Langmuir 1993,9, 192-196
192
N2 and H2O Adsorption on Combinations of Ti02 and Fe203 Albert B. Rives,’ Tanuj S. Kulkarni,+ and A m y L. Schwaner* Department of Chemistry, University of North Carolina at Greensboro, Greensboro, North Carolina 27412 Received July 20, 1992. In Final Form: November 9,1992
Simple experiments aimed at uncovering any changes in adsorption properties as F%O3is added to Tiorbased photocatalysts show that the Fe203 additione do very little to the N2 and H2O adsorption until the Fez03 content becomes higher than 80 wt 9%. Above 80 wt 9% Fe203, the amount of strongly adsorbed water begins to increase and it blocke the strong N2 physisorption. Strongly adsorbed water on surfaces of materials that are less than 80 wt 9% Fez03 has virtually no influence on N2 physisorption. X-ray photoelectron spectroscopic(XPS) analysis indicatesthat the surfaces do not become rich in Fe until the bulk Fez03 concentration goes above 80 wt 9%. T h e tendency for the surface to remain rich in T i 0 2 prevents significant adsorption changesfrom occurring as the Fez03 increases from low to moderate levels. Hence, the changes in photocatalytic properties that have been reported for T i 0 2 doped with Fez03 are not expected to stem from grow changes in adsorptive properties. are better electron traps than the Ti(IV) ions since the Fe(II1) ESR signal disappears before the Ti@) signal appears when heating the powders under vacuum.6 Despite the dissimilaritiesin the conclusionsof these papers, they both attribute changem in thephotophysicalpropertiof the materials to the presence of Fe(II1) in the TiO2. The adsorptive changes, however, have not been thoroughly examined for Fe in TiO2. The fact that the Fe in the conventionalcatalyst for ammoniasynthesisis thought to be responsible for activating N211 suggests that Fe additions to Ti02 may also influence N2 adsorption and activation. Not only might N2 adsorption be influenced by the presence of Fe in the TiO2, but so might H2O adsorption. Similarly,any influence the adsorption of one reactant might have on the adsorption of the other may also be affected. The purpose of the work reported here was to begin examining the relationship between Fez03 additions to Ti02 and the adsorption of N2 and H2O on these potentially important materials. Nitrogen physisorption at 77 K on Ti02 and Fez03 has received some attention from researchers trying to fully understand and model the physisorption process and the nature of the surfaces,12 but for the most part, the influences of metal additives have not been studied. Two noteworthy exceptions are found in the work of Burch and Flambard13and Hotta et al.“ Burch and Flambard show increased N2 physisorptionat 273 K on reduced Ti02 samples with Ni dispersed on them, while Hotta et al. show reduced N2 physisorption strength at 77 K when the hydroxyls of Fez03 are covered with a titanate coupling agent. Much has been studied and written about H2O adsorption on Ti02 and Fe203.1b1D The key features of H2O
Introduction In 1977 Schrauzer and Guth reported that Ti02 can be used to photosynthesize NH3 from N2 and H20.l They also reported that additions of 0.2-0.4 w t % Fez03 to the Ti02 increased the rate of NH3 formation as much as 6-fold.’ Since then, others have found increased rates for NH3 f ~ r m a t i o n using ~ - ~ Fe-doped TiO2-based photocatalysts. Indeed, a material developed by Khader et al. for use as a N2 reduction photocatalyst was made entirely of iron oxides.8 Additives, such as Fez03 in TiOz, can affect photocab alytic activity in two ways: (1)by modifyingthe catalyst’s photophysicalproperties,or (2) by modifyingthe catalyst’s adsorptiveproperties. In the case of Fez03in TiO2, changes in the photophysical properties have been uncovered and characterized by Gratzel and cO-workers?JOas well as Soria et al.6 On the one hand, Gratzel and co-workers found that the presence of Fe in colloidal Ti02 greatly increases the lifetime of the photogenerated electron-hole pair in colloidal TiO2. The electron is trapped at a surface Ti atom, converting it from Ti(1V) to Ti(III), and the hole is trapped at a bulk Fe atom, converting it from Fe(II1) to Fe(IV).BJOOn the other hand, however, Soriaet al.worked with Fe-doped powders and deduced that the Fe(II1) ions
* To whom correspondence should be addressed.
