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Effect of precursor of manganese supported on activated carbon catalysts for methanol decomposition
Catalysis Communications 5 (2004) 95–99 www.elsevier.com/locate/catcom
Effect of precursor of manganese supported on activated carbon catalysts for methanol decomposition S. Vankova a, T. Tsoncheva b
b,*
, D. Mehandjiev
c
a Department of Chemistry, University of Sofia, 1000, Sofia, Bulgaria Institute of Organic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl.9, Sofia 1113, Bulgaria c Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria
Received 1 October 2003; accepted 27 November 2003 Published online: 30 December 2003
Abstract Manganese supported on activated carbon materials obtained from aqueous or methanol solution of various manganese precursors are compared by X-ray diffraction, magnetic measurements and reduction with CO. The effect of the preparation conditions on their catalytic activity in methanol decomposition to CO and hydrogen is also studied and discussed. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Manganese on activated carbon; Methanol decomposition; Effect of precursor
1. Introduction It is known, that the preparation method used is one of the promising ways for the catalysts properties control [1–5]. In this relation, the state of the supported metal–metal oxides significantly depends on the nature of their corresponding precursors [6–10]. In our previous studies the favourable effect of copper ammonia precursor for the preparation of highly active copper supported on activated carbon catalysts for methanol decomposition has been reported [9,11]. On the contrary, lower catalytic stability was registered in the case of the catalysts, obtained by the corresponding nitrate precursor [11,12]. A well defined maximum in the conversion curves with the temperature increase was observed in this case. It has been also mentioned that the catalysts activity and selectivity in methanol decomposition could be improved by the deposition of copper or iron from organic medium [7,8,10,13]. At the same time, higher catalytic stability but significantly lower activity
at low temperatures was demonstrated for the manganese supported on activated carbon catalysts, prepared from the corresponding nitrate precursor [14]. In fact, the data on the catalytic behaviour of the manganese based materials in the methanol decomposition are rather insufficient [15–17]. The promotion effect of Mn on the Cu modified catalysts in this reaction has been reported. The aim of the present paper is to clear the effect of the precursor and the deposition medium used due to the preparation procedure on the state of manganese, supported on activated carbon. Special attention is paid on the catalytic behaviour of these materials in methanol decomposition to CO and hydrogen. In the last decades, this process gains a considerable interest as an alternative way for the preparation of efficient and ecological fuels [18–20].
2. Experimental 2.1. Materials
*
Corresponding author. Tel.: +359-2-979-3961; fax: +359-2-8700225. E-mail address: [email protected] (T. Tsoncheva). 1566-7367/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2003.11.019
The catalysts were prepared using activated carbon (AC) with low ash content (about 3–4%, obtained from apricot shell [21,22]. It was characterised with a specific
S. Vankova et al. / Catalysis Communications 5 (2004) 95–99
surface area (BET) of 910 m2 /g, pore volume )0.91 cm3 /g, mean pore radius )1.7 nm and mesopore volume )0.47 cm3 /g [23]. The incipient wetness technique was used for the manganese loading. Two grams of the AC were impregnated at 303 K with aqueous (W) or methanol (M) solutions of various manganese precursors: Mn(NO3 )2 4H2 O, [Mn(NH3 )6 ](NO3 )2 , Mn(CH3 -COO)2 2H2 O or MnCl2 4H2 O (pH 5.5–8.0). The manganese content of the solutions was controlled by method of complexometry. After that, the samples were dried at 393 K (for nitrate, chloride and acetate precursors) or at 313 K (for ammonia precursor), respectively. The decomposition of the precursors was carried out under vacuum at 573 K for 6 h (heating rate 0.6 K/min). More detailed description of the catalysts is given in Table 1. 2.2. Methods of investigation Textural characteristics were determined by lowtemperature (77.4 K) nitrogen adsorption. The specific surface area was estimated by BET method. The total manganese content in the samples was determined by atomic absorption technique with a Pye Unicam SP90V. X-ray powder diffraction patterns were obtained on TUR, Germany, diffractometer equipped with Cu Ka radiation source. The magnetic measurements were performed with Faraday type magnetic balance in a temperature range of 298–473 K. Temperature programmed reduction with CO (TPR) was carried out in a flow apparatus with 0.12% CO in Ar with a temperature rate of 14 K/min. The catalytic experiments were carried out in a flow type catalytic microreactor using Ar as a carrier gas with WHSV of 1.5 h 1 and methanol partial pressure of 1.57 kPa in the temperature programmed regime (2 K/min). GC analysis on Porapak Q and Molecular sieve columns were used. Carbon based material balance was done.
