Interaction of carbon monoxide with partially reduced ceria-zirconia supported catalysts
Metal oxide catalyst supports may contribute surface area, chemically active sites, or a combination to the catalytic reaction. Regarding cerium dioxide, the redox conversion between Ce3+ and Ce4+ is facile at conditions of industrial interest. When used as a catalyst support, this redox ability imparts CeO2 with the capability to store and release oxygen to the catalytic reaction under various conditions of oxidant partial pressure.
This work investigated the interaction of CO and O2 with ceria-based catalysts reduced to varying degrees. Studies performed using in situ Raman spectroscopy provide direct, unambiguous evidence of surface carbon deposits on CeO2, Ce0.75Zr0.25O2, and Pd catalysts following CO exposure. The simultaneous appearance of bands attributed to Ce3+ indicate that reduced Ce sites on the oxide supports are active for CO disproportionation (2CO ³ Cs + CO2).
In order to account for the increase in intensity of the carbon bands with increasing time of exposure to CO, we propose a mechanism for CO disproportionation on CeO2-x involving aggregated oxygen vacancy sites. At the aggregated vacancy, the degree of electron density localization controls the rates at which CO acts as an electron donor or acceptor to form an energetically activated complex. We show that aggregated electron vacancies provide active sites for the autocatalytic disproportionation of CO on CeO2-x.
After CO was disproportionated on the CeO2 surface, subsequent exposure to O2 at room temperature resulted in a decrease in the intensities of the carbon bands. At the same time, Raman modes characteristic of formate surface species appeared. We discuss the mechanism of formate formation on C/CeO2-x. Similar CO and O2 exposure studies performed using a complimentary technique, IR spectroscopy, indicate that site blocking by carbonates may play a role in the increased oxygen storage capacity of ceria-zirconia catalysts compared to ceria catalysts alone.
Advisor:James Miller; Julie d'Itri; Robert Parker; Sachin Velankar
School:University of Pittsburgh
School Location:USA - Pennsylvania
Source Type:Master's Thesis
Date of Publication:06/16/2008