[crt]

Hello,

I would have a question to heterogeneous catalysts. One finds commonly that heterogeneous catalysts need higher temperature than homogeneous catalysts to achieve a reasonable activity. What is the reason for this? Is there an explanation on molecular level?

thank you

crt

[Juan R.]

Time ago since I studied catalysis, but if I recall correctly a larger temperature would be needed for providing to the reactants the enough energy to be physically adsorbed or chemically bonded to the catalyst surface. Also notice that the solid is localized at some point and a higher temperature increase the average temperature of reactant molecules and thus the probability of that can met with the catalyst.

[Stepan]

Hello,

I would have a question to heterogeneous catalysts. One finds commonly that heterogeneous catalysts need higher temperature than homogeneous catalysts to achieve a reasonable activity. What is the reason for this? Is there an explanation on molecular level?

thank you

crt

I do not think this is always true.

In homogeneous analysis you are limited to how high you can rise the temperature by a boiling point of your solvent.  So, you mostly work at low temperature, let say <200C.

In heterogeneous catalysis (Gas-solid) you keep temperature high enough to keep products volatiles before they kill your catalyst and this would be > 200C. If products and reagents are volatile you can use low temperature. 20 years ago I made a solid catalyst for CO ->CO2 oxidation, which worked at 15C.

[crt]

Hey Guys, thanks for the answers.

Juan: physisorption does not require activation energy, however, chemisorption indeed does. But when a molecule is chemisorbed, you break the same bonds which you break with a homogeneous catalyst. However, there should be something here, what I do not see, I think...

If we keep the amount of the homogeneous catalyst and the active sites of a heterogeneous catalyst equal, I would expect the same results. That is, you only should increase the number of active sites (higher dispersion, metal loading, cat amount...) and keep the temperature low.

Stepan: I do not think it has to do anything with volatility. I think some industry guys were very happy if ammonia synthesis would run at room temperature. However, I can accept that high temperature is not _always_ needed.

There should be something around chemisorption vs steps usual in homogeneous catalysis (insertion, reductive elimination, oxidative addition...). Any idea?

Thanks again

crt

[Stepan]

Hey Guys, thanks for the answers.

Juan: physisorption does not require activation energy, however, chemisorption indeed does. But when a molecule is chemisorbed, you break the same bonds which you break with a homogeneous catalyst. However, there should be something here, what I do not see, I think...

If we keep the amount of the homogeneous catalyst and the active sites of a heterogeneous catalyst equal, I would expect the same results. That is, you only should increase the number of active sites (higher dispersion, metal loading, cat amount...) and keep the temperature low.

Stepan: I do not think it has to do anything with volatility. I think some industry guys were very happy if ammonia synthesis would run at room temperature. However, I can accept that high temperature is not _always_ needed.

There should be something around chemisorption vs steps usual in homogeneous catalysis (insertion, reductive elimination, oxidative addition...). Any idea?

Thanks again

crt

Ammonia at room temperature it would be nice..

But Fischer-Tropsh at room temperature would kill the catalyst at the fraction of the second. Mass transfer is a reason why Ethylene polymerization catalyst works a few second only. It gets covered with PE 

[crt]

Ok, diffusion seems believable. But is it the only thing? Can one compare activity in the two systems if not diffusion but the reaction is the rate limiting step?

The elementary steps are very different in heterogeneous and homogeneous catalysis. Is there anything what should connect for example temperature + adsorption + activity in heterogeneous catalysis and temperature + elementary steps + activity in homogeneous catalysis?

Have a nice day

[Stepan]

Beside the reagent mass transfer, I do not see other explanation for temperature difference between homogeneous and heterogeneous catalysis.

Heterogeneous catalysts work in liquid phase at low temperatures as good as homogeneous catalysts (let say hydrogenation on Ni). Pt catalyst burns hydrocarbons at room temperatures, Pd burns CO at room temperature - no problem.

Also, homogeneous catalysts are often too complex (let say enzymes or organometallic compounds), and do not stand elevated temperature.