Nickel-based perovskite catalysts: synthesis and catalytic tests in the production of syngas

Catalisadores à base de níquel com estrutura perovskita: síntese e testes catalíticos na produção de gás de síntese

Abstract

La_1-xSr_xNiO₃ (x= 0.0, 0.3 or 0.7) perovskite-type oxides were synthesized using the modified proteic gel method and using collagen as an organic precursor. Catalysts of La_1-xSr_xNiO₃/Al₂O₃ were obtained using the wet impregnation method. The synthesized catalysts were characterized by X-ray diffraction, surface area and temperature-programmed reduction. The catalysts were evaluated in the partial oxidation reaction of methane, and the levels of selectivity to CO, CO₂, H₂ and H₂O were determined. Among the catalysts studied, the catalyst LaNiO₃/Al₂O₃ had the highest methane conversion level (78%) and higher H₂ selectivity (55%).

Keywords:
La_1-xSr_xNiO₃; perovskite-type oxides; modified proteic gel method; partial oxidation of methane

Resumo

Perovskitas do tipo La_1-xSr_xNiO₃ (x= 0,0, 0,3 ou 0,7) foram sintetizadas utilizando o método do gel proteico modificado e usando colágeno como precursor orgânico. Catalisadores do tipo La_1-xSr_xNiO₃/Al₂O₃ foram obtidos utilizando-se o método da impregnação úmida. Os catalisadores sintetizados foram caracterizados por difratometria de raios X, área superficial e redução à temperatura programada. Os catalisadores foram avaliados na reação de oxidação parcial do metano, e os níveis de seletividade ao CO, CO₂, H₂ e H₂O foram determinados. Dentre os catalisadores estudados, o catalisador LaNiO₃/Al₂O₃ foi o que apresentou maior nível de conversão de metano (78%) e maior seletividade ao H₂ (55%).

Palavras-chave:
La_1-xSr_xNiO₃; óxidos do tipo perovskita; método do gel proteico modificado; oxidação parcial do metano

INTRODUCTION

Many of the studied catalysts for applications in chemical industry processes are based on mixed metal oxides. The specific preparation of these oxides, which are capable of performing complex functions, is one of the main research topics in the field of heterogeneous catalysis ¹. Among the mixed metal oxides, those with a perovskite structure constitute a family of crystalline ceramic materials of the ABO₃-type formula, which have recently been recognized as active catalysts, replacing noble metals ^1),(2. In these oxides, A is a large cation, such as alkali metals, alkaline earths and rare earths and B is a small cation, such as a transition metal. In general, they have well defined structures, the perovskite being ideal cubic type. However, these structures often appear distorted, especially for the orthorhombic and rhombohedral symmetries ³. It is possible to carry out partial substitutions of cations A and B without changing the fundamental structure of the perovskite ⁴. The effect of the partial replacement of the cation A, usually by a rare earth metal, and especially with elements with oxidation state other than +3, has been frequently studied ^5)-(7. However, interest in perovskite studies is promising in applications involving high temperatures due to their high stability. These materials have been considered important due to their electronic, optical, catalytic properties, among others ⁸.

The perovskites may present some structural defects, which can occur through the doping of the sites in the crystalline structure, generating excess or vacancy of oxygen ⁹. In some cases, doping with Sr at A-site can generate non-stoichiometric perovskites with excess oxygen and cation vacancies ¹⁰. Thus, the influence on the amount of Sr may influence the formation of the spinel structure, La₂NiO₄, which, like the perovskite structure, presents high thermal and mechanical stability ^10),(11. In addition, it offers resistance to wear related to the reaction conditions and is widely used in the study of solid-oxide fuel cell (SOFC) ^10),(12. In general, the perovskite properties are influenced by the synthesis method, calcination conditions (temperature, time and atmosphere) and substitutions at sites A and/or B. There are now several methods of obtaining ceramics oxides with perovskite type structure, such as the conventional powder technology, the Pechini method, the chelating precursor method, the proteic sol-gel method and the combustion synthesis process, among others ^{13), (14), (15}.

