Faraday Discuss. Chem. SOC., 1989, 87, 107-120 Copper-Zinc Oxide Catalysts Activity in Relation to Precursor Structure and Morphology David Waller, Diane Stirling and Frank S. Stone* School of Chemistry, University of Bath, Bath BA2 7AY Michael S. Spencer? ICI Chemicals & Polymers Ltd, Billingham Catalysis Centre, Clerieland TS23 1 L B Cu-Zn hydroxycarbonates have been studied as precursors of Cu-ZnO catalysts, with particular reference to the effect on catalyst activity of ageing the precursor prior to decomposition and reduction. The precursor obtained by precipitation from mixed nitrate solution (Cu/Zn molar ratio 2 : 1 ) at 333 K and pH 7.0 consisted of zincian malachite (Cu/Zn = 8.5: 1.5) and aurichalcite. The precursor was aged in the mother liquor at 333 K for various times.Characterisation by XRD, i.r., DTA, electron microscopy, EDAX and XPS showed that ageing led to loss of the aurichalcite and production of a more finely divided copper-enriched (Cu/Zn = 2 : 1) malachite phase. The unaged precursor yielded a catalyst of low activity for both methanol synthesis (studied at 50 bar and at 1 bar) and the reverse water-gas shift reaction. The aged precursor gave catalysts of much higher activity for both reactions. Increased ageing did not change the selectivity ratio for methanol synthesis cs. reverse shift in the CO,+ H, reaction at normal pressure. The success of the copper-zinc oxide/alumina catalyst for low-pressure (50- 100 bar) methanol synthesis has prompted a great deal of fundamental work on this reaction during the past ten years.The interest was largely triggered by an outstanding series of papers by Klier and co-workers, beginning in 1979.'-3 The combined effect has been to generate a dialogue unmatched for many years in catalysis. As amply documented in several authoritative controversial issues include both the mechanism of the reaction and the nature of the active sites on the catalyst. The principal active site for methanol synthesis over copper-zinc oxide is widely regarded as being a copper centre. There is controversy as to its environment and oxidation state variation during catalysis. Herman er al.' stimulated much interest with their proposal that isolated Cu' in solid solution in the surface of zinc oxide was the key element. However, the opinion of the ICI groupsX.' is that the active copper site is on a particle of copper metal.The activity of copper-zinc oxide is known to depend upon the catalyst precursor. This is the aspect on which the present work is focussed. The significance of characteris- ing the hydroxycarbonate precursor phases of Cu-ZnO catalysts was first highlighted by Herman et al.,' and particular attention has since been accorded to malachite and aurichalcite.'".'' Of the more detailed studies, the most recent are those of Porta et al., ' 2 3 ' 3 with which our own work has partly overlapped. Copper-zinc hydroxycarbonate precursors have been investigated at ten different Cu/Zn molar ratios from 100:O to 0: 100 and the phase analysis results, to be reported elsewhere," agree closely with those i- Present address: School of Chemistry and Applied Chemistry, Uni\ersitc of' U'ales College of C'drditf, Cardiff C F l 3TB.108 Copper- Zinc Oxide Catalysts of Porta et Our work has extended to measurements of the activity for the reverse water-gas shift reaction (CO? + H, -+ CO + H,O) for the Cu-ZnO catalysts derived from these precursors, and we have found that the highest activity is given by a 67:33, i.e.2 : 1, Cu/Zn ratio. On the basis of these results, we have selected the composition Cu/Zn = 2 : 1 for a detailed study of precursor ageing. Ageing of the precursor in the mother liquor after precipitation is often used as a stage in the preparation,” but this does not appear to have been studied systematically hitherto. The characterisation of the precursor can be made precise and the work follows through to the genesis of the catalyst and its activity in methanol synthesis under standard industrial conditions at 50 bar.Activity of the catalysts for the synthesis at 1 bar has also been studied, using in this case C02-H2 