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Chemical Vapor Deposition Synthesis of Carbon Nanomaterials over Lanthanum-Nickel Co-Loaded Catalyst Supported on Novel Radially Aligned Nano Rutile

Written By

Farai Dziike, Paul J. Franklyn and Nirmala Deenadayalu

Submitted: 12 March 2024 Reviewed: 15 April 2024 Published: 13 June 2024

DOI: 10.5772/intechopen.114995

Nanocomposites - Properties, Preparations and Applications IntechOpen
Nanocomposites - Properties, Preparations and Applications Edited by Viorica Parvulescu

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Nanocomposites - Properties, Preparations and Applications [Working Title]

Dr. Viorica Parvulescu and Dr. Elena Maria Maria Anghel

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Abstract

Deposition-precipitation using urea (DPU) method was efficiently used to load lanthanum and nickel catalyst nanoparticles onto the radially aligned nano rutile (RANR) support material to achieve a metal loading ranging from 1 to 10 wt. % La-Ni/RANR co-loaded supported catalysts. The PXRD peaks due to La occurred at 2θ values <30° and increased in intensities with an increase in La wt. % loading. The occurrence and distribution of the catalyst metal phases were analyzed using wavelength dispersive spectroscopy mapping (WDS) of the electrode probe microanalysis technique (EPMA). The reduction profiles showed TPR peaks that shifted to higher temperatures with an increase in metal wt. % loading. TEM micrographs of the La-Ni/RANR-supported catalysts showed that at different wt. % loadings, the particles deposited take different shapes and sizes with polydisperse La-Ni nanoparticles assuming a short rod-like structure at 1% wt loading. The La-Ni/RANR catalyst directly affected and influenced the nature of the carbon nanomaterials in CVD reactions under different parametric conditions of varied wt. % composition, temperature, flow rate, and time. It was concluded that the straight CNFs were catalyzed by the La end, while the coiled or twisted CNFs were catalyzed by the Ni end of this catalyst.

Keywords

  • nanomaterials
  • impregnation
  • rutile
  • crystallography
  • polymorph
  • and spectrometry

1. Introduction

The performance of metals used as catalysts varies greatly over a wide range of parameters, such as their particle size, the reaction medium, temperature, and support materials they are loaded on. A suitable method of metal loading has to be determined by considering factors such as pH, point of zero charge, particle size, and stability [1]. Titanium dioxide has been used in both its anatase and rutile phases as a metal nanoparticle support to produce catalyst materials [2, 3, 4, 5, 6, 7]. Radially aligned nano rutile (RANR) was used as an alternative support material in the preparation of La-Ni/RANR co-loaded metal catalyst for the synthesis of shaped carbon nanomaterials (SCNMs). This was of interest to see if the inclusion of two metals would produce a synergistic effect. Many methods for loading metal nanoparticles are available for the preparation of this catalyst. Lanthanum (La) and nickel (Ni) nanoparticles were loaded on RANR to give La-Ni/RANR co-loaded catalysts using a hydrothermal method and deposition-precipitation with urea (DPU) [8, 9]. The La-Ni/RANR co-loaded catalysts were explored for the synthesis of carbon nanofibers (CNFs) via a chemical vapor deposition (CVD) method. Previous analysis using PXRD revealed the occurrence of oxide phases of La, Ni, and the bimetallic La-Ni in the crystallographic phases La2O3, NiO, LaNiO3, and La2NiO4, respectively [10]. The reduction of the loaded metal oxides produced small-sized La-Ni nanoparticles. The small particle size range promoted high catalytic activity. Ni has a tendency of synthesizing coiled CNTs or twisted CNFs, while La has a tendency of synthesizing straight CNFs. PXRD, EPMA, and TPR analyses were useful techniques for determining the intrinsic properties and characteristics of the supported co-loaded catalysts. TEM, SEM, Raman, and TGA characterizations allowed detailed morphological studies of both the La-Ni/RANR catalysts and the carbon nanomaterials synthesized.

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2. Research background

Titania exists in three main polymorphs: Anatase, brookite, and the more thermodynamically stable rutile phase [11]. Anatase titanium dioxide (TiO2) has been used to support nanosized particles of different materials to produce different catalyst materials. Mono-, bi-, and poly-metallic type of catalysts have been previously prepared using TiO2via different methods, such as sol-gel, conventional impregnation, colloidal, Pechini, self-combustion, wet impregnation, and facile hydrothermal methods [2, 3]. Many previous works involving mono and heterogeneous catalysts was faced with various degrees of catalyst sintering and agglomeration. For example, supported gold is catalytically active when it is dispersed as small particles on an oxide support for carbon monoxide (CO) oxidation. However, deactivation sets in due to nanoparticles sintering and the formation of carbonates on the reactive sites. This phenomenon was postulated to be surmountable by preparing co-loaded catalysts [12].

Supported co-loaded catalysts have proved to exhibit significantly improved catalytic activity, enhanced stability, and synergetic effects both in their chemistry and performance [13]. Co-loaded metal nanoparticles in a catalyst have the capacity to induce geometric modifications of the respective metal surfaces rendering the catalysts more efficient. La-Ni co-loaded catalysts have previously been prepared on different supports, such as alumina and silica. Ni/Al2O3 has been prepared for use in steam reforming of ethanol and phenol [14, 15]. However, adding La to form La-Ni/Al2O3 catalyst stabilized the alumina support and improved the resistance to coking of the Ni/Al2O3 in various steam reforming reactions [16].

