0023-1584/05/4602- © 2005

MAIK “Nauka

/Interperiodica”0189

Kinetics and Catalysis, Vol. 46, No. 2, 2005, pp. 189–203. Translated from Kinetika i Kataliz, Vol. 46, No. 2, 2005, pp. 203–218.Original Russian Text Copyright © 2005 by Vorontsov, Kozlov, Smirniotis, Parmon.

Interest in heterogeneous photocatalytic processeshas grown in the past decade. Thus, the number of sci-entific publications on this subject matter issued since1991 is more than five thousands. Interest in these pro-cesses is due to several factors: First, photocatalytic orphotostimulated reactions can occur at high rates atroom temperature or even lower temperatures. Second,it was found that photocatalytic oxidation can degrade(mineralize) almost all organic compounds to inorganicproducts both in solutions and in a gas phase; this canbe used for effective purification of the environment.Third, photocatalysts can efficiently operate on irradia-tion with visible light, including sunlight; this providesan opportunity to use solar energy for performing use-ful processes such as hydrogen generation from water.Finally, a number of organic synthesis reactions thatcan be performed in a single step only by photocatalysisbecame known [1].

The majority of test heterogeneous photocatalysts aresemiconductors with various band gaps. Of these semi-conductors, titanium dioxide is the most commonlyused. This is due to its high catalytic activity, high chem-ical stability, inexpensiveness, and nontoxicity.

The principle of operation of semiconductor photo-catalysts is currently represented by a schematic dia-gram shown in Fig. 1 using titanium dioxide as anexample. The absorption of a light quantum in the bulk

of a semiconductor results in electron transfer from thevalence band to the conduction band. Then, the result-ing electron–hole pair can form an exciton, recombine,or undergo separation followed by trapping of chargecarriers at appropriate centers in the bulk of the semi-conductor or on its surface. Conduction electron andhole exhibit pronounced redox properties and enter intoreactions with the molecules or fragments of variouscompounds that occur on or at the surface of the semi-conductor. These primary reactions initiate the subse-

TiO

2

Photocatalytic Oxidation:I. Photocatalysts for Liquid-Phase and Gas-Phase Processes

and the Photocatalytic Degradation of Chemical WarfareAgent Simulants in a Liquid Phase

A. V. Vorontsov*, D. V. Kozlov*, P. G. Smirniotis**, and V. N. Parmon*

* Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090 Russia** University of Cincinnati, Cincinnati, OH 45221-0012, USA

Received April 1, 2004

Abstract

—The results of studies on the effect of the preparation procedure on the properties of TiO

2

-basedphotocatalysts and the kinetics and mechanism of the photocatalytic oxidation of organic water pollutants aresurveyed. The effects of calcination temperature, surface modification with platinum, and acid–base treatmentof the surface of titanium dioxide on its activity in model gas-phase and liquid-phase reactions are considered.Optimal catalyst preparation conditions were found in order to achieve maximum activity, and conceivable rea-sons for the effects of the above factors on the activity were revealed. The intermediate products and mecha-nisms of the photocatalytic and dark reactions of solutes that simulated chemical warfare agents in water areconsidered. All of the test simulants can undergo complete oxidation to form inorganic products in an aqueousTiO

2

suspension under irradiation with UV light. It was found that, in addition to oxidation, the dark steps ofhydrolysis also play an important role in the degradation of these substances. The low-frequency ultrasonictreatment (20 kHz) of a photocatalyst suspension in the course of the photocatalytic oxidation of dimethyl meth-ylphosphonate can accelerate the reaction because of the facilitated transport of reactants to the surface of pho-tocatalyst particles.

Conduction band

Valence band

e

–

Reduction

:

E

0

= –0.1

V NHE

Ä + e

–

→

Ä

–

B + h

+

→

B

+

E

0

= +3

V NHE

Oxidation:

h

ν

Fig. 1.

Principle of operation of a

TiO

2

catalyst:

e

–

and

h

+

are photogenerated electrons and holes, respectively;

E

0

arethe corresponding standard electrode potentials of thesespecies.

190

KINETICS AND CATALYSIS

Vol. 46

No. 2

2005

VORONTSOV

et al

.

quent chains of chemical transformations, which caninvolve both dark reactions and reactions with photoge-nerated conduction electrons and holes.

It is believed that the photocatalytic oxidation ofvarious substrates with molecular oxygen takes placeafter the primary reaction of photogenerated electronswith oxygen molecules, which results in a partial

reduction to

,

, and O

–

[2]. Photogenerated holesreact with water molecules to form extremely reactive

O

radicals and directly react with the oxidized sub-strate and the above charged oxygen species. The activeoxygen-containing species thus formed react with theoxidized substrate and initiate its further conversion upto the complete mineralization of the substrate.

To support the semiconductor nature of the photo-catalytic activity of

TiO

2

, Fig. 2 demonstrates theexperimental dependence of the quantum efficiency ofacetone oxidation on the wavelength of irradiationlight. It is well known that the band gap of

TiO

2

is~3.1 eV. It can be seen in Fig. 2 that the photocatalyticoxidation of acetone occurred with an almost constantquantum efficiency on short-wavelength irradiation andterminated on irradiation with light with photon energylower than 3.1 eV. Thus, the above action spectrum isconsistent with the statement that

TiO

2

photocatalysisis of a semiconductor nature.

O2– O2

2–

H.

In this work, we summarized the results of ourrecent experimental studies on the preparation of activephotocatalysts based on

TiO

2

by thermal treatment andsurface modification and the results of the photocata-lytic oxidation of substances that simulate chemicalwarfare agents in water.

EXPERIMENTAL

In this study, we used Degussa P25 titanium dioxidewith surface areas of 50 and 75 m

2

/g (70% anatase and30% rutile), Hombikat UV 100 (100% anatase; Sacht-leben Chemie GmbH) with a surface area of 340 m

2

/g,and independently prepared titanium dioxide samples(100% anatase). The oxidized organic substrates wereof at least 95% purity. As a rule, photocatalytic oxida-tion was performed with oxygen of prepurified air.

Test reactions for the determination of photocata-lytic activity in a gas phase were performed in flow-cir-culation and batch reactors equipped with opticalquartz windows for the irradiation of photocatalysts.The degradation reactions of chemical warfare agentsimulants were performed and the activity of photocat-alysts in the liquid-phase oxidation of phenol wasdetermined in batch reactors. Irradiation was per-formed with filtered or unfiltered light from a 1000-Wxenon lamp or with unfiltered light from a 1000-WDRSh mercury lamp.

The reactants and products were analyzed by gaschromatography, gas chromatography–mass spectrom-etry (GC–MS), and ion chromatography. Total organiccarbon (TOC) in aqueous solutions was determinedwith the use of an automated TOC analyzer (Shi-madzu). To determine nonvolatile substances by GC–MS, they were converted into volatile species by reac-tions with bis(trimethylsilyl)trifluoroacetamide(Supelco).

The UV and visible spectra were recorded with theuse of a UV-300 spectrophotometer (Shimadzu). In themeasurements of diffuse-reflectance spectra, MgO wasused as a standard substance. Upon irradiation withmonochromated light with

λ

= 313 nm, incident lightwas almost completely absorbed by the samples of pureand modified

TiO

2

. Therefore, the quantum yields of anumber of photocatalytic oxidation processes were cal-culated based on the intensity of incident light. Theintegrals of the diffuse-reflectance spectra of titaniumdioxide samples were calculated from the equation

I

=

(

λ

)

d

λ

, where

E

(

λ

)

is the reflectance (%) of light with

wavelength

λ

(nm); and

λ

1

= 300 nm and

λ

2

= 900 nm arethe limits of the range of measurement of diffuse-reflectance spectra.

