Role of ZrO₂ in Promoting the Activity and Selectivity of Co-Based Fischer–Tropsch Synthesis Catalysts

Journal Article · Tue Nov 17 00:00:00 EST 2015 · ACS Catalysis

DOI:https://doi.org/10.1021/acscatal.5b02205· OSTI ID:1418287

Johnson, Gregory R. ^[1]; Bell, Alexis T. ^[2]

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1. Univ. of California, Berkeley, CA (United States). Dept. of Chemical and Biomolecular Engineering
2. Univ. of California, Berkeley, CA (United States). Dept. of Chemical and Biomolecular Engineering; Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Chemical Sciences Division

The effects of Zr promotion on the structure and performance of Co-based Fischer-Tropsch synthesis (FTS) catalysts were investigated. Inclusion of Zr in the catalysts was found to increase the FTS turnover frequency and the selectivity to C₅₊ hydrocarbons and to decrease the selectivity to methane under most operating conditions. These improvements to the catalytic performance are a function of Zr loading up to an atomic ratio of Zr/Co = 1.0, above which the product selectivity is insensitive to higher concentrations of the promoter. Characterization of the Co nanoparticles by different methods demonstrated that the optimal Zr loading corresponds to half monolayer coverage of the Co surface by the promoter. Measurements of the rate of FTS at different pressures and temperatures established that the kinetics data for both the Zr-promoted and unpromoted catalysts are described by a two-parameter Langmuir-Hinshelwood expression. The parameters used to fit this rate law to the experimental data indicate that the apparent rate coefficient and the CO adsorption constant for the Zr-promoted catalysts are higher than those for the unpromoted catalyst. Elemental mapping by means of STEM-EDS provided evidence that Zr is highly dispersed over the catalyst surface and has limited preference for association with the Co nanoparticles. In situ X-ray absorption spectroscopy confirmed the absence of mixing between the Zr and Co in the nanoparticles. Here, these results suggest that Zr exists as a partial layer of ZrO₂ on the surface of the Co metal nanoparticles. Accordingly, it is proposed that Zr promotion effects originate from sites of enhanced activity at the interface between Co and ZrO₂. The possibility that ZrO₂ acts as a Lewis acid to assist in CO dissociation as well as to increase the ratio of CO to H adsorbed on the catalyst surface is discussed.

• Research Organization: Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: AC02-05CH11231; AC02-06CH11357
• OSTI ID: 1418287
• Journal Information: ACS Catalysis, Vol. 6, Issue 1; ISSN 2155-5435
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
cobalt
Fischer−Tropsch synthesis
heterogeneous catalysis
promotion
zirconium