Tuning the Size of Colloidal Ni Nanoparticles for CO2 Hydrogenation Catalysis

Abstract

Increasing demand for environmentally friendly produced fuels caused the interest in converting CO2 to useful products such as methane over Ni-based catalysts. Because the CO2 conversion (hydrogenation) over Ni catalysts is structure sensitive, it became essential to understand the role of the particle size and shape. It has been reported that silica-supported Ni nanoparticles around 2.5 nm particle size have the maximum catalytic activity.1,2 However, synthesizing small and monodisperse Ni nanoparticles (NP) below 3.0 nm was challenging with the ability to simultaneously deposit Ni NPs from the same batch on separate supports. Therefore, the ability to synthesize monodisperse Ni NPs around 2.5 nm is necessary for CO2 hydrogenation catalysis. The colloidal approach provides the possibility for high control in the particle size distribution by varying the reactants and reaction temperature during the synthesis of nanoparticles. However, ligand removal forms a well-known obstacle in colloidal synthesis. When the ligands are not appropriately removed, metal NPs could be inactive in the catalysis because of surface blockage. This study shows a new synthesis approach for colloidal Ni NPs, including ligand removal and deposition on the silica support for CO2 hydrogenation catalysis. Small colloidal Ni NPs were synthesized using the hot injection method. The Ni NP sizes were between 1.4 nm and 6.6 nm with a standard deviation of 13-30 %. The particle size was controlled by changing the ligands, reaction temperature and reactant ratios. Subsequently, the colloidal Ni NPs were deposited on the silica support and the ligands were removed by Meerwein’s reagent alkylation, as proven by operando FT-IR studies. The remaining ethyl groups from Meerwein’s reagent were removed during the catalyst's reduction. In the last part of this project, seven silica-supported Ni catalysts were prepared for CO2 hydrogenation catalysis and studied using operando FT-IR. Five Meerwein’s reagent-treated catalysts were highly active compared to a commercial catalyst, while untreated samples were not active in CO2 hydrogenation catalysis. Here, our colloidal approach provides a new synthesis method to synthesize Ni nanoparticles for CO2 hydrogenation catalysis that are smaller than reported, without P-containing ligands, monodisperse and catalytic active, and that can be deposited on separate supports simultaneously.