Recurring competitive reactions: Desorption of methane and molecular hydrogen from Cu(001)

Methane and molecular hydrogen desorption from a methyl and hydrogen exposed Cu(001) surface is investigated. Both gaseous products are observed nearly simultaneously within two temperature regimes separated by more than 100 K. The lower temperature desorption, at ∼325 K, is believed to result from two processes which compete for adsorbed atomic hydrogen: methyl reduction and associative hydrogen desorption. The higher-temperature competitive desorption is initiated after the onset of thermal decomposition of remaining methyl species, at ∼420 K. Kinetic simulations of the two presumed competing reactions are used to show observable and comparable methane and hydrogen evolution can occur in two temperature regimes, only with a precise balance of kinetic parameters, but fail to accurately reproduce the observed small differences in CH ₄ and H ₂ peak desorption temperatures. It is concluded that either the utilized desorption kinetics are inaccurate at low H _(a) coverages or rapid desorption, or the same reactions are not competitive at higher temperatures and an alternative active mechanism for product evolution must exist.