Carbon formation on nickel and nickel-copper alloy catalysts

Equilibrium, kinetic and morphological studies of carbon formation in CH4 + H-2, CO, and CO + H-2 gases on silica supported nickel and nickel-copper catalysts are reviewed. The equilibrium deviates in all cases from graphite equilibrium and more so in CO + CO2 than in CH4 + H-2. A kinetic model based on information from surface science results with chemisorption of CH4 and possibly also the first dehydrogenation step as rate controlling describes carbon formation on nickel catalyst in CH4 + H-2 well. The kinetics of carbon formation in CO and CO + H-2 gases are in agreement with CO disproportionation as rate determining step. The presence of hydrogen influences strongly the chemisorption of CO. Carbon filaments are formed when hydrogen is present in the gas while encapsulating carbon dominates in pure CO. Small amounts of Cu alloying promotes while larger amounts (Cu : Ni greater than or equal to 0.1) inhibits carbon formation and changes the morphology of the filaments ("octopus" carbon formation). Adsorption induced nickel segregation changes the kinetics of the alloy catalysts at high carbon activities. Modifications suggested in some very recent papers on the basis of-new results are also briefly discussed.