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Sulfate-modified zirconia catalysts: the role of surface electrification on the features of the sulfate adsorption process
ELSEVIER
Journal of Electroanalytical Chemistry 417 ( 19%) 193- 195
Short communication
Sulfate-modified zirconia catalysts: the role of surface electrification on the features of the sulfate adsorption process S. Ardizzone, E. Grassi Depurtment
of Physicnl
Chemistry
and Elecrrochemistry,
University
of Milan,
Via Go&i
19, I-20133
Milan,
Italy
Received 23 January 1996; revised 15 March 1996
Abstract Determinations of the adsorption of sulfates at the Z&,/solution interface have been performed at constant ionic strength (lo-* M KNO,) and at two different pH values (isotherm 1, pH 4.5; isotherm 2, pH 2.6). In both cases the adsorption isotherms presented a langmuirian shape; parameters describing the adsorption were obtained by processing data in the reciprocal Langmuir coordinates. The degree of packing of the adsorbed molecules and the nature of the forces supporting the adsorption are also discussed on the basis of literature results. Keywords:
Sulfated zirconia; Adsorption; Catalyst; Double layer
1. Introduction Large surface area zirconia powders have been used extensively as catalysts for several reactions, e.g. the syn-
thesis of hydrocarbons and methanol or the dehydration of alcohols to give olefins [l-3]. In recent years sulfate-modified zirconia catalysts have been developed. The treatment of the oxide (or of the oxide hydrous precursor) with sulfates results in a very strong increase in the acidity of the powders and in high catalytic activity for the isomerization of hydrocarbons or the conversion of alcohols 14-61. Besides affecting the acid character of the samples, sulfates produce several modifications of the features of the oxide, concerning: (i) phasecomposition, (ii) degreeof crystallinity, (iii) extent of surface area, (iv) degree of porosity, etc. Much work has been done and published regarding theselatter topics, and somecontroversial results have been reported, particularly on the role played by sulfateson the phasecomposition of ZrO, polymorphs and on the Lewis or Broensted acidity of the catalytic sites [7,8]. Further, notwithstanding the relevance of sulfatezirconia interactions, studies concerning the adsorption processitself are very limited [9,10]. Chokkaram et al. [IO] have reported curves of weight per cent sulfate adsorbedas a function of either initial or final Ii,SO, concentration in solution. The different points of each curve, however, having been obtained intrinsically at different pH and ionic OQ22-0728/96/$i5.00 PII SOO22.0728(96)04724-9
strength, pertain to different values of the electrochemical parametersof the oxide surface and may not be considered to identify a “true” adsorption isotherm. In the present work two adsorptionisothermsof H,SO, on ZrOz powders, obtained at constant pH and ionic strength, are reported and discussedwith the aim of bringing out some relevant evidence concerning the zirconiasulfate interactions.
2. Experimental All reagentswere of reagent grade purity and were used as received without further purification. Solutions and suspensionswere prepared with twice distilled water passedthrough a Milli-Q apparatus. The solid adsorbentemployed in the study was prepared by precipitation (KOH), at the boiling point, from an acidic ZrCl, solution. The pH of the suspensionwas never raised above pH 7 during the reaction. The reaction product, after purification by a dialysis procedure, was stored as a dry powder in a desiccator. By XRD it appearedto be composed mainly of tetragonal zirconia, although with a relatively low degree of crystallinity. The specific surface area determined by BET measurements was S = 216m2g-‘. The adsorption isotherms of H, SO, onto the zirconia samples were obtained under the following conditions:
Copyright 0 1996 Elsevier Science S.A. All rights reserved.
194
S. Ardiuone,
E. Grassi/Journal
of Electroanalytical
M KNO,; equilibration time 1 h; pH 4.5 for isotherm 1 and 2.6 for isotherm 2. At the end of the adsorption time the suspension was centrifuged in PEP nalgene tubes at 9OOOrev min-’ and the supematant was sampled for the evaluation of the residual sulfate concentration by direct titration with barium chloride in acetone + water solution (66% + 1% pyridine) using carboxyarsenazo as indicator Ill].
