Preparation and Characterization of Model Catalysts for the HCl Oxidation Reaction - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 206

d n 9 r 4 n g | 8

Figure 8.3 Schematic representation of the chlorination of RuO 2 (110). The oxygen

atoms are indicated as green balls and the Ru atoms as blue (bulk-
coordinated) and red (under-coordinated) balls. Ball-and-stick model of
bulk truncated RuO 2 (110) (left) revealing under-coordinated surface
atoms: bridging O atoms (O br ) and one-fold coordinatively unsaturated
Ru sites (Ru 1f-cus: red balls). Upon HCl exposure at higher temperatures
the stoichiometric surface transforms into a chlorinated surface (right)
where the bridging O br atoms are partly replaced by bridging chlorine
(Cl br ) atoms (shown in gray color). 7,101,102
Reprinted with permission from ref. 102. Copyright r 2010 American
Chemical Society.

layer of RuO 2 (110). A deeper reduction/chlorination of RuO 2 (110) has not
been observed under ultra-high vacuum (UHV)-typical conditions 7 nor at
higher pressures in the mbar range. 99,100 The maximum surface chlorination
of RuO 2 attained has been estimated to be 70-80%. 101
Surface chlorination of RuO 2 (110) proceeds via a multi-step process. The
adsorption energy of molecular HCl on RuO 2 (110) is only 40-60 kJ mol 1 .
Therefore, above T ¼ 200 K HCl can only be stabilized on the catalyst's
surface by dissociative adsorption in that the Cl binds to 1f-cus Ru forming
Cl ot and hydrogen is transferred to under-coordinated bridging O atoms
forming bridging (O br H) hydroxyl groups. The dissociative adsorption of HCl
takes place on RuO 2 (110) without activation barrier and is exothermic by
130 kJ mol 1 . Via a second hydrogen transfer by dissociative HCl adsorption
O br H is transformed to bridging water. Finally, bridging water is replaced
(either concerted or sequentially) by chlorine, a process which is activated by
140 kJ mol 1 as determined by DFT calculations. 101,102
In situ surface X-ray diffraction (SXRD) 100 revealed that chlorinated
RuO 2 (110) and RuO 2 (100) model catalysts are long-term stable under
reaction conditions where the gas feed p(HCl):p(O 2 ) was varied from 1 : 4 to
4 : 1 for pressures in the mbar range and temperatures as high as 850 K. Even
pure HCl exposure in the mbar range is not able to chemically reduce RuO 2
below 600 K, since the bridging oxygen positions are mainly populated by
chlorine, and on-top adsorbed chlorine blocks part of the under coordinated
Ru sites. Without the presence of under-coordinated surface oxygen further
HCl uptake is suppressed due to missing partners for the H-transfer.
Therefore, the RuO 2 (110) surface is resistant against pure HCl exposure in
the mbar range as long as the temperature is below 600 K. Above 650 K
chemical reduction of RuO 2 (110) to metallic Ru sets in. Substitution of a
bridging oxygen atom by a bridging chlorine (Cl br ) is associated with a

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