High Temperature Water Gas Shift Catalysts with Alumina

Alumina (Al2O3) was added as a component of conventional iron oxide based high temperature water gas shift (WGS) catalysts. The catalysts contained Fe-Al-Cr-CuO and were synthesized by coprecipitation. A series of catalysts were prepared with 5 to 50 wt% Al2O3, 8 wt% Cr2O3, 4 wt% CuO and the balance Fe2O3. One catalyst was prepared in which the chromia was replaced by alumina. All of the catalysts were compared to a reference WGS catalyst (88 wt% FeOx, 8 wt% Cr2O3, and 4 wt% CuO) with no alumina. The catalysts were characterized using temperature programmed reduction (TPR), surface area analysis using nitrogen physisorption, and scanning electron microscopy (SEM) with compositional analysis. The catalysts were also tested kinetically under WGS conditions.

Addition of 10 to 15 wt% alumina increased the catalyst activity and thermal stability, with approximately 15 wt% alumina addition being optimum, as this catalyst produced a reaction rate (normalized per mass) 74% higher than the reference catalyst. The effect of alumina addition was greater than the surface area increase alone, which suggests that alumina alters the activity of the iron oxide domains, likely through an increase in reducibility, as shown by the TPR results. This synergistic effect was only observed when both alumina and chromia were present. Alumina alone (as a replacement for chromia) was not as an effective stabilizer as chromia. Although both the aluminacontaining catalyst (without chromia) and the reference with chromia had similar initial surface areas (~160 m2/g), the alumina-containing catalyst retained only 74% as much surface area after reaction. Results from the catalysts with 50 wt% alumina suggest that the loss of catalytic activity is due also to the formation of aluminates.

Alumina addition to conventional high temperature water gas shift catalysts at concentrations of approximately 15 wt% increases CO conversion rates and increases thermal stability. If the alumina replaces the chromia content of the catalyst, the surface area after use is only 74% of the surface area of the comparable Fe-Cr-Cu catalyst. Hence, alumina alone cannot effectively stabilize Fe3O4.

When alumina is combined with chromia, varying effects were observed on the HT water gas shift catalysts. For the fresh unreduced catalysts, catalysts with both chromia and 10-15 wt% alumina have high fresh surface areas (over 200 m2/g) that are 20-30% higher than the catalyst without alumina. The surface area increase is larger than the proportional effect expected for the small amounts of alumina added. This suggests that the added alumina has an added structural effect on the other catalyst components. After use, these two catalysts retain higher surface areas (33.4 m2/g and 38.4 m2/g) compared to the other catalysts. Even the catalyst with the least amount of alumina, 83Fe-5Al-8Cr-4Cu, retained a surface area of 22.5 m2/g after use, which is higher than the reference Fe-Cr-Cu catalyst at 19.5 m2/g. Thus, alumina in Fe-Al-Cr-Cu catalysts significantly increase surface areas, especially for the used catalysts, indicating that thermal stability is also improved.

In the series of alumina-containing HT-WGS catalysts in this study, the optimum quantity of alumina addition is ~15 wt% (73Fe-15Al-8Cr-4Cu). This catalyst produced a CO conversion rate that was 73.8 % larger than the reference catalyst with no alumina (88Fe-8Cr-4Cu) at similar reaction conditions.