SELECTIVE BETA-ELIMINATION IN THE REACTION OF 2-PROPANOL TO ACETONE ON RH(111)-P(2X1)-O

The formation of acetone from 2-propanol and Rh(111)-p(2 x 1)-O has been investigated by temperature programmed reaction and X-ray photoelectron spectroscopies and isotopic labeling experiments under ultrahigh vacuum conditions. Some 2-propanol forms 2-propoxide on Rh(111)-p(2 x 1)-O below 250 K and selective beta (with respect to the metal in 2-propoxide) C-H bond breaking at approximately 270 K is the primary path for acetone evolution. A minor amount of reversible C-H bond activation is also observed. beta-carbon-hydrogen bond breaking is proposed to be the rate-limiting step for the initial acetone evolution from 2-propanol on Rh(111)-p(2 x 1)-O at high coverage based on kinetic isotope effects. The rate of acetone evolution is in part rate-limited by desorption, however, for low 2-propanol exposures. In addition, there is some oxygen exchange between the surface and the acetone at approximately 320 K. Combustion to H2O, CO and CO2 is a competing pathway. Irreversible gamma-C-H bond breaking primarily leads to combustion. The reactivity of 2-propanol on the (2 x 1)-O surface is dramatically different from that on clean Rh(111), where nonselective decomposition to CO and H-2 is induced. The inhibition of extensive, nonselective C-H and C-C bond breaking is a crucial factor in determining the selectivity for beta-dehydrogenation to produce acetone.