The effect of Au on TiO2 catalyzed selective photocatalytic oxidation of cyclohexane

Article abstract of DOI:10.1016/j.jphotochem.2010.10.027

Gold does not induce visible light activity of anatase Hombikat UV100 in the selective photo-oxidation of cyclohexane, as can be concluded from in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) measurements. Extremely small conduc

Full text of DOI:10.1016/j.jphotochem.2010.10.027

Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 326–332
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
The effect of Au on TiO2 catalyzed selective photocatalytic oxidation
of cyclohexane
Joana T. Carneiroa, Tom J. Savenijeb, Jacob A. Moulijna, Guido Mulc,∗
a Catalysis Engineering, ChemE, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands
b Opto-Electronic Materials, ChemE, Delft University of Technology, Julianalaan 136, 2628 BL, Delft, The Netherlands
c PhotoCatalytic Synthesis Group, Faculty of Science and Technology, University of Twente, Postbus 217, 7500 AE Enschede, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
Gold does not induce visible light activity of anatase Hombikat UV100 in the selective photo-oxidation
of cyclohexane, as can be concluded from in situ attenuated total reflectance Fourier transform infrared
(ATR-FTIR) measurements. Extremely small conductance values measured at 530 nm in Time Resolved
Microwave Conductivity (TRMC) experiments are in agreement with this conclusion. Upon UV activation,
gold enhances the initial rate of formation of surface adsorbed cyclohexanone, but is detrimental to the
overall production rate determined in a conventional top illuminated slurry reactor. The latter is the result
of strongly modified surface properties of TiO2 by gold deposition, affecting the adsorption/desorption
equilibrium of cyclohexanone.
Received 16 July 2010
Received in revised form
30 September 2010
Accepted 30 October 2010
Available online 5 November 2010
Keywords:
Au/TiO2
Visible-light
Thermal effects
ATR-FTIR
© 2010 Elsevier B.V. All rights reserved.
Photocatalysis
TRMC
1. Introduction
In the present work, we will first demonstrate that incipient
wetness impregnation of metallic gold nanoparticles stabilized by
Metal nanoparticles are of great interest because of their unique
electronic, optical and magnetic properties. The spectroscopy of
noble metal particles is associated with surface plasmon resonance,
which is a collective oscillation of free conduction band electrons
[1]. In particular, the strong plasmon absorbance of Au nanoparti-
cles has attracted attention [2,3] for a wide variety of applications.
Modification of TiO2 photocatalysts with Au has also been exten-
sively evaluated. Several authors observed visible light activity of
Au/TiO2 catalysts, tentatively explained by optical absorption of the
Au nanoparticles [4–7]. In photocatalytic wastewater degradation,
gold nanoparticles supported on TiO2 have also been found to pro-
mote the activity upon UV excitation. This is generally explained
by a catalytic effect of Au in transferring UV-photoexcited conduc-
tion band electrons of titania to the reacting substrate, typically
O2. The remaining free holes diffuse towards the titania surface
and are able to oxidize surface adsorbed species [8–13]. Some of us
have recently demonstrated, however, that this positive effect of
Au deposition can be over-compensated by the detrimental loss of
surface Ti-OH-groups during catalyst preparation, in particular if a
deposition–precipitation method is used [14].
poly(vinyl-pyrrolidone), leads to a smaller decrease in initial sur-
face activity upon UV excitation of Hombikat TiO2, as compared
to catalysts prepared by precipitation. As will be shown by in situ
ATR-FTIR measurements, Au also leads to enhanced adsorption of
cyclohexanone. To the best of our knowledge, the potential of vis-
ible light activation of Au/TiO2 to initiate cyclohexane oxidation
has not been previously assessed, and will be demonstrated not to
lead to the formation of cyclohexanone. Finally, the results will be
discussed on the basis of Time Resolved Microwave Conductivity
(TRMC) experiments.
2. Experimental
2.1. Catalyst preparation and characterization
Hombikat UV100 TiO2 (Sachtleben GmbH), H, was used as
received. It consists of 100% anatase as determined by XRD, and
has a surface area of 337 m2 g−1 with an average particle size of
7 nm [15]. This material was used to prepare a Au/TiO2 photo-
catalyst, AuH, by an incipient wetness impregnation method, using
a Au colloidal solution prepared as described by Lu et al. [16].
