High Reaction Barrier Research Articles

Perovskite oxides: application in oxygen evolution reaction.

Hydrogen, as a kind of green renewable energy, can greatly alleviate the current energy crisis and environmental pollution problems. As the focus and front of scientific research, splitting of water into hydrogen and oxygen by solar energy is considered to be one of the most ideal means to obtain the hydrogen energy. However, the oxygen evolution reaction (OER) is more difficult to occur, which greatly limits the application of water splitting. This is because OER is a 4 electron transfer reaction with a higher reaction barrier, requiring additional over-potential to drive the reaction. The additional over-potential will cause higher efficiency loss, thus reducing the practicability of the water splitting. The ideal OER catalyst should have an appropriate band gap, high efficiency in converting solar energy, chemical inertness and reduces the consumption of precious metals.

Adsorption and Decomposition of NH3 on Ni/Pt(111) and Ni/WC(001) Surfaces: A First-Principles Study

Density functional theory was used to study the NH3 behavior on Ni monolayer covered Pt(111) and WC(001). The electronic structure of the surfaces, and the adsorption and decomposition of NH3 were calculated and compared. Ni atoms in the monolayer behave different from that in Ni(111). More dz2 electrons of Ni in monolayer covered systems were shifted to other regions compared to Ni(111), charge density depletion on this orbital is crucial to NH3 adsorption. NH3 binds more stable on Ni/Pt(111) and Ni/WC(001) than on Ni(111), the energy barriers of the first N–H bond scission were evidently lower on Ni/Pt(111) and Ni/WC(001) than on Ni(111), these are significant to NH3 decomposition. N recombination is the rate-limiting step, high reaction barrier implies that N2 is produced only at high temperatures. Although WC has similar properties to Pt, differences of the electronic structure and catalytic activities are observed for Ni/Pt(111) and Ni/WC(001), the energy barrier for the rate-determined step increases on Ni/WC(001) instead of decreasing on Ni/Pt(111) when compared to Ni(111). To design cheaper and better catalysts, reducing the N recombination barrier by modifying Ni/WC(001) is a critical question to be solved.

Atomic Layer Deposition of Zinc Oxide: Diethyl Zinc Reactions and Surface Saturation from First-Principles

Zinc oxide thin films grown via atomic layer deposition have been under intense research for the past few years. Here, we present a comprehensive density functional theory study on the atomic layer deposition of zinc oxide. The adsorption of diethyl zinc and subsequent surface reactions are studied on an ideal (100) ZnO surface as well as on a stepped surface to compare ideal and nonideal surface structures. Our results show that diethyl zinc adsorbs and reacts rapidly on the surface to form monoethyl zinc. Our calculations also show that the initial ligand-exchange reactions are preferred on the planar surface over the step surface. Further reaction from monoethyl zinc to adsorbed zinc atoms has a high reaction barrier. We present two surface structures for the saturated zinc oxide surface at the end of the diethyl zinc pulse corresponding to low and high temperature approximations that are in good agreement with the experiments.

Comparison of Thiyl, Alkoxyl, and Alkyl Radical Addition to Double Bonds: The Unusual Contrasting Behavior of Sulfur and Oxygen Radical Chemistry.

High-level ab initio calculations have been used to compare prototypical thiyl, alkoxyl, and alkyl radical addition reactions. Thiyl radical addition to the sulfur center of thioketones is exothermic and rapid, occurring with negative enthalpic barriers and only weakly positive Gibbs free energy barriers. In stark contrast, alkoxyl radical addition to the oxygen center of ketones is highly endothermic and occurs with very high reaction barriers, though these are also suppressed. On the basis of analysis of the corresponding alkyl radical additions to these substrates and the corresponding reactions of these heteroatom radicals with alkenes, it suggested that addition reactions involving thiyl radicals have low intrinsic barriers because their unpaired electrons are better able to undergo stabilizing resonance interactions with the π* orbitals of the substrate in the transition state.

