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Reactivity of light hydrocarbons over noble-metal catalysts.
Podkolzin, S. G.
Ph.D. Thesis: University of Wisconsin, Madison, WI 53706, USA (2002)
Dissertation Abstracts International, B 63(4), 1958
(2002).
Abstract
Adsorption and reactions of light hydrocarbons (C2, C4) and
hydrogen were studied on surfaces of metal catalysts (Pt, Pt/Sn, Pt/Au, Pd and
Ni).
Catalysts were characterized with volumetric titration, calorimetry,
temperature-programmed desorption and infrared spectroscopy.
Kinetic data for acetylene and ethylene hydrogenation were collection under steady-state and transient conditions
over Pt and Pd catalysts.
Experimental data on modes of adsorption, energetics and reactivity of surface
species were consolidated with
molecular simulations.
The molecular simulations for co-adsorption of C2 species and
H2 and ethane hydrogenolysis on Pt were custom written in C++
using Monte Carlo algorithms
based on a lattice gas model, where energetics were dependent on types of sites and
proximity of neighboring surface species.
Parameters for the molecular simulations were estimated with density-functional theory
(DFT) calculations using
Dacapo
plane wave
code. DFT results were also used to elucidate details of molecular
reactions in acetylene hydrogenation over Pt and Pd catalysts and to propose an
explanation for observed experimental kinetic differences between these two
metals.
Address:
Department of Chemical Engineering, University of Wisconsin, Madison, WI
53706, USA.
Publisher:
Dissertation Abstracts International, Order No. DA3049354. (2002),
345 pages, CAN 138:127644, AN 2003:38654
View Presentation: 22 slides
Compromise between model accuracy and complexity:
 | Complexity of the reaction mechanism should be
justifiable. |
| Mechanism should not attempt to capture every reaction
detail, but only those that are essential for modeling experimental data. |
Combination of Density Functional Theory (DFT)
calculations with Monte Carlo simulations was used for modeling reaction
kinetics for C2 hydrocarbons on Pt and Pd surfaces.
Surface site approximations used in Monte Carlo simulations:
- competitive adsorption between C2Hx
and H;
- all C2Hx
hydrocarbons occupy roughly the same amount of surface space, regardless of
the number of surface bonds;
- pair-wise repulsive interactions
were defined and estimated with DFT:
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| Top view of multiple C2 surface species and atomic
hydrogen on Pt(111) |
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DFT estimates of pair-wise repulsive interaction between C2
hydrocarbon surface species and atomic hydrogen on Pt(111) |
Main stable stable species formed on adsorption of C2
hydrocarbons on metal surfaces
mouse over the picture changes the view side →
top.
Stable C2 surface species were characterized with infrared spectroscopy and
calorimetric measurements (click on a graph to enlarge)
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FTIR spectra of surface species formed on ethylene adsorption on
supported Pt catalysts (click on the graph to enlarge).
When ethylene is introduced at 173 K, only molecularly adsorbed (di-s-bonded
and p-bonded) species are formed. When the surface is heated
to 298 K or when ethylene is introduced at this higher temperature,
ethylidyne becomes the main surface species. Ethylidyne is formed by
ethylene dehydrogenation on the surface: CH2-CH2
(ads) → C-CH3 (ads) + H (ads). (View
animation of ethylene transformation to ethylidyne). |
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Differential heats of ethylene adsorption on supported Pt catalysts as
a function of surface coverage.
On ethylene adsorption at 173 K, di-s-bonded
and p-bonded species are formed with an average heat of about 120 kJ/mol.
The heat of adsorption declines quickly with coverage because of
non-equilibration with the support: ethylene adsorbs both on the metal and
on the support even at low coverage. At a higher temperature of 203
K, a better equilibration is observed, and platinum sites are mostly
titrated first (saturation coverage of about 45 mmol/g)
prior to ethylene adsorption on the support. At a still higher
temperature of 298 K, ethylene dehydrogenates on the surface with the
formation of mostly ethylidyne species, which is a more exothermic
reaction than the formation of molecularly adsorbed di-s-bonded
and p-bonded species. At a higher coverage at 298 K, ethylene disproportionates: ethylene reacts with surface hydrogen produced on
ethylidyne formation with the evolution of gas-phase ethane. |
Reactive intermediates in ethane hydrogenolysis and
corresponding transition states
mouse over changes the stable species to transition states C2HX → C2HX‡
with side and top views
Ethyl (CH2-CH3)
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Ethylidene (CH-CH3)
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Animation of DFT calculations and Monte Carlo simulations
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Surface sites on (111) surface plane: atop, bridge and three-fold.
Hexagonal lattice model slab. A small unit cell (2x2, 2x3 or 3x3) with
periodic boundary conditions was used in DFT calculations, and a larger
(42x42) periodic unit cell was used in Monte Carlo simulations with a
lattice gas model.
More
information on modeling CO adsorption.
More information on modeling ethylidyne (C-CH3) and hydrogen
co-adsorption.
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Monte Carlo trial moves in modeling co-adsorption of ethane and hydrogen
with the formation of ethylidyne (C-CH3) and atomic hydrogen
surface species: 1) ethane dehydrogenation into ethylidyne and hydrogen,
2) adsorption of the formed species, 3) hydrogen dissociation, 4)
adsorption of atomic hydrogen, 5) surface diffusion of ethylidyne and
hydrogen species, 6) desorption.
More information on modeling ethylidyne (C-CH3) and hydrogen
co-adsorption.
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Monte Carlo trial moves for insertion, removal and diffusion of ethylidyne
(C-CH3) and hydrogen surface species.
More information on modeling ethylidyne (C-CH3) and hydrogen
co-adsorption.
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Monte Carlo trial moves for kinetic modeling of ethane hydrogenolysis.
The pseudo steady-state background is determined by co-adsorption of
ethylidyne (C-CH3), hydrogen and di-s-bonded ethylene surface species. The
reaction rate is calculated using virtual insertion trial moves for
identified 3 transition state species: C2H5‡,
CH-CH3‡ and CH-CH2‡.
More information on modeling ethane hydrogenolysis.
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Acetylene adsorption with the formation of p-bonded
species.
More
information on the modes of acetylene adsorption on Pt(111).
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Multiple surface species formed by acetylene on the surfaces of platinum
and palladium.
More
information on the modes of acetylene adsorption on Pt(111).
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Transformation of a p-bonded
acetylene (CH-CH on an atop site) to a more stable vinylidene species
(C-CH2 on an fcc three-fold site).
More
information on the modes of acetylene adsorption on Pt(111).
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Transformation of di-s-bonded ethylene (on a
bridge site) to a more stable ethylidyne species and surface hydrogen.
More
information on ethylene adsorption on supported Pt.
More
information on ethylene adsorption on supported Pt-Au.
More
information on acetylene hydrogenation with ethylene formation on Pt(111).
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Hydrogen shuttling between hydrocarbon species on platinum and palladium
surfaces at high coverage. Initial geometry: C-CH2 and CH2-CH2,
final geometry: C-CH3 and CH-CH2.
More information on
hydrogen disproportionation between C2 species on Pt(111). |
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