# Durability of Ag-TiO2 Photocatalysts Assessed for the Degradation of Dichloroacetic Acid

Authors: Víctor M. Menéndez-Flores; Donia Friedmann; Detlef W. Bahnemann
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

The stability of Ag-TiO2 photocatalysts was examined for the photocatalytic degradation of dichloroacetic acid (DCA) as a function of the recycling times. The photocatalytic activity was investigated by measuring the rate of H+ ions released during the photodegradation of DCA and confirmed by measuring the total organic carbon removal. The photodegradation reactions were studied at pH 3 and pH 10 for a series of Ag-TiO2 photocatalysts as different with Ag loadings . All the Ag-TiO2 and bare TiO2 photocatalysts showed a decrease in photocatalytic activity on recycling for the DCA photodegradation reaction. The decrease in activity can be attributed to poisoning of active sites by Cl− anions formed during the photocatalytic DCA degradation.  The photocatalytic activity was, however, easily recovered by a simple washing technique. The reversibility of the poisoning is taken as evidence to support the idea that the recycling of Ag-P25 TiO2 photocatalysts does not have a permanent negative effect on their photocatalytic performance for the degradation of DCA. The choice of the preparation procedure for the Ag-TiO2 photocatalysts is shown to be of significant importance for the observed changes in the photocatalytic activity of the Ag-TiO2 particles.


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## Body

## 1. Introduction

The  presence  of  cocatalytic noble metal
deposits on metal oxide particles is often employed to enhance the photocatalytic
oxidation efficiency of processes through achieving a more efficient charge
separation [1]. Considerable work has been done on Ag-TiO2 modified
photocatalysts and their potential applications as enhanced photocatalysts. The
presence of Ag deposits on the TiO2 surface can help to efficiently
separate the electron hole pairs by attracting the conduction band
photoelectrons. This process has been shown to improve the overall efficiency for
a number of photocatalytic reactions [2]. Ag-TiO2 particles are
typically prepared by photodeposition with the titanium dioxide particles being
suspended in an aqueous solution containing the metal salt. When the suspension
is illuminated, metal ions are reduced by conduction band electrons to form the
metal deposits on the TiO2 surface. This process is described by the
series of reactions (1)–(3). Depending on
the preparation conditions, this photodeposition procedure typically yields
small metal deposits ranging from a few nanometers to tens of nanometers [2] (1)TiO2+hν→TiO2(e−,h+),(2)(TiO2)OH+h+→(TiO2)OH•+,(3)Ag++e−→Ag0. The size and dispersion of metal deposits on
TiO2 particles are critical in controlling their photocatalytic activity.
Given the strong effect of deposit size and dispersion, the development of
preparation methods which provide such close control over deposit size is
essential [3, 4]. An issue that is not often addressed is the stability of
these metal-doped particles and hence the effects, if any, on the longer-term
application potential and shelf-life stability of these particles. This is particularly
true for applications as photocatalysts, where these particles are subjected to
irradiation with UV(A) light. Reformation of surficial metal
clusters when the semiconductor particles are exposed to ultrabandgap
irradiation has often been reported [5, 6]. For example, while cluster
stabilization can be achieved by adsorption of metal clusters onto a solid
surface, it has been shown that the
rate of cluster growth on a quartz surface is accelerated by UV light [5].The stability of metal deposits on
photocatalysts under illumination may depend on a number of factors, such as, for
example, the type of metal, the deposit size, the photocatalytic reactions taking
place, and the reaction conditions such as solution pH. Additionally, the redox
potential can affect the reduction state of the supporting oxide. For example,
the efficiency of the cluster-oxide coupling has been found to strongly depend
on the band bending inTiO2 after silver deposition which in turn
varies with the reduction state of the oxide [7].Despite the large volume of studies focusing on
metal-depositedTiO2 photocatalysts, particularly Ag-TiO2,
the photocatalyst stability and recycling has been addressed only in a few
studies. Reproducibility tests were carried out on
Ag-doped TiO2 films prepared through a 2-step dipping and
illumination process using Degussa P25 as the photocatalyst. These tests proved
that the photocatalytic activity of the silver-modified films for the
degradation of methyl orange remains unchanged even after six consecutive
experiments with newly added pollutant quantities [8]. Xu et al. [9]
have studied recycled 0.5 atom%
Ag-deposited magnetic TiO2−SiO2−Fe3O4 photocatalysts and have also found that the photoreactivity for the degradation
of orange (II) was maintained after 3 recycles. However,
the total irradiation time was only 20 minutes which is relatively short.In a study by
Zhang, Ag-TiO2 photocatalysts were in fact
deactivated during the photocatalytic degradation of acetone in air [10], with a 73% reduction in activity during the fourth hour of
reaction. The deactivation was explained as being due to interactions between
partially oxidized organics and the Ag deposits, and possibly by the
aggregation of the silver nanoparticles, which was reflected in changes in the
absorption spectra of these particles. Regeneration was achieved by visible
light illumination and was attributed to the oxidation of Ag deposits to Ag+ ions, in the form of Ag2O, which were reduced to Ag0 upon
UV illumination. This study raised the issue of the nanometre level control of
feature size and interfeature spacing and the long-term stability of such
structures, hindering their full exploitation for device applications [11].Apart from photocatalytic applications, Sun et al. [12] have studied Ag-TiO2 cyanide sensors prepared by the
photodeposition method which were tested under mild conditions and found to
give good results using 0.1–10 μmol/L
cyanide at pH 9–12. The Ag-TiO2 sensors were found to be effective,
without the need for regeneration for up to one month. It must be said that the
available silver content was greater than the amount of cyanide to be detected.In this study, we have examined the durability
of Ag-TiO2 photocatalysts under practically relevant conditions. The model reaction was the photodegradation of dichloroacetic acid
(DCA). DCA is a known industrial pollutant the photodegradation of which has
been studied extensively, for example, by Bahnemann et al. [13]. Important parameters selected for an initial
exploration were the duration of illumination and the silver content. The poisoning of the photocatalyst surface by
chloride ions was explored and reactivation was investigated using a simple
washing technique. The deactivation of self-prepared colloidal TiO2 was compared to that of the commercially available P25 TiO2 photocatalysts.


