Hydrothermal Deposition as a Novel Method for the Preparation of Co-Mn Mixed Oxide Catalysts Supported on Stainless Steel Meshes: Application to VOC Oxidation

doi:10.21203/rs.3.rs-566024/v1

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Research Article

Hydrothermal Deposition as a Novel Method for the Preparation of Co-Mn Mixed Oxide Catalysts Supported on Stainless Steel Meshes: Application to VOC Oxidation

https://doi.org/10.21203/rs.3.rs-566024/v1

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published 21 Aug, 2021

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The aim of this work was to develop a novel method for the preparation of structured Co-Mn mixed oxide catalysts: deposition on stainless steel meshes by hydrothermal synthesis. The use of meshes enabled the deposition of a thin layer of the active phase, which significantly suppressed the influence of internal diffusion. Consequently, the prepared catalysts exhibited from 48 to 114 times higher catalytic activity in ethanol oxidation than the commercial pelleted Co-Mn-Al catalyst. Moreover, we have shown that their catalytic activity correlated with the amount of surface oxygen vacancies determined by XPS. Finally, the outstanding activity of the catalyst with Co:Mn ratio of 0.5 was ascribed to the mutual effect of high number of oxygen vacancies and exceptional redox properties.

Environmental Chemistry

Toxicology

Cobalt–manganese mixed oxides

Structured catalysts

Hydrothermal reaction

Stainless steel meshes

Ethanol total oxidation

Volatile organic compounds

Catalysts supported on stainless-steel meshes are flexible to be fit into any kind of reactor, while ensuring a low pressure drop and good heat transfer (Cybulski &Moulijn, 2006). Moreover, thin film of active phase deposited on metal meshes enables its high utilization in catalytic reaction due to low influence of the internal diffusion of reactants and products (Jiratova et al., 2019). Cobalt and manganese oxides show high catalytic activity in various oxidation reactions and the catalysts supported on metal meshes can be applied in any kind of a reactor especially that designated for fast reactions (Dvořáková et al., 2019). The Co–Mn oxides were investigated as catalysts for the total oxidation of volatile organic compounds (VOC) using various compounds, such as ethyl acetate and n–hexane (Tang et al., 2014), ethanol (Jirátová et al., 2009), toluene (Lamonier et al., 2007), and propanol (Castaño et al., 2015).

The oxides of transition metals can easily change the oxidation state and work in redox cycle. In Co₃O₄, the presence of both divalent and trivalent Co cations and good oxygen mobility through spinel structure is promising to catalyze the redox reactions. Many manganese oxides with different crystal structures and Mn cations in various oxidation states can be found (e.g., MnO, Mn₃O₄, Mn₂O₃, and MnO₂). Several Co–Mn mixed oxides were reported depending on the ratio of cations present in the products. The Co₂MnO₄ spinel has cubic structure whereas the Mn₂CoO₄ one is tetragonal (Lee et al., 2014). According to the thermodynamic study of Mn_1 + xCo_2−xO₄, both phases coexist for x between 0.3 and 0.9. The single cubic phase is stable below this range and tetragonal phase is stable for x > 0.9 (Aukrust &Muan, 1963). Another mixed oxide phase is trigonal with chemical formula Co_1 − zMn_1 + zO₃ (Masi et al., 2015). Tang et al. prepared mixed oxide matched with XRD reference pattern (Co,Mn)(Mn,Co)₂O₄ (PDF 00–018–0410). This formula implies the uncertainty in occupation of the metal cations in tetrahedral and octahedral sites (Tang et al., 2014).

Various methods can be applied for preparation of Co–Mn mixed oxides. Masi et al. prepared the Co–Mn mixed oxides by high–energy ball milling of single oxide powders and observed the impact of mechanochemical treatment by in situ XRD analysis of samples after different time of milling (up to 100 hours); they observed presence of aforementioned phases (Masi et al., 2015). Liang et al. prepared mixed oxides by citric acid–assisted sol–gel method and by coprecipitation of aqueous nitrate solution with ammonium hydroxide, observing larger surface area for the samples prepared by sol–gel method (Liang et al., 1998). Other authors used coprecipitation to prepare Co–Mn–Al hydrotalcite–like precursors to achieve good metal dispersion and thermal stability of the catalyst (Castaño et al., 2015, Jirátová et al., 2009, Lamonier et al., 2007). Lee et al. prepared mixed oxide powders via auto–combustion method, using glycine as a fuel (Lee et al., 2014). Tang et al. used acetate alcoholic solutions that were dried and then calcined to prepare mixed oxides (Tang et al., 2014). However, such methods have several disadvantages, e.g. high amount of chemicals needed for synthesis and thus negative impact on the environment.

