A detailed mechanism of the surface-mediated formation of PCDD/F from the oxidation of 2-chlorophenol on a CuO/Silica surface

Article abstract of DOI:10.1021/jp026045z

The formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) via a Cu(II)O-mediated reaction of 2-chlorophenol (2-MCP) has been studied in a packed bed reactor over a temperature range of 200-500 °C. Under oxidative conditions, the principle PCDD/F products were 1-monochlorodibenzo-p-dioxin (MCDD) > 4,6-dichlorodibenzofuran (DCDF) > dibenzo-p-dioxin (DD). EPR studies indicated the presence of a carbon-centered phenoxyl radical on the surface, which is attributed to chemisorption of 2-MCP at a copper oxide site followed by electron transfer to Cu(II) to form Cu(I) and a phenoxyl radical. The presence of a surface bound phenoxyl radical and the formation of MCDD, DCDF, and DD, which were also observed as the principle products of the gas-phase oxidation of 2-MCP, strongly suggest a surface-mediated mechanism involving many of the same radical and molecular species involved in the gas-phase formation of PCDD/F from 2-MCP. Reaction orders of 0.5-1.0 were observed for MCDD and DD formation, indicating an Eley-Rideal formation mechanism. Negative reaction orders were observed for DCDF formation, indicating a Langmuir-Hinshelwood formation mechanism. No highly chlorinated PCDFs were observed, suggesting a mechanism in which DCDF is desorbed from the surface before it can undergo additional chlorination. Highly chlorinated PCDDs were observed, which were consistent with a mechanism in which DD remained adsorbed to the surface and underwent additional chlorination. Chloro-o-quinone and chlorocatechol, which are precursors to semiquinone radicals, were also observed products. A detailed reaction mechanism accounting for all reported products is proposed.

Full text of DOI:10.1021/jp026045z

J. Phys. Chem. A 2003, 107, 4387-4395
4387
A Detailed Mechanism of the Surface-Mediated Formation of PCDD/F from the Oxidation
of 2-Chlorophenol on a CuO/Silica Surface
Slawomir Lomnicki† and Barry Dellinger*,†,‡
Department of Chemistry and Biodynamics Institute, Louisiana State UniVersity, Baton Rouge, Louisiana 70803
ReceiVed: April 30, 2002; In Final Form: February 12, 2003
The formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) via a Cu(II)O-mediated
reaction of 2-chlorophenol (2-MCP) has been studied in a packed bed reactor over a temperature range of
200-500 °C. Under oxidative conditions, the principle PCDD/F products were 1-monochlorodibenzo-p-
dioxin (MCDD) > 4,6-dichlorodibenzofuran (DCDF) > dibenzo-p-dioxin (DD). EPR studies indicated the
presence of a carbon-centered phenoxyl radical on the surface, which is attributed to chemisorption of 2-MCP
at a copper oxide site followed by electron transfer to Cu(II) to form Cu(I) and a phenoxyl radical. The
presence of a surface bound phenoxyl radical and the formation of MCDD, DCDF, and DD, which were also
observed as the principle products of the gas-phase oxidation of 2-MCP, strongly suggest a surface-mediated
mechanism involving many of the same radical and molecular species involved in the gas-phase formation
of PCDD/F from 2-MCP. Reaction orders of 0.5-1.0 were observed for MCDD and DD formation, indicating
an Eley-Rideal formation mechanism. Negative reaction orders were observed for DCDF formation, indicating
a Langmuir-Hinshelwood formation mechanism. No highly chlorinated PCDFs were observed, suggesting
a mechanism in which DCDF is desorbed from the surface before it can undergo additional chlorination.
Highly chlorinated PCDDs were observed, which were consistent with a mechanism in which DD remained
adsorbed to the surface and underwent additional chlorination. Chloro-o-quinone and chlorocatechol, which
are precursors to semiquinone radicals, were also observed products. A detailed reaction mechanism accounting
for all reported products is proposed.
I. Introduction
are present in the fly ashes in the exhaust and air pollution
control devices of most combustion systems,22,23 with copper
and iron ions believed to be the most active. However, for
surface-mediated processes, only semiglobal Eley-Rideal and
Langmuir-Hinshelwood mechanisms have been proposed.9,15,24,25
Because detailed reactions are not included in these mechanisms
and the PCDD/F forming reactions are side-reactions of other
surface process, there has been great confusion concerning the
role and implications of surface-catalyzed dioxin formation.
Does the surface-mediated reaction involve the same species
as the gas-phase reaction, with the surface sites being only a
medium for a low-temperature radical formation? Or, is the
mechanism entirely different, involving different reactants and
intermediates? This is one of the most intriguing questions
remaining unanswered.
