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Preparation and characterisation of cobalt containing layered double hypdroxides
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
PREPARATION AND CHARACTERISATION CONTAINING LAYERED DOUBLE HYDROXIDES
903
OF
COBALT
S.Kannan and C.S.Swamy Department of Chemistry, Indian Institute of Technology, Madras - 600 036, INDIA. SUMMARY A series of hydrotalcites of general formula Co2+-M3+-COs-HT (M s+ = A1,Fe and Cr) are prepared by coprecipitation technique. The influence of parameters such as preparation method, atomic ratio, supersaturation levels, aging and hydrothermal treatments are investigated to study their effect on the structure and texture of these materials. The obtained materials are characterised by X-ray diffraction, FT-IR studies, thermogravimetry-differential scanning calorimetry, transmission electron microscopy and BET surface area measurements. Thermal calcination of these materials resulted in the formation of high surface area non-stoichiometric spinels whose catalytic activity is studied using N20 decomposition reaction as the test reaction. The order of activity observed is Co-A1-COs-HT>Co-Fe-COs-HT>Co-Cr-CO 3HT. INTRODUCTION Layered double hydroxides commonly referred as hydrotalcite-like (HT-like) materials, consists of brucite-like (Mg(OH) 2) network, wherein the divalent ion is substituted by trivalent ion whose excess positive charge is compensated by anions, usually carbonate, which occupy the interlayer positions [1-5]. They are represented by the general formula
[M(II)I.xM(III)x(OH)2]x+Ax/nn'.mH20 where M(II) and M(III) are divalent and trivalent ions and A is the interlayer anion where water of crystallisation also finds a place. The physico-chemical properties of these materials are mainly characterised by the nature of metal ions and their composition [6,7]. Although a wide spectrum of metal ions have been incorporated into the network [8], reports available on cobalt containing hydrotalcites are scarce [9]. Thermal calcination of these materials resulted in the formation of stable, high surface area and non-stoichiometric mixed metal oxides employed in many catalytic transformations like steam reforming, methanol synthesis, higher alcohol synthesis
904 and N20 decomposition [10-12]. The physicochemical properties of these unusual solids are entirely different from the solids obtained by conventional ceramic routes. The synthesis of such materials for a desired catalytic reaction is the prime objective of solid state chemistry and catalysis [13]. The objective of the present investigation is to study the change in structure, stability and reactivity of cobalt containing hydrotalcites with various trivalent metal ions as a function of preparation methods and composition and characterising their thermally calcined products. EXPERIMENTAL The HT-like compounds are prepared by sequential precipitation wherein NaOH/Na2CO 3 mixture is added to the metal nitrate solutions at room temperature with increasing pH. These compounds are also prepared by coprecipitation under low supersaturation conditions wherein both the precipitants and metal nitrates are added simultaneously holding the pH between 9-10. The final pH of the solution was kept at 10 in both the cases. The slurry obtained is aged at 65~ for 24h, filtered, washed thoroughly with distilled water and dried at 80~ overnight. Hydrothermal treatments are performed at 110~ for 2 days in a teflon autoclave under autogenous conditions. The chemical compositions of these materials are determined by inductively coupled plasma emission spectrometry (Model 3410, ARL7) by dissolving the compounds in minimum amount of hydrochloric acid. X-ray diffraction of these samples are taken in Philips X-ray generator (Model PW 1330) using CoK a radiation (k = 1.7902A). The lattice parameters are calculated using least square fitting of the peaks mainly considering the peaks whose 2{}>40~ IR absorption spectra are recorded using FT-IR spectrometer (Perkin-Elmer Model 1760) in the form of KBr discs. TGDSC studies are carried out in Perkin-Elmer TG-DSC/7 at the heating rate of 10~ under nitrogen atmosphere. Surface area measurements, using BET method of adsorption of N 2 at 77K, are carried out in Carlo Erba Model 1800 automatic sorptometer. Catalytic tests are carried out in an all glass recirculatory static reactor. About lg of the precursor namely the hydrotalcite is employed for the catalytic studies. Thermal calcination of the material was done in vacuum to generate "in situ" mixed metal oxides which are active catalysts. The decomposition of N20 was carried out at 50 torr initial pressure of the gas in the temperature range 150~176 The details regarding the activation procedure is mentioned elsewhere [14]. R E S U L T S AND D I S C U S S I O N Table-1 shows the composition and the phase obtained for the various samples synthesised. The closeness in the values between calculated and measured composition indicates the completion of precipitation. XRD of the samples showed the single phase formation of liT-like phase exhibiting sharp and symmetric reflections
