Synthesis gas is an important feedstock for the petrochemical industry, and consists mainly of H2 and CO. It is widely produced from natural gas (CH4) and is used in a diverse range of industrial processes [1, 2]. The established techniques for producing syngas are effective, and depend on the different characteristics of the CH4 reforming reactions [3, 4] such as steam reforming, dry reforming, and partial oxidation reforming [5]. Among these natural reforming processes, in terms of global warming and growing energy needs, dry reforming or CO2 reforming of CH4 (CH4 + CO2 → 2CO + 2H2) has been shown to have significant environmental and economic advantages, because two harmful greenhouse gases are converted to green H2. This process is stimulating increasing interest, because it produces syngas with a low H2/CO ratio, which is preferable for liquid hydrocarbon production in the Fischer-Tropsch reaction [6, 7, 8].
Most current studies in this field concentrate on CO2 reforming of CH4 from natural gas, but other CH4 sources are also appropriate for the reaction [9]. One such alternative is coke oven gas (COG), which is a by-product of coke production from coal in the steel industry [10, 11]. The composition of COG is typically H2 (~58%-60%), CH4 (~23%-27%), CO (~5%-8%), and CO2 (less than 3%). Only a small fraction of the COG can be used as fuel in the coking plant; the rest is generally burnt off or even discharged into the atmosphere [12]. The environmental impact is large, because of the substantial greenhouse gas emissions [13, 14]. Much attention has therefore been given to the CO2 reforming of COG, which can provide a more sustainable solution for the transformation of this highly energetic gas and minimization of CO2 concentration [15]. The most significant feature of this alternative is the production of syngas with a H2/CO ratio close to 2, the ideal proportion for methanol synthesis, without any conditioning stages. Another interesting characteristic of CO2 reforming of COG is that the energy required for this endothermic reaction can be provided by the blast furnace, which produces a large volume of exhaust gas [16]. Furthermore, CO2 reforming of COG is a catalysed reaction for which the rapid deactivation of the catalyst caused by carbon deposition is a serious problem [17]. The development of catalysts with high activities and stabilities, and which inhibit carbon formation during the reforming reaction, is therefore important.
In recent decades, noble-metal-based catalysts have been reported to be less sensitive to coking than Ni-based catalysts. However, Ni-based catalysts still warrant investigation, because their low cost and high availability make them more affordable in commercial processes [18, 19]. In this respect, one strategy is to introduce Ni and/or Co in a defined structure or solid solution. Perovskite-type oxides (ABO3) with a well-defined structure have interesting redox properties and are accessible, enabling them to acquire very highly dispersed nanoscale metallic particles under a reducing atmosphere; this promotes the catalytic activity and diminishes coke formation [20, 21]. Another advantage of perovskite-type oxides is the wide range of possible substitutions of A and/or B cations, enabling combination of elements with different oxidation states [22].
Valderrama et al. [23] used the B-site-substituted perovskite-type oxide LaNi1−xCoxO3 as a catalyst precursor for the dry reforming of CH4. Ni0, Co0, and La2O2CO3 were formed in catalytic tests, and gave high activities and stabilities. Sutthiumpor et al. [24] found that a small amount of Sr with a promotional doping element in the A site gave a better catalytic performance, because of the presence of surface oxygen species.
However, there have been few studies of B-site-substituted perovskite-type oxides with a fixed A-site doping content. The main objective of this study was therefore to investigate the activities and stabilities of quaternary solid solutions, La0.6Sr0.4NixCo1−xO3, as catalysts for CO2 reforming of COG in a fixed-bed reactor. The effect of calcination temperature on the catalytic performance was also studied.
La0.6Sr0.4NixCo1−xO3 quaternary perovskite-type oxides were prepared using a sol-gel method. Stoichiometric quantities of La(NO3)3·6H2O, Sr(NO3)2, Ni(NO3)2·6H2O, and Co(NO3)2·6H2O were mixed and dissolved in the required amount of deionized water. Ethylenediaminetetraacetic acid (EDTA) and citric acid (CA) were added to the aqueous solution as complexing agents, in appropriate molar ratios (all metal ions/EDTA/CA = 1:1:1.5) [25, 26], and the pH was adjusted to 7-8 by ammonia titration. The resulting solution was stirred at near 100 °C with an electromagnetic agitator until evaporation of water had reduced the solution to a hyaloid gel. This gel was dried overnight at 120 °C, and calcined in air for 6 h, after heating to 850 °C at a rate of 5 °C/min.
