The Co-Pd (Cobalt-Palladium) System

Resistivity Testing of Palladium Dilution Limits in CoPd Alloys for Hydrogen Storage

Palladium satisfies most of the requirements for an effective hydrogen storage material with two major drawbacks: it has a relatively low gravimetric hydrogen density and is prohibitively expensive for large scale applications. Pd-based alloys should be considered as possible alternatives to a pure Pd. The question is how much one can dilute the Pd concentration in a variety of candidate materials while preserving the hydrogen absorption capability. We demonstrate that the resistivity measurements of thin film alloy samples can be used for a qualitative high-throughput screening and study of the hydrogen absorbing properties over the entire range of palladium concentrations. Contrary to palladium-rich alloys where additional hydrogen scattering indicates a degree of hydrogen content, the diluted alloy films respond by a decrease in resistance due to their thickness expansion. Evidence of significant hydrogen absorption was found in thin CoPd films diluted to just 20% of Pd.

Enhanced Performance of Bimetallic Co-Pd Catalysts Prepared by Mechanical Alloying

Bimetallic catalysts can provide enhanced performance, and Co-based catalysts in particular have been studied in various respects for their activity in the deposition of carbon nanofibers (CNFs). The majority of studies on CNF catalysis use co-precipitation to create alloys, but recent work has demonstrated the suitability of mechanical alloying (MA) by ball milling to reduce cost and increase catalytic activity. This work establishes the unique ability of MA to control the microstructure to produce bimetallic composites, which retain distinct metallic phases that improve catalytic activity. It is demonstrated that Co-Pd alloys reach a maximum in catalytic activity at an intermediate time of mechanical activation, where 30 min of milling outperformed samples milled for 5, 15, 60, and 240 min at a reaction temperature of 550 °C and a 1:4 C2H4:H2 reactant ratio. This indicates there is benefit to retaining the metals in distinct phases in close proximity. Ball milling provides a relatively simple and scalable method to achieve these unique microstructures, and in the optimal condition tested here, the activity toward carbon deposition is increased fourfold over prior work. Furthermore, the minimum temperature for deposition is also reduced. The characteristics of these materials, the effects of milling and annealing, and the underlying mechanisms of deposition are discussed.