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Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1
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Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1 N.A.A. Fatah a, A.A. Jalil a,b,*, S. Triwahyono c, N. Yusof c, C.R. Mamat c, S.M. Izan c, M.Y.S. Hamid a, I. Hussain c, R.H. Adnan c, T.A.T. Abdullah a,b, W. Nabgan a a
School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, 81310, UTM Johor Bahru, Johor, Malaysia b Centre of Hydrogen Energy, Institute of Future Energy, Universiti Teknologi Malaysia, 81310, UTM Johor Bahru, Johor, Malaysia c Faculty of Science, Universiti Teknologi Malaysia, 81310, UTM Johor Bahru, Malaysia
highlights
graphical abstract
Highly dispersed Co on KCC-1 enhanced the catalytic activity of CO methanation. 20Co/KCC-1 catalyst exhibited the highest
activity
on
CO
methanation. High intrinsic basicity of 20Co/ KCC-1 enhanced the activity of CO methanation. The 20Co/KCC-1 undergoes both types of associative and dissociative mechanism. Linear CO species on Co metal contributed to high CH4 products.
article info
abstract
Article history:
In this study, the conversion of CO into CH4 was investigated utilizing a series of cobalt
Received 5 November 2019
loaded on fibrous silica (KCC-1) catalysts (Co loading of 5e30 wt%), that were synthesized
Received in revised form
via microemulsion and impregnation techniques. FESEM-EDX and N2 physisorption
24 December 2019
demonstrated that the KCC-1 possessed a spherical structure with fibrous silica dendri-
Accepted 22 January 2020
meric morphology with a superior surface area of 861 m2g-1. A significant decreased in the
Available online xxx
catalyst surface area was noticed upon the addition of Co, suggesting a possible occurrence of KCC-1 pore blockage. Inversely, the number of basic sites on KCC-1 was enhanced after
Keywords:
the incorporation of Co, as observed by pyrrole adsorbed FTIR. At 523 K, bare KCC-1
CO methanation
exhibited a very low activity for CO methanation due to low basicity and the absence of
Fibrous silica KCC-1
surface active sites. The 20Co/KCC-1 demonstrated the best catalytic performance with
* Corresponding author. School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, 81310, UTM Johor Bahru, Johor, Malaysia. E-mail address: [email protected] (A.A. Jalil). https://doi.org/10.1016/j.ijhydene.2020.01.144 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Cobalt loading
72.7% yield of CH4 and 6.8% of CO2. These results were plausibly attributed to the high
Intrinsic basic site
intrinsic number of basic sites and high dispersion of Co on KCC-1 support. A detailed insitu FTIR spectroscopy study revealed that both types of associative and dissociative mechanism pathways significantly contributed to the high catalytic methanation activity. In addition to the dissociative mechanism, the linear CO species adsorbed on the Co metal by associative mechanism was also further hydrogenated to obtain the final CH4 products. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Pioneered by Sabatier and Senderens in 1902, catalytic CO methanation has become a pivotal reaction and has gained widespread attention by researchers [1e3]. The rising interest stems from the fact that the process is one of the simple way to eliminate the presence of CO from the hydrogen feed gas in the fuel cell industry [4]. The catalytic reaction also offers an advantage by producing synthetic natural gas, which is very useful as a substitute for diesel and fuel [5,6], as well as protecting the environment by reusing carbon sources [7]. Many reviews and research studies on CO methanation have been published, which have mainly focused on the aspects of catalyst development [8,9], thermodynamics [5], and reaction mechanisms [10,11]. Regardless of the thermodynamically favored CO methanation process, the presence of a catalyst is crucial in order to acquire an appropriate rate. In general, an active catalyst for CO methanation requires a good metal dispersion and antisintering characteristics. The most commonly used catalyst for the methanation reaction is Ni-based catalyst due to its low cost and high catalytic activity [12e14]. Other than Nibased catalyst, a cobalt (Co) supported catalyst has shown promising performance in CO methanation [15e17]. A Cobased catalyst was able to promote the CO hydrogenation reaction by providing an active phase for H2 dissociation, as well as by activating the CO molecule. Thus, the synergistic effect from the dual functionalities served by the Co metal, which are required for CO methanation, makes the performance of the CO-based catalysts strongly influenced by the metal particles size, as proposed by Bezemer and co-workers in their study of Co supported on carbon nanofiber (Co/ CNF) [18]. In this case, the authors pointed out that the minimum size for the cobalt particles required for CO hydrogenation was in the range of 6e8 nm to form an ideal domain of active sites, suggesting a significant influence of Co particle size for the CO hydrogenation reaction. Another aspect that is always gaining attention in catalytic CO methanation is the mechanism pathway of the reaction, regardless of the impact of the metal particle size obtained in the catalyst system. The mechanism pathway was reported differently for Co-based catalysts using different support materials. In a previous study, Yang et al. have claimed that the mechanism of 20 wt% Co supported on carbon nanotubes followed the hydrogen-assisted CO dissociation pathway [19]. While Tuxen et al. have demonstrated a clear dependence of Co nanoparticle size on the CO dissociation mechanism [20].
Since the support materials are considered to have an important effect on the existence state of the active metal (dispersion and morphology), wide-ranging research has been conducted on high surface area materials for the CO methanation [21,22]. Previously, the activity of CO methanation has been reported on bentonite [23], alumina [24], MCM-41 [25] and HZSM-5 [26]. In 2010, fibrous silica KCC-1 with a unique dendrimeric silica morphology and high surface area had been developed by Polshettiwar et al. [27]. After the development of KCC-1, the material has been applied as a support material in several catalytic reactions such as hydroisomerization of alkane [28], drug delivery [29], and CO2 capture [30]. However, there is no report regarding the application of Co supported KCC-1 catalyst for CO methanation. In this study, the potential of Co supported on KCC-1 with different Co loading for CO methanation has been investigated. The physicochemical properties of the catalysts were further studied to show its relationship with the catalytic activity. The mechanism pathway of CO methanation over the catalyst was elucidated using an in-situ FTIR spectroscopy.
Experimental Preparation of catalyst The synthesis of KCC-1 was conducted via modified microemulsion technique equipped with hydrothermal microwave method [28]. In the procedure, a mixture of cyclohexane and pentanol was prepared at room temperature, followed by a dropwise addition of tetraethyl orthosilicate (TEOS). Then, a separate mixture which consists of urea, cetylpyridinium bromide (CPB) and water was added into the former solution. The solution obtained was mixed until homogenous and transferred into a Teflon bottle, before was subjected to intermittent microwave irradiation (5 h, 393 K). Afterwards, the solid product was centrifuged, oven-dried (383 K, 12 h) and calcined (823 K, 6 h). The final product was denoted as KCC-1. Incipient wetness impregnation technique was applied to synthesized the Co/KCC-1 by varying the Co loading of 5, 10, 20, and 30 wt%. In the procedure, the as-synthesized KCC-1 was dispersed into the Co nitrate aqueous solution (Co(NO3)2.6H2O) mixture and was constantly mix at 333 K until all solution was dried. After that, the solid product was oven-dried (383 K, 12 h) and calcined (823 K, 3 h). The samples were labelled as xCo/KCC-1 (x ¼ 5, 10, 20 and 30 wt % of Co).
Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Characterization of physicochemical properties Crystallinity of the catalyst was identified by X-ray powder diffractometer (Bruker Advance D8) in the region of 2q ¼ 20e80 . The FESEM-EDX was used to study the structural morphology and the trace amount of the element in the catalysts (JEOL JSM-6701F). Textural characteristic of the catalysts was determined using N2 physisorption (SA3100 Beckman Coulter). Hydrogen temperature programmed desorption (H2-TPD) was conducted using Quantachrome Autosorb-1. Prior to the analysis, 300 mg of catalyst was reduced at 773 K for 4 h, followed by outgassing to remove the weakly physisorbed hydrogen. Then, the catalyst was cooled down to 293 K as initial temperature for the H2 chemisorption. FTIR spectroscopy provides information on a specific bond since molecules have specific frequencies of the internal vibration. In this study, FTIR analysis were conducted using an Agilent Cary 640 FTIR spectrometer equipped with a high-temperature stainless steel cell with CaF2 windows. Pyrrole adsorbed FTIR conducted at ambient temperature was applied to identify the intrinsic basicity of the catalysts. Furthermore, in situ FTIR spectroscopy was performed at room temperature by simultaneously adsorbed 2.67 kPa of CO and 7.99 kPa of H2 to the surface of 30 mg catalyst in pelletized form, followed by stepwise heating up to 623 K. Next, the proposed mechanism of the reaction was study based on the interaction of H2 or CO with pre-adsorbed CO or H2, respectively. The sample was treated in vacuum at 623 K for 1 h, followed by exposure to the of H2 or CO and subsequently exposed to 7.99 kPa of H2 or 2.67 kPa of CO, to monitor the interaction occurred from the ambient temperature up to 623 K with the increment of 50 K.
Catalytic activity measurement The catalytic testing was conducted using a fixed bed reactor at 1 bar pressure and temperature range from 423 to 573 K. Firstly, 200 mg catalyst was degassed under oxygen flow (Foxygen ¼ 100 ml/min) (773 K,1 h), reduced under H2 flow (Fhydrogen ¼ 100 ml/min) (773 K, 3 h) and reduced to 423 K. The actual pre-treatment and reaction temperatures were measured by using a thermocouple which was directly inserted into the catalyst bed. Upon reaching the reaction temperature, the H2 and CO was introduced simultaneously to the reactor (H2/CO ¼ 3) at specific gas hourly space velocity (GHSV) of 14500 ml g1 h1. The fraction of the products obtained from the outlet was using gas chromatography (Agilent 6090 N) coupled with a TCD detector. The analysis on the CO conversion, yield and selectivity of products were carried out using Eqs. (1)e(4). XCO ð%Þ ¼
Mx þ My 100 MCO þ Mx þ My
(1)
Sx ð%Þ ¼
Mx 100 Mx þ My
(2)
Yx ð%Þ ¼
XCO Sx 100
(3)
Rate ofCH4 formation ðmmolCH4 =g cat sÞ ¼
nx Wcat s
(4)
where XCO represents the CO conversion (%), S and Y are the selectivity and yield of x and y product (%) in which x and y product are a CH4 and CO2, respectively; M represents a mole of CO or product.
