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Degradation behaviour of a titanium supported RuO2TiO2 electrode
Surface Technology,
26 (1985)
DEGRADATION BEHAVIOUR RuO,-TiO, ELECTRODE TOMIYA KISHI, YOSHIHARU Faculty of Science hama 223 (Japan)
245
245 - 251
OF A TITANIUM
SUPPORTED
SUGIMOTO and TAKASHI NAGAI
and Technology,
Keio
University,
Hiyoshi
3-14-1, Kohoku-ku,
Yoko-
(Received March 4, 1985)
Summary
The degradation behaviour of a titanium supported RuOz-TiOz electrode in HCl-H,S04 solutions is investigated. In the early stage of electrolysis cracks on the electrode surface grow by dissolution of oxide in the strongly acidified medium, the proton concentration of which is controlled by the main electrolytic reactions. It is concluded that the titanium component in the oxide is responsible for the dissolution of the oxides in the cracks.
1. Introduction
To obtain more precise information on the stability and degradation of anode materials, the relatively early stage of degradation, before the well-defined degradation phenomenon of a sharp increase in anode potential [l] occurs, in a titanium supported RuO,-TiO, electrode was investigated. Changes in the electrode characteristics measured by cyclic voltammetry were analysed in relation to the nature and the rate of the main electrolytic reactions occurring simultaneously on the anode, and to the composition of the electrode.
2. Experimental details
Titanium supported RuO,-TiO, electrodes were prepared by the standard method (see ref. 2, p. 311): 1 M isopropanol solutions of RuCls and TiC13in various ratios were brushed on titanium plate which had been etched in 10% oxalic acid solution for 2 h at 80 “C. The coated film was dried at room temperature and pyrolysed at 350 “C for 10 min. The brushingpyrolysing procedure was repeated nine times and after the tenth time the film was pyrolysed at 450 “C for 1 h. The thickness of the film was about 6 pm and almost independent of the composition. The composition of the 0316-4583/85/$3.30
@ Elsevier Sequoia/Printed in The Netherlands
246
coated film ranged from 0.2 to 1 designated by x in the formula Ru,Til _ xO2. The crystal structure of the film, measured using the X-ray diffraction method, was of a r-utile type and its lattice constant changed almost linearly from that of pure RuO, to pure TiOz. The film surface had a well-known cracked, dried mud morphology (see ref. 3, p. 323). The electrolysis was conducted in 1 M HCl and 0.5 M H,SO,, and a mixture of these, i.e. under more severe conditions than those usually used in practice, so as to accelerate degradation of the electrode. Cyclic voltammograms of the electrode before and after electrolysis in 0.5 M H,S04 were analysed as a measure of the electrode characteristics. Surface investigations were also performed using X-ray microanalysis. The amount of metal components dissolved in the solution during long-term electrolysis was measured using atomic absorption analysis.
3. Results and discussion Typical changes of anode potential with time during electrolysis in 1 M HCI and 0.5 M H,S04 solutions under a more drastic condition, 2.0 A cmP2, than usual are shown in Fig. 1. Under oxygen evolution conditions, a typical degradation phenomenon of a sharp increase in potential is observed at shorter times than under chlorine evolution conditions, and in the former case detachment of a part of the cracked surface film is observed using microscopy. Before this final degradation occurs, the pseudo-capacitance of the electrode, characteristic of the RuO,-TiO, electrode [ 41, increases with the duration of electrolysis, This change in the pseudo-capacitance of the electrode is analysed in some detail. A typical cyclic voltammogram in the stationary state in 0.5 M H2S04 solution is shown in Fig. 2. Currents flowing in the potential range between
I 0
II 50
100 Time
150
200
250
I min.
0.4
0.8
I1.2
U lVvs.SHE
Fig. 1. Potential changes during electrolysis: j = 2.0 A cmp2 ; temperature, Fig. 2. Cyclic voltammograms sphere; temperature, 25 “C.
