Catalytic Decomposition Of NH3 Research Articles

Microwave-assisted ammonia decomposition over metal nitride catalysts at low temperatures

The negative environmental impact of fossil fuel-based energy systems has unveiled the need to develop a COx-free sustainable hydrogen (H2) economy. Employing a microwave-assisted route, a ternary metal nitride catalyst (i.e., Co2Mo3N), and a low-temperature-pressure NH3 decomposition process, this study investigated the possibility of developing a distributed H2 production process. This study not only explored lower cost-based catalyst systems but also the use of a microwave reactor to increase the energy efficiency of the process. Results from catalytic NH3 decomposition experiments, performed in microwave reactors on Co2Mo3N catalyst, demonstrated the peak energy efficiency (of ∼0.006 kgH2/kWh) at 400 °C in ambient pressure (with an NH3 conversion >90%) which was around ninety times (∼90 x) more efficient than a conventional system. Activation energy calculation also displayed a 20% less energy requirement for the microwave-based process (∼31 kJ mol−1) than the conventional system (∼37 kJ mol−1), indicating the advantage of the microwave-based process. Further microwave-assisted catalytic measurements and characterization of the Co2Mo3N catalyst, using x-ray diffraction (XRD) technique and scanning electron microscopic (SEM) images, revealed the excellent stability of this material at its peak performance (at 400 °C) and illustrated the potential of using this catalyst for a sustainable, economic, and energy-efficient approach for producing COx-free H2.

Multifunctional effect of copper in bimetallic Cu-M/Al2O3 catalysts (M = Fe, Co, and Ni) for NH3 decomposition

Ammonia is a promising hydrogen carrier, which can enhance energy transport efficiency when bulk NH3 is transported in a gas carrier vessel. At the end of the NH3 transport, the NH3 gas should be converted to H2 and N2 using NH3 decomposition catalyst, which has outstanding catalytic performance and low cost. The M/Al2O3 and Cu-M/Al2O3 catalysts (M = Fe, Co, and Ni) were test and characterized regarding catalytic NH3 decomposition because we expected the Cu addition could differently change the electronic structure and the desorption behaviors of active metal species. As our expectation, the added Cu modified the catalytic performance and characteristics, but these changes were different, depending on the kind of active metal species (Fe, Co, and Ni). The catalytic activity was improved for Fe/Al2O3 but worsened for Co/Al2O3 and Ni/Al2O3. In addition, the added Cu element acted as an electron donor in Cu-Fe/Al2O3 but as an electron acceptor in Cu-Co/Al2O3 and Cu-Ni/Al2O3 and influenced the desorption behaviors of the generated H and N atoms, particularly in the Cu-Fe/Al2O3, the added Cu alleviated the desorption behaviors of H and N atoms but suppress them in the Cu-Co/Al2O3 and Cu-Ni/Al2O3, which affected the catalytic performance of NH3 decomposition.

Ru dispersed on CeO2{1 0 0} facets boosting the catalytic NH3 decomposition for green H2 generation

A hydrothermal process was used to prepare CeO2 nanoparticles with mainly {100} facets and {110,100} facets, and a precipitation method was used to prepare CeO2 nanoparticles with mainly {111,100} facets for supporting Ru in NH3 decomposition studies. CeO2{100} promoted the formation of partially charged Ru (Ruδ+) species over Ru/CeO2{100}, leading Ru/CeO2{100} to perform the highest NH3 decomposition activity by desorbing re-combinative N2 efficiently. Density functional theory calculations have confirmed the Ru dispersed on terminated CeO2{100} with the high negative Bader charges for decreasing the formation energy of N2. In addition, the Cs addition significantly promotes the catalytic activity, with the optimal molar ratio of Ru observed at 1:1. 1.0Cs-Ru/CeO2{100} exhibited superior catalytic activity and durability compared to the others. This study proves that CeO2{100} can improve the electron donation to Ru, which is essential for enhancing N2 re-combinative desorption to elevate the NH3 decomposition activity.

NH3 decomposition for H2 production by thermal and plasma catalysis using bimetallic catalysts

Plasma catalysis has emerged as a promising approach for driving thermodynamically unfavorable chemical reactions. Nevertheless, comprehending the mechanisms involved remains a challenge, leading to uncertainty about whether the optimal catalyst in plasma catalysis aligns with that in thermal catalysis. In this research, we explore this question by studying monometallic catalysts (Fe, Co, Ni and Mo) and bimetallic catalysts (Fe-Co, Mo-Co, Fe-Ni and Mo-Ni) in both thermal catalytic and plasma catalytic NH3 decomposition. Our findings reveal that the Fe-Co bimetallic catalyst exhibits the highest activity in thermal catalysis, the Fe-Ni bimetallic catalyst outperforms others in plasma catalysis, indicating a discrepancy between the optimal catalysts for the two catalytic modes in NH3 decomposition. Comprehensive catalyst characterization, kinetic analysis, temperature program surface reaction experiments and plasma diagnosis are employed to discuss the key factors influencing NH3 decomposition performance.

