Redox Mechanisms of Silica-Supported Ni Particles: An X-Ray Absorption Fine Structure Investigation

Area of Science:

• Materials Science
• Catalysis
• Surface Chemistry

Background:

• Nickel (Ni) catalysts are crucial in various industrial processes.
• Understanding the redox behavior of NiO and metallic Ni is essential for catalyst design.
• Silica-supported Ni particles are widely used, but their redox mechanisms require detailed elucidation.

Purpose of the Study:

• To investigate the redox mechanisms of silica-supported Ni particles.
• To provide mechanistic insights into the interconversion between partially reduced NiO and partially oxidized metallic Ni.
• To elucidate the role of oxygen ion migration in these processes.

Main Methods:

• In situ X-ray absorption fine structure (XAFS) spectroscopy was employed.
• Experiments focused on surface oxidation and reduction of NiO and metallic Ni particles.
• Temperature-dependent reduction studies were conducted to analyze NiO layer thickness effects.

Main Results:

• Surface oxidation of reduced NiO particles showed incomplete metallic Ni coverage, with internal metallic Ni formation.
• NiO reduction initiates at specific surface sites, expelling O2- ions before full surface coverage.
• Metallic Ni oxidation progresses inward, forming a NiO layer, with reduction temperature increasing with NiO thickness.

Conclusions:

• Distinct redox mechanisms exist for NiO and metallic Ni, driven by oxygen ion migration direction.
• During reduction, O2- ions migrate to the surface along the NiO/Ni interface.
• This study clarifies the detailed mechanism of redox interconversion in solid Ni catalyst particles.