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

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 signicantly suppressed the inuence 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.

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 lm of the active phase and, consequently, the low in uence of internal diffusion limitations, this catalyst was more active in ethanol oxidation than the pelletized commercial Co 3 O 4 with 50 times higher content of cobalt oxide.
We reported formerly (Kovanda et al., 2013) that the e ciency 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 Co 2+ 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, H 2 -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.

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 nished 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 Te on 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.
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 lled 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 re ection 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. 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.

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 xed-bed reactor; details are given elsewhere (Soukup et al., 2012  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-de ned, 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 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; nally, well-de ned 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 All supported catalysts showed low surface area ranging from 0.16 to 0.71 m 2 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 m 2 g − 1 ( Table 1).
The results of H 2 -TPR measurements obtained in the temperature range from 25 to 900°C are shown in Fig. 3 and Table 1. TPR pro les (Fig. 3) re ected various phase composition and Co and Mn contents in the catalysts. The 1.0 sample containing only Co 3 O 4 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 Co 3+ to Co 2+ and Co 2+ to Co 0 (Calgaro &Perez-Lopez, 2017, Lin &Chen, 2004). Reduction of the 0 catalyst containing only Mn 2 O 3 proceeded in two steps with temperature maxima at 355 and 442°C. The reduction process 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 arti cial 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. 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 xing 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.  ion is a strong oxidizing agent and its disproportionation to give Mn 2+ and MnO 2 is reported.
Summarizing, based on the XPS results (Table 3) it can be concluded that Mn 3+ is prevailing on the surface of the examined catalysts. The Co 2+ is prevailing on the surface of most catalysts, with the exception of the 0.2 and 0.5 samples, where the concentration of Co 3+ 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.  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. %).

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 T 50 , 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 in uence 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 T 50 as the commercial pelleted Co-Mn-Al 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 T 90 (CO 2 ) 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 con rmed by H 2 -TPR (Fig. 3) In comparison with the pelletized commercial Co-Mn-Al mixed oxide catalyst, the supported Co-Mn mixed oxide catalysts showed signi cantly lower temperature needed to obtain 90% conversion of ethanol to CO 2 (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 CO 2 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 identi ed 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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