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Energetics of the adsorption-desorption cycles of oxygen on a titania-based catalyst
Journal of Molecular
Catalysis,
26 (1984)
105 - 115
105
ENERGETICS OF THE ADSORPTION-DESORPTION OXYGEN ON A TITANIA-BASED CATALYST
CYCLES OF
L. STRADELLA Istituto di Chimica Turin (Italy)
Generale
ed Inorganica,
Facoltci di Farmacia,
Via P. Giuria, g-10125
and E. PELIZZETTI Istituto VW)
di Chimica
Analitica,
Facoltci di Scienze
(Received September 8, 1983;accepted
M.F.N.,
Via P. Giuria, 5-10125
Turin
November 21,1983)
Summary
The adsorption-desorption of oxygen on pure TiOz and Ru02-loaded TiOa has been investigated at 305 K by means of a microcalorimetric technique. On pure TiOz the oxygen is adsorbed reversibly, and the differential heat suggests that the process proceeds through physically adsorbed molecules; loading with RuO, causes irreversible O2 chemisorption at low pressure while, at higher pressure, a process similar to that observed on pure TiOz occurs. The heats of exothermic desorption in the case of the loaded specimen may be interpreted by assuming the surface recombination of atomic oxygen.
Introduction
Numerous studies have been made in recent years of the use of semiconductor oxides such as TiO,, WOs and ZnO in photocatalytic and photosynthetic reactions [ 1 - 31. Of these processes, the photolytic generation of hydrogen and oxygen by visible light from water is of fundamental importance in the field of solar energy conversion [4 - 71. Initially, water splitting was achieved photochemically by using these n-type semiconductor oxides as electrodes [ 81. It now seems possible to use these semiconductors (in particular TiOJ in powder or colloidal form, and such systems appear to be much simpler and cheaper than the corresponding photochemical cells [9]. The oxidative and reductive reactions are believed to occur on the same particle which behaves as a ‘short-circuited cell’ with the simultaneous production of Hz and 0, [ 41. Interesting results have been obtained recently with a bifunctional redox catalyst based on TiOz particles loaded with RuO, and Pt [lo]. Since 0304-5102/84/$3.00
0 Elsevier Sequoia/Printed
in The Netherlands
106
the liberation of oxygen in the gas phase is not observed immediately after illumination, it is conceivable that adsorption and photo-uptake of oxygen take place at the surface. In the last decade, the photo-adsorption of oxygen [ll] and the interactions of differently prepared TiOz surfaces with water, hydrogen and other molecules have been extensively studied by using IR and ESR techniques (for a review see ref. 12). As far as the adsorption on n-type oxides at room temperature is concerned, discrimination between reversible molecular and irreversible ionic chemisorption seems to be well defined [13 - 161. Fewer studies, however, have been published on thermodynamic properties, such as the enthalpies and entropies of adsorption for these systems [17], or on attempts to characterize the chemical nature of active sites by means of test molecules [ 18 1. Moreover, the influence of controlled loading (e.g. RuOJ on the surface properties of semiconductors, such as the ease with which molecular oxygen is evolved, or on their catalytic or photocatalytic behaviour has not yet received sufficient attention. The present work deals with a preliminary investigation, using a microcalorimetric technique, of the energetics of oxygen adsorption-desorption cycles on pure titanium dioxide and on the same specimen loaded with ruthenium dioxide. Experimental The material studied was titanium dioxide powder (surface area = 5541 m* g-i) in two forms: (i) as a P-25 sample (72% anatase, 28% rutile) from Degussa (Germany); and (ii) as a P-25 sample of the same batch, loaded with 1% Ru02 according to the procedure already described [ 191. This latter sample will be indicated hereafter as P-25/RuO,. The specimens were heated in vacuum at 423 K for 1 h before each adsorption measurement, as this appeared to provide the best experimental conditions for oxygen photo-adsorption [20]. The oxygen used was of 99.95% purity supplied by SIO (Italy). Pressure changes in the volumetric system were measured by an electronic manometer (Datametrics, USA), employing a variable capacitance vacuum/pressure transducer. The whole apparatus was constructed with greaseless joints and stopcocks: a final vacuum of the order of 1O-4 N me2 was attainable. The experimental technique already described [21] employs a Tian-Calvet microcalorimeter connected to a volumetric apparatus and allows the simultaneous determination of the heat of adsorption and of the adsorption isotherm. Adsorption was performed by introducing small doses of O2 step by step into the calorimetric cells. Desorption was achieved by the successive expansion of the gas phase, in equilibrium with the adsorbate, into an empty calibrated volume. The final fractions of the adsorbate were removed reversibly at the experimental temperature by direct outgassing of the
107
adsorbent until no further desorption heat could be detected. Further adsorption-desorption cycles were performed to test the complete reversibility of the adsorption process. All the measurements were repeated at least twice to check reproducibility.
