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

The catalytic efficiency of CeO2 carriers and the catalytic properties of palladium (Pd) nanocrystals loaded on different morphology CeO2 carrier were studied for lean methane (CH4) combustion. The catalytic activity of simple CeO2 carriers was ranked in the order of rod-CeO2(r-CeO2) > cube-CeO2(c-CeO2) > octahedral-CeO2(o-CeO2). After the Pd species was loaded on CeO2, 90% of CH4 was converted at 348 °C catalyzed by Pd/octahedral-CeO2 (Pd/o-CeO2) with (1 1 1) crystal faces, which had much higher catalytic activity comparing with Pd loaded on c-CeO2 with (1 0 0) crystal faces and r-CeO2 with (1 1 0) and (1 0 0) crystal faces. The results of XRD, HRTEM, EDS, H2-TPR, XPS and elemental mapping showed that the Pd species in the form of Pd0 and PdO were highly dispersed on surface of all CeO2 carriers. Most importantly, quasi In-situ XPS results convinced that Ce3+/Ce4+ ion pairs played an important role in the catalytic combustion of low-concentration methane by adsorbing or releasing oxygen. Compared with r-CeO2 and c-CeO2, the concentration of Ce3+ on the surface of o-CeO2 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-CeO2 is not only related to the high dispersion of Pd0 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-CeO2 exposing.