mixtures, and parallel measurements of reverse shift activity have been made for comparison. Experimental Catalyst Preparation Cu-Zn nitrate solution with [Cu + Zn] = 1.5 mol dm-’ and Cu/Zn = 2 : 1 was pumped simultaneously with 1.5 mol dm -’ Na2C03 solution into a mixing vessel under conditions such that precipitation occurred at 333 K and a constant pH 7.0, followed by a collector vessel in which the stirred suspension could be aged at 333 K for specified times (0, 30, 75, 140 and 205 min, respectively).The slurry was then filtered and washed with de-ionized water until the Na content of the solid was < 160 ppm. The material was dried for 16 h at 363 K. The solid so formed will be referred to as the precursor. The precursor was heated in air at 623 K for 6 h to form the calcined oxide. This was pelletized, crushed and sieved to give a 600-85Opm fraction. The resulting material, reduced in hydrogen at ca. 520 K, constitutes the reduced catal-yst. Catalyst Characterisation ( a ) X-Ray Powder Diflraction Measurements were made with a Siemens Kristalloflex D500 diffractometer using Fe- filtered Co radiation [ h ( K a ) = 1.7902 A] step-scanning at 0.05 O intervals of 28 with a 4 s count at each step. Some samples wer? studied with a Philips diffractometer using Ni-filtered Cu radiation [ h ( K a ) = 1.5418 A].( b ) Electron Microscopy, Electron Diflraction and EDAX JEOL CXlOO and FX200 transmission electron microscopes equipped with LINK EDAX systems were used. Precursor samples were dispersed in ethanol and deposited on a nitrocellulose film supported on the microscope grid. The samples were carbon-coated before examination in the microscope. Copper grids were used for TEM and selected- area electron diffraction. Nylon grids in a Be holder were used for the energy-dispersive X-ray analysis (EDAX). ( c ) I . R. Spectroscopy A Perkin-Elmer 983 spectrometer equipped with a 3500 data station was used. Precursor samples were wet-ground in acetone, and dried on to NaCl plates. ( d ) U. K- Visible- N. I . R.Reflectance Spectra These were obtained with a Perkin-Elmer 330 spectrometer and a reflectance attachment employing BaSO, as standard. Solids were examined as sieved powders.D. Waller, D. Stirling, E S. Stone and M. S. Spencer 109 ( e ) Diflerential Thermal Analysis A DuPont thermograph was used, the ground sample (10 mg) being heated at 10 K min- ’. ( f) Surface Area Determination B.E.T. surface areas ( N2, 77 K) were obtained from isotherms determined gravimetrically with a Cahn RG microbalance. Copper metal areas (reduced catalyst) were determined using N 2 0 decomposition by the following methods. (i) Pulse method: the sample, H,-reduced (520 K, 16 h), cooled to 333 K, flushed in pure He, was subjected to 0.5 cm3 pulses of N20 in He as carrier until no further decomposition was registered.Integrated areas of N2 g.c. peaks were summed. (ii) Frontal analysis: The procedure of Chinchen et al.” (reactive frontal chromatography) was followed, using 333 K as the reaction temperature with 5% N,O in He as the reacting gas. ( g ) X.P.S. A VG Scientific ESCALAB mark I1 spectrometer was used with A1 X-rays (1486.6 eV). Samples were examined as pressed discs having been pre-outgassed at room temperature overnight in the preparation chamber. Binding energies were referred to the C 1s peak ( E , = 284.8 eV). Catalytic Measurements ( a ) High Pressure Activity testing for methanol synthesis was carried out at ICI Billingham, using a high-pressure microreactor rig equipped with on-line mass-spectrometric analysis. A syngas mixture (2.5% C02, 6.0% CO, 63% H2, 28.5% N2) at 50 bar and 513 K was used for the tests.