COx-free hydrogen production from ammonia decomposition over sepiolite-supported nickel catalysts

Sepiolite, a clay mineral, was utilized as a support for nickel-based catalysts for CO_x-free hydrogen production from ammonia decomposition. First, the physical and chemical properties of sepiolite were changed by calcining it at temperatures varying from 500 to 1000 °C, then nickel was impregnated on these calcined supports and tested for ammonia decomposition at various temperatures following reduction at 650 °C. Results indicated that even though the catalysts contained almost the same amount of nickel, they showed different hydrogen production performance. Detailed characterization of the catalysts before and after reaction illustrated that the support obtained by calcining sepiolite at 700 °C shows good basic properties with a high surface area offering a high degree of nickel dispersion. These properties lead to promising hydrogen production rates which are on par, if not higher, than most of the nickel-based catalysts prepared on supports, which are either not cheap or require tedious preparation procedures.     

Preparation of a novel bimodal catalytic membrane reactor and its application to ammonia decomposition for COx-free hydrogen production

A novel bimodal catalytic membrane reactor (BCMR) consisting of a Ru/γ-Al₂O₃/α-Al₂O₃ bimodal catalytic support and a silica separation layer was proposed. The catalytic activity of the support was successfully improved due to enhanced Ru dispersion by the increased specific surface area for the γ-Al₂O₃/α-Al₂O₃ bimodal structure. The silica separation layer was prepared via a sol–gel process, showing a H₂ permeance of 2.6 × 10⁻⁷ mol Pa⁻¹ m⁻² s⁻¹, with H₂/NH₃ and H₂/N₂ permeance ratios of 120 and 180 at 500 °C. The BCMR was applied to NH₃ decomposition for CO_x-free hydrogen production. When the reaction was carried out with a NH₃ feed flow rate of 40 ml min⁻¹ at 450 °C and the reaction pressure was increased from 0.1 to 0.3 MPa, NH₃ conversion decreased from 50.8 to 35.5% without H₂ extraction mainly due to the increased H₂ inhibition effect. With H₂ extraction, however, NH₃ conversion increased from 68.8 to 74.4% due to the enhanced driving force for H₂ permeation through the membrane.     

Enhanced hydrogen production in catalytic pyrolysis of sewage sludge by red mud: Thermogravimetric kinetic analysis and pyrolysis characteristics

The catalytic mechanism of red mud (RM) on the pyrolysis of sewage sludge was investigated. The thermogravimetric data were used to study the kinetic characteristics by using a discrete distributed activation energy model (DAEM) to clarify the effects of three main components (Fe₂O₃, Al₂O₃, SiO₂) in the RM on the pyrolysis of organic matters in sewage sludge. The modeling results showed that the pyrolysis of organic matters, especially at the higher temperature stage, was promoted by Fe₂O₃ and Al₂O₃ in the RM. Adding Fe₂O₃ or the RM alone could reduce the mean activation energy of sewage sludge pyrolysis by 13.9 and 20.1 kJ mol^?1, respectively. The modeling results were validated by pyrolysis experiments of raw sludge with different additives at 600, 700, 800, and 900 °C. The experimental results showed that the addition of Al₂O₃, Fe₂O₃ or the RM could produce more gas than the addition of SiO₂, especially at high temperatures. Fe₂O₃ and Al₂O₃ acted as catalysts in the tar decomposition by in-situ catalyzing the cracking of CC and CH bonds to produce more gases. Especially, Fe₂O₃ and Al₂O₃ increased the H₂ yield from sewage sludge pyrolysis at 700, 800, and 900 °C by 268.5 and 50.7%, 111.1 and 56.0%, 10.9 and 10.3%, respectively. The char obtained from pyrolysis of sewage sludge with the RM possessed magnetic property, which has various potential applications. The research indicates that the RM is an efficient catalyst in the pyrolysis of sewage sludge.     

