Catalysts For Oxidation Of Ammonia Research Articles

Active learning driven discovery of novel alloyed catalysts for selective ammonia oxidation

Alloy catalysts design faces the contradiction of small sample and huge screen space, especially in rational design with multi-objective and multi-constraints. Here, we conduct the active learning enabled innovative catalyst screening in alloy space composed of transition metal elements. First, the small dataset and large search space are constructed based on knowledge informed feature group. Then, the surrogate model based on Gaussian process regression are trained for predicting catalytic activity and N2 selectivity descriptors for selective catalytic oxidation of ammonia (NH3-SCO) process, merely based on the small dataset. Furthermore, coupling pareto solution and Bayesian optimization, the active learning driven material searching are performed in the huge alloy spaces with more than 1000 configurations. After 3 iterations with high throughput calculation on 30 alloyed configurations and further theoretical validation based on microkinetic and stability analysis, 9 ensemble configurations are considered as innovative alloy catalysts of NH3-SCO with reactivity and selectivity trade-off. Our study provides the reliable framework for catalyst design with multi-objective and multi-constraints when facing with small sample data and huge searching space and gives the specific guidance for rational design of NH3−SCO alloy catalysts.

Ammonia and toluene oxidation: Mutual activating effect of copper and cerium on catalytic efficiency

Copper and cerium containing hydrotalcite-like precursors Cux-Cey-Mg-Al were synthesized by mutual co-precipitation followed by calcination at 800 °C. The obtained mixed metal oxides were studied as catalysts for selective ammonia oxidation (NH3-SCO) and toluene combustion. Various techniques, such as temperature-programmed reduction with hydrogen, temperature-programmed desorption of toluene, scanning electron microscopy and reactive frontal chromatography were used to characterize the catalysts. Series 800-Cux-Ce2.5 showed the highest catalytic efficiency in both reactions, allowing complete NH3 and C7H8 conversion below 350 °C and 500 °C, respectively. CeO2 crystallites (12–18 nm), as well as dispersed CuO oxide forms were found in the most active samples. Both phases affect pollutants conversion, however, for ammonia oxidation the occurrence of Cu2+ is essential, while for toluene oxidation, the formation of Ce4+ is sufficient. Over the most active sample (800-Cu5-Ce2.5) the complete conversion in mutual oxidation of NH3 and toluene was achieved below 450 °C.

Designing ammonia oxidation catalysts via physical mixing of Pt/SiO2 and Cu/ZSM-5

Ammonia (NH3) is emerging as a carbon-neutral fuel alternative; however, its use poses environmental and health risks due to NH3 slip. The NH3-selective catalytic oxidation (NH3-SCO) process, which converts NH3 into N2 and H2O, offers a solution. However, it faces challenges with respect to the formation of byproducts, particularly N2O and NOx. This study focuses on designing NH3-SCO catalysts to enhance low temperature NH3 conversion efficiency while minimizing undesired byproducts. Pt/SiO2 catalysts were synthesized with adjusted Pt particle sizes to improve the NH3-SCO activity and were physically mixed with Cu/ZSM-5 to overcome the low N2 selectivity of Pt-based catalysts by utilizing the selective catalytic reduction (SCR) ability of Cu/ZSM-5. Pt1Cu3 catalyst, a physical mixture of Pt/SiO2 and Cu/ZSM-5 in a 1:3 mass ratio, was found to be the most effective, achieving complete NH3 oxidation at 250 ℃ while maintaining a high N2 selectivity of approximately 80%. Further investigation revealed that the performance of the Pt1Cu3 catalyst was significantly influenced by the arrangement and physical proximity of the Pt/SiO2 and Cu/ZSM-5 catalysts, with a closely contacted mixture showing synergistic effects that enhanced activity and selectivity. These results suggest that the Pt1Cu3 catalyst offers a promising solution for NH3 emission control by utilizing a balanced approach that combines SCO and SCR reactions.

