- Platinum catalyzed c-h activation and the effect of metal-support interactions
-
Catalytic C-H bond activation of methane and ethane on a series of silica supported platinum catalysts (Pt/SiO2) was studied by using hydrogen/deuterium (H/D) exchange. Kinetic experiments demonstrate that under the reaction conditions studied, the rate of C-H bond activation shows approximate first order dependence in alkane and inverse first order dependence in D2. The rate of C-H activation is affected by the presence of sodium on the silica support, where sodium-free supports have the fastest rates of C-H activation, as assessed by H/D exchange. CO adsorption and FTIR studies indicate that the Pt particles on the sodium-free support are more electron-deficient, having the most blue-shifted linear CO stretch, while sodium-containing supports are more electron-donating, having the most red-shifted linear CO stretch. It is proposed, based on the results described in this article and previous work in the literature, that more electron-donating supports cause the Pt particles to be more electron-rich and to adsorb D? (or H*) more strongly, thereby stabilizing the ground state and resting state of the catalyst, resulting in a decreased rate of C-H activation.
- Sattler, Aaron,Paccagnini, Michele,Lanci, Michael P.,Miseo, Sabato,Kliewer, Chris E.
-
p. 710 - 720
(2020/01/02)
-
- Catalytic Activation of Unstrained, Nonactivated Ketones Mediated by Platinum(II): Multiple C-C Bond Cleavage and CO Extrusion
-
The complexes [Pt(tolpy)Cl(L)] (tolpy = 2-(4-tolyl)pyridyl; L = dmso, dms, py, PPh3, CO) are precursors for the catalytic cleavage of C-C bonds and extrusion of CO from a series of unactivated ketones such as cyclohexanone; deuterium labeling experiments demonstrate the involvement of a transfer hydrogen step in the mechanism.
- Sarju, Julia P.,Dey, Debashish,Torroba, Javier,Whitwood, Adrian C.,Redeker, Kelly,Bruce, Duncan W.
-
supporting information
p. 4539 - 4542
(2019/11/29)
-
- Photocatalytic halohydrocarbon dehalogenation conversion method
-
The invention provides a photocatalytic halohydrocarbon dehalogenation conversion method which comprises the following steps: adding a photocatalyst quantum dot/rod into a solvent to obtain a solutionA; adding halohydrocarbon and an electronic sacrificial body into the solution A to obtain a solution B; utilizing a light source to irradiate the solution B and catalyzing the solution B to performhalohydrocarbon dehalogenation conversion. According to the photocatalytic halohydrocarbon dehalogenation conversion method disclosed by the invention, a nano quantum dot and a nano quantum rod are applied to dehalogenation conversion reaction of alkyl halide, alkenyl halide and alkyne halide for the first time; the reaction conditions are moderate, visible light is utilized as driving energy, a product is hydrocarbon compound, and the whole process has the advantages of environmental protection, conciseness and high efficiency. In addition, higher hydrocarbon of carbon chain growth can be generated after dehalogenation reaction, so that the method has potential application in preparation of higher hydrocarbon. According to the method disclosed by the invention, halohydrocarbon dehalogenation conversion and deuteration marking processes are jointly performed; hydrocarbon deuteration marking can be finished when a halohydrocarbon dehalogenation process is finished. The invention furtherprovides a method for performing deuteration marking on hydrocarbon.
- -
-
Paragraph 0107; 0108
(2019/04/02)
-
- Methylene migration and coupling on a non-reducible metal oxide: The reaction of dichloromethane on stoichiometric α-Cr2O3(0001)
-
The reaction of CH2Cl2 over the nearly-stoichiometric α-Cr2O3(0001) surface produces gas phase ethylene, methane and surface chlorine adatoms. The reaction is initiated by the decomposition of CH2Cl2 into surface methylene and chlorine. Photoemission indicates that surface cations are the preferred binding sites for both methylene and chlorine adatoms. Two reaction channels are observed for methylene coupling to ethylene in temperature-programmed desorption (TPD). A desorption-limited, low-temperature route is attributed to two methylenes bound at a single site. The majority of ethylene is produced by a reaction-limited process involving surface migration (diffusion) of methylene as the rate-limiting step. DFT calculations indicate the surface diffusion mechanism is mediated by surface oxygen anions. The source of hydrogen for methane formation is adsorbed background water. Chlorine adatoms produced by the dissociation of CH2Cl2 deactivate the surface by simple site-blocking of surface Cr3 + sites. A comparison of experiment and theory shows that DFT provides a better description of the surface chemistry of the carbene intermediate than DFT+U using reported parameters for a best representation of the bulk electronic properties of α-Cr2O3.
- Dong, Yujung,Brooks, John D.,Chen, Tsung-Liang,Mullins, David R.,Cox, David F.
-
-
- High-temperature Shilov-type methane conversion reaction: Mechanistic and kinetic studies
-
Traditional Shilov reactions (performed in aqueous solution with a PtCl2 catalyst) for methane conversion suffer from catalyst deactivation at high temperatures (> 100 °C), therefore only very low conversion rates have been achieved. In this paper, we show that Shilov-type C-H activations are achievable at much higher temperatures (~200 °C) by addition of concentrated aqueous solutions of Cl- to inhibit Pt catalyst precipitation. Various chloride-based ionic liquids also stabilized the Pt catalyst at mild reaction temperatures (~140 °C). Under high-pressure conditions (> 25.5 MPa), achieved using a specially designed sealed gold-tube reactor, very high methane conversion rates (> 90%) were obtained; this is attributed to the improved methane solubility in aqueous solution. Deuterium isotope (H/D) exchange between methane and water was used to examine the reaction reactivity and selectivity. Multiply D-substituted products were observed, indicating that multiple C-H activations occurred. A comprehensive network reaction that included all the chain reactions was set up to clarify the reactivities and product selectivities of the methane activation reactions. The reaction network consisted of a series of parallel first-order reactions, which can be described by the Arrhenius equation. The kinetic parameters such as the frequency factor, activation energies, and stoichiometric coefficients were obtained by fitting the experimental data. Because all four C-H bonds in a methane molecule are equivalent, multiple substitutions during methane conversion cannot be avoided. Our studies indicate that mono-substituted and di-substituted methane isotopologue generations have similar activation energies, suggesting that the highest mono-substitution selectivity cannot be greater than 50%.
