A Spectroscopic Study of Anatase Properties

Zeitschrift fur Physikalische Chemie Neue Folge,.Bd. 124, S. 211-222 (1981)© by Akademische Verlagsgesellschaft, Wiesbaden 1981

A Spectroscopic Study of Anatase Properties4. The Adsorption of Carbon Dioxide*

Claudio Morterra, Anna Chiorino and Flora Boccuzzi

Istituto di Chimica Fisica dell'Universita Corso Massimo d'Azeglio 48—

10125 Torino

and

Emilia Fisicaro

Centro Ricerche Pigmenti, Montedison, Spinetta Marengo (AL)

(Received July 7, 1980)

Anatase properties / C02 adsorption on Anatase

C02 chemisorption at ambient temperature on a via-chloride anatase preparation was

investigated by microgravimetry and IR spectroscopy. Besides a C02 species linearly held atsurface acidic sites, different types of carbonate-like species were formed, revealing the existenceof surface basic sites. The nature of surface carbonates varied on varying the sample activationtemperature. The overall chemisorbed amount was quite small, so indicating that very littlesurface basicity can be evidenced by a "soft" acid like C02. The activity of surface oxygens was

found to depend strongly on the hydration degree, as well as on the pre-adsorption of polarmolecules, so indicating that anatase surface basicity is influenced by the overall polarity of thesurface layer.

Die Adsorption von C02 bei Zimmertemperatur wurde an einem iiber Chlorid hergestelltenAnatas mil Hilfe von Mikrogravimetrie und IR-Spektroskopie untersucht. Neben einer linear an

sauren Oberflachenplatzen gebundenen C02-Species werden verschiedene Arten Karbonat-ahnlicher Species gebildet, die die Existenz basischer Oberflachenplatze deutlich machen. DieNatur der Oberflachenkarbonate iindert sich mit der Aktivierungstemperatur der Proben. Dieinsgesamt adsorbierte Menge ist niimlich klein, was zeigt, daB eine „weiche" Saure wie C02 nur

sehr wenig Obertlachenbasizitat nachweisen kann. Die Aktivitiit des Oberfliichensauerstoffshangt stark vom AusmaB der Hydratation und von der vorherigen Adsorption polarer Molekelnab. Dies zeigt, daB die Oberflachenbasizitat des Anatases durch die gesamte Polaritiit derOberflachenschicht bedingt ist.

* Part 1-

3 see ref. [1-

3].

212 Claudio Morterra, Anna Chiorino, Flora Boccuzzi and Emilia Fisicaro

Introduction

Carbon dioxide has been widely used as a probe molecule to check surfacebasicity of oxides, in view of the weak and reversible adsorption it originates,as well as of the great spectral changes it undergoes upon adsorption.Recently, a correlation has been found on several Al containing oxides[4—6] between cation coordination and carbonate-like structures formedupon C02 adsorption, as well as between surface hydration degree and basic-ity. The aim of this work is to study the basic properties of pure anatase, asrevealed by C02 adsorption, both to compare them with those of otheroxides, and to gain a preliminary knowledge of pure Ti02 surface, prior toinvestigate more complex Ti02 preparations (i.e. Ti02 with various additivesthat give it pigmentary characteristics).

In the recent years, other authors dealt with C02 adsorption on bothrutile and anatase, among which Yates [7], Primet [8], and Parfitt [9]. Somedisagreement exists amont these authors' findings, concerning the assign-ment of the IR bands of adsorbed species and the identification of adsorbingsites, so that a re-examination of the anatase/C02 system seemed to beneeded.

ExperimentalThe apparatus and experimental procedure of sample preparation and treatment have been

described in a previous paper [1] that the reader is referred to for details. In the present work, a

via-chloride preparation (TC) is dealt with, minor differences of chemical behaviour towardsC02 between this and other materials being discussed elsewhere. C02 was adsorbed on samplesfrom which Cl impurity was thermally eliminated and on which the so called vacuum-reductionwas avoided by 02 contact and evacuation [jj.

In all experiments reported here Ti02 surface water was completely exchanged with D20, toavoid the overlapping of carbonate bands with the bending mode of coordinated water at~ 1630 cm"1.