905
Table 1 Composition and phase obtained of the samples synthesised Sample Code
Composition
Preparation Method
Cat A Co-A1 Sequential Cat B Co-A1 Sequential Cat C Co-A1 Sequential Cat Dd Co-A1 Sequential Cat E Co-A1 Low super a Cat F Co-Fe Sequential Cat G Co-Fe Sequential Cat H Co-Fe Low super Cat I Co-Fe Low super Cat J Co-Cr Sequential Cat K Co-Cr Sequential Cat L Co-Cr Low super Cat M Co-Cr Low super a - Low supersaturation preparation method b - Calculated c - Observed d - Hydrothermally treated e - not determined f - HT + possibly hexagonal Co(OH) 2
M2+/M 3+ atomic ratio 2.0 b 2.5 3.0 n. e 3.0 2.0 3.0 2.0 3.0 2.0 3.0 2.0 3.0
2.0 c 2.5 3.0 3.0 1.6 2.8 2.3 3.6 1.9 2.8 1.9 3.4
Phase obtained HT HT HT HT HT HT f HT HT HT amorphous amorphous vw HT HT
for (003), (006), (110) and (113) planes and broad and asymmetric reflections for (102), (105) and (108) planes characteristic of clay minerals possessing layered structure. These materials have a rhombohedral 3Rm symmetry with a and c unit cell parameters calculated by least square fitting of the peaks. The lattice parameters and surface area of some of the hydrotalcites are given in Table-2. It can be inferred that the difference in the values of unit cell volume of Co-A1, Co-Fe and Co-Cr-COs-HTlcs are in good agreement with the ionic radii of the trivalent element [15]. Comparison of Cat A and Cat C indicated that as the composition increases (Co/A1 atomic ratio) both the lattice parameters increases with consequent increase in the unit cell volume. The increase in the lattice parameter 'a' can be attributed to higher ionic radii of Co2§ (0.74A) in comparison with A18+ (0.50A) and increase in 'c' parameter is due to reduced electrostatic interaction between the layer and the interlayer network. The preparation method significantly influence the crystallinity of the materials synthesised. Compounds synthesised under low supersaturation (LS) conditions are more crystalline than by sequential precipitation (SP). Furthermore, the crystallinity also s with increase in atomic r a t i o . Hydrothermal treatments increased the crystallinity of the material. This result is corroborated with the reduction in the surface area of the hydrothermally treated samples. However, hydrothermal treatments performed on Co-Fe-CO3-HTlcs resulted in the
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F T - I R spectra of the samples synthesised.
907 spinel formation indicating its thermal instability under these conditions. shows the XRD patterns of various samples synthesised.
Fig.1
Table 2 Unit cell parameters and surface area of the samples synthesised Sample Code Cat Cat Cat Cat Cat Cat Cat Cat Cat Cat
A B C D E G H I L M
Unit cell parameters a (A)
c (A)
V (A 3)
3.073 3.077 3.080 3.084 3.078 3.129 3.108 3.111 3.107 3.117
22.782 22.955 23.091 23.227 23.304 22.811 22.523 22.682 22.923 22.885
186.3 188.2 189.7 191.3 191.2 193.4 188.3 190.1 191.6 192.5
Surface area (m2/g) 64.8 27.7 69.4 42.5 35.0 88.7 60.7 72.2 250.7 155.6
The crystallinity of the material is also dependent on the nature of the trivalent metal ion present in the network. Co-AI-CO3-HT are more crystalline in comparison with Fe and Cr containing samples. In the case of Co-Cr-CO3-HT, preparation by sequential precipitation yielded amorphous material whereas preparation under low supersaturation resulted in a better crystalline material. In our Co-Fe containing samples, it is not completely possible to exclude the presence of hexagonal Co(OH) 2 prep_ared under sequential precipitation. These results indicated that presence of Al 3+ favours the formation of crystalline HTlc phase which is in accordance with the results observed by Clause et al for nickel containing hydrotalcites [7]. T E M results showed spherical to hexagonal platelets of thin and wide nature characteristic of these materials [3]. FT-IR absorption spectra of these materials, given in Fig.2, showed prominent bands around 3400cm "1,1630cm 1 and 1370cm" 1 corresponding to vOH stretching, vOH bending and v 3 carbonate stretching respectively, confirming the presence ofhydroxycarbonates. Absence of band around 3650cm'~indicates that all O H groups in the structure are hydrogen bonded and no free hydroxyl group is present [2]. Bands v 2 (out of plane deformation) and v 4 (in plane bending) of carbonate are observed around 870 and 680cm "I respectively. Differences noticed for all bands between observed vibrations of carbonate and free carbonate anion indicates even perturbation of anion in the interlayer. Bands observed at less than 1000cm "I are attributed to the lattice vibrations like M - O stretching and M - O - M bending vibrations [16]. A sharp band around 1600cm "I is observed for Fe and Cr containing samples suggests that it is not only due to water bending vibrations and but also due to the presence of bicarbonate anions [17].
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160 310 /,60 Temperature ('C) DSC traces of some of the hydrotalcites.