- Kang, Shujuan,Ma, Qisheng,Chen, Weiqun,Chen, Guanyi,Tang, Yongchun
-
p. 1777 - 1784
(2015/10/20)
-
- Highly efficient and stable photocatalytic reduction of CO2 to CH4 over Ru loaded NaTaO3
-
An efficient and stable photocatalytic activity was obtained over NaTaO3 by introducing an electron donor (H2) into the CO2 reduction process with water. Ru/NaTaO3 demonstrated the best activity (CH4 51.8 μmol h-1 g-1) and product selectivity in converting CO2 to CH4.
- Li, Mu,Li, Peng,Chang, Kun,Wang, Tao,Liu, Lequan,Kang, Qing,Ouyang, Shuxin,Ye, Jinhua
-
supporting information
p. 7645 - 7648
(2015/06/16)
-
- Iridium(iii) catalyzed trifluoroacetoxylation of aromatic hydrocarbons
-
A tridentate, NNC-tb (where NNC-tb = 2-(pyridin-2-yl)benzo[h]quinoline) ligated IrIII complex (NNC-tb)Ir(Ph)(4-MePy)(TFA), 11 along with analogues are very active for CH activation as evidenced by rapid catalytic H/D exchange between benzene and trifluoroacetic acid-d1 (DTFA). The complexes were examined with a variety of oxidants for the catalytic conversion of benzene to phenyltrifluoroacetate. Herein, the synthesis and characterization of (NNC-tb)Ir complexes is described along with the reactivity of these complexes towards arenes and alkanes.
- Bischof, Steven M.,Hashiguchi, Brian G.,Lokare, Kapil S.,Gunsalus, Niles,Yousufuddin, Mohammed,Periana, Roy A.
-
p. 35639 - 35648
(2014/12/10)
-
- On-stream regeneration of a sulfur-poisoned ruthenium-carbon catalyst under hydrothermal gasification conditions
-
Catalytic processes that employ Ru catalysts in supercritical water are capable of converting organics, such as wood waste or biosolids, into synthetic natural gas (CH4) with high efficiencies at relatively moderate temperatures of around 400 °C. However, Ru catalysts are prone to S poisoning and are quickly deactivated. As S is ubiquitous in raw biomass and technologies to remove S from hydrothermal biomass feeds are lacking, regeneration protocols that efficiently reactivate S-poisoned catalysts are required to realize efficient conversion processes and long catalyst lifetimes. In this work, we developed a method to remove S from a S-poisoned Ru catalyst under hydrothermal conditions through an oxidative treatment in the aqueous phase. By using in situ X-ray absorption spectroscopy under the reaction conditions, we show that Ru is oxidized by dilute H2O2 at low temperatures, which leads to the removal of adsorbed S species from the catalyst surface. By optimizing the regeneration conditions, it was possible to prevent oxidation of the catalyst carbon support, as revealed by ex situ TEM. This treatment led to a reactivation of the Ru catalyst with a significant increase in carbon-to-gas conversion and methane selectivity. Don't preach, bleach! Dilute hydrogen peroxide effects the reactivation of a sulfur-poisoned ruthenium catalyst under hydrothermal conditions. This mild oxidative treatment efficiently removes adsorbed sulfur from the ruthenium surface and restores the catalytic activity without corroding the catalyst support. Copyright
- Dreher, Marian,Steib, Matthias,Nachtegaal, Maarten,Wambach, Joerg,Vogel, Frederic
-
p. 626 - 633
(2014/03/21)
-
- Catalytic activity of systems based on supported potassium salts of transition metal carbonyl hydrides in hydrogen-deuterium exchange of hydrocarbons
-
The deposition of K2[Ru4(CO)13], K 2[Os3(CO)11], K2[Fe 2(CO)8], and K[Re(CO)5] onto graphite-like carbon Sibunit followed by the thermal decomposition of the supported carbonylmetallate in a flow of dihydrogen or argon affords systems capable of activating C-H bonds of methane, ethylene, and acetylene and of introducing them into hydrogen-deuterium exchange reactions. In the case of ethylene and acetylene, the isotope exchange proceeds at room temperature, while in the case of methane reaction temperatures not lower than 150 C are needed.
- Yunusov,Rummel,Herrmann,Kalyuzhnaya,Shur
-
p. 1191 - 1194
(2014/03/21)
-
- Hydrogen-deuterium exchange of methane on nickel and potassium promoted nickel prepared by the reduction of nickel oxide
-
The hydrogen-deuterium exchange reactions of methane in a deuterium stream were studied by a pulse experiment over a reduced nickel (Ni1373 prepared from nickel oxide calcined at 1373 K, and Ni773 prepared from nickel oxide calcined at 773 K) and K2O promoted reduced nickel (K-Ni1373 and K-Ni773). Ni1373 had a higher exchange activity than Ni773. The effects of the addition of K2O resulted in the decrease in the exchange activity due to the inhibition of step defect sites and/or the promotion of the change of nickel crystal structure. High activity of Ni1373 could be due to that the Ni1373 had the surface of a higher Ni(1 0 0)/Ni(1 1 1) ratio.
