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Investigation on the increased stability of the Ni–Co bi-metallic catalysts for the carbon dioxide reforming of methane
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Investigation on the increased stability of the Ni–Co bi-metallic catalysts for the carbon dioxide reforming of methane Anchittha Liua, Supareak Praserthdamb, Suphot Phatanasria,
⁎
a
Center of Excellence on Catalysis and Catalytic Reaction Engineering (CECC), Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok, 10330, Thailand b High-performance Computing Unit (CECC-HCU), Center of Excellence on Catalysis and Catalytic Reaction Engineering (CECC), Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand
ARTICLE INFO
ABSTRACT
Keywords: CO2 reforming Ni–Co catalysts HY zeolite ɤ-Al2O3-HY supported Bi-metallic catalysts
The carbon dioxide reforming of methane is one of the processes used to tackle global warming by reducing carbon dioxide, one of the most harmful greenhouse gases through the catalytic reaction with methane which produces syngas. Although the noble metals are good candidates for such a process, the cost and scarcity limit their uses. The non-noble metals on the other hand, with satisfactory activity and cost, are of interest except their stability against the coke formation, the main deactivation scheme during the dry reforming reaction. Therefore, the study of the enhanced stability found in the bi-metallic system of Ni catalysts via the introduction of Co was carried out in this work. The activity and coke resistance were studied on both mono and bi-metallic system of the Ni-based catalysts (10%wt) supported on γ-Al2O3-HY zeolite prepared via the sol-gel method. The activity testing was performed in a fixed-bed reactor with a unity feed volume ratio under atmospheric pressure at 973 K, where the Ni–Co bi-metallic system exhibits higher activity than both mono-metallic of either pure Ni or Co due to a higher number of active sites confirmed by the CO chemisorption. On the stability testing, the high stability was found on the bi-metallic system as verified by the time-on-stream testing. In addition, the higher reduction temperature for the bi-metallic system via the analysis using H2-TPR also suggested stronger metal-support interaction which may decrease the bonding strength of the coke on the surface leading to lower deactivation.
1. Introduction The greenhouse gas (GHG) emission contributes to one of the most concerned issues, where CO2 and CH4 are the main components of greenhouse gas. Utilizing these two main culprits via the CO2 reforming of CH4 or the dry reforming reaction is of interest since not only the GHG could be reduced but it also produces useful products, the syngas to be used as feed to many processes. The dry reforming reaction of CH4 (DRM) (shown in Eq. (1)) is an endothermic process which requires high temperature; thus, high energy supply. This process has many side reactions [1], in which it may be accompanied by a parallel reaction of the reverse water gas shift (RWGS) (Eq. (2)), methanation (Eq. (3)), carbon monoxide decomposition by the Boudouard reaction (Eq. (4)), which is the main cause in catalyst deactivation during DRM. In addition, the undesired methane decomposition reaction (Eq. (5)), and CO reduction (Eq. (6)) also occur [2]. CH4 + CO2 ↔ 2CO + 2H2 ΔH°298 = +247.3 kJ/mol
⁎
(1)
CO + H2O ↔ CO2 + H2 ΔH°298 = −41.0 kJ/mol
(2)
CO2 + 4H2 ↔ CH4 + 2H2O ΔH°298 = +165.0 kJ/mol
(3)
2CO ↔ C + CO2 ΔH°298 = −172.0 kJ/mol
(4)
CH4 ↔ C + 2H2 ΔH°298 = +75.0 kJ/mol
(5)
CO + H2 ↔ C + H2O ΔH°298 = −131.0 kJ/mol
(6)
Normally, the RWGS reaction always occurs during DRM. Therefore, to inhibit RWGS would be the key towards better catalysts which can produce syngas with high H2 selectivity and/or H2/CO ratio [1]. Due to that most catalysts are not stable at these high temperatures since the result of catalysts sintering and crystal structure collapsing [3,4]. Consequently, noble (Rh, Ru, Pd, Pt, etc.) and non-noble (Ni, Co, Fe, Cu, etc.) metals were the based suitable catalysts for DRM. The nonnoble metals have good catalytic performance than non-noble metals for a large number of studies. Because the cost-effectiveness many researchers have focused attention on the non-noble metal with high
Corresponding author. E-mail address: [email protected] (S. Phatanasri).
https://doi.org/10.1016/j.cattod.2019.07.047 Received 18 February 2019; Received in revised form 26 July 2019; Accepted 29 July 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Anchittha Liu, Supareak Praserthdam and Suphot Phatanasri, Catalysis Today, https://doi.org/10.1016/j.cattod.2019.07.047
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Al2O3 precursor and HY zeolite with Si/Al molar ratio = 100 (TOSHO). For the monometallic and bimetallic Ni–Co catalysts were prepared by the incipient wetness impregnation method using Ni(NO3)2.6H2O (Aldrich) and Co(NO3)2.6H2O (Aldrich) as metal precursors with different metal loading (10Ni, 10%Co, 3%Ni-7%Co, 5%Ni-5%Co, 7%Ni3%Co). After that, the catalysts were dried overnight at 110 °C and calcined under an airflow at 550 °C for 2 h. (In this present work, we design to study only the 10%Ni, 10%Co and 7%Ni-3%Co, but 3%Ni7%Co and 5%Ni-5%Co can study from our previous study [11].)
Table 1 Physical properties and chemical composition of catalysts. Catalysts
10%Ni/ γ-Al2O3-HY 10%Co/ γ-Al2O3-HY 7%Ni-3%Co/ γ-Al2O3-HY a b
N2 physisorption results BET surface area (m2/g)
Pore volumea (cm3/g)
Average pore diameterb (Å)
524.68 543.84 513.58
0.26 0.25 0.23
60.89 63.51 63.09
BJH desorption pore volume. BJH desorption average pore diameter.
