Photocatalytic oxidation of heptane in the gas-phase over TiO2

Article abstract of DOI:10.1016/S0045-6535(01)00115-1

VOC are important class of air pollutants, which are considered together with NO_x, SO_x, and particulates as the most important anthropogenic pollutants generated in urban and industrial areas. Gas-phase photocatalytic oxidation (PCO) of heptane over UV-illuminated TiO₂ was performed at ambient temperature in a batch reactor. Complete oxidation of heptane with nearly stoichiometric production of CO₂ and H₂O was obtained. The intermediates were propanal, butanal, 3-heptanone, 4-heptanone, and CO. The photocatalytic activity of TiO₂ could be sustained indefinitely due to the production of water in the system, which can replenish the consumed hydroxyl radicals. Reactive oxygen species, e.g., O₂/^-, O^-, O, and ^?OH, played important roles in the PCO of heptane. When hydroxyl radicals were consumed in the heterogeneous oxidation reactions, the surface must be continuously rehydrated if long-term catalytic activity is to be maintained.

Full text of DOI:10.1016/S0045-6535(01)00115-1

Chemosphere 46 +2002) 93±99
www.elsevier.com/locate/chemosphere
Photocatalytic oxidation of heptane in the
gas-phase over TiO2
*
Jing Shang , Yaoguo Du, Zili Xu
Department of Environmental Science and Engineering, Jilin University, Changchun 130023, People's Republic of China
Received 24 October 2000; received in revised form 26 March 2001; accepted 26 March 2001
Abstract
In this paper, gas-phase photocatalytic oxidation +PCO) of heptane over UV-illuminated TiO
ambient temperature in a batch reactor. Complete oxidation of heptane with almost stoichiometric production of CO
and H O was observed. The intermediates detected were propanal, butanal, 3-heptanone, 4-heptanone and carbon
monoxide. A scheme of the possible mechanism for PCO of heptane over TiO was suggested. Langmuir±Hinshelwood
kinetics equation was obtained from the results at di€erent initial concentrations of heptane, oxygen, moisture and light
intensity. The photocatalytic activity of TiO can be sustained inde®nitely. This can be attributed to the production
2
was carried out at
2
2
2
2
of water in the system, which can replenish the consumed hydroxyl radicals. Ó 2001 Elsevier Science Ltd. All rights
reserved.
Keywords: Photocatalytic oxidation mechanism; Reactive oxygen species; Reaction kinetics; Photocatalytic activity
1
. Introduction
1992; Yan et al., 1994; Herrmann, 1999; Assabane et al.,
000). Simultaneously, photocatalysis in gas-phase has
2
Volatile organic compounds +VOCs) are an im-
been attracting more and more attention +Obee and
Brown, 1995; Avila et al., 1998; Augugliaro et al., 1999).
As we know, alkanes, such as heptane, are among VOCs
in the air. Therefore, the studies on the gas-phase PCO
of alkanes become important.
portant class of air pollutants, which are considered
together with nitrogen oxides, sulfur oxides and particu-
lates as the most important anthropogenic pollutants
generated in urban and industrial areas +Avila et al.,
1
998). The term VOCs is used to identify all compounds
Djeghri et al. +1974) once reported photoinduced
containing carbon and present in the atmosphere, with
the exception of elemental carbon, carbon monoxide
and carbon dioxide +Augugliaro et al., 1999). VOCs
have drawn considerable attention in the last decade.
Among the methods to oxidize VOCs, heterogeneous
photocatalytic oxidation +PCO) is one of the most at-
tractive, due to the mild experimental conditions under
which the runs are usually carried out. Photocatalysis
has been largely used for photooxidation of many or-
ganic molecules in liquid-phase +Hustert and Zepp,
oxidation of C
ture. In general, they observed that alkanes ꢀC
formed ketones ꢀC 2nO† and other aldehydes ꢀC
2 8 2
±C alkanes on TiO at ambient tempera-
n
H
2n‡2
m
†
n
H
-
H
2mO† with 2 < m < n. If the alkane was branched, the
ketone was C 2mO with 3 < m < n. The reactivity of
di€erent types of carbon atoms followed the sequence:
m
H
C
tert > Cquat > Csec > Cprim. The carbon atom, which is
preferentially attacked by oxygen, is the one which pre-
sents the highest electron density together with the least
steric obstruction, but any of the carbon atoms of the
alkane molecule may be attacked. A proposed mecha-
nism for the oxidation of alkanes led to the formation of
an alcohol intermediate which in turn oxidized to an
aldehyde or ketone. The neutral atomic oxygen, O, is
*
Corresponding author. Fax: ꢀ86-431-8923907.
