The Effect of Sodium in Activity Enhancement of Nano-sized Pt/CeO2Catalyst for Water Gas Shift Reaction at Low Temperature
Dae-Woon Jeong, Hari S. Potdar, Ki-Sun Kim, Hyun-Seog Roh
Bulletin of the Korean Chemical Society (Print)
Water gas shift reaction (WGS) is a key step in H 2 production for fuel cell application. 1 Pt/CeO 2 is one of the most studied catalysts in WGS. 2 The Pt/CeO 2 catalysts are known to be bi-functional; in which both nano-sized Pt metal and CeO 2 supports adsorb and activate CO and H 2 O respectively. The activity/stability of Pt/CeO 2 catalysts strongly depends on the presence of oxidized species of Pt formed at the surface. 3 Recently, Zhai et al. reported that alkali ions (sodium and
... ) added in small amounts activate platinum adsorbed on irreducible supports such as alumina and silica for low temperature WGS. 4 The alkali ion-associated surface OH groups are activated by CO at low temperature in the presence of atomically dispersed Pt. 4 It is also known that the nano-sized CeO 2 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. 2 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/CeO 2 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/CeO 2 , which is confirmed by TPR. The nano-sized CeO 2 with high surface area ~117 m 2 /g was prepared from Ce(NO 3 ) 3 ·6H 2 O and 15% solution of KOH by a simple protocol without using any template by a simultaneous precipitation/digestion technique described earlier by our group. 2 Pt/CeO 2 catalyst was prepared by an incipient wetness impregnation method with Pt(NH 3 ) 4 (NO 3 ) 2 (99%, Aldrich). The amount of Pt loading was fixed at 1 wt %. Pt-Na/CeO 2 catalyst was prepared by an incipient wetness impregnation method with aqueous NaNO 3 (99%, Duksan) and Pt(NH 3 ) 4 (NO 3 ) 2 (99%, Aldrich) at the same time. The amount of sodium loading was fixed at 2%. The prepared catalysts were calcined at 500 o C for 6 h. Activity tests were carried out from 200 to 320 o C at 1 atm in a fixedbed micro-tubular quartz reactor. CO 2 selectivity is defined as follows: CO 2 selectivity = CO 2 formed /CO converted . Table 1 describes the characteristics of Pt-Na/CeO 2 and Pt/CeO 2 catalysts. The addition of sodium in Pt/CeO 2 catalyst Table 1. Characteristics of Pt-Na/CeO2 and Pt/CeO2 catalysts Catalyst S.A. a (m 2 /g) Dispersion b (%) Pt S.A. b (m 2 /g) Pt Size b (nm) 1Pt-2Na/CeO2 109 51.7 1.28 2.2 1Pt/CeO2 104 37.6 0.92 3.0 a Estimated from N2 adsorption at −196 o C. b Estimated from CO-chemisorption at 50 o C Figure 1. XRD patterns of Pt-Na/CeO2 and Pt/CeO2 catalysts. Figure 2. TPR patterns of (a) Pt-Na/CeO2 and (b) Pt/CeO2 catalysts.