Transition Metal Oxide Catalysts Research Articles

Photodriven Methane Conversion on Transition Metal Oxide Catalyst: Recent Progress and Prospects.

Methane as the main component in natural gas is a promising chemical raw material for synthesizing value-added chemicals, but its harsh chemical conversion process often causes severe energy and environment concerns. Photocatalysis provides an attractive path to active and convert methane into various products under mild conditions with clean and sustainable solar energy, although many challenges remain at present. In this review, recent advances in photocatalytic methane conversion are systematically summarized. As the basis of methane conversion, the activation of methane is first elucidated from the structural basis and activation path of methane molecules. The study is committed to categorizing and elucidating the research progress and the laws of the intricate methane conversion reactions according to the target products, including photocatalytic methane partial oxidation, reforming, coupling, combustion, and functionalization. Advanced photocatalytic reactor designs are also designed to enrich the options and reliability of photocatalytic methane conversion performance evaluation. The challenges and prospects of photocatalytic methane conversion are also discussed, which in turn offers guidelines for methane-conversion-related photocatalyst exploration, reaction mechanism investigation, and advanced photoreactor design.

Catalytic desorption of CO2‐loaded solutions of diethylethanolamine using bentonite catalyst

AbstractThe absorption of CO2 gas into aqueous alkanolamine solutions is the most advanced CO2 separation technology and a key challenge in this technique is the energy‐intensive process of solvent regeneration. The tertiary amine N,N′‐diethylethanolamine (or DEEA) is a candidate CO2‐capturing solvent with potential. To improve the energy efficiency of regeneration of DEEA, several catalysts were used for desorbing CO2 from loaded solutions of DEEA (2.5 M) at T = 363 K. Desorption trials were conducted in batch mode. The initial CO2 loading varied in the 0.3–0.35 mol CO2/mol DEEA range. The performance was analyzed by calculating the rate of CO2 desorption, cyclic capacity, and reduction in sensible energy. The amount of thermal energy needed for amine regeneration was significantly lowered by using nine transition metal oxide catalysts and the hierarchy was as follows: Al2O3 < MoO3 < V2O5 < TiO2 < MnO2 < ZnO < Cr2O3 < SiO2 < ZrO2. Among the metal oxides, Al2O3 increased desorption efficiency compared to blank runs by 89%. A clay‐based powder bentonite was also used as catalyst and its efficacy was compared with the metal oxides. This cheap and easily available bentonite catalyst was tuned through simple ion‐exchange with four acids (HCl, H3PO4, HNO3, and H2SO4). Upon treatment with H2SO4, bentonite remarkably increased desorption efficiency by 100%. Furthermore, bentonite catalyst treated with sulphuric acid (denoted here as Bt/H2SO4) was characterized by Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM), Fourier transform infrared spectrometery (FTIR), X‐ray diffraction (XRD), and ammonia temperature‐programmed desorption (NH3‐TPD). In this way, a comprehensive study on catalytic desorption of DEEA was performed.

Synergistic Effect between Fe<sup>4+</sup> and Co<sup>4+</sup> on Oxygen Evolution Reaction Catalysis for CaFe<sub>1−</sub><i><sub>x</sub></i>Co<i><sub>x</sub></i>O<sub>3</sub>

Chemical substitution is an effective way to improve electrocatalytic properties in transition metal oxides. We investigate the synergistic effect between Fe4+ and Co4+ ions on the catalytic activity for oxygen evolution reaction (OER) in the Fe-Co-mixed perovskite oxide CaFe1–xCoxO3. The OER activity of CaFe1–xCoxO3 is substantially increased by small amounts of Co (Fe) doping into CaFeO3 (CaCoO3), leading to the superiority compared to the pure Fe and Co perovskite oxides. The x dependences of the OER overpotential and specific activity for CaFe1–xCoxO3 (0.05 ≦ x ≦ 0.95) are expressed by constant offset from the weighted average between CaFeO3 and CaCoO3, which can be interpreted to be the synergistic effect between Fe4+ and Co4+ ions on OER activity. The absence of the optimum x for the highest activity for CaFe1–xCoxO3 contrasts with the volcano-like plots reported in various mixed-metal oxides. First-principle calculations using the special quasirandom structure models on CaFe1–xCoxO3 (x = 0.03–0.5) demonstrate that about half the amount of Fe4+ is electronically activated to possess smaller charge-transfer energies, corroborating the enhancement of catalytic activity in CaFe1–xCoxO3. These findings provide new insight into the synergistic effects in complex transition metal oxide catalysts.

