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Energetics of the interaction of hydrogen with loaded titanium dioxide
Journal of Molecular
Catalyszs,
35 (1986) 221 - 226
221
ENERGETICS OF THE INTERACTION OF HYDROGEN WITH LOADED TITANIUM DIOXIDE L. STRADELLA Zstztuto dz Chamzca Generale Turzn (Italy)
ed Znorganzca, Facoltci dz Farmacza,
Vza P Gzurza 9, 10125
E. PELIZZETTI Dzpartzmento Turzn (Italy)
dz Chzmzca Analztaca,
(Received August 21,1985,
Facoltci dz Sczenze
M F N,
Vza P Gzurza 5, 10125
accepted December 4, 1985)
Summary The heats of adsorption and desorption of hydrogen on a loaded titanium dioxide have been measured at 300 K by means of a Tian Calvet microcalorimeter. Differential heats of adsorption in the first run decrease from 40 to 3.5 kJ mol- ‘. Irreversibly adsorbed hydrogen is detected m all the adsorption-desorption cycles. Desorption shows differential heats that are always greater than the correspondmg adsorption ones. Thermokmetlc observations indicate diffusion of the adsorbate into the TiOz structure.
Introduction Titania-based semiconductors are typical photocatalysts proposed for the production of hydrogen m aqueous media [l]. On the other hand, this oxide is extensively used as a support or as a component in many commercial catalysts [ 21. The interaction of differently prepared TiOz samples with hydrogen has been studied by means of weight decrease, magnetic susceptibility, electrical conductance, IR and other spectroscopic techniques (for a review see [3]). However, few studies have been published on the thermodynamic properties of the chemisorbed state of hydrogen on titanium dioxide: there is a lack of information on the energetics of the different surface species related to heterolytic or homolytic dissociative adsorption [ 41. In this study, the interaction of hydrogen with loaded TiOz was investigated by means of adsorption microcalorimetry. Experimental The material studied was a P-25 sample from Degussa (surface area 55 m2 g-i; 72% anatase, 28% rutile), loaded with 1% Ru02 according to the 0304-5102/86/$3
50
@ Elsevler Sequoia/Printed
in The Netherlands
222
procedure described earlier [ 51. This sample will be indicated hereafter as P-25/Ru02. The calorimetric measurements were performed at 300 K employing a Calvet microcalorimeter connected to a volumetric apparatus, as elsewhere described [ 61. The specimen was heated in vacuum at 423 K for 1 h before each adsorption run, as these seem to be the optimum conditions for the photocatalytic process [ 71. The hydrogen used was of 99.95% purity supplied by SIO (Italy). A final vacuum of 10e4 Pa was attainable m the adsorption apparatus. All the measurements were repeated at least twice to check reproducibility.
Results and discussion Calorimetric and volumetric iso therms The calorimetric adsorption isotherms of three successive cycles for hydrogen at 300 K on P-25/Ru02 are shown in Fig. la. It must be noted that adsorption is pressure-dependent over the entire range examined: even a very small adsorbed amount yields a pressure of -100 Pa over the sample. In Fig. lb, the ‘apparent’ sorption isotherms in the same cycles are given. The term ‘apparent’ has been proposed owing to the unexpected difference m the trends of the curves in Fig. la and lb: if we draw the secondary and tertiary isotherms from the axes’ origin, the heats mcrease along successive cycles, and adsorbed amounts decrease. We suggest that this indicates the occurrence of a diffusion process of the adsorbate into the sample subsurface layers. We will discuss this fact extensively later.
0
2
4
6
6
lo
f&/k&
Fig. 1. Calorlmetrlc (a) and volumetric (b) adsorption Isotherms of Hz at 300 K on P-25/ RuOz 0, 1st adsorption run, 0, 2nd adsorption run after desorption of the first run, A, 3rd adsorption run after desorptlon of the second run.
223
Frg. 2 Differential heats of adsorptlon of Hz at 300 K on P-25/RuOz (1st cycle).