3. Results and discussion In Fig. 1 are presented data on methanol decomposition for the samples obtained by different manganese precursors. In Fig. 2 are compared the materials obtained by various precursors in organic or aqueous medium. Obviously, with the exception of the chloride loaded samples, all other materials do not differ essentially in their catalytic behaviour. Despite the nature of the precursor (Fig. 1) or deposition medium (Fig. 2) used, they exhibit a significant catalytic activity above 550 and at 650 K about 95–100% conversion is achieved. CO (about 90%) is the main registered carbon containing product in all cases. CO2 and small amounts of dimethyl ether (DME), methane and C2 –C4 hydrocarbons are also observed as by-products. In spite of this, the highest catalytic activity in all investigated temperature interval is observed for the 5.1Mn-N(W) (Figs. 1 and 2). This result is also confirmed by the variations in the manganese loading of the samples (Fig. 3(a) and (b)). 100 80 Conversion (mol%)
96
5.6Mn-C(W)
60
5.6Mn-A(W) 7.3Mn-Ac(W)
40 5.1Mn-N(W)
20 0 550
600
650 700 Temperature ( K)
750
800
Fig. 1. Temperature dependencies of methanol decomposition on manganese supported on activated carbon materials, obtained by various manganese precursors in aqueous medium.
Table 1 Composition, specific surface area, Weiss constants (H) and magnetic moments (leff ) of the manganese supported on activated carbon samples prepared in aqueous or methanol solution of various manganese precursors Sample
Precursor
Medium
Mn (wt%)
BET (m2 /g)
leff (BM)
1.6Mn-A(W) 3.8Mn-A(W) 5.6Mn-A(W) 2.1Mn-N(W) 5.1Mn-N(W) 9.4Mn-N(W) 7.1Mn-N(M) 7.3Mn-Ac(W) 7.3Mn-Ac(M) 5.6Mn-C(W) 7.6Mn-C(M)
Mn[(NH3 )6 ](NO3 )2 Mn[(NH3 )6 ](NO3 )2 Mn[(NH3 )6 ](NO3 )2 Mn(NO3 )2 Mn(NO3 )2 Mn(NO3 )2 Mn(NO3 )2 Mn(CH3 COO)2 Mn(CH3 COO)2 MnCl2 MnCl2
H2 O H2 O H2 O H2 O H2 O H2 O CH3 OH H2 O CH3 OH H2 O CH3 OH
1.6 3.8 5.6 2.1 5.1 9.4 7.1 7.3 7.3 5.6 7.6
682 730 474 832 448 851 472 810 700 448 339
b
a b
leff does not correlate with the Curie–Wiess low. Very low accuracy.
