Canonical Variational Transition State Theory Research Articles

The Homogeneous Gas-Phase Formation Mechanism of PCNs from Cross-Condensation of Phenoxy Radical with 2-CPR and 3-CPR: A Theoretical Mechanistic and Kinetic Study.

Chlorophenols (CPs) and phenol are abundant in thermal and combustion procedures, such as stack gas production, industrial incinerators, metal reclamation, etc., which are key precursors for the formation of polychlorinated naphthalenes (PCNs). CPs and phenol can react with H or OH radicals to form chlorophenoxy radicals (CPRs) and phenoxy radical (PhR). The self-condensation of CPRs or cross-condensation of PhR with CPRs is the initial and most important step for PCN formation. In this work, detailed thermodynamic and kinetic calculations were carried out to investigate the PCN formation mechanisms from PhR with 2-CPR/3-CPR. Several energetically advantageous formation pathways were obtained. The rate constants of key elementary steps were calculated over 600~1200 K using the canonical variational transition-state theory (CVT) with the small curvature tunneling (SCT) contribution method. The mechanisms were compared with the experimental observations and our previous works on the PCN formation from the self-condensation of 2-CPRs/3-CPRs. This study shows that naphthalene and 1-monochlorinated naphthalene (1-MCN) are the main PCN products from the cross-condensation of PhR with 2-CPR, and naphthalene and 2-monochlorinated naphthalene (2-MCN) are the main PCN products from the cross-condensation of PhR with 3-CPR. Pathways terminated with Cl elimination are preferred over those terminated with H elimination. PCN formation from the cross-condensation of PhR with 3-CPR can occur much easier than that from the cross-condensation of PhR with 2-CPR. This study, along with the study of PCN formation from the self-condensation 2-CPRs/3-CPRs, can provide reasonable explanations for the experimental observations that the formation potential of naphthalene is larger than that of 1-MCN using 2-CP as a precursor, and an almost equal yield of 1-MCN and 2-MCN can be produced with 3-CP as a precursor.

QM/MM Study of the H2 Formation on the Surface of a Water Ice Grain Doped With Formaldehyde: Molecular Dynamics and Reaction Kinetics

Formaldehyde has been widely observed in the icy mantle of interstellar grains. H2CO may be formed from successive hydrogenations of CO and may further contribute to the chemical complexity of the Interstellar medium (ISM) participating to heterogeneous reactions with colliding gas phase atoms. Within this context, Eley-Rideal and Langmuir-Hinshelwood rate constants of H2 formation on a formaldehyde doped amorphous water ice grain model of the ISM, were computed over a wide temperature range [15–2000 K]. We used classical molecular dynamics (MD) simulations to build the model of the H2CO doped ice surface. Then we studied theoretically by means of hybrid QM/MM ab initio and molecular mechanics methodology (ONIOM) H atoms abstraction from formaldehyde molecules and the H2 formation. Specifically, we investigate the reactivity of the gas phase H atom toward one formaldehyde molecule lying at one of the slab surfaces. The reaction path and the energetics are predicted, the mechanism is found to be exothermic by 14.89 kcal/mol and the barrier is 6.75 kcal/mol at the QM level CBS/DLPNO-CCSD(T)//ONIOM/aug-cc-pVTZ. We employ two approaches that take into account tunnelling and non-classical reflection effects by means of the Zero Curvature Tunnelling (ZCT), and the Small Curvature Tunnelling (SCT) which all provided comparable results to predict the kinetics of the reaction path. The rate constants show important quantum tunnelling effects at low temperatures when compared to rates obtained from the purely classical transition-state theory (TST) and from the canonical variational transition state theory (CVT). Corner cutting effects are highlighted in the SCT calculations by 4 to 5 orders of magnitude with respect to ZCT rate constants at low temperatures.

Large Pressure Effects Caused by Internal Rotation in the s-cis-syn-Acrolein Stabilized Criegee Intermediate at Tropospheric Temperature and Pressure.

