Vulcan XC72 catalyst

Fig. 8.8 Solid line Cyclic voltammogram of Pt/PrintexXE2 catalyst (63% metal loading, 2.128 jg Pt) prepared by the instant method. Dotted line Commercial Pt/Vulcan XC72 catalyst from ETEK (58% metal loading, 2.32pg Pt). Scan rate lOOmVs", electrolyte is Ar-saturated 0.5 M perchloric acid [59].

Aiming to deepen into the surface chemistry of typical PEMFC commercial catalysts, a widely used commercial sample of Pt (20wt.%) on Vulcan XC72 catalyst from E-TEK and Vulcan XC72 carbon black from Cabot Corp., have been used for the study and taken as reference materials. The surface diffusion processes associated with hydrogen and oxygen adsorbed species in this catalyst and in PEM fuel cell electrodes prepared with Vulcan XC72 have been examined in detail. 

The adsorption properties of the Pt/Vulcan XC72 catalyst have been evaluated by means of different procedures using H2 and CO as probe molecules. The influence of the support surface groups on the adsorption processes has been analyzed on the catalyst when subjected to different thermal treatments. To enlighten the participation of the surface groups of the support in the adsorptive properties of the catalyst, exchange experiments with isotopically labeled molecules have been performed. 

ADSORPTION-EXCHANGE PROPERTIES OF THE PT/VULCAN XC72 CATALYST... 

For the support material of electro-catalysts in PEMFC, Vulcan XC72(Cabot) has been widely used. This carbon black has been successfully employed for the fuel cell applications for its good electric conductivity and high chemical/physical stability. But higher amount of active metals in the electro-catalysts, compared to the general purpose catalysts, make it difficult to control the metal size and the degree of distribution. This is mainly because of the restricted surface area of Vulcan XC72 carbon black. Thus complex and careM processes are necessary to get well dispersed fine active metal particles[4,5]. 

Pt/Ru electrocatalysts are currently used in DMFC stacks of a few watts to a few kilowatts. The atomic ratio between Pt and Ru, the particle si2 e and the metal loading of carbon-supported anodes play a key role in their electrocatalytic behavior. Commercial electrocatalysts (e.g. from E-Tek) consist of 1 1 Pt/Ru catalysts dispersed on an electron-conducting substrate, for example carbon powder such as Vulcan XC72 (specific surface area of 200-250 m g ). However, fundamental studies carried out in our laboratory [13] showed that a 4 1 Pt/Ru ratio gives higher current and power densities (Figure 1.6). 

This complex reduction reaction leads to a relatively high overvoltage - at least 0.3 V- thus decreasing the cell voltage of the fuel cell by the same quantity. Pt-X binary catalysts (with X = Cr, Ni, Fe,. ..) give some improvements in the electrocatalytic properties compared vdth pure Pt dispersed on Vulcan XC72 [17]. 

Colloidal Pt/RuO c- (C5 0.4nm) stabilized by a surfactant was prepared by co-hydrolysis of PtCU and RuCls under basic conditions. The Pt Ru ratio in the colloids can be between 1 4 and 4 1 by variation of the stoichiometry of the transition metal salts. The corresponding zerovalent metal colloids are obtained by the subsequent application of H2 to the colloidal Pt/Ru oxides (optionally in the immobilized form). Additional metals have been included in the metal oxide concept [Eq. (10)] in order to prepare binary and ternary mixed metal oxides in the colloidal form. Pt/Ru/WO c is regarded as a good precatalyst especially for the application in DMECs. Main-group elements such as A1 have been included in multimetallic alloy systems in order to improve the durability of fuel-cell catalysts. PtsAlCo.s alloyed with Cr, Mo, or W particles of 4—7-nm size has been prepared by sequential precipitation on conductant carbon supports such as highly disperse Vulcan XC72 [70]. Alternatively, colloidal precursors composed of Pt/Ru/Al allow... 

Figure 12 Scheme of the preparation of colloidal Pt/Ru/Al PEMFC anode catalysts (>20% metal on Vulcan XC72 ) via the precursor concept. (From Ref. 66.)... 

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...

The work presented shows that an increase of the electrocatalytic activity can be obtained, if a suitable method for the catalyst synthesis is employed. In this sense, the Alcohol Reduction Method showed a positive effect, probably due to the good particle dispersion at the carbon surface and the suitable particle size distribution that this method produces. For the methanol oxidation results, an increase in the cell potential by PtRu/C electrocatalyst on Vulcan XC72 system was observed compared to the PtRu/C E-TEK formulation. This can be explained due to the better conductivity of this Carbon Suport, enhancing the speed of the electron transference in the Methanol Oxidation Reaction (MOR).These results can also be attributed to the good particle distribution at... 

Experimental The three catalyst precursors were synthesized using Pt- and Ru-chlorides as the starting material for the co-reduction with N(oct)4Bet3H (Cat.1), LiBet3H (Cat2), respectively and the acetylacetonate derivatives for Al(me)3 coreduction (Cat 3). The as-prepared colloidal catalyst precursors were supported on Vulcan XC72, as exempUfied for (Cat 1) in Fig. 2.21, and activated via conditioning at 300°C (cf Section 2.4.2.1). 

Results EDX indicated that all three catalysts had identical metal loadings and confirmed a uniform distribution of the bimetallic particles on the Vulcan XC72 surface. Particle size and Pt and Ru content were determined via XRD, as shown in Fig. 2.22. The first peak at 20=25° originates from Vulcan XC-72 carbon support while the other peaks are reflections of the face centered cubic (f c.c.) crystal lattice of Pt (vertical fines are pure Pt references). 

Case Study 2 Carbon (Vulcan XC72) Supported Bimetalhc Electrocatalysts. Preparation by the borate method of Pt-Cu colloidal catalysts having structural variations and comparative study of their electrochemical performance. 

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]. 

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... 

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