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CARBON NANJOSTRUCTURES FOR RENEWABLE ENERGY AND ECOLOGY
Nanosysiems: synthesis, properties, and applications
NOVEL CARBON NANOFIBERS WITH Ni-Mo AND Co-Mo NANOPARTICLES FOR HYDRODESULFURIZATION OF FUELS
Z. R. Ismagilov , A. E. Shalagina, O. Yu. Podyacheva, V. A. Ushakov, V. V. Kriventsov, D. I. Kochubey, A. N. Startsev
Member of International Editorial Board
Boreskov Institute of Catalysis Pr. Akad. Lavrentieva, 5, Novosibirsk, 630090, Russia Tel.: +7 (383) 330 6219; Fax: + 7 (383) 339 7352; E-mail: zri@catalysis.ru
Исмагилов Зинфер Ришатович
Сведения об авторе: профессор, доктор хим. наук, зав. лаборатории Института катализа СО РАН, Новосибирск.
Образование: факультет естественных наук Новосибирского Госуниверситета (1969 г.).
Область научных интересов: гетерогенный катализ, изотопные методы в катализе, катализ для защиты окружающей среды, каталитические процессы переработки углеводородного сырья, каталитическое сжигание топ-лив, технология производства оксида алюминия, технология производства блочных носителей и катализаторов, переработка радиоактивных отходов и ракетных топлив, каталитические микрореакторы, наноуглеродные материалы и катализаторы для водородной энергетики.
Публикации: более 500 публикаций и более 100 АС ССР и патентов РФ в соавторстве.
Methane decomposition over Ni-Mo-Al2O3 (10 wt. % Al2O3) and Co-Mo-Al2O3 (25 wt. % Al2O3) catalysts was studied at 550 and 500 °C. TEM, EDX and XRD investigations revealed localization of parent metal catalyst nanoparticles on the tips of the resulting carbon nanofibers (CNFs). The composition of these particles and the state of Ni, Co and Mo were examined by EXAFS spectroscopy. The catalytic activity of the isolated Ni-Mo and Co-Mo nanoparticles located on the tips of separate CNFs was tested in thiophene hydrodesulfurization (HDS) at 300 °C. The HDS activities of the CNF catalysts were found to be comparable with those of highly dispersed bimetallic sulfide catalysts supported on Sibunit carbon.
1. Introduction
Various carbon materials, such as charcoal, carbon black etc., are widely used as supports of metal catalysts. Currently, a new type of carbon materials — carbon nanofibers (CNFs) — excites a heightened interest [1-4]. Carbon nanofibers possess a number of valuable physical and chemical properties, namely: thermal stability, chemical inertness, electro- and thermal conductivity, high surface-to-volume ratio. CNFs are mainly the mes-oporous materials with the surface areas varying from 100 to 300 m2/g. Usually CNFs are synthesized by catalytic decomposition of carbon-containing compounds over iron sub-group metals (Ni, Co, Fe) and their alloys. This technique allows to produce CNF materials with predetermined struc-
tural and textural characteristics in a reproducible
manner. s
CNFs are devoid of some drawbacks peculiar ^
to microporous carbon supports, e. g. low mechan- *
ical stability, unsteadiness in oxidative environ- s
ment and microporosity which complicates reagent ^
access to the active centers of catalysts. As a con- §
sequence, CNF-based catalysts demonstrate im- |
proved activity as compared with traditional car- |
bon supports whatever processes are considered, 8
i. e. gas phase or liquid phase reactions. This mod- -j
ification of catalytic properties is believed to result g
from higher accessibility of deposited active com- ™
ponent to reactants [5-8] and from a new kind of 0 electronic interactions between catalyst particles and support surface [9, 10].
Supported metal catalysts are frequently subjected to deactivation processes at high tempera-
Статья поступила в редакцию 09.01.2007 г. The article has entered in publishing office 09.01.2007.
Z. R. Ismagilov, A. E. Shalagina, O. Yu. Podyacheva, V. A. Ushakov, V. V. Kriventsov, D. I. Kochubey, A. N. Startsev Novel carbon nanofibers with Ni—Mo and Co—Mo nanoparticles for hydrodesulfurization of fuels
ture. Thermal catalyst deactivation may be caused by sintering of active component which leads to an increase of metal particle size. One of the factors favourable for the migration of crystallite particles on support surface at high temperature is a weak strength of 'metal-support' interaction (5-15 kJ/mol). It was found that 'metal-support' interaction strength and, accordingly, thermal stability of supported catalysts decrease in the following order: Al2O3 > SiO2 > carbon [11]. One of the possible ways to prevent the sintering of active component may be a locking of metal particles in the structure of catalyst support. This approach was demonstrated for the metal-carbon filaments produced by pyrolysis of cellulose salts [12, 13]. The metal-carbon filaments revealed higher catalytic activity, selectivity and ther-mostability in hydrocarbon conversion [12] and dehydrogenation of alcohols [13] in comparison with the supported catalysts due to hard fixation of high dispersed metal phase (Ni, Co, Cu, Pt) in carbon matrix.