(6) Normal Pressure Activity testing at 1 bar was carried out in a laboratory-built flow system with g.c. analysis. The principal purpose of these experiments was to determine the activity for reverse shift as well as for methanol synthesis. The conversion has therefore been determined for two mixtures, ( i ) 5% C 0 2 , 55% Hz, 40% He and (ii) 10% C 0 2 , 10% H2, 80% He, the latter being more appropriate for reverse shift studies. In each case the procedure was to load the microreactor with 0.5 cm3 of calcined oxide and pre-reduce in 20% HZ, 80% He for 16 h at 508 K. The copper area of the catalyst was then determined in situ by N 2 0 reactive frontal chromatography at 333 K.After re-reduction at 508 K in the H2-He mixture the stream was switched to one of the two above C02-H2-He mixtures. The conversion was determined at various flow rates in order to ascertain whether conversion was proportional to the residence time. A. Characterisation of Precursors and Effect of Ageing ( a ) X - Ray Diflractometry Under the conditions of preparation described above, hydroxycarbonates may be expected as precipitates, notably zinc-containing malachite (‘zincian malachite’) [(Cu,_,,Zn,)CO,(OH),] and aurichalcite [(Cus~,yZn,)(C0,)2(OH),].1~ X-Ray analysis110 Copper- Zinc Oxide Catalysts 10 20 30 40 50 60 7c 80 201 O Fig. 1. X-Ray diffractograms of precursors (Fe-filtered Co K a radiation: A = 1.7902 A). Ageing times (min) in the mother liquor at 333 K: ( a ) 0, ( b ) 30, ( c ) 75, ( d ) 140, ( e ) 205.of our unaged precipitate [Cu/Zn = 2: 13 after drying confirmed that these two phases were present (fig. 1). Referring to reflections which do not overlap in the two phases, the malachite structure is identified by its (020), (120), (200) and (220) reflections at 28 values of 107.09, 20.34, 22.01 and 28.01 O, respectively (d-spacings of 6.023, 5.069, 4.689 and 3.699 A); correspondingly, the presence of aurichalcite is defined by its characteristic reflections (400), (511), (420) and (901) at 28 values of 15.11, 32.22, 35.81 and 39.81 O, respectively ( d = 6.809, 3.226, 2.909 and 2.629 A). There is no evidence for any other phase. The X-ray diffractograms of precursor which had been aged for 30, 75, 140 and 205 min in the mother liquor at 333 K before drying are also shown in fig.1. There are significant changes, namely: (i) disappearance of the aurichalcite pattern; (ii) broadening of the malachite reflections, e.g. (020), (120); (iii) small displacements in the peak positions, e.g. (020) to 28 = 17.01 O ( d = 6.054 A), (120) to 28 = 20.28 O ( d = 5.084 A), (200) to 28 = 22.13 O ( d =4.664 A ) and (220) to 28 = 28.05 O ( d = 3.694 A) as a result of ageing for 140 min. The largest amount of change occurs during the first 30 min of ageing. ( b ) Infrared Spectra The presence of malachite and aurichalcite phases in the unaged precursor, and the absence of other phases, is confirmed by the i.r. results shown in fig. 2. Furthermore, the disappearance of the aurichalcite on ageing is also clear.Note that the bands characteristic of aurichalcite at 1556, 1201 and 971 cm-' (which are at values where malachite does not absorb significantly) are all destroyed on ageing. There is a change in the OH-stretching region at 3300-3500 cm-' (not shown in fig. 2) whereby on ageing from 75 to 205 rnin the doublet which is a characteristic of pure malachite improves in resolution. ( c ) Diflerential Thermal Analysis DTA results for the precursors are shown in fig. 3. Aurichalcite gives a broad lower- temperature decomposition endotherm compared to malachite and in the unaged precur- sor manifests itself merely as a low-temperature tail (shaded region) on the malachiteFaraday Discuss. Chem. SOC., 1989, Vol. 87 Plate 1. Electron micrographs.( a ) Synthetic malachite ( x 94 000); ( b ) unaged precursor ( x 150 000); ( c ) precursor aged for 75 min ( x 250 000); ( d ) selected area electron diffraction pattern of platelet in centre of field in ( b ) . D. Waller, D. Stirling, F. S. Stone and M. S. Spencer (Facing p. 11 1)D. Waller, D. Stirling, F. S. Stone and M. S. Spencer 1 1 1 Fig. 2. 