CC BY
34
ISSN 2522-1841 (Online) AZERBAIJAN CHEMICAL JOURNAL № 2 2022 93
ISSN 0005-2531 (Print)
UDC 546.681
STUDY OF LOW-TEMPERATURE OXIDATION OF CARBON MONOOXIDE ON Cu-Mn-Fe CATALYTIC OXIDE SYSTEMS OBTAINED BY THE SOL-GEL
COMBUSTION METHOD
G.R.Azimova
M.Nagiev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
ezimova2015@gmail.com
Received 08.12.2021 Accepted 14.01.2022
The oxidation of carbon monoxide was studied on oxide Cu-Mn-Fe catalytic systems with metal ratio Cu:Mn:Fe=1:1:1; 2:1:1; 1:2:1 and 1:1:2 obtained by the sol-gel method with combustion. The results of X-ray phase analysis showed that the catalytic systems have a complex phase composition. Along with double oxides of manganese and iron, ferrites of copper-, manganese, copper manganite, manganese-substituted copper ferrites were formed (CuFe2O4; Mna43Fe2.57O4, Mna98Fe2.02O4; CuMn2O4; Cu1.2Mn1.8O4; Cu0.5Mn0.5Fe2O4). It has been found that depending on the ratio of metals in the catalyst, the reaction start temperature is in the temperature range of 120-1700C. And after 3-5 minutes after the start of the reaction, the conversion begins to increase sharply, reaching 100% within the next 5 minutes.
Keywords: oxidation, carbon monoxide, combustion sol-gel synthesis, oxides, spinels.
doi.org/10.32737/0005-2531-2022-2-93-99
catalysts, the temperature of the start of the reaction (the "ignition" temperature of the catalyst) lies in the range of 180 to 5000C. Given the fact that the CO oxidation reaction is exothermic and is accompanied by a temperature rise of 150-2000C, lower "ignition" temperatures for CO oxidation catalysts are an important economic and technical advantage.
We have previously revealed that copper-iron oxide compositions containing copper ferrite obtained by the sol-gel method with combustion are active in the oxidation of carbon monoxide at 23 0-2500C [15]. In view of the fact that manganese is one of the main components of oxidation catalysts, we assumed that by introducing manganese into the composition of copper ferrite by co-synthesis, it is possible to obtain compositions with improved oxidizing properties, which will allow the oxidation of the carbon monoxide to dioxide at lower temperatures. In addition, multi-component catalytic systems obtained by the solgel method with combustion are characterized by the presence of different phases, which allows surface oxygen to coordinate with different metal atoms of the structure, thereby exhibiting different reactivity.
Based on this, in this work, Cu-Mn-Fe
. . oxide compositions were synthesized by the sol-
widely in reviews [2, 15]. For different types of
Introduction
The development of catalysts for the oxidation of CO to CO2 is an important scientific and applied problem, because, on the one hand, it is considered a model for studying the mechanism of a heterogeneous catalytic process and on the other hand, it is of great practical importance for solving the problem of technogenic environmental pollution. In terms of the total mass, CO ranks first among the gases that pollute the air in large cities with developed industry and a large amount of transport.
For the synthesis of an active catalyst for the oxidation of carbon monoxide to carbon dioxide without the traditional use of noble metals, metal oxides Fe, Ni, Mn, Cu, Co, Cr, Ni, perov-skites, ferrites, complex systems based on mixed oxides Cu, Mn, Ce and Co are used [1-9].
In recent years, there have also been many works in the literature devoted to the oxidation of CO on catalysts with a spinel structure [3, 10-13]. Thus, in [3], it was shown that samples of a ferrospinel catalyst containing nickel, cobalt and copper are effective for the oxidative removal of carbon monoxide from automobile exhaust gases even at relatively lower temperatures (cold start). This issue was discussed more
gel method with combustion for research in the low-temperature oxidation of carbon monoxide.
Experimental part
Materials:
- Salts Fe(NO3^9H2O, Cu(NO3>3H2O, Mn(NO3)2 and citric acid C6H8O7H2O were used as precursors for the synthesis of Cu-Mn-Fe catalysts by the sol-gel method with combustion. Citric acid was used as a complexing agent and "fuel". The amount of citric acid was taken as 1 mol of acid per mol of metal.
