Washcoated Monolith Catalysts Research Articles

Origins of Bi-modal NO conversion behavior in NH3-SCR over Cu-chabazite revealed by mass transfer and surface kinetics analysis

A multi-site kinetic model for the NH3-SCR (Selective Catalytic Reduction) on a commercial Cu-Chabazite washcoated monolith catalyst was developed and validated using synthetic gas bench data. The model was used to demonstrate the overall controlling regimes in the reactor at temperature and space velocity ranges of 120–550 °C and 30,000–80,000 h−1, respectively. This was performed via characteristic times analysis and via selectively removing external and internal mass transport phenomena from the model. Latter resulted in a bi-modal NO conversion with a conversion trench in 230–430 °C range where main mass transfer limitations were found. Bi-modal NO conversion profile was due to the competition between NH3 adsorption/desorption reactions and Standard SCR for surface NH3 species and it can be masked depending on the ratio of Standard SCR rates to the rates of adsorption and desorption in the presence of mass transfer limitations which have positive effects towards NO conversion.

Chemical gradients in automotive Cu-SSZ-13 catalysts for NOx removal revealed by operando X-ray spectrotomography

Nitrogen oxide (NOx) emissions are a major source of pollution, demanding ever-improving performance from catalytic after-treatment systems. However, catalyst development is often hindered by limited understanding of the catalyst at work, exacerbated by widespread use of model catalysts rather than technical catalysts, and by global rather than spatially resolved characterization tools. Here we combine operando X-ray absorption spectroscopy with microtomography to perform three-dimensional chemical imaging of the chemical state of copper species in a Cu-SSZ-13 washcoated monolith catalyst during NOx reduction. Gradients in copper oxidation state and coordination environment, resulting from an interplay of NOx reduction with adsorption–desorption of NH3 and mass transport phenomena, were revealed at micrometre spatial resolution while simultaneously determining catalytic performance. Crucially, direct three-dimensional visualization of complex reactions on non-model catalysts is feasible only by the use of operando X-ray spectrotomography, which can improve our understanding of structure–activity relationships, including the observation of mass and heat transport effects. Obtaining spatially resolved spectroscopic information for catalysts under working conditions remains challenging. Here, an approach that combines X-ray absorption spectroscopy with microtomography is introduced and showcased for the selective catalytic reduction of NOx with ammonia over a Cu-SSZ-13 washcoated monolith catalyst.

Kinetic study of phenol oxidation in a trickle bed reactor over Al/Zr-pillared clay catalyst

Catalytic wet air oxidation of phenol was studied in a trickle bed reactor operated at 10 bar and temperatures between 120-160 °C over pillared clay wash-coated monolith catalyst. Different variables were studied including reaction temperature and liquid flow rate. The obtained results indicated that phenol conversion increase with an increase in temperature, whereas an increase in liquid flow rate has a negative effect on phenol conversion. Simple power law was used to model the kinetics of the process. When a simple power law model was used experimental results were predicted successfully with R2 = 0.9988 at 160 °C and the activation energy was 42.289 kJ/mol.

Optimal design of washcoated monolith catalyst for compact, heat -integrated ethanol reformers

Monolith reactors are extensively employed for adiabatic gas-solid reactions, primarily in environmental applications. They are emerging as an attractive application in catalytic reformers where hydrogen needs to be produced on-board. These uses of monoliths are based on the optimal configuration and geometry of the washcoat, i.e., the thin catalytic layer impregnated onto the inert backbone of the monolith. In this paper, we present an analysis of the influence of geometric parameters of the washcoated monolithic catalyst towards affecting the reactor performance of non-isothermal reactor operation. The reactor performance is accounted through ‘volumetric reactor activity’ which comprises of the transport-kinetic interactions under adiabatic reactor condition. The effects of different combinations of the radius of curvature and coating thickness on the reactor activity and effectiveness factor are investigated for different backbone geometries. Through this process, an optimal combination of (R, dh) for each backbone configuration is evaluated. The radius of curvature (R = 0) always gives highest reactor activity due to low mass transfer resistance obtained in corners. With the increase in washcoat loading, the reactivity increases and after a certain point, it decreases owing to the mass transfer resistance which starts dominating at beyond that point. Finally, a 3D monolith geometry is modeled, and its performance is evaluated through ethanol conversion and temperature variations while the reforming section is coupled with ethanol combustion. Parametric sensitivity analysis is also performed by varying the effective diffusivity, the reaction rate constant, reforming section inlet velocity and feed temperature.

