J. Chem. Soc., Faraday Trans. 1. 1989, 85(4), 895-906 Structural Investigation on a Spinel-related Zn/Cr = 1 Mixed-oxide SystemT Cinzia Cristiani and Pi0 Forzatti" Dipartimento di Chimica Industriule ed Ingegneria Chimica ' G. Natta' del Politecnico - Piazza Leonurdo da Vinci, 32, 20133 Milano, Itall? Maurizio Bellotto Dipartirnento di Ingegneria Meccunicu, Universita di Brescia - Via Vulotti, 9, 25060 Brescia, Italy A Zn-Cr mixed oxide of 1 / 1 composition has been characterized by powder X-ray diffraction, infrared spectroscopy, ultraviolet-visible diffuse reflec- tance spectroscopy, differential thermal analysis-thermal gravimetry, surface-area determination and chemical analysis. The behaviour of the system has been studied over a large temperature range and under different activation atmospheres.In addition to the model involving a single non- stoichiometric spinel phase, already proposed in the literature, a model involving a shell rich in ZnCrO, and a core constituted by ZnCr,O, and ZnO is discussed and shown to be consistent with the experimental evidence. Commercial high-temperature catalysts for the synthesis of methanol and mixtures of methanol and higher alcohols from CO and H, are usually based on pure and alkali- promoted mixed oxides of Zn and Cr, Most of the commercial catalysts present Zn/Cr ratios > 0.5. The catalytic activity has generally been attributed to a promoting effect of Cr because of physical effects related to the formation of ZnCr,O,, which either prevents the sintering of ZnO (regarded as the active phase)3 or acts as a support for ZnO.*- Alternatively the catalytic activity has been attributed to the ZnCr,O, spinel phase.' Recently a number of papers7-13 have appeared dealing with the structure and reactivity of Zn and Cr mixed oxides.In these catalysts the active species was reported to be a spinel-like phase, non-stoichiometric because of an excess of Zn. This excess of Zn was reported to be octahedrally coordinated, and to provide the catalytically active ~ i t e s . ~ * * * l ~ For an understanding of the nature of such a system the most interesting composition was found to be Zn/Cr = 1.' In fact this composition is reported to exhibit typical non-stoichiometry at the maximum degree, being monophasic, while at higher Zn loadings free ZnO is detected.However, the behaviour of the system, particularly at high calcination temperatures and under different activation atmospheres, was not fully clarified. The aim of this paper is to investigate the nature of a Zn/Cr = 1 mixed oxide. A number of techniques were used to obtain information on the valence state of the elements and their local coordination and on the structure of the present phases. The evolution of the sample has been followed in air, N, and H, up to temperatures higher than those reported by other authors. The target was to provide a better understanding of the phase transformations occurring in the system, during both activation and t This is Paper XV in the series Synthesis of Alcohols from Carbon Oxides and Hydrogen. 895896 Structural Investigation of a Zn-Cr Mixed Oxide catalytic reaction.A comparison with literature data is provided. A new model is discussed which is capable of interpreting the experimental evidence collected. Experiment a1 Preparation and Activation A Zn-Cr mixed-oxide sample with an atomic ratio Zn/Cr = 1/1 was prepared by dissolving Zn(NO,), * 6H,O and Cr(NO,), * 9H,O (Carlo Erba RPE). Zn and Cr were precipitated by addition of a 3 mol dm-, solution of (NH,),CO, at 333 K. Evolution of CO, was observed. The final pH of the supernatant liquor was 7.8. The resulting slurry was aged for 12 h at 353 K and then filtered and washed to remove NO, ions from the precipitate. The presence of NO; in the eluate was ascertained by qualitative analysis. After drying at 383 K a precursor was obtained, quoted as Z,C1-4A.Indeed all the Zn and Cr was precipitated, as confirmed by chemical analysis performed on the sample after calcination