Criegee intermediates are important atmospheric oxidants, and quantitative kinetics for stabilized Criegee intermediates are key parameters for atmospheric modeling but are still limited. Here we report barriers and rate constants for unimolecular reactions of s-cis-syn-acrolein oxide (scsAO), in which the vinyl group makes it a prototype for Criegee intermediates produced in the ozonolysis of isoprene. We find that the MN15-L and M06-2X density functionals have CCSD(T)/CBS accuracy for the unimolecular cyclization and stereoisomerization of scsAO. We calculated high-pressure-limit rate constants by the dual-level strategy that combines (a) high-level wave function-based conventional transition-state theory (which includes coupled-cluster calculations with quasiperturbative inclusion of quadruple excitations because of the strongly multiconfigurational character of the electronic wave function) and (b) canonical variational transition-state theory with small-curvature tunneling based on a validated density functional. We calculated pressure-dependent rate constants both by system-specific quantum Rice-Ramsperger-Kassel theory and by solving the master equation. We report rate constants for unimolecular reactions of scsAO over the full range of atmospheric temperature and pressure. We found that the unimolecular reaction rates of this larger-than-previously studied Criegee intermediate depend significantly on pressure. Particularly, we found that falloff effects decrease the effective unimolecular cyclization rate constant of scsAO by about a factor of 3, but the unimolecular reaction is still the dominant atmospheric sink for scsAO at low altitudes. The large falloff caused by the inclusion of the stereoisomerization channel in the master equation calculations has broad implications for mechanistic analysis of reactions with competitive internal rotations that can produce stable rotamers.

Heavy-Atom Tunneling in the Covalent/Dative Bond Complexation of Cyclo[18]carbon-Piperidine.

Recent quantum chemical computations demonstrated the electron-acceptance behavior of this highly reactive cyclo[18]carbon (C18) ring with piperidine (pip). The C18–pip complexation exhibited a double-well potential along the N–C reaction coordinate, forming a van der Waals (vdW) adduct and a more stable, strong covalent/dative bond (DB) complex by overcoming a low activation barrier. By means of direct dynamical computations using canonical variational transition state theory (CVT), including the small-curvature tunneling (SCT), we show the conspicuous role of heavy atom quantum mechanical tunneling (QMT) in the transformation of vdW to DB complex in the solvent phase near absolute zero. Below 50 K, the reaction is entirely driven by QMT, while at 30 K, the QMT rate is too rapid (kT ∼ 0.02 s–1), corresponding to a half-life time of 38 s, indicating that the vdW adduct will have a fleeting existence. We also explored the QMT rates of other cyclo[n]carbon–pip systems. This study sheds light on the decisive role of QMT in the covalent/DB formation of the C18–pip complex at cryogenic temperatures.

Experimental and Theoretical Studies of Trans-2-Pentenal Atmospheric Ozonolysis

We investigated the kinetics, mechanism and secondary organic aerosols formation of the ozonolysis of trans-2-pentenal (T2P) using four different reactors with Fourier Transform InfraRed (FTIR) spectroscopy and Gas Chromatography (GC) techniques at T = 298 ± 2 K and 760 Torr in dry conditions. The rate coefficients and branching ratios were also evaluated using the canonical variational transition (CVT) state theory coupled with small curvature tunneling (CVT/SCT) in the range 278–350 K. The experimental rate coefficient at 298 K was (1.46 ± 0.17) × 10−18 cm3 molecule−1 s−1, in good agreement with the theoretical rate. The two primary carbonyls formation yields, glyoxal and propanal, were 57 ± 10% and 42 ± 12%, respectively, with OH scavenger compared to 38 ± 8% for glyoxal and 26 ± 5% for propanal without OH scavenger. Acetaldehyde and 2-hydroxypropanal were also identified and quantified with yields of 9 ± 3% and 5 ± 2%, respectively, in the presence of OH scavenger. For the OH production, an upper limit of 24% was estimated using mesitylene as OH tracer. Combining experimental and theoretical findings enabled the establishment of a chemical mechanism. Finally, the SOA formation was observed with mass yields of about 1.5%. This work provides additional information on the effect of the aldehyde functional group on the fragmentation of the primary ozonide.

Effects of Anharmonicity, Recrossing, Tunneling, and Pressure on the H-Abstractions from Dimethylamine by Triplets O and O2.

Rate constants of the H-abstraction reactions from dimethylamine (DMA) by triplets O and O2 are theoretically determined with the canonical variational transition-state theory (CVT). By comparing the barrier heights and reaction energies obtained from different density-functional theory methods to those computed from the gold-standard method CCSD(T)/CBS(T-Q), we identify the M08-HX/ma-TZVP method as the best with a mean unsigned deviation of 1.0 kcal mol-1. On the basis of the optimized geometries and frequencies with the selected method, the rate constants are calculated using the CVT method combined with the multistructural torsional anharmonicity and small-curvature tunnelling (MS-CVT/SCT) options in the temperature range 200-2000 K. The calculations show that OH and HO2 are mainly produced from the direct abstraction from the C-H bond. The multistructural torsional anharmonicity has a large contribution to the rate constants, and the effects of recrossing and tunneling at the N-site are more important than those at C-site. Additionally, given the formation of reactant complex between DMA and triplet O, the H-abstraction channel is not favored at high pressure. Our calculations with both the Polyrate and MESS codes agree with the reported data within the uncertainty.