Catalytic ammonia decomposition to produce hydrogen: A mini-review

Hydrogen (H2) is considered one of the most promising alternative energy resources instead of fossil fuels, serving as a clean energy carrier. Ammonia (NH3) is regarded as a high percentage hydrogen carrier and utilized as an initial material for H2 production. As for the H2 production through NH3 decomposition, there are various catalytic processes particularly using Ru- and Pd-based catalysts. The Ru- and Ni-based NH3 decomposition catalysts have been studied due to synergetic effect of active metal phase in stabilizing unstable nitrogen intermediate species during NH3 decomposition reaction. The Pd catalyst-based membrane reactors were developed to enhance H2 separation from the product stream of NH3 decomposition. Integrated systems combining Ru catalyst and Pd-based membrane were also explored to achieve a high yield of H2 from NH3. This review summarizes recent literatures on catalytic NH3 decomposition to produce H2. It probably provides useful insights into designing large-scale H2 production processes.

NH3-Fed Patterned Electrode Solid Oxide Fuel Cell: Experimental Performance Characterization and Elementary Reaction Modeling

Ammonia-fueled solid oxide fuel cells (SOFCs) have attracted the focus of researchers due to the feature of no carbon emission. Understanding the reaction mechanism is vital for the design and optimization of NH3-fed SOFCs. However, the catalytic decomposition reactions involved in porous electrodes led to the difficulty to distinguish the anode reaction mechanism. In the present study, we utilized a patterned anode to get a designed triple phase boundary (TPB) and avoid the effects of porous electrodes. In the performance test, we found that the current density of the NH3-fed SOFC was approximately 20% of that of the H2-fed SOFC under a similar working position at 750℃. The difference may mainly resulted from the limit of catalytic decomposition of NH3 caused by the narrow reactive area in the patterned electrode. A one-dimensional elementary reaction model was further developed to validate this hypothesis and to provide additional evidence for the reaction mechanisms of the NH3-fed SOFC.

NH3-Fed Patterned Electrode Solid Oxide Fuel Cell: Experimental Performance Characterization and Elementary Reaction Modeling

Ammonia-fueled solid oxide fuel cells (SOFCs) have attracted the focus of researchers due to no carbon emissions in utilization. Understanding the reaction mechanism is vital for the design and optimization of NH3-fed SOFCs. However, the catalytic decomposition reactions involved in porous electrodes led to difficulty in distinguishing the anode reaction mechanism. In the present study, we utilized a patterned anode to obtain a designed triple phase boundary (TPB) and avoid the effects of porous electrodes. In the performance test, we found that the exchange current density of the NH3-fed SOFC was approximately 5% of that of the H2-fed SOFC under a similar working position at 700℃. The difference may mainly result from the limit of catalytic decomposition of NH3 caused by the narrow reactive area in the patterned electrode. A one-dimensional elementary reaction model was further developed to validate this hypothesis and to provide additional evidence for the reaction mechanisms of the NH3-fed SOFC.

Size-Dependent Activity of Iron Nanoparticles in Both Thermal and Plasma Driven Catalytic Ammonia Decomposition

Water-dispersible Fe3O4 nanoparticles with diameters of 4.2 ± 0.6, 6.1 ± 0.8, 8.1 ± 1, and 10.4 ± 1 nm were prepared through the polyol method and employed as the precursors of Fe3O4/Al2O3 catalysts to study the size-dependent activity. We identified that the activity of the catalysts in NH3 decomposition (driven by both thermal and dielectric barrier discharge plasma) increased with increasing Fe3O4 particle size. The turnover frequencies (TOFs) were increased from 0.9 to 5.8 s–1 with an increasing Fe3O4 precursor size from 4.2 to 10.4 nm during the thermocatalytic decomposition. A quite similar “particle size effect” was also observed for the plasma catalytic decomposition, although lower TOF was observed. Additionally, reaction-induced catalyst reconstruction was identified during the early-stage of the catalytic decomposition and can be attributed to the nitridation of FeOx to FexN. Our results provide new evidence for the “structure-sensitivity” of the catalytic NH3 decomposition.

