NH3 Conversion Research Articles

Dual confinement of LaCoOx modified Co nanoparticles for superior and stable ammonia decomposition

In this work, we report the rational design of amorphous LaCoOx modified Co nanoparticles confined in both nitrogen-doped carbon (NC) matrix and two-dimensional (2D) mesoporous silica (LaCoOx/Co@NC/SBA-15(2D)) via in situ confined pyrolysis of nano-sized La doped ZIF-67 in the mesopores of SBA-15(2D) as a novel dual-spatially-confined composite catalyst with superior activity and stability toward ammonia decomposition to produce hydrogen. It achieves a NH3 conversion efficiency close to 100 % at a high gas hourly space velocity (GHSV) of 60,000 gcat-1 h-1 with an encouraging H2 production rate of 446 mmol gCo-1 min-1 at 600 ℃ and can maintain stable catalytic activity for 200 h, almost outperforming all Co-based catalysts reported so far. Various characterizations and theoretical calculation results reveal that the outstanding ammonia decomposition performance mainly origins from the dual-spatially-confined configuration, fully exposed active sites and high mass transfer efficiency, and the synergy of amorphous LaCoOx, Co nanoparticles and NC matrix.

Enzymatic Electrosynthesis of Glycine from CO2 and NH3

AbstractEnzymatic electrosynthesis has gained more and more interest as an emerging green synthesis platform, particularly for the fixation of CO2. However, the simultaneous utilization of CO2 and a nitrogenous molecule for the enzymatic electrosynthesis of value‐added products has never been reported. In this study, we constructed an in vitro multienzymatic cascade based on the reductive glycine pathway and demonstrated an enzymatic electrocatalytic system that allowed the simultaneous conversion of CO2 and NH3 as the sole carbon and nitrogen sources to synthesize glycine. Through effective coupling and the optimization of electrochemical cofactor regeneration and the multienzymatic cascade reaction, 0.81 mM glycine was yielded with a highest reaction rate of 8.69 mg L−1 h−1 and faradaic efficiency of 96.8 %. These results imply a promising alternative for enzymatic CO2 electroreduction and expand its products to nitrogenous chemicals.

Enzymatic Electrosynthesis of Glycine from CO2 and NH3.

Enzymatic electrosynthesis has gained more and more interest as an emerging green synthesis platform, particularly for the fixation of CO2 . However, the simultaneous utilization of CO2 and a nitrogenous molecule for the enzymatic electrosynthesis of value-added products has never been reported. In this study, we constructed an in vitro multienzymatic cascade based on the reductive glycine pathway and demonstrated an enzymatic electrocatalytic system that allowed the simultaneous conversion of CO2 and NH3 as the sole carbon and nitrogen sources to synthesize glycine. Through effective coupling and the optimization of electrochemical cofactor regeneration and the multienzymatic cascade reaction, 0.81 mM glycine was yielded with a highest reaction rate of 8.69 mg L-1 h-1 and faradaic efficiency of 96.8 %. These results imply a promising alternative for enzymatic CO2 electroreduction and expand its products to nitrogenous chemicals.

Diagnostics-oriented Model for Automotive SCR-ASC

This paper presents a diagnostics-oriented aging model for combined Selective Catalytic Reduction (SCR) and Ammonia Slip Catalyst (ASC) system, along with a model-based on-board diagnostic (OBD) method applied to both test-cell data and on-road data from commercial trucks. The key challenge with model development was unavailability of NOx and NH3 measurements between SCR and ASC. Since it would have been very difficult to calibrate both SCR and ASC dynamics without any measurements between SCR and ASC, therefore ASC was modeled using static look-up tables to determine ASC’s NH3 conversion efficiency and its selectivity to NOx and N2O as a function of temperature and flow rate. The traditional three-state single-cell ordinary differential equation (ODE) model was used for SCR. Hot Federal Test Procedure (hFTP) was used to calibrate the model. Cold FTP (cFTP) and Ramped Mode Cycle (RMC) were used for validation. Results show that the SCR-ASC model can capture the aging signatures in tailpipe NOx, NH3, and N2O reasonably well for cFTP, hFTP, and RMC cycles in the testcell data. After slight re-calibration and combining with a simple model for commercial NOx sensor’s cross-sensitivity to NH3, the model works reasonably well for on-road data from commercial trucks. A model-based on-board diagnostic (OBD) method has been presented with enable conditions designed to detect operating conditions suitable for detecting aging signatures, while minimizing false positives and false negatives. The OBD method is applied to both test-cell and real-world truck data with commercial NOx sensors. Results on test-cell data demonstrate the challenges of robust SCR monitoring even on the limited data set used in this work. The model-based enable conditions are shown to be robust but extremely restrictive as the OBD gets enabled at very few points in the test-cell data. Application on truck data showed that the proposed OBD method can be implemented on commercial trucks with limited sensors. In the truck data, the enable conditions were satisfied on many more points than the test-cell data. Results on truck data show encouraging trends between relative degradation level and the number of miles on four trucks. In future work, these trends will be validated using more data from commercial trucks with known aging levels.