(Invited) High-Rate and High-Efficiency Molten Salt Li-O2 and Na-O2 Batteries Enabled by Nitrate Redox

The decarbonization of the electrical grid requires advancements in affordable, long-term energy storage to address the intermittency of energy sources like solar and wind, while the continued decarbonization of the transportation sector necessitates advancements in energy dense storage devices such as batteries, with low cost and made from abundant materials. Alkali metal-oxygen batteries can provide greater specific energy than Li-ion batteries and can potentially lower cost by reducing reliance on late transition metals like nickel and cobalt. In this talk, we share our recent work on molten-salt Li-O2 1 and Na-O2 2 batteries operating at 150-170 °C which can deliver high areal energy (33 mWh/cm2 geo) and power densities (19 mW/cm2 geo), with high energy efficiency (~90% at 5 mA/cm2 geo). We show that nitrate redox plays a critical role in these batteries, where the apparent four-electron oxygen reduction to form Li2O in Li-O2 batteries is facilitated by the electrochemical reduction of nitrate to nitrite, and subsequent chemical oxidation of nitrite to nitrate by molecular oxygen. In Na-O2 batteries, on the other hand, Na2O is further oxidized by O2 to Na2O2 to give an apparent two-electron oxygen reduction rection. We demonstrate the role of transition metal oxide catalysts for reducing the overpotential of nitrate redox in the positive electrode, as well as strategies for enhancing the power density and specific energy of these molten-salt metal-oxygen batteries. References (1) Zhu, Y. G.; Leverick, G.; Giordano, L.; Feng, S.; Zhang, Y.; Yu, Y.; Tatara, R.; Lunger, J. R.; Shao-Horn, Y. Nitrate-Mediated Four-Electron Oxygen Reduction on Metal Oxides for Lithium-Oxygen Batteries. Joule 2022, 6 (8), 1887–1903.(2) Zhu, Y. G.; Leverick, G.; Accogli, A.; Gordiz, K.; Zhang, Y.; Shao-Horn, Y. A High-Rate and High-Efficiency Molten-Salt Sodium–Oxygen Battery. Energy Env. Sci 2022, 15, 4636–4646.

Polyhedral Coordination Determined Co‐O Activity for Electrochemical Oxidation of Biomass Alcohols

AbstractEarth‐abundant transition metal oxides are promising electrocatalysts for oxidation of biomass alcohols. Here, CoO and Co3O4 are selected as representative cobalt oxide catalysts and grown on carbon fiber paper (CFP) electrodes to reveal the interplay between electronic structure and catalytic activity of catalysts for oxidation of glycerol, diols, and monohydric alcohols. In situ electrochemical tests elucidate that the CoO/CFP electrode has lower interfacial impedance, higher charge transfer, faster oxidation rate, and thereby the higher catalytic activity for alcohol oxidation than the Co3O4/CFP electrode. Especially for glycerol oxidation, the CoO/CFP electrode only requires 1.32 V to reach 10 mA cm−2, the potential is 120 mV lower than that for the Co3O4/CFP electrode. The CoO/CFP electrode can also produce value‐added products such as formate, acetate, and glycolate with high selectivity and efficiency at low energy consumptions from oxidation of biomass alcohols. Theoretical calculations further confirm the dominant role of octahedrally coordinated Co‐O sites in adsorption, activation, and oxidation of C3‐C1 alcohols. This work sheds light on the design of highly efficient transition metal oxide catalysts for oxidation of alcohols by populating octahedral sites in the crystal structure.