The trans/cis ratio of formic (HCOOH) and thioformic (HC(O)SH) acids in the interstellar medium

Context. Observations of the different isomers of molecules in the interstellar medium (ISM) have revealed that both low- and high-energy isomers can be present in space despite the low temperature conditions. It has been shown that the presence of these isomers may be due to tunneling effects. Aims. We carried out a theoretical study of the cis–trans isomerization reactions of two astrophysically relevant acids, formic acid (HCOOH) and thioformic acid (HC(O)SH), where the latter has recently been discovered in space. We also searched for these molecules towards the hot core G31.41+0.31 to compare their abundances with the expected theoretical isomerization results. Methods. We employed high-level ab initio calculations to study the reaction rate constants of the isomerization reactions. We used the canonical variational transition state theory with the multidimensional small curvature tunneling approximation in the temperature range of 10–400 K. Moreover, we used the spectrum obtained from the ALMA 3mm spectral survey GUAPOS (GUAPOS: G31 Unbiased ALMA sPectral Observational Survey), with a spectral resolution of ~0.488 MHz and an angular resolution of 1.′′2×1.′′2 (~4500 au), to derive column densities of HCOOH and HC(O)SH towards G31.41+0.31. Results. Our results demonstrate that these isomerizations are viable in the conditions of the ISM due to ground-state tunneling effects, which allow the system to reach the thermodynamic equilibrium at moderately low temperatures. At very low temperatures (Tkin ~ 10 K), the reaction rate constants for the cis-to-trans isomerizations are very small, which implies that the cis isomers should not be formed under cold ISM conditions. This is in disagreement with observations of the cis/trans isomers of HCOOH in cold cores where the cis isomer is found to be ~5–6% the trans isomer. At high temperatures (~150–300 K), our theoretical data not only match the observed behavior of the trans/cis abundance ratios for HCOOH (the cis form is undetected), but they support our tentative detection of the trans and – for the first time in the insterstellar medium – the cis isomer of HC(O)SH towards the hot molecular core G31.41+0.31 (with a measured trans/cis abundance ratio of ~3.7). Conclusions. While the trans/cis ratio for HC(O)SH in the ISM depends on the relative stability of the isomers, the trans/cis ratio for HCOOH cannot be explained by isomerization, and is determined by other competitive chemical processes.

On-The-Fly Kinetics of the Hydrogen Abstraction by Hydroperoxyl Radical: An Application of the Reaction Class Transition State Theory.

A Reaction Class Transition State Theory (RC-TST) is applied to calculate thermal rate constants for hydrogen abstraction by OOH radical from alkanes in the temperature range of 300–2500 K. The rate constants for the reference reaction C2H6 + ∙OOH → ∙C2H5 + H2O2, is obtained with the Canonical Variational Transition State Theory (CVT) augmented with the Small Curvature Tunneling (SCT) correction. The necessary parameters were obtained from M06-2X/aug-cc-pVTZ data for a training set of 24 reactions. Depending on the approximation employed, only the reaction energy or no additional parameters are needed to predict the RC-TST rates for other class representatives. Although each of the reactions can in principle be investigated at higher levels of theory, the approach provides a nearly equally reliable rate constant at a fraction of the cost needed for larger and higher level calculations. The systematic error is smaller than 50% in comparison with high level computations. Satisfactory agreement with literature data, augmented by the lack of necessity of tedious and time consuming transition state calculations, facilitated the seamless application of the proposed methodology to the Automated Reaction Mechanism Generators (ARMGs) programs.

The Mechanism and Kinetics Model of Degradation of Dicarboxylic Acids by Hydroxyl Radicals under Atmospheric Conditions.

The atmospheric degradation mechanism of dicarboxylic acids (DCAs) initiated by hydroxyl radicals has been theoretically investigated at the DLPNO-CCSD(T)/def2-TZVP//BH&HLYP/6-311++G(d,p) level of theory. In the presence of O2, the degradation of DCAs by hydroxyl radicals takes place through a two-step mechanism: the α-H elimination and the degradation of the peroxyl radical intermediate. The latter degradation mechanism is easy to proceed for the exothermic process of radical recombination. Therefore, the degradation rate of DCAs is determined by an α-H elimination step, which is accelerated in the case of long carbon-chain DCAs with a lower energy barrier. Canonical variational transition state theory has been employed to estimate the rate constants of the H-elimination step of the DCA degradation reaction by hydroxyl radicals over the temperature range of 220-1000 K.

