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

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.