In Fig. 2, the differential heats of adsorption for the first cycle, obtamed from the integral heat curves (not reported here for simplicity) are shown. The volumetric isotherm (1 cycle) exhibited (see Fig. lb) a Langmuir-like curve trend, but this adhesion is only formal, as we are dealing here with a heterogeneous surface. From the graph of Fig. 2, we may in fact distinguish three main types of interaction: (a) a few highly energetic sites (ml E.tmol g-l) are characterized by heats heats of the same magnitude were of chemisorption of -40 kJ mol-‘; observed on ZnO [ 81, on ZnO*Cr,Oa [ 91, on a Ni/SiOs catalyst [lo] and on Cu-Zn-Al catalyst [ 111; (b) an intermediate energy (10 kJ mol-‘) is shown by a family of more populated sites (-20 I.tmol g-l); (c) finally, a weak interaction occurs (-5 kJ mol-‘) corresponding to physical, non-specific adsorption. About 30% of the hydrogen is irreversibly adsorbed in the first cycle; this value is close to the fraction obtained for a similar catalyst [12] and might be due to hydride-type or hydroxyl-like bonds between hydrogen and the adsorbent, but these preliminary data do not allow an unambiguous assignment. From the differentiation of the curve of the integral desorptlon heats (not given here), we obtain a differential heat of desorption of 15.1 kJ mol-’ m the range between 2 and 25 kPa. This value is higher than the corresponding adsorption heat, so that an activated process may be assumed, for which an activation energy of 11.6 kJ mol-’ is obtained, if the desorption is limited to aspecific interaction. By our procedure we have a direct determmation of the desorption energy which results in values less than those obtained by TPD measurements [13] or by calculation of hydrogen diffusion [14] in a TiO, monocrystal. It is conceivable, however, that in our case, at 300 K, we are dealing with diffusion and desorption of molecular, weakly bound species.
224
0
10
20
30
40
50
60 PO lkPa
Fig 3. Calorlmetrlc adsorption-desorptlon cycles of Hz at 300 K on P-25/RuOz. 00, 1st adsorption-desorption run, g#, 2nd adsorption-desorptlon run, && 3rd adsorptlondesorptlon run
In Fig. 3, the three complete calorimetric adsorption-desorption cycles of Hz are collected. On such graphs we may primarily note the following: (1) a hysteresis loop is present m the first as well as m successive runs, i.e., adsorption-desorption runs follow energetically different paths; (ii) in each sorption run some process occurs yielding adsorbed species which are not evacuated at 300 K. Quite snnilar calorimetric isotherms have been obtained for H, on ZnO [15]. Although a slight decrease in the amount of this irreversible adsorbate can be detected in successive runs, the formation of this species does not seem to be exhausted even in the third run. This fraction might be related to the time of contact with the gas phase, so that the adsorption values of Fig. 3 should probably be considered as pseudo-equilibrium ones, although the system appeared to be in a stationary state as far as was detectable by our mstruments. Some kinetic data of the next section are consistent with such an interpretation.
Thermokinetlcs In Fig. 4 we show some typical calorimetric peaks of the first and of successive adsorption-desorption runs: it is clear that as the surface coverage increases, slow processes are established. It must be remembered that the equilibrium pressure was reached m all runs faster than the thermal equihbrium, as far as could be measured by our manometer. These data support the above-mentioned hypothesis that a Langmuir type model may not be applied to the Hz adsorption on TiOz, as the rate-determining step seems to be a slow diffusion, in the subsurface or into the bulk, which is roughly pressure independent. The heat evolution in the desorption run also obeys a parabolic law, and this fact may be further support for the validity of a diffusional mechanism. Hydrogen diffusion has already been suggested for a ZnO sample [15], for TiOz catalysts [ 12,161 and for several other oxides [ll, 17 - 191.