+ Current address: ENCAS Laboratories, Winston-Salem, NC 27107. Current address: Department of Chemistry, University of Texas at Austin, Austin, TX 78712. 1977,99,7189(1) Schrauzer, G. N.;Guth, T. D. J. Am. Chem. SOC. 7193. (2) Schrauzer,G. N.;Strampach, N.; Hui, L. N.; Palmer, M. R.; Salehi, J. Roc. Natl. Acad. Sci. U.S.A. 1988,80,3873-3876. (3) Schrauzer, C.N.;Guth, T. D.; Salehi, J.; Strampach, N.; Hui,L. N.; Palmer, M. R. In Homogeneous and HeterogeneousPhotocatalysis; Pelizzetti, E.,Serpone,N.,Eds.;D. Reidel: Dordrecht, The Netherlands, 1986, pp 609-620. (4) Radford, P. P.; Francis, C. G.J. Chem.SOC.,Chem. Commun. 1983, 1620-1621. (6) Palmisano, L.;Augugliaro, A.; Sclafani,A,; Schiavello,M. J. Phys. Chem. 1988,92,6710-6713. (6) Soria, J.; Conesa, J. C.; Augugliaro, V.; Palmisano,L.; Schiavello, M.; Sclafani, A. J. Phys. Chem. 1991, 95,274-282. (7) Schiavello, M.; Sclafani, A. In Photoelectrochemistry,Photocatalysis and Photoreactors; Schiavello, M., Ed.;D. Reidel: Dordrecht, T h e Netherlands, 1986; pp 603-1519 and references therein. (8) Khader, M. M.; Lichtin, N. N.; Vurens, G. H.; Salmeron, M.; Somorgai, G.A. Langmuir 1987,3, 303-304. (9) Moeer, J.; Griltzel, M.; Gallay, R. Helu. Chim. Acta 1987,70,16961606. (10) Griltzel, M.; Howe, R. F. J . Phys. Chem. ISSO, 94,2666-2672.
0743-7463/93/2409-0192$04.00/0
(11)Ertl,G. Proceedings of the Robert A. Welch Foundation Conferences on ChemicalResearch XXV. HeterogeneousCatalysb; Robert A. Welch Foundation: Houston, TX 1981; p 179-207. (12) (a) Day, R.E.;Parfitt, G.D. Trans. Faraday SOC. 1967,63,708716. (b)Furlong, D. N.; Rouquerol, F.; Rouquerol, J.; S i , K. 5. W. J. Chem. SOC., Faraday Trans. 1 1980,76,77&781. (c) Amati, D.; Kovate, E.Langmuir 1988,4,329-337. (13) Burch, R.;Flambard, A. R. J. Chem. SOC., Chem. Commun. 1981, 966-966. (14) Hota, Y.;Ozeki, S.;Suzuki, T.; Imai, J.; Kaneko, K. Langmuir 1991, 7, 2649-2663. (16) (a) Jones, P.; Hockey, J. A. Trans. Faraday SOC.1971,67,26792686. (b)Jackson, P.;Parfitt, G. D. TrOns. Faraday SOC. 1971,67,24692483. (c) Munera, G.;Stone, F. S. Discuss. Faraday Soc. 1971,62,206214. (18) Suda, Y.;Morimoto, T. Langmuir 1987,9,788-788. (17) Beck, D.D.; White, J. M.; Ratcliffe, C. T. J. Phys. Chem. 1988, 90,3123-3131. (8
1993 American Chemical Society
N2 and H f l Adeorption on no, and Fe203 adsorption on each of these materials are the same. Each exhibita three main states of adsorbed H2O; the most tightly bound species is dissociatively adsorbed as hydroxyls, the next most tightly bound species is nondiseociative with the oxygen atom coordinated to a surface metal ion, and the least tightly bound species is simply hydrogen bonded to the more tightly bound species. Presumably the 4-coordinate Ti atoms are needed to dissociate H2O on TiO2, while the 6-coordinate Ti atoms simply bind molecular H20.16J7 On Ti02 the least tightly bound species is reported to desorb at about 360 K,while the next desorbs somewhere around 480 K, and the most tightly bound species desorbs around 660 K.17 The role of H20 adsorption in the photochemistry over T i 0 2 is not