# Co-modified SBA-15 catalysts for waste gases purification processes - Effect of precursor

Bozhidar Grahovski, Jean-Luc Blin, Bénédicte Lebeau, Anton Naydenov, Ralitsa Velinova, Daniela Karashanova, Lionel Richaudeau, Hristo Kolev, Krasimir Tenchev, Silviya Todorova

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Bozhidar Grahovski, Jean-Luc Blin, Bénédicte Lebeau, Anton Naydenov, Ralitsa Velinova, et al. Co-modified SBA-15 catalysts for waste gases purification processes - Effect of precursor. Catalysis Today, 2025, 461, pp.115503. 10.1016/j.cattod.2025.115503. hal-05213149

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# Co-modified SBA-15 catalysts for waste gases purification processes - Effect of precursor

Bozhidar Grahovski<sup>a</sup>, Jean-Luc Blin<sup>b</sup>, Bénédicte Lebeau<sup>c,d</sup>, Anton Naydenov<sup>e</sup>,
Ralitsa Velinova<sup>e</sup>, Daniela Karashanova<sup>f</sup>, Lionel Richaureau<sup>b</sup>, Hristo Kolev<sup>a</sup>,
Krasimir Tenchev<sup>a</sup>, Silviya Todorova<sup>a,*</sup>

ARTICLE INFO

Keywords:

Co-modified SBA-15
Catalytic combustion
Butane, n-hexane oxidation
Mesoporous structure
Supported Co-Mn oxide catalysts

ABSTRACT

New data on the structural and catalytic properties of a series of mono-component cobalt and bi-component Co-Mn samples supported on SBA-15, prepared by different precursors are reported. The catalysts are characterized by SAXS, N₂-physisorption, XRD, TEM, TPR, XPS, UV-vis and FTIR methods and tested in combustion reactions of butane and n-hexane. It was established that the preparation of the Co-SBA-15 catalysts by different precursors does not significantly change the mesoporous structure. Co species are located in the mesopores, and their shrinkage occurs after the calcination to decompose of the cobalt precursor. Depending on the precursor used, different types of oxide phases are formed on the surface of the mesoporous support. Finely divided and weakly interacting with the support Co₃O₄ and CoSiO₃ are formed on the surface of cobalt samples prepared from both precursors. In addition to the mentioned two phases finely dispersed CoO and a silica-like phase are formed on the surface of the one-component sample prepared from cobalt nitrate. The highest activity of the bi-component Co-Mn sample obtained by a mixed solution of an acetate precursor is explained by very low crystallinity of the supported metal oxide phases and their highest reducibility.

# 1. Introduction

The abatement of volatile organic compounds (VOCs) in the waste gases is an important task in the areas of environmental protection. The European Commission presented the so-called Green Deal in December 2019, also known as the Green Pact, which is a set of policies aimed at making Europe climate neutral by 2050. The design of a catalytic system for complete oxidation of hydrocarbons is an important problem of the environmental catalysis. Cobalt oxide is reported as quite promising among the metal oxides for the removal of VOC's. The main drawback of the transition metal oxides catalysts is the deactivation as a result of aggregation. The problem could be tackled by application of supports with new properties, such as those of nanostructured porous oxides. Zeolites are one such type of carrier because of their specific pore

structure, acidic properties, high thermal stability and the ability to exchange ion. Another type of support that science is increasingly interested in is ordered mesoporous oxides such as silica. Among the different types of ordered mesoporous silicas, SBA-15 has attracted much interest because of its thicker pore walls and higher hydrothermal stability compared to MCM-41 materials [1].

Ordered mesoporous silica materials have drawn a lot of interest due to their huge surface area, uniform pore-size distribution, large pore size, and broad potential applications in separation, adsorption, and catalysis. The mesoporous support would produce stable, dispersed metal particles because of large surface area. As a result, there is a lot of room for further catalytic performance improvement. SBA-15, MCM-41, MCM-48, and KIT-6 silicas are used as supports for catalysts for CO and VOC oxidation. Recent findings on the degradation of volatile organic

<footnote><sup>a</sup> Institute of Catalysis, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bldg. 11, Sofia 1113, Bulgaria</footnote>

<footnote><sup>b</sup> Laboratoire Lorrain de Chimie Moleculaire L2CM, UMR7053, Faculté des Sciences et Technologies, Université de Lorraine/CNRS, CEDEX, Vandoeuvre-lès-Nancy F-54506, France</footnote>

<footnote><sup>c</sup> Institut de Science des Matériaux de Mulhouse, Université de Haute Alsace (UHA)/CNRS, IS2M, UMR 7361, CEDEX, Mulhouse F-68100, France</footnote>

<footnote><sup>d</sup> Universitéde Strasbourg, Strasbourg F-67000, France</footnote>

<footnote><sup>e</sup> Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bldg. 11, Sofia 1113, Bulgaria</footnote>

<footnote><sup>f</sup> Institute of Optical Materials and Technologies "Acad. Jordan Malinowski", Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bldg. 109, Sofia 1113, Bulgaria</footnote>

<footnote>* Corresponding author.
E-mail address: todorova@ic.bas.bg (S. Todorova).</footnote>
compounds by mesoporous silica materials and supported catalysts are discussed by W. Gao et al. [2]. The review examines mesoporous molecular sieve as a support for metal catalysts and their usage in catalytic oxidation of VOCs. The study summarized the impact of active component types, support structure, preparation method, precursors, and other factors on the valence state, dispersion, and loading of active species.

