Pt-based binary catalysts

Further, Pt-based binary catalysts, such as Pt/Cr and Pt/Ni [18], are less sensitive to the presence of alcohol than pure Pt, giving hope for the development of better catalysts for oxygen reduction. 

The major challenge for the electrocatalysis of ethanol is to facilitate its total oxida-tirai to CO2 at low potentials, which cannot be achieved by existing Pt-based binary catalysts. Adzic et al. developed the multifunctional ternary ft-Rh-Sn02 electrocatalyst which is effective in splitting the C-C bond in ethanol at room... 

It is well established that binary catalysts, with Pt as one of the central components, can exhibit a substantial resistance to CO. It has been found that the use of a second element, such as Ru [79,80], Fe [81], Ni [82], Mo [83], etc., in the form of alloy or co-deposit yields significant improvement in the CO-tolerance relative to pure Pt [84]. The superior CO tolerance is mainly attributed to the so-called "bifunctional" mechanism, i.e., the lower CO oxidation potential due to the addition of a second element. Among these various Pt-based binary catalysts, the most commonly used and the most promising catalyst is Pt-Ru/C, with the best atomic ratio being 1 1. Nevertheless, Pt-Ru/C does not provide tolerance against H2S poisoning of the anode [85]. 

V on Pt-based binary catalysts. After the adsorption of ethanol, the potential was first scanned in the negative direction. Left-hand panel. PtRu (ZSW, metal loading 40 pg, particle size 3-5 nm) middle panel. PtRh (ZSW, metal loading 40 pg, particle size 3-5 nm) right-hand panel PtsSn/C (DICP, metal loading 8 pg, particle size 2.1 nm). Scan rate 10 mV/s. Electrolyte flow rate 10 pL/s... 

On a basis of trial and error it was noticed that a practical fuel cell attains higher performance employing ternary platinum based materials than employing the binary catalysts. During the last decade, the global observation reveals an increasing of performance for the H2/CO oxidation as well as for the MOR when a third element was added to the best bimetallic catalyst, the Pt-Ru [57] or Pt-Sn [58] based material. An overview of the preparation and structural characteristics of Pt-based ternary catalysts [59] and their electrochemical performance [60] was presented by AntoUni. Therein, all the relevant works before 2007 are found. In summary, many ternary Pt-Ru-M catalysts (M = Wi Wox or W2C form. Mo, Ir, Ni, Co, Rh, Os, V) perform better than commercial standard Pt-Ru catalysts and/or Pt-Ru catalysts prepared by the same method than the ternary. 

Some work was carried to modify Pt-Ru catalysts by adding a third or even a fourth constituent. These Pt-Ru-based ternary or quaternary electrocatalysts such as Pt-Ru-Os, Pt-Ru-W, Pt-Ru-Ni, Pt-Ru-Mo, Pt-Ru-Pb, Pt-Ru02-Ir02, and Pt-Ru-Os-Ir showed improved catalytic activities than Pt-Ru binary catalysts [61, 80-85]. 

Platinum electrocatalysts are dispersed as small particles on high surface area conductive supports for effective use of costly Pt. The size of platinum particles, therefore, plays an important role in the oxygen reduction kinetics for fuel cell applications, in terms of both electrocatalytic activity and practical application of catalysts. Carbon-supported platinum shows a large surface area and increased catalytic activity. Alloy catalysts with various transition metals have been employed to increase the catalytic activity and reduce the cost. Various Pt-based alloy catalysts (binary, ternary, and quaternary alloy) have been tested over the last two decades. Many researchers have reported that Pt-based alloy catalysts show not only higher activity than Pt alone, but also exhibit good performance in the ORRs in PEFCs and DMFCs [100-108]. 

Due to the faeile poisoning effect of CO on Pt, many Pt-based binary alloys, such as Pt-Ru, Pt-Os, Pt-Sn, Pt-W, Pt-Mo, and so on, have been investigated as electrocatalysts for the methanol oxidation reaction (MOR) on flic DMFC anode. Among them, the Pt-Ru alloy has been found to be the most active binary alloy catalyst, and is commonly used in state-of-the-art DMFCs [32]. 

This review discusses the meehanism of CO tolerance and the development of CO-tolerant catalysts. The development of Pt-based binary/temaiy metallic clcctrocatalysts and Pt-ffee electrocatalysts is discussed. Useful information is also provided on characterization methods for the understanding of the mechanism of CO tolerance and the evaluation of anode electrocatalysts. 

From a general point of view, at ambient pressure and temperature, all electrooxidations of short-chain aliphatic alcohols (in acid) require the presence of the expensive precious metal Pt. However, as is well known, Pt is readily poisoned by CO-like intermediate species formed during methanol oxidation at low temperatures. It has been found that Pt-based binary or ternary catalysts, Pt-Mi, Pt-Mi-M2 (M = Ru, Sn, etc.) can improve the reaction kinetics of methanol electrooxidation based on the bifunctional effect (promoted mechanism by the second metal) [17,19,22,24,26] and/or on the tuned electronic properties of Pt (the intrinsic mechanism) [28,29,36]. The bifunctional effect is illustrated in the following equations ... 

One should note that poisoning of PEMFC anode catalysts by CO is also a severe problem as CO is found to some extent in most H2 gas supplies, as H2 is usually produced by steam reforming of CH4 (and CO is a by-product). It has been reported that a CO content as low as 10 ppm in H2 fuel will result in the poisoning of Pt electrocatalysts [74], As shown in Eqs. 17.8 and 17.9, the formation of OHads by water oxidation at the Pt surface is necessary for the oxidative removal of adsorbed CO. However, the formation of Pt-OH only occurs appreciably above 0.8 V vs. RHE [75]. This factor is considered to be the origin of the high overpotentials for the MOR and COOR and, often, a second metal that can provide oxide species at low potentials is added to Pt electrocatalysts to reduce such overpotentials. For example, Pt-based alloys containing elements such as Ru, Mo, W, and Sn have been used in attempts to speed up the electrocatalysis of methanol [70,76,77]. The Pt-Ru alloy (1 1 atomic ratio) is the most active binary catalyst and is most frequently used as the anode catalyst in DMFCs [78]. Ru is more easily oxidised than Pt and is able to form Ru-oxide adsorbates at 0.2 V vs. RHE, thereby promoting the oxidation of CO to CO2, as summarised in Eqs. 17.11-17.13 ... 

Okada et al. investigated the binary system Pt-Co(MPQH) (where MPQH mono-8-quinolyl-o-phenylenediamine) [89]. The catalysts were heat treated in Ar atmosphere for 2 h. The optimum heat treatment temperature was 873 K. The mass specific catalytic activity of the binary catalysts supported on carbon was 5 A gn compared to 40-60 A gpt for PtRu. It was also shown that the nature and structure of the ligand coordinating the Co had an effect on performance. Co(NH3)6Cl3 did not exert any catalytic effect when combined with Pt, while porphyrin based complexes of Co heat treated at 1073 K yielded approximately 20 to 30 mV lower overpotentials compared to catalyst formulatiorrs using Co(MPQH). 

Combinatorial methods have also been applied to the area of exploring methanol-tolerant Pt and non-Pt alloy eatalysts for DMFC cathodes. Liu et al. investigated a series of Pt-based and non-Pt binary alloys as ORR electrocatalysts by employing the high-throughput optical screening method. The catalyst arrays were prepared... 

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