Stagnation Flow Reactor Research Articles

Dimethyl ether low-temperature catalytic oxidation over Rh/Al2O3 in a stagnation-flow reactor

Dimethyl ether (DME) is a promising fuel for use in low-temperature portable hydrogen production, domestic applications, or diesel engines. It burns with less emissions than conventional fuels and has properties similar to LPG in terms of storage and transport, rendering it effective in many strategies for combating climate change. In this study we investigated the partial and total oxidation of DME over 5 wt% Rh/Al2O3 at low temperatures (215 to 320 °C), relevant to portable and domestic energy applications as well as the after-treatment systems of DME-powered engines. We captured the effects of temperature, flow rate, and inlet feed composition on the reactivity. For partial oxidation, we utilized the stagnation-flow reactor geometry to isolate the oxidation zone from the reforming zone. We discuss the reaction order with respect to DME and O2 and provide activation energy values under kinetics control. We also provide data where internal and external mass transfer limitations are present to examine the diffusive-convective transport near the catalyst surface, not easily done in three-dimensional environments such as packed beds. The experimental data we provide here pave the way for accurate kinetic modeling of DME partial and total oxidation on Rh/Al2O3, for reactor design and optimization as well as rational catalyst design.

Low-temperature CO oxidation over Rh/Al2O3 in a stagnation-flow reactor

This study provides thorough, novel experimental data for low-temperature CO oxidation on Rh/Al2O3 in a stagnation-flow reactor.

Reaction Kinetics of Catalytic Dry Reforming of Methane on Spinel-Derived Nickel Catalyst at Low-Temperature Using an in-Situ Stagnation-Flow Reactor

Dry reforming of methane (DRM) has been implemented to hydrocarbon-fueled ceramic fuel cells. This reduces their carbon footprint by utilizing carbon dioxide as a reactant. However, its application to low-temperature ceramic fuel cells has not been investigated, in which sluggish reforming kinetics and low durability induced by carbon coking should be resolved and overcome. In this study, DRM at low-temperature is examined by using spinel-derived nickel catalyst on aluminum oxide employed in a stagnation-flow reactor. With increasing calcination temperature, homogeneous mixture of nickel oxide and aluminum oxide show phase transition from mechanically mixed Ni/Al2O3 to chemically packed crystallite Ni/Al2O4 spinel structure. Due to their chemical structure, they have two different size of active nickel particle on aluminum oxide under reducing environment and show different catalytic activity. To elucidate the fundamental surface properties of fabricated catalysts, surface characterization technique such as XRD, TEM, BET, TPR, and CO-chemisorption are used. After surface characterization, their performance for DRM and reaction mechanism are examined by powder setup with gas chromatography and in-situ stagnation-flow reactor with mass spectrometer. The stagnation-flow reactor enables micro quartz-probe sampling of the compositional boundary layer in the vicinity of the catalyst surface. The vertical distance of micro quartz-probe from the catalytic surface is manipulated by a step-motor, and sampled gas are quantitatively analyzed with a quadrupole mass spectrometer. This configuration also makes it relatively simple to resolve numerically the complex reacting flow environment coupled with mass/momentum/energy transport phenomena. A numerical model can examine quantitatively the net production rate of each elementary step (48-step surface reaction mechanism) with varying operating conditions. Intrinsic reaction parameters such as a sticking coefficient of reactant adsorption, pre-exponential factor and activation energy of each reaction step are tuned by using experimental measurements. To determine the main reaction pathways and rate-determining steps, partial equilibrium, activation energy of C-H bond in methane and C-O bond in carbon dioxide, a sensitivity analysis with respect to reaction parameters is conducted. It is elucidated that the dry reforming of methane is pre-dominantly rate-determined by methane dehydrogenation steps. Especially, at low temperature region, an apparent activation energy of methane conversion increases sharply with the temperature, whereas that of carbon dioxide shows a trivial increase. The simulation results also show the dependence of thermal/fluidic effects on reaction kinetics including the catalyst temperature, reactant flow rates, mass/thermal diffusion in a porous catalyst layer. In this study, the detailed elementary reaction steps are developed, which enables predicting the effect of the reacting-flow environment on the overall reaction kinetics of dry reforming of methane.