Temperature-driven dynamic evolution process of Ag species on silver-loaded Zr0.2Sn0.8O2 catalyst for selective catalytic oxidation of ammonia

Silver-based catalysts are extensively utilized in the selective catalytic oxidation of ammonia (NH3-SCO). However, enhancing low-temperature activity often causes a decline in N2 selectivity, presenting a substantial challenge in balancing activity and N2 selectivity. In this work, we introduced zirconium into the SnO2 support, resulting in a significant enhancement of low-temperature activity by 200 °C for the Ag/Zr0.2Sn0.8O2 catalyst; yet, this also led to the formation of N2O at low temperatures and NOx at high temperatures. Characterizations such as XPS and in situ DRIFTS revealed that the dynamic changes of silver species driven by temperature had a profound impact on the reaction mechanism and guided the formation of different products. Below 200 °C, the predominance of Ag+ directed the imide mechanism, with the –HNO species being the key intermediate leading to the production of N2O. Above 300 °C, Ag0 became dominant, favoring the internal selective catalytic reduction (i-SCR) mechanism and the production of NOx. This work provides novel insights into improving N2 selectivity of Ag-based catalysts and contributes to a deeper understanding of the reaction mechanisms.

Active regeneration particulate filter operating cycle theoretical justification

Introduction (problem statement and relevance). Particulate filters or traps used in diesel vehicles to catch suspended particles require continuous and/or periodic regeneration in order to maintain operability. The order of diesel particulate filter loading and its regeneration can be presented in the form of an algorithm. Specific features of particulate filter functioning both in regular conditions and during active regeneration have been studied in a number of scientific papers, and the results obtained in them can be considered when developing the algorithm.The purpose of the work is development of the algorithm of the operating cycle of the diesel vehicle particulate filter including modes of filter loading, active regeneration and troubleshooting.Methodology and methods. Information from scientific literature regarding specific features of diesel vehicle exhaust gas aftertreatment systems operation, namely regarding particulate filters, has been collected under this study. The study assumes a noncatalytic diesel particulate filter that is contained in the system together with a diesel oxidation catalyst, nitrogen oxide reduction catalyst and ammonia oxidation catalyst. Particulate matter sensors, temperature sensors, pressure sensors, exhaust-gas flow sensors and flow sensors for fuel to burn the soot accumulated in the filter have been used as measuring instrumentation of system indicators.Results. The created algorithm of the particulate filter operating cycle included the loading mode with definition of parameters for switching to active regeneration, definition of the transition moment between the stages of deep filtration and filtration in the soot layer. Active regeneration consisted of a sequence of two controlled fuel injections for combustion of soot in the filter and occurrence of the temperature impulse between the injections with selection of the most suitable level of exhaust gas flow rate as well as the control over the filter temperature and content of oxygen. The problems of malfunctions of the treatment system were solved by controlling the fuel supply during regeneration.Scientific novelty. The peculiarities of processes in the particulate filter during active regeneration were considered in the algorithm development, including ensuring high efficiency of soot removal from the filter.Practical significance. The algorithm can be used to develop a procedure to perform bench and driving tests of diesel vehicles as well as it can be applied in a diesel engine during vehicles operation

Quantitative Chemical Mapping of Pt/Rh Gauze Catalysts for Ammonia Oxidation using Resonant X-ray Tomography

Quantitative Chemical Mapping of Pt/Rh Gauze Catalysts for Ammonia Oxidation using Resonant X-ray Tomography

Electrochemical oxidation of ammonia in water by Pt/NbC membrane-based catalytic nanofluid reactor

