On the Differential Thermal Analysis of the Platinum‐Oxygen System

Abstract

Differential thermal analyses (DTA) curves were obtained on preanodized and HNOs-passivated platinum (Pt) samples. In both cases two kinds of adsorption states for oxygen on Pt were detected as well as oxygen dissolved in the interior of the Pt metal. The DTA results agree well with the cathodic stripping pulse data obtained earlier.From x-ray diffraction analysis and vacuum fusion studies on samples of bright Pt passivated in air-saturated concentrated HNO3, it was concluded (1) that the Pt metal contained enough dissolved oxygen to be considered a Pt-O alloy. It was also found that oxygen could be made to diffuse through a thin Pt foil (2) under conditions of strong anodic polarization (6 mA/ cm ~ at 2V) of the back side of the Pt diaphragm in solutions of H2804 and that this anodization process can cause relatively large quantities of oxygen to be dissolved in the Pt lattice (3,4). In these studies three kinds of sorbed oxygen were distinguished: surface adsorbed oxygen, dermasorbed oxygen dissolved in the first two or three metallic layers, and absorbed oxygen dissolved in the bulk of the metal. The properties of Pt-O alloys made by passivation in HNO3 were somewhat different from those made by anodization (3, 5, 6). The HNQ-passivated Pt is much more stable than the Pt-O alloy obtained from anodization (6).Babenkova et al. (7) investigated the forms of hydrogen sorbed by palladium by measuring the desorption of hydrogen during a linearly programmed temperature rise experiment. They were able to find two forms of adsorbed hydrogen on the surface as well as the hydrogen dissolved in the bulk Pd.It seemed likely that a differential thermal analysis (DTA) of the platinum-oxygen system might enable one to observe the various forms of oxygen sorbed by Pt as well as to shed light on possible differences in the structure of Pt-O alloy formed by HNO3 passivation or by anodization.This report describes some DTA experiments made on Pt-O alloys. ExperimentalAll DTA curves were obtained with a furnace platform assembly with which analyses may be made on the sample in any controlled atmosphere at pressures ranging from 1.3 • 10 -6 to 2 atm. The reference sample was A1203 which is thermally inert in the temperature range of the scan, 0-1000~ The Pt-O alloy samples were made from small beads (0.11 cm in diameter) melted at the end of a Pt(99.99% pure) wire. These beads were cleaned by repeatedly heating to white heat in a H2 jet followed by quenching in concentrated HNO3. Afterward, three beads were mounted in a Teflon cell filled with concentrated HNO~ (70%) to soak for periods of time greater than 72 hr (6). Another set of three Pt beads were mounted in a Teflon cell filled with 2N H2SO4 solution and were anodized at 6 mA/cm 2 (2.00 • 0.05V) against a Pt gauze counterelectrode for periods of time greater than 24 hr. All Pt-O alloy samples were made at room temperature (24 ~ • 1~After formation of the Pt-O alloy, a bead was cut off of the wire and weighed ,-~37 mg) in a small Pt dish of * Electrochemical Society Active Mem...