# TiO2/SiO2 Composite Films Immobilized on Foam   Nickel Substrate for the Photocatalytic Degradation of Gaseous Acetaldehyde

Authors: Hai Hu; Wenjun Xiao; Jian Yuan; Jianwei Shi; Wenfeng Shangguan
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

The photocatalyst anatase titanium dioxide (TiO2) films were immobilized via sol-gel technique on foam nickel modified with SiO2 films as transition layer (indicated as TiO2/SiO2 films). The structural properties of TiO2/SiO2 composite films were characterized using TG-DSC, XRD, FE-SEM, and BET. The photocatalytic activities of immobilized TiO2/SiO2 films were tested through photocatalytic degradation reactions of gaseous acetaldehyde under ultraviolet (UV) light irradiation. The substrate modification with transition layer of SiO2 films improved the specific surface area (SSA) of substrate and the acetaldehyde adsorption on photocatalyst film enormously and resulted in great enhancement of photocatalytic activity and stability. The degradation ratio of gaseous acetaldehyde (concentration of 100 ppmv) on TiO2/SiO2 photocatalysts reached 100% after 100 minutes of irradiation. The photocatalytic activity of TiO2/SiO2 films reduced about 15% after 9 consecutive runs, which was more stable than that of TiO2 films, and was completely recovered after heating at 300°C for 1 hour.


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## Body

## 1. Introduction

Volatile organic compounds (VOCs) are one of the most
important causes of indoor air pollution, and their effective removal has become a focus in the field of indoor air
quality (IAQ). Formaldehyde is one of the major indoor VOCs pollutants and
exists extensively in modern building materials and furnishings. The removal of
formaldehyde is vital for improving IAQ and occupants' health due to the toxic
and carcinogenic risks [1].Photocatalysis, as an environmentally friendly
technology, has attracted much attention for its potential application in
utilizing the cheap and abundant solar energy. TiO2 semiconductor is a widely used
photocatalyst for the removal of all kinds of organic and inorganic pollutants
in air or water because of its stability, nontoxicity, low cost, and a
relatively satisfying activity [2–5]. However, TiO2 suspension system applied in
practice often suffers from the aggregation of suspended particles with
ultrafine size and the difficulties in separation of photocatalyst from
suspension after reaction [6, 7]. Photocatalyst immobilization technology overcomes the
above problems [8–10]. Many materials have been used as supports for
photocatalysts such as glass [11], stainless steel [12], porous alumina [13, 14], activated carbon [15, 16], zeolite [17, 18], ceramic foam [19–21], silica gel [22], and so on. All these materials have certain
drawbacks such as small SSA, low light transmittance, and so on. Foam metal
materials have been recently utilized in heterogeneous catalysis because their
uniform open-porous and reticulate structure provides the catalytic system
excellent gas-dynamic properties and sufficient contact of the gaseous
reactants with the catalyst surface [23], which show significant advantages in gas-phase
photocatalytic reactions [24, 25], so they are expected to be utilized as substrate in
remediation of indoor air pollution that mainly caused by VOCs.The immobilization process often results in decrease
of photocatalytic activity because of reduction in photocatalyst surface area.
Liu et al. [26, 27] used polyvinyl alcohol (PVA) as a binder to
immobilize TiO2 on support, but it might be
not stable and could react with positive holes to result in the decrease of
photocatalytic activity. So enhancing the photocatalytic activity of
immobilized TiO2 is of great significance for
its application in environmental purification. The combination of TiO2 and absorbents has been
reported to be effective for the enhancement of photocatalytic activity [28, 29]. Our previous studies
showed that the preheating treatment of the foam nickel substrate increased its
SSA and provided more sites for TiO2 loading resulting in the
enhancement of photocatalytic activity of the immobilized TiO2 [25]. In the present paper, foam nickel was modified by
coating SiO films possessing large SSA and
strong adsorbability as transition layers. Anatase TiO2 films were immobilized on the
modified substrate via sol-gel technique without any organic binder.
Acetaldehyde was used as a substitute for formaldehyde to test the
photocatalytic activity of TiO2/SiO2 films under UV irradiation
because it has similar physical and chemical properties and less toxicity than
formaldehyde.The aim of the work reported in this paper was to
investigate the effect of SiO2 transition layers coated on
substrate on surface properties and photocatalytic activity of immobilized TiO2. The effects of photocatalyst
preparation parameters along with their photocatalytic behavior in deactivation
and regeneration are presented.


## 2. Experimental

### 2.1. Composite TiO2/SiO2 Films Preparation

All chemicals were of analytical grade, provided by
China Medicine (Group) Shanghai Chemical Reagent Corporation, and were used
without any treatment. Precursor solutions for SiO2 and TiO2 films were prepared via
sol-gel processes. SiO2 sol was prepared as follows:
Ethyl silicate (TEOS) was dissolved in anhydrous ethanol under stirring for 1 hour at room temperature, and then the mixture of deionized water and
hydrochloric acid was added dropwise into the solution under vigorous stirring.
The resultant precursor solution was stirred at room temperature for 1 hour,
and then was sealed and placed in the dark for 24 hours. Finally, a uniform,
stable, and transparent sol of SiO2 was obtained through above
procedure. The final mole ratio of HCl, TEOS, C2H5OH, and H2O was 0.05:1:2:10 in the sol. The
synthetic procedure of TiO2 sol was described in earlier
literature [25]. Foam
nickel wafer (thickness of ca. 1.4 mm, porosity ≥ 95%, and Ni ≥ 99.9 wt.%) was used as
basic substrate materials. After ultrasonic treatment in ethanol to remove
grease, the foam nickel was washed with distilled water and dried at room
temperature.SiO2 and TiO2 films were prepared by a
dip-coating method. A piece of predried foam nickel was dipped in the SiO2 sol for minutes and then
rotated at high speed to form a wet gel film. After drying at room temperature
for 12 hours, the sample was calcined at 450°C for 2 hours in an air flow
oven. TiO2 films were deposited according
to the same procedure instead of calcining at 550°C for 1 hour. The TiO2/SiO2 double-layer films were
prepared by coating SiO2 layer on foam nickel firstly,
then depositing TiO2 layer over the SiO2 layer. For comparison, the
onefold TiO2 films immobilized on foam
nickel were synthesized as reported in earlier literature [25], in which the foam nickel
was preheated at 550°C for 10 minutes before
coating TiO2 films.The operations of dipping, drying, and heating in air
were repeated several times in order to obtain TiO2, SiO2, and TiO2/SiO2 films coated on foam nickel
with various coating cycles. The prepared TiO2, SiO2, and TiO2/SiO2 films will be denoted as T1,
T2, S1, S2, S1T1, S1T2, S2T1, and S2T2, where T and S indicate TiO2 and SiO2, respectively, and the numbers
followed indicate the coating cycles of the films. The arrange order of T and S
expresses the deposition order of TiO2 and SiO2 films. No obvious changes in
adhesion properties of the prepared films immobilized on foam nickel wafer and
mechanical properties of foam nickel were observed after calcination in the
study.


