Gibbs Free Energy Barrier Research Articles

Understanding the Role of Surface Oxygen in Hg Removal on Un-Doped and Mn/Fe-Doped CeO2 (111).

Effects of surface-adsorbed O and lattice O for the CeO2 (111) surface on Hg removal has been researched. In this work, periodic calculations based on density functional theory (DFT) were performed with the on-site Coulomb interaction. Hg is oxidized to HgO via the surface-adsorbed O by overcoming a Gibbs free energy barrier of 114.1 kJ·mol-1 on the CeO2 (111) surface. Mn and Fe doping reduce the activation Gibbs free energy for the Hg oxidation, and energies of 70.7 and 49.6 kJ·mol-1 are needed on Ce0.96 Mn0.04 O2 (111) and Ce0.96 Fe0.04 O2 (111) surfaces. Additionally, lattice O also plays an important role in Hg removal. Hg cannot be oxidized leading to the formation of HgO on the un-doped CeO2 (111) surface owing to the inertness of lattice O, which can be easily oxidized to HgO on Ce0.96 Mn0.04 O2 (111) and Ce0.96 Fe0.04 O2 (111) surfaces. It can be seen that both surface-adsorbed O and lattice O play important roles in removing Hg. The present study will shed light on understanding and developing Hg removal technology on un-doped and Mn/Fe-doped CeO2 (111) catalysts. © 2019 Wiley Periodicals, Inc.

Preparation, characterization and environmental application of the composite electrode TiO2‐NTs/SnO2—Sb with carbon aerogels

AbstractBACKGROUNDResearch on electrode materials with high electrochemical activity is of great importance for the electrochemical oxidation technique. In this work, an electrode comprising titanium dioxide nanotubes with tin oxide antimony (TiO2‐NTs/SnO2—Sb) with carbon aerogels (TSS‐CAs) was fabricated and used to degrade three isomers of dihydroxybenzenes.RESULTSIt was observed that the TSS‐0.5%CAs electrode prepared with 0.5 wt% CAs possessed prominent pore structure and a larger specific surface area of 180.83 m2 g−1. Electrochemical measurements revealed that the TSS‐0.5%CAs electrode had enhanced electrochemical activity, larger electroactive area, and higher oxygen evolution potential. Relevant operational parameters including initial contaminant concentration, pH, electrolyte concentration and current density had significant effects on the electrocatalytic performance of the TSS‐CAs electrode.CONCLUSIONIt was demonstrated that a proper content of carbon aerogel could improve the structure and electrochemical performance of the TSS‐CAs electrode. The removal of catechol was higher than that of hydroquinone and resorcinol during electrochemical oxidation, due to its lower Gibbs free energy barriers. Oxidation of intermediates of dihydroxybenzenes by the TSS‐CAs electrode also was confirmed. © 2019 Society of Chemical Industry

Highly Selective Photoreduction of CO2 with Suppressing H2 Evolution over Monolayer Layered Double Hydroxide under Irradiation above 600 nm

AbstractAlthough progress has been made to improve photocatalytic CO2 reduction under visible light (λ>400 nm), the development of photocatalysts that can work under a longer wavelength (λ>600 nm) remains a challenge. Now, a heterogeneous photocatalyst system consisting of a ruthenium complex and a monolayer nickel‐alumina layered double hydroxide (NiAl‐LDH), which act as light‐harvesting and catalytic units for selective photoreduction of CO2 and H2O into CH4 and CO under irradiation with λ>400 nm. By precisely tuning the irradiation wavelength, the selectivity of CH4 can be improved to 70.3 %, and the H2 evolution reaction can be completely suppressed under irradiation with λ>600 nm. The photogenerated electrons matching the energy levels of photosensitizer and m‐NiAl‐LDH only localized at the defect state, providing a driving force of 0.313 eV to overcome the Gibbs free energy barrier of CO2 reduction to CH4 (0.127 eV), rather than that for H2 evolution (0.425 eV).

Highly Selective Photoreduction of CO2 with Suppressing H2 Evolution over Monolayer Layered Double Hydroxide under Irradiation above 600 nm.

