(5an) Understanding the Catalytic Activity of Bi- and Monometallic Pd and Au Catalysts for Aqueous Reactions

Authors: 
Heck, K. N., Rice University


Understanding of the interactions between a catalytic surface with reactants and intermediates under reaction conditions can be used to tailor catalytic surfaces for increased activity or selectivity. Moreover, the feasibility of a catalyst for commercial use depends partly on the activity of the catalyst in the presence of possible deactivants in the reactant stream.

However, in situ spectroscopic characterization of heterogeneous catalysts in aqueous systems has long been a challenge. While methods such as electron energy loss spectroscopy have been used successfully to monitor to the adsorption of compounds to model catalyst sites, they rely on ultra high vacuum environments. While FTIR coupled with ATR techniques can offer more realistic conditions, the penetration depth of such spectroscopy is on the bulk level, and signals produced suffer from the strong absorption of the aqueous environment. Surface-enhanced Raman spectroscopy (SERS) using gold nanoshells, on the other hand, could provide the requisite sensitivity for analyzing aqueous-phase catalyzed reactions. Au nanoshells (NS's) are a class of SERS substrates, whose surface plasmon resonance (SPR) can easily be tuned by modifying core particle size and shell thickness, resulting in signals up to 1014 times greater than normal Raman alone.

To determine the applicability of SERS using Au NS's for aqueous systems, we examined two systems. Recent efforts have shown that glycerol, a major byproduct of biofuels production, can be converted over Au catalysts in alkaline media with great selectivity to fine chemicals. Additionally, previous studies show that Pd-on-Au (Pd/Au) nanoparticle (NP) catalysts have been shown to be 70x more effective than conventional Pd catalysts in the hydrodechlorination of chlorinated ethylenes in groundwater. While mechanisms exist for these two reactions, they there is little spectroscopic proof.

In-situ SERS monitoring of aqueous glycerol oxidation over Au NS's

In this study, we report the in-situ time-resolved identification of adsorbates and reaction intermediates in the basic aqueous-phase oxidation reaction of glycerol via surface-enhanced Raman spectroscopy (SERS) over a Au nanoshell (NS) catalyst. Coupled with batch experiments using a Au/C catalyst, we gain important insights into the effect of pH and O2 on the surface reaction.

In-situ SERS monitoring of aqueous HDC over Pd-on-Au NS's

We also report the synthesis of Pd-decorated Au NS's to mimic the HDC catalytic behavior of Pd/Au NPs during SERS analysis. SERS spectral analysis indicates that chemisorption and HDC surface reaction events of dichloroethylene (DCE) probe molecule can be observed. It appears that 1,1-DCE adsorbs to the Pd catalyst via pi-bonding with Pd atoms on the surface, for example. The results of this study highlight the promising use of SERS spectroscopy of metal-catalyzed reactions in water in situ.

Kinetic deactivation study of Pd/Au NPs with chloride and sulfide

Finally, to test the feasibility of Pd/Au NPs for use in the field, we examined the effect of two common Pd deactivants found in groundwater, chloride and sulfide. Results show that the catalyst is completely resistant to chloride, while monometallic Pd NPs and Pd/Al2O3 deactivate to 30% its initial rate at the highest chloride concentration tested. Additionally, the Pd/Au catalyst show improved resistance to deactivation of sulfide. While the Pd catalysts deactivated at the theoretical limit of S:Pdsurf of 0.5, the Pd/Au NPs showed resistances of up to S:Pdsurf of 1.5. The resistance is found to be compositionally dependent, with higher Pd wt% catalysts deactivating more rapidly. Additionally, the shift in the deactivation profile from a linear decrease in rate with increasing S:Pdsurf for low Pd wt% catalysts to a nonlinear profile with higher Pd wt% suggests the formation of different active sites made of larger Pd ensembles for the higher wt% catalyst.