The strong effect of support pore topology on the formation of copper [3–5], cobalt [6], iron [7] and bi-component copper and ceria [8], manganese [9] nanoparticles hosted in mesoporous silicas type SBA-15 and KIT-6 are illustrated in the investigation of the research group of Tsoncheva [3–9]. In work [7] the modification of SBA-15, KIT-5 and KIT-6 mesoporous silicas with different pore sizes with cobalt oxide was studied. It was established that the supports with larger mesopores facilitated the formation of easily reducible $Co_3O_4$ spinel particles. However, the interconnectivity of the mesopores and the uniformity of the channel dimensions also influenced the catalytic activity, implying mass-transfer effects, especially in the case of supports with cage-like mesopores.

In our previous study [10] we focused on examining the structural and catalytic characteristics of a series of single-component cobalt and manganese catalysts, as well as bi-component Co-Mn catalysts supported on SBA-15. It was found that modifying SBA-15 with Co, Mn, or both metals simultaneously does not significantly changed mesoporous structure; however, some pores become partially blocked by oxide, leading to a drop in specific surface area and pore volume. In the case of the mono-component Co-SBA-15, $Co_3O_4$ clusters are present on the surface and partially within the SBA-15 pore system. For Mn-SBA-15, manganese oxides tend to fill the channels of SBA-15, forming nanowires. The mixed Co-Mn oxide nanowires are generated within the channels of CoMn-SBA-15, alongside smaller nanoparticles that are aggregated outside the channels. The mesostructure, morphology of SBA-15, oxide phases, and oxide particle sizes remain unchanged after testing in complete *n*-hexane oxidation for all samples. Their resistance to agglomeration is attributed to the mesoporous framework.

The purpose of the research presented below is to study the influence of the cobalt source on the structural and textural characteristics of Co-SBA-15 and, accordingly, on the catalytic behavior in reactions important for environmental protection. Due to the fact that in our previous studies [10,11] we have found that the addition of manganese to cobalt catalysts has a promoting effect in the reaction mentioned above, the Co-Mn bi-component catalyst has also been studied.

# 2. Materials and methods

## 2.1. Catalysis preparation

The catalytic samples were obtained by the impregnation method. As a source of cobalt, two different salts were used: cobalt acetate Co($CH_3COO)_2.4H_2O$ (Merck, > 99 %) and cobalt nitrate Co($NO_3)_2.6H_2O$ (Sigma-Aldrich, assay 99.99 % trace metals basis). A mesoporous silicate sieve-SBA-15 was used as a catalytic support. The SBA-15 was prepared by a sol-gel route according to procedure described in [10]. The samples were obtained in the standard way: dissolving the cobalt salts in 6 ml of distilled water, suspending the carrier, homogenizing on a magnetic stirrer, drying. Each sample was coated with 5 wt% Co and recalculated according to the amount of carrier (SBA-15). The samples were calcined in a tube furnace, in a stream of air at a temperature of 450 °C for 3 h.

The mixed sample was prepared using cobalt acetate Co($CH_3COO)_2.4H_2O$ (Merck, > 99 %) and ($CH_3CO_2)_2Mn$ (Sigma-Aldrich, ≥ 99 %) to obtain a 2.5 wt%Mn-2.5 wt%Co-SBA-15 catalyst according to the procedure described above. The samples were denoted by 5 %Co-SBA-15 (acetate), 5 %Co-SBA-15 (nitrate) and 2.5 %Mn-2.5 %Co-SBA-15 (acetate).

## 2.2. Catalyst characterization

Data collection for small angle X-Ray scattering (SAXS) was done using an Anton Paar "SAXSess mc²" instrument that had a line-collimation system. The apparatus is coupled to an ID 3003 laboratory X-Ray generator (General Electric) with a sealed X-Ray tube (PANalytical, λ Cu, Kα = 0.1542 nm) and working at 50 mA and 40 kV. The catalyst samples were placed between two sheets of Kapton®. The X-ray beam scattering was recorded using a CCD detector (Princeton Instruments, 2084 x 2084 pixels array, 24 x 24 µm² pixel size) at a distance of 309 mm from the sample under investigation. The 2D image was obtained by using SAXQuant software (Anton Paar) into one-dimensional scattering intensities I(q) in regard to the magnitude of the scattering vector q = (4π/λ) sin(θ), where 2θ refers to the total scattering function angle. Due to the translucent beam-stop allowing the measurement of an attenuated primary beam at q = 0, all the measured intensities can be calibrated by normalizing the attenuated primary intensity. Data were corrected for the background scattering from the cell and for slit-smearing effects by a de-smearing procedure from SAXQuant software, using the Lake method.

Nitrogen adsorption-desorption isotherms were recorded at temperature of -196 °C within a wide relative pressure (p/p₀) range varying from 0.010 to 0.995 with a volumetric adsorption analyzer TRISTAR 3000 (Micromeritics). Before each measurement, the samples were degassed under vacuum (pressure of 0.13 mBar) at 25 °C for 16 h. The specific surface areas of the samples were estimated by the Bruker-Emmett-Teller (BET) method. The pore diameter and the pore size distribution were determined using data from the adsorption branch of the corresponding isotherm using the BJH (Barret-Joyner-Halenda) method.

The crystalline structure and phase composition of the samples were investigated by an Empyrean diffractometer (PANalytical), equipped with detector (Pixel 3D, PANalytical) using Cu Kα 45 kV-40 mA radiation in the 2θ range. The quantitative phase compositions of the samples were obtained by using the X'Pert HighScore software.

Temperature-programmed reduction (TPR) is described in [10].

X-ray photoelectron spectroscopy (XPS) was performed by using of ESCALAB MkII (VG Scientific) and the processing of the measured spectra has been described in [10].

The ultraviolet-visible (UV-vis) spectra were performed on a Thermo Scientific Evolution 300 spectrophotometer, connected with a Praying Mantis Diffuse Reflectance Accessory (Thermo Fisher Scientific). The infrared spectra were collected with Avatar 360 FTIR spectrometer at a spectral resolution of 2 cm⁻¹ and accumulation of 64 scans, using a KBr pellet technique.