Zhang et al. studied the catalytic performance of Ni-based catalysts using the support of γ-Al2O3·SiO2 and investigated the effect of the second metal of La, Y, Co, Cu, and Zr on the catalytic performance of Ni/Al2O3·SiO2 catalysts [17]. It was indicated that a low amount of La additives (5%) was enough to inhibit Ni crystal growth and enhance the reduction of nickel oxide. However, Garbarino and coworkers carried out a study on Ni/Al2O3 and Ni-La/Al2O3 catalysts for the steam reforming of ethanol and phenol. They discovered that Ni disperses on the pure support, while La disperses in a disordered way and concurrently reduces the acidity of the support [18]. Previous research reported on the effect of the rare-earth component of the RE/Ni catalyst on the formation and nanostructure of SWCNTs [19]. Several elements, including La, have been used together with Ni in bimetallic catalysts for the synthesis of SWCNTs [19, 20, 21, 22, 23]. The bimetallic catalysts have been found to have synergistic effects on the formation of SWNTs. The addition of rare-earth elements into the catalysts was often found to greatly improve the yield of SWNTs and even influence the nanostructure of the SWNTs, such as diameter and helicity [24]. Reports of the role of co-loaded metals in the bimetallic catalysts ranged from one metal (Y) being separated from the other (Ni) with only Ni catalyzing the growth process, through one metal associated with the CNM root development and the other supporting the growth of the CNMs to one metal serving as a precursor of a CNM nucleus and the other promotes growth. However, the exact roles played by catalyst metals in the formation of CNMs are still uncertain, and further studies are needed to obtain a clear understanding of growth by co-loaded or bimetallic catalysts.

In our previous study, nanosized particles of La and Ni were separately loaded on RANR to give La/RANR and Ni/RANR catalysts, respectively [25]. The catalysts were used in the synthesis of SCNMs via CVD [26]. The La/RANR catalyzed the synthesis of simple CNFs, while Ni/RANR synthesized twisted CNFs where it usually catalyzed synthesis of coiled carbon nanotubes (CNTs) over free-floating Ni nanoparticles or Ni loaded on other support materials such as alumina, silica, ferric oxides, and many others. Single transition metals such as Ni, Co, and Fe have been used in the preparation of SWCNTs [27, 28]. However, further research revealed that transitional mixed-metal components are more effective than single metals as catalysts. It was also found that incorporating a rare-earth element will give rise to a strong influence on both the quantity and the nanostructure quality of the SCNMs produced [29].

Here, we report on the catalytic performance of La and Ni nanoparticles supported simultaneously on RANR to give a La-Ni/RANR co-loaded catalyst, which has not previously been studied as a catalyst for the synthesis of carbon nanomaterials. The synergistic effect of the La and Ni nanoparticles in the co-loaded catalyst was compared and contrasted against the performance of the individual catalysts under uniform CVD conditions. The stability of the catalyst, the mechanism of CNMs growth, and the properties and characteristics of the “as synthesized CNMs” were explored with the hope of suggesting possible applications and scaling up for increased mass production.

In this work, we also report on the synthesis of SCNMs over co-loaded La-Ni/RANR catalysts in a CVD reaction. The metal catalyst ratios were maintained the same. However, the wt. % loading on the RANR support was varied from 1 to 10 wt. %. A parametric study was then performed in which the time of synthesis, temperature, and H2/C2H2 gas mixture flow rate were varied during the CVD synthesis. An optimal range of catalyst compositions and settings for the synthesis of SCNMs was determined from the different sets of parametric conditions.

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3. Experimental

3.1 Preparation of supported La-Ni/RANR co-loaded catalysts

The support was prepared in a hydrothermal method in which titanium tetrachloride (TiCl4), obtained from Sigma Aldrich was dispersed in water and refluxed for 24 h to produce RANR at 200°C [25]. The La-Ni/RANR co-loaded catalyst was prepared using a simultaneous DPU method of synthesis [26]. Both lanthanum trichloride (LaCl3) and nickel nitrate trihydrate (Ni(NO3)2·3H2O) were simultaneously and gradually added to a solution consisting of RANR suspended in a measured volume of deionized water. DPU was used to simultaneously load the La and Ni metals in their respective calculated weight percentages. Loadings of 1, 2, 5, 8, and 10 wt. % were achieved to give La-Ni/RANR catalysts. The DPU experiment was carried out at 80°C for 24 h with vigorous stirring.

3.2 Synthesis of CNFs

The setup of the CVD process used in the synthesis of CNFs in a horizontal quartz tube reactor. The co-loaded La-Ni/RANR catalyst was placed in a quartz boat positioned at the center of the furnace. The temperature maintained at the center of the furnace was varied at 300, 400, 500, 600, and 700°C. Acetylene gas (C2H2) was introduced into the tube reactor at a flow rate varied at 50, 75, and 100 mL min−1 with hydrogen set at the same flow rate being used as a carrier gas. The furnace’s temperature was ramped up at a heating rate of 10°C min−1 to the preset reaction temperature and was held at that temperature for a preset period of time. The time period over which the CVD reaction ran was varied at 30 min, 1 h, and 2 h. C2H2 and the carrier gas H2 were both introduced via their respective inlet tubes at a uniform preset flow rate. The furnace was allowed to cool down naturally to room temperature under hydrogen gas, which was allowed to flow throughout the duration of the reaction. The parametric study for the synthesis of CNFs was conducted as outlined in previous studies [25, 26].

