Improved CO₂ Hydrogenation on Ni–ZnO/MCM-41 Catalysts with Cooperative Ni and ZnO Sites

A set of Ni–ZnO/MCM-41 catalysts with different proportional Ni and ZnO loadings (up to 10 wt % in total) were synthesized for the carbon dioxide (CO₂) hydrogenation reaction. Ni nanoparticles and ZnO promoter were both loaded onto a MCM-41 support via the impregnation method. The catalysts were comprehensively characterized by Brunauer–Emmett–Teller, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and hydrogen temperature-programmed reduction measurements. Catalyst properties, including the porosity, Ni metal particle size, and Ni particle location, were all found to be influenced by the presence of the ZnO promoter. ZnO was suspected to improve Ni nanoparticle insertion into the MCM-41 mesoporous channels to form Ni–ZnO interfaces. Both CO and CH₄ were produced during the CO₂ hydrogenation reaction under the molar ratio of CO₂/H₂ at 1:3 at 350 °C. The CO₂ conversion rate and CO selectivity were found to increase as the reaction temperature increased from 350 to 700 °C. Among all studied materials, the catalyst containing 9 wt % Ni and 1 wt % Zn (denoted as sample E throughout the report) revealed the highest CO₂ conversion and selectivity toward CO (∼60% CO₂ conversion and 98.5% CO selectivity at 600 °C), while the catalyst containing 1 wt % Ni and 9 wt % Zn (denoted as sample A throughout the report) revealed the lowest activity (∼2.5% CO₂ conversion and 95% CO selectivity at 600 °C). This study illustrated that cooperative catalysis can be applied to tune the CO₂ hydrogenation reaction toward the reverse water–gas shift reaction for value-added CO production. When both Ni and ZnO are coupled in MCM-41, a hybrid system was designed and synthesized, in which Ni functions for H₂ dissociation and ZnO functions for CO₂ adsorption and accumulation.