Selecting oxygen source for partial oxidation of methane to methanol using microwave plasmas

The methanol selectivity in partial oxidation of methane in microwave plasma reactors is improved by using H₂O in the presence or absence of O₂. The use of H₂O₂ as an oxygen source has a similar effect, although it is less effective than H₂O. The addition of H₂ to the system has little effect on selectivity. Two pathways are suggested for the formation of methanol. One involves a CH₃O^* or CH₃O₂ ^* intermediate, while the other involves a direct combination of CH₃ ^* and OH^* radicals. The first pathway is favored in the presence of O₂ while the latter is favored in the presence of H₂O or H₂O₂. The best results are obtained for the CH4-O₂-H₂O system when methanol is formed through both pathways.     

Density Functional Theory Study for Adsorption of Oxygen and Water Molecules on 6H-SiC(0001) Surface

6H-SiC is an important semiconductor material. The 6H-SiC wafer is always exposed to a high-humidity environment and the effect from the absorbed water molecule and some relative adsorbates is not negligible. Here, the oxygen and water molecules absorbed on the 6H-SiC(0001) surface and the dissociation process were studied with density functional theory. On the 6H-SiC(0001) surface, absorbed O₂ is spontaneously dissociated into O^*, which is absorbed on a hollow site, and further transforms the 6H-SiC(0001) surface into SiO₂. The absorbed H₂O is spontaneously broken into OH^*and H^*, which are both absorbed on the top of the Si atom, and OH^* is further reversibly transformed into O^* and H^*. The H^* could saturate the dangling Si bond and change the absorption type of O^*, which could stabilize the 6H-SiC(0001) surface and prevent it from transforming into SiO₂.     

Theoretical studies of O2−:(H2O)n clusters

The interaction of superoxide ion O₂^? with up to four water molecules [O₂^?: (H₂O)_n, n = 1, 2, 4] has been investigated using ab initio molecular orbital theory. The binding energy of O₂^?: H₂O is calculated to be ?20.6 kcal/mol in good agreement with gas phase experimental data. At the MP3/6-31G^* level the O₂^?:H₂O complex has a C_2v structure with a double (cyclic) hydrogen bond between O₂^? and H₂O. A C_s structure with a single hydrogen bond is only 0.7 kcal/mol less stable. Interaction of H₂O with the doubly occupied π^* orbital of O₂^? is preferred slightly over interaction with the singly occupied π^* orbital. Natural bond orbital analysis suggests that both electrostatic and charge transfer interactions are important in anionic complexes. The charge transfer occurs predominantly in the O₂^? → H₂O direction and is important in determining the relative stabilities of the different structures and states. Singly and doubly hydrogen-bonded structures for the O₂^?: (H₂O)₂ and O₂^?: (H₂O)₄ clusters were found to be similar in stability and the increase in binding of the cluster becomes smaller as each additional water molecule is added to the cluster.     

Reaction Mechanism on the Synergistic Photodegradation of Dye X3B and Photoreduction of Dichromate in the Presence of Polyoxometalate

Photodegradation of a textile dye X3B and photoreduction of dichromate (Cr(VI)) in an acidic aqueous solution were studied under 320 nm cut-off UV light irradiation in the presence of two polyoxometalates (POM), H₃PW₁₂O₄₀ (PW), and H₄SiW₁₂O₄₀ (SiW). The reactions in POM-X3B-Cr(VI) system were faster than those in POM-X3B, POM-Cr(VI), and X3B-Cr(VI) systems. For all reactions, PW was more photoactive than SiW. The reaction rates were proportional to the initial concentration of each component. The effects of N₂, O₂, and air were small but regular, indicating Cr(VI) photoreduction by a reduced POM. Quenching experiments with H₂O₂ and ethanol revealed that X3B photodegradation mainly occurred through hydroxyl radical (OH). It was proposed that the production of OH and a reduced POM⁻ by the reaction between H₂O and excited POM^* was the rate determining step, with which all evidence could be well interpreted. Different effects of POM concentration in a two- or three-component system on the reaction rates suggested that the reaction between H₂O and excited POM^* was reversible.     

Thermal and kinetic analysis of uranium salts

The thermal decomposition kinetics of UO₂C₂O₄·3H₂O were studied by TG method in a flowing nitrogen, air, and oxygen atmospheres. It is found that UO₂C₂O₄·3H₂O decomposes to uranium oxides in four stages in all atmosphere. The first two stages are the same in the whole atmosphere that correspond to dehydration reactions. The last two stages correspond to decomposition reactions. Final decomposition products are determined with X-Ray powder diffraction method. Decomposition mechanisms are different in nitrogen atmosphere from air and oxygen atmosphere. The activation energies of all reactions were calculated by model-free (KAS and FWO) methods. For investigation of reaction models, 13 kinetic model equations were tested and correct models, giving the highest linear regression, lowest standard deviation, and agreement of activation energy value to those obtained from KAS and FWO equations were found. The optimized value of activation energy and Arrhenius factor were calculated with the best model equation. Using these values, thermodynamic functions (??H ^*, ??S ^*, and ??G ^*) were calculated.     

