Mesoporous chromia with ordered three-dimensional structures for the complete oxidation of toluene and ethyl acetate

Ce-based three-dimensional mesoporous microspheres with Mn homogeneous incorporation for toluene oxidation

Ce-based three-dimensional (3D) mesoporous microspheres with Mn homogeneous incorporation were synthesized. The CeMn-0.4, characterized by a Ce/Mn molar ratio of 6:4, demonstrated exceptional catalytic activity and stability. The formation of CeMn solid solution strengthened the Ce-Mn interaction, yielding higher concentrations of Ce3+ and Mn4+. Mn4+ initiated toluene preliminary activation owing to its robust oxidative properties, while Ce3+ contributed to oxygen vacancy generation, enhancing the activation of gaseous oxygen and lattice oxygen mobility. Integrating experiments and Density Functional Theory (DFT) calculations elucidated the oxygen reaction mechanisms. A portion of oxygen was converted into surface reactive oxygen species (Oads) that directly oxidized toluene. Additionally, the presence of oxygen vacancies promoted the participation of oxygen in toluene oxidation by converting it into lattice oxygen, which was crucial for the deep oxidation of toluene. Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFTS) indicated the accumulation of benzene-ring intermediates on the catalyst surface hindered continuous toluene oxidation. Thus, the abundant oxygen vacancies in CeMn-0.4 played a pivotal role in sustaining the oxidation process by bolstering the activation of gaseous oxygen and the mobility of lattice oxygen.

Cr2 O3 Promoted In2 O3 Catalysts for CO2 Hydrogenation to Methanol

Cr2 O3 was applied to study the modification of In2 O3 based catalysts for CO2 hydrogenation to methanol reaction. Combined with X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), etc., the structure of the catalysts was characterized. The reaction performances for CO2 hydrogenation to methanol were evaluated on a stainless-steel fix-bed reactor. The results showed that solid solutions were formed for the Cr2 O3 promoted In2 O3 catalysts. The important role of electronic interaction between Cr2 O3 and In2 O3 was revealed in the hydrogenation reaction. In1.25 Cr0.75 O3 sample exhibited the highest methanol yield, which was 2.8 times higher than that of pure In2 O3 . No deactivation was observed for In1.25 Cr0.75 O3 sample during the 50 hours of reaction. The improved catalytic performance may be due to the formation of the solid solutions and the highest amount of oxygen vacancies.

Enhancement of toluene oxidation performance over La1-xCoO3-δ perovskite by lanthanum non-stoichiometry

La_1-xCoO_3-δ catalysts with different non-stoichiometry of lanthanum ions were synthesized by using the sol-gel method, and their catalytic performance in toluene combustion was investigated. The results showed that the catalytic activity and stability of A-site nonstoichiometric La_1-xCoO_3-δ were improved to a certain extent compared with pure LaCoO₃ perovskite. Among them, the La_0.9CoO_3-δ catalyst gave the best catalytic performance for toluene oxidation. It achieved 90% toluene conversion at 205°C under the conditions of a WHSV (weight hourly space velocity) of 22,500 mL/(g·hr) and a 500 ppmV-toluene concentration. Various characterization techniques were used to investigate the relationship between the structure of these catalysts and their catalytic performance. It was found that the non-stoichiometric modification of the lanthanum ion at position A in LaCoO₃ changed the surface element state of the catalyst and increased the oxygen vacancy content, thus, combined with improved reducibility, improving toluene degradation on the catalyst.

Mesoporous catalysts for catalytic oxidation of volatile organic compounds: preparations, mechanisms and applications

Volatile organic compounds (VOCs) are mainly derived from human activities, but they are harmful to the environment and our health. Catalytic oxidation is the most economical and efficient method to convert VOCs into harmless substances of water and carbon dioxide at relatively low temperatures among the existing techniques. Supporting noble metal and/or transition metal oxide catalysts on the porous materials and direct preparation of mesoporous catalysts are two efficient ways to obtain effective catalysts for the catalytic oxidation of VOCs. This review focuses on the preparation methods for noble-metal-based and transition-metal-oxide-based mesoporous catalysts, the reaction mechanisms of the catalytic oxidations of VOCs over them, the catalyst deactivation/regeneration, and the applications of such catalysts for VOCs removal. It is expected to provide guidance for the design, preparation and application of effective mesoporous catalysts with superior activity, high stability and low cost for the VOCs removal at lower temperatures.

