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Applied Catalysis A: General 262 (2004) 105110

Intrinsic kinetics of photocatalytic oxidation of formicand oxalic acid on immobilised TiO2films

T.A. McMurray a, J.A. Byrne a,, P.S.M. Dunlop a, J.G.M. Winkelman b,B.R. Eggins a, E.T. McAdams a

a NIBEC, University o f Ulster at Jordanstown, Newtownabbey BT37 0QB, Northern Ireland, UKb Department of Chemical Engineering, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Received in revised form 12 November 2003; accepted 14 November 2003

Abstract

Titanium dioxide (TiO2) photocatalysis is a possible alternative/complementary technology to conventional water treatment methods. The

TiO2catalyst may be used as slurry or it may be immobilised onto a supporting substrate. With immobilised TiO2 films mass transfer problems

occur in most photocatalytic reactors, which results in a reduction of reactor efficiency and in the accuracy of measured catalyst efficiency and

kinetics. In order to determine the real intrinsic kinetics of photocatalytic reactions on immobilised TiO2films a stirred tank reactor (STR) was

used. The reactor incorporated a propeller and a baffle, thus providing good mixing and efficient mass transfer to the TiO2 film. Degussa P25

was immobilised onto borosilicate glass by a dip coating method and the kinetics of the photocatalytic degradation of the model pollutants,

formic acid and oxalic acid were investigated as a function of catalyst loading, initial pollutant concentration and propeller rotation speed. The

rate of degradation, of either acid, was not mass transfer limited at propeller speeds greater than 1000 rpm. The rate of formic acid degradation

was dependent upon catalyst loading up to a maximum loading above which a decrease in the degradation rate was observed.

The apparent quantum yield for the photocatalytic degradation was 5% for oxalic acid and 10% for formic acid. This compares very well

with usual reported apparent quantum efficiencies for photocatalysis which are 1%. The photocatalytic oxidation of both acids could be

described using a LangmuirHinshelwood kinetic model. 2003 Elsevier B.V. All rights reserved.

Keywords:Titanium dioxide; Photocatalysis; Formic acid; Oxalic acid; Kinetics

1. Introduction

Titanium dioxide (TiO2) photocatalysis is a possible al-

ternative/complementary technology to conventional water

treatment methods [13]. When TiO2 is illuminated with

light < 400 nm, electronhole pairs are generated. The

valence band holes are strongly oxidising and can react with

water or hydroxide ions at the interface to produce hydroxyl

radicals. Hoffman et al.[1] reviewed the evidence for both

direct hole transfer and indirect hole transfer via surface

bound hydroxyl radical, and significant body of literature

was found in support of both mechanisms. Both hydroxyl

radicals and valence band holes are powerful and indiscrim-

inate oxidizing species and will attack organic pollutants at

Corresponding author. Tel.: +44-289-36-8941;

fax: +44-289-36-6863.

E-mail addresses:[email protected] (T.A. McMurray),

[email protected] (J.A. Byrne).

the surface to yield carbon dioxide, water, and the respec-

tive mineral acids or salts [4]. Carraway et al. [5]reported

evidence for direct hole oxidation of tightly bound electron

donors such as formate at the semiconductor surface.

The catalyst may be used either as an aqueous slurry or

it may immobilised onto a supporting substrate [6]. Suspen-

sion or slurry type reactors have been reported to be efficient

due to the large surface area of catalyst available for reac-

tion and the efficient mass transfer within such systems[7].

However, due to the small particle size of the TiO2particles,

a post-treatment catalyst recovery stage involving microfil-

tration is necessary. Post-treatment catalyst recovery would

be undesirable at industrial scale as it would add to the cap-

ital and operating costs of the treatment process.

Alternatively, the TiO2 may be immobilised onto a sup-

porting substrate such as glass [8]. With an immobilised

system one can obtain a configuration in which all the cata-

lyst is illuminated, therefore the thickness of the supported

catalyst layer should be thin enough to enable the light to

0926-860X/$ see front matter 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2003.11.013


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106 T.A. McMurray et al. / Applied Catalysis A: General 262 (2004) 105110

reach all the catalyst[9].However, the use of immobilised

TiO2 films leads to mass transfer problems in most photo-

catalytic reactors which results in a reduction of reactor ef-

ficiency and in the accuracy of measured catalyst efficiency

and kinetics[10,11].

