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Hydrogenation of carbon monoxide over nanostructured systems: A mechanochemical approach
Applied Surface Science 257 (2011) 8165–8170
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Hydrogenation of carbon monoxide over nanostructured systems: A mechanochemical approach Gabriele Mulas a,∗ , Renato Campesi b , Sebastiano Garroni a , Francesco Delogu c , Chiara Milanese d a
Department of Chemistry, University of Sassari and INSTM, via Vienna 2, I-07100 Sassari, Italy JRC-IE, westerduinweg 3, Petten 1755 ZG, The Netherlands Department of Chemical Engineering and Materials, University of Cagliari, piazza d’Armi, I-09123 Cagliari, Italy d C.S.G.I & Department of Chemistry, Physical Chemistry Section, University of Pavia, Viale Taramelli 16, I-27100 Pavia, Italy b c
a r t i c l e
i n f o
Article history: Available online 20 April 2011 Keywords: Mechanochemical process Nanostructured catalysts Carbon monoxide hydrogenation
a b s t r a c t In this study we investigated the mechanochemical hydrogenation of carbon monoxide over nanostructured FeCo- and Mg2 Ni-based catalysts. To this aim powdered materials, prepared by mechanical alloying, were subjected to mechanical treatment under CO + H2 atmosphere. A methodology to evaluate the activity of the solid catalysts on an absolute basis was developed. Conversion data were, indeed, expressed as turnover frequency, TOF, and related to the occurrence of ball to powder collision events through the mechanochemical turnover frequency parameter, MTOF. Differences in the catalytic activity and selectivity were observed for the two FeCo-based studied systems, the solid solution Fe50 Co50 and its dispersion on TiO2 support. As for the Mg2 Ni system, we explored the possibility to estimate the specific role of hydrogen pre-activation step. The catalytic properties of the mechanically alloyed Mg2 Ni system were compared with the conversion data shown by the same system pre-hydrogenated and subsequently milled under CO atmosphere. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In the development of innovative solid catalysts a crucial role is recognized to be played by the crystallite size of active phases. The investigation of the properties of nanosized particles and nanostructured phases, in this field, dates back to times preceding the present development and widespread attention towards nanotechnologies. Besides the evident effects related to the increase in specific surface area, other factors as excited electronic states and thermodynamic metastability, allow to increase the chemical reactivity, positively influencing the catalytic properties of such systems [1]. Moreover, in addition to the preparation of new, highly reactive, nanosized phases, interest is addressed towards different processing techniques and methodologies, alternative to the classic ones, which could sustain and improve the activity of such systems during the catalytic tests. To this regard, the use of mechanical treatment represents a powerful tool to promote chemical transformation at the interface of solid–gas systems [2–4]. Although the transfer of kinetic energy is a well known process to activate chemical reactions, the quantitative evaluation of all the physical, as well as chemical aspects, involved in the so called
∗ Corresponding author. Tel.: +39 079 229524; fax: +39 079 212069. E-mail address: [email protected] (G. Mulas). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.03.024
‘mechanochemical process’ is far from being completely achieved [5–7]. Mechanochemistry is based on the study of the physical and chemical transformations induced by the application of mechanical stresses [2–4]. Tensile, compressive and shear stress, induced in a solid as a consequence of mechanical work, generate punctual defects, dislocations and deformations which all contribute to the achievement of topological and chemical disorder. It means that the mechanical treatment has a severe effect leading to the refinement of the microstructure, with formation of new surfaces mainly due to the reducing of crystalline grain size. The potential energy storage, together with the thermodynamic equilibrium displacement induced by mechanical input, contributes significantly to increase the chemical reactivity, resulting in the structural evolution of the system [2–4]. Reactions promoted by an intense transfer of energy, and consequent relaxation of highly excited states, cannot be described by classical thermodynamic laws. The elemental processes taking place during the mechanical treatment, indeed, are completely different to those occurring in thermally activated reactions. It has also to be taken into account that the effects generated during the transfer of mechanical energy depend on the intensity of the mechanical work, which has a direct influence on the structural properties and then on the surface reactivity. Therefore a deep investigation of the effects of mechanical work on nanostructured phases requires the detailed analysis, at
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the same time, of chemical properties of the surface (geometrical as well as electronic aspects) and of the bulk structural properties (microstructural deformation and atomic reordering), together with the study of the energy parameters linked to the intensity of the mechanical work. In this context, we propose a methodology to describe, on a quantitative basis, chemical processes at the gas–solid interface promoted by mechanical treatment. The experimental set up, as well as the developed procedures, were applied to the study of the hydrogenation process of carbon monoxide (CO), carried out over multi-component metal alloys and hydrides. This process, well known as Fischer–Tropsch (FT) synthesis, has had particular relevance in the industry in the post-war period [8–10], and nowadays it is receiving increasing attention for environmental issues and potential applicative purpose in a clean energy scenario. In a classical scheme of the FT reaction activated by conventional thermal treatment, hydrogen and carbon monoxide react over supported transition metal catalysts to yield saturated hydrocarbons, olefins and oxidized products. Such consideration prompted us to select, as catalysts, the cited systems for which recent literature data are available [11]. Conversion values observed during mechanochemical tests were related to the dynamic of the mechanical work, the chemistry of the surface, microstructure and kinetics aspects inferred to the heterogeneous processes.
a
b
2. Experimental The catalytic properties of different systems, i.e. Fe50 Co50 , FeCo supported on TiO2 (FeCo)/(TiO2 ) and Mg2 Ni were investigated. High-purity commercial powders were used as starting materials. All the samples were synthesized by mechanical treatment: as a general procedure, for each system 8 g of powder blend were sealed in a stainless steel vial and ball-milled by using a commercial Spex mixer/mill model 8000. The milling was performed under argon atmosphere with three stainless steel balls (3.5 g each) and a rotation speed of 875 rpm. Sample handling was performed under high-purity argon atmosphere inside a glove box device (MBraun), with O2 and H2 O levels below 0.1 ppm. The structural properties of the solid phases were analyzed by X Ray Powder Diffraction, XRPD, using a Rigaku DMax diffractometer equipped with Cu K␣ radiation tube and a graphite monochromator in the diffracted beam. The microstructural parameters were evaluated by least squares refinement procedures using the Rietveld method and the MAUD (Materials Analysis Using Diffraction) software [12]. The determination of phase relative abundance and crystallite size allowed to estimate specific surface area of the active phases [9] as well as the number of active sites. Surface atomic distribution data of elements were taken from the literature [13]. Furthermore a previously developed methodology [14], based on experimental evidence and theoretical determination of vial motion equations, allowed to evaluate parameters as impact frequency and kinetic energy transferred to the impacted powders during the mechanical treatment. Mechanochemical carbon monoxide hydrogenation runs were performed over 8 g of catalyst powders. 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 reservoir where the desired gaseous mixture was prepared. In particular, the powders of all the systems were exposed to a CO:H2 gaseous mixture in the molar ratio 1:3. Such stoichiometry was selected in order to maximize the selectivity towards a single product, i.e. methane. The Mg2 Ni system was also tested under different conditions: it was subjected
Fig. 1. CuK␣ XRPD patterns relevant to the synthesis of the nanostructured Fe–Co based systems. In the plot (a) the sequence refers to the synthesis of the Fe50 Co50 solid solution. 䊉 Fe bcc (JCPDS, 01-1252), Co hcp (JCPDS, 01-1277), Co fcc (JCPDS, 15-0806). In the plot (b) the preparation of the nanocomposite 10% wt Fe50 Co50 /TiO2 is shown: 䊉 Fe50 Co50 bcc, TiO2 , Rutile (JCPDS, 72-1148), TiO2 , Anatase (JCPDS, 84-1286), TiO2 , Brookite (JCPDS, 02-0514). Lower pattern is relevant to the as mixed powders, while the upper one refers to the mixture mechanically milled for 40 h.
