Insights into the Reactivity of Gd_2−xSr_xFe₂O₇ (x = 0 ÷ 0.4) in CO Radical Hydrogenation

Abstract

The effect of strontium substitution in the structure of the complex oxide Gd₂SrFe₂O₇ on the production of light olefins by CO hydrogenation was investigated. Perovskite-type oxides Gd_2−xSr_1+xFe₂O₇ (x = 0; 0.1; 0.2; 0.3; 0.4) were synthesized by sol–gel technology and characterized by XRD, Mössbauer spectroscopy, BET specific area, acidity testing, and SEM. The experimental data revealed a correlation between the state of iron atoms, acidity, and catalytic performance. It was found that with an increase in the content of Sr²⁺ in the perovskite phase, the basicity of the surface and the oxygen diffusion rate increased. This contributed to the CO dissociative adsorption, formation of active carbon, and its further interaction with atomic hydrogen.

1. Introduction

The development of new active and selectively acting catalytic systems remains one of the most important trends in modern chemistry. Oxygen-deficient complex oxides with a perovskite-type structure have attracted the attention of a huge number of researchers around the world as promising materials for creating electrochemical, catalytic, and magnetic devices [1,2,3,4,5]. Complex oxides with a layered perovskite-type structure have mixed oxygen, ionic and electronic conductivity and high stability. They are also used as catalysts for high-temperature processes [6,7,8,9,10].

Varying the composition of perovskite, including partial substitution or doping of the cation in the A-site and/or B-site, is an effective way to increase the stability and the oxygen and ionic conductivity, i.e., to improve the physical and chemical properties of perovskites for their further use in catalytic processes. Replacing the cation in the A-site leads to the changing of the size of the perovskite unit cell, and, consequently, the length of the B-O covalent bond in the structure. Cationic substitution in perovskite not only has a significant effect on catalytic activity due to the stabilization of unusual oxidation states, but it also promotes the simultaneous formation of structural defects responsible for oxygen mobility in the crystal lattice [1,3,11]. Doping the A-site of perovskite with alkali and alkaline earth metals leads to an increase in the mobility of lattice oxygen ions, reducing the process temperature in oxidation reactions [1]. It was shown in [12] that a better ability of the catalyst to adsorb CO₂ was realized due to the increase in basicity upon the introduction of Ca or Sr. This increased the rate of carbon removal, and thus the catalytic activity. The promoting effect of alkali and alkaline earth metals was also noted by the authors of [13]. It was found [14] that the doping of La³⁺ with low-valent strontium ions resulted is the appearance of an induced negative charge in the A-site. This was compensated by the formation of oxygen vacancies and/or oxidation of the metal in the B-position to higher oxidation states, thereby leading to the doped perovskite having a high electronic and ionic conductivity.

The catalytic properties of complex perovskite-type oxides, La_1−xSr_xCoO₃, in the selective Fischer–Tropsch synthesis of higher alcohols were studied in [15]. It was found that Sr substitution improved the catalyst’s reducibility and increasing strontium substitution beyond a threshold of 10 mol% Sr had an inverse effect on the catalytic performance in the syngas conversion. For x ≤ 0.1, the catalysts exhibited a stable rhombohedral structure, which led to good dispersion of active center Co⁰ during the reduction process and displayed a high catalytic activity. For x ≥ 0.2 the catalyst structure was gradually changed, from a rhombohedral to a less stable cubic structure, leading to Co⁰ sintering. This led to a decrease in the number of active centers and an improvement in the reducibility of samples due to the presence of strontium in the perovskite lattice. In the study of strontium-doped lanthanum cobaltites in the Fischer–Tropsch synthesis, it was found that SrCO₃ [16] formed on the surface of the samples after the reaction, with the amount of SrCO₃ increasing with an increase in the amount of strontium. It was also reported that this compound was inactive in the Fischer–Tropsch synthesis and coated the catalyst surface [17], blocking active sites, which adversely affected the catalytic activity and partially inhibited the process.

