Mechanism of carbon deposits removal from supported Ni catalysts

Abstract

Catalyst deactivation due to carbon deposition is a major issue in all reforming technologies. Because of the significant economic cost of catalyst replacement, catalyst regeneration is increasingly attracting attention. The regeneration mechanism of Ni catalysts, with respect to carbon removal, was investigated on support materials prepared by one-pot synthesis. The supports were classified based on their redox functionality: Al2O3, MgAl2O4 show no redox properties in contrast to MgFe0.09Al1.91O4 and CeZrO2 that have redox functionality.A Temporal Analysis of Products (TAP) setup was used to investigate the isothermal regeneration mechanism of Ni catalysts at 993 K by O2. Different mechanisms were distinguished depending on the redox functionality of the support material. Two consecutive steps occur on the support that have no redox properties (Al2O3 and MgAl2O4): metallic Ni is oxidized to form NiO (oxidation step), resulting in an initial local temperature increase of 50–60 K in total, enabling metal particle migration to carbon that was initially separated from the metal and subsequent oxidation through NiO lattice oxygen (reduction step). On the other hand, the mechanism of carbon removal by O2 from Ni catalysts on supports with redox properties does not require particle migration. Two parallel contributions are proposed: 1) Ni metal is oxidized to form NiO, where after lattice oxygen of NiO is used for the oxidation of carbon that is deposited upon the metals, 2) carbon oxidation through lattice oxygen that is provided by the support. No dependency of the carbon gasification mechanism on the exposed fraction of the metal (particle size in the nanoscale) or on the structure of the deposited carbon was concluded.