Adsorption and Desorption of NOx on Commercial Ceria-Zirconia (CexZr1-xO2) Mixed Oxides: A Combined TGA, TPD-MS, and DRIFTS study

The adsorption/desorption of NOx on commercial CexZr1-xO2 solid solutions (x = 1, 0.85, 0.69, 0.5, 0.21, 0.1, and 0) is reported in this paper. In the first part, the characterization of the ceria-zirconia is carried out by N-2 adsorption at 77 K, TEM, Raman, XRD, and DRIFTS in order to establish relationships between their bulk/surface properties and their NO and NO2 adsorption/desorption behavior. As deduced from thermogravimetric experiments, the adsorption of NO2 is promoted in comparison to that of NO with typical surface densities spreading between 3.1-4.6 NO2/nm(2) and 1.5-4.5 NO/nm(2) among the CexZr1-xO2 series. DRIFTS experiments reveal that NOx adsorption proceeds mainly via electron transfer and/or participation of surface oxygens and OH groups, inducing the formation of negatively charged surface species, such as hyponitrites, nitroxyl anions, nitrites, and nitrates. The mechanistic pathways promoting the formation of the different kinds of ad-NOx species are discussed in function of the catalyst pretreatment, the temperature, the nature of the reactive gas, and the ceria-zirconia composition.