It is known that carbon nanofibers and nan-otubes produced by catalytic decomposition of hydrocarbons contain catalyst nanoparticles located on the filament tips [1]. These high dispersed metal particles are protected from the sintering due to their location on separate carbon filaments. High dispersity and rigid fixation of the metal particles can lead to high catalytic activity and thermal stability of the systems. For example, it was shown that Fe-Mo catalytic particles located on the tips of carbon nanotubes possess high activity and thermal stability in methane decomposition [14].
As we demonstrated earlier [3, 4] the chemical composition and structure of bi/multi metallic nanoparticles tailored to CNFs can be regulated by proper selection of precursors and preparation procedure of parent catalyst for hydrocarbon decomposition. The objectives of the present work are the preparation of Co-Mo and Ni-Mo catalysts for methane decomposition, growing of CNFs containing Co-Mo or Ni-Mo nanoparticles on their tips, and measurement of catalytic activity in reaction of thiophene hydrodesulfurization (HDS).
2. Experimental
2.1. Catalyst preparation The Ni-Mo-alumina catalysts containing 10 wt. % Al2O3 as a binder were prepared by the coprecipitation method as described elsewhere [15, 16]. Coprecipitation of the components was carried out from the metal nitrate solution (nickel nitrate and aluminum nitrate) at 75 °C under vigorous stirring using as precipitants ammonium molybdate and sodium hydroxide. After ageing in mother liquor for 1.5 h the precipitates were filtered and carefully washed with distilled water until neutral pH. Washed samples were dried overnight in air at room temperature, then at 110 °C for 2 h and decomposed in flowing nitrogen at 350 °C for 2 h. The catalysts were reduced in a hydrogen flow (99.9 % purity) at 550 °C for 4 h. The reduced samples were passivated in situ by ethanol and dried at ambient conditions. Catalysts of the following compositions were prepared: 90Ni-10Al2O3, 87.5Ni-2.5Mo-10Al2O3, 85Ni-5Mo-10Al2O3, 80Ni-10Mo-10Al2O3, 70Ni-20Mo-10Al2O3, 45Ni-45Mo-10Al2O3 (Table 1).
To prepare the Co-Mo-alumina catalysts in the first step an oxide precursor of the Co-alumina catalyst with 25 wt. % of Al2O3 was produced by the way similar to described above for the Ni-Mo-Al2O3 catalysts. The Co-Al2O3 samples calcined in N2 flow were impregnated with ammonium mo-lybdate solution. The liquid/solid ratio was kept at incipient-wetness conditions. The catalysts were dried in air, reduced in hydrogen at 500 °C with subsequent passivation in situ by ethanol. Catalysts of the following compositions were prepared: 75Co-25Al2O3, 74Co-1Mo-25Al2O3, 73Co-2Mo-25Al2O3, 72Co-3Mo-25Al2O3, 70Co-5Mo-25Al2O3, 65Co-10Mo-25Al2O3 (Table 2).
2.2. Ni(Co)-Mo/CNF catalyst preparation The Ni(Co)-Mo/CNF catalysts were prepared by the methane decomposition over Ni-Mo-Al2O3 and Co-Mo-Al2O3 catalysts at 550 and 500 °C 2re3-spectively at 1 bar. The reduced and passivated catalyst in amount of 0.1 g was placed into the flow reactor with a fluidized catalyst bed and heated in H2 flow to the reaction temperature. Subsequently, methane of 99.99 % purity grade was
Table 1
Composition and performance of Ni— and Ni—Mo—Al2O3 catalysts in methane decomposition: T =550 °C, space velocity 90 l-h-1-gcat-1, methane pressure 1 bar
Catalyst
T"
Ni:Mo:Al2O3 (wt. %)
x (%)a
t (h)
G (g/gcat)
90Ni-10Al2O-
90:0:10
16
13
103
87. 5Ni-2.5Mo-10Al2 O3
87.5:2.5:10
15
50
85Ni-5Mo-10Al2O3
85:5:10
14
10
54
80Ni-10M0-10Al203
80:10:10
15
60
70Ni-20Mo-10Al203
70:20:10
15
11
59
45Ni-45Mo-10Al203
45:45:10
14
8
48
a «Stationary» methane conversion.
b Number before the element symbol indicates the content in the initial methane decomposition catalysts, wt. %.
