(786a) 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.