Methane decomposition to produce COx-free hydrogen and nano-carbon over metal catalysts: A review

Catalytic decomposition of methane (CDM) is a promising technology for producing CO_x-free hydrogen and nano-carbon, meanwhile it is a prospective substitute to steam reforming of methane for producing hydrogen. The produced hydrogen is refined and can be applied to the field of electronic, metallurgical, synthesis of fine organic chemicals and aerospace industries. However, the CDM for CO_x-free hydrogen production is still in its infancy. The urgent for industrial scale of CDM is more important than ever in the current situation of huge CO_x emission. This review studies CDM development on Ni-based, noble metal, carbon and Fe-based catalysts, especially over cheap Fe-based catalyst to indicate that CDM would be a promising feasible method for large hydrogen production at a moderate cheap price. Besides, the recent advances in the reaction mechanism and kinetic study over metal catalysts are outlined to indicate that the catalyst deactivation rate would become more quickly with increasing temperature than the CDM rate does. This review also evaluates the roles played by various parameters on CDM catalysts performance, such as metal loading effect, influences of supports, hydrogen reduction, methane reduction and methane/hydrogen carburization. Catalysts deactivation by carbon deposition is the prime challenge found in CDM process, as an interesting approach, a molten-metal reactor to continually remove the floated surface solid carbons is put forwarded in accordance to overcome the deactivation drawback. Moreover, particular CDM reactors using substituted heating sources such as plasma and solar are detailed illustrated in this review in addition to the common electrical heating reactors of fixed bed, fluidized bed reactors. The development of high efficiency catalysts and the optimization of reactors are necessary premises for the industrial-scale production of CDM.     

Production of hydrogen from methanol steam reforming using CuPd/ZrO2 catalysts – Influence of the catalytic surface on methanol conversion and CO selectivity

Electricity generation for mobile applications by proton exchange membrane fuel cells (PEMFCs) is typically hindered by the low volumetric energy density of hydrogen. Nevertheless, nearly pure hydrogen can be generated in-situ from methanol steam reforming (MSR), with Cu-based catalysts being the most common MSR catalysts. Cu-based catalysts display high catalytic performance, even at low temperatures (ca. 250 °C), but are easily deactivated. On the other hand, Pd-based catalysts are very stable but show poor MSR selectivity, producing high concentrations of CO as by-product. This work studies bimetallic catalysts where Cu was added as a promoter to increase MSR selectivity of Pd. Specifically, the surface composition was tuned by different sequences of Cu and Pd impregnation on a monoclinic ZrO₂ support. Both methanol conversion and MSR selectivity were higher for the catalyst with a CuPd-rich surface compared to the catalyst with a Pd-rich surface. Characterization analysis indicate that the higher MSR selectivity results from a strong interaction between the two metals when Pd is impregnated first (likely an alloy). This sequence also resulted in better metallic dispersion on the support, leading to higher methanol conversion. A H₂ production rate of 86.3 mmol h^?1 g^?1 was achieved at low temperature (220 °C) for the best performing catalyst.     

Thermo-photo synergic effect on methanol steam reforming over mesoporous Cu/TiO2–CeO2 catalysts

Methanol steam reforming (MSR) is considered as an effective method for hydrogen storage and generating high-quality hydrogen for fuel cells. In this work, mesoporous Cu/TiO₂–CeO₂ catalysts are proposed to achieve efficient MSR based on synergetic effects of thermal and photon energies. Optimal Ti/Ce molar ratio is found to be 2:1, for which excellent methanol conversion of 100% and extremely low CO selectivity of 2.2% are achieved for thermal catalysis. Further applying UV light (280–400 nm) irradiation while keeping the same temperature, the hydrogen production rate is enhanced to be 78.8 mmol/h/g from 58.6 mmol/h/g. The underlying mechanism is attributed to photogenerated electron hole pairs promoting the REDOX reaction of intermediate product methyl formate based on in situ diffuse reflectance infrared fourier transform spectra (DRIFTS). This work provides a new method to enhance methanol steam reforming performance via thermo-photo synergic effects, and paves a way for the development of direct solar driven MSR techniques.     

