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Preparation And Surface Characterisation Of Novel Ceria-Copper And Ceria-Manganese Mixed Oxides
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
75
Preparation And Surface Characterisation Of Novel Ceria-Copper And Ceria-Manganese Mixed Oxides Mafia Chdstophidou, and Chaffs R. Theochafis*, Porous Solids Group, Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus The paper presents surface texture and catalysis results for a series of copper and manganese-containing ceria samples. These samples, especially the 20% manganese containing one, had significantly higher surface area than the pristine ceria. The NO denox performance of the 20% copper containing sample indicated high selectivity towards nitrogen formation. 1. INTRODUCTION There has been much recent interest in the preparation of inorganic solids with controlled porosity in the microporous and mesoporous range, or with other desirable surface properties. The interest has somewhat shifted in the recent few years away from microporous towards mesoporous materials not least because of the increased interest in the use of porous materials in air or water pollution control. Ceria (CeO2), in particular, has attracted much interest because of its use as an additive in the socalled triodic automobile exhaust catalyst [1,2], but also because of its use as a catalyst in its own right, or as a catalyst support. Ceria acts as an excellent oxygen store [3-5] in the catalyst, which is thus rendered a very effective catalyst for combustion [6]. Moreover, addition of ceria to the automotive exhaust catalysts minimises the thermally induced sintering of the alumina support and stabilises the noble metal dispersion [7]. Ceria also enhances nitric oxide dissociation when added to various supported metal catalysts [8], which is another important function of the automotive exhaust catalyst. Recent investigations by Harrison et al have shown that ceria doped with certain lanthanides and promoted with copper and chromium have catalytic activities comparable to that of the noble metal catalysts [9]. In addition to the reactions described above which relate to the internal combustion engine emissions questions, the catalysed low temperature oxidative coupling of methane, the water gas shift reaction and many other catalytic reactions are also promoted by ceria [10-12]. A study of alkali and alkaline earth metal doped ceria
76 catalysts has shown that barium or calcium doped ceria were the most active catalyst for the oxidative coupling of CH4 [13]. Zhang and Baerns explained the observed dependence of C2 selectivity on the Ca content in terms of oxygen-ion conductivity [13]. Because of the promoting effects of ceria in many catalytic reactions, the preparation of high surface area and thermally stable ceria phases as well as the study of the parameters which control the structural, textural and redox properties of the material are of particular interest [14-15].
Ceria or metal supported on ceria have
been found to catalyse such diverse reactions as methanol oxidation to CO and HE [ 16], CO oxidation to CO2 [ 17], and reduction of NO and N20 to N2 [ 18]. In addition, several reports have appeared on the catalytic activity of mixed metal-cerium oxides.
At the University of Cyprus there is a long-standing interest in the synthesis and study of porous ceria [ 19-21 ]. Work so far has concentrated studying ways of enhancing and stabilising higher porosity, as well as studying the chemistry of the surface. Two strategies have been used, the insertion of controlled amounts of dopants on the one hand, and the use of organic matrices on the other. The mesoporosity of these solids is not intra-crystalline in the sense that the porosity of zeolites is, but depends upon aggregation of primary particles, and as such is susceptible to synthesis conditions such as pH and concentration of the cerium and other cations precursors used. In the present paper we present the results of an investigation into the use of copper and manganese ions as dopants. Copper was used because of reports in the literature about the catalytic potency of copper bearing ceria [22,23], and manganese because of it possesses a number of stable oxidation states. 2. EXPERIMENTAL
