Modification strategies for enhancing anti-coking of Ni-, Co-based catalysts during ethanol steam reforming: A review

Producing hydrogen from ethanol steam reforming (ESR) is a carbon-neutral and environment-friendly method, which has been expected to gradually reduce excessive emission of environmental pollution and over-exploitation of fossil resources. Low-cost nickel (Ni) and cobalt (Co) are considered the most promising active metals for industrial ESR catalysts, with the challenge that carbon deposition on such catalysts causes active site loss which limits their application. In this review, comprehensive knowledge on the ESR reaction mechanism and carbon deposition process were summarized. Based on understanding of the reaction mechanism, an anti-coking strategy keeping a balance between C–C bond scission and oxidation of hydrocarbon species was proposed. Two aspects of this strategy, including (i) enhanced C–C bond scission capability of metal, (ii) promoting effects of support for protecting the activity of metal particles and removing surface carbon, were particularly described. The revelation between the intermediate reaction and modification strategy enables the successful design of new and stable catalysts for improving anti-coking ability. This review not only shed light to the development of high-performance industrial ESR catalysts, but also contribute an innovative perspective to understand anti-coking mechanism for steam reforming of CH₃CHO, CH₃COOH, CH₃COCH₃, and even crude bio-oil.     

Steam reforming of methanol,ethanol and glycerol over nickel-based catalysts-A review

Depletion of non-renewable energy sources such as coal and natural gas is paving the way to generate alternative energy sources. Hydrogen, a very promising alternative energy has the highest energy density (143 MJ/kg) compared to any known fuel and it has zero air pollution due to the formation of water as the only by-product after combustion. Currently, 95% of hydrogen is produced from non-renewable sources. Hydrogen production from renewable sources is considered a promising route for development of sustainable energy production. Steam reforming of renewable sources such as methanol, ethanol and glycerol is a promising route to hydrogen production. This review covers steam reforming of these three alcohols using Ni-based catalysts with different supports. Chemistry of the steam reforming reactions is discussed. Hydrogen yield depends on operating conditions, the nature of active metal and the catalyst support. Supports play an important role in terms of hydrogen selectivity and catalyst stability because of their basic characteristics and redox properties. Synthesis of suitable catalysts that can suppress coke formation during reforming is suggested.     

Methane dry (CO2) reforming to syngas (H2/CO) in catalytic process: From experimental study and DFT calculations

Dry reforming of methane (DRM) is a promising reaction, it could convert two greenhouse gases CO₂ and CH₄ into syngas (CO and H₂) which could provide a mixed fuel for daily life or chemical feedstock for industrial application. Transition metals were widely applied in this process, however, single component of transition metal catalysts could not meet the stability, selectivity and activity demands simultaneously. And the coke formation on the catalysts is the major barrier to the commercialization of DRM. This review presents a systematic discussion and analysis of methane dry reforming to syngas in the catalytic process from both experimental study and density functional theory (DFT) calculation based on recent research. It includes catalytic performance test of activity, selectivity and stability in DRM on monometallic and bimetallic systems, and also gives the discussion of carbon formation in the former parts. The later parts focus on CH₄ and CO₂ activation over monometal and bimetal surface using DFT simulation. The rate determining step and reaction mechanisms involved in DRM are obtained based on thermodynamic analysis and microkinetic model. In the end, we give our outlook to the design and preparation of good performance catalysts as well as further theoretical simulation and analysis in DRM. This review could provide some useful information for going into methane dry reforming from both experimental application and atomic scale.     

Doping oxygen triggered electrocatalytic activity of carbon interpenetrating networks in acid electrolyte

The Fe–N–C catalysts may be promising candidates for replacing platinum group metal (PGM) catalysts to solve sluggish oxygen reduction reaction (ORR) kinetics in the proton exchange membrane fuel cells. However, the activity of Fe–N–C catalysts still has a certain gap compared with commercial Pt/C. Here, we provide a way to increase the intrinsic activity of Fe–N–C catalysts by designing active sites like ketone functional groups. A self-supporting interpenetrating network catalyst, composed of carbon nanotube (CNT) and carbon nanoparticle (CNP), is synthesized via multiple carbon sources (zinc-zeolitic imidazolate frameworks, polyaniline). The interpenetrating network features abundant ketone functional groups. The density functional theory (DFT) results prove that ketone groups can promote the ORR activity of FeN₄ active sites. This offers a new idea for improving the activity of Fe–N–C catalysts co-doped by oxygen and nitrogen in acidic systems.     

