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This manuscript reports a CO2 hydrogenation process in a catalytic laboratory‐scale packed‐bed reactor using an Fe/BZY15 (BaZr0.85Y0.15O3‐δ) catalyst to form hydrocarbons (e. g., CH4, C2+) at elevated pressure of 30 bar and temperatures in the range 270≤T≤375 °C. The effects of temperature, feed composition (i. e., CO2/H2 ratio, and residence time (i. e., Weight Hourly Space Velocity (WHSV) are studied to understand the relationship between CO2 conversion and carbon selectivity. Catalyst characterization elucidates the relationships between the catalyst structure, surface adsorbates, and reaction pathways. Thermodynamic analyses guide the experimental conditions and assist in interpreting results. While the feed composition and temperature influence the product distribution, the results suggest that the higher‐carbon (C2+) selectivity and yield depend strongly on residence time. The results suggest that the CO2 hydrogenation reaction pathway is similar to Fischer–Tropsch (FT) synthesis. The reaction begins with CO2 activation to form CO, followed by chain‐growth reactions similar to the FT process. The CO2 activation depends on the redox activity of the catalyst. However, the carbon chain growth depends primarily on the residence time. as is the case for the FT synthesis, high residence time (on the orders of hours) is required to achieve high C2+ yield. [ABSTRACT FROM AUTHOR]

Bifunctional basic‐metallic catalysts proved to be efficient for the selective hydrogenation of CO2 to methanol. The activity of these catalysts depends on the cooperative interaction between the amine groups and metallic sites, which is a function of amine group density, Pd particle perimeter length and the geometric properties of support pores. The pore width has the highest effect on the activity, increasing the methanol yield by about half an order of magnitude. Confining the space leads to a three‐dimensional utilization of the available metal surface sites compared to a two‐dimensional distribution of the bifunctional sites in larger pores, where the metal particle diameter is the decisive factor for the catalytic properties. [ABSTRACT FROM AUTHOR]

In order to understand deposit formation during urea selective catalytic reduction (SCR) resulting from isocyanic acid (HNCO) formation, the present study investigates the potential of HNCO hydrolysis by spatially resolved gas phase concentration profiles along a single catalyst channel of commercial Cu‐zeolite and V‐based SCR catalysts. The spatially resolved profiles, obtained in a special hot gas test rig via capillary technique, provide information on reaction rates of HNCO hydrolysis, NH3 adsorption and NO conversion, hereby revealing a better performance of the standard V‐based catalyst regarding the HNCO hydrolysis, which is attributed to the TiO2 support. [ABSTRACT FROM AUTHOR]

The NH3-SCR performance in the presence of short and long chain hydrocarbons (propylene and dodecane) as well as of formaldehyde was investigated for a series of V2O5-MoO3/TiO2 (VMoTi) and V2O5-WO3/TiO2 (VWTi) catalysts. The results demonstrate that vanadium oxides act as the main catalytically active component for NOx conversion, hydrocarbon (HC) and formaldehyde oxidation. Among VMoTi and VWTi materials with different vanadium loading, the catalysts containing 3 wt% V2O5 lead to the highest catalytic activity. The ability to simultaneously remove NOx, HC and HCHO over VMoTi is inferior to that of the conventional WO3-based catalyst. However, VMoTi exhibits higher CO2 selectivity, especially during oxidation of long-chain HCs, which represents an important advantage for a possible multifunctional catalyst. Apart from CO, formaldehyde was detected as byproduct during HC oxidation for both catalyst formulations. [ABSTRACT FROM AUTHOR]

The poisoning of Pd-Pt/Al2O3 and Pd-Pt/CeO2-ZrO2-Y2O3-La2O3 methane oxidation catalysts by SO2 was studied under conditions typical for lean burn gas engines. Regeneration of sulfur-poisoned catalysts was achieved by applying rich conditions at 500 and 550 °C. The presence of NOx resulted in a slower deactivation rate. While Pd-Pt/CeO2-ZrO2-Y2O3-La2O3 showed a superior catalytic activity, durability and regeneration ability compared to Pd-Pt/Al2O3 under NOx-free reaction conditions, its reactivation by a rich treatment was strongly inhibited if NOx was present during the aging and regeneration process. Operando X-ray absorption spectroscopy (XAS) was used to monitor the evolution of Pd and Pt during poisoning and regeneration. The studies show the formation of PdS and metallic Pd during reactivation of Pd-Pt/Al2O3, followed by transition to PdO after changing to lean reaction gas mixture. On the other hand, Pd species supported on CeO2-ZrO2-Y2O3-La2O3 could not be reduced under rich conditions and no regeneration occurred. [ABSTRACT FROM AUTHOR]

