- Email: [email protected]
Hydrogenation of carbon monoxide under mechanical activation conditions
Applied Catalysis A: General 366 (2009) 201–205
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Hydrogenation of carbon monoxide under mechanical activation conditions Francesco Delogu a,*, Gabriele Mulas b, Sebastiano Garroni b,1 a b
Dipartimento di Ingegneria Chimica e Materiali, Universita` degli Studi di Cagliari, piazza d’Armi, I-09123 Cagliari, Italy Dipartimento di Chimica, Universita` degli Studi di Sassari, via Vienna 2, I-07100 Sassari, Italy
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 May 2009 Received in revised form 6 July 2009 Accepted 7 July 2009 Available online 15 July 2009
The work focuses on the hydrogenation of carbon monoxide over solid catalysts undergoing mechanical activation by ball milling. The rate of carbon monoxide conversion at individual ball impacts was estimated by measuring the impact frequency, the mass of powders involved in individual impacts and their duration. The rate of the mechanochemical hydrogenation process was compared with the one of the corresponding thermal process. An enhancement of the catalyst activity under mechanical activation conditions is observed. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Mechanochemistry Carbon monoxide hydrogenation Catalysis
1. Introduction The mechanical activation (MA) of materials is based on the application of non-hydrostatic stresses to solid phases and their consequent deformation [1–3]. Pulling the crystal away from thermodynamic equilibrium, the mechanical deformation induces a local modification of the atomic coordination shells and a correlated enhancement of the chemical reactivity [4–6]. Such enhancement is related both to transient atomic rearrangement processes and to the generation of a steady-state population of lattice defects [4–6]. Also due to its ease of utilization and apparent simplicity, MA has been variously exploited in areas of application as different as powder metallurgy [3], mineral processing [7], organic synthesis [8–10] and hydrogen storage [11]. However, its intrinsic potential has been long overshadowed by the difficulty of directly investigating local chemical events at the surface or in the bulk of crystals undergoing high speed deformations [1–3]. Nevertheless, indirect insight has been obtained by a suitable mathematical model relating the kinetics of MA processes to local deformation events [12–14]. Thus, it has been possible to show that chemical transformations often proceed at higher rates under MA conditions rather than under isothermal ones [12–17]. The experimental evidences hitherto obtained concern solid– solid [12–14], solid–liquid [15] and solid–gas [16,17] reactions. It is
* Corresponding author. Tel.: +39 0706755073; fax: +390706755067. E-mail address: [email protected] (F. Delogu). 1 Present address: Departament de Fisica, Universitat Autonoma de Barcelona, 08193, Bellaterra, Spain. 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.07.011
therefore reasonable questioning whether MA could as well influence heterogeneous catalysis. Focusing precisely on this issue, the present work investigates the hydrogenation of carbon monoxide (CO) over a solid catalyst in powder form, a well known reaction with a variety of applications [18–22]. Aimed at pointing out possible mechanochemical effects, the hydrogenation process was carried out under both MA and isothermal conditions and the experimental findings suitably compared. Attention was focused on the early stages of the CO hydrogenation process. Experimental details and methodologies are given below. 2. Experimental methods For clarity, the characteristics of catalysts and the catalytic runs will be dealt with in separate subsections. A brief description of the procedures concerning MA is first given. 2.1. The mechanical treatment Catalyst powders were mechanically treated inside the stainless steel reactor of a commercial Mixer/Mill mod. 8000 (Spex CertiPrep Inc., Metuchen, NJ, USA). The reactor has a cylindrical inner chamber about 2 and 6 cm, respectively in radius and height, within which a powder mass mp of 8 g was introduced together with a stainless steel ball of 12 g. The reactor was then sealed in Ar and fixed to the mill mechanical arm. When the mill operates, the reactor moves along a relatively complex three-dimensional trajectory at a frequency n of about 14.6 Hz [23]. As a consequence, the ball inside repeatedly collides with the reactor walls. At the aforementioned mp and n values, the volume v* of powders trapped between the colliding surfaces is roughly equal to
202
F. Delogu et al. / Applied Catalysis A: General 366 (2009) 201–205
0.3 mm3. v* is quantity intrinsically determined by the powder and ball dynamics inside the reactor [23]. It keeps approximately constant provided that powder particles occupy at least the 1% of the reactor volume and that their average size keeps substantially constant during the treatment [24]. This is the case of the catalyst powders employed in the present study. The mass m* of powders trapped at collision depends on their density and on v* [23], but both v* and m* are insensitive to the powder microstructure [23]. With v* equal to about 0.3 mm3, the ball undergoes almost perfectly inelastic collisions lasting a time interval Dt* of about 1 ms [23–25]. Under such conditions, the ball trajectory becomes periodic and characterized by a frequency N of impacts of about 29.2 Hz and an average energy E transferred by the ball to the powders at impact equal to about 0.1 J [23–26]. About the 5% of E is retained by the ball after collision [25,26], the 40% is transformed into kinetic energy of powder particles escaping from the impact area [25,26] and the 3% is stored into defects [12–14,23]. Therefore, the amount of energy dissipated as heat amounts to about the 52% of E. It follows that, depending on their specific heat, trapped powders can undergo temperature rises DT* of about 50–150 K. This range of values agrees with indirect experimental DT* estimates [27] and is supported by numerical modeling [13]. Actually, deformation events due to the intense stresses generated by attrition of surface asperities can give rise to transient local temperatures as high as 1000 K [6,28]. Under such circumstances, electronic excitation can well take place. However, such temperatures are retained for times as short as 10 ns or less [6,28], so that possible mechanochemical effects at the surface of a catalyst should be mostly ascribed to local deformation events [6,28]. 2.2. The solid catalysts The catalysts consist of Co50Fe50 solid solution and Ni supported respectively on TiO2 anatase and monoclinic ZrO2 powders. Their chemical compositions are (Co50Fe50)0.2(TiO2)99.8 and Ni40(ZrO2)60. All the cited phases are nanocrystalline. The catalysts were prepared by mechanical treatment. This was performed as previously described and prolonged for 20 and 100 h respectively for (Co50Fe50)0.2(TiO2)99.8 and Ni40(ZrO2)60. No significant change in average particle size, total surface area and microstructure of powders occur during successive catalytic runs. Powders were handled under Ar atmosphere with O2, N2 and H2O content below 1 ppm. Catalyst powders were studied by X-ray diffraction (XRD) with a Rigaku D/Max diffractometer equipped with a Cu radiation tube and a graphite monochromator in the diffracted beam. Analyses were carried out under Ar atmosphere with a special sample holder. The XRD patterns are shown in Figs. 1 and 2. As for the (Co50Fe50)0.2(TiO2)99.8 catalyst, it can be seen that the mechanical treatment transform almost completely the anatase into brookite
Fig. 2. Experimental XRD pattern of the Ni40(ZrO2)60 catalyst. ( ) Ni fcc, (&) ZrO2 tetragonal, (*) ZrO2 monoclinic. The numerical profiles obtained from the Rietveld analysis are also shown.
