Carbon formation and CO methanation on silica-supported nickel and nickel–copper catalysts in CO + H2 mixtures

摘要

The steady-state rates of simultaneous carbon formation and methanation on silica-supported nickel and nickel-copper catalysts in CO + H 2 gas mixtures have been measured in the partial pressure range 10-30 kPa and the temperature range 583-873 K. The steady-state kinetic results for carbon formation can be explained by a model previously used for carbon formation in pure CO gas, but modified to take into account the influence of hydrogen. Comparisons between the results for Ni/SiO 2 , Ni 0.99 Cu 0.01 /SiO 2 , Ni 0.9 Cu 0.1 /SiO 2 , and Ni 0.75 Cu 0.25 /SiO 2 show that the specific rate of carbon formation is increased when 1 at.% of Cu is added to a nickel catalyst, but that the rate decreases when the Cu content is increased to 10 at.%, and that no carbon formation is seen for the Ni 0.75 Cu 0.25 /SiO 2 catalyst. The addition of a small amount of Cu, on the other hand, decreases the methanation rate while at higher Cu contents the rate goes up again and a maximum is observed at about 7 at.% Cu. Thus a simple ensemble model cannot explain the influence of copper on the rate of carbon formation or methanation. Comparison of carbon formation and methanation rates for the Ni/SiO 2 and the Ni 0.99 Cu 0.01 /SiO 2 catalysts indicates that the two reactions have different rate-controlling steps and that the carbon formation influences the methanation rate mainly through poisoning of the catalysts.