Steam reforming of ethanol on Rh/SiCeO2 washcoated monolith catalyst: Stable catalyst performance

The performance of a new Rh/CeSiO2 catalyst supported on a ceramic monolith for steam reforming (SR) of ethanol for hydrogen generation was investigated. It provides several advantages over a traditional pellet based catalyst in that it will reduce weight, size and pressure drop in the reactor. The effect of steam to ethanol molar ratio and temperature were first investigated on a powdered catalyst in order to establish the preferred reaction conditions to be used for tests on the monolith. The optimum temperature for coke free, high selectivity and stable catalyst operation was 1073 K at a steam to ethanol molar ratio of 3.5. The monolith supported catalyst was evaluated for aging stability, on/off performance and coke regeneration using steam gasification. After 96 h of SR of ethanol at 1028 K and water/ethanol molar ratio of 3.5 the monolith supported catalyst retained stable performance throughout the entire time on stream with the only products being H2, CO, CO2. Some coke formation was observed using Raman spectra, however, it did not cause any permanent deactivation. Regeneration via steam gasification at 973 K with 20% steam in N2 was successful for coke removal and complete catalyst regeneration.

Ni nanosheet-coated monolith catalyst with high performance for hydrogen production via natural gas steam reforming

A Ni nanosheet-coated FeCrAlloy monolith catalyst was successfully prepared using a deposition-precipitation-based “Korea Institute of Energy Research” (KIER) coating method, which provides a uniform coating of highly dispersive Ni catalyst on the metal substrate. The newly developed monolith catalyst showed a higher Ni dispersion and surface area on the substrate as compared to a washcoated monolith catalyst. The Ni nanosheet-coated monolith catalyst exhibited high activity and long-lasting stability at a high space velocity of GHSV=15,000h−1 during a natural gas steam reforming reaction for H2 production. This is mainly attributed to the fact that the uniform and highly dispersed Ni nanosheet catalyst improves the availability of surface active metal sites on the metal substrate.

Selective oxidation of ammonia to nitrogen on bi-functional Cu–SSZ-13 and Pt/Al2O3 monolith catalyst

The selective oxidation of NH3 to N2 on bi-functional mixed and dual-layer Cu–SSZ-13 and Pt/Al2O3 washcoated monolith catalysts is investigated under several reaction conditions to probe the effects of the added mass transfer barrier, in terms of the SCR loading (layer thickness), on the activity and selectivity of the catalyst. The combination of Cu–SSZ-13, itself shown to have high activity in reducing NOx to N2 with NH3, and Pt/Al2O3 is effective in converting the NOx produced by the very active but poorly selective precious metal catalyst. The data reveal a trade-off between NH3 conversion and N2 selectivity due to the mass transport barrier afforded by the added Cu–SSZ-13 top-layer: NH3 conversion decreases while N2 selectivity increases as Cu–SSZ-13 loading is increased. In the presence of feed NO, the NOx conversion of the dual-layer catalyst increases with an increase in the Cu–SSZ-13 washcoat loading. In the absence of feed NO, the dual-layer catalyst has a high selectivity to N2 due to the selective reaction in the Cu–SSZ-13 layer between NH3 from the bulk and counter-diffusing NOx formed in the underlying Pt/Al2O3. For the same catalyst loadings, the mixed washcoat catalyst gives a higher NH3 conversion at high space velocity than the dual-layer catalyst. This is attributed to NH3 conversion on Pt/Al2O3 sites not impeded by a SCR catalyst top-layer. In the presence of feed NO, the dual-layer catalyst shows a high NOx to N2 conversion as a result of the high SCR activity of the Cu–SSZ-13 top-layer. The mixed catalyst is more selective to NOx because the Pt catalyst in proximity to the SCR catalyst leads to the oxidation of NH3 back to NOx. Finally, we demonstrate that a hybrid catalyst having dual-layer and mixed catalyst attributes exhibits good performance by exploiting the beneficial effects of both catalyst types.

Experimental and modeling study of selective ammonia oxidation on multi-functional washcoated monolith catalysts

An experimental and modeling study of the selective oxidation of ammonia to N2 on washcoated multi-functional Pt/Al2O3+Fe–ZSM-5 monolith catalysts is reported. The catalysts are compared for a range of temperature, space velocity, and feed composition in the presence of H2O and CO2. The addition of the Fe–ZSM-5 is effective in converting the NOx produced by the poorly selective Pt/γ-Al2O3 catalyst. In the absence of feed NO, the dual-layer catalyst has a high selectivity to N2 due to selective reaction in the Fe–zeolite layer between NH3 from the bulk and counter-diffusing NOx formed in the underlying Pt/Al2O3. For T<300°C the mixed Fe–ZSM-5/Pt/Al2O3 catalyst has a lower N2O yield than the dual-layer Fe–ZSM-5/Pt/Al2O3 catalyst. This is attributed to NO migration and reduction on the Fe–ZSM-5. The mixed washcoat gives a lower N2 yield than the dual-layer due to Pt-catalyzed NH3 oxidation to NOx, which diffuses to the gas phase before its reduction. The mixed catalyst is less selective to N2 because the Pt catalyst in proximity to the SCR catalyst leads to the oxidation of NH3 back to NOx. Most of the trends are predicted by a 1+1 D reactor model that includes the Pt/Al2O3 ammonia oxidation and Fe–ZSM-5 SCR kinetics and the relevant transport processes. Kinetic parameters are estimated from single catalytic component experiments while the effective diffusivities in the layers are estimated from the simulation of multi-layer experiments in which inert top layers of γ-Al2O3 and Na–ZSM-5 were deposited onto an underlying Pt/Al2O3 layer. With the help of the model, explanations of the trends are provided.

