Planar oxide supported rhodium nanoparticles as model catalysts.
Sean M McClure1, M J Lundwall, D W Goodman
1Department of Chemistry, Texas A and M University, PO Box 30012, College Station, TX 77842-3012, USA.
Summary
Ethylene, carbon monoxide, and hydrogen reactions on Rh/SiO(2) catalysts show propionaldehyde production peaks at 2.5 nm Rh particle size. Smaller particles form carbonyl hydride species, reducing activity.
Area of Science:
- Heterogeneous catalysis
- Surface science
- Materials science
Background:
- Ethylene (C2H4) and carbon monoxide (CO) reactions with hydrogen (H2) over rhodium (Rh) catalysts are crucial in chemical synthesis.
- Understanding the influence of metal nanoparticle size on catalytic activity and selectivity is a key challenge in heterogeneous catalysis.
Purpose of the Study:
- To investigate the effect of Rh particle size on the C(2)H(4)/CO/H(2) reaction kinetics and mechanism on Rh/SiO(2) model catalysts.
- To elucidate the role of Rh particle size in propionaldehyde formation via the CO insertion pathway.
Main Methods:
- Kinetic reactivity measurements under near atmospheric conditions.
- Polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) under CO and reaction conditions.
- Analysis of Rh particle size distribution.
Main Results:
- Propionaldehyde turnover frequency (TOF) exhibited a maximum activity at Rh particle sizes around 2.5 nm.
- Small Rh particles (< 2.5 nm) showed the presence of Rh carbonyl species (Rh(CO)2, Rh(CO)H).
- Larger Rh particles were resistant to dispersion and carbonyl formation, while smaller particles showed reduced propionaldehyde formation due to carbonyl hydride species.
Conclusions:
- The particle size dependence of propionaldehyde production is attributed to the interplay between undercoordinated Rh sites and the formation of inhibitory carbonyl hydride species on smaller particles.
- Catalyst design targeting optimal particle sizes is crucial for maximizing propionaldehyde synthesis via CO insertion.
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