25 “C.
for various scanning rates in 0.5 M H*SOa: nitrogen atmo-
241
oxygen and hydrogen evolution reactions had a capacitive nature [ 51, and it is concluded that this is caused by the highly reversible redox reaction (1) in the surface region of the oxide [6], RuO, + H+ + e- = RuOOH
(1)
which is confirmed by the facts that the current density j* at 0.8 V, corresponding to a broad peak in current, is proportional to the sweep rate of the potential (see Fig. 3), where the true capacitive component of the double layer can be neglected [7], and that the characteristic charge Q* [4], as well as the pseudo-capacitance defined by j*/(sweep rate), is proportional to the content of RuOz component in the electrode, i.e. x, as shown in Fig. 4. The pseudo-capacitance C, is thus proportional to the number of active ruthenium sites in the surface region and, as shown in Fig. 5, the current density in the Tafel region of the chlorine evolution reaction corrected for unit C, is almost independent of X, whereas that of the oxygen evolution reaction increases with X, presumably owing to the fact that pair sites are used for this reaction [ 81. The pseudo-capacitance of the electrode increases with electrolysis time, as shown in Fig. 6, and this relation can be expressed by c P,t-
C&t=0 = /&i/2 (2) C,,t=o as shown in Fig. 7. In this equation, CPat denotes the value of C, at electrolysis time t, C,, t =o denotes that before electrolysis, and k is a proportionality constant. Increase in C, is deduced to be caused by an increase in the active surface area as measured by the active ruthenium sites, and the dependence on the square root of the time suggests that a diffusion process is rate controlling in this phenomenon. T
6-
X in Ru,Ti,,O,
Fig. 3. Dependence of peak current density j* on scanning rate, Fig. 4. Pseudo-capacitance in Ru,Til -xOz.
C, (I) and the characteristic
charge q* (0) as functions of x
248
/I!,;‘I ii 0
0.2
0.4 X
in
0.6
0.8
1.0
Ru,TI,_,O,
0
IO
20 30 40 Time/ min.
50
60
Fig. 5. Corrected electrolytic current density j’ as a function of x in Ru,Til _%02: 1, chlorine gas evolution (1 M HCl) at 1.35 V(SHE); 2, oxygen gas evolution (0.5 M HzS04) at 1.45 V(SHE). Fig. 6. Change in pseudocapacitance 3 V(SHE).
C, with electrolysis time: 0.6 M HCl-0.2
M HTSOo,
Microscopic observation of the carefully polished electrode surface indicated deepening of cracks and pit formation on the surface by electrolysis, besides uniform removal of the flat part of the surface. These phenomena develop to cause degradation of the electrode. From these deductions and observations, a model for the increase in the effective surface area can be proposed. The cracks on the electrode grow in the depth direction, as shown in Fig. 8, with diffusion of ionic species in the crack channels being the rate determining step, as in the case of cracking corrosion [9]. The length 1 of cracks can be represented by this simplified model as [ 91 co
l/2 01/2t'/2
2=2
i 2p 1
(3)
where, D, co, p are a diffusion constant, the concentration of the diffusion species at the bottom and the density of the oxide respectively. According to this model k is proportional to 2(~‘/2p)“~D 1’2. Growth in the depth direction would be caused by the concentration of a heavily defective region at the crack bottom [3] and acidification in narrow crack channels by electrolysis. Acidification that can accelerate the dissolution rate of the oxide, i.e. growth rate of the cracks, will be more intense as the relative oxygen evolution rate increases. In 1 M HCl-0.5 M H,SO,, mixed solutions, the proportionality constant k increases with the relative rate of the oxygen evolution reaction as shown in Figs. 9 and 10. As the decrease in pH value is larger in the case of the oxygen evolution reaction than in that of the chlorine evolution reaction, this correlation suggests that the diffusing species in the crack channels is the proton and it diffuses from the bottom to the top of the cracks. Proton concentration at the bottom of the cracks
249
0
.X-
log(t IminI Fig.
7. Time dependence of pseudo-capacitance
using eqn. (2): 3 V(SHE).
Fig. 8. Model for crack growth.
r
0.4 -
I
8 100
0
g 80
2
8
f60
L0” ‘; 40
0
B 20
8
O 0
-_;:; 0
0.2
0.4 Ccl-1
0.6 I
0.6
IO
M
Fig. 9. Dependence of k on the concentration
0
0.2
0.4
[cl-l
0.6 I
0.8
1.0
M
of Cl- ions in the solution: 3 V( SHE).