Polychlorinated dibenzo-p-dioxins and polychlorinated furans
(PCDD/F or “dioxins”) are among the most toxic known
environmental pollutants. It is well established that combustion
and other thermal processes are the principal origin of PCDD/
F,1-6 yet a detailed chemical mechanism that can account for
the majority of their formation has eluded researchers even
though it has been the subject of extensive research for the past
two decades.7-11 This research has resulted in different proposed
theories on how dioxins are formed: (i) gas-phase formation
from molecular precursors at temperatures >600 °C, (ii)
condensation reactions of precursors catalyzed by transition
metal oxides in “fly ash” at temperatures between 200 and 600
°C, and (iii) de novo oxidation and chlorination of elemental
carbon in soot between 200 and 600 °C.12-15
Although considerable progress has been made on the gas-
phase mechanism, it only accounts for about 30% of dioxin
emissions.16,17 Generally, it is accepted that dioxins can be
formed from identifiable precursors such as chlorophenols and
In an attempt to clarify some of these issues, we have
performed a detailed study of the copper-catalyzed reaction of
chlorinated phenols. Chlorinated phenols have been demon-
strated to be the predominant precursors in the surface-catalyzed
formation schemes of PCDD/F and are implicated as key
intermediates in the de novo pathway.14 Previous studies of
surface-catalyzed reactions of chlorinated phenols have answered
some questions but have also raised many issues due to the
selection of highly chlorinated congeners for experimental
study.9,26-29 In some cases, complex surfaces, including real
fly ash, were used.15,26,30,31 Consequently, we chose to study
the reaction of one of the simplest PCDD/F precursors,
2-chlorophenol, and use copper oxide supported on silica as a
surrogate for fly ash.32 In this paper, we present the reaction
kinetic results of the transformations of 2-chlorophenol over a
18
chlorobenzenes through the reactions of the keto and enol
forms of phenoxyl radicals.19-21 These radicals can combine
with other phenoxyl radicals, molecular chlorobenzenes, or
phenols to form PCDD/F.
With respect to the surface-mediated processes, which are
believed to account for on the order of 70% of all dioxin
formation, it is known that transition metal oxides and chlorides
play a significant role in dioxin formation.11 Transition metals
* To whom correspondence should be addressed. E-mail: barryd@lsu.edu.
† Department of Chemistry.
‡ Biodynamics Institute.
10.1021/jp026045z CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/14/2003

4388 J. Phys. Chem. A, Vol. 107, No. 22, 2003
Lomnicki and Dellinger
bed of copper oxide/silica under oxidative conditions and
propose detailed surface-mediated mechanisms that account for
the observed reaction products.
was very small and the kinetic analyses were performed at 250
°C where the 2-MCP conversion was low, we could assume a
differential mode of operation of the reactor. Thus, the expres-
sion for the reaction rate is
II. Experimental Section
C2-MCP ‚ x
The catalytic material (5% CuO/silica, m/m) was prepared
by the method of incipient wetness. A water solution of copper-
(II) nitrate (Aldrich) of concentration chosen so as to obtain
5% CuO/SiO2 system was used as the active phase precursor.
Silica powder (Aldrich, 500 m2/g) was introduced into the
equivalent volume of precursor solution for incipient wetness
to occur. The suspension was then well mixed and dried at 120
°C for 24 h.
R )
W ‚ t
where C2-MCP is the amount of 2-MCP (moles) introduced to
the system, x is a molar fractional conversion toward a particular
product, W is weight of catalyst (g), t is time on stream (h),
and R is the reaction rate (mol g-1 h-1). Studies to determine
whether the reactions are kinetically controlled or diffusion
controlled have been reported in a previous paper32 and indicated
that the reactor operates in the kinetic regime for the conditions
reported in this manuscript.
Electron Paramagnetic Resonance (EPR). EPR spectra of
2-MCP dosed CuO/silica powder were obtained using a
VARIAN E-109 EPR spectrometer. Samples were prepared as
follows: CuO/silica powder was placed in quartz tubes con-
nected to vacuum line and treated at 500 °C in air for 2 h. The
temperature was then decreased to 200 °C and samples were
evacuated to 1 × 10-2 atm. A reference sample was then sealed
under vacuum. The CuO/silica sample was exposed to 4 Torr
of 2-MCP at 200 °C for 2 min, evacuated to 1 × 10-2 atm and
sealed under vacuum. The EPR parameters were set at 100-
kHz, X-band; microwave frequency, 9.345 GHz; attenuation, 7
dB; modulation amplitude, 0.50 G; time constant, 0.032 s;
receiver gain 5-500, and scan time, 4 min (10 Hz). For the g
values/spin counts solid 1,1-diphenyl-2-picrylhydrazyl (DPPH,
∼50 µg) was used as a centerfield standard. EPR measurement
was performed at room temperature.
For the catalytic tests, 1 or 3 mg of a sample was placed
between the quartz wool plugs in a fused-silica reactor (1 mm
i.d.) in a well-characterized flow-reactor system setup. All
transfer lines were maintained at isothermal conditions at 225
°C. Prior to each flow-reactor experiment, the samples were
oxidized in situ at 500 °C for 1 h in 20% O2/He mixture at a
flow rate of 5.5 cm3/min to convert the copper nitrate to CuO.