at 673 K. Throughout the paper samples are identified with a notation indicating the activation temperature and the corresponding reaction atmosphere : e.g. Z,C,-llA indicates sample Z,C, heated at 1073 K in air. Sample ZlC,-4A was activated in air at temperatures extending from 673 to 1073 K, and in N, and H, atmospheres up to 673 and 773 K. Details of the temperature cycles of activation are summarized in table 1. Characterization The samples were characterized by means of powder X-ray diffraction, infrared spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy, differential thermal analysis-thermal gravimetry, surface-area measurements and chemical analysis.X-Ray powder diffraction patterns were collected using a Philips PW 1050/70 vertical goniometer with nickel-filtered Cu Ka radiation. Crystallite dimensions were evaluated using the Sherrer equation. Lattice parameters were calculated with a least-squares fitting routine, taking into account systematic errors produced by the goniometer. Quantitative estimates of multiphase samples were performed according to Klug and A1e~ander.I~ Infrared spectra were recorded on a Perkin-Elmer 147 spectrometer with the KBr pressed-disc technique. Ultraviolet-visible diffuse reflectance spectra were recorded by means of a Jasco UVIDEC-6 10 double-beam spectrophotometer equipped with integrating sphere (bandwidth 2 nm).Surface areas were determined by using a Carlo Erba Sorptomatic 1800 series instrument with a B.E.T. dynamic system. Differential thermal analysis-thermal gravimetry data were obtained with Mettler 2000 instrument. The chemical analysis was performed in order to determine the fraction of Cr present as Crv' in Z,C,-7A. 0.5 mol dm-, H,SO, was added at a quoted portion of the sample, under stirring and boiling for ca. 6 h (without evaporation of the acid solution) in order to dissolve the CrV1.l5 A yellow solution and a black residue (quoted as ZlC,7AR-4A) were separated by filtration. The amount of Crvl in the solution was determined by manganometric titration. A portion of ZlC,7AR-4A was etched and dissolved with a mixture of aqua regia and H,SO, by boiling for 24 h.The chromium present in the solution was again determined as Crv' by manganometric titration. Z1C,7AR-4A was also characterized as usual. Results Precursor The sample dried at 383 K ZlC,-4A is the matrix from which all subsequent samples have been derived. The X.r.d. powder pattern shows [fig. l(a)] only two very broad reflections owing to its microcrystallinity. In fact this sample has a very high surfaceC. Cristiani, P. Forzutti and M. Bellotto 897 Table 1. Activation-temperature cycles sample temperature cycle ZlC,-7A Z,C,-9A Z,C,-11A ZICl-7N ZlC,-8N Z,C1-7H Z,Cl-8H from r.t. to 673 K, 120 K h-l; at 673 K for 4 11 from r.t. to 673 K, 240 K h-l; from 673 to 873 K, 120 K h-l; at 873 K for 2 h from r.t. to 873 K, 240 K h-'; from 873 to 1073 K, 120 K h-l; at 1073 K for 1 h from r.t.to 673 K, 120 K h-l; at 673 K for 4 h from r.t. to 673 K, 240 K h-l; from 673 to 773 K, 60 K h-l; at 773 K for 3 h from r.t. to 673 K, 120 K h-l; at 673 K for 2.5 h from r.t. to 673 K, 240 K h-'; from 673 to 773 K, 60 K h-'; at 773 K for 2 h 0 0 A . 65 60 55 50 45 40 35 30 25 20 281" Fig. 1. Experimental X-ray powder diffraction patterns of (a) Z,C,-4A, (6) Z,Cl-7A, (c) Z,C,-9A and ( d ) Z,C,-llA, together with calculated patterns of a mixture of ZnCr,O, and ZnO: (i) f.w.h.m. = 2.0, (ii) f.w.h.m. = 0.6 and (iii) f.w.h.m. = 0.2". a, ZnO; 0, ZnCr,O,. area, i.e. = 252 m' g-l, which accounts for its small crystallite dimensions, in line with the broadness of the X.r.d. reflections. The i.r. spectrum [fig. 2(a)] gives evidence of bands characteristic of COi- (840, 1020, 1090, 1370 and 1480 cm-l) and possibly of NH; (1390 cm-').16 The diffuse reflectance spectrum [fig.3(a)] shows bands at 260, 370, 440, 580 and 700 nm. From literature data1' we can attribute the bands at ca. 440, 580 and 700 nm to CrII' d 4 transitions [fig. 4(b)], the band at 370 nm to 02- -+ Zn2+ charge transfer, as in ZnO [fig. 4(c)], and the bands at 370 and 270 nm to 02- + Cr6+ charge transfer. The oxidation of some CrIIl to Crv' is probably caused by aging the898 Structural Investigation of R Zn-Cr Mixed Oxide I 0 I I 1 1 I 1600 1200 MK) 400 wavenumber/cm-' slurry at 353 K in alkaline ~olution'~ and/or drying it at 383 K in air. The d.t.a.