Although progress has been made to improve photocatalytic CO2 reduction under visible light (λ>400 nm), the development of photocatalysts that can work under a longer wavelength (λ>600 nm) remains a challenge. Now, a heterogeneous photocatalyst system consisting of a ruthenium complex and a monolayer nickel-alumina layered double hydroxide (NiAl-LDH), which act as light-harvesting and catalytic units for selective photoreduction of CO2 and H2 O into CH4 and CO under irradiation with λ>400 nm. By precisely tuning the irradiation wavelength, the selectivity of CH4 can be improved to 70.3 %, and the H2 evolution reaction can be completely suppressed under irradiation with λ>600 nm. The photogenerated electrons matching the energy levels of photosensitizer and m-NiAl-LDH only localized at the defect state, providing a driving force of 0.313 eV to overcome the Gibbs free energy barrier of CO2 reduction to CH4 (0.127 eV), rather than that for H2 evolution (0.425 eV).

Theoretical understanding mechanisms and stereoselectivities of [2+2] cycloaddition of ketenes with ketimines catalyzed by bifunctional N-heterocyclic carbene

Reaction mechanism of NHC-catalyzed asymmetric [2+2] cycloaddition reaction between ketenes and isatin-derived ketimines for the formation of spirocyclic indolo-β-lactams has been investigated using density functional theory (DFT). The catalytic cycle for the title reaction included three possible mechanisms: the “ketene preferential mechanism” (mechanism A) and the “imine preferential mechanism” (mechanisms B and C). They have three similar steps: the first step is the nucleophilic attack of the NHC catalyst, the second one is ring-closure proceed, and the last one is catalyst regeneration and product formation. This work adopted the two different N-heterocyclic carbene catalysts (named as NHC-I and NHC-II) involved in experiment. The calculated results indicate that the Gibbs free energy barriers of mechanism A are remarkably lower than those of mechanisms B and C, and the reaction pathway leading to the SS-configured product has the lowest Gibbs free energy barrier, which agrees with the experiments. It is worth noting that in mechanism A, NHC-I has a free-rotating hydroxyl group which can form a hydrogen bond and ketene, which greatly reduces the energy of the enolate intermediate and affects the cycloaddition process, improving the stereoselectivity of the reaction. Furthermore, the special role of the catalysts and origin of stereoselectivity of the title reaction were also identified by global reactivity index and frontier molecular orbital analyses. The new insights obtained in this study might provide clues for understanding the reaction mechanism of the high stereoselective reaction catalyzed by bifunctional N-heterocyclic carbene catalysts.

Experimental and theoretical study on the regioselective synthesis and reaction of some bis- and poly(3-mercapto-1,2,4-triazin-5(4H)-one) derivatives

The synthetic utility of 3-thioxo-1,2,4-triazin-5-ones 1 and 4-amino-3-mercapto-1,2,4-triazin-5(4H)-ones 2 as building blocks for novel bis- and poly(1,2,4-triazines) via alkylation with the corresponding bis- and poly(halo) compounds was investigated. Prediction of the site of alkylation has been confirmed using spectroscopic data. Theoretical calculation using hybrid density functional method wb97xd and 6-311++g(d,p) as basis set was implemented to investigate the reaction products energetics parameters. The atomic charges were calculated and discussed in details using three different methods. The transition state search were implemented to estimate the Gibbs free activation energy barrier of the reaction to emphasis the experimental results.

Effect of acid strength on the formation mechanism of tertiary butyl carbocation in initial C4 alkylation reaction over H-BEA zeolite: A density functional theory study

Abstract As the first and essential step of the C4 alkylation reaction, the formation of tertiary butyl carbocation over H-BEA zeolite was first systematically studied utilizing density functional theory (DFT) calculation. Herein, two reaction pathways (i.e., skeletal isomerization and hydride transfer) were systematically compared involving the Gibbs free energy at 350 K. It is found that the overall apparent reaction barrier of the hydride transfer pathway is 76 kJ/mol, much lower than that of skeletal isomerization pathway (92 kJ/mol), indicating that the hydride transfer pathway is predominant over the H-[Al]-BEA. Moreover, effect of acid strength on reaction mechanism was further investigated over H-[Al]-BEA and H-[B]-BEA catalysts with stronger and weaker acidity, respectively. Results show that the hydrogen transfer route is still the dominant pathway to generate tertiary butyl carbocation when acid strength changes, which correlates well with the experimental results. However, rate-determining steps of skeletal isomerization and hydride transfer are changed, and the higher Gibbs free energy barriers are obtained by decreasing the acid strength. The insights in this work will be of great significance to understand the initial C4 alkylation mechanism and to design highly efficient catalysts.

