A facile fabrication of nanoflower-like Co<Subscript>3</Subscript>O<Subscript>4</Subscript> catalysts derived from ZIF-67 and their catalytic performance for CO oxidation

Herein, we reported a facile method for fabricating nanoflower-like Co₃O₄ catalysts via calcination treatment based on ZIF-67. The catalytic performances of the obtained Co₃O₄ catalysts were evaluated for the model reaction of CO oxidation. The results demonstrated that calcination temperature had a strong effect on the structure and catalytic reaction activity of Co₃O₄ catalyst. Co₃O₄ catalyst prepared at 400 °C (Co₃O₄-400) exhibited the optimum catalytic activity with a complete CO conversion temperature of 105 °C. This phenomenon was ascribed to the higher specific surface areas, smaller particle size, unique structure, good low-temperature reduction and higher abundances of surface Co²⁺ and adsorbed oxygen species. The addition of 1.0% water vapor had a negative effect on CO oxidation and the prepared Co₃O₄-400 catalyst presented long-term stability.     

Characterization and catalytic performance of Co/SBA-15 supported gold catalysts for CO oxidation

Cobalt-containing SBA-15 supported gold catalysts for low-temperature CO oxidation were prepared and characterized by N₂ adsorption/desorption, X-ray diffraction, transmission electron microscopy, inductively coupled plasma-atom emission spectroscopy and X-ray photoelectron spectroscopy techniques. The effects of cobalt and gold content on the catalyst activity were investigated in detail. Among them, 2% Au/40% Co/SBA-15 shows the highest activity, its complete conversion temperature for CO is at 273 K. It was believed that both the dispersion of Co₃O₄ and the high surface areas caused by SBA-15 contribute to the good activities of cobalt-containing SBA-15 supported gold catalysts. Furthermore, the strong metal-support interaction between gold and cobalt oxides is greatly related to the catalytic performance.     

Hierarchical laminated Al2O3 in-situ integrated with high-dispersed Co3O4 for improved toluene catalytic combustion

Hierarchical structure and surface properties of selective support afford some special effects on the catalytic activity, which could be tuned to achieve improved performance. Herein, using a combination of hydrothermal coprecipitation and thermal processing, we integrated highly-distributed Co₃O₄ spinel nanospecies on laminated hierarchically structured Al₂O₃ which could be used as a highly efficient VOCs treatment catalyst. Impressively, compared to Co-free Al₂O₃ counterpart (S_BET = 188.2 m²·g^?1), these obtained Co₃O₄ spinel functionalized catalysts are endowed with adjustable Co loading, optimized Co activity state, and obviously improved hierarchically structural properties (S_BET = 274.7 m²·g^?1). The microporous and mesoporous structures both existed in the obtained Al₂O₃, which is beneficial to the heterogeneous catalytic reaction process. The results reveal that the proper Co loading in the hierarchically structured Al₂O₃ could enable the rational modulation of catalytic activity in the combustion of toluene and exceeds the commercial 5 wt% Pd/C catalyst in the light of total catalytic oxidation ability. This developed heterogeneous Co₃O₄ compositing hierarchically structured Al₂O₃ provides a significant potential value for practical VOCs treatment.     

A study on the synthesis of nanostructured WC–10 wt% Co particles from WO<Subscript>3</Subscript>, Co<Subscript>3</Subscript>O<Subscript>4</Subscript>, and graphite

The formation of the nanostructured WC–10 wt% Co powder from WO₃, Co₃O₄, and graphite is studied. The effects of the processing parameters of high-energy ball milling, reduction in H₂ atmosphere, and carburization in Ar/CO atmosphere are investigated. The crystallite size of the as-synthesized WC is 30–40 and 40–50 nm for 900 and 1000 °C carburized powders, respectively. The powder is agglomerated with the size of the primary particles ranging from 50 to 700 nm. High-energy ball milling of WO₃–Co₃O₄–C powder mixtures leads to finer particle and crystallite sizes with larger surface area. Such milled powders can be reduced to nanostructured W at 570 °C and carburized to form WC at temperatures as low as 900 °C. Crystal growth has taken place during carburization, particularly at 1000 °C, which results in the formation of truncated triangular prisms and nanoplates of WC at 1000 °C.     

