Thermal instability of catalysts can sometimes change the nature of the catalyst, the catalyst is easily heated locally leading to clumping (ie small molecules combine to form large particles), the catalyst surface is reduced leading to reduced catalytic activity. Adding an activator will prevent the sintering process, helping to prolong the life of the contact surface.
Example 5: ZnO is an amorphous catalyst with quite high activity, but at 600 o C, ZnO becomes crystalline and the catalytic activity is completely lost. If Cr 2 O 3 is added , the catalyst (ZnO + Cr 2 O 3 ) remains in the amorphous form.
Increase chemical durability, reduce catalyst poisoning
3/ Classification of stimulants:
According to the new current perspective, stimulants are classified according to the following 3 functions:
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1) Textural promoter: has the role of preventing the increase in size of catalyst particles during use.
For example: Al 2 O 3 in Fe catalyst for NH 3 synthesis reaction Al 2 O 3 in catalyst for Pt/Al 2 O 3 reforming process
2) Electronic or structural promoter: is a substance that can enter the structural network of the catalytic active phase and change its electronic characteristics.
For example: K 2 O in Fe catalyst synthesizes NH 3
3) Poison - resistant promoter: is a substance that protects the active phase.
catalyst from being poisoned by impurities or by-products. For example, Re reduces coke formation on the Pt/Al 2 O 3 catalyst surface.
Thus, a catalyst may consist of an active catalytic phase and one or more activators.
active, such as catalysts for catalytic reforming processes in which:
- Pt: active catalyst
- Re: stimulant against poisoning
- Al 2 O 3 : geometric stimulant
4/ Some views explaining the phenomenon of stimulants:
1) When added to the catalyst, the stimulant will combine with the old active center and create a new active center with higher activity, creating more favorable conditions for the formation and decomposition of HCTG. Therefore, increasing the reaction rate. The amount of stimulant is small and optimal (small because the world has recognized the number of active centers is small and only equal to 10 -23 , meaning that if there is 1 active center, there will be 10 23 inactive centers)
2) The added stimulant will increase the speed of the slowest stage of the process, can increase the adsorption rate of this substance, decrease the adsorption rate of that substance, creating conditions for the main reaction to occur.
For example: NH 3 synthesis reaction : N 2 + 3H 2 2 NH 3
The catalyst is Fe, if the stimulant K 2 O is added, the activity increases many times. This is explained that when K 2 O is present, the adsorption rate of H 2 is increased, while the rate of N 2 is reduced moderately for the reaction to occur smoothly.
3) The stimulant changes the catalyst structure, making the crystal network on the catalyst surface more flexible and active.
There are two types of initiators: electronic initiators and hole initiators.
An electron initiator is an initiator with excess electrons, having less electron affinity than the catalyst; then the free electrons of the initiator will be given to the catalyst, stirring the crystal lattice of the catalyst, making the catalyst more flexible and active.
The defect activator is an electron-deficient activator, so it has a greater electron affinity than the catalyst. It will take the catalyst's electrons to make the catalyst surface more flexible, making the catalyst more active.
For example, the crystal lattice of Al 2 O 3
: oxygen bridge
: Al
4) The added stimulant changes the distance of the catalyst atoms to match the geometric equivalence factor.
5) The stimulant is a gas: The gas is introduced to create a gas film covering the catalyst surface and creating an active center on the surface. The gas becomes polarized, making the catalyst more active.
IV. Catalytic toxins 1/ Concept:
Catalytic poison is a substance that when added in very small amounts, the catalytic activity is greatly reduced.
to the point of being unusable in production.
Some common toxins:
Free Halogens: Cl 2 , I 2 , Br 2 ...
Mercury and Hg salt: HgCl 2 , Hg(CN) 2 ...
HCN, CO ...
Compounds with multiple valencies: because the valency is easily changed, it catalyzes unwanted reactions, i.e., causes catalyst poisoning.
- Compounds of S: H 2 S (S -2 ); H 2 SO 4 (S +6 ) ...
- Compounds of P: H 3 P (P -3 ); H 3 PO 4 (P +5 ) ...
- Compounds of N: NH 3 (N -3 ); thiofen (N +2 )
- Fe compounds: Fe o ; Fe +2 ; Fe +3 ...
Arsenic compounds: have a large number of free electrons, toxic to all catalysts and even humans.
