Solid phase photocatalytic reaction on the Soot/TiO2 interface: The role of migrating OH radicals

Article abstract of DOI:10.1021/jp026617f

The photocatalytic degradation of flame-deposited hexane soot on TiO₂ films was carried out under UV illumination (?? > 300 nm) in air in order to investigate the behavior of photooxidants at the solid/solid (soot/TiO₂) interface. The photodegradation of the immobile soot layer on TiO₂ film was monitored by performing soot weight loss measurement, scanning electron microscopic (SEM) analysis, and gas-chromatographic CO₂ production measurement. The bulk of soot layer with a??2 ??m thickness was completely oxidized to CO₂ over 30 h irradiation. The presence of O₂ was essential in the photocatalytic soot oxidation. The SEM images of the irradiated soot films revealed that the gap distance between the edges of soot on glass and TiO₂ domains continuously increased with UV illumination up to 80 ??m, implying that the active oxidants generated on the TiO₂ surface migrated across the gap into a remote soot domain to initiate the photocatalytic oxidation. The apparent activation energy for the photocatalytic soot oxidation was estimated to be 18.7 kJ/mol, which is largely ascribed to the diffusional activation of active oxidants because the reactant (soot) is immobile. Introducing water or 2-propanol vapor efficiently enhanced or reduced the photooxidation rate of soot since water or 2-propanol was the precursor or the scavenger of the main oxidant species, OH radical, respectively. The migrating nature of photooxidants (including OH radicals) should be taken into account in quantitative understanding of photocatalytic reaction mechanisms in all media (gas, liquid, and solid phases) in general.

Full text of DOI:10.1021/jp026617f

11818
J. Phys. Chem. B 2002, 106, 11818-11822
Solid Phase Photocatalytic Reaction on the Soot/TiO2 Interface: The Role of Migrating OH
Radicals
Myung Churl Lee and Wonyong Choi*
School of EnVironmental Science and Engineering, Pohang UniVersity of Science and Technology,
Pohang 790-784, Korea
ReceiVed: July 26, 2002; In Final Form: September 18, 2002
The photocatalytic degradation of flame-deposited hexane soot on TiO
illumination (λ > 300 nm) in air in order to investigate the behavior of photooxidants at the solid/solid
soot/TiO ) interface. The photodegradation of the immobile soot layer on TiO film was monitored by
performing soot weight loss measurement, scanning electron microscopic (SEM) analysis, and gas-
chromatographic CO
production measurement. The bulk of soot layer with ∼2 µm thickness was completely
oxidized to CO over 30 h irradiation. The presence of O was essential in the photocatalytic soot oxidation.
The SEM images of the irradiated soot films revealed that the gap distance between the edges of soot on
glass and TiO domains continuously increased with UV illumination up to 80 µm, implying that the active
oxidants generated on the TiO surface migrated across the gap into a remote soot domain to initiate the
2
films was carried out under UV
(
2
2
2
2
2
2
2
photocatalytic oxidation. The apparent activation energy for the photocatalytic soot oxidation was estimated
to be 18.7 kJ/mol, which is largely ascribed to the diffusional activation of active oxidants because the reactant
(soot) is immobile. Introducing water or 2-propanol vapor efficiently enhanced or reduced the photooxidation
rate of soot since water or 2-propanol was the precursor or the scavenger of the main oxidant species, OH
radical, respectively. The migrating nature of photooxidants (including OH radicals) should be taken into
account in quantitative understanding of photocatalytic reaction mechanisms in all media (gas, liquid, and
solid phases) in general.
Introduction
µm away from the TiO2 microdomains. This also demonstrated
the diffusing action of oxidizing species on the TiO2 surface.
TiO2 photocatalysis has been extensively studied for its
application to environmental remediation processes.1-4 Most of
the remediation power of TiO2 photocatalysts is mainly at-
tributed to the strong oxidation potential of the OH radicals that
are generated on the UV-illuminated TiO2 surface. The valence
band (VB) holes react with surface hydroxyl groups or water
molecules to produce OH radicals, which are usually considered
to remain bound to the surface and react with the substrate
molecule diffusing onto the surface. However, the possibility
On the other hand, the presence of diffusing OH radicals in an
aqueous suspension of TiO2 was recently suggested from a study
of photocatalytic degradation of tetramethylammonium ions.10
Unlike the photocatalytic reactions at the solid/liquid and
solid/gas interface where the reactants diffuse onto the il-
luminated surface site, the photocatalytic reaction at the solid/
solid interface is unique in that the reactants are immobile.
