Ni based catalysts promoted with cerium used in the steam reforming of toluene for hydrogen production

Ni_xMg_6?xAl_1.8Ce_0.2 (with 0 ≤ x ≤ 6) mixed oxides catalysts were prepared by hydrotalcite route. All the oxides were calcined at 800 °C and characterized by different physico-chemical methods. The catalysts are then reduced before their use in the steam reforming of toluene. The XRD and TG/DTA confirmed the formation of the hydrotalcite structure for the non-calcined samples. The N₂ adsorption/desorption results revealed that all catalysts correspond to mesoporous materials. The study by temperature programmed reduction (H₂-TPR) showed that the reducibility of the catalysts is influenced by the nickel content. The CO₂-TPD results showed that the catalyst with high magnesium content present the highest basicity. The Ni₂Mg₄Al_1.8Ce_0.2 shows the best toluene conversion among all the catalysts and it was then compared to a non-promoted catalyst. The spent catalysts were characterized by TPO, TG/DTA and XRD and they didn't reveal any coke formation.     

Catalysts evaluation for production of hydrogen gas and carbon nanotubes from the pyrolysis-catalysis of waste tyres

Six different types of catalysts (nickel, iron, and cobalt each supported by γ-Al₂O₃ and activated carbon) that were prepared via impregnation were used to produce hydrogen (H₂) and carbon nanotubes (CNTs) from the pyrolytic product of waste tyres. A two-stage pyrolytic-catalytic reactor was constructed, in which the waste tyre was pyrolyzed in the first pyrolysis reactor, and the resultant pyrolysis vapors underwent the reforming and upgrading step in the downstream catalytic reactor. The results showed that the interaction between the active metal and its support had a remarkable effect on the production of H₂ and CNTs. Compared with the series of γ-Al₂O₃ supported catalysts, all the activated carbon-supported catalysts showed higher H₂ yields and better CNTs quality. For the same catalyst support (γ-Al₂O₃ or activated carbon), the higher yield of H₂ and better quality of CNTs were obtained by the Ni catalysts, followed by the Fe catalysts and the Co catalysts. Among all the catalysts, Ni supported by activated carbon exhibited the best catalytic performance, producing the highest hydrogen yield (59.55 vol.%) and the best CNT quality. Further investigation about the influence of CH₄ and naphthalene as the carbon source on generated CNTs revealed that CH₄ led to longer CNT length and higher graphitization than naphthalene.     

Simultaneous production of hydrogen and carbon nanotubes from biogas: On the design of combined process

We introduced a novel combined process of CO₂ methanation (METH) and catalytic decomposition of methane (CDM) for simultaneous production of hydrogen (H₂) and carbon nanotubes (CNTs) from biogas. In this process, biogas is catalytically upgraded into CH₄-rich gas in METH reactor using Ni/CeO₂ catalyst, and the obtained CH₄-rich gas is subsequently decomposed into H₂ and CNTs in CDM reactor over CoMo/MgO catalyst. Among the three different process scenarios proposed, the combined process with a steam condenser equipped between METH and CDM reactors could greatly improve a CNTs productivity. The CNTs production yield increased by more than 2.5-fold, maximizing at 9.08 gCNTs/gCat with a CNTs purity of 90%. The deposited carbon product was characterized as multi-walled carbon nanotubes (MWCNTs) with a surface area of 136.0 m²/g, comparable with commercial CNTs of 199.8 m²/g. The remarkable I_G/I_D ratio of 2.18 confirms a superior portion of graphitic carbon in the synthesized CNTs upon the commercial CNTs with I_G/I_D = 0.74. Notably, the CH₄ conversion reached 94.5%, while the CO₂ conversion achieved 100%, resulting in the H₂ yield and H₂ purity higher than 90%. This combined process demonstrates a promising route for production of high quality CNTs and high purity H₂ with complete CO₂ conversion using biogas as abundant renewable energy resources. In addition, the test of raw biogas showed no deactivation of catalyst, justifying the implementation of the developed process for real biogas without purification.     

