Mechanism of HCl oxidation (Deacon process) over RuO2

Article abstract of DOI:10.1016/j.jcat.2008.01.020

The catalytic oxidation of hydrogen chloride to chlorine (Deacon reaction) is an attractive means to recover Cl₂ from HCl-containing waste streams in the chemical industry. Recently, RuO₂ supported on TiO₂ rutile has been industrially implemented for large-scale chlorine manufacture. However, detailed understanding of the reaction mechanism over claimed catalysts has not been acquired. Our tests in a fixed-bed reactor at ambient pressure concluded that RuO₂ powder is highly active for Cl₂ production. Characterization of the fresh and used RuO₂ samples by ex situ XRD and XPS revealed no appreciable alteration of the bulk structure and limited chlorination of the catalyst. Ab initio thermodynamics predicted that the initial state of the RuO₂(110) surface is partially over-oxidized, while the surface after reaction contains both oxygen and chlorine. DFT described the Deacon reaction with a Mars-van Krevelen type mechanism consisting of five steps: hydrogen abstraction from HCl, recombination of atomic chlorine, hydroxyl recombination, water desorption, and dissociative oxygen adsorption. An increased chlorine production was observed experimentally when increasing the feed O₂-to-HCl ratio. This can be ascribed both to the lower recombination energy of chlorine atoms to gas-phase Cl₂ at high coverages and the faster surface reoxidation due to higher partial oxygen pressures.

Full text of DOI:10.1016/j.jcat.2008.01.020

Journal of Catalysis 255 (2008) 29–39
www.elsevier.com/locate/jcat
Mechanism of HCl oxidation (Deacon process) over RuO2
a,∗
a
a
a,b,∗
Núria López , Jordi Gómez-Segura , Raimon P. Marín , Javier Pérez-Ramírez
a
Institute of Chemical Research of Catalonia (ICIQ), Avinguda Països Catalans 16, 43007 Tarragona, Spain
b
Catalan Institution for Research and Advanced Studies (ICREA), Passeig Lluís Companys 23, 08010 Barcelona, Spain
Received 20 August 2007; revised 16 January 2008; accepted 21 January 2008
Abstract
The catalytic oxidation of hydrogen chloride to chlorine (Deacon reaction) is an attractive means to recover Cl from HCl-containing waste
2
streams in the chemical industry. Recently, RuO supported on TiO rutile has been industrially implemented for large-scale chlorine manufacture.
2
2
However, detailed understanding of the reaction mechanism over claimed catalysts has not been acquired. Our tests in a fixed-bed reactor at
ambient pressure concluded that RuO powder is highly active for Cl production. Characterization of the fresh and used RuO samples by ex
2
2
2
situ XRD and XPS revealed no appreciable alteration of the bulk structure and limited chlorination of the catalyst. Ab initio thermodynamics
predicted that the initial state of the RuO (110) surface is partially over-oxidized, while the surface after reaction contains both oxygen and
2
chlorine. DFT described the Deacon reaction with a Mars–van Krevelen type mechanism consisting of five steps: hydrogen abstraction from
HCl, recombination of atomic chlorine, hydroxyl recombination, water desorption, and dissociative oxygen adsorption. An increased chlorine
production was observed experimentally when increasing the feed O -to-HCl ratio. This can be ascribed both to the lower recombination energy
2
of chlorine atoms to gas-phase Cl at high coverages and the faster surface reoxidation due to higher partial oxygen pressures.
2
©
2008 Elsevier Inc. All rights reserved.
Keywords: HCl oxidation; Chlorine; Deacon reaction; RuO ; Mechanism; Density Functional Theory; On-line Cl analysis; Ab initio thermodynamics
2
2
1
. Introduction
Elemental chlorine (Cl2) is widely used in the production
the main Cl2 production method, salt electrolysis, is energy in-
tensive and encompasses the disadvantage of producing caustic
soda. Thus, large-scale manufacturers aim at the development
of a chlorine recycling technology due to environmental and
economic reasons.
of a wide range of industrial and consumer products. Approxi-
mately 50% of the chlorine produced worldwide (ca. 50 Mton
per annum) is reduced through its use to HCl or chloride
salts [1]. An example of particular relevance is the phosgenation
of diamines to diisocyanates (Eq. (1)) as the first step to manu-
facture polyurethane resins and rubbers. This reaction leads to
An attractive route for chlorine recovery is the gas-phase
catalytic oxidation of HCl with air or oxygen (Eq. (2)), the
so-called Deacon reaction [2–4], due to the relative ease of ap-
plication and low electrical power and thermal requirements
when compared with electrolytic processes. The reaction is re-
4
mole of HCl per mole of diisocyanate.
0
−1
versible and exothermic (ꢀH = −28.4 kJmol HCl):
R(NH ) + COCl → R(NCO) + 4HCl.
The by-product HCl can be generally marketed as hydrochlo-
ric acid or used as a raw material in vinyl chloride monomer
(1)
2
2
2
2
4
HCl + O2 ↔ 2Cl2 + 2H2O.
(2)
From the various systems protected by Henry Deacon in the
(
VCM) production. However, as the VCM production growth
late 19th century, CuCl supported on pumice was the pre-
2
is slow, excess supply of HCl is foreseen. In these cases, this
highly toxic and corrosive acid must be neutralized. Besides,
ferred one. However, catalytic tests in a fixed-bed reactor at
7
03–773 K evidenced serious drawbacks for implementation:
low single-pass HCl conversion due to limited activity, rapid de-
activation due to volatilization of copper chloride above 673 K,
and corrosion problems due to the presence of unreacted HCl
and product H2O [4]. Many efforts have been undertaken since
*
Corresponding authors. Fax: +34 977 920 224.