Since the discovery of Fischer–Tropsch (FT) synthesis, iron-based catalysts have been used extensively in industrial FT plants [18,19]. The catalytic performance of iron catalysts depends on their active phase, transition metal promoter, alkali promoter, and support [20,21,22,23,24]. It has been shown that FT synthesis over iron catalysts is a structure-sensitive reaction [1,25,26,27], with the intrinsic reaction rate and product selectivity being functions of iron particle size. The promoted iron catalysts have, so far, shown one of the best performances in direct olefin synthesis from syngas. The particularly attractive features of iron catalysts for light olefin synthesis from syngas are summarized as follows:

-

Flexibility in terms of activity, selectivity, and reaction conditions;

-

High selectivity to olefins within the Anderson–Schulz–Flory hydrocarbon distribution;

-

Sensitivity of reaction rate and selectivity to promotion and support;

-

Activity in the water–gas shift (WGS) reaction and the possibility of using syngas with variable H₂/CO ratios;

-

Presence of different phases that are potentially active in FT synthesis;

-

Relative stability in the presence of syngas impurities compared to cobalt catalysts;

-

Lower cost compared to other metal catalysts.

Iron-based catalysts are very versatile with rich chemistry during their preparation, activation, and reaction. They can be adapted for various operating conditions of CO/CO₂ hydrogenation reactions [28,29]. Also, the structure of iron catalysts can significantly evolve as a function of the operating conditions [1,30]. Thus, during FT synthesis, iron species may be distributed among several phases, e.g., carbides, oxides, and metallic iron. The iron carbides may transform from one to another as a function of the operating conditions [1,2,6,13].

We have previously investigated GdB’_xFe_1−xO₃ (B’ = Co, Mn) and A_n+1Fe_nO_3n+1 (A = Gd, Sr, n = 1, 2,…,∞—number of perovskite layers) in the processes of carbon oxide adsorption, carbon oxide hydrogenation, and dry reforming of methane [31,32,33,34]. It was shown that the sol–gel method makes it possible to obtain samples with better catalytic characteristics compared to ceramic systems. A correlation between the number of perovskite layers (n = 1, 2,…,∞) in the layered oxide structure and its activity was observed. Comparison of the physicochemical and catalytic results of studies in the dry reforming of methane (DRM) and hydrogenation of carbon oxides made it possible to establish that the chemisorption processes for CO, CO₂, H₂, and CH₄ proceed on different types of centers. It has been established that the formation of Gd₂O₂CO₃ carbonate complexes occurs on Gd³⁺ ions (A-site), and transition metal ions in the B-site are responsible for the formation of atomic hydrogen. The nature of the 3d metal in GdBO₃ influences the ratio of molecular and atomic forms of hydrogen and its spillover on the surface of the catalyst. It was also found that GdCoO₃, after treatment in a reducing atmosphere at 900 °C and after 100 h of operation, was reduced to Gd₂Co₂O₅ and valence states of cobalt changed from Co³⁺ to Co²⁺, while for GdMnO₃ and GdFeO₃ there was little change in the structure and valence state [19,35].

There have been numerous studies of the acid–base properties of interacting with small carbon molecules [36,37], but not as many studies involving Fischer–Tropsch synthesis. In addition, the possibility of targeted variation of the cationic composition, their unique physicochemical properties, and their high thermal stability make perovskites promising for heterogeneous catalysis. In this regard, the main objectives of this work were to study the catalytic properties of complex layered oxides Gd_2−xSr_1+xFe₂O₇ (x = 0.1; 0.2; 0.3; 0.4) in CO hydrogenation and to establish a correlation between the composition of the complex oxide and its physicochemical and catalytic properties.

2. Results and Discussion

2.1. Characterization

Figure 1 shows X-ray diffraction patterns of synthesized complex oxides Gd_2−xSr_1+xFe₂O₇ (x = 0; 0.2) obtained by sol–gel technology (citrate–nitrate method) before and after catalysis. For the initial samples, it was found that the introduction of strontium, i.e., the transition from perovskite to perovskite-like layered structure in complex oxides Gd_2−xSr_1+xFe₂O₇, did not lead to a change in the type of crystal lattice—it remained as a tetragonal crystal lattice with the space group I4mmm [38].

The unit cell parameters, crystallite sizes, and the specific surface areas of iron-containing oxides are presented in

Table 1. Structural parameters, crystallite sizes, and S_BET of Gd_2−xSr_1+xFe₂O₇ (x = 0; 0.1; 0.2; 0.3; 0.4) samples.

Compound	Lattice Parameters (Å)	XRD Crystallite Size (nm)	Space Group	SBET (m2/g)
Gd2SrFe2O7	a = b = 3.891 c = 19.736	49.3	I4mmm [24]	4.4
Gd1.9Sr1.1Fe2O7	a = b = 3.888 c = 19.784	45.3	I4mmm [24]	5.0
Gd1.8Sr1.2Fe2O7	a = b = 3.887 c = 19.783	48.0	I4mmm [24]	4.8
Gd1.7Sr1.3Fe2O7	a = b = 3.885 c = 19.790	43.2	I4mmm [24]	4.8
Gd1.6Sr1.4Fe2O7	a = b = 3.884 c = 19.829	30.3	I4mmm [24]	1.9

.