2-Monochlorophenol (2-MCP) (Aldrich) was introduced into
20% O2/He reaction stream by a syringe pump through a
vaporizer maintained at 180 °C. The constant rate of the
injection was selected to obtain gas-phase concentration of
2-MCP of 88 ppm (v/v) to facilitate comparison of the results
to previous gas-phase experiments.32 The flow rate of the
reacting mixture was 5.5 cm3/min. For separate experiments in
which the contact time was varied, the flow rate of the reactant
was 1 cm3/min. The reaction was studied over a temperature
range of 225-500 °C. In some additional experiments, the
2-MCP concentration was varied from 29 to 177 ppm (v/v).
All reported results reflect 1-h on-stream experiments.
The effluent products from the reactor were trapped at the
head of capillary column (CP-Sil 8 phase 30 m, 0.25 mm i.d.,
0.25 film thickness and analyzed after completion of the reaction
using a GC-MS system (VARIAN SATURN 2000). The column
was temperature programmed from -60 to 290 C at 15 °C/
min. Mass spectra were obtained by running on full scale mode
(10 to 650 amu) for 26 min.
The spent catalyst bed was subjected to extraction with
methylene chloride in Micro Soxhlet extraction apparatus (Ace
Glass) for 24 h. The extract was then condensed and analyzed
via GC-MS for adsorbed reaction products. However, no
PCDD/F were detected in the extract, chlorinated naphthalenes
and phenols being the only compounds detected. Since no
PCDD/F were found to be present on the spent catalyst bed,
we can presume that all the products of interest were desorbed
into the gas phase.
PCDD/F concentrations were calculated based on the peak
area counts of chromatogram, based on the calibration made
using CIL (Cambridge Isotope Lab) standards. 2-MCP break-
through amounts were calculated based on the calibration using
reagent compound. The yields of PCDD/F were calculated using
the formula as follows:
III. Results
In a previous paper,32 we reported the copper oxide catalyzed
condensation of 2-MCP to form PCDD/F under pyrolytic
conditions. We found that PCDD/F formation is a side reaction
of 2-chlorophenol oxidation via the Mars-van Krevelen mech-
anism which involves transfer of oxygen atoms from the surface
to the adsorbate. Moreover, the data presented therein indicated
significant differences between the formation of PCDDs and
PCDFs, which in turn resulted in the conjecture that they are
formed according to two different mechanisms, Eley-Rideal
(E-R) and Langmuir-Hinshelwood (L-H), respectively. The
E-R mechanism involves the reaction between the adsorbed
species and a gas-phase molecule, while L-H is the reaction
between two surface-adsorbed species. However, on the basis
of the pyrolysis data alone, we could not construct a detailed
reaction mechanism. In combination with the following oxida-
tion data, a detailed mechanism can be proposed (vide Discus-
sion).
Figure 1 presents product data obtained with 1 mg of catalyst
(3 mg resulted in complete oxidation of 2-MCP and condensa-
tion products). The main PCDD/F products detected were the
same as under pyrolytic conditions: dibenzo-p-dioxin (DD),
1-monochloro dibenzo-p-dioxin (MCDD), and 4,6-dichloro-
dibenzofuran (DCDF). These are the same products expected
based on gas-phase radical-molecule and radical-radical
reactions.13,20,21,33-35 However, the gas-phase reactions only
occur above 600 °C.20,21 In our study, PCDD/F formation
occurred in the 200 to 500 °C range, which indicates the surface
mediated process. Between 300 and 500 °C, significant quanti-
ties of more highly chlorinated PCDD/F are formed; however,
MCDD is the dominant congener with a yield more than twice
out
PCDD/F
C
‚ 2
Y )
100%
in
(
)
C
2-MCP
out
PCDD/F
where C
is concentration of particular PCDD/F conge-
ners formed (in moles) and C
in
is inlet amount of 2-MCP
2-MCP
(in moles)
Experimental reaction rates were calculated assuming a simple
power law-type expression (R ) kCn). Since the catalyst bed

Mechanism of Catalytic PCDD/F Formation
J. Phys. Chem. A, Vol. 107, No. 22, 2003 4389
Figure 1. Gas-Phase PCDD/Fs products from oxidation of 2-MCP
over CuO(5%)/SiO2.
Figure 2. Kinetic rate order plot at 250 °C: 1 mg of CuO(5%)/SiO2,
88 ppm 2-CPh, 20% O2.
as high as other PCDD/F. Above 400 °C, MCDD yield
dramatically drops below those of DD and DCDF. For reaction
at >450 °C both DD and DCDF concentrations also decline
significantly.
again. The yields of CQ under oxygen rich conditions are
significantly higher than under pyrolysis.
The results of EPR experiments of 2-chlorophenol adsorption
over copper oxide/silica catalyst are presented in Figure 6A,B.
The CuO/Silica sample shows a broad signal originating from
Cu(II) complexes. When the sample was exposed to 4 Torr of
2-MCP at 200 °C for 2 min, trace A, sharp signal with a g-value
of 2.0028 appeared superimposed on the broad Cu(II) signal.
