Mechanism Of Selective Oxidation Research Articles

Steam oxidation of Cr-coated Sn-containing Zircaloy solid rod at 1000 °C

In this study, the steam oxidation behavior of Cr-coated Zry-4 alloy at 1000 °C and the various interesting phenomena observed during the oxidation process were investigated. During oxidation, Sn in the Zry-4 substrate diffused into the coating and precipitated in the Cr2O3 scale. The Kirkendall effect of these Sn-containing precipitates then generated microcavities. Furthermore, the outward diffusion behavior of Zr and the selective oxidation mechanism of the Zr particles were studied. Voids detected at the Cr2O3/Cr interface result mainly from vacancy condensation. The amorphization of Cr2O3 was also observed adjacent to the voids, and the amorphization mechanism was discussed.

Enzyme-like mechanism of selective toluene oxidation to benzaldehyde over organophosphoric acid-bonded nano-oxides

The completely selective oxidation of toluene to benzaldehyde with dioxygen, without the need to use H2O2, halogens, or any radical initiators, is a reaction long desired but never previously successful. Here, we demonstrate the enzyme-like mechanism of the reaction over hexadecylphosphate acid (HDPA)-bonded nano-oxides, which appear to interact with toluene through specific recognition. The active sites of the catalyst are related to the ability of HDPA to change its bonding to the oxides between monodentate and bidentate during the reaction cycle. This greatly enhances the mobility of the crystal oxygen or the reactivity of the catalyst, specifically in toluene transformations. The catalytic cycle of the catalyst is similar to that of methane monooxygenase. In the presence of catalyst and through O2 oxidation, the conversion of toluene to benzaldehyde is initiated at 70 °C. We envision that this novel mechanism reveals alternatives for an attractive route to design high-performance catalysts with bioinspired structures.

Unraveling the Mechanism of Ammonia Selective Catalytic Oxidation on Ag/Al2O3 Catalysts by Operando Spectroscopy

Ammonia leakage from the aftertreatment of diesel vehicles arouses serious environmental concerns, thus demanding a high-efficiency ammonia elimination catalyst. In the present work, the mechanism ...

Visible Light Driven VO2/g‐C3N4 Z‐Scheme Composite Photocatalysts for Selective Oxidation of DL‐1‐Phenylethyl Alcohol under Vis‐LEDs Irradiation and Aerobic Oxidation

AbstractA novel VO2/g‐C3N4 Z‐Scheme type heterostructure composite photocatalyst was fabricated by the ultrasonic‐assisted method. The newly synthesized photocatalyst has been systematically and comprehensively characterized and analyzed. Through the selective oxidation of DL‐1‐Phenylethyl alcohol under Vis‐LED irradiation and aerobic oxidation, the photocatalytic performance of VO2/g‐C3N4 photocatalyst was investigated. The results showed that the prepared VO2/g‐C3N4 nanocomposites exhibited excellent photocatalytic oxidation performance. Among them, the highest turnover frequency (TOF) was 0.42 mmol (DL‐1‐Phenylethyl alcohol)⋅mmol−1 (catalyst)⋅h−1, which was 5 times higher than the TOF of the original g‐C3N4. The effects of different wavelengths of LED monochromatic light, the composite ratio of VO2 and g‐C3N4, the reaction time, and the different benzyl alcohol substrates were studied. Also, catalyst life test was completed. Furthermore, a persuasive mechanism of photocatalytic selective oxidation of DL‐1‐Phenylethyl alcohol on VO2/g‐C3N4 photocatalyst was proposed through EPR and radical trapping experiments.

