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Education: Gasoline

Three-Way Catalysts: Aging, Causes of Failure and Deactivation

Three-Way Catalytic Converters (“Three-Way Catalysts”, “TWC”) are generally effective at achieving significant reductions of Carbon Monoxide, Hydrocarbons and Nitrogen Oxides. Unfortunately, the operating conditions to which Three-Way Converters are subjected often cause their catalysts to become thermally, chemically and/or mechanically deactivated. These causes of deactivation may occur separately or in combination, but their net effect is always the removal of active sites from the converter’s catalytic surface. Catalytic deactivation is broadly defined as a phenomenon in which the structure and state of the catalyst changes, leading to the loss of active sites on the catalyst’s surface, thereby causing a decrease in the catalyst’s performance. High temperatures and high temperature gradients, the presence of poisons and other impurities, as well as the fluctuation of gas phase composition and flow rates all increase the possibility of catalytic deactivation (22).

Thermal Deactivation by Thermal Degradation and Sintering

Thermal degradation of a Three-Way Catalyst begins at temperatures between 800° - 900° C, or in some cases, at lower temperatures depending upon the catalytic material. Thermal degradation is a physical process which leads to catalytic deactivation at high temperatures. Deactivation of this type is caused by a loss of catalytic surface area due to crystalline growth of the catalytic phase, the loss of washcoat area due to the collapse of the pore structure, and/or chemical transformations of catalytic phases to non-catalytic phases. The first two processes are typically referred to as sintering and the third process as Solid-solid Phase Transition at high temperatures.

Sintering: There are two models used to explain how sintering occurs: 1) the atomic migration model, and 2) the crystallite migration model.

In any case, the formulation and growth of crystals on the catalyst’s surface reduce the amount active sites which affect the oxidation of pollutive emissions. It should be mentioned that the rate of sintering increases exponentially with temperature and becomes increasingly pronounced above temperatures of 600°C.

Solid-solid Phase Transitions: are extreme forms of sintering that occur at very high temperatures and lead to the transformation of one crystalline phase into another. Phase transformations typically occur in the bulk washcoat and they dramatically decrease the surface area of the catalyst.

Active precious metals are commonly used as catalysts for the purification of exhaust gasses. Of all of the precious metals employed in TWCs—platinum, palladium and rhodium—rhodium has the greatest propensity to sinter at high temperatures. This leads to poor activity in the reduction ofNO_x, as rhodium is the most commonly used precious metal in a TWC’s reduction catalysts.

Redispersion: redispersion is a process that is the opposite of sintering. During redispersion, complex phenomena occur. Among them, particle sizes decrease and the surface area increases. In particular, the interaction between oxygen and precious metals may lead to the formation of species that are mobile on the catalyst’s surface and reverse the process of agglomeration (23).

Chemical Deactivation, Poisoning and Inhibition:

Accumulation of fuels and lubricants on catalytic surfaces reduces a catalyst’s effectiveness.

Poisoning is defined as a loss of catalytic activity due to the chemisorptions of impurities on the catalyst’s the active sites. Normally, a distinction is made between poisons and inhibitors. Poisons are substances that interact very strongly and irreversibly with the catalyst’s active sites, whereas the adsorption of inhibitors on the catalytic surface is weak and most-often reversible.

Catalytic converters are poisoned and/or inhibited by impurities contained in fuel and lubrication oils, or by metal shavings from the exhaust pipe. Even low levels of impurities are enough to completely cover a catalyst’s active sites. Of all poisons and inhi bitors, lead, sulfur, phosphorus, zinc, calcium and magnesium are the most common.

Fuel-based Poisons and Inhibitors:

Lead (Pb): Pb is arguably the most damaging catalytic poison. Catalytic converters are known to completely loose their catalytic capacity with only 10 refills of leaded gasoline, and the effects of lead on the catalyst are irreversible.

