More catalyst atoms or molecules exposed to the reactants. As the surface area increases the proportion of surface atoms with respect to the bulk increases markedly.
Thanks Dr Alan . But there are many catalytic reactions where the active catalytic component is not the support but the nanoparticles supported on it, in these cases also when the support surface area is increased there is an increase in the catalytic activity. The increase in surface atom to bulk atom will happen for the support but not for the active catalytic component ( for suppose the metal nanoparticle).
My concern is does better dispersion is the only reason of higher catalytic activity in these cases or any other intrinsic property of the supported nanoparticle also undergoes a change with the increase of the surface area of the support.
Good question. The easy answer is that catalytic activity is the rate of reaction. In the case of dispersed catalysts, that rate is proportional to the catalytic sites exposed, but I think you know this. Can the active sites dispersed on a surface have a higher activity than the same site in bulk? Yes. Anything that alters the near-environment of a catalytic site can alter its activity in my view. In the case of transition metals, this is certainly true. But, this problem gets complicated fast. More often than not, dispersing reagents on surfaces and exposing the product to special thermal conditions produces high activity for reasons that are never understood.
Dear Dr. Frank D Mango,
Yes I agree that anything that anything alter the near environment of the catalytic active site will alter its catalytic activity. Again you have mentioned that it is certainly true for the transition metals, then why it is not so certainly true for the metals belonging to the s block and p block elements ?
The factors that are altered around the the catalytic active site (cheifly the metal ion) is predominanly the geometry and the redox states of the metal ions. Change/distortion in geometry => better binding of substrates with the metal ions. Better binding=> ease of formation of a transition state (RD step) leading to formation of products. Increase in surface area increases the probability of exchange of metal ions (if it is favoured by other factors like the acidity or acid sites in the solids), but care should be taken care to have optimum loading of metal ions that enhance the diffusion (of substrate in and products out) properties of the solids, for a better catalytic activity
Regarding the metal ions, transition metal ions show variable valency as you will be knowing. Most of the catalytic processes are redox or electron transfer processes where the metal - substrate has to act as donor/ acceptor couple, and the metals can accept or donate electrons to form the intermediate radicals which quickly form products. The redox properties of transition metal ions are richer than than the s and p block elements.
As you said the metal nanoparticles are the active sites, but they are influenced by the supports in exposing their surfaces, increasing their stability, increasing the number of binding sites and so on.
Bikash, in simple terms its effect is very similar to increasing the concentration in a reaction, as the concentration increases the rate of the reaction increases due to an increase in the number of collisions.
Here, the larger surface area allows more reactants to reach the catalyst atoms/molecules. Thus more "collisions" take place and the rate becomes faster.
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