## 2. Experimental

### 2.1. Preparation of Ag-TiO2 and Colloidal
TiO2 Photocatalysts

The Ag-TiO2 photocatalysts were
prepared by a photodeposition process. Degussa P25 was used as the
TiO2 photocatalyst. P25 TiO2 consists of 70% anatase and
30% rutile with an average primary particle diameter of 55±5nm and
a BET surface area of 48m2/g. The photochemical experiments were
performed in a reactor (50 mL) with a plain quartz window
on which the light beam (λ≥320nm) was focused. Figure 1 provides a
schematic of the photocatalytic reactor setup. The reactor was equipped with a
magnetic stirring bar, a water-circulating jacket and three openings for pH
electrode, gas supply, and sample withdrawing. The photoreactor was filled with 50 mL of the aqueous suspension containing the TiO2 photocatalyst at a loading of 0.5 g/L. The photocatalyst slurry
was illuminated with UV(A) light for 20 minutes to oxidize any possible organic
carbon impurities on the TiO2 surface.Figure 1
Schematic presentation of the photocatalytic reactor setup.Silver ions in the form of
AgClO4 were then added to the TiO2 slurry at an initial
pH of approximately 6 to obtain the desired Ag+:Ti ratio. The
suspension was illuminated for 24 hours. A series of Ag-TiO2 photocatalysts with a respective Ag loading of 0.1, 0.2, 0.35, 0.5, 1, and 2 atom% was prepared. The samples were labelled as
xAg-TiO2, where x is the amount of Ag+ added given in
atom%.The prepared particles were characterized by electron
microscopy (emission transmission electron microscope JEOL JEM-2100F-UHR and emission
scanning electron microscope JEOL JSM-6700F). The STEM was supported by an
energy dispersive spectroscopic analysis (EDX) and elemental X-ray mapping
images provided information regarding the distribution of the various
components (TiO2 and Ag) and ions (Ti, Ag, Cl) before and after the
photoreaction.ColloidalTiO2 was prepared by a method described by Bahnemann [14] that involved
the dropwise addition of titanium tetraisopropoxide dissolved in 99.99% 2-propanol
to an aqueous hydrochloric acid solution of pH 1.5. The final concentrations of
TiO2 and 2-propanol were 1.5×10−2 and 1.2 M,
respectively. The reaction volume was 1 L. This mixture was stirred overnight employing
a magnetic stirrer. The final solution was optically transparent. Evaporation
under vacuum was used to collect the TiO2 powder which can be
resuspended in water and other polar solvents (e.g., alcohols) to form
transparent colloidal suspensions. UV-Vis absorbance measurements in the
wavelength range 200–700 nm showed the
steep increase in absorption below approximately 350 nm which is typical for nanosized
TiO2. The final TiO2 solution, whose photoactivity was
tested, was prepared by suspending 500 mg of this powder in 1 L of water.


### 2.2. Photocatalytic Degradation of DCA

After the photodeposition
reaction, the Ag-TiO2 photocatalysts were collected and sonicated
for 15 minutes before the addition of 1 mM DCA into the slurry. The pH was then
adjusted to either 3 or 10, by addition of 0.1 M HClO4 or 0.1 M NaOH,
respectively. The temperature of the suspension was maintained at 25C°. The suspension
was vigorously stirred without illumination for 30 minutes to attain adsorption
equilibrium of the DCA molecules on the photocatalyst surface and continuously
purged with air to ensure a constant O2 concentration throughout the
experiment. After this adsorption period, UV illumination was provided by a
high-pressure Xe-lamp (OSRAM HBO-500W). A UV(A)/Vis illumination was achieved employing a band-pass filter (WG 320) which eliminates
UV-radiation below λ=320nm. The intensity
of UV(A) illumination was 20 mW/cm2 at the entrance window of the
photoreactor as measured by a UV light meter (ultraviolet radiometer LT Lutron
UVA-365).ThepKa of DCA is
1.29 [15] and it thus exists in its anionic form in aqueous solutions at pH>2. The photocatalytic oxidation of one DCA anion results in the formation
of one proton, two CO2 molecules, and 2 Cl− ions (4). The
reaction pH was maintained constant using a pH-stat setup with the addition of HClO4 or NaOH as needed. (4)CHCl2COO−+O2⟶hν,TiO22CO2+H++2Cl−. The rate of the
photodegradation of DCA was followed by measuring the amount of OH− added to keep the pH constant and thus the amount of H+ formed using
the pH-stat technique as described by Bahnemann et al., 1993 [16]. Samples were taken at the beginning and at
the end of each run for total organic carbon (TOC) analysis. The reaction time
was 4 hours for each run. In total, 3 consecutive runs were preformed with the
same photocatalyst. 1 mM of DCA was injected at the beginning of each run. For
the third run, 4 mM Cl− was also added to selected experiments in
order to compare and investigate the effect of chloride ions on the photocatalytic
reaction rate.


