Catalyst poisoning

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. Lead in catalytic converters). Poisoning refers specifically to chemical deactivation, rather than other mechanism of catalyst degradation such as thermal decomposition or physical damage.

Poisoning Process

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. Poisoned sites can no longer accelerate the reaction with which the catalyst was supposed to catalyze. 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.

Common Catalyst Poisons

Organic functional groups and inorganic ions with lone pairs often have the ability to strongly adsorb to metal surfaces, thus prohibiting access to catalyst sites. Common catalyst poisons include the following: carbon monoxide, inorganic ions such as halide, cyanide, sulfide, sulfite, and phosphite and organic molecules such as nitriles, nitros, oximes and nitrogen-containing heterocycles. Agents vary their catalytic properties because of the nature of the transition metal.

Ruthenium, palladium, nickel, cobalt, and platinum are common catalysts used in many organic reactions. Some of the common poisoning reagents for each of the compounds are as follows:

Ruthenium

• Very resistant to poisoning.

Palladium

• Can be poisoned by amines, sulfides, thiols, lead and certain metal oxides.

Nickel and Cobalt

• Can be poisoned by amines, carbon monoxide, nitrogen monoxide, and halide ions.

Platinum

• Can be poisoned by amines, sulfides, thiols, aluminum, cobalt, and bismuth.

Selective Poisoning

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.

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 h_p 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, h_t 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 h_t. When h_t 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 h_t 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 h_t 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, h_t is the Thiele modulus for the unpoisoned case, and α is the fraction of the surface that is poisoned.

Benefits of Selective Poisoning

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. Poisoning can allow for selective intermediates to be isolated and final products with desirable stereochemistry to be achieved.

Examples

Palladium Catalyst

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.

Intentional Palladium Catalyst Poisoning

In some cases, a highly active catalyst can lead to undesirable secondary reactions with desired product. In some of these cases, the addition of a small amount of a catalyst poison increase the yield of the desired product by lower the catalyst activity. For example, in the classical "Rosenmund reduction" of an acyl chloride to the corresponding aldehyde, the palladium catalyst (over barium sulfate or calcium carbonate) is intentionally poisoned by the addition of sulfur or quinoline in order to lower the catalyst activity and thereby prevent further reduction of the aldehyde product to yield a primary alcohol. In the case of Lindlar's catalyst, palladium is poisoned with a lead salt to allow reduction of an alkyne to the corresponding alkene while preventing reduction of the alkene product to the corresponding alkane.

Hydrodesulfurization Catalysts

In the purification of crude petroleum products the process of hydrodesulfurization is utilized. Thiol containing hydrocarbons, such as thiophene, are reduced using H₂S in order to produce H₂S and different length chains of hydrocarbons. Common catalyst used are tungsten and molybdenum sulfide particles. By adding cobalt and nickel nuclei to either edge s or partially incorporating them into the crystal lattice structure can create more efficient catalyst. The synthesis of the catalyst creates a supported hybrid that prevents poisoning of the cobalt nuclei that may be unstable in the mono-nuclear form.

Catalytic Converter

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.

File:Magnaflow catalytic converter.JPG

Example of Catalytic Converter used in the Automotive Industry

Nickel

Raney nickel catalysts 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.

See also

• Hydrogen purity
• Reaction inhibitor
• Enzyme inhibitorfr:Poison de catalyseur