- For the synthesis of catalysts with a molar ratio of Cu:Mn:Fe = 1:1:1components, 2.4 g of Cu(NO3>3H2O, 2.5 g of Mn(NO3> 4H2O, 4 g of Fe (NO3)3-9H2O and 6.3 g citric acid were taken.
- For the synthesis of catalysts with a molar ratio of the Cu:Mn:Fe = 1:1:2 components, 2.4 g Cu(NO3^3H2O, 2.5 g Mn(NO3)2-4H2O, 8.0 g Fe(NO3)39H2O and 8.4 g citric acid were taken.
- For the synthesis of catalysts with a molar ratio of Cu:Mn:Fe = 1:2:1 component, 2.4 g of Cu(NO3)2-3H2O, 5.0 g of Mn(NO3MH2O, 4.0 g of Fe(NO3>9H2O and 8.4 g citric acid were taken.
- For the synthesis of catalysts with a molar ratio of the components Cu:Mn:Fe = 2:1:1, 4.8 g of Cu(NO3)2-3H2O, 2.5 g of Mn(NO3)2, 4.0 g of Fe(NO3V9H2O and 8.4 g citric acid were taken.
The precursors were dissolved in 10 ml of distilled water, mixed, and pH = 8 was adjusted with an ammonia solution. The mixture was stirred on a magnetic stirrer with heating until a gel formed. The resulting gel was placed in a drying oven heated to 175-1900C, in which it completely dried and then ignited.
Methods:
X-ray powder diffraction of the products was carried out on a Bruker D 2Phazer automatic diffractometer with a CuKa radiation source.
IR spectra were recorded on a Bruker FTIR Alfa spectrometer.
The measurement of the specific surface area of the samples was determined by low-temperature nitrogen adsorption using the multipoint BET method.
The obtained catalyst powders in the amount of 1 g were mixed with a binder - alu-
mogel, molded into granules, dried in the air, further heat treatment was carried out in a drying cabinet and a muffle furnace at temperatures of 135 and 5000C, respectively.
CO oxidation was carried out by the flow method at a CO:air = 1:3-5 (mol), a space velocity of 6000-12000 h-1. The analysis was carried out on a HXM chromatograph, in two columns with CaA and poropak Q sorbents.
Results and discussion
Samples of catalysts with different initial molar ratios of metals Cu: Mn:Fe = 1:1:1, 2:1:1, 1:2:1 and 1:1:2 were prepared by the sol-gel method with combustion. The results of X-ray powder diffraction showed that the catalysts obtained by the sol-gel method with combustion have a complex phase composition. Along with double oxides of manganese and iron (Mn3O4 and Fe3O4), manganese ferrites Mn0.98Fe2.02O4, copper CuFe2O4, copper manganite CuMn2O4, manganese-substituted copper ferrites (Cu05Mn05Fe2O4; Cu4.2Mn05O4) were formed, i.e. the resulting systems can be considered as solid solutions.
For example, Figures 1 and 2 show the diffraction pattern (general and expanded region of characteristic reflections) and the IR spectrum of the sample with the ratio Cu:Mn:Fe = 1:1:1.
In the literature, there are works devoted to the synthesis and study of the structure of the above-mentioned phases and their use as catalysts and adsorbents. The authors characterize them as spinel phases.
Thus, in [16], a nanospinel Cu0.5Mn0.5Fe2O4 was synthesized and characterized by FTIR, XRD, and SEM methods. In (B) [17], a spinel of the formula Cu12 Mn18 O4 was synthesized and the distribution of the valence of cations at room temperature was studied.
It was shown that all copper is present in the form of Cu2+ and there is a multivalent ground state of Mn with valence fractions: 55% Mn4+, 37% Mn3+ and 8% Mn2+. The spinel Mn098Fe202O4 was investigated in [18].
CuMn2O4 was obtained and studied in the oxidation of CO in [19]. In the IR spectrum (Figure 1), absorption peaks from 800 to 400 cm-1 characterize vibrations of Me-O bonds for tetra-hedrally coordinated ions [20-22].
