Effects of hydrogen transport on the kinetic regimes of 4-nitrophenol reduction by sodium borohydride

The reduction of 4-nitrophenol (4-NiP) by sodium borohydride is widely used to benchmark heterogeneous catalysts and is commonly simplified as a pseudo-first-order reaction, characterized by a single reaction rate constant. In reality, this reaction is more complex, as it is accompanied by hydrolysis of borohydride and concurrent hydrogenation of 4-NiP by produced hydrogen. This makes the local hydrogen concentration at catalytic sites an important, and so far overlooked, factor in shaping the apparent catalytic activity of heterogeneous catalysts. Re-examining benchmarking experiments on Pt-SiO2 supraparticles with different pore structures, we attribute contrasting kinetic behavior to distinct regimes of hydrogen transport: diffusive transport sustains high local concentrations of hydrogen and pseudo-first-order kinetics of 4-NiP hydrogenation, while bubble-mediated escape causes hydrogen loss, deviations from pseudo-first-order regime and incomplete conversion of 4-NiP. We propose a kinetic model that captures this transition and enables consistent interpretation of experimental data. More broadly, our analysis shows that apparent differences in activity observed in benchmarking experiments that use 4-NiP reduction by borohydride as a test reaction, can arise from hydrogen transport rather than intrinsic properties of the catalyst. This highlights the need to account for the hydrogen transport regime (bubbling/non-bubbling), when comparing catalyst performance across different experiments.