Sulfur Coverage Effects on the Reduction of Dilute Nitric Oxide at Platinum Black Gas Diffusion Electrodes.

To study the effect of sulfur coverages on porous platinum black gas diffusion during the reduction of nitric oxide in a quantitative manner, electrodes were prepared with fractional sulfur coverages between 0.0 and1.0 (based on total platinum surface sites as measured by electrochemical hydrogen adsorption). The deposited sulfur was obtained through reduction of adsorbed SO2 in a controlled system. The reduction of dilute streams of nitric oxide (3% in nitrogen) was performed in electrogenerative (iegalvanic) cells at positive potentials. At fractional sulfur coverages above 0.2, cell current began to diminish, though NO conversion could be controlled at a high level (80–95% in a single pass). The product selectively during NO reduction was markedly effected at higher sulfur coverages (θs > 0.2). Nitrous oxide formation was diminished, and ammonia production was greatly inhibited at these higher sulfur coverages. Hydroxylamine became the dominant product on sulfur-covered electrodes at lower voltages. Results from voltammetric experiments for NO reduction on a smooth bulk platinum electrode at comparable sulfur coverages did not reflect the results obtained with the platinum black-Teflon gas diffusion electrodes, illustrating some structure sensitivity. Some of the sulfur adsorbed on Pt-black gas diffusion electrodes is postulated to exist as sulfur doublets adsorbed between every other platinum atom.