3.3 Characterization of catalysts and carbon products

The crystallographic phases of co-loaded La-Ni/RANR catalysts were determined by obtaining powder X-ray diffraction (PXRD) patterns using Bruker D2 Phaser diffractometer in a 2θ range from 7 to 120° using Co Kα radiation wavelength (λ) of 1.79 nm. The “as-synthesized” CVD products were collected and analyzed by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and LRS. For TEM analysis, the samples were prepared by dispersing a small amount of the materials in methanol with slight sonication. The drops of the dispersed material were placed on a Cu grid. A FEI TEM Spirit operated at a voltage of 120 kV was used. For SEM analysis, the material removed from the tube reactor was directly mounted on carbon tape on a sample holder stub. A FEI ZEISS FIB was used for SEM observation. The average particle size was determined using ImageJ and statistical analysis on TEM and SEM micrographs. A minimum of 200 particles were counted on the micrographs and using the normal Gaussian curve, the particle sizes were statistically confirmed.

Raman spectroscopy was used to characterize the sp2 carbons from 0 to 3D, such as 3D graphite, 2D graphene, 1D carbon nanotubes, and 0D fullerenes. The 1D and 2D Laser Raman spectroscopy data were used to explain the crystallographic nature of the carbon nanomaterials synthesized in this work. The Bruker Senterra Laser Raman Spectrometer was used to determine the photon-phonon interactions in a material sample by measuring the Raman scattering intensity presented as a Raman signal. The illuminating wavelength was set at 785 nm in the Raman spectroscopy experiment of this work. High energy of UV photons tends to be destructive to solid-phase material samples, thus, near-IR lasers (660–830 nm) were used in this setup for the purposes of fluorescence suppression in Raman spectroscopy. This wavelength range was reported to offer the best balance between scattering efficiency, influence of fluorescence, detector efficiency, and availability of cost-efficient and compact, high-quality laser sources [30]. Energy-probe microanalysis (EPMA) was used to determine the chemical composition density of the carbon nanomaterials and the spatial distribution of materials in the La-Ni/RANR catalyst by EPMA WDS mapping and profiling.

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4. Results and discussion

Deposition-precipitation using urea (DPU) method efficiently loaded the metal nanoparticles on the RANR support to give La-Ni/RANR co-loaded-supported catalysts. Metal wt. % loading ranging from 1 to 10 wt. % were achieved. Detailed analysis and characterization of the catalysts using PXRD and EPMA revealed the crystallographic phases of the catalysts and the spatial distribution of the La-Ni nanoparticles on the RANR support, respectively. The morphology of the supported catalysts and the CVD products was analyzed using TEM and SEM from which the properties and characteristics of the catalyst materials and the carbon nanomaterials were studied in detail. The varied parametric conditions of synthesis produced carbonaceous materials of distinct crystallinity. LRS measured the degree of disorderliness and graphicity in the carbon matrix of the carbon nanomaterials. Thermogravimetric analysis (TGA) revealed intrinsic details on both the catalysts and CNMs through thermal stability determinations.

4.1 PXRD and EPMA analysis

The dominant peaks observed in the PXRD patterns in Figure 1 are due to the rutile phase of the RANR support. The major PXRD peak of the rutile phase occurs at 2θ 32.5° with crystal planes 110, while minor peaks occur at 101, 200, 111, 211, 220, and 002. This trend was exhibited in the whole metal percentage loading range. This is because the rutile is in great abundance as the catalyst support. A. E. Abasaeed et al. reported that the La and Ni phases exist as La2O3 and NiO, the thermodynamically stable phases, which occur at the reduction temperature of the respective oxide materials [31]. The peaks identified as La in Figure 1 may be ascribed to La(OH)3 and La2O3 phases. These phases reduce to La at 600°C to give a co-loaded catalyst La-Ni/RANR. The peaks due to La occurring at 2θ values <30° with the La PXRD major peak occurring at 2θ value 15° and are increasing in intensities with an increase in La wt. % loading as shown in Figure 1.

Figure 1.

PXRD patterns of La-Ni/RANR co-loaded catalysts.

The occurrence of individual La and Ni phases shows that the metal nanoparticles are not in a mixed-metal system. However, calcination and reduction are postulated to facilitate the formation of the active La and Ni metallic nanoparticles in the La-Ni/RANR co-loaded catalysts [32]. The occurrence and distribution of the catalyst metal phases were analyzed using wavelength dispersive spectroscopy mapping (WDS) by electrode probe microanalysis technique (EPMA) as shown in Figures 2 and 3.

Figure 2.

WDS spectrum profile of 10 wt. % La-Ni/RANR-supported catalyst.

Figure 3.

