# A Photocatalytic Active Adsorbent for Gas Cleaning in a Fixed Bed Reactor

Authors: Peter Pucher; Rabah Azouani; Andrei  Kanaev; Gernot Krammer
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

Efficient photocatalysis for gas cleaning purposes requires a large accessible, illuminated active surface in a simple and compact reactor. Conventional concepts use powdered catalysts, which are nontransparent. Hence a uniform distribution of light is difficult to be attained. Our approach is based on a coarse granular, UV-A light transparent, and highly porous adsorbent that can be used in a simple fixed bed reactor. A novel sol-gel process with rapid micro mixing is used to coat a porous silica substrate withTiO2-based nanoparticles. The resulting material posses a high adsorption capacity and a photocatalytic activity under UV-A illumination (PCAA = photocatalytic active adsorbent). Its photocatalytic performance was studied on the oxidation of trichloroethylene (TCE) in a fixed bed reactor setup in continuous and discontinuous operation modes. Continuous operation resulted in a higher conversion rate due to less slip while discontinuous operation is superior for a total oxidation to CO2 due to a user-defined longer residence time.


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## Body

## 1. Introduction

Many pollutants, smells, and odors as well as bacteria and pathogens exist in the atmosphere. They influence the
quality of life and they are sources of illnesses and allergies even if they
are at very low concentrations. An abatement
of these unwanted substances with state-of-the-art technology is either
technically complex or energetically expensive. The photocatalytic oxidation
technique represents a new and promising method for gas cleaning. Hence, the interest
in photocatalytic chemistry is rising worldwide as demonstrated by high numbers
of citations and patents [1]. However, upscaling and technical implementation
of photocatalysis are difficult, which is one of the reasons why industrial
realizations are still limited.The basic problem of the photocatalytic processes scale-up for commercial utilization is to provide
sufficient illuminated active surface area. A uniform distribution of light
inside the reactor and provision of a high number of active surface sites per
reactor volume unit are key parameters of a successful reactor design [2]. One
approach to meet these requirements is based on a fixed bed reactor with an
irradiated nanocoated fill. The used material should possess optimal optical
properties and a large inner surface with unhindered access by  pollutant to catalyst layer. Coarse-granular
wide-pore silica is an excellent candidate because it has  open porous structure and high-specific
surface area, and it satisfies UV-light transmission [3].In the present communication we report
on a new photocatalytic active adsorbent (PCAA) as a combination of TiO2-sol
nanoparticles immobilized on a UV-transparent coarse-granular porous silica
support. The UV-light-triggered degradation of trichloroethylene is
investigated and its kinetics is discussed.


## 2. Experimental

### 2.1. Preparation and Immobilization of Nanoparticles

The sol nanoparticles are prepared
in a sol-gel reactor using titanium tetraisopropoxide (TTIP) as a precursor.
This reactor has been described in [4]. In present experiments, the standard
operation conditions were conserved: two stock solutions of TTIP/propan-2-ol
and H2O/propan-2-ol (CTi=0.146 M, H = CH2O/CTi = 2.46)
were synchronously injected at high Reynolds numbers into the reactor container
through the static Hartridge-Roughton-type T-mixer by applying a dry nitrogen
gas pressure. The reactor temperature was maintained at 20.0°C. As a result,
metastable sol particles of size 2R = 6.0 nm with a narrow size distribution were
generated in the reactor. The sols slowly grow in size during the so-called induction period that
amounts for tind = 55 minutes. Particle immobilization was achieved
during the period of their relative stability: t<tind/2. The
support was immersed into the reactive colloid for ∼25 minutes, then withdrawn,
cleaned, and dried. The TiO2 nanoparticles are involved into their
bulk by capillary forces, then reacting on the internal surface of pores. Hence,
they are immobilized on the support surface. After the drying stage in a glove
box and then in an oven (80°C), a calcination at 450°C was performed typically
during 4 hours. The deposited catalyst mass on the porous substrate was measured
as described in [5]. A mean value of 0.55 wt.% has been obtained.


