Journal of Photochemistry and Photobiology A: Chemistry 87 (1995) 121-125

Photo-oxidation of sulfite ions in the presence of some iron oxides

Anas Ansari a, Jo& Peral a, X. Dom&nech ‘t*, Rafael Rodriguez-Clemente b, Ana Roig b, Elies Molins b

’ Dcpartament de Quimica, Univcrsirat Autnoma de Barcciona, 08193 Bcllatcwa, Spaitt b Institut de Cihcics dels Materials de Bancclona, CSIC, 08193 Bcllatcwa, Spain

Rccciwd 30 April 1994; ncccptcd 4 0etsbcr 1994

Tho photoentnlytie oxidation of sulfite ions over different iron oxides samples is investigated. For the different hematite samples tested, the experimental data of sulfite photo-oxidized vs. irradiation time can bc adjusted fairly well to a kinctic equation of the type dx/dt K(1 -x)/x, where x is the fraction of sulfite reacted at time t. The behaviour described by this equation corresponds to an heterogeneous process where the product of reaction is strongly adsorbed on the substrate. The rate constant K obtained for the different hematite samples can be related to their respective specific surface area. Fe(H) species were detected in solution when hematite suspensions were irradiated. These Fe(H) ions are assumed to be formed through photoreduction of lattice Fe(II1). On the contrary, for magnetite and maghemite suspensions no Fe(U) ions were detected in solution after irradiation. In the latter case, crystalline phase transformation is assumed to take place.

Keywords: Photo-oxidation; Sulfite ions; Iron axidcs

1. Introduction

Iron oxides are common materials that have the ability to photocatalyze some chemical reactions [l--7]. In those photocatalytic processes, the semiconductor particles, upon absorption of light of suitable energy, gcneratc electron-hole pairs which after migration to the semiconductor-electrolyte interface react with redox species in solution [g]. The bandgap of the different natural iron oxides lies around 2 eV 131, which points to the possibility of solar irradiation photoactivation of these semiconductor materials. Consequently, the iron oxides, which are common constituents of many natural environments, could play a non-negligible role in bio- geochemical cycles. Since the oxidation of SO2 by solid particles suspnnded in atmospheric water droplets seems to be a viable reaction pathway fclr the formation of acid rain [9], we have chosen sulfite as model compound in order to investigate the photocatalytic activity of different iron oxide samples. Particularly, we focus our attention in the study of the photocatalytic activity of

* Corresponding author.

some s;.nples of hematite, magnetite and mnghemitc, which are commonly present in different natural cn- vironmcnts.

2. Materials and methods

All chemicals used wcrc, at least, of reagent grade, Sodium sulfite (Merck) was used as sulfite source. Hematite, magnetite and maghemite were chosen as solid iron oxides phases. Three samples of a-FezO~ were used: (i) hematite A, synthesized by hydrolysis at high temperature (95 “C) of solutions of 0.02 M FeCl, and 0.0016 M I-ICI, in a vertical reactor with continuous flow (residence time 24 h), in a similar way to what has been described elsewhere [lo]; (ii) hematite I3, prepared by hydrolysis of a solution of 0.02 M FeC.1, and 0.0016 M WC1 in an oven at 100 “C, for 24 hours, following the method described by Matijcvis [I 13; hc- matite C was prepared in a similar way, but the eon- ccntration of I=ICl employed was 0.001 M. Magnetite was obtained by precipitative oxidation and it was used as the starting material for the obtention of mayhemite

lOlO-6030/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI lOlO-6030(94)03977-l

ELSEVIER Journal of Phoroehemistryand Pbotobiology A: Chemistry IO5 (1997) 123-127

The low-lying excited states of 1,6-methano [ lo] annulene

Lorenzo Catani 9 Cristina Gellini ‘, Pier Remigio Salvi O*, Giancarlo Marconi b ’ L&oratorio di Spettroscopia Molecolare. Dipam’menro di Chimica. Universitci di Firewe, Via G. Capponi 9. SO121 Fireme. Italy

