Effect of the Ca/P ratio of Ni-loaded hydroxyapatite on the catalytic decomposition of biomass tar at low temperatures

Abstract

Active tar decomposition catalysts are necessary to achieve the high-efficiency gasification of biomass at low temperatures (i.e., <600 °C). In this study, we prepared Ni-loaded catalysts using hydroxyapatite (HAp) supports with various Ca/P ratios (Ca/P = 1.40, 1.67, and 1.82) for use in biomass tar decomposition, and subsequently, investigated the effects of the Ca/P ratio on the tar decomposition activity for the various catalyst systems. The Ni-loaded HAp catalysts and HAp supports were characterized using N₂ adsorption measurements, X-ray diffraction, transmission electron microscopy combined with energy-dispersive X-ray spectroscopy, CO pulse adsorption measurements, and X-ray photoelectron spectroscopy. A higher Ca/P ratio in the HAp support led to a higher dispersion state of the loaded Ni particles. In a decomposition test of biomass tar at 450 °C, Ni/HAp possessing a higher Ca/P ratio exhibited superior catalytic activity, as measured by the amount of tar converted (tar conversion: 98.0%). The obtained gas composition was also influenced by the Ca/P ratio of the HAp support. Specifically, Ni/HAp containing a higher Ca/P ratio produced a methane-rich gas (8.1 mmol g_biomass⁻¹ CH₄), whereas Ni/HAp with a lower Ca/P ratio produced a hydrogen-rich gas (10.3 mmol g_biomass⁻¹ H₂). This difference can be attributed to the reverse dry reforming of methane at the interface between Ni and the HAp support. Steam reforming also effectively decreased the amounts of tar and deposited carbon and increased the gas yield. The tar conversion reached 99.7% using Ni/HAp containing the higher Ca/P ratio, with a steam-to-carbon ratio of 3 : 1. The investigated tar decomposition catalyst can improve biomass utilization and offers tunability via the Ca/P ratio.