The light flux intensity upon irradiation of a photo-catalyst in reactors was determined by standard ferriox-alate actinometry or by using radiant power meters. Theexperimental procedure was considered in more detailin the publications cited.

Eλ1

λ2∫

8

7

6

5

4

3

2

1

0

300 320 340 360 380 400 420

4.2 4.0 3.8 3.6 3.4 3.2 3.0

Quantum energy, eVQ

uant

um e

ffici

ency

, %

Wavelength, nm

Fig. 2.

Photocatalytic action spectrum of titanium dioxide(Hombikat UV 100) in the complete oxidation reaction ofacetone vapor in a batch reactor. Reactor volume, 400 ml;initial concentration of acetone vapor, 550–700 ppm; mono-chromated light intensity on the sample, 0.3–0.4 W/m

2

;sample surface area, 3 cm

2

; the light wavelength was variedwith an MDR-2 monochromator.

KINETICS AND CATALYSIS

Vol. 46

No. 2

2005

TiO

2

PHOTOCATALYTIC OXIDATION: I 191

RESULTS AND DISCUSSION

In this section, we consider the results of our studiesoriented towards the development of active photocata-lysts for liquid-phase and gas-phase oxidation as wellas photocatalytic water-treatment technology. Thesestudies involved preparation and characterization ofphotocatalysts and the kinetics and mechanisms of theoxidation of model substances.

1. Effect of the Preparation Procedure on the Activityof Photocatalysts Based on Titanium Dioxide

The preparation conditions of photocatalysts basedon titanium dioxide have an extremely significant effecton catalyst activity. The effects of preparation factorssuch as the calcination temperature of amorphous

TiO

2

samples, the procedure of supporting a platinum activa-tor onto crystalline

TiO

2

, and the acid–base treatment ofcrystalline

TiO

2

are considered below.

1.1. Effect of calcination temperature on theactivity of titanium dioxide.

The hydrolysis of tita-nium salts followed by thermal treatment of the result-ing amorphous precipitate of metatitanic acid is one ofthe most commonly used procedures for the prepara-tion of highly dispersed crystalline titanium dioxide.We found that the calcination temperature of samplesprepared by the hydrolysis of

TiCl

4

in an aqueous

NH

3

solution had a great effect on the activity of the samplesin the photocatalytic oxidation reaction of gaseous ace-tone [3]. As the calcination temperature was increasedfrom 320 to

500°ë

, the specific surface area of

TiO

2

decreased from 120 to 88 m

2

/g. In this case, accordingto XRD and TEM data, the average size of

TiO

2

crys-tallites increased from 11 to 17 nm. For all of the sam-ples, only crystalline anatase was present and an amor-phous phase was not detected. As the calcination tem-perature was increased to

450°ë

, the quantum yield ofacetone oxidation initially increased from 15 to 41%and then decreased to 32% at a calcination temperatureof

500°ë

.To find a correlation between the physicochemical

properties and the activity of

TiO

2

, we studied variousspectroscopic characteristics of the test samples of

TiO

2

. Thus, the EPR spectra of these samples alwaysexhibited a signal due to isolated paramagnetic NOmolecules on the surface and in the bulk of

TiO

2

[3].The intensity of this signal monotonically increasedwith calcination temperature. Obviously, the moleculesof NO are formed by the oxidation of residual

NH

3

molecules upon calcination of the samples, and they arenot directly related to changes in the photocatalyticactivity with calcination temperature.

A more interesting behavior was found in the mea-surement of the diffuse-reflectance spectra of the sam-ples in the visible and UV regions. All of the samplesexhibited similar spectra, which are characteristic ofsemiconductors with a band gap of ~3 eV: completelight absorption was observed in the short-wavelength

region, whereas the major portion of incident light wasalmost completely reflected at

λ

> 400 nm. It wasfound that reflection in the visible region of the spec-trum correlated with the activity of the samples. Fig-ure 3 shows that the integrals of diffuse-reflectancespectra (total whiteness) were linearly correlated withthe quantum yield of the complete oxidation of acetonevapor. The occurrence of this correlation was alsodemonstrated in other series of samples prepared atvarious calcination temperatures [4]. Thus, this corre-lation can be used for a rapid search for the most activesample in a series with different calcination tempera-tures. It is likely that the nature of the correlationobserved is related to competition between two oppo-site trends, which affect both the optical characteristicsand the photocatalytic properties of

TiO

2

, as the calci-nation temperature is increased: an increase in surfacecrystallinity and the removal of surface hydroxylgroups. An increase in the degree of crystallinity of

TiO

2

usually leads to an increase in photocatalyticactivity, whereas the removal of surface hydroxylgroups causes a decrease in activity.

1.2. Effect of supported platinum on the activityof titanium dioxide.

Surface modification with noblemetals, in particular, platinum, is a commonly used pro-cedure for improving the photocatalytic properties oftitanium dioxide. It is commonly believed that anincrease in the photocatalytic activity after supportingplatinum is primarily due to an improvement in photo-generated primary charge separation and a decrease in

15

Quantum yield, %

Integrated reflectance, nm %

45

46500

40

35

30

25

20

45000435004200040500

Fig. 3.

Dependence of the quantum yield of the photocata-lytic oxidation of acetone vapor in air on the integral of dif-fuse-reflectance spectra of titanium dioxide samples. Theprocedure used for calculating the integral of spectra isspecified in the Experimental section.

192

KINETICS AND CATALYSIS

Vol. 46

No. 2

2005

VORONTSOV

et al

.

the rate of charge recombination. Another reason forthe increase in activity can be the ordinary thermal cat-alytic activity of platinum, as both the starting sub-strates and their oxidation intermediates can undergodark oxidation on platinum particles. Noble metals canbe supported from their compounds onto

TiO

2

usingvarious techniques: by photocatalytic reduction in anaqueous, organic, or mixed suspension; by chemicalreduction with gaseous hydrogen at elevated tempera-tures; and by chemical reduction with soluble mildreducing agents at room temperature. A change in thesupporting procedure within a particular techniqueresulted in catalysts with strongly different activities.

Thus, in the photocatalytic deposition, metal parti-cles were deposited at

TiO

2

surface sites with anincreased density of photogenerated electrons. It wasfound that the pH of the starting solution of

H

2

PtCl

6

hada strong effect on the oxidation state of deposited plat-inum: the lower the pH value, the lower the oxidationstate of platinum. At pH < 4, platinum was depositedonly as

Pt

0

[5]. However, a decrease in pH stronglyinhibited the photocatalytic deposition of platinum.

To study the effect of the state of supported platinumon the photocatalytic activity of

TiO

2

, we prepared thesamples of

TiO

2

(Fluka) modified with

Pt(OH)

2

+ PtO

2,PtO2, and Pt0 using a photocatalytic technique and amechanical mixture of PtO2 and TiO2. Table 1 summa-rizes the results of the ESCA and TEM studies of theabove samples and the photocatalytic activity of thesesamples in the gas-phase oxidation of acetone and CO[6]. It can be seen that the photocatalytic depositiontechnique resulted in the formation of platinum parti-cles smaller than 7 nm. In this case, only the 0.8%Pt0/TiO2 sample exhibited higher activity in the photo-catalytic oxidation of acetone. The quantum yield of theprocess measured on this sample was 33%, whereas thequantum yield on the parent TiO2 was equal to 30%. Inthe oxidation of CO, all of the platinized samples werefound to be more active than pure TiO2. The 0.8%Pt0/TiO2 sample exhibited the greatest (twofold)

increase in the purely photocatalytic activity in CO oxi-dation, as compared with pure TiO2. The samples withhigh platinum contents were thermally active in COoxidation, so that it was impossible to determine theiractivity in a photocatalytic reaction path.