Fig. 1. Diffractogram of a catalyst with a ratio of metals Cu:Mn:Fe = 1:1:1.
Fig. 2. IR spectrum of the catalyst with the ratio of metals Cu:Mn:Fe = 1:1:1.
In this case, these are the 625 and 721 cm-1 bands. The absorption bands in the range 9001400 cm-1 (966.66, 1077.88, 1154.97, 1376.97, 1462.90) refer to bending vibrations of Me-OH bonds. The authors of [23] attribute the bands in the range 900-1100 cm-1 to the characteristics of the ferrite system.
The results of experiments in the oxidation of carbon monoxide on the catalysts we synthesized showed that the temperature of the starting of the reaction, at which the catalysts exhibit noticeable activity, lies in the temperature range of 120-1700C (Table). Already within the first 3-5 min after the start of the reaction, the conversion starts to grow sharply, reaching 100% within the next 5 min. At temperatures slightly above the specified range, a sharp increase in conversion is observed from the first minutes of the reaction, and the maximum conversion is achieved in a still short time - 6-8 min. The table also shows the values of the specific surface area of the synthesized samples. As can be seen from the Table, they are of the same order of magnitude and, depending on the
ratio of metals, fluctuate in the range of 12.419.5 m2/g. As noted above, the oxidation reaction of carbon monoxide is exothermic, and Figure 3 shows the nature of the rise in the reaction temperature with time on the studied catalyst samples.
Figure 4 shows the time dependence on the activity of catalysts with different Cu:Mn:Fe ratios at different reaction temperatures. As can be seen from Figure 3, with the same nature of the dependence, the full conversion of carbon monoxide in increasing temperature is achieved in less time.
So, on a catalyst with a molar ratio of metals Cu:Mn:Fe=1:1:1, the complete conversion of carbon monoxide to dioxide at a temperature of 1700C is achieved within 9 minutes, and at a temperature of 2000C - within 3-4 minutes (Figure 4, a). On a catalyst with a molar ratio of metals Cu:Mn:Fe=2:1:1, the time for complete conversion of carbon monoxide into dioxide is 10 min at 1450C and 6 minutes at 1700C (Figure 4, b).
The activity of multicomponent catalysts depends on many factors - chemical and phase composition, structure, dispersion, specific surface area, and the presence of defects in the structure. These factors are largely associated with the conditions for the synthesis of the catalyst.
The sol-gel combustion method can be considered as a type of SHS (self-propagating high-temperature synthesis) method, but proceeding at a lower temperature. Combustion occurs when a short-term thermal effect on the system is initiated, in which an exothermic reaction is initiated and further combustion occurs because of its heat release.
Temperature parameters and rate of CO oxidative conversion on Cu:Mn:Fe catalysts. CO:air ratio = 1:3, Vs of 10000 h-1
Component ratio Cu:Mn:Fe Specific surface area, m2/g Reaction start temperature, 0C Reaction "ignition" time, min Time to reach 100% conversion, min Reaction rate • 104, mol/sec g
1:1:1 19.5 170 5 9 1.22
1:2:1 12.4 155 4 10 1.22
1:1:2 13.5 120 5 7-8 1.03
2:1:1 19 145 4 8 1.03
Fig. 3. Nature of the temperature rises with time on Cu:Mn:Fe catalysts with different ratio of metals: 1 - (1:1:2), 2 - (2: 1:1), 3 - (1:2:1), 4 - (1:1:1). CO: air = 1:3, Vs = 10000 h-1.
5 10
t, min
a) b)
Fig. 4. Time dependence of the conversion of carbon monoxide at different temperatures on catalysts with the different ratio of metals: CO:air = 1:3 (mol), Vs = 10000 h-1: a) Cu:Mn:Fe=1:1:1 (1 - T=1700C, 2 - T=1800C, 3 - T=2000C). b) Cu:Mn:Fe=2:1:1 (1 - T=1450C; 2 - T=1700C).