Efficient nitrogen incorporation in GaAs using novel metal organic As–N precursor di-tertiary-butyl-arsano-amine (DTBAA)

III/V semiconductors containing small amounts of nitrogen (N; dilute nitrides) are discussed in the context of different solar cell and laser applications. The efficiency of these devices is negatively affected by carbon (C) incorporation, which comes either from the direct C–N bond in the N precursor unsymmetrical 1,1-dimethylhydrazine (UDMHy) used conventionally or from the alkyl groups of the conventional precursors for gallium (Ga), indium and arsenic (As) containing carbon. This C is incorporated together with the N due to the strength of the C–N bond. A further important issue in dilute nitride growth is the very low N incorporation efficiency in the crystal from UDMHy, which can be as little as 1% of the N supplied in the gas phase. Therefore, new metal organic chemicals have to be synthesized and their growth characteristics and suitability for dilute nitride growth have to be explored. This work presents the chemical di-tertiary-butyl-arsano-amine (DTBAA), which was synthesized, purified and tested as an N precursor for metal organic vapor phase epitaxy (MOVPE). Computational investigations show β-hydrogen and isobutane elimination to be the main reaction channel in the gas phase with high reaction barriers and absence of small fragments containing C as products. The loss of N via N2, as in UDMHy, can be excluded for unimolecular reactions of DTBAA. The Ga(NAs)/GaAs heterostructures were grown by MOVPE as initial test material and a systematic N incorporation study is presented in this paper. It is shown that high quality Ga(NAs) can be grown using DTBAA. The N incorporation was confirmed by high resolution X-ray diffraction and photoluminescence studies. All samples grown exhibit as grown room temperature photoluminescence and smooth surface morphologies. Furthermore, DTBAA shows extremely high N incorporation efficiency, which makes this molecule a very promising candidate for further research into dilute nitride material growth.

Catalytic Activity for Oxygen Reduction Reaction on CoN2-Graphene: A Density Functional Theory Study

The detailed catalytic mechanism of oxygen reduction reaction (ORR) on CoN2 embedded graphene (CoN2-G) has been investigated based on a density functional theory calculation. Pathways of four- and two-electron reductions are investigated for ORR. The most feasible four-electron reduction pathways are arranged in the following order: the reductions of O2(ads) into OOH(ads), OOH(ads) into O(ads) and H2O, then O(ads) into OH(ads), and finally OH(ads) into H2O. Free energy diagrams show that the elementary steps of ORR along the four-electron pathway are downhill at low electrode potential (up to 0.62 V vs. standard hydrogen electrode). The first reduction step, O2(ads)-to-OOH(ads) reaction has the highest reaction barrier (0.53 eV), acting as the kinetic rate-determining step. However, at a higher potential, the last reduction step, OH(ads)-to-H2O(ads) becomes uphill, and is the thermodynamic rate-determining step. Therefore, the first and the last reduction steps restrict the ORR performance together at high potential on CoN2-G catalysts. The two-electron reduction product H2O2 can be chemisorbed on the surface. However, the H2O2(ads) is unstable and easy to dissociate into OH species. Therefore, the two-electron ORR pathway is unfavorable on CoN2-G.

Elementary steps in Fischer–Tropsch synthesis: CO bond scission, CO oxidation and surface carbiding on Co(0001)

Dissociation of CO on a Co(0001) surface is explored in the context of Fischer–Tropsch synthesis on cobalt catalysts. Experiments show that CO dissociation can occur on defect sites around 330K, with an estimated barrier between 90 and 104kJmol−1. Despite the ease of CO dissociation on defect sites, extensive carbon deposition onto the cobalt surface up to 0.33ML requires a combination of high surface temperature and a relatively high CO pressure. Experimental data on the CO oxidation reaction indicate a high reaction barrier for the CO+O reaction, and it is argued that, due to the rather strong Co–O bond, (i) oxygen removal is the rate-limiting step during surface carbidization and (ii) in the context of Fischer–Tropsch synthesis, removal of surface oxygen rather than CO bond scission might be limiting the overall reaction rate.