The photocatalytic deposition of platinum ontoHombikat UV 100 titanium dioxide with a high specificsurface area at pH 1.5 resulted in a photocatalyst withother properties. As a result of platinizing, the quantumyield of oxidation of acetone vapor on the above photo-catalyst decreased from 62 to 39%, whereas the quan-tum yield of CO oxidation increased from 2 to 37% [7].It is likely that great differences in the properties of pla-tinized Fluka and Hombikat TiO2 samples were due toa considerable difference between the specific surfaceareas of these titanium dioxides: 15 and 340 m2/g,respectively.

The deposition of platinum onto titanium dioxidefrom an aqueous solution of H2PtCl6 with the use ofchemical reduction with sodium borohydride shouldresult in a photocatalyst exhibiting weaker contactbetween platinum particles and TiO2 than that in thecase of photocatalytic deposition. Indeed, the washingof a Pt/TiO2 sample that was prepared by chemicalreduction with distilled water resulted in the removal ofa considerable portion of supported platinum [8]. Acomparison between Pt/TiO2 samples with similar plat-inum contents prepared by either chemical reduction orphotoreduction demonstrated that the photocatalyticactivity of the chemically reduced sample in the oxida-tion of gaseous acetone at 40°ë was higher than that ofthe photoreduced catalyst by a factor of 1.5. Anincrease in the reaction temperature resulted in a con-tinuous increase in the activity of platinized samples; inthis case, the photocatalytic activities of the samples at100°ë also differed by a factor of 1.5. At 40°ë, the pho-toplatinized catalyst exhibited a lower photocatalyticactivity in the oxidation of acetone vapor, as comparedwith pure TiO2.

Table 1. Activity of platinized photocatalysts based on TiO2 (Fluka) prepared by the photoreduction of H2PtCl6

Phase composition Platinumconcentration, at. %

Platinumparticle size, nm

Quantum yieldof acetone oxidation*, %

Quantum yieldof CO oxidation*, %

PtO2 + Pt(OH)2/TiO2 1.2 0.8–1.6 2.8 (<0.5) 0.9 (<0.2)

Pt(OH)2/TiO2 1.5 0.5–7 5.5 (<0.5) 1.6 (0.9)

Pt0/TiO2 0.8 3 33 (1.4) 1.2 (<0.2)

PtO2 29 6 × 50 1.4 (<0.5) (32)

PtO2/TiO2,mechanical mixture

10 10–30 4.2 (<0.5) (34)

TiO2 0 TiO2 58–300 30 (<0.5) 0.6 (<0.2)

Note: Temperature, 40°C; concentration of acetone vapor, 550 ppm; concentration of CO, 4200 ppm; intensity of monochromated lightwith λ = 313 nm, 7 mW/cm2.

* The thermal activity of samples in the oxidation of acetone vapor, which was measured in the dark and, for convenience of compar-ison, expressed in the same units as the photocatalytic activity, is given in parentheses.

KINETICS AND CATALYSIS Vol. 46 No. 2 2005

TiO2 PHOTOCATALYTIC OXIDATION: I 193

The observed difference in the photocatalytic activ-ity of photoplatinized and chemically platinized sam-ples can be due to a difference in the interactions of tita-nium dioxide with supported platinum particles. In thephotoplatinized sample, the contact of supported parti-cles with the surface was more intimate; this fact mayadversely affect the photocatalytic activity because ofthe enhanced recombination of photogenerated chargecarriers.

Usually, improvement in the separation of photoge-nerated primary charges is considered among factorsresponsible for an increase in photocatalytic activityupon platinizing titanium dioxide. The greatest effect ofplatinum on this charge separation should be observedin photoplatinized TiO2 because platinum particles inthese samples are in intimate contact with the surface ofthe semiconductor. However, as demonstrated above,the occurrence of this contact actually resulted in adecrease in photocatalytic activity. By this it is meantthat photocatalytically deposited platinum particles,which are in intimate contact with the catalyst, may infact impair charge separation and accelerate chargerecombination. The following factors can also beresponsible for an increase in photocatalytic activity:(1) an improvement in the adsorption of a substrate onthe surface of a photocatalyst, (2) thermal catalyticreactions on the surface of platinum particles, and (3)photocatalytic reactions directly on the surface of plat-inum particles. The effects of all of the above three fac-tors were supported experimentally.

Thus, the constants of acetone adsorption from agas phase on platinized TiO2 were found to be higherthan those on pure TiO2 [8]. Thermal catalytic reac-tions were detected in the oxidation of acetone vaporaway from light [8], as well as in acetone oxidation ona platinized catalyst coated with a thick layer of pureTiO2 [9].

The photocatalytic oxidation of acetone directly onthe surface of platinum particles was also detected.Thus, Fig. 4 shows the temperature dependence of therate of complete acetone oxidation on platinized alu-mina under exposure to light and in the dark. It can beseen that the rates of oxidation under light and in thedark at 100 and 120°ë were essentially different.Because alumina is photocatalytically inactive underirradiation with soft UV light, the observed differencein the rates should be ascribed to a photostimulated oxi-dation reaction on the immediate surface of platinumparticles.

Impregnation with an aqueous solution of H2PtCl6followed by reduction with hydrogen at elevated tem-peratures (300–400°ë) is the most frequently usedmethod for the deposition of platinum onto various sup-ports. We studied the effect of the platinum thus depos-ited on the photocatalytic activity of Degussa P25 andHombikat UV 100 TiO2 in the oxidation reaction ofphenol dissolved in water [10]. In this case, thermalreactions on platinum did not play a considerable role

because the reaction was performed under thermostatedconditions at a reduced temperature (12°ë). It wasfound that an increase in photocatalytic activity wasobserved in the Hombikat TiO2, whereas platinizationof the Degussa TiO2 only decreased the activity(Table 2). In the platinization of the Hombikat TiO2, asample with 1 wt % platinum exhibited the greatestphotocatalytic activity. Nevertheless, the photocatalyticactivity of this sample was lower than the activity of thepure Degussa TiO2 by a factor of ~1.5. Transmissionelectron microscopy allowed us to find that the plati-num particles prepared as described above were welldispersed over the surface of TiO2 and measured0.7−1 nm. The total surface area of the platinum parti-cles deposited by the above method linearly increasedwith the amount of the supported metal [10]; thisresulted in close contact between platinum particlesand the catalyst and in an increase in the activity of theHombikat TiO2.

Unlike photoplatinization, the deposition of plati-num from solution followed by reduction with hydro-gen did not result in the precipitation of platinum parti-cles at sites with an increased density of photogener-ated electrons. Therefore, in this case, depositedplatinum particles could additionally improve chargeseparation. However, because the initial sample ofDegussa P25 TiO2 exhibited a very high efficiency ofphotogenerated charge separation, supported platinumdid not improve its photocatalytic activity. Platinizationmay also have an adverse effect because of the specific-

040

w × 10–10, mol/s

T, °C

6

80 140

18

15

12

9

3

60 100 120

1

2

Fig. 4. Rates of oxidation (w) of acetone vapor in air onPt/Al2O3 at various temperatures: (1) in the dark and (2)upon irradiation with UV light. The concentration of ace-tone vapor was 600 ppm, and the concentration of watervapor was 4500 ppm.

