# The Effect of Transition Metal Doping on the Photooxidation Process of Titania-Clay Composites

Authors: Judit Ménesi; Renáta Kékesi; László Kőrösi; Volker Zöllmer; André Richardt; Imre Dékány
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

Montmorillonite-TiO2 composites containing various transition metal ions (silver, copper, or nickel) were prepared, and their photocatalytic efficiencies were tested in the degradation of ethanol vapor at 70% relative humidity. Two light sources, UV-rich (λ = 254 nm) and visible (λ = 435 nm), were used. The kinetics of degradation was monitored by gas chromatography. It was established that, in the case of each catalyst, ethanol degradation was more efficient in UV-C (λ = 254 nm) than in visible light, furthermore, these samples containing silver or copper ions were in each case about twice more efficient than P25 TiO2 (Degussa AG.) used as a reference. In photooxidation by visible light, TiO2/clay samples doped with silver or copper were also more efficient than the reference sample, P25 TiO2. We show that doping metal ions can also be delivered to the surface of the support by ion exchange and significantly alters the optical characteristics of the TiO2/clay composite.


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## Body

## 1. Introduction

The photocatalytic  degradation of
volatile organic components (VOCs) by UV light in the presence of TiO2 catalysts
has been the subject of numerous publications [1–10]. TiO2 is one of the most efficient
photocatalysts; however, since semiconductor particles are
in need of the higher energy content of UV light, a mere 5% of sunlight is
utilized in the course of irradiation for excitation of the catalyst. Absorption
by photocatalysts in the visible range can be significantly improved by
incorporation of metal ions; however, photoactivity is highly dependent on the
chemical nature and the concentration of the doping  ion and on the method of
preparation. Dvoranová et al. [11]
incorporated cobalt, chrome, and manganese ions into the crystalline structure
of TiO2 using the sol-gel method. Although the presence of the
doping ions caused a significant absorption shift in the visible range relative
to pure P25 titanium dioxide, the photocatalytic activity of the catalysts
decreased considerably. Chiang et al. [12] prepared Degussa P25 TiO2 doped with CuO by photodeposition for
the purpose of liquid-phase degradations; the presence of Cu2+ ions,
however, again decreased photocatalytic efficiency. TiO2 doped with
CuO was also prepared by Slamet et al. [13], and enhanced photocatalytic effect was observed only at a single optimal
Cu(II) ion content. Copper is an efficient electron trap and is therefore
capable of inhibiting recombination of electron-hole pairs, thereby
significantly increasing efficiency.It
has been proven by many reports that the reaction rate is dependent on how the
component to be destroyed can be concentrated on the surface of TiO2 [14, 15]. TiO2-pillared clay minerals, these novel photocatalysts,
attract attention by their ability to accelerate the photocatalytic reaction by
their unique adsorption capabilities [16, 17]. This increased absorbability is
the result of the large specific surface area produced by the incorporation of
TiO2 particles among the silicate layers, creating a mesoporous
structure. Liu et al. [18]
obtained an Ag-TiO2/montmorillonite composite by hydrolysis of TiCl4 and, due to the large specific surface area and the effect of silver content to
improve light absorption, they observed an increased photooxidation activity as
compared to the reference material, TiO2. Kun et al. [19] established that
TiO2/montmorillonite nanocomposites pillared with TiO2 nanoparticles
exhibit outstanding catalytic activity in phenol degradation.Volatile organic components are
being widely utilized not only for industrial but also for household purposes,
leading to water and air pollution. Photocatalytic degradation of ethanol has
been discussed  in detail in a number of studies [20–22]. Nimlos et al. [23] studied the mechanism of degradation and established the following
sequence of products: ethanol →  acetaldehyde →  acetic
acid →  formaldehyde →  formic
acid →  CO2, with acetaldehyde as the main
intermediate. Sauer and Ollis studied deactivation of Degussa P25 TiO2 [24, 25]. In the presence of water vapor,
intermediates and/or products may accumulate on the surface of the catalyst and
deactivate it. Since water is one of the products of the reaction, the reaction
rate decreases continuously due to deactivation.Our
objective was the production of catalysts also efficient in the visible range.
As shown by the references above, incorporation of metal ions into TiO2 is
not always expedient; we therefore prepared composites with the transition
metal ions incorporated not directly on the surface or into the crystalline
structure of TiO2, but into the clay mineral acting as support. When
the photocatalyst TiO2 is mixed to or ground with the clay mineral
support, the result is a TiO2/layer silicate composite held together
by electrostatic forces [19]. We study the catalytic properties of the
ion-exchanged composite, obtained in this way in ethanol degradation on the
solid/gaseous interface at 70% relative humidity, to appropriately model local
environmental conditions in the photocatalytic reaction.