-t.g. analysis in N, and air up to 673 K [fig. 5(a) and (b)] shows two peaks (the first, endothermic, at 453-553 K and the second, exothermic, at 553-653 K) attributed to the decomposition and subsequent combustion of an ammoniacal compound [possibly (NH,),CrO,, qecomp = 453 K].The first endothermic peak at 453-553 K also has a possible contribution from the decomposition of Zn,(OH),(CO,), ( &ecomp z 538 K).lS The endothermic peak at ca. 373 K may be associated with dehydration of the sample. From the results so far it follows that the precursor may consist of a mixture of ammonium compounds and carbonates. Activation in Air Upon calcination up to 673 K sample Z1C,-7A is obtained. Its X.r.d. powder pattern shows a spinel-like phase [fig. l(b)], related to ZnCr,O, (JCPDS 22-1107). No other phase, namely ZnO (JCPDS 5-0664), is detectable. However, the broadness and asymmetry of the reflections prevents us from excluding the presence of ZnO.Fig. 1 (i) shows the calculated spectrum of a mixture of ZnO and ZnCr,O,. The Zn/Cr ratio of this mixture, resulting from ZnO and ZnCr,O,, is 1 / 1 and corresponds to that measured for sample Z,C,-1lA. The full width at half maximum (f.w.h.m.) is set to 2.0" in orderC. Cristiani, P. Forzatti and M . Bellotto 899 r 1 '-- I I I 1 1 1 1 ' 0 . m 250 350 450 550 650 750 850 wavelength /n m Fig. 3. Ultraviolet-visible diffuse reflectance spectra of (a) Z,C,-4A, (b) ZlC,-7A, (c) Z1C,-9A and (d) Z,C,-IIA. to simulate the plot in fig. l(6). From a rough visual comparison of fig. 1 (b) and (i) it is evident that the hypothesis that sample ZlC,-7A consists in the bulk of a mixture of ZnO apd ZnCr20, cannot be discarded.Particle-size determination gave a mean value of 50 A. This corresponds to a surface area of 126 m2 g-' (calculated assuming spheres with radius equal to the mean particle size), in line with the measured value of 104 m2 g-'. The lattice parameter of this spinel-like phase has been refined using all the resolved measured reflections, i.e. (1 1 l), (2 2 0), (3 1 l), (4 0 0), ( 5 1 1)/(3 3 3) and (4 4 0), and under the hypothesis that the sample is monophasic, i.e. the assumption was made that the line position is due only to the parameters of the spinel phase a n t is not affected by the overlapping lines of other phase!. The refined value was 8.40(2) A, higher than that reported for ZnCr,O, (a, = 8.328 A).However, in this case, the refined a, value is affected by the poor crystallinity of the sample and by the underlying hypothesis that the sample is monophasic. Because of this we cannot speculate further on this assumption. The i.r. spectrum [fig. 2 (b)] shows two absorptions at 500 and 600 cm-' attributed to the bending and stretching of Cr"'-O octahedra in ZnCr20,1g and a composite absorption in the range 85&930 cm-' attributed to the bending and stretching of CrV'-0 tetrahedra in ZnCrO, (a-phase).,' The diffuse reflectance spectrum [fig. 3 (b)] gives evidence of 02- -+ Zn2+ charge transfer at 370 nm as in ZnO and at 300 nm as in ZnCr,O, [fig. 4(a)]. 02- + Cr6+ charge transfer at 370 and 260 nm are also present while no bands due to CrIII d 4 transitions are evident [fig. 4(a) and (b)].CrI" d-d transitions are not evident in the u.v.