PdSO4 Surfaces in Methane Oxidation Catalysts: DFT Studies on Stability, Reactivity, and Water Inhibition

Although it is experimentally difficult to observe, PdSO4 is considered to be the culprit for the reduced activity of SO2-poisoned methane oxidation catalysts. Density functional theory (DFT) predicts that the formation of bulk PdSO4 is unlikely, which explains the lack of X-ray diffraction (XRD) evidence for the PdSO4 phase. Instead, experimental observations support the idea of PdSO4 being formed on PdO as thin films. Our study found PdSO4(110) and PdSO4(111), corresponding to PdO(100) and PdO(101), respectively, to be the most likely surfaces to be found on a poisoned catalyst. On these sulfate surfaces, PdSO4(111) contains coordinatively unsaturated palladium, which enables catalytic activity. The first C-H dissociation of methane on PdSO4(111) was found to be rather accessible with an energy barrier varying between 0.74 and 0.87 eV, values similar to those reported for metallic Pd. However, the presence of hydroxyl groups increases the barrier height. Methane oxidation is also hindered by an exceptionally strong water adsorption of -1.45 eV on the PdSO4(111) surface, which causes site blocking. A significantly strong adsorption energy causes the combination of surface hydroxyl groups to form adsorbed water that is energetically favorable. The results provide a theoretical justification for the observation that SO2-poisoned PdSO4/Al2O3 catalysts produce proper methane conversion under dry conditions but perform poorly under wet feed.