CO electro-oxidation

AU experiments to be described below are interpreted on the basis of the Langmuir-Hinshelwood (LH) mechanism for CO electro-oxidation suggested by GUman more than 40 years ago [Gihnan, 1964]. According to GUman s model, water needs to be activated on a free site on the surface, leading to surface-bonded OH ... 

Weaver and co-workers have carried out extensive smdies of CO electro-oxidation on Au single crystals [Chang et al., 1991 Edens et al., 1996]. Continuous oxidation of CO on Au starts at potentials where the formation of surface oxides or surface-bonded hydroxyl (OH) is not apparent from voltammetry. Weaver suggested the following mechanism ... 

The lower total activity for Rh electrodes may be partly due to increased CO poisoning and slower CO electro-oxidation kinetics compared with Pt electrodes, as demonstrated by the number of voltammetric cycles required to oxidize a saturated CO adlayer from Rh electrodes (see Section 6.2.2) [Housmans et al., 2004]. In addition, it is argued that the barrier to dehydrogenation is higher on Rh than on Pt, leading to a lower overall reaction rate [de Souza et al., 2002]. These effects may also explain the lower product selectivity towards acetaldehyde and acetic acid, which require the dehydrogenation of weakly adsorbed species. 

Koper MTM, Lebedeva NP, Hetmse CGM. 2002. Dynamics of CO at the solid/liquid interface studied by modebng and simulation of CO electro-oxidation on Pt and PtRu electrodes. Faraday Discuss 121 301-311. 

Figure 14.11 Dlustration of CO electro-oxidation at Pt-modified Ru(OOOl) (a) mixed, non-leactive adlayer (b) Pt-assisted formation of OH d at high local adsorbate coverages on the Ru areas (c) CO oxidation at the Pt islands. For simplicity, H is used instead of H30. ...

CO monolayer electro-oxidation on Pt/C has also been investigated with chron-oamperometry [Cherstiouk et al., 2003b Maillard et al., 2004a, 2007b]. Peaked current transients (Fig. 15.9) typical of CO electro-oxidation on polycrystalline... 

In addition, it sustains CO electro-oxidation at relatively low overpotential, and there are crystal face dependences for both the ORR and CO oxidation. Since Au is also a system that exhibits both particle size and support effects in heterogeneous catalysis, it provides an interesting model system for smdying such effects in electrocatalysis. 

There have been even fewer studies of CO electro-oxidation on supported Au particles than on supported Pt. One may expect, however, on the basis of the studies on extended Au surfaces, that there will be a structure sensitivity to the CO electrooxidation reaction on supported Au particles. 

CO electro-oxidation exhibits a strong particle size dependence on both carbon-and titania-supported Au catalysts a strong deactivation of the reaction is observed for particle sizes below about 3 nm. In the case of the titania supports, however, a distinct activation of the reaction is also evident. This manifests itself in a strong decrease in the overpotential for the reaction, and an increase in activity as the particle size decreases in the range 8-3 nm. The result is a maximum in the catalytic activity with particle size. 

Figure 6.27. Schematic of the CO electro-oxidation on a Pt or Pt alloy electrocatalyst in the presence of hydrogen. CO adsorption blocks the dissociative adsorption and oxidation of hydrogen.

The thermodynamic potential for this process lies close to OVrhe, however, CO electro-oxidation is characterized by a significant overpotential on platinum of about 0.8Vrhe- This overpotential is associated with the activation of the water (2) to produce an adsorbed oxidizing species that subsequently oxidized adsorbed CO through a Langmuir-Hinshelwood mechanism ... 

Eigure 16a shows CO stripping on Pt(lll), Pt(lll)-Ru (following spontaneous deposition), and Pt(lll)-Ru (where the spontaneously deposited ruthenium has been reduced in hydrogen). Only a very small reduction in overpotential for CO electro-oxidation is observed for Pt(lll)-Ru . The overpotential for CO electro-oxidation on the Pt(l 11) surface has been reduced, however, on the Pt(lll)-Ru surface, and the latter exhibits a doublet structure. This CO stripping result on Pt(l 11)-Ru is nearly identical to that found on the Pt(l 11)-Ru surface where the ruthenium was MVD deposited (Eigure 15) It was concluded that Pt(lll)-Ru ° was decorated islands of Ru . 

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