Formation of Active Sites of Carbon Nanofibers Growth in Self

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Kinetics, Catalysis, and Reaction Engineering

Formation of Active Sites of Carbon Nanofibers Growth in Self-Organizing NiPd Catalyst during Hydrogen-Assisted Decomposition of 1,2-Dichloroethane Yurii Bauman, Ilya V. Mishakov, Yuliya Vladimirovna Rudneva, Pavel E Plyusnin, Yury V. Shubin, Denis Korneev, and Aleksey A. Vedyagin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02186 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 22, 2018

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Industrial & Engineering Chemistry Research

Formation of Active Sites of Carbon Nanofibers Growth in Self-Organizing Ni-Pd Catalyst during Hydrogen-Assisted Decomposition of 1,2Dichloroethane Yurii I. Bauman†, Ilya V. Mishakov†‡, Yulia V. Rudneva§, Pavel E. Plyusnin§⊥, Yury V. Shubin§⊥, Denis V. Korneev#, Aleksey A. Vedyagin*†‡ †

Boreskov Institute of Catalysis SB RAS, pr. Ac. Lavrentieva 5, Novosibirsk 630090, Russian

Federation ‡

National Research Tomsk Polytechnic University, Lenin av. 30, Tomsk 634050, Russian

Federation §

Nikolaev Institute of Inorganic Chemistry SB RAS, pr. Ac. Lavrentieva 3, Novosibirsk 630090,

Russian Federation ⊥

#

Novosibirsk State University, Pirogova str. 2, Novosibirsk 630090, Russian Federation

Monash University, Melbourne, Victoria 3800, Australia

*Corresponding author E-mail: [email protected]; [email protected]

ACS Paragon Plus Environment

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: The Ni-Pd alloys synthesized by coprecipitation method were studied as precursors for the self-organizing particles catalyzing the growth of carbon nanofibers. Addition of palladium was found to enhance both the activity, stability and productivity of the catalyst in the hydrogen-assisted decomposition of 1,2-dichloroethane. Evolution of reflex (331) for 95Ni5Pd sample treated under the reaction conditions has revealed the formation of the interstitial solid solution phase NixPd1-xCδ. According to data of scanning electron microscopy, the formation of separated particles active in carbon growth takes place by 18th minute of exposure with reaction mixture. After 60 minutes of interaction the bulk Ni-Pd alloy undergoes complete disintegration with formation of dispersed active centers sized in a range of 0.4-0.6 microns. The elemental mapping of the formed particles showed that palladium is uniformly distributed within the bulk of active particles. The obtained carbon product is represented by prolonged carbon fibers with segmental structure.

KEYWORDS: Metal Dusting; Ni-Pd Alloys; Self-Organizing Catalyst; CCVD; 1,2Dichloroethane; Carbon Filaments; Segmental Structure

Introduction Nowadays, carbon nanofibers (CNFs) are recognized as an important nanomaterial attracting a much attention for their high potential in various fields of application.1-7 CNFs can be used as sorbents,2,3 supports for the different structured catalysts,3-6 catalytic composite materials,4,6 including the hierarchically structured ones.3,7 Among the variety of techniques used for the production of CNFs, the catalytic chemical vapor deposition (CCVD) is considered to be the most versatile and efficient method.8-10 The CCVD synthesis of carbon nanomaterials (CNM) is


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based on the decomposition of various hydrocarbons over the catalysts containing disperse metallic particles. The most widely used catalysts comprise such elements as Fe, Co and Ni;11 the latter should be mentioned as the most stable catalyst for processing of chlorinated hydrocarbons due to the highest thermodynamic resistance against formation of nickel chloride.12,13 Different metals (Cu, Mo, Pd etc.) can be used in composition with nickel to improve its catalytic performance in the CCVD synthesis and adjust the structural selectivity.14-16 One of the approaches to synthesize the Ni-catalyst for CCVD is based on utilization of the metal dusting (MD) phenomenon. Metal dusting (or carbon erosion) implies the corrosion of bulk metals and their alloys in carburizing atmosphere at a temperature range of 400-800 °C.17-19 The carbon transfers from the C-containing environment (CO, hydrocarbons etc.), dissolves into the metal and gets released in the form of graphite thus destroying the metal item.17 MD leads to disintegration of the metallic materials into a dust of nanoscale metal particles and carbon nanofilaments. This is therefore known as a negative process causing slow destruction of the industrial reactors made of steel and Ni-Cr alloys and resulting in their complete wastage.20 On the other hand, carbon erosion was recently shown to take place in the case of catalytic decomposition of chlorinated hydrocarbons over Ni-based systems.21,22 Being subjected to the action of an aggressive reaction atmosphere, the bulk Ni-based alloys undergo intensive corrosion leading to their full disintegration.21-23 This process accomplishes with formation of the disperse metallic particles functioning as active sites for the catalytic growth of the filamentous carbon. Such type of in situ formation of active catalyst derived from spontaneous disintegration of the bulk Ni-precursors allows one to talk about the “self-organizing catalyst” (SOC).24 The catalytic performance of Ni-based SOCs in the CCVD process can be influenced by the alloying of Ni with some other metals (Fe, Co, Cr, Pd). The palladium was recently found to be one of the


