J . Chem. SOC., Faraday Trans. I , 1985,81, 1121-1 132 Electrical Conductivity of Uranium-Antimony Oxide Catalysts BY STANISLAW E. GOLUNSKI~ AND THOMAS G. NEVELL* Department of Chemistry, Portsmouth Polytechnic, White Swan Road, Portsmouth PO1 2DT AND DAVID J. HUCKNALL Department of Chemistry, The University, Southampton SO9 5NH Received 5th April, 1984 The relative ionic and electronic contributions to the electrical conductivity of a uranium- antimony oxide catalyst and of USbO, have been determined from measurements of ax. and d.c. conductance. Under inert atmospheres (390-775 K) conduction in the catalyst (predominantly USb,O,, together with small proportions of Sb,O, and USbO,) is associated with both electronic and effectively charged atomic point defects. Only electronic conduction occurs in USbO,.Under oxygen (1 0-70 kPa, 493-682 K) both materials aren-typesemiconductorsat higher temperatures, but at lower temperatures semiconducting behaviour varies with the pressure of oxygen. Heating USbO, in oxygen induces an ionic contribution to conductivity. Ionic conduction in the catalyst is eliminated by heating in hydrogen or propene at 470 K but is restored by heating in oxygen. It is suggested that both charged oxygen vacancies and interstitial oxide ions are involved in interactions of gaseous components with uranium-antimony oxides. With alkenes, interstitial oxide ions give rise to the products of selective partial oxidation. The uranium antimonates, USb,O,, and USbO,, are regarded as essential com- ponents of highly active U-Sb-0 catalysts1 developed for the allylic oxidation of alkenes.2 Freshly prepared, these catalysts are highly active and selective but their performance can deteriorate rapidly under conditions of continuous ~peration.~ It has been established that certain oxide catalysts operate by the facile release and replenishment of lattice oxygen.Despite various investigations of the U-Sb-0 system, however, the precise role of lattice oxygen in selective allylic oxidation is unclear. Pendleton and T a y l ~ r , ~ for example, have reported that lattice oxygen participates in the oxidation of propene over a U-Sb-0 catalyst at 623-673 K, whereas the results of Keulks and c ~ w o r k e r s ~ ~ ~ indicate a different mechanism for this reaction over USb,O,, at 698 K. Delobel and coworkers6u~ have concluded that, although a redox mechanism is involved in selective oxidation over U-Sb-0 catalysts, in the temperature range 580-640 K the mobility of lattice oxygen is low.In the present work the electrical properties of a U-SbO catalyst (mainly USb,O,, with small amounts of USbO, and Sb,04) and USbO, have been investigated, yielding information concerning the nature of the oxygen involved in allylic oxidation in the presence of binary mixtures of uranium and antimony oxides. Present address: Wispers School, Haslemere, Surrey GU27 1AD. 11211122 ELECTRICAL CONDUCTIVITY OF U-Sb OXIDES EXPERIMENTAL MATERIALS Antimony(II1) oxide and chromium(Ir1) oxide (Aldrich Gold Label, stated purity 99.999 % ), sodium chloride (Fisons, Purity 99.9 ) and uranium(v1) dinitrate dioxide hexahydrate (B.D.H.Chemicals Ltd) were used without further purification. X-Ray diffraction (X.r.d.) and infrared spectroscopy (i.r.) showed that although the antimony(rI1) oxide was mainly the orthorhombic modification, valentinite, some unidentified material was also present. A uranium-antimony oxide catalyst (U: Sb = 1 : 3) was prepared according to the method of Grasselli and Callahan.la The product was shown (by X.r.d. and i.r.) to consist mainly of USb,O,, with relatively small amounts of USbO, and Sb204. Pure USbO, was prepared by the thermal decomposition of USb30,, in air (24 h at 1363 K). In experiments involving flowing gases, oxygen, nitrogen and hydrogen (B.O.C., purity 99.5 % or better) were used without further purification.In experiments with static systems, propene (Cambrian