Effect of CaO–ZrO 2 addition to Ni supported on γ-Al 2O 3 by sequential impregnation in steam methane reforming

Abstract

Ni catalysts supported on (CaO–ZrO 2)-modified γ-Al 2O 3 were prepared by sequential impregnation. The effects of varied CaO to ZrO 2 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–ZrO 2 as a promoter, each catalyst contained CaO–ZrO 2 at only 5%. γ-Al 2O 3 used as support was modified by CaO–ZrO 2 before the deposition of nickel oxide. The addition of CaO–ZrO 2 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 H 2, 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–ZrO 2 phases indicated that CaO and ZrO 2 reacted with each other to be monoclinic solid solution ZrO 2, CaZr 4O 9, CaZrO 3, and CaO corresponding to the phase diagram of CaO–ZrO 2. The existence of CaZrO 3 for 0.55 mol ratio of CaO/ZrO 2 enhanced activity in steam methane reforming because oxygen vacancies in CaZrO 3 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 H 2 yield. Moreover, the increasing amount of CaZrO 3 in the Ni catalysts significantly improved carbon gasification. However, the Ni catalysts with CaZrO 3 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.