### 2.2. Photocatalyst Characterization

The thermal gravimetric and differential scanning
calorimetry (TG-DSC) curves were recorded using a TGA 2050 thermogravimetric
analyzer (TA instruments, USA) and a DSC 404/LF instrument (Netzsch, Germany).
The TiO2 and SiO2 gel powders were heated in an
air atmosphere by a temperature raise of 10°C/min, which was the same
heating condition as the preparation of TiO2 and SiO2 films. The X-ray diffraction
(XRD) patterns were recorded using an X-ray diffractometer (Bruker, D8 ADVANCE)
operated at 40 kV and 40 mA utilizing a Cu Kα radiation
source (λ=0.15405nm). The surface morphology was observed using a
field-emission scanning electron microscope (FE-SEM; Sirion 200, Philips). The
SSA and the pore-size distribution were determined
by multipoint Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda
(BJH) procedures (Quantachrome Instruments, NOVA 1000). The samples of TiO2, SiO2, or SiO2/TiO2 films coated on foam nickel
were cut into small pieces before the BET and BJH measurements.


### 2.3. Photocatalytic Activity Tests

The photocatalytic activity of prepared materials was
evaluated by photocatalytic degradation of gaseous acetaldehyde. The process of
photocatalytic oxidation of acetaldehyde was conducted in a cylindrical glass
reactor, which was provided with a quartz window of ca.167.3cm2 and a volume of 1000cm3. The scheme of the photoreactor is presented in Figure 1. The initial concentration
of gaseous acetaldehyde (from Guang Ming Special Gas Co. Ltd., Dalian) injected
into the reactor was ca. 100 ppmv (parts per million by volume). The
photocatalyst immobilized on foam nickel substrate (ϕ=140mm) was placed on
the reactor bottom and was irradiated by a 15 W UV-light lamp with dominant
emission at 254 nm through the quartz window after the adsorption equilibrium
of gaseous acetaldehyde was reached. The bare-foam
nickel and foam nickel coated with SiO2 films were used for blank
test. The vertical distance between light source and photocatalyst was 25 cm.
The average light intensity striking on the film surface was about 400 μW cm-2, as measured by a UV
radiometer (made in the Photoelectric Instrument Factory of Beijing Normal
University) with the peak intensity of 254 nm. The concentrations of
acetaldehyde and CO2 in gas phase were measured by
a gas chromatograph (model GC9160, Ouhua instruments, Shanghai) equipped with a
Porapak-R column (2.0m×3.0mm) and a flame
ionization detector (FID) after converting CO2 to CH4 through a Ni-catalyst
methanizer. The temperatures of the Porapak-R column and methanizer were 100°C and 360°C, respectively. Nitrogen was
used as the carrier gas.Figure 1
Schematic diagram of photoreactor for acetaldehyde photocatalytic degradation.


## 2.1. Composite TiO2/SiO2 Films Preparation

All chemicals were of analytical grade, provided by
China Medicine (Group) Shanghai Chemical Reagent Corporation, and were used
without any treatment. Precursor solutions for SiO2 and TiO2 films were prepared via
sol-gel processes. SiO2 sol was prepared as follows:
Ethyl silicate (TEOS) was dissolved in anhydrous ethanol under stirring for 1 hour at room temperature, and then the mixture of deionized water and
hydrochloric acid was added dropwise into the solution under vigorous stirring.
The resultant precursor solution was stirred at room temperature for 1 hour,
and then was sealed and placed in the dark for 24 hours. Finally, a uniform,
stable, and transparent sol of SiO2 was obtained through above
procedure. The final mole ratio of HCl, TEOS, C2H5OH, and H2O was 0.05:1:2:10 in the sol. The
synthetic procedure of TiO2 sol was described in earlier
literature [25]. Foam
nickel wafer (thickness of ca. 1.4 mm, porosity ≥ 95%, and Ni ≥ 99.9 wt.%) was used as
basic substrate materials. After ultrasonic treatment in ethanol to remove
grease, the foam nickel was washed with distilled water and dried at room
temperature.SiO2 and TiO2 films were prepared by a
dip-coating method. A piece of predried foam nickel was dipped in the SiO2 sol for minutes and then
rotated at high speed to form a wet gel film. After drying at room temperature
for 12 hours, the sample was calcined at 450°C for 2 hours in an air flow
oven. TiO2 films were deposited according
to the same procedure instead of calcining at 550°C for 1 hour. The TiO2/SiO2 double-layer films were
prepared by coating SiO2 layer on foam nickel firstly,
then depositing TiO2 layer over the SiO2 layer. For comparison, the
onefold TiO2 films immobilized on foam
nickel were synthesized as reported in earlier literature [25], in which the foam nickel
was preheated at 550°C for 10 minutes before
coating TiO2 films.The operations of dipping, drying, and heating in air
were repeated several times in order to obtain TiO2, SiO2, and TiO2/SiO2 films coated on foam nickel
with various coating cycles. The prepared TiO2, SiO2, and TiO2/SiO2 films will be denoted as T1,
T2, S1, S2, S1T1, S1T2, S2T1, and S2T2, where T and S indicate TiO2 and SiO2, respectively, and the numbers
followed indicate the coating cycles of the films. The arrange order of T and S
expresses the deposition order of TiO2 and SiO2 films. No obvious changes in
adhesion properties of the prepared films immobilized on foam nickel wafer and
mechanical properties of foam nickel were observed after calcination in the
study.


