Coke Minimization during Conversion of Biogas to Syngas by

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Coke Minimization during Conversion of Biogas to Syngas by Bimetallic Tungsten-Nickel Incorporated Mesoporous Alumina Synthesized by the One-Pot Route Huseyin Arbag, Sena Yasyerli, Nail Yasyerli, Gulsen Dogu, Timur Dogu, Ilja Gasan Osojnik Crnivec, and Albin Pintar Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie504477t • Publication Date (Web): 12 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Industrial & Engineering Chemistry Research

Coke Minimization during Conversion of Biogas to Syngas by Bimetallic Tungsten-Nickel Incorporated Mesoporous Alumina Synthesized by the One-Pot Route

Huseyin Arbag1, Sena Yasyerli1, Nail Yasyerli1, Gulsen Dogu1,*, Timur Dogu2 Ilja Gasan Osojnik Črnivec3, Albin Pintar3,4 1

Department of Chemical Engineering, Gazi University, Ankara, Turkey 2

3

Dept. Chem. Eng., Middle East Technical University, Ankara, Turkey

Laboratory for Environmental Sciences and Engineering, National Institute of Chemistry, Ljubljana, Slovenia 4

Centre of Excellence for Low-Carbon Technologies, Ljubljana, Slovenia *Corresponding author: [email protected]

ABSTRACT Dry reforming of methane with CO2 was investigated over bi-metallic W and Ni incorporated mesoporous alumina catalysts prepared by the one-pot sol-gel route. Powdered materials were thoroughly characterized (N2-physisorption, XRD, XPS, SEM-EDX, TGA-DTA, TPH) prior and post catalytic runs, performed at 600 and 750 oC. High surface area W-Ni incorporated mesoporous alumina catalysts (SBET=178–192 m2/g) synthesized in this work showed excellent performance for the conversion of model biogas to synthesis gas. The Ni-W containing materials exhibited high catalytic activity, which was maintained throughout 150 hours TOS long-term operation at 750 oC. Increase of the W loading (0-10-15 wt. %) at fixed nickel amount (5 wt. %) resulted in prevented deactivation of the catalyst, most prominent at 600 °C, and minimization of coke formation on the surface of the catalyst. Tungsten incorporation was thus proven to significantly enhance and stabilize the overall catalyst performance.

KEYWORDS: dry reforming, mesoporous alumina, coke minimization, tungsten, nickel

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1. Introduction Production of alternative fuels and chemicals from bio-waste and alcohols attracted significant attention of researchers, due to decreasing availability of fossil fuel resources and environmental restrictions.1-6 Biogas, which is produced via anaerobic digestion of organic waste, is mainly composed of methane and carbon dioxide and can be further upgraded by means of catalytic transformation to synthesis gas.5,6 Synthesis gas or “syngas” is an attractive feedstock for the production of chemicals and synthetic fuels. Dry reforming of methane with carbon dioxide (Eq. 1) is the main limiting step for catalytic upgrading of biogas, therefore the development of active and stable heterogeneous catalysts with low coke formation features is required to successfully utilize two of the most abundant greenhouse gases (CH4 and CO2). CH4 + CO2 ↔ 2CO + 2H2

∆Ho298 = 247 kJ mol−1

(1)

Synthesis gas, which is produced as a result of CO2 reforming of methane, is a high-quality industrial feedstock. Examples of processes for further conversion of syngas gas to various fine chemicals involve methanol, dimethyl ether and Fischer-Tropsch synthesis pathways. According to dry reforming stoichiometry, hydrogen to carbon monoxide ratio is expected to be unity in the product stream. However, reverse water gas shift reaction (RWGS) (Eq. 2) occurs frequently during dry reforming of methane. Occurrence of RWGS side reaction causes some decrease in hydrogen yield and increase in CO concentration in the product stream.

CO2 + H2 ↔ CO+ H2O

∆Ho298 = 41.2 kJ mol−1

(2)

Other side reactions, such as methanation reaction (Eq. 3), steam reforming of methane (Eq. 4) and methane decomposition (Eq. 5) may also take place together with dry reforming reaction. Possible mechanisms of reforming reactions are available in the literature.1,2

CO2 + 4H2 ↔ CH4 + 2H2O

∆Ho298 = -164.9 kJ mol−1

(3)

CH4 + H2O↔CO + 3H2

∆Ho298 = 206 kJ mol−1

(4)

CH4 → C + 2H2

∆Ho298 = 75 kJ mol−1

(5)


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As reported by Cunha et al., catalytic decomposition of methane becomes significant at temperatures higher than 550-600 oC range, depending upon the catalyst composition.7,8 Another reaction, which may cause formation of significant amount of coke, is the Boudouard reaction (Eq. 6).9-13 Due to its exothermic nature, thermodynamics of this reaction is favorable at temperatures lower than 650 °C. ∆Ho298 = -173 kJ mol−1

2CO ↔ C + CO2

(6)

Coke formation on the surface of the catalyst can lead to catalyst deactivation, as well as clogging and complete plugging of the reforming reactor. Due to these reasons, coke formation has been considered as the main problem in conversion of biogas to syngas through dry reforming of methane and hence significant amount of research has been focused on this topic in recent years.14-16 Development of novel coke resistant catalysts for CO2 reforming of methane is a major issue for widespread use of biogas to syngas conversion process. Recent studies on dry reforming of methane were focused on development of new catalysts with high coking resistance and stable catalytic performance. Group VIII B metals, like Co, Ni, Pt, Ru, Rh, Pd, etc., were reported to show high catalytic activity in dry reforming of methane.17-24 Ni and Cu based catalysts were considered as promising catalysts for both steam and dry reforming reactions. While Ni catalyzes reforming, Cu is reported to catalyze water gas shift reaction4. Due to their high activity, availability and low cost, Ni based catalysts attracted major attention of researchers.15,20-22 However, the main problem of Ni based catalysts, supported on conventional microporous carriers, is the issue of severe coke formation on Ni particles and rapid deactivation of the catalyst. Effects of promoters, such as Ce, Mg, Ca, K, etc., on carbon formation over Ni, Co or Pt based catalysts were investigated in recent years.9,13,16,23,24 Ni-Co bimetallic catalysts and Ni based catalysts, which were promoted by small amounts of noble metals, were also synthesized and tested to improve their catalytic performances in dry reforming of methane.6,12,14,17,18,25-29 Effect of catalyst support material has been shown to be quite important from the point of view of catalytic performance and coke formation.18,19,30 Mesoporous catalyst supports were reported as less susceptible to catalyst deactivation due to coke formation than conventional microporous supports.18,28,29,31-33 In our recent studies, it was shown that the catalytic