E-mail addresses: nlopez@iciq.es (N. López), jperez@iciq.es
(
J. Pérez-Ramírez).
0021-9517/$ – see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2008.01.020

30
N. López et al. / Journal of Catalysis 255 (2008) 29–39
then for developing more efficient systems, culminating with
the Shell-Chlor process (Shell) in the 1960s and the MT-Chlor
process (Mitsui Toatsu Chemicals) in the late 1980s. The Shell-
Chlor process made use of a single fluidized-bed reactor with
a CuCl –KCl/SiO catalyst [5,6], but it was abandoned due
to limited HCl conversion and severe corrosion problems. The
MT-Chlor process utilizes a Cr2O3/SiO2 catalyst in a fluidized-
bed reactor at 623–673 K [7–9]. Presently, Mitsui Toatsu is
operating a 50 kton per annum plant based on the MT-Chlor
process at Omuta, Japan. In the late 1990s, Totsis et al. [10,11]
developed a dual fluidized-bed reactor system using copper-
based catalysts in order to achieve high HCl conversion to Cl2
with minimal corrosion. In the same line as Totsis et al., Nieken
and Watzenberger [12] demonstrated that periodic operation of
the Deacon process in a two-step fixed-bed reactor can over-
come corrosion problems.
Herein we have used experiments and Density Functional
Theory (DFT) to study the mechanism of HCl oxidation to Cl2
over RuO2 surfaces. Previous to the simulations, evidence that
polycrystalline RuO2 powder is active in the Deacon reaction
has been obtained at different feed HCl:O2 ratios by means of
catalytic tests in a continuous fixed-bed reactor at ambient pres-
sure. A reaction mechanism comprising five fundamental steps
has been identified and characterized. In particular, the effect of
surface coverage and the nature of the active oxygen species on
RuO2 have been analyzed. The fresh and used RuO2 samples
have been characterized in order to assess eventual changes in
the bulk and surface of the catalyst upon reaction.
2
2
2
. Experimental
2
.1. Catalyst characterization
A wider scope for industrialization of the one-step hy-
drochloric acid oxidation process has been realized through
the development by Sumitomo Chemicals of a process using
Ruthenium(IV) oxide, RuO2, was purchased from Sigma-
Aldrich. Prior to characterization and catalytic tests, the as-
received sample was heated in static air from 298 to 773 K
a RuO supported on TiO rutile catalyst in a fixed-bed re-
2
2
−
1
at 5 K min and kept isothermal for 5 h. Powder X-ray dif-
fraction (XRD) was measured in transmission using a Bruker
AXS D8 Advance diffractometer equipped with a Cu tube,
a Ge(111) incident beam monochromator (λ = 0.1541 nm), and
a Vantec-1 PSD. Data were recorded in the 2θ range of 5 to
actor configuration [13]. Not surprisingly, RuO2–TiO2 coated
titanium anodes are industrially used for chlorine evolution in
NaCl electrolytic cells [14,15]. The RuO2/TiO2 catalyst ex-
hibits high activity at low temperature and remarkable stabil-
ity, leading to Cl with higher purity than that obtained by
2
◦
◦
7
2
0 with an angular step size of 0.016 and a counting time of
NaCl electrolysis. Besides, the chlorine manufacturing cost is
claimed to be much lower than that of electrolysis and the
MT-Chlor process due to the low electric power consumption
and efficient heat recovery from the reaction. The Sumitomo
process can produce up to 400 kton of chlorine per annum in a
single reactor [16].
The vast number of patents claiming Deacon-type catalysts
and reactors contrasts with the scarce number of fundamental
studies dealing with mechanistic and kinetic aspects of the re-
action. Hisham and Benson [17] studied the basic thermochem-
istry of the Deacon reaction over a number of metal oxides. The
process was examined in two steps: (1) HCl absorption by the
metal oxide to form the metal (oxy)chloride and (2) the oxida-
.4 s per step. X-ray photoelectron spectroscopy (XPS) analy-
ses were obtained using a PHI 5500 Multitechnique System
from Physical Electronics equipped with an ultrahigh vacuum
−
9
−8
chamber (pressure between 5 × 10 and 2 × 10 Torr) and
monochromatic AlKα radiation at 1486.6 eV as the X-ray
source. Nitrogen adsorption–desorption isotherms at 77 K were
measured on a Quantachrome Autosorb 1MP analyzer. Before
analysis, the sample was degassed in vacuum at 393 K for 12 h.
The BET method [33] was applied to determine the total sur-
face area.
2
.2. Activity tests
tion of the latter by O to regenerate the metal oxide and free
2
The catalytic oxidation of HCl over RuO2 was studied in a
Cl2. CuO was concluded as the only system fulfilling the re-
quirements of a complete cycle below 700 K, although RuO2
was not included in the study. Exceptionally, Aglulin [18] in-
vestigated the kinetics of HCl oxidation over supported Cr2O3
catalysts in the presence of methane, although some of the steps
in the hypothesized mechanism cannot be considered as ele-
mentary.
