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Investigation of the water gas shift (WGS) reaction over promoted Pt catalysts

Besi Nefeli

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Summary

The Water – Gas Shift reaction (WGS) is an effective chemical process following the reactions of steam reforming of fuels in order to increase the concentration of the produced hydrogen (H2) and to eliminate carbon monoxide (CO) concentration. The commercially available catalysts, which are used nowadays, are mainly copper (Cu) and iron (Fe) catalysts which are sensitive to thermal treatment and oxidative reaction conditions. Previous studies in this field showed that noble metals like platinum (Pt), rhodium (Rh) and gold (Au) can replace conventional catalysts resulting in more active and stable catalysts. In the present thesis, Pt catalysts (0.5 wt.%) supported on calcined (at 600 oC) and non-calcined metal oxides including alumina, zirconia and ceria (Al2O3, ZrO2, CeO2) were synthesized employing the wet impregnation method and evaluated for the WGS reaction. Alkali (Li, Na, K, Cs) promoted Pt/Al2O3, Pt/ZrO2 and Pt/CeO2 catalysts were also synthesized by adding alkali metals on the support surface following the same method and tested with respect to their performance for the WGS reaction. Catalysts were tested in the temperature range of 200-600 oC using a feed stream simulating the outlet of a dry reforming of methane (CH4) reactor. Results showed that CeO2 is the most active support for the WGS reaction (Τ=480 oC, XCO = 66.9%) followed by ZrO2 and finally Al2O3. The calcination of the support has no influence on the CO conversion with the exception of Al2O3 where it was found that the calcination of the support resulted in slightly higher CO conversion at temperatures higher than 400 oC. The alkali promotion of Al2O3 and ZrO2 has minor influence on the catalytic activity, resulting in a small increase of the reaction rate for K and Li promoted Pt/Al2O3 catalysts and for the Na and Cs promoted Pt/ZrO2 catalysts. In contrast, the addition of alkalis on CeO2 support resulted in a shift of the CO conversion curve toward lower temperatures compared to the unpromoted Pt/CeO2 catalyst. Catalytic activity was found to increase following the order Pt/CeO2 < Pt/Li-CeO2 < Pt/Cs-CeO2 ~ Pt/K-CeO2 < Pt/Na-CeO2 with the optimum Pt/Na-CeO2 catalyst reaching the equilibrium conversion curve at 460 oC leading to XCO = 78%. In order to investigate the effect of synthesis method on the performance of Na promoted Pt/CeO2, a 0.5%Pt-0.059%Na/CeO2 catalyst was prepared with simultaneous addition of Pt and Na on CeO2 surface and compared with 0.5%Pt/0.059%Na-CeO2 catalyst where Pt addition was conducted after Na addition on CeO2 surface. Results showed that higher catalytic activity is achieved when Na is added on the support surface followed by Pt addition.

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