909 Cat C exhibited a doublet at 1380 and 1365cm "1 for v 3 stretching of carbonate, which can be attributed to lower symmetry of carbonate present in the interlayer (D3h symmetry distorted to C2v). This can also cause activation in v I mode observed around 1020cm "1 [18]. However, upon hydrothermal treatment (Cat D) a singl_e sharp band around 1365cm "1 is observed, indicating the enhanced ordering of CO82" ion in the interlayer. Nevertheless, this value is very much lower than that of free CO82" species [19] (1415cm'1), indicating that a strong electrostatic interaction exists between hydroxyl group and H20 molecules in the interlayer with carbonate species. In the case of Co-Fe and Co-Cr containing samples such doublets are observed even ai%er h~drothermal treatments, which clearly shows that a large degree of disorder of CO8~" species in the interlayer. This result is corroborated with X-ray results exhibiting low crystallinity of these samples. A closer examination of vOH band for aged samples indicated that as the M2+]M3+ ratio decreases (compare Cat A and Cat C) the band is shifted to lower wave numbers. This shift could be due to depletion of electron density around OH group bonded to A13+ ion by polarisation. Such shifts were not observed for Co-Fe and CoCr containing samples indicating the weak polaris~bility of Fe 8+ and Cr 8+ in comparison with A1~+. Fig.3 shows TG and its differential curve for some of the hydrotalcites. Most of the samples showed two stages of weight loss wherein the first weight loss occurring in the temperature range 150-250~ is attributed to the removal of physisorbed and interlayer water molecules and the second weight loss occurring in the temperature range 250-350~ ascribed for dehydroxylation between the sheets and decarbonation (loss of CO 2) leading to the destruction of the layered structure [20]. However, Cat C showed the second weight loss occurring in two stages. The first can be assigned for partial dehydroxylation within the layer and second is due to complete dehydroxylation and decarbonation[21]. In the case of Fe containing HTs irrespective of the preparation conditions, the release of interlayer water, structural water and CO 2 occur simultaneously in the temperature range 150-200~ However, for Co-Cr-HTs a better thermal stability is achieved for the samples prepared under LS conditions in comparison with SP conditions, owing to the better crystallinity of the former sample. For hydrothermally treated materials, the second weight loss splits into two peaks without affecting T 1. This could be due to better ordering in the interlayer space leading to step wise losses. Marchi et al [22] proposed that the presence of new peaks in the hydrothermally treated sample can be attributed to heterogeneity of the precipitate obtained. However, our X-ray results showed that the samples are more crystalline and single phase in nature. The net weight loss and transition temperatures for some of the samples are reported in Table-3. It can be clearly seen from the Table that as M 2+/M~+ ratio increases the transition temperatures T~ and To decreases. This can be explained by considering that upon M 3+ substitution ~by M 2~ in the network, the positive charge density of the layer increases and thus enhancing the electrostatic interaction between the layer and interlayer. DSC results, given in Fig.4, substantiated TG results showing two endotherms corresponding to two weight losses. The DSC transition temperature, although slightly higher than TG temperatures, showed a
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911 Table-3 TG transition temperature and net weight loss of the samples
Sample
Cat Cat Cat Cat Cat Cat Cat Cat Cat Cat Cat
A B C D E F G H I L M
Transition temperature (~ T1
T2
198 190 171 179 179 171 167 173 163 185 153
274 252,300 245 232,290 235 222 201 244 206 268 256
Net weight loss (%)
33.0 30.9 29.7 29.2 29.8 28.7 23.3 24.0 26.7 31.3 26.2
similar trend in the temperature which decreases with increase in M2+/M 3+ atomic ratio. Comparison of DSC curves of aged and hydrothermally treated samples showed an interesting observation that the curves are more intense for hydrothermally treated samples which is indicative of the higher crystallinity.
Characterisation of thermally calcined catalysts: Thermal calcination of these materials at 400~ in air are given in Fig.5. All the compounds showed non-stoichiometric spinel phase independent of M 3+ ion. The non-stoichiometry can be explained by the differences in the values obtained in lattice parameters and IR band positions between calcined catalysts and stoichiometric compounds. In the case of Co-A1-HTs', as the atomic ratio increases, more amount of Co304 is formed (by oxidation of Co2+). The band position shifts to higher frequency as the Co/A1 ratio increases indicative of the formation of solid solution. This result is substantiated by X-ray results showing that the lattice parameters calculated are intermediate between CoA120 4 and Co30 4 (8.105A and 8.084A). Similar behaviour is observed for Co-Cr and Co-Fe systems on variation with elemental composition. These results are in accordance with the results obtained by Busca et al [23] and Uzunova et al [24] respectively. However, detailed study on thermally calcined materials will be published elsewhere. Fig.6 shows the variation of X-ray pattern of Cat B with calcination temperature. As the temperature increases, the crystallinity of the obtained spinel increases as evidenced from the increase in the intensity and sharpness of the peaks. The lattice parameter calculated for these materials showed that as the temperature increases, approach of the stoichiometry is achieved. This result can be substantiated with surface area measurements (Fig.7), which decreases with increase in the
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913 calcination temperature. However, an initialincrease in the surface area is observed which could be due to 'crater'formation as observed by T E M [25],through which C O 2 exit.
