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Syngas production via CO2 reforming of methane using Co-Sr-Al catalyst
Accepted Manuscript Title: Syngas production via CO2 reforming of methane using Co-Sr-Al catalyst Author: Anis Hamza Fakeeha Muhammad Awais Naeem Wasim Ullah Khan Ahmed Sadeq Al-Fatesh PII: DOI: Reference:
S1226-086X(13)00219-0 http://dx.doi.org/doi:10.1016/j.jiec.2013.05.013 JIEC 1363
To appear in: Received date: Revised date: Accepted date:
25-2-2013 6-5-2013 10-5-2013
Please cite this article as: A.H. Fakeeha, M.A. Naeem, W.U. Khan, A.S. Al-Fatesh, Syngas production via CO2 reforming of methane using CoSr-Al catalyst, Journal of Industrial and Engineering Chemistry (2013), http://dx.doi.org/10.1016/j.jiec.2013.05.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Syngas production via CO2 reforming of methane using Co-Sr-Al catalyst
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Anis Hamza Fakeeha, Muhammad Awais Naeem, Wasim Ullah Khan, Ahmed Sadeq Al-Fatesh* Chemical Engineering Department, College of Engineering,
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* King Saud University P.O. Box 800, Riyadh 11421, Kingdom of Saudi Arabia
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* Corresponding author. Tel 009661-4676859 Fax 009661-4678770 Email [email protected]
Abstract
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The effect of addition of strontium in Co based catalysts during CO2 reforming of methane was investigated in the temperature range 500–700C. The Co/γ-Al2O3 supported catalysts with strontium as a promoter (0–2.25 wt%) were prepared by incipient wet impregnation method.
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Numerous techniques such as N2 adsorption-desorption isotherm, H2 temperature-programmed reduction (TPR), temperature-programmed desorption (TPD), X-ray diffraction (XRD),
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thermogravimetric analysis (TGA), Transmission Electron Microscopy (TEM), pulse
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chemisorption and temperature-programmed oxidation (TPO) were applied for characterization of fresh and spent catalysts. The results of characterizations and catalyst activity test revealed
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that introduction of Sr in Co/γ-Al2O3 catalyst had significant effect on stability and coke suppression. The Sr addition improves the metal support interaction as well as enhances the Lewis basicity of the catalyst. The improvement in basicity helps the chemisorption and dissociation of CO2 over the catalyst which in turn reduces carbon deposition. Key words: CO2/CH4 Reforming, Catalyst, Basicity, Coke, Stability, Sr, Co
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1. Introduction The global climatic variations and environmental protection regulations stress on the reduction of discharge of green-house gases into the atmosphere. Among the green-house gases, CO2 and
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CH4 emissions have been blamed the leading driving factor that originates the phenomenon of global warming [1-3]. Numerous research activities and technologies are in progress to mitigate and/or transform these gases into other valuable chemical products and fuels. The carbon dioxide
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or dry reforming of methane has gained special considerations from the last decade due to following reasons: (i) it produces syngas with H2/CO ratio near unity which is suitable for
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various industrial processes such as oxo- and Fischer-Tropsch synthesis and (ii) the process
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simultaneously utilizes two green-house gases i.e. CO2 and CH4 [4-7].
Although this unique idea has various environmental and economic inspirations, but unfortunately, no commercial processes for reforming of methane with CO2 is fully developed
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yet due to some complications of the process. The major deficiency of the dry reforming of methane (DRM) is the requirement of high operating temperature to get high conversions of both
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CH4 and CO2 due to the strong endothermic nature of the reaction. These harsh reaction conditions usually favor the sintering of active metal ensembles and deposition of huge quantity
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of carbon on the catalyst surface, resulting in catalyst deactivation and reactor blocking [8-10].
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The dry reforming reactions and the associated side reactions are as follows:
CO 2 CH 4
2CO 2H 2
CO 2 H 2
CO H 2 O
(1) (2)
2CO C CO2
(3)
C 2H 2
(4)
CH 4
The inhibition of carbon deposition over the catalyst surface is the ultimate challenge in CO2 reforming of methane. Numerous researchers reported group VIII transition metals’ better performance as active metal component for reaction (1). Among group VIII metals, noble metals show higher activity and resistance to carbon deposition [11, 12]. High cost and less abundance of noble metals present nickel and cobalt as an alternative. The main drawback of supported 2 Page 2 of 32
nickel catalysts is its deactivation due to carbon formation leading to reactor blockage. Long term industrial applications require catalysts having better stability. So a catalyst with slightly lower activity as compared to nickel and noble metals but being capable of performing for longer period of time is the right choice for dry reforming reaction. Recent studies revealed that
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supported cobalt catalysts show considerable activity for methane dry reforming process. Additionally cobalt based catalysts are reported to have higher resistance to carbon deposition.
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Taking into consideration comparable activity and coke resistance, cobalt can be a potential active component alternative to nickel [11, 13].
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In order to improve the stability and to avoid coke formation over cobalt based catalysts numerous approaches can be applied such as; use alkaline and/or rare earth metal oxides e.g.
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MgO, La2O3, Ce2O3 as a support or modify the catalysts by the addition of promoters e.g. K, Ce, Sm, Pr [14, 15]. In fact these promoters can complete their jobs as follows: (i) the promoters can
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increase metal support interaction (MSI) and active metal dispersion (ii) the promoters can change the acidic/basic character of catalyst [16].
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Q. Jing et al. [17] reported that 10wt% Sr loading in 5%Ni/SrO-SiO2 catalyst increases the Ni dispersion, metal-support interaction and CO2 adsorption which in turn improve the catalytic
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activity and stability for long term operation.
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K. Sutthiumporn et al. [18] studied the promotional effect of alkaline earth elements on Ni-La2O3 catalysts for dry reforming of methane. They revealed that Sr addition not only enhanced the CH4 and CO2 conversions but also reduced the carbon deposition on the catalyst surface. In the present research, a set of -Al2O3 supported Co-Sr catalysts, having different Sr loading, were prepared and tested in a tubular reactor for carbon dioxide reforming of methane in order to find a more promising catalyst with optimum Co/Sr metal ratio assuring high activity and stability with a decrease in coke deposition.
2. Experimental 2.1. Catalyst preparation The high surface area Alumina (γ-Al2O3) SA6175 supplied by Norton was used as the catalyst support throughout this investigation. The nitrate salts of Cobalt and Strontium i.e. 3 Page 3 of 32
(Co(NO3)2.6H2O) and (Sr(NO3)2 (Sigma Aldrich) were used as precursors for active metal and promoter respectively.
The Co/γ-Al2O3 catalysts (10wt% Co) were prepared by wet
impregnation method. The solution containing predetermined amount of (Co(NO3)2.6H2O) was prepared in double distilled water, then γ-Al2O3 was impregnated with the previously prepared
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solution of active metal salt. The Sr promoted catalysts with different Sr loading 0–2.25 wt% were prepared by co-impregnation of nitrate salts of the promoter and active metal with γ-Al2O3
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support using the same procedure mentioned above. After impregnation all the catalysts were dried overnight at 120°C and subsequently calcined at 600°C for 3 h.
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2.2. Catalyst Testing
Activity measurements were conducted at atmospheric pressure in a conventional flow apparatus
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consisting of a flow measuring and control system, a mixing chamber, a fixed bed tubular reactor and an on-line gas chromatograph. The tubular reactor (ID 13 mm) was electrically heated by a
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furnace with four independently heated zones; the temperature profile was measured using a thermocouple placed in an axial thermowell centered in the catalyst bed. This system was
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integrated to the GC (Varian Star 3400) and integrated line was heated with a heating tape to avoid the water condensation in the line. For each experiment the 0.6 gram weight of catalyst
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was used which was reduced under H2 flow (40 ml/min) at 550C for 2 h followed by N2 flow (30 ml/min) for 20 minutes prior to reaction. The reaction mixture CH4:CO2:N2 in the proportion
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17:17:2 at a total flow rate of 36 ml/min was used over catalyst bed for experiment. The effluent gases were analyzed on-line by the GC equipped with TCD detector. The two columns, Porapak Q and Molecular Sieve 5A, were used in series/bypass connections for the complete separation of reaction products. The comprehensive details of experimental setup and procedure are given elsewhere [19].
2.3. Catalyst characterization
The specific surface area of the fresh and used catalyst was measured with a Micromeritics Tristar II 3020 surface area and porosity analyzer from N2 adsorption desorption data at -196C. For each analysis, 0.3 g of catalyst was used. The degassing of the samples, before experiment, was done at 300C for 3 h to get rid of the moisture content and other adsorbed gases from the
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surface of catalyst. The pore distribution was calculated from adsorption branch of the corresponding nitrogen isotherm by applying the Barrett, Joyner, and Halenda (BJH) method. The temperature programmed reduction with hydrogen (H2-TPR), temperature programmed
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oxidation (TPO), pulse chemisorption and temperature programmed desorption (CO2-TPD) measurements were completed on a chemisorption apparatus (Micromeritics 2920 Auto Chem II). For H2-TPR, 40 mg of the sample was pretreated with high purity Argon (Ar) flow at 150°C
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for 30 min, followed by cooling it to room temperature. Then the sample was heated in a furnace up to 1000°C with a constant heating rate of 10°C/min using a flow of H2/Ar mixture (volume
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ratio, 10/90) under a flow rate of 40 ml/min. The signal of H2 consumption was monitored by a thermal conduction detector (TCD). The CO2 temperature programmed desorption (CO2-TPD)
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measurements were performed on fresh catalysts. The 70 mg of sample was first held at 200°C for 1 h under flowing He to remove physically adsorbed and/or weakly bound species. CO2
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adsorption was carried out at 50°C for 30 min by passing CO2/He mixed gas (volume ratio, 10/90) with a flow rate of 30 ml/min. The TPD signal was recorded by TCD with linear
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temperature increase up to 800°C at a rate of 10°C/min. In order to find the nature of carbon deposition on spent catalysts, TPO experiments were
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performed. The catalyst samples, used previously in the dry reforming reaction, were dried at 150°C for 30 min, under helium (He) flow (30 ml/min) and cooled down to room temperature,
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followed by an increase of temperature under O2/He (30 ml/min) flow with a temperature ramp of 10°C/min to 1000°C. The metal dispersion of the catalysts was measured by CO chemisorption at 50°C. Prior to the pulse chemisorption experiments, all samples were reduced under H2 flow (25 ml/min) at 800°C and subsequently flushed under He. Powdered X-ray diffraction (XRD) analysis of fresh and used catalyst was carried out using a Rigaku (Miniflex) diffractometer with a Cu Kα radiation operated at 40 kV and 40 mA. The scanning step and range of 2θ for analysis were 0.02 and 10-80 respectively. The amount of Coke deposition on the surface of spent catalysts was estimated by thermogravimetric analysis (TGA) in air atmosphere using EXSTAR SII TG/DTA 7300 (Thermogravimetric/Differential) analyzer. The sample of used catalyst, 20 mg in quantity, was heated from room temperature to 800C at a heating rate 20C /min. 5 Page 5 of 32
Transmission Electron Microscopy (TEM) of fresh and spent catalysts were carried out, using a JEOL-1011CX microscope operating at 80 kV accelerating voltage, to analyze the morphology of the deposited carbon and the average particle size of active metal crystal. Prior to TEM analysis the samples were first dispersed ultrasonically in hexane at room temperature. Then, a
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drop of the suspension was put on a lacey carbon-coated Cu grid to take images.
