Preparation and Characterization of Model Catalysts for the HCl Oxidation Reaction - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 202

For stability issues special model catalysts have to be designed with dis-
tinct morphologies in order to be able to visualize and follow structural
degradation of the model catalyst after reaction with scanning electron
microscopy (SEM). In stability studies we applied RuO 2 -based and CeO 2 -
based nanofibers as model catalysts (cf. Figure 8.1d). These model systems
bear the additional advantage of having large active surface areas of about
20-30 m 2 g 1 to conduct kinetic studies in a typical flow reactor set-up.

d n 9 r 4 n g | 8

8.3 Synthesis of Single Crystalline RuO 2 Films for
Gaining Molecular Information on Stability and
Activity
In order to produce single crystalline RuO 2 films with specific
surface orientations one can start from metallic Ru single crystals which
are cut along particular directions. A well-ordered crystalline RuO 2 film is
grown by exposing the single crystalline Ru surfaces to large amounts of
molecular oxygen (say 10 6 L, at 10 5 mbar; 1 L (where L means Langmuir)
corresponds to an exposure of 1.33 10 6 mbars) at temperatures in the
range from 600 to 750 K. This procedure leads to the growth of a 1-2 nm
thick RuO 2 film. On the Ru(0001) surface a single crystalline RuO 2 film in
(110) orientation is preferentially formed (cf. Figure 8.1a), 35,36 while on the
Ru ð 1010 Þ a high quality RuO 2 film in (100) orientation can be grown. 37,38
Occasionally other orientations are observed on the Ru(0001) and Ru ð 1010 Þ
single crystal surfaces, such as (101). 39,40 Unfortunately RuO 2 films thicker
than 3-5 nm can only be formed at higher surface temperatures, where
the oxide surface roughens considerably,

.

forming facets with various

orientations. 40
Thicker (and flat) single crystalline RuO 2 (110) films with variable thick-
ness of several 10 nm can be prepared on TiO 2 (110) single crystals serving as
templates. 41-43 Ru-carbonyls are frequently used as Ru-containing pre-
cursors and the actual oxidation process is assisted by an oxygen plasma.
The adsorption and decomposition of (Ru) 3 (CO) 12 over TiO 2 (110) has been
studied in great detail by XPS and RAIRS. 44,45 However, carbon contamin-
ation is unavoidable. At growth temperatures above 700 K Ti and Ru inter-
diffuse to form mixed Ru x Ti 2 x O 2 epitaxial films. This mixed oxide buffer-
film can be utilized to minimize the lattice mismatch between RuO 2 (110)
and TiO 2 (110), thereby producing flat single crystalline unstrained
RuO 2 (110) films of more than 20 nm thickness. 43 Very recently, RuO 2 islands
and closed films epitaxially grown on TiO 2 (110) could be prepared
(cf. Figure 8.1b) using an electron beam induced Ru evaporation as a carbon-
free Ru source. 46
Thin films of RuO 2 can be produced either by deposition of Ru and
subsequent oxidation of Ru films, or directly by deposition of RuO 2 . For
instance, ultra-thin highly textured Ru films in (0001) orientation can be