Reactions of NO2 with BaO/Pt(111) model catalysts: the effects of BaO film thickness and NO2 pressure on the formation of Ba(NOx)2 species

Abstract

The adsorption and reaction of NO₂ on BaO (<1, ∼3, and >20 monolayer equivalent (MLE))/Pt(111) model systems were studied with temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and infrared reflection absorption spectroscopy (IRAS) under ultra-high vacuum (UHV) as well as elevated pressure conditions. NO₂ reacts with sub-monolayer BaO (<1 MLE) to form nitrites only, whereas the reaction of NO₂ with BaO (∼3 MLE)/Pt(111) produces mainly nitrites and a small amount of nitrates under UHV conditions (P_NO2 ≈ 1.0 × 10⁻⁹ Torr) at 300 K. In contrast, a thick BaO (>20 MLE) layer on Pt(111) reacts with NO₂ to form nitrite–nitrate ion pairs under the same conditions. At elevated NO₂ pressures (≥1.0 × 10⁻⁵ Torr), however, BaO layers at all these three coverages convert to amorphous barium nitrates at 300 K. Upon annealing to 500 K, these amorphous barium nitrate layers transform into crystalline phases. The thermal decomposition of the thus-formed Ba(NO_x)₂ species is also influenced by the coverage of BaO on the Pt(111) substrate: at low BaO coverages, these species decompose at significantly lower temperatures in comparison with those formed on thick BaO films due to the presence of a Ba(NO_x)₂/Pt interface where the decomposition can proceed at lower temperatures. However, the thermal decomposition of the thick Ba(NO₃)₂ films follows that of bulk nitrates. Results obtained from these BaO/Pt(111) model systems under UHV and elevated pressure conditions clearly demonstrate that both the BaO film thickness and the applied NO₂ pressure are critical in the Ba(NO_x)₂ formation and subsequent thermal decomposition processes.