Enhancement of acidic sites in layered MnO2 for the highly efficient selective catalytic oxidation of gaseous ammonia

Ammonia (NH3), as a typical harmful gaseous pollutant in atmosphere, poses a serious threat to human health and ecological environment. It is of great significance to develop the cost-effective catalysts with both excellent selective catalytic oxidation (SCO) activity and N2 selectivity. In this study, the acidic sites of layered MnO2 were regulated and its NH3-SCO performance was investigated. The enhancement of acidic sites in layered MnO2 significantly improved the SCO of NH3, and the resulting MnO2 could achieve complete conversion of NH3 at as lower as 100 °C, even better than the reported noble-metal catalysts. Furthermore, the improvement in NH3-SCO activity can be attributed to the abundant active oxygen species and enhancement of acidic sites (especially stronger Brønsted acidic sites). On the one hand, more active oxygen species can enhance the catalytic oxidation of NH3. On the other hand, Brønsted acidic sites acted not only as adsorption sites but also reaction sites, the increase in the content and strength of acidic sites facilitated the adsorption and activation of NH3. This research reveals the functional role of acidic sites in NH3-SCO, and provides a suggestive for the rational design of catalysts with excellent N2 selectivity and low-temperature NH3-SCO activity.

Catalytic performance of sewage sludge char and ash in the decomposition of NH3

The present study was conducted to study the catalytic performance of sewage sludge char and ash on NH3 decomposition by using a fixed bed reactor. The results indicated that sewage sludge char exhibited strong activity towards NH3 decomposition, which was highly influenced by temperature and atmosphere. The influence of atmosphere on NH3 decomposition was investigated at 973 K, it showed that the char activity was greatly promoted with the presence of O2, and the major product of NH3 oxidation catalyzed by relatively fresh char was N2 rather than NO. In contrast, the NH3 decomposition was suppressed with H2O introducing, and there was a significant amount of NO being produced. Compared to char, the in-situ combusted ash showed relatively poor catalytic performance with respect to NH3 conversion. At 973 K, the NH3 conversion catalyzed by ash was dominated by the inhabitation effect of H2O with the presence of both O2 and H2O. However, when the temperature increased to 1128 K, O2 tends to take a dominant role to promote NH3 conversion. By XRD analysis of the fresh and spent char, it was proposed that iron phosphide (Fe2P) may be responsible for the high activities towards NH3 decomposition, following a cycle mechanism.

A Review on Numerical Simulation of Hydrogen Production from Ammonia Decomposition

Ammonia (NH3) is regarded as a promising medium of hydrogen storage, due to its large hydrogen storage density, decent performance on safety and moderate storage conditions. On the user side, NH3 is generally required to decompose into hydrogen for utilization in fuel cells, and therefore it is vital for the NH3-based hydrogen storage technology development to study NH3 decomposition processes and improve the decomposition efficiency. Numerical simulation has become a powerful tool for analyzing the NH3 decomposition processes since it can provide a revealing insight into the heat and mass transfer phenomena and substantial guidance on further improving the decomposition efficiency. This paper reviews the numerical simulations of NH3 decomposition in various application scenarios, including NH3 decomposition in microreactors, coupled combustion chemical reactors, solid oxide fuel cells, and membrane reactors. The models of NH3 decomposition reactions in various scenarios and the heat and mass transport in the reactor are elaborated. The effects of reactor structure and operating conditions on the performance of NH3 decomposition reactor are analyzed. It can be found that NH3 decomposition in microchannel reactors is not limited by heat and mass transfer, and NH3 conversion can be improved by using membrane reactors under the same conditions. Finally, research prospects and opportunities are proposed in terms of model development and reactor performance improvement for NH3 decomposition.

