Distinct Site Types Research Articles

Effect of photodecomposition on the infrared absorption of lead azide. II. Near infrared

The effects of photodecomposition on the near infrared intraazide ion absorption of lead azide films have been measured. The structure in the near ir is explained on the basis of the four types of azide lattice sites and the characteristic intraazide interatomic distances for each. The structure disappears during the early stage of the decomposition, indicating homogeneous destruction of the distinct types of sites in the thinner films. Changes in the near ir spectra are correlated with those in the far ir lattice modes and with those in the ultraviolet electronic spectra. The decomposition product has the optical properties of an amorphous material. The kinetics of the photodecomposition are analyzed and applied to the time dependence of the rate of nitrogen evolution.

Heterogeneous catalysed electron-exchange reactions—I. The thallium(I)-thallium(III) system

The slow, homogeneous, two-electron Tlqa+−Tlaq3+ exchange (t1/2 = 150 h at 35°C) is markedly catalysed by platinum black surfaces of high specific area (t1/2 = 3 min at 35°C). A detailed kinetic analysis reveals that the rate of the surface-catalysed reaction is described, over the bulk concentration range 10−4−10−1 M, by Rex = kexs[Tlads+][Tlads3+], where kexs = (1·5 ± 0·4) × 107 cm2/mol/s, ΔHs* = 11·1 ± 0·7 Kcal/mol and ΔSs* = + (10·2 ± 0·6) cal/deg/mol. The rate determining process is that of electron transfer between adsorbed reactants via the bulk platinum with an ‘inner-sphere’ transition state. The value of kexs varies only very slightly with large changes in the bulk reactant concentrations and hence with the formal system potential. A transfer coefficient, α = 0·1, is suggested by the data.204Tl tracer techniques have been used to measure independently the adsorbed surface concentrations, [Tlads+] and [Tlads3+]. These adsorbed concentrations are described quantitatively by independent Langmuir isotherms within the bulk concentration range 3 × 10−5 − 5 × 10−1 M. Adsorption in mixtures of Tlaq+and Tlaq3+ is additive and there is no evidence for competitive adsorption. Tlaq3+ is adsorbed more strongly and Tlaq+ and Tlaq3+ are considered to adsorb specifically on two distinct types of sites on the oxidized platinum surface.