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 b y reflection absorption IR spectroscopy and temperature programmed desorpti on. At 110 K, nondissociative molecular adsorption is observed in the monol ayer, with randomly oriented multilayers observed at higher exposures. On i ncreasing substrate temperature to 200 K, scission of the OH bond is observ ed 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 18 0 degrees angle. The alkoxide species is stable to 320 K, above which sciss ion of the alpha -CH bond occurs, with simultaneous formation and desorptio n of acetone. This selective dehydrogenation to acetone is the majority rea ction pathway on the surface and is critically controlled by the high barri er 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 mi nority 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.