Metal organic frameworks-assisted fabrication of CuO/Cu2O for enhanced selective catalytic reduction of NOx by NH3 at low temperatures

Probing the Atomistic Reaction Pathways in CuO/C Catalysts

CuOx/C catalysts have been used in the selective catalytic reduction of NOx because of the exceptional low-temperature denitration (de-NOx) activity. A fundamental understanding of the reaction between CuO and C is critical for controlling the component of CuOx/C and thus optimizing the catalytic performance. In this study, a transmission electron microscope equipped with an in situ heating device was utilized to investigate the atomic-scale reaction between CuO and C. We report two reaction mechanisms relying on the volume ratio between C and CuO: (1) The reduction from CuO to Cu2O (when the ratio is < ∼31%); (2) the reduction of CuO into polycrystalline Cu (when the ratio is > ∼34%). The atomistic reduction pathway can be well interpreted by considering the diffusion of O vacancy through the first-principle calculations. The atomic-scale exploration of CuO/C offers ample prospects for the design of industrial de-NOx catalysts in the future.

Heterojunctioned CuO/Cu2O catalyst for highly efficient ozone removal

In recent years, near surface ozone pollution, has attracted more and more attention, which necessitates the development of high efficient and low cost catalysts. In this work, CuO/Cu₂O heterojunctioned catalyst is fabricated by heating Cu₂O at high temperature, and is adopted as ozone decomposition catalyst. The results show that after Cu₂O is heated at 180°C conversion of ozone increases from 75.2% to 89.3% at mass space velocity 1,920,000 cm³/(g·hr) in dry air with 1000 ppmV ozone, which indicates that this heterojunction catalyst is one of the most efficient catalysts reported at present. Catalysts are characterized by electron paramagnetic resonance spectroscopy and ultraviolet photoelectron spectroscopy, which confirmed that the heterojunction promotes the electron transfer in the catalytic process and creates more defects and oxygen vacancies in the CuO/Cu₂O interfaces. This procedure of manufacturing heterostructures would also be applicable to other metal oxide catalysts, and it is expected to be more widely applied to the synthesis of high-efficiency heterostructured catalysts in the future.

MOF-Derived Co3O4 Nanoparticles Catalyzing Hydrothermal Deoxygenation of Fatty Acids for Alkane Production

Designing economical and nonprecious catalysts with a catalytic performance as good as that of noble metals is of great importance in future renewable bioenergy production. In this study, the metal-organic framework (MOF) was applied as a precursor template to synthesize Co₃O₄ nanoparticles with a carbon matrix shell (denoted as M-Co₃O₄). To select the synthesized optimal catalyst, stearic acid was chosen as the model reactant. The effects of catalyst dosage, methanol dosage, water dosage, temperature, and reaction time on catalytic efficiency were examined. Under the designed condition, M-Co₃O₄ exhibited high catalytic performance and the catalyst showed higher conversion of stearic acid (98.7%) and selectivity toward C8-C18 alkanes (92.2%) in comparison with Pt/C (95.8% conversion and 93.2% selectivity toward C8-C18). Furthermore, a series of characterization techniques such as scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption isotherms (Brunauer-Emmett-Teller (BET) method), and thermogravimetric analysis (TGA) was applied to investigate the physicochemical properties of the catalysts. Finally, we proposed that decarbonization (deCO) could be the presumably mechanistic pathway for the production of C8-C18 alkanes from the decomposition of stearic acid.

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.