Elimination of NH3 by Interfacial Charge Transfer over the Ag/CeSnOx Tandem Catalyst

Selective catalytic oxidation of NH3 is the most promising method for removing low-concentration NH3. However, achieving high activity and N2 selectivity remains a great challenge. A Ag/CeSnOx tandem catalyst with dual active centers was designed and synthesized, which couples the NH3 over-oxidation on the noble metal active sites with NOx reduction on the support. The tandem catalyst exhibited excellent NH3 selective catalytic oxidation (NH3–SCO) performance at 200–400 °C. Based on various characterization techniques and DFT calculations, it was identified that the silver species on the Ag/CeSnOx catalysts existed as AgO nanoparticles (AgO NPs), and the electrons on the support were more easily transferred to AgO NPs, which promoted the oxidation activity of AgO and the reduction performance of the CeSnOx support. The coupling between the AgO NPs and CeSnOx helped balance the NH3 oxidation rate and the NOx reduction rate. In addition, the uniform adsorption of gaseous NH3 on the oxidation and reduction sites was also demonstrated by theoretical calculations, which is a prerequisite for tandem catalysis. By in situ DRIFTS, we revealed that the NH3–SCO reaction over Ag/CeSnOx catalysts mainly follows the internal selective catalytic reduction mechanism. It was characterized by excessive oxidation of NH3 to NOx on AgO NPs. At a temperature lower than 200 °C, NOx was reduced to N2 by the adsorbed NH3 on the AgO. When the temperature was higher than 200 °C, NOx was reduced to N2 by NH3 or NH4+ adsorbed on the CeSnOx support. Therefore, the charge transfer at the Ag/CeSnOx catalyst interface and the coordination of atomic scale catalytic sites have realized the conversion of NH3 to N2 through NOx in a tandem catalytic mode.

N2-to-NH3 conversion by excess electrons trapped in point vacancies on 5f-element dioxide surfaces.

Ammonia (NH3) is one of the basic chemicals in artificial fertilizers and a promising carbon-free energy storage carrier. Its industrial synthesis is typically realized via the Haber-Bosch process using traditional iron-based catalysts. Developing advanced catalysts that can reduce the N2 activation barrier and make NH3 synthesis more efficient is a long-term goal in the field. Most heterogeneous catalysts for N2-to-NH3 conversion are multicomponent systems with singly dispersed metal clusters on supporting materials to activate N2 and H2 molecules. Herein, we report single-component heterogeneous catalysts based on 5f actinide dioxide surfaces (ThO2 and UO2) with oxygen vacancies for N2-to-NH3 conversion. The reaction cycle we propose is enabled by a dual-site mechanism, where N2 and H2 can be activated at different vacancy sites on the same surface; NH3 is subsequently formed by H- migration on the surface via associative pathways. Oxygen vacancies recover to their initial states after the release of two molecules of NH3, making it possible for the catalytic cycle to continue. Our work demonstrates the catalytic activities of oxygen vacancies on 5f actinide dioxide surfaces for N2 activation, which may inspire the search for highly efficient, single-component catalysts that are easy to synthesize and control for NH3 conversion.

Adsorption and catalytic oxidation of residual NH3 on coal ash after selective non-catalytic reduction in coal-fired boilers

Selective non-catalytic reduction (SNCR) with NH3 as the reducing agent is widely used for the denitrification of flue gas in coal-fired boilers, where fly ash significantly influences the conversion of the residual NH3 that does not participate in denitrification. However, there have been few studies on the exact nature of this influence, particularly the adsorption and reaction mechanisms of NH3 on fly ash. In this study, temperature-programmed desorption (TPD) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were used to study the mechanisms of NH3 adsorption and reactions over coal ash. In the absence of oxygen, in the temperature range of 50–450 °C, NH3 was adsorbed on the surface of the coal ash. The adsorption capacity of lignite ash was higher than that of anthracite ash. This difference was attributed to the large specific surface area and surface acidity of the lignite ash. However, between 450–850 °C, coal ash had a catalytic effect on NH3 decomposition and oxidation. Due to the high surface lattice oxygen content of lignite ash, its catalytic oxidative ability was superior to anthracite ash. Moreover, NH3 was first adsorbed over Lewis and Brønsted acid sites on the surface of coal ash and later underwent hydrogen abstraction to produce either the NH2 or the NH intermediate. The intermediates further reacted with the surface lattice oxygen of coal ash to produce NO and N2O. These results might be helpful for the management of NH3 residues from SNCR processes and the utilization of amino reducing agents in coal-fired boilers.

