Metal Oxide Catalysts Research Articles

Synthesis and Photodegradation of Bi2O3 and Pb-Bi2O3 Nanoparticles and Their Kinetic Study

The green synthesis method synthesized the bismuth oxide and lead-doped bismuth oxide nanoparticles using Ferula Asafoetida leaves extract. The lead-doped bismuth oxide showed greater degradation efficiency than undoped bismuth oxide. This greater efficiency was due to decreases in the band gap energy between the valence band and the conduction band of the metal oxide and reduced the chances of electron-hole pair recombination with the metal oxide catalyst. The synthesized nanoparticles were characterized by TGA, SEM, FT-IR, EDX, XRD, and UV- visible Spectrometer. XRD diffraction of Bi2O3 nanoparticles reflected a strong and sharp peak at 32.8˚ shows that Bi2O3 nanoparticles are in crystalline phase. The crystallite size of Bi2O3 nanoparticles is 13.433 nm, and Pb-doped Bi2O3 is 9.6 nm, calculated from the Debye-Scherrer equation. The synthesized Bi2O3 nanoparticles are round in shape with average size of ~ 90-100 nm While Pb doped Bi2O3 is ~ 75 f- 100 nm. The EDX spectra showed no additional peak for any impurities. The degradation rate of Malachite Green dye (MG) increased with the increase in contact time and temperature, while it decreased with increasing dye initial concentration and pH. Initially, the degradation efficiency of the bismuth oxide catalyst is increased with increasing catalyst amount, but after a certain amount of catalyst, it started decreasing as the catalyst amount was further increased. The irradiation time on photodegradation is deliberate, keeping other parameters steady at catalyst quantity 0.01 g at specific reaction conditions. Moreover, the dye showed an increase in degradation at 50 oC. The utmost degradation of 92% was observed for lead-doped bismuth oxide (Bi2O3) and 76% for undoped bismuth oxide (Bi2O3).

Engineering the Crystal Facet of Monoclinic NiO for Efficient Catalytic Ozonation of Toluene.

Modulating oxygen vacancies of catalysts through crystal facet engineering is an innovative strategy for boosting the activity for ozonation of catalytic volatile organic compounds (VOCs). In this work, three kinds of facet-engineered monoclinic NiO catalysts were successfully prepared and utilized for catalytic toluene ozonation (CTO). Density functional theory calculations revealed that Ni vacancies were more likely to form preferentially than O vacancies on the (110), (100), and (111) facets of monoclinic NiO due to the stronger Ni-vacancy formation ability, further affecting O-vacancy formation. Extensive characterizations demonstrated that Ni vacancies significantly promoted the formation of O vacancies and thus reactive oxygen species in the (111) facet of monoclinic NiO, among the three facets. The performance evaluation showed that the monoclinic NiO catalyst with a dominant (111) facet exhibits excellent performance for CTO, achieving a toluene conversion of ∼100% at 30 °C after reaction for 120 min under 30 ppm toluene, 210 ppm ozone, 45% relative humidity, and a space velocity of 120 000 h-1. This outperformed the previously reported noble/non-noble metal oxide catalysts used for CTO at room temperature. This study provided novel insight into the development of highly efficient facet-engineered catalysts for the elimination of catalytic VOCs.

In Situ X-ray Diffraction as a Basic Tool to Study Oxide and Metal Oxide Catalysts

X-ray diffraction (XRD) is a standard technique that is widely applied in heterogeneous catalysis to determine phase composition, atomic structure, and size of crystallites. This review is focused on the application of in situ XRD for studying the catalysts during their “lifetime” (under synthesis, activation, operation, and deactivation conditions), limiting the objects of research to oxide and metal oxide catalysts. Also included is a brief overview of modern techniques and instruments and the latest works illustrating different aspects of this technique in catalyst research. The main conclusion is that the field of heterogeneous catalysis research would benefit substantially from the application of in situ XRD for the structural, phase, and morphological characterization of solid catalysts. Even more useful information can be obtained if XRD is combined with other techniques that are more sensitive at length scales different from that of XRD.