and rutile. The Rietveld analysis [29] indicates that the average size of the Co50Fe50 coherent diffraction domains is around 20 nm. Brookite and rutile have instead average domain size of about 40 and 30 nm, respectively. In the case of the Ni40(ZrO2)60 catalyst, ZrO2 partially undergoes the monoclinic-to-tetragonal transition. Ni has coherent diffraction domains of average size around 40 nm, whereas both monoclinic and tetragonal ZrO2 domains are on the order of 10 nm. Experiments carried out with a Malvern Zetasizer nano s90 indicate that (Co50Fe50)0.2(TiO2)99.8 and Ni40(ZrO2)60 powders have average particle size of about 0.3 and 0.2 mm, respectively. The specific surface area Sp of the catalysts was measured by N2 adsorption according to the BET method [30]. The powders were transferred into the BET Fisons Instrument apparatus under Ar atmosphere, but Ar was successively replaced with N2 once the sample holder was connected with the device. (Co50Fe50)0.2(TiO2)99.8 and Ni40(ZrO2)60 powders exhibit Sp values roughly equal to 60 and 55 m2 g1. Isothermal adsorption curves have the typical shape of non-porous systems. The inhomogeneous character of catalyst powders, i.e. the presence of interfaces between metallic phase and support, was investigated by low-angle XRD. Full details will be reported elsewhere. It suffices here to report that supported Co50Fe50 and Ni domains, which represent the catalytically active phases, exhibit average size of about 30 and 50 nm, respectively. They have then specific surface area Sp,act of about 24 and 13 m2 g1 respectively. A few final words of comment about the catalysts. First, the mechanical treatment allows the transition of anatase to brookite and rutile roughly at room temperature, whereas about 1000 K are otherwise required [31]. Second, dispersing the Co50Fe50 phase on TiO2 by mechanical treatment avoids the oxidation of metallic species by reaction with the support, which occurs instead when the powders undergo a prolonged thermal treatment at 600 K or more. Specific experimental XRD analyses rule out the formation of oxidized Co50Fe50 phases. Yet, this does not mean that the interactions between metals and support are not important to catalysis. However, no investigation focusing on this issue has been here carried out. The same considerations also apply to the ZrO2supported Ni catalyst. 2.3. The mechanochemical and thermal CO hydrogenation processes
Fig. 1. Experimental XRD pattern of the (Co50Fe50)0.2(TiO2)99.8 catalyst. (*) Co50Fe50 bcc solid solution, (&) TiO2 rutile, (^) TiO2 brookite). The numerical profiles obtained from the Rietveld analysis are also shown.
CO hydrogenation was carried out under both MA and isothermal conditions on 8 g of catalyst powders. In the former case, powders were exposed to a 1:3 CO:H2 gaseous mixture. Experiments were carried out in batch inside a stainless steel cylindrical reactor with two pressure valves allowing the inlet and outlet of gases. The reactor was connected with an external gas tank where the desired gaseous mixture was prepared. The total pressure inside the reactor-tank system, measured by pressure transmitters, was set at 3 MPa. The reactor temperature was kept at 300 K and the frequency of reactor displacement at 14.6 Hz.
F. Delogu et al. / Applied Catalysis A: General 366 (2009) 201–205
Experiments under isothermal conditions were also carried out in batch. The catalyst powders were placed inside the reactor and exposed to a 1:3 CO:H2 gaseous mixture at 300 and 450 K. Trials were performed both with the reactor displacing at 14.6 Hz and with the reactor at rest, in both cases with no ball inside. No difference in the catalyst performances in the two cases was observed. Both as-synthesized catalyst powders and catalyst powders submitted to an activation procedure were used. Following previous work [20], in the latter case the catalysts were activated by exposing for 48 h the powders to a flow of gaseous H2 at a pressure of 1 MPa and at a temperature of 450 K. XRD analyses on H2-activated powders did not point out any microstructural change, so that H2 should only have reduced the metallic domains at the surface. CO hydrogenation was monitored by gas-chromatography (GC). A Fisons 8000 apparatus equipped with a HWD detector was employed to monitor CO and H2, whereas hydrocarbons were analyzed with a PerkinElmer 8600 apparatus equipped with a FID detector. Suitable standards were employed to perform quantitative analyses. The data obtained were then used to calculate the reaction mass balance. This points out that roughly all the C atoms of CO molecules converted are found in products. These essentially consist of simple hydrocarbons such as methane, ethane and propane. Traces of butane are also noted. The relative amounts of products change with the time of mechanical treatment. However, the initial stages of CO hydrogenation are characterized by a proportional decrease of CO and H2 partial pressures. Accordingly, CO hydrogenation proceeds with a stoichiometry of about 3 mol of H2 per 1 mol of CO. At least during the first 10 h of reaction, methane almost represents the only reaction product. The amounts of other hydrocarbons are negligible. For simplicity, attention was focused on the catalytic behavior of powders during the first stages of CO hydrogenation. In this way, CO conversion and methane formation can be considered almost synonymous terms. 3. Results The catalytic runs points out that only H2-activated catalysts promote the CO hydrogenation process under isothermal conditions. No reaction takes indeed place, at least in a time interval 10 h long, when as-synthesized catalysts are used. No H2 activation is indeed necessary for CO conversion to occur when CO hydrogenation is carried out under MA. In this case, also the as-synthesized powders catalyze the CO conversion. In addition, no difference between the performances of as-synthesized and H2-activated powders was observed under MA. The number n of CO moles reacted on (Co50Fe50)0.2(TiO2)99.8 powders is shown in Fig. 3 as a function of time t. Data regard the as-synthesized catalyst submitted to MA as well as the H2activated powders working at 300 and 450 K. The reaction rates
203
Fig. 4. The number n of CO moles reacted as a function of time t. Data refer to CO hydrogenation processes carried out over Ni40(TiO2)60 catalyst powders. Best-fitted lines are also shown.