Therefore, a more interesting method of the Co–Mn mixed oxide preparation is hydrothermal treatment of Co and Mn salts in aqueous solutions under temperatures higher than 100°C and corresponding pressure. Variation of preparation conditions can change physical chemical properties of the prepared products and, in the presence of a structural support, affect properties of the arising layers (e.g., their composition, adhesion to the support, etc.). We have shown recently that structured cobalt oxide catalysts supported on stainless steel meshes can be easily prepared by hydrothermal synthesis under mild conditions (Topka et al., 2020). Besides, they exhibited good performance and selectivity in ethanol oxidation, which was employed as a model volatile organic coumpound. Due to the formation of thin film of the active phase and, consequently, the low influence of internal diffusion limitations, this catalyst was more active in ethanol oxidation than the pelletized commercial Co₃O₄ with 50 times higher content of cobalt oxide. We reported formerly (Kovanda et al., 2013) that the efficiency of cobalt oxide in ethanol oxidation can be enhanced by preparing Co–Mn mixed oxide catalysts. Cai et al. demonstrated that Mn introduction leads to increased surface Co²⁺ concentration and active oxygen, which contributed to high catalytic activity in VOC oxidation (Cai et al., 2015). In a recent study (Zhang et al., 2020), the introduction of Mn increased reducibility and catalytic performance of cobalt oxides. The Co–Mn oxide catalysts were more active in toluene oxidation than their Co–Ni or Co–Cu analogues. However, the structured Co–Mn catalysts supported on stainless steel meshes were not studied yet.

The present work is focused on the preparation of structured Co–Mn mixed oxide catalysts with various Co/Mn molar ratios. The mixed oxides supported on stainless steel meshes were prepared by heating of carbonate precursors, which crystallized on the supports immersed in the aqueous solutions of Co and Mn nitrates during hydrothermal treatment in the presence of hydrolyzing urea. The physical–chemical properties of the catalysts were investigated by XRD, SEM, FTIR, H₂-TPR, and XPS. The catalytic activity of the prepared catalysts was examined in the total oxidation of ethanol and compared with that of the pelletized commercial Co–Mn–Al mixed oxide catalyst. The obtained physicochemical characteristics were correlated with activity and selectivity of the catalysts in ethanol oxidation, which was selected as a model reaction.

2.1 Catalysts preparation

Circular stainless steel meshes (composition in wt. % Fe 71, Cr 16, Ni 11, Mn 2, mesh size 0.40 mm, wire diameter 0.22 mm) with outer diameter of 25 mm were used as supports. Before deposition, the meshes were cleaned mechanically using a brush with detergent, then thoroughly washed in distilled water and the cleaning process was finished by degreasing in acetone for 10 min in an ultrasonic bath. Finally, the meshes were dried at room temperature in air. The supported mixed oxides with various Co:Mn molar ratios were prepared as follows: The meshes were placed into 75 ml of aqueous solution containing Co and Mn nitrates (total Co + Mn concentration of 0.10 mol l^− 1) and urea (0.50 mol l^− 1). The deposition was performed under hydrothermal conditions at 140°C for 120 h using sealed 100 ml Teflon lined stainless steel autoclaves. Then the meshes were taken out, rinsed with distilled water, dried at 60°C, and heated at 500°C for 4 h in air. The samples were labelled according to their nominal Co/(Co + Mn) molar ratios (0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1.0), what means that pure Mn oxide is labeled as 0, and pure Co oxide as 1.0. The pelletized commercial Co–Mn–Al mixed oxide catalyst (Astin 2–100, Czech Republic, Co:Mn:Al molar ratio of 4:1:1), labeled as Com, was used for comparison.

2.2 Catalyst characterization

Powder X–ray diffraction (XRD) patterns were recorded using a Bruker AXS D8 diffractometer with Co Kα radiation (λ = 0.179 nm) in 2θ range 10–80°, step size 0.02°. The qualitative analysis was performed with a HighScore Plus 4.0 software package (PANanalytical).

Surface morphology of the mixed oxides particles deposited on the stainless steel supports was observed using scanning electron microscope Tescan FERA 3.

Contents of Co and Mn in the deposited oxides were determined using the following way: Small amounts of the solids were gained by brushing of the meshes immersed in a container placed in an ultrasonic bath and filled with acetone. After the drying, the obtained solids were analyzed by energy–dispersive X–ray spectroscopy (EDX, Quantax 200, Bruker).

Surface area of the catalysts was determined from the adsorption isotherms of physisorbed krypton as described elsewhere (Šolcová et al., 2011), using a specially designed stainless steel vessel for measurements of bulky catalysts (Topka et al., CZ308606) enabling measurements of the surfaces of large samples (< 30 mm) in the device of ASAP, Micromeritics, USA.

FTIR (Fourier-transform infrared) spectrometer Avatar 360 (Nicolet) was used to measure infrared spectra of samples between 360 and 4000 cm^–1 (resolution 1.93 cm^–1, 300 scans, 1 s per scan). FTIR was used in a specular reflection mode to obtain spectra from the catalysts deposited on stainless steel meshes while pristine stainless steel mesh was used as a background. The spectrum of pelletized Co–Mn–Al mixed oxide catalyst was obtained in a common ATR mode when the pellet was crushed and the powder was pressed against the ZnSe crystal.

