NH3 Decomposition Research Articles

Bimetallic NixCo10-x/CeO2 as highly active catalysts to enhance mid-temperature ammonia decomposition: Kinetics and synergies

Catalytic ammonia decomposition is an attractive method to generate hydrogen at mid-temperatures (<700 °C) but must incorporate precious metals (Pd, Ru, etc.) to ensure high reactivity. Developing Ni-based catalysts to decompose ammonia can enhance its prospect for hydrogen generation. However, the catalytic activity of Ni is hardly satisfactory at mid-temperatures. In this work, we show the bimetallic NixCo10-x/CeO2 towards mid-temperature NH3 decomposition, with the metal loading of Ni and Co tuned. Kinetics study demonstrates that the NH3 decomposition reaction follows the Temkin Pyzhev mechanism and the synergy between Ni and Co can decrease the reaction orders regarding NH3 and increase the reaction orders regarding H2. Mechanistic results indicate that the recombinative N desorption limits the reaction rate. The synergy between Ni and Co can simultaneously decrease the energy barriers of the recombinative N desorption and mitigate the H2 poisoning effect. Therefore, Ni7.5Co2.5/CeO2 displays both high ammonia conversion (96.96%) and hydrogen formation rate (1947.9 mmol/(gcat.h)) at 650 °C. We hope the mechanism in this work can be used to guide the design of inexpensive catalysts to decompose ammonia at mid-temperatures.

Supporting high-loading Ni on SBA-15 as highly active and durable catalyst for ammonia decomposition reaction

Although supported Ni is generally considered the most active non-noble metal catalyst for decomposing NH3 to produce COx-free H2, its activity is not sufficient. Herein, supporting high-loading Ni on SBA-15 is explored to alleviate the low intrinsic activity issue of Ni. SBA-15 supports with tunable textual properties are synthesized to support Ni catalyst for NH3 decomposition. Characterization shows that Ni catalyst with a loading close to 40 wt% supported on SBA-15 with the largest specific surface area (Ni/SBA-15-80) exhibits a NH3 decomposition performance much better than those reported on other Ni-based NH3 decomposition catalysts, resulting from its favorable textural properties and high Ni loading. In addition, Ni/SBA-15-80 shows excellent catalytic stability, with no activity degradation over an 80-h NH3 decomposition test. This work reveals the importance of textural properties of support and Ni loading to NH3 decomposition performance and can provide a new idea for synthesizing high-performance NH3 decomposition catalysts.

Synergistic Effects of Keggin-Type Phosphotungstic Acid-Supported Single-Atom Catalysts in a Fast NH3-SCR Reaction

Fast selective catalytic reduction of nitrogen oxide with ammonia (NH3-SCR) (2NH3 + NO2 + NO → 2N2 + 3H2O) has aroused great interest in recent years because it is inherently faster than the standard NH3-SCR reaction (4NO + 4NH3 + O2 → 4N2 + 6H2O). In the present paper, the mechanism of the fast NH3-SCR reaction catalyzed by a series of single-atom catalysts (SACs), M1/PTA SACs (PTA = Keggin-type phosphotungstic acid, M = Mn, Fe, Co, Ni, Ru, Rh, Pd, Ir, and Pt), has been systematically studied by means of density functional theory (DFT) calculations. Molecular geometry and electronic structural analysis show that Jahn-Teller distortion effects promote an electron transfer process from N-H bonding orbitals of the NH3 molecule to the symmetry-allowed d orbitals (dxy and dx2-y2) of the single metal atom, which effectively weakens the N-H bond of the adsorbed NH3 molecule. The calculated free energy profiles along the favorable catalytic path show that decomposition of NH3 to *NH2 and *H species and decomposition of *NHNOH into N2 and H2O have high free energy barriers in the whole fast NH3-SCR path. A good synergistic effect between the Brønsted acid site (surface oxygen atom in the PTA support) and the Lewis acid site (single metal atom) effectively enhances the decomposition of NH3 to *NH2 and *H species. M1/PTA SACs (M = Ru, Rh, Pd, and Pt) were found to have potential for fast NH3-SCR reaction because of the relatively small free energy barrier and strong thermodynamic driving forces. We hope our computational results could provide some new ideas for designing and fabricating fast NH3-SCR catalysts with high activity.