rmech, rth,300 and rth,450 of the reactions carried out under MA and isothermal conditions at 300 and 450 K were estimated from the slope of the curves in Fig. 3. They amount to about 7.8 107, 3.3 107 and 8.7 104 mol h1, respectively. It follows that the rate of the MA process is apparently three orders of magnitude lower than the one of the reaction performed at 450 K, whereas it is comparable with the one of the reaction performed at 300 K. Under the hypothesis that only the metallic phase is catalytically active, the whole CO conversion must be ascribed to the 0.011 g of Co50Fe50 solid solution contained in 8 g of (Co50Fe50)0.2(TiO2)99.8 catalyst. The number n of CO moles reacted on of Ni40(ZrO2)60 powders is reported in Fig. 4 as a function of time t. As in the previous case, data concern the as-synthesized catalyst submitted to MA and the H2-activated powders submitted to isothermal conditions at 300 and 450 K. The observed CO conversion rates rmech, rth,300 and rth,450 are in this case equal respectively to about 2.4 105, 6.8 108 and 1.1 105 mol h1. Thus, the MA CO hydrogenation is about three orders of magnitude faster than the reaction carried out at 300 K, whereas its rate is comparable with the one of the reaction performed at 450 K. The content of catalytically active metallic phase in 8 g is here much higher than in the (Co50Fe50)0.2(TiO2)99.8 catalyst, amounting to 1.933 g. All the rmech, rth,300 and rth,450 values are reported in Table 1. It should be noted that the rth,300 and rth,450 conversion rates regard reactions performed in a moving batch reactor and cannot be directly compared with the ones obtained from catalytic runs with fixed bed catalysts working under flow conditions. Nevertheless, the comparison with a few literature values regarding the performances of similar catalysts working under conventional conditions can be useful to give an idea of the reliability of the data obtained in the present work. As for the (Co50Fe50)0.2(TiO2)99.8 catalyst, its performances can be compared for example with the ones exhibited by a (Co50Fe50)5(TiO2)95 catalyst [32]. This was tested at a temperature of 493 K and a total pressure of a 1:2 CO:H2 mixture equal to 1 MPa [34]. The CO conversion rate for this catalyst amounts to about 3.5 104 mol h1 [32]. It is then on the same order of magnitude of the rth,450 value here obtained. The Ni40(ZrO2)60 catalyst performances can be instead compared with the ones of a (Co11Ni7)(ZrO2)82 system tested under flow at temperature and pressure values of 520 K and 1 MPa, respectively with a 1:1 CO:H2 gaseous mixture [33]. Also in this case, the CO conversion rate on such catalyst under conventional conditions is on the same order of magnitude than rth,450. It follows that the rth,300 and rth,450 values obtained in the present work are satisfactorily in line with available literature data. 4. Discussion
Fig. 3. The number n of CO moles reacted as a function of time t. Data refer to CO hydrogenation processes carried out over (Co50Fe50)0.2(TiO2)99.8 catalyst powders. Best-fitted lines are also shown.
The experimental findings described above represent the starting point for a deeper analysis of the kinetic features of the
204
F. Delogu et al. / Applied Catalysis A: General 366 (2009) 201–205
Table 1 The rates, rmech and rth, and the specific rates, rsp;mech and rsp,th, of the reactions performed under mechanical and thermal activation conditions for the (Co50Fe50)0.2(TiO2)99.8 and Ni40(ZrO2)60 catalysts. rmech
rth,300
rth,450
Literature
(Co50Fe50)0.2(TiO2)99.8 Rate (mol h1)
7.8 107
3.3 107
9.4 103
3.5 104 [32]
Ni40(ZrO2)60 Rate (mol h1)
2.4 105
6.8 108
1.1 105
1.3 105 [33]
rsp;mech
rsp,th,300
rsp,th,450
(Co50Fe50)0.2(TiO2)99.8 Specific rate (mol g1 s1)
6.7 106
1.1 1011
3.0 108
Ni40(ZrO2)60 Specific rate (mol g1 s1)
1.2 104
2.3 1012
3.8 1010
CO hydrogenation processes carried out under MA conditions and a better comprehension of the MA ability of enhancing the rates of chemical transformations. It is first necessary worth noting that a direct comparison between the CO hydrogenation rates estimated for processes performed under MA and isothermal conditions is in principle wrong. In fact, the rates of CO hydrogenation under MA are referred to absolute time. However, this is not the best quantity to which refer the transformation. The reason is that most of time the ball simply travels between the opposite bases of the reactor, with no effect on the powders. Only at impact, non-hydrostatic stresses can indeed operate on them. It follows that the number n of impacts occurred inside the reactor is a more suited quantity to describe the kinetics of MA processes. Along this line, two other points are worth noticing. First, the CO conversion at any given individual impact between the ball and the reactor walls must be ascribed to the chemical action of the mass m* of catalyst powders involved in such impact. Indeed, assynthesized powders become catalytically active only under MA conditions. It follows that deformation processes taking place during the impact are necessary events for the activation of reactive sites at the catalyst surface. At the same time, it should be noted that the catalyst powders were also prepared by mechanical treatment, so that their nanocrystalline structure already possesses the highest possible amount of lattice defects allowed by repeated plastic deformation. Thus, the catalytic activity of assynthesized powders must not be ascribed to the steady-state population of lattice defects, but rather to the transient phenomena occurring during the deformation and the subsequent relaxation. Under such circumstances, since relaxation processes attain completion in time intervals of 1–10 ns [1–3,6,28], it can be assumed that transient phenomena have a lifetime t roughly equal to the duration Dt* of the impact. It is then possible to say that mechanochemical effects, if any, should operate on a time scale given by t. With such premises, it is relatively easy to work out a specific rate rsp;mech of CO hydrogenation under MA conditions referred to individual collisions. To such aim, rmech must be normalized to the impact frequency N, the powder mass m* trapped on the average at individual impacts and the average impact duration t. In formula, rsp;mech ¼ r mech N1 m1 t 1 . It is here worth remembering that all the aforementioned quantities were measured during experiments. In particular, N and t roughly amounted to 29.2 Hz and 1 ms, whereas m* was equal to about 1.1 and 1.9 mg for (Co50Fe50)0.2(TiO2)99.8 and Ni40(ZrO2)60 powders respectively. The specific CO conversion rates rsp;mech therefore amount to about 6.7 106 and 1.2 104 mol g1 s1, respectively for the (Co50Fe50)0.2(TiO2)99.8 and Ni40(ZrO2)60 catalyst powders. These rsp;mech values must be compared with the specific CO conversion rates rsp,th obtained under isothermal conditions.