How Chemoresistive Sensors Can Learn from Heterogeneous Catalysis. Hints, Issues, and Perspectives

The connection between heterogeneous catalysis and chemoresistive sensors is emerging more and more clearly, as concerns the well-known case of supported noble metals nanoparticles. On the other hand, it appears that a clear connection has not been set up yet for metal oxide catalysts. In particular, the catalytic properties of several different oxides hold the promise for specifically designed gas sensors in terms of selectivity towards given classes of analytes. In this review, several well-known metal oxide catalysts will be considered by first exposing solidly established catalytic properties that emerge from related literature perusal. On this basis, existing gas-sensing applications will be discussed and related, when possible, with the obtained catalysis results. Then, further potential sensing applications will be proposed based on the affinity of the catalytic pathways and possible sensing pathways. It will appear that dialogue with heterogeneous catalysis may help workers in chemoresistive sensors to design new systems and to gain remarkable insight into the existing sensing properties, in particular by applying the approaches and techniques typical of catalysis. However, several divergence points will appear between metal oxide catalysis and gas-sensing. Nevertheless, it will be pointed out how such divergences just push to a closer exchange between the two fields by using the catalysis knowledge as a toolbox for investigating the sensing mechanisms.

Catalytic synergy of Au@CeO2–rGO nanohybrids for the reductive transformation of antibiotics and dyes

Changes in morphology of Au@CeO₂–rGO nanohybrids demonstrated synergistic effects of the ternary components for reductive transformation of antibiotics and dyes.

Strategies for development of nanoporous materials with 2D building units

It has already been realized that two-dimensional (2D) materials carry a great potential in energy conversion and storage, gas storage, chemical sensing, and many other applications closely related to human life. These applications benefit from a key feature of 2D materials, namely the large specific surface area, which however can be diminished significantly due to the tendency of these materials to restack. In this review, we revisit the strategies - including soft and hard templating - that have been developed for generating nanoporosity in 3D materials and demonstrate their adaptation for 2D materials using carbon nitride and graphene materials as examples. Owing to the 2D nature of the building units, a new type of nanopore can be generated by perforating the basal planes. These in-plane nanopores are essential in many emerging applications of 2D materials such as semipermeable membranes; hence, their creation methods, including post-synthesis activation, ion bombardment, electron beam drilling, and nanolithography, are worthy of a critical review. Lastly, techniques for preventing the restacking by fabricating 2D-0D, 2D-1D, and 2D-2D layer-by-layer composite structures are discussed. The goal is to promote the use of these methods for creating nanoporosity in more 2D materials.

Enhanced photothermal catalytic degradation of toluene by loading Pt nanoparticles on manganese oxide: Photoactivation of lattice oxygen

Light-driven photothermocatalysis can provide enough energy to reach light-off temperature of VOCs on the surface of catalyst without auxiliary heat source. Herein, we synthesized noble-metal supported manganese oxide catalysts (xPt/MO) and studied their photothermal catalytic behavior of toluene degradation, where 1 Pt/MO (1 wt.% loading of Pt) and 2 Pt/MO (2 wt.% loading of Pt) exhibited more than 90 % of conversion and 70 % of mineralization under illumination of 200 mW/cm² light intensity with a value of 30,000 mL/(g·h) for weight hourly space velocity (WHSV), respectively. Comparison to pure MO, 1 Pt/MO owns a good photothermal catalytic stability for at least 60 h without obvious deactivation. The introduction of Pt promotes the crystallization of MO (verified by XRD and TEM analysis) and enhances the mobility of surface/sub-surface lattice oxygen (verified by O₂-TPD, H₂-TPR and CO consumption). It is proved that illumination not only supplies thermal energy to trigger the reaction of toluene oxidation but also further evoke more lattice oxygen on Pt/MO to participate in toluene decomposition.

