Anna Rokicińska

Patrycja Majerska

Marek Drozdek

Sebastian Jarczewski

Laetitia Valentin

Jianhong Chen

Adam Slabon

Stanislaw Dzwigaj
email: stanislaw.dzwigaj@upmc.fr

Piotr Kuśtrowski
email: piotr.kustrowski@uj.edu.pl

Impact of Mn addition on catalytic performance of Cu/SiBEA materials in total oxidation of aromatic volatile organic compounds

Keywords: catalytic combustion, volatile organic compounds, copper-containing catalysts, manganese-containing catalysts, BEA zeolite

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Many various technologies for the elimination of volatile organic compounds (VOCs), including adsorption, absorption and chemical transformation, have been developed because of the harmful nature of these organic pollutants [START_REF] Zhang | Low-temperature catalysis for VOCs removal in technology and application: A state-of-the-art review[END_REF]. However, the most efficient and economical way to remove VOCs emitted to the atmosphere at low concentrations seems to be catalytic combustion. This method allows for complete decomposition of VOCs already at low temperatures, therefore it does not lead to the formation of secondary impurities (such as NO x or dioxins) [START_REF] Zhang | Low-temperature catalysis for VOCs removal in technology and application: A state-of-the-art review[END_REF][START_REF] Kuśtrowski | Abatement of Volatile Organic Compounds Emission as a Target for Various Human Activities Including Energy Production[END_REF]. Due to a large variety of compounds classified as VOCs, different types of catalysts are used in the catalytic combustion. They usually contain an active phase based on precious metals or transition metal oxides. Noble metal-containing catalysts (e.g. Pt, Pd, Rh, Au, Ag) are extremely active in the total oxidation of VOCs even at low temperatures [START_REF] Liotta | Catalytic oxidation of volatile organic compounds on supported noble metals[END_REF][START_REF] Piumetti | Mesoporous manganese oxides prepared by solution combustion synthesis as catalysts for the total oxidation of VOCs[END_REF]. A disadvantage of these materials is, however, a high price of production and sensitivity to poisoning. Alternative metal oxide-based catalysts are much cheaper and stable, but exhibit lower efficiency compared to the noble metal-containing materials [START_REF] Li | Effect of Mn substitution on the promoted formaldehyde oxidation over spinel ferrite: Catalyst characterization, performance and reaction mechanism[END_REF]. The most commonly used oxide systems for the total oxidation of VOCs are: CuO [START_REF] Boycheva | Studies on non-modified and copper-modified coal ash zeolites as heterogeneous catalysts for VOCs oxidation[END_REF][START_REF] Natkański | Physicochemical properties of hydrogel template synthesized copper(II) oxide-modified clay influencing its catalytic activity in toluene combustion[END_REF], MnO x [START_REF] Shu | Catalytic oxidation of VOCs over Mn/TiO 2 /activated carbon under 185 nm VUV irradiation[END_REF][START_REF] Zhang | Influence of hydrothermal synthesis temperature on the redox and oxygen mobility properties of manganese oxides in the catalytic oxidation of toluene[END_REF], Cr 2 O 3 [START_REF] Somekawa | VOC decomposition over a wide range of temperatures using thermally stable Cr 6+ sites in a porous silica matrix[END_REF],

NiO [START_REF] Park | Toluene oxidation catalyzed by NiO/SiO 2 and NiO/TiO 2 /SiO 2 : Towards development of humidity-resistant catalysts[END_REF][START_REF] Jeong | Toluene combustion over NiO nanoparticles on mesoporous SiO 2 prepared by atomic layer deposition[END_REF], Fe 2 O 3 [START_REF] Jacquemin | Catalytic properties of beta zeolite exchanged with Pd and Fe for toluene total oxidation[END_REF][START_REF] Xie | Wet scrubber coupled with heterogeneous UV/Fenton for enhanced VOCs oxidation over Fe/ZSM-5 catalyst[END_REF] and Co 3 O 4 [START_REF] Li | Fabrication of mesoporous Co 3 O 4 oxides by acid treatment and their catalytic performances for toluene oxidation[END_REF][START_REF] Rokicińska | Co 3 O 4 -pillared montmorillonite catalysts synthesized by hydrogel-assisted route for total oxidation of toluene[END_REF][START_REF] Tian | Catalytic oxidation of hydrocarbons over Co 3 O 4 catalyst prepared by CVD[END_REF]. Two-component catalysts containing Co-Pt [START_REF] Todorova | Particle size and support effects on the complete benzene oxidation by Co and Co-Pt catalysts[END_REF][START_REF] Hu | Synthesis of a Highly Active and Stable Pt/Co 3 O 4 Catalyst and Its Application for the Catalytic Combustion of Toluene[END_REF], Co-Ce [START_REF] Todorova | Co 3 O 4 + CeO 2 /SiO 2 Catalysts for n-Hexane and CO Oxidation[END_REF][START_REF] Zhang | Cooperative Effect of the Ce-Co-O x for the Catalytic Oxidation of Toluene[END_REF], Co-Mn [START_REF] Wang | Template-free Scalable Synthesis of Flower-like Co 3-x Mn x O 4 Spinel Catalysts for Toluene Oxidation[END_REF][START_REF] Tang | Porous Mn-Co mixed oxide nanorod as a novel catalyst with enhanced catalytic activity for removal of VOCs[END_REF], Cu-Mn [START_REF] Vu | Catalytic oxidation of volatile organic compounds on manganese and copper oxides supported on titania, Reactors[END_REF][START_REF] Morales | Total oxidation of ethanol and propane over Mn-Cu mixed oxide catalysts[END_REF], Mn-Ce [START_REF] Tang | Preparation of hierarchical layer-stacking Mn-Ce composite oxide for catalytic total oxidation of VOCs[END_REF][START_REF] Luo | MnO 2 nanoparticles encapsuled in spheres of Ce-Mn solid solution: Efficient catalyst and good water tolerance for low-temperature toluene oxidation[END_REF], Cu-Ce [START_REF] Chen | Facile and Flexible Preparation of Highly Active CuCe Monolithic Catalysts for VOCs Combustion[END_REF][START_REF] Wang | A Novel Approach to High-Performance Aliovalent-Substituted Catalysts-2D Bimetallic MOF-Derived CeCuO x Microsheets[END_REF] or Ni-Mn [START_REF] Dong | Synergy of Mn and Ni enhanced catalytic performance for toluene combustion over Ni-doped α-MnO 2 catalysts[END_REF] were also described in the literature. They often showed higher catalytic activity compared to that observed for individual components. This beneficial effect is related to enhancement of mobility of lattice oxygen and/or facilitating the transport of electrons on the surface of catalyst.