225
Fig 4. Thermokmetic features. Adsorption thermograms related to: (a) 1st adsorption run, P, = 619 Pa, (b) 2nd adsorption run, Pe = 1 153 kPa; (c) 3rd adsorption run, Pe = 1 354 kPa, (d) desorption run (any cycle)
Conclusions The calorimetric study of the energetics of hydrogen adsorbed on P-25/RuO, has shown that two kinds of irreversibly adsorbed species (q*ff = -40 and 10 kJ mol-‘) are formed. In the successive adsorption-desorption runs, the irreversible fraction increases and a difference appears in the trend of calorimetric and volumetric isotherms. The interpretation we propose is based on a model that requires penetration of the adsorbate into the subsurface or the bulk of the solid, where further chemisorption occurs. Once ummpeded adsorption is attained, the kinetics are determined by a slow diffusion that allows hydrogen penetration into the TiOz channels. In this situation a Langmuirian behaviour does not seem to apply to the gas-solid interface. TGA and EPR measurements on the same Hz-TiO, system have been planned m our Inst.&.& to obtain further information about both the diffusional process and the nature of the more active sites.
References 1 2 3 4 5
M. Gratzel, Ber. Bunsenges Phys Chem., 84 (1980) 981. M. Matsuda and A. Kato, Appl. Catal, 8 (1983) 149. T. Wolkenstein, Adv Catal, 23 (1973) 157. M. L Knotek, Surf Scz., 91 (1980) L17. E Borgarello, J Kiwr, E. Pehzzettl, M. Vlsca and M. Gratzel, J Am Chem. Sot , 103 (1981) 6324. 6 G. Della Gatta, B. Fubmi and L. Stradella, J Chem Sot , Faraday Trans, 2 (1977) 73. 7 N D. Parkyns, m P. Hepple (ed.), Chemisorpt~on and Catalyszs, Proc. Conf Institute of Petroleum, London, 1970, p. 150
226 8 G Della Gatta, B. Fubmi and E. Giamello, in J. Rouquerol and K. S. W. Sing (eds ), Adsorption at the gas-sokd and hqurd-sohd Interfaces, Elsevier, Amsterdam, 1982, p. 331 9 W. E. Garner and F. J. Veal, J Chem. Sot., (1935) 1487. 10 G C Scburt, N H de Boer, G. J. H. Dorgelo and L L. van Reijen, m W. E Garner (ed.), Chemrsorptzon, Butterworths, London, 1957, p. 39 11 A. A. Dyallov and V. E. Ostrovski, Kmet. K&al, 25 (1984) 159 12 J. C Conesa, G Munuera, A. Munoz, V. Rives, J. Sanz and J. Sorra, m G. M. Palonk, S. J. Terchner and J. E. Germame (eds ), Spzllouer of Adsorbed Specres, Elsevier, Amsterdam, 1983, p 149. 13 T. Iwaki, J. Chem Sot, Faraday Trans 1, 79 (1983) 137 14 J. A. Bates, J. C. Wang and R. A. Perkms, Phys Rev. B , 19 (1979) 4130. 15 B Fubim, E Giamello, G. Della Gatta and G. Venturello, J Chem. Sot., Faraday Trans. 1, 78 (1982) 153. 16 Wang Hongh, Tang Sheng, Xre Maosong, Xlong Guoxmg and Guo Xiexlan, in B. Imelik, C. Naccache, G. Coudurler, N. Prahaud, P. Meriaudeau, P. Gallezot, G A. Martin and J. C Vedrme (eds ), Metal Support and Metal Additme Effects in Catalysis, Elsevler, Amsterdam, 1983, p. 19 17 J. Vitko, Jr., C. M Hastwig and P. L Mattern, in R. Pandelides (ed.), Proc Znt. Conf, New York, 1978, Pergamon Press, Oxford, 1978, p. 215 18 V. E. Ostrovsku and E G. Ingranova, Kinet Katal (USSR), 19 (1978) 681. 19 M. J. D Low and E. S. Argano, J Phys Chem , 70 (1966) 3115.