Taking into account that the whole powder charge mp participates in the reaction under isothermal conditions, rsp,th can be easily obtained by dividing rth by mp. For the (Co50Fe50)0.2(TiO2)99.8 catalyst, rsp,th,300 and rsp,th,450 are equal to about 1.1 1011 and 3.0 108 mol g1 s1 respectively. For the Ni40(ZrO2)60 catalyst, they are instead equal to about 2.3 1012 and 3.8 1010 mol g1 s1. All the different rsp;mech , rsp,th,300 and rsp,th,450 values are reported in Table 1 for comparison. It immediately appears that rsp;mech is respectively two and six orders of magnitude higher than the largest rsp,th values obtained for the (Co50Fe50)0.2(TiO2)99.8 and the Ni40(ZrO2)60 catalysts. The enhancement of the catalytic activity observed under MA conditions cannot be simply related to temperature and surface effects. In fact, although it is not reasonable to expect temperature rises larger than 150 K, the powders undergoing MA exhibit catalytic performances better than the ones of H2-activated catalyst powders submitted to isothermal conditions at 450 K. Regarding surface effects, both wide- and small-angle XRD analyses suggest that the coherent diffraction domains of the catalytically active phase have the same size and strain content before and after the catalytic runs. The same is true for the catalyst specific surfaces, as shown by BET analyses. It is thus more reasonable to relate the enhanced reactivity of powders under MA conditions to the nature and lifetime of local modifications of the crystalline structure due to the deformation events taking place at the impacts between the ball and the reactor walls. A final observation concerning the different performances of the (Co50Fe50)0.2(TiO2)99.8 and Ni40(ZrO2)60 catalysts is necessary. The specific CO conversion rates rsp;mech obtained under MA conditions apparently indicate that the Ni40(ZrO2)60 catalyst is much more effective than the (Co50Fe50)0.2(TiO2)99.8 one. However, it must be noted that the amount of catalytically active phase working at each impact in the two cases is different. This can be suitably taken into account by referring the specific CO conversion rate rsp;mech under mechanical activation to the mass mact of catalytically active phase and not to the total mass m* of catalyst powders involved in individual impacts. The result indicates that the CO hydrogenation reaction proceeds on the (Co50Fe50)0.2(TiO2)99.8 catalyst at a specific rate of about 4.8 103 mol g1 s1, whereas the corresponding value for Ni40(ZrO2)60 powders amounts to about 4.9 104 mol g1 s1. The (Co50Fe50)0.2(TiO2)99.8 catalyst exhibits therefore a higher activity than the Ni40(ZrO2)60 one, at least as far as the activity is referred to the amount of catalytically active phase available to the reaction. 5. Conclusions The experimental findings discussed in the present work point out that the MA has a marked effect on the performances of
F. Delogu et al. / Applied Catalysis A: General 366 (2009) 201–205
catalyst powders. A significant enhancement of the chemical reactivity of catalysts working under MA conditions is observed, which can be reasonably related to the deformation processes involving the powder amount trapped at individual impacts between the ball and the reactor walls. A satisfactory explanation of the experimental evidences obtained should rely on the intimate nature of such deformation processes, which is however quite obscure. For this reason, no definite rationale for the observed behavior is available. In spite of this, the rough order-of-magnitude estimates of the CO hydrogenation rates under MA conditions clearly define an interesting scenario which deserves deeper investigations. Acknowledgements Financial support has been provided by the University of Cagliari and the University of Sassari. References [1] [2] [3] [4]
G. Heinicke, Tribochemistry, Berlin, Akademie-Verlag, 1984. P. Yu Butyagin, Sov. Sci. Rev. B Chem. 14 (1989) 1. C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1. E.M. Gutman, Mechanochemistry of Materials, Cambridge International Science Publishing, Cambridge, 1998. [5] V.I. Levitas, O.M. Zarechnyy, J. Phys. Chem. B 110 (2006) 16035. [6] F. Delogu, G. Cocco, Phys. Rev. B 74 (2006) 035406. [7] P. Balaz, Mechanochemistry in Nanoscience and Minerals Engineering, Springer, Berlin, 2008.
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
205
B. Rodriguez, A. Bruckmann, C. Bolm, Adv. Synth. Cat. 349 (2007) 2213. G. Kaupp, Top. Curr. Chem. 254 (2005) 95. G. Kaupp, Cryst. Eng. Commun. 11 (2009) 388. B. Sakintuna, F. Lamari-Darkrim, M. Hirscher, Int. J. Hydrogen Energy 32 (2007) 1121. S. Garroni, F. Delogu, G. Mulas, G. Cocco, Scripta Mater. 57 (2007) 964. F. Delogu, Acta Mater. 56 (2008) 905. F. Delogu, Acta Mater. 56 (2008) 2344. A. Molnar, M. Varga, G. Mulas, M. Mohai, I. Bertoti, A. Lovas, G. Cocco, Mater. Sci. Eng. A 434 (2001) 1078. G. Mulas, F. Delogu, G. Cocco, J. Alloys Compd. 473 (2009) 180. F. Delogu, G. Mulas, Int. J. Hydrogen Energy 34 (2009) 3026. H.H. Storch, N. Golumbic, R.B. Anderson, The Fischer-Tropsch and Related Synthesis, New York, Wiley, 1951. J.R. Anderson, M. Boudart (Eds.), Catalysis Science and Technology, vol.1, Springer, Berlin, 1981. R.B. Anderson, The Fischer-Tropsch synthesis, Academic Press, New York, 1984. M.E. Dry, Catal. Today 71 (2002) 227. B.H. Davis, Catal. Today 71 (2002) 249. F. Delogu, C. Deidda, G. Mulas, L. Schiffini, G. Cocco, J. Mater. Sci. 39 (2004) 5121. G. Manai, F. Delogu, L. Schiffini, G. Cocco, J. Mater. Sci. 39 (2004) 5319. G. Manai, F. Delogu, M. Rustici, Chaos 12 (2002) 601. C. Deidda, F. Delogu, G. Cocco, J. Mater. Sci. 39 (2004) 5315. F. Delogu, G. Cocco, J. Alloys Compd. 58 (2008) 126. F. Delogu, G. Cocco, Phys. Rev. B 72 (2005) 014124. L. Lutterotti, R. Ceccato, R. Dal Maschio, E. Pagani, Mater. Sci. Forum 278 (1998) 281. J.M. Thomas, W.J. Thomas, Principles and Practice of Heterogeneous Catalysis, VCH, Weinheim, 1997. F. Delogu, J. Alloys Compd. 468 (2009) 22. D.J. Duvenhage, N.J. Coville, Appl. Catal. A: Gen. 153 (1997) 43. R. Sethuraman, N.N. Bakhshi, S.P. Katikaneni, R.O. Idem, Fuel Proc. Technol. 73 (2001) 197. J.R. Anderson, Structure of metallic catalysts, Academic Press, London, 1975.
