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Alexis T Bell - One of the best experts on this subject based on the ideXlab platform.
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selective oxidation and oxidative dehydrogenation of hydrocarbons on bismuth vanadium molybdenum oxide
Journal of Catalysis, 2015Co-Authors: Zheng Zhai, Xuan Wang, Rachel B Licht, Alexis T BellAbstract:Abstract A systematic investigation of the oxidative dehydrogenation of propane to Propene and 1- and 2-butene to 1,3-butadiene, and the selective oxidation of isobutene to methacrolein was carried out over Bi1−x/3V1−xMoxO4 (x = 0–1) with the aim of defining the effects of catalyst and reactant composition on the reaction kinetics. This work has revealed that the reaction kinetics can differ significantly depending on the state of catalyst oxidation, which in turn depends on the catalyst composition and the reaction conditions. Under conditions where the catalyst is fully oxidized, the kinetics for the oxidation of Propene to acrolein and isobutene to methacrolein, and the oxidative dehydrogenation of propane to Propene, 1-butene and trans-2-butene to butadiene are very similar—first order in the partial pressure of the alkane or alkene and zero order in the partial pressure of oxygen. These observations, together with XANES and UV–Vis data, suggest that all these reactions proceed via a Mars van Krevelen mechanism involving oxygen atoms in the catalysts and that the rate-limiting step involves cleavage of the weakest C H bond in the reactant. Consistent with these findings, the apparent activation energy and pre-exponential factor for both oxidative dehydrogenation and selective oxidation correlate with the dissociation energy of the weakest C H bond in the reactant. As the reaction temperature is lowered, catalyst reoxidation can become rate-limiting, the transition to this regime depending on ease of catalyst reduction and effectiveness of the reacting hydrocarbons as a reducing agent. A third regime is observed for isobutene oxidation at lower temperatures, in which the catalyst is more severely reduced and oxidation now proceeds via reaction of molecular oxygen, rather than catalyst lattice oxygen, with the reactant.
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oxidative dehydrogenation of propane over vanadia based catalysts supported on high surface area mesoporous mgal2o4
Journal of Catalysis, 2004Co-Authors: Owen R Evans, Alexis T Bell, Don T TilleyAbstract:Abstract The oxidative dehydrogenation of propane to Propene was investigated over a series of novel vanadia-based catalysts supported on high-surface-area magnesium spinel. A mesoporous MgAl 2 O 4 support was synthesized via a low-temperature sol–gel process involving the heterobimetallic alkoxide precursor, Mg[Al(O i Pr) 4 ] 2 . A high-purity catalyst support was obtained after calcination at 1173 K under O 2 atmosphere and active vanadia catalysts were prepared from the thermolysis of OV(O t Bu) 3 after grafting onto the spinel support. MgAl 2 O 4 -supported catalysts prepared in this manner have BET surface areas of 234–245 m 2 /g. All of the catalysts were characterized by X-ray powder diffraction, and Raman, solid-state NMR, and diffuse-reflectance UV–vis spectroscopy. At all vanadium loadings the vanadia supported on MgAl 2 O 4 exist as a combination of isolated monovanadate and tetrahedral polyvanadate species. As the vanadium surface density increases for these catalysts the ratio of polyvanadate species to isolated monovandate species increases. In addition, as the vanadium surface density increases for these catalysts, the initial rate of propane ODH per V atom increases and reaches a maximum value at 6 VO x /nm 2 . Increasing the vanadium surface density past this point results in a decrease in the rate of propane ODH owing to the formation of multilayer species in which subsurface vanadium atoms are essentially rendered catalytically inactive. The initial Propene selectivity increases with increasing vanadium surface density and reaches a plateau of ∼95% for the V/MgAl catalysts. Rate coefficients for propane ODH ( k 1 ), propane combustion ( k 2 ), and Propene combustion ( k 3 ) were calculated for these catalysts. The value of k 1 increases with increasing VO x surface density, reaching a maximum at about 5.5 VO x /nm 2 . On the other hand, the ratio ( k 2 / k 1 ) for V/MgAl decreases with increasing VO x surface density. The ratio ( k 3 / k 1 ) for both sets of catalysts shows no dependence on the vanadia surface density. The observed trends in k 1 , ( k 2 / k 1 ), and ( k 3 / k 1 ) are discussed in terms of the surface structure of the catalyst.