Some researchers have used synthetic routes similar to the polymer precursor method in which gelatin replaces ethylene glycol as a polymerization agent to obtain nanoparticles. This method consists in forming a complex metal chelate, including a chelating agent and a precursor metal, replacing ethylene glycol and citric acid or EDTA with gelatin. The method is distinguished by its simplicity and ability to obtain crystalline materials with high purity and good stoichiometric control. It is an alternative route to obtain oxides with high efficiency and low cost ¹⁶. Researchers showed that it is possible to obtain perovskites through the collagen-modified proteic gel method with formation of a single-phase material at 700 ºC ^6),(16. In this work, oxides of La_1-xSr_xNiO₃ (x= 0.0, 0.3 or 0.7) type were synthesized using the modified proteic gel method using collagen as an organic precursor. This method was chosen because it is reported as a low cost and relatively simple route to obtain some materials, which results in the production of pure materials with good stoichiometric control. Materials supported on alumina of La_1-xSr_xNiO₃/Al₂O₃ (x= 0.0, 0.3 or 0.7) type were also synthesized by the impregnation method. The activity of the catalysts was evaluated in the partial oxidation of methane and promising results are presented.

MATERIALS AND METHODS

Synthesis of perovskites: La_1-xSr_xNiO₃ (x= 0.0, 0.3 or 0.7) perovskite-type oxides were performed by the collagen-modified proteic gel method using nickel (Vetec, 97%), lanthanum (Vetec, 99.9%) and strontium (Synth, 99%) nitrates as the starting material and collagen (Santosflora, 95%) as chelating agent. The synthesis procedure consisted of the dissolution of the nickel(II) nitrate in distilled water under magnetic stirring at 30 °C for 30 min. Then, the solid lanthanum(III) nitrate was added to the system and kept under stirring for 30 min. For substituted oxides, lanthanum nitrate was added together with the strontium(II) nitrate in the molar ratios of 0.3 and 0.7. After this time, the solution was heated to 70 °C, and then the chelating agent was added. After addition of the collagen, the temperature was maintained at 70 °C for 1.5 h resulting in a viscous gel which was heat treated at 350 °C for 2 h at a heating rate of 10 °C.min^-1, resulting in a precursor powder which was calcined at 900 °C for 2 h using a 10 °C.min^-1 heating ramp. The samples treated at 350 °C were named LN-3 for LaNiO₃, L7S3N-3 for the sample La_0.7Sr_0.3NiO₃, and L3S7N-3 for the sample La_0.3Sr_0.7NiO₃. The calcined samples at 900 °C were named LN for LaNiO₃, L7S3N for the sample La_0.7Sr_0.3NiO₃, and L3S7N for the sample La_0.3Sr_0.7NiO₃. The calcined catalysts were supported on alumina (Merck, 99%) using the solvent impregnation method ¹⁶ starting with an ethanolic suspension of the perovskites. After the impregnation procedure, the samples were subjected to drying at 60 °C for 1 h. The supported catalysts were synthesized aiming to obtain 15% of the perovskite base in the support and were named LNA (LaNiO₃/Al₂O₃), L7S3NA (La_0.7Sr_0.3NiO₃/Al₂O₃), and L3S7NA (La_0.3Sr_0.7NiO₃/Al₂O₃).

