A systematic theoretical study the active sites of potassium promoter on the activity of water-gas shift reaction over Pt(1 1 1)

Abstract

Alkali metals play an important role as promoters in heterogeneous catalytic reactions, but the systematic study of the nature (or active sites) of promoter is far away from enough. Thepossible active sites of potassium promoter such as metallic potassium (K), hydroxyl species stabilized potassium (KOH), and partially oxidized potassium (K2O) species on the Pt-catalyzed water–gas shift reaction (WGSR) wassystematically theoretical studied in this work. The calculation results indicated that the adsorption strength follows the trend K2O/Pt(111) > KOH/Pt(111) ≈ K/Pt(111) > Pt(111), and the K2O additive enhances the adsorption properties largest because of more K ions can be availed to bind with adsorbed species. On the other hand, the promotion effect for WGSR with the order: KOH/Pt(111〉 K2O/Pt(111) > K/Pt(111) ≈ Pt(111), and the KOH species displays the promising promotion effect due to the facile of O–H/C–H bond cleavage as well as the C-O bond formation. Interestingly, it was shown that the high promotion effect of KOH (or K2O) on WGSR is related to the strong activation properties of water dissociation assisted by the OH (or O) group involved in KOH (or K2O). It might be pointed out that the metallic K additive atoms almost have no promotion effect on the Pt-catalyzed WGSR owing to the more difficult in the formation of C-O bond formation compared to that of pure Pt(111), and the active site of potassium promoter is the KOH-like species. The promotion properties of potassium on the adsorption strength of oxygen-containing species such H2O could be ascribed to the strong short-ranged direct interaction between the K-species through space. Microkinetic modeling analysis indicated that the carboxyl mechanism is the dominant mechanism of the whole WGSR, followed by the redox mechanism and formate mechanism. It is desirable for that the present work may extend to the alkali promotion character on WGSR to other situation like metal oxide-supported metal clusters.