NO 2 uptake under practically relevant conditions on BaO/Pt(1 1 1)

Abstract

The formation of nitrites and nitrates (Ba(NO x ) 2) under practically relevant conditions ( P NO 2 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 NO 2 at 500 K leads to the formation of a Ba(NO x ) 2 layer with small, disordered crystalline nitrate clusters. Under these conditions ( P NO 2 = 1.0 × 10 − 8 Torr and T = 500 K) only the top portion of the BaO layer converts to Ba(NO x ) 2 and the nitrites in this Ba(NO x ) 2 layer stay without converting completely to nitrates even after 100 min of NO 2 exposure. In the thermal decomposition of Ba(NO x ) 2, first nitrites decompose, releasing NO and then the decomposition of nitrates occurs via two pathways releasing NO 2 and NO + O 2. At 500 K and P NO 2 ≥ 1.0 × 10 − 7 Torr , first NO 2 reacts with BaO to form small disordered crystalline Ba(NO 3) 2 particles and then these particles agglomerate to form large, well-ordered (bulk-like) crystalline nitrates as the NO 2 exposure increases. The thermal decomposition of these well-ordered, bulk-like crystalline nitrate aggregates occurs in two steps releasing NO 2 and NO + O 2 in each step in two different temperature regions. NO 2 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.