Uniform one-dimensional hierarchical CoOx-N-C feather duster breaking the activity-stability trade-off for hydrogenation reactions

Carbon-based non-precious metal/oxide catalysts always face the dilemma of a trade-off between high activity and long-term stability in hydrogenation reactions, detrimental to their practical applications. Herein we propose a unique and uniform feather duster-type catalyst as a promising concept to solve this activity-stability trade-off. The feather duster is composed of both exposed and embedded active sites, that can facilitate substrate adsorption/activation and can be resistant against site leaching during the non-neutral aqueous phase reactions. We constructed a uniform one-dimensional hierarchical transition metal oxide-containing N-doped carbon (TMO-N-C), especially for CoOx-N-C (x = 1.0 or 1.3), based on a controlled depolymerization-reassembly strategy before morphology-conserved pyrolysis. The uniform, thermally stable feather duster-type precursor was induced in view of the interaction between cobalt 2-methylimidazolate framework (ZIF-67) polyhedron and zinc-trimesic acid (ZnBTC) fiber in water. And subsequent pyrolysis leads to a uniform CoOx-N-C nanoarchitecture with numerous nanorods planted on a single fiber. The resulting CoOx-N-C proves to be highly efficient for 4-nitrophenol reduction, delivering a high activity factor of 7.32 s−1 g−1 with exceptional long life over 10 cycles, thus ranking CoOx-N-C one of the best transition metal oxide catalysts for hydrogenation reactions. This research overcomes the dilemma that either high activity contributed by surface active sites or superior stability provided by embedded ones, offering valuable guidance for rational design of efficient metal/oxide catalysts.

Cation Substitution Induced d‐Band Center Modulation on Cobalt‐Based Spinel Oxides for Catalytic Ozonation

AbstractCo3O4 spinel is a promising transition metal oxide (TMO) catalyst for the catalytic ozonation of volatile organic compounds (VOCs). Herein, metal–organic frameworks (MOFs)‐derived Ni‐ and Mg‐ substituted Co3O4 catalysts retain similar spinel structures, but display improved and reduced ozonation performance of methyl mercaptan (CH3SH), respectively. Remarkably, the NiCo2O4 catalyst can still ≈90% removal of CH3SH after running for 20 h at room temperature under an initial concentration of 50 ppm CH3SH and 40 ppm O3, relative humidity of 60%, and space velocity of 300 000 mL h−1 g−1, exceeding the reported values. Experimental characterizations have unveiled that the substitution of Ni and Mg into the Co3O4 spinel altered surface acidity, oxygen species mobility, and Co2+/Co3+ ratio. The in situ Raman spectra reveal the dynamic formation Co(III)‐Oad* via the transformation of O3 into surface atomic oxygen (Oad*) and peroxide species (O2*). Theoretical calculations verify that Ni‐substitution increases nonuniform charges and Fermi density, leading to a moderate increase in d‐band center energy levels, thereby promoting O3 specific adsorption/activation to convert Oad*/O2* and •OH/1O2/•O2−, which contributes to eliminate CH3SH and prevent poisoning. The concept of tuning the d‐band center can provide valuable insights for the design of other catalysts for catalytic ozonation.

Transition Metal Oxides as Hydrogen Evolution Electrocatalyst: Scientometric Analysis

