Pyrolysis and Oxidation of Methane in a RF Plasma Reactor

The chemical kinetic effects of RF plasma on the pyrolysis and oxidation of methane were studied experimentally and computationally in a laminar flow reactor at 100 Torr and 373 K with and without oxygen addition into He/CH₄ mixtures. The formation of excited species as well as intermediate species and products in the RF plasma reactor was measured with optical emission spectrometer and Gas Chromatography and the data were used to validate the kinetic model. The kinetic analysis was performed to understand the key reaction pathways. The experimental results showed that H₂, C₂ and C₃ hydrocarbon formation was the major pathways for plasma assisted pyrolysis of methane. In contrast, with oxygen addition, C₂ and C₃ formation dramatically decreased, and syngas (H₂ and CO) became the major products. The above results revealed oxygen addition significantly modified the chemistry of plasma assisted fuel pyrolysis in a RF discharge. Moreover, an increase of E/n was found to be more beneficial for the formation of higher hydrocarbons while a small amount of oxygen was presented in a He/CH₄ mixture. A reaction path flux analysis showed that in a RF plasma, the formation of active species such as CH₃, CH₂, CH, H, O and O (¹D) via the electron impact dissociation reactions played a critical role in the subsequent processes of radical chain propagating and products formation. The results showed that the electronically excitation, ionization, and dissociation processes as well as the products formation were selective and strongly dependent on the reduced electric field.     

Influence of hydrocarbon gasses on PECVD a-C:H film deposition

Hydrogenated amorphous carbon layers (a-C:H) deposited at near room temperature by CH₄ and C₂H₂-Ar rf discharges have been studied. Discharge processes were investigated using growth kinetics and optical emission spectroscopy (OES). The role of plasma chemistry and of ion bombardment is discussed. Addition of argon, necessary to stabilize the C₂H₂ discharge, is found to enhance susbstantially gas phase processes such as dissociation, formation of atomic hydrogen and of CH, C₂, C₃ species (revealed by OES). It appears that rf plasma is very efficient for dissociation and ionization processes with threshold energies in the ten eV range. The layers' properties have been characterized by means of UV-Visible absorption, Fourier transform IR and Raman spectroscopies.     

Characterization of RF Discharge in Liquid n-Hexane and its Application to Synthesize Carbon Nano-Particles

The RF plasma discharge in liquid n-hexane is used to synthesize carbon nanoparticles. The results show that amorphous carbon nanoparticles with size of 5?C25?nm are the main product in the plasma in liquid n-hexane. Carbon nano-tubes of average diameter of 19?nm and length of 500?nm are also observed. The energy efficiency for carbon nanoparticles production is 2.9?mg/kJ which is more than 10 times larger than that of typical arc discharge synthesis method. The high speed observation indicates that the RF plasma in liquid has an alternating behaviour and it does not continuously emit light. The OES results show that H and C₂ are the main observed species in the plasma. Based on the OES data, it is shown that plasma temperature is 4,030?K and the electron density is 2.54?×?10²²?m^?3. Our results suggest that this method can be considered as a new route for carbon nanostructures production.     

Nanotubes in the V<Subscript>2</Subscript>O<Subscript>5</Subscript> · <Emphasis Type="Italic">n</Emphasis>H<Subscript>2</Subscript>O xerogel-hydroquinone-H<Subscript>2</Subscript>O system

Layered (C₆H₄)_0.11[VO_2.34(OH)_0.25] nanotubes (NTs) were prepared in the V₂O₅ · nH₂O-hydroquinone- H₂O system at 160–180°C for 5–7 days. Particle diameters were 40–60 nm; lengths do not exceed 1 μm, and the interlayer distance was 14.0 ± 0.1 Å. IR spectra, electrical conductivity, and thermal properties of nanotube powder were studied. The chemism of NT formation was determined, and a model of structure formation for these tubules was proposed.     