-visible diffuse reflectance spectrum, possibly because they are broad and thus smeared out, and/or because they are covered by the low-energy tail of the more intense charge-transfer bands.21 The larger half-width may indicate a greater antibonding character of the excited state compared to the stoichiometric spinel, i.e. a larger internuclear equilibrium distance. The band at 300 nm is not so evident as in the spectrum of ZnCr,O, [fig. 4(a)] because of the presence in sample Z,C,-7A of the 02- --+ Cr6+ charge-transfer absorptions. In contrast, the band at 300 nm is well evident in sample ZlC,-8H, as will be discussed later, owing to the absence of Crvl. On annealing this sample in air at 873 and 1073 K samples ZlC,-9A and Z,C,-1 1A are obtained.Their X.r.d. powder patterns evidence a progressive crystallization of ZnO (reflections at 28 = 31.88, 34.54, 36.62, 47.61 and 56.64") and of ZnCr,O, [fig. 1 ( c ) and (41. Fig. (ii) and (iii) show the calculated spectra equivalent to that already discussed inStructural Investigation of a Zn-Cr Mixed Oxide O.OO0 250 350 450 550 650 750 850 w avelengthhm Fig. 4. Ultraviolet-visible diffuse reflectance spectra of (a) ZnCr,O,, (b) Cr,O, and (c) ZnO. fig. l(i). Direct comparison of calculated and experimental X.r.d. patterns is in agreement with the hypothesis that samples Z,C,-9A and Z,C,-1 1A are constituted by a mixture of ZnO and ZnCr204. The f.w.h.m. are set to 0.6 and 0.2, respectively. Quantitative analysis of the pattern shows that all Zn in excess with respect to Zncr,04 is present as ZnO in sampleo Z,C,-l1A.The mean particle size increases to 100 A for sample Z,C,-9A and 259 A for sample Z,C,-llA. The refinedo lattice parameter decreases to a, = 8.35(1) A for sample Z,C,-9A and a, = 8.327(3) A for sample Zlcl- 11 A, thus approaching the value found for the stoichiometric ZnCr204 (a, = 8.328 A). The refinement was made using all measured reflections in these cases also. The surface- area value, calculated starting from crystallite dimensions, is 66 m2 g-' for Z,C,-9A and 27 m2 g-' for Z,C,-11 A, in line with the measured values of 47 and 19 m2 g-', respectively. The i.r. spectra [fig. 2(c) and (d)] indicate a sharpening of the bands attributed to the bending and stretching of CrII' octahedra and the appearance of a band attributed to ZnO (as a shoulder at 450 cm-').A progressive decrease, on increasing the calcination temperature, of the composite absorption in the range 850-930 cm-' associated with Crv' in a tetrahedral coordination is also evident. The diffuse reflectance spectra [fig. 3(c) and (d)] show an increase in the relative intensities of the absorptions corresponding to CrIII d-d transitions on increasing the calcination temperature. In order to quantify the amount of Crvl in Z,C,-7A a chemical analysis has been carried out. The etching conditions prevent oxidation of the CrIII eventually dissolved along with Crvl. Then manganometric titration detects only Crvl species.15 The Crvl dissolved by H,S04 etching is the 14.5% of the total Cr. This amount is in line with the one reported for a similar sample after etching with diluted CH,COOH.' The X.r.d.powder pattern of the residue (sample Z,C17AR-4A) shows a spinel-like phase whose crystallinity is comparable to that of sample Z,C,-7A. The i.r. spectrum of the residue shows no evidence of the composite absorption extending from 850 to 930cm-' associated with Crvl [fig. 2(e)]. The absorptions at 1150, 1050 and 870 cm-', which are not present in the i.r. spectrum of sample Z,C,-7A, are attributed to SO:- ions. The pair of bands at 500 and 600 cm-' do not differ significantly from those detected in sample Z,C,-7A. Chemical analysis confirms that the insoluble portion contains 85% of the total Cr. Thus the etching of sample Z,C,-7A occurs only on the surface and does not induce any structural modification of the bulk.It dissolves 14.5% of total Cr as Crvl;C. Cristiani, P. Forzatti and M . Bellotto 90 1 z 81 W 8j t 0,1 W 4 673 573 473 373 T/K < 673 573 473 373 T / K Fig. 5. Differential thermal analysis-thermal gravimetry up to 673 K of (a) Z1C,-4A in air and (b) Z1C,-4A in N,. (i) D.t.a., (ii) t.g. and (iii) d.t.g. the residual amount of Cr is present as Crl'* in the insoluble portion. No Crvl is detected in the residue. From the evidence collected so far, the evolution in air of the system under study can be summarized as