Abstract
Alumina-supported PdO catalysts are largely accepted as one of the effective catalysts for methane combustion reaction. We have examined the effects of addition of CeO2 to the PdO catalysts over chi-Al2O3 support on their microstructures by XAFS spectroscopy and X-ray diffraction technique. The addition of CeO2 improved the dispersiveness of PdO particles over the support and depressed the aggregation among the particles. In this study, we investigated the effect of addition of TiO2 to PdO-CeO2/chi-Al2O3 catalysts. TiO2 is well known as an n-type semiconductor and is expected to donate its electrons to PdO catalyst.
Addition of TiO2 to PdO in the PdO-CeO2/chi-Al2O3 catalysts lowered the starting temperature of methane oxidation from 250°C down to 200°C, and enhanced the activity significantly in the low temperature region below 350°C. Under methane atmosphere diluted by helium, some parts of PdO were already reduced to Pd metal in the TiO2-containing catalysts at 250°C, suggesting that the addition of TiO2 promotes a reduction of PdO. XPS results showed the observed binding energy of Pd3d in the TiO2-containing catalysts was lower than that in the TiO2-free catalysts. TiO2 undoubtedly donates its electrons to PdO, so that the bond strength of Pd-O should be weakened and lattice oxygens of PdO catalysts would easily react with methane even at as low temperature as 225°C.
