Oxygen-lean Conditions Research Articles

Oxidative Dehydrogenation of Propane over Nanostructured Mesoporous VOx/CexZr1-xO2 Catalysts

High-surface-area mesoporous CexZr1-xO2 materials synthesized through a surfactant-assisted approach of nanocrystalline particle assembly are utilized as a promising support for VOx-based catalysts. The catalytic properties of the resultant VOx/CexZr1-xO2 nanocatalysts are evaluated by the oxidative dehydrogenation of propane using a microreactor-GC system. It is indicated that the catalyst particles are on a nanoscale, having a mesoporous structure with uniform pore-size distribution and high surface area. The catalytic behavior of these mesoporous nanostructured VOx/CexZr1-xO2 catalysts for the oxidative dehydrogenation of propane reaction relies on the vanadia loading amount, the calcination temperature, the surface area and the Ce/Zr ratio of the supports, the particle size of active compounds, and the additional contribution to the propylene formation derives from the contribution of the catalytic dehydrogenation of propane under oxygen-lean conditions. The catalyst prepared with 8 wt% vanadia loading on Ce0.2Zr0.8O2 exhibits high and stable catalytic performance in the oxidative dehydrogenation of propane reaction. 

Coal oxidation characteristics and index gases of spontaneous combustion during the heating and cooling processes

In order to explore the changes during the coal cooling process in a closed fire zone (CFZ), the oxidation characteristics of the heating and cooling processes were thoroughly examined and compared. The heating and cooling processes under various oxygen concentrations of three coals were simulated with temperature-programmed experiment and coal spontaneous combustion experiment. Furthermore, the hottest spot, oxygen consumption, index gases, and oxidation kinetics between the gaseous production rate (f) and coal temperature (T), air supply (Q), oxygen concentration (CO2), and apparent activation energy (Ea) were analysed. The results revealed that the movements of the hottest spots did not overlap in the heating and cooling processes. Moreover, the oxygen consumption during the cooling process was higher than that during the heating process, and a lower oxygen concentration was associated with higher oxygen consumption, a condition in which coal strongly captured oxygen actively. The index gases during the heating and cooling processes notably differed, which were related to coal types, and during the forced heating process, the CO2/CO ratios of 1/3 coking coal and anthracite were higher than that of the forced cooling process, even under the oxygen-lean conditions, the CH4/C2H4 ratio of lignite during the forced cooling process was also notable. Moreover, the results in the cooling process of the temperature-programmed experiment in the carrier gases with 7% and 3% O2 were consistent with that of the spontaneous combustion experiment. Finally, the apparent activation energy at different stages of the heating and cooling processes was calculated, exhibiting inconformity, and apparent activation energy is lower in the cooling process, which is related to its higher oxygen consumption, and showing stronger oxidisability in the cooling process. In addition, through the results of coal oxidation kinetics, it is found that coal temperature affected the magnitude of apparent activation energy, the oxygen concentration impacted the oxidation activity of coal. These research results are valuable for informing the process of fire extinguishment and guiding the fire area unsealing in CFZ.

Methanol to hydrogen conversion on cobalt–ceria catalysts prepared by magnetron sputtering

Methanol oxidation catalyzed by bi-layered or mixed oxides of cobalt and cerium was studied at atmospheric pressure by temperature-programmed reaction technique. The catalysts were deposited in the form of thin films by magnetron sputtering from Co and CeO2 targets, forming double layer structures of CeO2/CoOx or CoOx/CeO2. Reaction selectivity and hydrogen production rates were monitored at O2:CH3OH reactant stoichiometries varied in the range 4:1–1:4, spanning from oxygen rich to oxygen lean reaction conditions. A complementary information in regard to thermal stability of the catalytic layers was obtained by X-ray photoelectron spectroscopy. A strong synergistic interaction between cerium and cobalt within the mixed oxide (driving dynamics of the Co2+/Co3+ and Ce4+/Ce3+ redox pairs and oxygen exchange) leads to a bi-functional catalyst. Particularly the CeO2/CoOx configuration proved to be significantly more active than the CoOx alone and more stable, especially under reducing environments. Further optimization of reactivity can be provided via control of the adlayer coverage.

