Nanoplasmonic Sensing of Transition Metal Catalyst Nanoparticle Oxidation States

Abstract

Abstract Catalysis is an important and key part of many processes both in fundamental research and in the applied chemical industry. The oxidation and reduction of tran- sition metal catalysts are common reactions that affect the catalytic performance of the corresponding nanoparticles. Hence it is very important to understand the role of the oxidation state in a catalytic process, as well as to control it in order to maximize catalyst performance towards the formation of the desired product(s). In this study copper and cobalt nanoparticle catalysts of sizes 67-190 nm were stud- ied in situ with direct and indirect plasmonic sensing in temperature-programmed oxidation and reduction experiments. The experimental study proved, together with computational simulations with the Modified Long Wavelength Approxima- tion, that the oxidation of copper yields a distinct change of the optical spectrum of the nanoparticles, and their exposure to 2 vol% O 2 in 200 ◦ C will mainly form Cu 2 O. The oxidation and reduction process of copper was also found to be reversible, and the temperature for the two processes was found to slightly decrease over the reac- tion cycles, due to structural changes of the particles. The outcome of the temperature-programmed oxidation and reduction experiments of cobalt nanoparticles was in several ways different from the copper ones. The most significant difference was that a broad dramatic change in extinction during the reduction process for the temperatures around 310-360 ◦ C was found. It was concluded that the effect was a combinatory result of a martensitic transformation, which simultaneously induced an increased oxidation rate of the cobalt oxides Co2O3 or CoO, as well as gives rise to a decrease of the conductivity.