Investigation of Zr, Gd/Zr, and Pr/Zr – doped ceria for the redox splitting of water

There is a renewed interest in CeO2 for use in solar-driven, two-step thermochemical cycles for water splitting. However, despite fast reduction/oxidation kinetics and high thermal stability of ceria, the cycle capacity of CeO2 is low due to thermodynamic limitations. In an effort to increase cycle capacity and reduce thermal reduction temperature, we have studied binary zirconium-substituted ceria (ZrxCe1-xO2, x = 0.1, 0.15, 0.25) and ternary praseodymium/gadolinium-doped Zr-ceria (M0.1Zr0.25Ce0.65O2, M = Pr, Gd). We evaluate the oxygen cycle capacity and water splitting performance of crystallographically and morphologically stable powders that are thermally reduced by laser irradiation in a stagnation flow reactor. The addition of zirconium dopant into the ceria lattice improves O2 cycle capacity and H2 production by approximately 30% and 11%, respectively. This improvement is independent of the Zr dopant level, up to 25%, suggesting that above 10% Zr dopant level, Zr might be displaced during the high temperature annealing process. The addition of Pr and Gd to the binary Zr-ceria mixed oxide, on the other hand, is detrimental to H2 production. A kinetic analysis is performed using a model-based analytical approach to account for effects of mixing and dispersion, and to identify the rate controlling mechanism of the water splitting process. We find that the water splitting reaction at 1000 °C and with 30 vol% H2O, for all doped ceria samples, is surface limited and best described by a deceleratory power law model (F-model), similar to undoped CeO2. Additionally, we used density functional theory (DFT) calculations to examine the role of Zr, Pr, and Gd. We find that the addition of Pr and Gd induce non-redox active sites and, therefore, are detrimental to H2 production, in agreement with experimental work. The calculated surface H2 formation step was found to be rate limiting, having activation barriers greater than bulk O diffusion, for all materials. This agrees with and further explains experimental findings.

Combining Planar Laser-Induced Fluorescence with Stagnation Point Flows for Small Single-Crystal Model Catalysts: CO Oxidation on a Pd(100)

A stagnation flow reactor has been designed and characterized for both experimental and modeling studies of single-crystal model catalysts in heterogeneous catalysis. Using CO oxidation over a Pd(100) single crystal as a showcase, we have employed planar laser-induced fluorescence (PLIF) to visualize the CO2 distribution over the catalyst under reaction conditions and subsequently used the 2D spatially resolved gas phase data to characterize the stagnation flow reactor. From a comparison of the experimental data and the stagnation flow model, it was found that characteristic stagnation flow can be achieved with the reactor. Furthermore, the combined stagnation flow/PLIF/modeling approach makes it possible to estimate the turnover frequency (TOF) of the catalytic surface from the measured CO2 concentration profiles above the surface and to predict the CO2, CO and O2 concentrations at the surface under reaction conditions.

Continuous on-sun solar thermochemical hydrogen production via an isothermal redox cycle

Solar thermochemical hydrogen production from water is a path towards a carbon-free sustainable hydrogen economy. Here, isothermal on-sun hydrogen production is demonstrated using active iron aluminate (hercynite) particles contained in dual fluidized bed reactors. The two fluidized beds were held in a single cavity solar receiver that was heated with a 10 kW high flux solar furnace. During 8 h of on-sun testing, 5.3 L of H2 were generated with an average productivity of 597 µmol H2/g using an intermittent process with optimized redox cycle times. Redox cycling was performed isothermally and continuously with equivalent oxidation and reduction times producing 547 µmol H2/g over two cycles. These results show excellent agreement with the H2 production measured in an 800X scaled-down electrically heated laboratory stagnation flow reactor and surpass on-sun H2 production of current benchmark materials. The effect of environmental variability on the incident solar radiation and hydrogen production is evaluated during on-sun testing. Finally, a discussion of the important factors for scalability of the process to commercial applications is provided, highlighting the importance of robust containment and active materials. This work links current materials research and development with commercial implementation in an on-sun process and demonstrates the viability of solar thermochemical hydrogen production that leverages continuous isothermal redox cycling.

