A first principles analysis of the hydrogenation of C1[sbnd]C4 aldehydes and ketones over Ru(0 0 0 1)

Abstract

The structure and degree of substitution of C1C4 oxygenate molecules can influence their chemisorption and reactivity on metal surfaces. Gradient-corrected periodic density functional theory calculations were carried out to analyze alkyl substituent effects on the hydrogenation of C1C4 aldehydes and ketones to their corresponding alcohols. All of these aldehydes along with acetone were found to adsorb in a di-σ η1η2(C,O) mode onto the Ru(0001) surface and result in rehybridization of the CO bond. Steric hindrance from two alkyl substituents on the carbonyl backbone of methyl ethyl ketone (MEK), however, prevents it from binding di-σ η1η2(C,O). It adsorbs instead atop a Ru atom in an η1(O) configuration through its oxygen atom. Hydrogenation of both aldehydes and ketones can occur through either a hydroxy or an alkoxy mechanism. The hydroxy route proceeds via the formation of the hydroxyalkyl intermediate (R1R2C*OH) by the addition of hydrogen to the oxygen of the carbonyl, whereas the alkoxy mechanism proceeds by the addition of hydrogen to the carbon end to form the alkoxy intermediate (R1R2CHO*). DFT calculations indicate that the activation barrier for the initial addition of hydrogen to the carbon to form the CH bond in the alkoxy mechanism is independent of the substituent groups that are attached to the carbon center as these groups are oriented away from the surface in the transition state and thus have little influence on the activation energies. The activation barriers for the addition of hydrogen to the oxygen of the carbonyl to form the OH bond in the hydroxy mechanism, however, was found to linearly correlate with the binding energy of the hydroxyalkyl intermediate that forms. This trend can be explained through the Brønsted–Evans–Polanyi relationship and the fact that both the hydroxyalkyl products and carbonyl reactants interact via their carbon centers and are correlated with one another. All of the carbonyls follow a similar trend in that the addition of hydrogen to the carbon of the carbonyl has a much lower barrier on Ru(0001) than the addition of hydrogen to the oxygen. The carbonyls thus readily react to form their alkoxy intermediates. Simple kinetic analyses and first-principle-based kinetic Monte Carlo simulations for formaldehyde over Ru(0001) show that the alkoxy is the most abundant surface intermediate and that the alkoxy route is more favorable than the hydroxy route.