3D Gold‐Modified Cerium and Cobalt Oxide Catalyst on a Graphene Aerogel for Highly Efficient Catalytic Formaldehyde Oxidation

A hard template method is used to prepare porous gold‐doped cerium and cobalt oxide (Au‐Ce_xCo_y) materials. A series of 3D Au‐Ce _xCo_y/graphene aerogel (GA) composites is then fabricated by a facile heating method. The obtained catalysts possess a well‐defined structure of ordered arrays of nanotubes and good performance in formaldehyde (HCHO) oxidation. The composition and surface elemental valence states of the catalysts are modulated by the Ce/Co molar ratio. The Au‐Ce_xCo_y catalyst and graphene oxide sheets are well compounded within 60 s through a diamine cross‐linker to form 3D Au‐Ce_xCo_y/GA composites. In addition, the resulting catalyst of 3 wt% Au‐Ce₃Co/GA achieves ≈55% conversion at room temperature and 100% conversion when the reaction temperature is raised at 60 °C. The synergistic effect between CeO₂ and Co₃O₄ promotes the migration of oxygen species and the activation of Au, which facilitates HCHO oxidation. The method used to prepare the 3D catalyst could be used to produce other catalytic materials with good replication of the template. In addition, these findings provide a simple method for rapid fabrication of catalyst/GA composites. The superior activity and stability of the 3D Au‐Ce₃Co/GA catalyst make it potentially applicable in HCHO removal.     

A true nanocomposite: Single crystalline Au nanoparticles on single crystalline Fe3O4 nanoparticles

An Au/Fe₃O₄ nanocomposite catalyst was fabricated through a simple deposition-precipitation method. The Au/Fe₃O₄ nanocomposite is a true nanocomposite that has single crystalline Au nanoparticles supported on single crystalline Fe₃O₄ nanoparticles. Lattice fringes from both Au and Fe₃O₄ single nanoparticles were simultaneously observed by transmission electron microscope (TEM). This nanocomposite catalyst showed much high activity in low temperature CO oxidation reaction. The Au/Fe₃O₄ nanocomposite catalyst reaches 100% CO conversion at 40 °C. In comparison, Au/commercial Fe₃O₄ catalyst needs 375 °C to convert CO. This Au/Fe₃O₄ nanocomposite is an ideal sample to study synergetic effect between the catalyst and the support at nanoscale.     

Metal-organic framework derived hexagonal layered cobalt oxides with {1 1 2} facets and rich oxygen vacancies: High efficiency catalysts for total oxidation of propane

A cobalt-based metal–organic framework was used as a precursor to synthesize Co₃O₄ catalysts exhibiting a hexagonal layered morphology by calcination at varying temperatures. Various characterization techniques, such as XRD, SEM, Raman, H₂-TPR, O₂-TPD and N₂ adsorption–desorption, were used to study the effects of calcination temperature on the grain size, surface area, and pore volume of the catalysts. The Co₃O₄ catalyst obtained by calcination at 350 °C (Co₃O₄-350) exhibited the highest catalytic activity for the total oxidation of propane. Furthermore, the small grain size and layered structure of Co₃O₄-350 allowed it to possess a high specific surface area, a highly exposed {1 1 2} facets, and abundant oxygen defects that facilitated a favorable low-temperature reducibility and oxygen mobility, thereby improving catalytic activity. This research offers a simple strategy for synthesis of Co₃O₄ with layered structure, highly exposed {1 1 2} facets and rich oxygen defects.     