Chemistry
417 (1996)
193-195
2.0
T = 25°C; lo-*
1.5 5 ;
1.0
i o_ 0.5 L
0t 0
3. Results and discussion
IO’ C / mol
Sulfate-modified catalysts are prepared, as a rule, by equilibrating the oxide powders directly with sulfuric acid of variable concentration. Consequently, the present adsorption isotherms were obtained in an acid pH range congruent with the literature data. The specific values of pH and ionic strength of the adsorption experiments were selected so as to produce significantly different conditions of surface charge and potential at the oxidelsolution boundarYThe point of zero charge (pzc) of the powders, determined by the classical procedure [ 121, was found to be 8.4, in agreement with the pzc values reported for zirconia not subjected to dehydration processes [ 13,141. The values of the oxide surface charge pertaining to the two isotherms are 15.8 p.Ccm-2 (isotherm 1) and 25.3pCcm-2 (isotherm 2) respectively. Investigations of the adsorption kinetics were performed by extending the duration of the experiments to times longer than 1 h; no appreciable variations in the sulfate bulk concentrations were observed. Fig. 1 presents adsorption isotherms of sulfates on zirconia samples, obtained under the experimental conditions discussed above. The shape of the two isotherms is very similar, first displaying a rising part and then leveling off to a plateau. The isotherms belong to the group classified in the literature as Langmuir-type isotherms.
0’ 0
1 10
1
20
1
30 IO’ C /mot
1
I
1
40
50
60
L-’
Fig. 1. Dependence of the surface excess r on H,SO, equilibrium concentration at constant KNO, concentration (IO-* M). 1, pH 4.5; 2, pH 2.6.
Fig. 2. Adsorption data expressed I- max = 2.40X 10m4 pm,, cme2, 10-4p,,cm-2, b= 1.10X10’.
1-l
in linearized b = 5.34X
Langmuir 104; 2,
coordinates. 1, T,,, = 3.80 X
Choice of a lower solution pH, i.e. increasing the positive surface charge and potential of the oxide surface, produces an increase in both the slope of the initial linear rising part and the value of the surface excess of the plateau. Fig. 2 reports adsorption results in the coordinates of the linearized Langmuir equations. The equation adopted in Fig. 2 is c/r= 1/t r,,, P ) + c/L where r is the surface excess, r,,, the maximum surface coverage and /3 the adsorption constant. The figure shows that the data, over all the investigated concentration range, are interpolated by straight lines with high correlation coefficients for both isotherms ( r, = 0.998; r2 = 0.999). This confirms the adherence of the experimental results to a Langmuir-type isotherm. This result is at variance with that observed by Chokkaram et al. [lo] on their adsorption data, which appeared not to be fitted by either Langmuir or Freundlich equations. This latter occurrence is possibly the result of the experimental conditions selected, i.e. the lack of invariance of pH and ionic strength for the different points of the isotherm. The processing of data in the linearized coordinates in Fig. 2 allows parameters describing the features of the adsorption process to be obtained. The density of the positive surface sites of the oxide plays a definite role in imposing the coverage by the anions. The ratio between the initial oxide surface charges at the two pH values (000.2/oo,1 = 1.60) reflects closely the ratio between the two r,,, values (rmax,Jrmax,, = 1.58). The proportional variation of the r,,, with a, is an indication of the relevant role played by electrostatic interaction in determining the adsorption. The experimental initial surface charges of the oxide substrates, although pertaining to relatively high ApH,,, values (isotherm 1, ApH,, = 3.9, isotherm 2, ApH,,, = 5.8) are in both cases much lower than the “maximum” charge which can be calculated on the assumption of a
S. Ardizzone,
E. Grassi/Journal
ofElectroamlyticul