### 2.2. Photocatalytic Reactor

The photocatalytic performance of the prepared PCAA was studied in a fixed bed plug flow reactor setup, depicted
in Figure1. Trichloroethylene (C2HCI3, TCE) was used as a
model pollutant at an inlet concentration varied from 25 to 300 ppmv. Purified-compressed
air and N2 acted as carrier gas, free of oil, CO/CO2, and
humidity. The overall gas flow of 1 l/min s.t.p. was controlled by mass flow
controllers. All experiments were carried out at ambient pressure and
temperature (∼1 bar, 23 ± 3°C) with dry gas. The inlet and outlet gas temperatures were monitored by two
type-K thermocouples. The coated material was filled in the stainless steel
reactor of a rectangular shape (fill size 50×76×11 mm3) covered by a
UV-A transparent glass window. One UV-A fluorescent tube (Sylvania BL350 8W,
peak emission at 350 ± 15 nm)
illuminated the material. Its performance was adjusted from 25% to 100% by an
electronic ballast (Philips EVG HF-R 114 TL5). This corresponds to the integral
intensity I varied from 0.68 to 5.35 mW/cm2, measured with a Solatell Sola-Scope 2000 setup.  The TCE
concentration was measured by a flame-ionization detector (FID TESTA 123),
which was calibrated with TCE gas before the measurements. A CO/CO2 gas analyzer (NDIR Hartmann&Braun URAS 14) measured the final products of
the photocatalytic TCE oxidation. The reactor could be bypassed to measure the
inlet concentrations.(a) Cut section of the photocatalytic reactor and (b) simplified flow sheet of the
setup.
(a)(b)


## 2.1. Preparation and Immobilization of Nanoparticles

The sol nanoparticles are prepared
in a sol-gel reactor using titanium tetraisopropoxide (TTIP) as a precursor.
This reactor has been described in [4]. In present experiments, the standard
operation conditions were conserved: two stock solutions of TTIP/propan-2-ol
and H2O/propan-2-ol (CTi=0.146 M, H = CH2O/CTi = 2.46)
were synchronously injected at high Reynolds numbers into the reactor container
through the static Hartridge-Roughton-type T-mixer by applying a dry nitrogen
gas pressure. The reactor temperature was maintained at 20.0°C. As a result,
metastable sol particles of size 2R = 6.0 nm with a narrow size distribution were
generated in the reactor. The sols slowly grow in size during the so-called induction period that
amounts for tind = 55 minutes. Particle immobilization was achieved
during the period of their relative stability: t<tind/2. The
support was immersed into the reactive colloid for ∼25 minutes, then withdrawn,
cleaned, and dried. The TiO2 nanoparticles are involved into their
bulk by capillary forces, then reacting on the internal surface of pores. Hence,
they are immobilized on the support surface. After the drying stage in a glove
box and then in an oven (80°C), a calcination at 450°C was performed typically
during 4 hours. The deposited catalyst mass on the porous substrate was measured
as described in [5]. A mean value of 0.55 wt.% has been obtained.


## 2.2. Photocatalytic Reactor

The photocatalytic performance of the prepared PCAA was studied in a fixed bed plug flow reactor setup, depicted
in Figure1. Trichloroethylene (C2HCI3, TCE) was used as a
model pollutant at an inlet concentration varied from 25 to 300 ppmv. Purified-compressed
air and N2 acted as carrier gas, free of oil, CO/CO2, and
humidity. The overall gas flow of 1 l/min s.t.p. was controlled by mass flow
controllers. All experiments were carried out at ambient pressure and
temperature (∼1 bar, 23 ± 3°C) with dry gas. The inlet and outlet gas temperatures were monitored by two
type-K thermocouples. The coated material was filled in the stainless steel
reactor of a rectangular shape (fill size 50×76×11 mm3) covered by a
UV-A transparent glass window. One UV-A fluorescent tube (Sylvania BL350 8W,
peak emission at 350 ± 15 nm)
illuminated the material. Its performance was adjusted from 25% to 100% by an
electronic ballast (Philips EVG HF-R 114 TL5). This corresponds to the integral
intensity I varied from 0.68 to 5.35 mW/cm2, measured with a Solatell Sola-Scope 2000 setup.  The TCE
concentration was measured by a flame-ionization detector (FID TESTA 123),
which was calibrated with TCE gas before the measurements. A CO/CO2 gas analyzer (NDIR Hartmann&Braun URAS 14) measured the final products of
the photocatalytic TCE oxidation. The reactor could be bypassed to measure the
inlet concentrations.(a) Cut section of the photocatalytic reactor and (b) simplified flow sheet of the
setup.
(a)(b)


