Influence of Carboxylic Acid on thePhotocatalytic Reduction of
Cr(VI) Using
Commercial TiO2
G. Colon, M. C. Hidalgo, and J. A. Navo*
Instituto de Ciencia de Materiales de Sevilla, Centro
MixtoCSIC-Universidad de Sevilla, Avda. Americo Vespucio s/n,
41092 Sevilla, Spain
Received May 28, 2001. In Final Form: July 23, 2001
IntroductionTitania has universally been recognized as one of
the
better photocatalysts in heterogeneous
photocatalysisapplications as it combines two important
complementaryfeatures for a photocatalyst: good UV absorption
efficiencyfor the light harvesting process and good
absorptioncapacities, due particularly to the density of OH-
groupsof amphoteric character. Many studies concern only
thecatalytic activity of titanium oxide with single
substrates.1There are few papers concerning decontamination
ofcomplex systems, which however are the actual situationof the
real environmental pollution.2,3Many waste streamsmay contain
mixtures of hazardous organic and inorganicspecies. In fact,
laboratory studies of these real mixturesare a complex problem to
resolve. H. Fu et al.4 reportedthe simultaneous degradation of
4-chlorophenol and Cr-(VI) in a synergic oxidation-reduction
reaction. Thespontaneous reaction between these two toxic
speciesunder environmental conditions is negligible. But Fu etal.
reported an important increase in the photocatalyticdegradation
when the species are present at the sametime. From the point of
view of the photocatalysis theory,both oxidation and reduction
reactions can take placesimultaneously by considering half redox
semireactionswith the electron-hole pairs photogenerated, avoiding
inthis way the possibility of recombination of these pairsand
therefore influenced synergically. Other interestingexamples were
reported by S. Malato5 using a S2O82- agent,and Y. B. Wang6 using
periodate and persulfate assacrificial oxidants.
The interest in using carboxylic acids as
complementaryreductants is that those compounds are considered
ascommon pollutants from industrial processes
(metallurgy,decontamination of nuclear plants and boilers, and
textileindustries).7 In our previous study,8 although the
increasein the conversion rates for the simultaneous reactionswith
respect to single ones was noteworthy, an importantdeactivation
process took place for commercial TiO2. Now,we study the effect of
different carboxylic acids in theCr(VI) photoreduction. We have
also carried out the same
experiences using N2 instead of O2 in order to state therole of
the gas used in the deactivation process observedin the
simultaneous reaction.
Experimental SectionCr(VI) solution was prepared by using
analytical grade K2-
Cr2O7 (Aldrich, 99%) at acid pH of about 1.5-2. H2SO4 was usedto
set the correct pH value. Salicylic acid (Aldrich, 99+%) andcitric
acid (Aldrich, 99+%) were prepared by dissolving the solidsin
deionized water. Both solutions had a concentration of 4 10-4 M.
For simultaneous degradation runs, a solution wasprepared by mixing
the corresponding volume of each startingsolution of the single
substrates. Therefore, the concentration ofthe new solution was in
this case 2 10-4 M in each pollutant.The pH of the resulting
solution was ca. 2. Degussa P25 andHombikat UV-100 (Sachtleben
chemie) commercial titaniumdioxides were employed as received.
All catalytic runs were performed in a Pyrex immersion
wellreactor. UV illumination of the reaction solutions was
carriedout by using a medium pressure 400 W Hg lamp supplied
byApplied Photophysics. Oxygen and nitrogen flow was employedin all
cases to produce a homogeneous suspension of the catalystin the
solution. Before each experiment, the catalysts (1 g/L)were settled
in suspension with the reagent mixture for 10 min.
The Cr(VI) concentrations were measured by means of UV-vis
spectroscopy, using the characteristic 290 nm band. About2 mL of
suspension was removed and filtered (Millipore Millex250.45 m
membrane filter) previously to UV-vis spectral analysis.
Results and Discussions
Structure and Texture of Catalysts. Wide charac-terization of
commercial TiO2 samples was reported in aprevious paper.8 Specific
surface areas of as receivedcommercial titanium dioxide samples
were determinedbymeansof theBrunauer-Emmett-Teller (BET)method.In
both cases, N2 isotherms could be assigned to type IIthough a small
hysteresis loop can be found. TiO2Hombikat presents an intermediate
shape between typesII and IV, with a hysteresis loop of namely type
H4. Frompore distribution curves, a great difference in the
poretexture of both oxides can be seen. Table 1 summarizessurface
and structural characterization for both catalysts.Worth noting is
the significantly higher value of the surfacearea for TiO2 Hombikat
UV-100 compared to DegussaP25. Pore volume is also greater than
that observed forDegussa P25, but the lower average pore
diameterindicates that Hombikat UV-100 particles are formingsmall
and compact aggregates, thus giving high surfacearea. On the other
hand, Degussa P25 can be thought toform rougher particles with a
porous surface, as wasobserved by the scanning electron microscopy
technique.8The morphology of the Degussa sample is clearly
differentfrom the Hombikat one.8 Thus, TiO2 Degussa presents
awrinkled surface. On the other hand, Hombikat particlesare mainly
spherical, formed by small subparticles highlyagglomerated.
