An analysis of methanol synthesis from CO and CO 2 on Cu and Pd surfaces by the bond-order-conservation-Morse-potential approach

Abstract

The mechanisms of methanol synthesis from CO and CO 2 on Cu(111) and Pd(111) have been analyzed using the BOC-MP (bond-order-conservation-Morse-potential) approach. The analysis was based on calculations of the heats of chemisorption, Q, for all adsorbed species and the activation barriers, Δ E ∗, for all elementary reactions believed to be involved in the synthesis of methanol from CO and CO 2. The relevant experimental values of Q and Δ E ∗, although scarce, agree well with the BOC-MP estimates. The formyl and formate routes to methanol were compared. On Cu(111), the activation barrier for hydrogenation of CO s to HCO s is found to be much larger than that for the desorption of CO s, which makes formyl formation non-competitive. By contrast, on Pd(111) the two barriers are calculated to be practically equal, making it very likely that formyl groups are formed. In the presence of OH s groups, formate formation via the reaction CO s + OH s → HCOO s is found to have a low activation barrier, particularly on Cu(111), where the formate route to methanol is preferred. The rate determining step in this case is projected to be the hydrogenolysis of formate groups to form formaldehyde and atomic oxygen. On Cu(111) the formate route also appears to be efficient for the hydrogenation of CO 2 to methanol, since the activation barrier for H s + CO 2,s → HCOO s is calculated to be smaller than that for desorption of CO 2,s. The reverse is true for Pd(111), which makes the formate route to methanol energetically unfavorable in this case. The mechanism of the WGS reaction has also been considered. It appears that the reaction does not proceed via the formate intermediate, and the rate-determining step for this reaction is projected to be the dissociation of water. On Cu(111), the reverse WGS reaction is found to be competitive with methanol formation. The BOC-MP projections are generally consistent with the observed features of hydrogenation of CO and CO 2 on Cu and Pd catalysts. Some apparent inconsistencies are pointed out and discussed.