1 Department of Physics, Technical University of Denmark2 Center for Atomic-scale Materials Design, Center, Technical University of Denmark
applied to catalytic oxidation of hydrogen halides
The results in this thesis are based on Density Functional Theory calculations. The catalytic activity of oxides and other compound materials are investigated. It is found that the adsorption energy of the molecules NH2, NH, OH and SH on transition metal nitride, oxide and sulfide surfaces scales linearly with the adsorption energy of their central N, O and S atoms. It is also found that they follow the same trend as in the case of adsorption of the same molecules on transition metals. The same type of scaling relations are also established between the adsorption energies of the halides (Cl, Br, and I) and OH on a wide range of rutile oxide surfaces. Furthermore, Brønsted-Evans-Polanyi (BEP) relations are found for the adsorption of a large number of molecules (including Cl, Br and I) on transition metal oxides. In these relations the activation energies scale linearly with the dissociative chemisorption energies. It turns out that the BEP relation for rutile oxides is almost coinciding with the dissociation line, i.e. no barrier exists for the reactive surfaces. The heterogeneous catalytic oxidation of hydrogen halides (HCl, HBr, and HI) is investigated. A micro-kinetic model is solved and the oxidation activities are compared at different coverage. Based on the obtained scaling relations two descriptors are identified that describe the reactions uniquely. By combining scaling with the micro-kinetic model, activity volcanoes for the three different oxidation reactions are derived. It is found that the commonly used RuO2 catalyst for HCl oxidation is the closest to optimal for all three oxidation processes. Finally, a study of transition metal substituted zeolites shows that relations similar to the above mentioned scaling applies for this type material.