Edited by: Fernando Bimbela, Universidad Pública de Navarra, Spain
Reviewed by: Jose Luis Pinilla, Instituto de Carboquímica (ICB), Spain; Ahmet Arisoy, Istanbul Technical University, Turkey; Jude Azubuike Onwudili, Aston University, United Kingdom
This article was submitted to Advanced Fossil Fuel Technologies, a section of the journal Frontiers in Energy Research
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In this work, we present the results of production of carbonaceous nanomaterials by decomposition of methane on a catalyst of Ni supported on a Biomorphic Carbon. The catalyst was prepared by thermal decomposition in a reductive atmosphere of vine shoots previously impregnated with the Ni precursor. In order to optimize the reaction productivity and selectivity, the effect of the main operational conditions (reaction temperature and feed composition) has been studied in a thermobalance. The main textural properties, BET area of 63 m2/g and 56% of microporosity, of the catalyst synthesized indicates that these materials are suitable for gas-phase reactions even in harsh conditions. Thus, the catalyst has proved to be active in the synthesis of carbon nanofibers and graphene related materials at elevated temperatures. The productivity, type, and quality of the carbonaceous nanomaterials obtained are deeply dependent on the operating conditions during the reaction. As an important fact, is has been obtained that the reaction temperature strongly affects the type of the nanomaterial produced. Thus, it is produced CNFs of bamboo type at temperatures until 850°C. Above this critical temperature, it is mainly obtained nanolayers of graphitic nature. The characterization results indicate that the highest quality graphenic materials were obtained operating at 950°C with 14.3% of CH4 and 14.3% of H2. The kinetic model used to analyze the experimental data is based on the more relevant stages of the mechanism of reaction.
The large number of applications found in recent decades for Carbonaceous Nanomaterials (CNMs), like carbon nanotubes, grapheme, and Graphene Related Materials (GRMs) like the so-called Few Layer Graphene (FLG), has led to develop numerous studies to know, develop and apply the excellent properties of these materials (Ji et al.,
The catalytic decomposition of hydrocarbons in vapor phase (CCVD) is one of the most important alternative production methods of these nanomaterials. This process shows several advantages such as easily scalability and cheaper production (Baddour and Briens,
On the other hand, the growing concern for the environment and the need to protect it, is leading to the investigation of new processes to obtain high added value products from renewable natural sources (El Achaby et al.,
The process based in the decomposition of lignocellulosic materials using a reducing or inert atmosphere at high temperatures and high heating rates is called biomorphic mineralization and allows converting structures formed by a biological process, e.g., wood and lignocellulosic biomass to inorganic materials with different potential applications (Mann,
The potential use of these carbonaceous materials obtained from lignocellulosic residue, e.g., vine shoot, as support of catalysts in different reactions of energy and/or environmental importance is of great interest, for example, in the above commented methane decomposition reaction to the simultaneous production of pure H2, free of CO and CO2, and of carbonaceous nanomaterials (CNMs).
In the present work, we study the kinetics of production of carbonaceous nanomaterials by decomposition of methane on a catalyst of Ni supported on a Biomorphic Carbon obtained by thermal decomposition of vine shoots. The influence of reaction temperature and feed composition on the quantity and the kind of CNMs obtained has been studied.
Finally, a key factor for the control in the selectivity of the CCVD process toward the type of carbonaceous nanomaterial desired is to know the reaction mechanism. The use of a kinetic model to analyse the CDM reaction data, based on the main stages of the mechanisms and which also consider the catalyst deactivation allows us optimizing the process (Villacampa et al.,
The vine shoots used in this work were supplied by a winery from the Denomination of Origin Somontano (Huesca, Spain). The material was prepared (milling and sieving) to obtain a homogeneous particle size distribution between 0.250 and 1.18 mm. The metal precursor used was Ni (II) Nitrate 6-hydrate supplied by Panreac Quimica SAU (ref.: 14144.1209).
