Low-temperature catalytic decomposition of hydrogen sulfide on metal catalysts under layer of solvent

When hydrogen sulfide decomposition {2 H₂S ? 2 H₂?+?S₂^(gas)} is carried out in the flow regime at room temperature on metal catalysts placed in a liquid capable of dissolving H₂S and sulfur, the reaction equilibrium can be significantly (up to 100%) shifted to the right yielding the desired product – hydrogen. The process efficiency was demonstrated using aqueous solutions of monoethanolamine (MEA), sodium carbonate, which is widely used in industry for H₂S absorption from tail gases, and aqueous hydrazine as examples. IR and Raman spectroscopy data demonstrated that sulfur obtained in the solutions is in the form of diatomic molecules. DFT calculations showed that diatomic sulfur forms weakly bound coordinative complexes with solvent molecules. Some problems related to sulfur accumulation and recovery from the solvents are discussed.     

The reaction thermodynamics of hydrogen sulfide decomposition into hydrogen and diatomic sulfur

To verify a hypothesis about the electronic state of diatomic gaseous sulfur formed during the low-temperature catalytic decomposition of hydrogen sulfide, we carried out some experiments to examine elemental sulfur dissociation. As shown, after heating at ～1000?K, elemental sulfur sealed in quartz ampoules with metal catalysts followed by quenching at room temperature did not produce any visible changes on solid sulfur. However, conversion of solid sulfur into gaseous diatomic sulfur can be realized via intermediate interaction of melted sulfur with hydrogen in the presence of Pt followed by decomposition of H₂S formed on the surface of the metal catalyst at room temperature. It is suggested that the conversion of the singlet sulfur atoms into the ground triplet state becomes feasible only on the surface of metal catalysts resulted from the dissociation of hydrogen sulfide into adsorbed atomic species.     

Pentalithium Ferrite (Li5FeO4) as Highly Active Material for Hydrogen Production in the Chemical Looping Partial Oxidation of Methane

In this work, the pentalithium ferrite was synthetized by solid-state method, and it was characterized by XRD and N₂ adsorption–desorption techniques. Then, H₂ production was obtained through a catalytic conversion process; chemical looping partial oxidation (CLPO) of methane using Li₅FeO₄ as multifunctional material. The catalytic decomposition of methane is an easy way to obtain clean energy, such as hydrogen, but in this process carbon deposition is also possible. The results showed that this material has the ability to convert methane to hydrogen, but it is also capable to donate oxygen atoms from its crystalline network, producing carbon monoxide and/or carbon dioxide, limiting the carbon deposition on the ceramic surface. In addition, it was demonstrated that this lithium-based ceramic produces hydrogen over a wide temperature range (550–900 °C), with a stable hydrogen production for 3 h at 825 °C. Furthermore, it was possible to achieve a cyclic test of methane decomposition with a pre-oxidation process between each cycle obtaining an outstanding increase in hydrogen production from 10% in cycle 1 to ~?100% in the cycle 10. This previous stage not only induces an increase in the decomposition of methane, but also avoids carbon deposition accompanied by the production of both CO_X compounds. Finally, it must be mentioned that Li₅FeO₄ is capable to chemisorb both carbon oxides produced, promoting high purity hydrogen production.

     

Catalytic processes and catalysts for production of elemental sulfur from sulfur-containing gases

The modern technologies for production of elemental sulfur are considered. It is demonstrated that along with the further wide application of the conventional Claus process with conventional alumina catalyst in the observable future some new trends which may significantly influence the technological picture of recovered sulfur manufacturing may be formulated: active development of Claus tail gas cleanup processes with the stress on replacement of subdewpoint Sulfreen-type processes by processes of hydrogen sulfide selective oxidation by oxygen; development of novel highly-efficient technologies for hydrogen sulfide decomposition to sulfur and hydrogen; application of new catalysts forms, first of all — at microfiber supports for Claus and H₂S oxidation processes; wider application of titania and vanadia catalysts at the newly constructed Claus units; development of technologies and catalysts for direct purification of H₂S-containing gases and for catalytic reduction of SO₂ for sulfur recovery from smelter gases. All these prospective routes are actively developed by Russian science and some of them are completely based on domestic developments in this area.     