Mesoporous cobalt monoxide-supported platinum nanoparticles: Superior catalysts for the oxidative removal of benzene

Mesoporous Co₃O₄ (meso-Co₃O₄)-supported Pt (0.53 wt.% Pt/meso-Co₃O₄) was synthesized via the KIT-6-templating and polyvinyl alcohol (PVA)-assisted reduction routes. Mesoporous CoO (meso-CoO) was fabricated through in situ reduction of meso-Co₃O₄ with glycerol, and the 0.18-0.69 wt.% Pt/meso-CoO samples were generated by the PVA-assisted reduction method. Meso-Co₃O₄ and meso-CoO were of cubic crystal structure and the Pt nanoparticles (NPs) with a uniform size of ca. 2 nm were well distributed on the meso-Co₃O₄ or meso-CoO surface. The 0.56 wt% Pt/meso-CoO (0.56Pt/meso-CoO) sample performed the best in benzene combustion (T_50% = 156 °C and T_90% = 186 °C at a space velocity of 80,000 mL/(g h)). Introducing water vapor or CO₂ with a certain concentration led to partial deactivation of 0.56 Pt/meso-CoO and such a deactivation was reversible. We think that the superior catalytic activity of 0.56 Pt/meso-CoO was intimately related to its good oxygen activation and benzene adsorption ability.

Suppressing Ammonia Re-Emission with the Aid of the Co3O4-NPs@KIT-6 Catalyst in Ammonia-Based Desulfurization

The re-emission of NH₃ and SO₂ caused by the decomposition of (NH₄)₂SO₃ is a crucial concern in ammonia-based desulfurization. In this study, a novel Co₃O₄-NPs@KIT-6 catalyst with a three-dimensional two-helix structure is proposed for converting (NH₄)₂SO₃ into (NH₄)₂SO₄. The oxidation rate of (NH₄)₂SO₃ with the catalyst was 7.5 times that without any catalyst, and this improvement was attributed to appropriately dispersed Co₃O₄ nanoparticles with a size of 4-10 nm that interacted with the KIT-6 support. Therefore, the number of active sites with substitution and hole defects was substantially increased, which is advantageous for high catalytic activities. Consequently, the amount of NH₃ and SO₂ re-emission during (NH₄)₂SO₃ oxidation was reduced by 43.9%, which considerably reduced potential environmental risks. The results of this study serve to advance ammonia desulfurization by improving the desulfurization efficiency, downsizing the oxidation tank, and generating considerable profit from efficient reclaiming of (NH₄)₂SO₄ as a fertilizer.

Investigation of synergistic effects and high performance of La-Co composite oxides for toluene catalytic oxidation at low temperature

Cobalt oxides have been considered as a kind of highly efficient catalyst for the oxidation of volatile organic compounds (VOCs). In this work, lanthanum-cobalt composite oxides were prepared by using the co-precipitation method, and toluene was used as the model compound. Diversified techniques including XRD, SEM, Raman spectra, XPS, H₂-TPR, and N₂ adsorption-desorption were applied to investigate the physicochemical properties of as-prepared materials. The composite catalysts showed different morphology including larger specific surface area and higher pore volume which would accelerate the adsorption of toluene and improve the amount of active sites on surface. Moreover, the addition of lanthanum could enhance the low-temperature reducibility, and it could be also beneficial to expose more Co³⁺ and adsorbed oxygen species on the surface of catalysts which could accelerate the oxidation of toluene and lower onset oxidation temperature. 0.05La-Co (with a molar ratio of lanthanum against cobalt is 0.05) showed the best catalytic performance. The complete conversion of toluene was achieved at 225 °C under the condition of toluene concentration = 1000 ppm and SV = 20,000 ml·g^-1·h^-1. Stability test over 0.05La-Co was conducted at 225 °C and it could maintain the 100% conversion of toluene for 720 min, indicating the excellent stability of as-prepared catalysts. Undoubtedly, lanthanum-cobalt composite oxide is a kind of promising material for the catalytic oxidation of VOCs.

