- Email: [email protected]
Steam reforming of ethanol using Cu-Ni supported catalysts
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
2147
Steam reforming of ethanol using Cu-Ni supported catalysts F. Marifio ! , M. Jobbagy 2, G. Baronetti 3 and M. Laborde ~Departamemo de Ingenieria Quimica (FI) Universidad de Buenos Aires Pabell6n de Industrias, Ciudad Universitaria, 1428 Buenos Aires, Argentina 2INQUIMAE (FCE y N) Universidad de Buenos Aires 3INCAPE (CONICET- FIQ) Universidad Nacional del Litoral The effect of calcination temperature (450-800~ and copper loading (lower and higher than 4wt%) on the catalytic behaviour and structure of Cu/Ni/K//y-A1203 catalysts were analysed. These catalysts were proven to be suitable for the steam reforming of ethanol. XPS, SEM-EDX, XRD and N20 chemisorption techniques were employed; catalytic activity is evaluated using the steam reforming of ethanol as the test reaction. Depending of both, copper loading and calcination temperature, a non-stoichiometric surface spinel can be obtained which has catalytic properties. The presence of a layered double hydroxide precursor is essential for the formation of this phase.
1. INTRODUCTION Previous works [1,2] have shown that copper-nickel catalysts, supported on y-AhO3 and doped with potassium hydroxide, are suitable for the production of synthesis gas from steam reforming of alcohol. These catalysts present acceptable activity and selectivity in the range of 250 - 300~ and atmospheric pressure. Copper is the active agent, nickel is added to favour carbon-carbon bond rupture and potassium is used to neutralise acidic sites of the support and, in this way, to avoid the diethyl ether production. In this work, the effect of calcination temperature is analysed for two catalysts, one with a copper loading lower than 4 wt% (C2KN sample) and the other with copper content higher than 4 wt% (C3KN sample). Nomenclature used is in agreement with that employed in a previous work [2]. XPS, SEM-EDX, XRD and N20 chemisorption techniques are employed; catalytic activity is evaluated from measurements carried out in a fixed bed catalytic reactor using the steam reforming of ethanol as test reaction.
2. EXPERIMENTAL y-AI203 spheres ~om Rh6ne-Poulenr (3 mm of diameter, specific area: 200 m2/g and porel volume: 0.44 ml/g), previously doped with KOH, were used as support. Catalysts were prepared by a co-impregnation method using an aqueous solution of copper and nickel nitrate as reported elsewhere [2]. Samples were dried overnight at room temperature and calcined at 450, 550, 625, 700 and 800~ for 5 h. Copper content was determined by Atomic Absorption
2148 analysis, employing a Varian-Techtmn equipment model AA-5. Catalysts with Cu content of 2,17 wt% (sample called C 2 K ~ and 4,75 wt% (sample called C 3 K ~ were prepared. Nominal Ni and K contents were 4 wt% and 0,15 wt% respectively for both samples. Powder X-ray spectra were obtained with a Siemens diffractometer Model D5000 using Cu K~ radiation, Ni filter and 40 Kv. Scanning Electron Microscopy (SEM) and EDX analysis were carried out in a Philips SEM 505 microscope (operating at 25kV) equipped with an EDAX Philips 505 microprobe. Before analysis, all samples were homogenised and covered with a very thin gold layer. X-Ray photoelectron spectra were obtained in a Shimadzu ESCA 750 Electron Spectrometer, using an AI Ka (1486,6 eV) as radiation source. Surface charging was observed for all the samples and binding energies of Cu 2p, Ni 2p and A12p were referred to C ls line at 284,6 eV. Copper surface area measurements were performed using the selective chemisorption of N20 over surface copper sites from CuO phase [3]. Previously, the sample was reduced "in situ" by a H2 / Nz stream at 180~ and then at 230~ N20 was fed in pulses to a reactor maintained at 40~ whereas N2 released during the reaction was evaluated using a TCD [2]. Steam reforming of ethanol was used as the test reaction. The reaction was carried out in a glass fixed-bed reactor at 300~ and 1 atrm Previously, the catalyst was reduced "in situ" by a H~ / N2 stream at 230~ Then, a liquid water-ethanol mixture (molar ratio 2.5" 1) was fed into the reactor (LHSV: 1.8 h"l ). The products were analysed by gas chromatography [2].