It was typical for all solid solutions that, with an increase in the content of strontium atoms, the parameter a decreased, while the parameter c increased. These changes in the lattice parameters are explained by the ionic sizes of the substituted atoms (the ionic radius of Sr²⁺ is 1.31 Å and that of Gd³⁺ is 1.107 Å), since an atom with a smaller ionic radius is replaced by an atom with a larger ionic radius. According to the data obtained by the method of low-temperature nitrogen adsorption (Table 1), all complex ferrites had a small specific surface of 1.4–4.8 m²/g, so it was quite difficult to determine the porosity. The specific surface area of solid solutions decreased with an increase in the content of strontium atoms; this can be easily explained by the synthesis temperature of these solid solutions. The Gd_1.9Sr_1.1Fe₂O₇ solid solution had the maximum surface area and was obtained at a minimum temperature of 1260 °C for 20 min. Solid solutions of Gd_1.7Sr_1.3Fe₂O₇ and Gd_1.8Sr_1.2Fe₂O₇ were synthesized at the same temperature, but with an annealing time of 1 h, so they had the same surface area. The complex ferrite Gd_1.6Sr_1.4Fe₂O₇ was obtained, single-phase only, at 1260 °C after 3 h of calcination, and the Gd_1.5Sr_1.5Fe₂O₇ solid solution was obtained single-phase after heating to 1400 °C for 10 min and this was the maximum calcination temperature, and therefore the minimum surface area.

The diffraction patterns of the Gd_2−xSr_1+xFe₂O₇ samples after catalysis did not differ from the initial ones. But, in addition to the peaks of complex oxides Gd_2−xSr_1+xFe₂O₇, there were peaks of quartz—SiO₂, which had been used as an inert additive to increase the volume of the catalyst. Thus, we can conclude that the phase composition of the catalyst after catalysis was unchanged.

Micrographs of Gd_2−xSr_1+xFe₂O₇ solid solutions (Figure 2) illustrate that the particle sizes were 150–300 nm. As an example, micrographs of solid solutions with x = 0; 0.2 are presented. It should be noted that the particles were closer to a more regular shape and had approximately the same size.

After catalytic tests, the samples showed slight particle agglomeration and the EDX spectra data confirmed the presence of carbon on the surface of spent catalysts (Figure 3).

The states of iron atoms in complex perovskite-like ferrites were studied using Mössbauer spectroscopy. All spectra were taken at room temperature.

The complete replacement of gadolinium by strontium, as well as the “embedding” of the SrO layer in GdFeO₃ (samples of SrFeO_3−δ and GdSrFeO₄) led to the appearance of an additional form of Fe⁴⁺ iron in contrast to gadolinium ferrite (Figure 4,

Table 2. Parameters of the Mössbauer spectra of solid solution Gd_2−xSr_1+xFe₂O₇ (x = 0; 0.1; 0.2; 0.3; 0.4) samples.

Compound	Fe Ion	Chemical Shift (mm/s)	Quadrupole Splitting, (mm/s)	Field (T)	Content (%)
Before Catalysis
Gd2SrFe2O7	Fe4+	0.07	-	-	13
	Fe+3	0.36	0.41	45.40	27
	Fe+3	0.36	0.37	43.53	23
	Fe+3	0.36	0.36	40.31	37
Gd2SrFe2O7	Fe+3	0.15	1.02	-	11
	Fe+3	0.35	0.40	46.44	70
	Fe+3	0.41	-0.0	49.90	19
Gd1.9Sr1.1Fe2O7	Fe4+	0.002	-	-	15
	Fe+3	0.34	0.42	45.76	27
	Fe+3	0.36	0.39	43.92	24
	Fe+3	0.38	0.35	41.20	34
Gd1.8Sr1.2Fe2O7	Fe4+	0.13	-	-	18
	Fe+3	0.34	0.43	45.15	24
	Fe+3	0.36	0.40	42.98	38
	Fe+3	0.48	0.48	38.16	20
Gd1.7Sr1.3Fe2O7	Fe4+	0.19	-	-	27
	Fe+3	0.37	0.43	43.73	39
	Fe+3	0.35	0.31	39.92	22
	Fe+3	0.50	0.42	34.38	12
Gd1.6Sr1.4Fe2O7	Fe4+	0.18	-	-	25
	Fe+3	0.36	0.39	41.87	63
	Fe+3	0.32	0.38	37.30	12
after catalysis
Gd2SrFe2O7	Fe+3	0.15	1.02	-	11
	Fe+3	0.35	0.40	46.44	70
	Fe+3	0.41	−0.0	49.90	19
Gd1.8Sr1.2Fe2O7	Fe+3	0.50	0.15	-	10
	Fe+3	0.37	0.43	46.50	80
	Fe+3	−0.02	−0.06	32.75	10

) [39].