8
9
Table 2
Composition and performance of Co— and Co—Mo—Al2O3 catalysts in methane decomposition: T = 500 °C, space velocity 45 l-h-1-gcat-1, methane pressure 1 bar
Sample Co:Mo:Al2O3 (wt. %) x (%) t (h) G (g/gcat)
75Co-25Al2O3 75:0:25 8 15 26
74Co-1Mo-25Al2O3 74:1:25 9 16 29
73 Co-2Mo-25Al2O3 73:2:25 9 24 41
72Co-3Mo-25Al2O3 72:3:25 9 20 37
70Co-5Mo-25Al2O3 70:5:25 9 15 26
65Co-10Mo-25Al2O3 65:10:25 7 11 15
introduced at atmospheric pressure and flow rate of 45-90 l h-1gcat-1.
2.3. Physicochemical characterization
The phase compositions of the parent Ni(Co)-Mo-Al2O3 catalysts and nanostructured Ni(Co)-Mo catalysts on CNFs were investigated by X-ray powder diffraction (XRD) to display the catalyst phase in the metallic state and to examine structural features of carbon phase. XRD patterns were recorded on a HZG-4 diffractometer using CuKa radiation. The average size of coherently scattering domains (CSD) was calculated from the line width following the Scherrer equation. The inter-layer distance d002 was determined from the position of diffraction line (002) corrected for the background and the Lorenz factor.
EXAFS method was used to characterize the composition of catalytic Ni(Co)-Mo nanoparticles after CNF growth and to study the local arrangement of the metals (Ni, Co and Mo). The EXAFS spectra of the Co-K, Ni-K and Mo-K edges were obtained at the EXAFS Station of Siberian Synchrotron Radiation Center. The storage ring VEPP-3 with the electron beam energy of 2 GeV and the average stored current of 90 mA was used as the source of radiation. The X-ray energy was monitored with a channel cut Si(111) monochromator. All the spectra were recorded under transmission mode using two ionization chambers as detectors. The EXAFS spectra were treated using the standard procedure [17] by «Viper» code [18]. The background was subtracted by extrapolating the pre-edge region onto the EXAFS region in the forms of Victoreen's polynomials. Three cubic splines were used to construct the smooth part of the absorption coefficient. The inflection point of the edge of the X-ray absorption spectrum was used as initial point (k = 0) of the EXAFS spectrum. The radial distribution functions (RDF), describing the local metal arrangements, were calculated from the EXAFS spectra in k3x(k) as modules of Fourier transforms at the wave number intervals of 3.0-12.0 A-1 and 3.5-13.0 A-1 for Co-K, Ni-K and Mo-K edges, respectively. Curve fitting procedure with EXCURV92 [19] code was employed to determine precisely the interatomic distances and coordination numbers in similar wave number intervals after preliminary Fourier filtering using the known XRD data of bulk compounds. Debye-Waller factors were fixed (all values were equal to 0.005 A2).
Structure and morphology of the Ni(Co)-Mo/CNFs were studied by transmission electron microscopy (TEM). TEM pictures were obtained by JEM-100CX microscope with lattice resolution 2 A and accelerating voltage 50-100 kV. High resolution TEM (HRTEM) micrographs were recorded with JEM-2010 instrument having lattice resolution of 1.4 A and accelerating voltage 200 kV.
Elemental analysis of metals in Ni(Co)-Mo/CNFs was performed by EDX method using the Energy-Dispersive X-ray Phoenix Spectrometer (EDAX) equipped with Si (Li) detector with energy resolution not worse than 130 eV. The EDX spectrometer was set at JEM-2010 microscope (JEOL).
Textural characteristics of the Ni(Co)-Mo/CNFs, such as BET specific surface area (^BET), pore size distribution, total pore volume (^pore) and average pore diameter (Dpore), were determined by the N2 adsorption at 77 K. The adsorption measurements were carried out using an ASAP-2400 instrument (Micromeritics).
2.4. Catalytic activity test
Catalytic activity of Ni(Co)-Mo/CNF catalysts was tested in a model reaction of thiophene hy-drodesulfurization at 300 °C under hydrogen pressure of 20 bars. Presulfidation of catalysts was carried out directly in the catalytic reactor by passing of the thiophene containing reaction mixture at 300 °C.