In short, metallic gold nanoparticles with poly(vinyl-pyrrolidone)
(PVP) were prepared following a methodology, which involved
the reduction of the metal salt by 120 mL anhydrous ethylene
∗
Corresponding author.
E-mail address: G.Mul@utwente.nl (G. Mul).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.10.027

J.T. Carneiro et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 326–332
327
glycol (Sigma–Aldrich) in the presence of PVP (0.8 g, PVP40T,
Sigma–Aldrich) at 353 K for 3 h. The obtained solution exhibited a
deep red color. For impregnation of TiO2, ∼4 mL of the Au solution
with a Au concentration of 2.6 mg mL−1 was used to impregnate
1 g of Hombikat. After the impregnation step the catalyst was dried
in a static air oven at 120 ◦C overnight, and used without further
calcination.
ICP analysis was performed to analyze the amount of Au present
in the AuH catalyst, using a ICP-OES PerkinElmer Optima 3000dv
instrument in the axial mode. The AAS instrument used was a
PerkinElmer Analyst 100. About 25 mg of the sample was shaken
well in a 25 mL mixture of 1% HF and 1.25% H2SO4 until complete
dissolution, followed by analysis. A standard solution of Au (range
0–30 mg L−1) was used for calibration.
Transmission electron microscopy (TEM) was performed on a
Philips CM30UT electron microscope with a FEG (field emission
gun) as the source of electrons operated at 300 kV. Samples were
mounted on a copper supported Quantifoil microgrid carbon poly-
mer.
UV/vis spectra of porous films of the catalysts, deposited by sol-
vent (water) evaporation under vacuum onto a 1 mm thick quartz
plate, were recorded on a Perkin-Elmer Lambda 900 spectrometer
equipped with an integrating sphere (Labsphere).
from ZEISS). The light intensity of the lamp used in the wavelength
absorption range of TiO2 (275–388 nm) is 2 × 10−7 Einst cm−2 s−1
,
determined at the position of the Pyrex window, i.e. where light is
˚
entering the reactor. Air dried over Molsieve 3 A, (Acros Organics)
and presaturated with cyclohexane, was bubbled through the TiO2
suspension at a rate of 30 mL min−1. During the reaction liquid was
withdrawn in aliquots of 0.2 mL and analyzed by GC. Organic com-
pounds were quantitatively analyzed twice using a flame ionization
detector and a Chrompack, CPwax52CB column. Hexadecane was
used as an internal standard. After reaction in the top illumination
reactor, the catalyst was filtered and TGA was performed in order
to determine the nature of surface adsorbed species.
The performance of the materials was also evaluated using
our home-built ATR-FTIR operando system and the procedure is
described in detail elsewhere [19]. Two different LED sets were used
with different wavelengths: 375 nm with 9 × 10−9 Einst cm−2 s−1
intensity, and 530 nm with 3 × 10−8 Einst cm−2 s−1 intensity. The
catalysts were coated on a ZnSe ATR crystal yielding an approx-
imate 1.0 ␮m thick coating. Oxygen saturated cyclohexane was
flown at 8.75 mL min−1 through the ATR cell. Prior to photo-
catalytic oxidation experiments, adsorption of cyclohexane on the
TiO2 was monitored for 90 min and a spectrum of adsorbed cyclo-
hexane on TiO2 was used as background for the photo-oxidation
experiments. The background and the sample spectra were aver-
aged from 64 and 32 spectra, respectively. Both reactor systems
were equipped with a cooling system to keep the reaction temper-
ature at 25 ◦C and to avoid cyclohexane evaporation.