Photocatalytic properties of two-dimensional graphene and layered transition-metal dichalcogenides based photocatalyst for photoelectrochemical hydrogen generation: An overview

Hydrogen production through photoelectrochemical (PEC) water-splitting process has drawn significant research attention because it is a promising clean source of energy for improving earth climate in the future. Two-dimensional (2D) graphene and transition metal dichalcogenides (TMDCs), as the core of the system, have become versatile materials for the development of photocatalyst due to their distinct optical, electrical, thermal and mechanical properties. TMDCs have received significant consideration because of low-cost and earth-abundant catalysts that can replace noble metals, such as Pt. Therefore, comprehensive discussions on the structure and properties of 2D graphene and layered TMDCs materials are presented. We also gather and review various fabrication methods for TMDCs-based and graphene-TMDCs-based photocatalysts that can affect the PEC performance and hydrogen evolution. The inherent limitations and several future trends on 2D graphene and layered TMDCs-based photocatalyst for PEC water-splitting application are also discussed.     

生物油水蒸气催化重整制氢研究进展

氢气作为一种环境友好的清洁能源,人们对它的关注度越来越高。生物油水蒸气催化重整制氢是未来制氢的一种可行性方案。本文综述了近年来生物油水蒸气重整制氢的研究进展。主要从重整制氢反应机理、热力学分析、催化重整催化剂、代表性的重整反应器方面进行讨论,指出催化重整中的主要问题是碳沉积导致催化剂失活。研制高活性、高稳定性、高选择性的催化剂是生物油催化重整制氢的关键。     

A novel thermally autonomous methanol steam reforming microreactor using SiC honeycomb ceramic as catalyst support for hydrogen production

Methanol steam reforming (MSR) is an attractive option for in-situ hydrogen production and to supply for transportation and industrial applications. This paper presents a novel thermally autonomous MSR microreactor that uses silicon carbide (SiC) honeycomb ceramic as a catalyst support to enhance energy conversion efficiency and hydrogen production. The structural design and working principle of the MSR microreactor are described along with the development of a 3D numerical model to study the heat transfer and fluid flow characteristics. The simulation results indicate that the proposed microreactor has a significantly low drop in pressure and more uniform temperature distribution in the SiC ceramic support. Further, the microreactor was developed and an experimental setup was conducted to test its hydrogen production performance. The experimental results show that the developed microreactor can be operated as thermally autonomous to reach its target working temperature within 9 min. The maximum energy efficiency of the microreactor is 67.85% and a hydrogen production of 316.37 ml/min can be achieved at an inlet methanol flow rate of 360 μl/min. The obtained results demonstrate that SiC honeycomb ceramic with high thermal conductivity can serve as an effective catalyst support for the development of MSR microreactors for high volume and efficient hydrogen production.     

Catalytic performance of a high-cell-density Ni honeycomb catalyst for methane steam reforming

The catalysis of methane steam reforming (MSR) by pure Ni honeycombs with high cell density of 2300 cells per square inch (cpsi) was investigated to develop efficient and inexpensive catalysts for hydrogen production. The Ni honeycomb catalyst was assembled using 30-μm-thick Ni foils, and showed much higher activity than that of a Ni honeycomb catalyst with cell density of 700 cpsi at a steam-to-carbon ratio of 1.36 and a gas hourly space velocity of 6400 h^?1 in a temperature range of 873–1173 K. Notably, the activity increased approximately proportional to the increasing geometric specific surface area of the honeycombs. The turnover rate of the Ni honeycomb catalyst was higher than that of supported Ni catalysts. The changes in chemical state of the Ni catalyst during hydrogen reduction and MSR reaction were analyzed by in situ X-ray absorption fine structure spectroscopy, which revealed that deactivation was mainly due to oxidation of the surface Ni atoms. These results demonstrated that the high-cell-density Ni honeycomb catalyst exhibits good performance for MSR reaction, and easy regeneration of the deactivated Ni honeycomb catalyst is possible only via hydrogen reduction.     

