Asymmetrical Double Flame Spray Pyrolysis-Designed SiO2/Ce0.7Zr0.3O2 for the Dry Reforming of Methane

Silica has the potential to enhance the performance of ceria-zirconia as a support for the dry reforming of methane; however, controlling the integration of silica with the ceria-zirconia using flame spray pyrolysis (FSP) is a significant challenge. To address this challenge, an asymmetrically variable double-FSP (DFSP) system was established to control the SiO₂ interaction with Ce_0.7Zr_0.3O₂. The engineered materials were then utilized as supports for Ni for the dry reforming of methane. Initially, silica formation during FSP synthesis was examined where it was revealed that, at a low precursor concentration (<1.5 M tetraethyl orthosilicate in xylenes), the physical characteristics of the silica varied differently in relation to what is typically encountered during FSP synthesis. Explicitly, on using a 0.5 M tetraethyl orthosilicate precursor, increasing the FSP feed rate provided an increase in the specific surface area from 217 m²/g at 3 mL/min to 363 m²/g at 7 mL/min. Adopting this knowledge on silica formation under these conditions, the asymmetrical DFSP system was then exploited to regulate the integration of ceria-zirconia with the silica. To restrict the silica from coating the particles during DFSP, the intersection distance along the silica flame was tuned from 18.5 to 28.5 cm, whereas the distance along the ceria-zirconia flame was fixed at 5 cm. It was found that at short intersection distances the ceria-zirconia provided sites for silica nucleation and growth, resulting in high surface-area silica encapsulating the ceria-zirconia. At large intersection distances, encapsulation of the ceria-zirconia by silica was suppressed. An enhanced oxygen storage capacity and basicity along with the small Ni sizes facilitated by the longer intersection distances produced the most selective catalyst for the dry reforming of methane.

Direct Single-Enzyme Biomineralization of Catalytically Active Ceria and Ceria-Zirconia Nanocrystals

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 (CeO_2-x) and ceria-zirconia (Ce_1-yZr_yO_2-x) nanocrystals for application as environmental catalysts. In contrast to typical anthropogenic synthesis routes, the crystalline oxide nanoparticles are formed at room temperature from an otherwise inert aqueous solution without the addition of a precipitant or additional reactant. 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 toward 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.

Synergy and Anti-Synergy between Palladium and Gold in Nanoparticles Dispersed on a Reducible Support

Highly active and stable bimetallic Au-Pd catalysts have been extensively studied for several liquid-phase oxidation reactions in recent years, but there are far fewer reports on the use of these catalysts for low-temperature gas-phase reactions. Here we initially established the presence of a synergistic effect in a range of bimetallic Au-Pd/CeZrO₄ catalysts, by measuring their activity for selective oxidation of benzyl alcohol. The catalysts were then evaluated for low-temperature WGS, CO oxidation, and formic acid decomposition, all of which are believed to be mechanistically related. A strong anti-synergy between Au and Pd was observed for these reactions, whereby the introduction of Pd to a monometallic Au catalyst resulted in a significant decrease in catalytic activity. Furthermore, monometallic Pd was more active than Pd-rich bimetallic catalysts. The nature of the anti-synergy was probed by several ex situ techniques, which all indicated a growth in metal nanoparticle size with Pd addition. However, the most definitive information was provided by in situ CO-DRIFTS, in which CO adsorption associated with interfacial sites was found to vary with the molar ratio of the metals and could be correlated with the catalytic activity of each reaction. As a similar correlation was observed between activity and the presence of Au⁰* (as detected by XPS), it is proposed that peripheral Au⁰* species form part of the active centers in the most active catalysts for the three gas-phase reactions. In contrast, the active sites for the selective oxidation of benzyl alcohol are generally thought to be electronically modified gold atoms at the surface of the nanoparticles.