1.r. spectra. M, malachite [Cu2C03(OH)J; A, aurichalcite [CU~Z~,(CO~)~(OH),] (standard synthetic samples); ( a ) - ( e ) precursors aged for 0, 30, 75, 140 and 205 min, respectively. endotherm, peaking at 600 K. The tail is absent after ageing. The most striking feature of fig. 3 is the pronounced shift of the malachite peak to higher temperatures on ageing. ( d ) Electron Microscopy, Electron Diflraction and EDAX At magnifications of the order of lo5, zinc-free malachite prepared under the same conditions as used for the Cu-Zn precursor has the morphology of ill defined crystallites, as shown in the electron micrograph of plate 1( a ) .The unaged Cu-Zn precursor contains very similar crystallites [plate l(b)], but they are accompanied by well defined platelets of entirely different habit. The platelets disappear on ageing. Plate l ( c ) is an electron micrograph of precursor aged for 75 min: the platelets are now much less evident and the dominant feature is the microcrystalline material characteristic of the malachite phase. Bearing in mind the X-ray and i.r. results, this would be consistent with the platelets being aurichalcite. Selected-area electron diffraction [plate 1 (d)] confirmed that the platelets are well crystallized aurichalcite, the pattern being identical with that illustrated for aurichalcite by Himelfarb et a1.l' Energy-dispersive X-ray analysis of selected areas of samples examined by electron microscopy enabled the relative amounts of copper and zinc in the precursor phases to be assessed.Copper and zinc are revealed at similar sensitivity by EDAX.I6 Fig. 4 ( a ) shows the EDAX pattern for an aurichalcite platelet in unaged precursor, showing K a and KP from Cu and Zn. The Cu KP component overlaps the Zn K a peak and appears as a shoulder on the high-energy side. Making allowance for this contribution to the peak height, it follows that the Zn content of the aurichalcite is lower than that of Cu.112 Copper- Zinc Oxide Catalysts 500 600 700 800 T I K Fig.-3.DTA of precursors. Ageing times (min): ( a ) 0, ( b ) 30, ( c ) 75, ( d ) 140, ( e ) 205. A c c Y .C m CI .- 7 8 9 1 0 7 8 9 1 0 7 8 9 1 0 energy/ keV Fig. 4. Energy-dispersive analysis of X-rays (EDAX) for precursors. ( a ) Platelet in unaged precursor, ( b ) small particles in unaged precursor, ( c ) small particles in precursor aged for 205 min. Fig. 4(6) shows the pattern for the other phase in the unaged precursor, namely that of a cluster of the small crystallites as seen in plate 1 (6). The presence of Zn is directly established, the Cu/Zn ratio being ca. 85: 15. Fig. 4(c) shows the EDAX pattern from the small crystallites present in aged precursor. The Zn content has clearly increased, and the Cu/Zn ratio is now close to the 2 : 1 value required from the known overallD.Waller, D. Stirling, F. S. Stone and M. S. Spencer A B I 1 I I I 930 940 950 96C 970 binding energy/eV 113 binding energy/eV Fig. 5. XPS of precursors. A. Cu 2 ~ , , ~ and Cu 2p,,* spectra of precursors ( a ) - ( e ) aged for 0, 30, 75, 140 and 205 min, respectively. B. Zn 2p,,, spectra of the precursors. composition. Several such patterns were registered from different regions of the sample, and no significant variation in the 2 : 1 Cu/Zn ratio was found. ( e ) X-Ray Photoelectron Spectroscopy X.p. spectra were measured to obtain information on the Cu/Zn ratio in the surface region of the precursor particles. The Cu 2p,/, and Zn2p,/, spectra for unaged and aged precursors are shown in fig.5 : for Cu the 2p region is also included. The satellite peaks for Cu confirm the oxidation state as Cd‘f The surface ratio (Cu/Zn),,, was obtained from the respective intensities of the 2p,/, signals, taking account of both the main and satellite peaks for Cu and noting the sensitivity factor of 0.88 recommended by Wagner et al.:” (Cu/zn)xPs = [ W U 2p,,*)/ Wn2p,,2)1(1/0.88). ( 1 ) The unaged precursor (mixture of zincian malachite and aurichalcite) yielded a (Cu/Zn),,, ratio of 2.6, whilst the aged precursor gave values of 2.0, 2.0, 2.1 and 2.2 for ageing times of 30, 75, 140 and 205 min, respectively. The value for the unaged precursor is not likely to be representative since the two phases present have such different morphology [plate 1 (b)]. The aged precursor (increasingly less aurichalcite) is increasingly free from this problem, and it is significant that for all of these the ratio is close to 2 : 1, the overall composition.The binding energy values and the actual profiles show no particular trend with ageing.114 Copper- Zinc Oxide Catalysts Table 1. Surface areas of calcined oxides and of reduced catalysts derived from unaged and aged precursors precursor ageing surface area of copper surface area of time/ min calcined oxide/m' g-' reduced catalyst/m' g-' 0 30 75 140 205 43 83 65 55 59 13 27 24 25 23 B. Characterisation of Calcined and Reduced Precursor The main aim of the present study is the identification of the changes occurring on precursor ageing and the effect on ultimate activity. Characterisation of the calcined and reduced precursor was therefore limited to XRD analysis, u.v.