THE SYNTHESIS AND CHARACTERIZATION OF THE SILVER COBALT CERIA COMPOSITE OXIDE CATALYSTS AND THE INVESTIGATION OF THE REACTION MECHANISM OF THE LOW TEMPERATURE SELECTIVE CO OXIDATION


Thesis Type: Doctorate

Institution Of The Thesis: Gazi Üniversitesi, Fen Bilimleri Enstitüsü, Turkey

Approval Date: 2007

Student: FİLİZ BALIKÇI

Consultant: FATMA ÇİĞDEM GÜLDÜR

Abstract:

In this study the silver cobalt ceria composite oxide catalysts were synthesized and characterized, and the kinetics of the low temperature CO oxidation were investigated. All catalysts were prepared by the co-precipitation method. The characterization, activity, selectivity and mechanism studies were done. X-Ray analysis showed that the mixed metaloxide catalysts which were calcined at 200°C showed amorphous phase structure and the crystal phase were obtained after calcination at 450°C. X-Ray photoelectron spectroscopy studies revealed that the oxidation state of the Ag, Co and Ce atoms on the surface are +1, +3 and +4 respectively for the catalysts which were calcined at 200°C. Catalysts composed of cobaltoxide gave the highest surface area results. The surface areas of the catalysts were decreased with increasing calcination temperature. All catalysts gave the average pore diameter in the mesopore range. The cobaltoxide phase in the catalysts structure caused an increase in the pore volume and average pore diameter. Temperature programmed reduction (TPRH2) studies showed that all of the catalysts were reduced after 100°C, ceriumoxide decreased the reduction ratio of the catalysts and shifted the reduction temperatures to the lower temperatures. Temperature programmed desorption studies (TPD-CO) showed that the desorption peaks which were due to the carbonates that was formed by the reaction of CO with the lattis oxygen. The pure oxide catalysts gave the one desorption peak and mixed metaloxide catalysts gave the more than one desorption peak at the temperature programmed oxidation (TPD-O2) studies. The low temperature CO oxidation activities of the catalysts decreased with increasing calcination temperature. The 50/50 Ag2O/Co3O4 and 50/50 Ag2O/CeO2 catalysts which were calcined at 200°C were found the most active catalysts for the low temperature CO oxidation. The 50/50 Ag2O/Co3O4 catalyst which was calcined at 200°C gave the best activity and 50/50 Co3O4/CeO2 catalyst c which was calcined at 200°C gave the best selectivity for the low temperature selective CO oxidation reaction. Deactivation studies showed that the 50/50 Ag2O/Co3O4 and 50/50 Co3O4/CeO2 catalysts which were calcined at 200°C did not loose their activity and selectivity for the low temperature selective CO oxidation. The three different adsorption gas mixtures were used at the FTIR adsorption studies. The surface carbonate species, CO2 (gas phase), CO⎯Ag+, CO⎯Co3+ adsorption bands were obtained. There was obtained that the lattis oxygen were used at the CO oxidation and the product CO2 adsorbed on the surface as in the carbonate forms. The OHadsorption bands were obtained when the adsorption gas mixture included H2. It is concluded that the CO oxidation reaction was occurred with the noncompetitive Langmuir_Hinselwood reaction mechanism at the interface of the mixed metaloxide catalysts.