Reducing NO to N2 in diesel engine exhaust is typically accomplished using a Cu/CHA catalyst. Although commercially available, Cu/CHA catalysts are still susceptible to degradation in performance via sulfur poisoning. In this study, we used different approaches to sulfur poisoning. We compared the results from these approaches to a fresh catalyst, one that had only undergone hydrothermal aging (HTA), i.e. no exposure to S, and an engine aged sample. For S poisoning, the catalysts were first exposed to SO2 at 400 degrees C then a desulfation phase at 575 degrees C. We tested samples after 1 cycle, 5 cycles and 100 cycles. Multiple cycles of SO2 exposure and desulfation led to an accumulation of S, and loss in multinuclear Cu species as well as reducible Cu. These factors led to a decrease in NOx conversion as well. Correlations were examined between the different Cu sites and NOx reduction, with the best being related to dynamic dimers. Most of the results suggest residual sulfate species (ZCuxSOy). The results demonstrate that a single sulfur exposure and regeneration process did not represent an engine-aged catalyst, in extent of SCR performance and in changes to Cu speciation.

Exploring the impact of periodic reaction conditions on hydrogen production in methane partial oxidation on Pt/Al 2 O 3 catalyst.

Designing robust catalysts for low-temperature oxidation is pertinent to the development of advanced combustion engines to meet increasingly stringent emissions limitations. Oxidation of CO, hydrocarbon, and NO pollutants over platinum-group catalysts suffer from strong inhibition due to their competitive adsorption, while coinage metals are generally slow at activating O2. Through computational screening, we discovered a PdCu alloy catalyst that completely oxidizes CO below 150 degrees C without inhibition by NO, propylene or water. This is attributed primarily to geometric effects and the presence of CO bound to Pd sites within the Cu-rich surface of the PdCu alloy. We demonstrate that the novel PdCu catalyst can be used in tandem with a PtPd catalyst to achieve sequential, inhibition-free, complete oxidation of CO in a two-bed system, while also achieving 50 % NO conversion below 120 degrees C. Moreover, neither water nor propylene adversely affect the low temperature CO oxidation activity. In contrast to typical PdPt alloys used in diesel oxidation catalysis, the rate of CO oxidation on a computationally identified PdCu alloy catalyst does not slow down when NO is present. To get the best of both worlds, a dual-bed configuration of PdCu for the oxidation of CO at low temperature, followed by PdPt to oxidize NO and hydrocarbons does the trick while using 50 % less platinum.image

Characterizing ceria-containing catalysts via common characterization techniques can be challenging. CO chemisorption is usually used to measure catalyst dispersion. However, the reaction between CO and ceria lattice oxygen leads to significant carbonate formation, resulting in an overestimation of the CO uptake and metal dispersion. Here, we propose a modified method to characterize ceria-containing catalysts which involves exposing the catalyst to CO2 prior CO adsorption to prevent further carbonate formation during CO chemisorption. We tested this method on Pd/CeO2 catalysts characterized by different ceria particle sizes and validated it via DRIFTS and CO oxidation kinetics. Validation conducted using CO oxidation kinetics showed that the TOFs derived by normalizing the reaction rates by the interfacial sites, which are the active sites, are similar for all the catalysts under investigation. Our results show that CO2-CO chemisorption can be a reliable technique for estimating metal dispersion of ceria-supported catalysts.

The nuclearity of Cu2+ species active in the low-temperature Reduction Half Cycle (RHC) of NOx Selective Catalytic Reduction with NH3 over Cu-CHA catalysts is controversial. In the past, transient CO to CO2 oxidation protocols have been used to titrate binuclear Cu2+ species, and identified NH3-solvated ZCu2+(OH)- ions as their precursors. However, the prior results relied on asymptotic extrapolation due to the very slow CO oxidation kinetics. We herein present the results from a prolonged (24 h) CO titration experiment under dry conditions over an industrial Cu-SSZ-13 catalyst: the results demonstrate the conversion of all ZCu2+(OH)- ions to binuclear Cu2+ complexes and agree well with the extrapolation of a shorter (90 min) experiment, based on the assumption of the CO oxidation rate being second order in ZCu2+(OH)-. These outcomes confirm the adequacy of short CO oxidation tests to titrate ZCu2+(OH)- ions and support a Cu2+ pair mediated RHC pathway.

