Mechanism Of Catalytic Oxidation Research Articles

Catalytic oxidation mechanisms of carbon monoxide over single and double vacancy Cr-embedded graphene

The detailed catalytic oxidation mechanisms of CO over Cr-embedded graphene have been investigated by means of M06-2X density functional. Both models with chromium atom embedded in single and double vacancy (Cr-SV and Cr-DV) in a graphene sheet were considered. It is found that the CO oxidation on Cr-SV-graphene prefers to Langmuir–Hinshelwood (LH) mechanism, while the Cr-DV-graphene shows good catalytic activity toward the CO oxidation via the Eley–Rideal (ER) mechanism. The present results imply that the low-cost Cr-embedded graphene is a prospective catalyst for CO oxidation at room temperature.

Electric field promoted ultra-lean methane oxidation over Pd-Ce-Zr catalysts at low temperature

A novel catalytic system combining oxidation catalyst with electric field was applied for low-temperature CH4 oxidation at low CH4 concentration (0.2% CH4). Compared with conventional catalytic process, significant improvement in activity was observed over PdCeZrOx catalyst in electric field. For example, the light-off temperature (T50) of CH4 (0.2% CH4) of 1 wt % PdCe0.75Zr0.25Ox catalyst was approximately 255 °C in an electric field with 3 mA electric current.The impact of electric field in CH4 oxidation over PdCeZrOx catalyst was thoroughly investigated. The XRD and XPS results revealed that electric field provided free electrons that facilitated the release of lattice oxygen in the ceria-zirconia solid solution, therefore reinforced Pd0 oxidation and the formation of palladium oxide (PdOx, x > 0) on catalyst surface. Electric field also enhanced the reducibility of the PdOx, which provided active oxygen for CH4 oxidation at low temperature. With in-situ FTIR technique, it was found that electric field greatly promotes the chemisorption and dehydrogenation of CH4, thereby accelerates the chemical reactions of the intermediate products, so as to promotes CH4 oxidation. The mechanism of catalytic oxidation of CH4 in electric field was proposed based on comprehensive investigations above.

INTENSIFICATION OF THE CYCLOHEXANE LIQUID PHASE OXIDATION PROCESS

Liquid-phase oxidation of cyclohexane to cyclohexanol and cyclohexanone was studied in the absence of solvents under an air pressure of 0.5-5 MPa, in the temperature range 115-150 °C, catalyzed by N-hydroxyphthalimide (N-HPI). It was established for the first time that the use of N-HPI as a catalyst in place of the conventionally used metal salts of variable valence allowed a 2-3-fold increase in the conversion of the initial hydrocarbon and selectivity from 70-75 to 90%. The combined use of N-HPI with cobalt(II) acetate results in an additional increase in the conversion of cyclohexane by 30-40%, the selectivity of cyclohexanol and cyclohexanone formation to 94-97%, which seems to be due to the synergistic effect between the two components of the catalyst. The mechanism of catalytic oxidation of cyclohexane to cyclohexanol and cyclohexanone is discussed. It has been suggested that N-HPI plays a dual role in the oxidation of cyclohexane: it catalyzes the conversion of cyclohexane to cyclohexanol and cyclohexanone and, on the other hand, promotes the conversion of cyclohexanol to cyclohexanone, thereby substantially reducing the formation of adipic acid and its esters, by-products of the reaction, and increases selectivity of oxidation. This also explains the unusually high (1.3-1.5 : 1) ketone: alcohol ratio in the oxidation products of cyclohexane in the presence of N-HPI. The high selectivity of the formation of the desired products, the conversion of cyclohexane, the moderate temperature, the available catalyst, suggest that this method of oxidizing cyclohexane to cyclohexanol and cyclohexanone may be of interest for further practical use.

