Nano and micro-TiO2 for the photodegradation of ethanol: experimental data and kinetic modelling

Article abstract of DOI:10.1039/c5ra05385d

With TiO₂-photocatalysis being an effective alternative to other more expensive Advanced Oxidation Processes (AOPs), the possibility of using micro-sized TiO₂ materials rather than the well-known nano-sized powders is an important goal in terms of both handling safety and cost saving. In this work the photodegradation of ethanol, used as a model VOC (Volatile Organic Compound) molecule, was investigated, comparing the efficiency of both commercial nano- and micro- sized TiO₂ samples. In all cases the same degradation pathway was observed, namely, a consecutive first-order reaction with acetaldehyde as an intermediate product and CO₂ and water as the final products. All photocatalysts were characterized by means of XRD, TEM, IR, BET and XPS analysis. A kinetic model was also developed considering the collected experimental data and a regression of both adsorption and kinetic constants was made using MATLAB software. The optimized parameters were used for simulating the experimental data using an ode15s algorithm.

Full text of DOI:10.1039/c5ra05385d

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Nano and micro-TiO2 for the photodegradation of
ethanol: experimental data and kinetic modelling
Cite this: RSC Adv., 2015, 5, 53419
Claudia L. Bianchi,ad Carlo Pirola,ad Federico Galli,ad Marta Stucchi, ad Sara Morandi,bd
*
Giuseppina Cerratobd and Valentino Capuccic
With TiO2-photocatalysis being an effective alternative to other more expensive Advanced Oxidation
Processes (AOPs), the possibility of using micro-sized TiO2 materials rather than the well-known nano-
sized powders is an important goal in terms of both handling safety and cost saving. In this work the
photodegradation of ethanol, used as a model VOC (Volatile Organic Compound) molecule, was
investigated, comparing the efficiency of both commercial nano- and micro- sized TiO2 samples. In all
cases the same degradation pathway was observed, namely, a consecutive first-order reaction with
acetaldehyde as an intermediate product and CO2 and water as the final products. All photocatalysts
were characterized by means of XRD, TEM, IR, BET and XPS analysis. A kinetic model was also developed
considering the collected experimental data and a regression of both adsorption and kinetic constants
was made using MATLAB software. The optimized parameters were used for simulating the experimental
data using an ode15s algorithm.
Received 26th March 2015
Accepted 8th June 2015
DOI: 10.1039/c5ra05385d
www.rsc.org/advances
Pollution abatement is something that is very important
nowadays, but worldwide research needs to develop new
1 Introduction
“green” technologies in order to limit energy consumption and
Photocatalysis is a well-known process in which a chemical
reaction occurs when a semiconductor is hit by radiation that
complies with its band gap. In particular, when the semi-
conductor is irradiated by a light source with an appropriate
wavelength, some of its electrons are transferred from the
valence band to the conduction band, creating electron–hole
pairs that are capable of causing reduction and oxidation
reactions.1
TiO2 is an excellent photocatalyst used for degrading organic
compounds, which are oxidized according to the aforemen-
tioned mechanism. In particular, UV irradiation induces the
formation of electron–hole pairs, whose charge carriers react
with chemical species such as water molecules (humidity) and
molecular oxygen present in the air to produce hydroxyl radicals
(cOH) and superoxide radical anions (O2cꢀ), respectively, which
contribute to the decomposition of organic molecules at the
TiO2 surface.1–3
save money, and also to take advantage of processes with no
environmental drawbacks.5 In
a sense, the TiO2-photo-
degradation of pollutants, in particular organic compounds,
nitrogen and sulphur oxides, is an effective alternative to the
much more expensive advanced oxidation processes (AOPs).6
Moreover, as most of human life spent in an indoor environ-
ment, and most of the soilage of the interior of buildings comes
from VOCs, their degradation is a very crucial point in order to
improve both air and human health quality.4,7
In this study, two different commercial TiO2 samples were
tested in the photodegradation reaction of ethanol, which was
chosen as a model molecule. Furthermore, considering it as an
important atmospheric pollutant, commonly used as an
industrial solvent and a fuel additive, its oxidation is of interest
because it is emitted from many industrial processes such as
breweries and bakeries. Ethanol emissions accounted for about
4% of the total VOC anthropogenic emissions in the UK in
1993.8
Moreover, the atmospheric concentrations of this pollutant
are rising as a consequence of the use of ethanol as a biofuel in
the automotive sector and it is well established that this fact
implies an increase in the atmospheric levels of acetaldehyde,
which is far more toxic and reactive in the atmosphere than
ethanol.9,10 Besides this, acetaldehyde, also present in the
ethanol photodegradation pathway, is an important indoor
pollutant itself, released by some building materials such as
polyurethane foams, and some consumer products such as
Chemical stability and also the high oxidizing power of the
photogenerated holes are the main characteristics that make
TiO2 the most used and investigated photocatalyst and also the
key factors for the pollutant oxidation.4
a
`
Dip. Chimica, Universita degli Studi di Milano, via C. Golgi 19, 20133 Milan, Italy.