Chemicals, purity 99%) and oxygen were condensed at 77 K and the middle fractions used. Hydrogen and nitrogen were stored after passage through a cold trap (77 K). APPARATUS AND PROCEDURE ELECTRICAL-CONDUCTIVITY MEASUREMENTS Measurements were made on polycrystalline samples, either compacted into discs or deposited on a conductivity probe, using a Wayne-Kerr Autobalance conductivity bridge B642 (1591.5 Hz) and a Keithley 610 C electrometer. m thick and 1.3 x m diameter) in a steel die (compacting pressure 10 tonne per 1.33 cm2). Electrical contact was made between two platinum electrodes to which pressure could be applied until a steady, constant value of conductance was obtained. Measurements were performed on samples heated to a temperature of 950 K under flowing atmospheres.’ In preliminary measurements to assess the reliability of the observations, a.c.and d.c. conductances were measured for well characterised samples of polycrystalline chromium(u1) oxide and sodium chloride. The results were in agreement with the literature*. and established that there were no substantial barrier layers at either the intergrain boundaries or the contacts between the electrodes and the sample. It was also shown that there were no significant temperature gradients in the samples. Conductivity Probe: This devicelo consisted of two platinum wires (3.8 x lo-, m diameter) separated by ca. m and wound concentrically along the length (1 x m) of a piece of woven silica tubing.Catalyst (ca. 0.008 g) was either electrodeposited on the probelo or applied as a suspension of cellulose nitrate in an ethanol+diethyl ether (50/50 v/v) mixture. In the latter method, solvent and cellulose nitrate were removed by gradual heating to 675 K under vacuum. The probe was mounted inside a cylindrical, acid-washed Pyrex reactor (200 cm3) which also contained powdered catalyst (0.78 g, surface area 2.2 m2). The reactor could be evacuated to Pa. Comparable changes in conductivity were observed when similar samples, either deposited on probes or compacted into discs, were heated under similar conditions. Discs: Powdered samples (0.6 g) were compacted under vacuum into discs (2 x X-RAY PHOTOELECTRON SPECTROSCOPY X.P.S. was performed using a VG Scientific ESCA 3 mark I1 spectrometer.Powdered catalysts were mounted in the instrument without further preparation and the spectra were obtained using magnesium Ka radiation. Details of the calibration of the spectrometer are discussed later.-7 - 8 h I d c 2 -9 \ v 80 - 1 c - 1 S . E. GOLUNSKI, T. G. NEVELL AND D. J. HUCKNALL -7 1.4 1 . 8 2 . 2 - 8 a -9 b I I 1.4 1.8 2 . 2 lo3 KIT 1123 1.5 2 .o 2 . 5 Fig. 1. U-Sb-0 catalyst under nitrogen: dependence of (i) a.c. and (ii) d.c. conductivity on temperature. (a) Compacted sample, first heating; (b) compacted sample, sixth heating; (c) sample on probe, after twenty cycles of heating (open points)/cooling (closed points). RESULTS VARIATION OF ELECTRICAL CONDUCTIVITY WITH TEMPERATURE u-sb-0 CATALYST When freshly prepared catalyst was heated in nitrogen [fig.1 (a)] or in oxygen, both a.c. and d.c. conductances increased exponentially with temperature. During initial heating/cooling cycles, the energy of activation for d.c. conduction (Ed,) decreased whilst that for a.c. (EaJ increased, the initial values depending on the method of measurement. However, these differences decreased rapidly with ageing [fig. 1 (b)] and, with the probe, stable behaviour was obtained after 15 cycles [fig. 1 (c)]. It was also observed that Ed, was constant between 293 and 673 K while E,, increased above a clearly defined temperature (q, table 1). USbO, There was no difference between the a.c. and d.c. conductances of polycrystalline USbO, in nitrogen at temperatures between 453 and 823 K. The conductance increased exponentially with temperature [fig.2 (a)] and was reproducible during repeated thermal cycling. These results suggested that