(442d) Reverse Micelle Synthesis of Bimetallic Catalysts

Control of particle size in heterogeneous catalysis is one of the key parameters for the achievement of specific surface sites, electronic properties, and structural characteristics leading to improved performance. One technique that can be utilized for increased control is micro-emulsion based catalyst synthesis, or micelle/reverse micelle synthesis. Since its first successful implementation in the early 1980s, this synthesis technique has been used in the preparation of noble-metal catalysts, nanoparticles with highly controlled size distributions, bimetallic systems, and oxide-based materials. The solutions, which consist of a water phase, an oil phase, and a surfactant enable the creation of nanosized reactors which provide the boundary for particle growth. The tunable parameters involved in micro-emulsion synthesis, such as water-to-surfactant ratio, nature of the surfactant, and nature of the reducing agent allow for tailoring specific size and composition of bimetallics to hone the activity for a given chemistry.

We have explored several catalytic systems using this synthetic approach, including NOx storage and reduction catalysts (NSR) for automotive exhaust applications, ammonia decomposition for hydrogen storage applications, and ethylene epoxidation. For example, Co/Pt/Ba NSR catalysts were synthesized in this manner and compared to their incipient wetness counterparts in a high-throughput reactor system. The nanoparticles were also analyzed using electron microscopy and elemental analysis techniques.

K promoted Ru catalysts are the standard catalysts for ammonia decomposition. For this reaction, catalysts prepared via incipient wetness will be compared with Ru-based and Ru-free bimetallics prepared via reverse micelles.

Recent modeling studies for ethylene epoxidation have identified several bimetallic catalysts that would increase the selectivity toward ethylene oxide. However, efficient production of these bimetallics while maintaining a consistent particle size can be difficult. The reverse micelle technique provides a possible solution of providing tight size and concentration distributions for the validation of DFT and microkinetic modeling results.