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Water and NO reversibly inhibit catalytic methane oxidation to CO2 over Pd/Al2O3 and Pd−Pt/Al2O3 at temperatures near 50 % conversion. Modulation‐excitation X‐ray absorption spectroscopy (ME‐XAS) was used to characterize the interaction of the inhibitors with the Pd and Pt components. Simulated X‐ray absorption near‐edge structure (XANES) spectra of Pd and Pt with various surface species were used to verify the findings of ME‐XAS measurements. Diffuse‐reflectance infrared Fourier‐transform spectroscopy (DRIFTS) allowed to correlate methane oxidation with surface formate species. An interaction of water with active Pd sites and water coverage of the Al2O3 support caused a drastic drop in formate production, correlated with the observed inhibition. In a similar manner, nitrite groups formed from gas‐phase NO inhibited the production of formates. At higher temperatures, the inhibition was reversed into a promoting effect by surface nitrates formed from NO that oxidized gas‐phase methane. This approach is not only interesting for emission control but in general for catalysts where dynamic structural changes occur. [ABSTRACT FROM AUTHOR]

An experimental and kinetic modeling study on the Ni-catalyzed conversion of methane under oxidative and reforming conditions is presented. The numerical model is based on a surface reaction mechanism consisting of 52 elementary-step like reactions with 14 surface and six gas-phase species. Reactions for the conversion of methane with oxygen, steam, and CO2 as well as methanation, water-gas shift reaction and carbon formation via Boudouard reaction are included. The mechanism is implemented in a one-dimensional flow field description of a fixed bed reactor. The model is evaluated by comparison of numerical simulations with data derived from isothermal experiments in a flow reactor over a powdered nickel-based catalyst using varying inlet gas compositions and operating temperatures. Furthermore, the influence of hydrogen and water as co-feed on methane dry reforming with CO2 is also investigated. [ABSTRACT FROM AUTHOR]

Abstract: The ignition (light-off) temperatures of catalytic oxidation reactions provide very useful information for understanding their surface reaction mechanism. In this study, the ignition behavior of the oxidation of hydrogen (H2), carbon monoxide (CO), methane (CH4), ethane (C2H6), and propane (C3H8) over Rh/alumina catalysts is examined in a stagnation-point flow reactor. The light-off temperatures are identified by means of the sudden increase of the catalyst temperature when linearly heating the catalyst for various fuel/oxygen ratios. For hydrogen and all hydrocarbons studied, the results show a rise of ignition temperature with increasing fuel/oxygen ratio, whereas the opposite trend is observed for the light-off of CO oxidation. Hydrogen oxidation, however, shows an opposite trend compared to previous investigations, performed on platinum [1,2]. [Copyright &y& Elsevier]

Abstract: The significance of gas-phase reactions in catalytic partial oxidation (CPOX) of isooctane at short contact times and high temperatures is studied experimentally and numerically to gain further understanding of hydrogen production by CPOX of logistic fuels for on-board applications. Special attention is given to the formation of coke precursors. CPOX of isooctane over a rhodium coated monolith with a molar inlet C/O ratio of 1.1 is used as reference case for a two-dimensional flow field description coupled with detailed surface and gas-phase reaction mechanisms. The results reveal catalyst coking and formation of coke precursors in the oxygen-free catalyst zone. Taking the product composition of the rich operated CPOX reactors (C/O=1.0−1.6) as inlet composition, homogeneous conversion in the gas-phase is studied in the temperature range from 873 to 1173K in a plug flow reactor. Conversion in the gas-phase is modeled by two detailed reaction mechanisms. Results show that most of the by-products and soot precursor species arise from unconverted fuel and not from additionally added hydrocarbons like ethylene. Both mechanisms well-predict all experimentally observed trends in gas-phase composition, both in axial reactor profiles and for different inlet compositions. The amount of soot precursors raises with increasing fuel feed corresponding to an increasing C/O ratio in CPOX experiments. [ABSTRACT FROM AUTHOR]

