Activation Barrier For Dissociation Research Articles

Pt3Zr alloy as a protective coating against oxidation and hydrogen attack on Zr-based components in nuclear reactors

The adsorption of intact and dissociated water molecules on the surfaces of the Pt3Zr alloy and pure Zr have been investigated by means of density functional theory simulations. In each case, a varying amount of water molecules was placed on the surface until saturation coverage was reached. For both surfaces, all the energy barriers for the partial and complete decomposition of water were calculated. The partial dissociation of H2O into OH and H, and the complete dissociation into O and two H atoms are significantly more difficult on Pt3Zr surfaces, as compared to pure Zr surfaces: the dissociative adsorption energies are smaller and the activation barriers for dissociation are larger in Pt3Zr. In addition, the recombination of H atoms into H2 molecules and desorption of those molecules is easier on the Pt3Zr surfaces. The results suggest that the use of the Pt3Zr alloy as a protective coating in Zr-based metallic components used in nuclear reactors can indeed improve their performance, since the alloyed Pt3Zr layers are much more resistant towards oxidation and H attack than pure Zr in the presence of hot water vapor.

C60Con complexes as hydrogen adsorbing materials

An active line of contemporary research is dedicated to the adsorption and storage of hydrogen on metal-doped carbon materials. Using density functional theory and van der Waals corrections we have studied molecular and dissociative adsorption of H2 on neutral and cationic C60Con complexes with n = 1–8. The Co atoms form compact clusters on the surface of the fullerene. Dissociative chemisorption of one H2 molecule is more stable than molecular adsorption on neutral C60Con, with the only exception of C60Co. When C60Con is ionized, the electronic charge deficit remains localized in the cobalt cluster. The molecular and dissociative adsorption features on cationic C60Con+ and neutral C60Con complexes are in general similar, but some differences can be highlighted. Molecular and dissociative H2 adsorption on C60Co2+ are competitive; in fact, molecular adsorption is slightly more stable as a result of the localized electronic charge deficit on the Co dimer. Another difference is that the dissociative adsorption energies on some of the neutral C60Con complexes are substantially larger than the dissociative adsorption energies on the corresponding C60Con+ cationic complexes by amounts between 0.2 and 0.5 eV. Activation barriers for dissociation of the adsorbed H2 molecule strongly depend on the charge state and cluster size. These barriers help to interpret the adsorption state (molecular or dissociated) of experimentally produced hydrogenated C60Con+ complexes. Hydrogen saturation has been studied in two cases. C60Co adsorbs three H2 units in molecular form, and C60Con adsorbs up to thirteen H2 units, four dissociated and nine in molecular form.

Is a Dissociation Process Underlying the Molecular Origin of the Debye Process in Monohydroxy Alcohols?

Herein, we investigated the molecular dynamics as well as intramolecular interactions in two primary monohydroxy alcohols (MA), 2-ethyl-1-hexanol (2EHOH) and n-butanol (nBOH), by means of broad-band dielectric (BDS) and Fourier transform infrared (FTIR) spectroscopy. The modeling data obtained from dielectric studies within the Rubinstein approach [Macromolecules2013, 46, 7525−7541] originally developed to describe the dynamical properties of self-assembling macromolecules allowed us to calculate the energy barrier (Ea) of dissociation from the temperature dependences of relaxation times of Debye and structural processes. We found Ea ∼ 19.4 ± 0.8 and 5.3 ± 0.4 kJ/mol for the former and latter systems, respectively. On the other hand, FTIR data analyzed within the van’t Hoff relationship yielded the energy barriers for dissociation Ea ∼ 20.3 ± 2.1 and 12.4 ± 1.6 kJ/mol for 2EHOH and nBOH, respectively. Hence, there was almost a perfect agreement between the values of Ea estimated from dielectric and FTIR studies for the 2EHOH, while some notable discrepancy was noted for the second alcohol. A quite significant difference in the activation barrier of dissociation indicates that there are probably supramolecular clusters of varying geometry or a ring-chain-like equilibrium is strongly affected in both alcohols. Nevertheless, our analysis showed that the association/dissociation processes undergoing within nanoassociates are one of the main factors underlying the molecular origin of the Debye process, supporting the transient chain model.