E-mail address: dashanshang@yahoo.com.cn +J. Shang).
0
045-6535/02/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved.
PII: S0045-6535+01)00115-1

94
J. Shang et al. / Chemosphere 46 ꢀ2002) 93±99
reactive species to form alcohols from alkanes. This O
species is assumed to be formed by the reaction between
by NaOH solution. The resultant solution was re¯uxed
until the color of the solution changed from yellow to
white. Then 5 ml deionized water was added and the
white precipitate was washed by ethanol. After drying,
the substrate was calcinated at 573 K. X-ray analysis
+D/Max-cA powder di€ractometer with nickel ®ltered
CuKa source) of the TiO particle revealed the presence
2
of anatase only and the mean crystallite size was esti-
mated to be 12 nm using the Scherrer equation +Ogawa
À
adsorbed oxygen species such as O2 and positive hole.
À
ESR measurements have detected the existence of O2 in
the presence of molecular oxygen on irradiated TiO
2
sample +Gravelle et al., 1971), and it is then quite possible
À
that O2 is able to combine with positive holes to form
free or trapped, neutral or ionic atomic oxygen species
+
Bickley et al., 1973). Herrmann et al. +1979) stated that
À
reactive oxygen species such as O and O2 played im-
portant roles in the PCO of isobutane. The mechanism
suggested involved O attack of isobutane to form acti-
vated intermediate, which was then transferred to ®nal
and Abe, 1981). The speci®c surface area of the TiO
2
2
particle was 60:5 m =g, as determined by BET method
+Micromeritics ASAP 2010).
products +acetone, CO
2
and H
The PCO of simple light alkanes +<C
2
O) by the aid of O
2
.
2
.2. Photocatalytic reaction
3
), especially
methane, into oxygen containing chemicals has also at-
tracted much attention +Gratzel et al., 1989; Suzuki
et al., 1990; Wada et al., 1993). For example, Wada et al.
The light source used in this investigation was a
0-cm-long 400 W UV lamp. The primary wavelength
distribution of this lamp is at 365 nm and the light in-
2
+
1 3
1993) investigated the PCO of simple C ±C alkanes at
2
tensity is 5:3 mW=cm at 15 cm away from the lamp.
temperatures from ambient to ca. 550 K. They observed
that the PCO of methane produced a small amount of
methanal together with carbon dioxide, and the PCO of
ethane produced ethanal. The reaction with propane
gave propanal and acetone. The yields of the products
were very small at ambient temperature, and increased
with increasing temperature up to 550 K. The species
involving adsorbed oxygen must be partially responsible
for deep oxidation of alkanes or intermediates to carbon
dioxide. They assumed that the PCO of light alkanes
consists of two steps: the former step is the reaction
between photogenerated reactive oxygen species and
alkanes, and the latter includes thermochemical con-
version of intermediates to the ®nal products and de-
sorption of the products from the catalyst bed.
During the experimental period, the light intensity could
be varied by changing the distance between the lamp and
the reactor. The batch reactor used was a 300 ml cy-
lindrical quartz tube. In the experiment, 0.1 g of TiO
2
particles was spread uniformly over the internal surface
of the reactor. After this, the reactor was vacuum-
packed and suitable amounts of heptane, oxygen and
moisture were injected into the reactor through a sam-
ple port. Then ultrapurity nitrogen was mixed with the
reactants in the reactor to one atmospheric pressure.
