Iron as recyclable metal fuel: Unraveling oxidation behavior and cyclization effects through thermogravimetric analysis, wide-angle X-ray scattering and Mössbauer spectroscopy

The carbon-free chemical storage and release of renewable energy is an important task to drastically reduce CO2 emissions. The high specific energy density of iron and its recyclability makes it a promising storage material. Energy release by oxidation with air can be realized by the combustion of micron-sized iron powders in retro-fitted coal fired power plants and in fixed-bed reactors under milder conditions. An experimental parameter study of iron powder oxidation with air was conducted based on thermogravimetric analysis in combination with wide-angle X-ray scattering and Mössbauer spectroscopy. In agreement with literature the oxidation was found to consist of a very fast initial oxidation of the outer particle layer followed by much slower oxidation due to diffusion of iron ions through the Fe2O3/Fe3O4 layer being the ratelimiting step. Scanning electron microscopy analysis of the iron particle before and after oxidation reveal a strong particle morphology transformation. This impact on the reaction was studied by cyclization experiments. Up to 10 oxidation-reduction cycles show that both, oxidation and reduction rates, increase strongly with cycling due to increased porosity.

Exploring the oxidation behavior of undiluted and diluted iron particles for energy storage: Mössbauer spectroscopic analysis and kinetic modeling

Iron is an abundant and non-toxic element that holds great potential as energy carrier for large-scale and long-term energy storage. While from a general viewpoint iron oxidation is well-known, the detailed kinetics of oxidation for micrometer sized particles are missing, but required to enable large-scale utilization for energy production. In this work, iron particles are subjected to temperature-programmed oxidation. By dilution with boron nitride a sintering of the particles is prevented enabling to follow single particle effects. The mass fractions of iron and its oxides are determined for different oxidation times using Mössbauer spectroscopy. On the basis of the extracted phase compositions obtained at different times and temperatures (600-700 °C), it can be concluded that also for particles the oxidation follows a parabolic rate law. The parabolic rate constants are determined in this transition region. Knowledge of the particle size distribution and its consideration in modeling the oxidation kinetics of iron powder has proven to be crucial.

Automated Generation of Microkinetics for Heterogeneously Catalyzed Reactions Considering Correlated Uncertainties

The study presents an ab-initio based framework for the automated construction of microkinetic mechanisms considering correlated uncertainties in all energetic parameters and estimation routines. 2000 unique microkinetic models were generated within the uncertainty space of the BEEF-vdW functional for the oxidation reactions of representative exhaust gas emissions from stoichiometric combustion engines over Pt(111) and compared to experiments through multiscale modeling. The ensemble of simulations stresses the importance of considering uncertainties. Within this set of first-principles-based models, it is possible to identify a microkinetic mechanism that agrees with experimental data. This mechanism can be traced back to a single exchange-correlation functional, and it suggests that Pt(111) could be the active site for the oxidation of light hydrocarbons. The study provides a universal framework for the automated construction of reaction mechanisms with correlated uncertainty quantification, enabling a DFT-constrained microkinetic model optimization for other heterogeneously catalyzed systems.

Hydrogen Production and Carbon Capture by Gas-Phase Methane Pyrolysis: A Feasibility Study

Invited for this month's cover is the research group of Olaf Deutschmann and the team of Patrick Lott at the Karlsruhe Institute of Technology. The Cover image shows how an electrically heated reactor converts methane from natural gas or biogas into gaseous hydrogen and elemental carbon by means of high-temperature pyrolysis. The transfer of this technology into industrial applications can be a valuable contribution towards a decarbonization of the chemical industry and the establishment of a hydrogen economy. The Research Article itself is available at 10.1002/cssc.202201720.

Hydrogen Production and Carbon Capture by Gas-Phase Methane Pyrolysis: A Feasibility Study

Using natural gas and sustainable biogas as feed, high-temperature pyrolysis represents a potential technology for large-scale hydrogen production and simultaneous carbon capture. Further utilization of solid carbon accruing during the process (i. e., in battery industry or for metallurgy) increases the process's economic chances. This study demonstrated the feasibility of gas-phase methane pyrolysis for hydrogen production and carbon capture in an electrically heated high-temperature reactor operated between 1200 and 1600 °C under industrially relevant conditions. While hydrogen addition controlled methane conversion and suppressed the formation of undesired byproducts, an increasing residence time decreased the amount of byproducts and benefited high hydrogen yields. A temperature of 1400 °C ensured almost full methane conversion, moderate byproduct formation, and high hydrogen yield. A reaction flow analysis of the gas-phase kinetics revealed acetylene, ethylene, and benzene as the main intermediate products and precursors of carbon formation.