A Single Pd Atom Stabilized on Boron‐Vacancy of h‐BN Nanosheet: A Promising Catalyst for CO Oxidation

AbstractThe possible reaction mechanisms for CO catalytic oxidation by O2 molecule on the Pd‐doped hexagonal boron nitride nanosheet (Pd‐BNNS) were studied using first‐principles calculations. The large adsorption energy of the Pd atom over the boron‐vacancy defect of BN nanosheet suggests that Pd‐BNNS could be stable under high temperatures. According to our results, the adsorption of CO over Pd‐BNNS is energetically preferable than that of O2. Three different reaction pathways of the CO oxidation are investigated comparably: the Eley‐Rideal (ER), the Langmuir‐Hinshelwood (LH) and the termolecular Eley‐Rideal (TER). Our results indicate that the CO oxidation reaction would like to take place via the TER mechanism due to its small activation energies. The calculated energy barrier for the rate‐determining step of the latter pathway is only 0.19 eV. Based on electronic structure analysis, such high catalytic activity of Pd‐BNNS can be related to the strong hybridization of the Pd‐4d and CO‐5σ/CO‐2π* states, which effectively activates the adsorbed CO molecules involved in the TER mechanism.

Insights into the Reaction Mechanism of Catalytic Wet Air Oxidation of Ammonia Over Bimetallic Ru–Cu Catalyst

The mechanism for catalytic wet air oxidation (CWAO) of ammonia to N2 over Ru–Cu/C catalyst is extensively studied by altering the initial pH, reaction temperature and atmosphere. It is found that N2 formation can be from the catalytically selective oxidation of ammonia or the disproportionation reaction between NH4+ and NO2−. The initial oxidation of ammonia determines the reaction mechanism, the evolution of pH and the distribution of various nitrogen species. Over-oxidation to nitrous acid lowers the pH of the solution due to dissociation of HNO2 to H+ and NO2−. With the decrease of pH, the concentration of NH4+ is increased and rapidly reacts with NO2− to form N2. The relatively lower pH also makes some nitrites be oxidized to NO3−. Enhancing the reaction of selective oxidation of ammonia to N2 increases the selectivity to N2 while limits the pH decrease and NO3− formation, since NO2− is more dominant to HNO2 at high pH and hardly oxidized to NO3−. The reaction temperature is one key factor to determine the reaction mechanism of CWAO of ammonia.

The preparation and photocatalytic performance of Bi2Fe4O9/NiFe2O4 composite photocatalyst

The magnetic n–p type Bi2Fe4O9/NiFe2O4 heterostructures with different mass fraction ratios were synthesized by a simple hydrothermal method. A variety of characterization tools involving X-ray diffraction, scanning electron microscope, ultraviolet–visible diffuse reflectance (DRS UV–Vis), fluorescence were applied to analyze the basic characteristics of Bi2Fe4O9/NiFe2O4. The photocatalytic oxidation capacities of Bi2Fe4O9/NiFe2O4 composite were studied through the degradation of methylene blue under UV light. The as-prepared samples exhibited more efficient photocatalytic activities than pure Bi2Fe4O9 and NiFe2O4, which could be attributed to the formation of the n–p junction between n-Bi2Fe4O9 and p-NiFe2O4. Simple magnetic test showed Bi2Fe4O9/NiFe2O4 had good magnetic recovering properties. Continuous photocatalytic experiments demonstrated that the heterojunction structure of Bi2Fe4O9/NiFe2O4 was stable. Based on the results of trapping agents experiments and fluorescence characterization, the catalytic oxidation mechanism of Bi2Fe4O9/NiFe2O4 composite catalysts were further studied, which indicated that ·OH and h+ played critical roles in the process of photocatalytic degradation.

A review of the preparation and applications of MnO2 composites in formaldehyde oxidation

This paper reviews several important preparation methods, and several kinds of MnO2 composites, which is efficient in formaldehyde catalytic oxidation. The preparation of MnO2 composites discussed in this article refer to sol–gel method, rheological phase reaction method, micro-emulsion method, chemical coprecipitation method, solid-phase synthesis and template method. After a presentation of methods for the preparation of MnO2, a review of the performance on formaldehyde catalytic oxidation over (I) manganese oxides; (II) metal–MnO2 oxide composites; (III) graphene–MnO2 composites is provided. The mechanisms for catalytic oxidation of HCHO over MnO2 composites, future directions and potential hotspots are also discussed to facilitate understanding.

