Theoretical and kinetic assessment of the mechanism of ethane hydrogenolysis on metal surfaces saturated with chemisorbed hydrogen

Flaherty, David W.; Hibbitts, David; Gürbüz, Elif I.; Iglesia, Enrique

doi:10.1016/j.jcat.2013.11.026

Cited by 63 publications

(221 citation statements)

References 69 publications

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“…Specifically, rates increased with increasing H 2 pressure for H 2 /C n H 2n ratios below 40, reached a maximum at 60−65 H 2 /C n H 2n , and then decreased at higher H 2 pressures with a functional dependence described by r ≈ (H 2 ) −λ (where λ > 0). These trends are similar to those previously reported for the hydrogenolysis of ethane, 17,21,23,24,29,30 n-alkanes, 16,29−31 and isoalkanes. 18,30 These λ values differ among cyclohexane reactants (1.5 < λ < 3.0, Table 2; uncertainties of ±0.2−0.5) but become nearly constant at the highest H 2 /C n H 2n ratios (250−275), indicating that λ depends on the number and structure of the alkyl substituents in the cyclohexane ring (methyl, isopropyl, and npropyl).…”

Section: Introductionsupporting

confidence: 91%

“…These represent a sequence of hypothetical steps chosen for convenience because their thermodynamic properties are known 47 or can be determined from theory or experiment. 17 For the stoichiometric reaction represented by eq 12, these steps include the (i) dehydrogenation of a gaseous cycloalkane to form a gaseous form of the C n H 2n−y intermediate (ΔG D ); (ii) desorption of γH* atoms to form the unoccupied sites required to bind C n H 2n−y (ΔG H ); (iii) adsorption of the gaseous C n H 2n−y species onto these sites (ΔG A ); and (iv) formation of the C−C cleavage transition state from the C n H 2n−y (γ*) species (ΔG R ). The resulting ΔG ⧧ values on H*-covered surfaces become…”

Section: C−c Bond Cleavage Transition States and Activation Enthalpiementioning

confidence: 99%

“…Figure 7 compares measured S ⧧ values and those determined from statistical mechanics estimates for cyclohexanes (MCH, n-PCH), isoalkanes (isobutane, 2-methylpentane), 18 and nalkanes (C 2 −C 8 ). 16,17 The measured S ⧧ values are calculated from ΔS ⧧ values (Table 3) using eq 16, with λ values from Table 3 and the restriction that y ≥ γ, which ensures that transition-state structures are consistent with measured rate data and the principles of bond order conservation. S ⧧ values for MCH and n-PCH are estimated from statistical mechanics with the assumptions that they possess no rotational entropy (i.e., S ⧧ 1D,rot = 0) and that S ⧧ 2D,trans , S ⧧ vib , and S ⧧ conf are identical for all C−C rupture transition states for a given alkyl cyclohexane reactant.…”

Section: Scheme 2 Thermochemical Cycle Representing Free-energy Chanmentioning

confidence: 99%

“…A combined experimental and theoretical study showed that the 1 C− 1 C bond in ethane cleaves on H*-covered Ir surfaces via a tetra-σ-bound α,β-coordinated intermediate (*CHCH*) with y and γ values of 4 and 2, respectively (i.e., λ = 3). 17 The value of λ for cleaving 2 C− 2 C and 2 C− 1 C bonds in n-alkanes (C 3 −C 10 ) is also equal to 3, 16 suggesting that the transition states for the hydrogenolysis of n-alkanes also resemble tetra-σ-bound α,β-coordinated surface intermediates. 3 C atoms, however, can lose only their single H atom and therefore can form only one C−M bond.…”

Section: Introductionmentioning

confidence: 99%

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Catalytic Ring Opening of Cycloalkanes on Ir Clusters: Alkyl Substitution Effects on the Structure and Stability of C–C Bond Cleavage Transition States