Catalytic activity and mechanism of selective catalytic oxidation of ammonia by Ag-CeO2 under different preparation conditions.

Given the problem of the high-temperature window of CeO2 catalyst activity, this study evaluated the catalytic properties of Ag/CeO2 prepared by changing the preparation methods and loadings. Our experiments showed that Ag/CeO2-IM catalysts prepared by the equal volume impregnation method could have better activity at lower temperatures. The Ag/CeO2-IM catalyst achieves 90% NH3 conversion at 200 °C, and the main reason is that the Ag/CeO2-IM catalyst has more vital redox properties, and the NH3 catalytic oxidation temperature is lower. However, its high-temperature N2 selectivity still needs to be improved and may be related to the less acidic sites on the catalyst surface. On both catalyst surfaces, the i-SCR mechanism governs the NH3-SCO reaction.

Revealing the origin of activity in phthalocyanine-based dual-metal sites towards electrochemical nitric oxide reduction.

Coordinated ligands play crucial roles in tuning the electrochemical nitrate reduction performance of phthalocyanine (Pc)-based dual atom catalysts. With the assistance of axial O ligands, fast NO to NH3 conversion can be realized on O-Ni2-Pc and O-Cu2-Pc. A 2-N product, N2O, can be synthesized on Co2-Pc, Cr2-Pc, O-Co2-Pc, and O-Fe2-Pc through N-N coupling with high NO coverage. ΔENO can be identified as a valid descriptor to support rational M2-Pc design.

NH3 decomposition over Ru/CeO2-PrOx catalyst under high space velocity conditions for an on-site H2 fueling station

From a commercialization perspective, NH3 decomposition catalysts used in on-site H2 fueling stations require a high NH3 conversion of over 99.5%, space velocity (SV) of 30,000 h−1 and reaction temperature of 500 °C. To achieve this target, we present Ru catalysts supported on CeO2−PrOx composites (Ru/CP) prepared by a coprecipitation method. The Ru/CP catalysts exhibited higher activity than the Ru/CeO2 and Ru/PrOx catalysts and achieved the target with Pr content (=Pr/(Ce+Pr)) of 33−67%. STEM observations and in situ X-ray absorption fine structure analysis revealed that an increase in the Pr content in the catalysts improved the Ru dispersion but inhibited the Ru metalation, owing to a strong “metal–support interaction”. Notably, 10% of Ru in the Ru/PrOx remains an oxide even when reduced at 550 °C. These results suggest that both Ru dispersion and metalation are enhanced at the optimal Pr content, maximizing the number of Ru active sites.

Numerical investigation of AdBlue film formation and NH3 conversion in generic SCR system using Eulerian stochastic fields method

In this paper, the Eulerian stochastic fields (ESF) method in a LES framework is applied to a generic selective catalytic reduction (SCR) configuration in order to retrieve seamlessly the effect of turbulence–chemistry interaction on the NH3-conversion and AdBlue film formation. The ESF method is based on the transport equation of a joint scalar filtered density function. The injection of AdBlue and relevant spray dynamics are modeled by the Lagrange-particle description where a computational parcel represents a cluster of real AdBlue droplets. Owing to the low injection pressure, weak atomization of AdBlue leads to intense droplet wall-impingement and film formation on the SCR duct wall and/or on the mixer elements, if present. A 2-D thin film approach is adopted to model the film formation and dynamics in combination with multi-regimes droplet–wall interaction model. After verification tasks against the reference data for simple gas phase reactive cases, the verified ESF method is coupled with a one-equation based LES turbulence model together with the Lagrange particle-tracking which is combined with the 2-D thin film approach to simulate the AdBlue injection and the film formation in the generic SCR configuration. At first, the assessment of the adopted LES mesh is carried out in terms of the so-called LES quality of index to determine the optimal mesh resolution. Next, to ensure the convergence with respect to the number of required ESF and mesh resolution, SCR simulations are performed by using various (2, 4, 6, 8, 12 and 16) stochastic fields and also with refined mesh. Thereby, the sensitivity of the predictive capability of the numerical tool is evaluated for the production of NH3 and HNCO with and without ESFs. This clearly indicates the importance of an accurate description of the turbulence–chemistry interaction to retrieve reliably the conversion of NH3. Finally, the film dynamics especially the evolution of the film thickness described with six required ESF and without ESF is compared with experimental data from the generic SCR configurations. To further demonstrate the potential of suggested numerical approach, a detailed numerical analysis is provided in terms of spray-impingement dynamics, scalar uniformity index, droplet life time and characteristics of the droplets prone to form solid deposits in the monolith channel.