Manganese was frequently added as the promoter of active phase for the VOCs combustion. It was found that its presence improves redox properties of a resulting oxide system. This effect is associated with the specific electronic configuration ([Ar] 3d 5 4s 2 ), which opens up the possibility of creating oxide systems with Mn atoms at different oxidation states, including MnO, Mn 3 O 4 , Mn 2 O 3 , and MnO 2 [START_REF] Xu | Gaseous Heterogeneous Catalytic Reactions over Mn-Based Oxides for Environmental Applications: A Critical Review[END_REF]. Depending on the type of support the catalytic activity of MnO x in the total oxidation of VOCs changes in the order: γ-Al 2 O 3 > SiO 2 > TiO 2 [START_REF] Jung | Effects of calcination and support on supported manganese catalysts for the catalytic oxidation of toluene as a model of VOCs[END_REF]. Furthermore, Santos et al. [START_REF] Santos | The role of lattice oxygen on the activity of manganese oxides towards the oxidation of volatile organic compounds[END_REF] revealed a relationship between redox properties and catalytic activity of MnO x . Easier reducible Mn 3 O 4 shows a significantly better catalytic performance than Mn 2 O 3 or MnO 2 with strongly bound lattice oxygen [START_REF] Piumetti | Mesoporous manganese oxides prepared by solution combustion synthesis as catalysts for the total oxidation of VOCs[END_REF][START_REF] Xu | Gaseous Heterogeneous Catalytic Reactions over Mn-Based Oxides for Environmental Applications: A Critical Review[END_REF][START_REF] Kim | Catalytic combustion of VOCs over a series of manganese oxide catalysts[END_REF][START_REF] Lyu | Catalytic removal of toluene over manganese oxide-based catalysts: a review[END_REF]. This suggests that the VOCs combustion over the MnO x catalysts takes place according to the Mars van Krevelen mechanism, i.e. an organic molecule is first oxidized by a lattice oxygen, which is then supplemented by gaseous O 2 [START_REF] Zhang | Influence of hydrothermal synthesis temperature on the redox and oxygen mobility properties of manganese oxides in the catalytic oxidation of toluene[END_REF]. Therefore, the availability and reactivity of surface oxygen species have a significant impact on the activity of MnO x [START_REF] Piumetti | Mesoporous manganese oxides prepared by solution combustion synthesis as catalysts for the total oxidation of VOCs[END_REF].

Mixed Cu-Mn oxides in bulky form [START_REF] Cao | Effect of loading content of copper oxides on performance of Mn-Cu mixed oxide catalysts for catalytic combustion of benzene[END_REF][START_REF] Aguilera | Cu-Mn and Co-Mn catalysts synthesized from hydrotalcites and their use in the oxidation of VOCs[END_REF][START_REF] Li | Self-Propagated Flaming Synthesis of Highly Active Layered CuO-δ-MnO 2 Hybrid Composites for Catalytic Total Oxidation of Toluene Pollutant[END_REF][START_REF] Luo | Copper modified manganese oxide with tunnel structure as efficient catalyst for low-temperature catalytic combustion of toluene[END_REF][START_REF] Xiao | Facile synthesis of homogeneous hollow microsphere Cu-Mn based catalysts for catalytic oxidation of toluene[END_REF] as well as deposited on various inorganic supports (e.g. Al 2 O 3 [START_REF] Wang | Enhanced catalytic toluene oxidation by interaction between copper oxide and manganese oxide in Cu-O-Mn/γ-Al 2 O 3 catalysts[END_REF][START_REF] Saqer | Catalytic oxidation of toluene over binary mixtures of copper, manganese and cerium oxides supported on γ-Al 2 O 3[END_REF][START_REF] Behar | New synthesis of nanosized Cu-Mn spinels as efficient oxidation catalysts[END_REF], TiO 2 [START_REF] Behar | New synthesis of nanosized Cu-Mn spinels as efficient oxidation catalysts[END_REF], ZrO 2 [START_REF] Behar | New synthesis of nanosized Cu-Mn spinels as efficient oxidation catalysts[END_REF] or mesoporous MCM-41 silica [START_REF] Li | Catalytic combustion of toluene on CuMn/MCM-41 catalysts: Influence of calcination temperature and operating conditions on the catalytic activity[END_REF]) were tested as catalysts in the total oxidation of aromatic VOCs. Wang et al. [START_REF] Wang | Enhanced catalytic toluene oxidation by interaction between copper oxide and manganese oxide in Cu-O-Mn/γ-Al 2 O 3 catalysts[END_REF] found that appropriate interaction between Cu and Mn introduced onto an Al 2 O 3 surface, which is influenced by a Cu/Mn molar ratio, results in enhanced activity. The highest conversions of toluene were achieved over the catalyst with the Cu/Mn molar ratio of 0.66. However, the optimal value of this parameter determined by Kim et al. [START_REF] Kim | Oxidation of toluene on γ-Al 2 O 3 supported copper-manganese catalysts[END_REF] for the same type of the support was 1.3. The observed differences were most likely related to the dispersion of active phase affecting the formation of various surface Cu-Mn species. On the other hand, the co-existence of Cu and Mn leads to an appearance of large number of surface defects, which together with increasing amount of active components enhance the catalytic performance in the benzene combustion [START_REF] Cao | Effect of loading content of copper oxides on performance of Mn-Cu mixed oxide catalysts for catalytic combustion of benzene[END_REF].

