Hydrogen from hydrogen sulfide: towards a more sustainable hydrogen economy

The decomposition of hydrogen sulfide (H₂S) with simultaneous hydrogen (H₂) generation offers a sustainable energy production option and an environmental pollution abatement strategy. H₂S is both naturally occurring and human-made. In the future, H₂S production is expected to increase due to increased heavy oil refining. Currently, H₂S is largely converted to sulfur and water using industrial processes such as the Claus process, however, it would be more useful and economical to convert H₂S to sulfur and H₂ instead. H₂ currently comes from the steam reforming of natural gas, which is an energy-intensive process. Because H₂ is a valued commodity and global consumption is expected to increase, alternative sources of H₂ and hydrogen conservation have become topics of active research. Alberta is an especially large consumer of H₂ due to its oil sands processing. H₂ from petroleum-based H₂S sources could be reused in petroleum upgrading, as a partial replacement of steam methane reforming. This review paper highlights some of the methods of H₂S utilization, such as partial oxidation, reformation and decomposition techniques and approaches that convert H₂S to sulfur, water and, more importantly, H₂. To date, almost no technologies exist that are suitable for converting H₂S to sulfur and H₂ for industrial-scale applications. Here, we survey the literature to identify the most promising approach.     

Liquid hydrogen production via hydrogen sulfide methane reformation

Hydrogen sulfide (H₂S) methane (CH₄) reformation (H₂SMR) (2H₂S + CH₄ = CS₂ + 4H₂) is a potentially viable process for the removal of H₂S from sour natural gas resources or other methane containing gases. Unlike steam methane reformation that generates carbon dioxide as a by-product, H₂SMR produces carbon disulfide (CS₂), a liquid under ambient temperature and pressure—a commodity chemical that is also a feedstock for the synthesis of sulfuric acid. Pinch point analyses for H₂SMR were conducted to determine the reaction conditions necessary for no carbon lay down to occur. Calculations showed that to prevent solid carbon formation, low inlet CH₄ to H₂S ratios are needed. In this paper, we analyze H₂SMR with either a cryogenic process or a membrane separation operation for production of either liquid or gaseous hydrogen. Of the three H₂SMR hydrogen production flowsheets analyzed, direct liquid hydrogen generation has higher first and second law efficiencies of exceeding 80% and 50%, respectively.     

Production of hydrogen from hydrogen sulfide assisted by dielectric barrier discharge

Hydrogen production by non-thermal plasma (NTP) assisted direct decomposition of hydrogen sulfide (H₂S) was carried out in a dielectric barrier discharge (DBD) reactor with stainless steel inner electrode and copper wire as the outer electrode. The specific advantage of the present process is the direct decomposition of H₂S in to H₂ and S and the novelty of the present study is the in-situ removal of sulfur that was achieved by operating DBD plasma reactor at ∼430 K. Optimization of various parameters like the gas residence time in the discharge, frequency, initial concentration of H₂S and temperature was done to achieve hydrogen production in an economically feasible manner. The typical results indicated that NTP is effective in dissociating H₂S into hydrogen and sulfur and it has been observed that by optimizing various parameters, it is possible to achieve H₂ production at 300 kJ/mol H₂ that corresponds to ∼3.1 eV/H₂, which is less than the energy demand during the steam methane reforming (354 kJ/mol H₂ or ∼3.7 eV/H₂).     

Multi-component optimisation for refinery hydrogen networks

Heavier crude oil, tighter environmental regulations and increased heavy-end upgrading in the petroleum industry are leading to the increased demand for hydrogen in oil refineries. Hence, hydrotreating and hydrocracking processes now play increasingly important roles in modern refineries. Refinery hydrogen networks are becoming more and more complicated as well. Therefore, optimisation of overall hydrogen networks is required to improve the hydrogen utilisation in oil refineries. Previous work over hydrogen management has developed methodologies for H₂ network optimisation, with a very simplistic assumption that all H₂ rich streams consist of H₂ and CH₄ only, which leads to a serious doubt of solution’s feasibility. To overcome the drawbacks in previous work, an improved modelling and optimisation approach has been developed. Light hydrocarbon production and integrated flash calculation are incorporated into a hydrogen consumer model. An optimisation framework is developed to solve the resulting NLP problem. A case study is carried out to demonstrate the effectiveness of the developed approach.     