’ Isrimto FRAEdel CNR. Via P. Gobetti 101.40129 Bologna. Italy

Received I5 July 19% accepted 18 O*ober 1996

Abstract

The electronic absorption SpCCtNIII of 1,6-methano[ IO]annulene in the region of the G*S, and So+& transitions was investigated at

mum and low temperature. By comparison with the one-photon spectrum of naphthalenc. and considering pseudoparity selection rukzs, t

strong 33 000 cm-’ band of the annulene derivative is assigned as the inductively perturbed So-+ (c ) electronic transition. The rich

vibronic structure of the So-+S, transition at low temperature is discussed in terms of Franck-Condon allowed transitions. The calculated intensities of the fundamental modes are in qualitative agreement with the experimental data. 8 I997 Elsevier Science S.A.

Keywords: Electronic absorption spectra; Low-lying excited stares: 1.1%Methanol lO]annuiene

1. Introduction

Aromaticity is a powerful guideline to the understanding

of the properties of conjugated cyclic r-electron systems [ 1.21. In the case of annulenes, i.e. the higher homologues of benzene with (4n + 2) T electrons, distortion from pla- narity may decrease the ?rdelocalization and isomerism may occur as a result of the structural flexibility [ 31. However, it is generally established [ 21 that a more rigid c framework, as supplied by bridgehead links between opposite C atoms or by the formation of triple bonds along the chain, favours the aromatic stabilization of the bond-equalized geometry over the cyclopolyenic arrangement of alternating single and dou- ble bonds.

There are good indications that the spectroscopy of bridged [ IO] -, [ l4]- and [ 181 -annulenes conforms to the aromatic behaviour. The subject has been thoroughly discussed in ear- lier studies [ 4-61. In this paper, we address the pseudoparity properties [ 71 ofexcited mclecular wavefunctions in bridged 1 ,6-methano[ lO]annulene (see Fig. 1) , due to the nature of altemant hydrocarbon, and the relation with the observedUV spectrum. Pseudoparity selection rules have been used for electronic assignment in simple aromatics [ 8-101 and may be particularly suitable in the discussion of low-energy tran- sitions of annulenes which remain unclear [ 61.

* Corresponding author.

Pubtisbed by Elsevier Science .%A. PllSlOtO-6030(96)04546-7

Apart from assignment, the excited states of bridgedannu- lenes have not received much attention so far. Their degree of aromaticity or, in other words, to what extent the excitation to virtual molecular orbitals (MOs) changes the cyclic struc- tural arrangement is an important piece of spectroscopic information in its own right as well as in relation to intra- molecular photoprocesses. The analysis of the !&+S,, absorption profiles may provide important clues on this point. If the electronic transition is allowed, the Franck-condon envelope of vibronic transitions depends, among other things,

on the position of the excited state potential well with respect to the ground state [ 1 I-131. To this end, we have measured the So-’ SI absorption spectrum of 1 &n&ano[ lo]- annulene at low temperature. The intensity profile of the vibronic bands indicates that. on excitation, a geometry change occurs towards a more delocalized structure.

f /Y

I & l

X

Fig. I. Molecularstmehxe of ldmethsao[ IO]annulcae.

124 A. &.wri et al. / J. Photochem. Photobids A: Chem. 87 (1995) 121~125

(Fe3+ -@- -Fe3’(0H’))+SO~-.ds--+

(Fe3+ -O*- - Fe3 + (OH-)) + SO;- a& (8)

The photogenerated conduction band electrons are highly energetic, thus reducing the adsorbed oxygen on the semiconductor and/or reducing the lattice Fe(III). This last process leads to the photocorrosion of the Fe203 [ 161:

Q?eS+ -0”” - Fe3 + (Cl,),,,) C e -

- Fti+(o2-)&) (IO)