The synthesis of the catalyst was carried out following different steps: First, the vine shoots were dried at 100°C overnight. Then, they were impregnated by incipient wetness with the Ni aqueous solution. After impregnation, the solid was dried at 80°C overnight. Finally, thermal decomposition was carried out. During this decomposition, Ni nanoparticles are generated, which catalyze partly the gasification of the carbonaceous support, generating greater porosity, BET area, and definitely better textural properties of the final catalyst. The main variables of this step are: atmosphere composition during de thermal treatment, temperature, duration time and heat rate to reach the final temperature (Cazaña et al.,
In order to know the type of carbon nanomaterials formed in the reaction and the textural and structural properties of the catalyst, different characterization techniques were used. A Mettler Toledo TGA/SDTA 851 analyzer, using 50 mL/min, was used to carry out thermogravimetric analyses in air (TGA-Air). This technique allows calculating the final percentage of Ni on the biomorphic carbon support obtained after the thermal decomposition of the vine shoots, knowing the impregnated initial amount of Ni and the final composition of the solid residue after the combustion in the TGA-Air experiment (NiO and ashes). Nitrogen adsorption–desorption isotherms at 77 K were developed to know the specific area and porosity of the catalyst. The equipment used was a TriStar 3000 instrument (Micromeritics Instrument Corp.). BET specific surface areas were measured from the adsorption branches in the relative pressure range of 0.01–0.10. The micropore volume estimation was made by means of the Dubinin-Radushkevich method (Dubinin and Radushkevich,
The CDM reaction was carried out at atmospheric pressure in a thermobalance (CI Electronics Ltd., UK, model MK2) operated as a continuous differential fixed-bed reactor (i.e., low reactant conversions, <10% methane conversion). Under these conditions a direct measurement of the reaction rate and therefore, of the catalyst activity is obtained. This experimental system allows continuous recording of the variations of sample weight and temperature during reaction. The reaction conditions were: sample weight: 25 mg; total flow-rate: 700 NmL/min.; temperature: 700–950°C; feed composition: %CH4: from 4.3 to 57.1%, 14.3% of H2 and N2 as balance. Blank experiments without catalyst are carried out to check the possible formation of carbonaceous deposits from the gas phase decomposition of methane, not observing the deposition of carbon in any case.
In order to describe the kinetics of carbonaceous material formation by catalytic methane decomposition, we have adapted a phenomenological model previously developed by our group (Rodríguez et al.,
Scheme of the mechanism of formation of carbonaceous nanomaterials by catalytic decomposition of hydrocarbons.
On the other side, given that the methane conversions attained at the reactor exit are very low (<10%), it is not necessary to consider the equilibrium reaction in the rate expression.
Regarding to the catalytic sites involved in the reaction, although the carbonaceous materials has been also used in the methane decomposition (Suelves et al.,
In addition to all these phenomena, it must be considered the deactivation of the catalyst. The decay of the catalyst activity is consequence of several factors like coke fouling of the external Ni surface, the falling of the methane diffusion rate through the nanolayers of graphite, the reconstruction of the Ni nanoparticles during the reaction, and the steric hindrance for the growth of CNTs, CNFs (Monzón et al.,
In spite of all (Monzón et al.,
The methane decomposition rate can be measured directly from the rate of carbon formation over the catalyst (
The term
In turn, the value of the
On the other side, the results shown in
The terms ψ
The term
For the particular case of very rapid carburization step, i.e., Ψ
In addition to the above considerations, it must be taken into account that there is a change of the type of carbonaceous nanomaterial formed depending of the reaction temperature, see
Nevertheless, at higher reaction temperatures, numerous nucleation points at the surface of the metallic nanoparticles are generated, creating many points for the exit of the dissolved carbon, which eventually leads to the formation of the graphenic materials surrounding the metallic nanoparticles observed at these conditions (see
In order to include this fact in the model, we have incorporated an additional parameter in the Equation 6 that allows modulating the effect of the diffusion time of carbon in each range of reaction temperature. This additional parameter,
In summary, the evolution of the carbon concentration is calculated as a function of the following parameters:
The values of
Where
In this expression,
One key advantage of the use of the MSC, unlike the SSR, is that it allows a rigorous statistical discrimination among several models with different number of parameters (Ward,
TGA-Air analyses for biomorphic carbon and Ni/BC catalyst.
