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Carbon dioxide catalytic conversion to nano carbon material on the iron–nickel catalysts using CVD-IP method
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Carbon dioxide catalytic conversion to nano carbon material on the iron–nickel catalysts using CVD-IP method Jiaquan Hu a,b, Zhanglong Guo a,c, Wei Chu a,b,c,∗, Le Li a, Tao Lin b,c,∗∗ a
College of Chemical Engineering, Sichuan University, Chengdu 610065, Sichuan, China Key Laboratory Green Chemistry & Technology of Ministry of Education (MOE), College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China c Institute of New Energy and Low-carbon Technology, Sichuan University, Chengdu 610065, Sichuan, China b
a r t i c l e
i n f o
Article history: Received 25 July 2015 Revised 16 September 2015 Accepted 16 September 2015 Available online xxx Keywords: Carbon dioxide utilization Catalytic capture Iron–nickel catalyst Chemical vapor deposition integrated process (CVD-IP) Solid-form nano carbon material
a b s t r a c t The over-consumption of fossil fuels resulted in the large quantity emission of carbon dioxide (CO2 ), which was the main reason for the climate change and more extreme weathers. Hence, it is extremely pressing to explore efficient and sustainable approaches for the carbon-neutral pathway of CO2 utilization and recycling. In our recent works with this context, we developed successfully a novel “chemical vapor deposition integrated process (CVD-IP)” technology to converting robustly CO2 into the value-added solid-form carbon materials. The monometallic FeNi0–Al2 O3 (FNi0) and bimetallic FeNix–Al2 O3 (FNi2, FNi4, FNi8 and FNi20) samples were synthesized and effective for this new approach. The catalyst labeled FNi8 gave the better performance, exhibited the single pass solid carbon yield of 30%. These results illustrated alternative promising cases for the CO2 capture utilization storage (CCUS), by means of the CO2 catalytic conversion into the solid-form nano carbon materials. © 2015 Science Press and Dalian Institute of Chemical Physics. All rights reserved.
1. Introduction World-wide human activities result in discharges of four longlived greenhouse gases: CH4 , CO2 , NO2 and halocarbons, wherein CO2 plays the most important role in the anthropogenic emissions. What’s more, CO2 is responsible for about 70% of the greenhouse effects [1–4]. Just now, the CO2 concentration in atmosphere is close to 400 ppm, which is significantly higher than the pre-industrial level of 280 ppm [1]. In recent years, the rapid rising of carbon dioxide level in the atmosphere has caused many kinds of extreme weathers to be frequently occurred, making the world economy and mankind to suffer from great challenges [5]. Consequently, it is crying-need for hunting effective approaches to reduce CO2 concentration rising in atmosphere and thus mitigate the global warming. To address this issue, world-wide scientists developed various CCUS technologies. At present, several kinds of the CO2 catalytic reduction were studied by chemical method [6], mineralization [7], photochemical method [8,9] and Electrocatalytic conversion [10].
∗ Corresponding author at: College of Chemical Engineering, Sichuan University, Chengdu 610065, Sichuan, China. Tel: +86 28 85400591. ∗∗ Corresponding author at: Key Laboratory Green Chemistry & Technology of Ministry of Education (MOE), College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China. Tel: +86 28 85400591. E-mail addresses: [email protected] (W. Chu), [email protected] (T. Lin).
Motiei et al. reported the technique that CO2 was reacted with metallic magnesium at 1000 °C and 10,000 bar to produce a small amount of CNTs and nested fullerenes [11]. His work indicated that CNTs could be synthesized from CO2 source. Lou et al. employed metallic lithium (Li) as the reductant, supercritical CO2 as the only carbon source to capture CO2 at an operating temperature 550 °C for 10 h [12]; and the result depicted that the product was the highly crystalline carbon nanotubes. Another interesting work was reported by Tamaura and Tabata [13]. Their team adopted more moderate condition, using oxygen deficient ferrites as the reaction medium or catalyst, to decompose carbon dioxide to carbon and oxygen. Taken together, the above-mentioned works are highly significant and motivating for alternative way of how to catalytically capture carbon dioxide and utilization. In recent years, a few CO2 -related researches and carbon materials investigations have been reported in our group [14–18]. For example, cyclic adsorption and desorption for CO2 separation and capture, dry reforming of methane with carbon dioxide, CO2 mineralization, synthesis of synthetic natural gas (SNG, methane) or methanol from CO2 selective hydrogenation. In 2011–2013, we have developed and reported a new alternative route: newly-developed “chemical vapor deposition integrated process (CVD-IP)” using CO2 as the only raw carbon source to yield solid-form nano carbon materials with good single-pass nano carbon yields [4]. The nano carbon materials gained from CVD-IP assume a crucial role in the evolution where improved technologies and devices are used for sustainable production and the
http://dx.doi.org/10.1016/j.jechem.2015.09.006 2095-4956/© 2015 Science Press and Dalian Institute of Chemical Physics. All rights reserved.
Please cite this article as: J. Hu et al., Carbon dioxide catalytic conversion to nano carbon material on the iron–nickel catalysts using CVD-IP method, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.09.006
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use of renewable energy and it can be utilized in many fields, for example, energy stories and energy conversion [19]. And more significantly, carbon dioxide is only the carbon resource. In this work, we extended our new approach to derive novel nano carbon materials by this CVD-IP technology to utilize carbon dioxide efficiently, with better economic feasibility. Typically, CO2 was introduced as the raw material and only carbon source into the reaction zone, where CO2 could be catalytically activated and then converted to solid-form nano carbon materials. 2. Experimental 2.1. Catalyst preparation The five Fe–Ni–Al based catalysts were prepared via citric acid combustion method using an aqueous solution of corresponding nitrate Fe (NO3 )3 •9H2 O, Ni (NO3 )2 •6H2 O, Al (NO3 )3 •9H2 O and citric acid. And the molar equivalent amount of citric acid is equal to the molar equivalent sum of Fe (III), Ni (II) and Al (III) amounts. In an example of preparation, solutions of nitrates and citric acid were mixed in the required mass ration. And the mixed solution was then stirred continuously for 2 h to form the homogeneous sol. After that, the sol was placed in a water bath pot at 80 °C, until the gel was formed. Next, the gel was dried in an oven at 120 °C for 12 h and then calcinated in a stove under air atmosphere to derive the final samples. In the catalyst samples, the Ni content varied from 0 wt%, 2 wt%, 4 wt%, 8 wt%, to 20 wt%, with the corresponding catalysts labeled as FNi0, FNi2, FNi4, FNi8 and FNi20, respectively. Accordingly, the synthetic carbon materials labeled as FNi0, FNi2, FNi4, FNi8 and FNi20.
