Toluene oxidation over mesoporous TiO2 in a combined process of wet-scrubbing and UV-catalysis

Ce/N @BC prepared based on plant metallurgy strategy: A novel activator of peroxymonosulfate for the degradation of sulfamethoxazole

A novel carbon catalyst was created based on plant metallurgy strategy for organic pollutants removal. Plants rich in CeO2 NPs in water were used as carbon precursors and pyrolyzed with urea to obtain Ce/N co-doped carbon catalysts, which were used in the degradation of sulfamethoxazole (SMX) by active peroxymonosulfate (PMS). The results showed that the Ce/N @BC/PMS system achieved to 94.5% degradation of SMX in 40 min at a rate constant of 0.0602 cm-1. The activation center of PMS is widely dispersed Ce oxide nanocrystals, and CeO2 NPs promote the formation of oxygen centered PFR with enhanced catalytic ability and longer half-life. In addition, N-doping facilitates the transfer of π-electrons within the sp2 carbon of biochar, increasing active sites and thus improving PMS activation efficiency. The degradation process was contributed to by both radical and non-radical activation mechanisms including 1O2 and direct electron transfer, with O2•- serving as 1O2's precursor. Through the DFT calculations, LC-MS and toxicological analyses, the degradation pathway of pollutants and the toxicity changes throughout the entire degradation process were further revealed, indicating that the degradation of SMX could effectively reduce ecological toxicity.

An efficient process for aromatic VOCs degradation: Combination of VUV photolysis and photocatalytic oxidation in a wet scrubber

The elimination of volatile organic compounds (VOCs) via vacuum ultraviolet (VUV) photolysis is greatly limited by low removal efficiency and gaseous byproducts generation, while photocatalytic oxidation of VOCs suffers from catalytic deactivation. Herein, a coupled process of gaseous VUV photolysis with aqueous photocatalytic oxidation with P25 as the catalyst was firstly proposed for efficient aromatic VOCs removal (VUV/P25). The removal efficiency of toluene reached 86.2% in VUV/P25 process, but was only 33.6% and 58.1% in alone gaseous VUV photolysis and aqueous ultraviolet photocatalytic oxidation (UV/P25) process, respectively. Correspondingly, the outlet CO₂ concentration in VUV/P25 process reached 132 ppmv. Toluene was firstly destructed by high-energy photons generated from gaseous VUV photolysis, resulting in its incomplete oxidation to form soluble intermediates including acids, aldehydes, esters. These soluble intermediates would be further degraded and mineralized into CO₂ in subsequent aqueous UV/P25 process. Notably, the concentrations of intermediates in VUV/P25 were much lower than those in VUV photolysis, indicating the synergy effect of VUV photolysis and UV/P25 process. The stability tests proved that VUV/P25 process maintained an excellent toluene degradation performance and P25 did not suffer from catalytic deactivation. In addition to toluene, the VUV/P25 system also achieved the efficient and sustainable degradation of styrene and chlorobenzene, suggesting its good application prospect in industrial VOCs treatment. This study proposes an efficient and promising strategy for deep oxidation of multiple aromatic VOCs in industries.

Elucidating the role of confinement and shielding effect over zeolite enveloped Ru catalysts for propane low temperature degradation

Volatile organic compounds (VOCs) are the main precursor for ozone formation and hazardous to human health. Light alkane as one of the typical VOCs is difficult to degrade to CO₂ and H₂O by catalytic degradation method due to its strong C-H bond. Herein, a series of ultrafine Ru nanoclusters (<0.95 nm) enveloped in silicalite-1 (S-1) zeolite catalysts were designed and prepared by a simple one-pot method and applied for catalytic degradation of propane. The results demonstrate that the enveloped Ru₁@S-1 catalyst has excellent propane degradation performance. Its T₉₅ is as low as 294 °C with moisture, and the turnover frequency (TOF) value is up to 5.07 × 10^-3 s^-1, evidently higher than that of the comparison supported catalyst (Ru₁/S-1). Importantly, Ru₁@S-1 exhibits superior thermal stability, water resistance and recyclability, which should be attributed to the confinement and shielding effect of the S-1 shell. The in-situ DRIFTS result reveals that the propane degradation over Ru₁@S-1 follows the Mars-van-Krevelen (MvK) mechanism, where the hydroxy from the framework of zeolite can provide the active oxygen species. Our work provides a new candidate and guideline for an efficient and stable catalyst for the low-temperature degradation of the light alkane VOCs.

Effects of sulfur poisoning on physicochemical properties and performance of MnO2/AlNi-PILC for toluene catalytic combustion

AlNi pillared clay (AlNi-PILC) was synthesized firstly, and then MnO₂ was supported via wetness impregnation from nitrate precursors. Sulphation was performed by in-situ decomposing ammonium sulfate with different concentrations over MnO₂/AlNi-PILC. Catalysts before and after sulfur poisoning were characterized by XRD, N₂ adsorption/desorption, HRTEM, XPS, H₂-TPR and NH₃-TPD. MnO₂/AlNi-PILC exhibited high catalytic activity, allowing the complete toluene combustion. Structure of the catalyst was obviously damaged after sulfur poisoning. (001) crystal plane strength of AlNi-PILC was decreased significantly. Meanwhile, the specific surface area and pore volume reduced with increase of sulfate concentration. Sulfur species were readily formed on the surface of poisoned catalyst and deposited in the pore structure of AlNi-PILC, which resulted in significant impacts on the structural stability, acidity and the number of active species. These changes were responsible for the decreased catalytic performance.

