Catalytic Oxidation of Naphthalene Using a Pt/Al2O3
Catalyst with Ozone
Paper # 90
Prepared by
Min-Hao Yuan,
Graduate Institute of Environmental Engineering, National Taiwan University, 71 Chou-Shan
Rd., Taipei 106, Taiwan
Ching-Yuan Chang,
Graduate Institute of Environmental Engineering, National Taiwan University, 71 Chou-Shan
Rd., Taipei 106, Taiwan
Je-Lueng Shie,
Department of Environmental Engineering, National I-Lan University, 1 Shen-Lung Rd., Sec.
1, I-Lan 260, Taiwan
Wen-Kai Du,
Graduate Institute of Environmental Engineering, National Taiwan University, 71 Chou-Shan
Rd., Taipei 106, Taiwan
Duu-Jung Lee,
Department of Chemical Engineering, National Taiwan University, 1 Roosevelt Rd., Sec. 4,
Taipei 106, Taiwan
Wen-Tien Tsai,
Department of Environmental Engineering and Science, Chia Nan University of Pharmacy
and Science, 60 Erh-Jen Rd., Sec. 1, Jen-Te, Tainan 717, Taiwan
ABSTRACT
This study investigated the application of catalytic oxidation process with ozone to
decompose polycyclic aromatic hydrocarbons (PAHs). The process is noted as
ozone-catalytic oxidation (OZCO). Naphthalene (Nap), which is the simplest and lowest
toxic PAH, was taken as a target compound. The emission of Nap to the atmosphere was
generally found from waste incineration or diesel engine combustion. Pt/Al2O3 catalyst was
used to facilitate OZCO process. The relationships between the conversion of Nap (XNap)
and related factors such as reaction temperature (T), space velocity (SV), and concentration
of ozone (CO3) were examined. Furthermore, the corresponding reaction mechanisms and
the roles of catalysts and ozone were discussed.
The results indicated that the required reaction temperatures for effective decomposition of
Nap decreased with the increase of inlet concentration of ozone (CO3,in) at the same
conversion level of Nap. Regarding the temperature at conversion of 50% (T50), there was
an approximate reduction of 20 K for the case of OZCO process with CO3,in of 1750 ppmv
(T50 = 460 K) compared to the process without ozone (T50 = 480 K). The value of
mineralization extent of Nap (MNap) with respect to the complete conversion of Nap to CO2
can reach as high as 95% at 490 K with CO3,in in the range of 900–1750 ppmv. Thus, OZCO
process is effective for the mineralization of Nap.
1
INTRODUCTION
Polycyclic aromatic hydrocarbons (PAHs) are classified as a particular group of contaminants
included in the lists of persistent organic pollutants (POPs) and environmental hormones.
Most of PAHs possess carcinogenicity and mutagenicity. The major emissions of PAHs are
from the incomplete combustion of waste incineration, operations of diesel engine and motor
vehicles, pyrolysis and gasification processes of coal and oil, and domestic combustion of
coal and wood, etc.1-3 Among the member of PAHs, naphthalene (Nap) is the most volatile
and simple structured compound and has been usually is taken as a target pollutant.
Furthermore, in urban atmosphere, it is the most abundant substance in the said group of
pollutants.4-5 A large portion of Nap was discharged into the atmosphere (90%), while a
small portion of it was discharged to water (5%) and soil (3%).6
The catalytic oxidation has been known as one of the most cost-effective technologies to
decompose volatile organic compounds (VOCs). It possesses high destruction efficiency,
economic benefits, and good applicability.7-10 As for catalysts, Pt/Al2O3 was employed for
catalytic oxidation and recognized as one of the most active catalysts for significant oxidation