Ni nanoparticles supported on mica for efficient decomposition of ammonia to COx-free hydrogen

A wet impregnation method is used to synthesize Ni nanocatalysts supported on naturally abundant mica nanosheets for decomposition of ammonia to CO_x-free hydrogen. The prepared catalysts are characterized by XRD, SEM, TEM, N₂ sorption, TG, XPS, H₂-TPR and NH₃/H₂-TPD techniques. The catalytic tests exhibit that the Ni/mica catalysts are highly active for ammonia decomposition. The two-dimensional structure of mica favors mass transportation, which can remarkably promote the catalytic process. The NH₃/H₂-TPD curves of Ni/mica show no desorption signals of NH₃ and H₂, indicating the presence of weak desorption energy barrier. The content of Ni in the catalysts is related to the crystallite size of Ni species, affecting their catalytic properties. As demonstrated in this study, the Ni/mica catalyst with 15% Ni loading amount exhibits the highest catalytic activity and long-term stability. At a high space velocity of 30000 cm³ g_cat^?1 h^?1, 97.2% ammonia conversion is achieved over the 15%Ni/mica catalyst at 650 °C, indicating the Ni/mica is a promising catalyst for ammonia decomposition.     

Effect of preparation methods on the catalyst performance of Co/MgLa mixed oxide catalyst for COx-free hydrogen production by ammonia decomposition

This work deals with the effect of catalyst preparation method of the mixed Co, Mg and La oxide catalysts on their structure and catalytic properties for ammonia decomposition. Two methods are used for catalysts preparations impregnation and co-precipitation (in air and in pure O₂ atmosphere), The Mg/La = 2 molar ratio and 5 wt% of cobalt content was maintained same in all catalysts. The catalyst performance was evaluated in the temperature range 300–550 °C at atmospheric pressure. The prepared catalysts were characterized by BET, XRD, TPR, XPS, CO₂-TPD and SEM techniques. No pronounced differences were observed in BET among the catalysts. It was found that the 5CML-OXY (5 wt%Co over MgLa catalyst prepared by co-precipitation method in oxygen atmosphere) has superior activity among the other catalysts. This could be attributed to availability of easily reducible cobalt species determined by TPR studies and enhanced interaction between Mg and La determined by SEM and XPS. The moderate basic site density determined by CO₂-TPD results was also increased in 5CML–OXY catalysts compared with other catalysts. These consequences are might be one of the reasons for enhanced activity of 5CML–OXY catalyst compared to other catalysts. Hence catalyst preparation by co-precipitation in oxygen atmosphere is the best method which might be one of the parameters that influenced on catalytic properties of the cobalt on MgOLa₂O₃ system, for ammonia decomposition.     

Ammonia decomposition on a highly-dispersed carbon-embedded iron catalyst derived from Fe-BTC: Stable and high performance at relatively low temperatures

Fe-BTC (iron 1,3,5-benzenetricarboxylic acid), a commercially available metal organic framework (MOF), was used as a sacrificial template to produce a series of carbon-embedded Fe catalysts upon its pyrolysis at different temperatures. The catalyst prepared by pyrolyzing Fe-BTC at 400 °C under flowing N₂ provided a high graphitic degree on the carbon support hosting highly dispersed Fe species at a Fe loading of 34 wt%. Performance measurements on ammonia decomposition to produce CO_x-free hydrogen showed that this catalyst provided an ammonia conversion of 73.8% at a space velocity of 6000 cm³ NH₃ h⁻¹ g_cat⁻¹ and at 500 °C for at least 120 h. This stable performance, exceeding that of some of the best non-noble metal catalysts, was associated with the presence of highly-dispersed Fe species at a significantly high Fe loading, embedded in a carbonaceous shell. The presence of the carbonaceous shell not only protected the active species against sintering, but also made them electron rich owing to its high level of graphitization.     

Improving hydrogen production from the hydrolysis of ammonia borane by using multifunctional catalysts

A novel multifunctional catalytic system has been developed for efficient hydrogen generation through the hydrolysis of ammonia borane. This system combines Pd NPs with acid sites and amines, which are both task-specific functionalities able to destabilize the N → B dative bond. The acidity of the support (zeolites of different structure and SiO₂/Al₂O₃ ratio) used to disperse the Pd NPs causes an increase in the hydrogen production rate. However, the positive effect of incorporating p-phenylenediamine in the catalyst is much more pronounced, causing a two-fold increase in the activity of the catalyst. The combined effect of the different functionalities yields excellent performance in the hydrolysis of ammonia borane, greatly enhancing the activity of the metal-based catalyst and reducing the activation energy of the catalyzed reaction.     