Characterization and evaluation of Fe–N–C electrocatalysts for oxygen reduction directly synthesized by reactive spray deposition technology

A novel method for synthesizing iron–nitrogen–carbon (Fe–N–C) electrocatalysts using a modified flame spray pyrolysis technique called reactive spray deposition technology (RSDT) is described. The physicochemical properties of the RSDT-synthesized catalyst are quantified through a series of experiments including nitrogen adsorption, electron microscopy, thermogravimetry, spectroscopy and electrochemical analysis with the overall aim of exploring opportunities to use RSDT as a single-step, scalable alternative to multi-step, energy-intensive furnace-based methods for synthesizing PGM-free electrocatalysts for oxygen reduction. The Fe–N–C is synthesized by pyrolyzing a liquid solution precursor mixture under oxygen lean conditions without the use of additional support material or heat treatment steps. Properties of critical importance to the performance of the Fe–N–C catalyst are discussed in detail with respect to properties of similar catalysts synthesized by multi-step methods reported in the literature. Materials characterization shows evidence of ORR active Fe–Nx sites, a high fraction of pyridinic nitrogen and carbon-encapsulated iron-rich particles. The existence of undesired amorphous carbon mixed with the catalytically active material is also observed and may require process development in future to remove. Rotating disk electrode analysis in alkaline media of the RSDT-synthesized Fe–N–C confirmed catalytic activity toward oxygen reduction, which is shown to follow a two-step reaction mechanism. While the activity of the Fe–N–C catalyst is lower than that of commercial Pt/C, it shows superior stability with a decrease in half-wave potential of only 5 mV after 4000 cycles in alkaline media, encouraging further investigation of this alternative flame-based synthesis route.

Intrinsic kinetic model for oxidative dehydrogenation of ethane over MoVTeNb mixed metal oxides: A mechanistic approach

An intrinsic kinetic model for the selective production of ethene via oxidative dehydrogenation of ethane over the M1 phase of MoVTeNb mixed metal oxides is presented. Formation of acetic acid, carbon monoxide and carbon dioxide has been incorporated using a holistic reaction mechanism. The proposed model is based on two different oxygen sites, namely, lattice oxygen causing carbon-hydrogen bond cleavage and electrophilic surface oxygen responsible for the formation of carbon-oxygen bonds. It is found that carbon dioxide exclusively originates from decarboxylation of acetate species, while ethene selectively reacts to CO. Consumption and formation of all species are well predicted by the proposed model. Sensitivity analyses demonstrate the strong impact of the initial carbon-hydrogen cleavage on the net ethene production rate. Moreover, regeneration of lattice oxygen sites is found to become rate-determining at oxygen-lean conditions.

Parametric analysis of torrefaction reactor operating under oxygen-lean conditions

A small-to medium-scale, mobile torrefaction system has the potential to improve the economics of biomass torrefaction and expand its deployment in decentralized, rural areas. In order to simplify the reactor design for deployment in these contexts, a torrefaction reactor prototype operating under oxygen-lean conditions was proposed and developed in our earlier study. The goal of this study is to carefully quantify some key performance metrics of the aforementioned oxygen-lean reactor design under more realistic conditions and compare these metrics with torrefaction under inert conditions. For each condition, we characterized the product yield, energy yield, and energy densification for different feedstock. By using mass closure and elemental analysis, we further calculated the composition in the solid and volatile components. We show some differences in the reactor's performance in comparison with existing literature data obtained under inert torrefaction conditions. In general, under an oxygen-lean environment and at similar temperature and residence time, slightly over-torrefied products with reduced solid mass and energy yield were obtained, which is consistent with results reported in prior studies. These sacrifices in the reactor performance should be weighed against the benefits of a simplified design that has greater potential in remote areas.

