1922
J. Phys. Chem. 1981, 85, 1922-1927
pulses do not affect appreciably the concentration of NO (Figure 6). We were unable to explain the absence of NO consumption as reactions of HNO in the gas phase or on the walls should not account for the stability of NO concentrati~n.~JO Because of this peculiar phenomenon, the reliability of the kNo value is not as good as for ko,, although this is not expressed in the estimated error in kNO. There remains an unresolved problem to be tackled. The observed decrease of k, with temperature lies within the range of experimental uncertainties but seems to be significant as measurements are reproducible. This behavior is expected for the fast reactions involved and implies that the activation energy is negligible, the temperature dependence being that of the frequency factor. Apparent negative activation energies have been observed for many radical-radical and radical-molecule reactions. The theory for this phenomenon has not yet been fully developed but should benefit from progress in unimolecular rate theory. The comparison between near-room-temperature data (-300-500 K) and high-temperature data ( w 1000-2000 K) is rather difficult because of the lack of reliable hightemperature data and the gap between the two temperature ranges. Flame studies by Peeters et a1.12 yielded a
value of ko, that is 10 times larger than ours, ko, = 5 X 10-l’ cm3molecule-’ s-’ at 1400 < T < 1800 K, while Westbrook et al.13 later obtained a lower value for ko, in their turbulent flow reactor experiment, ko, = (2.2 f 1.1)X 10-l2 cm3 molecule-‘ s-’ at T = 1100 K. This latter value fits well on our Arrhenius curve for ko, determined in the 300-500 K temperature range. Critical evaluation of the data shown in Table I indicates that k, values in the higher range are most reliable, and we recommend their use in atmospheric modeling. Despite the lack of accuracy in the ko, temperature-dependence study, use of our k,, value in stratospheric modeling is reasonable (ko, = (6.2 f 0.9) X 10-l2cm3 molecule-'^-^ at 228 K). This is to be compared with the lower value recommended in the recent compilation by Hampson14 cm3 molecule-’ s-l). ((5.1 f 1.3) X Acknowledgment. We thank Professor J. Joussot-Dubien for his encouraging interest in this work. (12)J. Peeters and G. Mahnen, Symp. (Int.) Combust., [Proc.], 14,133 (1973). (13)C. K. Westbrook, J. Creighton, and C. Lund, J. Phys. Chem., 81, 2542 (1977). (14)R. F. Hampson “Chemical Kinetic and Photochemical Data Sheets for Atmospheric Reactions”, Vol. 4,1980,p 47;Report No. FAAEE-80-17.
ESR Study of Copper Pentanedionate Solutions in Organic Solvents Adsorbed on Porous Silica Gels Giacomo Martini’ and M. Francesca Ottavlani Institule of Physical Chemlstry, UniversnY of Florence, 50121 Florence, Italy (Recelved: August 27, 1980; In Final Form: February 13, 1981)
The ESR spectra of Cu(acac)2introduced as a solution in organic solvents (CHC13,DMF, and pyridine) in different silica gels with pore diameters in the range 4-100 nm are used for the study of the adsorption of both the paramagnetic complex and solvent molecules on the surface where the g and the hyperfine splitting constant tensors are modified as a function of interaction with the silica gel substrate. Part of the Cu(acac)2complex inside the pores maintains liquidlike mobility, while a fraction dependent on the pore size, on the Cu(I1) concentration, and on the nature of the solvent gives rise to ESR spectra due to surface-adsorbed species. The ESR parameters and the relative intensity of the liquidlike and surface spectra allow us to suggest that (i) a surface silanol group is coordinated to Cu(acac)2in the apical position, (ii) the crystal-field strength of the = S i - O H group is low compared to that of CHC1, pyridine, and DMF, and (iii) the DMF molecules are adsorbed on the surface via hydrogen bonds more tightly than the pyridine molecules and CHC13is only loosely adsorbed.