Catalytic activity of N20 decomposition: Catalytic activity of N20 decomposition on various t h e m a l l y calcined catalysts are reported in Fig.8. The activity followed the order Co-A1-HT>Co-Fe-HT>>Co-CrHT. The low activity of Co-Cr catalysts can be attributed to low activity of n-type Cr20 3. Generally for N20 decomposition p-type oxides are more active t h a n n-type oxides owing to the cumulative adsorption in the former [26]. We presume that Cr ~+ segregates on the surface in Co-Cr catalysts, and thereby covering the active cobalt sites. Such segregation is also observed for Co-Fe-HT, although the extent of is small [24]. In the synthesis of hydrocarbons on Co-Cr catalysts, it is reported that even with high Co contents, a low activity is observed indicating that Co alone is not responsible for the activity [27]. With increase in the Co/A1 atomic ratio (compare CatA and Cat C), an increase in the activity is observed which could be explained by the fact that the number of adsorption centers increases on the surface. Our XPS results also confirmed that the Co/A1 surface composition increases with increase in the bulk composition although the former is lower in value. The activity of the present samples are compared with some of the most active catalysts reported in the literature [28]. Under our experimental conditions, Co-A1-HTs' showed appreciable conversion even at 150~ and Cat C showed 100% conversion at 250~ which is 100~ less than the most active catalyst reported so far. Analysis of the spent catalysts by XRD showed nonstoichiometric spinel type oxides. The high activity for Co-A1-HTs' can be explained based on the specific interaction between Co 2+ and Co 3+ ion with the support responsible for the generation of the active sites. However, a detailed study is required in understanding the nature of active centers. CONCLUSIONS The effect of preparation procedure, nature of trivalent metal ion and elemental composition on various physicochemical properties are summarised as follows: a. Compounds prepared under the composition range 2.0
914 ACKNOWLEDGEMENTS
The authors thank Air Products Chemicals Inc.,U.S.A for the research grant REFERENCES I,
2. 3. 4. 5. ,
7. ,
9. 10.
11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.
R.Allmann, Acta Crysallogr. 24 (1968) 972. S.Miyata, Clays Clay Miner., 23 (1975) 369. W.T.Reichle, Solid State Ionics, 22 (1986) 135. K.A.Corrado, A.Kostapapas and S.L.Suib, Solid State Ionics 26 (1988) 77. T.J.Pinnavaia, NATO ASI Ser., Ser.C (Zeolite Microporous Solids: Synthesis, Structure and Reactivity, 1992) p.91. W.T.Reichle, J. Catal, 94 (1985) 547. O.Clause, M.Gazzano, F.Trifiro,A.Vaccari and L.Zotorski, Appl. Catal., 73 (1991) 217. F.Cavani, F.Trifiro and A.Vaccari, Catal. Today, 11 (1991) 173. T.Sato, H.Fujita, T.Endo and M.Shimada, React. Solids, 5 (1988) 219. E.C.Kruissink, L.L.Van Reijen and J.R.H.Ross, J.Chem.Soc., Faraday Trans. I, 77 (1981)665. M.Piemontese, F.Trifiro, A.Vaccari, E.Foresti and M.Gazzano, in G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Prep. Catalysts V, Elsevier, Amsterdam, 1991 p.49. S.Kannan and C.S.Swamy, Appl. Catal.B., 3 (1994) 109. H.Yanagida, Angew. Chem. Int.(Eng) Edn., 27 (1988) 1389. J.Christopher and C.S.Swamy, J.Mol.Catal., 62 (1990) 69. R.D.Shannon and C.T.Prewitt, Acta CrystaUogr., B25 (1969) 925. M.J.Hernandez-Moreno, M_A.Ulibarri,J.L.Rendon and C.J. Serna, Phys. Chem. Miner., 12 (1985) 34. L.Pesic, S.Salipurovic, V.Markovic, D.Vucelic, W.Kagunya and W.Jones, J. Mater. Chem., 2 (1992) 1069. F.M.Labojos, V.Rives and M.A.Ulibarri, J.Mat.Sci., 27 (1992) 1546. K.Nakamoto, Infrared and R a m a n Spectra of Inorganic and Coordination Compounds, 3rd ed.,Wiley, N e w York, 1978. S.Kannan and C.S.Swamy, J.Mat.Sci.Lett., 11 (1992) 1585. F.Rey and V.Fornes, J.Chem.Soc., Faraday Trans. 88 (1992) 2233. A.J.Marchi, J.I.Di Cosimo and C.R.Apesteguia, in M.J. Phillips and M. Ternan (Editors), Proc. 9th Int. Congress on Catalysis, Ottawa, 1988, Vol.2, p.529. G.Busca, F.Trifiro and A.Vaccari, Langrnuir, 6 (1990) 1440 E.Uzunova, D.Klissurski, I.Mitov and P.Stefanov, Chem. Mater., 5 (1993) 576. W.T.Reichle, S.Y.Kang and D.S.Everhardt, J.Catal., 101 (1986) 352. P.Pomonis and J.C.Vickerman, J.Catal., 90 (1984) 305. F.Trifiro and A.Vaccari, in R.K. Grasselli and A.W. Sleight (Editors), Structure-ACtivity and Selectivity Relationships in Heterogenous Catalysis, Elsevier, Amsterdam, 1991, p.157. Y.Li and J.N.Armor, Appl. Catal.B., 1 (1992) L21.