Y-Doped BaCeO3 Perovskite-Supported Ru Catalysts for COx-Free Hydrogen Production from Ammonia: Effect of Strong Metal–Support Interactions

The development of highly efficient Ru-based catalysts for NH3 decomposition is necessary to enable the utilization of NH3 as a COx-free H2 carrier. Modulation of the interactions between the basic support materials and Ru particles significantly enhances the performance of Ru-based catalysts for NH3 decomposition. In this study, the strong metal–support interaction (SMSI) interface between the BaCeO3 perovskite support and Ru particles was controlled by yttrium (Y) doping in the range of 0–20 mol %. The substitution of Y3+ into Ce4+ sites improved the reducibility of the support for the removal of lattice oxygen atoms, which promoted the formation of the SMSI interface between the mobile oxygen-deficient suboxide supports and Ru particles. The Ru catalysts supported on Y-doped BaCeO3 exhibited superior activity compared to undoped and other representative Ru catalysts. Moreover, the SMSI shell encapsulating the Ru particles improved the stability of the Y-doped Ru catalyst. This comprehensive study demonstrates that the enhancement in catalytic performance is attributable to facilitated N2 desorption, which is the rate-limiting step for NH3 decomposition over Ru-based catalysts, via the formation of the SMSI interface. These research findings show that the control of the SMSI interface is a promising strategy for designing highly active catalysts to be used in NH3 decomposition.

Magnesium-promoted Ni/USY catalysts prepared via surfactant-assisted melt infiltration for ammonia decomposition

The dispersion of active metal species on the support material of a catalyst is crucial for efficient catalytic reactions. Surfactant-assisted melt infiltration (SAMI), in which catalytically active components are supported only by heating with a surfactant instead of a solvent, is a straightforward method for synthesizing highly dispersed catalysts. In this study, monometallic 10 wt% Ni catalyst and alkaline-earth-metal (Mg, Ca, Sr, and Ba)-doped Ni catalysts supported on the ultra-stable Y (USY) zeolite are prepared using SAMI for ammonia decomposition. The addition of SpanTM60 during the preparation of the Ni/USY catalyst improved its catalytic performance. Moreover, the Mg-promoted Ni/USY catalyst prepared via SAMI exhibited further enhancement in catalytic activity and stability, demonstrating the synergistic effect of the SpanTM60 surfactant and Mg promoter. Detailed characterization revealed that the improved performance of the surfactant-added Mg-promoted catalyst was due to the high dispersion of Ni and MgO particles and the enhanced acidic and basic properties, which affected the number of adsorption sites of NH3 and the kinetic rate-determining step during NH3 decomposition, respectively. The results showed that SAMI could be used to synthesize highly dispersed and active Ni catalysts on zeolites.

Special direct route for efficient transfer hydrogenation of nitroarenes at room temperature by monatomic Zr tuned α-Fe2O3

Determining the route of a heterogeneous catalytic reaction is an important step in studying the reaction mechanism as well as improving reaction activity. As for the widely reported transfer hydrogenation of nitroarenes to aniline, many reported works have made simple judgements about the reaction route based on the “direct route” and “condensation route” proposed by Haber in 1898. Herein, detailed mechanistic experiments and DFT calculations helped us identify a special direct reduction route that goes through Ph-NO2 → Ph-NHOH → Ph-NH2 without the widely recognized Ph-NO intermediate for the transfer hydrogenation of nitroarenes to anilines by monatomic Zr tuned α-Fe2O3. In this catalytic system, Zr0.025Fe0.975Oy-350 can achieve complete transfer hydrogenation of nitroarenes at room temperature, where the parent α-Fe2O3 is inactive. Accompanying spectroscopic characterizations and calculations illustrate that the incorporation of Zr into the crystal lattice of α-Fe2O3 can effectively reduce the electron cloud density of surface iron species. The resulting special Fe(3+α)+-O(2+β)--Zr(4-γ)+ structure has greatly enhanced Lewis acidity, that promotes the decomposition of N2H4·H2O to form active [H] for the reduction of nitroarenes. Moreover, the structure is also beneficial for the adsorption of nitroarene with weakening of the NO bond. The present work can be extended to the application of non-noble metal oxides in the transfer hydrogenation of nitrobenzene under mild condition, and provides a significant method for exploring the reaction process of a heterogeneous catalytic reaction.