## 2. Experimental

## 2.1. Materials

Starting materials for the
preparation of composites were Degussa P25 TiO2 and the fine
fraction of Na-montmorillonite EXM-838 (d<2 nm, Süd-Chemie AG, Munich, Germany).
The following nitrates were used for ion exchange in clay minerals: nickel
nitrate (Fluka Chemie GmbH, Buchs, Switzerland),
copper nitrate (LightTech, Dunakeszi, Hungary), and silver nitrate (LightTech, Dunakeszi, Hungary). Vapor phase degradation was performed using 99.8% anhydrous
ethanol (LightTech, Dunakeszi, Hungary).


## 2.2. Preparation of Transition Metal Exchanged Montmorillonite/TiO2 Composites

One gram of Na-montmorillonite
(EXM838) was left to swell in 100 mL of distilled water for 1 day. The next day,
1 mmol of Ag(I) nitrate or 0.5 mmol of Ni(II) and Cu(II) nitrate were added to
the montmorillonite swollen in distilled water, the volume of the medium was
brought up to 500 mL by the addition of distilled water, and the suspension was
left to stand in a hot air thermostat at 35°C for 2 days. The precipitate obtained was next
washed 3 times in distilled water, centrifuged and dried at 65°C. The composite was prepared by grinding, for
30 minutes in an agate mortar, a powder mixture consisting of 20% exchanged transition
metal-montmorillonite and 80% P25 TiO2. Metal contents relative to
the total mass of the catalyst were Ag: 2.16%, Cu: 0.64%, and Ni: 0.59%.


## 2.3. Structure Properties

The
XRD experiments (Figure1) were carried out in a Philips X-ray diffractometer
(PW 1830 generator, PW 1820 goniometer) with CuK-α radiation (λ = 0.1542 nm) 40 kV, 35 mA. The basal spacing (dL) was calculated from the  (001)
reflection via the Bragg equation using the PW 1877 automated powder
diffraction software. The analyses were carried out at ambient temperature in
the 1–15° (2Θ) ranges.
The 1–15° (2Θ) ranges are characteristic of the
structure of the layer silicate.X-ray diffraction (XRD) patterns of (a)
Me/clayand (b) Me/TiO2/clay
samples.
(a)(b)


## 2.4. Optical Properties

AMikropack Nanocalc 2000 spectrophotometer equipped
with an integrated sphere (ISP-50-8-R-GT) was used to record the diffuse
reflectance spectra of the samples. The spectra of the samples were analyzed
under ambient condition in the wavelength range of 350–700 nm. The
optical properties of transition metal-loaded titania/clay catalysts are listed
in Table 1.Table 1
The composition and optical properties
of the catalysts.
Metal
content
(mequ/g
clay)Metal
content (wt%)λg (nm)Eg (eV)TiO2 (P25)0.000.003903.18Ag/TiO2/clay1.002.164552.734013.09Ni/TiO2/clay1.000.594043.07Cu/TiO2/clay1.000.644093.03