Background:The development of cost-effective and high-activity hydrogen evolution reaction (HER) electrocatalysts is limiting the implementation of hydrogen production from electrochemical and photoelectrochemical water splitting, which is seen as a potential technology for clean energy production and long-term energy storage. Transition metal oxide catalysts, a large class of functional materials with variable elemental compositions and crystal shapes, have piqued the interest of scientists. Now, a scientific-based appraisal of the progress in this scientific field is required, as well as identification of the most promising materials and technologies, as well as present constraints and future commercialization chances.Methods:This article presents a scientometric analysis of transition metal oxides as hydrogen evolution electrocatalysts in the scientific field. 1421 publications from the Web of Science (WoS) database were extracted using a mix of relevant keywords and examined using multiple scientometric indexes utilizing Python and Anaconda Prompt, ScientoPy, and Citespace. It stated that the first point in this subject was in 1992, and that scientific progress has substantially increased since 2015.Results:Articles and reviews account for 83.8 percent and 13.4 percent of all items published in this category, respectively.Conclusion:The top two contributor countries were identified as China and the United States. The most active journal in this field is Journal of Materials Chemistry A.

Carbon cloth supporting spinel CuMn0.5Co2O4 nanoneedles with the regulated electronic structure by multiple metal elements as catalysts for efficient oxygen evolution reaction

Developing transition metal oxide catalysts to replace the noble metal oxide catalysts for efficient oxygen evolution reaction (OER) is essential to promote the practical application of water splitting. Herein, we designed and constructed the carbon cloth (CC) supporting spinel CuMn0.5Co2O4 nanoneedles with regulated electronic structure by multiple metal elements with variable chemical valences in the spinel CuMn0.5Co2O4. The carbon cloth not only provided good conductivity for the catalytic reaction but also supported the well-standing spinel CuMn0.5Co2O4 nanoneedles arrays with a large special surface area. Meanwhile, the well-standing nanoneedles arrays and mesoporous structure of CuMn0.5Co2O4 nanoneedles enhanced their wettability and facilitated access for electrolyte to electrochemical catalysis. Besides, the regulated electronic structure and generated oxygen vacancies of CuMn0.5Co2O4/CC by multiple metal elements improved the intrinsic catalytic activity and the durability of OER activity. Profiting from these merits, the CuMn0.5Co2O4/CC electrode exhibited superior OER activity with an ultralow overpotential of 189 mV at the current density of 10 mA⋅cm−2 and a smaller Tafel slope of 64.1 mV⋅dec−1, which was competitive with the noble metal oxides electrode. And the CuMn0.5Co2O4/CC electrode also exhibited long-term durability for OER with 95.3% of current retention after 1000 cycles. Therefore, the competitive OER activity and excellent cycling durability suggested that the CuMn0.5Co2O4/CC electrode is a potential candidate catalyst for efficient OER.

Investigating High-Performance Non-Precious Transition Metal Oxide Catalysts for Nitrogen Reduction Reaction: A Multifaceted DFT–kMC–LSTM Approach

Investigating High-Performance Non-Precious Transition Metal Oxide Catalysts for Nitrogen Reduction Reaction: A Multifaceted DFT–kMC–LSTM Approach

Enhanced oxygen evolution performance of iron-nickel oxide catalyst through dual-defect engineering

Transition metal oxides have been extensively investigated for oxygen evolution reaction (OER). While the introduction of oxygen vacancies (Vo) was found to be an effective way to enhance the electrical conductivity and the OER electrocatalytic activity of transition metal oxides, the oxygen vacancies are easily damaged during the long-term catalytic process, resulting in rapid decay of the electrocatalytic activity. Herein, we proposed the strategy of dual-defect engineering to enhance the catalytic activity and stability of NiFe2O4 by filling the oxygen vacancies of NiFe2O4 with phosphorus atoms. The filled P atoms could form coordination with iron and nickel ions to compensate the coordination number and optimize the local electronic structure, which not only enhances the electrical conductivity but also improves the intrinsic activity of the electrocatalyst. Meanwhile, the filling of P atoms could stabilize the Vo and thus improving the cycling stability of the material. The theoretical calculation further demonstrates that the improvement in conductivity and intermediate binding by P refilling remarkably contributes to enhancing the OER activity of NiFe2O4-Vo-P. Benefiting from the synergistic effect of filled P atoms and Vo, the derived NiFe2O4-Vo-P exhibits fascinating activity with ultra-low OER overpotentials of 234 and 306 mV at 10 and 200 mA cm−2, together with the good durability for 120 h at relatively high current density of 100 mA cm−2. This work sheds light on the design of high-performance transition metal oxide catalysts through defect regulation in the future.

