# Effect of Gold Dispersion on the Photocatalytic Activity of Mesoporous Titania for the Vapor-Phase Oxidation of Acetone

Authors: S. V. Awate; A. A. Belhekar; S. V. Bhagwat; R. Kumar; N. M. Gupta
Journal: International Journal of Photoenergy
 (2007-12-31)

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## Abstract

Mesostructured titanium dioxide photocatalyst, having uniform crystallite size (6–12 nm) and average pore diameter of∼4.2 nm, was synthesized by using a low-temperature nonsurfactant hydrothermal route, employing tartaric acid as a templating agent. Gold additions from 0.5 to 2 wt% were incorporated, either during the hydrothermal process or by postsynthesis wet impregnation. Compared to the impregnation-prepared samples, the samples synthesized hydrothermally contained smaller-size (≤1 nm) gold clusters occluded in the pores of the host matrix. Whereas CO2 and H2O were the main reaction products in UV-assisted vapor-phase oxidation of acetone using these catalysts, C2H6 and HCO2CH3 were also produced for higher acetone concentrations in air. The conversion of acetone was found to increase with decrease in the size of both TiO2 and gold particles. In situ IR spectroscopy revealed that titania and gold particles serve as independent adsorption and reaction sites for acetone and oxygen molecules. Acetone molecules adsorb exclusively at TiO2 surface, giving rise to a strongly adsorbed (condensed) state as well as to the formation of formate- and methyl formate-type surface species. Hydroxyl groups at titania surface participate directly in these adsorption steps. Nanosize gold particles, on the other hand, were primarily responsible for the adsorption and activation of oxygen molecules. Mechanistic aspects of the photochemical processes are discussed on the basis of these observations.


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## Body

## 1. Introduction

Concerted research has been devoted to
oxidative degradation of volatile organic contaminants via photocatalytic route,
with a view to find a viable solution to environmental pollution control [1–3].
Nevertheless, the low quantum yield and poor stability of presently available
photocatalytic materials have posed limitations on the large-scale application of
the above-mentioned process. These deficiencies arise mainly due to the
following reasons: (i) dependence on UV radiation due to large band gap, (ii) a
low rate of electron transfer to reactant molecules including oxygen, and (iii)
a high rate of recombination of electron-hole pair resulting in low quantum
yields. Various approaches have been attempted to enhance the efficiency of
photocatalytic materials, either by bringing out certain structural and
morphological modifications or by incorporation of certain cocatalysts. In the
case of widely studied titanium dioxide photocatalyst, following general strategies
have been adopted: (i) morphological changes in bulk TiO2  [1–8], (ii)
doping of TiO2  with a group VIII metal or a noble metal to increase
the lifetime of electron-hole pair and extending the light absorption to
visible region, [9–13], (iii) incorporation of TiO2  in certain
mesoporous materials in order to increase the exposed area [14–17], and (iv)
chemical substitution at Ti or O sites, so as to modify the band
characteristics and to shift the absorbance toward visible region [18–21]. In
spite of these recent developments, detailed knowledge on the precise role of
morphological properties of TiO2, and also that of a metal cocatalyst,
still eludes us.In
the recent studies reported by Bhattacharya et al. [14, 15], we demonstrated that
the nanosize TiO2  clusters dispersed in mesoporous MCM-41 silica
matrix exhibited a microenvironment that was different from that of the bulk
titania. The photocatalytic properties of these samples were found to exhibit
direct relationship with the number of chemisorption sites rather than to the
changes in electronic properties [15]. In the present study, we have adopted a
hydrothermal route for the preparation of bulk titanium dioxide samples, comprised
of nanosize crystallites that exhibit a mesoporous character. Gold additions from 0.5 to 2 wt%
of gold were doped in these samples by adopting two different approaches,
namely inclusion of Au during the hydrothermal step or alternatively its
dispersion over as-synthesized mesoporous TiO2  by insipient wet impregnation.
The samples were characterized for their structural and morphological properties
by using the techniques of powder X-ray diffraction (XRD), diffuse reflectance
UV-visible spectroscopy (DR UV-Vis), thermal analysis (TG/DTA), N2  adsorption, and transmission electron microscopy (TEM). Vapor-phase photo-oxidation
of acetone was employed as a representative and the reaction was carried out
both under visible and UV irradiations. In situ diffuse reflectance infrared Fourier-transform (DRIFT)
spectroscopy helped in probing the surface transient studies formed during the interaction
of acetone alone or in the presence of oxygen. The role of these transient
species in the overall photocatalytic process is elucidated on the basis of
these experimental results.


## 2. Experimental

### 2.1. Sample Preparation

The sample of mesoporous TiO2  was synthesized by following a low-temperature nonsurfactant hydrothermal route,
using tartaric acid as structure directing agent.  In this method, about 10 mL of Ti(IV)
isopropoxide (Aldrich Chemie, Steinheim, Germany) was added slowly to a mixture of 1N HCl
and tartaric acid (3.6 g) under stirring, with the HCl/alkoxide ratio being ~1.
The pH of the resultant mix was ~5. After complete hydrolysis of the alkoxide,
the mixture was transferred into a Teflon-lined autoclave and heated at 403 K
for 3 days followed by cooling to room temperature. The mass was centrifuged
and washed several times with distilled water. The white precipitate thus
achieved was dried at 373 K for 4-5 hours and then
calcined at 623 K for 3 hours to get a sample denoted as T1.The
samples of mesoporous TiO2  containing 0.5 to 2 wt% of gold were synthesized by
following two different methods. In method I, a required amount of chloroauric
acid (HAuCl4, Loba Chemie, Mumbai, India) was added to the mixture of alkoxide and
HCl during the hydrolysis step, as described above. After drying at 373 K, the
samples were calcined at different temperatures in the range of 525–773 K and are
denoted as H1–H6 in Table 1. In
method II, hydrothermally prepared titania powder (sample T1) was impregnated
with varying amounts of aqueous chloroauric acid solution, followed by drying
and calcination at an optimized temperature of 623 K (samples denoted as I1–I3). For some
comparative experiments, a sample of pure gold powder was prepared by hydrazine
reduction of aqueous chloroauric acid solution, followed by filtration, drying at
370 K, and then heating under vacuum (520 K for 3 hours, 750 K for 2 hours).Table 1
Physical properties of TiO2  and Au/TiO2  samples.
SampleCalcination Temp. (K)Au content (wt%)Surface area (m2g-1)Average crystallite size(d) (nm)Average gold particle size(e) (nm)Average pore diameter(f)  (nm)InitialFinalT1(a)623–-1099.6–-4.2H1(b)3751.00.6–-–-–-–-H2(b)5231.00.6214.45.2~1.03.5, 2.4H3(b)6231.00.6157.04.41.03.3, 1.6H4(b)7731.00.6–-10.8–-–-H5(b)6231.50.866.111.35.73.8H6(b)6232.01.365.08.7–-–-I1(c)6231.01.174.77.17.54.7I2(c)6231.51.735.77.0–-4.5I3(c)6232.02.139.47.09.04.5(a)TiO2 sample prepared by hydrothermal route.(b)Au/TiO2  with gold incorporated during hydrothermal process.(c)Au/TiO2  with gold dispersed over sample T1 by impregnation.(d)From XRD data using Scherrer’s equation.(e)From TEM results.(f)From N2 adsorption data.


### 2.2. Characterization

The
gold content in the synthesized samples was estimated by energy dispersive
analysis of X-rays (EDAX), using a PHOENIX
stereoscan-440
accessory attached to a scanning electron microscope (Leica, cambridge, UK). An Au–Cu metal coupon was used for
calibration purposes. The gold concentration in the samples, added initially
and the analyzed value in the final product, is given in Table1.Powder
XRD patterns were recorded in the wide-angle range on Rigaku, Miniflex (D Max
III VC) XRD machine (CuK∝ radiation) operated at 30 kV and 15 mA. Low-angle powder X-ray diffraction (LAXRD)
patterns were recorded over the scan range 2θ = 1.5 to 5° using a Philips X’Pert
Pro (M/S Panalytical) diffractometer using CuK∝ radiation
(λ = 1.5406 Å) operated at 40 kV and 30 mA.TEM
images were obtained on a JEOL-2010 CX microscope. The information about BET
surface area and pore characteristics of different samples was derived from
low-temperature nitrogen adsorption isotherms, recorded on a Quantachrome NOVA
1200 equipment, after pretreatment at 573 K under vacuum. The BJH formulation
was employed to obtain the pore-size distribution data. The samples were
further characterized by using DR UV-Vis spectroscopy (LAMBDA 650
spectrophotometer, Perkin-Elmer, Shelton, USA). About 3 wt% of a sample was mixed in
barium sulfate for these absorbance measurements. The simultaneous TG/DTA scans
were recorded in argon atmosphere for a representative sample H1, so as to monitor
various steps involved in sample synthesis.


### 2.3. Catalytic Activity

The experiments on photocatalytic
oxidation of acetone were conducted both under UV radiation and also under
visible light.  A 400 W mercury vapor
lamp (225–400 nm, peak at
365 nm) was employed for UV irradiation and a 500 W halogen lamp served as a
visible light source. Both light sources were housed in water cooled jackets to
cut off infrared radiation. An
amount of 25 mg of a sample was charged in a pyrex glass photoreactor
(100 mL capacity) for activity measurement where acetone vapor mixed in air (1–25 mol%) was
introduced after a brief pumping. The experiments were conducted at room
temperature under static condition, and the reaction products formed as a
function of radiation exposure were analyzed on a gas chromatograph (Shimadzu
model-R-15A, Porapaq-Q column, TCD detector). Qualitative analysis was also
conducted on some of the representative samples using a GC-MS (Shimadzu
model-R-15A) with a view to establish the identity of the reaction products
formed.


### 2.4. IR Study of Transient Surface Species

The IR spectra of surface species
formed during exposure of TiO2 and Au/TiO2 to acetone
were recorded in DRIFT mode using a Shimadzu SSU-8000 spectrophotometer. About
14 mg catalyst powder packed in the sample holder was activated in situ at 625 K for ~3 hours in a flowing stream (40 mL min-1) of pure N2, followed by cooling to room temperature and recording of a background spectrum.
The sample, maintained under nitrogen flow, was then exposed to several
successive pulses of either pure acetone vapor or acetone (10 mol%) + air, and
the IR spectra were recorded at each stage. The effect of raising the sample
temperature subsequent to saturation coverage was also monitored.  200 scans were coadded at a resolution of 4 cm-1 and the background spectrum of blank TiO2 was
compensated for recording of each plot.


## 2.1. Sample Preparation

The sample of mesoporous TiO2  was synthesized by following a low-temperature nonsurfactant hydrothermal route,
using tartaric acid as structure directing agent.  In this method, about 10 mL of Ti(IV)
isopropoxide (Aldrich Chemie, Steinheim, Germany) was added slowly to a mixture of 1N HCl
and tartaric acid (3.6 g) under stirring, with the HCl/alkoxide ratio being ~1.
The pH of the resultant mix was ~5. After complete hydrolysis of the alkoxide,
the mixture was transferred into a Teflon-lined autoclave and heated at 403 K
for 3 days followed by cooling to room temperature. The mass was centrifuged
and washed several times with distilled water. The white precipitate thus
achieved was dried at 373 K for 4-5 hours and then
calcined at 623 K for 3 hours to get a sample denoted as T1.The
samples of mesoporous TiO2  containing 0.5 to 2 wt% of gold were synthesized by
following two different methods. In method I, a required amount of chloroauric
acid (HAuCl4, Loba Chemie, Mumbai, India) was added to the mixture of alkoxide and
HCl during the hydrolysis step, as described above. After drying at 373 K, the
samples were calcined at different temperatures in the range of 525–773 K and are
denoted as H1–H6 in Table 1. In
method II, hydrothermally prepared titania powder (sample T1) was impregnated
with varying amounts of aqueous chloroauric acid solution, followed by drying
and calcination at an optimized temperature of 623 K (samples denoted as I1–I3). For some
comparative experiments, a sample of pure gold powder was prepared by hydrazine
reduction of aqueous chloroauric acid solution, followed by filtration, drying at
370 K, and then heating under vacuum (520 K for 3 hours, 750 K for 2 hours).Table 1
Physical properties of TiO2  and Au/TiO2  samples.
SampleCalcination Temp. (K)Au content (wt%)Surface area (m2g-1)Average crystallite size(d) (nm)Average gold particle size(e) (nm)Average pore diameter(f)  (nm)InitialFinalT1(a)623–-1099.6–-4.2H1(b)3751.00.6–-–-–-–-H2(b)5231.00.6214.45.2~1.03.5, 2.4H3(b)6231.00.6157.04.41.03.3, 1.6H4(b)7731.00.6–-10.8–-–-H5(b)6231.50.866.111.35.73.8H6(b)6232.01.365.08.7–-–-I1(c)6231.01.174.77.17.54.7I2(c)6231.51.735.77.0–-4.5I3(c)6232.02.139.47.09.04.5(a)TiO2 sample prepared by hydrothermal route.(b)Au/TiO2  with gold incorporated during hydrothermal process.(c)Au/TiO2  with gold dispersed over sample T1 by impregnation.(d)From XRD data using Scherrer’s equation.(e)From TEM results.(f)From N2 adsorption data.


