In any catalytic reaction, the noble metals prefer mostly than other metals because they are showing better activity as evidenced by literature reports. What is the reason for their better catalytic activity?
The noble-metals are called "noble " because they are much less reactive than the other metallic-state elements. therefore they remain in the metallic-state much longer and can act as catalysts in certain reactions. Choice depends upon the nature of the reaction. Pd, for example, absorbs hydrogen in the metallic state and is a good reducing agent in organic reactions- RC Ropp
The noble-metals are called "noble " because they are much less reactive than the other metallic-state elements. therefore they remain in the metallic-state much longer and can act as catalysts in certain reactions. Choice depends upon the nature of the reaction. Pd, for example, absorbs hydrogen in the metallic state and is a good reducing agent in organic reactions- RC Ropp
I agree with Ropp. As transition elements form multiple oxidation state, they are prone to react with reactants in a reaction undertaken. Again, being relatively much inert towards reactant, noble metals remain stable in their pristine metallic form (As they do not form various oxidation state). Hence used as catalyst just to enhance the rate of reaction without reacting with reactant molecules.
It wll be over simplification to conclude that the noble metals should always be preferred because of their comparative inertness and non existence of their variable oxidation states. There are quite a number of reactive transition metal ions belonging to the three complete transition series exhibiting variable oxidation states and yet act as very good catalysts. Permit me to speak that many a times, the existence of their variable oxidation states adds to their quality of acting as catalysts (viz. low activation enery).
Five main reasons regarding the role of noble metals (restricting to Pd, Pt) to act as better catalysts may be explained as follows:
[I] Noble metals are not at all susceptible to oxidation by atmospheric O2 while other reactive metal MAY become susceptible.
[II] Noble metals can be dispersed into nano scale particles much more easily than most of other transition metals which can easily adsorb H2/O2.
[III] Porocity of Pd or Pt is much more amenable toH2 adsorption.
[IV].The electrons present in the filled d-subshell (being more exposed) make dissociative adsorption of H2/O2 some what easier.
[V] It is very easy to prepare a MODIFIED CATALYST out of the noble Pd and Pt by impregnating the metal surface with carbon, silica and alumina which can make the reactions to occur at much softer reaction conditions that those of unmodified Pd and Pt catalysts. For example, hydrogenation of 2,4-DNT to 2,4 DAT occurs at 5Mpa /150C in MeOH solvent with Ni; with Pd/C modified catalyst( called carbon supported Pd), the reaction proceeds at 0.4Mpa/ 90C but without MeOH solvent while with Pd/Al2O3 modified catalyst(called alumina supported Pd) occurs at 0.5-0.4Mpa/72C without MeOH.
It is very interesting and helpful information. Some thing else to add is that in case of supported catalysts the oxidation/coupling/hydration/reduction reactions of HC can take place under ambient conditions very efficiently and cost effective because a very small amount of noble metal dispersed on solid support can give a very high re-activity.
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