Preparation and Characterization of Model Catalysts for the HCl Oxidation Reaction - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 201

conversion of HCl, one has to study stepped surface films with a periodic
array of steps. 30 If the interaction of the support with the catalytically active
RuO 2 component is dominating the catalytic performance, then one needs to
look at a layered system consisting of the active component RuO 2 (110)
coated for instance on TiO 2 (110) (cf. Figure 8.1b).
Single crystalline model catalysts allow for employing the whole battery of
surface science techniques to comprehend on the surface reaction on the
molecular level. 31,32 This encompasses the application of electron spec-
troscopy and electron diffraction for determining the electronic and atomic
structure of the surface. With well-prepared co-adsorption experiments of
the reactants one can gain deep insights into the elementary reaction steps.
With in situ techniques such as infrared spectroscopy (RAIRS), reaction
intermediates can be identified. All this kind of information can directly be
substantiated by ab initio calculations. The great drawback of this kind of
single crystalline model system is their small active surface area of about
1cm 2 which results only in a small overall conversion. Therefore, the kin-
etics of the actual reaction can hardly be studied under flow conditions
(neither in ultrahigh vacuum (UHV) nor in the mbar range) if the observed
turn-over number is not as high as with the CO oxidation reaction. Rather,
kinetic studies on single crystalline model catalysts need the use of a dedi-
cated corrosion resistant batch reactor with infinite residence time.
To overcome the problem with the small active surface area of single
crystalline model catalysts, one can use RuO 2 powder catalysts as shown in
Figure 8.1c. The active surface area of these model catalysts is high enough
(20-50 m 2 g 1 ) for reliable kinetic studies in flow reactor experiments. 32
Extensive sets of kinetic data are available for the HCl oxidation. 33 However,
the structure of RuO 2 powder catalysts is less well-defined than that of single
crystalline surfaces. Various surface orientations with low surface energies
are exposed, and the concentration of defects (steps, vacancies) on these
particles are much higher than on single crystalline RuO 2 (110) films. As-
suming that the HCl oxidation is structure sensitive, density functional
theory (DFT) calculations for the catalyzed reaction over RuO 2 powder
catalysts have to be performed for various low-surface-energy RuO 2 facets,
which in total is quite computer time consuming. 33 In addition, the low
surface energy (100) and (101) orientations undergo severe reconstructions
with so far unknown atomic structure 29,34 so that DFT simulations of bulk-
truncated surfaces are not meaningful. Reaction intermediates seen with
in situ experiments can hardly be identified with theoretical studies.
Besides activity, the stability of the catalyst is of major concern under such
harsh reaction conditions as encountered with the HCl oxidation. The ter-
minology 'stability' implies several aspects and needs careful consideration.
Regarding practical catalysis 'stability' addresses not only chemical stability,
but also morphological changes, e.g., transformation of the catalyst nano-
structure, grain structure, surface area, etc. Such morphological transfor-
mations can be unfavorable for catalysis, even if the chemical composition
of the catalyst is unaffected.

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