CaRMeN: An Improved Computer-Aided Method for Developing Catalytic Reaction Mechanisms

The software tool CaRMeN (Catalytic Reaction Mechanism Network) was exemplarily used to analyze several surface reaction mechanisms for the combustion of H2, CO, and CH4 over Rh. This tool provides a way to archive and combine experimental and modeling information as well as computer simulations from a wide variety of sources. The tool facilitates rapid analysis of experiments, chemical models, and computer codes for reactor simulations, helping to support the development of chemical kinetic models and the analysis of experimental data. In a comparative study, experimental data from different reactor configurations (channel, annular, and stagnation flow reactors) were modeled and numerically simulated using four different catalytic reaction mechanisms from the literature. It is shown that the software greatly enhanced productivity.

Kinetics and mechanism of solar-thermochemical H2 and CO production by oxidation of reduced CeO2

The combination of high thermal stability and fast reduction/oxidation kinetics has drawn attention to CeO2 for use in solar-driven, two-step thermochemical cycles for water and/or carbon dioxide splitting. In accordance with this renewed interest in CeO2, there is a need to better understand the gas splitting chemistry on the reduced oxide. In this study, we measured the H2 production rates over ceria powder, thermally reduced by laser irradiation, during oxidation by H2O and CO2 gases in a stagnation flow reactor. The reaction kinetics was extracted using a model-based analytical approach to account for the effects of mixing and dispersion in the reactor. We find the rigor of this approach necessary in order to identify the rate controlling mechanism and assign parameters to the kinetic model, and to successfully model the production of H2 and CO production over the entire envelope of the fuel curve. The water splitting reaction, in the range of 750–950 °C and 20–40 vol% H2O, can best be described by a first-order kinetic model with low apparent activation energy (29 kJ/mol); while the carbon dioxide splitting reaction, in the range of 650–875 °C and 10–40 vol% CO2, is a more complex surface-mediated phenomena.

Modeling pore processes for particle-resolved CFD simulations of catalytic fixed-bed reactors

Abstract The most rigorous description of catalytic fixed-bed reactors is particle-resolved CFD simulations. Pore processes can have a large effect toward reactor performance, since they limit internal mass transport. In this study, three pore models of different complexity are incorporated into the CFD software STAR-CCM+. Instantaneous diffusion, effectiveness-factor approach, and three-dimensional reaction–diffusion model are validated firstly with experimental data of CO oxidation in a stagnation-flow reactor from Karadeniz et al. (2013) . The 3D approach predicts the experiments with high accuracy over the entire temperature range followed by the computationally cheaper effectiveness-factor. In a second study, the effect of the three pore models is evaluated on a catalytic single sphere with Reynolds numbers varying between 10 and 2000. The local interplay between kinetics and transport phenomena are quantified. For all investigated cases internal mass transfer limitations are larger than external ones. This study can be applied for entire packed-bed simulations.

Surface Reaction Kinetics of the Oxidation and Reforming of Propane over Rh/Al2O3 Catalysts

AbstractA multi‐step surface reaction mechanism for partial oxidation and steam reforming of propane over Rh/Al2O3 catalysts is presented. The mechanism is also applicable to model reactions of the subsystems H2/CO/H2O/CO2/O2/CH4. A stagnation–flow reactor with a catalytically coated disk is used to determine the surface reaction rate and spatial concentration profiles on top of the catalytic plate using a micro‐probe sampling technique. The reactor configuration facilitates one‐dimensional modeling of coupled diffusive and convective transport within the gas‐phase boundary layer coupled with detailed heterogeneous chemistry models of the zero‐dimensional surface. The reaction system is studied at varying inlet concentrations and temperatures. The established reaction kinetics are furthermore tested by simulation of autothermal reforming of propane in an annular reactor previously described by Pagani.