Enhanced Catalytic Performance of La-Doped CoMn2O4 Catalysts by Regulating Oxygen Species Activity for VOCs Oxidation

Tuning the oxygen species activity of transition metal oxide catalysts is essential for harnessing environmental catalytic oxidation reactions, and the mobility of lattice oxygen and activation ability of gas oxygen over CoMn2O4 were well modulated by in situ solvothermal-induced lanthanum doping for VOCs oxidation. Lanthanum species were successfully doped into the spinel lattice, and the toluene oxidation rate of Co1Mn1.9La0.1 increased by 20.74 times compared with that of the undoped Co1Mn2 catalyst at 220 °C. With lanthanum doping, the content of oxygen vacancies was remarkably increased (1.4-fold) due to oxygen atoms in unusual bonding situations. Higher mobility and reactivity of lattice oxygen species were achieved by weakening the metal–oxygen bond strength, favoring the conversion of toluene to CO2 and H2O. Furthermore, doping lanthanum into the spinel lattice also promoted the activation and replenishment of gas oxygen. Moreover, the oxidation of anhydride species, as a rate-determining step, could be significantly accelerated due to lanthanum doping. This work provided a credible comprehension of the regulation of spinel oxygen species by lanthanum doping in VOCs abatement to improve the atmospheric environment.

Manganese dioxide supported on hollow graphitized carbon spheres for the catalytic oxidation of toluene: Improved adsorption and electron transfer

It would be beneficial to design transition metal-based catalysts having large surface areas and good electron transmission efficiency to replace expensive noble metal-based catalysts. In this work, hollow graphitized carbon spheres loaded with MnO2 nanosheets were prepared by a simple two step hydrothermal method. The large surface area and excellent conductivity promote the catalytic oxidation of toluene. The optimized catalyst degraded 90 % of 500 ppm-toluene at 180 °C and so out-performed some noble metal-based catalysts. The working mechanism and intermediates are also explained in detail. This study demonstrates that the adsorption capacity and conductivity of a carbon-based support prepared by a simple process can promote catalytic performance and is expected to assist in the development of high-performance transition metal oxide catalysts for volatile organic compound remediation.

Selectivity Control of Oxygen Reduction Reaction over Mesoporous Transition Metal Oxide Catalysts for Electrified Purification Technologies

Direct electrification of oxygen-associated reactions contributes to large-scale electrical storage and the launch of the green hydrogen economy. The design of the involved catalysts can mitigate the electrical energy losses and improve the control of the reaction products. We evaluate the effect of the interface composition of electrocatalysts on the efficiency and productivity of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), both mechanistically and at device levels. The ORR and OER were benchmarked on mesoporous nickel(II) oxide and nickel cobaltite (NiO and NiCo2O4, respectively) obtained by a facile template-free hydrothermal synthesis. Physicochemical characterization showed that both NiO and NiCo2O4 are mesoporous and have a cubic crystal structure with abundant surface hydroxyl species. NiCo2O4 showed higher electrocatalytic activity in OER and selectivity to water as the terminal product of ORR. On the contrary, ORR over NiO yielded hydroxyl radicals as products of a Fenton-like reaction of H2O2. The product selectivity in ORR was used to construct two electrolyzers for electrified purification of oxygen and generation of hydroxyl radicals.

Enhanced two-electron oxygen reduction for hydrogen peroxide production via fine-tuning the concentration of oxygen vacancies in MoO3−x

Producing hydrogen peroxide (H2O2) electrochemically is a possible alternative to industrial anthraquinone synthesis. Herein, α-MoO3−x catalysts with controllable oxygen vacancies for two-electron Oxygen Reduction (2e− ORR) electrocatalysis have been reported for the first time. Because of the finely tuned oxygen vacancy concentration of α-MoO3−x-400, it is particularly sensitive to the production of H2O2, providing very high selectivity in H2O2 production reaches as high as 93% with an average yield of 310 mmol gcat−1 h−1 over 12 h. In addition to providing us with the most abundant and attractive transition metal oxide (TMO) catalysts yet, this research provides a fresh approach to the rational development of effective TMO catalysts.