The chemical reagents used for the preparation of stock solutions were reagent grade and were used without further purification. The chemicals were in the form of the nitrate salts and were obtained commercially from Aldrich or Fluka. Pure ceria samples were prepared by precipitation of ceria from aqueous solutions containing 0.01M Ce 4+ by adding 1 M NH3 solution. The precipitate was dried at 523K. The surface acid sites concentration was measured using Hammett titrations, with phenolphthalein as indicator, and a 0.1M NaOH solution as base. Mixed oxide samples were prepared by the co-precipitation from aquatic solutions of the cations ([Ce(IV)] = 0.01 M) by adding equal volume of 1 M NH3 solution under
77 controlled conditions. The surface properties were investigated by nitrogen adsorption isotherm analysis and FTIR spectroscopy. The nitrogen adsorption isotherms were measured at 77 K using an ASAP 2000 analyser (Micromeritics) after outgassing the samples under vacuum (0.15Pa) at 423K. FTIR spectroscopy was carried out with a Shimadzu spectrophotometer (FTIR-8501) using both KBr and the DRIFTS method. The total pore volume referred to below, corresponds to the pore volume measured from the nitrogen isotherm, at p/p~
Thermogravimetric recordings were
obtained using a Shimadzu TGA apparatus in flowing air. The catalytic efficacy of ceria samples was measured vis ~ vis to oxidation of NO to N2. A continuous flow method was used, and measurements were carried out at various catalyst temperatures. The gaseous mixture used was 0.25% NO, 1% H2, 5% 02 and the rest helium. Mass flow controllers were used to control concentrations. The effluent gas synthesis was determined by quadrupole mass spectrometry. A Baker mass spectrometer was employed. 3. RESULTS Table 1 shows the adsorption isotherm data and acidity measurements for the samples under examination. Figures 1 and 2 show the nitrogen adsorption isotherms for the copper-containing and manganese-containing samples respectively. TABLE 1 Adsorption and Surface Acidity Measurements Pore Surface Acid Real Cu Content by Volume Sites Concn ICP (%) (cm3g"1) (mol/100g)
Sample
SBET (m2g-1)
CeO2 2% Cu 8% Cu 20% Cu 2% Mn 20% Mn
128 88 63 54 113 232
0.08 0.13 0.12 0.11 0.19 0.31
1.73 1.604 1.662 3.845 3.974
0 0.34 1.34 2.97 0 0
Figure 3 contains a graph showing the variation in N2 selectivity during catalysis runs for a variety of CuxCel_xO4 solids, and Figure 4 the corresponding percent conversions, as a function of temperature. Figures 5 and 6 contain the FTIR spectra for CuxCel_xO4and MnxCel.xO4 solids respectively.
78 4. Discussion
Examination of Figure 1 reveals that the FTIR spectrum of ceria and mixed metal oxide-ceria solids has three main features: first a manifold of broad bands centred 2%C u, B%C u & 2 0%C u
['
.
(,,)(' ' [ )2o.(c,,8" 2.,,c. . oc,,)
'
(c)
I.-
(b) (a) , 4000
,
3100
.
,
2 200
I
llcm
3'o0
'
' 400
Figure 1 FTIR spectrum of CeO2 containing (a) 2%, (b) 8% and (c) 20% Cu 2+ Around 3100 cm-1 second a band at 1630 crnl which is attributable to H-O-H bending mode and is indicative of the presence of molecular water in the sample, and thirdly a sharp peak at 1383 cm-1 which has been attributed to Ce-OH stretching and superimposed to higher and lower wavenumbers by a broad band, attributable to
2%Mn(d1) & 20%Mn(cl)
~
I-.-
~
(b)
f \ (a)
I 4000
I 31 O0
I
I 2200
I
I 1300
llcm Figure 2 FTIR spectrum of CeO2 containing 2%, and 20% Mn 2§
I 400
79 Ce-O-Ce modes which are characteristic of the fluorite structure of ceria [20]. The FTIR spectrum of pure CeO2 is characterised by a broad band at 1383 cm-1 which is narrower than that found in mixed oxide spectra, as that shown in Figure 1. This suggests that inclusion of the hetero-atoms in the ceria structure leads to a distortion of the lattice. No extra bands were observed in the FTIR spectra on inclusion of the hetero-atoms. Comparison of X-ray dit~actogrammes for pure and doped ceria, has indicated that there is a single phase, with cell parameters which are very similar to those of the pure phase. This apparent lack of distortion can be explained by the fact that the copper ion content found by ICP (see Table 1) was significantly lower than that in the mother liquor, and reached a maximum of only 2.97%. The significant differences between the spectra in Figure 1 were in the OH stretching frequencies: in the spectrum of pure ceria, only one band is observed [20] at 3400 cm-1, whereas the mixed copper-cerium oxide samples had three distinct peaks, at 3020, 3130 and 3400 cm-1. The fact that the intensity of the H-O-H bend band at 1640 cm-1 was very similar in the mixed oxide and pure ceria spectra, indicated that the differences between the pure and mixed oxides in the OH stretching bands, was due to differences in surface OH bands, attributable to Cu-OH groups, and may indicate that the heteroatoms are present as a dispersed microphase within the pores of the ceria phase.