The phase compositions and internal structures of the catalysts before and after reaction were identified by X-ray diffraction (XRD), using a Rigaku D/Max-2200 X-ray diffractometer. The scans (2θ) were conducted over the range 10° to 90° at a rate of 6°/min, using Cu Kα radiation (40 kV, 40 mA).
The surface areas and pore sizes of the solids were measured by N2 adsorption-desorption at −196 °C, using a Micromeritics ASAP 2010 adsorption meter; the samples (0.1-0.2 g) were degassed for 2 h at 200 °C before measurement.
Temperature-programmed reduction (TPR) studies were performed using a Micromeritics AutoChem II 2920 instrument equipped with a thermal conductivity detector (TCD) under a flow of a reducing H2 mixture. Samples (0.1 g) of fine catalyst were placed in a quartz-tube reactor of radius 5 mm, heated to 200 °C, and purged at constant temperature in an Ar atmosphere. A mixture of 5% H2 in Ar was then passed through the reaction tube at a flow rate of 30 mL/min; the temperature was increased to 1000 °C at 10 °C/min. The effluent gas was monitored by the TCD to detect variations in the H2 concentration, which indicate the possible compositions of subproducts.
The morphologies and elemental compositions of the catalysts, and metal dispersion and carbon deposition on the catalysts after the reaction were determined using field-emission scanning electronic microscopy (FE-SEM; JEOL JSM-6700F) and transmission electron microscopy (TEM; JEOL JEM 200CX).
Thermogravimetry-differential scanning calorimetry (TG-DSC) was performed, using a thermal analyser (NETZSCH STA 449 F3), to measure the amount of deposited carbon on the catalyst after reaction. In the TG-DSC process, a sample (20 mg) was exposed to a gas mixture consisting of 10% O2 in N2 at a flow rate of 40 mL/min, while the temperature was raised from ambient to 900 °C at a rate of 10 °C/min.
Temperature-programmed hydrogenation (TPH) was also performed using the Micromeritics AutoChem II 2920. A sample (50 mg) of each tested catalyst was placed in a quartz tube and heated to 950 °C at a rate of 10 °C/min under 10% H2 in Ar. The TPH tail gas was tested using a microreactor-mass spectrometer (Hiden Analytical, Catlab).
Catalytic activity tests were performed at atmospheric pressure in a 6-mm internal diameter quartz-tube reactor installed in a fixed-bed continuous flow system, as we have previously reported [27, 28]. The COG (31.5% CH4, 57.98% H2, 3.12% CO2, and 7.4% CO) was mixed with additional CO2 to obtain a feed reaction gas (24.54% CH4, 24.54% CO2, 5.76% CO, and 45.16% H2), with a 1:1 ratio of CH4 to CO2, determined by a mass flow meter. The catalyst (approximately 0.5 g) was placed in the reactor tube and reduced by a mixture of 15% H2 in N2 (30 mL/min) for 10 h, after heating from room temperature to 800 °C at a constant rate of 5 °C/min. After reduction, the gas flow was switched to the feed gas, at a rate of 100 mL/min, and the catalyst was tested at 800 °C for 24 h. The gas flow rates were controlled by mass flow controllers. The tail gas from the reactor was first filtered through a dehydrating agent and then analysed using the GC-9160 gas chromatograph with which the instrument was equipped.
In all experiments on the dry reforming of COG, the conversions, X, of CH4 and CO2 and the selectivities, S, of CO and H2 were separately calculated using the following equations [29, 30]:
Finand Fout represent the flow rates of the inlet and outlet gases, respectively.