For Gd_2−xSr_1+xFe₂O₇ samples, along with one line corresponding to Fe⁴⁺ atoms, there were three sextets characteristic of Fe⁺³, with Fe⁺³ atoms in three different environments, and all of them were magnetic, since they had magnetic splitting. The content of Fe⁴⁺ was maximum in the complex oxide with x = 0.3 (Table 2). The appearance of such an “exotic” iron oxidation state can be associated with the stabilization of the system due to the redistribution of electron density as a consequence of the introduction of a strontium ion with a lower oxidation state compared to gadolinium into the A-position of the perovskite structure. There was a change in the environment of iron atoms: one Fe atom had more gadolinium atoms, the second had more strontium atoms [39]. In addition to different environments of iron atoms, the crystal structure of the Gd_2−xSr_1+xFe₂O₇ solid solution was also stabilized due to the appearance of oxygen vacancies, and the formula of the solid solution should be more accurately written as Gd_2−xSr_1+xFe₂O_7−α, where α is close to 0.1.

The Mössbauer spectrum of the complex layered oxide Gd₂SrFe₂O₇ after the catalytic reaction differed from the Mössbauer spectrum of the initial oxide: there was no line corresponding to Fe⁴⁺ atoms. From Figure 4 and the data in Table 2, we can conclude that as a result of catalysis under the action of temperature and the reaction medium, changes occurred in the state of iron atoms and their local environment.

Thus, upon non-isovalent substitution of Gd³⁺→Sr²⁺ in the completely ordered Gd₂SrFe₂O₇ structure, where strontium atoms are in the perovskite layer and gadolinium atoms are in the rock-salt layer, partial substitution of Gd³⁺ atoms leads to charge compensation due to the formation of oxygen vacancies in a layer of rock salt [40]. In this case, some of the octahedrons become pyramids, and thus the environment of the Fe⁺³ atoms changes, which was proved by Mössbauer spectroscopy.

It is known that the properties of perovskite catalysts are affected by the acid–base state of its surface [41,42], so a series of experiments was carried out to test the electron-withdrawing properties of the synthesized ferrites. The results obtained for all the studied samples are presented in

Table 3. Fast and slow adsorption rates and total number of electron-acceptor centers.

Compound	W1, μmol/(g·min)		W2, μmol/(g·min)		Nmax, μmol/g
	Before Catalysis	After Catalysis	Before Catalysis	After Catalysis	Before Catalysis	After Catalysis
Gd2SrFe2O7	0.513	0.477	0.071	0.627	30.3	45.2
Gd1.9Sr1.1Fe2O7	-	0.448	-	0.339	27.4	33.9
Gd1.8Sr1.2Fe2O7	0.378	0.545	0.024	0.331	17.1	48.1
Gd1.7Sr1.3Fe2O7	0.397	0.673	0.054	0.378	28.3	36.7
Gd1.6Sr1.4Fe2O7	-	0.310	-	0.036	24.4	35.0

. For some initial samples of gadolinium and strontium ferrites, it was not possible to determine the adsorption rates and the value of N_max due to the occurrence of a suspension effect, which may have been due to the submicrocrystalline and nanocrystalline state of the ferrites.

The presence of two types of acid sites, differing in strength was established. Moreover, in the initial samples, fast adsorption centers predominated, the number of which increased with an increase in the content of Sr²⁺ and Fe⁴⁺ in the samples. At the same time, the total number of electron-withdrawing centers in the initial samples decreased when Gd was nonstoichiometrically replaced by Sr. After conducting catalytic tests for all samples, an increase in the rates of both slow and fast adsorption, as well as total acidity, was observed. This effect manifested itself most clearly on Gd_1.8Sr_1.2Fe₂O₇.

