Adsorption of cyclohexadiene, cyclohexene and cyclohexane on Pt(111)

The adsorption of 1,3-cyclohexadiene, 1,4-cyclohexadiene, cyclohexene and cyclohexane on Pt(111) was studied using ab initio density functional theory. For 1,3-cyclohexadiene three adsorption modes were distinguished: bridge 1,2-di-sigma/3,4-pi, hollow 1,4-di-sigma/2,3-pi and bridge 1,4-di-sigma/2,3-pi with adsorption energies of -155, -147 and -75 kJ/mol, respectively. Three stable adsorption modes were also identified for 1,4-cyclohexadiene: bridge quadra-sigma, hollow di-sigma/pi and bridge di-sigma with adsorption energies of -146 kJ/mol, -142 kJ/mol and -88 kJ/mol, respectively. Cyclohexene was found to adsorb in six modes: 4 di-sigma and 2 pi-adsorption modes. The preferred configuration was found to be boat di-sigma with an adsorption energy of -81 kJ/mol. The three other di-sigma adsorption modes have comparable adsorption energies, ranging from -64 to -69 kJ/mol. Molecular strain and C-Pt bonding energies are used to elucidate stability trends. Cyclohexane is found to adsorb only at the hollow site whereby the axial hydrogen atoms are positioned over surface Pt-atoms with an adsorption energy of -37 kJ/mol. The calculations correctly predict the weakening of the axial C-H bonds and provide a possible explanation for the large shift in the vibrational frequencies. (c) 2006 Elsevier B.V. All rights reserved.