Carbon-based nanocomposites in solid-state hydrogen storage technology: An overview

Hydrogen fuel is becoming a hot topic among the scientific community as an alternative energy source. Hydrogen is eco-friendly, renewable, and green. The synthesis and development of materials with great potential for hydrogen storage is still a challenge in research and needs to be addressed to store hydrogen economically and efficiently. Various solid-state materials have been fabricated for hydrogen energy storage; however, carbon-based nanocomposites have gained more attention because of its high surface area, low processing cost, and light weight nature. Carbon materials are easy to modify with various metals, metal oxides (MOs), and other organometallic frameworks because of the functional groups available on the surface and edges that increase the storage capacity of hydrogen. In addition, chemisorption is another way to enhance the hydrogen storage capacity of carbon-based nanocomposites. In this review, we discuss the success achieved thus far and the challenges that remain for the physical and chemical storage of hydrogen in various carbon-based nanocomposites. Various compositions of catalysts (eg, metal, MOs, alloy, metal organic frameworks) and carbon materials are designed for hydrogen storage. Superior energy storage in hybrids and composites as compared with pristine materials (catalysts or carbon nanotubes) is governed by the interaction, activation, and hydrogen adsorption/absorption mechanism of materials in the reaction profile. (Nano)composites comprising carbon material with metals, MOs, or alloys are important in this field, not only because of their potential for hydrogen sorption but also their significant cyclic stability and high efficiency upon successive adsorption-desorption cycles.     

Precisely deposited Pd on ZnO (002) facets derived from complex reduction strategy for methanol steam reforming

Pd/ZnO catalysts are promising for onboard hydrogen production through methanol steam reforming (MSR), but their selectivity still requires improving. In this research, a flower-like spherical ZnO material with mainly exposed (002) facet was prepared through surfactant-assisted hydrothermal method. The surfactant tartaric groups also serve as coordinating groups to adsorb Pd²⁺ to form a complex, which is further reduced to metallic Pd. By virtue of preferential adsorption of the surfactant on (002) facet, Pd particles are confined to (002) facet. The calcined PdO-ZnO-T-X catalyst shows highly dispersed PdO nanoparticles. Characterization results suggest that strong interaction between ZnO (002) facet and small PdO particles can facilitate complete PdO reduction and PdZn formation, and can effectively retard PdZn decomposition under oxidizing atmosphere. This research discloses structure-performance relationship of Pd/ZnO catalysts and provides a novel strategy for further improving their MSR performance.     

A review on nickel cobalt sulphide and their hybrids: Earth abundant,pH stable electro-catalyst for hydrogen evolution reaction

The production of hydrogen from electrocatalytic water splitting using a clean energy source has become a sustainable route to overcome the problems related to fossil fuels. Therefore, it is essential to develop a non-precious, stable and highly efficient electrocatalyst. Nickel and cobalt sulphide have gained much attention as hydrogen evolution reaction (HER) catalysts due to their pH stability, low cost and high activity. But the application of these sulphides is limited due to high over-potentials, low surface area and less catalytic active sites available for HER. Furthermore, Nickel and cobalt sulphide can be coupled with different functional components to enhance their catalytic activity. This comprehensive review focuses on the progress made so far to enhance the electrochemical properties of nickel and cobalt sulphides and their composites with various active materials. The comparative survey of their HER activities is made in terms of their electrocatalytic performance parameters.     

Methanol and ethanol steam reforming in membrane reactors: An experimental study

In this work a comparison between methanol steam reforming (MSR) reaction and ethanol steam reforming (ESR) reaction to produce hydrogen in membrane reactors (MRs) is discussed from an experimental point of view.     