194

KINETICS AND CATALYSIS Vol. 46 No. 2 2005

VORONTSOV et al.

ity of deposition by high-temperature reduction withhydrogen. Indeed, upon the deposition of platinum bychemical reduction, we found a considerable increasein the activity of the Degussa TiO2 in the oxidation ofdimethyl methylphosphonate dissolved in water.

1.3. Effect of the surface acidity of TiO2 on pho-tocatalytic oxidation. The change of the photocatalyticproperties of TiO2 by modifying surface acid–base sitesis a relatively new area of heterogeneous photocatalysis[11]. It is of importance that the reactions of photoge-nerated conduction electrons and holes are stronglyexothermal reactions and occur at a very high rate. It islikely that this rate does not limit the subsequent chem-ical transformations, which do not require light quanta.On the other hand, the dark steps of a reaction sequenceof the oxidized substrate can depend on surface acidity,and their rates can considerably vary depending onreaction conditions.

Table 3 summarizes data on changes in the proper-ties of Hombikat UV 100 titanium dioxide after treat-ment with aqueous solutions of sulfuric acid or sodiumhydroxide at 60°ë for 2 h. An increase in the concen-tration of sulfuric acid increased the amount of acidsites on the surface of TiO2 and decreased the amountof basic sites. On treatment with sodium hydroxide, thesame trend was observed with respect to basic sites.Acid–base treatment did not affect significantly the sur-face area and pore diameter of the test samples of TiO2.At the same time, acid treatment resulted in a signifi-cant increase in the photocatalytic activity of TiO2,whereas basic treatment resulted in a considerabledecrease in this activity. A clear correlation between thequantum efficiency of the deep oxidation of gaseousacetone and the number of acid sites on the surface ofTiO2 was observed.

A probable reason for the observed increase in pho-tocatalytic activity with increasing surface acidity ofTiO2 is the facilitated desorption of the resulting ëé2as the final reaction step of complete oxidation. Indeed,Fig. 5 shows that the adsorption isotherms of carbondioxide underwent a considerable change because ofthe treatment of titanium dioxide with a 1 M H2SO4solution. In this case, the adsorption of CO2 from dryair decreased by a factor of ~5. At high gas-phasehumidity, the decrease in adsorption was much less sig-nificant. This was likely due to the complete coverageof the hydrophilic surface of TiO2 with a water film athigh air humidity and the inability of carbon dioxidemolecules to displace adsorbed water from the surface.The high surface acidity facilitated the rapid degrada-tion of surface carbonates; thus, it can accelerate thisfinal step of complete oxidation.

The acid catalysis of one of the dark steps of com-plete oxidation can be another factor responsible for anincrease in the activity. This is exemplified by thehydrolysis of dimethyl methylphosphonate. A rate-lim-iting step catalyzed by an acid site can provide a

Table 2. Effect of the amount of supported platinum on theinitial quantum efficiency of phenol oxidation in an aqueoussuspension of the Pt/TiO2 photocatalyst

TiO2 Hombikat UV 100 TiO2 Degussa P25

platinumconcentra-tion, wt %

initial quantum efficiency, %

platinumconcentra-tion, wt %

initial quantumefficiency, %

0 0.12 0 0.88

0.5 0.26 0.1 0.79

1 0.43 0.25 0.63

1.5 0.36 0.5 0.59

2 0.31 1.5 0.48

3 0.34

Note: Quantum efficiency upon irradiation with a 200-W mercurylamp at 12.5 ± 0.5°C was calculated by dividing the initialrate of reaction by the incident flux of light quanta.

Table 3. Effect of acid–base pretreatment on the properties and photocatalytic activity of Hombikat UV 100 titanium dioxidein the oxidation of acetone vapor

Solution composi-tion for the treat-

ment of TiO2

Specific surfacearea SBET, m2/g Pore diameter, nm Amount of acid

sites, 10–4 mol/gAmount of basic sites, 10–3 mol/g

Quantum efficien-cy of oxidation

of acetone vapor, %

10 M H2SO4 – – 6.13 0.533 76

4 M H2SO4 341 5.0 5.10 0.639 71

1 M H2SO4 306 5.2 4.98 0.632 68

Untreated 347 4.9 3.69 0.982 62

1 M NaOH – – 3.00 1.12 44

4 M NaOH 355 5.0 2.60 1.29 30

10 M NaOH – – 1.74 1.32 17

Note: The photocatalytic tests were performed under the following conditions: temperature, 40°C; concentration of acetone vapor,500 ppm; concentration of water vapor, ~5000 ppm; irradiation with light from a 1000-W xenon lamp filtered using an interferencelight filter (λ = 334 nm).

KINETICS AND CATALYSIS Vol. 46 No. 2 2005

TiO2 PHOTOCATALYTIC OXIDATION: I 195

decrease in the apparent activation energy of oxidationand, consequently, accelerate the process of completeoxidation. Figure 6 demonstrates the effect of sulfuricacid on the apparent activation energy of the photocat-alytic oxidation of benzene vapor in a gas phase [12].An increase in the concentration of sulfuric acid usedfor the treatment of TiO2 resulted in a monotonicdecrease in the activation energy of the overall reaction.At a high acid concentration, the activation energy

approached a value close to 10 kJ/mol. Thus, the acidtreatment of a surface can decrease the activationenergy by a factor of higher than 4.

2. Kinetics and Reaction Pathsof Photocatalytic Oxidation

2.1. Complications of the apparent kinetics ofcatalytic and photocatalytic reactions in batch reac-tors. Both flow and batch reactors are used in studiesand practical applications of photocatalytic oxidation.Under steady-state conditions in flow reactors, theamounts of reactants adsorbed on a photocatalystremained constant; therefore, they can be ignored in amass balance of the observed photocatalytic reactions.In batch reactors, the concentrations of reactants andproducts changed in the course of the reaction, andadsorbed substances should be taken into account in amass balance. Thus, if a considerable fraction of reac-tants in a batch reactor occurs in an adsorbed state, thekinetics of changes in the measured concentrations ofthese substances in a gas or liquid phase can be signifi-cantly different from the case in which the mainamounts of the reactants occur in a homogeneous gas orliquid phase in the reactor. This difference can be par-ticularly important in the case of low-temperature het-erogeneous catalytic processes, which are characteris-tic of heterogeneous photocatalysis.

5

4

3

2

1

0 1000 2000 3000 4000

1

2

3

4

(a)

[CO2]ads, ×10–6 mol/g

0 1000 2000 3000 4000[Cé2], ppm

(b)

0.2

0.4

0.6

0.8

1.0

1.2

1

34

Fig. 5. Adsorption isotherms of gas-phase CO2 on (a) pureHombikat UV 100 TiO2 and (b) the above TiO2 treated withan aqueous 1 M H2SO4 solution. The isotherms were mea-sured at room temperature and at the following pressures ofwater vapor, ppm: (1) 0 (relative humidity of the gas phase,0%), (2) 7400 (25%), (3) 16000 (53%), and (4) 22200(74%).