of various VOCs,7 including Nap.11-12
Catalytic oxidation with ozone, noted as
ozone-catalytic oxidation (OZCO)13-15 or catalytic ozonation,16 was applied to the decompose
benzene, carbon monoxide, and other hydrocarbons (HCs) in the gas phase.13-23 Moreover,
this technique was used in the oxidation of coke for the continuous catalyst regeneration
(CCR) of naphtha reforming process.24-25 In general, the use of ozone as an oxidant is
particularly powerful at low VOC concentrations. The decomposition of ozone can form
highly reactive oxygen species which can oxidize VOCs at low temperatures.18
Nevertheless, Gervasini et al.21 pointed out that it is difficult to conclude a general trend of
destruction efficiencies of VOCs that have different chemical properties via OZCO.
Basically, ozone has distinct destruction efficiencies for certain aromatic and halogenated
VOCs, while is less effective for oxygenates.
Various types of metal oxide catalysts have been used for OZCO process. Einaga and
Futamura18 examined the catalytic activities of oxides such as Mn, Fe, Co, Ni, Cu, and Ag,
supported on γ
-alumina catalysts in the OZCO process for treating benzene and cyclohexane.
The results indicated that MnO2 is the most superior catalyst among the above metal oxides
tested. Similar findings were reported by Dhandapani and Oyama.26 Moreover, they
found that good linear relationship holds between the rates for decomposition of HCs and
ozone on several metal oxide catalysts. Thus, the enhancing effect of ozone on the
destruction of HCs is mainly governed by the decomposition rate of ozone over catalysts.
Chang et al.27 reported that Pt/Al2O3 gives higher decomposition rate of ozone than MnO2
catalyst at the same operation conditions. Therefore, Pt/Al2O3 is a more suitable catalyst for
OZCO process than MnO2.
In the present study, oxidation of Nap with ozone over Pt/Al2O3 catalyst was investigated in a
tubular fix-bed flow reactor at various operation conditions. The relationships between the
conversion (XNap) and mineralization extent (MNap) of Nap with related factors such as
reaction temperature (T), space velocity (SV) corresponding to particles of catalyst, and inlet
or initial concentration of ozone (CO3,in) were elucidated. In addition, the related reaction
mechanisms were also proposed.
2
EXPERIMENTAL
A schematic diagram of the fixed-bed catalytic reaction system is shown in Figure 1. The
system includes four parts: (1) feed gas A (eg., O2), (2) feed gas B (eg., N2), (3) OZCO, and
(4) analysis of effluent gas.
The feed gas A was O2 (99.99% O2, from Ching-Feng-Harng Co., Ltd., Taipei, Taiwan). It
flowed through a molecular sieve column (Supelco Co., Bellefonte, PA, USA) and then was
introduced to an ozone generator (Tairex Co., Taichung, Taiwan) to provide ozone. Two
mass flow controllers (HFC-202 and HFC-202B, Teledyne Hasting-Raydist, Hampton,
Virginia, USA) were used to control the volumetric inflow and outflow rates of the ozone
generator. Excess ozone gas was passed through the KI solution by the three-way value.
Nitrogen gas (99.99% N2, from Ching-Feng-Harng Co., Ltd.) from cylinder B was metered to
the system by a mass flow controller (HFC-202D, Teledyne Hasting-Raydist) and used as the
carrier gas to vaporize Nap. It passed through a conditioning bottle to buffer the gas flow
and then to another bottle full of solid Nap (purity > 99%, from Merck Ltd. Taiwan Branch,
Taipei, Taiwan) to generate Nap vapor. The concentration of Nap generated reached
