Strong Size Effects in Supported Ionic Nanoparticles: Tailoring the Stability of NO x Storage Catalysts

Abstract

Based on a well-defined model-catalyst approach, we study the particle size dependent properties of NO x storage materials. The single-crystal based model systems are prepared on an ordered Al 2 O 3 film, on which BaO nanoparticles are grown under ultrahigh-vacuum (UVH) conditions. Particle size and density are characterized by scanning tunneling microscopy (STM). The interaction with NO 2 is probed by molecular beam (MB) methods in combination with time-resolved IR reflection absorption spectroscopy (TR-IRAS). It is found that both, the stability and the formation kinetics of alumina supported barium nitrate nanoparticles show a strong dependence on particle size. Very small BaO particles are rapidly converted into nitrates, however, the resulting aggregates exhibit a strongly reduced thermal stability. Surface and bulk nitrate and nitrate features are identified by means of vibrational spectroscopy. It is concluded that the size dependencies are related to the formation and decomposition of surface-related BaNO x species the decomposition temperature of which can be tuned over an exceptionally large temperature interval. It is suggested that the stability of these surface NO x species is strongly modified by the underlying support.

Model NOx Storage Materials at Realistic NO2 Pressures

“…[25,31,37] Herein, we focus on particles prestabilized by annealing in O 2 . [26,29] We assumed that these particles approached the stoichiometry of barium aluminate (BaAl 2 O 4 ) upon annealing.…”

“…[16][17][18][19][20] More recently, the reaction of NO 2 with BaO deposits on Al 2 O 3 / NiAlA C H T U N G T R E N N U N G (110) were studied by Szanyi et al [21][22][23][24] and our group. [25][26][27][28][29][30][31] Some of the key results of these studies may be summarized as follows: A strong interaction with the support gave rise to interdiffusion of Al 3 + and Ba 2 + ions, resulting in the formation of barium aluminate-like nanoparticles. [29] As the exact stoichiometry was not established, we denoted these particles BaAl 2x O 1 + 3x .…”

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

“…Pioneering work on the interaction of NO 2 with model BaO surfaces was published by Bowker and co-workers, [16,32,33] Szanyi and co-workers [23,[34][35][36][37] and recently by us. [18,[38][39][40] Here we present the first model study on a "complete" NSR model system, involving the Ba storage compound and co-deposited noble metal nanoparticles on an alumina model support. By taking advantage of the full spectrum of surface-science methods, the structural and chemical properties of the model system were characterized and the degree of metal-oxide interaction controlled in great detail.…”

Oxidation of barium on the surface of nanothick chromium oxide films grown on the (110) molybdenum substrate

“…Both barium oxide and barium coexisted on the sample surface. In such situation barium oxide can serve as NO x storage and reduction (NSR) catalyst [13,14] and barium as a promoter. The results demonstrate a strong effect of thickness and surface morphology of chromium oxide films as well as of substrate-oxide interface condition on the film oxidizing ability with respect to barium.…”

“…This barium coverage decreases the work function of (110) molybdenum by %3 eV [14]. Then a 3 ML chromium film was deposited and oxidized on this surface.…”