The aim of this work is to discuss the catalytic performance of individual Cu and Mn oxides as well as bimetallic oxide systems with various Cu/Mn ratios deposited on the surface of dealuminated form of BEA zeolite (SiBEA) in the total oxidation of toluene. SiBEA was selected as the support due to its stability and presence of relatively wide channels which enable homogeneous distribution of the active phase over the whole surface (both inner and external) and possibility of work at elevated temperatures (without coking which would be intensified by the presence of Brønsted acid sites). In order to better understand the influence of properties of catalysts on their activity, a wide physicochemical characterization was performed, including determination of bulk and surface composition, phase composition, porosity, acidity, reducibility as well as the chemical nature and dispersion of the metal oxide active phase. The characterization measurements were also carried out for the materials after the catalytic process, proving that the reaction follows the Mars-van Krevelen mechanism, and the coexistence of Cu + /Cu 2+ and Mn 3+ /Mn 4+ pairs within one spinel structure allows for an increase in oxygen mobility, as well as a significant improvement in catalytic activity and stability.

Experimental

Catalyst preparation

Before the introduction of the active phase, the parent tetraethylammonium BEA (TEABEA) zeolite was dealuminated by treatment in a nitric acid solution (13 mol L -1 ) at 80 °C for 4 h, washed several times with distilled water and dried at 95 °C overnight. Then, the obtained siliceous BEA zeolite (SiBEA) with atomic Si/Al ratio of ca. 1000 was contacted with an aqueous solutions of copper nitrate (Cu(NO 3 ) 2 x 6 H 2 O), copper nitrate (Cu(NO 3 ) 2 x 6 with appropriate concentration of Cu and Mn (1 g of zeolite in 200 mL of solution) and stirred at 25 °C for 24 h. The concentrations of the metals salts were changed in such a way that from 2 to 10 wt % metal loading in the final catalysts obtained. Subsequently, the suspension was transferred to an evaporator and stirred under vacuum of a water pump at 60 °C until water was removed. The samples calcined at 550 °C for 3 h in air were labeled CuxMnySiBEA, where x refers to Cu wt % content, and y refers to Mn wt % content in the prepared solid.

Characterization

Chemical composition of samples was determined by X-ray fluorescence (XRF) using an ARL Quant'x (Thermo Scientific) spectrometer.

X-ray diffraction (XRD) patterns were collected in a D2 Phaser (Bruker) diffractometer with a CuKα X-ray radiation (λ = 1.54056 nm) in a 2 range of 5-90° with a step of 0.02°. UV-vis-DR spectra were recorded at ambient atmosphere on a Cary 5000 Varian spectrometer equipped with a double integrator with polytetrafluoroethylene as a reference.

Textural properties of the materials were determined by low-temperature adsorption of nitrogen at -196 °C using a Micromeritics ASAP 2020 sorptometer. Before the measurements, samples were outgassed at 250 °C for 5 h under vacuum. Specific surface areas (S BET and S Langmuir ) were calculated using the Brunauer-Emmett-Teller (BET) and Langmuir theory, respectively. Micropore volumes (V micro ) were determined using the t plot model, whereas total pore volumes (V total )the single point approach taking into account amounts of N 2 adsorbed at relative pressures p/p 0  0.99.

High-angle annular dark-field (HAADF) images, elemental mappings and EDX spectra were collected on a JEOL-2100F microscope in scanning transmission electron microscopy (STEM) mode. In order to eliminate an influence of copper element from a grid, SiO grids were used in this work. X-ray photoelectron spectroscopy (XPS) analyses were carried out in a system constructed by Prevac, equipped with a hemispherical analyzer (VG SCIENTA R3000) and a monochromatized aluminum source AlKα (E = 1486.6 eV). To compensate a charge on a surface of nonconductive samples, a low energy electron flood gun (FS40A-PS) was used.

Binding energies in the measured spectra were corrected by referring to a position of Si 2p peak (E b = 103.0 eV). Deconvolution was done after fitting the Shirley background using a peak shape described by the mixed function of Gauss and Lorentz (GL = 30). The spectra were processed using the CasaXPS software.

Temperature-programmed reduction (H 2 -TPR) measurements were performed in a home-made equipment. An amount of 0.05 g of a sample was placed in a quartz microreactor in a flow of Ar/H 2 mixture (95/5). The total flow rate of the reducing mixture was 22 mL min - 1 . The H 2 -TPR profiles were collected at a linear increase in temperature (heating rate of 10 °C•min -1 ) within the range of 100-900 °C using a thermal conductivity detector placed at the reactor outlet directly after a cold trap (-40 ±3 °C) used to remove produced water. The amount of hydrogen consumed during the reduction reaction was calculated based on calibration with CuO as a reference.

Acidic properties of samples were assessed by adsorption of pyridine (Py) followed by infrared spectroscopy (IR). The samples were pressed at ~1 ton cm -2 into thin wafers of ca. 10 mg cm -2 . Prior to the pyridine adsorption/desorption experiments, the wafers were placed inside an infrared cell and activated under static conditions in O 2 (1.6 10 4 Pa) at 450 °C for 3 h and outgassed at 300 °C (10 -3 Pa) for 1 h. Subsequently, pyridine was introduced (133 Pa) into the cell at room temperature. The IR spectra were collected on a Bruker Vector 22 spectrometer (resolution 2 cm -1 , 128 scans) after outgassing at 150 °C for 1 h. The resulting spectra were obtained after subtraction of the spectrum recorded before pyridine adsorption.

The quantification of acid sites was carried out as reported by Emeis [START_REF] Emeis | Determination of Integrated Molar Extinction Coefficients for Infrared Absorption Bands of Pyridine Adsorbed on Solid Acid Catalysts[END_REF].

