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 instance, by pyridine adsorption on A e r o ~ i l . ~It~is also 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
lD28
Busetto et ai.
The Journal of Physical Chemistry, Vol. 85, No. 13, 198 1
E)
D) C)
-
A) 56
40
24
Figure 2. ESR spectra at 293 K of MoS3 heated under vacuum for 3 h in the range 180-600 OC: (a) starting MoS3; (b) 180 O C ; (c) 240 OC; (d) 300 OC; (e) 400 OC; (f) 500 OC; (9) 600 O C ; (h) sample heated at 400 OC for 24 h.
28
8
8
Figure 1. X-ray diffractograms of MoS, heated at different temperatures: (A) starting MoS3; (B) 300 O C ; (C)400 OC; (D) 450 OC; (E) 500 O C .
relations have been established between catalytic activity and ESR signal i n t e n ~ i t y . ~ J ~Nevertheless, J~J~ very little work has been reported to ascertain the existence of Mo(V) both in sulfided molybdenum oxide systems20 and in synthetic M o S ~ . ~ ~ Recently the catalytic properties of synthetic MoSz of varying surface areas22have been investigated, and it was found that at least two different active centers are a t work in the hydrodesulfurization and hydrogenation (or isomerization) reactions. With the aim to obtain additional information on these catalysts, we now report an ESR study on the MoS3-MoS2 system which evidences the existence of two MOW) species together with polymeric sulfur. The g values and the variations of the signal intensities as a function of the treatment temperature of the samples can be useful in following the MOSS MoSz thermal decomposition.
-
Experimental Section Materials. All of the experiments described here were carried out with a single preparation of molybdenum trisulfide made by acidification of aqueous solutions of ammonium thiomolybdate. The dark brown product obtained by precipitation was washed with water and dried (11) M. Dufaux, M. Che, and C. Naccache, J. Chirn. Phys. Phys.-Chim. Biol., 67,527 (1970). (12) L. Burlamacchi, G. Martini, and E. Ferroni, Chem. Phys. Lett., 9,420 (1971). (13) L. Burlamacchi, G. Martini, and E. Ferroni, J. Chem. SOC.,Faraday Tram. 1 , 68, 1586 (1972). (14) K. S. Seshadri and L. Petrakis, J. Catal., 30, 195 (1973). (15) G. Martini, J. Magn. Reson., 15, 262 (1974). (16)L. Burlamacchi, G. Martini, F. Trifirij, and G. Caputo, J. Chem. Soc., Faraday Trans. 1 , 71,209 (1975). (17) W . K. Hall and M. Lojacono, h o c . I n t . Congr. Catal., 6th, 1976, paper A-I6 (1977). (18)N. Sontani, Reu. Phys. Chem. Jpn., 46, 1, 9 (1976). (19)L. Petrakis, P. L. Meyer, and T. B. Debies, J. Phys. Chen., 84, 1020 (1980). (20)K.S.Seshadri, F. E. Massoth, and L. Petrakis, J . Catal., 19,95 (1970). (21) P. Belongue and Y. V. Zanchetta, Reu. Chirn. Miner., 16, 565 (1979). (22) L. Busetto, A. Iannibello, F. Pincolini, and F. Trifirij, to be submitted for publication.
,
50G
,
Figure 3. 293 K ESR second derivative spectrum of MOSS heated under vacuum at 300 OC for 3 h.
under vacuum for 16 h. The residue was then washed several times with carbon disulfide, dried in vacuo overnight, and then heated for 3 h a t 120 "C. Samples of this material in amounts of 0.5-1.0 g were heated in a quartz tube under vacuum mmHg) a t temperature in the range 18MOO "C. Table I summarizes the compositions of the prepared samples given as Mo1=S2 since density measurements have shown that the products are to be regarded as molybdenum disulfide deficient in Figure 1 shows the X-ray diffractograms the carried out on a Phylips diffractometer using Cu K a radiation. EPR Spectra. The heated samples, after cooling, were transferred in air into Pyrex glass tubes (id., 2 mm), evacuated, and then sealed under vacuum. The ESR spectra were recorded on a Bruker Model 20Ott spectrometer operating a t the X band. Spectra at 77 K were obtained by using a liquid-nitrogen cold finger. Commercial MoS2 (Climax Molybdenum Co.) has also been used for comparison. Results The ESR spectra of the MoS3 samples heated under vacuum in the temperature range 180-600 "C are reported (23)J. MBring and A. LBvialdi, C.R. Hebd. Seances Acad. Sci., 213, 798 (1941).