Catalytic Transfer Hydrogenation with a Methandiide-Based Carbene Complex: An Experimental and Computational Study.

The transfer hydrogenation (TH) reaction of ketones with catalytic systems based on a methandiide-derived ruthenium carbene complex was investigated and optimised. The complex itself makes use of the noninnocent behaviour of the carbene ligand (M=CR2 →MH-C(H)R2 ), but showed only moderate activity, thus requiring long reaction times to achieve sufficient conversion. DFT studies on the reaction mechanism revealed high reaction barriers for both the dehydrogenation of iPrOH and the hydrogen transfer. A considerable improvement of the catalytic activity could be achieved by employing triphenylphosphine as additive. Mechanistic studies on the role of PPh3 in the catalytic cycle revealed the formation of a cyclometalated complex upon phosphine coordination. This ruthenacycle was revealed to be the active species under the reaction conditions. The use of the isolated complex resulted in high catalytic activities in the TH of aromatic as well as aliphatic ketones. The complex was also found to be active under base-free conditions, suggesting that the cyclometalation is crucial for the enhanced activity.

Room-Temperature, Low-Barrier Boron Doping of Graphene

Doping graphene with boron has been difficult because of high reaction barriers. Here, we describe a low-energy reaction route derived from first-principles calculations and validated by experiments. We find that a boron atom on graphene on a ruthenium(0001) substrate can replace a carbon by pushing it through, with substrate attraction helping to reduce the barrier to only 0.1 eV, implying that the doping can take place at room temperature. High-quality graphene is grown on a Ru(0001) surface and exposed to B2H6. Scanning tunneling microscopy/spectroscopy and X-ray photoelectron spectroscopy confirmed that boron is indeed incorporated substitutionally without disturbing the graphene lattice.

Direct photolysis of MeO-PBDEs in water and methanol: Focusing on cyclization product MeO-PBDFs

Polybrominated diphenyl ethers (PBDEs) and hydroxylated PBDEs can transform into polybrominated dibenzofurans (PBDFs) via photocyclization. However, it is unclear whether methoxylated PBDEs (MeO-PBDEs) can photocyclize to form MeO-PBDFs. In this study, 5-MeO-BDE-47, 5′-MeO-BDE-99 and 6-MeO-BDE-85 were selected as models to investigate their direct photolysis, especially photocyclization in two solvent environments (water and methanol) using simulated photochemical experiments and density functional theory (DFT) calculations. The experimental results showed that MeO-PBDEs had faster direct photolysis reactions and higher quantum yields in methanol, and MeO-PBDFs could only be formed in a methanol solution of 5-MeO-BDE-47. The DFT results indicated that the lowest excited triplet state MeO-PBDEs can form dibenzofurans via direct cyclization pathways. Intra-annular H-elimination was found to be the rate-determining step for most cyclization pathways with high reaction barriers (⩾19.7kcal/mol), while 5-MeO-BDE-47 was found to have a distinct pathway for which the rate-determining step is ring closure with a low barrier (13.8kcal/mol) in a methanol environment. For this pathway, H-elimination assisted by Br cleaved from an ortho-C–Br bond was observed with a 2.0kcal/mol barrier. Thus, the DFT results reasonably explained the experimental findings, and the photocyclization of MeO-PBDEs depended on the specific Br-substitution patterns and specific effects of the environmental media.