Manganese oxide on activated carbon with peroxymonosulfate activation for enhanced ciprofloxacin degradation: Activation mechanism and degradation pathway

Ciprofloxacin, a threat to the ecosystem, can be effectively eliminated through heterogeneous activation of peroxymonosulfate (PMS) by metallic oxide catalyst-based advanced oxidation processes (AOPs). Manganese oxide (MnOx) is one of the most promising metallic oxide catalysts and deserves in-depth research. Herein, a MnOx catalyst loaded on activated carbon (MnOx/AC) was synthesized for the PMS activation in ciprofloxacin degradation. Due to the anchoring effect of AC on MnOx, the MnOx/AC catalyst demonstrated excellent performance and good reusability in ciprofloxacin degradation. Characterizations indicated that redox circles among valence states of Mn(II, III, and IV) species on the catalyst surface triggers the generation of reactive oxygen species (ROS), including 1O2, HO and SO4−. Notably, predominant species was 1O2 (63.75%), which ascribed to the increment of Mn(II) and the consumption of lattice oxygen. Moreover, the PMS adsorption on the MnOx/AC catalyst was a crucial step in ciprofloxacin degradation, and the acidic condition was favored. Furthermore, seven intermediates were detected by liquid chromatograph mass spectrometer during the ciprofloxacin degradation, thus establishing the degradation pathway of the ciprofloxacin. This study provided insights into activation mechanism and degradation pathway for the ciprofloxacin degradation by MnOx in water treatment using PMS–AOPs reaction system.

Engineering metal oxide catalyst for volatile organic compounds oxidation

Engineering metal oxide catalyst for volatile organic compounds oxidation

Indoor water splitting for hydrogen production through electrocatalysis using composite metal oxide catalysts

This study explores an optimistic approach for large-scale hydrogen production by employing electrocatalysts based on nickel, cobalt, iron, and aluminum oxides as alternatives to costlier metals. This approach offers a cost-effective solution to electrolysis in water media for hydrogen production. This investigation is focused on the electrolysis process, engaging NiO–Al2O3–CoO–Fe2O3 in 1M solution of NaOH and KOH. The environmental and economic analyses are conducted to evaluate the overall effect and cost-effectiveness of the electrolysis process. These findings provide valuable insights into the performance, feasibility, and challenges of using oxides of aluminum, nickel, iron, and cobalt in electrolysis for hydrogen production. The structural and morphological analyses of metal oxides are conducted using XRD and SEM tools, which showed reduced crystallinity and open pore structure of the samples. Cyclic Voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS), and Linear Sweep Voltammetry (LSV) revealed a higher electrocatalytic activity, a larger electrochemical active surface area, a higher current density, and a high density of active sites of NiO–Al2O3–CoO–Fe2O3 composite. Electrode 1 of the composite catalyst produced 500 ml of hydrogen after 30 min of the process, while electrodes 2 and 3 produced 263 and 249 ml of hydrogen, respectively. This study also elucidated the electrocatalytic mechanism involved in water splitting using these composite materials.

Magnetically recyclable high-entropy metal oxide catalyst for aerobic catalytic oxidative desulfurization

High-entropy metal oxide catalysts have demonstrated exceptional performance in activating oxygen for catalytic oxidation reactions. However, ensuring their ease of cyclic usage is crucial to broadening their applications. To address this challenge, a magnetic and recyclable high-entropy metal oxide catalyst is designed and its potential in catalytic oxidative desulfurization of fuel oils with oxygen as the oxidant is also investigated. Through a mechanochemical-assisted calcination method, the magnetic high-entropy metal oxide catalyst (HEMO-900) is successfully synthesized, and the structure of the HEMO-900 catalyst is comprehensively characterized. In addition, it is found that the high-entropy structure facilitates charge modulation of the active components, thereby enhancing the oxygen activation performance. The HEMO-900 catalyst exhibits exceptional oxygen activation ability and catalytic oxidative desulfurization performance, and 96.9% of sulfur removal is achieved under optimal conditions. In addition, it achieves profound desulfurization of various fuel samples by efficiently oxidizing and eliminating diverse aromatic sulfur compounds. Additionally, the catalyst demonstrates outstanding magnetically recyclable properties, enabling rapid separation and recovery from the fuel oil phase through the application of an external magnetic field, making the HEMO-900 catalyst maintained a remarkable sulfur removal efficiency of 91.1% even in the 5th cycle. Therefore, this magnetically recyclable HEMO-900 catalyst possesses significantly promising research and application prospects in the field of catalytic oxidative desulfurization of fuel oils with oxygen as the oxidant, as well as some other heterogeneous catalysis.

