A Shock-Tube Study of the Rate Constant of PH3 + M PH2 + H + M (M = Ar) Using PH3 Laser Absorption.
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Phosphine (PH3) is a highly reactive and toxic gas. Prior experimental investigations of PH3 pyrolysis reactions have included only low-temperature measurements. This study reports the first shock-tube measurements of PH3 pyrolysis using a new PH3 laser absorption technique near 4.56 m. Experiments were conducted in mixtures of 0.5% PH3/Ar behind reflected shock waves at temperatures of 1460-2013 K and pressures of 1.3 and 0.5 atm. The PH3 time histories displayed two-stage behavior similar to that previously observed for NH3 decomposition, suggesting by analogy that the rate constant for PH3 + M PH2 + H + M (R1) could be determined. A simple three-step mechanism was assembled for data analysis. In a detailed kinetic analysis of the first-stage PH3 decomposition, values of k1,0 were obtained and best described by (in cm3mol-1s-1) k1,0 = 7.78 1017 exp(-80,400/RT), with units of cal, mol, K, s, and cm3. Agreement between the 1.3 and 0.5 atm data confirmed that the measured k1,0 was in the low-pressure limit. Agreement of the experimental k1,0 with ab initio estimates resolved the question of the main pathway of PH3 decomposition: it proceeds as PH3 PH2 + H instead of PH3 PH + H2.