Design and characterization of a stagnation flow reactor for heterogeneous microkinetic studies

By modelling of mass, momentum and thermal energy transport we critically investigated the design of a flexible testing cell. Its purpose is carrying out reactivity tests at high pressure (⩾1bar) on flat, disk-shaped active surfaces, under continuous gas flow. We revisited the stagnation-point flow arrangement to quantitatively evaluate the potential and the critical factors affecting the experimental information achievable. We investigated the constructive and the operating parameters. Specifically, geometry, contact time, temperature, rate of surface reaction, reactant diffusivity, carrier thermal conductivity, and heat of reaction were parametrically varied. Simulations confirm that reactions can become limited by reactant diffusion at high temperature (i.e. fast reactions), in a perfectly isothermal cell. However, the heat of reactions can easily distort the surface temperature distribution, as evidenced by 2D simulations; thus, the flow structure in the testing cell and the carrier heat conduction become the controlling factors, determining the reaction extent. Finally, a prototype was built and operated using H2 oxidation on a polycrystalline Pt disk, as probe catalyst and reaction. Measurements confirm the consistency of the simulation study, revealing the critical role played by the unexpected local temperature distribution on the active surface, largely prevailing on any mass diffusion limitation in determining the apparent activity of the surface.

Reactant utilization in CVD and ALD chambers

In both chemical vapor deposition (CVD) and atomic layer deposition (ALD) reactors, a substantial fraction of the reactants never react at the substrate and are pumped away. To quantify this, an analytic expression for reactant utilization is derived for an axisymmetric stagnation flow reactor. The utilization is found to depend on the dimensionless Peclet and Damkohler numbers. The highest utilization is seen for CVD processes in the mass transfer limit (Da → ∞). For finite reaction rates, CVD processes always have higher utilization than ALD processes.

Surface Reaction Kinetics of the Oxidation and Reforming of CH4 over Rh/Al2 O3 Catalysts

An 48-step sur face reaction mechanism with thermodynamic consistent kinetic data is presented for the catalytic conversion of the gaseous chemical system H2/O2/H2O/CO/CO2/CH4 over Rh/Al2O3 catalysts. Total and partial oxidation as well as steam reforming and dry reforming of methane over Rh catalysts is studied experimentally and numerically at varying temperature and composition. The results are used to extend the kinetic schemes we developed for H2 oxidation, CO oxidation kinetics, and the water-gas-shift reactions in former studies. Aside from the experiments in a stagnation-flow reactor presented here, we modeled a number of experiments from the literature to test the newly established kinetic scheme.

Fluidic effects on kinetic parameter estimation in lab-scale catalysis testing – A critical evaluation based on computational fluid dynamics

The influence of fluidic effects on two different kinetic parameter identifications in lab-scale catalysis testing was investigated using computational fluid dynamics. Firstly, the dry reforming of methane in a stagnation flow reactor with a detailed surface mechanism was simulated fully in three-dimensional. It is shown that the 3D simulations are not advantageous over the commonly used stagnation-flow boundary-layer problem description. This reactor setting is a valuable example of how fluidic effects on kinetic parameter estimation can be suppressed. Secondly, the oxidative coupling of methane in a fixed-bed reactor with a 10-step kinetic mechanism was simulated with a porous-media model. The experimental results could not be reproduced. The underlying plug-flow model for kinetic parameter identification fails in this highly exothermic reactor, because of significant radial temperature profiles and resulting radial concentration profiles. The correct prediction of temperature profiles is of major significance. This investigation highlights the importance of well defined reactor configurations in combination with spatially resolved temperature and concentration profiles for the determination of reliable kinetic parameters for highly exothermic or endothermic reactions.