F-doped Co3O4 with Pt-like activity and excellent stability for hydrogen evolution reaction in alkaline media

The development of Pt-like active and highly stable non-noble metal catalysts is vital for promoting hydrogen production by water electrolysis. Herein, we develop strong electronegative heteroatom fluorine (F) doped decoration Co3O4 (F-Co3O4) as an effective catalyst for hydrogen evolution reaction (HER) in 1 M KOH. As expected, the chemical states, specific surface area, and defects of Co3O4 are regulated without changing phase composition after F-doped decoration. As a result, a remarkable reduction in an overpotential by about 90 mV (@10 mA cm−2, 1 M KOH) for F-Co3O4 compared with Co3O4 is realized. It only needs an overpotential of 46 mV at 10 mA cm−2 is comparable to Pt/C. More importantly, F-Co3O4 also shows super stability up to 100 h. Therefore, our exploration provides a novelty heteroatom-doped decoration non-precious metal oxide catalysts for hydrogen evolution reactions and other applications.

Mechanism study on a novel Cu/Fe co-catalyzed heterogeneous particle mediated activation of hydrogen peroxide for efficient degradation of cyanides

The degradation of cyanide ions in cyanide containing wastewater is a major challenge in water treatment. This article uses a one-step hydrothermal method to prepare iron copper bimetallic oxide catalysts and applies them for the rapid degradation of cyanide (CN−). The experimental results show that under the optimal conditions, the removal rate of CN− in the bimetallic catalyst/H2O2 system is close to 100 % within 1 h, which is significantly better than the single metal oxide catalyst. Through characterization by SEM-EDS, XRD, XPS, and combined with catalytic performance experimental analysis, it was found that. There is a synergistic effect between Cu and Fe, which can effectively promote the cycling of metal ions and improve the generation efficiency of hydroxyl radicals (HO•) to a certain extent, thereby improving the degradation performance of CN−. In addition, ion chromatography was used to analyze the oxidation products and a possible pathway for CN− degradation was proposed. This provides new insights and ideas for the development of high-performance bimetallic oxide catalysts for cyanide degradation.

Enhancing ozone catalytic decomposition through acid treatment of α-MnO2 for improved activity and humidity resistance

Post-etching method using dilute acid solutions is an effective technology to modulate the surface compositions of metal-oxide catalysts. Here the α-MnO2 catalyst treated with 0.1 mol/L nitric acid exhibits higher ozone decomposition activity at high relative humidity than the counterpart treated with acetic acid. Besides the increases in surface area and lattice dislocation, the improved activity can be due to relatively higher Mn valence on the surface and newly-formed Brønsted acid sites adjacent to oxygen vacancies. The remnant nitro species deposited on the catalyst by nitric acid treatment is ideal hydrophobic groups at ambient conditions. The decomposition route is also proposed based on the DRIFTS and DFT calculations: ozone is facile to adsorb on the oxygen vacancy, and the protonic H of Brønsted acid sites bonds to the terminal oxygen of ozone to accelerate its cleavage to O2, reducing the reaction energy barrier of O2 desorption.

