Influence of the calcination conditions of the support on the activity of ruthenium-encapsulated porous hollow silica sphere catalysts for hydrogenation of carbon dioxide into formic acid

The present study investigated the influence of the calcination conditions of porous hollow silica spheres on the activity of a ruthenium-encapsulated porous hollow silica sphere catalyst for hydrogenation of carbon dioxide into formic acid. The hollow spheres were prepared at various calcination temperatures in air or in an argon flow. The amount of residual carbon content in the ruthenium-encapsulated hollow silica sphere catalysts increased with a decrease in the calcination temperature of the hollow silica sphere supports in air. Energy dispersive X-ray spectroscopy (EDS) and thermogravimetric (TG) analyses revealed that cetyltrimethylammonium bromide (CTAB) preferentially decomposed at calcination temperatures of up to 673 K, and most of the CTAB and carbon templates decomposed with the collapse of the hollow sphere catalyst particles in the catalysts calcined at 873 K. Moreover, the highest amounts of residual CTAB and carbon templates were found in the catalysts calcined in the argon flow. Differential thermal analysis (DTA), transmission electron microscopy (TEM), nitrogen sorption and X-ray diffraction (XRD) measurements showed that active ruthenium species were highly dispersed in the hollow spheres calcined in air, while a small amount of active ruthenium species with low dispersion were supported on the hollow spheres calcined in the argon flow. The catalyst calcined at 473 K exhibited the highest turnover number (TON) for formic acid formation (350 mol-HCOOH per mol-Ru), suggesting that the catalysts exhibited high activity not only owing to the high dispersion of the active species but also owing to the effective conduction of reaction heat by residual carbon species originating from CTAB in the nanospaces of the hollow spheres' shells.