Conversion Of Oxygen Research Articles

Catalytic conversion of carbohydrate-derived oxygenates over HZSM-5 in a tandem micro-reactor system

Effect of functionality, molecular size, and interactions of oxygenates over HZSM-5 were investigated to understand the reaction mechanism of catalytic fast pyrolysis of carbohydrates.

Rational design of mesoporous NiFe-alloy-based hybrids for oxygen conversion electrocatalysis

A simple and convenient strategy has been developed to prepare electrically conductive nitrogen-doped carbon modified mesoporous structure supported NiFe alloy nanocatalysts. The resulting electrocatalyst exhibits excellent electrocatalytic performance for both oxygen evolution reactions (OERs) and oxygen reduction reactions (ORRs).

Analysis of the catalytic steam gasification mechanism of biomass

The gasification of cellulose was here used to prove that the inner reaction of oxygen with carbon contained in the biomass was unavoidable during the gasification of biomass. It was also proven that the dehydrogenation reaction and conversion of oxygen in biomass to CO always caused the formation of residual carbon. Although, almost all of oxygen contained in the biomass could be converted into CO by the catalytic effect of ions, which in turn could be converted to CO2, but the CO2 was always methanized. This indicated that the hydrogen and syngas yield contained in the product gas of biomass could not practically achieve to the theoretical maximum hydrogen and syngas yield from biomass. The present study showed biomass gasification connected in series with coal gasification to be an excellent approach to large-scale conversion of biomass to hydrogen and syngas.

K-promoted NiMo catalysts supported on activated carbon for the hydrogenation reaction of CO to higher alcohols: Effect of support and active metal

This study investigates the effect of active metal and the nature and pre-treatment of the support in a series of K-promoted NiMo catalysts supported on activated carbon (AC) for the hydrogenation reaction of CO. The NiMo synergistic effect was found to be essential for the activation of CO. The use of activated carbon as support led to a three-fold increase in the selectivity toward higher alcohols, compared to the unsupported material, probably due to a reduction in the crystallinity of the active NiMoO4 phase and its partial transformation from alpha to beta form. Pre-treatment of the activated carbon prior to its use as support with HNO3 aqueous solution led to significant increase in CO conversion, related to the lower amount of ash and the much higher acidity of the acid treated sample. The use of a higher surface area acid-treated support also increases activity, with the KNiMo/AC2a catalyst exhibiting a 45% higher conversion at 280°C compared to KNiMo/AC1a. The improvement in activity can be ascribed to better dispersion of the active phase on the higher surface area AC support and therefore higher exposure of active NiOMo sites. Overall, the KNiMo/AC2a catalyst exhibited the optimum performance, with high CO conversion (25% at 280°C) and high production of oxygenates, with a space time yield to oxygenates of 141.5mg/gcatalyst/h at 280°C. A clear tendency of increasing higher alcohol production with decreasing acidity was evidenced for all K-promoted bimetallic NiMo catalysts.

Solid-State Method Toward PdO-CeO2 Coated Monolith Catalysts for Oxygen Elimination Under Excess Methane

Nanocrystalline PdO-CeO2 oxides were prepared by solid state grinding method, and then thermal treated at elevated temperatures. XRD, BET, Raman, XPS and TEM were employed to investigate the relationship between physicochemical characteristics and catalytic performances for the oxides. Homogeneous solid solutions are structurally stable up to 700 °C whilst segregation of Pd particles occurs at higher temperature. Monolith catalysts were obtained by wash-coating oxides onto cordierites, attempting to eliminate the scarce oxygen mixed in methane through methane oxidation reactions. Oxides calcined at 800 °C exhibited a higher activity during the first light-off experiment. However, activities improved dramatically for the other catalysts after the first ignition. It is apparent that small Pd particles at ceria surface were responsible for the activation of reactants at lower temperature, whereas well-dispersed Pd in ceria or solid solutions attributed to the total conversion of oxygen. Mars–van Krevelen mechanism was proposed with oxygen activation as the rate-limiting step on the PdCe-500 catalysts. PdO-CeO2 catalysts, prepared by solid-state method and wash-coated onto cordierites, exhibited a high activity for oxygen elimination under excess methane. Metallic Pd on the surface was regarded as the active sites for ignition while Pd2+ in the lattice was responsible for total oxygen conversion.