Mercury/oxygen reaction mechanism over CuFe2O4 catalyst

CuFe₂O₄ is regarded as a promising candidate of catalyst for Hg⁰ oxidation in industrial flue gas. However, the microcosmic reaction mechanism governing mercury oxidation on CuFe₂O₄ remains elusive. Herein, experiments and quantum chemistry calculations were conducted for understanding the chemical reaction mechanism of oxygen-assisted mercury oxidation on CuFe₂O₄. CuFe₂O₄ shows the optimal catalytic activity towards mercury oxidation at 150 ºC. The reactivity difference of different lattice oxygen species is associated with its atomic coordination environment. The lattice oxygen coordinating with two octahedral Cu atoms and a tetrahedral Fe atom shows higher catalytic activity towards mercury oxidation than other lattice oxygen atoms. The inverse spinel structure of CuFe₂O₄ is favorable for O₂ activation due to the Jahn-Teller effect, thereby promoting mercury oxidation. O₂ molecule preferably adsorbs on iron active site and dissociates into active oxygen species. Hg⁰ oxidation is a three-step reaction process: Hg⁰ adsorption, Hg(ads) → HgO(ads), and HgO desorption. The energy barrier of mercury oxidation by chemisorbed oxygen is lower than that of mercury oxidation by lattice oxygen. The chemisorbed oxygen preserves higher reactivity towards mercury oxidation than lattice oxygen. Hg(ads) → HgO(ads) is the rate-determining step of mercury oxidation by chemisorbed oxygen because of the higher energy barrier of 116.94 kJ/mol. This work could provide the theoretical guidance for the diversified structure design of highly-efficient catalysts used for elemental mercury oxidation.

Generation of multi-valence Cu x O by reduction with activated semi-coke and their collaboration in the selective reduction of NO with NH3

Multi-valence Cu_xO has been demonstrated to have high activity in the low-temperature selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). Here, Cu_xO was loaded onto activated semi-coke (ASC) for SCR, which has shown satisfactory low-temperature SCR activity. By virtue of the reduction property of carbon, the valence of Cu was regulated by simply adjusting the calcination temperature. The high concentration of Cu⁺ generated from the reduction of CuO by ASC during calcination can collaborate to form Cu²⁺/Cu⁺ circulation. After systematic characterization by XPS, H₂-TPD, and NH₃-TPR, it is revealed that abundant acidic sites and surface reactive oxygen species are formed on the surface of the catalysts. Further investigation with in situ DRIFTS confirms that the NH₃-SCR over the as-prepared CuO|Cu₂O-ASC catalysts simultaneously follows the Langmuir–Hinshelwood (L–H) and Eley–Rideal (E–R) pathways, attributed to the synergistic effects of Cu²⁺ and Cu⁺.

In this work, we systematically analyzed the structure–activity relationship and synergistic mechanism of activated semi-coke (ASC)-loaded multi-valence Cu_xO for the low-temperature NO-SCR process by NH₃.

Recent Advances in MnOx/CeO2-Based Ternary Composites for Selective Catalytic Reduction of NOx by NH3: A Review

Recently, manganese oxides (MnOx)/cerium(IV) oxide (CeO2) composites have attracted widespread attention for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia (NH3), which exhibit outstanding catalytic performance owing to unique features, such as a large oxygen storage capacity, excellent low-temperature activity, and strong mechanical strength. The intimate contact between the components can effectively accelerate the charge transfer to enhance the electron–hole separation efficiency. Nevertheless, MnOx/CeO2 still reveals some deficiencies in the practical application process because of poor thermal stability, and a low reduction efficiency. Constructing MnOx/CeO2 with other semiconductors is the most effective strategy to further improve catalytic performance. In this article, we discuss progress in the field of MnOx/CeO2-based ternary composites with an emphasis on the SCR of NOx by NH3. Recent progress in their fabrication and application, including suitable examples from the relevant literature, are analyzed and summarized. In addition, the interaction mechanisms between MnOx/CeO2 catalysts and NOx pollutants are comprehensively dissected. Finally, the review provides basic insights into prospects and challenges for the advancement of MnOx/CeO2-based ternary catalysts.

O3 oxidation excited by yellow phosphorus emulsion coupling with red mud absorption for denitration

Directing to unwieldiness NOx emitted by the industry, the removal of NOx was implemented using yellow phosphorus (P₄) emulsion and red mud slurry as composite absorbent. Where yellow phosphorus is considered to stimulate formation of the ecological ozone (O₃) from O₂, the oxidation of insoluble NO into water-soluble NOx species by O₃, and the red mud as a pH buffer can be used to maintain the pH of the absorption liquid in a range that better absorbs NOx. NO is finally converted into NO₂^- and NO₃^-, whereas the yellow phosphorus is mainly PO₄^3-. Single-factor influencing on the efficiency of denitration include the concentration of yellow phosphorus, reaction temperature, stirring intensity, gas flow rate, O₂ content, and red mud solid-liquid ratio were investigated. Response surface methodology (RSM) was used to optimize the process parameters. It was indicated that the removal rate of NOx can reach 99.3% under the optimal conditions. Moreover, the possible denitration reaction mechanism was also discussed.

