Biotechnological production of supported metal nanoparticles for catalysis

Abstract

Catalytic transformations are essential to industrial manufacturing and the modern global economy. However, in the context of the looming climate crisis and finite global resources, it is necessary to improve the sustainability of industrial chemical processes. The environmentally benign synthesis of metal nanoparticle catalysts with enhanced catalytic activity, selectivity, and recyclability is therefore highly desirable. Palladium (Pd) catalysts are crucial in the synthesis of essential feedstocks across many industries, but Pd is also a critical raw material that requires efficient and sustainable use. This thesis explores the biotechnological synthesis of Pd nanoparticles (bio-Pd), by the metal-reducing bacterium, Geobacter sulfurreducens, as alternative, sustainable heterogeneous catalysts. Crucially, bio-Pd is synthesised at ambient temperature, using renewable and environmentally benign materials, and has demonstrated high activity and selectivity for industrially important reactions, performing comparably to commercial Pd catalysts. However, a fundamental understanding of how to control the properties of biosynthesised nanoparticles is critically lacking. This project aims to develop control over bio-Pd physicochemical properties, leading to enhanced catalytic performance. This multidisciplinary research employed an array of advanced analytical techniques to probe how changes to biosynthesis parameters affected the size, morphology, oxidation state, and composition of bio-Pd nanoparticles. Varying the solution chemistry, using different Pd(II) salts and biological buffers, significantly altered Pd(II) speciation and subsequent microbe-metal interactions, resulting in changes to nanoparticle size, distribution, and the catalytic performance of bio-Pd. Challenging cells with distinct Pd loadings and electron donor conditions resulted in the deposition of bio-Pd at different cellular locations via different enzymatic reduction pathways. This affected the nanoparticles’ size, oxidation state, and surface accessibility to reactants, ultimately controlling the catalytic properties of bio-Pd. Bio-Pd synthesised under high Pd load was a highly active and reusable catalyst for the Suzuki reaction, maintaining 100% conversion over three reaction cycles. The novel discovery of 2 nm microbe-supported PdO nanoparticles is reported, which exhibited unprecedented bionanoparticle reaction rates for the reduction of 4-nitrophenol, outperforming a commercial Pd/C catalyst. The novel biosynthesis of bimetallic alloyed PdAg nanoparticles is reported, revealing the potential to fine-tune their bimetallic nanostructure using different metal molar ratios. Additionally, bio-PdAg exhibited enhanced catalytic activity over bio-Pd for 4-nitrophenol reduction, while efficiently minimising Pd utilization. Finally, an immobilization technique of bio-Pd on activated carbon supports was developed, while maintaining high catalytic efficiency. Overall, the findings from this project provide insights into controlling the properties of bionanoparticles for enhanced catalytic performance. Ultimately, these findings contribute towards the development of microbial biotechnologies regarding the improved sustainable use of precious metals.

Date of Award	19 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Jonathan Lloyd (Main Supervisor) & Sarah Haigh (Co Supervisor)