PtMo Catalysts

Of the alternative Ft formulations, the FtMo system has been the most studied in recent years. Work on bulk FtsMo alloys by Grgur, Markovic, and Ross showed similar CO tolerance to FtRu in the presence of H2. This tolerance was correlated with the ability of FtMo to oxidize CO at potentials as low as 0.05 V. However, unlike Ru but similar to Sn, the Mo appeared to oxidize CO just at neighboring Ft sites, with the majority of CO oxidized af potentials typical of pure Ft. The surface Mo atoms were found to be oxidized even at 0.0 V. Therefore, it was postulated that H2O dissociation to form OH was mediated by a Mo(IV)/(Vl) couple. Carbon-supported FtMo catalysts were reported to have better CO tolerance than FtRu in MEA testing up to CO concentrations of 100 ppm.i39 

Although great efforts have made to improved reformate-tolerant catalysts, no intrinsic reformate-tolerant catalysts have yet been discovered. The FtMo system appears to offer the greatest possibilities, especially at higher CO concentrations and at higher temperatures. However, the corrosion sensitivity of Mo over time needs to be addressed before these catalysts become practical systems. 

---

Tanoka, T., Yokata, K., Doi, H. et al. (1997) Selective catalytic reduction of NO over PtMo catalysts with alkaline or alkaline earth metal under lean static conditions, Chem. Lett. 409. 

Mo alloys of Pt have also been shown to enhance the CO tolerance of PEM fuel cell catalysts.Two peaks are often observed in the CO stripping volta-mmograms for PtMo catalysts the first at approximately 0.4 V vs RHE and the second at approximately 0.75 V. The first has been attributed to enhanced oxygen transfer from Mo oxy-hydroxide species on the surface of the catalyst particles. XANES at the Mo K and Pt L3 edges has provided support for the presence of such oxy-hydroxide species. Mukerjee et have shown that the... 

PtMo alloys are not as effective as PtRu for methanol, or ethanol, oxidation. As shown in Figure 29, the d band vacancy per Pt atom for the PtMo/C catalyst continues to increase until 0.6 V vs RHE, in contrast to the behavior of PtRu/C. ° The authors attribute this difference to the lack of removal of the Cl fragments from the particle surface by the oxy-hydroxides of Mo. However, the difference in the electrocatalytic activity of PtRu and PtMo catalysts may be attributed to ensemble effects as well as electronic effects. The former are not probed in the white line analysis presented by Mukerjee and co-workers. In the case of methanol oxidation, en-... 

Figure 2. (a). In situ XANES spectra for PtMo catalyst collected at the Mo k-edge (20,000 eV) at the indicated electrode potentials, (b). XANES Calibration curve constructed using the change in edge energy as a function of oxidation state of Mo. 

Ball S, Hodgkinson A, Hoogers G, Maniguet S, Thompsett D, Wong B. The proton exchange membrane fuel cell performance of a carbon supported PtMo catalyst operating on reformate. Electrochem Solid-State Lett 2002 5 A31-4. 

Platinum alloys offer better tolerance towards CO. Especially, PtRu and PtMo alloys show superior tolerance towards CO [62], Nonetheless, the performance of presently known catalysts is far from satisfactory. 

DBU DMC DMF EC EO EOS GSS ILs MBMTBP MEA MW PC PDMS PEG PEGda PEO PMPS PO PPG PPGda PTC PTHF PTMO PVP Diazabicyclo[5.4.0] -undec-7-ene Dimethylcarbonate Dimethylform amide Ethylene carbonate Ethylene oxide, oxyethylene Equation of state Gas-saturated solution Ionic liquids 2,2,-methylene-bis(4-methyl-6-tert-butylphenol) Monoethanolamine Molecular weight Propylene carbonate Polydimethylsiloxane Polyethylene glycol Poly(ethylene glycol) diacrylate Polyethylene oxide Poly(methylphenylsiloxane) Propylene oxide Poly(propylene glycol) Poly(propylene glycol) diacrylate Phase-transfer catalyst Poly(tetrahydrofuran) Polytetramethylene oxide Polyvinyl pyrrolidone... 

As is shown in the first part of the paper it is believed that Xe absorbs only in noncrystalline regions of polymers. Therefore it can be expected that crystalline domains form a diffusion barrier for Xe. For the right sizes of crystallites this would imply that the Xe diffusion coefficients are dependent on the diffusion time. Such effects have been found in some catalysts [25]. Attempts to detect similar phenomena in semi-crystalline polymers so far failed, possibly because the systems chosen here do not have the right internal crystalline structure (in the PBT/PTMO case) or the crystallites are too small (in the case of EPDM). Results on semi-crystalline non-elastomer polymers will be published elsewhere. 

As mentioned above, direct methanol oxidation and reformate tolerance represent two very challenging but significantly different electrocatalytic issues. This is despite the fact that poisoning by CO (or similar Ci moieties) is one of the critical aspects for both fuels. Binary catalysts such as PtSn, PtMo or PtRu offer superior performance but the precise reason for this is not known. At least 3 different mechanisms have been proposed, whereby the alloying M element ... 

Tomishige K, Nagasawa Y, Lee U, Iwasawa Y (1997) Performance and characterization of a [PtMo j/MgO catalyst The catalytic activity for NO-CO reactions and structural analysis by EXAFS. Bull Chem Soc Jpn 70 1607... 

Figure 18 Polarization curves for the electrooxidation of H2 containing 0.1% CO on the PtMo—4 1 and 3 1 catalyst RTLEs normalized by the alloy-specific surface area in the layer and compared to the curves for the bulk alloy RDEs. T = 333 K.

Yermakov (172) to account for the stability of PtMo/Si02 catalysts prepared by sequential decomposition of ally lie complexes. 

The above calculations have been extended to a number of other alloys, notably PtSn and PtMo as these have special interest in CO oxidation electrocatalysisOn PtSn, CO was found to interact only with the Pt sites, not with the Sn sites. This leaves the Sn sites available for the activation of water necessary to oxidize the chemisorbed CO, explaining why RSn is such an unusually good catalyst for the electrochemical oxidation of CO (at low potentials). ... 

A molecular perspective of reactions from quantum chemistry calculations is the first step toward a theoretical design of new electrodes (e.g. binary or even ternary alloys). While reaction mechanisms on Pt and Pt alloy surfaces are getting clearer, details of these mechanism still remain elusive. For example, bifunctional mechanism of CO oxidation on PtSn and PtMo has received very little theoretical attention. Loading effects (CO, OH or specifically adsorbed anions) on CO oxidation is also poorly understood and requires further investigation. Theoretical calculations are also required to understand catalyst reorganization. Details of these calculations are required for accurately modeling the macroscopic kinetics on well-defined electrode surfaces and ultimately designing nanocatalyst particles. 

Bimetallic PtMo alloy catalysts have thus attracted considerable attention as a result of their high catalytic activity in H2 oxidation using a CO/Hg Such enhanced performance was... 

CO oxidation through the formation of Ru-OH. The formation of Ru-OH is ca. 0.2 V lower than the formation of Pt-OH. The formation of Mo-OH occurs at an even lower potential, and thus PtMo is also a good CO-tolerant catalyst... 

A Novel PtMo ]/MgO Catalyst for Alkane-to-Alkene Conversion... 

---