2.2. Catalyst characterization The X-ray diffraction (XRD) was used to analyze the crystalline phase by using X-ray diffractometer SIEMENS D 5000 at Cu-Kα radiation between 20° and 80°. The N2-physisorption technique was used to determine the specific surface area of the catalyst (Brunauer–Emmett–Teller (BET) method) and to examine pore diameter and pore volume of the prepared catalyst (Barret-Joyner-Halenda (BJH) method) by using Micromeritics ASAP 2020. The scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX) was used to study morphology structure and agglomeration of the particles which is analyzed by the JEOL mode JSM-5800LV of scanning electron microscopy technique. The X-ray photoelectron spectroscopy (XPS) was used to determine the chemical state on the surface of catalysts using Kratos Amicus X-ray photoelectron spectrometer with Mg Kα X-ray as a source. The internal standard was the XPS spectra of C 1s line at a binding energy of 285.0 eV. The Micromeritics ChemiSorb 2750 Pulse Chemisorption System was used for the analysis of the hydrogen temperature-programmed reduction (H2-TPR). The sample was pre-treated at 500 °C for 1 h in N2. Afterward, the sample was heated with 25 ml/ min of 10% H2 in Ar mixture gas from 30 °C to 800 °C. The amount of H2 uptake was measured as a function of temperature using a Thermal Conductivity Detector (TCD). The as-spun alumina fibers are subjected to the thermogravimetric and differential thermal analysis (Diamond Thermogravimetric and Differential Analyzer, TA Instruments SDT Q600) to determine the carbon content in the sample in the temperature range from room (approximately 28 °C) to 1000 °C with a heating rate of 10 °C/min in 100 ml/min of airflow. Also, a Micromeritics ChemiSorb 2750 Pulse Chemisorption System was used to carry out the temperature-programmed desorption of carbon dioxide (CO2-TPD) in order to determine the basicity of catalysts. The samples were pretreated in helium flow at 500 °C for 1 h prior to the adsorption of carbon dioxide for 1 h to ensure saturation of such gas on the surface. Then, the sample was heated from 30 °C to 800 °C with a heating rate of 10 °C/ min. The profile of the desorbed CO2 was measured as a function of temperature using the TCD. The amount of metal active sites and the metal dispersion of the catalysts were evaluated by CO chemisorption
performance and low cost [5]. In an industrial, the Ni-based catalyst is popularly to use in DRM because of its availability, suitable activity, and low cost, although it is easier to coke formation than other noble metal, resulting in the decrease of the catalyst’s activity [6,7]. Furthermore, using second metal is one of the methods applied for developing carbon resistance of the Ni-based catalyst [8]. Addition of Co that is the second metal has been applied for developing the carbon resistance [9]. Therefore, the modification of Ni-based catalysts is possible to improve catalysts performance and anti-carbon deposition. Silicon, aluminum, and oxygen make crystalline solids structure that forms a connected framework as zeolites. Zeolites were variously used in many applications due to the improvement in thermal, high surface area, well-defined structure and high affinity for CO2 [10]. The use of Ni-based catalysts on zeolites in CO2 reforming of methane reaction has been studied by Apanee et al. [7]. They found that Ni/zeolite Y had better catalytic performance than the other types of studied zeolites (zeolite A, zeolite X, and ZSM-5). Moreover, 7 wt% Ni loading showed the best catalytic activity on each zeolite support. Madhi et al. [6] investigated the optimized composition of Ni–Co bimetallic catalyst over zeolite Y. The activity tests displayed that Ni7Co3/Y activity was higher than zeolite Y and Ni3Co7/Y catalyst at all temperatures. Therefore, it seems that the Ni-based catalysts over γ-Al2O3 and zeolite Y may have improved activity for CO2 reforming of methane reaction. In this work, the mono and bi-metallic system of the Ni-based catalysts over γ-Al2O3 and HY zeolite support was studied in terms of the catalytic properties and investigated the stability of the catalyst in CO2 reforming of methane reaction. 2. Experimental 2.1. Catalyst preparation The γ-Al2O3-HY zeolite supported catalyst was prepared by a sol-gel method using Alumina isopropoxide (Aldrich) and ethanol (Merck) as
Fig. 1. X-ray diffraction pattern of Ni–Co catalysts supported on γ-Al2O3-HY zeolite (a: 10%Ni/γ-Al2O3-HY zeolite, b: 10%Co/γ-Al2O3-HY zeolite, c: 7%Ni-3%Co/γAl2O3-HY zeolite). 2
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analysis using the Micrometritics Chemisorb 2750 fitted with a TCD and the ASAP 2101CV.3.00 software unit. The samples were reduced at 500 °C with 25 ml/min of H2 flow for 1 h. For consistency of the obtained CO peak, the 20 μl of CO was injected to the reactor at room temperature until three peaks of the same high were obtained for three consecutive calibrations. Finally, the CO adsorption on the catalyst was assumed that one CO molecule would bind one metal site only.