to a mechanochemical pre-hydrogenation process and then milled under pure CO. In all the mechanochemical runs under reactive atmosphere the total pressure inside the mechanochemical reactor, measured by pressure transmitters, was set at 0.6 MPa. Moreover, the reactor temperature was kept at 300 K and the frequency of reactor displacement at 14.6 Hz. The conversion rate and the selectivity of the CO hydrogenation were monitored by gas-chromatography (GC). Hydrogen and carbon monoxide gases were detected using a Fisons 8000 apparatus equipped with a Hot Wire Detector, whereas hydrocarbons were analyzed by a Perkin-Elmer 8600 apparatus equipped with a Flame Ionization Detector. Suitable gas standard mixtures were employed to quantify the hydrocarbons formation and CO conversion. 3. Results and discussion 3.1. FeCo based systems Our attention was initially addressed to the synthesis of the Fe50 Co50 system. The mechanical treatment of the pure elements induced an alloying reaction with the formation of a bcc solid solution, in agreement with literature data [15]. The structure evolution is shown in Fig. 1, plot a, where the XRPD patterns sequence refers to specimens subjected to different times of mechanical treatment.
G. Mulas et al. / Applied Surface Science 257 (2011) 8165–8170
a
b
Fig. 2. CO conversion, expressed as molar percentage, to hydrocarbons and methane observed in the catalytic test over the Co50 Fe50 (a), and (FeCo)/(TiO2 ) (b) systems.
The initial mixture, referred to as 0 h of milling, showed the coexistence of two Co allotropic phases, with fcc and hcp structure respectively, together with the Fe bcc phase. After 2 h of mechanical treatment, the fast transition of the Co fcc to Co hcp phase (the allotropic more stable phase) was achieved. For longer milling times, the Co reflections disappeared due to the dissolution of the Co atoms inside the Fe bcc lattice [15]. After 40 h of treatment the desired nanostructured conditions were approached, with average alloy crystallite size of 16 nm. A second system was prepared by a dispersion of 10 wt.% of the Fe50 Co50 solid solution over the anatase TiO2 phase. The pattern of the final structure induced by milling is displayed in Fig. 1 plot b, where the two patterns refer to the as mixed powders (lower trace, 0 h) and to the powders blend milled for 40 h (upper trace). The microstructural analysis indicates that nanostructured conditions were also reached by the TiO2 support, which underwent a phase evolution from anatase to rutile and brookite. The so prepared systems were then employed as catalysts in the CO hydrogenation under mechanochemical conditions. Powder catalysts were initially exposed to the gaseous mixture reagents under static conditions, i.e. with the mill at rest, but CO conversion was not observed. The starting of hydrocarbons synthesis occurred only under milling conditions, and kinetic behavior of the two investigated FeCo based systems was different. It is worth to mention that, for both systems, formation of the carbide species was not observed. Interestingly, in the supported system high selectiv-
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ity towards CH4 was achieved. Assuming that the TiO2 support is nonreactive, this behavior could be ascribed to the available source of C atoms on the surface as a result of the dispersion process of the active phase on the support. Data relevant to Fe50 Co50 solid solution are shown in Fig. 2, plot a. Conversion started only after an induction period which, under the present experimental conditions, was of about 6 h, and hydrocarbons formation increased almost linearly up to a maximum rate of r = 8.67 × 10−4 mol s−1 , while the amount of methane approached asymptotically a value of 3%. Different activity and selectivity values were displayed by the supported (FeCo)/(TiO2 ) system, as reported in Fig. 2, plot b. In this case CO conversion started right at the beginning of the milling action, and methane was the single observed product. The rate of methane formation increased up to a maximum value of r = 1.75 × 10−3 mol s−1 which was reached after about 50 h of treatment. Conversion rate then decreased and the total methane amount was about 4%. During the catalytic tests some evolution occurred in the structures [16]: in the Fe50 Co50 powders a reduced fraction of hcp Fe2 O3 (JCPDS 21165) was formed, while in the supported (FeCo)/(TiO2 ) system the fraction of TiO2 brookite evolved to rutile polymorph. It is to note that both the catalysts preserved the nanostructured conditions. Hereafter we focused on another aspect, i.e. to present the catalytic data of the studied systems on an absolute basis, which can allow a comparison among the results of different mechanochemical conditions and, more importantly, with the results of catalytic tests carried out under conventional thermal activation. It would also allow to evaluate the chemical conversion as a function of the effect of the transfer of mechanical work in heterogeneous catalytic reactions. In heterogeneous catalysis conversion rate and specific conversion rate are respectively expressed as: r = dnmol /dt(mol/s), and rs = dnmol /mp dt(mol/g· s) ; in the second equation the number of transformed moles per second are normalized to the catalyst mass, mp . However a parameter which can exhaustively characterise a catalytic reaction is the so called turnover frequency, TOF. It expresses the number of chemical transformation (or catalytic events) occurring per surface active site per unit of time. It is then necessary to know the absolute value of active sites of the catalytic system, Ns , which can be obtained by multiplying the surface atomic density ns = atoms/m2 to the total specific surface area St and to the total catalyst mass, mp . Ns [atoms] = (ns )(St )(mp ) =
atoms m2 m2
g
[g]
Then the TOF can be evaluated according to the following formula: TOF =
dnmol mp St ns dt
mol atoms · s
=
r mp St ns
mol atoms · s
Surface atomic density values, ns of close-packed structures can be obtained by geometrical studies of crystal planes, atomic populations and slipping directions, and data are available in the literature for the most commonly used metals [10]. It is also useful to underline that, for samples employed in conventional heterogeneous catalytic tests, St represents the external surface area exposed to gaseous reagents, which is commonly measured by physi- or chemi-sorptions techniques. However, when a mechanochemical processes is analyzed, and when the exposed catalysts surface is continuously refreshed, the maximum extension of specific surface area can be evaluated by summing the surface area of all the coherent diffraction domains exposed to reactive atmosphere as a consequence of powder grains refinement. For this reason, specific surface area St can be estimated resorting to microstructural characterization by diffraction techniques according to:
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Table 1 TOF, MTOF and MTE data for the FeCo-based studied catalysts.