Catalyszs,
35 (1986) 221 - 226
221
ENERGETICS OF THE INTERACTION OF HYDROGEN WITH LOADED TITANIUM DIOXIDE L. STRADELLA Zstztuto dz Chamzca Generale Turzn (Italy)
ed Znorganzca, Facoltci dz Farmacza,
Vza P Gzurza 9, 10125
E. PELIZZETTI Dzpartzmento Turzn (Italy)
dz Chzmzca Analztaca,
(Received August 21,1985,
Facoltci dz Sczenze
M F N,
Vza P Gzurza 5, 10125
accepted December 4, 1985)
Summary The heats of adsorption and desorption of hydrogen on a loaded titanium dioxide have been measured at 300 K by means of a Tian Calvet microcalorimeter. Differential heats of adsorption in the first run decrease from 40 to 3.5 kJ mol- ‘. Irreversibly adsorbed hydrogen is detected m all the adsorption-desorption cycles. Desorption shows differential heats that are always greater than the correspondmg adsorption ones. Thermokmetlc observations indicate diffusion of the adsorbate into the TiOz structure.
Introduction Titania-based semiconductors are typical photocatalysts proposed for the production of hydrogen m aqueous media [l]. On the other hand, this oxide is extensively used as a support or as a component in many commercial catalysts [ 21. The interaction of differently prepared TiOz samples with hydrogen has been studied by means of weight decrease, magnetic susceptibility, electrical conductance, IR and other spectroscopic techniques (for a review see [3]). However, few studies have been published on the thermodynamic properties of the chemisorbed state of hydrogen on titanium dioxide: there is a lack of information on the energetics of the different surface species related to heterolytic or homolytic dissociative adsorption [ 41. In this study, the interaction of hydrogen with loaded TiOz was investigated by means of adsorption microcalorimetry. Experimental The material studied was a P-25 sample from Degussa (surface area 55 m2 g-i; 72% anatase, 28% rutile), loaded with 1% Ru02 according to the 0304-5102/86/$3
50
@ Elsevler Sequoia/Printed
in The Netherlands
222
procedure described earlier [ 51. This sample will be indicated hereafter as P-25/Ru02. The calorimetric measurements were performed at 300 K employing a Calvet microcalorimeter connected to a volumetric apparatus, as elsewhere described [ 61. The specimen was heated in vacuum at 423 K for 1 h before each adsorption run, as these seem to be the optimum conditions for the photocatalytic process [ 71. The hydrogen used was of 99.95% purity supplied by SIO (Italy). A final vacuum of 10e4 Pa was attainable m the adsorption apparatus. All the measurements were repeated at least twice to check reproducibility.
Results and discussion Calorimetric and volumetric iso therms The calorimetric adsorption isotherms of three successive cycles for hydrogen at 300 K on P-25/Ru02 are shown in Fig. la. It must be noted that adsorption is pressure-dependent over the entire range examined: even a very small adsorbed amount yields a pressure of -100 Pa over the sample. In Fig. lb, the ‘apparent’ sorption isotherms in the same cycles are given. The term ‘apparent’ has been proposed owing to the unexpected difference m the trends of the curves in Fig. la and lb: if we draw the secondary and tertiary isotherms from the axes’ origin, the heats mcrease along successive cycles, and adsorbed amounts decrease. We suggest that this indicates the occurrence of a diffusion process of the adsorbate into the sample subsurface layers. We will discuss this fact extensively later.
0
2
4
6
6
lo
f&/k&
Fig. 1. Calorlmetrlc (a) and volumetric (b) adsorption Isotherms of Hz at 300 K on P-25/ RuOz 0, 1st adsorption run, 0, 2nd adsorption run after desorption of the first run, A, 3rd adsorption run after desorptlon of the second run.
223
Frg. 2 Differential heats of adsorptlon of Hz at 300 K on P-25/RuOz (1st cycle).