To obtain more information on the kinetics of the PCDD/F
formation processes, experiments with varying feed stream
concentration were performed. Figure 2 presents the natural
logarithmic relation of reaction rates versus concentration of
reactant (2-MCP) at 250 °C. Since PCDD/F formation is a
marginal process of chlorophenol oxidation, this particular
reaction temperature was chosen to avoid the effects of
oxidation/decomposition of both reaction products and 2-MCP
while preserving noticeable rates of PCDD/F formations. The
observed scattering of the data is a result of very low
concentration of analyzed products (ppb). The slope of the
regression is the reaction order for formation of each of the
products.
For the same reaction conditions (250 °C), the increase of
fuel/catalyst ratio (F/C) resulted in a significant change in the
product distribution (cf. Figure 3). With increasing F/C, a
significant decrease in DCDF yield can be observed, while DD
and MCDD show a moderate dependence. Simultaneously,
another product, 1′,2-dichloro-1-hydroxy-diphenyl ether (DCH-
DPE), appears whose yield gradually increases with F/C ratio,
slowly reaching a plateau for high F/C values.
Figure 3. Conversion as a function of the catalyst load, 250 °C, 20%
Figure 4 presents the temperature dependence of the congener
distribution of PCDDs for both pyrolytic and excess oxygen
conditions. Significant is the fact that for both conditions, no
other PCDF congeners than 4,6-DCDF were detected. On the
other hand, PCDDs are detected over the entire range of
chlorination, i.e., from non-chlorinated to octachlorinated. Under
pyrolytic conditions, higher chlorinated congeners disappear at
temperatures above 350 °C. In contrast, for excess oxygen
conditions, even more highly chlorinated PCDDs are detected
above 350 °C than below that temperature and the total PCDD
concentration is higher.
Figure 5 presents the yield of the reaction products, chloro-
o-quinone (CQ) and chloro-catechol (CC) as a function of
reaction temperature for both pyrolytic and excess oxygen
conditions. CC yields initially increased with increasing reaction
temperature and drastically decreased above 350 °C for both
pyrolytic and oxidative conditions. In contrast, CQ varies
significantly with reaction conditions. Under pyrolysis condi-
tions, only small changes with temperature are observed up to
450 °C, while under oxidative conditions the CQ yield steadily
increase with reaction temperature up to 400 °C before declining
O2
IV. Discussion
Eley-Rideal vs Langmuir-Hinshelwood Formation of
PCDD and PCDF? In our previous paper concerning the CuO-
catalyzed pyrolysis of 2-MCP, we discussed the differences in
mechanism of PCDD and PCDF formation.32 Pyrolytic condi-
tions resulted in a significant surface oxygen deficit. This
resulted in 2-MCP remaining adsorbed on the surface rather
than being oxidized. Since the DCDF yield increased as the
catalyst mass was decreased, we concluded that the DCDF yield
directly correlated with the concentration of surface species. This
argument is supported by the observation that the DCDF
maximum shifted ∼100 °C lower in temperature when the
catalyst mass was decreased by a factor of 3. This correlation
between the surface concentration of adsorbed phenolic species
and increased yield of DCDF was a foundation of the thesis
that PCDFs are formed according to Langmuir-Hinshelwood
mechanism under pyrolytic conditions. In contrast, the lack of
a similar dependence for PCDDs suggested that they may be
formed by an Eley-Rideal mechanism.

4390 J. Phys. Chem. A, Vol. 107, No. 22, 2003
Lomnicki and Dellinger
Figure 4. PCDD Congener distribution under pyrolytic and oxidative conditions. 1 mg of CuO(5%)/SiO2, 88 ppm 2-CPh.
TABLE 1: Comparison of Maximum PCDD/F Yields at
The data presented for excess oxygen conditions in this paper
further underline the differences in PCDD and PCDF formation,
as the PCDD/F yields presented in Figure 1 are very different
from those reported earlier under pyrolytic conditions. A
comparison of results under oxidative and pyrolytic conditions
is presented in Table 1. While the DCDF yield decreased by
only 25% with the addition of oxygen to the reaction system,
both DD and MCDD yields show significant variation. In
particular, the MCDD yield increased by a factor of 10. It is
interesting that, contrary to pyrolytic conditions, where no
specific maximum was found for MCDD on the temperature
dependence curve, a narrow window of high yields is present
under oxidative conditions. In contrast, the DD yields decreased
by a factor of 2 under oxidative conditions and the maximum
yield shifted 50 °C lower in temperature.
