The Effect of Sodium in Activity Enhancement of Nano-sized Pt/CeO 2 Catalyst for Water Gas Shift Reaction at Low Temperature

Water gas shift reaction (WGS) is a key step in H2 production for fuel cell application. Pt/CeO2 is one of the most studied catalysts in WGS. The Pt/CeO2 catalysts are known to be bi-functional; in which both nano-sized Pt metal and CeO2 supports adsorb and activate CO and H2O respectively. The activity/stability of Pt/CeO2 catalysts strongly depends on the presence of oxidized species of Pt formed at the surface. Recently, Zhai et al. reported that alkali ions (sodium and potassium) added in small amounts activate platinum adsorbed on irreducible supports such as alumina and silica for low temperature WGS. The alkali ion-associated surface OH groups are activated by CO at low temperature in the presence of atomically dispersed Pt. It is also known that the nano-sized CeO2 with hydroxyl species in contact with nano-sized platinum is able to stabilize these species to limited extent; which are known to be active species for WGS. In this study, we have demonstrated sodium has a beneficial effect on stabilizing only oxidized species of Pt at the surface, resulting in the improvement of the activity of Pt/CeO2 catalyst for WGS at low temperature. To the best of our knowledge, this is the first report that oxidized species of Pt are stabilized by Na in Pt/CeO2, which is confirmed by TPR. The nano-sized CeO2 with high surface area ~117 m /g was prepared from Ce(NO3)3·6H2O and 15% solution of KOH by a simple protocol without using any template by a simultaneous precipitation/digestion technique described earlier by our group. Pt/CeO2 catalyst was prepared by an incipient wetness impregnation method with Pt(NH3)4(NO3)2 (99%, Aldrich). The amount of Pt loading was fixed at 1 wt %. Pt-Na/CeO2 catalyst was prepared by an incipient wetness impregnation method with aqueous NaNO3 (99%, Duksan) and Pt(NH3)4(NO3)2 (99%, Aldrich) at the same time. The amount of sodium loading was fixed at 2%. The prepared catalysts were calcined at 500 C for 6 h. Activity tests were carried out from 200 to 320 C at 1 atm in a fixedbed micro-tubular quartz reactor. CO2 selectivity is defined as follows: CO2 selectivity = CO2 formed/CO converted. Table 1 describes the characteristics of Pt-Na/CeO2 and Pt/CeO2 catalysts. The addition of sodium in Pt/CeO2 catalyst