Sample thermal treatments were carried out in a in situ IR cell, allowing an ultimate vacuumof less than 10"3N m~2 to be reached.

Surface area was determined after each thermal treatment in a BET-like vacuum line with a

very small dead space and was calculated with the t-plot method.C02 adsorption isotherms were determined gravimetrically with a Sartorius 4102

electrobalance.All spectra were run on a Beckmann IR 12 double beam spectrophotometer.

Results

C02 adsorptionC02 chemisorption at 298 K on anatase strongly depends on pressure and

is almost completely reversible at the same temperature. Fig. 1 shows theadsorption isotherm on a sample highly dehydrated at 673 K.

A Spectroscopic Study of Anatase Properties, 4. 213

nrr|2

2 4 6 8KNrn2

Fig. 1. Adsorption isotherm at 298 K. of C02 on anatase dehydrated at 673 K. The asteriskindicates the amount remaining adsorbed after a 5min evacuation at 298 K. Ordinate: C02

molecules per nm2

1700 1600 1500 1400 1300 crrr1

Fig. 2. The IR spectrum in the 1730—1200 cm"1 range due to C02 adsorption (5.3 kNm 2)onto anatase dehydrated at various temperatures. (/) C02 allowed on the sample in equilibrium

with ~267N m"2 of D20. (2-5) After dehydration at 298, 373, 473, 673 K

On the contrary, C02 adsorption little depends on sample activationtemperature: the maximum uptake occurs on Ti02 dehydrated at ~473 K,that chemisorbs some 20 % more than samples almost completely hydrated


2800 2600 24CO cm"'

Fig. 3. The IR spectrum in the 2800 —2200cm 1 range due to CO, adsorption (5.3 kNm 2)onto anatase dehydrated at 373 K. (/) Background. (2) After CO, allowance

Table 1. IR bands of the species formed upon C02 adsorption on deulerated anatase. (The spectralranges indicated are those covered with increasing sample activation temperature)

Species IR active modes Assignment Sitesinvolved

A (see Scheme 5) 2345-2353 1 380 Coordinated I (Scheme 1)CO,

B (see Scheme 5) 2669

C (see Scheme 1)

D(see Scheme 2)

1555-1580 1425-1443 Bicarbonate III (Scheme 5)1670-1673 1243 Bidentate I (Scheme 1)

carbonate1 575-1 590 1 370- 1 323 Monodentate II (Scheme 2)

carbonate

(i.e. activated at ambient temperature) and samples completely dehydrated at-773 K.

The adsorption causes several changes in the 2800—

1 200cm"1 range ofthe IR spectrum of deuterated samples. Deuterated samples activated at


300 500 700 K

Fig, 4. The concentration trend of various C02 adsorbed species (C02 pressure is 5.3kNm~2)with anatase activation temperature. The letters correspond to the species described in Table 1

T> 473 K. also exhibit weak bands in the 3750—

3 700 cm"1 range, due toovertone and combination modes of linearly held C02 [3].

Fig. 2 shows the changes produced by C02 adsorption (5.3 kN irT2) inthe 1730—1 200cm"1 range of TC at various dehydration stages, whereasFig. 3 shows the changes produced in the 2800 —2200 cm"1 range of a TCsample dehydrated at 373 K. Several species are formed, whose spectralcharacteristics are summarized in Table 1 and whose dependence on TCactivation temperature is schematized in Fig. 4.

The main features are:

Species A. This is a chemisorbed species, in which C02 still maintains itslinear shape. It was discussed in a previous paper [3] and assigned to C02molecules interacting with coordinatively unsaturated (cus) cations, i.e.Lewis acid sites, whose asymmetric field weakly activates the IR-forbiddenTg+ mode at ~ 1 380cm"1.

Species B. This species, that forms at the expence of some free surface ODgroups and possesses an OD group absorbing at 2669 cm"1, was previouslyobserved by other authors [8,9] and assigned to surface bicarbonate-likestructures.

It is the only species formed to an appreciable extent when TC is inequilibrium with some water vapour at ambient temperature and is formeduntil some hydroxyl groups are left at the surface.