Kinetic and CFD Modeling of Exhaust Gas Reforming of Natural Gas in a Catalytic Fixed-Bed Reactor for Spark Ignition Engines

Fuel reforming is an attractive method for performance enhancement of internal combustion engines fueled by natural gas, since the syngas can be generated inline from the reforming process. In this study, 1D and 2D steady-state modeling of exhaust gas reforming of natural gas in a catalytic fixed-bed reactor were conducted under different conditions. With increasing engine speed, methane conversion and hydrogen production increased. Similarly, increasing the fraction of recirculated exhaust gas resulted in higher consumption of methane and generation of H₂ and CO. Steam addition enhanced methane conversion. However, when the amount of steam exceeded that of methane, less hydrogen was produced. Increasing the wall temperature increased the methane conversion and reduced the H₂/CO ratio.     

New thermochemical cycles for hydrogen production

The ease of decomposing some metal sulfates to oxides and sulfur trioxide is employed in the search for efficient closed cycle thermochemical methods for water splitting. The main features of the new processes are the production of O₂ through the decomposition of SO₂ and/or SO₃, the production of hydrogen by the decomposition of H₂S, reaction of H₂S with a metal, reaction of water with a metal, and/or reaction of water with sulfides.     

Hydrogen production from liquid hydrocarbon fuels for PEMFC application

Lack of efficient hydrogen storage intermediate has boosted the development of fuel processor or economic onsite hydrogen production techniques for application to proton exchange membrane fuel cell promptly. Aiming to develop onsite hydrogen production techniques for proton exchange membrane fuel cell application using nickel-based reforming catalysts and stainless steel reactors, in this paper, a novel process for H₂ production from liquid hydrocarbon fuels was proposed and experimentally demonstrated on a lab scale. The main operations involved prereforming, autothermal reforming, high temperature water gas shift, low temperature water gas shift and H₂ enrichment by Pd membrane. The results indicated that prereforming introduction prior to autothermal reforming suppressed undesired gas phase reactions efficiently and made reforming reactions perform catalytically and smoothly, which was confirmed by a stable 500 h time-on-stream test of both prereforming and autothermal reforming catalysts. The air distributed feed applied in autothermal reforming reactor coupled the endothermic steam reforming and exothermic catalytic combustion reactions over the catalyst closely, maintaining an appropriate temperature distribution curve for autothermal reforming catalyst bed. During the process of H₂ enrichment by highly H₂ permeable Pd composite membrane, concentration polarization played an important role.     

Co/CeO2-ZrO2 catalysts prepared by impregnation and coprecipitation for ethanol steam reforming

Co/CeO₂-ZrO₂ catalysts for the ethanol steam reforming were prepared by wet incipient impregnation and coprecipitation methods. These catalysts were characterized by nitrogen adsorption, TEM-EDX, XRD, H₂-TPR, and CO chemisorption techniques. It was found that the catalyst reducibility was influenced by the preparation methods; catalysts with different reduction behaviors in the pre-reduction showed different catalytic activities toward hydrogen production. The H₂-TPR studies suggested the presence of metal–support interactions in Co/CeO₂-ZrO₂ catalysts during their hydrogen pre-reduction, a necessary treatment process for catalysts activation. These interactions were influenced by the preparation methods, and the impregnation method is a favorable method to induce a proper metal–support effect that allows only partial reduction of the cobalt species and leads to a superior catalytic activity for the hydrogen production through ethanol steam reforming. At 450 °C, the impregnated catalyst gives a hydrogen production rate of 147.3 mmol/g-s at a WHSV of 6.3 h⁻¹ (ethanol) and a steam-to-carbon ratio of 6.5.     

