AbstractElectrocatalytic hydrogenation provides an alternate route for the hydrogenation of compounds under mild operation conditions, usually ambient conditions. Precious metals, due to their low overpotential, are the most active catalysts in both electrocatalytic hydrogenation reaction (ECR) and hydrogen evolution reaction (HER). However, due to their scarcity and high cost, various methods were explored to reduce the quantity of the precious metals loading on the catalyst without compromising their activity. Among these methods is the modification of the catalyst support.
Herein, we synthesised various Platinum, Rhenium and Copper supported on TiO2-C hybrid support catalysts and evaluated their catalytic activity in the electrocatalytic hydrogenation of benzaldehyde, furfural, and hydrogen evolution reaction. This study was based on three parts. In the first part, we synthesised Pt-based hybrid support catalysts with various TiO2 wt% loading and tested their electrocatalytic activity in the ECR of benzaldehyde to benzyl alcohol, and an optimum TiO2 wt% was established. In the second part, we evaluated the electrocatalytic activity of Pt, Re, and Cu on the optimum TiO2-C hybrid support catalyst in the ECR of furfural, and interestingly the earth-abundant metal, Cu-based catalysts outperformed the corresponding noble metals, Pt and Re based catalysts. We also explored the electrocatalytic activity of the Cu/TiO2-C under various reaction conditions in the ECR of furfural to optimise the process conditions. In the third part of the study, we study the hydrogen evolution reaction of the Pt supported on TiO2-C catalysts, which were found to have low faradaic efficiency in the ECR of benzaldehyde. The low faradaic efficiency of these catalysts is attributed to higher competing hydrogen evolution reactions. Their electrocatalytic activity in hydrogenation evolution reactions was further elucidated.
Benzaldehyde was converted to benzyl alcohol on Pt/C, Pt/TiO2 and Pt/TiO2-C catalysts by electrocatalytic hydrogenation derived by applied potential and the results from ECR were also compared with the conventional thermal hydrogenation method using molecular H2 in a pressurised reactor. The trend of the hydrogenation rates in electrocatalytic hydrogenation of the benzaldehyde based on the support materials with the same Pt metal loading was TiO2<C<TiO2-C. The rates prevailed with the amount of TiO2 loading, and the highest rate was achieved on Pt/TiO2-C catalysts with 16 wt% TiO2 loading. The activation energy for the electrocatalytic hydrogenation of benzaldehyde using the Pt/TiO2-C with optimum TiO2 loading catalyst was lower than that of Pt/C, 22 kJmol-1 and 32 kJmol-1 for Pt/ 16%TiO2-C and Pt/C respectively. The electrocatalytic hydrogenation rate on Pt/16%TiO2-C increased with the applied potential, while the faradaic efficiency was found to be optimum at moderate reduction potential and decreased at high reduction potential. In thermal hydrogenation of benzaldehyde at 1 bar H2 pressure and ambient temperature, the trend was different; as the rate increased with an increase in TiO2wt% loading. The thermal hydrogenation rate increased by 1.6 times as the TiO2 loading increased from 16wt% to 36wt%.
Having determined the best hybrid support with optimum TiO2 loading on carbon from the electrocatalytic hydrogenation of the benzaldehyde. We then synthesised Re/TiO2-C and earth-abundant metal, Cu/TiO2-C in addition to Pt/TiO2-C on the same optimum 16wt% TiO2 loading on carbon and 4wt% metal loading. The electrocatalytic activities of these catalysts were evaluated in the electrocatalytic hydrogenation of furfural. We synthesised copper-based TiO2-Carbon hybrid support catalyst (Cu/TiO2-C). A copper metal loading of 4 wt% was used in the synthesis of the catalyst. The copper-based TiO2-C hybrid support catalyst was found to be the most active in the electrocatalytic hydrogenation of furfural, with almost 100% selectivity of the furfuryl alcohol.
The catalytic activity of the Cu/TiO2-C catalyst was compared with that of Cu/C to ascertain the influence of TiO2 in modifying the carbon and enhancing the electrocatalytic hydrogenation of the furfural to furfuryl alcohol. The effects of the cathodic potential, electrolyte composition, temperature, and initial furfural concentrations. The Cu/TiO2-C was found to be more active than the Cu/C electrocatalyst.
Finally, we elucidated the electrocatalytic activity of the Pt/TiO2 with low faradaic efficiency in the electrocatalytic hydrogenation reaction. The electrocatalytic activities of these catalysts were evaluated in the hydrogen evolution reaction (HER). The Pt/TiO2-C hybrid support catalyst has a lower HER overpotential than the Pt/C, which signifies the faster formation of H2 at a lower reduction potential; also, a significant rise in the current density was obtained with optimum TiO2 wt% loading of 10% compared to the Pt/C, this might be attributed to the increased in the dispersion of catalyst as evidenced in the TEM analysis.
The hydrogen evolution reaction rate of the optimum TiO2 wt% loading on hybrid support catalyst was five times higher than that of Pt/C catalysts, with HER rate of 353 µmolg-1s-1 and 1710 µmolg-1s-1 for Pt/C and Pt/TiO2-C with TiO2 loading of 10 wt%, respectively, despite both having the same Pt metal loading of 4 wt%. However, the decrease in the electrocatalytic activity of the TiO2-C hybrid support at higher TiO2 wt% loading could be attributed to the increase in the resistance of the TiO2 loading, which will reduce the ionic transport at the electrode surfaces.
Thesis is embargoed until 31 July 2024.
|Date of Award
|Petroleum Technology Development Fund
|Haresh Manyar (Supervisor), Nancy Artioli (Supervisor) & Geetha Srinivasan (Supervisor)