Both C02 stretching modes of the bicarbonate move to higher wavenum-bers with increasing sample dehydration temperature, but the shift of the


asymmetric mode (at higher v) is less clearly seen, due to the overlapping withanother band.

Species C. This species, never observed in previous works, is characteristic ofhighly hydrated samples, but some surface unsaturation, brought about bydehydration, is needed for its formation, as it does not form at all on samplesstill in equilibrium with water vapour. The bands are sharp, symmetric, andof frequency quite constant with varying dehydration temperature, suggest-ing a well defined species, little influenced by lateral effects.

The spectral position of the two stretching modes, and their splitting ofsome 430 cm"1 strongly suggest the assignment to a bidentate carbonate [10],i.e. to a bridging structure between an anionic and a cationic vacancy in thesame coordination sphere, as shown in Scheme 1:

Species D. This is the only species observed on anatase in the early work byYates [7], but its variable spectral features were not observed, as C02adsorption was not studied on samples dehydrated at various temperatures.

The high frequency mode increases in frequency and the low frequencyone decreases on more and more dehydrated TC.

Also the shape and half-band width of the bands vary with varying sampledehydration temperature, especially for the low v mode. Consequently, inFig. 4 the curve relative to species D is reported as a broken line as so drasticchanges of spectral features are likely to be accompanied by some changes ofextinction coefficient. The quantitative meaning of curve D of Fig. 4 is thusrather questionable.

Spectral position and splitting of the two stretching modes are somewhatnon-specific of (C03)~2 groups [10]. Very similar bands, with a similar be-haviour with sample dehydration, were previously observed on a-Al203[5]. The assignment we propose is, like on a-Al203, to monodentatecarbonates, i.e. to C02 molecules coordinated to basic surface oxygens, asshown in Scheme 2:

o

,c.

1)

Site I Species C( stands for an anion vacancy)

O'2-

2) Ti C02

Site II Species D


1700 ieOO 1500 1400 1300 cm-1Fig. 5. The IR spectrum in the 1 730—1200cm" 1 range due to COz adsorption (curves 1) anddesorption (curves 2) on anatase dehydrated at 298 K (section a) and 773 K (section b).

Adsorption: 5.3kNm~2. Desorption: 5min at 298 K

The adsorbed amounts

The asterisk in Fig. 1 shows that a short pumping off at ambienttemperature eliminates chemisorbed C02 almost entirely. Fig. 5 indicatesthat, on both highly hydrated and dehydrated samples, monodentatecarbonates (species D) are the only species that partly remain after the room

temperature evacuation.If the gravimetric datum is used for a rough calculation of the integrated

extinction coefficient of the bands due to monodentate carbonates, it isdeduced that, on a fully dehydrated sample (Fig. 5 b) and under a pressure of5.3 kNm~2 they approximately represent 50 % of the total adsorbed amount,the other 50% being almost entirely due to linearly coordinated C02. Thisdatum is consistent with that reported in a previous paper devoted to anatasesurface acidity [3], in which it was observed that the amount of linearlycoordinated C02 was almost constant after a dehydration at 573

—

773 K andcorresponded to the amount of CO chemisorbable at the same sites(~ 0.6 molecules per nm2).

These data, though very approximate, also indicate that, on TC samplesdehydrated at temperatures in the 473

—

573 K range, the amount of sur-face bicarbonates formed under a pressure of 5.3kNm~2 is, very low(~ 0.2 molecules per nm2), as also suggested by the low intensity of the OD


band at 2669 cm 1 and by the known high extinction coefficient of C02stretching modes of bicarbonates [6].

Discussion

The complete dehydration of anatase occurs within 298 and ~ 773 K,through either of the mechanisms 3 and 4, where stands for an anionvacancy and cus stands for coordinatively unsaturated).

