COMPETITION BETWEEN C-O AND 1-C-H BOND SCISSION DURING DEOXYGENATION - THE REACTIONS OF 1-PROPANOL ON MO(110)

The reactions of 1-propanol on Mo(110) were investigated using temperature programmed reaction and high resolution electron energy loss and X-ray photoelectron spectroscopies. 1-Propanol forms 1-propoxide upon adsorption on Mo(110) at 120 K. An intensity analysis of the vibrational data indicates that the C-O bond vector is nearly perpendicular to the surface. Vibrational data also suggest that the C-O bond is weakened in 1-propoxide relative to 1-propanol. The 1-propoxide intermediate is stable up to 375 K, at which point it decomposes via three competing pathways: deoxygenation and dehydrogenation to form propene, the major hydrocarbon product; C-O bond hydrogenolysis to afford propane; and nonselective decomposition to gaseous dihydrogen, surface carbon, and surface oxygen. The reaction of isotopically-labeled 1-propanol shows that only the C-H bond at the 2 position (gamma to the metal and beta to the oxygen) in 1-propoxide is broken during propene formation. Carbon-oxygen bond scission is proposed to limit the rate of propene elimination. Conversely, dehydrogenation at the 1-carbon of 1-propoxide is proposed to limit the rate of nonselective decomposition. At the maximum coverage of 1-propoxide, approximately 60% of the adsorbed 1-propoxide forms hydrocarbons, whereas approximately 40% nonselectively decomposes. The reactions of 1-propanol are compared with 2-propanol and 1-propanethiol in an effort to delineate the controlling factors in deoxygenation kinetics.