a catalyst with high average pore diameter can improve active metals dispersion, pore structures, and provide higher active surface area per weight of the active metal [12,13]. Fig. 1 shows the XRD patterns of fresh catalysts. The XRD peaks of NiO at 2ɵ = 37.2°, 43.3°, and 62.8° can be seen, indicating the (111), (200), and (220) planes of NiO (JCPDS 01-073-1519), respectively. The XRD peaks of Co3O4 cubic phase at 36.9°,44.9°, and 65.3° indicate (311), (400), and (440) planes of Co3O4(JCPDS 01-076-1802), respectively. It demonstrated that Ni and Co species were well dispersed on catalysts surface due to the existing overlaps peaks of NiO and Co3O4 of 7%Ni-3%Co/γ-Al2O3-HY XRD pattern are difficult to differentiate [9]. The lower intensity of NiO and Co3O4 over 7%Ni-3%Co/γ-Al2O3-HY zeolite presented that the incorporation of Co species increased the dispersion of NiO, which is in the same way with the other researchers reported results [14,15]. Fig. 2 shows the morphology of fresh Ni–Co over Al2O3-HY zeolite catalysts which was studied by Scanning Electron Microscopy (SEM). Most catalysts had similar surface morphology and particle size as in accordance with BET pore diameter and pore volume results. The state of Ni and Co on monometallic and bi-metallic catalysts surface are detected by XPS analysis as shown in Fig. 3. In Fig. 3a shows Ni 2p3/2 XPS profile of 10%Ni/γ-Al2O3-HY zeolite and Fig. 3b shows Ni 2p3/2 XPS profile of 7%Ni-3%Co/γ-Al2O3-HY zeolite, the peak of Ni3+ are around 856.96 and 857.17 eV respectively [16], and the peak positions around 863 eV are the satellites [17]. While Co 2p3/2 XPS profile of 10%Co/γ-Al2O3-HY zeolite and 7%Ni-3%Co/γ-Al2O3-HY zeolite are shown in Fig. 3c and d respectively. For the Co particles covered with Co3O4 of 10%Co/γ-Al2O3-HY zeolite, the peaks were located at 782.99, 787.75 and 790.58 eV are assigned to Co3+, Co2+ and satellite respectively. Similarly, the peaks of 7%Ni-3%Co/γ-Al2O3-HY zeolite located at 782.6, 786.84 and 790.69 respectively [18,19]. In addition, the surface atomic composition of the catalysts with different phase compositions are shown in Table 2. It was found that Co3+ had more atomic concentration that Co2+ in 10%Co/γ-Al2O3-HY zeolite. The electron configuration of Co2+ and Co3+ are [Ar] 3d7 and [Ar] 3d7 4 s2,
2.3. Catalytic reaction study The DRM was carried out in a fix-bed reactor continuous-flow quartz reactor at atmospheric pressure. First, the catalyst was packed on quartz wool in the middle of the quartz tube reactor. Afterward, the catalyst was reduced at 500 °C for 1 h. with 50 ml/min of H2 flow before the reaction. Then, the 50 ml/min of N2 was purged to remove excess H2 prior to the heating of the catalyst to the reaction temperature of 700 °C. The feed volume ratio of 1:1 CH4:CO2 ratio was set for the reactor, in which the reaction testing is carried out for 3 h. with a total flow rate of 60 ml/min. Finally, the feed and products were analyzed for their compositions using the Thermal Conductivity Detector (TCD) type gas chromatograph (Shimadzu, GC-8A) equipped with Porapack-Q and Molecular sieve 5A packed column, in which the Argon was used as a carrier gas. 3. Results and discussion 3.1. Catalyst characterization One of the key factors affecting the catalyst’s activity is its surface area [6]. As a result, the catalyst surface area was measured by the BET method where the result is shown in Table 1. It was found that the specific surface area of the bi-metallic 7%Ni-3%Co/γ-Al2O3-HY was less than both mono-metallic systems: the 10%Ni/γ-Al2O3-HY and 10%Co/ γ-Al2O3-HY. This may be caused by the support’s pore blockage by both Ni and Co species. Also, from the previous study, it was found that using
Fig. 2. SEM images of fresh catalysts. (a: 10%Ni /γ-Al2O3-HY zeolite, b: 7%Ni-3%Co/γ-Al2O3-HY zeolite, c: 10%Co/γ -Al2O3-HY zeolite).
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Fig. 3. XPS spectra of Ni 2p for (a) 10%Ni/γ-Al2O3-HY zeolite, (b) 7%Ni-3%Co/γ-Al2O3-HY zeolite and Co 2p for (c) 10%Co/γ-Al2O3-HY zeolite, (d) 7%Ni-3%Co/γAl2O3-HY zeolite.
Table 2 The surface atomic composition of the Ni, Co, and Ni–Co over γ-Al2O3- HY catalysts with different phase compositions. Sample
10%Ni/ γ-Al2O3-HY 10%Co/ γ-Al2O3-HY 7%Ni-3%Co/ γ-Al2O3-HY
Atomic concentration (%) Ni3+
Co3+
Co2+
100 0 64.19
0 71.20 19.44
0 28.79 16.36
Table 3 CO chemisorption results of Ni, Co, and Ni–Co over γ-Al2O3- HY catalysts.
Atomic ratio Ni/Co
– – 1.79
Catalysts
Active sites (×1018)
%Dispersion
10%Ni/γ-Al2O3-HY 10%Co/γ-Al2O3-HY 7%Ni-3%Co/γ-Al2O3-HY
1.16 4.64 13.81
0.11 0.45 1.35
On the other hand, the metal dispersion is also confirmed by the EDX analysis. The Ni, Co, Si, and Al can be seen in this analysis illustrated in Fig. 4, which is consistent with the XRD results. The density of Ni and Co are shown as colored dots in the Fig. 4 of the bi-metallic was highest compared to the monometallic ones, in which this is consistent with the results calculated from the amount of adsorbed carbon monoxide from CO chemisorption. The peaks in H2 TPR profiles of the catalysts can be observed in Fig. 5, where it was used to investigate behavior during the reduction of the samples. Metal, support, and metal-support interaction affect the H2-TPR profiles, resulting in different reducibility and reduction temperature of catalysts. According to Afzal et al. [24], the reduction of the Ni2+ localized in the supercage and/or sodalite cavities are the reduction peaks found at low temperature, while at the high temperature the peak is the reduction of the Ni2+ localized in hexagonal cavities. According to H2-TPR profiles of catalysts, it was obviously seen that the bi-metallic Ni–Co catalysts exhibited two reduction peaks. From the previous work, the first peak is related to the reduction from Co3O4 to CoO which was observed at 337 °C and 367 °C for 10%Co/γ-Al2O3-HY and 7%Ni-3%Co/γ-Al2O3-HY, respectively. The second peak is related to a reduction from NiO to Ni and CoO to Co metallic that was located at 378 °C, 376 °C and 622 °C for 10%Ni/γ-Al2O3-HY, 10%Co/γ-Al2O3-
respectively. Normally, when the transition metals are ionized, the electrons were removed from the valence-shell s orbitals before they are removed from valence d orbitals due to the relative energies of the atomic orbitals. The 4s orbital had lower energy than the 3d orbitals and lower energy was more stable. For bi-metallic, Co3+ was decreased but still remaining more than Co2+ which may be caused by the effect of Ni [20–22]. The amount of the CO chemisorbed on the mono and bi-metallic at room temperature is shown in Table 3 which are used to calculate the number of the active site of the catalyst. The bi-metallic of 7%Ni-3%Co/ γ-Al2O3-HY has the highest number of the active site and metal dispersion. In addition, from the XRD result, the bi-metallic catalyst has lower intensity peak of NiO and Co3O4 than the monometallic catalyst. In the same ways, the bi-metallic catalyst had decreased the oxide form but increased the amount of metal due to the amount of CO adsorbed to the metal on the surface. While the dispersion of metal atoms on the catalyst surface can be illustrated by the ratio between the total number of metal atoms that is accessible to the adsorbate divided by the total metal atoms in the sample [23].