When adsorption reached equilibrium, as measured by
gas chromatograph +GC), the reaction was then started
by turning on the lamp. Subsequently, the content of
heptane in the reactor, obtained by a gastight syringe
from the sample port, was measured every 15 min by a
Varian 3700 GC equipped with a ¯ame ionization de-
tector and an SE-54 quart capillary column at 323 K.
Most researchers proceeding with the gas-phase PCO
of alkanes are primarily interested in partial oxidation,
and the oxidation mechanisms have been focused on.
However, few studies have been carried out on either
reaction kinetics or the activity of photocatalyst. In
this paper, we present a study of the gas-phase PCO
of heptane at ambient temperature over UV-illuminated
The measurement of O
2
was performed by a GC
equipped with a thermal conductivity detector +TCD)
and a molecular sieve stainless column at 323 K. The
contents of CO, CO
+
2 2
and H O were determined by GC
TCD) using molecular sieve +323 K), GDX-303 +323 K)
2
TiO powder. Complete PCO of heptane was observed,
and GDX-401 +383 K) column, respectively. The gas-
phase intermediates produced in the system were ana-
lyzed by a HP 6890 GC/HP 5973 MS and were identi®ed
by comparison to the National Institute of Standards
and the oxidation mechanism, reaction kinetics, as
well as photocatalytic activity of the catalyst were
investigated.
+
NIST) mass spectra library.
2
. Experimental
3
. Results and discussion
2
.1. Preparation of the TiO
2
photocatalyst
3
.1. The products of PCO of heptane
The PCO of heptane did not proceed in the absence
The nanocrystalline TiO
2
catalyst used in this study
was prepared by the method of alkoxide hydrolysis.
Ethanol was slowly added to 20 ml TiCl to form
TiꢀOC †4 and the produced HCl smoke was adsorbed
4
of either TiO
2
or UV light irradiation. However, when
2
H
5
the gas mixture containing heptane +0.4%, v/v, i.e.,

J. Shang et al. / Chemosphere 46 ꢀ2002) 93±99
95
3
1
6:6 g=m ) and oxygen +20%, v/v) entered the photo-
reactor with 0.1 g UV-illuminated TiO , the PCO of
heptane occurred. There were propanal, butanal, 3-
heptanone, 4-heptanone, CO, CO and H O formed
Taking into account the photocatalytic properties of n-
semiconductor TiO , the initial reaction steps on UV-
illuminated TiO can be summarized in the following
2
2
2
2
2
scheme +subscript ads represent adsorption):
during the PCO of heptane. Because the production of
butanal and 4-heptanone was little, we selected prop-
anal, 3-heptanone and CO as intermediates to follow the
variations of their concentrations. Fig. 1 shows the
variations of concentrations of heptane and by-products
hm
À
‡
TiO
2
! TiO
2
ꢀe ‡ h †
ads
ꢀ2†
ꢀ3†
ꢀ4†
ꢀ5†
ꢀ6†
ꢀ7†
ꢀ8†
2ads ‡ e ! OÀ2
À
O
O
2ads ! 2Oads
Oads ‡ e ! OÀ
À
2 2
including CO ; H O, propanal, 3-heptanone and CO
ads
under irradiation time. From Fig. 1, we can see that the
concentration of heptane decreased with the irradiation
À
‡
Å
OH ‡ h ! OH
ads
‡
Å
‡
H
2
O
ads ‡ h ! OH ‡ H
2 2
time. The concentrations of CO and H O increased
‡
‡
16ads
with increasing irradiation time and changed little when
heptane oxidized completely. Propanal and 3-heptanone
reached their maximum concentrations after 30 and 60
min of irradiation, respectively, and then decayed. CO
built up almost to its maximum yield while heptane
oxidized completely, and then degraded gradually.
C
7
H
16ads ‡ h ! C
7
H
When a photon with energy of hm matches or exceeds
the band-gap energy, E
g
, of TiO
2
, the conduction-band
‡
À
electrons ꢀe † and valence-band holes ꢀh † generate on
2
the surface of TiO . In the absence of the electron and
About 99.7% of stoichiometric CO
formed after 360 min of irradiation.