Efficient purification of His-tagged protein by superparamagnetic Fe3O4/Au–ANTA–Co2 + nanoparticles

Superparamagnetic Fe₃O₄/Au nanoparticles were synthesized and surface modified with mercaptopropionic acid (MPA), followed by conjugating Nα,Nα-Bis(carboxymethyl)-l-lysine hydrate (ANTA) and subsequently chelating Co^2 +. The resulting Fe₃O₄/Au–ANTA–Co^2 + nanoparticles have an average size of 210 nm in aqueous solution, and a magnetization of 36 emu/g, endowing the magnetic nanoparticles with excellent magnetic responsivity and dispersity. The Co^2 + ions in the magnetic nanoparticle shell provide docking site for histidine, and the Fe₃O₄/Au–ANTA–Co^2 + nanoparticles exhibit excellent performance in binding of a His-tagged protein with a binding capacity of 74 μg/mg. The magnetic nanoparticles show highly selective purification of the His-tagged protein from Escherichia coli lysate. Therefore, the obtained Fe₃O₄/Au–ANTA–Co^2 + nanoparticles exhibited excellent performance in the direct separation of His-tagged protein from cell lysate.     

Nanostructure SnO2 and supported Au catalysts: Synthesis,characterization, and catalytic oxidation of CO

Nanostructure tin dioxide (SnO₂) powders prepared by sol-gel dialytic processes using tin (IV) chloride and anhydrous alcohol as start materials, ammonia gas as catalyst of the formation of colloid solution and agent of removing Cl⁻, and by introducing dialytic processes to improve and accelerate the formation of gels. From the result of TG–DTA analyses, the dried samples were calcined at 673 K in air for 3 h. Tin dioxide nanoparticles were characterized by thermogravimetry and differential thermal analyses (TG–DTA), X-ray diffraction (XRD), nitrogen adsorption–desorption, X-ray photoelectron spectroscopy (XPS). The average particle size of the as-prepared tin dioxide was about 5 nm. The as-prepared SnO₂ possessed mesoporous structure and large surface area. The Au/SnO₂ catalysts for low-temperature CO oxidation were prepared by the deposition–precipitation method using as-prepared SnO₂ powders as the support. The Au/SnO₂ catalysts exhibited high catalytic activity for low-temperature CO oxidation. The nanostructure SnO₂ has promising applications in sensor, catalyst, catalytic support, mesoporous membranes, etc.     

Multiwalled carbon nanotubes-supported Nickel catalysts for the steam reforming of propane

Multiwalled carbon nanotubes (MWCNTs)-supported nickel catalysts with different metal-loading contents were synthesized trough deposition–precipitation (DP) method for its subsequent performance study on steam reforming reaction of propane. The metal-loading content was set at 5, 10, 20, and 25% of nickel. Results showed that 20 wt% nickel oxide over MWCNTs (20% NiO/MWCNTs) had the best performance, on the propane steam reforming reaction, in terms of H₂ conversion comparing with the rest of the NiO/MWCNTs catalysts (5, 10, 25 wt% Ni) and a nickel over alumina (Ni/Al₂O₃) commercial catalyst. The features of the NiO/MWCNTs catalysts were studied trough FT-IR, Raman spectroscopy, N₂ adsorption–desorption isotherms, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and field emission scanning electron microscopy measurements. The results evidenced that optimum relation between Ni content, Ni dispersion, and particle size played a main role in the catalyst performance, rendering the 20% NiO/MWCNT as the most promising, among the catalysts studied, for the steam reforming of propane.     