Results
Adsorption isotherms The calorimetric adsorption isotherms (i.e. the plots of the integral heats of adsorption (J g-l) versus the equilibrium pressures) for oxygen on P-25, at 305 K, over the pressure range 2 X lo3 - 6.5 X lo4 N m-’ are shown in Fig. 1. All the measurements were performed on specimens standardized according to the procedure described in the experimental section. The volumetric adsorption isotherms have not been reported for simplicity, since they were quite similar to the calorimetric ones. It should be noted that, despite the high sensitivity of the Tian-Calvet microcalorimeter, the first detectable heat emission was observed at 2 X lo3 N rnp2. Oxygen adsorption on P-25 at 305 K was completely reversible within experimental error and seemed to obey Henry’s law of adsorption; the results for five successive adsorption-desorption cycles exhibited good reproducibility. The corresponding calorimetric adsorption isotherms for oxygen on P-25/RuC2 are illustrated in Fig. 2; the volumetric isotherms (not reported
Fig. 1. Calorimetric adsorption isotherms for oxygen on P-25 at 305 K. 0 First adsorption run; •Isecond adsorption run after desorption of the first run; fl third adsorption run after desorption of the second run; 0 fourth adsorption run after desorption of the third run; n fifth adsorption run after desorption of the fourth run.
Fig. 2. Calorimetric adsorption isotherms for oxygen on P-25/RuOz at 305 K. 0 First adsorption run; @ desorption after the first adsorption run; 8 second adsorption run after desorption of the first run; 8 third adsorption run after desorption of the second run; + fourth adsorption run after desorption of the third run.
here) exhibited similar trends. In this case the first detectable heat emission occurred at 6 X lo2 N rnm2. From the results depicted in Fig. 2 the following may be deduced: (a) The first oxygen adsorption run exhibited a reversible and an irreversible branch. (b) The second (and successive) adsorption-desorption cycles were reversible over an extended range of pressures (from 1 X lo3 N rnd2 to 6 X lo4 N mp2). (c) Approximately 66% of the oxygen employed in the first cycle was adsorbed irreversibly. From a comparison of Fig. 1 with Fig. 2, the following deductions may be made: at the same equilibrium pressure the evolved heats are always smaller for the first run on P-25 than on the P-25/RuO, samples, and successive reversible isotherms on P-!i?5/RUo2 are quite similar to those on P-25. Heats of adsorp tion
Plots of the integral heats of adsorption as a function of the amount adsorbed (see Figs. 3 and 4) provide a satisfactory description of the evolution of the interaction energies for the two types of adsorbent. It is well known [21] that by differentiation of the curves shown in Figs. 3 and 4 it is possible to obtain the differential heats of adsorption, and these are summarized in Table 1.
109
Fig. 3. Plot of the integral heats of adsorption (J g-l) uersus the amounts adsorbed (pm01 g-l) for the P-25 sample. 0 First adsorption run; q second adsorption run after desorption of the first run; q third adsorption run after desorption of the second run; q fourth adsorption run after desorption of the third run; n fifth adsorption run after desorption of the fourth run.
0
I
I
1
5
‘0
15
I
20
I
25 n’ ~ld'(pol. j’J
Fig, 4. Plots of the integral heat of adsorption (J g-r) versus the amounts adsorbed (pm01 g-l) for the P-25/RuOz sample. 0 First adsorption run; 6 desorption after the first adsorption run; 8 second adsorption run after desorption of the first run; 8 third adsorption run after desorption of the second run; + fourth adsorption run after desorption of the third run.
1
P-25 P-25/Ru02
Sample
Energetics
TABLE
6.4 x 104 6.4 x lo4
Equilibrium pressure pe (N mw2) 250 210
Amounts adsorbed n, Wmol g-’ )
processes
adsorption
adsorption
Reversible
of differential
4.6 5.4
differential heats qdiff (kJ mol-l)
mounts adsorbed n, (ymol g-l ) 40
2 x 103
adsorption
equilibrium pressure pe (N mm2)
Irreversible
15.9
-
Differential heats qdiff (kJ mol-l)
0
z
111
As far as specific differences between P-25 and P-25/RuO, are concerned, the following observations may be made: (i) the P-25 sample exhibits reversible adsorption up to atmospheric pressure, which is characterized for all adsorption-desorption cycles by an average interaction energy of 4.6 kJ mol-i; (ii) two different adsorption processes apparently occur with P-25/Ru02 specimens: the first, which is irreversible at 305 K, is characterized by a differential heat of adsorption of 15.9 kJ mol-l; the second, which is reversible, has a weaker interaction energy (5.4 kJ mol-‘); (iii) the intermediate energy range is very limited (see Fig. 4), so that the site energy distribution on the P-25/RuO, surface appears to be quite simple with the transition from the irreversible to the reversible regimes implying few stable intermediate forms. A similar irreversible and reversible adsorption of dioxygen molecules has been observed on nickel and magnesium oxide at room temperature [411.