Metal active sites exchanged in zeolites dynamically change speciation under different reaction conditions, resulting in condition-dependent reactivity with gas molecules. This is of particular significance in chemical deactivation, such as sulfur poisoning of Cu-SSZ-13 zeolites used for NH3-assisted selective catalytic reduction (SCR) of NOx in diesel engine exhaust. Here, we employ computational techniques, combined with experiments and spectroscopies, to develop thermodynamic models for sulfur poisoning of zeolite framework-bound Cu dimers that form at high temperatures in Cu-SSZ-13 zeolites as a function of both the Al distribution and reaction conditions. Our results demonstrate that framework-bound Cu dimers that form under high temperature oxidative conditions are particularly susceptible to poisoning by SO2 and SO3. These Cu dimers react more exothermically with SOx species than Cu monomers, forming thermodynamically stable sulfur-containing Cu dimers that require high temperatures for catalyst regeneration.

Cu/SSZ-13 is a catalyst commonly used for selective catalytic reduction (SCR) of nitrogen oxides (NOx) in diesel engine exhaust. Standard configurations are based on SCR preceded by an oxidation catalyst, however, a close-coupled SCR configuration appears relevant for upcoming regulations addressing cold-start emissions. Two often discussed ion-exchanged Cu sites in Cu/SSZ-13 are denoted ZCuOH and Z2Cu, where Z represents an Al atom in the zeolite framework. Mild hydrothermal aging (HTA) causes Cu redistribution within the catalyst; ZCuOH sites transform into more stable Z2Cu species. In this work, the effect of mild HTA on NH3, NO, SO2, and CO oxidation is studied on a commercial Cu/SSZ-13 sample. Temperature-programmed reaction experiments, performed in a lab reactor, and previously published catalyst characterization data using H2-TPR, NH3-TPD, and DRIFTS were used to build and validate a mathematical model. It couples the kinetics of the oxidation reactions with ZCuOH and Z2Cu concentration evolution during mild HTA. The model uses two distinct rate coefficients for ZCuOH and Z2Cu sites, allowing prediction of the catalyst activity during aging. The results suggest that the rate coefficients for the studied reactions on Z2Cu are effectively nil, so the observed activity in NH3, NO, SO2, and CO oxidation can be attributed solely to ZCuOH sites.

Propene ammoxidation to acrylonitrile (ACN) over bismuth molybdate-based catalysts has been commercialized for more than 60 years. To meet forecasted growth, there is an opportunity for smaller-scale, decentralized ACN production. Forced dynamic operation (FDO), here referring to step changes in reactant inlet concentrations, has been used in low-volume production applications and has shown advantages in partial oxidation reactions. As a proof-of-concept study to evaluate FDO for acrylonitrile production, we applied periodic changes in inlet gas concentrations that varied between one phase containing all reactants and another that just contained O2, over an industrial bismuth molybdate-based catalyst. We varied the cycle period, duty cycle, and O2 concentration in the second phase and show that improved acrylonitrile yields can be obtained compared with those of steady-state operation under certain conditions. A correlation between lattice oxygen availability and FDO performance was observed.

