Facile Dehydrogenation of Ethane on the IrO2(110) Surface

Article abstract of DOI:10.1021/jacs.7b13599

Realizing the efficient and selective conversion of ethane to ethylene is important for improving the utilization of hydrocarbon resources, yet remains a major challenge in catalysis. Herein, ethane dehydrogenation on the IrO₂(110) surface is investigated using temperature-programmed reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. The results show that ethane forms strongly bound σ-complexes on IrO₂(110) and that a large fraction of the complexes undergo C-H bond cleavage during TPRS at temperatures below 200 K. Continued heating causes as much as 40% of the dissociated ethane to dehydrogenate and desorb as ethylene near 350 K, with the remainder oxidizing to CO_x species. Both TPRS and DFT show that ethylene desorption is the rate-controlling step in the conversion of ethane to ethylene on IrO₂(110) during TPRS. Partial hydrogenation of the IrO₂(110) surface is found to enhance ethylene production from ethane while suppressing oxidation to CO_x species. DFT predicts that hydrogenation of reactive oxygen atoms of the IrO₂(110) surface effectively deactivates these sites as H atom acceptors, and causes ethylene desorption to become favored over further dehydrogenation and oxidation of ethane-derived species. The study reveals that IrO₂(110) exhibits an exceptional ability to promote ethane dehydrogenation to ethylene near room temperature, and provides molecular-level insights for understanding how surface properties influence selectivity toward ethylene production.

Full text of DOI:10.1021/jacs.7b13599

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Facile Dehydrogenation of Ethane on the IrO(110) Surface
Yingxue Bian, Minkyu Kim, Tao Li, Aravind Asthagiri, and Jason F. Weaver
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b13599 • Publication Date (Web): 27 Jan 2018
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Facile Dehydrogenation of Ethane on the IrO2(110) Surface
Yingxue Bian1,†, Minkyu Kim2,†, Tao Li1, Aravind Asthagiri2, Jason F. Weaver1*
1Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA
2William G. Lowrie Chemical & Biomolecular Engineering, The Ohio State University, Columbus, OH 43210, USA
† Yingxue Bian and Minkyu Kim contributed equally to this work.
*To whom correspondence should be addressed, weaver@che.ufl.edu
Tel. 352-392-0869, Fax. 352-392-9513
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Abstract
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Realizing the efficient and selective conversion of ethane to ethylene is important for improving
the utilization of hydrocarbon resources, yet remains a major challenge in catalysis. Herein,
ethane dehydrogenation on the IrO2(110) surface is investigated using temperature programmed
reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. The results
show that ethane forms strongly-bound σ-complexes on IrO2(110) and that a large fraction of the
complexes undergo C-H bond cleavage during TPRS at temperatures below 200 K. Continued
heating causes as much as 40% of the dissociated ethane to dehydrogenate and desorb as
ethylene near 350 K, with the remainder oxidizing to COx species. Both TPRS and DFT show
that ethylene desorption is the rate-controlling step in the conversion of ethane to ethylene on
IrO2(110) during TPRS. Partial hydrogenation of the IrO2(110) surface is found to enhance
ethylene production from ethane while suppressing oxidation to COx species. DFT predicts that
hydrogenation of reactive oxygen atoms of the IrO2(110) surface effectively deactivates these
sites as H-atom acceptors, and causes ethylene desorption to become favored over further
dehydrogenation and oxidation of ethane-derived species. The study reveals that IrO2(110)
exhibits an exceptional ability to promote ethane dehydrogenation to ethylene near room
temperature, and provides molecular-level insights for understanding how surface properties
influence selectivity toward ethylene production.
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Introduction
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Developing catalysts that can directly convert ethane to ethylene is gaining increasing interest
due to the availability of light alkanes from shale gas as well as the increasing demand for
ethylene. The oxidative dehydrogenation (ODH) of ethane offers advantages over non-oxidative
processes and has been widely studied.1-3 The ODH of ethane occurs in the presence of oxygen
and involves the dehydrogenation of ethane to ethylene with concurrent oxidation of the released
hydrogen to water. The latter step makes the ODH of ethane an exothermic process for which
high conversion is thermodynamically favored at low temperature. Furthermore, the presence of
oxygen in the reactant stream minimizes catalyst deactivation by coking which can be a
significant problem in non-oxidative routes for ethane dehydrogenation. Various metal oxides as
well as alkali chlorides are effective in promoting the ODH of ethane and propane, with VOx-
based catalysts generally exhibiting the most favorable performance.1-9 However, the catalysts
that have been investigated to date do not achieve sufficient activity and selectivity to be utilized
at the industrial scale.
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Initial C-H bond cleavage is widely accepted as the rate-controlling step in the ODH of
ethane, and more generally in the catalytic processing of light alkanes.1 This situation presents a
challenge in developing catalysts that can selectively dehydrogenate ethane to ethylene because
the reaction steps that follow initial C-H bond cleavage occur rapidly and can be difficult to
control, particularly in the presence of oxygen. Recently, we have reported that CH4 undergoes
highly facile C-H bond activation on the IrO2(110) surface at temperatures as low as 150 K.10
We find that methane adsorbs as a strongly-bound σ-complex on IrO2(110) and that C-H bond
cleavage occurs by a heterolytic pathway wherein the adsorbed complex transfers a H-atom to a
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lattice oxygen atom, thus affording adsorbed CH3 and OH groups. Our results further show that
the resulting methyl groups react with the IrO2(110) surface via oxidation to COx and H2O as
well as recombination with adsorbed hydrogen to regenerate CH4, with these products desorbing
at temperatures above ~400 K during temperature programmed reaction spectroscopy (TPRS)
experiments.10 Key findings are that the initial C-H bond cleavage of CH4 is highly facile and
that subsequent reaction steps control the overall chemical transformations of methane on the
IrO2(110) surface. The ability of IrO2(110) to activate alkane C-H bonds at low temperature may
provide opportunities to develop catalysts that are capable of directly and efficiently
transforming light alkanes to value-added products.