Fig. 10. Relative rate of chlorine gas evolution to the total rate of gas evolution as a function of the concentration of Cl- ions in the solution: 3 V(SHE).
is controlled by the rate of the main electrolysis reactions as well as by their nature and therefore depends on the overpotential. The concentration can be simply combined with the overpotential 77under diffusion-controlled conditions as
(4) where cb denotes concentration in the bulk and other symbols have their usual meanings. Substituting for co in eqn. (3), the dependence of k on the overpotential is obtained :
250
210gh=210gk0+10gD+2.303-
nFv
(5)
RT
where k, is a constant containing the proportionality factor k, 2(c”/2p)“*. The experimental value of k depends on the electrode potential as expected from the above considerations, as shown in Fig. 11. The slope a(log k)/&I, however, is much lower than that expected from eqn. (5) owing presumably to the complex nature of the reactions between the reaction products and water which determine the pH value and to the potential drop in the crack channels. Possible dissolution reactions in the strongly acidic solution in the cracks are chemical
electrochemical
RuOz + 4H+ = Ru4+ + 2H,O
(6)
TiOz + H+ = Ti02+ + H,O
(7)
RuO, + 4H+ = Rus+ + 2H20 + 4e-
(3)
where Ru4+ and Rus+ ions are unstable and may readily be hydrolysed to form RuO, or H,RuOs. The constant k which may be interpreted as a rate constant for crack growth is influenced by the increase in the effective surface area corresponding to the number of active ruthenium sites, so is corrected for the amount of ruthenium component to obtain a relative rate constant corresponding to the real length, shown in Fig. 12. In Fig. 13, the rate of dissolution of the titanium component into the solutions during longer term electrolysis is plotted as a function of concentration of Cl- ions in the solution. It is revealed from these results that the dissolution of the titanium component in strong acid media will be responsible for the rate of crack growth. (The dissolution rate of the ruthenium component measured in the solution was subject to some uncertainty owing to the escape of ruthenium(VIII) into the gas phase.) Dissolution of the titanium component is confirmed by the relative decrease in titanium against ruthenium on the used electrode surface as measured by X-ray microanalysis.
04
0.6
X
inRu,Ti,$,
06
1.0
Fig. 11. Dependence of k on the potential :l M HCl. Fig. 12. Corrected value k ’ of k as a function of x in Ru,Til
_xO2 : 1 M HCl; 3 V(SHE).
251
60
Fig. 13. Dissolution rate rTi of the titanium component of Cl- ions in the solution: %V(SHE).
as a function of concentration
4. Conclusion In more drastic conditions than are usual, cracks on the surface grow by dissolution of the oxide film in the strongly acidified media. The titanium component is responsible for dissolution of the oxide, and in acidification the main electrolytic reactions play an important role. This phenomenon found in the early stage of electrolysis causes degradation of the Ru02TiOz film electrode. Acknowledgment The authors are grateful to Mr. H. Fujishiro for his support in ‘the experimental work. References 1 F. Hine, M. Yasuda, T. Noda, T. Yoshida and J. Okuda, J. Electrochem. Sot., 126 (1979) 1439. 2 S. Trassati and G. Lodi, in S. Trasatti (ed.), Electrodes of Conductive Metallic Oxides, Part A., Elsevier, Amsterdam, 1980, p. 311. 3 S. Trassati and G. Lodi, in S. Trasatti (ed.), Electrodes of Conductive Metallic Oxides, Part A., Elsevier, Amsterdam, 1980, p. 323. 4 S. Trasatti and W. E. O’Grady, in H. Gerischer and C. W. Tobias (eds.), Adv. Electrochem. & Electrochem. Eng., 12 (1981) 177. 5 G. Lodi, E. Sivieri, A. de Battisti and S. Trasatti, J. Appl. Electrochem., 8 (1978) 135. 6 D. Galizzioli, F. Tandardini and S. Trasatti, J. Appl. Electrochem., 4 (1974) 57. 7 B. E. Conway and H. Angerstein-Kozlowska, in A. D. Franklin (ed.), Electrocatalysis on Non-Metallic Surfaces, National Bureau of Standards, U.S. Government Printing Office, Washington, DC, 1976, p. 107. 8 L. I. Krishtalk, Electrochim. Acta, 26 (1981) 329. 9 G. P. Cherepanov, in J. O’M. Bockris, B. E. Conway, E. Yeager and R. E. White (eds.), Comprehensive Treatise of Electrochemistry, Plenum, New York, 1981, p. 333.