H20 D

/ /3) Ti- H20 + Ti

4)OH- OH" D 0(cus;Ti Ti - H20 * Ti Ti

They lead to the creation of an isolated anion vacancy in the former caseand of a coordinatively unsaturated anion-cation couple in the latter case.The above mechanisms are only schematic and cannot be thought of as reallyindependent, as surface hydroxyls and coordinated water are desorbed inmuch the same temperature range [1,3]. Besides, also the nature of thecreated sites is just schematic, the actual chemical structure likely being more

complex and possibly revealed by the interaction with suitable admolecules.All coordinatively unsaturated centres are, in principle, capable of

chemisorbing suitable molecules, to restore the surface layer, but surfacecoverages attained further to gas/solid interactions very seldom approachunity. In the case of no-d-electron metal oxides only water is capable ofbuilding up a full surface monolayer.

Carbon dioxide, that is able to interact with both acidic and basic centres,on anatase yields very low coverages in all cases. Anatase surface basicity,that is the main interest of the present work, so turns out to be very poor. Thepeculiar basicity of the surface layer of dehydrated anatase, compared withother oxides, was also revealed by the high percent of undissociatedmolecular water coordinated at the surface during the rehydration process[3].

Bidentate carbonates (species C, Scheme 1) are very labile and are onlyproduced on highly hydrated surfaces. This indicates that the basicity of theinvolved surface oxygens is not an intrinsic property of anatase, but isinfluenced by the overall polarity of the surface layer, so that on severelydehydrated surfaces C02 molecules preferably interact with the coordi-natively unsaturated cation of centres I in a linear form [3] rather than withthe anion-cation couple.


_,_-_i_._Lh_,_,_,_,_i_,_|_|_,_|2700 2500 2300 1700 1600 1500 1400 1300 1200

Fig. 6. Spectral changes due to incipient rehydration of anatase dehydrated at 673 K andequilibrated with 5.3kNm~2 of CO,, (b) Background, (•-• after contact with C02.(1

—

4) After the diffusion into the sample cell of very small D20 doses (D20 equilibrium pres-sures are lower than 13.3 Nm~2)

To confirm that the absence of bidentate carbonates on dehydratedsamples is due to the decreased basicity of surface oxygens, reversiblybrought about by dehydration, small D20 doses were allowed to diffuseslowly into a cell containing a highly dehydrated sample in contact with C02(see Fig. 6). The slow rehydration, monitored by the appearance of ODgroups (bands at 2800 —2400 cm"') and of coordinated D20 (band at~ 1 210 cm"l), causes several spectral changes, among which the formationof bidentate carbonates at the expense of some linear C02 species.

Also the experiment of Fig. 7 confirms that oxygen basicity is a functionof the overall surface layer polarity: C02 allowance onto a sampledehydrated at 473 K, and thus unable to form bidentate carbonates, causesthe production of fair amounts ofbidentates (the regular ones and a new type,with CO stretching mode at ~ 1710cm"1) if the dehydrated samplechemisorbed pyridine before contacting C02.

It is recalled here that Py chemisorbs in various ways on TC dehydratedsamples [3], yielding a rather high coverage, i.e. creating a large fraction of a

strongly polar surface layer, quite similar to that caused by water. Onremoving the major part of chemisorbed Py, surface covalence increasesagain, and bidentate carbonates do not form anymore upon C02 adsorption


A

03f

Q2f

O.lf

1800 1700 1600 1500 1400 300 cm",-1

Fig. 7. The IR spectrum in the 1800-1200cmrange due to C02 adsorption (5.3kNm~2)onto anatase that was dehydrated at 473 K. and then chemisorbed Pyridine. Before the contact

with O02, Py was desorbed at 298 K (curve /) and at 473 K (curve 2)

(curve 2 of Fig. 7), whereas the amount of linearly adsorbed C02 (species A)increases [3].

Fig. 7 shows that Py pre-adsorption completely inhibits the formation ofbicarbonates (species B), and that also after the removal of the vast majorityof Py, bicarbonates do not form upon C02 adsorption. This indicates that on

Ti02, as on other oxides [4], bicarbonates formation requires a suitable OHgroup with a nearby anionic vacancy, that would act as the first attack centre,as shown in Scheme 5:

It is also deduced that the anion vacancy in sites III belongs to the familyof the most acidic Lewis sites, as after Py evacuation at 473 K only the moststrongly coordinated Lewis species remain at the surface [3], but sites III arestill unable to form bicarbonates.