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Fig. 4. EDX analysis of all mono and bi-metallic catalysts. (a: 10%Ni/γ-Al2O3-HY zeolite, b: 7%Ni-3%Co/γ-Al2O3-HY zeolite, c: 10%Co/γ-Al2O3-HY zeolite).
carbon formation in CO2 reforming of methane. The CO2 desorption patterns of bi-metallic catalysts exhibit the highest peak compare to the monometallic catalysts, in which the first peak around 90 °C and the second around 655 °C attributed to the weak and strong basic sites,
HY and 7%Ni-3%Co/γ-Al2O3-HY in catalysts, respectively [25]. Fig. 6 displays the CO2 desorption patterns for all fresh catalysts. The CO2-TPD of the catalysts were used to determine the catalyst basicity which is a key property for determining the resistance against
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3.2. Catalyst performances The stability of the catalysts was tested at 700 °C for 180 min. Before the reaction, the prepared catalysts were reduced in flowing H2 for 1 h. The feed conversion and product selectivity were calculated based on the amount of H2 and CO product of Ni–Co/γ-Al2O3-HY for CO2 reforming of methane which was provided in Fig. 7. The CO2 conversion of the catalyst is higher than the CH4 conversion due to the reverse water-gas shift reaction [1]. The results showed that there was no significant deactivation for 7%Ni-3%Co/γ-Al2O3-HY and 10%Co/γ-Al2O3HY catalyst after 180 min meaning that during the activity testing the activity of 7%Ni-3%Co/γ-Al2O3-HY and 10%Co/γ-Al2O3-HY catalyst is stable while the 10%Ni/γ-Al2O3-HY catalysts were decreased verified by time-onsteam. This is due to the amount of coke deposition during DRM over all of the catalysts [8]. According to the CO chemisorption results, high amount of active sites and high metal dispersion of catalysts are important for a stable and active DRM catalyst [12,13]. Moreover, the 10%Co/γ-Al2O3-HY catalyst had lower activity than the other two catalysts due to lower activity of Co metal compared to Ni [29]. According to Tanios C. et al., the amount of deposited carbon can be reduced by the addition of Co. Nevertheless, the Co decreases in activity [30] same as results from the previous study that the activity was decreased when Co content increased [11]. Therefore, the catalytic performance on bi-metallic catalyst had a better activity trend than monometallic catalysts.
Fig. 5. The TPR profiles of the nickel-cobalt monometallic and bi-metallic catalysts over γ-Al2O3-HY zeolite.
3.3. Catalyst characterization after the performance test Thermogravimetric analysis with heating temperature ranging from room temperature to 1000 °C was used to investigate the possible coke deposition of the used catalysts as shown in Fig. 8. The moisture and other volatile contents may correspond to the weight loss at the temperature range up to 500–600 °C [31]. According to the result, the weight loss of 10%Ni/Al2O3-HY zeolite represented the higher weight loss (75%) than that of 7%Ni-3%Co/Al2O3-HY zeolite (65%) and 10%Co/Al2O3-HY zeolite represented the lowest weight loss that was no significant deactivation due to the Ni had a major role in coke accumulation [32] while Co decreased the amount of deposited carbon [30]. Although the used catalysts had considerable weight loss, the catalyst deactivation was not so significant to decrease the reaction activity as the deactivation of catalysts at reaction temperatures of 700 °C may be attributed by the carbon whisker-like species on nickel catalysts that caused the breakdown of catalyst and increased the ΔP rather than the deactivation of Ni surface [33].
Fig. 6. CO2-TPD profiles of the nickel-cobalt monometallic and bi-metallic catalysts over γ-Al2O3-HY zeolite.
Table 4 Basicity form CO2-TPD of Ni–Co bi-metallic catalysts over γ-Al2O3- HY catalysts. Catalysts
Total basic site, (μmol CO2/g cat)
10%Ni/γ-Al2O3-HY 10%Co/γ-Al2O3-HY 7%Ni-3%Co/γ-Al2O3-HY
5.0099 5.6194 15.3839
4. Conclusions
respectively. From the previous study, after having increased Ni/Co ratio the strong basic sites were increased. Therefore, the combining effect between the presence of Ni and missing Co3+ (from XPS results) may lead to the formation of the new active site. At this point also lead to enhance the basicity of the bimetallic catalysts [26]. The quantity of the basic site of the catalysts is shown in Table 4. The 7%Ni-3%Co /γAl2O3-HY showed the highest amount of basic sites. According to Jang et al., the mildly acidic CO2 activation that oxidizes the surface carbon can be accelerated by increasing the basicity of the catalysts. Therefore, the bi-metallic catalysts might activate CO2 on the catalyst surface, which would be provided from basicity higher than other catalysts [27,28].
In the DRM, high metal dispersion confirmed by SEM-EDX and high amount of metal active sites verified by the CO adsorption, on the Ni–Co bi-metallic system are the reasons behind the improved stability when Co is introduced. In addition, the weak bonding strength of the coke on the bi-metallic surface as determined by the H2-TPR analysis showing high reduction temperature suggested enhanced stability against coking due to the stronger metal-support interaction. Thus, the bi-metallic catalysts support over γ-Al2O3-HY zeolite exhibits higher activity and stability than the monometallic ones because (i) high metal dispersion, (ii) a large number of active sites, and (iii) strong metalsupport interaction between the Ni–Co bimetallic and γ-Al2O3-HY zeolite.
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Fig. 7. Catalysts performance of (A) CO2 conversions, (B) CH4 conversions, (C) H2 selectivity, and (D) CO selectivity of dry reforming of methane reaction.
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Fig. 8. Thermogravimetric analysis of the spent catalysts after 180 min of reaction.
Acknowledgments This research was financially supported by Center of Excellence on Catalysis and Catalytic Reaction Engineering (CECC), Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University.
