Effect of active metal loading on catalyst structure and performance in room temperature oxidation of acetone by ozone

Fundamental insights and recent advances in catalytic oxidation processes using ozone for the control of volatile organic compounds

The development of technologies for highly efficient treatment of emissions containing low concentrations of volatile organic compounds (VOCs) remains an important challenge. Catalytic oxidation with ozone (catalytic ozonation) is useful for the oxidative decomposition of VOCs, particularly aromatic compounds, under ambient temperature conditions. Only inexpensive transition metal oxides are required as catalysts, and Mn-based catalysts are widely used for catalytic ozonation. This review describes the oxidation reaction mechanisms, reaction pathways of aromatic hydrocarbons, and dependence of the catalytic ozonation activity on the reaction conditions. The reasons why Mn oxides are effective in catalytic ozonation are also explained. The structure of the catalytic active sites and the types of supporting materials contributing to the reaction are also discussed in detail, with the aim of establishing a VOC control technology. In addition, recent progress in catalytic oxidation processes using ozone as an oxidant has been outlined, focusing on catalyst materials and reaction conditions.

Catalytic ozonation of multi-VOCs mixtures over Cr-based bimetallic oxides catalysts from simulated flue gas: Effects of NO/SO2/H2O

Different Cr-based bimetallic oxides were prepared, and their catalytic performance was evaluated on the simultaneous removal of multi-VOCs mixtures (acetone, benzene, toluene, and o-xylene) by ozonation. Among them, Co-Cr catalyst stood out in catalytic ozonation of aromatic VOCs, and its activity on acetone conversion was promoted by raising the temperature and ozone concentrations, owing to lower crystallization, larger surface area, excellent redox and VOCs/CO2 desorption ability. Above 95% conversion of all multi-VOCs was achieved over the Co-Cr catalyst when the temperature was 100 °C and an excess ozone ratio λ (the ratio of actual moles of ozone to theoretical moles of ozone needed) was equal to 3. A competitive relationship was noticed during the removal process of four multiple VOCs, with significant inhibition of acetone conversion in the presence of aromatic VOCs, conceivably due to adsorption competition and byproducts accumulation. Effects of NO/SO2/H2O and respective reversibility were also investigated. The inhibition effects of NO/SO2/H2O on aromatic VOCs were far less than those on acetone. Further, the retarding effect of NO was reversible, attributing to physical adsorption competition, but the inhibition effect of SO2/H2O was irreversible, due to the blockage of active sites for VOCs removal. With the combination of scrubbing, multi-VOCs and NO/SO2 could be removed by catalytic ozonation simultaneously and efficiently. In-situ DRIFTS measurement was also conducted to investigate the adsorption and catalytic ozonation process of multi-VOCs mixtures, as well as under the presence of SO2/H2O, discovering the major intermediates, surface carboxylates and carboxylic acids.

Low-temperature degradation of toluene over Ag-MnOx-ACF composite catalyst

Volatile organic compounds (VOCs) have caused a serious threat to the atmosphere and human health. Therefore, it is of great significance to exploit effective catalytic materials for the safe and effective catalytic elimination of VOCs. Herein, Ag-MnO_x-ACF composite catalysts were constructed via a two-step impregnation strategy and used for catalytic toluene degradation. A remarkable low-temperature catalytic activity (T₁₀₀= 50℃), excellent stability, as well as CO₂ selectivity (80%) were achieved over the Ag-MnO_x-ACF catalyst. A series of characterizations indicated that the unique manganese defects structure of birnessite phase manganese oxide played an essential role for toluene oxidation, which was conducive to generating surface adsorbed oxygen. The higher ratio of Mn³⁺/Mn⁴⁺, abundant surface adsorbed oxygen and highly dispersed Ag species were determined to significantly facilitate toluene degradation. The mechanism of efficient degradation of toluene at low temperature was proposed. O₃ and H₂O molecules were activated via surface hydroxyl and Mn defects on Ag-MnO_x-ACF to produce sufficient •OH, enhancing the degradation performance of toluene. We provide a new idea for the catalytic oxidation of benzene VOCs at low even room temperatures.

Review on Catalytic Oxidation of VOCs at Ambient Temperature

As an important air pollutant, volatile organic compounds (VOCs) pose a serious threat to the ecological environment and human health. To achieve energy saving, carbon reduction, and safe and efficient degradation of VOCs, ambient temperature catalytic oxidation has become a hot topic for researchers. Firstly, this review systematically summarizes recent progress on the catalytic oxidation of VOCs with different types. Secondly, based on nanoparticle catalysts, cluster catalysts, and single-atom catalysts, we discuss the influence of structural regulation, such as adjustment of size and configuration, metal doping, defect engineering, and acid/base modification, on the structure-activity relationship in the process of catalytic oxidation at ambient temperature. Then, the effects of process conditions, such as initial concentration, space velocity, oxidation atmosphere, and humidity adjustment on catalytic activity, are summarized. It is further found that nanoparticle catalysts are most commonly used in ambient temperature catalytic oxidation. Additionally, ambient temperature catalytic oxidation is mainly applied in the removal of easily degradable pollutants, and focuses on ambient temperature catalytic ozonation. The activity, selectivity, and stability of catalysts need to be improved. Finally, according to the existing problems and limitations in the application of ambient temperature catalytic oxidation technology, new prospects and challenges are proposed.