conduction was essentially electronic, with a difference in energy between the valence and conduction bands (E, = 2&,) Of 106 kJ m0l-l. Following measurements of the isothermal conductivity of USbO, as a function of oxygen pressure (see below), the system was cooled in oxygen (90 kPa) which was then1124 ELECTRICAL CONDUCTIVITY OF U-Sb OXIDES Table 1. Activation energies for electrical conductivity ~~ ~ U-Sb-0 catalyst USbO, conditions ca. 1% ca. 9% ca. 15% under reduced reduced reduced under N2 (C,H,) (Hz) (HZ) NZ Tt/K 545 575 550 None None E,,/kJ mol-l E,,/kJ mol-1 34 40 37 54 53 61 74 78 67 61 39 54 53 67 74 78 - - - - -6.0 -8.0 1.5 2.0 2.5 Fig.2. USbO, under nitrogen: dependence of (i) a.c. and (ii) d.c. conductivity on temperature. (a) Compacted sample, first heating; (b) sample on probe, after heating/cooling cycles under oxygen. replaced by nitrogen (50 kPa). Subsequent observations revealed, over the temperature range studied, behaviour similar to that of the U-Sb-0 catalyst, with a.c. conductance exceeding d.c. conductance [fig. 2 (b)]. VARIATION OF ISOTHERMAL ELECTRICAL CONDUCTIVITY AS A FUNCTION OF OXYGEN PRESSURE Using materials deposited on probes, measurements were made of conductance, 0, as a function of the pressure of oxygen, Po,. Since changes in conductance were slow following adjustments to the oxygen pressure, a period of 24 h was allowed for equilibration before each reading was taken.Results fitted the general equation 0 K (Po*)+?S. E. GOLUNSKI, T. G. NEVELL AND D. J. HUCKNALL h - I C -s -7.39 v M - -7.40 1 - 7.3 8 . * - 5.82 n -5.84 - c: ’ -5.86 . v 0 00 - 5.88 1125 ~~ 4 .O 4.4 4.8 1% (Po*/Pa) Fig. 3. U-Sb-0 catalyst under oxygen: dependence of a.c. conductivity on partial pressure at (a) 493 and (b) 678 K. u-sb-0 CATALYST Only measurements of as. conductance were made (see Discussion). At 493 K and for pressures of oxygen in the range 10-1 5 kPa the value of q was + 0.035, but at higher pressures (30-70 kPa) the value of q was -0.050 [fig. 3(a)]. At 678 K, q was -0.08 at pressures of oxygen between 10 and 70 kPa [fig. 3(b)]. USbO, Both a.c. and d.c. conductances were measured. At 533 K, oac was characterised by exponents, q, of - 0.040 and + 0.100 for pressures of oxygen between 8 and 45 kPa and 45 and 90 kPa, respectively [fig.4(a)]. The values of q for d.c. conduction over the same ranges of pressure were considerably larger, the respective values being - 0.17 and +0.17. At 673 K, results were similar for oac and ode, giving an exponent of -0.025 [fig. 4(b)].1126 - 6.91 - 6.96 -7.20 n I - c: 2 -7.24 --- w Do - 7 . 2 8 ELECTRICAL CONDUCTIVITY OF U-Sb OXIDES -6.00 - 6 . 0 2 1 I I I 3.6 4.0 4.4 4.8 3.6 4.0 4.1 4.8 log ( P o p ) Fig. 4. USbO, under oxygen: dependence of (i) a.c. and (ii) d.c. conductivity on partial pressure at (a) 533 and (b) 673 K. EFFECT OF REDUCING ATMOSPHERES ON THE CONDUCTIVITY OF THE u-sb-0 CATALYST The electrical conductivity of the U-Sb-0 catalyst in the presence of reducing atmospheres was investigated using the probe over the temperature range 293-673 K.During exposure of the catalyst to propene or hydrogen, conditions were arranged such that, based on stoichiometric USb,O,, and the formation of water and propenal only, a predetermined proportion of lattice oxygen would be removed from the catalyst. After reduction, and in order to obtain repeatable initial values of a.c. and d.c. conductance, the catalyst was reoxidised by heating under oxygen (20 kPa) at 673 K for 24 h. Exposure of the catalyst at 673 K to propene equivalent to 1 % of the lattice oxygen caused the conductivity to increase by ca. 95%. The a.c. and d.c. measurements were identical above 575 K [fig. 5(a')], indicating a negligible ionic contribution to conduction, and the energy of activation for conduction was increased (table 1).Measurements were reproducible on reheating and similar results were obtained when the catalyst was heated under propene