(669d) Heterobimetallic Complexes As Controlled Precursors for Atomically Dispersed, Promoted Methane Oxidation Catalysts

Heterobimetallic
Complexes as Controlled Precursors for Atomically Dispersed, Promoted Methane
Oxidation Catalysts

Joseph Maalouf, Jay
Schwalbe, Joshua J. Willis, Eric A. Stach², Matteo Cargnello

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

²Center for Functional
Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USA

Thanks
to improvements in fracking technology, methane has become a cheap and abundant
commodity that is gaining a prominent position in the energy scenario. However,
there is a crucial need for improved methane combustion catalysts, as low
temperature methane oxidation can reduce emissions from natural gas leaks, and
reduce levels of toxic gases released from flaring, such as NO_x , SO_x
and CO. In this study, a method for producing a highly active catalyst that
combines both promotion and atomic dispersion is presented.

Promoted,
atomically disperse catalysts are extremely difficult to produce, especially
with traditional impregnation methods. These techniques rely on a probabilistic
adsorption of a compound onto a support, with no real control over distribution
and composition, but promotion often requires two metals to be in extremely
close vicinity to one another. The issue is only exacerbated when attempting to
achieve atomic dispersion, as this often requires extremely low weight
loadings, meaning that statistical occurrences of the metal and promoter
depositing onto the support with a high interface are extremely low.

Here,
we show that by using a pre-formed, atomically precise Palladium Cerium
heterobimetallic precursor, it is possible to achieve atomic dispersion and
keep the active phase (Pd) and the promotor (Ce) in close proximity. To fully
exploit this method, support engineering was also performed. Alumina was
functionalized with mixtures of triethoxy(octyl)silane (TEOOS) and
(3-mercaptopropyl) trimethoxysilane  (MPTMS) in a TEOOS to MPTMS ratio much
greater than one, in order to produce atomic scale islands of MPTMS, which bind
to the Pd in the complex, facilitating atomic dispersion and hindering clumping
via diffusion across the surface. Indirect evidence of atomic dispersion was
supported by HAADF-STEM characterization. These samples were then calcined to
remove any organic ligands and tested for activity, showing rates that were at
least 5 times higher than those obtained with catalysts prepared via
impregnation of the isolated Pd and Ce precursors.