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3. Results and Discussions
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3.1. Characterization of Catalysts 3.1.1. Textural Properties
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The nitrogen adsorption-desorption isotherms for 10wt% Co/γ-Al2O3 promoted and nonpromoted fresh catalysts are shown in Fig. 1. It can be depicted from plots that all N2 adsorption/desorption isotherms are matching to typical type IV adsorption curves and showing a
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hysteresis loop of the H1 type with ordered mesopores. The specific surface area, pore volume and average pore diameter for all Co based fresh and used catalysts are listed in Table 1. From
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the table it is observed that addition of Sr in the Co-alumina catalyst modifies the textural properties of the catalyst, which are evidenced from higher specific surface area and larger pore
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volume. In comparison to Sr promoted catalysts, the smallest pore volume, in case of non-
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promoted Co-alumina catalyst, is probably resulted due to partial blockage of the pores with bigger active metal particles. Whereas the larger pore volume in case of Sr promoted catalysts is indicating better dispersion of active metal over support which avoids blockage of pores due to sintering. From the Table 1 it is apparent that surface area and pore volume of spent catalyst are decreased after 6 h of reaction. In fact these changes are attributed to thermal sintering of active metal and/or carbon deposition over catalyst surface as evidenced by TEM (Fig. 6). It is obvious that the Sr promoted catalysts showed small decrease in surface area as compared to nonpromoted catalyst, after reaction, which indicates that Sr promoter has a certain effect on catalytic behavior of catalyst under the same reaction atmosphere. 3.1.2. Temperature Programmed Reduction (H2-TPR) It is believed that the active species in supported catalysts for dry reforming of methane are the reduced metallic clusters present at the surface of the catalyst. Temperature programmed 6 Page 6 of 32
reduction (TPR) is a very handy technique to find number of reducible species present in catalysts and, at the same time, to characterize the interaction between active metal and support. The TPR profiles of promoted and non-promoted fresh catalysts are presented in (Fig. 2) In the TPR pattern of non-promoted catalyst only one peak at temperature of 465°C is appeared which
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is attributed to the reduction of Co3O4 species having weak or no interaction with support. Generally, the composition of Co3O4 can be split as Co2+(Co23+)O4 in which Co2+ ions are
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located in the tetrahedral coordinated sites and Co3+ ions in the octahedral coordinated sites. In cobalt over alumina supported catalyst due to calcination at high temperatures some ion
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exchange between Co3O4 and Al2O3 occurred which lead to formation of Co-aluminate spinel species e.g., Co2AlO4 and CoAl2O4. Generally, these species have high reduction temperature as compared to Co3O4. Wang et al. [20] have categorized the TPR peaks for Co/Al2O3 catalysts into
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three different temperature regions: region I (<500°C), region II (500-900°C), and region III (>900°C). According to them, on the basis of reducibility sequence, the TPR peak in region I is
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ascribed to Co3O4, that in region II to Co2AlO4, and that in region III to CoAl2O4. It is apparent from Fig. 2 that TPR patterns of Sr promoted catalysts are different, both in broadness and
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intensity, from non-promoted catalyst. The difference in TPR pattern revealed that Sr addition has a definite effect on reduction behavior of these catalysts. As the Sr content is increased the
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main peak becomes bigger in size, which is most likely due to presence of more reducible species in catalyst. For 0.5wt% Sr promoted catalyst, two peaks are appeared with maxima
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centered at temperatures of 470°C and 650°C respectively, for this catalyst the first peak is attributed to reduction of Co3O4 while second broad peak is assigned to reduction of Co2AlO4 species. Whereas, for 1wt% Sr promoted catalyst, these two peaks are observed at slightly higher temperatures i.e., 480°C and 665°C respectively. In fact the positive shift in temperature is due to increase in metal support interaction. In the light of above results it can be concluded that addition of Sr not only improves the metal support interaction but also increases the number of reducible species. Actually the increase in number of reducible species is credited to better dispersion of active metal over catalyst surface which is also confirmed by pulse chemisorption (Fig. 7). Generally, in case of Co/Al2O3 catalyst, during the course of calcination at high temperature, due to very high metal support interaction cobalt aluminates are formed which are catalytically not favorable for methane reforming processes. On the other hand due to very weak metal support interaction at high calcination and/or reaction temperature sintering of active metal 7 Page 7 of 32
component becomes prominent which leads to catalyst deactivation. In short a balanced metal support interaction is preferred for better activity and stability. It is worthwhile to note that no reduction peak of CoAl2O4 was identified in all Sr promoted catalysts which is a good sign according to catalytically point of view. Moreover the formation of Co2AlO4 in Sr promoted
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catalysts is helpful in avoiding sintering by optimizing the metal support interaction, although Co2AlO4 has high reduction temperature but still it can be reduced at low temperature as
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compared to CoAl2O4. From TPR patterns it is clear that compared to all other Sr loadings 1.5wt% showed high metal support interaction, with formation of large amount of Co2AlO4,
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whereas 0.5wt% Sr loaded catalyst showed optimum metal support interaction due to formation of balanced amount of Co2AlO4 and presence of large number of reducible species. The balanced metal support interaction in case of 0.5wt% Sr promoted catalyst contributes to produce smaller
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crystallites/ensembles of active metal which in turn help to avoid sintering and carbon
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deposition. 3.1.3. X-ray diffraction (XRD)
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The XRD patterns of various promoted and non-promoted, fresh and used, Co based catalysts are presented in Figs. 3a and 3b respectively. It is hard to distinguish between Co3O4 and Co2AlO4
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crystalline phases from XRD diffractrogram due to superimposition of metal oxide and metalaluminate spinel phases. For both promoted and non-promoted fresh catalysts, the diffraction
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peaks appearing at 2θ = 31.3°; 2θ = 44.8°; 2θ = 55.6°; 2θ = 59.3° and 2θ = 65.2° correspond to Co3O4 and/or metal aluminate spinel phases (JCPDS: 00-042-1467/JCPDS: 00-038-0814), while the following diffraction peaks detected at 2θ = 19.3°; 2θ = 37.4°; 2θ = 46° and 2θ = 66.7° are attributed to γ-Al2O3 (JCPDS: 00-004-0875). The presence of metal aluminate spinel phases in fresh calcined catalysts is consistence with the TPR results (Fig. 2). It is quite worthy to note that in all Sr promoted catalysts no bulk peak for strontium was identified in diffractograms by the XRD, possibly due to higher dispersion of Sr in catalysts or it is present in small quantity which beyond the detection limit of XRD [17]. Although no bulk peak of Sr was identified in all catalysts but the formation of SrAl2O4, and/or similar kinds of aluminates, cannot be discarded during the course of calcination [21]. The formation of aluminate species was also reported by several researchers in literature for other alkaline earth metals (e.g., Mg and Ca). Koo et al. [22] reported that promotion of Ni/Al2O3 with MgO formed MgAl2O4 spinel phase in catalyst, which 8 Page 8 of 32
effectively prevents coke formation by increasing the CO2 adsorption due to the increase in basic strength of catalyst. In case of spent catalysts the diffraction peaks detected at 2θ = 43.8°; 2θ = 51.6° and 2θ = 75.8° are attributed to metallic Co (JCSPD: 00-015-0806) while the other diffraction peaks observed at 2θ = 37.4°; 2θ = 46° and 2θ = 66.7° are ascribed to γ-Al2O3
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(JCPDS: 00-004-0875). It can be seen from XRD patterns of used catalysts that at 2θ = 26° one extra diffraction peak appeared in case of non-promoted catalyst i.e. 0wt% Sr. Actually this
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diffraction peak is assigned to graphitic carbon which again indicated that non-promoted catalyst has highly suffered from coke deposition during reaction. The absence of this peak in the catalyst formation.
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3.1.4. Temperature programmed desorption (TPD)
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with 0.5wt% Sr promoter confirms its high resistance, during reaction, towards carbon
The basicity i.e. strength of the basic sites of the Sr promoted Co/Al2O3 fresh catalysts were
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evaluated by TPD with CO2 as a probe gas. The strength of the basic sites, usually in TPD, is reported in terms of temperature range where the chemisorbed CO2 on the basic sites is desorbed.
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According to adsorption-desorption phenomenon the adsorbate adsorbed on weaker basic sites desorbed at lower temperature while from stronger basic sites they desorbed at higher °
°
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temperature. Generally, the strength of basic sites is classified into weak (<200 C), medium °
°
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(200–400 C), strong (400–650 C) and very strong (>650 C) depending on the desorption temperature of CO2 [23]. The CO2-TPD profiles of promoted and non-promoted catalysts are presented in Fig. 4. From figure it can be depicted that Co-Al catalyst exhibits the least capacity °
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to adsorb CO2 with two desorption peaks centered at 95 C and 290 C respectively. These desorption peaks are due to presence of low and medium strength basic sites such as bicarbonates. These basic sites results due to the interaction between CO2 and the basic surface hydroxyl groups present over catalyst. On the other hand Sr promoted Co-Al catalysts are exhibiting more than two desorption peaks indicating the increase in basicity of catalysts. The Sr belongs to the alkaline earth metals group; thereby increasing Sr loading in catalysts raises the surface basicity of catalysts. For promoted catalysts, the addition of Sr not only expands the adsorption capacity of acidic CO2 gas but also enhances the basic character of the catalysts by slightly shifting and/or by creating extra desorption peaks at higher temperatures. In contrast to non-promoted catalyst the 0.5wt% Sr promoted catalyst shows four distinct CO2 desorption 9 Page 9 of 32
°
°
°
°
peaks with relative maxima centered at 95 C, 295 C, 385 C and 580 C respectively. The first and last peaks are assigned to weaker and stronger basic sites respectively while second and third peaks may be assigned to medium basic sites. On the contrary the 2.25wt% Sr promoted catalyst °
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°
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showed four distinct peaks with maxima centered at 110 C, 295 C, 580 C and 800 C
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respectively. In this case the first and second peaks can be assigned to weaker and medium strength basic sites while the last two peaks, at higher temperatures, are ascribed to strong and
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very strong basic sites respectively. It is well known that performance of catalyst is strongly influenced by Lewis basicity and strength of basic sites present in catalyst. Generally the strong
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basic sites are more favorable to suppress coke formation than weak and medium. However, it is tough to estimate the catalytic activity and coke resistance by the difference of basic strength
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because the catalytic performance also depends on the several other factors including active metal size, dispersion and reduction degree [22]. The results of basicity in terms of strength of basic sites, for promoted and non-promoted catalysts, are presented in Table 3. It is obvious from
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results that Sr promoted catalysts have more basic sites than non-promoted catalyst.
