Highly active SBA-15-confined Pd catalyst with short rod-like micro-mesoporous hybrid nanostructure for n-butylamine low-temperature destruction

Selective catalytic oxidation of DMF over Cu-Ce/H-MOR by modulating the surface active sites

Selective catalytic oxidation of the hazardous DMF exhaust gas presents a significant challenge in balancing oxidation activity and products selectivity (CO, NOx, N2, etc.). It is found that Cu/H-MOR demonstrates superior performance for DMF oxidation compared to CuO on other supports (γ-Al2O3, HY, ZSM-5) in terms of product selectivity and stability. The geometric and electronic structures of CuO active sites in Cu/H-MOR have been regulated by CeO2 promoter, leading to an increase in the ratio of active CuO (highly dispersed CuO and Cu+ specie). As a result, the oxidation activity and stability of the Cu/H-MOR catalyst were enhanced for DMF selective catalytic oxidation. However, excessive CuO or CeO2 content led to decreased N2 selectivity due to over-high oxidation activity. It is also revealed that Ce3+ species, active CuO species, and surface acid sites play a critical role in internal selective catalytic reduction reaction during DMF oxidation. The 10Cu-Ce/H-MOR (1/4) catalyst exhibited both high oxidation activity and internal selective catalytic reduction activity due to its abundance of active CuO specie as well as Ce3+ species and surface acid sites. Consequently, the 10Cu-Ce/H-MOR (1/4) catalyst demonstrated the widest temperature window for DMF oxidation with high N2 selectivity. These findings emphasize the importance of surface active sites modification for DMF selective catalytic oxidation.

Synergistic Catalytic Removal of NOx and n-Butylamine via Spatially Separated Cooperative Sites

Synergistic control of nitrogen oxides (NOx) and nitrogen-containing volatile organic compounds (NVOCs) from industrial furnaces is necessary. Generally, the elimination of n-butylamine (n-B), a typical pollutant of NVOCs, requires a catalyst with sufficient redox ability. This process induces the production of nitrogen-containing byproducts (NO, NO2, N2O), leading to lower N2 selectivity of NH3 selective catalytic reduction of NOx (NH3-SCR). Here, synergistic catalytic removal of NOx and n-B via spatially separated cooperative sites was originally demonstrated. Specifically, titania nanotubes supported CuOx-CeO2 (CuCe-TiO2 NTs) catalysts with spatially separated cooperative sites were creatively developed, which showed a broader active temperature window from 180 to 340 °C, with over 90% NOx conversion, 85% n-B conversion, and 90% N2 selectivity. A synergistic effect of the Cu and Ce sites was found. The catalytic oxidation of n-B mainly occurred at the Cu sites inside the tube, which ensured the regular occurrence of the NH3-SCR reaction on the outer Ce sites under the matching temperature window. In addition, the n-B oxidation would produce abundant intermediate NH2*, which could act as an extra reductant to promote NH3-SCR. Meanwhile, NH3-SCR could simultaneously remove the possible NOx byproducts of n-B decomposition. This novel strategy of constructing cooperative sites provides a distinct pathway for promoting the synergistic removal of n-B and NOx.

Tailored Synthesis of Catalytically Active Cerium Oxide for N, N-Dimethylformamide Oxidation

Cerium oxide nanopowder (CeO_x) was prepared using the sol-gel method for the catalytic oxidation of N, N-dimethylformamide (DMF). The phase, specific surface area, morphology, ionic states, and redox properties of the obtained nanocatalyst were systematically characterized using XRD, BET, TEM, EDS, XPS, H₂-TPR, and O₂-TPO techniques. The results showed that the catalyst had a good crystal structure and spherelike morphology with the aggregation of uniform small grain size. The catalyst showed the presence of more adsorbed oxygen on the catalyst surface. XPS and H₂-TPR have confirmed the reduction of Ce⁴⁺ species to Ce³⁺ species. O₂-TPR proved the reoxidability of CeO_x, playing a key role during DMF oxidation. The catalyst had a reaction rate of 1.44 mol g^-1_cat s^-1 and apparent activation energy of 33.30 ± 3 kJ mol^-1. The catalytic performance showed ~82 ± 2% DMF oxidation at 400 °C. This work's overall results demonstrated that reducing Ce⁴⁺ to Ce³⁺ and increasing the amount of adsorbed oxygen provided more suitable active sites for DMF oxidation. Additionally, the catalyst was thermally stable (~86%) after 100 h time-on-stream DMF conversion, which could be a potential catalyst for industrial applications.

