Engineering of the highly active low-temperature oxidation ceria based catalyst – surface structures and bulk doping

The interaction of palladium and ceria with formation of ionic palladium species is considered to be a main reason of high activity of Pd/CeO2 catalysts in CO and methane oxidation at low-temperatures (below ambient... [ view full abstract ]

The interaction of palladium and ceria with formation of ionic palladium species is considered to be a main reason of high activity of Pd/CeO₂ catalysts in CO and methane oxidation at low-temperatures (below ambient temperature for CO and less than 300^oC for methane). The formation of solid solutions Pd_xCe_1-xO_2-x-δ in Pd/CeO₂ catalysts have been proposed in literature, and the near-square-planar local environment of Pd²⁺ ion in ceria lattice was established experimentally in our work [1]. The bifunctional role of palladium in Pd/CeO₂ catalysts is proposed in literature; surface Pd²⁺ ions provide the substrate activation, while the bulk Pd²⁺ ions, serving as a dopants, increase the O^2- ions non-stoichiometry and mobility in parent ceria phase [2].

In this work we present a new evidence of Pd²⁺ ions incorporation into ceria lattice with the new superctructure formation. This structure was formed via the calcination of ultrasonically mixed nanodispersions of metallic palladium and highly defective ceria obtained via laser ablation technique which was accompanied by a sharp rise of the catalyst activity. Thus the formation of such structures gives grounds to suppose the crucial role of Pd²⁺ ions superstructures in the high catalyst activity rather than single Pd²⁺ ions in good agreement with the literature [3,4].

Fig.1. Pd₆Ce₂₆O₅₈ superstructure discovered in the laser ablated Pd-CeO₂ sample calcined at 600^oC and CO oxidation activity of the sample calcined at different temperatures.

We also made a successful attempt to generate surface structures by way of calcination of diluted solid solutions of Pd in ceria lattice which led to segregation of Pd²⁺ to subsurface layers and local enrichment. Moreover, additional doping of ceria with Ni²⁺ ions led to increase the PdNi/CeO₂ catalyst activity and thermal stability due to Ni²⁺ ions took the part of Pd²⁺ ions functions as activators of oxygen non-stoichiometry of ceria lattice.

Literature.

1. Gulyaev R.V., et.al, PCCP, 2014, 16, 13523-13539

2. M. S. Hegde et. al, Catalysis Today, 2015, 253, 40-50.

3. S. Colussi et. al, Angewandte Chemie International Edition, 2009, 48, 8481-8484.

4. P. Senftle et. al, ACS Catalysis, 2015, 5, 6187-6199.