## 2.1. Preparation of Ag-TiO2 and Colloidal
TiO2 Photocatalysts

The Ag-TiO2 photocatalysts were
prepared by a photodeposition process. Degussa P25 was used as the
TiO2 photocatalyst. P25 TiO2 consists of 70% anatase and
30% rutile with an average primary particle diameter of 55±5nm and
a BET surface area of 48m2/g. The photochemical experiments were
performed in a reactor (50 mL) with a plain quartz window
on which the light beam (λ≥320nm) was focused. Figure 1 provides a
schematic of the photocatalytic reactor setup. The reactor was equipped with a
magnetic stirring bar, a water-circulating jacket and three openings for pH
electrode, gas supply, and sample withdrawing. The photoreactor was filled with 50 mL of the aqueous suspension containing the TiO2 photocatalyst at a loading of 0.5 g/L. The photocatalyst slurry
was illuminated with UV(A) light for 20 minutes to oxidize any possible organic
carbon impurities on the TiO2 surface.Figure 1
Schematic presentation of the photocatalytic reactor setup.Silver ions in the form of
AgClO4 were then added to the TiO2 slurry at an initial
pH of approximately 6 to obtain the desired Ag+:Ti ratio. The
suspension was illuminated for 24 hours. A series of Ag-TiO2 photocatalysts with a respective Ag loading of 0.1, 0.2, 0.35, 0.5, 1, and 2 atom% was prepared. The samples were labelled as
xAg-TiO2, where x is the amount of Ag+ added given in
atom%.The prepared particles were characterized by electron
microscopy (emission transmission electron microscope JEOL JEM-2100F-UHR and emission
scanning electron microscope JEOL JSM-6700F). The STEM was supported by an
energy dispersive spectroscopic analysis (EDX) and elemental X-ray mapping
images provided information regarding the distribution of the various
components (TiO2 and Ag) and ions (Ti, Ag, Cl) before and after the
photoreaction.ColloidalTiO2 was prepared by a method described by Bahnemann [14] that involved
the dropwise addition of titanium tetraisopropoxide dissolved in 99.99% 2-propanol
to an aqueous hydrochloric acid solution of pH 1.5. The final concentrations of
TiO2 and 2-propanol were 1.5×10−2 and 1.2 M,
respectively. The reaction volume was 1 L. This mixture was stirred overnight employing
a magnetic stirrer. The final solution was optically transparent. Evaporation
under vacuum was used to collect the TiO2 powder which can be
resuspended in water and other polar solvents (e.g., alcohols) to form
transparent colloidal suspensions. UV-Vis absorbance measurements in the
wavelength range 200–700 nm showed the
steep increase in absorption below approximately 350 nm which is typical for nanosized
TiO2. The final TiO2 solution, whose photoactivity was
tested, was prepared by suspending 500 mg of this powder in 1 L of water.


## 2.2. Photocatalytic Degradation of DCA

After the photodeposition
reaction, the Ag-TiO2 photocatalysts were collected and sonicated
for 15 minutes before the addition of 1 mM DCA into the slurry. The pH was then
adjusted to either 3 or 10, by addition of 0.1 M HClO4 or 0.1 M NaOH,
respectively. The temperature of the suspension was maintained at 25C°. The suspension
was vigorously stirred without illumination for 30 minutes to attain adsorption
equilibrium of the DCA molecules on the photocatalyst surface and continuously
purged with air to ensure a constant O2 concentration throughout the
experiment. After this adsorption period, UV illumination was provided by a
high-pressure Xe-lamp (OSRAM HBO-500W). A UV(A)/Vis illumination was achieved employing a band-pass filter (WG 320) which eliminates
UV-radiation below λ=320nm. The intensity
of UV(A) illumination was 20 mW/cm2 at the entrance window of the
photoreactor as measured by a UV light meter (ultraviolet radiometer LT Lutron
UVA-365).ThepKa of DCA is
1.29 [15] and it thus exists in its anionic form in aqueous solutions at pH>2. The photocatalytic oxidation of one DCA anion results in the formation
of one proton, two CO2 molecules, and 2 Cl− ions (4). The
reaction pH was maintained constant using a pH-stat setup with the addition of HClO4 or NaOH as needed. (4)CHCl2COO−+O2⟶hν,TiO22CO2+H++2Cl−. The rate of the
photodegradation of DCA was followed by measuring the amount of OH− added to keep the pH constant and thus the amount of H+ formed using
the pH-stat technique as described by Bahnemann et al., 1993 [16]. Samples were taken at the beginning and at
the end of each run for total organic carbon (TOC) analysis. The reaction time
was 4 hours for each run. In total, 3 consecutive runs were preformed with the
same photocatalyst. 1 mM of DCA was injected at the beginning of each run. For
the third run, 4 mM Cl− was also added to selected experiments in
order to compare and investigate the effect of chloride ions on the photocatalytic
reaction rate.


