Characterization of surface changes at atmospheric pressures using optical reflectance

The feasibility of using visible optical reflectance to examine changes in a Pt surface produced by exposures to various reacting and non-reacting gases at atmospheric pressure is explored. Effects of heating a Pt ribbon to various temperatures alternately in O₂ and in NH₃, CO, or H₂ are examined, and the NH₃ and CO oxidation reactions are investigated. At room temperature typical changes of 1% can be detected with a precision greater than ± 0.01%. With NH₃-air the reflectivity increases upon heating in NH₃ and reaches a maximum at ∼600 K. Exposing the NH₃-covered surface to air produces a uniform decrease upon heating. In H₂ or CO, a large increase in reflectivity occurs at room temperature, and exposure to air produces a monotonic decrease upon heating. Dependences on wavelength, angle of incidence, and polarization were examined in detail for the NH₃-air system. The sensitivity was found to be greatest for p-polarization at short wavelengths. These results are generally consistent with changes in reflectivity expected for chemisorbed oxygen and other species on the Pt surface. In NH₃ and CO oxidation reactions, the precision was considerably reduced because of catalytic etching and ribbon bending at high temperatures. The former was reduced by switching between a pair of compositions on either side of the stoichiometric ratio which gave equal adiabatic temperatures, but catalytic etching produced continuous linear variations in light reflected, particularly in excess oxygen where volatile oxides promote catalytic etching.