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Hydrogenation of carbon monoxide under mechanical activation conditions Francesco Delogu a,*, Gabriele Mulas b, Sebastiano Garroni b,1 a b
Dipartimento di Ingegneria Chimica e Materiali, Universita` degli Studi di Cagliari, piazza d’Armi, I-09123 Cagliari, Italy Dipartimento di Chimica, Universita` degli Studi di Sassari, via Vienna 2, I-07100 Sassari, Italy
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 May 2009 Received in revised form 6 July 2009 Accepted 7 July 2009 Available online 15 July 2009
The work focuses on the hydrogenation of carbon monoxide over solid catalysts undergoing mechanical activation by ball milling. The rate of carbon monoxide conversion at individual ball impacts was estimated by measuring the impact frequency, the mass of powders involved in individual impacts and their duration. The rate of the mechanochemical hydrogenation process was compared with the one of the corresponding thermal process. An enhancement of the catalyst activity under mechanical activation conditions is observed. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Mechanochemistry Carbon monoxide hydrogenation Catalysis
1. Introduction The mechanical activation (MA) of materials is based on the application of non-hydrostatic stresses to solid phases and their consequent deformation [1–3]. Pulling the crystal away from thermodynamic equilibrium, the mechanical deformation induces a local modification of the atomic coordination shells and a correlated enhancement of the chemical reactivity [4–6]. Such enhancement is related both to transient atomic rearrangement processes and to the generation of a steady-state population of lattice defects [4–6]. Also due to its ease of utilization and apparent simplicity, MA has been variously exploited in areas of application as different as powder metallurgy [3], mineral processing [7], organic synthesis [8–10] and hydrogen storage [11]. However, its intrinsic potential has been long overshadowed by the difficulty of directly investigating local chemical events at the surface or in the bulk of crystals undergoing high speed deformations [1–3]. Nevertheless, indirect insight has been obtained by a suitable mathematical model relating the kinetics of MA processes to local deformation events [12–14]. Thus, it has been possible to show that chemical transformations often proceed at higher rates under MA conditions rather than under isothermal ones [12–17]. The experimental evidences hitherto obtained concern solid– solid [12–14], solid–liquid [15] and solid–gas [16,17] reactions. It is
* Corresponding author. Tel.: +39 0706755073; fax: +390706755067. E-mail address: [email protected] (F. Delogu). 1 Present address: Departament de Fisica, Universitat Autonoma de Barcelona, 08193, Bellaterra, Spain. 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.07.011
therefore reasonable questioning whether MA could as well influence heterogeneous catalysis. Focusing precisely on this issue, the present work investigates the hydrogenation of carbon monoxide (CO) over a solid catalyst in powder form, a well known reaction with a variety of applications [18–22]. Aimed at pointing out possible mechanochemical effects, the hydrogenation process was carried out under both MA and isothermal conditions and the experimental findings suitably compared. Attention was focused on the early stages of the CO hydrogenation process. Experimental details and methodologies are given below. 2. Experimental methods For clarity, the characteristics of catalysts and the catalytic runs will be dealt with in separate subsections. A brief description of the procedures concerning MA is first given. 2.1. The mechanical treatment Catalyst powders were mechanically treated inside the stainless steel reactor of a commercial Mixer/Mill mod. 8000 (Spex CertiPrep Inc., Metuchen, NJ, USA). The reactor has a cylindrical inner chamber about 2 and 6 cm, respectively in radius and height, within which a powder mass mp of 8 g was introduced together with a stainless steel ball of 12 g. The reactor was then sealed in Ar and fixed to the mill mechanical arm. When the mill operates, the reactor moves along a relatively complex three-dimensional trajectory at a frequency n of about 14.6 Hz [23]. As a consequence, the ball inside repeatedly collides with the reactor walls. At the aforementioned mp and n values, the volume v* of powders trapped between the colliding surfaces is roughly equal to
202
F. Delogu et al. / Applied Catalysis A: General 366 (2009) 201–205
0.3 mm3. v* is quantity intrinsically determined by the powder and ball dynamics inside the reactor [23]. It keeps approximately constant provided that powder particles occupy at least the 1% of the reactor volume and that their average size keeps substantially constant during the treatment [24]. This is the case of the catalyst powders employed in the present study. The mass m* of powders trapped at collision depends on their density and on v* [23], but both v* and m* are insensitive to the powder microstructure [23]. With v* equal to about 0.3 mm3, the ball undergoes almost perfectly inelastic collisions lasting a time interval Dt* of about 1 ms [23–25]. Under such conditions, the ball trajectory becomes periodic and characterized by a frequency N of impacts of about 29.2 Hz and an average energy E transferred by the ball to the powders at impact equal to about 0.1 J [23–26]. About the 5% of E is retained by the ball after collision [25,26], the 40% is transformed into kinetic energy of powder particles escaping from the impact area [25,26] and the 3% is stored into defects [12–14,23]. Therefore, the amount of energy dissipated as heat amounts to about the 52% of E. It follows that, depending on their specific heat, trapped powders can undergo temperature rises DT* of about 50–150 K. This range of values agrees with indirect experimental DT* estimates [27] and is supported by numerical modeling [13]. Actually, deformation events due to the intense stresses generated by attrition of surface asperities can give rise to transient local temperatures as high as 1000 K [6,28]. Under such circumstances, electronic excitation can well take place. However, such temperatures are retained for times as short as 10 ns or less [6,28], so that possible mechanochemical effects at the surface of a catalyst should be mostly ascribed to local deformation events [6,28]. 2.2. The solid catalysts The catalysts consist of Co50Fe50 solid solution and Ni supported respectively on TiO2 anatase and monoclinic ZrO2 powders. Their chemical compositions are (Co50Fe50)0.2(TiO2)99.8 and Ni40(ZrO2)60. All the cited phases are nanocrystalline. The catalysts were prepared by mechanical treatment. This was performed as previously described and prolonged for 20 and 100 h respectively for (Co50Fe50)0.2(TiO2)99.8 and Ni40(ZrO2)60. No significant change in average particle size, total surface area and microstructure of powders occur during successive catalytic runs. Powders were handled under Ar atmosphere with O2, N2 and H2O content below 1 ppm. Catalyst powders were studied by X-ray diffraction (XRD) with a Rigaku D/Max diffractometer equipped with a Cu radiation tube and a graphite monochromator in the diffracted beam. Analyses were carried out under Ar atmosphere with a special sample holder. The XRD patterns are shown in Figs. 1 and 2. As for the (Co50Fe50)0.2(TiO2)99.8 catalyst, it can be seen that the mechanical treatment transform almost completely the anatase into brookite
Fig. 2. Experimental XRD pattern of the Ni40(ZrO2)60 catalyst. ( ) Ni fcc, (&) ZrO2 tetragonal, (*) ZrO2 monoclinic. The numerical profiles obtained from the Rietveld analysis are also shown.