Production of ultra-dense hydrogen H(0): A novel nuclear fuel

Condensation of hydrogen Rydberg atoms (highly electronically excited) into the lowest energy state of condensed hydrogen i.e. the ultra-dense hydrogen phase, H(0), has gained increased attention not only from the fundamental aspects but also from the applied point of view. The physical properties of ultra-dense hydrogen H(0) were recently reviewed (Physica Scripta 2019 https://doi.org/10.1088/1402-4896/ab1276), summarizing the results reported in 50 publications during the last ten years. The main application of H(0) so far is as the fuel and working medium in nuclear particle generators and nuclear fusion reactors which are under commercial development. The first fusion process showing sustained operation above break-even was published in 2015 (AIP Advances) and used ultra-dense deuterium D(0) as fuel. The first generator giving a high-intensity muon flux intended for muon-catalyzed fusion reactors was patented in 2017, using H(0) as the working medium. Here, we first focus on the different nuclear processes using hydrogen isotopes for energy generation, and then on the detailed processes of formation of H(0). The production of H(0) employs heterogeneous catalysts which are active in hydrogen transfer reactions. Iron oxide-based, alkali promoted catalysts function well, but also platinum group metals and carbon surfaces are active in this process. The clusters of highly excited Rydberg hydrogen atoms H(l) are formed upon interaction with alkali Rydberg matter. The final conversion step from ordinary hydrogen Rydberg matter H(l) to H(0) is spontaneous and does not require a solid surface. It is concluded that the exact choice of catalyst is not very important. It is also concluded that the crucial feature of the catalyst is to provide excited alkali atoms at a sufficiently high surface density and in this way enabling formation and desorption of H(0) clusters. Finally, the relation to industrial catalytic processes which use H(0) formation catalysts is described and some important consequences like the muon and neutron radiation from H(0) are discussed.     

Thermal analysis of a micro tubular reactor for methanol steam reforming by optimizing the multilayer arrangement of catalyst bed for the catalytic combustion of methanolr

Packed bed tube reactors are commonly used for hydrogen production in proton exchange membrane fuel cells. However, the hydrogen production capacity of methanol steam reforming (MSR) is greatly limited by the poor heat transfer of packed catalyst bed. The hydrogen production capacity of catalyst bed can be effectively improved by optimizing the temperature distribution of reactor. In this study, four types of reactors including concentric circle methanol steam reforming reactor (MSRC), continuous catalytic combustion methanol steam reforming reactor (MSRR), hierarchical catalytic combustion methanol steam reforming reactor (MSRP) and segmented catalytic combustion reactor with fins (MSRF) are designed, modeled, compared and validated by experimental data. It was found that the maximum temperature difference of MSRC, MSRR, MSRP and MSRF reached 72.4 K, 58.6 K, 19.8 K and 11.3 K, respectively. In addition, the surface temperature inhomogeneity U_f and CO concentration of the MSRF decreased by 69.8% and 30.7%, compared with MSRC. At the same reactor volume, MSRF can achieve higher methanol conversion rate, and its effective energy absorption rate is 4.6%, 3.9% and 2.6% higher than that of MSRC, MSRR and MSRP, respectively. The MSRF could effectively avoid the influence of uneven temperature distribution on MSR compared with the other designs. In order to further improve the performance of MSRF, the influences of methanol vapor molar ratio, inlet temperature, flow rate, catalyst particle size and catalyst bed porosity on MSR were also discussed in the optimal reactor structure (MSRF).     