steady-state value at 333 K after one hour. The temperature of Nap vapor generator was
controlled at 333 K with a heating tape (HT 352, Electrothermal Engineering Ltd., Southend
on sea, Essex, UK) wrapped outside the generator to provide a constant Nap vapor
concentration of 600 ppmv. The other parts of setup following the Nap vapor generator to
the heating furnace were also wrapped with a heating tape keeping the temperature at 373 K
to prevent undesired condensation of Nap before sampling.
The instrumental parts of OZCO were composed of a tubular fixed-bed reactor (quartz), an
electrical heating furnace (San-Ja Electric and Machinery Ltd., Taipei, Taiwan), a
proportional integral derivative (PID) controller (H720, Eurothem Co., Taipei, Taiwan), two
chromel-alumel (K-type) thermocouples, and one data acquisition system (PICO, TC-08 (8
channels), Labview 5.0 (software), Karma Technology Ltd., Taipei, Taiwan). The mixture
of feed gases A and B, which was composed of O2, O3, N2, and Nap, was injected into the
catalytic reactor (quartz) maintained at the pre-set temperature. The composition of O2 in
the mixed gas was close to 21 vol.%. The commercial catalyst of DASH-220N employed
was obtained from N. E. Chemical Co., Japan. The catalyst consisted of 0.23 wt.% Pt
s
uppor
t
e
donγ
-Al2O3 with the bulk density of 770 g/L. The catalyst was ground and sieved
to 40-50 ASTM meshes (0.297–0.42 mm) to eliminate the intra-particle pore diffusion
resistance. Details of reaction system without ozone were described in our previous work. 11
The instruments for gas analysis included a GC-FID (HP 5890 series II, Hewlett Packard Inc.,
Palo Alto, California, USA) and monitoring devices of CO (T82, Industrial Scientific Corp.,
Oakdale, PA, USA) and CO2 (YES-206, YES Environment Technologies Inc., Delta,
BC, Canada). The GC-FID was used for the quantitative analyses of gaseous products such
as Nap and HCs. XNap of Nap was computed from the concentrations of the feed (CNap,in)
and effluent (CNap,out) streams. MNap of Nap was calculated from CNap,in and the
concentration of CO2 of effluent steam (CCO2,out). The applicable expressions for computing
XNap and MNap are as follows.
C Nap,in C Nap,out
X Nap 
C Nap,in
(1)
C
M Nap  CO2,out
10C Nap,in
(2)
3
The units of CCO2 and CNap in Eq. (2) are in vol.% . It is noted that a complete
mineralization of 1 mole of Nap produces 10 moles of CO2. Thus, MNap is relative to
complete conversion to CO2.
Figure 1: Schematic layout for the ozone-catalytic oxidation of Nap
Components: 1. cylinder A, 2. cylinder B, 3. molecular sieve columns, 4. mass flow controller,
5. ozone generator, 6. three-way valve, 7. naphthalene generator, 8. Y-type valve, 9.
sampling ports (to GC), 10. tubular fixed-bed reactor, 11. heating furnace, 12. thermocouple,
13. temperature controller, 14. CO monitor, 15. CO2 monitor, 16. KI solution, 17. vent to hood.
RESULTS AND DISCUSSION
Thermal Decomposition of O3 with and without Pt/Al2O3
Figure 2 (a) displays the temperature variations of residual fraction of O3 (1–XO3) with and
without Pt/Al2O3 catalyst. The results indicated that O3 was thermally decomposed starting
from about 390 K toward complete decomposition at 475 K. Therefore, the temperature for
pre-heating Nap generator at about 333 K did not affect the initial concentration of ozone
(CO3,in). The solid line of (a) was simulated using the kinetic model from Benson and
Axworthy.28 They reported that O3 is decomposed depending on T and the oxygen
concentration as follows.
4
k
1