and rutile. The Rietveld analysis [29] indicates that the average size of the Co50Fe50 coherent diffraction domains is around 20 nm. Brookite and rutile have instead average domain size of about 40 and 30 nm, respectively. In the case of the Ni40(ZrO2)60 catalyst, ZrO2 partially undergoes the monoclinic-to-tetragonal transition. Ni has coherent diffraction domains of average size around 40 nm, whereas both monoclinic and tetragonal ZrO2 domains are on the order of 10 nm. Experiments carried out with a Malvern Zetasizer nano s90 indicate that (Co50Fe50)0.2(TiO2)99.8 and Ni40(ZrO2)60 powders have average particle size of about 0.3 and 0.2 mm, respectively. The specific surface area Sp of the catalysts was measured by N2 adsorption according to the BET method [30]. The powders were transferred into the BET Fisons Instrument apparatus under Ar atmosphere, but Ar was successively replaced with N2 once the sample holder was connected with the device. (Co50Fe50)0.2(TiO2)99.8 and Ni40(ZrO2)60 powders exhibit Sp values roughly equal to 60 and 55 m2 g1. Isothermal adsorption curves have the typical shape of non-porous systems. The inhomogeneous character of catalyst powders, i.e. the presence of interfaces between metallic phase and support, was investigated by low-angle XRD. Full details will be reported elsewhere. It suffices here to report that supported Co50Fe50 and Ni domains, which represent the catalytically active phases, exhibit average size of about 30 and 50 nm, respectively. They have then specific surface area Sp,act of about 24 and 13 m2 g1 respectively. A few final words of comment about the catalysts. First, the mechanical treatment allows the transition of anatase to brookite and rutile roughly at room temperature, whereas about 1000 K are otherwise required [31]. Second, dispersing the Co50Fe50 phase on TiO2 by mechanical treatment avoids the oxidation of metallic species by reaction with the support, which occurs instead when the powders undergo a prolonged thermal treatment at 600 K or more. Specific experimental XRD analyses rule out the formation of oxidized Co50Fe50 phases. Yet, this does not mean that the interactions between metals and support are not important to catalysis. However, no investigation focusing on this issue has been here carried out. The same considerations also apply to the ZrO2supported Ni catalyst. 2.3. The mechanochemical and thermal CO hydrogenation processes
Fig. 1. Experimental XRD pattern of the (Co50Fe50)0.2(TiO2)99.8 catalyst. (*) Co50Fe50 bcc solid solution, (&) TiO2 rutile, (^) TiO2 brookite). The numerical profiles obtained from the Rietveld analysis are also shown.
CO hydrogenation was carried out under both MA and isothermal conditions on 8 g of catalyst powders. In the former case, powders were exposed to a 1:3 CO:H2 gaseous mixture. Experiments were carried out in batch inside a stainless steel cylindrical reactor with two pressure valves allowing the inlet and outlet of gases. The reactor was connected with an external gas tank where the desired gaseous mixture was prepared. The total pressure inside the reactor-tank system, measured by pressure transmitters, was set at 3 MPa. The reactor temperature was kept at 300 K and the frequency of reactor displacement at 14.6 Hz.
F. Delogu et al. / Applied Catalysis A: General 366 (2009) 201–205
Experiments under isothermal conditions were also carried out in batch. The catalyst powders were placed inside the reactor and exposed to a 1:3 CO:H2 gaseous mixture at 300 and 450 K. Trials were performed both with the reactor displacing at 14.6 Hz and with the reactor at rest, in both cases with no ball inside. No difference in the catalyst performances in the two cases was observed. Both as-synthesized catalyst powders and catalyst powders submitted to an activation procedure were used. Following previous work [20], in the latter case the catalysts were activated by exposing for 48 h the powders to a flow of gaseous H2 at a pressure of 1 MPa and at a temperature of 450 K. XRD analyses on H2-activated powders did not point out any microstructural change, so that H2 should only have reduced the metallic domains at the surface. CO hydrogenation was monitored by gas-chromatography (GC). A Fisons 8000 apparatus equipped with a HWD detector was employed to monitor CO and H2, whereas hydrocarbons were analyzed with a PerkinElmer 8600 apparatus equipped with a FID detector. Suitable standards were employed to perform quantitative analyses. The data obtained were then used to calculate the reaction mass balance. This points out that roughly all the C atoms of CO molecules converted are found in products. These essentially consist of simple hydrocarbons such as methane, ethane and propane. Traces of butane are also noted. The relative amounts of products change with the time of mechanical treatment. However, the initial stages of CO hydrogenation are characterized by a proportional decrease of CO and H2 partial pressures. Accordingly, CO hydrogenation proceeds with a stoichiometry of about 3 mol of H2 per 1 mol of CO. At least during the first 10 h of reaction, methane almost represents the only reaction product. The amounts of other hydrocarbons are negligible. For simplicity, attention was focused on the catalytic behavior of powders during the first stages of CO hydrogenation. In this way, CO conversion and methane formation can be considered almost synonymous terms. 3. Results The catalytic runs points out that only H2-activated catalysts promote the CO hydrogenation process under isothermal conditions. No reaction takes indeed place, at least in a time interval 10 h long, when as-synthesized catalysts are used. No H2 activation is indeed necessary for CO conversion to occur when CO hydrogenation is carried out under MA. In this case, also the as-synthesized powders catalyze the CO conversion. In addition, no difference between the performances of as-synthesized and H2-activated powders was observed under MA. The number n of CO moles reacted on (Co50Fe50)0.2(TiO2)99.8 powders is shown in Fig. 3 as a function of time t. Data regard the as-synthesized catalyst submitted to MA as well as the H2activated powders working at 300 and 450 K. The reaction rates
203
Fig. 4. The number n of CO moles reacted as a function of time t. Data refer to CO hydrogenation processes carried out over Ni40(TiO2)60 catalyst powders. Best-fitted lines are also shown.
