α-Alumina supported doped ceria catalysts for steam gasification and oxidation of radical coke

Abstract

A series of α-alumina supported doped (Mn, Gd, Sr) ceria catalysts is investigated for the gasification (steam) and oxidation (air) of industrial coke that was generated during ethane pyrolysis. The catalytic activity is evaluated in the presence of pure steam, air and steam–air mixtures. Gd, Mn and Sr dopants reduce the coke gasification temperature in pure steam by 65°C compared to the un-catalyzed coke gasification. Gd and Mn also decrease the coke oxidation temperature in air and help the dilute oxidation of coke in the presence of steam+air mixtures by decreasing the diluted oxidation temperature of coke by ∼125°C. The Mn doped ceria catalyst is able to resist coke formation, during in-situ coking experiments. Structural characterization of the α-alumina supported doped (Gd and Mn) ceria catalysts by extended X-ray absorption fine structure (EXAFS) and Raman spectroscopy shows the presence of Ce3+ species and oxygen vacancies, respectively, in the ceria catalysts. The catalytic activity of the Gd-doped sample appears to be related to the presence of increased oxygen vacancies, the smaller crystallite size and a higher catalyst surface area compared to catalysts with other dopants. For the Mn-doped ceria catalyst, the presence of increased oxygen vacancies is suggested to be the main reason for its improved activity. Exposure to steam at high temperatures imparts a negligible effect on the crystallite sizes of the doped ceria catalysts, as evidenced from X-ray diffraction, indicating negligible sintering and good thermal stability of these catalysts.