Density functional study of water-gas shift reaction on M3O3x/Cu(111)

Abstract

Density functional theory (DFT) was employed to study the water dissociation and water-gas shift (WGS) reaction on a series of inverse model catalysts, M₃O_3x/Cu(111) (M = Mg, Ti, Zr, Mo, W; x = 1, 2, 3). It has been found that the WGS reaction on Cu can be facilitated by introducing various oxides to lower the barrier of water dissociation. Accordingly, the calculated reaction energy for water dissociation was used as a scaling descriptor to screen the WGS activity of oxide–Cu model catalysts. Our calculations show that the activity towards water dissociation decreases in a sequence: Mg₃O₃/Cu(111) > Zr₃O₆/Cu(111) > Ti₃O₆/Cu(111) > W₃O₉/Cu(111), Mo₃O₉/Cu(111). It seems that Mg₃O₃/Cu(111) is the best WGS catalyst among the systems studied here, being able to dissociate water with no barrier. During the process, both Cu and oxides participate in the reaction directly. The strong M₃O_3x–Cu interaction is able to tune the electronic structure of M₃O_3x and therefore the activity towards water dissociation. Further studies of the overall WGS reaction on Mg₃O₃/Cu(111) show that water dissociation may not be the key step to control the WGS reaction on Mg₃O₃/Cu(111) and the removal of H from Mg₃O₃ can be problematic. The strong interaction between H and O from Mg₃O₃ blocks the O sites for further water dissociation and therefore the WGS reaction. Our study observes a very different behavior of oxide clusters in such small size from the bigger ones supported on Cu(111) and provides new insight into the rational design of the WGS catalysts.