1

PHOTOCATALYTIC REACTIONS OVER TiO2 SUPPORTED ON PORCELAIN SPHERES

GABRIELA PIPERATA, JORGE M. MEICHTRY AND MARTA I. LITTER∗

Unidad de Actividad Química, Centro Atómico Constituyentes, Comisión Nacional de Energía Atómica, Buenos Aires, Argentina

Av. Gral. Paz 1499, 1650 San Martín, Prov. de Buenos Aires, Argentina 541167727016 541167727886

litter@cnea.gov.ar

Abstract

Results of immobilisation of TiO2 on low-cost porcelain beads and the resulting photocatalytic

activities of the samples are presented. A very simple technique for immobilisation by immersion

in a TiO2 suspension, previously acidified to pH 2.5, followed by calcination at 450 oC, was used.

After a vigorous washing with a water jet, no detachment of the catalyst was observed at the end of

the fixation process. Reasons for the stability of the material are discussed. The crystallographic

nature of P-25 was not affected after attachment to the porcelain surface.

The beads were submitted for 100 h to turbulent regimes corresponding to Re numbers of 100 and

400 in a recirculating system. The effect of the recirculation flow was carefully examined to

evaluate the degree of detachment of the catalyst from the support and to assess how the treatment

influences on the activity of the supported catalyst. For this, photocatalytic tests were performed

on two model compounds, 4-chlorophenol and potassium dichromate in the presence of

ethylenediaminetetraacetic acid, and results were compared with those of untreated beads. A good

activity remained in the beads even after washing for more than 100 hours in a turbulent regime

(both Re 100 and 400). The activity of particles detached from the support by ultrasonication was

also compared with that of the pure precursor.

Key words: supported TiO2, heterogeneous photocatalysis, porcelain beads

Introduction

Heterogeneous photocatalysis (HP) for disinfection and detoxification of effluents

is an emergent technology already applied in industrialised countries [1, 2 and

references therein]. By irradiation of a semiconductor with UV light (below 400

∗ Corresponding author

2

nm), electron-hole pairs are created that can react with acceptors and donors

present in the suspension. Active species such as HO• radicals are formed,

provoking mineralization of organic pollutants [3]. Even toxic ionic metals can be

transformed in less noxious species [4].

So far, the most used photocatalyst is TiO2, especially in its commercial Degussa

P-25 powdered form. One of the most important drawbacks to achieve the

successful implementation of HP is that the photocatalyst must be separated from

the system after the treatment, and the cost of this separation stage may even

invalidate economically the technology [5, 6]. To solve this problem, several

attempts to immobilise TiO2 have been performed. For the immobilisation, two

techniques can be envisaged [6-9]: i) “in situ” synthesis of the oxide on the

support by a sol-gel, CVD technique or others; ii) direct impregnation of a

commercial or previously prepared TiO2 powder on the surface of the support.

The second one is the easiest and most economical procedure. The fixation is

achieved starting from a TiO2 suspension in an adequate dispersant followed by

sinterization, in some cases with the addition of ligand agents [10-12]. The

process by which the particles are fixed is not totally clear. Probably, electrostatic

interactions between particles and charge surfaces are involved, although covalent

interactions cannot be discarded when the adherence is extremely strong (for

example, with glass). There is no agreement on which method produces films with

better photocatalytic activity, and this depends generally on the nature and

experimental conditions of the system [6].

There is an important variety of materials that can be used to support TiO2. The

most studied are the ones based on SiO2, owing to their low cost and easy

availability [6]. The success of these coatings is based on the large adherence

between TiO2 and glass, attributed to the occurrence of some sintering in the

calcination process [13]. The thermal treatment is limited by the glass softening

temperature that, in the case of borosilicate, is next to 500 ºC. To overcome this

limitation, more resistant and then more expensive glasses must be used. Other

support materials include metals, ceramics, zeolites, plastics, cements, red bricks

and inorganic fibres [5, 7, 14-19].

The first prototypes of photoreactors with supported TiO2 were constructed by

simply covering the internal wall of glass tubes with powdered TiO2 [20-21]. To

have more surface area, beads, rings or glass fibres inside the tubes have been

3

filled [5]. These materials have been successfully used in commercial devices for

air purification [22]. Conversely to gas systems, the catalyst has been found

mechanically unstable in aqueous systems, generally submitted to strong abrasion

and turbulent regimes, which cause removal of the TiO2 coating with the time of

use. The geometrical shape of the support is a very important parameter for

fixation: whereas the fixation to a flat or cylindrical surface yields rather good

permanent adhesion, glass spherical shapes produce poor results, and TiO2 is

rapidly detached.