Antigenes de groupe sanguin : Synthese de glycopeptides Tn representant la partie N-terminale de la glycophorine humaine AN et AMc

This report describes the first synthesis of the triglycosylated pentapeptides H₂N—Leu—Se^*r—Th^*r—Th^*r—Gl^*u—OH et H₂N—Ser—Se^*r—Th^*r—Th^*r—Glu—OH, in which 1 represents the 2-acetamido-2-deoxy α-D-galactopyranosyl residues. These compounds constitute the antigenic aminoterminal portions of respectively human glycophorin A^N and A^Mc. The above compounds were obtained by a stepwise peptide coupling strategy in solution, beginning at the C terminus and proceeding toward the N-terminus, using amino acids or suitably protected and activated O-glycosyl-amino acids. Carbohydrate residues were introduced into the sequence as 2-azido-2-deoxy-α-D-galactopyranosyl-l-serine and l-thréonine derivatives obtained from the reducing sugar and amino acid by the trifluoromethanesulfonic anhydride procedure. Reduction of the azido functions followed by acetylation and catalytic hydrogenolysis afforded the above antigenic T_N glycosylated pentapeptides.     

Isotope effect in the formation of hydrogen peroxide by the sonolysis of light and heavy water

The kinetics of the formation of hydrogen peroxide by the sonolysis of light and heavy water in argon and oxygen atmospheres was investigated. The sonochemical reaction has a zero order with respect to hydrogen peroxide (H₂O₂, D₂O₂, or DHO₂). The measurement of the kinetic isotope effect (α), defined as the ratio of the reaction rates in H₂O and D₂O, carried out under argon gave a value of 2.2±0.3. The observed isotope effect decreases with an increase in the concentration of light water in H₂O−D₂O mixtures. No isotope effect is displayed in the oxygen atmosphere (α=1.05±0.10). The isotope effect is determined presumably by the mechanism of sonochemical decomposition of water molecules, which includes the H₂O−Ar^* and D₂O−Ar^* energy exchange (where Ar^* are argon atoms in the³P_2.0 excited state) in the nonequilibrium plasma generated by a shock wave, arising upon a cavitation collapse. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 645–649, April, 2000.     

H2-induced CO adsorption and dissociation over Co/Al2O3 catalyst

The activation of adsorbed CO is an important step in CO hydrogenation. The results from TPSR of pre-adsorbed CO with H2 and syngas suggested that the presence of H2 increased the amount of CO adsorption and accelerated CO dissociation. The H2 was adsorbed first, and activated to form H＊ over metal sites, then reacted with carbonaceous species. The oxygen species for CO2 formation in the presence of hydrogen was mostly OH^＊, which reacted with adsorbed CO subsequently via CO^＊＋OH^＊ → CO2^＊＋H^＊; however, the direct CO dissociation was not excluded in CO hydrogenation. The dissociation of C-O bond in the presence of H2 proceeded by a concerted mechanism, which assisted the Boudourd reaction of adsorbed CO on the surface via CO^＊＋2H^＊ → CH^＊＋OH^＊. The formation of the surface species （CH） from adsorbed CO proceeded as indicated with the participation of surface hydrogen, was favored in the initial step of the Fischer-Tropsch synthesis.     

Dissociative electron attachment to polyatomic molecules: Ion kinetic energy measurements

Dissociative electron attachment to SO₂, NO₂, NF₃ and H₂O₂ is studied in terms of the kinetic energies of the dominant fragment ions. The O^? data from SO₂ show that the two major resonances at 4.6 and 7.2 eV respectively have the same dissociation limit. Similarly, the resonances at 1.8 and 3.5 eV in the O^? channel in NO₂ appear to have same dissociation limit of NO (X ²Π) + O^?, while the resonance at 8.5 eV appears to dissociate to give NO (a ⁴Π_i) along with O^?. We find considerable internal excitation of the neutral fragments in all these cases along with that of NF₃, whereas the negative ion resonance in H₂O₂ appears to fragment almost like a diatomic system with very little internal excitation of the OH and OH^? fragments.     

