Beyond Geometric Effects: Particle Size-Dependent Electronic Promotion in Ru Catalysts for Ammonia Synthesis

Area of Science:

• Heterogeneous Catalysis
• Materials Science
• Surface Chemistry

Background:

• Traditional understanding of metal particle-size effects in catalysis focuses on geometric factors, assuming intrinsic site properties remain constant.
• The role of electronic effects, particularly size-dependent electronic promotion, in modulating catalytic activity has been less explored.

Purpose of the Study:

• To investigate the dual role of metal particle size in heterogeneous catalysis, encompassing both geometric and electronic effects.
• To elucidate how particle size influences the intrinsic properties of active sites through electronic promotion.

Main Methods:

• Utilized well-defined ruthenium (Ru) catalysts supported on multiwalled carbon nanotubes for ammonia synthesis.
• Separated geometric contributions (B5-like site density) from electronic promotion effects.
• Employed barium oxide (BaO) as an electronic promoter to study interfacial charge storage and capacitive effects.

Main Results:

• Without promoters, Ru catalyst activity and adsorption properties were invariant with particle size, aligning with geometric interpretations.
• With BaO promotion, smaller Ru particles exhibited enhanced electron density due to interfacial charge storage.
• This electronic enrichment tuned the intrinsic reactivity of B5-like sites, strengthening N2 activation and reducing hydrogen poisoning, leading to higher site-specific activity.

Conclusions:

• Metal particle size acts as a dual control parameter, influencing both site density and intrinsic site properties via electronic effects.
• Electronic promotion significantly modulates catalytic activity by altering the electronic structure of active sites.
• Findings provide new insights into the interplay between catalyst structure, charge distribution, and intrinsic catalytic activity in heterogeneous catalysis.