2.2. Catalytic Tests

The hydrogenation of carbon monoxide to various hydrocarbons was carried out to study the catalytic properties of Gd_2−xSr_1+xFe₂O_7. Analysis of reactant content in the gas phase near the catalyst surface showed that at room temperature there was intensive adsorption of CO. After adsorption equilibrium was established and up to 573 K, the composition of the gas phase (CO+H₂) changed little, while its transition to the catalytic temperature range was accompanied by the formation of CO₂, and its amounts formed on complex ferrites Gd_2−xSr_1+xFe₂O₇ during the reaction in the CO:H₂ = 1:2 ratio several times exceeded the amounts obtained in the hydrogen excess (Figure 5).

The CO conversions (Figure 6a) on all ferrites were 60–85% at the CO:H₂ = 1:2 ratio and varied insignificantly with increasing temperature. Increasing the hydrogen content in the reaction mixture to 1:4 (Figure 6b) led to a decrease in the CO conversion with increasing strontium content in the samples.

It should be noted that on the second day of the experiment, the same curve for all gadolinium ferrites was displayed as a line. A similar picture was clearly seen on the third day of testing. In the following days (days 4–6 of the experiment), no changes in catalytic activity were detected (Figure 6c).

The value of CO adsorption depends on the gadolinium–oxygen bond energy in Gd-O-Me, which will change as a result of a third distortion of the perovskite lattice upon non-isovalent substitution of Gd for Sr and a change in the oxygen–metal bond energy. Previously, it was shown that, on complex GdFeO₃ oxides, CO adsorption proceeded mainly on the A centers of perovskite [33] with the formation of Gd₂O₂CO₃ carbonate complexes, and the formation of CO₂ occurred as a result of the decomposition of these complexes with increasing temperature. The Boudouard mechanism, in which the surface O* species formed by CO* dissociation reacts with another CO* to form CO₂, plays a predominant role in CO₂ formation on the active χ-Fe₅C₂ phase [43]. Since the presence of iron carbides on the investigated catalysts was not confirmed, the formation of CO₂ by the Boudouard mechanism was unlikely. The formation of CO₂ is also possible in the water–gas shift (WGS) reaction (WGS): H₂O + CO→CO₂ +H₂ and in the interaction of the adsorbed CO_ads molecule with (O_S) perovskite oxygen [16].

Carrying out the reaction at ratios CO:H₂ = 1:2 and 1:4 showed that the products of the reaction were hydrocarbons C₁–C₅. The formation of all products began at 523 K and increased with increasing temperature. As an example, Figure 7 shows the temperature dependences of the rates of formation of C₂–C₅ hydrocarbons on a Gd_1.8Sr_1.2Fe₂O₇ sample, at a ratio of CO:H₂ = 1:2. For all samples, the highest formation rates were observed for methane, ethylene, and propylene. Comparison of the rates of product formation on the studied samples showed that the specific catalytic activity increased in the series:

Gd_1.6Sr_1.4Fe₂O₇ < Gd_1.9Sr_1.1Fe₂O₇ < Gd₂SrFe₂O₇ ≈ Gd_1.8Sr_1.2Fe₂O₇ ≈ Gd_1.7Sr_1.3Fe₂O₇

(1)

The distribution of hydrogenation products varied depending on the proportion of strontium in the catalyst structure and the composition of the reaction mixture (Figure 8).

For all catalysts, the proportion of methane among the reaction products was the largest. Moreover, the amount of methane formed changed slightly with an increase in the proportion of strontium in the samples up to x = 0.3. However, on the Gd_1.6Sr_1.4Fe₂O₇ sample, both at the ratio CO:H₂ = 1:2 and at CO:H₂ = 1:4, the proportion of methane exceeded 90%. The content of other C₂–C₅ paraffins for all samples, except for Gd_1.7Sr_1.3Fe₂O₇, was small (1–4%) and changed little depending on the initial composition of the reaction mixture. For Gd_1.6Sr_1.4Fe₂O₇ and Gd₂SrFe₂O₇ samples, varying the ratio of CO and H₂ had almost no effect on the distribution of reaction products. As for C₂–C₄ olefins, when the reaction was carried out at a ratio of CO:H₂ = 1:2, the introduction of strontium up to x = 0.3 in the amount of ethylene, propylene, and butylene changed little. The anomalous behavior of the Gd_1.7Sr_1.3Fe₂O₇ sample should be noted: at the ratio CO:H₂ = 1:2, the amounts of olefins were maximum, and when the reaction was carried out in an excess of hydrogen (CO:H₂ = 1:4), a significant amount of pentane was observed, comparable to the amount of methane.

