Performance of a NiFe2O4@Co Core-Shell Fischer-Tropsch Catalyst: Effect of Low Temperature Reduction

Alisa Govender; Ezra J Olivier; Sarah J Haigh; Daniel Kelly; Matthew Smith; Hendrik van Rensburg; Roy P Forbes; Eric van Steen

doi:10.1021/acsomega.0c04124

Performance of a NiFe₂O₄@Co Core-Shell Fischer-Tropsch Catalyst: Effect of Low Temperature Reduction

ACS Omega. 2020 Dec 16;5(51):32975-32983. doi: 10.1021/acsomega.0c04124. eCollection 2020 Dec 29.

Authors

Alisa Govender^{1

2}, Ezra J Olivier³, Sarah J Haigh⁴, Daniel Kelly⁴, Matthew Smith⁴, Hendrik van Rensburg⁵, Roy P Forbes⁶, Eric van Steen²

Affiliations

¹ Group Technology, Sasol South Africa (Pty) Ltd., P.O. Box 1, Sasolburg 1947, South Africa.
² Catalysis Institute, Department of Chemical Engineering, University of Cape Town, Private Bag, Rondebosch 7701, South Africa.
³ Centre for HRTEM, Nelson Mandela Metropolitan University, P.O. Box 77000, Port Elizabeth 6031, South Africa.
⁴ School of Materials, University of Manchester, Sackville Street, Manchester, M13 9PL, U.K.
⁵ Sasol UK, Purdie Building, North Haugh, St Andrews KY16 9ST, U.K.
⁶ School of Chemistry, University of the Witwatersrand, Private Bag 3, PO Wits, Johannesburg 2050, South Africa.

Abstract

In situ TEM gas-cell imaging and spectroscopy with in situ XRD have been applied to reveal morphological changes in NiFe₂O₄@Co₃O₄ core-shell nanoparticles in hydrogen. The core-shell structure is retained upon reduction under mild conditions (180 °C for 1 h), resulting in a partially reduced shell. The core-shell structure was retained after exposing these reduced NiFe₂O₄@Co₃O₄ core-shell nanoparticles to Fischer-Tropsch conditions at 230 °C and 20 bar. Slightly harsher reduction (230 °C, 2 h) resulted in restructuring of the NiFe₂O₄@Co₃O₄ core-shell nanoparticles to form cobalt islands in addition to partially reduced NiFe₂O₄. NiFe₂O₄ underwent further transformation upon exposure to Fischer-Tropsch conditions, resulting in the formation of iron carbide and nickel/iron-nickel alloy. The turnover frequency in the Fischer-Tropsch synthesis over NiFe₂O₄@Co₃O₄ core-shell nanoparticles reduced in hydrogen at 180 °C for 1 h was estimated to be less than 0.02 s^-1 (cobalt-time yield of 8.40 μmol^.g^-1.s^-1) with a C₅₊ selectivity of 38 C-%. The low turnover frequency under these conditions in relation to the turnover frequency obtained with unsupported cobalt is attributed to the strain in the catalytically active cobalt.