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Effective management of volatile organic compounds (VOCs) and carbon monoxide (CO) is critical to human health and the ecological environment. Catalytic oxidation is one of the most promising technologies for achieving efficient VOCs and CO emission control. Platinum group metal (PGM)-free catalysts are recently receiving sustainable attention in catalyzing VOCs and CO removal due to their low cost, superior catalytic activity, and excellent stability, but PGM-free catalysts face challenges in low-temperature catalytic efficiency. In this mini-review, starting with discussing the catalytic mechanism of VOCs and CO oxidation, we summarize the surface/interface modulation strategies of PGM-free catalysts to promote oxygen and VOCs/CO molecule activation for enhanced low-temperature oxidation activity, including oxygen vacancy engineering, heteroatom doping, surface acidity modification, and active interface construction. We highlight the currently remaining challenges and prospects of advanced PGM-free catalyst development for highly efficient VOCs and CO emission control in practical applications.

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Imines are essential intermediates in organic transformations, and is generally produced by dehydrogenative condensation of alcohols and amines with the assist of specialized catalysts and additives. Heterogeneous photocatalysis provides a sustainable platform for such process without the using of toxic oxidants, yet a functionalized photocatalyst with optimized co-adsorption of reactants needs to be developed to promote the stoichiometric oxidative condensation under ambient conditions. Here, we show that benzyl alcohol and aniline adsorb non-interferingly on the Fe node and the linker sites of the MIL-53(Fe) metal organic frameworks (MOFs), respectively. The co-adsorption of both reactants barely influences the reduction of molecular oxygen to generate oxygen radicals, resulting in efficient formation of benzaldehyde under visible light. Additionally, the weak adsorption of water together with surface acidity of the MIL-53(Fe) promote a rapid condensation of benzaldehyde with aniline and the depletion of generated water, achieving an efficient C-N bond creation for a wide range of substrates.

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It is a great challenge to remove VOCs and NOx simultaneously from flue gas in nonelectric industries. This study focuses on the construction of Fe-MnO2 catalysts that perform well in the simultaneous removal of toluene and NOx at low temperatures. Utilizing the Fe-induced phase transition of MnO2, Fe-MnO2-F&R catalysts with a composite morphology of nanoflowers and nanorods were successfully prepared that provided an abundant microporous structure to facilitate the diffusion of molecules of different sizes. Through in-depth investigation of the active sites and reaction mechanism, we discovered that Fe-induced phase transition could modulate the surface acidity of Fe-MnO2-F&R. The higher concentration of surface Mn4+ provided numerous Brønsted acid sites, which effectively promoted the activation of toluene to reactive intermediates, such as benzyl alcohol/benzoate/maleic acid. Simultaneously, Fe provided a large number of Lewis acid sites that anchor and activate NH3 species, thereby inhibiting NH3 nonselective oxidation. Furthermore, additional Brønsted acid sites were generated during the simultaneous reaction process, enhancing toluene activation. Consequently, the simultaneous removal of toluene and NOx was achieved through regulation of the physical structure and the concentration of acidic sites. The present work provides new insights into the rational design of bifunctional catalysts for the synergistic control of VOCs and NOx emissions.

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Achieving no or low polychlorinated byproduct selectivity is essential for the chlorinated volatile organic compounds (CVOCs) degradation, and the positive roles of water vapor may contribute to this goal. Herein, the oxidation behaviors of chlorobenzene over typical Mn-based catalysts (MnO2 and acid-modified MnO2) under dry and humid conditions were fully explored. The results showed that the presence of water vapor significantly facilitates the deep mineralization of chlorobenzene and restrains the formation of Cl2 and dichlorobenzene. This remarkable water vapor-promoting effect was conferred by the MnO2 substrate, which could suitably synergize with the postconstructed acidic sites, leading to good activity, stability, and desirable product distribution of acid-modified MnO2 catalysts under humid conditions. A series of experiments including isotope-traced (D2O and H218O) CB-TPO provided complete insights into the direct involvement of water molecules in chlorobenzene oxidation reaction and attributed the root cause of the water vapor-promoting effect to the proton-rich environment and highly reactive water-source oxygen species rather than to the commonly assumed cleaning effect or hydrogen proton transfer processes (generation of active OOH). This work demonstrates the application potential of Mn-based catalysts in CVOCs elimination under practical application conditions (containing water vapor) and provides the guidance for the development of superior industrial catalysts.