As it is known, the surface area as well as the nature of the TiO2 phase strongly

influences photocatalytic activity. Changes in the electronic and crystalline

structure of TiO2 due to interaction with the support may result in alteration of the

efficiency. The attachment of TiO2 to a surface decreases considerably the

available surface area and the amount of light reaching the particles. Also, mass

transfer limitations due to a poorer access or contact of the pollutant with the

catalyst are usually invoked. As a general behaviour, supported TiO2 is always

less active than suspended TiO2.

In this work, we discuss results of immobilisation of TiO2 on low-cost small

porcelain spheres where TiO2 has been impregnated by a simple technique. The

degree of detachment of the catalyst from the surface and the photocatalytic

activity on model compounds as 4-chlorophenol (4-CP) and potassium dichromate

(Cr(VI)) have been evaluated.

Experimental section

Materials and equipment

As support for TiO2, commercial porcelain beads according to 40685 DIN

Standard, Group 100 (Del Morro S.A., Argentina), 6 mm diameter were used.

Basically, this porcelain is a silicoaluminate containing vestiges of potassium.

TiO2 Degussa P-25 was a commercial sample, used as provided. K2Cr2O7 (Carlo

Erba), 4-CP (Fluka), Na2EDTA (Mallinkrodt) and all other chemicals were of

analytical reagent grade. Doubly distilled water was used for the preparation of all

solutions and for final washings. Diluted HClO4 was used for pH adjustments.

X-ray diffraction (XRD) patterns were obtained at room temperature with a

Philips PW-3710 diffractometer, using CuKα radiation.

4

Spectrophotometric measurements were performed with a Hewlett-Packard diode

array UV-visible spectrophotometer, model HP 8453 A.

Ultrasonications were performed with a Cleanson (25 KHz), ultrasonicator, model

CS-1109.

Impregnation procedure

Beads were impregnated with TiO2 according to the following procedure. Spheres

were introduced in a 0.1 M HNO3 solution for 2 h, then washed four times with

water and dried at 100 ºC. This step was followed by immersion in a 1M KOH

solution for 48 h, washing four times with water and drying again at 100 ºC. A

10% w/v TiO2 suspension, previously adjusted at pH 2.5 with HClO4, was

ultrasonicated for 30 min and the spheres were immersed in the suspension for 15

min. The excess of suspension was drained and the material was dried at 100 ºC

and calcined at 450 ºC for 2 h at a 3.3 ºC min-1 heating rate. At the end, the

spheres were rinsed for 15-30 min with a vigorous water jet to eliminate loose

particles, and dried at 100 ºC. The impregnation procedure was repeated six times.

Evaluation of the abrasion resistance

The mechanical stability of TiO2 on the porcelain spheres was tested by washing

with bidistilled water for more than 100 hours. For this sake, a recirculation set-up

was used, consisting of a 30-mm diameter, 140-mm length glass tube, in which

230 spheres were arranged as a fixed bed. From a reservoir, 500 mL of distilled

water was continuously recirculated by means of a peristaltic pump. Two

recirculation rates, 0.6 L min-1 and 2.4 L min-1, corresponding to Re 100 and Re

400 regimes, were tested. The Reynolds number for spherical beads can be

calculated according to [23]:

Re = Dp ρ v0 µ-1 (1)

where:

Dp: beads diameter

ρ: fluid density

v0: flow rate in an empty bed

µ: fluid viscosity

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For this geometry and configuration, a turbulent flow is defined for Re > 40.

Therefore, both recirculation rates correspond to turbulent regimes.

Periodically, samples of washings were taken and analysed

spectrophotometrically against pure water to evaluate the bleeding of TiO2 by

changes in the UV region due to the scattering caused by TiO2 particles. After

around 100 h of recirculation, a 45-mL sample was filtered through a 0.2 µm

Sartorius filter membrane and the membrane was analysed by XRD for a possible

solid residue.

Hereafter in this work, beads will be named “uncoated” (without TiO2),

“untreated” (with TiO2 but not submitted to the recirculation treatment), “Re 100”

and “Re 400”, respectively.