Characterization of perovskites: the thermogravimetric analyzes of the synthesized materials were carried out in a Shimadzu, TGA-50, in a nitrogen atmosphere, with a flow of 40 mL.min^-1, at a heating rate of 10 ºC.min^-1, in the temperature range from 30 to 900 °C using about 5 mg of sample. The Fourier transform infrared spectroscopy (FTIR) of the samples were recorded in the range of 4000 to 400 cm^-1 in a Varian, 640-IR, spectrophotometer, using the KBr pellet method. The X-ray diffractograms were obtained using a Rigaku, DMAX100, diffractometer, operated with CuKα and 2θ in the range of 10 to 60°, at a velocity of 2 °.min^-1, and step of 0.02°. Phase identification was performed by comparison with data from JCPDS (Joint Committee on Powder Diffraction Standards). The surface area of the catalysts was determined by N₂ physisorption at 77 K with the Quantachrome, Nova 1200, equipment. Temperature-programmed reduction (TPR) was performed using about 100 mg of sample in a Micromeritics, AutoChem II 2920, equipment. The reducing atmosphere of the reactor consisted of a mixture of 10.1% H₂ in N₂, with a flow of 50 mL.min^-1. The system was heated from 50 to 1000 °C using a heating rate of 10 °C.min^-1. A thermal conductivity detector was used to analyze the gas after water scraping. The catalytic tests were performed in a Micromeritics, Autochem II, reactor coupled to a Hiden, HPR20, mass spectrometer. The mass of catalyst used was 100 mg, and the reaction temperature was 900 °C under inert gas flow (He). The reactor containing the catalyst was subjected to 8 OSC (oxygen storage capacity) cycles with the combinations of 20 pulses of hydrogen and 10 pulses of oxygen as reducing and oxidizing, respectively. After the OSC cycles, two reaction cycles were programmed, each containing 15 pulses of methane as reducing agent and 15 pulses of oxygen as oxidizing agent. All pulses were monitored by the mass spectrometer. In order to quantify the components involved in the CSO and reaction processes, previous calibrations of the pure gas system were made as standard, simulating the same operating conditions and using quartz (SiO₂) as an inert solid in the reactor.

RESULTS AND DISCUSSION

The thermogravimetric curves of the thermally treated precursor powders at 350 °C are shown in Fig. 1. The precursor powder decomposition steps occurred at similar temperature ranges for all samples (LN-3, L3S7N-3 and L7S3N-3). It was possible to observe that the decomposition steps, in general, occurred in three or four temperature ranges. In the first temperature range, from 30 to 350 ºC, a mass loss occurred due to the release of water remaining of the synthesis process, and due to the initial decomposition of the remaining organic material. The largest percentage of mass loss observed for the LN-3 sample in comparison with other samples was due to this sample possibly having a greater amount of water remaining. In the second temperature range, from 350 to 580 ºC, for the LN-3 and L7S3N-3 samples the mass loss was related to the decomposition of the carboxylates, with subsequent formation of carbonates, which were decomposed in the temperature range between 580 and 800 °C. On the other hand, the sample L3S7N-3 showed a different decomposition profile of the carboxylates and carbonates in relation to the profile of the other samples. Furthermore, these decompositions occurred in the other temperature range. It is possible to observe that even at 900 ºC, the sample still show a mass loss, which can be attributed to the decomposition of carbonates formed ¹⁷. It was possible to observe a slight mass gain above 670 and 830 °C in the samples LN-3 and L7S3N-3, respectively, due to the oxidation reactions, as observed in other studies for perovskite of nickel-based synthesis ^16),(17.

Figure 1:
Thermogravimetric curves of the thermally treated precursor powders at 350 °C (LN-3, L3S7N-3 and L7S3N-3).
Figura 1:
Curvas termogravimétricas dos pós precursores tratados termicamente a 350 °C (LN-3, L3S7N-3 e L7S3N-3).

Fig. 2 shows the absorption spectra in the infrared region of samples heat treated at 350 °C for 2 h. All absorption spectra presented wide bands in the region between 3670-3280 cm^-1, which can be attributed to the stretching vibrations of the remaining O-H group of the synthesis ^18),(19. In the region of 1500-1340 cm^-1, bands were observed that can be attributed to the metal coordination by the carboxylate and amine groups of the collagen, since these vibrations are displaced when compared with the collagen spectrum bands (1637-1380 cm^-1). However, the characteristic bands between 1000-400 cm^-1 are related to the possible coordination of the carboxylate groups of the collagen with lanthanum, nickel or strontium ²⁰. The spectra of the heat-treated samples at 900 °C (result not shown) indicated the disappearance of the bands characteristic of the carboxylate groups and the amines present in the collagen. The samples L7S3N and L3S7N showed bands at 1455 and 857 cm^-1 which may be associated with the formation of strontium carbonate, being more intense in the sample L3S7N due to the higher concentration of strontium present in this sample ^21),(22.