E-mail: marta.stucchi@unimi.it
b
`
Universita di Torino & NIS Inter-departmental Centre, Torino, Italy
cGranitiFiandre SpA, Castellarano (MO), Italy
dConsorzio INSTM, Firenze, Italy
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cigarettes, adhesives, coatings and inks.11 In fact, ethanol is rst were recorded using a JEOL 3010-UHR instrument (acceleration
oxidized to acetaldehyde and then completely degraded to potential: 300 kV; LaB6 lament).
carbon dioxide and water.
Absorption/transmission IR spectra were obtained on a
The possibility of using the common pigmentary micro-sized Perkin-Elmer FT-IR System 2000 spectrophotometer equipped
TiO2 as a photocatalyst has already been proved,12 and, with a Hg–Cd–Te cryo-detector, working in the 7200–580 cmꢀ1
although a reduced crystal size ensures a higher surface area range at a resolution of 2 cmꢀ1 (number of scans ꢁ 20). Before
(that is positive in terms of photocatalytic activity), it is a the IR analysis, powdery samples were compressed in self-
disadvantage in terms of safety and product handling; in fact, supporting discs (of about 10 mg cmꢀ2) and placed in a
nano-powders can be very dangerous for health, as reported in homemade quartz cell, equipped with KBr windows, connected
several recent papers.13,14 A nanomaterial consists of one or to a conventional high-vacuum line (UHV). The spectra were
more components, present in various forms, that possesses at recorded at room temperature (RT) on the samples in air or aer
least a one-dimensional structure with an average diameter in prolonged outgassing at RT.
the 1–100 nm range. A quick search through the literature
For the characterization of the light absorption features and
shows that the bulk of the research articles on nanomaterials band-gap determinations, diffuse reectance spectra (DRS) of
genotoxicity were published within the past 3 years, even the powders were collected using a UV-vis scanning spectro-
though their cytotoxicity and carcinogenic responses, both in photometer (PerkinElmer, Lambda 35), which was equipped
vitro and in vivo, were assessed previously.15 As the development with a diffuse reectance accessory.
of nanotechnological applications continue to grow, it is
The XRD spectra were collected using a PW 3830/3020 X’ Pert
anticipated that there will be an even greater demand for safety, Diffractometer from PANalytical working Bragg–Brentano,
health and risk assessment studies in the forthcoming years.16 using the Cu Ka1 radiation (l ¼ 1.5406 A).
˚
In the present paper, two micrometric commercial samples
and the well-known nano-sized P25 by Evonik have been chosen
with a two-fold aim: i.e., not only to evaluate the semiconductor
2.2 Photocatalytic tests
efficiency, considering the different crystallite sizes, but also to Photocatalytic degradations were conducted in a cylindrical
verify the ethanol photodegradation pathway with all the tested glass reactor (diameter ¼ 200 mm, volume ¼ 5 L). The amount
materials.
of catalyst used in each test was 0.05 g and it was deposited on a
From a kinetic point of view, the whole process can be glass support as a thin lm. The latter is obtained by preparing
simplied considering a rst reaction on the catalyst surface, in a suspension of isopropanol and TiO2, and depositing it on the
which the adsorbed ethanol is converted to the adsorbed acet- glass slide through a Pasteur pipette.17 The reactor was initially
aldehyde, followed by a second reaction, in which the adsorbed charged with chromatographic air humidied at 40% RH. The
acetaldehyde is mineralized to carbon dioxide and water, starting ethanol concentration was set at 400 ppmV. Photon
following a Langmuir–Hinshelwood mechanism. The experi- sources were provided by a 500 W UV lamp (Jelosil model HG
mental data gathered were used for the regression of the char- 500) emitting in the 315–400 nm wavelength range (UV-A) at 30
acteristic kinetic parameters, assuming pseudo rst order Wmꢀ2 on the sample surface and each test lasted 4 h.18 The
kinetic constants.
ethanol photodegradation was monitored, using on-stream gas
chromatography (Agilent 3000 A microGC), directly connected
to the reactor, that automatically measures the internal
concentration of ethanol, acetaldehyde and carbon dioxide.