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Investigation on the increased stability of the Ni–Co bi-metallic catalysts for the carbon dioxide reforming of methane Anchittha Liua, Supareak Praserthdamb, Suphot Phatanasria,
⁎
a
Center of Excellence on Catalysis and Catalytic Reaction Engineering (CECC), Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok, 10330, Thailand b High-performance Computing Unit (CECC-HCU), Center of Excellence on Catalysis and Catalytic Reaction Engineering (CECC), Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand
ARTICLE INFO
ABSTRACT
Keywords: CO2 reforming Ni–Co catalysts HY zeolite ɤ-Al2O3-HY supported Bi-metallic catalysts
The carbon dioxide reforming of methane is one of the processes used to tackle global warming by reducing carbon dioxide, one of the most harmful greenhouse gases through the catalytic reaction with methane which produces syngas. Although the noble metals are good candidates for such a process, the cost and scarcity limit their uses. The non-noble metals on the other hand, with satisfactory activity and cost, are of interest except their stability against the coke formation, the main deactivation scheme during the dry reforming reaction. Therefore, the study of the enhanced stability found in the bi-metallic system of Ni catalysts via the introduction of Co was carried out in this work. The activity and coke resistance were studied on both mono and bi-metallic system of the Ni-based catalysts (10%wt) supported on γ-Al2O3-HY zeolite prepared via the sol-gel method. The activity testing was performed in a fixed-bed reactor with a unity feed volume ratio under atmospheric pressure at 973 K, where the Ni–Co bi-metallic system exhibits higher activity than both mono-metallic of either pure Ni or Co due to a higher number of active sites confirmed by the CO chemisorption. On the stability testing, the high stability was found on the bi-metallic system as verified by the time-on-stream testing. In addition, the higher reduction temperature for the bi-metallic system via the analysis using H2-TPR also suggested stronger metal-support interaction which may decrease the bonding strength of the coke on the surface leading to lower deactivation.
1. Introduction The greenhouse gas (GHG) emission contributes to one of the most concerned issues, where CO2 and CH4 are the main components of greenhouse gas. Utilizing these two main culprits via the CO2 reforming of CH4 or the dry reforming reaction is of interest since not only the GHG could be reduced but it also produces useful products, the syngas to be used as feed to many processes. The dry reforming reaction of CH4 (DRM) (shown in Eq. (1)) is an endothermic process which requires high temperature; thus, high energy supply. This process has many side reactions [1], in which it may be accompanied by a parallel reaction of the reverse water gas shift (RWGS) (Eq. (2)), methanation (Eq. (3)), carbon monoxide decomposition by the Boudouard reaction (Eq. (4)), which is the main cause in catalyst deactivation during DRM. In addition, the undesired methane decomposition reaction (Eq. (5)), and CO reduction (Eq. (6)) also occur [2]. CH4 + CO2 ↔ 2CO + 2H2 ΔH°298 = +247.3 kJ/mol
⁎
(1)
CO + H2O ↔ CO2 + H2 ΔH°298 = −41.0 kJ/mol
(2)
CO2 + 4H2 ↔ CH4 + 2H2O ΔH°298 = +165.0 kJ/mol
(3)
2CO ↔ C + CO2 ΔH°298 = −172.0 kJ/mol
(4)
CH4 ↔ C + 2H2 ΔH°298 = +75.0 kJ/mol
(5)
CO + H2 ↔ C + H2O ΔH°298 = −131.0 kJ/mol
(6)
Normally, the RWGS reaction always occurs during DRM. Therefore, to inhibit RWGS would be the key towards better catalysts which can produce syngas with high H2 selectivity and/or H2/CO ratio [1]. Due to that most catalysts are not stable at these high temperatures since the result of catalysts sintering and crystal structure collapsing [3,4]. Consequently, noble (Rh, Ru, Pd, Pt, etc.) and non-noble (Ni, Co, Fe, Cu, etc.) metals were the based suitable catalysts for DRM. The nonnoble metals have good catalytic performance than non-noble metals for a large number of studies. Because the cost-effectiveness many researchers have focused attention on the non-noble metal with high
Corresponding author. E-mail address: [email protected] (S. Phatanasri).
https://doi.org/10.1016/j.cattod.2019.07.047 Received 18 February 2019; Received in revised form 26 July 2019; Accepted 29 July 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Anchittha Liu, Supareak Praserthdam and Suphot Phatanasri, Catalysis Today, https://doi.org/10.1016/j.cattod.2019.07.047
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Al2O3 precursor and HY zeolite with Si/Al molar ratio = 100 (TOSHO). For the monometallic and bimetallic Ni–Co catalysts were prepared by the incipient wetness impregnation method using Ni(NO3)2.6H2O (Aldrich) and Co(NO3)2.6H2O (Aldrich) as metal precursors with different metal loading (10Ni, 10%Co, 3%Ni-7%Co, 5%Ni-5%Co, 7%Ni3%Co). After that, the catalysts were dried overnight at 110 °C and calcined under an airflow at 550 °C for 2 h. (In this present work, we design to study only the 10%Ni, 10%Co and 7%Ni-3%Co, but 3%Ni7%Co and 5%Ni-5%Co can study from our previous study [11].)
Table 1 Physical properties and chemical composition of catalysts. Catalysts
10%Ni/ γ-Al2O3-HY 10%Co/ γ-Al2O3-HY 7%Ni-3%Co/ γ-Al2O3-HY a b
N2 physisorption results BET surface area (m2/g)
Pore volumea (cm3/g)
Average pore diameterb (Å)
524.68 543.84 513.58
0.26 0.25 0.23
60.89 63.51 63.09
BJH desorption pore volume. BJH desorption average pore diameter.