In order to determine the real intrinsic kinetics of photo-

catalytic reactions on immobilized TiO2films, a stirred tankreactor (STR) was used. The reactor incorporated a pro-

peller and a baffle, thus providing good mixing and efficient

mass transfer to the TiO2 film. The kinetics of the photo-

catalytic degradation of model pollutants, formic acid and

oxalic acid, were investigated using TiO2films immobilized

on borosilicate glass. The effects of operational parameters

on the rate of degradation of the pollutants is reported, i.e.

propeller rotation speed, catalyst loading, and initial pollu-

tant concentration.

The organic pollutant for a standard test system should

be a recognised organic pollutant. It should be cheap,

easily analysed, water soluble, photochemically inactive,

non-volatile and readily and completely photomineralisedusing Degussa P25 TiO2as the semiconductor photocatalyst

[2]. Degussa P25 is reported to have a high photocatalytic

activity, which is due to the mixed phase of anatase and ru-

tile in P25 promoting charge-pair separation and inhibiting

recombination[1].

Formic acid and oxalic acid were used as the model pol-

lutants in this study because: (1) they are oxidised directly to

CO2 without the formation of any stable intermediate prod-

ucts; (2) they are intermediate products in the photocatalytic

degradation of other larger organic compounds; and (3) they

have been used previously in photocatalytic studies[1214].

2. Experimental

2.1. Immobilisation of TiO2

Borosilicate glass plates (110 mm 110mm) were

cleaned by sonication in hot detergent solution followed

by several rinses in distilled water. The plates were then

dried and weighed. TiO2 was dipcoated from a 5% TiO2(Degussa P25) methanol suspension with a constant with-

drawal rate 4.3 mm s1. The plates were dried after each

coat using an IR lamp. This procedure was repeated to pro-

duce plates with a range of TiO2 loadings. One side of the

coated plate was cleaned to remove the TiO2 and the plates

were annealed in air at 673 K for 1 h. Gravimetric analysis

of the plates was used to determine the TiO2 loading.

The borosilicate glass has a refractive index of 1.489 at

= 365 nm, which gives a corresponding loss due to re-

flection of a perpendicular beam of 3.8%.

2.2. Stirred tank reactor

A major concern in designing photocatalytic reactors is to

minimize mass transfer limitations within the reactor. Mass

Fig. 1. Schematic representation of a stirred rank reactor.

transfer limitations considerably decrease the accuracy of

measured catalyst efficiency and kinetics. A stirred tank re-

actor was used due to the simplicity of the system and the

ability to eliminate any mass transfer limitations. Good mass

transfer behaviour is obtained as turbulent flow within the

reactor can be created in order to transport the organic pol-

lutant towards the coated TiO2 plate and disperse O2 from

the headspace into the liquid.

A schematic representation of the STR is given inFig. 1

and corresponding dimensions inTable 1.

The coated TiO2glass plate was secured to the bottom of a

water-jacketed walled vessel creating a reservoir. A stainless

steel propeller was used in order to create a turbulent flow di-

rected towards the coated glass plate were the reaction takes

place and mass transfer rates should be maximised. The ro-

tation of the propeller was achieved using a homogenator

motor (Camlab Ltd., Tri-R Instruments model S63C) givingrotation speeds over the range 02500rpm. The homogena-

tor motor was calibrated using an optical tachometer. A

stainless steel baffle was used to increase mixing within the

reactor.

The catalyst was illuminated from below using two PL-S

9W/10 UV-A fluorescent lamps (Philips) with a stable output

between 350 and 400 nm (peak emission at 370 nm), which

were positioned at a distance of 2.5 cm away from the TiO2glass plate.

The light intensity entering the reactor was determined

by potassium ferrioxalate actinometry[15]. Oxygen (99.5%)

was added to the headspace of the reactor.