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.     

Poly (acrylamide-co-acrylonitrile) hydrogel-derived iron-doped carbon foam electrocatalyst for enhancing oxygen reduction reaction in microbial fuel cell

Developing advanced non-precious metal catalysts for oxygen reduction reaction (ORR) is critical for microbial fuel cells (MFCs). Fe–N–C catalysts are considered the best successor to platinum-based catalysts for ORR. Herein, we have synthesized environmental friendly, cost-effective Fe–N-doped carbon foam catalyst [Fe-embedded poly (acrylamide-co-acrylonitrile) hydrogel-based carbon foam(Fe@Am-co-An/CF)] by using Fe-embedded poly (Am-co-An) hydrogel for MFCs. Poly(Am-co-An) hydrogel is used as a carbon and nitrogen precursor. The synthesized catalysts are characterized by FTIR, SEM, TEM, XRD and XPS. Furthermore, four different catalysts based on different ratios of the metal such as Fe@Am-co-An/CF (1:22), Fe@Am-co-An/CF (2:22), Fe@Am-co-An/CF (3:22), and Am-co-An/CF have been prepared. Results indicate that the Fe@Am-co-An/CF (2:22) catalyst exhibits the highest power density (736.06 mWm^?2 at the current density of 1132.04 mAm^?2) compared to the other catalysts. The results of CV, LSV, EIS, and chronoamperometry indicate that Fe@Am-co-An/CF (2:22) is the most promising catalyst for ORR activity in MFCs.     

Development of nanosilica-based catalyst for syngas production via CO2 reforming of CH4: A review

The alarming global warming issue has sparked interest in researchers to mitigate greenhouse gas emissions via CO₂ reforming of CH₄ (CRM). Regrettably, the main drawback of CRM is catalyst deactivation because of coking and metal sintering. Therefore, exceptional resistance towards coking and sintering is crucial to formulate viable CRM catalysts. This article reviewed the latest development of nanosilica-based catalysts (mesoporous nanosilica, dendritic fibrous nanosilica, green nanosilica, and core@shell nanosilica) for CRM application. The physicochemical properties of nanosilica supports could be modulated by synthesis methods to improve their resistance towards coking and sintering. Furthermore, this review compiled the influence of catalytic properties of nanosilica supported catalysts, such as active metal dispersion, crystallite size, acid-basic properties, oxygen mobility, reducibility, porosity, and morphology on CRM. To conclude, nanosilica supports with strong metal-support interaction, homogeneous metal dispersion, appropriate crystallite size, and moderate acidity/basicity, exhibited satisfactory catalytic activity, thermal stability, and resistance towards coking and sintering. The fundamental study and depth understanding on this catalysis field is of worth in configuring robust catalysts for future industrial applications success of CRM reaction with superb activity and carbon resistance for CRM.     

H2-rich syngas production from biogas reforming: Overcoming coking and sintering using bimetallic Ni-based catalysts

Dry reforming of methane is a very appealing catalytic route biogas (mainly composed by greenhouse gases: carbon dioxide and methane) conversion into added value syngas, which could be further upgraded to produce liquid fuels and added value chemicals. However, the major culprits of this reaction are coking and active phase sintering that result in catalysts deactivation. Herein we have developed a highly stable bimetallic Ni–Rh catalyst supported on mixed CeO₂–Al₂O₃ oxide using low-noble metal loadings. The addition of small amounts of rhodium to nickel catalysts prevents coke formation and improves sintering resistance, achieving high conversions over extended reaction times hence resulting in promising catalysts for biogas upgrading.     

Reduction time effect on structure and performance of Ni–Co/MgO catalyst for carbon dioxide reforming of methane

Reducing catalysts with hydrogen is an important process for carbon dioxide reforming methane since metallic active sites are exposed and dispersed during the reduction process. In this work, Ni–Co/MgO catalysts were prepared for syngas production by using a multiple-impregnation method with a carbon dioxide reforming methane reaction. Activity evaluation showed that catalysts that reduced for 1 h exhibited superior catalytic activity with methane and carbon dioxide conversion at 92% and 97%, respectively, and the syngas ratio close to unity (0.98). The high activity is ascribed to the better metal dispersion (10.5%) and smaller active metal particle size (10 nm). Raman spectra analysis indicated that catalysts that reduced for a longer time possessed larger active metal particle size, and were more susceptible to the formation of graphite-like carbon deposits, which were difficult to be removed by the active oxygen species derived from carbon dioxide dissociation.     