2015

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Rates and locations of C−C cleavage during the hydrogenolysis of alkyl-cyclohexanes determine the isomeric products of ring opening and the yield losses from dealkylation. Kinetically relevant transition states for C−C rupture form by sequential quasi-equilibrated dehydrogenation steps that break C−H bonds, form C−metal bonds, and desorb chemisorbed H atoms (H*) from H*-covered surfaces. Activation enthalpies (ΔH ⧧ ), entropies (ΔS ⧧ ), and the number of H 2 (g) formed with transition states are larger for 3 C− x C rupture than for 2 C− 2 C or 2 C− 1 C cleavage for all cycloalkane reactants and Ir cluster sizes. 3 C− x C rupture transition states bind to surfaces through three or more C atoms, whereas those for less-substituted 2 C− 2 C bonds cleave via α,β species bound by two C atoms. 3 C− x C rupture involves larger ΔH ⧧ than 2 C− 2 C and 2 C− 1 C because the former requires that more C−H bonds cleave and H* desorb than for the latter two. These endothermic steps are partially compensated by C−metal bond formation, whereas the formation of additional H 2 (g) gives larger ΔS ⧧ . C−C rupture transition states for cycloalkanes have less entropy than those for C−C bonds in acyclic alkanes of similar size because C 6 rings decrease the rotational and conformational freedom. ΔH ⧧ values for all C−C bonds in a given reactant decrease with increasing Ir cluster size because the coordination of exposed metal atoms influences the stabilities of the H* atoms that desorb more than those of the transition states. ΔH ⧧ for 3 C− x C cleavage is more sensitive to cluster size because their transition states displace more H* than those for 2 C− 2 C or 2 C− 1 C bonds. These data and their mechanistic interpretation provide guidance for how surface coordination, reaction temperatures, and H 2 pressures can be used to control ring-opening selectivities toward desirable products while minimizing yield losses. These findings are consistent with trends for the hydrogenolysis of acyclic isoalkanes and seem likely to extend to C−X bond cleavage (where X = O, S, and N atoms) reactions during hydrotreating processes.

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Section: Introductionsupporting

confidence: 91%

Section: C−c Bond Cleavage Transition States and Activation Enthalpiementioning

confidence: 99%

Section: Scheme 2 Thermochemical Cycle Representing Free-energy Chanmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Catalytic Ring Opening of Cycloalkanes on Ir Clusters: Alkyl Substitution Effects on the Structure and Stability of C–C Bond Cleavage Transition States

2015

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“…Reactions that take place with cleavage of a C-C bond are structure-sensitive [3]. By studying the hydrogenolysis processes it is possible to discover new reasons for the effects of nanoparticle size, impurities, support, and the composition of bimetallic nanoparticles on their activity and also special features of the mechanisms of such processes [4]. As a rule, the effect of particle size on their activity in hydrogenolysis has been studied for the model hydrogenolysis of ethylene [3].…”

mentioning

confidence: 99%

Effect of the Size of Iron Nanoparticles on the Catalytic Activity and Selectivity of Fe/Cnt Nanocomposites in Hydrogenolysis of Ethylene