## 3. Results and Discussion

### 3.1. Surface Area and Pore Size

The uncoated substrate and
impregnated PCAA grains were analyzed for their total pore volume and pore size
distribution with Hg-intrusion method (Porotec Inc., Niederhofheimer Str. 55a, 65719 Hofheim, Germany), and their internal surface area with BET
method (STRÖHLEIN AREA-meter II). The nanoparticle coating process appears to
have no significant influence on both pore volume and surface area. Table1
summarizes the results for PCAA. The measured properties for coated and
uncoated materials were found to be within the range of deviation due to
inhomogeneities in the raw material. Thus, it can be concluded that hardly any
pores are blocked by incorporated TiO2 particles. The pore size
distribution of the substrate is broad enough to give sufficient room for
nanoparticles already at wide pore diameters. On the other hand, an increase of
the surface area due to the nanoparticles is low because of their low-mass load
(∼0.55 wt.%). Benmami et al. [5] have reported the specific surface of the
noncrystalline TiO2 nanoparticles in the range of 480 m2/g correlated to
their own mass. Being normalized on the total PCAA mass, this corresponds to an
active surface area of about 2.7 m2/g or just 0.8% of the substrate surface.Table 1
BET surface and pore characteristics of PCAA.
SubstrateMean grain size (mm)BET surface (m2/g)Most abundant pore diameter (nm)Pore volume (mm3/g)Porous silica granulate3-4330 ± 306 ± 0.7810 ± 70


### 3.2. Morphology Analysis of PCAA

EDX (Zeiss DSM 982 GEMINI), XRD (XRG 3000 INEL), and TEM (JEOL 200 keV) measurements were carried out to
characterize the immobilized catalyst nanoparticles on the inner substrate
surface. Beneath the typical peaks for the silica substrate (Si, O), the EDX
spectrum (Figure2) shows a small peak for titanium at about 4.5 keV. Because of
a low-mass loading of TiO2 nanoparticles embedded in amorphous
silica, the XRD patterns of PCAA showed no characteristic peaks. It was not
possible to identify the crystalline phase of TiO2 with this method.
High-resolution TEM measurements of PCAA bulk material have been performed to
investigate particle size and distribution of the nanoparticular TiO2 mater. Figure 3 evidences rare nonagglomerated crystalline domains of size in the
range between 5 and 10 nm. These rare crystallites are surrounded by amorphous
silica matrix visualized by low-contrast gray slabs and spots. A first
approximation of their crystalline
layer distances indicates the anatase phase.Figure 2
Electron microscope EDX spectrum of nano-TiO2 impregnated silica substrate PCAA (FELMI Graz).Figure 3
High-resolution TEM image of the TiO2 nanoparticles immobilized into the nanoporous silica substrate (LIMHP Paris).


### 3.3. UV-Absorption of the Nanocoated Substrates

The UV transmission of a substrate is one important property, since a large active area has to be illuminated in
practical applications. The substrate must not absorb photons at energies
sufficient to elevate an electron from the valence band to the conduction band
of the semiconductor. A custom-made setup consisting of a big volume quartz cell (55×35 mm2 window size) with adjustable spacing and an 8W UV-A tube (Sylvania BL350) light source was
used to measure the optical transparency dependence on the fill thickness (10–30 mm). The penetrating light was detected by a Chromex 250IS monochromator (grating 150 l/mm,
f = 30 cm, slit 20 μm) equipped with a CCD detector (Princeton). Figure 4(a) shows the spectral
dependence on the absorption shows a plateau
with no particular band structure. Because the silica band gap is large, we
assign this opacity to the light scattering on silica pores. On the
other hand, the opacity of TiO2-coated PCAA exhibits a
characteristic onset below ∼400 nm, which corresponds to the bulk TiO2 anatase absorption [6]. Short-wave UV-light is thereby scattered and absorbed
inside PCAA, while long-wave UV/VIS light is transmitted through a fill of
PCAA. This finding is documented by  Figure 4(b)) which clearly shows a red shift of
the normalized absorbance with increasing fill thickness.(a) Spectral dependence
of the absorbance of pure and nano-TiO2 impregnated silica
substrate. (b) Red-shift of the normalized absorbance with
increasing fill thickness of PCAA.
(a)(b)