Introduction The preparation and the structure of Cu(I1)-containing porous supports have been extensively studied by electron spin resonance (ESR) in the past years with the aim of elucidating the catalytic processes in which such systems are involved.’-7 Attention was mainly devoted to the localization and bonding features of the surface-adsorbed complexes. However, the study of the mobility of pure fluids or fluid mixtures into fully filled pores of porous
supports is also of particular relevance in such fields as heterogeneous catalysis, chromatography, and membrane science. Indeed, the flow rate is in part affected by the interactions of the flowing molecules with the pore walls. These interactions increase with decreasing pore sizes. Calorimetric techniques and adsorption isotherm techniques were extensively used in these researches.&’O Also infrared spectroscopy was widely used in spite of the limits imposed by experimental difficulties.”J2 The ESR of
(1)M. Boudart, E. G. Derouane, V. Indovina, and A. B. Walters, J. Catal., 39, 115 (1975). (2)Y. Mataunaga, Bull. Chem. SOC.Jpn., 34, 1291 (1961). (3)P. A. Berger and J. F. Roth, J. Phys. Chem., 71, 4307 (1967). (4)H. Lumbeck and J. Voigtliinder, J. Catal., 13, 117 (1969). (5)R. G. Herman and R. D. Flentge, J. Phys. Chem., 82,720(1978). (6) R. Deen, P. I. Th. Scheltus, and G. DeVries, J. Catal., 41, 218 (1976). (7)H.Tominaga, Y.Ono, and T. Keii, J . Catal., 40,197 (1975).
(8)J. J. Kipling, “Adsorption from Solutions of Electrolytes”, Academic Press, New York, 1965. (9)S. G. Ash, R. Brown, and D. H. Everett, J . Chem. Thermodyn., 5, 239 (1973). (10)A. C. Zettlemoyer and K. S. Narayan in “The Solid-Gas Interface”, E. A. Flood, Ed., Marcel Dekker, New York, 1967. (11)L. H. Little, “Infrared Spectra of Adsorbed Species”, Academic Press, New York, 1966. (12)C. H. Rochester, Adv. Colloid Interface Sci., 12,43 (1980).
0022-3654/81/2085-1922$01.25/00 1981 American Chemical Society
ESR of Adsorbed Cu(acac),
The Journal of Physical Chemistty, Vol. 85, No. 13, 1981 1923
suitable probes introduced as a liquid solution in porous supports was shown to be a powerful tool in studying the mobility of both the probe and the solvent molecules inside the pores Detailed information has been obtained either on the freezing properties of the intracrystalline fluid or on the properties of the surface-adsorbed species. Most of the previously reported work was carried out on water or aqueous mixtures, and the parahexmagnetic probes were hexaaquomanganese(II),14JsJ7 aaquocopper(II),16 ammonia-copper(II)18J9 and tetrahydroxycopper(II)20complexes. Because of the role of porous supports in gas, liquid, and thin-layer chromatography, it is of interest to study the adsorption properties of mixtures of organic liquids on porous supports. In this work we describe the use of ESR in studying the mobility and the surface adsorption on silica gels of C u ( a ~ a csolutions )~ in hydrophilic (N,N-dimethylformamide and pyridine) and hydrophobic solvents.
Experimental Section Four different silica gels (Merck adsorbent for chromatography) were used with the following pore diameters (in nm); 4 (surface area S = 650 m2/g; henceforth called S4), 6 (S = 400 m2/g; S6), 20 (S = 150 mz/g; SZO), and 100 ( S = 25 m2/g; SlOO). C u ( a ~ a cwas ) ~ prepared according to the procedure described by Jones.21 The crude product was crystallized from chloroform, washed with acetone and water, and dried a t 100 "C for 12 h. Anal. Calcd for C U ( C ~ H ~ O ~ ) ~ : C, 45.88; H, 5.39. Found: C, 45.69; H 5.32. Stock solutions M Cu(acac)z were prepared in reagent-grade of 5 X chloroform,pyridine, and N,iV-dimethylformamide (DMF). Silica gels were outgassed a t 200 "C for 2 h to remove adsorbed water, without a large