On the activity and stability of platinum/alumina catalyst during multiple deactivation and regeneration

Abstract

Catalyst surface characterisations have been carried out to investigate the role of dispersion on catalyst activity and to probe the occurrence of oscillations in coking levels with cycle number generally observed during multiple deactivation and regeneration schemes. The titrations were done, cycle by cycle, at 430°C after oxidation and at the same temperature (430°C) after reduction at 500°C. Results show the usually observed trend -that the dispersions after oxidation are higher than those after reduction. The average decline in dispersion from oxidation to reduction was calculated to be 39.25%. It was observed that the cycles with high toxic coke removal were characterised by high deactivation times. The deactivation times were still high even for cycles subsequent to those with low dispersion. At high dispersions the catalyst had short deactivation times, that is the small crystallites deactivate faster than large ones. The nature of reduceable coke and the efficiency of its removal is a much more determinant factor of catalyst activity than the level of metal dispersion. Thus prolonged toxic coke reduction at the high temperature of 500 "C, though resulting in an apparent lowering of dispersion, does not affect the quality of the catalyst. The dispersion before reduction could be reattained on oxidation. Hence reduction at 500°C did not introduce sintering.

Metal Redispersion Strategies for Recycling of Supported Metal Catalysts: A Perspective

“…In general, it has been reported that Pt dispersions of Pt/ Al 2 O 3 catalysts after treatment with oxygen are higher than the dispersions found after hydrogen treatments. 90 It has been reported that when increasing oxygen concentration during oxidative treatments of a Pt/Al 2 O 3 catalyst, PtO forms, which promotes the formation of Pt−Al 2 O 3 complexes and subsequent migration of Pt species into the bulk lattice of the support. 91 Upon reduction, these PtO and Pt−Al 2 O 3 species form Pt metal, resulting in a greater Pt dispersion being observed.…”

“…90 It has been reported that when increasing oxygen concentration during oxidative treatments of a Pt/Al2O3 catalyst, PtO forms, which promotes the formation of Pt-Al2O3 complexes and subsequent migration of Pt species into the bulk lattice of the support. 91 Upon reduction, these PtO and Pt-Al2O3 species form Pt metal, resulting in a greater Pt dispersion being observed.…”