## 2.2. Photocatalyst Characterization

The thermal gravimetric and differential scanning
calorimetry (TG-DSC) curves were recorded using a TGA 2050 thermogravimetric
analyzer (TA instruments, USA) and a DSC 404/LF instrument (Netzsch, Germany).
The TiO2 and SiO2 gel powders were heated in an
air atmosphere by a temperature raise of 10°C/min, which was the same
heating condition as the preparation of TiO2 and SiO2 films. The X-ray diffraction
(XRD) patterns were recorded using an X-ray diffractometer (Bruker, D8 ADVANCE)
operated at 40 kV and 40 mA utilizing a Cu Kα radiation
source (λ=0.15405nm). The surface morphology was observed using a
field-emission scanning electron microscope (FE-SEM; Sirion 200, Philips). The
SSA and the pore-size distribution were determined
by multipoint Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda
(BJH) procedures (Quantachrome Instruments, NOVA 1000). The samples of TiO2, SiO2, or SiO2/TiO2 films coated on foam nickel
were cut into small pieces before the BET and BJH measurements.


## 2.3. Photocatalytic Activity Tests

The photocatalytic activity of prepared materials was
evaluated by photocatalytic degradation of gaseous acetaldehyde. The process of
photocatalytic oxidation of acetaldehyde was conducted in a cylindrical glass
reactor, which was provided with a quartz window of ca.167.3cm2 and a volume of 1000cm3. The scheme of the photoreactor is presented in Figure 1. The initial concentration
of gaseous acetaldehyde (from Guang Ming Special Gas Co. Ltd., Dalian) injected
into the reactor was ca. 100 ppmv (parts per million by volume). The
photocatalyst immobilized on foam nickel substrate (ϕ=140mm) was placed on
the reactor bottom and was irradiated by a 15 W UV-light lamp with dominant
emission at 254 nm through the quartz window after the adsorption equilibrium
of gaseous acetaldehyde was reached. The bare-foam
nickel and foam nickel coated with SiO2 films were used for blank
test. The vertical distance between light source and photocatalyst was 25 cm.
The average light intensity striking on the film surface was about 400 μW cm-2, as measured by a UV
radiometer (made in the Photoelectric Instrument Factory of Beijing Normal
University) with the peak intensity of 254 nm. The concentrations of
acetaldehyde and CO2 in gas phase were measured by
a gas chromatograph (model GC9160, Ouhua instruments, Shanghai) equipped with a
Porapak-R column (2.0m×3.0mm) and a flame
ionization detector (FID) after converting CO2 to CH4 through a Ni-catalyst
methanizer. The temperatures of the Porapak-R column and methanizer were 100°C and 360°C, respectively. Nitrogen was
used as the carrier gas.Figure 1
Schematic diagram of photoreactor for acetaldehyde photocatalytic degradation.


## 3. Results and Discussion

### 3.1. Characterization of TiO2/SiO2 Composite Photocatalyst

Figure2 shows the TG-DSC thermograms of the room-temperature-dried TiO2 and SiO2 gels in an air atmosphere by
heating at the rate of 10°C/min to 800°C. In Figure 2(a), the weight loss of TiO2 gel ended before heating in
520°C. The exothermic peak at 514°C may be assigned to the phase
transition of amorphous TiO2 to anatase TiO2. Therefore, the calcination
temperature for TiO2 films was selected at 550°C in order to get the anatase
TiO2 films with large SSA. In Figure 2(b), the only obvious weight
loss process of SiO gel ended before heating in
100°C, which was assigned to evaporation
of H2O in the gel. No exothermic or
endothermic peak was observed, indicating that no phase transition of SiO2 occurred, and only amorphous
SiO2 was generated below 800°C. The synthetic conditions
for SiO2 films were selected at 450°C for 2 hours.TG-DSC curves of the room-temperature-dried TiO2 (a) and SiO2 (b) gels in an air atmosphere.
(a)(b)Figure3 shows the nitrogen adsorption-desorption curves measured by BET method
and the pore-size distribution calculated from desorption isotherms of SiO2 powder prepared by sol-gel method.
The SiO2 powder has enormous adsorption
ability and a SSA of 550.4 m2 g-1. The pore-size distribution of
SiO2 is very concentrated with an
average size of 3.5 nm, belonging to mesoporous materials. The SSAs of
bare-foam nickel, S1 and S2 are 0.12, 167.2, and 159.4 m2 g-1, respectively. The great
increment in the SSAs of S1 and S2 are attributed to the coating of SiO2 possessing large SSA over the
foam nickel. However, the repeated heating treatment of S2
resulted in a smaller SSA than the S1. The SiO2 film loaded on foam nickel as
transition layer contributes greatly to providing more active sites on
substrate for photocatalyst loading.Figure 3
N2 adsorption-desorption isotherms (a) and pore-size distribution (b) of SiO2 powder
obtained from sol-gel method.Figure4 shows the XRD patterns of foam nickel substrates that are without modification and modified
by SiO2 films for various coating
cycles. The diffraction peaks of Ni (JCPDS file no. 04-0850) but
not those of SiO2 are observed, which indicate the generation of
amorphous SiO2 films. The diffraction
intensity of Ni decreases with increasing coating cycles of SiO2 because the X-ray detection of
nickel is hindered by the SiO2 films coated on it. The
weight-increment ratios of SiO2 for S1 and S2 (i.e., the
percentage of the weight of loaded SiO2 compared with the weight of
foam nickel) are 19.4 wt.% and 34.8 wt.%, respectively.Figure 4
XRD patterns
of bare-foam nickel (a), S1 (b), and S2 (c).Figure5 shows the XRD patterns of immobilized photocatalysts of T1, S1T1, S2T1,
T2, S1T2, and S2T2. The diffraction peaks of anatase TiO2 (JCPDS file no. 21-1272)
increase with increasing TiO2 coating cycles, and appear
clearly at the second coating cycle. Loading the SiO2 transition layer also results
in peak intensity enhancement of anatase TiO2 indicating the amount increase
of loaded TiO2. Slight diffraction peaks of
NiO (JCPDS file no. 47-1049) generated from the oxidation of nickel during the
calcination are also observed.Figure 5
XRD patterns
of T1, S1T1, S2T1, T2, S1T2, and S2T2. (Symbol:•—anatase TiO2, ▴—NiO, ⋆—Ni.)The FE-SEM images (Figure6) display little difference in the framework
of substrate among S2, S2T2, and bare-foam nickel, but the surface of S2 and
S2T2 is rougher than that of bare-foam nickel after coating SiO2 and TiO2 films and heating treatment at
high temperature. The weight-increment ratios of TiO2 for S1T1 and S1T2 are 4.2 wt.% and 6.6 wt.%, respectively (see Table 1), which
indicate that the amount of loaded TiO2 increases with increasing
coating cycles of TiO2. The SSAs of S1T1, S1T2, S2T1,
and S2T2 are 70.6, 35.4, 56, and 32.9 m2 g-1, respectively. The major
distribution ranges of pore size are 3–5 nm for S1 and S2, and 3–10 nm for S1T1, S1T2, S2T1, and S2T2.Table 1
Comparison of surface characteristics and
photocatalytic behaviors of various TiO2 and TiO2/SiO2 films immobilized on foam
nickel.
SampleSurface characteristicsDegradation ratio of acetaldehyde inSSA of modified substrate(m2g−1)aSSA of the sample (m2 g-1)TiO2 weight ratio (%)b90 min(%)cT10.240.7535T20.240.9862S1T1167.270.64.242S1T2167.235.46.670S2T1159.456.04.180S2T2159.432.97.892aThe foam nickel substrate
was modified by preheating treatment (550°C for 10 minutes) for T1 and
T2, and coating SiO2 as transition layer for S1T1,
S1T2, S2T1, and S2T2 before loading TiO2. The S1 was regarded as
modified substrate for S1T1 and S1T2, and S2 as that of S2T1 and S2T2.
bCalculated according to R=(Wt/Wms)×100%, where R is the
weight-increment ratio of TiO2, Wt denotes the
weight of loaded TiO2, and Wms is the weight
of modified substrate.
cGaseous acetaldehyde in 100 ppmv of 1000 cm3. A UV-light lamp (λ=254nm,15W) was applied as
light source.FE-SEM images of bare-foam
nickel (a), S2 (b), and S2T2 (c). The scale is 200μm.
(a)(b)(c)