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In the present study, we investigated the dehydrogenation of ethane on the IrO2(110) surface.
We find that initial C-H bond cleavage of C2H6 occurs efficiently on IrO2(110) at low
temperature (~150 to 200 K) and that subsequent reaction produces C2H4 as well as COx species
during TPRS, with the C2H4 product desorbing between 300 and 450 K. We demonstrate that
partially hydrogenating the IrO2(110) surface to convert a fraction of the surface O-atoms to OH
groups enhances the conversion of C2H6 to C2H4 while suppressing extensive oxidation to COx
species. Our findings show that the controlled deactivation of surface O-atoms is an effective
means for promoting the selective conversion of ethane to ethylene on IrO2(110) at low
temperature.
Experimental Details
Details of the ultrahigh vacuum (UHV) analysis chamber with an isolatable ambient-pressure
reaction cell utilized in the present study have been reported previously.10 Briefly, the Ir(100)
crystal employed in this study is a circular disk (9 mm × 1 mm) that is attached to a liquid-
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nitrogen-cooled, copper sample holder by 0.015” W wires that are secured to the edge of crystal.
A type K thermocouple was spot welded to the backside of the crystal for temperature
measurements. Resistive heating, controlled using a PID controller that varies the output of a
programmable DC power supply, supports linearly ramping from 80 to 1500 K and maintaining
the sample temperature. Sample cleaning consisted of cycles of Ar+ sputtering (2000 eV, 1.5 ꢀA)
at 1000 K, followed by annealing at 1500 K for several minutes. The sample was subsequently
exposed to 5 × 10-7 Torr of O2 at 900 K for several minutes to remove surface carbon, followed
by flashing to 1500 K to remove final traces of oxygen.
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We generated an IrO2(110) film by exposing Ir(100) to 5 Torr of O2 (Airgas, 99.999%) for a
duration of 10 minutes (3 × 109 Langmuir) in the ambient-pressure reaction cell at a surface
temperature of 765 K. Our ambient-pressure reaction cell is designed to reach elevated gas
pressure while maintaining UHV in the analysis chamber.10 After preparation of the oxide film,
we lowered the surface temperature to 600 K, and then evacuated O2 from the reaction cell and
transferred the sample back to the UHV analysis chamber. We exposed the film to ~23 L O2
while cycling the surface temperature between 300 and 650 K to fill oxygen vacancies that may
be created during sample transfer from the reaction cell to the analysis chamber. This procedure
produces a high-quality IrO2(110) surface that has a stoichiometric surface termination, contains
~40 ML of oxygen atoms and is about 3.2 nm thick.10,11
The stoichiometric IrO2(110) surface consists of parallel rows of fivefold coordinated Ir
atoms and so-called bridging O atoms (see Supporting Information (SI)), each of which lacks a
bonding partner relative to the bulk and is thus coordinatively unsaturated (cus). Hereafter, we
refer to the fivefold coordinated Ir atoms as Ircus atoms and the bridging O-atoms as Obr atoms.
On the basis of the IrO2(110) unit cell, the areal density of Ircus atoms and Obr atoms is equal to
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37% of the Ir(100) surface atom density of 1.36 × 1015 cm-2. Since Ircus atoms are active
adsorption sites, we define 1 ML as equal to the density of Ircus atoms on the IrO2(110) surface.
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We studied the adsorption of C2H6 (Matheson, 99.999%) on clean and hydrogen pre-covered
IrO2(110) using TPRS. We delivered ethane to the sample from a calibrated beam doser at an
incident flux of approximately 0.0064 ML/s with the sample-to-doser distance set to about 15
mm to ensure uniform impingement of ethane across the sample surface. We prepared hydrogen
pre-covered IrO2(110) by exposing the surface to varying quantities of H2 at 90 K, followed by
heating to 380 K. We have recently reported that this procedure enhances the concentration of
HObr groups by promoting the hopping of H-atoms on Ircus sites to Obr.11 We estimate that ~0.075
to 0.15 ML of H2 adsorbs from the vacuum background during cooling of the initially clean
IrO2(110) surface, prior to a TPRS experiment. We collected TPRS spectra after ethane
exposures by positioning the sample in front of a shielded mass spectrometer at a distance of
about 5 mm and then heating at a constant rate of 1 K/s until the sample temperature reached 800
K. To ensure consistency in the composition and structure of the IrO2(110) layer, the surface was
exposed to 23.3 L of O2 supplied through a tube doser while cycling the surface temperature
between 300 and 650 K after each TPRS experiment. Initially, we monitored a wide range of
desorbing species to identify the main products that are generated from reactions of ethane on
IrO2(110), and found that the only desorbing species are C2H6, CO, CO2, C2H4, CH4 and H2O.