NOx Storage and Reduction with H2 on Pt/Rh/BaO/CeO2: Effects of Rh and CeO2 in the Absence and Presence of CO2 and H2O

The storage and reduction features of a family of Pt/Rh/BaO/CeO2/Al2O3 washcoated monolith catalysts are compared in terms of NOx conversion and product selectivity using H2 as the reductant. The catalyst composition, monolith temperature, regeneration time, and presence of H2O and CO2 in the feed were systematically varied to identify trends and to elucidate effects. In addition to cycling, experiments involving the reduction of a fixed amount of prestored NOx help to isolate differences in the regeneration features of the catalysts. The addition of both CeO2 and Rh to Pt/BaO increases the cycle-averaged NOx conversion and selectivity to N2, but oxygen storage on the ceria expectedly leads to the consumption of additional reductant during the regeneration. CeO2 is shown to be an inferior NOx storage component, but as a supplement to BaO, CeO2 provides the role of promoting the oxidation of NH3 to N2. The same stored oxygen is the likely cause for the enhanced NH3 oxidation. On the other hand, Pt/BaO is shown to be the most effective catalyst for converting NOx to NH3. Fixed NOx storage experiments show the existence of at least three rate-controlling regimes: one that is reductant-feed-rate-limited, another that is limited by NOx storage phase diffusion, and a third that has a chemical or textural origin. On Pt/BaO, the first two regimes are clearly distinguishable, whereas a more complex picture emerges for Pt/CeO2, with which a fraction of the stored NOx is kinetically inaccessible to reduction. Water inhibits the oxidation of NO on each catalyst, but the cycle-averaged NOx conversion is largely unaffected. In contrast, CO2 has only a minor effect on the conversion during steady state NO oxidation but significantly inhibits the NOx conversion during cyclic storage and reduction. This effect is attributed to the known higher stability of BaCO3 compared with BaO/Ba(OH)2. The conversion of H2 to the less effective reductant CO via reverse water gas shift chemistry is a contributing factor based on steady-state activity tests. This pathway is shown to be most important for the catalysts, containing Rh/CeO2, a known effective water gas shift catalyst. Building on the existing literature, most of the observed trends are interpreted in terms of the likely reaction pathways and transport processes. The findings are assessed in terms of identifying the catalyst best suited to the specific NOx trap application, be it a stand-alone reactor or one coupled with downstream NH3-based selective catalytic reduction.

Influence of the Substrate Properties on the Performances of NH3-SCR Monolithic Catalysts for the Aftertreatment of Diesel Exhaust: An Experimental and Modeling Study

The effects of structural and geometrical characteristics of wash-coated monolith catalysts on the NO-NO2/NH3 selective catalytic reduction (SCR) activity were experimentally investigated over the same Cu-exchanged zeolite system coated onto honeycomb cordierite substrates with different cell densities, lengths, washcoat loads, and channel shapes. A stacked configuration was also tested. Contrary to previous reports, it was found that both interphase and intraphase diffusional limitations appreciably affected the deNOx efficiency at intermediate to high temperatures, whereas entrance effects did not play a noticeable role in enhancing the NOx conversion. A two-phase 1D+1D dynamic mathematical model of SCR monolithic converters, which explicitly accounts for both gas/solid and intraporous mass-transfer resistances, successfully predicted all of the observed effects using a single set of rate parameters estimated from intrinsic kinetic runs performed over the same catalyst in powdered form, under diffusion-free conditions.

Kinetic and process study for ethanol reforming using a Rh/Pt washcoated monolith catalyst

The reforming of pure ethanol was studied over a bi-metallic precious metal (Rh/Pt) catalyst deposited on a ceramic monolith in order to analyze reforming process conditions. High ethanol conversion tests performed at low space velocities (<20,000h−1) confirmed that the catalyst could achieve 100% ethanol conversion to equilibrium concentrations of H2, CO, CO2 and CH4. Low conversion tests at high space velocities (≥50,000h−1) were conducted to produce an overall rate expression with an activation energy of 85kJmole−1. The reaction was found to have a 1.2 reaction order for ethanol and zero order for water for stoichiometric ethanol and water ratios. In addition, the impact of non-catalytic reactions was studied. The results showed that the catalyst was capable of reforming ethanol as well as the by-products from non-catalytic reactions at 500–700°C.This work is part 1 of a series to develop a process for steam reforming E85 (85% ethanol+15% gasoline) to generate hydrogen for a fuel cell.