## 3. Results and Discussion

For the
preparation of the Ag-TiO2 photocatalysts, the first indication of
the deposition of
metallic silver onto the TiO2 surface upon irradiation of the TiO2 and Ag+ suspensions was a color change of the particles from white to brown. A higher concentration of silver
ions corresponded to a higher intensity of this brownish color. According to the literature, the photodeposited Ag should be present in
metallic form, based on XRD
analysis of Ag/TiO2 particles prepared using 20.0 atom%Ag+ loading which confirmed the presence of metallic silver [17]. At
lower Ag content, metallic silver could not be detected since the mass of
silver in the sample was below the detection limit of the employed instrument
[17].In Figure2(a), the degradation of 1 mM DCA within
each run (followed as release of H+ ions during the degradation
process) using pure Degussa P25 as the photocatalyst is shown. Three
runs were consecutively carried out and a third run from a second series of experiments
is also presented here with 4 mM Cl− added before the illumination to
evaluate a possible poisoning effect of the photocatalyst’s active sites by chloride
ions. It can be clearly seen that both the reaction rate and yield are reduced
during each consecutive run with the most obvious inhibition being observed
upon the addition of chloride ions.(a) Degradation of DCA (shown
as release ofH+) using the photocatalyst Degussa P25 at
pH 3 in 3
consecutive runs, 1st run (—), 2nd run (–––), 3rd run
(⋯), a 3rd run (-⋅-) with addition
of 4 mM Cl− before the run started, and slope () used
for the determination of the photonic efficiency of each run with I≈3.39×10−2Einstein L−1h−1. The photocatalyst loading was 0.5 g/L. (b) Degradation of DCA (shown as
release of H+) using the photocatalyst Degussa P25at pH 3 in 3 consecutive runs with
intermittent washing between the runs, 1st run (—),
2nd run (–––), 3rd run (⋯), and slope ()
used for the determination of the photonic efficiency of each run with I≈3.39×10−2Einstein L−1h−1. The photocatalyst loading was 0.5 g/L.
(a)(b)As shown in Figure2(b), the
photocatalytic activity of Degussa P25 toward the degradation of DCA is almost
completely recovered once the catalyst is simply washed with pure water before
the next activity test. This clearly indicates the inhibitory effect of Cl− ions that was already obvious from the results presented in Figure 2(a).The effect of recycling the 0.35 Ag-TiO2 photocatalyst on the photocatalytic degradation rate of DCA is shown in Figure 3(a) and once again employing the washing technique in Figure 3(b). The
reaction conditions were the same as described previously for the experiment
with pure P25 TiO2. As can be seen from Figure 3(a), the reaction rate is found to be lowered after
the first run, and is lowered even more during the 3rd run. Once again, the
reaction rate is further reduced when this third run is carried out after the
addition of 4 mM Cl−. Moreover, the reaction seems to stop in this
case after about 20% of the initially present DCA has been degraded. This is
consistent with the respective TOC results shown in Figure 6 (vide infra).(a) Degradation of DCA (shown
as release ofH+) using the photocatalyst 0.35 Ag-TiO2 at
pH 3 in 3
consecutive runs, 1st run (—), 2nd run (–––), 3rd
run (⋯), a 3rd run (-⋅-) with addition
of 4 mM Cl− before the
run started, and slope ()
used for the determination of the photonic efficiency of each run with I≈3.39×10−2Einstein L−1h−1. The photocatalyst loading was 0.5 g/L. (b) Degradation of DCA (shown as release of H+) using the
photocatalyst 0.35 Ag-TiO2 at pH 3 in 3 consecutive runs with
intermittent washing between runs, 1st run (—),
2nd run (–––), 3rd run (⋯), and slope ()
used for the determination of the photonic efficiency with I≈3.39×10−2Einstein L−1h−1. The photocatalyst loading was 0.5 g/L.
(a)(b)In the next set of experiments, the aim was to
investigate whether the negative effect of recycling the Ag-TiO2 photocatalyst can also be avoided by a simple washing technique. Washing of the
photocatalysts was carried out by recovering the material by centrifugation of
the reaction slurry after each run and resuspending it in distilled water,
followed by the appropriate adjustment of pH. The results are shown in Figure 3(b).
As in the case of pure Degussa P25, it is obvious that this simple intermittent
washing technique between each reaction run leads to a remarkable improvement
in activity following the first and second recycles.The results shown in Figures2 and 3 clearly
show that a “clean” surface is a prerequisite for the photocatalyst to exhibit
its full activity independent of the absence or presence of cocatalysts such as
Ag-deposits. The detrimental effect of codissolved ions
such as Cl− is known since they are likely to adsorb onto the TiO2 surface and can disturb the adsorption of the organic compounds [18].The self-prepared colloidalTiO2 photocatalyst was tested with the same intention as pure Degussa P25 and P25-Ag
under the identical experimental conditions. Figure 4 shows the results of
these experiments. Obviously, the reaction rate and yield also decrease in
subsequent runs.  However,  this poisoning effect appears to be much less
pronounced for the colloidal particles as compared with pure or Ag-deposited
Degussa P25. Even with the further addition of 4 mM of Cl− ions before
the third run, no substantial
difference is observed  as
compared with a third run without the  addition of chloride ions. Thus the
capacity of the self-prepared colloidal particles to accommodate chloride ions
without any loss in photocatalytic activity appears to be very high.Figure 4
Degradation of DCA (shown as release ofH+) using the prepared
colloidal TiO2 photocatalyst at pH 3 in 3 consecutive runs without
intermittent washing between runs, 1st run (—),
2nd run (–––), 3rd run (⋯), a 3rd run (-⋅-) with addition
of 4 mM Cl− before the run started, and slope ()
used for the determination of the photonic efficiency of each run with I≈3.39×10−2Einstein L−1h−1. The photocatalyst loading was 0.5 g/L.The results of the TOC analysis performed at
