Carbon Overcoating of Supported Metal Catalysts for Improved Hydrothermal Stability

Abstract

Commercial mesoporous oxides, such as silica, are not stable in liquid-phase reactions, particularly aqueous-phase reactions at elevated temperatures, which are corrosive to oxide supports. We have shown previously that the hydrothermal stability of silica is significantly improved by coating the surface with thin carbon layers. Herein, we show that controlled pyrolysis of sugars also provides a facile approach for coating supported metal catalysts, leading to improved dispersion of the active metal phase in the hydrothermally aged catalyst. The carbon overcoats are synthesized at mild temperatures, resulting in an open structure, as revealed by 13 C NMR, which helps explain why the overcoats do not significantly block the active sites. We compare two approaches-depositing Pd on carbon-coated silica and depositing carbon overcoats on Pd/silica. The carbon overcoating approach leads to better performance after hydrothermal aging, as determined by using a probe reaction (CO oxidation) to quantify the number of active sites. The efficacy of the carbon overcoating was demonstrated by the improved stability of Pd/silica catalysts for aqueous phase acetone hydrogenation. Likewise, carbon-overcoated Cu/alumina catalyst was found to be more stable for aqueous-phase furfural hydrogenation compared with the uncoated catalyst.