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Cite This: ACS Catal. 2018, 8, 2651−2663

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Mechanism of Ethylene Oxychlorination on Ceria
Matthias Scharfe,† Marçal Capdevila-Cortada,‡ Vita A. Kondratenko,§ Evgenii V. Kondratenko,§
Sara Colussi,∥ Alessandro Trovarelli,∥ Núria López,*,‡ and Javier Pérez-Ramírez*,†

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†

Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 1,
8093 Zurich, Switzerland
‡
Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Av. Països Catalans 16,
43007 Tarragona, Spain
§
Leibniz-Institut für Katalyse e. V., Albert-Einstein-Straße 29a, 18059 Rostock, Germany
∥
Dipartimento Politecnico, Università di Udine, via del Cotonificio 108, 33100 Udine, Italy
S
* Supporting Information

ABSTRACT: Ethylene oxychlorination on CeO2 provides
ethylene dichloride (EDC) and the desired vinyl chloride
(VCM) in a single operation, in contrast to the traditional
process that requires two separate units. The origin of this
outstanding performance is unclear, and the mechanism has not
been discussed in detail. In the present work, we combine density
functional theory (DFT) with steady-state experiments and
temporal analysis of products (TAP) to close this gap. The
catalyst surface is found to contain CeOCl, while the bulk phase
is CeO2, regardless of the starting materials CeCl3, CeOCl, or
CeO2. Catalysis by different nanostructures highlights that the
CeO2(111) surface is more active than the (100) surface due to
the poisoning of the latter, while the selectivities are comparable.
In any case, the degree of oxygen removal from CeO2 and the
replenishment of the accordingly formed oxygen vacancies by Cl and its replenishment by Cl species lead to increased selectivity
to chlorinated products and decreased selectivity to carbon oxides. DFT and TAP studies reveal that the most likely pathway of
VCM formation takes place by a cascade reaction. First, EDC appears and then HCl is extracted in a concerted step to lead to
VCM. Such steps are a key characteristic of ceria. Other paths leading to minor products such as 1,2-dichloroethene (DCE) are
found possible by starting from VCM or EDC. CO is formed by combustion of chlorinated species, whereas CO2 can either stem
from further oxidation of CO or directly from ethylene. In summary, our work points out a rich complex behavior of the
chemistry of chlorinated compounds on the oxide surface, indicating that concerted steps and cascade reactions are possible for
these materials.
KEYWORDS: ethylene oxychlorination, mechanism, CeO2, oxychloride, oxygen vacancies, density functional theory,
temporal analysis of products
and/or improve the activity and selectivity.8−11 However, while
an EDC selectivity of 99% is achieved, issues surrounding active
phase volatilization or particle stickiness have not yet been fully
resolved.12 Nevertheless, the mechanism of ethylene oxychlorination on CuCl2-based catalysts has been unveiled by
numerous in situ and operando spectroscopy studies.13−17
Briefly, the process encompasses a three-step redox mechanism:
(i) chlorination of ethylene by reduction of CuCl2 to CuCl, (ii)
formation of a cupric oxychloride by oxidation of CuCl, and
finally (iii) closure of the catalytic cycle though rechlorination

1. INTRODUCTION
Polyvinyl chloride (PVC) constituted a 53 billion USD market
in 2015, growing with a compound annual growth rate of
5.3%.1 Its corresponding monomer, vinyl chloride (commonly
denoted as VCM), is produced through hydrochlorination of
acetylene2−4 or through energy-intensive thermal dehydrochlorination of 1,2-dichloroethane (known as ethylene
dichloride, EDC) with a low yield of 49−58%.5 EDC is, in
turn, formed either by Cl2 addition to ethylene or by ethylene
oxychlorination. The latter enables recycling of HCl from the
dehydrochlorination step in the balanced VCM process.5−7 To
date, the catalyst in ethylene oxychlorination comprises cupric
chloride (CuCl2) as the active phase, impregnated on a porous
support such as alumina, and is promoted by numerous alkali,
alkaline-earth, and rare-earth metals to reduce the copper loss
© 2018 American Chemical Society

Received: December 23, 2017
Revised: February 8, 2018
Published: February 15, 2018
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hydrothermally synthesized, with optimization of procedures
reported elsewhere.42−44 Briefly, 5.21 g of Ce(NO3)3·6H2O was
dissolved in 30 mL of distilled water to which 210 mL of 6.9 M
NaOH solution was added dropwise under vigorous stirring.
After precipitation, the suspension was transferred into a
Teflon-lined cylinder, sealed in a stainless-steel autoclave, and
kept at 453 K for 24 h. Subsequently, the suspension was
cooled to room temperature, centrifuged, and washed three
times with water and ethanol before the product was dried
overnight at 333 K and calcined in static air at 773 K for 3 h.
2.2. Catalyst Characterization. N2 sorption at 77 K was
measured in a Quantachrome Quadrasorb-SI analyzer. Prior to
the measurements, the samples were outgassed to 50 mbar at
573 K for 12 h. The Brunauer−Emmett−Teller (BET) method
was applied to calculate the total surface area, SBET, in m2 g−1.45
Oxygen storage capacity (OSC) measurements were performed
by thermogravimetric analysis (TGA) on a TA Instruments
Q500 apparatus. The sample (30 mg) was loaded on a
platinum pan and pretreated in N2 (60 cm3 STP min−1) at 573
K for 1 h, then the temperature was ramped up to 673 K and
equilibrated for 15 min. Thereafter, a flow with 4.5 vol % H2 in
N2 (60 cm3 STP min−1) was introduced and maintained for 2 h
(first OSC measurement) before purging with N2 for 5 min and
reoxidizing the sample with air (60 cm3 STP min−1) for 1 h.
After a second purge in N2 for 5 min, a second OSC
measurement was performed under identical conditions. Then,
two more OSC measurements were performed, following the
same procedure in order to ensure that no chlorine removal
would impair the measured OSC. The chlorine content in the
used samples was estimated by four independent X-ray
fluorescence (XRF) spectroscopy measurements using an
Orbis PC Micro-EDXRF analyzer with a Rh source (35 kV,
500 μA) and a silicon drift detector. A chlorine background
signal was determined in four fresh CeO2-873 at 0.3 atom %
and subtracted from all averaged measurements. Powder X-ray
diffraction (XRD) was measured using a PANalytical X’Pert
PRO-MPD diffractometer and Cu Kα radiation (λ = 0.15418
nm). The data were recorded in the 10−70° 2θ range with an
angular step size of 0.017° and a continuing time of 0.26 s per
step.
2.3. Catalytic Testing. The steady-state oxychlorination of
ethylene was investigated at ambient pressure in a continuousflow fixed-bed reactor (Scheme S1). The setup consists of (i)
mass flow controllers to feed C2H4 (PanGas, 3.5), HCl (Air
Liquide, 2.8, anhydrous), O2 (Messer, 19.96% in He), He
(PanGas, 5.0) as a carrier gas, and Ar (PanGas, 5.0) as an
internal standard, (ii) a syringe pump (Nexus 6000, Chemyx)
to feed EDC (Fluka, 99.5%), (iii) a vaporizer operated at 403 K
accommodating a quartz T-connector filled with glass beads to
vaporize EDC, (iv) an electrically heated oven hosting a quartz
microreactor equipped with a K-type thermocouple whose tip
reaches the center of the catalyst bed, (v) downstream heat
tracing to avoid any condensation of the reactants and
products, and (vi) a gas chromatograph coupled to a mass
spectrometer (GC-MS) for online analysis. The effluent stream
was neutralized by passing it through an impinging bottle
containing an aqueous NaOH solution (1 M). The catalyst
(Wcat = 0.5 g, particle size dp = 0.4−0.6 mm) was loaded in the
microreactor (10 mm inner diameter) and pretreated in He at
473 K for 30 min. Thereafter, a total flow (FT) of 100 cm3 STP
min−1 with a composition of C2H4:HCl:O2:Ar:He = 3:(1−6):
(1−6):3:(83.2−90), where Ar acted as internal standard and
He was used as carrier gas, was fed to the reactor at a bed