molecules
Sample
TOF
Fe50 Co50 (FeCo)/(TiO2 )
3.44 × 10−6 1.76 × 10−6
atoms×s
MTOF
molecules atoms×hit
9.51 × 10−6 4.8 × 10−2
MTE
molecules atoms×J
0.14 0.69
St = 6 × 104 / L (m2 g), where L and are respectively the average diameter of crystallites and the bulk density of constituent materials, under the assumption of domains with spherical shape [9]. In principle the TOF evaluation could be applied to a mechanochemical conversion, but further information can be gained by taking into account that transfer of mechanical energy is a discrete process, and that reactive events take place as a consequence of collisions. The modeling of the milling process previously reported [11] then allows to introduce a new parameter defined as mechanochemical turnover frequency, MTOF: MTOF =
r mhit Sn p,i t s
·
1 N
mol atoms · hit
where mhit is the fraction of powder subjected to mechanical work p,i at each impact and N is the impact frequency. The MTOF expresses the number of molecules transformed per impact and per number of surface atom on the active phase involved in the chemical process. By normalizing the MTOF to the impact energy E, we finally obtain another parameter, the yield per surface per energy unit, MTE : MTE =
r mhit Sn p,i t s
·
1 1 · N E
mol atoms · J
Applying the above considerations to the conversion rates of the Fe50 Co50 and (FeCo)/(TiO2 ) catalysts, we obtain the TOF and MTOF values reported in Table 1. Considering that the conversion rate was not constant but varied along the treatment, the estimation of TOF and MTOF was related to the maximum rate of conversion. It emerges that MTOFs are of the same order of magnitude of TOFs measured for the similar systems activated by thermal treatment [15]. Moreover, TOF values confirmed that the metallic surface sites of the (FeCo)/(TiO2 ) composite were able to convert a much larger number of CO molecules per time unit, although the supported system presented a lower number of metallic sites. The same difference was observed for MTOF data, parameter which characterizes the mechanochemical activity of the catalyst. Finally, the MTE suggested the yield of the mechanochemical reaction, estimating the fraction of CO molecules converted on each active surface site subjected to an input of external mechanical energy of a joule. 3.2. Mg2 Ni based systems The reaction mechanism of CO hydrogenation has been deeply studied in the past, and in a very simplified view, the process starts with CO chemisorption on metal surface followed by dissociation; then C–H bond formation can take place when chemisorbed C and H atoms diffusing on the surface, approach the proper bond distance and energy interaction. Moreover, it has been reported the efficacy of mechanical treatment in activating hydrogen ad- and ab-sorption processes in metals and alloys [17–21]. Taking into account the above consideration, we then explored the reactive properties, as CO conversion to methane under milling condition, of another system, i.e. Mg2 Ni, which has been widely studied as hydrogen storage material and for which different hydride phases, Mg2 NiH4 and Mg2 NiH0.3 , can be formed. The Mg2 Ni system was tested under different mechanochemical conditions, i.e. by exposing the powders to CO + H2 gas mixture, as
Fig. 3. CuKa XRPD patterns relevant to the Mg2 Ni system: as synthesized (trace a), after 100 h of milling under CO + H2 atmosphere (b), after mechanochemical hydrogenation for 44 h (c); and successively milled 72 h under CO atmosphere (d).
well as by separating the reactive process into two mechanochemical steps, a pre-hydrogenation followed by the treatment of the alloy hydrides under CO atmosphere. Mg2 Ni was synthesized from the elements by mechanical alloying followed by thermal treatment under ar atmosphere at 670 K to complete the alloying process. Relevant XRPD pattern, reported in Fig. 3 trace a, indicates the formation of the alloy together with a small fraction of the Ni richer MgNi2 phase. Such occurrence is probably due to some stoichiometry variation during the mechanical alloying, as a consequence of Mg powder sticking inside the milling apparatus. The thermal treatment step hindered the achievement of nanostructured conditions. When such powder was mechanically treated for 100 h under the CO + H2 gas mixture, a structural evolution occurred, and relevant XRPD pattern is reported in Fig. 3, trace b. Mg2 Ni has been almost completely converted in MgO (JCPDS 78-0430), MgNi3 C (JCPDS 41-0903), and Mg2 NiH0.3 . Phases abundance and microstructural features of the formed nanocrystalline phases are reported in Table 2. CO conversion did not start at the beginning of the mechanochemical process, but only after an induction period of more than 10 h of milling. Conversely, as reported in Fig. 4, graph a, a pressure drop of the CO + H2 mixture occurred during the early stage of the milling process. Such behavior appears similar to that observed in a previous study on NiZr based catalyst [22],
G. Mulas et al. / Applied Surface Science 257 (2011) 8165–8170
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Table 2 Microstructural parameters (crystallite size, microstrain and phase abundance) of the Mg2 Ni system after different mechanochemical treatments. Data are obtained from the Rietveld analysis of the patterns shown in Fig. 3. System
Phases
Crystallite size (nm)
Microstrain
Phase abundance (wt%)
Mg2 Ni Milled under CO:H2 (100 h) – Trace b in Fig. 3
MgO Mg2 NiH0.3 MgNi3 C
13.4 83.4 5.8
8.25 × 10−3 1.05 × 10−4 5.9 × 10−3
54.8 6.4 38.8
Mg2 Ni Milled under H2 (44 h) – Trace c in Fig. 3
Mg2 NiH4 Mg2 NiH0.3 Ni
27.5 70.0 40.0
9.02 × 10−3 1.13 × 10−3 8.05 × 10−3
32.2 31.9 28
Mg2 Ni Pre-hydrogenated and milled under CO, (72 h) – Trace d in Fig. 3
MgO NiO Ni3 C MgNi3 Cx Ni
11.9 3.0 25.7 29.0 21.2
7.4 × 10−3 6 × 10−3 4.9 × 10−3 6.8 × 10−3 8 × 10−3
51 7.4 28 5 8.5
and can be assumed to correspond to the sorption of gaseous reactants, and their activation after true formation of catalyst phases. In Fig. 4, graph b, the formation of hydrocarbons is reported as a function of the milling time. Methane and ethane were the main products. As cited above, to verify the role of the induction period, in a further experiment we pre-hydrogenated the Mg2 Ni system under dynamic condition, before the mechanochemical CO conversion.
a
In Fig. 3, trace c, the XRPD pattern of the hydrogenated system is shown, and microstructural features are listed in Table 2. The main phases are Mg2 NiH0.3 (JCPDS 40-1204), Mg2 NiH4 (JCPDS 390848) and metallic Ni (JCPDS 4-850), with a small fraction of MgH2 (JCPDS 12-0697) and MgO (JCPDS 78-430). The formation of such phases allowed to evaluate the ability of a mixture of hydrides, i.e. Mg2 NiH4 , Mg2 NiH0.3 and MgH2 , as well as of metallic Ni in the activation of CO.
a
b
b
Fig. 4. Pressure trend of the CO + H2 mixture as function of the milling time during the mechanical treatment over the Mg2 Ni system (a). The relative percentage of light hydrocarbons is plotted in the graph b.
