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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: 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 IglesiaAbstract: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: Alexis T Bell, Enrique Iglesia, 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, 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.
Zhen Zhao - One of the best experts on this subject based on the ideXlab platform.
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mn doping induced changes in pt dispersion and ptxmny alloying extent on pt mn dmsn catalyst with enhanced propane dehydrogenation stability
Journal of Catalysis, 2020Co-Authors: Xiaoqiang Fan, Dandan Liu, Xiaoying Sun, Ying Yang, Hongyang Liu, Jiangyong Diao, Zean Xie, Lian Kong, Xia Xiao, Zhen ZhaoAbstract:Abstract The development of highly stable propane dehydrogenation catalyst is significant with the growing extraction of shale gas in the world. A highly dispersed MnOx site decorated dendritic mesoporous silica nanoparticle support was prepared by simple in-situ emulsion method. It shows exceptionally ability to disperse and stabilize Pt clusters. The strong interaction between Pt and Mn formed and the electron transfer happened from Mn to Pt, which leads to an increase in the electron density of Pt. The high propane dehydrogenation activity, even for 100 h long-term test, was obtained on moderate Mn content catalyst. According to the experimental results and DFT calculation, the highly dispersed PtMn alloyed nanoparticles play an important role in enhancing the catalytic performance for propane dehydrogenation via a good balance on propane activation and propene desorption.
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ZnO Nanoparticles Encapsulated in Nitrogen-Doped Carbon Material and Silicalite-1 Composites for Efficient Propane Dehydrogenation.
iScience, 2019Co-Authors: Dan Zhao, Yajun Wang, Guiyuan Jiang, Zhen Zhao, Shanlei Han, Yaoyuan Zhang, Li RanjiaAbstract:Summary Non-oxidative propane dehydrogenation (PDH) is an attractive reaction from both an industrial and a scientific viewpoint because it allows direct large-scale production of propene and fundamental analysis of C-H activation respectively. The main challenges are related to achieving high activity, selectivity, and on-stream stability of environment-friendly and cost-efficient catalysts without non-noble metals. Here, we describe an approach for the preparation of supported ultrasmall ZnO nanoparticles (2–4 nm, ZnO NPs) for high-temperature applications. The approach consists of encapsulation of NPs into a nitrogen-doped carbon (NC) layer in situ grown from zeolitic imidazolate framework-8 on a Silicalite-1 support. The NC layer was established to control the size of ZnO NPs and to hinder their loss to a large extent at high temperatures. The designed catalysts exhibited high activity, selectivity, and on-stream stability in PDH. Propene selectivity of about 90% at 44.4% propane conversion was achieved at 600°C after nearly 6 h on stream.
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ZnO Nanoparticles Encapsulated in Nitrogen-Doped Carbon Material and Silicalite-1 Composites for Efficient Propane Dehydrogenation
Elsevier, 2019Co-Authors: Dan Zhao, Yajun Wang, Guiyuan Jiang, Shanlei Han, Yaoyuan Zhang, Ke Guo, Zhen ZhaoAbstract:Summary: Non-oxidative propane dehydrogenation (PDH) is an attractive reaction from both an industrial and a scientific viewpoint because it allows direct large-scale production of propene and fundamental analysis of C-H activation respectively. The main challenges are related to achieving high activity, selectivity, and on-stream stability of environment-friendly and cost-efficient catalysts without non-noble metals. Here, we describe an approach for the preparation of supported ultrasmall ZnO nanoparticles (2–4 nm, ZnO NPs) for high-temperature applications. The approach consists of encapsulation of NPs into a nitrogen-doped carbon (NC) layer in situ grown from zeolitic imidazolate framework-8 on a Silicalite-1 support. The NC layer was established to control the size of ZnO NPs and to hinder their loss to a large extent at high temperatures. The designed catalysts exhibited high activity, selectivity, and on-stream stability in PDH. Propene selectivity of about 90% at 44.4% propane conversion was achieved at 600°C after nearly 6 h on stream. : Chemistry; Catalysis; Nanoparticles Subject Areas: Chemistry, Catalysis, Nanoparticle
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dehydrogenation of propane over ptsnal sba 15 catalysts al addition effect and coke formation analysis
Catalysis Science & Technology, 2015Co-Authors: Jianmei Li, Zhen Zhao, Aijun Duan, Guiyuan JiangAbstract:A series of PtSnAl/SBA-15 catalysts were prepared by incipient-wetness impregnation and their catalytic performance was tested for propane dehydrogenation. The catalysts were characterized by XRF, XRD, BET, TEM, UV-vis DRS, NH3-TPD, O2-TPO, 27Al MAS-NMR, XPS and in situ Raman analyses. The addition of aluminum enhances the interaction of the Sn support and consequently stabilizes the oxidation state of Sn during the propane dehydrogenation reaction. The acid centers formed by aluminum addition show close contact with metal centers (Pt), which favors the synergistic effect of the bifunctional active centers. High catalytic performance over PtSnAl0.2/SBA-15 was obtained, and one-pass propane conversion and propene selectivity are 55.9% and 98.5%, respectively. Moreover, the in situ Raman results indicated the faster coke formation rate of PtSnAl0.4/SBA-15 than that of PtSnAl0.2/SBA-15, which may be accelerated by strong acid sites by excess aluminum addition.
