Catalyst poisoning

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Catalyst poisoning refers to the partial or total deactivation of a catalyst caused by exposure to a range of chemical compounds. Poisoning may be desirable when it results in improved selectivity (e.g. Lindlar's catalyst) but may be undesirable when the catalyst is rendered ineffective (e.g. in catalytic converters). Poisoning refers specifically to chemical deactivation, rather than other mechanism of catalyst degradation such as thermal decomposition or physical damage.[1]

Poisoning process[edit]

Poisoning involves compounds which bonds chemically to the active surface sites of a catalyst. This may have two effects: the total number of catalytic sites or the fraction of the total surface area that has the capability of promoting reaction always decreases, and the average distance that a reactant molecule must diffuse through the pore structure before undergoing reaction may increase.[2] Poisoned sites can no longer accelerate the reaction with which the catalyst was supposed to catalyze. [3] Large scale production of substances such as ammonia in the Haber–Bosch process include steps to remove potential poisons from the product stream.

The poisoning reaction should be viewed like any other chemical reaction between a gas phase reactant and the solid surface, where the poisoned sites are distributed throughout the catalyst pore structure as a function of poison diffusion into the catalyst and the rate of the poisoning reaction. At the two extremes, this gives rise to two scenarios. First, when the poisoning reaction rate is slow relative to the rate of diffusion, the poison will be evenly distributed throughout the catalyst and will result in homogeneous poisoning of the catalyst. Conversely, if the reaction rate is fast compared to the rate of diffusion, a poisoned shell will form on the exterior layers of the catalyst, a situation known as "pore-mouth" poisoning, and the rate of catalytic reaction may become limited by the rate of diffusion through the inactive shell.[2]

Selective poisoning[edit]

If the catalyst and reaction conditions are indicative of a low effectiveness factor, selective poisoning may be observed, which is a phenomenon where poisoning of only a small fraction of the catalyst surface gives a disproportionately large drop in activity. Mathematical models exist which describe the cases where the interaction of the poisoning process with the influence of the intraparticle diffusion on the rates of the primary and poisoning reactions leads to an interesting relations between observed catalytic activity and the fraction of surface poisoned.[2]

By combining a material balance over a differential element of pore length and the Thiele modulus, the equation is found:

 \eta =\frac{\tanh(h_p)}{h_p}

where η is the effectiveness factor of the poisoned surface and hp is the Thiele modulus for the poisoned case.

When the ratio of the reaction rate for the poisoned pore to the unpoisoned pore is considered, the following equation can be found:

 F =\frac{\tanh(h_t \cdot \sqrt{1-\alpha}) \cdot \sqrt{1-\alpha}}{\tanh(h_t)}

where F is the ratio of rates of poisoned and unpoisoned pores, ht is the Thiele modulus for the unpoisoned case, and α is the fraction of the surface that is poisoned.

The above equation simplifies depending on the value of ht. When ht is small, meaning that the surface is available, the equation becomes:

 F = 1 - \alpha

This represents the "classical case" of nonselective poisoning where the fraction of the activity remaining is equal to the fraction of the unpoisoned surface remaining.

When ht is very large, it becomes:

 F = \sqrt{1- \alpha}

In this case, the catalyst effectiveness factors are considerably less than unity, and the effects of the portion of the poison adsorbed near the closed end of the pore are not as apparent as when ht is small.

Delving further into the mathematical relationships of selective poisoning, or "Pore-Mouth" poisoning, looking at the steady-state conditions, the rate of diffusion of the reactant through the poisoned region is equal to the rate of reaction. The rate of diffusion is given by:

 Diffusion Rate = -\pi \cdot r_{avg}^2 \cdot D_c \cdot \frac{dC}{dx}

And the rate of reaction within a pore is given by:

 Reaction Rate = \eta \cdot \pi \cdot r_{avg} \cdot (1-\alpha) \cdot L_{avg} \cdot k_1'' \cdot C_c

Through further manipulation and substitution, the fraction of the catalyst surface available for reaction can be obtained from the ratio of the poisoned reaction rate to the unpoisoned reaction rate:

 F =\frac{r_{poisoned}}{r_{unpoisoned}}

or

 F = \frac{\tanh[(1-\alpha) \cdot h_t]}{\tanh(h_t)} \cdot \frac{1}{1 + \alpha \cdot h_t \cdot \tanh[(1-\alpha) \cdot h_t]}

where, as before, ht is the Thiele modulus for the unpoisoned case, and α is the fraction of the surface that is poisoned.[2]

Benefits of selective poisoning[edit]

Usually, catalyst poisoning is undesirable as it leads to a loss of usefulness of expensive noble metals or their complexes. However, poisoning of catalysts can be used to improve selectivity of reactions.

In the classical "Rosenmund reduction" of acyl chlorides to aldehydes, the palladium catalyst (over barium sulfate or calcium carbonate) is poisoned by the addition of sulfur or quinoline. This system reduces triple bonds faster than double bonds allowing for an especially selective reduction. Lindlar's catalyst is another example — palladium poisoned with lead salts.

Examples[edit]

An example can be seen with Raney nickel catalyst, which have reduced activity when it is in combination with mild steel. The loss in activity of catalyst can be overcome by having a lining of epoxy or other substances.

Poisoning of palladium and platinum catalysts has been extensively researched. As a rule of thumb, platinum (as Adams's catalyst, finely divided on carbon) is less susceptible. Common poisons for these two metals are sulfur and nitrogen-heterocycles like pyridine and quinoline.

A catalytic converter for an automobile can be poisoned if the vehicle is operated on gasoline containing lead additives. Fuel cells running on hydrogen must use very pure reactants, free of sulfur and carbon compounds.

References[edit]

  1. ^ Bartholomew, Calvin H. "Mechanisms of catalyst deactivation". Applied Catalysis A: General 212 (1-2): 17–60. doi:10.1016/S0926-860X(00)00843-7. 
  2. ^ a b c d Charles G. Hill, An Introduction To Chemical Engineering Kinetics and Reactor Design, John Wiley & Sons Inc., 1977 ISBN 0-471-39609-5, page 464
  3. ^ Jens Hagen, Industrial catalysis: a practical approach ,Wiley-VCH, 2006 ISBN 3-527-31144-0, page 197

See also[edit]