Synthesis and investigation of bimetallic Ni-Co/Al2O3 nanocatalysts using the polyol process

ABSTRACT

Ni-Co/Al₂O₃ catalysts with different Ni:Co ratios by weight were prepared using a simple polyol process. The activities of the catalysts were evaluated for the catalytic partial oxidation of methane (CPOM) in the temperature range of 600–800°C. Numerous techniques such as x-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analysis, inductively coupled plasma-mass spectroscopy (ICP-MS), thermogravimetric analysis (TGA), high-resolution transmission electron microscopy analysis (HRTEM), scanning electron microscopy analysis (SEM-EDS) and temperature-programmed oxidation (TPO) were applied to characterize fresh and spent catalysts. The XRD analysis confirmed that the loaded particles were metals and showed possible bimetallic nano-alloy Ni-Co formation for Ni- and Co-containing catalysts. The highest metal dispersion was 15.7% for the Ni_2.8Co_2.6/Al₂O₃ catalyst. The catalytic test results showed no correlation between metal dispersion and the metal particle size, and the activity decreased in the order of Ni_7.7/Al₂O₃ > Ni_2.8Co_2.6/Al₂O₃ ≈ Ni_3.8Co_1.5/Al₂O₃ > Ni_2.0Co_3.8/Al₂O₃ >> Co_6.8/Al₂O₃ under a flow rate of 157,500 L kg^?1 h^?1 with CH₄/O₂ = 2 (using air as an oxidant) at 800°C. The obtained results also showed that when the actual atomic Ni/Co ratio was 1.07 in the Al₂O₃-supported catalyst, the dispersion of the active sites appeared to be promoted by Co addition, and the catalytic activity was stable over a reaction time of 10 h. Among all the tested catalysts, the Ni_2.8Co_2.6/Al₂O₃ catalyst exhibited acceptable activity (75%) without coking.     

NiAl2O4 catalysts prepared by combustion reaction using glycine as fuel

The aim of this work is to evaluate the influence of glycine fuel used in a stoichiometric proportion and with a 10% and 20% excess of this fuel in the preparation of NiAl₂O₄ catalyst by combustion reaction. The powders were characterized by XRD, textural analysis by the BET nitrogen adsorption method, particle size distribution, and FTIR. The results show the presence of NiAl₂O₄ as a major phase and traces of NiO and Ni in all the catalysts studied here. The crystallite sizes were 22 nm in the stoichiometric composition and 18 and 9 nm, respectively, in the composition containing 10% and 20% excess glycine. The powder obtained from all the compositions presented morphological characteristics with irregular plate-shaped agglomerates. The increase in excess glycine caused the particle size in the three compositions to decrease to 59, 54 and 38 nm and the agglomerate size to increase to 7, 8 and 12 μm, respectively.     

Effect of fuel/oxidizer ratio and the calcination temperature on the preparation of microporous-nanostructured tricobalt tetraoxide

Microporous tricobalt tetraoxide, Co₃O₄, nanoparticles (NPs) clusters have been successfully fabricated using a simple but efficient controlled solution combustion route. Such a synthesis involves combustion reaction of cobalt nitrate with cetyl trimethylammonium bromide (CTAB). The combustion process has been analyzed by simultaneous thermal analysis. The resultant powders were characterized by means of X-ray diffraction technique (XRD), Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and nitrogen adsorption at −196 °C. The morphology and specific surface area of the obtained Co₃O₄ nanoparticles clusters have proved to be strongly dependent on the fuel (F)/oxidizer (O) molar ratio and the calcination temperature. It was found that both the crystallite size and the lattice parameter nanocrystalline Co₃O₄ increase with increasing the F/O molar ratio as well as the calcination temperature. X-ray diffraction confirmed the formation of CoO phase together with spinel Co₃O₄ using F/O ratio of 1. The concentration of such phase increases with increasing the F/O ratio. Moreover, when the calcination is applied at 900–1000 °C traces of CoO was obtained together with Co₃O₄ as a major phase.     

