Chemistry for Sustainable Development 20 (2012) 119�127 119

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Formation of Growth Centres of Carbon Nanofibresduring Self-Dispersing Ni-Containing Alloys:Studies by Means of Ferromagnetic Resonance

YU. I. BAUMAN, R. M. KENZHIN, A. M. VOLODIN, I. V. MISHAKOV and A. A. VEDYAGIN

Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences,Pr. Akademika Lavrentyeva 5, Novosibirsk 630090 (Russia)

E-mail: bauman@catalysis.ru

(Received June 24, 2011)

Abstract

Using ferromagnetic resonance and electron microscopy, dispersing bulk nickel and its alloys in thecourse of interaction with the reaction medium (1,2-dichloroethane and hydrogen) accompanied by theintense growth of carbon nanofibres was studied. It is demonstrated that in all the cases the formation ofrather uniform nickel nanoparticles occurs with the characteristic size of 200�300 nm. The efficiency ofthis method for studying the formation of nickel particles at the very initial stages of the reaction wasdemonstrated, including the induction period, when the use of electron microscopic studies is not efficientenough.

Key words: ferromagnetic resonance (FMR), 1,2-dichloroethane, catalytic decomposition, bulk alloys, nickel,chromium, catalytic corrosion, carbon nanofibres

Bauman Yu. I., Kenzhin R. M., Volodin A. M., Mishakov I. V. and Vedyagin A. A.

INTRODUCTION

Obtaining any structured materials as theresult of chemical reactions implies the presenceof self-organization processes those ultimatelydetermine the chemical composition, structureand morphology of these materials. Thus, theselective formation of either product could beexpected only within a fairly narrow range ofchemical reaction conditions. In particular, thisis true for catalytic processes involving theformation of carbon nanostructures.

The catalytic synthesis of nanostructuredcarbon materials (nanotubes and nanofibres) inmost cases is carried out with the use of metalsbelonging to the iron subgroup [1�3]. Themorphology and properties of the resultingcarbon nanomaterials are to a considerableextent dependent on the characteristics of

catalysts used such as size, the chemicalcomposition of a metal particle, the type ofcarrier, the composition of the gas phase aswell as the conditions of the catalytic reaction.Dispersed metallic nickel is easily oxidized inatmospheric air, so for such syntheses one usescatalysts wherein the initial particle of nickelmetal (or of its alloy) with the size required isformed in a reducing atmosphere from an oxideprecursor applied onto the dispersed carrier [3].

The use of halogenated compounds as pre-cursors for the synthesis of nanostructuredcarbon materials is connected with the necessi-ty for developing the methods of their dispo-sal. On the other hand, this could allow one todevelop a method of targeted producing car-bon nanofibres modified by halogens.

Of absolute interest is the development ofa technology for the synthesis of carbon nano-

120 YU. I. BAUMAN et al.

TABLE 1

Composition of raw materials for preparing the catalysts

Alloys Composition of alloys

according to State Standard GOST 10994�74, mass %

Ni Cr Fe Mn Al Si

Nickel (band) 99.9 � 0.1 � � �

Nichrome (0.1 mm wire) 75.5 23.0 1.5 � � �

Chromel (0.5 mm wire) 90.0 9.5 0.5 � � �

Alumel (0.5 mm wire) 94.5 0 0.5 2.0 2.0 1.0

materials without a stage of preparing support-ed catalysts. For these purposes one could useiron and nickel alloys. These materials in theform of foil and wire are manufactured on anindustrial scale and they are widely used in var-ious processes. It is obvious a bulk metal itself(or an alloy) is not able to initiate the processof growing carbon nanostructure materials. Thiscapability could be exhibited only by dispersedparticles. At the same time, there is a wellknown phenomenon of so-called catalytic cor-rosion of bulk metals and their alloys in thecourse of occurring Red-Ox catalytic reactions[4, 5]. Such reactions are generally accompa-nied by substantial �loosening� the metal sur-face with appearing particles characterized bya wide particle size distribution. To all appear-ance, these particles with suitable size could actas catalysts for the growth of carbon nanoma-terials (CNM).

Recently [6], we observed a phenomenon ofstructural and chemical self-organizing the cat-alysts in the course of CNM growth from chlo-ro-substituted hydrocarbon (1,2-dichloroethane).It was demonstrated that bulk nickel and itsalloys can be used as the precursor of a cata-lyst. These materials in the course of interac-tion with a reaction medium are dispersed toform a sufficiently homogeneous composition ofmetallic nickel particles with a typical size of200�300 nm. Further, these particles act as anefficient catalyst for the growth of CNM.

