On Pt alloy catalysts

The role of a seeond element in methanol oxidation on Pt alloy catalysts depends on how to lower the potential of water dissociation, to weaken the CO ehemisorption, and to direetly enhanee the electrooxidation of CO on Pt. It is obvious that none of the binary alloy catalyst, even the most used Pt-Ru catalyst, contains all of flic promoting effects simultaneously. The purpose of exploring ternary and quaternary Pt alloy catalysts is to combine all of the promoting effects into one alloy system to maximize the catalytic MOR activity. For example, Kim et al. [61] ascribed the enhancement of MOR activity on the Pt-Ru-Sn ternary alloy to the synergic effects of Ru as a water activator and Sn as an electronic modifier. Samjeske et al. [62] explained the synergic effect of Mo and Ru in the Pt-Ru-Mo ternary alloy by the way that Mo can shift the oxidation of weakly adsorbed CO on the Ru sites to lower potentials, facilitating the bi-functional effect. 

Various studies on Pt alloy catalysts have been carried out world over. Wieckowski et al. (2003) provides a comprehensive analysis of these studies. The final conclusion that is drawn is as follows. Alloying of Pt with transition metal elements increases the Pt d-band/atom vacancy and decreases the Pt-Pt bond distance. The extent of the change depends upon the electronegativity of the transition element. X-ray absorption... 

Colmenares L, Wang H, Jusys Z, Jiang L, Yan S, Sun GQ, Behm RJ. 2006. Ethanol oxidation on novel, carbon supported Pt alloy catalysts— Model studies under defined diffusion conditions. Electrochim Acta 52 221-233. 

Pt-alloy catalysts, 40 132-133 Pt microcrystal particle size on soot, 40 ... 

Mathias et al. showed that a 30% Ft alloy on a corrosion-resistant support gave an initial performance 50 mV below that of a standard 50% Pt/C catalyst. Although the Pt alloy catalyst showed much greater stability at 1.2 V, this large discrepancy in activity is unacceptable. Therefore, fhe challenge is to develop Pf (and Pf alloy) cafalysfs wifh high stabilify and similar activities to that shown by state-of-the-art Pt/C catalysts. 

More recently, Stamenkovic et al. [95,107] reported on the formation of Pt skins on Pt alloy electrocatalysts after high-temperature annealing. Pt skins were reported to exhibit strongly enhanced ORR activity. It was argued that the electronic properties of the thin Pt layer on top of the alloy alter its adsorption properties in such a way as to reduce the adsorption of OH from water and therefore to provide more surface sites for the ORR process (see Section 5.2 in Chapter 4 for a detailed discussion of skin catalysts, compare also Section 4.1.5 in the present Chapter). 

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. 

So far, various studies focused on developing catalyst materials with improved ORR activity, but only few reported the stability and durability of ORR catalysts. The study of accelerated durability tests (ADT) in conjunction with electron microprobe analysis (BMPA), LEED, and XRD techniques on Pt-based al-loys ° observed hd metal dissolution, diffusion of 3bulk oxides on the surface, and migration and agglomeration of Pt. Yu et al. compared the durability and activity of PtCo/C with Pt/C catalysts. Throngh determination of the electrochemically active sniiace area, mass, and specific activities with respeet to the potential cycles, they found the overall cell performance of PtCo/C is higher than that of Pt/C. They also concluded that the observed dissolution of Co has no severe impact on the cell performanee or membrane conductance. Additionally, Popov et al studied the stabihty of Pt M/C for X = 1,3 and M = V, Fe, Ni, Co. ADT analyses revealed that Pt/C has the lowest activity when eompared to Pt-alloy catalysts, and that the metal dissolntion is lower for a Pt M ratio of 3 1 than compared to a 1 1 ratio. Also, Pt-Ni showed a lower dissolution rate than the other considered Pt-M alloys. 

In conclusion by using rhenium as adsorbent instead of platinum, it is possible to achieve the ensemble control by sulfur passivation, at sulfur levels comparable to those applied for nickel and much lower than that which would have been required on a non-alloyed Pt-catalyst. A similar ensemble effect is achieved by alloying alone on Pt-Sn catalysts ... 

