The interaction of dissolved H with internally oxidized Pd–Rh alloys

Abstract

Homogeneous fcc Pd–Rh alloys have been internally oxidized in the atmosphere at several temperatures from 1023 to 1123 K forming oxide precipitates within a Pd matrix. As shown from electron diffraction patterns of the internally oxidized Pd 0.97Rh 0.03 alloy, the oxide that forms is a mixed oxide, PdRhO 2. Internally oxidized Pd–Rh alloys can be reduced with H 2 (573–623 K) forming PdRh precipitates within the Pd matrix. This is a technique for segregating components of a substitutional solid solution binary alloy. The segregated alloy can be returned to the homogeneous Pd–Rh alloy by annealing at an elevated temperature and thus, in contrast to the internal oxidation of, e.g., Pd–Al alloys, the process can be readily reversed. The oxidation and (reduction+H 2O loss) were monitored from weight changes. After internal oxidation/reduction “diagnostic” hydrogen isotherms (323 K) were measured to follow the extent of oxidation and to determine the extent of H trapped at the internal interfaces from the positive intercepts along the H/Pd axis in the dilute phase. The H capacities in the steeply rising hydride region of the H 2 isotherms for the internally oxidized alloys are smaller than for Pd unless the Pd in the PdRh precipitates is considered to be inactive for H 2 absorption for the calculation of H/Pd values. In the two-phase region the absorption plateau pressures were significantly greater than for Pd–H, however, they were much closer to the Pd plateau p H2 than to that of the unoxidized Pd–Rh alloys. The desorption plateaux were very close to those of Pd–H for X Rh=0.01 to 0.05 alloys and therefore it can be concluded that the matrix is pure Pd whose absorption plateaux are affected by the presence of the precipitates. The techniques of TEM, SEM and SANS (small angle neutron scattering) were employed to examine the alloys after internal oxidation both before and after reduction. SANS confirmed directly that internal precipitates form from the internal oxidation and that they are rather large with an appreciable fraction having diameters greater than 100 nm.