Photolysis tests were also performed for both pure ethanol
and acetaldehyde to monitor the possible molecule degradation
under UV light in the absence of a photocatalyst. The same
procedure was repeated using acetaldehyde. Adsorption tests
were also carried out by monitoring the concentration for 4
hours for both ethanol and acetaldehyde in the dark to obtain
the adsorption constants (K) separately, in order to reduce the
model parameters to be estimated. This approach has been
already used in the literature for the independent determina-
tion of adsorption and kinetic constants.19,20
2 Experimental
Ethanol is a Fluka product at a high purity grade (99.9%). Three
powdered commercial TiO2 samples were selected without
further treatment before their characterization and use: (i) a
nanometric sample by Evonik (P25), always used as a photo-
catalytic reference material and (ii) two micrometric samples
(1077 by Kronos and A-HR by Hunsdman), sold as pigments and
chosen with the following features: pure anatase, untreated
surfaces, an undoped material, and a powdery form.
2.1 Sample characterization
All the photocatalytic tests carried out on each powder have
The surface area of all the catalysts was determined by been repeated at least 5 times, exhibiting the same behavior for
conventional N2 adsorption (BET) at 77 K using a Sorptometer each run.
(Costech Mod. 1042).
The regression of both the adsorption and the kinetic
X-ray photoelectron spectra (XPS) were taken in an M-probe constants were made using MATLAB soware, minimizing the
apparatus (Surface Science Instruments). The source was sum of the squared errors between the calculated and the
monochromatic Al Ka radiation (1486.6 eV).
experimental data. The optimized parameters were used for
The morphology of the catalysts was inspected by means of simulating the experimental data using a differential equation
high-resolution transmission microscopy (HR-TEM): images solver able to handle stiff problems, i.e. the ode15s algorithm.21
53420 | RSC Adv., 2015, 5, 53419–53425
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grey box). In particular P25, differently from both the Kronos
and AH-R samples, exhibits a larger amount with high hetero-
geneity of free hydroxyls, the most important species for the
degradation reaction.18 On the other hand, different values of
the OH/Otot ratio result in very similar activity in case of both
the Kronos 1077 and AH-R powders. This is related to the fact
that the XPS analysis gives a result which refers to the entire
number of OH groups. The FTIR spectra (Fig. 2, section B)
provides evidence for a larger amount of H-bonded OH groups
for Kronos 1077, but very similar features of free OH for both
the samples.
3 Results and discussion
3.1 Morphological characterization
The morphological characteristics of all the used catalysts are
reported in Table 1.
XRD analysis conrms the well-known P25 phase composi-
tion, 80 : 20 in anatase/rutile ratio, and shows that anatase is
the unique polymorph for the other two samples, 1077 and A-
HR. One of the most critical drawbacks of both the micro-
metric samples is their low specic surface area. However, the
presence of the bare anatase polymorph is conrmed, and this
is a fundamental feature in terms of the photocatalytic
performance.
As for the general morphology of the various samples, low
magnication TEM images are displayed in Fig. 1. It is evident
that P25 totally reveals its nano-sized nature (average particle
size ꢁ 20–30 nm) if compared to both the 1077 and A-HR
powders (for which the average particles size is much larger,
lying in the 100–180 nm range). The high-resolution images
relative to all the samples (see the inset in each image reported
in Fig. 1) conrm the high crystallinity exhibited by the powders
and the phase composition, as well.