3. RESULTS AND DISCUSSION
In a previous paper [2], the effect of several copper loading (1.26, 2.17, 4.75, 6.36 wt%) on the catalytic behaviour and structure of Cu/Ni/~'-AI203 catalysts was discussed. Our experimental results, obtained by TPR and XRD, showed that CuO segregated phase was only observed for samples with copper loading higher than 4 wt%, in agreement with other authors [4, 5]. For this reason, in this work the attention will be focussed in samples with copper loading lower and higher than 4 wt%.
~---~CuC
Cu~204
~
CuO
81re Y
,.,,...,.,,.h,,,..,, i
N
35
'
i'l
36
37
i
'
38
t
'
'
39
40
2O Figure 1. XRD patterns for C2KN catalyst calcined at several temoeratures.
34
35
36
37 2e
38
39
40
Figure 2. XRD patterns for C3KN catalyst calcined at several temperatures
2149 XRD patterns for C2KN and C3KN catalysts calcined at different temperatures are shown in Figures 1 and 2 respectively. For C2KN, CuO reflections are not detected at any temperature; while for C3KN, the intensity of CuO reflections increases from 450~ to 625~ due to the growth of CuO erystallites; for higher temperatures CuO signals decrease and almost disappear. For both samples the central wide reflection increase above 700~ is assigned to an increase of the CuAI204 phase (it was verified that "/-A1203 calcined at temperatures between 450~ and 800~ does not change its diffraction pattern). It is known that CuA1204 is thermodynamically unstable in relation to CuO/7-A1203 at temperatures lower than 600~ [6]. It must be noted that NiO characteristic peak could not be observed in any sample; this fact, in addition to the preference of Ni 2+ ions to occupy oetahedral sites [6], indicates that nickel added should be always as NiAI204. SEM photographs of both catalysts calcined at 625~ showed similar pictures with a poor cristallinity as is indicated in Figure 3 for C2KN sample. An X-ray map, recorded with the Cu Ka line, showed a homogeneous Cu distribution, except for regions where high Cu concentrations are detected (Figure 3, A zone). The EDX analysis performed on this zone exhibits a strong Cu Ka line (Figure 4) suggesting the presence of CuO segregated phase in these regions. A similar picture for C3KN sample was obtained. On the other hand, for both samples calcined at 800~ Cu X-ray maps shows a homogeneous Cu distribution without Cu rich zones. In spite of XRD patterns of samples calcined at 625~ that only showed the presence of a segregated CuO phase for the catalyst with the highest Cu content (Figure 2), SEM-EDX
Figure 3. Scanning electron micrograph (x 7400) of C2KN sample calcined at 625~
2150 results indicate that CuO phase is detected for both catalysts. In order to explain this fact, XRD analysis of the catalyst precursors (samples just impregnated) was performed (Figure 5). The spectra corresponding to both catalyst precursors show the characteristic lines of layered double hydroxide (LDH),Cul.xAIx(OH)2NO3xH20 [7, 9 8]. Alejandre et al.[9], working with copper-aluminium mixed oxides samples, found that the 1.20 3.00 4.80 6.60 8.40 thermal decomposition of LDH E (keV) phase undergoes to an amorphous layered double oxide (LDO), Figure 4. EDX analysis of the A zone of Figure 3 CUl-xAlxO1.sx, which at photograph. temperatures higher than 400~ produces a CuO crystalline phase and an amorphous CuA1204 phase. It must be noted (Figure 5), that the intensity of LDH lines for C3KN precursor is noticeably larger than those for C2KN precursor. (the intensity of ~/-A1203 lines is used as reference). This fact would explain the detection of CuO lines in the XRD patterns of C3KN sample (Figure 2). In C2KN catalyst, the low amount of LDH precursor indicates a lower interaction Cu-AI. This fact would avoid the CuO crystalline phase formation, producing an amorphous CuO phase that was detected by SEM-EDX but not detected by XRD (Figure 1). The effect of calcination temperature on ethanol conversion and copper surface area (Scu) for both catalysts is shown in Figure 6. For C2KN both, conversion and Sc~, follow the same pattern and sharply decrease above 700~ However, the behaviour of C3KN catalyst is quite different. Although Scu values continuously decrease with calcination temperature, the activity values remain almost constant for the range of temperature studied. XPS data reveal that for all samples, Cu/A1 surface ratios are higher than corresponding bulk ones (Table 1). In addition, the values of Ni/A1 ratios for the surface and for the bulk are similar. All these results are in agreement with those reported by Hierl et al [7] indicating that the presence of Ni 2+ enhances the 0 20 40 60 80 segregation of Cu2+ ions to the surface. 2O Cu/A1 surface ratio of C2KN Figure 5. XRD patterns for C2KN and sample calcined at 800~ is clearly lower C3KN catalyst precursors. than that of the same sample calcined at 625~ (Table 1), which agrees with the _.
