Role of the exposed Pt active sites and BaO2 formation in nox storage reduction systems: a model catalyst study on BaOx/Pt(111)

Abstract

BaOx(0.5 MLE - 10 MLE)/Pt(111) (MLE: monolayer equivalent) surfaces were synthesized as model NOx storage reduction (NSR) catalysts. Chemical structure, surface morphology, and the nature of the adsorbed species on BaOx/Pt(111) surfaces were studied via X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED). For θBaOx < 1 MLE, (2 2) or (1 2) ordered overlayer structures were observed on Pt(111), whereas BaO(110) surface termination was detected for θBaOx = 1.5 MLE. Thicker films (θBaOx g 2.5 MLE) were found to be amorphous. Extensive NO2 adsorption on BaOx(10 MLE)/Pt(111) yields predominantly nitrate species that decompose at higher temperatures through the formation of nitrites. Nitrate decomposition occurs on BaOx(10 MLE)/Pt(111) in two successive steps: (1) NO(g) evolution and BaO2 formation at 650 K and (2) NO(g) + O2(g) evolution at 700 K. O2(g) treatment of the BaOx(10 MLE)/ Pt(111) surface at 873 K facilitates the BaO2 formation and results in the agglomeration of BaOx domains leading to the generation of exposed Pt(111) surface sites. BaO2 formed on BaOx(10 MLE)/Pt(111) is stable even after annealing at 1073 K, whereas on thinner films (θBaOx = 2.5 MLE), BaO2 partially decomposes into BaO, indicating that small BaO2 clusters in close proximity of the exposed Pt(111) sites are prone to decomposition. Nitrate decomposition temperature decreases monotonically from 550 to 375 K with decreasing BaOx coverage within θBaOx = 0.5 to 1.0 MLE. Nitrate decomposition occurs at a rather constant temperature range of 650700 K for thicker BaOx overlayers (2.5 MLE < θBaOx < 10 MLE). These two distinctly characteristic BaOx-coveragedependent nitrate decomposition regimes are in very good agreement with the observation of the so-called “surface” and “bulk” barium nitrates previously reported for realistic NSR catalysts, clearly demonstrating the strong dependence of the nitrate thermal stability on the NOx storage domain size.