Methane autothermal reforming with and without ethane over mono- and bimetal catalysts prepared from hydrotalcite precursors

Autothermal reforming of methane (in the presence or absence of ethane) over Ni and/or Rh catalysts prepared from hydrotalcites is reported. Surface and bulk structures of the reduced supported metal catalysts were characterized by various physicochemical techniques. XANES and EXAFS at the Ni and Rh K-edges indicate that the reduction of Ni ²⁺ and Rh ³⁺ in the support matrix with H ₂ was incomplete for all catalysts. NiRh alloy particles are formed in NiRh/MgAl. IR spectra of adsorbed CO indicate that the surface of the NiRh alloy particles is enriched in Ni. All catalysts hardly catalyzed coke formation during CH ₄ autothermal reforming. The surface concentration of reduced Ni was critical for the catalytic activity, i.e., the catalytic activity decreased in the order NiRh/MgAl > Ni/MgAl-2 > Rh/MgAl-1. However, Ni/MgAl-2 lost its activity, when cycling between 1073 and 773 K at very high space velocities (1.2×104 l/(hg)) through oxidation of metallic Ni. In contrast, NiRh/MgAl was relatively stable, while Rh/MgAl-1 did not deactivate. The enhanced stability of NiRh/MgAl by Rh against oxidation is attributed to H ₂ spillover from Rh in the NiRh alloy as well as to its high activity (larger fraction with reducing atmosphere). The activity of all catalysts in converting hydrocarbons increased in the presence of ethane, with ethane showing a high tendency to react preferentially. Ethane could be converted completely in the presence of partly converted methane. NiRh/MgAl and Rh/MgAl-1 showed stable activity at 1073 and 773 K, while Ni/MgAl-2 gradually deactivated at 773 K. The deactivation attributed to Ni oxidation was reduced by addition of C ₂H ₆.