We quantified desorption yields using established procedures as described in the SI.
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Computational Details
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All plane wave DFT calculations were performed using the projector augmented wave
pseudopotentials12 provided in the Vienna ab initio simulation package (VASP).13,14 The Perdew-
Burke-Ernzerhof (PBE) exchange-correlation functional15 was used with a plane wave expansion
cutoff of 450 eV. Dispersion interactions are modeled using the DFT-D3 method developed by
Grimme et al.16 We find that this method provides accurate estimates of the adsorption energies
of n-alkanes on PdO(101)17 and RuO2(110)18 in comparison with TPD-derived values; however,
the DFT-D3 calculations overestimate the adsorption energy of methane on IrO2(110).10 We find
that DFT-D3 calculations using the PBE functional also overestimate the binding energies of
C2H4 and C2H6 on IrO2(110). We compare the results of DFT-PBE calculations performed with
and without dispersion corrections in the SI (Table S1), and note that the predictions from both
methods support the conclusions of this study. We employed four layers to model the IrO2(110)
film, resulting in an ~12 Å thick slab with an additional 25 Å vacuum to avoid spurious
interactions normal to the surface. The PBE bulk lattice constant of IrO2 (a = 4.54 Å and c = 3.19
Å) is used to fix the lateral dimensions of the slab. The bottom two layers are fixed, but all other
lattice atoms are allowed to relax during the calculations until the forces are less than 0.05 eV/Å.
A 2 × 4 unit cell with a corresponding 2 × 2 × 1 Monkhorst-Pack k-point mesh is used. In
the present study, we define the binding energy, ꢀꢁ, of an adsorbed C2H6 molecule on the surface
using the expression,
ꢀꢁ = ꢂꢀꢃ ꢅ + ꢀꢇꢈꢉꢊꢋ − ꢀꢃ ꢅ /ꢇꢈꢉꢊ
ꢄ
ꢆ
ꢄ ꢆ
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where ꢀꢃ ꢅ /ꢇꢈꢉꢊ is the energy of the state containing the adsorbed C2H6 molecule, ꢀꢇꢈꢉꢊ is the
ꢄ
ꢆ
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energy of the bare surface, and ꢀꢃ ꢅ is the energy of an isolated C2H6 molecule in the gas phase.
ꢄ
ꢆ
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All reported binding energies are corrected for zero-point vibrational energy. From the equation
above, a large positive value for the binding energy indicates a high stability of the adsorbed
C2H6 molecule under consideration. We evaluated the barriers for C2H6 dehydrogenation on the
IrO2(110) surface using the climbing nudged elastic band (cNEB) method.19 Our DFT
calculations were performed for a single C2H6 molecule adsorbed within the 2 × 4 surface
model of IrO2(110), and corresponds to an C2H6 coverage equal to 12.5% of the total density of
Ircus atoms and 25% of the Ircus density within one Ircus row.
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Results and Discussion
TPRS of C2H6 adsorbed on IrO2(110)
Our TPRS results show that the IrO2(110) surface is highly reactive toward ethane as more than
90% of the C2H6 adsorbed on IrO2(110) oxidizes to CO, CO2 and H2O during TPRS at low initial
C2H6 coverages (Figure 1a). The CO2 and CO products desorb in TPRS peaks centered at 525
and 550 K, while H2O desorbs over a broader feature spanning temperatures from ~400 to 750
K. We also observe a small C2H6 TPRS peak at 110 K that arises from weakly-bound,
molecularly-adsorbed C2H6, likely associated with a minority surface phase.
At high initial C2H6 coverages, a fraction of the adsorbed C2H6 dehydrogenates to produce
C2H4 in addition to undergoing extensive oxidation to CO and CO2 (Figure 1b). Ethylene
desorption accounts for about 38% of the total amount of C2H6 that reacts during TPRS at
saturation of the initial C2H6 layer. The C2H4 TPRS feature resulting from C2H6 dehydrogenation
on IrO2(110) exhibits a maximum at 350 K and a shoulder centered at ~425 K, and most of the
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C2H4 desorbs at lower temperature than the CO and CO2 products. Assuming maximum values
of the desorption pre-factors (5.6 × 1018, 1.1 × 1019 s-1), we estimate that the C2H4 peak
temperatures of 350 and 425 K correspond to C2H4 binding energies of 132 and 162 kJ/mol,
respectively. Prior studies show that maximum desorption pre-factors are appropriate for
describing the desorption of small hydrocarbons from TiO2(110) and RuO2(110) surfaces,18,20
where the pre-factors are computed using a model based on transition state theory.21 We have
performed TPRS experiments following C2H4 adsorption on IrO2(110), and find that C2H4
desorbs in a broad feature spanning temperatures from ~150 to 500 K (see SI). The breadth of
this TPRS feature likely reflects a sensitivity of the C2H4 binding energy and configuration(s) to
the local environment, and will be addressed in a future study. Because the C2H4 TPRS feature
resulting from C2H6 dehydrogenation desorbs over a similar temperature range as C2H4 adsorbed
on IrO2(110), we conclude that C2H4 production from C2H6 on IrO2(110) is a desorption-limited
process.