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best promoters for catalytic activity and stability of Ni whereas the addition of Co and Cu was shown to have insignificant effect.25,26 It should be emphasized that palladium-based catalysts are widely applied for processing of different organochlorine compounds (for instance, via hydrodechlorination process) in both the liquid and gas phases.27-30 The main problem with these catalysts are their rapid deactivation (after few hours of operation), which is reported to be connected with the poisoning of active centers by organochlorine species.27-29 The deactivation process strongly depends on the nature of the used support and the organochlorine compound to be utilized.29 The key factors are believed to be the average metal particle size and the proportion of Pd in electro-deficient and zero-valent states. In general, the process of interaction of halogenated hydrocarbons with metallic catalyst can proceed via one of the three different routes depending on the reaction conditions. In hydrogen-deficient conditions at temperatures below 450 °C, the formation of metal chloride (for instance, NiCl2) with subsequent deactivation of the catalyst will be thermodynamically favored. Addition of excess hydrogen into the reaction mixture switches the process into the regime of hydrodechlorination, at which the substitution of halogen atoms with hydrogen occurs without damage of carbon skeleton of the initial molecule. The temperature of this route lies below 400 °C. Elevation of the reaction temperature up to 500 °C and above leads to decomposition of chlorinated substrate with formation of solid carbon deposits and release of gaseous HCl.12 Contrary to the hydrodechlorination conditions, the process of the hydrogenassisted decomposition of DCE goes via so-called “carbide cycle mechanism”.31 In this case, hydrogen plays a key role acting in the chemical bonding of the chlorine species being formed. Such stages as chlorination and hydrodechlorination of the surface of Ni particles are considered


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within this mechanism. Thereby, palladium species uniformly distributed within nickel particles are seemed to accelerate the mentioned stages. It should be also noted that the carbon nanomaterial produced via decomposition of chlorinated hydrocarbons is known to be characterized by unique structural and textural features.32-35 For example, the formation of the segmented CNFs (SCNFs) was earlier reported for the CCVD process of various chlorinated hydrocarbons over supported Ni-based catalysts.34 As it was recently found, bulk Ni-based alloys could be quite active in production of SCNFs via decomposition of 1,2-dichloroethane (DCE).35 Using the MD-based approach to the synthesis of self-organising Ni-catalysts, one could be able to enhance the productivity of the CCVD process towards the selective production of the carbon nanofibers with segmented structure. Taking into account all the mentioned above, the present research is aimed to study an effect of Pd addition and its content upon the catalytic performance of Ni-Pd system in the hydrogenassisted decomposition of C2H4Cl2 (DCE). The particular attention is paid to the regularities of MD process of the alloyed Ni-Pd precursors leading to a spontaneous emergence of the active centers for the CNF growth as well as to the structural peculiarities of the obtained carbon nanomaterial. Experimental Synthesis of Ni-Pd solid solution A series of Ni-Pd solid solutions was synthesized in accordance with earlier optimized coprecipitation method followed by the reduction of the resulted sediment in a hydrogen atmosphere.36 At the first stage, the certain amounts of the precursors, K2PdCl4 (Sigma-Aldrich)


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and Ni(NO3)2·6H2O (Pro Analysi), taken in a calculated ratio were dissolved in 100 ml of deionized water. The mixed solution of the precursor salts was added dropwise to an aqueous solution (0.1 M) of NaHCO3 (Purissimum) at vigorous stirring. The temperature of solution was maintained at level of 70 °C. The joint precipitation was conducted by feeding the 1 M aqueous solution of NaHCO3 at the controlled velocity in order to keep the рН within the region of 7.0 – 9.0 during the precipitation procedure. The resulted dark green sediment was then separated in the centrifuge, washed with distilled water and dried at temperature of 105 °C for 12 h. The dried sample was then subjected to a reduction procedure performed at 800 °C in a hydrogen flow (at a rate of 130 cm3/min) for 30 min. The obtained metallic powder of Ni-Pd alloy was cooled down to a room temperature in a He flow. The samples containing 3 and 5 wt.% of palladium were labeled as 97Ni-3Pd and 95Ni-5Pd, correspondingly. The described preparative route was implemented to prepare the reference 100% Ni sample free of Pd (labeled as 100Ni). Hydrogen-assisted decomposition of C2H4Cl2 The synthesized Ni-Pd alloys were studied in the process of hydrogen-assisted decomposition of 1,2-dichloroethane. DCE was selected as a model chlorinated hydrocarbon widely used as a precursor for the vinyl chloride monomer production. In order to investigate the formation of the active centers of the CNM growth in detail, a series of short-term experiments have been performed. The loading of the initial Ni-Pd alloy sample in each experiment was 100 mg. After being pretreated under the reductive conditions (H2, 500 °C, 30 min), the sample was brought to a contact with reaction environment (C2H4Cl2/H2/Ar) for 6, 18, 30 and 60 min. Resulted sample of the carbon product was cooled in an argon flow to a room temperature. The obtained samples were then studied by the powder XRD and SEM techniques.