The Direct Reduction of Iron Ore with Hydrogen

The steel industry represents about 7% of the world’s anthropogenic CO2 emissions due to the high use of fossil fuels. The CO2-lean direct reduction of iron ore with hydrogen is considered to offer a high potential to reduce CO2 emissions, and this direct reduction of Fe2O3 powder is investigated in this research. The H2 reduction reaction kinetics and fluidization characteristics of fine and cohesive Fe2O3 particles were examined in a vibrated fluidized bed reactor. A smooth bubbling fluidization was achieved. An increase in external force due to vibration slightly increased the pressure drop. The minimum fluidization velocity was nearly independent of the operating temperature. The yield of the direct H2-driven reduction was examined and found to exceed 90%, with a maximum of 98% under the vibration of ~47 Hz with an amplitude of 0.6 mm, and operating temperatures close to 500 °C. Towards the future of direct steel ore reduction, cheap and “green” hydrogen sources need to be developed. H2 can be formed through various techniques with the catalytic decomposition of NH3 (and CH4), methanol and ethanol offering an important potential towards production cost, yield and environmental CO2 emission reductions.

Enhanced NH3 decomposition for H2 production over bimetallic M(M=Co, Fe, Cu)Ni/Al2O3

Ammonia (NH3) has been regarded as an attractive energy source for hydrogen production via catalytic decomposition, especially over non-noble metal catalysts. Owing to the decent performance and low cost, nickel (Ni) based catalysts have been widely studied for catalytic decomposition of NH3 where the decomposition rates, can be further tailored by adding other metals. Herein, we synthesized bimetallic M(M = Co, Fe, Cu)Ni/Al2O3 catalysts delivering superior catalytic performance at a wide temperature range of 350–750 °C. Co-Ni/Al2O3 demonstrated nearly 100% conversion of NH3 decomposition to H2 at 700 °C and the gas hourly space velocity of 30,000 ml g−1 min−1, which was ~20.85% higher than that of intrinsic Ni/Al2O3. The structural characterizations indicated that Co could decrease the particle size and enhance the dispersion of active sites. The density functional theory calculations substantiated that the associative decomposition of N to generate N2 was the rate-determining step of the NH3 decomposition reactions. NH3 Temperature Programmed Desorption (TPD) results suggested that the enhanced catalytic performance was predominantly attributed to the Co that decreases the energy barrier for the associative decomposition of N to form N2. This study provides new insights in designing diverse catalysts for efficient NH3 decomposition for controllable H2 production.

Integration between energy and exergy analyses to assess the performance of furnace regenerative and ammonia decomposition systems

Energy and exergy analyses were performed on a glass furnace regenerator and hydrogen production via thermal catalytic ammonia decomposition. A novel integration between energy and exergy analyses was suggested to predict the performance of thermodynamic and hydrogen production systems. It was found that the heat recovered by the combustion air of the south side regenerator was higher than that on the north side because of the fouling effect of the deposit materials and the heat lost through the regenerator walls. The maximum energy and exergy efficiencies of the south side regenerator were 98% and 93%, respectively, while those of the north regenerator side were 96.3% and 80%, respectively. The integration between energy and exergy analyses was also applied to investigate the performance of H2 production from the catalytic NH3 decomposition system. The conversion rate results proved that an increase in the ammonia feeding pressure was unfavorable. Moreover, the maximum energy and exergy efficiencies of the NH3 thermal decomposition system were 73.5% and 13.34% at a feeding pressure of 100 kPa, respectively. Furthermore, the non-equilibrium of the system is identified from the integrated effectiveness and entropy generation number.

Novel hollow fiber membrane reactor for high purity H2 generation from thermal catalytic NH3 decomposition

Although storage of H2 in various liquid chemicals makes it very attractive for practical applications, production and purification of H2 from decomposition of these chemicals under relatively mild conditions is still challenging. In this work, we reported production of high-purity H2 from NH3 decomposition using a combined packed bed/membrane reactor loaded with Ru-based catalyst. Three types of membranes (modified MFI zeolite membrane, carbon molecular sieve membrane, and Pd/Ag membrane) were employed and compared for H2 production and purification from NH3 decomposition. All these membrane reactors exhibited high NH3 conversion of >99% with H2 recovery of >90% under pressurized NH3 feed of 7 bar. Nevertheless, H2 purity varied because of the different separation performance of these membranes. High H2 purity of >99.999% with <10 ppb NH3 concentration in permeate was achieved using high-quality Pd/Ag membrane reactor. These results suggest a feasible and highly energy efficient option for producing high-purity H2 from NH3 decomposition.

Kinetic and mechanistic analysis of NH3 decomposition on Ru(0001), Ru(111) and Ir(111) surfaces.

