H2O dissociation

Therefore, a bifUnctional mechanistic scheme, including the participation of both the metal (via the adsorption of CO) and the support (via the formation of "oxygen vacancies" which are active sites for the H2O dissociative adsorption) seems quite relevant to explain the specific behavior, for the NO r uction in the presence of water, of samples containing Zr02 Such active sites would be located at the met -support interface and are linked to the redox properties of the support... 

The symmetric pair of voltammetric peaks in the Ru(OOOl) base CV in the range 0.1-0.3 V (peaks B and B ), which is best seen for a lower potential limit of min = 0.1V, is tentatively assigned to (14.5), which can run reversibly in both directions. This assignment is based on the assumption that the surface is covered by 0.5 ML Oad at 0.3 V. Only for more negative potential limits, when OHad is further reduced to H2O and replaced by Hupd according to (14.1) and (14.2), does the re-oxidation of the adlayer require H2O dissociation according to (14.3) and (14.4). This provides a simple explanation why the pronounced hysteresis between OHad removal (peak A ) and reformation of OHad/Oad (peak A) is only observed when the potential is scanned to < 0.1 V. 

For the same reason, Ru(OOOl) modihcation by Pt monolayer islands results in a pronounced promotion of the CO oxidation reaction at potentials above 0.55 V, which on unmodified Ru(OOOl) electrodes proceeds only with very low reaction rates. The onset potential for the CO oxidation reaction, however, is not measurably affected by the presence of the Pt islands, indicating that they do not modify the inherent reactivity of the O/OH adlayer on the Ru sites adjacent to the Pt islands. At potentials between the onset potential and a bending point in the j-E curves, COad oxidation proceeds mainly by dissociative H2O formation/ OHad formation at the interface between the Ru(OOOl) substrate and Pt islands, and subsequent reaction between OHad and COad- The Pt islands promote homo-lytic H2O dissociation, and thus accelerate the reaction. At potentials anodic of the bending point, where the current increases steeply, H2O adsorption/OHad formation and COad oxidation are proposed to proceed on the Pt monolayer islands. The lower onset potential for CO oxidation in the presence of second-layer Pt islands compared with monolayer island-modified Ru(OOOl) is assigned to the stronger bonding of a double-layer Pt film (more facile OHad formation). 

Of the alternative Ft formulations, the FtMo system has been the most studied in recent years. Work on bulk FtsMo alloys by Grgur, Markovic, and Ross showed similar CO tolerance to FtRu in the presence of H2. This tolerance was correlated with the ability of FtMo to oxidize CO at potentials as low as 0.05 V. However, unlike Ru but similar to Sn, the Mo appeared to oxidize CO just at neighboring Ft sites, with the majority of CO oxidized af potentials typical of pure Ft. The surface Mo atoms were found to be oxidized even at 0.0 V. Therefore, it was postulated that H2O dissociation to form OH was mediated by a Mo(IV)/(Vl) couple. Carbon-supported FtMo catalysts were reported to have better CO tolerance than FtRu in MEA testing up to CO concentrations of 100 ppm.i39... 

Results obtained from careful examination of the pH effects on the potentiody-namic behavior of the Pt(lll) electrode in acidified perchlorate solutions [180] advocate in favor of the assumption that water and not Cl04 anions are involved in the unusual adsorption states in the doublelayer region of Pt(lll). Presumably, H2O dissociation and OH deprotonation are responsible for the two main electrode processes in the double-layer region of Pt(lll) in NaCl04 solutions. 

In pure water, H2O dissociates to H+ ions (protons) and OH ions (hydroxide) ... 

Mention should also be made of the possible effect of H2O dissociation (i.e., reaction [8]) to yield vapor phase H2, which was suggested by Horn et al. (78) as a factor in ceramic degradation. However, in the present system there is an additional source of O2 and, at thermodynamic equilibrium, K should be H2O-independent except for the noted H20-solubility effect. 

The calculations show that the substitutional Ru should be active for CH3OH dissociation. As mentioned earlier experimentally methanol does not bind at Ru because the sites are preferentially covered by H2O and OH. The Ru atoms in the surface provide nucleation sites only for OHgds formation. Like pure RUio, substituted Ru shows smaller and E for H2O dissociation than does Pt. The calculated D,. hc of H2O on (Pt3)(Pt4Ru3) is 0.31 eV, 0.5 eV less endothermic than on pure Pt (Fig. 6). The activation energy for H2O dissociation on (Pt3)(Pt4Ru3) is also smaller by 0.18 eV than on pure Pt, in agreement with earlier work of Anderson et al. [86]. Assuming that the pre-exponential factors A in the Arrhenius equation, k=A exp(-E /RT), are approximately the same for both pure Pt and mixed Pt-Ru clusters, a decrease of 0.18 eV in E would increase the rate of H2O dissociation reaction by a factor of 1000. 

H2O dissociation on pme Pt and on the Pt site in mixed Pt-Ru clusters is difficult. Ru sites more actively dissociate H2O [6,7]. In the present study we also find that Mo, W, and Re activate H2O more effectively than does Ru, partly because these metals (Mo, W, Re) adsorb OH strongly. COads(Pt) is removed by OHads(M), the principal oxidizing agent. However, strong OH adsorption introduces a barrier to CO oxidation. In this section, we examine the energetics of the surface reaction of COads(Pt) with OHads(M)- The scheme for calculating the combination energy (Ce,ads) is... 

Fig. 7. Schematic illustration of the calculated (a) H2O dissociation and activation energies at Pt and M sites, (b) COads(Pt) + OHads(M) combination and activation energies.

The theoretical study of CO adsorption and H2O dissociation on a series of mixed Pt-M surfaces [6,7,10] has provided a broad array of information necessary to understand the required characteristics of CO-tolerant binary catalysts. In... 

Cu, Zn, Ge, Zr, and Rh have high D> c and E for water dissociation, and thus are unsuitable as secondary metals. The enhanced promoting effect of Re, W, or Mo toward H2O dissociation are due to strong M-OH adsorption, and large Eads(OH) does not promote oxidative removal of CO. For M = Re, the activation energy for the COads + OHads combination reaction is the highest of all, and thus the reaction becomes the rate-determining step in CO removal. 

The best alloying metals are those that possess low activation energies for H2O dissociation and for COads + OHads — COOHads- Based on the energetics, with more emphasis on water activation and oxidant OH formation, we predict the most suitable alloying metals in CO electro-oxidation to be Mo, W, and Os, with Ru close behind. 

In pure water, then, the concentrations of these two species are equal since there are no other sources of H or OH except H2O dissociation ... 

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