Fig. 5. Mechanochemical pre-hydrogenated Mg2 Ni system: CO conversion into hydrocarbons, expressed as relative percentage and plotted as a function of the milling time, (graph a). In the graph b the percentage of formed light hydrocarbons is reported.
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Conversely when the Mg2 Ni powders were exposed to the CO + H2 mixture, hydrogen activation had still to occur on surface reactive sites to form atomic hydrogen, before the reaction with CO molecules could result in the conversion to hydrocarbons. Finally, a large difference in the kinetics emerged from the comparison of the MTOF data of the CO conversion over FeCo based systems and pre-hydrogenated Mg2 Ni alloy. Keeping into account the differences in the chemical processes, and under the presented assumption in evaluating the MTOF, the developed methodology allowed to quantify the better performance of pre-hydrogenated system in CO conversion. 4. Conclusions
Fig. 6. Percentage of methane formed as a function of milling time respectively over the pre-hydrogenated (, black squares) and as synthesized (䊉, red circles) Mg2 Ni system.
Fig. 3, pattern d, and Table 2 report the compositional and structural data of the sample described above after being milled up to 72 h under CO atmosphere. Again, a severe transformation took place during the process, leading to MgO (JCPDS 78-0430), Ni3 C (JCPDS 77-0194), NiO (JCPDS 78-0643), Ni (JCPDS 04-0850) and MgNi3 C (JCPDS 41-0903). Reduction in Ni content together with the concomitant formation of two carbide phases, Ni3 C (JCPDS 77-0194) and MgNi3 C (JCPDS 41-0903), suggests that CO is first chemisorbed by Ni and then it can react with the activated hydrogen atoms present on the surface of the hydride phases. It also seems that the reaction proceeded until all the available hydrogen could react, as confirmed by the lack of hydride phases after 72 h of milling treatment. As for the gaseous products, in Fig. 5, graphs a and b, the CO conversion, as well as the percentage of formed hydrocarbons, are plotted as a function of the milling time. CO conversion shows a sigmoid trend and the highest rate, r = 1.11 × 10−6 mol s−1 , was achieved after about 14 h of mechanical treatment. The main product is still the methane, and the hydrocarbons production varies as a function of the milling time according to the distribution reported in Fig. 5, graph b. Recognizing the catalytic effect of elemental Ni in the chemisorption of CO, as well as in activating H2 adsorption and dissociation, it can be assumed that a crucial role in the CO conversion over the hydride alloy was played by the Ni free fraction. We therefore determined the catalytic activity of Ni in the mechanical activation of CO in terms of MTOF. From the XRPD analysis it is possible to estimate the specific surface area of Ni as 9.49 m2 g−1 , and the number of free Ni atoms, Ns , in the whole mass: the result is 6.43 × 1015 atoms. On the other hand we evaluated the number of CO molecules activated at each impact which was equal to 4.05 × 1019 . Finally the ratio between this value and Ns yielded the MTOF which was equal to 6.3 × 103 , and corresponded to the number of CO molecules activated by Ni as a consequence of an impact event. As a confirmation of the fast kinetics of this pre-hydrogenated Mg2 NiHx system, a comparison of the conversion to methane for the two Mg2 Ni studied systems is shown in Fig. 6 as a function of milling time: it turns out that the pure alloy displayed a sort of incubation period (of about 20 h) before a noticeable amount of methane was formed, while the hydrogenated system during the same milling time reached the highest rate. Such different behavior can be explained by considering that in the case of the hydrogenated alloy the CO molecules could be directly converted by the activated hydrogen atoms present on the phases surface.
Quantitative evaluation of conversion data in heterogeneous processes at the solid–gas interface driven by mechanical energy input is a very difficult task. It requires the detailed analysis of surface properties and microstructural bulk characteristics of solid phases, together with the control of milling dynamics parameters. The methodology presented in this work allows to compare on an absolute basis mechanochemical conversion data with the results of heterogeneous catalytic tests carried out under conventional thermal activation conditions. Results of CO + H2 synthesis over FeCo based catalysts indicated that mechanochemical processes, characterized by mild conditions (room temperature, and near to atmospheric pressure), displayed conversion data similar or better than the corresponding thermally activated reactions, performed under severe conditions (400–800 K, 2–5 MPa). Further improvement in conversion data and kinetics was observed in the CO conversion over hydrides. The quantification of such effect allowed to gain some hints of the reaction mechanism, and may open some potential perspective for the application of the studied process in a future hydrogen-based energy scenario. Acknowledgements This work was funded by MIUR (Italian Ministry for University and Research) in the framework of the PRIN Project “Synthesis, characterization and functional evaluation of light hydrides-based nanostructured materials and nanoparticles for solid state hydrogen storage”, and University of Sassari. S.G. acknowledges his postdoctoral fellowship, supported by the Fondazione Banco di Sardegna. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
S.L. Scott, C.M. Crudden, C.W. Jones, Nanostructured Catalysts, Elsevier, 2003. G. Heinicke, Tribochemistry, Akademie-Verlag, Berlin, 1984. P. Yu Butyagin, Sov. Sci. Rev. B Chem. 14 (1989) 1. C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1. S. Garroni, F. Delogu, G. Mulas, G. Cocco, Scripta Mater. 57 (2007) 294. F. Delogu, Acta Mater. 56 (2008) 905. F. Delogu, Acta Mater. 56 (2008) 2344. R.B. Anderson, The Fischer-Tropsch Synthesis, Academic Press, Orlando, 1984. J.R. Anderson, M. Boudart, Catal. Sci. Technol., Berlin 1 (1981). J.M. Thomas, W.J. Thomas, Principles and Practice of Heterogeneous Catalysis, Weinheim, 1997. D.J. Duvenhage, N.J. Coville, Appl. Catal. A 153 (1997) 43. L. Lutterotti, R. Ceccato, R. Dal Maschio, E. Pagani, Mater. Sci. Forum 278 (1998) 281. J.R. Anderson, Structure of Metallic Catalysts, Academic Press Inc. Ltd., London, 1975. F. Delogu, M. Monagheddu, G. Mulas, L. Schiffini, G. Cocco, Int. J. Non-Equilibr. Process. 11 (2000) 235. C. Kuhrt, L. Schultz, J. Appl. Phys. 71 (1992) 1896. F. Delogu, G. Mulas, S. Garroni, Appl. Catal. A 366 (2009) 201. G. Mulas, L. Schiffini, G. Cocco, J. Mater. Res. 19 (2004) 3279. G. Mulas, F. Delogu, G. Cocco, J. Alloys. Compd. 473 (2009) 180. Y. Chen, J.S. Williams, J. Alloys Compd. 217 (1995) 181. J. Huot, G. Liang, R. Schulz, Appl. Phys. A 72 (2001) 187. L. Zaluski, A. Zaluska, J.O. Strom-Olsen, J. Alloys Compd. 217 (1995) 245. G. Mulas, L. Conti, G. Scano, L. Schiffini, G. Cocco, Mater. Sci. Eng. A 181 (182) (1994) 1085.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Hydrogenation of carbon monoxide over nanostructured systems: A mechanochemical approach Gabriele Mulas a,∗ , Renato Campesi b , Sebastiano Garroni a , Francesco Delogu c , Chiara Milanese d a
