Mechanistic Understanding of Alloy Effect and Water Promotion for Pd-Cu Bimetallic Catalysts in CO2 Hydrogenation to Methanol

Abstract

Density functional theory (DFT) calculations on Pd-Cu bimetallic catalysts reveal that the stepped PdCu(111) surface with coordinatively unsaturated Pd atoms exposed on the top is superior for CO 2 and H 2 activation and for CO 2 hydrogenation to methanol in comparison to the flat Cu-rich PdCu 3 (111) surface. The energetically preferred path for CO 2 to CH 3 OH over PdCu(111) proceeds through CO 2 * → HCOO* → HCOOH* → H 2 COOH* → CH 2 O* → CH 3 O* → CH 3 OH*. CO formation from CO 2 via a reverse water-gas shift (RWGS) proceeds more quickly than CH 3 OH formation in terms of kinetic calculations, in line with experimental observation. A small amount of water, which is produced in situ from both RWGS and CH 3 OH formation, can accelerate CO 2 conversion to methanol by reducing the kinetic barriers for O–H bond formation steps and enhancing the TOF. Water participation in the reaction alters the rate-limiting step according to the degree of rate control (DRC) analysis. In comparison to CO 2, CO hydrogenation to methanol on PdCu(111) encounters higher barriers and thus is slower in kinetics. Complementary to the DFT results, CO 2 hydrogenation experiments over SiO 2 -supported bimetallic catalysts show that the Pd-Cu(0.50) that is rich in a PdCu alloy phase is more selective to methanol than the PdCu 3 -rich Pd-Cu(0.25). Moreover, advanced CH 3 OH selectivity is also evidenced on Pd-Cu(0.50) at a specific water vapor concentration (0.03 mol %), whereas that of Pd-Cu(0.25) is not comparable. The present work clearly shows that the PdCu alloy surface structure has a major effect on the reaction pathway, and the presence of water can substantially influence the kinetics in CO 2 hydrogenation to methanol.