equivalent to 2% of the lattice oxygen. On heating the catalyst from 293 K in hydrogen (sufficient to remove 9% of the oxygen in the catalyst), a sharp increase in both a.c. and d.c. conductances occurred at ca. 475 K [fig. 5(b)]. The conductances were identical above 548 K, indicating no contribution from ionic conduction at higher temperatures. Subsequent cooling andS. E. GOLUNSKI, T. G. NEVELL AND D. J. HUCKNALL 1127 -5 -6 h d I c 2 , - 7 v 2 , -8 -9 -5 -6 - 7 - 8 -9 - 5 \P 1.5 2.0 2.5 1.5 2.0 2.5 103 KIT - 6 -7 -8 -9 1.5 2 .o 2.5 Fig.5. U-Sb-0 catalyst under reducing atmospheres: dependence of (i) a.c. and (ii) d.c. conductivity on temperature. (a) Propene equivalent to O,,,, admitted at 675 K (cooling, closed points; reheating, open points); (b) hydrogen equivalent to O,,,, admitted at 293 K, p first heating, q second heating; ( c ) hydrogen equivalent to O,.,, admitted at 675 K. reheating slightly increased a.c. and d.c. conductances over the entire temperature range. Finally, reduction equivalent to 15 % lattice oxygen using hydrogen eliminated the difference between ax. and d.c. conductances at temperatures > 455 K [fig. 5 (c)]. The activation energy for conduction in the reduced catalyst (54 kJ mol-l) was almost identical with that observed for USbO, under nitrogen (table 1).DISCUSSION Ionic point defects, which can contribute significantly to electrical conduction in metal oxides, are believed to play an important role in oxidation catalysis.ll9 l2 These defects may form during the preparation of the catalyst or subsequently as the result of internal or external eq~i1ibria.l~. l4 In addition, free electrons (e-) and electron holes (h+) may participate in electronic rearrangements involving adsorbed species and surface ions.ll According to Blumenthal and Seitz,15 a.c. conduction is associated with both electronic and ionic charge carriers, whereas d.c. conduction is due exclusively to the former. Conductance depends on the charge, mobility and concentration of defects. Ionic conduction may be due to cation or oxygen defects (charged vacancies, interstitial ions etc.) although TulleP suggests that, in oxides, conduction tends to be1128 ELECTRICAL CONDUCTIVITY OF U-Sb OXIDES controlled by the motion of ions in either the cation or oxygen sub-lattice.Electronic carriers can arise intrinsically or from the formation of ionic defects. Consideration of the various equilibria which may be set up involving a solid phase and molecular oxygen15-18 has, in the case of oxides such as Sn-Sb-019$20 and Cu-O,,l * 22 allowed oxygen-excessive and oxygen-deficient phases to be distinguished. An increase in conductance with ambient oxygen pressure, showing p-type semicon- duction, is consistent with the formation of interstitial oxide ions, Of- or OF, or with extension of the lattice and thus the creation of metal vacancies, VM.l69 l7 If equilibrium is assumed then for the former 40,(g) f OF + h+ (1) or +O,(g) f Of-+ 2h+.(2) [h+] = 2[0f-] (3) Using the requirement for electrical neutrality such as for eqn (2), expressions may be obtained which show that defect concentrations vary with oxygen pressure according to and [Or] cc (P02)0.25 [h+] cc (P02)0.25 or [Of-] cc (P02)0.167 ( 5 4 and [h+] oc (P02)0.167 ( 5 b) respectively, for the exclusive formation of either Or or Of-. In the latter case, since both metals in USb301, are reported to be in the oxidation state (v)*~* 6cf 23 then tO,(g) P V$ + 5h+ + 50; where 0; represents a lattice oxide ion; whence [V$] cc (Po2)0.2O8 (7 4 and [h+] cc (Po2)o.208. (7 b) According to these formulations, the concentration of both ionic and electronic defects show the same dependence on oxygen pressure.Opposite trends, showing n-type semiconduction, could be due to oxide ion vacancies or to interstitial metal ions.16* l7 Fot the former, concentrations of vacancies and electrons would be proportional to oxygen pressure to the power of - 