### 3.4. Photocatalytic Oxidation in Continuous Mode

First experiments were carried out under continuous gas feed and UV-A illumination. The photocatalytic performance
of PCAA was studied by the TCE degradation rate(rTCE) based on the
volume of the fixed bed (VFB). The experimental data of pollutant
concentration (in ppmv) at inlet (YIN) and outlet (YOUT)
and the total molar gaseous flow N˙g (in mol/s) were used to calculate the
experimental rate rTCE,exp as presented in(1)rTCE,exp(Y¯)=N˙gVFB⋅(YIN−YOUT)(molm3⋅s).Due to reactions, the TCE concentration
decreases downstream of the gas flow. Thus assuming Langmuir-Hinshelwood
reaction kinetics, this experimental rate is related to the logarithmic mean
concentrationY¯ between inlet and outlet:(2)Y¯=(YIN−YOUT)ln(YIN/YOUT)(ppmv).Among different kinetic studies
concerning the photocatalytic oxidation of TCE reported in literature (see,
e.g., Anderson et al. [7], Demeestere et al. [8], Doucet et al. [9]), we found that  none of them provided sufficient
information about the reaction rate dependence on light intensity over a wide
range of concentrations. Our experimental kinetic study was performed in
continuous operation mode for different inlet concentrations and varying light
intensity I. We developed a modified Langmuir-Hinshelwood kinetic model to
accommodate the influence of the concentration and light intensity on the TCE
decomposition rate rTCE:(3)rTCE(Y,I)=kI⋅KAd⋅Y1+KAd⋅Y⋅(1−e−C⋅(I/Imax))(molm3⋅s).The influence of the light intensity I is expressed by the exponential term of (3).
Thereby, the intrinsic light intensity I was related to the maximal intensity Imax (5.36 mW/cm2). A dimensionless adjustable parameter C, with no
further physical meaning, was used as a fit parameter. This exponential term was suggested to describe best the
observed saturation behavior over the applied concentration
range. Figure 5 illustrates this behavior in a 3D
surface fit. The points set for the experimental data obtained with (1)
and (2), while the surface presents the calculated model according to (3),
computed with the kinetic parameters of Table 2. The model satisfactorily
describes the measured data with a high-correlation coefficient r2 of 0.976.Table 2
Calculated kinetic parameters for the photocatalytic oxidation of TCE (25–300 ppm) on PCAA in
dependence of UV-A light intensity (0–5.35 mW/cm2, 11 mm fill thickness).
CatalystRatekI (mol·m−3·s−1)Adsorption constantKAd (ppm−1)Light constantC (1)Correlationr2 (1)PCAA0.01490.00850.3250.9769Figure 5
Experimental results (dots) and kinetic fit for the photocatalytic oxidation of TCE on PCAA in dependence of
concentration and light intensity.


### 3.5. Photocatalytic Oxidation in Discontinuous Mode

PCAA possesses a large inner surface and hence a high capacity for the adsorption of polar pollutant
molecules. In case of TCE, experiments resulted in the adsorption of about 1.4 mg/g at 100 ppmv (23°C) and no UV-A illumination. This adsorption capacity
combined with the photocatalytic activity can be used to run a reactor unit
equipped with PCAA in a discontinuous mode for gas cleaning purposes. In this
regime, first the adsorption step occurs of a pollutant from the raw gas stream
under no UV-A illumination, followed by a regeneration step with/without clean
air flush under UV-A illumination. Zou et al. [10] described the photocatalytic
oxidation of toluene on titania-silica pellets in alternating
adsorption/regeneration mode. A higher removal efficiency is generally expected
with an increased regeneration time.To test PCAA in discontinuous
operation conditions, we conducted experiments at 50 ppmv and 100 ppmv TCE
inlet concentration under two different conditions. Mode A was designed with
the adsorption timetad = 60 minutes and equal regeneration time treg = tad.
Mode B was designed with a twice longer regeneration time treg = 2·tad (120 minutes). During
regeneration, the reactor was flushed with 1 l/min s.t.p. clean air and UV-A
light was on with the maximum lamp performance (5.36 mW/cm2). Figure 6
shows the transient analyzer signals for Mode B at 50 ppmv TCE inlet
concentration. Since the fill volume was small (42 cm3), a certain
amount of TCE passed as slip the fixed bed reactor during the adsorption step
(UV-A lamps off). Around 50% of the incoming TCE molecules were emitted on this
stage. During the regeneration step only a very low TCE quantities were emitted
and the adsorbed TCE was stored in PCAA, being slowly oxidized by the
photocatalytic process until the next adsorption step.Figure 6
Transient TCE, CO, and CO2 concentration at reactor outlet for the discontinuous oxidation of TCE on PCAA
(50 ppm TCE inlet concentration, mode B: treg = 2·tad = 120 minutes).


