In Situ XPS and DRIFTS Studies On the Supported Co/CeO2-ZrO2 Catalysts for Ethanol Steam Reforming

In situ XPS and DRIFTS studies on the supported Co/CeO₂-ZrO₂ catalysts for ethanol steam reforming

Sean S.-Y. Lin,^a Do Heui Kim,^b and Su Y. Ha,^a,*

^aThe Gene and Linda Voiland School of Chemical Engineering and Bioengineering,

Washington State University, P.O. Box 642710, Pullman, WA 99164-2710, USA

^bInstitute for Interfacial Catalysis, Pacific Northwest National Laboratory,

PO Box 999, Richland, WA 99354, USA

The addition of zirconia improves the thermal stability of ceria as well as its capability of releasing/storing oxygen at the reaction temperatures for ethanol steam reforming (ESR). These properties allow the use of cerium-zirconium mixed oxide (CeO₂-ZrO₂) to support cobalt species for hydrogen production via ESR; the supported cobalt catalyst (10% Co/CeO₂-ZrO₂) has demonstrated a high hydrogen yield (4.9 mol H₂/ mol ethanol converted) at 450 ^oC under a steam-to-carbon (S/C) ratio of 6 by minimizing the production of methane and carbon monoxide. The maximum hydrogen production rate is 147 mmole/g-s measured at a WHSV_EtOH of 6.3. However, the catalytic activity and the selectivity were significantly influenced by the operating S/C ratio. The dual role of water in ESR was speculated not only to facilitate the water-gas-shift (WGS) reaction, but also to re-oxidize the pre-reduced cobalt specie during the reaction. The hydrogen temperature-programmed reduction (H₂-TPR) study suggested that CoO_x was formed over the pre-reduced Co/CeO₂-ZrO₂ catalysts when water was introduced at reforming temperatures.

In this study, in situ X-ray photoelectron spectroscopy (XPS) was used to investigate the effect of water and the S/C ratio on the surface composition of Co/CeO₂-ZrO₂ catalysts in ESR. A series of reduction and reaction processes for the catalysts were carried out inside the reaction chamber coupled with XPS. In addition, the surface chemistry of the catalysts treated under various reduction and reaction conditions was studied by an in situ diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) in order to explore the water-ethanol interactions over the catalyst surfaces and its effect on the catalytic selectivity.