H₂–TPR measurement was done using 10 mol. % H₂ and 20°C min^− 1 temperature ramp. Changes in H₂ concentration were detected with a catharometer. Reduction of the grained CuO (0.160–0.315 mm) was performed to calculate absolute values of the hydrogen consumed during reduction of the samples.

Surface elemental analysis was performed by X–ray Photoelectron Spectrometer Kratos ESCA 3400. As the samples were placed on a carbon tape, a carbon correction was not possible and all spectra were corrected to a metal bound oxygen (529.6 eV). Shirley background was subtracted and elemental compositions of the layers were calculated from the corresponding areas.

2.3 Catalytic experiments

The catalysts were tested in the oxidation of 770 ppm of ethanol in air at GHSV of 20 l g_cat^–1 h^–1 using a fixed–bed reactor; details are given elsewhere (Soukup et al., 2012). The T₅₀ temperature, at which 50 % of ethanol conversion was achieved, was used to compare the performance of the catalysts. Catalytic activity of the active phase was assessed as R₂₀₀ (amount of ethanol converted per gram of oxides and hour at 200°C). The T₉₀(CO₂) temperature, at which 90 % conversion of ethanol to CO₂ was achieved, was employed to compare the efficiency of the catalysts. The pelletized commercial Co–Mn–Al mixed oxide catalyst (Astin 2–100, Czech Republic, Co:Mn:Al molar ratio of 4:1:1) was tested under the same conditions for comparison.

3.1 Chemical, structural and textural properties of the catalysts

The XRD results confirmed that the precipitation of Co and Mn nitrates under hydrothermal conditions in the presence of hydrolyzing urea resulted in crystallization of Co and Mn carbonates on the supports. After calcination, the supported catalysts containing approximately 1–5 wt. % of active oxides deposited on the meshes were obtained (Table 1). Determination of the oxides composition by chemical analysis was difficult due to very low Co and Mn contents in the catalysts and simultaneous presence of Mn in the used stainless steel meshes. Therefore, the Co and Mn concentrations were determined by EDX in small amounts of the deposited oxides removed from the meshes by brushing. The Co/(Co + Mn) molar ratios in the prepared catalysts were calculated; these values corresponded approximately to the nominal ones, i.e., to the Co/(Co + Mn) molar ratios in the nitrate solutions used for the catalysts preparation (Table 1).

Table 1

Physicochemical characteristics of the catalysts.
Co/ (Co + Mn)^a	Oxides content (wt.%)	Weight Loss^b (%)	Co^c (at.%)	Mn^c (at.%)	Co/ (Co + Mn)^c	Particle size^d (µm)	S_BET (m² g_cat⁻¹)	S_BET (m² g_oxides⁻¹)	H₂–TPR^e (mmol g_oxides⁻¹)
0	4.97	22.8	-	-	0	16	0.71	14.3	3.4
0.2	3.28	22.5	8.1	31.6	0.20	15	0.38	11.6	3.0
0.4	2.55	6.8	14.6	20.4	0.42	9–15	0.27	10.6	3.5
0.5	1.14	34.1	8.4	8.7	0.49	13	0.27	23.7	5.3
0.6	2.95	29.2	23.7	19.1	0.55	4	0.48	16.3	5.1
0.8	1.91	21.0	29.8	8.2	0.78	7–20	0.49	25.7	6.0
1.0	1.34	3.6	-	-	1	7	0.16	11.9	12.9
Com	97.0^f	–	61.5^g	14.0^g	0.8	–	85	88	5.2
^a Nominal molar ratio
^b Weight loss after treatment in an ultrasonic bath related to the initial weight of the deposited oxides
^c Determined by EDX
^d Determined from SEM images
^e Hydrogen consumption in the temperature range 25–500°C, per gram of active phase
^f The Com catalyst contained 3 wt.% of graphite, which was added before pelletizing
^g Determined by chemical analysis

Adhesion of the deposited Co-Mn oxides to the stainless steel supports was examined by a simple test, when the catalysts samples immersed in acetone were treated in an ultrasonic bath for 10 min. After drying, the weight loss was determined and related to the initial weight of the oxides in the catalysts. The obtained values are summarized in Table 1; the deposited oxides showed quite good adhesion to the support; weight losses varied from 4 to 34 %, the highest being found for the 0.5 sample and the lowest for the 1.0 sample containing only cobalt oxide.

Powder XRD analysis of the supported samples showed the diffraction peaks at about 51 and 60° 2θ that corresponded to a Fe_0.65Cr_0.1Ni_0.25 phase (PDF 04–019–2390). They were ascribed to the stainless steel support. In the samples obtained after hydrothermal treatment of the stainless steel meshes in the solutions containing only Co or Mn nitrates, powder XRD analysis showed presence of CoCO₃ (spherocobaltite, PDF 04–008–8312) or MnCO₃ (rhodochrosite, PDF 04–011–3971). In the catalyst precursors prepared in the solutions containing mixtures of Co and Mn nitrates, both CoCO₃ and MnCO₃ were found as distinct phases. Only MnCO₃ diffraction lines were detected in the precursors with higher Mn content prepared using solutions with Co/(Co + Mn) molar ratios of 0.2 and 0.4. It is known that the Co and Mn cations are precipitated at slightly different pH values; pH of 7.5 and 8.5 were reported for precipitation of Co and Mn hydroxides in 0.01 mol l^− 1 solutions, respectively (Cavani et al., 1991). Therefore, former precipitation of Co cations to create CoCO₃ could be expected during urea hydrolysis resulting in gradual pH increase in the solutions.