Formation and reduction of NO from the oxidation of NH3/CH4 with high concentration of H2O

This study investigated the homogeneous formation and reduction of NO from the oxidation of CH4/NH3 to understand the effect of the high concentrations of H2O on NO emissions. The experimental results about the effects of excess oxygen ratio, gas compositions including H2O, CO2, NH3 and recycled NO, on NO formation and reduction were presented. The mechanism about the effects of high concentration of H2O and combined effect of H2O/CO2 on NO formation and reduction has been discussed. The results indicates that NO formation was markedly enhanced at high concentrations of H2O, which were mainly attributed to the increased OH radicals with steam addition at oxygen-rich conditions. By contrast, the radicals were consumed by reduction radicals, such as CO, H, and CHi radicals, under oxygen-lean conditions. NO concentrations followed the sequence of H2O/Ar > H2O/CO2 > CO2/Ar under both the oxygen-rich and oxygen-lean conditions, which meant that the highest and lowest NO emissions were recorded in Ar/H2O and CO2/Ar respectively. The combined H2O/CO2 concentration accordingly exerted an effective impact on NO reduction. The results about the effect of NH3 concentration in fuel on NO emission showed that NO emission increased with increasing NH3 concentration. Furthermore, NO reburning results implied that with increasing NO concentration, the total NO reduction increased, whereas the reduction ratio decreased.

A decentralized biomass torrefaction reactor concept. Part I: Multi-scale analysis and initial experimental validation

A new, simplified biomass torrefaction reactor concept that operates under oxygen-lean conditions is proposed as a potential way to downscale torrefaction reactors for small- and medium-scale applications. To verify the feasibility of the concept, a multi-scale analysis was conducted to understand the design requirements, underlying chemistry, intra-particle effects, and overall reactor-scale heat transfer. We demonstrate that the heat transfer within the reactor and the appropriate reactor height is largely determined by gas-phase advection. Finally, by implementing a laboratory-scale reactor and operating it under diverse conditions, we show that such a design can indeed satisfy the requirements for torrefaction. This lays the basis for the second part of this two-part paper, where we develop a detailed mathematical model for this concept. In future studies, we will also systematically define and map the performance metrics and reaction conditions in order to understand the scaling laws for potential commercialization of this concept.

A decentralized biomass torrefaction reactor concept. Part II: Mathematical model and scaling law

In Part I of the study, we proposed a simplified biomass torrefaction moving bed reactor design capable of decentralized, small-scale, and mobile deployment operated under an oxygen-lean condition. We built and validated a laboratory-scale test reactor. In the present study, we develop a mathematical description of the reactor and show that it produces reasonable fit to our experimental data. Contrary to many existing biomass gasifier studies, we demonstrate that at the small test-reactor scale, heat loss mechanism through the side wall is significant and cannot be ignored in the modeling. We further demonstrated that at the small test-reactor scale, the rapid axial thermal conduction plays a role in the heat transfer within the moving bed. Furthermore, by interrogating the scaling behaviors of the reactor, we show that as we scale up our current laboratory-scale reactor, at the same torrefaction severity, the mass yield of the torrefied biomass is predicted to increase by 10–20%, due to the decrease in relative heat losses at a larger scale. This study, therefore, seeks to understand and quantify some of the limitations of testing a scaled-down reactor prototype. The understanding gained in this study can both inform scaling laws for at-scale reactor designs, as well as point out areas of future work in order to develop a higher-fidelity description.

The Reasons for Nonlinear Phenomena in Oxidation of Methane over Nickel

The catalytic oxidation of methane over nickel foil is studied. It is shown that, under the oxygen-lean conditions in the regime of a flow-type reactor, nonlinear phenomena can appear in the form of self-sustained oscillations of the reaction rate and the catalyst temperature. To determine the reasons for self-sustained oscillations, X-ray diffraction and mass spectrometry in the operandо mode were used. It was found that the appearance of oscillations in the methane oxidation is due to periodical oxidation–reduction of the surface layer of nickel foil; metallic nickel has a higher catalytic activity than NiO. The oscillations of the catalyst temperature are determined by the occurrence of exothermic and endothermic processes associated with the reduction of nickel oxide and methane oxidation on the surface of metallic nickel.

Co-oxidation of methane (CH4) and carbon disulfide (CS2)