Diameter-controlled carbon nanotubes and hydrogen production

Simultaneous production of hydrogen and carbon nanomaterials over Ni-loaded ZSM-5 catalysts via catalytic decomposition of methane was investigated. The effects of nickel particle size and reaction temperature on the hydrogen production, catalyst deactivation and the morphologies of the carbon nanotubes were investigated. Two catalyst were prepared: Ni/ZSM-5(300) – predominant size of the Ni particles 30–60 nm and nNi/ZSM-5(300) predominant size of the Ni particles 10–20 nm.     

Carbon deposition behavior in steam reforming of bio-oil model compound for hydrogen production

Catalyst deactivation caused by coke formation is a bottleneck in steam reforming of bio-oil for hydrogen production. The investigation of carbon deposition behavior can make a contribution to the improvement of catalyst and the knowledge of reaction mechanism. In this paper, m-cresol (C₇H₈O, one of the organic compounds present in bio-oil) was chosen as model compound. The experiment was carried out on a commercial Ni/MgO catalyst. As a comparative test, m-cresol decomposition showed carbon deposition can be formed more easily under higher temperature. In steam reforming process, for the competition of carbon deposition and carbon elimination, a peak value of coking formation rate was obtained in a broad range of temperature (575–900 °C). The increase of steam to carbon ratio can favor the carbon elimination. Final coking formation rate curve was determined under optimal reaction conditions and the results showed the severity of carbon deposition maintained a very low level during the entire reaction time. Based on the distribution of reforming products, high temperature and sufficient water feeding can favor the carbon conversion from solid and liquid phase to gaseous phase. Unreacted m-cresol is the main organic compound detected in liquid condensate. Some secondary reactions can be deduced through the other compounds detected. The carbon deposition state on catalyst surface can be in the form of nanofiber, but their concrete shapes can be different due to different reaction conditions.     

Simultaneous production of hydrogen and carbon nanotubes from biogas: On the effect of Ce addition to CoMo/MgO catalyst

In this study, hydrogen and carbon nanotubes (CNTs) are simultaneously produced via a synergistic combined process of CO₂ methanation (METH) and chemical vapor deposition (CVD) processes using biogas as a feedstock. METH process could upgrade CO₂ containing biogas into CH₄-rich gas which then decomposed into H₂ and forming CNTs over CoMo/MgO catalyst by CVD process. The effects of Ce addition to CoMo/MgO were investigated. Comprehensive characterization confirms that all as-synthesized samples composed of well-aligned multi-walled carbon nanotubes (MWCNTs) with a narrow size distribution. The Ce addition improved CoMo dispersion on MgO, resulting in smaller and uniform CNTs. The small addition of Ce into CoMo/MgO catalyst could enhance the production CNTs yield. The higher Ce addition would, however, result in the CNTs yield decreased, attributed to a high basicity of CeO₂ surface and a large coverage of CeO₂ on the catalyst surface. The I_G/I_D increased with increased Ce addition, while the surface area monotonically decreased, attributed to a decrease in defects of nanotubes. In addition, this wisely combined process could result in a remarkable 100%CO₂ elimination, while high CH₄ conversion of 90% was obtained. The H₂ production yield could gain more than 30 vol% with respect to H₂ in the feed stream. The H₂ yield and purity in the effluent gas stream were approximately 90%.     

Evaluation of carbon nanotubes produced from toluene steam reforming as catalyst support for selective catalytic reduction of NOx

There is current interest in developing low cost, effective catalysts for the low temperature selective catalytic reduction (SCR) of nitrogen oxides (NO_x). In this work, we have applied carbon nanotubes (CNTs), produced as a by-product of hydrogen production from the steam reforming of toluene (as a representative hydrocarbon), as a catalyst support for a V₂O₅–WO₃ catalyst for SCR of NO_x. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis showed well dispersed metals on the surface of the CNTs. The V₂O₅–WO₃/CNT catalyst has exhibited NO_x reduction efficiency higher than 95% at reaction temperatures between 340 and 400 °C. However, there was a low NO_x reduction at SCR reaction temperature of less than 200 °C which is suggested to be due to the lack of Lewis acid sites, as determined from NH₃-TPD (temperature program desorption) analysis. Future work to lower the SCR reaction temperature with high NO_x efficiency is suggested.     