dehydroxylation taking Flgure 1. ESR spectra at 293 K of Cu(acac), after adsorption of a 5 X lo3 M Cu(I1) solution in chloroform on the S4, S6, S20, and SlOO place. After cooling, the silica gels (1g) were left in contact samples. The dashed line represents the ESR spectrum at 293 K of with the appropriately diluted stock solution (5 cm3), a 5 X M solution of Cu(acac), in unadsorbed chloroform. stored for 24 h, and then filtered very rapidly on a filter paper in order to avoid solvent evaporation as much as possible. The apparently dried powders were promptly inserted in quartz capillaries of 1-mm i.d. and sealed. ESR spectra were registered with a Bruker 200tt spectrometer operating in the X band. The 77 K spectra were obtained by simply using a liquid-nitrogen cold finger. Temperature variations above and below room temperan ture were obtained with the aid of the Bruker ST100/700 variable-temperature accessory. Results and Discussion The C u ( a c a ~complex )~ dissolved in organic solvents gave a room temperature the well-known four hyperfine line pattern with a line width dependent on the nuclear spin number.2z-26 At 77 K typical axial spectra were obtained (13)G. Martini and L. Burlamacchi, Chem. Phys. Lett., 41,129 (1976). (14)N. N. Tikhomirova, 1. V. Nikolaeva, E. N. Rosolovskaya, V. V. Demkin, and K. V. Topchieva, J. Catal., 40,61 (1975). (15)L.Burlamacchi, G. Martini, and M. F. Ottaviani, J.Chem. SOC., Faraday Trans. 2, 72, 324 (1976). (16)V. Bassetti, L. Burlamacchi, and G. Martini, J. Am. Chem. SOC., 101,5471 (1979). (17)G.Martini, J.Colloid Interface Sci., 80,39(1981);L.Burlamacchi and G. Martini, "Proceedings of the 2nd International Conference on Magnetic Resonance in Colloid and Interface Science", Reidel, Dordrecht, 1980, p 621. (18) G. Martini and L. Burlamacchi, J.Phys. Chem., 83,2505(1979). (19)G. Martini and V. Bassetti, J. Phys. Chem., 83, 2511 (1979). (20) M. F.Ottaviani and G. Martini, J. Phys. Chem., 84,2310(1980). (21) M. M. Jones, J.Am. Chem. SOC.,81, 3188 (1959). (22) B. R. McGarvey, J. Phys. Chem., 60, 71 (1956). (23) R. Wilson and D. Kivelson, J. Chem. Phys., 44, 4445 (1966). (24)I. Adato and I. Eliezer, J. Chem. Phys., 54, 1472 (1971). (25)T. Kogane, H.Yukawa, and R. Hirota, Chem. Lett., 477 (1974).
QLp ALP
li
2 14 5 5 5 G-gLC213 $2
38
-1
:
Flgure 2. ESR spectra at 293 K of Cu(acac), after adsorption of 5 X IO3 M Cu(acac), solutions in pyridine (left) and in DMF (right) on the S4 and S20 samples.
with ESR parameters dependent on the solvent which was used. The experimental and theoretical aspects of the ESR spectra of Cu(/3-diketonate)z complexes have been widely analyzed in the past literature,2z-26including unexpected high field transitions in the solid statez7and relaxation mechanisms in the fluid stateaZ3 Table I reports the magnetic parameters of Cu(acac), dissolved in the solvent used in this work together with the values obtained by other authors for the same systems and for other solvents. Clear relationships were previously established between the basicity (or Gutmann donor number28)of the solvent (26)E. Biqkowska, K. Leibler, and M. K. Kalinowski, Monatsh. Chem., 107, 865 (1976). (27)L. D. Rollmann and S.I. Chan, J. Chem. Phys., 50,3416(1969).
1924
The Journal of Physlcal Chemistry, Vol. 85, No. 13, 1981
Martini and Ottaviani
TABLE I : ESR Parameters of Cu(acac), in Solution CA)
solvent chloroform chloroform chloroform CHCl,/toluene (20%) 60% toluene/40% CHCl, pyridine pyridine pyridine DMF DMF DMF DMF toluene tetrahydrofuran a
(g)
g /I
g1
2.125
2.283 2.285 2.285 2.253 2.264 2.306 2.274 2.302 2.292
2.067 2.042 2.043
2.293 2.291 2.246 2.278
2.061 2.056 2.062 2.074
(+0.02)a (+0.003)b (*0.003)b
2.124 2.123 2.146 2.148 2.135 2.130 2.135 2.142
Values taken from the spectra at 2 9 8 K.