Sulfur (S): The presence of sulfur as oxide or sulfide invariably and often immediately decreases catalytic performance (24). Sulfur competes with other exhaust pollutants for space on the catalytic surface. During the combustion process, fuel sulfur oxidizes to SO₂ and SO₃. These compounds absorb onto the catalytic surface at low temperatures and react with alumina to form aluminum sulfates. These sulfates reduce the active surface of the wash coat and deactivate the catalyst. Deactivation of this type has the duel effect of reducing the converter’s overall performance as well as its oxygen storage capacity.

The impact of sulfur on vehicle-aged catalysts is typically irreversible under temperatures of 650°C. It should also be noted that even though catalytic purification efficiency is partially recoverable at higher temperatures, oxygen storage capacity is not.

Lube Oil Poisons and Inhibitors:

Lube oils can enter into the exhaust system by leaking through worn out piston rings, faulty valve seals, failed gaskets and/or warped engine components. Fouling occurs when lube oil emissions coat the catalyst with carbon soot. Carbon deposits prevent the catalytic converter from reducing harmful emissions, and they also reduce air flow. Reduced air flow increases engine backpressure and can force heat and exhaust gasses back into the engine compartment. In some cases, the engine may actually draw back exhaust gasses into the combustion chamber. Re-entrance of exhaust gases into the combustion chamber reduces subsequent combustion cycle efficiency. Reduced cycle efficiencies result in a loss of power, increased emissions, and overheating of engine components (25).

The process of a catalyst being coated with carbon soot is technically referred to as “coke formation”. In technical terms, coke formation is a phenomenon during which carbonaceous residues cover the catalyt’s active sites and decrease the catalyst’s active surface area. A primary cause of “pore blockage” is caused when coke formations are so large that carbon blocks the internal pores of the catalyst, thereby prohibiting airflow.

Phosphorus, zinc, calcium and magnesium are the most common impurities found in lubrication oils. Like sulfur, these substances accumulate on the catalyst’s surface and compete with other exhaust pollutants for surface area (26). These substances are generally regarded as catalyst inhibitors, rather than catalyst poisons. All of them however, decrease catalytic efficiency and can potentially cause harm to the engine.

Mechanical Deactivation:

Mechanical deactivation is caused by mechanical malfunction, improper operation of key components or physical damage being inflicted upon the TWC.

Converter Meltdown: Converters can literally melt down when conditions become so rich that raw fuel is discharged from the combustion chamber into the exhaust flow. Fuel in the exhaust flow can be ignited by a catalyst’s high temperatures. Burning fuel within the converter creates so much additional heat that the ceramic catalyst is unable to withstand the high temperatures and begins to melt. Melting causes the ceramic monolith to collapse and the converter to be destroyed. A melted ceramic converter may significantly block exhaust flow and cause irreparable damage to the engine.

Converter meltdown can also be caused by other malfunctions including: faulty oxygen sensors, incorrect fuel mixtures, worn spark plugs or plug wires, faulty check valves, incorrect ignition timing, faulty fuel injectors and other ignition malfunctions (27).

Deteriorated Spark Plugs or Spark Plug Wires: spark plugs that don’t fire, or misfire, can cause unburned fuel to be discharged into the exhaust system.

Improperly Operating Oxygen Sensor: an oxygen sensor failure can lead to incorrect readings of exhaust gasses. A faulty sensor can cause air / fuel ratios to be either too rich or too lean. A rich mixture can cause fuel to be discharged into the exhaust system. Lean mixtures produce conditions which diminish the rate at which hydrocarbons are oxidized.

Catalyst Fracture: fracture to the catalyst can be caused by the catalyst becoming loose or cracked. Once breakage occurs, pieces of the converter may dislodge and begin obstruct air flow. Airflow obstruction creates backpressure and increases heat in the exhaust system, which can ultimately lead to overheating.

The catalyst inside a TWC is made of lightweight ceramic that is protected by a durable insulating mat. This mat holds the catalyst in place and provides moderate protection against damage. Catalytic fracture can be caused by road debris striking the converter or from the protective mat becoming directly exposed to exhaust gasses. Causes of a catalytic fracture include road debris striking the converter, loose or broken converter hangers, stong vehicle impact with potholes and stresses of off-road driving (28).