Maximizing the production of hydrogen and carbon nanotubes: Effect of Ni and reaction temperature

With the objective of maximizing hydrogen and CNTs production, the catalytic cracking of naphtha has been carried out at progressive reaction temperatures i.e. from 600 to 750 °C. The ZSM-5 and nickel impregnated ZSM-5 were used as catalysts for cracking purpose in fluidization mode. The catalyst analysis imparted that impregnation of metallic nickel induces a strong adhesion on MFI structure of ZSM-5 associated with an enhancement in textural properties and acid density. In addition, the results disclose that the incorporation of nickel on ZSM-5 leads to increment in stability of catalyst which in turn pushes the yields of H₂, CNTs and conversion to greater values of 3.29%, 4.84% and 90%, respectively. The as-grown carbon structures over the catalyst surface were found to be multiwall carbon nanotubes confirmed by Raman spectra and TGA analysis where they exhibited high quality (I_D/I_G = 0.65) and purity, respectively, at 750 °C.     

Adsorption of hydrogen atoms onto the exterior wall of carbon nanotubes and their thermodynamics properties

In the present work, we present a systematic analysis of the chemisorption process pathway of hydrogen atoms onto the exterior wall of (5,5) carbon nanotubes using the ONIOM2 (B3LYP(6–31+G(d,p):UFF)) scheme, and we avoid the gross assumption of fixing any of the carbon atoms during the simulation. It is shown that the adsorption of hydrogen atoms onto the sidewall of CNTs are energetically favorable and the most stable state is to form two H–C σ-bonds while the original σ-bond between the carbon atoms is totally severed. In particular, we examined the molecular thermodynamics properties for the reaction at a range of temperatures from 77 K to 1000 K, and the results suggests that the reaction is possible at ambient temperature, but it is less favorable than that at lower temperatures.     

Renewable hydrogen production concepts from bioethanol reforming with carbon capture

This paper is assessing the hydrogen production from bioethanol at industrial scale (100000 Nm³/h hydrogen equivalent to 300 MW thermal) with carbon capture. Three carbon capture designs were investigated, one based on pre-combustion capture using chemical gas–liquid absorption and two based on chemical looping (one based on syngas and one using direct bioethanol looping). The carbon capture options were compared with the similar designs without carbon capture. The designs were simulated to produce mass and energy balances for quantification of key performance indicators. A particular accent is put on assessment of reforming technologies (steam and oxygen-blown autothermal reforming) and chemical looping units, process integration issues of carbon capture step within the plant, modelling and simulation of whole plant, thermal and power integration of various plant sub-systems by pinch analysis. The results for chemical looping designs (either syngas-based or direct bioethanol) show promising energy efficiency coupled with total carbon capture rate.     

Regenerable and durable catalyst for hydrogen production from ethanol steam reforming

In this work, perovskite-type oxides La_1−xCa_xFe_0.7Ni_0.3O₃ were prepared by using a citrate complex method. The catalysts were employed in the reactions of steam reforming of ethanol (SRE) and oxidative steam reforming of ethanol (OSRE) to produce hydrogen. A reduction-oxidation cycle was proposed to overcome the problems of active component sintering and carbon deposition encountered in SRE reaction. In the ex-situ reactions, highly dispersed surface nickel particles formed during the reduction of La_1−xCa_xFe_0.7Ni_0.3O₃, while during the introduction of an oxidative atmosphere these particles could be oxidized and restored back into the perovskite bulk. Owing to the existence of this segregation-incorporation cycle of nickel species in the perovskite oxides, the sintering of nickel particles under OSRE was found depressed effectively. Besides, this work proved that the oxygen in the feed is helpful to the elimination of deposited carbon. It seems promising for overcoming the problems of the active component sintering and carbon deposition in SRE reaction by regulating the redox ability of the perovskite-type oxides and the feed composition.     