2.036 2.068 2.086 2.071 2.053
104~11 10'~~
(*1)," G 79
(k3y cm-
(+2),, cm-
- 183 - 175
-17.5 -28.2 - 28.4
- 175
77
- 194 - 145 - 165 - 164 - 164 -175
-29
67
- 172 - 185
71.6
- 197 -176
-13.1 -7.5 - 15 - 19
57 56.6 66 65
ref this work 32 24 33 35 this work 24 32 this work 34 32 24 29 24
- 11.6 -3 -3.5 - 14.6
Values taken from the spectra at 77 K.
TABLE 11: ESR Parameters of Cu(acac), Adsorbed o n Silica Gels
system Cu(acac), -CHCl,-SIO, Cu(acac), -pyridine-SiO,
denom(g) ination (kO.01) LCa GCb ACb LPa GPb
2.12 2.14
( A)
g II
G
(k0.003)
A 1I (t0.0003), cm-'
2.280 2.253
-0.0186 -0.0198
2.058 2.063
-0.0018 -0.0029
0.86' 0.84
2.302 2.276
-0.0166 -0.0179
2.068 2.064
-0.0015
0.83' 0.82
LDa G D ~
2.13
2.292 2.269
-0.0175 -0.0187
2.061 2.053
-0.0017
0.84' 0.82
gl (+0.0002), (k0.003) cm-'
OLZ
77 55.5
APb
Cu(acac),-DMF-SiO,
A1
(tl),
68
AD^
Values taken from spectra at 298 K. Values taken from spectra at 77 K . 0 . 8 4 , and 0 . 8 3 for Cu(acac), in CHCl,, pyridine, and DMF, respectively.
and ESR parameters.".29s30 It was shown that g increases with the donor number of the solvent, while the opposite was observed for IAlll. This was attributed to the increasing dipolar field with increasing donor number of the solvent molecules (one or two) coordinated in the apical positions. The same behavior was shown by variously substituted copper acetylacetonates in c h l o r o f ~ r m . ~Experimental ~?~~ values of All, IAiil, decreased with the increase of the electron-drawing efficiency of the substituent on the ligand. ESR Spectra at Room Temperature. Figure 1reports the ESR spectra of C u ( a ~ a cin ) ~chloroform at 293 K. The spectra were obtained following adsorption of the complex M solution on narrow- (S4 and S6) and from a 5 X wide- (S20 and SlOO) pore supports; the ESR spectrum of C u ( a ~ a cin ) ~the unadsorbed solution is superimposed for reference. Figure 2 shows the 293 K ESR spectra of 5X M solutions of C u ( a ~ a cin ) ~pyridine and DMF adsorbed on S4 and S20 samples. (S6 and SlOO samples gave ESR spectra similar to those of S4 and S20 samples, respectively, and hence are not reported). Two distinct signals can be identified with chloroform and pyridine: (a) a liquidlike signal with the same isotropic (28! V. Gutmann,Electrochim.Acta, 21,661(1976);("Coordination Chemistry in Non-Aqueous Solutions", Springer-Verlag,West Berlin, 1968,Chapter 2. (29)S. Antosik, N. M. d. Brown, A. A. McConnell,and A. L. Porte, J. Chem. SOC.A. 545 (1969). (30)T. Ogata, T. Fugisawa, N. Tanaka, and H. Yokoi, Bull. Chem. SOC. Jpn., 49, 2759 (1976). (31)H.A. Kuska and M. T. Rogers,J . Chem. Phys., 43,1744(1965). (32)H.A. Kuska, M. T. Rogers, and R. E. Drullinger,J. Phys. Chem., 71, 109 (1967). (33)V. C. Swett and E. P. Dudek, J. Phys. Chem., 72, 1244 (1968). (34)N. D. Yordanov and D. Shopov, J. Inorg. Nucl. Chem., 38,137 (1976). (35)H.R. Gersmann and J. D. Swalen, J. Chem. Phys., 36, 3221 (1962).