Abstract: The oxidation state of the active metal is an important factor for catalyst stability under dry and steam reforming conditions. This work explores the correlation of the oxidation state of the active metal with the coking behavior of alumina‐supported cobalt and nickel catalysts from a thermodynamic point of view. To this end, the thermodynamics of the oxidation of Co/γ‐Al2O3 and Ni/γ‐Al2O3 were investigated by using calculations at both standard and technical reforming conditions. It is shown that oxidation of nickel by water or CO2 cannot occur spontaneously under reforming conditions regardless of participation of the alumina support material because of the positive Gibbs reaction energies. Cobalt, in contrast, is more easily oxidized and may form CoAl2O4 through interaction with the support. This phase may react with surface carbon to regenerate the catalyst after carbon formation through thermal cracking of methane. A Mars–van Krevelen type reaction scheme is proposed to explain the higher coking resistance of cobalt compared to nickel. [ABSTRACT FROM AUTHOR]

Water-gas-shift (WGS) and reverse water-gas-shift (RWGS) reactions are numerically investigated in a stagnation-flow on a porous Rh/Al2O3 catalyst. External and internal mass transfer effects are studied using three different models for the mass transport and chemical conversion inside the porous catalyst: the dusty-gas model, a set of reaction-diffusion equations, and the effectiveness factor approach. All three models are coupled with the boundary layer equations to describe the potential flow on the stagnation disc, and a multi-step surface reaction mechanism is implemented. The numerically predicted species profiles in the external boundary layer are compared with recently measured profiles. Internal mass transfer limitations are more significant than external ones in case of the 100 μm thick catalyst layer. The effects of catalyst structure (thickness, mean pore diameter, porosity, tortuosity) as well as flow rate and pressure on chemical conversion are discussed. [ABSTRACT FROM AUTHOR]

The correlation between thermal aging effects of three-way catalysts (TWC) and their spatial temperature profiles and CO conversion was studied. The study is based on temperature profiles within fresh and aged catalysts, as well as CO concentrations at the outlet of the samples. The highest deactivation was always observed at the catalyst inlet area. The modelling approach with separated reaction mechanisms for palladium and rhodium used here assumes a direct correlation between the temperature profiles resulting from the exothermal chemical reactions and the catalytically active surface area. The noble metal dispersion is determined by low temperature CO chemisorption measurements. It is used in simulation as the relevant parameter for quantitative prediction of the catalyst performance. [ABSTRACT FROM AUTHOR]

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Abstract: Internally reforming anode supported solid-oxide fuel cells (SOFC) running on humidified is studied numerically. The computational framework employs detailed multi-step models for heterogeneous chemistry for Ni catalysts. The electrochemistry is implemented using a modified Butler–Volmer setting based on elementary charge transfer kinetics. Transport through the porous matrix is modeled using the dusty gas model (DGM). Parameters required for the electrochemical model are adjusted to reproduce experimentally observed data. Obtained parameters are then used to model a co-flow planar single cell under internally reforming conditions assuming the interconnect walls as adiabatic. Numerous runs with varying inlet conditions of cathode stream are carried out to deduce the behavior of temperature and current density distribution in the cell. Results show that internal reforming generally leads to a temperature drop near the inlet boundary. [Copyright &y& Elsevier]

Abstract: This work focuses on the development and improvement of numerical tools for the simulation of catalytic monolith reactors. These simulations cover detailed descriptions of reaction mechanisms, transport in gas-phase and washcoats, fluid flow in single channels, and the entire reactor. In DETCHEMMONOLITH, the concept of building a transient model of the monolith reactor by the calculation of steady-state solutions of representative channels has been improved. Thus, simulations with inlet conditions varying in space and time become feasible. Furthermore, reactors with alternating channel properties can be considered. Another extension concerns storage catalysts that accounts for transient surface coverages of the storage medium. As example, three numerical studies are shown: a catalytic combustion device with alternating active and inactive channels, a catalytic combustion monolith with spatially varying inlet conditions, and a NO x storage catalyst with alternating inlet conditions. [Copyright &y& Elsevier]

Hydrogen production by catalytic partial oxidation and steam reforming of methane and propane towards synthesis gas are numerically investigated in stagnation-flow over a disc coated with a porous Rh/Al2O3 layer. A one-dimensional flow field is coupled with three models for internal diffusion and with a 62-step surface reaction mechanism. Numerical simulations are conducted with the recently developed computer code DETCHEMSTAG. Dusty-Gas model, a reaction-diffusion model and a simple effectiveness factor model, are alternatively used in simulations to study the internal mass transfer inside the 100 µm thick washcoat layer. Numerically predicted species profiles in the external boundary layer agree well with the recently published experimental data. All three models for internal diffusion exhibit strong species concentration gradients in the catalyst layer. In partial oxidation conditions, a thin total oxidation zone occurs close to the gas-washcoat interface, followed by a zone of steam and dry reforming of methane. Increasing the reactor pressure and decreasing the inlet flow velocity increases/decreases the external/internal mass transfer limitations. The comparison of reaction-diffusion and Dusty-Gas model results reveal the insignificance of convective flow on species transport inside the washcoat. Simulations, which additionally solve a heat transport equation, do not show any temperature gradients inside the washcoat. [ABSTRACT FROM AUTHOR]

