Surface Properties and Catalytic Performance of La0.8K0.2CuxMn1–xO3 for Simultaneous Removal of NOx and Soot

The influence of copper substitution in La_0.8K_0.2Cu_xMn_1–xO₃ on its catalytic performance for simultaneous removal of NO_x and soot under oxygen rich conditions was investigated. A series of catalysts was prepared and then characterized by XRD, SEM, BET, and XPS. The temperature programmed reaction (TPR) method was used to evaluate the catalytic performance of the catalysts. XRD results show that the partial substitution of Cu for Mn promotes the formation of perfect perovskites. SEM and BET results demonstrate that appropriate copper substitution enhances the porosity and increases the specific surface area, leading to conditions which are favorable for heterogeneous catalysis. XPS results indicate that a fraction of the Mn³⁺ is converted to Mn⁴⁺ on the addition of low levels of Cu. By correlation of the physicochemical properties and the catalytic performance, a large specific surface area, high porosity, high content of Mn³⁺ and synergistic effects of Mn³⁺ and Cu²⁺ are seen to favor the simultaneous catalytic removal of NO_x and soot.     

Effect of Catalysis on Plasma Assisted Catalytic Removal of Nitrogen Oxides and Soot

An active perovskite‐type catalyst (La_0.8K_0.2Cu_0.05Mn_0.95O₃) was prepared and characterized using XRD, BET, and SEM. Then, the effect of catalysis on plasma assisted catalytic removal of nitrogen oxides and soot was investigated by combining temperature programmed reaction (TPR) and the analysis of Fourier transform infrared spectroscopy (FT‐IR). When the C₃H₆ concentration in the feed gas is 0.27 %, the maximum NO_x removal rate increases from 43.5 % to 72.2 % after adding catalyst. FT‐IR results indicate that the addition of catalyst will promote the removal of NO_x, HC, and soot. There is still great amount of NO_x and HC remaining after plasma reaction, little NO_x and almost no HC after catalytic reaction, and no NO_x and HC after plasma assisted catalytic reaction.     

Dual‐bed Catalytic System for the Selective Reduction of NOx with Propene

For the removal of NO_x from exhausts containing excess oxygen, the selective catalytic reduction of NO_x using hydrocarbons (HC‐SCR) is highly interesting, especially for car applications (lean deNO_x) [1]. Two types of HC‐SCR catalysts can be distinguished. High‐temperature catalysts operate at temperatures above 573 K, showing moderate activity, and (mostly) low heat and poison resistance, but high N₂ selectivity. Low‐temperature catalysts, usually based on Pt, operate between 473 and 573 K, showing opposite features: very high activity, as well as heat and poison resistance, and a low N₂ selectivity by forming N₂O. Our strategy for developing an active and stable deNO_x system without N₂O emission is the implementation of a separate catalytic function for N₂O removal as a second stage.     

The Effect of Oxygen Concentration on the Reaction of NOx with Soot Over BaAl2O4

The effect of oxygen concentration on the catalytic reaction of NO_x with soot over BaAl₂O₄ has been studied by Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFTS). The introduction of O₂ into the NO flow can result in a reactant mixture of NO/NO₂/O₂. Increasing the O₂ concentration from 2 % to 5 % in the NO‐containing flow promotes the formation of NO₂ from the gas phase oxidation of NO. The reactant mixture with high O₂/NO flow, allows for the formation of greater amounts of nitrate species than that with low O₂/NO flow, which further promotes the reaction of soot with NO_x and leads to a high conversion efficiency of NO_x into N₂ and N₂O. In the absence of O₂, N₂O is not observed since the N₂O produced at high temperatures has reacted with soot before it can be detected.     