The high resolution transmission electron microscopy (HRTEM) studies were performed on a JEOL JEM 2100 instrument at an accelerating voltage of 200 kV.

## 2.3. Catalytic activity

The catalytic activity tests were carried out in a continuous-flow quartz-glass type of reactor under the following testing condition: catalyst bed volume of 0.5 cm³ (0.3 cm³ catalyst and 0.2 cm³ quartz-glass with the same particle size), irregular shaped particles with diameter of 0.45 ± 0.15 mm, reactor diameter of 6.0 mm. The gaseous hourly space velocity (GHSV<sub>STP</sub>) was fixed at 100,000 h⁻¹. The inlet concentrations of butane and *n*-hexane feed were 0.03 vol%, respectively, and the oxygen was at 16.0 vol%, no additional water vapor is added. All the feed gas mixtures were balanced to 100 % with nitrogen (4.6).

The gas analysis was performed using the gas-analyzers for CO/CO₂/O₂ (Maihak-Sick Mod. S 710, V.1.31, Hamburg, Germany) and THC-FID (analyzer for total organic content in gas phase, Thermo FID-TG, SK Elektronik GmbH, Leverkusen, Germany).
### 3. Results and discussion

#### 3.1. Characterization of the mesoporous catalysts before and after reaction

Whatever the catalyst, the typical Small angle X-Ray scattering (SAXS) pattern of SBA-15, which exhibits four reflection lines corresponding to (100), (110), (200) and (210) planes [10] of 2D hexagonal structure is obtained (Fig. 1). So, the hexagonal mesopore ordering is maintained upon the addition of the active phase for the combustion of butane and n-hexane. Comparing the SAXS pattern of 2.5 % Co-2.5 % Mn-SBA-15 with the ones of monometallic cobalt samples, the intensity of the reflection lines is lower for the 5 % Co-SBA-15 materials. This can indicate a partial filling of the SBA-15 mesopore with the oxide species. As was mentioned above the formation of mixed Co-Mn oxides nanowires were observed in our previous investigation concerning Co-Mn/SBA-15 [10]. After the reaction, except a shift to higher angle of the reflections lines due to the silica network contraction upon thermal treatment, no modification is observed on the SAXS patterns. This suggests that the hexagonal mesopore ordering is not disturbed during the combustion reaction.

Manometry nitrogen adsorption-desorption analysis shows that the bare SBA-15 presents a mixed type I/type IV isotherm [10]. Indeed, it is well known that SBA-15 contains both meso- and micropores. Upon the introduction of the Co species (Fig. 2), before reaction the type IV isotherm which is indicative of mesoporous materials as classified by the IUPAC [12] is maintained. Mainly H1 hysteresis loops are observed for

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** SAXS pattern of the bare SBA-15 (a) and of the catalysts before (BR) and after n-hexane oxidation (AR). b: 2.5 % Co-2.5 % Mn-SBA-15 (acetate)-BR; c: 2.5 % Co-2.5 % Mn-SBA-15 (acetate)-AR, d: 5 % Co-SBA-15 (nitrate)-BR, e: 5 % Co-SBA-15 (nitrate)-AR, f: 5 % Co-SBA-15 (acetate)-BR and g: 5 % Co-SBA-15 (acetate)-AR.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Nitrogen adsorption-desorption isotherms of the bare SBA-15 and of the catalysts before (BR) and after n-hexane oxidation (AR).</figcaption></figure>

the SBA-15 materials containing Co. After reaction for the catalyst prepared with Co acetate as precursor, the hysteresis loop is a combination of H1 and H2(b). Type H1 corresponds with the cylindrical mesopores of the SBA-15 support, while type H2(b) indicates partial pore plugging. The hybrid shape of hysteresis loops could be explained by a heterogeneous partial filling of the mesopores by the Co species. After the reaction for the catalyst prepared with Co nitrate only H1 hysteresis loop is observed Fig. 2.

The bare SBA-15 has a specific surface area of 982 m²/g, a mesopore volume of 0.99 cm³/g, and a diameter of 9.0 nm [10], as noted in Table 1. After the addition of Co, the specific surface area, the pore volume, and the mesopore diameter decrease (Table 1 and Fig. 3), suggesting that Co species are located in the mesopores but it is not excluded that pore shrinkage occurred upon the treatment at 450°C to decompose of the cobalt precursor. The decrease of pore size is weak, it can be due to the deposition of cobalt species on the pore surface or a results of the silica network condensation upon the thermal treatment at 450°C. Compared with the bare SBA-15, a part of the decrease in the specific surface area and the pore volume can be also attributed to the loss of some microporosity. The drop in the pore volume and specific surface area is more pronounced when nitrate is used as a precursor instead of acetate. After the reaction, except for the value of the mesopore diameter, a supplementary decrease of the specific surface area and the pore volume is detected.

From the SAXS and the manometry nitrogen adsorption-desorption

<caption>**Table 1**</caption>

Specific surface area (S<sub>BET</sub>), pore volume, and pore diameter of the catalysts before (BR) and after n-hexane oxidation (AR).