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performance of mesoporous silicate (MCM-41) supported Ni based catalysts could be improved by Ru and Rh modification.28,29 Mesoporous alumina is a more stable catalyst support than ordered mesoporous silicate structured materials (like MCM-41). Catalytic performances of mesoporous alumina supported Ni based catalysts were reported as being highly stable in our recent publication.9 Improvement of catalytic performance of Ni incorporated mesoporous materials, by impregnation of tungsten, was also illustrated in that work. Considering these initial promising results, a set of novel Ni-W incorporated bimetallic mesoporous alumina supported catalysts were synthesized by a one-pot route and a comprehensive and systematic investigation was performed in the present study, to investigate the effect of W on coke elimination.

2. Experimental 2.1 Synthesis and Characterization of the Catalytic Materials W and Ni incorporated bimetallic mesoporous alumina catalysts were synthesized following a one pot sol-gel procedure, which has been discussed elsewhere.34 Nickel nitrate hexahydrate (Ni(NO3)2.6H2O, Merck) and ammonium meta-tungstate hydrate salts were used as the Ni and W precursors, respectively. In this one-pot sol-gel synthesis procedure, Ni and W precursors were dissolved separately with stirring in deionized water. Aluminum isopropoxide was added into a separate container of deionized water, which was heated to 85 oC. Ni and W solutions were then added with stirring into the hot solvent solution. Nitric acid was added to start the hydrolysis reaction, resulting in a sol. Then, 1,3-butanediol was added, while stirring. The solution was stirred at room temperature for 24 h, and subsequently at 60 oC to form the gel. The product was then dried at 100 oC for 24 h and calcined at 800 oC for 6 h in a flow of dry air. Catalysts containing 5 wt. % Ni and 15, 10 and 0 wt. % W were denoted as 5Ni-15W-SGA, 5Ni-10WSGA and 5Ni-SGA, respectively. Synthesized materials were characterized by nitrogen adsorption-desorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) techniques and scanning electron microscopy, combined with Energy-dispersive X-ray spectroscopy (SEM-EDX). XRD. SEM and thermal analysis (TGA-DTA) of the used catalysts were also performed, to obtain relevant information about the amount and type of coke deposited on the catalytic materials. Temperature programmed hydrogenation (TPH) of the spent catalysts was also performed to gain further insight about the nature of coke formed on the catalyst surface.


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A Rigaku Ultima-IV instrument, which was available at the METU Central Laboratory, was used for the XRD analysis. BET surface area values and the pore size distributions of the synthesized materials were measured by means of the standard nitrogen adsorption–desorption technique, using a Quanta Chrome-Autosorb-1C sorptometer. XPS analysis was made by a SPECS EA300 instrument, also available at the METU Central Laboratory. Collected data were corrected for charge shifting, using standard C 1s binding energy of 284.5 eV. A QUANTA 400F Field Emission scanning electron microscope, coupled with EDX, was used to obtain SEM images of the fresh and the spent catalysts. Thermogravimetric and thermal flux analyses of the samples were investigated in a stream of dry air (30–900 °C, 10 °C/min heating ramp) by means of Perkin Elmer STA 6000 apparatus. Temperature programmed hydrogenation was performed by means of Perkin Elmer Pyris 1 thermo gravimetric analyzer (100–900 °C, 5 °C/min heating ramp) in a stream of diluted hydrogen (5 vol. % H2/N2). 2.2 Catalytic Reactions Most of the reaction tests for carbon dioxide reforming of methane were performed in a quartz tubular fixed bed reactor (I.D. 6 mm). These tests were performed at Gazi University, for a period of 4 hours for each run.34 A set of long-term experiments were also performed at the National Institute of Chemistry in Slovenia, for periods extending up to 150 hours. In shortperiod dry reforming tests, which were performed at Gazi University, 0.1 g of pelletized catalyst with a particle size range of 1–2 mm was loaded into the reactor and the activity tests were performed at 600 and 750 oC, at atmospheric pressure. Prior to the activity tests, catalysts were reduced in a flow of hydrogen for 3 h. Possible effect of diffusion resistance on the observed conversion values and dispersion effects within the reactor were tested in our earlier work, by performing two sets of experiments with the pelletized and powder Ni incorporated MCM41type catalysts (Ni-MCM-41).27,28 Results of those experiments had shown similar catalytic performances of powder and pelletized (1-2 mm) materials in dry reforming of methane, indicating negligible effects of pore diffusion and dispersion within the reactor on the observed rate. Considering that the pore dimensions of the catalysts used in the present study were larger than Ni-MCM-41 type materials used in the earlier work and also the activities of those catalysts were higher than the activity of the new catalysts, we do not expect any diffusion effect on the observed conversion values obtained in the present study.