Recent advancements in the cathodic catalyst for the hydrogen evolution reaction in microbial electrolytic cells

The microbial electrochemical technology is a foremost viable technology for hydrogen production from organic matter or wastewater catalyzed by electroactive microorganisms. Developing a high-efficient and low-cost cathode for hydrogen production is crucial for the practical applications of MEC. In the present article, cathode materials and catalysts for hydrogen evolution reaction (HER) in MECs are reviewed. There is an essential requirement of cost-effective HER catalysts for improving MEC performance and as the practical findings fell short of the ideal catalyst's expectations, the density functional theory (DFT) can give essential molecular knowledge and anticipate viable catalysts. Additionally, this article provides an overview of the development of density functional theory (DFT), as well as computer simulations for HER processes using DFT, and also computational designs and virtual screens of novel HER catalysts. The development of catalysts combined with DFT simulations offers significant advancements in the near future on the path to an ideal electrocatalyst in MEC.     

Graphene–CNT composite as catalyst support for microwave-assisted hydrogen releasing from liquid organic hydride

A series of composites comprised of graphene (rGO) and carbon nanotube (CNT) with various weight ratios of them have been synthesized. The catalytic performance of the Pt catalysts supported on the rGO–CNT composites has been evaluated in the dehydrogenation of liquid organic hydride (decalin) for hydrogen releasing and compared with that of Pt/rGO and Pt/CNT catalysts. Both microwave irradiation and conventional heating methods have been adopted for the reaction. The structural and surficial features of the composites and rGO as well as CNT have been characterized by means of several techniques. The thermal behaviour of different carbon materials under microwave irradiation has been measured. The results show that there is an optimal CNT weight content in the composites leading the Pt/rGO–CNT0.17 catalyst to the best performance that cannot be achieved by the other catalysts including Pt/rGO and Pt/CNT as well. This has been ascribed to the most plentiful interface formed between rGO and CNT of a proper content, which is beneficial to the deposition of the Pt nanoparticles having the highest catalytic activity. Additionally, the strong coupling effect of carbon materials with microwave irradiation gives rise to better catalytic performance in comparison to conventional heating due to its capability to induce higher reaction temperatures. Nevertheless, the intrinsic catalytic properties of the Pt catalysts supported on different carbon materials are independent on the heating modes.     

Methane decomposition to pure hydrogen and carbon nano materials: State-of-the-art and future perspectives

Hydrogen is a clean fuel widely used in fuel cells, engines, rockets and many other devices. The catalytic decomposition of methane (CDM) is a CO_x-free hydrogen production technology from which carbon nano materials (CNMs) can be generated as a high value-added byproduct for electrode, membranes and sensors. Recent work has focused on developing a low cost catalyst that could work without rapid deactivation by carbon deposition. In this review, the economic and environmental evaluation of CDM are compared with coal gasification, steam reforming of methane, and methanol steam reforming in terms of productivity, CO₂ emissions, and H₂ production and cost. CDM could be a favorable technology for on-site demand-driven hydrogen production on a small or medium industrial scale. This study covers the Fe-based, Ni-based, noble metal, and carbonaceous catalysts for the CDM process. Focusing on hydrogen (or carbon) yield and production cost, Fe-based catalysts are preferable for CDM. Although Ni-based catalysts showed a much higher hydrogen yield with 0.39 mol_H2/g_cat./h than Fe-based catalysts with 0.22 mol_H2/g_cat./h, the hydrogen cost of the former was estimated to be 100-fold higher ($0.89/$0.009). Further, the CDM performance on different types of reactors are detailed, whereas the molten-metal catalyst/reactor is suggested to be a promising route to commercialize CDM. Finally, the formation mechanism, characterization, and utilization of carbon byproducts with different morphologies and structures are described and analyzed. Versus other reviews, this review shows that cheap Fe-based catalysts (10 tons H₂/1 ton iron ore) and novel molten-metal reactors (95% methane conversion) for CDM are feasible research directions for a fundamental understanding of CDM. The CNMs by CDM could be applied to the waste water purification, lubricating oils, and supercapacitors.     