“full” dissociation of all the physically accessible surface OH groups. Therefore, considering the above reported r,,,/oO relation, it can be concluded that the adsorption film is, in both cases, in conditions which are far from the “full” occupancy of the surface sites. The “dilute” nature of the adsorption layer also finds support from the apparent lack of lateral repulsive interactions between the adsorbate molecules. In fact, besides the reported fit to the Langmuir “ideal” model, the experimental data have also been fitted (although with a less satisfying linear correlation) to a virial isotherm [15], showing a zero lateral interaction term. The values of maximum coverage r,,, obtained by the elaboration can be related to the BET specific surface area of the oxide in order to obtain the value of the available areas per adsorbed species. Calculations lead to a value of 0.692 nm’ (0.692 f 0.012 nm*) for isotherm 1 and 0.437 nm2 (0.437 rt 0.003 nm’) for isotherm 2. The size of the sulfates based on the bare crystallographic radius is 0.2nm2. As could have been expected, on the basis of the above considerations, in both cases the estimated available areas are higher than the geometrical size of the adsorbed species. A polar solvent like water is very likely to be present in the adsorbed monolayers formed at charged solid surfaces by ionic adsorbates. It is interesting to observe that the present datum of 0.43nm2 (isotherm 2) corresponds exactly to the 0.43nm2 value reported by Chokkaram et al. [lo] as the area occupied by sulfate anions in conditions corresponding to the saturation of the oxide surface (the conditions of saturation were derived from the trend of the particle surface area versus H,SO, concentration). Therefore, the adsorbed monolayers of sulfates onto zirconia remain hydrated, i.e. they could be classified as liquid-expanded, even in conditions of maximum saturation of the adsorbent. The values of the Gibbs energy of the adsorption calculated from the values of p are not high (AGY = -2.70kJmol-‘; AGZ = - 2.88 kJmol- ‘) and may be representative of a process supported by physical forces. The role played by either physical or chemical interactions in the present adsorption process is not clear-cut. Chemisorption. Sulfates are bivalent species and may be expected to show specific interactions with an oxide surface. The picture of the adsorption in a monolayer, obtained from the elaboration of the experimental data, might suggest partition of the species in the inner double layer and not in the diffuse region. Physisorption. The comparison between the two isotherms presented shows that the process is affected
Chemistry
417 (I9961
193-195
195
strongly by the attractive electrostatic interactions with the oxide surface. It is very likely that the process is both physi- and chemi-supported. Upon “washing”, the amount of sulfate adsorbed decreases (reversible adsorption physical forces) [10,16], but it never falls to zero even in the case of desorption tests performed at alkaline pH (negative surface charge of the substrate). This should indicate the presence of a not fully reversible chemisorption process [10,161.
Acknowledgements
Financial support from the MURST (Ministry of the University and of Scientific and Technological Research) (40% and 60% research funds) and the Italian National Research Council, Technological Research Committee (CNR, Comitato Ricerche Tecnologiche) is gratefully acknowledged.
References
Ill K. I21 T.
Tanabe, Mater. Chem. Phys.. 13 (1985) 347. Maehashi, K. Maymura, K. Domen, K. Aika and T. Onishi, Chem. Lett., ( 1984) 747. 131 M.Y. He and J.G. Eke&, J. Catal., 90 (1984) 17. Mater. Chem. [41 M. Bensitel, 0. Sam, J.C. Lavalley and B.A. Marrow, Phys., 19 (1988) 147. 151K. Tanabe, M. Misono, Y. Ono and H. Hattori, New Solids Acids and Bases, Elsevier, Amsterdam, 1989, p. 199. [61 K. Arata, M. Hino and N. Yamagata, Bull. Chem. Sot. Jpn., 63 (1990) 244. Catalysis Today, 20 L71 B.H. Davis, R.A. Keogh and R. Srinivasan, (1994) 219. [81 A. Clearfield, G.P.D. Serrette and A.H. Khazi-Syed. Catalysis Today, 20 (1994) 295. [9] F.R. Chen, G. Coudurier, J.F. Joly and J.C. Vedrine, in Symposium on Alkylation. Aromatization, Oligomerization of Short Chain Hydrocarbons over Heterogeneous Catalysts, American Chemical Society, New York, 1991. p. 878. [lo] S. Chokkaram, R. Srinivasan, D.R. Milbum and B.H. Davis, J. Colloid Interface Sci., 165 (1994) 160. [l 11 C.A. Watson (Ed.), Official and Standardized Methods of Analysis, The Royal Society of Chemistry, Cambridge, 3rd edn., 1994. [I21 S. Ardizzone and S. Carella, Mater. Chem. Phys., 31 (1992) 351. [I31 S. Ardizzone. G. Bassi and G. Liborio. Colloids Surf., 51 (1990) 207. [14] R. Paterson and H. Rahman, J. Colloid Interface Sci., 103 (1985) 106. [I51 W. Anderson and R. Parsons, in J.H. Schulman (Ed.), Proc. 2nd Int. Congress on Surface Activity, Vol. 3, Butterworths. London, 1957, p. 45. [16] L. Bodini, Thesis, University of Milan, 1995.
