Thermally stable Pt/Ti mesh catalyst for catalytic hydrogen combustion

In this study, platinum (Pt) supported on titanium (Ti) mesh catalysts for catalytic hydrogen combustion were prepared by depositing Pt as a thin-layer on metallic or calcined Ti mesh. The Pt thin-layer could be stabilized as uniformly distributed, near nano-sized particles on the surface of calcined Ti mesh by exposing the freshly sputtered Pt to hydrogen. Temperatures between 478 and 525 °C were reached during hydrogen combustion and could be maintained at a hydrogen flow rate of 0.4 normal liter (Nl)/min for several hrs. It was determined that Ti mesh calcination at ≥900 °C formed an oxide layer on the surface of Ti wires, which prevented significant Pt aggregation. X-ray photoelectron spectroscopy revealed that the surface of Ti mesh was fully converted to TiO₂ at ≥900 °C. Raman spectroscopy showed that the majority of TiO₂ was present in the rutile phase, with some minor contribution from anatase-TiO₂. The calcined Ti support was stable through all investigations and did not indicate any signs of degradation.     

Numerical investigation of premixed hydrogen/air combustion at lean to ultra-lean conditions and catalytic approach to enhance stability

Premixed combustion of hydrogen/air over a platinum (Pt) catalyst is numerically investigated in a planar channel burner with the aim of stabilising the flame at lean to ultra-lean conditions. A steady laminar species transport model is examined in conjunction with elementary heterogeneous and homogeneous chemical reaction schemes and validated against experimental results. A stability map is obtained in a non-catalytic burner for the equivalence ratios (φ) of 0.15–0.20, which serves as the basis for the catalytic flame analysis. Over the Reynolds numbers (Re) investigated in the non-catalytic burner, no flame is observed for φ ≤ 0.16, and flame extinction occurs at Re < 571 and Re < 381 for φ = 0.18 and 0.20, respectively. Moreover, a significant amount of unburned H₂ exits the burner in all cases. With the Pt catalyst coated on the walls, complete H₂ combustion is attained for 0.10 ≤ φ ≤ 0.20 where the contribution of gas phase (homogeneous) reaction increases with Re. Furthermore, radiation on the wall and at the inlet affects the combustion kinetics and flame temperature. Finally, NO_x emission is investigated under the same conditions and found to increase with equivalence ratio but has a negligible effect with the inflow Reynolds number.     

Development of a Pt/stainless steel mesh catalyst and its application in catalytic hydrogen combustion

In this paper, different methods to prepare a Pt/stainless steel mesh catalyst for catalytic hydrogen combustion are reported. Pt was deposited as a thin layer onto stainless steel mesh as a support matrix. Thermal treatment resulted in the formation of uniformly distributed near nano-sized Pt particles on the support's surface. The catalysts were evaluated for catalytic hydrogen combustion, and temperatures of between 420 and 520 °C were achieved and maintained at hydrogen flow rates of 0.2–0.4 Nl/min. SEM and EDX results indicate that stainless steel support calcination promoted the formation of a native oxide layer. This oxide layer stabilized Pt particles during hydrogen combustion procedures, effectively preventing significant Pt aggregation from occurring. It was also found that the Ni content of stainless steel promoted the catalytic hydrogen combustion reaction.     

Thermogravimetric kinetics on catalytic combustion of bituminous coal

The catalytic combustion and non-isothermal kinetics of bituminous coal by CeO₂, Fe₂O₃, and NiO were investigated. The exothermic characteristics during catalytic combustion of bituminous coal were determined. Based on the Coats-Redfern method by introducing the function of kinetics mechanisms, the activation energies and pre-exponential factors of catalytic combustion of bituminous coal were estimated iteratively by regression. It is found that the catalysts promoted the transport of oxygen to the coal or char surface and effectively improved the combustion characteristics of bituminous coal. Under the same experimental conditions, the exothermic values were significantly increased and the catalysts of composite oxides exhibited higher exothermic values than pure metal oxide catalysts. The metal oxides significantly reduced the activation energies of bituminous coal combustion. SEM analysis presented that combustion residues became more porous with the addition of the catalyst.     