Consequently, deeper mechanistic studies of the Deacon re-
action over ruthenium oxide are required in order to better
understand how the industrial catalyst works. This informa-
tion can also be beneficial for further catalyst optimization.
quartz fixed-bed reactor (8 mm i.d.) using 600 mg of sample
(
sieve fraction 200–300 µm), a total pressure of 1 bar, and a
−1 −1
.
weight-hourly space velocity (WHSV) was 16,600 ml g
h
The feed mixture contained 20 vol% HCl, 20–80 vol% O2, and
N2 as the balance gas. Hydrogen chloride (Praxair, purity 3.0
and water content <50 ppm), oxygen (Carburos Metálicos, pu-
rity 3.5), and nitrogen (Carburos Metálicos, purity 3.5) were
used without further purification. The individual gases were
introduced to the reactor by means of digital mass-flow con-
trollers. The material of all the lines in the set-up was Teflon®
in order to prevent corrosion problems, particularly downstream
of the reactor. Two protocols were applied for catalytic eval-
uation. Temperature-programmed reaction (TPR) was carried
out by ramping the temperature of the reactor block (Tfurnace)
RuO has received considerable attention over the past years
2
as model system for surface science and computational stud-
ies, and also as heterogeneous catalyst in oxidation of various
substrates, among others carbon monoxide [19–23], hydrogen
−
1
from 333 to 583 K at 10 K min in the reaction mixture. The
catalyst was pretreated in a 40 vol% O in N at 333 K for
[
24–26], ammonia [27], methane [28], alcohols [21,29–31], and
2
2
soot [32].
20 min prior to HCl admission. Isothermal tests were conducted

N. López et al. / Journal of Catalysis 255 (2008) 29–39
31
Fig. 1. Ball and stick model of different RuO surfaces. In the upper panel, oxygen and ruthenium atoms are represented by red and grey spheres, respectively. In
2
the lower panel the notation for the RuO (110) surface is presented in a p(2 × 1) supercell. (For interpretation of the references to color in this figure legend, the
2
reader is referred to the web version of this article.)
at Tfurnace = 573 K using molar HCl:O2 ratios in the feed of
:1, 1:2, 1:3, and 1:4. The temperature was ramped from 333
c = 3.140 Å and u = 0.305) are in good agreement with the ex-
perimentally determined values using single-crystal X-ray dif-
fraction [39]. The good accuracy of a similar approach when
compared to all electron calculations was tested in Ref. [40].
In a first step, the low-index (110), (101), (001), and (100)
surfaces were modeled (Fig. 1). The slabs contained at least
1
−
1
to 573 K at 10 K min in 20 vol% O2 in N2 and HCl was
subsequently introduced in the feed. The HCl:O2 ratio was var-
ied by increasing the inlet O2 concentration and the inlet HCl
concentration was fixed at 20 vol%. The flow of N2 was ad-
−
1
justed in order to keep a constant total flow (10 l STP h ).
During the tests, the catalyst bed temperature was continuously
recorded with a Fluke Digital Multimeter (model 187) equipped
with a K-type thermocouple.
5 layers of RuO and the vacuum space between the different
2
slabs was set to >10 Å, with a k-point sampling density of at
−
1
least 0.4 Å . The outermost 2 layers in the slabs were allowed
to relax. The surface energies, calculated for the stoichiometric
termination of each of the facets, were in agreement with pre-
vious estimates [41], leading to (110) as the most stable surface
Chlorine analysis was carried out using an Ocean Optics
miniature fiber optic spectrophotometer (model USB2000-UV-
VIS). Spectra were collected in the broad wavelength range of
−
2
with γ = 0.90 Jm
.
2
00–1000 nm every 10 s using a DT-MINI-2-GS Deuterium
The clean RuO (110) structure consists on alternating rows
2
Tungsten Halogen Light Source and a high-sensitivity Sony
ILX511 2048-element linear silicon CCD-array detector up-
graded for working in the UV spectral region. On-line Cl2
detection via optical absorbance measurements was achieved
by the continuous flow of product gases through a Z-flow cell
adopted as flow injection analysis (FIA) type assembly having
a 10-mm optical path length (FIAlab instruments). Water pro-
duced in the reaction was condensed using a reflux unit prior
of protruding oxygen atoms, usually denoted as O (bridge oxy-
gen) and rows of coordinatively unsaturated ruthenium atoms,
Rucus, along the [001] direction. For mechanistic studies, the
b
(110) surface of RuO has been represented by a 3-layer slab
2
separated by 15.7 Å, where each of the layers contains an
ORuO O sub-unit. Relaxation was only allowed in the two
2
first layers of the slab keeping the lower layer fixed. In order
to avoid spurious electrostatic interactions, the dipole arising
from the formation of an asymmetric surface was eliminated
from the vacuum. To study adsorption and reaction processes
the supercell employed was a p(2 × 1) reconstruction, lead-
to the detection cell. The carrier gas (N ) provided a reference
2
spectrum in optical absorbance processing.
3
. Computational details
ing to a coverage by H, Cl, or O of 0.50 ML. Tests with a
cus
larger supercell p(4 × 1) were performed to study the atomic
Density Functional Theory (DFT) was applied to study the
chlorine recombination to gas-phase Cl2 and the O2 adsorption.