In similar fashion, the FTIR spectra for Mn 2+ containing samples in Figure 2, showed that the samples showed two OH stretching bands at 3200 and 3400 cm-~, the latter band being attributable to OH stretch in molecular water. The differences between the spectra for CeO2, CuxCel.xO2 and MnxCel.,,O2 suggests that the extra bands are due to OH stretch of groups which are linked to the hetero-atom, suggesting the presence of the dispersed microphase. The differences in the surface acid sites concentration between the two mixed oxides (Table 1) confirms the presence of these surface groups. It is noteworthy that cation concentration had no effect on the acid site concentration, corroborating the probability o f a microphase. Figures 3 and 4 s u ~ i s e
the catalysis experiments results for the mixed copper-
cerium oxide and pristine ceria samples for the NO denox reaction. It can be seen from Figure 3 that the selectivity towards N2 was very high, and for one of the samples higher than for pristine ceria. However, as it can be seen from Figure 4 the
80 conversion rate achieved for these samples was not high, and significantly lower than those achieved by pristine ceria. lee r o
,m,
4=
.=.. m
=em
4-W
u o m o (/)
60
o c
~J
31
o
eM
(.I
Z 2o
ze=
.e
=o=
.o
Temperature
=o
do
I=
o
see
Z
(~
e
'~ee
25e
lIQ
Temperature
350
415
4SO
see
(OC)
Figure 3 Figure 4 Nitrogen selectivity in NO denox NO conversion for CuxCel_xO2samples reaction for CuxCe].xO2 samples (In both diagrams: O: 2% Cu, A 8%, square: 15% and inverted triangle, 20%)
100 , , = -=----
80
E6o
Vads-2%Cu Vdes-2%Cu Vads-8%Cu Vdes-8%Cu Vads-20%Cu Vdes-20%Cu
|
J
............
rt
e~
E
i
I
..........................
i'
o40
>o20
0
0 p/po0 dg
Figure 5 Nitrogen adsorption isotherms for CuxCel.xO2samples Figure 5 contains the nitrogen adsorption isotherms for the 20% Cu containing sample, whilst the surface properties of all samples are summarised in Table 1. The isotherms are of Type IV with hysteresis loops of the H4 type. There are no significant differences in the shape of the isotherm or the hysteresis loop, but as observed for other mixed oxides, there was a decrease in the SBET values for the
81
mixed oxide compared with the pure oxide, whilst there is a significant decrease in
SBET values with increasing copper ion inclusion. On the other hand, there is an increase in apparent pore volume for the mixed oxides compared with the pristine oxide. Significantly, there is no change with increasing hetero-atom concentration.
250 Vads-2%Mn Vdes-2%Mn Vads-20%Mn Vdes-20%Mn
200
! i t !
!
i
i
] l
>o
50
o
Figure 6 Nitrogen adsorption isotherms for
Mno.20Ceo.8oO2
samples
Figure 6 shows the nitrogen adsorption isotherm for the 20% manganese containing samples. It is noteworthy that these mixed oxides have a much higher apparent pore volume than the pristine sample. The solid with a 2% manganese content has a BET surface area comparable with the pristine sample, but the sample with 20% manganese has a significantly higher BET surface area (Table 1). These observations combined with those from the FTIR spectra suggest that the presence of the heteroatoms leads to a distortion of the lattice. The significantly higher value of surface acid sites concentration for the MnxCel.xO2 samples compared to the CuxCel.xO2 ones may be partly due to the higher apparent surface area for the former compare to the latter, but presumably is also linked to the differences in polarisation of the Mn-OH and CuOH groups, as well as the effect of the presence of the hetero-atoms to the properties of the Ce-OH groups. The significant increase in apparent surface area achieved for ceria with the controlled incorporation of hetero-atoms, as well as the interesting
82 catalytic activity exhibited by these solids, augurs well for the development of novel ceria-based solids with interesting properties. Acknowledgement We thank Dr I. Pashalidis and Ms G. Kyriakou for useful discussions, and the University of Cyprus and the European Union for financial assistance.