4.74 4.10
H (K) )112 )154
b
5.46 4.24 3.86
)286 )130 )51
a a a
5.44
28
S. Vankova et al. / Catalysis Communications 5 (2004) 95–99
analogues [9,11,12]. Here, the favourable role of ammonia precursor [9,11] and organic deposition medium [12] for the catalysts activity and stability in methanol decomposition has been reported. In order to clear the different effect of the metal precursor and deposition medium on the catalytic behaviour of copper and manganese loaded samples some additional investigations on their structure, magnetic and reduction properties are done. In Fig. 4 are presented X-ray diffraction patterns of the selected samples. XRD signals, typical for Mn3 O4 are observed in all cases and their intensities are the highest in the case of 5.6Mn-A (W). XRD signals with very low intensities, typical for MnO are also found in the spectra of acetate obtained materials. The effective magnetic moments (leff ) and Wiess constants (H) of the samples are shown in Table 1. The values of leff change in the range of 3.60–5.88 BM. According to [24,25] the calculated theoretical effective magnetic moment (leff ) for Mn(II), Mn(III) and Mn(IV) ions in octahedral crystal field is 5.92, 4.70 and 3.44 BM, respectively. So, the presence of Mn(II) as well as Mn(III) species could be concluded for all studied materials. The variations in the leff (Table 1), clearly show that the relative part of the reduced manganese species significantly changes with the nature of the manganese precursor used. Higher amount of Mn(II) could be expected for 5.1Mn-N(W) in comparison with its ammonia analogue. At the same time it is slowly affected by the precursor deposition medium. At the same time the values of leff for the acetate prepared samples do not correlate with the Curie–Weiss low. Hence, the presence of different manganese containing phases, characterized with various Weiss constants (H) could be assumed in this case. This result is also confirmed by the XRD data for the acetate samples, where the presence of MnO as well as Mn3 O4 is observed (Fig. 4). It is worth mentioning, that the highest values of leff is found for the sample characterized with the higher catalytic activity (5.1Mn-N (W)). In this case, a higher negative values of Wiess constant (H), typical for
100 5.1Mn-N(W)
Conversion (mol%)
80 7.1Mn-N(M)
60
7.3Mn-Ac(W) 7.3Mn-Ac(M)
40 5.6Mn-C(W) 7.6Mn-C(M)
20 0 550
600
650 700 Temperature (K)
750
800
Fig. 2. Methanol decomposition vs. temperature on manganese supported on activated carbon materials obtained from various manganese precursors. For the comparison their aqueous obtained analogues are given (dash lines).
Here, again higher catalytic activity is observed for the nitrate obtained materials in comparison with their ammonia analogues for all studied manganese concentrations. It is worth mentioning also that, despite the precursor used, a maximum in the catalysts activity with the manganese loading is observed and the best catalytic activity is registered at about 4–5 wt% manganese in the samples. No essential catalytic effect of the precursor deposition medium is established for nitrate and acetate based materials as well (Fig. 2). The chloride obtained samples differs significantly from their manganese containing analogues in their catalytic behaviour (Figs. 1, 2). Lower catalytic activity (up to 20 or 40% for 5.6Mn-C(W) and 7.6Mn-C(M), respectively) and well defined tendency for the CO selectivity decrease with the temperature increase is found in this case. The observed effects of the precursor on the catalytic behaviour of manganese modified materials are in contrast with the data obtained for their copper containing
80
80 2.1Mn-N(W) 5.1Mn-N(W)
40
9.4Mn-N(W)
Conversion (mol%)
100
Conversion (mol%)
100
60
(a)
1.6Mn-A(W)
60 3.8Mn-A(W)
40
5.6Mn-A(W)
20
20 0 500
97
550
600 650 700 Temperature (K)
0
750
(b)
550
600 650 Temperature (K)
700
750
Fig. 3. Effect of manganese loading on the catalytic activity of methanol decomposition on nitrate (a) or ammonia (b) obtained manganese/activated carbon materials.
98
S. Vankova et al. / Catalysis Communications 5 (2004) 95–99
Fig. 4. XRD patterns of manganese modified activated carbon samples obtained by various manganese precursors: (1) 5.1Mn-N(W); (2) 5.6MnA(W); (3) 5.6Mn-C(W); (4) 7.3Mn-Ac(M); (5) 7.3-Ac(W).
