摘要:
X-Ray absorption studies of amorphous Re& Simon J. Hibble” and Richard I. Walton Department of Chemistry, University of Reading, Whiteknights, PO Box 224, Reading, UK RG6 6AD Amorphous Re2S7 is formulated as Re3-S2 S-17 and a rhomboidal metal-metal bonded Re4SI6 unit proposed as the basic structural building block. The amorphous rhenium sulfide Re2S7 and its hydrate are used as catalysts in a number of hydrogenation reactions.1-3 The structure of this material is significant both because of these uses and as part of the wider interest in the structure of amorphous transition-metal ~halcogenides.4-~ The only direct structural information about the material is from an X-ray diffraction study by Diemann which showeg the presence of short rhenium-rhenium distances of ca.2.7 A.* Recently Miiller et al. used chemical extrusion to gain information on the structural building blocks present in Re2S7.9 They concluded that Re2S7 contained two types of Re4 cluster, rhomboidal and tetrahedral clusters, and contained sulfur in polysulfide groups and as S2-. We have attempted to gain information directly from the solid state to determine if these conclusions are justified. We have carried out EXAFS (ex- tended X-ray absorption fine structure) studies at the rhenium LIII-edge to gain information on the local coordination around rhenium and X-ray absorption studies at the sulfur K-edge to give information on the oxidation state of sulfur in Re2S7.T Unfortunately EXAFS studies at the sulfur K-edge are compli- cated by the close proximity of the Re MII-edge at 2682 eV.Experiments on the model compounds ReS2, VS4 and S were carried out at the same time. The Re LIII-edge EXAFS study of ReS2 gave coordination numbers and distances around rhenium in excellent agreement with the crystallographic values1° (see Table 1). The Fourier transform of the Re LIII-edge EXAFS data for Re2S7 clearly showed two shells and the refined values for a rhenium and sulfur shell are shown in Table 1. Fig. l(a) shows the k3-weighted EXAFS data for Re2S7 and the theoretical curve for our model. Fig. l(b) shows the Fourier transform. The refined Re-Re coordination number of 2.82 is consistent with the presence in Re2S7 of rhomboidal Re4 units with an average Re- Re coordination number of 2.5, tetrahedral Re4 units with an Re-Re coordination number of 3 or a mixture of these species.These experiments support the conclusions of Miiller as to the identity of the metal clusters in amorphous Re2S7. X-Ray absorption at the S K-edge was measured for Re2S7 and the model compounds a-S, VS4 and Re&, which contain 6 4 2 s C0 -2 -4 -6 1.o 0.8 $j0.6 3 .-c-e Q 0.4 0.2 0.0 1 2 3 4 5 RIA Fig. 1 Rhenium LIII-edge EXAFS data for Re2S7:(a)k3-weighted EXAFS (-) experimental and (---) theoretical, (b)the Fourier transform Table 1Coordination numbers, distances and Debye-Waller factors derived from the Re LIII-edge EXAFS studies of ReS2 and Re2S7 (crystallographically derived average distances and coordination numbers for Re& are shown in italics) Occupation Debye-Waller Discrepancy Shell numbera Distanceb/A factor/A2 index(‘/% ReS2 Ak = 15 A-1 S Re 5.96(22) 3.18(46) [61 2.362(2) 2.759(3) [2.398] 0.0134(5) 0.0154(11) 26.4 Nind = 7d [31 [2.803] Re2S7 S 5.02(20) 2.3 25(2) 0.0182(7) 24.9 Ak= 15A-I Re 2.82(37) 2.739(3) 0.0156( 1) Nind = 7 Errors quoted are statistical errors from the least-squares refinement.a From the ReS2 data and consideration of the behaviour of the fit-index with coordination number, we estimate the relative confidence limits for refined occupation numbers are +10-15% for the sulfur shell and +20%for the rhenium shell. h Errors in distances arise from systematic errors in the EXAFS experiment and data analysis, and limit the true accuracy of the distances to k0.02 A.R = { 11[XT(k) -XF(k)] Ik3dk/jl XF(k) I k3dk) x 100% where XT (k) and XB (k) are the theoretical and experimental EXAFS respectively.d Ak is the k-range used in the EXAFS analysis and Nindthe number of independent parameters refined. Chem. Commun., 1996 2135 only So, S-I and S-I1 respectively. The results are shown in Fig. 2. The position of the S K-edge shows that the lowest oxidation state of sulfur in Re2S7 is S-I. We suggest all sulfur is present as (S2)2-groups; this is consistent with our observation of an absorption band in the IR at 521 cm-1, which can be assigned as an S-S stretching mode of this group, and the absence of bands due to polysulfide groups {for example, in the complex [NH4]4[Re4(s2-)4(s32-)6], the S-S stretch occurs at 465 cm-1 (ref.ll)}. The simplest model for the structure of Re2S7 would assume that all sulfur is found as S-1 and would result in the