Soot Formation in Methane Pyrolysis Reactor: Modeling Soot Growth and Particle Characterization

Methane pyrolysis is a very attractive and climate-friendly process for hydrogen production and the sequestration of carbon as solid material. The formation of soot particles in methane pyrolysis reactors needs to be understood for technology scale-up calling for appropriate soot growth models. A monodisperse model is coupled with a plug flow reactor model and elementary-step reaction mechanisms to numerically simulate processes in methane pyrolysis reactors, namely, the chemical conversion of methane to hydrogen, formation of C-C coupling products and polycyclic aromatic hydrocarbons, and growth of soot particles. The soot growth model accounts for the effective structure of the aggregates by calculating the coagulation frequency from the free-molecular regime to the continuum regime. It predicts the soot mass, particle number, area, and volume concentration, along with the particle size distribution. For comparison, experiments on methane pyrolysis are carried out at different temperatures and collected soot samples are characterized using Raman spectroscopy, transmission electron microscopy (TEM), and dynamic light scattering (DLS).

Selective Catalytic Reduction with Hydrogen for Exhaust gas after-treatment of Hydrogen Combustion Engines

In this work, two palladium-based catalysts with either ZSM-5 or Zeolite Y as support material are tested for their performance in selective catalytic reduction of NOx with hydrogen (H2-SCR). The ligh-toff measurements in synthetic exhaust gas mixtures typical for hydrogen combustion engines are supplemented by detailed catalyst characterization comprising N2 physisorption, X-ray powder diffraction (XRD), hydrogen temperature programmed reduction (H2-TPR) and ammonia temperature programmed desorption (NH3-TPD). Introducing 10% or 20% TiO2 into the catalyst formulations reduced the surface area and the number of acidic sites for both catalysts, however, more severely for the Zeolite Y-supported catalysts. The higher reducibility of the Pd particles that was uncovered by H2-TPR resulted in an improved catalytic performance during the light-off measurements and substantially boosted NO conversion. Upon exposition to humid exhaust gas, the ZSM-5-supported catalysts showed a significant drop in performance, whereas the Zeolite Y-supported catalyst kept the high levels of conversion while shifting the selectivity from N2O more toward NH3 and N2. The 1%Pd/20%TiO2/HY catalyst subject to this work outperforms one of the most active and selective benchmark catalyst formulations, 1%Pd/5%V2O5/20%TiO2-Al2O3, making Zeolite Y a promising support material for H2-SCR catalyst formulations that allow efficient and selective NOx-removal from exhaust gases originating from hydrogen-fueled engines.

Optimizing Solid Oxide Fuel Cell Performance to Re-evaluate Its Role in the Mobility Sector

A sustainable, interconnected, and smart energy network in which hydrogen plays a major role cannot be dismissed as a utopia anymore. There are vast international and industrial ambitions to reach the envisioned system transformation, and the decarbonization of the mobility sector is a central pillar comprising a huge economic share. Solid oxide fuel cells (SOFCs) are one of the most promising technologies in the brigade of clean energy devices and have potentially wide applicability for transportation, due to their high efficiencies and impurity tolerance. To uncover future pathways to boost the cell's performance, we propose a detailed multiscale modeling methodology to evaluate the direct impact of cell materials and morphologies on commercial-scale system performance. After acquiring intrinsic electrokinetics decoupled from mass and charge transport of different anode and cathode materials via a half-cell model, a full cell model is employed to identify the most promising electrode combination. Subsequently, a scale-up to the system level is performed by coupling a 3-D kW-stack model to the balance of plant components while focusing on morphological optimization of the membrane electrode assembly (MEA). On optimally tailoring the MEA, model results demonstrate that an advanced cell design comprising a Ni fiber-CGO matrix structured anode and a LSCF-infiltrated CGO cathode could reach a stack power density of 1.85 kW L^-1 and a net system efficiency of 52.2% for operation at <700 °C, with manageable stack temperature gradients of <14 K cm^-1. The model-optimized power density is substantially higher than those of commercial stacks and surpasses industrial targets for SOFC-based range extenders. Thus, with further cell and stack development targeting the performance limiting processes elucidated in the paper, commercial SOFCs could, alongside range extenders, also act as prime movers in larger scale transport applications such as trucks, trains, and ships.

Experimental and numerical investigation of NO oxidation on Pt/Al2O3- and NOx storage on Pt/BaO/Al2O3-catalysts

Effects of temperature and inlet conditions on NO oxidation and NOx storage, as well as reduced NOx storage capacity over time – reflected by changes of measured NO concentration, which are reproduced by CFD using detailed reaction mechanisms.

