Evaluation of a method for characterizing coked and sulphided Rh/Al2O3 catalysts: study of the space and time dependence of sulphur deposition on Rh/Al2O3 in toluene steam reforming

Abstract

A simple and rapid chromatographic method for characterizing coked and sulphided Rh/Al2O3 catalysts was evaluated by studying the behaviour in an oxygen atmosphere of selected samples: RhA (fresh), RhAS (sulphided), RhAC (coked and RhACS (coked + sulphided).Electron microprobe analysis (EPMA) of the rhodium and sulphur profiles in these catalysts and IR analysis of sulphate ions on the alumina support were used jointly to correlate the results obtained by the chromatographic techniques (chemisorptions, titrations, thermoprogrammed reductions). Two alternative methods were tested: (i) selective oxidation of carbonaceous deposits by O2 pulses at medium temperature (320°C), which allows almost all of the sulphur to remain chemisorbed on the metal (method 1); the fractions of free metal surface (Rhf), of coked surface (RhC) and of sulphided surface (Rhs) are successively determined from H2 chemisorptions performed before and after the selective oxidation; (ii) oxidation by O2 pulses at high temperature (450°C), which results in carbon gasification and sulphur oxidation into the sulphate with correlative displacement from the metal to the support (method 2). A high-temperature reduction allows the sulphates to be reduced and, thus, sulphur to move back to metal. The second method (more rapid and reliable) was used for drawing ȁmapsȁ of carbon and sulphur distribution in space (along the bed axis) and in time as a function of the sulphur content in the feed, for catalysts used in toluene steam dealkylation at 460°C. A gradient of sulphur is evident in the catalyst bed; for instance, with 50 ppm of S in the feed, the sulphur coverage decreases from about 80% at the inlet to 20% at the outlet of the bed. Rhodium and sulphur profiles determined by EPMA on pellets located at different depths in the bed bear out these determinations. The sulphided fraction is generally also covered with carbon, particularly at the longest time on stream.