The textural characteristics were studied by N2 adsorption. The results for the Ni/BC catalyst give BET surface area of 63 m2/g a pore volume of 0.125 cm3/g with a percentage of micropore volume of 56%. For the biomorphic carbon obtained without previous impregnation with the Ni precursor, the results are 17 m2/g for the BET area, and 0.013 cm3/g of pore volume, with a 42% of micropores. Therefore, in this case, the addition of Ni enhances the development of the structure of the carbonaceous support, increasing both the area and the pore volume of the support.
The XRD patterns of the fresh catalyst and after reaction at 950°C can be observed in
XRD pattern of fresh catalyst
The peak obtained at 26 and 43°Corresponds to the plane (002) and (100), respectively, of the carbon formed after the decomposition step at 800°C, indicating that part of the biomorphic material formed during the preparation of the catalyst has an incipient graphitic structure. After reaction, these peaks mainly correspond with the accumulation of graphitic/graphenic material over the surface of the catalyst during the reaction.
TEM images of the fresh catalyst and the measured size distribution of the metallic particles are shown in
TEM images and Ni particle size distribution of fresh catalyst.
The Ni particle size distribution and the average diameter of the Ni particles were calculated with a number >500 particles, which confers enough significant statistical relevance. In addition, owing to the shape of the Ni particle size distributions obtained, it was decided to calculate the average diameter of the Ni particles using the following expression (Bergeret et al.,
On the other side, the large discrepancy observed between the values of the average Ni particle from Scherrer equation, i.e., the XRD results in
Raman spectra of fresh catalyst and after reaction at different temperatures and with different methane concentrations.
The productivity, measured as gC/gcat.min, selectivity and stability of the catalyst during the reaction have been studied as a function of the temperature and methane concentration in the feed.
In this regard,
Evolution of carbon content along time. Influence of reaction temperature. Red dotted lines: Model prediction.
Evolution of carbon content along time. Influence of CH4 concentration. Red dotted lines: Model prediction.
As regards the influence of the reaction temperature shown in
TEM images of Ni/BC catalyst after reaction at: 850°C
The increase in the methane concentration in the feed, see
The shape of all curves in
In the present case, both, the reaction temperature and the methane concentration, have similar effects on the evolution of the carbon concentration along time. Thus, the increase in the temperature and in the partial pressure of CH4 augments the initial reaction rate and the final amount of carbon deposited on the catalysts.
After the initial period of increasing rate, in all the cases it is observed a decay of the catalyst activity until to attain a residual rate characterized by a low value of the slope of the curves at the end of the reaction time.
In this point, is interesting to note that the XRD results in
Thus, TEM micrographs presented in
The wide distribution of Ni nanoparticle sizes (
At elevated temperatures of reaction, above 850°C, TEM images (
For the experiments at 900 and 950 the values of the I2D/IG and IG/ID ratios are ranged from 0.45 to 1.05 and 0.79 to 2.04, respectively, which indicate that the sample contains mostly graphite and FLG materials, in agreement with the results observed in TEM (Takenaka et al.,
In summary, at increasing reaction temperatures and/or methane concentrations, the methane cracking rate increases leaving a higher number of atoms of carbon on the Ni surface, which is quickly carburized. In these conditions, the amount of the carbon atoms dissolved on the metallic subsurface becomes very high, attaining a supersaturation state. Consequently, the driving force for the diffusion of the carbon atoms through the Ni nanoparticles is large, favoring a rapid precipitation of the carbon at the rear surface of the Ni nanoparticles to form carbonaceous nanomaterials (CMNs).