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effects of molybdena on the catalytic properties of vanadia domains supported on alumina for oxidative dehydrogenation of propane
Journal of Catalysis, 2004Co-Authors: Alexis T Bell, Enrique IglesiaAbstract:The oxidative dehydrogenation (ODH) of propane was investigated on vanadia dispersed on alumina containing a nominal polymolybdate monolayer (4.8 Mo/nm2). Dehydrogenation rates and selectivities on these catalysts were compared with those on vanadia domains dispersed on alumina. At a given vanadia surface density, ODH reaction rates per gram of catalyst were about 1.5–2 times greater on MoOx-coated Al2O3 than on pure Al2O3 supports. The higher activity of vanadia dispersed on MoOx-coated Al2O3 reflects the greater reducibility of VOx species as a result of the replacement of VOAl with VOMo bonds. The MoOx interlayer also increased the alkene selectivity by inhibiting propane and Propene combustion rates relative to ODH rates. This appears to reflect a smaller number of unselective V2O5 clusters when alkoxide precursors are used to disperse vanadia on MoOx/Al2O3 as compared to the use of metavanadate precursor to disperse vanadia on pure Al2O3. At 613 K, the ratio of rate coefficients for propane combustion and propane ODH was three times smaller on MoOx/Al2O3 than on Al2O3 supports. The ratio of rate constants for Propene combustion and propane ODH decreased by a similar factor.
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isotopic tracer studies of reaction pathways for propane oxidative dehydrogenation on molybdenum oxide catalysts
Journal of Physical Chemistry B, 2001Co-Authors: Kaidong Chen, Enrique Iglesia, Alexis T BellAbstract:Kinetic analysis and isotopic tracer studies were used to identify the elementary steps and their reversibility in the oxidative dehydrogenation of propane over ZrO2-supported MoOx catalysts. Competitive reactions of C3H6 and CH313CH2CH3 showed that Propene is the most abundant primary product, and that CO and CO2 are formed via either secondary combustion of Propene, or by direct combustion of propane. A mixture of C3H8 and C3D8 undergoes oxidative dehydrogenation without forming C3H8-xDx mixed isotopomers, suggesting that steps involving C−H bond activation are irreversible. Normal kinetic isotopic effects (kC-H/kC-D) were measured for propane dehydrogenation (2.3), propane combustion (1.6) and Propene combustion (2.1). These data indicate that the kinetically relevant steps in propane dehydrogenation and Propene combustion involve the dissociation of C−H bonds in the respective reactant. H−D exchange occurs readily between C3H6 and D2O or C3D6 and H2O, suggesting that OH recombination steps are reversi...
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kinetic isotopic effects in oxidative dehydrogenation of propane on vanadium oxide catalysts
Journal of Catalysis, 2000Co-Authors: Enrique Iglesia, Alexis T Bell, Kaidong ChenAbstract:Abstract Kinetic isotopic effects (KIEs) for oxidative dehydrogenation of propane were measured on 10 wt% V 2 O 5 /ZrO 2 . Normal KIEs were obtained using CH 3 CH 2 CH 3 and CD 3 CD 2 CD 3 as reactants for primary dehydrogenation (2.8) and combustion (1.9) of propane and for secondary combustion of Propene (2.6), suggesting that in all cases C–H bond dissociation is a kinetically relevant step. CH 3 CH 2 CH 3 and CH 3 CD 2 CH 3 reactants led to normal KIEs for dehydrogenation (2.7) and combustion (1.8) of propane, but to a very small KIE (1.1) for Propene combustion. These results show that the methylene C–H bond is activated in the rate-determining steps for propane dehydrogenation and combustion reactions. The rate-determining step in secondary Propene combustion involves the allylic C–H bond. In each reaction, the weakest C–H bond in the reactant is cleaved in the initial C–H bond activation step. The measured propane oxidative dehydrogenation KIEs are in agreement with theoretical estimates using a sequence of elementary steps, reaction rate expression, and transition state theory. The much smaller KIE for propane oxidative dehydrogenation (2.8) than the maximum KIE (6) expected for propane thermal dehydrogenation indicates the participation of lattice oxygen. The different KIE values for propane primary dehydrogenation and combustion suggest that these two reactions involve different lattice oxygen sites.