50

40

30

20

10

0

Ea, kJ/mol

2 4 6 8 10[H2SO4], mol/l

Fig. 6. Dependence of the apparent activation energy (Ea) ofthe photooxidation of benzene vapor in the air on the con-centration of sulfuric acid used for the modification ofHombikat UV 100 TiO2. Relative air humidity, 70–80%;room temperature; full UV-light intensity of a DRSh-1000lamp, 10 mW/cm2.

196

KINETICS AND CATALYSIS Vol. 46 No. 2 2005

VORONTSOV et al.

As the simplest example, we consider a photocata-lytic first-order reaction, which obeys the Langmuir–Hinshelwood equation and takes place with the partici-pation of large amounts of a catalyst [13]. Let usassume that Si reaction sites are irradiated with lightand the photocatalytic reaction occurs at these sites,whereas Sn reaction sites on the surface of the photocat-alyst are not illuminated and they operate as an adsor-bent. For simplicity, assume that the equilibrium con-stants of adsorption (K) are equal for illuminated anddarkened active sites. The total number of moles (n) ofan initial substance (a substrate of the photocatalyticreaction) in the test system obeys the balance equation

(1)

where c is the substrate concentration in a homoge-neous liquid or gas phase in the reactor, and V is the vol-ume of this phase.

On the other hand, the total substrate amount in thesystem changes only because of a surface reaction. Inthe simplest case, the rate of this change is described bythe Langmuir–Hinshelwood equation

(2)

n cVSi Sn+( )Kc

1 Kc+----------------------------,+=

dndt------

SikKc1 Kc+----------------,–=

where k is the apparent rate constant of the surface reac-tion.

Differentiating Eq. (1) with respect to time and thenequating it to (2), we can find the rate of change in thesubstrate concentration in a homogeneous phase in thereactor:

(3)

Thus, the occurrence of adsorption in the systemalways affects the apparent kinetics of changes in thesubstrate concentration in the volume of a batch reactor.When the main amount of the converted substrate is notadsorbed, the second term in the denominator of Eq. (3)can be ignored and the kinetics of changes in the sub-strate concentration in a homogeneous phase is reducedto the traditional Langmuir–Hinshelwood kinetics.

The above circumstance is of particular importancefor heterogeneous photocatalysis because a consider-able portion of the surface of a photocatalyst can beunilluminated under real conditions. Thus, this portionof the surface can participate only in adsorption pro-cesses. Evidently, the kinetics is affected by changes inthe fraction of the unilluminated portion of the photo-catalyst. Figure 7 shows concentration profiles calcu-lated from Eq. (3) at a fixed initial substrate amount inthe reactor depending on the amount of darkenedadsorption sites in the photocatalyst. It can be seen thatthe initial substrate concentration in a homogeneousphase in the reactor decreased with increasing area ofthe unilluminated photocatalyst surface, whereas thetime taken to reach a given low finite concentration ofthe substrate in the homogeneous phase increased.Although the kinetics of substrate concentration con-siderably changed, the experimental kinetic curves canbe adequately approximated by the simplest Langmuir–Hinshelwood kinetics. Nevertheless, the parametersobtained by a corresponding optimization of the exper-imental kinetic curves can differ from true values by asmall factor [13].

In the practical use of photocatalysis for the purifi-cation of a liquid or gas phase, the usual challenge is todecrease the concentration of the photooxidized sub-strate (a pollutant in air or water) to a level lower than acertain value (for example, the maximum permissibleconcentration (MPC) of the substrate) in the shortestpossible time. If the MPC is sufficiently high, the addi-tion of an amount of a heterogeneous photocatalystmay remove the pollutant from the bulk by adsorptioneven before oxidation. At the same time, at low MPCvalues, the time taken to reach a pollutant concentrationlower than the MPC can significantly increase with theused amount of a photocatalyst (Fig. 7).

2.2. Photocatalytic oxidation in solutions. Thephotocatalytic oxidation of compounds in solutions is

dcdt------

SikKc1 Kc+----------------–

VSi Sn+( )K

1 Kc+( )2-------------------------+

-----------------------------------.=

8

6

4

2

0

Concentration, mol/m3

0 3 6 9 12 15Time, s

12

3 4 5

3

4

5

21

Fig. 7. Calculated kinetics of consumption of a substance ina batch reactor according to the Langmuir–Hinshelwoodmodel at various amounts of adsorption sites on the surfaceof an inoperative catalyst, mol: (1) 0, (2) 1, (3) 2, (4) 5, and(5) 10. The amount of adsorption sites on a working catalystwas 1 mol, and the initial substance amount was 10 mol.The reactor volume was 1 m3. Arrows indicate the attain-ment of a desired final substrate concentration of5 mmol/m3.

KINETICS AND CATALYSIS Vol. 46 No. 2 2005

TiO2 PHOTOCATALYTIC OXIDATION: I 197

widely used for the purification and disinfection ofwater containing trace organic and inorganic pollutants.Unlike traditional catalytic processes, this oxidationusually does not require additional water heating and/orthe addition of reagents to the system. In this section,we consider the photocatalytic degradation of sub-stances that simulate highly toxic compounds: chemi-cal warfare agents.

2.2.1. Degradation of 2-phenethyl 2-chloroethyl sul-fide on TiO2. The test compound contains theSCH2CH2Cl group, which is present in the chemicalwarfare agent yperite (mustard gas). Because of thepresence of a phenyl substituent, the toxicity of the sim-ulant is low (LD50 of 850 mg/g in mice). Similarly toyperite, the simulant is sparingly soluble in water, andits dissolution is accompanied by hydrolysis. Figure 8illustrates the kinetics of hydrolysis and photocatalyticoxidation of the simulant [14]. It can be seen that, uponthe addition of the simulant to an aqueous suspensionof TiO2, the TOC concentration in solution slowlyincreased, and this increase was accompanied by adecrease in pH and an increase in the concentration ofchloride ions. Thus, the dissolution of the simulant inwater was accompanied by the hydrolysis of the C–Clbond.

Upon the onset of irradiation of the suspension withfull light from a medium-pressure mercury lamp, theTOC content rapidly decreased and the concentrationof sulfate ions in the aqueous phase of the suspensionincreased. After irradiation for approximately 4 h, theaqueous phase of the solution contained almost noorganic compounds, whereas the detected concentra-tion of sulfate ions (120 mg/l) corresponded to the com-plete mineralization of the simulant. To find the contri-bution of possible direct uncatalyzed photolysis of thesimulant, we studied its photocatalytic oxidation underexposure to light from a mercury lamp passed througha Pyrex filter with a transmission band at λ > 300 nm.Under these conditions, the direct light absorption bysimulant molecules was excluded because the redthreshold of its absorption is near 275 nm. In this case,photocatalytic oxidation occurred at a rate ~10 timeslower than that on irradiation with unfiltered light. Avery high rate of photolysis was observed upon the irra-diation of the system with full light from a mercurylamp in the absence of TiO2: the TOC concentration inthe aqueous phase decreased to almost zero in 185 min.Upon irradiation with filtered light, direct uncatalyzedphotolysis was not observed.