Influence of basic dopants on the activity of Ru/Pr6O11 for hydrogen production by ammonia decomposition

Basic oxides such as alkali metal oxides, alkaline earth metal oxides, and rare earth oxides were added to Ru/Pr₆O₁₁, and the activity of the catalysts with respect to hydrogen production by ammonia decomposition was investigated. Ru/Pr₆O₁₁ doped with alkali metal oxides, except for Li₂O, achieved higher NH₃ conversions than bare Ru/Pr₆O₁₁. Cs₂O, the most basic of the alkali metal oxides, was the most effective dopant. In contrast, other dopants with lower basicity than the alkali metal oxides achieved lower NH₃ conversions than bare Ru/Pr₆O₁₁. Changing the Cs/Ru molar ratio revealed that the best Cs/Ru ratio was 0.5–2; the reaction was effectively promoted without negative effects from coverage of the Ru surface by the Cs₂O. Varying the order of loading the Ru and Cs₂O onto Pr₆O₁₁ revealed that loading Ru onto Cs₂O/Pr₆O₁₁ was an effective way to enhance NH₃ conversion, and coverage of the Ru surface was reduced.     

Structure and catalytic properties of Ni/MWCNTs and Ni/AC catalysts for hydrogen production via ammonia decomposition

The structure and catalytic properties of nickel catalysts supported on multi-wall carbon nanotubes (MWCNTs) and on three different types of activated carbon (AC) were studied. The surface areas of AC carriers were defining the size of supported nickel particles. Large surface area of AC led to small Ni nanoparticles and high Ni dispersion. Turnover frequency (TOF_NH3) of ammonia decomposition decreased with decreasing of Ni particle size. The highest degree of ammonia conversion was observed on Ni/AC prepared by using of AC support with largest surface area. The catalytic activity of Ni/MWCNTs was much higher than catalytic activity of the studied Ni/AC catalysts. The synergic nickel-support interaction and special electronic conductivity properties of MWCNTs were responsible for high catalytic activity of Ni/MWCNTs catalyst.     

Production of COx-free hydrogen by the thermal decomposition of methane over activated carbon: Catalyst deactivation

Hydrogen, an environment-friendly energy source, is deemed to become strongly in demand over the next decades. In this work, CO_x-free hydrogen was produced by the thermal catalytic decomposition (TCD) of methane by a carbon catalyst. Deactivated catalysts at four-stage of progressive were characterized by nitrogen sorption and scanning electron microscopy. TCD of methane at 820 and 940 °C was about 13- and 8-folds higher than non-catalytic decomposition, respectively. High temperatures positively affected the kinetics of hydrogen production but negatively influenced the total amount of hydrogen and carbon products. The total pore volume was a good indicator of the total amount of hydrogen product. Catalyst activity was decreased because of the changes in the catalyst's textural properties within three ranges of relative time, that is, 0 to 45, 0.45 to 0.65, and 0.65 to 1. Models for specific surface area and total pore volume as functions of catalyst deactivation kinetics were developed.     

Supported Ni catalysts prepared from hydrotalcite-like compounds for the production of hydrogen by ammonia decomposition

Ni catalysts with high surface areas were prepared from Mg–Al hydrotalcite-like compounds containing Ni with different Mg-to-Al ratios for the production of hydrogen via ammonia decomposition. At atomic ratios of Mg-to-Al from 3:1 to 6:1, Ni reduction was greater than 90%, and the amounts of Ni⁰ exposed were kept at more than 200 μmol g^?1. With the increase in Mg-to-Al ratio, turnover frequency was increased due to an increase in the basicity of the support. Of the catalysts examined, Ni_MgAl(6:1) exhibited the highest NH₃ conversion, which was attributed to its relatively high basicity and large amount of Ni⁰ exposed.     

Microwave-assisted synthesis of high performance copper-based catalysts for hydrogen production from methanol decomposition

High performance copper-based catalysts (CuNiZnO/γ-Al₂O₃) for hydrogen production from methanol decomposition were successfully synthesized by the method of microwave-assisted thermal hydrolysis of urea. The ICP-OES, N₂ adsorption–desorption, SEM, TEM, XRD and H₂-TPR were applied to characterize the physicochemical properties of the prepared materials. These experiment results reveal that the higher thermal treatment temperature can improve the catalytic performance by enlarging the content of promoters (Zn and Ni) and promoting the dispersion of deposited particles. Moreover, the addition of Ni can significantly improve the catalytic performance of the copper-based catalysts, which is ascribed to the regulation of reaction process and inhibition of CuZn alloy. Among the obtained catalysts, the MW-Cu/Ni-95 developed at 95 °C exhibits a higher catalytic activity, which reaches 91.7% conversion at 250 °C. Most significantly, it shows the excellent catalytic performance as compared with the commercial catalyst under the same test conditions.     

