Multiple reaction states in CO hydrogenation on alumina-supported cobalt catalysts

Abstract

CO hydrogenation was studied on four (1, 3, 10 and 15 wt%) Co Al 2O 3 catalysts using the technique of temperature-programmed surface reaction (TPSR). Two distinct methane peaks (Peaks A and B) are observed for 3, 10 and 15% Co Al 2O 3 during TPSR of chemisorbed CO at room temperature, indicating the presence of two different reaction states or mechanisms for CO hydrogenation ( A and B). No methane peak is observed for 1% Co Al 2O 3 unless it is reduced at 1023 K. The more active A state, the relative population of which increases with increasing metal loading and increasing extent of reduction, probably corresponds to hydrogenation of atomic carbon on 3 D cobalt crystallites while the less active B state is assigned to decomposition on metal crystallites of a methoxy or formate species originally formed on the support from spilled-over hydrogen and carbon monoxide. TPSR spectra of hydrogen with carbon deposited by CO dissociation at 523 K show that the quantity of active α-carbon increases with increasing metal loading and correlates with the relative population of A sites. A linear correlation between logarithm of the steady-state methane turnover frequency and the relative population of A sites suggests that large variations in the steady-state CO hydrogenation rate with dispersion and metal loading observed for these catalysts may be explained in terms of variations in the distribution of reaction states for CO hydrogenation, i.e., a larger fraction of Reaction A at higher metal loadings and higher extents of reduction.