After calcination, Co₃O₄ (04–005–4386) and Mn₂O₃ (PDF 04–008–6383) were identified in the powder XRD pattern of the 1.0 and 0 catalysts, which contained only single Co and Mn oxides (Fig. 1). However, the Co–Mn mixed oxides were formed after calcination though the Co and Mn carbonates crystallized as separated phases on the surface of stainless steel meshes during their interaction with aqueous solutions of Co and Mn nitrates in presence of hydrolyzing urea. In the samples with lower Mn content, an incorporation of Mn cations into Co₃O₄ spinel lattice or direct CoMnO₃ formation could be anticipated. The pattern of the calcined 0.8 catalyst having relatively low Mn content contained Co₃O₄ accompanied by a cubic Co–Mn mixed oxide corresponding to Mn_1.9Co_1.1O₄ with spinel structure (PDF 04–009–3198) whereas Co₃O₄ together with CoMnO₃ (PDF 03–065–3696) were identified in the 0.6 catalyst. The powder XRD patterns of the 0.5 and 0.4 catalysts showed only CoMnO₃ as a crystalline mixed oxide deposited on the stainless steel support. Calcination of the precursors with low Co content prepared using solutions with Co/(Co + Mn) molar ratios of 0.4 and 0.2, in which Co and Mn carbonates were distinguished, led to transformation into various mixed oxides in dependence on Co and Mn contents. Only CoMnO₃ was found in the 0.4 sample obtained from the precursor, in which CoCO₃ diffraction lines were not detected; excess of Mn could be incorporated into an XRD–amorphous part. Tetragonal Mn₂CoO₄ spinel (PDF 04–007–4368) and Mn₅O₈ (PDF 04–008–3979) were found in the powder XRD pattern of the 0.2 catalyst with high Mn content. The powder X-ray pattern of the pelletized commercial catalyst (Com) is shown in the Supplementary material (Fig. S1). The catalyst contained a Co–Mn–Al mixed oxide with spinel structure. Slight amount of graphite (3 wt.%) was added to the mixed oxide before pelletizing.

Summarizing, the CoMnO₃ phase with rhombohedral ilmenite structure, indicating the presence of tetravalent Mn cations, was found in the 0.4, 0.5, and 0.6 catalysts (Fig. 1). Mixed oxides with spinel structure were detected in other Co-Mn catalysts; a non-perfect structure ordering with Co and Mn cations in various oxidation states could be expected in mixed oxides emerging after calcination of carbonate precursors at 500°C in oxidation environment (air).

In literature, slightly different composition was reported for the Co–Mn mixed oxides with Co:Mn molar ratio of 1:1 prepared by various methods. For example, Tang et al. prepared the (Co,Mn)(Mn,Co)₂O₄ phase by decomposition of oxalates (Tang et al., 2014). Other authors reported a mixture of tetragonal Mn₂CoO₄ and cubic MnCo₂O₄ oxides when applied ball milling (Masi et al., 2015) or sol–gel combustion method (Lee et al., 2014). Difference in the phase compositions of the obtained oxides could be caused by thermal treatment; aforementioned authors used higher temperatures and longer times during thermal treatment, resulting in the formation of different oxide phases.

The surface morphology of the as–obtained catalysts is documented by SEM images in Fig. 2. The stainless steel meshes were covered with rather big particles of Co–Mn oxides originating from crystals of carbonate precursors formed during hydrothermal reaction. The crystals of carbonate precursors had well–defined, roughly cubic shape (not shown here), which remained preserved after calcination. Sizes of the oxide particles changed in dependence on Co and Mn content (Table 1). The biggest particles with size of about 15 µm were found in the 0 and 0.2 catalysts with high Mn content and decreased with increasing Co content in the deposited oxides. The particles with different sizes of about 9 and 15 µm were observed in the SEM images of the 0.4 catalyst. Powder XRD pattern of this sample before calcination contained diffraction lines corresponding to MnCO₃ only (Fig. 1) and thus, presence of an amorphous Co component, making the precursor crystallization more difficult, could be expected. The 0.5 catalyst contained more uniform particles with size of 13 µm whereas the 0.6 sample showed the smallest particles (4 µm). In comparison with the 0.6 catalyst, the 0.8 and 1.0 ones with high Co content showed larger particles (about 7 µm) but some considerably larger particles were also observed in the 0.8 catalyst.