Simulation on thermoelectric device with hydrogen catalytic combustion

The micro-thermoelectric-generator based on catalytic combustion of hydrogen and oxygen was designed. With the application of general finite reaction rate model in CFD software of FLUENT, the effect of inlet parameters on the highest temperature difference between the hot and cold plate of the generator was studied. Results showed that, the temperature in the heating and cooling channel of the micro-thermoelectric-generator was uniform; with the increasing of inlet reactant temperature, the highest temperature difference increased, but the total efficiency of the generator decreased. Results can be used to the further design and optimization of micro-thermoelectric-generator based on hydrogen catalytic combustion.     

Experimental investigation on the kinetics of catalytic recombination of hydrogen with oxygen in air

Catalytic recombination of hydrogen with oxygen is one of the most attractive options to control the hydrogen concentration in air. The basic pre-requisite for the process design of any catalytic reactor is the knowledge of kinetic data. In the present study, the kinetic data for the catalytic recombination of hydrogen in presence of 0.5% Pd on alumina catalyst were generated using a packed bed reactor with complete recycle. The experiments were conducted using low concentration of hydrogen in air at different temperatures and the apparent rate constants were estimated assuming a first order reaction with respect to hydrogen. The resistances due to internal and external mass transfer were decoupled from the apparent kinetics and estimated separately. The activation energy and frequency factor were found out using the slope and intercept of the Arrhenius plot. The effect of different process parameters such as temperature, superficial velocity and the catalyst particle size on the overall reaction rate was also studied. The knowledge of the intrinsic kinetics along with the mass transfer can be easily extended for the design of catalytic recombination reactors during scale up.     

Pt modulates the electronic structure of Pd to improve the performance of Pd-based catalytic combustion catalyst

The electronic modulation between the catalytic active components can improve the catalytic activity and stability of the catalyst. The Pd-based catalysts can easily react with SO_X to form stable and inactive sulfates. In this paper, the Pd–Pt-based catalytic combustion catalyst was prepared by replacing part of Pd with a small amount of Pt. The storage tank VOCs catalytic combustion activity and the anti-SO₂ poisoning performance of the Pd–Pt-based catalyst and Pd-based catalyst were tested. The Pd 3d binding energy of each Pd-based catalyst was detected by XPS characterization, and the electronic structure changes of Pd active components was analyzed by the change of Pd 3d binding energy. The effect of electrons transfer between Pd and Pt on the improvement of catalytic combustion activity and SO₂ poisoning resistance of Pd-based catalysts was analyzed. The results show that the Pt addition can increase the electron cloud density of the Pd active components, and improve the performance of the Pd active components to adsorb and activate oxygen. The reaction of Pd and SO_X to form sulfate needs to gain electrons. The increase in the electron cloud density of the Pd active components in Pd–Pt-based catalyst makes it difficult for the Pd active components to adsorb SO_X and difficult to react with SO_X to form sulfate, thereby preventing the Pd active components from being poisoned and deactivated.     

Development of a powerful miniature hydrogen catalytic combustion powered thermoelectric generator

Rapid development of portable electronics promotes the R&D of micro/miniature power sources with high energy density. The high mass energy density and zero emission characteristic of hydrogen show a huge potential to develop powerful portable hydrogen-based power sources. A miniature hydrogen catalytic combustion powered thermoelectric generator (CCP-TEG) is designed and tested in detail. An outstanding catalytic core is prepared with a newly proposed method on the basis of combining H₂PtCl₆ solution and foamed transition metal. Such catalytic core is demonstrated to provide high combustion temperature, complete combustion, and sufficient heat flux for power generation. Several parameters including input power, equivalent ratio, cooling mode, and load resistance are investigated to clarify their influences on the combustion temperature, electric power, and various efficiencies (combustion, heat collection, TE, and overall efficiencies) of the hydrogen CCP-TEG. The developed hydrogen CCP-TEG is able to generate an electric power of 20.7 W with an overall efficiency of 2.04%, filling the research gap of generating large electric power (>10 W) with sufficiently high overall efficiency (>2%) in the research field of hydrogen CCP-TEG. The generated electric power and overall efficiency are much higher than those in previous hydrogen CCP-TEGs. The prepared catalytic core remains excellent functionality after running for 30 h, and the combustion temperature is as high as 918 K, which ensures the sufficiently high temperature difference for powerful power generation. This study is conducted to illustrate a concrete method on developing a powerful hydrogen CCP-TEG, and to identify further research directions.     

