Tracking the Seconds of a Clock Reaction: A Multiparametric Experimental Study on the Catalytic Reduction of Methylene Blue

Abstract

The catalytic reduction and subsequent reoxidation of methylene blue (MB) is a well-known reaction, commonly referred to as a “clock reaction”. Specifically, the reduction process is accompanied by a color change from intense blue to clear and can be readily monitored by UV–vis spectroscopy via the decrease in absorbance at λmax = 664 nm, providing facile access to valuable reaction kinetics. Unsurprisingly, the reduction of methylene blue has become a widely used probe for heterogeneous catalyst reactivity. Yet, despite its broad utility, the mechanism of reduction is not well-understood. Herein, we report an experimental study on the mechanism of MB reduction using Pt/C nanoparticles. Through a multiparametric study incorporating in situ probing of reaction kinetics, pH, dissolved oxygen, and variable catalyst and reductant loading, the multiple factors impacting the reaction are disentangled. We demonstrate that the reaction steps that limit the reduction of MB are the H2 evolution and reoxidation sequences. We limit these competitive reactions by optimizing reductant concentration and catalyst loading, N2 purging, and restriction of O2 redissolution. This achieves a highly competitive activity parameter of 25,740 min–1 gPt–1 L.