The impact of radiation heat transfer and radial heat losses in small-scale monoliths, as often used for testing catalysts or qualifying process conditions, are an important consideration to design and predict performance of commercial size reactors. The paper presents the 3D modeling of a honeycomb CPOX (Catalytic Partial Oxidation) reformer, including detailed surface chemistry for the conversion of methane on rhodium. The calculation domain comprises the flow region and two monoliths (one of them coated) which are positioned in a glass tube. For the simulations the software tool DUO (coupling between OpenFOAM and DETCHEM™) was used. The objective was to model the system without any boundary conditions for the temperature (aside from the inlet). As the temperature level is above 900 K solid body radiation has to be included. The comparison of the results with detailed experimental data shows that it is possible to reproduce the species concentrations and the temperature fields of the flow and solid structures well. The effect of radiation, leading to a heat transfer between the two monoliths, can clearly be indicated. However, this effect plays only a minor role with respect to the chemical conversion. The simulations capture the measured effect of radial heat removal on the conversion process in different channels inside the catalyst. [ABSTRACT FROM AUTHOR]

[Display omitted] • 3D modeling of an 86-particle fixed bed with CPOX and DRM chemistry. • ISAT and CA methods used to accelerate the calculation of reaction source terms. • CA method is simpler than other implementations of this acceleration technique. • ISAT and CA methods were added to DUO, which couples OpenFOAM and DETCHEM. • Speedup factors were 7x for CPOX and 35x for DRM, with acceptable calculation errors. Simulation of fixed-bed catalytic reactors is important for engineering industrial technologies. The large number of catalytically active surface faces makes the application of microkinetic models too time consuming for large 3D simulations that include fully resolved particle structures (e.g., ≫ 10′s of particles). Such simulations, though, are often required for small tube-to-particle diameter ratio configurations, where local flow effects are difficult to capture with 1D or 2D simulations. A common solution for speeding up reactive computational fluid dynamics (CFD) involves the use of chemistry acceleration methods, including In-Situ Adaptive Tabulation (ISAT) and Cell Agglomeration (CA). In the present contribution, DUO (DETCHEMTM + OpenFOAM coupling) CFD software has been extended to include ISAT and CA methods for surface reactions, in particular, using a CA method that is simpler than other implementations of this acceleration technique. Speedup factors and errors were tabulated for an 86 spherical particle 3D resolved fixed-bed reactor, considering the exothermic methane catalytic partial oxidation (CPOX) and the endothermic dry reforming of methane (DRM) chemistries. For CPOX, a speedup factor of ∼ 7x was achieved, while for DRM, a speedup of ∼ 35x was obtained, with both cases achieving an acceptable error in the solution. Although a tighter error tolerance with ISAT and CA is shown to reduce the overall calculation error, for some engineering situations this tradeoff in speedup and accuracy will be acceptable. Achieving usable results in around 0.5 – 2 h instead of around 20-hours (for these chemistries on this particular grid, run on a 400-core cluster) can vastly improve the feasibility of computational fluid dynamics for the analysis of fixed-bed catalytic reactors including microkinetics mechanisms. [ABSTRACT FROM AUTHOR]

Abstract: The catalytic partial oxidation (CPOX) of ethanol-blended fuels is studied over rhodium/alumina coated monoliths with ethanol/iso-octane mixtures as model systems. Blends containing 5–85vol.% ethanol are investigated, as well as the pure substances. All mixtures show a hydrogen yield of over 80% in millisecond contact times, with the highest yield for a blend with 10vol.% ethanol. For a high ethanol concentration in the blend, generally a high by-product formation is observed already at fuel lean conditions. The product yields cannot be linearly interpolated from the yields of the pure substances. In addition, the commercial fuels 95 RON gasoline and E85 are studied, revealing similar product yields in CPOX of the commercial fuels and the corresponding ethanol/iso-octane blends. Thus, two-component mixtures can be proposed as model system for the complex compositions of commercial fuel. [Copyright &y& Elsevier]