A Monkhorst–Pack [42] mesh was employed to perform the nu-
merical integration; the meshes contain 4 × 4 × 1 points for the
small supercell and reduced to 2 × 4 × 1 for the larger cell. The
barriers of relevant elementary steps have been calculated using
the CI-NEB [43].
mechanism of HCl oxidation to Cl2 on RuO2. Calculations
have been performed with the VASP code [34,35], employing
the RPBE functional [36], the inner electrons have been rep-
resented by PAW pseudopotentials [37], and the valence mono-
electronic states have been expanded in plane-waves with a cut-
off energy of 400 eV. RuO has a rutile structure [38] and is con-
sequently isostructural to the most stable TiO2 polymorph. The
unit cell parameters obtained with our set up (a = b = 4.569 Å,
RuO2 is a partially covalent oxide and thus formal charges
on Ru and O differ from the real charges of the atoms. RuO2
is thus reducible via O2 elimination but also the surface can be
2

32
N. López et al. / Journal of Catalysis 255 (2008) 29–39
Fig. 3. Absorbance due to Cl production in isothermal tests over RuO at
2
2
different molar HCl:O ratios. Conditions: feed mixture of 20 vol% HCl and
2
Fig. 2. Absorbance due to Cl production during temperature-programmed re-
2
−1
2
0–80 vol% O in N , T
= 573 K, GHSV = 16,600 h , P = 1 bar.
2
2
furnace
action of HCl with O over RuO . Conditions: feed mixture of 20 vol% HCl
2
2
−
1
and 40 vol% O in N , GHSV = 16,600 h , P = 1 bar. Inset: UV–vis spec-
2
2
feed HCl:O2 ratio of 1:2 and atmospheric pressure. The onset
temperature is around 500 K and the absorbance progressively
increases until reaching a plateau around 625 K with a value of
absorbance of 0.35. Acquisition data for absorption spectrum
using the combined-spectrum source ranging between 200–
410 nm (deuterium lamp) and 360–1000 nm (tungsten halogen
lamp) is unambiguously indicative of the selective detection of
pure chlorine in the UV spectral region, exhibiting the maxi-
mum absorbance at 330 nm (inset in Fig. 2) [48–50]. The satu-
ration of the detector was determined at 1.1 units of absorbance,
below which a linear relationship between the absorbance and
the Cl2 concentration was observed.
The influence of the HCl:O2 ratio on the Deacon activity of
RuO2 was investigated at Tfurnace = 573 K. As shown in Fig. 3,
the absorbance increases upon increasing the relative amount
of oxygen in the feed mixture. Interruption of O2 causes a
rapid drop of absorbance due to the diminished Cl2 produc-
tion. The beneficial effect of oxygen on the Cl2 production is
further discussed in Section 4.3 attending to simulation results
at different oxygen coverages. The single-pass HCl conversion
in the isothermal experiment at HCl:O2 = 1:2 was quantified
at ca. 70% by classical titration. To this end, the Cl2 concentra-
tion of the outlet gas was determined after passing through two
serial impingers over 10 min, each of which equipped with a
porous frit immersed into an aqueous solution of potassium io-
dide (20–30 wt%). The iodine thus obtained was titrated with
sodium thiosulfate (0.1 M).
trum at a particular temperature, showing the Cl band centered at 330 nm.
2
further oxidized in the Rucus centers. Under relevant pressures,
the surface Rucus centers on RuO2(110) can be partially cov-
ered [44]. We have employed ab initio thermodynamics [45,46]
to obtain the most likely composition of the RuO2(110) surface
at 573 K in the presence of O2 and Cl2. The thermodynamic
parameters for gas-phase oxygen or chlorine were taken from
the NIST reference tables [47]. In the initial state, only O2 is
in contact with the surface, thus several Ob and Ocus cover-
ages were calculated in a p(4 × 1) supercell and represented
as a function of the oxygen pressure. To represent the state of
the surface after reaction, a p(2 × 1) supercell was employed
where both O and Ru positions can be occupied by either
b
cus
O or Cl. In fact, Ob substitution by Cl is energetically possi-
ble, the reaction being exothermic by −0.15 eV. Moreover, the
Clb containing structure constitutes an energy minimum that
can be characterized by the lack of imaginary vibrational fre-
quencies. The electronic structure of Ru–Clb–Ru can be seen
as a three center-three electron bond with partially oxidized Ru
atoms although with a different atomic charge than that of the
clean RuO . This is possible due to the redox character of Ru
2
that can accept formal charges of +2, +3, and +4. In contrast,
double Cl substitution (i.e. 2Cl in the position of Ob) is an en-
dothermic process (by 2 eV) and thus not likely. This is similar
to what has been shown for the phase diagram of RuO2 in the
presence of CO and O, where CO and O can occupy both bridge
and cus positions [44]. The large number of configurations aris-
ing (21 for the p(2 × 1) supercell) when compared to the case
of oxygen (5 for p(4 × 1)) prevented us from using a more re-
alistic supercell.
Consequently, the high activity of RuO2-based catalysts in
the Deacon process claimed by Sumitomo [13] can be con-
firmed on the basis of the low onset temperature measured
in temperature-programmed reaction experiments and the rela-
tively high HCl conversion determined in steady-state tests over
unsupported ruthenium oxide.
4
. Results and discussion
4
.1. Deacon performance of RuO powder
4.2. Characterization of fresh and used RuO2
2
Fig. 2 shows the absorbance profile due to Cl2 production
The XRD pattern of the fresh RuO in Fig. 4 shows the char-
2
vs the temperature of the catalyst bed during a temperature-
acteristic reflections of ruthenium(IV) oxide (JCPDS 40-1290),
programmed reaction experiment over RuO2 powder at a molar
with (110) and (101) as the prevailing primary orientations.

