Mechanistic study of ethanol steam reforming on TM–Mo6S8 clusters: a DFT study

Abstract

The mechanism of ethanol steam reforming (ESR) on TM–Mo₆S₈ (TM = Pt, Pd) clusters is systematically investigated using a combination of the microscopic kinetic model, energetic span model (ESM) and d-band model under density functional theory (DFT) calculations. The results show that Pd–Mo₆S₈ shows the best performance both in terms of activity and stability, due to facile H₂ evolution from the cluster and less carbon coking. ESR is initiated by decomposition of ethanol: CH₃CH₂OH* → CH₃CH₂O* → CH₃CHO* → CH₃CO* + H* → CH₃* + CO* → CH₄* + CO*, followed by the water-gas shift (CO* + H₂O* → CO₂* + H₂*) reaction (WGSR) to produce CO₂ and H₂; the rate-determining step is the O–H bond scission of OH species. In other words, the WGSR, not the ethanol decomposition, is the bottleneck for the overall ethanol steam reforming process. Our results provide a mechanism and insight into the importance of hydroxyl groups to the ESR reaction over Pd–Mo₆S₈ catalysts, which may shed light on designing improved catalysts and is beneficial to the H₂ formation.