the presence of strong anti-ferromagnetic exchange interaction, is also registered. On the contrary, high values of leff , but very small positive values of H, which are typical for the presence of ferromagnetic interaction, are found for 7.6Mn-C(M). A significantly lower catalytic activity in comparison with the other samples is observed in this case. In our previous study the key role of catalytic active complex (CAC) in the heterogeneous catalysis was discussed [9,11,14,26,27]. It forms not only during the preparation procedure of the catalysts but also due to the influence of the reaction medium. In our opinion, the nature and the amount of CAC and also their stabilization determine the catalytic properties of the samples. In the case of metal–metal oxide supported on activated carbon catalysts it usually consists of metal species bonded not only between themselves but with the functional groups of the support as well. More over, depending on the nature of the supported active phase, the inclusion of metal ions in different oxidative state is also possible. In the case of manganese supported activated carbon the participation of Mn(II) species in the CAC formation was proved [14]. Here, the combined XRD and magnetic results could be an indication that this type of species present in all investigated samples. Probably, they are formed in the course of the catalysts preparation procedure due to the reduction effect of the support (AC). However, the variations in the amount and the state of Mn(II) with the preparation method used could be assumed. The lower XRD signals and the higher value of leff and H, combined with the higher catalytic activity and selectivity to CO for 5.1Mn-N(W) in comparison with its ammonia analogue could be an
indication for the easier formation of CACs in the nitrate obtained sample during the course of catalysts preparation. It is not excluded, this effect to be a result from the higher precursor dispersion of nitrate obtained samples. At the same time, the lower catalytic activity founded for the acetate samples could be a result from the CAC concentration decrease due to the formation of MnO in them. The observed maximum in the catalytic activity and leff with the precursor concentration for the nitrate and ammonia samples could be ascribed to the changes in the manganese reduction degree during the preparation procedure. It is not excluded, the extremum in the catalytic behaviour of chloride prepared
Fig. 5. TPR curves of manganese modified activated carbon samples obtained by various manganese precursors (1) 5.6Mn-A(W); (2) 7.3Ac(W); (3) 5.6Mn-C(W); (4) 5.1Mn-N(W).
S. Vankova et al. / Catalysis Communications 5 (2004) 95–99
materials to be due to the lower degree of MnCl2 decomposition under the preparation conditions used. This assumption is also confirmed by the significantly lower ABET for these samples and also by the calculated value of leff , which does not correlate with the Curie– Weiss low (Table 1). These conclusions are also confirmed by the TPR experiments (Fig. 5). A negligible reduction effect is registered only in the case of 5.1Mn-N(W) and it is in agreement with the assumption for the predominantly formation of Mn(II) even during the preparation procedure in it. The observed differences in the initial reduction temperature for the acetate, ammonia and chloride samples could be also an evidence for some variation in the manganese state (including the presence of different manganese containing phases) in them.
4. Conclusion The catalytic activity of manganese supported on activated carbon materials depends on the manganese precursor used. The sample obtained from aqueous solution of Mn(NO3 )2 exhibits the highest catalytic activity. In contrast with the corresponding copper loaded materials no favourable effect of the ammonia precursor or organic medium is confirmed for their manganese analogues.
Acknowledgements The authors acknowledge the National Science Fund of Bulgaria for the support of this work. Thanks are due to Dr. M. Christova for the TPR experiments and to M. Drebov for the samples preparation. References [1] Y. Liu, T. Hayakawa, T. Ishii, M. Kumagai, H. Yasuda, K. Suzuki, S. Hamakawa, K. Murata, Appl. Catal. A: General 210 (2001) 301.
