Environmental pollution from ozone and wastewaters containing dyes from various industries is an important problem for humanity. In this study, novel composite catalysts based on manganese carbonate ore from the Obrochishte deposit, Bulgaria, were used successfully in two environmentally relevant catalytic processes—the ozone decomposition and photocatalytic decolourization of Malachite Green (MG) dye under UV illumination. Manganese carbonate ore/NiO, manganese oxides, and silver-containing composites were synthesized via co-precipitation, followed by calcination at 500 °C or hydrothermal treatment at 160 °C, and then thermal treatment. The phase and elemental composition, structure, morphology, and textural characteristics of the obtained composites were investigated using powder X-ray diffraction analysis, wavelength-dispersive X-ray fluorescence, Fourier-transform infrared spectroscopy, scanning electron microscopy, nitrogen adsorption–desorption isotherms, and the BET method. The materials exhibit a mesoporous structure. The results established that the thermally treated MnCO₃ ore/NiO, manganese oxides, and Ag-containing composites demonstrate a higher catalytic efficiency for the removal of ozone (85%, 93%, and 99%) in comparison with hydrothermally treated analogues—79%, 66%, and 98%, respectively. The thermally treated manganese carbonate ore/silver-containing composite exhibits the highest photocatalytic ability (83% degree of decolourization of MG dye) compared to the other investigated catalysts.

Trace amounts of CO in H₂-rich gas can poison Pt electrodes in proton-exchange-membrane fuel cells, necessitating selective CO removal. Preferential oxidation of CO (PROX) offers an efficient route to oxidize CO while preserving H₂. Although noble-metal-based catalysts are widely used, their high cost has driven interest in non-precious alternatives. Co₃O₄–CeO₂ catalysts have emerged as particularly promising due to their high activity and stability. Two series of Co–Ce/SiO₂ catalysts were prepared via impregnation: in the first, Ce was introduced and calcined prior to Co deposition; in the second, Co and Ce nitrates were co-deposited from a mixed aqueous solution. The latter method enhances the interaction between Co₃O₄ and CeO₂, increasing the availability of surface oxygen species. Stability tests on the most active sample demonstrated remarkable durability, maintaining near-complete CO conversion over 100 h on dry stream.

Wastewater treatment has stimulated researchers to develop highly efficient catalysts for the degradation of organic pollutants. The present work focuses on the design and investigation of a titanium dioxide (TiO₂)-based photocatalytic system with improved catalytic ability under visible light. TiO₂/reduced graphene oxide (TiO₂/rGO) and squaraine dye (SQ) sensitized TiO₂/rGO (TiO₂/rGO/SQ) composites were synthesized by a facile solvothermal method. The incorporation of SQ aims to enhance the photocatalytic efficiency of the synthesized composites and extend their activity by utilizing the visible-light spectrum. The composite materials were characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), the Brunauer–Emmett–Teller (BET) surface area analysis, and UV–Vis diffuse reflectance spectroscopy (DRS). The synthesized composites were evaluated for the decolorization of the organic dyes Rhodamine B (RhB) and Fluorescein (FL) in aqueous solutions. The synthesized composites were evaluated for the decolorization of the organic dyes Rhodamine B (RhB) and fluorescein (FL) in aqueous solution. Under visible-light irradiation, TiO₂/rGO/SQ achieved 98% decolorization of FL and 86% decolorization of RhB within 180 min.

Tribocatalysis is an emerging approach for the oxidation of organic pollutants using frictional energy. ZnO nanorods were prepared by thermal decomposition of zinc acetate dihydrate and wet-chemically modified with Gd₂O₃, Dy₂O₃, or Er₂O₃. SEM, nitrogen sorption, XRD, ATR-FTIR, Raman spectroscopy, and XPS confirmed the formation of predominantly rod-like ZnO, preservation of its morphology and wurtzite structure after modification, and introduction of nanocrystalline RE₂O₃ phases. Tribocatalytic activity was evaluated under mechanical stirring using the anionic dye Reactive Black 5 (RB5) and the cationic dye Brilliant Green (BG), examining the effects of RE modification, stirring rate, friction pair, scavengers, and catalyst reuse. Removal increased with stirring rate and was consistently higher for RB5. At 800 rpm, Er:ZnO exhibited the highest activity toward both dyes, reaching approximately 81% RB5 removal (k_app ≈ 0.074 h⁻¹) and 56% BG removal (k_app ≈ 0.040 h⁻¹) after 24 h. COD removal reached 53–60% for RB5 and 27–43% for BG, indicating a reduction in the oxidizable organic load consistent with partial mineralization. ATR-FTIR revealed carbonate formation and PTFE transfer to recovered catalysts, highlighting surface changes and secondary contamination. The results demonstrate measurable tribocatalytic activity and show that the benefits of RE modification depend on the target dye.

In the present study the new manganese carbonate ore/copper and cobalt oxide-based catalysts were prepared using co-precipitation followed by calcination at 500◦C or by hydrothermal synthesis at 160◦C and then thermal treatment. The phase composition, structure, morphology, specific surface area and textural characteristics of the obtained composites were characterized by several methods as PXRD analysis, FTIR spectroscopy, SEM, nitrogen adsorption-desorption isotherms, and measurement of specific surface area. The PXRD analysis determined the average crystallite size of 10nm for Co3O4 and MnCo2O4 and 13–23nm for CuO phases. The synthesized composites exhibit mesoporous structure. The highest specific surface area (135m2/g) was established for thermally treated MnCO3 ore/cobalt oxide in comparison with the other investigated samples. The prepared materials were tested in two reactions – ozone decomposition and photocatalytic discolouration of Malachite Green (MG) dye as model pollutant under UV light. The results reveal that the hydrothermally treated MnCO3 ore/cobalt and copper oxide demonstrate higher catalytic ability about removal of ozone (87% and 63%) in comparison with the composites prepared by co-precipitation and calcination – 86% and 54%, respectively. Best results for discolouration of MG dye were achieved with MnCO3 ore/cobalt oxide-based photocatalyst (76%).