Sulfur Adsorption and Poisoning of Metallic Catalysts

Abstract

The catalytic activity of most transition metals is drastically reduced by the presence of hydrogen sulfide or other sulfur-containing compounds at extremely low concentration in the reagents. This poisoning effect is a major problem in many catalytic reactions, especially hydrogen reaction such as methanation of coal synthesis gas or reforming of naphthas. On the other hand, beneficial effects on the selectivity can be obtained by a partial and well-controlled poisoning of catalysts by sulfur. For example, an enhancement of the selectivity for heavier hydrocarbons in the CO + H₂ reaction has been reported [1–3]. Increases of the selectivity for partial hydrogenation of diolefins to the corresponding monoolefins were also observed [4, 5]. In the catalytic reforming of naphthas, partial poisoning by sulfur minimizes excessive hydrocracking.     

Biomass to hydrogen via catalytic steam reforming of bio-oil over Ni-supported alumina catalysts

Production of hydrogen (H₂) from catalytic steam reforming of bio-oil was investigated in a fixed bed tubular flow reactor over nickel/alumina (Ni/Al₂O₃) supported catalysts at different conditions. The features of the steam reforming of bio-oil, including the effects of metal content, reaction temperature, W_bHSV (defined as the mass flow rate of bio-oil per mass of catalyst) and S/C ratio (the molar ratio of steam to carbon fed) on the hydrogen yield were investigated. Carbon conversion (moles of carbon in the outlet gases to moles of the carbon feed) was also studied, and the outlet gas distributions were obtained. It was revealed that the Al₂O₃ with 14.1% Ni content gave the highest yield of hydrogen (73%) among the catalysts tested, and the best carbon conversion was 79% under the steam reforming conditions of S/C = 5, W_bHSV = 13 1/h and temperature = 950 °C. The H₂ yield increased with increasing temperature and decreasing W_bHSV; whereas the effect of the S/C ratio was less pronounced. In the S/C ratio range of 1 to 2, the hydrogen yield was slightly increased, but when the S/C ratio was increased further, it did not have an effect on the H₂ production yield.     

Electrochemical polishing of hydrogen sulfide from coal synthesis gas

An advanced process has been developed for the separation of H₂S from coal gasification product streams through an electrochemical membrane. This technology is developed for use in coal gasification facilities providing fuel for cogeneration coal fired electrical power facilities and molten carbonate fuel cell (MCFC) electrical power facilities. H₂S is removed from the syn-gas by reduction to the sulfide ion and hydrogen gas at the cathode. The sulfide ion migrates to the anode through a molten salt electrolyte suspended in an inert ceramic matrix. Once at the anode it is oxidized to elemental sulfur and swept away for condensation in an inert gas stream. The syn-gas is enriched with the hydrogen. Order of magnitude reductions in H₂S have been repeatedly recorded (100 ppm to 10 ppm H₂S) on a single pass through the cell. This process allows removal of H₂S without cooling the gas stream and with negligible pressure loss through the separator. Since there are no absorbents used, there is no absorption/regeneration step as with conventional technology. Elemental sulfur is produced as a byproduct directly, so there is no need for a Claus process for sulfur recovery. This makes the process economically attractive since it is much less equipment intensive than conventional technology.     

The multiple roles for catalysis in the production of H2

This review describes a number of different, largely catalytic approaches for producing H₂. Since a major fraction of the world's H₂ is produced by catalytic processes, involving multiple steps with different types of catalysts, it is clear that catalysis plays a critical role in the production of H₂. This review is focused on the use of catalysis for the current and future production of H₂. Some background will be provided to give a perspective of the dramatic change in the supply and demand for H₂ in the past decade, followed by a review of how it is produced commercially, with a view to how multiple types of catalysis contribute to the total process for H₂ production. Steam methane reforming, the major approach for H₂ manufacture, will be a focal point for most of the discussion in pointing out the large number of catalytic steps that are used in this major technology. Finally, some alternative catalytic approaches for H₂ production will be described.     

Catalytic performance of vanadia-doped alumina-pillared clay for selective oxidation of H2S

The catalytic oxidation of hydrogen sulfide over V₂O₅ supported on Al-pillared clay (V/Al-PILCs) was studied. The synthesized catalysts were examined using a variety of characterization techniques such as XRD, BET, XPS, ⁵¹V NMR, H₂-TPR and NH₃-TPD. A catalytic activity study performed by using V/Al-PILC catalysts showed that H₂S was successfully converted to elemental sulfur without considerable emission of sulfur dioxide. The H₂S conversion over V/Al-PILCs increased with the vanadia content up to 6 wt.%. This superior catalytic performance might be related to the uniform dispersion of vanadia species on the Al-PILC support.     