Therefore, the solid-phase photocatalytic reactions should be
8
mostly mediated by diffusing oxidants species. Little is known
that the OH radicals desorb from the surface and diffuse into
the bulk medium has been often raised.5
about the mechanisms and the role of diffusing OH radicals in
solid-phase photocatalytic reactions. In this study, we investi-
gated the solid-phase photocatalytic degradation reaction of
black carbon soot deposited on TiO2 film and demonstrated that
the OH radicals generated on the TiO2 surface migrated beyond
Recently, a few examples of diffusing OH radicals on
illuminated TiO2 have been reported from different studies.
Tatsuma et al.6,7 observed a remote oxidation of organic dye or
film that was separated from a TiO2-coated glass plate by a
small gap, which was ascribed to the action of active oxygen
species (most likely OH radicals) that were generated on the
TiO2 surface and transported through the gas phase. They
claimed that OH radicals traveled a distance up to 2.2 mm in
8
0 µm to mediate the solid-phase photocatalytic oxidation of
carbon soot. Mechanistic details and the role of migrating OH
radicals in the solid-phase photocatalytic reaction are discussed
in detail.
8
air to react with remote substrates. Cho and Choi reported that
the photodegradation of TiO2-blended PVC film developed
cavities around the imbedded TiO2 particles where the cavity
boundary was well separated by 1-2 µm from the particle
boundary, which was also attributed to diffusing OH radicals.
Experimental Section
Materials and Sample Preparation. The photocatalyst used
was Degussa P-25 TiO2 with a crystalline structure of primarily
anatase (80% anatase, 20% rutile), a B.E.T. surface area of ∼50
9
Haick and Paz observed a remote photodegradation of self-
2
m /g, and an average particle diameter of about 30 nm. The
assembled monolayers of aliphatic chains anchored to an inert
silicon surface whereas the chains were located as far as 20
2
TiO2 film was deposited on glass plates (area 0.55 or 3.24 cm )
using a dip-coating technique. A well-dispersed suspension (5
wt %) of TiO2 was prepared in distilled water and stirred for 2
h. A substrate glass plate was coated with TiO2 by dipping it in
*
Corresponding author e-mail: wchoi@postech.ac.kr; phone: +82-54-
2
79-2283; fax: +82-54-279-8299.
1
0.1021/jp026617f CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/19/2002

Role of Migrating OH Radicals
J. Phys. Chem. B, Vol. 106, No. 45, 2002 11819
the TiO2 suspension, dried under air, and then heated at 400 °C
for 30 min. The dip-coating procedure was repeated several
times until the thickness of the TiO2 film was approximately
∼
1 µm. One side of the TiO2-coated glass plate was cautiously
wiped out. Black carbon soot film (∼2 µm thickness) was
directly flame-deposited on the TiO2-coated glass plate by
burning n-hexane. The apparent UV absorption coefficient (ꢀλ)
of the hexane-soot film was estimated by measuring the
absorbance (or transmittance) of a soot-coated glass plate as a
function of the thickness of the soot film. The ꢀλ value of soot
4
-1
was determined to be approximately 2 × 10 cm at 350 nm.
Photolysis and Analysis of Soot. The soot photolysis
experiments were carried out under ambient air. When we
needed to monitor the CO2 generation from the photodegrading
soot on TiO2, a closed circulation reactor was used. It consisted
3
of a Teflon reactor (volume 21 cm ) with a quartz window (area
7
2
cm ) and a magnetically driven circulation pump (Takatsuki
2
Figure 1. Decrease of soot mass on TiO film as a function of UV
Co., SPP-3EBS) connected to the reactor. The air circulation
irradiation time. Two different modes of illumination (front vs back)
are compared for their degradation efficiencies. The initial soot thickness
was ∼2.2 µm.
