Equilibrium, kinetic and morphological studies of carbon formation
in CH4+H2, CO, and CO+H2 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+H2. 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+H2 well. The kinetics of
carbon formation in CO and CO+H2 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 ≥ 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.Center for Surface Reactivity