Heavy metals
2/ Symptoms of poisoning:
1) Poisoning caused by the catalyst itself
There are two types of catalysts: natural catalysts and synthetic catalysts.
Natural catalyst : study on the synthesis of artificial rubber Butadiene_1,3 from alcohol C 2 H 5 OH with amorphous aluminosilicate clay as catalyst
2 C 2 H 5 OH J CH 2 = CH - CH = CH 2
If the amorphous Aluminosilicate contains Fe, the yield of Butadiene is significantly reduced.
This proves that Fe is toxic to amorphous aluminosilicate catalysts.
Synthesis catalyst : in the process of preparing the catalyst, if the precipitation is too fast, it will drag along foreign substances, poisoning the catalyst.
2) Poisoning due to reactants brought in
When the reactant contains impurities such as sulfur compounds, phosphide, arsenide... then
The catalyst will adsorb these toxic substances first, making it impossible for the reactants to contact the catalyst surface. And this adsorption can be reversible or irreversible.
For example : Pt catalyst is poisoned by CO and CS 2 , but when blowing the reaction gas completely clean of CO and CS 2 , the catalytic activity is restored because the catalyst has been regenerated. But when Pt catalyst is poisoned by H 2 S and H 3 P , the activity of Pt cannot be easily regenerated. To regenerate it, a strong oxidant and high temperature are required.
Example : Preparation of NH 3 from N 2 and H 2 on Fe catalyst at 300at pressure, 450 o C
If the reaction mixture contains 0.5% O 2 or CO, the activity of Fe will decrease many times. Reduce Fe by blowing a mixture of clean N 2 + H 2 through Fe, O 2 will react with H 2 to form H 2 O, separating from the surface of Fe and the catalyst will start working again as before being poisoned.
But if there are Sulfur compounds adsorbed on the Fe surface, the activity of Fe decreases and when treated with a mixture of N 2 + H 2 , the catalyst does not return to its original state.
3) Poisoning due to substances produced during the reaction
It is poisoning due to carbon pocket formation or coke formation.
The phenomenon of carbon pocket formation often occurs due to paraffin cracking reactions that produce a large amount of carbon and will combine to create carbon pockets that block the catalyst capillary pores and at the same time cause the catalyst elements to be covered with a layer of carbon. When most of the catalyst surface is covered by those organic compounds, the catalyst activity decreases.
Similarly, coke formation is due to condensation of polycyclic compounds and plugging of catalyst pores.
To restore catalytic activity, steam or air blowing is often used.
High temperature to burn coke, carbon cleans catalyst surface.
C + H 2 O CO + H 2 C + O 2 CO 2
HC + O 2 CO 2 + H 2 O
This method is easy to implement, so the reactor and catalyst regeneration equipment are often arranged close together and work continuously.
4) Beneficial poisoning
Sometimes catalyst poisoning is used to increase catalyst selectivity. This phenomenon is called beneficial poisoning.
For example : hydrogenation reaction C 6 H 5 COCl in benzene solution on Pt catalyst to synthesize benzaldehyde C 6 H 5 CHO. The process occurs many consecutive reactions creating many by-products.
H 2
C6H5COCl
Benzoyl chloride
H 2
C6H5CHO
H 2
C6H5CH2OH
Benzyl alcohol
C6H5CH3
If a small amount of sulfur compound is added to the reaction solution, the process stops at the product benzaldehyde. This is because sulfur compounds poison alcohol and toluene .
In short, in production technology, the problem of how to reduce catalyst poisoning is very important.
Special attention is paid to increase reaction efficiency and increase catalyst working time.
3/ Factors affecting catalyst poisoning:
There are two major factors that influence catalyst poisoning.
A
250 0 C
230 0 C
210 0 C
190 0 C
170 0 C
130 0 C
Temperature: temperature has a great influence on catalyst poisoning. For example : oxidation reaction of SO 2 to SO 3
With catalyst: V2O5 and toxic substance : H2S
Carry out the reaction at thermal conditions A 0
different degrees, found that:
+ at 130 o C: activity is greatly reduced
+ at 250 o C: catalytic activity is restored to its original state
The reason is that at high temperatures the adsorption between V 2 O 5 and H 2 S is unstable, leading to desorption, so it does not affect the catalytic activity.