### 3.6. CO and CO2 Products Formation during Continuous and Discontinuous Operation Modes

A complimentary possibility in evaluation of the material photocatalytic performance is to follow the reaction
products formation. CO2 is the most desired final product of this
process. Table 3 compares the main figures of overall conversion efficiency,
oxidation rate COX/CTCE, and ratio of produced CO2 versus CO in discontinuous and continuous operation modes. This comparison
shows a much higher conversion efficiency in the continuous mode compared to
the discontinuous one. This is well supported by the high slip of our setup
during the adsorption step, as we defined conversion efficiency as (TCEIN−TCEOUT)/TCEIN. The main advantage of the discontinuous operation mode is the longer residence
time of the pollutant in PCAA. Hence, a much higher conversion of TCE to CO and
CO2 can be achieved. This is illustrated by the COX/CTCE ratio, which is computed out of a carbon balance for adsorbed/reacted TCE to
produced CO and CO2. Furthermore, a full oxidation to CO2 is preferred with longer regeneration time, which is illustrated by a higher CO2/CO
ratio for mode B. For all discontinuous tests, the computed oxidation rate was smaller than 1 which implies that a larger-quantity
pollutant was adsorbed than oxidized at the regeneration steps. Hence, the regeneration time was too short and the system was still in
an unsteady regime after  ∼15 hours operation
in mode B. Out of this finding, it can be concluded that TCE is to be converted
into chlorinated intermediates (e.g., DCAC [11, 12], Phosgene [11–13],
etc.) in a first fast reaction step. FTIR measurements proved the presence of
DCAC and phosgene in the off-gas stream of the photo-reactor. Since their high
polarity, these chlorinated intermediates can remain adsorbed onto the polar PCAA
surface and oxidized slowly to the final oxidation products including CO and CO2.
We assume a first-order kinetic reaction with the molar load of hydrocarbon in
PCAA as determining concentration. Figure 7 shows the transient outlet
concentration of CO/CO2 during a long-term regeneration step.
Thereby, the plotted lines were calculated with a first-order reaction scheme.
A detailed analysis of the reaction pathways was not subject of the present
communication.Table 3
Conversion efficiency, slip during adsorption, oxidation ratio, and CO2-to-CO
ratio for regeneration time equals adsorption time (treg = tad,
mode A), regeneration time twice adsorption time (treg = 2·tad, mode B), and continuous
operation (mode C).
TCE inlet ppmModeTotal TCE conversion (%)Slip during adsorption (%)COX/CTCE (%)CO2/CO (1)50A4551322.73B4449653.43C89—251.75100A3855302.04B4053483.05C73—281.61Figure 7
Transient CO, CO2 concentration at
reactor outlet for a long-term regeneration run. The measured data
points were fitted with a first-order kinetic fit (line).


## 3.1. Surface Area and Pore Size

The uncoated substrate and
impregnated PCAA grains were analyzed for their total pore volume and pore size
distribution with Hg-intrusion method (Porotec Inc., Niederhofheimer Str. 55a, 65719 Hofheim, Germany), and their internal surface area with BET
method (STRÖHLEIN AREA-meter II). The nanoparticle coating process appears to
have no significant influence on both pore volume and surface area. Table1
summarizes the results for PCAA. The measured properties for coated and
uncoated materials were found to be within the range of deviation due to
inhomogeneities in the raw material. Thus, it can be concluded that hardly any
pores are blocked by incorporated TiO2 particles. The pore size
distribution of the substrate is broad enough to give sufficient room for
nanoparticles already at wide pore diameters. On the other hand, an increase of
the surface area due to the nanoparticles is low because of their low-mass load
(∼0.55 wt.%). Benmami et al. [5] have reported the specific surface of the
noncrystalline TiO2 nanoparticles in the range of 480 m2/g correlated to
their own mass. Being normalized on the total PCAA mass, this corresponds to an
active surface area of about 2.7 m2/g or just 0.8% of the substrate surface.Table 1
BET surface and pore characteristics of PCAA.
SubstrateMean grain size (mm)BET surface (m2/g)Most abundant pore diameter (nm)Pore volume (mm3/g)Porous silica granulate3-4330 ± 306 ± 0.7810 ± 70