WDS spectral maps of 10 wt. % La-Ni/RANR-supported catalyst. Elemental quantitative compositional maps of (a) O, (b) Ti, (c) La, and (d) Ni of the bulk La-Ni/RANR mixed-metal catalyst.

The characteristic X-rays convolute with the instrumental response to produce a WDS X-ray spectrum as shown in Figure 2 for the 10 wt. % La-Ni/RANR-supported co-loaded catalysts. The X-ray spectrometer that measures the characteristic X-ray peak intensities was also set to measure the count rates of the characteristic X-rays. The WDS spectra in Figure 2 shows the peaks from the characteristic X-rays measured against the respective analyzing crystals to their Bragg diffraction characteristics. The elements Ti, La, and Ni show sharp high-intensity peaks, while oxygen has two low-intensity broad peaks. The distinct oxygen peaks are attributable to those from the TiO2 support and the residual oxygen in the La and Ni of the co-loaded metal oxide nanoparticles. It was also observed that there are no significant peaks due to intermediate phases. The spectral pattern is comparable to the intensities presented in the PXRD patterns of the La-Ni/RANR presented in Figure 1. The X-ray intensity distributions of the elements from an X-ray map permitted the generation of 2D and ternary scatter diagrams that present spatial information into concentration dimensions that display spatial relationships of elements or phases in materials [33].

WDS spectrometry was used to construct X-ray images of the sample showing the spatial distribution of each element in the La-Ni/RANR co-loaded catalyst. The element that generated higher counts from a more cross-sectional scattering resulted in a map showing high-density distribution of the element on the map. The WDS spectral maps in Figure 3 shows O, Ti, La, and Ni-rich phases in which other elements of the catalyst do not exist. The oxygen map in Figure 3(a) is less dense as compared to those of La, Ti, and Ni, a phenomenon correlated to Figures 1 and 2.

The circled areas in Figure 3(a)–(d) showed examples of regions of low (1) and high (2) concentrations of O, Ti, La, and Ni atoms, respectively. A closer look at the maps showed that the elements are dispersed and concentrated on different points in the maps. However, the La map shows a lot of brighter regions, a phenomenon attributable to its heavy atomic mass of 139 g mol−1. The elemental maps of nanosized metal particles could be developed on a 20 μm scale because the metals are supported on the μ-sized scale RANR. It can be concluded that both PXRD and EPMA analyses indicate that a single phase of the La-Ni/RANR was formed. TEM micrographs in Figure 4 also show how clustered the metal nanoparticles are on the RANR support at 10 wt. % loading.

Figure 4.

TPR curves of the La-Ni/RANR catalysts at different wt. % loadings calcined at 700°C: 150 mg; gas flow rate: 5% H2/Ar, 50 mL/min.

4.2 Temperature programmed reduction (TPR)

Reduction profiles of the La-Ni/RANR catalysts provided intrinsic information about the behavior and chemical interactions of the nanosized metal particles in the co-loaded system. Figure 5 shows the TPR profiles of the La-Ni/RANR catalysts at different wt. % loadings. The catalysts were all calcined at 700°C in air for 2 h. The reduction peaks shifted to higher temperatures with an increase in metal nanoparticles wt. % loading. The intensities also increased with wt. % loading. This implied that the higher the wt. % loading, the more homogeneous the metal-metal pairing and the orderliness in the La-Ni array. Consequently, the La-Ni nanoparticles attained higher crystallinity as confirmed by narrower and sharper PXRD peaks in Figure 1 and hence a higher reduction temperature of the La-Ni/RANR catalysts.

Figure 5.

SEM micrograph of RANR prepared using a hydrothermal method at 200°C over 24 h.

The reduction process completely reduced the oxide phases of the Ni and La to form the La-Ni co-loaded on RANR. The reduction profiles of catalysts at 1, 5, and 8 wt. % loading show a single peak at 500, 540, and 580°C, respectively. The 500°C reduction temperature for 1 wt. % La-Ni/RANR is comparable to that of plain RANR and the supported Ni/RANR. However, the shift in the peak positions to higher temperatures may be attributed to increased crystallinity of the metal oxides and mainly the increase in the oxygen in the metallic matrix of the catalysts, and not comparable to that of pure La/RANR at 600°C [25]. This consequently increased the reduction temperature as the wt. % loading increased as also confirmed by PXRD patterns showing increased intensities with the increase in metal wt. % loading [34].

The catalyst at 10 wt. % loading has a TPR profile showing two peaks. Detailed studies showed that the peak at around 300°C is not due to any of the metal oxide phases identified in the PXRD analysis, but it could be attributed to excess oxygen in the oxygen-saturated mixed-metal array, which translates to agglomeration or sintering on the support rutile nanorod surfaces of RANR [35]. The peak occurring around 680°C corresponds to the complete reduction of the mixed-metal oxides in a one-step process to give supported La-Ni/RANR mixed-metal catalysts [36]. The high reduction temperature observed at higher wt. % metal loading is predominantly a consequence of increased metal-metal interactions in the mixed-metal system. Morphological studies of the mixed-metal catalysts supported on the RANR using TEM and SEM give detailed information on the physical nature of the metal nanoparticles that influenced their intrinsic behavior and characteristics.

