Adsorption of 2-chlorophenol on Cu2O(111)â€“CuCUS: A first

Transcription

G Model
APSUSC-19790; No. of Pages 7
Applied Surface Science xxx (2010) xxx–xxx
Contents lists available at ScienceDirect
Applied Surface Science
journal homepage: www.elsevier.com/locate/apsusc
Adsorption of 2-chlorophenol on Cu2O(1 1 1)–CuCUS: A first-principles density
functional study
Mohammednoor Altarawneh a, Marian W. Radny b,*, Phillip V. Smith b, John C. Mackie c, Eric M. Kennedy c,
Bogdan Z. Dlugogorski c, Aloysius Soon d, Catherine Stampfl d
a
Chemical Engineering Department, Al-Hussein, Bin Talal University, P.O. Box 20, Ma’an, Jordan
School of Mathematical and Physical Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia
School of Engineering, Process Safety and Environment Protection Group, The University of Newcastle, Callaghan, NSW 2308, Australia
d
School of Physics, The University of Sydney, Sydney, New South Wales 2006, Australia
b
c
A R T I C L E I N F O
A B S T R A C T
Article history:
Available online xxx
First-principles density functional theory and a periodic-slab model have been utilized to investigate the
adsorption of a 2-chlorophenol molecule on a CuO(1 1 1) surface with a vacant Cu surface site, namely
Cu2O(1 1 1)–CuCUS. Several vertical and flat orientations have been studied. All of these molecular
configurations interact very weakly with the Cu2O(1 1 1)–CuCUS surface, an observation which also holds
for clean copper surfaces and the Cu2O(1 1 0):CuO surface. Hydroxyl-bond dissociation assisted by the
surface was found to be endoergic by 0.42–1.72 eV, depending predominantly on the position of the
isolated H on the surface. In addition, the corresponding adsorbed 2-chlorophenoxy moiety was found to
be more stable than a vacuum 2-chlorophenoxy radical by about 0.76 eV. Despite these predicted
endoergicities, however, we would predict the formation of 2-chlorophenoxy radicals from gaseous 2chlorophenol over the copper (I) oxide Cu2O(1 1 1)–CuCUS surface to be a feasible and important process
in the formation of PCDD/Fs in the post-flame region where gas-phase routes are negligible.
ß 2010 Elsevier B.V. All rights reserved.
Keywords:
2-Chlorophenol
PCDD/F
Polychlorodibenzo-p-dioxins
Polychlorodibenzofurans
CuO
DFT calculations
1. Introduction
It is widely accepted that copper surfaces enhance the
formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), or dioxins for short [1]. In particular, copper oxide
compounds are believed to be the most efficient catalysts for such
processes. Copper oxides operate through two pathways. The first
is the so-called de novo route which is characterized by the burnoff
of the carbon matrix (highly ordered carbon composites present in
the ash) together with chlorination and oxidation reactions
followed by the release of dioxins from the carbon matrix. The
second route is through catalyzing the self-coupling of direct
precursors such as chlorophenols, chlorophenoxy radicals and
chlorobenzenes [2,3]. The 2-chlorophenol molecule C6H5ClO is
composed of a benzene ring with a hydroxyl group and a chlorine
atom bonding to adjacent positions on the benzene ring.
The recent experimental work by Dellinger’s group has been
very useful in understanding the mechanisms underlying the role
of copper oxides in facilitating the condensation of chlorophenols
into dioxins. A central step in their experimentally supported
mechanisms was the fission of the hydroxyl bond and the
* Corresponding author. Tel.: +61 2 4921 5447; fax: +61 2 4921 6907.
E-mail address: [email protected] (M.W. Radny).
formation of a surface-bound oxygen-centered 2-chlorophenoxy
moiety [4,5]. Electron paramagnetic resonance (EPR) measurements suggested electron transfer from the copper oxide surface to
the adsorbed 2-chlorophenoxy radical, resulting in the formation
of a chlorophenolate. The proposed subsequent steps toward
the formation of dioxins take place via the Langmuir–Hinshelwood
(L–H) and Eley–Rideal (E–R) mechanisms [4,5]. The (L–H)
mechanism involves reaction between adsorbed species, while
the (E–R) mechanism involves reaction between a gaseous species
and an adsorbed species.
This study is part of our on-going effort to address the
interaction between chlorophenol and copper surfaces as the
initial and important step in understanding the catalytic effect of
copper. In recent papers, we have investigated theoretically the
interaction modes between a 2-chlorophenol molecule and the
clean (1 1 1) and (1 0 0) copper surfaces [6,7]. We have found that
non-dissociative modes of 2-chlorophenol interacting with these
clean copper surfaces, in either flat or vertical orientations, are very
weakly bound to the surfaces, whereas dissociation of the hydroxyl
group on these two surfaces is exoergic. The termination of the
clean surfaces was shown not to have any significant effect on the
general features of the interaction modes.