An overview of the methanol reforming process: Comparison of fuels,catalysts, reformers,and systems

One of the main challenges facing power generation by fuel cells involves the difficulties related to hydrogen storage. Several methods have been suggested and studied by researchers to overcome this problem. Among these methods, using fuel reformers as a component of the fuel cell system is a practical and promising alternative to hydrogen storage. Among many hydrogen carrier fuels used in reformers, methanol is one of the most attractive ones because of its distinctive properties. To design and improve of the methanol reformate gas fuel cell systems, different aspects such as promising market applications for reformate gas–fueled fuel cell systems, and catalysts for methanol reforming should be considered. Therefore, our goal in this paper is to provide a comprehensive overview on the past and recent studies regarding methanol reforming technologies, while considering different aspects of this topic. Firstly, different fuel reforming processes are briefly explained in the first section of the paper. Then properties of various fuels and reforming of these fuels are compared, and the characteristics of commercial reformate gas–fueled systems are presented. The main objective of the first section of the paper is to give information about studies and market applications related to reformation of various fuels to understand advantages and disadvantages of using various fuels for different practical applications. In the next sections of the paper, advancements in the methanol reforming technology are explained. The methanol reforming catalysts and reaction kinetics studies by various researchers are reviewed, and the advantages and disadvantages of each catalyst are discussed, followed by presenting the studies accomplished on different types of reformers. The effects of operating parameters on methanol reforming are also discussed. In the last section of this paper, methanol reformate gas–fueled fuel cell systems are reviewed. Overall, this review paper provides insight to researchers on what has been accomplished so far in the field of methanol reforming for fuel cell power generation applications to better plan the next stage of studies in this field.     

Efficient electrocatalyst of Pt–Fe/CNFs for oxygen reduction reaction in alkaline media

Transition metal iron-based catalysts are promising electrocatalysts for oxygen reduction reaction (ORR), and they have the potential to replace noble metal catalysts. The one-dimensional of carbon nanofibers with tubular structure can effectively promote the electrocatalytic activity, which facilitates electron transport. Herein, the Pt–Fe/CNFs were synthesized by electrospinning and subsequent calcination. Benefiting from the advantages of one-dimensional structure, Pt–Fe/CNFs-900 with fast electrochemical kinetics and excellent stability for ORR with excellent onset of 0.99 V, a low Tafel slope of 62 mV dec⁻¹ and high limiting current density of 6.00 mA cm⁻². Long-term ORR testing indicated that the durability of this catalyst was superior to that of commercial Pt/C in alkaline electrolyte. According to RRDE test, the ORR reaction process of Pt–Fe/CNFs-900 was close to four-electron transfer routes.     

A comprehensive review on improving the production of rich-hydrogen via combined steam and CO2 reforming of methane over Ni-based catalysts

During the last few decades, the global energy requirement is soaring significantly due to the rise of global population and economic development. This resulted in colossal release of CO₂ and CH_4, emissions into the atmosphere referred as greenhouse gases (GHGs), which poses a detrimental effects for the environment. One of the sustainable solutions to curb emissions of GHGs into the atmosphere is efficient utilization of syngas in order to produce useful chemicals and fuels. A comprehensive review is presented to highlight the capability of Ni-based catalysts in methane reforming through the application of both steam and dry routes referred to as bi-reforming of methane (BRM). Ni-based catalysts were found to support favorable reaction activity as they are cheaper than many exorbitant catalysts. The metal used for catalyst support exhibits higher stability and thermal resistance with improved resistance to coke formation. This review entails recent progresses in the development of Ni-based catalysts along with physical and kinetic aspects of the BRM process.     

Hydrogen generation by steam reforming of methanol over copper-based catalysts for fuel cell applications

This paper presents an investigation concerning the reforming of methanol over various base-metal oxide catalysts. Copper-based catalysts were effective for the steam reforming of methanol. The selectivity and conversion was studied in a flow reactor in the temperature interval 180–320°C. The active materials were impregnated on γ-alumina pellets using the wet impregnation method. The promoters used in the investigation were chromium (Cr), zinc (Zn) and zirconia (Zr). The copper content and promoter used played an important role in the catalyst's ability to selectively convert methanol at low temperatures. Catalysts with high copper contents generally gave higher conversions and selectivities for the steam reforming reaction. The use of ternary components generally increased the catalyst selectivity towards carbon dioxide. Zirconia had a positive influence on the catalytic performance at low temperatures. The possibilities for the use of reforming systems with copper-based catalysts in fuel cell applications are promising.     

