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

Abstract

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.