The significant differences in PCDD and PCDF formation
are also reflected in the reaction kinetic parameters (cf. Figure
2). The rate orders of MCDD and DD formation are in the range
of 0.5-1.0, which indicate the involvement of an adsorbed
species in the reaction mechanism. In contrast, the rate order
of the DCDF formation reaction is (-0.6). Negative rate orders
Pyrolytic and Oxidative Conditions
pyrolysis
oxidation
max yield (%}
product max yield (%)
T (°C)
T (°C)
400
325-350
375
DD
MCDD
DCDF
∼0.13
∼0.03
0.16
425-450
225-475
375
0.07
0.28
0.12
of catalytic reactions may occur if the Langmuir-Hinshelwood
mechanism is involved, i.e., when both reacting species are
adsorbed on the surface. The negative rate orders have been
observed previously for surface-catalyzed reaction of two
reagents36,37 and generally are a result of strong and competitive
adsorption effects. However, our situation is somewhat different
in that there is only one reactant in the gas phase and the product
is a result of condensation of the same two species. A reasonable
alternative explanation is a competitive reaction between the
adsorbed species and a gas-phase molecule (eq 1)
A + A* ) B
(1)
(2)
A* + A* ) AA
where A* represents the adsorbed surface species and A is the
gas phase molecule. In this case, increasing concentration of
the gas phase species A will reduce the concentration of the
surface (adsorbed) species A* due to an increase in the rate of
formation of B by reaction 1. Consequently, the rate of reaction
2 and formation of AA will decline because of the competitive
loss of A* due to reaction 1. This phenomenon has been
previously observed25 for the phenyl coupling reaction.
In our study, the formation curves of DCHDPE (cf. Scheme
1) and DCDF are mirror images (cf. Figure 3). This indicates
a competitive character of these two products and supports our
SCHEME 1: 1,2′-Dichloro-1′-hydroxy-diphenyl Ether
(DCHDPhE)
Figure 5. Chloro-quinones and chloro-catechols formation as a function
of temperature and reaction conditions.

Mechanism of Catalytic PCDD/F Formation
J. Phys. Chem. A, Vol. 107, No. 22, 2003 4391
earlier conjecture that DCDF is formed by a Langmuir-
Hinshelwood mechanism. At higher chlorophenol concentra-
tions, a competitive reaction occurs with gas phase 2-MCP
resulting in DCHDPE formation. This type of behavior is not
observed for PCDDs. Under pyrolytic conditions, we reported
that unlike the DCDF, whose yield increased and formation
maximum shifted toward lower temperatures with a decrease
of catalyst load, PCDD formation was not dependent on the
concentration of the surface sites.32 These two observations
suggest an Eley-Rideal mechanism for DD and MCDD
formation.
SCHEME 2: Chlorination by Surface Hypochlorite Ion
the decrease in DD under oxidative conditions, viz., it is being
chlorinated to form highly chlorinated PCDDs. The presence
of oxygen is also affecting the chlorination of other products,
i.e., significantly more polychlorinated phenols (PCPs) are
formed under oxidative conditions up to 375 °C. Above this
temperature PCP concentrations are comparable for both oxida-
tive and pyrolytic conditions. Dechlorination of reactants,
intermediates, or products was not observed, i.e., we have not
observed non-chlorinated phenols, benzenes, or dibenzofuran
in the gas-phase products of the process.
Various mechanisms of PCDD/F chlorination have been
proposed,15 including chlorination by Cl2 formed either in the
Deacon reaction, surface chloride decomposition, or direct
chlorination by metal chlorides.39 Recent studies have shown
that the surface chloride ions are very effective chlorinating
agents and exchange between the organic phase and surface
chlorides is fast.30,40 The presence of surface, metal chloride
species in our experiments are the result of reaction between
the surface hydroxides and oxides with HCl which is present
as a result of oxidation of 2-MCP.
The effect of oxygen is explained in terms of reoxidation of
surface copper (or other metal ion) since the lower oxidation
state of cations are retarding the chlorination process.41 However,
this explanation does not agree with the results obtained in our
experiments. Since 95% of the 2-MCP is completely oxidized
to CO and CO2 under pyrolytic conditions, there are enough
Cu(II) surface sites for 2-MCP oxidation such that chlorination
should not be limited. Under oxidative conditions, significant
chlorination is observed above 350 °C. Surface bound DD is
subject to two competitive processes: desorption and chlorina-
tion. With increasing reaction temperature, the rate of desorption
is fast enough to prevent chlorination above 350 °C. However,
under oxidative conditions, highly chlorinated PCDDs are
detected, and, above 350 °C, their concentration is even higher,
suggesting that the rate of chlorination is higher in the presence
of oxygen than pyrolysis. These two observations indicate that
in the presence of oxygen, other reagents may be formed with
more chlorinating power than chloride ions, e.g., surface
hypochlorite ions (cf. Scheme 2). In fact, it has been reported42
that the ortho- to para-chlorophenol ratio observed during the
chlorination of phenol over fly ash surfaces in the presence of
oxygen was exactly the same as those observed for chlorination
of phenol with tert-butyl hypochlorite. The formation of surface
hypochlorite species could explain the differences between the
degree of chlorination with and without oxygen present. In such
a case, surface-catalyzed chlorination of the adsorbed aromatic
species would proceed via electrophilic substitution of the
aromatic hydrogen atom (Scheme 3). The chlorinated aryl
compound can further undergo isomerization forming different
isomers. Additional experiments are indicated concerning the
The differences between the L-H and E-R mechanisms
should result in significant differences in the rate orders, i.e.,
the L-H reactions generally have lower reaction orders for
similar reactions,38 which is observed in our studies. These
mechanistic differences can explain observed differences be-
tween the PCDF and PCDD formation. An Eley-Rideal
mechanism was recently proposed for de novo formation of
PCDDs, where partially oxidized polynuclear aromatic hydro-
carbons (PAH) bonded to the carbon matrix reacted with a gas-
phase chlorophenol forming PCDDs.14
Surface Catalyzed Chlorination Reactions. Since the
MCDD formation rate order is slightly higher than for DD, we
expect stronger adsorption effects for DD formation. In fact,
our previous experiments suggested that DD remains adsorbed
on the surface, while MCDD is desorbed immediately after it
is formed.32 Accordingly, DD is more prone to additional surface
reactions such as chlorination. This conclusion may also well
be applied to explain the observed, unexpected differences in
MCDD and DD yields under oxidative conditions (Figure 1,
Table 1).