Low Temperature Ozonation of Acetone by Transition Metals Derived Catalysts: Activity and Sulfur/Water Resistance

Different transition metals (Cr/Fe/Mn/Co) derived catalysts supported on γ-Al2O3 were prepared by the isovolumetric impregnation method for catalytic ozonation of acetone (C3H6O), and their catalytic activities under industrial complex conditions were investigated. Among them, CrOx/γ-Al2O3 catalyst with Cr loading of 1.5%, abbreviated as Cr1.5%, achieved the best activity, benefitting from its larger surface area, larger proportion of Cr6+/Cr, more chemically desorbed oxygen species Oβ, appropriate acidity, and superiority of low-temperature reducibility. Simulated industrial conditions were used to investigate the applicability of Cr1.5% catalysts for catalytic ozonation of acetone. Results illustrated that the optimum temperature range was 120–140 °C, with molar ratio O3/C3H6O > 6. Different C3H6O initial concentrations had less effect over the activity of Cr1.5% catalysts, with little residual ozone, confirming the applicability of Cr1.5% catalysts in industrial application. The effects of sulfur/water vapor on catalytic activity were also investigated, and satisfactory resistance to sulfur or water vapor individually was obtained. Finally, in-situ DRIFTS measurement was carried out, to explore and illustrate mechanisms of acetone catalytic ozonation pathways and sulfur/water poisoning.

Normal temperature catalytic degradation of toluene over Pt/TiO2

Normal temperature catalytic ozonation is an effective method for the removal of volatile organic compounds (VOCs). A series of TiO₂-supported noble metal catalysts were synthesized by a facile impregnation method. The as-prepared catalysts were evaluated for the catalytic oxidation of toluene. It was determined that the 1 wt%Pt/TiO₂ exhibited outstanding performance that 65% conversion of toluene was achieved with the space velocity of 30,000 h^-1 even at room temperature (25°C). The structure-activity relationship of various catalysts was investigated via BET, XRD, SEM, TEM as well as XPS. The results indicated that the uniform dispersion of Pt nanoparticles, abundant surface adsorbed oxygen species as well as the strong interaction between Pt and TiO₂ favoured toluene degradation at normal temperature. Based on FT-IR, a simplified reaction scheme was proposed: toluene was first oxidized to benzoate species then alcohol species, ketones, carboxyl acids, which was finally degraded into CO₂ and H₂O. The low activation energy of 1 wt%Pt/TiO₂ determined to be 47 kJ mol^-1 also benefited for toluene degradation at ambient temperature.

Removal of VOCs by Ozone: n-Alkane Oxidation under Mild Conditions

Volatile organic compounds (VOCs) have a negative effect on both humans and the environment; therefore, it is crucial to minimize their emission. The conventional solution is the catalytic oxidation of VOCs by air; however, in some cases this method requires relatively high temperatures. Thus, the oxidation of short-chain alkanes, which demonstrate the lowest reactivity among VOCs, starts at 250–350 °C. This research deals with the ozone catalytic oxidation (OZCO) of alkanes at temperatures as low as 25–200 °C using an alumina-supported manganese oxide catalyst. Our data demonstrate that oxidation can be significantly accelerated in the presence of a small amount of O3. In particular, it was found that n-C4H10 can be readily oxidized by an air/O3 mixture over the Mn/Al2O3 catalyst at temperatures as low as 25 °C. According to the characterization data (SEM-EDX, XRD, H2-TPR, and XPS) the superior catalytic performance of the Mn/Al2O3 catalyst in OZCO stems from a high concentration of Mn2O3 species and oxygen vacancies.

Effective toluene oxidation under ozone over mesoporous MnOx/γ-Al2O3 catalyst prepared by solvent deficient method: Effect of Mn precursors on catalytic activity

In this study, the role of manganese precursors in mesoporous (meso) MnOx/γ-Al₂O₃ catalysts was examined systematically for toluene oxidation under ozone at ambient temperature (20 °C). The meso MnOx/γ-Al₂O₃ catalysts developed with Mn(CH₃COO)_2, MnCl₂, Mn(NO₃)₂.4H₂O and MnSO₄ were prepared by an innovative single step solvent-deficient method (SDM); the catalysts were labeled as MnO_x/Al₂O₃(A), MnO_x/Al₂O₃(C), MnO_x/Al₂O₃(N), and MnOx/Al₂O₃(S), respectively. Among all, MnO_x/Al₂O₃(C) showed superior performance both in toluene removal (95%) as well as ozone decomposition (88%) followed by acetate, nitrate and sulphated precursor MnO_x/Al₂O₃. The superior performance of MnO_x/Al₂O₃(C) in the oxidation of toluene to CO_x is associated with the ozone decomposition over highly dispersed MnO_x in which extremely active oxygen radicals (O^2-, O₂^2- and O^-) are generated to enhance the oxidation ability of the catalysts greatly. In addition, toluene adsorption over acid support played a vital role in this reaction. Hence, the properties such as optimum Mn³⁺/Mn⁴⁺ ratio, acidic sites, and smaller particle size (≤2 nm) examined by XPS, TPD of NH₃, and TEM results are playing vital role in the present study. In summary, the MnO_x/Al₂O₃ (C) catalyst has great potential in environmental applications particularly for the elimination of volatile organic compounds with low loading of manganese developed by SDM.