the end of each experimental run, that is after 4 hours of illumination, and
theH+ production efficiency in percent increase at the same time are
presented in Figures 5–7. These figures
provide a summary of the effect of recycling and washing of the photocatalysts with
respect to the mineralization of DCA. The TOC removal results are in fact in good
agreement with the results obtained following the release of H+, evincing
that the simple stoichiometry given in (4) is indeed correct.Figure 5
Removal
of DCA (TOC removal) after 4 hours of illumination using the photocatalyst
P25-TiO2 at pH 3 in 3 consecutive runs (1st run ▪,
2nd run , and 3rd run □),
the H+ production efficiency after 4 hours of illumination is also
shown (1st run ,
2nd run , and 3rd run ) in set A,
with chloride ion addition before the third run (set B), and with
intermittent washing between runs (set C). The photocatalyst loading was 0.5 g/L
and the light intensity I≈3.39×10−2Einstein L−1h−1.Figure 6
Removal
of DCA (TOC removal) after 4 hours of illumination using the photocatalyst 0.35
Ag-TiO2 at pH 3 in 3 consecutive runs (1st run ▪,
2nd run , and 3rd run □), the H+ production efficiency after 4 hours of illumination is also shown (1st run ,
2nd run , and 3rd run ) in set A,
with chloride ion addition in set B before the third run and with
intermittent washing between runs (set C). The photocatalyst loading was 0.5 g/L
and the light intensity I≈3.39×10−2Einstein L−1h−1.Figure 7
Removal
DCA (TOC removal) after 4 hours of illumination using the colloidal-TiO2 photocatalyst at pH 3 in 3 consecutive runs (1st run ▪,
2nd run , and 3rd run □) the H+ production efficiency after 4 hours of illumination is also shown (1st run ,
2nd run , and 3rd run ) in set A
and with chloride ion addition before the third run (set B). The photocatalyst loading
was 0.5 g/L and the light intensity I≈3.39×10−2Einstein L−1h−1.In Figure5, the results for pure Degussa P25 are
shown. The gradual decline in photoactivity following each recycle can be seen
from set A for the consecutive runs. In set B, the strong detrimental effect of
chloride ion addition before run 3 can be clearly seen. In set C, the photocatalyst
was collected after each run by centrifugation, resuspended in distilled water
at the correct pH and placed back in the reactor. However, for the pure P25
photocatalyst, the recovery of the photoactivity was not complete. Comparing
the results obtained from set A with those of set C, however, it is obvious
that the photoactivity after the washing technique is slightly higher than that
observed without washing.The results for the TOC removal using the 0.35
Ag-TiO2 photocatalyst are presented in Figure 6. These are also in
agreement with the DCA degradation results shown in Figures 3(a) and 3(b). The
results of set C show the successful application of this simple washing
technique resulting in an almost complete recovery of the photocatalytic
activity. While the amount of DCA degraded after 4 hours of illumination even
seems to slightly increase after each illumination/washing cycle (Figure 6, set
C), the initial rate of degradation is found to decrease (Figure 3(b)), in
particular, for the second run. In comparison with the results obtained for
pure Degussa P25, however, it is obvious that the photocatalytic activity of
Ag-TiO2 can be recovered much more rapidly by this simple
washing procedure. As will be shown below (in Figures 13 and 14), this might be
due to the fact that the chloride ions show a strong tendency to adsorb at the
silver clusters from where they can obviously be removed much easily than from
the bare TiO2 surface.For the colloidalTiO2 particles, the
analysis of the TOC removal after 4 hours of illumination also showed the
durability of this photocatalyst (the results are shown in Figure 7). In fact,
this photocatalyst showed a greater resilience to deactivation compared to pure
Degussa P25 and to the Ag-TiO2 photocatalysts. This is believed to
be due to the greater number of active sites provided by this photocatalyst for
the photocatalytic reaction, hence minimizing the observed poisoning by the
chloride ions. It is important to note that due to its colloidal nature, this
photocatalyst could not be separated by centrifugation and hence it could not
be washed between the recycles. Both its advantages and drawbacks need to be
taken into account when considering its application under real conditions.In the next set of experiments, the effect of
catalyst recycling on the photocatalytic DCA-degradation rates was examined for
a series of Ag-TiO2 photocatalysts varying the
amount of deposited Ag. These degradation experiments were carried out both under acidic (pH 3) and under basic (pH 10)
conditions. The aim was to identify reaction conditions or photocatalyst
composition, where the observed reduction in photonic efficiency due to
recycling can be minimized. Figure 8 provides a summary for the results
obtained for the measured photonic efficiencies (based on the initial degradation
rate of DCA as measured by the release of H+).Figure 8
Effect of recycling Ag-TiO2 photocatalysts on observed photonic efficiencies at pH 3 and pH 10
(1st run ▪,
2nd run , and 3rd run □), degradation of 1 mM DCA
without any extra addition of chloride ions or washing technique performed. The photocatalyst
concentration was 0.5 g/L with different silver loadings (atom%).
The light intensity at pH 3 was I≈3.39×10−2Einstein L−1h−1 and at pH 10 was I≈3.74×10−2Einstein L−1h−1.The photonic efficiencyζis defined as the ratio of the initial
degradation rate and the incident photon flux (see (5)),  where the initial
photocatalytic rate is calculated from the slope of the individual
concentration versus time profiles. This definition of the photonic efficiency
was first suggested by Serpone and Salinaro [19].
The incident photon flow per volumetric unit (I varied between 3.39×10−2 Einstein L−1h−1 and 3.74×10−2 Einstein L−1h−1) was calculated based upon the UV-A light
meter measurements [20], the irradiated surface area of 8.042cm2, and
the volume of the suspension (5×10−2L). (5)J0=I⋅λNA⋅h⋅c,ζ(%)=k⋅c0⋅VJ0⋅A⋅100 with J0= light flux [mol*s−1cm−2];