simply one of providing atoms for the products. Hydroxyls on the Ti02 surface are also important in the photoeorption of oxygen, with the most strongly bound hydroxyls utilized.2o Indeed,photosorptionof oxygen does not even take place unless the surface is hydro~ylated.21-~3 Consequently,in examining how Fez03 additions to Ti02 influence N2 and H20 adsorption, one needs to look at those influences on hydroxylated as well as non-hydroxylaM surfaces. Another important aspect of these materialsisthe crystal phase of the TiOz. Schrauzer and Guthl reported that anatase was photoactive while rutile was much less Unfortunately, the addition of Fez03 to anatase catalyzes the conversion to rutile. As they increased the Fez03level to 1%the amount of rutile also increased until the Ti02 was essentially all rutile. Schrauzer et aL2demonstrated that rutile does exhibit some activity and that their most active photocatalysta contain 20-40 % rutile? but anatase is the key T i 0 2 component of the active materials. In addition to the Ti02 phases, the phases in which the Fe resides are also important. For the first several percent added, the Fe makes asolid solution with the Fe occupying Ti sites (Le., it is a substitutional dopant). The phase diagram for Fe in rutile indicates that the solubility limit is about 3%, and it also indicates that pseudobrookite (FezTiOs)is present at concentrationsabove 3 % ,25though pseudobrookite has been found to be inactive in the photosynthesis of NH3.6 At roasting temperatures lower than those needed to fully make the pseudobrookite, Fe additions to Ti02 materials with large crystalliie regions also show the presence of a-Fe203.26 Materials showing the highest activity for the NH3 photosynthesis have Fe concentrations near 1% ,so long as anatase is the predominantTi02phase. Materials testsd by Soria et al.6 showed the highest activity with Fe levels as low as 0.2 % ,though other materials made in the same lab showed the highest activity at 1% Fe.S Radford and Francis' found a maximum reaction rate at an Fe level of (18) Cordoba, A,; Luque, J. J. Phys. Rev. B: Condens. Matter 1985, 31,8111-8117. (19) McCafferty, E.; Zettlemoyer, A. C. DiScws. Faraday SOC. 1971, 52,239-264. (20) Vihwannthan, V. Indian J. Chem. 1987, %A, 772-773. (21) Boomtra, A. H.; Muteaere, C. A. H. A. J. Phys. Chem. 1976,79, 1694-1698. Faraday (22) Munera, G.; Rives-Arnau,V.; Saucedo, A. J. Chem. SOC., Trans. I 1979, 75,736.747. (23) Bickley, R. I. In Photoelectrochemistry, Photocatalysis and Photoreactors; Schiavello, M.,Ed.;D. Reidel: Dordrecht, The Netherlands, 1985; pp 491-602. (24) See a h , (a) Wrighton, M. 5.; Gdey, D. S.; Wolczaneki, P. T.; Ellis, A. B.; Mom, D. L.; Linz, A. Proc. Natl. Acad. Sci. U.S.A. 1975,72, 1618-1622. (b) Kraeutler, B.; Bard, A. J. J. Am. Chem. SOC. 1978,100, 6985-6992. (c) Sclafeni,A.; P a l m i o , L.; Schiavello,M. J. Phys. Chem. 1990,94,829-832. (26) Bunill, L. A. J. Solid State Chem. 1974,10, 72-94. (26) Bickley,R.L;Leee,J.S.;Tilley,R.J.D.;Palmio,L.;Schiavello, M. J . Chem. Soc., Faraday Tram. 1992,88,377-383.