Figure 2:
Absorption spectra in the infrared region of the thermally treated precursor powders at 350 °C (LN-3, L3S7N-3 and L7S3N-3).
Figura 2:
Espectros de absorção na região do infravermelho dos pós precursores tratados termicamente a 350 °C (LN-3, L3S7N-3 e L7S3N-3).

The X-ray diffraction patterns of the catalysts La_1-xSr_xNiO₃ (x= 0.0, 0.3 or 0.7) are shown in Fig. 3. All samples showed the formation of the perovskite crystalline phase, according to the diffraction pattern of the lanthanum nickel structure, obtained in the literature (JCPDS). The perovskite phase was identified by the peaks of higher intensities in 2θ of 32.7º, 40.7°, 47.2º, 53.5º and 58.6º that are in accordance with the file JCPDS 33-0711, related to the presence of perovskite LaNiO₃, with rhombohedral structure and group space R-3m. In the case of the strontium-substituted samples, besides the LaNiO₃ phase, there were secondary phases related to nickel oxide (NiO), strontium oxide (SrO), and strontium carbonate (SrCO₃). In these results, it was observed that increasing the degree of substitution of lanthanum increased the formation of secondary phases. The presence of peaks with lower intensities in the formation of strontium oxide for the sample with the lowest degree of substitution (x=0.3) may be related to the lower strontium concentration when compared to the sample with the highest degree of substitution (x=0.7), where the peaks related to the secondary phases were more intense. Studies show that the NiO phase can be formed together with the perovskite structure ²³.

Figure 3:
X-ray diffraction patterns for La_1-xSr_xNiO₃ samples calcined at 900 °C: p - LaNiO₃; o - NiO; + - SrO; Δ - SrCO₃; and # - La₂NiO₄.
Figura 3:
Difratogramas de raios X das amostras La_1-xSr_xNiO₃ calcinadas a 900 ºC.

According to the XRD patterns of the LN, L7S3N and L3S7N samples, the formation of LaNiO₃ phase in the samples was observed, in which the nickel of the perovskite structure is in its Ni³⁺ oxidation state. However, in the samples in which the lanthanum is partially replaced by strontium (L7S3N and L3S7N samples), in addition to the formation of the perovskite phase, there were mixtures of other phases. The results also indicated that the increase of Sr²⁺ content (L3S7N sample) favored the formation of the spinel phase (La₂NiO₄), as observed in the sample L3S7N and by other authors ²⁴. The strontium accommodation in the structure of lanthanum nickel is more difficult than the same substitution in other similar perovskites ⁵. This same observation was related, for example, in solid solutions of La-Sr-Mn-O without the presence of segregated crystalline phase ⁷. However, segregated phases of oxides and carbonates were observed for the compositions with molar amounts of Sr²⁺ greater than 0.10 and an increase in the oxygen non-stoichiometry with increasing strontium. However, it was observed an increase in the catalytic activity of the catalysts when doped with strontium ^5),(7. Lanthanum (La³⁺) partial substitution by an element of lower charge, in this case strontium (Sr²⁺), changes the stoichiometry of the oxide that is caused by the imbalance of the charges, producing a mixture of the oxidation state of nickel (Ni²⁺/Ni³⁺) being compensated by the oxygen deficiency (vacancies) ⁵, which can generate an improvement in the catalytic activity. The formation of secondary phases may be related to the increase in the degree of imbalance, thus hindering the formation of the perovskite monophasic structure. The diffraction patterns of the samples supported on alumina are shown in Fig. 4. It is observed that the three main characteristic peaks of the perovskites remained, however, with less intensity when compared with peaks of the unsupported samples, in view of the lower content of perovskite on the stand.

Figure 4:
X-ray diffraction patterns for the prepared samples La_1-xSr_xNiO₃/Al₂O₃: p - perovskite.
Figura 4:
Difratogramas de raios X das amostras La_1-xSr_xNiO₃/Al₂O₃ preparadas: p - perovskita.

