On the feasibility of phosphide generation from phosphate reduction: The case of Rh, Ir, and Ag

The feasibility of reductive hydrogen annealing of metal phosphates as a pathway to phosphides has been explored for Rh, Ir and Ag. Phosphate precursors were prepared by combination of metal salts with ammonium monohydrogen phosphate in water. Rh and Ir precursors were isolated by solvent evaporation, whereas the reaction with silver acetate yielded Ag3(PO4) as a precipitate. Thermogravimetric analysis was employed using 5% H2 in Ar to ascertain appropriate reduction conditions for phosphide formation. Rh2P was formed as the sole crystalline phase from slow heating to 375 °C, although energy dispersive spectroscopy suggests amorphous phosphorus may be present as an impurity. Ir2P is best prepared from ramping to 600 °C in an inert atmosphere, followed by hydrogen annealing. Iridium metal is also present as an impurity, apparently arising from the iridium reagent or in the process of preparing the phosphate precursor. Ag3(PO4) does not yield a phosphide upon treatment with hydrogen, instead reducing directly to the metal at temperatures as low as 140 °C.