Metal-organic framework loaded manganese oxides as efficient catalysts for low-temperature selective catalytic reduction of NO with NH3

Benzene Oxidation in Air by an Amine-Functionalized Metal-Organic Framework-Derived Carbon- and Nitrogen-Loaded Zirconium Dioxide-Supported Platinum Catalyst

To learn more about the behavior of amine (NH2)-functionalized metal-organic framework (MOF)-derived noble metal catalysts in the removal of aromatic volatile organic compounds in air, benzene oxidation at low temperatures has been investigated using 0.2-, 0.8-, and 1.5%-platinum (Pt)/Universitetet i Oslo (UiO)-66-NH2. The benzene conversion (XB) of x%-Pt/UiO-66-NH2-R under dry conditions (175 °C) was 23% (x = 0.2%) < 52% (x = 0.8%) < 100% (x = 1.5%): 'R' suffix denotes reduction pretreatment using a hydrogen (10 vol %) and nitrogen mixture at 300 °C for the generation of metallic Pt (Pt0) sites and simultaneous partial MOF decomposition into carbon- and nitrogen-loaded zirconium dioxide. The prominent role of reduction pretreatment was apparent in benzene oxidation as 1.5%-Pt/UiO-66-NH2 did not exhibit catalytic activity below 175 °C (dry condition). The promotional role of moisture in benzene oxidation by 1.5%-Pt/UiO-66-NH2-R was evident with a rise in the steady-state reaction rate (r) at 110 °C (21 kPa molecular oxygen (O2)) from 1.3 × 10-3 to 5.0 × 10-3 μmol g-1 s-1 as the water (H2O) partial pressure increased from 0 to 1.88 kPa. In contrast, the activity was lowered with increasing RH due to catalyst poisoning by excess moisture (r (110 °C) of 6.6 × 10-04 μmol g-1 s-1 at 2.83 kPa H2O (21 kPa O2)). Kinetic modeling suggests that XB proceeds through the Langmuir-Hinshelwood mechanism on the Pt/UiO-66-NH2-R surface (dissociative O2 chemisorption and the involvement of two oxygen species in benzene oxidation). According to the density functional theory simulation, the carbon and nitrogen impurities are to make the first XB step (i.e., hydrogen migration from the benzene molecule to the substrate) energetically favorable. The second hydrogen atom from the benzene molecule is also extracted effectively, while the oxygen derived from O2 facilitates further XB. The Pt0 sites dissociate the O2 and H2O molecules, while the product of the latter, i.e., free hydrogen and hydroxyl, makes the subsequent XB steps energetically favorable.

MOF-based catalysts: insights into the chemical transformation of greenhouse and toxic gases

Metal-organic framework (MOF)-based catalysts are outstanding alternative materials for the chemical transformation of greenhouse and toxic gases into high-add-value products. MOF catalysts exhibit remarkable properties to host different active sites. The combination of catalytic properties of MOFs is mentioned in order to understand their application. Furthermore, the main catalytic reactions, which involve the chemical transformation of CH4, CO2, NOx, fluorinated gases, O3, CO, VOCs, and H2S, are highlighted. The main active centers and reaction conditions for these reactions are presented and discussed to understand the reaction mechanisms. Interestingly, implementing MOF materials as catalysts for toxic gas-phase reactions is a great opportunity to provide new alternatives to enhance the air quality of our planet.

Low-temperature oxidative removal of gaseous formaldehyde by an eggshell waste supported silver-manganese dioxide bimetallic catalyst with ultralow noble metal content