P PC)) (11)

in fact, I%(U) has been dctccted in sslutisn in some riments performed under illumination (see e(III), which has also been detected, could by homogeneous oxidation of Fe(II) in

h direct dissolution of the iron oxide excluded. Since lattice Fe(W) and adsorbed mpete to excavenge the electrons, a large

ncentratisn of iron species would be expected when oxygen is not present. This point has been confirmed in experiments performed under uitrogen atmosphere. The formation of 03’- species is the first step

a series of oxidants (e.g., H02’“, pi2 can oxidize Fe( II) or SQ2” [17]. Conversely, SCJ4’- has been detected in solution in

practically all the experiments. However, the observed amounts of S042- in solution nro lower than expected from the stoichiomotry of the process, Strong adsorption of sulfate anions on the semiconductor suP .ce can

emite and magnetite yield of sulfite phe

t of those iron oxides r than in presence of hematite. iation of a I mM sulfite solution,

after the PhotocataMic Drocess. The sr)ectrum corre-

-15 -10 -5 0 5 10 15

VELOCITY (mm/e)

Fig. 3. M6ssbaucr spectra at room tcmpcmturc. The dots rcprcscnt the cxpcrimcnrnl data and the solid lines computer fitted spcctm. (a) maghcmitc. und (b) maghcmitc after being used 8s photocatalyst. The spectrum could bc dccomposcd into three sub spectra that could bc nssigncd to P mixture of maghcmitc ( 43% and mngnctitc ( m 57%).

Tabk 4 Mlissbuucr spcctrnl paramctcrs of the maghcmitc sample alone and the mqhemitc + magnctitc snmplc obtained after photocatalysis

M6ssbaucr parnmctcrs at room tcmpcrnture

Passiblc site lsomcr A( tetrqonol) shifi a B(octnhcdral) (mm/s)

Qundrupalc Intcrnnl splitting magnetic

(mm/s) field (Tcsla)

y-Fc203 Fc”+(A+ B) 0.32fO.04 0 49.9f0.2 FC@J F+(A) 0.32 f 0.04 - 0.05 f 0.01 48.7 f 0.4

Fc*+.‘*(,) 0.62 f 0.02 -0.01 fO.O1 45.4 f 0.4

‘The isomer shift values arc reletivc to a-Fe.

sponds to what can be expected when analysing a mixture of maghemite and magnetite. The spectral parameters used to fit the subspectra assigned to magh- emite were those used in Fig. 3(a). The two other subspectra used for an optimal fit can be assigned to

A. Ansati er al. / 3. Phorochem. Phofobiol. A: Chem. 87 (1995) 121-125 125

magnetite, Fe(II)Fe(III),O,, where Fe(II1) are half in octahedral and half in tetrahedral sites, and Fe(I1) are only in octahedral sites, with continuous electron hop- ping between the Fe(I1) and Fe(II1) in octahedral positions [19], (Table 4). These results suggest that part of the yFe203 suffers a solid-state phase trans- formation to magnetite, Fe,O,+ It is noteworthy that if a sample contains a wide particle size distribution, the lines from particles with different sizes will not suffer the same isomer shift, and the line width of the Massbauer spectrum will then be broaden in an asym- metrical way, as it is most likely the case in our microcrystalline samples.

Indeed, the crystalline structures of maghemite and magnetite are very similar (inverse spinel) and, con- sequently the Fe(II1) in the octahedrical position can be readily red to l?e(II) without 8 1 lattice distortion [ 16) rly, Fe(II1) is reduced, instead of being rclcascd into the solution, it remains incsr- psratcd in the crystnllint: lattice.

4. Conclusions

The oxidation of sulfite in aerated water suspensions is photocatalyzed by the presence of hematite particles. The mechanism of oxidation seems to be related to the electron transfer between the S(IV) of the adsorbed sulfite and the Fe(II1) of the surface of the hematite particles, yielding both sulfate ions, which strongly adsorb on those surfaces, competing with the sulfite for the adsorption sites, and some Fe(I1) species which readily dissolve.

Maghemite and magnetite are less po&erful catalysts for the oxidation of sulfite. In those cases it seems that the reduction of Fe”+ to Fez” does not imply the dissolution of the second species. Instead, a sort of solid state phase change takes place where only the Fe3+ ions situated in octahedral sites become reduced and remain “in situ” leading to the local formation of the crystal structure of magnetite.

Acknowledgements

This work was financially supported by CICYT (Proj- ects MAT 92-0263~CO2-01 and AMB 92-1021X02- 01), for which we are grateful.

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