Roles of Oxygen Vacancies over Ni/CexZr1-xO2 Catalysts for Ethanol Steam Reforming

Abstract

The active sites over Ni/CexZr1-xO2 were studied for ethanol steam reforming (ESR). Oxygen vacancy is an essential active site affecting hydrogen production. In this study, to understand catalytic behavior depends on the lattice structures and temperatures over oxygen vacant sites, CexZr1-xO2 as the support was modified with Ce-rich (CZ) and Zr-rich (ZC). Two types of CexZr1-xO2 solid solution were used as support for preparing Ni/CZ (CZN) and Ni/ZC (ZCN) catalysts. ESR for all the catalysts was conducted at 550 ℃ and 750 ℃. The CZ catalyst contained higher oxygen vacancy not only on the surface but also in the bulk structure than the ZC catalyst. Ni contributes to the activity of the surface oxygen vacancy of both CZ and ZC catalysts by helping the reduction of the support. The oxygen vacancy in the bulk ZC structure completely disappeared due to the Ni impregnation whereas the bulk oxygen vacancy of CZ was still maintained. Oxygen vacancy generated active oxygen species via the steam activation and then, the active oxygen species simultaneously involved steam reforming, water gas shift reaction, and ethylene oxidation during ESR. From the reaction result, the lack of oxygen vacancy over ZC resulted in limited reaction routes due to limited active oxygen species. The ZC catalyst, when compared with CZ, exhibited the limitation of the ethylene oxidation at 550 ℃ and water gas shift reaction at 750 ℃, respectively. With the Ni species, even though the surface oxygen vacancy behavior over CZN and ZCN was the same, the oxygen-involved reaction was enhanced with the bulk oxygen vacancy of CZN, resulting in the highest H¬¬2 production of 1112.2 μmol/g•min for the CZN catalyst.