Cu-SSZ-13 is often studied as a selective catalytic reduction (SCR) of NOx catalyst for diesel after-treatment systems. Mild hydrothermal aging (HTA) is commonly known to cause changes in catalyst performance. In this study, a kinetic model of SCR over a Cu-SSZ-13 catalyst that includes mild HTA and its impact on the SCR reaction was developed. The migration of Cu species during HTA, and how this change in Cu distribution affects low-temperature and high-temperature NH3 oxidation and SCR reaction mechanisms, were considered. Flow reactor test results were used for parameter estimation and verification. Different characterization techniques, including H2 temperature-programmed reduction (H2-TPR), NO + NH3-TPR, NH3 temperature-programmed desorption (NH3-TPD), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), were used to characterize and quantify catalyst site densities. The model can simulate the migration of Cu species and the NOx reduction performance changes during mild hydrothermal aging as a function of aging temperature and time.

Methane oxidation under periodic conditions and the oxygen storage capacity of a bilayer Pt/Pd/Al2O3 over a Mn0.5Fe2.5O4 spinel catalyst were studied before and after SO2 exposure, and after simulated regeneration conditions. Prior to sulfur exposure, improvement in CH4 oxidation conversion under periodic conditions compared to steady-state conditions was observed. After sulfur exposure at 100 degrees C, there was a loss in CH4 oxidation performance and a loss of oxygen storage capacity of the spinel material. The extent of regeneration from sulfur poisoning depends on the ability to induce the decomposition of sulfate species, and while all regeneration methods tested in this study did improve CH4 conversion, regeneration methods under periodic conditions induced greater sulfur species desorption from the catalyst surface leading to improved CH4 conversion. Key regeneration parameters - temperature, feed composition, modulation amplitude and frequency - were optimized to induce S species decomposition and correlated to CH4 oxidation activity recovery.

Catalysts can undergo structural changes during the reaction, affecting the number and/or the shape of active sites. For example, Rh can undergo interconversion between nanoparticles and single atoms when CO is present in the reaction mixture. Therefore, calculating a turnover frequency in such cases can be challenging as the number of active sites can change depending on the reaction conditions. Here, we use CO oxidation kinetics to track Rh structural changes occurring during the reaction. The apparent activation energy, considering the nanoparticles as the active sites, was constant in different temperature regimes. However, in a stoichiometric excess of O2, there were observed changes in the pre-exponential factor, which we link to changes in the number of active Rh sites. An excess of O2 enhanced CO-induced Rh nanoparticle disintegration into single atoms, affecting catalyst activity. The temperature at which these structural changes occur depend on Rh particle size, with small particle sizes disintegrating at higher temperature, relative to the temperature required to break apart bigger particles. Rh structural changes were also observed during in situ infrared spectroscopic studies. Combining CO oxidation kinetics and spectroscopic studies allowed us to calculate the turnover frequency before and after nanoparticle redispersion into single atoms.

Interest in Pd/zeolites as automobile exhaust catalysts has driven research focused on understanding the chemical environment of Pd active centers. Here, we studied the effects of hydrothermal deactivation on a Pd/BEA hydrocarbon trap. Hydrothermal aging (HTA) led to decreased ethylene storage capacity and oxidation activity. Characterization by x-ray absorption spectroscopy (XAS) and NOx adsorption indicated that for lower Si/Al ratio samples, the amount of ion exchanged Pd increased after HTA. Temperature programmed desorption of NOx after NO exposure revealed two desorption peaks, with a high temperature peak that increased in area with increased aging temperature. Based on infrared spectroscopy, XAS and reactor results, the high temperature desorption peak is consistent with NO stored on Pd+. The Pd+ formed during NO adsorption, existing originally as [Pd(OH)]+. This species was less active in ethylene storage and oxidation than the original Pd2+ species. [Pd(OH)]+ stored NOx to a higher temperature than Pd2+.

Sulfur poisoning of Cu-SSZ-13 is a function of the catalyst's oxidation ability.

