction with pure H2 or with H2 plus added water. Formate-containing surface adlayers for these experiments were produced during steady-state catalysis in (a) H2:CO2 (with substantial formate coverage) and (b) moist H2:CO (with no IR visible formate species). These reaction conditions both produce methanol at steady state with relatively high rates. Adlayers containing formate were also produced by (c) formic acid adsorption. Various "titration" gases were used to probe these adlayers at modest temperatures (T = 410–450 K) and 6 bar total pressure. Methanol gas (up to ∼1% monolayer equivalent) was produced in "titration" from the H2:CO2 catalytic adlayers by H2 plus water, but not by dry hydrogen. The decay in the formate IR features accelerated in the presence of added water vapor. The H2:CO:H2O catalytic adlayer produced similar methanol titration yields in H2 plus water but showed no surface formate features in IR (less than 0.2% monolayer coverage). Finally, formate from formic acid chemisorption produced no methanol under any titration conditions. Even under H2:CO2 catalytic reaction conditions, isotope tracing showed that preadsorbed formate from formic acid did not contribute to the methanol produced. Although non-formate species exist during low-temperature methanol synthesis on copper which can be converted to methanol gas by titration with pure H2 plus water in sufficient quantities for that intermediate to be observable by IR, formate itself is only a "spectator" in this reaction and gives no observable methanol upon any titration we performed with H2 or H2 plus water.