Effects of CaO and ZrO₂ addition on methane steam reforming of Ni/Al₂O₃ catalyst

Abstract

Steam methane reforming (SMR) is a process widely used in industrial scale for H₂ production. Ni-based catalysts are generally used in SMR process since they offer low cost and high catalytic activity. However, Ni-based catalysts suffer from rapid deactivation resulting from carbon deposition. This research work focuses on the improvement to the stability of Ni catalysts. The addition of CaO–ZrO₂ promoter into Ni/Al₂O3 catalyst was expected to improve its stability. This work is divided into three main parts. First, effects of CaO to ZrO₂ mole ratios (0, 0.2, 0.35, 0.45, and 0.55) on the performance of Ni/γ-Al₂O₃ catalyst prepared by sequential impregnation are investigated. Ni/γ-Al₂O₃ catalyst containing CaO–ZrO₂ at 0.55 mole ratio provided the highest H₂ yield; however, it showed some amount of deposited carbon. The results were attributed to the appropriate mole ratio leading the formation of CaZrO₃ and then oxygen vacancies. The oxygen vacancies preferred to adsorb water molecules facilitating the reactions of carbon gasification and then water gas shift (WGSR). Secondly, preparation methods between co-impregnation and sequential impregnation of Ni/γ-Al₂O₃ catalyst at the appropriate mole ratio (0.55) are examined. The activity of co-impregnation–prepared catalyst was higher and more consistent than that of sequential impregnation–prepared catalyst. Nevertheless, at time on stream of 30 h the activity of co-impregnation–prepared catalyst significantly dropped due to the formation of encapsulating carbon. The results were attributed to the different preparation methods leading to their significant difference in physico-chemical properties and then catalytic activity. Finally, α-Al₂O₃ was used as supporter for preparing the (CaO–ZrO₂)- modified Ni catalyst through sequential impregnation. Loading percentage of CaOZrO₂ fixed at 0.5 mole ratio of CaO/ZrO₂ was varied from 0 to 15 wt%. The addition of CaO–ZrO₂ at 15 wt% indicated the highest H₂ yield and the small amount of deposited carbon. The results were attributed to the appropriate amount of formed CaZrO₃ nanoparticles leading to the favorable conditions for SMR reactions.