The values of
e 2 in
free solution are 0 . 8 6 ,
parameters of C u ( a ~ a c )in~ the unadsorbed solvents (henceforth called LC and LP for chloroform and pyridine, respectively); (b) a rather complex solidlike signal with partially resolved low-field components (AC and AP, in chloroform and pyridine, respectively) that will be discussed in the next section. When DMF solutions were adsorbed on wide- and narrow-pore silicas, practically only a liquidlike spectrum, LD, was observed, and only minor traces of an axial spectrum, AD, were identified in the S4 sample. LC, LP, and LD spectra are therefore due to Cu(aca& complex in free motion in the liquid filling the cavities. The intensity of spectra LC and LP with respect to AC and AF' increased with increasing pore diameter and Cu(I1) concentration, and they were practically the only ones M in observed on S20 and SlOO with [Cu(II)] I 5 X M in pyridine. chloroform and [Cu(II)] > 5 X A line-width analysis as a function of temperature was M Cuperformed only for SlOO samples containing ( a c a ~solutions )~ in pyridine and DMF, because the LP and LD spectra were practically the only signals observed in these samples. Within the experimental errors, the ESR width (not here reported) of each hyperfine line in the range 293-373 K was the same as in the unadsorbed solutions. The line-width behavior of Cu(acac)z in organic solvents has been analyzed in detail by Wilson and Kive l ~ o n The . ~ ~ anisotropic nuclear-electronic magnetic dipolar and g-tensor interactions together with spin rotational relaxation have been suggested to be the main sources of spin relaxation. Both mechanisms are dependent on the Debye reorientational correlation time TR = Vefn/(kT), where V,, is the effective volume of the relaxing probe regarded as a sphere and 7 is the solvent viscosity. The equal behavior of the line width of Cu(acac)2 in DMF and in pyridine, either in the adsorbed state on
The Journal of Physical Chemistry, Vol. 85,No. 13, 1987
ESR of Adsorbed Cu(acac), p .- - 0 0 0 2 9 gf'=
cm-'
qJ*'
i
2 064
gp = 2 2 5 3
2 063
gy = 2276
gdD = 2 269 A:b
A
/I
=-
0 O l E 3 cm a''
=-00187cm-'
v
in I
A,,
1925
= 2 C58
M Cu(acac), solution Flgure 3. ESR spectra at 77 K of a 5 X in chloroform after adsorption on the S4, S6, S20, and SlOO samples. M solution in Dashed line: ESR spectrum at 77 K of a 5 X chloroform -5% THF.
wide-pore silicas or in the unadsorbed solutions, means that the same correlation time dependence on temperature holds in both cases. Thus the adsorbed and confined liquids maintain the same mobility (i.e., viscosity) as in the free state in a large range of temperature. ESR Spectra at 77 K. Figure 3 shows the 77 K ESR M Cu(acac)z sosignals of silica gels containing 5 x lution in chloroform and the spectrum of the unadsorbed solution (with 5% added tetrahydrofuran). Figure 4 shows M Cu(acac)Ppyridine the S4 and S20 spectra of 5 X and DMF solutions. The liquidlike LC, LP, and LD spectra have converted into the axial signals GC, and GD, whose parameters (see Table 11) are very close to the ones of Cu(aca& in the unadsorbed solvents in the glassy state. The GC, GP, and GD spectra are therefore due to Cu(acac)ptrapped in the glassy matrix obtained by cooling the liquids inside the pores. Besides the GC and GP spectra, additional polycrystalline spectra with different axial magnetic parameters and fairly well-resolved low-field components were observed (spectra AC and AP) in the cases of chloroform and pyridine (Table 11). A spectrum similar to AC was observed by Yamada36 for Cu(acac)2 adsorbed from a chloroform solution onto X zeolite and silica gel after the solvent was evacuated. The relative intensity of GC, GP, AC, and AP spectra varied with pore diameter and Cu(I1) concentration exactly as at room temperature. The spectra AC and AP should be attributed to surface-adsorbed complexes. In silica gels containing DMF solutions, the surface-adsorbed complex was present (36)Y. Yamada, Bull. Chern. SOC.Jpn., 45, 60 (1972).