### 3.2. Photocatalytic Activity Test

#### 3.2.1. Acetaldehyde Adsorption

As shown in
Figure7, the TiO2/SiO2 composite films have intense
adsorption for gaseous acetaldehyde at the initial stage and reach adsorption
equilibrium in about 90 minutes after injecting gaseous acetaldehyde into the
reactor. The residual concentrations of acetaldehyde in gas phase for TiO2 films and TiO2 /SiO films are about 70 ppmv and 10 ppmv, respectively. The TiO2/SiO2 films have much higher
adsorbability for gaseous acetaldehyde than TiO2 films. It has been reported
that the adsorbability of adsorbent used in photocatalytic reaction had much
influence on the reaction rate, and the highest reaction rate was achieved
by supports that had moderate adsorbability, where
a high amount of absorbed reactants is available and could be easily
transferred to the loaded TiO2 particles [28, 29].Figure 7
Time courses
of concentrations of gaseous acetaldehyde caused by adsorption on bare-foam
nickel, T1, T2, S1T1, S1T2, S2T1, and S2T2.


#### 3.2.2. Acetaldehyde Photocatalytic Degradation

The degradation ratios of gaseous acetaldehyde were
calculated according to the equation,(1)Rt=CtCi, where Rt is the
degradation ratio of gaseous acetaldehyde, Ci denotes the
initial concentration of gaseous acetaldehyde (100 ppmv), and Ct is the
concentration of completely decomposed acetaldehyde during the reaction
calculated from the amount of CO2 evolution (the complete
photocatalytic decomposition of one acetaldehyde molecule yields two carbon
dioxide molecules) measured by a gas chromatograph. The degradation plots were
the average of three parallel tests.Blank reactions were performed in the dark or without
TiO2 (i.e., using the substrate
samples) under the same experimental conditions of the irradiation experiments.
Negligible degradation of acetaldehyde was observed in the absence of TiO2 or UV irradiation.Figure8 presents the photocatalytic decomposition of gaseous acetaldehyde on TiO2 and TiO2/SiO2 films immobilized on foam
nickel under UV light irradiation. The degradation ratios of gaseous
acetaldehyde increase very fast at the beginning of irradiation and then level
off. The photocatalytic activity at the initial stage of reaction is in
following order: S2T2 > S1T2 > S2T1 > T2 > S1T1 > T1, while the
photocatalytic activity of complete oxidation for total acetaldehyde shows the
following order: S2T2 > S2T1 > S1T2 > S1T1 > T2 > T1. The
degradation rate of acetaldehyde on onefold TiO2 films decreases with the
irradiation more rapidly than TiO2 films combining with SiO2 transition layer. The
photocatalytic activity of TiO2 was observably enhanced by
coating SiO2 films as transition layer.
Coating the SiO2 and TiO2 films repeatedly was also
advantageous to enhance the photocatalytic activity. The complete
photocatalytic decomposition of acetaldehyde was achieved after 100 minutes of
UV irradiation on S2T2, which was 3.6 times faster than that of S1T1.Figure 8
Time courses
of the photocatalytic decomposition of gaseous acetaldehyde over various TiO2 and TiO2/SiO2 films
immobilized on foam nickel under UV-light irradiation (λ=254nm,15W).From the results of adsorption (Figure7) and photocatalytic
reactions (Figure 8), it is found that the TiO2/SiO2 films show higher
adsorbability and degrade acetaldehyde more rapidly than the TiO2 films with the same TiO2 coating cycles. For instance,
the sample S2T1 decomposes acetaldehyde completely after 115 minutes of UV
irradiation, which is more than 3 times faster than T1. As well known, the
photocatalytic reaction occurs on the surface of photocatalyst, and the
photogenerated electrons and holes recombine very fast, so the separation of
interfacial charge carrier becomes possibly more effective when the donor or
acceptor is preadsorbed before the photocatalytic reaction. The SiO2 transition layer has a
moderate adsorbability to form a relatively higher-concentration environment of
gaseous acetaldehyde around TiO2, and then transfers the
acetaldehyde molecules rapidly to TiO2 films, thus results in a high
photocatalytic degradation rate.


