Typical crystal face effects of different morphology ceria on the activity of Pd/CeO2 catalysts for lean methane combustion

The catalytic efficiency of CeO 2 carriers and the catalytic properties of palladium (Pd) nanocrystals loaded on different morphology CeO 2 carrier were studied for lean methane (CH 4 ) combustion. The catalytic activity of simple CeO 2 carriers was ranked in the order of rod-CeO 2 (r-CeO 2 ) > cube-CeO 2 (c-CeO 2 ) > octahedral-CeO 2 (o-CeO 2 ). After the Pd species was loaded on CeO 2 , 90% of CH 4 was converted at 348 °C catalyzed by Pd/octahedral-CeO 2 (Pd/o-CeO 2 ) with (1 1 1) crystal faces, which had much higher catalytic activity comparing with Pd loaded on c-CeO 2 with (1 0 0) crystal faces and r-CeO 2 with (1 1 0) and (1 0 0) crystal faces. The results of XRD , HRTEM , EDS , H 2 -TPR, XPS and elemental mapping showed that the Pd species in the form of Pd 0 and PdO were highly dispersed on surface of all CeO 2 carriers. Most importantly, quasi In-situ XPS results convinced that Ce 3+ /Ce 4+ ion pairs played an important role in the catalytic combustion of low-concentration methane by adsorbing or releasing oxygen. Compared with r-CeO 2 and c-CeO 2 , the concentration of Ce 3+ on the surface of o-CeO 2 carrier was greatly increased when the Pd species were present, which indirectly reflected that the concentration of oxygen vacancies on the carrier surface also increased. Consequently, the high catalytic activity of Pd/o-CeO 2 is not only related to the high dispersion of Pd 0 and PdO species on the surface of the catalyst, but also closely related to the synergistic interaction between Pd species and (1 1 1) specific faces of o-CeO 2 exposing.