- Osawa, Tsutomu,Futakuchi, Takashi,Imahori, Tomoharu,Lee, I-Yin Sandy
-
experimental part
p. 68 - 71
(2010/06/20)
-
- Kinetic and theoretical study of the hydrodechlorination of CH 4- xClx (x = 1-4) compounds on palladium
-
The reaction kinetics of hydrodechlorination (HDCl) for a series of CH 4-xClx (x = 1-4) compounds were measured on a Pd/carbon catalyst. The rate of HDCl correlated with the C-Cl bond energy, suggesting scission of this bond in the molecularly adsorbed molecule is rate-determining. The measured reaction kinetics of the CH4-xClx compounds support a previously proposed Langmuir-Hinshelwood type reaction mechanism. Kinetic and isotope exchange experiments demonstrated the following: gas phase H2 and HCl are in equilibrium with surface H and Cl; adsorbed Cl is the most abundant surface intermediate; and irreversible scission of the first C-Cl bond is rate-determining. The overall hydrodechlorination reaction rate can be written as kKR-Cl[R-Cl]/(1 + KHCl[HCl]/KH2 1/2[H2]1/2). The activation energy of the rate-determining step was related linearly to the dissociation energy of the first C-Cl bond broken in a Broensted-Evans-Polanyi relationship. This behavior is in agreement with a previous study of CF3CF 3-xClx compounds. During the reaction of CH3Cl, CH2Cl2, and CHCl3 with deuterium, H-D exchange occurred in only 2%, 6%, and 9% of products, respectively. The increasing H-D exchange with Cl content suggests the steps which determine selectivity in these multipath, parallel reactions. The density functional theory (DFT)-calculated activation energies for the dissociation of the first C-Cl bond in the family of chlorinated methane compounds are in good agreement with the values extracted from kinetic modeling, suggesting that parameters estimated from DFT calculations may be used to estimate the reactivity of a particular chlorinated compound within a family of chlorocarbons.
- Chen, Nan,Rioux, Robert M.,Barbosa, Luis A. M. M.,Ribeiro, Fabio H.
-
experimental part
p. 16615 - 16624
(2011/10/30)
-
- Decarbonylation of ethanol to methane, carbon monoxide and hydrogen by a [PNP]Ir complex
-
The putative three-coordinate Ir(i) PNPPri(PNPPri = [N{2-P(CHMe2)2-4-MeC6H3} 2]-) pincer complex decarbonylates ethanol to yield methane, hydrogen and [PNPPri]Ir(CO). The mechanism involves the isolable trans-[PNPPri]Ir(H)(Me)(CO), which is susceptible to photochemical reductive elimination of methane.
- Melnick, Jonathan G.,Radosevich, Alexander T.,Villagran, Dino,Nocera, Daniel G.
-
supporting information; experimental part
p. 79 - 81
(2010/04/01)
-
- Reduction of alkyl halides by triethylsilane based on a cationic iridium bis(phosphinite) pincer catalyst: Scope, selectivity and mechanism
-
A highly efficient procedure for the reduction of a broad range of alkyl halides by triethylsilane based on a cationic iridium bis(phosphinite) pincer catalyst has been discovered and developed. This reduction chemistry is chemoselective and has unique selectivities compared with conventional radical-based processes and the aluminum trichloride/ triethylsilane (AlCl 3/Et3SiH) and triphenylmethyl tetrakis[pentafluorophenyl] borate/triethylsilane {[Ph3C] [B(C6F5) 4]/Et3SiH} systems. Reductions use three equivalents of triethylsilane relative to the halide and can be carried out with very low catalyst loadings and in a solvent-free manner, which may provide an environmentally attractive and safe alternative to many currently practiced methods for reduction of alkyl halides. Mechanistic studies reveal a unique catalytic cycle. The cationic iridium hydride 2,6-bis[di-(tert-butyl) phosphinyloxy)phenyl-(hydrido)iridium, (POCOP)IrH+ {POCOP= 2,6-[OP(t-Bu)2]2C6H3} binds and activates the silane. This complex serves as a potent silylating reagent to generate silyl halonium ions, Et3SiXR+, which are reduced by the neutral iridium dihydride to yield alkane product and regenerate the cationic (POCOP)IrH+, thus closing the catalytic cycle. All key intermediates have been identified by in situ NMR monitoring and kinetic studies have been completed. An application of this reduction system to the catalytic hydrodehalogenation of a metal chloride complex is also described.
- Yang, Jian,Brookhart, Maurice
-
scheme or table
p. 175 - 187
(2009/10/01)
-
- Isothermal pyrolysis of iodomethanes in gases
-
The fact was established that the pyrolysis of gaseous iodomethanes RI yields methane and non traces of recombination products R2. A pyrolysis mechanism was proposed and rate constants of limiting stages of the pyrolysis of iodomethane, trideuteroiodomethane, and diiodomethane over the range of 500-1500 K were determined. Pleiades Publishing, Ltd., 2009.