Investigation on catalytic performance and desorption behaviors of ruthenium catalysts supported on rare-earth oxides for NH3 decomposition

The basic property of the catalyst can influence the catalytic performance for the NH3 decomposition by alleviating associative desorption of N atom, known as a rate-determining step (RDS) in the Ru-based catalysts, but there were few experimental data to prove the alleviation of this step. Therefore, the Ru catalyst supported on rare-earth oxides and zirconium oxide were synthesized to compare the catalytic performance for NH3 decomposition and its dependence on the basic property of the Ru catalysts, including the other characterizations. The catalytic activities at 500 °C decreased in order of Ru/La2O3 > Ru/PrxOy > Ru/Sm2O3 ≅ Ru/Gd2O3 > Ru/Y2O3 > Ru/Yb2O3 > Ru/ZrO2 catalysts. This catalytic activity was related to the basic strength of the catalysts, which can influence the electron state of the impregnated Ru particles. The enhanced basic strength could modify the electronic state of the Ru particles on the support owing to electron donation from the oxide support to the Ru particles, and NH3-temperature programmed surface reaction (NH3-TPSR) confirmed that this enhanced electron donation could promote the desorption step of N atoms.

Hydrogen production via ammonia decomposition catalyzed by Ni/M–Mo–N (M = Ni, Co) bimetallic nitrides

This study demonstrates the significant improvement in NH3 decomposition using Ni-decorated M–Mo–N-based catalysts (M = Co and Ni) compared with conventional catalysts. Catalysts are prepared using a mixture of the corresponding metal salts and hexamethylenetetramine, and the impregnation method is used to decorate the Ni-particles on the catalysts. Among all the samples, 10 wt% Ni-decorated Co3Mo3N exhibits the highest NH3 conversion rate (71%) at 500 °C, and the performance remains stable for 30 h of long-term testing. According to the gas chromatography measurements, the H2/N2 ratio is approximately 3 in all cases, which is consistent with the theoretical value. X-ray photoelectron spectroscopy results show that Co3Mo3N possesses the highest NH3 conversion efficiency because of the weaker binding energy of Mo–N. Furthermore, Co3Mo3N exhibits a stronger Lewis acidity and higher NH3 decomposition, which is attributed to the easy breaking of the N–H bond on the Co3Mo3N surface.

Critical role of Cs doping in the structure and NH3 decomposition performance of Ru/MgO catalysts

The NH3 decomposition activity of Ru/MgO catalysts for the production of H2 decreases with increasing calcination temperature. Here, we found that this decrease in activity was the result of the interaction between the Ru and MgO support becoming stronger, causing formation of cationic Ru species. To mitigate this decrease in activity, we investigated the use of doping Ru/MgO catalysts with rare earth, alkali, or alkaline earth metals and found that Cs was particularly effective at enhancing catalytic activity, even at high calcination temperatures. Further examination revealed that doping with Cs weakened the interaction between the Ru and MgO support, resulting in the production of metallic Ru species. The metallic Ru produced by the doping of Cs enhanced the associative desorption of N atoms, which is the rate-limiting step of the NH3 decomposition reaction.

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.