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Decomposition of DCE has led to the formation of hydrogen chloride in the gas phase which was trapped at the reactor outlet using the concentrated NaOH solution. Besides HCl, the outlet gas contains traces of methane (resulted from the partial gasification of the deposited carbon) and C2 hydrocarbons (appeared as products of by-side hydrodechlorination reaction). Synthesis of self-organizing Ni-Pd/CNF catalysts The self-organizing Ni-Pd/CNF catalysts were obtained by interaction of DCE vapors with bulk Ni-Pd alloy that resulted in a rather quick disintegration of the latter. The kinetic measurement of the process was carried out in a gravimetric flow setup. The catalytic reactor was equipped with a quartz-spring McBain balances allowing one to follow the mass change of the sample during the reaction in the real-time regime.13 Prior to the catalytic experiment, each specimen of the NiPd alloy (2.00±0.02 mg) was pretreated at 500 °C in a reducing atmosphere (hydrogen flow) for 30 min. The reduced sample was then contacted with the reaction gas mixture containing DCE vapors (7.5 vol.%), excess of H2 (37.5 vol.%) and argon as a balance. The process was performed at 600 °C that was recently reported to be an optimal value for the bulk Ni-based systems.12 The self-organizing Ni-Pd/CNF catalysts were considered to be formed when the complete disintegration of bulk Ni-Pd alloy is reached. Thus, the total loading of metals in the formed catalyst was 10 wt.%, which corresponds to 9.5-9.7 wt.% of Ni and 0.3-0.5 wt.% of Pd. Note that the residual 90 wt.% of each catalyst is represented by the grown CNF. Characterization and measurements The chemical composition of the synthesized Ni-Pd alloys was determined by an atomic absorption spectroscopy (AAS) using a Solaar IC-3000 spectrophotometer. Metallic samples were dissolved in nitric acid prior to be analyzed by AAS.


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The phase composition of Ni-Pd samples was studied by a powder XRD analysis. XRD measurements were carried out at a room temperature on a Shimadzu XRD-7000 diffractometer (CuKα radiation, Ni filter on the reflected beam, scintillation detector with amplitude discrimination). The survey diffraction patterns were recorded in the step mode within the angular range of 2θ = 7–107 °, step 0.05 °. For precise determination of a lattice parameter, XRD data were collected in a far-angle region: 2θ = 140-147 °, step 0.02 °. The information available at the JCPDS data base was used as a reference.37 The lattice parameters were determined by the position of (331) reflex diffraction data (2θ ≈ 145 °) using the PowderCell 2.4 software. The volume-averaged crystallite sizes were estimated from the integral broadening of the (111), (200), (220) peaks using the WINFIT 1.2.1 program and the Scherrer formula.38,39 The same XRD technique was used to follow the changes in the phase composition of 95Ni-5Pd sample which was exposed to a contact with DCE vapors under the same reaction conditions for 6, 18, 30 and 60 min. Diffraction patterns were also recorded in 2 different modes: 2θ = 7–107 °, step 0.05 ° (survey diffractograms) and 2θ = 140-147 °, step 0.02 ° (to analyze the features of (311) reflex). The morphology and structure of the pristine Ni-Pd alloys as well as that of 95Ni-5Pd sample subjected to reaction for 6÷60 min were studied by a scanning electron microscopy (SEM) technique. The measurements were performed on a JEOL JSM6460 electron microscope with a resolution of 4 nm in a range of magnifications 5 ÷ 300,000×. The same technique was used to characterize the secondary structure of the produced carbon nanomaterials. The fine structure of the resulted carbon nanomaterials was examined by a transmission electron microscopy (TEM) using a JEM-1400 (Jeol, Japan) instrument (accelerating voltage 80 kV).