A synthetic exhaust gas bench was dynamically operated to investigate the impact of temperature, amplitude, split cycle, mean lambda, gas hourly space velocity, and oxygen storage capacity on average pollutant conversion and product selectivity of three-way catalysts in periodic operation. As temperature and amplitude increase and oxygen storage capacity decreases, the optimal frequency for maximum pollutant conversion increases. This is consistent with faster desorption of CO and O 2 from the catalyst, yielding free surface sites. Regarding the formation of secondary products, the optimal frequency for maximum pollutant conversion does not always correspond to minimal N 2 O and NH 3 emissions. The split cycle variation reveals the enhancement of C 3 H 8 and NO conversion after both lean-rich and rich-lean switches and C 3 H 6 and CO conversion after rich-lean switches at the optimal frequency. As periodic operation does not affect existing engine settings or operating conditions, it is a cost-effective control strategy for meeting future emission limits. [Display omitted] • Periodic operation enhances C 3 H 8 and NO conversion by 80%, and N 2 selectivity by 30% at 250 ∘C • Asymmetric periodic operation reveals conversion enhancement at optimal lean and rich half cycle • Optimal frequency increases with increasing temperature, amplitude and decreasing oxygen storage capacity • Observed optimal frequency decreases with increasing mixing upstream of the catalyst [ABSTRACT FROM AUTHOR]

• The presence of HCs and SO 2 significantly affects the NH 3 -SCR activity of a V-based SCR catalysts at pre-turbine position. • Depending on the HC used, NO x conversion decreases differently with simultaneous formation of some hazardous byproducts like HCHO and HCN. • Elevated pressures enhance the formation of S-containing deposits during prolonged SO 2 exposure at low temperatures. • Homogenous gas phase reactions lead to pronounced overconsumption of NH 3 but also tremendously enhance the NO oxidation to NO 2. With stricter regulations and decreasing exhaust gas temperature due to more efficient engine management, NO x abatement remains a critical issue despite many technical and catalytic advances. To counterbalance these challenges the positioning of exhaust gas aftertreatment systems in front of the turbocharger has gained significant interest in recent years. In this context, we report a systematic study on the impact of propylene, n -dodecane, and o -xylene as well as of SO 2 on the gas phase reactions and NH 3 -SCR activity of a conventional vanadium-based catalyst at increased pressure. An elevated pressure is shown to dramatically promote homogeneous reactions of hydrocarbons, which subsequently contribute to an increased NO oxidation but also to parasitic NH 3 overconsumption. The hydrocarbons are also faster catalytically oxidized and affect NO x reduction at all investigated temperatures. Their presence in the exhaust gas further leads to the formation of the toxic byproducts HCHO and HCN. Upon short term exposure to 20 ppm SO 2 in the feed gas, no significant catalyst poisoning was observed. However, for longer exposure times an enhanced formation of sulfur containing deposits was encountered at low temperatures for elevated pressures. Notably, their poisonous effect was reversible after heating to high temperatures. [ABSTRACT FROM AUTHOR]

[Display omitted] • CO 2 methanation over Ni-based catalysts studied with spatially-resolved experiments and modeling. • Strong gradients along catalyst bed cause oxidation and structural changes. • Model approach incorporates active catalytic surface area variation based on operando XAS data. • Combination of microkinetic modeling and operando experiments leads to better understanding of chemical kinetics. CO 2 methanation via the Sabatier reaction with (green) H 2 is promising due to its role in achieving a carbon–neutral energy balance in the context of Power-to-Gas technologies. Since Ni-based catalysts are relatively inexpensive compared to other metals and exhibit high catalytic activity, they are most commonly used. Due to the exothermic nature of the reaction, strong temperature and concentration gradients occur, which influence the catalyst structure. Thus, revealing the effects of structural changes of the catalyst along the reactor bed on local activity and selectivity is essential. A 1D packed-bed reactor model was used for numerical simulations, coupled with detailed microkinetics and mass transport limitations. The simulation results are compared with axially-resolved concentration and temperature profiles over 17 wt% Ni/γ-Al 2 O 3 and 17 wt% Ni 3.2 Fe/γ-Al 2 O 3 catalysts at oven temperatures of 623 K and 723 K. Using additional information from structural spatially-resolved synchrotron-based operando X-ray absorption spectroscopy studies, the oxidation state of Ni was considered in modeling the reactor by changing the catalytically active surface area along the reactor. Predicted surface coverages are compared with surface species experimentally determined by diffuse reflectance infrared Fourier transform spectroscopy. Overall, this study demonstrates the importance of combining modeling with spatially-resolved and temperature-dependent experiments to improve multiscale models and make predictions more accurate. [ABSTRACT FROM AUTHOR]