The dependence of the hydrocarbon chain growth factor α_i on the number of carbon atoms (i) in the chain of intermediate products is shown in Figure 9.

Regardless of the catalyst composition, the highest values were for α₁, i.e., the formation of hydrocarbons with two carbon atoms was most likely on the studied systems at CO:H₂ = 1:2, and on the Gd_1.7Sr_1.3Fe₂O₇ catalyst, this probability was maximum and reached 50%. An increase in the hydrogen content in the reaction mixture to CO:H₂ = 1:4 led to the fact that for the same sample the probability of formation of C₅ hydrocarbons became the maximum. The calculated values of the chain growth coefficients agreed with the values of selectivity for olefins.

The determination of selectivity for olefins showed that the studied samples had high values of S(C_nH_2n) when carrying out the reaction in the ratio CO:H₂ = 1:2 (Figure 10). Moreover, the non-isovalent substitution of gadolinium for strontium up to x = 0.3 contributed to the growth of S$\left(C_2^{=}-C_4^{=}\right).$ And when the reaction was carried out in an excess of hydrogen, on samples with an amount of strontium with x > 0.2, the selectivity for olefins significantly decreased.

Probably, with an excess of hydrogen in the reaction mixture, there is almost complete hydrogenation of the surface-active carbon, which arises as a result of dissociative adsorption of CO, occurs [34,44,45,46]. A decrease in the H₂ content leads to a decrease in the amount of atomic hydrogen on the catalyst surface and the appearance of CH_x radicals and their interaction with the formation of unsaturated hydrocarbons.

As mentioned above, the specific catalytic activity increased in the series:

Gd_1.6Sr_1.4Fe₂O₇ < Gd_1.9Sr_1.1Fe₂O₇ < Gd₂SrFe₂O₇ ≈ Gd_1.8Sr_1.2Fe₂O₇ ≈ Gd_1.7Sr_1.3Fe₂O₇,

(2)

and the highest selectivity for olefins was obtained on the last three. The low activity of Gd_1.6Sr_1.4Fe₂O₇ can be due to a number of factors. This sample had the smallest crystallite size and specific surface area, which was primarily due to prolonged annealing during synthesis. Also, unlike all other samples, the Fe⁺³ atoms in Gd_1.6Sr_1.4Fe₂O₇ are found only in two different environments.

According to Mössbauer spectroscopy data, in Gd_2−xSr_1+xFe₂O₇, iron is in the heterovalent state (Fe⁺³, Fe⁴⁺), and the amount of Fe⁴⁺ increases with an increase in the proportion of strontium in the samples. The number of electron-withdrawing centers and total acidity decreased in the same sequence. With an increase in the content of Sr²⁺ in the perovskite phase, the basicity of the surface, and the rate of diffusion of oxygen into the volume, increase due to an increase in the amount of rapidly exchanging oxygen [47,48]. This promotes the dissociative chemisorption of CO [31] and the formation of active carbon (graphite deposited directly on the catalyst surface) and its further interaction with atomic hydrogen to form CH_x radicals. Further recombination of CH_x into olefins becomes more likely, but polymerization into longer hydrocarbons was not observed. The presence of carbon on the surface was confirmed by EDX data. The preservation of catalytic characteristics for a long time and with an increase in temperature makes it possible to consider this carbon as active. The change in surface acidity and the absence of Fe⁴⁺ in the samples after catalysis indicate the formation of active sites under the action of the reaction medium. In accordance with the scheme presented in [11], in the case of a strontium-containing catalyst, the intermediate compound Ln₂O₂CO₃ is formed, which interacts with carbon particles or atomic hydrogen. Surface carbonates are formed on Gd atoms (A-site of the perovskite structure), while atomic hydrogen and CH_x radicals are formed on Fe atoms (B-site) [35,49]. Differences in the catalytic activity of ferrite samples can be associated both with the amount of weakly bound (superstoichiometric) oxygen, which is formed in cation-deficient lattices, and with different diffusion rates of weakly bound hydrogen (H_I) over the catalyst surface (spillover effect). An increase in the number of perovskite layers in complex oxides led [50] to a decrease in the activation energy for the desorption of weakly bound hydrogen; the Me^δ+—H₂ bond weakens, and therefore the ability of hydrogen to migrate over the surface and penetrate the volume increases. But, since samples with low acidity had the highest activity and selectivity, it can be assumed that an increase in surface basicity with increasing Sr²⁺ content in the perovskite phase, on the one hand, is favorable for the dissociative chemisorption of carbon monoxide, and, on the other hand, hinders the formation of atomic hydrogen. As a result, under conditions of hydrogen deficiency, the formation of unsaturated hydrocarbons occurs.

