Investigation of cooperative redox enhancements effects in the oxidation of 1,2-propanediol using gold and palladium catalysts

Alnahdi, Nouf 2025. Investigation of cooperative redox enhancements effects in the oxidation of 1,2-propanediol using gold and palladium catalysts. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

The use of heterogeneous bimetallic catalysts, particularly Au-Pd systems, in the aerobic oxidation of alcohols is of considerable academic and industrial significance and offers a cleaner, more sustainable approach. These catalysts demonstrate high activity and selectivity, making them suitable for a wide range of chemical transformations. This study focuses on a new bimetallic catalytic phenomenon, called Cooperative Redox Enhancement (CORE), which arises when spatially separated metals independently facilitate complementary half reactions, thereby achieving reaction rates exceeding those of either monometallic or alloyed systems. In this thesis, the CORE effect is systematically examined under various reaction parameters and explored between distinct catalytic species in the liquid and solid phases. The oxidation of 1,2-propanediol was used as a model reaction, an industrially relevant intermediate widely applied in pharmaceuticals, biofuels and food products. Monometallic (1 wt.% Au/C and 1 wt.% Pd/C) catalysts and their physical mixtures (PM) with Au:Pd ratios of 4:1, 1:1, and 1:4 were examined and systematically evaluated to elucidate the CORE effect. The reaction conditions were found to strongly impact the CORE. Stirring studies have shown that the conversion and CORE enhancement decreased with increased stirring, which was suggested to be result of moderate stirring promoted efficient electron transfer between the metals, whereas excessive agitation weakened interparticle contact and suppressed cooperative redox communication. It has been demonstrated i that the Au:Pd ratio governs the cooperative coupling of DH and OR, thereby controlling both activity and CORE enhancement. Temperature studies showed that sufficient Pd availability is crucial for temperature driven CORE benefits, and PM exhibiting lower activation energies than monometallic catalysts, confirming the role of CORE in reducing energy barriers and accelerating the reaction. Furthermore, Pd/C showed strong pressure sensitivity, whereas in PM, the presence of Au exhibited notable rate enhancement due to Au’s protective role in sustaining the Pd2+/Pd0 redox cycle and preventing overoxidation. Heterogeneous catalysts frequently experience metal leaching into the liquid phase, leading to a decline in catalytic performance. In this work, a stabilisation approach is proposed that exploits electrochemical coupling between spatially separated Au and Pd catalysts supported on carbon. This interaction regulates the oxidation state of Pd and effectively suppresses leaching under mildly alkaline conditions. However, at higher pH levels, the stabilising effect weakens, allowing a redox interplay between the liquid and solid phases to emerge. This dynamic process promotes continuous Pd oxidation-reduction cycling, resulting in a marked acceleration of the oxidation reaction. These outcomes provide new insights into bimetallic catalysis and extend the current understanding of the CORE effect by revealing redox behaviour not typically observed outside fully heterogeneous systems.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Chemistry
Date of First Compliant Deposit:	24 April 2026
Last Modified:	24 Apr 2026 16:02
URI:	https://orca.cardiff.ac.uk/id/eprint/186625

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