The activity measurements were conducted in a flow reactor with a fixed catalyst bed in a kinetic regime. The reaction mixture was composed by passing of hydrogen flow through the saturator filled with a mixture of cyclohexane and thiophene (9:1). The resulting molar composition of the reaction mixture was H2:C6H12:C4H4S = 2200:9:1. The inlet concentration of C4H4S and concentrations of the reaction products at the reactor outlet were determined by gas chromatography. The HDS reaction activity was calculated by the first-order equation relative to thiophene as ACC4H4S(mol)/
C4H4S = CC4H4S(inlet)-
((CM„ + CN.)(mol)-h), where ACC
'Mo 'C4H4S(
„(outlet) [20].
3. Results and discussion
3.1. Ni(Co)-Mo/CNF catalyst synthesis
It was shown earlier [16, 21] that considerable amount of filamentous carbon can be produced by methane decomposition at 500-550 °C over high-loaded nickel- and cobalt-containing catalysts with
Z. R. Ismagilov, A. E. Shalagina, O. Yu. Podyacheva, V. A. Ushakov, V. V. Kriventsov, D. I. Kochubey, A. N. Startsev Novel carbon nanofibers with Ni—Mo and Co—Mo nanoparticles for hydrodesulfurization of fuels
Fig. 1. Methane conversion as a function of the reaction
Reaction 1
Fig. 2. Methane conversion as a function of the reaction
time on the Ni—Al2O3 and Ni—Mo—Al2O3 catalysts at 55G °C
time on the Co—Al2O3 and Co—Mo—Al2O3 catalysts at 5GG °C
low percentage of binding «support», which is more correctly to be designated as «structural promoter» [22, 23] or «binder». The best performances were found for the coprecipitated 90 wt. % Ni-alumina and 60-75 wt. % Co-alumina catalysts, where car-
catalyst
bon yields reached 100 and 65g/g spondingly. Therefore, in the present study we used catalysts with the high total metal loadings: 90 wt. % for Ni-Mo-alumina (10 wt. % of Al2O3) and 75 wt. % for Co-Mo-alumina (25 wt. % of Al2O3). The Ni-Mo-Al2O3 and Co-Mo-Al2O3 catalysts 2wi3th molybdenum loading from 1 to 45 wt. % were prepared and studied with regard to their activity for the methane decomposition. The specific effect of molybdenum concentration in parent Ni-Mo-Al2O3 and Co-Mo-Al2O3 catalysts on carbon yield was also elucidated.
Performance of parent catalysts in CNF growing were compared on the basis of following parameters: methane conversion (x) by reaction time, catalyst lifetime before complete deactivation t and carbon capacity G (calculated as gram of carbon grown per gram of catalyst). As it is illustrated by Figs. 1 and 2, the dependencies of methane conversion on the reaction time plotted for Mo-containing catalysts in general are similar to those observed for monometallic Ni- and Co-alumina catalysts. The kinetic curves for all samples are characterized by three reaction stages: (1) induction period when methane conversion increases, (2) steady state when the conversion value is almost constant and (3) deactivation stage when methane conversion diminishes to zero.
Tables 1 and 2 summarize the catalytic properties of Ni-Mo-Al2O3 and Co-Mo-Al2O3 samples in the reaction of methane decomposition related to the catalyst composition. Relationships between the carbon capacity and the quantity of molybdenum introduced into Ni-Al2O3 and Co-Al2O3 catalysts are represented by Fig. 3. There are some differences in Mo concentration effect on catalytic behavior of Ni-Mo and Co-Mo samples. Introduction of small amount of
molybdenum (from 2.5 wt. %) into Ni-Al2O3 catalyst results in considerable decrease of carbon capacity due to decrease of the catalyst lifetime. The following increase of Mo concentration up to 45 wt. % does not affect G. In the case of Co-Mo catalysts, more complicated picture is observed: low concentrations of Mo (from 1 to 3 wt. %) lead to prolongation of catalyst lifetime. However, the further rise of Mo concentration (up to 10 wt. %) leads to essential decrease of catalyst lifetime and, consequently, to decrease of carbon capacity. At the same time, the maximum values of methane conversion are very close. The highest catalyst lifetime and, hence, the maximum of carbon capacity were observed when molybdenum concentration in Co-Mo-alumina sample was equal to 2 wt. % (Fig. 3). Thus, the synerge-tic effect in Co-Mo system at addition of 2 wt. % Mo is pronounced in an increase of catalyst lifetime and carbon capacity by a factor of 1.6 (Table 2).