2015

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It was established that increase in the size of the iron nanoparticles in Fe/CNT nanocomposites from 4 to 8 nm leads to increase of their activity and selectivity with respect to methane in the hydrogenolysis of ethylene. The activity and selectivity of the nanocomposites are significantly higher than those of macrocrystalline iron, and this may be due to differences in the thermal conductivity of the bulk iron and the carbon nanotubes.Hydrogenolysis reactions, which take place with cleavage of a C-C bond, lead to decrease in the chain length of aliphatic hydrocarbons and also to ring cleavage in cyclic hydrocarbons [1]. Such reactions are side processes and are undesirable in the reforming and isomerization of hydrocarbons [2,3]. Reactions that take place with cleavage of a C-C bond are structure-sensitive [3]. By studying the hydrogenolysis processes it is possible to discover new reasons for the effects of nanoparticle size, impurities, support, and the composition of bimetallic nanoparticles on their activity and also special features of the mechanisms of such processes [4]. As a rule, the effect of particle size on their activity in hydrogenolysis has been studied for the model hydrogenolysis of ethylene [3]. The hydrogenolysis of ethylene is of interest on account of the possibility of determining the effect of the size of the nanoparticles both on the activity of materials that contain such nanoparticles and on their selectivity in the process.Carbon nanotubes (CNTs) have relatively high chemical, thermal, and electrochemical stability, and this helps to extend the working life of catalysts that contain them. The accessibility of the surface and the possibility of controlling the porosity and the nature of the surface functional groups make carbon nanotubes more promising than activated carbon as catalyst material. Despite the lower specific surface area of carbon nanotubes compared with activated carbon, nanocomposites with deposited metal nanoparticles, including iron nanoparticles, on CNTs usually exhibit high activity in heterogeneous catalytic processes and particularly in hydrogenation and hydrogenolysis [5].Iron-containing catalysts are widely used in industry and particularly in the Fischer-Tropsch synthesis and in the synthesis of ammonia. At the present time catalysts with deposited nanoparticles of iron are being actively investigated in hydrogenation processes [6]. Study of the dependence of the effect of the size of the iron nanoparticles on the catalytic activity and selectivity of nanocomposites containing such nanoparticles are of both theoretical and practical interest in view of the fact that the establishment of such relationships may lead to the creation of novel catalytic systems with controllable activity and selectivity [7,8]. The literature does not contain any systematic investigations of hydrogenolysis reactions at iron-containing catalysts; this has led to interest in study of such systems and may lead to the discovery of new effects characteristic of iron but not of noble metals...

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Supra‐monolayer coverages on small metal clusters and their effects on H₂ chemisorption particle size estimates

Almithn

Hibbitts

2018

AIChE Journal

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H 2 chemisorption measurements are used to estimate the size of supported metal particles, often using a hydrogen-tosurface-metal stoichiometry of unity. This technique is most useful for small particles whose sizes are difficult to estimate through electron microscopy or X-ray diffraction. Undercoordinated metal atoms at the edges and corners of particles, however, make up large fractions of small metal clusters, and can accommodate multiple hydrogen atoms leading to coverages which exceed 1 ML (supra-monolayer). Density functional theory was used to calculate hydrogen adsorption energies on Pt and Ir particles (38-586 atoms, 0.8-2.4 nm) at high coverages (3.63 ML). Calculated differential binding energies confirm that Pt and Ir (111) single-crystal surfaces saturate at 1 ML; however, Pt and Ir clusters saturate at supra-monolayer coverages as large as 2.9 ML. Correlations between particle size and saturation coverage are provided that improve particle size estimates from H 2 chemisorption for Pt-group metals.

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Theoretical and kinetic assessment of the mechanism of ethane hydrogenolysis on metal surfaces saturated with chemisorbed hydrogen

Cited by 63 publications

References 69 publications

Catalytic Ring Opening of Cycloalkanes on Ir Clusters: Alkyl Substitution Effects on the Structure and Stability of C–C Bond Cleavage Transition States

Catalytic Ring Opening of Cycloalkanes on Ir Clusters: Alkyl Substitution Effects on the Structure and Stability of C–C Bond Cleavage Transition States

Effect of the Size of Iron Nanoparticles on the Catalytic Activity and Selectivity of Fe/Cnt Nanocomposites in Hydrogenolysis of Ethylene

Supra‐monolayer coverages on small metal clusters and their effects on H₂ chemisorption particle size estimates

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Theoretical and kinetic assessment of the mechanism of ethane hydrogenolysis on metal surfaces saturated with chemisorbed hydrogen

Cited by 63 publications

References 69 publications

Catalytic Ring Opening of Cycloalkanes on Ir Clusters: Alkyl Substitution Effects on the Structure and Stability of C–C Bond Cleavage Transition States

Catalytic Ring Opening of Cycloalkanes on Ir Clusters: Alkyl Substitution Effects on the Structure and Stability of C–C Bond Cleavage Transition States

Effect of the Size of Iron Nanoparticles on the Catalytic Activity and Selectivity of Fe/Cnt Nanocomposites in Hydrogenolysis of Ethylene

Supra‐monolayer coverages on small metal clusters and their effects on H2 chemisorption particle size estimates

Contact Info

Product

Resources

About

Supra‐monolayer coverages on small metal clusters and their effects on H₂ chemisorption particle size estimates