- Skorobogatov,Khripun,Rebrova
-
scheme or table
p. 2641 - 2651
(2010/06/14)
-
- Steam reforming of biomass based oxygenates-Mechanism of acetic acid activation on supported platinum catalysts
-
The activation of acetic acid during steam reforming reactions over Pt-based catalysts has been probed by decomposing CH3COOD over Pt/C. The product mixture contained CO2, CH4 and its D-analogs (CH4 - x Dx
- Matas Gueell,Babich,Seshan,Lefferts
-
body text
p. 229 - 231
(2009/02/07)
-
- H/D isotope exchange between methane and magic acid (HSO 3F-SbF5): An in situ NMR study
-
The kinetics of hydron exchange between methane and a series of DSO 3F-SbF5 superacids were measured by in situ 2H decoupled 1H NMR spectroscopy. The rates of exchange showed a strong dependence on antimony pentafluoride concentration, with the free energy of activation ΔG# (30°C) decreasing from 97 to 84 kJ mol -1 over the range of concentration 19 to 49 mol % SbF5. The constant free enthalpy of activation ΔH# (ca. 65 kJ mol-1) and the decreasing entropy of activation ΔS# seem to indicate that an increase in acidity of the superacid system does not substantially change the nature of the transition state but rather acts on its solvation.
- Walspurger, Stephane,Goeppert, Alain,Haouas, Mohamed,Sommer, Jean
-
p. 266 - 269
(2007/10/03)
-
- Effects of rhodium dispersion on catalytic behavior of Rh/active-carbon catalysts for H/D exchange reaction between and CH4 and D2
-
The H/D exchange reaction between CH4 and D2 was carried out over Rh/active-carbon catalysts, which were prepared from RhCl3 and Rh(NO3)3. In the case of the catalysts prepared from RhCl3, Rh species were homogeneously dispersed on the support from external surface to the inside of pores. Metallic particles of Rh were found to be the predominant species on the catalysts prepared from Rh(NO3)3 in the low Rh-loading region of 2 wt.%. The reaction rate per unit gram of catalyst and the product distribution in methane reflected well the Rh-dispersion on the catalysts. The catalysts which contained the highly dispersed Rh species as predominant species were found to be more active for the H/D exchange reaction than the catalysts with relatively large metal particles of Rh. On the former, the ratio of CH3D/CD4 was observed to be much higher than that on the latter.
- Sakagami,Ogata,Takahashi,Matsuda
-
p. 1930 - 1934
(2007/10/03)
-
- H/D exchange between CH4 and CD4 catalysed by a silica supported tantalum hydride, (?SiO)2Ta-H
-
The silica supported tantalum hydride (?SiO)2Ta-H 1, catalyses the H/D exchange reaction between CH4 and CD4 at 150 °C producing the statistical distribution of all methane isotopomers.
- Lefort, Laurent,Coperet, Christophe,Taoufik, Mostafa,Thivolle-Cazat, Jean,Basset, Jean-Marie
-
p. 663 - 664
(2007/10/03)
-
- Methane hydrogenation and confirmation of CHx intermediate species on NaY encapsulated cobalt clusters and Co/SiO2 catalysts: EXAFS, FTIR, UV characterization and catalytic performances
-
The adsorption of Co2(CO)8 onto dehydrated NaY powder under an N2 atmosphere predominantly yielded supported Co4(CO)12. The molecular cobalt carbonyl clusters and their decarbonylated products have been structurally characterized by in situ IR, extended X-ray absorption fine structure (EXAFS) and diffuse reflectance spectroscopies. The IR spectrum assigned to the species Co4(CO)12/NaY is shifted significantly from that observed for externally supported analogues on NaY and for this cluster in solution, which indicates that the cobalt carbonyl clusters occurring on the NaY are similar to those occurring in weakly basic solution. EXAFS coordination numbers (N) show that the successively decarbonylated samples maintain small cluster sizes, which depend on the temperature-programmed oxidation of the precursor. Methane hydrogenation was carried out on intrazeolitic cobalt clusters using a two-step process. It showed a relatively higher activity and selectivity to C2+ hydrocarbons in comparison with Co/SiO2 catalysts. On the basis of IR (νC-H 2960, 2880 cm-1 and δC-H 1520, 1393 cm-1) spectroscopy, mass spectrometry and reaction studies, one can conclude that CHx (x = 0, 1 or 2) surface carbonaceous species were generated by CH4 dissociation on the activated cobalt catalyst; the CH2 species was quite reactive, and propagated higher hydrocarbons.
- Shen, Guo-Cheng,Ichikawa, Masaru
-
p. 1185 - 1193
(2007/10/03)
-
- Enhancement of Selective Decomposition: Adsorption anf Reaction of Methanethiol on Carbon-Covered W(001)
-
Selective decomposition of methanethiol (CH3SH) on carbon-covered W(001) to produce mathane is enhanced by 75percent compared to the clean surface.The maximum enhancement requires only 0.25 monolayers (ML) of preadsorbed C.On a surface percovered with 0.8 ML of C, the methane desorbs in peaks at 460 and 550 K compared to 360 K on the clean surface, suggesting a greater stability in the C-S and C-H bonds.Increased intramolecular bond stability is confirmed by the temperature dependence of the S 2p and C ls soft X-ray photoemission.Methyl thiolate, CH3S, forms upon adsorption at 100 K.Chemisorbed methanethiol, which is not stable on the clean surface, is also observed between 100 and 300 K.The chemisorbed thiol decomposes to form additional thiolate.The thiolate reacts along three competing pathways.It undergoes rehydrogenation and desorbs as methanethiol, it selectively decomposes to form desorbed methane and adsorbed S, or it totally decomposes to form S, C, and desorbed H2.
- Mullins, D. R.,Lyman, P. F.
-
p. 5548 - 5555
(2007/10/02)
-
- Effect of Pt Particle Size on H/D Exchange of Methane over Alumina- and Zeolite-supported Catalysts
-
Deuterium exchange in methane was studied over a series of Pt/Al2O3 and Pt/mazzite catalysts.The reaction proceeds by a simple stepwise mechanism with CH3D being the primary product.Large and small metal particles located in zeolite and alumina catalysts have different catalytic activity with respect to deuterium exchange in methane.The rate of reaction per Pt atom was the same on the catalysts investigated for particles >15-20 Angstroem and considerably lower than the rate of reaction on samples with smaller Pt clusters.The activity of small clusters of Pt (diameter 15-20 Angstroem) depended on the nature of the support.
