Catalyst PtRu catalysts

Gavrilov AN, Savinova ER, Simonov PA, Zaikovskii VI, Cherepanova SV, TsirUna GA, Parmon VN. 2007. On the irrfluence of the metal loading on the stmcture of carbon-supported PtRu catalysts and their electrocatal3ftic activities in CO and methanol electrooxidation. Phys Chem Chem Phys 9 5476-5489. 

Jusys Z, Kaiser J, Behm RJ. 2002a. Composition and activity of high surface area PtRu catalysts towards adsorbed CO and methanol electrooxidation. A DBMS study. Electrochim Acta 47 3693-3706. 

Davies JC, Bonde J, Logadottir A, Nprskov JK, Chorkendorff I. 2005. The ligand effect CO desorption from PtRu catalysts. Euel Cells 4 429. 

These routes rely on the direct transformation of soluble molecular species into supported metal (or mixed metal) particles. One method that has recently become popular is the "polyol" method. This takes a solution of metal salts, the carbon support, and a polyalcohol such as ethylene glycol. On heating, the polyol acts as both stabilizer and reductant, forming reduced metal particles on the carbon. It has been used successfully to prepare Ft and PtRu catalysts. ... 

True bimetallic molecular precursors have been used to prepare PtRu catalysts. Steigerwalt, Deluga, and Lukehart impregnated the complex (77-C2H4) (Cl)Pt(77-Cl)2Ru(Cl)(773 r/2-2,7-dimethyloctadienediyl) on graphitic nanofibers via solvent evaporation. The deposited complex was subsequently decomposed by reductive annealing up to 650°C. 

As with CNTs, OMCs are often evaluated as supports for PtRu particles for MeOH oxidation. The range of materials tested to 2003 was reviewed by Chan et al., who found a number of examples that showed superior activity to conventional Ft and PtRu catalysts. More recent work—for example, use of PtRu catalysts derived from mesoporous Si02 spheres by Chai et al.—also showed enhancements over PtRu/XC72 catalysts for MeOH oxidation. ... 

An alternative approach in structuring a PtRu catalyst for CO tolerance has been reported by Brankovic, Wang, and Adzic,i who prepared coreshell Pt-Ru catalysts with submonolayer coverage of Pf (10-50%). In the presence of 100 ppm CO/H2, the 10% ML Pt catalyst showed much higher CO tolerance than a conventional PtRu catalyst with three times less Pt loading. 

The PtRu bimetallic system has been the catalyst of choice for MeOH oxidation in acid elecfrolyfes since its discovery by workers at Shell in the early 1960s2 In practice, PtRu lowers the overpotential for MeOH oxidation by >200 mV compared to pure Pt. The MeOH oxidation reaction on Pt and PtRu is probably the most studied reaction in fuel cell electrocatalysis due to its ease of sfudy in liquid electrolytes and the many possible mechanistic pathways. In recent years, the deposition of PtRu particles onto novel carbon supports and the novel PtRu particle preparation routes have proved popular as a means to demonstrate superiority over conventional PtRu catalysts. 

There have been many reports of variants of PtRu based on novel preparation chemistry or novel support materials showing superior activity to commercially available PtRu catalysts. These have been recently reviewed by Liu et al. One interesting feature of this work is that the PtRu atomic ratio used has been fixed at 1 1 (e.g., Chu and Gilman and Takasu et al. ). However, this ratio disagrees with the optimal ratios determined from bulk PtRu alloys. 

Although bulk- and surface-decorated samples agree broadly in terms of optimal Pt Ru surface ratios for MeOH oxidation, there is less agreement with practical PtRu catalysts, although the data are sparse. This would suggest that PtRu particles show Pt-segregated surfaces as predicted by theoretical calculations. 

Cyclic voltammograms of PtPb and PtRu catalysts in 0.1 M H2SO4, 0.5 M MeOH at room temperature. (Reprinted with permission from S. Maksimuk et ak, Journal of the American Chemical Society, 129,8684 (2007). Copyright 2007 American Chemical Society.)... 

