Preparation and Characterization of Model Catalysts for the HCl Oxidation Reaction - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 204

8.4 Atomic-Scale Properties of RuO 2 (110)
RuO 2 crystallizes in the rutile structure where the O atoms adopt sp 2 hy-
bridization, while the Ru atoms are coordinated to six O atoms forming a
slightly distorted octahedron (d 2 sp 3 hybridization of Ru). The bulk-truncated
RuO 2 (110) surface exposes two kinds of under-coordinated surface atoms.
These are the bridging oxygen atoms (O br ), which are coordinated to two
(instead to three) Ru atoms underneath. These are the 1f-cus Ru atoms,
i.e., one-fold under-coordinated Ru atoms, which are coordinated to five
(instead to six) O atoms (cf. Figure 8.2a). We may recall that RuO 2 is a me-
tallic oxide with electronic conductivity half of that of Ru itself, 77

d n 9 r 4 n g | 8

thereby

facilitating both the experimental and computational work. 28
Counting the formal charges of the Ru and O with þ 4 and 2, respect-
ively, the bulk-truncated RuO 2 (110) surface turns out to be auto compen-
sated in that the number of electrons missing at the surface O atoms are
compensated by the surplus electrons at the 1f-cus Ru sites (electron
counting rule 78 ). The actual charge on the Ru and O atoms in RuO 2 is
however much smaller as estimated by a Bader analysis based on DFT cal-
culations, 79 namely þ 1.74 for bulk-Ru and 0.87 for bulk-O, while 1f-cus Ru
carries a charge of þ 1.60 and O br 0.80. For comparison: Assuming that
each Ru-O bond polarizes 2/3 of a unit charge the formal charge of O br is
4/3 and that of the 1f-cus Ru is þ 10/3.

.

Figure 8.2 Ball and stick model of the stoichiometric RuO 2 (110) surface (a) and
the oxygen exposed RuO 2 (110) surface (b), where most of the under-
coordinated 1f-cus Ru site are occupied by on-top O (O ot ). The big green
balls are the oxygen atoms, the small blue and red balls are the Ru
atoms. At the stoichiometric RuO 2 (110) surface there are two types of
under-coordinated atoms, the bridging O atoms (O br ) and the 1f-cus Ru
site; 1f-cus stands for one-fold coordinatively unsaturated site.
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