β γ
α
625
FNi20
TCD singal (a.u.)
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655
FNi8 663
FNi4
697
FNi2
731
FNi0 100
300
500
700 800 o Temperature ( C)
800
Fig. 1. H2 -TPR profiles of FNix (x = 0, 2, 4, 8, 20) catalyst samples.
argon-ion laser was used for excitation. The instrument version is LabRAM HR800. 3. Results and discussion
2.2. Catalyst characterizations
3.1. H2 -TPR and XRD patterns analysis of the catalysts
The as-prepared fresh catalyst was pre-reduced in flowing H2 (50 mL/min) at 600 °C for 1 h and then executed X-ray diffraction (XRD) measurement. The XRD pattern of each reduced catalyst was measured on a DX-2700 instrument equipped with a Cu Kα radiation (incident wavelength λ = 0.15418 nm). The catalyst was placed on a quartz plate with a concave for sampling and scanned from 2θ value of 10°–85°, and the step-size of 0.03° and accumulation time of 0.5 s were adopted during scanning. The utilized beam voltage and current were 40 kV and 30 mA, respectively. The obtained patterns were analyzed using the MDI Jade 6.5 software with JCPDS data. Temperature-programmed reduction (TPR) experiments were performed on our own home-assembled equipment with a TCD analysis detector. Sample of 50 mg was placed in a quartz tube and then purged with 5 vol% H2 (balanced with N2 ) for 30 min at 100 °C. After that, the catalyst was heated to 800 °C (ramping rate = 5 °C/min) and kept at 800 °C for 60 min. The effluent gas flowed through a colorswitchable silica-gel trap to remove the moisture before being analyzed by a thermal conductivity detector (TCD) and the H2 consumption was calibrated using quantitative reduction of a standard CuO sample. In order to investigate the thermo-stability of the nano carbon material product, TG-DTG and DTA measurements were carried out for each carbon product sample. During the test, the air flow (50 mL/min) was applied as the carrier gas for reacting with the sample, at a heating rate of 10 °C/min in the temperature range of 50–800 °C. Transmission electron microscopy (TEM) images were collected on a Tecnai G2F20 transmission electron microscope of Sichuan University. Before the test, catalysts were ultrasonically dispersed in absolute ethanol for 30 min, and then deposited on a holey copper grid. Scanning electron microscope (SEM) images were obtained on a JEOL JSM-6700F system. The samples of produced nano carbon materials were investigated also by Raman spectroscopy. Radiation of 532 nm from an
The H2 -TPR measurements were performed to assess the reducibility of the catalyst. The data were illustrated in Fig. 1. On one hand, the overall reduction course for the pure Fe2 O3 to Fe includes two chemical processes, as follows [20,21]:
1/2Fe2 O3 + 1/6H2 → 1/3Fe3 O4 + 1/6H2 O
(1)
1/3Fe3 O4 + 4/3H2 → Fe + 4/3H2 O
(2)
During the TPR experiments, the reduction profiles displayed the expected two-step reduction process of Fe2 O3 to metallic iron. For the FNi0 sample without additive, there were mainly two reduction peaks at 410 °C and 731 °C, corresponding to α peak and β peak. There was also a small weak peak (γ peak) arose during isothermal step (at 800 °C). From Fig. 1, when Ni additive was included into the Fe/Al2 O3 system, the reduction curve significantly altered to lower temperature. It suggested that the Ni incorporation enhanced the Fe reducibility in an easier way. For the Fe–Ni bimetallic catalysts, the two main reduction peaks could be assigned to a two-step reduction of Fe2 O3 assisted by nickel. Namely the α peak at 300–475 °C, it was linked with the reduction step of Fe3+ in α -Fe2 O3 to Fe2+ in the mixed Fe2+/3+ of inverse spinel Fe3 O4 structure [22,23]. And the β peak, it was ranged from about 500 to 800 °C, while it was due to the reduction of Fe3 O4 to metallic Fe. With the Ni content increasing, a shift in the onset of the second reduction step was observed in the Ni-promoted catalyst. The peak temperature of β peak gradually shifted by about 8 °C to about 35 °C in the TPR profiles of the Nipromoted catalysts, suggesting the enhanced reducibility, while the β peak shape became much broader. This phenomenon suggested that there existed an interaction between Ni and Fe species, which attenuated the interaction formation between Fe and Al species, thus promoting the Fe reducibility. According to previous literatures [24,25], when nickel with less than 10 wt% loading amount was supported
Please cite this article as: J. Hu et al., Carbon dioxide catalytic conversion to nano carbon material on the iron–nickel catalysts using CVD-IP method, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.09.006
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Table 1. H2 uptake amounts of FNix (x = 0, 2, 4, 8, 20) catalysts measured by H2 -TPR.
α peak
FNi0 FNi2 FNi4 FNi8 FNi20
•
Intensity (a.u.)
H2 uptake (×10−4 mol)
Peak temperature (°C)
H2 uptake (×10−4 mol)
419 411 404 394 384
1.052 (29.23%) 1.085 (25.27%) 0.971 (23.97%) 0.818 (18.43%) 0.433 (7.59%)
731 697 663 655 625
2.546 (57.11%) 2.381 (56.02%) 2.219 (54.80%) 2.935 (66.13%) 4.206 (73.83%)
∇
NiFe 2O4 Fe3O4
⊕
Fe2O3
FNi0
110 Δ Fe-rich Fe-Ni alloy Θ ♦ Fe0.64Ni0.36
Θ Fe 311 ∇
27.86
211
200
28.97
Θ
Θ
•
FNi4
29.82
30 ∇
FNi2
β peak
Peak temperature (°C)
Carbon yield (%)
Sample
•
FNi8
24.40
21.71
0
2
20
10
ΔΘ FNi20
•
200 ♦
⊕
220 ♦
•
0 10
20
30
40
50
60
70
80
4
8
20
Ni content (wt%)
2 theta (degree) Fig. 3. Nano carbon yield in carbon products from CO2 catalytic conversion. Fig. 2. XRD patterns of FNix (x = 0, 2, 4, 8, 20) catalysts after reduction.