- Khodakov, Andrei,Barbouth, Nisso,Berthier, Yves,Oudar, Jacques,Schulz, Philippe
-
p. 569 - 574
(2007/10/02)
-
- Influence of Strong Metal-Support Interaction on Exchange with Deuterium and other Reactions of Hydrocarbons. Part 1. - Studies with Rh/TiO2 and Rh/SiO2
-
The changes in the catalytic properties of Rh/TiO2 caused by raising the reduction temperature from 473 to 773 K have been investigated for four reactions, the exchange with deuterium of methane and cyclopentane, and the hydrogenolysis of 2,2-dimethylpropane and methylcyclopentane.The so-called strong metal-support interaction (SMSI) brought about by the high-temperature reduction had least effect on the exchange of methane but reduced the rate of hydrogenolysis of 2,2-dimethylpropane by factors of 1E4 or 1E5.The SMSI was reversible and its influence on each of the reactions was eliminated by oxidation of the catalyst followed by low-temperature reduction.No marked changes in the catalytic behaviour of Rh/SiO2 resulted from increasing the reduction temperature.The results provide further evidence for the existence of a number of kinds of catalytic sites on Rh/TiO2.The sites for the more structure-sensitive reactions tend to be more seriously affected by SMSI and probably involve more metal atoms than the sites responsible for methane exchange.
- Costa Faro, Arnaldo da,Kemball, Charles
-
p. 741 - 748
(2007/10/02)
-
- Reactions of Hydrocarbons over Ru/SiO2: Exchange with Deuterium and the Onset of Hydrogenolysis
-
Exchange reactions with deuterium of a number of hydrocarbons, chosen to provide evidence about the nature of the intermediates, have been followed mass spectrometrically over a ruthenium-silica catalyst.The selected hydrocarbons were methane, propane, 2-methylpropane (2MP), 2,2-dimethylpropane (DMP), and 2,2,3,3-tetramethylbutane (TMB).Products from the reaction of propane and 2MP were analysed by deuterium NMR spectroscopy.The extent of multiple exchange increased with temperature and mechanisms involving αα- or αδ-adsorbed intermediates were favoured.The hydrogenolyses of DMP and TMB were followed at higher temperature with emphasis on the selectivity of the reaction and the change of the pattern of products with contact time.Subsidiary experiments to learn more about the course of the reactions with TMB were carried out with 2,2,3-trimethylbutane (TriMB) and 2,3-dimethylbutane (23DMB).There was evidence for a strong hydrocarbon-ruthenium interaction leading to a sequence of events with increase of temperature, relatively easy exchange but with self-poisoning, followed by moderately easy hydrogenolysis, again subject to self-poisoning.The sequence suggested the formation in turn of reversibly adsorbed hydrocarbon intermediates, more strongly adsorbed species leading to hydrogenolysis and finally carbonaceous residues.
- Brown, Ronald,Kemball, Charles
-
p. 585 - 594
(2007/10/02)
-
- Structure and Catalytic Activity of Alumina-Supported Pt-Co Bimetallic Catalysts. 2. Chemisorption and Catalytic Reactions
-
A series od Pt1-xCox/Al2O3 bimetallic catalysts have been characterized by temperature-programmed reduction (TPR), chemisorption of hydrogen and CO, deuterium exchange using both methanol and methane, and activity for the CO/H2 reaction.A Pt-assisted reduction mechanism over the entire range of composition was established by the TPR studies as well as by the chemisorption results.An enhanced metallic dispersion for the Pt-rich catalyst and formation of bimetallic particles on the Co-rich side was also indicated.In the CO hydrogenation over the Pt-rich catalysts the predominant products are methanol and dimethyl ether whereas on the Co-rich samples hydrocarbons and higher alcohols are produced.The mechanisms of deuterium exchange with methane and methanol are significantly different, the former being catalyzed solely by metallic sites while the latter utilizes both oxide and metallic sites for stepwise and multiple exchange, respectively.On the basis of the XPS data (preceding article) as well as the chemisorption results reported here, a surface model is introduced for interpretation of the catalytic results.
- Guczi, Lazlo,Hoffer, Tamas,Zsoldos, Zoltan,Zyade, Souad,Maire, Gilbert,Garin, Francois
-
p. 802 - 808
(2007/10/02)
-
- The Oxidative Coupling of Methane on Lithium Nickelate(III)
-
Kinetic studies and isotopic exchange measurements (CH4-CD4 and 16O2-18O2) of the oxidative coupling of methane over stoichiometric LiNiO2 indicate a redox mechanism involving lattice oxygen atoms.The formation of C2 products is second-order in methane, both in the presence and absence of gaseous oxygen.Methane is dissociatively adsorbed on Ni3+-O2- sites, and the rate-determining step is the coupling of adsorbed CH3.Reduction of the catalyst by methane forms NiO, over which deep oxidation occurs.Adsorbed oxygen or gaseous oxygen is responsible for the deep oxidation.
- Hatano, Masaharu,Otsuka, Kiyoshi
-
p. 199 - 206
(2007/10/02)
-
- H-D COUPLING CONSTANTS AND DEUTERIUM ISOTOPE EFFECTS ON THE PROTON CHEMICAL SHIFTS IN PARTIALLY DEUTERIATED METHANES
-
Very accurate frequency measurements on 500 MHz 1H NMR spectra of CH4, CH3D, CH2D2, and CHD3 are reported.There are relatively large (up to 10percent) solvent and temperature effects on the isotope shifts (2Δ=-13.998 to -15.555 ppb for CH3D), which are non-additive by ca 0.2 ppb/deuterium; 2JHD (-1.929 +/- 0.001 Hz, sign assumed, equivalent to 2JHH=-12.57 Hz) is almost independent of solvent, temperature, and the number of deuterons present.
