In this study, ultrasmall Ni nanoparticles (Ni NPs) were controllably supported in the cage-type mesopores of −COOH-functionalized mesoporous silica SBA-16 (denoted as Ni­(x)@S16C, where x is the Ni loading) via wet impregnation under alkaline conditions, followed by thermal reduction. The particle sizes of the Ni NPs ranged from 2.7 to 4.7 nm, depending on the Ni loading. Under the appropriate alkaline conditions (i.e., pH 9) deprotonation of the carboxylic acid groups on the cage-type mesopore surfaces endowed the effective incorporation of Ni2+ precursors via favorable electrostatic interactions, and thus well-dispersed Ni NPs confined in the cage-type mesopores of SBA-16 were achieved. The combination of the cage-type mesopores and the surface functionality provided dual beneficial features to confine the immobilized Ni NPs and to tune their particle sizes. The remarkably enhanced catalytic activities of the Ni­(x)@S16C materials for CO2 hydrogenation and CH4 formation were demonstrated. The cage-type SBA-16 support provided a positive effect for the Ni NPs to enrich the surface sites, which can strongly adsorb CO and CO2, thus leading to high catalytic rates for CO2 and CO hydrogenation. The reaction mechanism, catalytic kinetics, and active sites were investigated to correlate to the high reaction rate for CO2 hydrogenation to form CH4.

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Ni nanoparticles (around 4 nm diameter) were successfully supported on cage-type mesoporous silica SBA-16 (denoted as Ni@SBA-16) via wet impregnation at pH 9, followed by the calcination-reduction process. The Ni@SBA-16 catalyst with a very high Ni loading amount (22.9 wt %) exhibited exceptionally high CH4 selectivity for CO2 hydrogenation. At a nearly identical loading amount, the Ni@SBA-16 catalysts with smaller particle size of Ni NPs surprisingly exhibited a higher catalytic activity of CO2 hydrogenation and also led to a higher selectivity on CH4 formation than the Ni@SiO2 catalysts. This enhanced activity of the Ni@SBA-16 catalyst is suggested to be an accumulative result of the advantageous structural properties of the support SBA-16 and the well confined Ni NPs within the support; both induced a favorable reaction pathway for high selectivity of CH4 in CO2 hydrogenation.

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