Hydrogen, adsorption, platinum metal

Despite of this charge transfer many of these systems may be treated as ideally polarizable electrodes, if the adsorbed species are not transformed into a different component present inside the bulk phase. The latter condition is violated, for example, in the hydrogen adsorption at metals of the platinum group in which the adsorbed hydrogen atoms can be in equilibrium with protons in solution and hydrogen molecules in gas phase or hydrogen dissolved inside the metal. The latter system corresponds to perfectly polarizable electrodes, see Ref. [13] for further discussion. 

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). 

Karlberg GS. 2006. Adsorption trends for water, hydroxyl, oxygen, and hydrogen on transition-metal and platinum-skin surfaces. Phys Rev B 74 153414. 

The second most widely used noble metal for preparation of electrodes is gold. Similar to Pt, the gold electrode, contacted with aqueous electrolyte, is covered in a broad range of anodic potentials with an oxide film. On the other hand, the hydrogen adsorption/desorption peaks are absent on the cyclic voltammogram of a gold electrode in aqueous electrolytes, and the electrocatalytic activity for most charge transfer reactions is considerably lower in comparison with that of platinum. 

Figure 7.9 The mechanism for the hydrogenation of an alkene as catalyzed by finely divided platinum metal (a) hydrogen adsorption (b) adsorption of the alkene (c) and (d), stepwise transfer of both hydrogen atoms to the same face of the alkene (syn addition).

The admixture of lead to platinum has a similar effect (Fig. 5). At the same time, the aromatizing activity increases up to about 1 1 Pt Pt atomic ratio 24). With even more lead it scatters aroimd somewhat lower values 66). Electron donation from lead to platinum has been proved by infrared spectroscopy, so one may wonder whether lead is present as metal in the catalyst (75). The additive effect can also be interpreted by its creating hydrogen-deficient surface sites favorable for aromatization. When more lead is present than platinum (i.e., where no more continuous platinum surface is probable), the inverse correlation between hydrogen adsorptivity and activity ceases to exist. 

On the other hand, some of Suhrmann s electrical resistance measurements on nickel and platinum films and Eischens observations, of the effect of Hj on the infrared spectrum of chemisorbed CO on platinum, referred to earlier, suggest electron-transfer from hydrogen to the metal, i.e., adsorption of positive ions. 

According to long-lasting experimental efforts, the use of alloy catalysts that contain a less noble metal whose oxide exhibits low solubilities in acid electrolytes—in particular Sn and Bi are effective in this respect—enhance the catalytic activity of platinum. The rationale of this effect has been that the oxide of the nonnoble component at close atomic distance from the Pt surface atoms supplies by spillover the oxygen that is necessary to oxidize the adsorbed CO species. Today research and development turn more to Ru and lr or Rh, the more easily oxidizable platinum metals as alloying metals that seem to be at least as efficient as Bi and Sn and are certainly more stable than those in acidic environments—in particular if the anode potential becomes more anodic in cases of poor supply of fuel (158). The Pt-Ru anode exhibits a sizeable higher oxidation current for methanol and for adsorbed hydrogen than the Pt electrode, indication that a smaller part of the Pt electrode surface is blocked by CO adsorption. Still the catalytic activity is too low because the onset of the anodic peak of methanol oxidation is at a... 

After maximum coverage of platinum films upon hydrogen adsorption, three desorption peaks have been observed. The same peaks have been found for the alloys, but the relative populations of the various adsorption types were different. Here again, the peak corresponding to the larger heat of adsorption is most influenced, leading to the conclusion that it corresponds to hydrogen atoms bonded to several metal atoms. 

Recently, a nickel zeolite hydrogenation catalyst has been prepared by a novel route (94) involving the adsorption and decomposition of nickel carbonyl onto NaX, which would not be expected to result in the formation of acid sites. In general, the platinum metal-containing zeolites are more active than those containing other transition metals. For example, in zeolite Y the following activity series has been found,... 

Fig. 5. Hydrogen adsorption isotherms at 293 K with platinum-gold/Aerosil catalysts V, Pt 98, Au 2 mol %, 1.0 wt % metal 0.516 g catalyst A, Pt 90, Au 10 mol %, 0.9 wt % metal, 0.510gcatalyst , Pt 67, Au 33 mol %,0.9wt % metal, 0.500g catalyst O.Pt 15, Au 85 mol %, 1.0 wt % metal, 0.450 g catalyst standard pretreatment (cf. text). Filled symbols, amount of adsorbed hydrogen remaining after pumping at 293 K for 30 min, after equilibration at indicated pressure. Catalyst samples identified from corresponding symbols above. Within the limits of experimental accuracy, no adsorption could be detected on a Pt 0, Au 100 mol %, 1.0 wt % catalyst, using a 0.500 g sample (20).

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