Thermokinetics Different adsorption and desorption mechanisms may be deduced from the study of the kinetics of the process. To enable such an analysis, some typical heat emission peaks are illustrated in Fig. 5. It must be borne in mind that these graphs only illustrate the evolution of heat as a function of time and that the height of peaks a’ and b’ have been reduced by a factor of two to enable suitable graphic representation. No data relative to the gas phase have been reported, as in all cases the oxygen pressure attained the equilibrium value virtually instantaneously, small fluctuations observed over a period of time being in the range of the manometer sensitivity. I
I
I 4
time
(hours)
Fig. 5. Typical heat emission peaks (calorimetric deviation uersuS time) corresponding to the following processes: (a) oxygen activated adsorption on P-25/RuOz at pe = 5 X lo2 N mW2; (a’) instantaneous reversible oxygen adsorption on P-25 at pe = 2 X lo3 N rne2; (b) oxygen activated desorption from P-25/RuOz showing endo- and exo-thermic emissions; (b’) oxygen desorption from P-25.
112
Peaks such as a’ in Fig. 5 have been observed in all measurements with P-25 and at the higher equilibrium pressures (p, > 2 X lo3 N m-‘) during the first cycle with the P-25/RuO, samples, and for all subsequent cycles with these specimens. This peak must correspond to an instantaneous phenomenon since the deviation in the calorimetric curve exhibits a very sharp peak with a maximum at 1 min after the start of thermal emission. Beyond this maximum all subsequent peaks follow an exponential trend dependent upon the time constant of the instrument. These results confirm that all these measurements actually correspond to reversible processes. Curve a of Fig. 5 has been obtained only at low coverage (p, < 2 X lo3 N m-*) on P-%/h& samples. Such a broad calorimetric peak, which exhibits a maximum at 3 - 4 min after the start of the deviation, was followed by a slow emission of heat which persisted for 4 h which is typical of activated processes. As the equilibrium pressure on the adsorbent is attained instantaneously, surface rearrangement of the adsorbate (possibly involving movement towards the most energetic and dissociative sites) may be assumed. The formation of an adsorbed atomic phase will however be discussed in detail later. Further information may be deduced from an analysis of the heat emission peaks along the desorption branch. In the case of the P-%/ho2 sample, all the desorption peaks in the first cycle, except the last, are endothermic as expected. The curve obtained from the direct outgassing of the residual oxygen on P-%/b& left after the first cycle exhibits a considerable exothermic emission after a small initial endothermic trend. This fact provides independent proof that dissociative adsorption occurs on this solid, the recombination of the atoms probably taking place during desorption. The possible mechanism concerned with these processes will be discussed in the next section. All the desorption peaks, including the last, for the P-25 sample were normally endothermic, although for the last peak a small exothermic deviation also occurred (curve b’ in Fig. 5). This may be explained by assuming the existence of a small fraction of atomic oxygen on the P-25 surface.
Discussion Several experimental observations [22 - 24, 371 and rough estimations of the thermodynamic stability of the adsorbed species [25] indicate that adsorption of oxygen on the surface of n-type oxides follows the scheme: 02(ads.) --+
02-(ads.)
-
O-(ads.) -
O*-(lattice)
Obviously the precise calculation of the stabilization energy of an adsorbed species is quite difficult, since such calculation would involve a knowledge not only of lattice parameters such as the Madelung energy but also the exact positions of the atoms on the surface, the eventual presence of surface defects or impurities, etc.
113
The calorimetric and volumetric adsorption isotherms for oxygen on the P-25 samples (see, for example, Fig. 1) seem to provide the simplest type of isotherm, i.e. one that obeys Henry’s law. Similar isotherms have been found experimentally at room temperature and pressures up to atmospheric for O2 on silica gel [26], charcoal [29] and a variety of soil components [ 27, 281. The physisorption data for oxygen on silica at 273 K are comparable with those described in the present work [ 261. Although Henry’s law implies that both the gas and the adsorbed phase are perfect [30], it is still possible to observe the type of linearity exhibited in Fig. 1 [ 261. The small magnitude of the differential heats, the reversibility of the system and the kinetic trends observed lead to the conclusion that physically adsorbed molecular oxygen is formed at room temperature (a) from low pressures up to atmospheric on the P-25 sample and (b) at high pressures (ca. 2 X lo3 N m-2) in the first run on P-%/RU& and at all equilibrium pressures in subsequent runs with the same sample. The data of Table 1 may therefore be interpreted by assuming that the species characterized by the weaker interaction energies (4.6 and 5.4 kJ mol-l) correspond to physically adsorbed molecular oxygen. This electrically neutral molecular form has been identified by IR measurements on Ti02 [ 151, and by other methods on NiO [ 221 and Sn02 [ 161. The value of the heat of