formulation Re3.S2S-I7. This would produce an electron count of 14, which is appropriate for the electron-rich Re4 rhombus of Muller and the results of our rhenium LIII-edge EXAFS experiment. In Fig. 3 an Re4S16 unit is shown; this contains 4 p-q2-S2, 2 q2-S2 and 2 pS2 groups. The 2 p-S2 are shared between similar units and this model therefore produces the correct stoichiometry and fits all our experimental data. The average Re-S coordination number of 5.5 is closer to that found from our EXAFS study than the value of 6 produced by Muller's models.To produce an electron count of 12, which is appropriate for the Re4 tetrahedon, some sulfur would have to be in the -11 oxidation state. This is ruled out by the S K-edge absorption data. We therefore believe structural models for Re2S7 based on I I I I 2465 2470 2475 2480 2485 2490 Energy 1eV Fig. 2 Normalised sulfur K-edge absorption spectra: (a) a-S, (b) VS4, (c) Re2S7, (4ReS2 /.Re/ Fig. 3 Proposed Re4 rhomboidal structural building block for Re& Re4 tetrahedra to be less plausible than those based on Re4 rhombi. The results of our X-ray absorption study suggest that amorphous Re2S7 contains Re4 rhomboidal clusters which is in agreement with the chemical extrusion studies of Muller et al. The results of the sulfur K-edge absorption studies provide more direct information about the mode of bonding of sulfur than those obtained by chemical degradation.Although in the case of amorphous Re& there is some agreement between the results of EXAFS measurements carried out on the solid-state sample and the conclusions reached from chemical extrusion studies, this is not always the case. For example, in our EXAFS studies of amorphous MoS3l2 we found no evidence for the Mo~ triangles that Muller et al. postulate from their extrusion studies on this materiaLl3 We suggest that chemical extrusion might be a useful tool in investigating the structure of amorphous materials and may complement other techniques, but that it should be used with caution.We thank the EPSRC for the provision of EXAFS facilities and a studentship for R. I. W. and M. P. Beer and I. D. Fawcett for assistance with collecting the X-ray absorption data. Footnote t Re& was prepared by one of the methods described by Briscoe et al., namely the reaction between potassium perrhenate and sodium thiosulfate in an acidic solution.14 Rhenium LIII-edge EXAFS data and sulfur K-edge X-ray absorption spectra were recorded at the Daresbury SRS on stations 7.1 and 3.4, respectively, at room temperature with an electron-beam energy of 2 GeV and average beam current of 200 mA, as described in our previous work.15 The programs EXCALIB, EXBACK, and EXCURV92 were used to extract the EXAFS signal and analyse the data.16 Least-squares refinements of the structural parameters of the compounds were carried out against the k3-weighted EXAFS to minimise the discrepancy index. References 1 V.G. Pleshakov, K. D. Ambacheu, M. A. Ryashenteva, N. D. Sergeeva, M. V. Vener, L. A. Murugova, 0. V. Zvolinsky and N. S. Prostakov, Russ. Chem. Bull., 1994, 43, 1037. 2 C. Aretos and J. Vialle, in Rhenium, ed. B. W.Gonser, Elsevier, New York, 1962, p. 171. 3 H. S. Broadbent, L. H. Slaugh and N. L. Jarvis, J.Am. Chem. Soc., 1954, 76, 1519. 4 S. P. Cramer, K. S. Liang, A.J. Jacobson, C. H. Chang and R. R. Chianelli, tnorg. Chem., 1984, 23, 1215. 5 K. S. Liang, S. P. Cramer, A. J. Jacobson, C. H. Chang, J. P. de Neufville, R.R. Chianelli and F. Betts, J.Non-Cryst. Solids, 1980, 42, 345. 6 E. Diemann, Z. Anorg. Allg. Chem., 1977, 432, 127. 7 F. Z. Chien, S. C. Moss, K. S. Liang and R. R. Chianelli, Phys. Rev. B, 1984,29,4606. 8 E. Diemann, Z. Anorg. Allg. Chem., 1977, 431, 273. 9 A. Muller, E. Krickemeyer, H. Bogge, H. Ratajzcak and A. Armatage, Angew. Chem., 1994,106,800;Angew. Chem., tnt. Ed. Engl., 1994,33, 770. 10 H. H. Murray, S. P. Kelty and R. R. Chianelli, tnorg. Chem., 1994, 33, 4418. 11 A. Muller, E. Krickemeyer and H. Bogge, Angew. Chem., 1986,98,258; Angew. Chem., tnt. Ed. Engl., 1986, 25, 272. 12 S. J. Hibble, D. A. Rice, D. M. Pickup and M. P. Beer, Znorg. Chem., 1995,34, 5109. 13 A. Muller, E. Diemann, E.Krickemeyer, H. J. Walberg, H. Bogge and A. Armitage, Eur. J. Solid State tnorg. Chem., 1993, 30, 565. 14 H. V. A. Briscoe, P. L. Robinson and E. M. Stoddart, J. Chem. Soc., 1931, 1439. 15 S. J. Hibble, R. I. Walton and D. M. Pickup, J. Chem. Soc., Dalton Trans., 1996, 2245. 16 N. Binsted, J. W. Campbell, S. J. Gurman and P. C. Stephenson, EXAFS data analysis program, Daresbury Laboratory, 199 1. Received, 1st May 1996; Corn. 6103088B 2136 Chem. Commun., 1996
ISSN:1359-7345
DOI:10.1039/CC9960002135
出版商:RSC
年代:1996
数据来源: RSC