- Anet, Frank A. L.,O'Leary, Daniel J.
-
p. 2755 - 2758
(2007/10/02)
-
- Absoption and Reactions of Methanethiol on Clean and Modified Ni(110)
-
The reactions of methanethiol on clean and modified Ni(110) have been studied under ultrahigh-vacuum conditions by temperature-programmed reactions (TPR), including deuterium incorporation studies.Surface bound molecular fragments were identified by X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS).The TPR data indicate that the major products of the reactions of methanethiol with clean Ni(110) surfaces are methane and hydrogen.Methane desorbs in a reaction-limited peak at 276 K, which does not shift with methanethiol exposure.Hydrogen desorption occurs in several peaks depending on the exposure.The coverage dependence of the methane yield indicates a competition between decomposition and reaction to form methane.At low coverages, decomposition is the major pathway while at higher coverages methane formation dominates.Vibrational spectroscopy (HREELS) indicates the presence of the methyl thiolate intermediate at temperatures less than 200 K.X-ray photoelectron spectroscopy and deuterium incorporation experiments confirm this assignment.A mechanism has been proposed based on hydrogenolysis of the methyl thiolate species and is consistent with all of the data.The appropriate rate equations associated with this mechanism have been solved numerically to predict the TPR data, and qualitative agreement was achieved .Methanethiol reacts with sulfur- and oxigen-modified Ni(110) surfaces to produce methane, hydrogen, and, in the case of the oxidized surfaces , water.The major effect of the modifier was to enhance the formation of methane relative to decomposition.These observations can be explained by either electronic or structural effects.
- Huntley, D. R.
-
p. 6156 - 6164
(2007/10/02)
-
- Preparation and properties of metallacyclobutanes of nickel and palladium
-
Bis(phosphine)-3,3-dimethylnickela- and palladacyclobutanes have been prepared by intramolecular C-H insertion reaction of the corresponding dineopentyl metal complexes.Nickelacyclobutane complexes decompose when heated thereby undergoing competitive carb
- Miyashita, Akira,Ohyoshi, Mayumi,Shitara, Hiroaki,Nohira, Hiroyuki
-
p. 103 - 112
(2007/10/02)
-
- Isotopic Excange in the Sonolysis of Aqueous Solutions Containing D2 and CH4
-
Water was insonated under an argon atmosphere which contained various amounts of a D2-CH4 (2:1 vol percent) mixture.Maximum yield for the formation of CH3D, CH2D2, CHD3 and CD4 was observed at 40 vol percent argon.Ethane, ethylene, acetylene, and many higher hydrocarbons were also produced with about one third of the deuterated methane yield.In addition, some H/D exchange took place between D2 and H2O, and H2 was formed from water.Carbon monoxide and carbon were produced, too.A mechanism involving free radicals and atoms is discussed to explain these observations.
- Hart, Edwin J.,Fischer, Ch.-Herbert,Henglein, Arnim
-
p. 4166 - 4169
(2007/10/02)
-
- Mechanistic and Kinetic Study of Alkane Activation by Ti+ and V+ in the Gas Phase. Lifetimes of Reaction Intermediates
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The reactions of Ti+ and V+ with several deuterium-labeled alkanes are studied by using an ion beam apparatus.The dominant reactions observed for both of these metal ions are single and double dehydrogenations.Alkane loss recations are also observed for Ti+ but may be due to electronically excited states.The dehydrogenation mechanisms are investigated by using partially deuterated alkenes.The results are consistent with 1,2-eliminations for both V+ and Ti+, where deuterium scrambling may occur in the latter case.It is proposed that some 1,3-elimination of hydrogen also occurs in the reaction of Ti+ with n-butane.Although the dehydrogenation reactions of V+ and Ti+ appear to be similar to those of Ru+ and Rh+, there are some important differences in the reactivity of V+.Extensive adduct formation and large deuterium isotope effects are consistent with reaction intermediates which are relatively long-lived for V+ in comparison to Ti+, Ru+, and Rh+.Collisional stabilization studies are used to estimate dissociation rates of reaction intermediates formed when Ti+ and V+ interact with n-butane.The measured upper limits to the unimolecular decomposition rates are 1.47 X 105 s-1 and 1.23 X 107 s-1 for V+ and Ti+ respectively.Model RRKM calculations are able to reproduce these rates and provide an explanation of isotope effects observed when n-butane-d10 is employed as the neutral reactant.The slower rate for V+ is suggested to arise from the inability of V+ to form two strong ? bonds due to the 3d4 electronic configuration of the ground-state ion.This renders C-H bond insertion energetically much less favorable for V+ than for the other metal ions and limits the excitation energy of reaction intermediates.
- Tolbert, M. A.,Beauchamp, J. L.
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p. 7509 - 7517
(2007/10/02)
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- Reactions of Hydrocarbons on Alumina-supported Pt-Ir Bimetallic Catalysts - Part 1. - Exchange of Methane and Cyclopentane with Deuterium, and Hydrogenolysis of Butane, Pentane and Cyclopentane
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Exchange reactions with deuterium of cyclopentane (303 K) and of methane (473 K) and the hydrogenolysis of n-butane, n-pentane and cyclopentane have been studied on a series of alumina-supported Pt-Ir catalysts.Metal composition had little effect on the activity of the catalysts for the exchange reactions but influenced the character of the reactions in a regular manner.Rates of hydrogenolysis increased with iridium content, and the change was >100 for n-butane but only 10 for cyclopentane.The main reaction with cyclopentane was ring-opening, but the extent of further hydrogenolysis increased with iridium content in a manner that suggested the possible requirement of ensembles of three iridium atoms.