Oxophilicity as a Descriptor for NO Cleavage Efficiency over Group IX Metal Clusters.

Iridium and rhodium are group IX elements that can both catalytically reduce NO. To understand the difference in their reactivity toward NO, the adsorption forms of NO onto clusters of Ir and Rh are compared using vibrational spectra, recorded via infrared multiple-photon dissociation spectroscopy. The spectra give evidence for the existence of at least two specific adsorption forms. The main Ir6+NO isomer is one in which NO is dissociated, whereas one other is a local minimum structure in the reaction pathway leading to dissociative adsorption. In contrast to adsorption onto Rh6+, where less than 10% of the isomeric population was found in the global minimum associated with dissociative adsorption, a substantial fraction (about 50%) of NO dissociates on Ir6+. This higher efficiency is attributed to a considerably reduced activation barrier for dissociation on Ir6+. The key chemical property identified for dissociation efficiency is the cluster's affinity to atomic oxygen.

Water Poisons H2 Activation at the Au-TiO2 Interface by Slowing Proton and Electron Transfer between Au and Titania.

Understanding the dynamic changes at the active site during catalysis is a fundamental challenge that promises to improve catalytic properties. While performing Arrhenius studies during H2 oxidation over Au/TiO2 catalysts, we found different apparent activation energies (Eapp) depending on the feedwater pressure. This is partially attributed to changing numbers of metal-support interface (MSI) sites as water coverage changes with temperature. Constant water coverage studies showed two kinetic regimes: fast heterolytic H2 activation directly at the MSI (Eapp ∼ 25 kJ/mol) and significantly slower heterolytic H2 activation mediated by water (Eapp ∼ 45 kJ/mol). The two regimes had significantly different kinetics, suggesting a complicated mechanism of water poisoning. Density functional theory (DFT) showed water has minor effects on the reaction thermodynamics, primarily attributable to intrinsic differences in surface reactivity of different Au sites in the DFT model. The DFT model suggested significant surface restructuring of the TiO2 support during heterolytic H2 adsorption; evidence for this phenomenon was observed during in situ infrared spectroscopy experiments. A monolayer of water on the hydroxylated TiO2 surface increased the H2 dissociation activation barrier by ∼0.2 eV, in good agreement the difference in experimentally measured values. DFT calculations suggested H2 activation goes through a proton-coupled electron-transfer-like mechanism. During proton transfer to a basic support hydroxyl group, electron density is distributed through the gold nanorod and partially localized on the protonated support hydroxyl group. Water slows H2 activation by slowing this H+ transfer, forcing negative charge buildup on the Au and increasing the transition state energy.

Oxygen molecule dissociation on heteroatom doped graphdiyne

Developing metal-free electrocatalysts is vitally significant for oxygen molecule dissociation. Graphdiynes (GDY) doped with nonmetal atoms are designed and optimized as metal-free electrocatalysts, and their oxygen molecule dissociation catalytic performances are evaluated by density functional theory. The results of formation energies and cohesive energies reveal that the most favorable site for B, Si, P, S, As, Se and Te doping is the sp2-C atom at benzene ring (X-b), the most preferred sites for N and O doping is the sp-C atom nearest benzene ring at the chain (X-1 GDY). The O2 molecules are chemisorbed on B, N, O, Si, P, S-doped GDYs, the OO bonds increase from 1.23 Å to 1.39–1.57 Å. But, the O2 dissociation activation barrier on B, O, Si, P, S-doped GDY monolayers is up to 1.31–4.01 eV, indicating that this reaction is difficult to occur at room temperature. The O2 is physisorbed on As, Se and Te-doped GDY monolayers with a distance between O2 and GDY planes of 2.40–2.77 Å. The N-doped GDY has a small dissociation activation barrier of 0.90 eV, which may be a metal-free catalyst. The calculations promote further application of heteroatom doped GDYs on oxygen reduction reaction and other reactions.

