Friedemann Rohr

Interaction of NO2 with Model NSR Catalysts: Metal–Oxide Interaction Controls Initial NOx Storage Mechanism

Using scanning tunneling microscopy (STM), molecular-beam (MB) methods and time-resolved infrared reflection absorption spectroscopy (TR-IRAS), we investigate the mechanism of initial NO(x) uptake on a model nitrogen storage and reduction (NSR) catalyst. The model system is prepared by co-deposition of Pd metal particles and Ba-containing oxide particles onto an ordered alumina film on NiAl(110). We show that the metal-oxide interaction between the active noble metal particles and the NO(x) storage compound in NSR model catalysts plays an important role in the reaction mechanism. We suggest that strong interaction facilitates reverse spillover of activated oxygen species from the NO(x) storage compound to the metal. This process leads to partial oxidation of the metal nanoparticles and simultaneous stabilization of the surface nitrite intermediate.

Nitrite and nitrate formation on model NOx storage materials: on the influence of particle size and composition

A well-defined model-catalyst approach has been utilized to study the formation and decomposition of nitrite and nitrate species on a model NO(x) storage material. The model system comprises BaAl(2x)O(1+3x) particles of different size and stoichiometry, prepared under ultrahigh-vacuum (UHV) conditions on Al(2)O(3)/NiAl(110). Adsorption and reaction of NO(2) has been investigated by molecular beam (MB) methods and time-resolved IR reflection absorption spectroscopy (TR-IRAS) in combination with structural characterization by scanning tunneling microscopy (STM). The growth behavior and chemical composition of the BaAl(2x)O(1+3x) particles has been investigated previously. In this work we focus on the effect of particle size and stoichiometry on the reaction with NO(2). Particles of different size and of different Ba(2+) : Al(3+) surface ion ratio are prepared by varying the preparation conditions. It is shown that at 300 K the reaction mechanism is independent of particle size and composition, involving initial nitrite formation and subsequent transformation of nitrites into surface nitrates. The coordination geometry of the surface nitrates, however, changes characteristically with particle size. For small BaAl(2x)O(1+3x) particles high temperature (800 K) oxygen treatment gives rise to particle ripening, which has a minor effect on the NO(2) uptake behavior, however. STM shows that the morphology of the particle system is largely conserved during NO(2) exposure at 300 K. The reaction is limited to the formation of surface nitrites and nitrates, which are characterized by low thermal stability and completely decompose below 500 K. As no further sintering occurs before decomposition, NO(2) uptake and release is a fully reversible process. For large BaAl(2x)O(1+3x) particles, aggregates with different Ba(2+) : Al(3+) surface ion ratio were prepared. It was shown that the stoichiometry has a major effect on the kinetics of NO(2) uptake. For barium-aluminate-like particles with high Al(3+) concentration, the formation of nitrites and nitrates on the BaAl(2x)O(1+3x) particles at 300 K is slow, and kinetically restricted to the formation of surface species. Only at elevated temperature (500 K) are surface nitrates converted into well-defined bulk Ba(NO(3))(2). This bulk Ba(NO(3))(2) exhibits substantially higher thermal stability and undergoes restructuring and sintering before it decomposes at 700 K. For Ba(2+)-rich BaAl(2x)O(1+3x) particles, on the other hand, nitrate formation occurs at a much higher rate than for the barium-aluminate-like particles. Furthermore, nitrate formation is not limited to the surface, but NO(2) exposure gives rise to the formation of amorphous bulk Ba(NO(3))(2) particles even at 300 K.

Dauerhaltbarkeit von NOx-Nachbehandlungssystemen für Dieselmotoren

Als nachstes Ziel in der Weiterentwicklung von NOx-Speicherkatalysatoren steht die Erfullung der strengen Tier-2-Bin5-Grenzwerte im Fokus. Zur Erreichung dieses anspruchsvollen Ziels ist es notwendig, die Dauerhaltbarkeit des NOx-Speicherkatalysators weiter zu verbessern. Zwei Alterungsmechanismen sind in diesem Zusammenhang ausschlaggebend: die Vergiftung durch Schwefel und die thermische Schadigung des Speicherkatalysators. Beide Alterungsmechanismen sind nicht unabhangig voneinander, da der Katalysator wahrend den regelmasig durchzufuhrenden Entschwefelungen einer hohen thermischen Belastung ausgesetzt ist. Untersuchungen bei Umicore zeigen, wie durch eine deutliche Verbesserung des Entschwefelungsverhaltens, kombiniert mit einer verbesserten Entschweflungsstrategie des NOx-Speicherkatalysators, die thermische Belastung deutlich reduziert werden konnte und damit eine verbesserte Dauerhaltbarkeit gewahrleistet werden kann.

Long-term durability of NOx aftertreatment systems for diesel engines

Lean NOx trap (LNT) systems are currently being improved in order to comply with the more stringent Tier 2 Bin 5 standards. Improving the LNT’s durability is key in this endeavour. The main two aging mechanisms that need to be addressed are thermal deactivation and poisoning with sulphur. The two processes are linked because sulphur poisoning indirectly leads to thermal deactivation through regular desulfations of the catalyst. Those sulphur removal procedures expose the catalyst to considerable thermal stress that reduces its long term performance. Umicore research activities on improved LNT systems and data recorded in durability tests demonstrate substantially improved performance and durability. The main strategies employed were a reduction in the required desulfation temperature of the LNT as well as improved engine control and desulfation strategies.