Insight into the law and mechanism of selective oxidation of Q&P steel under different annealing parameters

In order to investigate the law and mechanism of selective oxidation of Quenching and Partitioning (Q&P) steel under different annealing parameters, the sensitivity of alloy elements added in Q&P steel to the annealing parameters was explored. Galvanizing experiments of QP steel were carried out at different annealing temperatures(770 °C and 730 °C) and different holding times(180 s and 300 s). The concentration change of selective oxidation element and the alloying element distribution in the coating were analyzed by Glow Discharge Optical Emission Spectrometer (GDOES), the morphology of surface oxides and inhibition layer were observed by Scanning Electron Microscope (SEM). The results showed that higher annealing temperature(770 °C) and longer annealing time(300 s) could both increase the concentration of Mn and Si on the surface of Q&P steel, improve the oxidation degree of steel surface, thin the inhibition layer, and lead to more leakage plating and deterioration of surface coating quality. The Mn depth profiles showing a parabolic trend and reaching its maximum value at about 20 nm below the surface, while the concentration of Si gradually decreases from the surface to the interior. The selective oxidation of Si and Mn are more sensitive to the annealing temperature than the annealing time.

Nucleation and growth of oxide particles on a binary Fe-Mn (1 wt. %) alloy during annealing

An Fe-Mn (1 wt. %) alloy was annealed at 800 °C in N2-5 vol. % H2 with a dew point of −40 °C. The nucleation and growth of external selective oxides of MnO were characterized as a function of ferrite grain orientation. Nucleation occurs successively on Fe{110}, Fe{111} and Fe{100} at 650–700 °C. The oxides are faceted and their shape, size and surface density, measured accurately, are strongly dependent on the ferrite grain orientation. The oxides grow as the annealing time increases. New oxides are formed after 180 s at 800 °C. The associated external selective oxidation mechanisms are discussed.

Z-scheme BiFeO3-CNTs-PPy as a highly effective and stable photocatalyst for selective oxidation of benzyl alcohol under visible-light irradiation

Z-scheme BiFeO3-CNTs-PPy photocatalysts were prepared via hydrothermal reaction and oxidative polymerization, and characterized by physico-chemical techniques to detect their structural and optical properties. The formed Z-scheme could efficiently enhance the separation of photo-generated electron-hole pairs, promote holes to directly participate in the oxidation reaction and avoid the formation of non-selective hydroxyl radicals. The photocatalytic performance of Z-scheme BiFeO3-CNTs-PPy photocatalysts was inspected by selective oxidation of benzyl alcohol under visible-light irradiation. In optimal condition, the highest conversion rate of benzyl alcohol was up to 83.2%, much higher than those of PPy (2.3%), BiFeO3 (15.2%), BiFeO3-CNTs (24.6%) and BiFeO3-PPy (29.4%), while the selectivity was above 99%. Simultaneously, BiFeO3-CNTs-PPy also presented good stability and could be recycled up to 5 times without remarkable loss of activity and change in structure. The relationship between solvents, reaction temperature, light wavelength, substituent type, and photocatalytic efficiency was investigated. The results showed that the best solvent and reaction temperature were ethyl acetate and 303 K, respectively. In the conversion of benzyl alcohol, 360−560 nm light played the most important role and the present of electron-withdrawing groups was beneficial to the selective oxidation. Moreover, according to the conclusions of the free radical capture experiments, the coumarin conversion experiments and electron spin resonance (ESR) spectra, the main active species in the photocatalytic process were determined. Based on these, the possible mechanism for photocatalytic selective oxidation of benzyl alcohol over BiFeO3-CNTs-PPy was proposed.

Effects of Oxygen Mobility in La–Fe-Based Perovskites on the Catalytic Activity and Selectivity of Methane Oxidation

The mechanism and structure requirements of selective and total oxidation of methane in a chemical looping process are both experimentally and theoretically examined on La1–xSrxFeO3−δ (x = 0, 0.2, ...

