Highly efficiency H2 production for real coal tar steam reforming over Ni-ca/H-Al catalyst: Effects of oxygen vacancy, CaO doping and synthesis methods

Abstract

Steam reforming of medium-low temperature coal tar (SRT) to produce H₂ is important for addressing the energy crisis and challenges of global climate change. However, current catalysts still suffer from low H₂ selectivity, low tar conversion and poor long-term stability due to the complex composition of real coal tar. Herein, a series of Ni-Ca/H-Al catalysts were prepared by different methods. The results showed that the acid pretreatment effectively removed the hydroxyl groups on the surface of γ-Al₂O₃ to form oxygen vacancies (Ov), and the NiO was mainly anchored in the Ov. The cit-Ni-2Ca/H-Al catalyst prepared by citric acid-assisted impregnation exhibited the highest H₂ yield (152.74 mmol/g_-tar), while CO₂ was only 22.11 mmol/g_-tar. Characterization showed that the addition of CaO improved the dispersion of NiO and increased the Ov concentration. The increase in Ov weakened the NiO bonds in NiO and facilitated the low-temperature reduction of NiO. The cit-Ni-2Ca/H-Al catalysts prepared by the citric acid-assisted impregnation method had better dispersion and strong basic sites compared with the impregnation and precipitation methods, leading to more active sites for CO₂ and H₂O adsorption, and improving the carbon build-up resistance and reusability of the catalysts. Py-GC/MS results showed the cit-Ni-2Ca/H-Al had excellent tar deoxidation ability and effectively promoted the breakage of CC bonds in furans, benzene ring branches, and CO bonds in aliphatic hydrocarbons. The cit-Ni-2Ca/H-Al catalyst exhibited high tar conversion (90.96%) and H₂ yield (146.27 mmol/g_-tar) without any treatment for at least 8 cycles. Interestingly, the H₂ yield reached 151.58 mmol/g_-tar by the simple regeneration of cit-Ni-2Ca/H-Al. This study provides a theoretical basis for efficient steam reforming of real coal tar for H₂ production and catalyst preparation.