Charge-distribution modulation of copper ferrite spinel-type catalysts for highly efficient Hg0 oxidation

Hg⁰ catalytic oxidation is an attractive approach to reduce mercury emissions from industrial activities. However, the rational design of highly active catalysts remains a significant challenge. Herein, the charge distribution modulation strategy was proposed to design novel catalysts: copper ferrite spinel-type catalysts were developed by introducing Cu²⁺ cations into octahedral sites to form electron-transfer environment. The synthesized catalysts with spinel-type stoichiometry showed superior catalytic performance, and achieved > 90 % Hg⁰ oxidation efficiency in a wide operation temperature window of 150-300 °C. The superior catalytic performance was closely associated with the mobile-electron environment of copper ferrite. Hg⁰ oxidation by HCl over copper ferrite followed the Eley-Rideal mechanism, in which physically adsorbed Hg⁰ reacted with active chlorine species. Density functional theory calculations revealed that octahedral Cu atom is the most active site of Hg⁰ adsorption on copper ferrite surface. Both direct oxidation pathway (Hg* → HgCl₂*) and HgCl-mediated oxidation pathway (Hg* → HgCl* → HgCl₂*) played important role in Hg⁰ oxidation over copper ferrite. HgCl₂* formation was identified as the rate-limiting step of Hg⁰ oxidation. This work would provide a new perspective for the development of admirable catalysts with outstanding Hg⁰ oxidation performance.

Asymmetric Supercapacitors Based on Hierarchically Nanoporous Carbon and ZnCo2O4 From a Single Biometallic Metal-Organic Frameworks (Zn/Co-MOF)

Metal-organic framework (MOF)-derived nanoporous carbons (NPCs) and porous metal oxide nanostructures or nanocomposites have gathered considerable interest due to their potential use in supercapacitor (SCs) applications, owing to their precise control over porous architectures, pore volumes, and surface area. Bimetallic MOFs could provide rich redox reactions deriving from improved charge transfer between different metal ions, so their supercapacitor performance could be further greatly enhanced. In this study, "One-for-All" strategy is adopted to synthesize both positive and negative electrodes for hybrid asymmetric SCs (ASCs) from a single bimetallic MOF. The bimetallic Zn/Co-MOF with cuboid-like structures were synthesized by a simple method. The MOF-derived nanoporous carbons (NPC) were then obtained by post-heat treatment of the as-synthesized Zn/Co-MOF and rinsing with HCl, and bimetallic oxides (ZnCo₂O₄) were achieved by sintering the Zn/Co-MOF in air. The as-prepared MOF-derived NPC and bimetallic oxides were utilized as negative and positive materials to assemble hybrid ASCs with 6 M KOH as an electrolyte. Owing to the matchable voltage window and specific capacitance between the negative (NPC) and positive (ZnCo₂O₄), the as-assembled ASCs delivered high specific capacitance of 94.4 F/g (cell), excellent energy density of 28.6 Wh/kg at a power density of 100 W/kg, and high cycling stability of 87.2% after 5,000 charge-discharge cycles. This strategy is promising in producing high-energy-density electrode materials in supercapacitors.

Understanding of NOx storage property of impregnated Ba species after crystallization of mesoporous alumina powders

The regulation of automobile exhaust gas, especially that concerning hazardous nitrogen oxide (called as NOx) becomes stricter year-by-year, which should be urgently corresponded for cleaning the NOx containing emission. According to surface affinity of γ-alumina to metal catalysts and its thermal stability, crystalline γ-alumina has been frequently utilized as catalyst supports showing relatively high specific surface area. From the viewpoint, we consider that highly porous alumina powders prepared using amphiphilic organic molecules are potential as such a catalyst support for improving NOx removing property. In this study, we report surface property of the mesoporous alumina powders against NOx molecules after crystallizing to its γ-phase and NOx storage property after impregnation of barium (Ba) acetate in the mesopores. Adsorption of NO with O₂ on mesoporous γ-alumina powders without Ba species were more likely to be bridging bidentate than chelating bidentate nitrates (NO₃^-) with comparing to commercially available γ-alumina powders. After impregnating the Ba species, admitted NO molecules were oxidized with enough O₂ and stored very strongly as ionic nitrate (NO₃^-) onto the Ba species even after heating at 500 °C. This preliminary study is helpful for designing mesoporous deNOx catalysts combined with unique storage/adsorption property.

