Why Do Catalysts Stop Working? Something Sticking Where It Should Not

Catalysis in industry usually happens at a solid surface, where reacting molecules attach at particular points called active sites, rearrange and leave. The number of those sites is limited, and anything that binds to one more strongly than the reactants do will occupy it permanently, which removes that site from service. A poison is therefore a substance that binds too well rather than one that is chemically aggressive, and the quantities needed are extremely small, since only the surface matters and the surface is a tiny fraction of the material. Parts per million of the wrong compound in a feed stream can deactivate a catalyst bed over time, which is why industrial processes spend considerable effort purifying inputs before they ever reach the reactor.

Poisoning is one of several mechanisms and distinguishing them matters for the remedy:

• •Poisoning, where a substance binds strongly to active sites and blocks them
• •Fouling or coking, where carbon or other material physically covers the surface
• •Sintering, where heat makes small particles merge into larger ones, reducing surface area
• •Leaching, where the active component dissolves into the reaction mixture and is carried away
• •Attrition, where the physical material breaks up under flow and mechanical stress
• •Some of these are reversible by regeneration and others are permanent

Several cases are widely encountered outside the laboratory. Vehicle catalytic converters are poisoned by lead, which is the direct reason leaded petrol had to be eliminated before converters could be fitted, and the two changes happened together for that reason rather than by coincidence. Sulphur compounds poison many metal catalysts and are removed from fuels and feed streams at substantial cost, which is one of the main jobs a refinery does. Ammonia synthesis catalysts are poisoned by oxygen-containing compounds, so the hydrogen feed must be scrupulously clean. Enzymes, which are biological catalysts, are inhibited by heavy metals binding to their active sites, which is part of why those metals are toxic, and a number of drugs work precisely by binding to and blocking an enzyme, which is deliberate poisoning applied usefully.

Deliberate deactivation turns out to be valuable in several places. Selective poisoning adds a substance that blocks the sites responsible for an unwanted side reaction while leaving the wanted reaction running, which is how certain industrial hydrogenations are stopped at a partial rather than a complete product, using a catalyst deliberately treated with lead and an organic compound for that purpose. Many pharmaceuticals are enzyme inhibitors and work by occupying an active site, with common painkillers, statins and several classes of antibiotic all operating that way. Some nerve agents and pesticides poison an enzyme essential to nerve function, which is the same chemistry applied lethally. And understanding what poisons a catalyst is how its mechanism is frequently worked out, since identifying which substances block it indicates what the active site must look like.

The countermeasures form a substantial part of process engineering. Guard beds placed upstream contain a sacrificial material that captures the poison before the feed reaches the expensive catalyst, and are replaced on a schedule. Feed purification removes the offending compounds at source. Catalyst formulations are modified to resist particular poisons, sometimes by adding a component that binds the poison preferentially. Regeneration by burning off deposited carbon in a controlled air stream restores fouled catalysts and is routine in refining, with some units running a continuous cycle of reaction and regeneration. Where deactivation is unavoidable, the reactor is designed so the bed can be replaced without extended shutdown. And selecting operating conditions that trade some activity for a longer life is frequently the economic answer rather than a technical failure.

Reactions happen at a limited number of active sites on a surface, so anything binding there more strongly than the reactants removes that site permanently. Parts per million are enough, which is why feeds are purified before reaching the reactor. Lead poisons vehicle catalytic converters, which is why leaded petrol had to go before converters could be fitted.