26 (1985)
DEGRADATION BEHAVIOUR RuO,-TiO, ELECTRODE TOMIYA KISHI, YOSHIHARU Faculty of Science hama 223 (Japan)
245
245 - 251
OF A TITANIUM
SUPPORTED
SUGIMOTO and TAKASHI NAGAI
and Technology,
Keio
University,
Hiyoshi
3-14-1, Kohoku-ku,
Yoko-
(Received March 4, 1985)
Summary
The degradation behaviour of a titanium supported RuOz-TiOz electrode in HCl-H,S04 solutions is investigated. In the early stage of electrolysis cracks on the electrode surface grow by dissolution of oxide in the strongly acidified medium, the proton concentration of which is controlled by the main electrolytic reactions. It is concluded that the titanium component in the oxide is responsible for the dissolution of the oxides in the cracks.
1. Introduction
To obtain more precise information on the stability and degradation of anode materials, the relatively early stage of degradation, before the well-defined degradation phenomenon of a sharp increase in anode potential [l] occurs, in a titanium supported RuO,-TiO, electrode was investigated. Changes in the electrode characteristics measured by cyclic voltammetry were analysed in relation to the nature and the rate of the main electrolytic reactions occurring simultaneously on the anode, and to the composition of the electrode.
2. Experimental details
Titanium supported RuO,-TiO, electrodes were prepared by the standard method (see ref. 2, p. 311): 1 M isopropanol solutions of RuCls and TiC13in various ratios were brushed on titanium plate which had been etched in 10% oxalic acid solution for 2 h at 80 “C. The coated film was dried at room temperature and pyrolysed at 350 “C for 10 min. The brushingpyrolysing procedure was repeated nine times and after the tenth time the film was pyrolysed at 450 “C for 1 h. The thickness of the film was about 6 pm and almost independent of the composition. The composition of the 0316-4583/85/$3.30
@ Elsevier Sequoia/Printed in The Netherlands
246
coated film ranged from 0.2 to 1 designated by x in the formula Ru,Til _ xO2. The crystal structure of the film, measured using the X-ray diffraction method, was of a r-utile type and its lattice constant changed almost linearly from that of pure RuO, to pure TiOz. The film surface had a well-known cracked, dried mud morphology (see ref. 3, p. 323). The electrolysis was conducted in 1 M HCl and 0.5 M H,SO,, and a mixture of these, i.e. under more severe conditions than those usually used in practice, so as to accelerate degradation of the electrode. Cyclic voltammograms of the electrode before and after electrolysis in 0.5 M H,S04 were analysed as a measure of the electrode characteristics. Surface investigations were also performed using X-ray microanalysis. The amount of metal components dissolved in the solution during long-term electrolysis was measured using atomic absorption analysis.
3. Results and discussion Typical changes of anode potential with time during electrolysis in 1 M HCI and 0.5 M H,S04 solutions under a more drastic condition, 2.0 A cmP2, than usual are shown in Fig. 1. Under oxygen evolution conditions, a typical degradation phenomenon of a sharp increase in potential is observed at shorter times than under chlorine evolution conditions, and in the former case detachment of a part of the cracked surface film is observed using microscopy. Before this final degradation occurs, the pseudo-capacitance of the electrode, characteristic of the RuO,-TiO, electrode [ 41, increases with the duration of electrolysis, This change in the pseudo-capacitance of the electrode is analysed in some detail. A typical cyclic voltammogram in the stationary state in 0.5 M H2S04 solution is shown in Fig. 2. Currents flowing in the potential range between
I 0
II 50
100 Time
150
200
250
I min.
0.4
0.8
I1.2
U lVvs.SHE
Fig. 1. Potential changes during electrolysis: j = 2.0 A cmp2 ; temperature, Fig. 2. Cyclic voltammograms sphere; temperature, 25 “C.
