(Non)formation of Methanol by Direct Hydrogenation of Formate on Copper Catalysts

Abstract

We have attempted to hydrogenate adsorbed formate species on copper catalysts to probe the importance of this postulated mechanistic step in methanol synthesis. Surface formate coverages up to 0.25 were produced at temperatures between 413 and 453 K on supported (Cu/SiO2) copper and unsupported copper catalysts. The adlayers were produced by various methods including (1) steady-state catalytic conditions in CO2−H2 (3:1, 6 bar) atmospheres and (2) exposure of the catalysts to formic acid. As reported in previous work, the catalytic surface at steady state contains bidentate formate species with coverages up to saturation levels of ∼0.25 at the low temperatures of this study. The reactivity of these formate adlayers was investigated at relevant reaction temperatures in atmospheres containing up to 6 bar H2 partial pressure by simultaneous mass spectrometry (MS) and infrared (IR) spectroscopy measurements. The yield of methanol during the attempted hydrogenation (“titration”) of these adlayers was insignificant (<0.2 mol % of the formate adlayer), even in dry hydrogen partial pressures up to 6 bar. Hydrogen titration of formate species produced from formic acid also failed to produce significant quantities of methanol, and attempted titration in gases consisting of CO-hydrogen mixtures or dry CO2 was also unproductive. The formate decomposition kinetics, measured by IR, was also unaffected by these changes in the gas composition. Similar experiments on unsupported copper also failed to show any methanol. From these results, we conclude that methanol synthesis on copper cannot result from the direct hydrogenation of (bidentate) formate species in simple steps involving adsorbed H species alone. Furthermore, experiments performed on both supported (Cu/SiO2) and unsupported copper catalysts gave similar results, implying that the methanol synthesis reaction mechanism involves only metal surface chemistry. Pre-exposure of the bidentate formate adlayer to oxidation by O2 or N2O produces a change to a monodentate configuration. Attempted titration of this monodentate formate/O coadsorbed layer in dry hydrogen produces significant quantities of methanol, although decomposition of formate to carbon dioxide and hydrogen remains the dominant reaction pathway. Simultaneous production of water is also observed during this titration as the copper surface is rereduced. These results indicate that coadsorbates related to surface oxygen or water-derived species may be critical to methanol production on copper, perhaps assisting in the hydrogenation of adsorbed formate to adsorbed methoxyl.

Hydrogenation of CO2 to methanol and CO on Cu/ZnO/Al2O3: Is there a common intermediate or not?

“…In the carboxyl mechanism, carboxyl decomposition (COOH ⁄ + ⁄ ¡ CO ⁄ + OH ⁄ ) is proposed as the RDS, where only one H atom is involved [8,30] ( ⁄ = adsorbed surface species or free surface sites). It is noted that the decomposition of formate (HCOO ⁄ ¡ OH ⁄ + CO ⁄ ), also with presence of a single H atom, has been discussed as an alternative RDS [31], but formate likely is a spectator in the shift chemistry [7,32]. Experimentally, we observe a slight evolution toward an inverse KIE with increasing pressure, which might point to a beginning change in mechanism of CO formation, possibly due to higher formate coverage (Fig.…”

“…In the recent literature reports, various hypotheses for the identity of the active sites and the nature of the reaction mechanism are presented, in particular with regard to the intermediates and precursors of methanol formation [6][7][8][9][10][11][12]. Different conclusions regarding the active site and the reaction mechanism have been drawn based on the different materials studied, such as clean and oxide-promoted Cu catalysts.…”

Frontiers, Opportunities, and Challenges in Biochemical and Chemical Catalysis of CO2 Fixation

“…Numerous studies have been aimed at identifying the elementary steps and rate-limiting process-(es). 86,88a,89a,90 …”

Recent development in catalytic technologies for methanol synthesis from renewable sources: A critical review

“…In the case of Cu active site, even though many studies have been conducted, the nature of the active site is yet to be fully understood and there are still controversies regarding the roles of active sites. Some researcher suggests that the catalytic activity is proportional to the surface area of metallic Cu (Cu 0 ) [85]. This is based on the fact that only Cu 0 was observed on the surface of Cu(1 0 0) [86] and that the catalyst activity increased as the Cu 0 increased up to Cu/ZnO ratio of 8 and then decreased [87].…”