Pulse-MS study of the partial oxidation of methane over Ni/La2O3 catalyst

The CH₄ direct oxidation reaction was studied at 600°C by the pulse-MS transient method over the Ni/La₂O₃ catalyst. Over the freshly prepared catalyst (which contains NiO), the CO selectivity and CH₄ conversion increased and attained constant values as the number of CH₄/O₂ pulses increased. Over the reduced catalyst (containing Ni), as the number of CH₄/O₂ pulses increased, the CO selectivity and CH₄ conversion decreased before they reached the same constant values as over the fresh catalyst. The CO selectivity increased as the residence time of the reactants shortened, implying that CO was directly generated without the preformation of CO₂. The activation energies of CH₄ dehydrogenation in the presence and absence of oxygen have been calculated using the bond-order conservation Morse-potential approach. The results indicate (1) the direct dehydrogenation steps are more likely to occur; (2) the transient oxygen species adsorbed on-top of the metal atoms promote dehydrogenation; (3) the oxygen species adsorbed on bridge or hollow sites do not promote dehydrogenation.