## 2.2. Characterization

The
gold content in the synthesized samples was estimated by energy dispersive
analysis of X-rays (EDAX), using a PHOENIX
stereoscan-440
accessory attached to a scanning electron microscope (Leica, cambridge, UK). An Au–Cu metal coupon was used for
calibration purposes. The gold concentration in the samples, added initially
and the analyzed value in the final product, is given in Table1.Powder
XRD patterns were recorded in the wide-angle range on Rigaku, Miniflex (D Max
III VC) XRD machine (CuK∝ radiation) operated at 30 kV and 15 mA. Low-angle powder X-ray diffraction (LAXRD)
patterns were recorded over the scan range 2θ = 1.5 to 5° using a Philips X’Pert
Pro (M/S Panalytical) diffractometer using CuK∝ radiation
(λ = 1.5406 Å) operated at 40 kV and 30 mA.TEM
images were obtained on a JEOL-2010 CX microscope. The information about BET
surface area and pore characteristics of different samples was derived from
low-temperature nitrogen adsorption isotherms, recorded on a Quantachrome NOVA
1200 equipment, after pretreatment at 573 K under vacuum. The BJH formulation
was employed to obtain the pore-size distribution data. The samples were
further characterized by using DR UV-Vis spectroscopy (LAMBDA 650
spectrophotometer, Perkin-Elmer, Shelton, USA). About 3 wt% of a sample was mixed in
barium sulfate for these absorbance measurements. The simultaneous TG/DTA scans
were recorded in argon atmosphere for a representative sample H1, so as to monitor
various steps involved in sample synthesis.


## 2.3. Catalytic Activity

The experiments on photocatalytic
oxidation of acetone were conducted both under UV radiation and also under
visible light.  A 400 W mercury vapor
lamp (225–400 nm, peak at
365 nm) was employed for UV irradiation and a 500 W halogen lamp served as a
visible light source. Both light sources were housed in water cooled jackets to
cut off infrared radiation. An
amount of 25 mg of a sample was charged in a pyrex glass photoreactor
(100 mL capacity) for activity measurement where acetone vapor mixed in air (1–25 mol%) was
introduced after a brief pumping. The experiments were conducted at room
temperature under static condition, and the reaction products formed as a
function of radiation exposure were analyzed on a gas chromatograph (Shimadzu
model-R-15A, Porapaq-Q column, TCD detector). Qualitative analysis was also
conducted on some of the representative samples using a GC-MS (Shimadzu
model-R-15A) with a view to establish the identity of the reaction products
formed.


## 2.4. IR Study of Transient Surface Species

The IR spectra of surface species
formed during exposure of TiO2 and Au/TiO2 to acetone
were recorded in DRIFT mode using a Shimadzu SSU-8000 spectrophotometer. About
14 mg catalyst powder packed in the sample holder was activated in situ at 625 K for ~3 hours in a flowing stream (40 mL min-1) of pure N2, followed by cooling to room temperature and recording of a background spectrum.
The sample, maintained under nitrogen flow, was then exposed to several
successive pulses of either pure acetone vapor or acetone (10 mol%) + air, and
the IR spectra were recorded at each stage. The effect of raising the sample
temperature subsequent to saturation coverage was also monitored.  200 scans were coadded at a resolution of 4 cm-1 and the background spectrum of blank TiO2 was
compensated for recording of each plot.


## 3. Results and Discussion

### 3.1. Morphological Features

The notations and morphological
properties of different samples are listed in Table1. Figures 1(a)–1(c) provide TEM
images of representative mesoporous TiO2 and Au/TiO2 samples. Comparative pore-size distribution in some of the samples, as measured
by N2-adsorption, is presented in Figures 2(a)–2(d). As seen in
these data, the surface area, crystallite shape and size, and the pore
characteristics of different samples depend considerably on the amount of gold
present and the mode of its doping. The gold-free TiO2 sample (T1),
prepared by hydrothermal route and calcined at 623 K, is comprised of almost round shape
crystallites, size varying from 6 to 12 nm and the majority being around 7.5 nm
in  cross-sectional diameter (Figure 1(a)).
The pore-size distribution in this sample falls in a narrow range (3 to 5 nm), with
the average pore-diameter being ~4.2 nm (Figure 2(a), Table 1). The inclusion
of a small amount of gold (1 wt%) during hydrothermal synthesis resulted in
more uniform and much smaller (1.5 to 3 nm) crystallites, as can be seen in Figure
1(b). The dark spots of high scattering contrast in the middle of these
crystallites may be related to the presence of gold particles entrapped in the
mesopores, the size of which is estimated at ≤1 nm. In addition to almost
spherical-shape TiO2 crystallites seen in Figure 1(b), the presence
of titania nanotubes (cross-sectional diameter of around 6 nm and length
varying from 15 to 70 nm) was also noticed in few TEM micrographs. No gold
particles were, however, noticeable in the vicinity of these nanotubes.TEM micrographs of mesostructured titania and
Au/TiO2 samples: (a) mesoporous TiO2 (T1), (b) Au/TiO2 (H3), and (c) sample I1.
(a)(b)(c)Pore-size distribution profiles in mesoporous
TiO2 and Au/TiO2 samples:  (a) T1, (b) H2, (c) H3, and (d) I3.
(a)(b)(c)(d)The size of the gold particles
increased considerably with the increase of gold loading and also when the
samples were prepared by using method II. For instance, the TEM micrograph of
sample H5 showed the presence of gold crystallites of size 3–8 nm, dispersed
prominently at the extraneous host surface. The size of the gold crystallites
was found to increase further in the samples synthesized by impregnation. Figure1(c) exhibits the typical TEM micrograph of sample I1, containing 1 wt% of Au.
Gold particles, having size in a wide range of 3 to 15 nm, are seen clearly in
this picture.The N2-adsorption results reveal
that the pore characteristics are affected considerably by the amount of gold
present in a sample and the method of preparation. Thus, sample T1 consisted of
uniform-size pores of average size 4.2 nm (Figure 2(a)), while samples H2 and
H3 reveal the presence of a bimodal pore system (Figures 2(b), 2(c)). In the
case of H2, the average size of mesopores is found to be ~3.5 nm instead of 4.2 nm and an equal number of pores in size range 1 to 3 nm are formed as a result
of gold inclusion (Figure 2(b)). Increase in calcination temperature to 623 K
for sample H3 resulted in further decrease in the size and number of larger-size
pores whereas those
of the smaller-size pores increased considerably (Figure 2(c)). For the samples
containing higher loading of Au and also in case of the samples prepared by
method II, the number of smaller-size pores is very small while the larger-size
pores are affected only marginally. A typical pore-size distribution pattern
for sample I3 is shown in Figure 2(d).The
results described above suggest that for the low loadings (≤1% Au), gold
crystallites are very small in size (≤1 nm) and are located preferably within
the mesopores of titania causing constriction, as is also indicated in the TEM
picture of Figure 1(b). For the higher loadings and in the case when samples were
synthesized by impregnation, the size of the gold particles increases
considerably, and resultantly they exist mainly at the extraneous surface of
host titania matrix (Figure 1(c)). This may lead to complete or partial blockage
of some of the pores while leaving others unaffected, as is indicated in the pore-size
distribution data of Figure 2(d).Our
results also show that by adopting a nonsurfactant approach [22], and by using
a small molecule such as tartaric acid for template, it is possible to
synthesize physically stable, very small and uniform-size (6–12 nm) titania
crystallites, as compared to several earlier attempts in this direction that
employed large-size templating agents, such as dodecylamin and poly ethylene
oxide (PEO) and obtained a material having a low surface area and a larger crystallite
size [23–25].


### 3.2. Powder XRD

Curves (a)–(e) in Figure3(a)
show the wide-angle powder XRD patterns of gold-containing TiO2 samples prepared by hydrothermal process followed by calcination under
different conditions. Curve (f) in this figure is for corresponding gold-free titania
sample T1. The prominent XRD lines appearing in curve (f) at 2θ values of 25.2,
36.8, 37.8, 48.1, 54.0, 55.1, and 62.7 degrees are the characteristic
reflections of anatase-phase TiO2 (JCPDS card 21-2172).  A considerable increase in the line widths,
particularly in curves (a)–(c), is a feature
arising due to decrease in particle size. The size of crystallites in different
samples, as calculated from the width of I100 reflection by using
Scherrer’s equation, is given in Table 1. The presence of small amount of gold
in our samples is indicated by a weak and broad XRD line appearing at 44.5° (d = 2.04) in curves (a)–(e), revealing
again the small size of Au crystallites. The X-ray diffraction patterns of
samples I1–I3 were found to be almost similar to those of sample T1, with no measurable change in
2θ value or in the line width (Figure 3(b)). A marginal change was, however,
observed in the relative intensity of two main reflections appearing at 2θ values
of 25.2 and 48.2 degrees.(a) Wide-angle powder
XRD patterns of mesoporous TiO2 and hydrothermally synthesized
Au/TiO2 samples: (a)
H1, (b) H2, (c) H3, (d) H4, (e) H5, and (f) T1.  (b) Comparative wide-angle powder XRD patterns of
impregnation-prepared Au/TiO2 and gold-free TiO2 samples:
(a) I1, (b) I2, (c) I3, and (d)
T1.
(a)(b)Figure4 presents the low-angle XRD patterns of some of the hydrothermally synthesized
samples. Data in this figure reveal that an appropriate thermal treatment is
vital for preparing a suitable mesoporous TiO2 matrix. Figure 4 shows
that no long-range mesoporous structure is formed at the stage of hydrothermal
treatment (curve (a)) and also after subsequent calcination at a low temperature
(~523 K) (curve (b)). This can be attributed to incomplete removal of condensed
moieties from the pores. A well-ordered mesoporous structure is observed only
on calcination of the sample at 623 K, as is reflected in the broad peak appearing
at ~1.8° in curve (c) of Figure 4. A similar low-angle XRD pattern
has been reported earlier for mesoporous titania [1]. At the same time, the
break down of this ordered structure is observed for the further increase in
calcination temperature, as is evident from the LAXRD pattern of sample H4,
shown in curve (d) of Figure 4.Figure 4
Low-angle powder XRD patterns of mesoporous Au/TiO2 samples synthesized by hydrothermal route and calcined at different
temperatures: (a) H1, (b) H2, (c) H3, and (d) H4.


