MeOH Oxidation Catalysts

It is likely that both mechanisms are active and dependent on potential. At low potentials (<200 mV) on PtRu, the bifunctional mechanism is not active because Ru is unable to dissociate adsorbed H2O to produce OH. However, above 250 mV, this does occur and CO oxidation by adsorbed OH becomes the dominant reaction in achieving CO tolerance. This is strongly related to the use of PtRu as a MeOH oxidation catalyst because CO oxidation is also the rate-determining step for this reaction. 

Eigure 3.9 shows temperatures for 50% conversion (T o) of CH3OH and its decomposed derivatives over Pt/y-Al203, Pd/y-Al203, and Au/a-Pe203 catalysts [52]. Eor MeOH oxidation, palladium is more active than platinum, while gold lies in between. These three catalysts are similarly active for the oxidation of HCHO and HCOOH. Catalytic oxidation at temperatures below 0°C can proceed over palladium and platinum for H2 oxidation, while it happens over gold for CO oxidation. 

Table 3.1 Catalytic activity of supported lb metal catalysts for MeOH oxidation [53]. ...

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. ... 

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. 

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. 

In the search for catalyst formulations superior to PtRu, many alternative Pt binary alloys have been investigated. In recent years, strong interest in PtOs alloys and bimetallics has been shown. Although Os does promote MeOH oxidation on Pt, it is somewhat less active than Ru, apart from high overpotentials (>0.50 V), where Os is less susceptible to overoxidation compared to... 

Cabucicchio et al.25 employed Raman and Mbssbauer spectroscopies to document the destruction of the active phase and subsequent reaction of the active metals with the support during MeOH oxidation over silica-supported Fe203-Mo03 catalyst. Burriesci et al. 6 also employed Mossbauer with ESCA to document the reduction of the active phase for a similar industrial catalyst and argue that excess M0O3 will help retain activity. 

FTIR studies on the Pd/TiN ethanol oxidation catalyst have also proved useful [91]. OH was again seen to form on the TiN support during reaction, which again reduces poisonous CO buildup on the Pd. Furthermore, XPS analysis of the material showed a metal-support interaction which results in the decrease of the Pd-CO bond, thereby further reducing its poisoning effects. These attributes make the liN-supported material a highly efficient catalyst support for both MeOH and EtOH oxidation. 

Methanol (MeOH) crossover from the anode to the cathode in the direct methanol fuel cell (DMFC) is responsible for significant depolarization of the Pt cathode catalyst. Compared to Pt-based catalysts, NPMCs are poor oxidation catalysts, of methanol oxidation in particular, which makes them highly methanol-tolerant. As shown in Fig. 8.25, the ORR activity of a PANI-Fe-C catalyst in a sulfuric acid solution is virtually independent of the methanol content, up to 5.0 M in MeOH concentration. A significant performance loss is only observed in 17 M MeOH solution ( 1 1 water-to-methanol molar ratio), a solution that can no longer be considered aqueous. The changes to oxygen solubility and diffusivity, as well as to the double-layer dielectric environment, are all likely to impact the ORR mechanism and kinetics, which may not be associated with the electrochemical oxidation of methanol at the catalyst surface. Based on the ORR polarization plots recorded at... 

Radiolytic deposition of Pt-Ru nanoparticles on polymer MWNT nanocomposites was performed by y-irradiation in aqueous solution at room temperature and ambient pressure. The three polymers used were poly(AAc), poly(MAc) and poly(VPBAc). The Pt-Ru nanoparticles were then deposited on to polymer-MWNT nanocomposites by the reduction of metal ions using y-irradiation to obtain polymer-MWNT with Pt-Ru nanoparticles. The catalysts obtained were then characterized by XRD, XPS, TEM and elemental analysis. The catalytic efficiency of the catalyst based on polymer-MWNT nanocomposites was examined for CO stripping and MeOH oxidation for use in a direct methanol fuel cell (DMFQ. The catalyst based on polymer-MWNT nanocomposites shows enhanced activity for the electrooxidation of CO and MeOH oxidation over that of a commercial E-TEK catalyst. 

Pt-M catalysts (M = Ru, Ni, Co, Sn and Au) based on polymer-MWNT nanocomposites were prepared using one-step y-irradiation. Two different types of functional polymers, poly(vinylphenylboronic acid) (PVPBAc) and polyvinylpyrrolidone (PVP), were used to prepare nanocomposites. The Pt-M catalysts obtained based on polymer-MWNT nanocomposites were then characterized by XRD, TEM and elemental analysis. The catalytic efficiency of the Pt-M catalysts based on polymer-MWNT nanocomposites was also examined for CO stripping and MeOH oxidation for use in a DMFC. The catalytic efficiency of the Pt-M catalyst based on polymer-MWNT nanocomposites for MeOH oxidation followed the order Pt-Sn > Pt-Co > Pt-Ru >Pt-Au>Pt-Ni catalysts. The CO adsorption capacity of the Pt-M catalyst based on polymer-MWNT nanocomposites for CO stripping decreased in the order Pt-Ru > Pt-Sn > Pt-Au > Pt-Co > Pt-Ni catalysts. 

Figure 11.4 Tafel plots for methanol oxidation on Pt + Ru/C coUoid catalysts with different atomic compositions at T = 298K (a) 1.0 M MeOH (h) 0.1 M MeOH (0.5 M H2SO4 sweep...

Other companies (e.g., Hoechst) have developed a slightly different process in which the water content is low in order to save CO feedstock. In the absence of water it turned out that the catalyst precipitates. Clearly, at low water concentrations the reduction of rhodium(III) back to rhodium(I) is much slower, but the formation of the trivalent rhodium species is reduced in the first place, because the HI content decreases with the water concentration. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilization of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives.8 The kinetics of the title reaction with respect to [MeOH] change if H20 is used as a solvent instead of AcOH.9 Kinetic data for the Rh-catalyzed carbonylation of methanol have been critically analyzed. The discrepancy between the reaction rate constants is due to ignoring the effect of vapor-liquid equilibrium of the iodide promoter.10... 

In the carbonylation of MeOH in the presence of Rh-exchanged zeolites, the Rhm ions are reduced to Rh1 ions, which lead to Rh-dicarbonyl and Rh-carbonyl-acetyl complexes.29-32 IrY and RhY zeolites catalyze the carbonylation of MeOH in the presence of a Mel promoter. The kinetics have been determined and IR spectra suggested that with the Ir catalyst the ratedetermining step was the addition of MeOH to the active species followed by migration of a Me coordinated to Ir. With the Rh catalyst, oxidative addition of Mel was the rate-determining step.33 A series of EXAFS measurements was made to determine the structural basis for... 

The reaction of alcohols with CO was catalyzed by Pd compounds, iodides and/or bromides, and amides (or thioamides). Thus, MeOH was carbonylated in the presence of Pd acetate, NiCl2, tV-methylpyrrolidone, Mel, and Lil to give HOAc. AcOH is prepared by the reaction of MeOH with CO in the presence of a catalyst system comprising a Pd compound, an ionic Br or I compound other than HBr or HI, a sulfone or sulfoxide, and, in some cases, a Ni compound and a phosphine oxide or a phosphinic acid.60 Palladium(II) salts catalyze the carbonylation of methyl iodide in methanol to methyl acetate in the presence of an excess of iodide, even without amine or phosphine co-ligands platinum(II) salts are less effective.61 A novel Pd11 complex (13) is a highly efficient catalyst for the carbonylation of organic alcohols and alkenes to carboxylic acids/esters.62... 

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