Therefore, the surface of these roundedparticles is full of small
very pores producing the higherspecific surface area exhibited by
Hombikat UV-100.
Regarding the structural feature, Degussa P25 showswell-defined
diffraction peaks corresponding to a phase
* Corresponding author. E-mail: [email protected].(1) Blake, D.
Bibliography of work on the heterogeneous photocatalytic
removal of hazardous compounds from water and air:
NationalRenewable Energy Laboratory: Golden, CO, 1999.
(2) Fallmann, H.; Krutzler, T.; Bauer, R.; Malato, S.; Blanco,
J. Catal.Today 1999, 54, 309.
(3) Mansilla, H. D.; Yeber, M. C.; Freer, J.; Rodriguez, J.;
Baeza, J.Water Sci. Technol. 1997, 35, 273.
(4) Fu, H.; Lu, G.; Li, S. J. Photochem. Photobiol., A 1998,
114, 81.(5) Malato, S.; Blanco, J.; Richter, C.; Braun, B.;
Maldonado, M. I.
Appl. Catal., B 1998, 17, 347.(6) Wang, Y. B.; Hong, C. S. Water
Res. 1999, 33, 2031.(7) Litter, M. I.; Navo, J. A. J. Photochem.
Photobiol., A 1994, 84,
183.(8) Colon, G.; Hidalgo, M. C.; Navo, J. A. J. Photochem.
Photobiol.
2001, 138, 79.
Table 1. Surface and Structural Characterization ofCatalysts
catalystSBET
(m2/g)St
(m2/g)Vp
(cm3/g)Dp()
XA(%)
Eg(eV)
TiO2 Degussa 51.0 47.0 0.15 315 80 3.5TiO2 Hombikat 289.0 282.0
0.34 35 100 3.5
7174 Langmuir 2001, 17, 7174-7177
10.1021/la010778d CCC: $20.00 2001 American Chemical
SocietyPublished on Web 10/04/2001
mixture of anatase and rutile. However, TiO2 Hombikatshowed a
rather amorphous diffraction pattern, with widerpeaks, indicating
less crystallinity. In this case, the onlyphase present is the
anatase one. Anatase molar fractionscalculated from X-ray
diffraction are summarized in Table1.
From UV-vis diffuse reflectance spectroscopy, band gapenergies
have been calculated (Table 1), being for bothphotocatalysts close
to 3.5 eV. Absorption is produced inboth cases at wavelengths lower
than 400 nm.
Photocatalytic Properties. Figure 1 shows the pho-tocatalytic
conversion for Cr(VI)photoreduction usingTiO2Degussa and Hombikat
under an oxygen flow. In bothcases, Cr(VI) photoreduction follows
similar kinetic pro-files. Both Degussa and Hombikat TiO2 arrive to
almostcomplete reduction, at the end of the experience.
Photonefficiencies calculated from these curves (Table 2)
indicatethat the reaction proceeds similarly using TiO2 Degussaand
Hombikat catalysts, being slightly higher for Hom-bikat TiO2.
(a) Cr(VI) and Salicylic Acid Simultaneous Photodeg-radation.
TiO2 Degussa P25 and Hombikat UV-100 weretested for the
simultaneous photodegradation of salicylicacid and Cr(VI), in the
presence of either oxygen ornitrogen flow. After 10 min in which
the photocatalystpowders reach the equilibrium with the substrates
to bephotodegraded, we have irradiated the suspension for 5h.
Figures 2 and 3 show the conversion value curves forTiO2 Degussa
P25 and Hombikat UV-100 tested for thisreaction in the presence of
O2 and N2. In the presence ofoxygen flow (Figure 2), the
photodegradation of Cr(VI)and salicylic acid seems to proceed very
rapidly. Photonefficiencies for the simultaneous photodegradation
presentvery high values with respect to single reactions.
However,maximum conversion observed at the end of the experi-ment
is not higher than 50% in the case of Degussa andbelow 80% for
Hombikat. Significant deactivation of thephotocatalyst is observed.
In our previous paper,8 we havetentatively explained this
deactivation process by the
formation of certain Cr-complexes that would attach atthe
surface of TiO2, blocking the photoprocesses. Thesemetal complexes
would arise from the intermediatephotodegradation products of
salicylic acid. In the case ofHombikat, it was stated that
different surface featuresimprove its photocatalytic properties
giving up higherconversionvalues.PhotoreductionofCr(VI) in
thepresenceof salicylic acid presents higher conversion rates than
inthe single one. In the single photoreduction of Cr(VI),dissolved
oxygen competes very strongly with Cr(VI)species for the
photogenerated electrons, suggesting thatits presence is
detrimental to the reduction of Cr(VI) toCr(III) as has been
reported by other authors.9Thus, wecarried out the same
photocatalytic experiences using apure nitrogen flow instead of
oxygen (Figure 3). In thiscase, conversion values are slightly
higher than thoseobtained in oxygen flow. Photon efficiencies
calculatedfrom conversion curve slopes (Table 2) indicate
howeverthat photoreactions carried out in the presence of pure
O2flow exhibit higher efficiencies, specially for the
Hombikatcatalyst. Using nitrogen as the flowing gas,
similarefficiencies have been calculated for both
catalysts,indicating that catalyst feature is not a determinant
factorin this case. Therefore, the formation of the metalcomplexes
that deactivate the surface photocatalystsshould be aided by the
presence of oxygen, the surfaceand structural features of the
photocatalyst being veryimportant in that case since the
deactivation process couldtake place. Thus, there is a double
function of oxygen in
(9) Alam, M.; Montalvo, R. Metall. Mater. Trans. B 1998, 28.
