Hello Rohini Singh,

An inhibitor slows down a reaction but it does not participate in the reaction and does not change its chemical composition (for example, the corrosion inhibitors). Whilst catalytic poisons (in the raw material or formed during the reaction) interact with the catalyst and change its composition. The poisoning can be reversible (the formation of coke on the catalyst, which can be burned) and irreversible (poisoning of the nickel catalyst by impurities of sulfur from the feed to form nickel sulphide).

BW

Anna

Inhibitor:

Inhibitor is a substance that decreases the rate of, or prevents, a reaction. An inhibitor can reduce the effectiveness of a catalyst in a catalysed reaction (either a non-biological catalyst or an enzyme). E.g., if a compound is so similar to (one of) the reactants that it can bind to the active site of a catalyst but does not undergo a catalytic reaction then that catalyst molecule cannot perform its job because the active site is occupied. When the inhibitor is released, the catalyst is again available for reaction.

Catalytic poisons : Substances which destroy the activity of the catalyst by their presence are known as catalytic poisons.

(i) For example, the presence of traces of arsenious oxide in the reacting gases reduces the activity of platinized asbestos which is used as catalyst i

(iii) The platinum catalyst used in the oxidation of hydrogen is poisoned by CO.

The catalyst inhibitors presence lowers catalytic activity, either by total or partial masking of the active sites or induced selective competition with the active sites. Inhibitors can be removed by rejuvenating the catalyst or design of task specific molecular shape selective catalysts. However, catalyst poison destroys the activity and cannot be recovered.

Catalyst inhibitor is used to reduce the catalytic activity of catalyst. This is desired and added to the reacting system. Whereas poison is undesired, and it is due to foreign undesired components.

A catalyst, as you know, is a substance that increases the rate of a chemical reaction. A catalyst inhibitor is a substance that decreases the rate of, or prevents, a chemical reaction, i.e. it opposes the effect of a catalyst. Example: A monomer (e.g. acrylic ester is sold with little amount of inhibitor in order to prevent undesirable polymerization. When we want to polymerize the monomer, we have to wash the inhibitor & then proceed).

A catalyst poison is one that causes total deactivation of the catalyst. Example: In hydrogenation of alkenes, a heterogeneous catalyst is used such as platinum or nickel. On industrial-scale, as time goes by carbon particles deposit on the metal's surface so the catalyst starts to lose its activity until it becomes poisoned or spent or exhausted. That is why an industry has to re-activate or regenerate the catalyst in order to re-use it.

Regeneration is not a big deal (i.e., there are easy methods to do that).

Catalyst inhibitors block access to the active sites of the catalyst, resulting in diffusion limitation, thereby lowering the rate of Reaction. Meanwhile, catalyst poisons afflict the catalyst active sites, resulting in catalyst deactivation.

Otherwise, this problem is approached in the case of solid catalysts, and differently in the case of enzymes - and there are no clear criteria here . If you look at books in the field of engineering of chemical reactions and in particular in the field of catalyst deactivation, there are authors who refer to catalyst poisoning as a separate way of deactivation, and there are those who have a different point of view. Every next book, we have a different approach to catalyst deactivation. Similarly, in the case of inhibition, and especially in its reflection in the kinetic equation (rate law). As an example I can give the synthesis of DME, where water may be an inhibitor for some catalysts, and at the same time is a reaction product that desorbs from the surface of the catalyst. So it may be an additional effect that slows down (the same as inhibition) the overall rate of the chemical reaction.

If I wanted to answer the question shortly, I would write: as a result of catalyst poisoning, the catalyst activity is completely lost, and therefore the rate of a given reaction is equal to the rate without its presence. As for the inhibition, we are dealing here with a reduction in the rate of the chemical reaction/s. And the reason may be both substrates and products.

A similar problem concerns enzymes. Although here, there is no poisoning in the sense of solid catalyst poisoning, but we are dealing with various types of inhibitions.