Langmuit, Vol. 8, No.1, 1993 193
about 1 % Fe, and Griitzel and co-workerseJOused up to 2 % Fe to demonstrate the long lifetimes of the electronhole pairs in their irradiated materials. In this work we report the resulta of a series of simple experiments involving N2 and H2O isotherms as well as thermal desorption spectrawhich were carried out on Ti0r based powders containing different amounta of FezO3. Experimental Section Sample Preparation. All TiO2-containing materials were made with anatase obtained from Aldrich. The crystallinephaee was greater than 99 % anatase, and the non-TiOz impurity was less than 0.05% Rutile sampleswere made by heatingthe anatase at 1273 K for approximately 24 h. Fez03 was made by heating Fe(NH4)2(SO4)26H20in air at 873 K for 3 h and then at 973 K for 2 h. Combinations of Fez03 and Ti02 were made by adding an appropriate amount of Fe(NHMSO4)2*6H20to anatase with enough water to make a thin paste, then heating at about 400 K until apparent dryness, and finally heating at 1150 K for 2 h. This is a different procedure than that used by Soria et alp96 and Bickley et al.,% who used FemNOs to impregnate the TiOz. We found it easier to obtain uniform samples with this Fe(I1) salt as compared to several Fe(II1) salts. The crystalline composition was determined with X-ray diffraction using a PhillipsElectronicInstruments diffractometer which uses a scintillation counter on a wide-angle goniometer as a detector. Surface composition of the samples was determined by X-ray photoelectron spectroscopic( X P S ) analysisperformed at the University on North Carolina at Chapel Hill in the lab of Professor Richard Linton. In the XPS analysis the Ti, Fe, and 0 surface concentrationswere determined using the Ti 2 ~ 3 1 2 Fe , 2p3p, and 0 1s peaks. Prior to the isotherms and thermal desorption experiments, samples were heated at 750-800 K firat in about 1bar of oxygen and then under an ultimate vacuum of about 10-8 Pa. Isotherms. Nz and HzO isotherms were carried out using a simplevolumetric systemequippedwithacapacitancemanometer (MKS Type 221A Baratron) for pressure measurement, and a 2-in. diffusion pump (Varian M2 with water-cooled baffle). Sample sizes ranged from 2 to 12 g. NZgas for adsorption had d less than 10 ppm impurity, mostly 02 and Ar,and was p through a Supelco OMI-1 purifier. HzO gas was obtained from triply distilled water further purified by several pump-thaw cycles. Thermal Desorption Spectra. Thermal desorption experiments were carried out in the same volumetric system as the isotherms,though samplesizes were much smaller,approximately 0.5 g. The small sample size minimizes thermal gradients a c r m the sample and also minimizes diffusional resistance for gas desorbing from the sample. Gas is adsorbed at a desired temperature (usuallyNz at 77 K), and then the sampleis pumped at the temperature with the diffusion pump for 1-5 h or until the pressure gets to about 10-3 Pa. The temperature is then allowed to rise, and the total pressure is monitored with a Varian 571 ionization gauge tube attached to a Varian 843 controller. The temperature is measured with a thermocouple in a well in the middle of the sample tube. Both temperature and pressure measurements are recorded and stored by computer using an IBM Data Acquisition and Control Adapter card. In the case of Nz adsorption at 77 K, desorption is achieved by simply removing the liquid Nz from around the sample and allowing the sample to come to room temperature. While the temperature rise is not linear, it is reproducible,and the presence or absence of desorption events can be used to identify the presence or absence of particularspecies on the surface. Similarly] the peak heights are only weakly dependent on the pumping time8(lessthan15%). Hence,oneissafeinusingthepeakheighta as a semiquantitative indication of the amounts of particular species adsorbed on the surface.
.
Results and Discussion Sample Characterization. Samples of Fez03in Ti02 were made as described in Experimental Section above. The range of compositions extended from pure T i 0 2 to
194 Langmuir, Vol. 9,No. 1, 1993 Table I. Phvrioal Promrtier of 5a111ple~ vol % of crystalline phases" surface area,
Rive8 et al.