Department of Chemistry, University of Sassari and INSTM, via Vienna 2, I-07100 Sassari, Italy JRC-IE, westerduinweg 3, Petten 1755 ZG, The Netherlands Department of Chemical Engineering and Materials, University of Cagliari, piazza d’Armi, I-09123 Cagliari, Italy d C.S.G.I & Department of Chemistry, Physical Chemistry Section, University of Pavia, Viale Taramelli 16, I-27100 Pavia, Italy b c
a r t i c l e
i n f o
Article history: Available online 20 April 2011 Keywords: Mechanochemical process Nanostructured catalysts Carbon monoxide hydrogenation
a b s t r a c t In this study we investigated the mechanochemical hydrogenation of carbon monoxide over nanostructured FeCo- and Mg2 Ni-based catalysts. To this aim powdered materials, prepared by mechanical alloying, were subjected to mechanical treatment under CO + H2 atmosphere. A methodology to evaluate the activity of the solid catalysts on an absolute basis was developed. Conversion data were, indeed, expressed as turnover frequency, TOF, and related to the occurrence of ball to powder collision events through the mechanochemical turnover frequency parameter, MTOF. Differences in the catalytic activity and selectivity were observed for the two FeCo-based studied systems, the solid solution Fe50 Co50 and its dispersion on TiO2 support. As for the Mg2 Ni system, we explored the possibility to estimate the specific role of hydrogen pre-activation step. The catalytic properties of the mechanically alloyed Mg2 Ni system were compared with the conversion data shown by the same system pre-hydrogenated and subsequently milled under CO atmosphere. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In the development of innovative solid catalysts a crucial role is recognized to be played by the crystallite size of active phases. The investigation of the properties of nanosized particles and nanostructured phases, in this field, dates back to times preceding the present development and widespread attention towards nanotechnologies. Besides the evident effects related to the increase in specific surface area, other factors as excited electronic states and thermodynamic metastability, allow to increase the chemical reactivity, positively influencing the catalytic properties of such systems [1]. Moreover, in addition to the preparation of new, highly reactive, nanosized phases, interest is addressed towards different processing techniques and methodologies, alternative to the classic ones, which could sustain and improve the activity of such systems during the catalytic tests. To this regard, the use of mechanical treatment represents a powerful tool to promote chemical transformation at the interface of solid–gas systems [2–4]. Although the transfer of kinetic energy is a well known process to activate chemical reactions, the quantitative evaluation of all the physical, as well as chemical aspects, involved in the so called
∗ Corresponding author. Tel.: +39 079 229524; fax: +39 079 212069. E-mail address: [email protected] (G. Mulas). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.03.024
‘mechanochemical process’ is far from being completely achieved [5–7]. Mechanochemistry is based on the study of the physical and chemical transformations induced by the application of mechanical stresses [2–4]. Tensile, compressive and shear stress, induced in a solid as a consequence of mechanical work, generate punctual defects, dislocations and deformations which all contribute to the achievement of topological and chemical disorder. It means that the mechanical treatment has a severe effect leading to the refinement of the microstructure, with formation of new surfaces mainly due to the reducing of crystalline grain size. The potential energy storage, together with the thermodynamic equilibrium displacement induced by mechanical input, contributes significantly to increase the chemical reactivity, resulting in the structural evolution of the system [2–4]. Reactions promoted by an intense transfer of energy, and consequent relaxation of highly excited states, cannot be described by classical thermodynamic laws. The elemental processes taking place during the mechanical treatment, indeed, are completely different to those occurring in thermally activated reactions. It has also to be taken into account that the effects generated during the transfer of mechanical energy depend on the intensity of the mechanical work, which has a direct influence on the structural properties and then on the surface reactivity. Therefore a deep investigation of the effects of mechanical work on nanostructured phases requires the detailed analysis, at
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the same time, of chemical properties of the surface (geometrical as well as electronic aspects) and of the bulk structural properties (microstructural deformation and atomic reordering), together with the study of the energy parameters linked to the intensity of the mechanical work. In this context, we propose a methodology to describe, on a quantitative basis, chemical processes at the gas–solid interface promoted by mechanical treatment. The experimental set up, as well as the developed procedures, were applied to the study of the hydrogenation process of carbon monoxide (CO), carried out over multi-component metal alloys and hydrides. This process, well known as Fischer–Tropsch (FT) synthesis, has had particular relevance in the industry in the post-war period [8–10], and nowadays it is receiving increasing attention for environmental issues and potential applicative purpose in a clean energy scenario. In a classical scheme of the FT reaction activated by conventional thermal treatment, hydrogen and carbon monoxide react over supported transition metal catalysts to yield saturated hydrocarbons, olefins and oxidized products. Such consideration prompted us to select, as catalysts, the cited systems for which recent literature data are available [11]. Conversion values observed during mechanochemical tests were related to the dynamic of the mechanical work, the chemistry of the surface, microstructure and kinetics aspects inferred to the heterogeneous processes.