O3 M X 
O O 2 M X
(3)
k2
k
3
O O3 
2O 2
(4)
where
MX = species of O3, O2, and N2,
[MX] = [O3] + 0.44[O2] + 0.41[N2]
(5)
with [ ] = concentration.
The kinetic equations according to Eqs. (3) and (4) are:
1  k 2 [O 2 ]
1



2


rO 3 2k 1 k 3 [O 3 ]
2k 1 [M X ][O 3 ]
(6)
rO3 = –d [O3]/dt, reaction rate of O3, [mol L-1 s-1]
(7)
k 1 4.61 1012 exp
24000 / RT 
, [L mol-1 s-1]
(8)
k 2 6 10 7 exp
600 / RT 
, [L2 mol-2 s-1]
(9)
where
, [L mol-1 s-1]
k 3 2.96 1010 exp
6000 / RT 
(10)
with units of concentration, time, and T in mol L-1, s, and K. The constant R is 8.314 J mol-1
K-1. Figure 2 (b) shows that ozone was completely decomposed on the catalyst surface as T
> 340 K with space velocity of 95,000 hr-1. The solid line of (b) was simulated applying the
kinetic expression of Chang et al.27 based on Eley-Rideal model which was used to describe
the catalytic decomposition of ozone on Pt/Al2O3 catalyst. The corresponding reactions are
as follows.
O3 + X → OX + O2
(11)
O3 + OX → X + 2O2
(12)
where X represents the active site of catalyst.
where
The corresponding rate equation is:
rO3 = -d [O3]/dt = k'[O3]
(13)
k ' 3681 exp
51100 / RT 
, [s-1]
(14)
The comparison of predicated and experimental results indicated good consistency.
5
Figure 2:
Thermal decomposition of ozone with and without Pt/Al2O3
1
1-XO3
0.8
0.6
(b)
(a)
0.4
0.2
0
250
300
350
400
T, K
450
500
550
(a). : without catalyst at CO3,in = 4300 ppmv, QG = 242.6 mL min-1, VR = 40 mL, tR = 9.9 s. (b).
: with Pt/Al2O3 catalyst at CO3,in = 1800 ppmv, SV = 95,000 hr-1. XO3: conversion of O3. T:
reaction temperature. CO3,in: initial concentration of O3. QG: gas flow rate. VR: volume of
reactor. tR: residence time of reactant gas in reactor (VR/QG). SV: space velocity. , :
experiments. Solid lines: model predictions. O3 was generated from O2 and then mixed with
N2 with volumetric ratio of O2 to N2 equal to 21/79.
OZCO Process of Nap over Pt/Al2O3
Figure 3 depicts the temperature variations of residual fraction (1–XNap) and mineralization
extent (MNap) of Nap via OZCO process over Pt/Al2O3 catalyst with CO3,in as a parameter.
The required reaction temperatures decreased with increase of CO3,in at the same conversion
level of Nap. Regarding the temperature at conversion of 50% (T50), there was about 20 K
reduction of T50 for the case of OZCO process with CO3,in of 1750 ppmv (T50 = 460 K),
compared to that of the process without ozone (T50 = 480 K). Furthermore, OZCO process
can effectively enhance the mineralization extent of Nap. The value of MNap can be as high
as 95% at 490 K with CO3,in in the range of 900 to 1750 ppmv.
Figure 4 presents the results of 1-XNap versus T at SV of 50,000 and 100,000 hr-1 for OZCO
process of Nap over Pt/Al2O3 catalyst. The corresponding contact times (tC = 1/SV) of the
reactant gas to particles of catalyst are 0.072 and 0.036 s, respectively. However, there were
no significant differences of XNap and of MNap with values of tC from 0.036 to 0.072 s. The
results indicated that the total volume of particles of catalyst is not a major influencing factor
compared to CO3,in.
6
100
100
80
80
60
60
40
40
20
20
0
400
420
440
460 480
T, K
500
520
MNap
Conversion (XNap) and mineralization extent (MNap) of Nap via OZCO
at SV = 100,000 hr-1
1-XNap
Figure 3:
0
540
, , , : residual Nap (1-XNap) at CO3,in of 0, 900, 1250, 1750 ppmv, respectively.
, , , : corresponding MNap. CNap,in = 600 ± 50 ppmv.
XNap and MNap of Nap via OZCO process at CO3,in = 1250 ppmv
100
100
80
80
60
60
40
40
20
20
MNap
1-XNap
Figure 4:
0
400
420
440
460
480
500
520
0
540
T, K
 and : residual Nap (1-XNap) at SV of 50,000 and 100,000 hr-1, respectively.
 and : corresponding MNap. CNap,in = 600 ± 50 ppmv.
7
The reaction mechanism of OZCO process over metal oxide catalysts was reported by
Mehandjiev et al.14-15 and Einaga and Futamura.19 They proposed that the reaction kinetic
equation of O3 can be described using Eley-Rideal model, while O2 is not adsorbed on the
active sites of catalyst. However, for the catalytic decomposition of O2 in air via Pt, the
dissociative chemisorption of O2 on Pt catalyst takes place because of the interactions of O
(2p) and Pt (6sp) and of O (2p) and Pt (5d).29 Furthermore, the aromatic compounds are
strongly adsorbed on the surface of Pt catalyst via the interaction between π-bonding of Pt
crystal and aromatic ring oriented parallelly to the plane.30-32 Therefore, according to the
findings of the above studies, a Langmuir-Hinshelwood model may be applicable for the
catalytic ozonation of aromatic compound via Pt catalyst. Thus, the mechanism of
ozonation of Nap over Pt/Al2O3 can be proposed as follows.
For adsorption:
k
aO

