Oxidation bimetallic catalysts

Sodium or potassium severely poisons Pt-Re catalysts but the manner in Which the alhali metal operates is not apparent. The present study was designed to use ESCA to determine the valence state of Re in Pt-Re bimetallic catalysts. The valence state would be determined in san les that had been reduced and transferred to the instrument without exposure to an oxidizing atmosphere. Catalysts with and without potassium would be examined. 

OH adsorption on Ru is a key factor that makes this metal the major component of various bimetallic catalysts for anode reactions. Ru-OH causes a signihcant inhibition of the ORR [Inoue et al., 2002]. In situ SXS data for the oxidation of Ru(OOOl) in acid... 

Waszczuk et al., 2001b Tong et al., 2002]. Because Ru is deposited as nanosized Ru islands of monoatomic height, the Ru coverage of Pt could be determined accurately. In that case, the best activity with regard to methanol oxidation was found for a Ru coverage close to 40-50% at 0.3 and 0.5 V vs. RHE. However, the structure of such catalysts and the conditions of smdy are far from those used in DMFCs. Moreover, the surface composition of a bimetallic catalyst likely depends on the method of preparation of the catalyst [Caillard et al., 2006] and on the potential [Blasini et al., 2006]. 

Alcohol oxidation by enzymes, 610-613 Alloy/bimetallic catalysts, 6-7, 70-71, 245-266, 317-337 Anderson-Newns Hamiltonian, 33-34 Anion adsorption effects, 143, 174-175, 208-239, 254, 281-283, 336, 525, 535-536... 

Comparing the TPSR results obtained with Co-HFER and Co/Pd-HFER catalysts, it is possible to verify that the introduction of palladium has a major importance for the improvement of the catalytic performance. The presence of Pd species and the redistribution of cobalt oxide species with the formation of Co-oxo cations can have a major role as catalytic sites for the lower temperature activation of CH4 with N02 and N2 formation. A conversion of 80 % of NOx into N2 is obtained with the bimetallic catalyst. 

The presence of 0.3 wt.% Pd on Co-HFER (3 wt.%) catalyst results on a very important increase of low-temperature interaction of CH4 with N02, as a consequence of both the presence Pd species (Pd2+ and PdO) and the cobalt oxides redistribution (formation of Co oxo-cations and decrease of cobalt oxide). With bimetallic catalyst, under oxygen excess conditions, an increase of 30 % in the NOx conversion to N2 is attained. 

TPR of supported bimetallic catalysts often reveals whether the two metals are in contact or not. The TPR pattern of the 1 1 FeRh/SiOi catalyst in Fig. 2.4 shows that the bimetallic combination reduces largely in the same temperature range as the rhodium catalyst does, indicating that rhodium catalyzes the reduction of the less noble iron. This forms evidence that rhodium and iron are well mixed in the fresh catalyst. The reduction mechanism is as follows. As soon as rhodium becomes metallic it causes hydrogen to dissociate atomic hydrogen migrates to iron oxide in contact with metallic rhodium and reduces the oxide instantaneously. 

Another useful bimetallic for fuel cell electrodes is Pt/Ru. Ruthenium is readily oxidized to Ru02 by calcination after it is impregnated. The PZC of ruthenium oxide is unknown. Propose a comprehensive sequence of experiments with which the SEA method can be applied for the synthesis of a Pt/Ru bimetallic catalyst supported on carbon. The goal is to have intimate contact between the Pt and Ru phases in the final, reduced catalyst. 

CO oxidation catalysis showed that, for all the supports, the bimetallic catalyst was more active at low temperatures than the corresponing monometallic and cometallic catalysts. Apparent activation energies for monometallic Pt and Au catalysts were very consistent, near 32 and 80 kJ/mole, respectively. The s)uiergism for PtieAuie catalysts also shows up in the apparent activation energies for these catalysts, which were consistently around 23 kJ/mole. 

Using the dendrimer route, it is possible to prepare supported catalysts not available via traditional routes. Dendrimer derived Pt-Au catalysts having compositions within the bulk miscibility gap can be prepared on several oxide supports. For all the supports studied, the bimetallic catalysts exhibited synergism with respect to mono- and cometallic catalysts for the CO oxidation and hydrocarbon NOx SCR reactions. The bimetallic Pt-Au catalysts also showed evidence of exchanging surface and subsurface atoms in response to strongly binding ligands such as CO. 

Preferential Catalytic Oxidation of CO. CO oxidation experiments (O2/CO =1) were conducted in a simulated gas stream of composition CO (9600 ppm) -b H2 (73.36 vol.%) -b CO2 (23.75 vol.%) -b CH4 (1.93 vol.%) (Table 11.7). The Pt catalyst is more active but less selective. The catalysts that contain more Pt were more active. A combination of Pt with Au forming a bimetallic catalyst enables preferential CO oxidation. It should be noted that the Pt-Au-MCM-41 catalysts are not deactivated in CO2. 

TABLE 11.7. Preferential CO oxidation over Au-Pt bimetallic catalysts" ... 

The surface structure and characteristics (density and acidity) of the hydroxyl groups presented in Fig. 13.21 (using CrystalMaker 2.1.1 software) give very useful information to understand the reactivity of the surface of the particles, particularly when adsorption of another complex is desired to synthesize a bimetallic catalyst, or to control the interaction with an oxide carrier (the deposition step). The isoelectric point calculated with the model (5.9) is in fair agreement with the experimental value (4.3). 

On the basis of the combined weight of the above results, we believe that bifunctional electrocatalytic properties may be operative for both MOR and ORR on the AuPt bimetallic nanoparticle catalysts depending on the nature of the electrolyte. For ORR in acidic electrolyte, the approaching of both the reduction potential and the electron transfer number for the bimetallic catalyst with less than 25%Pt to those for pure Pt catalyst is indicative of a synergistic effect of Au and Pt in the catalyst. For MOR in alkaline electrol)he, the similarity of both the oxidation potential and the current density for the bimetallic catalyst with less than 25%Pt to those for pure Pt catalyst is suggestive of the operation of bifunctional mechanism. Such a bifunctional mechanism may involve the following reactions ... 

Cg Dehydrocyclization. Arguments have been put forward that primary ring closure produces six-membered rings over three important catalyst types oxides, supported platinum, and bimetallic catalysts (107). The postulation of metal catalyzed Cg ring closure does not involve any definite suggestion whether its mechanism is direct or stepwise. ... 

In support of the conclusion based on silver, series of 0.2, 0.5, 1.0, 2.0, and 5.0 % w/w of platinum, iridium, and Pt-Ir bimetallic catalysts were prepared on alumina by the HTAD process. XRD analysis of these materials showed no reflections for the metals or their oxides. These data suggest that compositions of this type may be generally useful for the preparation of metal supported oxidation catalysts where dispersion and dispersion maintenance is important. That the metal component is accessible for catalysis was demonstrated by the observation that they were all facile dehydrogenation catalysts for methylcyclohexane, without hydrogenolysis. It is speculated that the aerosol technique may permit the direct, general synthesis of bimetallic, alloy catalysts not otherwise possible to synthesize. This is due to the fact that the precursors are ideal solutions and the synthesis time is around 3 seconds in the heated zone. 

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