Palladium (Pd) based catalysts are of increasing interest for the removal of volatile organic compounds (VOCs), such as trichloroethylene (TCE), from water by hydrogenolysis. Pd is often used in combination with promoter metals, such as gold (Au), which can increase TCE reduction rates by up to X-fold [1]. As a support, Granular activated carbon (GAC) has several advantages, including its capacity to absorb contaminates, which concentrates the reactants at the catalyst surface. It also provides a high surface area (porous) or anchoring Pd and Au nanoparticles (NPs). Previously, we have reported the synthesis of Pd/Au/GAC at 70°C to achieve the Pd and Au NPs on 20-40 mesh GAC catalyst, which has rapidly degraded TCE in laboratory batch experiments [2].To determine how temperature during synthesis affects the Pd and Au NP morphology and distribution on the GAC, and the reactivity of the resulting material, the catalysts were synthesized at two temperatures, 70°C and room temperature (RT), using a 2:1 molar ratio of Pd to Au precursor, with solvent acetone. These temperatures were chosen since 70°C is slightly hotter than the boiling point of acetone (56°C), and RT (22-25°C) is the most convenient temperature condition to maintain. SEM equipped with EDS was used to characterize the surface of the GAC grains. , the larger particles appear to be Au-rich, while the smaller particles tend to be Pd-rich. Our observations suggest that some of the formations are bimetallic, while others are elemental NPs or heterogeneous structures, further identification of which is the target of future work. We hypothesize that the difference in sizes of these NPs, between the catalysts synthesized with two different temperatures, is due to an increase in aggregation of Au NPs at the higher temperature.As-produced catalysts show no remarkable distinguishing features visible to the naked eye, as seen in Figure 3 (a). The effect of NPs' size on the activity of the catalyst was tested via degradation of TCE in a hydrogen-rich aqueous atmosphere. Samples were analyzed using a gas chromatographer with a flameionized detector (GC/FID), and are reported in Figure 3 (b). The RT samples degraded TCE faster than 70°C sample, possibly because the RT samples offer more of the catalytically active NP surface than their counter-part, despite the fact that the same amount of precursors were used for synthesizing both samples. Further investigations are ongoing towards better particle size control and aggregation, and its impact on degradation of contaminants. In general, NP aggregation control can have several benefits across different applications.
Adding Au to Pd nanoparticles (NPs) can impart high catalytic activity with respect to hydrogenation of a wide range of substances. These materials are often synthesized by reducing metallic precursors; hence, sonochemical and solvothermal processes are commonly used to anchor these bimetals onto thin supports, including graphene. Although similar NPs have been studied reasonably well, a clear understanding of structural characteristics relative to their synthesis parameters is lacking, due to limitations in characterization techniques, which may prevent optimization of this very promising catalyst. In this report, a strategic approach has been used to identify this structural and material synthesis correlation, starting with controlled sample preparation and followed by detailed characterization. This includes advanced scanning transmission electron microscopy and electron energy loss spectroscopy; the latter using a state-of-the-art instrumentation to map the distribution of Pd and Au, and to identify chemical state of the Pd NPs, which has not been previously reported. Results show that catalytic bimetal NP clusters were made of small zero-valent Pd NPs aggregating to form a shell around an Au core. Not only can the described characterization approach be applied to similar material systems, but the results can guide the optimization of the synthesis procedures.
The synthesis of palladium (Pd) and gold (Au) nanoparticles (NPs) for the treatment of groundwater has become a popular method because of its effectiveness and sustainability [1]. An effective synthesis method for the production of these NPs uses a combination of sonication and solvothermal processes [3],[4]. The as-made samples are designed to have a 2:1 Pd:Au molar ratio, and a 5wt% of Pd on carbon support, for comparison with corresponding commercial granular activated carbon (GAC) supported 5wt% Pd catalysts. In the past, we have shown that these as-made GAC supported Pd/Au NPs which have been fabricated in 24 hr were potent at degrading carcinogens like trichloroethylene (TCE) [3],[4]. Here, we have investigated the effect of formation time on carbon-supported Pd/Au NPs catalyst to potentially expedite the synthesis process. We synthesized samples in one-hour intervals between 0 and 24 hr and here compare the 1-hr and 24-hr samples. The samples were characterized using an FEI Tecnai F-20 TEM/STEM with EDS capabilities and a Shimadzu UV 3600 UV-Vis Spectrophotometer. Specifically, for TEM analysis, for better characterization, graphene was used as a support in the place of the denser, thicker GAC. In doing so, the surface of GAC is approximated to be similar to a highly defective surface of graphene. For any TCE degradation testing however, the as-made GAC-supported Pd/Au NPs have been used.
The contamination of groundwater by trichloroethylene (TCE) and other related contaminants is a compromise to the safety of drinking water all over the world. There have been many efforts to solve this issue, but all possible solutions have come at the cost of excessive waste or further contamination of the environment. The palladium (Pd), gold (Au) coated carbon supported catalyst makes use of current nanotechnology to provide an efficient method of degrading TCE to non-toxic levels of ethene and ethane. This thesis discusses the optimization of the novel solvothermal, green synthesis process of Pd/Au carbon supported nanoparticles (NPs) devolved by the Jiao Group at Portland State University. The pre-sonication of Au precursor, washing of the carbon supported NP samples, and reaction time of the samples were explored.Evidence suggested that sonicating the Au precursor did not, alone, synthesize nanoparticles, washing the catalysts did provide an effective method of ensuring nanoparticles adhered to their support, and that a reaction time of down to an hour may be just as effective as a reaction time of 24 hours. The preliminary results gathered here are promising results to suggest the claims mentioned above but will require further investigation for a definitive confirmation.
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