I= light intensity [J*s−1cm−2]; NA= Avogadro’s number [6.022⋅1023mol−1]; h= Planck constant [6.63⋅10−34Js]; c= light velocity [3⋅108m*s−1];
k= initial
rate constant [s−1]; A= area [cm2]; C0=initialDCA concentration [mol*L−1];  λ=
wavelength [nm];  V= volume [L] .In Figure8, the photonic efficiency measured
for each run is presented, comparing the fresh photocatalyst with the recycled ones
after the second and third runs. Figure 8 shows overall higher rates during the
first run at pH 3 compared to pH 10 both, for pure Degussa P25 and when silver
is photodeposited. This DCA degradation trend, although performed with a
different photocatalyst, is in agreement with earlier studies reported by
Bahnemann et al. [13], who
attributed the faster
rates observed under acidic conditions (pH 2.6) to the favorable DCA coordination through the
formation of bidentate complexes that could not be formed in alkaline medium at
pH 11.3. The absolute value of the photonic efficiency for the pure P25
photocatalyst obtained here is slightly lower than that reported previously [21],
where a photonic efficiency ζ=3.9% was measured at pH 3 under similar conditions.
This could, for example, be explained by a variation of the activity of such a
commercial product between different production batches which are well known
for most catalysts. It is interesting to note the influence of the presence of
chloride ions in the suspension. With the presence of 8 mM NaCl (2×2mM for
runs 1 and 2 plus 4 mM added before run 3) prior to run 3 the efficiency
decreases by approximately 50%, that is, the photocatalytic activity can be
considerably inhibited by the presence of anions such as chloride. A similar
yet less pronounced effect has been reported previously [21].In Table1, all photonic efficiency
values are presented that have been calculated using (5) for each
experiment. Previous reports ([21], Figure 4), concerning the photonic
efficiency of the degradation of DCA in the presence of P25, are consistent
with the photonic efficiency values obtained in this work. Here, the photonic efficiency
was found to be  between 2.83 and 3.69% at pH 3 (cf. ζ=3.9% at pH 3 [21]). Also, a decrease of the
photonic efficiency was previously noted at pH 10 (cf. ζ=0.6% at pH 10 [21]). The results obtained
here concerning the influence of Cl− can be compared with a set of
experiments carried out previously also [21], where the efficiency was reported
to decrease from ζ=3.8% with the number of subsequent experiments
to ζ=2.0% during run 3 and ζ=0.9% during run 6. Once again, the photonic
efficiency recovered after washing of the photocatalyst (reaching ζ=2.2% [21]) as confirmed in the present
results.Table 1
Photonic efficiency values of the employed photocatalysts (e.g., as given in Figures2 and 3).
pH 3pH 10PhotocatalystRateIntensityPhotonicRateIntensityPhotonic[0.5 g/L][mol/Lh][mol/Lh]Efficiency (%)[mol/Lh][mol/Lh]Efficiency (%)1.00E-041.00E-04Run 1Run 2Run 3Run 1Run 2Run 3Run 1Run 2Run 3Run 1Run 2Run 3P259.583.262.653.39E-022.830.960.783.902.601.703.74E-021.040.700.45P25-Ag%0.16.004.302.603.39E-021.771.270.774.862.391.953.74E-021.300.640.520.211.423.483.003.39E-023.371.030.892.841.841.243.74E-020.760.490.330.358.506.403.003.39E-022.511.890.892.160.950.953.74E-020.580.250.250.57.223.883.333.39E-022.131.151.984.203.202.593.74E-021.120.850.6915.353.903.533.39E-021.581.151.045.612.782.003.74E-021.500.740.5325.263.843.743.39E-021.551.131.103.122.161.483.74E-020.830.580.400.358.035.452.353.39E-022.371.610.69(4 mM Cl)0.3512.506.178.543.39E-023.691.822.52WashedP259.323.140.753.39E-022.750.930.22before 3rd run(4 mM Cl)P2512.309.379.313.39E-023.632.772.75WashedTiO211.409.076.733.39E-023.372.681.99colloidTiO2 colloid11.208.926.443.39E-023.312.631.90before 3rd run(4 mM Cl)A
closer inspection of the results shown in Figure8 reveals that while at pH 10
there is no clear relationship between the observed photonic efficiency and the
absence or presence of Ag on the photocatalyst, at pH 3, the fresh 0.2 Ag-TiO2 photocatalyst slightly outperforms pure Degussa P25. For the second run,
however, the situation has changed with 0.35 Ag-TiO2 now being the
best photocatalyst for the DCA degradation. Finally, in the third run all
tested catalysts exhibit almost the same photocatalytic activity (ζ≤1%). Apparently, the Cl− ions formed during the photocatalytic oxidation of dichloroacetate are able to
completely “neutralize” any enhancement in activity induced by the Ag clusters.
Based on the experimental evidence given in Figures 13 and 14 below, this is
explained by the preferred adsorption of chloride ions at these silver
clusters.In Figure9, a summary is provided of  the
results obtained for the measured TOC removal after 4 hours of illumination for
the same experiments shown in Figure 8. Figure 9 shows the TOC removal measured
for each run, comparing the fresh catalysts with the recycled photocatalysts
after the third run. Again, the results show the decline in activity on
recycling, with no particular trend with regard to the amount of silver
deposited or to the reaction pH. It is clear from both, the results in Figures
8 and 9, that while there is an optimum silver loading at 0.2 atom% at pH 3,
this 0.2 Ag-TiO2 photocatalyst has no clear advantage compared to
the other Ag-TiO2 samples after recycling. Finally, it should be
noted that while the initial degradation rate (used to calculate the ζ-values presented in Figure 8) seems to be
rather sensitive to the presence of the Ag-clusters (at least at pH 3), there is
no such dependence observed for the overall reaction yield after 4 hours of
illumination (cf. Figure 9).Figure 9
Effect of recycling Ag-TiO2 photocatalysts
on observed TOC removal at pH 3 and pH 10 (1st run ▪,
2nd run , and 3rd run □), degradation of 1 mM DCA
without any extra addition of chloride ions or washing technique performed.
The light intensity at pH 3 was I≈3.39×10−2 Einstein L−1h−1 and at pH 10 was I≈3.74×10−2Einstein L−1h−1.At pH 10, all the Ag-TiO2 photocatalysts
showed a similar performance regardless of silver content. Given that P25 TiO2 and Ag-TiO2 photocatalysts had a similar photoactivity after