Table I shows the specific surface area (SSA) results for samples La_1-xSr_xNiO₃ and La_1-xSr_xNiO₃/Al₂O₃ (x= 0.0, 0.3 or 0.7). The results showed surface area values for the LN materials that were concordant with the expected (~21 to 38 m².g^-1) ^6),(7. It was also possible to observe the effect of the insertion of strontium in the material from the specific surface area values of the prepared catalysts La_1-xSr_xNiO₃ (x= 0.0, 0.3 or 0.7). In general, it was observed that the addition of strontium caused an increase in the values of the specific surface area. This fact can be related to several factors, such as: formation of different crystallites related to different phases present in the material, as well as the substitution of a smaller cation (La³⁺ =0.106 nm) with a larger cation (Sr²⁺ =0.113 nm) ⁷. These substitutions may result in electronic imbalance depending on the load. The SSA values for the supported samples were increased in relation to the respective unsupported samples. This was probably due to the fact that alumina had a porous structure, which can impart a greater dispersion of the active species on the support and, consequently, result in higher SSA values ⁽9⁾. In addition, SSA values were the result of support and perovskite contributions.

Table I
Specific surface area (SSA) of the supported (La_1-xSr_xNiO₃/Al₂O₃) and unsupported (La_1-xSr_xNiO₃) catalysts.
Tabela I -
Área de superficial específica (SSA) para os catalisadores suportados (La_1-xSr_xNiO₃/Al₂O₃) e não suportados (La_1-xSr_xNiO₃).

Catalyst	LN	L7S3N	L3S7N	LNA	L7S3NA	L3S7NA
SSA (m2.g-1)	21	26	38	47	32	49

TPR profiles of the La_1-xSr_xNiO₃/Al₂O₃ (x= 0.0, 0.3 or 0.7) catalysts are shown in Fig. 5. Observing the reduction profile for the LNA catalyst, two reduction peaks were noted: the first at 387 °C and the second peak at 518 °C. These peaks corresponded to successive changes in the perovskite structure ⁷. The first peak referred to the reduction of Ni³⁺ to Ni²⁺ from the perovskite structure, corresponding to the formation of La₂Ni₂O₅ (Eq. A), and the second peak corresponded to the reduction of Ni²⁺ to Ni⁰, with formation of metallic nickel (Eq. B) ^15),(25. In addition, a third peak at 670 °C was observed for a possible formation of non-stoichiometric nickel aluminate (NiO-α-Al₂O₃) ²⁶, due to an interaction between NiO of the segregated phase with the support alumina (Eq. C):

$2{\mathrm{LaNiO}}_{3\left(s\right)}+{\mathrm{H}}_{2\left(g\right)}\to {\mathrm{La}}_2{\mathrm{Ni}}_2{\mathrm{O}}_{5\left(s\right)}+H_2{\mathrm{O}}_{\left(g\right)}$ (A)

${\mathrm{La}}_2{\mathrm{Ni}}_2{\mathrm{O}}_{5\left(\mathrm{s}\right)}+\; 2{\mathrm{H}}_{2\left(\mathrm{g}\right)}\to {\mathrm{La}}_2{\mathrm{O}}_{3\left(\mathrm{s}\right)}+\; 2{{\mathrm{Ni}}^0}_{\left(\mathrm{s}\right)}+\; 2{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{g}\right)}$ (B)

${\mathrm{NiO}}_{\left(\mathrm{s}\right)}+{\mathrm{H}}_{2\left(\mathrm{g}\right)}\to {{\mathrm{Ni}}^0}_{\left(\mathrm{s}\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{g}\right)}$ (C)

Figure 5:
Temperature-programmed reduction profiles for La_1-xSr_xNiO₃/Al₂O₃ catalysts.
Figura 5:
Perfis de redução à temperatura programada para os catalisadores La_1-xSr_xNiO₃/Al₂O₃.