follows. (1) The decomposition of the ammonium compounds and of the carbonates, constituents of sample Z1C,-4A, gives rise to sample Z,C,-7A.This sample may be constituted by a shell rich in Cr", formed by a phase related to ZnCrO, which contains 14.5 O/O of the total Cr, and by an inner core, formed by a spinel- like phase related to ZnCr,O,, and possibly ZnO. (2) When calcining to higher temperatures, we observe a decrease in the content of Cr", together with the crystallization of the present phases. The refined lattice parameter of the well crystallized spinel phase in sample Z,C,-llA is close to that reported in literature for ZnCr,O,. Activation in N, The samples ZlC,-7N and ZlC,-8N were obtained upon heating Z,C,-4A in N, at 673 and 773 K. The outcome of d.t.a.-t.g. analysis on sample ZlC1-4A in N, [fig. 5(a)] is similar to that previously discussed for sample ZlC,-4A in air. The X.r.d.powder patterns [fig. 6(a) and (b)] are consistent with the reflections of ZnCr,O,. The asymmetries and shoulders at 28 z 30 and 36" may be attributed to the presence o[ZnO, as already discussed. The mean crystallite dimension of sample ZlC,-8N is ca. 25 A. The Cr"'-O stretching at 600 cm-l and bending at 500 cm-l, typical of 11-111 type spinel phases, are detected in the i.r. spectra of the two samples ZlC,-7N and Z1C,-8N [fig. 7 (a) and (b)]. The composite absorption typical of tetrahedrally coordinated Crv' in the range 850-930 cm-' decreases on increasing the temperature from 673 to 773 K. Diffuse reflectance measurements on the sample Z1C,-8N [fig. 8(a)] are consistent with the presence of characteristics absorptions due to 0,- + Zn2+ and 02- -+ Cr6+ charge transfer (260, 340 and 360 nm) and due to Cr"' d-d transitions (450, 600 and 700 nm).Indeed, these absorptions are barely evident in the spectrum, but are seen by taking the second derivative.902 Structural Investigation of a Zn-Cr Mixed Oxide I 1 1 1 I I 1 I I I 65 60 55 50 45 40 35 30 25 20 291' Fig. 6. Experimental X-ray powder diffraction patterns of (a) ZlC,-7N, (b) Z,Cl-8N, (c) Z,C1-7H and ( d ) ZlC,-8H. a, ZnO; O,ZnCr,O,. 0 Fig. 7. Infrared spectra of (a) ZICl-7N, (b) ZlC,-8N, (c) Z,Cl-7H and ( d ) ZlC1-8H. 0, ZnCrzO,; ., ZnCrO,. The activating atmosphere is less oxidizing than air; thus there is a smaller amount of Crv' evidenced by both the i.r. and u.v.-visible diffuse reflectance spectra. In this case also, on increasing the activation temperature, we observed a decrease in the i.r.and u.v.-visible diffuse reflectance absorptions related to Crv*. Moreover, as compared to the samples calcined in air, the samples activated in N, are less crystallized.C. Cristiani, P. Forzatti and M. Bellotto (4 903 C c - 0.500 -8 4 \ - - d 1 I I I 1 0.000 Fig. 8. Ultraviolet-visible diffuse reflectance spectra of (a) Z1C,-8N and (b) Z,C,-8H. I I -1 TIK - 573 473 373 Fig. 9. Differential thermal analysis-thermal gravimetry up to 673 K of sample Z,C,-7A in H,. (a) D.t.a., (b) t.g. and (c) d.t.g. Activation in H, The samples ZlC,-7H and ZlC,-8H are obtained by the activation in H, of ZlC,-4A at 673 and 773 K. The X.r.d. patterns of sample Z,C1-7H [fig. 6(c)] indicate a very microcrystalline sample. Reflections consistent with those of a spinel-like phase are detectable.On increasing the temperature of annealing we observe [fig. 6 ( d ) ] the crystallization of the spinel-like phase and, to a much greater extent, of the ZnO phasz. The mean crystallite dimensions of the spinel-like phase in sample ZlC,-8H is ca. 25 A. The infrared spectra [fig. 7(c) and (d)] show bands at 500 and 600 cm.-l typical of octahedrally coordinated CrIII, but there is no evidence that Crv' is present. The diffuse reflectance spectrum of sample ZlC,-8H [fig. 8(b)] shows absorptions at 300 and 360 nm due to 02- -+ Zn2+ charge transfer, and absorptions at 450, 600 and 700 nm attributed to C P d-d transitions.904 Structural Investigation of a Zn-Cr Mixed Oxide Because of the reducing atmosphere in which the activation is performed, no Crvl is present in these samples.In fact, absorptions typical of Crvl in the i.r. and u.v.