3
-1
rate was about 200 cm min . Light illumination was carried
out using a 200-W mercury lamp. The soot-deposited TiO2 film/
glass was irradiated through a Pyrex glass cover to transmit
light with λ > 300 nm only. The light was incident on either
the soot-coated side (front irradiation) or the uncoated glass side
that initiated soot degradation. After more than 30 h of back
irradiation, the soot layer was completely oxidized, with the
black color of the soot-coated TiO2 film turning into white. The
cross-sectional SEM images of the soot layer on TiO2 film
(Figure 2) exhibit the progressive degradation of soot as the
irradiation time increases. The image shows that the soot layer
of ∼2 µm thickness completely disappeared after 32 h irradia-
tion, which corresponds to a soot oxidation rate of ∼65 nm/h.
The hexane soot layer shows very porous structure and has been
(back irradiation). The distance between the sample and the lamp
was 22 cm. The light flux onto the sample surface was measured
2
to be 1.5 mW/cm (300 nm < λ < 400 nm) using a power
meter (Newport 1815-C with a 818-UV silicon diode detector).
All the photolysis experiments were performed under air in a
lamp-housing box (35 cm × 30 cm × 35 cm) where the
temperature was maintained at 43-45 °C during irradiation.
When the effect of oxygen was investigated, the photolysis was
carried out under pure oxygen or helium atmospheres. A set of
duplicate or triplicate samples was tested under each experi-
mental condition. For the control experiment, the soot layer
deposited directly on a glass plate without TiO2 coating was
irradiated to compare its direct photodegradation with the
photocatalytic degradation of soot on TiO2. The loss of soot
mass due to photodegradation was monitored by weighing the
soot-coated glass plate before and after UV irradiation.
2
11
reported to have a B.E.T. surface area of 46 m /g, which is
very similar to that of P25 TiO2 used in this study. An
1
2
ellipsometric study by Remillard et al. reported that solid
stearic acid films (one or two layers with ∼30 Å thickness) on
TiO2 degraded with a removal rate of ∼22 nm/h under UV
illumination in air. Considering that the stearic acid films should
be much denser than the porous soot film, the photocatalytic
removal rates of the two different films are quite comparable
even though their thickness differs by a factor of 1000. Although
the oxidants should be produced on the surface of the TiO2 film,
a strict two-dimensional surface reaction at the soot/TiO2
interface cannot account for the complete degradation of the
soot bulk layer. This implies that the active oxidants generated
on the TiO2 surface desorbed and migrated into the bulk of the
soot layer. We believe that the migrating photooxidants played
a similar role in the photocatalytic degradation of stearic acid
The CO2 generated in the UV-illuminated reactor was
quantified using an on-line connected gas chromatograph (GC
Hewlett-Packard 6890) that was equipped with a flame ioniza-
tion detector (FID), a Porapak column, a CO2 methanizer (HP
G2747A), and a gas sampling valve. Images of the soot layer
on TiO2 film were obtained before and at regular time intervals
during UV illumination by using SEM (Hitach, S-2460N). The
cross-section of the soot/TiO2 film was imaged to measure the
soot thickness. The samples were coated in gold using a sputter
coater before SEM analysis. Raman spectroscopic analysis
1
2
layers on TiO2 films.
The oxidation of soot produced CO2 as shown in Figure 3.
The rates of CO2 production (or the soot oxidation rates) were
much higher under oxygen than air or helium, indicating that
the presence of O2 was essential for the soot oxidation. Under
the helium atmosphere, the CO2 generation proceeded until the
steady state was reached at about 30 min, which implied that
the trace oxygen that was adsorbed or trapped in the pore of
soot layer was depleted and further oxidation was inhibited. In
accordance with the mass monitoring experiment of Figure 1,
the soot layer directly deposited on the glass plate without TiO2
did not emit any CO2 at all under air and UV irradiation. The
CO2 production rates (Figure 3) decelerated gradually over time,
which seemed to be ascribed to depleting water vapor (or
adsorbed water) concentration in a close circulation reactor. The
presence of water vapor was essential for the efficient photo-
oxidation of soot (vide infra: Figure 8). In Figure 4, the rates
of CO2 production from three soot samples having different
thickness are compared between the front and back irradiation
(using a Renishaw system 3000 with an excitation wavelength
of 632.8 nm) of the flame-deposited black carbon soot showed
the presence of graphitic carbons.
