Lyudmila V. Moskaleva

The spin-conserved reaction CH+N2→H+NCN: A major pathway to prompt no studied by quantum/statistical theory calculations and kinetic modeling of rate constant

A new spin-conserved path for the CH(2H)+N2 reaction at temperatures relevant to prompt NO formation has been theoretically investigated by means of ab initio MO calculations at the G2M level of theory. The result of the calculation reveals that the CH(2H)+N2 reaction takes place primarily via the ground electronic doublet potential energy surface, producing H+NCN instead of the commonly assumed, spin-forbidden HCN+N(4S) products. The overall rate constant for NCN production has been computed by a multichamel canonical variational Rice-Ramsperger-Kassel-Marcus theory calculation for the temperature range 1500–4000 K at 0.5–2 atm pressure: k3=2.22×107 T1.48 exp (−11760/T) cm3/(mol·s). The theoretically predicted rate constant was found to be in good agreement with high-temperature shock tube data kinetically modeled with the new mechahism that includes NCN reactions. In addition, k, was also found to be consistent with the apparent rate constants previously modeled for prompt NO formation in several flamer studies.

Microscopic models of PdZn alloy catalysts: structure and reactivity in methanol decomposition

We review systematic experimental and theoretical efforts that explored formation, structure and reactivity of PdZn catalysts for methanol steam reforming, a material recently proposed to be superior to the industrially used Cu based catalysts. Experimentally, ordered surface alloys with a Pd : Zn ratio of approximately 1 : 1 were prepared by deposition of thin Zn layers on a Pd(111) surface and characterized by photoelectron spectroscopy and low-energy electron diffraction. The valence band spectrum of the PdZn alloy resembles closely the spectrum of Cu(111), in good agreement with the calculated density of states for a PdZn alloy of 1 : 1 stoichiometry. Among the issues studied with the help of density functional calculations are surface structure and stability of PdZn alloys and effects of Zn segregation in them, and the nature of the most likely water-related surface species present under the conditions of methanol steam reforming. Furthermore, a series of elementary reactions starting with the decomposition of methoxide, CH(3)O, along both C-H and C-O bond scission channels, on various surfaces of the 1 : 1 PdZn alloy [planar (111), (100) and stepped (221)] were quantified in detail thermodynamically and kinetically in comparison with the corresponding reactions on the surfaces Pd(111) and Cu(111). The overall surface reactivity of PdZn alloy was found to be similar to that of metallic Cu. Reactive methanol adsorption was also investigated by in situ X-ray photoelectron spectroscopy for pressures between 3 x 10(-8) and 0.3 mbar.

Unimolecular isomerization/decomposition of ortho-benzyne : ab initio MO/statistical theory study

High level molecular-orbital calculations have been carried out to investigate the potential energy surface for the o-benzyne decomposition to 1,3-butadiyne and acetylene as well as that for the isomerization sequence, ortho- to meta- to para-benzyne. The latter species can easily undergo Bergman decyclization. It is shown by statistical theory calculations that the isomerization channel may affect significantly the rate of o-benzyne disappearance in the thermal decomposition process, particularly, at T<2000 K. At 1000 K, the isomerization of o-C6H4 to its m- and p-isomers accounts for as much as 99% of the total disappearance rate. The first order rate coefficients for the production of 1,3-butadiyne, meta- and para-benzynes at 100 Torr, 1 atm and 10 atm pressures over the temperature range 1000–3000 K have been calculated for combustion applications.

Chapter 11 - Quantum chemistry with the Douglas-Kroll-Hess approach to relativistic density functional theory: Efficient methods for molecules and materials

We review the Douglas-Kroll-Hess (DKH) approach to relativistic density functional calculations for molecular systems, also in comparison with other two-component approaches and four-component relativistic quantum chemistry methods. The scalar relativistic variant of the DKH method of solving the Dirac-Kohn-Sham problem is an efficient procedure for treating compounds of heavy elements including such complex systems as transition metal clusters, adsorption complexes, and solvated actinide compounds. This method allows routine all-electron density functional calculations on heavy-element compounds and provides a reliable alternative to the popular approximate strategy based on relativistic effective core potentials. We discuss recent method development aimed at an efficient treatment of spin-orbit interaction in the DKH approach as well as calculations of g tensors. Comparison with results of four-component methods for small molecules reveals that, for many application problems, a two-component treatment of spin-orbit interaction can be competitive with these more precise procedures.

Ab Initio Chemical Kinetics for H + NCN: Prediction of NCN Heat of Formation and Reaction Product Branching via Doublet and Quartet Surfaces

The reaction of NCN with H atoms has been investigated by ab initio MO and RRKM theory calculations. The mechanisms for formation of major products on the doublet and quartet potential energy surfaces have been predicted at the CCSD(T) level of theory with the complete basis set limit. In addition, the heat of formation for NCN predicted at this rigorous level and those from five isogyric reactions are in close agreement with the best value based on the isodesmic process, (3)CCO + N2 = (3)NCN + CO, 109.4 kcal/mol, which lies within the two existing experimental values. The rate constants for the three possible reaction channels, H + NCN → CH + N2 (k(P1)), HCN + (4)N (k(QP1)), and HNC + (4)N (k(QP2)), have been calculated in the temperature range 298-3000 K. The results show that k(P1) is significantly higher than k(QP1) and k(QP2) and that the total rate constant agrees well with available experimental values in the whole temperature range studied. The kinetics of the reverse CH + N2 reaction has also been revisited at the CCSD(T)/CBS level; the predicted total rate constants at 760 Torr Ar pressure can be represented by kr = 4.01 × 10(-15) T(0.90) exp(-17.42 kcal mol(-1)/RT) cm(3) molecule(-1) s(-1) at T = 800-4000 K. The result agrees closely with the most recent experimental data and the best theoretical result of Harding et al. (J. Phys. Chem. A 2008, 112, 522) as well as that of Moskaleva and Lin (Proc. Combust. Inst. 2000, 28, 2393) evaluated with a steady-state approximation after a coding error correction made in this study.