information about how tightly Nz physisorbs on these materials with and without strongly adsorbed water on the surface. Strongly adsorbed water is defined, for theee mD1e mZ/g anatase rutile a-FenOs FezTiOs purpoees, as water that is not pumped off in a vacuum of anatase 9.6 100 Pa at mom temperature after the sample hae been rutile 1.0 100 exposed to about 2 kPa of water vapor. Thermal desorp2% Fez03 6.0 92 8 tion spectra (not shown) on the HzO-treated samples run 4% Fez03 6.3 91 9 from 300 to 800 K show desorption events at about 420 8% Fez03 6.1 84 13 and 660 K. These indicate that the strongly adsorbed 20% Fez03 5.5 39 37 9 15 water consists of the two most strongly bound states. 50% Fez03 5.2 42 38 20 80% Fez03 3.0 14 59 27 Figure 1 showsthe low-temperatureNz thermal desorp92% Fez03 4.2 88 12 tion spectra for characteristic samples, comparing des98% Fez03 2.4 100 orption from the surfaces with and without strongly 100% Fe203 10.6 100 adsorbedwater. On eachof the materials without strongly a Approximate volume percent of the given phases was calculated adsorbed water, a desorption maxima occurs at about 120 using the peak heights of the most intense reflections normalized by K,and this is attributed to the desorption of a strongly the number of electrons per unit volume of the phase. physisorbed Nz that is not desorbed during the pumping pure FezO3. This range is wider than that used by others at 77 K.27 putting Fe in Ti02 for purposes of photochemi~try,'-~~~~ Strongly adsorbed water has very little, if any, effect on but in order to clearly see the transformation of properties the Nz desorption spectra for samples with lees than 80% from one pure material to the other, we elected to cover Fe2O3. However, aa the bulk Fez03 concentration goes the entire range. The Fez03 concentration is described as from 80 wt % to pure Fez03, the Nz desorption at 120 K a weight percent of Fez03 in TiOz. This is convenient becomes much smaller on the surfaces with strongly scale since it extends from 0 to 100% as the samples go adsorbed water. In other words, the strongly adsorbed from pure Ti02 to pure Fez03; however, it should not be water blocks sites on the high Fez03 content materials taken as an indication of the phase of the Fe but rather that would otherwise strongly physisorb Nz. as a measure of the amount of Fe(II1) present with oxides To determine if the different roastingtemperatures used as the counterions. The results of Brunauer-Emmettin preparation of the pure Fez03 (973 K) and the Teller (BET) surface area measurements and X-ray combination samples (1150 K) affected these results, diffraction analyses are listed in Table I. another pure Fez03 sample was roasted at 1160 K for 2 h Even though the surface areas of these materials are and thermal desorption spectra were repeated. No difrelatively low, they are somewhat sensitive to the roasting ferences were seen in the thermal desorption spectra of time and temperature. However, one should not expect the two Fez03 samples, so we conclude that the observed this sensitivityto be reflected in the adsorption properties. behavior is truly a function of the material composition Experimentsdiecuseedbelow involvingthermal desorption and is not a function of the preparation variables. from pure Fez03as well aa XPS on the 80% Fez03samples While it is interesting that the strongly adsorbed water indicate that the properties displayed by these materials does not block the strong physisorption of Nz on the TiOr are not dependent on the surface area and preparation rich materials but doeson the Fe~Oa-richmaterials, it needs variables such as roast time, but are only dependent on to be clarified how much strongly adsorbed water exists the composition of the materials. on these surfaces. To do this, we simply carried out The crystal phase analysissupports the assertionsmade successiveisotherms and took the difference between the by Schrauer and Guth that Fe catalyzes the conversion two. The difference represents the amount of strongly of anatase to rutile.' The only Ti02 phase that shows up adsorbed HzO. Prior to the f i t isotherm the samples is rutile for the samples with 50% or greater Fe203. The were heated at about 750 K under a vacuum of 10-3 Pa in precise solubilityof Fez03in T i 0 2 is impoesibleto identify order to remove all adsorbed water and hydroxyle. The using the data in Table I, but if one adds the amount in isotherms were run at 298 K over the range of relative the cr-FezO3 phase plus 2/3 of the FezTiOs phase and pressures from 0 to ' 1 3 (0.93 kPa). Following the f i t subtracta that from the amount expected from the weight isotherm, the sample was pumped at 298 K from 2 to 24 percent, one fiinds that the solubility falls in the range of h, achieving a final vacuum of about Pa. The eecond 0 4 % . This is in line with the reported solubilityof Fez03 isotherm was run similarly to the fiit. The percent in rutile.% As expwted,~ pseudobrookite is the only phase decrease in the moles of adsorbed water at a relative containing both Fe and Ti; heating in 02 presumably pressure of l / 3 (0.93kPa) is taken to indicate the percent eliminates any ilmenite (FeTiOa), though XPS results of the surface covered by strongly adsorbed water. This discueeed below indicate there may be a surface phase percentage is plotted vs the samples' weight percent of resembling ilmenite for samples between 50 and 80 wt % Fez03 in Figure 2. One sees that the amount of water that FezOs. is strongly adsorbed and is not pumped off at room The simultaneous presence of more than two crystal temperature and 10-3 Pa is not simply a monotonic phases with different compositions suggests that these transition between the extremes of 100% Ti02 to 100% materials are not fully equilibrated. Presumably the FezO3. Nor does the strong water adsorption reflect the reaction between the Ti02 phase and the Fez03 phase to presence of any of the phases listed in Table I. Under the make FezTi06 is not complete for samples between 20% conditions of these experiments the anataae and rutile and 80% Fe208. This is similar to what Bickley et a1.H have about 10% of their surfaces covered by strongly saw in their impregnated samples roasted below 923 K adsorbed water, and the initial additions of Fez03have no which had large regions of crystdine TiO,. Nonetheless, effect on that fraction. However, as the Fez03 concenfrom these X-ray analyses and the XPS results one can deduce the important features of surface enrichment that (27) With rutile the desorption peak maximum falls at about 146 K, influence adsorption. but this simply reflects rutile'e difterent pore structure and ditauion Thermal DerorptionExperiments. Thermal desorpproper+. Theonsetofthedesorptionpeahbabouttheeamsfor~~ tion experiments were carried out in order to get qualitative and ruth, 80 K. ~
~~
~
Langmuir, Vol. 9, No.1, 1993 196
N2 and Xi& Adsorption on Ti02 and Fez03 ?
