Platinum hydrogen dissociation

Rosen and Schuldiner studied hydrogen dissociation and hydrogen atom accumulation on platinum and concluded that only a small fraction of the platinum atoms associated with hydrogen are active dissociation sites. 

Fig. 1.56. In the example depicted in this figure, a bifunctional catalyst like platinum on silicon oxide enables the isomerization of methyl-cyclopropane to buten-2 as well as its hydrogenation to butane [349]. The isomerization proceeds on the oxide support. Hydrogen dissociates on the metal particle and the H-atoms spillover onto the oxide support material where they are mobile to react with butene...

On the other hand, in the case of quinone as hydrogen acceptor, an electrochemical mechanism may be instrumental. On platinum, hydrogen molecules may not only dissociate but also ionize anodically and quinone may be reduced cathodically by uptake of electrons and hydrogen ions in accord with Vetter 123). Thus, one has the following tentative scheme in acid solution ... 

It has been proposed that the reaction mechanism for n-hexane and n-heptane isomerization over platinum promoted WZ (PtAVZ) at temperatures about 200 °C is a bifunctional non-classic one [9,10]. Active sites are formed by the interaction of hydrogen dissociated on platinum, migrating by spillover, with WOx surface species. Room temperature... 

Metal deposition on the electrocatalysts active sites can take place as well. Although elements such as chromium are used to enhance the activity of cathode catalysts, it must be in the alloyed form for such enhancement, while in the case of corrosion of metal plates, it will form an adlayer on the catalytic surface, blocking platinum atoms becoming inactive for oxygen or hydrogen dissociation. 

When hydrogen molecules are adsorbed on platinum they dissociate into hydrogen atoms that are covalently bonded (chemically adsorbed) to platinum atoms on the surface. In this case a different isotherm from that of Eq. (13.1-8) applies. Assume that a substance A2 dissociates to form two adsorbed A atoms that occupy two sites on the surface ... 

A major obstacle is related to the anode material. The active component in the anode is a highly dispersed metal supported on graphite that is pressed against the membrane. Platinum is chosen as the active metal because of its efficiency in dissociating hydrogen, but, unfortunately, platinum is also very sensitive towards trace amounts of impurities (e.g. CO) in the hydrogen gas. 

Let the electrolysis of dilute sulfuric acid (so-called electrolysis of water) with a platinum cathode and a platinum anode be considered next. Pure water is a very weak electrolyte and consequently a very poor conductor of electricity. It dissociates very slightly into H+ ions (it may be recalled that in fact, H+ ions does not remain as such but forms hydronium in H30+ by combining with a molecule of water, H+ + H20 H30+) and OFT ions. In the presence of little sulfuric acid (or for that matter any other strong electrolyte) the conductivity, i.e., ionization is greatly increased. The acidified water now contains H+ ions, OFT and SC3 ions. During electrolysis with platinum electrodes, H+ ions are attracted to the cathode, where each ion gains an electron and becomes a hydrogen atom ... 

The reason for this is that platinum is able to adsorb organic compounds dissociatively forming adsorbed hydrogen and organic residues and the oxidation mechanism of the latter involves extremely slow steps. 

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