Rhodium Catalyst Structural Changes during, and Their Impacts on the Kinetics of, CO Oxidation

Area of Science:

• Catalysis
• Materials Science
• Chemical Kinetics

Background:

• Catalysts undergo dynamic structural transformations during reactions, altering active site characteristics.
• Quantifying catalyst activity is complex when active sites change in number or shape during reaction conditions.
• Rhodium (Rh) catalysts can transform between nanoparticle and single-atom states, particularly in the presence of carbon monoxide (CO).

Purpose of the Study:

• To investigate the structural changes of Rh catalysts during CO oxidation.
• To correlate observed structural changes with variations in catalyst activity and turnover frequency.
• To understand the influence of reaction conditions, such as oxygen excess and temperature, on Rh nanoparticle stability and redispersion.

Main Methods:

• Utilizing CO oxidation kinetics to monitor Rh catalyst structural dynamics.
• Analyzing apparent activation energy and pre-exponential factors under varying reaction conditions.
• Employing in situ infrared spectroscopy to observe Rh structural transformations.
• Correlating kinetic data with spectroscopic observations.

Main Results:

• Apparent activation energy remained constant, indicating nanoparticles as stable active sites in certain regimes.
• Changes in the pre-exponential factor, particularly in excess O2, were linked to alterations in the number of active Rh sites.
• Excess O2 promoted CO-induced Rh nanoparticle disintegration into single atoms, influencing catalyst activity.
• The temperature for structural changes was dependent on Rh particle size, with smaller particles requiring higher temperatures for redispersion.

Conclusions:

• CO oxidation kinetics effectively track Rh structural changes, including nanoparticle to single-atom interconversion.
• Excess O2 and CO can induce Rh nanoparticle redispersion, affecting catalyst performance.
• Combining kinetic and spectroscopic methods enables accurate turnover frequency calculations before and after structural changes.