And one more thing, in the case of homogeneous catalysis, we can also deal with inhibition. Regards,

A catalyst poison essentially destroys the activities of a catalyst by primarily deactivating the catalyst. This destroyed activities (poisoning) could be due to presence of a specific product specie or prevailing process condition interfering with catalyst activity or collapse of catalyst active sites incase of a solid catalyst as a result of a catalyst poison.

A catalyst inhibitor limits or reduces the activities of a catalyst due to the prevailing condition or presence of a certain product specie.

Major differences

1. A catalyst inhibitor limits or reduces general or selective performance or activity of a catalyst

2. A catalyst poison destroys general performance of a catalyst.

I am open to more answers to improve my learning too.

If we are talking about catalyst poisoning, first of all we mean the irreversible process, which results in a complete loss of catalyst activity under the influence of small amount of substances called poisons. These can be, for example, sulfur compounds.

If, in turn, we are talking about inhibition, it is difficult to refer here from the point of view of a reversible phenomenon or not. What is emphasized here, it is the slowing down of the chemical reaction under the influence of the so-called inhibitor. The consequence, but only the consequence, may be the effect on selectivity understood as a change in the number of moles of the reference component for the main reaction (leading to a given product) with respect to to the change in the moles of the reference component in all reactions. Unfortunately, the reduction in the overall rate of chemical reaction can also be caused by mass transport phenomena. It is very difficult to distinguish when we dealing only with inhibition, and when we are dealing with the reaction rate limited by transport phenomena. If one of the products is suspected of being an inhibitor, it is added in various amounts to the substrate mixture during kinetic studies. However, the difficulties in interpretation of kinetic date increase if we deal with reversible reactions.

This is not, however, the end of the difficulties, because the loss of activity of solid catalyst under the influence of various factors (including poisons) can be immediate, but it can also take place gradually. Hence another kind of deactivation appears, namely the aging of the catalyst. But this is a completely different approach to the problem of deactivation. Regards,

Another statement and a new approach to inhibition and poisoning, However, it seems that the morphology changes are associated more with the aging of the catalyst and sintering under the influence of high temperatures than with poisoning. There are also such researchers who apart from the irreversible poisoning distinguish reversible poisoning. This is the case, for example, among those dealing with cracking or hydrocracking catalysts, where the reversible adsorption of certain nitrogen compounds is regarded as reversible poisoning. Chemisorption of other nitrogen compounds may also have an irreversible nature. Hence, the poisoning may, but does not have to "kill the catalyst" And besides, we can deal with coke deposits (coking) or heavy metals deposition. The term catalyst fouling is also used here.

There is no chance for any consensus in case of deactivation of catalyst.

Catalyst poison example is S on Pd or Pt catalysts. Catalyst activity is destroyed and can not be regenerated. A catalyst inhibitor example is CO on Pd catalyst since CO temporarily blocks active sites, so reduces activity and activity can be restored.

Everything depends on the criteria. Poisoning is regarded as deactivation of the catalyst, and inhibition is not. The inhibition may be a slowdown of the chemical reaction in a heterogeneous gas-solid system due to an excessive amount of the product. And this is not deactivation. It is easier to explain the inhibition by referring to enzymatic reactions. But here one can also distinguish several behaviors that differ from each other. Regards,

Inhibitors curb the catalyzing powers of a catalyst but do not react with any of the reactants but Poisons react with the catalyst and the reaction is irreversible.

Are you sure?. And what about inhibitors in enzymatic reactions? The enzyme is also a catalyst. Many textbooks also mention reversible poisoning in case of solid catalyst. There is no clarity and will not be here. Regards,

Inhibition is when a substance (feed, product, or inert/other) occupies the catalyst site to such a great extent that it prevents reactants from getting to the site and therefore "inhibits the reaction.

Poisons usually permanently block the sites but sometimes they can be removed during a regeneration (usually a burn with air or O2).

Tthe most common true poisons are arsenic, lead and cyanides -substances most people recognize as toxins/poisons.

