Probing the Surface of Ceria−Zirconia Catalysts Using NOx Adsorption/Desorption: A First Step Toward the Investigation of Crystallite Heterogeneity

Ce−Zr mixed oxides with a targeted composition of Ce 0.76 Zr 0.24 O 2 have been synthesized from nitrate precursors following different synthesis methods: coprecipitation, solid combustion synthesis with urea, citrate complexation, reverse microemulsion, and synthesis with an activated carbon fiber template. Though their real nominal compositions were nearly equal in most cases, the characterization by XRD, Raman, N 2 adsorption, FTIR, and XPS revealed distinct surface and bulk properties. The monitoring of NO + O 2 adsorption/desorption processes by in situ DRIFTS and TPD-MS allowed distinguishing the type, amount, and relative thermostabilities of nitrite/nitrate species on the different surfaces. A detailed analysis of these spectroscopic data was carried out in order to estimate the respective participation of Ce and Zr sites to the adsorption and further reaction of NO x on ceria−zirconia surfaces. In the most favorable cases, the adsorption of (NO + O 2 ) or NO 2 can be used to assess the relative enrichment of the different surfaces in Ce or Zr, but this method was found more difficult to apply than the FTIR of adsorbed methanol.