Effect of nickel microstructure on methane steam-reforming activity of Ni-YSZ cermet anode catalyst

The activity of nickel-yttria stabilized zirconia (Ni-YSZ) solid oxide fuel cell (SOFC) cermet anodes for the steam-reforming of methane has been investigated in the absence of electrochemical effects. The cermet was prepared by co-milling and sintering NiO and 5YSZ powders at 1375 °C in air. During the high-temperature sintering step, NiO dissolved into the YSZ particles to form a solid NiO-YSZ solution. During the subsequent catalyst reduction step, Ni exolved from the YSZ. As a result, many small Ni particles on the order of 10-20 nm formed at the surface of the YSZ. These small particles contributed significantly to the overall reforming activity, along with the large bulk Ni particles within the Ni-YSZ cermet. We observed high initial activity that decreased by as much as an order of magnitude with time on stream, until the anode catalyst reached a stable steady-state activity. The time to reach this stable activity was a function of the pretreatment and reaction conditions. Initial and lined-out activities and average turnover frequencies were obtained for both Ni-YSZ and bulk Ni, based on a rate expression that was first-order in methane and zero-order in steam. Comparative tests at 750 °C showed high initial activity on a per-Ni site basis with both materials, but these turnover rates declined over a period of a few hours. After lineout, there appeared to be a negligible effect of Ni particle size on turnover rate. These results indicate the presence of structure-sensitivity for methane reforming, but only with freshly calcined and reduced catalysts that may contain highly coordinatively unsaturated sites. There was an apparent structure-insensitivity with aged catalysts in which Ni particle sizes were generally ≥30 nm. Under reaction conditions with high space velocities and low methane conversions, the water-gas shift reaction did not establish thermodynamic equilibrium.