a
b
2. Experimental The catalytic properties of different systems, i.e. Fe50 Co50 , FeCo supported on TiO2 (FeCo)/(TiO2 ) and Mg2 Ni were investigated. High-purity commercial powders were used as starting materials. All the samples were synthesized by mechanical treatment: as a general procedure, for each system 8 g of powder blend were sealed in a stainless steel vial and ball-milled by using a commercial Spex mixer/mill model 8000. The milling was performed under argon atmosphere with three stainless steel balls (3.5 g each) and a rotation speed of 875 rpm. Sample handling was performed under high-purity argon atmosphere inside a glove box device (MBraun), with O2 and H2 O levels below 0.1 ppm. The structural properties of the solid phases were analyzed by X Ray Powder Diffraction, XRPD, using a Rigaku DMax diffractometer equipped with Cu K␣ radiation tube and a graphite monochromator in the diffracted beam. The microstructural parameters were evaluated by least squares refinement procedures using the Rietveld method and the MAUD (Materials Analysis Using Diffraction) software [12]. The determination of phase relative abundance and crystallite size allowed to estimate specific surface area of the active phases [9] as well as the number of active sites. Surface atomic distribution data of elements were taken from the literature [13]. Furthermore a previously developed methodology [14], based on experimental evidence and theoretical determination of vial motion equations, allowed to evaluate parameters as impact frequency and kinetic energy transferred to the impacted powders during the mechanical treatment. Mechanochemical carbon monoxide hydrogenation runs were performed over 8 g of catalyst powders. 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 reservoir where the desired gaseous mixture was prepared. In particular, the powders of all the systems were exposed to a CO:H2 gaseous mixture in the molar ratio 1:3. Such stoichiometry was selected in order to maximize the selectivity towards a single product, i.e. methane. The Mg2 Ni system was also tested under different conditions: it was subjected
Fig. 1. CuK␣ XRPD patterns relevant to the synthesis of the nanostructured Fe–Co based systems. In the plot (a) the sequence refers to the synthesis of the Fe50 Co50 solid solution. 䊉 Fe bcc (JCPDS, 01-1252), Co hcp (JCPDS, 01-1277), Co fcc (JCPDS, 15-0806). In the plot (b) the preparation of the nanocomposite 10% wt Fe50 Co50 /TiO2 is shown: 䊉 Fe50 Co50 bcc, TiO2 , Rutile (JCPDS, 72-1148), TiO2 , Anatase (JCPDS, 84-1286), TiO2 , Brookite (JCPDS, 02-0514). Lower pattern is relevant to the as mixed powders, while the upper one refers to the mixture mechanically milled for 40 h.
to a mechanochemical pre-hydrogenation process and then milled under pure CO. In all the mechanochemical runs under reactive atmosphere the total pressure inside the mechanochemical reactor, measured by pressure transmitters, was set at 0.6 MPa. Moreover, the reactor temperature was kept at 300 K and the frequency of reactor displacement at 14.6 Hz. The conversion rate and the selectivity of the CO hydrogenation were monitored by gas-chromatography (GC). Hydrogen and carbon monoxide gases were detected using a Fisons 8000 apparatus equipped with a Hot Wire Detector, whereas hydrocarbons were analyzed by a Perkin-Elmer 8600 apparatus equipped with a Flame Ionization Detector. Suitable gas standard mixtures were employed to quantify the hydrocarbons formation and CO conversion. 3. Results and discussion 3.1. FeCo based systems Our attention was initially addressed to the synthesis of the Fe50 Co50 system. The mechanical treatment of the pure elements induced an alloying reaction with the formation of a bcc solid solution, in agreement with literature data [15]. The structure evolution is shown in Fig. 1, plot a, where the XRPD patterns sequence refers to specimens subjected to different times of mechanical treatment.
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a
b
Fig. 2. CO conversion, expressed as molar percentage, to hydrocarbons and methane observed in the catalytic test over the Co50 Fe50 (a), and (FeCo)/(TiO2 ) (b) systems.
The initial mixture, referred to as 0 h of milling, showed the coexistence of two Co allotropic phases, with fcc and hcp structure respectively, together with the Fe bcc phase. After 2 h of mechanical treatment, the fast transition of the Co fcc to Co hcp phase (the allotropic more stable phase) was achieved. For longer milling times, the Co reflections disappeared due to the dissolution of the Co atoms inside the Fe bcc lattice [15]. After 40 h of treatment the desired nanostructured conditions were approached, with average alloy crystallite size of 16 nm. A second system was prepared by a dispersion of 10 wt.% of the Fe50 Co50 solid solution over the anatase TiO2 phase. The pattern of the final structure induced by milling is displayed in Fig. 1 plot b, where the two patterns refer to the as mixed powders (lower trace, 0 h) and to the powders blend milled for 40 h (upper trace). The microstructural analysis indicates that nanostructured conditions were also reached by the TiO2 support, which underwent a phase evolution from anatase to rutile and brookite. The so prepared systems were then employed as catalysts in the CO hydrogenation under mechanochemical conditions. Powder catalysts were initially exposed to the gaseous mixture reagents under static conditions, i.e. with the mill at rest, but CO conversion was not observed. The starting of hydrocarbons synthesis occurred only under milling conditions, and kinetic behavior of the two investigated FeCo based systems was different. It is worth to mention that, for both systems, formation of the carbide species was not observed. Interestingly, in the supported system high selectiv-
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ity towards CH4 was achieved. Assuming that the TiO2 support is nonreactive, this behavior could be ascribed to the available source of C atoms on the surface as a result of the dispersion process of the active phase on the support. Data relevant to Fe50 Co50 solid solution are shown in Fig. 2, plot a. Conversion started only after an induction period which, under the present experimental conditions, was of about 6 h, and hydrocarbons formation increased almost linearly up to a maximum rate of r = 8.67 × 10−4 mol s−1 , while the amount of methane approached asymptotically a value of 3%. Different activity and selectivity values were displayed by the supported (FeCo)/(TiO2 ) system, as reported in Fig. 2, plot b. In this case CO conversion started right at the beginning of the milling action, and methane was the single observed product. The rate of methane formation increased up to a maximum value of r = 1.75 × 10−3 mol s−1 which was reached after about 50 h of treatment. Conversion rate then decreased and the total methane amount was about 4%. During the catalytic tests some evolution occurred in the structures [16]: in the Fe50 Co50 powders a reduced fraction of hcp Fe2 O3 (JCPDS 21165) was formed, while in the supported (FeCo)/(TiO2 ) system the fraction of TiO2 brookite evolved to rutile polymorph. It is to note that both the catalysts preserved the nanostructured conditions. Hereafter we focused on another aspect, i.e. to present the catalytic data of the studied systems on an absolute basis, which can allow a comparison among the results of different mechanochemical conditions and, more importantly, with the results of catalytic tests carried out under conventional thermal activation. It would also allow to evaluate the chemical conversion as a function of the effect of the transfer of mechanical work in heterogeneous catalytic reactions. In heterogeneous catalysis conversion rate and specific conversion rate are respectively expressed as: r = dnmol /dt(mol/s), and rs = dnmol /mp dt(mol/g· s) ; in the second equation the number of transformed moles per second are normalized to the catalyst mass, mp . However a parameter which can exhaustively characterise a catalytic reaction is the so called turnover frequency, TOF. It expresses the number of chemical transformation (or catalytic events) occurring per surface active site per unit of time. It is then necessary to know the absolute value of active sites of the catalytic system, Ns , which can be obtained by multiplying the surface atomic density ns = atoms/m2 to the total specific surface area St and to the total catalyst mass, mp . Ns [atoms] = (ns )(St )(mp ) =
atoms m2 m2
g
[g]
Then the TOF can be evaluated according to the following formula: TOF =
dnmol mp St ns dt
mol atoms · s
=
r mp St ns
mol atoms · s
Surface atomic density values, ns of close-packed structures can be obtained by geometrical studies of crystal planes, atomic populations and slipping directions, and data are available in the literature for the most commonly used metals [10]. It is also useful to underline that, for samples employed in conventional heterogeneous catalytic tests, St represents the external surface area exposed to gaseous reagents, which is commonly measured by physi- or chemi-sorptions techniques. However, when a mechanochemical processes is analyzed, and when the exposed catalysts surface is continuously refreshed, the maximum extension of specific surface area can be evaluated by summing the surface area of all the coherent diffraction domains exposed to reactive atmosphere as a consequence of powder grains refinement. For this reason, specific surface area St can be estimated resorting to microstructural characterization by diffraction techniques according to:
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Table 1 TOF, MTOF and MTE data for the FeCo-based studied catalysts.