A model for copper oxides using a single isolated CuO dimer has
been shown to account for the main features of Dellinger’s
experimental model in terms of the formation of a chlorophenolate
0169-4332/$ – see front matter ß 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2010.01.101
Please cite this article in press as: M. Altarawneh, et al., Adsorption of 2-chlorophenol on Cu2O(1 1 1)–CuCUS: A first-principles density
functional study, Appl. Surf. Sci. (2010), doi:10.1016/j.apsusc.2010.01.101
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2
M. Altarawneh et al. / Applied Surface Science xxx (2010) xxx–xxx
moiety and the facile nature of the hydroxyl fission process [8].
However a more representative model has been presented in our
recent study of an extended Cu2O surface containing Cu–O bonds
in the outermost layer, namely the Cu2O(1 1 0):CuO surface [9].
While the non-dissociative modes of interaction of the 2chlorophenol were again found to be only weakly bound, the
formation of a 2-chlorphenoxy moiety on the Cu2O(1 1 0):CuO
surface was determined to be endoergic, in contrast to the clean
surfaces where it was exoergic.
In order to make conclusive statements about the interaction of
the 2-chlorophenol molecule with copper surfaces in general, and
copper oxide surfaces in particular, it is essential to study the
chemisorption of this molecule on another Cu2O surface. In this
paper, we present the results of a density functional theory (DFT)
study of the interaction between a 2-chlorophenol molecule and a
Cu2O(1 1 1) surface with a vacant surface Cu site, namely the
Cu2O(1 1 1)–CuCUS surface. This surface, and the Cu2O(1 1 0):CuO
surface, are predicted to be the most stable copper (I) oxide
surfaces under the temperature and pressure conditions appropriate to PCDD/F formation [10]. In addition, this surface contains a
defect site which might affect the reactivity of the surface. One of
our aims is to determine how the thermodynamics of the
chemisorption of a 2-chlorophenol molecule might depend on
the termination of the surface and the presence of a surface defect.
2. Computational methods
Geometry optimization and total energy calculations have been
carried out using the Vienna ab initio simulation package (VASP)
[11,12]. The generalized gradient approximation (GGA) for
exchange and correlation as developed by Perdew and Wang
(PW91) [13] was used to perform the spin-polarized calculations.
Projector augmented wave (PAW) potentials [12,14] are used to
represent the ionic potentials. A (2 2) surface unit cell
comprising four atomic layers has been employed in all calculations, together with a vacuum thickness of at least 10 Å to separate
each slab from its neighbouring images along the z-direction
(normal to the surface). The two top-most layers of our slab, in
addition to the molecule, were allowed to fully relax. The three
special k-points proposed for a hexagonal cell by Cunningham [15]
were used for integrations over the irreducible symmetry element
of the surface Brillouin zone (SBZ). The total energy was converged
to an accuracy of 1.0 10 5 eV, and the forces on each ion to an
accuracy of 0.01 eV Å 1. Any dipole effects along the z-direction
Fig. 1. (a) Optimized geometry for an ideal Cu2O(1 1 1) surface and (b) optimized geometry for Cu2O(1 1 1)–CuCUS. The dashed lines indicate the (1 1) surface unit cell.
Values in brackets are from Soon et al. [10]. All dimensions are in Å. The copper (oxygen) atoms are indicated by the larger (smaller) spheres.
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3. Results and discussion
indicated, while in Fig. 1b, our optimized geometry for the
Cu2O(1 1 1)–CuCUS surface is compared with the optimized
geometry of Soon et al. [10]. As indicated in Fig. 1b, there are
two distinct surface oxygen atoms for Cu2O(1 1 1)–CuCUS which we
distinguish as O1 and O2.
Apart from the missing surface Cu atoms, the geometries of the
two surfaces are quite similar, although the bond distances for the
ideal Cu2O(1 1 1) surface are consistently longer than those for the
Cu2O(1 1 1)–CuCUS surface with the defect. We also observe that the
bondlengths obtained from our VASP calculations for the (1 1)
Cu2O(1 1 1)–CuCUS surface are in very good agreement with the
values obtained previously by Soon et al. [10]. From the (1 1)
surface unit cell shown in Fig. 1, we constructed a (2 2) supercell
which we have used for all of our adsorption calculations (see Fig. 2).