Efficient Separation of n‐Butene and iso‐Butene by Flexible Ultramicroporous Metal‐Organic Frameworks with Pocket‐like Cavities

Efficient separation of n‐butene (n‐C₄H₈) and iso‐butene (iso‐C₄H₈) is of significance for the upgrading of C4 olefins to high‐value end products but remains one of the major challenges in hydrocarbon purifications owing to their similar structures. Herein, we report a flexible metal‐organic framework, MnINA (INA=isonicotinate), featuring one‐dimensional pore channels with periodically large pocket‐like cavities connected by narrow bottlenecks, for the first time for efficient n‐/iso‐C₄H₈ separation. MnINA with smaller pore size (4.62 Å) compared with CuINA (4.84 Å), exhibits steep adsorption isotherms and high capacity of 1.79 mmol g^?1 for n‐C₄H₈ (4.46 Å) through strong host‐guest interactions via C?H???π bonding. The narrow bottlenecks exert barriers for the large molecules of iso‐C₄H₈ (4.84 Å) within the gate‐opening pressure range of 0–0.1 bar. This gives rise to MnINA with excellent separation selectivity of 327.7 for n‐/iso‐C₄H₈ mixture. The adsorption mechanism for n‐C₄H₈ and the gate‐opening effect were investigated by dispersion‐corrected density functional (DFT‐D) theory, verifying the strong interactions between n‐C₄H₈ and the frameworks as well as the gate‐opening effect derived from the rotation of organic linkers. The breakthrough tests confirmed MnINA and CuINA can be promising candidates for n‐/iso‐C₄H₈ separation.     

Characterization of an argon-hydrogen microwave discharge used as an atomic hydrogen source. Effect of hydrogen dilution on the atomic hydrogen production

This work is devoted to the study of an argon-hydrogen microwave plasma used as an atomic hydrogen source. Our attention has focused on the effect of the hydrogen dilution in argon on atomic hydrogen production. Diagnostics are performed either in the discharge or in the post-discharge using emission spectroscopy (actinometry) and mass spectrometry. The agreement between actinometry and mass spectrometry diagnostics proves that actinometry on the Ha(656.3 nm) and Hβ(486.1 nm) hydrogen Balmer lines can be used to measure the relative atomic hydrogen density within the microwave discharge. Results show that the atomic hydrogen density is maximum for a gas mixture corresponding to the partial pressure ratioP _{H 2}/P _Ar range between 1.5 and 2. The variation of atomic hydrogen density can be explained by a change of the dominant reactive mechanisms. At a low hydrogen partial pressure the dominant processes are the charge transfers with recombinations between Ar⁺ and H₂ which lead to ArH⁺ and H ₂ ⁺ ion formation. Both ions are dissociated in dissociative electron attachment processes. At a low argon partial pressure the electron temperature and the electron density decrease with increasing partial pressure ratio. The dominant mechanisms become direct reactions between charged particles (e, H⁺, H ₂ ⁺ , and H ₃ ⁺ ) or excited species H(n=2) with H₂ producing H atoms.     

Studies on Properties of Tetra‐p‐nitro‐tetra‐O‐alkylcalix[4]arenes

Ibis paper reports the properties of the novel tetra‐p‐nitro‐tetra‐O‐alkyl‐calix[4]arenes (alkyl= n‐C₄H₉, 1; n‐C₈H₁₇ 2; n‐C₁₂H₂₅, 3; n‐C₁₆H₃₃, 4). X‐ray crystallographic analysis and ¹H NMR revealed that they exist as pinched‐cone conformation in crystal or cone conformation in solution. EFISH experiments at 1064 nm in CHCl₃, indicated that tetra‐p‐nitro‐tetra‐O‐butyl‐calix[4]arene (1) has higher hyperpolarizability β, values than the corresponding reference compound p‐nitro‐phenyl butyl ether, without red shift of the charge transfer band. Compounds 2, 3 and 4 with longer alkyl chains can form monolayer at the air/water.     

