Ethanol reforming on Co(0001) surfaces: a density functional theory study

A computational study using density functional theory is carried out to investigate the reaction mechanism of ethanol steam reforming on Co(0001) surfaces. The adsorption properties of the reactant, possible intermediates, and products are carefully examined. The reaction pathway and related transition states are also analyzed. According to our calculations, the reforming mechanism primarily consisting of dehydrogenation steps of ethanol, ethoxy, methanol, methoxy, and formic acid, is feasible on Co(0001) surfaces. It is also found that the reaction of formaldehyde yielding formic acid and hydrogen may not be an elementary reaction. The dehydrogenation of ethoxy possesses the highest barrier and is accordingly identified as the rate-determining step.     

Acrolein hydrogenation on Pt(211) and Au(211) surfaces: a density functional theory study

Partial hydrogenation of acrolein, the simplest α,β-unsaturated aldehyde, is not only a model system to understand the selectivity in heterogeneous catalysis, but also technologically an important reaction. In this work, the reaction on Pt(211) and Au(211) surfaces is thoroughly investigated using density functional theory calculations. The formation routes of three partial hydrogenation products, namely propenol, propanal and enol, on both metals are studied. It is found that the pathway to produce enol is kinetically favoured on Pt while on Au the route of forming propenol is preferred. Our calculations also show that the propanal formation follows an indirect pathway on Pt(211). An energy decomposition method to analyze the barrier is utilized to understand the selectivities at Pt(211) and Au(211), which reveals that the interaction energies between the reactants involved in the transition states play a key role in determining the selectivity difference.     

A density functional theory study of atomic oxygen and nitrogen adsorption over alpha-alumina (0001)

The interaction of atomic oxygen and nitrogen on the (0001) surface of corundum (alpha-alumina) is investigated from first-principles by means of periodic density functional calculations within the generalized gradient approximation. A large Al(2)O(3) slab model (18 layers relaxing 10) ended with the most stable aluminium layer is used throughout the study. Geometries, adsorption energies and vibrational frequencies are calculated for several stationary points for two spin states at different sites over an 1 x 1 unit cell. Two stable adsorption minima over Al or in a bridge between Al and O surface atoms are found for oxygen and nitrogen, without activation energies. The oxygen adsorption (e.g., E(ad) = 2.30 eV) seems to be much more important than for nitrogen (e.g., E(ad) = 1.23 eV). Transition states for oxygen surface diffusion are characterized and present not very high-energy barriers. The computed geometries and adsorption energies are consistent with similar adsorption theoretical studies and related experimental data for O, N or alpha-alumina. The present results along with our previous results for beta-cristobalite do not support the assumption of an equal E(ad) for O and N over similar oxides, which is commonly used in some kinetic models to derive catalytic atomic recombination coefficients for atomic oxygen and nitrogen. The magnitude of O and N adsorption energies imply that Eley-Rideal and Langmuir-Hinshelwood reactions with these species will be exothermic, contrary to what happens for beta-cristobalite.     

NO adsorption on MoS(x) clusters: a density functional theory study

The density functional theory (DFT) method has been used to investigate NO probe molecule adsorption on the stoichiometric (Mo(16)S(32)) and nonstoichiometric (Mo(16)S(34) and Mo(16)S(29)) clusters. The calculated adsorption energies indicate that the stoichiometric cluster has stronger NO affinity than the nonstoichiometric surfaces. It is also found that mononitrosyl adsorption is favored at low NO coverage, while dinitrosyl (germinal) and (NO)(2) dimer adsorption at high NO coverage are possible. Strong repulsive interaction has been found for the adsorbed dinitrosyl and (NO)(2) dimer species. In addition, the computed NO stretching frequencies for the mononitrosyl and dinitrosyl species agree well with the experimental data, while those of the dimer species are much lower than the suggested experimental data.     

