Reduction and Sulfidation Kinetics of Cerium Oxide and Cu-Modified Cerium Oxide

Abstract

The reducibility and H2S absorption capacity of cerium oxide and Cu-containing cerium oxide were examined in this work in the temperature range 623−923 K. The sorbents, prepared in bulk oxide form by the urea coprecipitation/gelation method, were composed of a cerium oxide backbone with a small amount of copper (5−40 atom %). Kinetics testing was conducted in a TGA apparatus. Temperature-programmed-reduction (TPR) experiments and in situ X-ray diffraction analysis were used to determine the reduction temperature and the various metal/oxide crystal phases. CuO in ceria was reduced to Cu metal at temperatures below 453 K. The reducibility of cerium oxide was increased by the presence of copper. A low Cu content (5 atom %) was sufficient to cause surface reduction of ceria at a temperature as low as 423 K. Sulfidation of prereduced samples was performed at 0.1 MPa in H2S−H2−N2 gas mixtures. The contribution of reduced cerium oxide to sulfidation was evident even at the low temperature 623 K. Thus, copper promotes both the reducibility and sulfidation of cerium oxide. The activation energy of the sulfidation reaction was calculated from initial sulfidation reaction rates over the temperature range 623−923 K. A low apparent activation energy of 16 ± 3 kJ/mol was found for the Cu contents 5 and 15 atom % in ceria, while the activation energy is 34 kJ/mol for the 40 atom % Cu−CeOx sorbent. This work has shown that copper-modified cerium oxide can be used as a sulfur sorbent over a wide temperature window.