Simultaneous Catalytic Removal of Nitrogen Oxides and Soot from Diesel Exhaust Gas over Potassium Modified Iron Oxide

Iron oxide modified by potassium, i.e. Fe_1.9K_0.1O₃, exhibits high catalytic performance for the simultaneous conversion of soot and NO_x into CO₂ and N₂. The present study shows that long‐time treatment of the catalyst leads to a drastic decrease in the activity, whereas even the aged catalyst maintains considerable activity. On the other hand, long‐time treatment causes selective N₂ formation, i.e. no more formation of the byproduct N₂O. This alteration of catalytic performance is likely due to agglomeration of the promoter potassium being present at the surface of catalyst. Detailed experiments were carried out with a more realistic diesel model exhaust gas to confirm that Fe_1.9K_0.1O₃ is a suitable catalyst for the simultaneous removal of soot and NO_x between 350 and 480 °C. It was assumed that (CO) intermediates, formed by the catalytic reaction of NO_x and oxygen with the soot surface, are the reactive species in NO_x‐soot conversion.     

Reduction of Nitrogen Oxides by Ammonia Over Iron‐Containing Catalysts

Selective catalytic reduction (SCR) of NO_x by NH₃ is an efficient technology for the removal of nitrogen oxides from diesel exhaust. It is a disadvantage that the active component in commercial catalysts, V₂O₅, is toxic and melts at ～650 °C. An alternative catalyst system, based on iron as the active component, was developed in this work. For this purpose, a number of supports were taken and impregnated with Fe₂O₃ and Fe₂O₃/WO₃ by an incipient wetness technique. The synthesized catalysts were tested in a diesel model exhaust using temperature programmed reaction. The screening accomplished to date, resulted in a 5.8 mol.‐% Fe₂O₃/Al₂O₃ catalyst that exhibited outstanding activity in the temperature range between 150–375 °C with selective formation of N₂. However, this catalyst was significantly deactivated by thermal aging at 600 °C. In contrast, the activity of the sample with 1.4 mol.‐% Fe₂O₃ and 0.8 mol.‐% WO_x showed very high thermal stability as well as selective N₂ production over the whole temperature regime, but it had lower SCR activity.     

Properties of BaAl2O4 in the Simultaneous Removal of Soot and NOx

The catalytic activity of BaAl₂O₄ in the simultaneous removal of soot and NO_x was evaluated by Temperature Programmed Reaction (TPR). It was found that BaAl₂O₄ could effectively catalyze the reaction of soot with NO_x under various reaction conditions. Compared with non‐catalytic combustion, ignition temperature, T_ig, and maximum combustion temperature, T_m, of soot decreased by more than 175 °C and 240 °C, respectively. Tight contact of soot with BaAl₂O₄, higher O₂ content and a lower flow rate of synthesized gases were beneficial to the catalytic reaction. It was confirmed by Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) analysis, that the reaction of soot with NO_2(ad) was the most important factor in simultaneous removal of soot and NO_x, in which NO_2(ad) existed on BaAl₂O₄ in the form of monodentate nitrate, bridged nitrate and ionic nitrate.     

Determination of Mechanism for Soot Oxidation with NO on Potassium Supported Mg‐Al Hydrotalcite Mixed Oxides

Soot oxidation with NO (in the absence of gas phase O₂) on potassium‐supported Mg‐Al hydrotalcite mixed oxides (K/MgAlO) was studied using a temperature‐programmed reaction and in situ FTIR techniques. Nitrite and the ketene group were identified as the reaction intermediates and thus a nitrite‐ketene mechanism was proposed in which surface active oxygen on K sites of K/MgAlO is transferred to soot by NO through nitrites. In the absence of gas phase O₂, soot oxidation with NO at lower temperatures (below 450 °C) is limited by the amount of active oxygen on the K sites. This kind of active oxygen is not reusable but can be replenished in the presence of gas phase O₂.     

RECP: A Reactor Designed to Test the Effect of a Catalyst on Soot Oxidation in a Diesel Particulate Filter

A new reactor designed to test soot combustion on a filter coated with an oxidation catalyst is described. It is designed to achieve screening investigations of catalysts in realistic conditions, i.e., close to those prevailing in a diesel particulate filter (DPF). In a DPF a soot layer is formed at the surface of a porous wall (filtration area) which may or may not be covered with a catalytic layer. In this new setup, the soot is deposited on a sample of a DPF which can be easily impregnated with oxidation catalysts. A model soot (commercial carbon black) is used for the investigation, and different procedures for the soot „deposit on the filter are tested.     

