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•Many Rh catalysts were tested for N2O decomposition under diesel conditions.•Rh/CeO2 is the only catalyst with good activity in a wet feed after aging.•Aging does not deactivate ...Rh/CeO2 but significantly increases its activity.•Removal O2 from the catalyst has been demonstrated to be the rate limiting step.
Numerous Rh catalysts were evaluated for N2O decomposition for automotive applications. Some Rh-containing spinel materials exhibit excellent fresh activities in the absence of H2O but become inactive after hydrothermal aging or when tested in a wet feed. Rh catalysts supported on zeolites can be very active in a dry feed even after aging but are extremely sensitive to H2O. Rh/CeO2 is an exceptional catalyst for this reaction in the presence of both H2O and O2. Hydrothermal aging (750 °C/20 h) significantly increases its activity. A similar activity enhancement was found by calcining the support before Rh impregnation. XPS results show a surface enrichment of Rh species on the aged Rh/CeO2 catalyst relative to the fresh catalyst. Aberration corrected STEM images reveal that Rh is buried in the bulk on the fresh catalyst and pulled out onto the surface of the support after thermal treatments. All catalysts are inhibited by H2O with the zeolite-based Rh catalysts being the worst. The aged Rh/CeO2 catalyst is less sensitive to H2O relative to others. DRIFTS data show that H2O sensitivity is related to catalyst hydrophilicity; a high coverage of OH groups on a catalyst reduces its N2O decomposition activity. H2-TPR results show that a Rh/CeO2 catalyst can be readily reduced at < 100 °C. On a reduced Rh/CeO2 catalyst, near complete N2O conversion can be obtained with a lean feed at 250 °C for a duration equivalent to its oxygen storage capacity. The N2O-DRIFTS experiments over a pre-reduced Rh/CeO2 catalyst show that Ce3+ sites are quickly oxidized to Ce4+ upon contacting N2O at room temperature, resulting N2 and adsorbed O, with the latter being an efficient oxidizer. Based on these results, a N2O decomposition mechanism is proposed for the Rh/CeO2 catalyst.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
A high-throughput (HT) screening platform developed at hte with the application focus on automotive catalysis is described. hte HT units are configured for performing steady-state testing, as well as ...dynamic tests with fast feed switches, such as lean/rich excursions for the evaluation of NOx storage capacity and efficiency of lean NOx traps (LNT), ammonia storage capacity for selective catalytic reduction (SCR), evaluation of oxygen storage capacity (OSC), as well as lambda sweep tests for screening of three-way catalysts (TWC). Even though catalysts are screened on a rather small scale (~100 mg powder), experience showed that dosing rather complex gas mixtures in concentrations close to that found in real exhaust for the given application is mandatory to generate relevant data. The objective of this work is to give additional insight into HT technology. In the industrial research laboratory, HT screening has matured to become a reliable approach for rapid screening of both reaction parameter spaces, as well as material properties relevant for exhaust gas catalyst development. Due to the speed of optimized screening involving 48 parallel reactors, automated handling of primary data is an imported requirement. Software for data reduction, like estimation of light-off temperature, needs to be robust and handle results for diverse sample libraries in an unattended fashion. In combination with the statistical design of experiment and multivariate data analysis, HT testing has become a valuable enhancement to automotive catalyst development.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
At hte the high throughput (HT) approach is applied in the field of environmental catalysis on a routine basis. Research programs for automotive applications require validated screening protocols for ...conditions relevant to engine exhaust as well as experimental measures to ensure quality control using statistical design of experiment. To illustrate the HT approach for a test protocol with dynamic feed switches in a 48-fold reactor, 15 model catalysts for lean NOx traps (LNT) were prepared and screened fresh and after 800 °C hydrothermal aging. In the fresh state, highest NOx efficiency was found at 350–450 °C. A ranking of BaO > SrO > CaO was found as the most active NOx storage components when used as dopants on alumina. 800 °C aging results in a severe performance loss. Using XRD and BET analysis, Pt sintering is identified as most likely cause. These findings agree well with results from conventional tests reported in the literature.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
A theoretical study of the oxidative addition of C−C vs C−H bonds to a rhodium(I) complex with PCP-type ligands has been carried out. Special attention has been paid to the effect of different bulky ...substituents at the phosphorus atoms of the chelate ligand. Therefore, B3LYP/lanl2dz+p//B3LYP/lanl2dz and ONIOM(B3LYP/lanl2dz+p:B3LYP/lanl2dz)//ONIOM(B3LYP/lanl2dz:HF/lanl1mb) methods have been utilized. According to the calculations, C−H activation is always the kinetically favored process (ΔΔE ⧧ 20 kJ·mol-1), though the C−C activation product is more stable (ΔΔE 20 kJ·mol-1). C−H addition is a reversible process; the product of the C−H activation can interconvert to the C−C activation product via an intermediate structure. Bulky substituents are found to increase the barrier for C−H activation relative to that for C−C activation. With additional ligands (e.g., phosphines), hexacoordinate complexes are formed. This is more favorable for the C−C activation products. Our calculations show that the activation reaction proceeds via complexes with a pentacoordinated rhodium atom. Thus, in the presence of donor ligands, the activation reaction is inhibited.