#### 3.2.3. Deactivation and Regeneration of the Photocatalyst

In order to evaluate the stability of photocatalyst,
consecutive photocatalytic reactions for acetaldehyde degradation were carried
out. As shown in Figures9 and 10, the photocatalytic activities of T2 and S2T2
films decrease gradually in subsequent runs. However, the deactivation of T2 is
faster than that of S2T2. It has been reported that the intermediates of
photocatalytic reaction adsorbed on the surface of photocatalyst would depress
the photocatalysis process [30]. The larger surface area of S2T2 provides more active
sites for photocatalytic reaction, which reduces the ratio of active sites
occupied by adsorption of reaction intermediates such as acetic acid, and so
on. Nevertheless, the complete reactivations of T2 and S2T2 films were achieved
by heating them at 300°C for 1 hour, no visible
deactivation caused by the exfoliation of photocatalyst during the reactions
was observed.Figure 9
Gaseous
acetaldehyde degradation rates on T2 for five consecutive runs and regenerated
T2 (heating at 300°C for 1 hour)
under UV-light irradiation (λ=254nm,15W).Figure 10
Gaseous
acetaldehyde degradation rates on S2T2 for nine consecutive runs and
regenerated S2T2 (heating at 300°C for 1 hour)
under UV-light irradiation (λ=254nm,15W).


#### 3.2.4. Reaction Mechanism for Acetaldehyde Photocatalytic Degradation

It is known that the basic process of photocatalysis
consists of UV light(λ<380nm) exciting an
electron from the valence band (VB) to the conduction band (CB) of the TiO2 semiconductor creating a hole
in the valence band. The electron-hole pairs, after migrating to the surface,
can in turn be trapped by surface-adsorbed molecules at different sites,
leading to oxidation and reduction processes. Under the experimental conditions
in this study (weak UV-illumination intensity),
most of the adsorbed acetaldehyde molecules should be oxidized to acetic acid
firstly. Then, the acetic acid is oxidized into carbon dioxide and water with
further illumination. The oxidative photodegradation process of acetaldehyde on
TiO2 in principle is described as
follows [30]: (2)O2+eCB−⟶O2•−;H2O+h+⟶•OH+H+;2CH3CHO+O2•−⟶2CH3COOH+e−;CH3CHO+2⋅OH⟶CH3COOH+H2O;CH3COOH+2O2•−⟶2CO2+2H2O+2e−;CH3COOH+8⋅OH⟶2CO2+6H2O.It has been established that the adsorbability of
photocatalyst is an important factor to determine its decomposition kinetics.
In cases of pollutants at low concentrations of ppmv level, it usually takes
long time to complete their decomposition [18, 31]. As for the as-prepared TiO2 and TiO2/SiO2 films, a considerable part of
target reactants and reaction intermediates are adsorbed around the surface of
TiO2/SiO2 films, while most
parts of them are dispersed in the gas phase on TiO2  films (Figure 11). It can be concluded that
the SiO2 interlayer has a large
capability of collecting target substances and degradation intermediates from
the gas phase onto the TiO2 films, thereby improves the
photocatalytic reaction rate.Figure 11
Schematic
diagram of acetaldehyde photocatalytic decomposition on TiO2 (a) and TiO2/SiO2 (b) films.