Effect of a single water molecule on ˙CH2OH + 3O2 reaction under atmospheric and combustion conditions.

The hydroxymethyl (˙CH2OH) radical is an important intermediate species in both atmosphere and combustion reaction systems. The rate coefficients for ˙CH2OH + 3O2 and (˙CH2OH + 3O2 (+H2O)) reactions were calculated using the Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) simulation and canonical variational transition state theory (CVT) between the temperature range of 200 to 1500 K based on the potential energy surface constructed using CCSD(T)//ωB97XD/6-311++G(3df,3pd). The results show that ˙CH2OH + 3O2 leads to the formation of CH2O and HO2 at temperatures below 800 K, and goes back to reactants at high temperature (>1000 K). When a water molecule is added to the reaction, the formation of CH2O and HO2 is favored at all temperatures. The calculated rate coefficient for the ˙CH2OH + 3O2 (2.8 × 10-11 cm3 molecule-1 s-1 at 298 K) is in good agreement with the previous experimental values (∼1 × 10-11 cm3 molecule-1 s-1 at 298 K). The rate coefficients for the water-assisted reaction (2.4 × 10-16 cm3 molecule-1 s-1 at 1000 K) is at least 3-4 orders of magnitude smaller than the water-free reaction (6.2 × 10-12 cm3 molecule-1 s-1 at 1000 K). This result is consistent with the similar types of reaction system. Our calculations also predict that the effect of a single water molecule favors the formation of CH2O in the combustion condition. However, the water-free reaction favors the formation of CH2O in the atmospheric condition. The current study helps to understand how a single water molecule changes the reaction mechanism and chemical kinetic behaviour under atmospheric and combustion conditions.

CH2 + O2: reaction mechanism, biradical and zwitterionic character, and formation of CH2OO, the simplest Criegee intermediate.

The singlet and triplet potential surfaces for the title reaction were investigated using the CBS-QB3 level of theory. The wave functions for some species exhibited multireference character and required the CASPT2/6-31+G(d,p) and CASPT2/aug-cc-pVTZ levels of theory to obtain accurate relative energies. A Natural Bond Orbital Analysis showed that triplet 3CH2OO (the simplest Criegee intermediate) and 3CH2O2 (dioxirane) have mostly polar biradical character, while singlet 1CH2OO has some zwitterionic character and a planar structure. Canonical variational transition state theory (CVTST) and master equation simulations were used to analyze the reaction system. CVTST predicts that the rate constant for reaction of 1CH2 + 3O2 is more than ten times as fast as the reaction of 3CH2 (X3B1) + 3O2 and the ratio remains almost independent of temperature from 900 K to 3000 K. The master equation simulations predict that at low pressures the 1CH2O + 3O product set is dominant at all temperatures and the primary yield of OH radicals is negligible below 600 K, due to competition with other primary reactions in this complex system.

An integrated protocol to study hydrogen abstraction reactions by atomic hydrogen in flexible molecules: application to butanol isomers.

This work presents a protocol designed to study hydrogen abstraction reactions by atomic hydrogen in molecules with multiple conformations. The protocol starts with the search and location of the conformers of the equilibrium structures using the TorsiFlex program. By a simple modification of the starting geometry of reactants, a Python script generates the input for the hydrogen abstraction transition states. Initially, the search of the stationary points (reactants and transition states) is carried out at a low-level employing firstly a preconditioned search and secondly a random search. The low-level conformers were reoptimized using a higher level electronic structure method. This information allows the evaluation of the multistructural harmonic-oscillator partition functions, which are corrected for zero-point energy anharmonicity by the hybrid degeneracy-corrected second-order vibrational perturbation theory and for torsional anharmonicity by the multistructural torsional method, as implemented in the MsTor program. The structural information of the stationary points is used by Pilgrim to evaluate the multipath canonical variational transition state theory thermal rate constants with multidimensional small-curvature corrections for tunneling. Therefore, the thermal rate constants include variational (recrossing) and tunneling effects in addition to the effect of multiple conformations on the thermal rate constants. These features grant the applicability of the method to a wide range of temperatures. The method was applied to each of the hydrogen abstraction sites of the four isomers of butanol. The methodology employed allowed us to calculate the thermal rate constants in the temperature range of 250-2500 K and to accurately fit them to analytical expressions. The variety of abstraction sites shows that the protocol is robust and that it can be employed to study hydrogen abstraction reactions in molecules containing carbon and oxygen as heavy atoms.