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In this study, a Co3O4 catalyst is synthesised using the chitosan-assisted sol-gel method, which simultaneously regulates the grain size, Co valence and surface acidity of the catalyst through a chitosan functional group. The complexation of the free -NH2 complex inhibits particle agglomeration; thus, the average particle size of the catalyst decreases from 82 to 31 nm. Concurrently, Raman spectroscopy, hydrogen temperature-programmed reduction, electron paramagnetic resonance spectroscopy and X-ray photoelectron spectroscopy experiments demonstrate that doping with chitosan N sources effectively modulates Co2+ to promote the formation of oxygen vacancies. In addition, water washing after catalyst preparation can considerably improve the low-temperature (below 250 °C) activity of the catalyst and eliminate the side effects of alkali metal on catalyst activity. Moreover, the presence of Brønsted and Lewis acid sites promotes the adsorption of C8H8. Consequently, CS/Co3O4-W presents the highest catalytic oxidation activity for C8H8 at low temperatures (R250 °C = 8.33 µmol g-1 s-1, WHSV = 120,000 mL hr-1∙g-1). In situ DRIFTS and 18O2 isotope experiments demonstrate that the oxidation of the C8H8 reaction is primarily dominated by the Mars-van Krevelen mechanism. Furthermore, CS/Co3O4-W exhibits superior water resistance (1- and 2- vol% H2O), which has the potential to be implemented in industrial applications.

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Pt-V bimetallic catalysts maybe promising substitutes to precious metal catalysts for selective catalytic oxidation (SCO) of NH3. But it remains a major challenge for Pt-V bimetallic catalysts to pursue high NH3 conversion rate and N2 selectivity simultaneously. In this work, both Cu and Er were adopted to modify V0.5/Pt0.04/TiO2 catalyst (denoted as V/PT), and the influences of Cu and Er doping amounts on NH3-SCO performance of V/PT catalysts were investigated systematically. The results indicated that the co-modification of Cu and Er imposed little influence on NH3 conversion efficiency, but significantly boosted N2 selectivity. Compared with other Cu-Er-modified V/PT catalysts, CEV/PT-4 catalyst exhibited outstanding NH3-SCO performance, which attained completely 100% NH3 conversion efficiency and > 90% N2 selectivity in the temperature range of 225-450 °C. It was significantly superior to the NH3-SCO performance of most previously reported catalysts. The characterization results indicated that the adequate doping amounts of Cu and Er resulted in an obvious enhancement on redox property and surface acidity of CEV/PT-4 catalyst. It also led to abundant Pt0 and surface chemisorbed oxygen species on catalyst surface, which facilitated the oxidation of NH3 to NOx and enhanced i-SCR reactions. In situ DRIFTS results showed that -NH2 species on the surface of CEV/PT-4 catalyst could actively react with nitrate species to generate N2 and H2O.

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The advanced approach for the preparation of the NH4 form of highly crystalline LSX zeolite under gentle drying conditions (40 °C, membrane pump dynamic vacuum) is discussed. Decationization of this form at moderate temperatures led to the formation of Brønsted acid sites (BASs), whose concentration and strength were characterized by IR spectroscopy. It was found that a maximum concentration of three BASs per unit cell can be achieved at 200 °C prior to the initiation of zeolite structure degradation. The proton affinity of BASs is unusual, and aspires 1240 kJ/mol, which is significantly higher compared to faujasites with higher moduli. The increase in temperature of the heat treatment (up to 300 °C) resulted in thermal decomposition of BASs and the manifestation of amorphous phase with corresponding Lewis acid sites (LASs) as well as terminal Si-OH groups. Both the destruction of BASs and formation of the LAS-containing amorphous phase are the key reasons for the significant decrease in the adsorption capacity in the micropore region revealed for the sample decationized at 300 °C.

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Developing efficient catalysts for the total oxidation of light alkane at low temperatures is challenging. In this study, superior catalytic performance in the total oxidation of light alkane was achieved by modulating the acidity and redox property of a Pt/CeO2 catalyst through phosphorus modification. Surface modification with phosphorus resulted in electron withdrawal from Pt, leading to platinum species with high valency and the generation of Brönsted acid sites, leading to increased acidity of the Pt/CeO2 catalyst. Consequently, the ability of the Pt/CeO2 catalyst to activate the C-H bond increased with increasing P content in the catalyst owing to the synergistic effect of Ptδ+-(CeO2-POx)δ- dipolar catalytic sites. In contrast, the redox property of the Pt/CeO2 catalyst weakened at first; subsequently, it was partially restored owing to the recovery of a part of the bare ceria surface with increasing P content. The turnover frequency in propane oxidation over the phosphate-modified Pt/CeO2 catalyst with a P/Ce atomic ratio of 0.06 was 10-fold higher than that over the unmodified Pt/CeO2 catalyst at 220 °C. This comprehensive study not only sheds light on the mechanism underlying the surface modification process but also offers a strategy for realizing higher catalytic activity in the total oxidation of light alkanes.