3.1. The Cu2O(1 1 1)–CuCUS surface
3.2. Non-dissociative structures
The geometry of the ideal Cu2O(1 1 1) surface was first
optimized using the (1 1) surface unit cell (see Fig. 1) and
calculated lattice constant for bulk Cu2O [9]. The structure of the
Cu2O(1 1 1)–CuCUS defect surface derived by Soon et al. [10] using
the localized atomic orbital DMOL software package was also
optimized using the same surface unit cell. The optimized
geometries for both surfaces are shown in Fig. 1. In Fig. 1a, the
optimized structure for the ideal Cu2O(1 1 1) surface is given, and
the position of the vacant Cu site in the Cu2O(1 1 1)–CuCUS surface
Several non-dissociative flat and vertical structures have been
considered for the adsorption of a 2-chlorophenol molecule on the
Cu2O(1 1 1)–CuCUS surface. The resulting optimized configurations
are shown in Figs. 2 and 3, respectively, and the corresponding
binding energies are reported in Table 1. From the calculated
binding energies and bond distances shown in Figs. 2 and 3, it is
clear that the 2-chlorophenol molecule is only very weakly bound
to the Cu2O(1 1 1)–CuCUS surface. The very weakly bound nature of
these structures is also evident from the fact that these structures
have been compensated by introducing a dipole vector with the
same value in the opposite direction. Binding energies have been
calculated using the geometries optimized with a 350 eV energy
cutoff. These binding energies were calculated as the difference
between the total energy of the optimized chemisorbed structure,
and the total energy of the non-interacting molecule and substrate
in the same supercell.
Convergence of our results with respect to the cut-off energy
and slab thickness is analogous to our previous study on the
Cu2O(1 1 0):CuO surface [9] where the binding energy was found
to differ by only 6% when using a cut-off energy of 500 eV and the
geometries were found not to change when using up to six layers.
Fig. 2. Optimized structures for flat adsorption of a 2-chlorophenol molecule on the Cu2O(1 1 1)–CuCUS surface. The dashed lines indicate the (2 2) supercell. Reported
distances are in Å. Only the first atomic layer is shown in the top views with the exception of O3 which is a second-layer atom. In this, and all subsequent figures, the carbon,
chlorine and hydrogen atoms are coloured dark grey, mid grey (green) and light grey, respectively. (For interpretation of the references to color in this figure legend, the reader
is referred to the web version of the article.)
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4
Fig. 3. Optimized structures for vertical adsorption of a 2-chlorophenol molecule on the Cu2O(1 1 1)–CuCUS surface. Reported distances are in Å. Only the first atomic layer is
shown in the top views.
exhibit a geometry very similar to that of an isolated 2chlorophenol molecule. The structure labelled ‘‘Vertical-4’’ in
Fig. 3 is observed to be the most stable structure with a binding
energy of 0.2 eV, most probably due to the existence of hydrogen
Table 1
Binding energies for the flat and vertical structures of 2-chlorophenol on the
Cu2O(1 1 1)–CuCUS surface.
Structure
Flat-1
Flat-2
Flat-3
Binding energy
eV
kcal/mol
0.12
0.10
0.11
2.77
2.30
2.54
Structure
Binding energy
eV
kcal/mol
Vertical-1
Vertical-2
Vertical-3
Vertical-4
0.07
0.06
0.07
0.20
1.61
1.38
1.61
4.61
bonding between the hydroxyl hydrogen and the O2 surface
oxygen atom (see Fig. 3). The lack of any of the ring distortions that
have been observed for other aromatics adsorbed on different
metallic surfaces [16] also highlights the very weak nature of the
interactions for 2-chlorophenol on the Cu2O(1 1 1)–CuCUS surface.
The fundamental shortcomings of the DFT–GGA method in
describing weakly bound structures suggest that the actual
binding energies for our flat and vertical structures may differ
from the values presented in Table 1 [7,17]. Recent work has, in
fact, shown that van der Waals long-range interactions are
necessary in order to adequately describe weakly adsorbed
systems, and can lead to significantly improved values for the
binding energies [17]. Such calculations are beyond the scope of
this work. Nonetheless, we can confidently state that all of these
non-dissociative structures, if stable, will only be weakly bound.
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3.3. Dissociative structures
3.3.1. Rupture of the O–H bond
Dissociation of the O–H bond of the chlorophenol molecule is a
crucial step in the condensation of chlorophenols into PCDD/Fs,
either in a purely gas-phase environment or via surface-catalyzed
reactions. The importance of this dissociation comes from the fact
that the activation energies for reaction steps involving the
chlorophenoxy radical (the moiety resulting from the dissociation)
are lower than for the steps involving the chlorophenol molecule.