on Al2 O3 with high temperature calcination, the Ni–Al2 O3 exhibited mainly one reduction peak located at 700–900 °C, i.e., in the form of the NiAl2 O4 spinel. Table 1 gives the quantitative reducibility results of the samples. It could be seen that the reduction degree of iron species was about 80% for the FNi0 sample. The trend for H2 consumption was distinctly shifted for the α and β peaks of the reduction of catalysts. With the Ni content increasing, the H2 reduction peak gradually increased for β peak but decreased for α peak. Especially, the H2 uptake for sample FNi20 was the more among the samples, suggesting that the promoter significantly improved the reduction of iron oxides after adding nickel additive. In order to authenticate the effect of Ni content on the crystalline phase of catalyst, XRD experiments have been executed. The XRD patterns of the freshly reduced catalysts in the range of 2θ = 10°– 85o were displayed in Fig. 2. A sharp peak was centered at around 2θ = 44.3°, could be observed on the samples, while it was linked with the iron metallic phase. Then, it was displaced gradually leftward with the increment of the Ni content. Based on documented results, this variation could be caused by the generation of strong interaction between the Fe and additive on reduced phase. This could cause an Fe-abundant Fe–Ni alloy formation. The mean grain diameter of the reduced iron could be derived via Scherer Equation using the diffraction line at 44.3°. The calculated mean iron granule diameter was 25.22 m for the FNi0 sample without Ni additive, and it decreased to 10.5–14.3 nm for the nickel promoted samples (FNi2, FNi4, FNi8, and FNi20). The Ni dopant could effectively downsize the iron grain, and such trend can be mainly associated with the strong synergy by the Fe–Ni alloying effect [26]. That is to say, the dispersion of active species was better by adding a certain amount of nickel additive. Meanwhile, the Fe°-assigned diffraction peaks centered at 64.8° and 82.1° were detected for the samples (Ni wt% = 0, 2, 4 and
8 wt%), while it was absent for sample FNi20. Interestingly, two diffraction peaks appeared at about 50.8° and 74.7° for the FNi20 sample. According to previous reports [27–29], these two peaks could be attributed to Ni-rich Fe–Ni alloy. This suggested that only when Ni content reached or excelled a certain quantity, the Ni-rich Fe–Ni alloy would be formed. Combining the TPR and XRD results, this synergy between Ni and Fe species could originate from the Fe–Ni alloying effects. It could be predicted that the enhanced reducibility would in situ produce more available Fe° active sites for the carbon species catalytic decomposition during the CVD-IP process, thus promoting the efficiency of CVDIP technology. 3.2. Nano-carbon materials production from CO2 conversion The monometallic Fe/Al2 O3 (FNi0) and the series of bimetallic FeNi/Al2 O3 (FN2, FN4, FN8 and FN20) catalysts were evaluated for the catalytic conversion of CO2 into carbon materials. The operating condition was moderately at 700 °C and 1 atm. And the computational formula for CO2 conversion was shown as follow:
Carbonyield (% ) =
Mc V × T ÷ 1000 ÷ 22.4 × 12
(3)
where, Mc represents the weight of carbon materials originating from CO2 , V represents volume flux for CO2 , and T is reaction time. As seen from Fig. 3, ∼24.45% gas carbon dioxide could be converted into solid carbon materials by CVD-IP technology when using monometallic catalyst FNi0. Unconventionally, after 2 wt% Ni added into catalyst, the activity was lower than monometallic catalyst FNi0. But, carbon yield gradually ascended with nickel contents increasing. When nickel content was 8 wt%, carbon yield reached a maximum of ca. 30.00%. However, when 20 wt% nickel was introduced or further increase of Ni content, carbon yield was not further increasing and exhibited very slight decline. According to previous characterization
Please cite this article as: J. Hu et al., Carbon dioxide catalytic conversion to nano carbon material on the iron–nickel catalysts using CVD-IP method, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.09.006
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100
Weight (%)
80
FNi0 FNi2 FNi4 FNi8 FNi20
60
40
20
0 0
100
200
300 400 500 600 Temperature (oC)
700
800
Fig. 4. TG curves of the produced nano carbon materials.
Fig. 5. DTA properties of the produced nano carbon materials.
of catalysts, the reason why carbon yield descends was that the interaction between active metal iron and nickel in the catalyst reduction process generated more nickel–iron alloy oxides, such as NiFe2 O4 , and then would expose the limited catalytic active sites on catalyst surfaces. 3.3. TG and DTA and SEM results of formed carbon products Thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA) were performed on the as grown nano carbon materials derived from only carbon resource-carbon dioxide. The TGA provided a straightforward characterization method with which the thermal stability of the carbon materials could be investigated. The experiments were conducted at a heating rate of 10 °C/min up to 800 °C in air 50 mL/min. According to reported Refs. [30,31], the weight loss from 300 °C to 450 °C could be attributed to the amorphous carbon burning during heat treatment while the graphite carbon would combust from 500 °C to 750 °C. In Fig. 4, a tiny tilt occurred with temperature elevation but an obvious weight loss phase didn’t arise. Interestingly, from Fig. 5, there was a broad and vulnerable peak centered at ca. 350 °C. Just as aforementioned, when temperature elevated to 300 °C, amorphous carbon included in samples began to be oxidated and liberating heat. With temperature continuing to rise, the graphite carbon combustion started slightly above 520 °C and completed at near 730 °C, but, there was a slight residue after 750 °C. Obviously,
we speculated that the final residue was catalyst active metal which was encapsulated in hollow carbon material on view of TEM pictures (Fig. 7). The DTA curves of the products indicated that an exothermal process, namely the oxidation of the carbon materials, occurred. As clearly seen, one notable exothermic peak at approximately 630 °C, preceded over that, amorphous carbon exothermic peak rose at 350 °C, was observed for all samples, which was due to the oxidation of graphite carbon [32]. According to the literatures, this nano carbon material consisted almost entirely of sp2 hybrid grapheme-type carbon [18]. According to the TG curves, there was a handful of weight loss occurring at around 400 °C, denoting that there was a bit of amorphous carbon existing in the samples. All in all, results here suggested that the CO2 -derived carbon materials were highly crystalline. The morphology of the as grown nano carbon materials was appraised with SEM, TEM. SEM micrographs of all carbon materials samples were displayed in the Fig. 6. The information about the carbon materials morphology was imparted from SEM images that all of carbon materials presented good growth orientation. These enlarge images showed that synthetic carbon materials possessed appearance of a unique structure