Elucidating the mass spectrum of the retronecine alkaloid using DFT calculations.

Pyrrolizidine alkaloids are natural molecules playing important roles in different biochemical processes in nature and in humans. In this work, the electron ionization mass spectrum of retronecine, an alkaloid molecule found in plants, was investigated computationally. Its mass spectrum can be characterized by three main fragment ions having the following m/z ratios: 111, 94, and 80. In order to rationalize the mass spectrum, minima and transition state geometries were computed using density functional theory. It was showed that the dissociation process includes an aromatization of the originally five-membered ring of retronecine converted into a six-membered ring compound. A fragmentation pathway mechanism involving dissociation activation barriers that are easily overcome by the initial ionization energy was found. From the computed quantum chemical geometric, atomic charges, and energetic parameters, the abundance of each ion in the mass spectrum of retronecine was discussed.

Quantum-Chemical DFT Study of Direct and H- and C-Assisted CO Dissociation on the χ-Fe5C2 Hägg Carbide.

The first step in the Fischer–Tropsch reaction is the production of C1 monomers by the dissociation of the C–O bond. Although Fe is the active metal, it is well known that under typical reaction conditions, it changes into various carbide phases. The Hägg carbide (χ-Fe5C2) phase is usually considered as the catalytically active phase. We carried out a comprehensive DFT study of CO dissociation on various surface terminations of the Hägg carbide, selected on their specific site topology and the presence of stepped sites. Based on the reaction energetics, we identified several feasible CO dissociation pathways over the Hägg carbide. In this study, we have compared the direct CO dissociation with H- and C-assisted CO dissociation mechanisms. We demonstrated that the reaction rate for CO dissociation critically depends on the presence and topology of interstitial C atoms close to the active site. Typically, the CO dissociation proceeds via a direct C–O bond scission mechanism on the stepped sites on the Fe carbide surface. We have shown a preference for the direct CO dissociation on the surfaces with a stepped character. The H-assisted CO dissociation, via a CHO intermediate, was preferred when the surface did not have a clear stepped character. We have also shown that activation barriers for dissociation are highly dependent on the surface termination. With a consistent data set and including migration corrections, we then compared the CO dissociation rates based on a simplified kinetic model. With this model, we showed that besides the activation energy, the adsorption energy of the CO, the C and the O species are important for the reaction rate as well. We found that the most active surface termination is a (111̅) surface cut in such a way that the surface exposes B5 sites that are not occupied by the C atoms. On these B5 sites, the direct CO dissociation presents the highest rate.

First-principles study of NO oxidation on Pt/CaTiO3 interface

A well-dispersed noble metal on perovskite support is expected to show good catalytic ability for NO oxidation under oxygen-rich conditions, which is an important step in reducing NO in lean burn exhaust gas. In this study, we investigated the catalytic oxidation of NO to NO2 on a Pt4/CaTiO3 (001) model to evaluate interfacial effect. Results showed that O2 preferred Pt–Ti interface sites, and that the decomposition of O2 required a dissociation activation barrier of 0.97eV. The decomposed O–O on the interface required nearly no activation barrier in the subsequent NO oxidation. However, the most rate-limiting step involved the desorption of the second formed NO2 from interfacial Pt4/CaTiO3 (001), which yielded a significant desorption barrier of 2.63eV. These findings can help understand the oxidation process of NO to NO2 on noble-metal–perovskite interface.

