Reaction Pathways over ZnZrO ₂ -Based Catalysts and Catalytic Sorbents

Reactive capture and conversion (RCC) is a process intensification approach that integrates CO₂ capture and hydrogenation within a single unit, removing the CO₂ purification and storage steps of traditional process flow schemes. This alters the catalytic step from a traditional steady-state (SS) flow process to a transient capture and conversion cycle, which could lead to product distributions distinct from those observed in conventional SS experiments. Such differences are investigated in the combined capture and hydrogenation of carbon dioxide to methanol over a ZnZrO₂ catalyst and a ZnZrO₂ + NaNO₃/Mg₃AlO <i>_x</i> catalytic sorbent (CS) using fixed-bed kinetic measurements, <i>in situ</i> diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and steady-state isotopic transient kinetic analysis-DRIFTS (SSITKA-DRIFTS). Under SS conditions, ZnZrO₂ produced methanol through sequential hydrogenation of HCOO* and CH₃O* intermediates. On the contrary, CO was attributed primarily to CO₂ dissociation at oxygen vacancies, as supported by isotopic shifts and measured reaction orders. For the CS, isotopic switching experiments suggested that monodentate carbonate species (CO₃ ^2-, abbreviated as m-CO₃ ^2-) act as active intermediates that can be hydrogenated to HCOO* and subsequently to CH₃O. Under RCC conditions, <i>in situ</i> DRIFTS and isotopic experiments reveal that m-CO₃ ^2- species formed during the CO₂ capture step follow two competing routes upon H₂ exposure: (i) direct hydrogenation to methane on the sorbent domain or (ii) migration of m-CO₃ ^2- to the ZnZrO₂ domain, where they are hydrogenated to methanol through the HCOO pathway. Overall, RCC enables carbonate hydrogenation routes not observed under SS cofeed conditions. Thus, the reaction pathways and rates during RCC can be different from operation under conventional SS conditions, and the product distribution is determined here by competition between carbonate hydrogenation on sorbent sites and migration to ZnZrO₂ for methanol synthesis.