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 NOx storage material. The model system comprises BaAl2xO1+3x particles of different size and stoichiometry, prepared under ultrahigh-vacuum (UHV) conditions on Al2O3/NiAl(110). Adsorption and reaction of NO2 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 BaAl2xO1+3x particles has been investigated previously. In this work we focus on the effect of particle size and stoichiometry on the reaction with NO2. Particles of different size and of different Ba2+ : Al3+ 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 BaAl2xO1+3x particles high temperature (800 K) oxygen treatment gives rise to particle ripening, which has a minor effect on the NO2 uptake behavior, however. STM shows that the morphology of the particle system is largely conserved during NO2 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, NO2 uptake and release is a fully reversible process. For large BaAl2xO1+3x particles, aggregates with different Ba2+ : Al3+ surface ion ratio were prepared. It was shown that the stoichiometry has a major effect on the kinetics of NO2 uptake. For barium-aluminate-like particles with high Al3+ concentration, the formation of nitrites and nitrates on the BaAl2xO1+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(NO3)2. This bulk Ba(NO3)2 exhibits substantially higher thermal stability and undergoes restructuring and sintering before it decomposes at 700 K. For Ba2+-rich BaAl2xO1+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 NO2 exposure gives rise to the formation of amorphous bulk Ba(NO3)2 particles even at 300 K.