Production of gas mixtures with regulated ratios between ethylene and carbon monoxide by the gas-phase oxidative cracking of light alkanes

It was found that, in the gas-phase oxidative cracking of C2-C5 light alkanes, the ratio between ethylene and CO in the products depends on both the residence time in a reactor and the process temperature. This is due to a change in the contributions of product formation and/or consumption channels with increasing the conversion of the reactants. However, the hydrocarbon/oxygen ratio is the main parameter responsible for the limiting ratio between these products reached in the region of deep conversions of both of the reactants. The channels of formation and, correspondingly, the composition of the main products of oxidative cracking change on going from ethane to n-pentane. In this case, the ethylene: CO ratio increases due to an increase in the concentration of ethylene in the products as the number of carbon atoms in the initial alkane molecule is increased at a constant alkane: oxygen ratio. In the oxidative cracking of the C2+ alkane constituents of natural gases, it is necessary to consider the influence of methane, which inhibits the oxidative conversion of heavier alkanes in comparison with their oxidation in an inert gas atmosphere. This leads to a significant decrease in the conversion of oxygen and an increase in the ethylene: CO ratio in the reaction products.

Hydrogen Production by Partial Oxidation of Methanol Over Gold Supported on Fe<SUB>2</SUB>O<SUB>3</SUB>

Partial oxidation of methanol (POM) to produce hydrogen was investigated over Au/Fe2O3 catalysts. The catalysts were prepared by inverse co-precipitation method. The influence of catalyst synthesis parameters such as precipitant, gold loading, calcination temperature and reaction parameter such as reaction temperature on POM reaction to produce hydrogen were investigated. The catalysts have been characterized by means of TGA, BET, XRD, TEM, SEM-EDS and XPS analyses. TGA and DTGA profiles clearly indicate that minimum decomposition temperature required to obtain Fe2O3 in Au/Fe2O3 catalysts is 660 K. SEM-EDS analysis confirms that Au and Fe in Au/Fe2O3 are homogeneously distributed over the agglomerate. A detailed microstuctural characterization of Au/Fe2O3 sample by XRD, TEM and XPS analyses has shown that nanometer sized gold particles with oxidized gold species is in predominant amount in the uncalcined catalysts sample. The mean particle size and metallic state of gold particles increase with increasing calcination temperature. A highly active Au/Fe2O3 catalyst is obtained by using Na2CO3 as precipitant with lower gold loading and calcination at 673 K. The catalytic behavior of the Au/Fe2O3 catalyst is related not only to the gold but also to the chemical state of the support. The effect of reaction temperature on the catalytic performance of the Au/Fe2O3 catalysts was studied in the temperature range of 423 to 523 K. Oxygen conversion was complete through out the examined temperature range. Methanol conversion increased with rise in temperature and attains 100% at 503 K; Hydrogen selectivity increased with rise in reaction temperature up to 523 K and then dropped off. The overall reactions involved are methanol combustion, partial oxidation, steam reforming and decomposition. CO produced by methanol decomposition and/or by reverse water gas shift is subsequently transformed into CO2 and H2 by the water gas shift and/or CO oxidation.

Lysine 362 in cytochrome c oxidase regulates opening of the K-channel via changes in pKA and conformation

The metabolism of aerobic life uses the conversion of molecular oxygen to water as an energy source. This reaction is catalyzed by cytochrome c oxidase (CcO) consuming four electrons and four protons, which move along specific routes. While all four electrons are transferred via the same cofactors to the binuclear reaction center (BNC), the protons take two different routes in the A-type CcO, i.e., two of the four chemical protons consumed in the reaction arrive via the D-channel in the oxidative first half starting after oxygen binding. The other two chemical protons enter via the K-channel in the reductive second half of the reaction cycle. To date, the mechanism behind these separate proton transport pathways has not been understood.In this study, we propose a model that can explain the reaction-step specific opening and closing of the K-channel by conformational and pKA changes of its central lysine 362. Molecular dynamics simulations reveal an upward movement of Lys362 towards the BNC, which had already been supposed by several experimental studies. Redox state-dependent pKA calculations provide evidence that Lys362 may protonate transiently, thereby opening the K-channel only in the reductive second half of the reaction cycle. From our results, we develop a model that assigns a key role to Lys362 in the proton gating between the two proton input channels of the A-type CcO.

