BIMETALLIC RUTHENIUM-GOLD CATALYSTS: CHARACTERIZATION AND CATALYTIC BEHAVIOR (ELECTRON MICROSCOPY, HETEROGENEOUS CATALYSTS, METAL-METAL INTERACTIONS).

摘要

Two series of Ru-Au bimetallic catalysts were investigated, one supported on SiO(,2) and the other on MgO. The catalysts were extensively characterized and the activity and selectivity patterns in the Fischer-Tropsch synthesis were studied.; The presence of Au on the catalyst surface influenced the preexponential factor but did not affect the apparent activation energy for CO hydrogenation. There was no significant influence of Au on either the product distribution or the olefin/paraffin ratio. Catalyst dispersion, however, markedly affected olefin/paraffin selectivity. IR bands of CO or NO adsorbed on Ru did not reveal any evidence for electronic effects.; The activity patterns in CO hydrogenation were similar to those reported previously in the hydrogenolysis of ethane. On SiO(,2), addition of Au led to a monotonic decline of two orders of magnitude in turnover numbers, while on MgO, the activity increased approximately 10 times at intermediate Au contents. The activity trends were similar in methanation as well as the production of higher hydrocarbons. This suggests that both the hydrogenolysis of ethane and the Fischer-Tropsch synthesis respond similarly to Au induced changes in surface structure.; Analytical electron microscopy was very useful in characterizing the differences between the SiO(,2) and MgO supported bimetallic catalysts. The data from this work showed how a typical bimetallic catalyst could be extremely non-uniform as regards the distribution of the two metals in particles of differing sizes. On SiO(,2), Ru existed in small, bimetallic, particles (< 4 nm) while the large (> 15 nm) particles were invariably monometallic Au. On MgO, Ru and Au were distributed over the entire size range, but the major contribution to Ru surface area came from medium (4-15 nm) and large (> 15 nm) size particles.; The drop in activity appears to be due to a geometric ensemble effect involving the presence of Au on the surface of Ru. The apparent increase in activity on MgO seems to be support related: Au counteracts the activity-suppressing effect of the MgO support on Ru, partially restoring the intrinsic activity of Ru.