New Aspects on the Mechanism of C₃H₆ Selective Catalytic Reduction of NO in the Presence of O₂ over LaFe_1–<i>x</i>(Cu, Pd)_<i>x</i>O_3−δ Perovskites

A series of LaFe_1–<i>x</i>(Cu, Pd)_<i>x</i>O_3−δ perovskites was fully characterized and tested for the selective catalytic reduction (SCR) of NO by C₃H₆ in the presence of O₂. The adsorbed species and surface reactions were investigated for mechanistic study by means of NO-temperature-programmed desorption (TPD), C₃H₆/O₂-TPD, and in situ diffuse reflectance Fourier transform spectroscopy, in order to discriminate the effects of copper and palladium partial substitutions. With respect to LaFeO₃, Cu²⁺ incorporation obviously improved SCR performance, due to its properties for C₃H₆ activation with an easy generation of partially oxidized active surface C_<i>x</i>H_<i>y</i>O_<i>z</i> species. The excellent catalytic activity at the low temperatures over LaFe_0.94Pd_0.06O₃ was attributed to the formation of reactive nitrites/nitrates, leading to a rapid reaction between adNO_<i>x</i> and C_<i>x</i>H_<i>y</i>O_<i>z</i> species, as well as a decreased occupation of the active sites by the inactive ionic nitrates. A mechanism was herein proposed with the formation of nitrite/nitrate and C_<i>x</i>H_<i>y</i>O_<i>z</i> surface species and the further organo nitrogen compounds (ONCs)/–CN/–NCO as important intermediates. Moreover, the acceleration of both formation of inactive ionic nitrate and deep oxidation of C₃H₆ contributed to a negative effect of O₂ excess for NO reduction, while Pd substitution significantly increased the O₂ tolerance ability