which these carbon materials were slim and like-fishbone. The bamboo like structure and bulged wall of the carbon materials could be clearly observed from the TEM images in Fig. 7. The TEM micrographs of the carbon materials indicated that the CO2 -derived carbon materials were carbon nanotubes (CNTs) with metallic nanoparticles encapsulated in CNTs. Meanwhile, from TEM pictures, the CNTs primarily possessed a bamboo-like and bouquetlike structure, distinct from the ordinary open-channel un-doped and/or well-shaped and smooth ektexine of CNTs. The bamboo-like structure of CNTs has previously been reported [33–36]. The mechanism, Terrones et al. proposed, for the growth of bamboo-like CNTs has explained how the special structure formed. In view of the mechanism, they suggested that during the growth of CNTs, on catalyst nanoparticles graphitic layers were produced over the catalyst surfaces, yielding a cone-shaped cup-like structure constituted by closely fitting nested grapheme components with various strains inside the cup. These tensions unbend with a sudden sliding of the cup, immediately followed by the generation of the next graphitic layer on the catalyst underneath the first cap. Such growth processes would not end only after the entire CNT was formed. Van Dommele et al. [37] reported another description, in which there was a direct correlation among the formation of capping, the inherent nature of catalysts, and the active metal species for growth of CNTs. According to their works, our iron-based catalyst for the CCUS with CVD-IP process facilitated the formation of a bamboo-like structure. In addition, there existed a great deal of CO2 and H2 O, and these molecules presented the nature of being weakly oxidative. As a result, in the process of the growth of carbon materials, these oxidants could effectively react with the formed amorphous carbon, and further etched or eroded the tube walls of CNTS formed in situ (as described in Eqs. 3 and 4), leading to a much lower fractions of amorphous carbons.
CO2 + C → 2CO
(4)
H2 O + C → CO + H2
(5)
3.4. Raman spectra results The degree of graphitization and structure of as grown nano carbon materials could be traced with Raman spectroscopy. Raman spectra of CNTs have several characteristic bands which imparted information on the extent of disordering and change in the electronic structure. The Raman spectra of as grown nano carbon materials on catalyst FNi0, FNi2, FNi4, FNi8 and FNi20 are shown in Fig. 8(a). As
Please cite this article as: J. Hu et al., Carbon dioxide catalytic conversion to nano carbon material on the iron–nickel catalysts using CVD-IP method, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.09.006
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Fig. 6. SEM images of samples. (a, b) FNi0, (c, d) FNi4.
Fig. 7. TEM images of samples. (a) FNi0, (b) FNi4.
been seen from the spectra, all spectra mainly exhibited two Raman bands, located at ∼1336 and ∼1564 cm–1 , named as D band and G band, respectively. In the spectrum of carbon materials grown on catalyst FNi8 and FNi20, the D’ band appeared at 1610 cm−1 as a shoulder of G band. The G-band results from C–C stretching in sp2 carbon networks and it could be found in all graphitic carbon Raman spectra. The G band intensity was linked with the degree of graphitization while the D-band and D’ band or other disorder-bands were spectral feature strongly associated with structural disorder of carbon in the carbon network. Especially, the D’ band was mainly due to six-membered ring structure and grapheme edges [38]. Based on the information, we concluded that our CCUS CVD-IP was a ro-
bust/effective strategy for obtaining the nano carbon materials with the higher degree of graphitization. Although the individual bands provide ample information about graphitic structures in the carbon materials, a quantitative measurement of the D-band over the G-band intensity ratio (Id /Ig ) is pervasively used as a valuator in combination with Raman spectra analysis of CNTs [39]. As an estimator of the graphite crystalline length and thus a survey of disorder, we calculated the ratio of Id /Ig of all samples and the results were shown in Fig. 8(b). Fig. 8(b) displays a luminously down trend in the Id /Ig ratio from 0.6859 to 0.5440 of samples FNi0, FNi2, FNi4 and FNi8, which indicated the less degree of disorder in CNTs and a higher degree of crystallinity in the CNTs structure produced. However, the Id /Ig
Please cite this article as: J. Hu et al., Carbon dioxide catalytic conversion to nano carbon material on the iron–nickel catalysts using CVD-IP method, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.09.006
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0.8 1564
(a)
(b)
Intensity (a.u.)
0.7 1610
Id / Ig
1336
0.6
0.5 FNi20 FNi8 FNi4
0.4
FNi2 FNi0
800
1000
1200
1400
1600
1800
2000
0.3
0
2
Raman shift (cm-1)
4
8
20
Ni content (wt%)
Fig. 8. (a) Raman spectra of synthesized nano carbon products, (b) Raman intensity ratio of the D-band and G-band.
ratio did not follow the expected trend but rose again after 20 wt% nickel was added into catalyst, forecasting more amorphous carbon or higher disorder in its structure. From FNi0 to FNi2, the Id /Ig ratio exhibited the most significant decline. This phenomenon could be explained by these catalyst active sites increasing after nickel was added into catalyst. From the H2 -TPR and XRD results, inclusion of nickel into the catalysts notably improved the catalyst reducibility, exposing the more available active sites of the reduced catalysts. Thus, the increased accessibility of active sites could decelerate the depositing rate of carbon species on the single active sites by means of the CVD pathway and consequently decreased the generation of amorphous carbon. 4. Conclusions In summary, the nano carbon materials were synthesized via a novel “chemical vapor deposition integrated process (CVD-IP)” technology based on the CO2 capture-utilization-storage (CCUS). The effects of the dopant content on different properties of CNTs were investigated. After adding the metallic nickel, the reducing property of the active metallic iron got significant improvement. At the CVD condition of 700 °C and 1 bar, when nickel content was 8 wt%, the nano carbon yield reached a good value of about 30%. The TEM and SEM results of typical samples revealed a mainly bamboo-like and bouquetlike structure; meanwhile the pictures showed that the CNTs products were the main solid-form carbon materials. Acknowledgments The authors acknowledge support for this project from the National Natural Science Foundation of China (21476145) and the National 973 Program of Ministry of Sciences and Technologies of China (2011CB201202). We would like to thank J. Deng, M. Liu and D. H. Xie for their assistances on TEM and Raman measurement; meanwhile, we also thank J Deng, H. Xu, Y. Y. Feng, and W. Yang for useful discussion and helps. Final, the authors thank China ChengDa Engineering Co., Ltd. References [1] T.R. Karl, K.E. Trenberth, Science 302 (5651) (2003) 1719. [2] S.-Y. Pan, E.E. Chang, P.-Ch. Chiang, Aerosol. Air Qual. Res 12 (5) (2012) 770.