of the oxychloride with HCl and water elimination.6,9,10,13,14,16,18−26
Recently, the great potential of CeO2 in ethylene oxychlorination was uncovered, revealing a stable bifunctional
character and enabling direct VCM and EDC production with
single-step yields up to 25%.27 Some of us put forward that this
VCM formation can be attributed to the material acidity,
correlating with the EDC dehydrochlorination performance of
CeO2, and that higher chlorination leads to higher EDC
selectivity and suppressed oxidation routes.27 Alternative
materials, also based on rare-earth elements, such as EuOCl,
were less active but highly selective toward VCM (75%) and
evidenced the selectivity−acidity link.28 However, a previous
study aiming to shed light on the complex ethylene
oxychlorination reaction network on oxide- or oxychloridebased materials used density functional theory (DFT) on RuO2
as a model catalyst due to the simple electronic structure.29
RuO2 is a poor oxychlorination catalyst, as it mostly forms CO2
and only small amounts of EDC and VCM during the reaction.
Nevertheless, investigation of 34 elementary steps over the
relatively simple surface revealed that oxidation processes are
thermodynamically favored, whereas chlorination processes are
kinetically controlled.29 In addition, the surface chlorine uptake,
i.e., the chlorine coverage, is a key factor determining the
selectivity of RuO2. Only high chlorine coverage allows VCM
displacement by C2H4, thus preventing the formation of VCMderived products; however at the same time it destabilizes
C2H4Cl intermediates, favoring the formation of EDC instead
of VCM.29
In this study, we investigate the mechanism of ethylene
oxychlorination over CeO2, as it was the most active under a
broad range of conditions and exhibited a strong effect of the
feed concentrations of HCl and O2 on the activity and
selectivity to chlorinated products.27,28 In addition, CeO2 shows
a correlation between dehydrochlorination performance and
surface acidity, the exact nature of which is as yet unknown.
Even though CeO2 has a more complex electronic structure
than RuO2, the computational setup based on DFT+U has
proven reasonably accurate.30−36 As our experiments reveal that
the active phase in oxychlorination is actually an oxychloride,
previous studies of the mechanism of oxychlorination on RuO2
and chlorination of RuO2 and CeO2 in the related HCl
oxidation potentially allow knowledge transfer to this work or
accompany our findings.29,32,37−40 We prove and complement
the theoretical insight by DFT with advanced transient kinetic
studies, uncovering the reaction pathways that lead to chlorinecontaining hydrocarbons and suggesting the routes for
combustion.

2. MATERIALS AND METHODS
2.1. Catalyst Preparation. Commercial CeO2 (SigmaAldrich, 99.5%) was calcined at 873 or 1023 K in static air using
a heating rate of 5 K min−1 and an isothermal step of 5 h prior
to its use in the catalytic studies. CeCl3 (ABCR, 99.9%) was
calcined at 423 K in static air using a heating rate of 5 K min−1
and an isothermal step of 5 h in order to desorb any adsorbed
species, while not transforming it to CeO2. The catalysts are
hereafter denoted as CeCl3/CeO2-x(-py)(-Ez), where x is the
calcination temperature in K, y the prereduction temperature in
K if applied, and Ez indicates the material used in ethylene
oxychlorination at a reaction temperature of z in K. CeOCl was
prepared by calcination at 1023 K, following the method
reported by Farra et al.41 CeO2 nanocubes (CeO2-NC) were
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an equilibrated catalyst sample (CeO2-873-E673) were studied
by temporal analysis of products (TAP), a transient pulse
technique with a time resolution of approximately 100 μs.46−48
The used TAP-2 system is equipped with an in-housedeveloped quartz-tube fixed-bed reactor (din = 6 mm, L = 40
mm) in which the catalyst (Wcat = 65 mg, particle size dp = 0.3−
0.5 mm) was packed between two layers of quartz of the same
particle size within the isothermal zone of the reactor. Then,
the reactor was evacuated stepwise to 10−5 Pa. The pulse
experiments were carried out at 673, 723, and 773 K using
C2H4:Ne = 1:1, C2H4:O2:Ne = 1:1:1, HCl:Ar = 1:1, Cl2:Ar =
1:1, and C2H4Cl2:Ar = 1:6 mixtures. An overall pulse size was
around 1016 molecules. Ne (Linde Gas, 5.0), Ar (Linde Gas,
5.0), O2 (Air Liquide, 4.8), C2H4 (Air Liquide, 3.5), EDC
(Sigma-Aldrich, 99.8%), and HCl (Air Liquide, 4.5, anhydrous)
were used for preparing reaction mixtures without additional
purification. A quadrupole mass spectrometer (HAL RC 301
Hiden Analytical) was used for quantitative analysis of feed
components and reaction products, where the following AMUs
were assigned for mass spectrometric compound identification:
70 (Cl2), 66 (EC), 64 (EC, EDC, VCM), 62 (EDC, VCM), 49
(EDC), 44 (CO2), 36 (HCl), 32 (O2), 29 (EC), 28 (EC, EDC,
CO2, CO, C2H4), 27 (EC, EDC, VCM, C2H4), 26 (EC, EDC,
VCM, C2H4), 22 (Ne), 18 (H2O), 2 (H2O, H2), 40 (Ar) and
20 (Ar, Ne). Pulses were repeated 10 times for each AMU and
averaged to improve the signal to noise ratio.
2.5. Density Functional Theory. All calculations were
performed with periodic boundary conditions at the density
functional theory level, implemented in the Vienna ab initio
simulation package (VASP, version 5.3.5),49−51 using the
Perdew−Burke−Ernzerhof (PBE) functional.52 Projector-augmented wave (PAW) pseudopotentials were used to consider
the inner electrons, whereas valence electronic states were
expanded in plane waves with an energy cutoff of 500 eV.53 In
order to reduce the self-interaction error, an effective Hubbard
potential (Ueff) of 4.5 eV was added to the Ce(4f) states.54
Spin-polarized calculations were performed when required. van
der Waals contributions were included by using the D3
dispersion correction method of Grimme et al.55 The criteria
for electronic and geometry optimization convergence were set
to 10−6 eV and 0.015 eV Å−1, respectively. The lattice
parameter of CeO2 was optimized with a dense 7 × 7 × 7 Γcentered k-point mesh and an energy cutoff of 700 eV, leading
to the lattice parameter acalc = 5.492 Å, which is in good
agreement (1.5% deviation) with the experimental value of aexp
= 5.411 Å.56 The investigated (111) surface was thereafter
modeled as periodically repeated slabs using a 3 × 3 × 1 Γcentered k-point mesh. The slabs were separated by 15 Å of
vacuum space on top and consisting of 3 × 3 unit cells with
three and six layers of Ce and O atoms, of which the upper two
Ce and four O layers were allowed to relax. Chlorination of the
surface was considered in the uppermost layer by replacement
of O atoms with Cl atoms. The regular O-terminated
CeO2(100) surface was modeled as a 2 × 2 supercell as
described in the literature with the same k-point sampling and
vacuum space as for the (111) surface.57 The climbing image
nudged elastic band (CI-NEB) method and corresponding
vibrational analysis were used to identify the transition states in
the reaction network.58,59 The structures, labeled with the same
names as described in this work, are freely accessible from the
ioChem-BD database.60,61 The vibrational, rotational, and
translational entropies of gas-phase molecules were calculated
using Gaussian 03W (Version 6.1),62 employing DFT with PBE

