Nox depolama-indirgeme kataliz işleminin BaOx/ Pt(111) ve BaOx/ TiO2/ Pt(111) model katalizörleri yardımıyla möleküler düzeyde anlaşılması

In this work, formation and decomposition pathways of of Ba(NO3)2 on BaO-BaO2 /Pt(111) surfaces were investigated at the molecular level for different BaO-BaO2coverages starting from small 2D islands of 0.5 MLE (MLE: monolayer equivalent) to thick multilayers of 10 MLE via temperature-programmed desorption (TPD), and X-ray Photoelectron Spectroscopy (XPS) and Low Energy Electron Diffraction (LEED). BaOxoverlayers with a surface coverage of ~ 1 MLE reveal long range ordering with (2×2) and/or (1×2) structures while BaOx films with a surface coverage of1.5 MLE yields a BaO(110) termination and thicker films ( ? 5 MLE) were observed to be amorphous. Saturation of thick (10 MLE) BaOxoverlayers with NO2 leads to the formation of nitrates. Nitrate thermal decomposition was demonstrated to proceed through nitrite intermediates. In TPD experimentstwo major pathways fornitrate decomposition were observed: 1) nitrate decomposition yielding only NO evolutionat ~650 K, and 2) nitrate decomposition withNO + O2evolutionat ~700 K. This multi-step decomposition behavior was explained by BaO2 formation during the first stage. The influence of the BaOxdepositionmethod on the morphology of the BaOx overlayers were established: when a thick BaOx layer is prepared using NO2 for Ba oxidation, BaOx overlayer efficiently wets the Pt(111) substrate forming a well-dispersed film. On the other hand, ifa thick BaOx layer is heated in O2 (to 873 K), BaOx overlayer agglomerates into 3D clusters, resulting in the formation of exposed (uncovered) Pt sites. BaOxoverlayers with uncoveredPt sitescan be ?cured? by nitration ? thermal decomposition procedures. When the BaOx layer coverage is below 2.5 MLE, nitrate decomposition temperature is observed at significantly lower temperatures, demonstrating the catalytic influence of the Pt sites facilitating the nitrate decomposition. It is proposed that initially, Ba(NO3)2 decomposes at the boundary/peripheral sites of the Pt/BaOx interface, followed by the nitrate decomposition originating from 2D BaOx islands, and eventually from the 3D BaOx agglomerates.Catalytic deactivation of TiO2-promoted NOx-storage reduction (NSR) catalysts due to thermal aging effects was investigated using a BaO/TiO2/Pt(111) model catalyst system. At room temperature, metallic Ba overlayers on TiO2/Pt(111) was found to be very reactive towards oxide ions on TiO2/Pt(111) resulting in the formation of BaOx and partial reduction of TiO2. Ba films adsorbed on TiO2/Pt(111) that are further oxidized in O2 at 523 K lead to BaO and BaO2 surface domains which can efficiently adsorb both NO2 and CO2. Thermal treatment of BaO-BaO2/TiO2/Pt(111) surface at T ? 300 K leads to a monotonic decrease in the surface Ba/Ti atomic ratio indicating the diffusion of BaO-BaO2 domains into the underlying TiO2 framework. Solid state reactions between BaOx and TiO2 particularly within 473-873K facilitate the formation of BaTiO3/Ba2TiO4/BaxTiyOz overlayers. After oxidation at higher temperatures (T > 873 K), surface becomes Ba-deficient and the enrichment of the surface with the Ti4+ sites results in a TiO2-terminated surface. Diffusion of BaOx into the TiO2 matrix and the enrichment of the surface with Ti sites drastically suppress the NO2 and CO2 adsorption/storage capacity of the model NOx storage system. These results reveal a direct evidence for the structural changes associated with the thermal deactivation of TiO2-promoted NSR catalysts.Key Words: NSR, model catalyst, BaO thin films, TiO2 thin films, NO2, UHV, BaOx / Pt(111), BaOx/ TiO2/ Pt(111), XPS, TPD, LEED.