Sulfur-containing species persist as important impurities in fossil fuels, affecting the combustion process of constituent hydrocarbons. This contribution reports the promotion effects of carbon disulfide (CS2) on oxidation of methane (CH4), with experiments conducted in a jet-stirred reactor (JSR), and explains the experimental findings from comprehensive kinetic modelling of CH4/CS2/O2 systems. This includes neat oxidation of CH4/O2 and CS2/O2 under stoichiometric conditions, as compared with co-oxidation of CH4/CS2/O2, containing different measures of CS2. Addition of small amounts of CS2 (50 ppm and 100 ppm) enhances the oxidation of CH4 (500 ppm) as characterised by a lower onset temperature (1300 K versus 1200 K and 1060 K). In contrast, the presence of CH4 delays the process of CS2 oxidation. Considering the similarity in the core charge of O and S, we propose the reactivity of S2, SO and in particular S towards CH4 to be responsible for the observed behaviours, in addition to an important effect of O radicals generated in oxidation of CS2 on engendering the oxidation of CH4. As reported in the literature, in analogy to O2 and O which oxidise CH4 into CO2, the S/S2/SO could also ‘sulfurdise’ CH4 into CS2 or COS under oxygen-lean conditions. Quantum chemistry calculations of the CH4 + O, CH4 + S, CH4 + S2, CH4 + SO and CH4 + O2 reactions further reveal the reactivity of O, O2, S, S2 and SO towards H abstraction from CH4. The sensitivity analysis of the kinetic modelling of the proposed co-oxidation reactions indicates that, the radicals formed during the CS2 conversion process promote the oxidation of CH4 at lower temperatures. However, the consumption of radicals in the CH4 oxidation also inhibits the decomposition of CS2 as observed in the experiments.

Manganese Oxide Thin Films on Au(111): Growth Competition between MnO and Mn3O4

The growth of manganese oxide on a Au(111) support has been examined over a wide range of preparation conditions and film thicknesses by means of scanning tunneling microscopy (STM), electron diffraction, and photoelectron spectroscopy. Two oxide polymorphs were found to coexist on the gold surface. Whereas MnO-type structures prevail at oxygen-lean preparation conditions, annealing in oxygen gives Hausmannite Mn3O4 as the dominant phase. Both polymorphs adopt square structures that only permit row-matched growth on the hexagonal gold, explaining the high tendency for oxide dewetting at elevated temperature. The manganese oxide films are thus polycrystalline and consist of a large number of submicrometer grains with different orientations and surface terminations. A variety of square and line patterns were identified on top of the oxide crystallites, all of them being compatible with either the primitive 5.8 × 5.8 A2 cell of Mn3O4(001) or an ordered MnO(100) vacancy structure. STM conductance spectroscopy...

Electrochemical and Spectroscopic Characterization of Non-Precious Metal Fe-N-C ORR Catalysts Synthesized By Direct Flame Spray Pyrolysis

The electrocatalyst is one of the major components of low temperature Proton Exchange Membrane (PEM) fuel cells, and despite being the focus of research and development for several decades, platinum nanoparticles supported on high surface area carbon remain the state-of-the-art for both acid and alkali based fuel cells. A major obstacle to the commercialization of PEM fuel cells is the cost associated with the use of expensive Pt group electrocatalysts on both the anode and cathode. DOE projections estimate that in high volume production, the cost of platinum alone could account for about 50% of the total cost of fuel cell power systems [1, 2]. There have been many attempts to alleviate this problem by reducing platinum loading on both electrodes. However, progress in reducing stack cost by reducing the platinum content has been offset by the increasing global price and limited availability of platinum. In addition, due to the sluggish oxygen reduction kinetics, the platinum loading on the cathode side when using pure hydrogen as a fuel source, is at least 3-5 times more than the loading on the anode side. Consequently, there is considerable interest in developing Pt-free Oxygen Reduction Reaction (ORR) catalysts that are cheaper to manufacture and more readily available than their Pt-based counterparts [3]. Nitrogen doped graphitic carbon is a promising candidate for Pt-free electrocatalysts. The incorporation of transition metals such as Fe or Co results in the formation of a metal-nitrogen coordination (M-Nx, where x is usually 2 or 4) which has been found to greatly enhance activity of non-precious metal catalysts towards oxygen reduction. M-N-C catalysts are typically synthesized through the pyrolysis of metal, nitrogen and carbon containing precursors in an inert or ammonia atmosphere between 800-1000°C followed by additional acid leaching and heat treatment steps [4-6]. The performance and durability of the catalyst under fuel cell operation conditions is dependent on the pore size distribution and active site density, both of which are significantly influenced by the synthesis process. Fe-N-C catalysts were synthesized for this study by a modified flame based deposition technique called Reactive Spray Deposition Technology (RSDT). The catalyst material was produced through the incomplete combustion of an atomized spray composed of anhydrous FeCl3 and cyanamide dissolved in a xylene-methanol-propane precursor mix. A nitrogen-gas sheath was also employed around the flame to reduce the entrainment of air into the flame. The oxygen lean conditions in the flame resulted in the formation of carbonaceous material due to the incomplete combustion of the organic precursors. XRD and Raman results indicated the presence of partially graphitic carbon with a disordered structure. The RSDT synthesis technique resulted in a morphology with mesoporous and macroporous features, as evidenced by SEM imaging and BET measurements (figures 1a and 1b). TGA analysis of the catalyst revealed the presence of amorphous carbon and soot mixed in with the graphitic material, suggesting further optimization of the synthesis process may be required. XPS analysis was performed to analyze the nature of the N-C bonding and detect the presence of the catalytically active Fe-Nx moieties (figure 1c). Electrochemical analysis of the catalyst will be conducted using Rotating Disk Electrode to measure the ORR activity and stability in acid medium. MEAs prepared from the Fe-N-C catalyst will be used to study the performance and durability under actual fuel cell operation conditions. Figure 1: (a) Secondary electron SEM image, and (b) BET analysis results of Fe-N-C catalyst synthesized by RSDT. (c) High resolution XPS scan of RSDT synthesized catalyst showing N1s peak along with table listing fraction of area under fitted peaks.