Catalytic combustion of methane over Pt and PdO-supported CeO<Subscript>2</Subscript>–ZrO<Subscript>2</Subscript>–Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts

Catalytic combustion of methane was investigated on Pt and PdO-supported CeO₂–ZrO₂–Bi₂O₃/γ-Al₂O₃ catalysts prepared by a wet impregnation method in the presence of polyvinylpyrrolidone. The catalysts were characterized by X-ray fluorescence analysis, X-ray powder diffraction, X-ray photoelectron spectra, transmission electron microscopy, and BET specific surface area measurements. The Pt/CeO₂–ZrO₂–Bi₂O₃/γ-Al₂O₃ and PdO/CeO₂–ZrO₂–Bi₂O₃/γ-Al₂O₃ catalysts were selective for the total oxidation of methane into carbon dioxide and steam, and no by-products such as HCHO, CO, and H₂ were obtained. The catalytic activities of the PdO/CeO₂–ZrO₂–Bi₂O₃/γ-Al₂O₃ catalysts were relatively higher than those of the Pt-supported catalysts, due to the facile re-oxidation of metallic Pd into PdO based on lattice oxygen supplied from the CeO₂–ZrO₂–Bi₂O₃ bulk. A decrease in the calcination temperature during the preparation process was found to be effective in enhancing the specific surface area of the catalysts, whereby particle agglomeration was inhibited. Optimization of the PdO amount and calcination temperature enabled complete oxidation of methane at temperatures as low as 320 °C on the 11.6 wt% PdO/CeO₂–ZrO₂–Bi₂O₃/γ-Al₂O₃ catalyst prepared at 400 °C.     

Interfacial activation of catalytically inert Au (6.7 nm)-Fe3O4 dumbbell nanoparticles for CO oxidation

Au nanoparticles epitaxially grown on Fe₃O₄ in Au (6.7 nm)-Fe₃O₄ dumbbell nanoparticles exhibit excellent stability against sintering, but display negligible catalytic activity in CO oxidation. Starting from various supported Au (6.7 nm)-Fe₃O₄ catalysts prepared by the colloidal deposition method, we have unambiguously identified the significance of the Au-TiO₂ interface in CO oxidation, without any possible size effect of Au. In situ thermal decomposition of TiO₂ precursors on Au-Fe₃O₄ was found to be an effective way to increase the Au-TiO₂ interface and thereby optimize the catalytic performance of TiO₂-supported Au-Fe₃O₄ dumbbell nanoparticles. By reducing the size of Fe₃O₄ from 15.2 to 4.9 nm, the Au-TiO₂ contact was further increased so that the resulting TiO₂-supported Au (6.7 nm)-Fe₃O₄ (4.9 nm) dumbbell particles become highly efficient catalysts for CO oxidation at room temperature.      

Au/LaVO4 Nanocomposite: Preparation, characterization, and catalytic activity for CO oxidation

The size of the gold particles is a very important parameter to get active catalysts. This paper reports a novel colloidal deposition method to prepare Au/LaVO₄ nanocomposite catalyst with monodispersed Au colloids and uniform LaVO₄ nanoplates in nonpolar solvent. Monodispersed Au colloids with tunable size (such as 2, 5, 7, 11, 13, and 16 nm) and LaVO₄ nanocrystals with well-defi ned shapes were pre-synthesized assisted with oleic acid/amine. During the following immobilization process, the particle size and shape of Au and LaVO₄ were nearly preserved. As-prepared Au/LaVO₄ nanocomposite showed high catalytic activity for CO oxidation at room temperature. Since sizes of gold particles were well-defi ned before the immobilization process, size effect of gold particles was easy to be investigated and the results show that 5-nm Au/LaVO₄ nanocomposite has the highest activity for CO oxidation. This synthetic method can be extended further for the preparation of other composite nanomaterials.     