<table><thead><tr><th>Sample</th><th>BET Surface Area (m²/g)</th><th>Pore Volume (cm³/g)*</th><th>Pore Diameter (nm)</th></tr></thead><tbody><tr><td>SBA15</td><td>982</td><td>0.99</td><td>9.0</td></tr><tr><td>5 %Co-SBA15 (acetate)-BR</td><td>497</td><td>0.83</td><td>8.6</td></tr><tr><td>5 %Co-SBA15 (acetate)-AR</td><td>310</td><td>0.44</td><td>8.3</td></tr><tr><td>5 %Co-SBA15 (nitrate)-BR</td><td>184</td><td>0.32</td><td>7.6</td></tr><tr><td>5 %Co-SBA15 (nitrate)-AR</td><td>120</td><td>0.21</td><td>7.7</td></tr><tr><td>2.5 % Co-2.5 % Mn-SBA-15 (acetate)-BR</td><td>357</td><td>0.59</td><td>8.8</td></tr><tr><td>2.5 % Co-2.5 % Mn-SBA-15 (acetate)-AR</td><td>274</td><td>0.39</td><td>8.3</td></tr></tbody></table>

<footnote>* Values obtained from BJH method applied to the adsorption branch of the isotherm.</footnote>
<figure><img src="image_4.png" /><figcaption>Fig. 3. Pore size distribution (PSD) of the catalysts before (BR) and after n-hexane oxidation (AR).</figcaption></figure>

analyses, it can be concluded that the type of Co precursor does not affect the mesopore ordering but it influences the textural properties. Partial replacement of cobalt with manganese at bi-component Co-Mn sample leads to a more significant decrease in the specific surface area and pore volume. The decrease in surface area in the two-component sample may also be due to the formation of nanowires along the pores. A similar phenomenon was found by us in our previous studies [10]. Before and after the reaction, considering the error on the measurement (≈ 5%), the mesopore diameters of both samples are the same.

## 3.2. Characterization of the supported metal species before and after reaction

Fig. 4 shows the X-ray diffraction patterns of the synthesized samples before and after reaction. All samples show an amorphous halo characteristic of silicon oxide. Broad and very low intensity reflections for Co₃O₄ and CoO are evidenced by the X-ray patterns of the catalyst obtained by impregnation with Co(NO₃)₂·6H₂O, demonstrating the development of finely divided oxide nanoparticles.

The presence of broad and low intense peaks characteristics of Co₃O₄ are clearly observed for 5 %Co-SBA-15 (acetate) BR (black line), showing that finely dispersed oxide particles are formed on the surface of this sample as well. No changes were observed in the X-ray patterns

<figure><img src="image_5.png" /><figcaption>Fig. 4. XRD patterns of Co-SBA-15 prepared by impregnation from A. Co(NO₃)₂·6H₂O, B. from Co(CH₃COO)₂·4H₂O and C. of 2.5 %Co-2.5 %Mn-SBA-15 prepared by impregnation from a mixture of Co(CH₃COO)₂·4H₂O and (CH₃COO)₂Mn.</figcaption></figure>
after catalytic reaction.

The broad and very weakly intense peak at 2θ= 38.6° is observed in the bi-component MnCo sample before the reaction, most likely indicating the beginning of MnO₂ crystallization (Fig. 4C). After the catalytic test, very low intensity and broad peaks are observed in the region of mixed Co-Mn oxides. The formation of finely divided non-stoichiometric spinels is obtained during the decomposition of supported mixed cobalt and manganese nitrates at low temperatures (between 250 °C and 400 °C) in our previous investigation, using the same support and nitrates as sources of cobalt and manganese [10].

To investigate the reduction behavior of the prepared catalysts, an H₂-TPR was carried out (Fig. 5). The TPR experiment was conducted at temperatures up to 700 °C because the maximum temperature at which the catalytic experiment was run never exceeded this one. For the sample obtained from nitrate, two intense peaks are observed in the interval 550–700 °C. Carrero et al. [13] state that cobalt oxide species in close proximity to the SBA-15 support can be identified by reduction peak above 500° C. Several studies relate the hydrogen consumption above 600 ⁰C in the TPR profiles of the cobalt-modified mesoporous materials, to the reduction of silicates generated by the interaction of highly dispersed CoO phases with the support [14]. That is, in our case, we can claim that when cobalt is deposited from a cobalt nitrate, a cobalt oxide phase strongly interacting with the support, and silicate-like phases are formed on the surface.

According to the authors in [15] the decomposition of cobalt nitrate proceeds with the release of nitrogen dioxide, water, and oxygen: 3Co(NO₃)₂ · 6H₂O → Co₃O₄ + 6NO₂ + O₂ + 18H₂O.

It has been found that the nature of cobalt species on the surface of silica depends on the concentration of NOx at the surface and in the gas phase during nitrate decomposition [16]. High concentrations of NOx oxidize the intermediate Co²⁺ ions to Co₃O₄, while in the absence of NOx, cobalt ions react with silica to form surface silicates. We assume that under the conditions in which the calcination was carried out, i.e. in

<figure><img src="image_6.png" /><figcaption>Fig. 5. TPR spectra of single- and bi-component catalysts.</figcaption></figure>

an air stream, a substantial fraction of NOx is carried away by the air stream, thereby reducing the concentration of NOx at the surface, promoting Co silicate formation.

The formation of CoO and silicates during the decomposition of cobalt nitrate deposited on SiO₂ was established by Puskas [17]. Authors found that during the decomposition of cobalt nitrate deposited on a high surface area SiO₂, due to the migration of SiO₂, a part of CoO is encapsulated by SiO₂, which prevents its subsequent oxidation to Co₃O₄. The extensive migration of silica is attributed to strong attraction between SiO₂ and CoO species, inducing the removal of silicic acid or silica molecules from the silica surface. The loss of a surface area is also explained with interaction of SiO₂ and CoO species. Most likely, similar phenomena take place in our case when nitrate is used as a precursor. As can be seen from the BET measurements, the strongest decrease in the surface area is found for the sample obtained from cobalt nitrate.

Although in the sample obtained from nitrate by impregnation, broad and low intensity peaks for Co₃O₄ are observed in the XRD pattern, in the temperature interval of Co₃O₄ reduction, there is no clearly defined hydrogen consumption. One low intensity and broad wave is also distinguishable at around 250 °C. Obviously, the Co₃O₄ is formed in a very small amount. It is well established that during the decomposition of cobalt nitrate, nitrogen oxides and CoO are formed, which is oxidized in the next stage to Co₃O₄. However, if the CoO species become “catched” by the attachment of silicon dioxide molecules, these CoO-xSiO₂ complexes cannot participate in Co₃O₄ formation [17], leading to its low concentration of the surface.