"m,
. A
I
A || 9 ~,u
2151 decrease of Scu and activity values observed -~-S Cu between these calcination 350 C2KN " 0.8 temperatures. But for 300 sample C3KN, Cu/A1 ~SCu 250 surface ratio and catalytic 0.6 C3KN activity do not change ~=200 tbetween 625~ and 0.4 150 Conversion 800~ while Scu is C2KN notably lower at 800~ It 100 0.2 must be noted that Scu -A50 was determined taking Conversion ' I ' I ' I ' I ' I ' I ' 0 into account only the C3KN 500 600 700 800 surface copper sites ~om Calcination Temperature (~ CuO phase. Then, other copper species with Figure 6. Effect of calcination temperature on copper surface catalytic properties could area and conversion for C2KN and C3KN samples. be present on the surface. Lo Jacono et al [10] have shown that Cu2+/ ~/-A1203 systems, with a copper content higher than 3.87 wt% and calcined at approximately 600~ consist of a dispersion of copper ions on the alumina surface, which form a structure related to that of the spinel CuA1204 and called "surface spinel". These authors detected the incipient formation of this surface spinel by its X-ray diffraction lines at d=2.856 A y d= 2.436 A. Recently, Alejandre et al [11 ] claim that the intensity of this lines increases with copper content and calcination temperature. For C3KN sample, XRD lines of CuO phase disappear at temperatures higher than 700~ and the central wide reflections increase above this temperature (Figure 2). Besides, it must be noted that the characteristic lines of the surface spinel are detected (XRD pattern not shown). Meanwhile, Cu/A1 surface ratio is the same for this sample calcined at 625~ and 800~ Therefore, it can be postulated that the called "surface spinel" is formed in C3KN catalyst calcined at 800~ Moreover, this surface spinel would have catalytic properties. The formation of this "surface spinel" could be favoured by the presence of crystalline CuO coming from the thermal decomposition of the LDH precursor due to a greater amount of this specie in the C3KN sample with respect to the C2KN sample (Figure 5). 400
==
Table 1. Comparison between C2KN and C3KN samples calcined at 625~ and 800~
XPS (Cu/AI) surface molar ratio Calculated (Cu/AI) bulk molar ratio Copper surface area, Scu (m2/gcu) Ethanol conversion (%)
C2KN sample 625~ 800~ 0.076 0.038 0.019 181.72 98.80 0.76 0.29
C3KN sample 625~ 800~ 0.067 0.075 0.042 1 1 0 . 2 1 59.89 0.81 0.74
2152 The effect of calcination temperature on C2KN catalyst is different. A decrease of the Cu/Al molar surface ratio (Table 1) and an increase of the X-ray diffzaction lines in the metal aluminate zone (Figure 1) are observed for this catalyst calcined at 800~ with respect to the sample calcined at 625~ This fact indicates the formation of bulk CuA1204 phase, which has worse catalytic properties as is revealed by the ethanol conversion drop (Figure 6).