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Carbon dioxide catalytic conversion to nano carbon material on the iron–nickel catalysts using CVD-IP method Jiaquan Hu a,b, Zhanglong Guo a,c, Wei Chu a,b,c,∗, Le Li a, Tao Lin b,c,∗∗ a
College of Chemical Engineering, Sichuan University, Chengdu 610065, Sichuan, China Key Laboratory Green Chemistry & Technology of Ministry of Education (MOE), College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China c Institute of New Energy and Low-carbon Technology, Sichuan University, Chengdu 610065, Sichuan, China b
a r t i c l e
i n f o
Article history: Received 25 July 2015 Revised 16 September 2015 Accepted 16 September 2015 Available online xxx Keywords: Carbon dioxide utilization Catalytic capture Iron–nickel catalyst Chemical vapor deposition integrated process (CVD-IP) Solid-form nano carbon material
a b s t r a c t The over-consumption of fossil fuels resulted in the large quantity emission of carbon dioxide (CO2 ), which was the main reason for the climate change and more extreme weathers. Hence, it is extremely pressing to explore efficient and sustainable approaches for the carbon-neutral pathway of CO2 utilization and recycling. In our recent works with this context, we developed successfully a novel “chemical vapor deposition integrated process (CVD-IP)” technology to converting robustly CO2 into the value-added solid-form carbon materials. The monometallic FeNi0–Al2 O3 (FNi0) and bimetallic FeNix–Al2 O3 (FNi2, FNi4, FNi8 and FNi20) samples were synthesized and effective for this new approach. The catalyst labeled FNi8 gave the better performance, exhibited the single pass solid carbon yield of 30%. These results illustrated alternative promising cases for the CO2 capture utilization storage (CCUS), by means of the CO2 catalytic conversion into the solid-form nano carbon materials. © 2015 Science Press and Dalian Institute of Chemical Physics. All rights reserved.
1. Introduction World-wide human activities result in discharges of four longlived greenhouse gases: CH4 , CO2 , NO2 and halocarbons, wherein CO2 plays the most important role in the anthropogenic emissions. What’s more, CO2 is responsible for about 70% of the greenhouse effects [1–4]. Just now, the CO2 concentration in atmosphere is close to 400 ppm, which is significantly higher than the pre-industrial level of 280 ppm [1]. In recent years, the rapid rising of carbon dioxide level in the atmosphere has caused many kinds of extreme weathers to be frequently occurred, making the world economy and mankind to suffer from great challenges [5]. Consequently, it is crying-need for hunting effective approaches to reduce CO2 concentration rising in atmosphere and thus mitigate the global warming. To address this issue, world-wide scientists developed various CCUS technologies. At present, several kinds of the CO2 catalytic reduction were studied by chemical method [6], mineralization [7], photochemical method [8,9] and Electrocatalytic conversion [10].
∗ Corresponding author at: College of Chemical Engineering, Sichuan University, Chengdu 610065, Sichuan, China. Tel: +86 28 85400591. ∗∗ Corresponding author at: Key Laboratory Green Chemistry & Technology of Ministry of Education (MOE), College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China. Tel: +86 28 85400591. E-mail addresses: [email protected] (W. Chu), [email protected] (T. Lin).
Motiei et al. reported the technique that CO2 was reacted with metallic magnesium at 1000 °C and 10,000 bar to produce a small amount of CNTs and nested fullerenes [11]. His work indicated that CNTs could be synthesized from CO2 source. Lou et al. employed metallic lithium (Li) as the reductant, supercritical CO2 as the only carbon source to capture CO2 at an operating temperature 550 °C for 10 h [12]; and the result depicted that the product was the highly crystalline carbon nanotubes. Another interesting work was reported by Tamaura and Tabata [13]. Their team adopted more moderate condition, using oxygen deficient ferrites as the reaction medium or catalyst, to decompose carbon dioxide to carbon and oxygen. Taken together, the above-mentioned works are highly significant and motivating for alternative way of how to catalytically capture carbon dioxide and utilization. In recent years, a few CO2 -related researches and carbon materials investigations have been reported in our group [14–18]. For example, cyclic adsorption and desorption for CO2 separation and capture, dry reforming of methane with carbon dioxide, CO2 mineralization, synthesis of synthetic natural gas (SNG, methane) or methanol from CO2 selective hydrogenation. In 2011–2013, we have developed and reported a new alternative route: newly-developed “chemical vapor deposition integrated process (CVD-IP)” using CO2 as the only raw carbon source to yield solid-form nano carbon materials with good single-pass nano carbon yields [4]. The nano carbon materials gained from CVD-IP assume a crucial role in the evolution where improved technologies and devices are used for sustainable production and the
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use of renewable energy and it can be utilized in many fields, for example, energy stories and energy conversion [19]. And more significantly, carbon dioxide is only the carbon resource. In this work, we extended our new approach to derive novel nano carbon materials by this CVD-IP technology to utilize carbon dioxide efficiently, with better economic feasibility. Typically, CO2 was introduced as the raw material and only carbon source into the reaction zone, where CO2 could be catalytically activated and then converted to solid-form nano carbon materials. 2. Experimental 2.1. Catalyst preparation The five Fe–Ni–Al based catalysts were prepared via citric acid combustion method using an aqueous solution of corresponding nitrate Fe (NO3 )3 •9H2 O, Ni (NO3 )2 •6H2 O, Al (NO3 )3 •9H2 O and citric acid. And the molar equivalent amount of citric acid is equal to the molar equivalent sum of Fe (III), Ni (II) and Al (III) amounts. In an example of preparation, solutions of nitrates and citric acid were mixed in the required mass ration. And the mixed solution was then stirred continuously for 2 h to form the homogeneous sol. After that, the sol was placed in a water bath pot at 80 °C, until the gel was formed. Next, the gel was dried in an oven at 120 °C for 12 h and then calcinated in a stove under air atmosphere to derive the final samples. In the catalyst samples, the Ni content varied from 0 wt%, 2 wt%, 4 wt%, 8 wt%, to 20 wt%, with the corresponding catalysts labeled as FNi0, FNi2, FNi4, FNi8 and FNi20, respectively. Accordingly, the synthetic carbon materials labeled as FNi0, FNi2, FNi4, FNi8 and FNi20.