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a)
b)
0.004
0.003
0.002
0.001
0.000
0.004
0.003
0.002
0.001
0.000
TPRS
C2H6 + IrO2(110)
TPRS
C2H6 + IrO2(110)
[C2H6]o ~ 0.27 ML
[C2H6]o ~ 0.11 ML
H2O
H2O
CO2
C2H6
σ-complexes
CO
CO2
C2H4
CO
C2H6
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Temperature (K)
Temperature (K)
Figure 1: TPRS spectra of C2H6, C2H4, CO, CO2 and H2O obtained after adsorbing C2H6 on IrO2(110) at 90 K to
reach initial C2H6 coverages of a) 0.11 ML and b) 0.27 ML.
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A new C2H6 TPRS feature centered at 185 K emerges after the TPRS features generated by
the CO, CO2, C2H4, H2O products first saturate at a total C2H6 coverage near 0.20 ML (SI), with
this TPRS feature developing two maxima at ~150 and 175 K as its desorption yield begins to
saturate (Figure 1b). The C2H6 TPRS peak at 110 K grows only slowly as the total C2H6
coverage increases to about 0.35 ML, but a separate peak at 120 K intensifies sharply thereafter
(see Fig. S2 in the SI). The C2H6 TPRS feature at 150-185 K is consistent with the desorption of
relatively strongly-bound C2H6 σ-complexes adsorbed on the Ircus atoms of IrO2(110). Using
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Redhead analysis with a maximum value of the desorption pre-factor (5.9 × 1017 s-1), we predict
a binding energy of 65 kJ/mol for the C2H6 TPRS peak at 185 K. We also estimate a saturation
coverage of ~0.30 ML for C2H6 σ-complexes on IrO2(110), based on the amount of C2H6 that
desorbs above ~135 K plus the total amount that reacts. Our estimate agrees to within about 20%
of the saturation coverage of C2H6 σ-complexes on RuO2(110).18 Because the σ-complexes serve
as dissociation precursors (see below), our TPRS results reveal that C2H6 C-H bond cleavage
occurs readily on IrO2(110) at temperatures between ~150 and 200 K, i.e., in the same range as
desorption of the C2H6 σ-complexes. We are unaware of other materials that exhibit such high
activity toward promoting the C-H bond activation of C2H6.
We have recently shown that IrO2(110) is exceptionally active in promoting CH4 C-H bond
cleavage at temperatures as low as 150 K.10 The present results demonstrate a similarly high
reactivity of IrO2(110) toward C2H6 activation. Our prior study shows that CH4 initially adsorbs
on Ircus atoms and undergoes C-H bond cleavage by a heterolytic pathway involving H-atom
transfer to a neighboring Obr atom, producing CH3-Ircus and HObr groups. We found that the
energy barrier for CH4 bond cleavage is nearly 10 kJ/mol lower than the binding energy of the
CH4 σ-complex, resulting in near unit dissociation probability for CH4 on IrO2(110) at low
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temperature and coverage. The resulting CH3 groups are oxidized by the surface to CO, CO2 and
H2O that desorb in TPRS features that are similar to those observed in the present study for C2H6
oxidation on IrO2(110). This similarity suggests that common reaction steps control the rates of
CO, CO2 and H2O production during the oxidation of CH4 and C2H6 on IrO2(110), after initial C-
H bond cleavage. We previously reported that CH4 oxidation to CO, CO2 and H2O is favored at
low CH4 coverage, but that recombinative desorption of CH4 competes with oxidation at higher
initial CH4 coverage.10 Our current results demonstrate that C2H6 also preferentially oxidizes
during TPRS when the initial C2H6 coverage is sufficiently low. A key difference is that C2H6
dehydrogenates to C2H4 on IrO2(110) at high initial C2H6 coverage rather than recombinatively
desorbing, and generates C2H4 at relatively low temperature (~300 to 450 K).
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We show below that the coverage of HObr groups plays a decisive role in determining the
branching between C2H6 oxidation and C2H4 production. The proposed steps for C2H6 activation
and subsequent dehydrogenation on IrO2(110) are the following,
Initial C2H6 dissociation vs. desorption:
C2H6(ad) → C2H6(g)
C2H6(ad) + Obr → C2H5(ad) + HObr
C2H5(ad) + Obr → C2H4(ad) + HObr
C2H4(ad) + Obr → C2H3(ad) + HObr
C2H4(ad) → C2H4(g)
C2H5 dehydrogenation:
C2H4 dehydrogenation vs. desorption:
Ethane initially adsorbs in a molecular state C2H6(ad) and forms a σ-complex by datively
bonding with Ircus atoms, and a competition between dissociation and desorption of the C2H6(ad)
species determines the net probability of initial C-H bond cleavage. Our TPRS results show that
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dissociation of the C2H6(ad) species is strongly favored over desorption at low C2H6 coverages.
Since dissociation of the C2H6(ad) species requires an Obr atom, a decrease in the coverage of Obr
atoms via conversion to HObr groups may be mainly responsible for C2H6 dissociation reaching
saturation during TPRS beyond a critical C2H6 coverage. After initial dissociation, the resulting
C2H5(ad) species can dehydrogenate to C2H4(ad) species, and the C2H4(ad) species can either
desorb or further dehydrogenate via H-atom transfer to an Obr atom. Again, the coverage of Obr
atoms decreases with increasing C2H6 coverage because an increasing fraction of the Obr atoms is
converted to HObr groups via dehydrogenation of the C2H6-derived species. According to the
proposed reaction steps, C2H4 desorption should become favored as the Obr atom coverage
decreases.
