On the role of Alkaline Earth Metal Oxides in Environmental Catalysis

Doctoral thesis, 2005

In heterogeneous catalysis, metal oxides have various roles: as catalysts, as support material for catalytically active metal particles, as promoters, as stabilizers, or as storage materials. Although their main application is as support material, their role as promoter or storage material becomes increasingly important as emission legislations are becoming more stringent. One such case is the catalytic cleaning of emissions from lean burn gasoline and diesel engines. Running the engine at lean conditions implies that the presently employed catalytic concepts are incapable of continuous NOx reduction. In this thesis, density functional theory (DFT) has been used to study the role of alkaline earth metal oxides, both as a storage material and as a support material for catalytically active metals. The main focus has been NOx storage in barium containing NOx storage and reduction (NSR) catalysts. As the storage material is experimentally ill-defined, different models for the storage material have been investigated (BaO(100), (BaO)9 cluster and BaCO3(110)). An atomistic understanding of species formed upon NO2 exposure to the storage material is presented. The mechanistic understanding comprises a pair-wise adsorption of NO2 with enhanced stability compared with single NO2 adsorption. Moreover, for NO2 storage in BaCO3, the pair-wise mechanism is shown to be crucial for effective surface decarbonation. Ab initio molecular dynamics calculations for NO2 adsorption on BaO(100) have been performed to study the dynamics of adsorbed NO2 molecules. Nitrites and nitrates are found to be mobile on the short time scale of the simulations (~4 ps). The role of oxides as model support material has been studied considering Pt atom, dimer and film adsorption on MgO(100) and BaO(100). For MgO(100), Pd, Ag and Au adsorption have been studied also. Both the metal/metal-oxide bond and support effects on the CO-metal bond upon CO adsorption have been explored. The support is found to prepare Pt for CO adsorption, resulting in both an enhanced Pt-CO bond and support-PtCO bond. However, this effect is only observed for mono-layer metal adsorption. Finally, the CO oxidation reaction is studied over a MgO supported Au nanoparticle. Special focus is given to adsorption and reaction at low-coordinated Au sites at the Au/MgO perimeter and to the effects of dopants. Keywords DFT, adsorption, catalysis, NOx storage, CO oxidation, supported catalysts, metal oxide, BaO, MgO, BaCO3, Au, slab, cluster