molecules
Sample
TOF
Fe50 Co50 (FeCo)/(TiO2 )
3.44 × 10−6 1.76 × 10−6
atoms×s
MTOF
molecules atoms×hit
9.51 × 10−6 4.8 × 10−2
MTE
molecules atoms×J
0.14 0.69
St = 6 × 104 / L (m2 g), where L and are respectively the average diameter of crystallites and the bulk density of constituent materials, under the assumption of domains with spherical shape [9]. In principle the TOF evaluation could be applied to a mechanochemical conversion, but further information can be gained by taking into account that transfer of mechanical energy is a discrete process, and that reactive events take place as a consequence of collisions. The modeling of the milling process previously reported [11] then allows to introduce a new parameter defined as mechanochemical turnover frequency, MTOF: MTOF =
r mhit Sn p,i t s
·
1 N
mol atoms · hit
where mhit is the fraction of powder subjected to mechanical work p,i at each impact and N is the impact frequency. The MTOF expresses the number of molecules transformed per impact and per number of surface atom on the active phase involved in the chemical process. By normalizing the MTOF to the impact energy E, we finally obtain another parameter, the yield per surface per energy unit, MTE : MTE =
r mhit Sn p,i t s
·
1 1 · N E
mol atoms · J
Applying the above considerations to the conversion rates of the Fe50 Co50 and (FeCo)/(TiO2 ) catalysts, we obtain the TOF and MTOF values reported in Table 1. Considering that the conversion rate was not constant but varied along the treatment, the estimation of TOF and MTOF was related to the maximum rate of conversion. It emerges that MTOFs are of the same order of magnitude of TOFs measured for the similar systems activated by thermal treatment [15]. Moreover, TOF values confirmed that the metallic surface sites of the (FeCo)/(TiO2 ) composite were able to convert a much larger number of CO molecules per time unit, although the supported system presented a lower number of metallic sites. The same difference was observed for MTOF data, parameter which characterizes the mechanochemical activity of the catalyst. Finally, the MTE suggested the yield of the mechanochemical reaction, estimating the fraction of CO molecules converted on each active surface site subjected to an input of external mechanical energy of a joule. 3.2. Mg2 Ni based systems The reaction mechanism of CO hydrogenation has been deeply studied in the past, and in a very simplified view, the process starts with CO chemisorption on metal surface followed by dissociation; then C–H bond formation can take place when chemisorbed C and H atoms diffusing on the surface, approach the proper bond distance and energy interaction. Moreover, it has been reported the efficacy of mechanical treatment in activating hydrogen ad- and ab-sorption processes in metals and alloys [17–21]. Taking into account the above consideration, we then explored the reactive properties, as CO conversion to methane under milling condition, of another system, i.e. Mg2 Ni, which has been widely studied as hydrogen storage material and for which different hydride phases, Mg2 NiH4 and Mg2 NiH0.3 , can be formed. The Mg2 Ni system was tested under different mechanochemical conditions, i.e. by exposing the powders to CO + H2 gas mixture, as
Fig. 3. CuKa XRPD patterns relevant to the Mg2 Ni system: as synthesized (trace a), after 100 h of milling under CO + H2 atmosphere (b), after mechanochemical hydrogenation for 44 h (c); and successively milled 72 h under CO atmosphere (d).
well as by separating the reactive process into two mechanochemical steps, a pre-hydrogenation followed by the treatment of the alloy hydrides under CO atmosphere. Mg2 Ni was synthesized from the elements by mechanical alloying followed by thermal treatment under ar atmosphere at 670 K to complete the alloying process. Relevant XRPD pattern, reported in Fig. 3 trace a, indicates the formation of the alloy together with a small fraction of the Ni richer MgNi2 phase. Such occurrence is probably due to some stoichiometry variation during the mechanical alloying, as a consequence of Mg powder sticking inside the milling apparatus. The thermal treatment step hindered the achievement of nanostructured conditions. When such powder was mechanically treated for 100 h under the CO + H2 gas mixture, a structural evolution occurred, and relevant XRPD pattern is reported in Fig. 3, trace b. Mg2 Ni has been almost completely converted in MgO (JCPDS 78-0430), MgNi3 C (JCPDS 41-0903), and Mg2 NiH0.3 . Phases abundance and microstructural features of the formed nanocrystalline phases are reported in Table 2. CO conversion did not start at the beginning of the mechanochemical process, but only after an induction period of more than 10 h of milling. Conversely, as reported in Fig. 4, graph a, a pressure drop of the CO + H2 mixture occurred during the early stage of the milling process. Such behavior appears similar to that observed in a previous study on NiZr based catalyst [22],
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Table 2 Microstructural parameters (crystallite size, microstrain and phase abundance) of the Mg2 Ni system after different mechanochemical treatments. Data are obtained from the Rietveld analysis of the patterns shown in Fig. 3. System
Phases
Crystallite size (nm)
Microstrain
Phase abundance (wt%)
Mg2 Ni Milled under CO:H2 (100 h) – Trace b in Fig. 3
MgO Mg2 NiH0.3 MgNi3 C
13.4 83.4 5.8
8.25 × 10−3 1.05 × 10−4 5.9 × 10−3
54.8 6.4 38.8
Mg2 Ni Milled under H2 (44 h) – Trace c in Fig. 3
Mg2 NiH4 Mg2 NiH0.3 Ni
27.5 70.0 40.0
9.02 × 10−3 1.13 × 10−3 8.05 × 10−3
32.2 31.9 28
Mg2 Ni Pre-hydrogenated and milled under CO, (72 h) – Trace d in Fig. 3
MgO NiO Ni3 C MgNi3 Cx Ni
11.9 3.0 25.7 29.0 21.2
7.4 × 10−3 6 × 10−3 4.9 × 10−3 6.8 × 10−3 8 × 10−3
51 7.4 28 5 8.5
and can be assumed to correspond to the sorption of gaseous reactants, and their activation after true formation of catalyst phases. In Fig. 4, graph b, the formation of hydrocarbons is reported as a function of the milling time. Methane and ethane were the main products. As cited above, to verify the role of the induction period, in a further experiment we pre-hydrogenated the Mg2 Ni system under dynamic condition, before the mechanochemical CO conversion.