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The feed stream of the reactor was composed of methane, carbon dioxide and argon with 1/1/1 molar ratio. Total flow rate of this stream was adjusted to 60 Nml/min. A gas chromatograph (Perkin-Elmer Autosystem XL), which was equipped with a thermal conductivity detector and a Carbosphere column, was used for the evaluation of the composition of the product stream. This chromatograph was connected on-line to the reactor outlet stream and analysis of the product stream was made at certain intervals. Catalytic performances of the synthesized materials were evaluated in terms of CO2 and CH4 fractional conversions and H2, CO selectivity values, with respect to converted CH4. Selectivity and fractional conversion definitions are given below:

CH 4 Conversion =

H 2 Selectivity =

(CH

4 ( in )

− CH 4 ( out )

CH 4 (in ) H 2( out )

(CH

4 ( in )

− CH 4 ( out )

)

)

;

;

CO2 Conversion =

CO Selectivity =

(CO

2 ( in )

− CO2 ( out )

)

CO2(in ) CO( out )

(CH

4 ( in )

− CH 4 ( out )

)

Long-term stability tests were performed with two of the synthesized catalysts for 150 h, in a fully automated computer-controlled fixed-bed reactor unit (PID Eng&Tech Microactivity reactor), installed at the Laboratory for Environmental Sciences and Engineering, National Institute of Chemistry, Slovenia. In these tests, 0.5 g of the powdered catalyst was mixed with 2.833 g of inert SiC and loaded into the tubular quartz reactor (I.D. 10 mm). These time-onstream tests (TOS) were performed at 750 oC and atmospheric pressure. An equimolar mixture of CH4 and CO2 was fed to the reactor at a 100 Nml/min total flow rate. The reactor unit was coupled to a gas chromatograph (Agilent Technologies, model 7890A), to allow online analysis of the discharged gas stream composition.

3. Results and Discussions 3.1 Catalyst Characterization Physical properties of the synthesized catalysts are given in Table 1. The surface area values of the sol–gel alumina (SGA) supported Ni and Ni-W catalysts were all in the range of 178-192 m2/g. The BET surface area of pure mesoporous alumina, which was prepared by a similar sol-gel route (SGA), was reported as 192 m2/g in our recent publication.9 Results proved


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that Ni and W incorporation into SGA through a one-pot procedure did not cause significant reduction in BET surface area of the synthesized materials. Increase of the W content of the catalyst caused slight decrease in the BET surface area and in the micropore volume, as well. All materials were found to have average pore diameters in the mesoporous range of 11-13 nm. Nitrogen adsorption/desorption isotherms of the synthesized catalysts were all Type IV, according to IUPAC classification, with sharp hysteresis loops of adsorption and desorption branches (Fig. 1). Such isotherms indicated the formation of mesoporous structures with ordered pores. As shown in Fig. 2, these catalytic materials had quite narrow pore size distributions, with most of the pores being in the 8-15 nm range. XRD patterns of the Ni and Ni-W incorporated calcined and reduced mesoporous alumina catalysts prepared by the one-pot sol-gel procedure showed typical peaks corresponding to γAl2O3, at 2θ values of 37.6, 39.5, 45.8, 60.8 and 66.8o (Fig. 3). The peaks observed at 2θ values of 44.5, 51.8 and 76.4o in the XRD spectra of the reduced materials correspond to reduced nickel clusters.9,28 However, in the case of calcined materials, no peaks corresponding to metallic Ni or NiO were observed. All of the peaks observed in the XRD pattern of calcined 5Ni-15W-SGA catalyst belong to γ-Al2O3 (Fig. 3). As it was reported in our earlier publications, nickel was expected to be in the form of NiO in the calcined materials.27,28 However, no peaks were observed for NiO or WOx in the XRD patterns of the calcined materials synthesized in this work. These results indicated that crystal sizes of NiO and WOx clusters within the calcined catalysts synthesized in the present work, were smaller than the detection limit of XRD analysis. However, as a result of the reduction step, larger Ni clusters were formed, which were clearly seen in the XRD patterns of the reduced materials. Crystal dimensions of the metallic Ni within the reduced and spent materials were estimated as being in the range of 13-16 nm, from the Scherrer equation (Table 2). Results indicated that, although the crystal dimension of nickel compounds increased significantly as a result of the reduction step, no appreciable change was observed after the dry reforming reactions performed at 600 and 750 oC. These observations were also supported by the SEM-EDX mapping results of the calcined, reduced and the spent catalysts in the Supporting Information. We did not observe any sharp peaks corresponding to either WOx or W in the XRD spectra of the synthesized materials (Fig. 3). Main characteristic peaks of WOx (WO3, W20O58) were expected at 2θ ranges of 23.2-24.4o and 33.3-34.2o. Reduction of WO3 to W20O58 and further


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reduction to metallic tungsten is reported at high temperatures.35 Therefore, the main diffraction peak of metallic tungsten was expected to appear at a 2θ value of 40.2o, coinciding with the XRD peak of pure γ-Al2O3. The absence WOx peaks in the XRD spectra of the synthesized materials can thus be explained with high dispersion of tungsten oxide within the lattice of the prepared materials, yielding particles with dimensions of WOx that were too small for the detection with XRD analysis. To obtain a better understanding of tungsten compounds, which were present on the surface of the synthesized catalytic materials, XPS analysis was also performed. For the reduced 5Ni-10W-SGA sample (Fig. 4), a wide band corresponding to the tungsten compounds was observed in the range of 32–39 cm-1. Binding energies of WO3, WO2, W and Al2(WO4)3 are 35.8, 32.8, 31.3 and 35.8 cm-1, respectively. Deconvolution of the wide band observed in the range of 32-39 cm-1 indicated that the main tungsten compounds were WO3 and Al2(WO4)3 on the surface of 5Ni-10W-SGA. Al2(WO4)3 was reported to have limited stability, as it was reported to decompose to AlWO4 and WO3-x by partial loss of oxygen.36 This was considered to contribute to the redox ability of the synthesized materials. Typical SEM images of 5Ni-10W-SGA are given in Fig.5. As shown in this figure, the shape of this material was quite irregular and particle dimensions were in the range of 3-20 µm.