3. Materials and Methods

3.1. Catalyst Preparation

The synthesis of Gd_2−xSr_1+xFe₂O₇ (x = 0; 0.1; 0.2; 0.3; 0.4) complex oxides was performed using the citrate–nitrate sol–gel technique [32,51]. Stoichiometric amounts of Sr(NO₃)₂ (99.5%, Vecton, St. Petersburg, Russia), Gd(NO₃)₃·6H₂O (99.9%, Vecton, St. Petersburg, Russia), Fe(NO₃)₃·9H₂O (98%, Vecton, St. Petersburg, Russia) nitrates were dissolved in a flask with distilled water, then placed on a magnetic stirrer with a thermostat. Citric acid was added to the obtained solution of salts with constant stirring. After complete dissolution of the citric acid, ammonia solution was added to establish pH ~6. Then the solution was evaporated at a temperature of ~120 °C until the self-ignition of the resulting gel and the formation of black powder. Then, the temperature was raised to 450 °C and the resulting powder was calcined for 2 h.

The obtained Gd_2−xSr_1+xFe₂O₇ powders were ground in an agate mortar, pressed into tablets, and calcined in an oven at the following temperature regimes:

-

Gd₂SrFe₂O₇ and Gd_1.9Sr_1.1Fe₂O₇—at 1523 K for 20 min;

-

Gd_1.7Sr_1.3Fe₂O₇ and Gd_1.8Sr_1.2Fe₂O₇—at 1523 K for 1 h;

-

Gd_1.6Sr_1.4Fe₂O₇—at 1523 K for 3 h.

3.2. Characterization

Powder X-ray diffraction (XRD) was used to determine the phase composition and structural data of the obtained compounds. The analysis was carried out using a Rigaku MiniFlex II diffractometer (Rigaku Corporation, Tokyo, Japan) with CuK_α radiation. Diffraction patterns of all investigated compounds were obtained under the same conditions sufficient to detect all characteristic reflections: the range of angles was 2θ = 10–60°, the scanning speed was 5°/min. The obtained diffraction patterns were analyzed by comparing the present reflections with the data of the ICDD-PDF2 database; the data obtained allowed us to determine the phase composition of the studied compounds. Crystallite sizes were estimated using the Scherrer equation:

$$D=\frac{0.94\lambda }{\beta cos\theta }$$

(3)

where λ is the wavelength, CuKα = 1.5406 Å, θ is the Bragg angle, and β is the width of the reflection at half height.

The morphology of the samples was studied using a ZeissMerlin scanning electron microscope (Carl Zeiss AG, Oberkochen, Germany) with an accelerating voltage of 10 kV, which operates in a low vacuum and does not require sputtering for nonconductive samples. A Carl Zeiss Supra 40VP scanning electron microscope (Carl Zeiss AG, Oberkochen, Germany) at 20 kV was also used.

The state of iron atoms was determined by Mössbauer spectroscopy on a WISSEL spectrometer (WissEL—Wissenschaftliche Elektronik GmbH, Starnberg, Germany). Isotope ⁵⁷Co in the Rh matrix with an activity of 10 mKu was used as a radioactive source and all measurements were performed under standard conditions in the absorption geometry. An α-Fe foil was used to calibrate the velocity scale and the zero position of the chemical shift.

The specific surface area was determined by the method of low-temperature nitrogen adsorption at T = 77 K on a Nova 4200e (Quantachrome) and QuadrasorbSI devices (Quantachrome Instruments, Boynton Beach, FL, USA); the samples were preliminarily degassed at 300 °C for 5 h. The obtained adsorption–desorption isotherms were used to estimate the specific surface area of the samples by the BET method.