β γ
α
625
FNi20
TCD singal (a.u.)
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655
FNi8 663
FNi4
697
FNi2
731
FNi0 100
300
500
700 800 o Temperature ( C)
800
Fig. 1. H2 -TPR profiles of FNix (x = 0, 2, 4, 8, 20) catalyst samples.
argon-ion laser was used for excitation. The instrument version is LabRAM HR800. 3. Results and discussion
2.2. Catalyst characterizations
3.1. H2 -TPR and XRD patterns analysis of the catalysts
The as-prepared fresh catalyst was pre-reduced in flowing H2 (50 mL/min) at 600 °C for 1 h and then executed X-ray diffraction (XRD) measurement. The XRD pattern of each reduced catalyst was measured on a DX-2700 instrument equipped with a Cu Kα radiation (incident wavelength λ = 0.15418 nm). The catalyst was placed on a quartz plate with a concave for sampling and scanned from 2θ value of 10°–85°, and the step-size of 0.03° and accumulation time of 0.5 s were adopted during scanning. The utilized beam voltage and current were 40 kV and 30 mA, respectively. The obtained patterns were analyzed using the MDI Jade 6.5 software with JCPDS data. Temperature-programmed reduction (TPR) experiments were performed on our own home-assembled equipment with a TCD analysis detector. Sample of 50 mg was placed in a quartz tube and then purged with 5 vol% H2 (balanced with N2 ) for 30 min at 100 °C. After that, the catalyst was heated to 800 °C (ramping rate = 5 °C/min) and kept at 800 °C for 60 min. The effluent gas flowed through a colorswitchable silica-gel trap to remove the moisture before being analyzed by a thermal conductivity detector (TCD) and the H2 consumption was calibrated using quantitative reduction of a standard CuO sample. In order to investigate the thermo-stability of the nano carbon material product, TG-DTG and DTA measurements were carried out for each carbon product sample. During the test, the air flow (50 mL/min) was applied as the carrier gas for reacting with the sample, at a heating rate of 10 °C/min in the temperature range of 50–800 °C. Transmission electron microscopy (TEM) images were collected on a Tecnai G2F20 transmission electron microscope of Sichuan University. Before the test, catalysts were ultrasonically dispersed in absolute ethanol for 30 min, and then deposited on a holey copper grid. Scanning electron microscope (SEM) images were obtained on a JEOL JSM-6700F system. The samples of produced nano carbon materials were investigated also by Raman spectroscopy. Radiation of 532 nm from an
The H2 -TPR measurements were performed to assess the reducibility of the catalyst. The data were illustrated in Fig. 1. On one hand, the overall reduction course for the pure Fe2 O3 to Fe includes two chemical processes, as follows [20,21]:
1/2Fe2 O3 + 1/6H2 → 1/3Fe3 O4 + 1/6H2 O
(1)
1/3Fe3 O4 + 4/3H2 → Fe + 4/3H2 O
(2)
During the TPR experiments, the reduction profiles displayed the expected two-step reduction process of Fe2 O3 to metallic iron. For the FNi0 sample without additive, there were mainly two reduction peaks at 410 °C and 731 °C, corresponding to α peak and β peak. There was also a small weak peak (γ peak) arose during isothermal step (at 800 °C). From Fig. 1, when Ni additive was included into the Fe/Al2 O3 system, the reduction curve significantly altered to lower temperature. It suggested that the Ni incorporation enhanced the Fe reducibility in an easier way. For the Fe–Ni bimetallic catalysts, the two main reduction peaks could be assigned to a two-step reduction of Fe2 O3 assisted by nickel. Namely the α peak at 300–475 °C, it was linked with the reduction step of Fe3+ in α -Fe2 O3 to Fe2+ in the mixed Fe2+/3+ of inverse spinel Fe3 O4 structure [22,23]. And the β peak, it was ranged from about 500 to 800 °C, while it was due to the reduction of Fe3 O4 to metallic Fe. With the Ni content increasing, a shift in the onset of the second reduction step was observed in the Ni-promoted catalyst. The peak temperature of β peak gradually shifted by about 8 °C to about 35 °C in the TPR profiles of the Nipromoted catalysts, suggesting the enhanced reducibility, while the β peak shape became much broader. This phenomenon suggested that there existed an interaction between Ni and Fe species, which attenuated the interaction formation between Fe and Al species, thus promoting the Fe reducibility. According to previous literatures [24,25], when nickel with less than 10 wt% loading amount was supported
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Table 1. H2 uptake amounts of FNix (x = 0, 2, 4, 8, 20) catalysts measured by H2 -TPR.
α peak
FNi0 FNi2 FNi4 FNi8 FNi20
•
Intensity (a.u.)
H2 uptake (×10−4 mol)
Peak temperature (°C)
H2 uptake (×10−4 mol)
419 411 404 394 384
1.052 (29.23%) 1.085 (25.27%) 0.971 (23.97%) 0.818 (18.43%) 0.433 (7.59%)
731 697 663 655 625
2.546 (57.11%) 2.381 (56.02%) 2.219 (54.80%) 2.935 (66.13%) 4.206 (73.83%)
∇
NiFe 2O4 Fe3O4
⊕
Fe2O3
FNi0
110 Δ Fe-rich Fe-Ni alloy Θ ♦ Fe0.64Ni0.36
Θ Fe 311 ∇
27.86
211
200
28.97
Θ
Θ
•
FNi4
29.82
30 ∇
FNi2
β peak
Peak temperature (°C)
Carbon yield (%)
Sample
•
FNi8
24.40
21.71
0
2
20
10
ΔΘ FNi20
•
200 ♦
⊕
220 ♦
•
0 10
20
30
40
50
60
70
80
4
8
20
Ni content (wt%)
2 theta (degree) Fig. 3. Nano carbon yield in carbon products from CO2 catalytic conversion. Fig. 2. XRD patterns of FNix (x = 0, 2, 4, 8, 20) catalysts after reduction.