Understanding the Reactivity of Mn-Oxo Porphyrins for Substrate Hydroxylation: Theoretical Predictions and Experimental Evidence Reconciled.

The Mn-oxo porphyrin (MnOP) mechanism for substrate hydroxylation is computationally studied with the aim to better understand reactivity in these systems. Theoretical studies suggest Mn(V)OP species to be very reactive intermediates with thermally accessible reaction barriers represented by low-spin/high-spin-crossover occurring in the Mn(V)OP oxidant, and kinetics for selected Mn(V)OP species indeed find high reactivity. On the other hand, MnOP complexes lead to modest yields in hydroxylation reactions of several different substrates, implying low rate constants and high reaction barriers. The resolution of this inconsistency is very important to understand the reactivity of Mn-oxo porphyrins and to improve the catalytic conditions. In this work we use the toluene hydroxylation by the Mn(V)OP(H2O)+ complex as a case study to gain deep insight into the reaction mechanism. Minimum energy crossing point (MECP) results on the H-abstraction process from toluene indicate a first crossover from a singlet to a triplet spin state of the Mn(V)OP(H2O)+ species with a thermally accessible barrier, followed by a very facile H-abstraction by the triplet complex. Issues concerning (i) the validation of the level of the density functional theory employed (BP86) to describe the singlet-triplet energy gap in the Mn(V)OP(H2O)+ system versus highly accurate DMRG-CASPT2/CC calculations, and (ii) the influence of the axial ligand (X = none, Cl-, CH3CN, OH-, and O2-) on MnOP reactivity, which models the different experimental conditions, are addressed. The ligand trans influence mainly controls the reactivity through the singlet-triplet energy gap modulation, with the porphyrin ruffling distortion also finely tuning it. Finally, a stepwise model for the H-abstraction process is proposed which allows a direct comparison between the calculated and experimentally measured Gibbs free activation energy barriers ( Zhang et al. J. Am. Chem. Soc. 2005 , 127 , 6573 - 6582 ). The low yields in catalysis are shown not to be due to low reactivity of Mn(V).

Study on gas reaction path in GaN HVPE

By density functional theory of quantum chemistry, the gas reaction mechanism in GaN HVPE growth is extensively studied, especially the reaction of GaCl with NH3, the elimination of H2 or Cl2 from GaCl3:NH3, and the formation of H radicals caused by the evaporation of liquid gallium and its subsequent reactions. By calculations of Gibbs free energies and energy barriers at different temperatures, the reaction probabilities are determined both thermodynamically and kinetically. The results show that Δ G >0 for the whole six possible reactions between GaCl and NH3, which indicates that those reactions are unfavorable. For the three reactions with elimination of H2 or Cl2 from GaCl3:NH3, the reactions are also unfavorable. By thermodynamic calculation of Ga phase change, when T >1200 K, the Ga vapor pressure rises rapidly. Thus, the gaseous Ga in gas phase reaction cannot be neglected. The gaseous Ga reacts with HCl to form GaCl and H radicals, and H radicals will finally result in the amino compounds GaClNH2 and GaCl(NH2)2, which are also unfavorable to further react with NH3. Thus, we conclude that, in addition to GaCl, GaClNH2 and GaCl(NH2)2 are also the most likely Ga-containing molecules in the HVPE reactor, which will provide both GaN surface growth and nanoparticles.