As it has been commented above, both the type of CMN formed, and their rate of growth are consequence of a subtle equilibrium between the rates of nucleation and of segregation-precipitation at the exit points of a metallic nanoparticle. If the nucleation rate is low, it is favored the formation of CNTs and CNFs because, once the precipitation of the carbon atoms begins, rapidly the concentration inside the nanoparticle falls below its solubility level, and a steady grow of CNFs is observed, see TEM pictures in
On the contrary, if the rate of nucleation is high, the number of exit points can be very large, and the drainage of the carbon atoms occurs in parallel at all the point of the external surface of the Ni nanoparticles, forming graphenic layers surrounding these nanoparticles. The accumulation of these graphenic layers finally forms the observed graphitic nanoplatelets; see TEM pictures in
As it has been considered in the development of the kinetic model, the decrease of the reaction rate is the consequence of the catalyst deactivation. However, given that there is a notorious change on the reaction mechanism with the reaction temperature, see
At high reaction temperatures, the causes of catalyst decay can be related to the complex phenomena involved during the diffusion of the reactants through the layers of graphite, the reconstruction and encapsulation of the metallic nanoparticles during the reaction, and the steric hindrance to the growth of graphitic nanomaterials in the form of layers (Cazaña et al.,
With respect to the results of the application of the kinetic model to the experimental data, in
Arrhenius plot of the kinetic parameters. pCH4 = 0.1428 atm.
Relating to the effect of the temperature,
The effect of the
Influence of CH4 concentration on the kinetic parameters.
Finally, in
Influence of temperature and methane concentration on the kinetic parameter
Accordingly, and given that all the experiments done to analyse the influence
A catalyst of Ni (24%wt.) supported on Biomorphic Carbon residues, has been synthesized using vine shoots residues, which are an important waste in viniculture. The preparation method involves a simple one-step thermal decomposition stage. The main textural properties, BET area of 63 m2/g and 56% of microporosity, of the catalyst synthesized indicates that these materials are suitable for reactions which participate reagents in gas phase, even in the harsh conditions used during the decomposition of methane.
Thus, the catalyst has proved to be active in the synthesis of carbon nanofibers and graphene related materials by catalytic decomposition of methane at elevated temperatures.
The productivity, type and quality of the carbonaceous nanomaterials obtained are deeply dependent on the operating conditions during the reaction.
The results of this study indicate that nanocarbonaceous material productivity increases with the reaction temperature and the partial pressure of methane because the decomposition rate of methane increases under these operating conditions.
As an important fact, is has been obtained that the reaction temperature strongly affect the type of the nanomaterial produced. Thus, at temperatures until 850°C it is produced CNFs of bamboo type. Above this critical temperature, it is mainly obtained nanolayers of graphitic nature. This fact is a consequence of the change on the balance of diffusion-nucleation-precipitation of the dissolved carbon atoms inside the Ni nanoparticles. At low temperatures the process is governed by the precipitation forming CNFs. At elevated temperatures, the high nucleation rates attained determine the formation of nanolayers surrounding the Ni nanoparticles.
The kinetic model used to analyze the experimental data is based on the effect of each of the main stages of the mechanism of reaction. The values of the parameters have a clear physical meaning and their evolution with the temperature and the methane concentration is in agreement with the model hypothesis.
The characterization results indicate that the highest quality graphenic materials were obtained operating at 950°C with 14.3% of CH4 and 14.3% of H2. Under these conditions, the Raman spectra and TEM images have shown that the nanocarbonaceous material consists mainly of few layer graphene and graphitic nanolayers surrounding the Ni nanoparticles. The nanoplatelets, are eventually exfoliated during the reaction resulting in separated layers of graphene.
All datasets generated for this study are included in the manuscript and the supplementary files.
MA, FC, JV, and VS performed the experiments. NL, ER, and AM contributed to the planning, the interpretation of results and the writing of the manuscript.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
We acknowledge financial support from MINECO (Madrid, Spain) FEDER, Projects ENE2017-82451-C3 y ENE2013-47880-C3.