Preparation and Characterization of Model Catalysts for the HCl Oxidation Reaction - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 200

may affect the performance of a catalytic reaction. This effect is called
structure sensitivity in catalysis research. 22
In order to gain atomic scale insights into a heterogeneously catalyzed
reaction system one has to resort on model catalysis. 23,24 The chosen model
system should be related as closely as possible to the considered effect of the
real reaction system. 25 Besides model catalysts also simple model reactions
are in the focus of model catalysis. In surface science the most frequently
studied reaction system is the CO oxidation reaction; 26 simply in the sense
that only activity counts and stability of catalyst is not of major concern. For
the case of RuO 2 (110), the CO oxidation has been thoroughly discussed in
previous papers 27,28 and found to be structure insensitive. 29
Here we will focus on the heterogeneously catalyzed HCl oxidation. Vari-
ous model catalysts can be envisioned for the Sumitomo process: single
crystals of RuO 2 , single crystalline RuO 2 (110) films, supported RuO 2 (110)
films of various thicknesses on a rutile TiO 2 (110) substrate, micro-scale
RuO 2 powder, and RuO 2 nanofibers (cf. Figure 8.1). 13 The proper choice of
the model system is dictated by the scientific question in mind when taking
off with the scientific project. If one is interested in a molecular level
understanding of the reaction mechanism, then one should retain to single
crystal RuO 2 (110) or RuO 2 (100) films with well-defined surface structure of
the catalytically active sites provided by one-fold under-coordinated surface
atoms (cf. Figure 8.1a). If steps and kinks are important in the catalytic

d n 9 r 4 n g | 8

.

Figure 8.1 Various types of model catalysts forming a hierarchy of increasing
complexity exemplified by a RuO 2 -based catalyst. (a) Single crystalline
RuO 2 (110) films grown on a single crystalline Ru(0001) surface. On the
atomic scale, the oxide film is well ordered exposing under-coordinated
Ru and O sites. (b) Separated single crystalline RuO 2 (110) islands
(height: 2 nm, lateral size 10 nm) are evenly distributed across the
TiO 2 (110) surface and dense RuO 2 films of 10 nm thickness on
TiO 2 (110). (c) Micro-scale RuO 2 powder exposing various facets in vari-
ous orientations, predominantly along the (110) orientation. (d) Poly-
crystalline RuO 2 -based nanofibers revealing a well-defined cylinder
morphology. Morphological alteration under harsh reaction conditions
should easily be visible in scanning electron microscopy (SEM).