Effect of Poly(vinyl alcohol) Concentration on the Micro/Mesopore Structure of SBA15 Silica

In this work, a series of micro/mesoporous SBA15 silica materials were synthesized using P123 and poly(vinyl alcohol) (PVA) as co-templates. The pore structure of the prepared SBA15 was observed to be a function of the PVA concentration. When the amount of PVA was relatively small, the specific surface area, micropore volume, and pore wall thickness of the synthesized SBA15 were considerably large. By contrast, when a large amount of PVA was added, the pore wall thickness was greatly reduced, but the mesopore volume and size increased. This is because the added PVA interacted with the polyethylene oxide (PEO) in the shells of the P123 micelles. Furthermore, when the amount of PVA was increased, the core polypropylene oxide (PPO) block also increased, owing to the enhanced aggregation of the P123/PVA mixed micelles. This research contributes to a basic comprehension of the cooperative interactions and formation process underlying porous silica materials, assisting in the rational design and synthesis of micro/mesoporous materials.

Rationally Engineering a CuO/Pd@SiO2 Core-Shell Catalyst with Isolated Bifunctional Pd and Cu Active Sites for n-Butylamine Controllable Decomposition

Volatile organic amines are a category of typical volatile organic compounds (VOCs) extensively presented in industrial exhausts causing serious harm to the atmospheric environment and human health. Monometallic Pd and Cu-based catalysts are commonly adopted for catalytic destruction of hazardous organic amines, but their applications are greatly limited by the inevitable production of toxic amide and NO_x byproducts and inferior low-temperature activity. Here, a CuO/Pd@SiO₂ core-shell-structured catalyst with diverse functionalized active sites was creatively developed, which realized the total decomposition of n-butylamine at 260 °C with a CO₂ yield and N₂ selectivity reaching up to 100% and 98.3%, respectively (obviously better than those of Pd@SiO₂ and CuO/SiO₂), owing to the synergy of isolated Pd and Cu sites in independent mineralization of n-butylamine and generation of N₂, respectively. The formation of amide and short-chain aliphatic hydrocarbon intermediates via C-C bond cleavage tended to occur over Pd sites, while the C-N bond was prone to breakage over Cu sites, generating NH₂· species and long free-N chain intermediates at low temperatures, avoiding the production of hazardous amide and NO_x. The SiO₂ channel collapse and H⁺ site production resulted in the formation of N₂O via suppressing NH₂· diffusion. This work provides critical guidance for a rational fabrication of catalysts with high activity and N₂ selectivity for environmentally friendly destruction of nitrogen-containing VOCs.

Achieving acetone efficient deep decomposition by strengthening reactants adsorption and activation over difunctional Au(OH)Kx/hierarchical MFI catalyst

Realizing the simultaneous adsorption and activation of O₂ and reactants over supported noble metal catalysts is crucial for the oxidation of organic hydrocarbons. Herein, we report a facile one-step ethylene glycol reduction method to synthesize difunctional Au(OH)K_x sites, which were anchored on a hierarchical hollow MFI support and adopted for acetone decomposition. The alkali ion-associated adjacent surface hydroxyl groups were coordinated with Au nanoparticles, resulting in partially oxidized Au¹⁺ sites with improved dispersion. The results obtained from exclusive ex situ and in situ experiments illustrated that the proper content of K and hydroxyl groups significantly enhanced the adsorption of surface O₂ and acetone molecules around the Au sites simultaneously, whereas the excess K species inhibited the catalytic performance by blocking the pore structure and decreasing the acidity of catalysts. The Au(OH)K_0.7/h-MFI catalyst exhibited the highest efficiency for acetone oxidation, over which 1500 ppm acetone can be completely oxidized at just 280 °C with an extremely low activation energy of 32.5 kJ mol^-1. The carbonate species were detected as the main intermediates during acetone decomposition over the difunctional Au(OH)K_x sites through a Langmuir - Hinshelwood (L - H) mechanism. This finding paves the way for designing and constructing efficient functional active sites for the complete oxidation of hydrocarbons.

Magnesium-Modified Co3O4 Catalyst with Remarkable Performance for Toluene Low Temperature Deep Oxidation

Co3O4, MgCo2O4 and MgO materials have been synthesized using a simple co-precipitation approach and systematically characterized. The total conversion of toluene to CO2 and H2O over spinel MgCo2O4 with wormlike morphology has been investigated. Compared with single metal oxides (Co3O4 and MgO), MgCo2O4 with the highest activity has exhibited almost 100% oxidation of toluene at 255 °C. The obtained results are analogous to typical precious metal supported catalysts. The activation energy of toluene over MgCo2O4 (38.5 kJ/mol) is found to be much lower than that of Co3O4 (68.9 kJ/mol) and MgO ((87.8 kJ/mol)). Compared with the single Co and Mg metal oxide, the as-prepared spinel MgCo2O4 exhibits a larger surface area, highest absorbed oxygen and more oxygen vacancies, thus highest mobility of oxygen species due to its good redox capability. Furthermore, the samples specific surface area, low-temperature reducibility and surface adsorbed oxygenated species ratio decreased as follows: MgCo2O4 > Co3O4 > MgO; which is completely in line with the catalytic performance trends and constitute the reasons for MgCo2O4 high excellent activity towards toluene total oxidation. The overall finding supported by ab initio molecular dynamics simulations of toluene oxidation on the Co3O4 and MgCo2O4 suggest that the catalytic process follows a Mars–van Krevelen mechanism.

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.