Thermogravimetric analysis (TGA) of the catalysts was carried
out on a TGA/SDTA851e thermobalance (Mettler-Toledo). The sam-
ple powders were heated from 298 to 1073 K at a heating rate of
10 K min−1, in a flow of 100 mL min−1 of He or air, respectively.
2.2. Time Resolved Microwave Conductivity experiments
3. Results
For the Time Resolved Microwave Conductivity (TRMC) mea-
surements, a porous film of the catalysts was deposited by solvent
(water) evaporation onto a 1 mm thick quartz plate. The TRMC tech-
nique is based on the measurement of the normalized change of
microwave power reflected by a sample after illumination by a laser
pulse at a variable wavelength. In this work two wavelengths were
chosen: 300 nm (UV-light) and 530 nm (visible light). The intensity
of the laser pulse was varied using a set of metallic neutral density
filters. The normalized change in microwave power reflected by the
sample, ꢀP(t)/P, is caused by a change of the conductance induced
by the laser pulse, ꢀG(t), which correlates with the product of the
charge carrier formation and the sum of electron and hole mobili-
ties, ꢁꢂꢃi. A full description of the microwave circuit and the data
analysis is given elsewhere [17]. The time-resolved microwave sig-
nal obtained on excitation with a nanosecond laser pulse, can be
characterized by two stages. Up to approximately 30 ns, the sig-
nal is dictated by the instrumental response time. After this initial
stage, the signal decays due to trapping or recombination of charge
carriers. As the signal decay is not exponential, the general decay
shape is characterized by the halftime, ꢄ1/2, defined as the period
involved to reduce to half of its maximum value. Due to their higher
3.1. Characterization
Fig. 1a shows the UV/vis spectra of both materials under inves-
tigation. The absorbance, FA, is the relative amount of photons
actually attenuated by the catalyst layer, defined as
I0 − IT − IR
FA
=
I0
where I0, IT and IR are the incident, transmitted and reflected light
intensities, respectively. For the purple material AuH, the UV/vis
spectrum shows a broad absorbance with a maximum at ∼530 nm,
which is assigned to the plasmon resonance absorption of metallic
Au nanoparticles. This optical transition is the result of collective
oscillations of electrons at the nano-particle surface at a specific
frequency [20]. This transition appears as a broad band in the optical
absorbance spectrum due to the thickness of the AuH sample which
is of several microns.
The particle size distribution of the Au nanoparticles deposited
on the TiO2 is depicted in Fig. 1b. The distribution shows that most
of the Au particles are <4 nm. The largest particles are ∼16 nm. A
TEM micrograph revealing ∼6 nm Au particles on the TiO2 surface
is shown in the inset. The image also shows that H consists of nano-
crystallites inter-connected to form a three-dimensional porous
network, and that this TiO2 sample does not consist of well defined
separated crystals, as discussed previously [21]. The Au loading was
determined to be 0.8 wt.% by ICP analysis.
mobility, electrons contribute far more to the photoconductance in
ꢀ
TiO2 nanoparticles [17] than positive charges and, therefore
ꢃi
is assumed to be close to the electron mobility, ꢃ∼e .
2.3. Photo-activity measurements
To evaluate catalyst performance in the selective oxidation of
cyclohexane upon UV illumination, reactions were carried out in
a top illumination reactor (TIR), as described previously [14]. In
a typical experiment 100 mL of cyclohexane containing 1 g L−1 of
catalyst was used (slurry system). The catalysts were dried for 1 h
at 120 ◦C to remove adsorbed water and impurities, prior to sus-
pension. The solution was illuminated from the top of the reactor
through a Pyrex window that cuts off the highly energetic UV radia-
tion[18]. Ahigh pressuremercury lampof50 Wwas used (HBO50W
3.2. Activity determination in the top illumination reactor
Previous work showed that the main products of photocat-
alytic cyclohexane oxidation are cyclohexanone and cyclohexanol,
with a ketone over alcohol selectivity higher than 98% [14,15,22].
Cyclohexane conversion is below 2%, and CO2 contributions, due to
complete oxidation, are not significant (cyclohexanone and cyclo-
hexanol are reported to be produced with selectivities >95% relative
to CO2) [15]. While radical chemistry in solution can lead to other

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J.T. Carneiro et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 326–332
Fig. 2. Cyclohexanone production profiles in the TIR for H and AuH. The cyclohex-
anone value reaches a plateau after prolonged time of reaction, as a result of loss of
activity of the catalyst. The inset shows the cyclohexanone/cyclohexanol selectivity
during reaction for the two materials.
period for both materials under investigation. The AuH material
showed an increase in cyclohexanol selectivity [9,14].
