Deuterium tracing studies and microkinetic analysis of ethylene hydrogenation over platinum

Goddard, Scott A.; Cortright, Randy D.; Dumesic, James A.

doi:10.1016/0021-9517(92)90148-b

Cited by 43 publications

(23 citation statements)

References 19 publications

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“…The chemisorption of specific reactants and of probe molecules are also used to titrateand thus to count and characterizebinding sites at catalytic surfaces. , Chemisorbed H atoms (H*), formed via H 2 dissociation, are attractive titrants for metal catalysts because their binding is relatively strong but yet reversible and give hydrogen-to-surface-metal (H:M surf ) stoichiometries that depend on surface structure and cluster size to a lesser extent than other titrants (e.g., CO*, O*). Dihydrogen is also a ubiquitous reactant in the hydrogenation or hydrogenolysis of many substrates, ,,− such as alkanes, − alkenes, − CO, − arenes, organosulfur compounds , and NO x . , Accurate thermodynamic data for H 2 chemisorption at temperatures and coverages relevant to catalysis are essential to probe the nature of catalytic surfaces, the relevance and accuracy of thermodynamic adsorption parameters derived from mechanistic interpretations of kinetic data for relevant reactions, and the adequacy of Langmuirian models for molecular binding and surface reactions.…”

Section: Introductionmentioning

confidence: 99%

Hydrogen Chemisorption Isotherms on Platinum Particles at Catalytic Temperatures: Langmuir and Two-Dimensional Gas Models Revisited

2019

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Density functional theory (DFT) and dihydrogen chemisorption uptakes at temperatures relevant to catalysis are used to determine and interpret adsorption enthalpies and entropies over a broad range of chemisorbed hydrogen (H*) coverages (0.1 ML to saturation) on Pt nanoparticles (1.6, 3.0, and 9.1 nm mean diameters) and Pt(111) surfaces. Heats of adsorption decrease by 30 kJ mol −1 as H 2 coverage increases from nearly bare (0.1 ML) to saturated (∼1 ML) surfaces, because of the preferential saturation of lowcoordination surface atoms by H* at low coverages. Such surface nonuniformity also leads to stronger binding on small Pt particles at all coverages (e.g., 47 kJ mol −1 on 1.6 nm, 40 kJ mol −1 on 3.0 nm, and 37 kJ mol −1 on 9.1 nm particles all at 0.5 ML), because their surfaces expose a larger fraction of low-coordination atoms. As H* approaches monolayer coverages, H*−H* repulsion leads to a sharp decrease in binding energies on all Pt particles. Measured H* entropies decrease with increasing H* coverage and decreasing particle size, but their values (35−65 J mol −1 K −1 ) are much larger than for immobile H* species at all coverages and particle sizes. Two-dimensional gas models in which mobile H* adsorbates move rapidly across uniform (ideal), excluded-area, or DFT-predicted nonuniform potential energy surfaces all give H* entropies ≥30 J mol −1 K −1 , qualitatively consistent with measured values. Larger H* entropies are predicted for uniform (ideal) PES while smaller H* entropies are predicted using nonuniform PES. DFT-derived barriers for H* diffusion between hcp and fcc 3-fold sites on Pt( 111) surface are about 6 kJ mol −1 , a value similar to thermal energies (kT) at temperatures of catalytic relevance, consistent with fast diffusion in the time scale of adsorption−desorption events and isotherm measurements. Mobile adsorbate entropy models are therefore essential for accurate estimates of adsorbate coverages, particularly because vibrational frequencies analyzed by harmonic oscillator approximations lead to large underpredictions of entropies, resulting in DFT-predicted adsorption free energies that are too large to result in high coverageseven at conditions where H* is known to cover the surface. The nonuniformity of Pt surfaces, repulsion among coadsorbates, and high adsorbate mobility starkly contrast with the requirements for Langmuirian descriptions of binding and reactions at surfaces, despite the ubiquitous use and success in practice of the resulting equations in describing adsorption isotherms and reaction rates on surfaces. Such fortuitous agreement is a likely consequence of the versatility of their functional form, taken together with the limited range in pressure and temperature in adsorption and kinetic measurements. The adsorption and kinetic constants derived from such data, however, would differ significantly from theoretical estimates that rigorously account for surface coordination, adsorbate mobility, and coadsorbate repulsion.