Journal of Electroanalytical Chemistry 417 ( 19%) 193- 195
Short communication
Sulfate-modified zirconia catalysts: the role of surface electrification on the features of the sulfate adsorption process S. Ardizzone, E. Grassi Depurtment
of Physicnl
Chemistry
and Elecrrochemistry,
University
of Milan,
Via Go&i
19, I-20133
Milan,
Italy
Received 23 January 1996; revised 15 March 1996
Abstract Determinations of the adsorption of sulfates at the Z&,/solution interface have been performed at constant ionic strength (lo-* M KNO,) and at two different pH values (isotherm 1, pH 4.5; isotherm 2, pH 2.6). In both cases the adsorption isotherms presented a langmuirian shape; parameters describing the adsorption were obtained by processing data in the reciprocal Langmuir coordinates. The degree of packing of the adsorbed molecules and the nature of the forces supporting the adsorption are also discussed on the basis of literature results. Keywords:
Sulfated zirconia; Adsorption; Catalyst; Double layer
1. Introduction Large surface area zirconia powders have been used extensively as catalysts for several reactions, e.g. the syn-
thesis of hydrocarbons and methanol or the dehydration of alcohols to give olefins [l-3]. In recent years sulfate-modified zirconia catalysts have been developed. The treatment of the oxide (or of the oxide hydrous precursor) with sulfates results in a very strong increase in the acidity of the powders and in high catalytic activity for the isomerization of hydrocarbons or the conversion of alcohols 14-61. Besides affecting the acid character of the samples, sulfates produce several modifications of the features of the oxide, concerning: (i) phasecomposition, (ii) degreeof crystallinity, (iii) extent of surface area, (iv) degree of porosity, etc. Much work has been done and published regarding theselatter topics, and somecontroversial results have been reported, particularly on the role played by sulfateson the phasecomposition of ZrO, polymorphs and on the Lewis or Broensted acidity of the catalytic sites [7,8]. Further, notwithstanding the relevance of sulfatezirconia interactions, studies concerning the adsorption processitself are very limited [9,10]. Chokkaram et al. [IO] have reported curves of weight per cent sulfate adsorbedas a function of either initial or final Ii,SO, concentration in solution. The different points of each curve, however, having been obtained intrinsically at different pH and ionic OQ22-0728/96/$i5.00 PII SOO22.0728(96)04724-9
strength, pertain to different values of the electrochemical parametersof the oxide surface and may not be considered to identify a “true” adsorption isotherm. In the present work two adsorptionisothermsof H,SO, on ZrOz powders, obtained at constant pH and ionic strength, are reported and discussedwith the aim of bringing out some relevant evidence concerning the zirconiasulfate interactions.
2. Experimental All reagentswere of reagent grade purity and were used as received without further purification. Solutions and suspensionswere prepared with twice distilled water passedthrough a Milli-Q apparatus. The solid adsorbentemployed in the study was prepared by precipitation (KOH), at the boiling point, from an acidic ZrCl, solution. The pH of the suspensionwas never raised above pH 7 during the reaction. The reaction product, after purification by a dialysis procedure, was stored as a dry powder in a desiccator. By XRD it appearedto be composed mainly of tetragonal zirconia, although with a relatively low degree of crystallinity. The specific surface area determined by BET measurements was S = 216m2g-‘. The adsorption isotherms of H, SO, onto the zirconia samples were obtained under the following conditions:
Copyright 0 1996 Elsevier Science S.A. All rights reserved.
194
S. Ardiuone,
E. Grassi/Journal
of Electroanalytical
M KNO,; equilibration time 1 h; pH 4.5 for isotherm 1 and 2.6 for isotherm 2. At the end of the adsorption time the suspension was centrifuged in PEP nalgene tubes at 9OOOrev min-’ and the supematant was sampled for the evaluation of the residual sulfate concentration by direct titration with barium chloride in acetone + water solution (66% + 1% pyridine) using carboxyarsenazo as indicator Ill].