Carbon-Free, Binder-Free MnO2@Mn Catalyst for Oxygen Reduction Reaction

Reasonable design and feasible preparation of low-cost and stable oxygen reduction reaction (ORR) catalysts with excellent performance play a key role in the development of fuel cells and metal-air batteries. A 3D porous superimposed nanosheet catalyst composed of metal manganese covered with MnO2 nanofilms (P-NS-MnO2@Mn) was designed and synthesized by rotating disk electrodes (RDEs) through one-step electrodeposition. The catalyst contains no carbon material. Therefore, the oxidation and corrosion of the carbon material during use can be avoided, resulting in excellent stability. The structural and composition characterizations indicate that the nanosheets with sharp edges exist on the surface of the wall surrounding the macropore (diameter ∼ 5.07 μm) and they connect tightly. Both the nanosheets and the wall of the macropore are composed of metal manganese covered completely with MnO2 film with a thickness of less than 5 nm. The half-wave potential of the synthesized P-NS-MnO2@Mn catalyst is 0.86 V. Besides, the catalyst exhibits good stability with almost no decay after a 30 h chronoamperometric test. Finite element analysis (FEA) simulation reveals the high local electric field intensity surrounding the sharp edges of the nanosheets. Density functional theory (DFT) calculations reveal that the novel nanosheet structure composed of MnO2 nanofilms covered on the surface of the Mn matrix accelerates the electronic transfer of the MnO2 nanofilms during the ORR process. The high local electric field intensity near the sharp edge of the nanosheets effectively promotes the orbital hybridization and strengthens the adsorbing Mn-O bond between the active site Mn in the nanosheets and the intermediate OOH* during the ORR process. This study provides a new strategy for preparing transition metal oxide catalysts and a novel idea about the key factors affecting the catalytic activity of transition metal oxides for the ORR.

Construction of Nitrogen‐Doped Carbon Functionalized Ni(OH)2 for Selective CO2 Photoreduction

AbstractPhotoreduction of CO2 is a sustainable way of producing carbon‐neutral fuels. However, the development of robust catalyst for the selective reduction of CO2 to value‐added chemicals remains a challenge. Using a photochemical approach, encapsulation of nitrogen‐doped carbon and Ni(OH)2 is to form a hybrid (Ni(OH)2/NC) for photocatalytic CO2 reduction. The as‐synthesized Ni(OH)2/NC catalyst exhibits enhanced selectivity and activity for CO production under visible light. A CO evolution rate of 14.93 μmol h−1 with 84 % selectivity is achieved, outperforming the bare Ni(OH)2 (8.51 μmol h−1 evolution rate and 45 % selectivity for CO). The structures and physicochemical properties of the Ni(OH)2/NC were carefully analyzed by various characterization techniques and density functional theory (DFT) calculations. Results suggest that the NC modification can modulate the electronic structures of Ni(OH)2, and thus consolidate the adsorption and activation of CO2. Furthermore, the interface charge transfer between photosensitizer and Ni(OH)2 can also be promoted after NC modification. As a result, the Ni(OH)2/NC catalyst exhibits superior activity for CO2‐to‐CO photoreduction. Moreover, NC‐encapsulated NiO (NiO/NC) was successfully prepared under the same conditions. This NiO/NC catalyst also exhibits enhanced selectivity and activity for CO2 reduction with visible light, suggesting the effectiveness of carbon‐modification for managing CO2 reduction. This work provides a new avenue to modulate the selectivity and activity of CO2 reduction over transition metal hydroxides or oxide catalysts.