Bicarbonates are seen to form abundantly in the first stages of TCrehydration (Fig. 6), most likely at the expense of some of the linearlycoordinated species that are desorbed upon water contact. This confirms thenature and the role of sites III, and suggests that, when dehydroxylated(dehydroxylated sites III can be either Ti ions carrying two anion vacancies or

sites I), sites III are favoured centres for rehydration.As for monodentate carbonates, dehydration/rehydration experiments

allow a rationalization of the spectral changes of Fig. 2. In fact, it seems

logical to admit that there are two such species. A first one, that prevails on

o, OH

Site III Species A Species B


highly dehydrated samples, has the symmetric C02 stretching mode sharpand centered at 1320— 1 330cm"1 and the asymmetric mode at 1 590 cm"1(e.g., see curve 5). On rehydration, the two modes would decrease theirsplitting (the high v mode shifts downwards, and the low v one upwards, as

already observed [10]), this variable splitting of the two coupled modesalways occurring on varying the covalence of the CO bond (i.e. on varying thedouble bond character of the C02 bonds).

A second monodentate carbonate, characteristic of highly hydratedsamples, possesses a broader and stronger symmetric mode centered atfrequencies as high as 1 370 cm"1 (e.g. see curve 2), and the asymmetric modeat ~ 1 575 cm-1, i.e. at lower v than the other monodentate (the actualspectral position is rather uncertain, due to the overlapping with abicarbonate mode). In the first stages of dehydration, most water desorbsaccording to the mechanism of Scheme 3 [1], so that the latter monodentatecarbonate is tentatively ascribed to a coordinatively unsaturated oxygen,whose basicity is primarily induced by a un-dissociated water moleculecoordinated at the nearby cation, carrying a coordinative vacancy (seeScheme 6):

There are two arguments in favour of this assignment:(i) C02 adsorption onto a-Al203 yields a monodentate species at nearly

identical frequencies, and this species was previously assigned to a structurelike that of Scheme 6, with an octahedrally coordinated Al ions as the cationiccentre [5];

(ii) C02 adsorption onto preadsorbed Py (see Fig. 7) yields two monoden-tate species with symmetric C02 stretching mode at 1 320 and ~ 1 385 cm"1respectively. The former species is the monodentate carbonate typical ofhighly dehydrated surfaces and exhibits the same v shifts previously describedas a function of the overall surface polarity, whereas the latter is only formedon samples with preadsorbed Py and is typical of high Py coverages, so that itcan be ascribed to a structure like that of Scheme 7:

o 2-

6)

Ti'

7) \Ti'"

This is regarded as a site in which oxygen basicity is induced by thecoordination of Py onto the nearby cation.


The easy elimination at 473 K of the Py responsible for the formation ofthis monodentate carbonate suggests that the involved centre possesses a

weaker Lewis acidity than, for instance, sites III [3].

References

1. C. Morterra, A. Chiorino, A. Zecchina and E, Fisicaro, Gazz. Chim. Ital. 109 (1979) 683.2. C. Morterra, A. Chiorino, A. Zecchina and E. Fisicaro, Gazz. Chim. Ital. 109 (1979) 691.3. C. Morterra, G. Ghiotti, E. Garrone and E. Fisicaro, J. Chem. Soc. Faraday Trans. I 76

(1980) 2102.4. C. Morterra, G. Ghiotti, F. Boccuzzi and S. Coluccia, J. Catal. 51 (1978) 299.5. C. Morterra, S. Coluccia, G. Ghiotti and A. Zecchina, Z. Phys. Chem. Neue Folge 104

(1977) 275.6. C. Morterra, A. Zecchina, S. Coluccia and A. Chiorino, J. Chem. Soc. Faraday Trans. I 73

(1977) 1544.7. D. J. C. Yates, J. Phys. Chem. 65 (1961) 746.8. M. Primet, P. Pichat and M. V. Mathieu, J. Phys. Chem. 75 (1971) 1216.9. P. Jackson and G. D. Parfitt, J. Chem. Soc. Faraday Trans. I (1972) 896.

10. E. Borello, Fundamental Principles in Heterogeneous Catalysis (NATO Adv. Study Inst.,Venice, 1971).

A Spectroscopic Study of Anatase Properties

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