The role of multiple gas—solid collisions in the catalytic decomposition of formic acid

Abstract

An improved version of a low pressure free-molecular flow reactor is described. Use of a macroscopic, geometrically well-defined analog of a catalyst pore enables one to combine the multiple gas—solid collision effects of a high pressure supported catalyst with the detailed flow characterization available in a vacuum system. Simulations of the molecular trajectories within the reactor produce a relationship between an observed integral conversion and the probability of reaction when a gas molecule strikes the surface; low reaction probabilities are integrated experimentally in a manner which makes them readily observable. The Pt and Cu catalysed decomposition of formic acid was studied in two different reactor geometries at temperatures between 300°C and 450°C. The probability of reaction per collision on these surfaces was found to be independent of the collisional history and could be described by the relationships: Pr = 1.12 × 10 4 exp (− 16000/ R gT ) (platinum) Pr = 2.52 × 10 5 exp (− 21600/ R gT ) (copper). These probabilities are consistent with single collision experiments performed on various crystals and wires if the accommodation coefficient for the single collision is less than unity. No information on the molecular mechanism of decomposition was obtained.