Exceptional Activity for Methane Oxidation With Catalysts Prepared By Modular Assembly of Subunits

Exceptional activity for methane oxidation with catalysts prepared by modular assembly of subunits

M. Cargnello,^1,2 J. J. Delgado Jaén,³ J. C. Hernández Garrido,³ K. Bakhmutsky,⁴ T. Montini,¹
J. J. Calvino Gamez,³ R. J. Gorte,⁴ P. Fornasiero¹

¹ Department of Chemical and Pharmaceutical Sciences, ICCOM-CNR, Consortium INSTM, University of Trieste, via L. Giorgieri 1, 34127 Trieste, Italy

 ² Department of Chemistry, University of Pennsylvania, 231 S. 34^th Street, Philadelphia, PA 19104, USA

³ Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química Inorgánica, Facultad de Ciencias, Universidad de Cádiz, Campus Río San Pedro, 11510 Puerto Real, Cádiz (Spain)

³ Department of Chemical and Biomolecular Engineering, University of Pennsylvania, 311A Towne Building, 220 S. 33^rd  Street, Philadelphia, PA 19104, USA

There is a critical need for improved methane-oxidation catalysts to both reduce emissions of methane, a green-house gas that will soon be regulated, and significantly enhance the efficiency of power generation in gas turbines. Presently available, emissions-control catalysts are notoriously ineffective at reducing CH₄ in exhaust streams. High-temperature combustion also results in the emission of toxic nitrogen oxides (NO_x) and CO. Combustion of CH₄ promoted by heterogeneous catalysts could utilize the available energy of methane at lower temperatures, increasing efficiency and limiting emissions by drastically reducing the required temperatures. Materials for this application must also be catalytically and mechanically stable at high reaction and flame temperatures. PdO_x is recognized as one of the best catalysts for catalytic CH₄ oxidation. Unfortunately, Pd-based catalysts tend to deactivate, both due to the loss of active surface by sintering and due to the transformation into the less active but thermodynamically favored metallic Pd phase at temperatures above 600 °C. Although alumina is a commonly employed support, both experimental and theoretical studies reveal that ceria (CeO₂) can improve the catalytic activity of supported Pd due to the stabilization of PdO_x species. Materials that could simultaneously enhance the performance of Pd-based catalysts at low temperatures and limit deactivation at elevated temperatures would greatly improve the viability of catalytic CH₄ combustion processes.

In this contribution, we report on a novel, hierarchical design of core-shell type catalysts that is inspired by the concepts of supramolecular chemistry. Two active building blocks, Pd and CeO₂, are prepared separately, then self-assemble and organize in solution to form supramolecular core-shell structures held together by metal ion-ligand coordination chemistry. We exploit the pre-organization of the functionalized Pd@CeO₂ core-shell structures to disperse single units onto a modified, catalytically inert alumina carrier. Transmission Electron Microscopy (TEM) investigations demonstrate that it is indeed possible to deposit single structures where the metal-promoter interaction is maintained even after severe thermal treatments at temperatures up to 850 °C. The special configuration of the hierarchical catalyst gives rise to exceptionally high and stable performance for the catalytic combustion of methane with reduced amounts of Pd and ceria. The particular geometry appears to stabilize the active phase of the catalyst, not only preventing agglomeration of palladium oxide particles during the catalytic reaction but also preventing the PdO_x from being transformed to Pd at its usual transition temperature.