In Fig. 2, the differential heats of adsorption for the first cycle, obtamed from the integral heat curves (not reported here for simplicity) are shown. The volumetric isotherm (1 cycle) exhibited (see Fig. lb) a Langmuir-like curve trend, but this adhesion is only formal, as we are dealing here with a heterogeneous surface. From the graph of Fig. 2, we may in fact distinguish three main types of interaction: (a) a few highly energetic sites (ml E.tmol g-l) are characterized by heats heats of the same magnitude were of chemisorption of -40 kJ mol-‘; observed on ZnO [ 81, on ZnO*Cr,Oa [ 91, on a Ni/SiOs catalyst [lo] and on Cu-Zn-Al catalyst [ 111; (b) an intermediate energy (10 kJ mol-‘) is shown by a family of more populated sites (-20 I.tmol g-l); (c) finally, a weak interaction occurs (-5 kJ mol-‘) corresponding to physical, non-specific adsorption. About 30% of the hydrogen is irreversibly adsorbed in the first cycle; this value is close to the fraction obtained for a similar catalyst [12] and might be due to hydride-type or hydroxyl-like bonds between hydrogen and the adsorbent, but these preliminary data do not allow an unambiguous assignment. From the differentiation of the curve of the integral desorptlon heats (not given here), we obtain a differential heat of desorption of 15.1 kJ mol-’ m the range between 2 and 25 kPa. This value is higher than the corresponding adsorption heat, so that an activated process may be assumed, for which an activation energy of 11.6 kJ mol-’ is obtained, if the desorption is limited to aspecific interaction. By our procedure we have a direct determmation of the desorption energy which results in values less than those obtained by TPD measurements [13] or by calculation of hydrogen diffusion [14] in a TiO, monocrystal. It is conceivable, however, that in our case, at 300 K, we are dealing with diffusion and desorption of molecular, weakly bound species.
224
0
10
20
30
40
50
60 PO lkPa
Fig 3. Calorlmetrlc adsorption-desorptlon cycles of Hz at 300 K on P-25/RuOz. 00, 1st adsorption-desorption run, g#, 2nd adsorption-desorptlon run, && 3rd adsorptlondesorptlon run
In Fig. 3, the three complete calorimetric adsorption-desorption cycles of Hz are collected. On such graphs we may primarily note the following: (1) a hysteresis loop is present m the first as well as m successive runs, i.e., adsorption-desorption runs follow energetically different paths; (ii) in each sorption run some process occurs yielding adsorbed species which are not evacuated at 300 K. Quite snnilar calorimetric isotherms have been obtained for H, on ZnO [15]. Although a slight decrease in the amount of this irreversible adsorbate can be detected in successive runs, the formation of this species does not seem to be exhausted even in the third run. This fraction might be related to the time of contact with the gas phase, so that the adsorption values of Fig. 3 should probably be considered as pseudo-equilibrium ones, although the system appeared to be in a stationary state as far as was detectable by our mstruments. Some kinetic data of the next section are consistent with such an interpretation.
Thermokinetlcs In Fig. 4 we show some typical calorimetric peaks of the first and of successive adsorption-desorption runs: it is clear that as the surface coverage increases, slow processes are established. It must be remembered that the equilibrium pressure was reached m all runs faster than the thermal equihbrium, as far as could be measured by our manometer. These data support the above-mentioned hypothesis that a Langmuir type model may not be applied to the Hz adsorption on TiOz, as the rate-determining step seems to be a slow diffusion, in the subsurface or into the bulk, which is roughly pressure independent. The heat evolution in the desorption run also obeys a parabolic law, and this fact may be further support for the validity of a diffusional mechanism. Hydrogen diffusion has already been suggested for a ZnO sample [15], for TiOz catalysts [ 12,161 and for several other oxides [ll, 17 - 191.