In the TPR profile of the sample obtained from cobalt acetate, a very weakly intense peak at 308 °C and a tail above 700 °C are observed. The first one is attributed to the reduction of finely dispersed and weakly interacting with the support Co₃O₄ oxide particles [10]. The consumption of hydrogen above 700 °C is attributed to the reduction of silicate phases [18]. The formation of cobalt silicates upon impregnation of SBA-15 with Co(CH₃COO)₂·4H₂O was established by Szegedi et. al. [19]. The surface charge of the silica support has a significant impact on the adsorption of ions when it is impregnated with an ionic solution [20]. The amphoteric nature of silanol groups allows them to exist as SiOH, SiO⁴⁻, or SiOH₂⁻ species, depending on the impregnation solution's pH. Consequently, the pH of the impregnation solution has a significant influence on the mechanism of cobalt ion deposition. Cobalt silicates and hydrosilicates are formed when cationic exchange occurs between positively charged cobalt ions and negatively charged silica in an impregnation solution with a pH > 5 [19]. pH of used solution in our case is 7. The formation of CoSiO₃ is clearly visible from TEM images (Fig. 10). The other explanation for the formation of cobalt silicates when using cobalt acetate as a precursor is the fact that the decomposition of cobalt acetate in air is a highly exothermic reaction. The other explanation for the formation of cobalt silicates when using cobalt acetate as a precursor is the fact that the decomposition of cobalt acetate in air is a highly exothermic reaction [21]. The reduction process for the Co-Mn catalysts begins at a lower temperature than the mono component one. Three overlapping reduction peaks are evident for the bi-component sample: 233 ⁰C, 306 ⁰C, and 450 ⁰C. The authors of reference [22] state that the low temperature peak can be linked to highly scattered surface Mn⁴⁺ species that are easily reducible and originate from the nano Co-Mn oxide (CoₓMnᵧOz (s)+H₂(g) → Mn₃O₄ - Co₃O₄ (s) + H₂O(g)). The reduction of Mn₃O₄ and Co₃O₄ (Mn₃O₄-Co₃O₄ (s)+H₂(g) → MnO-CoO(s) + H₂O(g) may be the cause of the second reduction peak [23,24]. The hydrogen consumption at 450 °C could be assigned to reduction of Co²⁺ to metallic cobalt since the process of Mn²⁺ → Mn₀ needs temperature higher than 1200°C [10]. In mixed oxide systems, phases reducing in the silicate reduction interval are not observed.

UV-vis spectra give additional information about cobalt species present in Co-SBA-15 catalyst (Fig. 6). The spectra can reveal details about the chemical environment and oxidation state of metals. The peaks are broad and improperly resolved. The bands at 524 nm, 590 nm
<figure><img src="image_7.png" /><figcaption>**Fig. 6.** UV–VIS spectra of 5 %Co-SBA-15 (acetate), 5 %Co-SBA-15 (nitrate) and 2.5 %Co-2.5 %Mn-SBA-15 (acetate) catalysts before (BR) and after *n*-hexane oxidation (AR).</figcaption></figure>

and 645 nm are visible in the UV–vis spectra of all samples before and after reaction. The presence of this triplet is characteristic of T ⁴A₂ (F)→⁴T₁ (P) transition characteristic of isolated Co²⁺ ions in tetrahedral coordination [25]. Additionally, the samples’ blue colour gives further support to this identification. The triplet peaks with the same position are characteristic of the electronic d-d transitions for Co²⁺ ions with pure tetrahedral coordination in Co₂SiO₄ silicates [26].

Similar data were obtained from Szegedi et. al [25] when cobalt was deposited on SBA-16. The data we received also gives us reason to claim that cobalt species may be incorporated as a single Co²⁺ site into the silica framework of SBA-15 and participates in strong interactions with the silica matrix. These Co²⁺ species most likely occur as an amorphous silicates phase. Additional evidence for this is the presence of the band around 340 nm, which is visible in all monocomponent cobalt samples. According to literature data, it refers to Co²⁺ in cobalt silicate [27]. The formation of silicate-like phases was suggested by TPR datas above.

The broad adsorption peaks at around 410 nm and 760 nm is seen in UV–vis spectra of the sample prepared by impregnation from nitrate, and it is attributed to the octahedrally coordinated Co³⁺ [28]. Data on

the absorption bands of d-d transitions of cobalt and manganese in various chemical conditions are provided in [29]. Because the catalysts contain both cobalt and manganese, two transition metals, identifying the bands is fairly difficult. An additional trouble is created by the low intensity and strong overlap of the bands. The absorption band around 410 nm in Uv-vis spectra of monocomponent cobalt samples and at 440 nm in the bi-component Co-Mn one, can be attributed to octahedrally coordinated Co³⁺ in the mixed spinel oxide Co₃O₄. The intensity of the peaks in bi-component CoMn-SBA-15 sample is much lower and broader. Very likely the band at 498 nm is the result of the presence of Mn⁴⁺ [29]. In the case of the bi-component catalysts, the correct attribution is very difficult due to the weak intensity of these bands and their strong overlap.

X-ray photoelectron spectroscopy (XPS) was used for studying at the surface oxidation states of Co and Mn. An example of a Co2p₁/₂ curve fitting before and after reaction is shown in Fig. 7.