temperature (T) of 423−773 K and pressure (P) of 1 bar. Note
that relatively low feed concentrations were selected to prevent
corrosion, enable safe handling, and minimize the formation of
hot spots in the catalyst bed due to the high reaction
exothermicity. The standard C2H4:HCl:O2:He = 3:4.8:3:89.2
feed was chosen on the basis of the literature.12,27 Prior to the
analysis of the reaction mixtures, the catalysts were equilibrated
for at least 1 h under each condition. The gas composition at
the reactor outlet, containing reactants (C2H4, O2, HCl) and
products (EDC, VCM, DCE, EC, CO, CO2), was determined
online using a gas chromatograph equipped with a GSCarbonPLOT column coupled to a mass spectrometer (Agilent
GC 7890B, Agilent MSD 5977A) with a triple-axis detector and
an electron multiplier. A representative chromatogram is
depicted as an inset in Scheme S1. In the HCl oxidation
tests, the Cl2 production was quantified by offline iodometric
titration (using a Mettler Toledo G20 Compact Titrator) of
triiodide, formed by purging the Cl2-containing reactor outlet
through an aqueous KI (Sigma-Aldrich, 99.5%) solution (0.1
M), with 0.01 M sodium thiosulfate solution (Sigma-Aldrich,
99.99%). The conversion of HCl in HCl oxidation experiments,
X(HCl), was calculated using eq 1
X(HCl) =

2xCl 2,outlet
x HCl,inlet

× 100%

(1)

where xHCl,inlet and xCl2,outlet denote the volumetric concentration
of HCl and Cl2 at the reactor inlet and outlet, respectively. The
conversion of ethylene in ethylene oxychlorination experiments, X(C2H4), was calculated according to eq 2
xC H ,inlet − xC2H4,outlet
X(C2H4) = 2 4
× 100%
xC2H4,inlet
(2)
where xC2H4,inlet and xC2H4,outlet denote the volumetric concentration of ethylene at the reactor inlet and outlet, respectively.
The selectivity of a reaction product j, S(j), was calculated
according to eq 3
S ( j) =

xj/nj
∑ xj/nj

× 100%

(3)

where xj and nj denote the volumetric concentrations of
product j at the reactor outlet and the corresponding
stoichiometric factor with respect to the number of carbon
atoms, respectively (e.g., C2H4 + 2O2 → 2CO + 2H2O, nCO =
2).
The carbon mass balance error εC was determined using eq 4
εC =

xC2H4, j ,inlet − ∑ xC2H4, j ,outlet /nC2H4, j ,outlet
xC2H4, j ,inlet

× 100%
(4)

where xC2H4,j and nC2H4,j denote the concentration of ethylene or
product j at the reactor inlet or outlet and the corresponding
stoichiometric factor with respect to the number of carbon
atoms, respectively. Each catalytic data point reported is an
average of at least three measurements. The carbon mass
balance in all catalytic tests closed at 96% or higher. After the
tests, the catalyst bed was quenched to room temperature in He
flow.
2.4. Temporal Analysis of Products. Overall reaction
pathways of product formation in ethylene oxychlorination over
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Figure 1. Ethylene conversion (X) and selectivity (S) versus temperature ramping up (arrow 1, open symbols) and down (arrow 2, solid symbols)
over (a) CeCl3-423, (b) CeOCl-1023, and (c) CeO2-1023. Other conditions: C2H4:HCl:O2 = 3:4.8:3, Wcat = 0.5 g, Ftot = 100 cm3 STP min−1, P = 1
bar. XRD patterns of fresh (bottom) (d) CeCl3-423 (green), (e) CeOCl-1023 (blue), and (f) CeO2-1023 (black) with corresponding reference
patterns of CeCl3 (ICDD PDF 77-0154 and 01-0149), CeOCl (ICDD PDF 73-5027), and CeO2 (ICDD PDF 73-6318) and used catalysts (top,
red) that all show only the CeO2 phase corresponding again to ICDD PDF 73-6318. (g−i) HRTEM images (left) and elemental maps (right, where
red depicts O and green Cl, respectively) of used catalysts corresponding to (a)−(c).

functionals, a cc-pVQZ basis set, the RMS force criterion set to
10−5, and the temperature set to 673.15 K. The Gibbs free
energy was calculated by taking the vibrational entropy into
account, as implemented in Gaussian and described by
Ochterski.63

mainly chlorinates on the surface or only to some extent in the
near-subsurface region at O2:HCl feed ratios greater than
0.75,32 1,37 or 2,64 constituting a rather large spread.
As the ethylene oxychlorination reaction conditions used are
in general closer to those reported in the first two publications,
ceria was previously proven to be stable in oxychlorination of
ethylene,27 and the XRD patterns of catalysts used in Figure
1d−f only show the presence of the oxide phase, we estimate
that the bulk material keeps the oxide nature while the
catalytically active surface consists of an oxychloride. In
addition, key DFT calculations show that the energy required
to displace one Cl atom from the surface to a subsurface
position is 3 eV,32 indicating that this is an unlikely event. The
CeOCl and CeO2 samples used exhibit mostly octahedral
morphologies, whereas the CeO2 formed from CeCl3 shows
less developed crystals, likely due to insufficient oxygen
availability during such a transformation. However, this insight
into the composition of the surface does not yet allow
conclusions about which facet of chlorinated ceria should be
the model surface for further studies. Therefore, we investigated
the performance of polycrystalline ceria and CeO2 nanocubes
(CeO2-873-E673 and CeO2-NC-E673), exposing predominantly the (111) and (100) surfaces, respectively. Figure S1
shows that there are no significant differences in selectivity
between the nanocubic and polycrystalline samples when they
are compared at equal conversion (Figure S2 depicts the
relationship of conversion and product distribution of our
standard sample). The only difference is the slightly decreased
combustion in favor of higher selectivity to chlorinated
hydrocarbons of CeO2-NC-E673, which can be explained by
easier chlorination of the (100) surface in comparison to the
(111) surface, evidenced by a much higher net (Table S1) and
per surface area (Figure S1) chlorine uptake. The latter is likely
also a reason for the reduced activity, together with the lower
surface area of the nanocubes used (19 versus 39 m2 g−1).