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3.1.5. TGA and Temperature Programmed Oxidation (TPO)
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The TGA was used to quantify the carbon formation over promoted and non-promoted spent catalysts. In Table 2, the TGA data of both promoted and non-promoted spent catalysts is
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presented. From the TGA data it can be observed that when Sr was introduced in Co-Al catalyst the carbon deposition became lesser. The highest weight loss of 13.9 wt%, due to carbon gasification, was shown for non-promoted catalyst while among all the Sr promoted spent catalysts the smallest weight loss was observed for 0.5wt% Sr promoted catalyst (5.1 wt%). In order to find the nature or type of carbon deposition over spent catalysts, the TPO experiments were performed. The results showed that the rate and type of carbon deposited mainly depends on Sr loading. Normally Co-based catalysts suffer from several forms of the carbon e.g. atomic carbon, amorphous carbon and graphitic carbon. Oxidative atmosphere gasify these carbons to CO2 at different temperature ranges: atomic carbon < 250°C, amorphous carbon 250~600°C and graphitic carbon > 600°C [24]. The TPO profiles for the Co-xSr-Al spent catalysts are presented in Fig. 5. The TPO curves of all promoted and non-promoted spent catalysts showed only one peak in temperature range of 250–540°C, indicating amorphous type of carbon deposition, except for 0.75wt% Sr promoted catalyst which showed two peaks with maxima centered at 10 Page 10 of 32
505°C and 650°C. For this catalyst, the first peak can be attributed to amorphous type carbon while second peak is assigned to graphitic type carbon. The broadness of second peak is indicating the combustion of carbon species of different crystalline forms, e.g. carbon nanofibers (CNFs) and carbon nanotubes (CNTs), etc. From Fig. 5, it is clear that total peak area of
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0.75wt% Sr promoted catalyst is small, since both peaks have very little intensity, as compared to non-promoted catalyst which indicates less amount of carbon formation. The smallest peak
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area, having maxima centered at 310°C, was observed with 0.5wt% Sr, which indicates that it remained stable and hardly suffered from coke formation during reaction as confirmed by TGA
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(5.1 wt% loss). Overall from the TPO patterns it can be concluded that addition of Sr as a promoter in Co-Al catalyst has a significant effect on coke suppression. TPO results are in good
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agreement with the TGA results (Table 2).
3.1.6. Transmission Electron Microscopy (TEM) and CO Pulse Chemisorption
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The TEM images of both Sr promoted and non-promoted, fresh and spent, catalysts are shown in Fig. 6. Figures depict that Co metal particles more uniform distribution of Co metal particles was
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achieved in case of 0.5wt% Sr promoted catalyst as compared to non-promoted catalyst. Highly dispersed Co species are actually formed in Sr promoted catalysts due to the existence of a strong
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interaction between the Co oxide species and support (TPR Fig. 2). The smaller cobalt crystallites were observed for 0.5wt% Sr promoted fresh catalyst, where particles up to 16 nm
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were found (Fig. 6c). In contrast, the non-promoted Co-Al fresh catalyst contains bigger particles up to 45 nm (Fig. 6a). Whisker type of carbon, observed in non-promoted Co-Al spent catalyst (Fig. 6b), is responsible for catalyst deactivation but 0.5wt% Sr promoted spent catalyst showed no significant coke deposition (Fig. 6d). The active metal dispersion and the average metal particle size for the catalysts estimated by CO pulse chemisorption are shown in Fig. 7. From figure, it can be observed that addition of Sr in catalyst not only increases the active metal dispersion but also reduces the metal particle size. The metal dispersion and active metal particle size both are very important factors in controlling the carbon deposition. Similar results were reported by Yu et al. [21] that addition of alkaline earth metals in cobalt based catalysts could increase the dispersion of cobalt and decrease the coke formation. The highest active metal dispersion and smallest metallic crystallite size were attained for 0.5wt% Sr catalyst, which were responsible for its better stability and resistance towards coking. While among all the tested 11 Page 11 of 32
catalysts lowest dispersion and biggest active metallic size were observed in case of nonpromoted catalyst, which made it favorable for carbon formation. 3.2. Catalytic Activity and Stability
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The results of initial (after 30 min) catalytic activity at different reaction temperatures (500, 600 and 700°C) by adding Sr promoter in Co-Al catalysts are presented in Fig. 8 and Table 2. The
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results revealed that the conversions of both CH4 and CO2 were increased with rise in reaction temperature which confirms the endothermicity of dry reforming reaction [25]. Moreover all
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catalysts showed CH4 conversion lower than the CO2 conversion, indicating the occurrence of reverse water gas shift (RWGS) reaction [26, 27]. The existence of RWGS reaction (CO2 + H2 ↔ H2O + CO; ΔH298 = +41 kJ/mol) resulting in smaller H2/CO ratio, less than unity, because it
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consumes some amount of H2 produced in reaction and increases the CO2 conversion. Several mechanistic studies have proposed that during dry reforming reaction, CH4 is
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decomposed first on the active metal sites to yield reactive surface carbonaceous species, which are then oxidized to CO by the oxygen that originate from CO2. In view of that, the rate of
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carbon deposition on the catalyst surface is dependent on the relative rates of the formation of
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carbon deposits and its oxidative removal. The large amount of carbon deposition will take place when the rate of the oxidative removal is slower than the rate of formation [12, 28]. To develop a
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stable and active catalyst is a challenge in dry reforming reactions. Generally, the catalyst stability can be associated with the phenomenon of deactivation due to carbon deposition, sintering and/or loss of active surface due to its oxidation under reaction conditions. Most of the times all these phenomena occur side by side but in some cases one of these dominates and causes severe deactivation [29, 30].
In order to compare catalytic activity, 700°C was fixed as the reaction temperature for estimation of the long-term stability of the catalysts in this study. The influence of Sr promoter on the performance of Co-Al catalyst, at 700°C reaction temperature for 6 h time-on-stream (TOS), in terms of CH4, CO2 conversions and H2/CO ratio is shown in Table 2 and Fig. 9 respectively. It is depicted from data that at the beginning of the reaction the CH4 conversion for non-promoted catalyst was 84.4%, but after 6 h of reaction it reached to 82.1%. This decrease in activity is attributed to the formation of a huge amount of coke deposits on the surface of the catalyst as 12 Page 12 of 32
verified by TGA, TPO, TEM and XRD patterns of spent catalysts (Table 2, Figs. 5, 6 and 3b). Contrarily, the CH4 conversion over 0.5wt% Sr promoted catalyst was 82.4% at the start of the reaction and nearly remained at a constant of 81.2%, with a small decline after 6 h of reaction. The slight decrease in conversion, with deactivation factor 1.45, is signifying its higher stability
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as compared to non-promoted catalyst which has deactivation factor 2.72. The improved stability and resistance against coking of Sr-promoted catalysts are actually credited to their higher
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basicity and better metal support interaction. All other Sr promoted catalysts has also shown stable conversion of methane as compared to non-promoted catalyst, revealing that Sr has a
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significant effect on stability of catalysts. Recently Yu et al. [31] has studied the promotional effect of Sr on Co/Al2O3 catalyst during partial oxidation of methane and they revealed that addition of Sr (>2% mass fraction) has a great influence on activity and stability of catalyst.
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During their investigation they had used 20wt% loading of Co but in comparison to dry reforming, especially for cobalt based catalyst, the use of high loading is not recommended [14].
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Ruckenstein et al. [26] studied the effect of carbon deposition and catalytic deactivation as a function of Co loading during CO2 reforming of methane and they revealed that the stability of
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Co/-Al2O3 catalyst is strongly dependent on the Co loading. They observed that highest amount of carbon deposition and maximum deactivation occurred when 20wt% loading of Co was used.
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In fact the difference in the performance of same catalyst in partial oxidation and dry reforming
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of methane could be due to the presence of Sr in the former reaction and/or difference of the thermodynamicity of these reaction (former exothermic while later endothermic) or difference in both reactions mechanisms [32]. Moreover, for all promoted catalysts, it was observed that the presence of a promoter leads to a lower coke deposition with a slight decrease in methane conversion. The decrease in methane conversion in case of promoted catalysts may be attributed to the partial coverage of fraction of Co active sites by Sr during impregnation due to which the exposed metal surface for reaction is decreased and/or formation of cobalt aluminate species in catalysts during calcination [14]. In comparison at small loadings this phenomenon is less prominent while at higher loadings it becomes more significant e.g. for 2.25wt% loading (Table 2). Similar results were reported for Sr and other alkaline earth metal promoters in literature. Rynkowski et al. [33] reported that addition of a moderate amount of strontium (up to 0.5%) in Ni/La2O3 catalyst improves its stability and resistance towards coke formation but slightly decreases the catalytic activity. We reported in a previously published work [34] that the addition 13 Page 13 of 32
of Ca exhibited negligible carbon deposition at the expense of small reduction in catalytic activity. The results of CO2 conversions for all promoted and non-promoted catalysts are shown in Table 2. The data indicated that all Sr promoted catalysts presented the CO2 conversion higher than the
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corresponding methane conversions. The higher activation of CO2 in case of Sr promoted catalysts can be ascribed to higher basicity of these catalysts. It is well known fact that basic
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catalysts enhanced the CO2 activation in dry reforming process [33]. On the basis of CO2 conversions the catalytic activity followed the order: 2.25%SrCo > 1.5%SrCo > 1%SrCo >
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0.5%SrCo > 0.25%SrCo > 0%SrCo. Comparatively, the highest loss in CO2 conversion was observed for non-promoted catalysts, whereas all other Sr promoted catalysts showed relatively stable performance. The high and stable CO2 activation, in case of Sr promoted catalysts, can be
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considered asa key factor which favored the coke suppression by gasifying it.