Total oxidation of methanol on Cu(110): a density functional theory study

The partial and total oxidation of methanol on clean and oxygen-precovered Cu(110) has been studied by periodic density functional theory calculations within the generalized gradient approximation. Reaction paths including the geometry and the energetics of several reaction intermediates and the activation barriers between them have been determined, thus creating a complete scheme for methanol oxidation on copper. The calculations demonstrate that the specific structure of oxygen on copper plays an important role in both the partial and the total oxidation of methanol. For lower oxygen concentrations on the surface, the partial oxidation of methanol to formaldehyde is promoted by the presence of oxygen on the surface through the removal of hydrogen in the form of water, which prevents the recombinative desorption of methanol. At larger oxygen concentrations, the presence of isolated oxygen atoms reduces the C-H bond breaking barrier of adsorbed methoxy considerably, thus accelerating the formation of formaldehyde. Furthermore, oxygen also promotes the formation of dioxymethylene from formaldehyde, which then easily decays to formate. Formate is the most stable reaction intermediate in the total oxidation. Thus the formate decomposition represents the rate-limiting step in the total oxidation of methanol on copper.     

Oxidation of carbon monoxide on Rh(111): a density functional theory study

The adsorption of CO on the oxygen precovered Rh(111) and the oxidation of CO are investigated with density functional theory. The preferred adsorption site and the possible reaction channel are determined. The experimentally observed inertness of the (2 x 2)-O-CO structure is explained by the reaction barrier and the stability of the reactants.     

Interplay of hydrogenation and dehydrogenation in isoindoline and indoline isomers: a density functional theory study

Under the conditions of palladium-catalyzed formate reduction, the isomers isoindoline and indoline undergo distinct hydrogenation and dehydrogenation processes to form 4,5,6,7-tetrahydroisoindole and indole, respectively. In terms of resonance energy, the reduction of isoindoline is accompanied by a loss of aromaticity, whereas the dehydrogenation of indoline occurs with a gain in aromaticity. To rationalize why isoindoline and indoline, under the same conditions of palladium catalysis, form different products, we used density functional theory calculations to investigate the mechanisms of the two reaction pathways. Both processes are initiated through direct oxidative insertion (OxIn) of Pd(0) into the aliphatic C-H bond at the methylene group, followed by beta-hydride elimination to form the isoindole and indole. Because isoindole is much less stable relative to indole, it undergoes further hydrogenation on its benzene moiety to form the final product, 4,5,6,7-tetrahydroisoindole. Our theoretical findings rationalize the experimental observations.     

四唑互变异构反应的密度泛函理论(DFT)研究

运用11种密度泛函理论方法对四唑互变异构反应进行了计算研究。结果表明,B3LYP-DFT法与从头算的优化几何和能量最为吻合;在6-31^*基组下B3LYP计算的IR频率与MP2/6-311G^*^*计算结果相差很小;用未经校正的B3LYP计算频率求得的产物(2H-四唑)的热力学性质与实测结果也完全一致;由此推荐B3LYP-DFT法适合于对四唑化合物作系统研究。     

Substituent effects on benzdiyne: a density functional theory study

Density functional theory (DFT) studies were performed to investigate the effect of substituents on the properties of benzdiyne derivatives. Twelve substituted benzdiynes-C(6)X(2), where X = F, Cl, Br, Me, CF(3), CN, OH, NO(2), NH(2), OMe, NMe(2), and Ph-were considered along with the unsubstituted 1,4-benzdiyne. The structures, vibrational frequencies, and IR intensities of these benzdiynes were studied with a popular three-parameter hybrid density functional (B3LYP) combined with the split-valence 6-31G(d) basis set and Dunning's correlation-consistent polarized triple-zeta (cc-pVTZ) basis set. The relative stabilities of the substituted benzdiynes were studied with the help of reaction energies of isodesmic reactions, which showed that the electron-withdrawing groups destabilized the benzdiynes more than they did the corresponding benzenes, whereas the electron-donating groups stabilized the benzdiynes more than they did their benzene counterparts. Correlation analyses revealed that field/inductive effects played a more important role than did resonance effects. The changes in atomic charges and spin populations due to the substituents were also studied. The asymmetric nu(Ctbd1;C) stretching modes obtained were close to the 1500-cm(-)(1) mark. Reinvestigation of the experimental results supported these results; a weak IR band at 1486 cm(-)(1) was assigned to this asymmetric stretching mode in C(6)(CF(3))(2) F. Some other benzdiynes also had large IR intensity values for their asymmetric nu(Ctbd1;C) vibrational modes due to the coupling with other vibrational modes. Heats of formation for the substituted benzdiynes were obtained from the reaction energies calculated at the B3LYP/cc-pVTZ level of theory.     