3. RESULTS AND DISCUSSION
3.1. Effect of Material Properties. In order to choose the
right model catalyst for this study, the first step was to identify
which type of material is present during the reaction by
correlating steady-state activity with the bulk-phase composition. Figure 1a−c depicts the conversion of ethylene and
selectivity to various products in ethylene oxychlorination on
CeCl3, CeOCl, and CeO2. The temperature was cycled from
423 to 723 K and backward in order to achieve the strongest
hysteresis behavior, as it can be expected that CeCl3 transforms
to the oxide under an oxygen-containing atmosphere at high
temperatures. The material that behaves quite differently from
the rest during up- and downcycling is CeCl3, which shows first
only formation of ethyl chloride before the conversion drops to
almost zero until 723 K, where about 58% conversion is
reached. At this temperature, the product distribution is similar
to that of CeOCl and CeO2, while the activity is about the same
as that of CeOCl and a bit higher than that of CeO2. However,
most importantly, the cooling part of the cycle is qualitatively
the same in all three materials, suggesting a transformation of
the different starting materials to one and the same phase. As
CeOCl and CeO2 do not exhibit such a hysteresis behavior, it is
likely that this common phase is an oxychloride, which either
exists from the beginning in CeOCl or forms easily and quickly
from CeO2 through chlorination already at low temperatures.
XRD evidences the prevalence of bulk CeO2 in all samples after
the test (Figure 1d−f), whereas elemental mapping (Figure
1g−i) shows the presence of oxygen and chlorine throughout
all samples. Previous studies of HCl oxidation report that CeO2
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Figure 2. (a) Conversion and selectivity of ethylene oxychlorination at 673 K on CeO2-873 without pretreatment and with a prereduction in 5 vol %
H2 at 873 K for 2 h. The right axis indicates the chlorine content per surface area of the used catalysts (see also Table S1). Other conditions:
C2H4:HCl:O2 = 3:4.8:3, Wcat = 0.5 g, Ftot = 100 cm3 STP min−1, P = 1 bar, time on stream 2 h. The insets schematically depict the principle of
chlorination, where the top part shows CeO2 with a low or high amount of vacancies formed by prereduction, which leads to low or high chlorination
(yellow Ce, red O, green Cl). (b, c) HRTEM images and (d, e) elemental mapping of CeO2-873-E673 and CeO2-873-p873-E673. The scale bars in
the micrographs represent 100 nm.

or bulk of CeO2 is modified by the introduction of chlorine,
which can then alter the way CeO2 interacts with H2/O2 during
OSC measurements. Detection of chlorine traces by XRF (0.7
atom %) in the sample even after four reduction/oxidation
cycles indicates that chlorine modifies the material permanently. This is also evidenced by the OSC value, which stays
constant at ca. 50 μmol O2 g−1 after three cycles. The literature
supports the presence of nonremovable chlorine in ceria,
suggesting that CeOCl patches which form on the surface can
grow into subsurface layers at temperatures at or above 673
K.67,68 Furthermore, Cl was found to remain in the lattice until
1173 K under reducing conditions.69 Interestingly, prereduction of CeO2 with 5 vol % H2 in He at 873 K has the same
effect on the OSC as the oxychlorination reaction carried out at
673 K (CeO2-873-p873 and CeO2-873-E673 both exhibit an
OSC of exactly 50 μmol O2 g−1). Furthermore, the prereduced
sample after reaction CeO2-873-p873-E673 also exhibits an
OSC of 53 μmol O2 g−1. This corroborates the assumption that
the incorporation of nonremovable chlorine in the bulk does
facilitate vacancy formation in other areas of CeO2, as also
observed in a study on chlorine poisoning of Ce/Zr mixed
oxide catalysts.70 However, further investigations of these
aspects would go beyond the scope of this work. In conclusion,
vacancies and chlorination are strongly linked and cannot be
discussed separately. While the existence of vacancies seems
important to allow a certain extent of surface chlorination
which suppresses combustion and allows the reaction to take
place, too many vacancies, leading to too high chlorination, are
detrimental to the performance. Furthermore, even though it
cannot be excluded that chlorine atoms from the surface diffuse
into subsurface layers, substantial subsurface chlorine populations are unlikely, as they correspond to highly endothermic
processes. Taking all of the above into account, a stoichiometric
CeO2(111) surface as reference and the same surface with one
defect, i.e., one vacancy that can be filled by one chlorine atom,
became the surfaces of choice for the DFT investigations.
3.2. Reaction Mechanism. Initial calculations of the two
model surfaces showed the existence of three distinct
adsorption sites, of which the stoichiometric surface exhibits