The glass wall of the reactor consisted of a cooling jacket

connected to a thermostatic bath for temperature regulation

Table 1

Specifications of stirred tank reactor

Dimensions of STR

Outer diameter (including cooling jacket) (m) 1.00 101

Inner diameter (excluding cooling jacket) (m) 8.50 102

Height (m) 9.50 102

Reactor volume (m3) 5.40 104

Illuminated catalyst area (m2) 5.67 103

Height of propeller from bottom (m) 4.00 102


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T.A. McMurray et al. / Applied Catalysis A: General 262 (2004) 105110 107

of the system (Grant FH5 flow heater and a Grant FC25

flow cooler). The reactor was kept at a constant temperature

(20 2 C) throughout the experiments.

2.3. Determination of relative mass transfer coefficient

Borosilicate glass was cleaned, weighed and coated withbenzoic acid by pouring molten benzoic acid over the plate.

Benzoic acid melts between 122 and 123C, and partly

sublimes at this temperature. On contact with the glass

plate, the benzoic acid immediately solidified, yielding a

thick uniform layer of benzoic acid on the plate. Once

cooled, a circumference matching that of the base of the re-

actor was etched using a knife and the excess removed. The

remaining benzoic acid layer, conforming to the catalyst

film geometry, was smoothed using a hot cloth and rinsed

with distilled water. The plate was weighed to determine

the amount of benzoic acid present. Experiments were car-

ried out at propeller rotational speeds of 1000, 1500 and2000 rpm. The benzoic acid coated plate was placed in the

reactor and the reactor was filled with 200 cm3 of water. A

2.0 cm3 sample was taken immediately, then every 30 s in

the first 2 min, and then every minute there after for a total

of 10 min. The benzoic acid concentration was determined

by UV absorption at 272 nm (Perkin-Elmer, Lambda 11).

2.4. Photocatalytic experiments

The UV lamps were allowed to stabilise for 20 min prior

to commencing experiments. Aqueous solution (200 cm3) of

formic acid or oxalic acid, with the desired initial concen-

tration, was added to the reactor and the headspace purged

with O2. The propeller was switched on and adjusted to the

required speed. A 1.5 cm3 sample was taken immediately

(t= 0 s) and every 15 min thereafter, usually for 2 h.

2.5. Analysis

The concentration of formic acid and oxalic acid were

determined by Ion Exclusion HPLC with an Aminex

HPX-87H Ion Exclusion Column (300 mm 7.8 mm

i.d., Bio-Rad). Conditions were as follows: mobile phase

was 1 103 mol dm3 H2SO4 pH 1.5 at a flow rate of

0.8 cm3 min1, column temperature was 30 C, injection

volume was 100l for formic acid and 20 l for oxalic

acid, UV detection at = 210 nm.

3. Results and discussion

3.1. Relative mass transfer coefficient

The mass transfer coefficient was estimated at several ro-

tational stirrer speeds by performing experiments with ben-

zoic acid.Fig. 2shows the concentration of benzoic acid as

Fig. 2. Benzoic acid concentration vs. time at different propeller rotational

speeds.

Table 2

Mass transfer coefficients calculated for benzoic acid at varying propeller

rotational speeds

Propeller speed (rpm) k1 (m s1) 105

1000 2.48

1500 3.54

2000 3.90

a function of time for the three propeller speeds studied, i.e.

1000, 1500 and 2000 rpm.

To determine the mass transfer coefficient (k1) from the

concentration (C) versus time (t) curve the balancedEq. (1)

was used:

VdC

dt

= k1A(Cs C) (1)

where V is the volume of water present in the reactor, A

the area covered with benzoic acid, and Cs the solubility

of benzoic acid in water (the saturated concentration of

benzoic acid in water over solid benzoic acid).

Using the data, the mass transfer coefficient for benzoic

acid in this reactor was determined for each propeller speed

(Table 2).