The O–H fission process is a central part of kinetic models
addressing the formation of PCDD/Fs both homogeneously and
heterogeneously.
5
On the clean copper surfaces, the formation of a 2-chlorophenoxy moiety is exoergic by 1.76 eV and 1.41 eV, respectively,
relative to the clean Cu(1 0 0) and Cu(1 1 1) surfaces and a 2chlorophenol molecule in the gas phase. In contrast, this fission
process was found to be endoergic by 0.36 eV on the Cu2O(1 1 0):CuO surface [9]. However, both the copper oxide surface and the
Cu(1 0 0) surface are predicted to require similar total energies
when the energy barriers involved in actually forming the 2chlorophenoxy moieties are taken into account.
For the Cu2O(1 1 1)–CuCUS surface, we have considered three
configurations that could result from the O–H fission process.
These configurations are represented by the structures D1, D2 and
D3 in Fig. 4 and their corresponding reaction energies are given in
Fig. 4. Chemisorption of a 2-chlorophenoxy radical on the Cu2O(1 1 1)–CuCUS surface in the optimized structures D1, D2 and D3. Distances are in Å. Only the first atomic layer
is shown in the top views with the exception of the O3 which is a second-layer atom. A representative reaction is given below each structure.
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Table 2
Reaction energies for the formation of D1–D7 structures. A positive sign indicates
that the reaction is endoergic.
Structure
D1
D2
D3
Reaction energy
eV
kcal/mol
+0.42
+1.72
+0.52
+9.69
+39.66
+12.00
Structure
D4
D5
D6
D7
Reaction energy
eV
kcal/mol
+2.46
+1.54
0.03
+2.87
+35.52
+35.15
0.69
+66.21
Table 2. These structures are less stable than a vacuum 2chlorophenol molecule and the clean surface by 0.42 eV, 1.72 eV
and 0.52 eV, respectively. In structure D1, the hydroxyl H is
attached to a second-layer oxygen O3 atom. The distance between
the phenolic oxygen (the oxygen atom in the 2-chlorophenol
molecule) and the nearest surface copper atom is 2.02 Å and the
C–O bond length in the adsorbed 2-chlorophenoxy moiety is 1.31 Å,
0.06 Å larger than the 1.25 Å equilibrium distance of a free 2chlorophenoxy radical. In structure D2, the hydroxyl H is attached to
an O1 atom, while in structure D3 it is attached to an O2 atom.
As the geometries of the adsorbed 2-chlorophenoxy moieties
are very similar in structures D1, D2 and D3, we believe that the
noticeable differences in their overall energies originate from the
different positions of the hydroxyl H on the surface. To verify this
hypothesis, we have calculated the binding energy for the
adsorbed H and the 2-chlorophenoxy moiety for each of these
three structures. The adsorbed H was found to be more stable than
a vacuum H by 2.93 eV, 1.70 eV and 2.76 eV for the D1, D2 and D3
structures, respectively. This indicates that O1 is the least favoured
position for the adsorbed hydroxyl H. By contrast, the adsorbed 2chlorophenoxy moiety was found to be more stable than a vacuum
2-chlorophenoxy radical by about 0.76 eV for all three structures. It
Fig. 5. Structures D4–D7. In structures D5 and D6 only the chlorine atom has dissociated from the 2-chlorophenol molecule, while for structures D4 and D7 both the hydroxyl
OH group and the chlorine atom have dissociated. Distances are in Å. Only the first atomic layer is shown in the top views, although the second-layer O3 oxygen atom has also
been included for D4 and D5. A representative reaction is given below each structure.
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thus follows that the significant difference between the overall
stability of structure D2, and structures D1 and D3, is directly
attributable to the difference in the binding energy of their
chemisorbed H. Combining the above results also leads us to
conclude that the energy required to form a free chlorophenoxy
radical via interaction of a 2-chlorophenol molecule with the
Cu2O(1 1 1)–CuCUS surface varies from 1.18 eV (for D1) to 2.48 eV
(for D2).
The behavior of the Cu2O(1 1 1)–CuCUS surface in forming a 2chlorophenoxy moiety is similar to that of the Cu2O(1 1 0):CuO
surface in terms of the calculated binding energies for hydroxyl H
fission on the two surfaces. As we have seen, these D1–D3 reaction
processes are endoergic. However, it is well known that such
reactions can occur at sufficiently high temperatures. The gasphase formation of a 2-chlorophenoxy radical through the reaction
of a chlorophenol molecule with the radical pool is negligible in the
post-flame region (150–500 8C). The energy of 0.42–1.72 eV
associated with forming a 2-chlorophenoxy moiety on these
copper (I) oxide surfaces via the D1–D3 reaction processes is,
however, sufficiently low to ensure that these surfaces would play
an important role in the production of resonance stabilized 2chlorophenoxy radicals [18–20]. The importance of these surfaces
in the catalytic formation of PCDD/Fs might also come from
facilitating the condensation of two molecules and/or radicals into
PCDD/Fs compounds (including possibly lowering the net activation energies with respect to the gas-phase process), but this
warrants further investigation.