2.2. Catalyst characterization The X-ray diffraction (XRD) was used to analyze the crystalline phase by using X-ray diffractometer SIEMENS D 5000 at Cu-Kα radiation between 20° and 80°. The N2-physisorption technique was used to determine the specific surface area of the catalyst (Brunauer–Emmett–Teller (BET) method) and to examine pore diameter and pore volume of the prepared catalyst (Barret-Joyner-Halenda (BJH) method) by using Micromeritics ASAP 2020. The scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX) was used to study morphology structure and agglomeration of the particles which is analyzed by the JEOL mode JSM-5800LV of scanning electron microscopy technique. The X-ray photoelectron spectroscopy (XPS) was used to determine the chemical state on the surface of catalysts using Kratos Amicus X-ray photoelectron spectrometer with Mg Kα X-ray as a source. The internal standard was the XPS spectra of C 1s line at a binding energy of 285.0 eV. The Micromeritics ChemiSorb 2750 Pulse Chemisorption System was used for the analysis of the hydrogen temperature-programmed reduction (H2-TPR). The sample was pre-treated at 500 °C for 1 h in N2. Afterward, the sample was heated with 25 ml/ min of 10% H2 in Ar mixture gas from 30 °C to 800 °C. The amount of H2 uptake was measured as a function of temperature using a Thermal Conductivity Detector (TCD). The as-spun alumina fibers are subjected to the thermogravimetric and differential thermal analysis (Diamond Thermogravimetric and Differential Analyzer, TA Instruments SDT Q600) to determine the carbon content in the sample in the temperature range from room (approximately 28 °C) to 1000 °C with a heating rate of 10 °C/min in 100 ml/min of airflow. Also, a Micromeritics ChemiSorb 2750 Pulse Chemisorption System was used to carry out the temperature-programmed desorption of carbon dioxide (CO2-TPD) in order to determine the basicity of catalysts. The samples were pretreated in helium flow at 500 °C for 1 h prior to the adsorption of carbon dioxide for 1 h to ensure saturation of such gas on the surface. Then, the sample was heated from 30 °C to 800 °C with a heating rate of 10 °C/ min. The profile of the desorbed CO2 was measured as a function of temperature using the TCD. The amount of metal active sites and the metal dispersion of the catalysts were evaluated by CO chemisorption
performance and low cost [5]. In an industrial, the Ni-based catalyst is popularly to use in DRM because of its availability, suitable activity, and low cost, although it is easier to coke formation than other noble metal, resulting in the decrease of the catalyst’s activity [6,7]. Furthermore, using second metal is one of the methods applied for developing carbon resistance of the Ni-based catalyst [8]. Addition of Co that is the second metal has been applied for developing the carbon resistance [9]. Therefore, the modification of Ni-based catalysts is possible to improve catalysts performance and anti-carbon deposition. Silicon, aluminum, and oxygen make crystalline solids structure that forms a connected framework as zeolites. Zeolites were variously used in many applications due to the improvement in thermal, high surface area, well-defined structure and high affinity for CO2 [10]. The use of Ni-based catalysts on zeolites in CO2 reforming of methane reaction has been studied by Apanee et al. [7]. They found that Ni/zeolite Y had better catalytic performance than the other types of studied zeolites (zeolite A, zeolite X, and ZSM-5). Moreover, 7 wt% Ni loading showed the best catalytic activity on each zeolite support. Madhi et al. [6] investigated the optimized composition of Ni–Co bimetallic catalyst over zeolite Y. The activity tests displayed that Ni7Co3/Y activity was higher than zeolite Y and Ni3Co7/Y catalyst at all temperatures. Therefore, it seems that the Ni-based catalysts over γ-Al2O3 and zeolite Y may have improved activity for CO2 reforming of methane reaction. In this work, the mono and bi-metallic system of the Ni-based catalysts over γ-Al2O3 and HY zeolite support was studied in terms of the catalytic properties and investigated the stability of the catalyst in CO2 reforming of methane reaction. 2. Experimental 2.1. Catalyst preparation The γ-Al2O3-HY zeolite supported catalyst was prepared by a sol-gel method using Alumina isopropoxide (Aldrich) and ethanol (Merck) as
Fig. 1. X-ray diffraction pattern of Ni–Co catalysts supported on γ-Al2O3-HY zeolite (a: 10%Ni/γ-Al2O3-HY zeolite, b: 10%Co/γ-Al2O3-HY zeolite, c: 7%Ni-3%Co/γAl2O3-HY zeolite). 2
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analysis using the Micrometritics Chemisorb 2750 fitted with a TCD and the ASAP 2101CV.3.00 software unit. The samples were reduced at 500 °C with 25 ml/min of H2 flow for 1 h. For consistency of the obtained CO peak, the 20 μl of CO was injected to the reactor at room temperature until three peaks of the same high were obtained for three consecutive calibrations. Finally, the CO adsorption on the catalyst was assumed that one CO molecule would bind one metal site only.