Total Oxidation of Propane over a Ru/CeO2 Catalyst at Low Temperature

Ruthenium (Ru) nanoparticles (∼3 nm) with mass loading ranging from 1.5 to 3.2 wt % are supported on a reducible substrate, cerium dioxide (CeO₂, the resultant sample is called Ru/CeO₂), for application in the catalytic combustion of propane. Because of the unique electronic configuration of CeO₂, a strong metal-support interaction is generated between the Ru nanoparticles and CeO₂ to stabilize Ru nanoparticles for oxidation reactions well. In addition, the CeO₂ host with high oxygen storage capacity can provide an abundance of active oxygen for redox reactions and thus greatly increases the rates of oxidation reactions or even modifies the redox steps. As a result of such advantages, a remarkably high performance in the total oxidation of propane at low temperature is achieved on Ru/CeO₂. This work exemplifies a promising strategy for developing robust supported catalysts for short-chain volatile organic compound removal.

Au-Pd/mesoporous Fe2O3: Highly active photocatalysts for the visible-light-driven degradation of acetone

Three-dimensionally ordered mesoporous Fe₂O₃ (meso-Fe₂O₃) and its supported Au, Pd, and Au-Pd alloy (xAuPd_y/meso-Fe₂O₃; x=0.08-0.72wt.%; Pd/Au molar ratio (y)=1.48-1.85) photocatalysts have been prepared via the KIT-6-templating and polyvinyl alcohol-protected reduction routes, respectively. Physical properties of the samples were characterized, and their photocatalytic activities were evaluated for the photocatalytic oxidation of acetone in the presence of a small amount of H₂O₂ under visible-light illumination. It was found that the meso-Fe₂O₃ was rhombohedral in crystal structure. The as-obtained samples displayed a high surface area of 111.0-140.8m²/g and a bandgap energy of 1.98-2.12eV. The Au, Pd and/or Au-Pd alloy nanoparticles (NPs) with a size of 3-4nm were uniformly dispersed on the surface of the meso-Fe₂O₃ support. The 0.72wt.% AuPd_1.48/meso-Fe₂O₃ sample performed the best in the presence of 0.06mol/L H₂O₂ aqueous solution, showing a 100% acetone conversion within 4hr of visible-light illumination. It was concluded that the good performance of 0.72wt.% AuPd_1.48/meso-Fe₂O₃ for photocatalytic acetone oxidation was associated with its ordered mesoporous structure, high adsorbed oxygen species concentration, plasmonic resonance effect between AuPd_1.48 NPs and meso-Fe₂O₃, and effective separation of the photogenerated charge carriers. In addition, the introduction of H₂O₂ and the involvement of the photo-Fenton process also played important roles in enhancing the photocatalytic activity of 0.72wt.% AuPd_1.48/meso-Fe₂O₃.

Tuning the micromorphology and exposed facets of MnOx promotes methyl ethyl ketone low-temperature abatement: boosting oxygen activation and electron transmission

MnO_x nanowires with highly exposed {101} facets of Mn₃O₄ possess excellent low-temperature activity and stability for methyl ethyl ketone destruction.