Selecting substituent elements for LiMnPO4 cathode materials combined with density functional theory (DFT) calculations and experiments

LiMnPO4 cathode material has a high voltage platform and matches the existing electrolyte window, and thus researchers are constantly shifting their focus from LiFePO4 to LiMnPO4. However, LiMnPO4 has lower electron (ion) conductivity than LiFePO4, and besides, its delithiated phase MnPO4 will suffer thermal decomposition at lower temperatures more easily than FePO4. In order to effectively solve the above problems by elements substitution, DFT calculations are employed to screen for suitable dopants from a series of transition metals including Fe, Mg, Ni, V, Nb, Ti. Properties such as electronic structure, atomic Bader charge, O2 evolution Gibbs free energy, average voltages, and lithium ion diffusion energy barrier were evaluated. Based on the calculation, Fe is the most effective doping element because Fe doping is able to reduce the band gap of the material and improve the electronic conductivity, suppress the O2 evolution reaction of the delithiation phase and improve the thermal stability. The reason for such a situation is that Fe can form a stronger covalent bond with the surrounding O atoms to bind the escape of O. Fe doping reduces ion diffusion energy barrier to promote lithium ion diffusion. Electrochemical tests show that Fe doping can improve the electrochemical properties of LiMnPO4.

Mechanistic Insights into Protonated Diamines-catalyzed Decarboxylation of Oxaloacetate

The chemical mechanisms of protonated diamines-catalyzed decarboxylation of oxaloacetic acid anions in water solutions have been studied by using density functional theory. The calculated results show that the activated Gibbs free energy of the decarboxylation step is the highest in the whole diamine-catalytic processes for OA2-, and protonated ethylenediamine (ENH+) is the best catalyst of the five diamines, which is consistent with the study of Thalji et al. However, for OA-, different with OA2-, the dehydration step is the rate-determining one except 1,3-diaminopropane, and protonated 1,4- diaminobutane is the best catalyst of the five catalysts. The results also indicate that the second amino group participates in the reaction as the proton acceptor or proton donor, and it assists in decarboxylation by hydrogen bonds, decreasing the active Gibbs free energy barrier of the whole catalytic process. These results provide insight into the precise catalytic mechanism of several enzymes whose reactions are known to proceed via an imine intermediate.

Two-dimensional π-conjugated metal-organic nanosheets as single-atom catalysts for the hydrogen evolution reaction.

The design and synthesis of electrocatalysts with controllable active sites are highly desirable for producing hydrogen. Inspired by the experimental realization of two-dimensional metal-organic framework (2D-MOF) nanosheets, a computational screening was performed to find out the optimal single-atom catalysts for the hydrogen evolution reaction (HER) from 36 2D-MOFs composed of transition metal and benzene-like molecular ligands. Through first-principles calculations, six configurations were identified as potential catalysts for the HER with performance comparable to those of Pt-based catalysts due to the near-zero Gibbs free energy and favourable activation barriers originating from their appropriate d-orbital bonding mechanism with atomic hydrogen.

Formation of OH radicals from the simplest Criegee intermediate CH2OO and water

Hydroxyl (OH) radicals can convert nitrogen and sulfur oxides to nitric and sulfuric acid, respectively, and oxidize nonmethane hydrocarbons to organic acids, which play an important key role in the environmental chemistry. The famous Criegee intermediates (CIs) are also key species in atmospheric chemistry. The most important loss paths that control CIs levels are suggested to be the reaction with water vapor. To study the reaction mechanism of the simplest CI (CH2OO) with H2O, stationary species and saddle points on the potential energy surface (PES) of CH2OO + H2O have been investigated at CCSD(T)/AUG-cc-pVTZ//B3LYP/6-311++G(2d,2p) level of theory. The computational results indicate that the most feasible channel for the reaction of CH2OO with H2O produces HOCH2O and hydroxyl radicals (OH). Because the highest Gibbs free energy barrier is only 10.5 kcal/mol, this path will be active in atmosphere and OH radical can be produced in this process. Besides, we find a transition state which has not been reported before and explained the mechanism of the formation of a key intermediate. We also scan the formation PES of OH radical from an intermediate and find a loose transition state, which illustrates the mechanism of the producing of OH radical in the reaction of CH2OO with H2O. Here we present a new way of producing OH radicals.