Influence of nitrogen vacancies on selective oxidation of aromatic alcohols on g-C3N4: A comparative DFT study

The development of new metal-free catalysts that effectively oxidise alcohols to aldehydes or ketones is one of the fundamental goals of research on environmental photocatalysis. In this study, the influence of nitrogen vacancies on the catalytic process was investigated. We performed calculations using the density functional theory (DFT) to elucidate the mechanism of selective oxidation of benzyl alcohol to benzaldehyde with molecular O2 on g-C3N4. Through modelling of the pathways of the reaction, we found that vacancy defects strongly promote the formation of aldehyde. Our results clearly depicted that there are different interaction patterns that determine the reaction energy profiles for defective and defect-free systems. Furthermore, we estimated the energy barrier for the postulated rate-limiting step for a few alcohols by using the climbing image nudge elastic band (CI-NEB) method. The obtained values agreed very well with the benzaldehyde yield reported by Ding et al. (2018) [23]. In addition to the energetic effects, the electronic properties of the nitrogen-vacancy-containing g-C3N4 were also discussed. Our results demonstrated the underlying mechanism of selective oxidation of alcohols to aldehydes on both the catalysts in a comprehensive and systematic manner. Thus, by knowing the individual reaction steps, further increase in the substrate conversion may be attained by modification of the catalyst structure.

Mechanism of selective and complete oxidation in La2O3-catalyzed oxidative coupling of methane

The catalytic mechanism and reaction network of oxidative coupling of methane over La2O3 are thoroughly investigated by density functional theory calculations.

Insight into the active site and reaction mechanism for selective oxidation of methane to methanol using H2O2 on a Rh1/ZrO2 catalyst

Five-coordinated Rh leads to the over-oxidation of CH4, while four-coordinated Rh stabilizes CH3 and facilitates methanol formation via the CH3OOH intermediate.

H2S-Selective Catalytic Oxidation to Sulfur over Iron Oxide Sorbent Supported on Semi-Coke

Iron oxides modified by semi-coke sorbents (referred to as FeSC) were prepared by the hydrothermal impregnation method and were used for the selective catalytic oxidation of H₂S to sulfur. The physicochemical properties of samples were characterized by Brunauer–Emmett–Teller analysis, scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetry (TG), and X-ray photoelectron spectroscopy. The results showed that iron oxide could be highly loaded on semi-coke with a microporous structure and high specific surface area. Fe₂O₃ was considered the only major active component on the surface of the sorbent. Fe7SC stably maintained the catalytic performance at 180 °C with the longest breakthrough time and highest release sulfur capacity. There was no benefit to desulfurization with excess oxygen in the feed gas. Through further analysis by TG, elemental sulfur was confirmed to be the main product of the catalytic oxidation of H₂S. Besides, a possible mechanism of H₂S selective catalytic oxidation over sorbent was proposed, in which the key step was the oxidation of FeS into S⁰ and Fe³⁺ by O₂. Moreover, sorbent deactivation was mainly caused by pore structure blockage by the sulfur product.

Direct oxidation esterification of methacrolein with methanol: Oxygen vacancy promotion of Zr‐doped Au/CeO2 nanorods

AbstractA series of Zr‐doped CeO2 nanorods supported by Au nanoparticle catalysts with different morphologies were prepared for methyl methacrylate (MMA) synthesis via direct oxidation esterification of methacrolein (MAL) with methanol. Catalytic activities under mild conditions (0.3 MPa; 80°C) were evaluated in the kettle reactor. Different characterization methods, such as XRD, Raman, N2 adsorption, ICP, TEM, SEM, XPS, and H2‐TPR techniques, were applied to study the relationship between catalytic activity and structural characteristics, especially the form of Zr doped into the lattice of CeO2 and the promotion mechanism of selective oxidation. The influence of Au supported on Ce0.6Zr0.4O2 nanorods (Au/CZ‐R) was also discussed. The results exhibited that the highest catalytic activity for the oxidative esterification with a MAL conversion of 99% and a MMA selectivity of 74% was achieved. The MMA synthesis performance correlated well with the surface oxygen vacancies, in particular for the active oxygen species around the Au particles.

