Biomineralization and bioinspired synthesis of ceria and transition metal-oxide solid solution nanocrystals

Abstract Biomineralization is an intriguing approach to the synthesis of functional inorganic materials for energy applications whereby biological systems are engineered to mineralize inorganic materials and control their structure over multiple length scales under mild reaction conditions. Herein we demonstrate a single enzyme mediated biomineralization route to synthesize crystalline, catalytically active, quantum confined ceria (CeO2-x) and ceria-zirconia (Ce1-yZryO2-x) nanocrystals and a bio-inspired ligand mediated method for the synthesis of copper doped ceria (Ce1-yCuyO2-x) and gallium oxide. In contrast to typical synthesis routes, the crystalline oxide nanoparticles are formed at room temperature from an otherwise inert aqueous solution. An engineered form of silicatein, rCeSi, as a single enzyme not only catalyzes the direct biomineralization of the nanocrystalline oxides, but also serves as a templating agent to control their morphological structure. The biomineralized nanocrystals of less than 3 nm in diameter are catalytically active towards carbon monoxide oxidation following an oxidative annealing step to remove carbonaceous residue. The introduction of zirconia into the nanocrystals leads to an increase in Ce(III) concentration, associated catalytic activity, and the thermal stability of the nanocrystals.Our ligand mediated synthesis of crystalline oxide is enabled through ligand exchange prior to pH adjustment to prevent the precipitation of the hydroxide phase. By producing particles at room temperature, dopant exsolution and particle growth by sintering can be minimized and/or controlled. Using our methodology, copper dopant concentrations of up to 35 mol % could be produced in 1.7 nm diameter ceria nanocrystals. The resulting materials showed high catalytic activity towards both the water gas shift reaction (WGS) and CO oxidation, with improved performance following the trend of increasing copper content. In comparison to pure ceria nanocrystals, the WGS activation energy decreased from 89.0 to 49.2 kJ mol-1 and the CO oxidation light-off temperature decreased from 262 to 159°C at a space velocity of 25,000 h-1 upon doping with 35 mol % copper. Using this ligand mediated synthesis gallium oxide nanocrystals can also be produced with crystal sizes of 2.7 ± 0.5 nm. These gallium oxide crystals when used with an Al2O3 support show activity towards the ethane dehydrogenation reaction and show reduced deactivation due to coke when compared to a TiO2 supported catalyst. Reaction with platinum and gallium oxide nanocrystals supported on alumina reached 26% and maintained 11% conversion after 48 hours (at 500°C and WHSV = 0.5 h-1). Our results show the complementary effect of gallium oxide nanoparticles on alumina when utilized in platinum dehydrogenation system. This effect is due to the ability of gallium oxide on alumina to limit coke build-up.