References 1. G.J.K. Acres, in "Perspectives in Catalysis", (Eds J.M. Thomas and K.I.Zamaraev), Blackwell Scientific Publications, London, (1992) 2. J.G. Nunan, H.J. Robota, M.J.Cohn and S.A. Bradley, J. Catal., 133 (1992) 309 3. D. Terribile, A. Trovarelli, C. de Leitenburg and G. Dolcetti, Chem. Mater., 9 (1997) 2676 4. T. Bunluesin, R.J. Gorte and G.W. Graham, Applied. Catal. B-Environmental, 15 (1998) 107 5. V.P. Zhdanov and B. Kasemo, Applied Surface Science, 135 (1998) 297 6. Se H. Oh and C.C. Eickel, J. Catal., 115 (1988) 543 7. Thi X.T. Sayle, S.C. Parker and C.Richard A. Catlow, J. Phys. Chem., 98 (1994) 13625 8. A. Takami, T. Takemoto, H. Iwakuni, K. Yamada, M. Shigetsu, and K. Komatsu, Catal. Today, 35 (1997) 75 9. P.G. Harrison, W. Azelee, A.T. Mubarak, C. Bailey, W. Daniell, Studies in Surface Science and Catalysis, 116, 495, (1998) 10. W. Liu, C. Wadia, and M. Flytzani-Stephanopoulos, Catal. Today, 28 (1996) 391 11. P.K. Rao, K.S.R. Rao, S.K. Masthan, K.V. Narayana, T. Rajiah, and V.V. Rao, Applied. Catal. A-General, 163 (1997) 123 12. A. Naydenov, B. Stoyanova, and D. Mehandjiev, Journal of Molecular Catalysis A-Chemical, 98 (1995) 9 13. Z.-L. Zhang and M. Baems, J. Catal., 135 (1992) 317 14. J.Z. Shyu, W.H.Weber and H.S.Gandhi, J. Phys. Chem., 92 (1988) 4964 15. D. Terribile, A. Trovarelli and G. Dolcetti, J of Catalysis., 178 (1998) 299 16. W.J. Shen and Y. Matsumura, Phys Chem Chem Phys 2,1519, (2000) 17. S.H. Overbury, D.R. Mullins, and D.R. Huntley, J Catal 186, 296, (1999) 18. E.S. Putna, R.J. Gone, J.M. Vohs, J Catal 178, 598, (1998) 19. I. Pashalidis and C.R. Theocharis, Second European East West Workshop on Chemistry and Energy, Sintra, Portugal, March 1995 20. I. Pashalidis, and C. R. Theocharis, in (Eds K.K. Unger, G. Kreysa and J.P. Baselt), "Studies in Surface Science and Catalysis", Elsevier Science Publishers, 128, 643- 652, (2000) 21. G. Kyriakou, I. Paschalidis and C.R. Theocharis, Fundamentals of Adsorption, 7, In the Press 22. A.Martinez-Arias, M. Femandez-Garcia, J. Sofia, J Catal 182, 367, (1999) 23. J. Soria, J.C. Conesa, A. Martinezarias, Solid State Ionics 63-5, 755, (1993)
75
Preparation And Surface Characterisation Of Novel Ceria-Copper And Ceria-Manganese Mixed Oxides Mafia Chdstophidou, and Chaffs R. Theochafis*, Porous Solids Group, Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus The paper presents surface texture and catalysis results for a series of copper and manganese-containing ceria samples. These samples, especially the 20% manganese containing one, had significantly higher surface area than the pristine ceria. The NO denox performance of the 20% copper containing sample indicated high selectivity towards nitrogen formation. 1. INTRODUCTION There has been much recent interest in the preparation of inorganic solids with controlled porosity in the microporous and mesoporous range, or with other desirable surface properties. The interest has somewhat shifted in the recent few years away from microporous towards mesoporous materials not least because of the increased interest in the use of porous materials in air or water pollution control. Ceria (CeO2), in particular, has attracted much interest because of its use as an additive in the socalled triodic automobile exhaust catalyst [1,2], but also because of its use as a catalyst in its own right, or as a catalyst support. Ceria acts as an excellent oxygen store [3-5] in the catalyst, which is thus rendered a very effective catalyst for combustion [6]. Moreover, addition of ceria to the automotive exhaust catalysts minimises the thermally induced sintering of the alumina support and stabilises the noble metal dispersion [7]. Ceria also enhances nitric oxide dissociation