Fully ion-exchanged Pd/SSZ-13 model passive NOx adsorbers (PNA) exhibit significant NOx storage capability, achieving NOx-to-Pd ratios of 1, and NOx desorption at higher temperatures, where downstream NOx reduction catalysts are active, under simulated exhaust conditions. However, CO has been found to induce PNA degradation, which limits the potential for practical application. In this study, in an attempt to understand the consequences of limiting CO exposure, we integrated a model Pt/Al2O3 diesel oxidation catalyst (DOC) with a Pd/SSZ-13 PNA and performed NOx adsorption and temperature-programmed desorption (TPD) cycles with the PNA, the DOC, and the DOC+PNA integrated system. Despite the high initial NOx-to-Pd ratio, Pd/SSZ-13 experienced significant degradation over 15 adsorption and desorption cycles when including CO, with the NOx-to-Pd ratio dropping from 0.98 to 0.75. CO oxidation over the DOC+PNA integrated system lights off at significantly lower temperature compared to the PNA, limiting the PNA CO exposure at temperatures above 200 degrees C. As a result, the durability of the DOC+PNA integrated system is enhanced, and only a 0.02 decrease in NOx-to-Pd ratio was observed over the 15 test cycles. A further benefit with integration of the PNA with the DOC was a lower temperature NOx release, within a more practical temperature window. This is due to the enhanced oxidation activity of the DOC+PNA integrated system and consequently an early onset of NO2 formation, which was found to trigger the low temperature NOx release. Low temperature NOx adsorption and TPD experiments with controlled exposure of CO and NO2 reveal two types of NOx storage mechanisms, one of which is destabilized by the presence of NO2, leading to the evolution of a lower temperature NOx release. Overall, integrating the PNA with a highly active low temperature CO oxidation catalyst was beneficial by lowering the NOx release temperature window and leading to significantly less NOx capacity loss.

Pd-based zeolite materials have gained significant attention as passive NOx adsorbers (PNAs) for diesel-engine cold-start NOx mitigation due to their ability to store NOx at low temperatures. Pd/ZSM-5 is a promising PNA candidate, however, the NO adsorption mechanism over this material is not well understood. This study combines flow reactor experiments and surface spectroscopy to investigate NO adsorption under a variety of conditions. The state of hydration of the Pd cations played an important role in determining the impact of H2O and CO concentration on the PNA performance. Below 150 degrees C, the inhibition effect of H2O on NO adsorption was mitigated by CO. Due to dehydration, neither H2O nor CO had an impact on NO uptake at 150 degrees C, where maximum NO storage capacity of the PNA was also observed. The observed gas composition and temperature effects on NO adsorption and formation of surface intermediates ultimately inform a proposed mechanism.

Hydrocarbon traps for exhaust emissions control adsorb hydrocarbons in low temperature exhaust and release them as the exhaust warms up. In this work, a Pd/BEA hydrocarbon trap was tested under lean exhaust conditions using ethylene and dodecane as model hydrocarbons. Ethylene uptake was partially inhibited by CO and H2O when fed separately. When both were added, the loss in ethylene uptake was 90% relative to the condition with no H2O or CO. Dodecane uptake was unchanged under all conditions tested. During a temperature ramp, ethylene desorbed and was combusted to CO2 and H2O over active Pd centers. Further, oxidation light-off of dodecane generated an exotherm which caused rapid desorption of the remaining hydrocarbon species from the zeolite. For both hydrocarbons, CO co-feed led to a decreased oxidation light-off temperature, and therefore lower desorption temperature. By pretreating the catalyst in CO and H2O at 80 °C, and even after removing CO from the feed, the enhanced oxidation light-off behavior was observed. DRIFTS characterization shows that some form of oxidized Pd was reducible to Pd0 by CO at 80 °C only in the presence of H2O. Further, this reduction appears reversible by high temperature oxygen treatment. We speculate that this reduced Pd phase serves as the active site for low temperature hydrocarbon oxidation.

