Transition Metal Oxides Supported on TiO2 as Catalysts for the Low-Temperature Selective Catalytic Reduction of NOx by NH3

The conversion of NOx and the yield of N2O during NH3-SCR-DeNOx below 473 K over TiO2-supported transition metal oxide catalysts with equal loading of 20 wt.-% decreases in the following order of the supported oxides: MnOx > CuOx > CoOx > FeOx > NiOx > CeOx. The storage capacity for NH3, characterized by the acid site density of the catalyst, is not directly correlated with the catalytic activity. Rather, the temperature range for the reduction of the supported transition metal oxides as determined by TPR-H2 is the main governing factor for high NH3-SCR-DeNOx activity, especially in the temperature range below 473 K. At the same time, oxidation temperature range and the density of Lewis acid sites govern the formation of N2O. The decomposition of NH4NO3 as an intermediate in the NH3-SCR-DeNOx reaction is determined by the redox property of TMO-based catalysts, which further influences both the windows of the decomposition temperature and the yield of N2O. The correlation between the redox properties and the activity for NH3-SCR-DeNOx was confirmed for a series of MnOx-CeOx/TiO2-SiO2 mixed transition metal oxide catalysts as a promising combination of the less active and more selective CeOx with less selective and highly active MnOx. The linear correlation between reduction temperature range and the NH3-SCR-DeNOx activity indicates that the found relation can be transferred to other supported transition metal-containing catalysts for low-temperature NH3-SCR-DeNOx.