Practically the same products were detected in bothdirect photolysis and photocatalytic oxidation. Styrene,benzaldehyde, and benzeneacetaldehyde were predom-inant among the volatile products of degradation. Thenonvolatile products that remained in the aqueousphase contained 2-phenethyl 2-hydroxyethyl sulfide,2-phenethyl 2-hydroxyethyl sulfone, 2-phenethyl-sulfinic acid, benzeneacetic acid, benzoic acid, etc.Considerable amounts of 2-phenethyl disulfide,

2-phenethylsulfinic acid, and 2-phenethyl trisulfidewere extracted from the surface of the photocatalystafter the reaction. Figure 9 demonstrates possible reac-tion paths of the photostimulated degradation of thesimulant, which follow from the nature of degradationproducts. After a dark step (which does not require theparticipation of light) of hydrolysis of the carbon–chlo-rine bond, the photocatalytic or photolytic cleavage ofcarbon–sulfur bonds took place, followed by the oxida-tion and hydrolysis of the products up to the formationof the final inorganic substances: sulfuric acid, carbondioxide, and hydrogen chloride.

2.2.2. Effect of ultrasound on the photocatalytic oxi-dation of dimethyl methylphosphonate. The photocata-lytic oxidation of substrates in aqueous suspensionsusually occurs at much lower rates than the correspond-ing gas-phase processes. Nevertheless, in the oxidationof heteroatomic organic substances, a liquid-phase pro-cess is usually characterized by lower deactivation ofthe photocatalyst. This is due to the fact that the finalinorganic products of oxidation desorb into solutionand do not block surface reaction centers. It is wellknown that ultrasound can initiate a number of oxida-tion reactions and accelerate photocatalytic reactions ina liquid phase.

We studied the effect of 20-kHz ultrasound on thephotocatalytic degradation of dimethyl methylphos-

0

200

[TOC], [Cl–], [SO4–2], mg/l

Reaction time, min

30

1800

120

170516004000

3

4

5

6

7

60

90

1

2

3

4

Fig. 8. Kinetics of changes in the concentrations and pH ofan aqueous phase in the hydrolysis and subsequent photo-catalytic degradation of a yperite simulant (2-phenethyl2-chloroethyl sulfide) in an aqueous suspension of Hom-bikat UV 100 TiO2: (1) TOC, (2) chloride ions, (3) pH, and(4) sulfate ions. Initial simulant concentration, 250 mg/l;TiO2 concentration, 250 mg/l; UV irradiation with a 450-Wmercury lamp started after 1705 min.

198

KINETICS AND CATALYSIS Vol. 46 No. 2 2005

VORONTSOV et al.

phonate in a batch reactor [15]. Figure 10 shows thecurves that characterize TOC concentration changes insolution under various oxidation conditions. It can beseen that the oxidation of dimethyl methylphosphonatein a suspension of the Hombikat UV 100 TiO2 photo-catalyst did not occur under the action of only ultra-sound (curve 1). In the photocatalytic oxidation of dim-ethyl methylphosphonate in the absence of ultrasound(curve 3), the TOC concentration decreased at an aver-age rate of 0.15 mg l–1 min–1. Upon the simultaneousultrasonication and UV irradiation of the suspension,the oxidation of dimethyl methylphosphonate was con-siderably accelerated, and the rate of TOC consumptionwas as high as 0.34 mg l–1 min–1. The following factorsmay be responsible for the positive effect of ultrasound:(1) an increase in the dispersity of TiO2 particles in thesuspension, (2) a facilitated transport of reactants due to

improved stirring of the suspension, and (3) a change inthe reaction mechanism. Curve 2 in Fig. 10 was mea-sured under conditions of short-time exposures to ultra-sound at regular intervals for dispersing the photocata-lyst. In this case, the reaction rate of dimethyl meth-ylphosphonate oxidation was somewhat lower than thatin the photocatalytic oxidation without ultrasonication.Consequently, an increase in the dispersity of TiO2 isnot the main reason for the positive effect of ultrasound.Indeed, the intensification of stirring of the suspensionon photocatalytic oxidation resulted in a small increasein the rate of dimethyl methylphosphonate oxidation(curve 4). Thus, a conceivable reason for the positiveeffect of ultrasound consists in an acceleration of reac-tant transport to the surface of TiO2.

To study the possible effect of ultrasound on themechanism of photocatalytic oxidation, we identified

S Cl

S OH

O

SH

SS

SS

O

O

CHOCOOH CHO

SO2H

Hydrolysis of the C–Cl bond

Cleavage of the C–S bond

CH3CHOHSCH2CH2OH

HO2SCH2CH2OH

CO2 HCl H2SO4

Fig. 9. Conceivable reaction scheme of the degradation of 2-phenethyl 2-chloroethyl sulfide in an aqueous suspension of HombikatUV 100 TiO2 under UV irradiation.

KINETICS AND CATALYSIS Vol. 46 No. 2 2005

TiO2 PHOTOCATALYTIC OXIDATION: I 199

the degradation products of dimethyl methylphospho-nate and measured their relative concentrations. Wedetected five reaction products: the three main productswere phosphate ions, methyl methylphosphonate, andmethylphosphonate, and the two other products, dime-thyl phosphate and methyl phosphate, were present inlower concentrations.

Figure 11 shows the reaction scheme of the photo-catalytic oxidation of dimethyl methylphosphonate,which was constructed based on the experimental dataon reaction products. It is believed that hydroxyl radi-cals generated on TiO2 initiate the oxidation of the ini-tial substrate and detected intermediates. After this ini-tiation, several subsequent steps of oxidation rapidlyoccur with the participation of atmospheric oxygen andwithout the participation of light. We constructed akinetic model for the oxidation scheme proposed. Theadequacy of this model was checked against experi-mental kinetics of the photocatalytic oxidation of dim-ethyl methylphosphonate and the intermediates andproducts of this oxidation [15]. It was found that theoptimized values of all of the rate constants used in thismodel for photocatalytic oxidation in the presence ofultrasound were higher than those in the absence ofultrasound. As expected, this can be easily explained by

an accelerated reactant transport under the action ofultrasound.

The kinetics of the photocatalytic oxidation of dim-ethyl methylphosphonate in the absence of ultrasoundis adequately described by a reaction scheme without astep of the direct oxidation of dimethyl methylphos-phonate to phosphate ions with constant kS. However,this step should be added in order to describe satisfac-torily the reaction kinetics in the presence of ultra-sound. Thus, the action of ultrasound results in theappearance of an additional oxidation reaction path,which can be related, for example, to a considerableacceleration of the transport of reactants into the poresof the photocatalyst. If the starting compound (dime-thyl methylphosphonate) enters into the micropores ofthe photocatalyst, it can occur there up to completephotocatalytic oxidation. Therefore, in terms of thedetection of the concentration of the starting substratein a homogeneous aqueous phase, the complete oxida-tion of dimethyl methylphosphonate to phosphate ionswithin pores without the release of intermediate prod-ucts into the bulk of the solution appears as a step withrate constant kS (Fig. 11).

2.2.3. Photocatalytic degradation of other chemicalwarfare agent simulants. We also studied the conver-sion of diethylphosphoramidate, pinacolyl methylphos-phonate, and 2-(butylamino)ethanethiol [16], whichsimulate the chemical structures of the chemical war-fare agents tabun, soman, and VX, respectively.