The Journal of Physical Chemistry, Vol. 85, No. 73, 7987
ESR of Decomposition of MOSS
TABLE 11: ESR Parameters of Sulfur Radicals Adsorbed or Imbedded into Solids solid radical g, g2
MoS,-MOS, Mo-AI ,O,
CO-Mo- A1,0 MgO zeolite 3A K C1
sulfur long chain sulfur long chain sulfur short chain s 3sulfur long chain s 9 -
2.04 8 2.050 2.053 2.04 3 2.04 7 2.0499
/
Flgure 4. ESR peak height as a function of temperature of decomposition of the three different absorptions: (0) Moo3+;(H) MoS3+; (0) sulfur radical.
in Figure 2, which, in addition, shows the spectrum of the starting M o S ~obtained as described in the experimental section and the signal obtained from a sample heated under vacuum at 400 for 24 h. From the analysis of the relative signal intensity of the various lines at different temperatures, three absorptions can be recognized: (i) Signal A is an absorption due to a species with approximately axial symmetry with g, = 1.93 and g I = 1.88. (ii) Signal B is an absorption attributed to Mo-d species (see below). This absorption shows different magnetic parameters as a function of the decomposition temperature: below 300 "C, g, = 1.97 and gll = 1.95 (henceforth called signal BJ, and above 300 "C, g, = 199 and gli = 2.01 (signal BJ. (iii) Signal C is an orthorhombic absorption with gl = 2.048, g, = 2.034, and g3 = 2.004. The features of this signal are better understood from a spectral analysis in second derivative, as Figure 3 shows. The dependence of the signal intensities (in arbitrary units) on the temperature is shown in Figure 4, and, although the values reported are affected by considerable uncertainty, the trends observed can give useful information on the nature and the fate of the various paramagnetic species present in our system. The maximum intensity of signal A is at -240 "C and then decreases with increasing temperature. Signal B follows a different trend; in fact it is higher for the starting MoS2 sample and drops to very low values at -300 "C (signal Bl); at -325 "C it gains in intensity and then decreases with increasing temperature (signal B2). In the latter temperature range, the observed absorptions associated with signal B are much less resolved. Nevertheless, the magnetic parameters of signal Bz can be easily obtained from the better-resolved spectrum of the sample tempered for 24 h at 400 "C (spectrum h in the Figure 2). Finally signal C follows the same trend of signal A, with a maximum at -300 "C and then falling in a very narrow temperature range (300-325 "C). If the heating is carried out in air, only signal A shows a great increase in intensity, whereas signals B and C do not appreciably change either in shape or in intensity. No changes are also observed if air is admitted over the prepared samples at room temperature. Moreover, spectra taken at 77 K do not show appreciable differences in the line-shape resolution for all of the observed absorptions,
2.034 2.029 2.030 2.004 2.032 2.0319
g3
ref
2.004 1.998 2.000 2.004 2.010 2.00 26
this work 20 25 26 27 28
1929
and no additional lines are detected at low temperature. The commercial MoSz shows only a narrow signal (AH = 4 G, g = 2.006) that can be attributed to lattice vacancies and/or to carbonaceous radicals. The three-g-value signal C is not observed after treatment of commercial MoS2with elemental sulfur both in CCl., solution and at 400 "C in the vapor phase.