The relative mass transfer coefficient for formic acid and

oxalic acid (Table 3)was then determined using the diffu-

sion coefficients for benzoic acid, formic acid and oxalic

acid (Eq. (2)), where the exponent is valid for turbulent

conditions[16].

k1,oxalic/formic =

D0oxalic/formic

D0benzoic

0.67k1,benzoic (2)

Table 3

Calculated relative mass transfer coefficients for formic acid and oxalic

acid

Propeller

speed (rpm)

k1 formic acid

(m s1) 105k1 oxalic acid

(m s1) 105

1000 3.83 3.25

1500 5.47 4.65

2000 6.01 5.11


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Fig. 3. Effect of catalyst loading and apparent quantum yield on formic

acid degradation rate.

The diffusion coefficient of benzoic acid (7.821010 m2 s1)

and formic acid (1.49109 m2 s1) were determined using

the methods of Wilke-Chang and Le Bas [17]and the dif-fusion coefficient for oxalic acid used (1.17 109 m2 s1)

was taken from Kulas et al. [11].

3.2. Catalyst loading

In the dip coating procedure it was found that the catalyst

loading was directly proportional to the number of dips with

3.9 104 kg m2 of TiO2 deposited per dip.

The rate of degradation of formic acid was measured as

a function of catalyst loading in order to determine the op-

timum catalyst loading. The initial concentration of formic

acid was 5.3 mol m3

and the degradation was observedto follow zero-order kinetics. Formic acid (99%) was de-

graded after 90 min of illumination using a catalyst load-

ing of 1.17 102 kg m2. For the lowest catalyst load-

ing (1.2 103 kg m2), 91% degradation was achieved in

240 min. The rate of degradation increased with increas-

ing catalyst loading, up to an optimum loading of approx-

imately 1.17 102 kg m2. It was found any further in-

crease above this catalyst loading caused a decrease in the

rate of degradation (seeFig. 3).This trend has been observed

with other workers[18,19]and can be explained by the fact

that as the catalyst film becomes too thick the TiO2will be-

gin to effectively mask itself with the total irradiation being

absorbed by only the initial layers of catalyst. The maxi-

mum rate, under the conditions of these experiments, will

be achieved when all of the incident light is absorbed by the

catalyst film. This occurs, forI0( = 370 nm)= 3.5 104

Einstein m2 s1, when the catalyst loading is approxi-

mately 1.17 102 kg m2 (seeFig. 3).

3.3. Rate as a function of propeller rotation speed

The rate of degradation was examined as a function of

propeller speed to determine if it was mass transfer limited.

Experiments at three different propeller rotations, i.e. 2000,

Fig. 4. Effect of propeller rotational speed on formic acid and oxalic acid

degradation rate.

1500 and 1000 rpm were performed.Fig. 4shows the con-

centration versus time plot for the different rotation speeds

for formic acid and oxalic acid. The rates for each of thepropeller speeds were very similar with a coefficient of vari-

ance of 2% for formic acid and 3% for oxalic acid (Table 4).

Therefore it was concluded that the photocatalytic degrada-

tion of formic acid and oxalic acid was not mass transfer

limited under the conditions of the experiments.

3.4. Rate as a function of formic acid concentration

The degradation of pollutants can be described by the

LangmuirHinshelwood kinetic model [1,4,20]. Assump-

tions for the LH model have been described by Fox and

Dulay[21]suggesting that: (1) only one substrate can bindat each surface site; (2) at equilibrium the number of surface

adsorption sites is fixed; (3) there is no interaction between

adjacent adsorbed substrates; (4) the rate of surface absorp-

tion of the substrate is larger than the rate of any subsequent

chemical reactions; and (5) the heat of absorption by the

substrate is identical for each site and is independent of sur-

face coverage. Two situations can exist: pseudo zero-order

kinetics and pseudo first-order kinetics. In the STR the

degradation rate of formic acid and oxalic acid appeared to

follow pseudo zero-order kinetics (seeFig. 5).

Table 4Determined rates for formic acid and oxalic acid at varying propeller

rotational speeds

rpm Rate

(mol m2 s1) 105R2 Data

points

Formic acid

1000 3.03 0.946 7

1500 2.94 0.990 7

2000 3.06 0.986 7

Oxalic acid

1000 1.59 0.991 13

1500 1.53 0.993 13

2000 1.48 0.990 13


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T.A. McMurray et al. / Applied Catalysis A: General 262 (2004) 105110 109

Fig. 5. Effect of initial concentration and apparent quantum yield on

formic and oxalic acid degradation rate.

