Effects of Hydrogen Transport on the Kinetic Regimes of 4-Nitrophenol Reduction by Sodium Borohydride

The reduction of 4-nitrophenol (4-NiP) with sodium borohydride is widely used to benchmark heterogeneous catalysts, yet its kinetics are commonly oversimplified as pseudo-first-order. In reality, borohydride hydrolysis and hydrogenation by dissolved hydrogen proceed concurrently, making hydrogen transport a decisive factor in shaping apparent activity. Re-examining data on Pt-SiO2 supraparticles with different pore structures, we attribute contrasting kinetic behavior to distinct regimes of hydrogen transport: diffusive transport sustains pseudo-first-order kinetics, while bubble-mediated escape causes hydrogen loss and incomplete conversion. 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 during 4-NiP benchmarking can arise from hydrogen transport rather than intrinsic properties of the catalyst, underscoring the need to account for transport effects when comparing catalyst performance.

Tatiana Nizkaia, Philipp Groppe, Valentin Müller, Jens Harting, Susanne Wintzheimer, Paolo Malgaretti · 2025