Platinum for hydrogen

The electro-catalytic oxidation of hydrogen, and reduction of oxygen, at carbon supported platinum based catalysts remain essential surface processes on which the hydrogen PEM fuel cell relies. The particle size (surface structure) and promoting component (as adsorbate or alloy phases) influence the activity and tolerance of the catalyst. The surface chemical behavior of platinum for hydrogen, oxygen, and CO adsorption is considered, in particular with respect to the influence of metal adsorbate and alloy components on close packed and stepped (defect) platinum surfaces. Dynamical measurements (employing supersonic molecular beams) of the... 

The best performance was achieved with platinum for hydrogen evolution and iridium for oxygen evolution. The catalyst loading was between 0.5 and 3 mg cm . Cobalt clathrochelates were also tested for oxygen evolution, but their performance is much lower [51]. Typical voltages at 80-90 °C and 0.1 MPa are given in Table 8.7. 

The conditions for hydrogenation of alkynes are similar to those employed for alkenes In the presence of finely divided platinum palladium nickel or rhodium two molar equivalents of hydrogen add to the triple bond of an alkyne to yield an alkane... 

Other elements have atoms that can have different ratios of protons to neutrons. Indeed, hydrogen actually consists of three types of atoms. All hydrogen atoms have the same number of protons (one for hydrogen), giving each a mass of 1 Dalton, but some atoms of hydrogen also contain one neutron in the nucleus as well as the proton (mass of 2 Da), while yet others have two neutrons with each proton (mass of 3 Da). Thus hydrogen has three naturally occurring isotopes of mass 1, 2, and 3 Da. Chemically, there are only small differences between the reactivities of the different isotopes for any one element. Thus isotopes of palladium aU react in the same way but react differently from all isotopes of platinum. 

Transition-metal organometallic catalysts in solution are more effective for hydrogenation than are metals such as platinum. They are used for reactions of carbon monoxide with olefins (hydroformyla-tion) and for some ohgomerizations. They are sometimes immobihzed on polymer supports with phosphine groups. 

Standard Test Methodfor Surface Area of Catalysts. (D3663—78) Standard Test Method for Hydrogen Chemisorption on Supported Platinum on Alumina Catalysts. (D3908-80) American Society for Testing and Materials (ASTM), Philadelphia, PA. 

A variety of catalysts including copper, nickel, cobalt, and the platinum metals group have been used successfully in carbonyl reduction. Palladium, an excellent catalyst for hydrogenation of aromatic carbonyls is relatively ineffective for aliphatic carbonyls this latter group has a low strength of adsorption on palladium relative to other metals (72,91). Nonetheless, palladium can be used very well with aliphatic carbonyls with sufficient patience, as illustrated by the difficult-to-reduce vinylogous amide I to 2 (9). 

Palladium, platinum, and Raney nickel 7,126) all have been used successfully under mild conditions for hydrogenation of the azido function. In especially sensitive molecules, subambient temperature may prove advantageous. Reduction of methyl 3, 5-dihydroxy-4 -methoxy-7-(3-azido-3-carboxypropoxy)flavanone (32) in aqueous alkali proved capricious, The major product (33) was contaminated by several other products when reagents were mixed and hydrogenated at room temperature or above, but by the... 

The product (12.1 mg) obtained in the above step was dissoived in 0.3 mi of water, to which was then added a catalytic quantity (about 5 mg) of platinum oxide. Hydrogenation was made with hydrogen gas at a pressure of 35 kg/cm for 1.5 hours. The reaction solution was filtered to remove the catalyst, and the filtrate was concentrated to dryness, giving the desired product 3, 4 -dideoxykanamycin B in the form of its monocarbonate. The yield was 11.5 mg (95%). -f 110° (c 1, water). The overall yield of 3, 4 -dideoxykanamycin B... 

The platinum(IV) compound that has shown most promise is carboplatin (paraplatin), which received FDA approval in 1990. Features to note in its structure are the use of hydroxy and carboxylate groups to improve water solubility. As noted above, the ammine ligand has been found to need at least one hydrogen, possibly for hydrogen-bonding to phosphate groups in the DNA (Figure 3.116). 

