Mechanisms and Controlling Characteristics of the Catalytic Oxidation of Methane. Technical Progress Report, June 15, 1990--December 14, 1992 PDF Download
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Languages : en
Pages : 13
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We have demonstrated in this work (1) that methane is readily activated at mild conditions (100°C, 1 torr) over a relatively noble metal, Pd. This was observed using a stepped and kinked Pd(679) crystal (1), and other crystal faces are now being investigated to establish whether the cracking of the C-H bond of methane on Pd is structure sensitive or structure insensitive. Oxygen chemisorption is extremely structure sensitive: weakly bonded, highly reactive oxygen overlayers form on Pd(100) surface (2), while strongly bonded, moderately reactive oxygen overlayers form on Pd(111) and Pd(679). Reaction of the weakly bonded oxygen with surface carbide gives rise to CO2 over clean Pd(100) but to CO over halogen-doped Pd(100) (3--5). The effect of halogens is primarily ensemble-controlling, or oxygen-supply restricting, but long range influence of surface Cl on the strength of the Pd-O bond has also been observed (3). Because the overall chemistry of methane activation with the subsequent oxidation gives rise to the very important oxidative reforming CH4 + 1/2 O2 -->{sub Pd/Cl} CO + 2 H2, Pd/Cl we plan to continue our study of this reaction in detail over Pd(100) (completed), Pd(111) (initiated), Pd(311) (initiated), Pd(110) (to be initiated), and Pd(679) (completed), without and with the halogen modifiers.
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Languages : en
Pages : 13
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We have demonstrated in this work (1) that methane is readily activated at mild conditions (100°C, 1 torr) over a relatively noble metal, Pd. This was observed using a stepped and kinked Pd(679) crystal (1), and other crystal faces are now being investigated to establish whether the cracking of the C-H bond of methane on Pd is structure sensitive or structure insensitive. Oxygen chemisorption is extremely structure sensitive: weakly bonded, highly reactive oxygen overlayers form on Pd(100) surface (2), while strongly bonded, moderately reactive oxygen overlayers form on Pd(111) and Pd(679). Reaction of the weakly bonded oxygen with surface carbide gives rise to CO2 over clean Pd(100) but to CO over halogen-doped Pd(100) (3--5). The effect of halogens is primarily ensemble-controlling, or oxygen-supply restricting, but long range influence of surface Cl on the strength of the Pd-O bond has also been observed (3). Because the overall chemistry of methane activation with the subsequent oxidation gives rise to the very important oxidative reforming CH4 + 1/2 O2 -->{sub Pd/Cl} CO + 2 H2, Pd/Cl we plan to continue our study of this reaction in detail over Pd(100) (completed), Pd(111) (initiated), Pd(311) (initiated), Pd(110) (to be initiated), and Pd(679) (completed), without and with the halogen modifiers.
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Category : Power resources
Languages : en
Pages : 654
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Category :
Languages : en
Pages : 15
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Progress has included (1) construction and installation of an ultraviolet photoelectron spectrometer (UPS) with power supply and pumping system that has been added as an attachment to the SCIENTA ESCA-300 instrument, (2) modification of the high resolution electron energy loss spectrometer (HREELS) to produce a stable ultra high vacuum (UHV) environment for initial experiments with a Pd(311) single crystal, (3) construction of a separate high vacuum system for preparation of surface doped model catalysts by chemical vapor deposition and pretreatment of a Pd(100) single crystal in this system, (4) carried out detailed experiments of methane activation and oxidation on Pd(679) using a high pressure reaction cell contained in a third ultra high vacuum system, (5) completion of adsorption/desorption studies of H2, CO, and O2 on Pd(679), (6) utilized angle-resolved XPS to probe the diffraction characteristics and structure of the Pd(100) surface, (7) determination of the electronic surface structure of Pd(100) using angle-resolved UPS, and (8) computational analysis of oxygen overlayers on the PD(100) surface. Each of these is discussed in further detail below.