In Fig. 3a, a set of TGA profiles is shown for the samples before
(fresh) and after (spent) the photocatalytic reaction performed in
the TIR, expressed as the derivative in time of the weight loss of
the materials at a given reference temperature. In the profiles, two
peaks at 60 ◦C and 240 ◦C are present, which are in agreement with
previously reported data [21]. The first desorption peak at 60 ◦C
corresponds to adsorbed water, which after reaction is less for all
the samples. Surprisingly, the weight loss at 240 ◦C is considerably
higher for the spent AuH catalyst, Fig. 3b, as compared to the spent
H sample, Fig. 3a. Because this peak is accompanied by an exother-
mic effect, this is assigned to (hydro)carbon containing adsorbates
formed during reaction. The total weight loss increases in the fol-
lowing order: Hspent < AuHspent < AuH < H.
Fig. 1. (a) UV/vis attenuation spectra of the studied materials in terms of absorbed
fraction (FA). (b) Particle size distribution of the Au nanoparticles deposited on TiO2,
AuH. The inset shows a TEM micrograph of the AuH material with an Au nanoparticle
of ∼6 nm in size.
3.3. Time Resolved Microwave Conductivity
products, in view of the applied wavelengths (>275 nm) this is not
expected to occur, as is confirmed by our GC analyses.
Fig. 4 shows the TRMC transients obtained upon excitation
of the samples at 300 nm and 530 nm wavelength at laser pulse
intensities of 24 ␮J cm−2 and of ∼0.1 mJ cm−2, respectively. A high
laser intensity was used, because of the small signal obtained
for the samples at 530 nm at lower intensities. The maximum
incident-intensity normalized photo-conductance of the differ-
ent samples follows the order H (300 nm) > AuH (300 nm) > H
(530 nm) > AuH (530 nm). In the case of 530 nm excitation, the AuH
and H catalysts show a positive region until 30 ns, after which
for AuH a negative conductance value (imaginary conductance) is
Cyclohexanone formation upon photo-oxidation of cyclohex-
ane is shown in Fig. 2e for the two catalysts. The shape of the
profiles, i.e. the presence of a plateau, is the result of catalyst deac-
tivation, induced by carbonate and carboxylate formation on the
catalyst surface [19]. The amount of cyclohexanone formed as well
as its initial reaction rate is slightly higher for the H material when
compared to AuH. In addition to cyclohexanone production, the
inset shows the cyclohexanone/cyclohexanol selectivity defined as
ncyclohexanone/(ncyclohexanol + ncyclohexanone) × 100 during the reaction
a
b
0
-1x10-4
-2x10-4
-3x10-4
0
240oC
240oC
-1x10-4
-2x10-4
-3x10-4
60oC
50
fresh
spent
fresh
spen
60oC
50
t
100
150
Tr (oC)
200
250
300
100
150
200
250
300
Tr (oC)
Fig. 3. TGA profiles under a He flow as weight loss vs reference temperature of the fresh and spent materials collected after reaction in the TIR and of the fresh catalysts for
(a) H and (b) AuH.

J.T. Carneiro et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 326–332
329
a
b
2.0
300 nm
530 nm
H
0
-5
1.5
1.0
0.5
0.0
H
-10
-15
AuH
AuH
0
100
200
300
400
1ns
100ns
10µs
Time (ns)
Time (s)
Fig. 4. Incident intensity-normalized conductance (ꢀG/I0) transients obtained for the samples, H and AuH, studied at (a) 300 nm and (b) 530 nm laser wavelength (note the
different Y axis scale). It should be taken into account that for the AuH transients at 530 nm, the typically used equations for conductance calculation [9] are no longer valid,
in view of the negative values.
and 1000 cm−1. The C O stretching mode assigned to bulk cyclo-
hexanone can be followed by the band at 1714 cm−1, and the one
corresponding to the adsorbed analogue by the band at 1690 cm−1
[19]. The development of the spectra of H, Fig. 5a, shows qual-
itatively the same bands as AuH, Fig. 5b, but the intensities are
different, especially in the area where the peaks of adsorbed and
bulk cyclohexanone are present. A persisting higher intensity of the
1690 cm−1 band in the case of AuH is observed.
In Fig. 6 the peak height evolution during photocatalytic oxida-
tion of cyclohexane with 375 nm irradiation for two absorptions
assigned to the OH-groups sites, 3633 cm−1, referred to as (Ti)2-
OH, and the OH stretching vibration of adsorbed water molecules
centred at 3375 cm−1 is shown. Generally, in the beginning of the
reaction the water amount at the catalyst surface increases and
after ∼20 min starts to decrease, eventually becoming negative
until a minimum plateau is reached. The water amount formed dur-
observed at longer lifetimes. For H the conductance approaches
zero.