17O and 183W NMR studies of the paratungstate anions in aqueous solutions

¹⁷O (40.7 MHz) and ¹⁸³W (12.5 MHz) NMR spectra of aqueous Na₁₀[H₂W₁₂O₄₂]·27H₂O (1), Na₆[W₇O₂₄]·14H₂O (2) and (NH₄)₆[Mo₇O₂₄]·nH₂O solutions, as well as of 2, 1 and 0.1 M Na₂WO₄ and 2 M Li₂WO₄ solutions acidified up to P = 0.5, 1 and 1.14 have been measured. The composition of the W₇O₂₄^6? anion remains unchanged (2), its structure being similar to that of Mo₇O₂₄^6?183W NMR spectrum shows three resonances with the chemical shifts + 269.2, ?98.8 and ?178.9 ppm relative to WO₄^2? and intensity ratio 1:4:2. “Paratungstate A” produced during polycondensation of WO₄^2? at P ? 1.17 is identical with heptatungstate W₇O₂₄^6?. The [H₂W₁₂O₄₂]^10?183W NMR spectrum in the acidified 2 M Li₂WO₄ solution has four resonances with the chemical shifts in the range - 105–145 ppm and intensity ratio 1:2:1:2. As suggested by NMR data, the H₂W₁₂O₄₂^10? ? W₇O₂₄^6? transformations occur, which depend upon concentration and temperature.     

A First-Principles Study of CO2 Hydrogenation on a Niobium-Terminated NbC (111) Surface

As promising materials for the reduction of greenhouse gases, transition-metal carbides, which are highly active in the hydrogenation of CO₂, are mainly considered. In this regard, the reaction mechanism of CO₂ hydrogenation to useful products on the Nb-terminated NbC (111) surface is investigated by applying density functional theory calculations. The computational results display that the formation of CH₄, CH₃OH, and CO are more favored than other compounds, where CH₄ is the dominant product. In addition, the findings from reaction energies reveal that the preferred mechanism for CO₂ hydrogenation is thorough HCOOH^*, where the largest exothermic reaction energy releases during the HCOOH^* dissociation reaction (2.004 eV). The preferred mechanism of CO₂ hydrogenation towards CH₄ production is CO₂^*→t,c-COOH^*→HCOOH^*→HCO^*→CH₂O^*→CH₂OH^*→CH₂^*→CH₃^*→CH₄^*, where CO₂^*→t,c-COOH^*→HCOOH^*→HCO^*→CH₂O^*→CH₂OH^*→CH₃OH^* and CO₂^*→t,c-COOH^*→CO^* are also found as the favored mechanisms for CH₃OH and CO productions thermodynamically, respectively. During the mentioned mechanisms, the hydrogenation of CH₂O^* to CH₂OH^* has the largest endothermic reaction energy of 1.344 eV.     

Kinetic isotope effect in hydrogen isotope exchange between water and phosphines

The kinetic parameters of the tritium exchange between^*H₂O(^*D₂O) and (CH₃)₂PH in the gas phase and between^*H₂O(^*D₂O) and (C₆H₅)₂PH in liquid were measured. Both reactions appeared to be heterogeneous ones. The^*HH/^*DD kinetic isotope effects were estimated and compared with the results of exchange between methanol and phosphines. The differences in isotope effects are explained by the change of the symmetry of the four center cyclic transition complex resulting from the various type of solvation of this complex.     

Changes in the inner coordination sphere as the primary chemical act of excited Gd3+aq

Photoexcited Gd(OH₂)³⁺₈ frees two water molecules to give ^*Gd(OH₂)³⁺₆ in an entropy-controlled process (ΔS^O ～ 9.5 cal/mol deg, ΔH^O ～ 0.32 kcal/mol, ΔG^O ～ ?2.5 kcal/mol) and secondary excited-state processes. This inner-sphere change may be rationalised by different 4f-shell contraction and 5p-shell expansion in the lowest and excited levels of the 4f⁷ configuration.     

Electron transfer oxidation kinetics of Na2[Mo2O4EDTA] by S2O82− ion in dilute acid medium

Na₂(Mo₂^vO₄EDTA).4H₂O crystals have been prepared in pure form. Kinetics for the oxidation of the compound by S₂O₈²⁻ have been studied spectrophotometrically in dilute sulphuric acid medium. The effects of hydrogen ion concentration, metal ion concentration, S₂O₈²⁻ ion concentration, and temperature on the process have been studied. Rate equations have been derived to explain the experimental observations. On the basis of the observations, suitable mechanisms have been suggested. The kinetic parameters E^*, ΔH^≠, and ΔS^≠ of the process have also been determined. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 269–274, 1997     

Non-empirical SCF and Cl Study of the ground and excited states of thioformaldehyde