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Prior to being deposited onto TEM grid, the CNM samples were mixed with deionized water (without using ultrasound treatment). Thus obtained CNM/H2O suspensions allowed one to preserve rather brittle structure of the segmented carbon filaments. The local chemical composition of the active catalytic particles was studied by means of a scanning transmission electron microscopy (STEM) combined with an energy dispersive spectrometer (EDX). STEM micrographs were obtained on a НT-7700 microscope, Hitachi (Japan), which was equipped with an EDX spectrometer XFlash 6T/60, Bruker (Germany). The produced CNM samples were analyzed by a low-temperature adsorption technique (BET method). The N2 adsorption isotherms were measured at 77 K on an ASAP-2400 instrument (Micromeritics, USA). Based on the obtained isotherms, the following textural parameters were determined: specific surface area (SSA), m2/g; pore volume (Vpore), cm3/g; average pore diameter (D), Å. Results and Discussion X-ray diffraction analysis (XRD) of pristine Ni-Pd alloys The phase composition of the pure nickel sample and the synthesized Ni-Pd alloys was identified by means of an X-ray diffraction analysis. The corresponding survey XRD patterns are given in Fig. 1A. All the samples exhibit the presence of five reflexes detected within 2θ = 40-100 ° region, which are typical for the face-centered cubic lattice (fcc). One can see that the increase of Pd content in the Ni-Pd alloy causes the characteristic shift of the reflexes towards the low-angle region, regarding to the plane nickel (100Ni sample).


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Kα (1)

B Kα (2)

A

95Ni-5Pd

(222)

(220)

(311)

(331)

(200)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(111)


95Ni-5Pd 97Ni-3Pd

97Ni-3Pd 100Ni

100Ni

40

50

60

70

80

90

100

141

142

2θ, degree

143

144

145

146

2θ, degree

Figure 1. X-ray diffraction profiles for pure nickel and Ni1-xPdx alloys: (a) – survey patterns; (b) – reflex (331) in far angles. The significant change of the reflex position can be clearly observed from the far-angle XRD patterns plotted in Fig. 1B. Peaks recorded within the interval of 2θ = 141-147 ° are attributed to the (331) reflex (Fig. 1B). The absence of any extra reflexes on the XRD profiles along with the evidenced shift of peaks position allows one to conclude that the applied preparative technique provides the formation of the single-phase Ni-Pd alloys. The prepared samples are represented by the isomorphic solid solution based on fcc crystal lattice of nickel. The information about the composition of the synthesized Ni-Pd alloy samples, their lattice parameters and the calculated average crystallite sizes is summarized in Table 1. The lattice parameter (a) is seen to be noticeably increased (from 3.524 to 3.538 Å) due to the addition of Pd into the alloy composition. It is worth noting that the chemical composition of the studied Ni-Pd samples could be defined from the XRD data (Table 1) in accordance with the previously developed approach.25 The estimated values of Pd content are seen to be in appropriate agreement with the results of the chemical analysis (AAS method). Table 1. Pd content and crystal parameters of the prepared Ni and Ni-Pd samples.


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Content of Pd Specified Sample content of Pd, by AAS, name wt. % wt.%

XRD data a, Å

at.% Pd

wt.% Pd

ACS*, nm

100Ni

-

-

3.524(1)

-

-

82

97Ni-3Pd

3

2.8

3.532(1)

1.7

3.0

81

95Ni-5Pd

5

5.0

3.538(1)

3.6

6.3

73

* – average crystallite size. Kinetic study on hydrogen-assisted decomposition of DCE Synthesized samples of Ni-Pd solid solution were studies as precursors for the self-organizing Ni-Pd catalysts. The interaction of the bulk Ni-Pd alloy with the carburizing atmosphere (containing C2H4Cl2 vapors and H2) resulted in a rapid disintegration of the metallic material forced by the phenomenon of metal dusting (carbon erosion).22 Carbon erosion leads to emergence of the dispersed metallic particles acting as centers for the catalytic growth of the carbon nanostructures. The kinetic curves for the process of CNM accumulation in course of DCE decomposition are shown in Fig. 2. First section of the kinetic curve (0÷20 min) showing the obvious delay in the carbon deposition is known as the induction period (IP). The duration of IP could be conventionally estimated as the period of time needed for sample to reach 100% of weight gain. As it follows from Fig. 2, the duration of IP is similar for all the samples, thus indicating no significant effect of the Pd presence. Nevertheless, the comparison of kinetic curves for the Ni-Pd samples within the active stage of CNM deposition (20÷90 min) reveals the remarkable impact of Pd on the catalytic performance of nickel. Thus, an introduction of 5 wt.% Pd into Ni lattice enhances the rate of the carbon deposition in ~ 1.5 times (Fig. 2).


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1800 1 - 95Ni-5Pd 2 - 97Ni-3Pd 3 - 100Ni

1600

Weight Gain, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1400

1

1200

2

1000

3

800 600 400 200 0 0

20

40

60

80

Time, min

Figure 2. Comparative kinetic studies on carbon nanomaterial growth over pure nickel and NiPd alloys via hydrogen-assisted decomposition of DCE at 600 °C. The strong positive effect of Pd addition upon the catalytic activity and stability of Ni-based system in hydrogen-assisted decomposition of DCE is vividly illustrated by the diagram presented in Fig. 3.