Catalytic modification of conventional SOFC anodes with a view to reducing their activity for direct internal reforming of natural gas

When using natural gas as fuel for the solid oxide fuel cell (SOFC), direct internal reforming lowers the requirement for cell cooling and, theoretically, offers advantages with respect to capital cost and efficiency. The high metal content of a nickel/zirconia anode and the high temperature, however, cause the endothermic reforming reaction to take place very fast. The resulting drop of temperature at the inlet produces thermal stresses, which may lower the system efficiency and limit the stack lifetime. To reduce the reforming rate without lowering the electrochemical activity of the cell, a wet impregnation procedure for modifying conventional cermets by coverage with a less active metal was developed. As the coating material copper was chosen. Copper is affordable, catalytically inert for the reforming reaction and exhibits excellent electronic conductivity. The current density–voltage characteristics of the modified units showed that it is possible to maintain a good electrochemical performance of the cells despite the catalytic modification. A copper to nickel ratio of 1:3 resulted in a strong diminution of the catalytic reaction rate. This indicates that the modification could be a promising method to improve the performance of solid oxide fuel cells with direct internal reforming of hydrocarbons.     

Iron-gelatin aerogel derivative as high-performance oxygen reduction reaction electrocatalysts in microbial fuel cells

Currently, precious based materials are known as highly efficient and widely used catalysts for oxygen reduction reaction (ORR). The expensive price and scarce resource of precious metals have stimulated researchers to explore low-cost and high-performance non-precious metal catalysts. Gelatin is a promising precursor to prepare cost-effective and high-performance catalysts because of abundant micropores and nitrogen self-doping sites after pyrolysis. Herein, we developed a new highly active ORR catalyst (G/C–Fe-2) containing Fe–N coordination sites and Fe/Fe₃C nanoparticles. G/C–Fe-2 exhibited excellent ORR electrocatalytic activity (onset potential: 0.21 V, and limiting current density:7.36 mA cm^?2) and high-performance in air-cathode MFCs (Output voltage: 660 mV, and maximum power density: 560 mW m^?2). It is significant for synthesizing low-cost and high-activity ORR electrocatalysts through this strategy.     

Catalyst modification strategies to enhance the catalyst activity and stability during steam reforming of acetic acid for hydrogen production

A high energy content (∼122 MJ/kg H₂) and presence of hydrogen-bearing compounds abundance in nature make hydrogen forth runner candidate to fulfill future energy requirements. Biomass being abundant and carbon neutral is one of the promising source of hydrogen production. In addition, it also addresses agricultural waste disposal problems and will bring down our dependency on fossil fuel for energy requirements. Biomass-derived bio-oil can be an efficient way for hydrogen production. Acetic acid is the major component of bio-oil and has been extensively studied by the researchers round the globe as a test component of bio-oil for hydrogen generation. Hydrogen can be generated from acetic acid via catalytic steam reforming process which is thermodynamically feasible. A number of nickel-based catalysts have been reported. However, the coke deposition during reforming remains a major challenge. In this review, we have investigated all possible reactions during acetic acid steam reforming (AASR), which can cause coke deposition over the catalyst surface. Different operating parameters such as temperature and steam to carbon feed ratio affect not only the product distribution but also the carbon formation during the reaction. Present review elaborates effects of preparation methods, active metal catalyst including bimetallic catalysts, type of support and microstructure of catalysts on coke resistance behavior and catalyst stability during reforming reactions. The present study also focuses on the effects of a combination of a variety of alkali and alkaline earth metals (AAEM) promoters on coke deposition. Effect of specially designed reactors and the addition of oxygen on carbon deposition during AASR have also been analyzed. This review based on the available literature focuses mainly on the catalyst deactivation because of coke deposition, and possible strategies to minimize catalyst deactivation during AASR.     

