Promotional effects of chemisorbed oxygen and hydroxide in the activation of C–H and O–H bonds over transition metal surfaces

Abstract

Electronegative coadsorbates such as atomic oxygen (O⁎) and hydroxide (OH⁎) can act as Brønsted bases when bound to Group 11 as well as particular Group 8–10 metal surfaces and aid in the activation of X–H bonds. First-principle density functional theory calculations were carried out to systematically explore the reactivity of the C–H bonds of methane and surface methyl intermediates as well as the O–H bond of methanol directly and with the assistance of coadsorbed O⁎ and OH⁎ intermediates over Group 11 (Cu, Ag, and Au) and Group 8–10 transition metal (Ru, Rh, Pd, Os, Ir, and Pt) surfaces. C–H as well as O–H bond activation over the metal proceeds via a classic oxidative addition type mechanism involving the insertion of the metal center into the C–H or O–H bond. O⁎ and OH⁎ assist C–H and O–H activation over particular Group 11 and Group 8–10 metal surfaces via a σ-bond metathesis type mechanism involving the oxidative addition of the C–H or O–H bond to the metal along with a reductive deprotonation of the acidic C–H and O–H bond over the M–O⁎ or M–OH⁎ site pair. The O⁎- and OH⁎-assisted C–H activation paths are energetically preferred over the direct metal catalyzed C–H scission for all Group 11 metals (Cu, Ag, and Au) with barriers that are 0.4–1.5eV lower than those for the unassisted routes. The barriers for O⁎- and OH⁎-assisted C–H activation of CH4 on the Group 8–10 transition metals, however, are higher than those over the bare transition metal surfaces by as much as 1.4eV. The C–H activation of adsorbed methyl species show very similar trends to those for CH4 despite the differences in structure between the weakly bound methane and the covalently adsorbed methyl intermediates. The activation of the O–H bond of methanol is significantly promoted by O⁎ as well as OH⁎ intermediates over both the Group 11 metals (Cu, Ag, and Au) as well as on all Group 8–10 metals studied (Ru, Rh, Pd, Os, Ir, and Pt). The O⁎- and OH⁎-assisted CH3O–H barriers are 0.6 to 2.0eV lower than unassisted barriers, with the largest differences occurring on Group 11 metals. The higher degree of O⁎- and OH⁎-promotion in activating methanol over that in methane and methyl is due to the stronger interaction between the basic O⁎ and OH⁎ sites and the acidic proton in the O–H bond of methanol versus the non-acidic H in the C–H bond of methane. A detailed analysis of the binding energies and the charges for O⁎ and OH⁎ on different metal surfaces indicates that the marked differences in the properties and reactivity of O⁎ and OH⁎ between the Group 11 and Group 8–10 metals is due to the increased negative charge on the O-atoms (in O⁎ as well as OH⁎) bound to Group 11 metals. The promotional effects of O⁎ and OH⁎ are consistent with a proton-coupled electron transfer and the cooperative role of the metal-O⁎ or metal-OH⁎ pair in carrying out the oxidative addition and reductive deprotonation of the acidic C–H and O–H bonds. Ultimately, the ability of O⁎ or OH⁎ to act as a Brønsted base depends upon its charge, its binding energy on the metal surface (due to shifts in its position during X–H activation), and the acidity of the H-atom being abstracted.