Pd-based catalysts promoted by hierarchical porous Al2O3 and ZnO microsphere supports/coatings for ethyl acetate highly active and stable destruction

Developing economical and active materials is of great significance for VOC purification. Here, hierarchical porous Al₂O₃ and ZnO microspheres (Al₂O₃-pm and ZnO-pm) were synthesized by a facile hydrothermal strategy. The urchin-like Al₂O₃-pm and flower-like ZnO-pm possess high specific surface area (especially; external surface area) obviously boost the dispersion of Pd with 29.3 % and 30.1 % over Pd/Al₂O₃-pm and Pd/ZnO-pm, respectively, over 3.4 times higher than those of commercial Al₂O₃- and ZnO-supported counterparts. Pd/Al₂O₃-pm possesses excellent activity and CO₂ yield in ethyl acetate (EA) degradation, with TOF reaches 7.76 × 10^-3 s^-1 at 160 °C under GHSV of 50,000 h^-1. Moreover, Pd/Al₂O₃-pm exhibits satisfied performance in EA-contained binary VOCs oxidation and has high long-term stability under both dry and humid conditions. Both Pd sites and Brønsted acid sites participated in reaction process and initially react with EA to form ethylene and ethanol, respectively. Larger amount Brønsted acid sites over Pd/Al₂O₃-pm promote ethanol formation and C-C cleavage, resulting in different CO₂ yields and EA activation mechanisms. The coating greatly enhances Pd dispersion over Pd supported monolithic catalyst, endowing its desired activity and stability even with a much lower Pd loading. This work promotes the potential application of noble-metal-based monolithic materials in VOC degradation.

Synergistic effects of Cu species and acidity of Cu-ZSM-5 on catalytic performance for selective catalytic oxidation of n-butylamine

In this work, a series of Cu-ZSM-5 catalysts with different SiO₂/Al₂O₃ ratios (25, 50, 100 and 200) were synthesized and investigated in n-butylamine catalytic degradation. The n-butylamine can be completely catalytic degradation at 350°C over all Cu-ZSM-5 catalysts. Moreover, Cu-ZSM-5 (25) exhibited the highest selectivity to N₂, exceeding 90% at 350°C. These samples were investigated in detail by several characterizations to illuminate the dependence of the catalytic performance on redox properties, Cu species, and acidity. The characterization results proved that the redox properties and chemisorption oxygen primarily affect n-butylamine conversion. N₂ selectivity was impacted by the Brønsted acidity and the isolated Cu²⁺ species. Meanwhile, the surface acid sites over Cu-ZSM-5 catalysts could influence the formation of Cu species. Furthermore, in situ diffuse reflectance infrared Fourier transform spectra was adopted to explore the reaction mechanism. The Cu-ZSM-5 catalysts are the most prospective catalysts for nitrogen-containing volatile organic compounds removal, and the results in this study could provide new insights into catalysts design for VOC catalytic oxidation.

Fabrication of MnOx-CeO2/cordierite catalysts doped with FeOx and CuO for preferable catalytic oxidation of chlorobenzene

A series of MnO_x-CeO₂ catalysts with MO_x doping (M = Cu, Fe, Co and La) supported on cordierite were synthesized by the citric acid complex method, showing preferable catalytic oxidation of chlorobenzene. The distribution of active oxides, surface areas, as well as the structural morphology of M-MnO_x-CeO₂ catalysts varied with the different Mn/Ce and M/Mn molar ratios. Meanwhile, physicochemical properties of these catalysts were characterized by XRD, BET, SEM, TEM, H₂-TPR and IR. More importantly, the catalytic oxidation routes were also investigated where the process was from chlorobenzene to CO₂, H₂O, HCl and other by-products for the FeO_x-MnO_x-CeO₂ and CuO-MnO_x-CeO₂ catalysts. The CuO-MnO_x-CeO₂ catalysts showed a higher chlorobenzene conversion, and the measured light-off temperature T₉₀ was approximately 400°C. However, a large amount of chloropropane as main by-products was observed. For the FeO_x-MnO_x-CeO₂ catalysts, more carbon monoxide could be found with inadequate oxidation. Comparative analyses of two catalysts indicated that the main cause of the oxidation activities and mechanisms were different in the oxidation capacity and water absorbability of FeO_x and CuO. Nevertheless, all of these catalysts did not exhibit any deactivation due to chloride with a high reaction temperature, with chloride transformed to form HCl in the off-gas stream.

Revealing the unexpected promotion effect of diverse potassium precursors on α-MnO2 for the catalytic destruction of toluene

The alkali metal potassium has the functions of structure promotion and electronic modulation in metal oxides.

A citric acid-assisted deposition strategy to synthesize mesoporous SiO2-confined highly dispersed LaMnO3 perovskite nanoparticles for n-butylamine catalytic oxidation

A citric acid-assisted deposition strategy was applied to synthesize mesoporous SiO₂-confined highly dispersed LaMnO₃ perovskite nanoparticles with optimum catalytic performance and N₂ selectivity.