Results and discussion Physicochemical properties The XRD diffractograms of KCC-1 and 20Co/KCC-1 in the wideangle region are plotted in Fig.1A and C, respectively. In particular, both catalysts exhibited a broad hump located at 2q ¼ 20 e28 , suggesting the reflection of the amorphous silica phase [31]. However, 20Co/KCC-1 showed a lower peak intensity, which indicated that the KCC-1 structure was markedly disturbed by the introduction of Co. It was noted that five new peaks emerged at 2q ¼ 31.4, 34.6, 44.6, 59.5, and 75.5 , which reflected the presence of a Co3O4 phase [32]. The peaks were distinguishable due to the high crystallinity of Co metal, as obtained by the Scherrer formula in Table 1. The structural morphology of the catalysts was obtained by FESEM analysis, while the elemental configuration of the metal loaded was observed by FESEM-EDX. Fig. 1A depicts the FESEM illustration of KCC-1, which possessed a porous and uniform spherical structure with a unique dendrimeric morphology [29]. The distance between each unique dendrimer allowed the reactant to easily diffuse, thus facilitating high catalytic performance. Fig. 1B demonstrates the particle size distribution of bare KCC-1, which was mainly distributed in the range of 700e1000 nm. In Fig. 1C, the FESEM image of 20Co/KCC-1 illustrated a similar morphology with the pristine KCC-1. Fig. 1DeF shows the elemental mapping of 20Co/KCC1, where the Co metal was represented as the red spot on the structure of KCC-1 (Fig. 1E). Based on the image, it can be inferred that the Co metal was well dispersed on the surface of the KCC-1. Nitrogen physisorption analysis is the most widely used technique to gather insight into the textural properties of solid materials. Fig. 2 illustrates the N2 physisorption isotherms of the studied catalysts. All KCC-1 based catalysts exhibited type IV isotherms coupled with H3 hysteresis, that commonly observed in mesostructured materials [33]. The hysteresis of the KCC-1 based catalysts was corresponded to the capillary condensation of nitrogen vapour in the slit like pores which originated from the aggregation of particles in plate form [34]. Besides, the existence of intraparticle and interparticle pores in KCC-1 can be verified by the capillary condensation phases located at P/PB ¼ 0.4 and 0.9, respectively. The isotherm and hysteresis loop were identical for bare KCC-1 and Co/KCC-1, demonstrating the same classification as a mesoporous material (Fig. 2BeE). However, at higher P/PB, the second step of capillary condensation was slightly decreased with an increasing number of Co loading, which suggests a possibility of pores blockage by the Co. The textural properties data obtained from the N2 physisorption of all catalysts are listed in
Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Fig. 1 e (A) Wide-angle XRD patterns and FESEM image of KCC-1 catalyst, (B) particle size distributions of KCC-1 catalyst, (C) wide-angle XRD patterns and FESEM image of 20Co/KCC-1 catalyst and (DeF) elemental mapping images of 20Co/KCC-1 catalyst.
Table 1. It was observed that KCC-1 possessed a high surface area of 861 m2g-1 and an average pore width of 6.95 nm. The incorporation of Co has resulted in decreased specific surface area, suggesting a possible blockage of catalysts pores by deposition of Co metal. The introduction of 5 wt% of Co has slightly decreased the average pore diameter into 6.92 nm, suggestion a possible deposition of the metal on the pore mouth. However, further increased of Co loading to 10 wt% has resulted in bigger average pore diameter of 7.04 nm, which plausibly due to the combination of two or more original destructed pores. In particular, further addition of Co up to 30 wt% has decreased the average pore diameter, suggesting a metal blockage of the formerly formed pores. These results are concomitant with the result obtained in the total surface area (Table 1), which demonstrated that the accumulation of
Co metal might occur on the pores of KCC-1 while increasing the Co loading during the synthesis procedure [35]. Detailed information regarding the bonding between KCC1 and Co metal was identified by FTIR spectroscopy, as depicted in Fig. 3A. The KCC-1 exhibited three main bands attributed to asymmetric SieOeSi vibration (1084 cm1), SieOeSi bending (796 cm1) and symmetric stretching of SieOeSi (460 cm1) [36]. A typical absorbance band of SieOeSi vibration was also detected on the Co-doped catalysts, with an additional band positioned at approximately 966 cm1, which ascribed to the vibration of SieOH stretching [37]. In particular, the peak intensity decreased upon raising the Co loading, which was plausibly due to the fractional substitution of the SieOH and SieOeSi by a SieOeCo bond [38]. All Co-loaded KCC-1 showed double bands at 570 and 665 cm1, which
Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Table 1 e Textural and vibrational properties of various cobalt loaded on KCC-1. Catalysts Total Surface Area (m2 g1)a KCC-1 5Co/KCC1 10Co/ KCC-1 20Co/ KCC-1 30Co/ KCC-1 a b c d e f
Average Pore Diameter dCo3 O4 (nm)c NeH Stretching Peak (nm)b Aread
Ea (kJ mol1)e
Co dispersion (%) f
861 620
6.95 6.92
e 10.7
275.1 570.9
e 97.43
e 10.5
556
7.04
11.5
575.1
82.76
11.6
507
6.90
14.9
748.3
62.01
22.3
370
6.69
18.7
651.7
81.55
18.2
Obtained from BET method. Obtained from NLDFT method. The crystallite size of Co3O4 in the catalyst calculated from XRD using Scherer equation. Obtained from gaussian curve fitting based on Fig. 4. Ea values were determined in the range of 523e673 K. Obtained from H2-TPD.
Fig. 2 e N2 physisorption isotherm and NLDFT pore size distribution of (A) KCC-1, (B) 5Co/KCC-1, (C) 10Co/KCC-1, (D) 20Co/ KCC-1, and (E) 30Co/KCC-1.
agreed to CoeO vibrations of the octahedrally coordinated Co3þ and tetrahedrally coordinated Co2þ, respectively [33]. The appearance of these bands inferred that the Co metal was successfully incorporated into the KCC-1 support. The FTIR spectra of evacuated catalyst at 673 K in the hydroxyl range of 3800e3400 cm1 are plotted in Fig. 3B. All catalysts possessed three bands that were ascribed to the terminal hydroxyl groups (3740 cm1), internal hydroxyl groups (3710 cm1) and H-bonded hydroxyl groups (3655 cm1). It was noted that the intensity of the 3470 cm1 band was decreased concomitantly with increasing Co loading, indicating a probable change in the silica framework owing to the interaction with Co3O4 particles [40]. Fig. 3C illustrates the FTIR spectra of adsorbed pyrrole in the region that corresponded to ring-stretching vibrations. Three intense peaks corresponded to non-dissociated pyrrole (1531 and 1423 cm1) and d(OH) (1625 cm1) were observed in all catalysts. A notable decrease was observed in the peak intensity upon increasing the Co loading from 5 to 30 wt%, suggesting a possible replacement of metal into the framework of the silica-based catalyst [41].
The measurement of the intrinsic basicity in all catalysts and its strength was obtained by pyrrole adsorbed FTIR, as illustrated in Fig. 4. All catalysts exhibited signals ascribed to the perturbed NeH stretching in the pyrrole molecule (C4H4NH) in the range of 3650e3250 cm1, which resulted from the interaction of pyrrole with the basic sites in the catalyst (framework oxygen) [42]. An absorption band ascribed to the NeH group from the pyrrole molecules in the gas form was observed at 3530 cm1 (dotted line), while the interaction of the NeH group with the p-system in a different pyrrole molecule can be noted at 3420 cm1, ascribing to the physically adsorbed pyrrole in a liquid-like phase (asterisk symbol) [43]. The amount of basic sites in all catalysts was determined based on the peak area under the band agreeing to NeH stretching which was located at 3450 cm1 (KCC-1), 3447 cm1 (5Co/KCC-1), 3430 cm1 (10Co/KCC-1), 3422 cm1 (20Co/KCC-1), and 3436 cm1 (30Co/KCC-1). The area of the NeH stretching peak of the catalysts was obtained by Gaussian curve-fitting and listed in Table 1. An increasing trend in the number of basic sites was observed after
Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Fig. 3 e (A) IR-KBr spectra, (B) IR spectra after activated at 673 K and (C) IR spectra of pyrrole adsorbed on activated catalysts after outgassing at room temperature in the range of 2200e1300 cm-1 of (a) KCC-1, (b) 5Co/KCC-1, (c) 10Co/KCC-1, (d) 20Co/ KCC-1, and (e) 30Co/KCC-1.