adsorption for this molecular species is of the same order of magnitude as the heat of condensation of oxygen (6.8 kJ mol-l). Gas molecules are probably bound to the surface by van der Waals type forces, so that the adsorption process’is probably controlled by nonspecific interactions. The greater values observed in the case of P-25/Ru02 may be explained by assuming some contribution by weak bonds between the surface cations (c.u.s.) and molecular oxygen. The overlapping of metal atomic orbitals and the orbitals of molecular oxygen, together with the possible structures of the adsorbed species have all been well described by the application of crystal-ligand field theory [ 3 11. The calorimetric and volumetric isotherms obtained at 305 K for the adsorption of oxygen on P-%/&I& samples may formally be described as of the Langmuir type, exhibiting two branches, the former being pressureindependent while the latter is pressure-dependent. As already stated, analyses of the thermokinetics and heats of adsorption also suggest that at least two kinds of adsorbed species are formed when oxygen contacts a titanium dioxide surface. The formation of different surface species on semiconducting oxides has been assessed by the same calorimetric technique during the adsorption of oxygen at room temperature on NiO [32,33] and ZnO [34], and by means of a TPD chromatogram during the interaction of Sn02 with the same gas [ 3 51. Much work has been performed emplOyihg IR and ESR techniques (a recent review is given in ref. 12) to estimate the chemical nature of the sites involved in the adsorption or photo-adsorption of oxygen on titanium dioxide. A common feature of Ti02 systems adsorbing oxygen in the form of the superoxide ion (02-) or the monoatomic ion (O-) species is the small
114
concen~ation of donor centres capable of tran~itting electrons to the adsorbed molecules. As a result of the thermal treatment used in this work, both the TiOz samples studied were hydroxylated [ZO] ; however the absence of the higher differential heats of adsorption for the P-25 sample (see Table 1) suggests that the role of surface hydroxyl groups in the formation of chemisorbed oxygen species is probably not very important [36]. As noted above, the last desorption peak for the P-25/RuO, sample exhibits a considerable exothermic phenomenon. This may be inte~reted by assuming a wide energy distribution for the dissociative sites and the possibility that some chemisorbed species are reversibly adsorbed at 305 K. The present calorimetric data do not however enable a choice to be made between the formation of a paramagnetic 02- species, dissociative chemisorption or the coexistence of both mechanisms. In fact, if the former mechanism was important, the exothermic path in the desorption process could be explained by assuming the geometrical re~~gement of surface complexes along the adsorbent, the rehyb~d~ation of bonding orbitals on adsorption sites, and so on. The nature of the active sites on P-25/RuO, has not been definitively authenticated in this preliminary work, even if the ruthenium cations, which probably exist in different valency states as a result of the thermal treatment under vacuum employed, appear to be a determining factor. The formation of RuO, types of surface complex has been previously suggested to explain the chemiso~tion of oxygen on RuOz [38]. It should not be forgotten that a possible contribution arising from some Ti3+ ions which could exist on anatase surfaces under the dehydration conditions employed in this work has been demonstrated by pyridine adsorption measurements [ 181. In conclusion, it is clear that loading with RuOz affects the electronic structure of the surface of anatase creating new electronic levels close to the surface; similar features have been detected with titanium dioxide doped with niobium 1393. One should also not exclude the existence of active sites for the chemiso~tion of oxygen, such sites arising from particular surface defects (e.g. steps, grain boundaries or dislocations) for which direct experimental evidence has recently been obtained from chemiluminescence and piezoelectricity measurements [ 401. Further studies, including an examination of titanium dioxide specimens loaded with different amounts of RuO, linked with additional calorimetric, IR and EPR measurements, are needed to provide detailed information about the nature and the number of the ions involved in the donating electronic levels and about the corresponding chemisorb~ oxygen species.
References 1 T. Wolkenstein, Adv. Catal., 23 (1973) 157. 2 A. J. Bard, J. Phys. Chem., 86 (1982) 172.
115 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