3.2 Activity Test Results Most of the catalytic activity tests of the synthesized catalysts were performed within a reaction period of four hours, at 600 and 750 oC. Results of the 4 h activity tests obtained at 600 showed that the catalyst containing no tungsten (5Ni-SGA) was highly active, giving quite high CO2 and CH4 conversion values at the start of these tests (Fig. 6). However, fractional conversion values of CO2 and CH4 showed decreasing trends within the reaction period of four hours. Although tungsten incorporation to the Ni based mesoporous alumina catalyst caused some decrease of activity towards dry reforming of methane, it also caused stabilization of 5NiSGA catalyst. In fact, quite stable CO2 and CH4 conversions were obtained with 5Ni-10W-SGA and 5Ni-15W-SGA materials (Fig. 6). As it was reported in the literature, stabilization of catalytic performances of Ni based catalysts for reforming and methane decomposition reactions could also be achieved by Cu addition into the catalyst structure.4,8


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Another important improvement achieved by tungsten incorporation was the approach of CO2 and CH4 fractional conversion values to each other. Due to the contribution of reverse water gas shift reaction (Eq. 2), fractional conversion values of CO2 obtained with these catalysts were higher than the fractional conversion values of CH4. The ratio of fractional conversions of CO2 and CH4 was about 1.9 with 5Ni-SGA, at the end of the four hour reaction test. However, this ratio was less than 1.4 with 5Ni-15W-SGA (Fig. 5). Similar conclusions were derived from the hydrogen and carbon monoxide selectivity values reported in Fig. 7. CO selectivity was shown to increase as a function of time with the 5Ni-SGA catalyst, at 600 oC. However, more stable CO and H2 selectivities were obtained with the tungsten incorporated bimetallic catalytic materials. CO/H2 molar ratio obtained over 5Ni-SGA, at the end of the four hour test was about 1.7 at 600 o

C. However, this ratio was about 1.4 with 5Ni-15W-SGA. Therefore, both conversion and

selectivity results indicated that, the catalyst ability to suppress the occurrence of RWGS reaction was enhanced, and the catalyst operation was stabilized as a result of tungsten incorporation. Short-term catalytic performance experiments performed at 750 oC showed higher CH4 and CO2 conversion values than the corresponding values obtained at 600 oC (Fig. 8). At this reaction temperature, near-equilibrium conversions were observed. Results obtained at 750 oC were also highly stable and CO/H2 ratios of about 1.25 were obtained (Fig. 9). Methane conversion values obtained at 750 oC with the catalysts synthesized in this work are compared with some of the dry reforming results published in the literature in Table 3. As shown in this table, methane conversion values obtained with the catalysts synthesized in this study were quite comparable with the literature results. Although tungsten incorporation into SGA supported Ni catalyst caused some decrease of methane conversion values, it caused significant improvement in suppressing the occurrence of RWGS reaction and achieving more stable catalytic performance. As it has been discussed in Section 3.3 of this manuscript, tungsten incorporation also caused significant improvement for minimization of coke formation. A set of 150 h TOS experiments were also performed at 750 oC over the bimetallic nickeltungsten incorporated mesoporous alumina catalysts (5Ni-10W-SGA and 5Ni-15W-SGA). CO, H2, CO2, CH4 and H2O concentrations at the reactor outlet were highly stable in these TOS experiments (Fig. 10). The value of CO/H2 molar ratio obtained in these TOS experiments was about the same as the ratio obtained in the short-period activity tests. Also, no deactivation of W incorporated catalysts were observed during these TOS tests.


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3.3 Coke Elimination due to Tungsten Incorporation Coke formation on the surface of the catalysts during dry reforming tests was determined by means of thermal (TGA-DTA) and temperature programmed hydrogenation (TPH) analyses of the used catalysts. XRD analyses of the used catalysts were also performed to observe crystalline form of coke, as well as morphological changes (if any) in the catalysts during dry reforming reaction tests. TGA of the used catalysts were performed in a flow of dry air at a heating rate of 10 oC per minute. These analyses were made with the used catalysts, after 4 h of reaction period. The main contributors to the formation of carbonaceous species on the catalyst surface are the dissociation of methane (CH4 → C + 2H2) and Boudouard reaction (2CO → C + CO2). Methane decomposition is expected to gain importance at temperatures higher than 550-600 o

C.7,8 while the contribution of Boudouard reaction becomes more significant at temperatures

lower than 650 oC. Nickel particle size and dispersion were also reported to have significant role on the type, location and amount of carbonaceous matter over the catalyst.9,12,37,38 As shown in the TGA results of the spent catalysts after four hours of reaction tests performed at 600 oC (Fig. 11), weight loss due to coke removal decreased with an increase in tungsten amount of the catalyst. In fact, the amount of coke was only about 2 wt. % in the case of spent 5Ni-15W-SGA catalyst at the end of 4 h reaction period, at 600 oC. As discussed above in relation to methane and CO2 conversion results, catalytic performance of this material was also highly stable. Although methane decomposition might also have some contribution to coke formation at 600 oC, main contributor was expected as the Boudouard reaction, at this temperature. Stable operation and low coke formation indicated an important feature of coke formation suppression of the catalysts by combination of tungsten/nickel incorporation, as the equilibrium of Boudouard reaction was quite high at 600 °C. There was almost no coke formation over 5Ni-15W-SGA catalyst at the higher reaction temperature of 750 oC (Fig. 12), where equilibrium constrains do not allow for the occurrence of Boudouard reaction. In fact, the weight loss observed over 400 oC in the TGA analysis that can be accounted to carbon accumulation was less than 1 wt. % for this catalyst. It is furthermore important to note that the weight loss observed due to oxidation of coke over the used 5Ni-SGA was about 7 wt. %. In order to see the exothermic oxidation trend of carbonaceous material