## 2.5. Photocatalytic Reactions

Photooxidation
of ethanol was performed in the reactor (volume: ca. 700 mL) at 25 ± 0.1°C. The
experimental setup is described in our previous paper [26]. The photoreactor (Figure 3)
consists of two concentrically positioned tubes, namely, an inner quartz tube
and an outer Pyrex glass tube. The light sources of the reactor were two types
of 15 W low-pressure mercury lamp  (LightTech, Dunakeszi,  Hungary), namely, a GCL307T5L/CELL type (light source 1)
with characteristic emission wavelength at λmax = 254 nm
and a GCL303T5 S#2 type (light source 2) with characteristic emission
wavelength at λmax = 435 nm. The
catalyst was sprayed onto the outer side of the inner quartz tube from 10 to 20 (m/v)% aqueous dispersion using N2 stream. The surface of the catalyst film was 44.8 cm2, and
the catalyst mass per unit surface was 0.310±0.03 mg/cm2. The film thickness was ca. 0.7–1.0 μm. The reactor was filled
with dry synthetic air ([H2O] <
5 ppm) to a final pressure of 760 Torr. After the delivery of 10 μL ethanol and 12 μL water, the system was left
to stand for 30 minutes for evaporation of the components and establishment of
the adsorption equilibrium. The reaction starts with switching on the lamp.
Sampling from the gas phase (1 mL) was performed at selected time intervals, and
the composition was analyzed in a gas chromatograph (Shimadzu GC-14B) equipped
with a thermal conductivity (TCD) and a flame ionization detector (FID). The
temperature of the column (HayeSep Q, length 2 m) was 140°C. A sample volume of
1 mL was introduced through a six-way valve to the online GC used for
quantitative analysis of ethanol, acetaldehyde, and CO2. A membrane pump inserted into the experimental
assembly circulates the gas mixture to be degraded through the flow meter, the reactor,
and the sampler of the gas chromatograph. The flow rate of the gas mixture in
the measuring system was 375 mL/min. The initial concentration of ethanol was 0.25 mmol dm-3 at relative humidity of ~70%.