Nonstoichiometric Perovskite Oxides for Solar Thermochemical H2 and CO Production

Perovskite oxides (ABO3) are a largely unexplored class of materials in solar fuel applications. In this paper we examine the use of nonstoichiometric perovskite-type oxides in a two-step, solar-thermochemical water or carbon dioxide splitting cycle. We find that O2 begins to evolve during thermal reduction from a Sr- and Mn-doped LaAlO3 fully 300°C lower than that of CeO2, and that these compounds will split both H2O and CO2. The yield of H2 and CO is significantly greater than CeO2, a benchmark material in solar fuels research, at a thermal reduction temperature 150°C below that commonly reported for CeO2. In addition, the perovskite redox kinetics compare favorably to CeO2, which is known for its rapid reaction rates. We also find that an Fe- doped CaTiO3 is redox active and will split H2O, though the performance of this material is similar to that of CeO2. Finally, we introduce an experimental protocol that combines an ideal stagnation-flow reactor with detailed numerical modeling to effectively deconvolve intrinsic material behavior from interference induced by physical processes occurring inside the flow reactor. This method utilizes rate information contained within the entire time domain of the oxidation reaction, and assigns rate- governing processes to the material within the context of solid-state kinetic theory.

Kinetics and mechanism of solar-thermochemical H2 production by oxidation of a cobalt ferrite–zirconia composite

Accurate knowledge of water splitting kinetics is essential for the design and optimization of high-temperature thermochemical cycles for solar-driven fuel production, but such crucial data are unavailable for virtually all redox materials of potential practical value. We describe an investigation of the redox activity and oxidation kinetics of cobalt ferrite, a promising material for this application that is representative of a broader class of metal-substituted ferrites. To enable repetitive cycling, ferrites must be supported on another oxide to avoid sintering and deactivation. Consequently, we synthesized a composite material using atomic layer deposition of cobalt and iron oxides on zirconia, a commonly used ferrite “support”, to create a well-controlled, uniformly distributed composition. Our results show that the support is not an innocent bystander and that dissolved iron within it reacts by a different mechanism than embedded iron oxide particles in the matrix. Samples were thermally reduced at 1450 °C under helium and oxidized with steam at realistic process temperatures ranging from 900 °C to 1400 °C. Experiments within a fluid-dynamically well-behaved stagnation-flow reactor, coupled with detailed numerical modelling of the transient H2 production rates, allow us to effectively deconvolve experimental artefacts from intrinsic material behaviour over the entire time domain of the oxidation reaction. We find that second-order reaction and diffusion-limited mechanisms occur simultaneously at different oxidation rates and involve iron in two separate phases: (1) reduced Fe dissolved in the ZrO2 support and (2) iron oxide located at the interface between embedded ferrite particles and the zirconia matrix. Surprisingly, we also identified a catalytic mechanism occurring at the highest temperatures by which steady-state production of H2 and O2 occurs. The results reported here, which include Arrhenius rate constants for both oxidation mechanisms, will enable high-fidelity computational simulation of this complex, but promising approach to renewable fuel production.

Kinetics of hydrogen oxidation on Rh/Al2O3 catalysts studied in a stagnation-flow reactor

A newly established stagnation flow reactor with analysis of spatially resolved concentrations profiles is presented as useful tool for the investigation of heterogeneously catalyzed gas-phase reactions. The simplicity of this laboratory-scale reactor enables detailed modeling of the diffusive and convective transport within the one-dimensional gas-phase boundary-layer coupled with elementary-step homogeneous and heterogeneous reaction mechanisms. This set-up is applied to study the kinetics of hydrogen oxidation over Rh/Al2O3. By combining experimental and modeling results for a wide range of temperature and fuel/oxygen ratios, a thermodynamically consistent set of kinetic data for a 12-step surface reaction mechanism is derived. The applicability of the mechanism is further tested by the model prediction of experimentally derived ignition temperatures in a stagnation flow reactor and oxygen conversion in H2-rich hydrogen oxidation in an annular flow reactor at varying flow rate and temperature.