3.4. ATR-FTIR
In the in situ ATR-FTIR spectra of the reaction performed
under 375 nm irradiation, two regions are of interest [19]. The
high wavenumber region, where stretching vibrations of OH-
groups (3637 cm−1) and adsorbed water are to be expected
(3350–3380 cm−1), and the low wavenumber region, where cyclo-
hexanone bands (1689 cm−1 and 1714 cm−1), carboxylate (taken
between 1589 and 1544 cm−1, [23,24]), and carbonate bands
(1403 cm−1, [24,25]) appear. The spectral development during
reaction in the low wavenumber region is shown in Fig. 5, for both
materials under study. It is apparent that in the early stages of reac-
tion a broad range of infrared absorptions develops between 1800
a
b
-1
1403 cm
-1
1403 cm
0.017
0.017
-1
1575 cm
-1
1575 cm
240 min
240 min
-1
1714 cm
1690 cm
0.010
-1
0.010
-1
1690 cm
1714 cm
-1
0.5 min
0.5 min
0
1800
0
1800
1200
1600
1400
1600
1400
1200
Wavenumbers (cm-1)
Wavenumbers (cm-1)
Fig. 5. ATR-FTIR spectra of cyclohexane photocatalytic oxidation on (a) H and (b) AuH coated on a ZnSe crystal with light irradiation wavelength of 375 nm. The spectra were
recorded between 0.5 and 240 min of reaction time. The wavenumbers of the most important bands, representing product formation at the catalyst surface, are depicted.

330
J.T. Carneiro et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 326–332
a
b
2x10-3
water
(Ti)2-OH
0
-2x10-3
-4x10-3
0
50
100
150
200
250
0
50
100
150
200
250
t (min)
t (min)
Fig. 6. Time evolution of the selected peaks, adsorbed water centred at 3375 cm−1 and bridged (Ti)2-OH group at 3633 cm−1, from the spectra measured with the ATR-FTIR
system for (a) H and (b) AuH with light irradiation at 375 nm.
ing reaction is higher in the case of the AuH material and also the
OH-groups decrease (are covered) more significantly for the AuH
sample as compared to H.
4. Discussion
4.1. UV-photocatalysis
Using deconvolution of the peaks assigned to bulk and desorbed
cyclohexanone [21], the time profiles shown in Fig. 7 were obtained.
For both materials the time profiles show that in the first 10 min
of reaction the amount of adsorbed cyclohexanone increases at a
high rate followed by a slower increase. Bulk cyclohexanone also
increases, but at a slower rate. The initial rate of adsorbed cyclohex-
anone is faster for the AuH catalyst, Fig. 7b, compared to H, Fig. 7a. In
the case of AuH there is a well defined plateau of adsorbed cyclohex-
anone between 20 and 120 min followed by a continuous increase.
In the case of AuH the bulk cyclohexanone formation rate increases
after ∼120 min of reaction. The H profiles show that the adsorbed
cyclohexanone reaches a plateau after 100 min of reaction, while
the bulk cyclohexanone amount increases continuously.
Fig. 8 shows the spectral evolution during reaction time under
530 nm light irradiation. None of the bands correspond to species
formed when the reaction is performed under UV-light irradi-
ation. Considering the very low absorption intensities, we can
state that cyclohexane selective oxidation is not taking place at
530 nm light irradiation for both materials. The bands present
at 1124 and 1162 cm−1 (left shoulder in the previous peak) can
be assigned to cyclohexane adsorption. The bands at 1450 cm−1
and 1260 cm−1 are characteristic of the scissoring and twisting
vibrations, respectively, of CH2 groups in cyclohexane [19]. These
bands are increasing in both materials except for the absorption
at 1450 cm−1 for AuH, Fig. 8b, which is decreasing. The decreasing
band at 1640 cm−1 corresponds to the removal of water from the
surface, already present in the catalyst layer at the beginning of the
reaction.
First it is important to note that as a result of Au deposition
on Hombikat UV100, the amount of surface bound water and OH
groups decreases (Fig. 3), diminishing the initial reaction rate of
production of cyclohexanone, Fig. 2. The negative effect of Au depo-
sition observed in Fig. 3, is not as extensive as previously reported
[14]. The explanation is based on the different synthesis methods:
the impregnation method to deposit Au nanoparticles on the TiO2
surface used in this work apparently did not induce as much sur-
face modifications as the deposition–precipitation method used
previously [14].
