---
title: "Catalyst masking"
description: "Catalyst masking is the deposition of a thin blanket of fine ash on the face of an SCR catalyst that physically blocks ammonia and NOx molecules from reaching the underlying active sites. Gas continues to flow through the catalyst cells, but the active surface area is shadowed and reaction efficiency falls."
canonical_url: "https://sylio.co/glossary/catalyst-masking"
last_updated: "2026-06-28T02:29:36.959Z"
---

**Catalyst masking** is the deposition of a thin blanket of fine ash on the face of an [SCR catalyst](/glossary/selective-catalytic-reduction) that physically blocks ammonia and NOx molecules from reaching the underlying active sites. Gas continues to flow through the catalyst cells, but the active surface area is shadowed and reaction efficiency falls.

## How masking differs from related failure modes

<table>
<thead>
  <tr>
    <th>
      Failure mode
    </th>
    
    <th>
      Mechanism
    </th>
    
    <th>
      Reversible?
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      <strong>
        Masking
      </strong>
    </td>
    
    <td>
      Ash blanket on the active surface
    </td>
    
    <td>
      Yes — cleaning restores activity
    </td>
  </tr>
  
  <tr>
    <td>
      <a href="/glossary/catalyst-pluggage">
        Pluggage
      </a>
    </td>
    
    <td>
      Particles physically block catalyst channels
    </td>
    
    <td>
      Sometimes (depends on hardness)
    </td>
  </tr>
  
  <tr>
    <td>
      <a href="/glossary/catalyst-poisoning">
        Poisoning
      </a>
    </td>
    
    <td>
      Chemical species bind to active sites
    </td>
    
    <td>
      Usually no — catalyst replacement
    </td>
  </tr>
</tbody>
</table>

Masking is the most operationally manageable of the three because it responds to cleaning.

## What deposits cause masking

- Calcium-rich fly ash (Western US sub-bituminous, biomass)
- Ammonium-salt films on tail-end SCRs
- Sub-micron silica from biomass fuels
- Iron-oxide carry-over from blast-furnace or sinter-plant SCR applications

## Sonic horns and masking control

[Sonic horns](/glossary/sonic-horn) positioned upstream of each catalyst layer continuously dislodge the developing ash blanket before it consolidates. Combined with periodic steam [sootblowing](/glossary/sonic-sootblower), this two-tier cleaning typically restores catalyst activity by 10–30% within months of installation.

## Related terms

- [Selective Catalytic Reduction (SCR)](/glossary/selective-catalytic-reduction)
- [Catalyst pluggage](/glossary/catalyst-pluggage)
- [Catalyst poisoning](/glossary/catalyst-poisoning)
- [Honeycomb catalyst](/glossary/honeycomb-catalyst)
- [Sonic horn](/glossary/sonic-horn)