Enrique Iglesia - One of the best experts on this subject based on the ideXlab platform.
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effects of molybdena on the catalytic properties of vanadia domains supported on alumina for oxidative dehydrogenation of propane
Journal of Catalysis, 2004Co-Authors: Alexis T Bell, Enrique IglesiaAbstract:The oxidative dehydrogenation (ODH) of propane was investigated on vanadia dispersed on alumina containing a nominal polymolybdate monolayer (4.8 Mo/nm2). Dehydrogenation rates and selectivities on these catalysts were compared with those on vanadia domains dispersed on alumina. At a given vanadia surface density, ODH reaction rates per gram of catalyst were about 1.5–2 times greater on MoOx-coated Al2O3 than on pure Al2O3 supports. The higher activity of vanadia dispersed on MoOx-coated Al2O3 reflects the greater reducibility of VOx species as a result of the replacement of VOAl with VOMo bonds. The MoOx interlayer also increased the alkene selectivity by inhibiting propane and Propene combustion rates relative to ODH rates. This appears to reflect a smaller number of unselective V2O5 clusters when alkoxide precursors are used to disperse vanadia on MoOx/Al2O3 as compared to the use of metavanadate precursor to disperse vanadia on pure Al2O3. At 613 K, the ratio of rate coefficients for propane combustion and propane ODH was three times smaller on MoOx/Al2O3 than on Al2O3 supports. The ratio of rate constants for Propene combustion and propane ODH decreased by a similar factor.
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isotopic tracer studies of reaction pathways for propane oxidative dehydrogenation on molybdenum oxide catalysts
Journal of Physical Chemistry B, 2001Co-Authors: Kaidong Chen, Enrique Iglesia, Alexis T BellAbstract:Kinetic analysis and isotopic tracer studies were used to identify the elementary steps and their reversibility in the oxidative dehydrogenation of propane over ZrO2-supported MoOx catalysts. Competitive reactions of C3H6 and CH313CH2CH3 showed that Propene is the most abundant primary product, and that CO and CO2 are formed via either secondary combustion of Propene, or by direct combustion of propane. A mixture of C3H8 and C3D8 undergoes oxidative dehydrogenation without forming C3H8-xDx mixed isotopomers, suggesting that steps involving C−H bond activation are irreversible. Normal kinetic isotopic effects (kC-H/kC-D) were measured for propane dehydrogenation (2.3), propane combustion (1.6) and Propene combustion (2.1). These data indicate that the kinetically relevant steps in propane dehydrogenation and Propene combustion involve the dissociation of C−H bonds in the respective reactant. H−D exchange occurs readily between C3H6 and D2O or C3D6 and H2O, suggesting that OH recombination steps are reversi...
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kinetic isotopic effects in oxidative dehydrogenation of propane on vanadium oxide catalysts
Journal of Catalysis, 2000Co-Authors: Enrique Iglesia, Alexis T Bell, Kaidong ChenAbstract:Abstract Kinetic isotopic effects (KIEs) for oxidative dehydrogenation of propane were measured on 10 wt% V 2 O 5 /ZrO 2 . Normal KIEs were obtained using CH 3 CH 2 CH 3 and CD 3 CD 2 CD 3 as reactants for primary dehydrogenation (2.8) and combustion (1.9) of propane and for secondary combustion of Propene (2.6), suggesting that in all cases C–H bond dissociation is a kinetically relevant step. CH 3 CH 2 CH 3 and CH 3 CD 2 CH 3 reactants led to normal KIEs for dehydrogenation (2.7) and combustion (1.8) of propane, but to a very small KIE (1.1) for Propene combustion. These results show that the methylene C–H bond is activated in the rate-determining steps for propane dehydrogenation and combustion reactions. The rate-determining step in secondary Propene combustion involves the allylic C–H bond. In each reaction, the weakest C–H bond in the reactant is cleaved in the initial C–H bond activation step. The measured propane oxidative dehydrogenation KIEs are in agreement with theoretical estimates using a sequence of elementary steps, reaction rate expression, and transition state theory. The much smaller KIE for propane oxidative dehydrogenation (2.8) than the maximum KIE (6) expected for propane thermal dehydrogenation indicates the participation of lattice oxygen. The different KIE values for propane primary dehydrogenation and combustion suggest that these two reactions involve different lattice oxygen sites.
