Fischer-Tropsch synthesis over cobalt catalysts supported on zirconia-modified alumina

The effect of adding zirconia to the alumina support on supported cobalt Fi scher-Tropsch catalysts has been studied. At 5 bar and H-2:CO ratio 9:1 zir conia addition to the support leads to a significant increase in both activ ity and selectivity to higher hydrocarbons as compared to the unmodified ca talysts, Reducibility and cobalt dispersion on the other hand are not impro ved by the presence of zirconia compared to the unmodified catalysts. SSITK A measurements have been performed in order to determine the intrinsic acti vity per active site. At constant temperature, zirconia-modified and unmodi fied catalysts showed basically the same intrinsic activity. Similar result s were obtained with a noble metal (Pt) promoted catalyst. The promoting ef fect appears to be mainly due to coverage effects rather than a change in t he intrinsic activity of the active sites. The turnover frequencies were fo und to be independent of pressure bur strongly temperature dependent. Howev er, the increase in turnover frequency did not account for the entire incre ase in reaction rate with temperature. This indicates that also the coverag e of reactive intermediates increases with increasing temperature. (C) 2000 Elsevier Science B.V. All rights reserved.