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Category : Government reports announcements & index
Languages : en
Pages : 1832
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Author:
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Category :
Languages : en
Pages : 15
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Progress has included (1) construction and installation of an ultraviolet photoelectron spectrometer (UPS) with power supply and pumping system that has been added as an attachment to the SCIENTA ESCA-300 instrument, (2) modification of the high resolution electron energy loss spectrometer (HREELS) to produce a stable ultra high vacuum (UHV) environment for initial experiments with a Pd(311) single crystal, (3) construction of a separate high vacuum system for preparation of surface doped model catalysts by chemical vapor deposition and pretreatment of a Pd(100) single crystal in this system, (4) carried out detailed experiments of methane activation and oxidation on Pd(679) using a high pressure reaction cell contained in a third ultra high vacuum system, (5) completion of adsorption/desorption studies of H2, CO, and O2 on Pd(679), (6) utilized angle-resolved XPS to probe the diffraction characteristics and structure of the Pd(100) surface, (7) determination of the electronic surface structure of Pd(100) using angle-resolved UPS, and (8) computational analysis of oxygen overlayers on the PD(100) surface. Each of these is discussed in further detail below.
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Category : Science
Languages : en
Pages : 676
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Category :
Languages : en
Pages : 19
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Sulfate anion was used to modify the surface basicity of 1 wt% Sr/La2O3, and catalytic tests were carried out to probe the selective oxidation of methane to C2 coupling products over these catalysts. Over a range of reaction temperatures of 500--700°C, most of the catalytic tests were carried out with a 1 wt% SO4 2−/1 wt% Sr/La2O3with a CH4/air = 1/1 reactant mixture at 1 atm and with a gas hourly space velocity (GHSV) = 70,000 l/kg catal/hr. The sulfated catalyst showed the largest improved catalytic effect at 500°C. Compared to the activity of the nonsulfated Sr/La2O3, the sulfated catalyst resulted in enhancement of the methane conversion, the C2 selectivity, and the yield of C2 products. In situ laser Raman spectroscopy was used to characterize the surface of sulfated and nonsulfated catalysts. Preliminary results indicate that the sulfate anion preferentially bonded to the Sr rather than to the La ions. The promoting effect of the acidic sulfate on the catalytic activity of basic Sr/La2O3 seems to be due to the inhibition of carbonate formation on the surface Sr ions.
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Category :
Languages : en
Pages : 17
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Support effects on catalytic reactions, especially of highly exothermic oxidation reactions, can be very significant. Since we had shown that a MoO3/SiO2 catalyst, especially when used in a double bed configuration with a Sr/La2O3 catalyst, can selectively oxidize methane to formaldehyde, the role of the SiO2 support was investigated. Therefore, partial oxidation of methane by oxygen to form formaldehyde, carbon oxides, and C2 products (ethane and ethene) has been studied over silica catalyst supports (fumed Cabosil and Grace 636 silica gel) in the 630-780°C temperature range under ambient pressure. When relatively high gas hourly space velocities (GHSV) were utilized, the silica catalysts exhibit high space time yields (at low conversions) for methane partial oxidation to formaldehyde, and the C2 hydrocarbons were found to be parallel products with formaldehyde. In general, the selectivities toward CO were high while those toward CO2 were low. Based on the present results obtained by a double catalyst bed experiment, the observations of product composition dependence on the variation of GHSV (i.e. gas residence time), and differences in apparent activation energies of formation of C2H6, and CH2O, a reaction mechanism is proposed for the activation of methane over the silica surface. This mechanism can explain the observed product distribution patterns (specifically the parallel formation of formaldehyde and C2 hydrocarbons).
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Category :
Languages : en
Pages : 19
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The objective of this research is to develop the scientific background for direct catalytic oxidation of methane over oxides that are doubly doped with transition metal ions. The desired process aims at employing of a double redox mechanism, where one redox couple is utilized for activation of oxygen and another for the conversion of CH3 radicals to carbocations via electron transfer reaction. The latter species can react with surface OH− groups to form methanol or formaldehyde. To establish the foundations for such a process, two groups of the catalysts, one containing dispersed redox centers (Cu{sup I}/Fe{sup III}/ZnO and Cu{sup I}/Sn{sup IV}/ZnO) and a second of delafossite-type oxides containing concentrated redox centers (CuFeO2, CuCoO2) were synthesized and chemically analyzed for composition. For the sake of comparison, undoped ZnO treated in the same way as doped zinc oxide catalysts was also prepared. The samples were characterized by X-ray diffraction, BET surface area measurements and preliminarily by scanning electron microscopy. A catalytic testing unit and reactor to study the title reaction were designed and constructed.
Author: J. R. H. Ross
Publisher:
ISBN: 9789282632178
Category : Catalysis
Languages : en
Pages : 144
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