Studies of HCl oxidation and trimethoxybenzene oxyhalogenation have previously shown similar results, where nanooctahedra exhibited a higher activity per surface area in
comparison to nanocubes.64,65 However, the chlorine uptake
was therein reported higher on the octahedra than on the
cubes, in contrast with our findings, which is most likely linked
to the different conditions and absence of ethylene.64
Furthermore, while the stabilities of the two different CeO2
morphologies under the reaction conditions are comparable, as
evidenced by HRTEM analysis of fresh and used samples
(Figure S1b−e), the (111) surface is generally more stable from
a thermodynamic point of view.66
However, as the effect of oxygen vacancies and their possible
chlorination is yet unclear, we conducted experiments with in
situ prereduced CeO2, in order to create vacancies before
exposing the material to our reaction conditions. These
vacancies can be filled by oxygen or chlorine during the
reaction, as depicted in the schemes in Figure 2a (see Table S1
for surface areas and chlorine contents), while chlorination is
preferred by 0.89 eV (vide infra). In turn, the higher degree of
chlorination leads to less activity, less combustion, and
improved EDC selectivity of CeO2-873-p873-E673 with respect
to the reference sample CeO2-873-E673 (Figure 2a). Even
though the structure is preserved in both cases (HRTEM
images of used catalysts show no significant differences and
octahedra are still clearly visible), elemental mapping also
evidences much more chlorine in CeO2-873-p873-E673 than in
CeO2-873-E673 (Figure 2b−e), confirming the XRF measurements. Therefore, vacancies are at least partially healed by
incorporation of chlorine rather than oxygen under the reaction
conditions, thus creating a surface oxychloride structure and
constituting an important factor determining the selectivity.
The number of vacancies that form under reaction conditions
was determined by measurement of the oxygen storage capacity
(OSC), a technique widely applied in the literature.66
In general, an increase in the OSC of used materials in
comparison to fresh CeO2 (50 μmol O2 g−1 for CeO2-873-E673
versus 34 μmol O2 g−1 for CeO2-873) is observed. This
increase in OSC would suggest that, during reaction, the surface
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two (lattice oxygen and lattice cerium atoms) and the defective
surface exhibits an oxygen vacancy in addition. HCl adsorbs
dissociatively on CeO2(111), where the hydrogen proton binds
to a lattice oxygen and the chlorine anion adsorbs either on top
of a Ce atom (Eads = −1.40 eV) or in an oxygen vacancy (Eads =
−3.16 eV). As the difference in adsorption energy is relatively
high, it is plausible that, once vacancies exist, chlorine will
predominantly fill them, in accordance with high Cl activation
barriers in HCl oxidation.32 In contrast to the (111) surface,
HCl adsorption on the regular O-terminated (100) surface57 is
much stronger (Eads(HCl) = −3.64 eV and Eads(2HCl) = −6.08
eV) and Cl evolution is very energy intensive (2.64 eV) and
thus unlikely. Therefore, once HCl adsorbs on CeO2(100), Cl
is very likely to stay on that surface, thus poisoning it, which
explains the relative inertness of this surface. Similar to HCl
adsorption on CeO2(111), even though nondissociative,
chlorinated hydrocarbons also occupy up to three adsorption
sites simultaneously. While ethylene adsorbs on top of a lattice
cerium, chlorinated hydrocarbons occupy one lattice oxygen
and neighboring Ce atoms or vacancies (Figures S3 and S4).
The same holds for chemisorbed intermediates, where lattice
oxygen creates a bond to one C atom and the potential chlorine
ends orient so that they occupy Ce atoms or vacancies (vide
infra). A comparison of the adsorption energies of ethylene and
chlorinated products on a stoichiometric, an oxygen vacant, and
a chlorinated surface (Figure 3 and Table S2) shows that

Our previous work proposes a sequential mechanism of
ethylene oxychlorination to EDC and its dehydrochlorination
to VCM, as depicted in dashed black lines in Figure 4, which
represents the starting point of a transition state investigation
on a stoichiometric and defective surface. However, some
elementary steps in this pathway are highly unlikely due to high
barriers for the formation of EDC (stoichiometric 1.98 eV,
defective 2.77 eV, step 3 in Table S2). In addition, step 14
rendered unfeasible, as the removal of one chlorine atom always
also involves the stripping of the second chlorine atom, leading
to a surface-bound C2H3. Therefore, this pathway needs to be
discarded, but an alternative consecutive mechanism could be
imagined by ceria enabling concerted steps,71 i.e., the
combination of individual steps into one elementary step, as
depicted by solid black arrows in Figure 4. Herein, path 1a
combines the addition of one chlorine atom from a vacancy and
another chlorine atom that is adsorbed on top of a cerium atom
to ethylene. The following path 7a represents the simultaneous
abstraction of H and Cl from EDC, directly leading to VCM.
Figure 5 and Table 1 report the reaction profile along these
paths.
In fact, step 1a was found to consist of two individual steps
(reaction coordinates 2−6, steps 1a-1 and 1a-2) with barriers of
1.16 and 0.60 eV, as the chlorine addition is not fully symmetric
and thus a non-surface-bound C2H4Cl intermediate with only
real frequencies exists. In other words, after ethylene and HCl
adsorption from the gas phase, ethylene attacks the chlorine on
top of a cerium atom (reaction coordinates 2−4), while in the
second part, this intermediate directly picks up the chlorine
from a vacancy (left inset in Figure 5). Nevertheless, this path is
still greatly favored over the two individual steps with a surfacebound C2H4Cl intermediate or the situation with two
chlorinated vacancies (Ea > 3 eV). Therefore, it cannot be
considered a single elementary step, most likely because it
encompasses the rotation that creates the reacting intermediate.
On the other hand, the third transition state at reaction
coordinate 7 (right inset in Figure 5) involves a truly
simultaneous stripping of one chlorine by a vacancy and one
hydrogen by a lattice oxygen, resulting in a barrier of only 0.48
eV. In contrast to the previous steps, this H−Cl extrusion does
not involve a rotation, allowing the creation of a largely
stabilized six-membered ring, formed by the surface and the
transition state structure. Plots of the charge density (insets in
Figure 5) evidence this hypothesis. However, EDC is the
dominating product in comparison to VCM as experimentally
observed, which can be explained by the lower ΔG value for
EDC desorption in comparison to the barrier for its
transformation to VCM.
In mechanistic investigations through density functional
theory, selectivity is retrieved by the comparison of the desired
path against potential side routes leading to the same or
secondary products. Such an alternative route to form VCM
could be offered by dehydrogenation of C2H5Cl (EC) or its
corresponding intermediate C2H4Cl. However, while the latter
is in principle possible with a barrier of 1.68 eV (path 4 in
Figures 4 and 6 and Table 2), EC formation itself is highly
unlikely. While the stoichiometric surface prohibits EC
formation completely, the defect surface allows at least a very
easy transformation of a surface alcohol (C2H4ClOH) to EC
(Ea = 0.04 eV). However, such a surface alcohol formation is
strongly endothermic (Er = 1.5 eV) and thus the formation of
EC can be excluded (path 2, 2a, 5, 6, 8 in Figure 4 and Figure
S13 and Tables S3 and S4).

Figure 3. Adsorption energies of ethylene and chlorinated products on
a stoichiometric surface (stoich) and two defect surfaces with either
one oxygen vacancy (1 vac) or one incorporated chlorine atom (1 Cl).

ethylene is generally weakly adsorbed and EDC is always the
most strongly adsorbed compound, with the exception of the
case with one oxygen vacancy, where EC adsorbs with the same
energy. DCE exhibits a slightly weaker adsorption, similar to
the case for EC, while VCM is always the least strongly bound
product. Therefore, EDC, EC, and DCE are more likely to take
part in further transformations in comparison to VCM if the
barriers are reasonably low. This is very desirable, however,
VCM is only on the stoichiometric surface less strongly
adsorbed than ethylene and the adsorption energies of all
products on the oxygen vacant surface are very close. The latter
can be explained by the fact that the presence of a vacancy acts
as an ideal trapping site for the chlorinated end of each
molecule and therefore other contributions are less important
(Figure S4). Nevertheless, the result of this comparison is not
in line with the experimentally observed selectivity (Figures 1
and 2 and Figure S2) and therefore not sufficient to unravel the
mechanism of ethylene oxychlorination on CeO2.
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Figure 4. Schematic reaction network showing the main pathway (solid black) leading to EDC and VCM and side pathways leading either to VCM
(light blue) or chlorinated side products (orange, magenta, purple, blue, gray). Solid (dashed) boxes indicate desired (undesired) products. For the
sake of clarity, all truly reversible reactions are displayed in a single direction. The arrow labels indicate the transferred atom and the reaction number
corresponding to Table S2 and Figures S5−S12. The main path leading to the desired product is highlighted by the black thick arrows. Dashed
arrows represent pathways that were found to be highly unlikely. Surface-bound intermediates are depicted with a surface excerpt of two Ce and one
O atom. The barrier of surface Cl diffusion (step 5) was 0.40 eV.

