(771f) Modified Zirconia As Gold Catalyst Support for Low-Temperature Methanol Steam Reforming

Modified Zirconia as Gold Catalyst Support for Low-Temperature Methanol Steam Reforming

Chongyang Wang, Howard Saltsburg, Maria Flytzani-Stephanopoulos^*

Department of Chemical and Biological Engineering, Tufts University, 4 Colby Street, Medford

MA 02155, United States

Nanoscale zirconia (ZrO₂) is widely investigated for its catalytic relevance both as catalyst support and catalyst. Surface acidity/basicity of zirconia is proved to be a critical factor for developing strong interaction between catalyst support and metals. Pristine zirconia with strong Lewis acids catalyzes the methanol steam reforming reaction (SRM) through η²-adsorption of the methoxy group, which produces CO as an undesired intermediate, requiring further processing (through the water-gas shift reaction) to maximize the yield of hydrogen. In this study, nanoscale zirconia was modified with zinc oxide (ZnO) at different amounts, and examined as support of gold catalysts for the SRM reaction. The gold precursor adsorbed more efficiently on the modified zirconia when using the anion adsorption synthesis method due to the higher point of zero charge (PZC) of the modified zirconia. Better dispersion of gold was also achieved on the ZnO-modified zirconia, which in turn improved the catalyst activity. Notably, the new method can be used to control the selectivity over both the low and moderately high temperatures (175ºC to 375ºC) by modulating the whole surface acidity/basicity of the catalyst. As with other gold catalysts, the η¹-adsorption of methoxy group is favored, producing CO₂ and H₂ as exclusive products of SRM.

To investigate the modification of zirconia, surface acidity titration has been conducted with probing molecules (isopropanol) both in temperature-programmed surface reaction (TPSR) and temperature-programmed desorption (TPD) modes. SRM-TPSR and CH₃OH-TPSR were performed on supports and gold-added catalysts to unravel the effects of surface acidity/basicity on the selectivity of methanol steam reforming reaction, and to understand the reaction mechanism of methanol steam reforming reaction on modified and pristine zirconia. Steady-state SRM reaction activity tests were also carried out on the bare and gold-loaded zirconia surfaces. In summary, the interaction between catalyst and support, and dispersion and stability of gold are enhanced by modification of the surface acidity of zirconia, and the overall catalyst selectivity to hydrogen is greatly improved.