on Al2 O3 with high temperature calcination, the Ni–Al2 O3 exhibited mainly one reduction peak located at 700–900 °C, i.e., in the form of the NiAl2 O4 spinel. Table 1 gives the quantitative reducibility results of the samples. It could be seen that the reduction degree of iron species was about 80% for the FNi0 sample. The trend for H2 consumption was distinctly shifted for the α and β peaks of the reduction of catalysts. With the Ni content increasing, the H2 reduction peak gradually increased for β peak but decreased for α peak. Especially, the H2 uptake for sample FNi20 was the more among the samples, suggesting that the promoter significantly improved the reduction of iron oxides after adding nickel additive. In order to authenticate the effect of Ni content on the crystalline phase of catalyst, XRD experiments have been executed. The XRD patterns of the freshly reduced catalysts in the range of 2θ = 10°– 85o were displayed in Fig. 2. A sharp peak was centered at around 2θ = 44.3°, could be observed on the samples, while it was linked with the iron metallic phase. Then, it was displaced gradually leftward with the increment of the Ni content. Based on documented results, this variation could be caused by the generation of strong interaction between the Fe and additive on reduced phase. This could cause an Fe-abundant Fe–Ni alloy formation. The mean grain diameter of the reduced iron could be derived via Scherer Equation using the diffraction line at 44.3°. The calculated mean iron granule diameter was 25.22 m for the FNi0 sample without Ni additive, and it decreased to 10.5–14.3 nm for the nickel promoted samples (FNi2, FNi4, FNi8, and FNi20). The Ni dopant could effectively downsize the iron grain, and such trend can be mainly associated with the strong synergy by the Fe–Ni alloying effect [26]. That is to say, the dispersion of active species was better by adding a certain amount of nickel additive. Meanwhile, the Fe°-assigned diffraction peaks centered at 64.8° and 82.1° were detected for the samples (Ni wt% = 0, 2, 4 and
8 wt%), while it was absent for sample FNi20. Interestingly, two diffraction peaks appeared at about 50.8° and 74.7° for the FNi20 sample. According to previous reports [27–29], these two peaks could be attributed to Ni-rich Fe–Ni alloy. This suggested that only when Ni content reached or excelled a certain quantity, the Ni-rich Fe–Ni alloy would be formed. Combining the TPR and XRD results, this synergy between Ni and Fe species could originate from the Fe–Ni alloying effects. It could be predicted that the enhanced reducibility would in situ produce more available Fe° active sites for the carbon species catalytic decomposition during the CVD-IP process, thus promoting the efficiency of CVDIP technology. 3.2. Nano-carbon materials production from CO2 conversion The monometallic Fe/Al2 O3 (FNi0) and the series of bimetallic FeNi/Al2 O3 (FN2, FN4, FN8 and FN20) catalysts were evaluated for the catalytic conversion of CO2 into carbon materials. The operating condition was moderately at 700 °C and 1 atm. And the computational formula for CO2 conversion was shown as follow:
Carbonyield (% ) =
Mc V × T ÷ 1000 ÷ 22.4 × 12
(3)
where, Mc represents the weight of carbon materials originating from CO2 , V represents volume flux for CO2 , and T is reaction time. As seen from Fig. 3, ∼24.45% gas carbon dioxide could be converted into solid carbon materials by CVD-IP technology when using monometallic catalyst FNi0. Unconventionally, after 2 wt% Ni added into catalyst, the activity was lower than monometallic catalyst FNi0. But, carbon yield gradually ascended with nickel contents increasing. When nickel content was 8 wt%, carbon yield reached a maximum of ca. 30.00%. However, when 20 wt% nickel was introduced or further increase of Ni content, carbon yield was not further increasing and exhibited very slight decline. According to previous characterization
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100
Weight (%)
80
FNi0 FNi2 FNi4 FNi8 FNi20
60
40
20
0 0
100
200
300 400 500 600 Temperature (oC)
700
800
Fig. 4. TG curves of the produced nano carbon materials.
Fig. 5. DTA properties of the produced nano carbon materials.
of catalysts, the reason why carbon yield descends was that the interaction between active metal iron and nickel in the catalyst reduction process generated more nickel–iron alloy oxides, such as NiFe2 O4 , and then would expose the limited catalytic active sites on catalyst surfaces. 3.3. TG and DTA and SEM results of formed carbon products Thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA) were performed on the as grown nano carbon materials derived from only carbon resource-carbon dioxide. The TGA provided a straightforward characterization method with which the thermal stability of the carbon materials could be investigated. The experiments were conducted at a heating rate of 10 °C/min up to 800 °C in air 50 mL/min. According to reported Refs. [30,31], the weight loss from 300 °C to 450 °C could be attributed to the amorphous carbon burning during heat treatment while the graphite carbon would combust from 500 °C to 750 °C. In Fig. 4, a tiny tilt occurred with temperature elevation but an obvious weight loss phase didn’t arise. Interestingly, from Fig. 5, there was a broad and vulnerable peak centered at ca. 350 °C. Just as aforementioned, when temperature elevated to 300 °C, amorphous carbon included in samples began to be oxidated and liberating heat. With temperature continuing to rise, the graphite carbon combustion started slightly above 520 °C and completed at near 730 °C, but, there was a slight residue after 750 °C. Obviously,
we speculated that the final residue was catalyst active metal which was encapsulated in hollow carbon material on view of TEM pictures (Fig. 7). The DTA curves of the products indicated that an exothermal process, namely the oxidation of the carbon materials, occurred. As clearly seen, one notable exothermic peak at approximately 630 °C, preceded over that, amorphous carbon exothermic peak rose at 350 °C, was observed for all samples, which was due to the oxidation of graphite carbon [32]. According to the literatures, this nano carbon material consisted almost entirely of sp2 hybrid grapheme-type carbon [18]. According to the TG curves, there was a handful of weight loss occurring at around 400 °C, denoting that there was a bit of amorphous carbon existing in the samples. All in all, results here suggested that the CO2 -derived carbon materials were highly crystalline. The morphology of the as grown nano carbon materials was appraised with SEM, TEM. SEM micrographs of all carbon materials samples were displayed in the Fig. 6. The information about the carbon materials morphology was imparted from SEM images that all of carbon materials presented good growth orientation. These enlarge images showed that synthetic carbon materials possessed appearance of a unique structure which these carbon materials were slim and like-fishbone. The bamboo like structure and bulged wall of the carbon materials could be clearly observed from the TEM images in Fig. 7. The TEM micrographs of the carbon materials indicated that the CO2 -derived carbon materials were carbon nanotubes (CNTs) with metallic nanoparticles encapsulated in CNTs. Meanwhile, from TEM pictures, the CNTs primarily possessed a bamboo-like and bouquetlike structure, distinct from the ordinary open-channel un-doped and/or well-shaped and smooth ektexine of CNTs. The bamboo-like structure of CNTs has previously been reported [33–36]. The mechanism, Terrones et al. proposed, for the growth of bamboo-like CNTs has explained how the special structure formed. In view of the mechanism, they suggested that during the growth of CNTs, on catalyst nanoparticles graphitic layers were produced over the catalyst surfaces, yielding a cone-shaped cup-like structure constituted by closely fitting nested grapheme components with various strains inside the cup. These tensions unbend with a sudden sliding of the cup, immediately followed by the generation of the next graphitic layer on the catalyst underneath the first cap. Such growth processes would not end only after the entire CNT was formed. Van Dommele et al. [37] reported another description, in which there was a direct correlation among the formation of capping, the inherent nature of catalysts, and the active metal species for growth of CNTs. According to their works, our iron-based catalyst for the CCUS with CVD-IP process facilitated the formation of a bamboo-like structure. In addition, there existed a great deal of CO2 and H2 O, and these molecules presented the nature of being weakly oxidative. As a result, in the process of the growth of carbon materials, these oxidants could effectively react with the formed amorphous carbon, and further etched or eroded the tube walls of CNTS formed in situ (as described in Eqs. 3 and 4), leading to a much lower fractions of amorphous carbons.