Metal‐Free Activation of Enthalpically Strong Bonds: Unraveling the Potential of Hitherto Unexplored Singlet Carbenes

AbstractDensity functional theory calculations have been carried out on a number of ambiphilic and electrophilic carbenes (both experimentally known and computationally designed) to understand their reactivity profile towards the activation of thermally robust bonds. Based on the calculated values of activation energy barriers, it was found that for ammonia (NH3), most of the molecules favor an electrophilic mode of activation. On the other hand, even though the electrophilic pathway is found to be more favorable than the nucleophilic one for activation of phosphine (PH2Ph) by majority of the carbenes, the barrier heights for both pathways are calculated to be comparable for diamidocarbene (DAC), cyclic amino aryl carbenes (CAArCs) and bicyclic alkyl amino carbene (BICAAC). In agreement with their better electron donation and acceptance abilities, the hitherto unexplored BICAAC, CAArCs and cyclic alkyl amido carbenes (CAAmC) are predicted to be more effective than cyclic alkyl amino carbenes (CAACs) towards the activation of both ammonia and phosphine. For the activation of silane (SiH3Ph), a hydride transfer pathway is found to be more favorable than the proton transfer pathway. Further, the calculated values of total Gibbs free energies and activation energy barriers for the splitting of N−H, P−H and Si−H bonds by a majority of the molecules are found to be comparable to those of the experimentally evaluated ones, implying that these known‐yet unexplored‐carbenes may be considered as suitable candidates for the activation of such enthalpically strong bonds.magnified image

The mechanism of nanoparticle precipitation induced by electron irradiation in transmission electron microscopy

Ru nanoparticles (NPs) can precipitate in Ru doped SiO2 amorphous thin films, triggered by electron irradiation in (scanning) transmission electron microscope ((S)TEM). A new mechanism was introduced to interpret the formation of metal NPs in (S)TEM. The induced electric field by electron irradiation, which originates from charging due to ionizations and excitations of atom electrons, can reduce the Gibbs free energy barrier for nucleation of metal particles. Furthermore, the directional ion drifting driven by the electric forces may accelerate the kinetic process of metal particle precipitation.

Region selectivity of nanometer scale crystallization behavior in metallic glass

It has been known that the internal structure of metallic glass is composed of elastic region and fragile region. However, it is not clear whether the distribution of these regions has an effect on the occurrence of crystallization behavior. Here, the molecular dynamics simulation is employed to investigate the nanometer scale crystallization behavior in metallic glass. A regional selective characteristic is revealed for the occurrence of crystallization behavior in the metallic glasses, which is different from that in the melt, showing the unique nature of crystallization behavior in amorphous system. It is observed that the crystallization behavior occurs first in the elastic region dominated by the icosahedral network, rather than in the fragile region dominated by the low-population polyhedron. This phenomenon can be attributed to that the Gibbs free energy barrier between the BCC structure and the low-population polyhedron in the fragile region is higher than the free energy barrier between the BCC structure and the icosahedral network.

Droplet Growth Dynamics during Atmospheric Condensation on Nanopillar Surfaces

ABSTRACTThe Gibbs free energy barrier for heterogeneous nucleation of a condensed droplet on a rough surface changes significantly with changes of humidity content in the condensing environment. The influence of environmental factors (ambient temperature and relative humidity) and substrate characteristics (topology, surface chemistry, and substrate temperature) on atmospheric condensation phenomenon is very important to elucidate the condensed droplet wetting state and condensate harvesting applications. Condensation from the humid air has been reported for plain silicon and fabricated nanopillar surfaces to facilitate condensate harvesting. Droplet growth and size distributions were recorded for 90 min. Spherical droplets condensed on the silicon surfaces and irregular-shaped droplets were observed on the nanopillar surfaces due to the pinning effect of the pillars. The effect of droplet pinning on coalescence events has been described based on the energy balance for the condensed droplets. A mathematical model reveals that certain dimensional combinations (pillar pitch, pillar diameter, and pillar height) of the nanopillar geometry are required to exhibit the pinning mechanism for condensed droplets. Regeneration of droplets was observed at void spaces generated from coalescence events. The growth of individual droplets was tracked over multiple time and length scales, starting from nucleation to get further insight into the direct growth and coalescence mechanisms.Abbreviation: ESEM: Environmental Scanning Electron Microscope; HCP: Hexagonal Closed-Packed; MPL: Microsphere Photolithography; RH: Relative Humidity

Photosensitized Membrane Permeabilization Requires Contact-Dependent Reactions between Photosensitizer and Lipids.

