The effects of the addition of calcium phosphate on catalytic activities for ammonia decomposition on CoMo-based catalysts

Abstract

In this study, wet impregnation method and sol–gel technique were used to prepare CoMo-based catalysts supported on two types of porous silicas containing calcium phosphate. The prepared catalysts were characterized using various techniques, such as N₂ adsorption–desorption measurement, scanning electron microscope, X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), differential thermal analysis, and X-ray photoelectron spectroscopy. The catalysts were tested for ammonia decomposition to produce CO_x-free hydrogen. The results of the XRD and FT-IR measurements elucidated that the crystal structure of calcium phosphate on the silica supports was attributed to calcium hydroxyapatite (HAp). The addition of calcium phosphate changed the porous structures of the catalysts, and decreased their BET surface areas and total pore volumes. Furthermore, the catalytic activity of the CoMo-based catalysts was influenced by the amount of calcium phosphate added. It was observed that the ammonia conversion at 873 K was increased by a maximum of 12.6% when 0.5 wt% of calcium phosphate was added to the catalyst, while a further addition of calcium phosphate beyond 1.0 wt% inhibited the ammonia conversion. From this study, it was concluded that HAp could work as a promoter for ammonia decomposition.