a
In Fig. 3, trace c, the XRPD pattern of the hydrogenated system is shown, and microstructural features are listed in Table 2. The main phases are Mg2 NiH0.3 (JCPDS 40-1204), Mg2 NiH4 (JCPDS 390848) and metallic Ni (JCPDS 4-850), with a small fraction of MgH2 (JCPDS 12-0697) and MgO (JCPDS 78-430). The formation of such phases allowed to evaluate the ability of a mixture of hydrides, i.e. Mg2 NiH4 , Mg2 NiH0.3 and MgH2 , as well as of metallic Ni in the activation of CO.
a
b
b
Fig. 4. Pressure trend of the CO + H2 mixture as function of the milling time during the mechanical treatment over the Mg2 Ni system (a). The relative percentage of light hydrocarbons is plotted in the graph b.
Fig. 5. Mechanochemical pre-hydrogenated Mg2 Ni system: CO conversion into hydrocarbons, expressed as relative percentage and plotted as a function of the milling time, (graph a). In the graph b the percentage of formed light hydrocarbons is reported.
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Conversely when the Mg2 Ni powders were exposed to the CO + H2 mixture, hydrogen activation had still to occur on surface reactive sites to form atomic hydrogen, before the reaction with CO molecules could result in the conversion to hydrocarbons. Finally, a large difference in the kinetics emerged from the comparison of the MTOF data of the CO conversion over FeCo based systems and pre-hydrogenated Mg2 Ni alloy. Keeping into account the differences in the chemical processes, and under the presented assumption in evaluating the MTOF, the developed methodology allowed to quantify the better performance of pre-hydrogenated system in CO conversion. 4. Conclusions
Fig. 6. Percentage of methane formed as a function of milling time respectively over the pre-hydrogenated (, black squares) and as synthesized (䊉, red circles) Mg2 Ni system.
Fig. 3, pattern d, and Table 2 report the compositional and structural data of the sample described above after being milled up to 72 h under CO atmosphere. Again, a severe transformation took place during the process, leading to MgO (JCPDS 78-0430), Ni3 C (JCPDS 77-0194), NiO (JCPDS 78-0643), Ni (JCPDS 04-0850) and MgNi3 C (JCPDS 41-0903). Reduction in Ni content together with the concomitant formation of two carbide phases, Ni3 C (JCPDS 77-0194) and MgNi3 C (JCPDS 41-0903), suggests that CO is first chemisorbed by Ni and then it can react with the activated hydrogen atoms present on the surface of the hydride phases. It also seems that the reaction proceeded until all the available hydrogen could react, as confirmed by the lack of hydride phases after 72 h of milling treatment. As for the gaseous products, in Fig. 5, graphs a and b, the CO conversion, as well as the percentage of formed hydrocarbons, are plotted as a function of the milling time. CO conversion shows a sigmoid trend and the highest rate, r = 1.11 × 10−6 mol s−1 , was achieved after about 14 h of mechanical treatment. The main product is still the methane, and the hydrocarbons production varies as a function of the milling time according to the distribution reported in Fig. 5, graph b. Recognizing the catalytic effect of elemental Ni in the chemisorption of CO, as well as in activating H2 adsorption and dissociation, it can be assumed that a crucial role in the CO conversion over the hydride alloy was played by the Ni free fraction. We therefore determined the catalytic activity of Ni in the mechanical activation of CO in terms of MTOF. From the XRPD analysis it is possible to estimate the specific surface area of Ni as 9.49 m2 g−1 , and the number of free Ni atoms, Ns , in the whole mass: the result is 6.43 × 1015 atoms. On the other hand we evaluated the number of CO molecules activated at each impact which was equal to 4.05 × 1019 . Finally the ratio between this value and Ns yielded the MTOF which was equal to 6.3 × 103 , and corresponded to the number of CO molecules activated by Ni as a consequence of an impact event. As a confirmation of the fast kinetics of this pre-hydrogenated Mg2 NiHx system, a comparison of the conversion to methane for the two Mg2 Ni studied systems is shown in Fig. 6 as a function of milling time: it turns out that the pure alloy displayed a sort of incubation period (of about 20 h) before a noticeable amount of methane was formed, while the hydrogenated system during the same milling time reached the highest rate. Such different behavior can be explained by considering that in the case of the hydrogenated alloy the CO molecules could be directly converted by the activated hydrogen atoms present on the phases surface.
Quantitative evaluation of conversion data in heterogeneous processes at the solid–gas interface driven by mechanical energy input is a very difficult task. It requires the detailed analysis of surface properties and microstructural bulk characteristics of solid phases, together with the control of milling dynamics parameters. The methodology presented in this work allows to compare on an absolute basis mechanochemical conversion data with the results of heterogeneous catalytic tests carried out under conventional thermal activation conditions. Results of CO + H2 synthesis over FeCo based catalysts indicated that mechanochemical processes, characterized by mild conditions (room temperature, and near to atmospheric pressure), displayed conversion data similar or better than the corresponding thermally activated reactions, performed under severe conditions (400–800 K, 2–5 MPa). Further improvement in conversion data and kinetics was observed in the CO conversion over hydrides. The quantification of such effect allowed to gain some hints of the reaction mechanism, and may open some potential perspective for the application of the studied process in a future hydrogen-based energy scenario. Acknowledgements This work was funded by MIUR (Italian Ministry for University and Research) in the framework of the PRIN Project “Synthesis, characterization and functional evaluation of light hydrides-based nanostructured materials and nanoparticles for solid state hydrogen storage”, and University of Sassari. S.G. acknowledges his postdoctoral fellowship, supported by the Fondazione Banco di Sardegna. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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