The role of alkali-metal promotion on CO oxidation over PdO/SnO2 catalysts

Sol-gel prepared PdO/SnO2 and Na-PdO/SnO2 catalysts were investigated by X-ray photoelectron spectroscopy (XPS), diffuse reflectance IR (DRIFTS), temperature-programmed reaction spectroscopy (TPRS) and impulse techniques. Alkali-metal promotion was observed to enhance CO oxidation rate and decreases light-off temperatures of PdO/SnO2. XPS analysis revealed that alkali-metal promotion resulted in the segregation of Pd atoms and promoted the oxygen storage capacity of the catalysts. The presence of super-oxide species was also observed over Na-PdO/SnO2 catalyst and the formation of a relatively stable ternary oxide of sodium and palladium (NaxPd3O4) was postulated. Surface titration experiments utilizing pulse technique showed that alkali-metal-promoted sample was able to preserve part of its CO oxidation activity even after reduction pre-treatment and that oxygen diffusion from bulk of the catalyst effectively promoted the activity of Na-PdO/SnO2 sample at 150 deg C.