Experimental and kinetic studies of NO reduction by ammonia injection into high-temperature reducing atmosphere

Experiments were conducted to investigate the NOx reduction performance by ammonia injection into a high-temperature reducing atmosphere and the integrated effects of temperature and oxygen concentration were discussed in detail. Ammonia oxidation was trivial at lower temperatures and the presence of oxygen facilitated the replenishment of OH radicals, enhancing the NOx reduction efficiency. The replenishment of OH radicals and ammonia oxidation were simultaneously promoted at higher temperatures. Therefore, trace amount of oxygen was preferred for higher temperatures to suppress the NOx formation from ammonia oxidation. A maximum NOx reduction efficiency of over 95 % was achieved at 1573 K and 0.0 vol% O2 with normalized stoichiometric ratio of 1.5 and residence time of 0.71 s. Furthermore, rate-of-production analysis was performed to provide insights into NH3/NO reaction pathways in the absence of oxygen. Results showed that the NOx reduction process was initiated by thermal decomposition of NH3 and CO2 acted the role of O2 when oxygen is absent, which could provide O atoms to react with H for the replenishment of OH radicals. The relatively high activation energy of NH3 decomposition accounted for the shift of the temperature window towards higher temperatures in the absence of oxygen.

High thermal stability Si-Al based N-carrier for efficient and stable chemical looping ammonia generation

Chemical looping ammonia generation (CLAG) is a promising NH3 production technology due to its potential for high yield of NH3 and low CO2 emission footprint. However, the stabilities of the existing N-carriers are problematic during the cyclic N-sorption/desorption. This research was designed to overcome that challenge through use of elemental doping and depositing the N-carrier on a support. The experimental results show that the N-sorption/desorption performance of γ-Al2O3 N-carriers decreased with cycling due to the deterioration of the pore structure and phase transformation. A Si-modified alumina-based N-carrier, with a Si: Al molar ratio of 1:20 prepared by co-precipitation, was much more resistant to high-temperature phase transition and collapse of pore structure, resulting in highly stable cyclic N-sorption/desorption performance. Use of ZrO2 as a support for the Si-modified Al-based N-carrier further reduced NH3 decomposition and thus increased the NH3 yield and selectivity by 85.6% and 53.6% compared to that obtained with the pure γ-Al2O3 N-carrier.

Hydrogen production by cracking of ammonium hydroxide using liquid-phase plasma on the modified TiO2 photocatalysts

Ammonia is a promising material as a direct source of green hydrogen production. This paper reports a method for mass production of hydrogen from liquid NH3(NH4OH) through a photocatalytic decomposition reaction using liquid plasma. In this reaction, the highest hydrogen production rate was observed in the TiO2 photocatalyst doped with N and metal ions as a photocatalyst sensitive to visible light with a low bandgap. At this time, the hydrogen production rate was obtained as about 142 L/g∙h. This is due to the high photoactivity of the visible light-sensitive photocatalyst in liquid plasma emitting strong visible light and ultraviolet light. The H2 production rate obtained from the decomposition of liquid NH3 by plasma discharge to the catalyst was higher than the H2 production rate obtained from the NH3 electrolysis process.

Catalytic ammonia decomposition to COx-free hydrogen over ruthenium catalyst supported on alkali silicates

A series of alkali metal silicates A2SiO3 (A = Li, Na and K) were synthesized by a sol–gel method route and employed as support for Ru nanoparticles to catalyze NH3 decomposition. The NH3 conversion can be ranked as the order of Ru/K2SiO3 > Ru/Na2SiO3 > Ru/Li2SiO3 > Ru/SiO2 at the same conditions, and Ru/K2SiO3 displays the highest NH3 conversion of 60.5% with the TOF H2 of 2.03 s−1 under the weight hourly space velocity (GHSV) of 30,000 mL.gcat-1.h−1 at 450 oC. The combination results of different characterization indicate that the formation of alkali metal silicates can increase the strength and number of basic sites, and the strong electronic metal-support interaction (EMSI) would appear between Ru particles and oxygen vacancies in A2SiO3 structure at reduction conditions. The presence of EMSI can result in the higher Ru dispersion and enhanced electronic density of Ru particles over A2SiO3, greatly facilitating the decomposition of NH3 to COx-free hydrogen.