Hydrogen sulfide removal process embedded optimization of hydrogen network

Due to the trend in tighter environmental regulations on heavier crude oil processing, hydrogen has become an important strategic resource in modern refineries. Refiners have to improve the efficiency of hydrogen distribution networks to satisfy the increasing demand of hydrogen. Consequently, plenty of work has been focusing on optimizing hydrogen reuse and purification schemes, which is known as hydrogen network integration (HNI). In refineries, hydrogen purification techniques include hydrocarbon removal units and hydrogen sulfide (H₂S) removal units. Hydrocarbon removal units such as membrane separation and pressure swing adsorption (PSA) are frequently employed in the HNI study. However, the possibility of integrating H₂S removal units into HNI study has been overlooked until recently. H₂S removal units are usually modeled as mass exchangers and independently studied as mass exchange networks (MEN). In the present work, an improved modeling and optimization approach has been developed to integrate H₂S removal units into HNI. By introducing a desulfurization ratio, R_dspl,i′$R_{d{s}_{pl,i^{\prime }}}$, simplified MEN is incorporated into hydrogen distribution network. Total annual cost (TAC) is employed as the optimizing object to investigate the tradeoffs between hydrogen distribution network cost and MEN cost. Pressure constraints and impurity concentrations are considered, and cost equations are established to determine the installation of new equipments in order to synthesis an economical network. A practical case study is used to illustrate the application and effectiveness of the proposed method.     

Modelling of hydrogen production from hydrogen sulfide in geothermal power plants

Geothermal power plants emit high amount of hydrogen sulfide (H₂S). The presence of H₂S in the air, water, soils and vegetation is one of the main environmental concerns for geothermal fields. There is an increasing interest in developing suitable methods and technologies to produce hydrogen from H₂S as promising alternative solution for energy requirements. In the present study, the AMIS technology is the invention of a proprietary technology (AMIS^® - acronym for “Abatement of Mercury and Hydrogen Sulfide” in Italian language) for the abatement of hydrogen sulphide and mercury emission, is primarily employed to produce hydrogen from H₂S. A proton exchange membrane (PEM) electrolyzer operates at 150 °C with gaseous H₂S sulfur dimer in the anode compartment and hydrogen gas in the cathode compartment. Thermodynamic calculations of electrolysis process are made and parametric studies are undertaken by changing several parameters of the process. Also, energy and exergy efficiencies of the process are calculated as % 27.8 and % 57.1 at 150 °C inlet temperature of H₂S, respectively.     

Elevated temperature pressure swing adsorption using LaNi4.3Al0.7 for efficient hydrogen separation

The application of LaNi₅ based alloys as adsorbent for hydrogen separation and purification has been proposed for a long time. However, the actual utilization is limited by the poor CO tolerance of the alloys at atmospheric temperature. In this study, an elevated temperature vacuum pressure swing adsorption (ET-VPSA) method for H₂ separation using hydrogen storage material LaNi_4.3Al_0.7 is proposed and demonstrated to be energy efficient. Elevating the working temperature results in improved CO tolerance of LaNi_4.3Al_0.7, making it possible for the alloy to be used in more situations. An ET-VPSA model was built to explore the correlations between product H₂ purity, recovery rate, feed gas composition, cycle duration and counter-current blow down (CD) pressure. The results show that H₂ recovery rate of ET-VPSA reaches 95% while it is usually 85% or lower for regular pressure swing adsorption (PSA). The energy efficiency of these two separation methods is evaluated by methanol reforming-proton exchange membrane fuel cell system models which contain PSA or ET-VPSA as H₂ purification unit. A larger net power generation amount indicates less energy loss during H₂ purification process. Although the vacuum pump will lead to extra energy consumption, benefiting from higher H₂ recovery rate, the net efficiency of the system with ET-VPSA is 0.475, still higher than that with PSA (0.448).     

A parametric investigation of hydrogen energy potential based on H2S in Black Sea deep waters

In this study, a parametric investigation is carried out to estimate the hydrogen energy potential depending on the quantities of H₂S in Black Sea deep waters. The required data for H₂S in Black Sea deep waters are taken from the literature. For this investigation, the H₂S concentration and water layer depth are taken into account, and 100% of conversion efficiency is assumed. Consequently, it is estimated that total hydrogen energy potential is approximately 270 million tons produced from 4.587 billion tons of H₂S in Black Sea deep waters. Using this amount of hydrogen, it will be possible to produce 38.3 million TJ of thermal energy or 8.97 million GWh of electricity energy. Moreover, it is determined that total hydrogen potential in Black Sea deep waters is almost equal to 808 million tons of gasoline or 766 million tons of NG (natural gas) or 841 million tons of fuel oil or 851 million tons of natural petroleum. These values show that the hydrogen potential from hydrogen sulphur in Black Sea deep water will play an important role to supply energy demands of the regional countries. Thus, it can be said that hydrogen energy reserve in Black Sea is an important candidate for the future hydrogen energy systems.     

