Kinetic Studies on the Pyrolysis of H₂S

Abstract

The kinetics and the mechanism of the thermal decomposition of H2S and subsequent reactions have been studied. The rate constant for the initiation reaction H2S + M → products (1) was determined by a shock tube−infrared emission spectroscopy at temperatures 2740−3570 K to be k 1 = 10-10.44±0.31 exp[−(268.6±18.4)kJ mol-1/RT] cm3 molecule-1 s-1, which is about one-fifth to one-tenth of the recent results reported by Woiki and Roth (J. Phys. Chem. 1994, 98, 12958) and Olschewski et al. (J. Phys. Chem. 1994, 98, 12964). An ab initio (MRCI+Q) calculation suggested that a spin-forbidden product channel (→S(3P) + H2) is energetically favorable compared to a H−S bond fission channel; that is, the singlet−triplet intersystem crossing occurs at an energy lower than the dissociation threshold for HS + H by about 17 kJ mol-1. The present rate constant for reaction 1 could be well reproduced by an unimolecular decomposition theory with the calculated energy for the crossing and with a reasonable collision parameter, βc. The rate constants for important subsequent reactions, S(3P) + H2 → products (3) and S(3P) + H2S → products (4), were also determined by a laser photolysis−shock tube−atomic resonance absorption spectrometry method: k 3 = 10-9.58±0.16 exp[−(82.5±4.0) kJ mol-1/RT] (1050−1660 K) cm3 molecule-1 s-1, and k 4 = 10-9.86±0.17 exp[−(30.9±4.1) kJ mol-1/RT] (1050−1540 K) cm3 molecule-1 s-1. The ARAS measurement of H atoms revealed that the main products for reaction 3 are HS + H at pressures below 2 atm.

Kinetic and modeling studies of the reaction S+H2S

“…The data of Shiina et al [8] align well with the present work, especially after correction for secondary reactions in their experiments, and are also accurately modeled. It is apparent that most of the reaction in those experiments is in fact via the triplet channel.…”

“…We also considered experimental data from Shiina et al [8] at higher temperatures (1050-1660 K). In this case, the formation of SH in the flash photolysis was assumed to be negligible, as stated by the authors [8], but the contribution of secondary reactions is now predicted to be significant, with the observed rates of S-atom consumption being as much as 1.7 times greater than the rate of H2S+S.…”

“…Shiina et al [8] found reaction (1) to proceed with a barrier close to the endothermicity of (2) and suggested that formation of SH+SH occurs via a triplet-singlet crossing and insertion. This possibility was confirmed in high-level theoretical work [7] which showed that the low singlet barrier relative to the direct abstraction reaction (1) on the triplet surface would make the surface crossing route especially important at low temperatures.…”

Chemical kinetics mechanism for oxy-fuel combustion of mixtures of hydrogen sulfide and methane

“…For the reaction H 2 S + S = SH + SH, two duplicate reactions corresponding to different reaction channels were defined in the original sulfur mechanism [35]. We determined the uncertainties in the pre-exponential factors of each of these by varying them and comparing the resulting fit to the original data of Gao et al [49] and Shiina et al [50]. The limits on the variation of the pre-exponential factor that were chosen are +20%/-40% for the first of the two duplicate reactions and ±25% for the second reaction.…”

“…The pyrolysis of H 2 S has been the subject of a number of experimental (e.g., [43,55]) and modeling (e.g., [50,52]) studies. Compared to the experimental data of Hawboldt et al [43] (targets 6-7), H 2 S conversion predicted by the basic mechanism tends to be too slow for most temperatures (see Fig.…”