Theoretical Studies on the Mechanism of the Michael Addition Reaction Catalyzed by a Thiourea‐Cinchona‐Amine: Triple Activation

AbstractThis paper attempts to illuminate the mechanism of the catalytic Michael addition of 2,4‐pentanedione with acrylaldehyde mediated by a thiourea‐cinchona‐amine catalyst via density functional theory calculations. The calculated results show that the C−C coupling easily occurs once the electronegative and electropositive sites of the reactants are generated by the catalyst. However, the previously proposed mechanism with the formation of an imine from the carbonyl group of reactants and the amine group of catalyst fails due to high reaction barriers for both the formation of the imine and the hydrolysis of the imine with water in order to regenerate the catalyst. Instead, the reaction pathways going through the protonation of the amine in the catalyst and formation of hydrogen bonds between the reactants and catalyst have smaller barriers. Our study will be helpful in designing new catalysts or extending the substrate scope of the reaction.

β-Hydrogen Elimination Mechanism in the Absence of Low-Lying Acceptor Orbitals in EH2(t-C4H9) (E = N-Bi).

The β-hydrogen elimination reactions of group 15 alkyl compounds at the example of EH2(t-C4H9) (element E = N-Bi) were investigated and compared to the group 13 example of GaH2(t-C4H9). With the aid of extensive density functional theory based analysis of atomic and electronic structures at the transition state, we can derive three distinct reaction classes. The gallium compound follows the well-known β-hydride route with participation of an empty p orbital at the metal in a concerted, synchronous fashion, exhibiting a low barrier. For compounds with group 15 elements, we find highly nonsynchronous reactions with high reaction barriers. In the case of nitrogen, a proton-like H atom is transferred via attack of the nitrogen nonbonding electron pair. For the heavier homologues (P-Bi), E-Cα bond breaking occurs first and the H atom does not carry charge at the transition state. The reaction barrier in group 15 homologues is thus determined by the E-Cα bond strength down the group. The results enable a rationale for ligand design for precursors involved in chemical vapor-phase deposition processes because a good ligand needs to stabilize the positive charge at Cα.

A Density Functional Theory Study on Mechanism of Electrochemical Oxygen Reduction on FeN3-Graphene

Fe/N/C catalysts are proven to be highly active for oxygen reduction reaction (ORR). The detailed kinetic and thermodynamic ORR behavior on FeN3-G (three-coordinated FeN3 center embedded in graphene) based on density functional theory is investigated in this work. The results show that O2 dissociation mechanism with a high active barrier is unfavorable. All ORR intermediates except H2O2 can be chemisorbed stably on the surface. H2O2 is easily dissociated, which indicates that a two-electron pathway is impossible. This conclusion is further supported by the calculations of OOH dissociation reaction barriers, which are extremely low. The results prove that FeN3-G catalysts promote four-electron ORR. Given the high adsorption energy of the surface toward O2, the O2(ads)-to-OOH(ads) reaction has the highest reaction barrier in the whole reduction steps, which functions as the kinetic rate-determining step. However, free energy diagrams show that reduction of OH into H2O remains uphill for all positive electrode potential vs. the normal hydrogen electrode. The high endothermic ΔG value of H2O formation indicates this step is the most sluggish one and implies the highest resistance for the whole ORR. The FeN3-G is not a well catalyst for ORR.

Theoretical study on the reaction mechanism of carbon dioxide reduction to methanol using a homogeneous ruthenium(II) phosphine catalyst

Quantum-chemical calculations using the density functional TPSS were carried out for the catalytic reduction of carbon dioxide to methanol using a ruthenium catalyst and hydrogen gas. Preparation of the active species as well as the catalytic cycle were modeled on the quantum-chemical level with solvent effects included by means of a continuum solvation model. Outer as well as inner sphere mechanisms were considered to gain insight into the details of carbon dioxide reduction using a ruthenium(II) catalyst. The overall Gibbs free reaction energy for CO2+3 H2→MeOH+H2O is computed to be −13.0kJ/mol. The highest reaction barrier (112.4kJ/mol) is found for the outer sphere hydrogen transfer from the active ruthenium species to carbon dioxide via a five-membered transition state structure.

