1.1 Why platinum:
Many electrochemical experiments were performed on platinum and palladium surface because of some of its properties as follows,
1. Electronic and geometric factors:
On theoretical grounds, maximum activity in a hydrogenation process is expected to be shown by metals in group VIII of the periodic table and especially by metals in the palladium and platinum triads [9]. For easy chemical reaction of gases on the metal surface it requires vacant ‘d’ orbital which can accept e- from the reactants. If more number of vacant ‘d’ orbitals are present then there may be difficulty for dehydrogenation process because gases are strongly chemisorbed. VIII group elements contain one e- in their vacant ‘d’ orbital. This is called electronic factor of elements. Second condition is metal atoms should have spaced and ordered arrangement. Palladium and platinum have arranged atomic structure is called its geometric factor. These two factors determine group VIII elements have affinity towards hydrogenation reaction.
1. Platinum and palladium both have same cubic closed pack (CCP) crystal structure. Indicates that hydrogenation occur over the surfaces may give similar results. [Further experiments show nearly similar diffusion coefficients of hydrogen on metal surfaces].
2. Heat of
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The chemisorption energy is typically in the range of EChem ≈ 20 - 150 KJ/mol H2 and thus significantly higher than the respective energy for physisorption which is in the order of 4-6 kJ/mol H2. This hydride formation is exothermic, as entropy of product is lower than metal and hydrogen gas and the reverse reaction is endothermic. Therefore hydrogen desorption requires heat supply. Hydrogen gas or hydrogen atoms are charged to metal from an electrolyte. Following schematic diagram shows the process