Ab initio SCF and Cl calculations are reported for ground and various low-lying Rydberg and valence excited states of thioformaldehyde H₂CS. A double-zeta basis of near Hartree-Fock quality is employed in this work and the importance of polarization functions is also assessed. The calculations indicate uniformly larger CX bond lengths in this system than for H₂CO in the corresponding electronic states; they also lind potential minima for H₂CS non-planar nuclear conformations in the (n,π^*) and (π,π^*) excited states but in each case the calculated inversion barriers are seen to be smaller than those encountered in formaldehyde. The vertical transition energies to the various excited states studied are also found to be significantly smaller in H₂CS than in H₂CO but the order of electronic states is concluded to be virtually identical for the two systems. The lowest-lying excited states are the ^3,1(n,π^*) species calculated at 1.84 and 2.17 eV respectively; the first two allowed transitions are indicated to be the Rydberg species (n,s_R) and (n,px_R) at 5.83 and 6.62 eV. These are followed by the two allowed transitions σ → π^* and π → π^* at 7.51 and 7.92 eV respectively, both well below the first ionization limit in H₂CS. The much smaller splitting between the ^3,1(π,π^*) species in H₂CS than in H₂CO is attributed to the relatively diffuse charge distribution of the sulfur atom compared to that of oxygen.     

Study on the extraction of U(IV) relevant to PUREX process

The paper presents a radiokinetic study on the appearance and growth of^*Fe₂S₃,^*Fe(OH)₃,^*Fe₂(C₂O₄)₃,^*Fe(IO₃)₃ crystals in a colloidal medium of agar and gelatine. The values of the diffusion constants through gels of⁵⁵⁺⁵⁹Fe³⁺ radioactive cations and of the rate of global growth process of these crystals in agar or gelatine were calculated using the experimental data. A new method for the determination of the starting time for the complex nucleation process was proposed. The formation rate of crystals under study decreases in the order:^*Fe(OH)₃>^*Fe(IO₃)₃>^*Fe₂S₃>^*Fe₂(C₂O₄)₃, in agar medium and^*Fe(OH)₃>^*Fe(IO₃)₃>^*FeC₂O₄)₃>^*FeS₃, in gelatine medium.     

Collisional quenching of K* (4p2p) and K* (5p2p by H2O,CF4, and CH4

Rate constants are reported for collisional quenching of K^* (4p^2p and K^* (5p^2p) by H₂O, CF₄ and CH₄. The K^* (4p^2p or K^*(5p^2p) is produced by photodissociation of K1 vapor at 2450 Å or 1925 Å, respectively. As for Na^* (3p^2p, H₂O, CF₄, and CH₄ are very inefficient at quenching K^* (4p^2p); however, they are very efficient at quenching K^* (5p^2p).     

金(I)配合物催化乙烯加氢的反应机理

采用密度泛函理论B3LYP方法, 对两类金(I)配合物AuX (X=F, Cl, Br, I)和AuPR₃⁺(R=F, Cl, Br, I, H, Me,Ph)催化C₂H₄加氢反应的机理进行了理论研究. 计算显示Au(I)配合物对C₂H₄氢化具有较好的催化效果, 其作用下的加氢反应存在“活化H―H键后再与C₂H₄反应”和“活化C＝C键后再与H₂反应”两种途径, 前者的活化能较后者低90-120 kJ·mol^-1, 因而具有明显的能量优势. 研究表明AuPR₃⁺ 的催化能力明显强于AuX. 此外, X/PR3基团供、吸电子能力的变化对配合物的催化能力也具有较为显著的影响. 电子结构分析显示Au(I)配合物在C₂H₄ 加氢反应中不仅能够削弱H―H、C＝C 键的强度, 还使H₂ σ_H―H^*、C₂H₄ π_C＝C^* 轨道能级下降, 从而缩小了π_C＝C-σ_H―H^*或σ_H―H-π_C＝C^*轨道间的能级差, 促进了C₂H₄-H₂反应中的电子离域, 从而降低禁阻反应发生的难度.σ_H―H^*、π_C＝C^*轨道能级改变量与加氢反应活化能E_a的降低值之间存在较好的一致性关系, 因此使上述轨道能级下降幅度越大的Au(I)配合物可以获得较好的催化效果.     

Bridged bimetallic complexes of molybdenum tris(dimethylpyrazolyl)borates

Summary The syntheses of [MoL^*(NO)(OR)NHC₆H₄NH₂)], [MoL^*(NO)I(NHC₆H₄NHMoL^*(NO)(OR)] (L^*=tris(3,5-dimethylpyrazolyl)borate; R=Me, Et,n-Pr,i-Pr,n-Bu and C₅H₁₁), and [{MoL^*(NO)(OR)}₂NHC₆H₄NH] (R=Me, Et andn-Pr) are described, the compounds being characterised by elemental analyses, i.r. and¹H n.m.r. spectroscopy.