In order to investigate the properties ofmetal particles in the dispersed state it is ap-propriate to use the method of the ferromag-netic resonance (FMR). In many cases it is ap-plied for studying the systems whose composi-tion involves metallic nickel, iron and cobalt,the metals those exhibit ferromagnetism [7�12].

Measuring the FMR spectra can be performedusing standard electron spin resonance (ESR)spectrometers within three-centimetre wave-length range in the magnetic fields with theinduction value ranging within 0�5000 Gs. Thepresence of strong exchange interaction in fer-romagnetics and their great anisotropy, as wellas a number of other factors make it very dif-ficult to interpret the FMR spectra in polydis-perse substances as compared to the interpre-tation of ESR spectra. Nevertheless, in the caseof monodisperse systems the FMR spectra (af-ter appropriate calibration) could be the sourceof information about the geometry and disper-sion level of ferromagnetic particles, their con-centration, as well as (in case of alloys) con-cerning their chemical composition.

The aim of the present work consisted inthe investigation of the formation and evolu-tion of Ni-containing catalysts in the courseof growing carbon nanofibres from 1,2-dichlo-roethane (DCE) using the methods ferromag-netic resonance and electron microscopy. Asstarting materials we used bulk metallic nick-el and standardized alloys with chromium, alu-minum, etc.

EXPERIMENTAL

Materials

As the catalyst precursors, we used thefollowing bulk metals and alloys: nickel belt(NP2E), nichrome (Ch20N80), chromel and alumel.In the experiments we used high purity argon andhydrogen, chemical purity grade 1,2-dichloro-ethane, as well as concentrated ÍCl and HNO3.

Table 1 demonstrates the composition ofstarting materials for preparing catalysts.

SELF-DISPERSING NI-CONTAINING ALLOYS 121

Testing the samples

Immediately before the experiment, thesamples of bulk alloys were subjected to a pre-liminary treatment (activation). The activationwas carried out in two ways: a) by etching in amixture of concentrated HCl and HNÎ3 at aratio of 3 : 1, respectively, for 3 min, b) alter-nate impact by oxidizing and reducing media,so-called Red-Ox activation).

Studying the kinetics of catalytic DCE de-composition with the formation of carbonnanofibres was carried out with the use of aflow reactor equipped with a Mac Ben microbal-ance [13].

After loading into the reactor and establish-ing a preset temperature mode the samples wereheld in a flow of hydrogen. The reactor wasthen fed with the reaction mixture of the fol-lowing composition, vol. %: DCE vapour � 7.5,hydrogen � 37, argon � the rest. The changein mass of the sample was registered at an in-terval of 2 min. After the experiment the sam-ples were cooled in a flow of argon to attain aroom temperature.

FMR measurement technique

The FMR spectra of the samples were reg-istered using an experimental setup based onan ERS-221 ESR spectrometer described in [14]at a room temperature. The processing of thespectra obtained was performed with the useESR_CAD software we developed earlier.

Metals when they are placed into the reso-nance cavity of the ESR spectrometer absorbmicrowave power very strongly, which preventsthe spectrometer from operation. In this connec-tion, the methodological feature of operationwith such highly absorbing samples should con-sist in using a very small portion of the sample(less than 1 mg). However, in addition to a lowmass, the samples should be homogeneous inorder to the amount of metal appeared within atest ampoule would correspond to its averagecontent in the catalyst. For this purpose, we per-formed a preliminary stage of homogenizing asufficiently large (about 10 mg) sample by grind-ing in a mortar; further from this mass we tooka weighed sample portion 0.5�1 mg) to be placedinto a quartz ampoule for the measurements.

RESULTS AND DISCUSSION

Assessment of the applicability of the FMRmethod for studying the catalysts based on bulknickel and its alloys

For all the systems studied, the FMR spec-tra of metallic nickel appeared the most infor-mative. Simple measurements at a room tem-perature, allowed carrying out a screening ofseveral sample series to reveal basic patterns inthe formation of the active metal component.

Figure 1 demonstrates typical changes inFMR spectra after the treatment of differentbulk nickel-based alloys in the reaction mix-ture (DCE + H2). It is seen that for all the ma-terials, there are significant changes observedin the FMR spectra after carbonization: theproduct obtained is characterized by a wide sin-gle line with a typical value g ~ 2.3 inherent innickel, with the width ranging within 500�1100Gs (Table 2).

The nickel-chromium alloy (see Fig. 1, a) doesnot exhibit any FMR signal at a room temper-ature, which could be caused by a high con-tent of chromium therein (more than 10 mass%). The Curie temperature, i. e., the domainof existing the ferromagnetic state of thesealloys ranges within a low temperature region(�100 °C or lower) [15].

