Heterogeneous reaction mechanism of elemental mercury oxidation by oxygen species over MnO2 catalyst

MnO₂-based catalysts have attracted great attention in the field of elemental mercury (Hg⁰) catalytic oxidation because of their superior catalytic performance and wide temperature window. Quantum chemistry calculations based on density functional theory (DFT) combined with periodic slab models were carried out to investigate the heterogeneous mechanism of Hg⁰ oxidation by oxygen species (gas-phase O₂, chemisorbed oxygen, and lattice oxygen) on MnO₂ surface. The results indicate that Hg⁰ and HgO are chemically adsorbed on MnO₂ surface with the adsorption energies of ?69.50 and ?226.48?kJ/mol, respectively. The adsorption of O₂ on MnO₂ surface belongs to chemisorption. O₂ can decompose on MnO₂ surface with an energy barrier of 97.46?kJ/mol to produce two atomic adsorbed oxygen. The perpendicular adsorbed O₂ and dissociative adsorbed O₂ are more favorable for Hg⁰ catalytic oxidation than lattice oxygen, and perpendicular adsorbed O₂ is the most active oxygen for Hg⁰ oxidation. The reaction pathway of Hg⁰ oxidation by perpendicular adsorbed O₂ includes three reaction steps: Hg⁰?→?Hg(ads)?→?HgO(ads)?→?HgO. The third step (HgO(ads)?→?HgO) is endothermic by 168.17?kJ/mol with an energy barrier of 179.48?kJ/mol, and it is the rate-limiting step of the whole Hg⁰ oxidation reaction.     

Adsorption of water molecules on yttrium barium cuprate superconductors

Gravimetry and thermogravimetric analysis were used to study the adsorption of water molecules on the high temperature superconductor YBa₂Cu₃O₇ at room temperature. It was found that water adsorption subdivides into surface adsorption and bulk adsorption, which starts after the formation at the surface of a physically bound water layer no less than 65–100 Å thick. During bulk adsorption, H₂O molecules diffuse from this surface layer to the lattice, where they form four bound states with desorption temperatures of ～208, 330, 370, and 775°C and heats of formation of 38, 99, 72, and 68 kJ/mol, respectively, and mainly occupy interstitial sites of the intermediate layers. The presence of molecules in the lattice does not affect either the superconducting transition temperature or resistance to direct current; however, it results in an increase in the surface resistance. The resistance to direct current increases due to the formation of dielectric inclusions of other phases.     

Mechanism and kinetics of Cu2O oxidation in chemical looping with oxygen uncoupling

In chemical looping with oxygen uncoupling, oxygen carrier (OC) circulates between the fuel and air reactors to release and absorb O₂ repeatedly. In order to assess the re-oxidation characteristic of Cu-based OC in the air reactor from the microscopic mechanism and macroscopic kinetics perspective, DFT calculations and isothermal oxidation experiments were conducted. In DFT calculations, Cu₂O(111) surface was chosen as the objective surface to explore the oxygen uptake as well as the atomic transportation pathways, and to determine the rate-limiting steps basing on the energy barrier analyses. It was found that the energy barrier of the surface reaction step (0.96?eV) is smaller than that of the ions diffusion step (1.61?eV). Moreover, the Cu cations outward diffusion occurs more easily than O anions inward diffusion, which confirmed the epitaxial growth characteristic of Cu₂O oxidation. The isothermal oxidation experiments were conducted in a thermogravimetric analyzer (TGA), and about 3.5?mg CuO@TiO₂-Al₂O₃ particles within the diameter range of 75–110?µm were tested between 540 and 600 °C, where the internal and external gas diffusion effects were eliminated. Mixtures of 5.2-21.0?vol.% O₂ in N₂ were adopted as the gas agent for oxidation. Based on the understandings obtained from DFT calculations, a simple mathematical model with unknown parameters of the surface reaction process (mainly the activation energy, E_k) and ions diffusion process (mainly the activation energy, E_D) was established to describe the overall oxidation process in TGA experiments. Eventually, these unknown parameters were determined as E_k=?50.5?kJ/mol and E_k=?79.2?kJ/mol via global optimization. With the attained parameters, simulations reproduced the experimental results very well, which demonstrated that this simplification model, where grain is converted almost layer by layer but different from the feature of the shrinking core model is able to accurately describe the overall oxidation process of Cu₂O.     

