Sulfur release from a model Pt/Al2O3 diesel oxidation catalyst: Temperature-programmed and step-response techniques characterization Original Research Article

The S release, or desulfation process, of a model Pt/Al2O3 diesel oxidation catalyst (DOC) was investigated using temperature-programmed techniques and step-response methods. During the temperature-programmed experiments, the sulfur loading, H2 concentration and gas composition, in terms of H2O and/or CO2 presence, were systematically investigated. The results show that desulfation is promoted as the gas environment changes from oxidizing to inert and then to reducing conditions. Compared to CO, H2 is more active in a dry environment, and the presence of H2O further promotes the desulfation while CO2 has no obvious effect. Changing the H2 concentration influences the desulfation products, with higher H2 concentrations generating larger amounts of H2S. The data indicate that the desulfation process can be viewed as a stepwise reduction of sulfates to SO2 and then to H2S. Meanwhile, the sulfur loading also affects the SO2/H2S ratio due to the distribution of the sulfur species, and a relatively medium sulfur loading (equivalent to 3 g/L) yields the largest SO2/H2S ratio. The results of the step-response methods show that the desulfation process has a low kinetic dependence on H2. Furthermore, the apparent reaction order with respect to sulfur is temperature dependent, and decreases with increasing temperature. These results suggest that desulfation is mass-transfer limited by the diffusion of sulfur species to the active Pt sites or hydrogen tot he sulfur sites. The apparent activation energy for desulfation was initially 59.1 kJ/mol, but decreased as more S was released to 39.0 kJ/mol due to mass-transfer limitation.