The possible reasons of irreversible deactivation of Pt/sulfated zirconia catalysts: structural and surface analysis

Abstract

Three samples of Pt/sulfated zirconia were compared: a calcined one (‘calc.’); one after in n-hexane reactions at 1 bar pressure (‘used’) and one totally deactivated in high-pressure runs (‘deact.’). Neither X-ray diffraction and electron microscopy nor X-ray photoelectron spectroscopy showed any marked change between the first two states. However, a complete transformation of zirconia into well-crystallized tetragonal modification with a marked redispersion of Pt was observed after total deactivation. Ion scattering spectroscopy showed that a large fraction of these small Pt particles has been buried deep under support layers. The surface fraction of Pt could be reduced approaching Pt 0 state by in-situ H 2 treatment in all cases. The amount of carbon increased with progressing deactivation, however, no significant differences were observed in its chemical state showing graphite and aliphatic polymers as the main component, along with oxygenated carbon components, including carboxyl groups. The predominant sulfur component was S +6 with no S −2 present in any of the cases. However, small amounts of S +4 appeared in the ‘deact.’ sample, its amount being enhanced by in-situ hydrogenation. Accumulated carbon may have caused the decrease of activity and isomer selectivity in the ‘used’ catalyst as compared with the ‘calc.’ sample. These were more conspicuous at higher temperatures and lower hydrogen pressures. The main effect of surface C in the ‘used’ state may have been the hindering of ‘hydrogen transfer’ between metal and acid sites, necessary for high isomerization selectivity. Final deactivation, in turn, can be due to structural rearrangement, involving recrystallization and ‘burying’ most Pt under zirconia layers, both effects leading to the disruption of the most active metal–acid ensembles. As a consequence, an oxidative starting of the reaction became more significant, leading to a partial reduction of S +6 to S +4 to a detectable extent. This reduction was analogous to that observed with a Pt-free sulfated zirconia which underwent a much shorter and less severe exposure to n-hexane. By dissociating H 2 and promoting hydride transfer to adjacent acidic sites Pt ensures thus an acidic start of isomerization, preventing an extensive S +6 → S +4 reaction. Final deactivation can thus be related to the lack of (sufficient) Pt on the surface.