The acid–base properties of the catalysts were determined using UV spectroscopy of pyridine adsorption. An SF-103 single-beam scanning spectrophotometer (U-Therm International (H.K.) Limited, Hong Kong) was used to measure concentrations in liquids. The pyridine adsorption spectra in the ultraviolet region of a blank pyridine solution in octane and solutions of adsorption systems with supports and catalyst were recorded at room temperature for 60 min. There were no changes in absorption maxima at 253 nm (analytical absorption band) with pyridine concentration. This was performed using a calibration curve plotted against the optical density of solution D and pyridine concentration. Equation (3) was used to compute Gibbsian adsorption (G, mol/g):

$$G=\frac{\left(C_0-C_t\right)·V}{m}=\frac{\left(D_0-D_t\right)·V}{m·\epsilon ·l}$$

(4)

where V (solution volume) is 10 mL, m (sample mass) is 0.1 g, D₀ and D_t are equal to the optical density of pyridine at maximum absorption prior to and during the adsorption process, l (cuvette length) is 1 cm, and $\epsilon$ is the molar absorption coefficient (extinction, ${\epsilon }_{Py}=2·{10}^6\mathrm{L}·{\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}·{\mathrm{c}\mathrm{m}}^{-1}$).

From the kinetic dependences of pyridine adsorption, the rates of this process, which characterize the strength of acid sites, were determined. The rate of fast adsorption W₁ (stronger centers of fast adsorption) was determined from the tangent of the slope of the tangent to the initial part of the kinetic dependence. The rate of slow adsorption W₂ on centers of the second type (weak centers of slow adsorption) was determined from the slope of the following sections.

3.3. Catalytic Activity Tests

Catalyst performance of the samples in the carbon monoxide hydrogenation reaction was evaluated in a quartz tubular microreactor containing 0.1 g of the sample diluted in 0.5 g of quartz to avoid the formation of hot spots in the catalytic bed alongside increasing the volume. Activity tests were carried out in the temperature range 523–708 K under atmospheric pressure with the feed composition CO:H₂ = 1:2 and CO:H₂ = 1:4 and total flow rate of 1.5 L·h⁻¹, which corresponds to GHSV of 8700 h⁻¹. Gaseous effluent was analyzed online using a gas chromatograph instrument (Crystal 5000.2 (Chromatec Instruments, Yoshkar-Ola, Mari El, Russia), a column of stainless steel filled with Porapack Q, argon as a carrier gas) fitted with thermal conductivity and flame ionization detectors.

The catalytic characteristics were calculated using the following equations:

$$X_i,\%=\frac{n_{int}-n_{out}}{n_{int}}\times 100$$

(5)

$$S_i=\frac{R_i}{\sum R_i}·100\%,$$

(6)

$$R_i=\frac{{n_i}_{\mathrm{o}ut}\omega }{Vm{S}_{BET}}$$

(7)

where n_int и n_out is the initial amount of substance and mole yield of products, ∑n_i is the mole yield of hydrocarbon reaction products (mole), Ri is the rate of formation of the i-th reaction product per 1 g catalyst (mol/h·g), ΣR_i is the sum of yields of the i-th reaction products per 1 g catalyst (mol/h·g), ω is volumetric rate of the reaction mixture (L/h), V is the volume of the chromatograph loop (0.153 × 10⁻³ L), and m is the mass of the catalyst, g.

The carbon-chain growth factor α_i was calculated using Equation (7) [52]:

$${\alpha }_i=\frac{{\sum }_{k>i}\frac{Y_k}{k}}{{\sum }_{k\ge i}\frac{Y_k}{k}},$$

(8)

where ${\alpha }_i$ is carbon-chain growth factor for the intermediate with the number of carbon atoms I, and Y_k is the yield of the component with the number of carbon atoms k.

The coefficient α1 is the probability of the addition of CO to the intermediate containing one carbon atom to form an intermediate with two carbon atoms, α₂ is the probability of the next step of the addition of CO to the intermediate with two carbon atoms, etc.

4. Conclusions

In conclusions, complex perovskite-type oxides Gd_2−xSr_1+xFe₂O₇ (x = 0; 0.1; 0.2; 0.3; 0.4), prepared by the sol–gel method were studied as catalysts for hydrogenation of carbon monoxide. The effect of gadolinium partial nonisovalent substitution by strontium on the physicochemical and catalytic properties in the process was investigated. It was found that the iron atoms in the samples were in the heterovalent state Fe⁴⁺/Fe³⁺⁺, which was compensated by oxygen vacancies. It has been suggested that Cd³⁺ and Fe³⁺ in the composition of Gd-O-Me are active centers, while the presence of Sn²⁺ somewhat complicates the process and shifts it towards the formation of olefins. The formation of catalytically active centers under the action of the reaction medium was found. It was also found that varying the composition of Gd_2−xSr_1+xFe₂O₇ complex oxides led to changes in the oxygen–metal bond energy in Gd-O-Me, in the ratio of metals in different oxidation states, and in the acid–base surface properties, which was reflected in the adsorption and catalytic characteristics of the complex oxides.