Pt/Ru atomic ratio

In this reaction mechanism, three or four Pt sites are involved in methanol dissociation, whereas only one Ru site is involved in water activation, so that the best Pt/Ru atomic ratio is between 3 1 and 4 1 [15]. 

Electrocatalysts Pt(wt.%) Ru(wt.%) Pt Ru atomic ratio Pt Ru atomic ratio EDX Particle size (nm) Scherrer ... 

According to the bifunctional mechanism at least three Pt atoms are needed to adsorb one methanol molecule and activate it to form a Pt-(CO)ad while mily one Ru atom is needed to activate one water molecule to form Ru-OH. Thus, the best atomic ratio of Pt to Ru should be 4 1. However, as mentioned above, the best MOR activity is found for 1 1 Pt-Ru atomic ratio. Actually, the proportion 1 1 Pt-Ru provides the smaller CO oxidation onset potential [78]. Therefore, this scenario indicates that the CO oxidation is the rate determining step of MOR on Pt-Ru catalyst. So, it is not a surprise to find the best Pt-Ru proportion for MOR being equal to the best Pt-Ru proportion for the CO oxidation. 

Pt-Ru catalysts also received much attention in ethanol oxidation [40 3]. Camara et al. [42] investigated the EOR activity of Pt-Ru electrodeposits as a function of their atomic composition and demonstrated that the catalytic activity of Pt-Ru is strongly dependent on the Ru content. The optimum Pt Ru composition was 3 2. At low Ru concentration, there are insufficient Ru sites to effectively assist the oxidation of adsorbed residues, and the oxidation current remains almost at the levels obtained for pure Pt. This finding is in line with the observation from Lamy et al. [27] showing a poor activity with the Pt Ru atomic ratio of 4 1. Ru concentrations higher than ca 40 % caused a decrease in current, and this effect can be rationalized in terms of inhibition of ethanol adsorption, presumably due to the diminution of Pt sites. 

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

Furthermore, an interesting aspect of the Ru effect relates to the effect of temperature and the optimum Pt Ru ratio. Gasteiger et al. showed that dissociative methanol adsorption can occur on Ru sites as well, but it is a temperature-activated process [94]. Therefore, at low temperatures (e.g., 298 K) a higher Pt Ru atomic ratio (above 1 1) is required to facilitate the dissociative adsorption and dehydrogenation of methanol preferentially on Pt, whilst at high temperatures (e.g., 333 K and above) a surface richer in Ru is beneficial (e.g., 1 1 at. ratio) since Ru becomes active for chemisorption and the rate determining step switches to flie reaction between COad and OHad [94]. 

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.)...

Highly active PtRu nano-alloys supported on Vulcan XC72 were also produced by colloidal methods, including the co-reduction of Pt(acac)2 and Ru(acac)2 (where acac = acetylacetonate) with 1,2-hexadecanediol in octyl ether with oleylamine and oleic acid surfactants controlling the particle growth [105]. The average particle size was 2.4 nm 0.5 nm and the Pt Ru atomic ratio was 1 1. The mass activity of the PtRu nano-alloy at 0.45 V vs. SHE was 32.9 mA mg compared to 11.7 mA mg" for the commercial E-TEK Inc. catalyst [105]. 

Support Catalyst load (mg cm eora) Pt Ru atomic ratio Catalyst surface area per geometric area of the support (m m geom)... 

Figure 10.23. Bright-field TEM micrograph of the nanocomposite (left) and plot of Pt Ru atomic ratios measured by HR-EDS for individual metal alloy nanoparticles (right) [167]. (Reprinted with permission from Chem Mater 2003 15 3320-5. Copyright 2003 American Chemical Society.)...

Several approaches have been considered to optimize the electrochemical behaviour of bimetallic catalysts. For example, some authors have studied the effect of PtRu catalyst structure on methanol and CO electro-oxidation, " whereas other authors " have focused on the optimal Pt/Ru atomic ratio. 

Dubau et al. showed that, for a given Pt/Ru atomic ratio, alloying Pt and Ru did not necessarily lead to the most active catalyst for the methanol oxidation reaction, but that dispersing Pt and Ru metals in strong interactions on the same carbon grains (decoration of platinum particles by smaller ruthenium particles) led to higher current densities for the electro-oxidation of methanol for potentials lower than 0.5 V vs RHE. An electrocatalytic enhancement of methanol oxidation at Pt particles decorated by Ru compared with the alloy compounds of the same composition was also observed by other authors. " ... 

During methanol oxidation the efficient catalyst must allow a complete oxidation to CO2. Currently, carbon-supported Pt-Ru catalysts have been shown to be promising among the best candidates available for electrochemical oxidation of methanol at anodes of DMFCs [13,14]. A Pt/Ru atomic ratio of 1 1 was reported as the preferred atomic composition [62,63]. 

Lukehart and coworkers [26] successfully prepared a Pt-Ru/graphitic carbon nanofiber nanocomposite exhibiting high relative performance as a direct methanol fuel cell anode catalyst Multistep deposition and reactive decomposition of a single-soiuce molecular precursor of Pt and Ru metal on herringbone graphitic carbon nanofibers affords a Pt-Ru/GNF nanocomposite containing Pt-Ru alloy nanoclusters widely dispersed on the GNF support The nanocomposite has a total metal content of 42 wt% with a bulk Pt/Ru atomic ratio of about 1 1 and... 

The Pt Ru atomic ratios were obtained by Energy dispersive X-ray analysis (EDX) using a scanning electron microscope (SEM) Phillips XL30 with a 20 keV electron beam and equipped with ED AX DX-4 microanaliser. 

PtRu/C electrocatalysts were prepared by hydrothermal carbonization process using different carbon sources (Table 1). In the reaction conditions, the carbohydrates and/or their products of degradation can act as reducing agent of Pt(IV) and Ru(III) ions, which acts as catalysts of the carbonization process [2], The carbonization yields of the as-synthesized materials were in the range of 65-75 wt%. After thermal treatment at 900°C a weight loss of about 50-60 wt% was observed for all prepared materials and the Pt Ru atomic ratios and the PtRu loadings were similar to the nominal values. 

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