Chemistry
417 (1996)
193-195
2.0
T = 25°C; lo-*
1.5 5 ;
1.0
i o_ 0.5 L
0t 0
3. Results and discussion
IO’ C / mol
Sulfate-modified catalysts are prepared, as a rule, by equilibrating the oxide powders directly with sulfuric acid of variable concentration. Consequently, the present adsorption isotherms were obtained in an acid pH range congruent with the literature data. The specific values of pH and ionic strength of the adsorption experiments were selected so as to produce significantly different conditions of surface charge and potential at the oxidelsolution boundarYThe point of zero charge (pzc) of the powders, determined by the classical procedure [ 121, was found to be 8.4, in agreement with the pzc values reported for zirconia not subjected to dehydration processes [ 13,141. The values of the oxide surface charge pertaining to the two isotherms are 15.8 p.Ccm-2 (isotherm 1) and 25.3pCcm-2 (isotherm 2) respectively. Investigations of the adsorption kinetics were performed by extending the duration of the experiments to times longer than 1 h; no appreciable variations in the sulfate bulk concentrations were observed. Fig. 1 presents adsorption isotherms of sulfates on zirconia samples, obtained under the experimental conditions discussed above. The shape of the two isotherms is very similar, first displaying a rising part and then leveling off to a plateau. The isotherms belong to the group classified in the literature as Langmuir-type isotherms.
0’ 0
1 10
1
20
1
30 IO’ C /mot
1
I
1
40
50
60
L-’
Fig. 1. Dependence of the surface excess r on H,SO, equilibrium concentration at constant KNO, concentration (IO-* M). 1, pH 4.5; 2, pH 2.6.
Fig. 2. Adsorption data expressed I- max = 2.40X 10m4 pm,, cme2, 10-4p,,cm-2, b= 1.10X10’.
1-l
in linearized b = 5.34X
Langmuir 104; 2,
coordinates. 1, T,,, = 3.80 X
Choice of a lower solution pH, i.e. increasing the positive surface charge and potential of the oxide surface, produces an increase in both the slope of the initial linear rising part and the value of the surface excess of the plateau. Fig. 2 reports adsorption results in the coordinates of the linearized Langmuir equations. The equation adopted in Fig. 2 is c/r= 1/t r,,, P ) + c/L where r is the surface excess, r,,, the maximum surface coverage and /3 the adsorption constant. The figure shows that the data, over all the investigated concentration range, are interpolated by straight lines with high correlation coefficients for both isotherms ( r, = 0.998; r2 = 0.999). This confirms the adherence of the experimental results to a Langmuir-type isotherm. This result is at variance with that observed by Chokkaram et al. [lo] on their adsorption data, which appeared not to be fitted by either Langmuir or Freundlich equations. This latter occurrence is possibly the result of the experimental conditions selected, i.e. the lack of invariance of pH and ionic strength for the different points of the isotherm. The processing of data in the linearized coordinates in Fig. 2 allows parameters describing the features of the adsorption process to be obtained. The density of the positive surface sites of the oxide plays a definite role in imposing the coverage by the anions. The ratio between the initial oxide surface charges at the two pH values (000.2/oo,1 = 1.60) reflects closely the ratio between the two r,,, values (rmax,Jrmax,, = 1.58). The proportional variation of the r,,, with a, is an indication of the relevant role played by electrostatic interaction in determining the adsorption. The experimental initial surface charges of the oxide substrates, although pertaining to relatively high ApH,,, values (isotherm 1, ApH,, = 3.9, isotherm 2, ApH,,, = 5.8) are in both cases much lower than the “maximum” charge which can be calculated on the assumption of a
S. Ardizzone,
E. Grassi/Journal
ofElectroamlyticul
