Selective oxidation of CO with modified Pd/ZnO catalysts in the presence of H2: Effects of additives and preparation variables

Supported Pd catalysts on various support materials, ZnO, Ga2O3, SiO2, CeO2, In2O3, and Al2O3, exhibit poor activity and selectivity for the selective oxidation of CO in H2-rich gas with O2. The most active catalyst is Pd/ZnO among the six samples examined. The Pd species exist in the form of PdZn alloy in this catalyst and the alloy is more effective for the reaction compared with metallic Pd species of the other catalysts. The activity of Pd/ZnO catalyst can be significantly improved by the addition of alkali compounds, in particular Cs and Rb. These additives were shown to enhance the CO oxidation but suppress the undesired H2 oxidation. With the optimized Cs-modified Pd/ZnO catalyst (Cs/Pd = 3), the CO concentration of a mixture of CO/O2/H2/CO2 (0.5/2.0/23/7.0) can be reduced to a 40 ppm level at 433 K, while controlling the H2 conversion level to <10%. Temperature programmed desorption measurements indicate that the addition of Cs affects the adsorption/desorption behavior of CO and H2 on PdZn alloy particles in the Pd/ZnO catalyst but not the size of PdZn alloy crystallites. The Cs promoter is effective to weaken the strength of CO adsorption on the catalyst and increase the amount of H2 weakly interacting with them in the presence of both CO and H2, resulting in the enhanced selective CO oxidation activity of the Cs-modified Pd/ZnO catalyst.