Results and Discussion
Photocatalytic Degradation of Soot on TiO2. Figure 1
compares the time-dependent profiles of soot degradation
between the front and back irradiation modes under air. Direct
photolytic degradation of soot without TiO2 was not observed
at all. The soot degraded slower under the front irradiation than
the back irradiation. This is due to the fact that the soot layer
blocked some fraction of incident UV in the front irradiation
mode with fewer photons reaching the underlying TiO2 surface.
On the other hand, the back irradiation provided unattenuated
light intensity onto the TiO2 film and generated more oxidants

11820 J. Phys. Chem. B, Vol. 106, No. 45, 2002
Lee and Choi
Figure 2. Cross-sectional SEM images of soot-coated TiO
2
films on a glass plate. The UV light was illuminated from the backside. (a) Before
illumination; (b) 18 h illuminated; (c) 32 h illuminated.
Figure 3. CO
as a function of the illumination time (front irradiation mode). The
photocatalytic CO generation rates under air, oxygen, and helium
atmospheres are compared. The soot area was 0.55 cm , and its
thickness was ∼2.4 µm.
2
evolution from the photocatalytic degradation of soot
Figure 4. Comparison of the initial photocatalytic soot oxidation (or
CO generation) rates with the front or back illumination mode. The
samples 1, 2, and 3 (in this order) have a successively thicker coating
2
2
2
of soot (1.7, 2.3, and 3.9 µm, respectively) on the same TiO
2
-coated
2
glass plate. The soot area was 0.55 cm .
modes. Front irradiation produced less CO2 than the back
irradiation, which is consistent with the weight loss data of
Figure 1. It should be noted that the CO2 production rates
exhibited opposite dependences on the soot thickness between
the two different irradiation modes. Under the front irradiation,
a thicker soot layer blocked more UV light with a reduced rate
of CO2 production. However, the fact that the back irradiation
generated more CO2 with thicker soot coating cannot be
explained provided that the soot oxidation occurred only at the
soot/TiO2 interface. The amount of soot in direct contact with
the TiO2 surface should be independent of the soot thickness.
This can be accounted for only by assuming that the soot
oxidation proceeded both at the soot/TiO2 interface and in the
bulk of soot layer. Photooxidants generated on the TiO2 film
surface should diffuse into the porous soot layer where more
CO2 was produced in a thicker soot layer. Since d[CO2]/dt
continuously increased with increasing soot thickness up to ∼4
µm under the back irradiation, the oxidants (most likely OH
radicals) should migrate beyond this thickness.
images. Figure 5a illustrates the process of a gap (d) develop-
ment by migrating oxidants (e.g., OH radicals) between the
edges of TiO2 and soot domains on a glass plate. The SEM
images in Figure 5b,c show that the photodegradation of the
soot layer on an inert glass surface started from the TiO2/soot
interface and continued to penetrate into the soot/glass domain
with increasing gap distance, d. As shown in Figure 6, the gap
distance between the edges of soot and TiO2 domains continu-
ously increased with UV irradiation beyond 80 µm. This result
clearly verifies that the active oxidant species formed on the
irradiated TiO2 surface desorbed and migrated across the glass
surface to reach the soot domain. The rate of the gap distance
development seemed to gradually decelerate with time, which
indicated that the number of photooxidants that reached the soot
domain through migration decreased with increasing d. Our
observation is quite similar to that reported by Haick and Paz,9
who could photodegrade octadecyltrichlorosilane chains on a
silicon surface in the presence of adjacent TiO2 microdomains.
Both observations confirm that the surface diffusion of active
oxidants on TiO2 is an efficient mechanism through which
photocatalytic reactions proceed. It should also be considered
Evidence for Migration of OH Radicals and Their Role.
To assess how far the oxidants migrate away from the TiO2
surface, the photocatalytic degradation of a soot layer near the
edge of TiO2 film was monitored by taking a series of SEM
6,7
that the diffusion of active oxidants through the air could play

Role of Migrating OH Radicals
J. Phys. Chem. B, Vol. 106, No. 45, 2002 11821
Figure 6. Increase of the gap distance d (see Figure 5a) as a function
of UV irradiation time.
