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

Abstract

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.

Kinetics of hydrogen activation on ceria–zirconia

“…It is possible that hydroxyl groups bound to a single Ce atom exist, but the corresponding peak was not well-resolved. The assignment of the other peaks is analogous to the spectrum of pure ceria, namely, bridging hydroxyl groups (II-A) (3668-3660 cm À1 ) [57,58], bridging hydroxyl groups next to oxygen vacancies (II-B) (3640-3634 cm À1 ) [57,58], and oxyhydroxide groups (3505-3495 cm À1 ) [55,57]. Consistent with the observations for pure ceria (Fig.…”

“…8b). The peak at 3712 cm À1 is attributed to stretching vibrations of singly bound hydroxyl groups (I), and the peak at 3650 cm À1 is assigned to bridging hydroxyl groups (II-A) [29,[55][56][57]. The shoulder at 3631 cm À1 is attributed to bridging hydroxyl groups next to oxygen vacancies (II-B) [55,56,58].…”

Pt/Ce Pr1−O2 (x= 1 or 0.9) NO storage–reduction (NSR) catalysts

“…The inferior NSE obtained at 200 °C compared to 300 °C is consistent with our previous observations for ceria-based LNT catalysts and reflects either less complete regeneration of NOx storage sites at 200 °C (compared to 300 °C) and/or a more severe kinetic limitation with respect to NO oxidation to NO2 at 200 °C. Inferior NOx storage at 400 °C can be attributed to the relative thermal instability of surface cerium nitrates at this temperature, as documented in the literature [38,39].…”

Insights into the role of a structural promoter (Ba) in three-way catalyst Pd/CeO2–ZrO2 using in situ DRIFTS

“…NO x adsorption. 33 After stoichiometric O 2 is injected into the NO x /Ar gas, the bands related to nitrites significantly decrease in intensity, especially over Pd/CZB5. 8A and B.…”