4. CONCLUSIONS In summary, it can be postulated that two different phenomena occur during the calcination stage in catalysts with a copper loading higher than 4 wt%: a) the growth of CuO crystallites, and b) the increase of the copper-support interaction, producing a surface spinel phase. This phase has catalytic properties that counterbalance the loss of activity due to the first phenomena. The presence of the LDH in this catalyst precursor is responsible of this behaviour . . . . On the other hand, for catalysts with a copper loading lower than 4 wt% an amorphous CuO phase highly dispersed is produced which is not detected by X-ray diffraction, whatever the calcination temperature is. Nevertheless, SEM-EDX analysis shows the presence of this phase at 625~ When the calcination temperature increases, the high dispersion of CuO particles avoids their growth and favours the diffusion of copper ions into the alumina matrix. Then, a bulk spinel without catalytic properties is produced and, as a consequence, a sharp decrease of catalytic activity is observed.
Acknowledgements The authors thanks the Universidad de Buenos Aires for the financial support and the JICA for the donation of the ESCA spectrometer. References 1. C.Luengo, G.Ciampi, C.Steckelberg and M.Laborde, Int.J.Hyd. Energy 17 (9) (1992) 677. 2. F.Mariflo, E.Cerrella, S.Duhalde, M.Jobbagy and M.Laborde, Int.J.Hyd. Energy 23 (12) (1998) 1095. 3. Th. Osinga, B. Linsen and W. Van Beek, J.Catal. 7 (1967) 277 4. R. Friedman and J. Freemm~ J.Cata155 (1978) 10 5. Ch. Sivaraj and P. Kantarao, Appl.Cat. 45 (1988) 103. 6. R. Hierl, H. Knozinger and H. Urbach, J.Catal. 69 (1981) 475 7. F. Cavani, F. Trifiro and A. Vaccari, Catal. Today, 11 (1991) 173 8. H. P. Boehm, J. Steinle and C. Vieweger, Angew. Chem. Int., 4 (1977) 265 9. A. Alejandre, F.Medina, P.Salagre, X.Correig and J.Sueiras, Chem. Mater., 11 (1999), 939. 10. M. Lo Jacono, A. Cimino and M. Inversi, J. Catal., 76 (1982) 320. 11. A. Alejandre, F. Medina, A. Fortuny, P. Salagre and J. E. Sueiras, Appl. Catal. B: Environmental, 16 (1998) 53
2147
Steam reforming of ethanol using Cu-Ni supported catalysts F. Marifio ! , M. Jobbagy 2, G. Baronetti 3 and M. Laborde ~Departamemo de Ingenieria Quimica (FI) Universidad de Buenos Aires Pabell6n de Industrias, Ciudad Universitaria, 1428 Buenos Aires, Argentina 2INQUIMAE (FCE y N) Universidad de Buenos Aires 3INCAPE (CONICET- FIQ) Universidad Nacional del Litoral The effect of calcination temperature (450-800~ and copper loading (lower and higher than 4wt%) on the catalytic behaviour and structure of Cu/Ni/K//y-A1203 catalysts were analysed. These catalysts were proven to be suitable for the steam reforming of ethanol. XPS, SEM-EDX, XRD and N20 chemisorption techniques were employed; catalytic activity is evaluated using the steam reforming of ethanol as the test reaction. Depending of both, copper loading and calcination temperature, a non-stoichiometric surface spinel can be obtained which has catalytic properties. The presence of a layered double hydroxide precursor is essential for the formation of this phase.
1. INTRODUCTION Previous works [1,2] have shown that copper-nickel catalysts, supported on y-AhO3 and doped with potassium hydroxide, are suitable for the production of synthesis gas from steam reforming of alcohol. These catalysts present acceptable activity and selectivity in the range of 250 - 300~ and atmospheric pressure. Copper is the active agent, nickel is added to favour carbon-carbon bond rupture and potassium is used to neutralise acidic sites of the support and, in this way, to avoid the diethyl ether production. In this work, the effect of calcination temperature is analysed for two catalysts, one with a copper loading lower than 4 wt% (C2KN sample) and the other with copper content higher than 4 wt% (C3KN sample). Nomenclature used is in agreement with that employed in a previous work [2]. XPS, SEM-EDX, XRD and N20 chemisorption techniques are employed; catalytic activity is evaluated from measurements carried out in a fixed bed catalytic reactor using the steam reforming of ethanol as test reaction.