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180 160

Yield of CNM, g/gNi

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


140

100Ni 97Ni-3Pd 95Ni-5Pd

120 100 80 60 40 20 0 3

4

5

Time, h

Figure 3. Catalytic performance of pure nickel and Ni-Pd alloys in the hydrogen-assisted decomposition of DCE at 600 °C. The diagram shows the yield of carbon nanomaterial (related to 1 g of Ni) produced for 3, 4 and 5 hours of reaction. While the reference 100Ni sample appears to reach its limit of CNM yield after 4 h (~ 40 g/gNi) due to the plausible deactivation, the Ni-Pd specimens exhibited rather steady growth in productivity towards CNM (Fig. 3). The yield of carbon product achieved for 5 h of process was found to be as high as 160 g/gNi for both 97Ni-3Pd and 95Ni-5Pd alloys. The similar stabilizing effect has been recently reported for the model Ni-Cr system, while the addition of Co and Cu was shown to have negligible influence on the performance of Ni.26 It thus can be concluded that the introduction of Pd in moderate amounts (3-5 wt.%) permits one to boost the performance of nickel in CCVD of chlorinated hydrocarbons due to promotion of the catalytic activity and considerable improvement of its resistance to deactivation.


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Carbon erosion of the 95Ni-5Pd alloy sample. SEM data On the next stage the research was focused on investigation of the peculiarities of the carbon erosion process leading to a spontaneous formation of the active particles responsible for the accumulation of CNM. The 95Ni-5Pd alloy shown earlier as the most effective catalyst precursor was taken for the study. The morphology of the pristine 95Ni-5Pd alloy can be seen from SEM images presented in Fig. 4.

Figure 4. SEM images for the prepared 95Ni-5Pd sample. Magnifications are 1,000× (A) and 10,000× (B). As it was already mentioned, this precursor material was synthesized by the coprecipitation of Ni and Pd salts with subsequent reduction of the dried sediment in a hydrogen atmosphere at 800 °C. The prepared sample is represented by a powder of grey color with a typical metallic glitter. As it follows from Fig. 4, the 95Ni-5Pd alloy consists of rather large (tens of microns) secondary particles, which are composed of sintered roundish grains having the size of 1-5 µm. The wellexpressed grain boundaries can be clearly observed within the structure of the sintered


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aggregates (Fig. 4B). The specific surface area of the initial 95Ni-5Pd alloy was measured to be ~ 0.5 m2/g. In order to study the evolution of 95Ni-5Pd precursor in course of its contact with reaction mixture in more detail, the sample was subjected to a short-term exposure during 6, 18 and 30 min. The structure and morphology of the resulted specimens were examined by SEM technique. The corresponding SEM micrographs at different magnifications are given in Figs. 5A-J. The SEM micrographs for the samples held under the reaction conditions for 60 and 120 min, which corresponds to a full wastage of the alloy, are also presented for comparison. As seen from Fig. 5A, a short-time treatment (6 min) did not practically make any apparent changes to the structure of the alloy, although tiny nuclei of carbon filaments protruded from the inter-grain boundaries might be discerned (Fig. 5B).


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Figure 5. SEM micrographs of the 95Ni-5Pd alloy sample exposed to the contact with reaction mixture C2H4Cl2/H2/Ar at 600 °C for 6 min (A, B), 18 min (C, D), 30 min (E, F), 60 min (G, H) and 120 min (I, J). Images (C), (G), (H) and (J) are the bright-field SEM micrographs. Changes became more obvious after 18 min of interaction: bright-field image in Fig. 5C reveals the appearance of the dispersed metallic particles (indicated as white contrast spots) resulted from progressive disintegration of the Ni-Pd alloy. The clusters of early-grown carbon filaments can be found on the alloy’s surface (Fig. 5D). By the 18th minute of interaction, the amount of deposited carbon is still very small, which corresponds to the stage of induction period on the kinetic curve (Fig. 2). An increase in exposure time to 30 min results in formation of the abundant coverage from the carbon nanostructured product (Fig. 5E,F). Rate of the carbon accumulation demonstrates rapid growth, thus indicating the transition from induction period to the stage of the intensive CNM deposition. Further expansion of the interaction time to 60 min leads to a complete degradation of the bulk alloy’s framework. After the full wastage, the metallic component in the resulted 60-min sample is only represented by the ensemble of the active particles that could be clearly seen from the