Ammonia production from nitrogen-rich biomass gasification: Nitrogen transformation from model amino acids

Ammonia (NH3) is currently being explored as a sustainable energy carrier. Ammonia production from nitrogen-rich biomass gasification provides a sustainable, distributed, and flexible process to efficient storage and conversion of renewable energy. Nitrogen is primarily in the form of proteins (amino acids) in biomass. Three amino acids, glutamic acid (Glu), glycine (Gly), and phenylalanine (Phe), with distinct structures, were screened as N-containing model compounds to assess the effects of operating conditions on nitrogen chemistry. Results indicate that chemical structure has asignificantimpact on thermal decomposition properties and N-conversion selectivity of amino acids. Gly possesses the highest NH3-N yield (∼56.4%), because the simple amino structure easily detaches as NH3. Nevertheless, Phe, as the representative of aromatic amino acids, has the lowest NH3/HCN ratio. Additionally, it is interesting to find some phenomena in common for the aromatic amino acids and aliphatic amino acids. The introduction of steam is proven to enhance the hydrogenation of char-N, the thermal cracking of tar-N, and the hydrolysis of HCN into NH3, thus promoting the NH3 selectivity. The NH3-N yield increases initially and then suppresses at 700–900 °C, which is impacted by the extent of NH3 decomposition reaction. High steam content can contribute to the enhanced availability of H radicals, consequently resulting in significant release of NH3.

Shielding Protection by Mesoporous Catalysts for Improving Plasma-Catalytic Ambient Ammonia Synthesis.

Plasma catalysis is a promising technology for decentralized small-scale ammonia (NH3) synthesis under mild conditions using renewable energy, and it shows great potential as an alternative to the conventional Haber–Bosch process. To date, this emerging process still suffers from a low NH3 yield due to a lack of knowledge in the design of highly efficient catalysts and the in situ plasma-induced reverse reaction (i.e., NH3 decomposition). Here, we demonstrate that a bespoke design of supported Ni catalysts using mesoporous MCM-41 could enable efficient plasma-catalytic NH3 production at 35 °C and 1 bar with >5% NH3 yield at 60 kJ/L. Specifically, the Ni active sites were deliberately deposited on the external surface of MCM-41 to enhance plasma–catalyst interactions and thus NH3 production. The desorbed NH3 could then diffuse into the ordered mesopores of MCM-41 to be shielded from decomposition due to the absence of plasma discharge in the mesopores of MCM-41, that is, “shielding protection”, thus driving the reaction forward effectively. This promising strategy sheds light on the importance of a rational design of catalysts specifically for improving plasma-catalytic processes.

Phase, microstructure evolution, and periodic density functional theory study of reduction and nitridation of V2O3 with clean ammonia gas

Vanadium nitride (VN) has a wide range of application because of its excellent properties, which include high hardness, outstanding wear resistance, and good electrical conductivity. This study investigated the mechanism for the reduction and nitridation of V2O3 with clean ammonia gas, using both experimental and density functional theory (DFT) studies. The experimental results indicated that V2O3 could easily be converted to VN in an ammonia atmosphere at 500–800 °C. The reaction pathway to form VN was V2O3 → VNxO1–x → VN. Increasing the reaction temperature was conducive to an increase in the N content of VNxO1–x. A DFT study systematically revealed the adsorption of NHx and H on the V2O3 (0001) surface. The results showed that with the dissociation of NH3, its adsorption energy on the surface of vanadium oxide became higher. The whole reaction process could be divided into NH3 decomposition on the surface and the formation of H2O(g). Both of these were endothermic reactions, and the reaction step of generating H2O(g) needed a higher temperature. The bonding of V3c–N facilitated the desorption of O3c atoms to form H2O(g), which explained why VN can be prepared by the reaction of NH3 and V2O3.

Evaluation of Durability Performance of a Ru/MgO Catalyst for Ammonia Decomposition at an on-Site Hydrogen Fueling Station

Since ammonia (NH3) decomposition catalysts for on-site hydrogen fueling stations must possess at least 10 years of durability, an activity/durability evaluation technique that can estimate catalyst activity after several thousands of hours was developed based on short-term durability tests in the laboratory. It consists of a series of repetitive activity/durability tests: (a) thermal treatments in NH3 at a predetermined endurance temperature for a given time, followed by (b) activity measurements at a reaction temperature of 500 °C. The technique was utilized for accelerated-deterioration durability evaluation of a Ru/MgO catalyst. It was discovered that the NH3 conversion decreases almost linearly, as a function of the endurance time at all endurance temperatures. Furthermore, the Arrhenius equation was employed to explore the relationship between the deterioration rate and endurance temperature. This technique can be considered a crucial tool for predicting the durability of NH3 decomposition catalysts employed in H2 fueling stations.