2. EXPERIMENTAL y-AI203 spheres ~om Rh6ne-Poulenr (3 mm of diameter, specific area: 200 m2/g and porel volume: 0.44 ml/g), previously doped with KOH, were used as support. Catalysts were prepared by a co-impregnation method using an aqueous solution of copper and nickel nitrate as reported elsewhere [2]. Samples were dried overnight at room temperature and calcined at 450, 550, 625, 700 and 800~ for 5 h. Copper content was determined by Atomic Absorption
2148 analysis, employing a Varian-Techtmn equipment model AA-5. Catalysts with Cu content of 2,17 wt% (sample called C 2 K ~ and 4,75 wt% (sample called C 3 K ~ were prepared. Nominal Ni and K contents were 4 wt% and 0,15 wt% respectively for both samples. Powder X-ray spectra were obtained with a Siemens diffractometer Model D5000 using Cu K~ radiation, Ni filter and 40 Kv. Scanning Electron Microscopy (SEM) and EDX analysis were carried out in a Philips SEM 505 microscope (operating at 25kV) equipped with an EDAX Philips 505 microprobe. Before analysis, all samples were homogenised and covered with a very thin gold layer. X-Ray photoelectron spectra were obtained in a Shimadzu ESCA 750 Electron Spectrometer, using an AI Ka (1486,6 eV) as radiation source. Surface charging was observed for all the samples and binding energies of Cu 2p, Ni 2p and A12p were referred to C ls line at 284,6 eV. Copper surface area measurements were performed using the selective chemisorption of N20 over surface copper sites from CuO phase [3]. Previously, the sample was reduced "in situ" by a H2 / Nz stream at 180~ and then at 230~ N20 was fed in pulses to a reactor maintained at 40~ whereas N2 released during the reaction was evaluated using a TCD [2]. Steam reforming of ethanol was used as the test reaction. The reaction was carried out in a glass fixed-bed reactor at 300~ and 1 atrm Previously, the catalyst was reduced "in situ" by a H~ / N2 stream at 230~ Then, a liquid water-ethanol mixture (molar ratio 2.5" 1) was fed into the reactor (LHSV: 1.8 h"l ). The products were analysed by gas chromatography [2].
3. RESULTS AND DISCUSSION
In a previous paper [2], the effect of several copper loading (1.26, 2.17, 4.75, 6.36 wt%) on the catalytic behaviour and structure of Cu/Ni/~'-AI203 catalysts was discussed. Our experimental results, obtained by TPR and XRD, showed that CuO segregated phase was only observed for samples with copper loading higher than 4 wt%, in agreement with other authors [4, 5]. For this reason, in this work the attention will be focussed in samples with copper loading lower and higher than 4 wt%.
~---~CuC
Cu~204
~
CuO
81re Y
,.,,...,.,,.h,,,..,, i
N
35
'
i'l
36
37
i
'
38
t
'
'
39
40
2O Figure 1. XRD patterns for C2KN catalyst calcined at several temoeratures.