A study on the catalytic hydrogenation of N-ethylcarbazole on the mesoporous Pd/MoO3 catalyst

Mesoporous MoO₃ shows an apparent activity in the catalytic hydrogenation of N-ethylcarbazole (NEC), where a significant amount of tetrahydro-N-ethylcarbazole (4H-NEC) and perhydro-N-ethylcarbazole (PNEC) are detected with the hydrogen uptake of 0.97 wt% after 6 h when the temperature rises to 220 °C. 0.5 wt% Pd/MoO₃ catalyst shows a superior catalytic efficiency than the traditional precious metal catalysts 0.5 wt% Ru/Al₂O₃ and 0.5 wt% Pd/Al₂O₃, especially in the conversion of Octahydro-N-ethylcarbazole (8H-NEC) to PNEC. The hydrogenation mechanism of MoO₃ is completely different from the traditional precious metal catalysts. With the presence of a small amount of Pd, the breaking of HH bond is greatly accelerated, result in the promotion of hydrogen spillover rate and the increase of the concentration of hydrogen molybdenum bronze H_xMoO₃, which improves the catalytic efficiency of the MoO₃ catalyst. Rise the temperature also helps increasing the concentration of H in H_xMoO₃.     

Optimization of a counter-flow microchannel reactor using hydrogen assisted catalytic combustion for steam reforming of methane

The objective of this study is to optimize a microchannel reactor using hydrogen assisted catalytic combustion for steam reforming of methane. Hydrogen assisted catalytic combustion does not require preheating because the catalytic combustion of hydrogen occurs at room temperature. After start-up by hydrogen catalytic combustion, fuels of hydrogen and methane were changed to methane. The geometric configuration of the counter-flow reactor was optimized by the simulation model under steady state condition. The hydrogen flow rate in the counter-flow reactor was also optimized by transient simulations using the response surface methodology. As a result, the counter-flow reactor showed extremely short start-up time because of the optimized configuration and the optimized hydrogen flow rate. Hot spots were avoided because of the hydrogen shut-off after start-up. The operating characteristics of the counter-flow reactor were compared with those of the co-flow reactor.     

Ignition propensity of hydrogen/air mixtures impinging on a platinum stagnation surface

An experimental investigation into the ignition characteristics of lean pre-mixed hydrogen/air mixtures is conducted using a stagnation-point flow configuration against a platinum surface, with a special emphasis on the determination of potential fire safety hazards associated with hydrogen release in the presence of a catalyst. Two distinct regimes are observed for this system – catalytic surface reactions and gas-phase ignition. It is demonstrated that depending on mixture equivalence ratio, catalytic surface reactions can be initiated with or without surface heating. When significant surface heat is released via catalytic reactions, gas-phase ignition can be induced, greatly increasing the apparent danger of hydrogen leaks in the presence of a platinum surface. The critical surface temperatures leading to catalytic ignition for hydrogen/air mixtures over a platinum surface are further investigated over a range of equivalence ratios and stretch rates. It is shown that ultra-lean hydrogen/air mixtures can be catalytically ignited even in the absence of external heat addition, suggesting that hydrogen leakage in the presence of a platinum surface may pose a fire safety risk even at room temperature. Furthermore, even without a transition to gas-phase ignition, the surface temperature that can be sustained with surface reactions alone may contribute to component degradation or itself pose a safety hazard.     

Experimental parametric investigation of platinum catalysts using hydrogen fuel

The aviation organization is creating awareness for the overall reduction of NOx emissions by up to 80% in the near future. This motivates to conduct research on the current state of art, catalytic stabilized combustion chamber using hydrogen. This was achieved by performing an experimental parametric investigation of Platinum catalysts in two phases. Firstly, the design of three diverse configurations of mixers and was investigated experimentally and numerically. The chosen mixer was implemented in the parametric study of five different Pt catalysts varying in geometric and material properties. This was executed at unpressurized and NOx emission solely due to the catalytic reaction was examined for varying thermal power and air/fuel ratios. Furthermore, temperatures were recorded. Additionally, CFD simulation was accomplished and compared with the measurement data. The overall least NOx achieved was 7.5  ppm at 5 kW for the metal catalyst. The result of this work proposed suitable catalyst for the development of a combined combustor configuration (including catalyst and combustion chamber) which will be intended for small aircraft engine applications.     