MCDD is not subject to subsequent surface reactions and its
yield increases compared to pyrolytic conditions, which directly
reflects the effect of oxygen on the rate of formation. The
addition of oxygen could also increase DD formation; however,
since it remains adsorbed on the surface it can undergo a series
of additional surface-catalyzed oxidation reactions. This thesis
is supported by the observation that DD has a maximum yield
under pyrolytic conditions at 450 °C, but has an almost
undetectable yield at the same temperature under oxidative
conditions. Moreover, the chlorination process can contribute
to the decline of the DD yield. In fact, formation of higher
chlorinated congeners through the chlorination process is
associated with the adsorbed DD (vide infra), and a significant
increase in the total PCDD yield compared to pyrolytic
conditions (Figure 4) is observed. The majority of the PCDD
congeners are highly chlorinated PCDDs. These effects can
justify the observed “formal” decrease of DD yield under
oxidative conditions.
The effect of oxygen on the degree of chlorination of PCDD/F
needs more attention. Higher chlorinated PCDFs were not
detected under oxidative or pyrolytic conditions which we
attribute to immediate desorption of the initially formed DCDF.
In contrast, more highly chlorinated PCDDs were formed under
oxidative than pyrolytic conditions (cf. Figure 4). However,
formation of highly chlorinated PCDDs is suppressed above 350
°C under pyrolytic conditions in which the rate of desorption
is higher than the rate of chlorination.32 Conversely, under
oxidative conditions, the chlorination process is even more
effective above 350 °C than below this temperature. Considering
that, under pyrolytic conditions, DD desorption becomes
significant at 350 °C (sharp increase of DD yield), it can be
concluded that under oxidative conditions the rate of chlorination
of PCDDs is greater than the rate of desorption. This explains
SCHEME 3: Formation of Surface Hypochlorite Species

4392 J. Phys. Chem. A, Vol. 107, No. 22, 2003
Lomnicki and Dellinger
SCHEME 4: Chloro-quinone (CQ), Chloro-catechol
(CC), and PCDD
identification and nature of the species responsible for chlorina-
tion under oxygen-rich conditions.
Intermediates. Under pyrolytic conditions, two possible
intermediates in the formation of PCDD/F were recognized, i.e.,
3-chloro-1,2-benzendiol (3-chlorocatechol or CC) and 1,4-
cyclohexadiene-4-chloro-5,6-one (chloro-o-quinone or CQ).
Similar products have been previously reported as intermediates
of the phenol oxidation processes. In particular, during the
photocatalytic oxidation of phenols in solution, catechols and
quinones were detected.43-45 Because CQ and CC both contain
ortho oxygens atoms, they are likely intermediates in the
formation of oligomers of 2-MCP (Scheme 4). The data obtained
under pyrolytic conditions indicate a correlation between the
CC and PCDD formation, i.e., both DD and CC increase with
temperature until 350 °C, where significant increase in DD and
drastic decline in CC yields are observed.
CQ may be a product of further oxidation of CC, or they
may share a common precursor.46 Figure 5 presents a compari-
son of CC and CQ formation as a function of temperature under
pyrolytic and oxidative conditions. The yield of CC is slightly
increased in the presence of oxygen, while the overall shapes
of the pyrolysis and oxidation curves are otherwise similar. In
contrast, the presence of molecular oxygen increases the CQ
yields almost 2 orders of magnitude in the low-temperature
region and a factor of 10 above 350 °C. Since CQ is a product
of either CC oxidation or they share a common precursor, one
might anticipate that CC would be also formed at a higher rate
when oxygen is present in the stream and therefore have a higher
concentration. However, the lack of a significant increase in
the CC concentration in the effluent indicates that it can undergo
further transformations. PCDD yields increase significantly
under oxidative conditions (contrary to PCDF which are not
affected by the presence of oxygen). Thus, we conclude that
CC is indeed involved in the formation of the PCDDs, which
is supported by the fact that at temperatures where PCDD yields
increase sharply (above 350 °C), the CC concentration drops
drastically.
Figure 6. EPR spectra of CuO(5%)/SiO2 before and after 2-MCP
adsorption.
associated chlorophenoxyl radical and M(n+)-1 site. Such
48
interaction was proposed for the phenol adsorption over a
Co-Fe-O system.