34
35
36
37 2e
38
39
40
Figure 2. XRD patterns for C3KN catalyst calcined at several temperatures
2149 XRD patterns for C2KN and C3KN catalysts calcined at different temperatures are shown in Figures 1 and 2 respectively. For C2KN, CuO reflections are not detected at any temperature; while for C3KN, the intensity of CuO reflections increases from 450~ to 625~ due to the growth of CuO erystallites; for higher temperatures CuO signals decrease and almost disappear. For both samples the central wide reflection increase above 700~ is assigned to an increase of the CuAI204 phase (it was verified that "/-A1203 calcined at temperatures between 450~ and 800~ does not change its diffraction pattern). It is known that CuA1204 is thermodynamically unstable in relation to CuO/7-A1203 at temperatures lower than 600~ [6]. It must be noted that NiO characteristic peak could not be observed in any sample; this fact, in addition to the preference of Ni 2+ ions to occupy oetahedral sites [6], indicates that nickel added should be always as NiAI204. SEM photographs of both catalysts calcined at 625~ showed similar pictures with a poor cristallinity as is indicated in Figure 3 for C2KN sample. An X-ray map, recorded with the Cu Ka line, showed a homogeneous Cu distribution, except for regions where high Cu concentrations are detected (Figure 3, A zone). The EDX analysis performed on this zone exhibits a strong Cu Ka line (Figure 4) suggesting the presence of CuO segregated phase in these regions. A similar picture for C3KN sample was obtained. On the other hand, for both samples calcined at 800~ Cu X-ray maps shows a homogeneous Cu distribution without Cu rich zones. In spite of XRD patterns of samples calcined at 625~ that only showed the presence of a segregated CuO phase for the catalyst with the highest Cu content (Figure 2), SEM-EDX
Figure 3. Scanning electron micrograph (x 7400) of C2KN sample calcined at 625~
2150 results indicate that CuO phase is detected for both catalysts. In order to explain this fact, XRD analysis of the catalyst precursors (samples just impregnated) was performed (Figure 5). The spectra corresponding to both catalyst precursors show the characteristic lines of layered double hydroxide (LDH),Cul.xAIx(OH)2NO3xH20 [7, 9 8]. Alejandre et al.[9], working with copper-aluminium mixed oxides samples, found that the 1.20 3.00 4.80 6.60 8.40 thermal decomposition of LDH E (keV) phase undergoes to an amorphous layered double oxide (LDO), Figure 4. EDX analysis of the A zone of Figure 3 CUl-xAlxO1.sx, which at photograph. temperatures higher than 400~ produces a CuO crystalline phase and an amorphous CuA1204 phase. It must be noted (Figure 5), that the intensity of LDH lines for C3KN precursor is noticeably larger than those for C2KN precursor. (the intensity of ~/-A1203 lines is used as reference). This fact would explain the detection of CuO lines in the XRD patterns of C3KN sample (Figure 2). In C2KN catalyst, the low amount of LDH precursor indicates a lower interaction Cu-AI. This fact would avoid the CuO crystalline phase formation, producing an amorphous CuO phase that was detected by SEM-EDX but not detected by XRD (Figure 1). The effect of calcination temperature on ethanol conversion and copper surface area (Scu) for both catalysts is shown in Figure 6. For C2KN both, conversion and Sc~, follow the same pattern and sharply decrease above 700~ However, the behaviour of C3KN catalyst is quite different. Although Scu values continuously decrease with calcination temperature, the activity values remain almost constant for the range of temperature studied. XPS data reveal that for all samples, Cu/A1 surface ratios are higher than corresponding bulk ones (Table 1). In addition, the values of Ni/A1 ratios for the surface and for the bulk are similar. All these results are in agreement with those reported by Hierl et al [7] indicating that the presence of Ni 2+ enhances the 0 20 40 60 80 segregation of Cu2+ ions to the surface. 2O Cu/A1 surface ratio of C2KN Figure 5. XRD patterns for C2KN and sample calcined at 800~ is clearly lower C3KN catalyst precursors. than that of the same sample calcined at 625~ (Table 1), which agrees with the _.
