How to Characterize Supported PdOx Catalysts by CO-FTIR Spectroscopy: Importance of Surface Reduction and Particle Size

The palladium oxide phase (PdO x ) plays a critical role in catalytic methane oxidation and CO oxidation reactions. Precise characterization of supported PdO x by CO-FTIR spectroscopy is often accompanied by the reduction of PdO x by CO. Lowering the CO partial pressure to 0.05% CO/N 2 (0.5 mbar) and the adsorption temperature to −130 °C is a proper condition to inhibit the reduction completely. Then CO-FTIR spectroscopy was conducted on a series of supported PdO x catalysts. We find that CO adsorbed on coordinatively unsaturated Pd atoms (Pd cus ) on the PdO x surface gives three adsorption bands at ∼2145 cm –1 . Three types of surface Pd cus atoms with different degrees of coordination unsaturation (types I, II, and III) exist in supported PdO x catalysts; their ν CO are located at 2133, 2145, and 2160 cm –1, respectively. The order of coordination unsaturation of surface Pd cus should be type III < type II < type I. Since ν CO of surface Pd cus is between ν CO of Pd 2+ (CO) and Pd + ( CO ), the effective charge of surface Pd cus is 1 < n < 2. For small PdO x clusters in zeolite-supported PdO x catalysts, surface Pd cus atoms with more coordination unsaturation (i.e., step, edge, and corner sites) are more abundant, leading to greater IR intensity of adsorbed CO.