RuO2 surface phases

Stability of RUO2 Surface Phases. Reuter and Scheffler first outline the theory and methodology of ab initio thermodynamics in a study applied to Ru02 in equilibrium with an O2 atmosphere. The authors consider three possible surface terminations of RuO2(110). The RuO2(110)-Obridge sutfacc is predicted to be the most stable because it has the lowest number of uncoordinated O atoms and has no net dipole. The second... 

In this context, it is noteworthy that Over and co-workers (46,164-167) found the same types of Mars-van Krevelen mechanism for CO oxidation for ruthenium. Although the active surface was not characterized directly under high-pressure conditions in these investigations, it was found for the ruthenium(0 0 01) surface, which forms a RuO2(l 1 0) thin film in an oxygen-rich environment, that the activity of ruthenium as an oxidation catalyst is in fact primarily related to the RUO2 phase. 

Fig. 5.14. Top view of the RuO2(110) oxide surface explaining the location of the two prominent adsorption sites (coordinatively unsaturated, cus and bridge, br). Also shown are perspective views of the four steady state adsorption phases present in Fig. 5.15 (Ru = light large spheres, O = dark medium spheres, C = white small...

Properties Solid solution rather than mixed phase at >40% RuOj, TiO2 was in rutile form anatase was present at lower RUO2 concentrations for BET specific surface area, see Table 3.1458 [1750]. 

One method to render silica aerogels conductive is to conformally coat them with a secondary conductive phase. This was demonstrated by Rolison and co-workers by solution-phase treatment of silica aerogels with RUO4 at low temperatures, which subsequently decomposes to plate RUO2 on the aerogel surface (19). 

Fig. 3.10 Scanning electron microscopy images of (a) bare n-Ta3N5 eiectrode surface, (b) n-TaaNs eiectrode with eiectrodeposited Pt particies, (c) n-Ta3N5 eiectrode with Ir02 eiectrodeposited from a coiioidai Ir02 solution prepared from an acidic condensation method [28], and (d) n-Ta3N5 electrode with vapor phase deposited RUO2 from decomposition of RUO4 [29]...

Kim YD, Over H, Krabbes G, Ertl G (2000) Identification of RUO2 as the active phase in CO oxidation on oxygen-rich ruthenium surfaces. Top Catal 14 95-100... 

An example of a pressure-gap system is CO oxidation on rutheninm. Rntheninm does not exhibit any activity for CO oxidation at low pressnres however, at pressures in the several Torr regime, Ru has the highest activity of all relevant transition metals (i.e., Pt, Pd, Or, Rh, Ru) [16]. In 2000, a surface science study by Over et al. revealed that the active phase at high pressures is not the metal Ru(OOOl) surface, instead the catalytically highly active phase for CO oxidation is a RUO2 film,... 

Over et al. performed in situ surface X-ray diftiaction experiments (SXRD, see Box 8.1) [19, 20]. The combination of online reaction product analysis with SXRD allowed them to correlate the turn over frequency (TOF—number of reaction products per site per second) for CO oxidation with the structure of the catalytic surface. Figure 8.3b shows that, in the mbar regime, two distinct phases can be present the RUO2 phase and a non-oxidic phase. At tanperatures below 550 K, both phases have a nearly identical activity for temperatures above 550 K, the oxide phase has the higher activity, showing that the oxide is indeed the active phase under these conditions. 

As the amount of Se increases in Ru-Se, a Ru metallic form on the surface increases and thereby the ORR activity gets improved [175]. If RUO2 is the predominant phase, Se incorporation leads to the formation of RuSe2 which is not an active catalyst [176]. The activity of Ruj-Scy/C also depends upon the pH and the maximum was obtained at pH=8 [177]. Above pH 8 the activity decreases because of the presence of oxide/hydroxide adlayers on the Ru substrate which inhibit the ORR. Se becomes metallic when interacting with Ru catalysts due to charge transfer from Ru to Se. This charge transfer renders Ru less favorable for high activity and stability of the catalyst [178]. 

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