PREPARATION AND CHARACTERISATION CONTAINING LAYERED DOUBLE HYDROXIDES
903
OF
COBALT
S.Kannan and C.S.Swamy Department of Chemistry, Indian Institute of Technology, Madras - 600 036, INDIA. SUMMARY A series of hydrotalcites of general formula Co2+-M3+-COs-HT (M s+ = A1,Fe and Cr) are prepared by coprecipitation technique. The influence of parameters such as preparation method, atomic ratio, supersaturation levels, aging and hydrothermal treatments are investigated to study their effect on the structure and texture of these materials. The obtained materials are characterised by X-ray diffraction, FT-IR studies, thermogravimetry-differential scanning calorimetry, transmission electron microscopy and BET surface area measurements. Thermal calcination of these materials resulted in the formation of high surface area non-stoichiometric spinels whose catalytic activity is studied using N20 decomposition reaction as the test reaction. The order of activity observed is Co-A1-COs-HT>Co-Fe-COs-HT>Co-Cr-CO 3HT. INTRODUCTION Layered double hydroxides commonly referred as hydrotalcite-like (HT-like) materials, consists of brucite-like (Mg(OH) 2) network, wherein the divalent ion is substituted by trivalent ion whose excess positive charge is compensated by anions, usually carbonate, which occupy the interlayer positions [1-5]. They are represented by the general formula
[M(II)I.xM(III)x(OH)2]x+Ax/nn'.mH20 where M(II) and M(III) are divalent and trivalent ions and A is the interlayer anion where water of crystallisation also finds a place. The physico-chemical properties of these materials are mainly characterised by the nature of metal ions and their composition [6,7]. Although a wide spectrum of metal ions have been incorporated into the network [8], reports available on cobalt containing hydrotalcites are scarce [9]. Thermal calcination of these materials resulted in the formation of stable, high surface area and non-stoichiometric mixed metal oxides employed in many catalytic transformations like steam reforming, methanol synthesis, higher alcohol synthesis
904 and N20 decomposition [10-12]. The physicochemical properties of these unusual solids are entirely different from the solids obtained by conventional ceramic routes. The synthesis of such materials for a desired catalytic reaction is the prime objective of solid state chemistry and catalysis [13]. The objective of the present investigation is to study the change in structure, stability and reactivity of cobalt containing hydrotalcites with various trivalent metal ions as a function of preparation methods and composition and characterising their thermally calcined products. EXPERIMENTAL The HT-like compounds are prepared by sequential precipitation wherein NaOH/Na2CO 3 mixture is added to the metal nitrate solutions at room temperature with increasing pH. These compounds are also prepared by coprecipitation under low supersaturation conditions wherein both the precipitants and metal nitrates are added simultaneously holding the pH between 9-10. The final pH of the solution was kept at 10 in both the cases. The slurry obtained is aged at 65~ for 24h, filtered, washed thoroughly with distilled water and dried at 80~ overnight. Hydrothermal treatments are performed at 110~ for 2 days in a teflon autoclave under autogenous conditions. The chemical compositions of these materials are determined by inductively coupled plasma emission spectrometry (Model 3410, ARL7) by dissolving the compounds in minimum amount of hydrochloric acid. X-ray diffraction of these samples are taken in Philips X-ray generator (Model PW 1330) using CoK a radiation (k = 1.7902A). The lattice parameters are calculated using least square fitting of the peaks mainly considering the peaks whose 2{}>40~ IR absorption spectra are recorded using FT-IR spectrometer (Perkin-Elmer Model 1760) in the form of KBr discs. TGDSC studies are carried out in Perkin-Elmer TG-DSC/7 at the heating rate of 10~ under nitrogen atmosphere. Surface area measurements, using BET method of adsorption of N 2 at 77K, are carried out in Carlo Erba Model 1800 automatic sorptometer. Catalytic tests are carried out in an all glass recirculatory static reactor. About lg of the precursor namely the hydrotalcite is employed for the catalytic studies. Thermal calcination of the material was done in vacuum to generate "in situ" mixed metal oxides which are active catalysts. The decomposition of N20 was carried out at 50 torr initial pressure of the gas in the temperature range 150~176 The details regarding the activation procedure is mentioned elsewhere [14]. R E S U L T S AND D I S C U S S I O N Table-1 shows the composition and the phase obtained for the various samples synthesised. The closeness in the values between calculated and measured composition indicates the completion of precipitation. XRD of the samples showed the single phase formation of liT-like phase exhibiting sharp and symmetric reflections
905
Table 1 Composition and phase obtained of the samples synthesised Sample Code