### 3.3. TG/DTA Studies

TG/DTA
curves in Figure5 correspond to as-synthesized Au (1%)/TiO2 (sample
H1), where after hydrothermal treatment the sample was subjected to drying in
air at 375 K without further calcination. The differential TG scan (curve (b)) indicates
weight loss in two major steps: at 400 K due to release of adsorbed water and
at 510 K because of the decomposition of tartaric acid used as a templating
agent. These assignments were validated by independently recording the thermal
data for pure tartaric acid.  Closely
spaced dual endotherms appearing at 510 K and 525 K in DTA plot (curve (c))
suggest that the decomposition of tartaric acid and the release of the reaction
products may occur in two different steps. A small TG peak appearing at 645 K
(curve (b)) and the corresponding endotherms in curve (c) are related to
conversion of gold to its metallic state.Figure 5
Thermal analysis data on hydrothermally prepared uncalcined
Au/TiO2 gel (sample H1): (a) TG, (b) DTG, and (c) DTA.The
data of thermal analyses presented above in conjunction with low-angle XRD
results of Figure4 help us to demonstrate that the removal of tartaric acid at
an appropriate temperature subsequent to hydrothermal treatment and drying of
gel is a vital step for preparing mesoporous titania. Thus, no N2 adsorption or mesoporosity are observed in sample H1, dried at 375 K (Table 1)
but subjected to no further calcination. Removal of tartaric acid begins at 470 K but remains incomplete on calcination at 523 K (Figure 5, curve (b)), and the
sample H2 therefore exhibits nitrogen adsorption corresponding to mesoporous
materials (Figure 2(b)), but no long-range mesoporous network is formed yet (Figure
4, curve (b)). Calcination at 623 K enables complete removal of tartaric acid and
thus gives rise to a long-range mesoporous network (Figure 4, curve (c)).  Further, calcination at 750 K or above
resulted in the collapse of porous titania network, as is revealed in curve (d)
of Figure 4, recorded for sample H4.


### 3.4. DR UV-Vis Spectra

The presence of gold was
found to have a considerable influence on the DR UV-Vis spectrum of TiO2,
depending upon its amount and the method of sample preparation. Figures 6(a) and
6(b) exhibit these data for Au/TiO2 samples synthesized by using
methods I and II, respectively. Comparative data on sample T1 are given in
curve (e) of Figure 6(a). As is noticed in Figure 6(a), absorbance in UV region
increases considerably in case of Au/TiO2 samples synthesized by
method I, as compared to sample T1 (curve (e)). The intensity of this
absorbance band, however, decreases progressively on calcination at higher
temperatures (curve (c)), and also for higher loadings of Au (curve (d)). We
also observe a broad absorption band in the entire visible region of spectrum
in the case of samples containing gold (cf. Figure 6(a), curve (e)). The
intensity of the visible region absorbance (400–800 nm) is also
found to decrease marginally with the increasing gold content.Diffuse reflectance UV-Vis
spectra of (a) hydrothermally prepared Au/TiO2 and TiO2 samples: (a) H2, (b) H3, (c)
H4, (d) H6, and (e)
T1, and (b) impregnation-prepared Au/TiO2 samples: (a)
I1, (b) I2, and (c) I3.
(a)(b)The
intensity of UV-region absorbance band for samples I1–I3, synthesized
by impregnation, was found to be almost similar to that of T1. However, the visible
region absorbance was much higher for these samples, which may be attributed to
the absorbance by the gold crystallites existing at the external surface (Figure6(b)).The
results presented above thus clearly reveal a direct relationship between the
intensity of absorbance bands, both in UV and visible regions, and the crystallite size
of titania as well as that of the dispersed gold. In both cases, the higher the crystallite size
is, the lower the value of the absorbance will be. The visible region
absorbance in Figures6(a), 6(b)  is a
characteristic feature associated with the nanostructured gold where the weak
and broad band appearing at 550–600 nm is a well-reported
phenomenon arising due to plasmon resonance [26].


### 3.5. Photocatalytic Activity

When
acetone vapor + air were fed to the reactor at room temperature and in the
absence of radiation, no reaction occurred over mesoporous TiO2 (sample T1) as well as over Au/TiO2 samples prepared by the two
methods described above. Similarly, no products were formed in the presence of
visible light.The reaction products formed
during photo-oxidation of acetone under UV irradiation were mainly carbon
dioxide and water. Small quantities of ethane and methyl formate (H–COO–CH3,
IUPAC name methyl
methanoate) were produced only for higher concentrations of acetone in air. The
catalytic activity and yield of these products depended on various factors,
such as radiation dose, concentration of acetone in reactant mixture, and the
pore characteristics of the catalyst samples. The presence of gold also resulted
in considerable enhancement of the activity of a catalyst, where both the
content and the size of Au particles played an important role. The highlights
of these results are presented below.


#### 3.5.1. TiO2

Curves (a)–(c) in Figure7
show the yield of CO2 as a function of UV irradiation, when a
mixture of acetone and air, containing varying concentrations of acetone, was reacted
over mesoporous TiO2 (sample T1). As seen in these data, yield of CO2 corresponds to 100% conversion for acetone concentration of 2.7 mol% or less,
the value of t1/2 (time taken for 50% conversion of acetone) being
around 30 minutes (curve (a)). The yield of CO2 decreased
progressively along with the simultaneous formation of other reaction products
for increasing acetone: air mol ratios in the reaction mixture. For instance,
in the case of acetone concentration at 10 mol%, conversion to CO2 was limited to ~25% even after ~2 hours of reaction time (curve (c)). As
mentioned above, ethane and methyl formate were the other reaction products
formed for higher molar concentrations of acetone in air, as detected in the
GC-MS analysis of reaction products. Curve (d) in Figure 7 shows the
representative data on ethane yield for the experiment carried out with 13 mol%
of acetone in air. The yields of methyl formate and water formed in the
reaction were not very reproducible because of their condensing nature, and no
attempt is therefore made to compile the quantitative data on these products.Figure 7
Yield of CO2 (curves
(a)–(c)) and C2H6 (curve (d)) as a function of time, when mesoporous TiO2 (sample
T1) was used for the UV-induced vapor-phase photocatalytic oxidation of acetone
present at varying concentrations in air. (a) 2.7, (b) 5.3, (c) 10, and (d) 13 mol%.


#### 3.5.2. Au / TiO2

The
reaction rate was much higher in the case of gold containing TiO2 samples, with the activity depending upon the gold content and the morphology
of TiO2.  Figure 8 presents
the yield of CO2 during photo-oxidation of acetone as a function of
irradiation time for the experiments conducted on hydrothermally prepared
sample H3 and for the different compositions of reactant mixture. As seen in
the results of curves (a)–(c) in Figure 8,
not only the conversion but the rate of the reaction also depended upon acetone
concentration, the lower the concentration the faster being the reaction, as
expected. Lower conversions were observed for the reaction mixture containing a
higher concentration of acetone (Figure 8, curve (d)), along with the formation
of other reaction products as in the case of TiO2.Figure 8
Yield of CO2 as a function of
UV-radiation dose when hydrothermally synthesized Au/TiO2 catalyst
H3(curves (a)–(d)) and H4
(curve (e)) were employed for vapor-phase photo-oxidation of acetone present at
different concentrations in air. (a) 1.1, (b) 2.5, (c) 4.8, (d) 13, and (e) 3.9 mol%.Calcination
of a sample at higher temperatures resulted in considerable loss of catalytic
activity.  Comparative data on reaction
of acetone (3.9 mol%) + air over sample H4, that is sample calcined at 773 K
instead of 623 K (sample H3), are shown in curve (e) of Figure8.  As seen in Figure 8 curve (e), less than 60% of
acetone gets converted to CO2 in this case, even for a lower
concentration of acetone in air. These results show a strong relationship
between the catalytic activity, surface area, and the size of gold particles,
which are affected considerably on calcination.Au/TiO2 samples H5 and H6, with a higher gold content of 0.8 and 1.3 wt%, respectively
(see Table 1), showed lower catalytic activity as compared to sample H3
containing ~0.6 wt% Au. For instance, samples H5 and H6 gave rise to a maximum
yield of CO2 at ca. 70% and 80%, respectively, during the reaction
of acetone (3.5 mol%) + air  for about 80
minutes. Similarly, the samples synthesized by impregnation (I1–I3) showed
lower catalytic activity in terms of reaction rate as compared to the corresponding
hydrothermally synthesized samples. Figure 9 presents typical results on dose-dependent
CO2 yields for reaction of acetone (~3.4 mol%) + air over samples
I1–I3.  A progressive increase in
activity is observed as a function of gold content in these data. Thus, while
almost 100% conversion of acetone was observed in case of samples containing
1.5 or 2 wt% of Au (curves (b) and (c)), sample I1 gave only ca. 60% conversion
even after two hours of reaction time. At the same time, the value of t1/2 is about 20 to 30 minutes in case of data in Figure 9 curves (b) and (c), as
compared to 8 to 13 minutes in case of sample H3 having lower gold content (Figure
8).Figure 9
Yield of CO2 as a function of
UV-radiation dose during the vapor-phase photo-oxidation of acetone, (3.2–3.5 mol% in air)
at room temperature over impregnation-prepared Au/TiO2 samples I1(curve (a)), I2 (curve (b)), and I3 (curve (c)).These
results thus provide evidence that the activity of a catalyst is governed by
multiple factors. In addition to surface area and gold content, morphology of
TiO2 and the size of the gold particles also contribute
significantly to the overall catalytic activity. The poorer activity of samples
H5 and H6 as compared to H3 may be attributed to the increase in gold
crystallite size with increasing gold content (TEM evidence). The extraordinary
catalytic and optical properties of smaller-size (<2 nm) gold particles are now
a well-reported phenomenon [27, 28]. It is known that the gold catalysts are
structure sensitive and the binding states of reactant molecules over Au
nanoparticles dispersed over metal oxide supports are found to be different
than those over
bulk gold or gold films. It is suggested that the gold-metal oxide interfaces
act as sites for the activation of molecules such as O2, CO, hydrocarbons,
and so forth [27]. In some of the studies, these unique properties of gold nanoparticles
are attributed to their electronic structure [29].  We have demonstrated earlier that even a
marginal increase in gold particle size as a result of high-temperature calcination
may lead to a considerable decrease in the adsorption and catalytic activity of
Au/Fe2O3 catalysts [30].


### 3.6. Infrared Spectroscopy

The results
of our in situ DRIFT
spectroscopy studies, where pure acetone vapor or acetone + air was dosed over representative
catalyst samples after appropriate pretreatment, are presented in the following
subsections.