4.3 Transmission and scanning electron microscopy

The morphology of the RANR support is characterized by spherical shapes formed from the nucleation of rutile nanorods around a central core to give a radially aligned rutile nanorods cluster of RANR microspheres. Figure 4 shows the heterogeneously sized RANR with radii ranging from 50 to 600 nm. The micrograph suggests a tightly bound rutile nanorods network held at the center of the RANR structure. The RANR exhibit high-density mesoporous surfaces, and this is expected to promote accessibility of the inner surfaces of the nanorods network. Figure 6 shows TEM micrographs of RANR-supporting La-Ni nanoparticles of the La-Ni/RANR-supported catalysts. The catalysts in micrographs of Figure 6 showed that at different wt. % loadings, the particles deposited on the support take different shapes and sizes. Figure 6(a) is a micrograph of 1 wt. % La-Ni/RANR showing polydisperse La-Ni nanoparticles on the RANR support, assuming a short rod-like structure. The particle size of the nanoparticles ranged from 2 to 10 nm with an average size of 4.9 nm.

Figure 6.

TEM micrographs of La-Ni nanoparticles co-loaded on RANR to give (a) 1 wt. % La-Ni/RANR, (b) 10 wt. % La-Ni/RANR, and (c) 8 wt. % La-Ni/RANR supported catalysts.

The morphology of the La-Ni nanoparticles changed at a higher percentage composition. Figure 6(b) is a TEM micrograph of 10 wt. % La-Ni/RANR showing La-Ni nanoparticles of different morphologies and sizes. The catalyst is showing agglomeration on the support surface suggesting extensive polydispersion of the La-Ni nanoparticles. This is a sharp difference at lower wt. % loadings as shown in Figure 6(c), which are characterized by less agglomeration and polydispersion of the La-Ni nanoparticles. Previous studies elucidated the presence of spherical particles of Ni in the 5 wt. % Ni/RANR, with a diameter range of 1–6 nm with an average diameter of 4 nm, while that of La, ranged from 2 to 8 nm with an average diameter of 7 nm [25, 26]. This is within the particle size range of the low wt. % loading of 1 wt. % La-Ni/RANR. It was observed that wt. % loading, the particle size range of La-Ni/RANR shifted to a longer range as presented in Figure 7(a). This is comparable to the particle size distribution of the La-Ni nanoparticles on RANR, which has a narrow range and a small average particle size of about 5 nm at 5–10 wt %.

Figure 7.

Size distribution analysis of (a) La-Ni nanoparticles supported on the RANR support and (b) rutile nanorods of the RANR support.

The narrow range of particle sizes of the La-Ni mixed-metal catalysts is the synergistic effect of both the unique topology on RANR support and the DPU method of metal loading used. This synergy supports the loading of a very narrow range of nanosized metal particles on the support. The defects on which the metal particles sit on the RANR nanorods are yet to be studied to understand the actual mechanism of metal deposition. However, Figure 7 compares the particle size distribution of the La-Ni mixed-metal particles relative to the RANR nanorod thickness.

The particle size analysis in Figure 7(a) shows a wider size range than the range of thickness of the rutile nanorods of the RANR support. However, the average size of the metal particles is about 6 nm, while that of the rutile nanorods is about 7 nm. The few La-Ni particles with sizes greater than 10 nm may be attributed to agglomeration during DPU process and the subsequent calcination. This explains what is exhibited in Figure 6(b) in which the whole RANR sphere appears to be completely covered by La-Ni nanoparticles on its surface. The agglomerated particles of La-Ni and Ni appear as dark spots on the catalyst material. These properties and characteristics of the La-Ni nanoparticles and the supported La-Ni/RANR catalyst directly affected and influenced the nature of the carbon nanomaterials synthesized over these catalysts in CVD reactions. Figure 8 shows SEM micrographs of the carbon nanomaterials synthesized over La-Ni/RANR under different parametric conditions of varied wt. % composition, temperature, flow rate, and time. Percentage weight composition was varied at 0.5–10 wt. % and a temperature of 300–700°C over a time range of 30 min–2 h with a flow rate of 50–100 mL min−1.

Figure 8.

SEM micrographs of CNFs synthesized over 0.5–10 wt. % La-Ni/RANR at 500°C, 1 h, 75 mL min−1.

Morphological studies of the CVD products synthesized over the La-Ni/RANR catalysts performed using SEM revealed that the carbon nanomaterials are fibrous in nature. The CNFs are a mixture of both straight long stretching fibers and twisted or coiled fibers. Figure 8(a) shows how the CNFs densely cover the RANR structure supporting the La-Ni nanoparticles. The CNFs also grew radially assuming the arrangement of the radially aligned nanorods of the RANR support. It was postulated that the actual morphology of each individual CNF may have been directly attributed to the surface or end of the La-Ni nanoparticle over which it grew. This implied that the straight CNFs (Figure 8(d)), were catalyzed by the La end of the co-loaded La-Ni/RANR catalyst, while the coiled or twisted CNFs (Figure 8(f)) were catalyzed by the Ni end of this catalyst. This is in correlation to the observed CNMs synthesized over the mono-catalysts of La/RANR and Ni/RANR reported in the previous publications [25, 26]. The parameters varied during the CVD reaction had distinct influences on the products obtained. Temperature and wt. % loading significantly affected the yield due to the relative temperature of catalytic decomposition to the C2H2/H2 gas and hence degree of conversion over time periods. Figure 9 presents TEM and SEM micrographs of CNFs synthesized at different temperatures.

