Intermediate Sensing during Catalytic CO2 Hydrogenation with Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy

Abstract

CO2 hydrogenation reactions will accelerate the transition to CO2 capture and utilization, leading to a carbon-neutral society. CO2 hydrogenation catalyzed by nickel nanoparticle can form methane, which can be used as synthetic gas or as feedstock in the chemical industry. Unravelling the reaction pathway and key intermediates of this CO2 hydrogenation reaction is key for the rational design of the catalyst. CO2 methanation has previously been extensively studied with infrared (IR) spectroscopy to unravel its reaction mechanism. However, IR spectroscopy has its limitations since it is insensitive to certain intermediates due to selection rules. In contrast, Raman spectroscopy is a promising technique, as it is complementary to IR. In particular, Raman spectroscopy can detect low energy vibrations, such as metal-carbon and metal-oxygen intermediates. These vibrations are difficult to observe in IR as they fall in the far IR and fingerprint region, and can therefore give crucial additional information about the reaction pathway. Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy (SHINERS) is a promising structure sensitive technique, which uses Shell-Isolated Nanoparticles (SHINs) to enhance the weak Raman signals caused by low Raman scattering probability. During the research reported in this thesis, a model system was developed and tested to study the CO2 methanation reaction with Raman spectroscopy, using SHINs for signal enhancement of the energy intermediate vibrations on the catalyst surface. The developed system consisted of a combination of a gold wafer and SHINs to create optimal Raman signal enhancement spots (hotspots), where the catalyst of interest was positioned. The system was built up by depositing catalyst nickel nanoparticles (Ni NPs) with VSPARTICLE spark ablation onto a coated gold wafer. After catalyst deposition, SHINs were drop-casted onto the wafer. The gold wafer was coated with silica or titania to prevent participation in the CO2 methanation reaction. In situ Raman mapping detected spatial inhomogeneities in signal enhancement on the sample and located the hotspots with highest Raman signal. Three different adsorption geometries of CO adsorbed on Ni in the high Raman wavenumber region were observed which correspond to vibrations seen in operando Fourier-transform infrared spectroscopy literature data. Also, two preliminary nickel-carbon vibrations of CO adsorbed on nickel were detected in the low Raman wavenumber region, suggesting that the CO2 methanation followed a carbide reaction pathway. This study shows that the model system is a showcase of how SHINERS can be used to study CO2 hydrogenation reactions with Raman spectroscopy and holds promise for the study of catalytic surface reactions, and elucidation of the adsorption geometry of key reaction intermediates in CO2 hydrogenation reactions.