NSTI Nanotech 2009

Using Gold to Enhance Palladium Catalytic Activity for Trichloroethene Reductive Dechlorination

Y.-L. Fang, P.J.J. Alvarez, J.T. Miller, M.S. Wong
Rice University, US

Keywords: nanoparticle catalysis, palladium, gold, trichloroethene, hydrodechlorination

Abstract:

Trichloroethene (TCE) is one of the most common hazardous organic contaminants found in groundwater. This chlorinated solvent has been widely used in various fields including electronics manufacturing, metal degreasing, dry cleaning and chemical intermediates; therefore, lots of TCE has entered the environment through leakage, disposal and other pathways. TCE has been linked to liver damage, impaired pregnancies, and cancers in humans. Compared to conventional physical displacement methods such as air-stripping and carbon adsorption, remediation through the catalytic reductive dechlorination of TCE molecules is a more effective and desirable approach. In our previous work, palladium-on-gold colloidal nanoparticles (Pd/Au NPs, 4 nm) have been shown to catalyze the TCE hydrodechlorination (TCE HDC) reaction in water, at room temperature, and in the presence of hydrogen, with the most active Pd/Au NPs (k = 1900 L/gPd/min) found to be >70 times more active than conventional mono-metallic Pd supported on alumina (k = 26 L/gPd/min) on a per-Pd atom basis. Of note, our Pd/Au NPs exhibited volcano-like catalytic activity with different surface coverage. We proposed that Au enhances Pd activity through electronic and/or geometric effect. In this talk, we will address nanostructure evidence of our Pd/Au NPs to correlate it with reaction activity profile. We believe that geometric effect is the dominant effect for Au enhancement. The bimetallic Pd-on-Au nanostructure was determined by extend X-ray absorption fine structure (EXAFS) spectroscopy. The results showed that our Pd/Au NPs had Au-core and Pd-shell and most of surface Pd atoms were metallic. Pd atoms on Au NP surfaces exhibited better stability of zero oxidation state (metallic form) than pure Pd NPs which showed higher tendency to be oxidized. In addition, we will discuss results from diffuse reflectance infra-red Fourier transform spectroscopy (DRIFTS), utilizing carbon monoxide (CO) as a probe molecule, which can provide structure information of small Pd aggregates on Au surfaces. Kinetic analysis of TCE HDC reaction with Pd/Au NPs and Pd NPs will also be presented. This analysis verified a proposed reaction mechanism, and revealed information about the nature of active sites of different catalysts (Pd/Au NPs and Pd NPs).
 
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