## 3. Results

Showing the XRD pattern, we can establish that for the
sodium montmorillonite, characteristic Bragg peak at 2Θ∘ = 7.02∘ appears and the basal  distance is d001 = 1.276 nm. After the exchange of Ag(I) ions, the
intensity of this  reflection is lower and d001 = 1.108 nm because the
layered structure will be partially disoriented. However, in the case of Cu(II)- and Ni(II)-ions
modified montmorillonite samples, the basal distance is 1.336 nm and 1.445 nm,
respectively (Figure 1(a)); that means the nickel and copper ions can be
incorporated between the silicate layers, and the two valent  cations can close the
layers together producing well-oriented lamellar structures. After mixing these modified transition metal
montmorillonits with TiO2 powder, the intensity of the clay
reflections will be very small, and the interlayer distance will increase to d001 = 1.651 nm (Ag/TiO2/clay), d001 = 1.453 nm (Ni/TiO2/clay),
and d001 = 1.367 nm (Cu/TiO2/clay), showing the possibility
of the partial incorporation of any TiO2 nanoparticles between the
lamellae (Figure 1(b)). The formation of this suggested structures is presented in
Figure 2. The photocatalytic
efficiency of montmorillonite/TiO2 composites was tested in the
degradation of ethanol vapor at 70% relative humidity. The reference catalyst
was pure Degussa P25 TiO2. The degradations were also repeated
without catalyst with both light sources; it was found that ethanol
concentration did not decrease in either case, indicating that self-photolysis
does not take place in the vapor studied. The decrease in ethanol concentration
and the formation of the main intermediate acetaldehyde were monitored in the
course of the measurements (Figures 4(a), 4(b) and 5(a), 5(b)). The kinetics of gaseous
CO2 formation was also studied (Figures 4(c), 5(c)). The results of the
degradations are summarized in Table 2. In order to characterize catalysts, we
introduced the parameter “w,” which means the amount of ethanol destroyed
by irradiation with the given light source in unit time in a reactor volume of
1 dm3, normalized to a catalyst film surface of 1 m2
(Figure 6). The
parameter w is calculated by the following equation:(1)w=amount of degraded EtOH(mmol/dm3)time (min)*surface(m−2).Table 2
Photocatalytical characterization of the TiO2 (P25)
and the transition metal nanocomposite samples.
“Light source 1” (λ = 254 nm)“Light source 2” (λ = 435 nm)ΔcEtOHkefficiencyΔcEtOHkefficiency(mmol dm-3)
10 min(*10-4s-1)%(mmol dm-3)
10 min(*10-4s-1)%TiO2 (P25)0.0726.581000.0171.43100Ag/TiO2/clay0.21134.942930.0322.31188Ni/TiO2/clay0.0766.351050.0151.2288Cu/TiO2/clay0.14013.961940.0271.88159Schematic drawing of preparation of
Me-ion exchanged montmorillonite/TiO2 composites.Figure 3
Schematic drawing of the photoreactor.Ethanol concentration changes under (a)
UV-Vis irradiation (light source 1), (b) kinetic curves of acetaldehyde,and (c) CO2 formation.
(a)(b)(c)Ethanol concentration changes under (a)
visible irradiation (light source 2), (b) kinetic curves of acetaldehyde, and
(c) CO2 formation.
(a)(b)(c)Amount of degraded ethanol in unit
time (1 minute), normalized to a catalyst film surface of 1 m2(w).
(a)(b)The measured ΔcEtOH values are normalized to 1 g of
TiO2 catalyst and summarized in Figure 7. Table 2 also includes the
rate constants (k) calculated from the slopes of the first-order kinetic
functions ln(c/c0) versus time. The constants unambiguously indicate
that the reactions studied follow first-order kinetics (Figures 8(a), 8(b)).Decreasing of the ethanol
concentrations is normalized to 1 g of TiO2 in unit time (10 minutes).
(a)(b)Determination of rate constants (k)
with ln(c/c0) versus t (sec) function by using (a) “light source 1” and (b) “light source 2”
lamps.
(a)(b)All in all, it can be established
that, in the case of each catalyst, ethanol is destroyed much more efficiently
by irradiation with the light source rich in UV-C (λ = 254 nm) than in visible
light. The reason for this is well known from the literature: the band gap
energy of TiO2 is Eg = 3.2 eV (λg = 387 nm),
allowing excitation by light source 1 [27–30]. In the course
of photooxidation by both light sources, samples containing silver or copper
exhibit higher rates than does the reference TiO2 sample. The
composite containing nickel ions degrades ethanol at a higher rate only in the
light of the source rich in UV-C, whereas its efficiency in visible light is
identical with that of the reference sample, P25 TiO2. Efficiencies (Table 2) are calculated from changes in concentration during a 10-minute irradiation
time; ethanol degradation effected by the reference sample Degussa P25 TiO2 is taken as 100%. The activity of the silver-containing TiO2/clay
composite during irradiation with light source 1 is threefold, and that of the
copper-containing composite is twofold
over P25 TiO2. This extraordinary effect is attributed, on the one
hand, to excellent adsorption of ethanol by the layer silicate, allowing fast
sorption of ethanol to the TiO2 surface via diffusion [18]. On the
other hand, the doping silver and copper, presumably present in the system in
the form of silver and copper oxide nanoparticles, respectively [29, 30],
significantly decrease the rate of electron-hole recombination on the TiO2 surface, increasing catalytic activity. This outstandingly high activity,
however, is somewhat decreased in visible light (light source 2), but even in
this case, silver and copper degrade ethanol at rates nearly 1.5-fold faster
than does P25 TiO2. We assume that the reason for this is an
increased photon absorbance in the visible wavelength range (λ = 400–700 nm) on
the composites prepared by us, as demonstrated by the absorbance spectra shown
in Figure 9. The spectrum of Ni/TiO2/clay and Cu-loaded TiO2/clay
displays similar absorbance at wavelengths shorter than 410 nm, while Cu/TiO2/clay
absorbs more photons whose wavelengths are larger than 410 nm, resulting in the
color of light green. The sample of Ag/TiO2/clay possesses an
additional broadened absorption peak at about 450 nm in the visible region, that
indicates the brown color of the deposited silver can improve the light
absorption of Ag/TiO2/clay and increase its photocatalytic activity.
In other words, the efficiency of degradation in visible light can be explained
by the excellent adsorption properties of the layer silicate and the red shift
of photon absorbance at higher wavelengths.Figure 9
Diffuse reflectance UV-Vis spectra
of exchanged Na-montmorillonite/TiO2 composites.


## 4. Conclusions

Photocatalytic utilization of titanium dioxide
and its composites with clay mineral support was studied in a vapor phase
oxidation reaction. Since results on direct doping of TiO2 with
transient metals reported in the literature did not show an unambiguous
increase in efficiency, we used 20% montmorillonite support exchanged with
silver, copper, or nickel ions and mixed/ground with P25 TiO2 in
powder form. The amounts of metal normalized to the total mass of the catalyst
were Ag: 2.16%, Cu: 0.64%, and Ni: 0.59%. The composite obtained was used
for the preparation of films on the surface of the glass reactor, where photooxidation
was carried out. The rate of photooxidation by irradiation with the light
source rich in UV-C (λ = 254 nm) increased significantly as a result of the
introduction of silver or copper included in the layer silicate, and the rate
of the photooxidation process in visible light was also higher than that
measured in the presence of the reference sample P25 TiO2.



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