Studies of the interactions of H/sub 2/ and CO with Pd/SiO/sub 2/ promoted with Li, Na, K, Rb, and Cs

The interactions of CO and H/sub 2/ with Pd/SiO/sub 2/ promoted with Li, Na, K, Rb, and Cs have been investigated using temperature-programmed desorption and temperature-programmed surface reaction. Introduction of the promoter following the preparation of the Pd/SiO/sub 2/ catalyst causes a small increase (similarly ordered 7%) in the dispersion of the Pd particles. Reduction of the promoted catalysts removes a significant quantity of oxygen from the promoter, but only a small portion of the promoter appears to cover the Pd particles. Alkali promotion of Pd/SiO/sub 2/ does not significantly influence the amounts of H/sub 2/ and CO that can be adsorbed on the metal. The alkali promoters have a slight influence on the distribution of H/sub 2/ adstates but cause a significant change in the distribution of CO adstates. For low reduction temperatures, alkali promotion of Pd/SiO/sub 2/ decreases the activity of Pd for the dissociation of CO. However, increasing the reduction temperature increases the CO dissociation activity of the promoted samples due to the increased degree of reduction of the promoter. This enhanced dissociation activity is reversed by the reoxidation of the promoter by the H/sub 2/O or CO/sub 2/ formed under reaction conditions. The nascent carbon formedmore » by CO dissociation on alkali-promoted Pd/SiO/sub 2/ is less reactive than that on unpromoted Pd/SiO/sub 2/, and the promoted catalysts have a lower activity for CH/sub 4/ synthesis relative to that for unpromoted Pd/SiO/sub 2/ decreasing in the order: unpromoted > Li > Na > K > Rb > Cs. 49 references.« less