Carl F. Melius

Kinetic and thermodynamic issues in the formation of aromatic compounds in flames of aliphatic fuels

We have formulated a chemical kinetic model to predict the growth of higher hydrocarbons in a lightly sooting C2H2/O2/Ar flame. The predictions of the model compare favorably with the experimental results of Bastin et al. (Twenty-Second Combustion Symposium). Analysis of the mechanism shows that reactions of 1CH2 play a central role in forming the C3 and C4 hydrocarbons that ultimately lead to ring formation. Several possibilities are considered for the cyclization reaction. The most likely candidates involve reaction complexes in which the hydrogen atoms are not optimally placed. This point is discussed in some detail. We argue that the “first ring” is most likely formed by reaction of two propargyl radicals, C3H3 + C3H3 ⇐ C6H5 + H or C3H3 + C3H3 ⇐ C6H6.

Aromatic and polycyclic aromatic hydrocarbon formation in a laminar premixed n-butane flame

Experimental and detailed chemical kinetic modeling work has been performed to investigate aromatic and polycyclic aromatic hydrocarbon (PAH) formation pathways in a premixed, rich, sooting, n-butane–oxygen–argon burner stabilized flame. An atmospheric pressure, laminar flat flame operated at an equivalence ratio of 2.6 was used to acquire experimental data for model validation. Gas composition analysis was conducted by an on-line gas chromatograph/mass spectrometer technique. Measurements were made in the main reaction and post-reaction zones for a number of low molecular weight species, aliphatics, aromatics, and polycyclic aromatic hydrocarbons (PAHs) ranging from two to five-fused aromatic rings. Reaction flux and sensitivity analysis were used to help identify the important reaction sequences leading to aromatic and PAH growth and destruction in the n-butane flame. Reaction flux analysis showed the propargyl recombination reaction was the dominant pathway to benzene formation. The consumption of propargyl by H atoms was shown to limit propargyl, benzene, and naphthalene formation in flames as exhibited by the large negative sensitivity coefficients. Naphthalene and phenanthrene production was shown to be plausibly formed through reactions involving resonantly stabilized cyclopentadienyl and indenyl radicals. Many of the low molecular weight aliphatics, combustion by-products, aromatics, branched aromatics, and PAHs were fairly well simulated by the model. Additional work is required to understand the formation mechanisms of phenyl acetylene, pyrene, and fluoranthene in the n-butane flame.

Reaction mechanisms in aromatic hydrocarbon formation involving the C5H5 cyclopentadienyl moiety

The quantum chemical BAC-MP4 and BAC-MP2 methods have been used to investigate the reaction mechanisms leading to polycyclic aromatic hydrocarbon (PAH) ring formation. In particular we have determined the elementary reaction steps in the conversion of two cyclopentadienyl radicals to naphthalene. This reaction mechanism is shown to be an extension of the mechanism occurring in the H atomassisted conversion of fulvene to benzene. The net reaction involves the formation of dihydrofulvalene, which eliminates a hydrogen atom and then rearranges to form naphthalene through a series of ring closures and openings. The importance of forming the -CR(·)-CHR-CR′=CR″- moiety, which can undergo rearrangement to form three-carbon atom ring structures, is illustrated with the C4H7 system. The ability of hydrogen atoms to migrate around the cyclopentadienyl moiety is illustrated both for methyl-cyclopentadiene, C5H5CH3, and dihydrofulvalene, C5H5C5H5, as well as for their radical species, C6H7 and C5H5C5H4. The mobility of hydrogen in the cyclopentadienyl moiety plays an important role both in providing resonance-stabilized radical products and in creating the -CR(·) CHR-CR′=CR″- moiety for ring formation. The results illustrate the radical pathway for converting five-membered rings to aromatic six-membered rings. Furthermore, the results indicate the important catalytic role of H atoms in the aromatic ring formation process.

The potential energy surface of the HO2 molecular system

Abstract The potential energy surface for the non-linear H + O 2 → HO * 2 — OH + O reaction has been calculated using the multi-configuration self-consistent-field configuration interaction (MC SCF Cl) method. The resulting HO 2 surface has a small barrier (⪅0.1 eV) on the incoming channel at an H—O—O angle of ≈ 120°. The outgoing channel (endothermic) does not have an activation barrier. An analytic form for the potential energy surface is presented for use in molecular scattering.

Aromatic and polycyclic aromatic hydrocarbon formation in a premixed propane flame

Experimental and detailed chemical kinetic modeling has been performed to investigate aromatic and polycyclic aromatic hydrocarbon (PAH) formation pathways in a premixed, rich, sooting, propane-oxygen-argon burner stabilized flame. An atmospheric pressure, laminar flat flame operated at an equivalence ratio of 2.6 was used to acquire experimental data for model validation. Gas composition analysis was conducted by an on-line gas chromatograph/mass spectrometer (GC/MS) technique. Measurements were made in the main reaction and post-reaction zones for a number of low molecular weight species, aliphatics, aromatics, and polycyclic aromatic hydrocarbons (PAHs) ranging from two to five-fused aromatic rings Reaction flux and sensitivity analysis were used to help identify the important reaction sequences leading to aromatic and PAH growth and destruction in the propane flame. Benzene formation was shown to be dominated by the propargyl recombination reaction. A secondary benzene formation pathway occurred from ...