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Product yields as a function of the C2H6 coverage
Figure 2 shows the initial and reacted TPRS yields of C2H6 σ-complexes as a function of the
initial C2H6 coverage on IrO2(110) as well as the yields of C2H6 that converts to C2H4 vs.
oxidizing to COx species. We set the total reacted yield of C2H6 equal to the sum of the C2H4
yield plus one half of the yield of CO + CO2, where the factor of one half converts the COx yield
to the amount of C2H6 that oxidizes, and we define the initial amount of C2H6 σ-complexes as
equal to the reacted C2H6 yield plus the amount of C2H6 that desorbs in the TPRS feature above
~135 K. Our results show that 90 to 100% of the strongly-bound C2H6 reacts during TPRS as the
C2H6 coverage increases to ~0.25 ML, at which point the yield of reacted C2H6 begins to plateau
toward a value of 0.20 ML and the yield of C2H6 σ-complexes that desorb concurrently
increases. The reacted C2H6 yield corresponds to about 67% of the adsorbed C2H6 complexes at
saturation. Our results demonstrate that a large quantity of C2H6 reacts on IrO2(110) during
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TPRS, and thus support the conclusion that initial C-H activation and subsequent reaction occur
on the crystalline terraces of IrO2(110).
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0.4
Yield vs C2H6 coverage
C2H6 + IrO2(110)
C2H6 σ-complex
0.3
0.2
0.1
0.0
C2H6 (reacted)
COx (x 0.5)
C2H4
0.0
0.1
0.2
0.3
0.4
0.5
C2H6 coverage (ML)
Figure 2: TPRS product yields as a function of the initial coverage of C2H6 adsorbed on IrO2(110) at 90 K,
including the initial coverage of C2H6 σ-complexes (desorbed + reacted), the reacted yield of C2H6, the C2H4
yield and the yield of ethane that oxidizes (0.5*COx).
Our results further show that C2H6 oxidation is strongly favored at low coverage, and that C2H4
production initiates at moderate coverage as the COx yield begins to saturate. The yield of
oxidized ethane increases nearly to saturation with increasing C2H6 coverage to about 0.15 ML,
and thereafter plateaus at a value of about 0.12 ML. Ethylene production first becomes evident at
a C2H6 coverage above 0.10 ML and increases toward a plateau value as the total C2H6 coverage
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rises to ~0.30 ML. The maximum C2H4 yield is equal to 0.08 ML at saturation of the C2H6 σ-
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complexes, and represents a large fraction (~38%) of the C2H6 that reacts on IrO2(110). The
evolution of the product yields with the C2H6 coverage suggests that the availability of Obr atoms
plays a decisive role in determining the reaction pathways that adsorbed C2H6 molecules can
access on IrO2(110). Notably, our current results show that the COx yield saturates at an
Obr:C2H6 ratio close to five; however, the actual minimum Obr:C2H6 ratio needed to promote
C2H6 oxidation to COx may be less than five because background H2 adsorption converts ~0.15
to 0.25 ML of the initial Obr atoms to HObr groups prior to the C2H6 TPRS experiment.
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Enhanced selectivity for C2H4 production on H-covered IrO2(110)
We find that the selectivity toward C2H4 production from C2H6 can be enhanced by pre-
hydrogenating the IrO2(110) surface. Figure 3a compares TPRS traces of the 27 and 44 amu
fragments obtained after adsorbing ~0.14 ML of C2H6 on clean IrO2(110) vs. an IrO2(110)
surface with an estimated H-atom pre-coverage of 0.32 ML. The 27 amu TPRS trace exhibits
well-separated features arising from C2H6 and C2H4, and the 44 amu feature alone is sufficient
for representing the change in COx production because surface hydrogenation causes similar
changes in the CO and CO2 TPRS features.
Our results show that pre-hydrogenating the surface to a moderate extent (<~0.4 ML) causes
the CO2 TPRS peak to diminish, while the C2H4 TPRS feature intensifies and skews toward
lower temperature, with the maximum shifting from 445 to 350 K. Pre-hydrogenation also
causes a C2H6 TPRS peak at ~175 K to gain intensity, whereas this peak is negligible after
generating a moderate C2H6 coverage on clean IrO2(110) (SI). These changes show that pre-
hydrogenating IrO2(110) suppresses C2H6 oxidation to COx species but enhances C2H4
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production when the H-atom pre-coverage is moderate. The concurrent increase in the C2H6
TPRS peak at 175 K correlates with the decrease in COx TPRS yields, and thus demonstrates that
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surface pre-hydrogenation causes a fraction of the adsorbed C2H6
than oxidize. This behavior provides further evidence that adsorbed C2H6
precursors to reaction and that dissociation involves H-atom transfer to Obr atoms.