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kinetics and mechanism of oxidative dehydrogenation of propane on vanadium molybdenum and tungsten oxides
Journal of Physical Chemistry B, 2000Co-Authors: Kaidong Chen, And Alexis T Bell, Enrique IglesiaAbstract:The effect of cation identity on oxidative dehydrogenation (ODH) pathways was examined using two-dimensional VOx, MoOx, and WOx structures supported on ZrO2. The similar kinetic rate expressions obtained on MoOx and VOx catalysts confirmed that oxidative dehydrogenation of propane occurs via similar pathways, which involve rate-determining C−H bond activation steps using lattice oxygen atoms. The activation energies for propane dehydrogenation and for Propene combustion increase in the sequence VOx/ZrO2 < MoOx/ZrO2 < WOx/ZrO2; the corresponding reaction rates decrease in this sequence, suggesting that turnover rates reflect C−H bond cleavage activation energies, which are in turn influenced by the reducibility of these metal oxides. Propane ODH activation energies are higher than for Propene combustion. This leads to an increase in maximum alkene yields and in the ratio of rate constants for propane ODH and Propene combustion as temperature increases. This difference in activation energy (48−61 kJ/mol) be...
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isotopic tracer and kinetic studies of oxidative dehydrogenation pathways on vanadium oxide catalysts
Journal of Catalysis, 1999Co-Authors: Kaidong Chen, Andrei Y Khodakov, Jun Yang, Alexis T Bell, Enrique IglesiaAbstract:Kinetic analysis and isotopic tracer studies were used to identify elementary steps and their reversibility in the oxidative dehydrogenation of propane on VOx/ZrO2 catalysts with VOx surface densities between 1.6 and 6 VOx/nm2. Competitive reactions of C3H6 and CH313CH2CH3 showed that CO forms via secondary combustion of Propene intermediates. CO2 formed via this reaction and also via the direct combustion of propane. Reactions of 18O2/C3H8 mixtures on supported V216O5 led to the preferential initial appearance of lattice 16O atoms in all oxygen-containing products, as expected if lattice oxygens were required for the activation of C–H bonds. Isotopically mixed O2 species were not detected during reactions of C3H8–18O2–16O2 reactant mixtures. Therefore, dissociative O2 chemisorption steps are irreversible. Similarly, C3H8–C3D8–O2 reactants undergo oxidative dehydrogenation without forming C3H8−xDx mixed isotopomers, suggesting that C–H bond activation steps are also irreversible. Normal kinetic isotopic effects (kC–H/kC–D=2.5) were measured for primary oxidative dehydrogenation reactions. Kinetic isotope effects were slightly lower for propane and Propene combustion steps (1.7 and 2.2, respectively). These data are consistent with kinetically relevant steps involving the dissociation of C–H bonds in propane and Propene. C3H6–D2O and C3D6–H2O cross exchange reactions occur readily during reaction; therefore, OH recombination steps are reversible and nearly equilibrated. These isotopic tracer results are consistent with a Mars–van Krevelen redox mechanism involving two lattice oxygens in irreversible C–H bond activation steps. The resulting alkyl species desorb as Propene and the remaining O–H group recombines with neighboring OH groups to form water and reduced V centers. These reduced V centers reoxidize by irreversible dissociative chemisorption of O2. The application of pseudo-steady-state and reversibility assumptions leads to a complex kinetic rate expression that describes accurately the observed water inhibition effects and the kinetic orders in propane and oxygen when surface oxygen and OH groups are assumed to be the most abundant surface intermediates.
Kaidong Chen - One of the best experts on this subject based on the ideXlab platform.