Steam reforming of technical bioethanol for hydrogen production

Essentially all work on ethanol steam reforming so far has been carried out using simulated bioethanol feedstocks, which means pure ethanol mixed with water. However, technical bioethanol consists of a lot of different components including sugars, which cannot be easily vaporized and steam reformed. For ethanol steam reforming to be of practical interest, it is important to avoid the energy-intensive purification steps to fuel grade ethanol. Therefore, it is imperative to analyze how technical bioethanol, with the relevant impurities, reacts during the steam reforming process. We show how three different distillation fractions of technical 2nd generation bioethanol, produced in a pilot plant, influence the performance of nickel- and ruthenium-based catalysts during steam reforming, and we discuss what is required to obtain high activity and long catalyst lifetime. We conclude that the use of technical bioethanol will result in a faster catalyst deactivation than what is observed when using pure ethanol–water mixtures because of contaminants remaining in the feed. However, the initial activity of the catalysts are not affected by this, hence it is important to not only focus on catalyst activity but rather on the lifetime of the catalyst.     

Gliding arc plasma reforming of toluene for on-board hydrogen production

Hydrogen is considered as a clean and promising fuel, and hydrogen production on-board has attracted widespread research attention. In this work, a gliding arc discharge (GAD) plasma reactor was utilized to reform toluene at room temperature and atmosphere pressure. The performance of hydrogen production through oxidative reforming with toluene as raw material under different input power, oxygen to carbon molecular ratio (O/C), residence time and argon addition was investigated. The optimal yields of H₂ and CO (48.6% and 44.3%) were obtained under the condition of the input power of 32 W, the O/C of 0.68, the residence time of 18.4 s and 10 vol% Ar addition. By analyses of spectrum lines and GC-MS, the plasma reforming mechanism of toluene was proposed. It is believed that N₂(B³Πg) and Ar* could increase the formation of reactive oxygen species (O⁺, O (¹D), O and so on), and N₂(B³Πg) could impact directly the reforming of toluene.     

Tight-binding study of hydrogen adsorption on palladium decorated graphene and carbon nanotubes

In this work we report a theoretical study on the atomic and molecular hydrogen adsorption onto Pd-decorated graphene monolayer and carbon nanotubes by a semi-empirical tight-binding method. We first investigated the preferential adsorption geometry, considering different adsorption sites on the carbon surface, and then studied the evolution of the chemical bonding by evaluation of the overlap population (OP) and crystal orbital overlap population (COOP). Our results show that strong C–Pd and H–Pd bonds are formed during atomic hydrogen adsorption, with an important role in the bonding of C 2p_z and Pd 5s, 5p_z and 4d_z² orbitals. The hydrogen storage mechanism in Pd-doped carbon-based materials seems to involve the dissociation of H₂ molecule on the decoration points and the bonding between resultant atomic hydrogen and the carbon surface.     

Release of chemisorbed hydrogen from carbon nanotubes: Insights from ab-initio molecular dynamics simulations

The dynamics and energetics related to the release of chemisorbed hydrogen from small-diameter single-walled carbon nanotubes is investigated by first-principles molecular dynamics simulations. Our results suggest a possible route for thermally-activated desorption of hydrogen from the nanotube sidewall, leading to formation of molecular H₂, and shed light on the basic mechanisms of the reversible storage of hydrogen in carbon nanotubes. In agreement with recent experiments, simulations indicate carbon nanotubes as suitable materials for the reversible storage of hydrogen. Moreover, calculations point to the restoration of the π bond patterning of the sidewall as the driving force for the desorption of hydrogen from carbon nanotubes.     

High efficient production of hydrogen from crude bio-oil via an integrative process between gasification and current-enhanced catalytic steam reforming

High efficient production of hydrogen from the crude bio-oil was performed in the gasification-reforming dual beds. A recently developed electrochemical catalytic reforming method was applied in the downstream reforming bed using NiCuZnAl catalyst. Production of hydrogen from the crude bio-oil through both the single gasification and integrative gasification-reforming processes was investigated. The maximum hydrogen yield of 81.4% with carbon conversion of 87.6% was obtained through the integrative process. Hydrogen is a major product (∼73 vol%) together with by-products of CO₂ (∼26 vol%) as well as very low content of CO (<1%) and a trace amount of CH₄ through the integrative route. In particular, the deactivation of the catalyst was significantly depressed by using the integrative gasification-reforming method, comparing to the direct reforming of the crude bio-oil. The mechanism and evaluation for the downstream electrochemical catalytic reforming were also discussed. The integrative process with higher hydrogen yield and carbon conversion, potentially, would be a useful route to produce hydrogen from the crude bio-oil.     