σ
-complexes to desorb rather
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σ-complexes serve as
a)
b)
Yield vs. H2 pre-exposure
C2H6 + IrO2(110)-H
[C2H6]o ~ 0.13 ML
TPRS (27 + 44 amu)
C2H6 + IrO2(110)-H
0.20
0.15
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clean
H-precovered
[C2H6]o ~ 0.14 ML
reacted
CO2 (x 0.5)
COx (x 0.5)
C2H4
C2H6
-complex
σ
C2H4
0.0
0.2
0.4
0.6
0.8
1.0
100
200
300
400
500
600
700
Initial H coverage (ML)
Temperature (K)
Figure 3: a) TPRS traces of m/z = 27 and 44 obtained after adsorbing ~0.14 ML of C2H6 at 90 K on a (nominally)
clean IrO2(110) surface (blue) and an IrO2(110) surface with a hydrogen pre-coverage of 0.32 ML (red). The 27
and 44 amu traces are represented by thick vs. thin lines. b) Total reacted C2H6 yield, oxidized C2H6 yield
(0.5*COx) and C2H4 yield obtained as a function of the hydrogen pre-coverage during TPRS for a C2H6 coverage
of ~0.13 ML.
Figure 3b shows how the total TPRS yield of reacted C2H6 as well as the yields of the C2H4
and COx reaction products evolve as a function of the initial H-atom coverage on IrO2(110), for
an initial C2H6 coverage of 0.13
± 0.015 ML. We estimate that the nominally clean IrO2(110)
surface was covered by ~0.15 ML of H-atoms prior to ethane adsorption. Our results show that
the total yield of reacted C2H6 decreases monotonically with increasing H-atom pre-coverage,
indicating that initially converting Obr atoms to HObr groups suppresses C2H6 activation on
IrO2(110). The COx yield decreases sharply and continuously from a value of 0.11 to 0.01 ML as
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the H-atom coverage increases to about 1 ML. In contrast, however, the C2H4 yield increases
from ~0.03 to 0.045 ML with increasing H-atom coverage to ~0.32 ML and thereafter decreases,
reaching a final value of 0.005 ML at saturation of the initial H-atom layer. The C2H4 yield
begins to fall below its value on the (nominally) clean IrO2(110) surface when the initial H-atom
coverage starts to exceed 0.5 ML. These changes represent a nearly threefold increase in the
selectivity for C2H4 production, as measured by the ratio of ethane that converts to ethylene vs
COx species, i.e., selectivity equals 2[C2H4]/[COx]. The selectivity increases to 0.75 at moderate
C2H6 coverage (~0.13 ML) and hydrogen pre-coverage, and is thus slightly higher than the
selectivity of 0.65 achieved at high C2H6 coverage on nominally clean IrO2(110). It may be
possible to achieve even higher selectivity for C2H4 production by performing experiments at
high C2H6 coverage on partially-hydrogenated IrO2(110). The evolution of product yields with
increasing H-coverage demonstrates that Obr atoms are needed to promote the initial C-H
activation of C2H6 on IrO2(110) as well as further dehydrogenation and that the controlled
deactivation of Obr atoms by hydrogenation provides a means to enhance reaction selectivity to
favor the conversion of ethane to ethylene.
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Pathways for C2H6 dehydrogenation on IrO2(110)
We examined several possible C2H6 adsorption configurations (see Fig. S4 in SI) and predict that
C2H6 forms a strongly-bound σ-complex on IrO2(110) by adopting a flat-lying geometry along
the Ircus row in which each CH3 group forms a H-Ircus dative bond (a 2η1 configuration) and the
C2H6 molecule effectively occupies two Ircus sites. This staggered 2η1 configuration is similar to
that predicted by Pham et al. but they report an eclipsed C2H6 2η1 configuration,22 which we find
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to be less stable than the staggered configuration by ~9 kJ/mol (Fig. S4). We have previously
reported that C2H6 complexes on PdO(101) and RuO2(110) also preferentially adopt the 2η1
configuration.18,23,24
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Figure 4a shows the energy diagram computed using DFT-D3 for the sequential
dehydrogenation of C2H6 to C2H4 on IrO2(110), followed by either C2H4 desorption (red) or
C2H4 dehydrogenation to adsorbed C2H3. DFT-D3 predicts that the 2η1 C2H6 complex achieves a
binding energy of 107 kJ/mol on clean IrO2(110) and that the barrier for C-H bond cleavage via
H-transfer to an Obr atom is only 38 kJ/mol. According to the calculations C2H6 dehydrogenation
to produce C2H5-Ircus and HObr species is exothermic by about 97 kJ/mol, and the barrier for
reaction is significantly lower than the binding energy of the adsorbed C2H6 complex (38 vs. 107
kJ/mol). We find that DFT-PBE calculations without dispersion corrections underestimate the
C2H6 binding energy on IrO2(110), but still predict that the C2H6 dissociation barrier is lower
than the desorption barrier (Table S1). Our calculations thus predict that C2H6 C-H bond
cleavage is strongly favored over molecular desorption on clean IrO2(110) such that all adsorbed
C2H6 molecules will dissociate at low temperature, provided that Obr atoms are available for
reaction. This prediction agrees well with our experimental finding that C2H6 dissociates on
IrO2(110) with near unit probability at low C2H6 coverages (Figure 2).
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a)
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C2H6 + IrO2(110)
0
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-50
107
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C2H4(g)
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-100
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-200
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C2H6(ad)
C2H5(ad)
C2H4(ad)
C2H3(ad)
C2H4(g)
b)
C2H6 + IrO2(110)–2HObr
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-150
-200
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C2H6(ad
C2H3(ad)
-250
-300
C2H5(ad)
C2H4(ad)
Figure 4: Energy pathways for the dehydrogenation of C2H6 adsorbed on IrO2(110) as computed using DFT-D3
for surfaces initially containing (a) zero and (b) two HObr groups. The final reaction step compares the energy
changes for C2H4 desorption (red) vs. dehydrogenation to a C2H3(ad) species (black). A comparison of the
energetics for these pathways with and without D3 can be found in Table S1 in the SI.