Ordered Large-Pore Mesoporous Cr2O3 with Ultrathin Framework for Formaldehyde Sensing

A series of ordered mesoporous chromium oxides (Cr₂O₃) were synthesized by first replicating bicontinuous cubic Ia3d mesoporous silica (KIT-6), then a controlled mesostructural transformation from Ia3d to I4₁32 symmetry during the replication from KIT-6 to Cr₂O₃ was achieved by reducing the pore size and interconnectivities of KIT-6, accompanied with an increase in pore size from 3 to 12 nm and a decrease in framework thickness from 8.6 to 5 nm of the resultant Cr₂O₃ replicas. The gas-sensing behavior of the Cr₂O₃ replicas toward formaldehyde (HCHO) was systematically investigated. Ordered mesoporous Cr₂O₃ with both large accessible pores (12 nm) and an ultrathin framework (5 nm) exhibits the best sensing performance, with a response (R_gas/R_air = 119) toward 9 ppm of HCHO 4.4 times higher than that (R_gas/R_air = 27) of its counterpart with small pores and a thick framework. Moreover, it possesses excellent selectivity for detecting HCHO over other interference gases such as CO, benzene, toluene, p-xylene, NH₃, H₂S, and moisture. The significantly enhanced sensing performance of ordered large-pore mesoporous Cr₂O₃ with ultrathin framework suggests its great potential for the selective detection of HCHO.

Facile preparation of ordered mesoporous MnCo2O4 for low-temperature selective catalytic reduction of NO with NH3

Ordered mesoporous MnCo2O4 nanomaterials were successfully prepared through the nanocasting route using SBA-15 and KIT-6 as hard templates. These mesoporous nanomaterials were characterized using XRD, BET, TEM, NH3-TPD, H2-TPR, NO-TPD, XPS and DRIFT. The low temperature selective catalytic reduction (SCR) activity of NO with NH3 was investigated, which revealed that 3D-MnCo2O4 using KIT-6 as a template can totally clean all NO over a wide temperature range of 100-250 °C with a gas hourly space velocity (GHSV) of 32,000 h(-1), while 2D-MnCo2O4 with SBA-15 as a template had 95% conversion rate at the same condition. 3D-MnCo2O4 showed the best performance to clean NO due to its typical three-dimensional porous structure, large specific surface area, abundant active surface oxygen species and Lewis acid sites. All the results indicate that a novel, cheap catalyst for catalytic removal of NO can be designed by controlling the morphology at the nanoscale.

Catalytic Oxidation of Formaldehyde Over Mesoporous MnOx-CeO2 Catalysts

Formaldehyde is regarded as the major indoor air pollutant. Because of harmful effect on human health, its emission abatement is of significant practical interest. We report here excellent low-temperature catalytic performances of mesoporous MnO x - CeO 2 catalysts in the process of formaldehyde oxidation. These MnO x - CeO 2 catalysts were synthesized by a "nanocasting" method using SBA-15 as hard template. TEM images showed that the as-fabricated MnO x - CeO 2 composites possess well-ordered mesoporous architectures. Results of catalytic tests revealed that mesoporous MnO x - CeO 2 nanocomposites have excellent low-temperature catalytic activity for formaldehyde oxidation, the temperature for 100% formaldehyde conversion can be as low as 65°C over these noble-metal-free mesoporous catalysts. The excellent catalytic performance is attributed to their ordered mesoporous structures that expose abundant active sites to formaldehyde molecules.

Ultraselective and sensitive detection of xylene and toluene for monitoring indoor air pollution using Cr-doped NiO hierarchical nanostructures

Ultraselective and sensitive detection of xylene and toluene with minimum interferences of other indoor air pollutants such as benzene, ethanol, and formaldehyde is achieved using NiO hierarchical nanostructures doped with Cr. Pure and 1.15-2.56 at% Cr-doped NiO flower-like hierarchical nanostructures assembled from nanosheets are prepared by a simple solvothermal reaction and their gas sensing characteristics toward o-xylene and toluene gases are investigated. The 1.15 at% Cr-doped NiO hierarchical nanostructures show high responses to 5 ppm of o-xylene and toluene (ratio of resistance to gas and air = 11.61 and 7.81, respectively) and negligible cross-responses to 5 ppm of benzene, formaldehyde, ethanol, hydrogen, and carbon monoxide. However, pure NiO nanostructures show low responses to 5 ppm of o-xylene and toluene (ratio of resistance to gas and air = 2.01 and 1.14, respectively) and no selectivity toward any specific gas is observed. Significant enhancement of the response and selectivity to o-xylene and toluene is attributed to the decrease in the hole concentration in NiO and the catalytic oxidation of methyl groups by Cr doping.