Partial Oxidation of Methane to Syngas Over γ-Al2O3-Supported Rh Nanoparticles: Kinetic and Mechanistic Origins of Size Effect on Selectivity and Activity

A series of supported Rh/γ-Al2O3 catalysts with an overall metal loading of 0.005 wt % was synthesized by impregnation of γ-Al2O3 with a toluene solution containing colloidally prepared well-defined (1.1, 2.5, 2.9, 3.7, and 5.5 nm) Rh nanoparticles (NP). The size of NP was not found to change after their deposition on γ-Al2O3 and even after performing partial oxidation of methane (POM) to synthesis gas at 1073 K for 160 h on stream. Apparent CO formation turnover rates and CO selectivity strongly decrease with an increase in this size. Contrarily, the overall scheme of POM is size-independent, i.e. CO and H2 are mainly formed through reforming reactions of CH4 with CO2 and H2O at least under conditions of complete oxygen conversion. The size effect on the activity and selectivity was related to the kinetics of interaction of CH4, O2, and CO2 with Rh/γ-Al2O3 as concluded from our microkinetic analysis of corresponding transient experiments in the temporal analysis of products reactor. The rate constants of...

Partial oxidation of methane in a temperature-controlled dielectric barrier discharge reactor

We studied the relative importance of the reduced field intensity and the background reaction temperature in the partial oxidation of methane in a temperature-controlled dielectric barrier discharge reactor. We obtained important mechanistic insight from studying high-temperature and low-pressure conditions with similar reduced field intensities. In the tested range of background temperatures (297<T<773K), we found that the conversion of methane and oxygen depended on both the electron-induced chemistry and the thermo-chemistry, whereas the chemical pathways to the products were overall controlled by the thermo-chemistry at a given temperature. We also found that the thermo-chemistry enhanced the plasma-assisted partial oxidation process. Our findings expand our understanding of the plasma-assisted partial oxidation process and may be helpful in the design of cost-effective plasma reformers.

Use of a DC discharge in a plasma reactor with a rotating ground electrode for production of synthesis gas by partial oxidation of methane

A recently designed rotating ground electrode plasma reactor has been used to produce synthesis gas by partial oxidation of methane at room temperature and atmospheric pressure without a catalyst. Experiments were performed to investigate the effects of feed flow rate, feed composition, and the presence of nitrogen in the feed. It was shown that increasing the O2-to-CH4 molar ratio reduces conversion of the reagents at low feed flow rates but promotes conversion at high feed flow rates. Increasing the O2-to-CH4 molar ratio also results in further methane partial oxidation reactions at high feed flow rates. Use of air instead of pure oxygen increases the conversion of methane and oxygen but reduces selectivity for the main products (H2 and CO). This reactor can be used to produce a wide variety of H2-to-CO molar ratios for different applications under stable conditions.

On the stability of conventional and nano-structured carbon-based catalysts in the oxidative dehydrogenation of ethylbenzene under industrially relevant conditions

Relevant carbon-based materials, home-made carbon–silica hybrids, commercial activated carbon, and nanostructured multi-walled carbon nanotubes (MWCNT) were tested in the oxidative dehydrogenation of ethylbenzene (EB). Special attention was given to the reaction conditions, using a relatively concentrated EB feed (10vol.% EB), and limited excess of O2 (O2:EB=0.6) in order to work at full oxygen conversion and consequently avoid O2 in the downstream processing and recycle streams. The temperature was varied between 425 and 475°C, that is about 150–200°C lower than that of the commercial steam dehydrogenation process. The stability was evaluated from runs of 60h time on stream. Under the applied reactions conditions, all the carbon-based materials are apparently stable in the first 15h time on stream. The effect of the gasification/burning was significantly visible only after this period where most of them fully decomposes. The carbon of the hybrids decomposes completely rendering the silica matrix and the activated carbon bed is fully consumed. Nano structured MWCNT is the most stable; the structure resists the demanding reaction conditions showing an EB conversion of ∼30% (but deactivating) with a steady selectivity of ∼80%. The catalyst stability under the ODH reaction conditions is predicted from the combustion apparent activation energies.

Hydrogen production by steam reforming of bio-oil/bio-ethanol mixtures in a continuous thermal-catalytic process

The feasibility of the steam reforming of bio-oil aqueous fraction and bio-ethanol mixtures has been studied in a continuous process with two in-line steps: thermal step at 300 °C (for the controlled deposition of pyrolytic lignin during the heating of the bio-oil/bio-ethanol feed) followed by steam reforming in a fluidized bed reactor on a Ni/α-Al2O3 catalyst. The effect of bio-ethanol content in the feed has been analyzed in both the thermal and reforming steps, and the suitable range of operating conditions (temperature and space-time) has been determined for obtaining a high and steady hydrogen yield. Higher ethanol content in the mixture feed improves the reaction indices and reduces coke deposition. Operating conditions of 700 °C and space-times higher than 0.23 gcatalyst h (gbio-oil+EtOH)−1 are suitable for attaining almost fully conversion of oxygenates (bio-oil and ethanol) and hydrogen yields above 93%, with low catalyst deactivation.

