What is the role of catalyst in various reactions?

[jd12345]

There are many reactions in which i don't understand what the role of catalyst is.

Reactions like hydrogenation of alkenes , use of Ni i understand - it kinda breaks the hydrogen-hydrogen bond and provide it for the alkene.
In contact process i.e. SO2 + 1/2O2 ----> SO3 catalyst is V2O5. I understand this as V2O5 reacts with SO2 to provide oxygen and then oxidses

But i don't understand catalyst in reactions like :- habers process i.e. N2 + 3H2 ----> 2NH3
catalyst is Fe but what is its role?
similarly CO + 2H2 ---> CH3OH catalyst is cobalt but what is its use??
CH3OH ----> HCHO + H2 catalyst is heated copper but how does it remove hydrogen

Plz tell me what are the role of catalyst in these 3 reactions -
There are many other reactions where catalyst are used and i don't understand but its not possible to ask them all so if you can tell me in general too it'll be very helpful

 

[Mike H]

The processes you mention are all cases of heterogeneous catalysis. You have a catalyst (typically a solid in many applications) and the reactants adsorb to the surface. The bond weakens, such as in N₂, with a triple bond between the nitrogen atoms. You also can gain an increase in effective concentration (instead of the reactants just floating around in the gas or liquid phase, they're all now adsorbed to the catalyst surface).

Be sure to read the link, and I would Google "heterogeneous catalysis." There's plenty of information out there, as it's both an example of really interesting chemistry but also very practical chemistry.

 

[jd12345]

how do we determine which metal to use in which reaction?
example: in habers process iron is used. This might be because it adsorbs nitrogen very efficiently. But what makes it such a good adsorbent of nitrogen?

 

[Borek]

how do we determine which metal to use in which reaction?

Experimentally.

From what I know selecting catalysts is still kind of a voodoo.

 

[jd12345]

okay, but the question is :- why do certain metals adsorb specific gases better than others? Iron is used in habers process, why?
There must be some reason!

 

[xplosiv3s]

okay, but the question is :- why do certain metals adsorb specific gases better than others? Iron is used in habers process, why?
There must be some reason!

Possibly due to the Iron having few variable oxidation states that makes it more suitable than say, Manganese.

 

[Mike H]

For the Haber-Bosch process, at least, there's an element of pure practicality. Back in the day, using iron was cheaper/less troublesome than getting an adequate amount of ruthenium or osmium (which are better catalysts, although I think there was also an issue with catalytic lifetime/catalyst poisoning, but don't quote me on that). The process is also sensitive to the surface topography.

Like Borek said - catalyst design still has vast amounts of voodoo in it. While there are methods to investigate the detailed molecular processes at work, there is still a long way to go before we can design catalysts off the top of our head and have them work just as we expect.

 

[jd12345]

ok thank you