Process integration of sulfur combustion with claus SRU for enhanced hydrogen production from acid gas

The commercial Claus sulfur recovery process is intended for treating H₂S present in acid gas by recovering sulfur. During this process, hydrogen present in H₂S is inadvertently converted to low grade steam. In the current study, an improved technique for recovering hydrogen and sulfur from acid gas containing H₂S was developed using Aspen HYSYS®. Hydrogen production by thermal decomposition of H₂S was achieved in the tubes of a waste heat exchanger connected in-series with a reaction furnace and followed by Claus sulfur recovery unit (SRU). The energy requirement for the decomposition reaction was supplied through elemental sulfur combustion in the reaction furnace. While H₂S decomposition was defined by a kinetic model in a plug flow reactor, sulfur combustion and H₂S-SO₂ combustion processes were described using Sulsim? Sulfur Recovery model in Aspen HYSYS®. A commercial Claus sulfur recovery unit (SRU) located in Abu Dhabi was considered for process development. Two different process integration schemes differing in hydrogen recovery layout design were analyzed. Based on various performance indicators, including hydrogen and sulfur yields, H₂S conversion rate, and sulfur combustion rate, the most feasible process configuration for maximizing overall process efficiency was identified. The proposed integrated process has the capability for generating hydrogen yield as high as 33% and a simultaneous sulfur recovery of nearly 99%. In addition, the developed processes can significantly curtail the handling load on catalytic section by 11.3% and 16%, respectively, in terms of catalyst bed volume.     

Modeling and optimal design of cyclic processes for hydrogen purification using hydride-forming metals

Hydrogen at high purity degrees can be obtained by using the well-known Pressure Swing Adsorption (PSA) process. In this paper, a Pressure Swing Absorption (PSAb) alternative operating batch wise is analyzed. An optimal design of cyclic processes for hydrogen purification using hydride-forming metals as absorption material is addressed. The selected case study is a thermo-chemical treatment process that consumes high purity hydrogen to reduce oxides and generates a waste stream that contains residual H₂. PSAb process is fed with this hydrogen-poor stream; and high purity hydrogen recovery levels are obtained. A mathematical model based on an energy integrated scheme is presented to develop the optimal process design and to obtain optimal operating conditions. Various optimized solutions are compared by modifying key parameters or restriction equations. Thus, an interesting trade-off between H₂ recovery and system size is analyzed. Large systems operate at large cycle times, obtaining up to 98% of H₂ recovery in the order of hours, whereas small systems can recover up to 60% of H₂ in short cycles of a few seconds.     

High efficient separation of H2/CH4 using ZIF-8/glycol-water slurry: Process modelling and multi-objective optimization

Hydrogen (H₂) is expected to play a vital role in future global energy system. The efficient and low-energy consumption process for H₂/CH₄ separation from hydrogen rich industrial off-gas is still a key challenge. The absorption-adsorption process for H₂/CH₄ separation using ZIF-8/glycol-water slurry is a promising alternative technique due to its high separation efficiency, mild operation conditions and continuous operation mode. We proposed two process configurations: a decompression desorption process A to obtain 99.5 mol% H₂; and a process B combined with decompression and H₂ stripping desorption to attain 99.99 mol% H₂. The detailed process modelling and multiple objective optimizations for two processes are conducted to determine optimal operation conditions, stream characteristics, and unit energy requirements. Results show that the H₂ recovery ratio and total unit energy consumption reaches 99.70% and 0.3876 kW·h/Nm³ for Process A; 99.47% and 0.4608 kW·h/Nm³ for Process B, respectively. It indicates the novel process can simultaneously achieve high purity and high H₂ recovery with low energy consumption.     