Figure 12 shows the kinetics of decrease in the con-centration of TOC during the oxidation of the abovesubstances in an aqueous suspension of Hombikat UV100 TiO2 in a batch reactor under exposure to light froma xenon lamp. At equal initial concentrations of thesimulants (250 mg/l), the highest and lowest rates ofcomplete oxidation were observed with pinacolylmethylphosphonate and 2-(butylamino)ethanethiol,respectively. It can be seen that all of the simulants

10

0 50

[TOC], mg/l

Reaction time, min

20

100 150 200 250 300

30

40

50

60

70

80

90

100

1

2

34

5

Fig. 10. Kinetics of changes in the concentration of TOC inthe oxidation of dimethyl methylphosphonate in an aqueoussuspension of Hombikat UV 100 TiO2: (1) under the actionof 20-kHz ultrasound, (2) under UV irradiation with ultra-sonication at regular intervals in the dark, (3) under UV irra-diation, (4) under UV irradiation with stirring with a mag-netic stirrer, and (5) under UV irradiation with ultrasonica-tion. In all cases, a flow of air (500 cm3/min) caused stirringto prevent the sedimentation of TiO2 from the suspension.Irradiation was performed with unfiltered light from a450-W mercury lamp.

PH3CO OCH3

O

CH3

PH3CO OCH3

O

OH

PH3CO OH

O

OH

PHO OH

O

OH

PHO OH

O

CH3

PH3CO OH

O

CH3

k4

OH•

OH• k5 OH• k2

3OH•kS

k7

OH•

k6

OH•k3

OH•

OH•k1

Fig. 11. Detected products and conceivable reaction pathsin the photocatalytic degradation of dimethyl methylphos-phonate in an aqueous suspension of Hombikat UV 100TiO2. The reaction conditions are specified in Fig. 10.

200

KINETICS AND CATALYSIS Vol. 46 No. 2 2005

VORONTSOV et al.

under discussion underwent complete mineralization toinorganic substances.

In the course of the photocatalytic degradation ofdiethylphosphoramidate, an increase in the concentra-tions of ammonium, nitrate, and phosphate ions in theaqueous phase of the suspension was detected. Nitriteions in a low concentration were also detected. Aftercompletion of the photocatalytic oxidation of the simu-lant, the concentration of nitrite ions decreased to avery low value. Three fourths of the nitrogen in thestarting simulant was converted into ammonia as aresult of oxidation. Among volatile reaction products,acetaldehyde and ammonia were detected, whereasnonvolatile products contained seven compounds.

The composition of the products and kinetic data onthe photooxidation allowed us to propose a scheme ofreaction paths, which is shown in Fig. 13. All of theproducts specified in this reaction scheme weredetected experimentally. The main reaction paths arethe photocatalytic oxidation of the α-carbon atom fol-lowed by the hydrolysis of the resulting anhy-dride/hemiacetal and the cleavage of the phosphorus–nitrogen bond, which mainly leads to ammonia. In thecourse of the subsequent reactions, the photocatalyticand dark oxidation of other moieties of the substratemolecule and the formation of inorganic products takeplace.

In the oxidation of pinacolyl methylphosphonate,methylphosphonic acid was the main nonvolatile reac-tion intermediate, whereas acetone was the main vola-tile product. An additional 11 volatile products, whichwere formed by the oxidation of the pinacolyl residueof the parent substance, were found in lower concentra-tions. Among nonvolatile products, an additional 8 sub-stances were detected aside from methylphosphonicacid. Figure 14 demonstrates the scheme of the initialreaction paths of the photocatalytic oxidation of pinac-olyl methylphosphonate. It is believed that the dark oxi-dation of the simulants under consideration is initiatedby the reaction of a substrate with the photocatalyti-cally generated hydroxyl radical. Next, the resultingalkyl radicals react with molecular oxygen to formalkylperoxide radicals; alcohols and aldehydes can beformed by subsequent conversion of the latter radicals.

The pinacolyl methylphosphonate molecule con-tains three types of carbon atoms, which can be readilyattacked by hydroxyl radicals (Fig. 14). Thus, in thephotocatalytic oxidation of the CH group, the constitu-ent hydrogen is replaced by the hydroxyl group. In thiscase, the simulant molecule is converted into a hemiac-etal, which, as is well known, is readily hydrolyzed inaqueous solutions. This hydrolysis results in meth-ylphosphonic acid and pinacolyl alcohol.

The photocatalytic oxidation of the methyl groupsof a tert-butyl moiety results in the formation of corre-sponding alcohol, aldehyde, and acid, whereas the oxi-dation of the methyl group bound to phosphorus prima-rily gives pinacolylphosphoric acid. Finally, all of theintermediate products are oxidized to carbon dioxideand phosphoric acid.

The most interesting products were found in thephotocatalytic oxidation of 2-(butylamino)ethanethiol.This simulant contains the S–CH2CH2–N moiety,which is characteristic of the chemical warfare agentVX. In the photocatalytic oxidation of this substrate inan aqueous suspension of TiO2, an increase in the con-centrations of sulfate and ammonium ions wasobserved. The concentration of nitrite ions passedthrough a maximum at reaction times of ~100 min andthen decreased, whereas the concentration of nitrateions remained low during the entire course of the reac-tion (650 min). Although the TOC concentrationdecreased by only 29% in 650 min, the concentration ofthe resulting sulfate ions corresponded to a 66% con-version of the total sulfur of the parent substance. Wedetected 22 volatile and 14 nonvolatile intermediates.The most interesting products were volatile heterocy-clic compounds containing sulfur and nitrogen in therings, such as 2-propyldihydrothiazole S-oxide, 2-pro-pylthiazoline, 2-propylthiazole, and 2-propylthiazoli-dine.

Figure 15 shows the scheme of the initial reactionpaths of this simulant, which was constructed based onthe composition of the main products detected. As inthe case of other sulfur-containing compounds, one of

[TOC], mg/l

Reaction time, min

120

1000800600400200

90

60

30

00

1

2

3

Fig. 12. Kinetics of changes in the concentration of TOC inthe photocatalytic degradation of substances that simulatechemical warfare agents in an aqueous suspension of Hom-bikat UV 100 TiO2: (1) 2-(butylamino)ethanethiol, (2) pina-colyl methylphosphonate, and (3) diethylphosphoramidate.The initial concentrations of all of the substances and TiO2were 250 mg/l.

KINETICS AND CATALYSIS Vol. 46 No. 2 2005

TiO2 PHOTOCATALYTIC OXIDATION: I 201

C2H5O P

O

OC2H5

NH2

C2H5O P

O

OH

NH2

C2H5O P

O

OC2H5

OH

C2H5O P

O

OCH2CH2OH

NH2

C2H5O P

O

OH

OH

C2H5O P

O

OCH2CH2OH

OH

HOCH2CH2 P

O

OCH2CH2OH

OH

Oxidation of α-C1 Oxidation of NH2 Hydrolysis of P-NH2 Oxidation of β-C1

NO3– NH3

CH3CHO

Oxidation of α-C1 Oxidation of β-C1Cleavage of the

P-NH2 bond

NO3– NH3,

CH3CHO NO3– NH3,

Oxidation of β-C2

Oxidation of α-C1

HOCH2COOH

Oxidation of α-C2

CH3COOH

CH3CHO

Cleavage of theP-NH2 bond

CO2 NH3 NO3– H3PO4

Fig. 13. Detected products and a conceivable scheme of the main reaction paths in the photocatalytic degradation of diethylphos-phoramidate in an aqueous suspension of Hombikat UV 100 TiO2.

P

O

O

OH

POH

O

O

OH

P

O

O

OH OHPH3C

O

OH

OH

P

O

O

OH

O

P

O

O

OH

O

OH

O

Oxidation of (CH3)3Oxidation of CH

Oxidation of CH

CO2 H3PO4

Fig. 14. Detected products and a conceivable scheme of the main initial reaction paths in the photocatalytic degradation of pinacolylmethylphosphonate in an aqueous suspension of Hombikat UV 100 TiO2.

