Effect of Rhodium Distribution on Thermal Stability of Nanoporous Palladium–Rhodium Powders

Powders of nanoporous palladium and palladium alloy particles are of potential value for storage of hydrogen isotopes, as long as the pores remain stable over a useful range of temperatures and chemical environments. Rhodium alloys are known to have enhanced hydrogen storage and improved thermal stability versus pure palladium. However, the distribution of rhodium on pore and particle surfaces is critical to this. Pores are more ordered and thermally stable in rhodium-rich regions. Treatment of particles at elevated temperature under reducing conditions can cause rearrangement of Rh and Pd at the surface, but not a major change in Rh distribution throughout the particle. Heating in the presence of hydrogen causes more rapid pore rearrangement than heating in vacuum subsequent to hydrogen exposure, suggesting a direct chemical influence of hydrogen on mobility of surface atoms. These results provide a clear path to future improvements in the stability of nanoporous metals in reducing atmospheres.