"m,
. A
I
A || 9 ~,u
2151 decrease of Scu and activity values observed -~-S Cu between these calcination 350 C2KN " 0.8 temperatures. But for 300 sample C3KN, Cu/A1 ~SCu 250 surface ratio and catalytic 0.6 C3KN activity do not change ~=200 tbetween 625~ and 0.4 150 Conversion 800~ while Scu is C2KN notably lower at 800~ It 100 0.2 must be noted that Scu -A50 was determined taking Conversion ' I ' I ' I ' I ' I ' I ' 0 into account only the C3KN 500 600 700 800 surface copper sites ~om Calcination Temperature (~ CuO phase. Then, other copper species with Figure 6. Effect of calcination temperature on copper surface catalytic properties could area and conversion for C2KN and C3KN samples. be present on the surface. Lo Jacono et al [10] have shown that Cu2+/ ~/-A1203 systems, with a copper content higher than 3.87 wt% and calcined at approximately 600~ consist of a dispersion of copper ions on the alumina surface, which form a structure related to that of the spinel CuA1204 and called "surface spinel". These authors detected the incipient formation of this surface spinel by its X-ray diffraction lines at d=2.856 A y d= 2.436 A. Recently, Alejandre et al [11 ] claim that the intensity of this lines increases with copper content and calcination temperature. For C3KN sample, XRD lines of CuO phase disappear at temperatures higher than 700~ and the central wide reflections increase above this temperature (Figure 2). Besides, it must be noted that the characteristic lines of the surface spinel are detected (XRD pattern not shown). Meanwhile, Cu/A1 surface ratio is the same for this sample calcined at 625~ and 800~ Therefore, it can be postulated that the called "surface spinel" is formed in C3KN catalyst calcined at 800~ Moreover, this surface spinel would have catalytic properties. The formation of this "surface spinel" could be favoured by the presence of crystalline CuO coming from the thermal decomposition of the LDH precursor due to a greater amount of this specie in the C3KN sample with respect to the C2KN sample (Figure 5). 400
==
Table 1. Comparison between C2KN and C3KN samples calcined at 625~ and 800~
XPS (Cu/AI) surface molar ratio Calculated (Cu/AI) bulk molar ratio Copper surface area, Scu (m2/gcu) Ethanol conversion (%)
C2KN sample 625~ 800~ 0.076 0.038 0.019 181.72 98.80 0.76 0.29
C3KN sample 625~ 800~ 0.067 0.075 0.042 1 1 0 . 2 1 59.89 0.81 0.74
2152 The effect of calcination temperature on C2KN catalyst is different. A decrease of the Cu/Al molar surface ratio (Table 1) and an increase of the X-ray diffzaction lines in the metal aluminate zone (Figure 1) are observed for this catalyst calcined at 800~ with respect to the sample calcined at 625~ This fact indicates the formation of bulk CuA1204 phase, which has worse catalytic properties as is revealed by the ethanol conversion drop (Figure 6).
4. CONCLUSIONS In summary, it can be postulated that two different phenomena occur during the calcination stage in catalysts with a copper loading higher than 4 wt%: a) the growth of CuO crystallites, and b) the increase of the copper-support interaction, producing a surface spinel phase. This phase has catalytic properties that counterbalance the loss of activity due to the first phenomena. The presence of the LDH in this catalyst precursor is responsible of this behaviour . . . . On the other hand, for catalysts with a copper loading lower than 4 wt% an amorphous CuO phase highly dispersed is produced which is not detected by X-ray diffraction, whatever the calcination temperature is. Nevertheless, SEM-EDX analysis shows the presence of this phase at 625~ When the calcination temperature increases, the high dispersion of CuO particles avoids their growth and favours the diffusion of copper ions into the alumina matrix. Then, a bulk spinel without catalytic properties is produced and, as a consequence, a sharp decrease of catalytic activity is observed.
Acknowledgements The authors thanks the Universidad de Buenos Aires for the financial support and the JICA for the donation of the ESCA spectrometer. References 1. C.Luengo, G.Ciampi, C.Steckelberg and M.Laborde, Int.J.Hyd. Energy 17 (9) (1992) 677. 2. F.Mariflo, E.Cerrella, S.Duhalde, M.Jobbagy and M.Laborde, Int.J.Hyd. Energy 23 (12) (1998) 1095. 3. Th. Osinga, B. Linsen and W. Van Beek, J.Catal. 7 (1967) 277 4. R. Friedman and J. Freemm~ J.Cata155 (1978) 10 5. Ch. Sivaraj and P. Kantarao, Appl.Cat. 45 (1988) 103. 6. R. Hierl, H. Knozinger and H. Urbach, J.Catal. 69 (1981) 475 7. F. Cavani, F. Trifiro and A. Vaccari, Catal. Today, 11 (1991) 173 8. H. P. Boehm, J. Steinle and C. Vieweger, Angew. Chem. Int., 4 (1977) 265 9. A. Alejandre, F.Medina, P.Salagre, X.Correig and J.Sueiras, Chem. Mater., 11 (1999), 939. 10. M. Lo Jacono, A. Cimino and M. Inversi, J. Catal., 76 (1982) 320. 11. A. Alejandre, F. Medina, A. Fortuny, P. Salagre and J. E. Sueiras, Appl. Catal. B: Environmental, 16 (1998) 53