Analysis and modeling of char particle combustion with heat and multicomponent mass transfer

A char combustion model suitable for a large-scale boiler/gasifier simulation, which considers the variation of physical quantities in the radial direction of char particles, is developed and examined. The structural evolution within particles is formulated using the basic concept of the random pore model while simultaneously considering particle shrinkage. To reduce the computational cost, a new approximate analytical boundary condition is applied to the particle surface, which is approximately derived from the Stefan–Maxwell equations. The boundary condition showed reasonably good agreement with direct numerical integration with a fine grid resolution by the finite difference method under arbitrary conditions. The model was applied to combustion in a drop tube furnace and showed qualitatively good agreement with experiments, including for the burnout behavior in the late stages. It is revealed that the profiles of the oxygen mole fraction, conversion, and combustion rate have considerably different characteristics in small and large particles. This means that a model that considers one total conversion for each particle is insufficient to describe the state of particles. Since our char combustion model requires only one fitting parameter, which is determined from information on the internal geometry of char particles, it is useful for performing numerical simulations.

Micro scale dynamic modeling of LSM/YSZ composite cathodes

Abstract In this study, a detailed reaction mechanism for oxygen reduction on LSM/YSZ is applied to dynamic one-dimensional micro-scale modeling of LSM–YSZ composite cathodes. The model incorporates the effects of cathode microstructural properties on the local transport phenomena and electrochemistry inside the cathode. The reaction mechanism used in the model features two parallel routes for bulk gas oxygen conversion to electrolyte lattice oxygen, namely two-phase boundary (2 PB) and three-phase boundary (3 PB) pathways. The model predicts field distributions of local thermodynamic values, over-potential, Faradaic current and other parameters relevant to cathode performance. Electrochemical impedance simulations are performed in the time domain to analyze the contribution of specific mechanistic steps to the overall impedance. A parametric study was performed to assess the sensitivity of the model predictions to selected model parameters and cathode micro-structural properties. Results reveal that the relative contribution from 3 PB and 2 PB pathways to the total charge transfer is dependent upon operating conditions. Parametric sensitivities observed in the impedance results are compared with polarization simulations to assess the physical plausibility of the proposed reaction and transport model. Simulation results are further compared to experimental findings reported in literature.

Synthesis and substantially enhanced gas sensing sensitivity of homogeneously nanoscale Pd- and Au-particle decorated ZnO nanostructures

We successfully prepared Pd- and Au-nanoparticle-decorated ZnO nanostructures (Pd–ZnO and Au–ZnO) by combining sputtered Pd and Au ultrathin films on ZnO surfaces and by postannealing in vacuum. The Pd and Au nanoparticles were homogeneously distributed on ZnO surfaces without substantial aggregation. The ranges of the Pd- and Au-nanoparticle diameters were 4–11nm and 5–8nm, respectively. The photoluminescence spectra of the Pd–ZnO and Au–ZnO exhibited a quenched UV emission-band intensity and an increased visible-light emission band intensity compared with those of pure ZnO nanostructures. The results of X-ray photoelectron spectroscopy analysis in this work showed that the Pd–ZnO obtained a higher amount of chemisorbed oxygen and enhanced selectivity to acetone compared with the Au–ZnO. The gas responses of the ZnO nanostructured sensors were substantially improved with noble-metal nanoparticle decoration. The enhanced adsorbed oxygen and molecule-ion conversion rate in using nanoscale noble-metal particle decoration might account for the observed results.