#### 3.6.1. Adsorption of Acetone

Curve (b)
in Figure10 exhibits the C–H stretching
(3100–2800 cm-1)
vibrational bands formed over hydrothermally prepared Au/TiO2 sample
H3 on exposure to 0.2 μmol g-1 of acetone at room temperature and
recorded after the lapse of ~10
minutes. The intensity of this band remained unchanged when more amount
of acetone (up to ~1 μmol g-1) was dosed over the sample in form of
successive pulses. Comparative IR spectrum recorded for acetone vapor in this
region is shown in curve (a) of this figure. The ν(C–H) bands of Figure
10(b) show the absence of Q-branch splitting of vibrational-rotational bands
associated with acetone vapor (curve (a)). Also, instead of a sharp band at
2970 cm-1 (band width ~5 cm-1) in case of acetone vapor
(curve (a)), we observe a considerably broadened band  at
2973 cm-1, the corresponding value of band width (full width
at half maximum, FWHM) being ~18 cm-1 for acetone in its adsorbed
state (curve (b)). This band was found to be quite stable on thermal
activation, as seen from the relative absorbance values in spectra (c) and (d)
of Figure 10, recorded on rise in sample temperature to 320 K and 360 K subsequent
to saturation coverage. The intensity of this band decreased only marginally
even after raising the sample temperature to 470 K.Figure 10
C–H stretching
region infrared spectra of sample H3 (hydrothermally synthesized Au/TiO2), exposed to  0.2 μmol of acetone vapor
and recorded after the lapse of 10 minutes (curve (b)) and subsequent heating
at 320 K (curve (c)) and 360 K (curve (d)). Curve (a) shows corresponding
vibrational bands of acetone vapor. The numbers given in the parentheses
indicate the absorbance value of the particular IR band.Figure11 presents corresponding
plots in 2000–1000 cm-1 region (νC=O, νC=C, νC–C, and δCH
regions). Curve (a) shows again a comparative spectrum of acetone vapor. In the
CO stretching region, we observe a broad band centered at ~1701 cm-1 and a shoulder band at ~1738 cm-1 in curve (b) of Figure 11,
recorded soon after adsorption of acetone over sample H3. Instead, a doublet at
1738 and 1725 cm-1 is observed in case of acetone vapor (curve (a)),
arising due to the Fermi resonance between the fundamental CO vibration and an
overtone. With an increase in contact time, the intensity of 1738 cm-1 band decreases further while the frequency of the new band formed at 1701 cm-1 shifts to a lower value of 1695 cm-1 (Figure 11, curve (c)). As in the
case of C–H stretching
bands, the ν(CO) vibrational bands were found to be fairly stable on raising
the sample temperature up to 470 K after acetone adsorption. Spectra (d) and
(e) in Figure 11 show the representative plots obtained at sample temperatures
of 320 K and 350 K, respectively.Figure 11
Stretching C=O, C=C,
C–C, and deformation CH region vibrational bands developed on sample H3 after
exposure to 0.2μmol (14 μmol g-1) of acetone vapor and recorded after
a delay time of 2 minutes (curve (b)) and 15 minutes (curve (c)) and subsequent
raising of sample temperature to 320 K (curve (d)) and 360 K (curve (d)). Curve
(a) shows corresponding vibrational bands of acetone vapor. The numbers given
in the parentheses indicate the absorbance value of the particular IR band.


#### 3.6.2. Adsorption of Acetone + Air

The vibrational bands similar to those in Figures10 and 11 were observed on interaction of acetone (10 mol%) + air at room
temperature, the only difference being a considerable increase in the intensity
of the bands. Figure 12 exhibits IR spectra of H3 sample at saturation
coverage, recorded 10 minutes after exposure (curve (a)) and after subsequent
rise in sample temperature to 320 (curve (b)) and 350 K (curve (c)). As seen
from the absorbance values given in Figure 12, the intensity of the vibrational
bands in these spectra is greater by a factor of almost 2, compared to the
corresponding experiments performed using acetone alone (Figure 11). These results
indicate that the presence of oxygen facilitated the reaction of acetone over
catalyst surface to form certain strongly adsorbed and thermally stable species
(Figures 12(curve (b)), 12(curve (C))). We may also mention that the spectral features similar to
those in Figures 10, 11, and 12 were observed in the case of impregnation-prepared
Au/TiO2 samples having a different gold content.Figure 12
Stretching C=O, C=C, C–C, and deformation CH
region vibrational bands developed on sample H3 after exposure  to
acetone vapor + air and recorded after a delay time of 10 minutes (curve
(a)) and on subsequent raising of sample temperature to 320 K (curve (b)) and
370 K (curve (c)). Curve (d) shows comparative vibrational bands of methyl
formate vapor adsorbed over H3 under nitrogen flow at room temperature. The
numbers given in the parentheses indicate the absorbance value of the
particular IR band.Adsorption of acetone over TiO2 gave rise to the vibrational bands similar again to those shown in Figures 10
and 11, described above. The IR bands were found to be quite stable on heating
of the sample up to 470 K under nitrogen flow, as in the case of results
obtained using Au/TiO2.


#### 3.6.3. Hydroxyl Region Bands

Curve (a) in Figure13 presents the
hydroxyl region bands of Au/TiO2 (sample H3), exposed to 0.2 μmol g-1 of acetone and recorded soon after. As seen in this figure, the spectral
developments in ν(CH) or ν(CO) regions are accompanied with simultaneous
removal of IR bands of different hydroxyl groups at titania surface, as
indicated by the negative absorbance values (peaks pointing downwards) of ν(OH)
bands at 3710, 3672, and 3600 cm-1 in Figure 13. The presence of
overlapping multiple vibrational bands in 3900–3400 cm-1 region,
representing differently coordinated hydroxyl groups over the titania surface, has
been reported earlier [31]. These results reveal a strong adsorbent-adsorbate
interaction involving the hydroxyl groups, irrespective of their coordinative
environment.Figure 13
Hydroxyl
region IR spectra of sample H3 after room temperature adsorption of 14μmol g-1 of acetone vapor and recorded after the lapse of 15 minutes (curve (a)) and
subsequent heating at 320 K (curve (b)), 350 K (curve (c)), and 380 K (curve (d)).
The numbers given in the parentheses indicate the absorbance value of the
particular IR band.Based upon the above-mentioned
information about the shape, width, and intensity, the IR absorption bands in
Figures10 and 11 may be attributed to an overlap of closely spaced vibrational
bands, arising due to multiple adsorbed species. In some of the  previously reported studies [32, 33], acetone
is found to adsorb in molecular state over TiO2 at a lower coverage
while at a higher coverage it undergoes an Aldol condensation reaction followed
by dehydration to yield 4-methyl-3-penten-2-one ((CH3)2C=CHCOCH3),  commonly known as mesityl oxide (MSO). The
absence of Q-branch splitting and broadening of the ν(C–H) band shown in Figure 10 is in conformity
with the presence of a condensed state of acetone. The IR absorption bands
appearing at 2973 and 2931 cm-1 in Figure 10 and at 1702, 1475, and
1364 cm-1 in Figure 11 may be assigned to this strongly adsorbed
state of acetone molecules (cf. Figures 10 and 11, curve (a)). The presence of
a weak shoulder at 1738 cm-1 in Figure 11 (curve b) and its removal
with increase in contact time (curve c) indicate that the transformation of acetone vapor
to its condensed state is a slow and activated process. We, however, observe no
formation of MSO, both for adsorption of acetone alone or acetone + air. As
mentioned by Zaki et al. [33], the mesityl oxide is distinguishable from
acetone by a νC=C band at 1634 cm-1 and νC–C band at 1294 cm-1.
No such bands are, however, observed in Figures 11 and 12, thus ruling out the
formation of MSO under experimental conditions of this study.Theν(CH3) vibration at
2973 cm-1 (Figure 10), νC=O vibration at 1695 cm-1, and
several weak absorption bands in 1900–1100 cm-1 region (Figure 12) may
be assigned to methyl formate (methyl methanoate) in its adsorbed state, as
reported in one of our earlier studies [15]. This identification is validated
by a study of Lochař et al. [34] where similar bands were observed for
adsorption of methyl formate over metal oxide surfaces. To further confirm the
assignment of absorption bands in Figures 11 and 12, methyl formate vapor was
dosed over sample H3 at room temperature and the representative spectrum thus
obtained is shown in curve (d) of Figure 12, which matches well with the IR
bands shown in Figures 11 and 12 (curves (a)–(c)). We may also
mention that the formation of methyl formate was detected clearly in our GC-MS
analysis of reaction products formed in photo-oxidation of acetone over TiO2.
A difference in the relative intensity and the shift in frequency of some of
the bands in curves (c) and (d) of Figure 12 may be attributed to the
difference in the binding of methyl formate groups, formed in situ and that
adsorbed in a molecular form. We may also mention that the νas(COO)+
δ(CH) combination bands of adsorbed mono- and bidentate formate (HCOO−) species
give rise to absorption bands at 2970 cm-1 and 2936 cm-1 whereas corresponding  ν(CH) vibrations
appear at 2896 cm-1 and 2856 cm-1. Similarly, in the
lower IR region, the bands at 1575 cm-1 and 1376 cm-1 are
assigned to νas(COO) and νs(COO) vibrations of the
bidentate formate species while the bands at 1542 cm-1 and 1363 cm-1 arise due to such vibrations of monodentate formate species [15, 35–38]. A
considerable increase of absorbance in 2950–2850 cm-1 region and a
large width of 2973 cm-1 band in Figure 10, and similarly the
presence of various unresolved overlapping bands in 1800–1000 cm-1 region (Figures 11 and 12), help us to conclude that in addition to methyl
formate, HCOO− species
may also form on interaction of acetone and subsequent thermal activation.The active role of formate and
methyl formate groups in photocatalytic reactions over titania and other oxide
surfaces has been amply demonstrated by several research groups [15, 32, 33, 38, 39]. For instance, Wu et al. [39] demonstrated that the methoxy-, ethoxy-,
and formate-type surface groups are photo-oxidized on TiO2 under
UV-illumination, and the rate of photo-oxidation depends on the binding mode of
these species at catalyst surface, the monodentate species being more reactive
as compared to their bidentate counterparts. Simultaneous FTIR and photocatalysis
studies conducted in our laboratory have also exhibited the involvement of such
adsorbed species in the photo-oxidation of methanol over TiO2/MCM
and UOx/MCM catalyst surfaces [15, 38]. El-Maazawi et al. [32]
proposed a reaction mechanism, where formation of O− (ads)
species from gas-phase O2 and its reaction with adsorbed acetone are shown to play an
important role.To summarize, the results of our
study reveal that the adsorption of acetone over nanostructured TiO2 or Au/TiO2 may occur in several modes, giving rise to a condensed
and strongly adsorbed state of molecular acetone and methyl formate and formate types of surface
adsorbed species. The IR results show that the host matrix and the gold
particles may serve as independent adsorption and reaction sites for acetone
and oxygen molecules. Since similar IR absorbance bands are observed on
adsorption over TiO2 and Au/TiO2 and also with or without
the presence of air in reaction mixture, it is apparent that the adsorption of
acetone occurs primarily at titania surface. The spectral features in ν(OH)
region  (Figure 13) reveal that the Ti–OH
groups may directly interact with acetone molecules. This process is reversed
on thermal activation, as is apparent from the absorbance values shown in Figure
13 (curves (b)–(d)). Since
acetone + air adsorption over Au/TiO2 gave rise to higher intensity
of the IR absorption bands, as described above, we may conclude that the
adsorption and activation of O2 molecules at Au sites plays an
important role. This phenomenon, promoted only by nanosize Au particles,
explains the high catalytic activity of sample H3 as compared to other samples
having higher gold content but larger-size crystallites.The overall photocatalytic process
would therefore involve the interaction of these primary adsorbed species with
the electron-hole pair generated on band gap excitation of TiO2,
giving rise to formation of various ion-radicals, free radicals, and surface
bonded transient species. Various possible photochemical steps may subsequently
prevail, leading thereby to the formation of different reaction products, as
has been reported widely (see, e.g., [40]).


