A Periodic Density Functional Theory Study of the Dehydrogenation of Methanol over Pt(111)

Nonlocal gradient-corrected periodic density functional theory calculations were used to examine the dehydrogenation of methanol to CO over the Pt(111) surface. Two decomposition routes were examinedone involving the activation of the O−H bond of methanol to form the methoxide intermediate and the other involving C−H bond activation to form the hydroxymethyl intermediate. These intermediates can subsequently react to form formaldehyde, formyl, and finally CO on the surface. Although these pathways are interesting because of their potential relevance for methanol fuel cells, under UHV conditions, we find that methanol will more likely desorb than react on Pt(111). The barriers for C−H and O−H bond activation were found to be much higher than the measured heat of desorption. This is consistent with experimental evidence. Our results indicate that methanol adsorbs weakly at the atop site on Pt. At 25% surface coverage, the hydroxymethyl (CH2OH), methoxide (CH3O), formaldehyde (HCHO), and formyl (HCO) species that can form adsorb at the atop, hollow, di-σ, and hollow (η2−η-C,O) sites, respectively. CO and atomic hydrogen adsorb in the three-fold-hollow sites. The chemisorption energies of CH3OH, CH2OH, CH3O, HCHO, HCO, CO, and H in their most favorable adsorption sites on the Pt surface were predicted to be −43, −209, −161, −49, −237, −168, and −269 kJ/mol, respectively. The computed adsorption energies were used to calculate the overall reaction energies for the proposed series of elementary steps in the metal-catalyzed dehydrogenation of methanol. The C−H bond activation of methanol to form the hydroxymethyl intermediate was calculated to be exothermic by −16 kJ/mol. This path is thermodynamically favored over the competing path involving the activation of the O−H bond of methanol to form methoxide, which was found to be endothermic by +65 kJ/mol. The activation barrier for the dehydrogenation of methanol to form the hydroxymethyl intermediate was found to be 50 kJ/mol lower than the barrier to form methoxide. Both barriers are nevertheless too high, and therefore, methanol desorbs rather than reacts over ideal Pt(111) in a vacuum. The dehydrogenation of the hydroxymethyl intermediate to formaldehyde is endothermic by +37 kJ/mol, whereas the dehydrogenations of formaldehyde to formyl and of formyl to CO were both found to be exothermic at −63 and −80 kJ/mol, respectively. CO was found to be strongly bound on the metal surface (ΔE ads = −168 kJ/mol) and is,