Microwave assisted heating has been used for the synthesis of palladium–gold alloy nanoparticles (NPs), using a polyol co-reduction method. Various compositions of homogeneously alloyed poly-vinylpyrrolidone (PVP) capped PdxAu100-x (x = 10-90) NPs were synthesized under microwave heating (µwI). Homogeneous mixing between the two constituent metals and successful tuning of various compositions for PdAuNPs could be achieved at 150 °C under µwI using short reaction times (≤ 300 s). Mechanistic studies of PdAuNPs formation showed that small Pd nanoclusters are formed initially into which rapid incorporation of Au atoms occur to yield the alloy NPs. Heterogeneous catalysts were prepared by supporting the as synthesized PdAuNPs on amorphous silica (a-SiO2). Model gas-phase cyclohexene hydrogenation catalysis was studied and a comparison of activity vs. % Pd resulted in a volcano like behavior for PdAuNPs. The most optimum composition for cyclohexene hydrogenation reaction was found to be Pd59Au41NPs. Theoretical studies were performed to validate the experimental observation using density functional theory (DFT). DFT calculations accounting for local ensemble effects and long range electronic effects closely matched with experimental catalytic results. A comparison between conventional heating (CvH) and µwI for the synthesis of PdAuNPs showed that µwI consistently yields in more size-focused and catalytically superior NPs. Ligand quantification of PdAuNPs was performed using solution state 1H-NMR spectroscopy and showed that when compared to µwI, CvH leads in a higher degree of PVP incorporation. Lastly, the most active PdAuNPs were supported on titania and α-alumina in addition to a-SiO2 and absence of any significant strong metal-support interaction (SMSI) was confirmed by comparative catalytic testing. PdAu alloy NPs supported on a-SiO2 were tested for solution phase nitrite reduction in water as well. Compared to pure Pd NPs, PdAu alloy NPs showed better nitrite reduction activity. Volcano like behavior was again observed between nitrite reduction activity and % Pd content of the alloy NPs. PdAu catalysts were also tested for sulfide fouling. Experimental results showed that incorporation of Au into the alloy NPs appreciably decreased the extent of sulfide fouling. DFT calculations showed that S2− binding to the surface of alloy NPs is weakened in the presence of Au. Post-sulfide fouling analyses showed no noticeable structural changes in PdAu alloy NPs.

Microwave assisted synthesis and catalytic studies of palladium–gold alloy NPs

254th National Meeting (2017)

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