Quench-induced surface engineering of NiO/NiSex hybrid for highly enhanced electrochemical oxygen evolution reaction

To achieve sustainable production of hydrogen fuel through electrochemical water splitting, efficient and low-cost electrocatalysts for the kinetically sluggish oxygen evolution reaction (OER) are urgently desirable. Herein, a calcination-quenching strategy to synthesize core-shell NiO@NiSex nanorods (NiSex is the core and NiO is the shell) with tailored surface compositions and structures is reported. In the calcination step, a NiO@NiSex nanostructure supported on nickel foam (NF) (denoted as NiO@NiSex/NF) was formed via oxidative calcination of highly conductive NiSe in situ grown on NF (denoted as NiSe/NF). Subsequently, the as-obtained NiO@NiSex/NF electrode at the high temperature state was rapidly placed in a cold iron salt solution to achieve the quenching process. The results conclusively demonstrated that the quenching process resulted in iron doping and vacancies generation on the surface of NiO, effectively reconfiguring the desired surface of the catalyst, thus giving rise to notably enhanced electrocatalytic performance for OER in alkaline media. As a result, the quenched-engineered NiO@NiSex/NF electrode requires ultralow overpotentials of 231 and 265 mV to yield current densities of 10 and 100 mA cm−2, respectively, and presents a very low Tafel slope of 28.9 mV dec−1 as well as excellent durability, with the performance superior to most of metal oxide catalysts reported to date. This work extends the use of quenching chemistry in fabrication of high-performance metal oxide catalysts for energy related applications.

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.

Improved high-temperature sintering resistance and light alkanes oxidation activity by La modification on Co3O4 nanocatalyst

In order to improve low-temperature activity and high-temperature sintering resistance of cobalt-based catalyst in catalytic degradation of emitted light alkanes, a strategy of lanthanum modification on Co3O4 nanocrystalline is proposed in this work, aiming at efficiently degrading methane emitted from the oil and natural gas industry. The La-Co3O4 nanocatalysts were prepared by co-precipitation method, and the effects of La-doping on the structural features and catalytic activity of Co3O4 catalyst for methane oxidation were investigated. The results show that the addition of lanthanum forms a composite catalyst containing La2O3 phase and Co3O4 phase, and the interaction increases the active species, which can significantly improve the low-temperature activity of the catalyst. Among them, 5 % La-Co3O4-F catalyst shows the best catalytic activity with 48 °C decrease of the temperature of 90 % methane conversion (T90) compared with pure Co3O4. Furthermore, with thermal ageing treatment at 750 °C for 100 h, Lanthanum modification can significantly slow down the agglomeration growth and the decrease of specific surface area of cobalt oxide in long-term running, and the formation of perovskite structure (LaCoO3) on Co3O4 nanocrystal surface increases the mobility of lattice oxygen, further inhibiting the sintering deactivation of Co3O4 catalyst. The catalyst doped with 5 % La still showed better performance after thermal ageing, which is 127 °C lower than T90 of pure Co3O4. Moreover, this advantage of La doping can also be observed in the catalytic oxidation of other typical hydrocarbons such as ethane and propane. This work provides a promising strategy toward the design of advanced non-precious metal oxide catalysts for practical catalytic oxidation application with excellent high-temperature sintering resistance.

Molten Salt-Derived RuO2 Nanocrystals and Nanowires: Unveiling Correlations of Morphology, Microstructure, and Electrocatalytic Performance.

Ruthenium oxide (RuO2), due to its comparable binding energy with *H and cost-effectiveness against Pt, has emerged as a pivotal electrocatalyst for oxygen evolution reaction (OER). In the present study, RuO2 nanocrystals (NCs) and nanowires (NWs) were obtained by a molten salt process and the morphology, crystal structure, and local bonding features were examined by using electron microscopy and X-ray absorption spectroscopy. From the electrochemical measurement, both RuO2 NCs and NWs exhibit favorable stability and activity toward oxygen evolution reaction in an alkali medium, althought NCs exhibit higher activity, which is likely attributed to the larger surface area and the high local structural disorder. The theoretical calculation reveals that RuO2 NWs with a primary (110) orientation show a higher overpotential due to its d-band center's proximity to the Fermi level versus (101). The present work suggests that the molten salt process could be an efficient method for producing metal oxide catalysts with tailorable geometry and performances.