Curvature effect of O2 adsorption and dissociation on SiC nanotubes and nanosheet

The structural characteristics of catalyst such as size and shape are very important to the catalytic activity and selectivity. In this work, based on density functional theory, O2 adsorption and dissociation on SiC nanotubes and nanosheet are studied systematically. The results show that O2 adsorption and dissociation are highly sensitive to the surface curvature of SiC nanomaterials. Both the O2 adsorption energy and dissociation activation barrier energy increase as the curvature varying from negative to positive. These results may open a new avenue for modulating reaction dynamics via surface curvature.

Covalent Hypercoordination: Can Carbon Bind Five Methyl Ligands?

AbstractC(CH3)5+ is the first reported example of a five‐coordinate carbon atom bound only to separate (that is, monodentate) carbon ligands. This species illustrate the limits of carbon bonding, exhibiting Lewis‐violating “electron‐deficient bonds” between the hypercoordinate carbon and its methyl groups. Though not kinetically persistent under standard laboratory conditions, its dissociation activation barriers may permit C(CH3)5+ fleeting existence near 0 K.

Covalent hypercoordination: can carbon bind five methyl ligands?

C(CH3)5(+) is the first reported example of a five-coordinate carbon atom bound only to separate (that is, monodentate) carbon ligands. This species illustrate the limits of carbon bonding, exhibiting Lewis-violating "electron-deficient bonds" between the hypercoordinate carbon and its methyl groups. Though not kinetically persistent under standard laboratory conditions, its dissociation activation barriers may permit C(CH3)5(+) fleeting existence near 0 K.

Rotational Effects on the Dissociative Adsorption and Abstraction Dynamics of O2/Al(111)

Focussing on the dynamical aspects of the initial sticking of O2/Al(111), we perform quantum dynamical calculations under the influence of an orientationally anisotropic potential energy surface. We discuss the effect of molecular rotation on the O2 dissociative adsorption. Our calculation results reveal that despite the absence of activation barrier for dissociation with parallel polar orientation, the incoming molecule is reflected from the surface with high probability. For small translational energy, the molecules are trapped by a potential energy dip which exists at a metastable site for dissociation through abstraction, and this is also seen as rotational resonance. We also show that our calculation results agree with the experimentally observed incident angle dependent sticking probability.

Kinetic stabilities of bis-terpyridine complexes with iron(ii) and cobalt(ii) in organic solvent environments

Though 2,2′:6′,2′′-terpyridine is commonly used as a ligand for forming metallo-supramolecular assemblies in a variety of solvent environments, little is known about how the properties of the medium influence its behavior. We have studied the kinetic stabilities of bis terpyridine complexes with iron(II) and cobalt(II), and found variations in decay rate between commonly employed solvents of more than five orders of magnitude for iron, and thirty-fold for cobalt. While numerous factors likely contribute to these effects, a clear correlation between the decay rates of the iron complexes and the solubility of terpyridine suggests that favorable solvent–ligand interactions may lower the activation barrier for dissociation by stabilizing the transition state. These results illustrate the importance of solvent selection when designing metallo-supramolecular assemblies based on terpyridine, and also provide insights for further tuning the dynamic properties of these complexes.

Geometric and Electronic Confinement Effects on Catalysis

We report a density functional theory-based study of the electronic and geometric effects exerted on molecules confined between transition-metal surfaces. We first investigate changes in adsorption energies and activation barriers for dissociation of O2 and NO2 on Pt(111) and Fe(111), respectively. It is found that the energy barrier for dissociation of an O2 molecule adsorbed on the bridge site when the molecule is confined between two Pt (111) surfaces is 35% lower than that on a single surface, and the barrier becomes negligible when the molecule adsorbs with its axis forming and angle of 60 or 90° with the planes of the surfaces. In the case of NO2 confined between Fe (111) surfaces, the decrease is from 5 to 12% in the first deoxygenation and 10–15% in the second deoxygenation reaction. For both confined molecules, there is a substantial charge transfer from the metal surfaces that induces bond weakening, thus facilitating dissociation. It is also found that the dissociations of CO, NO, and O2 on var...

