DFT studies for cleavage of C-C and C-O bonds in surface species derived from ethanol on Pt(111)

Results from self-consistent periodic DFT calculations were used to study the relative stabilities and reactivities of surface species on Pt(I 11) derived by subsequent removal of hydrogen atoms from ethanol. Within each C2OHx isomeric set, the lowest energy surface species (with respect to gaseous ethanol and clean Pt(I 11) slabs) are ethanol, 1-hydroxyethyl (CH3CHOH), I-hydroxyethylidene (CH3COH), acetyl (CH3CO), ketene (CH2CO), ketenyl (CHCO), and CCO species. The energies of these species are -27, -28, -55, -84, -82, -88, and -53 kJ/mol, respectively, where the corresponding H atoms removed from ethanol are adsorbed on separate Pt(I 11) slabs. Transition states for C-C and C-O bond cleavage reactions were calculated for the most stable intermediates and for intermediates leading to exothermic bond cleavage reactions. A linear correlation between the energies of transition state and the energies of corresponding surface species was used to estimate transition-state energies of remaining reaction intermediates. The I-hydroxyethylidene (CH3COH) species has the lowest energy transition state (42 kJ/mol) for C-O bond cleavage, and the adsorbed ethylidyne (CCH3) and hydroxyl product species lead to a favorable energy change for this C-O bond cleavage reaction (-38 kJ/mol). The ketenyl (CHCO) species has the lowest energy transition state (4 kJ/mol) for C-C bond cleavage, and the adsorbed CO and methylidyne (CH) product species lead to a very exothermic energy change for this reaction (-144 kJ/mol). Results from DFT calculations, combined with transition state theory, predict that the rate constant for C-C bond cleavage in ethanol is faster than for C-O bond cleavage on Pt(l 11) at temperatures higher than about 550 K. In addition, the calculated value of the rate constant for C-C bond cleavage in ethanol is predicted to be much higher than for C-C bond cleavage in ethane on Pt(I 11). Similarly, the rate of C-C bond cleavage in ethanol is predicted to be much higher than for C-0 bond cleavage in carbon monoxide on Pt(I 11). (C) 2003 Elsevier Inc. All rights reserved.