Figure 5. Reaction profile and corresponding optimized structures for ethylene oxychlorination and EDC dehydrochlorination on a defective
CeO2(111) surface (solid black pathway, i.e. steps 1a and 7a in Figure 4 and Table S2) including the Gibbs free energy difference for desorption. The
reaction coordinates are described in Table 1. The blue glow in the structures highlights a vacancy. The insets depict one individual (left) and a
concerted (right) transition state. The yellow overlay represents the charge density (isosurface level 0.2309 |e−|Å−3). Color code: oxygen, red;
cerium, yellow; carbon, black; chlorine, green; hydrogen, white.

and the necessary surface intermediate would not exist.
Furthermore, even on a stoichiometric surface, step 7a exhibits
only a barrier of 0.61 eV, which is 0.56 eV lower than the
formation of the necessary intermediate (step 7), making this
path even more unlikely. Both alternative pathways to yield
DCE (blue and gray in Figure 4, steps 9, 10, 12, 15) with VCM
or EDC as a starting point exhibit relatively high barriers.

Formation of the undesired byproducts cis- and trans-DCE is
possible through steps 11 and 13 from the surface-bound
intermediate C2H3Cl2 (Figure S14 and Table S5), favoring the
cis isomer, which reflects the experimental observation.
However, this mechanism is rather unlikely, as it only works
on a stoichiometric surface or patches where no vacancy is near.
Otherwise, the concerted step 7a would directly lead to VCM
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Table 1. Intermediates, State Descriptions, Energies with Respect to Gas-Phase Reactants (ΔE), Reaction Energies with Respect
to the Previous Intermediate (Er), Activation Energies (Ea), and Imaginary Frequencies of Transition States (νi),
Corresponding to Figure 5a
intermediate

state description

ΔE (eV)

1
2
3
4
5
6
7
8

C2H4(g) + 2 HCl(g) + □ + 2* + 2■
C2H4* + Cl□ + Cl* + 2 H■
C2H4*- -Cl□ + Cl* + 2 H■
C2H4Cl*□ + Cl* + 2 H■
C2H4Cl*□- -Cl* + 2 H■
C2H4Cl2*□* + 2 H■
C2H3Cl**- -Cl□- -H■ + 2 H■
C2H3Cl** + Cl□ + 3 H■

0
−4.90
−3.74
−3.99
−3.39
−3.70
−3.22
−5.72

Ea (eV)

νi (cm−1)

1.16

254

0.60

239

0.48

Er (eV)

612

−4.90
0.91
0.29
−1.94

a

The symbol ■ denotes a lattice oxygen, □ an oxygen vacancy, and * a lattice cerium, respectively. Multiple symbols indicate occupation of multiple
sites by the adsorbate. In transition states, the added or stripped atom is indicated with a double dash (- -).

Figure 6. Reaction profile and corresponding optimized structures for direct VCM formation from ethylene on a defective CeO2(111) surface (light
blue pathway, i.e. steps 1 and 4 in Figure 4 and Table S2) including the Gibbs free energy difference for desorption. The reaction coordinates are
described in Table 2. The blue glow in the structures highlights a vacancy. The color code is the same as in Figure 5.

Table 2. Intermediates, State Descriptions, Energies with Respect to Gas Phase Reactants (ΔE), Reaction Energies with Respect
to the Previous Intermediate (Er), Activation Energies (Ea), and Imaginary Frequencies of Transition States (νi), corresponding
to Figure 6a
intermediate

state description

ΔE (eV)

1
2
3
4
5
6

C2H4(g) + HCl(g) + □ + * + 2■
C2H4* + Cl□ + H■ + ■
C2H4*- -Cl□ + H■ + ■
C2H4Cl*□ + H■ + ■
C2H3Cl*□- -H■ + H■
C2H3Cl*□ + 2 H■

0
−3.79
−2.61
−3.38
−1.71
−2.99

Ea (eV)

νi (cm−1)

1.18

278

1.67

Er (eV)

450

−3.79
−0.40
−0.39

a

The symbol ■ denotes a lattice oxygen, □ an oxygen vacancy, and * a lattice cerium. Multiple symbols indicate occupation of multiple sites by the
adsorbate. In transition states, the added or stripped atom is indicated with a double dash, - -.

(chlorinated) gas-phase hydrocarbon is not possible on a
lattice oxygen on the CeO2(111) surface. Furthermore, the
adsorption energy of HCl is 0.89 eV higher than that of O2,
resulting in the preferential adsorption of HCl. As O2 activation
leaving active O atoms on the surface is unlikely, the lateral
oxidation processes have not been investigated further.
3.3. Direct Chlorination and Gas-Phase Contributions.
From previous studies, we know that CeO2 exhibits
considerable HCl oxidation activity at conditions close to our
reaction conditions,28,32 prompting an investigation of gasphase and direct chlorination contributions. As depicted in
Figure 8a,b, direct chlorination, i.e., addition of Cl2 to ethylene,
or formation of VCM through replacement of H by Cl reaches
about 50% conversion over quartz particles and about 60%

However, due to strong VCM and even stronger EDC
adsorption on a vacancy, the blue and gray paths, both favoring
the cis isomer, seem to represent the more likely mechanism of
DCE formation (Figure 7 and Tables 3 and 4). Counterbalancing this process is only the fact that, even though VCM
or EDC is strongly adsorbed on a vacancy, it can easily be
displaced by HCl or O2 adsorption.
Oxidation processes were studied by investigating the energy
barriers of chemisorption of ethylene and VCM on a molecular
O2 in a vacancy, which were both very low (0.88 and 1.01 eV).
However, the probability of such a setting existing is extremely
low, as O2 would easily split up when another vacancy is
present (barrier of 0.80 eV), refilling of two vacancies (surface
and subsurface) is possible,32 and chemisorption of a
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Figure 7. Reaction profile and corresponding optimized structures for DCE formation from VCM (end of step 4, Figure 6) on a defective
CeO2(111) surface through steps 9, 10, and 12 in Figure 4 and Table S2 (left), with the reaction coordinates described in Table 3. Reaction profile
and corresponding optimized structures for DCE formation from EDC (end of step 1a-2, Figure 5) on a defective CeO2(111) surface through step
15 (right), with the reaction coordinates described in Table 4. The color code is the same as in Figure 5.