The results of our EPR studies on 2-MCP adsorption over
copper oxide/silica catalyst are presented in Figure 6. The CuO/
silica sample displays a broad signal originating from Cu(II)
complexes. When the sample was exposed to 4 Torr of 2-MCP
at 200 °C for 2 min, a new, sharp signal with a g-value of 2.0028
appeared superimposed on the broad copper signal (cf. Figure
6b), which we attribute to a radical formed as a result of
chlorophenol interactions with copper oxide surface. Similar
interaction was reported earlier for various chlorophenol
isomers.53,54 Signals with similar parameters were observed for
adsorption of 4-, and 3-chlorophenol, DD and 4-chloroanisole
over Cu(II) smectites.54-56 The authors of the latter have
attributed this signal to the formation of the keto form of a
radical with the electron delocalized on the ortho and para
positions of the ring. This explanation is also plausible in our
case, since oxygen centered radicals easily convert to their
respective keto forms,8,13,21,34 and they have much higher
g-values.57
This experiment confirms that upon adsorption of 2-chloro-
phenol over copper oxide, the interaction of adsorbed phenolate
species with the surface results in formation of phenoxyl radical.
This type of radical is resonance stabilized with the radical center
delocalized. The most favorable positions for the electron to
be localized are the oxygen atom and the ortho and para
positions in the ring.8,13 Taking into the account the above
discussion, the proposed adsorption mechanism is presented in
Scheme 5.
A very interesting conclusion can be drawn by comparison
of our results with the EPR studies of 2,4,6-trichlorophenol
adsorbed on copper oxide.58 The authors of this work have
detected a radical signal with g ) 2.0063 and attributed it to
adsorbed phenoxyl radical species. This is in contrast with our
case, where the g-value was 2.0028. On the basis of the g-value
we attribute our signal from 2-MCP to a carbon centered radical,
while the signal observed for 2,4,6-TCP was attributed to an
oxygen-centered radical. This difference suggests that if ortho
and para positions in the ring are substituted (this is true at least
for chlorine) the equilibrium between different mesomers is
shifted toward the oxygen-centered phenoxyl radicals. When
ortho or para positions are not substituted, the carbon centered,
Detailed Reaction Mechanism. The first step of every
catalytic reaction requires adsorption of the reactant on the
catalytic surface. Phenol and chlorophenol adsorption has been
studied extensively. It is generally believed that phenols adsorb
on the surfaces through H2O elimination at surface oxide and
hydroxyl sites resulting in surface phenolate formation.31,47-50
However, the subsequent steps in their surface reaction are not
well understood. Some authors propose that phenolates withdraw
electrons from the metal site leading to carbanion formation.51
However, since chlorophenols are known to be electron
donors,52 it is more likely that the electron transfer is from
chlorophenolate to the metal cation site (Mn+) forming surface-

Mechanism of Catalytic PCDD/F Formation
J. Phys. Chem. A, Vol. 107, No. 22, 2003 4393
SCHEME 5: Adsorption of 2-MCP on Copper (II)
Oxide
that the reaction involves the latter. This species can further
react via two pathways: desorption followed by hydrogen
elimination as copper is reduced to Cu(0) and form CQ, or
tautomerization to surface species.8 Species 8 results in the
formation of CC, and Cu(0) is again formed. Species 8 can also
be subject to chlorination. In fact, trace quantities of dichloro-
catechol were detected in the gas phase effluent. However, since
the concentration of the latter is very low, it is reasonable to
assume that species 8 transforms rapidly to other surface
associated structures, preventing chlorination of surface-bound
catechol.
CC is an intermediate in PCDD formation, which is reflected
in correlation of CC and DD yields. Species 8 can react with
gas-phase 2-MCP (cf. Scheme 8). There are two possible modes
of attack, each of them leading to different surface species (9
and 11) and, as a consequence, different reaction products. In
Scheme 8a, MCDD is formed as it is desorbed from the surface
following hydrogen transfer to the surface via the same cyclic
pathway proposed in Scheme 5. In contrast in Scheme 8b, the
formation of DD involves ring closure via HCl elimination, and
DD remains attached to the surface. The attachment of DD to
the surface explains its lower concentration than DCDF or
MCDD as well as the presence of higher PCDDs (but not higher
chlorinated PCDFs), i.e., DD is chlorinated, while it is chemi-
sorbed to the surface. At temperature >350 °C the C-O-Cu
bond is likely to rupture, producing gas-phase DD (after
hydrogen insertion), which explains the drastic increase in DD
yield above this temperature.
keto form of the radical appears to be more stable on the surface
(cf. Scheme 5).
This finding has implications for the mechanism of formation
of PCDDs and PCDFs. Two different surface mechanisms are
involved in PCDD/F formation (vide supra); PCDFs are formed
according to a Langmuir-Hinshelwood pathway and PCDDs
are produced as a result of an Eley-Rideal mechanism. The
surface species (3) presented in Scheme 5 is a surface equivalent
for the gas-phase, carbon-centered radical that is a PCDF
precursor.21,34 We can expect that reaction of two surface keto-,
carbon-centered radicals will result in the formation of PCDFs
(cf. Scheme 6). As a result of surface radical-radical interaction
they undergo recombination and tautomerization leading to
species.4 The adsorbed species eliminates a hydrogen to form
It is interesting to note that DCHDPE formation increases as
DCDF concentrations decrease. We believe this is the result of
the reaction between the surface bound radical (3) with the gas-
phase chlorophenol (Scheme 9).