Electrochemical oxidation has garnered increasing attention for its potential in treating ammonia in wastewater. However, achieving both high conversion and high nitrogen selectivity simultaneously, while maintaining a short hydraulic retention time (HRT), remains a significant challenge. In this study, we constructed an efficient niobium carbide (NbC) nanoporous membrane reactor by incorporating metal platinum as a catalyst for the electrochemical oxidation of ammonia. The electrocatalytic oxidation process occurred within the nanochannels of the membrane. This design allowed for enhanced single-pass removal of ammonia due to the short diffusion distance on the nanometer scale, which subsequently increased the mass transfer efficiency. Remarkably, we achieved highly controllable pore sizes in the NbC membrane, ranging from 200 to 8.3 nm by dip-coating method. Without the addition of any oxidants, the single-pass removal of ammonia reached 100 % when the applied potential was set at 4 V vs. SCE on a membrane with a pore size of 8.3 nm, and an HRT of 249 s. The primary product obtained from this process was nitrogen, displaying a selectivity of 88.09 %, with a small percentage of nitrate production (selectivity of 11.91 %). By decreasing the applied potential or increasing the pore size, the selectivity of nitrogen can be further improved, reaching 100 %. However, this adjustment may lead to a decrease in the conversion of ammonia. DFT results have revealed that the production of nitrogen on the Pt (111) surface occurs through the NH4+→NH3 → NH2 → N → N2 pathway. This finding contributes to the understanding of the electrochemical process for ammonia removal in water, and provides a viable option for the design of efficient nanomembrane reactors.

Surface States Governing the Activity and Selectivity of Pt-Based Ammonia Slip Catalysts for Selective Ammonia Oxidation

Surface States Governing the Activity and Selectivity of Pt-Based Ammonia Slip Catalysts for Selective Ammonia Oxidation

Enhancing activity and non-deactivating stability on N-modified TiO2 catalyst for visible-light photocatalytic oxidation of ammonia at room temperature

Nitrogen modified TiO2 catalysts (N-TiO2(1)) with high photocatalytic activity and non-deactivating stability were prepared by using urea as nitrogen source. Its efficiency of photocatalytic oxidation of NH3 (NH3-PCO) remained above 80 %, peaking at 88 % at room temperature over 72 h, while N2 selectivity also improved noticeably, stabilizing at approximately 87.1 %. Conversely, the NH3 conversion on pure TiO2 plateaued at approximately 67 % within 72 h, peaking at only 72.8 %, with nitrogen selectivity not exceeding 55 %. N element was successfully doped into TiO2 lattice system in the form of O-Ti-N, which reduced the band gap energy from 3.18 eV to 3.07 eV and improved the light utilization rate. The water molecules formed during the reaction contribute to the in-situ reaction to continually generate hydroxyl radicals (·OH), thereby conferring greater reaction stability. By-products NOx- and NOx are formed during the reaction process, while NOx will be also converted to N2 by initial selective catalytic reduction (i-SCR) mechanism. These findings present a novel perspective for enhancing the stability of NH3-PCO active agents and have substantial applications within the parameters of air pollution control.

Development of a Robust Electrochemical Ammonia Electrolysis Protocol Using in Operando Quantitative Analysis

Ammonia electrolysis is an eco-friendly technology that shows promise for producing clean hydrogen. Compared to water electrolysis, ammonia electrolysis only requires a low external voltage of 0.06 V to produce hydrogen. This research proposes a simple protocol for evaluating the practical properties of electrocatalysts for ammonia electrolysis using in-operando gas chromatography. The protocol allows for reliable and efficient comparison and evaluation of new ammonia oxidation catalysts, including distinguishing between competitive oxidation reactions and monitoring them in real-time. Additionally, the author investigated the correlation between physical or electrochemical surface areas and stability performances to estimate practical stability based on half-cell analysis. The use of this rigorous protocol should aid in evaluating the practical performance of ammonia oxidation and contribute to identifying viable pathways for the practical electrochemical oxidation of ammonia to hydrogen.

Lattice-Stabilized Chromium Atoms on Ceria for N2O Synthesis.