N. López et al. / Journal of Catalysis 255 (2008) 29–39
33
Fig. 5. Ab initio thermodynamics diagram of the surface composition of
RuO (110) as a function of the O pressure at 573 K.
2
2
Fig. 4. X-ray diffraction patterns of the fresh and used RuO samples. Inset:
2
Wulff construction of the powder with the calculated surface energies for the
facets in Fig. 1.
nium oxychloride such as RuOCl2 was observed after reaction.
This indicates that the bulk of the catalyst was not altered by
the Deacon test. The BET surface area of the fresh catalyst
We have performed calculations on the (110), (100), (101), and
2
−1
2
−1
(
001) facets of RuO2 (see Fig. 1). The (110) facet is the lowest
(10 m g ) decreased to 2 m g in the used one, suggesting
in energy and thus found in the largest extension when em-
ploying the Wulff construction [51] to build the structure of
a nanoparticle (inset of Fig. 4). Moreover, in the TiO2 rutile
used as the support for RuO2 in the Sumitomo catalyst [13,16],
the (110) surface is the most stable too [52]. Accordingly, fur-
ther DFT mechanistic studies concentrated on the RuO2(110)
surface. The choice of the (110) facet for calculations and
the polycrystalline sample for catalytic tests can be justified.
Temperature-programmed reaction studies over RuO2(110) in
UHV and polycrystalline RuO2 at high-pressure conditions are
virtually identical [21], indicating that the reaction pathways
are very much alike and therefore the active centers on RuO2
are the same. In addition, it is also known that the efficiency
for CO oxidation hardly depends on the orientation of RuO2
some degree of sintering of the RuO particles. X-ray pho-
toelectron spectroscopy was conducted in order to investigate
changes in the nature of the ruthenium species and to deter-
mine the extent of chlorination of the surface. The core level
peaks of Ru 3d, O 1s, and Cl 2p are illustrated in Fig. 6 for the
2
used RuO catalyst, i.e. after Deacon reaction. The binding en-
2
ergy of ruthenium and oxygen (Ru 3d5/2 at 281 eV, Ru 3d3/2 at
285 eV, and O 1s at 529 eV) is characteristic of polycrystalline
RuO [21] and was identical in the fresh and used samples. This
2
suggests that the Deacon reaction does not induce significant
changes in the oxidation state of surface ruthenium sites in the
tested period. The distinctive feature of the fresh and used RuO2
specimens was the presence of chlorine in the latter sample, as
shown in Fig. 6 by the Cl 2p3/2 (198 eV) and Cl 2p1/2 (199.5 eV)
peaks. This demonstrates a certain degree of surface chlorina-
tion caused by HCl oxidation, although we cannot conclusively
distinguish the nature of chlorine, i.e. whether it is as molecu-
lar chlorine solvated, hydrochloric acid solvated, or in the form
of a surface oxychloride species. In any case, the minor amount
of chlorine is limited to the catalyst surface. Accordingly, the
use of an oxide, rather than a chlorinated compound, for DFT
modelling is justified. In line with XPS analysis, application
of other characterization methods such as far-infrared and Ra-
man spectroscopies, temperature-programmed desorption cou-
pled to mass spectrometry, and chemical analysis detected vir-
[
53], at least not when the (110), (100), and (101) facets are
compared, since all three low-energy surfaces contain five-fold
coordinated ruthenium atoms, Rucus (see Fig. 1). In addition,
the results from ab initio thermodynamics shown in Fig. 5 indi-
cate that under the experimental conditions, a partial occupation
of the Rucus atoms on the surface is likely. The state of the ini-
tial active surface in the presence of the gas-phase O2 pressures
contains all the Ob sites occupied and coverage of Ocus in the
range of 0.25–0.75 ML.
The used RuO2 sample resulting from temperature-pro-
grammed reaction experiments was characterized ex situ in or-
der to assess changes due to the reaction. It should be recalled
tually no chlorine in the used RuO catalyst. The practically
2
−
1
that the catalyst was subjected to a ramp of 5 K min from 333
to 583 K in a mixture of 20 vol% HCl, 40 vol% O2, and 40 vol%
N2 and the final temperature was kept for 1 h. Thereafter, the re-
actor was rapidly cooled down to room temperature in N2 flow.
The X-ray diffraction pattern of the used sample did not expe-
rience apparent changes with respect to the fresh one (Fig. 4).
The position and relative intensity of the reflections of the RuO2
phase were very similar and in particular no crystalline ruthe-
unaltered state of RuO after the Deacon reaction highlights the
2
stability of this catalyst and likely explains its successful im-
plementation as compared to other oxides based on copper and
chromium, where extensive volatilization upon formation of the
corresponding (oxy)chlorides has been observed. Future work
will consider application of operando spectroscopic techniques
to monitor the Deacon reaction. This approach proved success-
ful to assess the chlorination at both surface and bulk levels of

34
N. López et al. / Journal of Catalysis 255 (2008) 29–39
Fig. 6. XPS spectra of Ru 3d (left), O 1s (center) and Cl 2p (right) core level electrons of the RuO catalyst used in the Deacon reaction.
2
lanthanide oxide-based catalysts during the destruction of CCl4
in the presence of steam [54].
have studied the reaction network at distinct degrees of surface
oxidation and chlorination.
