Distinct Kinetic Roles of Bifunctional and Bimetallic Ru–Pt/TiO2 Catalysts in N -Ethylcarbazole Hydrogenation for Hydrogen Storage

Abstract

To improve the hydrogen storage efficiency of N-ethylcarbazole (NEC) for liquid organic hydrogen carrier (LOHC) applications, understanding its catalytic hydrogenation kinetics and pathways is critical. In this study, monometallic catalysts (Ru/TiO2 and Pt/TiO2), a bifunctional Ru–Pt/TiO2 catalyst synthesized by a sequential incipient wetness (SIW) impregnation method, and a bimetallic Ru–Pt/TiO2 catalyst synthesized by a chemical reduction method were investigated through experimental evaluation and kinetic modeling to reveal the influence of the catalyst system on NEC hydrogenation pathways. Under mild conditions (162 °C, 60 bar), the bifunctional SIW 1 wt.% Ru – 1 wt.% Pt/TiO2 achieved 96.3% NEC conversion with a hydrogen uptake of 4.7 wt.%, outperforming both the bimetallic catalyst, which achieved up to 77% NEC conversion and 3.6 wt.% hydrogen uptake at 165 °C and 60 bar, and all monometallic catalysts across the examined temperature range. 12H-NEC isomer distribution indicated a temperature-dependent shift toward the most thermodynamically stable 12[H] A isomer for Ru-containing catalysts, with ∼150 °C identified as the transition point at which the less stable 12[H] B isomer ceased to dominate, whereas Pt/TiO2 consistently favored the formation of 12[H] A regardless of temperature. Kinetic modeling showed that the bifunctional Ru–Pt catalyst exhibited the lowest activation energies for the initial hydrogenation step (EA,1 = 13.6 ± 0.4 kJ/mol) and the direct NEC-to-12H-NEC pathway (EA,4 = 50.9 ± 0.5 kJ/mol). In contrast, the bimetallic Ru–Pt catalyst exhibited the lowest EA,2 (39.5 ± 0.4 kJ/mol) and EA,3 (3.7 ± 0.3 kJ/mol) among all tested catalysts, kinetically favoring the conversion of 4H-NEC to 8H-NEC and 8H-NEC to 12H-NEC. These findings demonstrate the critical role of the catalyst active sites in governing NEC hydrogenation kinetics and guide the design of efficient LOHC hydrogen storage systems.