Hydrotalcites with vanadium,effective catalysts for steam reforming of toluene

The steam reforming of toluene has been studied on three catalysts with vanadium (0.9, 1.75, 3%) derived from hydrotalcites precursors (Mg/Al molar ratio 3) in the temperature range 400–500 °C. Catalysts were characterized by BET, XRD, SEM, TEM, FT-IR and then a correlation between physico-chemical characteristics and catalytic activity for toluene steam reforming has been done. The results showed that the catalyst with 3% V, with polyvanadate species, achieves the best catalytic activity, with a toluene conversion of 77.5%, at 500 °C, and a H₂ composition of 57%.     

Electrochemical behaviour of single walled carbon nanotubes – Hydrogen storage and hydrogen evolution reaction

The electrochemical behaviour of single walled carbon nanotubes (SWCNT) related to the mechanism involved in the hydrogen electrode reaction applying electrochemical and spectroscopic techniques is studied. Cyclic voltammetry applied to electrodes containing different percentages of SWCNT demonstrates that this material can behave as efficient capacitor and that the hydrogen electrode reaction develops through the H-electrosorption followed by the formation of molecular H₂ and its evolution. Also, SWCNT are able to storage hydrogen within their porous structure. This is confirmed through the galvanostatic charge and discharge experiments. Electrochemical impedance spectroscopy allowed calculating the real area that takes part in the electrode reaction and the main and valuable conclusion is that the hydrogen electrode reaction consists of a simple charge transfer reaction and that the H adatom relaxation or diffusion processes can be disregarded. Furthermore, a model proposed for their structure which was validated through impedance experiments confirms those conclusions. Results of Raman spectra allowed identifying the nature of the electrodes confirming that after purification the material is composed of single walled carbon nanotubes.     

Chemisorption,physisorption and hysteresis during hydrogen storage in carbon nanotubes

The question of chemisorption versus physisorption during hydrogen storage in carbon nanotubes (CNTs) is addressed experimentally. We utilize a powerful measurement technique based on a magnetic suspension balance coupled with a residual gas analyzer, and report new data for hydrogen sorption at pressures of up to 100 bar at 25 °C. The measured sorption capacity is less than 0.2 wt.%, and there is hysteresis in the sorption isotherms when multi-walled CNTs are exposed to hydrogen after pretreatment at elevated temperatures. The cause of the hysteresis is then studied, and is shown to be due to a combination of weak sorption – physisorption – and strong sorption – chemisorption – in the CNTs. Analysis of the experimental data enables us to calculate separately the individual hydrogen physisorption and chemisorption isotherms in CNTs that, to our knowledge, are reported for the first time here. The maximum measured hydrogen physisorption and chemisorption are 0.13 wt.% and 0.058 wt.%, respectively.     

High hydrogen content syngas for biofuels production from biomass air gasification: Experimental evaluation of a char-catalyzed steam reforming unit

This work investigates the performance of a reformer reactor for the upgrading of syngas and char derived from a pilot-scale air gasifier. The proposed setup represents a circular approach for the production of hydrogen-rich syngas from air gasification. Specifically, the reforming-unit was operated under a range of temperatures (from 700 °C to 850 °C) and steam flow rates and for each the improvement in producer gas composition and reducing species yield is evaluated. The results highlight that an increase in hydrogen concentration is obtained at higher temperature, moving from 16.2% to 21.3%, without using steam, and to 45.6%, with steam injection on the char-bed, while CO concentration did not follow a monotonic behavior. Moreover, the gas quality index, defined as a ratio between reducing species and inert species, delivered the highest values at the highest temperatures and steam flow rates. These results provide a guide for future gas quality optimization studies.