## 3.2. Morphology Analysis of PCAA

EDX (Zeiss DSM 982 GEMINI), XRD (XRG 3000 INEL), and TEM (JEOL 200 keV) measurements were carried out to
characterize the immobilized catalyst nanoparticles on the inner substrate
surface. Beneath the typical peaks for the silica substrate (Si, O), the EDX
spectrum (Figure2) shows a small peak for titanium at about 4.5 keV. Because of
a low-mass loading of TiO2 nanoparticles embedded in amorphous
silica, the XRD patterns of PCAA showed no characteristic peaks. It was not
possible to identify the crystalline phase of TiO2 with this method.
High-resolution TEM measurements of PCAA bulk material have been performed to
investigate particle size and distribution of the nanoparticular TiO2 mater. Figure 3 evidences rare nonagglomerated crystalline domains of size in the
range between 5 and 10 nm. These rare crystallites are surrounded by amorphous
silica matrix visualized by low-contrast gray slabs and spots. A first
approximation of their crystalline
layer distances indicates the anatase phase.Figure 2
Electron microscope EDX spectrum of nano-TiO2 impregnated silica substrate PCAA (FELMI Graz).Figure 3
High-resolution TEM image of the TiO2 nanoparticles immobilized into the nanoporous silica substrate (LIMHP Paris).


## 3.3. UV-Absorption of the Nanocoated Substrates

The UV transmission of a substrate is one important property, since a large active area has to be illuminated in
practical applications. The substrate must not absorb photons at energies
sufficient to elevate an electron from the valence band to the conduction band
of the semiconductor. A custom-made setup consisting of a big volume quartz cell (55×35 mm2 window size) with adjustable spacing and an 8W UV-A tube (Sylvania BL350) light source was
used to measure the optical transparency dependence on the fill thickness (10–30 mm). The penetrating light was detected by a Chromex 250IS monochromator (grating 150 l/mm,
f = 30 cm, slit 20 μm) equipped with a CCD detector (Princeton). Figure 4(a) shows the spectral
dependence on the absorption shows a plateau
with no particular band structure. Because the silica band gap is large, we
assign this opacity to the light scattering on silica pores. On the
other hand, the opacity of TiO2-coated PCAA exhibits a
characteristic onset below ∼400 nm, which corresponds to the bulk TiO2 anatase absorption [6]. Short-wave UV-light is thereby scattered and absorbed
inside PCAA, while long-wave UV/VIS light is transmitted through a fill of
PCAA. This finding is documented by  Figure 4(b)) which clearly shows a red shift of
the normalized absorbance with increasing fill thickness.(a) Spectral dependence
of the absorbance of pure and nano-TiO2 impregnated silica
substrate. (b) Red-shift of the normalized absorbance with
increasing fill thickness of PCAA.
(a)(b)