The employed method of stainless steel coating with Co and Mn carbonate precursors led to crystallization of big particles with roughly cubic shape. Gradually increasing pH in the solution due to urea hydrolysis resulted in precipitation of Co and Mn cations and crystallization of tiny particles well adhering to the support surface, followed by growth of these particles with increasing reaction time; finally, well–defined big crystals were obtained. The conditions of hydrothermal reaction affect strongly size and morphology of the prepared products; for example, formation of urchin–like hierarchical CoMn₂O₄ nanostructures (Venkateswarlu et al., 2017), pseudo–spherical particles of Co₃O₄ and Mn_xCo_3−xO₄ core-shell nanoparticles (Bian, 2018), or three–dimensional mesoporous Co₃O₄ flowers constructed by many nanoneedles (Liao et al., 2018) was reported.

All supported catalysts showed low surface area ranging from 0.16 to 0.71 m² g^− 1 (calculated per gram of the catalyst, i.e. including the stainless steel meshes). For better comparison of the catalysts, their surface area was recalculated per gram of the active oxides (Co, Mn, and/or Co–Mn ones). The surface area of the active oxides changed from 11.6 to 25.7 m² g^− 1 (Table 1).

The results of H₂–TPR measurements obtained in the temperature range from 25 to 900°C are shown in Fig. 3 and Table 1. TPR profiles (Fig. 3) reflected various phase composition and Co and Mn contents in the catalysts. The 1.0 sample containing only Co₃O₄ showed one main reduction peak with T_max at 395°C and a shoulder at somewhat lower temperature. This peak was ascribed to the reduction of Co³⁺ to Co²⁺ and Co²⁺ to Co⁰ (Calgaro &Perez-Lopez, 2017, Lin &Chen, 2004). Reduction of the 0 catalyst containing only Mn₂O₃ proceeded in two steps with temperature maxima at 355 and 442°C. The reduction process included Mn₃O₄ as an intermediate; reduction of MnO₂/Mn₂O₃ to Mn₃O₄ followed by reduction of Mn₃O₄ to MnO was reported (Kapteijn et al., 1994, Wu et al., 2016). The reduction of MnO₂/Mn₂O₃ can proceed also in one step without Mn₃O₄ intermediate formation (Arena et al., 2001) depending on the conditions of TPR experiments. Incorporation of Mn into cobalt oxide manifested in broadening of temperature range, in which the reduction of the catalysts proceeded. The TPR experiments revealed reduction of some other mixed oxide components formed from carbonate precursors during calcination. The highest number of such components was observed with the 0.4 and 0.5 catalysts. Four distinguished peaks were observed in TPR profile of the 0.4 catalyst with T_max at 335, 400, 490, and 593°C. In the TPR profile of the 0.5 catalyst, the reduction peaks were overlapping and a hint of new low–temperature peak can be seen at 227°C. This temperature maximum can be ascribed to the reduction of adsorbed oxygen species (Chen et al., 2017, Luo et al., 2018) and/or to Co⁴⁺ reduction, which can be formed by oxidation of surface Co³⁺ according to equation ½ O₂ + Co³⁺ → O⁻ + h (Co⁴⁺), where h is a positive hole, i.e., orbital without electron (Kaczmarczyk et al., 2016). The finding agrees well with the published values of T_max for Co⁴⁺ reduction at about 200°C (Jung et al., 2019). Reduction of the commercial Co–Mn–Al mixed oxide catalysts (the Com sample) proceeded in two temperature regions, the first between 100 and 500°C and the other between 500 and 900°C. Both regions consist of some overlapping reduction peaks. Li et al. observed a similar TPR profile of Co–Mn–Al mixed oxide obtained at 500°C from a coprecipitated precursor (Li et al., 2019).

The FTIR spectra of the examined catalysts are presented in Fig. 4. The spectra were taken in the range from 4000 to 400 cm^− 1 but their analysis was possible only in the range from 400 to 900 cm^− 1 due to interferences from the meshes forming artificial bands at wavenumbers higher than 900 cm^− 1. In the range from 400 to 900 cm^− 1, the stretching vibrations of metal–oxygen bonds can be found. Intensity of the bands is very low due to low concentration of active components in the supported catalysts; nevertheless, some interesting features can be recognized in the obtained spectra. Stretching vibrations at 554 and 659 cm^− 1 are typical for Co₃O₄ with spinel structure (Bentley et al., 1968, Jirátová et al., 2017). In the FTIR spectrum of the 1.0 catalyst, the bands 580, 648, and 667 cm^− 1 were observed. These bands correspond to Co₃O₄, with variations between 560–574 cm^− 1 for the first band and 659–661 cm^− 1 for the second one. The first band ν1 at 580 cm^− 1 was attributed to the spinel lattice vibrations connected with Co cations in octahedral sites. However, it seems that the first band is composed of more than one component. The second band ν2 at about 658 cm^− 1 was ascribed to vibrations of bonds between oxygen and Co in tetrahedral sites. The FTIR spectrum of the 0 catalyst containing only Mn oxide showed very broad but distinct band with maximum at 719 cm^− 1. This band can be attributed to the Mn–O bond (Dubal et al., 2010, Wu et al., 2017). In the Co–Mn mixed oxide catalysts, similar stretching vibrations of Co–O and Mn–O bonds at 568 and 656 cm^− 1 were observed (Habibi &Bagheri, 2017) and ascribed to the vibrations of Co cations in octahedral and tetrahedral oxygen environment related to the Co–O and/or Mn–O bonds in spinels, respectively (Martin de Vidales et al., 1995, Ramachandran et al., 2015). The band at 860 cm^− 1 can be attributed to δ(C–O) vibrations, coming likely from the rest of MnCO₃ remaining in the catalysts even after calcination (Udayabhanu et al., 2017).