The rate law is shown in Eq. (3), where R is the initial

rate of the disappearance of formic acid and [S] is the initial

concentration.K is the Langmuir adsorption constant and k

is the rate proportionality constant.

R =kK[S]

1 + K[S] (3)

The usual method for obtaining values for K and k is to

plot a double reciprocal plot of initial rate (R) versus initial

concentration [S] (Fig. 6; rate data shown inTable 5). Such

a plot should be linear if the LH rate form is representative.

The intercept of this line corresponds to 1/kand the slope is

equal to 1/kK. Therefore, ifkis independent of reactant, the

intercepts should be equal for all reactants degraded in the

same reactor and under the same conditions[22]. For formic

acid,kwas determined to be 3.77 105 molm2 s1, and

K to be 2.23m2 mol1. For oxalic acid, k was determined

to be 1.99 105 molm2 s1 andKto be 0.88 m2 mol1.

The rate constant k for formic acid is close to twice that

measured for oxalic acid and therefore k, in this instance, is

not independent of the reactant.

Fig. 6. Double reciprocal of rate vs. initial concentration for formic and

oxalic acid.

Table 5

Initial rate of degradation for different initial concentrations of acid

Initial concentration (molm3) Rate (mol m2 s1) 105

Formic acid

5.20 3.45

4.20 3.36

3.10 3.332.80 3.13

0.52 2.62

Oxalic acid

5.00 1.52

4.00 1.69

3.00 1.52

2.00 1.20

1.00 0.95

3.5. Quantum efficiency

Potassium ferrioxalate actinometry was carried out todetermine the light intensity falling upon the TiO2 film.

The actinometer solution conforms to reactor dimensions

and only measures the light that enters the reactor. The

method used in this case is as that of Hatchard and Parker

as given in Calvert and Pitts [15]. The incident photon

flux on the TiO2 coated support was determined to be

3.5 104 Einstein m2 s1. The apparent quantum yield

(app) of the reaction can be defined as the initial degrada-

tion rate (mol m2 s1) of pollutant divided by the photon

flux (I0) (Einstein m2 s1)(Eq. (4)).

app=

rate

I0 (4)

app increases with catalyst loading (Fig. 3) and reaches

a maximum corresponding to the optimum film thickness.

The maximum appmeasured were 10% for formic acid and

5% for oxalic acid (seeFig. 5).This is an important finding

as normally reportedappvalues for photocatalytic systems

are ca. 1%[3].

4. Conclusions

The kinetics of the photocatalytic degradation of oxalic

and formic acid on immobilized TiO2 films were investi-

gated under conditions of high mass transfer using a novel

stirred tank reactor. The effect of operational parameters

on the rate of degradation were investigated, i.e. propeller

rotation speed, catalyst loading, and initial pollutant con-

centration. It was found that the rate of degradation was not

significantly dependent upon the propeller rotation speed

and therefore not mass transfer limited. The degradation

rate increased with increasing catalyst loading until an op-

timum was reached above which, the rate decreased. Both

oxalic and formic acid degradation kinetics were found to

obey a LangmuirHinshelwood type kinetic model. The


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maximum apparent quantum yield for oxalic acid was 5%

and that for formic acid was 10%, under the conditions

of the experiments. This compares well with other studies

reporting ca. 1% for photocatalytic reactions. Further work

will investigate the effect of light intensity, pH and ionic

strength on the degradation kinetics.

Acknowledgements

The authors would like to thank Degussa for supplying

samples of P25, Henk Giller, Philips lighting, The Nether-

lands, for supplying UV lamps, the engineering technical

staff of the University of Ulster for reactor construction,

the European Commission for funding under the 4th and

5th Framework Programmes for PCATIE ENV4-CT97-0632

and PEBCAT EUK1-CT-2000-00069 respectively. Also to,

the Department of Higher and Further Education Training

and Employment, Northern Ireland, for funding T.A. Mc-

Murray.

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