Three-dimensional ordered macroporous bismuth vanadates: PMMA-templating fabrication and excellent visible light-driven photocatalytic performance for phenol degradation

Three-dimension ordered macroporous (3D-OM) bismuth vanadates with a monoclinic crystal structure and high surface area (18-24 m(2) g(-1)) have been prepared using ascorbic acid (AA)- or citric acid (CA)-assisted poly(methyl methacrylate) (PMMA)-templating strategy with bismuth nitrate and ammonium metavanadate as the metal sources, HNO(3) as the pH adjuster and ethylene glycol and methanol as the solvent. The materials were characterized by a number of analytical techniques. The photocatalytic performance of the porous BiVO(4) samples was evaluated for the degradation of phenol in the presence of a small amount of H(2)O(2) under visible light illumination. The effects of the initial phenol concentration and the H(2)O(2) amount on the photocatalytic activity of the photocatalyst were examined. It is shown that the chelating agent, AA or CA, and the amount in which it is added had a significant impact on the quality of the 3D-OM structure, with a "(Bi + V):chelating agent" molar ratio of 2:1 being the most appropriate. Among the as-prepared BiVO(4) samples, the one with a surface area of ca. 24 m(2) g(-1) showed the best visible light-driven photocatalytic performance for phenol degradation (phenol conversion = ca. 94% at phenol concentration = 0.1 mmol L(-1) and in the presence of 0.6 mL H(2)O(2)). A higher phenol conversion could be achieved within the same reaction time if the phenol concentration in the aqueous solution was lowered, but an excess amount of H(2)O(2) was not a favorable factor for the enhancement of the catalytic activity. It is concluded that the excellent photocatalytic activity of 3D-OM BiVO(4) is due to the high quality 3D-OM structured BiVO(4) that has a high surface area and surface oxygen vacancy density. We are sure that the 3D-OM material is a promising photocatalyst for the removal of organics from wastewater under visible light illumination.

Manganese oxides with rod-, wire-, tube-, and flower-like morphologies: highly effective catalysts for the removal of toluene

Nanosized rod-like, wire-like, and tubular α-MnO(2) and flower-like spherical Mn(2)O(3) have been prepared via the hydrothermal method and the CCl(4) solution method, respectively. The physicochemical properties of the materials were characterized using numerous analytical techniques. The catalytic activities of the catalysts were evaluated for toluene oxidation. It is shown that α-MnO(2) nanorods, nanowires, and nanotubes with a surface area of 45-83 m(2)/g were tetragonal in crystal structure, whereas flower-like spherical Mn(2)O(3) with a surface area of 162 m(2)/g was of cubic crystal structure. There were the presence of surface Mn ions in multiple oxidation states (e.g., Mn(3+), Mn(4+), or even Mn(2+)) and the formation of surface oxygen vacancies. The oxygen adspecies concentration and low-temperature reducibility decreased in the order of rod-like α-MnO(2) > tube-like α-MnO(2) > flower-like Mn(2)O(3) > wire-like α-MnO(2), in good agreement with the sequence of the catalytic performance of these samples. The best-performing rod-like α-MnO(2) catalyst could effectively catalyze the total oxidation of toluene at lower temperatures (T(50%) = 210 °C and T(90%) = 225 °C at space velocity = 20,000 mL/(g h)). It is concluded that the excellent catalytic performance of α-MnO(2) nanorods might be associated with the high oxygen adspecies concentration and good low-temperature reducibility. We are sure that such one-dimensional well-defined morphological manganese oxides are promising materials for the catalytic elimination of air pollutants.