Similar synergism in Co-Mo system was discovered recently for the production of single-walled carbon nanotubes (SWCNTs) by CH4 decomposition and CO disproportionation [24, 25]. It was established that molybdenum addition to cobalt
Fig. 3. Dependence of carbon capacity on molybdenum concentration in Ni-Mo-alumina (10 wt. % of Al2O3) and Co-Mo-alumina (25 wt. % of Al2O3) catalysts
catalysts could noticeably increase the yield of SWCNTs and also improve their quality. To explain this fact several hypotheses were proposed. In the first instance, stabilization of Co nanoparticles may take place owing to decoration of their edges by MoO3 or Mo2C generated during the catalyst preparation and in the reaction course, correspondingly. The second explanation [25, 26] is based on the idea that Mo2C serves as an active center for promoting the methane aromatization [27]. In that case, Mo introduced into Co catalyst may provide the neighbouring Co sites with intermediate aromatic species increasing yield of carbon na-notubes [25, 26]. However, it is unlikely that this assumption could be completely applied to our results as the reaction temperature of 500 °C might be insufficient for the catalytic aromatization of methane to occur [28]. At the same time, the first hypothesis regarding the stabilization of high dispersed cobalt particles by Mo compounds can explain the results observed in the present study, namely, an increase of catalyst lifetime when Mo is added. Decorating of Co particles by oxide or/and carbide of Mo reduces their aggregation at high temperature and, as a consequence, catalyst lifetime increases. On the other hand, too high Mo content may weaken the interaction between Co particles and support [25] or decrease cobalt-containing active surface and, as a result, reduce catalyst stability.
For Ni-Mo catalysts no synergetic effect was observed, addition of molybdenum resulted in substantial decrease of carbon deposition (Fig. 3). It is known that doping of nickel catalyst by small quantity of Mo can improve resistance of the catalyst to coking in steam [29] and CO2 [30] hydrocarbon reforming. Morphology of carbon deposits on Ni and Ni-Mo catalysts is reported to be similar. However, essential reduction of coking rate was observed. The following reasons for decrease of carbon deposition by molybdenum doping were proposed in [29]: (1) formation of Ni-Mo alloy, which diminishes carbon solubility in catalytic particle and, as a consequence, inhibits nucleation of filamentous carbon; (2) segregation of Mo on the surface of crystallites in the oxidative atmosphere; (3) decrease of dehydrogenation extent of dissociating 'C^H^' fragments and easier gasification of hydrocarbon units due to possibility of variation of Mon+ oxidation state during the reaction [31].
For the production of Ni(Co)-Mo/CNF catalysts the reaction time of 2 h was defined as optimum taking into account the steady-state level of activity of Ni(Co)-Mo catalysts in the area of this time, and substantial total metal content in the resulting samples.
It is well known that HDS catalysts consist of MoS2 promoted by nickel or cobalt, and normally the ratio between Mo:Ni(Co) is equal 2 [20]. It is difficult to produce the Ni(Co)-Mo/CNF catalysts with the same ratio of Mo to Ni(Co), since molybdenum is inactive in methane decomposition. So, we chose the catalysts with the highest Mo concentration available in our experiments. As an example, 70Ni-20Mo-10Al203 and 65Co-10Mo-25Al203 catalysts will be discussed in some details.
3.2. XRD studies of Ni(Co)-Mo/CNFs
The XRD patterns of the Ni(Co)-Mo/CNF samples are given in Fig. 4. Two phases, namely graphitic carbon and metallic nickel in face centered cubic (fcc) structure, are clearly identified in the XRD spectrum of the Ni-Mo sample. Comparison of diffractograms of the Ni-Mo/CNF samples with the parent catalysts shows that the lattice constant and the mean crystallite size of nickel (D311 = 8.4-9.1 nm) did not change during CNF growth. Increased value of nickel lattice parameter in both samples (0.354-0.356 nm instead of 0.352 nm [PDF ICDD 4-850]) may be assigned to the formation of an alloy on the basis of fcc lattice of Ni. In addition, the Ni-Al spinel (which was found in 90Ni-10Al2O3 [15]) was not detected for Ni-Mo/CNFs. In the2ca3se of Co-Mo/CNF sample peaks corresponding to graphitic carbon, high dispersed metallic cobalt in fcc structure and a spinel structure, like (Co, Al)[Al, Co, Mo]2O4, with a lattice parameter 8.143 A and crystallite size ~4 nm were observed (Fig. 4). The peak at ~47.5° which can be assigned to metallic cobalt in closed-packed hexagonal structure was found. All these phases with the exception of graphitic carbon were observed in the XRD spectra of parent Co-Mo-alumina catalysts before CNF growth.