Poly­[[[(1,10‐phenanthroline‐κ2N,N′)­zinc(II)]‐μ3‐5‐hydroxy­isophthalato‐κ4O,O′:O′′:O′′′] monohydrate]

In the title compound, {[Zn(C₈H₄O₅)(C₁₂H₈N₂)]·H₂O}_n or {[Zn(OH‐BDC)(phen)]·H₂O}_n (where OH‐H₂BDC is 5‐hydroxy­isophthalic acid and phen is 1,10‐phenanthroline), the Zn atoms are coordinated by two N atoms from the phen ligands and by four O atoms from hydroxy­isophthalate ligands in a highly distorted octahedral geometry, with Zn—O distances in the range 2.042 (4)–2.085 (5) Å and Zn—N distances of 2.133 (5) and 2.137 (5) Å. The {[Zn(OH‐BDC)(phen)]·H₂O}_n infinite zigzag polymer forms a helical chain of [Zn₂(OH‐BDC)₂]_n units. Face‐to‐face π–π interactions (3.60–3.75 Å) occur between two phen rings belonging to the same helical chain. Consolidation of the packing structure is achieved by O—H⋯O hydrogen‐bonding interactions between the carboxyl­ate O atoms, the hydroxyl group and the water mol­ecule, forming two‐dimensional sheets.     

catena‐Poly[[[pyridinecopper(II)]bis[μ3‐4‐(2‐oxidobenzylideneamino)benzoato]] dimethylformamide disolvate], a polymer composed of dimeric dicopper building units

The title compound, {[Cu(C₁₄H₉NO₃)(C₅H₅N)]·C₃H₇NO}_n or {[Cu₂L₂(py)₂]·2DMF}_n [py is pyridine, L is 4‐(salicylideneamino)benzoate and DMF is dimethylformamide], is composed of dimeric dicopper [CuL(py)]₂ building units, which are interlinked into a one‐dimensional chain through the formation of Cu—O_COO bonds. The dimeric unit is centrosymmetric, containing two Cu^II atoms linked by bridging phenolate O atoms into a Cu₂O₂ plane with a chelating Cu—O bond length of 1.927 (2) Å and a bridging Cu—O bond length of 2.440 (2) Å. Interchain C—H...O and π–π stacking interactions are responsible for an extensive three‐dimensional structure in which the resulting channels are filled by DMF solvent molecules.     

Neutral and ion chemistry in a C2H4-SiH4 discharge

This paper reports on a mass spectrometric study of the neutral and ionic species in a low-pressure rf discharge sustained in a C₂H₄-SiH₄ mixture diluted in helium. It is shown that C₂H₄ is readily decomposed into C₂H ₂ ^* and C₂H₃. The formation of secondary products such as C₄H₂, C₄H₄, and C₄H₆ is observed and confirms the presence of C₂H₂ in the discharge. Methylsilane (CH₃SiH₃) and ethylsilane (C₂H₅SiH₃) are also synthesized in this discharge. It is also observed that the major ions C₂H ₄ ⁺ , C₃H ₅ ⁺ , SiH ₃ ⁺ , Si₂H ₄ ⁺ , SiCH ₃ ⁺ , SiC₂H ₃ ⁺ , and SiC₂H ₇ ⁺ are not representative of the direct ionization of neutral species. Their formation is thus interpreted on the basis of ion-molecule reactions.     

Contradictory ageing behaviour and optical property of iodine doped and H2SO4 doped pulsed DC plasma polymerized aniline thin films

Polymerization of aniline in the presence of H₂SO₄ and iodine vapour is carried out using pulsed DC glow discharge plasma. The as prepared iodine doped plasma polymerized aniline films are found to be highly crystalline due to some crystal structure formed by the iodine incorporated into the film and the ex-situ H₂SO₄ doped films are found to be amorphous. However, the structure of the iodine doped films transforms to amorphous one while that of the ex-situ H₂SO₄ doped films transforms to crystalline one due to the effect of ageing. The optical band gap of the samples decreases depending upon the level of doping and annealing, having minimum optical band gap for iodine doped plasma polymerized aniline. Due to ageing, the optical property of the H₂SO₄ doped film is found to become superior while for iodine doped films a negligible change of the optical property has been noticed even after six months of ageing.     