Novel tetrasubstituted zinc phthalocyanine-attapulgite composites for efficient catalytic oxidation of styrene with tert-butyl hydroperoxide as oxidant

Novel tetrasubstituted zinc phthalocyanine-attapulgite ([ZnPc(OC10H14N)4]-ATP) composites were prepared by an ultrasonic impregnation method. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS), Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy. The prepared [ZnPc(OC10H14N)4]-ATP composites are used as the catalyst for styrene oxidation reaction to obtain the major products of styrene oxide and benzaldehyde, and the effects of solvent, oxidant, temperature, time and catalyst dosage on the reaction were investigated. The results showed that the [ZnPc(OC10H14N)4]-ATP composites exhibited high catalytic activity in the styrene reaction with the solvent being DMF and the oxidant being TBHP. The highest styrene conversion (84.13%) was obtained under the optimum conditions of 6 h, 100 °C and 20 mg of catalyst. Additionally, the possible mechanism of selective oxidation of styrene by [ZnPc(OC10H14N)4]-ATP composites was also proposed.

Preparation of MnOx Supported LiOH Activated Soybean Oil Sludge Catalyst and Its Analysis in Denitration Mechanism of Selective Catalytic Oxidation (SCO)

Treatment with selective catalytic oxidation (SCO) is an effective technology applied recently for conversion of nitrogen oxides pollution control. In order to solve the problems of high cost and difficulties in practical application of SCO catalyst, it was put forward using the solid waste sludge from soybean oil plant as catalyst carrier to prepare denitration catalyst. The sludge was treated by alkaline activation and then MnOx-based sludge was prepared by impregnation. Finally, MnOx-based sludge was calcined in the muffle furnace. The effects of activation and calcination conditions on catalyst activity were investigated. Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the activity of the sludge based denitration catalyst, and the structure and activity of the sludge based denitration catalyst were furtherly confirmed. According to the achieved results, (1) after activated by LiOH with a mass concentration of 15% for 4 hours, the surface of the sludge catalyst has more alkali functional groups, making the denitration of sludge catalyst the best; (2) the MnOx-based catalyst calcined in the muffle furnace with calcination temperature of 450 °C for 4 hours has obvious denitration efficiency.

NH3 -Driven Benzene C-H Activation with O2 that Opens a New Way for Selective Phenol Synthesis.

Catalytic benzene C-H activation toward selective phenol synthesis with O2 remains a stimulating challenge to be tackled. Phenol is currently produced industrially by the three-steps cumene process in liquid phase, which is energy-intensive and not environmentally friendly. Hence, there is a strong demand for an alternative gas-phase single-path reaction process. This account documents the pivotal confined single metal ion site platform with a sufficiently large coordination sphere in β zeolite pores, which promotes the unprecedented catalysis for the selective benzene hydroxylation with O2 under coexisting NH3 by the new inter-ligand concerted mechanism. Among alkali and alkaline-earth metal ions and transition and precious metal ions, single Cs+ and Rb+ sites with ion diameters >0.300 nm in the β pores exhibited good performances for the direct phenol synthesis in a gas-phase single-path reaction process. The single Cs+ and Rb+ sites that possess neither significant Lewis acidic-basic property nor redox property, cannot activate benzene, O2 , and NH3 , respectively, whereas when they coadsorbed together, the reaction of the inter-coadsorbates on the single alkali-metal ion site proceeds concertedly (the inter-ligand concerted mechanism), bringing about the benzene C-H activation toward phenol synthesis. The NH3 -driven benzene C-H activation with O2 was compared to the switchover of the reaction pathways from the deep oxidation to selective oxidation of benzene by coexisting NH3 on Pt6 metallic cluster/β and Ni4 O4 oxide cluster/β. The NH3 -driven selective oxidation mechanism observed with the Cs+ /β and Rb+ /β differs from the traditional redox catalysis (Mars-van Krevelen) mechanism, simple Langmuir-Hinshelwood mechanism, and acid-base catalysis mechanism involving clearly defined interaction modes. The present catalysis concept opens a new way for catalytic selective oxidation processes involving direct phenol synthesis.