We find that the adsorbed C2H5 group on IrO2(110) can also dehydrogenate by a low energy
pathway wherein the CH3 group transfers a H-atom to an Obr atom, resulting in an adsorbed C2H4
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species and a HObr group located in the opposing row from the initial HObr group (Figure 4a).
DFT-D3 predicts an energy barrier of 52 kJ/mol for this reaction and an exothermicity of 75
kJ/mol. The barrier for C2H5 dehydrogenation is relatively low because the CH3 group maintains
a H-Ircus dative interaction that weakens one of the C-H bonds. The C2H4 product adopts a
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bidentate geometry in which a C-Ircus σ-bond forms at each CH2 group (i.e., di-σ configuration).
Our calculations predict that the C2H4 species needs to overcome a barrier of 189 kJ/mol to
desorb vs. a barrier of 68 kJ/mol to dehydrogenate via H-transfer to an Obr atom, affording an
adsorbed C2H3 species and a third HObr group. The calculations thus predict that C2H4
dehydrogenation is strongly favored over C2H4 desorption when Obr atoms are available to serve
as H-atom acceptors. This prediction is consistent with our experimental observation that C2H6
oxidation occurs preferentially over C2H4 production at low initial C2H6 and HObr coverages.
Figure 4b shows the computed pathway for C2H6 dehydrogenation on IrO2(110) when two of
the four accessible Obr atoms are initially hydrogenated to HObr groups. For these calculations,
we hydrogenated Obr atoms located in opposing rows, with each next to a different CH3 group of
the C2H6 complex (Figure 4b). Our calculations predict that hydrogenation of the two Obr atoms
destabilizes the C2H6 σ-complex on IrO2(110) by about 22 kJ/mol. We have recently reported
that the hydrogenation of Obr atoms also destabilizes H2 complexes on IrO2(110).11 Our
calculations also predict that the energy barriers are nearly the same for C2H6 and C2H5
dehydrogenation on the initially clean IrO2(110) vs. pre-hydrogenated IrO2(110)-2HObr surfaces
when reaction occurs by H-transfer to an Obr atom (Figures 4a, b).
Sequential dehydrogenation of C2H6 to C2H4 on the initial IrO2(110)-2HObr surface converts
all four of the accessible Obr atoms to HObr groups, and causes C2H4 desorption to become
favored over further dehydrogenation because HObr groups are much less reactive than Obr
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atoms. The energy barrier for C2H4 dehydrogenation via H-transfer to a HObr group is 152
kJ/mol, compared with 68 kJ/mol for C2H4 dehydrogenation to an Obr atom. In addition, the
reverse reaction features an energy barrier of only 5 kJ/mol so the H2Obr species would rapidly
transfer a H-atom to C2H3 to regenerate the adsorbed C2H4 and HObr species. Our DFT
calculations thus indicate that C2H4 desorption is favored over dehydrogenation when all of the
accessible Obr atoms are hydrogenated to HObr. This prediction agrees well with our
experimental findings that pre-hydrogenation of IrO2(110) promotes the conversion of C2H6 to
C2H4 while suppressing C2H6 oxidation, and that C2H4 production begins to occur on initially
clean IrO2(110) only at moderate initial C2H6 coverages.
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Discussion
Our results show that C2H6 activation is highly facile on IrO2(110) at temperatures below 200 K,
and that further dehydrogenation produces C2H4 that desorbs between 300 and 450 K. Based on
comparison with reference TPRS data (SI), we conclude that C2H4 desorption is the rate-limiting
step in the conversion of C2H6 to C2H4 on IrO2(110) during TPRS. Our DFT calculations support
these conclusions as they predict that the barrier for C2H6 C-H bond cleavage on clean IrO2(110)
is lower than that for C2H4 desorption by at least 100 kJ/mol. Indeed, we find that the IrO2(110)
surface is exceptionally active in promoting alkane C-H bond cleavage - we estimate a barrier
between 35 and 40 kJ/mol for ethane activation on IrO2(110), and our prior work reveals an even
lower barrier of 28 kJ/mol for CH4 activation on IrO2(110).10 In fact, our DFT results predict that
initial C-H bond cleavage has the lowest barrier among the reaction steps involved in C2H6
conversion to C2H4 on IrO2(110).
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In contrast to IrO2(110), initial C-H bond activation is the rate-determining step in the ODH
of alkanes on most other oxides. Supported vanadium-oxide based catalysts have been widely
studied due to their favorable performance in promoting the ODH of ethane and propane.1,9
While the specific values can depend on multiple factors, barriers for ethane C-H bond cleavage
on VOx-based catalysts lie in a range from about 120 to 150 kJ/mol,5,25,26 and reactors are
operated at temperatures between 700 and 900 K to achieve optimal rates and selectivity of
alkene production from ethane and propane.2 According to DFT, ethylene desorption is the rate-
determining step for the conversion of C2H6 to C2H4 on IrO2(110) under TPRS conditions
because the dehydrogenation of adsorbed C2H6 and C2H5 groups are both facile processes on
clean IrO2(110) and the C2H4 product binds strongly. From our TPRS data, we estimate that the
barrier for C2H4 desorption from IrO2(110) lies between about 130 and 165 kJ/mol, and is thus
close to the values reported for C2H6 C-H activation barriers on VOx-based catalysts. However,
since the entropy of activation is much larger for C2H4 desorption compared with ethane C-H
bond cleavage, C2H4 desorbs from IrO2(110) at lower temperature relative to the temperatures at
which VOx-based catalysts would achieve comparable rates of ethane conversion to ethylene.