Efficient Co‐Cu/MS Catalyst from the Sequential Adsorption and Carbonization of Melamine Foam for the Selective Oxidation of 5‐Hydroxymethylfurfural (HMF)

AbstractPolyethylenefuranoate, a suitable alternative for polyethylene terephthalate, can be synthesized from biomass‐derived 2,5‐furandicarboxylic acid (FDCA), which is obtained via the selective oxidation of 5‐hydroxymethylfurfural (HMF) over noble or non‐noble metal oxide catalysts. However, the FDCA yields using non‐noble metal catalysts can be improved. Hence, in this study, we developed a series of Co‐ and Cu‐embedded melamine sponge (MS)‐derived carbon catalysts (xCo‐Cu/MS) for HMF oxidation, wherein Co and Cu oxides with different Co : Cu ratios were embedded in carbonized MS. The metals were adsorbed on the MS, followed by carbonization. This strategy allowed the uniform distribution of metal clusters in the carbon texture, which enhanced the catalytic activity for HMF conversion and improved the FDCA yield. The good distribution and small crystal size of Co3O4 significantly increased the active surface area, enhancing the catalytic activity. Compared to Co‐Cu/MS catalysts, the 4Co‐Cu/activated carbon catalyst synthesized via impregnation afforded the maximum FDCA yield (93 %) with 100 % HMF conversion at 170 °C and 7 h. Moreover, oxidative regeneration using H2O2 proved to be the best regeneration method for the MS catalysts, with only a 5 % decrease in the FDCA yield after three cycles.

Ruthenium-nickel oxide derived from Ru-coupled Ni metal–organic framework for effective oxygen evolution reaction

Developing Ru-based catalysts with both high activity and stability for the oxygen evolution reaction (OER) is very significant in the water electrolysis technique. Here, a bimetallic metal oxide catalyst (RuO2-NiO) derived from Ru-coupled Ni metal–organic framework (Ni-MOF) by using terephthalic acid as the ligand was successfully synthesized through a facile ultrasonic treatment and subsequent thermal annealing approach; and the effective role of the coupled effect between RuO2 and NiO in stabilizing Ru was found significant in OER catalysis. A relatively small d-band center due to the electronic structure regulation and synergistic effect of the heterostructure was found to cause weakened adsorption of surface oxygen species. Theoretical calculations demonstrated that the electronic modulation between RuO2 and NiO can significantly accelerate the dissociation of water and modulate the chemical adsorption of oxygen intermediates on the catalyst, thereby enhancing the OER activity of the catalyst. The optimized catalyst of RuO2-NiO afforded a current density of 10 mA cm−2 at low overpotentials of 210 mV toward OER, and good catalytic stability, kinetics and efficiency were also discussed. This remarkable catalytic performance can be attributed to the unique sheet-like structure and porous morphology of the catalyst with increased exposure of active sites and the coupling effect between RuO2 and NiO for moderate binding strength to the intermediates. This study showed an effective approach for bimetallic oxide catalyst fabrication and their applications in energy conversion reactions.

Developments in Titanium-Based Alkali and Alkaline Earth Metal Oxide Catalysts for Sustainable Biodiesel Production: A Review.

Biodiesel represents a biodegradable, environmentally friendly, and renewable alternative to fossil fuels. Despite more than three decades of research, significant obstacles still hinder the widespread production of biodiesel. This current review elucidates both the potential and the existing challenges associated with homogeneous and heterogeneous catalysts in catalyzing biodiesel production, with a particular focus on alkali analogues, alkaline earth metal oxides, and titania-based catalysts. In particular, a comprehensive analysis is presented concerning alkali and alkaline earth-based titania (TiO2 ) catalysts. Among these, the alkaline earth metal oxides, including lithium, calcium, and strontium when combined with titanium-based catalysts, exhibit superior catalytic activity compared to other metal oxides, owing to their heightened basicity. Consequently, this review offers a thorough and up-to-date insight into the potential of titania-based heterogeneous catalysts for advancing biodiesel production.