Chromel (see Fig. 1, b) exhibits a weak FMRsignal inherent in dispersed nickel particles.More likely, its presence is connected with someheterogeneity in the alloy as well as with thepresence of nickel particles therein. Alumel (seeFig. 1, c) and bulk nickel metal (see Fig. 1, d)

TABLE 2

Ratio C/Ni in the final product and FMR signal width

Samples* C/Ni, mass % ∆H, Gs

C/nichrome-550 315 540

C/nichrome-700 59 820

C/chromel-700 1360 730

C/alumel-700 1250 780

C/nickel-550 270 1100

* Figures in the n ame of samples denote thetemperature of obtaining the catalyst, °Ñ.

122 YU. I. BAUMAN et al.

exhibit strongly anisotropic ferromagnetic reso-nance spectra inherent in large metal particleswith large deviations from spherical symmetry.

Among all the systems under consideration,of the greatest interest are catalysts based onnichrome. In the original state, the nichromedoes not exhibit any FMR signal, thus appear-ing metallic nickel particles would be distinctlyvisible even at a very low amount.

One could expect that appearing the ferro-magnetic nickel and its further dispersing wouldbe accompanied by significant changes in the

FMR spectra. Indeed, the FMR spectra inher-ent in all the carbonized samples differ to a con-siderable extent from those inherent in the cor-responding initial materials (see Fig. 1).

It should be noted that in all the cases, thereare similar, quite symmetric signal peaks areobserved with a wide single line and a typicalvalue g ~ 2.3 inherent in nickel. This indicates theformation of spherically symmetric particles ofmetallic nickel sufficiently uniform in size.

These results indicate that upon the expo-sure of the reaction medium irrespective of

Fig. 1. FMR spectra of nichrome (a), chromel (b), alumel (c) and nickel (d): 1 � before contacting with the reactionmedium; 2, 3 � after the carbonization in the reaction mixture (DCE + H2) at 550 and 700 °Ñ, respectively; temperatureof recording the spectra is equal to 20 °Ñ.

SELF-DISPERSING NI-CONTAINING ALLOYS 123

Fig. 2. TEM images of nickel-containing alloys after carbonizing the samples resulting from the contact with the reactionmedium (DCE + H2) at 550 °C: à � nickel band, b � nichrome, c � alumel, d � chromel.

the nature of the precursor there is the for-mation of similar-size particles of nickel metalobserved those are included in the compositionof the resultant carbon product to exhibit sim-ilar FMR spectra. In this case the relative con-centration of such particles could be assessedfrom the amplitude of FMR spectrum.

The FMR data are in a good agreement withthe results of electron microscopy studies ofthe samples after carbonization (Fig. 2). It isclearly seen that the carbon-caused erosion ofnickel alloys with different initial compositionresults in appearing the reactive particles ofapproximately the same size. The size of themost of the particles observed those catalyzethe growth of carbon fibres ranges within 200�300 nm. An exception is presented by chromel(see Fig. 2, d) that is also characterized by thepresence of fine particles smaller than 100 nm.

The experimental results presented indicatethat the formation of uniform dispersed nickel

particles for all the cases is caused by the ex-traction of its nanoparticles from bulk metal(alloy) in the course of the reaction with chlo-rine-containing compounds in a hydrogen at-mosphere. The subsequent growth of carbonoccurs just on such kind of nickel nanoparti-cles. The similarity of FMR spectra and TEMdata for all the catalysts investigated indicatethat they contain dispersed nickel particles sim-ilar in size and shape. Thus, in the course ofprocessing the bulk metals, a process of evi-dent self-organizing the catalyst takes placein the reaction mixture. It could be assumedthat this phenomenon is caused by the pres-ence of halogens and hydrogen in the reac-tion medium. Red-Ox processes occurring insuch a medium result in etching the impuri-ties and the formation of a developed surfaceof the nickel metal as a supplier of nanopar-ticles required for the growth of carbonnanofibres.

124 YU. I. BAUMAN et al.

Fig. 3. FMR spectra for original nichrome (1) and fornichrome samples obtained during the induction periodafter the interaction with the reaction medium (DCE +H2) at 550 °Ñ for 3 (2) and 16 min (3). The temperature ofspectra registration is equal to 20 °Ñ.

Fig. 4. Catalytic surface corrosion of nichrome wire underthe action of the reaction medium (DCE + Í2) at 550 °Ñ(SEM data): a � initial sample; b, c � after 7 and 16 mintreatment, respectively.

