The Formation and Stability of Adsorbed Formyl as a Possible Intermediate in Fischer-Tropsch Chemistry on Ruthenium.

Abstract

Density functional theory has been used to study the formation and stabilization of the chemisorbed formyl species, HCO, on the Ru(001) surface. It has been found that the η1-HCO species attached to a Ru atom by a single Ru-C bond is unstable, and that it will convert to more strongly bound η2- and η3-HCO surface species. These more highly coordinated species decompose readily to produce H(a) and CO(a) via low activation energy reactions, indicating that the observation of HCO(a) species in any bonding mode at temperatures above 90 K is unlikely. Additionally, we found that abstraction of H from HCO(a) upon atomic H bombardment is another low-barrier reaction mechanism leading to the formation of adsorbed CO and desorption of an H2 molecule. Using a stepped Ru(109) surface, containing atomic steps periodically located every 22 Å along the Ru(001) surface, we have bombarded chemisorbed CO with atomic H at T < 170 K in an attempt to produce and spectroscopically observe adsorbed HCO species by infrared reflection absorption spectroscopy. These experiments have been unsuccessful, and instead we have observed the surface crowding of CO(a) by H(a) species, and the displacement of CO(a) at high H(a) coverages. Both the experimental and theoretical results support the conclusion that the formyl species can be easily dissociated on the Ru surface. These studies throw into question previous measurements indicating that the interaction of atomic hydrogen with CO is effective in producing stable adsorbed formyl species on Ru.