苯酚低温催化水蒸气重整制氢

以Raney Ni为催化剂,在温和条件下(523～723 K)实现了苯酚催化水蒸气重整制氢反应。研究表明,反应温度、液体空速和原料浓度等反应条件是影响苯酚转化率和H₂选择性的重要因素,较高的反应温度和较低的液体空速有利于提高苯酚转化率,但不利于提高H₂选择性。对比苯酚水相重整制氢过程发现,尽管水蒸气重整反应温度相对较高,且需要汽化原料使反应在气相中进行,但该过程具有比水相重整更高的H₂选择性(93%～100%)。此外,Raney Ni催化剂上苯酚水蒸气重整反应与现有的文献结果比较还具有反应条件温和、催化剂稳定性好(60h)以及CO含量低(CO/CO₂摩尔比为0.01～0.2)等优点。将该技术应用于工业含酚有机废水的资源化处理制备的H₂可以直接作为氢源使用。     

An analysis of the ignition of premixed gases by a hot spherical surface with catalytic reforming reaction☆

A study of the unsteadiness problem of the ignition of static premixed gases that contain CH₄ and steam by a catalytic hot sphere and a non-catalytic hot sphere were conducted, and a comparison between calculated and experimental results was done in the paper. The catalytic reforming reaction of CH₄ with steam on the surface of the sphere produced a small amount of H₂, CO and CO₂, at the same time there occur oxidizing reactions of CH₄, H₂ and CO in the space. Both experimental and calculated results show that a small quantity of H₂ produced by catalytic reforming reaction can greatly reduce the ignition temperature. In traditional catalytic combustion precious metals is applied to catalyse oxidizing reaction between oxygen and fuel to reduce ignition temperature. In this paper, a study on a ‘indirect’ catalytic combustion is conducted. The cheap catalytic material of Ni with rare earth is used and reforming reaction between steam and fuel is catalyzed, so hydrogen is generated on the surface of hot sphere and utilized to improve combustion. Calculation indicates that the high reactivity and high diffusivity of H₂ remarkably affect ignition.     

Selective oxidation of H<Subscript>2</Subscript>S to sulfur over CeO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> catalyst

The catalytic oxidation of hydrogen sulfide (H₂S) to elemental sulfur was studied over CeO₂-TiO₂ catalysts. The synthesized catalysts were characterized by various techniques such as X-ray diffraction, BET, X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption of ammonia, and scanning electron microscopy (SEM). Catalytic performance studies of the CeO₂-TiO₂ catalysts showed that H₂S was successfully converted to elemental sulfur without considerable emission of sulfur dioxide. CeO₂-TiO₂ catalysts with Ce/Ti=1/5 and 1/3 exhibited the highest H₂S conversion, possibly due to the uniform dispersion of metal oxides, high surface area, and high amount of acid sites.     

The use of carbon nanotubes with and without nitrogen doping as support for ruthenium catalysts in the ammonia decomposition reaction

Ru catalysts were supported on two different carbon materials, multiwall carbon nanotubes and bamboo-like carbon nanotubes doped with nitrogen, which were synthesized by catalytic chemical vapour deposition of C₂H₂/H₂/N₂ or C₂H₂/NH₃/H₂/N₂, respectively, over Fe/SiO₂ catalyst. All the carbon supports and/or the prepared Ru catalysts were characterized by several techniques including transmission electron microscopy, X-ray photoelectron spectroscopy, N₂ adsorption isotherms and CO chemisorption. The Ru catalysts were tested in the catalytic ammonia decomposition reaction. High yields towards hydrogen production were achieved. Carbon nanotubes were heated in an inert atmosphere at temperatures up to 1773 K in order to study the effects of such support treatments on the ammonia decomposition reaction. The elimination of acidic groups from the surfaces, prior to catalyst preparation, and/or the surface graphitization of the materials produced a higher catalytic activity during the reaction. The catalytic activity of Ru particles was significantly improved when supported on carbon nanotubes doped with nitrogen.     