Cu,Mg and Co effect on nickel-ceria supported catalysts for ethanol steam reforming reaction

Ethanol steam reforming is a promising reaction which produces hydrogen from bio and synthetic ethanol. In this study, the nano-structured Ni-based bimetallic supported catalysts containing Cu, Co and Mg were synthesized through impregnation method and characterized by XRD, BET, SEM, TPR and TPD analysis. The prepared catalysts were tested in steam reforming of ethanol in the S/C = 6, GHSV of 20,000 mL/(g_cat h) at the temperature range of 450–600 °C. Among the xNi/CeO₂ (x = 10, 13, 15 wt%) catalyst, the sample containing 13 wt% Ni with surface area of 64 m²/g showed the best performance with 89% ethanol conversion and 71% H₂ selectivity as well as low CO selectivity of 8% at 600 °C and The addition of Cu, Mg, and Co to catalyst structure were evaluated and it was found that the nature of second metal has a strong influence on the catalyst selectivity for H₂ production. Considering to results of TPR analysis, the 13Ni–4Cu/CeO₂ catalyst showed proper reduction which caused in better activity. On the other side based on TPD analysis, the more basic property of 13Ni–4Mg/CeO₂ bimetallic catalyst provided a better condition to methane steam reforming, leading to lower CH₄ selectivity and consequently more H₂ production. The 13Ni–4Cu/CeO₂ exhibited the highest activity and lowest selectivity towards ethanol conversion and CO production about 99% and 4%, while the 13Ni–4Mg/CeO₂ catalyst possessed the highest H₂ selectivity and lowest CH₄ selectivity about 74% and 1% respectively at 600 °C. The Ni–Cu and Ni–Mg bimetallic catalysts shows good stability with time on stream.     

Synthesis of Ni@Al2O3 nanocomposite with superior activity and stability for hydrogen production from plastic-derived syngas by CO2-sorption-enhanced reforming

Catalytic dry (CO₂) reforming of plastic-derived syngas is a promising method of producing hydrogen-rich syngas and reducing greenhouse gases. The development of catalysts with high activity and stability is critical for this reaction. In this study, we fabricated core-shell structured Ni@Al₂O₃ catalysts with different shell thicknesses using advanced polyol and sol-gel methods. The effects of different Al/Ni ratios on the activity and stability of the catalysts in the CO₂ reforming reaction were investigated. The main challenge for CO₂ reforming of methane is carbon deposition. In the developed catalysts, the mesoporous Al₂O₃ coating outside the Ni core enhances the stability. However, the interaction between the core and the shell strongly affects the catalyst activity and product selectivity in the reaction. The catalyst with an Al/Ni ratio of 2 exhibited the highest methane conversion of up to 88% and the lowest carbon deposition, compared to the congeners with Al/Ni ratios of 1 and 3.     

Mesoporous silica supported Ni-based catalysts for methane dry reforming: A review of recent studies

Dry reforming of methane (DRM) emerged as a green alternative to utilize major greenhouse gases (GHG's) and produce syngas. However, the main limitations of DRM are sintering and coking of catalyst. In recent decade the Ni-based catalysts has been meticulously studied for DRM reaction since noble metal-based catalysts can't be commercialized due to economic constraints. The catalyst support, synthesis techniques, effect of promoters can significantly affect the catalyst performance. Mesoporous silica (MS) supported catalysts emerged as a promising material due to their enhanced applications in catalysts with significantly high surface areas, thermal stability, and easy availability. Till date no separate study has been carried out focusing the MS as catalyst support for DRM. This comprehensive review highlights the recent advances in MS supported catalysts for DRM. Further, it focuses on reaction mechanism, reaction kinetics, thermodynamics, effect of catalyst support, synthesis route and effect of promoters on DRM performance.     

Effects of noble metal-doping on Ni/La2O3–ZrO2 catalysts for dry reforming of coke oven gas

A series of noble metal (Ru, Pd, Ag) doped Ni catalysts supported on La₂O₃–ZrO₂ mixed oxide were prepared using the sol–gel method and evaluated for use in dry reforming of coke oven gas (COG). The catalysts were investigated by means of N₂ adsorption–desorption, XRD, H₂-TPR, TPH, TEM and TG–DSC. TPH analysis revealed that two carbonaceous species formed on the used catalysts and that the low-temperature carbon species was sufficiently active for the reforming reaction. TEM observations indicated that highly dispersed and small metal particles were formed, suppressing coke deposition and improving catalytic performance. The test results indicated that the addition of trace amounts of noble metals effectively promotes catalytic activity. The 0.1Ru–10Ni/8LZ catalyst showed the highest performance among the bimetallic catalysts, because of the strong synergetic effect between Ru and Ni via the formation of a Ru–Ni alloy, which will be promising catalysts in the catalytic dry reforming of COG.