Formic acid decomposition at low pressures on flashed and unflashed platinum wires

Abstract

The catalytic activity of unflashed wires at formic acid reactant pressures of 10 −4 torr and less was found to vary considerably from one specimen of wire to another. Flashing in vacuum or reduction in hydrogen at 970 °K increased the activity of wires considerably, in some cases by as much as 500 times at 970 °K. Hydrogen reduction was, however, characterized by a rapid loss of activity of the wire during continuous reaction at 970 °K, whereas a flashed wire gave a steady activity at this and higher temperatures, corresponding to a reaction probability per collision of close to unity. The decay of activity could be attributed to saturation of the surface region by carbon deposited either from the decomposition of formic acid, or the cracking of residual gases. The retardation order at 527 °K with respect to hydrogen was minus one on a freshly flashed wire, but zero on a wire which had been used for formic acid decomposition. This retardation order may therefore reflect the extent of carburization of the surface. The decay of activity with time of reaction at low reaction temperatures subsequent to flashing a wire is postulated to be governed by a dynamic equilibrium involving removal of carbonaceous matter from the surface by reaction with water vapor, and diffusion of carbon through the bulk metal. This equilibrium is described in terms of a Langmuir type of isotherm.