Cobalt Carbon Bonding during Carbon Hydrogasification Revealed by 59Co Internal Field NMR

Abstract

Cobalt catalysts are involved in many chemical processes crucial to the energy transition and the shift from oil to biosourced chemicals. Thanks to the ferromagnetic character of cobalt metal, these catalysts can be studied by nuclear magnetic resonance (NMR) in the internal magnetic field of the particle. Using carbon hydrogasification as an example, this work illustrates the potential of Co-59 internal field (IF) NMR to study the Co crystallite phase transitions and to distinguish quantitatively, within the limits of the skin effect, different Co-C intermediates present inside a sample containing cobalt and carbon. Here, the main Co-C intermediates evidenced are a Co/C solid solution and a Co3C phase. When using a ball milling process for carbon hydrogasification, a high amount of Co-C intermediates forms, and the reaction rate is increased by several orders of magnitude compared to a classic catalytic reaction (in a fixed-bed reactor). In this work, the time evolution of the Co-59 IF NMR are compared to the reaction rate of carbon hydrogasification over the course of the ball milling process. We find a direct relationship between the amount of Co-C intermediates inside the sample and the reaction rate. This confirms that, like for a carbon hydrogasification reaction under classic conditions, the carbon dissociation and formation of Co-C bonds is the rate-determining step.