Igor V Koptyug - One of the best experts on this subject based on the ideXlab platform.
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selective single site pd in hydrogenation catalyst for production of enhanced magnetic resonance signals using parahydrogen
Chemistry: A European Journal, 2018Co-Authors: Dudari B Burueva, Kirill V Kovtunov, Andrey V Bukhtiyarov, Danila A Barskiy, I P Prosvirin, Igor S Mashkovsky, G N Baeva, Valerii I Bukhtiyarov, Aleksandr Yu Stakheev, Igor V KoptyugAbstract:Pd-In/Al2 O3 single-site catalyst was able to show high selectivity (up to 98 %) in the gas phase semihydrogenation of Propyne. Formation of intermetallic Pd-In compound was studied by XPS during reduction of the catalyst. FTIR-CO spectroscopy confirmed single-site nature of the intermetallic Pd-In phase reduced at high temperature. Utilization of Pd-In/Al2 O3 in semihydrogenation of Propyne with parahydrogen allowed to produce ≈3400-fold NMR signal enhancement for reaction product propene (polarization=9.3 %), demonstrating the large contribution of pairwise hydrogen addition route. Significant signal enhancement as well as the high catalytic activity of the Pd-In catalyst allowed to acquire 1 H MR images of flowing hyperpolarized propene gas selectively for protons in CH, CH2 and CH3 groups. This observation is unique and can be easily transferred to the development of a useful MRI technique for an in situ investigation of selective semihydrogenation in catalytic reactors.
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Parahydrogen-Induced Polarization Study of the Silica-Supported Vanadium Oxo Organometallic Catalyst
2018Co-Authors: Vladimir V. Zhivonitko, Ivan V. Skovpin, Kai C. Szeto, Mostafa Taoufik, Igor V KoptyugAbstract:Parahydrogen can be used in catalytic hydrogenations to achieve substantial enhancement of NMR signals of the reaction products and in some cases of the reaction reagents as well. The corresponding nuclear spin hyperpolarization technique, known as parahydrogen-induced polarization (PHIP), has been applied to boost the sensitivity of NMR spectroscopy and magnetic resonance imaging by several orders of magnitude. The catalyst properties are of paramount importance for PHIP because the addition of parahydrogen to a substrate must be pairwise. This requirement significantly narrows down the range of the applicable catalysts. Herein, we study an efficient silica-supported vanadium oxo organometallic complex (VCAT) in hydrogenation and dehydrogenation reactions in terms of efficient PHIP production. This is the first example of group 5 catalyst used to produce PHIP. Hydrogenations of propene and Propyne with parahydrogen over VCAT demonstrated production of hyperpolarized propane and propene, respectively. The achieved NMR signal enhancements were 200–300-fold in the case of propane and 1300-fold in the case of propene. Propane dehydrogenation in the presence of parahydrogen produced no hyperpolarized propane, but instead the hyperpolarized side-product 1-butene was detected. Test experiments of other group 5 (Ta) and group 4 (Zr) catalysts showed a much lower efficiency in PHIP as compared to that of VCAT. The results prove the general conclusion that vanadium-based catalysts and other group 4 and group 5 catalysts can be used to produce PHIP. The hydrogenation/dehydrogenation processes, however, are accompanied by side reactions leading, for example, to C4, C2, and C1 side products. Some of the side products like 1-butene and 2-butene were shown to appear hyperpolarized, demonstrating that the reaction mechanism includes pairwise parahydrogen addition in these cases as well
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 IglesiaAbstract: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: Alexis T Bell, Enrique Iglesia, 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, 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.