## 3.1. Characterization of TiO2/SiO2 Composite Photocatalyst

Figure2 shows the TG-DSC thermograms of the room-temperature-dried TiO2 and SiO2 gels in an air atmosphere by
heating at the rate of 10°C/min to 800°C. In Figure 2(a), the weight loss of TiO2 gel ended before heating in
520°C. The exothermic peak at 514°C may be assigned to the phase
transition of amorphous TiO2 to anatase TiO2. Therefore, the calcination
temperature for TiO2 films was selected at 550°C in order to get the anatase
TiO2 films with large SSA. In Figure 2(b), the only obvious weight
loss process of SiO gel ended before heating in
100°C, which was assigned to evaporation
of H2O in the gel. No exothermic or
endothermic peak was observed, indicating that no phase transition of SiO2 occurred, and only amorphous
SiO2 was generated below 800°C. The synthetic conditions
for SiO2 films were selected at 450°C for 2 hours.TG-DSC curves of the room-temperature-dried TiO2 (a) and SiO2 (b) gels in an air atmosphere.
(a)(b)Figure3 shows the nitrogen adsorption-desorption curves measured by BET method
and the pore-size distribution calculated from desorption isotherms of SiO2 powder prepared by sol-gel method.
The SiO2 powder has enormous adsorption
ability and a SSA of 550.4 m2 g-1. The pore-size distribution of
SiO2 is very concentrated with an
average size of 3.5 nm, belonging to mesoporous materials. The SSAs of
bare-foam nickel, S1 and S2 are 0.12, 167.2, and 159.4 m2 g-1, respectively. The great
increment in the SSAs of S1 and S2 are attributed to the coating of SiO2 possessing large SSA over the
foam nickel. However, the repeated heating treatment of S2
resulted in a smaller SSA than the S1. The SiO2 film loaded on foam nickel as
transition layer contributes greatly to providing more active sites on
substrate for photocatalyst loading.Figure 3
N2 adsorption-desorption isotherms (a) and pore-size distribution (b) of SiO2 powder
obtained from sol-gel method.Figure4 shows the XRD patterns of foam nickel substrates that are without modification and modified
by SiO2 films for various coating
cycles. The diffraction peaks of Ni (JCPDS file no. 04-0850) but
not those of SiO2 are observed, which indicate the generation of
amorphous SiO2 films. The diffraction
intensity of Ni decreases with increasing coating cycles of SiO2 because the X-ray detection of
nickel is hindered by the SiO2 films coated on it. The
weight-increment ratios of SiO2 for S1 and S2 (i.e., the
percentage of the weight of loaded SiO2 compared with the weight of
foam nickel) are 19.4 wt.% and 34.8 wt.%, respectively.Figure 4
XRD patterns
of bare-foam nickel (a), S1 (b), and S2 (c).Figure5 shows the XRD patterns of immobilized photocatalysts of T1, S1T1, S2T1,
T2, S1T2, and S2T2. The diffraction peaks of anatase TiO2 (JCPDS file no. 21-1272)
increase with increasing TiO2 coating cycles, and appear
clearly at the second coating cycle. Loading the SiO2 transition layer also results
in peak intensity enhancement of anatase TiO2 indicating the amount increase
of loaded TiO2. Slight diffraction peaks of
NiO (JCPDS file no. 47-1049) generated from the oxidation of nickel during the
calcination are also observed.Figure 5
XRD patterns
of T1, S1T1, S2T1, T2, S1T2, and S2T2. (Symbol:•—anatase TiO2, ▴—NiO, ⋆—Ni.)The FE-SEM images (Figure6) display little difference in the framework
of substrate among S2, S2T2, and bare-foam nickel, but the surface of S2 and
S2T2 is rougher than that of bare-foam nickel after coating SiO2 and TiO2 films and heating treatment at
high temperature. The weight-increment ratios of TiO2 for S1T1 and S1T2 are 4.2 wt.% and 6.6 wt.%, respectively (see Table 1), which
indicate that the amount of loaded TiO2 increases with increasing
coating cycles of TiO2. The SSAs of S1T1, S1T2, S2T1,
and S2T2 are 70.6, 35.4, 56, and 32.9 m2 g-1, respectively. The major
distribution ranges of pore size are 3–5 nm for S1 and S2, and 3–10 nm for S1T1, S1T2, S2T1, and S2T2.Table 1
Comparison of surface characteristics and
photocatalytic behaviors of various TiO2 and TiO2/SiO2 films immobilized on foam
nickel.
SampleSurface characteristicsDegradation ratio of acetaldehyde inSSA of modified substrate(m2g−1)aSSA of the sample (m2 g-1)TiO2 weight ratio (%)b90 min(%)cT10.240.7535T20.240.9862S1T1167.270.64.242S1T2167.235.46.670S2T1159.456.04.180S2T2159.432.97.892aThe foam nickel substrate
was modified by preheating treatment (550°C for 10 minutes) for T1 and
T2, and coating SiO2 as transition layer for S1T1,
S1T2, S2T1, and S2T2 before loading TiO2. The S1 was regarded as
modified substrate for S1T1 and S1T2, and S2 as that of S2T1 and S2T2.
bCalculated according to R=(Wt/Wms)×100%, where R is the
weight-increment ratio of TiO2, Wt denotes the
weight of loaded TiO2, and Wms is the weight
of modified substrate.
cGaseous acetaldehyde in 100 ppmv of 1000 cm3. A UV-light lamp (λ=254nm,15W) was applied as
light source.FE-SEM images of bare-foam
nickel (a), S2 (b), and S2T2 (c). The scale is 200μm.
(a)(b)(c)


## 3.2. Photocatalytic Activity Test

### 3.2.1. Acetaldehyde Adsorption

As shown in
Figure7, the TiO2/SiO2 composite films have intense
adsorption for gaseous acetaldehyde at the initial stage and reach adsorption
equilibrium in about 90 minutes after injecting gaseous acetaldehyde into the
reactor. The residual concentrations of acetaldehyde in gas phase for TiO2 films and TiO2 /SiO films are about 70 ppmv and 10 ppmv, respectively. The TiO2/SiO2 films have much higher
adsorbability for gaseous acetaldehyde than TiO2 films. It has been reported
that the adsorbability of adsorbent used in photocatalytic reaction had much
influence on the reaction rate, and the highest reaction rate was achieved
by supports that had moderate adsorbability, where
a high amount of absorbed reactants is available and could be easily
transferred to the loaded TiO2 particles [28, 29].Figure 7
Time courses
of concentrations of gaseous acetaldehyde caused by adsorption on bare-foam
nickel, T1, T2, S1T1, S1T2, S2T1, and S2T2.


### 3.2.2. Acetaldehyde Photocatalytic Degradation

The degradation ratios of gaseous acetaldehyde were
calculated according to the equation,(1)Rt=CtCi, where Rt is the
degradation ratio of gaseous acetaldehyde, Ci denotes the
initial concentration of gaseous acetaldehyde (100 ppmv), and Ct is the
concentration of completely decomposed acetaldehyde during the reaction
calculated from the amount of CO2 evolution (the complete
photocatalytic decomposition of one acetaldehyde molecule yields two carbon
dioxide molecules) measured by a gas chromatograph. The degradation plots were
the average of three parallel tests.Blank reactions were performed in the dark or without
TiO2 (i.e., using the substrate
samples) under the same experimental conditions of the irradiation experiments.
Negligible degradation of acetaldehyde was observed in the absence of TiO2 or UV irradiation.Figure8 presents the photocatalytic decomposition of gaseous acetaldehyde on TiO2 and TiO2/SiO2 films immobilized on foam
nickel under UV light irradiation. The degradation ratios of gaseous
acetaldehyde increase very fast at the beginning of irradiation and then level
off. The photocatalytic activity at the initial stage of reaction is in
following order: S2T2 > S1T2 > S2T1 > T2 > S1T1 > T1, while the
photocatalytic activity of complete oxidation for total acetaldehyde shows the
following order: S2T2 > S2T1 > S1T2 > S1T1 > T2 > T1. The
degradation rate of acetaldehyde on onefold TiO2 films decreases with the
irradiation more rapidly than TiO2 films combining with SiO2 transition layer. The
photocatalytic activity of TiO2 was observably enhanced by
coating SiO2 films as transition layer.
Coating the SiO2 and TiO2 films repeatedly was also
advantageous to enhance the photocatalytic activity. The complete
photocatalytic decomposition of acetaldehyde was achieved after 100 minutes of
UV irradiation on S2T2, which was 3.6 times faster than that of S1T1.Figure 8
Time courses
of the photocatalytic decomposition of gaseous acetaldehyde over various TiO2 and TiO2/SiO2 films
immobilized on foam nickel under UV-light irradiation (λ=254nm,15W).From the results of adsorption (Figure7) and photocatalytic
reactions (Figure 8), it is found that the TiO2/SiO2 films show higher
adsorbability and degrade acetaldehyde more rapidly than the TiO2 films with the same TiO2 coating cycles. For instance,
the sample S2T1 decomposes acetaldehyde completely after 115 minutes of UV
irradiation, which is more than 3 times faster than T1. As well known, the
photocatalytic reaction occurs on the surface of photocatalyst, and the
photogenerated electrons and holes recombine very fast, so the separation of
interfacial charge carrier becomes possibly more effective when the donor or
acceptor is preadsorbed before the photocatalytic reaction. The SiO2 transition layer has a
moderate adsorbability to form a relatively higher-concentration environment of
gaseous acetaldehyde around TiO2, and then transfers the
acetaldehyde molecules rapidly to TiO2 films, thus results in a high
photocatalytic degradation rate.


