Effect of Impregnation Procedures of Nitrilotriacetic Acid on the Fischer-Tropsch Synthesis Activity and Surface Structure of Co/SiO₂ Catalyst

Abstract

In order to make clear the promoting effects of modification of an aqueous Co nitrate solution with some chelating agents reported previously, the present study has investigated the FTS activity and surface structure of the catalysts prepared by a stepwise impregnation of the aqueous nitrilotriacetic acid (NTA) and Co nitrate solutions. The order of the stepwise impregnation of these reagents was varied, and obtained results were compared with those for the catalysts prepared by the co-impregnating method. Promoting effects of modification with NTA were clearly observed, irrespective of the order of the stepwise impregnation of the aqueous NTA and Co nitrate solutions. In particular, the catalyst prepared by the stepwise impregnation of the aqueous NTA solution followed by the aqueous Co nitrate solution showed an identical FTS activity to the catalyst prepared by the co-impregnating method. In temperature programmed reduction profiles of these catalysts after calcinations, both low temperature (ca. 500 K) and high temperature (900-1100 K) peaks were observed irrespective of the impregnating methods. The intensity of the high temperature peak, however, was evidently weaker for the catalyst prepared by the stepwise impregnation of the aqueous NTA solution followed by the aqueous Co nitrate solution. Furthermore, diffuse reflectance FT-IR measurement on the catalysts after the drying step showed that NTA forms a complex with Co²⁺ except for the catalyst prepared by the stepwise impregnation of the aqueous NTA solution followed by the aqueous Co nitrate solution. Some of NTA were free from the complex formation on this catalyst. Besides, unlike the catalyst prepared by the co-impregnating method, Co loading was able to be changed when the catalyst was prepared by the stepwise impregnation method, which resulted in higher FTS activities. From these results, it was suggested that the promoting effect of modification with NTA is brought by the formation of highly dispersed Co species after the calcinations, which is originated from the NTA-Co²⁺ complex.