Potential and Limitations of Natural Chabazite for Selective Catalytic Reduction of NOx with NH3

The potential of the natural chabazite for the selective catalytic reduction (SCR) of NOx with NH₃ is evaluated in the present work. Activity tests were performed under technically relevant reaction and temperature conditions for the fresh and hydrothermally aged catalysts. The natural chabazite before and after alkaline removal as well as after iron and copper addition were compared. The structural as well as surface and bulk properties were elucidated by a variety of complementary characterization techniques, i.e. XRD, XPS, EPR, BET, NH₃‐TPD, ex situ and in situ XAS. The results indicate that an important facet for using the natural chabazite for the standard and fast SCR reactions is the removal of alkaline metals, which at the same time also leads to a partial change of the structure and the size of the iron‐containing particles. The performance and especially the hydrothermal stability can be further improved by copper addition.     

Formation and stability of barium aluminate and cerate in NOx storage-reduction catalysts

The formation and stability of BaAl₂O₄ and BaCeO₃ in Pt-Ba/Al₂O₃ and Pt-Ba/CeO₂ based NO_x storage-reduction (NSR) catalysts has been investigated using kinetic measurements, X-ray diffraction, thermal analysis and X-ray absorption spectroscopy. In as-prepared state, the Ba-component in the NSR catalysts was made up of amorphous BaO and BaCO₃. The formation of BaAl₂O₄ started above 850 °C, whereas the formation of BaCeO₃ was already observed at 800 °C and was faster than that of BaAl₂O₄. The stability of BaAl₂O₄ and BaCeO₃ in various liquid and gaseous atmospheres was different. BaAl₂O₄ was rapidly hydrated at room temperature in the presence of water and transformed to Ba(NO₃)₂ and γ-alumina in the presence of HNO₃, whereas BaCeO₃ was decomposed to much lower extent under these conditions. Interestingly, BaCeO₃ was transformed to Ba(NO₃)₂/CeO₂ in the presence of NO₂/H₂O at 300–500 °C. Also, the presence of CO₂ led to decomposition of barium cerate, which has important consequences for the catalyst ageing under NO_x-storage conditions and can be exploited for regeneration of thermally aged NSR-catalysts.     

Solvent Influence on the Hydrodeoxygenation of Guaiacol over Pt/SiO2 and Pt/H‐MFI 90 Catalysts

Second generation biofuels are produced in the bioliq® process at the Karlsruhe Institute of Technology via gasification of pyrolysis oil and synthesis of gasoline from the emerging synthesis gas. An alternative strategy is the direct upgrading of the pyrolysis oil by hydrodeoxygenation (HDO). The present study reports on the HDO of guaiacol as one of the phenolic compounds strongly abundant in such mixtures. Special focus was laid on the solvent influence using Pt‐based catalysts. Higher HDO ability was seen using nonpolar solvents and acidic supports. Characterization of the catalysts before and after the test showed that the solvent did not only influence the reactivity, but also the catalyst stability.     

Flame-synthesized LaCoO3-supported Pd: 2. Catalytic behavior in the reduction of NO by H2 under lean conditions

A 0.5 wt% Pd/LaCoO₃, prepared by flame-spray pyrolysis (FP), was tested as catalyst for the low-temperature selective reduction of NO by H₂ in the presence of excess O₂. In particular, the effect of the precalcination and prereduction temperature on catalytic activity was compared with that of a similar Pd/LaCoO₃ sample prepared by impregnation with a Pd solution of FP-prepared LaCoO₃. The FP-made catalyst allowed full NO conversion at 150 °C, with 78% selectivity to N₂, thus outperforming the catalytic behavior of the corresponding sample prepared by impregnation. The higher activity of the FP-made catalyst has been attributed to the formation of segregated Co metal particles, not present in the impregnated sample, formed during the precalcination at 800 °C, followed by reduction at 300 °C. Two reaction mechanisms can be deduced from the temperature-programmed experiments. The first of these, occurring at lower temperatures, indicates cooperation between the Pd and Co metal particles, with formation of active nitrates on cobalt, successively reduced by hydrogen spillover from Pd. The second, occurring at higher temperature, allows 50% conversion of NO, with >90% selectivity to N₂, and involves N adatoms formed by dissociative NO adsorption over Pd. Prereduction at 600 °C led to a slight increase in catalytic activity, due to the formation of a PdCo alloy, which is more stable on reoxidization compared with Pd alone. Moreover, the cooperative reaction mechanism seems to be favored by the proximity of Co and Pd in metal particles.     