25 “C.
for various scanning rates in 0.5 M H*SOa: nitrogen atmo-
241
oxygen and hydrogen evolution reactions had a capacitive nature [ 51, and it is concluded that this is caused by the highly reversible redox reaction (1) in the surface region of the oxide [6], RuO, + H+ + e- = RuOOH
(1)
which is confirmed by the facts that the current density j* at 0.8 V, corresponding to a broad peak in current, is proportional to the sweep rate of the potential (see Fig. 3), where the true capacitive component of the double layer can be neglected [7], and that the characteristic charge Q* [4], as well as the pseudo-capacitance defined by j*/(sweep rate), is proportional to the content of RuOz component in the electrode, i.e. x, as shown in Fig. 4. The pseudo-capacitance C, is thus proportional to the number of active ruthenium sites in the surface region and, as shown in Fig. 5, the current density in the Tafel region of the chlorine evolution reaction corrected for unit C, is almost independent of X, whereas that of the oxygen evolution reaction increases with X, presumably owing to the fact that pair sites are used for this reaction [ 81. The pseudo-capacitance of the electrode increases with electrolysis time, as shown in Fig. 6, and this relation can be expressed by c P,t-
C&t=0 = /&i/2 (2) C,,t=o as shown in Fig. 7. In this equation, CPat denotes the value of C, at electrolysis time t, C,, t =o denotes that before electrolysis, and k is a proportionality constant. Increase in C, is deduced to be caused by an increase in the active surface area as measured by the active ruthenium sites, and the dependence on the square root of the time suggests that a diffusion process is rate controlling in this phenomenon. T
6-
X in Ru,Ti,,O,
Fig. 3. Dependence of peak current density j* on scanning rate, Fig. 4. Pseudo-capacitance in Ru,Til -xOz.
C, (I) and the characteristic
charge q* (0) as functions of x
248
/I!,;‘I ii 0
0.2
0.4 X
in
0.6
0.8
1.0
Ru,TI,_,O,
0
IO
20 30 40 Time/ min.
50
60
Fig. 5. Corrected electrolytic current density j’ as a function of x in Ru,Til _%02: 1, chlorine gas evolution (1 M HCl) at 1.35 V(SHE); 2, oxygen gas evolution (0.5 M HzS04) at 1.45 V(SHE). Fig. 6. Change in pseudocapacitance 3 V(SHE).
C, with electrolysis time: 0.6 M HCl-0.2
M HTSOo,
Microscopic observation of the carefully polished electrode surface indicated deepening of cracks and pit formation on the surface by electrolysis, besides uniform removal of the flat part of the surface. These phenomena develop to cause degradation of the electrode. From these deductions and observations, a model for the increase in the effective surface area can be proposed. The cracks on the electrode grow in the depth direction, as shown in Fig. 8, with diffusion of ionic species in the crack channels being the rate determining step, as in the case of cracking corrosion [9]. The length 1 of cracks can be represented by this simplified model as [ 91 co
l/2 01/2t'/2
2=2
i 2p 1
(3)
where, D, co, p are a diffusion constant, the concentration of the diffusion species at the bottom and the density of the oxide respectively. According to this model k is proportional to 2(~‘/2p)“~D 1’2. Growth in the depth direction would be caused by the concentration of a heavily defective region at the crack bottom [3] and acidification in narrow crack channels by electrolysis. Acidification that can accelerate the dissolution rate of the oxide, i.e. growth rate of the cracks, will be more intense as the relative oxygen evolution rate increases. In 1 M HCl-0.5 M H,SO,, mixed solutions, the proportionality constant k increases with the relative rate of the oxygen evolution reaction as shown in Figs. 9 and 10. As the decrease in pH value is larger in the case of the oxygen evolution reaction than in that of the chlorine evolution reaction, this correlation suggests that the diffusing species in the crack channels is the proton and it diffuses from the bottom to the top of the cracks. Proton concentration at the bottom of the cracks
249
0
.X-
log(t IminI Fig.
7. Time dependence of pseudo-capacitance
using eqn. (2): 3 V(SHE).
Fig. 8. Model for crack growth.
r
0.4 -
I
8 100
0
g 80
2
8
f60
L0” ‘; 40
0
B 20
8
O 0
-_;:; 0
0.2
0.4 Ccl-1
0.6 I
0.6
IO
M
Fig. 9. Dependence of k on the concentration
0
0.2
0.4
[cl-l
0.6 I
0.8
1.0
M
of Cl- ions in the solution: 3 V( SHE).