The surface composition in the near–surface region of the catalysts and chemical state of the elements were determined by XPS. As a carbon tape was used for fixing of the samples to a holder, it could manifest itself in a higher concentration of C. Thus, the calibration of the spectra was carried out according to a metal bound oxygen (529.6 eV). Binding energies of core level electrons are shown in the Supplementary material (Table S1) and the surface concentrations of the components are summarized in Table 2. In addition to the main components (Co, Mn, O) given in Table 2, Fe was also detected on the catalysts surfaces, likely coming from a slight erosion of stainless steel meshes during hydrothermal treatment in the solutions containing hydrolyzing urea.

Table 2

Surface composition of the catalysts (at. %) determined by XPS.
Co/(Co + Mn)^a	C 1s	O 1s	Co 2p	Mn 2p	Fe 2p
0	47.77	37.97	0.00	12.76	1.50
0.2	53.00	35.71	2.40	7.60	1.30
0.4	56.92	32.51	4.35	4.41	1.82
0.5	52.38	37.70	3.74	2.18	4.01
0.6	58.38	33.10	5.18	2.73	0.62
0.8	50.49	37.56	7.02	1.79	3.14
1.0	66.04	28.41	3.38	0.00	2.16
^a Nominal molar ratio

Deconvolution of the Co 2p, Mn 2p and O 1s peaks is reported in Table 3. The Co 2p peak consisted of the two maxima, which corresponded to Co 2p_1/2 and Co 2p_3/2 orbitals. The spin-orbital splitting (15.2 ± 0.05 eV) as well as broad satellite structure (Biesinger et al., 2011) points to the presence of Co²⁺ and Co³⁺ (Hagelin-Weaver et al., 2004). The peak at 780 eV was assigned to tetrahedral Co²⁺, while the peaks at 781.3 and 783.5 eV were ascribed to octahedral Co³⁺. This is in line with the literature data (Langell et al., 2000) and our earlier works (Chromčáková et al., 2015, Obalová et al., 2013, Obalová et al., 2007), although a reverse approach was also reported (Iablokov et al., 2015).

Table 3. Deconvolution of Co 2p, Mn 2p and O 1s peaks, molar ratios of Co²⁺/Co³⁺, Mn³⁺/Mn⁴⁺, and surface oxygen vacancies O_v/(Co+Mn) from XPS spectra.

The Mn 2p_3/2 peak centered at 642.3–641.9 eV was ascribed to Mn₃O₄ and MnO₂ (Kim &Shim, 2010). Determination of the manganese oxidation state was done as reported previously (Obalová et al., 2013, Pacultova et al., 2019). The Mn 2p_3/2 spectrum was fitted by two peaks corresponding to Mn³⁺ (the component with lower binding energy) and Mn⁴⁺ (the component with higher binding energy). The Mn³⁺ ion is a strong oxidizing agent and its disproportionation to give Mn²⁺ and MnO₂ is reported. Summarizing, based on the XPS results (Table 3) it can be concluded that Mn³⁺ is prevailing on the surface of the examined catalysts. The Co²⁺ is prevailing on the surface of most catalysts, with the exception of the 0.2 and 0.5 samples, where the concentration of Co³⁺ is higher by 20 or 10 %, respectively. Table 4shows a relation between the surface and bulk concentrations of the catalyst components. Compared to the bulk, surface of the examined catalysts is enriched with cobalt, which is mostly pronounced in the 0.5 catalyst. Surface concentration of manganese is suppressed, again mostly in the 0.5 catalyst.

Table 4

Relative concentration (in %) of Co and Mn in the catalysts determined by EDX and XPS and relation of surface to bulk concentrations of individual components.
Co/(Co + Mn)^a	EDX		XPS		XPS/EDX
Co/(Co + Mn)^a	Co	Mn	Co	Mn	Co	Mn
0	-	-	0.00	100.00	-	-
0.2	20.48	79.52	24.02	76.08	1.17	0.96
0.4	41.51	58.49	49.71	50.40	1.20	0.86
0.5	49.32	50.68	63.28	36.89	1.28	0.73
0.6	55.31	44.69	65.57	34.56	1.19	0.77
0.8	78.34	21.66	79.68	20.32	1.02	0.94
1.0	-	-	99.71	0.00	-	–
^a Nominal molar ratio

The deconvolution of oxygen spectra revealed three peaks with binding energies of about 529.8, 531.5, and 533.0 eV. The first peak at 529.8 eV (O_l) was attributed to the lattice oxygen in the metal oxides, the second O_v centered at 531.5 eV corresponded to the oxygen vacancies, and the third Ow around 532.3 eV was assigned to the hydroxyl species of surface adsorbed water (Feng et al., 2016, Wang et al., 2017). The number of oxygen vacancies belonging to Co and Mn oxides (Table 3) was calculated as the ratio of the oxygen vacancies O_v concentration to the sum of Co and Mn concentrations (in at. %).

