Propan-2-ol on Ni(111): identification of surface intermediates and reaction products

The adsorption and reaction of propan-2-ol on Ni(1 1 1) has been followed by reflection absorption IR spectroscopy and temperature programmed desorption. At 110 K, nondissociative molecular adsorption is observed in the monolayer, with randomly oriented multilayers observed at higher exposures. On increasing substrate temperature to 200 K, scission of the OH bond is observed with formation of a 2-propoxide surface species which is adsorbed with C, site symmetry and oriented upright with the metal-O-C field close to a 180 degrees angle. The alkoxide species is stable to 320 K, above which scission of the alpha -CH bond occurs, with simultaneous formation and desorption of acetone. This selective dehydrogenation to acetone is the majority reaction pathway on the surface and is critically controlled by the high barrier to alpha -CH bond activation which ensures remarkable stability for the 2-propoxide intermediate. As a result, selective dehydrogenation occurs at a sufficiently high enough temperature so that acetone desorption competes very effectively with unselective decomposition to CO, H and CxHy. Acetone is, therefore, evolved in a reaction-limited process at 340 K, while the minority non-selective decomposition pathway evolves H: and CO in desorption-limited processes. The ease of bond breaking (O-H > alpha -CH > alpha -CC) identified for C-1 and C-2 alcohols on Ni(1 1 1) seems also to be valid for C-3 alcohol chemistry on this surface. (C) 2001 Elsevier Science B.V. All rights reserved.