Enthalpy of formation of LiNiO2, LiCoO2 and their solid solution,LiNi1−xCoxO2

LiCoO₂, LiNiO₂ and their solid solution, LiNi_1−xCo_xO₂, are important cathode materials for lithium ion batteries. Samples in this system were synthesized by solid state reaction of Co₃O₄, NiO and Li₂CO₃ or LiOH·H₂O. Their lattice parameters were determined by Rietveld refinement. High temperature drop solution calorimetry in molten 3Na₂O·4MoO₃ and 2PbO·B₂O₃ solvents at 974 K was performed to determine the enthalpy of formation from the constituent oxides plus oxygen and the enthalpy of mixing in the solid solution series. There are approximately linear correlations between the lattice parameters, the enthalpy of formation from oxides (Li₂O, NiO and CoO) plus O₂ and the Co content in the compounds. The solid solution of LiCoO₂ and LiNiO₂ is almost ideal, showing a small positive enthalpy of mixing. The enthalpy of formation of LiCoO₂ from oxides (Li₂O, NiO and CoO) and oxygen at 298 K is −142.5±1.7 kJ/mol (from sodium molybdate calorimetry) or −140.2±2.3 kJ/mol (from lead borate calorimetry). That of LiNiO₂ is −56.2±1.5 kJ/mol (from sodium molybdate calorimetry) or −53.4±1.7 kJ/mol (from lead borate calorimetry). The cobalt compound is thus significantly more stable than its nickel analogue. The phase assemblage LiCoO₂, Li₂O and CoO is seen at a lower oxygen pressure at constant temperature than the assemblage Co₃O₄/CoO, reflecting the stabilization of Co(III) in the ternary Li–Co–O system.     

Weakly interacting molecular clusters of CO with H2O,SO2, and NO+

Intermolecular interactions in three dimers, CO···H₂O, CO···SO₂, and CO···NO⁺, were studied at the CCSD(T) level of theory, using a series of the augmented correlation consistent polarised basis sets. Interaction energy and its components as well as vibrational spectra for local minima were computed using both harmonic and anharmonic approximations. While CO···H₂O and CO···SO₂ are weakly bound with the binding energies ?7.4 and ?6.4 kJ/mol, CO···NO⁺ is much more stable with the binding energy of ?32.8 kJ/mol corresponding to ΔG = ?4.7 kJ/mol at 254 K.     

Reaction mechanism of elemental mercury oxidation to HgSO4 during SO2/SO3 conversion over V2O5/TiO2 catalyst

Experiments and density functional theory calculations were conducted to uncover the reaction chemistry of Hg⁰ oxidation during SO₂/SO₃ conversion over V₂O₅/TiO₂ catalyst. The results show that SO₂ promotes Hg⁰ oxidation over V₂O₅/TiO₂ catalyst with the assistance of oxygen. The promotional effect is dependent on the reaction temperature, and is associated with the bimolecular reaction between Hg⁰ and SO₃ over V₂O₅/TiO₂ catalyst. SO₂ can be oxidized to SO₃ which has high oxidation ability for Hg⁰ oxidation. SO₂/SO₃ conversion proceeds through a three-step reaction process in the sequence of SO₂ adsorption → SO₂ oxidation → SO₃ desorption. SO₂ oxidation presents an activation energy barrier of 223.84 kJ/mol. HgSO₄ species is formed from the bimolecular reaction between Hg⁰ and SO₃ over V₂O₅/TiO₂ catalyst. Hg⁰ oxidation by SO₃ over V₂O₅/TiO₂ catalyst occurs through three reaction pathways, which are energetically favorable for HgSO₄ formation. SO₂* → SO₃* is identified as the rate-determining step of HgSO₄ formation. During Hg⁰ oxidation by SO₃ over V₂O₅/TiO₂ catalyst, HgSO₄ desorption is a highly endothermic reaction process and requires a higher external energy. The proposed skeletal reaction network can be used to well understand the reaction mechanism of Hg⁰ oxidation during SO₂/SO₃ conversion over V₂O₅/TiO₂ catalyst.     

Point-defect properties of and sputtering events in the {001} surfaces of Ni3Al I. Surface and point-defect properties

Constant-area and fully relaxed molecular dynamics methods are employed to study the properties of the surface and point defects at and near {001} surfaces of bulk and thin-film Ni, Al and Ni₃Al respectively. The surface tension is larger than the surface energy for all {001} surfaces considered in the sequence: Al (1005?mJ?m^?2)<?Ni₃Al (mixed Ni–Al plane outermost, 1725?mJ?m^?2)<?Ni₃Al (all-Ni-atoms plane outermost, 1969?mJ?m^?2)<?Ni (1993?mJ?m^?2). For a surface of bulk Ni₃Al crystal with a Ni–Al mixed plane outermost, Al atoms stand out by 0.0679?Å compared with the surface Ni atoms and, for the all-Ni-atoms surface, Al atoms in the second layer stand out by 0.0205?Å compared with Ni atoms in the same layer. Vacancy formation energies are about half the bulk values in the first layer and reach a maximum in the second layer where the atomic energy is close to the bulk value but the change in embedding energy of neighbouring atoms before and after vacancy formation is greater than that in the bulk. Both the vacancy formation energy and the surface tension suggest that the fourth layer is in a bulk state for all the surfaces. The formation energy of adatoms, antisite defects and point-defect pairs at and near {001} surfaces of Ni₃Al are also given.     