Figure 9.

Twisted CNFs synthesized over 8 wt. % La-Ni/RANR at 1 h period and a flow rate of 75 mL min−1 comparing temperature as a factor affecting CNFs growth per unit time.

Detailed studies showed similar trends at the whole wt. % loading, flow rate, and time range. Temperature had the most significant influence due to its direct effect on the decomposition of C2H2. The higher the temperature, the more efficiently the C2H2 was decomposed, and hence the more catalytic conversion of the carbon generated into CNFs as shown in Figure 9. A closer look at the CNFs synthesized using TEM revealed that the fibers have twisted and straight chain morphologies.

TEM analysis of the CVD products also revealed that the La-Ni/RANR catalyzed the synthesis of straight heterogeneous CNFs from C2H2/H2 under different sets of parametric conditions. Figure 10 showed that the fibers are of a wide range of lengths and diameters. Figure 10(b),(c) show fibers of different thicknesses, while Figure 10(d) shows a single fiber stretching for approximately 10 μm in length. The metal particle at the tip of the fiber plays a crucial role in determining the diameter of the synthesized fibers [26].

Figure 10.

TEM micrographs of CNFs synthesized over La/RANR at (a) 1 wt. %, (b) 2 wt. %, (c) 5 wt. %, and (d) 10 wt. % all at 700°C, 2 h, 100 mL min−1.

The CNFs synthesized over the La-Ni/RANR catalysts show a diameter range that is not comparable to the average particle size of the La-Ni nanoparticles of about 7 nm. This is because the metal catalyst particles have a tendency to agglomerate on the support surface as proved in Figures 2,3, and 6. This set the precedence for the growth of thick CNFs. Moreover, the catalyst particles drift away from the point of support during CNF growth. Consequently, the metal catalyst particles undergo sintering during high-temperature synthesis and give rise to the synthesis of even high-diameter CNFs as shown by the data presented in Figure 11. The fibers have a wide diameter range of 50–500 nm and an average diameter of about 175 nm. It can be concluded that the high diameter range is attributable to a widespread polydispersion of the La-Ni nanoparticles in the catalysts coupled with a systematic increase in wt. % loading that promoted agglomeration and sintering resulting in the synthesis of CNFs of a high diameter range. The properties and characteristics of the CNFs synthesized over the co-loaded La-N/RANR were similar to those synthesized over the mono-loaded catalysts La/RANR and Ni/RANR at the whole parametric range. The following sections summaries the trends observed for the La-Ni/RANR co-loaded catalysts.

Figure 11.

Fiber thickness distribution analysis of CNFs synthesized over 8 wt. % La-Ni/RANR catalysts at 1 h and a flow rate of 75 mL min−1.

4.4 Percentage weight loading

The catalytic performance of the La-Ni/RANR was assessed by varying the percentage weight loading of the La-Ni nanoparticles on the RANR support. The catalysts were varied from 0.5 to 10 wt. %. Each catalyst was used under the same set of parametric conditions. Two main observations were made. Firstly, the La-Ni nanoparticle size increased with an increase in wt. % loading. This was because the RANR support remained the same in terms of its morphology, topology, and the heterogeneous points at which loaded nanoparticles were deposited. As the percentage weight increased, the particles’ proximity increased such that they became more susceptible to agglomerate at higher reaction temperatures. The catalyst materials tended to sinter and increased in particle size. At 10 wt. % loading, the La-Ni nanoparticles coverage density on the support is significantly high and that TEM analysis showed a complete cover of the RANR support (Figure 6(b)). At lower wt. % loading, there is less or little agglomeration of La-Ni nanoparticles on the RANR support. However, it was observed that agglomeration took place as the La-Ni nanoparticles drifted away from the RANR support during CVD reaction.

The second observation was made on the influence of wt. % loading on the synthesis of CNFs in the CVD process. At all wt. % loading, the catalysts synthesized a mixture of straight and twisted CNFs with heterogeneous thickness and length under the whole range of parametric conditions. The general observation made was that average fiber diameter tended to increase with the increase in wt. % loading. Since wt. % loading determined the coverage density of the La-Ni on the RANR support, and the fiber coverage density on the RANR support was directly proportional to the degree of La-Ni nanoparticles distribution on the RANR support (Figure 9). It may be concluded that the percentage weight loading only may not be used to characteristically explain the CNFs produced in the CVD process.

4.5 Effect of temperature on CNFs synthesis over La-Ni/RANR

Temperature plays a crucial role in assisting in the decomposition of the C2H2 used as the carbon source and providing the activation energy for the synthesis of the CNFs. However, detailed studies revealed that temperature also affects the properties and characteristics of the CNFs produced [19, 26, 32]. It is expected that the outer diameter of the CNFs formed becomes larger with an increase in temperature. This is because increased reaction temperatures promote sintering or agglomeration of catalyst nanoparticles and a consequential gradual increase in particle size due to heat-induced aggregation during the reaction process [37]. This was observed in reactions involving 10 wt. % loading because of a high density of catalyst particles at close proximity and readily aggregate to form agglomerated particles that facilitate the synthesis of thicker fibers [38]. However, at lower metal loadings, this phenomenon was minimal. TEM and SEM micrographs show how the catalyst particles agglomerate on the support surface and are further confirmed by TPR analysis. The degree of graphitization and crystallinity of the CNFs increases with an increase in temperature as expounded in Raman studies [25]. At 700°C the fiber thickness and crystallinity were observed to be high than in low-temperature reactions at 300–500°C.

