Initial Steps in the Selective Catalytic Reduction of NO with NH3 by TiO2-Supported Vanadium Oxides

Electronic structure calculations at the density functional theory/ B3LYP level (selectively benchmarked by CCSD(T)) were performed on neutral and protonated monomer and dimer clusters of vanadium oxide (VxOy) on a cluster model of a TiO2 support to predict the first steps in the mechanism of the selective catalytic reduction (SCR) of nitric oxide by ammonia. The vanadium cluster structures are based on experimental NMR measurements. The first step is Lewis acid-base addition of NH3 to a vanadium site for the neutral and formation of an NH4+ site on the protonated surface. Different proton transfer pathways, which depend on the initial neutral or protonated sites coupled with addition of NO lead to the formation of NH2NO surface species. The mechanisms can be complicated involving many different pathways for the proton transfers, especially for the initial protonated surface. The addition of the doublet NO leading to the formation of NH2NO leads to a reduction of a vanadium and transfer of the spin to this site. NH2NO desorbs and then undergoes gas-phase rearrangements to form the final products N-2 + H2O. The barrier heights for the gas-phase rearrangement process leading to final product formation are comparable in a number of cases to the barrier heights on the catalyst and may represent the rate-determining step. The reaction on the cluster model with a reduced vanadium(+4) proceeds by different paths with addition of NO leading to a second reduced vanadium site. The predicted pathways are consistent with the available experimental data and show that the complete SCR mechanism is very complicated as additional H2O molecules will be removed from the surface by the addition of O-2 to fully regenerate the catalyst and oxidize V back to the formal +5 state.