Comparative study of structural properties and NOx storage-reduction behavior of Pt/Ba/CeO2 and Pt/Ba/Al2O3

Differences in the NO_x storage-reduction (NSR) behavior of Pt/Ba/CeO₂ and Pt/Ba/Al₂O₃ have been identified and traced to their different chemical and structural properties. The results show that Pt/Ba/CeO₂ exhibits inferior NO_x storage and, particularly, reduction (regeneration) activity compared to the Al₂O₃ supported catalyst. The incomplete reduction of the stored NO_x-species in Pt/Ba/CeO₂ seems to be caused by a faster and more profound reoxidation of Pt particles during the lean period as evidenced by in situ X-ray absorption spectroscopy. Interestingly, the reduction activity could be significantly improved by a pre-reduction step at mild conditions. Exposure of the Pt/Ba/CeO₂ catalyst to reducing H₂ atmosphere in the temperature range 300–500 °C lead to a moderate increase of Pt particle size which beneficially influenced the regeneration activity. In contrast, pre-reduction at temperatures above 500 °C was unfavorable and resulted in a severe decrease of the regeneration activity, probably due to migration of the partially reduced CeO₂ onto the surface of Pt particles.     

Gold-Catalyzed Aerobic Oxidation of Benzyl Alcohol: Effect of Gold Particle Size on Activity and Selectivity in Different Solvents

The effect of the size of gold particles deposited on CeO₂ and TiO₂ supports on their catalytic behavior in the aerobic oxidation of benzyl alcohol in different solvents (mesitylene, toluene, and supercritical carbon dioxide) has been investigated. The size of supported gold particles deposited via a colloidal route was in the range 1.3–11.3 nm, as determined by means of EXAFS and HAADF-STEM measurements. The catalytic performance of the supported gold catalysts in the different solvents revealed a significant effect of the gold particle size. Optimal activity was observed for catalysts with medium particle size (ca. 6.9 nm) whereas smaller and bigger particles showed inferior activity. Identical trends for the activity–particle size relationship were found using Au/CeO₂ and Au/TiO₂ for the reaction at atmospheric pressure in conventional solvents (mesitylene, toluene) as well as under supercritical conditions (scCO₂). Selectivity to benzaldehyde was only weakly affected by the gold particle size and mainly depended on reaction conditions. In supercritical CO₂ (scCO₂) selectivity was higher than in the conventional solvents under atmospheric pressure. All catalysts tested with particle sizes ranging from 1.3 to 11.3 nm showed excellent selectivity of 99% or higher under supercritical conditions.     

Abgasnachbehandlung in mobilen Systemen: Stand der Technik,Herausforderungen und Perspektiven

Looking at exhaust‐gas after‐treatment systems in its entirety leads to further improvement of emission control devices and to the accomplishment of future challenges such as lower legislative emission limits, new fuels, and more efficient engine concepts. This article provides an overview on the state‐of‐the art mobile exhaust‐gas after‐treatment devices. Current and future challenges are discussed in the light of present approaches such as hierarchical modeling reaching from DFT computations of molecular processes to CFD simulation of entire lines of exhaust‐gas cleaning devices, close‐to‐reality emission control test benches and aging procedures, on‐board diagnostics, and catalyst characterization at operating conditions and preferentially all length scales. In future, knowledge‐based robot‐controlled preparation and dynamic models coupled with information from real operation will significantly support research and development.     

Oscillatory CO Oxidation Over Pt/Al2O3 Catalysts Studied by In situ XAS and DRIFTS

Fresh and mildly aged Pt/Al₂O₃ model diesel oxidation catalysts with small and large noble metal particle size have been studied during CO oxidation under lean burn reaction conditions to gain more insight into the structure and oscillatory reaction behaviour. The catalytic performance, CO adsorption characteristics using in situ DRIFTS and oxidation state using in situ XAS were correlated. Stable and pronounced oscillations only occurred over the catalyst with smaller particle sizes. Characteristic for this catalyst are low-coordinated surface Pt sites (more corner and edge atoms) which seem to become oxidized at elevated temperature as evidenced by in situ DRIFTS and in situ XAS. In situ XAS further uncovered that the oxidation of the Pt surface starts from the end of the catalyst bed and the oxidation state oscillates like the catalytic activity.