## 3.4. Photocatalytic Oxidation in Continuous Mode

First experiments were carried out under continuous gas feed and UV-A illumination. The photocatalytic performance
of PCAA was studied by the TCE degradation rate(rTCE) based on the
volume of the fixed bed (VFB). The experimental data of pollutant
concentration (in ppmv) at inlet (YIN) and outlet (YOUT)
and the total molar gaseous flow N˙g (in mol/s) were used to calculate the
experimental rate rTCE,exp as presented in(1)rTCE,exp(Y¯)=N˙gVFB⋅(YIN−YOUT)(molm3⋅s).Due to reactions, the TCE concentration
decreases downstream of the gas flow. Thus assuming Langmuir-Hinshelwood
reaction kinetics, this experimental rate is related to the logarithmic mean
concentrationY¯ between inlet and outlet:(2)Y¯=(YIN−YOUT)ln(YIN/YOUT)(ppmv).Among different kinetic studies
concerning the photocatalytic oxidation of TCE reported in literature (see,
e.g., Anderson et al. [7], Demeestere et al. [8], Doucet et al. [9]), we found that  none of them provided sufficient
information about the reaction rate dependence on light intensity over a wide
range of concentrations. Our experimental kinetic study was performed in
continuous operation mode for different inlet concentrations and varying light
intensity I. We developed a modified Langmuir-Hinshelwood kinetic model to
accommodate the influence of the concentration and light intensity on the TCE
decomposition rate rTCE:(3)rTCE(Y,I)=kI⋅KAd⋅Y1+KAd⋅Y⋅(1−e−C⋅(I/Imax))(molm3⋅s).The influence of the light intensity I is expressed by the exponential term of (3).
Thereby, the intrinsic light intensity I was related to the maximal intensity Imax (5.36 mW/cm2). A dimensionless adjustable parameter C, with no
further physical meaning, was used as a fit parameter. This exponential term was suggested to describe best the
observed saturation behavior over the applied concentration
range. Figure 5 illustrates this behavior in a 3D
surface fit. The points set for the experimental data obtained with (1)
and (2), while the surface presents the calculated model according to (3),
computed with the kinetic parameters of Table 2. The model satisfactorily
describes the measured data with a high-correlation coefficient r2 of 0.976.Table 2
Calculated kinetic parameters for the photocatalytic oxidation of TCE (25–300 ppm) on PCAA in
dependence of UV-A light intensity (0–5.35 mW/cm2, 11 mm fill thickness).
CatalystRatekI (mol·m−3·s−1)Adsorption constantKAd (ppm−1)Light constantC (1)Correlationr2 (1)PCAA0.01490.00850.3250.9769Figure 5
Experimental results (dots) and kinetic fit for the photocatalytic oxidation of TCE on PCAA in dependence of
concentration and light intensity.


## 3.5. Photocatalytic Oxidation in Discontinuous Mode

PCAA possesses a large inner surface and hence a high capacity for the adsorption of polar pollutant
molecules. In case of TCE, experiments resulted in the adsorption of about 1.4 mg/g at 100 ppmv (23°C) and no UV-A illumination. This adsorption capacity
combined with the photocatalytic activity can be used to run a reactor unit
equipped with PCAA in a discontinuous mode for gas cleaning purposes. In this
regime, first the adsorption step occurs of a pollutant from the raw gas stream
under no UV-A illumination, followed by a regeneration step with/without clean
air flush under UV-A illumination. Zou et al. [10] described the photocatalytic
oxidation of toluene on titania-silica pellets in alternating
adsorption/regeneration mode. A higher removal efficiency is generally expected
with an increased regeneration time.To test PCAA in discontinuous
operation conditions, we conducted experiments at 50 ppmv and 100 ppmv TCE
inlet concentration under two different conditions. Mode A was designed with
the adsorption timetad = 60 minutes and equal regeneration time treg = tad.
Mode B was designed with a twice longer regeneration time treg = 2·tad (120 minutes). During
regeneration, the reactor was flushed with 1 l/min s.t.p. clean air and UV-A
light was on with the maximum lamp performance (5.36 mW/cm2). Figure 6
shows the transient analyzer signals for Mode B at 50 ppmv TCE inlet
concentration. Since the fill volume was small (42 cm3), a certain
amount of TCE passed as slip the fixed bed reactor during the adsorption step
(UV-A lamps off). Around 50% of the incoming TCE molecules were emitted on this
stage. During the regeneration step only a very low TCE quantities were emitted
and the adsorbed TCE was stored in PCAA, being slowly oxidized by the
photocatalytic process until the next adsorption step.Figure 6
Transient TCE, CO, and CO2 concentration at reactor outlet for the discontinuous oxidation of TCE on PCAA
(50 ppm TCE inlet concentration, mode B: treg = 2·tad = 120 minutes).