Fig. 4. XRD diffractograms of Ni-Mo/CNFs (70Ni-20Mo-10Al2O3) (a) and Co-Mo/CNFs (65Co-10Mo-25Al2O3) (b) (Co* refers to metallic Co in hexagonal structure)
The molybdenum-containing phases were not found by XRD in both parent Ni-Mo-Al2O3 and Co-Mo-Al2O3 catalysts after each of the preparation steps (coprecipitation, calcination, reduction) as well as after CNF growth. However, diffraction lines assigned to NiMoO4 phase were detected in the XRD patterns of Ni-Mo-Al2O3 catalysts calcined in nitrogen flow at 700 °C. Appearance of diffraction peaks related to NiMoO4 phase is likely to be caused by sintering of finely dispersed molybdate particles in low temperature samples that were invisible by XRD method. Furthermore, reaction between metallic Ni (or Ni-Mo alloy) and MoOx species at high temperature may results in formation of NiMoO4 phase.
Z. R. Ismagilov, A. E. Shalagina, O. Yu. Podyacheva, V. A. Ushakov, V. V. Kriventsov, D. I. Kochubey, A. N. Startsev Novel carbon nanofibers with Ni—Mo and Co—Mo nanoparticles for hydrodesulfurization of fuels
XRD measurements reveal graphitic nature of carbon in Ni-Mo/CNFs and Co-Mo/CNFs. Interplanar distance d002 determined from the XRD data was found to be 0.342-0.343 nm, that is higher than in ideal graphite (0.338 nm [PDF ICDD 41-1487]) and equal to d002 of CNFs produced on 90Ni-10Al203 [32] and 75Co-25Al2O3 [16] catalysts. Coherent scattering region values along the c-axis of carbon filament Lc (i. e., along the lines perpendicular to basal plane of graphite) were 4.6-5.3 nm. High d002 with low Lc values are typical for turbostratic carbon formed by graphite networks stacked in a random manner [33].
3.3. Microstructure of Ni(Co)-Mo/CNFs
TEM images of Ni(Co)-Mo/CNF samples proved filamentous structure of the deposited carbon. Figs. 5 and 6 show HRTEM photographs of carbon nanofibers grown on Ni-Mo-Al2O3 and Co-Mo-Al2O3 catalysts, respectively. It is obvious that formation of CNFs goes under 'tip-growth' mechanism for carbon filament growth [34] leading to location of catalytic particle on fiber tip. In the case of Ni-Mo-Al2O3 sample (Fig. 5) 'herringbone' fibers with graphite layers arranged at an angle of 45° to c-axis of filament grow from drop-shaped catalytic particles of 25-30 nm in size. Fiber diameter is equal to the catalytic particle size. Such structure and morphology are typical for CNFs formed on Ni-alu-mina catalysts. In the same way, structure of the carbon filaments grown on Co-Mo-Al2O3 catalysts (Fig. 6) is similar to that of CNFs produced by 75Co-25Al203 catalyst [16]. Carbon nanofibers have a hollow-like core structure with the turbostratic graphite layers stacked up at small angle to fiber axis ~15°. Outer diameter of the fiber ranges between 20 and 25 nm and diameter of inner channel is about 5 nm.
Elemental composition of catalyst nanoparti-cles located on CNF tips was determined by EDX spectroscopy in addition to the XRD analysis registered Ni and Co metal phases. According to EDX data, the catalyst nanoparticles consist of both nickel (or cobalt) and molybdenum (Figs. 5, 6).
3.4. EXAFS studies of Ni(Co)-Mo/CNFs
Fig. 7 shows the curves of radial distribution functions (RDF) describing the local arrangements of Ni and Co in Ni-Mo/CNF and Co-Mo/CNF samples, respectively. Both the spectra have almost identical shape, corresponding to fcc structure, and show that nickel and cobalt are present mainly as metallic phases. Indeed, from Table 3 it is seen that calculated set of the Me-Me distances
Fig. 5. HRTEM image (on the top) of Ni-Mo/CNFs (70№-20Mo-10A1203) exhibiting 'herringbone' structure with nanoparticle at the tip. EDX spectrum (at the bottom) of the nanoparticle shows that qualitative ratio between Ni and Mo is saved after CNF growth
Fig. 6. HRTEM image (on the top) of Co-Mo/CNFs (65Co-10Mo-25Al203) exhibiting hollow-like core structure with nanoparticle at the tip. EDX spectrum (at the bottom) of the nanoparticle demonstrates that it contains both Co and Mo in the same qualitative ratio as the parent catalyst before CNF growth
(Me = Ni or Co), as compared with those for reference metallic Co and Ni, prove this assumption. The lower coordination numbers and higher first Me-Me distances in comparison with the reference data from ICSD database (Table 3) can be explained by some distortion of the metal clusters rather than small particle size, since according to TEM data the catalytic particles are 20-30 nm in size. Such distortion may be caused by some interaction of metal clusters with carbon, molybdenum or alumina. Perhaps, both Co(Ni) and Co(Ni)-Mo phases are presented.