For the used catalyst, the RuO2(110) structure shall be in
equilibrium with both Cl2 and O2. In this case the ab initio
thermodynamics are much more complex, the configurations
included in the search are reported in top Fig. 7 while the energy
diagram is shown in bottom Fig. 7. The most likely configura-
tions at relevant O2 and Cl2 pressures contain bridging sites
occupied mostly with oxygen, the complete substitution of Ob
atoms by Clb is energetically unfavored (by more than 1 eV
with respect to the clean surface), thus suggesting that com-
plete chlorination of the surface is indeed unlikely. The Rucus
sites are also partially occupied, however, in this position Cl
is more likely than O. Although partial substitution of Ob by
Cl is energetically favored under equilibrium conditions, care
must be taken since the elimination of Ob from the surface
is highly endothermic (1.95 eV with respect to gas-phase O2)
and the kinetics of replacement can not compete with other
less energy demanding processes like O2 desorption from Rucus
sites.
HCl + O* + * ↔ OH* + Cl*,
Cl* + Cl* ↔ Cl2 + 2*,
OH* + OH* ↔ H2O* + O*,
H2O* ↔ H2O + *,
(3)
(4)
(5)
(6)
(7)
O2 + 2* ↔ 2O*.
4
.3.1. Hydrogen abstraction from hydrogen chloride
The first step in the mechanism is the hydrogen abstraction
from HCl leading to adsorbed atomic chlorine and hydroxyl
species (Eq. (3)). Three dissociation scenarios were investi-
gated by using different oxygen coverages (Fig. 8). For the
regular (110) surface, the bridge oxygen (Ob) can act as the ba-
sic center to dissociate hydrogen chloride. In the initial state,
the HCl molecule is bonded through electrostatic interaction
to Ob. This adsorbed state is determined −0.48 eV below the
gas-phase reactants. For this structure, the relevant geomet-
ric parameters are d(Ob–H) = 1.780, d(H–Cl) = 1.358, and
d(Cl–Rucus) = 2.668 Å. The H–Cl distance is already longer
than that in the gas-phase molecule, i.e. d(H–Cl) = 1.274 Å.
Dissociation of the HCl is exothermic by −0.98 eV and leads
4
.3. Reaction mechanism
The mechanism of the overall Deacon reaction (Eq. (2))
on RuO2(110) can be structured in five steps: hydrogen ab-
straction from hydrogen chloride by adsorbed atomic oxygen
to a bridging hydroxyl group (O H) and a chlorine atom ad-
sorbed on the Rucus position (denoted as Clcus). The geometric
parameters in the final state are d(Ob–H) = 0.992, d(H–Cl)
b
(
Eq. (3)), recombination of surface chlorine atoms and des-
=
2.278, and d(Cl–Rucus) = 2.361 Å. At the transition state,
orption as gas-phase Cl2 (Eq. (4)), recombination of surface
hydroxyl groups (Eq. (5)), water desorption (Eq. (6)), and dis-
sociative oxygen adsorption for surface regeneration (Eq. (7)).
Although the above reaction scheme appears relatively simple,
the complexity of the real process is large due to the distinct
nature of the oxygen species on the surface and the different
oxygen and chlorine coverage. Accordingly, each of the simpli-
fied steps in the reaction can occur in different scenarios. For
instance, O* in Eqs. (3) and (5) can be either Ocus or Ob and
each case has to be computed separately. In consequence, we
Ea = 0.05 eV, the hydrogen atom is midway between the oxy-
gen and the chlorine atoms, i.e. d(Ob–H) = 1.702, d(H–Cl) =
1
.371, and d(Cl–Rucus) = 2.672 Å. The basic features of this
elementary step are not affected by the presence of Clcus atoms
in the Ru neighboring to the Rucus active site, top right Fig. 8.
The HCl adsorption is reduced to −0.12 eV and dissociation is
slightly less exothermic (−0.93 eV) compared to the clean sur-
face, while the dissociation barrier is even smaller than 0.05 eV.
When a partially O-covered Rucus exists on the surface (see
bottom Fig. 8), an alternative reaction path is feasible. In the

N. López et al. / Journal of Catalysis 255 (2008) 29–39
35
Fig. 7. Schematic models included in the ab initio thermodynamics diagram of the surface composition of RuO (110) as a function of the partial O and Cl
2
2
2
pressures at 573 K.
initial configuration, HCl can point towards the Ocus atom lead-
ing to a hydroxyl group on a Rucus site (Ocus–H) and a chlorine
atom on a neighboring Rucus site (Clcus). The initial state is
stabilized by −0.47 eV with respect to the gas-phase molecule
and the relevant distances are d(Ocus–H) = 1.659, d(H–Cl) =
RuO2(110)-Ocus or Clcus and thus can easily occur at any con-
dition studied here. In good correspondence, Hisham and Ben-
son [17] concluded that the chlorination step is fast over most
single oxides.
1
.356, and d(Cl–Rucus) = 2.641 Å. The Ocus-assisted H ab-
4
.3.2. Chlorine recombination
straction is exothermic by −0.75 eV, slightly smaller than the
Ob-assisted H abstraction (−0.98 eV). In the final state, the
d(Ocus–H) = 0.994 and d(Cl–Rucus) = 2.358 Å. The transition
state is 0.11 eV over reactants and the distances are d(Ocus–H)
Recombination of adsorbed atomic chlorine (Eq. (4)) has
been studied for several conditions (Fig. 9). First, a regular sur-
face with a p(2 × 1) unit cell was employed with both Ru
cus
sites covered by Cl . However, a chlorine atom can substi-
cus
=
1.684, d(H–Cl) = 1.351, and d(Rucus–Cl) = 2.640 Å.
