How to Characterize\nSupported PdO<i><sub>x</sub></i> Catalysts by CO-FTIR Spectroscopy:\nImportance of Surface\nReduction and Particle Size

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