Evidence of Structure Sensitivity in the Fischer–Tropsch Reaction on Model Cobalt Nanoparticles by Time‐Resolved Chemical Transient Kinetics

• Published in: Angewandte Chemie International Edition 2017-06, Vol.56 (26), p.7415-7419
• Main Authors: Ralston, Walter T., Melaet, Gérôme, Saephan, Tommy, Somorjai, Gabor A.
• Format: Article
• Language: English
• Subjects: Atmospheric pressure; Carbon; Carbon monoxide; Catalysts; Cobalt; cobalt nanoparticles; Colloid chemistry; Crude oil; Fischer-Tropsch process; Fischer–Tropsch; Hydrocarbons; Hydrogenation; Kinetics; Nanoparticles; Reaction kinetics; Sensitivity; surface chemistry; time-resolved measurements; X-ray absorption spectroscopy
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201701186

The Fischer–Tropsch process, or the catalytic hydrogenation of carbon monoxide (CO), produces long chain hydrocarbons and offers an alternative to the use of crude oil for chemical feedstocks. The observed size dependence of cobalt (Co) catalysts for the Fischer–Tropsch reaction was studied with colloidally prepared Co nanoparticles and a chemical transient kinetics reactor capable of measurements under non‐steady‐state conditions. Co nanoparticles of 4.3 nm and 9.5 nm diameters were synthesized and tested under atmospheric pressure conditions and H2/CO=2. Large differences in carbon coverage (ΘC) were observed for the two catalysts: the 4.3 nm Co catalyst has a ΘC less than one while the 9.5 nm Co catalyst supports a ΘC greater than two. The monomer units present on the surface during reaction are identified as single carbon species for both sizes of Co nanoparticles, and the major CO dissociation site is identified as the B5‐B geometry. The difference in activity of Co nanoparticles was found to be a result of the structure sensitivity caused by the loss of these specific types of sites at smaller nanoparticle sizes. Size it up: Size‐dependent activity of cobalt nanoparticles in Fischer–Tropsch synthesis is correlated with loss of carbon monoxide dissociation sites on small nanoparticles. Transient kinetic experiments and synchrotron spectroscopy reveal greater carbon accumulation on large nanoparticles. Carbon monoxide dissociation sites occur on face‐centered cubic cobalt (221) steps.