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bright-field SEM images shown in Fig. 5G,H. The major fraction of both 60-min and 120-min samples is a carbon product deposited mainly in the form of long filaments (Fig. 5G-J). It is noteworthy that the active metallic particles of as-formed self-organizing catalyst are found to be primarily incorporated into the structure of the grown carbon fibers. XRD study on carbon erosion of 95Ni-5Pd alloy An ex-situ powder X-ray diffraction analysis was used in order to investigate the peculiarities of the carbon erosion process resulting in the formation of the catalytic active particles as well as the initial stage of the carbon fibers generation. The described above 95Ni-5Pd alloy subjected to interaction with DCE (600 °C) at different exposure time (6, 18, 30 and 60 min) was studied by XRD method. The recorded XRD patterns are shown in Fig. 6. All the diffraction profiles exhibit a presence of the intensive reflexes corresponding to the facecentered cubic lattice of Ni-Pd solid solution. The broadened peak of carbon phase (002) at 2θ ~ 26 ° along with reflexes attributed to NiO phase can be observed for the samples treated more than 18 min (Fig. 6). The intensity of these peaks keeps growing with an increase of the exposure time of the samples in the reaction medium. The appearance of NiO phase could be explained by the partial oxidation of the active metallic particles (due to a contact with atmospheric air) that are known to have pyrophoric properties as a result of their small size. The XRD patterns in the far-angle area (reflex (331)) for the pristine 95Ni-5Pd alloy and that reacted with DCE for 6, 18, 30 and 60 min are compared in Fig. 7. It is well seen that a shorttime exposure in carburizing atmosphere causes a significant shift of the peak (331) towards the low-angle region along with its considerable broadening. The observed broadening of (331) reflex might be caused by uneven distribution of carbon in the bulk of alloyed Ni-Pd particles as


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well as by microstress of the crystal lattice that emerges due to inclusion of C atoms into the interstitial space.

- Carbon phase

(111)

- NiO phase

(200)

60 min

30 min

18 min

6 min

0 min

20

30

40

50

2θ , degree

Figure 6. X-ray diffraction profiles for the 95Ni-5Pd alloy sample treated under reaction conditions (C2H4Cl2/H2/Ar, 600 °C) for 6, 18, 30 and 60 min.


19


(331)

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Kα1 Kα2 60 min

30 min 18 min 6 min 0 min

140

141

142

143

144

145

146

147

2θ, degree

Figure 7. Evolution of reflex (331) for the 95Ni-5Pd sample treated under reaction conditions (C2H4Cl2/H2/Ar, 600 °C) for 6, 18, 30 and 60 min. The values of the lattice parameters determined from XRD data for the samples exposed to the reaction conditions at different exposure times are listed in Table 2. The lattice parameter was found to be increased by 0.002-0.004 Å in comparison with non-treated specimen. Table 2. Impact of exposure time (t) on the lattice parameter (a) of the model 95Ni-5Pd alloy. Experimental conditions: C2H4Cl2/H2/Ar, 600 °C. Number of sample

Exposure time (t), min

Lattice parameter (a), Å

∆a, Å

1

0

3.537(1)

-

2

6

3.540(2)

0.003

3

18

3.541(2)

0.004


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4

30

3.541(2)

0.004

5

60

3.541(2)

0.004

The observed increase in value of the lattice parameter (a) along with the corresponding shift of the peaks to the low-angle region can be explained by the entry of carbon into the alloy’s crystal lattice. Such introduction of C atoms results in formation of the interstitial solid solution phase, which might be designated as NixPd1-xCδ. Formation of nonstoichiometric carbide at 600–700 °C corresponds to a carbon solubility in Ni and Pd reported in the literature.40-44 It has to be emphasized that the lattice parameter did not show any changes with the increase of exposure time within 18÷30 min range. The observed fact indicates that the concentration of carbon in NiPd alloy probably achieves its limit for the 18-min sample and remains steady during the subsequent interaction. Characterization of the active centers of the CNM growth As demonstrated above, rapid and full degradation of the bulk Ni-Pd alloy exposed to reaction with DCE leads to spontaneous formation of the dispersed catalytic particles. The structure and morphology of the active catalytic centers resulted from self-disintegration of 95Ni-5Pd alloy were explored by means of microscopic techniques (STEM+EDX, TEM). The obtained results are presented in Figs. 8 and 9. Fig. 8 shows the TEM micrographs of the typical active centers of CNM growth. First metallic particle (Fig. 8A) has flattened shape. It catalyzed the formation of two thick fibers in opposite directions and one thin filament from the lateral face. At the same time, Fig. 8B demonstrates quite symmetric triangle-shaped crystal of Ni-Pd alloy associated with three filaments similar in


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their diameter. The size of metallic particles belongs to a submicron range of 0.4÷0.8 µm, which agrees well with earlier reported data for carbon erosion of the bulk Ni-based alloys under the same conditions.26,45 Thus, the observed metallic particles derived from the disintegration of the precursor Ni-Pd alloy were found to be well-facetted and characterized with rather symmetric geometry. They can catalyze the simultaneous growth of several carbon filaments (2-5).