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isotopic tracer studies of reaction pathways for propane oxidative dehydrogenation on molybdenum oxide catalysts
Journal of Physical Chemistry B, 2001Co-Authors: Kaidong Chen, Enrique Iglesia, Alexis T BellAbstract:Kinetic analysis and isotopic tracer studies were used to identify the elementary steps and their reversibility in the oxidative dehydrogenation of propane over ZrO2-supported MoOx catalysts. Competitive reactions of C3H6 and CH313CH2CH3 showed that Propene is the most abundant primary product, and that CO and CO2 are formed via either secondary combustion of Propene, or by direct combustion of propane. A mixture of C3H8 and C3D8 undergoes oxidative dehydrogenation without forming C3H8-xDx mixed isotopomers, suggesting that steps involving C−H bond activation are irreversible. Normal kinetic isotopic effects (kC-H/kC-D) were measured for propane dehydrogenation (2.3), propane combustion (1.6) and Propene combustion (2.1). These data indicate that the kinetically relevant steps in propane dehydrogenation and Propene combustion involve the dissociation of C−H bonds in the respective reactant. H−D exchange occurs readily between C3H6 and D2O or C3D6 and H2O, suggesting that OH recombination steps are reversi...
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kinetic isotopic effects in oxidative dehydrogenation of propane on vanadium oxide catalysts
Journal of Catalysis, 2000Co-Authors: Enrique Iglesia, Alexis T Bell, Kaidong ChenAbstract:Abstract Kinetic isotopic effects (KIEs) for oxidative dehydrogenation of propane were measured on 10 wt% V 2 O 5 /ZrO 2 . Normal KIEs were obtained using CH 3 CH 2 CH 3 and CD 3 CD 2 CD 3 as reactants for primary dehydrogenation (2.8) and combustion (1.9) of propane and for secondary combustion of Propene (2.6), suggesting that in all cases C–H bond dissociation is a kinetically relevant step. CH 3 CH 2 CH 3 and CH 3 CD 2 CH 3 reactants led to normal KIEs for dehydrogenation (2.7) and combustion (1.8) of propane, but to a very small KIE (1.1) for Propene combustion. These results show that the methylene C–H bond is activated in the rate-determining steps for propane dehydrogenation and combustion reactions. The rate-determining step in secondary Propene combustion involves the allylic C–H bond. In each reaction, the weakest C–H bond in the reactant is cleaved in the initial C–H bond activation step. The measured propane oxidative dehydrogenation KIEs are in agreement with theoretical estimates using a sequence of elementary steps, reaction rate expression, and transition state theory. The much smaller KIE for propane oxidative dehydrogenation (2.8) than the maximum KIE (6) expected for propane thermal dehydrogenation indicates the participation of lattice oxygen. The different KIE values for propane primary dehydrogenation and combustion suggest that these two reactions involve different lattice oxygen sites.
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kinetics and mechanism of oxidative dehydrogenation of propane on vanadium molybdenum and tungsten oxides
Journal of Physical Chemistry B, 2000Co-Authors: Kaidong Chen, And Alexis T Bell, Enrique IglesiaAbstract:The effect of cation identity on oxidative dehydrogenation (ODH) pathways was examined using two-dimensional VOx, MoOx, and WOx structures supported on ZrO2. The similar kinetic rate expressions obtained on MoOx and VOx catalysts confirmed that oxidative dehydrogenation of propane occurs via similar pathways, which involve rate-determining C−H bond activation steps using lattice oxygen atoms. The activation energies for propane dehydrogenation and for Propene combustion increase in the sequence VOx/ZrO2 < MoOx/ZrO2 < WOx/ZrO2; the corresponding reaction rates decrease in this sequence, suggesting that turnover rates reflect C−H bond cleavage activation energies, which are in turn influenced by the reducibility of these metal oxides. Propane ODH activation energies are higher than for Propene combustion. This leads to an increase in maximum alkene yields and in the ratio of rate constants for propane ODH and Propene combustion as temperature increases. This difference in activation energy (48−61 kJ/mol) be...