## 3.1. Morphological Features

The notations and morphological
properties of different samples are listed in Table1. Figures 1(a)–1(c) provide TEM
images of representative mesoporous TiO2 and Au/TiO2 samples. Comparative pore-size distribution in some of the samples, as measured
by N2-adsorption, is presented in Figures 2(a)–2(d). As seen in
these data, the surface area, crystallite shape and size, and the pore
characteristics of different samples depend considerably on the amount of gold
present and the mode of its doping. The gold-free TiO2 sample (T1),
prepared by hydrothermal route and calcined at 623 K, is comprised of almost round shape
crystallites, size varying from 6 to 12 nm and the majority being around 7.5 nm
in  cross-sectional diameter (Figure 1(a)).
The pore-size distribution in this sample falls in a narrow range (3 to 5 nm), with
the average pore-diameter being ~4.2 nm (Figure 2(a), Table 1). The inclusion
of a small amount of gold (1 wt%) during hydrothermal synthesis resulted in
more uniform and much smaller (1.5 to 3 nm) crystallites, as can be seen in Figure
1(b). The dark spots of high scattering contrast in the middle of these
crystallites may be related to the presence of gold particles entrapped in the
mesopores, the size of which is estimated at ≤1 nm. In addition to almost
spherical-shape TiO2 crystallites seen in Figure 1(b), the presence
of titania nanotubes (cross-sectional diameter of around 6 nm and length
varying from 15 to 70 nm) was also noticed in few TEM micrographs. No gold
particles were, however, noticeable in the vicinity of these nanotubes.TEM micrographs of mesostructured titania and
Au/TiO2 samples: (a) mesoporous TiO2 (T1), (b) Au/TiO2 (H3), and (c) sample I1.
(a)(b)(c)Pore-size distribution profiles in mesoporous
TiO2 and Au/TiO2 samples:  (a) T1, (b) H2, (c) H3, and (d) I3.
(a)(b)(c)(d)The size of the gold particles
increased considerably with the increase of gold loading and also when the
samples were prepared by using method II. For instance, the TEM micrograph of
sample H5 showed the presence of gold crystallites of size 3–8 nm, dispersed
prominently at the extraneous host surface. The size of the gold crystallites
was found to increase further in the samples synthesized by impregnation. Figure1(c) exhibits the typical TEM micrograph of sample I1, containing 1 wt% of Au.
Gold particles, having size in a wide range of 3 to 15 nm, are seen clearly in
this picture.The N2-adsorption results reveal
that the pore characteristics are affected considerably by the amount of gold
present in a sample and the method of preparation. Thus, sample T1 consisted of
uniform-size pores of average size 4.2 nm (Figure 2(a)), while samples H2 and
H3 reveal the presence of a bimodal pore system (Figures 2(b), 2(c)). In the
case of H2, the average size of mesopores is found to be ~3.5 nm instead of 4.2 nm and an equal number of pores in size range 1 to 3 nm are formed as a result
of gold inclusion (Figure 2(b)). Increase in calcination temperature to 623 K
for sample H3 resulted in further decrease in the size and number of larger-size
pores whereas those
of the smaller-size pores increased considerably (Figure 2(c)). For the samples
containing higher loading of Au and also in case of the samples prepared by
method II, the number of smaller-size pores is very small while the larger-size
pores are affected only marginally. A typical pore-size distribution pattern
for sample I3 is shown in Figure 2(d).The
results described above suggest that for the low loadings (≤1% Au), gold
crystallites are very small in size (≤1 nm) and are located preferably within
the mesopores of titania causing constriction, as is also indicated in the TEM
picture of Figure 1(b). For the higher loadings and in the case when samples were
synthesized by impregnation, the size of the gold particles increases
considerably, and resultantly they exist mainly at the extraneous surface of
host titania matrix (Figure 1(c)). This may lead to complete or partial blockage
of some of the pores while leaving others unaffected, as is indicated in the pore-size
distribution data of Figure 2(d).Our
results also show that by adopting a nonsurfactant approach [22], and by using
a small molecule such as tartaric acid for template, it is possible to
synthesize physically stable, very small and uniform-size (6–12 nm) titania
crystallites, as compared to several earlier attempts in this direction that
employed large-size templating agents, such as dodecylamin and poly ethylene
oxide (PEO) and obtained a material having a low surface area and a larger crystallite
size [23–25].


## 3.2. Powder XRD

Curves (a)–(e) in Figure3(a)
show the wide-angle powder XRD patterns of gold-containing TiO2 samples prepared by hydrothermal process followed by calcination under
different conditions. Curve (f) in this figure is for corresponding gold-free titania
sample T1. The prominent XRD lines appearing in curve (f) at 2θ values of 25.2,
36.8, 37.8, 48.1, 54.0, 55.1, and 62.7 degrees are the characteristic
reflections of anatase-phase TiO2 (JCPDS card 21-2172).  A considerable increase in the line widths,
particularly in curves (a)–(c), is a feature
arising due to decrease in particle size. The size of crystallites in different
samples, as calculated from the width of I100 reflection by using
Scherrer’s equation, is given in Table 1. The presence of small amount of gold
in our samples is indicated by a weak and broad XRD line appearing at 44.5° (d = 2.04) in curves (a)–(e), revealing
again the small size of Au crystallites. The X-ray diffraction patterns of
samples I1–I3 were found to be almost similar to those of sample T1, with no measurable change in
2θ value or in the line width (Figure 3(b)). A marginal change was, however,
observed in the relative intensity of two main reflections appearing at 2θ values
of 25.2 and 48.2 degrees.(a) Wide-angle powder
XRD patterns of mesoporous TiO2 and hydrothermally synthesized
Au/TiO2 samples: (a)
H1, (b) H2, (c) H3, (d) H4, (e) H5, and (f) T1.  (b) Comparative wide-angle powder XRD patterns of
impregnation-prepared Au/TiO2 and gold-free TiO2 samples:
(a) I1, (b) I2, (c) I3, and (d)
T1.
(a)(b)Figure4 presents the low-angle XRD patterns of some of the hydrothermally synthesized
samples. Data in this figure reveal that an appropriate thermal treatment is
vital for preparing a suitable mesoporous TiO2 matrix. Figure 4 shows
that no long-range mesoporous structure is formed at the stage of hydrothermal
treatment (curve (a)) and also after subsequent calcination at a low temperature
(~523 K) (curve (b)). This can be attributed to incomplete removal of condensed
moieties from the pores. A well-ordered mesoporous structure is observed only
on calcination of the sample at 623 K, as is reflected in the broad peak appearing
at ~1.8° in curve (c) of Figure 4. A similar low-angle XRD pattern
has been reported earlier for mesoporous titania [1]. At the same time, the
break down of this ordered structure is observed for the further increase in
calcination temperature, as is evident from the LAXRD pattern of sample H4,
shown in curve (d) of Figure 4.Figure 4
Low-angle powder XRD patterns of mesoporous Au/TiO2 samples synthesized by hydrothermal route and calcined at different
temperatures: (a) H1, (b) H2, (c) H3, and (d) H4.


## 3.3. TG/DTA Studies

TG/DTA
curves in Figure5 correspond to as-synthesized Au (1%)/TiO2 (sample
H1), where after hydrothermal treatment the sample was subjected to drying in
air at 375 K without further calcination. The differential TG scan (curve (b)) indicates
weight loss in two major steps: at 400 K due to release of adsorbed water and
at 510 K because of the decomposition of tartaric acid used as a templating
agent. These assignments were validated by independently recording the thermal
data for pure tartaric acid.  Closely
spaced dual endotherms appearing at 510 K and 525 K in DTA plot (curve (c))
suggest that the decomposition of tartaric acid and the release of the reaction
products may occur in two different steps. A small TG peak appearing at 645 K
(curve (b)) and the corresponding endotherms in curve (c) are related to
conversion of gold to its metallic state.Figure 5
Thermal analysis data on hydrothermally prepared uncalcined
Au/TiO2 gel (sample H1): (a) TG, (b) DTG, and (c) DTA.The
data of thermal analyses presented above in conjunction with low-angle XRD
results of Figure4 help us to demonstrate that the removal of tartaric acid at
an appropriate temperature subsequent to hydrothermal treatment and drying of
gel is a vital step for preparing mesoporous titania. Thus, no N2 adsorption or mesoporosity are observed in sample H1, dried at 375 K (Table 1)
but subjected to no further calcination. Removal of tartaric acid begins at 470 K but remains incomplete on calcination at 523 K (Figure 5, curve (b)), and the
sample H2 therefore exhibits nitrogen adsorption corresponding to mesoporous
materials (Figure 2(b)), but no long-range mesoporous network is formed yet (Figure
4, curve (b)). Calcination at 623 K enables complete removal of tartaric acid and
thus gives rise to a long-range mesoporous network (Figure 4, curve (c)).  Further, calcination at 750 K or above
resulted in the collapse of porous titania network, as is revealed in curve (d)
of Figure 4, recorded for sample H4.


## 3.4. DR UV-Vis Spectra

The presence of gold was
found to have a considerable influence on the DR UV-Vis spectrum of TiO2,
depending upon its amount and the method of sample preparation. Figures 6(a) and
6(b) exhibit these data for Au/TiO2 samples synthesized by using
methods I and II, respectively. Comparative data on sample T1 are given in
curve (e) of Figure 6(a). As is noticed in Figure 6(a), absorbance in UV region
increases considerably in case of Au/TiO2 samples synthesized by
method I, as compared to sample T1 (curve (e)). The intensity of this
absorbance band, however, decreases progressively on calcination at higher
temperatures (curve (c)), and also for higher loadings of Au (curve (d)). We
also observe a broad absorption band in the entire visible region of spectrum
in the case of samples containing gold (cf. Figure 6(a), curve (e)). The
intensity of the visible region absorbance (400–800 nm) is also
found to decrease marginally with the increasing gold content.Diffuse reflectance UV-Vis
spectra of (a) hydrothermally prepared Au/TiO2 and TiO2 samples: (a) H2, (b) H3, (c)
H4, (d) H6, and (e)
T1, and (b) impregnation-prepared Au/TiO2 samples: (a)
I1, (b) I2, and (c) I3.
(a)(b)The
intensity of UV-region absorbance band for samples I1–I3, synthesized
by impregnation, was found to be almost similar to that of T1. However, the visible
region absorbance was much higher for these samples, which may be attributed to
the absorbance by the gold crystallites existing at the external surface (Figure6(b)).The
results presented above thus clearly reveal a direct relationship between the
intensity of absorbance bands, both in UV and visible regions, and the crystallite size
of titania as well as that of the dispersed gold. In both cases, the higher the crystallite size
is, the lower the value of the absorbance will be. The visible region
absorbance in Figures6(a), 6(b)  is a
characteristic feature associated with the nanostructured gold where the weak
and broad band appearing at 550–600 nm is a well-reported
phenomenon arising due to plasmon resonance [26].


## 3.5. Photocatalytic Activity

When
acetone vapor + air were fed to the reactor at room temperature and in the
absence of radiation, no reaction occurred over mesoporous TiO2 (sample T1) as well as over Au/TiO2 samples prepared by the two
methods described above. Similarly, no products were formed in the presence of
visible light.The reaction products formed
during photo-oxidation of acetone under UV irradiation were mainly carbon
dioxide and water. Small quantities of ethane and methyl formate (H–COO–CH3,
IUPAC name methyl
methanoate) were produced only for higher concentrations of acetone in air. The
catalytic activity and yield of these products depended on various factors,
such as radiation dose, concentration of acetone in reactant mixture, and the
pore characteristics of the catalyst samples. The presence of gold also resulted
in considerable enhancement of the activity of a catalyst, where both the
content and the size of Au particles played an important role. The highlights
of these results are presented below.


### 3.5.1. TiO2

Curves (a)–(c) in Figure7
show the yield of CO2 as a function of UV irradiation, when a
mixture of acetone and air, containing varying concentrations of acetone, was reacted
over mesoporous TiO2 (sample T1). As seen in these data, yield of CO2 corresponds to 100% conversion for acetone concentration of 2.7 mol% or less,
the value of t1/2 (time taken for 50% conversion of acetone) being
around 30 minutes (curve (a)). The yield of CO2 decreased
progressively along with the simultaneous formation of other reaction products
for increasing acetone: air mol ratios in the reaction mixture. For instance,
in the case of acetone concentration at 10 mol%, conversion to CO2 was limited to ~25% even after ~2 hours of reaction time (curve (c)). As
mentioned above, ethane and methyl formate were the other reaction products
formed for higher molar concentrations of acetone in air, as detected in the
GC-MS analysis of reaction products. Curve (d) in Figure 7 shows the
representative data on ethane yield for the experiment carried out with 13 mol%
of acetone in air. The yields of methyl formate and water formed in the
reaction were not very reproducible because of their condensing nature, and no
attempt is therefore made to compile the quantitative data on these products.Figure 7
Yield of CO2 (curves
(a)–(c)) and C2H6 (curve (d)) as a function of time, when mesoporous TiO2 (sample
T1) was used for the UV-induced vapor-phase photocatalytic oxidation of acetone
present at varying concentrations in air. (a) 2.7, (b) 5.3, (c) 10, and (d) 13 mol%.