-visible diffuse reflectance spectra are not evident. In order to investigate further the CrII' + CrIV transformation the sample Z,C,-7A was reduced under H, on a thermogravimetric balance up to 673 K. An exothermic peak is observed at 523-573 K with an associated weight loss of ca. 1.2% (fig. 9). The weight loss is entirely associated with the CrV1 -+ Cr'" reduction. This is in agreement with the calculated value of 1.44 Oh, assuming that the amount of Crvl is 14.5 O/O of the total Cr, as was derived from the chemical analysis. Accordingly, the i.r. spectrum obtained following the d. t.a.-t.g. experiment shows no evidence of absorptions characteristic of Crvl coordinated tetrahedrally.Discussion The data previously reported show evidence of a spinel-like phase for all the samples investigated. ZnO is detected as a separate phase in some cases. Other authors previously reported the spinel-like phase to be non-stoichiometric for similar samples prepared in a slightly different way. 7-13 The evidence of this non-stoichiometry was found primarily in the X.r.d. powder patterns. The non-stoichiometry was explained in terms of octahedrally coordinated ZnI' ions. The structure was regarded as intermediate between a spinel and a rock-salt structure. This hypothesis was supported by a profile-fitting analysis of the X-ray powder pattern,' and occupancy factors were given for octahedral and tetrahedral Zn" and for octahedral CrIII ions.', Studies of H, heterolytic dissociation and CO absorptionlo* l2 provide evidence of surface sites different from pure ZnO and stoichiometric ZnCr,O,. In addition, the high catalytic activity of the Zn/Cr = 1/1 system was related to this non-stoichiometric spinel phase.The decrease in activity with time was attributed to the metastable behaviour of the system, which evolves to stoichiometric ZnCr,O, with segregation of Zn0.7y89 l1 Moreover, the width of e.s.r. signals indicated a distribution of CrI'I-CrIII distances suggesting a non-ordered system.12 Even if the model proposed, considering a non-stoichiometric spinel phase, is in a good agreement with experimental data, other models can be taken into account. In fact profile-fitting analysis of such a microcrystalline pattern is no definite evidence, and the authors themselves could not exclude the presence of small amounts of amorphous ZnO,ll even if this was not present in the profile-fitting model.Moreover, the absorption of probe molecules and catalytic activity can be related to surface properties and are not direct evidence of a bulk non-stoichiometric spinel phase. Furthermore, the presence of Crvl was not accounted for. From the data reported, no definite evidence of non-stoichiometry or stoichiometry of the spinel phase emerges for our sample. The data may suggest an alternative model for the investigated Zn/Cr = 1 mixed oxide, in which the sample is thought to be constituted by a shell rich in ZnCrO, and a core of ZnCr,O, and ZnO.In this model surface Zn", C P and Crvl ions may be located in octahedral and tetrahedral sites in a non-ordered fashion. Note that ZnCrO, has a crystal structure closely related to the spinel structure : in fact it is constituted by the same oxygen lattice, within which Zn is located in the octahedral and Cr in the tetrahedral holes. Thus the presence of octahedrally coordinated Zn" is justified by the simultaneous presence of tetrahedrally coordinated Cr". In the bulk, Zn" ions not accommodated in the stoichiometric spinel phase are present as ZnO. The sample is never monophasic, and ZnO is not detected at 673 K in air by X.r.d. analysis because of the poor crystallinity of the phases and the large width of the peaks. On increasing the calcination temperature the amount of Crvl decreases, probably through the decomposition of ZnCrO, to ZnCr,O, and ZnO.The appearance of ZnOC. Cristiani, P . Forzatti and M. Bellotto 905 in the X.r.d. spectra