Recent advances in the chemical oxidation of gaseous volatile organic compounds (VOCs) in liquid phase

The chemical oxidation of gaseous volatile organic compounds (VOCs) in liquid phase may possess great advantages in its high removal efficiency, mild conditions, good reliability, wide applicability, and little potential secondary pollution, which has aroused extensive research interests in the past decade. This Overview Article summarizes the latest achievements to eliminate VOCs by chemical oxidation in liquid phase including gas-liquid mass transfer, homogeneous/heterogeneous oxidation, electrochemical oxidation, and coupling technologies. Important research contributions are highlighted in terms of mass transfer, catalytic materials, removal/mineralization efficiency, and reaction mechanism to evaluate their potential industrial applications. The current challenges and future strategies are discussed from the viewpoint of the deep degradation of refractory VOC substrates and their intermediates. It is anticipated that this review will attract more attention toward the development and application of chemical oxidation methods to clear complex industrial organic exhaust gas.

Recent progresses in the synthesis of MnO2 nanowire and its application in environmental catalysis

Nanostructured MnO₂ with various morphologies exhibits excellent performance in environmental catalysis owing to its large specific surface area, low density, and adjustable chemical properties. The one-dimensional MnO₂ nanowire has been proved to be the dominant morphology among various nanostructures, such as nanorods, nanofibers, nanoflowers, etc. The syntheses and applications of MnO₂-based nanowires also have become a research hotspot in environmental catalytic materials over the last two decades. With the continuous deepening of the research, the control of morphology and crystal facet exposure in the synthesis of MnO₂ nanowire materials have gradually matured, and the catalytic performance also has been greatly improved. Differences in the crystalline phase structure, preferably exposed crystal facets, and even the length of the MnO₂ nanowires will evidently affect the final catalytic performances. Besides, the modifications by doping or loading will also significantly affect their catalytic performances. This review carefully summarizes the synthesis strategies of MnO₂ nanowires developed in recent years as well as the influences of the phase structure, crystal facet, morphology, dopant, and loading amount on the catalytic performance. Besides, the cutting-edge applications of MnO₂ nanowires in the field of environmental catalysis, such as CO oxidation, the removal of VOCs, denitrification, etc., have been also summarized. The application of MnO₂ nanowire in environmental catalysis is still in the early exploratory stage. The gigantic gap between theoretical investigation and industrial application is still a great challenge. Compared with noble metal based traditional environmental catalytic materials, the lower cost of MnO₂ has injected new momentum and promising potential into this research field.

This review summarizes the synthesis strategies for MnO₂ nanowire and the influences of the phase structure, crystal facet, metal doping, and interface effect on its performance in various environmental catalysis processes.

Efficient α-MnO2 with (2 1 0) facet exposed for catalytic oxidation of toluene at low temperature: A combined in-situ DRIFTS and theoretical investigation

The α-MnO₂ catalysts with (1 1 0), (2 1 0) and (3 1 0) crystal facets exposed were prepared via hydrothermal method and studied for the catalytic oxidation of toluene. Some characterization technologies and DFT theoretical calculation were combined to analyze the as-synthesized catalysts. The α-MnO₂ catalyst exposed with the (2 1 0) plane displayed best catalytic performance and attained complete toluene conversion at 140 °C. The O₂-TPD and XPS results exhibited the amount of surface lattice oxygen on α-MnO₂-210 catalyst was largest. Lower accumulation and faster disintegration of intermediates which was characterized by in-situ DRIFTS could be discovered on the surface of α-MnO₂-210 catalyst. The results of DFT calculation showed that the unique atomic arrangement of α-MnO₂-210 catalyst enhanced the charge separation and conversion, promoting the formation of active oxygen and the activation of toluene. The E_a of α-MnO₂-210 catalyst was 24.75 kJ mol^-1, lowest among the three catalysts. This work highlights the facet effects on catalytic property and provides new insight into the understanding of catalytic oxidation reaction mechanism of toluene.

Solar oxidation of toluene over Co doped nano-catalyst

Cobalt (Co) co-doped TiO₂ photo-catalysis were synthesized, characterized and tested toward solar photocatalytic oxidation of toluene (TOL). A multi-technique approach was used to characterize and relate the photo-catalytic property to photo-oxidation performance. Adding Co to TiO₂ significantly changed crystal size and surface morphology (surface area, pore-volume, and pore size), reduced the bandgap energy of TiO₂ and improved the solar photo-oxidation of TOL. Up to 96.5% of TOL conversion (%TN_conv) was achieved by using Co-TiO₂ compared with 28.5% with naked TiO₂. The maximum %TN_conv was achieved at high hydraulic retention time (HRT) ≥ 100 s, Co content in the photo-catalyst of 5 wt% and relative humidity (%RH) of 50%. The mechanism of TOL solar oxidation was related to the concentration of OH• and •O₂^-. radicals produced from the generated electrons and holes on the surface of Co-TiO₂. The products formed during the photo-catalytic oxidation of TOL were mainly CO₂ and water, and minor concentration of benzene and benzaldehyde. Overall, the Co-TiO₂ could be used as a potential photo-catalyst for the oxidation of toluene in gas-phase streams on an industrial scale.