The cyclohexanone/cyclohexanol selectivity of the AuH catalyst
is lower than obtained with H. This is expected, since upon UV-
light activation the electrons in the conduction band of TiO2 are
transferred to the Au nanoparticles [9], and the induced charge sep-
aration will increase the concentration of OH-radicals at the TiO2
surface, consecutively generating a higher transient concentration
of cyclohexyl hydroperoxide radicals. This increases the probabil-
ity of the reaction of two cyclohexyl hydroperoxide radicals to yield
cyclohexanol [14,22]. Since the preparation procedure of AuH was
different as compared to reported in Ref. [9], new TRMC mea-
surements at 300 nm were performed, Fig. 4a, and as expected,
the conductance signals obtained lead to the same conclusions
reported previously [9]. The photoconductance values and electron
lifetimes in TiO2 are lower under UV-light excitation when Au is
present, due to the fact that upon formation of an electron–hole
pair, electrons will migrate to the Au nanoparticles. Ti-OH sites
around the perimeter of Au particles have been previously iden-
a
b
0.5
0.5
0.4
0.3
0.2
0.1
0.0
0.4
0.3
0.2
ads
ads
0.1
0.0
bulk
bulk
0
50
100
150
200
250
0
50
100
150
200
250
t (min)
t (min)
Fig. 7. Bulk and adsorbed cyclohexanone time profiles obtained from peak deconvolution until 240 min of reaction for (a) H and (b) AuH with light irradiation wavelength
of 375 nm.

J.T. Carneiro et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 326–332
331
a
b
-1
0.006
0.004
0.006
0.004
-1
1450 cm
1124 cm
-1
1260 cm
-1
1340 cm
1260 cm
1124 cm
-1
1040 cm
-1
-1
1040 cm
-1
-1
0.002
1380 cm
0.002
0.000
0.000
-1
1390 cm
-0.002
-0.004
-0.006
-0.008
-0.010
-0.002
-0.004
-0.006
-0.008
-0.010
-1
1450 cm
-1
1640 cm
-1
1640 cm
1700
1500
1300
1100
1700
1500
1300
1100
Wavenumbers (cm-1)
Fig. 8. ATR-FTIR spectra of cyclohexane photocatalytic oxidation on (a) H and (b) AuH coated on a ZnSe crystal with light irradiation wavelength of 530 nm. The spectra were
recorded between 0.5 and 90 min of reaction time. The wavenumbers of the most important peaks are depicted.
tified as more reactive in ‘thermal’ applications of Au/TiO2. We
believe these sites are also very effective in photocatalysis, since
these are the most likely to be benefiting from the presence of Au
in making charge separation more efficient.
According to a previously proposed reaction mechanism, sur-
face adsorbed water is formed upon formation of a cyclohexyl
radical (i.e. OH-radical oxidation of cyclohexane), Eq. (1) [22]. The
profiles shown in Fig. 7 reveal that more water is formed in the case
of the AuH material. Furthermore, the higher water content is also
reflected by the decreasing intensity of the (Ti)2-OH sites, which is
more significant for AuH than for H.
overlapping bands in the spectral range where absorptions of cyclo-
hexanol are expected, renders identification difficult. This region is
composed of a multitude of absorptions assigned to various carbon-
ate and carboxylate species [19,22]. Furthermore, and according to
the light intensities used in both reactors, which is considerably
higher in the TIR experiment, the yields are likely too small to be
detected.
4.2. Visible-light photocatalysis
Based on observations in femtosecond transient absorption
spectroscopy involving a visible excitation pulse in combination
with an IR probe, Furube et al. observed plasmon induced elec-
tron transfer from 10 nm Au nanoparticles to TiO2 [26]. These
plasmon electrons in the conduction band of TiO2 are claimed to
reduce molecular oxygen at the TiO2 surface efficiently, while the
remaining electron deficit in the Au nanoparticles should be able
to oxidize organic compounds [4]. This model has been used to
explain the photocatalytic activity of Au/TiO2 in methylene blue
degradation upon visible light activation [5]. If the steps indicated
above would be effective, 530 nm laser activation of Au/TiO2 should
result in electron mobility in TiO2. The photo-conductance values
presented in Fig. 5 show that in Hombikat with and without Au
promotion electron mobility is in fact very small: ∼1000 times
lower at 530 nm as compared to activation at 300 nm. The thus
apparent absence of electron transfer from Au to TiO2 is in line
with previous studies using time resolved optical measurements
to analyze photo-activation of gold nano-particles, which report
sub-nanosecond recombination of plasmon excited states [26]. The
TRMC equipment used for our study is limited to the nanosecond
timescale, so this recombination process is not observed.