Phase Equilibrium of TiO2 Nanocrystals in Flame-Assisted Chemical Vapor Deposition.

Nano-scale titanium oxide (TiO2 ) is a material useful for a wide range of applications. In a previous study, we showed that TiO2 nanoparticles of both rutile and anatase crystal phases could be synthesized over the size range of 5 to 20 nm in flame-assisted chemical vapor deposition. Rutile was unexpectedly dominant in oxygen-lean synthesis conditions, whereas anatase is the preferred phase in oxygen-rich gases. The observation is in contrast to the 14 nm rutile-anatase crossover size derived from the existing crystal-phase equilibrium model. In the present work, we made additional measurements over a wider range of synthesis conditions; the results confirm the earlier observations. We propose an improved model for the surface energy that considers the role of oxygen desorption at high temperatures. The model successfully explains the observations made in the current and previous work. The current results provide a useful path to designing flame-assisted chemical vapor deposition of TiO2 nanocrystals with controllable crystal phases.

Insight of DFT and ab initio atomistic thermodynamics on the surface stability and morphology of In2O3

In2O3 catalysts show remarkable activity and selectivity in methanol synthesis from CO2 hydrogenation. In order to get insight into the surface stability of this catalyst, density functional theory and ab initio atomistic thermodynamics method were used to investigate the surface free energies of various facets as a function of oxygen chemical potential, as well as the influences of temperature, pressure and gas compositions. The results show that the (111) facet presents lowest surface free energy under oxygen-rich condition, while the indium-terminated (100) facet is the most stable one under oxygen-lean condition. Moreover, we applied Wulff construction to determine the equilibrium shape of In2O3 with different oxygen chemical potentials. The equilibrium shape under oxygen-lean condition is cubic, which only expose (100) facet, while, the equilibrium shape under oxygen-rich condition is octahedron, which only expose (111) facet. Meanwhile, the results agree well with what is observed experimentally. It is further predicted that Wulff shape of In2O3 exists in a truncated octahedron morphology in which the (100) surface becomes predominant plane under CO2 hydrogenation reaction conditions.

Simultaneous Reduction of Diesel Particulate and NOx Using a Catalysis-Combined Nonthermal Plasma Reactor

