Recyclable NiMnO_x/NaF catalysts: Hydrogen generation via steam reforming of formaldehyde

In this paper, a low-cost and efficient NiMnO_x/NaF catalyst that can be recycled for re-preparation was developed by sol–gel method for formaldehyde steam reforming to produce hydrogen, aiming at the high cost of commercial reforming catalyst and the difficulty of recycling after deactivation and abandonment. Results showed that the NiMnO_x/NaF catalyst could efficiently steam reforming formaldehyde to produce hydrogen. Among them, the catalyst with 5 wt% NiMnO_x had better hydrogen production performance, achieving 100% hydrogen yield, 100% formaldehyde conversion and 100% carbon balance from 500 to 600 ℃. Moreover, 5%-NiMnO_x/NaF catalyst exhibited excellent stability at 500 ℃ for 28 h and the hydrogen production performance of the recycled catalyst was restored to the level of the initial fresh sample. Combining analysis of FE-SEM, N₂ adsorption/desorption, XRD, H₂-TPR, in situ DRIFTS, XPS, and DFT calculations, it was found that the surface micro-structure of the NaF carrier was dense, and the specific surface area of the NiMnO_x/NaF catalyst was very small, breaking through the limitation of low specific surface area catalysts that cannot obtain high-quality catalytic activity. Its steam reforming of formaldehyde to produce hydrogen was a fast surface reaction process. Ni and Mn species combined with NaF to form the Na_0.67Ni_0.33Mn_0.67O₂ phase, and F atom had a strong induction effect on Ni atom, which was conducive to weakening the Ni-O bond, thus improving the low-temperature redox performance of Ni²⁺. In the process of reforming formaldehyde to produce hydrogen, the NiMnO_x/NaF catalyst could quickly adsorb and activate formaldehyde and H₂O molecules, and promoted the formation of formate intermediates through the adsorbed hydroxyl groups on the surface, and then rapidly produced H₂ and CO₂.