DFT Calculations for the HONO Elimination Process of CL-20 Conformers

The HONO elimination process is regarded to be an important initial decomposition process of energetic nitramines. Four CL-20 conformers based on the ε-CL-20 were obtained by the optimization at the m062x/cc-pvtz level in this study, and the Transition State (TS) and Intrinsic Reaction Coordinate (IRC) calculations were carried out at the same level. In addition, the rate coefficients and activation energy of the HONO elimination process were evaluated using conventional transition state theory (TST) and canonical variational transition state theory (CVT) with Eckart and small-curvature tunneling (SCT) methods to correct the transmission coefficients for the quantum tunneling effect. The calculation results have shown that the HONO elimination process concerning the nitro groups located on six numbered rings is the hardest to happen, and it seems that the longer distance between nitro groups and the adjacent hydrogen atom would result in the higher barrier energy; the HONO elimination process is most likely to happen for the axial positioning of nitro groups located on five numbered rings and most unlikely to happen for the ones located on six numbered rings; CL-20 II and CL-20 IV conformers are the most unstable one and most stable one concerning the reaction difficulty of the HONO elimination process.

Temperature dependent kinetics for the reaction between OH radicals and (E)- and (Z)- CHF = CHCl: A dual-level computational study

The temperature dependent kinetics for the reactions between (E)/(Z)- CHF = CHCl and OH radicals were studied using CCSD(T)/cc-pVTZ//M06-2X/6–31 + G(d,p) level of theory. The rate coefficients of the studied reaction paths were calculated by Canonical Variational Transition state theory followed by small curvature tunnelling corrections from 200 to 400 K. Various reaction pathways involved in both the reactions were exothermic and spontaneous. The Arrhenius expressions for the title reactions were obtained as k(E)-CHF=CHCl = 5.73 × 10-19 T2.44 exp (622.1/T) and k(Z)-CHF=CHCl = 2.22 × 10-18 T2.17 exp (803.6/T) cm3molecule-1s−1 respectively.

Theoretical study of mechanisms and kinetics of reactions of the O(3P) atom with alkyl hydroperoxides (ROOH) where (R = CH3 & C2H5)

An analysis of O(3P) reaction mechanisms and kinetics with CH3OOH and CH3CH2OOH has been examined. Potential energy diagrams for the named reactions have been evaluated at the W1 and CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ levels of theory. For the two reactions, a total of seven abstraction channels have been described. The reaction enthalpies and standard reaction Gibbs free energy are calculated at the W1 level. The kinetic study is conducted within the temperature range of 210–350 K using transition state theory (TST), TST with Wigner correction (TST/W), and TST with zero curvature tunneling (TST/ZCT). The rate coefficients were also calculated using canonical variational transition state theory (CVT), CVT with zero curvature tunneling (CVT/ZCT), and CVT with small curvature tunneling (CVT/SCT). Branching ratios have been computed using the CVT/SCT rate coefficients. Hydrogen abstraction from the hydroxyl group is identified as a significant pathway in both reactions.

Theoretical studies of the reactions of 1,2-Ethanediol with triplet oxygen and hydrogen atoms

An direct kinetics study of the (CH2OH)2 + O(3P) and (CH2OH)2 + H reactions was performed over the temperature range of 300–1500 K. The geometries of all stationary points were optimised at the M06-2X/cc-pVQZ level of theory. The electronic energies were refined by CCSD(T)/CBS and QCISD(T)/cc-pVTZ methods. The rate coefficients were calculated using canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) correction. The results showed that the channels of hydrogen abstraction from the CH2 group were most kinetically favourable at the temperature less than 1300 K and have positive temperature dependence. The OH-transfer channels were ignorable.