### 3.5.2. Au / TiO2

The
reaction rate was much higher in the case of gold containing TiO2 samples, with the activity depending upon the gold content and the morphology
of TiO2.  Figure 8 presents
the yield of CO2 during photo-oxidation of acetone as a function of
irradiation time for the experiments conducted on hydrothermally prepared
sample H3 and for the different compositions of reactant mixture. As seen in
the results of curves (a)–(c) in Figure 8,
not only the conversion but the rate of the reaction also depended upon acetone
concentration, the lower the concentration the faster being the reaction, as
expected. Lower conversions were observed for the reaction mixture containing a
higher concentration of acetone (Figure 8, curve (d)), along with the formation
of other reaction products as in the case of TiO2.Figure 8
Yield of CO2 as a function of
UV-radiation dose when hydrothermally synthesized Au/TiO2 catalyst
H3(curves (a)–(d)) and H4
(curve (e)) were employed for vapor-phase photo-oxidation of acetone present at
different concentrations in air. (a) 1.1, (b) 2.5, (c) 4.8, (d) 13, and (e) 3.9 mol%.Calcination
of a sample at higher temperatures resulted in considerable loss of catalytic
activity.  Comparative data on reaction
of acetone (3.9 mol%) + air over sample H4, that is sample calcined at 773 K
instead of 623 K (sample H3), are shown in curve (e) of Figure8.  As seen in Figure 8 curve (e), less than 60% of
acetone gets converted to CO2 in this case, even for a lower
concentration of acetone in air. These results show a strong relationship
between the catalytic activity, surface area, and the size of gold particles,
which are affected considerably on calcination.Au/TiO2 samples H5 and H6, with a higher gold content of 0.8 and 1.3 wt%, respectively
(see Table 1), showed lower catalytic activity as compared to sample H3
containing ~0.6 wt% Au. For instance, samples H5 and H6 gave rise to a maximum
yield of CO2 at ca. 70% and 80%, respectively, during the reaction
of acetone (3.5 mol%) + air  for about 80
minutes. Similarly, the samples synthesized by impregnation (I1–I3) showed
lower catalytic activity in terms of reaction rate as compared to the corresponding
hydrothermally synthesized samples. Figure 9 presents typical results on dose-dependent
CO2 yields for reaction of acetone (~3.4 mol%) + air over samples
I1–I3.  A progressive increase in
activity is observed as a function of gold content in these data. Thus, while
almost 100% conversion of acetone was observed in case of samples containing
1.5 or 2 wt% of Au (curves (b) and (c)), sample I1 gave only ca. 60% conversion
even after two hours of reaction time. At the same time, the value of t1/2 is about 20 to 30 minutes in case of data in Figure 9 curves (b) and (c), as
compared to 8 to 13 minutes in case of sample H3 having lower gold content (Figure
8).Figure 9
Yield of CO2 as a function of
UV-radiation dose during the vapor-phase photo-oxidation of acetone, (3.2–3.5 mol% in air)
at room temperature over impregnation-prepared Au/TiO2 samples I1(curve (a)), I2 (curve (b)), and I3 (curve (c)).These
results thus provide evidence that the activity of a catalyst is governed by
multiple factors. In addition to surface area and gold content, morphology of
TiO2 and the size of the gold particles also contribute
significantly to the overall catalytic activity. The poorer activity of samples
H5 and H6 as compared to H3 may be attributed to the increase in gold
crystallite size with increasing gold content (TEM evidence). The extraordinary
catalytic and optical properties of smaller-size (<2 nm) gold particles are now
a well-reported phenomenon [27, 28]. It is known that the gold catalysts are
structure sensitive and the binding states of reactant molecules over Au
nanoparticles dispersed over metal oxide supports are found to be different
than those over
bulk gold or gold films. It is suggested that the gold-metal oxide interfaces
act as sites for the activation of molecules such as O2, CO, hydrocarbons,
and so forth [27]. In some of the studies, these unique properties of gold nanoparticles
are attributed to their electronic structure [29].  We have demonstrated earlier that even a
marginal increase in gold particle size as a result of high-temperature calcination
may lead to a considerable decrease in the adsorption and catalytic activity of
Au/Fe2O3 catalysts [30].


## 3.5.1. TiO2

Curves (a)–(c) in Figure7
show the yield of CO2 as a function of UV irradiation, when a
mixture of acetone and air, containing varying concentrations of acetone, was reacted
over mesoporous TiO2 (sample T1). As seen in these data, yield of CO2 corresponds to 100% conversion for acetone concentration of 2.7 mol% or less,
the value of t1/2 (time taken for 50% conversion of acetone) being
around 30 minutes (curve (a)). The yield of CO2 decreased
progressively along with the simultaneous formation of other reaction products
for increasing acetone: air mol ratios in the reaction mixture. For instance,
in the case of acetone concentration at 10 mol%, conversion to CO2 was limited to ~25% even after ~2 hours of reaction time (curve (c)). As
mentioned above, ethane and methyl formate were the other reaction products
formed for higher molar concentrations of acetone in air, as detected in the
GC-MS analysis of reaction products. Curve (d) in Figure 7 shows the
representative data on ethane yield for the experiment carried out with 13 mol%
of acetone in air. The yields of methyl formate and water formed in the
reaction were not very reproducible because of their condensing nature, and no
attempt is therefore made to compile the quantitative data on these products.Figure 7
Yield of CO2 (curves
(a)–(c)) and C2H6 (curve (d)) as a function of time, when mesoporous TiO2 (sample
T1) was used for the UV-induced vapor-phase photocatalytic oxidation of acetone
present at varying concentrations in air. (a) 2.7, (b) 5.3, (c) 10, and (d) 13 mol%.


## 3.5.2. Au / TiO2

The
reaction rate was much higher in the case of gold containing TiO2 samples, with the activity depending upon the gold content and the morphology
of TiO2.  Figure 8 presents
the yield of CO2 during photo-oxidation of acetone as a function of
irradiation time for the experiments conducted on hydrothermally prepared
sample H3 and for the different compositions of reactant mixture. As seen in
the results of curves (a)–(c) in Figure 8,
not only the conversion but the rate of the reaction also depended upon acetone
concentration, the lower the concentration the faster being the reaction, as
expected. Lower conversions were observed for the reaction mixture containing a
higher concentration of acetone (Figure 8, curve (d)), along with the formation
of other reaction products as in the case of TiO2.Figure 8
Yield of CO2 as a function of
UV-radiation dose when hydrothermally synthesized Au/TiO2 catalyst
H3(curves (a)–(d)) and H4
(curve (e)) were employed for vapor-phase photo-oxidation of acetone present at
different concentrations in air. (a) 1.1, (b) 2.5, (c) 4.8, (d) 13, and (e) 3.9 mol%.Calcination
of a sample at higher temperatures resulted in considerable loss of catalytic
activity.  Comparative data on reaction
of acetone (3.9 mol%) + air over sample H4, that is sample calcined at 773 K
instead of 623 K (sample H3), are shown in curve (e) of Figure8.  As seen in Figure 8 curve (e), less than 60% of
acetone gets converted to CO2 in this case, even for a lower
concentration of acetone in air. These results show a strong relationship
between the catalytic activity, surface area, and the size of gold particles,
which are affected considerably on calcination.Au/TiO2 samples H5 and H6, with a higher gold content of 0.8 and 1.3 wt%, respectively
(see Table 1), showed lower catalytic activity as compared to sample H3
containing ~0.6 wt% Au. For instance, samples H5 and H6 gave rise to a maximum
yield of CO2 at ca. 70% and 80%, respectively, during the reaction
of acetone (3.5 mol%) + air  for about 80
minutes. Similarly, the samples synthesized by impregnation (I1–I3) showed
lower catalytic activity in terms of reaction rate as compared to the corresponding
hydrothermally synthesized samples. Figure 9 presents typical results on dose-dependent
CO2 yields for reaction of acetone (~3.4 mol%) + air over samples
I1–I3.  A progressive increase in
activity is observed as a function of gold content in these data. Thus, while
almost 100% conversion of acetone was observed in case of samples containing
1.5 or 2 wt% of Au (curves (b) and (c)), sample I1 gave only ca. 60% conversion
even after two hours of reaction time. At the same time, the value of t1/2 is about 20 to 30 minutes in case of data in Figure 9 curves (b) and (c), as
compared to 8 to 13 minutes in case of sample H3 having lower gold content (Figure
8).Figure 9
Yield of CO2 as a function of
UV-radiation dose during the vapor-phase photo-oxidation of acetone, (3.2–3.5 mol% in air)
at room temperature over impregnation-prepared Au/TiO2 samples I1(curve (a)), I2 (curve (b)), and I3 (curve (c)).These
results thus provide evidence that the activity of a catalyst is governed by
multiple factors. In addition to surface area and gold content, morphology of
TiO2 and the size of the gold particles also contribute
significantly to the overall catalytic activity. The poorer activity of samples
H5 and H6 as compared to H3 may be attributed to the increase in gold
crystallite size with increasing gold content (TEM evidence). The extraordinary
catalytic and optical properties of smaller-size (<2 nm) gold particles are now
a well-reported phenomenon [27, 28]. It is known that the gold catalysts are
structure sensitive and the binding states of reactant molecules over Au
nanoparticles dispersed over metal oxide supports are found to be different
than those over
bulk gold or gold films. It is suggested that the gold-metal oxide interfaces
act as sites for the activation of molecules such as O2, CO, hydrocarbons,
and so forth [27]. In some of the studies, these unique properties of gold nanoparticles
are attributed to their electronic structure [29].  We have demonstrated earlier that even a
marginal increase in gold particle size as a result of high-temperature calcination
may lead to a considerable decrease in the adsorption and catalytic activity of
Au/Fe2O3 catalysts [30].


## 3.6. Infrared Spectroscopy

The results
of our in situ DRIFT
spectroscopy studies, where pure acetone vapor or acetone + air was dosed over representative
catalyst samples after appropriate pretreatment, are presented in the following
subsections.


### 3.6.1. Adsorption of Acetone

Curve (b)
in Figure10 exhibits the C–H stretching
(3100–2800 cm-1)
vibrational bands formed over hydrothermally prepared Au/TiO2 sample
H3 on exposure to 0.2 μmol g-1 of acetone at room temperature and
recorded after the lapse of ~10
minutes. The intensity of this band remained unchanged when more amount
of acetone (up to ~1 μmol g-1) was dosed over the sample in form of
successive pulses. Comparative IR spectrum recorded for acetone vapor in this
region is shown in curve (a) of this figure. The ν(C–H) bands of Figure
10(b) show the absence of Q-branch splitting of vibrational-rotational bands
associated with acetone vapor (curve (a)). Also, instead of a sharp band at
2970 cm-1 (band width ~5 cm-1) in case of acetone vapor
(curve (a)), we observe a considerably broadened band  at
2973 cm-1, the corresponding value of band width (full width
at half maximum, FWHM) being ~18 cm-1 for acetone in its adsorbed
state (curve (b)). This band was found to be quite stable on thermal
activation, as seen from the relative absorbance values in spectra (c) and (d)
of Figure 10, recorded on rise in sample temperature to 320 K and 360 K subsequent
to saturation coverage. The intensity of this band decreased only marginally
even after raising the sample temperature to 470 K.Figure 10
C–H stretching
region infrared spectra of sample H3 (hydrothermally synthesized Au/TiO2), exposed to  0.2 μmol of acetone vapor
and recorded after the lapse of 10 minutes (curve (b)) and subsequent heating
at 320 K (curve (c)) and 360 K (curve (d)). Curve (a) shows corresponding
vibrational bands of acetone vapor. The numbers given in the parentheses
indicate the absorbance value of the particular IR band.Figure11 presents corresponding
plots in 2000–1000 cm-1 region (νC=O, νC=C, νC–C, and δCH
regions). Curve (a) shows again a comparative spectrum of acetone vapor. In the
CO stretching region, we observe a broad band centered at ~1701 cm-1 and a shoulder band at ~1738 cm-1 in curve (b) of Figure 11,
recorded soon after adsorption of acetone over sample H3. Instead, a doublet at
1738 and 1725 cm-1 is observed in case of acetone vapor (curve (a)),
arising due to the Fermi resonance between the fundamental CO vibration and an
overtone. With an increase in contact time, the intensity of 1738 cm-1 band decreases further while the frequency of the new band formed at 1701 cm-1 shifts to a lower value of 1695 cm-1 (Figure 11, curve (c)). As in the
case of C–H stretching
bands, the ν(CO) vibrational bands were found to be fairly stable on raising
the sample temperature up to 470 K after acetone adsorption. Spectra (d) and
(e) in Figure 11 show the representative plots obtained at sample temperatures
of 320 K and 350 K, respectively.Figure 11
Stretching C=O, C=C,
C–C, and deformation CH region vibrational bands developed on sample H3 after
exposure to 0.2μmol (14 μmol g-1) of acetone vapor and recorded after
a delay time of 2 minutes (curve (b)) and 15 minutes (curve (c)) and subsequent
raising of sample temperature to 320 K (curve (d)) and 360 K (curve (d)). Curve
(a) shows corresponding vibrational bands of acetone vapor. The numbers given
in the parentheses indicate the absorbance value of the particular IR band.