Modulating surface cation vacancies of nickel-cobalt oxides as efficient catalysts for lithium-oxygen batteries

• The Ni-Co oxide (NCO) with cationic vacancies is developed by a facile approach. • The electronic modulation induced by cationic vacancies can enhance the electrocatalytic activity for ORR and OER. • The NCO-350-based LOB exhibits remarkable discharge capacity and cycle life. Rechargeable lithium-oxygen batteries (LOBs) have received incremental attention owing to their high energy density and applicability to mobile devices and electric vehicles. However, the lack of robust, low-cost, and environmentally benign bifunctional catalysts is a major impediment to the commercial application. The introduction of vacancies is one of the effective strategies to enhance the performance of cathode catalysts for lithium-oxygen batteries, but the preparation is complicated. In this work, needle-like microsphere cathode catalysts of nickel-cobalt oxide containing cationic vacancies are constructed by controlling the annealing temperature. It is demonstrated that the presence of cationic vacancies can modulate the electronic structure of the catalyst, reduce the energy barrier for the oxygen electrode reaction, meanwhile enhance the bifunctional catalytic activity. Impressively, the nickel-cobalt oxide-based LOB with cationic vacancies exhibits large specific capacity (12205 mAh g –1 at 200 mA g –1 ) and good durability. This work provides worthwhile insight into the formation and catalytic enhancement mechanism of transition metal oxide catalysts with cationic vacancies, and to some extent, the creation of efficient and low-cost oxygen electrocatalysts for LOBs.

Effective hydrodeoxygenation bio-oil via natural zeolite supported transition metal oxide catalyst

Bio-oil from biomass pyrolysis is promising to be used as a sustainable biofuel and high-value-added chemical. However, the presence of high acid, water, and oxygenate causes corrosive properties, low higher heating value (HHV), and instability of the bio-oil component. Therefore, refining the bio-oil is essential to improve its quality. In this study, we introduced natural zeolite (HZ) impregnated with transition metal oxide (TMO) to refine the bio-oil using the hydrodeoxygenation method (HDO) at various catalyst ratios and temperatures. We find that ZnO/HZ 5% wt. shows the best catalytic performance, with the conversion of organic phase reaching ∼ 50%. The refined bio-oil from Fe2O3, ZnO, and CuO has high-quality physicochemical properties with carbon, oxygen, water level, and HHV values are 37–52%, 40–53%, 8–27%, and 17–21 MJ/kg, respectively. This result represents a high catalytic performance for the hydrodeoxygenation process of bio-oil using natural zeolite-based transition metal oxide for better and low-cost biofuel production.

Emission Control of Toluene in Iron Ore Sintering Using Catalytic Oxidation Technology: A Critical Review

Iron ore sintering flue gas containing large amounts of volatile organic compounds (VOCs) can form secondary photochemical smog and organic aerosols, thus posing a serious threat to human health and the ecological environment. Catalytic combustion technology has been considered as one of the most prospective strategies for VOC elimination. This paper focuses on a review of studies on catalytic removal of typical VOCs (toluene) on transition metal oxide catalysts in recent years, with advances in single metal oxides, multi-oxide composites, and supported metal oxide catalysts. Firstly, the catalytic activities of a series of catalysts for toluene degradation are evaluated and compared, leading to an analysis of the key catalytic indicators that significantly affect the efficiency of toluene degradation. Secondly, the reaction pathway and mechanism of toluene degradation are systematically introduced. Considering the site space and investment cost, the conversion of VOC pollutants to harmless substances using existing selective catalytic reduction (SCR) systems has been studied with considerable effort. Based on the current development of simultaneous multi-pollutant elimination technology, the interaction mechanism between the NH3-SCR reaction and toluene catalytic oxidation on the surface is discussed in detail. Finally, views on the key scientific issues and the challenges faced, as well as an outlook for the future, are presented. This overview is expected to provide a guide for the design and industrial application of NO/VOC simultaneous removal catalysts.