Structures and properties of π Br‐bond in complexes C2H4−nFn?BrF (n = 0–2)

Using the counterpoise‐corrected potential energy surface method, the stationary structures of the π Br‐bond complexes C₂H_4‐nF_n? BrF (n = 0–2) with all real frequencies have been obtained at MP2/aug‐cc‐pVDZ level. The order of the π Br‐bond length is 2.625 Å (C₂H₄? BrF) < 2.714 Å (C₂H₃F? BrF) < 2.751 Å (g‐C₂H₂F₂? BrF) < 2.771 Å (trans‐C₂H₂F₂? BrF) < 2.778 Å (cis‐C₂H₂F₂? BrF). The interaction energies (E_int) are, respectively,‐5.9 (C₂H₄? BrF),‐4.4 (C₂H₃F? BrF),‐3.7 (g‐C₂H₂F₂? BrF),‐3.1 (cis‐C₂H₂F₂? BrF),‐2.8 kcal/mol (trans‐C₂H₂F₂? BrF), at the CCSD (T)/aug‐cc‐pVDZ level, which include larger electron correlation contributions (E_corre). The order of E_corre is‐3.40 (C₂H₄? BrF),‐3.60 (C₂H₃F? BrF),‐3.85 (g‐C₂H₂F₂? BrF),‐3.86 (cis‐C₂H₂F₂? BrF),‐3.88 kcal/mol (trans‐C₂H₂F₂? BrF). The earlier results show above that the F substituent effect elongates the π Br‐bond, reduces the E_int, and increases the E_corre contribution of the interaction energy. Interestingly, the interaction energy of the cis‐C₂H₂F₂? BrF structure with longer interaction distance is larger than that of the corresponding trans‐C₂H₂F₂? BrF structure with shorter interaction distance. This reason comes from a special secondary interaction between lone pairs of Br atom with positive charge and some atoms (H, C) with positive charges of C₂H₂F₂ in the cis‐C₂H₂F₂? BrF structure. Comparing with corresponding C₂H_4‐nF_n? ClF and C₂H_4‐nF_n? HF, the C₂H_4‐nF_n? BrF system has the larger E_int in which main contribution comes from the larger E_corre, representing the larger dispersion interaction. The larger E_corre contribution of the E_int of π Br‐bond can be used to understand that the π Br‐bond is shorter and stronger than corresponding π Cl‐bond. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Gas-phase electron diffraction studies of the molecular structures of pentafluoroethane,C2F5H,and monofluoroethane,C2H5F

The molecular structures of C₂F₅H and C₂H₅F have been studied using gas-phase electron diffraction data collected on the Balzers KDG2 instrument. The following values for the main independent geometrical parameters were obtained (r_a values with e.s.d. in parentheses): in C₂F₅H, C-C = 1.525(4) Å, C-F(CHF₂) = 1.347 Å, C-F(CF₃) = 1.327 Å [C-F(av.) = 1.335(2) Å], ∠CCF(av.) = 110.0(2)°; in C₂H₅F, C-C = 1.502(5) Å, C-F = 1.397(4) Å, C-H = 1.097(2) Å. ∠CCF = 110.4(2)°, ∠CCH(av.) = 113.6(4)°. Evidence is presented to show that the electron diffraction data for C₂H₅F are not compatible with values for the bond angles deduced spectroscopically.     

Mechanistic Studies on the Gas‐Phase Dehydrogenation of Alkanes at Cyclometalated Platinum Complexes