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[2] G. Mul, A.S. Hirschon, Catal. Today 65 (2001) 69. [3] W.J. Shen, Y. Matsumura, Phys. Chem. Chem. Phys. 2 (2000) 1519. [4] B. Coq, F. Figueras, J. Molec. Catal. A: Chemical 173 (2001) 117. [5] Y. Matsumura, K. Tanaka, N. Tode, T. Yazawa, M. Haruta, J. Mol. Catal. A: Chemical 152 (2000) 157. [6] J. Llorea, P.R. de la Piscina, J. Sales, N. Homs, Chem. Commun. (2001) 641. [7] C. Minchev, R. K€ ohn, T. Tsoncheva, M. Dimitrov, M. Fr€ oba, Stud. Surf. Sci. Catal. 135 (2001) 157. [8] C. Minchev, R. Koehn, T. Tsoncheva, M. Dimitrov, I. Mitov, D. Paneva, H. Huwe, M. Froeba, Stud. Surf. Sci. Catal. B 142 (2002) 1245. [9] T. Tsoncheva, S. Vankova, D. Mehandjiev, Fuel 82 (2003) 755. [10] K. Hadjiivanov, T. Tsoncheva, M. Dimitrov, C. Minchev, H. Kn€ ozinger, Appl. Catal. A: General 241 (2003) 218. [11] T. Tsoncheva, R. Nickolov, S. Vankova, D. Mehandjiev, Canad. J. Chem. 81 (2003) 1096. [12] T. Tsoncheva, R. Nickolov, Y. Neinska, C. Minchev, D. Mehandjiev, Bulg. Chem. Commun. 32 (2000) 218. [13] T. Tsoncheva, Tz. Venkov, M. Dimitrov, C. Minchev, K. Hadjiivanov, J. Molec. Catal. A: Chemical 209 (2003) 125. [14] D. Mehandjiev, S. Vankova, T. Tsoncheva, Oxid. Commun. 25 (2002) 441. [15] R. Zhang, Y. Sun, S. Peng, Stud. Surf. Sci. Catal. 130 (2000) 2129. [16] W.H. Cheng, Appl. Catal. A: General 130 (1995) 13. [17] W.H. Cheng, C.Y. Shiau, T.H. Liu, H.L. Tung, J.F. Lu, C.C. Hsu, Appl. Catal. A: General 170 (1998) 215. [18] W.H. Cheng, H.H. Kung, in: W.H. Cheng, H.H. Kung (Eds.), Methanol Production and Use, Marcel Dekker, New York, 1994 (Chapter 1). [19] A. Yildiz, K. Pekmez, in: Y. Y€ ur€ um (Ed.), Hydrogen Energy System, NATO ASI Series., Ser. E: Appl. Sci., vol. 295, Kluwer Academic Publishers, Netherlands, 1995, p. 195. [20] L. Pettersson, K. Sj€ ostr€ om, Combust Sci. Tech. 80 (1991) 265. [21] V. Minkova, M. Razvigorova, M. Goranova, L. Ljutzkanov, G. Angelova, Fuel 70 (1991) 713. [22] D. Mehandjiev, N. Stankova, P. Dimitrova, J. Colloid, Interface Sci. 53 (2000) 230. [23] R. Nickolov, T. Tsoncheva, R. Mehandjiev, Fuel 81 (2002) 203. [24] F.E. Mabbs, D.J. Machin, in: . In: Magnetism and Transition Metal Complexes, Chapman and Hall, London, 1973, p. 153. [25] D. Mehandjiev, S. Angelov, in: . In: Magnetochemistry of Solid State, Nauka I Iskustvo, Sofia, 1979, p. 135. [26] D. Mehandjiev, in: L. Petrov, Ch. Bonev, G. Kadinov (Eds.), Proceedings of the Ninth International Symposium on Heterogeneous Catalysis, Bulgaria, 2000, p. 19. [27] T. Tsoncheva, R. Nickolov, R. Mehandjiev, React. Kinet. Catal. Lett. 72 (2001) 383.
Effect of precursor of manganese supported on activated carbon catalysts for methanol decomposition S. Vankova a, T. Tsoncheva b
b,*
, D. Mehandjiev
c
a Department of Chemistry, University of Sofia, 1000, Sofia, Bulgaria Institute of Organic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl.9, Sofia 1113, Bulgaria c Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria
Received 1 October 2003; accepted 27 November 2003 Published online: 30 December 2003
Abstract Manganese supported on activated carbon materials obtained from aqueous or methanol solution of various manganese precursors are compared by X-ray diffraction, magnetic measurements and reduction with CO. The effect of the preparation conditions on their catalytic activity in methanol decomposition to CO and hydrogen is also studied and discussed. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Manganese on activated carbon; Methanol decomposition; Effect of precursor
1. Introduction It is known, that the preparation method used is one of the promising ways for the catalysts properties control [1–5]. In this relation, the state of the supported metal–metal oxides significantly depends on the nature of their corresponding precursors [6–10]. In our previous studies the favourable effect of copper ammonia precursor for the preparation of highly active copper supported on activated carbon catalysts for methanol decomposition has been reported [9,11]. On the contrary, lower catalytic stability was registered in the case of the catalysts, obtained by the corresponding nitrate precursor [11,12]. A well defined maximum in the conversion curves with the temperature increase was observed in this case. It has been also mentioned that the catalysts activity and selectivity in methanol decomposition could be improved by the deposition of copper or iron from organic medium [7,8,10,13]. At the same time, higher catalytic stability but significantly lower activity
at low temperatures was demonstrated for the manganese supported on activated carbon catalysts, prepared from the corresponding nitrate precursor [14]. In fact, the data on the catalytic behaviour of the manganese based materials in the methanol decomposition are rather insufficient [15–17]. The promotion effect of Mn on the Cu modified catalysts in this reaction has been reported. The aim of the present paper is to clear the effect of the precursor and the deposition medium used due to the preparation procedure on the state of manganese, supported on activated carbon. Special attention is paid on the catalytic behaviour of these materials in methanol decomposition to CO and hydrogen. In the last decades, this process gains a considerable interest as an alternative way for the preparation of efficient and ecological fuels [18–20].
