Continuous synthesis of metallic and oxidic palladium nanoparticles with tunable sizes for catalytic applications

Air pollution is of particular importance in modern society due to its negative effects on human health, urban quality of life and the environment and is part of the UN Sustainability Goals. Catalytic converters have changed the course of the transport industry in efforts to significantly reduce emissions in vehicles. More specifically, three-way catalysts are the most widely used catalysts for gasoline engines and use platinum group metals, such as palladium, as the active catalyst materials and oxides such as cerium dioxide as support. Palladium and palladium-based cerium dioxide catalysts have been extensively studied in three-way catalysis due to their excellent catalytic properties. However, ambiguity in mechanistic knowledge persists, such as identifying catalytic active sites, optimal metal oxidation states and nanoparticle sizes for the different reactions, as well as understanding the dynamic behaviour of this system. Furthermore, differences between systems operating under realistic conditions using a full gas mixture, as opposed to model conditions conventionally used for catalytic evaluation, contribute towards the complexity of this system. Difficulties in precise metal nanoparticle control and the procession of multiple reactions simultaneously, poses challenges in decoupling the effect of nanoparticle size and metal oxidation state on catalytic activity, respectively. In this study, the continuous synthesis of palladium and palladium oxide nanoparticles for their use in three-way catalysts for air pollution remediation is presented. Coiled flow inverter microreactors are used to obtain PdO and Pd nanoparticles of various sizes and the optimised novel syntheses subsequently constitute the catalyst preparation method of Pd-based cerium dioxide catalysts. The catalysts are evaluated for their catalytic performance under model and realistic conditions and an attempt to decouple the effects of nanoparticle size and initial oxidation state on activity is made. This thesis demonstrates the successful synthesis of small, monodisperse PdO and Pd nanoparticles in continuous flow in the size range between ~2.5-5 nm, which maintain their colloidal stability even in the absence of in-situ stabilisers. Palladium oxide is synthesised via the hydrolysis of a metal precursor using base, whereas palladium is synthesised via the reduction of various metal precursors using sodium borohydride. In the synthesis of PdO nanoparticles through the hydrolysis of palladium dinitrate dihydrate, precursor concentration is observed to have the biggest effect on size change via manipulation of nucleation and growth rates, allowing for particle size tuning between ~2.3-4.2 nm with a narrow particle size distribution. Using this synthetic method, fast nucleation to form polyhydroxo intermediate species leads to small nanoparticles which are stable in solution, as measured by large absolute zeta potential values, through accumulation of electric surface charge. The colloidal stability of PdO nanoparticles in aqueous solution is sensitive to pH changes and the nanoparticles remain most stable in the highly basic pH region. In the synthesis of metallic Pd nanoparticles, reduction using a strong reducing agent, such as sodium borohydride, leads to the fast nucleation and formation of nanoparticles as small as ~3.2 nm. Amongst the parameters studied for the size tuning of Pd, precursor concentration and early mixing had the biggest effect on size through manipulation of nucleation and growth rates, allowing for the size tuning of nanoparticles in the size range between ~3.2-4.9 nm with a narrow size distribution. Colloidal stability for Pd nanoparticles in solution is a major challenge of this study. Precursors with varying counter ligands and post synthetic, as well as in-situ stabilisers, were investigated to optimise the synthesis. Ultimately, strong ligand stabilisation of counter ligands in precursors is most significant in the resulting colloidal stability of nanoparticles and amongst the nitrate, citrate, chloride and nitrosyl counter ligands studied here, nitrosyl ligands offer the best stabilisation for Pd nanoparticles. The synthesised PdO and Pd nanoparticles of different sizes are deposited using a pH-controlled method onto an industrially-supplied cerium dioxide support, including only the active metal and support and excluding any additional promoters. The model catalysts are characterised and catalytically tested under model conditions for carbon monoxide oxidation, as well as under realistic conditions for all three-way catalytic reactions of carbon monoxide oxidation, hydrocarbon oxidation and nitrous oxide reduction. Decoupling the effect of nanoparticle size and oxidation state of palladium on activity is the main focus of this thesis. The effect of ageing on catalytic activity is also briefly explored herein. Overall, characterisation of catalysts suggests a mostly homogeneous distribution of small nanoparticles via the pH-controlled Pd deposition on CeO2 in Pd/CeO2 catalysts prepared with potassium nitrosyl tetra-nitro-nitrato palladate and PdO/CeO2 catalysts prepared using palladium dinitrate dihydrate but severe aggregation in Pd/CeO2 catalysts prepared using palladium citrate. The catalytic evaluation demonstrates the overall superior performance of PdO/CeO2 over Pd/CeO2, both under model and realistic conditions, with activation of PdO/CeO2 for CO oxidation at ~75oC. This is largely attributed to the high reducibility (<100oC with H2) of Pd species in PdO/CeO2, which is suggested to be the primary factor contributing to the initial activation of catalysts for carbon monoxide and hydrocarbon oxidation and nitrous oxide reduction and also determines the initial optimal particle size. Catalysts with smaller Pd particle sizes show superior performance after Pd is reduced, using pre-treatments or in the second TWC test, due to more available catalytic active sites.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• Kappelou, Anastasia

Advisor dc:contributor.advisor

• Torrente-Murciano, Laura

• nanotechnology
• heterogeneous catalysis
• palladium
• microfluidics

Licence

• https://www.repository.cam.ac.uk/bitstreams/32a214ad-3341-46cd-a6be-5323a06123ea/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.124911

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/395380