In the ion/molecule reactions of the cyclometalated platinum complexes [Pt(L? H)]⁺ (L=2,2′‐bipyridine (bipy), 2‐phenylpyridine (phpy), and 7,8‐benzoquinoline (bq)) with linear and branched alkanes C_nH_2n+2 (n=2–4), the main reaction channels correspond to the eliminations of dihydrogen and the respective alkenes in varying ratios. For all three couples [Pt(L? H)]⁺/C₂H₆, loss of C₂H₄ dominates clearly over H₂ elimination; however, the mechanisms significantly differs for the reactions of the “rollover”‐cyclometalated bipy complex and the classically cyclometalated phpy and bq complexes. While double hydrogen‐atom transfer from C₂H₆ to [Pt(bipy? H)]⁺, followed by ring rotation, gives rise to the formation of [Pt(H)(bipy)]⁺, for the phpy and bq complexes [Pt(L? H)]⁺, the cyclometalated motif is conserved; rather, according to DFT calculations, formation of [Pt(L? H)(H₂)]⁺ as the ionic product accounts for C₂H₄ liberation. In the latter process, [Pt(L? H)(H₂)(C₂H₄)]⁺ (that carries H₂ trans to the nitrogen atom of the heterocyclic ligand) serves, according to DFT calculation, as a precursor from which, due to the electronic peculiarities of the cyclometalated ligand, C₂H₄ rather than H₂ is ejected. For both product‐ion types, [Pt(H)(bipy)]⁺ and [Pt(L? H)(H₂)]⁺ (L=phpy, bq), H₂ loss to close a catalytic dehydrogenation cycle is feasible. In the reactions of [Pt(bipy? H)]⁺ with the higher alkanes C_nH_2n+2 (n=3, 4), H₂ elimination dominates over alkene formation; most probably, this observation is a consequence of the generation of allyl complexes, such as [Pt(C₃H₅)(bipy)]⁺. In the reactions of [Pt(L? H)]⁺ (L=phpy, bq) with propane and n‐butane, the losses of the alkenes and dihydrogen are of comparable intensities. While in the reactions of “rollover”‐cyclometalated [Pt(bipy? H)]⁺ with C_nH_2n+2 (n=2–4) less than 15 % of the generated product ions are formed by C? C bond‐cleavage processes, this value is about 60 % for the reaction with neo‐pentane. The result that C? C bond cleavage gains in importance for this substrate is a consequence of the fact that 1,2‐elimination of two hydrogen atoms is no option; this observation may suggest that in the reactions with the smaller alkanes, 1,1‐ and 1,3‐elimination pathways are only of minor importance.     

Benefits of <Emphasis Type="Italic">n</Emphasis>-butanol,as a biofuel,in reducing the levels of soot precursors issued from the combustion of benzene flames

Although oxygenated fuel additives are effective in reducing soot emissions, the extent to which molecular structure of the oxygenate plays a role in soot reduction has remained unclear and controversial. To gain a deeper insight in this field, a detailed chemical kinetic modeling approach was used to examine the phenomenon of suppression of sooting by the addition of oxygenated hydrocarbon species to the fuel. For this task, the PREMIX code in conjunction with Chemkin II and models resulting from the merging of validated kinetic schemes describing the oxidation of the components of the n-butanol-benzene mixtures were used to investigate the effect of n-butanol addition on the formation?depletion of acetylene recognized as soot precursor in flames under fuel-rich conditions. The first part of this study treats the dependence of the soot precursor amounts on n-butanol percentage in the fuel mixture, whereas the second part defines the key reaction mechanisms responsible for the observed reduction in C₂H₂ and consequently in polycyclic aromatic hydrocarbons and soot amounts induced by the oxygenate additive. The principal objective of the current study was to obtain fundamental understanding of the mechanisms through which the oxygenate compound affects the soot precursor amounts. The modeling results indicated that there was a dramatic decrease in the acetylene peak height with the addition of the oxygenated addtitive. This finding was found to be due to the increase in the C₂H₂ consumption rates induced by n-butanol addition. Finally, the modeling results provided evidence that n-butanol played a role in changing acetylene formation mechanism by enhancing the role of C₃H₄P, C₃H₄ and aC₃H₅ and by eliminating the role of C₆H₄, C₅H₅, C₅H₆, H₂CCCCH, C₄H₂, C₅H₄O, C₂H, CHCHCHO, H₂C₄O and C₄H₄.     