2. Experimental 2.1. Materials
*
Corresponding author. Tel.: +359-2-979-3961; fax: +359-2-8700225. E-mail address: [email protected] (T. Tsoncheva). 1566-7367/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2003.11.019
The catalysts were prepared using activated carbon (AC) with low ash content (about 3–4%, obtained from apricot shell [21,22]. It was characterised with a specific
S. Vankova et al. / Catalysis Communications 5 (2004) 95–99
surface area (BET) of 910 m2 /g, pore volume )0.91 cm3 /g, mean pore radius )1.7 nm and mesopore volume )0.47 cm3 /g [23]. The incipient wetness technique was used for the manganese loading. Two grams of the AC were impregnated at 303 K with aqueous (W) or methanol (M) solutions of various manganese precursors: Mn(NO3 )2 4H2 O, [Mn(NH3 )6 ](NO3 )2 , Mn(CH3 -COO)2 2H2 O or MnCl2 4H2 O (pH 5.5–8.0). The manganese content of the solutions was controlled by method of complexometry. After that, the samples were dried at 393 K (for nitrate, chloride and acetate precursors) or at 313 K (for ammonia precursor), respectively. The decomposition of the precursors was carried out under vacuum at 573 K for 6 h (heating rate 0.6 K/min). More detailed description of the catalysts is given in Table 1. 2.2. Methods of investigation Textural characteristics were determined by lowtemperature (77.4 K) nitrogen adsorption. The specific surface area was estimated by BET method. The total manganese content in the samples was determined by atomic absorption technique with a Pye Unicam SP90V. X-ray powder diffraction patterns were obtained on TUR, Germany, diffractometer equipped with Cu Ka radiation source. The magnetic measurements were performed with Faraday type magnetic balance in a temperature range of 298–473 K. Temperature programmed reduction with CO (TPR) was carried out in a flow apparatus with 0.12% CO in Ar with a temperature rate of 14 K/min. The catalytic experiments were carried out in a flow type catalytic microreactor using Ar as a carrier gas with WHSV of 1.5 h 1 and methanol partial pressure of 1.57 kPa in the temperature programmed regime (2 K/min). GC analysis on Porapak Q and Molecular sieve columns were used. Carbon based material balance was done.
3. Results and discussion In Fig. 1 are presented data on methanol decomposition for the samples obtained by different manganese precursors. In Fig. 2 are compared the materials obtained by various precursors in organic or aqueous medium. Obviously, with the exception of the chloride loaded samples, all other materials do not differ essentially in their catalytic behaviour. Despite the nature of the precursor (Fig. 1) or deposition medium (Fig. 2) used, they exhibit a significant catalytic activity above 550 and at 650 K about 95–100% conversion is achieved. CO (about 90%) is the main registered carbon containing product in all cases. CO2 and small amounts of dimethyl ether (DME), methane and C2 –C4 hydrocarbons are also observed as by-products. In spite of this, the highest catalytic activity in all investigated temperature interval is observed for the 5.1Mn-N(W) (Figs. 1 and 2). This result is also confirmed by the variations in the manganese loading of the samples (Fig. 3(a) and (b)). 100 80 Conversion (mol%)
96
5.6Mn-C(W)
60
5.6Mn-A(W) 7.3Mn-Ac(W)
40 5.1Mn-N(W)
20 0 550
600
650 700 Temperature ( K)
750
800
Fig. 1. Temperature dependencies of methanol decomposition on manganese supported on activated carbon materials, obtained by various manganese precursors in aqueous medium.