In this case, various physical and chemical transformations occur - melting, chemical reaction, diffusion which affect the formation of the composition and structure.
Catalysts were synthesized by the sol-gel method with combustion are multiphase oxide systems of variable composition, therefore, they are characterized by the presence of all types of defects in solids: a) point defects - vacancies, implanted atoms (ions)]; b) extended defects
(dislocations) extending over many unit cells; c) electronic defects, representing local violations of the charge distribution [24].
It is known, that solids with a defective surface have a higher catalytic and adsorption activity as compared to the structure of a perfect crystal. Using various methods of synthesis, it is possible to raise the defectiveness of the outer surface of crystals or even create lacunar structures by selectively extracting certain cations from the crystal lattice [25].
Allowing for the fact that the catalytic systems we synthesized also contain individual oxide phases. To determine the degree of their participation in low-temperature CO oxidation, experiments were carried out with individual oxides of copper, manganese, and iron, also obtained by the sol-gel method with combustion. It revealed that on iron oxide the reaction proceeds at a noticeable rate at temperatures above 4000C, on manganese oxide - above 3500C, and on copper oxide, at 2500C the conversion is about 80%.
As noted above, we have previously [15] found that copper-iron oxide compositions obtained by the sol-gel method with combustion and containing copper ferrite, are active in the oxidation of carbon monoxide at 230-250° C.
In the catalytic systems studied in this work, the addition of manganese to the composition of copper ferrite promotes lower-temperature oxidation of CO. As is known, in the oxidation of various compounds, including carbon monoxide, with molecular oxygen on d-element oxides, the reaction rate largely depends on the formation or breaking of the oxygen-metal bond.
Therefore, the main criterion for choosing oxidation catalysts is the strength of oxygen binding to the active site of the surface. In ferrites, transition metals can be located in tetrahe-dral and octahedral positions (pores). Their arrangement in one or another position depends on the diameter and charge of the cation, but mostly on the electronic configuration of the cations (degree of filling of the 3d and 4d shells) and the electrostatic field of the lattice. Proceeding from a large number of theoretical and experimental works, cations, according to their tendency to occupy octa-pores, are ar-
3+
ranged in the following row (at *T = 0): Cr , Ni2+, Mn3+, Cu2+, Al3+, Cu+, Fe2+, Co2+, Fe3+, Mn . Cations on the left Al are more inclined to occupy octahedral pores, while cations
3+ 0+
from Al to Fe can occupy both tetra- and octa-pores [24]. The placement of transition metal ions in octahedral vacancies leads to the fact that the energy of the Me-O bond decreases which results in easier electronic transition and thereby a rise in the rate of the oxidation reaction.
Besides, in the presence of oxide phases in the synthesized Cu-Mn-Fe catalysts, as well as various metal ferrites, the surface oxygen can coordinate with different metal atoms of the structure, thereby exhibiting different reactivity, which suggests the possibility of the reaction proceeding in one stage (mechanism of Lang-muir-Hinshelwood), and in two stages (mechanism of Mars-van-Krevelen) [2].
In addition, owing to the presence of various oxide and metal ferrite phases in the synthesized Cu-Mn-Fe catalysts, the surface oxygen can coordinate with different metal atoms of the structure, thereby exhibiting different reactivity which suggests the possibility of the reaction proceeding both in one stage (Lang-muir-Hinshelwood mechanism), and in two stages (Mars-van-Crevelen mechanism). In the first case, carbon monoxide adsorbed on the catalyst reacts with adsorbed oxygen (a fused mechanism). According to the Mars-van-Kre-velen mechanism, in the first stage the reaction proceeds between the oxidized catalyst and CO:
Kat - O + CO = CO2 + Kat -In the second stage the reduced catalyst is oxidized with oxygen from the gas phase:
2Kat - + O2 = 2 Kat-O, The active form of oxygen is lattice oxygen and during the reaction, there is an alternate reduction and oxidation of the catalyst.