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formed on the surface of the spent catalysts, DTA profiles were also obtained (Fig. 13, a and b). As shown in the DTA profile of the used 5Ni-15W-SGA after the reaction test at 600 oC, two exothermic peaks were observed at about 650 and 450 oC, corresponding to the oxidation of two different carbon species on the surface. Intensities of these peaks became extremely small for the spent catalyst obtained after the 750 oC reaction test (Fig. 13a). On the other hand, the DTA profile of used 5Ni-SGA catalysts at the reaction temperature of 750 oC (Fig. 13b) showed a sharp exothermic peak, which was observed at about 650 oC, with a strong shoulder located at about 450 oC. Incorporation of tungsten to the catalyst caused significant reduction of the intensity of the DTA peaks. The sharpness of the peak observed for 5Ni-SGA at about 650 oC was significantly decreased by tungsten incorporation and a broad peak with a low intensity was observed in the DTA of the spent 5Ni-15W-SGA. These results indicated the formation of two different types of carbon species on the catalyst surface. Amorphous carbonaceous species were considered to be oxidized in air at relatively low temperatures (decomposition maximum at 400 °C). However, at higher temperatures, the weight loss in TGA could be attributed to gasification of different types of filamentous carbon (peaks at 500 and 650 °C) to CO or CO2.14,32,33,39 Instability of the CH4 and CO2 conversion values observed during dry reforming tests over 5NiSGA at 600 oC was due to significant amount of coke formed over the active sites of this catalyst. Operation at higher temperatures and incorporation of tungsten into 5Ni-SGA was shown to cause significant reduction in coke formation and significant improvement in the catalytic stability. Positive effect of tungsten compounds on coke minimization is considered to be mainly due to their good redox properties. Mechanisms involving redox reactions were proposed in the literature.40 Cunha et al. showed that Raney-type Ni and Co catalysts were quite good catalysts for decomposition of methane into hydrogen and filamentous carbon7 at temperatures higher that 550 oC. They also showed that Cu addition into Raney-type Ni catalysts improved catalytic stability in methane decomposition.8 Presence of Cu in the Ni-Cu alloyed Raney-type catalyst was also shown to be quite beneficial for the formation of more ordered graphitic carbon at 600 o

C. XRD analysis results of the spent catalysts are given in Fig. 14. In the XRD patterns of the

spent 5Ni-SGA, after dry reforming tests performed at 600 and 750 oC, a new peak was observed at a 2θ value of about 26o. This peak corresponds to crystalline carbon deposited on the catalyst


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surface. The intensity of this peak was stronger in the XRD pattern of used 5Ni-SGA after the reaction test at 600 oC. These results were consistent with the TGA results. As the reaction temperature was increased, coke formation was decreased. Coke formed at high temperatures might have further reacted with CO2 or H2O. As shown in Fig. 14a, incorporation of tungsten to the catalyst structure caused significant minimization of the carbon peak in the XRD of spent 5Ni-10W-SGA and complete elimination in the XRD of spent 5Ni-15W-SGA, demonstrating the suppressed coke formation with the emphasis on decreased filamentous deposits. For both reaction temperatures, no carbon peak was observed in the XRD profiles of the spent 5Ni-15WSGA, clearly indicating the positive effect of W incorporation for coke resistance. These results also supported the conclusions reached from the TGA–DTA results. Apparently, tungsten oxide plays a role of reduction of coke during dry reforming of methane. Redox reactions occurring on the tungsten oxide sites are considered to be responsible for the elimination of coke formation. As shown in Fig. 10, both 5Ni-10W-SGA and 5Ni-15W-SGA showed highly stable catalytic performances within a reaction period of 150 hours, at 750 oC. Although formation of some coke was observed after the time-on-stream tests (less than 10 wt. %), no deactivation of the catalysts was observed within this reaction period. This is mainly because of the type of carbon (filamentous) formed on the catalyst surface during these time-on-stream tests. Comparison of the SEM images of the fresh 5Ni-10W-SGA and the spent catalyst after the 150 h time-onstream test (at the same magnification) (Fig. 15, a and b) showed formation of some carbon filaments accumulated at certain regions on the catalyst surface. Line SEM-EDX analysis of the carbon containing region of the used catalyst (Fig. 15c) clearly showed the presence of high amount of carbon at this location. The ratio of intensities of the EDX signals for W and Ni present on the catalyst surface was shown as being around 2 in this figure, which was in agreement with the value of the synthesized material. In order to observe the type of carbon formed on the catalyst surface, SEM image of the coke accumulated region of the surface of the used 5Ni-10W-SGA was also obtained at a higher magnification (Fig. 16). Formation of carbon filaments on the surface of the used catalyst was clearly seen in this figure. Analysis of the dimensions of these carbon filaments showed that their diameters were in the range of 20–50 nm. Some additional peaks were observed at 2θ values of 35.7 and 48.2o in the XRD patterns of the tungsten incorporated spent catalysts obtained after reaction at 750 oC in the 4 h reaction


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tests (Fig. 14). These peaks showed formation of tungsten carbide crystals within the structure of these catalysts during dry reforming of methane, especially at elevated temperatures. Apparently, coke formed on the catalyst surface reacted with WOx and as a result, tungsten carbide was formed. Oxidation through dissociative adsorption of CO2 and carbidation reactions, were expected to take place on the tungsten sites of these catalytic materials.40 This redox cycle helps to minimize coke formation on the Ni-W incorporated bimetallic mesoporous catalysts synthesized in the present work. Formation tungsten carbide was more clearly observed in the XRD patterns of the spent catalyst after the 150 h TOS test performed at 750 oC (see Supporting Information). To see if there was any change in the pore structure of the synthesized catalysts after 150 h reaction tests, nitrogen adsorption/desorption analyses of the spent catalysts were also obtained. N2 adsorption/desorption isotherms of the fresh and spent 5Ni-10W-SGA showed that the catalyst kept its mesoporous structure even after 150 h of activity test (Fig. 17). Some decrease of pore volume was observed after the 150 h tests. BET surface area of the catalyst decreased from 190. to 129 m2/g, as the smaller pores could have been sintered or filled with the produced coke. Still, the observed level of coke formation did not induce any catalyst deactivation or reactor plugging, enabling unhindered long-term operation. In terms of catalyst regeneration, exothermic oxidation processes generate much heat on the surface of the catalyst and can cause severe catalyst deterioration. In order to examine the possibility of catalyst regeneration by endothermic methanation, temperature programmed hydrogenation (TPH) analysis of the spent 5Ni-10W-SGA and 5Ni-15W-SGA catalysts was performed (Fig. 18). In these experiments, two-step coke removal profile were observed, similar to TGA/DTA curves. As the filamentous carbon doesn’t reduce easily due to a high degree of graphitization, most of the carbon formed on the surface of both used catalysts was removed through reaction with hydrogen at temperatures higher than 600 oC.