4.6 Gas flow rates and time variation

Detailed studies on the effect of gas flow rate and time variation revealed that both parameters affect the CVD products in similar ways. At low gas flow rates or short time periods, very little carbon is available for deposition on the La-Ni nanoparticle surfaces [26]. This implied that low-yield carbon materials are produced and are mainly amorphous. This phenomenon is not affected by temperature variation because the little carbon deposited undergoes incomplete hybridization and thus contributes to a disordered carbon matrix. The carbon materials consist of a mixture of sp2 and sp3 hybrid carbons that do not completely close the graphite rings of the carbon material and hence amorphous carbon products [39]. At high flow rates and short time periods, amorphous carbon rapidly deposits on the catalyst material, and the carbon is inadequately crafted into disordered products. But at low flow rates and long time periods, there is insufficient carbon for the complete synthesis of CNFs [26]. It was observed that at high flow rates and long time periods, the actual properties and characteristics of the CVD products are subject to temperature. This is because flow rate and time only determine how much carbon is deposited on the catalyst material. It is the nature of the catalysts and temperature that gives the CNF their intrinsic properties and characteristics by influencing the mechanism in which the CNFs are synthesized during the CVD reaction. It may be concluded that time or flow rate only may not be adequately used to account for the acquired qualities of the synthesized CNFs.

4.7 LRS analysis of CNFs synthesized over La-Ni/RANR

LRS studies enabled the determination of detailed intrinsic characteristics and properties of “as-synthesized” carbon nanomaterials. Raman spectroscopy is a technique that determines the vibrational modal points of the respective peaks of the Raman spectrum of molecules [40]. The diameter distribution may be determined from relative peak intensities while the spectral lines reveal the nature of the carbon nanomaterials prepared by analyzing their tangential modes, G-band (1590 cm−1) and compared to either the low-frequency peaks, D-band (1320 cm−1), and or radial breathing modes (140–190 cm−1) [41].

Detailed studies of Raman measurements performed on CNFs synthesized over the whole range of La-Ni/RANR catalysts showed a common trend in the D- and G-band peaks.

The Raman spectra did not show low-frequency peaks due to carbon confirming that the catalysts did not synthesize single-walled carbon nanotubes (SWCNTs) and hence all the materials produced are CNFs [42]. Figure 12 shows that the G-band is at 1600 cm−1, while the D-band is at around 1325 cm−1. This suggests that the La-Ni/RANR catalysts have a strong consistent influence on the synthesis of the carbon network in both the straight and twisted CNFs [43]. Figure 12(a) shows the variation of peak intensities at different time periods of CNF synthesis. It can be observed that the intensities of both the D- and G-band peaks are increasing with an increase in time. This is because as time increased, there was increased catalyst exposure to the carbon source. The carbon network is sufficiently knit into a more crystalline graphitic form giving rise to a high-intensity G-band peak [44]. However, disorderliness occurs within the carbon matrix as a result of limited carbon interaction with the catalyst surface resulting in the D-band peak. The carbon is incorporated as amorphous carbon and causes defects in the CNF’s matrix as is common in multiwall carbon nanotubes (MWCNTs) and defects in SWCNTs [45]. It can be concluded that the CNFs are more graphitic but with considerable disorderliness as confirmed by the G- and D-band peak intensities, respectively.

Figure 12.

Laser Raman spectra of CNFs (a) synthesized over 5 wt. % La-Ni/RANR catalysts at different time periods, (b) synthesized over 10 wt. % La-Ni/RANR catalysts at different temperatures under uniform conditions (2 h, 100 mL min−1).

Temperature supports enhanced crystallinity in the synthesis of carbon nanomaterials above the direct influence of the catalyst over which carbon growth is taking place. Figure 12(b) presents Raman spectra of CNFs synthesized at different temperatures with all other parametric conditions held constant. The D- and G-bands are at 1325 and 1600 cm−1, respectively, with the G-band more intense than the D-band peak. The Raman peak intensities also increased with an increase in temperature from 300 to 700°C. The spectra show that there is no significant shifting in the main radial breathing mode peak positions over the 300–600°C temperature range. At 700°C, the D-band peak shifted from 1325 cm−1 to around 1375 cm−1 and this may be explained in terms of temperature-enhanced crystallinity as a consequence of the low degree of disorderliness in the amorphous carbon [38].

This peak shifting observed may indicate that the diameter of the CNFs is not affected by temperature but by the particle size of the catalyst nanoparticles [25]. However, the D-band peak shift to the right at 700°C showed relative purity tendencies in the CNFs. This is because there is less disorderliness in the carbon matrix of the CNFs, the more crystalline and graphitic they are, the pure the product. This is also comparable to the yield of the CNFs with temperature. It was observed that as temperature increased, the yield also increased because higher temperatures facilitate efficient decomposition of the C2H2 carbon source and make it readily available at the catalyst reaction surface for CNF synthesis. It may be concluded that there was no synergistic effect observed in La-Ni/RANR co-loaded catalysts.