The use of bottle caps as submerged aerated filter medium

In this study, a submerged aerated filter (SAF) using bottle caps as a support medium was evaluated. The system was fed with effluent from an upflow anaerobic sludge blanket system at ETE 2-South wastewater treatment plant, under different volumetric organic load rates (VOLRs). The population of a particular nitrifying microbial community was assessed by fluorescent in situ hybridization with specific oligonucleotide probes. The system showed an average removal of chemical oxygen demand (COD) equal to 76% for VOLRs between 2.6 and 13.6 kg COD m(-3)_media.day(-1). The process of nitrification in conjunction with the removal of organic matter was observed from applying VOLRs lower than 5.5 kg COD m(-3)_media.day(-1) resulting in 78% conversion of NH4(+)-N. As the applied organic load was reduced, an increase in the nitrifying bacteria population was observed compared with total 4'-6-diamidino-2-phenylindole (DAPI) stained cells. Generally, SAF using bottle caps as a biological aerated filter medium treating wastewater from an anaerobic system showed promising removal of chemical oxygen demand (COD) and conversion of NH4(+)-N.

Understanding the role of TiO2crystal structure on the enhanced activity and stability of Ru/TiO2catalysts for the conversion of lignin-derived oxygenates

Although Ru catalysts supported on reducible oxides such as TiO2 hold significant promise for the deoxygenation of biomass derived oxygenates, a significant drawback is their instability under oxidation conditions necessary for catalyst regeneration. In this contribution, the role of TiO2 crystal structure on resistance to metal particle sintering during calcination treatments at 400 and 500 °C is investigated. The resulting impact of the calcination temperature and TiO2 support phase for the conversion of guaiacol at 400 °C under atmospheric pressure of hydrogen over supported Ru catalysts is presented. Results suggest that the rutile TiO2 phase plays an important role in stabilizing Ru particles during calcination pretreatment in comparison with anatase supported Ru catalysts. Furthermore, rates normalized to the area of the support and the Ru suggest that the high activity of Ru/TiO2 systems for guaiacol conversion is attributed to defect sites created by hydrogen spillover from the Ru metal to the reducible TiO2 as opposed to only the sites located at the Ru/TiO2 interface.

Conversion of biomass-derived small oxygenates over HZSM-5 and its deactivation mechanism

HZSM-5 catalyst deactivation was studied using aqueous feed mixtures containing ethanol, ethanol + acetic acid, ethanol + ethyl acetate, or ethanol + acetaldehyde in a fixed bed reactor at 360 °C and 300 psig. Compared to an ethanol alone experiment, addition of other oxygenates reduced catalyst life in the order of: ethyl acetate < acetic acid < acetaldehyde. Based on the liquid product and spent catalyst analyses from the individual ethanol, acetaldehyde, acetic acid, and ethyl acetate feeds, the presence of acetaldehyde appears to produce high molecular weight aromatic compounds which deactivate the catalyst through a pore-blocking mechanism. Acetic acid deactivates the catalyst through an active site poisoning mechanism or strong adsorption of acetate intermediates on the active sites (hydroxyl groups). Ethanol deactivates the catalyst primarily through its pore-blocking mechanism, but the rate of ethanol deactivation is orders of magnitude slower than that of acetaldehyde. Ethyl acetate hydrolyzes to form acetic acid and ethanol which deactivate the catalyst through their respective mechanisms. In addition, each functional group of oxygenates requires different active sites/catalysts and different operating conditions due to competitive adsorptions on active sites for their conversion to the desired products. Therefore, it is necessary to pre-treat the mixture of oxygenates to produce a feed stream containing the same or similar functional group compounds before converting the feed stream to hydrocarbon compounds over the HZSM-5 catalyst.

Characterization and Evaluation of Prepared Fe2O3/Al2O3 Oxygen Carriers for Chemical Looping Process

Alumina supported Fe2O3 was prepared to evaluate its feasibility as an oxygen carrier for chemical looping using a thermogravimetric analyzer (TGA) and fixed bed reactor. Fe2O3/Al2O3 particles containing 60 wt% Fe2O3 and sintered at 1300°C demonstrated reasonable oxygen conversion. The reduction mechanism of Fe2O3/Al2O3 oxygen carriers was proposed based on the thermogravimetric analysis and XRD characterization. FeAl2O4 is formed and can serve as a supporting material as well as oxygen carrier in practical operations by using a moving bed reactor. The prepared Fe2O3/Al2O3 oxygen carriers demonstrated high CO2 yields in relation to syngas and methane combustion in the fixed bed reactor operated at temperatures higher than 500 and 850°C, respectively. Moreover, hydrogen generation was demonstrated to be feasible by steam oxidation with reduced Fe2O3/Al2O3 oxygen carriers for experiments conducted at temperatures above 800°C. The crush strength and TGA of Fe2O3/Al2O3 were examined, and the results showed that they had an appropriate crush strength and reasonable reactivity as oxygen carriers for use in a chemical looping process.