A Shock-Tube Study of the Rate Constant of PH3 + M ⇄ PH2 + H + M (M = Ar) Using PH3 Laser Absorption

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 124; no. 37; pp. 7380 - 7387
• Main Authors: Mulvihill, Clayton R, Juárez, Raquel, Mathieu, Olivier, Petersen, Eric L
• Format: Journal Article
• Language: English
• Published: American Chemical Society 17.09.2020
• Subjects: A: Kinetics, Dynamics, Photochemistry, and Excited States
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.0c04917

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 μm. 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 k 1,0 were obtained and best described by (in cm3·mol–1·s–1) k 1,0 = 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 k 1,0 was in the low-pressure limit. Agreement of the experimental k 1,0 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.