0.167, and for the latter (Mf+) the exponent of oxygen pressure would be -0.208. ELECTRICAL CONDUCTION IN THE u-sb-0 CATALYST The catalyst exhibited both electronic and ionic conduction under oxygen or inert atmospheres at temperatures between 293 and 773 K [fig. 1 (b)]. Changes were observed in ionic conduction, however, which were not reflected in the electronic conduction. Since there was no corresponding phase modification,la it is probable that these changes were associated with different mechanisms of ionic conduction.Correspondingly, the measured exponents q in the dependence of conductance on oxygen pressure showed n-type semiconduction above the transition temperature and complex behaviour below this temperature. Numerical values of q were much lower than those deduced above.S. E. GOLUNSKI, T. G. NEVELL AND D. J. HUCKNALL 1129 The simultaneous presence of oxide ion vacancies and interstitial oxide ions has been proposed for several oxides of u r a n i ~ m . ~ , - ~ ~ For U,O, at high temperatures, a decrease in q from 0.25 to -0.167 with oxygen pressure from loF2 to lo5 kPa2' was attributed to the initial formation, at low pressures, of interstitial oxide ions near the surface, despite the high concentration of vacancies.Filling of these vacancies occurred at higher pressures. These defects have also been observed using diffraction techniques.26 The present results indicate that the U-Sb-0 catalyst interacts with oxygen in a similar way, but they do not allow the effective charge of the ionic defects to be determined. The formation of interstitial oxygen species which diffuse only slowly to vacant sites is consistent with the relatively slow rates of reoxidation of the catalyst by atmospheric oxygen after static experiments on the oxidation of alkenes.lo The large increase in conductivity accompanying reduction of the catalyst may be due in part to charge transfer associated with adsorption, but this does not account for the corresponding elimination of ionic conduction.The two observations are, however, consistent with the removal of interstitial oxide ions by processes which increase the concentration of free electrons in the catalyst. Thus, in early stages, the following overall processes could be involved (assuming Of-) H,(g) +Of- e H,O(g) + 2e- CH,-CH=CH,(g) + 0;- + 0; + CH,=CH-CHO(g) + H,O(g) + Vi+ + 4e- (9) where 0; represents a lattice oxide ion and Vi+ an oxide ion vacancy. For hydrogen, the mechanism could involve reaction with superficial lattice oxide ions to leave vacancies which are then filled by interstitial oxide ions. Alternatively, hydrogen could be dissociatively adsorbed. In the allylic oxidation of propene, the antimony ions at which symmetrical adsorbed intermediates are formed1" would be oxide ion vacancies (Vi+) and at least one of the following steps would involve interstitial oxide ions: C,H,(g) + 0; + Vi+ -+ OH(ads) + C,H,(ads) + 2h+ C,H,(ads) + 0; -+ C,H,(ads) + OH(ads) C,H,(ads) + Of- + Vg+ -+ C,H,(ads) + OH(ads) (10) (1 1 4 (1 1 b) C,H,(ads) + Of- -+ C,H,O(g) + Vg+ + 4e- C,H,(ads) + 0; -+ C,H,O(g) + 2Vi+ + 4e- (124 (12b) (13) 20H(ads) -+ H,O(g) + V;+ + 0; + 2e-.ELECTRICAL CONDUCTION IN USbO, Under inert atmospheres, electronic processes were the only means of conduction in freshly prepared USbO, at temperatures between 453 and 823 K. The similarity of values of Eg in USbO, and the reduced U-Sb-0 catalyst indicates that electronic conduction is associated with structural features common to the two antimonates. From d.c. measurements at 533 K it appears that evacuation of USbO, brings about the formation of lattice oxide ion vacancies.Subsequent exposure to low pressures of oxygen allows these to be filled: 0, evac V;++2e-+2O2(g)e0; (14) and under higher pressures interstitial oxide ions and associated positive holes are (1 5 ) formed : iO,(g) s O:-+ 2h+.1130 ELECTRICAL CONDUCTIVITY OF U-Sb OXIDES - 2 Do 1 1 385 390 39 5 4 00 