The partial addition of strontium to the material resulted in significant modifications in the reduction profiles and the appearance of new peaks. These modifications may be indicative of a change in the structural composition of the material. One of the observed changes is in relation to the inversion in the intensities of the first and second peaks in relation to the sample that did not contain strontium. Similar results were reported in ²⁶. For the samples L7S3NA and L3S7NA, the maxima at 356 and 372 °C were attributed to several simultaneous steps, such as the reduction of Ni³⁺ to Ni²⁺ from the perovskite structure with formation of La₂Ni₂O₅ phase (Eq. A). This is then reduced resulting in Ni⁰ (Eq. B) and reduction of NiO (due to the secondary phase) in Ni⁰ (Eq. C). The second peak with maximum at 565 °C for L7S3NA, and at 417 °C with an intermediate peak at 388 °C for L3S7NA, can be attributed to the reduction of Ni²⁺ to Ni⁰ for the remaining NiO, suggesting that Ni metal was not completely reduced between 356 and 372 °C.

The LNA, L7S3NA and L3S7NA catalysts were evaluated on the reaction of partial methane oxidation. The catalytic activity of the catalysts was based on methane conversion and the conversion calculation was carried out through the material balance for the carbon, neglecting the coke. In this way, all carbon entering the system in the form of methane must exit as unconverted CO₂, CO and CH₄. Methane conversion and selectivity to CO, CO₂, H₂ and H₂O, as well as H₂/CO ratio, were analyzed. Conversion and selectivity calculations were performed based on the Eqs. D to H:

$\mathrm{C}(\%)=100·\frac{{\mathrm{C}}_{\mathrm{CH}4}^0-{\mathrm{C}}_{\mathrm{CH}4}^{\mathrm{f}}}{{\mathrm{C}}_{\mathrm{CH}4}^0}$ (D)

where C(%) represents methane conversion, ${\mathrm{C}}_{\mathrm{CH}4}^0$ is the amount of methane matter entering the reactor and ${\mathrm{C}}_{\mathrm{CH}4}^{\mathrm{f}}$ is the amount of methane matter in the reactor outlet. All the catalysts evaluated presented selectivity for syngas production. Figs. 6 and 7 show the results of conversion and selectivity. According to the results, it is possible to observe that the LNA sample had a methane conversion of 78%. However, the conversion decreased with increasing strontium content in the samples with conversions of 53 and 38% for samples L7S3NA and L3S7NA, respectively. The catalysts selectivity to the CO and CO₂ products were calculated by:

$\mathrm{CO}(\%)=100·\frac{{\mathrm{CO}}^{\mathrm{f}}}{{\mathrm{CO}}^{\mathrm{f}}+{\mathrm{CO}}_2^{\mathrm{f}}}$ (E)

${\mathrm{CO}}_2(\%)=100·\frac{{\mathrm{CO}}_2^{\mathrm{f}}}{{\mathrm{CO}}^{\mathrm{f}}+{\mathrm{CO}}_2^{\mathrm{f}}}$ (F)

where CO^f represents the amount of carbon monoxide matter in the reactor outlet and CO₂ ^f the amount of carbon dioxide matter in the reactor outlet. The selectivity of the catalysts to the products H₂ and H₂O were calculated through:

${\mathrm{H}}_2(\%)=100·\frac{{\mathrm{H}}_2^{\mathrm{f}}}{{\mathrm{H}}_2^{\mathrm{f}+}{\mathrm{H}}_2{\mathrm{O}}^{\mathrm{f}}}$ (G)

${\mathrm{H}}_2\mathrm{O}(\%)=100·\frac{{\mathrm{H}}_2{\mathrm{O}}^{\mathrm{f}}}{{\mathrm{H}}_2^{\mathrm{f}+}{\mathrm{H}}_2{\mathrm{O}}^{\mathrm{f}}}$ (H)