3.2 Kinetic parameters regression
The degradation reaction is followed considering this
pathway:14
ꢀ
k1
/
ð1Þ CH3CH2OHads CH3COHads
k2
ð2Þ CH3COHads /2CO2 þ H2O
Photocatalyzed reactions in which a gas phase reactant is
involved are usually interpreted using the Langmuir–Hinshel-
BET surface area data are consistent with the above consid- wood mechanism,25 where only the reactant molecules previ-
erations. In fact, a particle size located in the nano-size range, as ously adsorbed on the catalytic surface take part in the reaction.
in case of P25, implies a much higher surface area and vice versa This kinetic interpretation is well-known for photocatalyzed
the opposite trend is expected for the micro-sized powders (see reactions, as reported in some previous papers.26,27
Table 1, column 2 and 3).
For the regression of the kinetic parameters in the present
The surface state of the TiO2 particles was analyzed by XPS. paper, two consecutive rst order reactions were considered, as
No signicant differences can be observed in the Ti 2p region shown in the scheme reported in the introduction section. The
concerning the binding energies (BEs). The peak Ti 2p3/2 is differential equation system that characterizes the system is
always regular and the BE (see Table 1, column 4) compares well reported here:
8
with the data for Ti(IV) reported in the literature for TiO2
dne
dt
Kene
>
>
materials.22 The analysis of the oxygen peaks reveals the pres-
ence of more than one component, which can be attributed to
lattice oxygen in TiO2 (529.9 eV) and to surface OH species
(>531.5 eV).19
¼ ꢀ k1
>
>
1 þ Kene þ Kana
>
>
>
<
dna
k1Kene ꢀ k2Kana
¼ þ
>
dt
1 þ Kene þ Kana
Kana
>
>
>
>
>
>
:
dn
CO2
The hydrophilicity/hydrophobicity characteristics of the
photocatalyst surface plays a crucial role in determining the
ethanol adsorption and thus the nal photocatalytic activity, at
least in the degradation of pollutants.23 The sixth column of
Table 1 reports the OH/Otot surface ratio, which refers to a
quantitative measure of the hydrophilicity/hydrophobicity of
the TiO2 surface, estimated using XPS (Fig. 2).23,24 On one hand,
P25 material exhibits the lowest OH/Otot ratio, and its better
activity is surely related to its higher surface area, which ensures
a bigger amount of surface OH groups, as evidenced by the FTIR
spectra (see Fig. 2, section B) in the region of absorption bands
related to H-bonded and free hydroxyls (these last evidenced by
¼ þ2k2
dt
1 þ Kene þ Kana
Ke and Ka (molꢀ1) are the adsorption constant for ethanol (e)
and acetaldehyde (a), respectively, and k1 and k2 (mol minꢀ1) are
the rst order kinetic constants. As can be observed, the
adsorption of CO2 was not considered.
The adsorption constant (Ki) was obtained considering
reversible adsorption for both ethanol and acetaldehyde. The
differential equation used for this regression is:
dni
¼ ꢀkads; ini þ kdes; iðn0; i ꢀ niÞ
dt
Table 1 Main features of the selected TiO2 samples
BET surface area
Average crystallite
size (nm)
Band gap
(eV)
Sample
Anatase/Rutile composition
(m2 gꢀ1
)
XPS Ti 2p3/2 (eV)
XPS OH/Otot
P25
1077
A-HR
80% anatase 20% rutile
100% anatase
100% anatase
52
11
12
26
130
130
458.4
458.4
458.0
0.14
0.32
0.14
3.21
3.15
3.15
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Fig. 1 TEM images of the various TiO2 samples.
for either ethanol or acetaldehyde, n0,i is the moles of the seems to have no relation with the OH/O surface ratio of the
component charged in the reactor and ni is the moles of the catalyst.
component at a certain time. The adsorption constant is then
calculated by dividing kads by kdes aer the regression.
In Fig. 4–6 the experimental data obtained in the photo-
catalytic runs performed using the different catalysts have been
reported, together with the calculated trend using the opti-
mized kinetic parameters.
The regressed kinetic parameters are reported in Table 3:
The simulated results, obtained with the optimized kinetic
parameters, exhibit a good t for the test performed using both
P25 and the micrometric samples. This was also conrmed by
the low value of the sum of the squared errors (SSE). It is
possible to observe the great difference between the nanometric
(P25) and micrometric (AHR and 1077) samples. In fact, the
numerical values of both the kinetic constants are larger for
P25, in agreement with its superior catalytic performance.
