Chemistry catalysts- transition metalsWatch
Hello! Can you explain to me why some transition metals act as great heterogeneous catalysts and some don't? What determines whether they'll be good or bad catalysts? I know that it depends on adsorption strength, but how do we know which transition metal is adsorbed more?
On a basic level you are looking for a 'Goldilocks zone' of adsorption.
If adsorbtion is too weak, you never have enough substrate absorbed on a surface to allow reaction to progress at a reasonable rate. If adsorption is too strong then your products don't get released from your catalyst and that can rapidly lead to poisoning of your catalyst which is again useless.
Likewise these factors can be tuned, for example by changing partial pressure of reactants, or my altering Temperature. Increasing T increases reaction rate on the surface, but also decrease the amount of substrate absorbed on the surface at any given time so it's a trade off in this situation.
On top of that you have to account for he fact that the exact mechanism of surface based catalysis isn't fully understood and many possible mechanisms exist (Langmuir-Hinshelwood and Eley-Rideal being most common). This again leads to different considerations when carrying out catalysis.
Other factors that you might what to be clued up on include the surface structure (eg steps at an atomic level often lad to the most active sites) and also the fact that species may adsorb to the surface in more than one way (chemisorption and physisorption, dissociation and non-dissociative, Langmuir vs BET isotherm type absortion etc. etc.).
A final thing to consider is that different metals have different affinities for different substrates.
As a very rough guide, you can use a metals behaviour in organometallic chemistry and in complex ions to get some idea of how well it might bind some substrates. For example Platinum is likely to bind CO very well (extended d orbital make for efficient pi back donation and hence a strong synergic bond) but Ti is likely to be less good at binding CO. Ti, you would think, would be a much better at absorbing Oxygen, because Ti-O bonds are ubiquitous in nature mainly due to their stability.
Another way to see how well substrates bind is to measure (or look up) binding energies for different species on different metals.
I've tried to give a brief but relatively comprehensive overview of the factors involved, if you want to find out more I'd recommend you try to get hold of an introductory text on Hetergeneous catalysis.
I find the following oxford chemistry primers to be quite a good introduction for an undergraduate sort of level:
Surfaces- Garry Attard and Colin Barnes
The Basis and Applications of Heterogeneous Catalysis- Michael Bowker