Effect of lanthanum on the properties of copper, cerium and zirconium catalysts for preferential oxidation of carbon monoxide

摘要

Catalysts based on copper and cerium oxides are active and selective in preferential oxidation of carbon monoxide (CO-PROX) and then considered as promising for obtaining hydrogen with acceptable purity for proton exchange membrane fuel cell. However, the low specific surface area of cerium oxide leads to the production of large copper particles, which do not contribute to the catalyst activity. Aiming to find more efficient catalysts for this reaction, the effect of lanthanum on the properties of catalysts based on copper, cerium and zirconium was studied in this work. The samples (CuO-Ce_(1-x)La_xO_(2-x/2) and CuO-Ce_(0.5-x/2)Zr_(0.5-x/2)La_xO_(2-x/2); x = 0.0; 0.05; 0.10) were prepared by precipitation in alkaline medium and heated under air flow at 450 °C. They were characterized by 1CP-OES, Fourier transform infrared spectroscopy, X-ray diffraction, Raman spectroscopy, specific surface area and porosity measurements, temperature programmed reduction, oxygen storage capacity measurements and isotopic exchange with oxygen-18. The catalysts were evaluated in CO-PROX from 150 to 300 ℃. The zirconium-containing samples presented tetragonal structure while the zirconium-free ones showed face-centered cubic structure. Lanthanum and copper were incorporated into ceria lattice, while zirconia and ceria formed solid solutions. Zirconium increases the specific surface area by decreasing the particle crystal size and also created mesoporosity. Lanthanum decreased the interaction between copper and ceria while zirconium did the opposite. On the other hand, lanthanum increased the oxygen storage capacity and oxygen mobility whereas zirconium decreased it. Lanthanum did not improve the activity and selectivity of the catalysts in CO-PROX. Moreover, zirconium increased the activity, by increasing the interaction between cerium and copper and then increasing the interface where the reaction occurs. All catalysts were resistant against deactivation by water and the activity and selectivity could be recovered after poisoning by carbon dioxide.