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.