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IJS, KILJ, NUK, PNG, UL, UM
The development of advanced emission control systems to meet the strict regulations requires efficient and flexible material screening capabilities. Here, a high throughput test unit is described. ...Two case studies demonstrate the rapid screening of relevant parameter spaces and material functionalities which can be used in product development. One involves steady‐state testing of hydrocarbon oxidation in Diesel aftertreatment systems, while the other shows the evaluation of oxygen storage capacity in the optimization of three‐way catalysts for gasoline engines.
Hydrocarbon oxidation is an important functionality of exhaust gas purification systems. In the development of such systems, material screening for the evaluation of heterogeneous catalysts is an essential part. Here, a high throughput screening method is developed, offering a correlation between catalyst composition and performance.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Complex multi-element lead structures of mixed metal oxides that may be identified as hits during high throughput experimentation (HTE) campaigns, can be deconvoluted retrospectively on the basis of ...simple binary and ternary oxides as illustrated in the current example of a hit found in an ammoxidation reaction. On the basis of the performance of the simple binary and ternary mixed metal oxides structure property relationships can be established, that give insight into the roles of the different components of the complex mixed metal oxides and may also help in establishing a reaction mechanism and converting the hit into a development candidate.
The mechanism of competitive intramolecular C−C and C−H bond activation in PCN pincer complexes of rhodium(I) has been studied computationally. Experimentally this system exhibits exclusive cleavage ...of an aryl−methyl C−C bond; no products corresponding to C−H bond cleavage in the methyl group have been observed. The calculated relative energies indicate that the C−C activation product is the most stable one (ΔE < −50 kJ·mol-1 relative to the C−H activation product) and that its formation is irreversible. C−H activation is not kinetically forbidden, but it is fast and reversible. Because of the weak Rh−N bond several intermediates on the C−H activation paths are accessible; therefore low concentrations of these species are a plausible reason for the experimental result. Further, this study reveals that the unique preference of C−C bond activation in the PCN system results from insufficient stabilization of the C−H activation product due to strain. To model the complete ligand including all bulky substituents used in the experiment, the ONIOM method has been employed, using the B3LYP/lanl2dz level for the inner layer and the HF/lanl1mb level to model substituent effects.
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In this theoretical study on the Heck reaction we explore the feasibility of an alternative pathway that involves a PdII/PdIV redox system. Usually, the catalytic cycle is formulated based on a ...Pd0/PdII mechanism. We performed quantum chemical calculations using density functional theory on a model system that consisted of diphosphinoethane (DPE) as a bidentate ligand and the substrates ethylene and phenyl iodide to compare both mechanisms. Accordingly, the PdII/PdIV mechanism is most likely to occur in the equatorial plane of an octahedral PdIV complex. The energy profiles of both reaction pathways under consideration are largely parallel. A major difference is found for the oxidative addition of the C−I bond to the palladium centre. This is a rate‐determining step of the PdII/PdIV mechanism, while it is facile for a Pd0 catalyst. The calculations show that intermediate ligand detachment and re‐attachment is necessary in the course of the oxidative addition to PdII. Therefore, we expect the PdII/PdIV mechanism to be only feasible if a weakly coordinating ligand is present.
In a theoretical study the mechanism of the Heck reaction has been explored. Density functional theory calculations on a model system show that major steps of a PdII/PdIV and a Pd0/PdII pathway are structurally similar. The major difference is found for the oxidative addition step, which has a high barrier in the PdII/PdIV system (two intermediates in this step are shown here). All other steps require comparable activation energies.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
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