Controlling Metal-Oxide Reducibility for Efficient C-H Bond Activation in Hydrocarbons.

Knowing the structure of catalytically active species/phases and providing methods for their purposeful generation are two prerequisites for the design of catalysts with desired performance. Herein, we introduce a simple method for precise preparation of supported/bulk catalysts. It utilizes the ability of metal oxides to dissolve and to simultaneously precipitate during their treatment in an aqueous ammonia solution. Applying this method for a conventional VOx -Al2 O3 catalyst, the concentration of coordinatively unsaturated Al sites was tuned simply by changing the pH value of the solution. These sites affect the strength of V-O-Al bonds of isolated VOx species and thus the reducibility of the latter. This method is also applicable for controlling the reducibility of bulk catalysts as demonstrated for a CeO2 -ZrO2 -Al2 O3 system. The application potential of the developed catalysts was confirmed in the oxidative dehydrogenation of ethylbenzene to styrene with CO2 and in the non-oxidative propane dehydrogenation to propene. Our approach is extendable to the preparation of any metal oxide catalysts dissolvable in an ammonia solution.

Catalytic conversion of lignin into monoaromatic hydrocarbons over a Ni/Al hydrotalcite-derived catalyst

Lignin is the most abundant aromatic biopolymer. Thus, preparation of BTX (benzene, toluene and xylenes) by catalytic fast pyrolysis (CFP) of lignin is attractive. Ni-Al mixed metal oxide (MMO) catalysts, derived from pyrolyzing Ni-Al layered double hydroxides (LDH), were prepared for the catalytic fast pyrolysis of lignin. The effects of catalyst Ni/Al ratio, catalyst dosage and pyrolysis temperature on the liquid products were investigated by Py-GC/MS. The Ni3Al3-MMO produced the best liquid product distribution from lignin at 700 ℃. The selectivity of Ni3Al3-MMO for aliphatic hydrocarbons and monocyclic aromatic hydrocarbons (MAHs) was 18.0% and 78.2%, respectively. The selectivity to polyaromatic hydrocarbons (PAHs) was 3.8%. No phenols and phenolic derivatives were produced. This is related to the mesoporous catalytic structure (pore size of 14.37 nm), large external surface area (134.74 m2/g) and strong acidity (4.92 mmolNH3/g) of Ni3Al3-MMO. In addition, a mechanistic study was performed with the lignin model compound (guaiacol) showing that Ni3Al3-MMO exhibits strong deoxygenation activity for aromatic C–O bond cleavage of lignin.

Ionizing radiation-assisted in-situ synthesis of nitrogen-doped graphene oxide-supported nano Fe3O4 for PMS activation to degrade emerging pollutants

This study firstly reported the in-situ preparation of nitrogen-doped graphene oxide-supported nano Fe3O4 (Fe3O4@NRGO) by ionizing radiation. The conversion of Fe(II) to iron oxide was modulated by absorbed dose in solution, which regulated the catalytic activity of Fe3O4/NRGO. When the initial concentration of Fe(II) was 0.5 mM and the absorbed dose was 10 kGy, the as-prepared catalyst (Fe3O4@NRGO-10) showed superior catalytic activity for activating peroxymonosulfate (PMS) to degrade phenol. In the presence of 0.4 mM PMS and 0.5 g/L Fe3O4@NRGO-10, the pseudo first-order kinetic rate of phenol degradation reached 0.0286 min−1. Sulfate radicals, hydroxyl radicals and high-valent iron oxo made major contribution to phenol degradation. The superior catalytic activity of Fe3O4@NRGO-10 might be attributed to the synergistic effect of NRGO and Fe3O4, in which NRGO promoted the conversion of Fe(III) to Fe(II). Fe3O4@NRGO-10 exhibited good stability in the cycling experiments. In addition to phenol, the Fe3O4@NRGO-10/PMS system could degrade sulfamethoxazole, bisphenol A and atrazine, indicating the wide applicability. This study reported a facile way to prepare carbon-supported metal oxide catalysts and convenient way to modulate the catalytic activity by absorbed dose.