A kinetic study of the gas-phase thermolysis of tetraborane(10)

Abstract

The kinetics of thermal decomposition of tetraborane(10) have been investigated by a mass spectrometric technique in the pressure range 0.86–38.69 mmHg and temperature range 40.3–77.9 °C (313.4–351.0 K). The initial rate of consumption of B4H10 follows the first-order rate equation –d[B4H10]/dt= 6.4 × 1011 exp(–99 400/RT)[B4H10], with no evidence for the -order dependence claimed in the most recent literature. In the initial homogeneous gas-phase reaction, B5H11is produced with an order slightly greater than unity, and at a rate of 0.4–0.5 mole per mole of B4H10 consumed; hydrogen is also produced, with first-order kinetics, together with non-volatile solid hydride. Significant induction periods are observed in the build-up of species other than B5H11(i.e. B2H6, B6H12, and B10H14), indicating that they are the result of reactions subsequent to those involved in the initial stages. The initial reaction is interpreted in terms of the three-step mechanism, (1a), (2), and (3), in which (1a) is rate-limiting. A possible fourth step (5) may be an additional minor reaction favoured at low pressures. B4H10⇌{B4H8}+ H2(1a), {B4H8}+ B4H10→ B5H11+{B3H7}(2), {B3H7}+{B3H7}→{‘B6H14 – 2m’}(polymerizes)+mH2(3), {B4H8}+{B4H8}→{‘B8H16 – 2n’}(polymerizes)+nH2(5)