Abstract Thermally stable, ordered surface alloys of Sn and Pt that isolate threefold Pt, twofold Pt, and single-Pt atom sites can be produced by controlled deposition of Sn onto Pt single crystals and annealing. The strnctnre was established by characterization with several techniques, including ALISS, XPD, LEED, and STM. Chemisorption and catalysis studies of these well-defined, bimetallic surfaces also define the overall chemical reactivity of Pt-Sn alloys, clarify the role of a second-metal component in altering chemistry and catalysis on Pt alloys, and develop general principles that describe the reactivity and selectivity of bimetallic alloy catalysts. 

Mechanisms for ORR on pure Pt have been proposed but still remain elusive [43 8]. As one of the possible mechanisms, Yeager et al. considered that the ratedetermining step (r.d.s.) in acid electrolytes is an adsorption step of O2 molecule as side-on or bridged types accompanied by electron transfer [49]. Let us consider this mechanism on the alloy catalysts (see Figure 12 [42]), where the r.d.s. is... 

The vulnerability of Pt and Pt alloy catalysts to poisoning by trace contaminants at operation temperatures typical for a PEFC is well documented and is of clear concern in the design of a power system based on a PEFC stack. Sources of contaminants include both fuel and air feed streams as well as processes derived from chemical instability of cell component(s). As to the feed streams, polishing of anode feed streams generated by fuel processing upstream the cell should leave very low levels of CO to be dealt with effectively within the cell (see Sect. 8.3.7.1), whereas any traces of sulfur or ammonia have to be perfectly eliminated upstream the anode... 

Further enhancement of the specific activity of PEFC air cathode catalysts has been achieved by moving on from carbon-supported Pt to carbon-supported Pt alloy catalysts [41, 78-81]. The gain in activity per unit mass of Pt in moving over from Pt to PtCo alloy cathode catalysts is demonstrated in Fig. 43 (explanation for this enhancement in activity has been given above). With the rise by factor... 

Since Pt dissolution is favored by high electrode potential, relative humidity, and temperature, the possibility to limit the risk of electrocatalyst aging is based on the use of Pt-alloy catalyst instead of pure platinum, at least for the cathode, which is characterized by higher potential with respect to anode, and by adoption of operative conditions not too severe in terms of humidity and temperature. While this last point requires interventions on the membrane structure, the study of catalyst materials has evidenced that a minor tendency to sintering can be obtained by the addition of non-noble metals, such as Ni, Cr, or Co, to the Pt cathode catalyst [59, 60], suggesting a possible pathway for future work. On the other hand also the potential application of non-platinum catalysts is under study, in particular transition metal complexes with structures based on porphyrines and related derivatives have been proposed to substitute noble metals [61], but their activity performance is still far from those of Pt-based catalysts. 

Figure 5.9 shows the voltammetric behavior of Pt/C (a) and Pd-Co-Pt/C (b) towards ORR in the presence and absence of methanol. As can be also observed, the presence of 0.5 mol methanol causes a negative shift of 50 mV in the halfwave potential, in contrast to Pt/C for which there is a severe loss of activity. The linear scan voltammograms of the methanol oxidation on all the investigated materials in 0.5 mol H2SO4 + 0.5 mol L CH3OH solution, showed that the current densities of the methanol oxidation reaction on Pd-Co-X alloy catalysts (X = Au, Ag, Pt) diminish to values much lower than for Pt/C catalyst, and the onset of methanol oxidation occurs at more positive potentials, demonstrating the lowered MOR activity of the Pd-Co-Pt alloy catalysts. 

Antolini and Gonzalez recently proposed a simple empirical model to evaluate the contribution of alloyed and non-aUoyed platinum and tin to the ethanol oxidation reaction on Pt-Sn/C catalysts for DEFC [194]. On the basis of the model, the ethanol oxidation on partially alloyed catalysts occurs through a dual pathway mechanism, separately involving the PtsSn and Pt-SnOx phases. The model, validated by experimental data, can predict the performance of a DEFC by varying the Sn content and/or the degree of alloying of Pt-Sn/C catalysts used as the anode material (Fig. 8.20). 

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