rmech, rth,300 and rth,450 of the reactions carried out under MA and isothermal conditions at 300 and 450 K were estimated from the slope of the curves in Fig. 3. They amount to about 7.8 107, 3.3 107 and 8.7 104 mol h1, respectively. It follows that the rate of the MA process is apparently three orders of magnitude lower than the one of the reaction performed at 450 K, whereas it is comparable with the one of the reaction performed at 300 K. Under the hypothesis that only the metallic phase is catalytically active, the whole CO conversion must be ascribed to the 0.011 g of Co50Fe50 solid solution contained in 8 g of (Co50Fe50)0.2(TiO2)99.8 catalyst. The number n of CO moles reacted on of Ni40(ZrO2)60 powders is reported in Fig. 4 as a function of time t. As in the previous case, data concern the as-synthesized catalyst submitted to MA and the H2-activated powders submitted to isothermal conditions at 300 and 450 K. The observed CO conversion rates rmech, rth,300 and rth,450 are in this case equal respectively to about 2.4 105, 6.8 108 and 1.1 105 mol h1. Thus, the MA CO hydrogenation is about three orders of magnitude faster than the reaction carried out at 300 K, whereas its rate is comparable with the one of the reaction performed at 450 K. The content of catalytically active metallic phase in 8 g is here much higher than in the (Co50Fe50)0.2(TiO2)99.8 catalyst, amounting to 1.933 g. All the rmech, rth,300 and rth,450 values are reported in Table 1. It should be noted that the rth,300 and rth,450 conversion rates regard reactions performed in a moving batch reactor and cannot be directly compared with the ones obtained from catalytic runs with fixed bed catalysts working under flow conditions. Nevertheless, the comparison with a few literature values regarding the performances of similar catalysts working under conventional conditions can be useful to give an idea of the reliability of the data obtained in the present work. As for the (Co50Fe50)0.2(TiO2)99.8 catalyst, its performances can be compared for example with the ones exhibited by a (Co50Fe50)5(TiO2)95 catalyst [32]. This was tested at a temperature of 493 K and a total pressure of a 1:2 CO:H2 mixture equal to 1 MPa [34]. The CO conversion rate for this catalyst amounts to about 3.5 104 mol h1 [32]. It is then on the same order of magnitude of the rth,450 value here obtained. The Ni40(ZrO2)60 catalyst performances can be instead compared with the ones of a (Co11Ni7)(ZrO2)82 system tested under flow at temperature and pressure values of 520 K and 1 MPa, respectively with a 1:1 CO:H2 gaseous mixture [33]. Also in this case, the CO conversion rate on such catalyst under conventional conditions is on the same order of magnitude than rth,450. It follows that the rth,300 and rth,450 values obtained in the present work are satisfactorily in line with available literature data. 4. Discussion
Fig. 3. The number n of CO moles reacted as a function of time t. Data refer to CO hydrogenation processes carried out over (Co50Fe50)0.2(TiO2)99.8 catalyst powders. Best-fitted lines are also shown.
The experimental findings described above represent the starting point for a deeper analysis of the kinetic features of the
204
F. Delogu et al. / Applied Catalysis A: General 366 (2009) 201–205
Table 1 The rates, rmech and rth, and the specific rates, rsp;mech and rsp,th, of the reactions performed under mechanical and thermal activation conditions for the (Co50Fe50)0.2(TiO2)99.8 and Ni40(ZrO2)60 catalysts. rmech
rth,300
rth,450
Literature
(Co50Fe50)0.2(TiO2)99.8 Rate (mol h1)
7.8 107
3.3 107
9.4 103
3.5 104 [32]
Ni40(ZrO2)60 Rate (mol h1)
2.4 105
6.8 108
1.1 105
1.3 105 [33]
rsp;mech
rsp,th,300
rsp,th,450
(Co50Fe50)0.2(TiO2)99.8 Specific rate (mol g1 s1)
6.7 106
1.1 1011
3.0 108
Ni40(ZrO2)60 Specific rate (mol g1 s1)
1.2 104
2.3 1012
3.8 1010
CO hydrogenation processes carried out under MA conditions and a better comprehension of the MA ability of enhancing the rates of chemical transformations. It is first necessary worth noting that a direct comparison between the CO hydrogenation rates estimated for processes performed under MA and isothermal conditions is in principle wrong. In fact, the rates of CO hydrogenation under MA are referred to absolute time. However, this is not the best quantity to which refer the transformation. The reason is that most of time the ball simply travels between the opposite bases of the reactor, with no effect on the powders. Only at impact, non-hydrostatic stresses can indeed operate on them. It follows that the number n of impacts occurred inside the reactor is a more suited quantity to describe the kinetics of MA processes. Along this line, two other points are worth noticing. First, the CO conversion at any given individual impact between the ball and the reactor walls must be ascribed to the chemical action of the mass m* of catalyst powders involved in such impact. Indeed, assynthesized powders become catalytically active only under MA conditions. It follows that deformation processes taking place during the impact are necessary events for the activation of reactive sites at the catalyst surface. At the same time, it should be noted that the catalyst powders were also prepared by mechanical treatment, so that their nanocrystalline structure already possesses the highest possible amount of lattice defects allowed by repeated plastic deformation. Thus, the catalytic activity of assynthesized powders must not be ascribed to the steady-state population of lattice defects, but rather to the transient phenomena occurring during the deformation and the subsequent relaxation. Under such circumstances, since relaxation processes attain completion in time intervals of 1–10 ns [1–3,6,28], it can be assumed that transient phenomena have a lifetime t roughly equal to the duration Dt* of the impact. It is then possible to say that mechanochemical effects, if any, should operate on a time scale given by t. With such premises, it is relatively easy to work out a specific rate rsp;mech of CO hydrogenation under MA conditions referred to individual collisions. To such aim, rmech must be normalized to the impact frequency N, the powder mass m* trapped on the average at individual impacts and the average impact duration t. In formula, rsp;mech ¼ r mech N1 m1 t 1 . It is here worth remembering that all the aforementioned quantities were measured during experiments. In particular, N and t roughly amounted to 29.2 Hz and 1 ms, whereas m* was equal to about 1.1 and 1.9 mg for (Co50Fe50)0.2(TiO2)99.8 and Ni40(ZrO2)60 powders respectively. The specific CO conversion rates rsp;mech therefore amount to about 6.7 106 and 1.2 104 mol g1 s1, respectively for the (Co50Fe50)0.2(TiO2)99.8 and Ni40(ZrO2)60 catalyst powders. These rsp;mech values must be compared with the specific CO conversion rates rsp,th obtained under isothermal conditions.