when added to various supported metal catalysts [8], which is another important function of the automotive exhaust catalyst. Recent investigations by Harrison et al have shown that ceria doped with certain lanthanides and promoted with copper and chromium have catalytic activities comparable to that of the noble metal catalysts [9]. In addition to the reactions described above which relate to the internal combustion engine emissions questions, the catalysed low temperature oxidative coupling of methane, the water gas shift reaction and many other catalytic reactions are also promoted by ceria [10-12]. A study of alkali and alkaline earth metal doped ceria
76 catalysts has shown that barium or calcium doped ceria were the most active catalyst for the oxidative coupling of CH4 [13]. Zhang and Baerns explained the observed dependence of C2 selectivity on the Ca content in terms of oxygen-ion conductivity [13]. Because of the promoting effects of ceria in many catalytic reactions, the preparation of high surface area and thermally stable ceria phases as well as the study of the parameters which control the structural, textural and redox properties of the material are of particular interest [14-15].
Ceria or metal supported on ceria have
been found to catalyse such diverse reactions as methanol oxidation to CO and HE [ 16], CO oxidation to CO2 [ 17], and reduction of NO and N20 to N2 [ 18]. In addition, several reports have appeared on the catalytic activity of mixed metal-cerium oxides.
At the University of Cyprus there is a long-standing interest in the synthesis and study of porous ceria [ 19-21 ]. Work so far has concentrated studying ways of enhancing and stabilising higher porosity, as well as studying the chemistry of the surface. Two strategies have been used, the insertion of controlled amounts of dopants on the one hand, and the use of organic matrices on the other. The mesoporosity of these solids is not intra-crystalline in the sense that the porosity of zeolites is, but depends upon aggregation of primary particles, and as such is susceptible to synthesis conditions such as pH and concentration of the cerium and other cations precursors used. In the present paper we present the results of an investigation into the use of copper and manganese ions as dopants. Copper was used because of reports in the literature about the catalytic potency of copper bearing ceria [22,23], and manganese because of it possesses a number of stable oxidation states. 2. EXPERIMENTAL
The chemical reagents used for the preparation of stock solutions were reagent grade and were used without further purification. The chemicals were in the form of the nitrate salts and were obtained commercially from Aldrich or Fluka. Pure ceria samples were prepared by precipitation of ceria from aqueous solutions containing 0.01M Ce 4+ by adding 1 M NH3 solution. The precipitate was dried at 523K. The surface acid sites concentration was measured using Hammett titrations, with phenolphthalein as indicator, and a 0.1M NaOH solution as base. Mixed oxide samples were prepared by the co-precipitation from aquatic solutions of the cations ([Ce(IV)] = 0.01 M) by adding equal volume of 1 M NH3 solution under
77 controlled conditions. The surface properties were investigated by nitrogen adsorption isotherm analysis and FTIR spectroscopy. The nitrogen adsorption isotherms were measured at 77 K using an ASAP 2000 analyser (Micromeritics) after outgassing the samples under vacuum (0.15Pa) at 423K. FTIR spectroscopy was carried out with a Shimadzu spectrophotometer (FTIR-8501) using both KBr and the DRIFTS method. The total pore volume referred to below, corresponds to the pore volume measured from the nitrogen isotherm, at p/p~