?
0
W
21 v)
v)
2
a 75
Temperature ( K ) n
?
175
?
.
0
100 125 150 Temperature ( K )
?
0
W
W
23
21
.
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.
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75 100 125 150 175 125 150 175 Temperature ( K ) Temperature ( K ) Figure 1. RepresentativeNz thermal desorptionspectra comparing water-free (solid lines) and water-treated (dashed lines) surfaces of (top, left) anataae, (top, right) 20% Fez03 in TiOz, (bottom, left) 92% Fez03 in TiOz, and (bottom, right) a-FerOa. 75
5 v, M
1 1 0
100
20
40
60
80
100
Weight Percent Fe203
Figure 2. Percent of adsorbed water that is strongly bound vs the w t % FezO,.
tration increasesbeyond 80wt % the percent of the surface covered by strongly adsorbed water approaches 36 % These values are decidedlysmaller than the 48 % found on Fez03 by Watanabe and Setom and less than the 43 % found on rutile and the 61% found on Fez03by Morimoto et al.29 But one should expect considerable absolute variations since McCafferty and Zettlemoyerlgas well as another experiment by Morimoto et aL30find a value of 30% for Fe203, close to the one found here. Thermal desorptions of our samples from 300 to 800 K show that the overwhelmingmajority of this strongly adsorbed water is molecular in nature; this is determined by the fact that the desorption peak around 420 K which has been attributed to molecular water bound to the surface is more than an order of magnitude greater than any peak near 620 K which has been attributed to the desorption of water arising from the recombination of hydroxyls. Comparing Figures 1and 2, one sees that the decline in the strong N2 physisorption is associatedwith the increase in the coverage of strongly adsorbed water. Presumably
.
(28) Watanabe,H.;Seta, J. Bull. Chem. SOC.Jpn. 1988,61,3067-3072. (29) Morimoto,T.; Nagao, M.;Tokuda, F. J. Phye. Chem. 1969, 79,
243-u8. - ._ - .-.
(SO)Morimoto,T.;Nngao, M.;Tokuda,F.Bull. Chem.SOC.Jpn. 1968,
41,1638-1637.
the sitesthat stronglyadsorb water also strongly physisorb N2. However, the strong N2 physisorption is not strong enough to displacethe adsorbed water, and it is effectively blocked by the strongly adsorbed water. Just because the strongly adsorbed water blocke the strong N2 physisorption in the high Fez03 content materials, it should not be concludedthat N2 failsto physisorb altogether on these materials. Only the strongly physisorbed N2 is blocked. Indeed the BET isothermsare very similar on the high Fez03 content materials with and without stronglyadsorbed water. However, even with the BET isothermsthe extent of the reduction in N2 adsorption with strongly adsorbed water is dwernable. There is a decrease of about 7% in the number of moles of NZthat is required for forminga monolayer of N2 over Fez03when comparing the water-free vs the water-treated samples, but with the samples with less that 80% Fez03 the differenceis less than 2 % Clearly, the strongly adsorbed water does not eliminate or even greatly reduce the extent of N2 physisorption on the water-treated FezOs-rich samples,but rather it eliminatesor weakensthe adsorption at the stronger binding sites which are more readily hydrated or hydroxylated. Since the H2O adsorption data and the N2 desorption data are essentially constant for e 8 0 wt % FezOs, the clear implication is that a surface phase is formed which remains fundamentally unchanged as the Fez03 concentration is increased. Above 80 wt % Fez03the compition of the surface gradually becomes more like pure Fe2O3. To verify this suspicion, XPS experimenta were carried out to measure the Ti and Fe content of the samples’surfaces. Figure 3 shows the measured surface Fe content as a function of the bulk Fe content. The Fe content is expressed as a percentage of the total metal atoms, Le., metal atom percent. Since the molecular weight of T i 0 2 is very nearly half that of Fe203, the weight percent Fez03 and the metal mole percent Fe are essentiallythe same for the bulk material. Samples with lesa than 20 w t % Fe show a small surface enrichment in Fe, but Ti ie still the predominant metal atom. As the bulk Fe concentration
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196 hngmuir, Vol. 9, No. 1, 1993
Rives et al.