where H₂ ^f represents the amount of hydrogen matter in the reactor outlet and H₂O^f the amount of water matter in the reactor outlet. Fig. 7 shows the selectivity data. It was observed that the catalysts had a low CO₂ selectivity, suggesting that they had greater resistance to a possible formation of carbon deposits. The main desirable products are CO and H₂, in a ratio closer to 1:2, which corresponds to the partial oxidation reaction of methane: CH₄+½O₂→CO+2H₂²⁷. Among the catalysts, LNA was the most selective to hydrogen, reaching a selectivity value of 55%, whereas L7S3NA and L3S7NA reached lower values, 46 and 26%, respectively. In relation to the selectivity to CO and CO₂ (Fig. 7), they evidenced greater selectivity to CO in relation to CO₂, a desirable factor, since the desired product is the synthesis gas. Among the catalysts, the most selective to CO was L3S7NA, however, it obtained low selectivity to H₂, which makes it undesirable to be an input in the Fischer-Tropsch synthesis reaction. Among the catalysts, the LNA was the most selective to H₂ with a value of 55%, while the others reached lower values, mainly L3S7NA with 26% H₂ selectivity. However, L3S7NA was more selective to CO with a value of 74%. Fig. 6 shows the conversion results of CH₄ and H₂/CO ratio. It was found that the catalysts obtained a decrease in catalytic activity when the lanthanum was partially replaced by strontium. In this case, it was desirable that the activity would increase with the partial replacement of the lanthanum with strontium, a fact that was not verified, suggesting that the formation of segregated materials obtained in samples L7S3NA and L3S7NA could have made difficult the conversion of CH₄. In general, the catalyst that obtained the best conversion and a better H₂/CO ratio of 2 was LNA. It suggested that it has adequate reason to be an input into the Fischer-Tropsch synthesis reaction.

Figure 6:
Conversion data and H₂/CO ratio for LNA, L7S3NA and L3S7NA catalysts.
Figura 6:
Dados de conversão e razão H₂/CO para os catalisadores LNA, L7S3NA e L3S7NA.

Figure 7:
Selectivity to CO, CO₂, H₂ and H₂O for the catalysts LNA, L7S3NA and L3S7NA.
Figura 7:
Seletividade a CO, CO₂, H₂ e H₂O para os catalisadores LNA, L7S3NA e L3S7NA.

CONCLUSIONS

The results obtained in this work showed that the use of low cost and non-toxic organic materials such as collagen as a chelating agent in the synthesis of mixed oxides with perovskite structure is promising. According to thermogravimetric curves and absorption spectra in the infrared region, all residual organic material remaining from the synthesis was decomposed to 800 °C, except for samples with 70% of lanthanum substitution, whose temperature was close to 900 °C. X-ray diffraction patterns for the calcined materials revealed the formation of perovskite in all samples. However, there was the formation of secondary phases with increasing lanthanum substitution content. The perovskite phase was also identified in the supported material (La_1-xSr_xNiO₃/Al₂O₃) together with the alumina phase. The specific surface area analyses showed that through the synthesis method used values of desirable specific surface area are obtained when compared to other synthesis methods. According to the temperature-programmed reduction profiles, it was observed that the reduction of the materials was favored with the increase in the degree of substitution of the lanthanum. The complete reduction of nickel in the supported samples occurred at a temperature below 700 °C. The catalysts showed catalytic activity in the methane conversion, and the highest conversion level was 78% for the LaNiO₃/Al₂O₃ (LNA) catalyst. For the catalysts La_0.7Sr_0.3NiO₃/Al₂O₃ (L7S3NA) and La_0.3Sr_0.7NiO₃/Al₂O₃ (L3S7NA), the conversion was 53 and 38%, respectively. The selectivity to H₂ was also higher for the LNA catalyst, 55%. The values of ratio of H₂/CO were 2.0, 1.0 and 2.9 for the catalysts LNA, L7S3NA and L3S7NA, respectively. In general, all the catalysts tested showed satisfactory results in methane conversion. However, the LNA catalyst presented the best result, since the catalysts L7S3NA and L3S7NA showed a decrease in catalytic activity when the lanthanum was partially replaced by strontium.

ACKNOWLEDGEMENTS

The authors are thankful to CNPq and CAPES for their financial support.

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Publication Dates

• Publication in this collection
  Jul-Sep 2018

History

• Received
  31 July 2017
• Reviewed
  28 Dec 2017
• Accepted
  22 Feb 2018