Ru provides sites for water activation as well as having an electronic effect on the Pt atoms, such that CO is less strongly adsorbed. In situ XAS measurements have been used to determine the structure of PtRu catalysts, to assess the magnitude of any electronic effect that alloy formation may have on the Pt component of the catalyst, and to provide evidence in support of the bifunctional mechanism. 

The analysis of the EXAFS of alloy catalyst particles is inherently more complicated than that of single metals. In the case of PtRu catalysts there is an added complication that the backscattering from Pt and Ru neighbors at similar distances interfere with one another, giving rise to beats in the EXAFS data. This phenomenon was first described by McBreen and Mukerjee ° for a poorly alloyed 1 1 atomic ratio PtRu/C catalyst. The presence of beats in the EXAFS data is more apparent in the EXAFS obtained at the Pt L3 edge for a well mixed 1 1 PtRu/C catalyst than in that of a poorly mixed catalyst of the same composition, as shown in Figure 27 compare panels a and c. Pandya et al. showed that the beats occur because the difference in the backscattering phase shifts from Pt and Ru is... 

Neto and co-workers examined the ex situ Pt L3 EXAFS for a series of PtRu catalyst powders in air of varying nominal composition from 90 10 through to 60 40 atom %. The catalysts were prepared using a formic acid reduction method developed by the authors which resulted in very poorly alloyed particles, even after heat treatment to 300 °C under a hydrogen atmosphere. Unfortunately, the authors were not able to obtain Ru K edge data to identify the local structure of the Ru in their catalysts. 

The applied electrode potential has been shown to have an effect on both the XANES and EXAFS of PtRu catalysts. The variations of the Pt d band vacancy per atom, (/7j)t,s, with potential over the range 0.0—0.54 V vs RHE for both the poorly mixed 1 1 PtRu/C catalyst investigated by McBreen and Mukerjee ° and a well mixed 1 1 PtRu/C catalyst studied by Russell et al. were less than that for a pure Pt/C catalyst. McBreen and Mukerjee attributed this difference to a reduction in the adsorption of hydrogen on the Pt sites of the alloy catalyst. The results also provide evidence of an electronic effect upon alloying Pt with Ru. The effects on the Ru XANES were much less significant, but some evidence of a change to a higher oxidation state at potentials above 0.8 V was observed. ... 

The importance of collecting such data in situ is illustrated by the work of Lin et al. ° and O Grady et al. Lin et al. found that a commercial PtRu catalyst consisted of a mixed Pt and Ru oxide, in contrast to the catalyst prepared in their own laboratory. However, the data were collected ex situ in air. O Grady et al. showed that even a commercial unsupported PtRu catalyst showed heavy oxidation... 

PtRu catalysts with controlled atomic ratios were prepared by adjusting the nominal concentrations of platinum and ruthenium salts in the solution, whereas different mean particle sizes could be obtained by adjusting some electric parameters of the deposition process, e.g., ton (during which the current pulse is applied) and toff (when no current is applied to the electrode), as determined by different physicochemical methods (XRD, EDX, and TEM) [40], Characterization by XRD led to determine the crystallite size, the atomic composition and the alloy character of the PtRu catalysts. The atomic composition was confirmed using EDX, and TEM pictures led to evaluate the particle size and to show that PtRu particles formed small aggregates of several tens of nanometers (Figure 9.10). 

The theoretical cell voltage of a DMFC at standard conditions is 1.20 V. The materials used in DMFCs are similar to those in PEMFCs. Pt, PtRu, and Nafion membrane are used as cathode catalyst, anode catalyst, and proton transfer membranes, respectively. However, the catalyst loading in a DMFC is much higher than the loading used in H2/air fuel cells, because both side reactions are slow (Pt loadings 4 mg/cm2 for a DMFC, 0.8 mg/cm2 for a H2/air fuel cell). 

Methanol oxidation in a DMFC is more difficult than H2 oxidation in a PEMFC, and the kinetics is slow, even using state-of-the-art PtRu catalysts. The role of Ru in methanol oxidation is to provide oxygenated species to oxidize the CO formed on Pt catalytic sites at low potentials. The mechanism can be written as follows ... 