Influence of the oxidative/reductive treatments on Pt/CeO2 catalyst for hydrogen iodide decomposition in sulfur–iodine cycle

A Pt/CeO₂ catalyst was prepared by sol–gel method. The as-received sample was successively oxidized, reduced and re-oxidized. The samples were characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and hydrogen iodide (HI) catalytic decomposition activity evaluation for the sulfur–iodine cycle. The oxidative/reductive atmosphere affected the structure and performance of the catalyst by the strong metal-support interaction (SMSI). It was suggested that a migration of Ce⁴⁺ from the bulk to the surface occurred during the reductive treatment. The diffusion process was reversed when the atmosphere was changed to an oxidative one. The decoration or encapsulation of Pt by ceria support changed with the atmosphere, and affected the activity of the catalyst. At temperature below 400 °C, the reduced sample exhibited the best activity. After then the activity of the reduced and re-oxidized samples tends to be the similar, but still better than the as-received and oxidized samples. The surface and interfacial Pt⁰ sites were both considered as the effective factors. Models were constructed to describe the diffusion of Ce⁴⁺ and oxygen vacancies as well as the possible shell-core structure of Pt crystallites and the decoration/encapsulation by ceria support.     

Engineering of the N-doped carbon support for improved performance of supported Pd catalysts in hydrogen production from gas-phase formic acid

Development of N-doped Pd/C catalysts for hydrogen production from gas-phase formic acid is a challenge. To elucidate the efficient routes of nitrogen insertion on the surface of a mesoporous carbon support, the latter was treated with melamine (Mel), dicyandiamide or NH₃ at 673 and 823 K. Pyrolysis of the melamine/carbon mixture taken in a 1:2 ratio provides an increase in the reaction rate by a factor of 5. The inserted N-sites strongly interact with Pd leading to the formation of highly dispersed Pd nanoparticles (∼1.6 nm) and active atomically dispersed Pd²⁺ species. With a further increase of the Mel/C ratio, the number of surface N-sites decreases due to occupation of carbon support pores with a g–C₃N₄–type residue. This provides a decrease in the Pd dispersion leading to lower reaction rates. Therefore, melamine is an efficient N precursor. The considered synthesis of N-doped catalysts could be scaled.     

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.     

Theoretical analysis of the direct decomposition of methane gas in a laminar stagnation-point flow: CO2-free production of hydrogen

In this work, a theoretical analysis is developed to predict the decomposition temperature of methane gas, CH₄, in a planar stagnation-point flow over a catalytic carbon surface. Hydrogen is produced (without CO₂ as a byproduct) by means of a heterogeneous reaction mechanism, which is modeled with five heterogeneous reactions, including adsorption and desorption reactions. The mass species, momentum, and energy conservation equations for the gas phase are solved, taking into account that the temperature of decomposition is characterized by the Damköhler number. Therefore, the critical temperature conditions for the catalytic thermal decomposition are found by using a high activation energy analysis for the desorption kinetics of the adsorbed hydrogen component, H(s)$\mathrm{H}\left(s\right)$. Specifically, the numerical estimations show that, for increasing values of the velocity gradient associated with the stagnation flow, the temperature of decomposition grows, depending on the surface coverages of the product species.     

Preparation of TiO2/MoSe2 heterostructure composites by a solvothermal method and their photocatalytic hydrogen production performance

TiO₂/MoSe₂ composite photocatalysts were prepared by a solvothermal method using different MoSe₂ loadings on the surface of TiO₂ to improve the catalytic activity. Upon increasing the MoSe₂ loading, the fluorescence intensity of the catalyst gradually decreased, indicating that the MoSe₂ loading increased the utilization of photogenerated carriers in TiO₂ and improved the catalytic activity. The formic acid produced by the oxidation of lactic acid and methanol dissociated into oxygen-containing anions that were electrostatically adsorbed on the catalyst surfaces. During photocatalysis, the photogenerated electrons on TiO₂ were transferred to MoSe₂, which separated the photogenerated electrons and holes and increased the quantum efficiency. Therefore, the hydrogen production rate of the composite catalyst was higher than that of pure TiO₂, with a quantum efficiency of about 36.8%.     