### 3.2.3. Deactivation and Regeneration of the Photocatalyst

In order to evaluate the stability of photocatalyst,
consecutive photocatalytic reactions for acetaldehyde degradation were carried
out. As shown in Figures9 and 10, the photocatalytic activities of T2 and S2T2
films decrease gradually in subsequent runs. However, the deactivation of T2 is
faster than that of S2T2. It has been reported that the intermediates of
photocatalytic reaction adsorbed on the surface of photocatalyst would depress
the photocatalysis process [30]. The larger surface area of S2T2 provides more active
sites for photocatalytic reaction, which reduces the ratio of active sites
occupied by adsorption of reaction intermediates such as acetic acid, and so
on. Nevertheless, the complete reactivations of T2 and S2T2 films were achieved
by heating them at 300°C for 1 hour, no visible
deactivation caused by the exfoliation of photocatalyst during the reactions
was observed.Figure 9
Gaseous
acetaldehyde degradation rates on T2 for five consecutive runs and regenerated
T2 (heating at 300°C for 1 hour)
under UV-light irradiation (λ=254nm,15W).Figure 10
Gaseous
acetaldehyde degradation rates on S2T2 for nine consecutive runs and
regenerated S2T2 (heating at 300°C for 1 hour)
under UV-light irradiation (λ=254nm,15W).


### 3.2.4. Reaction Mechanism for Acetaldehyde Photocatalytic Degradation

It is known that the basic process of photocatalysis
consists of UV light(λ<380nm) exciting an
electron from the valence band (VB) to the conduction band (CB) of the TiO2 semiconductor creating a hole
in the valence band. The electron-hole pairs, after migrating to the surface,
can in turn be trapped by surface-adsorbed molecules at different sites,
leading to oxidation and reduction processes. Under the experimental conditions
in this study (weak UV-illumination intensity),
most of the adsorbed acetaldehyde molecules should be oxidized to acetic acid
firstly. Then, the acetic acid is oxidized into carbon dioxide and water with
further illumination. The oxidative photodegradation process of acetaldehyde on
TiO2 in principle is described as
follows [30]: (2)O2+eCB−⟶O2•−;H2O+h+⟶•OH+H+;2CH3CHO+O2•−⟶2CH3COOH+e−;CH3CHO+2⋅OH⟶CH3COOH+H2O;CH3COOH+2O2•−⟶2CO2+2H2O+2e−;CH3COOH+8⋅OH⟶2CO2+6H2O.It has been established that the adsorbability of
photocatalyst is an important factor to determine its decomposition kinetics.
In cases of pollutants at low concentrations of ppmv level, it usually takes
long time to complete their decomposition [18, 31]. As for the as-prepared TiO2 and TiO2/SiO2 films, a considerable part of
target reactants and reaction intermediates are adsorbed around the surface of
TiO2/SiO2 films, while most
parts of them are dispersed in the gas phase on TiO2  films (Figure 11). It can be concluded that
the SiO2 interlayer has a large
capability of collecting target substances and degradation intermediates from
the gas phase onto the TiO2 films, thereby improves the
photocatalytic reaction rate.Figure 11
Schematic
diagram of acetaldehyde photocatalytic decomposition on TiO2 (a) and TiO2/SiO2 (b) films.


## 3.2.1. Acetaldehyde Adsorption

As shown in
Figure7, the TiO2/SiO2 composite films have intense
adsorption for gaseous acetaldehyde at the initial stage and reach adsorption
equilibrium in about 90 minutes after injecting gaseous acetaldehyde into the
reactor. The residual concentrations of acetaldehyde in gas phase for TiO2 films and TiO2 /SiO films are about 70 ppmv and 10 ppmv, respectively. The TiO2/SiO2 films have much higher
adsorbability for gaseous acetaldehyde than TiO2 films. It has been reported
that the adsorbability of adsorbent used in photocatalytic reaction had much
influence on the reaction rate, and the highest reaction rate was achieved
by supports that had moderate adsorbability, where
a high amount of absorbed reactants is available and could be easily
transferred to the loaded TiO2 particles [28, 29].Figure 7
Time courses
of concentrations of gaseous acetaldehyde caused by adsorption on bare-foam
nickel, T1, T2, S1T1, S1T2, S2T1, and S2T2.