4. Conclusions W-Ni incorporated bimetallic mesoporous alumina catalysts were successfully synthesized following a one-pot sol-gel route. Activity test results proved minimization of coke formation during dry reforming of methane, as a result of tungsten incorporation into Ni based mesoporous alumina. No coke formation was observed over the catalyst containing 5 % Ni and 15 % W (5Ni-


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15W-SGA) during short-term operation. During long-term 150 hour stability test, 10 wt. % of mainly filamentous carbon build-up was observed. However, both 5Ni-15W-SGA and 5Ni-10WSGA catalysts showed very stable catalytic performances during these time-on-stream tests. It was concluded that incorporation of tungsten into the Ni containing mesoporous alumina catalysts, which were synthesized by the one-pot sol-gel route, caused highly stable catalytic performance in dry reforming of methane. Coke formation was minimized due to the redox ability of WOx.

Acknowledgement TUBITAK (111M449), Gazi Univ. (06/2010-38) funds, collaboration with Laboratory for Environmental Sciences and Engineering, National Institute of Chemistry, Slovenia (bilateral project ARRS-TUBITAK BI-TR/12-14-002) and Central Laboratory of Middle East Technical University are acknowledged.

Supporting Information Fig S1giving XRD pattern of spent catalyst after 150 h TOS test at 750 oC, and Fig S2 giving SEM-EDX mappings of nickel and tungsten compounds in the calcined, reduced and spent catalysts are available free of charge via the Internet at http://pubs.acs.org/.

References

(1)

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(2)

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(3)

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(4)

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(5)

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(6)

Izquierdo, U.; Barrio, V. L.; Bizkarra, K.; Gutierrez, A. M.; Arrabi, J. R.; Gartzia, L.; Banuelos, J.; Lopez-Arbeloa, I.; Cambra, J. F. Ni and Rh-Ni Catalysts Supported on Zeolites L for Hydrogen and Syngas Production by Biogas Reforming Processes. Chem. Eng. J. 2014, 238, 178-188.

(7)

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(8)

Cunha, A. F.; Orfao, J. J. M.; Figueiredo. Methane decomposition on Ni-Cu alloyed Raney-type catalysts. Int. J. Hydrogen Energy 2009, 34, 4763-4772.

(9)

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Table captions: Table 1. Physical properties of the catalysts. Table 2. Crystal sizes of reduced and used 5Ni-15W-SGA catalysts. Table 3. Comparison of some of published methane dry reforming results with our work.


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Table 1. Physical properties of the catalysts.

Sample

Metal content in

BET surface

Pore size

Pore

Micro pore

synthesis

area (m2/g)

(nm)

volume

volume

(cm3/g)

(cm3/g)

solution 5Ni-SGA 5Ni-10W-SGA

5Ni-15W-SGA

5 wt. % Ni 5 wt. % Ni 10 wt. % W 5 wt. % Ni 15 wt. % W

192

11

0.55

0.084

190

13

0.59

0.083

178

11

0.56

0.075


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Table 2. Crystal sizes of reduced and used 5Ni-15W-SGA catalysts Sample

Crystal Size of Ni, nm

Calcined 5Ni-15W-SGA

-

Reduced 5Ni-15W-SGA

15.7

Used 5Ni-15W-SGA after 4 h reaction test at 600oC

12.8

Used 5Ni-15W-SGA after 4 h reaction test at 750oC

14.1 o

Used 5Ni-15W-SGA after 150 h reaction test at 750 C

14.6


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Table 3. Comparison of some of published methane dry reforming results with our work. Researcher

Catalyst

Reaction Condition

Djinovic et al.14

Ni-Co/CeO2-ZrO2

Hou et al.17

Rh/Al2O3

Ozkara et al.23

Pt/ZrO2

Ozkara et al.24

Co-Ce/ZrO2

Verykios X. E.41

Ni/La2O3

Laosiripojana et al.42

CeO2

Lovell et al.43

Ni-MCM-41

Izquierdo et al.44

Ni-MgO

Liu et al.31

5Ni/MCM-41

This work

5Ni-SGA

This work

5Ni-10W-SGA

This work

5Ni-15W-SGA

Arbag et al.28

Ni-MCM-41

This work

5Ni-SGA

T: 750 oC P: 1.2 atm CH4/CO2: 1/1 WHSV: 37,000 ml/(gcat. h) T:800 oC P: 1 atm CH4/CO2: 1/1 WHSV: 60,000 ml/(gcat.h). T: 700 oC P: 1 atm CH4/CO2: 1/1 WHSV: 15,600 mL/(gcat.h) T: 650 oC P: 1 atm CH4/CO2: 1/1 WHSV: 60,000 mL/(gcat.h) T:750 oC, P: 1 atm CH4/CO2/He= 1/1/0 WHSV: 60,000 mL/(gcat.h) T:900 oC, P: 0.06 atm CH4/CO2/He= 1/1/0 WHSV: 72,000 mL/(gcat.h) T:750 oC, P: 1 atm CH4/CO2/Ar= 1/1/1 GHSV: 12,230 h-1 T:800 oC, P: 1 atm CH4/CO2/Ar= 3/2/0 T:750 oC P: 1 atm CH4/CO2/He= 1/1/2 WHSV: 50,000 mL/(gcat.h) T: 750 oC P: 1 atm CH4/CO2/Ar = 1/1/1 WHSV: 36,000 ml/(gcat. h) T: 750 oC P: 1 atm CH4/CO2/Ar = 1/1/1 WHSV: 36,000 ml/(gcat. h) T: 750 oC P: 1 atm CH4/CO2/Ar = 1/1/1 WHSV: 36,000 ml/(gcat. h) T: 600 oC P: 1 atm CH4/CO2/Ar= 1/1/1 WHSV: 36,000 ml/(gcat. h) T: 600 oC P: 1 atm CH4/CO2/Ar = 1/1/1 WHSV: 36,000 ml/(gcat. h)