Reaction mechanism of ammonia oxidation over RuO2(1 1 0): A combined theory/experiment approach

Combining state-of-the-art density functional theory (DFT) calculations with high resolution core level shift spectroscopy experiments we explored the reaction mechanism of the ammonia oxidation reaction over RuO₂(1 1 0). The high catalytic activity of RuO₂(1 1 0) is traced to the low activation energies for the successive hydrogen abstractions of ammonia by on-top O (less than 73 kJ/mol) and the low activation barrier for the recombination of adsorbed O and N (77 kJ/mol) to form adsorbed NO. The NO desorption is activated by 121 kJ/mol and represents therefore the rate determining step in the ammonia oxidation reaction over RuO₂ (1 1 0).     

硫酸(氢)根吸附层对Pd(111)电极上甲酸氧化反应的影响研究

本文使用循环伏安法和电势阶跃法分别研究了添加和不添加Na₂SO₄的0.1 mol/LH₂SO₄+0.1 mol/LHCOOH溶液中Pd(111)电极上甲酸氧化反应(FAO)的动力学行为,并与同样条件下0.1 mol/LHClO₄中的动力学行为进行比较. 加入0.05 mol/L或者0.1 mol/LNa₂O₄后,在相同的电位下负向扫描的FAO电流比正向扫描的显著减小. 本文推测在(SO₄^*_ad)_m+[(H₂O)_n-H₃O⁺]或(SO₄^*_ad)_m+[Na⁺(H₂O)_n-H₃O⁺]吸附层相转变电势以正的电位, 这个吸附层的结构可能随着电位的增加或Na₂SO₄的加入变得更加致密和稳定. 因此,破坏或者脱附致密的硫酸(氢)根吸附层变得更加困难,使得FAO 动力学在较高电位和随后的负扫电位受到明显的抑制.     

Fe-Cr-Al合金氧化膜形成机理电子理论研究

通过自编软件建立了Fe-Cr-Al合金表面、氧化膜/基体界面模型,采用递归法计算了合金元素在Fe-Cr-Al合金表面、氧化膜/基体界面的环境敏感镶嵌能、亲和能、结合能、态密度等电子结构参数.从电子层次系统研究了Fe-Cr-Al合金氧化膜的形成机理、稀土元素和杂质硫对氧化膜形成过程及黏附性的影响机理.研究表明Fe-Cr-Al合金中Al的偏聚驱动力远大于Y,Cr.氧化初期氧从合金表面向合金内部扩散,合金内部Al向合金表面扩散,使合金形成富铝、氧表面层;氧与Al间的亲和力较大（亲和能低）,氧原子容易与Al结合生成Al₂O₃保护膜;合金中加入Y后,Y在合金表面偏聚,抑制Al向合金表面扩散,氧化膜的横向生长得到有效控制,从而避免氧化膜皱褶形貌的发生,提高氧化膜的黏附性;合金内部的S通过扩散汇集在基体/氧化膜界面,S使界面区原子的总能增高,总态密度降低,减小了界面的稳定性,进而削弱氧化膜与合金基体的结合力. 关键词： 电子结构 高温氧化 Fe-Cr-Al合金     

Roles of γ-Fe2O3 in fly ash for mercury removal: Results of density functional theory study

First-principle calculations based on density function theory (DFT) are used to clarify the roles of γ-Fe₂O₃ in fly ash for removing mercury from coal-fired flue gases. In this study, the structure of key surface of γ-Fe₂O₃ is modeled and spin-polarized periodic boundary conditions with the partial relaxation of atom positions are employed. Binding energies of Hg on γ-Fe₂O₃ (0 0 1) perfect and defective surfaces are calculated for different adsorption sites and the potential adsorption sites are predicted. Additionally, electronic structure is examined to better understand the binding mechanism. It is found that mercury is preferably adsorbed on the bridge site of γ-Fe₂O₃ (0 0 1) perfect surface, with binding energy of −54.3 kJ/mol. The much stronger binding occurs at oxygen vacancy surface with binding energy of −134.6 kJ/mol. The calculations also show that the formation of hybridized orbital between Hg and Fe atom of γ-Fe₂O₃ (0 0 1) is responsible for the relatively strong interaction of mercury with the solid surface, which suggests that the presently described processes are all noncatalytic in nature. However, this is a reflection more of mercury's amalgamation ability.     