Reduction of CO2 Emission from Off-Gases of Steel Industry by Dry Reforming of Methane

In a novel process, CO₂ and CH₄ from the off‐gases of the coke oven and blast furnace are used in homogeneous reforming of those greenhouse gases to valuable syngas, a mixture of H₂ and CO. Synthetic mixtures of the off‐gases from those large apparatuses of steel industry are fed to a high‐temperature, high‐pressure flow reactor at varying temperature, pressure, residence time, and mixing ratio of coke oven gas (COG) to blast furnace gas (BFG). In this study, a maximal reduction of 78.5 % CO₂ and a CH₄ conversion of 95 % could be achieved at 1350 °C, 5.5 bar, and a COG/BFG ratio of 0.6. Significant carbonaceous deposits were formed but did not block the reactor tube in the operational time window allowing cyclic operation of the process. These measurements were based on prior thermodynamic analysis and kinetic predictions using an elementary‐step reaction mechanism.

Spatially and Temporally Resolved Measurements of NO Adsorption/Desorption over NOx-Storage Catalyst

The two-dimensional (2D) temporal evolution of the NO-concentration over a NOx-storage catalyst is investigated in situ with planar laser-induced fluorescence (PLIF) in an optically accessible parallel wall channel reactor. Signal accumulated phase-correlated 2D-recordings of repetitive adsorption/desorption cycles are obtained by synchronizing the switching of the NO gas flow (on/off) with the laser and detection system, thereby significantly increasing the signal-to-noise ratio. The gas compositions at the reactor outlet are additionally monitored by ex-situ analytics. The impacts of varying feed concentration, temperature and flow velocities are investigated in an unsteady state. Transient kinetics and the mass transfer limitations can be interpreted in terms of the NO concentration gradient changes. The technique presented here is a very useful tool to investigate the interaction between surface kinetics and the surrounding gas flow, especially for transient catalytic processes.

Emission of Toxic HCN During NOx Removal by Ammonia SCR in the Exhaust of Lean-Burn Natural Gas Engines

Reducing greenhouse gas and pollutant emissions is one of the most stringent priorities of our society to minimize their dramatic effects on health and environment. Natural gas (NG) engines, in particular at lean conditions, emit less CO2 in comparison to combustion engines operated with liquid fuels but NG engines still require emission control devices for NOx removal. Using state-of-the-art technologies for selective catalytic reduction (SCR) of NOx with NH3 , we evaluated the interplay of the reducing agent NH3 and formaldehyde, which is always present in the exhaust of NG engines. Our results show that a significant amount of highly toxic hydrogen cyanide (HCN) is formed. All catalysts tested partially convert formaldehyde to HCOOH and CO. Additionally, they form secondary emissions of HCN due to catalytic reactions of formaldehyde and its oxidation intermediates with NH3 . With the present components of the exhaust gas aftertreatment system the HCN emissions are not efficiently converted to non-polluting gases. The development of more advanced catalyst formulations with improved oxidation activity is mandatory to solve this novel critical issue.

Unravelling the Different Reaction Pathways for Low Temperature CO Oxidation on Pt/CeO2 and Pt/Al2O3 by Spatially Resolved Structure-Activity Correlations

Spatially resolved operando HERFD-XANES (high energy resolution fluorescence detected X-ray absorption near edge structure) complemented by CO concentration gradient profiles along the catalyst bed (SpaciPro) was used to identify the dominant reaction paths for the low and high temperature CO oxidation on Pt/CeO₂ and Pt/Al₂O₃. At low temperatures, features associated with CO adsorption on Pt were found for both catalysts. During the oxidation reaction light-off, the evolution of the spectral and catalytic profile diverged along the catalyst bed. The CO oxidation rate was high on Pt/CeO₂ from the beginning of the catalyst bed with CO being adsorbed on Pt, whereas low CO conversion due to strong CO poisoning was found on Pt/Al₂O₃. This correlation of the CO concentration gradient with unique insight by HERFD-XANES gave direct proof of the crucial contribution of the Pt-CeO₂ perimeter sites overcoming the CO self-inhibition effect at low temperatures.

Thermodynamics and reaction mechanism of urea decomposition

A new reaction mechanism of urea decomposition is proposed after a strict treatment of thermodynamics in all phases.

CaRMeN: a tool for analysing and deriving kinetics in the real world

This paper presents the concepts of an open software tool (CaRMeN) that can be used to rapidly analyse and derive models, in particular chemical kinetics. The software automates the workflow of comparing model vs. experiment, which must currently be done manually and is thus a time-consuming and error-prone task. The capabilities of the software are illustrated through a case study. Experimental data for the conversion of methane over rhodium catalysts in a wide range of conditions and experimental setups are numerically simulated using five different mechanisms from the literature. The applicability of the mechanisms as well as differences between flow and diffusion models are evaluated. The results show that no single mechanism reliably predicts the chemical conversions of all of the experiments. Although the software was initially developed for chemical kinetics applications, it can also be extended to run any simulation code, and can therefore be applied in other scenarios.