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isotopic tracer and kinetic studies of oxidative dehydrogenation pathways on vanadium oxide catalysts
Journal of Catalysis, 1999Co-Authors: Kaidong Chen, Andrei Y Khodakov, Jun Yang, Alexis T Bell, Enrique IglesiaAbstract:Kinetic analysis and isotopic tracer studies were used to identify elementary steps and their reversibility in the oxidative dehydrogenation of propane on VOx/ZrO2 catalysts with VOx surface densities between 1.6 and 6 VOx/nm2. Competitive reactions of C3H6 and CH313CH2CH3 showed that CO forms via secondary combustion of Propene intermediates. CO2 formed via this reaction and also via the direct combustion of propane. Reactions of 18O2/C3H8 mixtures on supported V216O5 led to the preferential initial appearance of lattice 16O atoms in all oxygen-containing products, as expected if lattice oxygens were required for the activation of C–H bonds. Isotopically mixed O2 species were not detected during reactions of C3H8–18O2–16O2 reactant mixtures. Therefore, dissociative O2 chemisorption steps are irreversible. Similarly, C3H8–C3D8–O2 reactants undergo oxidative dehydrogenation without forming C3H8−xDx mixed isotopomers, suggesting that C–H bond activation steps are also irreversible. Normal kinetic isotopic effects (kC–H/kC–D=2.5) were measured for primary oxidative dehydrogenation reactions. Kinetic isotope effects were slightly lower for propane and Propene combustion steps (1.7 and 2.2, respectively). These data are consistent with kinetically relevant steps involving the dissociation of C–H bonds in propane and Propene. C3H6–D2O and C3D6–H2O cross exchange reactions occur readily during reaction; therefore, OH recombination steps are reversible and nearly equilibrated. These isotopic tracer results are consistent with a Mars–van Krevelen redox mechanism involving two lattice oxygens in irreversible C–H bond activation steps. The resulting alkyl species desorb as Propene and the remaining O–H group recombines with neighboring OH groups to form water and reduced V centers. These reduced V centers reoxidize by irreversible dissociative chemisorption of O2. The application of pseudo-steady-state and reversibility assumptions leads to a complex kinetic rate expression that describes accurately the observed water inhibition effects and the kinetic orders in propane and oxygen when surface oxygen and OH groups are assumed to be the most abundant surface intermediates.
Haijun Jiao - One of the best experts on this subject based on the ideXlab platform.
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surface carbon hydrogenation on precovered fe 110 with spectator coverage dependent chain initiation and propagation
Journal of Physical Chemistry C, 2019Co-Authors: Xiaodong Wen, Haijun JiaoAbstract:Experimentally, surface carbon hydrogenation on metallic iron forms CH4, ethylene/ethane, and Propene/propane, while previous density functional theory studies showed that these reactions are highly endothermic. This disagreement can be attributed to the shortcoming of low coverage of surface carbon atoms and the underestimated role of surface hydrogen. On a p(4 × 4) Fe(110) surface with 0.25 ML carbon coverage (4 C atoms) and other free sites filled with H atoms, we studied surface C hydrogenation with spectators and found that the formation of CH4, ethylene/ethane, and Propene/propane becomes exothermic. Coupling of CH + CH and CH3C + CH to acetylene and propylene is favored thermodynamically. Next, CHCH and CH3CCH can be hydrogenated into CH2CH and CH3CHCH, and the subsequent hydrogenation of CH2CH and CH3CHCH determines the formation and selectivity of alkenes and alkanes. The most important surface species is carbide HC for chain initiation or CH3C (RC for higher homolog) for chain propagation. This ...
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surface carbon hydrogenation on precovered fe 110 with spectator coverage dependent chain initiation and propagation
The Journal of Physical Chemistry, 2019Co-Authors: Xiaodong Wen, Haijun JiaoAbstract:Experimentally, surface carbon hydrogenation on metallic iron forms CH₄, ethylene/ethane, and Propene/propane, while previous density functional theory studies showed that these reactions are highly endothermic. This disagreement can be attributed to the shortcoming of low coverage of surface carbon atoms and the underestimated role of surface hydrogen. On a p(4 × 4) Fe(110) surface with 0.25 ML carbon coverage (4 C atoms) and other free sites filled with H atoms, we studied surface C hydrogenation with spectators and found that the formation of CH₄, ethylene/ethane, and Propene/propane becomes exothermic. Coupling of CH + CH and CH₃C + CH to acetylene and propylene is favored thermodynamically. Next, CHCH and CH₃CCH can be hydrogenated into CH₂CH and CH₃CHCH, and the subsequent hydrogenation of CH₂CH and CH₃CHCH determines the formation and selectivity of alkenes and alkanes. The most important surface species is carbide HC for chain initiation or CH₃C (RC for higher homolog) for chain propagation. This model of high coverage with spectators may help understand the mechanism of heterogeneous catalysis under real conditions.
