I am aware that the catalysts enhance the rates of reaction towards equilibria, but according to principle of microscopic reversibility, it equally enhances the rate of reverse reaction too provided the conditions. then why hunt for new catalysts by trial and error of all elements and their combinations.
Does this mean Rh wire guaze which oxidises NH3 to NOx, can also helpful in deNOx and ammonia synthesis?
sorry if it sounds absurd, just want to know whether it's just theoretical or practised in industries depending on the necessities. Need explainations or papers, Thanks in advance
This is possible, but depends on a lot of contidions. The key is the strcuture of the concerned catalyst.
As Jie Shan Qiu says structure is the key. If the reaction is structure insensitive then the law of microscopic reversibility should hold and it does. You can show this using isotope exchange processes for example in propane dehydrogenation at ~600C in the presence of deuterium. Thermodynamics will allow ~50% yield of propene. If you examine the extent of deuterium inclusion in both propane and propene it is clear that the forward and backward reactions are taking place simultaneously. However if the reaction is structure sensitive then the answer is not so simple.
On your point about why not use the same catalyst for forward and reverse reactions, the catalyst may not be stable under the conditions required by thermodynamics to get good yields. An example would be acetylene hydrogenation to ethylene over Pd, at the temperature required to get a decent yield for the reverse reaction the Pd will have sintered making it unsuitable.
Thank you Professor, For dehydrogenations at such high temperatures(600 C), i read that catalytic effect is not predominant because homogeneous (thermal) reactions are equally effective as heterogenous (catalysed) reactions and industries mostly prefer homogeneous thermal reactions in such cases.
However, isotopic studies reveal good example of the equilibria established between forward and reverse reactions. thank you for explaining the effect of catalyst structure in selective hydrogenations
Yet, we have the principle of microscopic reversibility emphasizing the rate acceleration towards and away from the equlibria. Do we have any examples in history of catalysis where this principle is applied in search for a novel catalyst for the target reaction. i want to know the way the researchers approach in choosing a catalyst.
thanks again
Catalysts work by lowering the energy of the transition state of a reaction (the highest potential energy point during a reaction), this essentially means that the catalyst must 'bind' to the transition state, and knowing this allows you to rationally approach its design.
You first need to isolate the transition state and then find/design a molecule to bind to it. However, transition states are not stable (by definition), so instead you can start by finding a stable molecule that 'looks like' the predicted transition state.
For a particular example, the formation/hydrolysis of esters from/to organic acids proceeds via a transition state RO(R)C(OH)O- that looks a little like a phosphonate ester RO(R)P(OH)O. While the former is not isolable, the later is, and the interactions of specific phosphonate esters with candidate catalyst molecules can be probed spectroscopically or by X-ray etc. Strongly binding candidate catalysts can then be tested against the target esters/acids. (recognising that the phosphonate ester will not have been an exact mimic of the actual transition state, so some fine tuning will be necessary).
Thank you Sir, that's useful information,
however we are choosing the catalyst depending on the known or analogous reactions. if the pioneers have chosen a less appropriate catalyst or group of catalysts, the effect will be cascading to whomever approach using those ideas. we have many elements which are functional and few bifunctional and their combinations will further have wide activities.
If we have a fundamental principle or set of clues, a lot of effort can be saved. I thought the microscopic reversibility was one of such concept. but its hardly used in practice. I am eager to know if there are any ground principles for catalyst selection. i will be happy with any such article or info., it helps a lot.
- Badges
- Science topic
- Similar topics
- Methodology
- Research Methodology
- Research Papers