Fig. 10. Relative rate of chlorine gas evolution to the total rate of gas evolution as a function of the concentration of Cl- ions in the solution: 3 V(SHE).
is controlled by the rate of the main electrolysis reactions as well as by their nature and therefore depends on the overpotential. The concentration can be simply combined with the overpotential 77under diffusion-controlled conditions as
(4) where cb denotes concentration in the bulk and other symbols have their usual meanings. Substituting for co in eqn. (3), the dependence of k on the overpotential is obtained :
250
210gh=210gk0+10gD+2.303-
nFv
(5)
RT
where k, is a constant containing the proportionality factor k, 2(c”/2p)“*. The experimental value of k depends on the electrode potential as expected from the above considerations, as shown in Fig. 11. The slope a(log k)/&I, however, is much lower than that expected from eqn. (5) owing presumably to the complex nature of the reactions between the reaction products and water which determine the pH value and to the potential drop in the crack channels. Possible dissolution reactions in the strongly acidic solution in the cracks are chemical
electrochemical
RuOz + 4H+ = Ru4+ + 2H,O
(6)
TiOz + H+ = Ti02+ + H,O
(7)
RuO, + 4H+ = Rus+ + 2H20 + 4e-
(3)
where Ru4+ and Rus+ ions are unstable and may readily be hydrolysed to form RuO, or H,RuOs. The constant k which may be interpreted as a rate constant for crack growth is influenced by the increase in the effective surface area corresponding to the number of active ruthenium sites, so is corrected for the amount of ruthenium component to obtain a relative rate constant corresponding to the real length, shown in Fig. 12. In Fig. 13, the rate of dissolution of the titanium component into the solutions during longer term electrolysis is plotted as a function of concentration of Cl- ions in the solution. It is revealed from these results that the dissolution of the titanium component in strong acid media will be responsible for the rate of crack growth. (The dissolution rate of the ruthenium component measured in the solution was subject to some uncertainty owing to the escape of ruthenium(VIII) into the gas phase.) Dissolution of the titanium component is confirmed by the relative decrease in titanium against ruthenium on the used electrode surface as measured by X-ray microanalysis.
04
0.6
X
inRu,Ti,$,
06
1.0
Fig. 11. Dependence of k on the potential :l M HCl. Fig. 12. Corrected value k ’ of k as a function of x in Ru,Til
_xO2 : 1 M HCl; 3 V(SHE).
251
60
Fig. 13. Dissolution rate rTi of the titanium component of Cl- ions in the solution: %V(SHE).
as a function of concentration
4. Conclusion In more drastic conditions than are usual, cracks on the surface grow by dissolution of the oxide film in the strongly acidified media. The titanium component is responsible for dissolution of the oxide, and in acidification the main electrolytic reactions play an important role. This phenomenon found in the early stage of electrolysis causes degradation of the Ru02TiOz film electrode. Acknowledgment The authors are grateful to Mr. H. Fujishiro for his support in ‘the experimental work. References 1 F. Hine, M. Yasuda, T. Noda, T. Yoshida and J. Okuda, J. Electrochem. Sot., 126 (1979) 1439. 2 S. Trassati and G. Lodi, in S. Trasatti (ed.), Electrodes of Conductive Metallic Oxides, Part A., Elsevier, Amsterdam, 1980, p. 311. 3 S. Trassati and G. Lodi, in S. Trasatti (ed.), Electrodes of Conductive Metallic Oxides, Part A., Elsevier, Amsterdam, 1980, p. 323. 4 S. Trasatti and W. E. O’Grady, in H. Gerischer and C. W. Tobias (eds.), Adv. Electrochem. & Electrochem. Eng., 12 (1981) 177. 5 G. Lodi, E. Sivieri, A. de Battisti and S. Trasatti, J. Appl. Electrochem., 8 (1978) 135. 6 D. Galizzioli, F. Tandardini and S. Trasatti, J. Appl. Electrochem., 4 (1974) 57. 7 B. E. Conway and H. Angerstein-Kozlowska, in A. D. Franklin (ed.), Electrocatalysis on Non-Metallic Surfaces, National Bureau of Standards, U.S. Government Printing Office, Washington, DC, 1976, p. 107. 8 L. I. Krishtalk, Electrochim. Acta, 26 (1981) 329. 9 G. P. Cherepanov, in J. O’M. Bockris, B. E. Conway, E. Yeager and R. E. White (eds.), Comprehensive Treatise of Electrochemistry, Plenum, New York, 1981, p. 333.