Thermogravimetric recordings were
obtained using a Shimadzu TGA apparatus in flowing air. The catalytic efficacy of ceria samples was measured vis ~ vis to oxidation of NO to N2. A continuous flow method was used, and measurements were carried out at various catalyst temperatures. The gaseous mixture used was 0.25% NO, 1% H2, 5% 02 and the rest helium. Mass flow controllers were used to control concentrations. The effluent gas synthesis was determined by quadrupole mass spectrometry. A Baker mass spectrometer was employed. 3. RESULTS Table 1 shows the adsorption isotherm data and acidity measurements for the samples under examination. Figures 1 and 2 show the nitrogen adsorption isotherms for the copper-containing and manganese-containing samples respectively. TABLE 1 Adsorption and Surface Acidity Measurements Pore Surface Acid Real Cu Content by Volume Sites Concn ICP (%) (cm3g"1) (mol/100g)
Sample
SBET (m2g-1)
CeO2 2% Cu 8% Cu 20% Cu 2% Mn 20% Mn
128 88 63 54 113 232
0.08 0.13 0.12 0.11 0.19 0.31
1.73 1.604 1.662 3.845 3.974
0 0.34 1.34 2.97 0 0
Figure 3 contains a graph showing the variation in N2 selectivity during catalysis runs for a variety of CuxCel_xO4 solids, and Figure 4 the corresponding percent conversions, as a function of temperature. Figures 5 and 6 contain the FTIR spectra for CuxCel_xO4and MnxCel.xO4 solids respectively.
78 4. Discussion
Examination of Figure 1 reveals that the FTIR spectrum of ceria and mixed metal oxide-ceria solids has three main features: first a manifold of broad bands centred 2%C u, B%C u & 2 0%C u
['
.
(,,)(' ' [ )2o.(c,,8" 2.,,c. . oc,,)
'
(c)
I.-
(b) (a) , 4000
,
3100
.
,
2 200
I
llcm
3'o0
'
' 400
Figure 1 FTIR spectrum of CeO2 containing (a) 2%, (b) 8% and (c) 20% Cu 2+ Around 3100 cm-1 second a band at 1630 crnl which is attributable to H-O-H bending mode and is indicative of the presence of molecular water in the sample, and thirdly a sharp peak at 1383 cm-1 which has been attributed to Ce-OH stretching and superimposed to higher and lower wavenumbers by a broad band, attributable to
2%Mn(d1) & 20%Mn(cl)
~
I-.-
~
(b)
f \ (a)
I 4000
I 31 O0
I
I 2200
I
I 1300
llcm Figure 2 FTIR spectrum of CeO2 containing 2%, and 20% Mn 2§
I 400
79 Ce-O-Ce modes which are characteristic of the fluorite structure of ceria [20]. The FTIR spectrum of pure CeO2 is characterised by a broad band at 1383 cm-1 which is narrower than that found in mixed oxide spectra, as that shown in Figure 1. This suggests that inclusion of the hetero-atoms in the ceria structure leads to a distortion of the lattice. No extra bands were observed in the FTIR spectra on inclusion of the hetero-atoms. Comparison of X-ray dit~actogrammes for pure and doped ceria, has indicated that there is a single phase, with cell parameters which are very similar to those of the pure phase. This apparent lack of distortion can be explained by the fact that the copper ion content found by ICP (see Table 1) was significantly lower than that in the mother liquor, and reached a maximum of only 2.97%. The significant differences between the spectra in Figure 1 were in the OH stretching frequencies: in the spectrum of pure ceria, only one band is observed [20] at 3400 cm-1, whereas the mixed copper-cerium oxide samples had three distinct peaks, at 3020, 3130 and 3400 cm-1. The fact that the intensity of the H-O-H bend band at 1640 cm-1 was very similar in the mixed oxide and pure ceria spectra, indicated that the differences between the pure and mixed oxides in the OH stretching bands, was due to differences in surface OH bands, attributable to Cu-OH groups, and may indicate that the heteroatoms are present as a dispersed microphase within the pores of the ceria phase.