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3.3. Effect of Sr on Carbon Deposition
During the dry reforming of methane a dynamic redox process is established, due to the coexistence of reductive and oxidative atmosphere, by which a portion of Co0 is oxidized to Co–
d
O first then it is again reduced to Co0 by carbonaceous species. Generally when the reductive
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atmosphere dominates, a huge amount of carbon is accumulated while, in contrast, when the oxidative atmosphere dominates, the number of active metal sites is dropped due to their
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oxidation. Both of these phenomena give rise to severe catalytic deactivations. Consequently, the catalysts will accomplish a stable performance as the balance is developed between the formation of carbonaceous species and its oxidative gasification. There are several origins for the formation of carbon over the catalyst surface, depending on operating conditions and the properties of catalyst. The two important properties of a catalyst that affect the carbon deposition are catalyst surface structure and surface acidity. Usually the coke deposition occurs more easily on bigger particles than smaller ones. It is reported that the carbon deposition can be avoided or even stopped by reducing metal particle sizes in catalysts. Based on the modeling and experimental interpretations, no carbon formation happens with active metal crystallite size below 6 nm [35]. Moreover, carbon formation is favored by acidic supports. It has been suggested that carbon deposition can be mitigated when the metal is supported on a metal oxide having a strong Lewis basicity [36, 37]. 14 Page 14 of 32
Actually the addition of Sr plays dual role in preventing the carbon deposition on catalyst surface during dry reforming of methane: (i) it enhances the basicity of catalyst surface, which in turn increases the ability of the catalyst to chemisorb CO2. This increased quantity of adsorbed CO2
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further reacts with the surface carbonaceous species such as CHx (0∼3), formed during reaction,
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to produce CO and resulting in the reduction of carbon deposition via reverse Boudouard
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reaction: (2CO⇌ CO2 + C), (ii) it helps to resist the active metal particles from excessive growth
at high temperatures i.e., dividing the active metal particles into smaller ensembles which are
d
less prone to coking.
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The reduction temperature of SrO is high, therefore the introduction of Sr in Co-Al catalyst moved the reduction peak of cobalt oxide towards higher temperatures due to enhanced metal-
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support interaction. This strong metal-support interaction leads to formation of metal support spinel species which favor to resist the carbon formation. On the basis of whisker carbon formation mechanism, for cobalt based catalysts, the adsorbed carbon atoms first dissolve into the cobalt crystallites. Then the diffusion of carbon atoms through the metal occurs, followed by precipitation at the rear of the cobalt particle, which results in formation of polymeric carbon filament. On the other hand the strong interaction between metal and support in catalysts leads to low or no carbon diffusion into cobalt lattice, and thus results in suppression of whiskers carbon formation [38].
4. Conclusion The Sr promoted cobalt-alumina catalysts were prepared and employed in dry reforming of methane. The obtained results revealed that the addition of Sr reduced the coke deposits and slightly decreased the methane conversion. This higher tendency towards coke resistance of 15 Page 15 of 32
promoted catalysts is credited to enhancement in the metal support interaction, improvements in basicity of the catalysts and smaller active metal crystallites. The Sr promoter boosts the adsorption and dissociation of CO2 over the catalyst surface which resulted in the formation of oxygen intermediates near the interaction between Sr and cobalt, where coke deposits are
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gasified subsequently. In comparison with Co-Al catalyst the minimum carbon deposition and the best stability were attained by the 0.50wt% Sr promoted catalyst. These findings were
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confirmed by XRD, TEM, TPR, TPD, pulse chemisorption, TGA and TPO.
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5. Acknowledgement
The authors gratefully acknowledge their appreciation to the Deanship of Scientific Research at
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KSU for funding the work through the research group Project # RGP-VPP-119.
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References
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[4] Z. Yan, Z. Wang, D. B. Bukur, D. W. Goodman, J. Catal. 268 (2009) 196–200. [5] H. Er-rbib, C. Bouallou, F. Werkoff, Energy Procedia. 29 (2012) 156–165. [6] B. Steinhauer, M. R. Kasireddy, J. Radnik, A. Martin, Appl. Catal. A. 366 (2009) 333–341. [7] S. A. Chattanathan, S. Adhikari, S. Taylor, Int. J. Hydrogen Energy. 37 (2012) 18031–18039. [8] Z. Hou, Jing Gao, J. Guo, D. Liang, H. Lou, X. Zheng, J. Catal. 250 (2007) 331–341. [9] W. Shen, H. Momoi, K. Komatsubara, T. Saito, A. Yoshida, S. Naito, Catal. Today. 171 (2011) 150–155.
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[10] S. Y. Foo, C. Cheng, T. H. Nguyen, A. A. Adesina, Int. J. Hydrogen Energy. 37 (2012) 17019–17026. [11] P. Ferreira-Aparicio, A. Guerrero-Ruiz, R. Rodro˜Aˆguez, Appl. Catal. A. Gen. 170 (1998)
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[14] A. W. Budiman, S. H. Song, T. S. Chang, C. H. Shin, M. J. Choi, Catal. Surv. Asia. 16 (2012) 183–197.
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[15] S. Y. Foo, C. K. Cheng, T. H. Nguyen, A. A. Adesina, J. Mol. Catal. A: Chem. 344 (2011) 28–36.
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[16] Y. H. Wang, H. M. Liu, B. Q. Xu, J. Mol. Catal. A: Chem. 299 (2009) 44–52. [17] Q. Jing, H. Lou, J. Fei, Z. Hou, X. Zheng, Int. J. Hydrogen Energy. 29 (2004) 1245–1251.
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d
[18] K. Sutthiumporn, S. Kawi, Int. J. Hydrogen Energy. 36 (2011) 14435–14446. [19] A. S. Al-Fatesh, A. A. Ibrahim, A. H. Fakeeha, M. A. Soliman, M. R. Siddiqui, A. E.
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Abasaeed, Appl. Catal. A. 364 (2009) 150–155. [20] H. Y. Wang, E. Ruckenstein, Catal. Lett. 75 (2001) 13-18. [21] C. Yu, W. Weng, Q. Shu, X. Meng, B. Zhang, X. Chen, X. Zhou, J. Nat. Gas Chem. 20 (2011) 135–139. [22] K. Y. Koo, H. S. Roh, Y. T. Seo, D. J. Seo, W. L. Yoon, S. B. Park, Appl. Catal. A. Gen. 340 (2008) 183–190.
[23] Istadi, N. A. S. Amin, J. Mol. Catal. A: Chem. 259 (2006) 61–66. [24] Z. Hao, Q. Zhu, Z. Jiang, B. Hou, H. Li, Fuel Process. Technol. 90 (2009) 113–121. [25] C. Song, W. Pan, Catal. Today. 98 (2004) 463–484.
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[29] A. M. Gadalla, B. Bower, Chem. Eng. Sci. 43 (1988) 3049–3062.
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[27] N. Laosiripojana, W. Sutthisripok, S. Assabumrungrat, Chem. Eng. J. 112 (2005) 13–22.
[30] K. Takanabe, K. Nagaoka, K. Nariai, K. Aika, J. Catal. 230 (2005) 75–85.
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[31] C. Yu, X. Zhou, W. Weng, J. Hu, C. Xi-rong, L. Wei, J. Fuel Chem. Tech. 40 (2012) 1222– 1229.
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[32] R. Jin, Y. Chen, W. Li, W. Cui, Y. Ji, C. Yu, Y. Jiang, Appl. Catal. A. Gen. 201 (2000) 71– 80.
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[33] J. Rynkowski, P. Samulkiewicz, A. K. Ladavos, P. J. Pomonis, Appl. Catal. A. Gen. 263 (2004) 1–9.
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d
[34] A. S. A. Al-Fatesh, A. H. Fakeeha, A. E. Abasaeed A E, Chin. J. Catal. 32 (2011) 1604–1609.
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[36] R. Bouarab, O. Akdim, A. Auroux, O. Cherifi, C. Mirodatos, Appl. Catal. A. 264 (2004) 161–168.
[37] R. Bouarab, O. Cherifi, A. Auroux, Thermochim. Acta. 434 (2005) 69–73. [38] L. Ji, S. Tang, H. C. Zeng, J. Lin, K. L. Tan, Appl. Catal. A. 207 (2001) 247–255.
18 Page 18 of 32
Table 1: Textural properties of Sr-promoted 10wt%Co/γ-Al2O3 catalysts
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Table captions
Table 2: Catalytic performance of Sr-promoted 10wt%Co/γ-Al2O3 catalysts
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Table 3: Estimation of basic sites in Sr-promoted 10wt%Co/γ-Al2O3 catalysts
Figure captions
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Fig. 1 Nitrogen adsorption-desorption isotherms for promoted and non-promoted fresh catalysts. Fig. 2 H2-TPR profiles for fresh catalysts with different Sr content.
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Fig. 3a XRD patterns for promoted and non-promoted fresh catalysts.
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Fig. 3b XRD patterns for promoted and non-promoted spent catalysts.
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Fig. 4 CO2-TPD profiles for fresh catalysts with different Sr content.
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Fig. 5 TPO profiles for spent catalysts with different Sr content. Fig. 6 TEM micrographs of promoted and non-promoted Co-Al catalysts (a) fresh 0.5wt% Sr; (b) used 0.5wt% Sr; (c) fresh 0.5wt% Sr; (d) used 0.5wt% Sr Fig. 7 Active metal dispersion and Co particle size obtained from CO chemisorption as a function of Sr content. Fig. 8 Variations of CH4 & CO2 conversions and H2/CO ratio with Sr loadings at ●=500°C, ■=600°C
Fig. 9 H2/CO ratio versus time on stream for catalysts with different Sr content at 700oC
19 Page 19 of 32
Table 1: Textural properties of Sr-promoted 10wt%Co/γ-Al2O3 catalysts
Sr (wt%)
Used
Vp
Dp
SBET
Vp
Dp
(m2/g)
(cm3/g)
(nm)
(m2/g)
(cm3/g)
(nm)
0
165.6
0.507
11.35
137.5
0.489
13.24
0.25
176.9
0.541
11.35
0.514
12.94
0.50
177.5
0.534
11.34
161.6
0.532
12.83
0.75
176.9
0.533
11.22
156.3
0.511
12.91
1.0
175.5
0.527
11.32
154.5
0.497
13.11
1.5
176.1
0.528
11.24
153.1
0.495
12.89
2.25
175.2
0.524
11.14
151.3
0.515
13.04
us
cr
SBET
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Fresh
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d
M
an
153.6
Ac ce p
SBET: BET surface area; Vp: total pore volume; Dp: average pore diameter calculated by BJH method
20 Page 20 of 32
%Conversion
Coke c (wt%)
2.72
13.9
CH4 b
CO2 a
CO2 b
0
84.4
82.1
82.2
79.5
0.25
83.3
81.3
82.5
80.3
2.40
8.2
0.50
82.4
81.2
83.1
81.8
1.45
5.1
0.75
81.4
79.6
1.0
80.9
79
1.5
80.5
78.6
2.25
80.1
us 80.8
2.21
5.9
83.8
81.7
2.34
6.7
84.3
82.2
2.36
7.9
84.9
82.6
2.49
8.8
M
82.8
te
78.1
cr
CH4 a
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%DF
d
Sr (wt%)
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Table 2: Catalytic performance of Sr-promoted 10wt%Co/γ-Al2O3 catalysts
Reaction temperature = 700°C; a: after 30 min; b: after 360 min; c: Estimated by TGA;
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DF= [(Initial CH4 conversion – Final CH4 conversion)/Initial CH4 conversion] x100;
Table 3: Estimation of basic sites in Sr-promoted 10wt%Co/γ-Al2O3 catalysts Sr (wt%)
Weak
Medium
Strong + Very strong
Total (µmol/g)
0
29.340
160.548
-
189.888
0.5
22.291
189.096
1.104
212.491
1.0
31.437
192.349
-
223.786
1.5
28.698
206.021
5.121
239.840
2.25
37.698
187.911
17.560
243.169
21 Page 21 of 32
ip t cr us an M d te Ac ce p
Fig. 1 Nitrogen adsorption-desorption isotherms for promoted and non-promoted fresh catalysts.
