Role of the Exposed Pt Active Sites and BaO2 Formation in NO_x Storage Reduction Systems: A Model Catalyst Study on BaO_x/Pt(111)

摘要

BaO_x(0.5 MLE - 10 MLE)/Pt(111) (MLE: monolayer equivalent) surfaces were synthesized as model NO_x storage reduction (NSR) catalysts. Chemical structure, surface morphology, and the nature of the adsorbed species on BaO_x/Pt(111) surfaces were studied via X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED). For θ_(BaO_x)< 1 MLE, (2 X 2) or (1 X 2) ordered overlayer structures were observed on Pt(111), whereas BaO(llO) surface termination was detected for θ_(BaO_x) - 1-5 MLE. Thicker films (θ_(BaO_x) ≥ 2.5 MLE) were found to be amorphous. Extensive N02 adsorption on BaO_x( 10 MLE) /Pt (111) yields predominantly nitrate species that decompose at higher temperatures through the formation of nitrites. Nitrate decomposition occurs on BaO_x(l0 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 BaO_x(l0 MLE)/ Pt( 111) surface at 873 K facilitates the BaO2 formation and results in the agglomeration of BaO_x domains leading to the generation of exposed Pt( 111) surface sites. BaO2 formed on BaO_x(10 MLE)/Pt(111) is stable even after annealing at 1073 K, whereas on thinner films (θ_(BaO_x) = 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 BaO_x coverage within θ_(BaO_x)= 0-5 to 1.0 MLE. Nitrate decomposition occurs at a rather constant temperature range of 650-700 K for thicker BaO_x overlayers (2.5 MLE < θ_(BaO_x) < 10 MLE). These two distinctly characteristic BaO_x-coverage-dependent 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 NO_x storage domain size.