T. Freund and W. P. Gomes, Catal. Rev., 3 (1969) 1. M. Grlitzel, Ber. Bunsenges. Phys. Chem., 84 (1980) 981. M. S. Wrighton,Acc. Chem. Res., 12 (1979) 303. A. Heller,Acc. Chem. Res., 14 (1981) 154. G. Porter and M. D. Archer, Interdisc. Sci. Rev., 1 (1976) 119. A. Fujishima and K. Honda, Nature (London), 238 (1972) 37. J. Kiwi, E. Borgarello, E. Pelizzetti, M. Vista and M. GrHtzel, in A. Harriman and M. A. West (eds.), Photogeneration of Hydrogen, Academic, London, 1982. E. Borgarello, J. Kiwi, E. Pelizzetti, M. Vista and M. Griitzel, J. Am. Chem. SOC., 103 (1981) 6423. A. H. Boonstra and A. H. A. Mutsaers, J. Phys. Chem., 79 (1975) 1694. G. D. Parfitt, Progr. Surf. Membr. Sci., 11 (1976) 181. R. P. Marcellini, R. E. Rano and S. J. Teichner, Actes 2” Congr. Intern. Cotaiyse, Paris, 1960, Editions Techniq, Paris, 1961, p. 289. K. Kuchynka and K. Klier, Collect. Czech. Chem. Commun., 28 (1963) 148. A. A. Davydov and M. P. Komarova, Symposium Adsorbirovanny kislorod u katalize, Institute of Catalysis, Academy of Science USSR, Novosibirsk, 1972, Preprint No. 19. T. A. Gundriser and A. A. Davydov, React. Kinet. Catal. Lett., 3 (1975) 63. P. C. Gravelle, Adu. Catal., 22 (1972) 250. C. Morterra, G. Ghiotti and E. Garrone, J. Chem. Sot., Faraday Trans. 1, 76 (1980) 2102. E. Borgarello, J. Kiwi, E. Pelizzetti, M. Vista and M. GrPtzel, J. Am. Chem. Sot., 103 (1981) 6324. N. D. Parkyns, in P. Hepple (ed.), Chemisorption and C&al. Proc. Conf., Institute of Petroleum, London, 1970, p. 150. G. Della Gatta, B. Fubini and L. Stradella, J. Chem. Sot., Faraday Trans. 2, 73 (1977) 1040. A. Bielanski and M. Najbar, J. Cotal., 25 (1972) 398. W. Kauzig and M. H. Cohen, Phys. Rev. Lett., 3 (1959) 509. G. N. Keulks, J. Cotal., 10 (1970) 232. D. G. Tuek, J. Inorg. Nucl. Chem., 26 (1964) 1525. D. M. Young and A. D. Crowell, Physical Adsorption of Gases, Butterworths, London, 1962, p. 106. P. M. Emmett, S. Brunauer and S. Lovak, Soil Sci., 45 (1938) 57. M. Weissmann and W. Z. Neumann, 2. Pelerniir. Dung, 40 (1935) 49. A. Magnus and W. KaIberer, 2. Anorg. Chem., 164 (1927) 357. M. M. Rowley and W. B. Innes, J. Phys. Chem., 45 (1941) 158. G. Henrici-OlivB and S. OlivB, Coordination and Catalysis, Verlag Chemie, Weinheim, 1977, p. 266. R. M. Dell and F. S. Stone, Trans. Faraday Sot., 50 (1954) 501. P. C. Gravelle and S. J. Teichner, Adu. Catal., 20 (1969) 167. T. J. Barry and K. Klier, Discuss. Faraday Sot., 31 (1961) 219. N. Yamazoe, J. Fuchigami, M. Kishikawa and T. Seiyama, Surface Sci., 86 (1979) 335. H. P. Bohem and M. Z. Herrmann, 2. Anorg. Allg. Chem., 352 (1967) 156. K. Nakamoto, IR Spectra of Znorganic and Coordination Compounds, Wiley, New York, 1963, p. 84. P. G. Dickens and M. B. Sutcliffe, Trans. Faraday Sot., 60 (1964) 1272. T. J. Gray and N. Lowery, Discuss. Faraday Sot., 52 (1971) 132. J. Marris, B. Kasemo and E. Tornquist, Chem. Phys. Lett., 52 (1977) 538. B. Charman and R. M. Dell, Trans. Faraday Sot., 59 (1963) 453.
Catalysis,
26 (1984)
105 - 115
105
ENERGETICS OF THE ADSORPTION-DESORPTION OXYGEN ON A TITANIA-BASED CATALYST
CYCLES OF
L. STRADELLA Istituto di Chimica Turin (Italy)
Generale
ed Inorganica,
Facoltci di Farmacia,
Via P. Giuria, g-10125
and E. PELIZZETTI Istituto VW)
di Chimica
Analitica,
Facoltci di Scienze
(Received September 8, 1983;accepted
M.F.N.,
Via P. Giuria, 5-10125
Turin
November 21,1983)
Summary
The adsorption-desorption of oxygen on pure TiOz and Ru02-loaded TiOa has been investigated at 305 K by means of a microcalorimetric technique. On pure TiOz the oxygen is adsorbed reversibly, and the differential heat suggests that the process proceeds through physically adsorbed molecules; loading with RuO, causes irreversible O2 chemisorption at low pressure while, at higher pressure, a process similar to that observed on pure TiOz occurs. The heats of exothermic desorption in the case of the loaded specimen may be interpreted by assuming the surface recombination of atomic oxygen.
Introduction
Numerous studies have been made in recent years of the use of semiconductor oxides such as TiO,, WOs and ZnO in photocatalytic and photosynthetic reactions [ 1 - 31. Of these processes, the photolytic generation of hydrogen and oxygen by visible light from water is of fundamental importance in the field of solar energy conversion [4 - 71. Initially, water splitting was achieved photochemically by using these n-type semiconductor oxides as electrodes [ 81. It now seems possible to use these semiconductors (in particular TiOJ in powder or colloidal form, and such systems appear to be much simpler and cheaper than the corresponding photochemical cells [9]. The oxidative and reductive reactions are believed to occur on the same particle which behaves as a ‘short-circuited cell’ with the simultaneous production of Hz and 0, [ 41. Interesting results have been obtained recently with a bifunctional redox catalyst based on TiOz particles loaded with RuO, and Pt [lo]. Since 0304-5102/84/$3.00
0 Elsevier Sequoia/Printed
in The Netherlands
106
the liberation of oxygen in the gas phase is not observed immediately after illumination, it is conceivable that adsorption and photo-uptake of oxygen take place at the surface. In the last decade, the photo-adsorption of oxygen [ll] and the interactions of differently prepared TiOz surfaces with water, hydrogen and other molecules have been extensively studied by using IR and ESR techniques (for a review see ref. 12). As far as the adsorption on n-type oxides at room temperature is concerned, discrimination between reversible molecular and irreversible ionic chemisorption seems to be well defined [13 - 161. Fewer studies, however, have been published on thermodynamic properties, such as the enthalpies and entropies of adsorption for these systems [17], or on attempts to characterize the chemical nature of active sites by means of test molecules [ 18 1. Moreover, the influence of controlled loading (e.g. RuOJ on the surface properties of semiconductors, such as the ease with which molecular oxygen