Chemical kinetics of two-step thermochemical decomposition of hydrogen sulfide over nickel sulfide

The study develops a multistep reaction mechanism for a two-step thermochemical cycle that produces hydrogen from H2S decomposition, using nickel sulfide as a catalyst. Two mechanisms were investigated: 1) a gas-phase mechanism that occurs without a Ni catalyst to determine the contribution of thermal decomposition to the overall reaction without a catalyst, and 2) a microkinetic model of the catalytic Ni surface phase to examine the key parameters affecting the overall cycling process and H2S conversion efficiency. Both mechanisms were validated using CHEMKIN-Pro software to analyze the experimental data and trends. The study showed that gas-phase H2S conversion was not kinetically significant below 850 degrees C. The microkinetic model showed that most H2S was transformed into H2 and NiS products when the sulfurization temperature reached 500 degrees C. Additionally, the site fraction of NiS decreased while the number of free active nickel sites increased as the regeneration temperature increased from 500 degrees C to 800 degrees C. A temperature limit of 700 degrees C was set for the regeneration step due to the agglomeration behavior of nickel sulfide particles. (c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.