Direct Measurements of the Reaction OH + CH_2O-> HCO + H_2O at High Temperatures

摘要

The reaction of hydroxyl [OH] radicals with formaldehyde [CH_2O] was studied at temperatures ranging from 934 K to 1670 K behind reflected shock waves at an average total pressure of 1.6 atm.OH radicals were produced by shock-heating tert-butyl hydroperoxide [(CH_3)_3-CO-OH],while 1,3,5-trioxane |(CH_2O)_3] was used in the preshock mixtures to generate reproducible levels of CH_2O.OH concentration time-histories were inferred from laser absorption using the well-characterized R_1(5) line of the OH A-X (0,0) band near 306.7 nm.Detailed error analyses,taking into account both experimental and mechanism-induced contributions,yielded uncertainty estimates of +-25% at 1595 K and +-15% at 1229 K for the rate of the reaction between CH_2O and OH.These uncertainties are substantially lower than the factor of two uncertainty currently used for this reaction at high temperatures.The rate constants were fit with the recent low-temperature measurements of Sivakumaran et al.(Phys Chem Chem Phys 2003,5,4821-4827) to the three-parameter form shown below;this fit reconciles experimental data on the title reaction at low,intermediate,and high temperatures (200-1670 K).k_2 = 7.82 x 10~7T~(L63)exp(531/T(K)](cm~3mol~(-1) s~(-1))The reaction of OH with CH_2O was also studied using quantum chemical methods at the CCSD(T) level of theory using the 6-311 ++G(d,p) basis set.The transition state for the H-atom metathesis reaction was located,and reaction rate coefficients were calculated.Reasonable agreement with the experimental measurements was obtained.The decomposition rate of tert-butyl hydroperoxide to a tert-butoxy radical and an OH radical was measured,and fit to the following form k_5 = 2.50x 10~(15)exp(-21640/T(K))(s~(-1)) Uncertainty limits for k_5 were estimated to be +-25% in the 900-1000 K temperature range,a marked reduction from the factor of 2-3 uncertainty currently recommended for this reaction in the literature.