225
Fig 4. Thermokmetic features. Adsorption thermograms related to: (a) 1st adsorption run, P, = 619 Pa, (b) 2nd adsorption run, Pe = 1 153 kPa; (c) 3rd adsorption run, Pe = 1 354 kPa, (d) desorption run (any cycle)
Conclusions The calorimetric study of the energetics of hydrogen adsorbed on P-25/RuO, has shown that two kinds of irreversibly adsorbed species (q*ff = -40 and 10 kJ mol-‘) are formed. In the successive adsorption-desorption runs, the irreversible fraction increases and a difference appears in the trend of calorimetric and volumetric isotherms. The interpretation we propose is based on a model that requires penetration of the adsorbate into the subsurface or the bulk of the solid, where further chemisorption occurs. Once ummpeded adsorption is attained, the kinetics are determined by a slow diffusion that allows hydrogen penetration into the TiOz channels. In this situation a Langmuirian behaviour does not seem to apply to the gas-solid interface. TGA and EPR measurements on the same Hz-TiO, system have been planned m our Inst.&.& to obtain further information about both the diffusional process and the nature of the more active sites.
References 1 2 3 4 5
M. Gratzel, Ber. Bunsenges Phys Chem., 84 (1980) 981. M. Matsuda and A. Kato, Appl. Catal, 8 (1983) 149. T. Wolkenstein, Adv Catal, 23 (1973) 157. M. L Knotek, Surf Scz., 91 (1980) L17. E Borgarello, J Kiwr, E. Pehzzettl, M. Vlsca and M. Gratzel, J Am Chem. Sot , 103 (1981) 6324. 6 G. Della Gatta, B. Fubmi and L. Stradella, J Chem Sot , Faraday Trans, 2 (1977) 73. 7 N D. Parkyns, m P. Hepple (ed.), Chemisorpt~on and Catalyszs, Proc. Conf Institute of Petroleum, London, 1970, p. 150
226 8 G Della Gatta, B. Fubmi and E. Giamello, in J. Rouquerol and K. S. W. Sing (eds ), Adsorption at the gas-sokd and hqurd-sohd Interfaces, Elsevier, Amsterdam, 1982, p. 331 9 W. E. Garner and F. J. Veal, J Chem. Sot., (1935) 1487. 10 G C Scburt, N H de Boer, G. J. H. Dorgelo and L L. van Reijen, m W. E Garner (ed.), Chemrsorptzon, Butterworths, London, 1957, p. 39 11 A. A. Dyallov and V. E. Ostrovski, Kmet. K&al, 25 (1984) 159 12 J. C Conesa, G Munuera, A. Munoz, V. Rives, J. Sanz and J. Sorra, m G. M. Palonk, S. J. Terchner and J. E. Germame (eds ), Spzllouer of Adsorbed Specres, Elsevier, Amsterdam, 1983, p 149. 13 T. Iwaki, J. Chem Sot, Faraday Trans 1, 79 (1983) 137 14 J. A. Bates, J. C. Wang and R. A. Perkms, Phys Rev. B , 19 (1979) 4130. 15 B Fubim, E Giamello, G. Della Gatta and G. Venturello, J Chem. Sot., Faraday Trans. 1, 78 (1982) 153. 16 Wang Hongh, Tang Sheng, Xre Maosong, Xlong Guoxmg and Guo Xiexlan, in B. Imelik, C. Naccache, G. Coudurler, N. Prahaud, P. Meriaudeau, P. Gallezot, G A. Martin and J. C Vedrme (eds ), Metal Support and Metal Additme Effects in Catalysis, Elsevler, Amsterdam, 1983, p. 19 17 J. Vitko, Jr., C. M Hastwig and P. L Mattern, in R. Pandelides (ed.), Proc Znt. Conf, New York, 1978, Pergamon Press, Oxford, 1978, p. 215 18 V. E. Ostrovsku and E G. Ingranova, Kinet Katal (USSR), 19 (1978) 681. 19 M. J. D Low and E. S. Argano, J Phys Chem , 70 (1966) 3115.