(Non) formation of methanol by direct hydrogenation of formate on copper catalysts
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 413K and 453K 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) by exposure of the catalysts to formic acid. As reported in earlier 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 were also unproductive. The formate decomposition kinetics, measured by IR, were 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 only involves 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 re-reduced. These results indicate that co-adsorbates 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.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 990552
- Report Number(s):
- PNNL-SA-74992; 30209; TRN: US201020%%410
- Journal Information:
- Journal of Physical Chemistry C, 114(40):17205-17211, Vol. 114, Issue 40; ISSN 1932-7447
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
10 SYNTHETIC FUELS
CARBON DIOXIDE
CATALYSTS
CHEMISTRY
COPPER
FORMATES
FORMIC ACID
GASES
HYDROGEN
HYDROGENATION
KINETICS
MASS SPECTROSCOPY
METHANOL
MIXTURES
OXIDATION
OXYGEN
PARTIAL PRESSURE
REACTION KINETICS
SPECTROSCOPY
SYNTHESIS
TITRATION
Methanol synthesis
copper catalysts
SSITKA
hydrogenation
Environmental Molecular Sciences Laboratory