Effects of hydrogen addition on methane catalytic combustion in a microtube

The objective of this paper is to study hydrogen-assisted catalytic combustion of hydrocarbon on a microscale experimentally. In the experiment, neither methane nor ethane can be ignited by itself, but hydrogen can be ignited and burn steadily in this tube. It is found that there is no significant difference between hydrogen added to the hydrocarbon and hydrogen alone as fuel without the platinum thermocouple, but the temperature will increase and the efficiency of methane combustion will increase considerably when the platinum thermocouple was put into the microtube. Methane can burn steadily without adding hydrogen after ignited by hydrogen. It can be concluded that the addition of hydrogen to hydrocarbon is favorable to ignition and the platinum thermocouple catalyzes the hydrocarbon combustion. The experiment result showed that the added hydrogen acts as an assistant for ignition and expands the range for methane steady burn. After igniting, methane can burn steadily alone at catalytic condition. This is useful for optimization microcombustion fuel.     

Consideration of boundary conditions of porous catalyst in numerical simulation with elementary reaction scheme of catalytic combustion

In many actual applications of catalytic combustion, catalysts are coated on the channel and have a porous structure. Although numerical simulation for the optimal design of a reactor requires boundary conditions, it is not easy to obtain the concentration of radical species in the experiment; in addition, the measurement within a porous structure is even more difficult. This research considers the requirements imposed on the boundary conditions of a porous catalyst in which diffusion and reaction occur, particularly in the case with an elementary reaction scheme of hydrogen oxidation on platinum. When the concentration fluxes are given at the interface between the gas stream and the catalyst layer, reaction formulae restrict the number and type of boundary conditions. On the other hand, when the concentration is set as the boundary condition, slight differences in concentration may significantly affect the overall reaction rate. On the basis of the above findings, the features of several models were summarized.     

Experimental studies on syngas catalytic combustion on Pt/Al2O3 in a microreactor

Synthesis gas (a mixture of CO and H₂) oxidation is studied over a supported Pt/Al₂O₃ catalyst in a novel microreactor fabricated for studying the intrinsic chemical kinetics of highly exothermic reactions. CO was found to significantly inhibit H₂ oxidation. In contrast, H₂ addition promotes CO oxidation at low mole fractions but has a small promoting effect at high hydrogen mole fractions. As a result, the apparent reaction order of H₂ changes from positive to zero. The change in hydrogen reaction order is associated with hysteresis. Possible mechanisms for the observed behavior are discussed.     

Effects of platinum stagnation surface on the lean extinction limits of premixed methane/air flames at moderate surface temperatures

A stagnation flow reactor was used to study the effects of platinum on the lean flammability limits of atmospheric pressure premixed methane/air flames at moderate stagnation surface temperatures. Experimental and computational methods were used to quantify the equivalence ratio at the lean extinction limit (?_ext) and the corresponding stagnation surface temperature (T_s). A range of flow rates (57–90 cm/s) and corresponding strain rates were considered. The results indicate that the gas-phase methane/air flames are sufficiently strong relative to the heterogeneous chemistry for T_s conditions less than 750 K that the platinum does not affect ?_ext. The computational results are in good agreement with the experimentally observed trends and further indicate that higher reactant flow rates (>139 cm/s) and levels of dilution (>∼10% N₂) are required to weaken the gas-phase flame sufficiently for surface reaction to play a positive role on extending the lean flammability limits.     

Investigation of influence of detailed chemical kinetics mechanisms for hydrogen on supersonic combustion using large eddy simulation

Five detailed hydrogen combustion chemical kinetics mechanisms coupled with a partially stirred reactor (PaSR) combustion model were applied with large eddy simulation (LES) to study the influence of detailed mechanisms on supersonic combustion in a model scramjet combustor. The LES predictions of five detailed mechanisms for velocity, temperature, and combustor wall pressure show reasonable agreement with experimental results. Examining the effects on the distributions of temperature and species in supersonic combustion reveals that the supersonic flame structure is affected by detailed mechanisms. The different detailed mechanisms have a strong influence on the combustion efficiency, volume of the subsonic region, and subsonic combustion heat release rate in the combustor. Moreover, the total heat release in the computational domain for the five detailed chemical kinetics mechanisms is quite different. The subsonic combustion is dominant in the combustor for all detailed mechanisms. An analysis of the important reactions for H₂O, HO2, and OH is performed, revealing the reasons for differences in temperature and species distributions among the different detailed mechanisms in the combustor.     