Figure 1(a) shows the XRD patterns of the synthesized La0.6Sr0.4NixCo1−xO3 solids calcined at 850 °C. The results show that whatever the degree of Ni substitution (x), the samples prepared using the sol-gel method retain good crystallinity in the perovskite structure, without the presence of any impurity phases. The substitution of Ni for Co slowly prompts changes in the diffraction peak intensities; the intensity of the diffraction peak corresponding to the NiO phase increases with increasing Ni content, as reported previously [31]. As can be seen from Table 1, which summarizes the Brunauer-Emmett-Teller (BET) specific surface areas and the main characteristics of the catalysts, the formation of different perovskite phases after calcination is determined by the degree of substitution [32]. For x < 0.5, La0.6Sr0.4CoO3 and LaSrCoO4 are obtained, and the NiO phase is not apparent; for x ≥ 0.5, NiO and La1.5Sr0.5NiO4 are also observed. The peak structures show a gradual change from LaSrCoO4 to La1.5Sr0.5NiO4 as the value of x increases.
To investigate the effect of the partial substitution of B cations, the angular area between 31° and 34°, which contains the most intense diffraction peaks of La0.6Sr0.4NixCo1−xO3, was magnified, as shown in Fig. 1(b). The diffraction intensity of the peak at around 31.5° increases with increasing Ni content, as a result of lattice distortion, which shows the formation of La1.5Sr0.5NiO4. However, the peak at about 33.2° does not show any significant shift in symmetry, suggesting the formation of a solid solution containing LaSrCoO4 and La1.5Sr0.5NiO4. Table 1 also shows that the specific surface areas of the La0.6Sr0.4NixCo1−xO3 catalysts vary from 2.91 to 6.42 m2/g, depending on the substitutions, although this has no direct relationship with the catalytic activities of the samples.
The cell parameters for the hexagonal and rhombohedral symmetries of La0.6Sr0.4NixCo1−xO3 solids are shown in Table 2. These were calculated for each x value, indexing the most intense diffraction peaks in Fig. 1 in the planes corresponding to (110) and (024) of La0.6Sr0.4CoO3 (JCPDS 89-5719) to an ideal hexagonal system; the distorted rhombohedral parameters were also calculated using the equations reported by Valderrama et al. [33, 41]. The rhombohedral parameters decrease with Ni content from 5.4075 to 5.3824 Å, which contributes to the decrease in the B-O distance in the perovskite-type structure [23], confirming solid solution formation among these quaternary systems.
Figure 2 shows the H2-TPR profiles of the fresh catalysts, and the corresponding H2 consumptions for different peaks are shown in Table 3. There are several reduction peaks, arising from the formation of different Ni and Co intermediate phases, and the final reduction temperature and H2 consumption decrease with increasing Ni content. The two samples with x < 0.5 have three H2 consumption peaks, corresponding to the reduction of Co ions with different valences. The first peak, at 300 °C, corresponds to the reduction of La0.6Sr0.4CoO3, the second peak to the reduction of NiO, and the third to the reduction of LaSrCoO4. The reduction process is consistent with the results reported by Valderrama et al. [33]:
La0.6Sr0.4CoO3 +1.1H2 → 0.4LaSrCoO4 + 0.6Co + 0.1La2O3 + 1.1H2O (1)
NiO + H2→ Ni + H2O (2)
2LaSrCoO4 + 3H2 → 2Co + La2O3 + 2SrO + 3H2O (3)
For x ≥ 0.5, the transition peak of La0.6Sr0.4CoO3 disappears with increasing degree of substitution, and two reduction peaks are observed. The first peak appears at a lower temperature, around 350-400 °C, corresponding to free NiO, which is more readily reduced by H2. The second peak arises mainly from the reduction of La0.6Sr0.4NiO3 to La2O3, SrO, and Ni0.
The La0.6Sr0.4NixCo1−xO3 XRD profiles after reduction by H2 at 800 °C are shown in Fig. 3. The results are in keeping with the TPR analysis. The formation of the active species Ni0 and Co0 over La2O3 and SrO promotes high metal dispersion without formation of a Ni-Co alloy, as shown in Table 1. This is a crucial factor in resistance to the formation of carbon deposits in the CO2 reforming of COG.