FMR investigation of self-dispersion processfor nichrome

The original nichrome does not exhibit FMRsignals at a room temperature (see Fig. 1, a),however its interaction with the DCE results inthe formation of quite uniform nickel particlescharacterized by a very intense signal. The sen-sitivity of the spectrometer under use allowsregistering the signals (and the amount ofnanoparticles) whose intensity is two orders ofmagnitude lower than that for the particles ofthe samples investigated. Thus, the FMR meth-od allows one to detect the formation of nickelnanoparticles within the initial parts of the re-action kinetic curves, when the process of theirformation on the surface of the origin alnichrome is just beginning.

It should be noted that studying the phe-nomena those occur during the induction peri-od inherent in the process of self-dispersingbulk metals, is extremely difficult task for elec-tron microscopy.

Figure 3 demonstrates FMR spectra for aseries of samples obtained at short contact timeswith DCE vapour, i. e., during the inductionperiod. To detect any changes in the mass of

the sample in the reaction using the gravimet-ric method (Mac Ben microbalance) is failed.However, from the FMR spectra presented onecould clearly see that appearing the signals typ-ically inherent in dispersed nickel particles canbe registered for nichrome samples after thecontact with the reaction medium within a fewminutes, when there is no carbon material inthe volume and all the processes occur on thesurface of nichrome.

SELF-DISPERSING NI-CONTAINING ALLOYS 125

Fig. 6. FMR spectra of nichrome samples obtained via theinteraction with the reaction medium (DCE + H2) at 550 °Ñduring the induction period (the temperature of spectraregistration equal to 20 °Ñ): 1 � initial sample, 2 � afterpreliminary etching with acids, 3 � bulk nickel sample.

Fig. 5. Effect of the preliminary treatment of nichrome(a) and nickel (b) on the duration of the induction period:1 � without pre-treatment, 2 � mixture of acids, 3 �Red-Ox processing.

This is in a good agreement with the SEMdata (Fig. 4), those demonstrate that after 7 minof the treatment the surface of the nichromewire exhibit rather distinct traces of the cata-lyst corrosion, the nickel metal crystallizes fromthe alloy (contrasting white spots), with theformation of the rudiments of carbon fibres.After contacting with the reaction mixture for16 min almost the entire surface exhibit a uni-form covering with carbon fibres (see Fig. 4).

In [6] we demonstrated that the kinetics ofthe accumulation of a carbon product in thereaction mixture (DCE + H2) at 550 °Ñ with theuse of bulk nickel alloys as the catalyst pre-cursor exhibits a pronounced induction periodthat could be reduced either after treating thesurface with a mixture of acids, or after theRed-Ox treatment.

Figure 5 demonstrates the typical kineticcurves inherent in this process for nichrome andnickel. It is clearly seen (see Fig. 5, a) that inthe case of nichrome the effect of pre-treat-

ment is especially considerable. At the sametime, the final state of the nichrome sampleafter preliminary activation of the surface,according to the induction period, is close tothe state of initial bulk nickel (see Fig. 5, b).

In the course of the induction period, theformation of dispersed nickel particles is ob-served for nichrome with typical FMR spectra(see Figs. 3 and 6, curve 1). At the same time,the sample of nichrome after preliminary treat-ment with acids under similar conditions dem-onstrates an intense strongly anisotropic FMRspectrum (see Fig. 6, curve 2) inherent in thebulk of metallic nickel (curve 3) rather than infine nickel particles of, which spectrum is notobserved for original nichrome. To all appear-ance, the formation of bulk nickel could occurat the stage of preliminary treating thenichrome with acids at the expense of the dif-ference in the dissolution rates of the metals.Further, the material obtained has been char-acterized by the induction period value inher-ent in the metallic nickel (see Fig. 5, b).

It should be noted that the porous nickel,so-called Raney nickel, uses to be prepared viaa similar procedure by etching from the nick-el-aluminum alloy with alkali. There are no data

126 YU. I. BAUMAN et al.

Fig. 7. FMR spectra of nichrome-based catalysts obtainedafter the interaction with the reaction medium (DCE + H2).Reaction temperature, °Ñ: 450 (1), 600 (2), 650 (3), 720(4), 750 (5).

concerning the use of the FMR method for thestudies of such processes in the literature. It isobvious that the same procedure could be ap-plied in order to activate alumel.