To estimate the amount of TiO2 attached to the beads, 21 of them were

ultrasonicated (25 KHz) for 30 min in 10 mL of bidistilled water. Ultrasonication

at this frequency did not provoke any deterioration of the beads. The beads were

withdrawn from the suspension, rinsed and dried at 100 ºC, and the mass of TiO2

was calculated from differences in weight before and after the detachment. This

procedure was repeated three times and the results averaged. The amount of TiO2

deposited on untreated beads was 2.1 mg TiO2/g spheres (≈ 0.8 mg TiO2/cm2).

For “Re 100”, 1.7 mg TiO2/g spheres was calculated (≈ 0.7 mg TiO2/cm2) and for

Re 400, 0.6 mg TiO2/g spheres (≈ 0.3 mg TiO2/cm2). The suspensions obtained

from the ultrasonication procedure were used for photocatalytic experiments, as

described later.

Photocatalytic tests

Photocatalytic irradiations were carried out in a 30-mL cylindrical cell, open to

air, irradiated from the top, using high-pressure xenon arc lamps (Osram XBO,

150 or 450 W). The set-up was thermostatted at 25 ºC. The cell was provided with

a 300-nm cut off filter, avoiding wavelengths lower than 300 nm. The IR fraction

of the incident light was removed by a 50-mm cylindrical water filter.

The solution containing the model pollutant (4-CP or Cr(VI)) at a fixed

concentration and pH, together with TiO2, either supported or in suspension, was

introduced in the cell. In the case of supported TiO2, a layer of 21 beads was

evenly arranged on the bottom of the cell. When suspended TiO2 was used, the

suspension was first ultrasonicated for 15 min and magnetically stirred during the

6

run. Prior to irradiation, solutions or suspensions were kept in the dark for 30 min,

to assure substrate-surface equilibrium. No significant differences in

concentrations were observed after this dark period. Once the irradiation was

initiated, periodical 500 µL samples were taken and diluted to 5 mL for analysis.

Suspensions were filtered through a 0.45 µm MSF filter before analysis. At least,

duplicated runs were carried out for each condition, averaging the results. Photon

flows per unit volume were determined with ferrioxalate [24], using the same

volume of actinometric solution as those used in the experiments to keep the same

conditions.

For 4-chlorophenol, 25 mL (corresponding to a 2.8-cm height of the solution in

the cell) of a 0.2 mM solution at initial pH 3 were irradiated. 4-CP degradation

was followed by the decrease of the peak at 225 nm. During the photocatalytic

reaction, some intermediates (phenol, hydroquinone, among others) can be

formed. Under our conditions, the amount of them is very low and their

absorption coefficient at 225 nm is lower than that of 4-CP [25, 26]. This validates

our measurement methodology.

For chromium (VI) experiments, 20 mL (corresponding to a 2.5-cm height of the

solution in the cell) of 0.4 mM K2Cr2O7, together with 0.9 mM Na2EDTA as a

sacrificial synergetic agent [4], was used. Initial pH was adjusted to 2. Cr(VI)

measurement was performed spectrophotometrically at 249 nm [27].

The concentration of TiO2 in the different experiments is indicated in the legends

of the figures. For the case of supported TiO2, the values correspond to equivalent

concentrations in suspension, calculated from the estimated amount of TiO2 in the

beads.

Results and discussion

Preparation of the impregnated beads

A very simple procedure was used to impregnate the porcelain beads with TiO2. A

pretreatment with HNO3 solution (pH 1) for 2 h, allowed to eliminate any

previous impurity of the surface to improve further treatments. As the surface of

beads was very smooth, an alkaline treatment on the porcelain by immersion in

KOH solution was done. A 10% w/v TiO2 suspension, adjusted at pH 2.5 with

HClO4, was ultrasonicated to break possible aggregates. The supports were

7

introduced then in this suspension and kept for a time as short as 15 min. At this

pH value and TiO2 concentration, the deposition of the catalyst is facilitated by

electrostatic attraction. Use of another mineral acid than HClO4 could have left

anions on the surface that would have reduced the photocatalytic activity of the

semiconductor [12]. Calcination at 450 ºC for 2 h at a controlled temperature

ramp was performed. In this way, beads were resistant to crack. Finally, the

material was rinsed vigorously with water to eliminate loose particles. Previous

experiments in our laboratory had shown that the activity of the beads were higher

when two TiO2 layers were applied compared to only one impregnation. In this

work, we decided to apply 6 layers, but this number should be still optimised.