“full” dissociation of all the physically accessible surface OH groups. Therefore, considering the above reported r,,,/oO relation, it can be concluded that the adsorption film is, in both cases, in conditions which are far from the “full” occupancy of the surface sites. The “dilute” nature of the adsorption layer also finds support from the apparent lack of lateral repulsive interactions between the adsorbate molecules. In fact, besides the reported fit to the Langmuir “ideal” model, the experimental data have also been fitted (although with a less satisfying linear correlation) to a virial isotherm [15], showing a zero lateral interaction term. The values of maximum coverage r,,, obtained by the elaboration can be related to the BET specific surface area of the oxide in order to obtain the value of the available areas per adsorbed species. Calculations lead to a value of 0.692 nm’ (0.692 f 0.012 nm*) for isotherm 1 and 0.437 nm2 (0.437 rt 0.003 nm’) for isotherm 2. The size of the sulfates based on the bare crystallographic radius is 0.2nm2. As could have been expected, on the basis of the above considerations, in both cases the estimated available areas are higher than the geometrical size of the adsorbed species. A polar solvent like water is very likely to be present in the adsorbed monolayers formed at charged solid surfaces by ionic adsorbates. It is interesting to observe that the present datum of 0.43nm2 (isotherm 2) corresponds exactly to the 0.43nm2 value reported by Chokkaram et al. [lo] as the area occupied by sulfate anions in conditions corresponding to the saturation of the oxide surface (the conditions of saturation were derived from the trend of the particle surface area versus H,SO, concentration). Therefore, the adsorbed monolayers of sulfates onto zirconia remain hydrated, i.e. they could be classified as liquid-expanded, even in conditions of maximum saturation of the adsorbent. The values of the Gibbs energy of the adsorption calculated from the values of p are not high (AGY = -2.70kJmol-‘; AGZ = - 2.88 kJmol- ‘) and may be representative of a process supported by physical forces. The role played by either physical or chemical interactions in the present adsorption process is not clear-cut. Chemisorption. Sulfates are bivalent species and may be expected to show specific interactions with an oxide surface. The picture of the adsorption in a monolayer, obtained from the elaboration of the experimental data, might suggest partition of the species in the inner double layer and not in the diffuse region. Physisorption. The comparison between the two isotherms presented shows that the process is affected
Chemistry
417 (I9961
193-195
195
strongly by the attractive electrostatic interactions with the oxide surface. It is very likely that the process is both physi- and chemi-supported. Upon “washing”, the amount of sulfate adsorbed decreases (reversible adsorption physical forces) [10,16], but it never falls to zero even in the case of desorption tests performed at alkaline pH (negative surface charge of the substrate). This should indicate the presence of a not fully reversible chemisorption process [10,161.
Acknowledgements
Financial support from the MURST (Ministry of the University and of Scientific and Technological Research) (40% and 60% research funds) and the Italian National Research Council, Technological Research Committee (CNR, Comitato Ricerche Tecnologiche) is gratefully acknowledged.
References
Ill K. I21 T.
Tanabe, Mater. Chem. Phys.. 13 (1985) 347. Maehashi, K. Maymura, K. Domen, K. Aika and T. Onishi, Chem. Lett., ( 1984) 747. 131 M.Y. He and J.G. Eke&, J. Catal., 90 (1984) 17. Mater. Chem. [41 M. Bensitel, 0. Sam, J.C. Lavalley and B.A. Marrow, Phys., 19 (1988) 147. 151K. Tanabe, M. Misono, Y. Ono and H. Hattori, New Solids Acids and Bases, Elsevier, Amsterdam, 1989, p. 199. [61 K. Arata, M. Hino and N. Yamagata, Bull. Chem. Sot. Jpn., 63 (1990) 244. Catalysis Today, 20 L71 B.H. Davis, R.A. Keogh and R. Srinivasan, (1994) 219. [81 A. Clearfield, G.P.D. Serrette and A.H. Khazi-Syed. Catalysis Today, 20 (1994) 295. [9] F.R. Chen, G. Coudurier, J.F. Joly and J.C. Vedrine, in Symposium on Alkylation. Aromatization, Oligomerization of Short Chain Hydrocarbons over Heterogeneous Catalysts, American Chemical Society, New York, 1991. p. 878. [lo] S. Chokkaram, R. Srinivasan, D.R. Milbum and B.H. Davis, J. Colloid Interface Sci., 165 (1994) 160. [l 11 C.A. Watson (Ed.), Official and Standardized Methods of Analysis, The Royal Society of Chemistry, Cambridge, 3rd edn., 1994. [I21 S. Ardizzone and S. Carella, Mater. Chem. Phys., 31 (1992) 351. [I31 S. Ardizzone. G. Bassi and G. Liborio. Colloids Surf., 51 (1990) 207. [14] R. Paterson and H. Rahman, J. Colloid Interface Sci., 103 (1985) 106. [I51 W. Anderson and R. Parsons, in J.H. Schulman (Ed.), Proc. 2nd Int. Congress on Surface Activity, Vol. 3, Butterworths. London, 1957, p. 45. [16] L. Bodini, Thesis, University of Milan, 1995.