Thermal decomposition of diborane. Part I. The decomposition mechanism at low conversion and temperature and the inhibiting effect of accumulated hydrogen

Abstract

A gas recirculating device was used to trap the condensible products at –130 °C at the exit of the reaction vessel. The reaction order was three halves in diborane concentration. The Arrhenius parameters, admittedly low, are: log A/(cm^3/2 mol^–1/2 s^–1)= 4·72 ±0·14 and E_a= 42·47 ± 1·17 kJ mol^–1. The reaction, however, is considered to be truly homogeneous since neither coating nor changes in surface:volume ratio altered the rate constant found in an aged reaction vessel. The decomposition was also studied in a static system. For reaction times shorter than 200 s a marked decrease of the rate constant with time was observed. After this interval the rate constant stabilizes and remains almost constant and the figures found are similar to those reported in previous studies. A competitive reaction between an intermediate and hydrogen (inhibition) or diborane is suggested to account for the initial decrease of the rate constant. The decomposition of products (principally B₅H₁₁) is held, responsible for the observed constancy of the rate constants after the first 10 min of reaction.