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

TL;DR

This paper investigates how hydrogen transport mechanisms influence the apparent kinetics of 4-nitrophenol reduction, revealing that transport effects can mimic or obscure intrinsic catalytic activity, and proposes a model to interpret these effects.

Contribution

It introduces a kinetic model that accounts for hydrogen transport regimes, clarifying how transport effects impact observed catalytic kinetics in 4-NiP reduction.

Findings

Hydrogen transport regimes significantly affect apparent reaction kinetics.

Bubble-mediated hydrogen escape leads to incomplete conversion.

Transport effects can cause misinterpretation of catalyst activity.

Abstract

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.