SOLID SOLUBILITY IN Cu5Gd1-xCax SYSTEM: STRUCTURE, STABILITY, HYDROGENATION AND CATALYSIS OF OXYGEN REDUCTION

Abstract

We studied the effects of substituting gadolinium in the compound Cu5Gd with Ca by investigating the phase stability and crystal structure of the resulting new compounds. For rapidly quenched materials produced by melt spinning, the crystal structure was always hexagonal P6/mmm, irrespective of the Ca addition (x) in alloys with the formula Cu5Gd1-xCax, indicating good solid solubility under these conditions, which was additionally confirmed by Vegard’s law. Slower cooling upon arc-melting process caused the phase separation into Cu4Gd and CuCa. Using TEM, we investigated rapidly solidified ribbons of Cu5Gd0.5Ca0.5 and observed co-existence of Cu–Ca secondary phase with a matrix phase having the nominal stoichiometric composition. The oxygen level in this sample was found to be within 2–5 at. %, which was attributed to surface oxidation during or after TEM sample preparation. Upon hydrogenation, the crystal structure of all samples changed from hexagonal to cubic (F-43m), which is the thermodynamically stable polymorph of Cu5Gd compound. Strong catalytic activity of water formation from gaseous H2 and O2 was coincidentally discovered during dehydrogenation experiment, thus making this material as potential candidate for zero-platinum oxygen reduction catalyst.