Model NO x Storage Materials at Realistic NO 2 Pressures

The interaction of NO 2 with single‐crystal‐based model NO x storage materials, consisting of barium aluminate nanoparticles on Al 2 O 3 /NiAl(110), are investigated by time‐resolved infrared reflection absorption spectroscopy (TR‐IRAS) at realistic NO 2 partial pressures up to 1.75 mbar. The data is compared to spectra obtained under ultrahigh vacuum (UHV) conditions on the same model system. At 300 K, the NO 2 uptake at pressures around 1 mbar proceeds through rapid initial formation of surface nitrites and nitrates, similar to that under UHV conditions. The vibrational spectra of the surface species formed at realistic NO 2 pressures are comparable to those for species formed under UHV conditions. Beyond the formation of surface species, the formation of bulk nitrates occurs, but is kinetically strongly hindered. At a very low rate, the formation of a disordered barium bulk nitrate is detected. At 500 K, this kinetic hindrance is overcome and the available Ba 2+ is quantitatively converted to bulk Ba(NO 3 ) 2 . The IRAS spectrum of these Ba(NO 3 ) 2 particles differs characteristically from those obtained for nitrate multilayers formed upon incomplete conversion under UHV conditions. In addition to the formation of bulk Ba(NO 3 ) 2 , a more weakly bound disordered nitrate species is formed. This species gives rise to a dynamic NO 2 uptake and release well below the decomposition temperature of bulk Ba(NO 3 ) 2 . The experiments show that model studies under UHV conditions mainly provide information on the initial reaction mechanism, whereas the observation of actual bulk NO x storage phases requires experiments at realistic temperatures and pressures.