Nickel Nanoparticles Anchored on Activated Attapulgite Clay for Ammonia Decomposition to Hydrogen

Ammonia decomposition to hydrogen technique is an effectively way to solve the problems associated with the storage and transportation of hydrogen, but the development of a high-performance catalyst for ammonia decomposition is a great challenge. Ni species supported on activated attapulgite clay (AATP) is prepared by a homogeneous precipitation method for ammonia decomposition to COx-free H2. The structural properties of the Ni/AATP catalysts are characterized by thermogravimetric analysis, X-ray diffraction, scanning and transmission electron microscopy, H2 temperature-programmed reduction, and N2 sorption technique. It is revealed that the porous structure and high surface area of rod-like symmetric AATP results in highly dispersed NiO particles because the presence of a strong interaction between AATP and NiO particles. In particular, the Si-OH in AATP can react with Ni species, forming Si-O-Ni species at the interface between Ni and AATP. The Ni/AAPT catalysts are used for ammonia decomposition, the 20%-Ni/ATTP catalyst shows a 95.3% NH3 conversion with 31.9 mmol min−1 gcat−1 H2 formation rate at 650 °C. This study opens a new way to utilize natural minerals as an efficient support of catalysts towards ammonia decomposition reaction.

Insights into the influence of water molecules on selective catalytic ozonation of gaseous ammonia into nitrogen on cryptomelane-type manganese oxide using in-situ DRIFTS

Catalytic ozonation is an environmentally friendly technology for the removal of gaseous NH3 due to high NH3 conversion and high N2 selectivity at ambient temperature. However, the influence mechanism of ubiquitous water vapor on catalytic ozonation of NH3 is unclear. In this study, cryptomelane-type manganese oxide (OMS-2) catalyst was prepared and tested for catalytic ozonation of NH3 in different relative humidity. The results showed that water vapor significantly decreased the catalytic activity, which was due to the inhibition of water on NH3 adsorption on Lewis acid sites and O3 decomposition on oxygen vacancies, as well as the combination of water with active oxygen species (O22− and Oatom). And the effect of water vapor on NH3 conversion was more significant than O3 decomposition because more Mn–OH were involved in the O3 decomposition under humid conditions. Combining in-situ DRIFTS results with the performance of NH3 oxidation, it is found that L-2 acid sites (the peak of NH3 adsorption on Lewis acid sites at 1188 cm−1) were the main active sites for adsorption and activation of NH3 in the early stage of catalytic reaction; as the reaction progressed, L-2 acid sites were gradually occupied by water and more Brønsted acid sites participated in the catalytic reaction. This work deepened the understanding of the reaction process for selective catalytic ozonation of NH3, and provided theoretical guidance for the design of efficient hydrophobic catalysts to eliminate gaseous NH3 pollution.

Designing Reactive Bridging O2- at the Atomic Cu-O-Fe Site for Selective NH3 Oxidation.

Surface oxidation chemistry involves the formation and breaking of metal-oxygen (M-O) bonds. Ideally, the M-O bonding strength determines the rate of oxygen absorption and dissociation. Here, we design reactive bridging O2- species within the atomic Cu-O-Fe site to accelerate such oxidation chemistry. Using in situ X-ray absorption spectroscopy at the O K-edge and density functional theory calculations, it is found that such bridging O2- has a lower antibonding orbital energy and thus weaker Cu-O/Fe-O strength. In selective NH3 oxidation, the weak Cu-O/Fe-O bond enables fast Cu redox for NH3 conversion and direct NO adsorption via Cu-O-NO to promote N-N coupling toward N2. As a result, 99% N2 selectivity at 100% conversion is achieved at 573 K, exceeding most of the reported results. This result suggests the importance to design, determine, and utilize the unique features of bridging O2- in catalysis.

