Autori: Peter J. Miedziak, Zirong Tang, Thomas E. Davies, Dan I. Enache, Jonathan K. Bartley, Albert F. Carley, Andrew A. Herzing, Christopher J. Kiely, Stuart H. Taylor and Graham J. Hutchings

Editorial: Journal of Materials Chemistry, 19, p.8619–, 2009.

Rezumat:

CeO2 has been prepared from an acetate precursor by an antisolvent precipitation technique using
supercritical CO2. The supercritically synthesized ceria support was used to prepare Au-Pd based
catalysts for the selective oxidation of alcohols in solvent-free conditions using molecular oxygen as
oxidant. The supercritically precipitated catalyst demonstrated high activity for alcohol oxidation, and
it was much more active than catalysts prepared using a CeO2 support derived from the acetate through
a non-supercritical synthesis route. The bimetallic Au-Pd supported catalyst was considerably more
active than monometallic catalysts containing Au and Pd only. HAADF imaging and STEM-XEDS
mapping showed that both Au and Pd metallic components were intimately mixed and uniformly
highly dispersed over the supercritical nanocrystalline CeO2 support spheres. In contrast, the Au-Pd
catalyst on the non-supercritical CeO2 support showed discrete uniform Au-Pd alloy particles with
a size range of 50 to 150 nm. The homogeneous alloy particles were Au-rich and Pd-deficient as
compared with the preparation ratio and a low number of highly dispersed Pd was also associated with
the support. XPS data for the ceria supported catalysts confirmed the differences of metal dispersion
and identified that in both cases the surface species present were Au0 and Pd2+. On extended re-use the
non-supercritical Au-Pd/CeO2 catalyst showed sequential deactivation. On the contrary, the
supercritical Au-Pd/CeO2 catalyst showed a significant increase of activity, and it was only during the
third re-use that the activity was marginally lower than the fresh catalyst. After use the spherical
morphology of nanocrystalline supercritical CeO2 started to increasingly break down, until the
morphology started to resemble the CeO2 prepared by the non-supercritical route. Simultaneously
there was an increase of the metal particle size on the supercritically prepared CeO2 support, as discrete
Au-rich and Pd-rich bimetallic particles were formed, and there was a decrease of the metal content.
XPS confirmed the loss of metal on use and showed that there was reduction of the ceria surface during
use. The increase of activity with a corresponding loss of metal components indicates that the fresh
catalyst contains Au and Pd in inactive forms, whilst the active sites have extremely high turnover
frequencies.

Cuvinte cheie: supercritical nanocrystalline CeO2, bimetallic Au-Pd supported catalyst, alcohols selective oxidation

URL: http://pubs.rsc.org/en/Content/ArticleLanding/2009/JM/b911102f