Enhancement of heat and mass transfer characteristics of metal hydride reactor for hydrogen storage using various nanofluids

The execution of metal hydride reactor (MHR) for storage of hydrogen is greatly affected by thermal effects occurred throughout the sorption of hydrogen. In this paper, based on different governing equations, a numerical model of MHR filled by MmNi_4.6Al_0.4 is formed using ANSYS Fluent for hydrogen absorption process. The validation of model is done by relating its simulation outcomes with published experimental results and found a good agreement with a deviation of less than 5%; hence present model accuracy is considered to be more than 95%. For extraction or supply of heat, water or oil is extensively used in MHR during the absorption or the desorption process so as to improve the competency of the system. Since nanofluid (mixture of base fluid and nanoparticles) has a higher heat transfer characteristics, in this paper the nanofluid is used in place of the conventional heat transfer fluid in MHR. Further the numerical model of MHR is modified with nanofluid as heat extraction fluid and results are presented. The Al₂O₃/H₂O, CuO/H₂O and MgO/H₂O nanofluids are selected and simulations are carried out. The results are obtained for different parameters like nanoparticle material, hydrogen concentration, supply pressure and cooling fluid temperature. It is seen that 5 vol% CuO/H₂O nanofluid is thermally superior to Al₂O₃/H₂O and MgO/H₂O nanofluid. The heat transfer rate improves by the increment in the supply pressure of hydrogen as well as decrement in temperature of nanofluid. The CuO/H₂O nanofluid increases the heat transfer rate of MHR up to 10% and the hydrogen absorption time is improved by 9.5%. Thus it is advantageous to use the nanofluid as a heat transfer cooling fluid for the MHR to store hydrogen.     

Performance of gas-phase photocatalytic reactors on hydrogen production

Three types of high-performance photocatalytic reactors were developed for gas-phase photocatalytic hydrogen (H₂) production from hydrogen sulphide (H₂S) and effective photocatalytic decomposition of gaseous H₂S at a very low concentration is investigated. In this paper, three lab-scale photocatalytic reactors viz., packed bed photocatalytic reactor, catalyst coated fixed bed photocatalytic reactor and catalyst dispersed photocatalytic reactors were developed to study the performance of reactors on hydrogen production. The novel photocatalyst (CdS + ZnS)/Fe₂O₃ and the optimized catalyst dosage, H₂S gas flow rate, pollutant concentration, light irradiations were used. The experimental result indicates that packed bed photocatalytic reactor can effectively splits the H₂S into hydrogen (i.e. 98%) and rapidly decompose H₂S toward zero concentration than the other two reactors. Hence the bench-scale photocatalytic reactor was fabricated in packed bed reactor and the maximum hydrogen conversion achieved from hydrogen sulphide was found to be 98%.     

Optimization of two bio-oil steam reforming processes for hydrogen production based on thermodynamic analysis

Thermodynamics of hydrogen production from conventional steam reforming (C-SR) and sorption-enhanced steam reforming (SE-SR) of bio-oil was performed under different conditions including reforming temperature, S/C ratio (the mole ratio of steam to carbon in the bio-oil), operating pressure and CaO/C ratio (the mole ratio of CaO to carbon in the bio-oil). Increasing temperature and S/C ratio, and decreasing the operating pressure were favorable to improve the hydrogen yield. Compared to C-SR, SE-SR had the significant advantage of higher hydrogen yield at lower desirable temperature, and showed a significant suppression for carbon formation. However excess CaO (CaO/C > 1) almost had no additional contribution to hydrogen production. Aimed to achieve the maximum utilization of bio-oil with as little energy consumption as possible, the influences of temperature and S/C ratio on the reforming performance (energy requirements and bio-oil consumption per unit volume of hydrogen produced, Q_D/H2 (kJ/Nm³) and Y_Bio-oil/H2 (kg/Nm³)) were comprehensively evaluated using matrix analysis while ensuring the highest hydrogen yield as possible. The optimal operating parameters were confirmed at 650 °C, S/C = 2 for C-SR; and 550 °C, S/C = 2 for SE-SR. Under their respective optimal conditions, the Y_Bio-oil/H2 of SE-SR is significant decreased, by 18.50% compared to that of C-SR, although the Q_D/H2 was slightly increased, just by 7.55%.     

Bioproduction of hydrogen from food waste by pilot-scale combined hydrogen/methane fermentation

A pilot-scale two-phase hydrogen/methane fermentation system generated 3.9 L biogas per unit time and reactor volume from food waste, of which the fraction of H₂ was approximately 60% at a hydraulic retention time (HRT) of 21 h. As substrate, 90% of the carbohydrates in the organic compounds were consumed, based on COD removal efficiency, and the hydrogen yield was approximately 1.82 (H₂-mol/glucose-mol). The maximum decomposition rate coefficient of hydrogen fermentation was observed at an HRT of 21 h, indicating that reducing HRTs improves hydrogen production. Over 80% of the methane was produced in the methane fermentation tank and the predominant fraction of organic acids after methane fermentation comprised acetic acid. Based on our economic evaluation, two-phase hydrogen/methane fermentation has greater potential for recovering energy than methane-only fermentation.     

