Synthesis and Application of Lanio₃ Perovskite-Type Nanocatalyst with Zr for Carbon Dioxide Reforming of Methane

Introduction

Synthesis gas (syngasH₂/CO) is a mixture of H₂ and CO that is an important feed used for the petrochemical industry like Fisher-Tropeshprocess. Generally, in syngas cycle with different ratio of H₂/CO, variety of products is obtained.

Similarly, several groups have carried out research on the production of syngas from methane. (e.g. the partial oxidation of methane (POM), reaction 1), steam reforming of methane (SRM), reaction 2) and dry reforming of methane (DRM), reaction 3 [1].

POM: CH₄+ ½ O_2 →CO +2H₂(ΔH_298K = -22.6 kJ/mol)                          (1)

SRM: CH₄+ H₂O   →       CO +3H₂(ΔH_298K = 225.4 kJ/mol)                  (2)

DRM: CH₄+ CO₂   →     2CO +2H₂(ΔH_298K = 260.5 kJ/mol)                  (3)

Syngas economy is considered to be a good process to reduce CO₂ and CH₄ emission which are both greenhouse gases [2].

In recent years considerable researchers have been focused on DRM reaction to produce syngas [3-5].A lower H₂/CO ratio is produced in dry reforming of methane, which favors to liquid fuel production when compared with steam reforming and partial oxidation of methane [6-7]. The reactions that take place in this process are endothermic:

CO₂ + H₂ ↔ H₂O + CO                (ΔH_298K = 41 kJ/mol)                     (4)

Catalysts play special role in this reaction, nowadays a considerable attention is focused on the activity and stability of the catalysts.

Recently,provskite oxide structures (ABO₃) have been studied widely as catalysts in dry reforming of methane reactions [2]. In the provskite-type oxides, A is (lanthanides, alkali metals and earth alkali metals) and B is (transition metals) where thermal resistance is controlled by Acations and catalytic activity is controlled with B redox cations [1]. However, many researchers have doped transition metals in A-site; and lanthanides, alkali metals and alkali earth metals in B-site [8-10].

There are studies reported on the beneficial effect of noble metal addition such as many studies have been carried out with the aim to synthesize complex mixed metallic oxides with the perovskite structure.A lot of papers have reported different doping level of Ni in B-site of perovskite type oxides that are successful in catalytic activity for reforming of methane [11-14].

For examples, Goldwasser et al. have investigated the influence of Ru partial substitution by Ni and La partial substitution by Ca in LaRuO₃ and LaRu_0.8Ni_0.2O₃, respectively, in the combined reforming of methane (CRM) with CO₂ and O₂, in DRM processes [15]. In another work,Jahangiri et al. have reported the behavior of La_1–xSm_xNiO₃ and LaNi_1–xCo_xO₃ as catalyst precursors in CRM process [16&17]. In addition, they studied the performance of LaNi_1-xFe_xO₃perovskite-type oxides with various substitution degrees (x) in the CRM process [18].In the present study, La_1-xZr_xNiO₃ perovskiteswere synthesizedwith different substitution degrees(x) and characterized with conventional methods for investigating the stability and activity of them as a catalyst in the DRM process.

Experimental

Materials and Method

In this research, La(NO₃)₃.6H₂O,(Merck, >99/9%), ZrO(NO₃)₂.6H₂O, (Aldrich, 99/99%), Ni(NO₃)₂.6H₂O (Merck, >99%), citric acid (Merck, 99/5%) and ethylene glycol (99%) were used for synthesis of La_1-xZr_xNiO₃nanocatalysts. Appropriate stoichiometric of La, Ni nitrates and Zirconium oxynitrate solution (1M) were mixed and stirred for 40 minutes, then citric acid and ethylene glycol with molar ratio of 1 were added to thissolution. The mixture was stirred for 8 hours in 80 °C. The resulting gel was dried at110 °C for 24 h and calcined in two steps: 1st at 350 °C for 0.5 h and 2nd at 800 °C for 2 h.

Equipment

The prepared sampleswere characterized by X-ray diffraction (using 3003 PTS diffractometer), with a copper anode (CuK_amonochromatized radiation source, λ= 1.5405Å) to ascertainformation of the perovskite structure,phase purityand crystallite size determination. The morphology of the prepared samples was studied by scanning electron microscopy (SEM, using Philips XL30 microscope). BET method was applied for measuring the surface area of the samples using N₂ at -200°C(on a Tristar 3000 apparatus) and inductivity coupled plasma (ICP) emission spectroscopy was used (on a Perkin-Elmer ICP/5500 apparatus) for quantitative analysis of the samples. Temperature programmed reduction (TPR, on a 29000 apparatus) was performed for measuring of hydrogen consumption.

