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10.1007_bf02062074

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10.1007_bf02062074
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  React. Kinet. Catal. Lett., Vol. 37, No. 2, 301-305 1888) ESR INVESTIGATIONS ON ZnO 7-A1203 CATALYTIC SYSTEM I,V. Nicolescu a, M. Chipar~ b, C. Pascu a, D. Barb~ I. Untea a, M. Floarea a and E. Chiril~ c apolytechnic Institute of Bucharest, Chemical Technology Faculty, Polizu - 1, bInstitute for Physics and Technology of Materials, CInstitute of Chemical and Biochemical Energetics, Bucharest, Romania Received April 4, 1987 Accepted November 16, 1882 ESR investigations on ZnO doped with various amounts of y-AI203 thermally treated,are reported. Experimen- tal data are related to the electronic structure of ZnO-y-AI203 and to desulfurization specific activity for natural gas. HpHBO~HTC~ Hccne~OBaHHe MeTO~OM 9HP ZnO, HaHeceHHoPo Ha pasnHqHble Ko~nqeCTBa TepMHqeCKH o6pa6OTaHHOFO y- AI203 . ~KcHepHMeHTa~bH~e ~aHHble O6~CH~DTCH 9neKT- pOHHb~ CTpOeHHeM ZnO/y-AI203 H cpaSHHSamTCH CO cne- HH~HqeCKO~ aKTHBHOCTbm K HpoHeccy ~eCynb~ypHpOB~HHS HpHpO~HOFO Fa3a. The Zn0 as well as the ZnO-T-AI203 system exhibit semicon- ducting features, which may be related to catalytic activity under various reaction conditions [1-3]. ESR spectroscopy may be used to investigate the presence of active centers a.c.) in the ZnO-y-AI203 system. EXPERIMENTAL ESR spectra of the Zn0-y-Al203 system as well as of the Akad~miai Kiadd, Budapest  NICOLESCU et al : ESR INVESTIGATIONS pure ZnO and pure y-AI203 components, have been recorded using a JES-ME-3X spectrometer, operating in X band (about 9 GHz). Measurements have been performed in air, at roan temperature. The samples have been measured in the temperature range 300-500 K, using a JES-VT-3X variable temperature accessory. In order to estimate accurately the g value, freshly recrystallized DPPH has been used as field marker. The catalysts have been prepared bv the reaction in a wet phase, among ZnO, hexahydrated aluminium nitrate and acid am- monium carbfmahe. The products have been thermally treated at 773- -1073 K. Samples have been specified by various molar ratios between X-AI203 and ZnO, denoted by r (r=0.05-0.5). The activity test has been performed with carbon sulfide at 673 K, in a re- circulating system during 10 hours. The specific activity (SA wt.%) is expressed by the amount of sulfur retained on ZnO-~-AI203 9 RESULTS AND DISCUSSION Pure ZnO thermally trea~ed at 773 K, unlike 7-A1203, gives a narrow asymmetrical resonance line, characterized by a g value close to g= 2.0 (Fig. i). Resonance line parameters (the peak to peak resonance line width Hpp in Tesla, g value and shape factor K), are given in Table i. Shape factor K, has been estimated using the slope method [4] and a.c. concentra- tion C has been derived using the formula C K I H 2 PP where I is the resonance line intensity. As may be noticed from Fig. 2, curve i, C reaches a maxim~n for r=0.1. The resonance line width dependence on mole ratio r (Table i) supports this result. Increasing the amount of y-Al203 introduced in the system from r=0 to r=0.1, the reso- nance line width is narrowed due to the a.c. concentration increase (which enhances exchange interactions, responsible 302  NICOLESCU et al : ESR INVESTIGATIONS a ZnO b r 0.05 ------ TA-? t3 r -0.10 r ----.... TAo 773 d r 0.25 --'-- TA- 3 e r-0.50 TA 7n f r 0.10 TA 973 , ,4xlO 4T Fig. I. ESR spectra of the ZnO-y-AI203 system Table 1 Resonance line parameters for samples thermally treated at 773 K Sample Hpp 0- 4T) K g ZnO 6.8 1.09 2.0054 ZnO-y-Al203 r=0.05) 7.0 1.16 2.0047 ZnO-y-Al203 r=0.10) 3.8 1.44 2.0043 ZnO-y-Al203 r=0.25) 5.0 1.37 2.0044 ZnO-y-AI203 r=0.50) 7.2 i.ii 2.0045 y-AI203 - _ _ for resonance line narrowing and reduce the relative weight of dipolar interactions leading to a Lorentzian line [i,4]). As the mole ratio r is further increased, C decreases and the 303  NICOLESCU et al : ESR INVESTIGATIONS 5 20 10 5 0 \ \ 1.00 =1 0.75 ~ o.so (.) 0.25 | i l 0.1 ~2 0.3 0.4 0.5 ~6 r Fig. 2. The dependence of relative a.c. concentration C i) and of the specific activity SA 2) on the desulfurization process of natural gas on the mole ratio r for the sample thermally treated at 773 K resonance line width increases, as dipolar interactions become dominant. These features are supported by the resonance line shape K) dependence on r. The dependence of C for r=0.1 on thermal treatment temper- ature Table 2) shows that C a.c.) is maximum for the sample calcined at 773 K. Above 973 a defected spinel phase develops, in which probably the a.c. are located. Table 2 Resonance line parameters for samples thermally treated at various temperatures T A r=0.1O) T A K) Hpp 10-4T) K g C r.u.) 773 3.8 1.44 2.0043 1.00 873 4.0 1.14 2.0044 0.9~ 973 3.4 1.09 2.0052 0.88 1073 1.6 1.30 2.0063 1.71 304
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