Why does the activity of carbon monoxide oxidation catalyst increase after reduction by hydrogen ?

02 August 2023 1 2K Report

Many noble metal catalysts require hydrogen pretreatment for the oxidation of carbon monoxide. What is the purpose of this?

Joshua Depiver

The hydrogen pretreatment of many noble metal catalysts for oxidising carbon monoxide (CO) is a well-known process that enhances catalytic activity. The reasons for this are multifaceted and include changes to the catalyst surface, oxidation state, and active site accessibility. Here's a breakdown of why hydrogen reduction might lead to an increase in the activity of a CO oxidation catalyst:

Removal of Surface Oxides: Noble metals like Pt, Pd, and Rh can form surface oxides that block active sites. The reduction with hydrogen (H2) can remove these oxides, freeing up more surface area and exposing fresh active sites for CO oxidation.

Changing the Oxidation State: Some noble metal catalysts work more efficiently in a particular oxidation state. Hydrogen reduction can change the oxidation state of the metal, possibly rendering it more active for the particular reaction.

Redistribution of Active Sites: The interaction of hydrogen with the catalyst surface may lead to a redistribution of the active sites, creating more favourable sites for CO oxidation.

Enhanced Dispersion: Reduction with hydrogen can lead to a higher dispersion of the metal particles on the support, thereby increasing the surface area available for the reaction. This, in turn, can enhance the catalytic activity.

Modifying Electronic Properties: Hydrogen can modify the electronic structure of the metal, possibly leading to an increased affinity for CO or oxygen, depending on the particular mechanism of the reaction. This could change the rate at which intermediates are formed or products are released, leading to a higher catalytic activity.

Activation of the Support: If the catalyst is supported on a material like a metal oxide, hydrogen reduction can also modify the properties of the support itself, leading to a synergistic effect that enhances CO oxidation.

Cleaning the Surface: Hydrogen can also remove impurities and contaminants from the catalyst's surface, which may hinder the catalytic activity.

In summary, hydrogen pretreatment is not a one-size-fits-all method, and the actual mechanism can depend on the specific catalyst, support, reaction conditions, and other factors. It often requires careful characterization and understanding of the catalyst system to utilize hydrogen pretreatment effectively. Research in this area continues, and modern surface science and catalysis techniques are providing more insights into these complex processes.

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