C6H12+OH• → C6H• + H2O
(1)
11
The fact that water is formed in higher amounts over the AuH
material suggests a higher rate of formation of surface adsorbed
cyclohexanone, which is in agreement with the data shown in Fig. 7
for the first ∼25 min of reaction.
The trends shown in Fig. 7, can be explained by a two site adsorp-
tion model discussed in previous work, where one site gives strong
adsorption, in the early stages of the reaction, and the other weaker
adsorption at later stages of the reaction [21]. Fig. 7b suggests
that this model is also valid for the AuH material, demonstrating
that the first plateau of adsorption is even clearer than for H. This
might be related to the sites in close proximity of the Au particles,
which are proposed to be more reactive than ‘remote’ sites. The
final absorption values for adsorbed cyclohexanone are compara-
ble. This is expected, since the number of OH-groups involved is
practically the same. It should be noted that the presence of Au
seems to enhance the reactivity of the most active sites of TiO2,
yielding predominantly adsorbed cyclohexanone. This is confirmed
by the thermogravimetrical results presented in Fig. 4 where the
peak at 240 ◦C increased more significantly for AuH as compared to
H. Although the existence of two types of sites for cyclohexanone
adsorption is in agreement with the model reported previously for
this reaction, the chemical nature of the sites is yet unknown.
The ATR-FTIR results shown in Fig. 5 are inconclusive with
respect to cyclohexanol formation. Although this compound is
detected in the TIR, in the ATR-FTIR analysis the complexity of the
The striking negative feature (>30 ns) observed for AuH upon
visible light excitation is also important to discuss. Due to the
high visible light laser intensities, some Au nanoparticles might
be melting under laser pulse excitation, as can be concluded from
the following. Considering the value of the heat capacity for Au
(0.126 J g−1 K−1), a particle diameter of 2 nm, and the energy of the
laser pulse, 0.1 mJ cm−2, the temperature rise, ꢀT, within the Au

332
J.T. Carneiro et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 326–332
nanoparticle would be ∼600 K. This means that the actual temper-
ature in the Au nanoparticle is ∼900 K. According to the melting
point of Au particles with sizes below ∼2.5 nm diameter which is
reported to be below 700 K [27], the Au is in the liquid state at 300 K
upon illumination. The melting phenomena of the Au nanoparticles
explain the negative conductance values observed. This is due to the
change of the real dielectric constant value of the sample, result-
ing in a shift of the resonance frequency of the loaded cavity, and
the negative signal intensities [28]. In conclusion, reporting photon
fluxes in studies on Au promoted photocatalysis is very important,
to exclude contributions of thermo-catalysis.
In view of the above evaluation, a plasmon absorption induced
electron transfer from Au to TiO2 is not expected. Indeed, Fig. 8
does not provide evidence for the formation of cyclohexanone. Only
a minor decrease in the amount of adsorbed water is observed,
possibly related to the above mentioned heat effects. The positive
spectral contributions in Fig. 8 are assigned to adsorbed cyclo-
hexane exchanging with initially present surface adsorbed water.
Since the time-scale for electron–hole recombination from the Au
nanoparticles to the TiO2 under visible light radiation is in the fs
range, promoting TiO2 with Au does not have a significant influence
on the surface photo-chemistry, since the important time-scale for
photocatalytic processes is usually in the ␮s time range.
visible light induced photocatalysis of Au promoted TiO2 reported
in the literature might be the consequence of these thermal effects,
rather than the previously proposed electronic effects. The lifetime
of the plasmon photon excited state is likely too short to initiate
such effects.
References
[1] U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer-Verlag,
Berlin (1995).
[2] S. Link, M.A. El Sayed, J. Phys. Chem. B 103 (1999) 8410–8426.