-visible-n.i.r.diffuse reflectance spectroscopy and surface area determination. The unaged precursor and also samples of the precursor aged for 30, 75, 140 and 205 min as previously described were converted to calcined oxide by heating at 623 K for 6 h in air. X-Ray diffraction of the products showed the presence of CuO and ZnO, but no other phase. The oxide from aged precursor showed greater line-broadening than that from the unaged precursor, signifying smaller particle size. The effect of ageing in producing a more finely divided calcined oxide was confirmed by surface area measurements (table 1). Interestingly, the 30 min aged precursor gave oxide with the highest specific surface area, almost twice that from the unaged material.The high copper content (Cu/Zn = 2 : 1 ) necessarily resulted in the calcined oxide being very black. However, even without dilution with inert material, u.v.-visible-n.i.r. DRS readily revealed the CuO absorption edge near 850 nm. There was no absorption at 1400-1500 nm, the region characteristic of Cu" in tetrahedral coordination in Zn0.I8 No spectral difference between oxide from unaged and aged precursor could be discerned. Calcined oxide was treated at ca. 520 K in H7 to give the reduced Cu/ZnO used for activity tests. The only significant new characterisation required was the copper surface area. To observe the effect on the metal surface area of ageing the precursor from which the catalyst was derived, the N 2 0 pulse method was applied to the respective samples immediately after reduction under standard conditions.The resulting surface areas are shown in table 1, from which it is seen that the surface area variation observed for calcined oxide is reflected in the copper area of reduced catalyst. C. Catalytic Activity for High pressure Methanol Synthesis The activities of the five reduced Cu/ZnO catalysts listed in table 1 were compared to that of a standard ICI methanol synthesis catalyst (Cu/ZnO/A1,03) and relative activities were determined per unit weight of catalyst. Results are shown in fig. 6. There is a striking increase in activity as a result of deriving the catalyst from a precursor which has been aged. Beyond 30min ageing the activity depends very little on ageing time, the variations being close to those expected for an activity which is proportional to copper surface area.The unaged precursor, by contrast, yields a catalyst whose activity is well below expectations on that basis. D. Catalytic Activity for CO, Conversion at Normal Pressure Substantial evidence has now that the route to methanol synthesis is via C02 hydrogenation. It was of interest, therefore, to study both methanol synthesisD. Waller, D. Stirling, F. S. Stone and M. S. Spencer 1 .o L fi v 0,5 - - I 1 1 I - 0 115 Fig. 6. Relative activity in high-pressure methanol synthesis at 50 bar and 513 K for reduced catalysts as a function of precursor ageing time: data refer to activity after 3 h on stream. 1.0( 0.5 0 0 0.05 0.10 0.15 0.20 0.25 I/(space velocity)- I / s Fig.7. Methanol formation as a function of reciprocal space velocity for reaction of C 0 2 and H2 (C02/H2/He = 1 : 11 : 8) at 453 K and 1 bar for catalysts derived from unaged precursor (a) and precursor aged for different times, uiz. V, 30 min; 0, 75 min; 0, 140 min; A, 205 min.116 Copper- Zinc Oxide Catalysts 0.05 0.10 015 0.20 0.25 (space velocity)-'/s Fig. 8. CO formation by the reverse water-gas shift readion as a function of reciprocal space velocity for reaction of C 0 2 and H2 (C02/H,/He = 1 : 11 : 8) at 453 K and 1 bar. Symbols as for fig. 7. and the reverse shift reaction using C02+ H2 as reactants, and