Impact of Soot on NOx Adsorption over Cu-Modified Hydrotalcite-Derived Lean NOx Trap Catalyst

The impact of soot on NO_x adsorption was studied over a Cu-modified hydrotalcite-derived lean NO_x trap catalyst in a NO + O₂ atmosphere. Powder X-ray diffraction, scanning electron microscopy, Raman scattering spectroscopy, and X-ray photoelectron spectroscopy were used to characterize the surface properties of the pure catalyst and the soot/catalyst mixture. The adsorbed NO_x species on the samples were evaluated by in situ diffuse reflectance Fourier transform spectroscopy. The soot coverage decreases the available adsorption sites on the surface of the catalyst, and a portion of active oxygen species are consumed by the soot oxidation during He pretreatment process. The NO_x adsorption on two catalyst samples simultaneously undergoes two routes: the "nitrite route" and the "nitrate route". The "nitrite route" is more dominant than the "nitrate route". During NO_x adsorption, the soot oxidation weakens the NO oxidation to NO₂, and the released CO₂ competes with NO_x on the adsorption sites. Moreover, the temperature-programmed desorption tests indicate that the presence of soot reduces the NO_x storage capacity of the catalyst and shifts the NO desorption peak to the lower temperature range by 50 °C.