Fig. 4 e IR spectra of pyrrole adsorbed at (a) room temperature on (A) KCC-1, (B) 5Co/KCC-1, (C) 10Co/KCC-1, (D) 20Co/KCC-1, and (E) 30Co/KCC-1 after outgassing at (b) room temperature, (c) 323 K, (d) 373 K, (e) 423 K, (f) 473 K and (g) before exposure to pyrrole.
increasing the loading of Co metal from 5 to 20 wt%. In particular, the KCC-1 and 20Co/KC-1 exhibited the lowest (275.1) and the highest (748.3) peak areas in the number of basic sites, respectively. Further addition of Co loading to 30 wt% has led to a decrease in the peak area (651.7), which was probably due to the agglomeration of metal, thus inhibiting the interaction of the H-donor with the basic oxygen in the catalyst. In this case, it can be concluded that the amount of basic sites for all catalysts were in order of KCC-1 < 5Co/ KCC-1 < 10Co/KCC-1 < 30Co/KCC-1 < 20Co/KCC-1. Based on the pyrrole adsorbed FTIR, the strength of basic sites in the catalyst can be measure by the bathochromic shift of the NeH stretching vibration Dѵ(NH) [44]. As shown in Fig. 4, the shift in NeH stretching vibration for bare KCC-1 was ѵ(NH) ¼ 80 cm1, ѵ(NH) ¼ 83 cm1 for 5Co/KCC-1, ѵ(NH) ¼ 100 cm1 for 10Co/KCC-1, ѵ(NH) ¼ 108 cm1 for 20Co/KCC-1 and ѵ(NH) ¼ 94 cm1 for 30Co/KCC-1. Thus, the
order of basic strength for the catalysts was in order of KCC-1 < 5Co/KCC-1 < 30Co/KCC-1 < 10Co/KCC-1 < 20Co/KCC-1. However, the 30Co/KCC-1 catalyst possessed the weakest basic site strength which was plausibly due to the poor ability of 30Co/KCC-1 to form/preserve its surface active sites and/or metal agglomeration during the synthesis [45].
Catalytic performance The pristine KCC-1 and Co loaded KCC-1 were tested for CO methanation at 423e573 K under atmospheric pressure, and the results obtained are presented in Fig. 5. A very low activity was observed for the bare KCC-1, which might be attributed to the absence of metal as active sites [46]. The presence of metal acts as dissociation or adsorption sites for H2 and CO in catalytic methanation. However, the absence of support material also does not give any catalytic activity, and therefore it can be
Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Fig. 5 e (A) CO conversion, (B) Rate of CH4 formation as a function of the reaction temperature and mass ratio of CO/H2 ¼ 1/3 and (C) Product yield of CO methanation at 573 K as a function of time-on-stream. KCC-1 (,), 5Co/KCC-1 (△), 10Co/KCC-1 (✰), 20Co/KCC-1 (), and 30Co/KCC-1 (⋄).
concluded that both metal and support material are complementary to each other in CO methanation. Besides, the catalytic activity of all Co-loaded KCC-1 was gradually improved as the reaction temperature was raised from 423 K to 573 K. It was observed that 30Co/KCC-1 showed CO conversion started from 523 K and reached 95.5% at 573 K. Furthermore, 20Co/ KCC-1 reached 79.5% conversion at 573 K. Meanwhile, 10Co/ KCC-1 and 5Co/KCC-1 only gave CO conversion of 10.3% and 6.9%, respectively. Fig. 5C shows the distribution of CH4 and CO2 obtained from the CO methanation over Co-loaded KCC-1 at 573 K. It was observed that only slight production of CH4 and CO2 were obtained from the bare KCC-1. The yields of CH4 and CO2 were gradually and significantly increased when the loading of Co was raised from 5 wt% to 20 wt%. The yield of CH4 and CO2 obtained over 5Co/KCC-1, 10Co/KCC-1, 20Co/KCC-1 and 30Co/ KCC-1 were 6.6% and 0.3%, 9.8% and 0.5%, 72.7% and 6.8%, and 72.5% and 23.0%, respectively. In particular, a slight decreased in the yield of CH4 was noticed in 30Co/KCC-1 with an enhancement in CO2 yield. The result was plausibly due to the agglomeration of Co metal that might have deteriorated the active sites for the dissociation of H2 and hydrogenation of CO at the surface of the catalyst. Besides, rapid deactivation of 30Co/KCC-1 might have originated from a poor ability to create or to preserve the active basic sites as demonstrated by IR adsorbed pyrrole analysis. The 20Co/KCC-1 showed the best performance in CO methanation, which is possibly due to the occurrence of an appropriate amount of intrinsic basic sites and the optimum amount of Co loading. The Arrhenius equation was applied to determine the apparent activation energy (Ea) for all Co/KCC-1 catalysts and the values obtained are listed in Table 1. The Ea values for 5Co/KCC-1, 10Co/KCC-1, 20Co/KCC-1 and 30Co/
KCC-1 were determined as 97.4 kJ/mol, 82. kJ mol1, 62.0 kJ/ mol and 81.5 kJ/mol, respectively. However, the value of Ea for KCC-1 could not be determined as it exhibited very low activity in the CO methanation reaction, compared to the Co supported KCC-1. Based on the table, 20Co/KCC-1 exhibited the lowest activation energy (Ea), which was plausibly due to the well dispersed Co metal and suitable intrinsic basic sites. Thus, 20Co/KCC-1 possessed good catalytic activity for CO methanation reaction among the Co/KCC-1 type catalysts. The obtained Ea is comparable with those observed for the silica supported catalyst as reported by Furman et al.(2015), in which the Ea for RueSiO2 is 78 ± 1 kJ/mol for the CO methanation reaction [47]. The stability tests of all Co supported KCC-1 catalysts are presented in Fig. S3, where the conversion of CO was plotted as a function of time. The reaction was conducted at 573 K, H2/CO ¼ 3 and GHSV of 14500 ml g1 h1. It was observed that most of the Co/KCC-1 catalyst could retain their CO conversion after 72 h, except for the 30Co/KCC-1. The significant reduction in CO conversion over 30Co/KCC-1 was observed after 42 h, which plausibly due to the metal sintering. This is in agreement with the calculated metal crystallite size (Table 1), where 30Co/KCC-1 exhibited the largest Co3O4 crystallite size of 18.7 nm. Based on the literature reviews in Table 2, the catalytic activity of the Co-loaded KCC-1 is comparable with the recently studied catalysts. The CO methanation over 20 wt% Co loaded ZrO2 produced only 30% CO conversion and 54.4% selectivity of CH4 [48]. Despite low reaction temperature (473 K) and the high number of Co loaded, the yield of CH4 (16.3%) was lower than 10Co/KCC-1 (47.9%). This result was due to the low reducibility of Co metal in Co/ZrO2 which led to a lower surface of the active Co0 phase. In addition, the high activity can be observed on 10% Ni/MCM-41 with 100% CO
Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Table 2 e Comparison study of Co-promoted catalyst for CO methanation. Catalyst
Catalytic performance [%]
Reaction conditions
CO conversion
CH4 selectivity
CH4 yield
Temperature [K]
Pressure [MPa]
0.1 6.9 10.3 79.3 95.5 30.0 99.8 100.0 87.0
100 96.3 95.3 92.6 75.9 54.4 89.2 85.0 14.0
0.1 6.6 9.8 73.4 72.5 16.3 89.0 85.0 12.2
573 573 573 573 573 473 673 623 533
0.1 0.1 0.1 0.1 0.1 0.1 1.0 0.1 1.0
KCC-1 5Co/KCC-1 10Co/KCC-1 20Co/KCC-1 30Co/KCC-1 20% CoeZrO2 10% Ni/SBA-15 10% Ni/MCM-41 10% Co/SiO2
Reference
This This This This This [48] [50] [49] [51]
study study study study study
conversion, 85% of CH4 selectivity and 85% of CH4 yield at 623 K [49]. It was reported that the addition of nickel into MCM-41 has increased the metal dispersion and upsurge the catalyst basicity. On the other hand, 10%Ni/SBA-15 also possessed high catalytic activity in CO conversion (99.8%) with a high yield of CH4 (89.0%), which is much higher than 20Co/ KCC-1 obtained in this study [50]. This might be due to the presence of 1.0 MPa pressure in this reaction which helps to enhance the catalytic performance. Furthermore, the application of 10%Co/SiO2 on CO methanation reaction was also reported, which produced 87.0% CO conversion, 14% CH4 selectivity and 12.2% yield of CH4 at 493 K [51]. These results were much lower than that of 20Co/KCC-1 which gave 97.4% CO conversion, 90.3% selectivity of CH4 and 87.8% yield of CH4 at 673 K. This result was plausibly attributed to the high surface area of 20Co/KCC-1 that facilitated the dispersion of Co metal, and provided more accessible active sites [52].
information on the species that might be formed during the mechanism of CO methanation. By flowing the CO gas, two peaks were noted at 2170 and 2110 cm1 which can be assigned as gaseous CO. While no peak was observed during the flow of H2 gas. Fig. 6A illustrates the FTIR spectra of the pre-adsorbed CO on 20Co/KCC-1 after exposure to H2. An intense peak was noted at 1625 cm1, which can be attributed to atomic hydrogen [43]. At 523 K, intensification of two bands was notified at 2360 and 2340 cm1, which can be assigned to gaseous CO2, along with the formation of two peaks at 2110 and 2170 cm1, which were attributed to the gaseous CO. According to the thermodynamics study, the formation of byproduct CO2 might be originated from the following reaction [5,54]: 2CO þ 2H2 /CO2 þ CH4
(5)
Proposed mechanism
2CO / CO2 þ C
(6)
The reaction mechanism pathway of CO methanation is an ongoing topic for discussion that is often disputed by most researchers who are experts in the heterogeneous catalysis area. Although there are several studies on the mechanism of CO methanation, it still becomes a crucial topic to be resolved. Based on previous studies, the proposed mechanism pathway for CO methanation can be divided into associative and dissociative schemes. The associative scheme involves the combination of adsorbed H2 and CO to produce intermediates (COH, CHO and CHOH), which subsequently undergo the dissociation of a CeO bond. While the dissociative scheme involves the direct dissociation of adsorbed CO to form a surface carbon (C), that acts as an intermediate before the adsorption of H2 occurs to undergo the methanation reaction [53]. In this current study, the interaction between CO and H2 were studied to reveal the plausible route of the reaction mechanism for the CO methanation reaction for KCC-1 and 20Co/KCC-1 catalyst via in-situ FTIR adsorption spectroscopy. The mechanistic reaction pathway for CO methanation was clarified based on in situ FTIR for the interaction of H2 or CO with pre-adsorbed CO or H2. The 20Co/KCC-1 catalyst was chosen due to its good performance in terms of CH4 yield obtained from the catalytic reaction part. Prior to the in-situ FTIR, a blank test reaction (without the presence of a catalyst) of gaseous CO, H2 and CO2 was conducted to obtain important
CO þ H2O / CO2 þ H2
(7)
In this study, the formation of CO2 was plausibly due to the reversed methane reforming (Equation (5)) and Boudouard reaction of CO (Equation (6)) rather than water gas shift reaction (Equation (7)), since the formation of CO2 was observed at temperature lower than 723 K. Besides, two peaks were also notified at 1510 and 1420 cm1, which corresponded to the asymmetric and symmetric stretching vibration of the carbonate species in bidentate form, respectively. Specifically, the peaks intensity of the carbonate species was significantly increased by increasing the temperature up to 623 K. This was plausibly due to the dissociation of gaseous CO, followed by its migration to form an intermediate (COOad) before further hydrogenation with H2 to produce CH4. Meanwhile, the study on the interaction of CO with preadsorbed H2 was elucidated based on Fig. 6B. It was observed that after the temperature was increased from 323 to 623 K, the band at 2170 and 2110 cm1 which can be attributed to gaseous CO, were reduced with a concomitant increase of gaseous CO2 peaks at 2360 and 2340 cm1. The peak for gaseous CO2 was gradually intensified at a higher temperature, suggesting a possible formation of an undesired CO2 byproduct that originated from the reversed methane reforming.
Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Fig. 6 e Evaluation of FTIR spectra of (A) interaction of H2 with pre-adsorbed CO and (B) interaction of CO with pre-adsorbed H2 on 20Co/KCC-1. The samples were (a) adsorption of H2 or CO at room temperature and heating up to (b) 323 K, (c) 373 K, (d) 423 K, (e) 473 K, (f) 523 K, (g) 573 K and (h) 623 K.
Furthermore, a sharp peak was observed at 1625 cm1, which was attributed to the atomic hydrogen peak. The intensity of this peak was slightly increased while increasing the temperature up to 623 K due to the high ability of the catalyst to dissociate H2 molecule to form atomic H [43]. Besides, a minor peak that corresponded to the SieOeSi vibration of the KCC-1 support could be observed at 1870 cm1 [55]. Meanwhile, the existence of the peak at 1970 cm1 indicates the adsorption of atomic H on the surface of the catalysts [56,57]. Moreover, the intensity of the peaks at 1510 and 1420 cm1 was increased with a gradual increase of temperature up to 623 K, which was plausibly due to the strong ability of atomic H to dissociate gaseous CO to produce CH4. In order to determine the main CO methanation mechanism pathway of 20Co/KCC-1 and the role of Co metal in the reaction, the in situ FTIR of CO þ H2 was conducted on the KCC1 and 20Co/KCC-1. The spectra obtained are presented in Fig. 7. In can be notified that both catalysts exhibited two peaks assigned to CO2 at 2360 and 2340 cm1. However, these peaks were more intense in KCC-1 compared to 20Co/KCC-1 at higher temperature range (523e623 K) which was plausibly
due to the presence of oxygen vacancy property possessed by KCC-1 that acted as an adsorption site for CO2 produced from the reversed methane reforming and the CO disproportionate reaction [54]. In particular, the peaks were formed earlier in 20Co/KCC-1 (starting at 373 K), suggesting that the presence of Co metal has enabled the dissociation of both CO and H2 molecules at low temperatures. Based on Fig. 7A and B, both catalysts also exhibited a peak at 1870 cm1 which can be assigned to the SieOeSi vibration in KCC-1 support [55]. However, this peak was smaller in the 20Co/KCC-1 catalyst, which could be due to the partial substitution of SieOeSi with the Co metal to form the SieOeCo vibration. Besides, both catalysts also possessed a very sharp and narrow peak at 1625 cm1 which can be deduced to be atomic hydrogen. The peak was noticeably more intense in 20Co/KCC-1 compared to KCC-1, suggesting the role of Co metal to dissociate H2 molecule to form atomic H [58]. Besides, both catalysts showed the existence of three peaks at 2170, 2110 and 1970 cm1 which indicated the adsorption band of linear or bridging gaseous CO on the surface of catalysts, respectively [56,57]. In KCC-1, those peaks were distinctively increased up to 473 K,
Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Fig. 7 e Evaluation of the FTIR spectra of adsorbed gases (CO þ H2) on (A) KCC-1 and (B) 20Co/KCC-1, (C) CH4 band at 3014 cm1 and (D) CeH deformation mode at 1300 cm-1 in 20Co/KCC-1. The catalysts were heated up to (a) room temperature, (b) 323 K, (c) 373 K, (d) 423 K, (e) 473 K, (f) 523 K, (g) 573 K, and (h) 623 K.
followed by a consistent reduction until 623 K, which can be explained by the emergence of a new peak at 2340 cm1, assigned to gaseous CO2. However, based on Fig. 7B, the intensity of gaseous CO peaks were significantly reduced as the temperature was increased till 573 K and had nearly disappeared at 623 K, which may be due to the adsorption of gaseous CO on the surface of Co metal in bridging and tri-fold configurations [56]. Furthermore, a new peak can be notified at 2050 cm1 that corresponded to the interaction of Co0 metal with a carbonyl species (Co0 e CO) on 20Co/KCC-1 [59]. However, the peak at 2050 cm1 shifted to 2020 cm1 at higher temperatures due to the destabilization of the Co0eCO bond on Co0 sites [43,58]. The intensity of the peak at 2020 cm1 significantly increased up to 623 K, which was plausibly due to the dissociation of gaseous CO to form adsorbed carbon (Cad)and adsorbed oxygen (Oad) species on the surface of the Co metal [10].
Previously, Panagiotopoulou et al. reported on the adsorption of gaseous CO onto the surface of RueTiO2 catalyst in which the gaseous CO was dissociated before hydrogenation with H atom to form methane [60]. In addition, two new peaks at 1510 and 1350 cm1 appeared on 20Co/KCC-1, which corresponded to the asymmetric and symmetric stretching vibrations of the surface bidentate carbonate species (vsCOO and vasCOO), respectively [54]. Those peaks only appeared on 20Co/KCC-1 due to the migration of the Cad and Oad species that had occurred from the surface of the Co metal to surface of supporting materials [43]. In particular, the intensity of the peak at 1350 cm1 was more visible, suggesting a progressive formation of bidentate carbonate species as an intermediate on the surface of KCC-1. While, the dissociation of H2 was expected to occur simultaneously and started to hydrogenate the intermediate of bidentate carbonate species to form CH4, which was confirmed by the presence of sharp peaks at 3014
Scheme 1 e Plausible reaction mechanism of CO methanation over Co/KCC-1. Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Table 3 e Mechanism pathway of carbon monoxide methanation over homogeneous catalysts. Catalyst
Mechanism pathway
Amount of metal
Reference
RueTiO2 Ni/ɤAl2O3 Ru (0001) Ru/Al2O3
Associative þ Dissociative Dissociative Dissociative Associative
5 wt% of Ru 50 wt% of Ni e 5 wt% of Ru
[60] [61] [62] [63]
and 1300 cm1 (Fig. 7C and D), which corresponded to the peak of the CH4 band and CeH deformation modes, [which can be attributed to gaseous CO2, become more distinct with increasing temperatures starting from 523 K to 623 K. Furthermore, from the in situ FTIR adsorption spectra of CO þ H2 (Fig. 7A and B), it can be concluded that the mechanism pathway of the CO methanation reaction over KCC-1 catalyst followed the associative scheme. In the case of 20Co/KCC-1, both the associative and dissociative types of mechanisms were observed during the reaction. The dissociative mechanism was distinctly observed starting from the temperature range of 523 Ke623 K, where the formation of CH4 was initially observed at 3014 cm1. Nevertheless, the associative mechanism of CO methanation also significantly contributed to the formation of CH4, as the adsorbed species of COeCO0 in linear form was also observed in 20Co/KCC-1 (Scheme 1). According to the literature, the divergences in the CO methanation mechanisms between the associative and dissociative methanation are closely associated with the reaction conditions and the composition of catalyst component [10]. Table 3 summarizes several types of previously reported catalysts and their corresponding mechanism pathway in CO methanation. A previous DRIFT study on CO methanation using Ni/ɤAl2O3 revealed that the catalyst portrayed a dissociation mechanism where the C and H atomic species were formed by the dissociation of CO and H2 on Ni [61]. Besides, a dissociative CO mechanism was also observed with a flat Ru (0001) surface as the catalyst [62]. In contrast, the associative CO mechanism has been reported on several types of catalyst with low metal loadings, such as Ru/TiO2 [60] and Ru/Al2O3 [63].
Conclusion Highly dispersed Co loaded on KCC-1 catalyst with dendrimeric morphology was successfully synthesized for CO methanation. The catalyst with a 20 wt% of Co exhibited superior performance in the reaction with a 72.7% yield of CH4 and low formation of CO2 compared to other Co loading. The superior performance of the 20Co/KCC-1 could be explained by its high amount of basic sites, as well as the well-dispersed Co metal on the KCC-1 that could avoid metal sintering. A detailed in-situ FTIR spectroscopy study revealed that both the associative and dissociative mechanism pathways made significant contributions to the high catalytic methanation activity. In addition to the dissociative mechanism, the linear CO species adsorbed on the Co metal by the associative mechanism was further hydrogenated to obtain the final CH4 products.
Acknowledgement This work was supported by the Professional Development Research University grant from Universiti Teknologi Malaysia (Grant No.04E07) and Research University Grant from Universiti Teknologi Malaysia (Grant No.19H04).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.01.144.