A novel ion transport membrane reactor for fundamental investigations of oxygen permeation and oxy-combustion under reactive flow conditions

Ion transport membrane (ITM) reactors present an attractive technology for combined air separation and fuel conversion in applications such as syngas production, oxidative coupling or oxy-combustion, with the promise of lower capital and operating costs, as well higher product selectivities than traditional technologies. The oxygen permeation rate through a given ITM is defined by the membrane temperature and oxygen chemical potential difference across it. Both of these parameters can be strongly influenced by thermochemical reactions occurring in the vicinity of the membrane, though in the literature they are often characterized in terms of the well mixed product stream at the reactor exit. This work presents the development of a novel ITM reactor for the fundamental investigation of the coupling between fuel conversion and oxygen permeation under well defined fluid dynamic and thermodynamic conditions, including provisions for spatially resolved, in-situ investigations. A planar, finite gap stagnation flow reactor with optical and probe access to the reaction zone is used to facilitate in-situ measurements and cross-validation with detailed numerical simulations. Using this novel reactor, baseline measurements are presented to elucidate the impact of the sweep gas fuel (CH4) fraction on the oxygen permeation and fuel conversion. In addition, the difference between well-mixed gas compositions measured at the reactor outlet and those measured in the vicinity of the membrane surface are discussed, demonstrating the unique utility of the reactor.

Flow Structure and Heat Transfer in a Stagnation Flow CVD Reactor

An experimental study is undertaken to investigate the flow structure and heat transfer in a stagnation flow chemical vapor deposition (CVD) reactor at atmospheric pressure. It is critical to develop models that predict flow patterns in such a reactor to achieve uniform deposition across the substrate. Free convection can negatively affect the gas flow as cold inlet gas impinges on the heated substrate, leading to vortices and disturbances in the normal flow path. This experimental research will be used to understand the buoyancy-induced and momentum driven flow structure encountered in an impinging jet CVD reactor. Investigations are conducted for various operating and design parameters. A modified stagnation flow reactor is built where the height between the inlet and substrate is reduced when compared with a prototypical stagnation flow reactor. By operating such a reactor at certain Reynolds and Grashof numbers, it is feasible to sustain smooth and vortex free flow at atmospheric pressure. The modified stagnation flow reactor is compared with other stagnation flow geometries with either a varied inlet length or varied heights between the inlet and substrate. Comparisons are made to understand the impact of such geometric changes on the flow structure and the thermal boundary layer. In addition, heat transfer correlations are obtained for the substrate temperature. Overall, the results obtained provide guidelines for curbing the effects of buoyancy and for improving the flow field to obtain greater film uniformity when operating a stagnation flow CVD reactor at atmospheric pressure.

Hydrogen Production via Chemical Looping Redox Cycles Using Atomic Layer Deposition-Synthesized Iron Oxide and Cobalt Ferrites

Iron oxide (γ-Fe2O3) and cobalt ferrite (CoxFe3−xO4) thin films were synthesized via atomic layer deposition (ALD) on high surface-area (50 m2 g−1) m-ZrO2 supports. The oxide films were grown by sequentially depositing iron oxide and cobalt oxide, adjusting the number of iron oxide to cobalt oxide cycles to achieve a desired stoichiometry. High resolution transmission electron microscopy and X-ray diffraction indicate that the films are crystalline and have a thickness of ∼2.5 nm. Raman spectroscopy was used to confirm the predominance of the spinel phase in the case of cobalt ferrite. Films were chemically reduced at 600 °C using mixtures of H2, CO, and CO2. The evolution of oxide phases as a function of time during this reduction was observed using in situ X-ray diffraction, showing that γ-Fe2O3 are reduced only to FeO, while CoxFe3−xO4 are reduced all the way to a Co/Fe alloy. Subsequent water splitting measurements in a stagnation flow reactor yielded peak H2 rates exceeding virtually all of those rep...