Figure 1. Photocatalytic conversions for Cr(VI)
photoreductionusing O2 flow for TiO2 Degussa P25 and TiO2 Hombikat
UV-100.
Table 2. Photon Efficiencies for Cr(VI) Photoreductionin the
Presence of Carboxylic Acid
photocatalyst atmosphere single salicylic acid citric acid
Degussa O2 8.5 32.5 34.7N2 20.6 37.4
Hombikat O2 10.6 40.4 31.9N2 23.3 43.1
Figure 2. Photocatalytic conversions for Cr(VI) photoreductionin
the presence of salicylic acid and using O2 flow for (a)
TiO2Degussa P25 and (b) TiO2 Hombikat UV-100.
Notes Langmuir, Vol. 17, No. 22, 2001 7175
the reaction: one positive, increasing the photon efficien-cies,
and one negative, leading to the formation of speciesthat would
turn off the photoreaction.
(b) Cr(VI) and Citric Acid Simultaneous Photodegra-dation.
Figure 4 shows the photocatalytic conversions forTiO2 Degussa and
Hombikat for the photodegradation ofCr(VI) in the presence of
citric acid. In both cases,conversions for the photocatalytic
reduction of Cr(VI)arrive almost to the complete reduction to
Cr(III). Nosignificant differences can be observed when the
reactionis carried out under N2, although in both cases
conversionvalues seem to be slightly higher. When these results
arecompared with those reported in the above section, it isclear
that the nature of the carboxylic acid is ratherimportant in the
reduction of Cr(VI). When the nonaro-matic oligocarboxylic acid is
used as the sacrificial agent,TiO2 does not suffer any deactivation
process, andtherefore surface characteristic is not the
determinantfeature of the photocatalysts. Citric acid improves
thephoton efficiency of Cr(VI) photoreduction, specially usingN2 as
the flowing gas. However, results obtained forDegussa are not so
different for N2 and O2. The presenceof oxygen would be involved in
this reaction, in the waythat it participates actively in the
photodegradation,competing directly with the Cr(VI) reduction. Only
in thecase of the Hombikat photocatalyst is there a greatinfluence
using nitrogen gas flow, the value being highenough with respect to
the value calculated from theconversion curve in oxygen.
The mechanism of photodegradation of aromatics israther complex.
There are many studies concerning the
mechanism followed by these organic compounds duringtheir
photocatalytic oxidation.10,11 Aromatic compoundstrend to form
quinone/hydroquinone and polyhydroxyderivative (mainly dihydroxy)
species at the first stagesof photodegradation, while the first
step in the oxidationof nonaromatic compounds is the formation of a
doublebond by a dehydration process11 and successive crackingof the
molecule into a shorter one. Therefore, it would besupposed that
the formation of this derivative species fromthe aromatic ring
leads to the formation of metal com-plexes, probably at the
surface, that deactivate thecatalyst. Photoreduction of Cr(VI) in
the presence of citricacid does not generate this kind of
complexes, or at leastcomplexes created are not so harmful for
photocatalystactivity.
Structural and morphological comparison of two com-mercial TiO2
photocatalysts (Degussa P25 and HombikatUV-100) revealed that both
oxides present differentcrystalline structure as well as surface
features. Nosignificant differences can be found in the light
absorptiveproperties.
The nature of the carboxylic acid used as the sacrificialagent
seems to have a very important role in thephotoreduction of Cr(VI).
Oligocarboxylic acid with aro-matic character seems to induce the
deactivation of thephotocatalyst. Meanwhile, a nonaromatic
carboxylic acidsuch as citric acid leads to higher conversion rates
without
(10) Pelizzetti, E.; Minero, C. Electrochim. Acta 1993, 38,
47.(11) Herrmann, J. M.; Tahiri, H.; Guillard, C.; Pichat, P.
Catal. Today
1999, 54, 131.
Figure 3. Photocatalytic conversions for Cr(VI) photoreductionin
the presence of salicylic acid and using N2 flow for (a)
TiO2Degussa P25 and (b) TiO2 Hombikat UV-100.
Figure 4. Photocatalytic conversions for Cr(VI) photoreductionin
the presence of citric acid and using N2 and O2 flow for (a)TiO2
Degussa P25 and (b) TiO2 Hombikat UV-100.
7176 Langmuir, Vol. 17, No. 22, 2001 Notes
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