Reactions of hydrogen peroxide vapor dissociated in a microwave plasma

Analysis of the plasma emission from a low-pressure microwave cavity discharge through flowing hydrogen peroxide vapor showed that both H and OH were produced in proportions which varied with the applied power. When the dissociated vapor was condensed at 195 K only water was obtained; at 77 K, H₂O₂ and H₂O₄ were also obtained. Their formation could not be increased by increasing the H atom or OH radical concentration in the plasma. When the reaction time of the dissociated vapor between the plasma exit and the cold surface was increased, the rate of H₂O₂ formation increased mostly at the expense of water formation. It appears that, as in the case of the reaction of H with O₂, the rate of H₂O₂ formation is dependent on the concentration of O₂ produced in the spatial afterglow by the gas-phase reactions of the hydroxyl radicals.     

Evidence for long-lived excited states of [CnH2]2+ carbodications

The energetics, structures, stabilities and reactivities of[C_nH₂]²⁺ ions have been investigated using computational methods and experimental mass spectrometric techniques. Spontaneous decompositions of [C_nH₂]²⁺ into [C_nH]⁺ + H⁺ products, observed for ions with odd-n values, have been explained by invoking the formation of excited triplet states. Even-n [C_nH]⁺ ions possess triplet ground states with low-lying excited states, whereas odd-n ions have triplet states with energies several eV above ground singlet states. Radiationless transitions of vibrationally excited long-lived triplet state ions into singlet state continua are suggested as possible mechanisms for spontaneous deprotonation processes of odd-n [C_nH₂]²⁺ ions. Evidence for these long-lived excited states has been obtained in bimolecular single electron transfer reactions.     

活性氮与碘乙烷反应化学发光研究

在流动余辉装置上, 利用N₂空心阴极放电制备活性氮, 研究了活性氮与碘乙烷(C₂H₅I)反应的化学发光. 在620～820 nm波长范围内观察到了较强的发射光谱, 拟合得到的光谱常数表明它来源于NI(b¹Σ^＋→X³Σ^－)跃迁, 并对35个谱峰进行了振动归属. 最后讨论了活性氮中主要成分与C₂H₅I反应的可能过程, 结合辅助性实验分析表明, 活性氮中的N(²P)与C₂H₅I直接反应很可能产生激发态NI(b¹Σ^＋)自由基. 这是利用化学反应直接产生激发态NI(b¹Σ^＋)的首次报道, 观察到的激发态最高振动能级为v'＝6.     

Poly­[[bis­(pyridine‐κN)copper(II)]‐μ3‐5‐hydroxy­isophthalato‐κ3O:O′:O′′]

In the title compound, [Cu(C₈H₄O₅)(C₅H₅N)₂]_n or [Cu(OH‐BDC)(py)₂]_n (where OH‐H₂BDC is 5‐hydroxy­isophthalic acid and py is pyridine), the Cu atoms are coordinated by two N atoms from the pyridine ligands and by three O atoms from hydroxy­isophthalate ligands in a highly distorted triangular bipyramidal environment, with Cu—O distances in the range 1.941 (4)–2.225 (5) Å and Cu—N distances of 2.014 (6) and 2.046 (6) Å. The [Cu(OH‐BDC)]_n two‐dimensional network is built up from interlocking 22‐, 15‐ and eight‐membered rings via sharing of Cu atoms and O—H⋯O hydrogen bonds. Consolidation of the packing structure is achieved by edge‐ or point‐to‐face C—H⋯π interactions and offset or slipped π–π stacking interactions.     

Steric control in the formation of amine or amide complexes from [Mo(No){HB(3,5-Me2C3N2H)3}I2]

The reaction between [Mo(NO){HB(3,5-Me₂C₃N₂H)₃}I₂] and pyrrolidine or piperidine produces both the amine complexes [Mo(NO){HB(3,5-Me₂C₃N₂H)₃} I(NHC_nH₂n)] (n = 4 or 5) and the amide complexes [Mo(NO){HB(3,5-Me₂C₃N₂H)_3} I(NC_nH_2n)]. The role of steric factors in the formation of these compounds is discussed.