Effect of Porous Catalyst Support on Plasma-Assisted Catalysis for Ammonia Synthesis.

We report on the effect of catalyst support particle porosity on the conversion of NH3 synthesis from N2 and H2 in a coaxial dielectric barrier discharge (DBD) plasma reactor. The discharge was created using an AC applied voltage with the reactor at room temperature and near atmospheric pressure (550 Torr). Two different particles of almost equal diameter (∼1.5 mm)─porous silica (SiO2) ceramic beads (average pore size: 8 nm) and smooth, nonporous soda lime glass beads─were compared in the DBD reactor. As the pore size in the SiO2 particles was smaller than the Debye length, penetration of the plasma into the pores of the particles was unlikely; however, reactive species generated in the plasma outside the particles could diffuse into the pores. The N2 conversion and energy yield of NH3 increased with applied voltage for both particle types, and these values were consistently higher when using the SiO2 beads. Discharge and plasma properties were estimated from Lissajous plots and using calculations with the BOLSIG+ software. The effect of these two different catalyst supports on the physical properties of the discharge was negligible. High resolution optical emission spectra revealed that the concentrations of N2+, atomic N, and atomic H (Hα, Hβ) in the plasma discharge were lower with the porous SiO2 beads than with the glass beads at every applied voltage tested. This indicates that these active species participate in heterogeneous reactions at support particle surfaces and that the larger surface area presented by the porous particles led to higher rates of depletion of these intermediates and a higher rate of ammonia synthesis.

Selective Epitaxial Growth of Ca2NH and CaNH Thin Films by Reactive Magnetron Sputtering under Hydrogen Partial Pressure Control.

Calcium compounds with N and H are promising catalysts for NH3 conversion, and their epitaxial thin films provide a platform to quantitatively understand the catalytic activities. Here we report the selective epitaxial growth of Ca2NH and CaNH thin films by controlling the hydrogen partial pressure (PH2) during reactive magnetron sputtering. We find that the hydrogen charge states can be tuned by PH2: Ca2NH containing H- is formed at PH2 < 0.04 Pa, while CaNH containing H+ is formed at PH2 > 0.04 Pa. In situ plasma emission spectroscopy reveals that the intensity of the Ca atomic emission (∼422 nm) decreases as PH2 increases, suggesting that Ca reacts with H2 and N2 to form Ca2NH at lower PH2, whereas at higher PH2, CaHx is first formed on the target surface and then sputtered to produce CaNH. This study provides a novel route to control the hydrogen charge states in Ca-N-H epitaxial thin films.

Rational tuning towards B-sites (B = Mn, Co, Al) on CoB2O4 binary oxide for efficient selective catalytic oxidation of ammonia

NH3 catalytic oxidation technology (NH3-SCO) is promising for the direct conversion of NH3 to N2 and H2O. Currently, balancing its NH3 catalytic oxidation activity and N2 selectivity remains a challenge. Herein, the redox properties and the surface oxygen vacancies of CoB2O4 were modulated to enhance the activity of the catalyst by using three elements (Co, Mn, Al) as the B-site elements of the CoB2O4 spinel. Eventually, CoMn2O4 synthesized with Mn as the B-site element possessed more surface oxygen vacancies and generated Mn3+-Mn4+ pairs that could facilitate the redox properties compared with CoAl2O4 and CoCo2O4 due to the interaction between Co and Mn, which in turn enhanced its catalytic activity together and enabled the complete conversion of NH3 at 150 °C. Additionally, using Mn as the B-site element of CoB2O4 also facilitated the generation of the Bronsted acid sites on the catalyst surface, which was beneficial to its N2 selectivity (65 % at 150 °C). In situ DRIFTs indicated the existence of two reaction mechanisms over CoMn2O4: the i-SCR mechanism with bidentate nitrate as the primary reactive intermediate species and the hydrazine mechanisms with –NH2 and N2H4 as the main intermediate species.