to examine in each case the effect of precursor ageing on the activity of the subsequently derived catalyst.The same set of prepared solids was used as for the high-pressure studies, the samples being loaded into the reactor as calcined oxide and reduced in situ, as described in the Experimental section. Temperature, C02/H2 ratio and flow rate (space velocity) were selected such that it was possible to measure the activity for methanol synthesis and reverse shift in one and the same experiment. Under our experimental conditions, the rate of the reverse shift reaction is not limited by equilibrium approach below 500 K. By contrast, equilibrium for the methanol synthesis reaction C0,+3H, S CH,OH+H,O at normal pressure is achieved at ca. 473 K, the methanol yield being ca. 0.2%. It is therefore necessary to go to lower temperatures in order to measure the rate of methanol formation under conditions which are not equilibrium-limited.However, the problem of sensitivity soon becomes acute: in our case the methanol yield fell below the limit of detectability of the analysis system below 433 K. The temperature of 453 K was therefore chosen as a good compromise. The reaction mixture used was CO,/H,/He = 1:11:8. The basis of the comparison between catalysts for methanol synthesis activity was the determination of an effective rate constant given by the slope of the plot of CH30H concentration in the effluent gas vs. reciprocal space velocity. The results obtained are shown in fig. 7. The catalyst derived from unaged precursor has extremely low activity for methanol synthesis under these conditions. An optimum activity is reached for catalyst produced from precursor aged for 140 min.D.Waller, D. Stirling, E S. Stone and M. S. Spencer 117 0 0.2 0 0.LO 0.60 (space velocity)- ' / s Fig. 9. CO formation by the reverse water-gas shift reaction as a function of reciprocal space velocity for reaction of C02 and H2 (CO,/H,/He = 1 : 1 : 8) at 473 K and 1 bar. Symbols as for fig. 7. The corresponding results for conversion to CO (the reverse shift reaction) in the C02/H2/He 1 : 11 : 8 mixture at 453 K are shown in fig. 8. The maximum activity is attained also in this case for catalyst derived from the precursor which had been aged for 140min. As for methanol synthesis, the unaged precursor gives a catalyst of correspondingly poor activity. Finally, the reverse shift reaction was studied per se using an equimolar C02/H2 mixture (C02/H2/He = 1 : 1 : 8).Fig. 9 shows results obtained at 473 K. The plots of effluent CO concentration us. reciprocal space velocity showed some curvature in this case, but the characteristics were again similar with regard to the relative activities of the five catalysts. The copper surface areas of the catalysts were determined after reduction but prior to the above experiments by the method of N 2 0 frontal analysis using the same flow system as was employed for the catalysis. The specific areas obtained were 15, 26, 27, 21 and 20 m2 g-' for the reduced catalysts whose precursors had been aged at 0, 30, 75, 140 and 205 min, respectively. The values for the last two catalysts are lower than expected (cJ table 1). Discussion Structure and Morphology of the Catalyst Precursor As initially formed, the hydroxycarbonate precursor is made up of two phases, one with malachite structure and the other aurichalcite (fig.1). The d-spacings of the malachite phase show that it contains both copper and zinc. The values are shifted from those of118 Copper- Zinc Oxide Catalysts laboratory-prepared pure-Cu malachite, for which Porta et a1.’