One-Pot Synthesis of a Highly Active and Stable Ni-Embedded Hydroxyapatite Catalyst for Syngas Production via Dry Reforming of Methane

Hydroxyapatite (HAP) is widely used as a loaded catalyst support due to its excellent ion exchange properties. In this study, a series of Ni-based hydroxyapatite catalysts were prepared by a novel method, one-pot co-precipitation (denoted as Ni/HAP-OP), for dry reforming of methane (DRM) testing. The textural, structure, and physicochemical properties of the catalysts were studied by inductively coupled plasma spectroscopy (ICP), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), the Brunauer-Emmett-Teller (BET) method, scanning transmission electron microscopy (STEM), and hydrogen temperature-programmed reduction (H-2-TPR), and the spent catalysts were analyzed by thermogravimetric analysis (TGA), Raman, and transmission electron microscopy (TEM). The Ni/HAP-OP catalysts exhibit a typical hydroxyapatite structure. The Ni-0.5/HAP-OP catalyst has the highest dispersion of Ni among the prepared catalysts, and most of the added Ni is in the form of Ni2+ [I] and Ni2+ [II] in the lattice of hydroxyapatite. When Ni is post-impregnated (Ni-0.5/HAP), almost half of the Ni is present in the form of NiO. The different coordination configurations of Ni are confirmed to play an important role in the DRM reaction. Naturally, the Ni-0.5/HAP-OP catalyst shows excellent catalytic activity and catalytic stability in the 200 h stability test compared with Ni-0.5/HAP, with stable CH4 (about 94%) and CO2 (95%) conversion. The deactivation is only 0.014 and 0.017% h(-)(1), respectively, and the H-2/CO ratio in the syngas is always maintained close to 1.0. Analyses of the spent catalysts reveal that amorphous carbon deposits are observed around the metallic Ni on the Ni-0.5/HAP catalyst, and Raman analysis indicates the presence of both amorphous and graphitic carbon deposits. In contrast, only trace amounts of graphitic carbon are present in the Ni-0.5/HAP-OP catalyst after 200 h of the DRM reaction. The Ni-based hydroxyapatite catalysts prepared by the one-pot co-precipitation method exhibit high activity and stability for the DRM reaction thanks to the confinement of Ni particles and the inhibition of carbon deposition.