Identifying MnVII-oxo Species during Electrochemical Water Oxidation by Manganese Oxide

SummaryIdentifying surface active intermediate species is essential to reveal the catalytic mechanism of water oxidation by metal-oxides-based catalysts and to develop more efficient catalysts for oxygen-oxygen bond formation. Here we report, through electrochemical methods and ex situ infrared spectroscopy, the identification of a MnVII = O intermediate during catalytic water oxidation by a c-disordered δ-MnOx with an onset-potential-dependent reduction peak at 0.93 V and an infrared peak at 912 cm−1. This intermediate is proved to be highly reactive and much more oxidative than permanganate ion. Therefore, we propose a new catalytic mechanism for water oxidation catalyzed by Mn oxides, with involvement of the MnVII = O intermediate in a resting state and the MnIV−O−MnVII = O as a real active species for oxygen-oxygen bond formation.

Catalytic supercritical water destructive oxidation of tributyl phosphate: Study on the effect of operational parameters

Catalytic destructive oxidation of tributyl phosphate (TBP) in supercritical water (SCW) medium was examined. Different nanocatalysts were synthesized using SCW method and their abilities to destruct the TBP in the aqueous solution were compared in order to pick out the best catalyst for the subsequent experiments. The effect of main operational variables, including reaction temperature, time and pressure, and oxidant concentration was investigated in order to determine optimal conditions. A practical mechanism for catalytic oxidation of TBP in SCW was also proposed based on the results of various analyses. The results demonstrated that among all of the catalysts, CeO2 provided the best result in the oxidation process, by which the TOC removal efficiency was increased by 22% with respect to noncatalytic oxidation. In addition, temperature and pressure showed an influential effect on the TBP decomposition efficiency, while the effect of oxidant can be neglected.

Flexible 3D Fe@VO2 core-shell mesh: A highly efficient and easy-recycling catalyst for the removal of organic dyes

Nowadays, it is extremely urgent to search for efficient and effective catalysts for water purification due to the severe worldwide water-contamination crises. Here, 3D Fe@VO2 core-shell mesh, a highly efficient catalyst toward removal of organic dyes with excellent recycling ability in the dark is designed and developed for the first time. This novel core-shell structure is actually 304 stainless steel mesh coated by VO2, fabricated by an electrophoretic deposition method. In such a core-shell structure, Fe as the core allows much easier separation from the water, endowing the catalyst with a flexible property for easy recycling, while VO2 as the shell is highly efficient in degradation of organic dyes with the addition of H2O2. More intriguingly, the 3D Fe@VO2 core-shell mesh exhibits favorable performance across a wide pH range. The 3D Fe@VO2 core-shell mesh can decompose organic dyes both in a light-free condition and under visible irradiation. The possible catalytic oxidation mechanism of Fe@VO2/H2O2 system is also proposed in this work. Considering its facile fabrication, remarkable catalytic efficiency across a wide pH range, and easy recycling characteristic, the 3D Fe@VO2 core-shell mesh is a newly developed high-performance catalyst for addressing the universal water crises.

Catalytic decomposition of HCN on copper manganese oxide at low temperatures: Performance and mechanism

The development of innovative treatment technology for HCN at low temperatures is vital for the control of HCN pollution. Here, a copper manganese oxide (Cu-Mn-O) catalyst was prepared for the decomposition of HCN at temperatures from 80 to 200 °C. The results showed that the Cu-Mn-O catalyst exhibited excellent catalytic performance for HCN conversion. X-ray photoelectron spectroscopy analysis revealed that Mn3+ manifested highly catalytic activity and was largely responsible for HCN decomposition. The N-contained products of HCN contained NH3, NO/NO2, N2O, and N2, thereby suggesting the concurrent catalytic oxidation and hydrolysis during HCN decomposition on the catalyst. The catalytic oxidation mechanism characterized by in situ diffuse reflectance infrared Fourier transform manifested that four N-contained intermediates (i.e., -CN, -NH2, =NH and -NCO) were produced; subsequent the oxidation of these intermediates resulted in the formation of final product and/or oxidative species NO+. The reaction of NO+ with the N-contained intermediates also generated the final conversion products. Catalytic intermediate formamide plays a critical role in the hydrolysis of HCN, and its hydrolysis leads to the formation of NH3. Multiple cycle experiments demonstrate the long-term stability of the Cu-Mn-O catalyst. These results indicate that catalytic decomposition of HCN based on the Cu-Mn-O catalyst at low temperatures may be an efficient approach for the treatment of tail gases containing HCN.