ChemInform Abstract: The Bond-Order Conservation Approach to Chemisorption and Heterogeneous Catalysis: Applications and Implications

Publisher Summary For chemisorption phenomena on transition metal surfaces, including surface reactivity, the bond-order conservation-Morse potential (BOC-MP) model appears to provide such a framework in which the intricacies of real phenomena are coherently interrelated. The BOC-MP model is a simple, truly “back-of-the-envelope’’ model that can be directly used by practitioners in the field. It efficiently describes and interrelates a wide variety of chemisorption phenomena. Most important, the model maps out metal surface reactions providing insight into both regularities and details. In principle, any metal surface reaction can be treated this way. The only requirement is to retain the rigor and simplicity of the model projections. This analytic model proved to be efficient for treating energetics of atomic and diatomic adsorbates on transition metal surfaces, particularly, the heat of chemisorption and activation barriers for dissociation and recombination. Recently, the BOC-MP model has been extended to treat energetics of polyatomic adsorbates as well, which made it possible to analyze mechanisms of catalytic heterogeneous reactions of practical importance. The chapter surveys major applications and implications of BOC-MP modeling to chemisorption and heterogeneous catalysis.

Dissociation of carbonic acid: Gas phase energetics and mechanism from ab initio metadynamics simulations

A comprehensive metadynamics study of the energetics, stability, conformational changes, and mechanism of dissociation of gas phase carbonic acid, H2CO3, yields significant new insight into these reactions. The equilibrium geometries, vibrational frequencies, and conformer energies calculated using the density functional theory are in good agreement with the previous theoretical predictions. At 315 K, the cis-cis conformer has a very short life time and transforms easily to the cis-trans conformer through a change in the O=C-O-H dihedral angle. The energy difference between the trans-trans and cis-trans conformers is very small (approximately 1 kcal/mol), but the trans-trans conformer is resistant to dissociation to carbon dioxide and water. The cis-trans conformer has a relatively short path for one of its hydroxyl groups to accept the proton from the other end of the molecule, resulting in a lower activation barrier for dissociation. Comparison of the free and potential energies of dissociation shows that the entropic contribution to the dissociation energy is less than 10%. The potential energy barrier for dissociation of H2CO3 to CO2 and H2O from the metadynamics calculations is 5-6 kcal/mol lower than in previous 0 K studies, possibly due to a combination of a finite temperature and more efficient sampling of the energy landscape in the metadynamics calculations. Gas phase carbonic acid dissociation is triggered by the dehydroxylation of one of the hydroxyl groups, which reorients as it approaches the proton on the other end of the molecule, thus facilitating a favorable H-O-H angle for the formation of a product H2O molecule. The major atomic reorganization of the other part of the molecule is a gradual straightening of the O=C=O bond. The metadynamics results provide a basis for future simulation of the more challenging carbonic acid-water system.

Synthesis and Characterization of trans-[Co((BA)2en)(amine)(CN)] and trans-K[Co((BA)2en)(CN)2] Complexes:Indication of Cobalt-cyanid π -Back Bonding Interaction