Catalytic testing

Catalytic combustion of toluene and benzene was conducted in a quartz microreactor with an inner diameter of 8.0 mm. A catalyst (0.1 g, particle size 160-315 μm) was inserted into the middle of reactor on a quartz wool plug. Before a catalytic run, the catalyst was degassed at 500 °C for 30 min in flowing air (100 mL•min -1 ). Then, the reactor was cooled to 200 °C and dosing aromatic hydrocarbon at concentration of 1000 ppm began. The organic substrate was introduced into the feed using a thermostated scrubber filled with liquid compounds kept at temperature ensuring its vapor pressure at the assumed level. The catalytic activity was studied at 200, 250, 275, 300, 325, 350, 400, 450 and 500 °C. The reactor was maintained at each temperature during three analyzes of reaction products (ca. 70 min), and then heated to a higher temperature at a heating ramp of 10 °C•min -1 . The reaction products were analyzed using a Bruker 450 gas chromatograph equipped with two capillary columns (Porapak S and Chromosorb WAW-DMCS), two flame ionization detectors, a thermal conductivity detector and a methanizer.

In addition, stability tests in the toluene combustion were carried out maintaining identical experimental conditions with the only difference that after degassing the reactor was cooled to 300 °C. Then, at this temperature toluene dosing was turned on (optionally 5 vol% of water vapor was additionally introduced into the feed using a syringe pump), and the reaction products were analyzed periodically during the next 24 h.

Results and discussion

Structural and textural properties of the Cu-Mn/BEA catalysts

BEA-supported catalysts containing different copper to manganese molar ratios for the total oxidation of VOCs were synthesized by the conventional wet impregnation method.

Regardless of the amount of transition metal(s) introduced, the reflections attributed to the BEA structure (among them the most intense diffraction lines are centered at 7.5° (101), 21.4° (300), 22.4° (302), 25.3° (304) and 27.1° (006) [PDF 00-056-0467]) are identified in the XRD patterns of all calcined materials (Fig. 1) [START_REF] Higgins | The framework topology of zeolite beta[END_REF]. In addition to the above mentioned reflections, in the XRD patterns the features typical of metal oxide phases are found. For the samples lacking a second transition metal, i.e. Cu10SiBEA and Mn10SiBEA, the diffraction lines typical of the CuO or Mn 2 O 3 phases, respectively, are observed. For the sample containing 10 wt% of Cu, the (002) and ( 111 It should be assumed that, as in previously presented papers [START_REF] Wang | Shape evolution of Cu-doped Mn 3 O 4 spinel microcrystals: influence of copper content[END_REF][START_REF] Xiong | The role of the Cu dopant on a Mn 3 O 4 spinel SCR catalyst: Improvement of low-temperature activity and sulfur resistance[END_REF], for the mixed oxide systems with a Cu/Mn molar ratio < 0.5 the Cu-doped Mn 3 O 4 phase with the tetragonal hausminnite structure is formed due to the Jahn-Teller distortion. Its high dispersion, as in the case of Mn 2 O 3 in the Mn10SiBEA sample, causes that no intense diffraction lines attributed to this phase are noticeable in the XRD pattern collected for Cu2Mn8SiBEA. After the introduction of higher amount of Cu, the distortion of spinel lattice disappears. Therefore, the two-component, Cu-rich catalysts (Cu4Mn6SiBEA, Cu5Mn5SiBEA, Cu6Mn4SiBEA, and Cu8Mn2SiBEA) show the reflections at ca. 36.0°, 57.8° and 63.5° corresponding to the hkl Due to the very small size of the crystallites (estimated using the Scherrer equation based on the position of the (311) reflection) within the range from 2.7 nm (Cu8Mn2SiBEA) to 8.7 nm (Cu6Mn4SiBEA), it is difficult to precisely determine the lattice parameters. Nevertheless, it can be noticed that they differ slightly between the samples (a = b = c = 0.828±0.001 nm), but no clear correlation between the calculated values and the relative content of Cu and Mn is found. These discrepancies are rather expected, because in the (Cu x Mn 3-x ) 1-δ O 4 spinel structure the metal cations can be incorporated in different positions at various oxidation states (i.e. Cu + , Cu 2+ , Mn 2+ , Mn 3+ , Mn 4+ ). As determined by the N 2 adsorption measurements, the starting SiBEA material exhibits a type I isotherm (according to the IUPAC classification) characteristic for microporous materials (Fig. 4). The formally calculated S BET value for this support is 686 m 2 /g (S Langmuir = 910 m 2 /g) and the V total -0.740 cm 3 /g (Table 1). It should be noted that only ca. 35% of the V total is generated by micropores. The remaining pores are mainly large interparticle voids, whose presence is manifested by an increase in the amount of adsorbed nitrogen observed at the highest relative pressures (p/p 0 > 0.9). The introduction of the transition metal-containing active phase results in a decrease in the S BET and the S Langmuir by 22-31 %. Simultaneously, the V micro drops by 24-33% and the V total by 32-40%. Therefore, it is clearly visible that the formed oxide crystallites partially block the pores of the support. However, they affect more significantly the macropores than the micropores, indicating a preferential location of the oxide crystallites in the interparticle spaces. Such a conclusion is rational due to the sizes of these crystallites significantly higher than the pore diameters in the SiBEA zeolite.