is due to a better crystallization of all the phases, with a consequently smaller f.w.h.m. and better resolution of the peaks. This hypothesis is supported by the calculated spectra of fig. 1, in which the different crystallization of the phases is simulated by varying the f.w.h.m. Fig. 1 (i), like fig. 1 (b), does not show clearly the presence of ZnO. The samples activated in N, show the same behaviour as those calcined in air. There is a trend towards improved crystallization on increasing the temperature. At the same temperatures, however, crystallite dimensions are smaller compared with the samples calcined in air.The inert atmosphere leads to a smaller CrV' content in these samples. With regard to the samples activated in H,, the reducing atmosphere causes the complete absence of Crv' even at 673 K. Increasing the annealing temperature to 773 K, rapid crystallization of ZnO is detected, while the spinel-like phase behaves as it does in the samples activated in N,. In this case the reduction of Crv' to Cr"' may explain the observed phase transformations, and be the cause of the different crystallization rates of ZnO and ZnCr,O,. The reduction of the surface layers of ZnCrO, may result in a structural situation that is different from a mixture of ZnO and ZnCr,O,. The interaction of the two phases to form a solid solution and/or to give a defective phase may justify the surface behaviour reported by other authors and discussed above.Conclusions The basic features of the Z,C, oxide system under study can be summarized as follows. (1) At 373 K the sample is microcrystalline and may be formed of a mixture of ammoniacal compounds and carbonates. (2) On heating in air at 673 K a spinel-like phase is obtained. Crvl is also present in the surface layers. Calcination at higher temperatures brings about a decrease in the amount of Crv' and the crystallization of all the phases present. (3) Activation in N, shows the same trend on increasing the temperature. A smaller amount of Crv' is present in the structure because of the annealing atmosphere. (4) The activation in H, brings about a more rapid crystallization of ZnO, formed through the reduction of ZnCrO, to ZnCr,O,.The behaviour of the system may be explained hypothesizing non-stoichiometry of the spinel phase in line with previous suggestions. '-13 An alternative model is considered here, which is also consistent with the experimental data. In this model the sample is constituted by a shell rich in ZnCrO, and a core of ZnCr,O, and ZnO. This work was performed under a contract from Progetto Finalizzato Energetica 11. Thanks are due to Dr N. Ferlazzo and to Prof. M. Zocchi for stimulating discussion and criticism. References 1 G. Natta in Catalysis, ed. P. H . Emmet (Reinhold, New York, 1953), vol. 111, chap. 8. 2 G. Natta, U. Colombo and I. Pasquon, in Catalysis, ed. P. H. Emmet (Reinhold, New York, 1953), 3 G.Natta and P. Corradini, Proc. Znt. Symp. React. Solids, Gothenburg, 1952, p. 619. 4 J. E. Germain, J. Bigourd, J. P. Beaufils, B. Gras and L. Pousolle, Bull. SOC. 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Alexander, in X-Ray Diflraction Procedures (Wiley, New York, 1974), p. 531. 15 I. M. Kolthoff and P. J. Elving, in Treatise on Analytical Chemistry (Interscience, New York, 1959). 16 R. A. Nyquist and R. 0. Kogel, in IR Spectra of Inorganic Compounds (Academic Press, New York, 17 C. K. Jnrgensen, in Absorption Spectra and Chemical Bonding in Complexes (Pergamon Press, Oxford, 18 P. Forzatti, C. Cristiani, N. Ferlazzo, L. Lietti, E. Tronconi, P. L. Villa and I. Pasquon, J. Catal., 1988, 19 J. Preudhomme and P. Tarte, Spectrochim. Acta, 1971, 27, 1817. 20 P. P. Cord, P. Courtine and G. Pannetier, Spectrochim. Acta, 1972, 28, 1601. 21 F. A. Cotton and G. Wilkinson, in Adoanced Inorganic Chemistry (Interscience, New York, 1966), Faraday Trans. 1, 1987, 83, 2213. Trans. I , 1988, 84, 1405. 1423. 1971). 1962), chap. 13. 111, 120. p. 713. Paper 8/01823E; Received 9th May, 1988