The local Mo environments in the Co-Mo/CNF and Ni-Mo/CNF samples are clearly different as shown in Fig. 8. In the case of Ni-Mo/CNF sample it can be concluded that Mo is present mainly as a Ni-Mo metallic phase. It should be noted that fitting by the use of Mo-Mo distances only is failed because it gives very short Mo-Mo distances ~2.6 A which are not presented in Mo reference (Table 3). The contribution of «oxidized» phase is quite small because only the short Mo-O distance with low coordination number is observed. For the Co-Mo/ CNF sample, analysis of the EXAFS data obtained
Fig. 7. Radial distribution function (RDF) curves of Co K-edge in Co-Mo/CNFs (65Co-10Mo-25Al2O3) (solid line) and Ni K-edge in Ni-Mo/CNFs (70Ni-20Mo-10Al2O3) (dotted line) after 2 hours of CNF growth
R-5.Â
Fig. 8. RDF curves obtained at the K-edge of Mo for Co-Mo/CNFs (65Co-10Mo-2 5Al2O3) (solid line) and Ni-Mo/ CNFs (70Ni-20Mo-10Al203) (dotted line) after 2 hours of
CNF growth 2 3
at Mo K-edge indicates the presence of a Mo-O contribution at 1.75 À in a large quantity (Fig. 8). Nevertheless, the presence of small amount of a Co-Mo phase cannot be excluded since the Mo-Co distance ~2.5 À was observed with a small coordination number (Table 3). The question concerning the structure of Mo distorted oxidizing phase is still open for discussion, and detail analysis will be carried out in future. But it should be noted that this structure is more corresponded to distorted MoO2 structure.
3.5. Textural properties of Ni(Co)-Mo/CNFs Textural properties of Ni(Co)-Mo/CNFs presented in Table 4 show that these samples are mesoporous with pores of 7.5-23.9 nm in average size depending on the parent catalyst composition. The BET surface area varies between 92 and
117m2/g. It should be noted that textural characteristics of Ni(Co)-Mo/CNFs are close to those of Ni(Co)/CNFs.
3.6. Catalytic performance of Ni(Co)-Mo/CNF catalysts in thiophene hydrodesulfurization At the beginning of experiments the conversion decreases by a factor of 1.4-1.8 for Ni-Mo/CNFs, whereas for Co-Mo/CNFs by a factor of 2.5-3.5 (Fig. 9). It is obvious that Ni-Mo/CNF catalysts are more active than Co-Mo/CNFs, the steady state thiophene conversion on these catalysts differ by two fold. These data are in a good agreement with literature claiming that Ni-Mo systems are usually more active than Co-Mo ones [35]. It is remarkable that activity of our samples is comparable with that of highly dispersed sulfide bimetallic catalysts prepared via anchoring of the metal com-
Table 3
Structural parameters of Co-Mo/CNFs (65Co-10Mo-25Al2O3) and Ni-Mo/CNFs (70Ni-20Mo-10Al2O3), as obtained from the EXAFS data analysisa
Sample Ni(Co) K-edge Mo K-edge
R (А) N R (А) N R (А) N R (А) N
Ni-Mo/CNFs (70Ni-20Mo) 2.52 7.3 1.73 0.8 2.48 2.6 2.70 1.6
Co-Mo/CNFs (65Co-10Mo) 2.49 5.5 1.75 2.3 2.52 1.0 2.56 1.3
Ni b 2.49 12
Co b 2.42 12
Mo b 2.72 8
a In the table, R (A) represents the coordination distance for each respective bond, and N are the coordination numbers for the local arrangement of metals (Ni, Co or Mo). b Data taken from ICSD database (1997).