## 3.6. CO and CO2 Products Formation during Continuous and Discontinuous Operation Modes

A complimentary possibility in evaluation of the material photocatalytic performance is to follow the reaction
products formation. CO2 is the most desired final product of this
process. Table 3 compares the main figures of overall conversion efficiency,
oxidation rate COX/CTCE, and ratio of produced CO2 versus CO in discontinuous and continuous operation modes. This comparison
shows a much higher conversion efficiency in the continuous mode compared to
the discontinuous one. This is well supported by the high slip of our setup
during the adsorption step, as we defined conversion efficiency as (TCEIN−TCEOUT)/TCEIN. The main advantage of the discontinuous operation mode is the longer residence
time of the pollutant in PCAA. Hence, a much higher conversion of TCE to CO and
CO2 can be achieved. This is illustrated by the COX/CTCE ratio, which is computed out of a carbon balance for adsorbed/reacted TCE to
produced CO and CO2. Furthermore, a full oxidation to CO2 is preferred with longer regeneration time, which is illustrated by a higher CO2/CO
ratio for mode B. For all discontinuous tests, the computed oxidation rate was smaller than 1 which implies that a larger-quantity
pollutant was adsorbed than oxidized at the regeneration steps. Hence, the regeneration time was too short and the system was still in
an unsteady regime after  ∼15 hours operation
in mode B. Out of this finding, it can be concluded that TCE is to be converted
into chlorinated intermediates (e.g., DCAC [11, 12], Phosgene [11–13],
etc.) in a first fast reaction step. FTIR measurements proved the presence of
DCAC and phosgene in the off-gas stream of the photo-reactor. Since their high
polarity, these chlorinated intermediates can remain adsorbed onto the polar PCAA
surface and oxidized slowly to the final oxidation products including CO and CO2.
We assume a first-order kinetic reaction with the molar load of hydrocarbon in
PCAA as determining concentration. Figure 7 shows the transient outlet
concentration of CO/CO2 during a long-term regeneration step.
Thereby, the plotted lines were calculated with a first-order reaction scheme.
A detailed analysis of the reaction pathways was not subject of the present
communication.Table 3
Conversion efficiency, slip during adsorption, oxidation ratio, and CO2-to-CO
ratio for regeneration time equals adsorption time (treg = tad,
mode A), regeneration time twice adsorption time (treg = 2·tad, mode B), and continuous
operation (mode C).
TCE inlet ppmModeTotal TCE conversion (%)Slip during adsorption (%)COX/CTCE (%)CO2/CO (1)50A4551322.73B4449653.43C89—251.75100A3855302.04B4053483.05C73—281.61Figure 7
Transient CO, CO2 concentration at
reactor outlet for a long-term regeneration run. The measured data
points were fitted with a first-order kinetic fit (line).


## 4. Conclusion

A photocatalytic active adsorbent (PCAA) was prepared by coating a coarse granular porous silica
with TiO2 nanoparticles in a sol-gel reactor. The used substrate
acts as adsorbent possessing a high inner surface (BET = 330 m2/g) with an open
wide pore structure. A high-resolution TEM image revealed a particle size of
5–10 nm. The crystalline particles are isolated embedded in the amorphous
silica matrix. Experimental test for the photocatalytic oxidation of
trichloroethylene (TCE) was conducted in a fixed-bed reactor setup under dry
conditions. A study of influence of the pollutant concentration and UV-A light
intensity was performed in a continuous operation mode. An adapted
Langmuir-Hinshelwood kinetic model was found to satisfactorily describe the
measured TCE destruction rate. The continuous operation mode resulted in a
higher TCE conversion efficiency but a lower oxidation ratio to CO and CO2 than the discontinuous operation mode. The main problem of a discontinuous
operation was the slip of pollutant during the adsorption step. This problem
can be solved by a longer length of the PCAA fill. The CO and CO2 production rates were found to be directly dependent on the amount of pollutant
or intermediate adsorbed in PCAA rather than on the inlet concentration.PCAA can be regenerated by the photocatalytic oxidation process. This opens up new and
alternative ways in photocatalytic reactor design and gas cleaning technology.
Our results indicate a large material potential for applications at low
pollutant concentrations and small to medium gas flow rates. Recent experiments
indicate a photo-induced activity of the uncoated substrate for the destruction
of TCE developed after some UV-A illumination time. This currently unknown
process maybe driven by a chlorine radical reaction. Further experiments with
photocatalytic oxidation of methanol on PCAA are under way.



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*Source: 759736-2008-05-22.xml*