MoB/g-C3N4 Interface Materials as a Schottky Catalyst to Boost Hydrogen Evolution

Proton adsorption on metallic catalysts is a prerequisite for efficient hydrogen evolution reaction (HER). However, tuning proton adsorption without perturbing metallicity remains a challenge. A Schottky catalyst based on metal-semiconductor junction principles is presented. With metallic MoB, the introduction of n-type semiconductive g-C3 N4 induces a vigorous charge transfer across the MoB/g-C3 N4 Schottky junction, and increases the local electron density in MoB surface, confirmed by multiple spectroscopic techniques. This Schottky catalyst exhibits a superior HER activity with a low Tafel slope of 46 mV dec-1 and a high exchange current density of 17 μA cm-2 , which is far better than that of pristine MoB. First-principle calculations reveal that the Schottky contact dramatically lowers the kinetic barriers of both proton adsorption and reduction coordinates, therefore benefiting surface hydrogen generation.

AB INITIO MO CALCULATIONS FOR THE REACTIONS OF NH2 WITH H2, H2O, NH3 AND CH4 : PREDICTION OF ABSOLUTE RATE CONSTANTS AND KINETIC ISOTOPE EFFECTS

Abstract Ab initio calculations for the NH 2 +H 2 , NH 2 +CH 4 , NH 2 +NH 3 , and NH 2 +H 2 O reactions are performed using the G2M method. Obtained energy barriers and molecular parameters are used for the TST calculations of absolute rate constants. The G2M method is shown to predict the activation energies within 2–3 kcal/mol. Adjustment of the calculated energy barrier by such an amount usually leads to good agreement between observed and predicted rate constants. Fitted three-parameter expressions for theoretical rate constants of the title reactions are obtained and recommended for practical applications. The calculations show significant isotope effects for the NH 2 +RH reactions. The primary isotope effect in the reactions of NH 2 with D 2 , CD 4 , ND 3 , and D 2 O is due to the absence of tunnelling and the increase of the quantum mechanical barriers because of differences in zero-point energies and results in the significant (up to 25 times) decrease of the reaction rates, especially, at low temperatures. The secondary isotope effect observed for the ND 2 reactions is opposite; the rate constants become slightly higher (up to two times at 300 K), since the quantum chemical barriers decrease.

The heat of formation of gaseous PuO22+ from relativistic density functional calculations

Using a set of model reactions, we estimated the heat of formation of gaseous PuO2(2+) from quantum-chemical reaction enthalpies and experimental heats of formation of reference species. To this end, we carried out relativistic density functional calculations on the molecules PuO(2)2+, PuO2, PuF6, and PuF4. We used a revised variant (PBEN) of the Perdew-Burke-Ernzerhof gradient-corrected exchange-correlation functional, and we accounted for spin-orbit interaction in a self-consistent fashion. As open-shell Pu species with two or more unpaired 5f electrons are involved, spin-orbit interaction significantly affects the energies of the model reactions. Our theoretical estimate for the heat of formation DeltafH degree 0(PuO2(2+),g), 418+/-15 kcal mol-1, evaluated using plutonium fluorides as references, is in good agreement with a recent experimental result, 413+/-16 kcal mol-1. The theoretical value connected to the experimental heat of formation of PuO2(g) has a notably higher uncertainty and therefore was not included in the final result.

Elastic polarizable environment cluster embedding approach for water adsorption on the α-Al2O3(0001) surface. A density functional study

Low coverage water adsorption on the α-Al2O3(0001) surface has been studied with a generalized-gradient density functional approach using embedded cluster and periodic slab methodologies. An advanced cluster embedding method in an elastic polarizable environment (EPE), which enables an accurate description of the adsorption-induced substrate relaxation, has been applied systematically at various density functional levels: PW91, BP, and PBEN. In addition, periodic slab model calculations based on the PW91 functional were carried out for varying surface supercell sizes, (2 × 2) and (3 × 3), which compare very well with the corresponding embedded-cluster results. In agreement with two recent studies employing integrated MO + MO (IMOMO) embedded cluster and periodic Car–Parrinello BLYP methodologies, our calculations predict the 1,2-dissociative adsorption to be about 10 kcal mol−1 more favorable than molecular adsorption; however, at variance with the latter study, we predict 1,4-dissociative adsorption to be least favorable. Analysis of adsorbate-induced relaxation renders the interaction energy with the unrelaxed substrate in the 1,4-dissociative case negative (unbound complex), thus rationalizing the smallest (by absolute value) interaction energy. Our best estimates for binding energies, at the PBEN level, for molecular, 1,2-dissociative, and 1,4-dissociative adsorption are −22.5, −31.2, and −17.2 kcal mol−1, respectively.

Computational study of the kinetics and mechanisms for the reaction of H atoms with c-C5H6

The H+ c -C 5 H 6 reaction has been studied at the modified Gaussian-2 level of theory. Statistical theory calculations have been performed using a theoretical potential energy surface and molecular parameters. It has been shown that the reaction can occur by both the abstraction and the addition mechanisms. Theoretical rate constants have been calculated for the low-lying product channels and relative contributions of the addition and abstraction channels have been assessed. The previously neglected role of openchain intermediates in the evaluation of the reaction kinetics has been suggested and the corresponding rate constants have been recommended for inclusion in the modeling of the H+ c -C 5 H 6 reaction.