Table 4
Textural properties of CNFs
Sample
Ni/CNFs (90Ni)
Ni-Mo/CNFs (85Ni-5Mo)
Ni-Mo/CNFs (70Ni-20Mo)
Co-Mo/CNFs (75Co)
Co-Mo/CNFs (73Co-2Mo)
Co-Mo/CNFs (65Co-10Mo)
£bet (m /g)
102
97
92
116
117
108
e (cm3/g)
0.26
0.19
0.17
0.56
0.70
0.48
Dpore (nm)
9.9
7.6
7.5
20.1
23.9
17.9
Z. R. Ismagilov, A. E. Shalagina, O. Yu. Podyacheva, V. A. Ushakov, V. V. Kriventsov, D. I. Kochubey, A. N. Startsev Novel carbon nanofibers with Ni-Mo and Co-Mo nanoparticles for hydrodesulfurization of fuels
plexes on the surface of Sibunit carbon material [20, 35] (Table 5).
4. Conclusions
Methane decomposition over Ni-Mo-Al2O3 (10 wt. % Al2O3) and Co-Mo-Al2O3 (25 wt. % Al2O3) catalysts produces carbon nanofibers containing parent metal catalyst nanoparticles of 20-30 nm in size located at the CNF tips.
The catalyst particles consist of nickel (cobalt) in metallic state and high-dispersed molybdenum-containing phase. The nanofiber structure is similar to that of CNFs resulted from the methane decomposition over Ni-Al2O3 and Co-Al2O3 catalysts. Carbon nanofibers with a 'herringbone' structure are formed on the Ni-Mo-Al2O3 catalysts, while hollow-like fibers are grown on the samples.
50
40
v 30
20
□ - Ni-Mo/CNFs (85Ni-5Mo)
л -Ni-Mo/CNFs (70Ni-20Mo)
о-Co-Mo/CNFs (73Co-2Mo)
o- Co-Mo/CNFs (65Co-10Mo)
- ^^ А Л- t. Л Л
I --—Щ>------ф--v——v 1 . 1 1 1 , 1
Co-Mo-Al2O3
0 20 40 60 80 100 120
Reaction time, min
Fig. 9. Hydrodesulfurization of thiophene over Ni-Mo/CNF and Co-Mo/CNF catalysts
Table 5
Stationary catalytic activity in thiophene hydrodesulfurization at 300 °C
a Stationary thiophene conversion.
b Stationary molar catalytic activity, ACC H S(mol)/((CMo+CN1)(mol)xh) [35].
Sample Composition (wt . %) a (%) a MCA b
Ni (Co) Mo Al2O3 С
Ni-Mo/CNFs (85Ni-5Mo) 5.88 0.35 0.69 93.08 26 47
Ni-Mo/CNFs (70Ni-20Mo) 3.83 1.09 0.55 94.53 18 10
(0.6Ni-4.0Mo)/Sibunit [35] 0.6 4.0 0 95.4 42
Co-Mo/CNFs (73Со-2Мо) 15.19 0.42 5.20 79.19 14 7
Co-Mo/CNFs (65Со-10Мо) 15.56 2.39 5.98 76.07 12 6
(1.1 Co-8.0Mo)/Sibunit [35] 1.1 8.0 0 90.9 22
Introduction of molybdenum into cobalt catalyst results in increase of lifetime of Co-Mo system and CNF yield. Synergism effect is observed when the molybdenum addition is equal to 23 wt. %. On the contrary, in Ni-Mo catalyst the presence of molybdenum leads to decrease of activity of Ni-Mo catalyst in respect to CNF growth. Both Ni-Mo/CNFs and Co-Mo/CNFs are the graphitic mesoporous materials with the specific surface area between 92-117 m2/g.
The samples of Ni(Co)-Mo/CNFs were tested as Ni-Mo and Co-Mo catalysts in thiophene hy-drodesulfurization and showed HDS activity com-s parable with that of the high dispersed sulfide [j bimetallic catalysts supported on Sibunit carbon * material. The approach described in this study, S i.e. preparation of isolated Ni-Mo and Co-Mo ^ nanoparticles each located on the tips of separate g CNFs, is advantageous and may be used in the
CD
| close future for the design of new carbon support-
I ed nanocatalysts for different applications.
0
1
§• Acknowledgements
i
S We are grateful to Dr. L. B. Avdeeva for dis-
™ cussion of the research results, Dr. E. M. Moroz for valuable remarks, Dr. A. L. Chuvilin for providing of TEM images and O. G. Abrosimov for EDX measurements.
This research has been carried out under the support by INTAS (05-1000005-7726), NWO-RFBR (047.017.028) and SB RAS Integrated Project No. 4.5 grants.
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