3.2 Activity and selectivity in the ethanol total oxidation

The prepared catalysts were tested in the total oxidation of ethanol. Ethanol conversions over the examined catalysts in dependence on reaction temperature are demonstrated in Fig. 5 and characteristic values describing catalytic activity and selectivity of the prepared catalysts are summarized in Table 5. Temperatures T₅₀, at which 50 % conversion of ethanol was achieved, varied in the range from 185°C for the 0.2 catalyst to 207°C for the 1.0 catalyst. The deposition of Co–Mn oxide thin layer enabled its high utilization in ethanol oxidation due to practically negligible influence of internal diffusion. Despite more than 85 times lower content of Co–Mn oxides in the catalytic bed, the 0.2 catalyst exhibited practically the same catalytic performance in terms of T₅₀ as the commercial pelleted Co–Mn–Al catalyst (Table 5). Specific catalytic activity R₂₀₀, expressed as mmol of ethanol converted per gram of oxides and hour at 200°C, was chosen to compare better the performance of examined catalysts containing various amounts of active Co–Mn oxides. The highest specific activity was found for the 0.5 catalyst, despite it showed slightly higher T₅₀ temperature than the 0.2 sample. In order to evaluate the stability of the supported catalysts with time on stream, four consecutive catalytic experiments were performed. While the R₂₀₀ of 34.2 mmol·g_oxides⁻¹·h^− 1 was achieved for the 0.5 catalyst in the first experiment, this value reached 35.9 mmol·g_oxides⁻¹·h^− 1 in the fourth experiment and the deactivation of the catalyst was not observed.

Table 5

Catalytic properties of the catalysts with various Co/(Co + Mn) molar ratios deposited on stainless steel meshes compared to commercial Co–Mn–Al mixed oxide pellets (Com). Reaction conditions: 8 meshes in the bed, 770 ppm of ethanol in air, reaction temperature increase 2°C min^− 1 from 50 to 400°C, GHSV 20 l g^− 1h^− 1.
Co/ (Co + Mn)^a	T₅₀ (°C)	T₉₀(CO₂) (°C)	R₂₀₀^b (mmol g_oxides⁻¹h^− 1)	AcA_max^c (ppm)	T_max^d (°C)	CO_max^e (°C)	T_max^f (°C)	Relative activity^g
0	200	248	6.7	485	226	37	245	22
0.2	185	244	14.5	566	217	95	236	48
0.4	195	251	14.7	601	227	149	249	49
0.5	195	253	34.2	609	225	163	256	114
0.6	194	264	15.0	506	221	108	235	50
0.8	189	254	23.6	619	221	156	247	79
1.0	207	260	20.6	579	243	76	261	69
Com	189	353	0.3	20	164	0	-	1
^a Nominal molar ratio
^b Catalytic activity in mmol of ethanol converted per gram of oxides and hour at 200°C
^c Maximum concentration of acetaldehyde during the experiment
^d Temperature at which the maximum concentration of acetaldehyde was achieved
^e Maximum concentration of carbon monoxide during the experiment
^f Temperature at which the maximum concentration of carbon monoxide was achieved
^g Catalytic activity in terms of R₂₀₀ related to that of the Com catalyst

Typical evolution of reaction byproducts during ethanol oxidation is shown in Fig. 6. Acetaldehyde was detected as the main byproduct of ethanol oxidation over all supported catalysts. Maximum acetaldehyde concentrations over the supported catalysts were achieved at the temperatures ranging from 217 to 243°C and varied from 485 ppm for the 0 catalyst to 619 ppm for the 0.8 one (Table 5). Evolution of acetaldehyde over the pelleted Co–Mn–Al mixed oxide catalyst (the Com sample) was considerably lower (up to 20 ppm only) and the maximum of its formation occurred at lower temperature (164°C). The maximum evolution of CO was found over the most active 0.5 catalyst (163 ppm), while no CO was detected over the Com catalyst. Importantly, the T₉₀(CO₂) temperature needed to attain 90% conversion of all carbon components to carbon dioxide was much lower with the supported catalysts (from 244 to 260°C) than with the Com sample (353°C).

