Catalytic Activity of Microbially Formed Palladium Nanoparticles
Materials Science and Engineering
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The applications of palladium are wide and varied, but characteristics such as its catalytic nature allow it to be widely used in chemical and electrochemical industries. However, with the cost of palladium on the rise, alternative methods have been investigated to accommodate demands. Due to the enhanced catalytic nature of nanoparticles, palladium nanoparticles have been investigated as a viable cost reduction strategy to incorporate into catalytic systems. Common techniques for the formation of nanoparticles involve either physical or chemical methods. However, more economical and non-toxic alternatives exist in the form of biological methods. In this study, microbially-formed palladium nanoparticles were synthesized using a solution of sodium tetrachloropalladate and the metal reducing properties of Clostridium pasteurianum BC1. These palladium nanoparticles were then purified through a simple centrifugation process and characterized for their morphology, chemical, and electrocatalytic characteristics. Heat treatments and immobilized microbial cultures were also utilized to improve the catalytic performance. Morphology, size, and composition of palladium nanoparticle samples were determined using scanning electron microscopy, transmission electron microscopy, dynamic light scattering analysis, energy dispersive X-ray spectroscopy, and X-ray diffraction. Electrochemical behavior was investigated via cyclic voltammetry using a traditional three electrode set-up consisting of a modified glassy carbon working electrode, a platinum wire counter electrode, and a Ag│AgCl reference electrode in a potassium hydroxide solution.Scanning electron microscopy and transmission electron microscopy revealed consistent spherical morphology throughout all samples. Dynamic light scattering analysis revealed the average size of palladium nanoparticles formed using suspended cultures to be approximately 20 nm. The average size of palladium nanoparticles formed using immobilized cultures was found to be 15 nm. Energy dispersive X-ray spectroscopy and X-ray diffraction showed the nanoparticles in the non-heat treated samples consist only of palladium, while palladium and palladium oxides were present in heat treated samples. Finally, cyclic voltammetry revealed that palladium nanoparticles formed using suspended microbial cultures performed poorly with respect to abiotic controls, in terms of mass activity with average mass activities of 7 mAmg-1 and 93 mAmg-1, respectively. An increase in palladium nanoparticle catalytic performance was observed after utilizing immobilized microbial cultures to synthesize the nanoparticles via sodium alginate gel entrapment. The average mass activities of nanoparticles formed using suspended cultures and immobilized cultures was measured to be 7 mAmg-1 and 130 mAmg-1, respectively. Further improvements in catalyst performance were explored using heat treatment methods by heating palladium nanoparticle samples at 400 °C. Comparatively, it was found that the electrochemical activity observed in heat treated palladium nanoparticles formed using immobilized microbial cultures greatly exceeded that of heat treated palladium nanoparticles formed in suspended microbial cultures with average mass activities of 177 mAmg-1 and 43 mAmg-1, respectively.To our knowledge, this is the first study that evaluates the electrochemical catalytic activity of microbially-formed palladium nanoparticles. The results of this study aim to support the use of nanoparticles formed using facile and environmentally-friendly microbial synthesis methods as a suitable alternative as opposed to standard physical and chemical synthesis methods.