Factors in forming CO and CO2 over a cermet of Ni-gadolinia-doped ceria with relation to direct methane SOFCs

Temperature-programmed self de-coking tests were carried out to study the factors in forming CO and CO2 over Ni-gadolinia-doped ceria (GDC), Fe-GDC, and Ni-Fe-GDC cermet catalysts with relation to direct methane SOFCs. It was found that the bulk lattice oxygen plays a very important role in forming CO and CO2. CO formation is generally associated with O species coming from the bulk lattice oxygen while CO2 formation generally is associated with the surface OH species. With Ni:GDC = 3:5 in weight, a very large amount of CO is formed with relatively negligible CO2 formation. With 1 wt% Ni, preferential CO2 formation can be realized below 700 °C. The activity of CH4 decomposition over Fe is much lower than that over Ni, but CO2 formation is favored over Fe. A large promotion of the CO2 formation activity appears with the addition of 0.1 wt% Fe into 1 wt% Ni and a synergistic effect is demonstrated. In general, CO2 formation is favored below 700 °C while CO formation favored above 800 °C. However, with Fe addition, CO2 formation can be favored over Ni-GDC at 1000 °C due to the high-temperature redox property of the Fe species. Thus, for preferential CO2 formation, highly dispersive Ni distribution with Fe addition is recommended for the anode cermet of intemediate-temperature SOFCs.