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

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, H-2 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, H-2 is more active in a dry environment, and the presence of H2O further promotes the desulfation while CO2 has no obvious effect. Changing the H-2 concentration influences the desulfation products, with higher H-2 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 H-2. 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. (C) Published by Elsevier B.V.