Moreover, for this sample the k2 constant is one order of
magnitude larger than the corresponding k1 constant; this
means that, using nano-sized catalysts, the degradation of
acetaldehyde is very fast with respect to the conversion of
ethanol. The situation is completely different when the reac-
tions are catalysed by the micrometric samples. In fact, using
these catalysts, both the numerical values of the k1 and k2
kinetic constants and the numerical value of the constants
between the 1077 and AHR samples are very similar. The
3.3 Photolysis and adsorption tests
In order to determine the real role of the photocatalytic process,
both photolysis and simple ethanol absorption were
investigated.
The photolysis contribution is negligible, and it can be
concluded that both ethanol and its oxidized intermediate
product, i.e. acetaldehyde, are not sensitive to the bare UV light,
and their concentrations remain stable for the entire test.
The role of absorption is different, as displayed in Fig. 3a
(P25) and b (Kronos 1077) (AH-R sample gave very similar
results to the 1077 adsorption and is not reported for the sake of
clarity).
The optimized adsorption constants calculated from these
experimental data are reported in Table 2:
The results indicate that P25 possesses a higher adsorption
constant for ethanol if compared to that exhibited by both
micrometric samples. On the contrary, acetaldehyde results
show an opposite trend and the adsorption of these species
Fig. 2 O1s XPS spectra (A) and FTIR spectra (B) recorded at RT in air for (a) P25, (b) A-HR, (c) 1077.
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Fig. 3 Adsorption runs of (a) acetaldehyde and (b) ethanol on P25 (B) and 1077 (C). Dotted lines are the calculated curves using the regressed
adsorption constants.
Table 2 Adsorption constants regressed from experimental data
Adsorption constant K [molꢀ1
]
Component
Ethanol
Acetaldehyde
P25
0.2373
0.0786
1077
0.1891
0.1680
A-HR
0.1884
0.1809
Fig. 6 Experimental ethanol photodegradation tests performed using
the A-HR catalyst. Empty points represent acetaldehyde. Dotted lines
are the simulated trends obtained using the regressed kinetic constant.
Table 3 Regressed kinetic constant for all the photocatalysts
k1
k2
[mol minꢀ1
]
[mol minꢀ1
]
SSE
Fig. 4 Experimental ethanol photodegradation tests performed using
the P25 catalyst. Empty points represent acetaldehyde. Dotted lines
are the simulated trends obtained using the regressed kinetic constant.
Sample
P25
1077
AHR
0.68
0.34
0.39
10.41
0.42
0.42
3.91 ꢂ 10ꢀ9
1.69 ꢂ 10ꢀ9
6.88 ꢂ 10ꢀ10
analysis of the kinetic elaboration gives us important informa-
tion about the rate of reaction using either the nanometric or
micrometric samples. The rate of VOC degradation is in general
increased if catalysed by a nanometric sample with respect the
micrometric ones. This phenomenon is particularly important
for some light compounds (i.e., acetaldehyde). Nevertheless, the
catalytic properties of micrometric samples are conrmed from
both the direct experimental conversion and this kinetic inter-
pretation. In particular, considering the degradation of ethanol,
the rate of its conversion is not so different for either the
nanometric or micrometric samples.
The FTIR data obtained for the TiO2 powders, either before
and aer the photodegradation processes, indicate that ethanol
leaves some residues (traces) onto the surface of the supports,
independently of their micro- or nanosized nature. See for
Fig. 5 Experimental ethanol photodegradation tests performed using
the 1077 catalyst. Empty points represent acetaldehyde. Dotted lines are
the simulated trends obtained using the regressed kinetic constant.
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model, the possibility to exploit the photocatalytic potential of
the pigmentary TiO2 is proved, contributing to the already
useful photocatalytic processes. The micrometric TiO2, which is
less dangerous and much less expensive with respect to the
nanometric P25, is an active photocatalyst, being able to
degrade VOCs into CO2. Also, the rate of reaction and the kinetic
constants are lower with respect to P25. The good tting
between the experimental and simulated results conrms the
assumption of a consecutive rst order reaction mechanism
degradation pathway that is not inuenced by the TiO2 crys-
tallite dimension.
Acknowledgements
This research was supported by LIFE+ Environment Policy and
Governance project LIFE13 ENV/IT/000140.
Fig. 7 Absorbance FTIR spectra relative to fresh and used P25 (top
spectra) and 1077 (bottom) TiO2 samples in the ethanol degradation
reaction. The differences between the spectra of the samples before
and after ethanol degradation are reported in red.
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