Surface acidity of the Cu-Mn/BEA catalysts

Acidity of the samples was studied by FT-IR after adsorption of pyridine (Fig. 5). charge of the framework is counterbalanced by H + ions attached to lattice oxygen atoms connecting adjacent silicon and aluminum atoms. Such protons play a role of Brønsted acid sites. The investigated SiBEA zeolite was initially subjected to the dealumination process, which in a consequence resulted in an efficient elimination of majority of H + ions from the BEA structure. Therefore, in the FTIR spectrum of SiBEA the band at 1545 cm -1 , typical of pyridine molecules interacting with the Brønsted acid sites, is not observed. The absence of this band is also noticeable for SiBEA modified with Cu and/or Mn. On the other hand, for the parent SiBEA zeolite the narrow intense band at 3736 cm -1 related to isolated internal silanol groups and of the broad band at 3530 cm -1 due to H-bonded SiOH groups are observed (Fig. 5). The latter band reveals the formation of vacant T-atom sites associated with silanol groups, in line with earlier data for SiBEA zeolites [START_REF] Dzwigaj | Incorporation of vanadium species in a dealuminated β zeolite[END_REF][START_REF] Dzwigaj | Role of silanol groups in the incorporation of V in β zeolite[END_REF][START_REF] Trejda | Methanol oxidation on VSiBEA zeolites: Influence of V content on the catalytic properties[END_REF]. Upon impregnation of the SiBEA zeolite with the copper nitrate and manganese nitrate solutions as well as co-impregnation of both solutions the intensity of the silanol bands at 3736 and 3520 cm -1 is reduced, particularly that at 3520 cm -1 corresponding to the H-bonded SiOH groups (Fig. 5), suggesting that the silanol groups interact with the copper and manganese species [START_REF] Dzwigaj | Probing Different Kinds of Vanadium Species in the VSiβ Zeolite by Diffuse Reflectance UV-Visible and Photoluminescence Spectroscopies[END_REF][START_REF] Dzwigaj | Evidence of Three Kinds of Tetrahedral Vanadium (V) Species in VSiβ Zeolite by Diffuse Reflectance UV-Visible and Photoluminescence Spectroscopies[END_REF][START_REF] Hajjar | Two kinds of framework Al sites studied in BEA zeolite by X-ray diffraction, Fourier Transform Infrared Spectroscopy, NMR techniques, and V probe[END_REF]. The discussed deficiency of Brønsted acidity is compensated in the Cu-and/or Mn-doped samples by the presence of Lewis acid centers. Their relatively high surface concentration is manifested by three bands at 1449-1451 cm -1 , 1488-1490 cm -1 and 1607-1610 cm -1 [START_REF] Barzetti | Pyridine and ammonia as probes for FTIR analysis of solid acid catalysts[END_REF]. A clear correlation between the type of doped metal and the number of the generated Lewis acid centers is found (Table 1). And so, for the unmodified SiBEA support the presence of Lewis acidity is negligible, while the highest numbers of such sites are observed for the Cu-rich materials.

From this point of view, the doping with Mn encourages the appearance of a lower number of Lewis acid centers. It should be kept in mind that gaseous oxygen (being typical Lewis base) participating in the catalytic oxidation process can be easily adsorbed on the Lewis acid centers and converted to various reactive surface species, such as O 2-, O 2 2-and O - [START_REF] Niu | Preparation of metal-doped Cu-Mn/HTS-1 catalysts and their mechanisms in efficient degradation of toluene[END_REF].

Coordination of transition metal active sites in the Cu-Mn/BEA catalysts

Coordination of transition metal species dispersed on the surface of BEA zeolite was examined by UV-vis-DR spectroscopy.

Figure 6. UV-vis-DR spectra of SiBEA before and after deposition of the Cu-Mn containing active phase.

In the spectra presented in Figure 6, features typical of the ligand-to-metal chargetransfer (CT) transitions between the surface O 2-and metal ions are observed at low wavelengths. The CT band is more distinct for the Cu-rich samples, where it appears around 280 nm with a shoulder at higher wavelengths due to the formation of Cu 2+ -O 2--Cu 2+ clusters and highly dispersed CuO nanocrystallites. An increase in the Mn content leads to widening of this band as well as a loss of its intensity. Obviously, the Mn-containing species are not so highly ordered as copper-containing ones, which may be due to the presence of many low crystalline Mn oxide and hydroxide forms with varying degrees of dispersion. Similar conclusions can be drawn after analyzing the recorded UV-vis-DR spectra in the range of d-d transitions (above 600 nm). For the Mn10SiBEA material no intense band is found there, because the d-d transitions originated from Mn n+ occur below 500 nm. On the other hand, the UV-vis-DR spectrum of Cu10SiBEA shows a wide band centered at 820 nm, which can be attributed to the 2 E g → 2 T 2g spin-allowed transitions of Cu 2+ cations situated in the distorted octahedral symmetry of CuO [START_REF] Kondratowicz | Novel CuO-containing catalysts based on ZrO 2 hollow spheres for total oxidation of toluene[END_REF]. For the samples with coexisting Mn, another band is distinguished at ca. 680 nm. Considering the appearance of (Cu x Mn 3-x ) 1-δ O 4 spinel phase in these samples (confirmed by XRD), the presence of this band confirms the location of Cu 2+ in the tetrahedral sites of this partly inverse spinel structure [START_REF] Wąskowska | CuMn 2 O 4 : properties and the high-pressure induced Jahn-Teller phase transition[END_REF]. Unfortunately, multiplet splitting observed for the Mn 2p peak excludes the reliable discussion of Mn oxidation states. The Mn 3s region, often used for this purpose [START_REF] Ilton | XPS determination of Mn oxidation states in Mn (hydr)oxides[END_REF], is not also helpful in resolving this issue due to the low concentration of Mn in the tested samplesthe intensity of the measured Mn 3s signals is hardly distinguished from the background level.