is evolved, or on their catalytic or photocatalytic behaviour has not yet received sufficient attention. The present work deals with a preliminary investigation, using a microcalorimetric technique, of the energetics of oxygen adsorption-desorption cycles on pure titanium dioxide and on the same specimen loaded with ruthenium dioxide. Experimental The material studied was titanium dioxide powder (surface area = 5541 m* g-i) in two forms: (i) as a P-25 sample (72% anatase, 28% rutile) from Degussa (Germany); and (ii) as a P-25 sample of the same batch, loaded with 1% Ru02 according to the procedure already described [ 191. This latter sample will be indicated hereafter as P-25/RuO,. The specimens were heated in vacuum at 423 K for 1 h before each adsorption measurement, as this appeared to provide the best experimental conditions for oxygen photo-adsorption [20]. The oxygen used was of 99.95% purity supplied by SIO (Italy). Pressure changes in the volumetric system were measured by an electronic manometer (Datametrics, USA), employing a variable capacitance vacuum/pressure transducer. The whole apparatus was constructed with greaseless joints and stopcocks: a final vacuum of the order of 1O-4 N me2 was attainable. The experimental technique already described [21] employs a Tian-Calvet microcalorimeter connected to a volumetric apparatus and allows the simultaneous determination of the heat of adsorption and of the adsorption isotherm. Adsorption was performed by introducing small doses of O2 step by step into the calorimetric cells. Desorption was achieved by the successive expansion of the gas phase, in equilibrium with the adsorbate, into an empty calibrated volume. The final fractions of the adsorbate were removed reversibly at the experimental temperature by direct outgassing of the
107
adsorbent until no further desorption heat could be detected. Further adsorption-desorption cycles were performed to test the complete reversibility of the adsorption process. All the measurements were repeated at least twice to check reproducibility.
Results
Adsorption isotherms The calorimetric adsorption isotherms (i.e. the plots of the integral heats of adsorption (J g-l) versus the equilibrium pressures) for oxygen on P-25, at 305 K, over the pressure range 2 X lo3 - 6.5 X lo4 N m-’ are shown in Fig. 1. All the measurements were performed on specimens standardized according to the procedure described in the experimental section. The volumetric adsorption isotherms have not been reported for simplicity, since they were quite similar to the calorimetric ones. It should be noted that, despite the high sensitivity of the Tian-Calvet microcalorimeter, the first detectable heat emission was observed at 2 X lo3 N rnp2. Oxygen adsorption on P-25 at 305 K was completely reversible within experimental error and seemed to obey Henry’s law of adsorption; the results for five successive adsorption-desorption cycles exhibited good reproducibility. The corresponding calorimetric adsorption isotherms for oxygen on P-25/RuC2 are illustrated in Fig. 2; the volumetric isotherms (not reported
Fig. 1. Calorimetric adsorption isotherms for oxygen on P-25 at 305 K. 0 First adsorption run; •Isecond adsorption run after desorption of the first run; fl third adsorption run after desorption of the second run; 0 fourth adsorption run after desorption of the third run; n fifth adsorption run after desorption of the fourth run.
Fig. 2. Calorimetric adsorption isotherms for oxygen on P-25/RuOz at 305 K. 0 First adsorption run; @ desorption after the first adsorption run; 8 second adsorption run after desorption of the first run; 8 third adsorption run after desorption of the second run; + fourth adsorption run after desorption of the third run.
here) exhibited similar trends. In this case the first detectable heat emission occurred at 6 X lo2 N rnm2. From the results depicted in Fig. 2 the following may be deduced: (a) The first oxygen adsorption run exhibited a reversible and an irreversible branch. (b) The second (and successive) adsorption-desorption cycles were reversible over an extended range of pressures (from 1 X lo3 N rnd2 to 6 X lo4 N mp2). (c) Approximately 66% of the oxygen employed in the first cycle was adsorbed irreversibly. From a comparison of Fig. 1 with Fig. 2, the following deductions may be made: at the same equilibrium pressure the evolved heats are always smaller for the first run on P-25 than on the P-25/RuO, samples, and successive reversible isotherms on P-!i?5/RUo2 are quite similar to those on P-25. Heats of adsorp tion
Plots of the integral heats of adsorption as a function of the amount adsorbed (see Figs. 3 and 4) provide a satisfactory description of the evolution of the interaction energies for the two types of adsorbent. It is well known [21] that by differentiation of the curves shown in Figs. 3 and 4 it is possible to obtain the differential heats of adsorption, and these are summarized in Table 1.
109
Fig. 3. Plot of the integral heats of adsorption (J g-l) uersus the amounts adsorbed (pm01 g-l) for the P-25 sample. 0 First adsorption run; q second adsorption run after desorption of the first run; q third adsorption run after desorption of the second run; q fourth adsorption run after desorption of the third run; n fifth adsorption run after desorption of the fourth run.