Hierarchically porous γ-Al2O3 nanosheets: Facile template-free preparation and reaction mechanism for H2S selective oxidation

A template-free strategy has been developed for the controlled synthesis of hierarchically porous γ-Al2O3 with three dimensional (3D) “cluster-like” architectures assembled from 2D nanosheets (∼15 nm thick). This unique structure endows the material with large active surface area, high accessibility and ready mass transport ability, capable of promoting the catalytic performance. As a result, the designed γ-Al2O3 exhibits high activity for selective oxidation of H2S to elemental sulfur. All the porous γ-Al2O3 samples show outstanding sulfur selectivity (∼100%) below 200 °C. At higher temperatures, the H2S conversion increases while the sulfur selectivity decreases. The maximum sulfur yield (93%) can be achieved over the optimal sample at 240 °C, which is much higher than those of commercial alumina and most of the reported alumina-based catalysts. The structure–activity relationship of the catalysts has been systematically studied. It is demonstrated that the surface acidity of the γ-Al2O3 has a great influence on catalytic activity. By means of pyridine and H2S in situ FTIR, the nature of the active sites as well as the reaction mechanism of H2S selective catalytic oxidation over γ-Al2O3 is revealed, providing important insights into the design of alumina-based materials for the removal of sulfur-containing compounds.

Adsorption and surface reaction pathway of NH3 selective catalytic oxidation over different Cu-Ce-Zr catalysts

The adsorption species and reaction mechanism for NH3 selective catalytic oxidation (NH3-SCO) over series of Cu-Ce-Zr catalysts prepared by citric acid sol–gel (SOL), homogeneous precipitation (HP) and incipient wetness impregnation (IW) methods were systematically investigated by in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT) and Mass Spectroscopy (MS). The redox characteristics of catalysts were determined by the synergistic effect among each catalyst components. The reaction pathway on all of three catalysts applied on the NH3-SCO were investigated and compared through the formation of intermediate species and final products. The Cu-Ce-Zr-SOL catalyst with the best catalytic activity presented the excellent adsorption ability of NHx and NOx species. A large amount of intermediate species over Cu-Ce-Zr-SOL surface facilitated the reaction. On the other hand, the most of NH3 was located in Brønsted acid sites in the form of NH4+ over Cu-Ce-Zr-HP which was stable during the reaction. The larger CuO particle and a vast of B acid sites on Cu-Ce-Zr-IW showed the remarkably negative effect for the NH3 adsorption and activation. Moreover, less amounts of intermediates formed on Cu-Ce-Zr-HP and Cu-Ce-Zr-IW. That resulted in the catalytic performance of those two catalysts were inferior to Cu-Ce-Zr-SOL. However, the reaction pathway of all three catalysts was all following to iSCR route. The adsorbed NH3 was activated to form NHx and HNO intermediate species. With the increase of temperature, NHx species would be further oxidized to nitrate species, which could further react with NHx. It was also observed that the catalytic activity of surface oxygen species was higher than gaseous oxygen on Cu-Ce-Zr catalyst, and it had priority to react with NHx. The gaseous oxygen took part in the reaction and facilitated the formation of N2 as the temperature was relatively higher.

Mechanism of Aerobic Alcohol Oxidation Mediated by Water‐Soluble CuII‐TEMPO Catalyst in Water: A Density Functional Theory Study

AbstractThe catalytic mechanism for the aerobic alcohol oxidation in the alkaline water solution catalyzed by a CuII/2,2,6,6‐tetramethylpiperidinyl‐1‐oxy (TEMPO) catalyst system, ([(L)CuII(Hen)(H2O)] (H3L: 3‐(5‐Chloro‐2‐hydroxy‐3‐sulfophenylhydrazo) pentane‐2,4‐dione; en: Ethylenediamine), is presented by density functional theory (DFT) calculations. Four pathways (path A, path B, path C and path D) are presented. Our calculations demonstrate that path A is the favourable pathway and zwitterionic property of the catalyst is not likely affect the efficiency of alcohol oxidation. In path A, the catalytic cycle consists of catalyst activation, substrate oxidation and catalyst regeneration parts. The calculated turn‐over frequency (TOF=3.89 h−1) is in line with the experimental result (TOF=5.40 h−1). It is also found that the H atom migrates from alkoxide to the oxygen atom of TEMPO in the TOF determining transition state (TDTS).