Insight into the effect of morphology on catalytic performance of porous CeO2 nanocrystals for H2S selective oxidation

Shape-specific CeO2 nanocrystals (rods, cubes, spheres and nanoparticles) with well-defined crystal facets and hierarchically porous structure were successfully synthesized and used as model catalysts to study the structure-dependent behavior and reaction mechanism for H2S selective oxidation over ceria-based catalysts. It is deduced that the defect sites and base properties of CeO2 are intrinsically determined by the surface crystal facets. Among the nanocrystals, CeO2 nanorods with well-defined {110} and {100} crystal facets exhibits superb catalytic activity and sulfur selectivity. The high reactivity for H2S selective oxidation is attributed to the high concentration of surface oxygen vacancies which are beneficial for the conversion of lattice oxygen to active oxygen species. Besides, the presence of hierarchically porous structure of CeO2 nanorods hinders the formation of SO2 and sulfate, ensuring good sulfur selectivity and catalyst stability. Through a combined approach of density-functional theory (DFT) calculations and in situ DRIFTS investigation, the plausible reaction mechanism and nature of active sites for H2S selective oxidation over CeO2 catalysts have been revealed.

Characterization of the crack initiation and propagation in Alloy 600 with a cold-worked surface

The crack initiation on a cold-worked surface of Alloy 600, exposed to simulated pressurized water reactor primary water, was mechanistically studied through high-resolution characterization. Mechanical polishing introduced a thin recrystallization layer on the specimen surface, which lead to the crack initiation along highly-deformed recrystallization grain boundaries after preferential oxidation. Intergranular crack propagation occurred once the initiation cracks met the matrix grain boundaries under the external loading and the residual stress introduced by the prior 20% cold working. The controlling mechanism of SCC crack propagation was believed to be an intergranular selective oxidation mechanism.

Phenyl Ring Transfer Mechanism of Styrene Selective Oxidation to Phenyl Acetaldehyde on Gold Catalysts from Density Functional Theory (DFT) Studies

Olefin selective oxidation is one of the most important reactions in the modern chemical industry. In this work, we systematically studied the reaction mechanism of styrene selective oxidation to benzaldehyde, styrene epoxide, and phenyl acetaldehyde on various model O atom preadsorbed gas-phase gold clusters AuNq (N = 16, 28, 34, 55; q = 0, ±1) and flat Au(111) surfaces by means of density functional theory (DFT) calculations. We proposed a new reaction channel, namely, the styrene first interacts with the oxygen-adsorbed gold catalysts to form a five-membered ring known as oxametallacycle intermediate (OMME), and then the phenyl group linked to the α-C atom transfers to the β-C atom and forms phenyl acetaldehyde. This new mechanism is different from the hydrogen atom transfer mechanism proposed previously. The DFT calculation results showed that the new reaction pathway for the formation of phenyl acetaldehyde is energetically feasible over the gold clusters as well as the Au(111) surface and exhibits h...

A DFT Study on the Selective Oxidation of Ethane Over Pure SBA-15 and SBA-15-supported Vanadium Oxide

The selective oxidation of ethane over pure SBA-15 and V/SBA-15 were theoretically studied by density functional theory. The cluster models of pure SBA-15 and V/SBA-15 were proposed. The structure properties of these two models were calculated and were found to be in good agreement with experimental values. The catalytic reaction pathways for the ethane oxidation to acetaldehyde and ethylene were determined. Our results show that the hydroxyl groups on pure SBA-15 can activate the gas-phase O2 to form a peroxide species, which acts as the active site for the selective oxidation of ethane. The formation of ethylene is much more preferred than that of acetaldehyde over pure SBA-15. For V/SBA-15, the peroxide species also acts as the active center. The energy barrier of C–H bond activation over V/SBA-15 is by 14.63 kJ/mol lower than that over pure SBA-15. The formation of acetaldehyde is preferred than that of ethylene over V/SBA-15. On the basis of our results, the reaction mechanisms of ethane selective oxidation over pure SBA-15 and V/SBA-15 were systematically compared and discussed. The theoretical results in this study are in good agreement with our previous experimental results. They can reasonably explain the catalytic nature of pure SBA-15 and the effect of vanadium, opening new perspectives in the understanding of the chemistry of SBA-15.