Options for net zero emissions hydrogen from Victorian lignite. Part 1: Gaseous and liquefied hydrogen

This two-part paper investigates the feasibility of producing export quantities (770 t/d) of blue hydrogen meeting international standards, by gasification of Victorian lignite plus carbon capture and storage (CCS). The study involves a detailed Aspen Plus simulation analysis of the entire production process, taking into account fugitive methane emissions during lignite mining. Part 1 focusses on the resources, energy requirements and greenhouse gas emissions associated with production of gaseous and liquefied hydrogen, while Part 2 focusses on production of ammonia as a hydrogen carrier.In this study, the proposed process comprises lignite mining, lignite drying and milling, air separation unit (ASU), dry-feed entrained flow gasification, gas cooling and cleaning, sour water-gas shift reaction, acid gas removal, pressure swing adsorption (PSA) for hydrogen purification, elemental sulphur recovery, CO₂ compression for transport and injection, hydrogen liquefaction, steam and gas turbines to generate all process power, plus an optional post-combustion CO₂ capture step. High grade waste heat is utilised for process heat and power generation. Three alternative process scenarios are investigated as options to reduce resource utilisation and greenhouse gas emissions: replacing the gas turbine with renewable energy from off-site wind turbines, and co-gasification of lignite with either biomass or biochar. In each case, the specific net greenhouse gas intensity is estimated and compared to the EU Taxonomy specification for sustainable hydrogen.This is the first time that a coal-to-hydrogen study has quantified the greenhouse gas emissions across the entire production chain, including upstream fugitive methane emissions. It is found that both gaseous and liquefied hydrogen can be produced from Victorian lignite, along with all necessary electricity, with specific emissions intensity (SEI) of 2.70 kg CO₂-e/kg H₂ and 2.73 kg CO₂-e/kg H₂, respectively. These values conform to the EU Taxonomy limit of 3.0 kg CO₂-e/kg H₂. This result is achieved using a Selexol™ plant for CO₂ capture, operating at 89.5%–91.7% overall capture efficiency. Importantly, the very low fugitive methane emissions associated with Victorian lignite mining is crucial to the low SEI of the process, making this is a critical advantage over the alternative natural gas or black coal processes.This study shows that there are technical options available to further reduce the SEI to meet tightening emissions targets. An additional post-combustion MDEA CO₂ capture unit can be added to increase the capture efficiency to 99.0%–99.2% and reduce the SEI to 0.3 kg CO₂-e/kg H₂. Emissions intensity can be further reduced by utilising renewable energy rather than co-production of electricity on site. Net zero emissions can then be achieved by co-gasification with ≤1.4 dry wt.% biomass, while a higher proportion of biomass would achieve net-negative emissions. Thus, options exist for production of blue hydrogen from Victorian lignite consistent with a ‘net zero by 2050’ target.     

Investigation of a novel & integrated simulation model for hydrogen production from lignocellulosic biomass

Process simulation and modeling works are very important to determine novel design and operation conditions. In this study; hydrogen production from synthesis gas obtained by gasification of lignocellulosic biomass is investigated. The main motivation of this work is to understand how biomass is converted to hydrogen rich synthesis gas and its environmentally friendly impact. Hydrogen market development in several energy production units such as fuel cells is another motivation to realize these kinds of activities. The initial results can help to contribute to the literature and widen our experience on utilization of the CO₂ neutral biomass sources and gasification technology which can develop the design of hydrogen production processes. The raw syngas is obtained via staged gasification of biomass, using bubbling fluidized bed technology with secondary agents; then it is cleaned, its hydrocarbon content is reformed, CO content is shifted (WGS) and finally H₂ content is separated by the PSA (Pressure Swing Adsorption) unit. According to the preliminary results of the ASPEN HYSYS conceptual process simulation model; the composition of hydrogen rich gas (0.62% H₂O, 38.83% H₂, 1.65% CO, 26.13% CO₂, 0.08% CH₄, and 32.69% N₂) has been determined. The first simulation results show that the hydrogen purity of the product gas after PSA unit is 99.999% approximately. The mass lower heating value (LHV_mass) of the product gas before PSA unit is expected to be about 4500 kJ/kg and the overall fuel processor efficiency has been calculated as ～93%.     

