(734a) Structure-Property Relationships of Palladium Catalyzed Methane Complete Combustion Using Uniform Nanoparticles

Structure-Property
Relationships of Palladium Catalyzed Methane Complete Combustion using Uniform
nanoparticles

Joshua J. Willis, Emmett
Goodman,Matteo Cargnello

¹Department
of Chemical Engineering and SUNCAT Center for Interface Science and Catalysis,
Stanford University, Stanford, CA 94305, USA

There is a critical
need for reducing emissions of methane, the second most prevalent greenhouse
gas. Homogeneous combustion of methane, as in flaring, results in the release
of toxic gases such as NO_x, SO_x and CO, and current
methods of catalytic emission control are still ineffective at low temperatures
(<300˚C) and in the presence of steam. Improving methane combustion
catalysts would affect several technologies that have to deal with methane emissions.
While many materials have been investigated for their methane combustion
activity, palladium is widely accepted as one of the best metal catalysts.
Unfortunately, it is unclear which phase of palladium (Pd metal, Pd oxide or a
combination of both) is most active for this reaction; furthermore, there is a
need to utilize Pd to the best possible extent given its precious nature. To
gain a fundamental understanding of this system and provide insights into the
active sites to further improve the activity and stability of Pd-based
catalysts, control over the size, shape and composition of the metal
nanocrystals is essential.

In this study, highly monodisperse palladium nanocrystals,
prepared via high-temperature colloidal synthesis, are used to investigate the
structure-activity relationships of palladium-catalyzed methane complete
combustion in the low size regime (2-8nm). By deposition of the nanocrystals
onto high-surface area supports (Al₂O₃, CeO₂,
MgO and SiO₂; all with similar surface areas) and activation using a
fast thermal annealing process, highly monodisperse, stable catalysts are
obtained. These materials are used to draw structure-activity relationships
under realistic methane complete combustion conditions. Kinetic
characterization demonstrates that the activity is dependent on the oxidation
state of the Pd phase, which is also affected by the size of the Pd
nanocrystals. Furthermore, the effect of steam is systematically investigated
as a function of size and support. These new observations are made possible
through the use of tightly controlled Pd nanocrystals. Our work provides clear
elements for improving the activity of Pd-based combustion catalysts and in
general a framework for understanding structure-activity relationships using highly
uniform catalysts under realistic reaction conditions.