Figure 4. ESR spectra at 77 K of Cu(acac), after adsorption of 5 X 10" M Cu(acac), solutions in pyridine (left)and in DMF (right) on the 54 and 520 samples.
in the narrow pores (S4 sample) with a very low intensity and only with low Cu(I1) concentration (I5 X M). The 77 K spectra in the case of DMF were generally dominated by the signal due to Cu(aca& in the glassy matrix (spectrum GD). The ESR spectra of the surface-adsorbed complexes were always broader than those of Cu(acac)zin the glassy solvents. The chloroform solution adsorbed on the widepore systems gave rise to a large unstructured signal superimposed on the axial AC and GC spectra whose intensity increased with increasing Cu(I1) concentration. This signal closely resembles the one observed in crystallized chloroform (obtained by slowly cooling the unadsorbed solution), and it can be attributed to Cu(acac)z clusters segregated upon solvent crystallization. This means that an actual crystallization of the adsorbed liquid occurs only for chloroform adsorbed on wide-pore silicas. No similar spectra were observed by adsorption of DMF and pyridine solutions. Adsorbed Species and Silica-Solvent Interactions. The GC, GP, and GD spectra do not require further discussion since their characteristics are the ones of the copper pentanedionates analyzed in the previous l i t e r a t ~ r e . ~ ~ . ~ . ~ ~ * ~ More important are spectra AC, AP, and AD, since they can give details on the adsorption mechanism of either the paramagnetic species or the solvent molecules. The stereochemistry and the coordination of the Cu(aca& adsorbed complexes can be inferred from their magnetic parameters. These complexes were not readily removed from the surface by washing with the appropriate solvent, and hence they can be considered tightly bonded to the surface. The AC, AP, and AD spectra were indeed observed even at room temperature. From thermodynamical considerations, the Cu(acac)z molecule will be adsorbed on sites with the lowest energy, i.e., the highest differential adsorption heat. These sites are independent of each other since exchange-narrowed ESR spectra arising from the interaction among neighbor surface Cu(I1) ions were not observed even with the highest concentration used. A similar result was found in the adsorption of ammoniacopper complexes on the same silica gels.l9 The appreciable width of the 77 K spectra, which was almost independent of the concentration, may therefore be due to surface irregularities that lead to small changes in the spectral parameters. Spectra AC, AP, and AD can be approximately considered as axial spectra and interpreted on the basis of the
1926
Martini and Ottaviani
The Journal of Physical Chemistry, Voi. 85, No. 73, 1981
following spin hamiltonian: 7f = gllPH$z
+ g,P(H3, + HySJ + A I I ~ S+, A,(I,S, + I$,)
(1)
although an actual difference in the g,, g,, A,, and A, values cannot be ruled out since it could be masked under the broad perpendicular components. The surface-adsorbed Cu(acac)2 species must differ from the free Cu(acac)2,as shown by the differences in the magnetic parameters (Table 11). In particular, increased IAlll and decreased gll were observed for the adsorbed complexes with respect to the unadsorbed ones. The trend gll(CHC13) gll(DMF) gg(pyridine) IAIIJ(CHCM > IAilI(DMF) > lAgl(pyridine) was maintained. The lAill variations between free and adsorbed complexes were of the same order of magnitude in all three cases, within the limits of the experimental error. We suggest that the surface-adsorbed species are generated by replacement of a solvent molecule in the apical position with a surface silanol group: x
I CGa *H :o/
I were X is CHC1, DMF, or pyridine, respectively. Stability considerations rule out the possibility of a bridging on a pair of sites with a complete replacement of the solvent molecules. This should lead to ESR signals with approximately the same parameters in all cases. The decrease of gll and the increase of IAlll, according to the established relationships between donor number of the coordinated molecules and the magnetic parameters,24*,30indicate that the crystal field arising from the =Si-OH group is appreciably lower than that from CHCl,, DMF, and pyridine, with a resultant crystal-field intensity order of pyridine > DMF > CHC13 > =Si-OH The donor number of =Si-OH should be very low because the magnetic parameters of the AC spectrum are intermediate between those of Cu(aca& in chloroform32 and in toluene.29 The toluene donor number is near zero.% The IAlil, gll, and g, parameters can be used to evaluate the degree of covalency a2of the u bond in the equatorial p1ane.B,32v35137*The following approximate equation can be used uniterated: All = P[-a2(y7