We investigated the catalytic NH3 decomposition on Ru and Ir metal surfaces using density functional theory. The reaction mechanisms were unraveled on both metals, considering that, on the nano-scale, Ru particles may also present an fcc structure, hence, leading to three energy profiles. We implemented thermodynamic and kinetic parameters obtained from DFT into microkinetic simulations. Batch reactor simulations suggest that hydrogen generation starts at 400 K, 425 K and 600 K on Ru(111), Ru(0001) and Ir(111) surfaces, respectively, in excellent agreement with experiments. During the reaction, the main surface species on Ru are NH, N and H, whereas on Ir(111), it is mainly NH. The rate-determining step for all surfaces is the formation of molecular nitrogen. We also performed temperature-programmed reaction simulations and inspected the desorption spectra of N2 and H2 as a function of temperature, which highlighted the importance of N coverage on the desorption rate.

Experimental investigation and modeling of boundary layer flashback for non-swirling premixed hydrogen/ammonia/air flames

Carbon free fuels such as hydrogen/ammonia blends show a promising potential to become sustainable and renewable fuels for gas turbines and other combustion systems. One interesting aspect about these blends is the possibility to adjust different combustion properties like the laminar burning velocity or ignition delay time by changing the ratio between H2 and NH3. Such fuel blends can be produced via partial catalytic decomposition of NH3. However, such mixtures can lead to flame instabilities such as flashback, especially if the hydrogen content is high. In the present study, the boundary layer flashback of premixed hydrogen/ammonia/air mixtures is investigated experimentally for non-swirling flows at normal temperature (293 K) and normal pressure (101 kPa). A new experimental setup for boundary layer flashback investigation with a fully automated measurement procedure is introduced. With preliminary studies, the influence of various measurement procedures on the flashback limits is firstly investigated. For a broad flashback study, the data of 351 flashback experiments are collected. The ammonia content in H2/NH3 fuel mixtures is varied from 0 vol% to 50 vol% in 10 vol% steps. The fuel–air equivalence ratio is ranging from 0.38 to 1.17. As the ammonia content is increasing, the mean flow velocities at flashback are exponentially decreasing. Additionally, theoretical modeling is performed. A model is derived based on the concept of the critical velocity gradient which is able to predict the measured data with high accuracy. For two exemplary cases with H2/air and 80% H2/20% NH3/air mixtures, the process of boundary layer flashback is investigated in detail with low and high speed direct imaging and image post-processing. During the flashback onset of H2/NH3/air flames a separate reaction of H2 followed by the reaction of NH3 can be observed. Also, a flame-oscillation between fused silica tube and burner head with approximately 10 Hz was observed. Furthermore, indications for an adverse pressure gradient based on the flame propagation speed is seen. Details about the flame structure during the flashback process of H2/NH3/air flames are shown. During the upstream flame propagation of H2/NH3/air flames, high frequency oscillations with about 830 Hz of the leading flame tip are observed, which are assumed to be related to thermoacoustic instabilities.

Kinetic-model-based design of industrial reactor for catalytic hydrogen production via ammonia decomposition

A two-dimensional reactor model based on reaction kinetics and heat and mass transfer for catalytic NH3 decomposition was constructed. A reactor design that minimizes the effect of endothermic reaction was investigated. It is difficult to design a single-tube reactor for NH3 treatment at a flow rate of 10 m3/h because heat transfer from the reactor wall cannot keep up with the fast consumption of heat by the reaction, even under low space velocity conditions. The target conversion was achieved by decreasing the NH3 supply rate. A reactor with a packed bed of suitable length was designed. Industrial-scale NH3 treatment can be achieved using a multi-tube reactor.

Computationally aided, entropy-driven synthesis of highly efficient and durable multi-elemental alloy catalysts.

Multi-elemental alloy nanoparticles (MEA-NPs) hold great promise for catalyst discovery in a virtually unlimited compositional space. However, rational and controllable synthesize of these intrinsically complex structures remains a challenge. Here, we report the computationally aided, entropy-driven design and synthesis of highly efficient and durable catalyst MEA-NPs. The computational strategy includes prescreening of millions of compositions, prediction of alloy formation by density functional theory calculations, and examination of structural stability by a hybrid Monte Carlo and molecular dynamics method. Selected compositions can be efficiently and rapidly synthesized at high temperature (e.g., 1500 K, 0.5 s) with excellent thermal stability. We applied these MEA-NPs for catalytic NH3 decomposition and observed outstanding performance due to the synergistic effect of multi-elemental mixing, their small size, and the alloy phase. We anticipate that the computationally aided rational design and rapid synthesis of MEA-NPs are broadly applicable for various catalytic reactions and will accelerate material discovery.

