A systematic theoretical study of the water gas shift reaction on the Pt/ZrO2 interface and Pt(111) face: key role of a potassium additive

摘要

In the present work, density functional theory (DFT) calculations were performed to explore the reaction mechanism and activity of the water gas shift reaction (WGSR) on a clean and K-promoted Pt-40 nanorod supported by the ZrO2 (Pt-40/ZrO2) model, as well as on a clean and K-modified Pt(111) surface. The calculation results show that the carboxyl mechanism is responsible for the WGSR with H2O dissociation as the common key step on all models. We found that the Pt-40/ZrO2 model is more active toward the WGSR compared to the Pt(111) surface due to the central roles of the support and interface on Pt-40/ZrO2 which can facilitate H2O dissociation by strengthening OH binding at the transition state (TS) and final state (FS). More importantly, it is noticed that the addition of K can enhance the activity of the WGSR on the Pt-40/ZrO2 model by reducing the apparent activation energy of the whole reaction as well as the energy barriers of the steps, i.e., H2O and COOH dissociation for the dominant carboxyl pathway. The origin of the K promotion effect on H2O and COOH dissociation is that K can greatly stabilize the dissociated oxygenated species at the TS by the direct K-O bonding. Nevertheless, the K adatom would hinder the progress of the WGSR on the Pt(111) surface with one key reaction of H2O dissociation slightly promoted and another vital reaction of COOH formation strongly poisoned by K addition. The stronger promotion effect of K on H2O dissociation on Pt-40/ZrO2 than that on Pt(111) is attributed to the smaller stabilization effect of K on H2O on Pt-40/ZrO2, indicating that the K effect on the dissociation reaction is structure sensitive. Moreover, the K effect is also sensitive to the reaction type since K promotes nearly all the dissociation reactions but neutrally affects or even inhibits the kinetics of the association reactions on both Pt-40/ZrO2 and Pt(111) models, caused by the same or even higher stabilization effects on the ISs from K compared to the corresponding TSs. The functional mechanism expounded in this work could be applicable to other electropositive additives like Na and Cs in the activation of the WGSR.