Composition
Preparation Method
Cat A Co-A1 Sequential Cat B Co-A1 Sequential Cat C Co-A1 Sequential Cat Dd Co-A1 Sequential Cat E Co-A1 Low super a Cat F Co-Fe Sequential Cat G Co-Fe Sequential Cat H Co-Fe Low super Cat I Co-Fe Low super Cat J Co-Cr Sequential Cat K Co-Cr Sequential Cat L Co-Cr Low super Cat M Co-Cr Low super a - Low supersaturation preparation method b - Calculated c - Observed d - Hydrothermally treated e - not determined f - HT + possibly hexagonal Co(OH) 2
M2+/M 3+ atomic ratio 2.0 b 2.5 3.0 n. e 3.0 2.0 3.0 2.0 3.0 2.0 3.0 2.0 3.0
2.0 c 2.5 3.0 3.0 1.6 2.8 2.3 3.6 1.9 2.8 1.9 3.4
Phase obtained HT HT HT HT HT HT f HT HT HT amorphous amorphous vw HT HT
for (003), (006), (110) and (113) planes and broad and asymmetric reflections for (102), (105) and (108) planes characteristic of clay minerals possessing layered structure. These materials have a rhombohedral 3Rm symmetry with a and c unit cell parameters calculated by least square fitting of the peaks. The lattice parameters and surface area of some of the hydrotalcites are given in Table-2. It can be inferred that the difference in the values of unit cell volume of Co-A1, Co-Fe and Co-Cr-COs-HTlcs are in good agreement with the ionic radii of the trivalent element [15]. Comparison of Cat A and Cat C indicated that as the composition increases (Co/A1 atomic ratio) both the lattice parameters increases with consequent increase in the unit cell volume. The increase in the lattice parameter 'a' can be attributed to higher ionic radii of Co2§ (0.74A) in comparison with A18+ (0.50A) and increase in 'c' parameter is due to reduced electrostatic interaction between the layer and the interlayer network. The preparation method significantly influence the crystallinity of the materials synthesised. Compounds synthesised under low supersaturation (LS) conditions are more crystalline than by sequential precipitation (SP). Furthermore, the crystallinity also s with increase in atomic r a t i o . Hydrothermal treatments increased the crystallinity of the material. This result is corroborated with the reduction in the surface area of the hydrothermally treated samples. However, hydrothermal treatments performed on Co-Fe-CO3-HTlcs resulted in the
906 ii
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o >,
Cat
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,,
20
0
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29 (degrees) Fig. 1 XRD patterns of some of the hydrotalcites prepared. ..
Fig.2
F T - I R spectra of the samples synthesised.
907 spinel formation indicating its thermal instability under these conditions. shows the XRD patterns of various samples synthesised.
Fig.1
Table 2 Unit cell parameters and surface area of the samples synthesised Sample Code Cat Cat Cat Cat Cat Cat Cat Cat Cat Cat
A B C D E G H I L M
Unit cell parameters a (A)
c (A)
V (A 3)
3.073 3.077 3.080 3.084 3.078 3.129 3.108 3.111 3.107 3.117
22.782 22.955 23.091 23.227 23.304 22.811 22.523 22.682 22.923 22.885
186.3 188.2 189.7 191.3 191.2 193.4 188.3 190.1 191.6 192.5
Surface area (m2/g) 64.8 27.7 69.4 42.5 35.0 88.7 60.7 72.2 250.7 155.6
The crystallinity of the material is also dependent on the nature of the trivalent metal ion present in the network. Co-AI-CO3-HT are more crystalline in comparison with Fe and Cr containing samples. In the case of Co-Cr-CO3-HT, preparation by sequential precipitation yielded amorphous material whereas preparation under low supersaturation resulted in a better crystalline material. In our Co-Fe containing samples, it is not completely possible to exclude the presence of hexagonal Co(OH) 2 prep_ared under sequential precipitation. These results indicated that presence of Al 3+ favours the formation of crystalline HTlc phase which is in accordance with the results observed by Clause et al for nickel containing hydrotalcites [7]. T E M results showed spherical to hexagonal platelets of thin and wide nature characteristic of these materials [3]. FT-IR absorption spectra of these materials, given in Fig.2, showed prominent bands around 3400cm "1,1630cm 1 and 1370cm" 1 corresponding to vOH stretching, vOH bending and v 3 carbonate stretching respectively, confirming the presence ofhydroxycarbonates. Absence of band around 3650cm'~indicates that all O H groups in the structure are hydrogen bonded and no free hydroxyl group is present [2]. Bands v 2 (out of plane deformation) and v 4 (in plane bending) of carbonate are observed around 870 and 680cm "I respectively. Differences noticed for all bands between observed vibrations of carbonate and free carbonate anion indicates even perturbation of anion in the interlayer. Bands observed at less than 1000cm "I are attributed to the lattice vibrations like M - O stretching and M - O - M bending vibrations [16]. A sharp band around 1600cm "I is observed for Fe and Cr containing samples suggests that it is not only due to water bending vibrations and but also due to the presence of bicarbonate anions [17].
