(767f) VOx surface Catalyst for Low Cost, High Performance Hydrogen Permeable Vanadium Membranes

Vanadium based hydrogen permeable membranes suffer from two intrinsic issues: embrittlement at high hydrogen lattice concentrations and catalytically inactive surfaces for hydrogen dissociation/reassociation. To solve these issues catalytic coatings are used to activate the surface, while careful pressure and temperature limitations during operation prevent embrittlement. Pd dominates the field as a surface catalyst, yet it suffers from interdiffusion with vanadium limiting operating temperatures to <400 °C. Unfortunately, embrittlement becomes a significant issue at these low temperatures due to vanadium’s exothermic hydrogen solubility dependence. Thus, it is desirable to use an alternative catalyst that can limit interdiffusion and allow for operation at higher temperatures and hydrogen pressures. In this area, fabrication of a hydrogen dissociation active VO_x species in situ on vanadium foil surfaces by oxidation in air followed by reduction under hydrogen has been demonstrated. The advantage of this method is manufacturing without the need for expensive Pt-group metals or vacuum equipment. Our experiments have confirmed that 100% selective hydrogen flux through the membrane is achievable, and we observed an initial spike in flux that reached 0.35 mol m^-2 s^-1 at a transmembrane pressure of 200 kPa. However, the hydrogen flux declined over 15 hours before stabilizing around 0.07 mol m^-2 s^-1. Moreover, thermal stability was only witnessed at 550 °C; increasing or decreasing temperature severely limited hydrogen transport. Nevertheless, at 550 °C we observed excellent stability over 120 hours and measured a permeability of 1.72 × 10^-8 mol m^-1 s^-1 Pa^-0.5. This talk will explain the chemistry of VO_x surface catalysts, discuss the membrane performance, and describe shortcomings associated with this unique system.