0
I
I
1
5
‘0
15
I
20
I
25 n’ ~ld'(pol. j’J
Fig, 4. Plots of the integral heat of adsorption (J g-r) versus the amounts adsorbed (pm01 g-l) for the P-25/RuOz sample. 0 First adsorption run; 6 desorption after the first adsorption run; 8 second adsorption run after desorption of the first run; 8 third adsorption run after desorption of the second run; + fourth adsorption run after desorption of the third run.
1
P-25 P-25/Ru02
Sample
Energetics
TABLE
6.4 x 104 6.4 x lo4
Equilibrium pressure pe (N mw2) 250 210
Amounts adsorbed n, Wmol g-’ )
processes
adsorption
adsorption
Reversible
of differential
4.6 5.4
differential heats qdiff (kJ mol-l)
mounts adsorbed n, (ymol g-l ) 40
2 x 103
adsorption
equilibrium pressure pe (N mm2)
Irreversible
15.9
-
Differential heats qdiff (kJ mol-l)
0
z
111
As far as specific differences between P-25 and P-25/RuO, are concerned, the following observations may be made: (i) the P-25 sample exhibits reversible adsorption up to atmospheric pressure, which is characterized for all adsorption-desorption cycles by an average interaction energy of 4.6 kJ mol-i; (ii) two different adsorption processes apparently occur with P-25/Ru02 specimens: the first, which is irreversible at 305 K, is characterized by a differential heat of adsorption of 15.9 kJ mol-l; the second, which is reversible, has a weaker interaction energy (5.4 kJ mol-‘); (iii) the intermediate energy range is very limited (see Fig. 4), so that the site energy distribution on the P-25/RuO, surface appears to be quite simple with the transition from the irreversible to the reversible regimes implying few stable intermediate forms. A similar irreversible and reversible adsorption of dioxygen molecules has been observed on nickel and magnesium oxide at room temperature [411.
Thermokinetics Different adsorption and desorption mechanisms may be deduced from the study of the kinetics of the process. To enable such an analysis, some typical heat emission peaks are illustrated in Fig. 5. It must be borne in mind that these graphs only illustrate the evolution of heat as a function of time and that the height of peaks a’ and b’ have been reduced by a factor of two to enable suitable graphic representation. No data relative to the gas phase have been reported, as in all cases the oxygen pressure attained the equilibrium value virtually instantaneously, small fluctuations observed over a period of time being in the range of the manometer sensitivity. I
I
I 4
time
(hours)
Fig. 5. Typical heat emission peaks (calorimetric deviation uersuS time) corresponding to the following processes: (a) oxygen activated adsorption on P-25/RuOz at pe = 5 X lo2 N mW2; (a’) instantaneous reversible oxygen adsorption on P-25 at pe = 2 X lo3 N rne2; (b) oxygen activated desorption from P-25/RuOz showing endo- and exo-thermic emissions; (b’) oxygen desorption from P-25.
112
Peaks such as a’ in Fig. 5 have been observed in all measurements with P-25 and at the higher equilibrium pressures (p, > 2 X lo3 N m-‘) during the first cycle with the P-25/RuO, samples, and for all subsequent cycles with these specimens. This peak must correspond to an instantaneous phenomenon since the deviation in the calorimetric curve exhibits a very sharp peak with a maximum at 1 min after the start of thermal emission. Beyond this maximum all subsequent peaks follow an exponential trend dependent upon the time constant of the instrument. These results confirm that all these measurements actually correspond to reversible processes. Curve a of Fig. 5 has been obtained only at low coverage (p, < 2 X lo3 N m-*) on P-%/h& samples. Such a broad calorimetric peak, which exhibits a maximum at 3 - 4 min after the start of the deviation, was followed by a slow emission of heat which persisted for 4 h which is typical of activated processes. As the equilibrium pressure on the adsorbent is attained instantaneously, surface rearrangement of the adsorbate (possibly involving movement towards the most energetic and dissociative sites) may be assumed. The formation of an adsorbed atomic phase will however be discussed in detail later. Further information may be deduced from an analysis of the heat emission peaks along the desorption branch. In the case of the P-%/ho2 sample, all the desorption peaks in the first cycle, except the last, are endothermic as expected. The curve obtained from the direct outgassing of the residual oxygen on P-%/b& left after the first cycle exhibits a considerable exothermic emission after a small initial endothermic trend. This fact provides independent proof that dissociative adsorption occurs on this solid, the recombination of the atoms probably taking place during desorption. The possible mechanism concerned with these processes will be discussed in the next section. All the desorption peaks, including the last, for the P-25 sample were normally endothermic, although for the last peak a small exothermic deviation also occurred (curve b’ in Fig. 5). This may be explained by assuming the existence of a small fraction of atomic oxygen on the P-25 surface.
Discussion Several experimental observations [22 - 24, 371 and rough estimations of the thermodynamic stability of the adsorbed species [25] indicate that adsorption of oxygen on the surface of n-type oxides follows the scheme: 02(ads.) --+
02-(ads.)
-
O-(ads.) -
O*-(lattice)
Obviously the precise calculation of the stabilization energy of an adsorbed species is quite difficult, since such calculation would involve a knowledge not only of lattice parameters such as the Madelung energy but also the exact positions of the atoms on the surface, the eventual presence of surface defects or impurities, etc.