Figure 7 demonstrates the FMR spectra ofthe samples of catalysts based on nichromeobtained as the result of the contact with 1,2-dichloroethane vapour at different temperatures.It is evident that in the working temperaturerange (700 °Ñ) the FMR spectra exhibit a dom-inated signal from dispersed nickel (see Fig. 7,curves 1�4). At the same time, the FMR spec-tra of the catalyst formed at 750 °Ñ exhibit theFMR signal inherent in bulk nickel metal (seeFig. 7, curve 5).

CONCLUSION

In general, the data obtained by means ofthe FMR method indicate that under the con-ditions of occurring the catalytic reaction of1,2-dichloroethane decomposition on bulk al-loys there is a self-organization of a catalystobserved to form fine nickel particles close insize to each other, the shape of the particlesbeing close to spherically symmetric one. The

particle sizes, taking into account the electronmicroscopy data, range within 200�300 nm.

Significant is the fact that the same disper-sion level of particles can be obtained both thefrom nickel band, and from all of the nickel-based alloys such as nichrome, chromel, alumel.In the cases when the acidic treatment resultsin preliminary etching an associated metal fromthe alloy, the formation of a porous solid nick-el is observed with FMR spectra inherent inbulk nickel. After that, the further process ofthe formation of identical dispersed nickel par-ticles (the active component of catalysts) is ob-served significantly faster to occur.

The FMR method appeared a rather infor-mative one for qualitative and quantitativestudying the self-dispersion processes for bulknickel alloys at the initial stage. For such sam-ples one could obtain information both aboutthe process of forming the bulk porous nickelfrom the alloy, and concerning the process ofthe further transformation into homogeneousdispersed nickel particles.

It should be noted that the effect of dis-persing nickel metal in the course of catalyticoxidation reaction is known sufficiently long inthe literature as �catalyst corrosion� [4, 5]. Itcould be believed that appearing the dispersedparticles of nickel in the formation of cata-lysts from bulk nickel and its alloys also occursdue to alternating oxidation-reduction reactionsto proceed, where HCl plays the role of an ox-idant, and hydrogen serves as a reducing agent.In the case of nickel only a dispersion processoccurs, whereas in the case of nickel alloys atfirst porous nickel is formed leaving the alloystructure due to the differences between themetals composing the alloy in the reactivity andstability of intermediates under formation.

Acknowledgement

The authors are grateful to the researchers ofthe IC of the SB RAS A. N. Serkova and V. I.Zaikovskaya for help in the electron microscopystudies.

REFERENCES

1 Baker R. T. K., Carbon, 27, 3 (1989) 315.2 Buyanov R. A., Chesnokov V. V., Kataliz v Prom-sti,

2 (2006) 3.

SELF-DISPERSING NI-CONTAINING ALLOYS 127

3 Chesnokov V. V., Buyanov R. A., Usp. Khim., 7 (2000) 675.4 Schmid B., Aas N., Grong O., Odegard R., Appl. Cat. A.,

215, 1�2 (2001) 257.5 Gladkiy A. Yu., Kaichev V. V., Ermolaev V. K.,

Bukhtiyarov V. I., Parmon V. P., Kinetika i Kataliz,46, 2 (2005) 1.

6 Bauman Yu. I., Mishakov I. V., Buyanov R. A., VedyaginA. A., Volodin A. M., Kinetika i Kataliz, 52, 4 (2001) 557.

7 Vonsovskiy S. V. (Ed.), in: Ferromagnitny Rezonans,Gos. izd-vo fiz.-mat. lit., Moscow, 1961, p. 17.

8 Slinkin A. A., Usp. Khim., 37, 8 (1968)1521.9 Sharma V. K., Baiker A., J. Chem. Phys., 75, 12 (1981) 5596.

10 Sharma V. K., Baiker A., Synthetic Metals, 11, 1 (1985) 1.

11 Slinkin A. A., Kucherov A. V., Yuffa A. Ya., StakheevA. Yu., Fedorovskaya E. A., Kinetika i Kataliz, 28, 5(1987) 1261.

12 Yulikov M. M., Abornev I. S., Martyanov O. N., YudanovV. F., Isupov V. P., Chupakhina L. E., Tarasov K. K.,Mitrofanova N. P., Kinetika i Kataliz, 45, 5 (2004) 776.

13 Mishakov I. V., Chesnokov V. V., Buyanov R. A.,Pakhomov N. A., Kinetika i Kataliz, 42, 4 (2001) 598.

14 Volodin A. M., Sobolev V. I., Zhidomirov G. M., Kinetikai Kataliz, 39, 6 (1998) 844.

15 Kikoin I. K. (Ed.), Tablitsy Fizicheskikh Velichin(Handbook), Atomizdat, Moscow, 1976, p. 525.