High throughput investigation of supported catalysts for carbon oxides-free hydrogen production from ammonia decomposition.

摘要

Ammonia decomposition catalysts were investigated for the zero-emission, on-board generation of hydrogen for fuel cell vehicles. A potassium-promoted ruthenium (Ru) catalyst supported on gamma-Al2O3 was synthesized via incipient wetness impregnation. A calcination step (oxidation) led to the formation of an anisotropic potassium ruthenate with a Hollandite-type crystal structure (KRu4O8). Advanced characterization techniques were used to determine that a catalyst activation step (reduction) caused reduction of the oxide to form a network of metallic ruthenium (Ru) "nanowires," comprised of crystallite segments with irregular surfaces. Ammonia chemistry is structure sensitive on Ru, with significant rates of reaction occurring only where a three-fold hollow of Ru atoms is next to a step edge, known as a "B5 site". For the Ru nanowires, both the grain boundaries between crystallites as well as irregularities in the surfaces provided locations for B5 sites, explaining the high activity of the catalyst. Modeling of the Hollandite transformation suggested that the kinetics occurred via Avrami-type nucleation and growth, modified to account for differences in Ru packing density between the two phases.;Because Hollandite was identified as a precursor to the active phase for ammonia decomposition, a design of experiments around Hollandite formation conditions was undertaken to maximize activity. Using high throughput reactor testing and electron microscopy, it was found that by decreasing the catalyst calcination temperature from 550°C down to 350°C, Hollandite crystal diameter could be reduced from 23 nm to 12 nm, resulting in a higher turnover frequency (TOF).;A microemulsion synthesis technique for model ammonia decomposition catalysts was developed, featuring a linear correlation between microemulsion water concentration and resulting Ru particle size. Catalysts were created whose average particle size ranged from 2--7 nm with a narrow size distribution. The optimum TOF corresponded to an average particle size of 3.5 nm, which showed excellent agreement with published models predicting the optimal size for B5 active sites. Microemulsion synthesis was also used to synthesize bimetallic formulations, and it was found that Ru-Ni increased the hydrogen production rate (normalized to Ru loading), leading to decreased precious metal usage.