Catalytic decomposition of DCOOD studied by reactive scattering of a modulated DCOOD nozzle beam by a polycrystalline platinum surface in ultra high vacuum

Abstract

The catalytic decomposition of formic acid by a polycrystalline platinum surface was studied by use of modulated molecular beam techniques with mass spectrometric phasesensitive detection. Kinetic information about elementary surface reaction steps was obtained. The formation of CO 2 was found to be a monomolecular, whereas that of D 2 was a bimolecular process. The resulting reaction mechanism may be described as follows: ▪ The rate constants in dependence from the surface temperature t 0 are η = 7.1 × 10 3 exp(− 9.9 RT 0 kcal/mole), k d 1 = 6.3 × 10 7 exp(− 10.2 RT 0 kcal/mole) ( sec −1), k d 2 = 3.2 × 10 −8 exp(− 9.3 RT 0 kcal/mole) ( particles −1 sec −1cm 2). The sticking probability η is provided by the temperature dependence of the intensity of the nonreactive scattered formic acid molecules; the rate constants k d 1 and k d 2 are derived from the measured phase shift between reactive and nonreactive scattered particles. From the phase angle ϑ, the average surface residence time τ of the intermediates is computed: 3.7 ≳ τ DCOO ≳ 0.41 msec (418 ≲ T 0 ≲ 505 K), 31.8 ≳ τ D ≳ 11.6 msec (418 ≲ T 0 ≲ 460 K). The difference between τ D and τ DCOO is because of the different molecularity of desorption.