Low-Temperature Growth of Reactive Pyrochlore Nanostructures on Zirconia-Supported Ceria: Implications for Improved Catalytic Behavior

Abstract

The use of a catalyst support for the design of nanoscale heterogeneous catalysts based on cerium oxide offers vast possibilities for future catalyst development, particularly with regard to an increased focus on the use of renewable biogas and an emerging hydrogen economy. In this study, zirconia-supported ceria catalysts were synthesized, activated by using different thermochemical treatments, and characterized by way of temperature-programmed reduction (TPR), oxygen storage capacity, X-ray diffraction, electron microscopy, and luminescence spectroscopy using Eu3+ as a spectroscopic probe. Through reduction–oxidation pretreatment routines, reactive pyrochlore structures were created at temperatures as low as 600 °C and identified through TPR and electron microscopy experiments. A structural relationship and alignment of the crystal planes is revealed in high-resolution scanning transmission electron microscopy experiments through the digital diffraction patterns. Low-temperature pretreatment induces the formation of reactive pyrochlore domains under retention of the surface area of the catalyst system, and no further morphological changes are detected. Furthermore, the formation of pyrochlore domains achieved through severe reduction and mild reoxidation (SRMO) treatments is reversible. Over multiple alternating SRMO and severe reduction and severe reoxidation (SRSO) treatments, europium spectroscopy and TPR results indicate that pyrochlore structures are recreated over consecutive treatments, whenever the mild oxidation step at 500 °C is the last treatment (SRMO, SRMO–SRSO–SRMO, etc.).

Supported Ce/Zr pyrochlore monolayers as a route to single cerium atom catalysts with low temperature reducibility

“…In conclusion, the improvement of the reducibility of the sample at low temperature can be attributed to the presence of the Ce/Zr pyrochlore monolayer in the case of SO-SRMO sample, in good agreement with previously reported results. 18 , 20 , 21 …”

“…Zirconia- and yttria-stabilized zirconia (YSZ)-supported ceria systems have been recently investigated. 18 , 19 , 20 Temperature-programmed reduction treatments have revealed that the reducibility of YSZ-supported ceria can be largely enhanced after a so-called strong reduction mild oxidation (SRMO) treatment, which consists in a reduction at 900°C in a flux of H 2 (5% in Ar) for several hours followed by an oxidation at 500°C in O 2 (5% in He). The redox behavior of this SRMO-treated YSZ-supported catalyst is also superior to that of the bulk Ce/Zr mixed oxide of the same composition submitted to the same treatment.…”