Modification of NaAlH4 properties using catalysts for solid-state hydrogen storage: A review

Energy is an essential requirement in our daily lives. Currently, most of our energy demands are fulfilled by fossil fuels. After 20 years, non-renewable fossil fuels are estimated to plummet rapidly. The world will face energy shortage and will seek for a new environmental method of energy generation for transportation, economy and application. Hydrogen is a fascinating energy carrier that is considered as ‘hydrogen economy’ for the future. The key challenge in developing the hydrogen economy is the context of hydrogen storage. Storing hydrogen via the solid-state method has received special attention and consideration because of its safety and larger storage capacity. A light complex hydride, NaAlH₄, is considered as an attractive material for solid-state hydrogen storage owing to its high hydrogen capacity, bulk in availability and low cost. Sluggish sorption kinetics and poor reversibility have driven research into various catalysts to enhance its hydrogen storage properties. This review article examines the development of different catalysts and their effects on the hydrogen storage properties of NaAlH₄. The addition of catalyst offers synergistic catalytic effect on the dehydrogenation performance of NaAlH₄. Doping NaAlH₄ with catalyst promote promising results such as lower decomposition temperature, improved kinetics and reduced activation energy. Superior performance on the dehydrogenation performance of NaAlH₄ doping with the catalyst may be due to the nanosized catalyst particle and in situ formed active species that may serve as nucleation sites at the surface of the NaAlH₄ matrix and benefiting the kinetics properties of NaAlH₄.     

Theoretical evaluation of various configurations of silica membrane reactor in methanol steam reforming using CFD method

The main purposes of this work was to evaluate from a theoretical point of view the performance of silica membrane reactors (MRs) in various configurations for generating hydrogen via methanol steam reforming (MSR) reaction using a two dimensional computational fluid dynamic (CFD) method, presenting details about molar fractions of gas species, velocity and pressure distributions at the simulated conditions. The CFD model was firstly validated and, then, used for the simulations, achieving an acceptable agreement between numerical outcomes and experimental data. The simulations were realized for MSR reaction carried out in three types of silica MRs, namely: 1) silica MR with cocurrent flow pattern (MR1); 2) silica MR with countercurrent flow pattern (MR2); 3) silica MR with countercurrent flow pattern including a water gas shift (WGS) reaction stage in the permeate side (MR3), meanwhile comparing the results with a traditional reactor (TR). The influence of several operating parameters (reaction temperature and pressure, and feed flow rate) on the performance of the aforementioned silica MRs in terms of methanol conversion, hydrogen yield and CO-selectivity was evaluated and the results compared with an equivalent TR. The simulations via CFD method indicated the MR3 results to be the best solution over the other MR proposed configurations and the TR as well, presenting the best simulation results at 10 bar of transmembrane pressure, 513 K, SF = 6, GHSV = 6000 h⁻¹ and feed molar ratio = 3/1 with CO selectivity ≤0.04%, methanol conversion and hydrogen yield >90%.     

Recent progress in CoP-based materials for electrochemical water splitting

Over the past years, transition metal phosphides (TMPs)-based materials have attracted extensive attention as noble-metal-free electrocatalysts for hydrogen evolution reactions and oxygen evolution reactions because of their high conductivity, high electrocatalytic activity, and stability. As a typical TMP, CoP-based materials are widely adopted as electrocatalysts in electrochemical water splitting owing to their unique physical properties, such as a P-rich component and superior charge transferability. Very recently, the research efforts focus on using various effective strategies to further enhance the electrocatalytic performance of CoP-based catalysts. However, no guideline for systematically improving their electrocatalytic activity exists currently. Hence, this review summarizes recent advances in optimizing the electrocatalytic performance of CoP-based materials from four aspects, namely, morphology design, construction of heterointerface, defect engineering, and atomic doping. Finally, this study briefly proposes the existing challenges and the future working directions for designing advanced CoP-based electrocatalysts and other non-previous metal electrocatalysts.