22 Page 22 of 32
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Fig. 2 H2-TPR profiles for fresh catalysts with different Sr content.
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ip t cr us an M d te Ac ce p
F ig. 3a XRD patterns for promoted and non-promoted fresh catalysts. 24 Page 24 of 32
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Fig. 3b XRD patterns for promoted and non-promoted spent catalysts.
25 Page 25 of 32
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Fig. 4 CO2-TPD profiles for fresh catalysts with different Sr content.
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200
300
400
500
d
100
M
an
us
cr
TCD Signal (a.u.)
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0% Sr 0.25% Sr 0.5% Sr 0.75% Sr 1.0% Sr 1.5% Sr 2.25% Sr
600
700
800
900
1000
o
te
Temperature ( C)
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Fig. 5 TPO profiles for spent catalysts with different Sr content.
27 Page 27 of 32
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Fig. 6 TEM micrographs of promoted and non-promoted Co-Al catalysts (a) fresh 0.5wt% Sr; (b) used 0.5wt% Sr; (c) fresh 0.5wt% Sr; (d) used 0.5wt% Sr
28 Page 28 of 32
ip t cr us an M
Ac ce p
te
d
Fig. 7 Active metal dispersion and Co particle size obtained from CO chemisorption as a function of Sr content.
29 Page 29 of 32
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te
d
Fig. 8 Variations of CH4 & CO2 conversions and H2/CO ratio with Sr loadings at ●=500°C, ■=600°C
30 Page 30 of 32
ip t cr us an M d
Ac ce p
te
Fig. 9 H2/CO ratio versus time on stream for Co-Al catalysts with different Sr content at 700oC
31 Page 31 of 32
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Ac ce p
te
d
M
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Graphical Abstract
32 Page 32 of 32
S1226-086X(13)00219-0 http://dx.doi.org/doi:10.1016/j.jiec.2013.05.013 JIEC 1363
To appear in: Received date: Revised date: Accepted date:
25-2-2013 6-5-2013 10-5-2013
Please cite this article as: A.H. Fakeeha, M.A. Naeem, W.U. Khan, A.S. Al-Fatesh, Syngas production via CO2 reforming of methane using CoSr-Al catalyst, Journal of Industrial and Engineering Chemistry (2013), http://dx.doi.org/10.1016/j.jiec.2013.05.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Syngas production via CO2 reforming of methane using Co-Sr-Al catalyst
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Anis Hamza Fakeeha, Muhammad Awais Naeem, Wasim Ullah Khan, Ahmed Sadeq Al-Fatesh* Chemical Engineering Department, College of Engineering,
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* King Saud University P.O. Box 800, Riyadh 11421, Kingdom of Saudi Arabia
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* Corresponding author. Tel 009661-4676859 Fax 009661-4678770 Email [email protected]
Abstract
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The effect of addition of strontium in Co based catalysts during CO2 reforming of methane was investigated in the temperature range 500–700C. The Co/γ-Al2O3 supported catalysts with strontium as a promoter (0–2.25 wt%) were prepared by incipient wet impregnation method.
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Numerous techniques such as N2 adsorption-desorption isotherm, H2 temperature-programmed reduction (TPR), temperature-programmed desorption (TPD), X-ray diffraction (XRD),
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thermogravimetric analysis (TGA), Transmission Electron Microscopy (TEM), pulse
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chemisorption and temperature-programmed oxidation (TPO) were applied for characterization of fresh and spent catalysts. The results of characterizations and catalyst activity test revealed
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that introduction of Sr in Co/γ-Al2O3 catalyst had significant effect on stability and coke suppression. The Sr addition improves the metal support interaction as well as enhances the Lewis basicity of the catalyst. The improvement in basicity helps the chemisorption and dissociation of CO2 over the catalyst which in turn reduces carbon deposition. Key words: CO2/CH4 Reforming, Catalyst, Basicity, Coke, Stability, Sr, Co
1 Page 1 of 32
1. Introduction The global climatic variations and environmental protection regulations stress on the reduction of discharge of green-house gases into the atmosphere. Among the green-house gases, CO2 and
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CH4 emissions have been blamed the leading driving factor that originates the phenomenon of global warming [1-3]. Numerous research activities and technologies are in progress to mitigate and/or transform these gases into other valuable chemical products and fuels. The carbon dioxide
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or dry reforming of methane has gained special considerations from the last decade due to following reasons: (i) it produces syngas with H2/CO ratio near unity which is suitable for
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various industrial processes such as oxo- and Fischer-Tropsch synthesis and (ii) the process
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simultaneously utilizes two green-house gases i.e. CO2 and CH4 [4-7].
Although this unique idea has various environmental and economic inspirations, but unfortunately, no commercial processes for reforming of methane with CO2 is fully developed
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yet due to some complications of the process. The major deficiency of the dry reforming of methane (DRM) is the requirement of high operating temperature to get high conversions of both
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CH4 and CO2 due to the strong endothermic nature of the reaction. These harsh reaction conditions usually favor the sintering of active metal ensembles and deposition of huge quantity
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of carbon on the catalyst surface, resulting in catalyst deactivation and reactor blocking [8-10].
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The dry reforming reactions and the associated side reactions are as follows:
CO 2 CH 4
2CO 2H 2
CO 2 H 2
CO H 2 O
(1) (2)
2CO C CO2
(3)
C 2H 2
(4)
CH 4
The inhibition of carbon deposition over the catalyst surface is the ultimate challenge in CO2 reforming of methane. Numerous researchers reported group VIII transition metals’ better performance as active metal component for reaction (1). Among group VIII metals, noble metals show higher activity and resistance to carbon deposition [11, 12]. High cost and less abundance of noble metals present nickel and cobalt as an alternative. The main drawback of supported 2 Page 2 of 32
nickel catalysts is its deactivation due to carbon formation leading to reactor blockage. Long term industrial applications require catalysts having better stability. So a catalyst with slightly lower activity as compared to nickel and noble metals but being capable of performing for longer period of time is the right choice for dry reforming reaction. Recent studies revealed that
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supported cobalt catalysts show considerable activity for methane dry reforming process. Additionally cobalt based catalysts are reported to have higher resistance to carbon deposition.
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Taking into consideration comparable activity and coke resistance, cobalt can be a potential active component alternative to nickel [11, 13].
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In order to improve the stability and to avoid coke formation over cobalt based catalysts numerous approaches can be applied such as; use alkaline and/or rare earth metal oxides e.g.
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MgO, La2O3, Ce2O3 as a support or modify the catalysts by the addition of promoters e.g. K, Ce, Sm, Pr [14, 15]. In fact these promoters can complete their jobs as follows: (i) the promoters can
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increase metal support interaction (MSI) and active metal dispersion (ii) the promoters can change the acidic/basic character of catalyst [16].
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Q. Jing et al. [17] reported that 10wt% Sr loading in 5%Ni/SrO-SiO2 catalyst increases the Ni dispersion, metal-support interaction and CO2 adsorption which in turn improve the catalytic
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activity and stability for long term operation.
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K. Sutthiumporn et al. [18] studied the promotional effect of alkaline earth elements on Ni-La2O3 catalysts for dry reforming of methane. They revealed that Sr addition not only enhanced the CH4 and CO2 conversions but also reduced the carbon deposition on the catalyst surface. In the present research, a set of -Al2O3 supported Co-Sr catalysts, having different Sr loading, were prepared and tested in a tubular reactor for carbon dioxide reforming of methane in order to find a more promising catalyst with optimum Co/Sr metal ratio assuring high activity and stability with a decrease in coke deposition.
2. Experimental 2.1. Catalyst preparation The high surface area Alumina (γ-Al2O3) SA6175 supplied by Norton was used as the catalyst support throughout this investigation. The nitrate salts of Cobalt and Strontium i.e. 3 Page 3 of 32
(Co(NO3)2.6H2O) and (Sr(NO3)2 (Sigma Aldrich) were used as precursors for active metal and promoter respectively.
The Co/γ-Al2O3 catalysts (10wt% Co) were prepared by wet
impregnation method. The solution containing predetermined amount of (Co(NO3)2.6H2O) was prepared in double distilled water, then γ-Al2O3 was impregnated with the previously prepared
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solution of active metal salt. The Sr promoted catalysts with different Sr loading 0–2.25 wt% were prepared by co-impregnation of nitrate salts of the promoter and active metal with γ-Al2O3
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support using the same procedure mentioned above. After impregnation all the catalysts were dried overnight at 120°C and subsequently calcined at 600°C for 3 h.
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2.2. Catalyst Testing
Activity measurements were conducted at atmospheric pressure in a conventional flow apparatus
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consisting of a flow measuring and control system, a mixing chamber, a fixed bed tubular reactor and an on-line gas chromatograph. The tubular reactor (ID 13 mm) was electrically heated by a
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furnace with four independently heated zones; the temperature profile was measured using a thermocouple placed in an axial thermowell centered in the catalyst bed. This system was
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integrated to the GC (Varian Star 3400) and integrated line was heated with a heating tape to avoid the water condensation in the line. For each experiment the 0.6 gram weight of catalyst
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was used which was reduced under H2 flow (40 ml/min) at 550C for 2 h followed by N2 flow (30 ml/min) for 20 minutes prior to reaction. The reaction mixture CH4:CO2:N2 in the proportion
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17:17:2 at a total flow rate of 36 ml/min was used over catalyst bed for experiment. The effluent gases were analyzed on-line by the GC equipped with TCD detector. The two columns, Porapak Q and Molecular Sieve 5A, were used in series/bypass connections for the complete separation of reaction products. The comprehensive details of experimental setup and procedure are given elsewhere [19].