Decomposition studies of NH3 and ND3 in presence of H2 and D2 with Pt/Al2O3 and Ru/Al2O3 catalysts

In the fusion reactor ITER, ammonia will be produced as a result of the interaction between the hydrogen isotopes used as fuel and nitrogen used to spread the power loads of a larger area. As part of the fuel management in ITER, NQ3 (NQ3, Q = H, D, T) will have to be decomposed using a palladium membrane reactor. The decomposition of pure NH3 and ND3 was studied in this work using commercial platinum (Pt) and ruthenium (Ru) catalysts on alumina (0.5 wt% loading), in a conventional reactor configuration (i.e., without a palladium membrane). With Pt/Al2O3, decomposition fractions larger than 90% were achieved with NH3 above 800 K using the lowest flow-to-mass ratio (FNH3/g-cat) of 0.015 sccm g−1. However, with the increase of FNH3/g-cat to 0.220 sccm g−1, similar decompositions were achieved only at ≈1000 K. In contrast, with Ru/Al2O3 decomposition fractions above 90% were attained already below 700 K, regardless of FNH3/g-cat. With both catalysts the decomposition of NH3 was found to be more efficient than that of ND3 at a wide range of temperatures, thus evidencing the existence of isotopic effect. A strong inhibition of both NH3 and ND3 in presence of, respectively, H2 and D2 with Pt/Al2O3 was observed. This effect was stronger at lower temperatures and larger hydrogen partial pressures. The inhibition effect with Ru/Al2O3 was less pronounced and it was suppressed at 629 K. Isotopic exchange reactions with equimolar mixtures of NH3-D2 and ND3-H2 revealed that the most and least abundant isotopologue are, respectively, NH2D and ND3. At the relevant temperature window in which the PMR will be operated (673–823 K), the Ru-based catalyst exhibits superior performances in terms of decomposition rates, negligible isotopic and inhibition effects. A slight reduction of the performances with this catalyst was observed with 0.200 sccm g−1. This work suggests that 0.5 wt% Ru/Al2O3 is the most suitable catalyst to be used during ITER operation.

Photoelectrochemical and photocatalytic studies by applying an organic p-n bilayer for the selective oxidation of ammonia to dinitrogen

• Platinum-loaded MWCNT was newly fabricated as a co-catalyst for NH 3 oxidation. • The selective oxidation of NH 3 to N 2 occurred at the cocatalyst-loaded organo-photoanode. • The overall decomposition of NH 3 was photoelectrochemically and photocatalytically accomplished. • Even when Pt was subjected to a high potential, its stable catalysis for the NH 3 oxidation was verified. • By embedding the co-catalyst in a Nafion membrane, the disorder of Pt was avoided at the high applied potential. Ammonia (NH 3 ) is a pollutant that needs to be treated properly. Its oxidative degradation to dinitrogen (N 2 ) involves the transfer of six electrons, and thus, an active catalyst must be applied to accomplish the selective formation of N 2 from NH 3 . In this work, polycrystalline Pt was newly prepared on a multi-walled carbon nanotube (MWCNT) supporter, which was used as a co-catalyst by embedding it in a Nafion® membrane (i.e., Nf[Pt/MWCNT]). The 10 nm-sized Pt particles were well characterized by means of transmission electron microscopy and electrochemistry. We have found that an organic p-n bilayer, comprising fullerene ( n -type) and phthalocyanine (p-type), works as a photoanode capable of oxidation at the phthalocyanine surface. When applying Nf[Pt/MWCNT] as the co-catalyst at the organo-bilayer, the selective NH 3 oxidation was confirmed to occur stably in terms of photoelectrochemistry and photocatalysis, particularly under visible light irradiation.