Nano-sized PtRu catalysts supported on carbon have been synthesized from inverse micro emulsions and emulsions using H2PtClg (0.025 M)/RuCl3 (0.025 M)/NaOH (0.025 M) as the aqueous phase, cyclohexane as the oil phase, and NP-5 or NP-9) as the surfactant, in the presence of carbon black suspended in a mixture of cyclohexane and NP-5-I-NP-9 [164]. The titration of 10% HCHO aqueous solution into the inverse micro emulsions and emulsions resulted in the formation of PtRu/C catalysts with average particle sizes of about 5 nm and 20 nm respectively. The RuPt particles were identified by X-ray diffraction. X-ray photoelectron, and BET techniques. All of the catalysts prepared show characteristic diffraction peaks pertaining to the Pt fee structure. XPS analysis... 

It can also be observed from this figure that Sn-containing catalyst is a more effective catalyst for the oxidation of CO than that containing Ru, as a lower onset potential of the oxidation wave is obtained with the former catalyst. It has also to be noted that PtSn catalysts are less active towards methanol electrooxidation than PtRu catalysts (see Section IV. 1). ° However, adsorbed CO species are proposed as reaction intermediates of methanol electro-oxidation, which seems to lead to a paradoxical behavior of PtSn based catalysts. In CO stripping experiments, a negative shift of the onset potential for the oxidation of adsorbed CO on PtSn also occurs. " On the basis of in situ infrared spectroscopy studies coupled with electrochemical measurements, Mo-... 

However, the presence of two CO oxidation peaks seems to indicate that two different zones of oxidation exist on PtRu catalysts. In situ infrared reflectance spectroscopy can again be used to confirm this hypothesis. 

This quantitative analysis allows a comparison in the product yield, Wq, for the ethanol oxidation reaction between different catalysts. In the discussed example, the three catalysts considered present close yields, with a low CO2 production (for Pt/C and PtRu/C catalysts CO2 is only produced during the positive going scan), whereas acetaldehyde and acetic acid both present a product yield of 60-70 % and 30-40 %, respectively. A shght increase in the acetaldehyde yield can be observed for the PtRu catalyst, leading to a lower Faradic efficiency for the ethanol oxidation reaction, compared to that obtained on Pt/C and PtsSn/C catalysts. 

Considering the importance of Ru island size, recent studies have been conducted by our group to determine the aging mechanism of some commercially available PtRu catalysts age. PtRu black electrocatalysts were obtained from Tanaka and Johnson-Matthey (referred to hereafter as Tk and JM respectively) and were observed ageing by potential cychng between 0-0.8 V vs. RHE. Both materials (1 1 Pt Ru) were found to have slightly... 

Recent reports [22, 23] have demonstrated better CO tolerance with higher loadings (1-2 mg/cm ) PtRu catalysts in PEFC anodes, particularly at cell current densities lower than 200 mA/cm. In contrast, a thin-fihn anode catalyst of very low PtRu loading, prepared as a composite of carbon-supported PtRu (0.15 mg/cm ) and recast ionomer [14], did not exhibit lower losses when 5-20 ppm CO was introduced into the hydrogen feed stream [21]. The same PtRu catalyst was successful, however, in... 

Hsin Yu L, Hwang Kuo C, Yeh C-T (2007) Poly(vinylpyrrolidone)-modified graphite carbon nanofibers as promising supports for PtRu catalysts in direct methanol fuel cells. J Am Chem Soc 129 9999... 

Since on pure platinum, methanol oxidation is strongly inhibited by poison formation, bimetallic catalysts such as PtRu or PtSn, which partially overcome this problem, have received renewed attention as interesting electrocatalysts for low-temperature fuel cell applications, and consequently much research into the structure, composition, and mechanism of their catalytic activity is now being undertaken at both a fundamental and applied level [62,77]. Presently, binary PtRu catalysts for methanol oxidation are researched in diverse forms PtRu alloys [55,63,95], Ru electrodeposits on Pt [96,97], PtRu codeposits [62,98], and Ru adsorbed on Pt [99]. The emphasis has recently been placed on producing high-activity surfaces made of platinum/ruthenium composites as a catalyst for methanol oxidation [100]. 

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