Role of the exposed Pt active sites and BaO 2 formation in NO x storage reduction systems: A model catalyst study on BaO x/Pt(111)
Journal of Physical Chemistry C
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Please cite this item using this persistent URLhttp://hdl.handle.net/11693/21657
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 BaOx < 1 MLE, (2 × 2) or (1 × 2) ordered overlayer structures were observed on Pt(111), whereas BaO(110) surface termination was detected for θ BaOx = 1.5 MLE. Thicker films (θ BaOx ≥ 2.5 MLE) were found to be amorphous. Extensive NO 2 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(10 MLE)/Pt(111) in two successive steps: (1) NO(g) evolution and BaO 2 formation at 650 K and (2) NO(g) + O 2(g) evolution at 700 K. O 2(g) treatment of the BaO x(10 MLE)/Pt(111) surface at 873 K facilitates the BaO 2 formation and results in the agglomeration of BaO x domains leading to the generation of exposed Pt(111) surface sites. BaO 2 formed on BaO x(10 MLE)/Pt(111) is stable even after annealing at 1073 K, whereas on thinner films (θ BaOx = 2.5 MLE), BaO 2 partially decomposes into BaO, indicating that small BaO 2 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 BaOx = 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 BaOx < 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. © 2011 American Chemical Society.
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