**Table 2** displays the cobalt surface atomic concentrations as well as the peak positions of the samples both before (BR) and after (AR) the reaction. For cobalt containing samples, peaks with defined binding energies (BEs) of either 795.2–796 eV (Co³⁺) or 797.1–798 eV (Co²⁺), respectively, was measured [30,31]. A peak with a binding energy in the interval 797.1–798 eV (Co²⁺) is present in all cobalt-containing samples, confirming the occurrence of Co²⁺ ions. Table 2 makes this clear: prior to the reaction, the Co²⁺ is the only species present on the surface

<caption>**Table 2**</caption>

Surface atomic concentrations of cobalt in 5 %Co-SBA-15 (acetate), 5 %Co-SBA-15 (nitrate) and 2.5 %Co-2.5 %Mn-SBA-15 (acetate).

<table><thead><tr><th></th><th>Co2p<sub>1/2</sub> BR</th><th>Co2p<sub>1/2</sub> AR</th></tr></thead><tbody><tr><td colspan="3"><strong>5 % Co-SBA-15 (acetate)</strong></td></tr><tr><td>Concentration, at%</td><td>1.29</td><td>1.04</td></tr><tr><td>BE, eV</td><td>797.5</td><td>797.4</td></tr><tr><td>Concentration, at%</td><td>Co<sup>2+</sup>(1.29)</td><td>Co<sup>2+</sup>(1.04)</td></tr><tr><td colspan="3"><strong>5 % Co-SBA-15 (nitrate)</strong></td></tr><tr><td>Concentration, at%</td><td>0.56</td><td>0.51</td></tr><tr><td>BE, eV</td><td>796.0</td><td>796.0</td></tr><tr><td>Concentration, at%</td><td>Co<sup>3+</sup>(0.04)</td><td>Co<sup>3+</sup>(0.07)</td></tr><tr><td colspan="3"><strong>2.5 % Co-2.5 % Mn-SBA-15 (acetate)</strong></td></tr><tr><td>Concentration, at%</td><td>0.63</td><td>0.58</td></tr><tr><td>BE, eV</td><td>797.5</td><td>794.7</td></tr><tr><td>Concentration, at%</td><td>Co<sup>2+</sup>(0.63)</td><td>Co<sup>3+</sup>(0.06)</td></tr></tbody></table>

<figure><img src="image_8.png" /><figcaption>**Fig. 7.** X-ray photoelectron spectra of Co2p1/2 of 5 %Co-SBA-15 (acetate) and 5 %Co-SBA-15 (nitrate) and 2.5 %Co-2.5 %Mn-SBA-15 (acetate) catalysts before (BR) and after *n*-hexane oxidation (AR).</figcaption></figure>
of the sample prepared from acetate and it predominates on the surface of the catalyst prepared by impregnation from nitrate. It is evident that the deposition of Co from Co(CH₃COO)₂·4H₂O promotes the development of surface Co²⁺ ions. Table 2 also shows that the surface cobalt concentration changes very little after the reaction. This observation may be related to the stabilization of the various cobalt species within the SBA-15 framework.

The Mn2p peaks are measured in the energy range 636–658 eV consisting of both Mn2p₃/₂ and Mn2p₁/₂ core levels (Fig. 8). Table 3 shows the distribution of manganese ions on the surface before and after reaction. Before reaction, manganese is present as Mn⁴⁺, while after reaction, both oxidation states are observed. These data agree with those obtained from X-ray diffraction, which showed that before the reaction the manganese was most likely in the form of MnO₂, and after the reaction in mixed oxides. Uv-vis spectra is also in support of the statements above.

These XPS results reveal a noteworthy evolution of surface oxidation states upon reaction. In the case of the bi-component 2.5 %Co–2.5 %Mn-SBA-15 sample, a distinct appearance of Mn³⁺ species (BE ≈ 640.3 eV) is observed alongside Mn⁴⁺ after the catalytic test, indicating a redox transformation that may facilitate enhanced oxygen mobility. Simultaneously, Co³⁺ species become detectable on the surface, with a notable binding energy shift exceeding 1 eV compared to the monometallic systems. This shift suggests a transition toward a higher oxidation state or stronger metal-support interaction in the presence of Mn.

The correlation between the appearance of Mn³⁺ and the increased Co³⁺ signal supports the hypothesis that manganese promotes the activation and stabilization of oxidized cobalt species, which are crucial in redox-type catalytic cycles such as those following the Mars–van Krevelen mechanism. These electronic effects, combined with improved reducibility observed in TPR profiles, explain the superior activity of the Co–Mn catalyst, further supported by the reaction rate constants

<figure><img src="image_9.png" /><figcaption>**Fig. 8.** X-ray photoelectron spectra of Mn2p of 2.5 %Co- 2.5 %Mn-SBA-15 (acetate) catalyst before (BR) and after n-hexane oxidation (AR).</figcaption></figure>

<table><caption>**Table 3**

Surface atomic concentrations of manganese in 2.5 % Co-2.5 % Mn-SBA-15.</caption><thead><tr><th></th><th>Mn2p<br>before reaction</th><th>Mn2p<br>after reaction</th></tr></thead><tbody><tr><td colspan="3"><strong>2.5 % Co–2.5 % Mn-SBA–15</strong></td></tr><tr><td>Concentration, at%</td><td>0.52</td><td>0.47</td></tr><tr><td>BE, eV</td><td>642.2</td><td>640.3</td></tr><tr><td>Concentration, at%</td><td>Mn⁴⁺ (0.52)</td><td>Mn³⁺ (0.15)</td><td>Mn⁴⁺ (0.32)</td></tr></tbody></table>

<table><caption>**Table 4**

Calculated reaction parameters of 5 %Co-SBA-15 (acetate), 5 %Co-SBA-15 (nitrate) and 2.5 % Co-2.5 % Mn-SBA-15 (acetate).</caption><thead><tr><th>Sample</th><th>Rate constant, butane at 356 °C (first order kinetics) s<sup>-1</sup></th><th>Rate constant, hexane at 345 °C (first order kinetics) s<sup>-1</sup></th></tr></thead><tbody><tr><td>5 %Co-SBA15- (acetate)</td><td>9.9</td><td>9.9</td></tr><tr><td>5 %Co-SBA15- (nitrate)</td><td>4.0</td><td>4.2</td></tr><tr><td>2.5 % Co–2.5 % Mn-SBA–15 (acetate)</td><td>13.4</td><td>21.0</td></tr></tbody></table>

presented in Table 4.