Catalytic partial oxidation (CPOX) is a promising technologyforreforming of liquid hydrocarbon fuels to hydrogen or synthesis gasfor use in fuel cells. The addition of a certain amount of the tailgas of the fuel cell stack to the reformer inlet feed can increaseoverall efficiency and lead to higher H2and CO selectivitiesand reduce coke formation. The effect of carbon dioxide or steam addition(1, 5, 10, 20, and 30 vol% of the total flow) on the performance ofa CPOX reformer operated with isooctane as fuel surrogate is systematicallystudied over a wide range of C/O feed ratios (0.72–1.79) usinga Rh/alumina honeycomb catalyst. The specific impact of the coreactantsH2O and CO2on reformer behavior can be interpretedby the water gas shift (WGS) chemistry. Production of H2and CO2increases with H2O addition at theexpense of CO and H2O. Opposite trends are observed incase of CO2addition. Tail gas recycling reduces formationof soot precursors up to 50% compared to the corresponding fuel feedwithout coreactants. However, tail-gas recycling shifts the formationof soot precursors toward lower C/O ratios. [ABSTRACT FROM AUTHOR]

Jet-fuel surrogates with varying composition and sulfur content were partially oxidized over a Rh/Al 2 O 3 honeycomb catalyst. The surrogates consisted of a blend of n-dodecane, 1,2,4-trimethylbenzene, and benzothiophene to represent model mixtures of the main chemical properties of jet fuels. The experiments were performed under quasi-autothermal conditions for three different C/O-ratios in a set-up, specifically designed for fuels with high boiling points. The product distribution significantly changes with sulfur addition showing increased water formation, undesired by-products and a decline in fuel conversion. A steady increase of by-product formation over time on stream leads to coke deposition for fuels with ≥50 mg S per kg fuel. The interplay between sulfur and coke, caused deactivation and its impact on syngas selectivity, fuel conversion, and by-product formation is discussed. [ABSTRACT FROM AUTHOR]

The catalytic partial oxidation of methane over a honeycomb-structured Rh/Al 2 O 3 coated catalyst is studied at a molar C/O ratio of unity and short-contact times experimentally and numerically. Axial species and temperature profiles inside the catalytic monolith are measured by a capillary sampling technique. A detailed numerical analysis of the flow, temperature and species concentration profiles is conducted using the new software tool DUO, which is an interface for the coupling of the computational tools OpenFOAM ® and DETCHEM™. This interface enables the integration of detailed surface chemistry into OpenFOAM ® and allows the time-efficient calculation of structured catalysts and other catalytic reactors with multiple combined fluid and solid regions including heterogeneous reactions. In comparison with experimental data, spatial profiles of species and temperature are analyzed. The thermal boundary conditions are shown to have a strong impact on reliable predictions. Even though the characteristics of the influence of radial heat loss on the conversion can be governed, the use of a radiation model is recommended. [ABSTRACT FROM AUTHOR]

Highlights: [•] CFD modeling of suction probe technique for partial oxidation in catalytic monoliths. [•] Strong influence of the probe on concentration profiles (dependent on its position). [•] Correction of measurement errors only possible for isothermal conditions. [•] Good agreement between measurements and calculations that take probe into account. [ABSTRACT FROM AUTHOR]

Abstract: A pseudo-transient numerical model is used for the simulation of a multi-functional catalytic plate reactor (CPR). The work mainly addresses the problems associated with on-board reforming for solid-oxide fuel cells. Heat management is achieved by indirectly coupling partial oxidation with reforming. Water management is achieved by partially recycling the anode stream from a solid-oxide fuel cell. The model uses detailed heterogeneous chemistry for reforming and oxidation reactions occurring on the catalyst beds. [ABSTRACT FROM AUTHOR]

The objective of this investigation is a better understanding of transient processes in catalytic monoliths. As an example, the light-off of the partial oxidation of methane to synthesis gas (H2 and CO) on a rhodium/alumina catalyst is studied experimentally and numerically.Methane/oxygen/argon mixtures are fed at room temperature and atmospheric pressure into a honeycomb monolith, which is preheated until ignition occurs. The exit gas-phase temperature and species concentrations are monitored by a thermocouple and mass spectroscopy, respectively. In the numerical study, the time-dependent temperature distribution of the entire solid monolith structure and the two-dimensional laminar reactive flow fields in the single monolith channels are simulated. A multi-step heterogeneous reaction mechanism is used, and the surface coverage with adsorbed species is calculated as function of the position in the monolith. During light-off, complete oxidation of methane to water and carbon dioxide occurs initially. Then, synthesis gas selectivity slowly increases with rising temperature. [Copyright &y& Elsevier]