Catalytic Activity Evaluation

Catalytic activity of the samples was studied under 600 to 800 °C. The reactants and products were analyzed by a gas chromatograph (Model 6890N, Agilent Technologies) provided with the thermal conductivity detector (TCD). The experimental tests for the catalyst activity study were carried out using the feed gases (CH₄, CO₂, N₂ and H₂) with ultra-high pure grade (>99.999%) in a micro-reactor equipped with mass flow controllers (Model 5850, Brooks Instrument).

The temperature of micro-reactor was measured and controlled by two thermocouples (Ni–Cr, K-Type, 0.5 mm diameter) and two PID thermo-controllers (Model JumoiTRON08). The catalyst (0.4 g for all cases) was loaded in the middle of the reactor, and the feed gases at a total flow rate of 100 ml/min (WHSV=15 l/(h.g), CH₄/CO₂= 1/1) under atmospheric pressure were introduced into the reactor as the defaultprior to activity measurements.Catalyst precursors were reduced in situ in a flow of 20% H₂ in N₂ (total flow rate of 50 ml/min) at 700 °C for 2 h to generate the metal phase. In all tests, the performances wereevaluated by conversions. The CH4 and CO2 conversions, H2 and CO yields and H2/CO ratio are defined as follow[7]:

Results and Discussions

Crystalline Structure

The XRD patterns of the synthesizedsamples are shown in Fig. 1. The XRD patterns of Zr doped LaNiO₃indicated that Zr doping level (x) in these samples were 0.2 with formation of well-crystallized perovskite structure without any impurities(JCPDS No: 33-710).For Zr doped LaNiO₃sample, beyond x=0.2 additional peaks are presented which belongs to ZrO₂, La₂Zr₂O₇ and NiO compounds.

Figure 1: XRD patterns of La1-xZrxNiO3nanocatalysts Click here to View Figure

 

For calibrating the position of the reflections, a quartz standard was used. Since all the prepared samples could be indexed with cubic symmetry of the ABO₃ type, the cell parameter (a) in the unit cell of these compounds was calculated using the following equation:

The obtained results are shown in Table 1. The results indicatethatby increasing the amountof Zr in the La_1-xZr_xNiO₃ samples, the cell parameter decreases.

Table 1: Cell parameter (a) of the prepared samples obtained from XRD data

La1-xZrxNiO3
X	Cell parameter (a) (Å)
0	3.888
0.1	3.867
0.2	3.864
0.3	3.855
0.5	3.854

The crystallite size of the preparedsamples calculated by the Scherrer equation using the intense peak is presented in Table 2 [19].Thistableshows that the preparedparticlesare appeared in nanoscale.

Table 2: Elemental analysis^*, BET surface areas and crystallite size of some calcinedLa_1-xZr_xNiO₃perovskites (calculated by Scherrer equation)

La1-xZrxNiO3
X	La (wt.%)	Zr (wt.%)	Ni (wt.%)	SA (m2/g)	D (nm)
0	**(3.07)8.96	(00.0)00.0	(29.7) 30.2	3	50
0.1	65.0 (64.8)	4.5(4.8)	30.5(30.4)	5	53
0.3	50.1	16.8	33.1	3	45
0.5	38.3	29.6	32.1	2	43
* Calculated by ICP		** Nominal values are in parentheses

 

Chemical Analysis and Surface Area Measurements

The chemical composition (wt.%) and surface area for some prepared catalysts are given in Table 2.As it is shown, the experimental data for Ni, Zrand La (wt.%) are close to the nominal values (reported in parentheses). BET surface areas of the catalysts are found in the range of 3-5m²/g. Since preparation of perovskite oxides by modified citrate sol-gel method, require long exposure to high temperatures, leading to low surface area solids is not surprising. Similar behavior has also been reported by Pahlevanzadeh et al. [17].

Morphology

The SEM images of the samples are shown in Fig 2. These images have shown that the synthesized particles were in nanoscale which is nearly in spherical shape. For
La_1-xZr_xNiO₃ samples, morphology study of the particles with doping level up to 0.2 is uniform which indicate that these samples are crystallized in mono phase perovskite.

Figure 2: SEM images of the La1-xZrxNiO3 (x=0 (A), x=0.1 (B), x= 0.2 (C), x=0.3 (D) and x=0.5 (E)). Click here to View Figure

 

TPR profiles of the prepared samples are shown in Fig.3. TPR study of the samples displays two main steps of hydrogen consumption. The first one indicates the reduction of Ni³⁺ to Ni²⁺ with a maximum consumption of hydrogen in around 382°C [18]. The second step can be related to Ni²⁺ to Ni⁰ reduction in around 570°C [20]. Thistrend is usual for perovskite type samples and confirmsthatnickelispresent in the B-site. In addition, for Zr doped LaNiO₃ sample, both steps shifted to higher temperatures which can be due to the presence of Ni-Zr-O system [21]. This result confirms the doping level of Zr which is obtained by XRD patterns.