3.3.2. Formation of phenyl and benzyne moieties
Single and double dehydrogenation of benzene to produce
phenyl and benzyne moieties, respectively, have been investigated
over a clean copper surface. The dehydrogenation process was
found to be endoergic [21]. The formed phenyl and benzyne
moieties can serve as direct precursors for the formation of
polyaromatic compounds through the so-called ‘‘Ullmann reactions’’ [22]. As the chlorine–carbon bond is weaker than the
hydrogen–carbon bond in the 2-chlorophenol molecule [20], we
have also considered the possible dissociation of the chlorine atom
and its subsequent adsorption on the Cu2O(1 1 1)–CuCUS surface.
Two resultant structures labelled D5 and D6 are shown in Fig. 5. In
structure D5, the dissociated chlorine atom forms an ionic bond
with a surface copper atom with a bond distance of 2.21 Å. The
unpaired carbon atom attaches to a second-layer oxygen atom and
forms a semi-hydroquinone structure. The newly formed C–O
bond length is 1.35 Å. The formation of structure D5 is associated
with substantial deformation of the surface and the overall process
is endoergic by 1.54 eV. In structure D6, the unpaired carbon atom
created by the dissociation of the chlorine atom is attached to a
surface oxygen atom via a distance of 1.41 Å. The formation of
structure D6 is slightly exoergic by 0.03 eV.
Finally, we have considered the formation of a benzyne moiety
through the dissociation of both the hydroxyl group and the chlorine
atom. Two possible structures, D4 and D7, are shown in Fig. 5. In
structure D4, the two unpaired carbons that are created by the loss of
the chlorine atom and the hydroxyl group are attached to a surface
copper atom and a second-layer O3 oxygen atom. The formation of
structure D4 is associated with a substantial endoergicity of 2.46 eV.
In structure D7, the formed benzyne moiety stands upright at the
bridge position connecting two C atoms with two identical copper–
carbon distances of 1.97 Å. The formation of structure D7 is also
highly endoergic by 2.87 eV. In both D4 and D7, the chlorine atom
forms a single ionic bond with a surface copper atom.
The formation energies calculated for structures D4–D7 are
very similar to those for the analogous structures on the
Cu2O(1 1 0):CuO surface, which is the other quite stable copper
(I) oxide surface [9]. These results are interesting as our previous
7
work has shown that the formation of phenyl and benzyne
moieties constitutes the most thermodynamically accessible
pathways for the interaction of a 2-chlorophenol molecule with
clean copper surfaces [6,7]. In view of the negligible importance
of gas-phase decomposition pathways for the 2-chlorophenol
molecule in the post-flame region (150–500 8C), these surface
dissociative structures (D4–D7) provide feasible exit channels for
the 2-chlorophenol molecule, despite the considerable endoergicity of structures D4, D5 and D7.
4. Conclusions
In this paper, we have extended our DFT investigations of the
interaction of 2-chlorophenol, a direct precursor for the formation
of PCDD/Fs, with copper surfaces, to the Cu2O(1 1 1)–CuCUS defect
surface. We have found that the overall behavior of the
Cu2O(1 1 1)–CuCUS surface is very similar to that of Cu2O(1 1 0):CuO surface, both in terms of the very weak interaction of the
molecular configurations, and the endoergic nature for the
formation of dissociated structures. Of all of the dissociation
structures, resulting from the cleavage of H, OH, Cl, or both OH and
Cl, only one of the Cl-dissociated structures was found to be
exothermic, and this was only marginally so. In order to fully
account for the role of copper (I) oxide surfaces in catalyzing PCDD/
F formation, we thus believe that potential energy surfaces for the
L–H and E–R mechanisms on these surfaces should be investigated,
and the net activation energies compared with those of the purely
gas-phase processes. This warrants further investigation.
Acknowledgements
This research has been supported by a grant from the Australian
Research Council. The authors also acknowledge access to the
computational facilities of the Australian Partnership of Advanced
Computing (APAC) via their Merit Allocation Scheme.
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Adsorption of 2-chlorophenol on Cu2O(111)â€“CuCUS: A first

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