Theoretical investigations on the OH radical mediated kinetics of cis- and trans-CH3CF=CHF and CH3CH=CF2 over temperature range of 200-400K

Global reactivity for the OH-radical mediated kinetics of three Hydrofluoroolefins (HFOs), cis-CH3CF=CHF, trans-CH3CF=CHF and CH3CH=CF2 was evaluated at the CCSD(T)/cc-pVTZ//M062x/6-31G+(d,p) level of theory. The overall kinetics was computed using the Canonical Variational Transition State Theory (CVT) in combination with Small Curvature Tunneling (SCT) and Interpolated Single Point Energy (ISPE) corrections over the temperature range of 200 and 400 K. The total rate coefficients (k, in units of cm3 molecule−1 sec−1) for the reaction of OH radical with these HFOs at 298 K were computed to be, k298Kcis−CHF=CFCH3+OH=1.69×10−12, k298Ktrans−CHF=CFCH3+OH=3.12×10−12, and k298KCF2=CHCH3+OH=5.40×10−12. To further corroborate the computed kinetics, thermodynamic parameters and branching ratios were also computed for all the reaction pathways. Atmospheric lifetimes, Radiative forcing (RF), Global Warming Potentials (GWP) and Photochemical Ozone Creation Potentials (POCP) of these HFOs were also calculated and are presented in this manuscript.

Theoretical study of the reactions of triplet oxygen atom with 1-propanol and 2-propanol

The reactions of triplet oxygen atom (O(3P)) with 1-Propanol and 2--Propanol were characterized at CCSD(T)/CBS//M06-2X/cc-pVTZ level of theory. The rate coefficients were calculated by the canonical variational transition state theory with the small curvature tunneling correction. The calculated total rate coefficients agreed with the available reference values. The dominant channel in the 1-C3H7OH + O(3P) reaction is hydrogen-abstraction from the α-CH2 group at temperature less 450 K or hydrogen-abstraction from β-CH2 groups at temperatures higher than 500 K; the dominant channel in the 2-C3H7OH + O(3P) reaction is hydrogen-abstraction from the CH group at temperature less than 1200 K.

Kinetics of the Reactions of Ozone with Halogen Atoms in the Stratosphere

It is well established that reaction cycles involving inorganic halogens contribute to the depletion of ozone in the atmosphere. Here, the kinetics of O3 with halogen atoms (Cl, Br, and I) were investigated between 180 and 400 K, expanding the temperature range relative to prior studies. Canonical variational transition state theory including small curvature tunneling correction (CVT/SCT) were considered, following the construction of the potential energy surfaces. MRCI + Q/aug-ano-pVTZ//MP2/aug-cc-pV(T + d)Z and MRCI + Q/aug-ano-RCC-VTZP//MP2/aug-cc-pV(T + d)Z levels of theory were used to calculate the kinetic parameters. Calculated rate coefficients were used to fit the Arrhenius equations, which are obtained to be k1 = (3.48 ± 0.4) × 10−11 exp[(−301 ± 64)/T] cm3 molecule−1 s−1, k2 = (3.54 ± 0.2) × 10−11 exp[(−990 ± 35)/T] cm3 molecule−1 s−1 and k3 = (1.47 ± 0.1) × 10−11 exp[(−720 ± 42)/T] cm3 molecule−1 s−1 for the reactions of O3 with Cl, Br, and I atoms, respectively. The obtained rate coefficients for the reactions of O3 with halogen atoms using CVT/SCT are compared to the latest recommended rate coefficients by the NASA/JPL and IUPAC evaluations. The reactivity trends and pathways of these reactions are discussed.

DFT study on the interactions between HCN and NO over char

A noteworthy part of HCN conversion is in reference to its potential behavior in NO reduction. In this work, a comprehensive density functional theory (DFT) study was carried out to obtain new insight into the mechanisms of HCNNO reaction on the char surface. A model modified with the hydroxyl group contributes to a better understanding of NO reduction with HCN oxidation. The adsorption modes are found to influence the adsorption energies and reaction paths significantly. For single adsorption, the HCN adsorption in dual-site mode is the most thermally favorable. The co-adsorption of HCN and NO is dramatically more significant than the sum of the corresponding single adsorption, mainly due to the reaction of adsorbates on the surface. In addition, the microscopic reaction paths for CO and N2 formation are summarized based on the most energetically stable adsorption and co-adsorption structures separately. The canonical variational transition-state theory (CVT) is adopted here to provide fruitful kinetic implications in the temperature range of 500–1800 K. The analysis indicates that the occurrence of HCN oxidation decreases the reaction energy barriers evidently, which is also responsible for the rates increasing. The co-adsorption mechanism is favorable for NO conversion, especially for the release of CO. The findings from this work are beneficial for better understanding and illustrating the ultra-low NOx emission processes. The co-adsorption mechanism that was always neglected before is strongly suggested in future computational studies.