2
and H
2
O had been
hole scavengers, most of them recombine with each
other within a few nanoseconds. If the scavengers or
surface defects are present to trap the electrons or holes,
À
‡
e ±h recombination can be prevented and the subse-
quent reactions caused by the electrons and holes may
be dramatically enhanced +Park et al., 1999). In this
3
.2. Mechanism aspects
From the results illustrated above, we know that the
PCO of heptane led to the formation of intermediates
case, oxygen adsorbed on the surface of TiO
signi®cant role in the reaction with electrons, with
2
plays a
such as aldehydes and ketones, and ®nally CO
2
and
O were produced. The PCO process of heptane can be
À
À
O ; O and O being the products of this electron trans-
2
H
written as
2
fer +reactions +3)±+5)) +Peral and Ollis, 1992). At the
same time, photogenerated holes are trapped by hy-
droxyl ions or water on the surface, producing hydroxyl
radicals +reactions +6) and +7)), which play important
TiO2;hm
TiO2;hm
C
7
H16 ‡ 11O
2
! intermediates ! 7CO
2
‡ 8H
2
O
ꢀ
1†
roles in photocatalytic reactions +Park et al., 1999).
À
Moreover, reactions among O , O and H
Å
2
O
to the formation of OH and HO ads, and HO
ads give rise
2
Å
Å
is
2
2ads
Å
another source of OH +Pellizzetti and Minero, 1993;
Augugliaro et al., 1999). In the gas phase, organic sub-
strates can themselves act as adsorbed traps for the
photogenerated holes +reaction ++8)), since water mole-
cules are not the predominant species in contact with
the catalyst in the gas phase. Although in the presence of
Å
water vapor, OH groups are present on the TiO
2
surface
and their contribution to photooxidation cannot be
discarded, the preliminary adsorption of organic com-
pounds is a very important prerequisite for highly e-
cient detoxi®cation +Fox and Dulay, 1993). In fact, in a
2
photocatalytic reaction, TiO photogenerated valence-
band holes +ꢀ3.1 V vs NHE at pH 0) are capable of
oxidizing any organic compound +Alberici and Jardim,
1
997).
No alcohol was detected in the present experiment,
but it was shown previously +Djeghri et al., 1974;
Djeghri and Teichner, 1980) that the ®rst step of the
PCO of alkanes was the formation of this intermediate
whose dehydration or oxidation proceeded generally
with a higher rate than its formation from alkanes. In
Fig. 1. Degradation of heptane +a), evolution of CO
O +c), and formation and decay of propanal +d), 3-heptanone
e) and CO +f) under irradiation +d, e and f were depicted in
2
+b) and
H
2
+
3
inset). Heptane concentration ˆ 16:6 g=m , O
2
ˆ 20% +v/v),
2
without water, UV ˆ 5:3 mW=cm .

96
J. Shang et al. / Chemosphere 46 ꢀ2002) 93±99
À
this case, reactive oxygen species such as O or O can
‡
kKC
1 ‡ KC
r ˆ
;
ꢀ14†
attack C
7
H16ads or C
7
H
to form C
7
H15OHads
.
1
6ads
The production of 3-heptanone and 4-heptanone in-
dicates the formation of 3-heptanol and 4-heptanol,
which can be attacked by hydroxyl radicals to produce
3
where r is the reaction rate ꢀg=m min†, C is the con-
3
centration of reactant ꢀg=m †, k is the rate constant
3
3
ꢀ
g=m min†, and K is the adsorption constant ꢀm =g†.
corresponding heptanone and H
+
2
O +reactions +9) and
Inverse of the equation gives
10)). 3-heptanol or 4-heptanol can also proceed with the
dehydration reaction to form 3-heptene, which can be
further oxidized to propanal and butanal +reactions
1
1
1
ˆ
‡ k
:
ꢀ15†
r
kKC
+
11)±+13)). The mechanism suggested here is consistent
with that of Blake and Grin +1988), which involved the
hydroxyl radicals attack of 1-butanol to give butanal
and water, or a dehydration leading to the formation of
If the assumed L±H form is valid, then a plot of 1=r vs
1=C should be linear.
