Structure Sensitivity of the Water–Gas Shift Reaction over Cu/CeO₂: A Combination of DFT, MF-MKM, and kMC

Abstract

The structure sensitivity of the water–gas shift reaction (WGSR) over Cu/CeO 2 (111) and Cu(111) was systematically investigated through a comprehensive approach employing density functional theory (DFT) calculations, mean-field microkinetic modeling (MF-MKM), and kinetic Monte Carlo (kMC) simulation. Utilizing constrained minima hopping technology, the most stable structural configurations of Cu 1 /CeO 2 (111), Cu 4 /CeO 2 (111), Cu 10 /CeO 2 (111), and Cu nanorod [Cu rod /CeO 2 (111)] catalysts were identified. The MF-MKM results revealed that the activity of the WGSR followed the order Cu 4 > Cu 10 > Cu rod > Cu 1 > Cu(111), indicating a pronounced structure sensitivity with increasing Cu n cluster size. Specifically, the reaction activity first increases and then decreases with the increase of copper loading, and Cu 4 shows the highest catalytic activity. These findings align with experimental observations, showing that the reaction rate increases with copper coverage up to 0.4 ML, reaches a maximum value, and then decreases. Moreover, it was shown that Cu 4 /CeO 2 (111) has a preference for the carboxyl pathway. It was proposed that the active sites for Cu/CeO 2 (111) are likely to be smaller copper clusters such as the Cu 4 cluster rather than single Cu atoms (limiting for the carboxyl dissociation) or larger Cu clusters (disfavored for the H 2 O dissociation). The high activity of Cu 4 /CeO 2 (111) can be attributed to its effective activation of the O–H bonds and C–O bond formation ability. The more oxidized Cu state like Cu 1 /CeO 2 or less oxidized Cu state like Cu rod /CeO 2 is not the active site for the WGSR, and the moderate oxidation state like Cu 4 /CeO 2 (111) (Bader charge per copper atom is ca. 0.3|e|, which approximately corresponds to the formal Bader charge of Cu + ) is the active site. The combination of Cu 0 and Cu + serves as the active site for the adsorption of CO and the formation of COOH, making it the active center for Cu rod and Cu 10 in catalyzing the WGSR. Importantly, compared with MF-MKM, kMC gives a more uniform distribution of surface coverage (neither too high nor too low) and thus gains more reasonable simulation results. Our findings provide important insights into the design and optimization of Cu/CeO 2 catalysts by controlling the Cu coverage and Cu n cluster size to enhance the activity and selectivity of the catalyst.

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