+ K O ) + (911 - 2.002) + 3/?(gl - 2.002) + a ]
degree are also expected for the out-of-plane ?r bond which is more sensitive to axial ligation. However the calculation of this bonding parameter requires the exact values of the ligand-field transition energies which are not easily obtainable. On the basis of the previous considerations, it seems reasonable that surface adsorption of C u ( a ~ a c should )~ depend on the replacement facility of the coordination solvent molecules with surface =Si-OH groups. Experimentally we found that on silica having the same pore size the fraction of surface-adsorbed Cu(acac)2was higher with chloroform than with pyridine and dimethylformamide and in the last solution the adsorbed Cu fraction was always very small. Therefore, it resulted in the following order of surface adsorption: C u ( a c a ~ ) ~ ( C H C>l ~Cu(acac)2(pyridine) ) > Cu(aca&(DMF) This order agrees quite well with the differences in the gll values between free and adsorbed complexes (Aq = 0.030, 0.030, and 0.023 in chloroform, pyridine, and DMF, respectively) which result from all of the interactions, solvent and substrate, in the presence of solvated complex. Thus, a competition exists between Cu(aca& and the solvent molecules with the surface ESi-OH according to the adsorption equilibria =Si-OH-(solvent) + Cu(a~ac)~-(solvent)~ e ~Si-OH-Cu(acac)2~(solvent) + 2(solvent) The ESR data indicate that the surface adsorption of the solvent is higher for DMF than for pyridine, being the lowest in the case of chloroform. The polar molecules of DMF and pyridine can be adsorbed on the ESi-OH groups through a hydrogen bond: /H
I
I1
The hydrogen bond in structure I is expected to be stronger than that in structure II.39 However, two additional structures with hydrogen bonds can be hypothesized, although their contribution to adsorption should be smaller:
(2)
where P = 0.036 cm-’ and KO is the Fermi contact term for the free ion (k, = 0.43).% For free complexes the value 0.04 has been used for the numerical value a in eq 2, while the value 0.016 has been used for the adsorbed complex because that is the appropriate value for copper ions with oxygen at the tetragonal ligand positions. The a2values calculated from eq 2 (Table 11) indicate that substitution of the Si-OH in the axial position tends to produce more covalent bonding between the copper and the planar ligands. It was indeed reported that larger values of IAlll and lower values of gll lead to relatively smaller a2,i.e., more covalent coordinate Variations in the covalency (37)A. H. Maki and B. R. McGarvey, J. Chem. Phys., 29,31 (1958). (38)D. Kivelson and R. Neiman, J. Chem. Phys., 35, 149 (1961).
IV
In principle, the adsorption through hydrogen bonds of the carbonyl group of a single DMF molecule on pair sites of silanol groups should be considered. This adsorption is not of great relevance since it was shown that a pair-site adsorption on silica of ethyl acetate or cyclohexanone occurs only in the low coverage condition^.^^^^ Thus the (39)M.D. Josten and L. J. Shad, “Hydrogen Bonding”, Marcel Dekker, New York, 1974.
J. Phys. Chem. 1981, 85,1927-1930
possibility of a t least three different adsorption ways (11, 111, and IV) as compared with the unique possibility (I) may account for the apparently higher adsorption of DMF with respect to pyridine. Because of the basic nature of pyridine, a true acid-base reaction with the surface should also be taken into account (eq 3). However, IR specSi-OH
-
+ NC5H5
Si-0-
+ +HNC5H5
(3)
troscopy has shown that pyridine is adsorbed on surface silanol groups without the formation of pyridinium ions;42 i.e., the silica surface has a very low Bronsted acidity. The hydrogen bond is therefore largely favored, as shown, for It~is also instance, by pyridine adsorption on A e r o ~ i l . ~ known that silicic acid adsorbs DMF and other aliphatic (40)S. N. W. Cross and C. H. Rochester, J. Chem. Soc., Faraday Trans. 1 , 75, 2865 (1979). (41)A. D. Buckland, C. H. Rochester, D.-A. Trebilco, and K. Wigfield, J. Chem. Soc., Faraday Trans. 1, 74,2393 (1978). (42)L. Kubelkova and P. Jiru, Collect. Czech. Chem. Commun., 37, 2853 (1972). (43)D. M. Griffiths, K. Marshall, and C. H. Rochester, J. Chem. SOC., Faraday Trans., 70,400 (1974).
