Simultaneous measurements of gas-phase (by mass spectrometry) and surface species (by infrared spectroscopy) during catalysis and/or during surface reaction transients allows firm association of surface species with product formation, particularly when isotope methods are used in conjunction. Such results from two reactions involving methanol are presented here - (1) methanol synthesis on copper and (2) methanol dehydration on surface tungsten oxide species. The mechanisms of both reactions are central to the kinetics and have attracted considerable research. Methanol synthesis: On supported Cu(10wt%)/SiO2, a 13C-formate adlayer was produced from 13C formic acid chemisorption. Under subsequent catalytic reaction conditions in 6 bar H2-12CO2 the IR-detected high coverage by surface formate slowly switches to the 12C isotope. While substantial 13C formate remains however, the methanol product contains only the 12C istope. Formate is therefore conclusively shown to be a "spectator" species in methanol synthesis on copper under these conditions. Methanol dehydration: Atomic layer deposited (ALD) tungsten oxide WOx/SiO2 catalysts, were prepared by solution impregnation techniques. During methanol dehydration catalysis on WOx/SiO2, prominent surface species, previously assigned to Lewis-bound methanol and methoxy [3], are observed. The stabilities of these adlayer species were observed under both (1) inert conditions (flowing Ar) and (2) during flowing methanol/DME production using CD3/CH3 isotope discrimination (methanol partial pressure 1.5 mbar. The methyl groups remained intact during these experiments. While the species were very stable in inert gas and survived overnight at temperatures up to 300C, the measured replacement times were shorter under catalytic conditions. The replacement time for methoxy was ~1000 s and that for L-methanol was ~2000 s. The transient in the DME product transient was similar to the L-methanol at ~2000 s. The relative roles of reactive renewal versus simple displacement reactions will be discussed. References G.A. Olah, A. Goeppert, G.K. S. Prakash, J. Org. Chem. 74 (2009) 487-498. Y. Yang, R.S. Disselkamp, C.T. Campbell, J. Szanyi, C.H.F. Peden, J.G. Goodwin, Jr., Rev. Sci. Instrum., 77 (2006) 94-104. Wachs. I.E.; Deo, G;, Jehng, J-M.; Kim, D. S.; Hu, H. ACS Symposium Series (1996), 638, 292-299.

Spectator or Intermediate? Simultaneous MS/FTIR studies of methanol catalysis

249th National Meeting (2015)

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