Mechanism of partial oxidation of methane over a nickel-calcium hydroxyapatite catalyst

The mechanism of partial oxidation of methane to synthesis gas over a nickel-calcium hydroxyapatite catalyst was studied by employing pulse experiments for the powder catalyst and by measuring temperature profiles of the activated, washcoated monolith catalyst. The pulse study showed that after the catalyst was partially reduced, carbon deposition occurred to a great extent and CO was predominantly produced over CO 2. Temperature profiles of the monolith catalyst showed that the highest temperature difference between the furnace and the monolith became smaller as the furnace temperature increased. We propose that the reaction occurs primarily via the pyrolysis mechanism or direct dissociation of methane. Adsorbed CO (CO s) is a common intermediate and it is rapidly desorbed to produce CO(g), especially at high temperature, or converted to CO 2(g), especially at low temperature. The observation that the fully reduced catalyst exhibited lower activity suggests that both metallic Ni and partially oxidized nickel are required in order to exhibit high activity and selectivity.

Selective catalytic reduction of NOx with propene over double wash-coat monolith catalysts

Selective catalytic reduction (SCR) among the approaches for alleviating NOx emission was much attracted. Zeolites have the advantage for adsorption of propene, and noble metal catalyst has the advantage for oxidation of NO to NO 2. Pt (or Au)/Al 2O 3 (or SiO 2) were used as the lower layer of double wash-coated monolith catalysts. Zeolites (H-mordenite or ZSM-5) were coated as the upper layer. The catalytic performance of the double wash-coated catalyst was, remarkably, improved. Also, temperature window was shifted to lower temperature and broadened. It was known that the combined noble metal monolith catalyst with zeolite was very effective in removing NOx by SCR with hydrocarbons.

Dynamics of storage and reaction in a monolith reactor: lean NO [formula omitted] reduction

The NO x storage and reduction performance of a series of supported Pt/BaO washcoated monolith catalysts is investigated ( y %Pt/ x %BaO/Al 2 O 3 ; y = 2 –2.6%; x = 0 –25%). Storage experiments over a range of exposure (storage) times and feed flow rates (space velocities) show the importance of the NO oxidation to NO 2 reaction. At high space velocities, kinetic limitations reduce the NO conversion and NO x trapping efficiency (fraction of NO x trapped). Higher trapping efficiencies are achieved at lower space velocities when the NO oxidation approaches the equilibrium conversion limit. The NO x trapping efficiency increases with barium loading, but the extent of increase depends on the exposure time. Cycle-averaged NO x conversions and N 2 selectivities exceeding 80% are achieved over a range of operating conditions, including cycle time, space velocity, and reductant (propylene) concentration. The NO x conversion exhibits a maximum at an intermediate cycle time, and the location and magnitude of the maximum depends on the barium loading. The conversion also achieves a maximum at an intermediate barium loading, indicating that increases in dynamic storage capacity of the catalyst do not guarantee improved reduction performance. The dependence of the trapping efficiency on the exposure (storage) time provides a good estimate for the lean storage time needed to achieve a prescribed cycle-averaged NO x conversion during cycling. The dependence of the cycle-averaged NO x conversion on barium loading parallels that of the short-time NO x storage, indicating the importance of the NO x trapping step in the overall monolith catalyst performance.

Supported honeycomb monolith catalysts for high-temperature ammonia decomposition and H2S removal

Catalysts that have potential in simultaneous removal of H2S and NH3 decomposition were developed. The monolith supports of high surface area and acceptable mechanical strength based on titania and silica–alumina precursors were prepared and tested. Preparation routine and composition of Mn, Fe and Cu oxides supported honeycomb catalysts have been optimized. Impregnated and washcoated monolith catalysts were tested in ammonia high-temperature decomposition.

The Sulfur and Water Resistance of Modified Washcoated Zeolite-Exchanged Monolith Catalysts for SCR of NOx with Propane

The method of preparation of modified Cu-substituted zeolite DeNOx catalysts washcoated on monolith ceramics has been developed. Non-modified and modified monolith catalysts were tested in DeNOx reaction with propane. It was shown that the catalyst modified by cerium and containing titania together with H-ZSM-5 and Al2O3 in the washcoating layer demonstrates high level of activity, and its resistance during multiple cycles of poisoning by sulfur compounds and water at 400 and 500°C is also high.