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Hematite (α-Fe2O3) is a promising candidate for NH3 selective catalytic reduction (NH3-SCR) of NOx due to its good sulfur resistance. However, the activity of pure α-Fe2O3 is very low. In this work, α-Fe2O3 obtained excellent N2 selectivity and medium-high temperature activity via a simple surface sulfation method. The α-Fe2O3-350 (sulfated at 350 °C) sample showed an NO conversion rate of ~ 100% in the range of 275-350 °C and exhibited excellent H2O and SO2 resistance ability at 300 °C. Furthermore, pure α-Fe2O3 was used as a model catalyst to fully uncover the effect of sulfation on FeOx-based catalysts in NH3-SCR reactions. Structural characterization indicated that the degree of surface sulfation of the catalyst would be deepened with increasing temperature, and the states of sulfate species on α-Fe2O3 changed from surface sulfates to bulk-like sulfates. Although sulfation treatment reduced the redox properties of α-Fe2O3, it significantly increased its surface acidity and thus the activity. Excessive bulk-like sulfates induced a decrease in activity. Sulfation inhibited the adsorption of NOx on the α-Fe2O3 catalyst surface and reduced the thermal stability of nitrates at medium-high temperature. Thus, the Langmuir-Hinshelwood (L-H) mechanism was inhibited, and the reaction mainly followed the Eley-Rideal (E-R) mechanism.

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In this study, ternary mixed oxide catalysts containing Al2O3-MoO3-MgO and Al2O3-MoO3-WO3 were prepared with a changing ratio of MoO3:Al2O3 between 0.05 and 20.00. All catalysts showed 100% selectivity towards propene during the conversion of propan-2-ol at temperatures between 220 and 400 °C. The catalysts prepared from WO3 possessed very strong acid sites, which cause higher catalytic activity than catalysts prepared from MgO. Besides, the ratio of MoO3:Al2O3 was found to be directly proportional to the conversion yield for all catalysts. XRD results show that whole MgO reacted with Al2O3 and MoO3 to form amorphous MgMoO4 and MgAl2O4 phases during catalyst preparation. Furthermore, WO3 reacted only with Al2O3 to form Al2(WO4)3 and WO3 phase was also detected in the final product. The higher surface acidity and catalytic activity of Al2O3-MoO3-WO3 catalyst referred to this WO3 phase within the structure.

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In this paper, sulfated ZrO2 were synthesized via precipitation and impregnation method, and the promoting effects of support sulfation on selective catalytic reduction (SCR) performance of CeO2/ZrO2 catalysts were investigated. The results revealed that sulfated ZrO2 could significantly enhance the SCR activity of CeO2/ZrO2 catalysts in a wide temperature range. Especially when S/Zr molar ratio was 0.1, CeO2/ZrO2-0.1S catalyst exhibited a large operating temperature window of 251 âˆ¼ 500 °C and its N2 selectivity was 100 % in the temperature range of 150 âˆ¼ 500 °C. Moreover, CeO2/ZrO2-0.1S catalyst possessed a superior low-temperature activity over 0.1S-CeO2/ZrO2 catalyst. After exposing to 100 ppm SO2 for 15 h, a high NO conversion efficiency of CeO2/ZrO2-0.1S catalyst (90.7 %) could still be reached. The characterization results indicated that ZrO2 treated with a proper dosage of sulfate acid was beneficial to enlarge the specific surface area greatly. Sulfated ZrO2 was also in favor of promoting the transformation of CeO2 from crystalline state to highly-dispersed amorphous state, and inhibiting the transformation of ZrO2 from tetragonal to monoclinic phase. It could also enhance the total surface acidity greatly with an increase in both Brønsted acid sites and Lewis acid sites, thus significantly improving NH3 adsorption on catalyst surface. Besides, the promoting effect of support sulfation on SCR performance of CeO2/ZrO2 catalysts was also related with the enhanced redox property, higher Ce3+/(Ce3++Ce4+) ratio and abundant surface chemisorbed labile oxygen. The in-situ DRIFTS results implied that nitrate species coordinated on the surface of CeO2/ZrO2-0.1S catalyst could participate in the Selective catalytic reduction with ammonia (NH3-SCR) reactions at either medium or high temperature, suggesting that both Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms might be followed in SCR reactions.

