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

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 tot ally deactivated in high-pressure runs ('deact.'). Neither X-ray diffractio n and electron microscopy nor X-ray photoelectron spectroscopy showed any m arked change between the first two states. However, a complete transformati on of zirconia into well-crystallized tetragonal modification with a marked redispersion of Pt was observed after total deactivation. Ion scattering s pectroscopy showed that a large fraction of these small Pt particles has be en buried deep under support layers. The surface fraction of Pt could be re duced approaching Pt-0 state by in-situ H-2 treatment in all cases. The amo unt of carbon increased with progressing deactivation, however, no signific ant differences were observed in its chemical state showing graphite and al iphatic polymers as the main component, along with oxygenated carbon compon ents, including carboxyl groups. The predominant sulfur component was S+6 w ith no S-2 present in any of the cases. However, small amounts of S+4 appea red in the 'deact.' sample, its amount being enhanced by in-situ hydrogenat ion. Accumulated carbon may have caused the decrease of activity and isomer selectivity in the 'used' catalyst as compared with the 'calc.' sample. Th ese were more conspicuous at higher temperatures and lower hydrogen pressur es. The main effect of surface C in the 'used' state may have been the hind ering of 'hydrogen transfer' between metal and acid sites, necessary for hi gh isomerization selectivity. Final deactivation, in turn, can be due to st ructural rearrangement, involving recrystallization and 'burying' most Pt u nder zirconia layers, both effects leading to the disruption of the most ac tive metal-acid ensembles. As a consequence, an oxidative starting of the r eaction became more significant, leading to a partial reduction of S+6 to S +4 to it detectable extent. This reduction was analogous to that observed w ith a Pt-free sulfated zirconia which underwent a much shorter and less sev ere exposure to n-hexane. By dissociating H-2 and promoting hydride transfe r 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. (C) 1999 Elsevie r Science B.V. All rights reserved.