[3] J.J. Mock, M. Barbic, D.R. Smith, D.A. Schultz, S. Schultz, J. Chem. Phys. 116 (2002)
6755–6759.
[4] E. Kowalska, R. Abe, B. Ohtani, Chem. Commun. (2009) 241–243.
[5] X.Z. Li, F.B. Li, Environ. Sci. Technol. 35 (2001) 2381–2387.
[6] R.S. Sonawane, M.K. Dongare, J. Mol. Catal. A: Chem. 243 (2006) 68–76.
[7] V. Rodriguez-Gonzalez, R. Zanellac, G. del Angela, R. Gomeza, J. Mol. Catal. A:
Chem. 281 (2008) 93–98.
[8] H.X. Li, Z.F. Bian, J. Zhu, Y.N. Huo, H. Li, Y.F. Lu, J. Am. Chem. Soc. 129 (2007)
4538–4539.
[9] J.T. Carneiro, T.J. Savenije, G. Mul, Phys. Chem. Chem. Phys. 11 (2009)
2708–2714.
[10] S.C. Chan, M.A. Barteau, Langmuir 21 (2005) 5588–5595.
[11] A. Orlov, D.A. Jefferson, N. Macleod, R.M. Lambert, Catal. Lett. 92 (2004) 41–47.
[12] S. Sakthivel, M.V. Shankar, M. Palanichamy, B. Arabindoo, D.W. Bahnemann, V.
Murugesan, Water Res. 38 (2004) 3001–3008.
[13] T. Sreethawong, S. Yoshikawa, Catal. Commun. 6 (2005) 661–668.
[14] J.T. Carneiro, C.C. Yang, J.A. Moma, J.A. Moulijn, G. Mul, Catal. Lett. 129 (2009)
12–19.
[15] P. Du, J.A. Moulijn, G. Mul, J. Catal. 238 (2006) 342–352.
[16] P. Lu, T. Teranishi, K. Asakura, M. Miyake, N. Toshima, J. Phys. Chem. B 103
(1999) 9673–9682.
[17] J.E. Kroeze, T.J. Savenije, J.M. Warman, J. Am. Chem. Soc. 126 (2004) 7608–7618.
[18] P. Du, J.T. Cameiro, J.A. Moulijn, G. Mul, Catal. Appl. A 334 (2008) 119–128.
[19] A.R. Almeida, J.A. Moulijn, G. Mul, J. Phys. Chem. C 112 (2008) 1552–1561.
[20] Y. Tian, T. Tatsuma, J. Am. Chem. Soc. 127 (2005) 7632–7637.
[21] J.T. Carneiro, J.A. Moulijn, G. Mul, J. Catal. 273 (2010) 199–210.
[22] J.T. Carneiro, A.R. Almeida, J.A. Moulijn, G. Mul, Phys. Chem. Chem. Phys. 12
(2010) 2744–2750.
[23] P.Z. Araujo, C.B. Mendive, L.A.G. Rodenas, P.J. Morando, A.E. Regazzoni, M.A.
Blesa, D. Bahnemann, Colloids Surf. A 265 (2005) 73–80.
[24] D.J. Yates, J. Phys. Chem. 65 (1961) 746–753.
[25] P.A. Connor, K.D. Dobson, A.J. McQuillan, Langmuir 15 (1999) 2402–2408.
[26] A. Furube, L. Du, K. Hara, R. Katoh, M. Tachiya, J. Am. Chem. Soc. 129 (2007)
14852–14853.
5. Conclusions
Upon UV-light activation, the effective separation of
electron–hole pairs induced by Au nanoparticles results in an
enhanced concentration of hydroxyl radicals on the TiO2 surface,
and thus a higher initial rate of surface cyclohexanone formation.
In addition, Au was found to decrease the desorption rate of
cyclohexanone, which leads to lower production of cyclohexanone
dissolved in cyclohexane, as analyzed using a top illumination
reactor. Visible light activation of Au/TiO2 photocatalysts does not
initiate the studied reaction, in agreement with the absence of
evidence for electron injection from Au into the conduction band
of TiO2. The TRMC data suggest the temperature of the Au particles
might increase significantly upon light activation. We propose that
[27] P. Buffat, J.P. Borel, Phys. Rev. A 13 (1976) 2287–2298.
[28] M.P. Dehaas, J.M. Warman, Chem. Phys. 73 (1982) 35–53.