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As a heavy metal, Pb is one component in coal-fired flue gas and is widely considered to have a strong negative effect on catalyst activity in the selective catalytic reduction of NOx by NH3 (NH3-SCR). In this paper, we investigated the deactivation mechanism of the Mo-Ce/Zr-PILC catalyst induced by Pb in detail. We found that NO conversion over the 3Mo4Ce/Zr-PILC catalyst decreased greatly after the addition of Pb. The more severe deactivation induced by Pb was attributed to low surface area, lower amounts of chemisorbed oxygen species and surface Ce3+, and lower redox ability and surface acidity (especially a low number of Brønsted acid sites). Furthermore, the addition of Pb inhibited the formation of highly active intermediate nitrate species generated on the surface of the catalyst, hence decreasing the NH3-SCR activity.

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This paper investigates the catalytic properties of the iridium catalysts supported on modified silicalite-1. Post-synthesis modification of silicalite-1, with solutions of ammonium compounds (NH4F and NH4OH), appeared to be an efficient method to generate the acidic sites in starting support. The modification of support led not only to changes in its acidity but also its porosity-formation of additional micro- and mesopores. The novel materials were used as supports for iridium. The iridium catalysts (1 wt.% Ir) were characterized by N2 adsorption/desorption measurements, temperature-programmed reduction with hydrogen (TPR-H2), H2 chemisorption, transmission electron microscopy (TEM), temperature-programmed desorption of ammonia (TPD-NH3), X-ray photoelectron spectroscopy (XPS) and tested in the hydrogenation of toluene reaction. The catalytic activity of iridium supported on silicalite-1 treated with NH4OH (higher porosity of support, better dispersion of active phase) was much higher than that of Ir supported on unmodified and modified with NH4F silicalite-1.

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We report on a tunable solid-state approach to modify the acidity of cotton substrates using citric, oxalic, and fumaric acids. The first stage of the method involves soaking the cotton swatches in an ethanolic saturated solution of the corresponding acid. After drying, the carboxylation reaction proceeds at high temperature (T > 100 °C) and in solid state. We quantified the effect of temperature and reaction time on the solid-state carboxylation reaction, which allowed us to tune the carboxylation degree and the acidity of the surface. We characterized the modified cotton by performing adsorption isotherms and by determining the kinetics of adsorption of a cationic dye: methylene blue (MB). We found that the MB uptake kinetics varied as a function of the acidic strength of the surface, which is closely related to the strength of the acid used for surface modification. The proposed solid-state cotton carboxylation procedure allows us to achieve sustainable cotton modification, which constitutes a starting point for several applications using cotton as the substrate.

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The presence of ultrafine plastics particles and its potential to concentrate and transport organic contaminants in aquatic environments have become a major concern in recent years. Specifically, the uptake of hazardous chemicals by plastics particles may affect the distribution and bioavailability of the chemicals. In this study, the adsorption of tetracycline (TC), an antibiotic frequently found in aquatic environments, on high-density polyethylene (PE) particles with the average size of 45 µm, was investigated. The PE particles were characterized for surface acidity for the first time. Results showed that pH controls the surface charge of PE particles. TC adsorption onto PE particles was rapid as expected following the pseudo-second-order rate law (r2 > 0.99). Polar forces in addition to specific chemical interactions, such as hydrogen bonding and hydrophophilicity controlled TC adsorption onto PE particles. Parameters, including pH, dissolved organic matter, ionic strength, major cations and anions affected TC adsorption onto PE micro-particles. Results indicated that PE particles can function as a carrier of antibiotics in the aquatic environment, which potentially imposes ecosystem and human health risks.

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CuSO4/TiO2 catalysts with high catalytic activity and excellent resistant to SO2 and H2O, were thought to be promising catalysts used in Selective catalytic reduction of nitrogen oxides by NH3. The performance of catalysts is largely affected by calcination temperature. Here, effects of calcination temperature on physicochemical property and catalytic activity of CuSO4/TiO2 catalysts were investigated in depth. Catalyst samples calcined at different temperatures were prepared first and then physicochemical properties of the catalyst were characterized by N2 adsorption-desorption, X-ray diffraction, thermogravimetric analysis, Raman spectra, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption of NH3, temperature-programmed reduction of H2 and in situ diffuse reflectance infrared Fourier transform spectroscopy. Results revealed that high calcination temperature had three main effects on the catalyst. First, sintering and anatase transform into rutile with increase of calcination temperature, causing a decrement of specific surface area. Second, decomposition of CuSO4 under higher calcination temperature, resulting in disappears of Brønsted acid sites (S-OH), which had an adverse effect on surface acidity. Third, CuO from the decomposition of CuSO4 changed surface reducibility of the catalyst and favored the process of NH3 oxidation to nitrogen oxides (NOx). Thus, catalytic activity of the catalyst calcined under high temperatures (≥600°C) decreased largely.