Catalyzing aldehyde hydrosilylation with a molybdenum(VI) complex: a density functional theory study

[MoCl(2)O(2)] catalyzes the hydrosilylation reaction of aldehydes and ketones, as well as the reduction of other related groups, in apparent contrast to its known behavior as an oxidation catalyst. In this work, the mechanism of this reaction is studied by means of density functional theory calculations using the B3LYP functional complemented by experimental data. We found that the most favorable pathway to the first step, the Si--H activation, is a [2+2] addition to the Mo=O bond, in agreement with previous and related work. The stable intermediate that results is a distorted-square-pyramidal hydride complex. In the following step, the aldehyde approaches this species and coordinates weakly through the oxygen atom. Two alternative pathways can be envisaged: the classical reduction, in which a hydrogen atom migrates to the carbon atom to form an alkoxide, which then proceeds to generate the final silyl ether, or a concerted mechanism involving migration of a hydrogen atom to a carbon atom and of a silyl group to an oxygen atom to generate the silyl ether weakly bound to the molybdenum atom. In this Mo(VI) system, the gas-phase free energies of activation for both approaches are very similar, but if solvent effects are taken into account and HSiMe(3) is used as a source of silicon, the classical mechanism is favored. Several unexpected results led us to search for still another route, namely a radical path. The energy involved in this and the classical pathway are similar, which suggests that hydrosilylation of aldehydes and ketones catalyzed by [MoCl(2)O(2)] in acetonitrile may follow a radical pathway, in agreement with experimental results.     

Pt adsorption on the PbTiO3(110) polar surface: a density functional theory study

The monolayer (ML) and submonolayer Pt on both terminations of PbTiO₃(110) polar surface have been studied by using density functional theory (DFT) with projector‐augmented wave(PAW) potential and a supercell approach. The most favored ML Pt arrangements on PbTiO and O₂ terminations are the hollow site and the short‐bridge site, respectively. By examining the geometries of different ML arrangements, we know that the dominant impetus for stability of the favored adsorption site for PbTiO termination is the Pt–Ti interaction (mainly from covalent bonding), while that for O₂ termination is the Pt–O interaction (mainly from ionic bonding). In addition, the appearance of the gap electronic states in the outermost layers of each termination indicates that a channel for charge transfer between adsorbed layer and substrate is formed. Moreover, the interface hybridization between Pt 5d and O 2p orbitals is also observed, especially for ML Pt on O₂ termination. The stability sequences for various arrangements of 1/2 ML Pt adsorption conform well with those of ML Pt adsorption, and the most stable arrangement is energetically more favorable than the corresponding ML coverage in the view of adsorption energy maximization. The behavior, i.e. the increase in adsorption energy with decrease in coverage, indicates that Pt? Pt interactions weaken those between Pt and the substrate. Copyright © 2009 John Wiley & Sons, Ltd.     

CO2 adsorption on TiO2(101) anatase: a dispersion-corrected density functional theory study

Adsorption, diffusion, and dissociation of CO(2) on the anatase (101) surface were investigated using dispersion-corrected density functional theory. On the oxidized surface several different local minima were identified of which the most stable corresponds to a CO(2) molecule adsorbed at a five-fold coordinated Ti site in a tilted configuration. Surface diffusion is characterized by relatively small activation barriers. Preferential diffusion takes place along Ti rows and involves a cartwheel type of motion. The presence of a bridging oxygen defect or a surface interstitial Ti atom allows creation of several new strong binding configurations the most stable of which have bent CO(2) structures with simultaneous bonding to two surface Ti atoms. Subsurface oxygen vacancy or interstitial Ti defects are found to enhance the bonding of CO(2) molecules to the surface. CO(2) dissociation from these defect sites is calculated to be exothermic with barriers less than 21 kcal/mol. The use of such defects for catalytic activation of CO(2) on anatase (101) surface would require a mechanism for their regeneration.     