Table 3. Intermediates, State Descriptions, Energies with Respect to Gas-Phase Reactants (ΔE), Reaction Energies with Respect
to the Previous Intermediate (Er), Activation Energies (Ea), and Imaginary Frequencies of Transition States (νi),
Corresponding to Figure 7a
intermediate

state description

ΔE (eV)

Er (eV)

1
2
3
4
5
6

C2H3Cl*□ + 2 H■ + ■
C2H2Cl*□- -H■ + 2 H■
C2H2Cl*□ + 3 H■ + * + ■ + HCl(g)
C2H2Cl*□ + 4 H■ + Cl*
c/t-C2H2Cl*□- -Cl* + 4 H■
c/t-C2H2Cl2*□* + 4 H■

−2.99
−1.51
−4.40
−5.57/−5.94
−3.84/−4.04
−4.65/−4.59

νi (cm−1)

−1.41
−1.17/−1.54

Ea (eV)
1.48

1499

1.73/1.90

376/365

0.92/1.35

a

The symbol ■ denotes a lattice oxygen, □ an oxygen vacancy, and * a lattice cerium. Multiple symbols indicate occupation of multiple sites by the
adsorbate. In transition states, the added or stripped atom is indicated with a double dash, - -.

Table 4. Intermediates, State Descriptions, Energies with Respect to Gas-Phase Reactants (ΔE), Reaction Energies with Respect
to the Previous Intermediate (Er), Activation Energies (Ea), and Imaginary Frequencies of Transition States (νi),
Corresponding to Figure 7a
state description

ΔE (eV)

Er (eV)

1
2
3

C2H4Cl2*□* + 2 H■ + 2■
C2H2Cl2*□*- -H■- -H■ + 2 H■
c-C2H2Cl2*□* + 4 H■

−3.75
−2.12
−4.65

Ea (eV)

νi (cm−1)

1.63

intermediate

601

−0.90

a

The symbol ■ denotes a lattice oxygen, □ an oxygen vacancy, and * a lattice cerium. Multiple symbols indicate occupation of multiple sites by the
adsorbate. In transition states, the added or stripped atom is indicated with a double dash, - -.

beaten by its lower counterpart (1.32 eV) of direct VCM
formation through a one-step dehydrogenation−Cl addition,
where one hydrogen is stripped from ethylene by one Cl while
the other Cl transfers to the C2H3 radical. The exothermicity of
this reaction (−1.31 eV) is however lower than for EDC
formation, implying that EDC dehydrochlorination is endothermic by 0.70 eV. In addition, the latter presents a very high
energy barrier of 2.47 eV, excluding the dehydrogenation
pathway in the gas phase. Even EDC-, HCl-, or O2-assisted
dehydrochlorination, i.e., the presence of one of the above
molecules in the close surroundings of EDC, did not
significantly change the barrier. Prompted by this result, we
conducted gas-phase dehydrochlorination of EDC with and
without cofed HCl or O2, supporting the calculations, as no
conversion could be observed until 723 K (6% only for cofed
HCl; no conversion otherwise, Figure S15c). Thus, reactions in
the gas phase predominantly yield VCM, as direct VCM

conversion over CeO2-873 at 673 K. In both cases, VCM is the
dominant product. In order to ensure that quartz particles were
inert and did not catalyze the reaction, a pure gas-phase test was
conducted in an empty reactor, yielding the exact same
performance (Figure S15a). Considering that, under equivalent
conditions, the degree of HCl conversion into Cl2 reaches 10 or
5% over CeO2-873 or CeO2-NC (Figure 8c and Figure
S15b)and comparing the oxychlorination activity (Figure
8d) it can be expected that at 673 K about 6−8% in total might
stem from gas-phase direct chlorination. Figure 8e depicts the
gas-phase activation energies of EDC and VCM formation from
ethylene in which direct addition of Cl2 to ethylene to form
EDC yields an energy barrier of 1.41 eV while being exothermic
by −2.01 eV. In comparison to this direct addition, the
literature also reports the possibility of radical-assisted
mechanisms for direct halogenation, e.g., hydroxyl-assisted
direct iodination of aromatics, highlighting the rich chemistry
offered by halogenation reactions.72 The former barrier is only
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Figure 8. Conversion (X) and selectivity (S) versus temperature in steady-state catalytic tests of (a) direct chlorination with C2H4:Cl2 = 3:2.4 on
quartz and (b) CeO2-873, (c) HCl oxidation with HCl:O2 = 4.8:3 on CeO2-873, and (d) oxychlorination on CeO2-873 with C2H4:HCl:O2 = 3:4.8:3.
Other conditions: Wcat = 0.5 g, Ftot = 100 cm3 STP min−1, P = 1 bar. (e) Reaction profile for gas-phase ethylene chlorination and subsequent EDC
dehydrochlorination (black) and gas-phase direct VCM formation from ethylene (light blue).

Figure 9. Normalized transient responses of (a) Cl2, VCM, EDC, and CO2 upon simultaneous pulsing of HCl:Ar = 1:1 and C2H4:O2:Ne = 1:1:1 at
773 K and (b) VCM, EDC, and CO2 upon simultaneous pulsing of Cl2:Ar = 1:1 and C2H4:O2:Ne = 1:1:1 at 773 K.

VCM, black arrows in Figure 4). As the same sequence of
product formation was also observed in the absence of gasphase O2 (Figure S16), the above statement should be valid for
a broad range of coverage by oxygen and chlorine species, thus
supporting the validity of our chosen defect model surface. In
addition, this experiment also revealed a correlation between
the amount of formed chlorine-containing products (EDC and
VCM) and the amount of detected Cl2 (Figure S17),
suggesting that high chlorine coverage favors both the
formation of EDC/VCM and desorption of Cl2. This statement
is in line with the hypothesis that both processes take place in
parallel and increased temperature, and thus less poisoning
through faster Cl2 evolution, leads to an increased yield of EDC
and VCM.
In order to check if the source of chlorine, i.e. HCl or Cl2,
influences the pathways of EDC and VCM formation, we
performed experiments with simultaneous pulsing of Cl2, C2H4,
and O2. Figure 9b depicts the obtained height-normalized
transient responses of EDC, VCM, and CO2. The order of
appearance of these products indicates that EDC is the first
chlorinated hydrocarbon formed from ethylene, which is in
agreement with tests using HCl as chlorine source. Therefore,
the order of the reaction steps including chlorination of C2H4
to EDC and subsequent dehydrochlorination of this product to
VCM does not depend on the source of chlorine. This
statement is in line with the observation in our calculations that