Reinhard Schomacker - One of the best experts on this subject based on the ideXlab platform.
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high performance vox n tiox m sba 15 catalysts for the oxidative dehydrogenation of propane
Catalysis Science & Technology, 2014Co-Authors: Carlos A Carrero, Arne Dinse, Robert Schlogl, Annette Trunschke, Markus Kauer, Till Wolfram, Neil G Hamilton, Reinhard SchomackerAbstract:Grafted VxOy catalysts for oxidative dehydrogenation of propane (ODP) have been studied due to their potential high performance and as model catalysts in the past. We report on a positive synergetic effect capable of considerably enhancing the Propene productivities above reported performances. The most productive catalysts were found at metal loadings (V + Ti) close to the monolayer coverage. The 4V/13Ti/SBA-15 catalyst presented a considerably high productivity (6–9 kgPropene kgcat−1 h−1). Moreover, with this catalyst, Propene productivity only slightly decreased as a function of propane conversion, indicating that Propene combustion toward COx occurs more slowly in comparison to other catalysts exhibiting high Propene productivities. A detailed kinetic analysis of the 4V/13Ti/SBA-15 catalyst revealed that high vanadia and titania dispersions are required for high Propene productivity.
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oxidative dehydrogenation of propane on silica sba 15 supported vanadia catalysts a kinetic investigation
Journal of Molecular Catalysis A-chemical, 2009Co-Authors: Arne Dinse, Sonia Khennache, Benjamin Frank, Christian Hess, Rita Herbert, Sabine Wrabetz, Robert Schlogl, Reinhard SchomackerAbstract:Silica (SBA-15) supported vanadium oxide was used for a kinetic study of the oxidative dehydrogenation of propane in a fixed bed reactor. Prior to this study, spectroscopic characterization using a variety of techniques such as FTIR spectroscopy, Raman spectroscopy, DR UV-Vis spectroscopy and X-ray Phototelectron Spectroscopy revealed the absence of bulk vanadia and a high dispersion of active surface sites for the investigated catalyst. The kinetic data evaluation was based on a formal kinetic approach. Calorimetric measurements were used to determine the heat of adsorption of propane on the catalyst. The data indicate that the primary combustion of propane is negligible. Reaction orders of one for the propane dehydrogenation and Propene combustion indicate the participation of these species in the respective rate determining step. The zero reaction order determined for the catalyst reoxidation reveals a participation of lattice oxygen in this reaction step. Higher activation energies of propane dehydrogenation as compared to the Propene combustion indicate the participation of the weaker allylic C-H bond of Propene in the rate determining step of the Propene combustion. This results in higher Propene selectivites at elevated temperatures. Kinetic parameters, including apparent and real activation energies and the equilibrium constant of the propane adsorption allowed for a comparison with theoretical predictions and show a good agreement.
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oxidative dehydrogenation of propane over low loaded vanadia catalysts impact of the support material on kinetics and selectivity
Journal of Molecular Catalysis A-chemical, 2008Co-Authors: Arne Dinse, Benjamin Frank, Christian Hess, Daniela Habel, Reinhard SchomackerAbstract:The influence of the support material of low loaded (< 2 V nm ²) vanadia catalysts on selectivities, activation energies and turn over frequencies in the oxidative dehydrogenation of propane and the combustion of Propene was investigated. CeO2, TiO2, Al2O3, ZrO2 and SiO2 supported catalysts were prepared by saturation wetness impregnation in toluene. Characterization with temperature programmed reduction and Raman spectroscopy revealed a high dispersion of surface vanadia species for all investigated catalysts. The impact of heat and mass transfer limitations on the catalytic performance has been thoroughly excluded. Selectivities towards Propene as well as activation energies strongly depend on the support material. For all catalysts, Propene selectivity increases with temperature. Deconvolution of the reaction network of ODP into decoupled reactions of different reactants for at least three of the catalysts is not possible, because of a significant impact of the oxidation state of the catalyst on the reaction. Except for the CeO2 supported catalyst, the contribution of the bare support material on the activity can be neglected.