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Our TPRS results show that the desorption of ethylene from IrO2(110) occurs at lower
temperature during TPRS than the reaction-limited desorption of H2O and COx species resulting
from ethane oxidation (Figure 1). A possible implication is that low temperature operation can
enable IrO2 catalysts to promote the conversion of ethane to ethylene at high rates while
minimizing COx production. However, the higher desorption temperature of H2O compared with
C2H4 suggests that H2O desorption could be a rate-controlling step in the IrO2-promoted
conversion of C2H6 to C2H4 under steady-state conditions. While further study is needed, our
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results suggest possibilities for achieving efficient and selective conversion of ethane to ethylene
at low temperature using IrO2-based catalysts.
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Our results also demonstrate that partial hydrogenation of the IrO2(110) surface enhances
ethane conversion to ethylene while suppressing extensive oxidation to COx species. We find
that HObr groups are significantly less active than Obr atoms as H-atom acceptors, and, as a
result, hydrogenating a fraction of the Obr atoms limits the extent to which adsorbed
hydrocarbons can dehydrogenate and causes C2H4 desorption to become favored over further
dehydrogenation and extensive oxidation. This behavior provides a viable explanation of the
evolution of TPRS product yields with increasing C2H6 coverage. At low C2H6 coverage enough
Obr atoms are available to allow each C2H6 molecule to extensively dehydrogenate, and produce
intermediates that oxidize to COx species with further heating. With increasing C2H6 coverage,
the extent to which C2H6 molecules dehydrogenate becomes limited because a larger fraction of
Obr atoms convert to HObr groups and deactivate. Consistent with this interpretation, our
experiments demonstrate that the selectivity toward ethane conversion to ethylene can be
enhanced by partially hydrogenating the IrO2(110) surface prior to adsorbing ethane. This
finding may have broad implications for developing methods by which to modify the selectivity
of IrO2 catalysts. In particular, our results demonstrate that controllably deactivating a fraction of
the reactive O-atoms of IrO2 is an effective approach for promoting the partial dehydrogenation
of ethane over extensive oxidation.
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Summary
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We investigated the dehydrogenation of ethane on the stoichiometric IrO2(110) surface using
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TPRS and DFT calculations. Our results show that ethane forms strongly-bound
IrO2(110) and that a large fraction of the adsorbed complexes undergo C-H bond cleavage below
200 K during TPRS. Our DFT calculations predict that ethane -complexes on IrO2(110)
dissociate by a heterolytic mechanism involving H-atom transfer to a neighboring Obr atom, and
that the barrier for C-H bond cleavage is lower than the binding energy of the C2H6 -complex.
σ-complexes on
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σ
σ
We find that the resulting ethyl groups react with the IrO2(110) surface via oxidation to COx
species and H2O as well as dehydrogenation to C2H4, with the C2H4 product desorbing between
300 and 450 K. Both DFT calculations and TPRS experiments show that C2H4 desorption is the
rate-limiting step in the conversion of C2H6 to C2H4 on IrO2(110) during TPRS. Our
experimental results demonstrate that partially hydrogenating the IrO2(110) surface enhances the
conversion of ethane to ethylene while suppressing ethane oxidation to COx species. According
to DFT, converting a fraction of the Obr atoms to HObr groups causes C2H4 desorption to become
favored over further dehydrogenation because HObr groups are poor H-atom acceptors compared
to Obr atoms. Our findings reveal that the IrO2(110) surface exhibits an unusual ability to
promote the dehydrogenation of ethane to ethylene near room temperature during TPRS, and
demonstrate that controlled deactivation of Obr atoms is an effective way to promote ethylene
production from ethane on IrO2(110).
Supporting Information
Structural representation of the IrO2(110) surface; Measurement of product yields; TPRS traces
for C2H6 as a function of coverage on IrO2(110); TPRS traces for C2H4 adsorbed on IrO2(110) at
90 K; Configurations of C2H6 adsorbed on IrO2(110) as predicted with DFT; Comparison of
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DFT-PBE calculations with and without dispersion-corrections for C2H6 dehydrogenation on
IrO2(110). This material is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgements
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We acknowledge the Ohio Supercomputing Center for providing computational resources. We
gratefully acknowledge financial support for this work provided by the Department of Energy,
Office of Basic Energy Sciences, Catalysis Science Division through Grant DE-FG02-
03ER15478.
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ToC Graphic
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Clean IrO2(110)
C2H6 + IrO2(110)
TPRS (27 + 44 amu)
C2H6 + IrO2(110)-H
clean
COx
H-precovered
[C2H6]o ~ 0.14 ML
CO2 (x 0.5)
C2H6(ad)
C2H4(ad)
C2H6
-complex
H-covered IrO2(110)
σ
C2H4
C2H4
100
200
300
400
500
600
700
Temperature (K)
C2H6(ad)
C2H4(ad)
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