A strong interaction between Co and Mn components in the Co–Mn mixed oxide catalysts was confirmed by H₂–TPR (Fig. 3). Incorporation of Mn changed reduction behavior of the cobalt oxide. While the 1.0 catalyst exhibited the reduction maximum at 395°C and the 0 catalyst containing only Mn oxide showed low–temperature maximum at 355°C, enhanced reducibility was observed for the supported Co–Mn mixed oxides. The H₂–TPR profiles of 0.2, 0.4, 0.6, and 0.8 samples revealed shift of the low–temperature maximum reduction to 335°C. Furthermore, the 0.5 catalyst exhibited a shoulder at about 227°C, which was tentatively ascribed to reduction of Co⁴⁺ components (Mitran et al., 2020). Thus, the high catalytic activity of the 0.5 catalyst could be connected with its improved reduction behavior. It was reported recently that the reduction of Co-Mn mixed oxides occurs in two steps (Bulavchenko et al., 2018). In the first step, (Mn,Co)₃O₄ is reduced to form (Mn,Co)O solid solutions; Co₂MnO₄ oxide starts to be reduced to Co_0.6Mn_0.4O first and then Mn–rich Mn₂CoO₄ is transformed into Mn_0.6Co_0.4O. In the second step, the (Mn,Co)O solid solutions are reduced to metallic cobalt and MnO. Similar behavior can be observed for other Co–Mn oxide catalysts.

We reported previously a good correlation between H₂–TPR profiles and performance of the catalysts in ethanol oxidation (Gaálová et al., 2019, Topka et al., 2019, Topka &Klementová, 2016). The above-mentioned correlation of catalysts reducibility with catalytic performance of Co–Mn mixed oxide catalysts points to the redox (Mars – van Krevelen) reaction mechanism. This idea is supported by the XPS results shown in Fig. 7. They indicate that, regardless of the catalyst composition, the catalytic activity (shown here as relative to the Com catalyst) correlates with the amount of surface oxygen vacancies. Surface oxygen vacancies have been recently reported to play a crucial role in VOC oxidation (Lin et al., 2018, Yang et al., 2019). These defect sites on the surface of metal oxides adsorb oxygen from the air and transform it into catalytically active species. Similarly to our results, Mitran et al. recently observed that replacing of Co³⁺ with Mn⁴⁺ in Co–Mn oxides up to Mn_0.15Co_2.85 leads to the formation of active sites for ethanol oxidation (Mitran et al., 2020). Interaction of Co³⁺ and Mn⁴⁺ with lattice oxygen created oxygen vacancies, which served as catalytically active sites.

Precipitation of Co and Mn nitrates in the presence of hydrolyzing urea under hydrothermal conditions resulted in crystallization of Co and Mn carbonates on the surface of stainless steel meshes; supported catalysts containing 1.1–5 wt. % of Co–Mn mixed oxides were obtained after calcination of the carbonate precursors at 500°C in air. In ethanol oxidation, the supported catalysts exhibited catalytic performance comparable to the pelleted commercial Co–Mn–Al mixed oxide catalyst. However, their catalytic activity, related to the content of active Co and Mn components, was 48–114 times higher. Regardless of catalysts composition, their catalytic activity correlated with the amount of surface oxygen vacancies determined by XPS. Outstanding catalytic activity was revealed for the catalyst with Co/(Co + Mn) molar ratio of 0.5, which contained only CoMnO₃ as a crystalline phase. This catalyst showed the best redox properties in the H₂–TPR measurements and the highest amount of surface oxygen vacancies determined by XPS.

In comparison with the pelletized commercial Co–Mn–Al mixed oxide catalyst, the supported Co–Mn mixed oxide catalysts showed significantly lower temperature needed to obtain 90% conversion of ethanol to CO₂ (244–260°C vs. 353°C). This is an important result because the primary goal of the catalytic reaction is complete VOC oxidation to carbon dioxide (and water) at as low temperature as possible. A decrease of about 100°C is important also from the economical point of view: lower operating temperature of a catalyst needed to obtain a complete reactant conversion to CO₂ means lower operating cost.

The formation of byproducts, namely acetaldehyde and carbon monoxide, was observed over the supported Co-Mn mixed oxide catalyst; the maximum concentrations were detected at about 220–260°C in dependence on the catalysts composition. No acetaldehyde was identified at temperatures higher than 270°C. In contrast, the pelletized commercial Co-Mn-Al mixed oxide catalyst exhibited only low acetaldehyde formation and no formation of carbon monoxide. It was very likely due to internal diffusion effects and, consequently, slower transfer and longer contact time of the reactant and reaction intermediates inside the pellets.

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Not applicable.

Availability of data and materials

All data generated or analysed during this study are included in this published article and its supplementary material.

Competing interests

The authors declare that they have no competing interests.

Funding

The authors thank the Czech Science Foundation for the financial support (project 21–04477S).

Authors' contributions

FK designed the study, supervised the synthesis and wrote part of the manuscript. KJ supervised the catalytic measurements and wrote part of the manuscript. MD prepared the catalysts. JB performed the catalytic tests. MK performed the characterization of the catalysts. PT wrote part of the manuscript and finalized the manuscript. All authors read and approved the final manuscript.

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Hydrothermal Deposition as a Novel Method for the Preparation of Co-Mn Mixed Oxide Catalysts Supported on Stainless Steel Meshes: Application to VOC Oxidation

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Abstract

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1. Introduction

2. Experimental

2.1 Catalysts preparation

2.2 Catalyst characterization

2.3 Catalytic experiments

3. Results And Discussion

3.1 Chemical, structural and textural properties of the catalysts

3.2 Activity and selectivity in the ethanol total oxidation

4. Conclusions

5. Declarations

6. References

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