Figure 3: TPR profiles of prepared La1-xZrxNiO3(calcined in air at 800 °C) Click here to View Figure

 

Catalytic Activity

Catalytic activity of the prepared samples in the temperature range of 600-800 °C has been studied in DRMprocess. The CH₄and CO₂conversions and product yields in the presence of La_1-xZr_xNiO₃ in different temperatures are presented in Fig. 4. These figures show that CH₄and CO₂ conversions increase by increasing thetemperature in the presence of all catalysts. However, the CH₄ conversions are lower than those of CO₂. This behavior could be due to consuming of CO₂ in reaction (4). In addition, both the H₂and CO yields increase with increasing the reaction temperature in the presence of the prepared catalysts, while the CO yields are higher than the H₂yields. This effect is more notable at higher temperatures, which are in consistent with Aghabozorg et al. study [16].

In the presence of La_1-xZr_xNiO₃ samples with doping levels of x=0.3 and 0.5, there is no outstanding difference in the CH₄and CO₂conversions and product yields for each temperature. These results confirm that the mono phase pesrovskite structure shift to other metal oxide phases for doping level of x= 0.3 and 0.5 which is in consistent with XRD and TPR results.

Catalytic activity study of the prepared samples also indicate that H₂/CO ratio in the presence of the catalyst with doping level of x=0 and x= 0.1 is ~1, while this ratio for upper doping level is less than 1. In other words, it can be concluded that the reactions (3) and (4) are the main occurring reactions. These results indicate that perovskite structures are favorable for producing more H₂ that is in accordance with Valderrama et al. research [22].

Fig. 5 indicates the CH₄ and CO₂ conversions and H₂ and CO yields versus time at 750 °C for La_1-xZr_xNiO₃ samples. This figure shows that the CH₄, CO₂ conversions and H₂, CO yields in the presence of the pure perovskite are more than those of catalysts containing more than one phase. However, these parameters for LaNiO₃ are more observable than that of the samples containing Zr. The partial substitution of La³⁺ by Zr⁴⁺ in La_1-xZr_xNiO₃ structure is undesirable for Ni reduction and leads to decreasing the mobility of the oxygen toward the surface of the solid[22].

Fig. 5 (H₂ and CO yields) indicates that by letting time elapse, the yields in the presence of
La_1-xZr_xNiO₃are decreased. However, since the extent of yield decrease roughly follows the same way for both CO and H₂ yields, the H₂/CO ratio remain constant (Fig. 6).

Figure 4: CH4 and CO2 conversions, and H2 and CO yields as a function of the reaction temperatureforLa1-xZrxNiO3samples in DRM process (CH4/CO2= 1/1, WHSV=15 l/(h.g)).   Click here to View Figure

Figure 5: CH4 and CO2 conversions, H2 and CO yields as a function of the reaction time for La1-xZrxNiO3samples at 750 °C in DRM process (CH4/CO2= 1/1, WHSV=15 l/(h.g)).  Click here to View Figure

Figure 6: H2/CO ratios for La1-xZrxNiO3 as a function of the reaction time in DRM process (CH4/CO2= 1/1, WHSV=15 l/(h.g)).   Click here to View Figure

 

Conclusions

The XRD patterns of Zr doped LaNiO₃indicated that Zr doping level (x) in these samples was 0.2 with formation of well-crystallized perovskite structure in nanometer scale without any impurities.
The cell parameter calculation obtained from XRD data showed that, by increasing the amount of Zr in the La_1-xZr_xNiO₃ samples, the cell parameter decreased.
TPR study of the prepared samples displayed two main steps of hydrogen consumption. The first one indicated the reduction of Ni³⁺ to Ni²⁺. The second step was related to Ni²⁺ to Ni⁰ reduction in around 570°C. Also TPR analysis revealed that by increasing the amount of Zr inthe preparedsamples reduction process became difficult which could be due to presence of different forms of oxides other than perovskite type in the samples.
CH₄and CO₂ conversions increased by increasing thetemperature in the presence of all catalysts. However, the CH₄ conversions were lower than that of CO₂.
The CH₄and CO₂conversions and H₂and CO yields for the pure perovskite catalysts were more than those catalysts containing more than one phase.
Global warming by greenhouse gases such as carbon dioxide (CO₂) and methane (CH₄) is one of the most important problems in all over the world. In recent years, many researches aimed to solve this problem and promote hydrogen production technology such as CO₂ reforming of methane generally called (DRM). In this study we investigate La_1-xZr_xNiO₃nanostructures and its catalytic behavior indry reforming of methane and supposed to find out more applicable perovskitenanocatalyst in this process.

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