Effects of ferric and manganese precursors on catalytic activity of Fe-Mn/TiO2 catalysts for selective reduction of NO with ammonia at low temperature

Fe-Mn/TiO₂ catalysts were prepared through the wet impregnation process to selective catalytic reduction of NO by NH₃ at low temperature, and series of experiments were conducted to investigate the effects of key precursors on their SCR performance. Ferric nitrate, ferrous sulfate, and ferrous chloride were chosen as Fe precursors while manganese nitrate, manganese acetate, and manganese chloride as Mn precursors. These precursors had been commonly used to prepare Fe-Mn/TiO₂ catalysts by numerous researchers. The results showed that there were distinct differences in NO conversion efficiencies at low temperature of catalysts prepared with different precursors. Catalysts prepared with ferric nitrate and manganese nitrate precursors exhibited the best catalytic performance at low temperature, while three kinds of catalysts prepared with manganese chloride precursors exhibited significantly low catalytic activity. All catalysts were characterized by XRD, SEM, H₂-TPR, NH₃-TPD, and XPS. The results indicated that when the catalysts were prepared with manganese nitrate or manganese acetate as precursors, Mn⁴⁺ contents and O_β/(O_β + O_α) ratios decreased in an order of ferric nitrate > ferrous sulfate > ferrous chloride, which was consistent with the change of catalytic activities of the corresponding catalysts at low temperature. It can be found that the excellent catalytic performance of Fe(A)-Mn(a)/TiO₂ was ascribed to high redox property and enrichment of Mn⁴⁺species and surface chemical labile oxygen groups.

Facet effect of Co3O4 nanocatalysts on the catalytic decomposition of ammonium perchlorate

Crystal facets can affect the catalytic decomposition of ammonium perchlorate, but the underlying mechanisms have long remained unclear. Here, we use the nanorods, nanosheets and nanocubes of Co₃O₄ catalysts exposing {110}, {111} and {100} facets as model systems to investigate facet effects on catalytic AP decomposition. The peak temperature of high temperature decomposition (HTD) process (T_HTD) of AP by nanorods, nanosheets and nanocubes Co₃O₄ decrease from 437.0 °C to 289.4 °C, 299.9 °C and 326.3 °C, respectively, showing obvious facet effects. We design experiments about AP decomposition under different atmospheres to investigate its mechanism and verify that the accumulation of ammonia (NH₃) on AP surface can inhibit its decomposition and that the facet effects are related to the adsorption and oxidation of NH₃. The binding energies of NH₃ on the {110}, {111} and {100} planes calculated via density functional theory (DFT) are -1.774 eV, -1.638 eV, and -1.354 eV, respectively, indicating that the {110} planes are more favorable for the adsorption of NH₃. Moreover, the {110} planes are readily to form CoNO structure, which benefits the further oxidation of the NH₃.

Highly efficient WO3-FeOx catalysts synthesized using a novel solvent-free method for NH3-SCR

WO₃-FeO_x catalysts with various WO₃ contents were synthesized through a facile solvent-free method, satisfying the selective catalytic reduction of NO (NH₃-SCR). Strikingly, the optimum 30 %WO₃-FeO_x catalyst with the largest surface area exhibited the most outstanding catalytic activity, achieving the nearly 100 % NO_x removal efficiency in a wide temperature window between 225-500 °C, which was better than that of Fe-W series catalysts reported in other studies. In addition, Raman and XPS results proved that the introduction of WO₃ altered the electronic environment of Fe₂O₃, inducing the formation of Fe₃O₄ (Fe²⁺) and surface adsorbed oxygen. In situ DRIFTS demonstrated that the interaction between WO₃ and Fe₂O₃ not only promoted the adsorption capacity of NH₃ on the catalyst, but also contributed to the formation of adsorbed NO_x species. NO_x reduction reaction on WO₃-FeO_x catalyst proceeded via the Eley-Rideal and Langmuir-Hinshelwood mechanism synchronously. All of these factors, jointly, accounted for the superior catalytic activity and N₂ selectivity of WO₃-FeO_x catalysts.