Discussion The results of the ESR study on the system MoS3-MoS2 will be discussed on the basis of the spectral parameters and correlated with structural and analytical data obtained on the same samples by investigating the irreversible thermal decomposition MOSS MoSz in the temperature range 180-600 0C.22 For a better understanding of the ESR properties of the system, the discussion will be developed by focusing attention on each of the three signals, A, B, and C, which will be analyzed separately. Signal A . The magnetic parameters of this absorption are temperature independent, and signal A can be unambigously attributed to the Moo3+species in axial symmetry in a square pyramidal coordination. The features of this signal have been widely investigated in the past litera~ u ~ ~ . The ~ J observation ~ J ~ J ~that the signal-A intensity is higher in the samples heated in air is in accord with the above attribution. Signal A is also present in the starting M o S ~before and after treatment at 120 "C, and it can be due to a very low degree of contamination with the corresponding oxygenated species which are known to give the Mo(V) signal even if the samples have been previously not extensively reduced.15 The intensity variations with thermal treatment (Figure 4) can be assigned to a progressive transformation Mo(V1) --* MOW) --* Mo(IV), and the trend observed in this case also is in agreement with the findings previously reported both on Moo3 and on Moo3-containing compound^.'^ Signal B. If it is assumed that both the B1 and B2 absorptions are attributable to a M O P +species, an explanation of the differences in the magnetic parameters and of the signal intensities should arise from an analysis of the variation in the composition and in the structure that MOSSundergoes by heating at varying temperatures. As Table I shows, the composition of the decomposed M o S ~changes from Moo.elS2 to M00.91S2 when the temperature is raised from 300 to 600 "C, indicating a progressive loss of sulfur in this temperature range. The X-ray diffractograms indicate that only above 300 "C (line B in Figure 1) do the first vague bands appear, and their positions correspond to those of MoS,; 3,4 lines D and E (450 and 500 "C, Figure 1)evidence a greater order of the atomic arrangement, but the material is still far from crystallinity as it results from the broad peaks. Thus signal B1 may be attributed to a MoS3+species with a symmetry largely determined by the MoS3 structure. The observed magnetic parameters indicate that the coordination around Mo(V) should be approximately square pyramidal with partial double-bond Mo=S character. This structural attribution is in agreement with other ESR absorptions in Mc=O3'-containing materials for which the IB,) wave function was suggested to be the ground state.12 On the
-
J. Phys. Chem. 1981, 85, 1930-1933
1930
other hand, a recent X-ray determination on noncrystalline MOSS has shown the presence of molybdenum both in tetragonal pyramidal and in tetrahedral symmetry.24 Above -300 "C the symmetry of the MoS3+species may be determined by the MoS2 structure that becomes the prevalent phase. The fact that gllfor signal Bzis -2.0 and greater than g, suggests a large rearrangement of the structural parameters. We suggest that in this case the ground state could be lAl), i.e., largely a d,z Mo atomic orbital, as expected for gll 2.0. The observation that signal Bzbecomes more resolved with increasing temperature of treatment is also in agreement with the increased structural order a shown by X-ray data. It should be noted that the discontinous trend in the signal intensity in the range 300-350 "C corresponds to the temperature range in which the decomposition MOSS MoSz is known to take place in the largest extent. Signal C. The three-g-value signal (gl = 2.048, g, = 2.034, g3 = 2.004) must be attributed to an unsymmetrical and strongly immobilized species since paramagnetic species in rotational and/or librational motions adsorbed on a surface should give partially averaged values of the anisotropic parameters. This is also proved by the fact that the g values and the overall line shape do not depend on the registration temperature from 293 to 77 K. The observed values of the g tensor components and the spectrum shape (relatively sharp line even at 293 K) rule out their attribution to Mo in lower oxidation degrees. Spectra similar to that here reported have been previously observed in various sulfur-containing systems (Table 11). For example, Seshadri et a1.20have observed on sulfided molybdema-alumina catalysts a triplet centered around g = 2.0 that was assigned to sulfur atom chains adsorbed on the surface. Lojacono et al.,25 in studying the ESR properties of sulfided molybdenum-cobalt-alumina catalysts, have assigned the observed three-g-value signal to short sulfur chains strongly attached to the surface. These
-
(24)E.Diemann, 2.Anorg. Allg. Chem., 432,127 (1977). (25)M.Lojacono, J. L. Verbeek, and G. C. A. Schuit, Proc. Int. Congr. 5th, 1972,1409 (1973).
