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摘要
The objective of this paper is to study and clarify the role of selected supports (both reducible and non-reducible) on the activity, selectivity and stability of RhPt-based catalyst for diesel reforming. Autothermal reforming (AIR) of low-sulfur diesel (S similar to 6 ppm, C/H similar to 6.43 (w/w)), H2O/C similar to 2.5, O-2/C similar to 0.49, was tested at bench scale to detect differences in activity for catalysts consisting of 1 wt% Rh and 1 wt% Pt supported on alumina, ceria-zirconia (17.5 wt% ceria), silica and titania. Promoters in the form of MgO. Y2O3, La2O3, CeO2 and ZrO2, ranging from 4 wt% to 10 wt%, were also added onto the supports to detect differences in catalyst activity in terms of diesel conversion, CO2 selectivity, and hydrogen and ethylene production. All metals were added sequentially onto the support by the incipient wetness technique and washcoated on 400 cpsi cordierite monolithic carriers with dimensions d = 17.8 mm, l=30.5 mm. The product gas analysis, using FTIR and NDIR, showed that RhPt/CeO2-ZrO2 was found to be most active for AIR of diesel since a fuel conversion close to 98% was obtained. Furthermore, the catalyst activity of the unpromoted samples, in terms of diesel conversion, increased in the following order: RhPt/SiO2 < RhPt/TiO2 < RhPt/Al2O3 < RhPt/CeO2-ZrO2. The addition of promoters was found to be insignificant as well as having a negative impact on the catalyst performance in most cases, except for the alumina-promoted sample. The addition of 10 wt% La2O3 on RhPt/Al2O3 was found to enhance diesel conversion, hydrogen productivity as well as lower the ethylene concentration from 3700 ppm to less than half that value. The latter observation was confirmed by O-2-TPO analysis of aged powder samples where lower loads of coke were present than on the La-promoted sample. The morphology, surface and bulk properties of RhPt/CeO2-ZrO2 were closely examined in order to provide a possible correlation between the activity and characterization results. N-2-BET analysis showed that the surface area of RhPt/CeO2-ZrO2 was 64 m(2)/g, while the silica samples exhibited the highest area, similar to 137-185 m(2)/g. Hence, the difference in the surface areas was not enough to explain the trends observed in the activity measurements. XRD analysis of RhPt/CeO2-ZrO2 showed crystalline phases characteristic of zirconia, most likely tetragonal. Also, the diffractogram did not reveal any Rh or Pt peaks indicating that the noble metal particles are highly dispersed on the support. In contrast, peaks ascribed to metallic Pt (similar to 30-46 nm) were clearly visible on the XRD patterns taken from all the other supported samples. H-2-TPR analysis of RhPt/CeO2-ZrO2 showed reduction peaks ascribed to RhiOx species as well as a minor hydrogen spillover effect on the support to be present at T=120 degrees C and 450 degrees C, respectively. Also, the hydrogen consumption of the RhiOx species was the highest compared to the other supported RhPt samples. TEM analysis performed on fresh RhPt/CeO2-ZrO2 showed that the RhiOx and Pt particles were highly dispersed on the support, both with particle sizes in the vicinity of similar to 5-15 nm. Rh species was found on ceria and zirconia, while Pt was present mainly on the ceria layer possibly in the form of Pt-O-Ce bonds. H-2-chemisorption analysis measured at T similar to 40 degrees C shows similar Rh dispersion results. To summarize, the higher activity results of RhPt/CeO2-ZrO2 for AIR of diesel, compared to other supported catalysts, may be ascribed to the higher reducibility of RhiOx species as well as the superior Rh and Pt dispersion. Also, the support contribution, in particular ceria, is believed to promote water gas-shift activities as well as reduce coke deposits on the catalyst surface.
- 出版日期2011-8-11