Renewable carbohydrates are a potential high-density hydrogen carrier

The possibility of using renewable biomass carbohydrates as a potential high-density hydrogen carrier is discussed here. Gravimetric density of polysaccharides is 14.8 H₂ mass% where water can be recycled from PEM fuel cells or 8.33% H₂ mass% without water recycling; volumetric densities of polysaccharides are >100 kg of H²/m³. Renewable carbohydrates (e.g., cellulosic materials and starch) are less expensive based on GJ than are other hydrogen carriers, such as hydrocarbons, biodiesel, methanol, ethanol, and ammonia. Biotransformation of carbohydrates to hydrogen by cell-free synthetic (enzymatic) pathway biotransformation (SyPaB) has numerous advantages, such as high product yield (12 H₂/glucose unit), 100% selectivity, high energy conversion efficiency (122%, based on combustion energy), high-purity hydrogen generated, mild reaction conditions, low-cost of bioreactor, few safety concerns, and nearly no toxicity hazards. Although SyPaB may suffer from current low reaction rates, numerous approaches for accelerating hydrogen production rates are proposed and discussed. Potential applications of carbohydrate-based hydrogen/electricity generation would include hydrogen bioreactors, home-size electricity generators, sugar batteries for portable electronics, sugar-powered passenger vehicles, and so on. Developments in thermostable enzymes as standardized building blocks for cell-free SyPaB projects, use of stable and low-cost biomimetic NAD cofactors, and accelerating reaction rates are among the top research & development priorities. International collaborations are urgently needed to solve the above obstacles within a short time.     

Optimization of hydrogen production from three reforming approaches of glycerol via using supercritical water with in situ CO2 separation

A pathway for hydrogen production from supercritical water reforming of glycerol integrated with in situ CO₂ removal was proposed and analyzed. The thermodynamic analysis carried out by the minimizing Gibbs free energy method of three glycerol reforming processes for hydrogen production was investigated in terms of equilibrium compositions and energy consumption using AspenPlus™ simulator. The effect of operating condition, i.e., temperature, pressure, steam to glycerol (S/G) ratio, calcium oxide to glycerol (CaO/G) ratio, air to glycerol (A/G) ratio, and nickel oxide to glycerol (NiO/G) ratio on the hydrogen production was investigated. The optimum operating conditions under maximum H₂ production were predicted at 450 °C (only steam reforming), 400 °C (for autothermal reforming and chemical looping reforming), 240 atm, S/G ratio of 40, CaO/G ratio of 2.5, A/G ratio of 1 (for autothermal reforming), and NiO/G ratio of 1 (for chemical looping reforming). Compared to three reforming processes, the steam reforming obtained the highest hydrogen purity and yield. Moreover, it was found that only autothermal reforming and chemical looping reforming were possible to operate under the thermal self-sufficient condition, which the hydrogen purity of chemical looping reforming (92.14%) was higher than that of autothermal reforming (52.98%). Under both the maximum H₂ production and thermal self-sufficient conditions, the amount of CO was found below 50 ppm for all reforming processes.     

Evidence for H2S gas as an intermediate species in the reaction mechanism of trapping hydrogen by cobalt disulfide

Cobalt sulfide prepared by aqueous precipitation using Na₂S and a Co(II) salt is known to trap hydrogen at room temperature and low pressure. The importance of oxidation of the primary CoS precipitate with atmospheric oxygen with respect to its efficiency as a hydrogen absorber is demonstrated. This stage of oxidation produces a mixture of two solid phases: a partially crystallized cobalt hydroxide Co(OH)₂ and an amorphous cobalt sulfide CoS₂ with a Co(OH)₂/CoS₂ molar ratio of 1 as predicted by thermodynamics. This biphasic product is probably the basic cobalt sulfide CoSOH considered in older and even more recent work. This product traps molecular hydrogen with a H₂/Co molar ratio of 0.5 whereas unoxidized CoS precipitate traps almost no hydrogen (H₂/Co = 0.025). Moderate acidic treatment of the absorber at room temperature leads to the selective dissolution of Co(OH)₂. The remaining cobalt sulfide has CoS₂ stoichiometry and reacts with hydrogen to form H₂S gas and CoS. We showed that H₂S released is reactive toward bases: CoS or Na₂S were formed when H₂S reacted with Co(OH)₂ or NaOH, respectively. This proves that the hydrogen trapping reaction mechanism implies H₂S as an intermediate species.