PtRu catalysts preparation

The Case of PtRu Catalyst Preparation and the Optimum Pt Ru Atomic Ratio... 

The previous sections indicated that at present PtRu remains the most effective binary catalyst for methanol oxidation. A significant amount of work has been carried out and various theoretical and experimental techniques have been brought to bear in order to reveal the details of the Pt-Ru catalytic/co-catalytic effect. For DMFC performance enhancement, which is the prevalent point of view adopted in this review, the situation is further complicated since in addition to intrinsic kinetic effects, the anode performance depends in a synergistic and often poorly understood manner on the PtRu catalyst preparation method, PtRu atomic ratio, surface morphology (e.g., roughness), presence and type of support, operating anode potential range, methanol concentration, and temperature. 

Both PtRu/MgO catalysts prepared from cluster precursor and organometallic mixture were active for ethylene hydrogenation. The apparent activation energy of the former catalyst obtained from the Arrhenius plot during -40 to -25°C was 5.2 kcal/mol and that of the latter catalyst obtained during -50 to -30°C was 6.0 kcal/mol. The catalytic activity in terms of turn over frequency (TOP) was calculated on the assumption that all metal particles were accessible for reactant gas. Lower TOP of catalyst prepared from cluster A at -40°C, 57.3 x lO" s" was observed probably due to Pt-Ru contribution compared to that prepared from acac precursors. 

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. 

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. 

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 influence of the applied potential on the XAS of PtRu fuel cell catalysts is also apparent in data collected under fuel cell conditions. Viswanathan et al. reported XANES data obtained at both the Pt L3 and Ru K edges for a 1 1 PtRu/C catalyst prepared as a Nafion bound MEA. They found that both the Pt and Ru were metallic in both the freshly prepared ME As and ME As under operating conditions. 

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). 

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... 

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... 

Recent work by Lukehart et al. has demonstrated the applicability of this technique to fuel-cell catalyst preparation [44g,h]. Through the use of microwave heating of an organometallic precursor that contains both Pt and Ru, PtRu/Vulcan carbon nanocomposites have been prepared that consist of PtRu alloy nanoparticles highly dispersed on a powdered carbon support [44g]. Two types of these nanocomposites containing 16 and 50 wt.% metal with alloy nanoparticles of 3.4 and 5.4 nm, respectively, are formed with only 100 or 300 s of microwave heating time. The 50 wt.% supported nanocomposite has demonstrated direct methanol fuel-cell anode activity superior to that of a 60 wt.% commercial catalyst in preliminary measurements. 

Figure 2 X-ray difflactograms of the different a) Pt und b) PtRu carbon supported catalysts prepared in Vulcan XC72, Vulcan XC72R, Carbon Pearl and commercial...

Table 2.6 Characterization of the prepared PtRu catalysts Pt content (total, EDX surface, XRD), mean particle sizes d,...

Fig. 7.8 Voltammograms (A) and in-situ infrared spectra (B) recorded from PtRu catalyst particles immobilized on a polycrystalline Au electrode according to the procedure outlined in Fig. 7.7. Measurements were performed in 0.05 M I-I2SO4. PtRu particles were prepared by electroless deposition of Ru on Pt-black to an Ru atom...

Watanabe et al. " reported a preparation procedure for a highly dispersed PtRu catalyst through co-deposition of colloidal Pt and Ru oxides on carbon in aqueous media, followed by a reduction with... 

By the Bonnemann method, PtRu catalysts with well-defined, completely alloyed particles and a very narrow particle size distribution (< 3 nm) were obtained, and showed comparable activity with that of state of the art commercially available catalyst. To simplify the preparation steps and avoid using chloride containing stabilizers, Paulus et developed a modified route by using organoaluminium molecules (e.g. A1(CH3)3) as both the reductive agent and stabilizer. 

Tsuji M, Kubokawa M, Yano R, Miyamae N, Tsuji T, Jun M-S, Hong S, Lim S, Yoon S-H, Mochida I (2006) Fast preparation of PtRu catalysts supported on carbon nanofibers by the microwave-polyol method and their application to fuel cells. Langmuir 23 387-390... 

MWNT and high surface area mesoporous carbon xerogel were prepared and used as supports for mono-metallic Pt and bi-metaUic PtRu catalysts by J.L. Figueiredo [32]. A remarkable increase in the activity was observed when PtRu catalysts were supported on the oxidized xerogel. From the XPS results, it had been shown that the oxidized support helps to maintain the metals in the metalUc state, as required for the electro-oxidation of methanol. 

PtRu catalysts with MCMB as support [35] showed lower polarization characteristics than that with CB as support. Pt-Ru nanoparticles (1.6 nm) were supported on carbon nanotubes (200nm diameter, 8-10 um length) obtained by carbonization of PPy on an alumina membrane [36]. The amount and morphology of Pt nanoparticles depend on the types of carbon nanomaterlals, Including GNFs or CNTs [37]. Surfactant-stabilized Pt and Pt/Ru electrocatalysts for PEMFC had been prepared and investigated by X. Wang [38]. 

Figure 4.14. Arrhenius plot for methanol electrooxidation at 0.5 V vs. RHE using colloidal PtRu catalyst supported on Vulcan XC72. Electrolyte 1 M CH3OH - 0.5 M H2SO4. Scan rate 1 mV s . Pt Ru atomic ratios 2.33 1, , o 4 1 and Pt/C [96]. (With kind permission from Springer Science+Business Media Journal of Applied Electrochemistry, Elecfrooxidation of methanol at platinum-ruthenium catalysts prepared from colloidal precursors atomic composition and temperature effects, 33, 2003, 419-49, Dubau L, Coutanceau C, Gamier E, Leger J-M, Lamy C, figure 11.)...

Carbon particle supported (typically Vulcan XC72) non-oxide ternary combinations of PtRu with elements such as W, Mo, Sn were also investigated by a number of authors. Again the catalyst preparation technique in conjunction with the elemental composition had a big impact on the catalytic activity. Gotz and... 

The method of catalyst preparation (especially in the case of alloys) and the catalyst interaction with the polymer support matrix play important roles in determining the resultant electrocatalytic effect. The catalyst/support couple PtSn/PAni (0.5 pm thickness) with PtSn synthesized by electroreduction at 0.1 V vs. RHE was found to be an effective catalytic system for formic acid oxidation, lowering the anode potential by over 100 mV compared to pure Pt/PAni and PtRu/PAni [324]. Moreover, the oxidation of formic acid on PtSn/PAni commences at low potentials, in the hydrogen adsorption region, around 0.1-0.2 V vs. RHE. 

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