### 3.6.2. Adsorption of Acetone + Air

The vibrational bands similar to those in Figures10 and 11 were observed on interaction of acetone (10 mol%) + air at room
temperature, the only difference being a considerable increase in the intensity
of the bands. Figure 12 exhibits IR spectra of H3 sample at saturation
coverage, recorded 10 minutes after exposure (curve (a)) and after subsequent
rise in sample temperature to 320 (curve (b)) and 350 K (curve (c)). As seen
from the absorbance values given in Figure 12, the intensity of the vibrational
bands in these spectra is greater by a factor of almost 2, compared to the
corresponding experiments performed using acetone alone (Figure 11). These results
indicate that the presence of oxygen facilitated the reaction of acetone over
catalyst surface to form certain strongly adsorbed and thermally stable species
(Figures 12(curve (b)), 12(curve (C))). We may also mention that the spectral features similar to
those in Figures 10, 11, and 12 were observed in the case of impregnation-prepared
Au/TiO2 samples having a different gold content.Figure 12
Stretching C=O, C=C, C–C, and deformation CH
region vibrational bands developed on sample H3 after exposure  to
acetone vapor + air and recorded after a delay time of 10 minutes (curve
(a)) and on subsequent raising of sample temperature to 320 K (curve (b)) and
370 K (curve (c)). Curve (d) shows comparative vibrational bands of methyl
formate vapor adsorbed over H3 under nitrogen flow at room temperature. The
numbers given in the parentheses indicate the absorbance value of the
particular IR band.Adsorption of acetone over TiO2 gave rise to the vibrational bands similar again to those shown in Figures 10
and 11, described above. The IR bands were found to be quite stable on heating
of the sample up to 470 K under nitrogen flow, as in the case of results
obtained using Au/TiO2.


### 3.6.3. Hydroxyl Region Bands

Curve (a) in Figure13 presents the
hydroxyl region bands of Au/TiO2 (sample H3), exposed to 0.2 μmol g-1 of acetone and recorded soon after. As seen in this figure, the spectral
developments in ν(CH) or ν(CO) regions are accompanied with simultaneous
removal of IR bands of different hydroxyl groups at titania surface, as
indicated by the negative absorbance values (peaks pointing downwards) of ν(OH)
bands at 3710, 3672, and 3600 cm-1 in Figure 13. The presence of
overlapping multiple vibrational bands in 3900–3400 cm-1 region,
representing differently coordinated hydroxyl groups over the titania surface, has
been reported earlier [31]. These results reveal a strong adsorbent-adsorbate
interaction involving the hydroxyl groups, irrespective of their coordinative
environment.Figure 13
Hydroxyl
region IR spectra of sample H3 after room temperature adsorption of 14μmol g-1 of acetone vapor and recorded after the lapse of 15 minutes (curve (a)) and
subsequent heating at 320 K (curve (b)), 350 K (curve (c)), and 380 K (curve (d)).
The numbers given in the parentheses indicate the absorbance value of the
particular IR band.Based upon the above-mentioned
information about the shape, width, and intensity, the IR absorption bands in
Figures10 and 11 may be attributed to an overlap of closely spaced vibrational
bands, arising due to multiple adsorbed species. In some of the  previously reported studies [32, 33], acetone
is found to adsorb in molecular state over TiO2 at a lower coverage
while at a higher coverage it undergoes an Aldol condensation reaction followed
by dehydration to yield 4-methyl-3-penten-2-one ((CH3)2C=CHCOCH3),  commonly known as mesityl oxide (MSO). The
absence of Q-branch splitting and broadening of the ν(C–H) band shown in Figure 10 is in conformity
with the presence of a condensed state of acetone. The IR absorption bands
appearing at 2973 and 2931 cm-1 in Figure 10 and at 1702, 1475, and
1364 cm-1 in Figure 11 may be assigned to this strongly adsorbed
state of acetone molecules (cf. Figures 10 and 11, curve (a)). The presence of
a weak shoulder at 1738 cm-1 in Figure 11 (curve b) and its removal
with increase in contact time (curve c) indicate that the transformation of acetone vapor
to its condensed state is a slow and activated process. We, however, observe no
formation of MSO, both for adsorption of acetone alone or acetone + air. As
mentioned by Zaki et al. [33], the mesityl oxide is distinguishable from
acetone by a νC=C band at 1634 cm-1 and νC–C band at 1294 cm-1.
No such bands are, however, observed in Figures 11 and 12, thus ruling out the
formation of MSO under experimental conditions of this study.Theν(CH3) vibration at
2973 cm-1 (Figure 10), νC=O vibration at 1695 cm-1, and
several weak absorption bands in 1900–1100 cm-1 region (Figure 12) may
be assigned to methyl formate (methyl methanoate) in its adsorbed state, as
reported in one of our earlier studies [15]. This identification is validated
by a study of Lochař et al. [34] where similar bands were observed for
adsorption of methyl formate over metal oxide surfaces. To further confirm the
assignment of absorption bands in Figures 11 and 12, methyl formate vapor was
dosed over sample H3 at room temperature and the representative spectrum thus
obtained is shown in curve (d) of Figure 12, which matches well with the IR
bands shown in Figures 11 and 12 (curves (a)–(c)). We may also
mention that the formation of methyl formate was detected clearly in our GC-MS
analysis of reaction products formed in photo-oxidation of acetone over TiO2.
A difference in the relative intensity and the shift in frequency of some of
the bands in curves (c) and (d) of Figure 12 may be attributed to the
difference in the binding of methyl formate groups, formed in situ and that
adsorbed in a molecular form. We may also mention that the νas(COO)+
δ(CH) combination bands of adsorbed mono- and bidentate formate (HCOO−) species
give rise to absorption bands at 2970 cm-1 and 2936 cm-1 whereas corresponding  ν(CH) vibrations
appear at 2896 cm-1 and 2856 cm-1. Similarly, in the
lower IR region, the bands at 1575 cm-1 and 1376 cm-1 are
assigned to νas(COO) and νs(COO) vibrations of the
bidentate formate species while the bands at 1542 cm-1 and 1363 cm-1 arise due to such vibrations of monodentate formate species [15, 35–38]. A
considerable increase of absorbance in 2950–2850 cm-1 region and a
large width of 2973 cm-1 band in Figure 10, and similarly the
presence of various unresolved overlapping bands in 1800–1000 cm-1 region (Figures 11 and 12), help us to conclude that in addition to methyl
formate, HCOO− species
may also form on interaction of acetone and subsequent thermal activation.The active role of formate and
methyl formate groups in photocatalytic reactions over titania and other oxide
surfaces has been amply demonstrated by several research groups [15, 32, 33, 38, 39]. For instance, Wu et al. [39] demonstrated that the methoxy-, ethoxy-,
and formate-type surface groups are photo-oxidized on TiO2 under
UV-illumination, and the rate of photo-oxidation depends on the binding mode of
these species at catalyst surface, the monodentate species being more reactive
as compared to their bidentate counterparts. Simultaneous FTIR and photocatalysis
studies conducted in our laboratory have also exhibited the involvement of such
adsorbed species in the photo-oxidation of methanol over TiO2/MCM
and UOx/MCM catalyst surfaces [15, 38]. El-Maazawi et al. [32]
proposed a reaction mechanism, where formation of O− (ads)
species from gas-phase O2 and its reaction with adsorbed acetone are shown to play an
important role.To summarize, the results of our
study reveal that the adsorption of acetone over nanostructured TiO2 or Au/TiO2 may occur in several modes, giving rise to a condensed
and strongly adsorbed state of molecular acetone and methyl formate and formate types of surface
adsorbed species. The IR results show that the host matrix and the gold
particles may serve as independent adsorption and reaction sites for acetone
and oxygen molecules. Since similar IR absorbance bands are observed on
adsorption over TiO2 and Au/TiO2 and also with or without
the presence of air in reaction mixture, it is apparent that the adsorption of
acetone occurs primarily at titania surface. The spectral features in ν(OH)
region  (Figure 13) reveal that the Ti–OH
groups may directly interact with acetone molecules. This process is reversed
on thermal activation, as is apparent from the absorbance values shown in Figure
13 (curves (b)–(d)). Since
acetone + air adsorption over Au/TiO2 gave rise to higher intensity
of the IR absorption bands, as described above, we may conclude that the
adsorption and activation of O2 molecules at Au sites plays an
important role. This phenomenon, promoted only by nanosize Au particles,
explains the high catalytic activity of sample H3 as compared to other samples
having higher gold content but larger-size crystallites.The overall photocatalytic process
would therefore involve the interaction of these primary adsorbed species with
the electron-hole pair generated on band gap excitation of TiO2,
giving rise to formation of various ion-radicals, free radicals, and surface
bonded transient species. Various possible photochemical steps may subsequently
prevail, leading thereby to the formation of different reaction products, as
has been reported widely (see, e.g., [40]).


## 3.6.1. Adsorption of Acetone

Curve (b)
in Figure10 exhibits the C–H stretching
(3100–2800 cm-1)
vibrational bands formed over hydrothermally prepared Au/TiO2 sample
H3 on exposure to 0.2 μmol g-1 of acetone at room temperature and
recorded after the lapse of ~10
minutes. The intensity of this band remained unchanged when more amount
of acetone (up to ~1 μmol g-1) was dosed over the sample in form of
successive pulses. Comparative IR spectrum recorded for acetone vapor in this
region is shown in curve (a) of this figure. The ν(C–H) bands of Figure
10(b) show the absence of Q-branch splitting of vibrational-rotational bands
associated with acetone vapor (curve (a)). Also, instead of a sharp band at
2970 cm-1 (band width ~5 cm-1) in case of acetone vapor
(curve (a)), we observe a considerably broadened band  at
2973 cm-1, the corresponding value of band width (full width
at half maximum, FWHM) being ~18 cm-1 for acetone in its adsorbed
state (curve (b)). This band was found to be quite stable on thermal
activation, as seen from the relative absorbance values in spectra (c) and (d)
of Figure 10, recorded on rise in sample temperature to 320 K and 360 K subsequent
to saturation coverage. The intensity of this band decreased only marginally
even after raising the sample temperature to 470 K.Figure 10
C–H stretching
region infrared spectra of sample H3 (hydrothermally synthesized Au/TiO2), exposed to  0.2 μmol of acetone vapor
and recorded after the lapse of 10 minutes (curve (b)) and subsequent heating
at 320 K (curve (c)) and 360 K (curve (d)). Curve (a) shows corresponding
vibrational bands of acetone vapor. The numbers given in the parentheses
indicate the absorbance value of the particular IR band.Figure11 presents corresponding
plots in 2000–1000 cm-1 region (νC=O, νC=C, νC–C, and δCH
regions). Curve (a) shows again a comparative spectrum of acetone vapor. In the
CO stretching region, we observe a broad band centered at ~1701 cm-1 and a shoulder band at ~1738 cm-1 in curve (b) of Figure 11,
recorded soon after adsorption of acetone over sample H3. Instead, a doublet at
1738 and 1725 cm-1 is observed in case of acetone vapor (curve (a)),
arising due to the Fermi resonance between the fundamental CO vibration and an
overtone. With an increase in contact time, the intensity of 1738 cm-1 band decreases further while the frequency of the new band formed at 1701 cm-1 shifts to a lower value of 1695 cm-1 (Figure 11, curve (c)). As in the
case of C–H stretching
bands, the ν(CO) vibrational bands were found to be fairly stable on raising
the sample temperature up to 470 K after acetone adsorption. Spectra (d) and
(e) in Figure 11 show the representative plots obtained at sample temperatures
of 320 K and 350 K, respectively.Figure 11
Stretching C=O, C=C,
C–C, and deformation CH region vibrational bands developed on sample H3 after
exposure to 0.2μmol (14 μmol g-1) of acetone vapor and recorded after
a delay time of 2 minutes (curve (b)) and 15 minutes (curve (c)) and subsequent
raising of sample temperature to 320 K (curve (d)) and 360 K (curve (d)). Curve
(a) shows corresponding vibrational bands of acetone vapor. The numbers given
in the parentheses indicate the absorbance value of the particular IR band.


