PB-05
80 120 160 200 240
0 10 20 30 40 50 60 70 80 90
CO Conversion (%)
Temperature (℃) a
80 120 160 200 240
0 20 40 60 80 100
0 20 40 60 80 b 100
Temperature (℃)
CO2 Selectivity (%) O2 Conversion (%)
Highly active IrFe/Al
2O
3catalyst for preferential CO oxidation:
Studied by Microcalorimetry
Wansheng Zhang, Aiqin Wang, Lin Li, Xiaodong Wang, Tao Zhang*
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian,116023 Keywords: CO oxidation, PROX, Ir-Fe catalyst, Iridium, Microcalorimetry
Preferential oxidation (PROX) of CO by O2 in H2-rich stream has drawn much attention over the last decade due to its potential application in fuel cell technology.
Various catalyst systems, including supported noble metals (Pt, Ru and Rh), highly dispersed gold particles, as well as CuO/CeO2, have been proved to be active for this reaction. In contrast, very few studies have been reported on Ir catalysts probably because of the rare and expensive Ir resources. In this work, we designed a novel and efficient catalyst system IrFe/Al2O3, focusing on the promoting role of Fe species.
IrFe/Al2O3 was prepared by successive incipient wetness impregnation. Ir content in all the catalyst samples was fixed at 1wt.%, and Fe content was to give Fe/Ir atomic ratio of 5/1. For comparison, Ir/Al2O3 and Ir/Fe2O3 were also prepared by the same method. Prior to the test of catalytic activity, the catalyst sample was in-situ reduced with H2 at 300℃ for 2 h. The effluent gas was on-line analyzed by a gas chromatograph (Angilent GC-6890) equipped with a TCD detector. Microcalorimetric measurements of CO, O2 and H2 adsorption were performed using a BT 2.15 heat-flux calorimeter.
The catalytic performances of the four samples are shown in Fig. 1. The CO conversion and CO2 selectivity of IrFe/Al2O3 were much higher than those of Iridium singly supported on either γ-alumina or iron oxide in a wide temperature range (80~220℃). Such difference in catalytic activity with Fe introduction to Ir/Al2O3 was particularly remarkable at a wide temperature range, which is very important for fuel cell applications, especially with the high CO2 selectivity.
Fig. 1. CO conversions (a), O2 conversions (filled symbol) and CO2 selectivities (open symbol) (b) versus reaction temperature over IrFe/Al2O3 (■,□), Ir/Al2O3 (●,○), Ir/Fe2O3 (◆,◇), Fe/Al2O3 (▲).
The IrFe/Al2O3, Ir/Al2O3 and Fe/Al2O3 samples were characterized by XRD, HRTEM, H2-TPR and Microcalorimetry. The results of XRD and HRTEM characterization indicated that both FeOx and Ir species were highly dispersed on Al2O3 support, without any visible FeOx or Ir particles. The reduction of Ir species on Fe/Al2O3 became easier than that on Al2O3, together with the partial reduction of Fe2O3, testified clearly by the characterization result of H2-TPR.
Microcalorimetry analysis demonstrated that when Ir was supported on the pre- formed Fe/Al2O3, the strength of CO or H2 adsorption and distribution of active sites on the IrFe/Al2O3 catalyst were different from Ir/Al2O3. The addition of Fe caused the
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great enhancement of the strength and saturation uptake of O2 adsorption on the IrFe/Al2O3 catalyst. It was demonstrated that the sandwich structure (IrFe/Al2O3) allowed metallic Ir sites exposed on the surface and accessible for CO adsorption, while did not interfere with the O2 adsorption and activation on the FeOx species.
Thus, a non-competitive adsorption L-H mechanism has been proposed where CO adsorbed on Ir sites and O2 adsorbed on FeOx sites; the reaction may take place at the interface of Ir and FeOx or via a spill-over process.
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