202

KINETICS AND CATALYSIS Vol. 46 No. 2 2005

VORONTSOV et al.

C1HS C2

HN

C3C4

C5C6

NH

SS

HN

N

S

NH

S

O

N

S

S

HN NH2CHO

NH

SO2H

NH

SO3H

NH

COOH

Ring closure Oxidation of C2 Oxidation of C3 Oxidation of sulfur

CO2 NH3 NO3– H2SO4

Fig. 15. Detected products and conceivable main reaction paths of the dark and photocatalytic reactions of 2-(butylamino)ethaneth-iol in the course of oxidation in an aqueous suspension of Hombikat UV 100 TiO2.

the main reaction paths is the oxidation of the sulfuratom. In this case, it rapidly occurred in the absence ofUV irradiation and led to a disulfide. The subsequenttransformations were initiated by the photocatalystunder irradiation and involved ring closures, the oxida-tion of carbon atoms bound to the nitrogen atom, and adeeper oxidation of the sulfur atom in the disulfide. Theintroduction of a hydroxyl group into the simulant mol-ecule at carbon atoms adjacent to the nitrogen atomresulted in the formation of aminols, which, as is wellknown, undergo hydrolysis to form correspondingamines and aldehydes. The oxidation of the disulfidesulfur atom led to a thiosulfonate, which undergoeshydrolysis with the formation of sulfinic acid. Next,this acid was oxidized to sulfonic and sulfuric acids.Finally, all of the reaction paths of 2-(butyl-amino)ethanethiol oxidation resulted in the formationof inorganic products: carbon dioxide, ammonia, sulfu-ric acid, and nitric acid.

CONCLUSIONS

Thus, we found that the photocatalytic activity oftitanium dioxide can exhibit a maximum at a certaincalcination temperature. In this case, the photocatalyticactivity of samples calcined at various temperaturescorrelates with their whiteness (that is, the integral ofdiffuse-reflectance spectra).

Among photoreduced Pt/TiO2 samples, a catalystwith platinum in a zero oxidation state exhibits thegreatest activity in the photocatalytic oxidation of ace-tone vapor. The activity of Pt/TiO2 samples prepared bythe chemical reduction of H2PtCl6 is higher than that ofthe samples prepared by photocatalytic reduction. Theenhanced activity of Pt/TiO2, as compared with that ofpure TiO2, results from dark reactions and photoreac-tions on the surface of platinum, better charge separa-tion, and greater adsorption of reactants.

The activity of TiO2 in the photocatalytic oxidationreactions of acetone and benzene vapors increases afterthe treatment of the catalyst surface with sulfuric acid.In this case, a correlation between the activity and thenumber of acid sites on the surface of TiO2 is observed.Moreover, an acceleration of photocatalytic oxidationon TiO2 treated with the acid is accompanied by adecrease in the activation energy of the reaction and thefacilitated desorption of ëé2, which is the final oxida-tion product.

The presence of a large amount of adsorbed reagentsin a photocatalytic batch reactor significantly affectsthe kinetics of their consumption and can exert either apositive or negative effect on water and air purification.

The TiO2 photocatalytic oxidation of sulfur-con-taining organic compounds can be accompanied bydirect photolysis under exposure to soft UV light. In thephotocatalytic oxidative degradation of the chemical

KINETICS AND CATALYSIS Vol. 46 No. 2 2005

TiO2 PHOTOCATALYTIC OXIDATION: I 203

warfare agent simulants 2-phenethyl 2-chloroethyl sul-fide, diethylphosphoramidate, pinacolyl methylphos-phonate and 2-(butylamino)ethanethiol, many volatileand nonvolatile products are detected. In the photocat-alytic oxidation of the last-named simulant, heterocy-clic intermediate products are formed. The dark hydrol-ysis of intermediate products is an important step in thedegradation of the simulants. Inorganic substances arethe final products of the photocatalytic oxidation of thesimulants.

The action of 20-kHz ultrasound on a suspension ofTiO2 did not cause the oxidation of dimethyl meth-ylphosphonate dissolved in this suspension. However,it significantly accelerated the photocatalytic oxidationof this compound because of an accelerated transport ofreactants to the surface of the photocatalyst.

ACKNOWLEDGMENTSThis work was supported by the Russian Foundation

for Basic Research (project no. 02–03-08002) and theprogram “Leading Scientific Schools of Russia” (grantno. NSh 1484.2003.3). Vorontsov acknowledges thesupport of the Foundation for the Support of DomesticScience. This study was supported in part by the “Rus-sia in Flux” program of the Academy of Finland (grantno. 208134).

REFERENCES1. Kisch, H. and Lindner, W., Chem. Unserer Zeit, 2001,

vol. 4, p. 250.2. Coronado, J. M., Maira, A.J., Conesa, J.C., Yeung, K.L.,

Augugliaro, V., and Soria, J., Langmuir, 2001, vol. 17,p. 5368.

3. Vorontsov, A.V., Altynnikov, A.A., Savinov, E.N., andKurkin, E.N., J. Photochem. Photobiol., A, 2001,vol. 144, nos. 2–3, p. 193.

4. Vorontsov, A.V., Cand. Sci. (Chem.) Dissertation,Novosibirsk: Catalysis Center, 1998.

5. Xi, C., Chen, Z., Li, Q., and Jin, Z., J. Photochem. Pho-tobiol., A, 1995, vol. 87, p. 249.

6. Vorontsov, A.V., Savinov, E.N., and Jin, Z., J. Photoch.Photobiol., A, 1999, vol. 125, nos. 1–3, p. 113.

7. Vorontsov, A.V., Savinov, E.N., Barannik, G.B.,Troitsky, V.N., and Parmon, V.N., Catal. Today, 1997,vol. 39, p. 207.

8. Vorontsov, A.V., Stoyanovz, I.V., Kozlov, D.V., Sima-gina, V.I., and Savinov, E.N., J. Catal., 2000, vol. 189,p. 360.

9. Kennedy, J.C. and Datye, A.K., J. Catal., 1998, vol. 179,p. 375.

10. Sun, B., Vorontsov, A.V., and Smirniotis, P.G., Lang-muir, 2003, vol. 19, p. 3151.

11. Kozlov, D., Bavykin, D., and Savinov, E.N., Catal. Lett.,2003, vol. 86, p. 169.

12. Kozlov, D.V., Panchenko, A.A., Bavykin, D.V., Savi-nov, E.N., and Smirniotis, P.G., Russ. Chem. Bull.,2003, vol. 52, p. 1100.

13. Vorontsov, A.V. and Savinov, E.N., Chem. Eng. J., 1998,vol. 70, no. 3, p. 231.

14. Vorontsov, A.V., Panchenko, A.A., Savinov, E.N., Lion, C.,and Smirniotis, P.G., Environ. Sci. Technol., 2002,vol. 36, no. 23, p. 5261.

15. Chen, Y.-C., Vorontsov, A.V., and Smirniotis, P.G., Pho-tochem. Photobiol. Sci., 2003, vol. 2, no. 6, p. 694.

16. Vorontsov, A.V., Davydov, L., Reddy, E.P., Lion, C.,Savinov, E.N., and Smirniotis, P.G., New J. Chem., 2002,vol. 26, no. 6, p. 732.