Rate Coefficient Determinations for H + NO2 → OH + NO from High Pressure Flow Reactor Measurements.

Rate coefficients for the reaction H + NO2 → OH + NO (R1) have been determined over the nominal temperature and pressure ranges of 737-882 K and 10-20 atm, respectively, from measurements in two different flow reactor facilities: one laminar and one turbulent. Considering the existing database of experimental k1 measurements, the present conditions add measurements of k1 at previously unconsidered temperatures between ∼820-880 K, as well as at pressures that exceed existing measurements by over an order of magnitude. Experimental measurements of NOx-perturbed H2 oxidation have been interpreted by a quasi-steady state NOx plateau (QSSP) method. At the QSSP conditions considered here, overall reactivity is sensitive only to the rates of R1 and H + O2 + M → HO2 + M (R2.M). Consequently, the ratio of k1 to k2.M may be extracted as a simple algebraic function of measured NO2, O2, and total gas concentrations with only minimal complication (within measurement uncertainty) due to treatment of overall gas composition M that differs slightly from pure bath gas B. Absolute values of k1 have been determined with reference to the relatively well-known, pressure-dependent rate coefficients of R2.B for B = Ar and N2. Rate coefficients for the title reaction determined from present experimental interpretation of both laminar and turbulent flow reactor results appear to be in very good agreement around a representative value of 1.05 × 10(14) cm(3) mol(-1) s(-1) (1.74 × 10(-10) cm(3) molecule(-1) s(-1)). Further, the results of this study agree both with existing low pressure flash photolysis k1 determinations of Ko and Fontijn (J. Phys. Chem. 95 3984) near 760 K as well as a present fit to the theoretical expression of Su et al. (J. Phys. Chem. A 106 8261). These results indicate that, over the temperature range considered in this study and up to at least 20 atm, net chemistry due to stabilization of the H-NO2 reaction intermediate to form isomers of HNO2 may proceed at negligible rates compared to R1.

Uncertainty analysis in the use of chemical thermometry: a case study with cyclohexene.

A general method to evaluate the absolute uncertainties in temperatures derived using chemical thermometry is developed and applied to the retro Diels-Alder reaction of cyclohexene. Experiments from previous studies of this reaction are reanalyzed to establish the minimum absolute uncertainty limit. Chemical thermometry results are compared with thermocouple measurements in experiments performed in a flow reactor at 6.1 atm pressure and at temperatures from 957 to 978 K . Using conservative uncertainty estimates, our analysis yields absolute (1σ) uncertainties of temperature through chemical thermometry using this reaction greater than ±20 at 1000 K. Neither more refined experimental techniques nor computational theory is likely to refine rate correlation parameters sufficiently to reach the absolute temperature uncertainties often reported in the literature for chemical thermometry using the retro Diels-Alder reaction of cyclohexene. Published chemical thermometry uncertainty estimates typically have not quantitatively considered the absolute uncertainties of the original data from which the reference rate correlations were based.

Dehydration rate measurements for tertiary-butanol in a variable pressure flow reactor.

Fundamentally, the dehydration reaction of tertiary-butanol is frequently used as an internal standard for relative rate studies of other decomposition reactions. We report here a study using radical trappers to isolate this path in tertiary-butanol pyrolysis experiments conducted in the Princeton variable pressure flow reactor between 658 and 980 K. A novel technique that determines the rate constant value by applying a global least-squares fit incorporating all experimental species (tertiary-butanol, isobutene, and water) evolution data is developed and applied to yield six rate constant values at two reaction pressures (6.1 and 18 atm) and at temperatures between 949 and 980 K. Data from previously reported studies are reanalyzed to evaluate their "absolute" uncertainties, and new Arrhenius parameters are derived based upon the present and previous measurements. The recommended rate constant (uncertainties) for the dehydration reaction is k = 2.88(0.91) × 10(7)T(2.21(0.10)) s(-1) exp(-62.4(0.9) kcal mol(-1)/RT). The new correlation is in excellent agreement with other independent experimental and theoretical studies appearing in the literature.

Functionalized graphene sheet colloids for enhanced fuel/propellant combustion.

We have compared the combustion of the monopropellant nitromethane with that of nitromethane containing colloidal particles of functionalized graphene sheets or metal hydroxides. The linear steady-state burning rates of the monopropellant and colloidal suspensions were determined at room temperature, under a range of pressures (3.35-14.4 MPa) using argon as a pressurizing fluid. The ignition temperatures were lowered and burning rates increased for the colloidal suspensions compared to those of the liquid monopropellant alone, with the graphene sheet suspension having significantly greater burning rates (i.e., greater than 175%). The relative change in burning rate from neat nitromethane increased with increasing concentrations of fuel additives and decreased with increasing pressure until at high pressures no enhancement was found.

Computational singular perturbation analysis of two-stage ignition of large hydrocarbons.

Computational singular perturbation (CSP) analysis has been used to gain understanding of the complex kinetic behavior associated with two-stage ignition of large hydrocarbon molecules. To this end, available detailed and reduced chemical kinetics models commonly used in numerical simulations of n-heptane oxidation phenomena are directly analyzed to interpret the underlying fundamental steps leading to two-stage ignition. Unlike previous implementations of the CSP methodology, temperature is included as one of the state variables so that factors controlling ignition can be unambiguously determined. The analyzed models show differences in the factors contributing to the initial development and shutdown of the first ignition stage. However, during the second stage, both models show the importance of the degenerate branching decomposition of hydrogen peroxide, which contradicts some previous interpretations of this phenomenon.