Low temperature combustion (LTC) diesel engines are more fuel efficient and have coincident lower temperature exhaust gas and higher CO and hydrocarbon exhaust gas concentrations than today's standard diesel engine. To meet regulations, diesel oxidation catalysts (DOCs) will need to be improved and optimized to handle these higher CO and hydrocarbon concentrations emitted and the simultaneous lower exhaust temperatures. Pt-Pd bimetallic catalysts are often used as oxidation catalysts. Here, the mechanistic effects of water on CO and C3H6 oxidation were studied over model monometallic Pt and Pd catalysts, and a bimetallic 1:1 Pt-Pd/ gamma-Al2O3 catalyst. Water in the reaction mixture improved CO oxidation light-off for the monometallic Pd and Pt-Pd catalysts and had a negative impact on light-off over the monometallic Pt catalyst. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that more CO adsorbed on the Pd and Pt-Pd catalysts when water was present, particularly at the particle/support interface, while the opposite occurred on the Pt catalyst. For C3H6 oxidation, water in the reaction showed a negative impact on the light-off temperature for the Pd catalyst and a positive effect for the Pt catalyst. The Pt-Pd catalyst showed intermediate light-off behavior between the two monometallic catalysts. In terms of selectivity, slightly more CO and acetic acid were formed over the Pd and PtPd catalysts when water was in the reaction mixture, while less CO was formed over the Pt catalyst, but overall, the selectivity towards certain partial oxidation products was not greatly affected with the addition of water to the reaction mixture.

Metal ions exchanged on zeolites represent a unique bridge between heterogeneous solid materials and homogeneous inorganic chemistry. The complexing of exchanged metal ions with H2O or NO, is of pa...

The three-way NOx storage catalyst (TWNSC) combines components from a conventional three-way catalyst (TWC) and NOX storage and reduction (NSR) catalyst to improve NOX emissions while exploiting the higher fuel economy of lean-burn gasoline vehicles. The performance of a commercial monolithic TWNSC was studied to understand the NOX trapping and reduction performance over a range of conditions with emphasis on identifying conditions leading to optimal performance in terms of a standalone TWNSC or one coupled with a downstream selective catalytic reduction (SCR) device. Using H2, CO, and C3H6 in various combinations, the impact of cycle timing (cycle time, rich duty fraction), reductant and O2 feed concentrations, and feed temperature on NOX conversion and product (N2, NH3, N2O) selectivities was determined. Steady state experiments were conducted to assess catalyst activity and selectivity and to help interpret the phenomena observed during lean-rich switching. Cycling experiments reveal maxima in the NOX conversion and ammonia-to-NOx ratio (ANR) at distinct, intermediate cycle times. The existence of operating conditions giving these maxima depends on the reductant type, feed temperature, and O2 feed concentration. For example, a large disparity in the lean/rich ratio (stoichiometric number) of the lean and rich feeds tends to lead to a NOx conversion maximum. Where possible, the data trends are interpreted in terms of known performance features of the TWC and NSR catalysts. The study findings provide guidance for optimizing the TWNSC formulation and operation strategy.

There have been ongoing research efforts focused on layering or zoning different washcoats/active metals on the catalysts constituting diesel aftertreatment systems: the diesel oxidation catalyst (DOC), the selective catalytic reduction (SCR) catalyst, the lean NOX trap (LNT), the ammonia slip catalyst (ASC), and the diesel particulate filter (DPF). This review paper aims to shed insight into the state-of-the-art research on catalyst design in this area and how these catalyst designs may evolve to tackle engine emission reductions in the future. First, we discuss the motivation for zoning or layering catalysts and pioneering work on three-way catalyst (TWC) design for reducing gasoline engine emissions; then, we focus on the catalytic systems used for diesel exhaust aftertreatment. The configuration of the aftertreatment systems for diesel engines generally consist of an oxidation catalyst for hydrocarbon (HC), CO, and NO oxidation (over the DOC), a NOX reduction catalyst (over one or combined SCR/LNT/ASC catalysts), and a particulate matter (PM) filter (using a DPF). The research to date consistently demonstrates that zoning and layering catalyst regions leads to improved performance and/or smaller system volumes required.