a catalyst with high average pore diameter can improve active metals dispersion, pore structures, and provide higher active surface area per weight of the active metal [12,13]. Fig. 1 shows the XRD patterns of fresh catalysts. The XRD peaks of NiO at 2ɵ = 37.2°, 43.3°, and 62.8° can be seen, indicating the (111), (200), and (220) planes of NiO (JCPDS 01-073-1519), respectively. The XRD peaks of Co3O4 cubic phase at 36.9°,44.9°, and 65.3° indicate (311), (400), and (440) planes of Co3O4(JCPDS 01-076-1802), respectively. It demonstrated that Ni and Co species were well dispersed on catalysts surface due to the existing overlaps peaks of NiO and Co3O4 of 7%Ni-3%Co/γ-Al2O3-HY XRD pattern are difficult to differentiate [9]. The lower intensity of NiO and Co3O4 over 7%Ni-3%Co/γ-Al2O3-HY zeolite presented that the incorporation of Co species increased the dispersion of NiO, which is in the same way with the other researchers reported results [14,15]. Fig. 2 shows the morphology of fresh Ni–Co over Al2O3-HY zeolite catalysts which was studied by Scanning Electron Microscopy (SEM). Most catalysts had similar surface morphology and particle size as in accordance with BET pore diameter and pore volume results. The state of Ni and Co on monometallic and bi-metallic catalysts surface are detected by XPS analysis as shown in Fig. 3. In Fig. 3a shows Ni 2p3/2 XPS profile of 10%Ni/γ-Al2O3-HY zeolite and Fig. 3b shows Ni 2p3/2 XPS profile of 7%Ni-3%Co/γ-Al2O3-HY zeolite, the peak of Ni3+ are around 856.96 and 857.17 eV respectively [16], and the peak positions around 863 eV are the satellites [17]. While Co 2p3/2 XPS profile of 10%Co/γ-Al2O3-HY zeolite and 7%Ni-3%Co/γ-Al2O3-HY zeolite are shown in Fig. 3c and d respectively. For the Co particles covered with Co3O4 of 10%Co/γ-Al2O3-HY zeolite, the peaks were located at 782.99, 787.75 and 790.58 eV are assigned to Co3+, Co2+ and satellite respectively. Similarly, the peaks of 7%Ni-3%Co/γ-Al2O3-HY zeolite located at 782.6, 786.84 and 790.69 respectively [18,19]. In addition, the surface atomic composition of the catalysts with different phase compositions are shown in Table 2. It was found that Co3+ had more atomic concentration that Co2+ in 10%Co/γ-Al2O3-HY zeolite. The electron configuration of Co2+ and Co3+ are [Ar] 3d7 and [Ar] 3d7 4 s2,
2.3. Catalytic reaction study The DRM was carried out in a fix-bed reactor continuous-flow quartz reactor at atmospheric pressure. First, the catalyst was packed on quartz wool in the middle of the quartz tube reactor. Afterward, the catalyst was reduced at 500 °C for 1 h. with 50 ml/min of H2 flow before the reaction. Then, the 50 ml/min of N2 was purged to remove excess H2 prior to the heating of the catalyst to the reaction temperature of 700 °C. The feed volume ratio of 1:1 CH4:CO2 ratio was set for the reactor, in which the reaction testing is carried out for 3 h. with a total flow rate of 60 ml/min. Finally, the feed and products were analyzed for their compositions using the Thermal Conductivity Detector (TCD) type gas chromatograph (Shimadzu, GC-8A) equipped with Porapack-Q and Molecular sieve 5A packed column, in which the Argon was used as a carrier gas. 3. Results and discussion 3.1. Catalyst characterization One of the key factors affecting the catalyst’s activity is its surface area [6]. As a result, the catalyst surface area was measured by the BET method where the result is shown in Table 1. It was found that the specific surface area of the bi-metallic 7%Ni-3%Co/γ-Al2O3-HY was less than both mono-metallic systems: the 10%Ni/γ-Al2O3-HY and 10%Co/ γ-Al2O3-HY. This may be caused by the support’s pore blockage by both Ni and Co species. Also, from the previous study, it was found that using
Fig. 2. SEM images of fresh catalysts. (a: 10%Ni /γ-Al2O3-HY zeolite, b: 7%Ni-3%Co/γ-Al2O3-HY zeolite, c: 10%Co/γ -Al2O3-HY zeolite).
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Fig. 3. XPS spectra of Ni 2p for (a) 10%Ni/γ-Al2O3-HY zeolite, (b) 7%Ni-3%Co/γ-Al2O3-HY zeolite and Co 2p for (c) 10%Co/γ-Al2O3-HY zeolite, (d) 7%Ni-3%Co/γAl2O3-HY zeolite.
Table 2 The surface atomic composition of the Ni, Co, and Ni–Co over γ-Al2O3- HY catalysts with different phase compositions. Sample
10%Ni/ γ-Al2O3-HY 10%Co/ γ-Al2O3-HY 7%Ni-3%Co/ γ-Al2O3-HY
Atomic concentration (%) Ni3+
Co3+
Co2+
100 0 64.19
0 71.20 19.44
0 28.79 16.36
Table 3 CO chemisorption results of Ni, Co, and Ni–Co over γ-Al2O3- HY catalysts.
Atomic ratio Ni/Co
– – 1.79
Catalysts
Active sites (×1018)
%Dispersion
10%Ni/γ-Al2O3-HY 10%Co/γ-Al2O3-HY 7%Ni-3%Co/γ-Al2O3-HY
1.16 4.64 13.81
0.11 0.45 1.35
On the other hand, the metal dispersion is also confirmed by the EDX analysis. The Ni, Co, Si, and Al can be seen in this analysis illustrated in Fig. 4, which is consistent with the XRD results. The density of Ni and Co are shown as colored dots in the Fig. 4 of the bi-metallic was highest compared to the monometallic ones, in which this is consistent with the results calculated from the amount of adsorbed carbon monoxide from CO chemisorption. The peaks in H2 TPR profiles of the catalysts can be observed in Fig. 5, where it was used to investigate behavior during the reduction of the samples. Metal, support, and metal-support interaction affect the H2-TPR profiles, resulting in different reducibility and reduction temperature of catalysts. According to Afzal et al. [24], the reduction of the Ni2+ localized in the supercage and/or sodalite cavities are the reduction peaks found at low temperature, while at the high temperature the peak is the reduction of the Ni2+ localized in hexagonal cavities. According to H2-TPR profiles of catalysts, it was obviously seen that the bi-metallic Ni–Co catalysts exhibited two reduction peaks. From the previous work, the first peak is related to the reduction from Co3O4 to CoO which was observed at 337 °C and 367 °C for 10%Co/γ-Al2O3-HY and 7%Ni-3%Co/γ-Al2O3-HY, respectively. The second peak is related to a reduction from NiO to Ni and CoO to Co metallic that was located at 378 °C, 376 °C and 622 °C for 10%Ni/γ-Al2O3-HY, 10%Co/γ-Al2O3-
respectively. Normally, when the transition metals are ionized, the electrons were removed from the valence-shell s orbitals before they are removed from valence d orbitals due to the relative energies of the atomic orbitals. The 4s orbital had lower energy than the 3d orbitals and lower energy was more stable. For bi-metallic, Co3+ was decreased but still remaining more than Co2+ which may be caused by the effect of Ni [20–22]. The amount of the CO chemisorbed on the mono and bi-metallic at room temperature is shown in Table 3 which are used to calculate the number of the active site of the catalyst. The bi-metallic of 7%Ni-3%Co/ γ-Al2O3-HY has the highest number of the active site and metal dispersion. In addition, from the XRD result, the bi-metallic catalyst has lower intensity peak of NiO and Co3O4 than the monometallic catalyst. In the same ways, the bi-metallic catalyst had decreased the oxide form but increased the amount of metal due to the amount of CO adsorbed to the metal on the surface. While the dispersion of metal atoms on the catalyst surface can be illustrated by the ratio between the total number of metal atoms that is accessible to the adsorbate divided by the total metal atoms in the sample [23].