% CH4 conversion

48

57

75

50

90

31

92 62.2

70

70

60

53

28

~ 22


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Figure captions: Figure 1. Nitrogen adsorption-desorption isotherms of the synthesized catalytic materials. Figure 2. Pore size distributions of 5Ni-SGA, 5Ni-10W-SGA and 5Ni-15W-SGA catalysts. Figure 3. XRD patterns of reduced 5Ni-SGA, 5Ni-10W-SGA, 5Ni-15W-SGA and calcined 5Ni15W-SGA catalysts (*: γ-Al2O3, +: Ni) Figure 4. XPS analysis of 5Ni-10W-SGA sample. Figure 5. SEM images of 5Ni-10W-SGA sample. Figure 6. Fractional conversions of a) CO2 and b) CH4 over 5Ni-SGA, 5Ni-10W-SGA and 5Ni15W-SGA catalysts in short-term 4 h TOS tests at Tr = 600 oC. Figure 7. CO and H2 selectivities with respect to converted CH4 over 5Ni-SGA, 5Ni-10W-SGA and 5Ni-15W-SGA catalysts in short-term 4 h TOS tests at Tr = 600 oC. Figure 8. Fractional conversions of a) CO2 and b) CH4 over 5Ni-SGA, 5Ni-10W-SGA and 5Ni15W-SGA catalysts in short-term 4 h TOS tests at Tr = 750 oC. Figure 9. CO and H2 selectivities with respect to converted CH4 over 5Ni-SGA, 5Ni-10W-SGA and 5Ni-15W-SGA catalysts in short-term 4 h TOS tests at Tr = 750 oC. Figure 10. Composition of the gas mixture at the reactor outlet for long-term 150 h TOS tests at Tr = 750 oC using a) 5Ni-10W-SGA and b) 5Ni-15W-SGA catalysts (mcat = 0.5 g, mSiC = 2.833 g).


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Figure 11. TGA analysis of used 5Ni-SGA, 5Ni-10W-SGA and 5Ni-15W-SGA catalysts used in short-term 4 h TOS tests at Tr = 600 oC. Figure 12. TGA analysis of used 5Ni-SGA and 5Ni-15W-SGA catalysts in short-term 4 h TOS tests at Tr = 750 oC. Figure 13. Comparison of DTA profiles of a) spent 5Ni-15W-SGA catalyst in short-term 4 h TOS tests at Tr = 600 and 750 oC; b) spent 5Ni-SGA and 5Ni-15W-SGA catalysts in short-term 4 h TOS tests at Tr = 750 oC. Figure 14. XRD patterns of used 5Ni-SGA, 5Ni-10W-SGA and 5Ni-15W-SGA catalysts in short-term 4 h TOS tests at a) Tr = 600 oC, b) Tr = 750oC (¤: C; *: γ-Al2O3; +: Ni; Ж: WC). Figure 15. SEM photographs of a) fresh and b) used 5Ni-10W-SGA after long-term 150 h TOS test at Tr = 750 oC; c) line SEM-EDX analysis results of the used 5Ni-10W-SGA catalyst. Figure 16. SEM image of coke containing region of spent 5Ni-10W-SGA catalyst after longterm 150 h TOS test. Figure 17. N2 adsorption/desorption isotherms of spent catalysts after long-term 150 h TOS tests. Figure 18. TPH analysis of used catalysts after long-term 150 h TOS test at Tr = 750 oC.


24

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

25


400 Desorption Adsorption

300 5Ni-10W-SGA

Volume (cm3/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5Ni-15W-SGA 5Ni-SGA

200

100

0 0.0

0.2

0.4

0.6

0.8

1.0

P/Po

Figure 1. Nitrogen adsorption-desorption isotherms of the synthesized catalytic materials.


26

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2.50

2.00 5Ni-10W-SGA dV/dlog(d)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5Ni-15W-SGA

1.50

5Ni-SGA

1.00

0.50

0.00 1

10

100

Pore Size, nm

Figure 2. Pore size distributions of 5Ni-SGA, 5Ni-10W-SGA and 5Ni-15W-SGA catalysts.


27


*

* + * * *

+

*

*

+

5Ni-SGA Reduced

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5Ni-10W-SGA Reduced

5Ni-15W-SGA Reduced 5Ni-15W-SGA Calcined

10

20

30

40

50

60

70

80

2θ

Figure 3. XRD patterns of reduced 5Ni-SGA, 5Ni-10W-SGA, 5Ni-15W-SGA and calcined 5Ni15W-SGA catalysts (*: γ-Al2O3, +: Ni)


28

Page 29 of 43

Al2(WO4 )3 Al2(WO4)3 WO3 WO3

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Al2 (WO4 )3 37.0 eV

WO3 35.1 eV

30

32

34 36 Binding Energy (eV)

38

40

Figure 4. XPS analysis of 5Ni-10W-SGA sample.


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 43

Figure 5. SEM images of 5Ni-10W-SGA sample.


30

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0.5

Fractional Conversion

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4 0.3 0.2 0.1 0.0 0

30

60

90 120 Time, min

CO2

150

180

210

240

CH4

a)5Ni-15W-SGA

b) 5Ni-15W-SGA

a)5Ni-10W-SGA

b) 5Ni-10W-SGA

a) 5Ni-SGA

b) 5Ni-SGA

Figure 6. Fractional conversions of a) CO2 and b) CH4 over 5Ni-SGA, 5Ni-10W-SGA and 5Ni15W-SGA catalysts in short-term 4 h TOS tests at Tr = 600 oC.