A Kinetic Investigation of the Initialization of Catalyzed Chain Growth of Styrene: The Reaction of Cp*2ZrCl2 with Dibenzylmagnesium

The system Cp*2ZrCl2 and dibenzyl magnesium in toluene, which is a model system for initialization in the catalyzed chain growth (CCG) of styrene using a bifunctional organometallic chain transfer agent, is studied via time‐resolved NMR spectroscopy. Based on these data, a mechanism of initialization is proposed. The experimental concentration profiles of the individual species can successfully be modeled to the proposed kinetic scheme. Refined kinetic coefficients of high significance can be obtained by applying analytical methods using pseudo‐first‐order rate approximations. It is found that the adduct of the Zr catalyst and the Mg compound only slowly transforms via a ligand exchange reaction, yielding the alkylated Zr species. The temperature dependence of these reactions indicates a high reaction barrier of this type of rearrangement, which constitutes the core reaction of CCG in terms of molecular‐weight control. image

Computational Study of Carbostannylation Implicating Bimetallic Catalysis Involving “AuI–Vinyl–PdII” Species

We have investigated computationally the gold and palladium cocatalyzed reaction of alkynes with vinylstannane. Our work has involved a careful and thorough exploration of different mechanistic possibilities. We find that palladium acting alone as a catalyst leads to a very high reaction barrier, consistent with the experimental observation that there is no reaction in the presence of just palladium. However, the involvement of a gold(I) complex lowers the reaction barrier considerably, and the vinylstannylation reaction can proceed with a modest activation energy of about 10 kcal/mol. Our key finding is that the introduction of the gold complex avoids the formation of high-energy structures involving vinyl species in a trans arrangement on palladium. Our work confirms the role of intermediates containing both palladium and gold as suggested by Blum. For the gold–palladium cocatalyzed reaction, we also investigated an alternative mechanism suggested by Blum. With some modifications, this mechanism has a slightly higher reaction barrier, but if it does occur, then we predict a strong dependency on the counterion, in agreement with related experimental findings.

Theoretical study on copper-catalyzed reaction of hydrosilane, alkyne and carbon dioxide: A hydrocarboxylation or a hydrosilylation process ?

The hydrocarboxylation and hydrosilylation processes proposed in the copper-catalyzed reaction among carbon dioxide, diphenylacetylene and HSi(OEt)3 were comparatively studied with the aid of density functional theory calculations. Our study is to explore why the reaction preferred a hydrocarboxylation rather than a hydrosilylation process. It was found that the σ bond metathesis between Cu–C and H–Si involved in the hydrosilylation process had a significantly high reaction barrier in the presence of CO2 (47.4 kcal/mol). Instead, CO2 insertion and the subsequent σ bond metathesis between Cu–O and H–Si involved in the hydrocarboxylation process were confirmed kinetically feasible, consistent with the experimental facts.

Methane CH Activation by Palladium Complexes with Chelating Bis(NHC) Ligands: A DFT Study

Palladium bis(NHC) complexes are able to catalytically activate and to promote the oxidation of methane. The conversion of methane to methyl trifluoroacetate catalyzed by the palladium bis(NHC) complex dibromido(1,1′-dimethyl-3,3′-methylenediimidazolin-2,2′-ylidene)palladium(II) was investigated by a DFT study on the B3LYP/6-311++G(d,p)//B3LYP/6-31G(d) level of theory in combination with solvent and dispersion corrections. According to the DFT calculations the reaction proceeds by a palladium(II–II–IV) pathway without formation of palladium(0). A second redox cycle via a bromide–bromine shuttle, which mediates the oxidation of palladium(II) to palladium(IV), is proposed. Although CH activation by palladium(0) is predicted to be a very facile process, this pathway is very unlikely, due to the extraordinarily high reaction barrier for reductive elimination. CH activation by palladium(IV) has been investigated as well and might be a viable pathway comparable in energy to an activation by palladium(II) with o...

