Fischer-Tropsch synthesis over activated-carbon-supported cobalt catalysts: Effect of Co loading and promoters on catalyst performance

The effects of Co loading and K, Ce, and Zr promoters on initial activities and selectivities of activated-carbon-supported cobalt (Co/AC) Fischer-Tropsch synthesis (FTS) catalysts, were studied under conditions of 240 degreesC, 2.4 MPa, 650 h(-1), and H-2/CO = 2:1 in a fixed-bed reactor. The reduction and metal dispersion properties of the catalysts were investigated through temperature-programmed reduction (TPR) and CO chemisorption. Detectable hydrocarbons up to C-20 were formed on the Co/AC catalysts. Co loading affected initial syngas conversion and CH4 selectivity in the following order: 20%Co > 15%Co > 10%Co > 7%Co, whereas it had an opposite effect on content of C-12-C-20 in the liquid organic product. K, Ce, and Zr promoters remarkably changed the initial catalytic performance of the Co/AC catalysts. Addition of K to the Co/AC catalyst significantly decreased the FTS activity and CH4 selectivity, but increased the water gas shift (WGS) activity. Addition of Cc improved the Co/AC catalyst activity, accompanied by high CH4 selectivity. Neither CO2 nor CH4 selectivity changed greatly on the Zr-promoted Co/AC catalyst even though the Zr promoter increased the FTS activity. The TPR and CO adsorption studies showed that K, Ce, and Zr promoters improved Co dispersion and interaction between Co oxide and the AC surface, which might be the reasons for the subsequent catalyst performance, especially for the Ce- and Zr-promoted catalysts. The TPR study also showed that the reduction steps of the reductively decomposed Co/AC catalysts by pure H-2 are Co3O4 --> CoO --> Co.