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Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1 N.A.A. Fatah a, A.A. Jalil a,b,*, S. Triwahyono c, N. Yusof c, C.R. Mamat c, S.M. Izan c, M.Y.S. Hamid a, I. Hussain c, R.H. Adnan c, T.A.T. Abdullah a,b, W. Nabgan a a
School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, 81310, UTM Johor Bahru, Johor, Malaysia b Centre of Hydrogen Energy, Institute of Future Energy, Universiti Teknologi Malaysia, 81310, UTM Johor Bahru, Johor, Malaysia c Faculty of Science, Universiti Teknologi Malaysia, 81310, UTM Johor Bahru, Malaysia
highlights
graphical abstract
Highly dispersed Co on KCC-1 enhanced the catalytic activity of CO methanation. 20Co/KCC-1 catalyst exhibited the highest
activity
on
CO
methanation. High intrinsic basicity of 20Co/ KCC-1 enhanced the activity of CO methanation. The 20Co/KCC-1 undergoes both types of associative and dissociative mechanism. Linear CO species on Co metal contributed to high CH4 products.
article info
abstract
Article history:
In this study, the conversion of CO into CH4 was investigated utilizing a series of cobalt
Received 5 November 2019
loaded on fibrous silica (KCC-1) catalysts (Co loading of 5e30 wt%), that were synthesized
Received in revised form
via microemulsion and impregnation techniques. FESEM-EDX and N2 physisorption
24 December 2019
demonstrated that the KCC-1 possessed a spherical structure with fibrous silica dendri-
Accepted 22 January 2020
meric morphology with a superior surface area of 861 m2g-1. A significant decreased in the
Available online xxx
catalyst surface area was noticed upon the addition of Co, suggesting a possible occurrence of KCC-1 pore blockage. Inversely, the number of basic sites on KCC-1 was enhanced after
Keywords:
the incorporation of Co, as observed by pyrrole adsorbed FTIR. At 523 K, bare KCC-1
CO methanation
exhibited a very low activity for CO methanation due to low basicity and the absence of
Fibrous silica KCC-1
surface active sites. The 20Co/KCC-1 demonstrated the best catalytic performance with
* Corresponding author. School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, 81310, UTM Johor Bahru, Johor, Malaysia. E-mail address: [email protected] (A.A. Jalil). https://doi.org/10.1016/j.ijhydene.2020.01.144 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Cobalt loading
72.7% yield of CH4 and 6.8% of CO2. These results were plausibly attributed to the high
Intrinsic basic site
intrinsic number of basic sites and high dispersion of Co on KCC-1 support. A detailed insitu FTIR spectroscopy study revealed that both types of associative and dissociative mechanism pathways significantly contributed to the high catalytic methanation activity. In addition to the dissociative mechanism, the linear CO species adsorbed on the Co metal by associative mechanism was also further hydrogenated to obtain the final CH4 products. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Pioneered by Sabatier and Senderens in 1902, catalytic CO methanation has become a pivotal reaction and has gained widespread attention by researchers [1e3]. The rising interest stems from the fact that the process is one of the simple way to eliminate the presence of CO from the hydrogen feed gas in the fuel cell industry [4]. The catalytic reaction also offers an advantage by producing synthetic natural gas, which is very useful as a substitute for diesel and fuel [5,6], as well as protecting the environment by reusing carbon sources [7]. Many reviews and research studies on CO methanation have been published, which have mainly focused on the aspects of catalyst development [8,9], thermodynamics [5], and reaction mechanisms [10,11]. Regardless of the thermodynamically favored CO methanation process, the presence of a catalyst is crucial in order to acquire an appropriate rate. In general, an active catalyst for CO methanation requires a good metal dispersion and antisintering characteristics. The most commonly used catalyst for the methanation reaction is Ni-based catalyst due to its low cost and high catalytic activity [12e14]. Other than Nibased catalyst, a cobalt (Co) supported catalyst has shown promising performance in CO methanation [15e17]. A Cobased catalyst was able to promote the CO hydrogenation reaction by providing an active phase for H2 dissociation, as well as by activating the CO molecule. Thus, the synergistic effect from the dual functionalities served by the Co metal, which are required for CO methanation, makes the performance of the CO-based catalysts strongly influenced by the metal particles size, as proposed by Bezemer and co-workers in their study of Co supported on carbon nanofiber (Co/ CNF) [18]. In this case, the authors pointed out that the minimum size for the cobalt particles required for CO hydrogenation was in the range of 6e8 nm to form an ideal domain of active sites, suggesting a significant influence of Co particle size for the CO hydrogenation reaction. Another aspect that is always gaining attention in catalytic CO methanation is the mechanism pathway of the reaction, regardless of the impact of the metal particle size obtained in the catalyst system. The mechanism pathway was reported differently for Co-based catalysts using different support materials. In a previous study, Yang et al. have claimed that the mechanism of 20 wt% Co supported on carbon nanotubes followed the hydrogen-assisted CO dissociation pathway [19]. While Tuxen et al. have demonstrated a clear dependence of Co nanoparticle size on the CO dissociation mechanism [20].
Since the support materials are considered to have an important effect on the existence state of the active metal (dispersion and morphology), wide-ranging research has been conducted on high surface area materials for the CO methanation [21,22]. Previously, the activity of CO methanation has been reported on bentonite [23], alumina [24], MCM-41 [25] and HZSM-5 [26]. In 2010, fibrous silica KCC-1 with a unique dendrimeric silica morphology and high surface area had been developed by Polshettiwar et al. [27]. After the development of KCC-1, the material has been applied as a support material in several catalytic reactions such as hydroisomerization of alkane [28], drug delivery [29], and CO2 capture [30]. However, there is no report regarding the application of Co supported KCC-1 catalyst for CO methanation. In this study, the potential of Co supported on KCC-1 with different Co loading for CO methanation has been investigated. The physicochemical properties of the catalysts were further studied to show its relationship with the catalytic activity. The mechanism pathway of CO methanation over the catalyst was elucidated using an in-situ FTIR spectroscopy.
Experimental Preparation of catalyst The synthesis of KCC-1 was conducted via modified microemulsion technique equipped with hydrothermal microwave method [28]. In the procedure, a mixture of cyclohexane and pentanol was prepared at room temperature, followed by a dropwise addition of tetraethyl orthosilicate (TEOS). Then, a separate mixture which consists of urea, cetylpyridinium bromide (CPB) and water was added into the former solution. The solution obtained was mixed until homogenous and transferred into a Teflon bottle, before was subjected to intermittent microwave irradiation (5 h, 393 K). Afterwards, the solid product was centrifuged, oven-dried (383 K, 12 h) and calcined (823 K, 6 h). The final product was denoted as KCC-1. Incipient wetness impregnation technique was applied to synthesized the Co/KCC-1 by varying the Co loading of 5, 10, 20, and 30 wt%. In the procedure, the as-synthesized KCC-1 was dispersed into the Co nitrate aqueous solution (Co(NO3)2.6H2O) mixture and was constantly mix at 333 K until all solution was dried. After that, the solid product was oven-dried (383 K, 12 h) and calcined (823 K, 3 h). The samples were labelled as xCo/KCC-1 (x ¼ 5, 10, 20 and 30 wt % of Co).
Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Characterization of physicochemical properties Crystallinity of the catalyst was identified by X-ray powder diffractometer (Bruker Advance D8) in the region of 2q ¼ 20e80 . The FESEM-EDX was used to study the structural morphology and the trace amount of the element in the catalysts (JEOL JSM-6701F). Textural characteristic of the catalysts was determined using N2 physisorption (SA3100 Beckman Coulter). Hydrogen temperature programmed desorption (H2-TPD) was conducted using Quantachrome Autosorb-1. Prior to the analysis, 300 mg of catalyst was reduced at 773 K for 4 h, followed by outgassing to remove the weakly physisorbed hydrogen. Then, the catalyst was cooled down to 293 K as initial temperature for the H2 chemisorption. FTIR spectroscopy provides information on a specific bond since molecules have specific frequencies of the internal vibration. In this study, FTIR analysis were conducted using an Agilent Cary 640 FTIR spectrometer equipped with a high-temperature stainless steel cell with CaF2 windows. Pyrrole adsorbed FTIR conducted at ambient temperature was applied to identify the intrinsic basicity of the catalysts. Furthermore, in situ FTIR spectroscopy was performed at room temperature by simultaneously adsorbed 2.67 kPa of CO and 7.99 kPa of H2 to the surface of 30 mg catalyst in pelletized form, followed by stepwise heating up to 623 K. Next, the proposed mechanism of the reaction was study based on the interaction of H2 or CO with pre-adsorbed CO or H2, respectively. The sample was treated in vacuum at 623 K for 1 h, followed by exposure to the of H2 or CO and subsequently exposed to 7.99 kPa of H2 or 2.67 kPa of CO, to monitor the interaction occurred from the ambient temperature up to 623 K with the increment of 50 K.
Catalytic activity measurement The catalytic testing was conducted using a fixed bed reactor at 1 bar pressure and temperature range from 423 to 573 K. Firstly, 200 mg catalyst was degassed under oxygen flow (Foxygen ¼ 100 ml/min) (773 K,1 h), reduced under H2 flow (Fhydrogen ¼ 100 ml/min) (773 K, 3 h) and reduced to 423 K. The actual pre-treatment and reaction temperatures were measured by using a thermocouple which was directly inserted into the catalyst bed. Upon reaching the reaction temperature, the H2 and CO was introduced simultaneously to the reactor (H2/CO ¼ 3) at specific gas hourly space velocity (GHSV) of 14500 ml g1 h1. The fraction of the products obtained from the outlet was using gas chromatography (Agilent 6090 N) coupled with a TCD detector. The analysis on the CO conversion, yield and selectivity of products were carried out using Eqs. (1)e(4). XCO ð%Þ ¼
Mx þ My 100 MCO þ Mx þ My
(1)
Sx ð%Þ ¼
Mx 100 Mx þ My
(2)
Yx ð%Þ ¼
XCO Sx 100
(3)
Rate ofCH4 formation ðmmolCH4 =g cat sÞ ¼
nx Wcat s
(4)
where XCO represents the CO conversion (%), S and Y are the selectivity and yield of x and y product (%) in which x and y product are a CH4 and CO2, respectively; M represents a mole of CO or product.