2.3. Catalyst characterization
The specific surface area of the fresh and used catalyst was measured with a Micromeritics Tristar II 3020 surface area and porosity analyzer from N2 adsorption desorption data at -196C. For each analysis, 0.3 g of catalyst was used. The degassing of the samples, before experiment, was done at 300C for 3 h to get rid of the moisture content and other adsorbed gases from the
4 Page 4 of 32
surface of catalyst. The pore distribution was calculated from adsorption branch of the corresponding nitrogen isotherm by applying the Barrett, Joyner, and Halenda (BJH) method. The temperature programmed reduction with hydrogen (H2-TPR), temperature programmed
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oxidation (TPO), pulse chemisorption and temperature programmed desorption (CO2-TPD) measurements were completed on a chemisorption apparatus (Micromeritics 2920 Auto Chem II). For H2-TPR, 40 mg of the sample was pretreated with high purity Argon (Ar) flow at 150°C
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for 30 min, followed by cooling it to room temperature. Then the sample was heated in a furnace up to 1000°C with a constant heating rate of 10°C/min using a flow of H2/Ar mixture (volume
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ratio, 10/90) under a flow rate of 40 ml/min. The signal of H2 consumption was monitored by a thermal conduction detector (TCD). The CO2 temperature programmed desorption (CO2-TPD)
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measurements were performed on fresh catalysts. The 70 mg of sample was first held at 200°C for 1 h under flowing He to remove physically adsorbed and/or weakly bound species. CO2
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adsorption was carried out at 50°C for 30 min by passing CO2/He mixed gas (volume ratio, 10/90) with a flow rate of 30 ml/min. The TPD signal was recorded by TCD with linear
d
temperature increase up to 800°C at a rate of 10°C/min. In order to find the nature of carbon deposition on spent catalysts, TPO experiments were
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performed. The catalyst samples, used previously in the dry reforming reaction, were dried at 150°C for 30 min, under helium (He) flow (30 ml/min) and cooled down to room temperature,
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followed by an increase of temperature under O2/He (30 ml/min) flow with a temperature ramp of 10°C/min to 1000°C. The metal dispersion of the catalysts was measured by CO chemisorption at 50°C. Prior to the pulse chemisorption experiments, all samples were reduced under H2 flow (25 ml/min) at 800°C and subsequently flushed under He. Powdered X-ray diffraction (XRD) analysis of fresh and used catalyst was carried out using a Rigaku (Miniflex) diffractometer with a Cu Kα radiation operated at 40 kV and 40 mA. The scanning step and range of 2θ for analysis were 0.02 and 10-80 respectively. The amount of Coke deposition on the surface of spent catalysts was estimated by thermogravimetric analysis (TGA) in air atmosphere using EXSTAR SII TG/DTA 7300 (Thermogravimetric/Differential) analyzer. The sample of used catalyst, 20 mg in quantity, was heated from room temperature to 800C at a heating rate 20C /min. 5 Page 5 of 32
Transmission Electron Microscopy (TEM) of fresh and spent catalysts were carried out, using a JEOL-1011CX microscope operating at 80 kV accelerating voltage, to analyze the morphology of the deposited carbon and the average particle size of active metal crystal. Prior to TEM analysis the samples were first dispersed ultrasonically in hexane at room temperature. Then, a
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drop of the suspension was put on a lacey carbon-coated Cu grid to take images.
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3. Results and Discussions
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3.1. Characterization of Catalysts 3.1.1. Textural Properties
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The nitrogen adsorption-desorption isotherms for 10wt% Co/γ-Al2O3 promoted and nonpromoted fresh catalysts are shown in Fig. 1. It can be depicted from plots that all N2 adsorption/desorption isotherms are matching to typical type IV adsorption curves and showing a
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hysteresis loop of the H1 type with ordered mesopores. The specific surface area, pore volume and average pore diameter for all Co based fresh and used catalysts are listed in Table 1. From
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the table it is observed that addition of Sr in the Co-alumina catalyst modifies the textural properties of the catalyst, which are evidenced from higher specific surface area and larger pore
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volume. In comparison to Sr promoted catalysts, the smallest pore volume, in case of non-
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promoted Co-alumina catalyst, is probably resulted due to partial blockage of the pores with bigger active metal particles. Whereas the larger pore volume in case of Sr promoted catalysts is indicating better dispersion of active metal over support which avoids blockage of pores due to sintering. From the Table 1 it is apparent that surface area and pore volume of spent catalyst are decreased after 6 h of reaction. In fact these changes are attributed to thermal sintering of active metal and/or carbon deposition over catalyst surface as evidenced by TEM (Fig. 6). It is obvious that the Sr promoted catalysts showed small decrease in surface area as compared to nonpromoted catalyst, after reaction, which indicates that Sr promoter has a certain effect on catalytic behavior of catalyst under the same reaction atmosphere. 3.1.2. Temperature Programmed Reduction (H2-TPR) It is believed that the active species in supported catalysts for dry reforming of methane are the reduced metallic clusters present at the surface of the catalyst. Temperature programmed 6 Page 6 of 32
reduction (TPR) is a very handy technique to find number of reducible species present in catalysts and, at the same time, to characterize the interaction between active metal and support. The TPR profiles of promoted and non-promoted fresh catalysts are presented in (Fig. 2) In the TPR pattern of non-promoted catalyst only one peak at temperature of 465°C is appeared which
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is attributed to the reduction of Co3O4 species having weak or no interaction with support. Generally, the composition of Co3O4 can be split as Co2+(Co23+)O4 in which Co2+ ions are
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located in the tetrahedral coordinated sites and Co3+ ions in the octahedral coordinated sites. In cobalt over alumina supported catalyst due to calcination at high temperatures some ion
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exchange between Co3O4 and Al2O3 occurred which lead to formation of Co-aluminate spinel species e.g., Co2AlO4 and CoAl2O4. Generally, these species have high reduction temperature as compared to Co3O4. Wang et al. [20] have categorized the TPR peaks for Co/Al2O3 catalysts into
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three different temperature regions: region I (<500°C), region II (500-900°C), and region III (>900°C). According to them, on the basis of reducibility sequence, the TPR peak in region I is
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ascribed to Co3O4, that in region II to Co2AlO4, and that in region III to CoAl2O4. It is apparent from Fig. 2 that TPR patterns of Sr promoted catalysts are different, both in broadness and
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intensity, from non-promoted catalyst. The difference in TPR pattern revealed that Sr addition has a definite effect on reduction behavior of these catalysts. As the Sr content is increased the
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main peak becomes bigger in size, which is most likely due to presence of more reducible species in catalyst. For 0.5wt% Sr promoted catalyst, two peaks are appeared with maxima
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centered at temperatures of 470°C and 650°C respectively, for this catalyst the first peak is attributed to reduction of Co3O4 while second broad peak is assigned to reduction of Co2AlO4 species. Whereas, for 1wt% Sr promoted catalyst, these two peaks are observed at slightly higher temperatures i.e., 480°C and 665°C respectively. In fact the positive shift in temperature is due to increase in metal support interaction. In the light of above results it can be concluded that addition of Sr not only improves the metal support interaction but also increases the number of reducible species. Actually the increase in number of reducible species is credited to better dispersion of active metal over catalyst surface which is also confirmed by pulse chemisorption (Fig. 7). Generally, in case of Co/Al2O3 catalyst, during the course of calcination at high temperature, due to very high metal support interaction cobalt aluminates are formed which are catalytically not favorable for methane reforming processes. On the other hand due to very weak metal support interaction at high calcination and/or reaction temperature sintering of active metal 7 Page 7 of 32
component becomes prominent which leads to catalyst deactivation. In short a balanced metal support interaction is preferred for better activity and stability. It is worthwhile to note that no reduction peak of CoAl2O4 was identified in all Sr promoted catalysts which is a good sign according to catalytically point of view. Moreover the formation of Co2AlO4 in Sr promoted
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catalysts is helpful in avoiding sintering by optimizing the metal support interaction, although Co2AlO4 has high reduction temperature but still it can be reduced at low temperature as
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compared to CoAl2O4. From TPR patterns it is clear that compared to all other Sr loadings 1.5wt% showed high metal support interaction, with formation of large amount of Co2AlO4,
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whereas 0.5wt% Sr loaded catalyst showed optimum metal support interaction due to formation of balanced amount of Co2AlO4 and presence of large number of reducible species. The balanced metal support interaction in case of 0.5wt% Sr promoted catalyst contributes to produce smaller
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crystallites/ensembles of active metal which in turn help to avoid sintering and carbon
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deposition. 3.1.3. X-ray diffraction (XRD)
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The XRD patterns of various promoted and non-promoted, fresh and used, Co based catalysts are presented in Figs. 3a and 3b respectively. It is hard to distinguish between Co3O4 and Co2AlO4
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crystalline phases from XRD diffractrogram due to superimposition of metal oxide and metalaluminate spinel phases. For both promoted and non-promoted fresh catalysts, the diffraction
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peaks appearing at 2θ = 31.3°; 2θ = 44.8°; 2θ = 55.6°; 2θ = 59.3° and 2θ = 65.2° correspond to Co3O4 and/or metal aluminate spinel phases (JCPDS: 00-042-1467/JCPDS: 00-038-0814), while the following diffraction peaks detected at 2θ = 19.3°; 2θ = 37.4°; 2θ = 46° and 2θ = 66.7° are attributed to γ-Al2O3 (JCPDS: 00-004-0875). The presence of metal aluminate spinel phases in fresh calcined catalysts is consistence with the TPR results (Fig. 2). It is quite worthy to note that in all Sr promoted catalysts no bulk peak for strontium was identified in diffractograms by the XRD, possibly due to higher dispersion of Sr in catalysts or it is present in small quantity which beyond the detection limit of XRD [17]. Although no bulk peak of Sr was identified in all catalysts but the formation of SrAl2O4, and/or similar kinds of aluminates, cannot be discarded during the course of calcination [21]. The formation of aluminate species was also reported by several researchers in literature for other alkaline earth metals (e.g., Mg and Ca). Koo et al. [22] reported that promotion of Ni/Al2O3 with MgO formed MgAl2O4 spinel phase in catalyst, which 8 Page 8 of 32
effectively prevents coke formation by increasing the CO2 adsorption due to the increase in basic strength of catalyst. In case of spent catalysts the diffraction peaks detected at 2θ = 43.8°; 2θ = 51.6° and 2θ = 75.8° are attributed to metallic Co (JCSPD: 00-015-0806) while the other diffraction peaks observed at 2θ = 37.4°; 2θ = 46° and 2θ = 66.7° are ascribed to γ-Al2O3
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(JCPDS: 00-004-0875). It can be seen from XRD patterns of used catalysts that at 2θ = 26° one extra diffraction peak appeared in case of non-promoted catalyst i.e. 0wt% Sr. Actually this
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diffraction peak is assigned to graphitic carbon which again indicated that non-promoted catalyst has highly suffered from coke deposition during reaction. The absence of this peak in the catalyst formation.
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3.1.4. Temperature programmed desorption (TPD)
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with 0.5wt% Sr promoter confirms its high resistance, during reaction, towards carbon
The basicity i.e. strength of the basic sites of the Sr promoted Co/Al2O3 fresh catalysts were
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evaluated by TPD with CO2 as a probe gas. The strength of the basic sites, usually in TPD, is reported in terms of temperature range where the chemisorbed CO2 on the basic sites is desorbed.