Particulate matter (PM) and NO x emitted from diesel engines is simultaneously reduced by a barrier-type catalyst-packed nonthermal plasma (NTP) application, driven by a pulse high-voltage power supply under oxygen-lean conditions. Catalyst particles of γAl2O3 and Ag/γAl2O3 with a diameter of 2–4 mm are used as packed pellets and carbon PM is loaded among the pellets. The reduction results are compared with those in a previous study using a noncatalytic BaTiO2 pellet packed bed reactor. NO x is reduced by N radicals, and PM is incinerated by oxygen radicals induced either by NO x or ozone (O3) reduction at elevated local temperatures among the pellets. Since CO and CO2 are generated, the carbon PM is actually combusted under oxygen-lean conditions, resulting in the simultaneous removal of PM and NO x with NTP-assisted catalysts. It is found that in the presence of the catalysts, PM removal calculated from CO and CO2 generation increases with an increase in the oxygen concentration. The maximum PM removal energy efficiency of 0.92 g(C) /kWh is observed for O2 = 2%. Furthermore, NO x removal decreases with an increase in the oxygen concentration. The maximum NO x removal energy efficiency of 14.2 g(NO2 ) /kWh is observed for O2 = 0%. Compared with the experimental results obtained without the catalyst pellets, the NO x removal energy efficiency increases by a maximum of 47% for O2 = 2%. When comparing the present results with those in the previous study using BaTiO3 pellets, NO x removal energy efficiency is larger, but the PM removal is lower in the case of catalyst packed-bed reactor.

Computational analysis of the early stage of cuprous oxide sulphidation: a top-down process

ABSTRACTThe initial steps of Cu2O sulphidation to Cu2S have been studied using plane-wave density functional theory at the PBE-D3+U level of sophistication. Surface adsorption and dissociation of H2S and H2O, as well as the replacement reaction of lattice oxygen with sulphur, have been investigated for the most stable (111) and (100) surface facets under oxygen-lean conditions. We find that the (100) surface is more susceptible to sulphidation than the (111) surface, promoting both H2S adsorption, dissociation and the continued oxygen–sulphur replacement. The results presented in this proceeding bridge previous results from high-vacuum experiments on ideal surface to more realistic corrosion conditions and set the grounds for future mechanistic studies. Potential implications on the long-term final disposal of spent nuclear fuel are discussed.This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems.

Changes in Thermal Kinetics Characteristics during Low-Temperature Oxidation of Low-Rank Coals under Lean-Oxygen Conditions

In the present work, we derive the heat-flow and mass-loss curves for the oxidation of three low-rank coals under Oxygen-Lean conditions using thermogravimetry and differential scanning calorimetry. Relationships between the exothermic character of the low-temperature oxidation (the stage before the volatile combustion of coal) and the oxygen concentration were analyzed. Kinetic parameters for low-temperature coal oxidation under oxygen-lean conditions were obtained, based on a derived Arrhenius equation. The results demonstrate that there is a critical oxygen concentration above which the full oxidative combustion reaction of low-rank coal will be reached under oxygen-lean conditions of continuous ventilation; if the oxygen concentration exceeds the critical value, the temperature at which coal oxidative combustion reaction is completed is pushed back as the oxygen concentration decreases. The amount of heat released during low-temperature oxidation decreases with decreasing oxygen concentration, but the...

Experimental investigation and numerical simulation of CO oxidation with HCl addition

Chlorine is rich in biomass and some coals and significantly released as HCl(g) during combustion process, and this promotes the recombination of free radicals OH, H, O, and HO2 and further influences CO oxidation. In this paper, the inhibition effect of HCl on CO oxidation is investigated by experiment and numerical simulation. The experiment is operated under different reaction zone temperatures and equivalence ratios in an entrained flow reactor. And the numerical simulation is conducted by CFD software coupled with reduced reaction mechanism. The results indicate that HCl addition obviously inhibits CO oxidation under oxygen-rich condition, and the inhibition enhances as HCl concentration rises, but weakens as temperature increases. Under chemical equivalent and oxygen-lean conditions, the inhibition effect is limited.

Selective and Stable Ethylbenzene Dehydrogenation to Styrene over Nanodiamonds under Oxygen-lean Conditions.

For the first time, significant improvement of the catalytic performance of nanodiamonds was achieved for the dehydrogenation of ethylbenzene to styrene under oxygen-lean conditions. We demonstrated that the combination of direct dehydrogenation and oxidative dehydrogenation indeed occurred on the nanodiamond surface throughout the reaction system. It was found that the active sp(2)-sp(3) hybridized nanostructure was well maintained after the long-term test and the active ketonic carbonyl groups could be generated in situ. A high reactivity with 40% ethylbenzene conversion and 92% styrene selectivity was obtained over the nanodiamond catalyst under oxygen-lean conditions even after a 240 h test, demonstrating the potential of this procedure for application as a promising industrial process for the ethylbenzene dehydrogenation to styrene without steam protection.