Figure 4 shows the catalytic activities and stabilities of the La0.6Sr0.4NixCo1−xO3 catalysts obtained from CO2 reforming of COG at 800 °C; the averages of all catalytic parameters in the process are summarized in Table 4. As expected, the conversions of CH4 and CO2 and the selectivities for CO and H2 are improved greatly by increasing the degree of Ni substitution to 1, suggesting that the reforming reaction is favoured by the presence of Ni. This behaviour arises because metallic Ni0 has better activity than Co0 for the suppression of carbon deposition during the reforming reaction, and the reduction of polyvalent Co ions requires higher temperatures, according to the TPR profiles [34]. An increase of x from 0 to 0.3 significantly improves the catalytic activity, with the CH4 conversion increasing from 50.5% to 87.2%. This indicates that Ni doped in the samples has a synergetic effect, which accelerates Co activation and eliminates the initial induction period. Furthermore, the CH4 conversions shown in Table 4 are a little smaller than the CO2 conversions. Valderrama et al. [23] reported that the presence of Sr slightly promoted the side reactions of CO2 reforming of COG, including the reverse water-gas shift reaction; this not only increases the CO2 conversion but also gives rise to higher selectivity for CO by the participation of H2 and CO2 [29]. Furthermore, there are no changes in the activities over other catalysts during 20 h runs, indicating excellent stability.
The effect of calcination temperature on the catalytic activities of the La0.6Sr0.4NiO3 catalysts was investigated. The results are shown in Fig. 5, and the average values are listed in Table 5. It is evident that the CH4 and CO2 conversions of all the catalysts do not have large temperature gradients, with the values ranging from 89.9% to 92.8%. However, the results still show different catalytic activities. When the temperature is increased from 600 to 800 °C, the conversions of CH4 and CO2 increase from 91.6% and 98.6%, respectively, to 92.8% and 99.8%. The higher calcination temperature leads to the formation of a more strongly interacting perovskite structure, which requires a higher temperature for reduction. This decreases the content of free NiO and causes the metal Ni particles from the perovskite structure to be highly dispersed on the support, leading to better catalytic activity. However, when the temperature rises to 900 °C, the conversions of CH4 and CO2 fall to 90.7% and 98.7%, respectively, suggesting that the catalytic activity is related not only to the degree of metal dispersion but also to the amount of carbon deposited, which is determined by the interactions. When the calcination temperature is over 800 °C, the phase structure of the catalyst changes with the disappearance of La2O3, which leads to a decrease in metallic Ni in the crystalline phase, and therefore a decrease in the catalytic activity, as reported by Zhang et al. [35, 36].
Figure 6 shows the XRD patterns of the La0.6Sr0.4NixCo1−xO3 catalysts after reaction; Ni0, Co0, SrO, La2O3, SrCO3, and La2O2CO3 phases are observed. Previous research has shown that an important CO2 adsorption produces SrCO3 and La2O2CO3 in the CO2 reforming of COG [37]. The observation of the La2O2CO3 phase here provides corroborating evidence that it is an important reaction intermediate and has a significant impact on the reforming process. It is evident that La2O3 doped with Sr provides oxygen vacancies, which promote the oxidation of CH4, enabling La2O2CO3 to be formed more readily, as reported by Fierro et al. [38]. The La2O2CO3 phase and CH4 interact to regenerate La2O3, which inhibits coke deposition and contributes to the high catalytic activity. Furthermore, the XRD analyses show that the presence of Ni0 and Co0 can be attributed to the reducible atmosphere (CO-H2). These phases are highly dispersed in the lattice of the perovskite crystal, also preventing the formation of deposits [39].
Microstructural analyses of the La0.6Sr0.4NixCo1−xO3 solids after reaction were performed using SEM; the images are shown in Fig. 7. The surface structures of the tested catalysts with different degrees of Ni substitution differ significantly. Some agglomeration of metallic particles and whisker carbon species is evident, suggesting the formation of deposits of different qualities after the reforming reaction. TEM was used to further observe the dispersion of the metallic particles and coke formation over the catalysts; the images are shown in Fig. 8. It can be seen that the La0.6Sr0.4CoO3 catalyst, which has the lowest activity, has few carbon nanotubes. When x = 0.5, a large number of carbon nanotubes are observed. However, for La0.6Sr0.4NiO3, the number of carbon nanotubes is smaller, indicating that the increase in their formation with increasing degree of Ni substitution occurs only within a certain range of x. The metallic particles are uniformly dispersed on the matrix solid and are not covered by the whisker carbon, which indicates that the perovskite-type catalyst precursors have stable structures and good catalytic activities under severe reaction conditions [40, 41].