Therefore, the most important information about the state of the surface can be obtained from the analysis of XPS Cu 2p spectra consisting two main regions (Cu 2p 3/2 and Cu 2p 1/2 ) due to spin-orbit coupling (Fig. 7A). In the Cu 2p 3/2 region of the Cu10SiBEA and Cu8Mn2SiBEA catalysts, two peaks at 932.8 ± 0.1 eV and 935.0 ± 0.3 eV (attributed to Cu 2+ ions surrounded by O 2-and OH -ions, respectively) with very intense satellite features at ~942 eV and 944 eV are found [START_REF] Kondratowicz | Novel CuO-containing catalysts based on ZrO 2 hollow spheres for total oxidation of toluene[END_REF]. An increase in the Mn content results in a gradual decrease in the number of superficial hydroxyl-bound Cu 2+ species (Fig. 7B) and an appearance of additional peak at 930.5 eV corresponding to Cu + [START_REF] Zhou | Porous Cu-Mn-O catalysts fabricated by spray pyrolysis method for efficient CO oxidation[END_REF]. The presence of Cu 0 was excluded based on the analysis of the L 3 M 4.5 M 4.5 Auger spectra [START_REF] Biesinger | Advanced analysis of copper X-ray photoelectron spectra[END_REF]. The described finding confirms that both Cu 2+ and Cu + ions co-exist in the created (Cu x Mn 3-x ) 1-δ O 4 spinel phase. The latter ones appear together with Mn 4+ as a result of redox equilibrium Cu 2+ + Mn 3+ = Cu + + Mn 4+ , which was previously discussed by Papavasiliou et al. [START_REF] Papavasiliou | Combined steam reforming of methanol over Cu-Mn spinel oxide catalysts[END_REF]. The highest content of Cu + is reached for the Cu5Mn5SiBEA sample with equimolar participation of Cu and Mn in the active phase (Fig. 7B). In Figure 8A the H 2 -TPR profiles recorded for the SiBEA-supported catalysts are shown. Evidently, the samples containing only one type of transition metal (Cu or Mn) differ significantly in their reducibility. The reduction of Cu10SiBEA begins at a temperature as low as 130 °C and reaches its maximum rate at ca. 250 °C. This process is so fast that usually postulated two stages of CuO reduction (Cu 2+ → Cu + followed by Cu + → Cu 0 [START_REF] Cao | Effect of loading content of copper oxides on performance of Mn-Cu mixed oxide catalysts for catalytic combustion of benzene[END_REF][START_REF] Tabakova | CO and VOCs Catalytic Oxidation Over Alumina Supported Cu-Mn Catalysts: Effect of Au or Ag Deposition[END_REF]) overlap to give one reduction peak. Nevertheless, on its high-temperature side at about 280 °C, a shoulder appears, which indicates the presence of harder reducible Cu forms. The latter reduction effect most likely comes from the Cu n+ ions and CuO x clusters present in the non-framework positions in the channels and on the surface of zeolite. These fine moieties are stabilized by interactions with the negatively charged zeolite framework. On the other hand, the reduction profile of the Mn10SiBEA sample is much more complex. Generally, the reduction of manganese phases requires definitely higher temperatures than for the coppercontaining species and starts at ca. 180 °C. Two maxima at higher temperatures (425 °C and 520 °C) are attributed to the two-stage reduction of Mn 2 O 3 (Mn 2 O 3 → Mn 3 O 4 followed by Mn 3 O 4 → MnO), as previously observed by Yang et al. [START_REF] Yang | Enhancement effect of acid treatment on Mn 2 O 3 catalyst for toluene oxidation[END_REF]. In turn, the reduction peak at 360 °C shows that much easier reducible, surficial Mn specimens are also present on the Mn10SiBEA surface.

Reducibility of the

In the case of the preparations containing Cu and Mn simultaneously, the reduction starts at a temperature similar to that found for Cu10SiBEA. However, the main reduction peaks gradually widen and move to higher temperatures. It can be noticed that the amount of easily reducible Cu species decreases with the increasing total Cu loading. Their presence is manifested by the shoulder at approx. 230 °C observed mainly for the samples with the low Cu loadings. This effect is supported by analyzing the initial rate of H 2 consumption versus inverse temperature (Figure 8B), which is the highest for Cu2Mn8SiBEA. Nevertheless, it can be concluded that the presence of manganese strongly stabilizes the oxidation state of copper ions (Cu 2+ and Cu + ), which occupy the positions in the Mn 3 O 4 and/or Cu x Mn 3-x ) 1-δ O 4 structures as confirmed by XRD and XPS studies. On the other hand, the reduction of Mn n+ ions becomes noticeably easier compared to that of Mn 2 O 3 dominating in the Mn10SiBEA catalyst. Suffice it to say that for all copper-manganese samples the reduction process finishes essentially below 450 °C. The analogous effect promoting the reduction of Mn n+ ions in the environment of Cu n+ ions coexisting in one spinel phase was previously described by other researchers [START_REF] Morales | Total oxidation of ethanol and propane over Mn-Cu mixed oxide catalysts[END_REF][START_REF] Papavasiliou | Combined steam reforming of methanol over Cu-Mn spinel oxide catalysts[END_REF]. This mutual interaction of Mn n+ and Cu n+ ions changing the reduction mechanism can also be important for catalytic activity.

Catalytic activity of the Cu-Mn/BEA catalysts in VOCs combustion

The developed SiBEA-based materials were tested as catalysts in the total oxidation of toluene, representing a wide group of aromatic VOCs. Figure 9A shows the toluene conversions as a function of reaction temperature. After the introduction of transition metal (Cu and/or Mn), the catalytic activity of SiBEA zeolite increases drastically. In the case of these catalysts, the toluene conversion is observed already at 250 °C, which is in strong contrast to the parent SiBEA (showing low activity starting from 400 °C). This catalytic parameter increases with raising the reaction temperature. An influence of the type and content of the transition metal on the toluene conversion is clearly visible. At lower temperatures (250 and 275 °C) it correlates with the Cu content -the higher Cu loading, the higher toluene conversion (but not higher than 11%). However, an interesting effect is noted at 300 °C, where the catalysts are grouped into those containing a single transition metal and others containing Cu and Mn simultaneously. Double promotion has a clear beneficial effect on the catalytic activity in the toluene combustion. In a narrow temperature window (up to The positive effect of mixing Mn 3+ and Cu 2+ cations in one oxide structure deposited on SiBEA is related to the possible electron migration between both metal sites. High B electronegativity of copper promotes the electron transfer from manganese, and the redox cycle of Cu 2+ /Cu + activating oxygen appears [START_REF] Zhang | Effect of the A-site cation over spinel AMn 2 O 4 (A = Cu 2+ , Ni 2+ , Zn 2+ ) for toluene combustion: Enhancement of the synergy and the oxygen activation ability[END_REF]. The formed oxygen vacancies, which facilitate adsorption of oxygen species, remain stable on the surface of active phase above 275 °C [START_REF] Hu | Toluene catalytic combustion over copper modified Mn 0.5 Ce 0.5 O x solid solution sponge-like structures[END_REF]. Generally, manganese oxides show tendency to the formation of surface defects, which can be additionally induced by chemical treatment (e.g., acid leaching [START_REF] Yang | Enhancement effect of acid treatment on Mn 2 O 3 catalyst for toluene oxidation[END_REF]). Such vacancies can adsorb and activate gaseous oxygen, and subsequently improve mobility of formed surficial oxygen species. Most importantly, the adsorbed oxygen species are more reactive in the toluene combustion and easier to be replenished compared to lattice oxygen [START_REF] Lyu | Catalytic removal of toluene over manganese oxide-based catalysts: a review[END_REF]. The involvement of lattice oxygen is very often considered in the total oxidation of VOCs in the so-called Mars van-Krevelen mechanism [START_REF] Xiao | Facile synthesis of homogeneous hollow microsphere Cu-Mn based catalysts for catalytic oxidation of toluene[END_REF][START_REF] Liu | The promoting effect of S-doping on the NH 3 -SCR performance of MnO x /TiO 2 catalyst[END_REF]. According to this mechanism, which is also assumed for the catalysts studied in this work, in the first step toluene molecules react with oxygen species built into the structure of the active phase surface. During the combustion reaction, CO 2 and H 2 O molecules as well as reduced active sites are produced. In the second step, the reduced active sites react with O 2 molecules present in the gas phase reactivating the catalyst. For such a mechanism to be valid, a transition metal oxide phase must not interact too strongly with a support. This arrangement is obtained when using dealuminated BEA zeolite.