a distinct tendency for the surfaces of the moderately Ferich materials to be enriched in Ti.
L
20 40 60 80 100 Metal atom % Fe Bulk Figure 3. Surface Fe concentration,as determined by XPS,vs bulk Fe concentration, as determined by the stoichiometry of preparation. The dashed line represents the expected surface Fe concentration based on the bulk Fez03 concentration. The uncertainties are within the size of the symbols marking the experimental points, f2 metal atom ?6 Fe.
goes above 50 wt 7% Fe203, the surface remains approximately 50 metal atom % Fe, but above 80 metal atom % in the bulk the surface Fe content approaches 100%.In other words, samples between 50 and 80 wt % Fez03have surfacesenriched in Ti. Considering the thermal desorptions and the isotherms, the surface Ti's are presumably covering the Fe sites that would strongly physisorb Nz. The approximate 1:1 TkFe ratio in the samples that are 50-80 wt 9% Fez03 suggests that the surface is ilmenitelike. However, there is no compellingreason to think the Fe on the surface should be Fe(II), as it would be in ilmenite, since the sample had been heated in oxygen. It seem more likelythat there is a layer of Ti02 encapsulating a pseudobrookite phase-reminiscent of the Ti02 encapsulation of Pt particles in the strong metal support interaction.31 To demonstrate that the XPS resulta reflect relatively equilibrated samples and are not a strong function of a preparation variable such as roast time, the 80 wt % Fez03 sample was reheated for 2 h at 1150 K and retested. The X P S results were virtually identical in the twoexperimenta, so we conclude that our observations are not strongly dependent on the preparation variables and that there is (91) Belton, D.M.; Sun,Y.-M.; White, J. M. J. Phys. Chem. 1984,88, 5172-5176.
Conclusions These experimenta show clearly that the combinations of Ti02 and Fez03 have surfaces that are rich in Ti until the Fez03 concentration reaches at least 80 wt 7%. Only when the Fez03 content goes above 80 wt % does the surface begin to resemble Fez03 and do the adsorptive characteristics of pure Fez03 begin to show up. Those characteristicsinclude increased strong adsorptionof HzO and blocking of the strong Nz physisorption by the strongly adsorbed HzO. Our resulta are in line with those of Hotta et al." who have covered someof the hydroxyls on Fez03with titanate couplingreagents and found slightlyreduced BET surface areas and weaker Nz physisorption. In their case,however, the titanates were only allowed to go on the surface, and they were not supposed to exist as the oxides, but rather as alkoxides. If their deductions are correct, one would predict that in our case the surface Ti's block Fe-centered sites that would otherwiee tightly bind adsorbed water. These Fe-centered sitee also strongly physisorb Nz, but in a manner similar to that of TiOz. Consequently, in the absence of stronglyadsorbed water little differenceis seen in the nature of the strong NZphysieorption as a function of Fez03content. The HzO adsorbed on them Fe-centered sites apparently prevents Nz from physisorbing strongly. On the Ti-rich surfacesthere is a smaller tendency to form strongly adsorbed water, and consequently,the strong Nz physisorption is not disrupted significantly. The tendency for the surface to remain rich in Ti02 preventa significant adsorption changes from occurring as the Fez03 concentration increases from 0 to 80 wt 9%. Hence, the changes in photocatalytic properties that have been for Ti02 doped with Fez03 are not expected to stem from gross adsorptivechanges, but rather are expectedto be dominated by the photophysicalchanges involvingthe lifetime of the photogeneratedelectron-hole pair.6,7,9,10 Acknowledgment. This research was supported by grants from the Research Corp. and the UNCG Research council.
