Kentaro Tsuchiya

Studies on the oxidation mechanism of H2S based on direct examination of the key reactions

By conducting an excimer laser photolysis (193 and 248 nm) behind shock waves, three elementary reactions important in the oxidation of H2S have been examined, where, H, O, and S atoms have been monitored by the atomic resonance absorption spectrometry. For HS + O2 products (1), the rate constants evaluated by numerical simulations are summarized as: k1 = 3.1 × 10−11exp|-75 kJ mol−1/RT| cm3molecule−1s−1 (T = 1400-1850 K) with an uncertainty factor of about 2. Direct measurements of the rate constants for S + O2 SO + O (2), and SO + O2 SO2 + O (3) yield k2 = (2.5 ± 0.6) × 10−11 exp|-(15.3 ± 2.5) kJ mol−1/RT| cm3molecule−1s−1 (T = 980-1610 K) and, k3 = (1.7 ± 0.9) × 10−12 exp|-(34 ± 11) kJ mol−1/RT| cm3molecule−1s−1 (T = 1130-1640 K), respectively. By summarizing these data together with the recent experimental results on the H(SINGLE BOND)S(SINGLE BOND)O reaction systems, a new kinetic model for the H2S oxidation process is constructed. It is found that this simple reaction scheme is consistent with the experimental result on the induction time of SO2 formation obtained by Bradley and Dobson.

Novel Products from C6H5 + C6H6/C6H5 Reactions

To date only one product, biphenyl, has been reported to be produced from C(6)H(5) + C(6)H(6)/C(6)H(5) reactions. In this study, we have investigated some unique products of C(6)H(5) + C(6)H(6)/C(6)H(5) reactions via both experimental observation and theoretical modeling. In the experimental study, gas-phase reaction products produced from the pyrolysis of selected aromatics and aromatic/acetylene mixtures were detected by an in situ technique, vacuum ultraviolet (VUV) single photon ionization (SPI) time-of-flight mass spectrometry (TOFMS). The mass spectra revealed a remarkable correlation in mass peaks at m/z = 154 {C(12)H(10) (biphenyl)} and m/z = 152 {C(12)H(8) (?)}. It also demonstrated an unexpected correlation among the HACA (hydrogen abstraction and acetylene addition) products at m/z = 78, 102, 128, 152, and 176. The analysis of formation routes of products suggested the contribution of some other isomers in addition to a well-known candidate, acenaphthylene, in the mass peak at m/z = 152 (C(12)H(8)). Considering the difficulties of identifying the contributing isomers from an observed mass number peak, quantum chemical calculations for the above-mentioned reactions were performed. As a result, cyclopenta[a]indene, as-indacene, s-indacene, biphenylene, acenaphthylene, and naphthalene appeared as novel products, produced from the possible channels of C(6)H(5) + C(6)H(6)/C(6)H(5) reactions rather than from their previously reported formation pathways. The most notable point is the production of acenaphthylene and naphthalene from C(6)H(5) + C(6)H(6)/C(6)H(5) reactions via the PAC (phenyl addition-cyclization) mechanism because, until now, both of them have been thought to be formed via the HACA routes. In this way, this study has paved the way for exploring alternative paths for other inefficient HACA routes using the PAC mechanism.

Reaction rates of atomic oxygen with straight chain alkanes and fluoromethanes at high temperatures

Abstract Rates of reaction of atomic oxygen ( 3 P) with straight chain alkanes (C 1 –C 5 ) and fluoromethanes (CH 3 F) and CHF 3 ) have been measured at temperatures above 850 K by a laser flash photolysis-shock-tube technique coupled with atomic resonance absorption spectrometry. Measured rate constants agreed well with a recent TST calculation for O+CH 4 and O+C 2 H 6 . However, a small deviation from the TST calculation was found for O+C 3 H 8 and larger alkanes. Preferable modified Arrhenius expressions for these reactions are presented. A good Evans-Polanyi correlation was found for the reactions investigated in the present study.

Reaction rates of O(3P) atom with fluoroethanes at 1000-1400 K

Abstract The overall rate constants for reactions of O ( 3 P ) with three fluoroethanes, O ( 3 P )+ CH 3 CH 2 F → OH + CH 2 CH 2 F / CH 3 CHF (1), O ( 3 P )+ CH 2 FCH 2 F → OH + CHFCH 2 F (2), and O ( 3 P )+ CH 3 CHF 2 → OH + CH 2 CHF 2 / CH 3 CF 2 (3), were measured by laser flash photolysis in shock heated sample gases at 1000–1400 K. The rate constants were obtained in the simple Arrhenius form, k=Aexp(−Ea/RT), where A=2.19×10−9, 1.24×10−9 and 7.43×10 −9 cm 3 molecule −1 s −1 and Ea=58.0, 57.0 and 82.7 kJ mol−1, and uncertainty factors for k at 2σ level are F=1.36, 1.27 and 1.19, respectively. The effect of substitution of the fluorine atom on the overall rate constant is discussed in comparison with the corresponding rates for a series of alkanes and fluoromethanes.

Chain reaction mechanism in hydrogen/fluorine combustion.

Vibrationally excited species have been considered to play significant roles in H2/F2 reaction systems. In the present study, in order to obtain further understanding of the chain reaction mechanism in the combustion of mixtures containing H2 and F2, burning velocities for H2/F2/O2/N2 flames were measured and compared to that obtained from kinetic simulations using a detailed kinetic model, which involves the vibrationally excited species, HF(ν) and H2(ν), and the chain-branching reactions, HF(ν > 2) + F2 = HF + F + F (R1) and H2(ν = 1) + F2 = HF + H + F (R2). The results indicated that reaction R1 is not responsible for chain branching, whereas reaction R2 plays a dominant role in the chain reaction mechanism. The kinetic model reproduced the experimental burning velocities with the presumed rate constant of k2 = 6.6 × 10(-10) exp(-59 kJ mol(-1)/RT) cm(3) s(-1) for R2. The suggested chain-branching reaction was also investigated by quantum chemical calculations at the MRCI-F12+CV+Q/cc-pCVQZ-F12 level of theory.

Improvement of chemical kinetic data at high temperatures by piston actuated shock tube, excimer laser photolysis, and atomic resonance absorption spectrometry

Qualitative improvement of chemical kinetic data at high temperatures has been achieved, to a great extent, by use of a combination of the piston actuated shock tube, the excimer laser photolysis, and the atomic resonance absorption spectrometry. Some of the important studies on elementary reactions performed recently in the University of Tokyo are demonstrated.