113
The calorimetric and volumetric adsorption isotherms for oxygen on the P-25 samples (see, for example, Fig. 1) seem to provide the simplest type of isotherm, i.e. one that obeys Henry’s law. Similar isotherms have been found experimentally at room temperature and pressures up to atmospheric for O2 on silica gel [26], charcoal [29] and a variety of soil components [ 27, 281. The physisorption data for oxygen on silica at 273 K are comparable with those described in the present work [ 261. Although Henry’s law implies that both the gas and the adsorbed phase are perfect [30], it is still possible to observe the type of linearity exhibited in Fig. 1 [ 261. The small magnitude of the differential heats, the reversibility of the system and the kinetic trends observed lead to the conclusion that physically adsorbed molecular oxygen is formed at room temperature (a) from low pressures up to atmospheric on the P-25 sample and (b) at high pressures (ca. 2 X lo3 N m-2) in the first run on P-%/RU& and at all equilibrium pressures in subsequent runs with the same sample. The data of Table 1 may therefore be interpreted by assuming that the species characterized by the weaker interaction energies (4.6 and 5.4 kJ mol-l) correspond to physically adsorbed molecular oxygen. This electrically neutral molecular form has been identified by IR measurements on Ti02 [ 151, and by other methods on NiO [ 221 and Sn02 [ 161. The value of the heat of adsorption for this molecular species is of the same order of magnitude as the heat of condensation of oxygen (6.8 kJ mol-l). Gas molecules are probably bound to the surface by van der Waals type forces, so that the adsorption process’is probably controlled by nonspecific interactions. The greater values observed in the case of P-25/Ru02 may be explained by assuming some contribution by weak bonds between the surface cations (c.u.s.) and molecular oxygen. The overlapping of metal atomic orbitals and the orbitals of molecular oxygen, together with the possible structures of the adsorbed species have all been well described by the application of crystal-ligand field theory [ 3 11. The calorimetric and volumetric isotherms obtained at 305 K for the adsorption of oxygen on P-%/&I& samples may formally be described as of the Langmuir type, exhibiting two branches, the former being pressureindependent while the latter is pressure-dependent. As already stated, analyses of the thermokinetics and heats of adsorption also suggest that at least two kinds of adsorbed species are formed when oxygen contacts a titanium dioxide surface. The formation of different surface species on semiconducting oxides has been assessed by the same calorimetric technique during the adsorption of oxygen at room temperature on NiO [32,33] and ZnO [34], and by means of a TPD chromatogram during the interaction of Sn02 with the same gas [ 3 51. Much work has been performed emplOyihg IR and ESR techniques (a recent review is given in ref. 12) to estimate the chemical nature of the sites involved in the adsorption or photo-adsorption of oxygen on titanium dioxide. A common feature of Ti02 systems adsorbing oxygen in the form of the superoxide ion (02-) or the monoatomic ion (O-) species is the small
114
concen~ation of donor centres capable of tran~itting electrons to the adsorbed molecules. As a result of the thermal treatment used in this work, both the TiOz samples studied were hydroxylated [ZO] ; however the absence of the higher differential heats of adsorption for the P-25 sample (see Table 1) suggests that the role of surface hydroxyl groups in the formation of chemisorbed oxygen species is probably not very important [36]. As noted above, the last desorption peak for the P-25/RuO, sample exhibits a considerable exothermic phenomenon. This may be inte~reted by assuming a wide energy distribution for the dissociative sites and the possibility that some chemisorbed species are reversibly adsorbed at 305 K. The present calorimetric data do not however enable a choice to be made between the formation of a paramagnetic 02- species, dissociative chemisorption or the coexistence of both mechanisms. In fact, if the former mechanism was important, the exothermic path in the desorption process could be explained by assuming the geometrical re~~gement of surface complexes along the adsorbent, the rehyb~d~ation of bonding orbitals on adsorption sites, and so on. The nature of the active sites on P-25/RuO, has not been definitively authenticated in this preliminary work, even if the ruthenium cations, which probably exist in different valency states as a result of the thermal treatment under vacuum employed, appear to be a determining factor. The formation of RuO, types of surface complex has been previously suggested to explain the chemiso~tion of oxygen on RuOz [38]. It should not be forgotten that a possible contribution arising from some Ti3+ ions which could exist on anatase surfaces under the dehydration conditions employed in this work has been demonstrated by pyridine adsorption measurements [ 181. In conclusion, it is clear that loading with RuOz affects the electronic structure of the surface of anatase creating new electronic levels close to the surface; similar features have been detected with titanium dioxide doped with niobium 1393. One should also not exclude the existence of active sites for the chemiso~tion of oxygen, such sites arising from particular surface defects (e.g. steps, grain boundaries or dislocations) for which direct experimental evidence has recently been obtained from chemiluminescence and piezoelectricity measurements [ 401. Further studies, including an examination of titanium dioxide specimens loaded with different amounts of RuO, linked with additional calorimetric, IR and EPR measurements, are needed to provide detailed information about the nature and the number of the ions involved in the donating electronic levels and about the corresponding chemisorb~ oxygen species.
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