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Fig. 4. EDX analysis of all mono and bi-metallic catalysts. (a: 10%Ni/γ-Al2O3-HY zeolite, b: 7%Ni-3%Co/γ-Al2O3-HY zeolite, c: 10%Co/γ-Al2O3-HY zeolite).
carbon formation in CO2 reforming of methane. The CO2 desorption patterns of bi-metallic catalysts exhibit the highest peak compare to the monometallic catalysts, in which the first peak around 90 °C and the second around 655 °C attributed to the weak and strong basic sites,
HY and 7%Ni-3%Co/γ-Al2O3-HY in catalysts, respectively [25]. Fig. 6 displays the CO2 desorption patterns for all fresh catalysts. The CO2-TPD of the catalysts were used to determine the catalyst basicity which is a key property for determining the resistance against
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3.2. Catalyst performances The stability of the catalysts was tested at 700 °C for 180 min. Before the reaction, the prepared catalysts were reduced in flowing H2 for 1 h. The feed conversion and product selectivity were calculated based on the amount of H2 and CO product of Ni–Co/γ-Al2O3-HY for CO2 reforming of methane which was provided in Fig. 7. The CO2 conversion of the catalyst is higher than the CH4 conversion due to the reverse water-gas shift reaction [1]. The results showed that there was no significant deactivation for 7%Ni-3%Co/γ-Al2O3-HY and 10%Co/γ-Al2O3HY catalyst after 180 min meaning that during the activity testing the activity of 7%Ni-3%Co/γ-Al2O3-HY and 10%Co/γ-Al2O3-HY catalyst is stable while the 10%Ni/γ-Al2O3-HY catalysts were decreased verified by time-onsteam. This is due to the amount of coke deposition during DRM over all of the catalysts [8]. According to the CO chemisorption results, high amount of active sites and high metal dispersion of catalysts are important for a stable and active DRM catalyst [12,13]. Moreover, the 10%Co/γ-Al2O3-HY catalyst had lower activity than the other two catalysts due to lower activity of Co metal compared to Ni [29]. According to Tanios C. et al., the amount of deposited carbon can be reduced by the addition of Co. Nevertheless, the Co decreases in activity [30] same as results from the previous study that the activity was decreased when Co content increased [11]. Therefore, the catalytic performance on bi-metallic catalyst had a better activity trend than monometallic catalysts.
Fig. 5. The TPR profiles of the nickel-cobalt monometallic and bi-metallic catalysts over γ-Al2O3-HY zeolite.
3.3. Catalyst characterization after the performance test Thermogravimetric analysis with heating temperature ranging from room temperature to 1000 °C was used to investigate the possible coke deposition of the used catalysts as shown in Fig. 8. The moisture and other volatile contents may correspond to the weight loss at the temperature range up to 500–600 °C [31]. According to the result, the weight loss of 10%Ni/Al2O3-HY zeolite represented the higher weight loss (75%) than that of 7%Ni-3%Co/Al2O3-HY zeolite (65%) and 10%Co/Al2O3-HY zeolite represented the lowest weight loss that was no significant deactivation due to the Ni had a major role in coke accumulation [32] while Co decreased the amount of deposited carbon [30]. Although the used catalysts had considerable weight loss, the catalyst deactivation was not so significant to decrease the reaction activity as the deactivation of catalysts at reaction temperatures of 700 °C may be attributed by the carbon whisker-like species on nickel catalysts that caused the breakdown of catalyst and increased the ΔP rather than the deactivation of Ni surface [33].
Fig. 6. CO2-TPD profiles of the nickel-cobalt monometallic and bi-metallic catalysts over γ-Al2O3-HY zeolite.
Table 4 Basicity form CO2-TPD of Ni–Co bi-metallic catalysts over γ-Al2O3- HY catalysts. Catalysts
Total basic site, (μmol CO2/g cat)
10%Ni/γ-Al2O3-HY 10%Co/γ-Al2O3-HY 7%Ni-3%Co/γ-Al2O3-HY
5.0099 5.6194 15.3839
4. Conclusions
respectively. From the previous study, after having increased Ni/Co ratio the strong basic sites were increased. Therefore, the combining effect between the presence of Ni and missing Co3+ (from XPS results) may lead to the formation of the new active site. At this point also lead to enhance the basicity of the bimetallic catalysts [26]. The quantity of the basic site of the catalysts is shown in Table 4. The 7%Ni-3%Co /γAl2O3-HY showed the highest amount of basic sites. According to Jang et al., the mildly acidic CO2 activation that oxidizes the surface carbon can be accelerated by increasing the basicity of the catalysts. Therefore, the bi-metallic catalysts might activate CO2 on the catalyst surface, which would be provided from basicity higher than other catalysts [27,28].
In the DRM, high metal dispersion confirmed by SEM-EDX and high amount of metal active sites verified by the CO adsorption, on the Ni–Co bi-metallic system are the reasons behind the improved stability when Co is introduced. In addition, the weak bonding strength of the coke on the bi-metallic surface as determined by the H2-TPR analysis showing high reduction temperature suggested enhanced stability against coking due to the stronger metal-support interaction. Thus, the bi-metallic catalysts support over γ-Al2O3-HY zeolite exhibits higher activity and stability than the monometallic ones because (i) high metal dispersion, (ii) a large number of active sites, and (iii) strong metalsupport interaction between the Ni–Co bimetallic and γ-Al2O3-HY zeolite.
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Fig. 7. Catalysts performance of (A) CO2 conversions, (B) CH4 conversions, (C) H2 selectivity, and (D) CO selectivity of dry reforming of methane reaction.
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Fig. 8. Thermogravimetric analysis of the spent catalysts after 180 min of reaction.
Acknowledgments This research was financially supported by Center of Excellence on Catalysis and Catalytic Reaction Engineering (CECC), Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University.
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