Oxidation enthalpies for reduction of ceria surfaces

The thermodynamic properties of surface ceria were investigated through equilibrium isotherms determined by flow titration and coulometric titration measurements on high-surface-area ceria and ceria supported on La-modified alumina (LA). While the surface area of pure ceria was found to be unstable under redox conditions, the extent of reduction at 873 K and a P(O₂) of 1.6 × 10⁻²⁶ atm increased with surface area. Because ceria/LA samples were stable, equilibrium isotherms were determined between 873 and 973 K on a 30-wt% ceria sample. Oxidation enthalpies on ceria/LA were found to vary with the extent of reduction, ranging from −500 kJ/mol O₂ at low extents of reduction to near the bulk value of −760 kJ/mol O₂ at higher extents. To determine whether +3 dopants could affect the oxidation enthalpies for ceria, isotherms were measured for Sm⁺³-doped ceria (SDC) and Y⁺³-doped ceria. These dopants were found to remove the phase transition observed in pure ceria below 973 K but appeared to have minimal effect on the oxidation enthalpies. Implications of these results for catalytic applications of ceria are discussed.     

Soot low-temperature combustion on Cu–Zr/ZSM-5 catalysts in O2/He and NO/O2/He atmospheres

Zirconium doped Cu/ZSM-5 catalysts were prepared and characterized in this investigation. Catalytic activity during soot combustion was determined in both O₂/He and NO/O₂/He atmospheres by temperature-programmed oxidation. The use of zirconium reduces the temperature of maximum soot oxidation rate by 229 °C in O₂/He atmosphere and 270 °C in NO/O₂/He atmosphere. The promoting effect of zirconium is discussed in terms of surface dispersion, enrichment of active components, and creation of oxygen vacancies where molecular oxygen or NO_x is adsorbed forming basic surface oxygen species active for soot oxidation. The NO₂ formed at the copper–zirconium interface sites leads to the ignition temperature being significantly decreased to 93 °C, which is inside the exhaust temperature range of diesel engines. To understand the combustion reaction kinetics, the activation energy and reaction order of soot combustion were evaluated. According to the Redhead method, the activation energy for non-catalyzed reaction is 164 kJ/mol under the O₂/He atmosphere. For the Cu/ZSM-5 and Cu–Zr/ZSM-5, the activation energies under the O₂/He atmosphere (134–151 kJ/mol) are slightly higher than those under the NO/O₂/He atmosphere (128–135 kJ/mol). The Freeman–Carroll method is suitable to describe the soot combustion in the NO/O₂/He atmosphere, with the activation energies for the catalysts in the range of 97–112 kJ/mol and the average value of reaction order equal to 1.36.     

Nb-Al合金高温氧化机理

通过自编软件建立了铝氧化膜与基体铌界面的原子集团模型,用递归法计算了合金的原子埋置能、原子结合能等电子参数,从电子层面分析铌合金高温氧化机理.研究表明:铝通过晶界扩散偏聚在合金表面,并与氧结合生成致密的Al2O3氧化膜,阻挡氧向铌基体扩散.晶界和稀土元素能提高氧化膜与基体间的原子结合能,增加其界面的结合强度,加强氧化膜与基体铌间的黏附性.因此,通过在合金中添加稀土元素或细化合金晶粒均能提高铌合金的抗高温氧化性能.     

O2、 CO2和H2O在UN(001)表面化学吸附的密度泛函理论研究

采用广义梯度近似GGA,修正Perdew-Burke-Ernzerhof交换-关联泛函,以及周期性切片模型对O₂、CO₂和H₂O在UN(001)表面的化学吸附行为进行非自旋极化水平的密度泛函理论计算. 在四个对称性化学位置条件下,对化学吸附能与分子和UN(001)表面之间距离的关系曲线进行优化. 结果表明O₂、CO₂和H₂O分子的最稳定吸附位置分别为桥式平行、空心平行和桥式H     

Oxidation of hydrogen sulfide and corrosion of stainless steel in gas mixtures containing H<Subscript>2</Subscript>S,O<Subscript>2</Subscript>, H<Subscript>2</Subscript>O,and CO<Subscript>2</Subscript>