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This study investigated thermal treatment of red mud (RM) and its effect on phase composition, surface property, and sorption capacity exemplified by phosphate. Dehydration (∼600 °C), decomposition of carbonate minerals (700 °C-800 °C), and silicate/aluminate formation (900 °C-1000 °C) occurred upon thermal treatment of RM. Grain growth and vitrification that rendered initial morphology changes and decreased the specific surface area of RM from 26.5 to 4.1 m2/g when treated from 600 to 1000 °C, respectively. Surface acidity, i.e., intrinsic acidity constant and surface acidity density, decreased as well after thermal treatment at 600 °C due to burnouts of organics then increased upon further elevated-temperature treatment because of phase transformation. Thermal activation enhanced phosphate adsorption density (µmol/m2). Multilayer sorption aided by leached metal ions was responsible for phosphate immobilization.

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Corresponding the high presence of 3-monochloropropane-1,2-diol esters (3-MCPDE) and glycidyl esters (GE) in refined palm oil, this paper re-evaluated degumming and bleaching processes of physical palm oil refining to reduce the amount of said contaminants. Separation-free water degumming was incorporated into the process, and this significantly (p < 0.05) reduced the esters content without compromising other oil qualities. Different types of bleaching earth (BE) were dosed at 0.5, 1.0 and 1.5% to investigate their effects on the esters formation. Results showed that different type of BE had their own optimal dosage for minimum esters formation. Surface acidity was confirmed as the key performance determinant of BE in mitigation of 3-MCPDE and GE in oil rather than the porosity profile. Specifically, BE with high acidity should be avoided, but slightly acidic BE (pH ≃ 5) was found to provide the greatest reduction of esters as compared to natural and neutral activated BE.

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Activated alumina (AA) has been extensively applied in the defluorination of industrial wastewaters and groundwater. Although the dissolution of AA due to formation of fluoroaluminate complexes (AlFx3-x), especially in acidic condition, has been observed, its role on fluoride uptake by alumina has not been discussed in any previous literature, most of which consider F- as the sole adsorbed species. The present study described the effect of fluoroaluminate complexes on fluoride adsorption onto alumina. Results indicated that fluoroaluminate complexes, major fluoride species at pH < 6, were responsible for total fluoride adsorbed. Free fluoride ions were adsorbed mainly in the alkaline pH region, e.g., pH > 6. The dissolution of AA during defluorination was measured and analyzed by the thermodynamic solubility model. The surface concentration of F- and AlFx3-x were calculated considering electrostatic interactions. Characterization of fluoride-laden AA by XPS revealed that the fraction of surface Al-F species decreased with pH, which suggested the transition of the surface fluorinated species to that of free fluoride ions. The stability constants of four surface complexes, namely, AlOH-FAl2+, AlOH-F2Al+, AlOH2+-F- and AlOH-F-, were 106.88, 105.36, 102.72 and 102.36, respectively. Obviously fluoroaluminate complexes exhibited stronger chemical bonds with the surface hydroxy species than free fluoride.

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Cr3+-doped TiO2 nanoparticles (Ti-Cr) were synthesized by microwave-assisted sol-gel method. The Ti-Cr catalyst was characterized by X-ray diffraction, ultraviolet-visible diffuse reflectance spectroscopy, N2 adsorption-desorption analysis, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, photoluminescence spectroscopy, X-ray photoelectron spectroscopy (XPS) and zetametry. The anatase mesoporous Ti-Cr material exhibited a specific surface area of 54.5 m2/g. XPS analysis confirmed the proper substitution of Ti4+ cations by Cr3+ cations in the TiO2 matrix. The particle size was of average size of 17 nm for the undoped TiO2 but only 9.5 nm for Ti-Cr. The Cr atoms promoted the formation of hydroxyl radicals and modified the surface adsorptive properties of TiO2 due to the increase in surface acidity of the material. The photocatalytic evaluation demonstrated that the Ti-Cr catalyst completely degraded (4-chloro-2-methylphenoxy) acetic acid under visible light irradiation, while undoped TiO2 and P25 allowed 45.7% and 31.1%, respectively. The rate of degradation remained 52% after three cycles of catalyst reuse. The higher visible light photocatalytic activity of Ti-Cr was attributed to the beneficial effect of Cr3+ ions on the TiO2 surface creating defects within the TiO2 crystal lattice, which can act as charge-trapping sites, reducing the electron-hole recombination process.

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