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According to adsorption-desorption phenomenon the adsorbate adsorbed on weaker basic sites desorbed at lower temperature while from stronger basic sites they desorbed at higher °
°
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temperature. Generally, the strength of basic sites is classified into weak (<200 C), medium °
°
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(200–400 C), strong (400–650 C) and very strong (>650 C) depending on the desorption temperature of CO2 [23]. The CO2-TPD profiles of promoted and non-promoted catalysts are presented in Fig. 4. From figure it can be depicted that Co-Al catalyst exhibits the least capacity °
°
to adsorb CO2 with two desorption peaks centered at 95 C and 290 C respectively. These desorption peaks are due to presence of low and medium strength basic sites such as bicarbonates. These basic sites results due to the interaction between CO2 and the basic surface hydroxyl groups present over catalyst. On the other hand Sr promoted Co-Al catalysts are exhibiting more than two desorption peaks indicating the increase in basicity of catalysts. The Sr belongs to the alkaline earth metals group; thereby increasing Sr loading in catalysts raises the surface basicity of catalysts. For promoted catalysts, the addition of Sr not only expands the adsorption capacity of acidic CO2 gas but also enhances the basic character of the catalysts by slightly shifting and/or by creating extra desorption peaks at higher temperatures. In contrast to non-promoted catalyst the 0.5wt% Sr promoted catalyst shows four distinct CO2 desorption 9 Page 9 of 32
°
°
°
°
peaks with relative maxima centered at 95 C, 295 C, 385 C and 580 C respectively. The first and last peaks are assigned to weaker and stronger basic sites respectively while second and third peaks may be assigned to medium basic sites. On the contrary the 2.25wt% Sr promoted catalyst °
°
°
°
showed four distinct peaks with maxima centered at 110 C, 295 C, 580 C and 800 C
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respectively. In this case the first and second peaks can be assigned to weaker and medium strength basic sites while the last two peaks, at higher temperatures, are ascribed to strong and
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very strong basic sites respectively. It is well known that performance of catalyst is strongly influenced by Lewis basicity and strength of basic sites present in catalyst. Generally the strong
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basic sites are more favorable to suppress coke formation than weak and medium. However, it is tough to estimate the catalytic activity and coke resistance by the difference of basic strength
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because the catalytic performance also depends on the several other factors including active metal size, dispersion and reduction degree [22]. The results of basicity in terms of strength of basic sites, for promoted and non-promoted catalysts, are presented in Table 3. It is obvious from
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results that Sr promoted catalysts have more basic sites than non-promoted catalyst.
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3.1.5. TGA and Temperature Programmed Oxidation (TPO)
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The TGA was used to quantify the carbon formation over promoted and non-promoted spent catalysts. In Table 2, the TGA data of both promoted and non-promoted spent catalysts is
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presented. From the TGA data it can be observed that when Sr was introduced in Co-Al catalyst the carbon deposition became lesser. The highest weight loss of 13.9 wt%, due to carbon gasification, was shown for non-promoted catalyst while among all the Sr promoted spent catalysts the smallest weight loss was observed for 0.5wt% Sr promoted catalyst (5.1 wt%). In order to find the nature or type of carbon deposition over spent catalysts, the TPO experiments were performed. The results showed that the rate and type of carbon deposited mainly depends on Sr loading. Normally Co-based catalysts suffer from several forms of the carbon e.g. atomic carbon, amorphous carbon and graphitic carbon. Oxidative atmosphere gasify these carbons to CO2 at different temperature ranges: atomic carbon < 250°C, amorphous carbon 250~600°C and graphitic carbon > 600°C [24]. The TPO profiles for the Co-xSr-Al spent catalysts are presented in Fig. 5. The TPO curves of all promoted and non-promoted spent catalysts showed only one peak in temperature range of 250–540°C, indicating amorphous type of carbon deposition, except for 0.75wt% Sr promoted catalyst which showed two peaks with maxima centered at 10 Page 10 of 32
505°C and 650°C. For this catalyst, the first peak can be attributed to amorphous type carbon while second peak is assigned to graphitic type carbon. The broadness of second peak is indicating the combustion of carbon species of different crystalline forms, e.g. carbon nanofibers (CNFs) and carbon nanotubes (CNTs), etc. From Fig. 5, it is clear that total peak area of
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0.75wt% Sr promoted catalyst is small, since both peaks have very little intensity, as compared to non-promoted catalyst which indicates less amount of carbon formation. The smallest peak
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area, having maxima centered at 310°C, was observed with 0.5wt% Sr, which indicates that it remained stable and hardly suffered from coke formation during reaction as confirmed by TGA
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(5.1 wt% loss). Overall from the TPO patterns it can be concluded that addition of Sr as a promoter in Co-Al catalyst has a significant effect on coke suppression. TPO results are in good
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agreement with the TGA results (Table 2).
3.1.6. Transmission Electron Microscopy (TEM) and CO Pulse Chemisorption
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The TEM images of both Sr promoted and non-promoted, fresh and spent, catalysts are shown in Fig. 6. Figures depict that Co metal particles more uniform distribution of Co metal particles was
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achieved in case of 0.5wt% Sr promoted catalyst as compared to non-promoted catalyst. Highly dispersed Co species are actually formed in Sr promoted catalysts due to the existence of a strong
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interaction between the Co oxide species and support (TPR Fig. 2). The smaller cobalt crystallites were observed for 0.5wt% Sr promoted fresh catalyst, where particles up to 16 nm
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were found (Fig. 6c). In contrast, the non-promoted Co-Al fresh catalyst contains bigger particles up to 45 nm (Fig. 6a). Whisker type of carbon, observed in non-promoted Co-Al spent catalyst (Fig. 6b), is responsible for catalyst deactivation but 0.5wt% Sr promoted spent catalyst showed no significant coke deposition (Fig. 6d). The active metal dispersion and the average metal particle size for the catalysts estimated by CO pulse chemisorption are shown in Fig. 7. From figure, it can be observed that addition of Sr in catalyst not only increases the active metal dispersion but also reduces the metal particle size. The metal dispersion and active metal particle size both are very important factors in controlling the carbon deposition. Similar results were reported by Yu et al. [21] that addition of alkaline earth metals in cobalt based catalysts could increase the dispersion of cobalt and decrease the coke formation. The highest active metal dispersion and smallest metallic crystallite size were attained for 0.5wt% Sr catalyst, which were responsible for its better stability and resistance towards coking. While among all the tested 11 Page 11 of 32
catalysts lowest dispersion and biggest active metallic size were observed in case of nonpromoted catalyst, which made it favorable for carbon formation. 3.2. Catalytic Activity and Stability
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The results of initial (after 30 min) catalytic activity at different reaction temperatures (500, 600 and 700°C) by adding Sr promoter in Co-Al catalysts are presented in Fig. 8 and Table 2. The
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results revealed that the conversions of both CH4 and CO2 were increased with rise in reaction temperature which confirms the endothermicity of dry reforming reaction [25]. Moreover all
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catalysts showed CH4 conversion lower than the CO2 conversion, indicating the occurrence of reverse water gas shift (RWGS) reaction [26, 27]. The existence of RWGS reaction (CO2 + H2 ↔ H2O + CO; ΔH298 = +41 kJ/mol) resulting in smaller H2/CO ratio, less than unity, because it
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consumes some amount of H2 produced in reaction and increases the CO2 conversion. Several mechanistic studies have proposed that during dry reforming reaction, CH4 is
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decomposed first on the active metal sites to yield reactive surface carbonaceous species, which are then oxidized to CO by the oxygen that originate from CO2. In view of that, the rate of
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carbon deposition on the catalyst surface is dependent on the relative rates of the formation of
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carbon deposits and its oxidative removal. The large amount of carbon deposition will take place when the rate of the oxidative removal is slower than the rate of formation [12, 28]. To develop a
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stable and active catalyst is a challenge in dry reforming reactions. Generally, the catalyst stability can be associated with the phenomenon of deactivation due to carbon deposition, sintering and/or loss of active surface due to its oxidation under reaction conditions. Most of the times all these phenomena occur side by side but in some cases one of these dominates and causes severe deactivation [29, 30].
In order to compare catalytic activity, 700°C was fixed as the reaction temperature for estimation of the long-term stability of the catalysts in this study. The influence of Sr promoter on the performance of Co-Al catalyst, at 700°C reaction temperature for 6 h time-on-stream (TOS), in terms of CH4, CO2 conversions and H2/CO ratio is shown in Table 2 and Fig. 9 respectively. It is depicted from data that at the beginning of the reaction the CH4 conversion for non-promoted catalyst was 84.4%, but after 6 h of reaction it reached to 82.1%. This decrease in activity is attributed to the formation of a huge amount of coke deposits on the surface of the catalyst as 12 Page 12 of 32
verified by TGA, TPO, TEM and XRD patterns of spent catalysts (Table 2, Figs. 5, 6 and 3b). Contrarily, the CH4 conversion over 0.5wt% Sr promoted catalyst was 82.4% at the start of the reaction and nearly remained at a constant of 81.2%, with a small decline after 6 h of reaction. The slight decrease in conversion, with deactivation factor 1.45, is signifying its higher stability
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as compared to non-promoted catalyst which has deactivation factor 2.72. The improved stability and resistance against coking of Sr-promoted catalysts are actually credited to their higher
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basicity and better metal support interaction. All other Sr promoted catalysts has also shown stable conversion of methane as compared to non-promoted catalyst, revealing that Sr has a
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significant effect on stability of catalysts. Recently Yu et al. [31] has studied the promotional effect of Sr on Co/Al2O3 catalyst during partial oxidation of methane and they revealed that addition of Sr (>2% mass fraction) has a great influence on activity and stability of catalyst.
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During their investigation they had used 20wt% loading of Co but in comparison to dry reforming, especially for cobalt based catalyst, the use of high loading is not recommended [14].
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Ruckenstein et al. [26] studied the effect of carbon deposition and catalytic deactivation as a function of Co loading during CO2 reforming of methane and they revealed that the stability of
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Co/-Al2O3 catalyst is strongly dependent on the Co loading. They observed that highest amount of carbon deposition and maximum deactivation occurred when 20wt% loading of Co was used.
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In fact the difference in the performance of same catalyst in partial oxidation and dry reforming
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of methane could be due to the presence of Sr in the former reaction and/or difference of the thermodynamicity of these reaction (former exothermic while later endothermic) or difference in both reactions mechanisms [32]. Moreover, for all promoted catalysts, it was observed that the presence of a promoter leads to a lower coke deposition with a slight decrease in methane conversion. The decrease in methane conversion in case of promoted catalysts may be attributed to the partial coverage of fraction of Co active sites by Sr during impregnation due to which the exposed metal surface for reaction is decreased and/or formation of cobalt aluminate species in catalysts during calcination [14]. In comparison at small loadings this phenomenon is less prominent while at higher loadings it becomes more significant e.g. for 2.25wt% loading (Table 2). Similar results were reported for Sr and other alkaline earth metal promoters in literature. Rynkowski et al. [33] reported that addition of a moderate amount of strontium (up to 0.5%) in Ni/La2O3 catalyst improves its stability and resistance towards coke formation but slightly decreases the catalytic activity. We reported in a previously published work [34] that the addition 13 Page 13 of 32
of Ca exhibited negligible carbon deposition at the expense of small reduction in catalytic activity. The results of CO2 conversions for all promoted and non-promoted catalysts are shown in Table 2. The data indicated that all Sr promoted catalysts presented the CO2 conversion higher than the
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corresponding methane conversions. The higher activation of CO2 in case of Sr promoted catalysts can be ascribed to higher basicity of these catalysts. It is well known fact that basic
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catalysts enhanced the CO2 activation in dry reforming process [33]. On the basis of CO2 conversions the catalytic activity followed the order: 2.25%SrCo > 1.5%SrCo > 1%SrCo >
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0.5%SrCo > 0.25%SrCo > 0%SrCo. Comparatively, the highest loss in CO2 conversion was observed for non-promoted catalysts, whereas all other Sr promoted catalysts showed relatively stable performance. The high and stable CO2 activation, in case of Sr promoted catalysts, can be
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considered asa key factor which favored the coke suppression by gasifying it.