These assumptions were confirmed by the results of special experiments on the conversion of carbon monoxide. In the first series of experiments, the reaction on the catalyst was carried out without an air supply. In the second series of experiments, the catalyst was preliminarily purged with an inert gas at the reaction temperature and then the experiment was conducted without an air supply. In the third series of experiments, after purging the catalyst with inert gas, carbon monoxide was fed into the reaction medium, i.e. the reaction proceeded in an inert gas atmosphere. The reaction temperature varied in the range 165-2800C.
The results showed that in all experiments a weak conversion of carbon monoxide was observed, as evidenced by a slight rise in the reaction temperature of 12-180C during the
first 3 minutes. Then the temperature dropped very slowly and after about 20 minutes it reached the initial one to indicate the reaction termination. After the air was supplied to the reaction space did the reaction begin to proceed intensively, reaching 100% conversion within 5-7 minutes.
Conclusion
Cu-Mn-Fe catalytic oxide systems were synthesized by the sol-gel combustion method and studied in the reaction of oxidation of carbon monoxide to dioxide.
It has been found that in these catalytic systems the oxidation of carbon monoxide to dioxide proceeds at low temperatures of 120-1700C.
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ZOL-GEL YANMA ÜSULU ÏLO ALINAN Cu-Mn-Fe KATALÏTÏK SÏSTEMLORIN ϧTÎRAKI ÏLO KARBON MONOOKSiDiN AÇAGI TEMPERATURLU OKSiDLOSMOSi
G.R.Ozimova
Karbon monooksidin oksidlsçmssi reaksiyasi zol-gel yanma usulu ils alinmiç Cu-Mn-Fe (1:1:1; 2:1:1; 1:2:1 и 1:1:2) oksid katalitik sistemlsrinin içtiraki ils tsdqiq olunub. Rentgen faza analizinin nsticslsri göstsrdi ki, katalitik sistemlsr mürskksb faza tsrkibins malikdir. Manqan vs dsmirin oksidlari ils yanaçi, mis-, manqan ferritlari, mis manqanit, manqanla svszlsnmiç mis ferritlsri smsls gslir (CuFe2O4; Mna43Fe2.57O4, Mno.98Fe2.02O4; CuMn2O4 ; Cui.2MnL8O4; Cuo.5Mno.5Fe2O4). Müsyysn edilmiçdir ki, reaksiyanin baçlama temperaturu katalizatorda metallarin nisbstindsn asili olaraq 120-1700C temperatur intervalinda olur. Reaksiya baçlandiqdan sonra ilk 3-5 dsqiqs srzinds çevrilms ksskin çskilds artmaga baçlayir, sonraki 5 dsqiqs srzinds 100%-s çatir.
Açar sözlar: oksidh§m3, ёзт qazi, yanma sol-gel sintezi, oksidbr, çpinelhr.
ИССЛЕДОВАНИЕ НИЗКОТЕМПЕРАТУРНОГО ОКИСЛЕНИЯ МОНООКСИДА УГЛЕРОДА НА ПОЛУЧЕННЫХ ЗОЛЬ-ГЕЛЬ МЕТОДОМ С ГОРЕНИЕМ ОКСИДНЫХ Cu-Mn-Fe
КАТАЛИТИЧЕСКИХ СИСТЕМАХ
Г.Р.Азимова
Исследовано окисление монооксида углерода на оксидных Cu-Mn-Fe каталитических системах, с соотношением металлов Cu:Mn:Fe=1:1:1; 2:1:1; 1:2:1 и 1:1:2, полученных золь-гель методом с горением. Результаты рентгенофазового анализа показали, что каталитические системы обладают сложным фазовым составом. Наряду с двойными оксидами марганца и железа образуются ферриты меди, марганца, манганит меди, марганецзамещенные ферриты меди (CuFe2O4; Mn043Fe257O4, Mn0.98Fe2.02O4; CuMn2O4; CuL2MnL8O4; Cu05Mn05Fe2O4). Установлено, что температура начала реакции находится в интервале температур 120-1700С в зависимости от соотношения металлов в катализаторе. А после 3-5 минут после начала реакции конверсия резко начинает расти, достигая 100% в течение следующих 5 минут.
Ключевые слова: окисление, монооксид углерода, золь-гель синтез с горением, оксиды, шпинели.