- Garden, David,Kemball, Charles,Whan, David A.
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p. 3113 - 3124
(2007/10/02)
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- Activation of C-H Bonds in Saturated Hydrocarbons. H-D Exchange between Methane and Benzene catalysed by a Soluble Iridium Polyhydride System
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The soluble iridium pentahydride (i-Pr3P)2IrH5 (activated by t-Bu-CH=CH2) catalyses H-D exchange between C6D6 and CH4 under mild conditions.
- Cameron, Charles J.,Felkin, Hugh,Fillebeen-Khan, Tauqir,Forrow, Nigel J.,Guittet, Eric
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p. 801 - 802
(2007/10/02)
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- Reactions of FeCH2(+) and CoCH2(+) with Aliphatic Alkanes in the Gas Phase. Activation of C-H and C-C Bonds by Naked Transition-Metal Carbene Ions
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Gas-phase reactions of the title carbenes with several aliphatic alkanes using Fourier transform mass spectrometry (FTMS) ,are described.CoCH2(+) reacts with alkanes larger than methane whereas FeCH2(+) reacts with alkanes larger than ethane.Both FeCH2(+) and CoCH2(+) react predominantly by initial C-H bond insertion with some C-C bond insertion also observed.As a consequence of facile carbene-alkyl coupling, C-C bond cleavage processes proceed predominantly with elimination of the original carbene incorporated into the departing alkane neutral.In addition a small amount of C-C bond formation product is also observed.Finally, a greater degree of H/D scrambling is observed for CoCD2(+) than for FeCD2(+).
- Jacobson, D. B.,Freiser, B. S.
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p. 4373 - 4378
(2007/10/02)
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- Transition-Metal-Carbene Chemistry. Structure, Thermodynamics, and Reactivity of RhCH2+ in the Gas Phase
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The first studies invoving a second-row transition-metal methylidene ion, RhCH2+, in the gas phase using Fourier transform mass spectrometfy (FTSM) are described.Product distribution for the reactions of RhCH2+ with hydrogen, methane, ethane, ethene, propene, and cyclopropane are reported.The methylene bond strength, D0(Rh+-CH2), was bracketed between 92 and 95 kcal/mol from which D0(Rh+-CH2) = 94 +/- 5 kcal/mol is assigned.Structural studies clearly indicate formation of a methylidene-rhodium complex instead of a hydrido-metyhylidene species.Collisional activation of RhCH2+ yields both RhC+ and Rh+, suggesting that α-hydride abstraction in RhCH2+ can occor.RhCH2+ reacts readily with both hydrogen and methane and represents the first example of methane activation by a cationic mononuclear transition-metal complex in the gas phase.The activation energy for methane elimination from Rh(H)(CH3)+ appears to be less than 5 kcal/mol. α-Hydride abstraction from Rh(CH3)+ resulting in methane elimination is found to be facile.Reactions of Rh+ with cyclopropane and RhCH2+ with ethene suggest that β-hydride elimination from a rhodacyclobutane intermediate occurs readily.Therefore, the rhodacyclobutane+ ring is belived to have significant puckering.Finally, metathesis products are observed for reactions of RhCD2+ with both ethene and propene.The results of this work are compared to earlier studies on first-row MCH2+ ions.
- Jacobson, D. B.,Freiser, B. S.
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p. 5870 - 5876
(2007/10/02)
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- Exchange Reactions of Hydrocarbons on Silica-supported Rh-Pt Bimetallic Catalysts
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A series of silica-supported rhodium-platinum catalysts containing 10E-1 mol metal g-1 silica have been shown to be well dispersed by temperature-programmed reduction.Exchange reactions with deuterium of methane (463 K), 2,2-dimethylpropane (323 K) and n-butane (311 K) were followed using most of the catalysts in the series.Two patterns of activity against composition curve were found.For methane and 2,2-dimethylpropane, i.e. molecules containing only primary C-H bonds which cannot form α,β-diadsorbed species, platinum was as effective if not more effective than rhodium.However, for the exchange of n-butane (and also propane) rhodium was some 20-30 times more active than platinum.The extent of multiple exchange increased with rhodium content.There was some evidence for the importance of electronic effects from the variations of activity and selectivity with metal composition.
- Oliver, James A.,Kemball, Charles,Brown, Ronald,Jamieson, Edward W.
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p. 1871 - 1882
(2007/10/02)
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- Photocatalytic Hydrogenation of Alkynes and Alkenes with Water over TiO2. Hydrogenation Accompanied by Bond Fission
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Photocatalytic reactions of alkynes and alkenes with water have been investigated over TiO2 powder.The major photoformed products are those formed by hydrogenation accompained by C=C or CC bond fission.Oxidation products such as CO and CO2 are also formed.From the change in the product yields with the remaining amount of water adsorbed as well as the pressure of water vapor it is concluded that water molecules and not surface OH- groups are responsible for the reaction and also that vacant sites for the alkyne (or alkene) adsorption are necessary for the occurrence of the reaction.The fission of the C=C or CC bond observed with the hydrogenation is attributed to the interaction of the alkynes (or alkenes) with the trapped e-(Ti3+) and h+(.OH) pairs.It is suggested that a close association of photoformed electron and hole pairs plays a significant role in the bond fission.