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3.3. Effect of Sr on Carbon Deposition
During the dry reforming of methane a dynamic redox process is established, due to the coexistence of reductive and oxidative atmosphere, by which a portion of Co0 is oxidized to Co–
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O first then it is again reduced to Co0 by carbonaceous species. Generally when the reductive
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atmosphere dominates, a huge amount of carbon is accumulated while, in contrast, when the oxidative atmosphere dominates, the number of active metal sites is dropped due to their
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oxidation. Both of these phenomena give rise to severe catalytic deactivations. Consequently, the catalysts will accomplish a stable performance as the balance is developed between the formation of carbonaceous species and its oxidative gasification. There are several origins for the formation of carbon over the catalyst surface, depending on operating conditions and the properties of catalyst. The two important properties of a catalyst that affect the carbon deposition are catalyst surface structure and surface acidity. Usually the coke deposition occurs more easily on bigger particles than smaller ones. It is reported that the carbon deposition can be avoided or even stopped by reducing metal particle sizes in catalysts. Based on the modeling and experimental interpretations, no carbon formation happens with active metal crystallite size below 6 nm [35]. Moreover, carbon formation is favored by acidic supports. It has been suggested that carbon deposition can be mitigated when the metal is supported on a metal oxide having a strong Lewis basicity [36, 37]. 14 Page 14 of 32
Actually the addition of Sr plays dual role in preventing the carbon deposition on catalyst surface during dry reforming of methane: (i) it enhances the basicity of catalyst surface, which in turn increases the ability of the catalyst to chemisorb CO2. This increased quantity of adsorbed CO2
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further reacts with the surface carbonaceous species such as CHx (0∼3), formed during reaction,
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to produce CO and resulting in the reduction of carbon deposition via reverse Boudouard
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reaction: (2CO⇌ CO2 + C), (ii) it helps to resist the active metal particles from excessive growth
at high temperatures i.e., dividing the active metal particles into smaller ensembles which are
d
less prone to coking.
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The reduction temperature of SrO is high, therefore the introduction of Sr in Co-Al catalyst moved the reduction peak of cobalt oxide towards higher temperatures due to enhanced metal-
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support interaction. This strong metal-support interaction leads to formation of metal support spinel species which favor to resist the carbon formation. On the basis of whisker carbon formation mechanism, for cobalt based catalysts, the adsorbed carbon atoms first dissolve into the cobalt crystallites. Then the diffusion of carbon atoms through the metal occurs, followed by precipitation at the rear of the cobalt particle, which results in formation of polymeric carbon filament. On the other hand the strong interaction between metal and support in catalysts leads to low or no carbon diffusion into cobalt lattice, and thus results in suppression of whiskers carbon formation [38].
4. Conclusion The Sr promoted cobalt-alumina catalysts were prepared and employed in dry reforming of methane. The obtained results revealed that the addition of Sr reduced the coke deposits and slightly decreased the methane conversion. This higher tendency towards coke resistance of 15 Page 15 of 32
promoted catalysts is credited to enhancement in the metal support interaction, improvements in basicity of the catalysts and smaller active metal crystallites. The Sr promoter boosts the adsorption and dissociation of CO2 over the catalyst surface which resulted in the formation of oxygen intermediates near the interaction between Sr and cobalt, where coke deposits are
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gasified subsequently. In comparison with Co-Al catalyst the minimum carbon deposition and the best stability were attained by the 0.50wt% Sr promoted catalyst. These findings were
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confirmed by XRD, TEM, TPR, TPD, pulse chemisorption, TGA and TPO.
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5. Acknowledgement
The authors gratefully acknowledge their appreciation to the Deanship of Scientific Research at
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KSU for funding the work through the research group Project # RGP-VPP-119.
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Ac ce p
[36] R. Bouarab, O. Akdim, A. Auroux, O. Cherifi, C. Mirodatos, Appl. Catal. A. 264 (2004) 161–168.
[37] R. Bouarab, O. Cherifi, A. Auroux, Thermochim. Acta. 434 (2005) 69–73. [38] L. Ji, S. Tang, H. C. Zeng, J. Lin, K. L. Tan, Appl. Catal. A. 207 (2001) 247–255.
18 Page 18 of 32
Table 1: Textural properties of Sr-promoted 10wt%Co/γ-Al2O3 catalysts
ip t
Table captions
Table 2: Catalytic performance of Sr-promoted 10wt%Co/γ-Al2O3 catalysts
us
cr
Table 3: Estimation of basic sites in Sr-promoted 10wt%Co/γ-Al2O3 catalysts
Figure captions
an
Fig. 1 Nitrogen adsorption-desorption isotherms for promoted and non-promoted fresh catalysts. Fig. 2 H2-TPR profiles for fresh catalysts with different Sr content.
M
Fig. 3a XRD patterns for promoted and non-promoted fresh catalysts.
d
Fig. 3b XRD patterns for promoted and non-promoted spent catalysts.
te
Fig. 4 CO2-TPD profiles for fresh catalysts with different Sr content.
Ac ce p
Fig. 5 TPO profiles for spent catalysts with different Sr content. Fig. 6 TEM micrographs of promoted and non-promoted Co-Al catalysts (a) fresh 0.5wt% Sr; (b) used 0.5wt% Sr; (c) fresh 0.5wt% Sr; (d) used 0.5wt% Sr Fig. 7 Active metal dispersion and Co particle size obtained from CO chemisorption as a function of Sr content. Fig. 8 Variations of CH4 & CO2 conversions and H2/CO ratio with Sr loadings at ●=500°C, ■=600°C
Fig. 9 H2/CO ratio versus time on stream for catalysts with different Sr content at 700oC
19 Page 19 of 32
Table 1: Textural properties of Sr-promoted 10wt%Co/γ-Al2O3 catalysts
Sr (wt%)
Used
Vp
Dp
SBET
Vp
Dp
(m2/g)
(cm3/g)
(nm)
(m2/g)
(cm3/g)
(nm)
0
165.6
0.507
11.35
137.5
0.489
13.24
0.25
176.9
0.541
11.35
0.514
12.94
0.50
177.5
0.534
11.34
161.6
0.532
12.83
0.75
176.9
0.533
11.22
156.3
0.511
12.91
1.0
175.5
0.527
11.32
154.5
0.497
13.11
1.5
176.1
0.528
11.24
153.1
0.495
12.89
2.25
175.2
0.524
11.14
151.3
0.515
13.04
us
cr
SBET
ip t
Fresh
te
d
M
an
153.6
Ac ce p
SBET: BET surface area; Vp: total pore volume; Dp: average pore diameter calculated by BJH method
20 Page 20 of 32
%Conversion
Coke c (wt%)
2.72
13.9
CH4 b
CO2 a
CO2 b
0
84.4
82.1
82.2
79.5
0.25
83.3
81.3
82.5
80.3
2.40
8.2
0.50
82.4
81.2
83.1
81.8
1.45
5.1
0.75
81.4
79.6
1.0
80.9
79
1.5
80.5
78.6
2.25
80.1
us 80.8
2.21
5.9
83.8
81.7
2.34
6.7
84.3
82.2
2.36
7.9
84.9
82.6
2.49
8.8
M
82.8
te
78.1
cr
CH4 a
an
%DF
d
Sr (wt%)
ip t
Table 2: Catalytic performance of Sr-promoted 10wt%Co/γ-Al2O3 catalysts
Reaction temperature = 700°C; a: after 30 min; b: after 360 min; c: Estimated by TGA;
Ac ce p
DF= [(Initial CH4 conversion – Final CH4 conversion)/Initial CH4 conversion] x100;
Table 3: Estimation of basic sites in Sr-promoted 10wt%Co/γ-Al2O3 catalysts Sr (wt%)
Weak
Medium
Strong + Very strong
Total (µmol/g)
0
29.340
160.548
-
189.888
0.5
22.291
189.096
1.104
212.491
1.0
31.437
192.349
-
223.786
1.5
28.698
206.021
5.121
239.840
2.25
37.698
187.911
17.560
243.169
21 Page 21 of 32
ip t cr us an M d te Ac ce p
Fig. 1 Nitrogen adsorption-desorption isotherms for promoted and non-promoted fresh catalysts.
22 Page 22 of 32
ip t cr us an M d te
Ac ce p
Fig. 2 H2-TPR profiles for fresh catalysts with different Sr content.
23 Page 23 of 32
ip t cr us an M d te Ac ce p
F ig. 3a XRD patterns for promoted and non-promoted fresh catalysts. 24 Page 24 of 32
ip t cr us an M d te Ac ce p
Fig. 3b XRD patterns for promoted and non-promoted spent catalysts.
25 Page 25 of 32
ip t cr us an M d te
Ac ce p
Fig. 4 CO2-TPD profiles for fresh catalysts with different Sr content.
26 Page 26 of 32
200
300
400
500
d
100
M
an
us
cr
TCD Signal (a.u.)
ip t
0% Sr 0.25% Sr 0.5% Sr 0.75% Sr 1.0% Sr 1.5% Sr 2.25% Sr
600
700
800
900
1000
o
te
Temperature ( C)
Ac ce p
Fig. 5 TPO profiles for spent catalysts with different Sr content.
27 Page 27 of 32
ip t cr us an M d te Ac ce p
Fig. 6 TEM micrographs of promoted and non-promoted Co-Al catalysts (a) fresh 0.5wt% Sr; (b) used 0.5wt% Sr; (c) fresh 0.5wt% Sr; (d) used 0.5wt% Sr
28 Page 28 of 32
ip t cr us an M
Ac ce p
te
d
Fig. 7 Active metal dispersion and Co particle size obtained from CO chemisorption as a function of Sr content.
29 Page 29 of 32
ip t cr us an M Ac ce p
te
d
Fig. 8 Variations of CH4 & CO2 conversions and H2/CO ratio with Sr loadings at ●=500°C, ■=600°C
30 Page 30 of 32
ip t cr us an M d
Ac ce p
te
Fig. 9 H2/CO ratio versus time on stream for Co-Al catalysts with different Sr content at 700oC
31 Page 31 of 32
ip t
Ac ce p
te
d
M
an
us
cr
Graphical Abstract
32 Page 32 of 32