## 3.2.2. Acetaldehyde Photocatalytic Degradation

The degradation ratios of gaseous acetaldehyde were
calculated according to the equation,(1)Rt=CtCi, where Rt is the
degradation ratio of gaseous acetaldehyde, Ci denotes the
initial concentration of gaseous acetaldehyde (100 ppmv), and Ct is the
concentration of completely decomposed acetaldehyde during the reaction
calculated from the amount of CO2 evolution (the complete
photocatalytic decomposition of one acetaldehyde molecule yields two carbon
dioxide molecules) measured by a gas chromatograph. The degradation plots were
the average of three parallel tests.Blank reactions were performed in the dark or without
TiO2 (i.e., using the substrate
samples) under the same experimental conditions of the irradiation experiments.
Negligible degradation of acetaldehyde was observed in the absence of TiO2 or UV irradiation.Figure8 presents the photocatalytic decomposition of gaseous acetaldehyde on TiO2 and TiO2/SiO2 films immobilized on foam
nickel under UV light irradiation. The degradation ratios of gaseous
acetaldehyde increase very fast at the beginning of irradiation and then level
off. The photocatalytic activity at the initial stage of reaction is in
following order: S2T2 > S1T2 > S2T1 > T2 > S1T1 > T1, while the
photocatalytic activity of complete oxidation for total acetaldehyde shows the
following order: S2T2 > S2T1 > S1T2 > S1T1 > T2 > T1. The
degradation rate of acetaldehyde on onefold TiO2 films decreases with the
irradiation more rapidly than TiO2 films combining with SiO2 transition layer. The
photocatalytic activity of TiO2 was observably enhanced by
coating SiO2 films as transition layer.
Coating the SiO2 and TiO2 films repeatedly was also
advantageous to enhance the photocatalytic activity. The complete
photocatalytic decomposition of acetaldehyde was achieved after 100 minutes of
UV irradiation on S2T2, which was 3.6 times faster than that of S1T1.Figure 8
Time courses
of the photocatalytic decomposition of gaseous acetaldehyde over various TiO2 and TiO2/SiO2 films
immobilized on foam nickel under UV-light irradiation (λ=254nm,15W).From the results of adsorption (Figure7) and photocatalytic
reactions (Figure 8), it is found that the TiO2/SiO2 films show higher
adsorbability and degrade acetaldehyde more rapidly than the TiO2 films with the same TiO2 coating cycles. For instance,
the sample S2T1 decomposes acetaldehyde completely after 115 minutes of UV
irradiation, which is more than 3 times faster than T1. As well known, the
photocatalytic reaction occurs on the surface of photocatalyst, and the
photogenerated electrons and holes recombine very fast, so the separation of
interfacial charge carrier becomes possibly more effective when the donor or
acceptor is preadsorbed before the photocatalytic reaction. The SiO2 transition layer has a
moderate adsorbability to form a relatively higher-concentration environment of
gaseous acetaldehyde around TiO2, and then transfers the
acetaldehyde molecules rapidly to TiO2 films, thus results in a high
photocatalytic degradation rate.


## 3.2.3. Deactivation and Regeneration of the Photocatalyst

In order to evaluate the stability of photocatalyst,
consecutive photocatalytic reactions for acetaldehyde degradation were carried
out. As shown in Figures9 and 10, the photocatalytic activities of T2 and S2T2
films decrease gradually in subsequent runs. However, the deactivation of T2 is
faster than that of S2T2. It has been reported that the intermediates of
photocatalytic reaction adsorbed on the surface of photocatalyst would depress
the photocatalysis process [30]. The larger surface area of S2T2 provides more active
sites for photocatalytic reaction, which reduces the ratio of active sites
occupied by adsorption of reaction intermediates such as acetic acid, and so
on. Nevertheless, the complete reactivations of T2 and S2T2 films were achieved
by heating them at 300°C for 1 hour, no visible
deactivation caused by the exfoliation of photocatalyst during the reactions
was observed.Figure 9
Gaseous
acetaldehyde degradation rates on T2 for five consecutive runs and regenerated
T2 (heating at 300°C for 1 hour)
under UV-light irradiation (λ=254nm,15W).Figure 10
Gaseous
acetaldehyde degradation rates on S2T2 for nine consecutive runs and
regenerated S2T2 (heating at 300°C for 1 hour)
under UV-light irradiation (λ=254nm,15W).


## 3.2.4. Reaction Mechanism for Acetaldehyde Photocatalytic Degradation

It is known that the basic process of photocatalysis
consists of UV light(λ<380nm) exciting an
electron from the valence band (VB) to the conduction band (CB) of the TiO2 semiconductor creating a hole
in the valence band. The electron-hole pairs, after migrating to the surface,
can in turn be trapped by surface-adsorbed molecules at different sites,
leading to oxidation and reduction processes. Under the experimental conditions
in this study (weak UV-illumination intensity),
most of the adsorbed acetaldehyde molecules should be oxidized to acetic acid
firstly. Then, the acetic acid is oxidized into carbon dioxide and water with
further illumination. The oxidative photodegradation process of acetaldehyde on
TiO2 in principle is described as
follows [30]: (2)O2+eCB−⟶O2•−;H2O+h+⟶•OH+H+;2CH3CHO+O2•−⟶2CH3COOH+e−;CH3CHO+2⋅OH⟶CH3COOH+H2O;CH3COOH+2O2•−⟶2CO2+2H2O+2e−;CH3COOH+8⋅OH⟶2CO2+6H2O.It has been established that the adsorbability of
photocatalyst is an important factor to determine its decomposition kinetics.
In cases of pollutants at low concentrations of ppmv level, it usually takes
long time to complete their decomposition [18, 31]. As for the as-prepared TiO2 and TiO2/SiO2 films, a considerable part of
target reactants and reaction intermediates are adsorbed around the surface of
TiO2/SiO2 films, while most
parts of them are dispersed in the gas phase on TiO2  films (Figure 11). It can be concluded that
the SiO2 interlayer has a large
capability of collecting target substances and degradation intermediates from
the gas phase onto the TiO2 films, thereby improves the
photocatalytic reaction rate.Figure 11
Schematic
diagram of acetaldehyde photocatalytic decomposition on TiO2 (a) and TiO2/SiO2 (b) films.


## 4. Conclusions

TiO2/SiO2 films were immobilized on foam
nickel substrate by sol-gel technique. The photocatalytic activity and
stability of TiO2 were greatly enhanced by
coating SiO2 films as transition
layer. The SiO2 transition layer possessed
large SSA and the TiO2/SiO2 composite film showed much
stronger adsorption of gaseous acetaldehyde than the onefold TiO2 film. The repeated coating of
SiO2 transition layer and TiO2 photocatalyst increased the
amount of TiO2 immobilized on substrate,
which was helpful for accelerating the photocatalytic reaction. It was
suggested that the use of inert transition layer possessing large SSA and
appropriate adsorbability on support was favorable for enhancing the activity
of immobilized TiO2 photocatalyst.



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*Source: 679421-2008-04-10.xml*