Although the general mechanisms of lipid oxidation are known, the chemical steps through which photosensitizers and light permeabilize lipid membranes are still poorly understood. Herein we characterized the products of lipid photooxidation and their effects on lipid bilayers, also giving insight into their formation pathways. Our experimental system was designed to allow two phenothiazinium-based photosensitizers (methylene blue, MB, and DO15) to deliver the same amount of singlet oxygen molecules per second to 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine liposome membranes, but with a substantial difference in terms of the extent of direct physical contact with lipid double bonds; that is, DO15 has a 27-times higher colocalization with ω-9 lipid double bonds than MB. Under this condition, DO15 permeabilizes membranes at least 1 order of magnitude more efficiently than MB, a result that was also valid for liposomes made of polyunsaturated lipids. Quantification of reaction products uncovered a mixture of phospholipid hydroperoxides, alcohols, ketones, and aldehydes. Although both photosensitizers allowed the formation of hydroperoxides, the oxidized products that require direct reactions between photosensitizer and lipids were more prevalent in liposomes oxidized by DO15. Membrane permeabilization was always connected with the presence of lipid aldehydes, which cause a substantial decrease in the Gibbs free energy barrier for water permeation. Processes depending on direct contact between photosensitizers and lipids were revealed to be essential for the progress of lipid oxidation and consequently for aldehyde formation, providing a molecular-level explanation of why membrane binding correlates so well with the cell-killing efficiency of photosensitizers.

Origin of efficient oxygen reduction reaction on Pd monolayer supported on Pd-M (M=Ni, Fe) intermetallic alloy

Intermetallic alloy has become a hot topic in heterogeneous catalysis due to improved activity and stability. We report here the results of Pd monolayer supported on face-centered tetragonal (fct) Pd-M (M = Ni or Fe) intermetallic alloy as cathode materials by first principle calculations. Adsorbed O2 is found to undergo hydrogenation process into OOH instead of direct dissociative mechanism. The oxygen reduction reaction (ORR) proceeds via four-electron transfer pathway on both surfaces. The enhanced activities are ascribed to moderate adsorption strength for ORR intermediates due to compression strain and substrate electron transfer. The dissolution potential analysis indicates that both Pd/PdNi(111) and Pd/PdFe(111) are stable under operating condition. Gibbs free energy analysis and barrier calculations suggest that the proposed Pd monolayer structures are promising for cathode material. These results can be useful in designing intermetallic Pd-M alloy as ORR electrocatalysts.

Theoretical insight into reaction mechanisms of 2,4-dinitroanisole with hydroxyl radicals for advanced oxidation processes.

The detailed degradation mechanism of an insensitive explosive, 2,4-dinitroanisole (DNAN), in advanced oxidation processes (AOPs) was investigated computationally at the M06-2X/6-311 + G(d,p)/SMD level of theory. Results obtained show that the addition-elimination reaction is the dominant mechanism. The phenol products formed can continue to be oxidized to benzoquinone radicals that are often detected by experiments and may be the initial reactants of ring-opening reactions. The H-abstraction reaction is an unavoidable competing mechanism; the intermediate generated can also undergo the process of addition-elimination reaction. The nitro departure reaction involves not only hydroxyl radical (•OH), but also other active substances (such as •H). More importantly, we found that AOP technology can easily degrade DNAN, similar to TNT and DNT. Thus, this method is worth trying in experiments. The conclusions of this work provide theoretical support for such experimental research. Graphical abstract Possible pathways of degradation by •OH radicals in advanced oxidation processes (AOPs) of the typical insensitive explosive 2,4-dinitroanisole (DNAN) were investigated by density functional theory (DFT) methods. Based on the Gibbs free energy barriers and intermediates, the dominant reaction mechanism was determined. The conclusions will be helpful in utilizing AOP technology to remove DNAN pollution.