The presence of Lewis acid sites in the Cu-rich samples additionally promotes the appearance of reactive forms of chemisorbed oxygen, which are more strongly stabilized under reducing conditions in the Cu-Mn/SiBEA samples compared to Cu/SiBEA, as confirmed by the results of H 2 -TPR studies. Hence, the catalysts based on mixed Cu-Mn systems show excellent catalytic properties at higher reaction temperatures (> 275 °C), in which the surface of one component Cu10SiBEA is deeply reduced and its regeneration with molecular oxygen is hindered. The presence of two kinds of Jahn-Teller ions (Mn 3+ and Cu 2+ ) in the spinel structure additionally facilitates solid state charge transfer redox system (Cu 2+ + Figure 10. Comparison of high-resolution XPS Cu 2p and C 1s spectra for fresh and spent

Cu6Mn4SiBEA. The samples after the reaction of toluene combustion stopped at 275 or 300 °C were immediately transferred to the XPS spectrometer.

If we look at selectivity of the tested Cu-Mn/SiBEA catalysts, we find that next to the expected products of complete oxidation of toluene (CO 2 and H 2 O), the formation of minor amounts of benzene is observed, which occurs especially at low temperatures (Fig. 9B). CO and other by-products formation is negligible. The appearance of benzene as a main byproduct is reasonable due to the formation of benzoyl species (postulated as the former step of the toluene combustion mechanism) that may crack. At low temperatures, when the rate of consecutive steps is not high enough, probability of their decomposition is relatively high.

Therefore, for all Cu-loaded SiBEA catalysts the presence of benzene among products is observed only below 300 °C (for less active Mn10SiBEA and SiBEA benzene is produced also at higher temperatures). The selectivity to benzene is lower, the catalyst is more active in the toluene combustion. In addition, the developed Cu-loaded SiBEA catalysts are also active in the combustion of benzene, and therefore C 6 H 6 released as a result of the decomposition of benzoyl groups may be burned after re-adsorption in a further part of the catalytic bed.

To recognize the catalytic activity of the studied materials in the total oxidation of benzene, we performed tests for this reaction under analogous conditions as in the case of the toluene combustion. The obtained results in the form of benzene conversion for three selected catalysts are shown in Figure 11. A correlation of catalytic activity with the copper content is found within the whole operating temperature window. The reaction starts at ca. 250 °C and accelerates with increasing temperature reaching 100% conversion around 450 °C. Thus, it can be concluded that benzene molecules are more difficult to oxidize over the tested catalysts compared to toluene molecules. This effect is quite reasonable due to different reaction mechanisms. It is generally believed that the presence of a methyl group in a toluene molecule facilitates oxidation of an aromatic ring compared to a benzene molecule, in which an aromatic ring has to be dissociated directly [START_REF] Genuino | Gas-Phase Total Oxidation of Benzene, Toluene, Ethylbenzene, and Xylenes Using Shape-Selective Manganese Oxide and Copper Manganese Oxide Catalysts[END_REF]. Finally, we studied stability of the SiBEA-supported catalysts in the toluene combustion performed at 300°C. In Figure 12 changes in the toluene conversion during 24 h time-onstream are compared for two chosen samples -Cu5Mn5SiBEA and Cu10SiBEA. test, but it quickly reaches a stable level just slightly lower (by about 3%) than that achieved in the environment without a significant excess of water vapor.

Conclusion

The developed modification method allows obtaining high dispersion of Cu and/or Mn oxide phases on a BEA zeolite support. By changing the Cu/Mn ratio in the mother solution,

  ) diffraction lines characteristic for the monoclinic structure of CuO nanoparticles appear at 35.6° and 38.8°, respectively [PDF 00-041-0254]. In turn, in the case of the Mn10SiBEA catalyst the peak at 32.9° is indexed as the (222) reflection of the cubic Mn 2 O 3 phase [PDF 00-041-1442]). Other reflections most likely exhibit too low intensity to be clearly detected in the recorded diffractograms, that may indicate a high degree of manganese oxide dispersion and its low crystallinity.

Figure 1 .

 1 Figure 1. X-ray diffraction patterns of SiBEA before and after deposition of the Cu-Mn

Figure 2 .

 2 Figure 2. HAADF survey of Cu2Mn8SiBEA, corresponding elemental mappings and EDX

Figure 3 .

 3 Figure 3. HAADF survey of Cu8Mn2SiBEA, corresponding elemental mappings and EDX

Figure 4 .

 4 Figure 4. N 2 adsorption-desorption isotherms of SiBEA before and after deposition of the Cu-

Figure 5 .

 5 Figure 5. FT-IR spectra of the SiBEA-based materials pre-adsorbed with pyridine.

Figure 7 .

 7 Figure 7. High-resolution XPS Cu 2p spectra (A) and distribution of various surface Cu forms

B

  photoemission from the core levels of all elements distributed in the structure (i.e., Si, O, Cu

Figure 9 .

 9 Figure 9. Catalytic performance of SiBEA before and after deposition of the Cu-Mn

Figure 11 .

 11 Figure 11. Conversion of benzene over Cu10SiBEA, Cu5Mn5SiBEA and Mn10SiBEA vs.

Figure 12 .

