Why do some heterogeneous catalysts need a support, and others not?

You seem confused.

A heterogeneous catalyst is one that does not dissolve in the reaction solvent. if Pd/C dissolved in (insert solvent here) it wouldn't make a very good heterogeneous catalyst.

Perhaps the question you should ask is "what effect does the catalyst support have on the solubility of the catalyst?"

Quote: Originally posted by Mesa

You seem confused. A heterogeneous catalyst is one that does not dissolve in the reaction solvent. if Pd/C dissolved in (insert solvent here) it wouldn't make a very good heterogeneous catalyst. Perhaps the question you should ask is "what effect does the catalyst support have on the solubility of the catalyst?"

No no no . I understand the difference between heterogenous and homogenous. What I was asking is why do these catalysts seem more active in their heterogenous form (in comparison to be dissolved in the homogenous state). Palladium Chloride for instance is soluble in water, but Pd/C not so much. Would the surface area of the palladium molecules not be maximized in the totally dissolved form, versus on the solid support? There are plenty of various metals that are capable of dissolving hydrogen like Palladium and Platinum, but why are they never used in their dissolved form?

The form of the catalyst should not have too much of an effect on its interaction with hydrogen. Whether Pd/C vs PdCl, Platinum Oxide vs Platinum Black. The hydrogen isn't really forming a bond with the molecule(despite being called Metal-Hydrides), but rather it is adsorbing and dissolving into the lattice. Hence why Palladium for instance is thought to be able to absorb 900x its molarity in hydrogen. Some metals can do even more.


When a reaction is put under pressure the hydrogen is going to dissolve into the water/solvent just as it would in the air above the water. Ideally the hydrogen should interact with and absorb into the Palladium/Platinum particles just as easily, if not more easily in their totally dissolved form than when they are on a solid support. It is the internal lattice of the catalytic metal that weakens the H2 bond so I am not entirely clear why these catalysts are rarely used in homogenous form. Is it simply because they are harder to recover where as in heterogenous form they can be filtered out? Or are they for some unknown reason less active?

[Edited on 13-2-2014 by Electra]

Quote: Originally posted by Electra

Quote: Originally posted by Mesa   You seem confused. A heterogeneous catalyst is one that does not dissolve in the reaction solvent. if Pd/C dissolved in (insert solvent here) it wouldn't make a very good heterogeneous catalyst. Perhaps the question you should ask is "what effect does the catalyst support have on the solubility of the catalyst?" No no no . I understand the difference between heterogenous and homogenous. What I was asking is why do these catalysts seem more active in their heterogenous form (in comparison to be dissolved in the homogenous state). Palladium Chloride for instance is soluble in water, but Pd/C not so much. Would the surface area of the palladium molecules not be maximized in the totally dissolved form, versus on the solid support? There are plenty of various metals that are capable of dissolving hydrogen like Palladium and Platinum, but why are they never used in their dissolved form? The form of the catalyst should not have too much of an effect on its interaction with hydrogen. Whether Pd/C vs PdCl, Platinum Oxide vs Platinum Black. The hydrogen isn't really forming a bond with the molecule(despite being called Metal-Hydrides), but rather it is adsorbing and dissolving into the lattice. Hence why Palladium for instance is thought to be able to absorb 900x its molarity in hydrogen. Some metals can do even more. When a reaction is put under pressure the hydrogen is going to dissolve into the water/solvent just as it would in the air above the water. Ideally the hydrogen should interact with and absorb into the Palladium/Platinum particles just as easily, if not more easily in their totally dissolved form than when they are on a solid support. It is the internal lattice of the catalytic metal that weakens the H2 bond so I am not entirely clear why these catalysts are rarely used in homogenous form. Is it simply because they are harder to recover where as in heterogenous form they can be filtered out? Or are they for some unknown reason less active? [Edited on 13-2-2014 by Electra]