Porous Medium Model in Computational Fluid Dynamics Simulation of a Honeycombed SCR DeNOx Catalyst

The commercial computational fluid dynamics software FLUENT was used to simulate the flow and chemical reaction process of a honeycombed SCR DeNOx catalyst. In the calculation model, the porous medium model was applied to describe the wall body region of the honeycombed catalyst and the ordinary flow model in order to define the honeycombed channel region. From the comparison between the two gas diffusion rates within catalyst pore structure and gas medium, their effects on the SCR DeNOx reaction rate were analyzed. A correction factor of the chemical reaction rate was created to modify the porous medium model in order to accurately calculate the chemical reaction process of the catalyst model. The study results indicated that the combination between porous medium model and reaction rate correction factor could compensate the deviation of calculated reaction rate and the interfacial diffusion problems.     

Catalytic Oxidation of Methanol to Formaldehyde in a Continuous Fluidized‐Bed Reactor

Partial oxidation of methanol to formaldehyde by using a mixture of ferric and molybdenum oxides as the reaction catalyst at 280–330 °C has been studied in a continuous fluidized bed reactor. The reactor was a cylindrical tube of 20 mm in i.d. and 36 mm in o.d. placed vertically and connected to a truncated coneshaped cyclone separator. The catalyst was prepared by the precipitation method using aqueous solutions of ammonium heptamolybdate and ferric nitrate. The effect of certain parameters, such as temperature, superficial gas velocity and feed flow rates, on the extent of oxidation reaction has been investigated. The maximum size of the catalyst particles was 990 μm, therefore, neither external nor internal diffusion was expected to be effective in the process. The experimental data were correlated with three classes of hydrodynamic models presented for fluidized systems. The best correlation was obtained with compartment type models.     

Selective Catalytic Reduction of NO by Ammonia over Raney‐Ni Supported Cu‐ZSM–5 I. Catalyst Activity and Stability

Composite materials containing Raney Ni and Cu‐ZSM‐5 are highly active catalysts for the selective catalytic reduction (SCR) of NO by NH₃. Their catalytic properties were studied with particular attention to the influence of moisture and SO₂ in the feed, and to effects of catalyst shaping operations. Composite materials (16–20 wt‐% zeolite) were prepared by mixing the components, with different degree of segregation in the resulting pressed particles, or by growing ZSM‐5 crystallites on the surface of leached Raney Ni, which were then exchanged with Cu ions. Catalytic tests were performed with 1000 ppm NO, 1000 ppm NH₃, 2 % O₂ in He, at 3–6.5 · 10⁵ h^–1 (related to zeolite component). With physical mixtures, the catalytic behaviour strongly depended on the mixing strategy, particles containing both Ni and zeolite being inferior to mixed Ni‐only and zeolite‐only particles. The SCR activity was promoted by 2 % H₂O in the feed, SO₂ (200 ppm) was a moderate poison at low temperatures, but indifferent or slightly promoting at high temperatures. A catalyst prepared from ZSM‐5 grown on Raney Ni, which was ranked intermediate in dry feed, was promoted to excellent performance in H₂O and SO₂ containing feed at T > 700 K and was stable for 38 h at 845 K. The results suggest that SCR catalysts containing highly active zeolites should be produced avoiding shaping operations e.g. by use of zeolite crystallites grown on wire packings.     

Modeling of Kinetic Expressions for the Reduction of NOx by Hydrogen in Oxygen‐Rich Exhausts Using a Gradient‐Free Loop Reactor

The reduction of NO_x by hydrogen under lean conditions is investigated in a gradient‐free loop reactor. Using this computer‐controlled reactor, the reaction rates can be measured under exact isothermal conditions. Systematic variation of the input concentrations of hydrogen, nitric oxide, oxygen as well as reaction temperature provides a complete data set of reaction rates for the given reaction system. A number of kinetic rate expressions were evaluated for their ability to fit the experimental data by using toolboxes of MATLAB. The temperature influence on reaction rate constants and adsorption equilibrium constants were correlated simultaneously using Arrhenius and van’t Hoff equations, respectively. The kinetic rate expression based on a Langmuir‐Hinshelwood‐type model describes the data and the model can be improved by introducing a correction term in square root of hydrogen partial pressure over the range of conditions investigated.     