Figure 8. TEM images of the segmented carbon fibers obtained via hydrogen-assisted decomposition of DCE over the 95Ni-5Pd alloy sample at 600 °C. Both the flattened (A) and symmetric triangle-shaped (B) Ni-Pd particles are well seen. The local chemical composition of the metallic particles was studied by the STEM technique combined with EDX analysis. Fig. 9A demonstrates the STEM image of the active metallic crystallite resulted from the disintegration of the 95Ni-5Pd alloy during the hydrogen-assisted decomposition of DCE at 600 °C. This particle plays the role of the catalytic center for CNM formation combining the functions of decomposition of C2H4Cl2 molecules and the nucleation and growth of the carbon fibers. It is seen to be attached to several carbon filaments having rather disordered structure (Fig. 9A).


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Figure 9. STEM image of the cartography zone (A) and elemental mappings of Ni (B) and Pd (C) for the 95Ni-5Pd5 alloy sample treated under reaction conditions for 2 h. The mapping data for nickel and palladium contained in the composition of studied particle are presented in Fig. 9B,C. As follows from the obtained images, the active center of CNM growth is composed of two principle metals – Ni and Pd. It has to be noted that both the metals are seen to be localized within the boundaries of the examined metallic crystal showing no any inclusions that might be usually present inside the structure of carbon filaments (Fig. 9B,C). It is also well seen that Pd demonstrates very uniform character of distribution throughout the alloy particle (Fig. 9C). Structure of the carbon product The structure of the carbon filaments resulted from the hydrogen-assisted decomposition of DCE can be well seen from microscopic images presented in Figs. 8-11. It should be noted that the obtained carbon product is totally represented by very long carbon fibers (Fig. 10A) with incorporated active particles. Active metallic particles can be clearly seen from Fig. 10A as


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bright contrast spots. Interestingly that Fig. 10B demonstrates the example of so-called octopuslike structure, when the particle is seen to be attached to five carbon filaments simultaneously.

Figure 10. SEM micrographs showing the segmented structure of the two-dimensional (A) and multidimensional (B) grown carbon filaments obtained via the hydrogen-assisted decomposition of DCE at 600 °C during 2 h. The fine structure of the broken edge of segmented carbon filament can be discerned from image presented in Fig. 11A. It is appeared to be composed by the ensemble of smaller “bricks” of graphene having the size of about 20-30 nm. It is worth noting that the cross-section of this fiber reveals the nicely facetted geometry, which was presumably translated by the shape of motherparticle catalyzing the formation of fiber.


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Figure 11. Structural peculiarities of the segmented carbon filaments obtained via hydrogenassisted decomposition of DCE over the 95Ni-5Pd alloy sample at 600 °C during 2 h. A) – SEM micrograph; (B) – TEM micrograph. It can be thus concluded that the observed carbon fibers are shown to possess the well-expressed segmental organization.46 The structure of the segmented carbon nanofibers (or SCNFs) is composed from the blocks of densely and loosely packed graphenes that are arranged with regular interchange. This fact is illustrated by the TEM image given in Fig. 11B. The formation of SCNFs is assumed to be caused by the discrete character of carbon diffusion and subsequent graphite nucleation, which are thought to be hindered by the chemisorbed chlorine species present on the metallic surface.25,36 Textural characteristics of CNM The textural parameters of the produced CNM samples measured by adsorption method (BET) as well as the apparent density values are given in Table 3. One can see that the addition of Pd causes the noticeable increase in specific surface area of the carbon product. SSA of carbon nanomaterial synthesized on 95Ni-5Pd alloy was measured to be ~ 400 m2/g. At the same time,


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the pore diameter demonstrates the opposite tendency (decrease from 87 to 66 Å), while the porosity of CNM remains almost the same (0.50-0.55 cm3/g). Table 3. Impact of Pd concentration in the 95Ni-5Pd alloys on textural characteristics and apparent density of the produced CNM. Reaction conditions: DCE/H2/Ar, 600 °C, 2 h. No. 1 2 3