* cite = 5.993 A, d,,, = 5.055 A, dzoo = 4.699 A and d,,, = 3.693 A, in the direction of decreased unit-cell size and by amounts which imply 10-20% Zn in solid solution. The effect of ageing the precursor in the mother liquor at 333 K is rapid disappearance of the X-ray reflections of aurichalcite (fig. l ) , leaving a matured malachite phase with d -spacings suggesting a Zn content increased to 30-40%, which is the solubility limit.12714 The i.r. data (fig. 2) establish that loss of the aurichalcite reflections is not due to fragmentation or deterioration in the quality of the crystals, but is a genuine destruction of the phase. The SOH band at 879cm-I in the Cu-malachite reference spectrum is displaced to 863 cm-’ in the 140 min aged precursor: this band is sensitive to Zn content14 and indicates [Zn] > 20%, consistent with the X-ray results. The peak temperature of the DTA endotherm (fig. 3) also reflects the presence of zinc in the malachite phase. The work of Porta et a1.,I2 confirmed by our own has shown that the malachite endotherm (ca. 575 K in Cu malachite) moves up in temperature on incorporating Zn.Even more significant is the continuing trend with sharpening of the peak. We interpret this as evidence not only that the Zn content increases on ageing but also that Zn confers a higher lattice energy. We recall here the parallel decrease in size of the unit cell and the X.P.S. evidence of Porta et al.” for increased covalency. The electron microscopy and EDAX results (plate 1 and fig. 4) corroborate and extend the above findings. The two phases in the precursor as initially formed are distinguished absolutely (plate 1 ) . The aurichalcite platelets are found by EDAX to have a Cu/Zn ratio rather greater than unity. This is consistent with their Cu content being close to 6O%, the limit found in mineral samples, and that expected if aurichalcite has 60% of its cations in octahedral sites and 40% in tetrahedral sites (by analogy with its close analogue hydrozincite, Zn,( CO,),( OH),), the former being occupied by Cu” and the latter by Zn*+.EDAX (fig. 4) also established that the malachite component of the unaged precursor definitely contains zinc, and moreover that Cu/Zn * 85 : 15, in agreement with XRD. The results illustrated in plate 1 and fig. 4 also provide important insight into the changes induced by ageing. As the relatively zinc-rich aurichalcite platelets disappear, the malachite phase takes on a more microcrystalline appearance [plate 1( c ) at x250 000), consistent with the increased X-ray line-broadening. This speaks against growth of the pre-existing Cu/Zn 85 : 15 crystallites by coating with highly zinc-rich material.The EDAX analysis of the aged malachite microcrystallites, sampled widely, shows the 2 : 1 Cu/Zn ratio consistent with the Zn solubility limit” and with the overall ratio pre-defined by the preparation conditions. The homogeneity of the aged zincian malachite, and confirmation that the particles are not formed of a highly copper-rich core and a highly zinc-rich surrounding is provided by the X.P.S. results (fig. 5 ) . The Cu 2p3,, spectra confirm the oxidation state as CU” in the surface (presence of the satellite and the EB of the main peak at 934.7 eV, invariant with ageing). More importantly, however, the Cu/Zn ratio for the surface region of the aged particles, as derived from the Cu 2p3,, and Zn 2p,/, intensities, is close to 2: 1 , the value which EDAX shows is the overall ratio for the particles.The conclusions about the precursor may now be summarised. Starting from the Cu/Zn 2 : 1 aqueous solution, malachite (Cu/Zn = 85 : 15) and aurichalcite (Cu/Zn = 60:40) are rapidly precipitated. Ageing at 333 K causes solution of the aurichalcite simultaneously with nucleation of Cu/Zn 67 : 33 (i.e. 2 : 1 j malachite, or possibly deposi- tion of this material on the smallest of the 85 : 15 malachite crystallites. This is accom- panied by growth of the 2 : 1 malachite (high lattice energy) as stable small crystallites at the expense of the less stable and significantly larger 85 : 15 crystallites, the extra zinc being continuously supplied by the dissolving aurichalcite, the structure which has the least stability (fig.3).D. Waller, D. Stirling, F. S. Stone and M. S. Spencer 119 On conversion to oxide, the ageing effect manifests itself as textural promotion (decrease in particle size, increase in surface area) and this follows through to reduced catalyst in respect of Cu area (table 1). The short ageing (30min) shows the largest effect on the basis of surface area. However, the real test is catalyst activity, and we address this in the next section. Effect of