Dear Rick, This time I can not agree with you, because the description you provided is more like fouling and in particular coking ("usually a burn with air or O2") than inhibition or poisoning. Moreover, the inhibition (slow down in reaction rate} in the case of solid catalysts, excluding porous bodies with immobilized enzymes, is often caused by transport phenomena. But as I wrote above, there are many different criteria for deactivation and inhibition.

There is, of course, a number of catalyst poisons, at which even a low concentration, the catalyst is completely and irreversibly poisoned. This applies to metal catalysts but not only. Regards,

Miroslaw,

I think of fouling as a blockages that cause maldistribution. A burn will sometimes remove a blockage (e.g. if it is coke blocking a pore) but sometimes it will not and dumping and reloading catalyst is often necessary, especially if he blockage is caused by scale/metals instead of hydocarbons.

Coking/carbon disposition is the most common way for commercial Hydrotreating, Catalytic Cracking and Hydrocracking catalyst to lose actiity. Historically these catalyst were :regenerated" with a burn but recently the trend has been to dump catalyst and send it to an offsite regeneration facitity for screnning and coke burning. Arsenic and lead are posons that will permanently deactivate most catalyst and cause them to be unregenerable via burn.

I think of an inhibitor as a material that competes with reactant for catalyst sites inhibition is reversable. If the inhibitor is removed from the feed the rate of the desired reaction will increase. An inhibitor may also increase the rate of coking. If the calayst site is coked up it will usually require a burn(with air or O2) to remove the coke. that has closed up catalysyt pores and/or blocked active sites

.

Poisoning and coking are forms of catalyst deactivation that usually require a stoppage either for regeneration or catalyst replacement.You seem to disagree with the comment it can sometimesbe removed during a regeneration (usually a burn with air or O2). I agree it is not really a "poison" if you can remove it with air/ burn.

All catalyst can be "regenerated" if you take extreme enough measures.

For my purposes - in the refining indusrty- catalyst is not regenerable if activity can not be recovered with a simple burn or other relatively minor procedures like a wash with a solvent.

However even then the active metals can be reclamed and used to make new catalyst.

Rick,

I deliberately referred to your statements on the basis of partial negation to trigger further discussion. Each statement can be taken literally, but it does not have to be.

However, I added that both inhibition and deactivation can be considered from the point of view of various criteria. There is the approach of J.B. Butt, there is the approach of G. Froment, there is the approach of B.W. Wojciechowski, but there are also a number of other approaches including Japanese scientists. One can also mention parallel deactivation, consecutive deactivation as well as consecutive-competing one. Moreover, the phenomena of deactivation (in case of enzymes - inactivation) or inhibition can not be separated from specific chemical processes. Another approach concerns the oil refinery (you mention a few processes), other ammonia production and still other organic synthesis. Once, I tried to find a universal division, but I did not succeed. There was no consensus on different types of deactivation and, in my opinion, it will never be. Regards,

Catalyst poisoning deactivate the catalyst reaction. Catalyst inhibition prevent the reaction of the catalyst. This varies depending on the type of medium and the state of the catalyst in question.

This is what IUPAC says:

" Traces of impurities in the fluid to which the catalystis exposed can adsorb at the active sites and reduce oreliminate catalytic activity. This is called poisoning andthe effective impurity is called a poison. If adsorption ofpoison is strong and not readily reversed, the poisoning iscalled permanent. If the adsorption of the poison isweaker and reversible, removal of the poison from thefluid phase results in restoration of the original catalyticactivity. Such poisoning is called temporary. If adsorptionof the poison is still weaker and not greatly preferred toadsorption of reactant, the reduction in rate occasionedby the poison may be called competitive inhibition orinhibition. Here, of course, the poison may be present inmuch larger than trace amounts. There are, of course, nosharp boundaries in the sequence permanent poisoning,temporary poisoning, competitive inhibition."

It can be consulted at:

http://publications.iupac.org/pac/pdf/1976/pdf/4601x0071.pdf