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 1375oC 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 contribute significantly to the overall reforming activity, along with the large bulk Ni particles within the Ni-YSZ cermet. We have observed high initial activity that decreases by as much as an order of magnitude with time on stream, until the anode catalyst reaches a stable steady state activity. The time to reach this stable activity is a function of the reaction conditions and feed gas composition. Higher temperature, hydrogen partial pressure, and space velocity all accelerated the deactivation rate at a constant steam-to-carbon ratio of 3. Initial and lined out activities and average turnover frequencies were obtained for both Ni-YSZ and bulk Ni, based on a rate expression that is first order in methane and zero order in steam. Comparative tests at 750oC show high initial activity on a per-Ni site basis with both materials, but these turnover rates decline over a period of a few hours. Following lineout, there appears 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 is an apparent structure insensitivity with aged catalysts where Ni particle sizes are generally 50 nm and greater. Under reaction conditions that employ high space velocities and low methane conversions, the water-gas-shift reaction does not establish thermodynamic equilibrium.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 947037
- Report Number(s):
- PNNL-SA-60597; JCTLA5; AA2530000; TRN: US200904%%372
- Journal Information:
- Journal of Catalysis, 258(2):356-365, Vol. 258, Issue 2; ISSN 0021-9517
- Country of Publication:
- United States
- Language:
- English
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