The composition of volatile and solid products of oxidation of hydrogen sulfide and stainless steel in gas mixtures containing H₂S, O₂, H₂O, and CO₂ has been determined using mass spectrometry, x-ray diffraction analysis, and scanning electron microscopy. It has been shown that holding an H₂S–O₂ mixture at 301 K results in prevailing formation of elemental sulfur and iron sulfides in the form of porous hygroscopic crust on the reactor wall surface. Formation of gas-phase sulfur causes self-acceleration of the oxidation of hydrogen sulfide; the resulting water triggers corrosion of the reactor wall. Heating of the resulting sulfur-sulfide crust in O₂ medium is accompanied by formation of SO₂ and heat release at T > 508 K. After heating of the H₂S–CO₂ mixture to 615 K, H₂ and COS were found in the volatile reactants; no noticeable corrosion of the reactor wall has been detected. It has been established that addition of O₂ to the H₂S–CO₂ mixture and its heating to 673 K leads to formation of ferrous sulfates. The mechanisms of the observed processes are discussed.     

Influence of oxalate anions on manganese electrodeposition in sulfate solution

The influences of oxalate anions on manganese electrodeposition in sulfate solution were investigated on the basis of cathode current efficiency, characterization of SEM-EDX and XRD, solution chemistry calculation, thermodynamics and electrochemical test. The experimental results show that the range of (NH₄)₂C₂O₄ was adjusted from 0 mol/L to 4.8?×?10^?3 mol/L. And 1.5?×?10^?3 mol/L (NH₄) ₂C₂O₄ was suitably used with initial pH 7.0. The characterization of SEM indicates that oxalate anions can improve the morphology of electrodeposited films. The electrodeposited films containing manganese were characterized and determined by EDX and XRD. The solution chemistry calculation of catholyte and oxalate anions shows that the main active species are MnSO₄, Mn(SO₄)2? 2, Mn²⁺, Mn(SO₄)C₂O2? 4, MnC₂O 4, Mn(NH₃)²⁺, and C₂O2? 4. The reaction trend between C₂O2? 4 and Mn²⁺ ions is confirmed by computation of reaction energy. Electrochemical test analysis indicates oxalate anions increase the overpotentials of hydrogen evolution reaction and manganese electrodeposition.     

Formation of combustible gases during interaction of tungsten and zirconium with supercritical fluid H2O/CO2

Oxidation of bulk samples of tungsten (923 K) and zirconium (773 and 873 K) by H₂O/CO₂ supercritical fluid (molar ratio [CO₂]/[H₂O] = 0.17–0.26) at a pressure of about 300 atm is investigated. Oxidation produces monoclinic WO₃, monoclinic W₁₉O₅₅, monoclinic ZrO₂, H₂, CO, CH₄, and carbon (on the surface of tungsten oxide). Differences in oxidation mechanisms for tungsten and zirconium are revealed. CO₂ molecules take part in the oxidation of tungsten only after oxide formation in reaction with H₂O. Zirconium is oxidized fully, and oxidation of tungsten terminates in the formation of the oxide layer at the metal surface.     

DFT study on the mechanism of trimolecular radical reactions: isomerisation in small clusters

ABSTRACT

Structural and thermodynamic properties of 48 trimolecular clusters containing one radicl and two protic molecules (H₂O, NH₃, H₂O₂, CH₃OH, HOCl) were studied at B3LYP/6-311++G(3df,3pd) level of theory. These radical-clusters have non-cyclic structures and are stabilised via two inter-molecular hydrogen bonding interactions. The calculated enthalpies of formation of the radical-clusters were generally in the range of ?30 to ?50 kJ/mol. The calculated activation energies (E_a) of the intra-cluster hydrogen transfers were smaller than 70 kJ/mol. Also, structures and thermodynamics of 15 cyclic molecular clusters as well as multi-hydrogen transfers in them were investigated. The results showed that the stability of the cyclic clusters and activation energies of the multi-hydrogen transfers depend on the cluster size.     

Experimental determination of U diffusion in α-Ti

α-spectrometry was used in order to measure the diffusion of U in bulk α-Ti in the temperature range 863–1123?K (540–850?°C). A straight Arrhenius plot was found, giving diffusion parameters Q?=?297?kJ/mol and D ₀?=?5?×?10^?3?m²/s, which are similar to the α-Ti self-diffusion ones, when measured in Ti samples with a similar impurity content than presently. This behaviour is compatible with the hypothesis of U diffusing via a vacancy-assisted mechanism in the α-Ti lattice and contrasts with older results in which the activation energy is almost a third the self-diffusion one, even lower than the vacancy formation energy.