Fig. 2 shows under the condition of a ®xed oxygen
3
1
-butene and water. It has also been observed that when
content of 20% +v/v), i.e., 265:6 g=m , and without
water, the variation of inverse of reaction rates ꢀ1=r†
the corresponding alcohols and alkenes are photooxi-
dized as reactants, the same ketones and aldehydes are
produced +Djeghri and Teichner, 1980).
under inverse of heptane concentrations ꢀ1=C †. From
h
Fig. 2, we can see that the experimental data are in good
agreement with this L±H rate form. Fig. 3 shows the
Å
3
CH
3
CH
CH
CH CH
CH CH
CHOHCH
2
CHOHCH
CH COCH
CHOHCH
CH COCH
CH CH
2
CH
CH
CH
2
CH
2
CH3ads ‡ 2 OH
variation of inverse of reaction rates ꢀ1=r† of 16:6 g=m
!
CH
3
2
2
2
CH
2
CH3ads ‡ 2H
2
O
O
ꢀ9†
heptane under di€erent inverse of oxygen concentrations
ꢀ1=CO2 †. The linear plot indicates that the kinetics of
oxygen also ®ts the L±H form. Fig. 4 shows the varia-
Å
3
2
2
2
2
CH3ads ‡ 2 OH
!
CH
3
2
2
2
CH
2
CH3ads ‡ 2H
2
ꢀ10†
3
tion of inverse of reaction rates ꢀ1=r† of 16:6 g=m
3
CH
2
2
2
2
CH3ads
heptane under di€erent concentrations of water ꢀC
†
w
3
ÀH2O
ranging from 3.7 to 22:4 g=m . The linear plot of 1=r
!
CH
3
CH
2
CH@CHCH
CH
2
CH
2
CH3ads
CH3ads
ꢀ11†
ꢀ12†
ꢀ13†
w
with C indicates that water in the gas feed inhibits the
CH
3
CH
2
CH
2
CHOHCH
2
2
CH3ads
heptane oxidation rate. Widely di€ering rate in¯uences
of water have been reported for di€erent gas-phase
photocatalytic oxidation reactions. Dibble and Raupp
ÀH2O
!
CH
3
CH
CH@CHCH
CH
2
CH@CHCH
2 2
CH
CH
3
CH
2
2
CH CH3ads
2
+
in water for H
1990) found the rate of TCE oxidation to be zero-order
À3
O
!
CH
3
2
CHOads ‡ CH
3
CH
2
CH
2
CHOads
2
O mole fractions below 10 and to be-
come strongly inhibitory with a -3-order rate depen-
À3
dence for water mole fractions between 5 Â 10 and
The resulting aldehydes and ketones can be oxidized
Å
À2
5
found that the PCO rate of trace toluene in air was
 10 . In contrast, Ibusuki and Takeuchi +1986)
to acids under the function of OH and O
2
+Sopyan and
Murasawa, 1994). CO
ther photodecarboxylation. The photodecarboxylation
of carboxylic acid in the presence of TiO powder in gas-
2
can then be derived from fur-
2
phase regime is well known +Sato, 1983; Augugliaro
et al., 1986; Muggli and Falconer, 1999). The mecha-
Å
nism involved OH radical attack, and also the direct
discharge of carboxylate ions by photogenerated holes
should also be taken into account. As for CO, reactive
À
À
oxygen species such as O , O and O have been sug-
gested as the adsorbed species responsible for the PCO
2
2
of CO to form CO +Sato and Kadowaki, 1987; Lins-
ebigler et al., 1996; Dickinson et al., 1999).
3
.3. Kinetics of the PCO of heptane
Langmuir±Hinshelwood +L±H) rate expression has
been widely used in liquid-phase photocatalysis and has
been found of utility in gas-phase photocatalysis +Peral
and Ollis, 1992; Alberici and Jardim, 1997; Vorontsov
et al., 1999). If the concentrations of oxygen and
water remain constant, this equation can be simpli®ed as
Fig. 2. The variation of inverse of reaction rates under di€erent
inverse of heptane concentrations. O ˆ 20% +v/v), without
2
2
water, UV ˆ 5:3 mW=cm .