Particle size and support effects on the complete benzene oxidation by Co and Co–Pt catalysts

Monometallic cobalt and bimetallic Co–Pt samples of various particle sizes have been prepared using SiO₂ and synthetic kenyaite (layered silicate) as a support. They are characterized by elemental analysis, XRD, TPR, and XPS. Cobalt is introduced by two methods—classical impregnation and ammonia method. The ammonia method of preparation leads to the formation of finely dispersed Co₃O₄ on both supports. Besides, hardly reducible cobalt silicate phases appear predominantly on the SiO₂ support. The Co₃O₄ particle size varies between 5 and 20 nm, depending on the support. The monometallic Co samples prepared by ammonia method on both supports are more active in benzene combustion than the impregnated ones due to the finer dispersion of the easily reducible Co₃O₄. Addition of Pt improves the activity and the promoting effect is more evident for the impregnated sample. This is explained with the synergy effect of cobalt oxide species and Pt. The less promoting effect of Pt on the catalytic activity of the bimetallic kenyaite-supported samples is attributed to the stronger interaction between the Co oxide phase and Pt during the preparation process.     

Facile Synthesis of Co<Subscript>3</Subscript>O<Subscript>4</Subscript>/CoO Nanoparticles by Thermal Treatment of Ball-Milled Precursors

Co₃O₄/CoO nanoparticles have been synthesized by a simple method which is based on the ball-milling and calcination of cobalt acetate and citric acid. The samples were characterized using X-ray diffraction, transmission electron microscope, and Fourier transform infrared spectroscopy. The results show that Co₃O₄ nanoparticles with an average particle size of ∼40 nm can be obtained by calcination of ball-milled precursors at relatively low temperature (350 °C) for 3 hours. It should be noted that it is possible to control the size of Co₃O₄ particles by calcination temperature, calcination time and also by ball-milling duration using this method. Meanwhile, the pure CoO nanoparticles were obtained successfully by thermal decomposition of Co₃O₄ at 950 °C and quickly quenching to liquid nitrogen.     

Control of Catalytic Activity of Nano‐Au through Tailoring the Fermi Level of Support

The catalytic properties of nanometals are strongly dependent on their electronic states which, are influenced by the interaction with the supports. However, a precise manipulation of the electronic interaction is lacking, and the nature of the interaction is still ambiguous. Herein, using Au/ZnFe_xCo_2?xO₄ (x = 0–2) as a model system with continuously tuned Fermi levels of supports, the electronic structure of the Au catalyst can be precisely controlled by changing the Fermi level of the support, which arises from the charge redistribution between the two phases. A higher Fermi level of ZnFe₂O₄ support makes nano‐Au negatively charged and thus facilitates the oxidation of CO, and in contrast, a lower Fermi level of ZnCo₂O₄ support makes nano‐Au positively charged and is preferential to the oxidation of benzyl alcohol. This work represents a solid step towards exploration of advanced catalysts with deliberate design of electronic structure and catalytic properties.     

Synthesis and structure characterization of Ru nanoparticles stabilized by PVP or γ-Al2O3

Uniform and stable Ru nanoparticles were synthesized by reduction of RuCl₃ in ethylene glycol (EG) in the presence of poly(N-vinyl-2-pyrrolidone) by using microwave-assisted solvothermal method. The obtained materials were characterized by UV-vis, FT-IR, XPS, XRD and TEM techniques, and used as precursors of heterogeneous metal colloid catalysts. Characterization by TEM showed that as-prepared PVP-stabilized Ru nanoparticles have small average diameters (below 2 nm) and narrow size distributions (1-3 nm). Diffraction data confirmed that a crystallite size is around 2.0 nm. A colloidal Ru/γ-Al₂O₃ catalyst was obtained by two different methods: immobilization of the PVP-stabilized Ru colloid on the support or by in situ deposition of Ru colloid, e.g., reduction of RuCl₃ with EG in the presence of the γ-alumina. It was found that both synthesis methods produced the Ru/γ-Al₂O₃ catalysts with narrow size distributions of metallic nanoparticles, that are distributed uniformly over the support. However, only in situ preparation of the colloidal Ru/γ-Al₂O₃ catalyst results in chlorine free system with high activity for hydrogen chemisorption. The H₂ uptake on the Ru(PVP)/γ-Al₂O₃ catalyst was very low because the ruthenium surface was strongly occluded with a thin layer of polymer molecules.