Figure 7. Arrhenius plot for the photocatalytic CO
2
production from
2
the degradation of soot on TiO . The soot area was 0.55 cm , and the
2
irradiation was from the front side.
in water,13 and 14.1 kJ/mol for ethylene in air.14 Although the
apparent activation energy in typical photocatalytic reactions
is often ascribed to the adsorption/desorption process of reactants
1
5
and products on the photocatalyst surface, it is most probably
due to the diffusion process of active oxidants in the present
case. The immobile reactant (soot) and the easily desorbing
product (CO2) should have negligible contribution to the
apparent activation energy. A similar Ea value of 0.24 eV for
the photocatalytic degradation of the aliphatic chain monolayer
Figure 5. Remote photocatalytic degradation of soot near the edge of
the TiO domain. (a) Schematic illustration of the photocatalytic
2
degradation of the soot layer near the borderline of the soot and TiO
2
domains where a gap distance d develops between the edges of TiO
2
and the soot domain with illumination time. The SEM images show
the developing gap at (b) 6 h and (c) 12 h irradiation. The initial soot
thickness was ∼0.7 µm, and the irradiation was from the front side.
9
immobilized on TiO2 film has also been reported.
Since the migrating active oxidants at the soot/TiO2 interface
a role in the oxidation of the soot layer. Although the through-
air diffusion mechanism was shown to be far less contributing
than the surface diffusion mechanism in the remote degradation
are most likely to be OH radicals as in other photocatalytic
1-4
remediation systems,
the effect of water vapor, which is a
precursor of hydroxyl radical, on the soot oxidation rate was
investigated and is shown in Figure 8. The initial addition of
water vapor rapidly increased the CO2 production rate, and the
water content above 20% relative humidity was saturated with
respect to the soot oxidation rate, which was a typical water-
concentration dependence found among gas-phase photocatalytic
9
of the aliphatic chain monolayer, the degradation of the soot
layer above the TiO2 film should be mediated by the diffusing
oxidants into the pores of bulk soot layer. The through-air
diffusion within the soot layer thickness (less than a few
micrometers) could be an efficient mechanism as well.
1
6
The temperature effect on the soot photooxidation rate was
investigated by measuring d[CO2]/dt at the reactor temperature
of 323, 348, and 368 K. By plotting the logarithm of the CO2
generation rate versus the reciprocal temperature (Figure 7), an
apparent Arrhenius activation energy (Ea) for the soot photo-
oxidation was calculated to be 18.7 kJ/mol (or 0.19 eV). This
value is comparable with typical activation energies of photo-
catalytic oxidation of organic molecules: 10 kJ/mol for phenol
oxidation reactions. The reactivity of OH radicals can be
4
d,10
quenched with the addition of hydroxyl radical scavengers.
17
Since 2-propanol is an efficient OH radical scavenger, the rates
of soot photooxidation were rapidly reduced with the addition
of 2-propanol vapor as shown in Figure 9. On the basis of the
above results, we conclude that the migrating active photooxi-
dants in the photocatalytic soot oxidation reaction are primarily
OH radicals.

11822 J. Phys. Chem. B, Vol. 106, No. 45, 2002
Lee and Choi
TiO2 surface and migrated across a gap of inert glass surface
into a soot domain to initiate remote photocatalytic oxidation.
The OH radicals were found to diffuse away as far as 80 µm
from the TiO2 domain. The photocatalytic oxidation of soot on
TiO2 led to the complete degradation of a soot layer with CO2
production. Both surface diffusion and through-air diffusion
should be operative in order to accomplish the photooxidation
of the bulk of soot layer with a few micrometers thickness. Since
the degradation of the immobile soot layer should be carried
out by mobile oxidants, migrating OH radicals seem to be the
dominant mechanism in solid-phase photocatalytic degradation
reactions. This finding confirms the recent observations6
-10
of
the migrating behavior of active photooxidants generated on
TiO2 and proposes that the diffusing nature of photooxidants
should be taken into account in a quantitative understanding of
photocatalytic reaction mechanisms in all media (gas, liquid,
and solid).
Acknowledgment. We appreciate the financial support from
the Korea Science and Engineering Foundation (KOSEF)
through the Center for Integrated Molecular Systems and the
Brain Korea 21 project.
Figure 8. Effect of humidity on the photocatalytic oxidation rate of
soot in the front irradiation mode. The soot area was 3.24 cm .
2
References and Notes
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(
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(
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