J. Shang et al. / Chemosphere 46 ꢀ2002) 93±99
97
cules on the surface of catalyst to lower the reaction
rate.
From the analyses of Figs. 2±4 above, we can assume
the following L±H rate form, which describes reaction
rate concentration dependences:
ꢀ
ꢁꢀ
ꢁ
K
h
C
h
K 2
1 ‡ KO2 C 2
O
C 2
O
r ˆ k
:
ꢀ16†
1
‡ K
h
C
h
‡ K
w
C
w
O
3
3
Values of k ˆ 0:315 g=m min; K
h
3
ˆ 0:295 m =g; K
w
ˆ
3
0
:098 m =g and K
O
2
ˆ 0:0024 m =g were then calcu-
lated from Figs. 2±4. From the L±H rate expression of
heptane PCO described above, we can see that the ad-
sorptions of heptane and oxygen were on di€erent types
of sites and there were competitive adsorptions between
heptane and water vapor for the same site, assuming that
the major parameters were concentrations of heptane,
oxygen and the introduced water and no in¯uence of
reaction products on reaction rate.
Fig. 3. The variation of inverse of reaction rates under di€erent
inverse of oxygen concentrations. Heptane concentration ˆ
3
2
1
6:6 g=m , without water, UV ˆ 5:3 mW=cm .
The reaction rate constant is a function of the light
intensity with the following relationship +Peral and Ollis,
1
992):
a
k ˆ k
0
I ;
ꢀ17†
where k
0
is the rate constant independent of light in-
2
a
3
tensity, g=ꢀm minꢀmW=cm † †, I is the light intensity,
2
mW=cm , and a is the order constant. By substituting
the above relationship to the expression of +16) under
®xed initial concentrations of heptane, oxygen and hu-
midity, the rate expression should be
0
a
r ˆ k I ;
ꢀ18†
Fig. 4. The variation of inverse of reaction rates under di€erent
3
water concentrations. Heptane concentration ˆ 16:6 g=m , O
2
ˆ
2
2
0% +v/v), UV ˆ 5:3 mW=cm .
enhanced by water vapor, increasing almost linearly
with water vapor content between 0% and 60% relative
humidity. In the present study, water vapor concentra-
3
tions of 3.7±22:4 g=m are clearly inhibitory for heptane
photooxidation. The inhibitive e€ect of water can be
explained as +Peral and Ollis, 1992; Wang et al., 1998):
+
1) there is a competition for adsorption between hep-
tane and water vapor; +2) water vapor molecules may
block heptane adsorption sites and +3) the increased
content of water vapor can destroy the equilibrium
between consumption +due to the consumption of
hydroxyl free radicals) and adsorption of water +to
replenish the hydroxyl free radicals) to keep stable re-
action rate, with more adsorption of water vapor mole-
Fig. 5. The linear regression of log r vs log I. Heptane con-
3
centration ˆ 4:15 g=m ; O ˆ 20% +v/v), without water.
2

98
J. Shang et al. / Chemosphere 46 ꢀ2002) 93±99
where
Acknowledgements
ꢀ
ꢁꢀ
ꢁ
K
h
C
h
K
O2
C
O2
k0 ˆ k
This work was supported by the National Science
Foundation of China.
0
:
ꢀ19†
1
‡ K
h
C
h
‡ K
w
C
w
1 ‡ KO2
C
O2
Fig. 5 shows the plot of log r vs log I. A straight line with
0
slope a ˆ 0:58 and intercept log k ˆ À1:154. Since
References
C
K
h
; C
w
; C
O
; K
h
; K
w
and K
O
2
are known, rate constant
2
0
can be determined. The value of a ˆ 0:58 indicated
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.4. Photocatalyst activity
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2
7
1±87.
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1
TiO
by others that the catalytic activity of TiO
2
can be sustained inde®nitely. It has been reported
could be
2
6
maintained inde®nitely under an abundance of water
vapor +Dibble and Raupp, 1990; Peral and Ollis, 1992;
Obee and Brown, 1995; Alberici and Jardim, 1997). It is
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