Under dark/low temperature (DLT) conditions, the oxidative removal of gaseous formaldehyde (FA) was studied using eggshell waste supported silver (Ag)-manganese dioxide (MnO₂) bimetallic catalysts. To assess the synergistic effects between the two different metals, 0.03%-Ag-(0.5-5%)-MnO₂/Eggshell catalysts were prepared and employed for DLT-oxidation of FA. The steady-state FA oxidation reaction rate (mmol g^-1 h^-1), when measured using 100 ppm FA at 80 °C (gas hourly space velocity (GHSV) of 5308 h^-1), varied as follows: Ag-1.5%-MnO₂/Eggshell-R (9.4) > Ag-3%-MnO₂/Eggshell-R (8.1) > Ag-1.5%-MnO₂/Eggshell (7.5) > Ag-5%-MnO₂/Eggshell-R (7.2) > Ag-1.5%-MnO₂/CaCO₃-R (6.8) > MnO₂-R (6) > Ag-0.5%-MnO₂/Eggshell-R (3.2) > Ag/Eggshell-R (2.6). (Here, 'R' denotes hydrogen-based thermochemical reduction pretreatment.) The temperature required for 90% FA conversion (T₉₀) at the same GHSV exhibited a contrary ordering: Ag/Eggshell-R (175 °C) > Ag-0.5%-MnO₂/Eggshell-R (123 °C) > Ag-5%-MnO₂/Eggshell-R (113 °C) > MnO₂-R (99 °C) > Ag-1.5%-MnO₂/Eggshell (96 °C) > Ag-3%-MnO₂/Eggshell-R (93 °C) > Ag-1.5%-MnO₂/Eggshell-R (77 °C). The eggshell catalyst outperformed the ones made of commercial calcium carbonate due to the presence of defects in the former. The MnO₂ co-catalyst enhances the catalytic activities through the capture and activation of atmospheric oxygen (O₂) with rapid catalytic regeneration. Also, MnO₂ favorably captures the hydrogen of the adsorbed FA molecules to make the oxidation pathway thermodynamically more favorable.

Progress in Metal-Organic Framework Catalysts for Selective Catalytic Reduction of NOx: A Mini-Review

Nitrogen oxides released from the combustion of fossil fuels are one of the main air pollutants. Selective catalytic reduction technology is the most widely used nitrogen oxide removal technology in the industry. With the development of nanomaterials science, more and more novel nanomaterials are being used as catalysts for the selective reduction of nitrogen oxides. In recent years, metal-organic frameworks (MOFs), with large specific surface areas and abundant acid and metal sites, have been extensively studied in the selective catalytic reduction of nitrogen oxides. This review summarizes recent progress in monometallic MOFs, bimetallic MOFs, and MOF-derived catalysts for the selective catalytic reduction of nitrogen oxides and compares the reaction mechanisms of different catalysts. This article also suggests the advantages and disadvantages of MOF-based catalysts compared with traditional catalysts and points out promising research directions in this field.

Synthesis of metal–organic frameworks (MOFs) and their application in the selective catalytic reduction of NOx with NH3

This review summarizes the synthesis, applications for the NH3-SCR and methods for strengthening the water resistance and thermal stability of MOF catalysts.

Preparation of Quasi-MIL-101(Cr) Loaded Ceria Catalysts for the Selective Catalytic Reduction of NOx at Low Temperature

At present, the development of novel catalysts with high activity Selective Catalytic Reduction (SCR) reaction at the low temperature is still a challenge. In this work, the authors prepare CeO2/quasi-MIL-101 catalysts with various amounts of deposited ceria by a double-solvent method, which are characterized by X-ray diffraction (XRD), Fourier Transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and so on. The results show that the increase of Ce content has a great influence on the catalytic property of the catalyst. The introduction of Ce can promote the conversion between Cr3+ and Cr5+ and increase the proportion of lattice oxygen, which improves the activity of the catalyst. However, the catalyst will be peroxidized when the content of Ce is too high, resulting in the decline of the catalytic activity. This experiment indicates that CeO2/quasi-MIL-101 plays a significant role in the NH3-SCR process at the low temperature when the loading of Ce is 0.5%. This work has proved the potential of this kind of material in NH3-SCR process at the low temperature, providing help for subsequent studies.

Novel metal-organic framework supported manganese oxides for the selective catalytic reduction of NOx with NH3: Promotional role of the support

A novel MnOx@MIL-125(Ti) catalyst was constructed for the selective catalytic reduction (SCR) of NOx with NH₃, in which MIL-125(Ti), a kind of metal-organic frameworks (MOFs), was used as the support because of the structural feature, large surface area and high thermal stability. The SCR results showed that MnOx@MIL-125(Ti) exhibited high deNOx ability and N₂ selectivity in a wide operating temperature range. Moreover, it exhibited better SO₂ resistance than MnOx/TiO₂(P25). Characterization results revealed that MIL-125(Ti) had resulted in a high dispersion of MnOx and a strong metal-support interaction for MnOx@MIL-125(Ti), which could promote the formation of the abundant Mn⁴⁺ and surface chemical oxygen, facilitating the activation of the reactants. In situ DRIFTS results suggested that NH₃-SCR over MnOx@MIL-125(Ti) followed both Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanism. In addition, NO^- species were proved to be the important intermediates which were involved in SCR reaction.