## 3.6.2. Adsorption of Acetone + Air

The vibrational bands similar to those in Figures10 and 11 were observed on interaction of acetone (10 mol%) + air at room
temperature, the only difference being a considerable increase in the intensity
of the bands. Figure 12 exhibits IR spectra of H3 sample at saturation
coverage, recorded 10 minutes after exposure (curve (a)) and after subsequent
rise in sample temperature to 320 (curve (b)) and 350 K (curve (c)). As seen
from the absorbance values given in Figure 12, the intensity of the vibrational
bands in these spectra is greater by a factor of almost 2, compared to the
corresponding experiments performed using acetone alone (Figure 11). These results
indicate that the presence of oxygen facilitated the reaction of acetone over
catalyst surface to form certain strongly adsorbed and thermally stable species
(Figures 12(curve (b)), 12(curve (C))). We may also mention that the spectral features similar to
those in Figures 10, 11, and 12 were observed in the case of impregnation-prepared
Au/TiO2 samples having a different gold content.Figure 12
Stretching C=O, C=C, C–C, and deformation CH
region vibrational bands developed on sample H3 after exposure  to
acetone vapor + air and recorded after a delay time of 10 minutes (curve
(a)) and on subsequent raising of sample temperature to 320 K (curve (b)) and
370 K (curve (c)). Curve (d) shows comparative vibrational bands of methyl
formate vapor adsorbed over H3 under nitrogen flow at room temperature. The
numbers given in the parentheses indicate the absorbance value of the
particular IR band.Adsorption of acetone over TiO2 gave rise to the vibrational bands similar again to those shown in Figures 10
and 11, described above. The IR bands were found to be quite stable on heating
of the sample up to 470 K under nitrogen flow, as in the case of results
obtained using Au/TiO2.


## 3.6.3. Hydroxyl Region Bands

Curve (a) in Figure13 presents the
hydroxyl region bands of Au/TiO2 (sample H3), exposed to 0.2 μmol g-1 of acetone and recorded soon after. As seen in this figure, the spectral
developments in ν(CH) or ν(CO) regions are accompanied with simultaneous
removal of IR bands of different hydroxyl groups at titania surface, as
indicated by the negative absorbance values (peaks pointing downwards) of ν(OH)
bands at 3710, 3672, and 3600 cm-1 in Figure 13. The presence of
overlapping multiple vibrational bands in 3900–3400 cm-1 region,
representing differently coordinated hydroxyl groups over the titania surface, has
been reported earlier [31]. These results reveal a strong adsorbent-adsorbate
interaction involving the hydroxyl groups, irrespective of their coordinative
environment.Figure 13
Hydroxyl
region IR spectra of sample H3 after room temperature adsorption of 14μmol g-1 of acetone vapor and recorded after the lapse of 15 minutes (curve (a)) and
subsequent heating at 320 K (curve (b)), 350 K (curve (c)), and 380 K (curve (d)).
The numbers given in the parentheses indicate the absorbance value of the
particular IR band.Based upon the above-mentioned
information about the shape, width, and intensity, the IR absorption bands in
Figures10 and 11 may be attributed to an overlap of closely spaced vibrational
bands, arising due to multiple adsorbed species. In some of the  previously reported studies [32, 33], acetone
is found to adsorb in molecular state over TiO2 at a lower coverage
while at a higher coverage it undergoes an Aldol condensation reaction followed
by dehydration to yield 4-methyl-3-penten-2-one ((CH3)2C=CHCOCH3),  commonly known as mesityl oxide (MSO). The
absence of Q-branch splitting and broadening of the ν(C–H) band shown in Figure 10 is in conformity
with the presence of a condensed state of acetone. The IR absorption bands
appearing at 2973 and 2931 cm-1 in Figure 10 and at 1702, 1475, and
1364 cm-1 in Figure 11 may be assigned to this strongly adsorbed
state of acetone molecules (cf. Figures 10 and 11, curve (a)). The presence of
a weak shoulder at 1738 cm-1 in Figure 11 (curve b) and its removal
with increase in contact time (curve c) indicate that the transformation of acetone vapor
to its condensed state is a slow and activated process. We, however, observe no
formation of MSO, both for adsorption of acetone alone or acetone + air. As
mentioned by Zaki et al. [33], the mesityl oxide is distinguishable from
acetone by a νC=C band at 1634 cm-1 and νC–C band at 1294 cm-1.
No such bands are, however, observed in Figures 11 and 12, thus ruling out the
formation of MSO under experimental conditions of this study.Theν(CH3) vibration at
2973 cm-1 (Figure 10), νC=O vibration at 1695 cm-1, and
several weak absorption bands in 1900–1100 cm-1 region (Figure 12) may
be assigned to methyl formate (methyl methanoate) in its adsorbed state, as
reported in one of our earlier studies [15]. This identification is validated
by a study of Lochař et al. [34] where similar bands were observed for
adsorption of methyl formate over metal oxide surfaces. To further confirm the
assignment of absorption bands in Figures 11 and 12, methyl formate vapor was
dosed over sample H3 at room temperature and the representative spectrum thus
obtained is shown in curve (d) of Figure 12, which matches well with the IR
bands shown in Figures 11 and 12 (curves (a)–(c)). We may also
mention that the formation of methyl formate was detected clearly in our GC-MS
analysis of reaction products formed in photo-oxidation of acetone over TiO2.
A difference in the relative intensity and the shift in frequency of some of
the bands in curves (c) and (d) of Figure 12 may be attributed to the
difference in the binding of methyl formate groups, formed in situ and that
adsorbed in a molecular form. We may also mention that the νas(COO)+
δ(CH) combination bands of adsorbed mono- and bidentate formate (HCOO−) species
give rise to absorption bands at 2970 cm-1 and 2936 cm-1 whereas corresponding  ν(CH) vibrations
appear at 2896 cm-1 and 2856 cm-1. Similarly, in the
lower IR region, the bands at 1575 cm-1 and 1376 cm-1 are
assigned to νas(COO) and νs(COO) vibrations of the
bidentate formate species while the bands at 1542 cm-1 and 1363 cm-1 arise due to such vibrations of monodentate formate species [15, 35–38]. A
considerable increase of absorbance in 2950–2850 cm-1 region and a
large width of 2973 cm-1 band in Figure 10, and similarly the
presence of various unresolved overlapping bands in 1800–1000 cm-1 region (Figures 11 and 12), help us to conclude that in addition to methyl
formate, HCOO− species
may also form on interaction of acetone and subsequent thermal activation.The active role of formate and
methyl formate groups in photocatalytic reactions over titania and other oxide
surfaces has been amply demonstrated by several research groups [15, 32, 33, 38, 39]. For instance, Wu et al. [39] demonstrated that the methoxy-, ethoxy-,
and formate-type surface groups are photo-oxidized on TiO2 under
UV-illumination, and the rate of photo-oxidation depends on the binding mode of
these species at catalyst surface, the monodentate species being more reactive
as compared to their bidentate counterparts. Simultaneous FTIR and photocatalysis
studies conducted in our laboratory have also exhibited the involvement of such
adsorbed species in the photo-oxidation of methanol over TiO2/MCM
and UOx/MCM catalyst surfaces [15, 38]. El-Maazawi et al. [32]
proposed a reaction mechanism, where formation of O− (ads)
species from gas-phase O2 and its reaction with adsorbed acetone are shown to play an
important role.To summarize, the results of our
study reveal that the adsorption of acetone over nanostructured TiO2 or Au/TiO2 may occur in several modes, giving rise to a condensed
and strongly adsorbed state of molecular acetone and methyl formate and formate types of surface
adsorbed species. The IR results show that the host matrix and the gold
particles may serve as independent adsorption and reaction sites for acetone
and oxygen molecules. Since similar IR absorbance bands are observed on
adsorption over TiO2 and Au/TiO2 and also with or without
the presence of air in reaction mixture, it is apparent that the adsorption of
acetone occurs primarily at titania surface. The spectral features in ν(OH)
region  (Figure 13) reveal that the Ti–OH
groups may directly interact with acetone molecules. This process is reversed
on thermal activation, as is apparent from the absorbance values shown in Figure
13 (curves (b)–(d)). Since
acetone + air adsorption over Au/TiO2 gave rise to higher intensity
of the IR absorption bands, as described above, we may conclude that the
adsorption and activation of O2 molecules at Au sites plays an
important role. This phenomenon, promoted only by nanosize Au particles,
explains the high catalytic activity of sample H3 as compared to other samples
having higher gold content but larger-size crystallites.The overall photocatalytic process
would therefore involve the interaction of these primary adsorbed species with
the electron-hole pair generated on band gap excitation of TiO2,
giving rise to formation of various ion-radicals, free radicals, and surface
bonded transient species. Various possible photochemical steps may subsequently
prevail, leading thereby to the formation of different reaction products, as
has been reported widely (see, e.g., [40]).


## 4. Conclusions

The
results of the present study demonstrate that the structural and morphological properties
of Au/TiO2 have a direct relationship with the activity of these
photocatalysts. The role of a noble metal cocatalyst in semiconductor mediated
photo-oxidation of organic compounds has been debated widely, and, in general,
two different concepts have been accepted. According to the one theory, the
enhancement of photocatalytic activity is attributable to an increase in charge
separation efficiency, and the other is that the metal particles lead to the
shift in quasi-Fermi level of the semiconductor-metal composite to more
negative potentials [41]. We observe no considerable shift in the UV-region
absorbance band of Au/TiO2 samples. Our IR spectroscopy results, on
the other hand, suggest that the titania and gold particles serve as
independent adsorption and reaction sites for acetone and oxygen molecules. Acetone
molecules adsorb exclusively at TiO2 surface, giving rise to a
condensed state as well as to the formation of formate- and methyl formate-type
surface species.  Based on the dependence
of catalytic activity on gold particle size, we may infer that the Au sites are
primarily responsible for the adsorption and activation of oxygen molecules. This
is in agreement with the well-reported adsorption properties of small-size
(<2 nm) gold particles at Au/support interfaces, as discussed above. Since
our Au/TiO2 samples showed no activity under visible light
irradiation, it is evident that the direct absorption of radiation by gold
particles as seen in Figure 6 may play an insignificant role. The band gap
absorbance in TiO2 resulting in the generation of e−/h+ pair may therefore solely govern the photochemical steps. Overall, the increase
in the exposed active surface sites (TiO2 and Au) may influence both
the chemisorption of the reactant molecules and the utilization efficiency of
the photoinduced electron-hole pair.



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*Source: 789149-2008-03-12.xml*