NO2 uptake under practically relevant conditions on BaO/Pt(1 1 1)

The formation of nitrites and nitrates (Ba(NOx)2) under practically relevant conditions (PNO2PNO2 up to 1.0 Torr and T = 500 K) and their thermal decomposition on BaO (>20 monolayer equivalent (MLE))/Pt(1 1 1) were studied using temperature programmed desorption (TPD), infrared reflection absorption (IRA), and X-ray photoelectron (XP) spectroscopies. The exposure of BaO to 1.0 × 10−8 Torr NO2 at 500 K leads to the formation of a Ba(NOx)2 layer with small, disordered crystalline nitrate clusters. Under these conditions (PNO2=1.0×10−8 TorrPNO2=1.0×10−8 Torr and T = 500 K) only the top portion of the BaO layer converts to Ba(NOx)2 and the nitrites in this Ba(NOx)2 layer stay without converting completely to nitrates even after 100 min of NO2 exposure. In the thermal decomposition of Ba(NOx)2, first nitrites decompose, releasing NO and then the decomposition of nitrates occurs via two pathways releasing NO2 and NO + O2. At 500 K and PNO2≥1.0×10−7 TorrPNO2≥1.0×10−7 Torr, first NO2 reacts with BaO to form small disordered crystalline Ba(NO3)2 particles and then these particles agglomerate to form large, well-ordered (bulk-like) crystalline nitrates as the NO2 exposure increases. The thermal decomposition of these well-ordered, bulk-like crystalline nitrate aggregates occurs in two steps releasing NO2 and NO + O2 in each step in two different temperature regions. NO2 pressure ≥1.0 × 10−5 Torr is required for the complete oxidation of initially formed nitrites to nitrates and the full nitration of the BaO layer at 500 K sample temperature.

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► At PNO2<10−7 TorrPNO2<10−7 Torr disordered Ba(NO3)2 and Ba(NO2)2 form at 500 K.
► At PNO2<10−5 TorrPNO2<10−5 Torr ordered, bulk-like Ba(NO3)2 forms at 500 K.
► The disorder/order Ba(NO3)2 phase transition is reversible.
► Two-step Ba(NO3)2 decomposition.