Precursor Ageing on Ultimate Catalyst Activity The most important single result to emerge is the very poor activity of catalyst derived from unaged precursor, whether for methanol synthesis (fig. 6 and 71, or reverse shift (fig. 8 and 9). It is not simply a matter of smaller copper area: the area-normalised activity relative to catalyst from aged precursor is lower.Thus aurichalcite is an undesired precursor component. Its presence in the precursor affects the normal pressure methanol synthesis (fig. 7) more than the high-pressure synthesis (fig. 6), possibly because of a lowered propensity for keeping copper sufficiently reduced in the steady state. By contrast, precursor consisting solely of a malachite phase which has been stabilised by having zinc incorporated to the solubility limit (the aged precursor) leads to a catalyst of very high relative activity (fig. 6), close to that of standard commercial catalyst. Ageing for more than 30 min has little effect on steady-state activity. The advantage of longer ageing shows up in the activity and selectivity studies for C 0 2 conversion at 1 atmf (fig.7-9). The maximum activity is not observed until the precursor has been aged for 75min or more. The linear plots of fig. 7 and 8 enable direct comparisons of the rates of methanol synthesis and CO formation to be made. The ratio of the rate constants (slopes of the plots) shows the reverse shift reaction to be ca. three times as fast as methanol synthesis, in spite of the unfavourable 1 : 1 1 C02/ H2 ratio for the former. The reactions proceed by different mechanisms,2’ but it is interesting that the pattern of the effect of precursor ageing is the same for both, and also for the reverse shift under conventional 1 : 1 COz/H2 conditions (fig. 9). The dissociative adsorption of C 0 2 , yielding oxidised copper, may be the key to this.The precursor effect on the catalytic reactions does not follow expectations based solely on the copper surface area. The activity for reverse shift is a better guide to methanol synthesis activity than the N,O-determined copper area. The authors acknowledge the support of this work by the S.E.R.C. and by ICI Chemicals & Polymers, Ltd. References 1 R. G. Herman, K. Klier, G. W. Simmons, B. P. Finn, J. B. Bulko and T. P. Kobylinski, J . Catal., 1979, 56, 407. 2 J . B. Bulko, R. G. Herman, K. Klier and G. W. Simmons, J. Phjvs. Chew., 1979, 83, 3118. 3 S. Mehta, G. W. Simmons, K. Klier and R. G. Herman, J. Catal., 1979, 57, 339. 4 H. H. Kung, Catal. Rev. Sci. Eng., 1980, 22, 235. 5 K. Klier, Adu. Catul., 1982, 31, 243. 6 K. Klier, App. Sucf Sci., 1984, 19, 267. 7 G. C . Chinchen, P. J. Denny, J. R. Jennings, M. S. Spencer and K. C . Waugh, Appl. Catal., 1988, 36, 8 S. P. S. Andrew, 7th Int. Congr. Catal., Post-Congr. Symp., Osaka, paper 12, 1980. 9 G. C . Chinchen, K. C. Waugh and D. A. Whan, Appl. Cutal., 1986, 25, 101. 1. 10 P. B. Himelfarb, G. W. Simmons, K. Klier and R. G. Herman, J. Caral., 1985, 93, 442. 1 I M. H . Stacey and M. D. Shannon, in Reactivity of Solids, ed. P. Rarret and L. Dufour (Elsevier, Amsterdam, 1985). p. 713. 12 P. Porta, S. D e Rossi, G. Ferraris, M. Lo Jacono, G. Minelli and G. Moretti, J. Caral., 1988, 109, 367. 13 P. Porta, G. Fierro, M. Lo Jacono a n d G. Moretti, Catal. Today, 1988, 2, 675. -t 1 atm = 101 325 Pa.120 Copper- Zinc Oxide Catalysts 14 F. S. Stone and D. Waller, to be published. 15 G. C. Chinchen, C. M. Hay, H. D. Vandervell and K. C. Waugh, J. Catal., 1987, 103, 79. 16 G. Cliff and G. W. Lorimer, Roc. 5th Eur. Congr. Electron Microscopy (Inst. Physics, London, 1972), 17 C. D. Wagner, L. E. Davis, M. V. Zeller, J. A. Taylor, R. H. Raymond and L. H. Gale, SurJ Interface 18 F. H. Chapple and F. S. Stone, Proc. Br. Ceram. SOL:, 1964, I , 45. 19 G. Liu, D. Willcox, M. Garland and H. H. Kung, J. Catal., 1985, 96, 251. 20 G. C. Chinchen, P. J. Denny, D. G. Parker, M. S. Spencer and D. A. Whan, Appl. Caral., 1987,30, 3 3 3 . 21 G. C. Chinchen, M. S. Spencer, K. C. Waugh and D. A. Whan, J. Chem. SOC., Faraday Trans. 1, 1987, p. 140. Anal., 1981, 3, 211. 83. 2193. Paper 9/004761; Received 27th January, 1989