Structure sensitive reactions over supported ruthenium catalysts during Fischer-Tropsch synthesis

Abstract

Highly dispersed ruthenium catalysts can be prepared on alumina by aqueous impregnation of ruthenium. EXAFS at the K-edge showed that this type of catalyst, after calcination and reduction, consisted of ruthenium particles, which were about 0.8 nm in size. When highly dispersed on alumina, ruthenium appears to catalyze the water-gas shift reaction, which occurs subsequent to Fischer-Tropsch synthesis. The hydrocarbons produced had low olefinicity, possibly because ofin situ production of hydrogen via the water-gas shift reaction. Highly dispersed ruthenium was not stable on alumina during Fischer-Tropsch synthesis. The ruthenium agglomeration on the alumina surface, as well as overall ruthenium loss from the catalyst, was attributed to the formation of a volatile ruthenium carbonyl species. Catalysts with about 85% of the ruthenium in the form of 3–7 nm particles were prepared on alumina by reverse micelle impregnation of ruthenium. These larger particles were stable against ruthenium carbonyl formation and, therefore, did not exhibit ruthenium agglomeration or loss of ruthenium. Catalysts with 3–7 nm ruthenium particles displayed a higher turnover number for hydrocarbon synthesis, higher olefinicity, and chain-growth probability and did not exhibit water-gas shift activity in contrast to ruthenium particles which were about 0.8 nm in size. The CO disproportionation measurements showed much less CO dissociation over highly dispersed ruthenium relative to 3–7 nm ruthenium particles. This phenomenon is consistent with the low activity, the low chain-growth probability and may also relate to the tendency to form ruthenium carbonyl that is observed with small ruthenium particles. The apparent water-gas shift activity of highly dispersed ruthenium can be explained by the low CO dissociation efficiency as well as by the proposed ability to dissociate the water molecule.