Density functional theory study of CHx (x=1-3) adsorption on clean and CO precovered Rh(111) surfaces

CH(x) (x=1-3) adsorptions on clean and CO precovered Rh(111) surfaces were studied by density functional theory calculations. It is found that CH(x) (x=1-3) radicals prefer threefold hollow sites on Rh(111) surfaces, and the bond strength between CH(x) and Rh(111) follows the order of CH(3)相似文献   

Multicoefficient density functional theory (MC-DFT)

We have systematically tested the performance of several pure and hybrid versions of density functional methods on different types of molecular energies by combining energies calculated using more than one basis sets. Most hybrid functionals show important performance improvement as compared to methods using only a single basis set. The results suggest that, in many cases, scaling the basis set corrections is also important for density functional theory calculation. The best method, the B1B95 functional using the cc-pVDZ/cc-pVTZ/aug-cc-pVDZ basis set combination, achieves an average accuracy of 1.76 kcal/mol on a database of 109 atomization energies, 38 hydrogen-transfer barrier heights, 38 non-hydrogen-transfer barrier heights, 13 ionization potentials, and 13 electron affinities.     

The oxo exchange reaction mechanism of americyl(VI): a density functional theory study

Journal of Radioanalytical and Nuclear Chemistry - This work concerned the oxo exchange of americyl(VI) ([AmO2]2+) in alkaline solution by means of B3LYP calculations. Four possible reaction...     

Theoretical study of hydrated copper(II) interactions with guanine: a computational density functional theory study

Optimization of the hydrated Cu(II)(N7-guanine) structures revealed a number of minima on the potential energy surface. For selected structures, energy decompositions together with the determination of electronic properties (partial charges and electron spin densities) were performed. In the complexes of guanine with the bare copper cation and that with the monoaqua ligated cation, an electron transfer from guanine to Cu(II) was observed, resulting in a Cu(I)-guanine(+) type of complex. Conformers with two aqua ligands are borderline systems characterized by a Cu partial charge of +0.7e and a similar value of the spin density (0.6e) localized on guanine. When tetracoordination of copper was achieved, only then the prevailing electron spin density is unambiguously localized on copper. The energetic preference of diaqua-Cu-(N7,O6-guanine) over triaqua-Cu-(N7-guanine) was found for the four-coordinate structures. However, the energy difference between these two conformations decreases with the number of water molecules present in the systems, and in complexes with five water molecules this preference is preserved only at DeltaG level where thermal and entropy terms are included.     

The reaction pathways for HSCH3 adsorption on Au(111): a density functional theory study

Density functional theory was used to investigate the reaction pathways for HSCH(3) adsorption on Au(111) at low coverage. A molecular adsorbed state was found with the S atom bond on Top sites (E approximately -0.38 eV) and molecular adsorption is nonactivated. The H-SCH(3) dissociation process is energetically less favorable and becomes slightly exothermic only when surface relaxation is considered (DeltaE approximately -0.2 eV). All the reaction pathways present a sizable activation energy barrier, with the lowest being approximately 0.52 eV (0.41 eV taking into account slab relaxation). In the corresponding saddle point of the potential energy surface, the S atom of the methylthiolate molecule is placed on Top sites and the H near a Bridge site. The high barrier obtained explains the complete absence of reactive methanethiol dissociation found in recent experiments.     