tute an O in the RuO structure (Cl ), being able to recombine
with Clcus atom at a Rucus site. The recombination of two Clcus
starts by the approach of one of the chlorine atoms to the sec-
b
2
b
In summary, the barriers for hydrogen abstraction from
HCl are small over both p(2 × 1)-RuO2(110) and p(2 × 1)-

36
N. López et al. / Journal of Catalysis 255 (2008) 29–39
ond followed by evolution of gas-phase Cl2. The direct process
the latter process is not feasible under the experimental condi-
tions applied in the experiments.
d
is endothermic by E = 1.56 eV on the clean surface. The Clcus
+
Clb recombination is much more endothermic, 3.20 eV. Thus
For the Clcus + Clcus reaction, the effect of excess Ocus
and Clcus on the neighboring Rucus sites has been studied in
the p(4 × 1) supercell varying the amount of extra Xcus =
Ocus and/or Clcus on the surface: θX = 0, 0.25, and 0.50 ML
cus
(
Fig. 10). For the clean RuO2(110) surface, a desorption energy
d
E = 1.80 eV was found. The presence of Ocus (Clcus) signifi-
cantly lowered the energy required for Cl + Cl recombination,
from 1.80 eV at θO = 0 to 1.26 eV at θO = 0.50 ML or
cus
cus
1
.37 eV at final θClcus = 0.50 ML. Thus the endothermicity of
the reaction can be largely affected by the quality of the surface,
i.e. the degree of oxidation or chlorination. This implies a pos-
itive effect of Ocus and/or Clcus on Cl2 production due to easier
chlorine recombination. This fact might (at least partially) ac-
count for the increased Cl2 production at higher O2 pressures
in Fig. 3.
Regarding technical issues, Cl2 recombination calculated in
the small p(2×1) supercell (open circle at θO = 0 in Fig. 10)
cus
d
leads a value of E = 1.56 eV. This is due to the presence of
spurious lateral repulsions in this cell and indicates that one
should take precautions when using small supercells to study
this process. This is critical here, as chlorine recombination is
the most energetically demanding step in the proposed Deacon
scheme. Finally, the absolute values for adsorption/desorption
energies are known to be affected by systematic errors due to
inaccuracies in the functional [36]. However, the dependence
of the recombination energy with the total coverage found in
Fig. 10 should be seen as a derivative of the energy and is thus
less affected by systematic errors. In conclusion, the trend in
Fig. 10 reflects a physical situation.
4
.3.3. Water formation
Fig. 8. Schematic representation of the initial and final states for hydrogen ab-
straction from HCl on RuO (110) at two different oxygen/chlorine coverages.
2
H2O is formed by recombination of surface hydroxyl groups
followed by desorption (Eqs. (5) and (6)). Firstly, H is trans-
Fig. 9. Schematic representation of the initial states for atomic chlorine recombination on RuO (110). In this case a p(4 × 1) cell is employed and three different
2
extra O coverages.

N. López et al. / Journal of Catalysis 255 (2008) 29–39
37
ferred from ObH to Ocus. In a second step, hydroxyl recombi-
nation was studied. These processes are visualized in Fig. 11.
Since Ob atoms are difficult to remove from the RuO2(110) sur-
face, recombination has only been studied as ObH + OcusH
to yield Ob and a water molecule sitting at the Rucus site fol-
lowed by water desorption. The same process can also take
place between two hydroxyl groups on the Rucus sites, hydroxyl
recombination and water desorption was also examined for this
case (vide infra).
gen and oxygen atoms are d(Ob–H) = 1.264 and d(Ocus–H) =
1.210 Å, respectively. The Ocus–Rucus bond is distorted to assist
hydrogen transfer and the corresponding distance is 1.849 Å.
At the final state, the d(Ob–H) is enlarged to 2.453 Å and the
hydroxyl group distance is d(Ocus–H) = 0.978 Å, while the
bonding distance in the Rucus–OcusH group is 1.942 Å. If chlo-
rine is present in the neighboring Rucus site the energy for the
reaction is −0.03 eV while the barrier is 0.48 eV; thus the effect
of Clcus is very small.
Hydrogen transfer from the Ob–H to the Ocus site is only
exothermic by −0.01 eV. At the initial state the d(Ob–H) =
The second hydrogen transfer can be described as follows:
Ob–H + Ocus–H → Ob + H2Ocus (Fig. 11, bottom). The en-
ergy at the transition state is 0.38 eV higher than reactants and
the overall process is slightly endothermic, 0.27 eV. In the final
structure a water molecule is sitting at a Rucus site. The desorp-
tion energy of the water molecule is 0.67 eV. If a Clcus atom is
present in the neighboring Rucus site the energies are slightly af-
fected, the hydrogen transfer process is exothermic by 0.16 eV
due to the formation of a Cl–H2O interaction in the final state,
and the barrier is lowered to 0.08 eV. However, this is compen-
sated by the largest desorption energy of water, 0.90 eV. The
recombination of two hydroxyl groups in neighboring Rucus
sites (Ocus–H + Ocus–H → Ocus + H2Ocus) can be also con-
sidered (not shown in Fig. 11). This may correspond to the high
oxygen coverage regime. At the initial state the H–Ocus distance
is 1.878 Å while at the transition state the H atom is placed at
0
.982 Å, i.e. typical from a hydroxyl group. The d(Ocus–H) =
.149 Å and the d(Ocus–Rucus) = 1.752 Å. The hydrogen trans-
2
fer is hindered by a barrier of 0.55 eV (0.56 eV for the inverse
barrier). At the transition state, the distances between the hydro-
1
.287 and 1.183 Å from both Ocus atoms. The transition state is
only 0.24 eV higher in energy than reactants and the process
is exothermic by −0.11 eV. Water desorption in this case is
endothermic by 0.90 eV. The larger endothermicity found for
water desorption is associated with the formation of hydrogen
bonds. Our results leading to small barriers for hydroxyl re-
combination are in agreement with previous calculations and
experimental results [24–26].