Understanding the Active Sites of Ag/Zeolites and Deactivation Mechanism of Ethylene Catalytic Oxidation at Room Temperature

Microporous Ag/zeolite (ZSM-5, Beta, Y, and Mordenite) catalysts were found to be potentially good catalytic materials for ethylene oxidation at room temperature. These catalysts were evaluated under both dry and humid atmospheres, achieving 100% conversion of 100 ppm of ethylene mineralized into CO2 at 25 °C. Moreover, the zeolite framework type and relative humidity had a significant effect on catalytic stability. Pyridine Fourier transform infrared spectra (FTIR) and solid-state hydrogen-1 (1H) magic angle spinning nuclear magnetic resonance (MAS NMR) studies revealed that Bronsted acid sites were the active sites in Ag/zeolites; the deactivated Ag/zeolites had no available Bronsted acid sites. When the number of Bronsted acid sites of Ag/ZSM-5-humid and Ag/Beta-humid were reduced, the catalytic activities of Ag/ZSM-5-humid and Ag/Beta-humid decreased. Temperature-programmed oxidation mass spectrometry (TPO-MS) and water adsorption/desorption results indicated that H2O adsorbed on Bronsted acid sites l...

Study of catalytic ozonation for tetracycline hydrochloride degradation in water by silicate ore supported Co3O4.

Tetracycline hydrochloride (TCH) degradation by cobalt modified silicate ore (CoSO) catalytic ozonation in aqueous solution was investigated. CoSO catalyst was synthesized by an impregnation method using Co(NO3)2 as the precursor and natural silicon ore (SO) as the support. The key catalyst preparation conditions (i.e., impregnation concentration, calcination temperature and time) were optimized. The activity and stability of CoSO catalyst and its catalytic ozonation mechanism for TCH degradation were studied. The results showed that Co3O4 was successfully coated on the silicon ore and the CoSO catalyst was highly efficient in catalytic ozonation for TCH degradation. The TCH removal by CoSO/O3 could reach 93.2%, while only 69.3% by SO/O3 and only 46.0% by O3 alone at 25 min. The reaction of TCH degradation followed pseudo-first order kinetics. TOC removal rate by CoSO/O3 was 2.0 times higher than that by SO/O3, and 3.5 times higher than that by O3 alone. The reaction conditions (TCH initial concentration, catalyst concentration, pH and temperature) for catalytic ozonation were systematically investigated. The possible mechanism for the CoSO catalytic ozonation process was proposed, where hydroxyl radical oxidation mainly accounted for the substantial TCH degradation. Furthermore, CoSO showed great durability and stability after seven reaction cycles.

Preparation of Ni/NiO-C catalyst with NiO crystal: catalytic performance and mechanism for ethanol oxidation in alkaline solution

Highly active and durable catalysts are imperative for the development of alcohol oxidation research. Herein, a Ni/NiO-C catalyst with hollow NiO crystal can be used in ethanol oxidation. The hollow NiO nanoparticles are absolutely coated in the amorphous carbon and the generated NiO crystal remarkably raises the electrocatalytic performances of catalysts. Among all samples, the Ni/NiO-C3 catalyst shows the highest current density in 0.5 M ethanol solution. This is mainly derived from the synergistic effect of the crystallographic form, carbon coating, and hollow structure. Besides, the reaction mechanism of Ni/NiO-C3 electrode is controlled by charge transfer in high-concentration ethanol. With the concentration of ethanol decreased, a diffusion control mechanism is observed on Ni/NiO-C3 electrode. The attenuation rate of current density is only 0.1% after 3600 s by chronoamperometry in 0.1 M ethanol aqueous solution, indicating that the Ni/NiO-C3 electrode has a superior stability. Moreover, Ni/NiO-C3 electrode exhibits an rapid current response in 0.1 mM ethanol solution and a linear relationship between 0.69 and 40.17 mM, which provides a new idea for the research of fuel cell type ethanol sensor.