THERMODYNAMIC CONSIDERATIONS ON THE PURIFICATION OF H2SO4 AND HIX PHASES IN THE IODINE-SULFUR HYDROGEN PRODUCTION PROCESS

The reaction equilibrium and phase equilibrium in H₂SO₄ and HIx phases produced by the Bunsen reaction of the iodine-sulfur thermochemical hydrogen production process were examined using a chemical process simulator, ESP, with a thermodynamic database based on the mixed solvent electrolyte model. At temperatures lower than ca. 110°C, the reaction of HI and H₂SO₄ produced elemental sulfur in both phases. At higher temperatures, the reverse Bunsen reaction occurred, and SO₂ was produced in the H₂SO₄ phase. In the HIx phase, conversely, SO₂ formation predominated in a narrow temperature range and H₂S was produced with the increase in temperature. The presence of N₂ gas lowered the temperature of the predominant reaction change. A feed of O₂ for purification was proposed to suppress the consumption of objective components in the H₂SO₄ phase purification, and an O₂ feed to the HIx phase for the suppression of H₂S and S impurities was proposed by the simulation.     

Production of hydrogen by unmixed steam reforming of methane

Unmixed steam reforming is an alternative method of catalytic steam reforming that uses separate air and fuel–steam feeds, producing a reformate high in H₂ content using a single reactor and a variety of fuels. It claims insensitivity to carbon formation and can operate autothermally. The high H₂ content is achieved by in situ N₂ separation from the air using an oxygen transfer material (OTM), and by CO₂ capture using a solid sorbent. The OTM and CO₂ sorbent are regenerated during the fuel–steam feed and the air feed, respectively, within the same reactor. This paper describes the steps taken to choose a suitable CO₂-sorbent material for this process when using methane fuel with the help of microreactor tests, and the study of the carbonation efficiency and regeneration ability of the materials tested. Elemental balances from bench scale experiments using the best OTM in the absence of the CO₂ sorbent allow identifying the sequence of the chemical reaction mechanism. The effect of reactor temperature between 600 and on the process outputs is investigated. Temperatures of 600 and under the fuel–steam feed were each found to offer a different set of desirable outputs. Two stages during the fuel–steam feed were characterised by a different set of global reactions, an initial stage where the OTM is reduced directly by methane, and indirectly by hydrogen produced by methane thermal decomposition, in the second stage, steam reforming takes over once sufficient OTM has been reduced. The implications of these stages on the process desirable outputs such as efficiency of reactants conversion, reformate gas quality, and transient effects are discussed.     

Characteristics of hydrogen production by immobilized cyanobacterium Microcystis aeruginosa through cycles of photosynthesis and anaerobic incubation

The characteristics of hydrogen production using immobilized cyanobacterium Microcystis aeruginosa were studied through a two-stage cyclic process. Immobilized cells were first grown photosynthetically under CO₂ and light, followed by anaerobic H₂ production in the absence of light and sulfur. M. aeruginosa was capable of generating H₂ under immobilized conditions, and the use of immobilized cells allowed the maintenance of stable production and sped up the changes in culture conditions for cyclic two-stage operation. M. aeruginosa was also capable of utilizing exogenous glucose as a substrate to generate hydrogen and 30 mM concentration proved to be optimal. The externally added glucose improved H₂ production rates, total produced volume and the lag time required for cell adaptation prior to H₂ evolution. The rate of hydrogen evolution was increased as temperature increased, and the maximum evolution rate was 48 mL/h/L and 34.0 mL/h/L at 42 °C and 37 °C, respectively. The optimal temperature for hydrogen production was 37–40 °C because temperatures higher than 42 °C resulted in cell death. In order to continue repeated cycles of H₂ production, at least two days of photosynthesis under conditions with light, CO₂, and sulfur should be allowed for cells to recover H₂ production potential and cell viability.