Influence of steel industrial wastes on burnout rate and NOx release during the pulverized coal catalytic combustion

In this work, 3 kinds of coal, 5 kinds of wastes from steel industry were chosen, orthogonal combustion experiments were carried out in a lab-scale drop tube furnace. Burnout rate and NO_x release were selected as evaluation indexes to evaluate the catalytic combustion and denitration effect. The catalytic combustion performance was discussed in term of the X-ray diffraction (XRD) and Scanning electron microscopy (SEM) of combustion residues. The results showed that NO_x content and burnout rate of different coal varied greatly; some additives could change the apparent form of combustion residues, which indicated that the additives played good roles in combustion process. This paper will not only achieve purpose of improving coal burning efficiency and energy saving, but also turning wastes into valuables and protect the environments.     

Gas phase chemistry in catalytic combustion of methane/air mixtures over platinum at pressures of 1 to 16 bar

The gas-phase combustion of fuel-lean methane/air premixtures over platinum was investigated experimentally and numerically in a laminar channel-flow catalytic reactor at pressures 1 bar?p?16 bar. In situ, spatially resolved one-dimensional Raman and planar laser induced fluorescence (LIF) measurements over the catalyst boundary layer were used to assess the concentrations of major species and of the OH radical, respectively. Comparisons between measured and predicted homogeneous (gaseous) ignition distances have led to the assessment of the validity of various elementary gas-phase reaction mechanisms. At low temperatures (900 K?T?1400 K) and fuel-to-air equivalence ratios (0.05?φ?0.50) typical to catalytic combustion systems, there were substantial differences in the performance of the gaseous reaction mechanisms originating from the relative contribution of the low- and the high-temperature oxidation routes of methane. Sensitivity analysis has identified the significance of the chain-branching reaction CHO + M = CO + H + M on homogeneous ignition, particularly at lower pressures. It was additionally shown that C2 chemistry could not be neglected even at the very fuel-lean conditions pertinent to catalytic combustion systems. A gas-phase reaction mechanism validated at 6 bar?p?16 bar has been extended to 1 bar?p?16 bar, thus encompassing all catalytic combustion applications. A reduced gas-phase mechanism was further derived, which when used in conjunction with a reduced heterogeneous (catalytic) scheme reproduced the key catalytic and gaseous combustion characteristics of the full hetero/homogeneous reaction schemes.     

Graphene-anchored Ni6MnO8 nanoparticles with steady catalytic action to accelerate the hydrogen storage kinetics of MgH2

Transition metal-based oxides have been proven to have a substantial catalytic influence on boosting the hydrogen sorption performance of MgH₂. Herein, the catalytic action of Ni₆MnO₈@rGO nanocomposite in accelerating the hydrogen sorption properties of MgH₂ was investigated. The MgH₂ + 5 wt% Ni₆MnO₈@rGO composites began delivering H₂ at 218 °C, with about 2.7 wt%, 5.4 wt%, and 6.6 wt% H₂ released within 10 min at 265 °C, 275 °C, and 300 °C, respectively. For isothermal hydrogenation at 75 °C and 100 °C, the dehydrogenated MgH₂ + 5 wt% Ni₆MnO₈@rGO sample could absorb 1.0 wt% and 3.3 wt% H₂ in 30 min, respectively. Moreover, as compared to addition-free MgH₂, the de/rehydrogenation activation energies for doped MgH₂ composites were lowered to 115 ± 11 kJ/mol and 38 ± 7 kJ/mol, and remarkable cyclic stability was reported after 20 cycles. Microstructure analysis revealed that the in-situ formed Mg₂Ni/Mg₂NiH₄, Mn, MnO₂, and reduced graphene oxide synergically enhanced the hydrogen de/absorption properties of the Mg/MgH₂ system.