 12 Figure 12. Stability tests in toluene combustion over Cu5Mn5SiBEA and Cu10SiBEA at

  materials containing pure CuO, pure Mn 2 O 3 or mixed oxide systems, mainly of the (Cu x Mn 3- x ) 1-δ O 4 spinel structure, were obtained. The introduction of the transition metal oxide particles into the BEA porous structure resulted in only slight decrease in texture parameters (specific surface area, volume of micropores, total pore volume) of the support. The observed changes in porosity in combination with the average size of the formed oxide phase particles evidently suggest the location of the active phase nanocrystallites in interparticle voids of BEA. The parent BEA zeolite was initially dealuminated, therefore the support used for modification was essentially free of acid sites. Nevertheless, the introduction of the transition metal oxide phases led to the appearance of Lewis acid centers, the concentration of which increased with raising Cu content. During the thermal treatment, Cu 2+ and Mn 3+ cations exhibiting the Jahn Teller effect were partially transformed into more stable Cu + and Mn 4+ ions. Their presence guaranteed stabilization of the system under reducing conditions, which, in combination with the presence of numerous surface vacancies, resulted in the high catalytic activity and stability of the developed catalysts in the total oxidation of toluene. of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-12-023/08).

 C Table 1 .

 C1 Transition metal content, textural properties and concentration of acid sites for the studied BEA-based materials.

						Textural parameters		Lewis acidity
		Mn	Cu	Cu/Mn			t-plot
	Sample	content [wt %]	content [wt %]	molar ratio	S BET [m 2 /g]	S Langmuir [m 2 /g]	external surface area	V micro [cm 3 /g]	V total [cm 3 /g]	PyL ca. 1450 cm -1 [µmol/g]
							[m 2 /g]
	SiBEA	-	-	-	686	910	102	0.258	0.740	0.00
	Cu10SiBEA	0.0	9.8	-	530	706	87	0.195	0.499	150.96
	Cu8Mn2SiBEA	2.4	7.7	2.88	483	630	78	0.174	0.501	n.m.
	Cu6Mn4SiBEA	3.5	6.1	1.56	477	634	77	0.176	0.446	n.m.
	Cu5Mn5SiBEA	5.3	4.5	0.76	507	675	81	0.188	0.496	104.04
	Cu4Mn6SiBEA	5.6	4.2	0.67	509	675	75	0.192	0.499	n.m.
	Cu2Mn8SiBEA	8.7	1.9	0.20	499	663	75	0.188	0.502	n.m.
	Mn10SiBEA	10.0	-	-	504	670	77	0.189	0.475	68.20

Cu-Mn/BEA catalysts

  Figure 8. H 2 -TPR profiles (A) and initial H 2 consumption rate as a function of inverse temperature (B) for SiBEA before and after deposition of the Cu-Mn containing active phase.

			A					SiBEA Cu10SiBEA
								Cu8Mn2SiBEA
								Cu6Mn4SiBEA
								Cu5Mn5SiBEA
	signal [a.u.]						Cu4Mn6SiBEA Cu2Mn8SiBEA Mn10SiBEA
	TCD
		100	200	300	400	500	600	700	800	900
					Temperature [°C]
	Cu×s)]	10 11						Cu10SiBEA Cu8Mn2SiBEA
	-4 ×mol/(mol	7 8 9						Cu6Mn4SiBEA Cu5Mn5SiBEA Cu4Mn6SiBEA Cu2Mn8SiBEA
	consumption [10	3 4 5 6
	H 2	2
	Initial	0 1	B
		2.10	2.12	2.14	2.16	2.18		2.20	2.22
						1000/T [K -1 ]

x ) 1-δ O 4 cubic phase [PDF 01-076-2296].

°C), complete conversion of the aromatic substrate is achieved over all transition metalloaded catalysts.

Acknowledgements

A.R. would like to thank for the financial support obtained under the French Government Scholarship program. The research was partially carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework

Mn 3+ = Cu + + Mn 4+ ) [START_REF] Behar | New synthesis of nanosized Cu-Mn spinels as efficient oxidation catalysts[END_REF]. Therefore, in the temperature window of catalytic performance up to 300 °C the valency of the surface Cu is almost completely preserved, as confirmed by the XPS measurements carried out for the Cu6Mn4SiBEA catalyst after the toluene combustion process interrupted at 275 or 300 °C (Fig. 10). In both spent samples the intense component at 930.5 eV assigned to Cu + is observed. On the other hand, an increase in the participation of Cu 2+ -OH -species is found, especially after the reaction at 275 °C. Thus, surface hydroxylation occurs under the moisture-rich reaction conditions. Furthermore, slightly higher contents of surface carbon appear in these samples after the catalytic run (Fig. 10). The fresh Cu6Mn4SiBEA catalyst contains 0.9 at.% of C, whereas after the reaction this amount increases to 3.7 at.% (the process stopped at 275 °C) or 2.8 at.% (the process stopped at 300 °C). This is not surprising, since the oxidation of toluene most likely undergoes through several steps: (i) adsorption of an aromatic hydrocarbon molecule onto an active site with the formation of benzoyl species by abstraction of H from a methyl group, (ii) transformation of benzoyl into intermediates, such as benzyl alcohol, benzaldehyde, and benzoate species, (iii) reaction with oxygen species to produce intermediates without aromatic ring, such as maleic anhydride and carboxylate species, and finally (iv) the deepest oxidation to CO 2 and H 2 O. Some of these intermediates may remain on the surface, creating a relatively small amount of carbon deposit. This reaction mechanism was confirmed by DRIFT studies carried out by various authors for Cu-Mn-O catalysts [START_REF] Zhang | Insight into the enhanced performance of toluene removal from simulated flue gas over Mn-Cu oxides modified activated coke[END_REF][START_REF] Wei | The influence of annealing temperature on copper-manganese catalyst towards the catalytic combustion of toluene: The mechanism study[END_REF][START_REF] Dong | Tuning oxygen vacancy concentration of MnO 2 through metal doping for improved toluene oxidation[END_REF].