 2OX
O2 2X 

k'aO
k
aZ

 OX O 2
O3 X 

k 'aZ
k aN

 NapX
Nap X 

k'aN
(15)
(16)
(17)
For surface reaction:
k
sN

 PX X
NapX OX 

k'sN
(18)
For desorption:
k dP

 P X
PX 

k'dP
(19)
Overall reaction {(15) + (16) + 3 × [(17) + (18) + (19)]}:

 3P
O3 3Nap 

(20)
where P represents product of the reaction.
If the reaction of Eq. (16) is predominant compared to the reaction of Eq. (15), then the above
equations can be simplified as follows.

 OX + O2
O3 + X 

(16)

 NapX
Nap + X 

(17)

 PX + X
NapX + OX 

(18)

 P+X
PX 

(19)
8
The overall reaction becomes

 P + O2
O3 + Nap 

Assume the surface reaction (Eq. (18)) is irreversible and controlling.
reaction rate (rs) of Nap is as
(21)
Then, the surface
rs = ksN qm O3 N
(22)
rs = –
d [Nap]/dt, reaction rate of Nap, or moles of Nap reacted per gram of
catalyst per time, [mol s-1 (g-cat.)-1]
(23)
where
ksN = reaction rate constant of Nap, [s-1]
qm = concentration corresponding to a complete coverage of monomolecular layer on
catalyst, [mol (g-cat.)-1]
O3 = fraction of surface coverage of O3, [-]
N = fraction of surface coverage of Nap, [-]
According to the Langmuir-Hinshelwood model,33-35 the expressions of O3 and N are as
follows.
O3 = K O3 C O3 /(1 + K O3 C O3 + KNCN + KPCP + K O 2 C O 2 )
(24)
N = KNCN/(1 + K O3 C O3 + KNCN + KPCP + K O 2 C O 2 )
(25)
where
K O3 , KN, KP, K O 2 = adsorption equilibrium constants of O3, Nap, product, O2,
[L mol-1]
C O3 , CN, CP, C O 2 = concentrations of O3, Nap, product, O2, [mol L-1]
The last two terms in the denominators of Eqs. (24) and (25) are included to take into account
the possibilities that the product species and O2 may be adsorbed on the catalyst. If they
were not adsorbed, the corresponding terms can be omitted. Equations (24) and (25) then
become:
O3 = K O3 C O3 /(1 + K O3 C O3 + KNCN)
(26)
N = KNCN/(1 + K O3 C O3 + KNCN)
(27)
For a conventional expression of reaction rate (rc) with unit in moles per volume of reaction
fluid per time, Eq. (22) may be rewritten as
rc = –dCNap/dt = krc O3 N
(28)
where
9
CNap or CN = concentration of Nap in bulk fluid, [mol L-1]
krc = reaction rate constant of Nap, [mol L-1 s-1]
The unit of rc is mol L-1 s-1.
CONCLUSIONS
The catalytic oxidation of naphthalene (Nap) over Pt/Al2O3 catalyst in the presence of ozone
noted as OZCO process can not only decrease the reaction temperature but also increase the
mineralization extent of Nap (MNap). The reaction temperature decreased as the inlet ozone
concentration CO3,in increased at the same conversion level of Nap (XNap). The temperature
(T50) at 50% conversion of Nap for OZCO process with CO3,in at 1750 ppmv (T50 = 460 K)
decreased about 20 K compared to that for the process without ozone (T50 = 480 K). With
CO3,in in the range of 900 –1750 ppmv, the value of MNap can be as high as 95% at 490 K.
ACKNOWLEDGEMENT
The authors would like to thank the National Science Council of Taiwan for the financial
support of this research under Contract No. NSC 93-2211-E-002-015.
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KEY WORDS
Polycyclic aromatic hydrocarbons (PAHs), naphthalene (Nap), ozone-catalytic oxidation,
ozone, Pt catalyst, alumina
12