Reactivity of ceria, Gd- and Nb-doped ceria to methane

The reactivity of methane with ceria, and ceria doped with either 10 cation% Gd, 1.4 cation% Nb, 5 cation% Nb has been investigated using temperature-programmed reaction (TPRx) and isothermal reaction at 900 degreesC in 5% methane in argon. The influence of the calcination atmosphere on the Nb-doped ceria has also been studied. All the cerias showed activity for conversion of methane to hydrogen and carbon monoxide with only small amounts of carbon deposition. Methane conversion is controlled by surface reaction with an activation energy of 165 +/- 10 kJ mol(-1). Gd doping was found to have a relatively minor effect on the reaction rate, but reaction with Nb-doped ceria was significantly slower. XPS results suggested that this is due to segregation of Nb to the ceria surface on the samples calcined in air, however, the XPS results were more complex for the samples calcined in N-2/H-2 mixture. Nb-doped ceria was capable of oxidising carbon deposits at lower temperatures than ceria or Gd-doped ceria. (C) 2002 Elsevier Science B.V. All rights reserved.