Pt catalysts monolayers

Figure 6.21. Schematics of currently pursued Pt-based electrocatalyst concepts for the ORR. (A) Pt bulk alloys (B) Pt alloy monolayer catalyst concepts (C) Pt skin catalyst concept (D) De-alloyed Pt core-shell catalyst concept.

The Pt alloy monolayer nanoparticle catalysts (e.g., Pt-Re layer on Pd cores) showed a clearly improved specific (Pt surface normalized) ORR activity their Pt mass-based electrocatalytic activity, however, exceeded that of pure Pt catalysts by an impressive factor of 18 x— 20 x. Their noble metal (Pt, Re, and Pd) mass-based activity improvement was still about a factor 4x. The Tafel slope in the 800-950 mV/RHE range suggested that the surface accumulation of Pt-OH species is delayed on the Pt monolayer catalyst. The enormous increase in Pt mass-based activity is obviously due to the small amount of Pt metal inside the Pt monolayer. 

Pt alloy monolayer catalysts exhibited even more active ORR behavior compared to Pt monolayer catalysts. To understand this phenomenon computational DFT studies were carried out. The hypothesis to be tested was that, for instance, Ru metal atoms in the Pt—Ru monolayer are OH-covered and could inhibit the adsorption of additional OH on neighboring surface sites (adsorbate-adsorbate repulsion effect). A very similar hypothesis was put forward about three years earlier by Paulus et al. [105] who postulated that Co surface atoms might exhibit a so-called common-ion effect, that is, they could repel like species from neighboring sites. A combined computational-experimental study finally confirmed this hypothesis [123] If oxophilic atoms such as Ru or Os were incorporated into the Pt monolayer catalysts, the formation of adjacent surface OH was delayed, if not inhibited. Oxo-phobic atoms, such as Au, displayed the opposite effect, would not inhibit Pt—OH formation, and were found to be detrimental to the overall ORR activity. 

While the stability of the monolayer Pt alloy catalyst concept was initially unclear and therefore threatened to make the monolayer catalyst concept a questionable longer term solution, a very recent discovery seems to lend support to the claim that Pt monolayer catalyst could be made into stable catalyst structures Zhang et al. [94] reported the stabilizing effect of Au clusters when deposited on top of Pt catalysts. The presence of Au clusters resulted in a stable ORR and surface area profile of the catalysts over the course of about 30,000 potential cycles. X-ray absorption studies provided evidence that the presence of the Au clusters modified the Pt oxidation potentials in such a way as to shift the Pt surface oxidation towards higher electrode potentials. 

The classic sensitizer dye employed in DSC is a Ru(II) bipyridyl dye, cis-bis(isothiocyanato)-bis(2,2/-bipyridyl-4,4/-dicarboxylato)-Ru(II), often referred to as N3 , or in its partially deprotonated form (a di-tetrabutyl-ammonium salt) as N719. The structure of these dyes are shown in 2 and 26. The incorporation of carboxylate groups allows immobilization of sensitizer to the film surface via the formation of bidendate coordination and ester linkages, whilst the (- NCS) groups enhance the visible light absorption. Adsorption of the dye to the mesoporous film is achieved by simple immersion of the film in a solution of dye, which results in the adsorption of a dye monolayer to the film surface. The counter electrode is fabricated from FTO-coated glass, with the addition of a Pt catalyst to catalyze the reduc-... 

Juang and Liu [74,75] presented that the interfacial tensions between water/ -hexane and water/toluene in the synthesis of ether-ester compounds by PTC could be measured. These two-phase systems contained PT catalyst, an aqueous phase reactant, and/or alkali. The interfacial data could be well described by the Gibbs adsorption equation coupled with the Langmuir monolayer isotherm. 

Fig. 18.4 (a) Initial three CVs of the PtCus catalyst annealed at 600 °C during electrochemical dealloying compared to the CV of a commercial Pt catalyst (reprint with permission from ref [27]). (b) Diagrammatic illustration of how the critical dissolution potential of a Cu monolayer depends on the composition of its subsurface layer (reprint with permission from ref [40])... 

The present chapter summarized the fundamental aspects and recent advances in electrocatalysts for the oxidation reactions of H2/CO, methanol, and ethanol occurring at fuel cell anodes emphases were placed on the state-of-the-art Pt-Ru- and Pt-Sn-based catalytic systems. Pt-based catalysts are still considered to be the most viable for the anodic reactions in acidic media. The major drawback however, is the price and limited reserves of Pt. To lower the Pt loading, the core-shell strucmre comprising Pt shells is more beneficial than the alloy structure, since all the Pt atoms on the nanoparticle surfaces can participate in the reactions (and those in cores do not) particularly, the Pt submonolayer/monolayer approach would be an ultimate measure to minimize the Pt content [30-35]. The architectures in nanoscale also have a significant effect on the reactivity and durability [54, 94] and thus should be explored continuously in the future. As for the ethanol oxidation, Rh addition is shown to enhance the selectivity towards C-C bond splitting [70, 71] however, Rh is even more expensive than Pt, and thus less expensive constituents replacing Rh are necessary to be found. 

Papageorgopoulos and de Bruijn showed that the CO-type speeies (e.g., CO, -COH, -COOH) formed from CO2 did not form a eomplete monolayer on Pt catalysts, and its oxidation occurred at less positive potentials than that of a CO adlayer formed directly from CO [30], They carried out the adsorption processes at 18 mV and believed that the adsorption of the former CO-t5tpe species from CO2 included linear, bridge, and multiple bonds, while that of CO was exclusively linear. The CO oxidative stripping peaks became much smaller when the CO-type species was made on PtRu catalysts, indicating that less CO was formed on PtRu than on Pt. Such an experiment indicates that the reverse water-gas shift reaction occurs to a smaller extent when PtRu is used as the anode catalyst than when Pt is used. 

One valuable chemisorption technique is the hydrogen-oxygen titration reaction. First proposed by Benson and Boudart for Pt catalysts [59], it represents the titration of an oxygen monolayer by hydrogen near 300 K and its stoichiometry for Pt was predicted based on equations 3.1 and... 

---