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909 Cat C exhibited a doublet at 1380 and 1365cm "1 for v 3 stretching of carbonate, which can be attributed to lower symmetry of carbonate present in the interlayer (D3h symmetry distorted to C2v). This can also cause activation in v I mode observed around 1020cm "1 [18]. However, upon hydrothermal treatment (Cat D) a singl_e sharp band around 1365cm "1 is observed, indicating the enhanced ordering of CO82" ion in the interlayer. Nevertheless, this value is very much lower than that of free CO82" species [19] (1415cm'1), indicating that a strong electrostatic interaction exists between hydroxyl group and H20 molecules in the interlayer with carbonate species. In the case of Co-Fe and Co-Cr containing samples such doublets are observed even ai%er h~drothermal treatments, which clearly shows that a large degree of disorder of CO8~" species in the interlayer. This result is corroborated with X-ray results exhibiting low crystallinity of these samples. A closer examination of vOH band for aged samples indicated that as the M2+]M3+ ratio decreases (compare Cat A and Cat C) the band is shifted to lower wave numbers. This shift could be due to depletion of electron density around OH group bonded to A13+ ion by polarisation. Such shifts were not observed for Co-Fe and CoCr containing samples indicating the weak polaris~bility of Fe 8+ and Cr 8+ in comparison with A1~+. Fig.3 shows TG and its differential curve for some of the hydrotalcites. Most of the samples showed two stages of weight loss wherein the first weight loss occurring in the temperature range 150-250~ is attributed to the removal of physisorbed and interlayer water molecules and the second weight loss occurring in the temperature range 250-350~ ascribed for dehydroxylation between the sheets and decarbonation (loss of CO 2) leading to the destruction of the layered structure [20]. However, Cat C showed the second weight loss occurring in two stages. The first can be assigned for partial dehydroxylation within the layer and second is due to complete dehydroxylation and decarbonation[21]. In the case of Fe containing HTs irrespective of the preparation conditions, the release of interlayer water, structural water and CO 2 occur simultaneously in the temperature range 150-200~ However, for Co-Cr-HTs a better thermal stability is achieved for the samples prepared under LS conditions in comparison with SP conditions, owing to the better crystallinity of the former sample. For hydrothermally treated materials, the second weight loss splits into two peaks without affecting T 1. This could be due to better ordering in the interlayer space leading to step wise losses. Marchi et al [22] proposed that the presence of new peaks in the hydrothermally treated sample can be attributed to heterogeneity of the precipitate obtained. However, our X-ray results showed that the samples are more crystalline and single phase in nature. The net weight loss and transition temperatures for some of the samples are reported in Table-3. It can be clearly seen from the Table that as M 2+/M~+ ratio increases the transition temperatures T~ and To decreases. This can be explained by considering that upon M 3+ substitution ~by M 2~ in the network, the positive charge density of the layer increases and thus enhancing the electrostatic interaction between the layer and interlayer. DSC results, given in Fig.4, substantiated TG results showing two endotherms corresponding to two weight losses. The DSC transition temperature, although slightly higher than TG temperatures, showed a
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911 Table-3 TG transition temperature and net weight loss of the samples
Sample
Cat Cat Cat Cat Cat Cat Cat Cat Cat Cat Cat
A B C D E F G H I L M
Transition temperature (~ T1
T2
198 190 171 179 179 171 167 173 163 185 153
274 252,300 245 232,290 235 222 201 244 206 268 256
Net weight loss (%)
33.0 30.9 29.7 29.2 29.8 28.7 23.3 24.0 26.7 31.3 26.2
similar trend in the temperature which decreases with increase in M2+/M 3+ atomic ratio. Comparison of DSC curves of aged and hydrothermally treated samples showed an interesting observation that the curves are more intense for hydrothermally treated samples which is indicative of the higher crystallinity.
Characterisation of thermally calcined catalysts: Thermal calcination of these materials at 400~ in air are given in Fig.5. All the compounds showed non-stoichiometric spinel phase independent of M 3+ ion. The non-stoichiometry can be explained by the differences in the values obtained in lattice parameters and IR band positions between calcined catalysts and stoichiometric compounds. In the case of Co-A1-HTs', as the atomic ratio increases, more amount of Co304 is formed (by oxidation of Co2+). The band position shifts to higher frequency as the Co/A1 ratio increases indicative of the formation of solid solution. This result is substantiated by X-ray results showing that the lattice parameters calculated are intermediate between CoA120 4 and Co30 4 (8.105A and 8.084A). Similar behaviour is observed for Co-Cr and Co-Fe systems on variation with elemental composition. These results are in accordance with the results obtained by Busca et al [23] and Uzunova et al [24] respectively. However, detailed study on thermally calcined materials will be published elsewhere. Fig.6 shows the variation of X-ray pattern of Cat B with calcination temperature. As the temperature increases, the crystallinity of the obtained spinel increases as evidenced from the increase in the intensity and sharpness of the peaks. The lattice parameter calculated for these materials showed that as the temperature increases, approach of the stoichiometry is achieved. This result can be substantiated with surface area measurements (Fig.7), which decreases with increase in the
912
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913 calcination temperature. However, an initialincrease in the surface area is observed which could be due to 'crater'formation as observed by T E M [25],through which C O 2 exit.