Results and discussion Physicochemical properties The XRD diffractograms of KCC-1 and 20Co/KCC-1 in the wideangle region are plotted in Fig.1A and C, respectively. In particular, both catalysts exhibited a broad hump located at 2q ¼ 20 e28 , suggesting the reflection of the amorphous silica phase [31]. However, 20Co/KCC-1 showed a lower peak intensity, which indicated that the KCC-1 structure was markedly disturbed by the introduction of Co. It was noted that five new peaks emerged at 2q ¼ 31.4, 34.6, 44.6, 59.5, and 75.5 , which reflected the presence of a Co3O4 phase [32]. The peaks were distinguishable due to the high crystallinity of Co metal, as obtained by the Scherrer formula in Table 1. The structural morphology of the catalysts was obtained by FESEM analysis, while the elemental configuration of the metal loaded was observed by FESEM-EDX. Fig. 1A depicts the FESEM illustration of KCC-1, which possessed a porous and uniform spherical structure with a unique dendrimeric morphology [29]. The distance between each unique dendrimer allowed the reactant to easily diffuse, thus facilitating high catalytic performance. Fig. 1B demonstrates the particle size distribution of bare KCC-1, which was mainly distributed in the range of 700e1000 nm. In Fig. 1C, the FESEM image of 20Co/KCC-1 illustrated a similar morphology with the pristine KCC-1. Fig. 1DeF shows the elemental mapping of 20Co/KCC1, where the Co metal was represented as the red spot on the structure of KCC-1 (Fig. 1E). Based on the image, it can be inferred that the Co metal was well dispersed on the surface of the KCC-1. Nitrogen physisorption analysis is the most widely used technique to gather insight into the textural properties of solid materials. Fig. 2 illustrates the N2 physisorption isotherms of the studied catalysts. All KCC-1 based catalysts exhibited type IV isotherms coupled with H3 hysteresis, that commonly observed in mesostructured materials [33]. The hysteresis of the KCC-1 based catalysts was corresponded to the capillary condensation of nitrogen vapour in the slit like pores which originated from the aggregation of particles in plate form [34]. Besides, the existence of intraparticle and interparticle pores in KCC-1 can be verified by the capillary condensation phases located at P/PB ¼ 0.4 and 0.9, respectively. The isotherm and hysteresis loop were identical for bare KCC-1 and Co/KCC-1, demonstrating the same classification as a mesoporous material (Fig. 2BeE). However, at higher P/PB, the second step of capillary condensation was slightly decreased with an increasing number of Co loading, which suggests a possibility of pores blockage by the Co. The textural properties data obtained from the N2 physisorption of all catalysts are listed in
Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Fig. 1 e (A) Wide-angle XRD patterns and FESEM image of KCC-1 catalyst, (B) particle size distributions of KCC-1 catalyst, (C) wide-angle XRD patterns and FESEM image of 20Co/KCC-1 catalyst and (DeF) elemental mapping images of 20Co/KCC-1 catalyst.
Table 1. It was observed that KCC-1 possessed a high surface area of 861 m2g-1 and an average pore width of 6.95 nm. The incorporation of Co has resulted in decreased specific surface area, suggesting a possible blockage of catalysts pores by deposition of Co metal. The introduction of 5 wt% of Co has slightly decreased the average pore diameter into 6.92 nm, suggestion a possible deposition of the metal on the pore mouth. However, further increased of Co loading to 10 wt% has resulted in bigger average pore diameter of 7.04 nm, which plausibly due to the combination of two or more original destructed pores. In particular, further addition of Co up to 30 wt% has decreased the average pore diameter, suggesting a metal blockage of the formerly formed pores. These results are concomitant with the result obtained in the total surface area (Table 1), which demonstrated that the accumulation of
Co metal might occur on the pores of KCC-1 while increasing the Co loading during the synthesis procedure [35]. Detailed information regarding the bonding between KCC1 and Co metal was identified by FTIR spectroscopy, as depicted in Fig. 3A. The KCC-1 exhibited three main bands attributed to asymmetric SieOeSi vibration (1084 cm1), SieOeSi bending (796 cm1) and symmetric stretching of SieOeSi (460 cm1) [36]. A typical absorbance band of SieOeSi vibration was also detected on the Co-doped catalysts, with an additional band positioned at approximately 966 cm1, which ascribed to the vibration of SieOH stretching [37]. In particular, the peak intensity decreased upon raising the Co loading, which was plausibly due to the fractional substitution of the SieOH and SieOeSi by a SieOeCo bond [38]. All Co-loaded KCC-1 showed double bands at 570 and 665 cm1, which
Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Table 1 e Textural and vibrational properties of various cobalt loaded on KCC-1. Catalysts Total Surface Area (m2 g1)a KCC-1 5Co/KCC1 10Co/ KCC-1 20Co/ KCC-1 30Co/ KCC-1 a b c d e f
Average Pore Diameter dCo3 O4 (nm)c NeH Stretching Peak (nm)b Aread
Ea (kJ mol1)e
Co dispersion (%) f
861 620
6.95 6.92
e 10.7
275.1 570.9
e 97.43
e 10.5
556
7.04
11.5
575.1
82.76
11.6
507
6.90
14.9
748.3
62.01
22.3
370
6.69
18.7
651.7
81.55
18.2
Obtained from BET method. Obtained from NLDFT method. The crystallite size of Co3O4 in the catalyst calculated from XRD using Scherer equation. Obtained from gaussian curve fitting based on Fig. 4. Ea values were determined in the range of 523e673 K. Obtained from H2-TPD.
Fig. 2 e N2 physisorption isotherm and NLDFT pore size distribution of (A) KCC-1, (B) 5Co/KCC-1, (C) 10Co/KCC-1, (D) 20Co/ KCC-1, and (E) 30Co/KCC-1.
agreed to CoeO vibrations of the octahedrally coordinated Co3þ and tetrahedrally coordinated Co2þ, respectively [33]. The appearance of these bands inferred that the Co metal was successfully incorporated into the KCC-1 support. The FTIR spectra of evacuated catalyst at 673 K in the hydroxyl range of 3800e3400 cm1 are plotted in Fig. 3B. All catalysts possessed three bands that were ascribed to the terminal hydroxyl groups (3740 cm1), internal hydroxyl groups (3710 cm1) and H-bonded hydroxyl groups (3655 cm1). It was noted that the intensity of the 3470 cm1 band was decreased concomitantly with increasing Co loading, indicating a probable change in the silica framework owing to the interaction with Co3O4 particles [40]. Fig. 3C illustrates the FTIR spectra of adsorbed pyrrole in the region that corresponded to ring-stretching vibrations. Three intense peaks corresponded to non-dissociated pyrrole (1531 and 1423 cm1) and d(OH) (1625 cm1) were observed in all catalysts. A notable decrease was observed in the peak intensity upon increasing the Co loading from 5 to 30 wt%, suggesting a possible replacement of metal into the framework of the silica-based catalyst [41].
The measurement of the intrinsic basicity in all catalysts and its strength was obtained by pyrrole adsorbed FTIR, as illustrated in Fig. 4. All catalysts exhibited signals ascribed to the perturbed NeH stretching in the pyrrole molecule (C4H4NH) in the range of 3650e3250 cm1, which resulted from the interaction of pyrrole with the basic sites in the catalyst (framework oxygen) [42]. An absorption band ascribed to the NeH group from the pyrrole molecules in the gas form was observed at 3530 cm1 (dotted line), while the interaction of the NeH group with the p-system in a different pyrrole molecule can be noted at 3420 cm1, ascribing to the physically adsorbed pyrrole in a liquid-like phase (asterisk symbol) [43]. The amount of basic sites in all catalysts was determined based on the peak area under the band agreeing to NeH stretching which was located at 3450 cm1 (KCC-1), 3447 cm1 (5Co/KCC-1), 3430 cm1 (10Co/KCC-1), 3422 cm1 (20Co/KCC-1), and 3436 cm1 (30Co/KCC-1). The area of the NeH stretching peak of the catalysts was obtained by Gaussian curve-fitting and listed in Table 1. An increasing trend in the number of basic sites was observed after
Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Fig. 3 e (A) IR-KBr spectra, (B) IR spectra after activated at 673 K and (C) IR spectra of pyrrole adsorbed on activated catalysts after outgassing at room temperature in the range of 2200e1300 cm-1 of (a) KCC-1, (b) 5Co/KCC-1, (c) 10Co/KCC-1, (d) 20Co/ KCC-1, and (e) 30Co/KCC-1.
