Kinetic and mechanistic investigation of pH-dependent benzaldehyde hydrogenation over Pt/FL-MoS2 in aqueous media

Aqueous-phase hydrogenation of biomass-derived oxygenates is an important route for the production of renewable chemicals, but the role of pH and of the catalyst–water interface in hydrogen-transfer mechanisms is still not fully understood. In this work, benzaldehyde hydrogenation to benzyl alcohol over 5%Pt/FL-MoS2 was investigated in aqueous media at different pH values by combining catalytic tests, kinetic analysis, adsorption measurements, isotopic labelling experiments and H2/D2O isotopic exchange studies. The catalyst was characterized by XRD and N2 physisorption, showing that the few-layer MoS2 structure was preserved after Pt deposition and that the material retained a relatively high surface area. Catalytic tests revealed a strong pH dependence, with the highest activity under acidic conditions and a marked decrease in activity at near-neutral and basic pH. Kinetic analysis showed positive but sub-unity reaction orders in H2 and weak dependence on benzaldehyde concentration, while adsorption measurements indicated that benzaldehyde adsorption alone cannot explain the observed pH trend. Isotope experiments showed moderate H2/D2 kinetic isotope effects but stronger H2O/D2O solvent isotope effects. GC-MS analysis revealed negligible deuteration of residual benzaldehyde and significant deuterium incorporation into benzyl alcohol when D2O was used. Together with H2/D2O exchange measurements, these results indicate that molecular hydrogen activation and interfacial hydrogen/deuterium exchange are coupled during the reaction. Overall, benzaldehyde hydrogenation over Pt/FL-MoS2 is not adequately described by a simple non-exchanging Langmuir–Hinshelwood mechanism but is more consistently interpreted as an O-first hydrogenation sequence strongly coupled to the Pt–MoS2–water interface. The results are consistent with an interfacial proton–electron transfer contribution, although a rapidly exchanging aqueous-phase Langmuir–Hinshelwood pathway cannot be excluded.