A density functional theory study on pelargonidin

A complete conformational analysis on the isolated and polarizable continuum model (PCM) modeled aqueous solution cation, quinonoidal, and anion forms of pelargonidin, comprising the diverse tautomers of the latter forms, was carried out at the B3LYP/6-31++G(d,p) level. The results indicate that the most stable conformer of cationic and quinonoidal forms of pelargonidin are completely planar in the gas phase, whereas that of the anionic form is not planar. In contrast, PCM calculations show that the plane of the B ring is slightly rotated with regard to the AC bicycle in the most stable conformer of the cation and quinonoidal form. The most stable conformers of the cation, both in gas phase and aqueous solution, display anti and syn orientations for, respectively, C2-C3-O-H and C6-C5-O-H dihedral angles, whereas syn and anti orientation of hydroxyls at 7 and 4' positions are nearly isoenergetic. The most stable tautomer of quinonoidal pelargonidin is obtained by deprotonating hydroxyl at C5 in gas phase but at C7 according to PCM. Also, the most stable tautomer of the anion is different in gas phase (hydrogens are abstracted from hydroxyls at C5 and C4') and PCM simulation (C3 and C5). Tautomeric equilibria affect substantially the geometries of the AC-B backbone providing bond length variations that basically agree with the predictions of the resonance model. Most of the conformers obtained display an intramolecular hydrogen bond between O3 and H6'. Nevertheless, this interaction is not present in the most stable anions. Ionization potentials and O-H bond dissociation energies computed for the most stable conformers of cation, quinonoidal, and anion forms are consistent with an important antioxidant activity.     

Mechanisms of difluoroethylene ozonolysis: a density functional theory study

We present a density functional theory (DFT) study on the mechanisms of gas-phase ozonolysis of three isomers of difluoroethylene, namely, cis-1,2-difluoroethylene, trans-1,2-difluoroethylene, and 1,1-difluoroethylene. MPW1K/cc-pVDZ and BHandHLYP/cc-pVDZ methods are employed to optimize the geometries of stationary points as well as the points on the minimum energy path (MEP). The energies of all the points were further refined at the QCISD(T)/cc-pVDZ and QCISD(T)/6-31+G(df,p) levels of theory with zero-point energy (ZPE) corrections. The ozone-cis-1,2-difluoroethylene reaction is predicted to be slower than the ozone-trans-1,2-difluoroethylene reaction. The enhanced reactivity of trans-1,2-difluoroethylene relative to the cis isomer is similar to the reactions of ozone with cis- and trans-dichloroethylene. The ozone-1,1-difluoroethylene reaction is predicted to be slower than the ozone-trans-1,2-difluoroethylene reaction. These results are in agreement with experimental studies. The calculated mechanisms indicate that in ozone-difluoroethylene reactions the yields of OH might be trivial, which is different from the reactions of ozone with unsaturated hydrocarbons.     

Experimental and theoretical characterization of Co(III) and Zn(II) complexes with phenol-based hexa-dentate macro-acyclic ligands: synthesis,density functional theory and time-dependent density functional theory studies

Macro-acyclic complexes with two dissimilar coordination sites: one includes six coordination set (N₄O₂) and the other a tetradentate donor set (N₂O₂) of the type [Co^IIIL^en]ClO₄ and [Co^IIIL^tn]ClO₄ (where L^en = N,N′-bis(pyridine-2-ylmethyl)-N,N′-bis(5-bromo-3-formyl-2-hydroxybenzyl)-1,2-diaminoethane and L^tn = N,N′-bis(pyridine-2-ylmethyl)-N,N′-bis(5-bromo-3-formyl-2-hydroxybenzyl)-1,3-diaminoepropane) have been synthesized and characterized. Characterization result revealed that in the synthetic procedure of [Co^IIIL^en]ClO₄ only an isomer (trans-pyridine/cis-phenolate) among three possible geometrical isomers is formed. However, synthesis of its counterpart, [Co^IIIL^tn]ClO₄ resulted in the formation of a mixture of geometrical isomers (trans-pyridine/cis-phenolate and cis-pyridine/cis-phenolate). The computational studies of these complexes demonstrated that in [Co^IIIL^tn]⁺ an equilibrium exists between two possible geometrical isomers as result of a small energy difference (0.72 kcal mol^?1), whereas in [Co^IIIL^en]⁺ the difference is (2.71 kcal mol^?1). Furthermore, [Zn^IIL^en] has been studied computationally and experimentally by IR, UV–Vis spectroscopy techniques. The results showed that it has a similar structure to [Co^IIIL^en]⁺ complex. Electronic spectra of the complexes were analyzed and the absorption bands were assigned through the density functional theory (DFT) and time-dependent density functional theory (TD–DFT) studies procedures. The molecular orbital diagrams of the complexes were also determined.