Preparation and Characterization of Model Catalysts for the HCl Oxidation Reaction - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 216

d n 9 r 4 n g | 8

Figure 8.8 Zr 0.20 Ce 0.80 O 2 nanofibers used for a HCl oxidation reaction, before (A)
and after (B) reaction. A reaction mixture: HCl/O 2 /Ar ¼ 2/2/3 at a flow of
15 mL min 1 (STP) at T ¼ 703 K for 60 hours on stream was used. 135

substantially smaller as revealed by broader reflections in the XRD data. 135
The BET surface area of the Zr 0.20 Ce 0.80 O 2 nanofibers before and after reaction
is about 30 m 2 g 1 . Obviously, no substantial sintering took place during the
reaction.

8.9 Conclusions and Outlook
Model catalysis is the approach to gain in-depth information of a catalyzed
reaction such as the harsh HCl oxidation reaction (Deacon process). With
surface sensitive experiments on single crystalline RuO 2 (110) and RuO 2 (100)
films we have gained molecular level understanding of the reaction mech-
anism and on the surface chlorination process. Analogous experiments on
single crystalline CeO 2 surfaces (such as CeO(111) are completely missing.
With the use of nanofibers, i.e., model catalyst with well-defined cylin-
drical morphology, morphological changes caused by the HCl oxidation re-
action can be studied and followed. We could demonstrate that the addition
of Ti is able to stabilize RuO 2 -based catalysts, while the addition of Zr is able
to stabilize CeO 2 -based catalysts.
However, whilst nanofibrous CeO 2 -based materials are quite ideal model
systems to study stability issues, they are not that suitable as model to study
strategies to combine enhanced stability with optimized activity of nano-
structured CeO 2 or Zr 1 x Ce x O 2 d , as the surface area is only of the order of
ca. 20-30 m 2 g 1 . Here, CeO 2 133 and Zr 1 x Ce x O 2 d 134 in the form of meso-
porous architectures appears as a promising alternative, featuring surface
areas of up to 250 m 2 g 1 and possessing a well-defined porosity in terms of
the pore shape and pore size distribution. These materials have been re-
ported recently in the form of both, thin films and powders, with pore sizes
of ca. 10-20 nm and quite regular mesoscopic order. 133,134,136
Aside from crystalline nanoparticles, 133 CeO 2 was synthesized in the form
of mesoporous powders and films. 134,136-139 Only very few other crystalline
oxides (most notably TiO 2 ) have been prepared in such a diversity of
nanostructures and morphologies. This flexibility in morphological control
using sol-gel chemistry thus enables the synthesis of basically any

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