Hydrogen production from the methanolysis of ammonia borane by Pd-Co/Al2O3 coated monolithic catalyst

The aim of this study is to enable high hydrogen production yield from catalytic methanolysis of ammonia borane (AB) in the presence of a cordierite type ceramic monolithic. The monolithic channel surfaces were coated with Al₂O₃ by wash-coating method and then this layer was impregnated with 1 wt%Pd-2 wt%Co bimetallic catalyst. SEM-EDX and multi-point BET analysis were used in order to characterize the catalyst. The experimental studies were conducted in a continuous flow type reactor, which was used for the first time in this study. The reactions were carried on low temperature (40 °C), and with various AB feed concentrations and flow rates. It was found that the highest hydrogen production yield (88.5%) was obtained from AB flow rate of 3.3 mL/min, and AB feed concentration of 0.1 wt%. It was concluded that Pd-Co/Al₂O₃ coated monolithic, which is a stable, active and low-cost catalyst, was a very promising catalyst for on-board hydrogen production from the methanolysis of ammonia borane.     

Modelling the hydrodynamics and kinetics of methane decomposition in catalytic liquid metal bubble reactors for hydrogen production

Bubble reactors using molten metal alloys (e.g, nickel-bismuth and copper-bismuth) with strong catalytic activity for methane decomposition are an emerging technology to lower the carbon intensity of hydrogen production. Methane decomposition occurs non-catalytically inside the bubbles and catalytically at the gas-liquid interface. The reactor performance is therefore affected by the hydrodynamics of bubble flow in molten metal, which determines the evolution of the bubble size distribution and of the gas holdup along the reactor height. A reactor model is first developed to rigorously account for the coupling of hydrodynamics with catalytic and non-catalytic reaction kinetics. The model is then validated with previously reported experimental data on methane decomposition at several temperatures in bubble columns containing a molten nickel-bismuth alloy. Next, the model is applied to optimize the design of multitubular catalytic bubble reactors at industrial scales. This involves minimizing the total liquid metal volume for various tube diameters, melt temperatures, and percent methane conversions at a specified hydrogen production rate. For example, an optimized reactor consisting of 891 tubes, each measuring 0.10 m in diameter and 2.11 m in height, filled with molten Ni_0·27Bi_0.73 at 1050 °C and fed with pure methane at 17.8 bar, may produce 10,000 Nm³.h^?1 of hydrogen with a methane conversion of 80% and a pressure drop of 1.6 bar. The tubes could be heated in a fired heater by burning either a fraction of the produced hydrogen, which would prevent CO₂ generation, or other less expensive fuels.     

High performance of bulk Mo2N and Co3Mo3N catalysts for hydrogen production from ammonia: Role of citric acid to Mo molar ratio in preparation of high surface area nitride catalysts

Hydrogen production from ammonia decomposition was studied using a series of unsupported high surface area molybdenum nitride (Mo₂N) and cobalt promoted molybdenum nitride (3%Co-Mo₂N) catalysts prepared with citric acid (CA) as a chelating agent. To elucidate the influence of citric acid amount in preparation conditions on the structure and catalytic activity, we prepared catalysts with different citric acid to Mo molar ratios i.e. CA/Mo = 1, 2, 3 and 4. The catalytic activity was evaluated in the temperature range of 300–600 °C at atmospheric pressure. The catalytic activity of the tested samples has changed in the following order of CA/Mo atomic ratio of 1 < 2 < 3 > 4. Therefore, the catalyst prepared by using CA/Mo ratio = 3 showed the highest catalytic activity. BET, XRD, XPS, SEM and TEM-EDS techniques were been used to characterize the catalysts. The increased activity of Mo₂N-3:1 and 3%Co-Mo₂N-3:1 catalysts was due to increased surface area, decreased particle size and increased relative proportions of Mo₂N and Co₃Mo₃N phases. The ammonia conversion for 3%Co-Mo₂N catalyst was increased from 75 to 97% at 550 °C with the increase of CA/Mo ratio from 1 to 3. This enrichment of activity in 3%Co-Mo₂N-3:1 catalyst is due to increased dispersion of Co₃Mo₃N microstructure on γ-Mo₂N platelets confirmed by SEM and TEM results. No deactivation was observed for any catalysts investigated in this study for ammonia decomposition.