The interaction of hydrocarbons (HC) and nitric oxide (NO) over noble metal catalysts for exhaust gas after-treatment of lean-operated combustion engines can lead to secondary emissions, namely the formation of nitrous oxide (N 2 O), which is a strong greenhouse gas calling for N 2 O reduction concepts. By means of a series of light-off tests over state-of-the-art Pt-Pd oxidation catalysts, this study identifies the most critical catalyst operation regimes that should be avoided in order to minimize N 2 O levels. Especially unsaturated HCs react with NO to form significant amounts of N 2 O between 150 °C and 350 °C; an increasing HC/NOx ratio generally promotes N 2 O formation, whereas the NO oxidation reaction is increasingly inhibited. Since low space velocities and fast catalyst heating allow for minimizing N 2 O levels, active heating of catalytic converters during cold start and phases of low exhaust gas temperatures may efficiently reduce the formation of N 2 O in real-world applications. [Display omitted] • Interaction of hydrocarbons and NO over Pt-Pd results in significant N 2 O formation. • Hydrocarbon type, concentration, temperature, and space velocity as main factors. • Unsaturated hydrocarbons and increasing hydrocarbon/NOx ratio promote N 2 O formation. • Catalyst heating allows to minimize low-temperature N 2 O formation. • Hydrocarbons inhibit NO oxidation at higher temperatures. [ABSTRACT FROM AUTHOR]

This paper presents a model extension to dual-layer monolithic catalysts from a one-dimensional (1D) ( Balakotaiah, 2008; Joshi et al., 2009 ) mathematical model used for single-layer monolithic catalysts. The novel transient non-isothermal model computes average concentrations inside the bulk flow and in each layer of washcoat at every computational grid point along the monolith channel. The model has the capability that physical (e.g. diffusivity, pore radius, and thickness) and chemical (reaction kinetics) properties of the two layers of the washcoat can be different from each other. The relatively low computational cost offers real-time simulations of multi-layer monolithic catalysts. Analytically derived expressions for transverse mass transfer coefficients between the bulk flow and washcoat layers are presented for single-layer and dual-layer catalysts with circular cross-section, which can be easily extended for multi-layer catalysts. In addition, three expressions are presented for calculation of three internal Sherwood numbers, which are used in the dual-layer model, based on the values of Thiele moduli of the washcoat layers. These expressions are especially useful for calculation of the mass transfer coefficients for dual-layer catalysts with non-circular cross-sections. Application limitations of the 1D multi-layer model with respect to physical and chemical operating conditions are discussed. The derived model is applied for an automotive aftertreatment catalyst and the results of this model are compared with the results of a detailed 2D model (selective catalytic reduction/platinum group metals dual-layer catalysts). [ABSTRACT FROM AUTHOR]

• Support material strongly impacts sulfur poisoning of Pd-based catalysts. • Rich regenerative treatment results in formation of PdS. • Catalyst reactivation by decomposition of PdS and formation of PdO.. • Presence of water during rich treatment enhances efficiency of regeneration. Pd-Pt/Al 2 O 3 and Pd-Pt/CeO 2 -ZrO 2 -Y 2 O 3 -La 2 O 3 methane oxidation catalysts were investigated under typical lean-burn gas engine conditions with respect to sulfur poisoning and during reactivation, particularly under the most efficient rich conditions. Sulfation of the noble metal and the support material led to pronounced catalyst deactivation. A pronounced transfer of sulfates to the support, particularly alumina, only partially protects the noble metal. In situ X-ray absorption spectroscopy gave insight into the nature and evolution of Pd species during regenerative treatment. Although palladium sulfate that formed during poisoning is decomposed at approximately 200 °C in oxygen-free conditions, support regeneration requires higher temperatures, which result in PdS formation above 400 °C. Despite its high stability under rich conditions, PdS decomposition by oxidation in lean atmosphere leads to the recovery of the activity. In addition, water vapor that is present during the rich regeneration exhibited a beneficial effect on the regeneration with higher catalytic activity after regeneration. [ABSTRACT FROM AUTHOR]