The FT-IR spectra of the pure SBA-15 and Co-SBA-15 samples are shown in Fig. 9. Strong absorption bands at 1078 cm⁻¹, 800 cm⁻¹, and 460 cm⁻¹ for the SBA-15 are in good agreement with the SiO₂ bond structure and can be attributed to the bending modes of the Si–O–Si bond, the symmetric stretch vibration of the Si–O–Si, and the asymmetric stretching vibration of the Si–O–Si in the SiO₄ tetrahedron [32].

The band at 960 cm⁻¹ may originate from Si–OH [32]. This band decreases in intensity and shifts to the lower wave numbers for both mono component cobalt samples. A similar phenomenon has been observed in the SiO₂–Nb₂O₅ systems by Francisco and has been attributed to the formation of Si–O–Nb [33]. It is likely that the formation of Si–O–Co bond is observed in our case, especially since the silicates has been demonstrated by both TPR and TEM.

Two characteristic bands are typical for Co₃O₄: in the range of 550–600 cm⁻¹ (v1) and the other in the range of 650–700 cm⁻¹ (v2) [34]. The first one is linked to the octahedral Co³⁺–O bond’s stretching vibration, and the second one is linked to the tetrahedral Co²⁺–O bond’s stretching vibration.

The absence of the band for Co³⁺–O in the FTIR spectra of the sample prepared from acetate could be another conformation that cobalt mainly forms surface silicate and CoO during the applied preparation condition. The band in this interval for bicomponent Co-Mn sample is not visible. A shift to lower frequencies was observed in our previous investigation

<figure><img src="image_10.png" /><figcaption>**Fig. 9.** FT-IR spectra of the SBA-15, 5 %Co-SBA-15 (acetate) and 5 %Co-SBA-15 (nitrate) samples.</figcaption></figure>
[35] for the bands in the ranges of 550–600 cm⁻¹ (v1) and 650–700 cm⁻¹ (v2) in the IR spectrum of the bi-component CoMn sample, indicating a decrease in the strength of Co–O bond.

The samples 5%Co-SBA-15 (acetate) and 5%Co-SBA-15 (nitrate) before reaction were analyzed by TEM (Fig. 10 and 11). The typical morphology of the SBA-15 support with the channel structure is visualized in Figs. 10 a), 11 a) and 11 c). The inner diameter of the pores is determined by high resolution (HR) TEM mode and presented in Fig. 11 f). The presence of particles with higher electron contrast in the same BF TEM images (Fig. 10 a) and 11 a) and c)) suggests the formation of a crystalline phase, evidenced by Selected Area Electron Diffraction (SAED) patterns (Figs. 10 c) and 11 d)) and HRTEM micrographs (Figs. 10 b) and d) and 11 e). The SAED pattern in Fig. 11 b) reveals the presence of amorphous areas in the samples. The morphology of the dark elongated formation, that appear along the cylindrical mesopores, enclosed in the orange square (Fig. 11 c)), supports the idea for partial filling of the mesopores. The hexagonal organizing of cylindrical mesopores observed by SAXS is confirmed by TEM also.

The analysis of samples before reaction performed by high resolution TEM identified the interplanar distances corresponding to different crystal planes of CoSiO₃ in the samples prepared from both precursors. A reflection attributed to (311) crystal planes of Co₃O₄ is also possible (Fig. 10d) for 5%Co-SBA-15 (acetate) sample.

The results from TEM analysis are in accordance with above-mentioned assumptions of the formation of a silicate phase, both in the samples obtained from nitrates and those from acetate. When acetate is used as a precursor, the formation of Co₃O₄ is also suggested. Puskas et al. [17] consider that silicates are formed during TPR experiment. Our data show that they are already formed during calcination, because the TEM measurements are performed on fresh samples, after calcination.

Based on the results discussed in this section it can be concluded that:

The hexagonal mesopore ordering is maintained upon the addition of the active phase and it is not changed during the combustion reaction. Co species are located in the mesopores and that their shrinkage occurs after the treatment at 450°C aiming to decompose the cobalt precursor. The drop in the pore volume and specific surface area is more pronounced when nitrate is used as precursor instead of acetate. The usually strong decrease in the surface area of the samples prepared from

nitrate could be attributed to silica migration and weak bond formation between CoO and SiO₂ [17].

Finely dispersed and weakly interacting with the support Co₃O₄ and CoSiO₃ are formed on the surface of cobalt samples prepared from both precursors. On the surface of the sample obtained from acetate, particles of finely dispersed CoO and silicate-like phases are also formed. Co²⁺ and Co³⁺ are present simultaneously on the surface of 5%Co-SBA-15 (nitrate), with the concentration of the latter being very low. Co²⁺ is only species on the surfaces of the catalysts prepared from acetate.

When the Co–Mn catalyst is prepared from a mixed acetate solution, the finely divided oxides (cobalt, manganese, and mixed Co-Mn), which is reduced at a lower temperature are formed.

### 3.3. Catalytic activity

Fig. 12 A and B illustrates the temperature dependence of the complete oxidation of butane and *n*-hexane over the mono- and bi-component catalysts. On all samples under investigation, H₂O and CO₂ were the only reaction products.

The order of activity is as follows: 2.5 % Co-2.5 % Mn-SBA-15 (acetate) > 5 % Co-SBA-15 (acetate) > 5 % Co-SBA-15 (nitrate). In order to explain the difference in the catalytic behavior, the rate constant for both reactions and for all studied sample are calculated (Table 4).