A Frontier Orbital Study with ab Initio Molecular Dynamics of the Effects of Solvation on Chemical Reactivity: Solvent-Induced Orbital Control in FeO-Activated Hydroxylation Reactions

Solvation effects on chemical reactivity are often rationalized using electrostatic considerations: the reduced stabilization of the transition state results in higher reaction barriers and lower reactivity in solution. We demonstrate that the effect of solvation on the relative energies of the frontier orbitals is equally important and may even reverse the trend expected from purely electrostatic arguments. We consider the H abstraction reaction from methane by quintet [EDTAH(n)·FeO]((n-2)+), (n = 0-4) complexes in the gas phase and in aqueous solution, which we examine using ab initio thermodynamic integration. The variation of the charge of the complex with the protonation of the EDTA ligand reveals that the free energy barrier in gas phase increases with the negative charge, varying from 16 kJ mol(-1) for [EDTAH4·FeO](2+) to 57 kJ mol(-1) for [EDTAHn·FeO](2-). In aqueous solution, the barrier for the +2 complex (38 kJ mol(-1)) is higher than in gas phase, as predicted by purely electrostatic arguments. For the negative complexes, however, the barrier is lower than in gas phase (e.g., 45 kJ mol(-1) for the -2 complex). We explain this increase in reactivity in terms of a stabilization of the virtual 3σ* orbital of FeO(2+), which acts as the dominant electron acceptor in the H-atom transfer from CH4. This stabilization originates from the dielectric screening caused by the reorientation of the water dipoles in the first solvation shell of the charged solute, which stabilizes the acceptor orbital energy for the -2 complex sufficiently to outweigh the unfavorable electrostatic destabilization of the transition-state relative to the reactants in solution.

Insight into the active phase of CO oxidation on Ni/Pt and NiO1 − x/Pt model catalysts from a first principles investigation

CO oxidation on bimetallic and metal oxide has drawn much attention in the past years due to its importance both technologically and theoretically, but the active phase as well as the detailed reaction mechanism on the bimetallic surface oxide (i.e., a sandwich-like surface structure) are still unclear. In this work, the CO oxidation on the various Pt–Ni model catalysts [including Pt(111), Pt/Ni/Pt(111), Ni/Pt(111), NiO1−x/Pt(111) and NiO1−x/Pt/Ni/Pt(111)] was studied by performing the density functional theory calculations. It was found that the CO oxidation reaction would process with a higher reaction barrier on metals at lower oxygen coverage via the Langmuir-Hinshelwood (L-H) mechanism, whereas CO oxidation reaction would take place with a lower barrier at higher oxygen coverage on metals or in the presence of molecular oxygen/CO (on NiO1−x-like systems) via the Eley-Rideal mechanism. The calculation results show that the activation energy of CO oxidation follows the order: Pt(111) (0.75eV)>Pt/Ni/Pt(111) (0.69eV)>Ni/Pt(111) (0.47eV at 1ML oxygen), which is in general agreement with the experimental observations. On the surface oxide NiO1−x/Pt(111) and NiO1−x/Pt/Ni/Pt(111) systems, it was found that the molecular CO can subtract the surface lattice oxygen to form CO2 spontaneously through the Eley-Rideal mechanism on NiO1−x/Pt/Ni/Pt(111), whereas such kinetic behavior cannot occur on the NiO1−x/Pt(111) system, suggesting the high reactivity of CO oxidation on NiO1−x/Pt/Ni/Pt(111). The possible reason was analyzed by the magnitude of surface oxygen vacancy formation energy, namely NiO1−x/Pt/M/Pt(111) with relatively low vacancy formation energy as compared to that of NiO1−x/Pt(111) (3.46 vs 4.51eV). Moreover, we extend the above study to a more general case in which the subsurface metals in NiO1−x/Pt/M/Pt(111) system including VIII group metals like Fe/Co/Ni and the IB group metals like Cu, and it was found that the molecular CO can subtract the surface lattice oxygen atom to form CO2 spontaneously via the E-R reaction mechanism for all these NiO1−x/Pt/M/Pt(111) systems.