% Pd (wt.) 0 3 5

SSA, m2/g 325 340 392

V(pore), cm3/g 0.50 0.53 0.55

D(pore), Å 87 73 66

Apparent density, g/ml 0.12 0.02 0.03

It has to be also noted that the increase in Pd content in Ni-Pd alloy has led to a significant drop of apparent density of the carbon product (from 0.12 to 0.02-0.03 g/ml) suggesting this type of CNM to be an extremely fluffy material (Table 3). Such an unusual macroscopic features of the carbon nanomaterial produced on the Ni-Pd catalysts can be explained by highly porous and loose texture of CNM caused by the growth of rather straight and long carbon filaments (Fig. 10A). Conclusion The developed preparative technique based on coprecipitation with subsequent high-temperature reduction in hydrogen allowed us to obtain Ni-Pd solid solutions, whose formation was confirmed by XRD. The synthesized Ni-Pd alloys have been shown to serve as precursors for the self-organizing catalytic system active in the processing of chlorinated hydrocarbons (1,2dichloroethane) into carbon nanomaterial. Introduction of Pd was found to have distinct positive impact upon both the catalytic performance and resistance of the catalyst to deactivation. Due to


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the strong stabilizing effect of Pd addition, the obtained Ni-Pd catalysts have demonstrated very high productivity (~ 160 g of CNM per 1 g of Ni for 5 h of reaction) in comparison with the reference 100Ni sample (> 50 g/gNi). Recently, we have reported on the impact of metal M in the composition of Ni-M alloyed systems on catalytic performance of self-organized catalysts studied under the similar reaction conditions.26 Among the investigated additives (M = Co, Cu, Fe, Cr), only chromium was shown to have a stabilizing effect on the process of CNF growth over the nickel-based catalysts. Thus, the yield of carbon over 95Ni-5Cr catalyst was near 90 g/gNi (5 h of reaction under the same conditions) that is in 1.8 times less if compare with 95Ni-5Pd catalyst presented here. The formation of the dispersed metallic particles acting as centers for the catalytic growth of CNM has been studied in detail. For this purpose, the 95Ni-5Pd alloy was subjected to a contact with reaction mixture for various exposure times (6, 18, 30, 60 min) in order to follow the evolution of its composition and structure. Interaction of 95Ni-5Pd precursor with DCE vapors results in a fast dissolution of carbon into the structure of Ni-Pd alloy, thus causing its further disintegration. According to XRD patterns recorded in far angles, the carbon transfer to the bulk of alloy leads to an obvious appearance of a new phase, which is likely represented by the interstitial solid solution NixPd1-xCδ. The lattice parameters for the Ni-Pd samples exposed to reaction conditions were found to be increased by 0.002-0.004 Å as compared with the pristine 95Ni-5Pd alloy. The formation of nonstoichiometric carbide phase was shown to be accompanied by a rapid degradation of the bulk Ni-Pd alloys resulting in emergence of the disperse metallic particles. All the formed active crystals were found to catalyze the growth of carbon filaments at least in two opposite directions. EDX mapping of the active particle has


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revealed that Pd is distributed quite uniformly throughout the bulk of submicron metallic crystal implying no possible redistribution of the components in course of the metal dusting process. The results of microscopic studies (SEM, TEM) indicated that the produced CNM is mainly represented by long carbon filaments of submicron diameter. The prevailing structural type of the carbon fibers can be named as segmental one, which is assumed to reflect the discrete character of the carbon deposition process due to poisoning effect of chlorine. The segmented CNM resulted from hydrogen-assisted decomposition of C2H4Cl2 over Ni-Pd alloys is characterized by comparatively high specific surface area (SSA = 320-400 m2/g) and pore volume (Vpore ~ 0.5 cm3/g). Thus, the developed micro-dispersed precursors for the Ni-Pd catalysts can be proposed for purposeful CCVD-based synthesis of segmented carbon filaments possessing unique structural and textural features.

AUTHOR INFORMATION Corresponding Author Aleksey A. Vedyagin; Boreskov Institute of Catalysis SB RAS, Novosibirsk 630090, Russian Federation; E-mail: [email protected]; Phone: +7 383 3269660; Fax: +7 383 3307453 ORCID: 0000-0002-6930-936X Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources


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Russian Foundation for Basic Research; Ministry of Science and High Education. ACKNOWLEDGMENT This study was supported by the Russian Foundation for Basic Research (project #16-33-60074 mol_a_dk) and Ministry of Science and High Education (project AAAA-A17-117041710079-8). The analysis of experimental results was partly carried out at Tomsk Polytechnic University within the framework of Tomsk Polytechnic University Competitiveness Enhancement Program. ABBREVIATIONS CNF, carbon nanofibers; CCVD, catalytic chemical vapor deposition; CNM, carbon nanomaterials; MD, metal dusting; SOC, self-organizing catalyst; SCNF, segmented carbon nanofibers; DCE, 1,2-dichloroethane; XRD, X-ray diffraction; SEM, scanning electron microscopy; AAS, atomic absorption spectroscopy; STEM, scanning transmission electron microscopy; EDX, energy dispersive spectrometry; SSA, specific surface area; IP, induction period; BET, Brunauer–Emmett–Teller.

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Formation of Active Sites of Carbon Nanofibers Growth in Self

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