- Anpo, Masakazu,Aikawa, Norikazu,Kodama, Sukeya,Kubokawa, Yutaka
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p. 2569 - 2572
(2007/10/02)
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- Trinuclear Osmium Clusters as Models for Intermediates in Carbon Monoxide Reduction Chemistry. 1. Stepwise Reduction of CO to a μ-CH2 Ligand on an Os3 Cluster Face
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Treatment of Os3(CO)12 in tetrahydrofuran (THF) with K at 0 deg C gives an unstable formyl complex K (1).Acidification of THF solutions of 1 with aqueous 20percent H3PO4 yields the new cluster Os3(CO)11(μ-CH2) which has been spectroscopically characterized.Attempted alkylation of 1 with BF4 also gave Os3(CO)11(μ-CH2).Methane is produced when Os3(CO)11(μ-CH2) is heated under an H2 atmosphere, but when the mixture is heated in the absence of H2, CO loss occurs to yield H2Os3(CO)9(μ3-CCO), which has been spectroscopically characterized.The mechanism of the - to Os3(CO)11(μ-CH2) transformation is discussed as well as the possible relevance of these results to CO reduction on metal surfaces.
- Steinmetz, Guy R.,Morrison, Eric D.,Geoffroy, Gregory L.
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p. 2559 - 2564
(2007/10/02)
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- METHYLENE SPECIES AS PIVOTAL PRECURSORS FOR HYDROCARBON FORMATION REACTIONS ON COBALT CATALYSTS.
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The presence of CH//2 species on a carbon-deposited cobalt catalyst is demonstrated by the disproportionate formation of CH//2D//2 in the reaction with D//2. Hydrogenation of a **1**3C-deposited Co/C catalyst at 430 degree C yielded methane and a trace amount of ethylene, and it was found that the **1**3C content was substantially different in methane (31%) from that in ethylene (83%). If ethylene was brought into contact with the **1**3C-deposited Co/C catalyst at 180 degree C, (**1**3C//1)-propene was predominantly formed via a homologation reaction.
- Tanaka,Nakagawa,Yaegashi,Aomura
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p. 239 - 246
(2008/10/08)
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- ELECTRONIC EFFECTS OF ADDITIVES ON THE SELECTIVITY IN CO/H2 REACTION OVER NICKEL CATALYSTS
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It was found with CO/H2 reaction over unsupported Ni catalysts that the C2+/CH4 selectivity decreased in the order; Ni-boride > Raney-Ni > Decomposed-Ni ca.Ni-phosphide.This suggests that both the activity and selectivity increase as the electron density on Ni metal increases.The H/C atomic ratio of the surface carbon species deposited during the reaction was also dependent on the catalyst.
- Okamoto, Yasuaki,Matsunaga, Etsuo,Imanaka, Toshinobu,Teranishi, Shiichiro
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p. 565 - 568
(2007/10/02)
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- Reactions of n-Butane with Hydrogen and of Methane with Deuterium on Silica-supported Palladium/Silver Catalysts at High Temperatures
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The reaction of n-butane with hydrogen and the exchange of methane with deuterium were studied at 738 K on a series of well homogenized silica-supported palladium/silver alloy catalyst.Different patterns of activity with catalyst composition were observed for each of the main modes of reaction of n-butane - dehydrogenation, demethanation and hydrogenolysis.A comparison of the rates and character of these reactions with the information on methane exchange provides evidence about the probable rate-determining steps of the various processes.
- Haining, Ian H. B.,Kemball, Charles,Whan, David A.
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p. 3011 - 3031
(2007/10/02)
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- Very Low-Pressure Pyrolysis of Chloroethane-2,2,2-d3. Kinetics of DCl Elimination and the Effect of CO2 Inert Bath Gas
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The thermal unimolecular decomposition of chloroethane-2,2,2-d3 has been studied by means of very low-pressure pyrolysis (VLPP) over the temperature range 975-1213 K (gas/wall collisions only) and at 1173 K with CO2 as a collision bath gas.The experimental results are consistent with the reaction proceeding via the four-center elimination of DCl.No firm evidence could be found for the three-center elimination of HCl under the experimental conditions employed.Extrapolated high-pressure rate coefficients obtained via the application of RRKM theory with the incorporation of gas/wall energy-transfer efficiencies consistent with a recent VLPP study of chloroethane-2d1 are given by 1013.6 exp(-243 kJ mol-1/RT) s-1.Fitting of the pressure-dependent data by solution of the integrodifferential reaction-diffusion master equation gives a gas/gas average downward collision energy transfer value of ca. 1040 cm-1 for CO2/CD3CH2Cl collisions.
- Francisco, Joseph S.,Steinfeld, Jeffrey I.,King, Keith D.,Gilbert, Robert G.
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p. 4106 - 4108
(2007/10/02)
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- REACTIONS OF METHYLSILANE AT THE SURFACES OF NICKEL, RHODIUM AND TUNGSTEN
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The chemisorption and reactions of methylsilane at the surfaces of films of nickel,rhodium and tungsten at 195 K and above have been investigatedto determine how the metal-catalysed reactions of silanesdiffer from those of alkanes.Methylsilane chemisorption is accompanied byformation of gas-phase hydrogen and methane andf isotopic studies have shown that adsorption occurs through the silicon atom.Mutual exchange between CH3SiH3 and CH3SiD3 is catalysed by all three metals at 195 K.Exchange is confined to the silyl group and some poisoning occurs.Exchange between CH3SiH3 and D2 is catalysed by the metals investigated but substantial poisoning of the reaction is observed.Surfaces poisoned by this reaction retion activity for mutual exchange.Mechanisms of exchange are proposed.
- Bradshaw,David I.,Moyes,Richard B.,Wells,Peter B.
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p. 979 - 987
(2007/10/02)
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