1927
acid amides through hydrogen bondingaU The structure of the solvent layers near the surface will be affected by the surface interactions, and this will influence the overall mobility up to a certain distance from the surface. It is not easy to evaluate this distance in the present case, but, from the variation of the ESR line width of C ~ ( a c a cas ) ~a function of temperature, it is concluded that most of the liquid filling the pores larger than 20 nm has the same mobility of the free solvent. In the case of CHC13, both hydrogen-bond and acid-base reactions have a very low probability of occurrence, and strong adsorption of solvent molecules cannot be found. The CHC13 molecule behaves, in fact, more as a proton donor than as an acceptor. The influence on the surface potential is effective only at short distances, as proved by the actual cryatallization of the solvent in the large-pore systems.
Acknowledgment. Thanks are due to the National Council of Researches (CNR) for financial support. (44)G. Lagaly, Adu. Colloid Interface Sci., 1 1 , 105 (1979).
Electron Spin Resonance of Paramagnetic Species as a Tool for Studying the Thermal Decomposition of Molybdenum Trisulfide Luigl Busetto, Angelo Vaccari,
Istituto di Chimica Industriale. Universiti di Bologne, 40 136 Bologna. Italy and Giacomo Martini'
Istituto di Chimica Fisica, Universiti di Firenze, 50 121 Firenze, Italy (Received: November 78, 1980; In Final Form: March 5, 7981)
ESR has been used in the study of the irreversible thermal decomposition of molybdenum trisulfide to disulfide in the range 180-600 "C. Three paramagnetic species were observed: (a) M0S3+,whose magnetic parameters were different depending on which phase, either MOSSor MoS2, was predominant; (b) sulfur chain radical, due to loss of sulfur during the decomposition; (c) Moo3+,due to a very low degree of contamination of the system with oxygenated Mo species. The nature of these species is discussed. The intensity and the line shape of signals for a and b as a function of the treatment temperature of the samples were used to follow the decomposition steps. The ESR results have been correlated with X-ray and analytical data.
Introduction Molybdenum sulfides have received considerable attention because of their extensive applications as catalysts in industrial processes, as dry lubricants, and as lubricant additives.' Since the effectiveness both as catalysts and as lubricants will depend on the structure, the crystal perfection, and the texture of the compounds in question, the crystallinity of synthetic molybdenum sulfides in the composition range MoS3-MoS2, together with the irreversible thermal decomposition2 of molybdenum trisulfide to disulfide, has been studied by several authors mainly by X-ray diffra~tion,"~ differential thermal analysis, and therm~gravimetry.~ (1) 0.Weisser and S.Lauda, ''Sulfide Catalysts: Their Properties and Applications", Pergamon-Vieweg, Oxford, 1973. (2)W. Biltz and A. Kocher, Z . Anorg. Allg. Chem., 248, 172 (1941). (3)J. C.Wildervanck and G. Jellinek, Z . Anorg. Allg. Chem., 328,309 (1964). (4)P. Ratnasamy and A. J. Leonard, J . Catal., 26, 352 (1972). 0022-3654/81/2085-1927$01.25/0
T A B L E I : Analyticil Data of Heated MoS, composition composition of M O , - ~ S ~ , of MO,-~S~, temp,"C 1-x temp,"C 1-x
300 3 50 400
0.80 0.83 0.85
450 500 600
0.87 0.91 0.91
In addition molybdenum, particularly pentavalent molybdenum, has been studied by electron spin resonance (ESR) in several host material^,^'^ and a number of cor(5)E. Ya. Rode and B. A. Lebedev, Russ. J. Inorg. Chem. (Engl. Transl.),6, 608 (1961). (6) R. T. Kai, Phys. Reo., 128, 151 (1962). (7)G. K. Boreskov, V. A. Dzis'ko, V. M. Emel'yanova, Y. I. Pecherskaya, and V. B. Kazanskii, Dokl. Akad. Nauk SSSR, 150,829 (1963). (8)J. Masson and J. Nechtshein, Bull. SOC.Chim. Fr., 3933 (1968). (9)J. M. Peacock, M. J. Sharp, A. J. Parker, P. G. Ashmore, and J. A. Hockey, J. Catal., 15, 379 (1969). (10)K.S.Seshadri and L. Petrakrs, J. Phys. Chem., 74,4102(1970).
0 1981 American Chemical Society
