Cadmium photosensitised decomposition of propane at 265°C and at λ= 3261 Å

Abstract

A detailed analytical investigation of the cadmium photosensitised decomposition of propane at 265°C and λ= 3261 Å has been made. The major products detected and estimated were methane, propylene, dimethylbutane (DMB), ethylene and hydrogen. Minor products detected were traces of ethane and isobutane. The major products were found to form initially at accelerated rates with time. (CH₄/H₂), (C₃H₆/H₂) and (C₂H₄/CH₄) ratios were found to decrease with conversion or time in a given run at a given reactant pressure, while (DMB/H₂) ratio was found to increase. The ratio of the rate constants (k₁/k_b) of the reactions, viz., Cd(³P₁)+ C₃ H₈→ CdH + i- C₃H₇(1) Cd(³P₁)→ Cd(¹S₀)+hv(b) was found to be 3.30 × 10² dm³ mol^–1 at zero cadmium concentration in the reaction vessel. If the radiative lifetime of Cd(³P₁) be 2 × 10^–6 s, then k₁ becomes 1.68 × 10⁸ dm³ mol^–1 s^–1, which gives 0.015 Ų as the quenching cross section of propane per molecule, in agreement with the value of Steacie and LeRoy (i.e., 0.012 Ų). If (1/k_b) be 1.0 × 10^–4 s, as reported by Strausz et al., then k₁ becomes 3.30 × 10⁶ dm³ mol^–1 s^–1, which gives 0.000 29 Ų as the quenching cross section. The disproportionation/combination rate constant ratios, (k_d/k_c), of isopropyl radicals at 265°C was found to be unity.