In similar fashion, the FTIR spectra for Mn 2+ containing samples in Figure 2, showed that the samples showed two OH stretching bands at 3200 and 3400 cm-~, the latter band being attributable to OH stretch in molecular water. The differences between the spectra for CeO2, CuxCel.xO2 and MnxCel.,,O2 suggests that the extra bands are due to OH stretch of groups which are linked to the hetero-atom, suggesting the presence of the dispersed microphase. The differences in the surface acid sites concentration between the two mixed oxides (Table 1) confirms the presence of these surface groups. It is noteworthy that cation concentration had no effect on the acid site concentration, corroborating the probability o f a microphase. Figures 3 and 4 s u ~ i s e
the catalysis experiments results for the mixed copper-
cerium oxide and pristine ceria samples for the NO denox reaction. It can be seen from Figure 3 that the selectivity towards N2 was very high, and for one of the samples higher than for pristine ceria. However, as it can be seen from Figure 4 the
80 conversion rate achieved for these samples was not high, and significantly lower than those achieved by pristine ceria. lee r o
,m,
4=
.=.. m
=em
4-W
u o m o (/)
60
o c
~J
31
o
eM
(.I
Z 2o
ze=
.e
=o=
.o
Temperature
=o
do
I=
o
see
Z
(~
e
'~ee
25e
lIQ
Temperature
350
415
4SO
see
(OC)
Figure 3 Figure 4 Nitrogen selectivity in NO denox NO conversion for CuxCel_xO2samples reaction for CuxCe].xO2 samples (In both diagrams: O: 2% Cu, A 8%, square: 15% and inverted triangle, 20%)
100 , , = -=----
80
E6o
Vads-2%Cu Vdes-2%Cu Vads-8%Cu Vdes-8%Cu Vads-20%Cu Vdes-20%Cu
|
J
............
rt
e~
E
i
I
..........................
i'
o40
>o20
0
0 p/po0 dg
Figure 5 Nitrogen adsorption isotherms for CuxCel.xO2samples Figure 5 contains the nitrogen adsorption isotherms for the 20% Cu containing sample, whilst the surface properties of all samples are summarised in Table 1. The isotherms are of Type IV with hysteresis loops of the H4 type. There are no significant differences in the shape of the isotherm or the hysteresis loop, but as observed for other mixed oxides, there was a decrease in the SBET values for the
81
mixed oxide compared with the pure oxide, whilst there is a significant decrease in
SBET values with increasing copper ion inclusion. On the other hand, there is an increase in apparent pore volume for the mixed oxides compared with the pristine oxide. Significantly, there is no change with increasing hetero-atom concentration.
250 Vads-2%Mn Vdes-2%Mn Vads-20%Mn Vdes-20%Mn
200
! i t !
!
i
i
] l
>o
50
o
Figure 6 Nitrogen adsorption isotherms for
Mno.20Ceo.8oO2
samples
Figure 6 shows the nitrogen adsorption isotherm for the 20% manganese containing samples. It is noteworthy that these mixed oxides have a much higher apparent pore volume than the pristine sample. The solid with a 2% manganese content has a BET surface area comparable with the pristine sample, but the sample with 20% manganese has a significantly higher BET surface area (Table 1). These observations combined with those from the FTIR spectra suggest that the presence of the heteroatoms leads to a distortion of the lattice. The significantly higher value of surface acid sites concentration for the MnxCel.xO2 samples compared to the CuxCel.xO2 ones may be partly due to the higher apparent surface area for the former compare to the latter, but presumably is also linked to the differences in polarisation of the Mn-OH and CuOH groups, as well as the effect of the presence of the hetero-atoms to the properties of the Ce-OH groups. The significant increase in apparent surface area achieved for ceria with the controlled incorporation of hetero-atoms, as well as the interesting
82 catalytic activity exhibited by these solids, augurs well for the development of novel ceria-based solids with interesting properties. Acknowledgement We thank Dr I. Pashalidis and Ms G. Kyriakou for useful discussions, and the University of Cyprus and the European Union for financial assistance.
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