A comparison of the rate constants for the butane and *n*-hexane oxidation reactions at a fixed temperature shows that the values for the 5%Co-SBA15-(acetate) sample are more than twice as high as those for the 5%Co-SBA15-(nitrate) sample. The higher catalytic activity can be explained by the higher specific surface area of 5%Co-SBA15-(acetate), which helps to increase the access of the reactants to the active sites. Another factor is the formation, although in a small amount, of an easily reducible Co₃O₄ phase. One of the possible explanations for the lowest activity for a nitrate prepared sample is the complete blocking of the pores up to 6 nm. The blocking influences the access of the reagents to the catalytic surface, which leads to a decrease in the catalytic activity.

As can be seen from Fig. 12 the most active catalyst in both investigated reactions is the two-component one. This is in line with our previous studies [10,34,35] according to which the supported on SiO₂ or

<figure><img src="image_11.png" /><figcaption>**Fig. 10.** Bright Field (BF) TEM image of 5%Co-SBA-15 (acetate) sample (a), HRTEM images (c and d), the corresponding SAED patterns with phase identification (c), according Crystallography Open Database (COD) Entry #96–810–3719 for CoSiO₃ and Entry #96–900–5901 for Co₃O₄.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Fig. 11. BF TEM image of 5 %Co-SBA-15 (nitrate) sample (a), the corresponding SAED patterns (d) and HRTEM images (e and f) with phase identification according to Crystallography Open Database (COD) Entry #96–810–3719 for CoSiO₃.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 12. Temperature dependence of A. butane and B. *n*-hexane combustion.</figcaption></figure>

SBA-15 bicomponent CoMn catalysts are more active than the mono-component one. It is well known that in most cases the oxidation of VOCs in the presence of oxide catalysts proceeds through Mars-van Krevelen mechanism [36]. This mechanism was established in our previous study concerning mixed cobalt-manganese catalysts supported on SiO₂ [35]. The Mars-van Krevelen mechanism states that the solid oxidizes the VOCs and that the organic molecule's introduction of oxygen species comes from the crystal lattice. As a result, the lattice oxygen mobility and the catalyst reducibility can be linked to catalytic behavior. Oxygen mobility in the lattice correlates with the reducibility of catalysts. Data from TPR measurements show the lowest reduction temperature of Co-Mn sample which means a correspondingly higher mobility of oxygen in the oxide lattice. The low crystallinity for the bicomponent sample can be suggested from the XRD patterns, and as a consequence, an increase in surface oxygen species can be assumed. In accordance with Morales [37], oxygen can be adsorbed onto vacancies resulting from structural defects linked to low crystallinity in the oxide. Calculated reaction parameters (Table 4) also confirm the highest activity of the bi-component sample in both studied reactions.

## 4. Conclusion

The catalysts' performance of mono-component Co and bi-component Co-Mn catalysts supported on SBA-15 prepared by different precursors, in combustion of butane and *n*-hexane demonstrate high selectivity to H₂O and CO₂.

The results from the instrumental methods showed that the hexagonal mesopore ordering is preserved upon adding of the active phase and is not changed throughout the combustion reaction. Co species are located in mesopores. When nitrate has been used as the precursor rather than acetate, the decrease in the pore volume and specific surface area is more noticeable.

Depending on the precursor used, different types of oxide phases are formed on the surface of the mesoporous support. Finely divided and weakly interacting with the support Co₃O₄ and CoSiO₃ are formed on the surface of cobalt samples prepared from both precursors. In addition to the mentioned two phases finely dispersed CoO and a silica-like phase are formed on the surface of the one-component sample prepared from cobalt nitrate.

The observed highest catalytic activity for bi-component Co-Mn sample in both studied reaction is explained with formation of easy-to-
reduce oxide phases.

## Author contributions

S.T., J.-L.B., B.L., A.N., and R.V.: results analysis, manuscript preparation; conceptualization and discussion; A.N. and R.V. catalytic test; H.K. performed and discussed XPS analysis; D. K.: HRTEM measurements and analysis; B.L., L.R. and J.-L.B.: BET, SAXS, and XRD analysis; K.T. H₂-TPR measurements; B.G. sample synthesis; S.T., J.-L.B. supervision; S.T., A.N., and R.V. funding acquisition. All authors contributed to discussion of the manuscript. All authors have read and agreed to the published version of the manuscript.

## CRediT authorship contribution statement

**Lionel Richaudeau:** Data curation. **Hristo Kolev:** Writing – original draft, Data curation. **Ralitsa Velinova:** Writing – review & editing, Writing – original draft, Investigation, Data curation. **Daniela Karashanova:** Writing – original draft, Data curation. **Bozhidar Grahovski:** Investigation, Data curation. **Krasimir Tenchev:** Formal analysis, Data curation. **Silviya Zhivova Todorova:** Writing – review & editing, Writing – original draft, Project administration, Methodology, Investigation, Funding acquisition, Conceptualization. **Jean-Luc Blin:** Writing – review & editing, Writing – original draft, Methodology, Conceptualization. **Bénédicte Lebeau:** Writing – review & editing, Writing – original draft, Project administration, Funding acquisition, Conceptualization. **Anton Naydenov:** Writing – review & editing, Writing – original draft, Investigation, Conceptualization.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

The authors thank to the bilateral projects BULGARIA – FRANCE (Contracts Nº KP-06-Rila/1 of Bulgarian Science Fund) for financial support.

Research equipment of Distributed Research Infrastructure INFRA-MAT, part of the Bulgarian National Roadmap for Research Infrastructure, supported by the Bulgarian Ministry of Education and Science, was used in this investigation.

## Data availability

No data was used for the research described in the article.

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