authors did not exclude the possible existence of Sy. This latter species has been elegantly identified on the surface of partially hydroxylated MgO treated with elemental sulfur at 400 "C, and the g values together with the 33S hyperfine structure have indicated that the S3- ions are librating in the plane of the three sulfur atoms.26 Furthermore, Dudzik and Preston2' have reported a three-gvalue signal that was assigned to long sulfur chains in the lattice of zeolites impregnated with sulfur vapor at 400 "C. By analogy we assign signal C to sulfur atom chains long enough to preclude the interaction of the unpaired electrons located on the extremities. Such a species is probably not only adsorbed on the surface because the intensity of signal C is practically insensitive to oxygen adsorption, washing with CCll and treatment for 2 h in flowing helium. It disappears only when the samples are used in catalytic hydrotreating,22 but under these conditions the A and B signals are also largely removed. On this basis we assume that the sulfur radical chains in our system are largely "hidden" in the MoS3-MoSz bulk. Again the intensity variations of signal C with the decomposition temperature can be used as a diagnostic method for following the irreversible thermal decomposition MOSS MoSz. Thus in the range 250-300 "C, where MOSSbegins to decompose, the signal intensity is a t its maximum, and then it drastically drops in the range 300-350 "C and tends to stabilize above 450-500 "C,where MoS3 is presumed to be totally decomposed to MoS2,as indicated by the constant values of the Mo/S ratio in the latter range of temperatures (Table I).
-
Acknowledgment. Thanks are due to Dr. F. Pincolini for the sample preparation. The National Council of Research (CNR) and the Minister0 della Pubblica Istruzione have provided partial support for this study. (26)J. H.Lunsford and D. P. Johnson, J. Chem. Phys., 58, 2079 (1973). (27)Z.Dudzik and K. F. Preston, J. Colloid Interface Sci.,26, 374 (1968). (28)J. R. Morton, Proc. Colloq. Ampere, 15, 299 (1969);J. Chem. Phys., 43,3418 (1965). 1~~
~~I
Phase Transitions in the Layer Structure Compounds Zr(HPO,)(n-C, Hzn+,PO4) Shoji Yamanaka, Karunori Sakamoto, and Makoto Hattori Department of Applied Chemistry, Faculty of Engineering, Hlroshima University, Hiroshima 730, Japan (Recelved: November 5, 1980; In Final Form: March 11, 198 1)
The thermal behavior of the layer structure compounds Zr(HP0)4(n-C,Hzn+lP04)(n = 1-18) has been studied by X-ray diffraction and calorimetric measurements. Compounds with n 2 7 have structural phase transitions in the temperature range from 45 to 95 "C accompanied by an increase in basal spacing. The transition temperature and the transition entropy increase linearly with the number of carbon atoms (n)in the alkyl chain. The entropy change would be mainly due to the increase in conformational freedom of the alkyl chains. The increase in the entropy change per CH2group is 0.66R (R is the gas constant), indicating that the alkyl bilayers sandwiched by zirconium phosphate layers are in a quasi liquid state in the high temperature phases.
Introduction Zirconium bis(mon0hydrogen orthophosphate) is an insoluble ion exchanger with a layer structure. The monoand dihydrate, Zr(HP0.J2.Hz0 and Zr(HP04)2.2Hz0,are known and are called a-and y-zirconium phosphate, re-
spectively. The crystal structure of the a form was analYzed by Clearfield and Smith.' Each Phosphate layer consists of Z r 0 6 octahedra lying on a Plane and POdOH) (1)A. Clearfield and G. D. Smith, Inorg. Chem., 8, 431 (1969).
0022-3654/81/2085-1930$01.25/00 1981 American Chemical Society