formation is favored over EDC formation, while EDC
dehydrochlorination is nonexistent.
3.4. Temporal Analysis of Products. In an attempt to
corroborate the theoretical reaction scheme described above,
we conducted transient experiments with submillisecond time
resolution on pre-equilibrated CeO2-873-E673. Figure 9a
depicts the height-normalized transient responses of Cl2,
EDC, VCM, and CO2, recorded after simultaneous pulsing of
HCl, C2H4, and O2. Herein, Cl2 was the first product detected
at the reactor outlet followed by EDC, VCM, and finally CO2.
The same product formation sequence was observed in the
absence of gas-phase O2 (i.e., when only HCl and C2H4 were
simultaneously pulsed) (Figure S16). Notably, neither radicals
nor EC was observed in these experiments, in agreement with
the results of steady-state experiments and DFT. Therefore, the
pink (8) and orange pathways (2 and 5, 6) of EC formation
and its dehydrogenation to VCM in Figure 4 do not contribute
to the overall reactivity. However, this does not necessarily
exclude the second step of dehydrogenation (path 4 in Figure
4), which could still be part of the direct VCM formation from
ethylene, even though it is not favored. Insight into this
circumstance was indirectly derived from analyzing the order of
appearance of EDC and VCM (Figure 9a), which suggests that
C2H4 is initially converted into EDC followed by dehydrochlorination of the latter to VCM. This experimental finding
supports the pathways predicted by DFT (i.e. C2H4 → EDC →
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was chosen as a model catalyst. We revealed that the formation
of VCM most likely proceeds through dehydrochlorination of
EDC which is greatly facilitated on the defect surface, while
pure dehydrogenation pathways that require C2H5Cl as an
intermediate could be excluded. The major finding, enabling
easy EDC to VCM transformation and thus making the
proposed pathway feasible, is the ability of ceria to perform
concerted elementary steps. Most importantly, of the three
types of theoretical concerted steps, HCl, H2, or Cl2 addition or
abstraction, only HCl abstraction was found dominating and
responsible for the excellent dehydrochlorination behavior of
ceria, while H2 abstraction is feasible. The cause for this easy
concerted HCl abstraction can be found in the formation of a
hexagonal structure that is formed by the electron densities of
the involved C, Cl, H, O, and Ce atoms. Furthermore,
formation of other polychlorinated byproducts such as DCE is
possible from either EDC or VCM, with concerted H
abstraction, or through individual steps from VCM, given that
EDC or VCM is adsorbed on a vacancy. In other words,
chlorinated vacancies facilitate the formation of EDC and its
dehydrochlorination to VCM, staying chlorinated at the end of
the process. However, VCM that is formed through an
alternative, yet less important, direct path without EDC as
intermediate leads to a non-chlorine-filled vacancy, acting as an
anchoring site and increasing its adsorption, which then enables
DCE formation. Given that the cis isomer is the only
experimentally observed product, and EDC also occupies a
vacancy after its formation, EDC appears to be more likely to
transform to DCE in comparison to VCM. These observations
are supported by transient kinetic studies that evidenced the
oxychlorination−dehydrochlorination pathway by revealing
that EDC is always formed as the first product, regardless of
the chlorine source, followed by VCM. In addition, separate
EDC dehydrogenation experiments substantiated this path,
while the absence of ethyl chloride intermediates excluded pure
dehydrogenation processes for VCM formation. Furthermore,
we revealed that CO mostly stems from the combustion of
chlorinated hydrocarbons, whereas ethylene yields only CO2.
Gas-phase chlorination, mostly yielding VCM, was found to be
non-negligible, yet not governing, while EDC dehydrogenation
is impossible in the gas phase.
The synergies obtained by the extensive combination of
computational, steady state, and transient kinetic analysis in this
study allow us to shed light on a complex reaction network of
an industrially highly relevant reaction on CeO2, providing a
basis for further development and simulation of oxide- or
oxychloride-based catalysts for oxychlorination reactions.

both HCl and Cl2 immediately dissociate upon adsorption,
regardless of vacancies being present or not.
In addition, the catalytic ability to dehydrochlorinate EDC
was tested in separate experiments, resulting in the transient
responses of EDC, VCM, CO, and CO2 as shown in Figure 10.
The first observed product was VCM, further supporting the
conclusion about an intermediate role of EDC upon conversion
of HCl and C2H4 to VCM.

Figure 10. Normalized transient responses of VCM, EDC, CO, and
CO2 upon pulsing of EDC:Ar = 1:6 at 773 K.

Even though combustion products (CO and CO2) account
for the smallest fraction of products, they are yet substantial
and an understanding of their formation is important for
improving the catalyst performance. CO was not detected when
C2H4 and O2 were simultaneously pulsed with HCl or Cl2
(Figure 9). This is also valid for the ethylene oxidation reaction
in the absence of Cl-contacting reactants (Figure S18).
Although it cannot be completely excluded that CO was
formed in tiny amounts, which are below the detection limit,
the main unselective pathway of C2H4 conversion is its
oxidation to CO2. CO and CO2 were observed when EDC
reacted with the catalyst in the absence of gas-phase O2. It is
also possible that these carbon oxides originated from VCM.
Thus, both ethylene and its chlorinated products are
responsible for the formation of COx in the course of ethylene
oxychlorination. In addition, lattice oxygen species appear to
actively participate in these undesired processes.

4. CONCLUSIONS
The combination of computational analysis by the application
of density functional theory with steady-state experiments and
investigation of transient kinetics by temporal analysis of
products revealed the mechanism of ethylene oxychlorination
on CeO2. Experimental results suggest that the catalytic
material consists of an oxide, supporting a surface oxychloride
during reaction conditions. In addition, surface alterations by
formation and chlorination of vacancies are an important factor
determining the selectivity. While empty vacancies decrease the
selectivity to EDC or VCM, facilitating their further transformation to DCE, chlorinated vacancies enable the formation
of EDC in the first place, as evidenced by an increase of
selectivity to desired products with higher surface chlorination.
Different facets of ceria ((111) and (100)) did not show
significant performance differences except for lower activity of
the polar (100) surface, which was related to lower overall
surface area and higher chlorination (poisoning) of the sample.
Thus, the most stable (111) surface of CeO2 with one vacancy

■

ASSOCIATED CONTENT

S
* Supporting Information

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.7b04431.
Scheme of oxychlorination setup, additional characterization and catalytic data, optimized geometries of
adsorbed species, and initial, transition state, and final
geometries of elemental steps, additional reaction
profiles, and additional TAP data (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail for N.L.: nlopez@iciq.es.
*E-mail for J.P.-R.: jpr@chem.ethz.ch.
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Marçal Capdevila-Cortada: 0000-0003-4391-6580
Alessandro Trovarelli: 0000-0002-1396-4031
Núria López: 0000-0001-9150-5941
Javier Pérez-Ramírez: 0000-0002-5805-7355
Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS
This work was sponsored by Swiss National Science
Foundation (project no. 200021−156107). The authors
acknowledge Evgeniya Vorobjeva for HRTEM microscopy.

■

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