Table 1 Composition, specific surface area, Weiss constants (H) and magnetic moments (leff ) of the manganese supported on activated carbon samples prepared in aqueous or methanol solution of various manganese precursors Sample
Precursor
Medium
Mn (wt%)
BET (m2 /g)
leff (BM)
1.6Mn-A(W) 3.8Mn-A(W) 5.6Mn-A(W) 2.1Mn-N(W) 5.1Mn-N(W) 9.4Mn-N(W) 7.1Mn-N(M) 7.3Mn-Ac(W) 7.3Mn-Ac(M) 5.6Mn-C(W) 7.6Mn-C(M)
Mn[(NH3 )6 ](NO3 )2 Mn[(NH3 )6 ](NO3 )2 Mn[(NH3 )6 ](NO3 )2 Mn(NO3 )2 Mn(NO3 )2 Mn(NO3 )2 Mn(NO3 )2 Mn(CH3 COO)2 Mn(CH3 COO)2 MnCl2 MnCl2
H2 O H2 O H2 O H2 O H2 O H2 O CH3 OH H2 O CH3 OH H2 O CH3 OH
1.6 3.8 5.6 2.1 5.1 9.4 7.1 7.3 7.3 5.6 7.6
682 730 474 832 448 851 472 810 700 448 339
b
a b
leff does not correlate with the Curie–Wiess low. Very low accuracy.
4.74 4.10
H (K) )112 )154
b
5.46 4.24 3.86
)286 )130 )51
a a a
5.44
28
S. Vankova et al. / Catalysis Communications 5 (2004) 95–99
analogues [9,11,12]. Here, the favourable role of ammonia precursor [9,11] and organic deposition medium [12] for the catalysts activity and stability in methanol decomposition has been reported. In order to clear the different effect of the metal precursor and deposition medium on the catalytic behaviour of copper and manganese loaded samples some additional investigations on their structure, magnetic and reduction properties are done. In Fig. 4 are presented X-ray diffraction patterns of the selected samples. XRD signals, typical for Mn3 O4 are observed in all cases and their intensities are the highest in the case of 5.6Mn-A (W). XRD signals with very low intensities, typical for MnO are also found in the spectra of acetate obtained materials. The effective magnetic moments (leff ) and Wiess constants (H) of the samples are shown in Table 1. The values of leff change in the range of 3.60–5.88 BM. According to [24,25] the calculated theoretical effective magnetic moment (leff ) for Mn(II), Mn(III) and Mn(IV) ions in octahedral crystal field is 5.92, 4.70 and 3.44 BM, respectively. So, the presence of Mn(II) as well as Mn(III) species could be concluded for all studied materials. The variations in the leff (Table 1), clearly show that the relative part of the reduced manganese species significantly changes with the nature of the manganese precursor used. Higher amount of Mn(II) could be expected for 5.1Mn-N(W) in comparison with its ammonia analogue. At the same time it is slowly affected by the precursor deposition medium. At the same time the values of leff for the acetate prepared samples do not correlate with the Curie–Weiss low. Hence, the presence of different manganese containing phases, characterized with various Weiss constants (H) could be assumed in this case. This result is also confirmed by the XRD data for the acetate samples, where the presence of MnO as well as Mn3 O4 is observed (Fig. 4). It is worth mentioning, that the highest values of leff is found for the sample characterized with the higher catalytic activity (5.1Mn-N (W)). In this case, a higher negative values of Wiess constant (H), typical for
100 5.1Mn-N(W)
Conversion (mol%)
80 7.1Mn-N(M)
60
7.3Mn-Ac(W) 7.3Mn-Ac(M)
40 5.6Mn-C(W) 7.6Mn-C(M)
20 0 550
600
650 700 Temperature (K)
750
800
Fig. 2. Methanol decomposition vs. temperature on manganese supported on activated carbon materials obtained from various manganese precursors. For the comparison their aqueous obtained analogues are given (dash lines).