Fig. 4 e IR spectra of pyrrole adsorbed at (a) room temperature on (A) KCC-1, (B) 5Co/KCC-1, (C) 10Co/KCC-1, (D) 20Co/KCC-1, and (E) 30Co/KCC-1 after outgassing at (b) room temperature, (c) 323 K, (d) 373 K, (e) 423 K, (f) 473 K and (g) before exposure to pyrrole.
increasing the loading of Co metal from 5 to 20 wt%. In particular, the KCC-1 and 20Co/KC-1 exhibited the lowest (275.1) and the highest (748.3) peak areas in the number of basic sites, respectively. Further addition of Co loading to 30 wt% has led to a decrease in the peak area (651.7), which was probably due to the agglomeration of metal, thus inhibiting the interaction of the H-donor with the basic oxygen in the catalyst. In this case, it can be concluded that the amount of basic sites for all catalysts were in order of KCC-1 < 5Co/ KCC-1 < 10Co/KCC-1 < 30Co/KCC-1 < 20Co/KCC-1. Based on the pyrrole adsorbed FTIR, the strength of basic sites in the catalyst can be measure by the bathochromic shift of the NeH stretching vibration Dѵ(NH) [44]. As shown in Fig. 4, the shift in NeH stretching vibration for bare KCC-1 was ѵ(NH) ¼ 80 cm1, ѵ(NH) ¼ 83 cm1 for 5Co/KCC-1, ѵ(NH) ¼ 100 cm1 for 10Co/KCC-1, ѵ(NH) ¼ 108 cm1 for 20Co/KCC-1 and ѵ(NH) ¼ 94 cm1 for 30Co/KCC-1. Thus, the
order of basic strength for the catalysts was in order of KCC-1 < 5Co/KCC-1 < 30Co/KCC-1 < 10Co/KCC-1 < 20Co/KCC-1. However, the 30Co/KCC-1 catalyst possessed the weakest basic site strength which was plausibly due to the poor ability of 30Co/KCC-1 to form/preserve its surface active sites and/or metal agglomeration during the synthesis [45].
Catalytic performance The pristine KCC-1 and Co loaded KCC-1 were tested for CO methanation at 423e573 K under atmospheric pressure, and the results obtained are presented in Fig. 5. A very low activity was observed for the bare KCC-1, which might be attributed to the absence of metal as active sites [46]. The presence of metal acts as dissociation or adsorption sites for H2 and CO in catalytic methanation. However, the absence of support material also does not give any catalytic activity, and therefore it can be
Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Fig. 5 e (A) CO conversion, (B) Rate of CH4 formation as a function of the reaction temperature and mass ratio of CO/H2 ¼ 1/3 and (C) Product yield of CO methanation at 573 K as a function of time-on-stream. KCC-1 (,), 5Co/KCC-1 (△), 10Co/KCC-1 (✰), 20Co/KCC-1 (), and 30Co/KCC-1 (⋄).
concluded that both metal and support material are complementary to each other in CO methanation. Besides, the catalytic activity of all Co-loaded KCC-1 was gradually improved as the reaction temperature was raised from 423 K to 573 K. It was observed that 30Co/KCC-1 showed CO conversion started from 523 K and reached 95.5% at 573 K. Furthermore, 20Co/ KCC-1 reached 79.5% conversion at 573 K. Meanwhile, 10Co/ KCC-1 and 5Co/KCC-1 only gave CO conversion of 10.3% and 6.9%, respectively. Fig. 5C shows the distribution of CH4 and CO2 obtained from the CO methanation over Co-loaded KCC-1 at 573 K. It was observed that only slight production of CH4 and CO2 were obtained from the bare KCC-1. The yields of CH4 and CO2 were gradually and significantly increased when the loading of Co was raised from 5 wt% to 20 wt%. The yield of CH4 and CO2 obtained over 5Co/KCC-1, 10Co/KCC-1, 20Co/KCC-1 and 30Co/ KCC-1 were 6.6% and 0.3%, 9.8% and 0.5%, 72.7% and 6.8%, and 72.5% and 23.0%, respectively. In particular, a slight decreased in the yield of CH4 was noticed in 30Co/KCC-1 with an enhancement in CO2 yield. The result was plausibly due to the agglomeration of Co metal that might have deteriorated the active sites for the dissociation of H2 and hydrogenation of CO at the surface of the catalyst. Besides, rapid deactivation of 30Co/KCC-1 might have originated from a poor ability to create or to preserve the active basic sites as demonstrated by IR adsorbed pyrrole analysis. The 20Co/KCC-1 showed the best performance in CO methanation, which is possibly due to the occurrence of an appropriate amount of intrinsic basic sites and the optimum amount of Co loading. The Arrhenius equation was applied to determine the apparent activation energy (Ea) for all Co/KCC-1 catalysts and the values obtained are listed in Table 1. The Ea values for 5Co/KCC-1, 10Co/KCC-1, 20Co/KCC-1 and 30Co/
KCC-1 were determined as 97.4 kJ/mol, 82. kJ mol1, 62.0 kJ/ mol and 81.5 kJ/mol, respectively. However, the value of Ea for KCC-1 could not be determined as it exhibited very low activity in the CO methanation reaction, compared to the Co supported KCC-1. Based on the table, 20Co/KCC-1 exhibited the lowest activation energy (Ea), which was plausibly due to the well dispersed Co metal and suitable intrinsic basic sites. Thus, 20Co/KCC-1 possessed good catalytic activity for CO methanation reaction among the Co/KCC-1 type catalysts. The obtained Ea is comparable with those observed for the silica supported catalyst as reported by Furman et al.(2015), in which the Ea for RueSiO2 is 78 ± 1 kJ/mol for the CO methanation reaction [47]. The stability tests of all Co supported KCC-1 catalysts are presented in Fig. S3, where the conversion of CO was plotted as a function of time. The reaction was conducted at 573 K, H2/CO ¼ 3 and GHSV of 14500 ml g1 h1. It was observed that most of the Co/KCC-1 catalyst could retain their CO conversion after 72 h, except for the 30Co/KCC-1. The significant reduction in CO conversion over 30Co/KCC-1 was observed after 42 h, which plausibly due to the metal sintering. This is in agreement with the calculated metal crystallite size (Table 1), where 30Co/KCC-1 exhibited the largest Co3O4 crystallite size of 18.7 nm. Based on the literature reviews in Table 2, the catalytic activity of the Co-loaded KCC-1 is comparable with the recently studied catalysts. The CO methanation over 20 wt% Co loaded ZrO2 produced only 30% CO conversion and 54.4% selectivity of CH4 [48]. Despite low reaction temperature (473 K) and the high number of Co loaded, the yield of CH4 (16.3%) was lower than 10Co/KCC-1 (47.9%). This result was due to the low reducibility of Co metal in Co/ZrO2 which led to a lower surface of the active Co0 phase. In addition, the high activity can be observed on 10% Ni/MCM-41 with 100% CO
Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Table 2 e Comparison study of Co-promoted catalyst for CO methanation. Catalyst
Catalytic performance [%]
Reaction conditions
CO conversion
CH4 selectivity
CH4 yield
Temperature [K]
Pressure [MPa]
0.1 6.9 10.3 79.3 95.5 30.0 99.8 100.0 87.0
100 96.3 95.3 92.6 75.9 54.4 89.2 85.0 14.0
0.1 6.6 9.8 73.4 72.5 16.3 89.0 85.0 12.2
573 573 573 573 573 473 673 623 533
0.1 0.1 0.1 0.1 0.1 0.1 1.0 0.1 1.0
KCC-1 5Co/KCC-1 10Co/KCC-1 20Co/KCC-1 30Co/KCC-1 20% CoeZrO2 10% Ni/SBA-15 10% Ni/MCM-41 10% Co/SiO2
Reference
This This This This This [48] [50] [49] [51]
study study study study study
conversion, 85% of CH4 selectivity and 85% of CH4 yield at 623 K [49]. It was reported that the addition of nickel into MCM-41 has increased the metal dispersion and upsurge the catalyst basicity. On the other hand, 10%Ni/SBA-15 also possessed high catalytic activity in CO conversion (99.8%) with a high yield of CH4 (89.0%), which is much higher than 20Co/ KCC-1 obtained in this study [50]. This might be due to the presence of 1.0 MPa pressure in this reaction which helps to enhance the catalytic performance. Furthermore, the application of 10%Co/SiO2 on CO methanation reaction was also reported, which produced 87.0% CO conversion, 14% CH4 selectivity and 12.2% yield of CH4 at 493 K [51]. These results were much lower than that of 20Co/KCC-1 which gave 97.4% CO conversion, 90.3% selectivity of CH4 and 87.8% yield of CH4 at 673 K. This result was plausibly attributed to the high surface area of 20Co/KCC-1 that facilitated the dispersion of Co metal, and provided more accessible active sites [52].
information on the species that might be formed during the mechanism of CO methanation. By flowing the CO gas, two peaks were noted at 2170 and 2110 cm1 which can be assigned as gaseous CO. While no peak was observed during the flow of H2 gas. Fig. 6A illustrates the FTIR spectra of the pre-adsorbed CO on 20Co/KCC-1 after exposure to H2. An intense peak was noted at 1625 cm1, which can be attributed to atomic hydrogen [43]. At 523 K, intensification of two bands was notified at 2360 and 2340 cm1, which can be assigned to gaseous CO2, along with the formation of two peaks at 2110 and 2170 cm1, which were attributed to the gaseous CO. According to the thermodynamics study, the formation of byproduct CO2 might be originated from the following reaction [5,54]: 2CO þ 2H2 /CO2 þ CH4
(5)
Proposed mechanism
2CO / CO2 þ C
(6)
The reaction mechanism pathway of CO methanation is an ongoing topic for discussion that is often disputed by most researchers who are experts in the heterogeneous catalysis area. Although there are several studies on the mechanism of CO methanation, it still becomes a crucial topic to be resolved. Based on previous studies, the proposed mechanism pathway for CO methanation can be divided into associative and dissociative schemes. The associative scheme involves the combination of adsorbed H2 and CO to produce intermediates (COH, CHO and CHOH), which subsequently undergo the dissociation of a CeO bond. While the dissociative scheme involves the direct dissociation of adsorbed CO to form a surface carbon (C), that acts as an intermediate before the adsorption of H2 occurs to undergo the methanation reaction [53]. In this current study, the interaction between CO and H2 were studied to reveal the plausible route of the reaction mechanism for the CO methanation reaction for KCC-1 and 20Co/KCC-1 catalyst via in-situ FTIR adsorption spectroscopy. The mechanistic reaction pathway for CO methanation was clarified based on in situ FTIR for the interaction of H2 or CO with pre-adsorbed CO or H2. The 20Co/KCC-1 catalyst was chosen due to its good performance in terms of CH4 yield obtained from the catalytic reaction part. Prior to the in-situ FTIR, a blank test reaction (without the presence of a catalyst) of gaseous CO, H2 and CO2 was conducted to obtain important
CO þ H2O / CO2 þ H2
(7)
In this study, the formation of CO2 was plausibly due to the reversed methane reforming (Equation (5)) and Boudouard reaction of CO (Equation (6)) rather than water gas shift reaction (Equation (7)), since the formation of CO2 was observed at temperature lower than 723 K. Besides, two peaks were also notified at 1510 and 1420 cm1, which corresponded to the asymmetric and symmetric stretching vibration of the carbonate species in bidentate form, respectively. Specifically, the peaks intensity of the carbonate species was significantly increased by increasing the temperature up to 623 K. This was plausibly due to the dissociation of gaseous CO, followed by its migration to form an intermediate (COOad) before further hydrogenation with H2 to produce CH4. Meanwhile, the study on the interaction of CO with preadsorbed H2 was elucidated based on Fig. 6B. It was observed that after the temperature was increased from 323 to 623 K, the band at 2170 and 2110 cm1 which can be attributed to gaseous CO, were reduced with a concomitant increase of gaseous CO2 peaks at 2360 and 2340 cm1. The peak for gaseous CO2 was gradually intensified at a higher temperature, suggesting a possible formation of an undesired CO2 byproduct that originated from the reversed methane reforming.
Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Fig. 6 e Evaluation of FTIR spectra of (A) interaction of H2 with pre-adsorbed CO and (B) interaction of CO with pre-adsorbed H2 on 20Co/KCC-1. The samples were (a) adsorption of H2 or CO at room temperature and heating up to (b) 323 K, (c) 373 K, (d) 423 K, (e) 473 K, (f) 523 K, (g) 573 K and (h) 623 K.
Furthermore, a sharp peak was observed at 1625 cm1, which was attributed to the atomic hydrogen peak. The intensity of this peak was slightly increased while increasing the temperature up to 623 K due to the high ability of the catalyst to dissociate H2 molecule to form atomic H [43]. Besides, a minor peak that corresponded to the SieOeSi vibration of the KCC-1 support could be observed at 1870 cm1 [55]. Meanwhile, the existence of the peak at 1970 cm1 indicates the adsorption of atomic H on the surface of the catalysts [56,57]. Moreover, the intensity of the peaks at 1510 and 1420 cm1 was increased with a gradual increase of temperature up to 623 K, which was plausibly due to the strong ability of atomic H to dissociate gaseous CO to produce CH4. In order to determine the main CO methanation mechanism pathway of 20Co/KCC-1 and the role of Co metal in the reaction, the in situ FTIR of CO þ H2 was conducted on the KCC1 and 20Co/KCC-1. The spectra obtained are presented in Fig. 7. In can be notified that both catalysts exhibited two peaks assigned to CO2 at 2360 and 2340 cm1. However, these peaks were more intense in KCC-1 compared to 20Co/KCC-1 at higher temperature range (523e623 K) which was plausibly
due to the presence of oxygen vacancy property possessed by KCC-1 that acted as an adsorption site for CO2 produced from the reversed methane reforming and the CO disproportionate reaction [54]. In particular, the peaks were formed earlier in 20Co/KCC-1 (starting at 373 K), suggesting that the presence of Co metal has enabled the dissociation of both CO and H2 molecules at low temperatures. Based on Fig. 7A and B, both catalysts also exhibited a peak at 1870 cm1 which can be assigned to the SieOeSi vibration in KCC-1 support [55]. However, this peak was smaller in the 20Co/KCC-1 catalyst, which could be due to the partial substitution of SieOeSi with the Co metal to form the SieOeCo vibration. Besides, both catalysts also possessed a very sharp and narrow peak at 1625 cm1 which can be deduced to be atomic hydrogen. The peak was noticeably more intense in 20Co/KCC-1 compared to KCC-1, suggesting the role of Co metal to dissociate H2 molecule to form atomic H [58]. Besides, both catalysts showed the existence of three peaks at 2170, 2110 and 1970 cm1 which indicated the adsorption band of linear or bridging gaseous CO on the surface of catalysts, respectively [56,57]. In KCC-1, those peaks were distinctively increased up to 473 K,
Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Fig. 7 e Evaluation of the FTIR spectra of adsorbed gases (CO þ H2) on (A) KCC-1 and (B) 20Co/KCC-1, (C) CH4 band at 3014 cm1 and (D) CeH deformation mode at 1300 cm-1 in 20Co/KCC-1. The catalysts were heated up to (a) room temperature, (b) 323 K, (c) 373 K, (d) 423 K, (e) 473 K, (f) 523 K, (g) 573 K, and (h) 623 K.
followed by a consistent reduction until 623 K, which can be explained by the emergence of a new peak at 2340 cm1, assigned to gaseous CO2. However, based on Fig. 7B, the intensity of gaseous CO peaks were significantly reduced as the temperature was increased till 573 K and had nearly disappeared at 623 K, which may be due to the adsorption of gaseous CO on the surface of Co metal in bridging and tri-fold configurations [56]. Furthermore, a new peak can be notified at 2050 cm1 that corresponded to the interaction of Co0 metal with a carbonyl species (Co0 e CO) on 20Co/KCC-1 [59]. However, the peak at 2050 cm1 shifted to 2020 cm1 at higher temperatures due to the destabilization of the Co0eCO bond on Co0 sites [43,58]. The intensity of the peak at 2020 cm1 significantly increased up to 623 K, which was plausibly due to the dissociation of gaseous CO to form adsorbed carbon (Cad)and adsorbed oxygen (Oad) species on the surface of the Co metal [10].
Previously, Panagiotopoulou et al. reported on the adsorption of gaseous CO onto the surface of RueTiO2 catalyst in which the gaseous CO was dissociated before hydrogenation with H atom to form methane [60]. In addition, two new peaks at 1510 and 1350 cm1 appeared on 20Co/KCC-1, which corresponded to the asymmetric and symmetric stretching vibrations of the surface bidentate carbonate species (vsCOO and vasCOO), respectively [54]. Those peaks only appeared on 20Co/KCC-1 due to the migration of the Cad and Oad species that had occurred from the surface of the Co metal to surface of supporting materials [43]. In particular, the intensity of the peak at 1350 cm1 was more visible, suggesting a progressive formation of bidentate carbonate species as an intermediate on the surface of KCC-1. While, the dissociation of H2 was expected to occur simultaneously and started to hydrogenate the intermediate of bidentate carbonate species to form CH4, which was confirmed by the presence of sharp peaks at 3014
Scheme 1 e Plausible reaction mechanism of CO methanation over Co/KCC-1. Please cite this article as: Fatah NAA et al., Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.144
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Table 3 e Mechanism pathway of carbon monoxide methanation over homogeneous catalysts. Catalyst
Mechanism pathway
Amount of metal
Reference
RueTiO2 Ni/ɤAl2O3 Ru (0001) Ru/Al2O3
Associative þ Dissociative Dissociative Dissociative Associative
5 wt% of Ru 50 wt% of Ni e 5 wt% of Ru
[60] [61] [62] [63]
and 1300 cm1 (Fig. 7C and D), which corresponded to the peak of the CH4 band and CeH deformation modes, [which can be attributed to gaseous CO2, become more distinct with increasing temperatures starting from 523 K to 623 K. Furthermore, from the in situ FTIR adsorption spectra of CO þ H2 (Fig. 7A and B), it can be concluded that the mechanism pathway of the CO methanation reaction over KCC-1 catalyst followed the associative scheme. In the case of 20Co/KCC-1, both the associative and dissociative types of mechanisms were observed during the reaction. The dissociative mechanism was distinctly observed starting from the temperature range of 523 Ke623 K, where the formation of CH4 was initially observed at 3014 cm1. Nevertheless, the associative mechanism of CO methanation also significantly contributed to the formation of CH4, as the adsorbed species of COeCO0 in linear form was also observed in 20Co/KCC-1 (Scheme 1). According to the literature, the divergences in the CO methanation mechanisms between the associative and dissociative methanation are closely associated with the reaction conditions and the composition of catalyst component [10]. Table 3 summarizes several types of previously reported catalysts and their corresponding mechanism pathway in CO methanation. A previous DRIFT study on CO methanation using Ni/ɤAl2O3 revealed that the catalyst portrayed a dissociation mechanism where the C and H atomic species were formed by the dissociation of CO and H2 on Ni [61]. Besides, a dissociative CO mechanism was also observed with a flat Ru (0001) surface as the catalyst [62]. In contrast, the associative CO mechanism has been reported on several types of catalyst with low metal loadings, such as Ru/TiO2 [60] and Ru/Al2O3 [63].
Conclusion Highly dispersed Co loaded on KCC-1 catalyst with dendrimeric morphology was successfully synthesized for CO methanation. The catalyst with a 20 wt% of Co exhibited superior performance in the reaction with a 72.7% yield of CH4 and low formation of CO2 compared to other Co loading. The superior performance of the 20Co/KCC-1 could be explained by its high amount of basic sites, as well as the well-dispersed Co metal on the KCC-1 that could avoid metal sintering. A detailed in-situ FTIR spectroscopy study revealed that both the associative and dissociative mechanism pathways made significant contributions to the high catalytic methanation activity. In addition to the dissociative mechanism, the linear CO species adsorbed on the Co metal by the associative mechanism was further hydrogenated to obtain the final CH4 products.
Acknowledgement This work was supported by the Professional Development Research University grant from Universiti Teknologi Malaysia (Grant No.04E07) and Research University Grant from Universiti Teknologi Malaysia (Grant No.19H04).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.01.144.
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