5
species as one of the Cu(I) ions is oxidized back to Cu(II).
V. Conclusions
Ring closure then occurs via a cyclic transition state that results
in regeneration of the surface hydroxyl group on Cu(II) in a
catalytic cycle. The desorption of 4,6-PCDF from the surface
as it is formed explains the lack of observation of more highly
chlorinated PCDFs.
CC and CQ are also formed from species 3, but by a parallel
reaction pathway. Species 3 reacts with the terminal oxygen
ion of the copper oxide surface (cf. Scheme 7) forming species
7. A similar surface process has been previously reported;48
however, the proposed mechanism involved bonding between
a carbon center and reduced metal. Since the metal sites are
coordinated to more than one terminal oxygen ligand, we believe
The results of this study have multiple implications for
copper-catalyzed PCDD/F formation from chlorinated phenols.
(i) The major PCDD/F products are the same as those
predicted and observed for the gas-phase reaction of 2-MCP,
i.e., MCDD > DCDF > DD. This is attributable to a general
transferrability of the gas-phase radical-radical and radical-
molecule mechanisms of formation to the surface. PCDD/F were
formed as low as 225 °C and reached a maximum in the 325-
475 °C temperature range.
(ii) Upon 2-monochlorophenol adsorption on CuO/silica
catalyst, an EPR signal with a g-value of 2.0028 was observed.
SCHEME 6: Surface-Mediated Formation of DCDF

4394 J. Phys. Chem. A, Vol. 107, No. 22, 2003
Lomnicki and Dellinger
SCHEME 7: Surface-Mediated Chloro-catechol and Chloro-quinone Formation
On the basis of comparison to other studies in the literature, it
is assignable as the carbon-centered, keto-mesomer of phenoxyl
radical. This further supports the contention that surface-assisted
radical reactions are active in PCDD/F formation.
under pyrolytic conditions, highly chlorinated PCDDs were not
observed due to DD desorption prior to chlorination.
(vii) Chlorination is enhanced in the presence of oxygen
(versus pyrolysis) due to the formation of surface hypochlorite
species.
(viii) Chlorinated catechol and chlorinated quinone are formed
via competitive side reactions. Adsorbed catechol is an inter-
mediate in formation of PCDDs. Both catechols and quinones
are precursors to semiquinone type radicals that are known to
reduce molecular oxygen to superoxide.
This work also has implications for the possible involvement
of other species, i.e., chlorinated benzenes, in the formation of
PCDD/F. It is reasonably well established that the exhaust of
incinerators contain higher concentrations of chlorinated ben-
zenes than chlorinated phenols.59-61 On the basis of our
proposed mechanism of formation, this suggests significant
consequences. It is possible, even likely, that chlorobenzenes
will adsorb on transition metal surfaces by HCl elimination to
again form surface phenoxyls. If gas-phase chlorophenols
are also present, surface-catalyzed PCDF formation is limited
by the competition with the fast reaction of surface bound
phenoxyls with gas-phase chlorophenols that lead to side-
products such as DCHDPE. However, if chlorobenzenes are the
dominant gas-phase species, there will be minimal competition
with the Langmuir-Hinshelwood surface reaction to form
PCDFs because the reaction of surface-bound chlorophenoxyl
(iii) A negative reaction order was observed for DCDF
formation, indicating a Langmuir-Hinshelwood mechanism of
formation that involved two surface adsorbed chlorophenols.
Positive reaction orders were observed for MCDD and DD
formation. For the latter two, an Eley-Rideal mechanism
involving one surface adsorbed and one gas-phase chlorophenol
molecules can explain the observed product distribution.
(iv) Increasing the concentration of 2-MCP reduced the yield
of DCDF. This is attributable to the more rapid rate of the
competing reaction of the surface adsorbed chlorophenoxyl with
gas phase 2-MCP than the reaction between two surface bound
species. As a result dichloro-hydoxy diphenyl ether (DCHDPE)
is formed.
(v) The only observed PCDF was 4,6-dichloro-dibenzofuran.
This was attributed to a mechanism of formation in which DCDF
was immediately desorbed from the surface upon formation such
that no additional chlorination could occur.
(vi) Highly chlorinated congeners of PCDD were observed
in addition to MCDD and DD. Two different pathways of their
formation were proposed. Like DCDF, MCDD is immediately
desorbed upon formation. DD remains chemisorbed and under-
goes chlorination. In contrast, at higher temperature (>350 °C)
SCHEME 8: Surface-Mediated MCDD and DD Formation

Mechanism of Catalytic PCDD/F Formation
J. Phys. Chem. A, Vol. 107, No. 22, 2003 4395
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