Taking into account that the whole powder charge mp participates in the reaction under isothermal conditions, rsp,th can be easily obtained by dividing rth by mp. For the (Co50Fe50)0.2(TiO2)99.8 catalyst, rsp,th,300 and rsp,th,450 are equal to about 1.1 1011 and 3.0 108 mol g1 s1 respectively. For the Ni40(ZrO2)60 catalyst, they are instead equal to about 2.3 1012 and 3.8 1010 mol g1 s1. All the different rsp;mech , rsp,th,300 and rsp,th,450 values are reported in Table 1 for comparison. It immediately appears that rsp;mech is respectively two and six orders of magnitude higher than the largest rsp,th values obtained for the (Co50Fe50)0.2(TiO2)99.8 and the Ni40(ZrO2)60 catalysts. The enhancement of the catalytic activity observed under MA conditions cannot be simply related to temperature and surface effects. In fact, although it is not reasonable to expect temperature rises larger than 150 K, the powders undergoing MA exhibit catalytic performances better than the ones of H2-activated catalyst powders submitted to isothermal conditions at 450 K. Regarding surface effects, both wide- and small-angle XRD analyses suggest that the coherent diffraction domains of the catalytically active phase have the same size and strain content before and after the catalytic runs. The same is true for the catalyst specific surfaces, as shown by BET analyses. It is thus more reasonable to relate the enhanced reactivity of powders under MA conditions to the nature and lifetime of local modifications of the crystalline structure due to the deformation events taking place at the impacts between the ball and the reactor walls. A final observation concerning the different performances of the (Co50Fe50)0.2(TiO2)99.8 and Ni40(ZrO2)60 catalysts is necessary. The specific CO conversion rates rsp;mech obtained under MA conditions apparently indicate that the Ni40(ZrO2)60 catalyst is much more effective than the (Co50Fe50)0.2(TiO2)99.8 one. However, it must be noted that the amount of catalytically active phase working at each impact in the two cases is different. This can be suitably taken into account by referring the specific CO conversion rate rsp;mech under mechanical activation to the mass mact of catalytically active phase and not to the total mass m* of catalyst powders involved in individual impacts. The result indicates that the CO hydrogenation reaction proceeds on the (Co50Fe50)0.2(TiO2)99.8 catalyst at a specific rate of about 4.8 103 mol g1 s1, whereas the corresponding value for Ni40(ZrO2)60 powders amounts to about 4.9 104 mol g1 s1. The (Co50Fe50)0.2(TiO2)99.8 catalyst exhibits therefore a higher activity than the Ni40(ZrO2)60 one, at least as far as the activity is referred to the amount of catalytically active phase available to the reaction. 5. Conclusions The experimental findings discussed in the present work point out that the MA has a marked effect on the performances of
F. Delogu et al. / Applied Catalysis A: General 366 (2009) 201–205
catalyst powders. A significant enhancement of the chemical reactivity of catalysts working under MA conditions is observed, which can be reasonably related to the deformation processes involving the powder amount trapped at individual impacts between the ball and the reactor walls. A satisfactory explanation of the experimental evidences obtained should rely on the intimate nature of such deformation processes, which is however quite obscure. For this reason, no definite rationale for the observed behavior is available. In spite of this, the rough order-of-magnitude estimates of the CO hydrogenation rates under MA conditions clearly define an interesting scenario which deserves deeper investigations. Acknowledgements Financial support has been provided by the University of Cagliari and the University of Sassari. References [1] [2] [3] [4]
G. Heinicke, Tribochemistry, Berlin, Akademie-Verlag, 1984. P. Yu Butyagin, Sov. Sci. Rev. B Chem. 14 (1989) 1. C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1. E.M. Gutman, Mechanochemistry of Materials, Cambridge International Science Publishing, Cambridge, 1998. [5] V.I. Levitas, O.M. Zarechnyy, J. Phys. Chem. B 110 (2006) 16035. [6] F. Delogu, G. Cocco, Phys. Rev. B 74 (2006) 035406. [7] P. Balaz, Mechanochemistry in Nanoscience and Minerals Engineering, Springer, Berlin, 2008.
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
205
B. Rodriguez, A. Bruckmann, C. Bolm, Adv. Synth. Cat. 349 (2007) 2213. G. Kaupp, Top. Curr. Chem. 254 (2005) 95. G. Kaupp, Cryst. Eng. Commun. 11 (2009) 388. B. Sakintuna, F. Lamari-Darkrim, M. Hirscher, Int. J. Hydrogen Energy 32 (2007) 1121. S. Garroni, F. Delogu, G. Mulas, G. Cocco, Scripta Mater. 57 (2007) 964. F. Delogu, Acta Mater. 56 (2008) 905. F. Delogu, Acta Mater. 56 (2008) 2344. A. Molnar, M. Varga, G. Mulas, M. Mohai, I. Bertoti, A. Lovas, G. Cocco, Mater. Sci. Eng. A 434 (2001) 1078. G. Mulas, F. Delogu, G. Cocco, J. Alloys Compd. 473 (2009) 180. F. Delogu, G. Mulas, Int. J. Hydrogen Energy 34 (2009) 3026. H.H. Storch, N. Golumbic, R.B. Anderson, The Fischer-Tropsch and Related Synthesis, New York, Wiley, 1951. J.R. Anderson, M. Boudart (Eds.), Catalysis Science and Technology, vol.1, Springer, Berlin, 1981. R.B. Anderson, The Fischer-Tropsch synthesis, Academic Press, New York, 1984. M.E. Dry, Catal. Today 71 (2002) 227. B.H. Davis, Catal. Today 71 (2002) 249. F. Delogu, C. Deidda, G. Mulas, L. Schiffini, G. Cocco, J. Mater. Sci. 39 (2004) 5121. G. Manai, F. Delogu, L. Schiffini, G. Cocco, J. Mater. Sci. 39 (2004) 5319. G. Manai, F. Delogu, M. Rustici, Chaos 12 (2002) 601. C. Deidda, F. Delogu, G. Cocco, J. Mater. Sci. 39 (2004) 5315. F. Delogu, G. Cocco, J. Alloys Compd. 58 (2008) 126. F. Delogu, G. Cocco, Phys. Rev. B 72 (2005) 014124. L. Lutterotti, R. Ceccato, R. Dal Maschio, E. Pagani, Mater. Sci. Forum 278 (1998) 281. J.M. Thomas, W.J. Thomas, Principles and Practice of Heterogeneous Catalysis, VCH, Weinheim, 1997. F. Delogu, J. Alloys Compd. 468 (2009) 22. D.J. Duvenhage, N.J. Coville, Appl. Catal. A: Gen. 153 (1997) 43. R. Sethuraman, N.N. Bakhshi, S.P. Katikaneni, R.O. Idem, Fuel Proc. Technol. 73 (2001) 197. J.R. Anderson, Structure of metallic catalysts, Academic Press, London, 1975.