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Section: Introductionmentioning

confidence: 99%

Hydrogen Chemisorption Isotherms on Platinum Particles at Catalytic Temperatures: Langmuir and Two-Dimensional Gas Models Revisited

2019

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“…Molecular hydrogen (H 2 ) is a common reagent in metal‐catalyzed reactions, such as the hydrogenation of unsaturated C = C bonds and the reduction of CO, NO, or N 2 ; it is also used to cleave C–C bonds in alkane hydrogenolysis, C–O bonds in hydrodeoxygenation, and C–S bonds in hydrodesulfurization . H 2 readily dissociates on most noble metal catalysts, including Pt and Ir, at ambient or near‐ambient conditions.…”

Section: Introductionmentioning

confidence: 99%

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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“…12−17 These distributions of alkane isotopologues have in general been accounted for by multiple olefin−alkyl interconversions within the Horiuti− Polanyi mechanism, but deviations from what can be justified that way have been reported and explained via the recollection of additional mechanistic steps, which have included the disproportionation of adsorbed olefins and/or alkyl moieties, the cross reaction between adsorbed olefins and alkyls or between either of those species and molecular deuterium, the initial dehydrogenation of adsorbed olefins to form vinyl surface intermediates, and the adsorption and reaction of hydrogen in different types of competitive and noncompetitive adsorption sites. 4,15,18,19 All of these mechanistic possibilities were debated in the early literature but have been almost forgotten because of the lack of compelling evidence to make a clear case for their need to explain the experimental isotopologue distributions.Particularly relevant to our discussion is the proposal by Bond of a "direct addition" mechanism (graphical abstract, bottom pathway), which he introduced to justify the excess in C n H 2n D 2 production sometimes seen in the C n H 2n + D 2 conversions. 4,13,20 To specifically probe this step, we have designed an experiment based on the use of high-flux effusive molecular beams where olefin hydrogenation and H−D isotope exchange reactions are catalyzed by a platinum surface under…”

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confidence: 98%

“…Much of the knowledge available on this mechanism has been generated by experiments using isotope labeling. It has been well established that when deuterium is used instead of hydrogen in the reaction mixture, the resulting alkanes display a wide range of isotope substitutions. − These distributions of alkane isotopologues have in general been accounted for by multiple olefin–alkyl interconversions within the Horiuti–Polanyi mechanism, but deviations from what can be justified that way have been reported and explained via the recollection of additional mechanistic steps, which have included the disproportionation of adsorbed olefins and/or alkyl moieties, the cross reaction between adsorbed olefins and alkyls or between either of those species and molecular deuterium, the initial dehydrogenation of adsorbed olefins to form vinyl surface intermediates, and the adsorption and reaction of hydrogen in different types of competitive and noncompetitive adsorption sites. ,,, All of these mechanistic possibilities were debated in the early literature but have been almost forgotten because of the lack of compelling evidence to make a clear case for their need to explain the experimental isotopologue distributions.…”

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confidence: 99%

Direct Addition Mechanism during the Catalytic Hydrogenation of Olefins over Platinum Surfaces

Dong

Ebrahimi

Tillekaratne

et al. 2016

J. Phys. Chem. Lett.

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The mechanism of the hydrogenation of olefins catalyzed by metal surfaces was probed by using isotope labeling in conjunction with a high-flux effusive molecular beam setup capable of sustaining steady-state conversion under well-controlled ultrahigh vacuum (UHV). The unique conditions afforded by this instrument, namely, a single collision regime and impinging frequencies equivalent to pressures in the mTorr range, led to the clear identification of two competing pathways: a multiple H-D isotope exchange channel explained by the well-known Horiuti-Polanyi mechanism but with an unusually high probability for β-hydride elimination from the alkyl surface intermediate (versus its reductive elimination to the alkane), and a direct addition route that produces dideuterated alkanes selectively. The latter may follow an Eley-Rideal mechanism involving an adsorbate (either the olefin or the hydrogen/deuterium atoms resulting from dissociative adsorption of H2/D2) and a gas-phase molecule (the other reactant), or, alternatively, it could reflect the limited diffusion of the hydrogen atoms on the surface under catalytic conditions because of site blocking by the islands of strongly bonded carbonaceous (alkylidyne) layers present during catalysis. Regardless, our data clearly show that the distribution of alkane isotopologues obtained from the conversion of olefins with deuterium can deviate significantly from statistical expectations.

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Deuterium tracing studies and microkinetic analysis of ethylene hydrogenation over platinum

Cited by 43 publications

References 19 publications

Hydrogen Chemisorption Isotherms on Platinum Particles at Catalytic Temperatures: Langmuir and Two-Dimensional Gas Models Revisited

Hydrogen Chemisorption Isotherms on Platinum Particles at Catalytic Temperatures: Langmuir and Two-Dimensional Gas Models Revisited

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

Direct Addition Mechanism during the Catalytic Hydrogenation of Olefins over Platinum Surfaces

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Deuterium tracing studies and microkinetic analysis of ethylene hydrogenation over platinum

Cited by 43 publications

References 19 publications

Hydrogen Chemisorption Isotherms on Platinum Particles at Catalytic Temperatures: Langmuir and Two-Dimensional Gas Models Revisited

Hydrogen Chemisorption Isotherms on Platinum Particles at Catalytic Temperatures: Langmuir and Two-Dimensional Gas Models Revisited

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

Direct Addition Mechanism during the Catalytic Hydrogenation of Olefins over Platinum Surfaces

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