Fig. 10. Recombination energy of chlorine atoms to gas-phase Cl as a function
2
of the coverage Xcus = O, Cl on RuO (110).
2
Fig. 11. Schematic representation of the initial and final states for hydrogen transfer for water formation at the Rucus site on RuO (110). Top: O –H + Ocus → O
2
b
b
+
OcusH; and bottom O –H + OcusH → O + H Ocus.
b
b
2

38
N. López et al. / Journal of Catalysis 255 (2008) 29–39
HCl molecule and as a hydroxyl reservoir in water formation.
However, structural oxygen in ruthenium oxide (Ob) cannot
be removed as water (H2Ob) and therefore cannot oxidize by
itself hydrogen chloride. In contrast, Ocus can act as proton
scavenger, forming hydroxyl groups on the surface and water
molecules. The binding energy of the latter is low enough so
they can desorb under reaction conditions.
5
. Conclusions
DFT calculations in combination with experiments have
Fig. 12. Schematic representation of the initial and final states for O dissocia-
2
been used to study the HCl oxidation to Cl2 (Deacon process)
over RuO2. Activity tests in a fixed-bed reactor at ambient pres-
sure demonstrated that RuO2 powder is a highly active catalyst
for Cl2 production. Characterization of the fresh and used RuO2
revealed no appreciable alteration of the bulk structure and lim-
ited chlorination of the sample, in agreement with ab initio
thermodynamics. The Deacon reaction has been described by
a Mars–van Krevelen type reaction mechanism consisting of
five elementary steps: hydrogen abstraction from HCl, recom-
bination and desorption of atomic chlorine, hydroxyl recombi-
nation, water desorption, and oxygen adsorption. Atomic chlo-
rine recombination to yield gas-phase Cl2 is the most energy-
demanding step. In addition, surface reoxidation can be blocked
by high surface coverages. Reaction conditions (mainly partial
pressures and temperature) will determine whether the rate-
determining step of the Deacon process is chlorine recombina-
tion or oxygen readsorption. Further clarification of this aspect
requires more specific mechanistic and kinetic studies. Finally,
we have experimentally observed a larger Cl2 production upon
increasing the feed O2:HCl ratio. Based on our calculations, this
trend is tentatively attributed to both the increase of O coverage
on the surface at higher partial O2 pressures and the reduced
energy for chlorine formation at high coverages.
tion on RuO (110).
2
4
.3.4. Surface reoxidation
In order to complete the catalytic cycle, the surface Ocus
atoms on the surface need to be regenerated by dissociative
adsorption of gas-phase O2 (Eq. (7)). This is similar to what
happens for CO oxidation on this surface and has been quoted
as Mars–van Krevelen mechanism [19]. We have experimen-
tally observed that the Cl2 production fades away when no O2
is available in the feed gas (see Fig. 3). The O2 dissociation
on the Rucus sites leading to two Ocus atoms has been studied.
The O2 molecule has been adsorbed on different configurations
over the Rucus rows of the RuO2 surface. The O2cus tilted is
adsorbed by −0.47 eV and can be considered as the precursor
for dissociation (Fig. 12, left). In the O2cus configuration, the
−
O–O distance is 1.307 Å, thus close to a superoxo O species,
2
the shortest O–Rucus distance is 2.044 and the longest 2.498 Å,
from the second O to the free Rucus. The transition state struc-
ture is located 0.38 eV higher in energy than the initial state.
In the structure, the O–Rucus distance is 1.891 Å while the sec-
ond O–Rucus is 1.883 Å and the O–O distance is enlarged to
1
.670 Å. The final structure is 0.76 eV more stable than the
O2cus and consists in 2Ocus. The effect of surface coverage has
been explored in a similar way as for Cl2 formation, i.e. in a
p(4 × 1) supercell and with various Clcus and Ocus coverages.
The adsorption energy of the precursor state O2cus molecule
is reduced with respect to the clean surface, < −0.20 eV. The
transition state lays about 0.4 eV higher in energy than the pre-
cursor state, while the final state is about −0.65 eV lower in
energy than the reactants. The surface coverage effect on O2
dissociation is to reduce the binding energy of the precursor to
the surface.
Finally, in the reaction network, O2 readsorption is the only
step requiring two empty Rucus sites. As we have seen in the
computational characterization of the surface (Section 3), two
neighboring empty Rucus sites might not be abundant at high
Cl2 and O2 pressures. Consequently, surface oxidation may be-
come a difficult step in the reaction. This latter aspect might be
partially responsible for the dependence observed between the
Cl2 production over RuO2 and the partial O2 pressure in the
feed mixture (Fig. 3).
Acknowledgments
Financial support from the Spanish MEC (projects CTQ-
2
006-01562/PPQ, CTQ2006-00464BQU, and Consolider-
Ingenio 2010, grant CSD2006-003) and the Generalitat de
Catalunya is acknowledged. The authors thank BSC and RES-
CesViMa for computing time.
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