Nickel nanocatalysts prepared via continuous supercritical hydrothermal synthesis for supercritical water gasification

This study demonstrates the use of continuous supercritical hydrothermal synthesis (CSHS) for producing active and stable nickel nanocatalysts employed in supercritical water gasification (SCWG), which generates hydrogenrich syngas from biomass. The investigation focused on synthesizing a catalyst material comprising active nickel and a magnesium/aluminum-mixed additive using CSHS. The impact of synthesis temperature on the properties of the product was initially assessed by synthesizing a series of Ni-Mg-Al mixed particles at temperatures ranging from 350 to 500 degrees C under a constant pressure of 25 MPa. Needle -like nanocrystal bundles mainly composed of AlOOH were successfully obtained, exhibiting high specific surface areas (88-119 m 2 /g) and demonstrating highly dispersed Ni, Mg, and Al elements as confirmed by TEM-EDS analysis. Increasing the temperature of CSHS resulted in higher product crystallinity and an increased conversion rate of Ni 2+ precursor, from 87.4 % at 350 degrees C to 95.9 % at 500 degrees C. Another series of Ni/Mg-Al-O catalysts with varying Mg/Al contents was synthesized at 500 degrees C and 25 MPa, and evaluated for SCWG of 5 wt% glucose at 500 degrees C. Due to the well -dispersed components, these catalysts exhibited remarkable and identical activities, resulting in a significant increase in H 2 yield from 0.9 mol/mol without a catalyst to 3.4-3.9 mol/mol, as well as an enhancement in carbon gasification efficiency from 24.6 % to 71.3-72.9 %, when loaded with a catalyst loading of 10 wt% glucose. An optimized composition of Mg/Al was determined, comprising 30 % Mg 2+ and 40 % Al 3+ species, due to the formation of an appropriate quantity of Mg -Al crystalline species through the CSHS approach. This catalyst exhibited the highest specific surface area and demonstrated remarkable stability in terms of crystalline and pore structures during the SCWG use. We conclude that CSHS is an effective approach for synthesizing active and stable metal nanocatalysts with excellent property stability for SCWG applications.