4 0 5 binding energylev I I I I J 5 3 5 5 4 0 5 4 5 5 5 0 55 5 binding energy/eV Fig. 6. X-ray photoelectron spectra of U-Sb-0 active and deactivated catalysts (measured binding energies corrected with reference to C lsl/2 at 285.3 eV). (a) U 4factive (U 4fat 383.7 and 394.4 eV); (b) U 4fdeactivated [U 4fbinding energies (a)]; ( c ) Sb 3d deactivated (Sb 3d5,2 at 534.5 eV and Sb 3d3,2 at 544.0 eV); (d) Sb 3d active (Sb 3d5,, at 533.4 eV and Sb 3d3,, at 542.8 eV).Although changes in ionic conduction would be expected to show the same dependence on oxygen pressure, this was much less marked, which may reflect the relative immobility of atomic defects at this temperature. The value of -0.03 at 673 K for the exponent q (both a.c. and d.c.) is again not consistent with a simple mechanism, indicating the possible presence of both charged oxygen vacancies and interstitial oxide ions which do not interact. That some of these defects are retained on cooling would explain the distinct ionic contribution to the conductivity of USbO, observed subsequently.S. E. GOLUNSKI, T. G. NEVELL AND D. J. HUCKNALL 1131 CATALYSIS BY URANIUM ANTIMONATES The performance of uranium antimonate catalysts depends on the method of preparation, In the absence of gas-phase oxygen, pure USb,O,, reacts only slightly with propene to form propenal,6a whereas the catalyst containing small amounts of Sb,O, and USbO, oxidises alkenes selectively in much larger amounts.la Reoxidation of the impure material is relatively slow and changes in conductivity reflect the extent of reduction.1° The present results show that interstitial oxide ions make a major contribution to the conductivity of the U-Sb-0 catalyst. Although it has been suggestedlC that, in this catalyst, oxygen vacancies provide sites for the adsorption of alkenes during selective allylic oxidation, the possible role of interstitial oxide ions has not been reported previously. The reaction of the adsorbed hydrocarbon with such ions, formed by the interaction of the catalyst with gaseous oxygen, is consistent both with observations indicating a redox mechanismlC9 and results which suggest the involvement of adsorbed oxygen.The progressive deterioration in the performance of the catalyst during continuous use is likely to be caused by the depletion of these ions, the replacement of which from gas-phase oxygen appears to be slow. By implication, interstitial oxide ions in a fresh catalyst are associated with the impurities Sb,O, and/or USbO,. The particular importance of antimony with respect to the catalytic behaviour has also been shown by X-ray photoelectron spectroscopy (fig. 6, spectra uncorrected for surface charging29).Whereas the binding energies associated with U 4f were identical in fresh catalyst and catalyst deactivated by reduction with propene, those for Sb 3d3/, and Sb 3d,,, were increased significantly in the deactivated catalysts. Calculation of the binding energies in insulating or semiconducting samples requires calibration of the spectrometer. At the time, the above results were corrected for charging by reference to the C lsl,2 peak, a technique also used by Delobel et aZ.6c It has been however, that this is an unsatisfactory internal standard since the deposits which give rise to it are in electrical equilibrium with the specimen. Although further work appears to be necessary on X.P.S. of the U-Sb-0 system, the importance of Sb in the catalytic system is clear.Finally, note that the binding energies for the U 4f peaks correspond exactly to those reported by -411en31 for yUO,. This is unexpected and requires further investigation. The mechanisms by which interstitial oxide ions are formed or reformed remain unclear. However, for Sn-Sb-0 catalysts32 evidence points to the significant catalytic effects of superficial antimony ions and to the importance of the impurity Sb,O,. 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