Divacancy-nitrogen/boron-codoped graphene as a metal-free catalyst for high-efficient CO oxidation

Metal-free catalysts have attracted wide attention due to their potentially catalytic applications. We performed density functional theory (DFT) calculations to investigate the different reaction mechanisms for CO catalytic oxidation on single-atom B incorporated into divacancy-nitrogen-doped graphene sheet (B-GN4). The calculated results show that the B atom can be strongly trapped at the active center of GN4 sheet and the formed B-GN4 configuration can be stable enough at high temperature. Based on the strong adsorption and significant activation of the reactants (CO and O2), the possible catalytic reactions of CO oxidation on the B-GN4 sheet are comparably studied through the Eley−Rideal (ER) and Langmuir–Hinshelwood (LH) mechanisms. In the sequential reactions, the dissociative adsorption of an O2 molecule as a starting step and following by the ER reactions (2O + 2CO → 2CO2) have much smaller energy barriers than the traditional ER and LH mechanisms. Besides, the dissociative reaction of CO molecule on the B-GN4 sheet, the formation process and surface activity of B-C codoped GN4 sheet (BC-GN4) are analyzed. This result indicates that the B-GN4 sheet as an anode material can promote the CO oxidation reaction, which provides a comprehensive understanding of graphene-based metal-free catalyst.

Homogeneous electrocatalytic water oxidation catalyzed by a mononuclear nickel complex

Three nickel (II) complexes of o-phenylenebis(N′-methyloxamidate) (L1), o-phenylene(N′-methyloxamidate)oxamate (L2) and o-phenylenebis(oxamate) (L3) with tetradentate ligands are synthesized and characterized. These four-coordinate complexes, (Me4N)2[NiLi] (i = 1–3 for complex 1–3), are investigated for electrocatalytic water oxidation in basic phosphate buffer solution. Experimental results show that (Me4N)2[NiL1] (1) is a homogeneous catalyst for electrochemical water oxidation. However, under the same condition, 2 and 3 decompose into NiOx nanoparticles, which acts as precatalyst for the electrocatalytic water oxidation. The catalytic mechanism for electrocatalytic water oxidation by 1 is proposed using cyclic voltammetry experiments, kinetic isotope effect and Pourbaix diagram. By constructing the relationship between the molecular structure and stability of these catalysts, nitrogen atom is found to be more beneficial than carboxyl group for the stability of nickel based homogeneous electrochemical water oxidation catalysts.

A Bis(µ‐chlorido)‐Bridged Cobalt(II) Complex with Silyl‐Containing Schiff Base as a Catalyst Precursor in the Solvent‐Free Oxidation of Cyclohexane

A new bis(µ‐chlorido)‐bridged cobalt(II) complex [Co2(µ‐Cl)2(HL2)4][CoCl4] (1), where HL2 is a silyl‐containing Schiff base, was synthesised. The structure of this compound was established by X‐ray crystallography revealing a zwitterionic form adopted by the organic ligand. The temperature dependence of the magnetic susceptibility and the field dependence of the magnetisation indicate the presence of ferromagnetic interactions between paramagnetic d7 cobalt(II) centres (SCo = 3/2). The exchange coupling parameter J(Co1–Co2) = +7.0 cm–1 extracted from broken‐symmetry (BS) DFT calculations agrees well with the value of +8.8 cm–1 determined from the experimental data by fitting them with the Hamiltonian . Electrochemical studies indicate that complex 1 is inefficient as a catalyst in electrochemical reduction of protons. One of the reasons is the low stability of the complex in solution. In contrast, 1 acts as an effective homogeneous (pre)catalyst in the microwave‐assisted neat oxidation of cyclohexane with aqueous tBuOOH (TBHP). The possible mechanism of catalytic oxidation and other advantages of using 1 in the oxidation of cycloalkanes are discussed.