The structure of vitamin B12 (cyano cobalamine) was determined in the laboratory of Hodgkin and co-workers in the mid-1960s. In the equatorial plane the central six coordinate cobalt (III) ion is surrounded by four pyrole nitrogen atoms of a corrin ligand. While one of the axial sites is occupied by a 5,6-dimethyl benzimidazol-α-Dribofuranose-3-phosphate, the second axial ligand position is occupied by a cyanide group. The vitamin can be converted in vivo to active alkylcobalamin coenzymes. These alkylcobalamin are know naturally occurring compounds with a biologically active carbon-metal bond. The macrocyclic tetradentate ligands with four nitrogen donor or Schiff base ligands (N2O2) which coordinated to four equatorial position octahedral structure of cobalt (III) center are considered as models for vitamin B12. 6-10 The effect of axial ligation (amine) on properties of cyano group with an organic monoanion ligand has played an important role in our knowledge of the properties of vitamin B12 and of model compounds. The coordination of the amine ligand in trans position can lead to cyano activation barrier of dissociation. A variety of physical methods such as infrared and NMR have been used to study of π-back bonding from the cobalt (III) metal center into cyano ligand. Infrared and NMR spectroscopy allows one to investigation of the effect amines on the cyano ligand π-back bonding in complexes. These results were indicative of the existence of dπ-pπ back bonding in cyano cobalt species (Scheme 1). The effect of trans axial ligands on the cyanide chemical shifts and stretching properties can influence the relative stabilization of I and II species by the axial ligand. In this paper, we report result of H NMR and IR study of a series of trans-(Schiff base)CN(amine)cobalt(III) complexes with different axial ligands (Scheme 2), which permits a more detailed analysis of these properties.

Microkinetic modelling of the formation of C 1 and C 2 products in the Fischer–Tropsch synthesis over cobalt catalysts

The heats of adsorption of different C 1 and C 2 molecules assumed to be present during the initial steps of the Fischer–Tropsch synthesis and activation energies for elementary steps envisioned to occur in the synthesis are calculated for Co by using the unity bond index-quadratic exponential potential (UBI-QEP) method. The preexponential factors for the elementary steps are calculated from transition-state theory, and the rate constants are calculated according to the Arrhenius equation. The activation barrier for hydrogenation of CO is found to be lower compared to hydrogen assisted dissociation of CO, which has a smaller activation barrier than direct dissociation of CO. The reaction steps with high activation barriers are eliminated. Based on this elimination two sets of elementary steps for formation of C 1 and C 2 alkenes and alkanes in the Fischer–Tropsch synthesis are established: one based on hydrogen assisted CO dissociation (carbide mechanism) and one based on CO hydrogenation (CO insertion mechanism). In addition, one mechanism of producing CO 2 from the water–gas shift reaction is proposed. The resulting mechanisms are combined and used in the microkinetic model, which are fitted to experimental results at methanation conditions ( T = 483 K or 493 K, p = 1.85 bar and H 2/CO = 10) over a Co/Al 2O 3 Fischer–Tropsch catalyst. A good tuning is obtained by adjusting the C–Co and H–Co binding strengths. The microkinetic modelling based on these assumptions indicates that CO is mainly converted through hydrogenation of CO and that C 2 compounds are mainly produced by insertion of CO into a metal–methyl bond. Thus, from the surface coverages and reaction rates predicted by the microkinetic modelling the mechanism can be further reduced to only include the CO insertion mechanism. Hydrogenation of CHO to CH 2O is found to be the rate determining initiation step, and insertion of CO into a metal–methyl bond is found to be the rate determining step for chain growth. By using the UBI-QEP method for calculation of activation energies, the activation barriers for dissociation of CO and hydrogenation of surface carbon are found to be too large for the carbide mechanisms to occur. However, experimental data or another theoretical method is necessary in order to support or disprove the calculated activation energies in this work.

Catalytic activity of noble metals promoting hydrogen uptake

The engineering of pure and metal alloy catalysts for hydrogen absorption is needed to improve the kinetics of hydrogen-related devices. We introduce a new route to search for alloys that can yield superior catalytic behavior for hydrogen absorption, using an optical technique to measure the catalytic activity for hydrogen sorption of thin films. The catalytic activity of the noble metals Pd, Ni, Cu, Ag, Pt, and Au is studied as a function of hydrogen pressure and temperature. The rate-limiting step is identified by the pressure dependence and the normal isotope effect for hydrogen absorption. The measured rates and activation energies are correlated to such physical properties as activation barrier for dissociation and heat of solution. The observed compensation effect is explained in the framework of interplay between surface and bulk processes. From the experimentally derived model, a guiding principle for the search for catalysts promoting hydrogen absorption is drawn.