Investigating nanoscale catalysts from primary growth to catalytic activity

Tierney, George (2022) Investigating nanoscale catalysts from primary growth to catalytic activity. University of Southampton, Doctoral Thesis, 289pp.

Record type: Thesis (Doctoral)

Abstract

The ability to mass produce tailored nanoparticles, e.g. uniform sizes and surface site types, and accessibilities, has wide reaching implications for both academic and industrial research. Sol-immobilisation is proven to offer an easy to control route for the preparation of metallic nanoparticles, where a colloidal solution of nanoparticles is preformed (stabilised using polyvinyl alcohol) and anchored to a support material. However, as yet, there is limited understanding of how systematic variations to the synthesis parameters influence the fundamental nucleation and growth steps in nanoparticle formation. Systematic changes to the solvent of synthesis, through the addition of C₁-C₄ linear and branched chain alcohols, were employed in the preparation of metallic Pd colloids. It was discovered using spectroscopic techniques (UV-Vis, IR and XAFS) and TEM imaging that the greatest control of nanoparticle growth was achieved in solutions of equal MeOH and H₂O parts per volume. Additionally, Pd colloids prepared in solutions of > 50 vol. % MeOH saw a drastic decrease in their achieved metal loading. This issue was remedied by removing the acidification step during immobilisation. The resultant influence of this updated procedure on Pd nanoparticle properties was characterised using the previously listed techniques along with thermal decomposition methods (TGA, TPR, TPD). From this, it was established that the synthesis of Pd nanoparticles in MeOH caused increased layering of the stabiliser around the nanoparticle and support surfaces, suppressing the spill over of H₂. The performance of these non-acidified PVA-capped Pd nanoparticles were investigated for furfural hydrogenation. The limiting of H₂ spill over was shown to be highly effective in switching off the acid-catalysation pathway to acetals over the TiO₂ support surface, making it selective for the desired hydrogenation products. In addition, the mechanisms of colloidal Au nucleation and growth were investigated via novel XAFS experiments. A proof of concept study for in situ XAFS measurements was first approached by measuring solutions of preformed Au colloids prepared under varied synthesis parameters: synthesis temperature = 1, 25, 50 and 75 °C, and [Au] = 50, 100 and1000 µM. Theestablished relationship between synthesis temperature and nanoparticle size was found to be consistent in colloidal and supported Au NP systems. The influenceof the [Au] on XAFS nanoparticle size, however, was only observed after immobilisation of the colloid, something which has not previously been reported. Building on the successful acquisition of colloidal XAFS, a continuous microfluidic system was designed and implemented to measure the in situ reduction of HAuCl₄ ([Au] = 100 µM) to Au⁰ nanoparticles. Issues concerning the flow regime of the cell, metal deposition on the reactor walls, and X-ray compatibility were all considered prior to in situ measurements. XAFS data acquired during reduction showed that under mild reducing conditions nanoparticle nucleation occurs on a 10² ms time scale, and that complete reduction of the Au species occurs over the course of 3.5 seconds. Furthermore, this novel work proves that time-resolved studies of colloidal nanoparticle nucleation and growth are feasible.