---
title: "Catalyst poisoning"
description: "Catalyst poisoning is the chemical deactivation of SCR catalyst active sites by trace species in the flue gas. Unlike masking (physical blanket) or pluggage (channel blockage), poisoning is a chemical process that binds molecules to the catalyst's vanadium, tungsten or titanium active centres. Cleaning cannot reverse it; the affected layer must be regenerated off-site or replaced."
canonical_url: "https://sylio.co/glossary/catalyst-poisoning"
last_updated: "2026-06-28T02:29:37.057Z"
---

**Catalyst poisoning** is the chemical deactivation of [SCR catalyst](/glossary/selective-catalytic-reduction) active sites by trace species in the flue gas. Unlike [masking](/glossary/catalyst-masking) (physical blanket) or [pluggage](/glossary/catalyst-pluggage) (channel blockage), poisoning is a chemical process that binds molecules to the catalyst's vanadium, tungsten or titanium active centres. Cleaning cannot reverse it; the affected layer must be regenerated off-site or replaced.

## Common poisons

<table>
<thead>
  <tr>
    <th>
      Poison
    </th>
    
    <th>
      Source
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      Arsenic
    </td>
    
    <td>
      Coal-fired flue gas, especially sub-bituminous
    </td>
  </tr>
  
  <tr>
    <td>
      Alkali metals (K, Na)
    </td>
    
    <td>
      Biomass, agricultural-residue and waste-fuel ash
    </td>
  </tr>
  
  <tr>
    <td>
      Phosphorus
    </td>
    
    <td>
      Animal-fat biofuels, sewage-sludge co-firing
    </td>
  </tr>
  
  <tr>
    <td>
      Calcium
    </td>
    
    <td>
      Wet limestone scrubbers upstream, biomass
    </td>
  </tr>
  
  <tr>
    <td>
      Sulphur trioxide (high concentration)
    </td>
    
    <td>
      SO₂ + V₂O₅ oxidation at high SCR temperature
    </td>
  </tr>
  
  <tr>
    <td>
      Lead and zinc
    </td>
    
    <td>
      Waste-to-energy, some industrial off-gas streams
    </td>
  </tr>
</tbody>
</table>

## Mitigation

- Fuel selection / blending to control fuel-bound poison content
- Guard layers (sacrificial top catalyst layers protecting layers below)
- Catalyst formulation tuned to expected poisons (e.g. alkali-resistant for biomass)
- [Catalyst regeneration vs replacement](/glossary/catalyst-regeneration-vs-replacement) campaigns to extend catalyst life

## Related terms

- [Selective Catalytic Reduction (SCR)](/glossary/selective-catalytic-reduction)
- [Catalyst masking](/glossary/catalyst-masking)
- [Catalyst regeneration vs replacement](/glossary/catalyst-regeneration-vs-replacement)