Probing Nitrogen Chemistry: a Theoretical Study for Important Reactions of NxHy, HCN and HNCO Oxidation
Session chaired by Dr. Agustin ValeraMadina
Ammonia (NH3) is a carbon free energy carrier with relatively high energy density, but its combustion produces nitrogen oxides (NOx) and other harmful pollutants. Therefore, understanding its oxidation chemistry is necessary to develop industrial combustion applications. Diazene (N2H2), diazenyl radical (NNH), amidogen radical (NH2), hydrogen cyanide (HCN) and isocyanic acid (HNCO) are important intermediates in the NH3 combustion, and HCN and HNCO are highly toxic compounds [1]. However, there are very few experimental and modeling work of HCN [2] and HNCO [3] oxidation in literature, and limited fundamental studies were found regarding the rate coefficients determination of the key reactions for the two compounds’ oxidation. Therefore, this study aims to systematically perform high level quantum chemical calculations for the rate constants of important reactions of N2H2 (N2H2, NNH and NH2), HCN and HNCO oxidation, as well as the thermochemistry of species involved. NxHy System: NNH + M ↔ N2 + H + M N2H2 + H ↔ NNH + H2 N2H2 + O ↔ NNH + OH NH2 + H ↔ NH + H2 NH2 + OH ↔ NH + H2O HCN System: HCN + M ↔ HNC + M HCN + H ↔ NC + H2 HCN + O ↔ NC + OH HCN + OH ↔ NC + H2O HCN + HO2 ↔ NC + H2O2 HCN + O2 ↔ NC + HO2 HNCO System: HNCO + H ↔ NCO + H2 HNCO + O ↔ NCO + OH HNCO + OH ↔ NCO + H2O Regarding the calculation method, the M062X [4] method with the 6311++G(d,p) [5, 6] basis set was used for the geometry optimizations, vibrational frequency calculations and also the hindered rotation treatments for lower frequency modes. All vibrational frequencies and zero point vibrational energies (ZPVEs) were scaled by 0.983 and 0.9698 respectively, which was recommended for the M062X functional by Zhao and Truhlar [4] The electronic single point energies (SPEs) were calculated at the CCSD(T)/ccpVXZ level of theory (where X = T and Q) [7, 8], and the resulting SPEs were extrapolated to the complete basis set (CBS) limit using the following formula: [9, 10] ECBS = ECCSD(T)/ccpVQZ + (ECCSD(T)/ccpVQZ  ECCSD(T)/ccpVTZ) * 44 / (54 – 44) The internal rotations that correspond to low frequency torsional modes were scanned in 10 degree increments as a function of dihedral angle using the M062X/6311++G(d,p) method. This method was also used to perform intrinsic reaction coordinate (IRC) calculations [11] on each transition state (TS) to ensure it was connected to the desired reactants and products. The quantum mechanical tunneling was taken into account for an unsymmetrical Eckart barrier model [12]. For the MultiWell [13] program suite calculation, the Lamm module was used to calculate both external rotational constants and reduced moment of inertia for the hindered internal rotations. The calculated results were then fitted to truncated Fourier series, which were further used as 1D hindered internal rotation input in the Thermo module. The highpressure limit (HPL) rate coefficients were finally calculated by the Thermo module as a function of temperature (298.15 – 2000 K) based on canonical transition state theory (TST) [14]. The calculated rate coefficients were fitted to a modified Arrhenius expression as a function of temperature: k = A (T/Tref)n exp(E/RT) Where A is the Afactor, T is the temperature in units of Kelvin, Tref = 1 K, n is the temperature exponent at 1 K, and E is related to the activation energy (by Ea = E + nRT). As to the quantum chemical methods for the thermodynamic properties calculation, the average atomization formation enthalpies for all of the nitrogen containing species were carried out using a combined compound method CBSAPNO/G3/G4 [1517], which was found to yield results approaching “chemical accuracy” (arbitrarily, ≈ 4 kJ mol–1 or 1 kcal mol–1) when benchmarked against enthalpy of formation values in the Active Thermochemical Tables (ATcT).[1820] The thermochemical values of interest (enthalpy of formation, entropy and heat capacity) were calculated as a function of temperature (298.15−3000 K), and these resulting values were fitted to NASA polynomials [21] using the Fitdat utility in ANSYS CHEMKINPRO [22]. Eventually, the rate constants of 14 reactions and thermodynamic properties of 9 species were systematically carried out. The rate coefficients have been comprehensively compared based on 1) different systems: N2H2 (N2H2, NNH and NH2), HCN and HNCO systems; 2) different reaction types: abstraction, decomposition and isomerization reactions. Both reaction rates and thermochemistry have been compared with the experimental and theoretical results in literature, and excellent agreement was obtained.

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