What is needed for an industrial catalyst?
- High stability
- High selectivity
- Ease of availability
I did not say high activity, since the kind of activity needed depends on the application. A highly active catalyst can affect the selectivity. Also, if the activity is lower than needed, it can be elevated using promoters.
A catalysts loses all this due to the following reasons:
Catalysts if reusable, can undergo wear and tear due to the above mentioned reasons. Poisoning can occur due to impurities in the product streams, or the products itself can poison it. Sulfur is a notorious catalyst poison. It haunts the petroleum industry. It bonds to the active sites of the catalyst. Oxygen and water, are catalysts poisons for iron catalyst in ammonia synthesis.
Selective poisoning is deliberate poisoning of highly active catalysts that are so active that they catalyze undesired side reactions. Lindlar’s catalyst is one such example. Lindlar’s catalyst is a palladium catalyst poisoned with traces of lead and quinoline, that reduce its activity such that it can only reduce alkynes, not alkenes.
Coke is what gets deposited on a catalyst during oil refining. It deactivates it. The catalyst can be reactivated by burning it off.
Catalyst deactivation due to coke formation is an important technological and economic problem in petroleum refining and in the petrochemical industry. Remedies to catalyst deactivation are sought by a variety of strategies involving modification of catalyst surface composition such as the use of polymetallic catalysts and/or by manipulation of the reaction environment which often limits the yield due to thermodynamic constrains (i.e., high hydrogen pressures, etc.). In the limit, when the activity reaches unacceptable limits, regeneration by burning off carbon residues can usually be attained, Regeneration can take place in situ, as in fixed-bed reactors, or in an adjacent reactor to which the catalysts is transported to, such as in moving-bed reactors or in fluidized-bed reactors. In the first case intermittent operation is required, whereas in the second case the operation is continuous, but a second regeneration reactor is required. The choice of the proper process cycle is an economic optimization problem constrained by catalysts cost, operational and regenerational cost, and by the value of the final product [1–5]. Process optimization under catalyst decay is an engineering problem that requires a knowledge of the catalyst deactivation kinetic. – E. E. Wolf & F. Alfani, Catalysis Reviews: Science and Engineering, Volume 24, Issue 3, 1982
Decomposition of a catalyst occurs when it is exposed to temperatures it cannot handle. It renders it thermally, structurally and chemically unstable.
Deactivation mechanisms: A) Coke formation, B) Poisoning, C) Sintering of the active metal particles, and D) Sintering and solid-solid phase transitions of the washcoat and encapsulation of active metal particles (cf. Suhonen 2002).
(Image: Oulu University Library)
Although regenerating a catalyst brings it back to life, but in due course it keeps losing its activity bit by bit if not entirely, which is when it is eventually replaced. A catalyst, like diamond, is not forever. The worth of the catalyst can be found by knowing its TOF and TON.
TOF is its turn over frequency and TON is its turn over number.
TOF is the amount of reactions per unit time occurring at the reaction center.
TON is TOF upto the death of the catalyst, which can be shown numerically as follows:
TON = TOF * lifetime of the catalyst