Hydrogen production via catalytic decomposition of NH3 using promoted MgO-supported ruthenium catalysts

Catalytic decomposition of NH3 to high purity hydrogen offers a promising strategy for fuel cells, but presents challenges for high hydrogen yields at comparatively low temperatures due to the lack of efficient catalysts. Here, we report the facile preparation of ultra-fine ruthenium (Ru) species dispersed on MgO, which show excellent activity and high temperature stability for NH3 decomposition reaction. The hydrogen yield of the prepared Ru/MgO catalysts reaches ca. 2,092 mmol H2 gRu–1 min–1 at 450 °C, far exceeding that of the previously reported most reactive Ru-based catalysts and the same chemical composition samples prepared by other approaches. Various characterization techniques containing X-ray absorption fine structure (XAFS), in-situ diffuse reflectance infrared Fourier transform spectroscopy ( in-situ DRTFTS) and temperature-programmed reduction/desorption (TPR/TPD) were carried out to explore the structure-function relation of the prepared Ru/MgO catalysts. We found that the Ru species interact strongly with the MgO support, which can efficiently protect the Ru species and MgO support from agglomerating during NH3 decomposition test, maintaining the stability of the catalysts.

Oxidation of Ammonia by Ilmenite under Conditions Relevant to Chemical-Looping Combustion

The oxidation of ammonia to NOx and N-2 was investigated under conditions that pertain to the fuel reactor during the chemical-lopping combustion of coal with ilmenite. The catalytic decomposition of NH3, the oxidation of NH3 over ihnenite, and the reduction of NO by reduced ilmenite and NH3 were studied experimentally. The catalytic decomposition of NH3, NO reduction by NH3, and reduced ilmenite were found to be important for N-2 formation. NH3 oxidation over ilmenite was the only way in which NO could be formed in this systerri, with around 18% the NH3 being converted to NO at 850-950 degrees C, The oxidation of NH3 was only slightly influenced by the reactor temperature but was strongly influenced by the concentrations of NH3 and syngas. NO formation was,promoted by high concentrations of NH3 and decreased by high concentrations of syngas. The selectivity of the NH3 toward-NO formation was favored at low concentrations of NH3 and syngas. The conversion of NH3 was complete in most cases, although 15-25% of the NH3 was not converted when the inlet syngas concentration increased to levels higher than 10-30%.

Ammonia Decomposition for Hydrogen Production in Catalytic Microchannels with Slip/Jump Effects

The rarefaction effects on the catalytic decomposition of NH3 in ruthenium–coated planar microchannels is numerically simulated in the Knudsen number range 0.015-0.03. A colocated finite–volume method is used to solve the governing equations. A concentration jump model derived from the kinetic theory of gases is employed to account for the concentration discontinuity at the reactive walls. A detailed surface reaction mechanism for ammonia decomposition on ruthenium along with a multi-component species diffusion model are used to study the effects of concentration jump coupled with velocity slip and temperature jump on the walls. The velocity-slip, temperature-jump and concentration-jump boundary conditions have miscellaneous effects on flow, temperature and species concentration fields. The results suggest that the velocity-slip boundary condition only slightly influences the species distribution at the edge of the Knudsen layer as well as inside the channel, while the temperature-jump boundary condition affects the heat and mass transfer characteristics the most. The concentration-jump effect, on the other hand, can counter balance the temperature-jump effects in some cases.

Experimental and Theoretical Investigation of Molybdenum Carbide and Nitride as Catalysts for Ammonia Decomposition

Constant CO(x)-free H2 production from the catalytic decomposition of ammonia could be achieved over a high-surface-area molybdenum carbide catalyst prepared by a temperature-programmed reduction-carburization method. The fresh and used catalyst was characterized by N2 adsorption/desorption, powder X-ray diffraction, scanning and transmission electron microscopy, and electron energy-loss spectroscopy at different stages. Observed deactivation (in the first 15 h) of the high-surface-area carbide during the reaction was ascribed to considerable reduction of the specific surface area due to nitridation of the carbide under the reaction conditions. Theoretical calculations confirm that the N atoms tend to occupy subsurface sites, leading to the formation of nitride under an NH3 atmosphere. The relatively high rate of reaction (30 mmol/((g of cat.) min)) observed for the catalytic decomposition of NH3 is ascribed to highly energetic sites (twin boundaries, stacking faults, steps, and defects) which are observed in both the molybdenum carbide and nitride samples. The prevalence of such sites in the as-synthesized material results in a much higher H2 production rate in comparison with that for previously reported Mo-based catalysts.