Total Oxidation of Chlorinated Hydrocarbons on A1–xSrxMnO3 Perovskite‐Type Oxide Catalysts – Part II: Catalytic Activity

The influence of the kind of A‐site cation in A_1–xSr_xMnO₃ perovskites (A = La, Pr, Nd, Di [didymium]) on the catalytic activity in the total oxidation of methane, chloromethane, dichloromethane, and trichloroethylene has been studied. In contrast to methane, the total oxidation of chlorinated hydrocarbons (CHC) is connected with a reversible catalyst deactivation and the formation of byproducts at low reaction temperatures. For the catalysts calcined at 600 and 800 °C, resp., the catalytic activity is determined mainly by specific surface area, amount of oxide admixtures and crystallinity of the perovskite. DiMnO₃ showed the highest and PrMnO₃ catalysts the lowest catalytic activity in the total oxidation of methane and CHC. Partial substitution of A by Sr leads to an enhancement of the catalytic activity in the total oxidation of methane, but not in the total oxidation of CHC.     

Total Oxidation of Chlorinated Hydrocarbons on A1–xSrxMnO3 Perovskite‐Type Oxide Catalysts – Part I: Catalyst Characterization

Thermoanalytical measurements (DTG‐DTA‐MS), X‐ray diffraction (XRD), temperature‐programmed reduction (TPR), redox titration and X‐ray photoelectron spectroscopy (XPS) were used to characterize A_1–xSr_xMnO₃ perovskite catalysts (A = La, Nd, Pr, Di [didymium]). The catalyst samples were investigated before and after interaction with chloromethane in the temperature range between 300 and 650 °C. XRD and TPR measurements revealed the presence of oxide admixtures in samples calcined at 600 and 800 °C, resp., in air. Crystallinity of the samples and the amount of oxide admixtures depend on the kind of A‐site cations. Interaction of the perovskite samples with chlorinated hydrocarbons at reaction temperatures leads to a decrease of the specific surface areas; the perovskite structure is preserved. Redox titration and TPR measurements showed that the Mn(IV) content in the perovskites increases by partial substitution of La by Sr and decreases after interaction with chloromethane.     

Progress in Understanding Plasma‐Propellant Interaction

To explore the phenomena of the plasma‐propellant interaction (PPI) and to point out the role of radiation, a series of experiments have been performed at the U.S. Army Research Laboratory. For this, the plasma radiation to the propellant has been separated from all other energy transfer mechanisms. The results clearly indicate that the observed structural damage of translucent JA2 samples is due to radiation only. An other aim of this work is to clarify some misunderstandings in analyzing plasma‐propellant interaction. By variation of the electrical pulse length it will be shown that electrical energy is in no way a useful parameter to characterize propellant response to variable plasma igniter systems.     

Computer Simulation of the Performance of a Downer‐Regenerator CFB for the Partial Oxidation of n‐Butane to Maleic Anhydride

A reactor model for a downer‐regenerator circulating fluidized‐bed (CFB) during the partial oxidation of n‐butane to maleic anhydride is presented. Upflow reactors (risers) suffer from severe solids back mixing and gas‐solids‐separation, in comparison down flow reactors exhibit a more uniform gas‐solids flow and reduced backmixing, resulting in narrower residence time distributions. Due to the sensitivity of the VPO catalyst to over‐reduction, downer reactors present an interesting alternative to riser reactors. The reactor models for the downer and the regenerator fluidized‐bed are coupled with reduction and oxidation kinetics for the catalyst, respectively. The influence of the solids residence time distributions for the combined system of both reactors on the oxidation state of the catalyst is explored by a novel newly developed oxygen loading distribution. Simulation results suggest the limited solids‐flux in downers restrict the maximum butane concentrations, while the scale‐up is predicted to be uncritical.