Deactivation and regeneration of Pt containing sulfated zirconia and sulfated zirconia
Active, inactive, coked and resulfated SZ and Pt/SZ samples were investigated by XRD, TG, TPE, TPD and IR measurements. As test reaction conversion of n-heptane at 200 °C and atmospheric pressure was chosen. The active catalysts exhibit the pure tetragonal phase, have a sulfate content corresponding to a surface coverage of more than half a monolayer and show Brønsted acidity. It is assumed that the active species consist of pyrosulfate groups, which can oxidize hydrogen (alkanes) to water (and alkenes) by decomposing into sulfate groups and adsorbed SO2.The atmosphere of the first activation step of Pt/SZ does not seem to have any influence on the performance of Pt/SZ. For regeneration an oxidative atmosphere, a temperature of 500 °C and reduction in hydrogen at 200 °C is necessary to restore the catalytic activity of Pt/SZ completely. If, however, regeneration of Pt/SZ is done in He or N2, a remarkable loss of SO2 was observed and the catalyst became irreversibly inactive. The sulfur species, which are more weakly bonded to the surface after a reductive step are evolved at lower temperatures (between 300 and 600 °C) in an inert gas atmosphere. This species are essential for catalytic activity, whereas the sulfate groups, which are removed at temperatures higher than 600 °C, are inactive for n-alkane conversion.The inactive SZ and Pt/SZ samples exhibit the tetragonal and monoclinic structure, have lost approximately 40% of their sulfate groups and possessed only Lewis acid sites. If an inactive Pt/SZ sample was resulfated Brønsted acidity was regenerated and the sample was active, however to a lesser extent than a fresh catalyst.The changes occurring during coking did not affect the textural properties; no changes of the crystal structure were detected by XRD. Almost all coking was observed in the first minutes of reaction. Regeneration in air reestablished the activity whereas regeneration in an inert atmosphere led to a loss of sulfate groups and therefore to inactivity.
Journal: Applied Catalysis A: General - Volume 301, Issue 1, 10 February 2006, Pages 115–122