[H 2 S]
Stimulants and carriers: when toxins appear, the stimulants and carriers will first adsorb the toxins, releasing the active center of the catalyst.
4/ Active center of catalyst:
H 2 O 2
Pt
H 2 O + ½ O 2
When studying reactions:
It was found that an extremely small amount of toxic H2S was enough to cause a sharp decrease in the activity of Pt. From there, the question arose as to why, with tens of millions of catalyst atoms, only a small amount of toxic substance could cause the catalyst to lose its activity. This proved that not all catalyst atoms catalyze the reaction, but only a very small portion of the atoms catalyze the reaction. That number of active atoms is called the active center.
When studying the adsorption of gases on the surface of solid catalysts, people found that: each catalyst atom on the surface can adsorb a gas atom and that catalyst atom is called the adsorption center ; then it will form a layer of gas atom adsorption on the catalyst surface.
Thus, the number of adsorption centers and active centers are not equivalent.
If we accept that: activated adsorption centers are the type of centers that have the effect of creating conditions for chemical reactions to proceed, then the number of activated adsorption centers is called the number of catalytic active centers. And thus, the number of activated adsorption centers accounts for a very small part compared to the general adsorption centers.
With different reactions and different catalysts, the number of active catalytic centers is also different.
The world recognizes the number of active catalytic centers as 10-23 .
Up to now, the recognition of the existence of an active catalytic center has not been fully recognized. What is an active center? What effect does it have on the reactants? Up to now, opinions have not been unified, but it can be generally defined: "Active centers are centers with large free valence, so they can adsorb reactants and carry out the reaction."
Here are a few theories that explain the activity center.
1/ Taylor theory:
In 1962, Taylor did an experiment: in a beaker containing Ni particles, add I2 solution . Observing under a microscope, it was seen that some spots were stained but then gradually spread and the color was uneven.
To explain this phenomenon, he did another experiment: adsorbing CO on the Ni surface, he found: the Ni surface is not flat but convex . Explanation:
At position 1: Ni has 3 free valence atoms
⎜
Ni 1
⎜⎜⎜⎜
Ni Ni 3 Ni Ni 2
⎜⎜⎜⎜
Ni Ni 4 Ni Ni
⎜⎜⎜⎜
Ni Ni Ni Ni
⎜⎜⎜⎜
Taylor diagram of Ni surface
2: Ni has 2 free valences 3: Ni has 1 free valence 4: Ni has 0 free valences
Thus, on the Ni surface, they are all Ni atoms but they have different free valences so they have different chemical capacities.
According to Taylor, at position 1, Ni has 3 free valences so it can combine with 3 CO atoms to form Ni(CO) 3 on the Ni surface.
Similarly, at position 2: Ni(CO) 2 is formed
at position 3: NiCO formation
at position 4: nothing formed.
From here we can see that the active center of Ni only occupies a very small part compared to the total number of Ni atoms. And Taylor concluded: position 1 is the most active center (but in fact position 1 is the strongest adsorption center).
Taylor's view only gives a preliminary concept of the active mind, but does not provide an analytical method to identify and distinguish it as the active mind.
2/ Dankov theory:
In complete contrast to Taylor, Dankov considered the active centers to be concave and negative points.
Dankov's diagram is as follows:
5
1
4
2
3
According to Dankov, there are 5 locations on the metal surface:
If concave at position 1: there are 5 free surfaces
If concave at position 2: there are 4 free surfaces
If the concave is at position 3: there are 3 free surfaces
If concave at position 4: there are 2 free surfaces
If concave at position 5: there is 1 free surface
At position number 1: CO is adsorbed by 5 free surfaces
stretching the C=O bond towards 5 free surfaces. Similarly at positions 2, 3, 4, CO is adsorbed by 4, 3, 2 free surfaces
Thus, the more concave the position, the more bonds are formed between a CO atom and the free surfaces. Specifically, position number 1 has the strongest adsorption capacity and highest activity.
Dankov proposed a different image of the active center from Taylor but also did not explain clearly about the active center. According to Dankov, position 1 is the most active because it can create 5 bonds. But creating many bonds does not mean having many possibilities for chemical reactions.
To explain the active center of a catalyst more fully, it is necessary to study other factors such as binding energy, geometric bonding, electron exchange in the bond... (explained by catalysis theories).