The specific amount of carbon deposited was determined using TG-DSC. Figure 9(a) and (c) show an obvious weight gain for the samples, caused by Ni oxidation, from 300 to 500 °C. Varying degrees of weight loss then occur, because of oxidation of carbon species on the used catalysts, from 500 to 700 °C. The amount of carbon deposited ranges from 0.7% to 28.4%, depending on the degree of substitution. La0.6Sr0.4CoO3 has the least amount of deposited carbon, because of its low catalytic activity, although the amount of carbon is not maximum on La0.6Sr0.4NiO3, confirming the SEM and TEM results. The effects of calcination temperature on the carbon deposits are shown in Fig. 9(c) and (d). When the temperature is increased from 600 to 800 °C, the amount of deposited carbon decreases from 23.4% to 16.5%, but then rises again to 24.7% at 900 °C. These results indicate that a low calcination temperature leads to instabilities in the crystal structure, and a high temperature leads to sintering of metallic Ni particles, promoting the formation of carbon deposits.
TPH analysis was performed to further study the type of carbon deposited; the results are shown in Fig. 10(a) and the mass spectrum is shown in Fig. 10(b). These results indicate the formation of different carbon species, as we have previously reported [42]. The first peaks (230 and 270 °C) are related to removal of active carbon species such as CHx (x = 1, 2, 3) [43]. The negative peak at 330-350 °C is assigned to the reaction of deposited carbon species with H2 at the Ni/La2O2CO3 interface to produce CH4, as shown in Fig. 10(b). Slagtern et al. [44] found that CHx can react with H2 to form CH4. The second peaks observed in the TPH profiles (500 and 610 °C) are related to whisker-type carbon and graphitic carbon, largely originating from CH4 decomposition [45], as observed in the SEM and TEM images. The second negative peak, at 750-800 °C, arises from formation of C2H4, as the mass spectrum shows. The carbon corresponding to the peak at 793 °C probably originates from CH4 decomposition and CO disproportionation [46]. The TPH analysis also indicates that CH4 decomposition is a reversible process, and that high concentrations of H2 in the COG can efficiently inhibit carbon deposition in the reforming reaction.
Figure 11 shows a schematic diagram of the surface reaction over the reduced La0.6Sr0.4NixCo1−xO3 perovskite catalysts. Both CH4 and CO2 activation are assumed to occur directly on the metallic surface to form adsorbed hydrogen and carbon atoms, and an important CO2 adsorption occurs on the La2O3-SrO matrix, producing SrCO3 and La2O2CO3, the active species for CO2 reforming of COG. The reaction of carbonate species with surface carbon species occurs at the metal sites, converting the carbon species to CO [6]. In this way, the elimination and production of CHx and graphitic carbon on the surface of the catalysts reach equilibrium. The following scheme summarizes the related elementary steps:
CH4 + M (M=Ni, Co) → C-M + 2H2 (4)
La2O3 + CO2 → La2O2CO3 (5)
SrO + CO2 → SrCO3 (6)
La2O2CO3 + C-M → La2O3 +2CO + M (7)
The catalytic activity and stability are therefore not affected by the formation of carbon deposits, mainly for the following reasons: (1) the presence of La2O2CO3 inhibits carbon formation by reacting with CH4 to reform CO; (2) the strong interactions between the highly dispersed metallic particles and the support enable the catalyst to maintain high activity; and (3) COG containing abundant H2 can also reduce the formation of carbon deposits by inhibiting decomposition of CH4.
A series of La0.6Sr0.4NixCo1−xO3 catalysts were prepared using a sol-gel method, and tested in CO2 reforming of COG. The results indicate that partial substitution of Ni for Co improves the catalytic activity and stability, because metallic Ni0 has better activity than Co0 in the reforming reaction. The active Ni0 and Co0 species, which are nanoscale particles stabilized by the Sr doping, are highly dispersed on the matrix of the solid La2O3-SrO and suppress coke formation. The La0.6Sr0.4NiO3 calcined at 800 °C displays the best catalytic activity and anti-coking ability. The XRD results indicate that La2O2CO3 is a reaction intermediate that plays a very important role in the reforming reaction by inhibiting the formation of carbon deposits, accelerating the oxidation of CH4 and regeneration of La2O3 under the severe reaction conditions used.
2015, Vol. 36 Issue (7): 915-924
PDF (1083 KB)
2015, Vol. 36 Issue (7): 915-924
PDF (1083 KB)