The impregnation treatment did not affect the crystallographic composition of the

TiO2 coated onto the spheres, as it will be discussed later.

Evaluation of the abrasion resistance under different recirculation regimes

The adherence of TiO2 to porcelain spheres was tested by washing with bidistilled

water in two vigorous turbulent regimes for more than 100 hours. After this

treatment, washings were analysed for the presence of TiO2 particles,

spectrophotometrically as well as by DRX after filtration. Concerning “Re 100”,

no TiO2 was observed in any case in the washings or as a solid residue, but

actually, some detachment was detected by difference of weight before and after

treatment in the turbulent regime (2.1 against 1.7 mg TiO2/g spheres). In the case

of “Re 400”, a much lower amount of TiO2 remained after the treatment (0.6 mg

TiO2/g spheres) and a very weak but visible peak of anatase was detected by DRX

as a solid residue in the membrane used for filtration.

It was verified that neither the turbulent recirculation nor the impregnation

procedure had affected the crystallographic composition of the coated TiO2. For

this sake, Re 100 beads were submitted to ultrasonication for 30 min and a DRX

spectrum of the detached TiO2 powder was obtained. Comparison of this spectrum

with that of pure P-25 (not shown), indicated that both samples presented a similar

anatase:rutile ratio (74% anatase), calculated from the intensity of the main

diffraction peaks ((101) anatase and (110) rutile), according to ref. [28].

8

On the other hand, it was proven that on this type of beads, made of porcelain,

TiO2 presented a higher adherence than that on Pyrex glass spheres, tested

previously by us in preliminary experiments.

Photocatalytic experiments

To evaluate the activity of the coated beads, two photocatalytic model systems,

tested previously in our and other laboratories were used: a) degradation of 4-CP;

b) reduction of Cr(VI) in the presence of excess of ethylenediaminetetraacetic acid

(EDTA). No attempts to compare the results of the different experiments after

normalization by TiO2 mass have been made.

Figure 1 shows 4-CP degradation with different TiO2-containing beads at pH 3.

Untreated beads as well as beads submitted to both Re regimes are compared,

together with TiO2 in suspension at the same concentration as in the untreated

beads (0.7 g l-1). As expected, immobilized TiO2 presented lower efficiency than

that of the suspended catalyst at the same concentration. As it can be seen,

although the activity is decreased after the turbulent recirculation treatment, it is

worthwhile to remark that the treatment was prolonged for more than 100 h. The

activity loss was higher with the highest Re number. Of course, the amount of

TiO2 in the reaction volume was also lower due to the previous detachment.

However, both Re 100 and Re 400 beads show still some efficiency.

⟨Figure 1⟩ As the photocatalytic degradation of 4-CP with immobilised TiO2 was found a

slow system to compare the activities of the samples, another very well known

and faster photocatalytic reaction, reduction of Cr(VI) in the presence of EDTA at

pH 2 [4], was tested. Results are shown in Figure 2. In the absence of TiO2, a

minor photochemical reaction between Cr(VI) and EDTA takes place. The results

were similar to those obtained for 4-CP, Cr(VI) reduction efficiency decreasing in

immobilised TiO2 compared to that in suspension and in “Re 100” and “Re 400”

with respect to “untreated”.

⟨Figure 2⟩ Activities of “Re 100” and a suspension prepared from TiO2 detached from “Re

100” by ultrasonication are compared in Figure 3, together with a P-25 suspension

of the same concentration. As expected, due to a lower surface area, mass transfer

9

limitations and a lower amount of light reaching the beads only from the top, the

efficiency was lower for the immobilised catalyst. It must be taken into account

also that suspensions were magnetically stirred in contrast with experiments done

with beads. TiO2 detached from “Re 100” shows a lower activity compared with

that of the precursor suspension. This lower activity can be explained by the

presence of some impurities coming from the support [17].

⟨Figure 3⟩ Same experiments were performed using “Re 400” and results are presented in

Figure 4. As in the previous case, fixed TiO2 presents a lower activity than TiO2

in suspension. It is remarkable that here the activity of detached TiO2 is almost

identical to the activity of P-25. We think that a more profound washing than in

“Re 100” has removed impurities, increasing the activity.