Catalytic activity of N20 decomposition: Catalytic activity of N20 decomposition on various t h e m a l l y calcined catalysts are reported in Fig.8. The activity followed the order Co-A1-HT>Co-Fe-HT>>Co-CrHT. The low activity of Co-Cr catalysts can be attributed to low activity of n-type Cr20 3. Generally for N20 decomposition p-type oxides are more active t h a n n-type oxides owing to the cumulative adsorption in the former [26]. We presume that Cr ~+ segregates on the surface in Co-Cr catalysts, and thereby covering the active cobalt sites. Such segregation is also observed for Co-Fe-HT, although the extent of is small [24]. In the synthesis of hydrocarbons on Co-Cr catalysts, it is reported that even with high Co contents, a low activity is observed indicating that Co alone is not responsible for the activity [27]. With increase in the Co/A1 atomic ratio (compare CatA and Cat C), an increase in the activity is observed which could be explained by the fact that the number of adsorption centers increases on the surface. Our XPS results also confirmed that the Co/A1 surface composition increases with increase in the bulk composition although the former is lower in value. The activity of the present samples are compared with some of the most active catalysts reported in the literature [28]. Under our experimental conditions, Co-A1-HTs' showed appreciable conversion even at 150~ and Cat C showed 100% conversion at 250~ which is 100~ less than the most active catalyst reported so far. Analysis of the spent catalysts by XRD showed nonstoichiometric spinel type oxides. The high activity for Co-A1-HTs' can be explained based on the specific interaction between Co 2+ and Co 3+ ion with the support responsible for the generation of the active sites. However, a detailed study is required in understanding the nature of active centers. CONCLUSIONS The effect of preparation procedure, nature of trivalent metal ion and elemental composition on various physicochemical properties are summarised as follows: a. Compounds prepared under the composition range 2.0
914 ACKNOWLEDGEMENTS
The authors thank Air Products Chemicals Inc.,U.S.A for the research grant REFERENCES I,
2. 3. 4. 5. ,
7. ,
9. 10.
11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.
R.Allmann, Acta Crysallogr. 24 (1968) 972. S.Miyata, Clays Clay Miner., 23 (1975) 369. W.T.Reichle, Solid State Ionics, 22 (1986) 135. K.A.Corrado, A.Kostapapas and S.L.Suib, Solid State Ionics 26 (1988) 77. T.J.Pinnavaia, NATO ASI Ser., Ser.C (Zeolite Microporous Solids: Synthesis, Structure and Reactivity, 1992) p.91. W.T.Reichle, J. Catal, 94 (1985) 547. O.Clause, M.Gazzano, F.Trifiro,A.Vaccari and L.Zotorski, Appl. Catal., 73 (1991) 217. F.Cavani, F.Trifiro and A.Vaccari, Catal. Today, 11 (1991) 173. T.Sato, H.Fujita, T.Endo and M.Shimada, React. Solids, 5 (1988) 219. E.C.Kruissink, L.L.Van Reijen and J.R.H.Ross, J.Chem.Soc., Faraday Trans. I, 77 (1981)665. M.Piemontese, F.Trifiro, A.Vaccari, E.Foresti and M.Gazzano, in G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Prep. Catalysts V, Elsevier, Amsterdam, 1991 p.49. S.Kannan and C.S.Swamy, Appl. Catal.B., 3 (1994) 109. H.Yanagida, Angew. Chem. Int.(Eng) Edn., 27 (1988) 1389. J.Christopher and C.S.Swamy, J.Mol.Catal., 62 (1990) 69. R.D.Shannon and C.T.Prewitt, Acta CrystaUogr., B25 (1969) 925. M.J.Hernandez-Moreno, M_A.Ulibarri,J.L.Rendon and C.J. Serna, Phys. Chem. Miner., 12 (1985) 34. L.Pesic, S.Salipurovic, V.Markovic, D.Vucelic, W.Kagunya and W.Jones, J. Mater. Chem., 2 (1992) 1069. F.M.Labojos, V.Rives and M.A.Ulibarri, J.Mat.Sci., 27 (1992) 1546. K.Nakamoto, Infrared and R a m a n Spectra of Inorganic and Coordination Compounds, 3rd ed.,Wiley, N e w York, 1978. S.Kannan and C.S.Swamy, J.Mat.Sci.Lett., 11 (1992) 1585. F.Rey and V.Fornes, J.Chem.Soc., Faraday Trans. 88 (1992) 2233. A.J.Marchi, J.I.Di Cosimo and C.R.Apesteguia, in M.J. Phillips and M. Ternan (Editors), Proc. 9th Int. Congress on Catalysis, Ottawa, 1988, Vol.2, p.529. G.Busca, F.Trifiro and A.Vaccari, Langrnuir, 6 (1990) 1440 E.Uzunova, D.Klissurski, I.Mitov and P.Stefanov, Chem. Mater., 5 (1993) 576. W.T.Reichle, S.Y.Kang and D.S.Everhardt, J.Catal., 101 (1986) 352. P.Pomonis and J.C.Vickerman, J.Catal., 90 (1984) 305. F.Trifiro and A.Vaccari, in R.K. Grasselli and A.W. Sleight (Editors), Structure-ACtivity and Selectivity Relationships in Heterogenous Catalysis, Elsevier, Amsterdam, 1991, p.157. Y.Li and J.N.Armor, Appl. Catal.B., 1 (1992) L21.