4.8 Thermogravimetric analysis (TGA)

Synthesis of CNFs over the La-Ni/RANR was measured using thermogravimetric analysis (TGA). The technique allowed the quantification of the amount of carbon deposited on the surface of the catalyst during the CVD synthesis of CNFs [46]. The analysis confirmed how the reaction parameters influenced and affected the production of CNFs over the catalyst materials. Figure 13 presented decomposition profiles of analyses at different time periods of CNFs synthesis (Figure 13(a)) and at different temperatures (Figure 13(b)). Time was varied at 30 min, 1 h, and 2 h. The profile of products synthesized at 30 min showed a weight loss of about 35%. This increased to 45 and 50% for 1 and 2 h, respectively. This implied that the longer the reaction time, the more CNFs that were synthesized over the catalyst material. This was because the catalyst was sufficiently exposed to the carbon depositing on its surface and allowed the adequate synthesis of a more crystalline carbon matrix of the CNFs. The CNFs synthesized at longer time periods tend to have a dense coverage around the catalyst (Figure 10(a)). Coupled with high temperatures, they tend to be long and have a thicker outside diameter and hence a high weight loss during TGA analysis as shown in Figure 13(a).

Figure 13.

TGA profiles of CNFs (a) synthesized over 1 wt. % La-Ni/RANR catalysts at 600°C and 75 mL min−1at different time periods, (b) synthesized over 10 wt. % La-Ni/RANR catalysts at different temperatures under uniform conditions (2 h, 100 mL min−1).

There is a measurable degree of correlation between the purity of the CNFs and the TGA data [47, 48]. The insert in Figure 13(a) shows differential thermogravimetric curves (DTG) for the corresponding time variation TGA analyses. All the DTG profiles have major peaks above 550°C which are ascribed to the deposition of coke with different degrees of crystallinity and graphicity over the La-Ni/RANR catalysts [49]. The DTG peak of CNFs at 30 min shows a single peak at 650°C, while that at 2 h is at 680°C. The 1 h profile shows multiple peaks, which are attributed to different phases of carbon such as monoatomic carbon and amorphous carbon, which is highly unstable and reactive, and hence decomposes at low temperatures <550°C [50]. The residual weight is due to the La-Ni/RANR catalyst materials.

The comparison of CNFs synthesized at different temperatures with all other conditions held constant using TGA revealed the quality of the CNFs and also the yield of the CVD reaction. Figure 13(b) illustrates the effect of temperature on the quality of CNFs synthesized by the La-Ni/RANR catalysts. Previous research reported that low-temperature TGA and TPO peaks are due to the combustion of amorphous carbon species, while peaks at higher temperatures could be attributed to graphitic carbon. The profile for products at 300°C showed a weight loss of about 8 and 92% being due to residual catalyst material. This implied that at 300°C, very little carbon material is deposited on the catalysts and the temperature is too low to support the synthesis of crystalline graphitic carbon material. At 400°C, there is increased yield but the carbon materials are predominantly amorphous carbon that quickly burnt out at 400–580°C with a weight loss >80%. Reactions at higher temperatures ranging from 500 to 700°C show high graphitization, increased yield, and weight loss along this temperature range (Figure 13(b)). We can conclude that temperature and wt. % determines the amount of carbon deposition on the La-Ni/RANR catalyst and the quality of the CNFs synthesized.

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5. Conclusions

A two-step procedure comprising DPU synthesis, followed by reduction process, affords the preparation of La-Ni nanoparticles supported on RANR to give La-Ni/RANR mixed-metal catalysts. TPR and PXRD results proved and confirmed the mechanism of formation of the supported La-Ni/RANR catalyst through DPU and reduction. The wt. % loading affected the sizes of the La-Ni nanoparticles. The La-Ni/RANR catalyzed the synthesis of a mixture of long straight chain CNFs and twisted CNFs using a simple CVD method. The La-Ni/RANR catalyst did not show an intrinsic synergistic effect in its catalytic effect during the synthesis of CNFs. The different metallic ends catalyzed fibers of different morphologies as was observed for the mono-loaded La/RANR and Ni/RANR catalysts. Reaction parametric conditions influenced the quality, properties, and characteristics of the CNFs synthesized over the La-Ni/RANR catalysts. Raman and TGA analyses revealed that the CNFs were more graphitic but with considerable disorderliness as confirmed by the G- and D-band peak intensities and TGA profiles, respectively. Temperature and wt. % loading was proved to be crucial parameters that determine the amount of carbon deposition on the La-Ni/RANR catalyst and the quality of the CNFs synthesized.

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Acknowledgments

The authors would like to acknowledge the Durban University of Technology for supporting the publication of this work through its Office of the Research and Postgraduate Support Directorate by funding this project. Gratitude is extended to the Department of Applied Chemistry in the Faculty of Applied Sciences at the Durban University of Technology for the environment and material resources for executing this research work.

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Written By

Farai Dziike, Paul J. Franklyn and Nirmala Deenadayalu

Submitted: 12 March 2024 Reviewed: 15 April 2024 Published: 13 June 2024

© 2024 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.