Sorption of oxygen from solution by noble metals: II. Nitric acid-passivated bright platinum

Abstract

The nature of the adsorbed films produced on platinum electrodes treated with concentrated nitric acid was studied by means of constant current stripping pulses as a function of the time of passivation and the contact with atmospheric oxygen. It is concluded that the dissolved oxygen of the Pt-O alloy does not come from the HNO 3. Instead, the HNO 3 treatment makes the Pt more accommodating to the sorption of oxygen (in the manner of an oxygen sponge), and the Pt then sorbs oxygen from the surroundings producing the Pt-O alloy. The presence of the oxygen in the Pt lattice modifies the electronic structure of the Pt, generating an electrode material with different catalytic and electronic properties from untreated Pt. In the presence of oxygen, the HNO 3 treatment produces a Pt-O alloy electrode with a complete monolayer of Pt-O adsorbed on the surface. Such an electrode exhibits the reversible oxygen potential in O 2-saturated, 2 N H 2SO 4 solution. The rest potential is a linear function of the degree of surface coverage. No more than a monolayer is adsorbed by this procedure, but the equivalent of two layers may be formed by anodizing the Pt-O alloy electrode. Apparently the Pt-O sites are converted to PtO 2 sites which are unstable at potentials below 1500 mV in the presence of Pt metal. The Pt-O alloy structure is rather stable and is converted to that of untreated Pt by heating the metal in the vicinity of its melting point.