31


3.5 3.0 2.5 Selectivity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 43

2.0 1.5

CO

1.0 0.5

H2

a) 5Ni-15W-SGA

b) 5Ni-15W-SGA

a) 5Ni-10W-SGA

b) 5Ni-10W-SGA

a) 5Ni-SGA

b) 5Ni-SGA

0.0 0

30

60

90

120

150

180

210

240

Time, min

Figure 7. CO and H2 selectivities with respect to converted CH4 over 5Ni-SGA, 5Ni-10W-SGA and 5Ni-15W-SGA catalysts in short-term 4 h TOS tests at Tr = 600 oC.


32

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1.0

Fractional Conversion

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8 0.6 0.4 0.2 0.0 0

30

60

90 120 Time, min

CO2

150

180

210

240

CH4

a)5Ni-15W-SGA

b) 5Ni-15W-SGA

a) 5Ni-10W-SGA

b) 5Ni-10W-SGA

a) 5Ni-SGA

b) 5Ni-SGA

Figure 8. Fractional conversions of a) CO2 and b) CH4 over 5Ni-SGA, 5Ni-10W-SGA and 5Ni15W-SGA catalysts in short-term 4 h TOS tests at Tr = 750 oC.


33


2.5

2.0

Selectivity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 43

1.5

1.0

CO

0.5

H2

a) 5Ni-15W-SGA

b) 5Ni-15W-SGA

a) 5Ni-10W-SGA

b) 5Ni-10W-SGA

b) 5Ni-SGA

a) 5Ni-SGA

0.0 0

30

60

90

120

150

180

210

240

Time, min

Figure 9. CO and H2 selectivities with respect to converted CH4 over 5Ni-SGA, 5Ni-10W-SGA and 5Ni-15W-SGA catalysts in short-term 4 h TOS tests at Tr = 750 oC.


34

100

Concentration (Vol. %)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CO CO H2 H 2 CO2 CO 2 CH CH44 H H2O 2O

80 60 40 20 0 0

30

60

90

120

150

Time on Stream, h

a) 100

Concentration (Vol. %)

Page 35 of 43

CO CO H2 H2 CH 4 CH4 CO2 CO2 H2O H2O

80 60 40 20 0 0

30

60

90

120

150

Time on Stream, h

b) Figure 10. Composition of the gas mixture at the reactor outlet for long-term 150 h TOS tests at Tr = 750 oC using a) 5Ni-10W-SGA and b) 5Ni-15W-SGA catalysts (mcat = 0.5 g, mSiC = 2.833 g).


35


5Ni-SGA

5Ni-10W-SGA

5Ni-15W-SGA

0 -5

Weight Loss %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 43

-10 -15 -20 -25 -30 0

200

400

600

800

1000

Temperature, o C

Figure 11. TGA analysis of used 5Ni-SGA, 5Ni-10W-SGA and 5Ni-15W-SGA catalysts used in short-term 4 h TOS tests at Tr = 600 oC.


36

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5Ni-SGA

5Ni-15W-SGA

0 -5 -10

Weight Loss %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-15 -20 -25 -30 0

200

400

600

800

1000

Temperature, o C

Figure 12. TGA analysis of used 5Ni-SGA and 5Ni-15W-SGA catalysts in short-term 4 h TOS tests at Tr = 750 oC.


37


15

o C reaction test after 600 5Ni-15W-SGA @ Rxn 600 oC o C reaction test 5Ni-15W-SGA @ after 750 Rxn 750 oC

Heat Flow (mW)

10

5

0 0

200

400 Temperature,

600

800

1000

800

1000

oC

a) 5Ni-SGA

5Ni-15W-SGA

15

10

Heat Flow (mW)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 43

5

0 0

200

400 Temperature,

600 oC

b) Figure 13. Comparison of DTA profiles of a) spent 5Ni-15W-SGA catalyst in short-term 4 h TOS tests at Tr = 600 and 750 oC; b) spent 5Ni-SGA and 5Ni-15W-SGA catalysts in short-term 4 h TOS tests at Tr = 750 oC.


38

Page 39 of 43

* + * *

Intensity (a.u.)

* 5Ni-15W-SGA

+

*

50

60

*

+

¤

5Ni-10W-SGA

* 5Ni-SGA

10

20

30

40

70

80

2θ

a) * + * Ж*

+

*

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5Ni-15W-SGA

Ж

+

*

¤

* 5Ni-SGA

10

20

30

40

50

60

70

80

2θ

b) Figure 14. XRD patterns of used 5Ni-SGA, 5Ni-10W-SGA and 5Ni-15W-SGA catalysts in short-term 4 h TOS tests at a) Tr= 600 oC, b) Tr= 750 oC (¤: C; *: γ-Al2O3; +: Ni; Ж: WC).


39


a)

b)

250

Al

200

Counts (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 43

150

100

50

O W C Ni

0 0

1

2

3

4 5 Distance (µm)

6

7

8

c) Figure 15. SEM photographs of a) fresh and b) used 5Ni-10W-SGA after long-term 150 h TOS test at Tr = 750 oC; c) line SEM-EDX analysis results of the used 5Ni-10W-SGA catalyst.


40

Page 41 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 16. SEM image of coke containing region of spent 5Ni-10W-SGA catalyst after longterm 150 h TOS test.


41


400 Desorption Adsorption 300

Volume (cm3 /g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 43

200

100 5Ni-10W-SGA 5Ni-10W-SGA-150h 0 0.0

0.2

0.4

0.6

0.8

1.0

P/Po

Figure 17. N2 adsorption/desorption isotherms of spent catalysts after long-term 150 h TOS tests.


42

Page 43 of 43

0

0

5Ni-15W-SGA

5Ni-10W-SGA -2

% Weight Loss

% Weight Loss

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-4 -6 -8

-2 -4 -6 -8

0

200

400 600 800 o Temperature, C

1000

0

200

400 600 800 Temperature, oC

1000

Figure 18. TPH analysis of used catalysts after long-term 150 h TOS test at Tr = 750 oC.


43

Coke Minimization during Conversion of Biogas to Syngas by

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