⟨Figure 4⟩

Conclusions

Our results indicate that a very good adherence of TiO2 is obtained onto this

porcelain beads compared with Pyrex glass spheres. This aspect is important as

this is one of the main drawbacks when spherical geometries are used to fix the

catalyst by impregnation. We have then obtained a cost effective material by a

very simple technique, in comparison with sol-gel or other preparative synthesis

of TiO2 on the support.

We have noticed that the adherence of TiO2 to the support is affected by a

turbulent regime in a recirculation system. However, Reynolds number

corresponding to 100 would be still suitable for application in a fixed bed reactor

without significant detachment of the catalyst. Further experiments are needed to

determine the optimal Re number that guarantees a turbulent regime without TiO2

detachment.

Although immobilised TiO2 on porcelain beads does not exhibit a very good

photocatalytic activity for Cr(VI) reduction and 4-CP degradation under the

conditions explored in this paper compared with TiO2 in suspension, the new

material would able to be applied in a fixed bed in which, with an optimum flow

rate, geometry and other parameters, a better global performance of the system

would be obtained.

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Further studies are underway to optimise the fixation time and the number of

TiO2 layers to be applied on the support.

Acknowledgements

We thank G. Leyva for taking DRX spectra. Work performed as part of Comisión

Nacional de Energía Atómica CNEA P5-PID-36-4 Program, Agencia Nacional de

Promoción de la Ciencia y la Tecnología, PICT98-13-03672 ANPCYT project

and Consejo Nacional de Investigaciones Científicas y Técnicas, PIP662/98

CONICET project. G.P. thanks CNEA-CONICET for a fellowship to perform this

work. M.I.L. is a member of CONICET.

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Legends to the figures

Figure 1: 4-CP degradation with immobilised and suspended TiO2. Conditions: [4-CP] = 0.2 mM;

pH0 3; T = 25oC; open to air; P0 = 12 µeinstein s-1 l-1; λ > 300 nm. a) “untreated”, [TiO2] = 0.7 g l-

1; b) P-25 suspension, [TiO2] = 0.7 g l-1 ; c) “Re 100”, [TiO2] = 0.5 g l-1; c) “Re 400”, [TiO2] =

0.15 g l-1.

Figure 2: Cr(VI) reduction with suspended TiO2 and beads submitted to different treatments.

Conditions: [Cr(VI)] = 0.2 mM; [EDTA] = 0.9 mM; pH0 2; T = 25oC; open to air; P0 = 18

µeinstein s-1 l-1; λ > 300 nm. a) P-25 suspension, [TiO2] = 0.9 g l-1; b) “uncoated”, [TiO2] = 0; c)

“untreated”, [TiO2] = 0.9 g l-1; d) “Re 100”, [TiO2] = 0.6 g l-1; e) “Re 400”, [TiO2] = 0.2 g l-

1.Figure 3: Cr(VI) reduction efficiencies of immobilised and suspended TiO2. The equivalent

concentration of TiO2 in all cases is 0.6 g l-1. Conditions: [Cr(VI)] = 0.2 mM; [EDTA] = 0.9 mM;

pH0 2; T = 25oC; open to air; P0 = 3.6 µeinstein s-1 l-1; λ > 300 nm. a) “Re 100”; b) TiO2 detached

from “Re 100” after ultrasonication; c) P-25 suspension.

Figure 4: Cr(VI) reduction efficiencies of immobilised and suspended TiO2. The equivalent

concentration of TiO2 in all cases is 0.2 g l-1. Conditions: [Cr(VI)] = 0.2 mM; [EDTA] = 0.9 mM;

pH0 2; T = 25 oC; open to air; P0 = 3.6 µeinstein s-1 l-1; λ > 300 nm. a) “Re 400”; b) TiO2 detached

from “Re 400” after ultrasonication; c) P-25 suspension

0

0.2

0.4

0.6

0.8

1

0 1 2 3Time (h)

[4-C

P]/[4

-CP]

0

4

(c)

(b)

(a)

(d)

Figure 1, Piperata et al.

13

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100Time (min)

[Cr(

VI)]/

`[Cr(

VI)]

0

120

(b)

(d)(c)

(a)

(e)

Figure 2, Piperata et al.

14

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100 120Time (min)

[Cr(

VI)]/

[Cr(

VI)] 0

(b)(c)

(a)

Figure 3, Piperata et al.

15

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100 120Time (min)

[Cr(

VI)]/

[Cr(

VI)]

0

(b)

(a)

(c)

Figure 4, Piperata et al.References

16