Effect of CaO–ZrO2 addition to Ni supported on γ-Al2O3 by sequential impregnation in steam methane reforming

Ni catalysts supported on (CaO–ZrO2)-modified γ-Al2O3 were prepared by sequential impregnation. The effects of varied CaO to ZrO2 mole ratios at 0, 0.20, 0.35, 0.45, and 0.55 on the activity and stability of the modified Ni catalysts were studied. As a result of using CaO–ZrO2 as a promoter, each catalyst contained CaO–ZrO2 at only 5%. γ-Al2O3 used as support was modified by CaO–ZrO2 before the deposition of nickel oxide. The addition of CaO–ZrO2 at an optimum ratio was expected to improve the stability of Ni catalysts due to the decrease of carbon formation resulting from carbon gasification. All the fresh catalysts were characterized by ICP, XRD, BET surface area, TGA in H2, and TPR before catalytic testing in steam methane reforming at 600 °C. The spent catalysts were examined by TEM and TGA to observe the catalysts deactivation. The identification of CaO–ZrO2 phases indicated that CaO and ZrO2 reacted with each other to be monoclinic solid solution ZrO2, CaZr4O9, CaZrO3, and CaO corresponding to the phase diagram of CaO–ZrO2. The existence of CaZrO3 for 0.55 mol ratio of CaO/ZrO2 enhanced activity in steam methane reforming because oxygen vacancies in CaZrO3 greatly preferred the water adsorption creating the favorable conditions for carbon gasification and, then, water gas shift. The prominence and continued existence of these two reactions on the Ni catalysts leads to the particular increase of H2 yield. Moreover, the increasing amount of CaZrO3 in the Ni catalysts significantly improved carbon gasification. However, the Ni catalysts with CaZrO3 showed whisker carbon after catalytic testing; this carbon specie has not been tolerated in steam methane reforming. Therefore, these results significantly differed from the hypothesis.