Here, again higher catalytic activity is observed for the nitrate obtained materials in comparison with their ammonia analogues for all studied manganese concentrations. It is worth mentioning also that, despite the precursor used, a maximum in the catalysts activity with the manganese loading is observed and the best catalytic activity is registered at about 4–5 wt% manganese in the samples. No essential catalytic effect of the precursor deposition medium is established for nitrate and acetate based materials as well (Fig. 2). The chloride obtained samples differs significantly from their manganese containing analogues in their catalytic behaviour (Figs. 1, 2). Lower catalytic activity (up to 20 or 40% for 5.6Mn-C(W) and 7.6Mn-C(M), respectively) and well defined tendency for the CO selectivity decrease with the temperature increase is found in this case. The observed effects of the precursor on the catalytic behaviour of manganese modified materials are in contrast with the data obtained for their copper containing
80
80 2.1Mn-N(W) 5.1Mn-N(W)
40
9.4Mn-N(W)
Conversion (mol%)
100
Conversion (mol%)
100
60
(a)
1.6Mn-A(W)
60 3.8Mn-A(W)
40
5.6Mn-A(W)
20
20 0 500
97
550
600 650 700 Temperature (K)
0
750
(b)
550
600 650 Temperature (K)
700
750
Fig. 3. Effect of manganese loading on the catalytic activity of methanol decomposition on nitrate (a) or ammonia (b) obtained manganese/activated carbon materials.
98
S. Vankova et al. / Catalysis Communications 5 (2004) 95–99
Fig. 4. XRD patterns of manganese modified activated carbon samples obtained by various manganese precursors: (1) 5.1Mn-N(W); (2) 5.6MnA(W); (3) 5.6Mn-C(W); (4) 7.3Mn-Ac(M); (5) 7.3-Ac(W).
the presence of strong anti-ferromagnetic exchange interaction, is also registered. On the contrary, high values of leff , but very small positive values of H, which are typical for the presence of ferromagnetic interaction, are found for 7.6Mn-C(M). A significantly lower catalytic activity in comparison with the other samples is observed in this case. In our previous study the key role of catalytic active complex (CAC) in the heterogeneous catalysis was discussed [9,11,14,26,27]. It forms not only during the preparation procedure of the catalysts but also due to the influence of the reaction medium. In our opinion, the nature and the amount of CAC and also their stabilization determine the catalytic properties of the samples. In the case of metal–metal oxide supported on activated carbon catalysts it usually consists of metal species bonded not only between themselves but with the functional groups of the support as well. More over, depending on the nature of the supported active phase, the inclusion of metal ions in different oxidative state is also possible. In the case of manganese supported activated carbon the participation of Mn(II) species in the CAC formation was proved [14]. Here, the combined XRD and magnetic results could be an indication that this type of species present in all investigated samples. Probably, they are formed in the course of the catalysts preparation procedure due to the reduction effect of the support (AC). However, the variations in the amount and the state of Mn(II) with the preparation method used could be assumed. The lower XRD signals and the higher value of leff and H, combined with the higher catalytic activity and selectivity to CO for 5.1Mn-N(W) in comparison with its ammonia analogue could be an
indication for the easier formation of CACs in the nitrate obtained sample during the course of catalysts preparation. It is not excluded, this effect to be a result from the higher precursor dispersion of nitrate obtained samples. At the same time, the lower catalytic activity founded for the acetate samples could be a result from the CAC concentration decrease due to the formation of MnO in them. The observed maximum in the catalytic activity and leff with the precursor concentration for the nitrate and ammonia samples could be ascribed to the changes in the manganese reduction degree during the preparation procedure. It is not excluded, the extremum in the catalytic behaviour of chloride prepared
Fig. 5. TPR curves of manganese modified activated carbon samples obtained by various manganese precursors (1) 5.6Mn-A(W); (2) 7.3Ac(W); (3) 5.6Mn-C(W); (4) 5.1Mn-N(W).
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materials to be due to the lower degree of MnCl2 decomposition under the preparation conditions used. This assumption is also confirmed by the significantly lower ABET for these samples and also by the calculated value of leff , which does not correlate with the Curie– Weiss low (Table 1). These conclusions are also confirmed by the TPR experiments (Fig. 5). A negligible reduction effect is registered only in the case of 5.1Mn-N(W) and it is in agreement with the assumption for the predominantly formation of Mn(II) even during the preparation procedure in it. The observed differences in the initial reduction temperature for the acetate, ammonia and chloride samples could be also an evidence for some variation in the manganese state (including the presence of different manganese containing phases) in them.
4. Conclusion The catalytic activity of manganese supported on activated carbon materials depends on the manganese precursor used. The sample obtained from aqueous solution of Mn(NO3 )2 exhibits the highest catalytic activity. In contrast with the corresponding copper loaded materials no favourable effect of the ammonia precursor or organic medium is confirmed for their manganese analogues.
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