Oxygen inhibition of NO reduction may operate by its greater affinity for hydrogen and CO. Several authors have found that when the molar ratio of 02/C0 is between the stoichiometric ratio of 0.5 and 0.7, the rate of reduction of NO rapidly falls to zero. They concluded that NO reduction can proceed only with an excess of hydrogen and CO 38, 39, 102). Jones et al. have found that below the temperature of 400°F, hydrogen prefers to attack NO instead of oxygen over platinum and palladium. This preference does not extend above 400°F 103). 

Figure 10.5. Evolution of the intrinsic catalytic activity of platinized platinum for the hydrogenation of maleic acid (Cm=10 3 M, 299 K, 0.5 M HC104), as a function of potential, ( ) spontaneously set potential, (I, ) imposed potential in absence, ( ) and in presence ( ) of H2 (1 atm).5 Reprinted with permission from Elsevier Science.

The photo-Kolbe reaction is the decarboxylation of carboxylic acids at tow voltage under irradiation at semiconductor anodes (TiO ), that are partially doped with metals, e.g. platinum [343, 344]. On semiconductor powders the dominant product is a hydrocarbon by substitution of the carboxylate group for hydrogen (Eq. 41), whereas on an n-TiOj single crystal in the oxidation of acetic acid the formation of ethane besides methane could be observed [345, 346]. Dependent on the kind of semiconductor, the adsorbed metal, and the pH of the solution the extent of alkyl coupling versus reduction to the hydrocarbon can be controlled to some extent [346]. The intermediacy of alkyl radicals has been demonstrated by ESR-spectroscopy [347], that of the alkyl anion by deuterium incorporation [344]. With vicinal diacids the mono- or bisdecarboxylation can be controlled by the light flux [348]. Adipic acid yielded butane [349] with levulinic acid the products of decarboxylation, methyl ethyl-... 

It was quickly seen from studies on platinum single crystals that voltammograms for hydrogen adsorption and desorption differ somewhat among the different faces and between the single-crystal faces and polycrystalline platinum. Despite these differences, though, they have common traits as weU. The areas under these curves,... 

Cyclic voltammetry is perhaps the most important and widely used technique within the field of analytical electrochemistry. With a theoretical standard hydrogen electrode at hand, one of the first interesting and challenging applications may be to try to use it to make theoretical cyclic voltammograms (CVs). In following, we set out to do this by attempting to calculate the CV for hydrogen adsorption on two different facets of platinum the (111) and the (100) facets. 

Poisoning of platinum fuel cell catalysts by CO is undoubtedly one of the most severe problems in fuel cell anode catalysis. As shown in Fig. 6.1, CO is a strongly bonded intermediate in methanol (and ethanol) oxidation. It is also a side product in the reformation of hydrocarbons to hydrogen and carbon dioxide, and as such blocks platinum sites for hydrogen oxidation. Not surprisingly, CO electrooxidation is one of the most intensively smdied electrocatalytic reactions, and there is a continued search for CO-tolerant anode materials that are able to either bind CO weakly but still oxidize hydrogen, or that oxidize CO at significantly reduced overpotential. 

Takasu Y, Eujii Y, Yasuda K, Iwanaga Y, Matsuda Y. 1989. Electrocatalytic properties of ultra-fine platinum particles for hydrogen electrode reaction in an aqueous solution of sulfuric acid. Electrochim Acta 34 453-458. 

It is very well known that Pt is one of the best metal catalysts for hydrogen as well as for organic oxidations. Nevertheless, a comparison of the electrochemical behavior of hydrogen and any of these organic substances shows large differences. While hydrogen establishes its reversible thermodynamic potential with platinum in an aqueous acidic solution very quickly, the reversible potential of the other fuels could never be experimentally observed. 

Further evidence for surface effects upon the stereochemistry of electrochemical reduction of ketones comes from the discovery that the nature of the cathode material may effect stereochemistry. Reduction of 2-methylcyclo-hexanone affords pure trans-2-methylcyclohexanone at mercury or lead cathodes, a mixture of cis and trans alcohols (mostly trans) at nickel, and pure cis alcohol at copper 81 >. Reduction could not be effected at platinum presumably hydrogen evolution takes place before the potential necessary for reduction of the ketone can be reached. 

The activity of metals other than platinum for skeletal reactions of larger molecules is not well documented, particularly in a mechanistic sense. Carter, Cusumano, and Sinfelt (157) have recently studied the reaction of n-heptane on a series of group VIII metals in the form of hydrogen-reduced (300°C) metal powders. The nature of the reaction pathways is summarized in Table IX. Although many metals have been... 

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