Fe and Ni AB initio effective potentials for use in molecular calculations

Abstract We present effective potentials to replace the Ar core electrons of Fe and Ni. These effective potentials are obtained from ab initio ground state wavefunctions of Fe and Ni and are tested by comparing with ab initio SCF calculations for excited states of Fe, Fe + , Fe 2+ , Fe 3+ , Ni, Ni + , Ni 2+ , and the FeH + molecule.

Experimental and modeling investigation of aromatic and polycyclic aromatic hydrocarbon formation in a premixed ethylene flame

Experimental and detailed chemical kinetic modeling has been performed to investigate aromatic and polyaromatic hydrocarbon formation pathways in a rich, sooting, ethylene-oxygen-argon premixed flame. An atmospheric pressure, laminar flat flame operated at an equivalence ratio of 2.5 was used to acquire experimental data for model validation. Gas composition analysis was conducted by an on-line gas chromatograph/mass spectrometer (GC/MS) technique. Measurements were made in the flame and post-flame zone for a number of low molecular weight species, aliphatics, aromatics and polycyclic aromatic hydrocarbons (PAHs) ranging from two to five-aromatic fused rings. The modeling results show the key reaction sequences leading to aromatic and polycyclic aromatic hydrocarbon growth involve the combination of resonantly stabilized radicals. In particular, propargyl and 1-methylallenyl combination reactions lead to benzene and methyl substituted benzene formation, while polycyclic aromatics are formed from cyclopentadienyl radicals and fused rings that have a shared C{sub 5} side structure. Naphthalene production through the reaction step of cyclopentadienyl self-combination and phenanthrene formation from indenyl and cyclopentadienyl combination were shown to be important in the flame modeling study. The removal of phenyl by O{sub 2} leading to cyclopentadienyl formation is expected to play a pivotal role in the PAH or soot precursor growth process under fuel-rich oxidation conditions.

Theoretical characterization of the minimum energy path for the reaction H+O2→HO2*→HO+O

The potential energy surface for the reaction H+O2→HO*2 →HO+O has been characterized in the vicinity of the minimum energy path using CASSCF/contracted CI calculations with a basis set which is triple zeta valence quality plus three sets of polarization functions. CASSCF/CI calculations were carried out along the CCI minimum energy path. The latter calculation shows essentially no barrier for addition of an H atom to O2, in agreement with predictions made in earlier studies. The potential surface for recombination of OH and O is complicated by a crossing, at rOO ≈5.5a0, between the surface for electrostatic (OH dipole–O quadrupole) interaction and that for the formation of an O–O chemical bond. This surface crossing results in a small (≈0.5 kcal/mol) barrier.

High‐temperature photochemistry and BAC‐MP4 studies of the reaction between ground‐state H atoms and N2O

The H+N2O reaction has been investigated using the high‐temperature photochemistry (HTP) technique. H(1 2S) atoms were generated by flash photolysis of NH3 and monitored by time‐resolved atomic resonance fluorescence with pulse counting. The bimolecular rate coefficient for H‐atom consumption, leading essentially to N2+OH, from 390 to 1310 K is found to be given by k1(T)=5.5×10−14 exp(−2380 K/T)+7.3×10−10 exp(−9690 K/T) cm3 molecule−1 s−1; the accuracy is assessed as approximately 25% at the 2σ confidence level. Above 750 K, k1 closely follows the Arrhenius behavior of the second term alone. Distinct curvature is evident below 750 K. k1 is compared to theoretical BAC‐MP4 predictions and good agreement is found for a model involving rearrangement of an HNNO intermediate coupled with tunneling through an Eckart potential barrier, which dominates at the lower temperatures. The branching ratio for the channel leading to NH+NO is discussed in the context of recent thermochemical information and a maximum rate ...

Electronic properties of metal clusters (Ni13 to Ni87) and implications for chemisorption

Abstract First principles calculations of the electronic properties of Ni clusters (up to Ni 87 ) are reported. It is found that the ionization potential (IP) converges to bulk values (work function) by Ni 43 , whereas the electron affinity (EA) is off by 2.5 eV, even for Ni 87 . The conduction band of ∼16 eV appears converged by Ni 87 . It is found that the electron density for surface atoms is significantly lower than the bulk value. The significance of these results for chemisorption on small metallic clusters and for modelling of chemisorption on bulk surfaces is discussed.