CO2 + C → 2CO
(4)
H2 O + C → CO + H2
(5)
3.4. Raman spectra results The degree of graphitization and structure of as grown nano carbon materials could be traced with Raman spectroscopy. Raman spectra of CNTs have several characteristic bands which imparted information on the extent of disordering and change in the electronic structure. The Raman spectra of as grown nano carbon materials on catalyst FNi0, FNi2, FNi4, FNi8 and FNi20 are shown in Fig. 8(a). As
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Fig. 6. SEM images of samples. (a, b) FNi0, (c, d) FNi4.
Fig. 7. TEM images of samples. (a) FNi0, (b) FNi4.
been seen from the spectra, all spectra mainly exhibited two Raman bands, located at ∼1336 and ∼1564 cm–1 , named as D band and G band, respectively. In the spectrum of carbon materials grown on catalyst FNi8 and FNi20, the D’ band appeared at 1610 cm−1 as a shoulder of G band. The G-band results from C–C stretching in sp2 carbon networks and it could be found in all graphitic carbon Raman spectra. The G band intensity was linked with the degree of graphitization while the D-band and D’ band or other disorder-bands were spectral feature strongly associated with structural disorder of carbon in the carbon network. Especially, the D’ band was mainly due to six-membered ring structure and grapheme edges [38]. Based on the information, we concluded that our CCUS CVD-IP was a ro-
bust/effective strategy for obtaining the nano carbon materials with the higher degree of graphitization. Although the individual bands provide ample information about graphitic structures in the carbon materials, a quantitative measurement of the D-band over the G-band intensity ratio (Id /Ig ) is pervasively used as a valuator in combination with Raman spectra analysis of CNTs [39]. As an estimator of the graphite crystalline length and thus a survey of disorder, we calculated the ratio of Id /Ig of all samples and the results were shown in Fig. 8(b). Fig. 8(b) displays a luminously down trend in the Id /Ig ratio from 0.6859 to 0.5440 of samples FNi0, FNi2, FNi4 and FNi8, which indicated the less degree of disorder in CNTs and a higher degree of crystallinity in the CNTs structure produced. However, the Id /Ig
Please cite this article as: J. Hu et al., Carbon dioxide catalytic conversion to nano carbon material on the iron–nickel catalysts using CVD-IP method, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.09.006
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Fig. 8. (a) Raman spectra of synthesized nano carbon products, (b) Raman intensity ratio of the D-band and G-band.
ratio did not follow the expected trend but rose again after 20 wt% nickel was added into catalyst, forecasting more amorphous carbon or higher disorder in its structure. From FNi0 to FNi2, the Id /Ig ratio exhibited the most significant decline. This phenomenon could be explained by these catalyst active sites increasing after nickel was added into catalyst. From the H2 -TPR and XRD results, inclusion of nickel into the catalysts notably improved the catalyst reducibility, exposing the more available active sites of the reduced catalysts. Thus, the increased accessibility of active sites could decelerate the depositing rate of carbon species on the single active sites by means of the CVD pathway and consequently decreased the generation of amorphous carbon. 4. Conclusions In summary, the nano carbon materials were synthesized via a novel “chemical vapor deposition integrated process (CVD-IP)” technology based on the CO2 capture-utilization-storage (CCUS). The effects of the dopant content on different properties of CNTs were investigated. After adding the metallic nickel, the reducing property of the active metallic iron got significant improvement. At the CVD condition of 700 °C and 1 bar, when nickel content was 8 wt%, the nano carbon yield reached a good value of about 30%. The TEM and SEM results of typical samples revealed a mainly bamboo-like and bouquetlike structure; meanwhile the pictures showed that the CNTs products were the main solid-form carbon materials. Acknowledgments The authors acknowledge support for this project from the National Natural Science Foundation of China (21476145) and the National 973 Program of Ministry of Sciences and Technologies of China (2011CB201202). We would like to thank J. Deng, M. Liu and D. H. Xie for their assistances on TEM and Raman measurement; meanwhile, we also thank J Deng, H. Xu, Y. Y. Feng, and W. Yang for useful discussion and helps. Final, the authors thank China ChengDa Engineering Co., Ltd. References [1] T.R. Karl, K.E. Trenberth, Science 302 (5651) (2003) 1719. [2] S.-Y. Pan, E.E. Chang, P.-Ch. Chiang, Aerosol. Air Qual. Res 12 (5) (2012) 770.
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Please cite this article as: J. Hu et al., Carbon dioxide catalytic conversion to nano carbon material on the iron–nickel catalysts using CVD-IP method, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.09.006