(246g) Interaction of Hydrogen with Dilute Transition Metal Atoms Dispersed in Supported Liquid Metal Droplets

Supported metal catalysts consisting of small amounts of a catalytically active metal dissolved in a liquid metal matrix (typically Ga) and dispersed in the form of droplets on an inert support material are active for hydrocarbon dehydrogenation. Ga has high temperature H₂ solubility and permeability, providing opportunities for the development of Ga-based systems for process intensification of equilibrium-limited reactions. SiO₂ supported Pd-Ga catalysts with varying Pd contents (molar ratios of 1:20, 1:40, 1:60 and 1:80) were prepared using a facile ultrasoncation-based procedure. H₂ adsorption isotherms revealed a significant H₂ uptake for all catalysts at elevated temperatures (150 – 550 °C), with Pd content of the catalysts having minimal impact on the H₂ uptake. Thermodynamic analysis (van’t Hoff) demonstrated an endothermic heat consistent with associative H₂ absorption, consistent with the increase in absorption with increasing temperature. Transient measurements of absorption suggest a fast surface adsorption followed by slow absorption into the bulk. The kinetics of the catalysts were explored using the probe reaction of H₂-D₂ scrambling. The rate of HD production was found to be significantly higher with increasing Pd-content, however the activation energy was minimally affected. This suggests higher Pd content leads to an increased number of active sites but has a limited effect on the identity of the active sites, suggestive of single site catalyst behavior. Varying the partial pressures of H₂ and D₂ revealed the mechanism was consistent with non-competitive adsorption and that there was a significant kinetic isotope effect (KIE), found to be around 1.5 in the case of the catalyst with a 1:20 molar ratio. The results here provide insight into the fundamental interactions of H₂ with these unique liquid metal catalysts, which will aid in the optimization of their performance for H₂-based reactions where simultaneous separation is advantageous.