recycling, this study demonstrates that at least for the photodegradation of
DCA, there was no advantage in depositing Ag on the TiO2 surface if
the intention is to recycle the photocatalyst with no intermittent washing. For
practical applications, where recycling is part of a normal assumed procedure [22],
this finding is of great consequence.TEM analysis of the photocatalysts was carried out with the aim to
examine if any visible morphological changes had occurred to the photocatalyst
samples after the second recycle. The TEM analysis was combined with an
EDXS-REM from which elemental maps of Ti, Ag, and Cl were obtained to observe their
distribution in the samples. As a blank comparison, Figure10 presents an EDX spectrum of Degussa P25 before the
photodeposition of silver or the photodegradation of DCA. Figure 11 shows the same sample after Ag was deposited on the
surface. The results are presented
for the 0.2 Ag-TiO2 sample which showed the highest initial activity
at pH 3 and the greatest decline in performance after 2 recycles. Figure 12
shows the EDX spectrum after the photodegradation of DCA in three consecutive
runs. The presence of Cl− ions detected, indicative of the poisoning
of the photocatalyst surface.Figure 10
EDXS-spectrum of Degussa P25 prior to the photodeposition of silver or the
photodegradation of DCA.Figure 11
EDXS
spectrum of 0.2 Ag-TiO2 photocatalyst prior to the photodegradation
of DCA.Figure 12
EDXS
spectrum of 0.2 Ag-TiO2 photocatalyst after photodegradation of DCA
in three consecutive runs.Electron microscopy
analysis of 0.2 Ag-TiO2 photocatalyst particles before the DCA
photodegradation reaction. (a) STEM image of particles, (b) corresponding Ti X-ray
map, and (c) corresponding Ag X-ray map.
(a)(b)(c)Electron microscopy analysis of 0.2
Ag-TiO2 photocatalyst particles after the DCA photodegradation
reaction. (a) STEM image of particles, (b) corresponding Ag X-ray map,
and (c) corresponding Cl X-ray map.
(a)(b)(c)In Figure13, the STEM image and the
elemental maps of Ti and Ag in the 0.2 Ag-TiO2 are shown. These show
the distinct silver deposits present in the samples. The size of the silver
deposits in 0.2 Ag-TiO2 sample is larger than that typically
presented in the literature [2]. This is believed to be due to the
comparatively long irradiation time of 24 hours during the photodeposition
reaction. Figure 14 shows the STEM image of the same 0.2 Ag-TiO2 photocatalyst sample after the DCA photodegradation reaction. The corresponding
X-ray maps of Ag and Cl confirm the presence of the Cl− ions and the
close association of Ag with the Cl− ions.Based on the above electron
microscopy results, it is clear that following the DCA degradation, there is a
considerable amount ofCl− ions on the photocatalysts surface.It is believed that the chloride
ions are merely physically adsorbed onto the surface of the photocatalyst
rather than reacting with the silver deposits. This was supported by the fact
that the photocatalysts were found to regain their activity by a simple washing
procedure. When considering the
preparation of the Ag-TiO2 particles by photodeposition, the silver
salt (AgClO4) was added at slightly acidic pH (pH 6) and the
titration to the reaction pH followed afterward. Hence at pH 10, the silver
ions are expected to stay adsorbed on the surface and there will be hardly any
external formation of AgOH precipitates. Similarly, at pH 3 the silver ions are
expected to remain adsorbed onto the TiO2 surface. This is believed
to have minimized the reaction between Cl− ions and Ag+ or AgOH as the photodegradation reaction proceeded.If, on the other hand, the titration of the
titania to pH 3 or pH 10 was to be done before the silver was added, different
results would have been expected. At pH 3, there would have been a lower amount
of silver adsorbed onto the titania surface since bothAg+ and the
titania surface are positively charged. At pH 10, AgOH will most certainly form
adjacent to the titania particles. When Cl− anions are formed in solution, these are expected to
react with the Ag+ ions or AgOH to form the silver halides (silver
chloride has a solubility of 0.8 ppm (Ksp(AgCl)=1.6×10−10)
which is pH independent, so most of the silver chloride would have precipitated). Upon UV exposure,
the silver salts/compounds are
expected to be reduced to silver atoms (as in the photographic process). This would have quite a significant impact on
the photolytic formation of the silver deposits on the titania surfaces and the
subsequent photocatalytic behavior of the material.It is also often reported that small
photodeposited Ag particles grow due to Oswald ripening as the photodeposition
reaction proceeds. For example, at a silver loading of  0.39 wt%, after 60 seconds
of irradiation, the size of the Ag deposits was 3 nm which increased as the
irradiation time was doubled with some larger Ag deposits (23 nm) being formed [23]. TEM analysis
was carried out on the 0.2 Ag-TiO2 photocatalysts before and after
recycling to check, if
the silver deposits were in fact reforming after multiple exposures. The results presented in Figure 15
demonstrate that there was no observable change in the size of the Ag deposits.
This was as expected since the preparation process allowed 24 hours of
illumination for the deposition of the silver particles. As mentioned earlier,
the initial Ag deposits were in fact much larger than Ag-TiO2 photocatalysts which are typically studied. Such studies often employ much
shorter irradiation times (30 minutes to few hours) [2]. For our case, the
intention was to mimic real conditions which in fact can involve long periods
of illumination as several pilot plant studies on photocatalysis have
demonstrated [18, 22]. Hence
the procedure for preparing the Ag-TiO2 photocatalyst particles is
critical in determining the nature of the Ag-TiO2 particles that are
formed and the subsequent changes and/or interactions which may occur as the
photocatalytic reaction proceeds under real conditions.TEM images
of 0.2 Ag-TiO2 photocatalyst (a) before and (b) after recycle.
(a)(b)


