NATURAL GAS CONVERSION V Studies in Surface Science and Catalysis, Vol. 119 A. Parmaliana et al. (Editors) o 1998 Elsevier Science B.V. All rights reserved. 107 Modified alumina supports for cobalt Fischer-Tropsch catalysts F. Rohr b, A. Holmen a'b, K.K. Barbo a, P. Warloe a, E.A. Blekkan a'b aDepartment of Industrial Chemistry, Norwegian University of Science and Technology (NTNU), N-7037 Trondheim, Norway bSINTEF Applied Chemistry, N-7037 Trondheim, Norway* The effect of promotion with zirconia on alumina supported cobalt FT-catalysts has been studied. The promoted catalysts show an increase in activity by up to 50 % compared to the unpromoted systems in CO hydrogenation at 5 bar total pressure. The zirconia is introduced as an addition to the support before impregnating with the active metal. For a constant zirconia loading the activity increases with increasing cobalt loading. The increase is asymptotic towards high cobalt loadings as opposed to the unpromoted catalysts which show a linear dependency with cobalt amount. No increase in the cobalt reducibility is observed with TPR-experiments. Neither does hydrogen chemisorption detect any increase in the cobalt dispersion. SSITKA data recorded at low pressure reveals that the overall gain in the catalytic activity is due to an increased coverage with reactive intermediates, and is not caused by an increase of the intrinsic turnover frequency. 1. Introduction Supported cobalt catalysts are the preferred catalysts for the Fischer-Tropsch synthesis of long chain paraffins from syngas made from natural gas. Alumina is much used as a support for cobalt catalysts due to favourable mechanical properties, but cobalt supported on alumina has a limited reducibility due to a strong interaction between the support and the cobalt oxides [1-3]. This can to a certain extent be overcome by promotion with easily reducible metal promoters, e.g. Pt [2,4]. The Fischer-Tropsch synthesis is generally believed to be structure insensitive and the specific activities at practical conditions are reported to be independent of the support [5]. Catalytic modifiers or promoters can influence the activity or the selectivity of catalysts, either through structural modification or through chemical or electrostatic interaction with the catalytically active material. Zr is reported to be a promoter for Fischer-Tropsch synthesis over Co/SiO2 [6], where Zr leads to improved activity. In the present work we investigate the possibility of changing the support interaction between cobalt and alumina by adding zirconia to the alumina support and studying the changes in the catalytic properties of cobalt supported on this new material. The use of SSITKA (Steady State Isotopic Transient Kinetic Analysis) [7-9] allows the measurement of intrinsic turnover frequencies as well as the surface coverage with * The authors are grateful for the financial support for this work from the EU through contract no. JOF3 CT950016