ReaxFF molecular dynamics simulation of nickel catalysed gasification of cellulose in supercritical water

Abstract

Reactive force field (ReaxFF) molecular dynamic simulation was performed to elucidate the mechanism of Ni-catalysed supercritical water gasification of cellulose considering the effects of temperature and cellulose to water ratio. Simulations showed that Ni could decrease the activation energy of C–C and C–O bond cleavage, promoting the depolymerisation and ring-opening process of cellulose. The yields of gaseous products increase with the increasing temperature. H2 yield mainly depends on H free radical number, which can be generated from cellulose dehydrogenation and water splitting reactions. These two reactions were promoted on Ni surface, leading to an increase in H2 yield. In the presence of Ni catalyst, water plays a limited role in providing H free radicals to produce H2, while the hydrogen atoms in cellulose are the primary source of H2 generation. Meanwhile, reducing the concentration of O•H could enhance H2 production as the combination of O•H and H• is a H radical consumption process. Small organic fragments would be absorbed on the Ni surface, where they undergo deoxygenation via the cleavage of C–O bonds, resulting in a decrease in CO and CO2 yields. The increase in water mass fraction would promote the H2 yield as more H radical would be produced due to water splitting reaction. Moreover, the addition of water would occupy the Ni active sites and prevent the adsorption of organic fragments. These dissociative fragments are prone to produce more CO. The carbon deposition on the Ni surface results in the deactivation of the catalyst. Simulation results suggested that carbon deposition and permeation increase with increasing temperature. In contrast, the increase in water mass fraction can favour carbon elimination from the catalyst surface.