## 4. Conclusions

In this study, it was possible to demonstrate
the drop in photocatalytic activity after 3 consecutive runs for the
degradation of DCA, using Ag-TiO2 photocatalysts under both acidic
and basic conditions. The aim was to test these photocatalysts under realistic
process conditions which may involve photocatalyst recycle, and extended
periods of illumination (up to 12 hours). Interestingly, a minimal reduction in activity was observed for high-surface area,
colloidal TiO2. This is
believed to be due to the greater number of active sites in this sample which
made it more resilient to poisoning by the released chloride ions during the
photodegradation of DCA.The photocatalytic activity was easily
recovered by a simple washing technique, which involved the collection of the
photocatalyst particles by centrifugation and their resuspension in a chloride-free
solution. The reversibility of the poisoning by the chloride ions provides
evidence to the assumption  that the recycling of Ag-P25TiO2 photocatalysts
(for a total 12 hours of illumination) does not have a permanent negative
effect on their photocatalytic performance for the degradation of DCA. This was
clearly affected by the choice of preparation procedure which minimized the
interaction of the chloride ions with free Ag+ ions or AgOH
precipitates in solution, hence avoiding the formation of AgCl. The formation
of AgCl in the system would have lead to complex photolytic reactions, whereby
the overall performance for the photodegradation of DCA would have been
difficult to predict.



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*Source: 280513-2008-10-27.xml*