The development of selective catalysts for direct conversion of ammonia into nitrous oxide, N2O, will circumvent the conventional five-step manufacturing process and enable its wider utilization in oxidation catalysis. Deviating from commonly accepted catalyst design principles for this reaction, reliant on manganese oxide, we herein report an efficient system comprised of isolated chromium atoms (1 wt %) stabilized in the ceria lattice by coprecipitation. The latter, in contrast to a simple impregnation approach, ensures firm metal anchoring and results in stable and selective N2O production over 100 h on stream up to 79% N2O selectivity at full NH3 conversion. Raman, electron paramagnetic resonance, and in situ UV-vis spectroscopies reveal that chromium incorporation enhances the density of oxygen vacancies and the rate of their generation and healing. Accordingly, temporal analysis of products, kinetic studies, and atomistic simulations show lattice oxygen of ceria to directly participate in the reaction, establishing the cocatalytic role of the carrier. Coupled with the dynamic restructuring of chromium sites to stabilize intermediates of N2O formation, these factors enable catalytic performance on par with or exceeding benchmark systems. These findings demonstrate how nanoscale engineering can elevate a previously overlooked metal into a highly competitive catalyst for selective ammonia oxidation to N2O, paving the way toward industrial implementation.

Ammonia as a Marine Fuel towards Decarbonization: Emission Control Challenges

Decarbonization of the maritime sector to achieve ambitious IMO targets requires the combination of various technologies. Among alternative fuels, ammonia (NH3), a carbon-free fuel, is a good candidate; however, its combustion produces NOx, unburnt NH3 and N2O—a strong greenhouse gas (GHG). This work conducts a preliminary assessment of the emission control challenges of NH3 application as fuel in the maritime sector. Commercial catalytic technologies are applied in simulated NH3 engine exhaust to mitigate NH3 and NOx while monitoring N2O production during the reduction processes. Small-scale experiments on a synthetic gas bench (SGB) with a selective-catalytic reduction (SCR) catalyst and an ammonia oxidation catalyst (AOC) provide reaction kinetics information, which are then integrated into physico-chemical models. The latter are used for the examination of two scenarios concerning the relative engine-out concentrations of NOx and NH3 in the exhaust gas: (a) shortage and (b) excess of NH3. The simulation results indicate that NOx conversion can be optimized to meet the IMO limits with minimal NH3 slip in both cases. Excess of NH3 promotes N2O formation, particularly at higher NH3 concentrations. Engine-out N2O emissions are expected to increase the total N2O emissions; hence, both sources need to be considered for their successful control.

Bi-functional two-dimensional cobalt silicate catalyst for selective catalytic oxidation of ammonia

We have newly synthesized the zeolitic cobalt silicate catalysts via a simple hydrothermal treatment of borosilicate MWW precursor with Co precursor solution. As the hydrothermal temperature increased, a structural transformation occurred, shifting from three-dimensional (3D) to two-dimensional (2D) delaminated MWW layers (DML), and further to 2D amorphous phyllosilicate, accompanied by a continuous increase in Co content. The synthesized bi-functional 2D cobalt silicate facilitates the selective catalytic oxidation of NH3 (NH3-SCO), a process that requires both surface acidity and redox characteristics. In particular, Co-DML-160 prepared by hydrothermal treatment at 160 °C exhibited the coexistence of Co3O4 and framework Co species resulting in a strong redox ability and high Lewis acidity, respectively, due to their synergetic effect.

Computational design and experimental validation of monometallic-doped SnO2 catalysts for selective catalytic oxidation of ammonia

Due to the intricate structure of catalysts, traditional catalyst design relies on iterative trial-and-error experiments. We have systematically established a catalyst design strategy to evaluate the performance of NH3-SCO reactions from the perspective of DFT calculations. Specifically, we used the catalyst formation energy as a stability descriptor and the adsorption energies of NH3, NH2, and O p-band center as performance descriptors, we identified the four most promising catalysts among 45 kinds of doped SnO2 catalysts. Subsequently, experimental validation was performed to demonstrate the outstanding consistency between the DFT-driven descriptors and the stability and catalytic performance of the catalyst. Particularly, the Ce doping resulted in a 175 °C reduction in the T90 compared to the SnO2 catalyst. It is noteworthy that Ce doping promotes the cycling between oxygen vacancies and lattice oxygen, which contributes primarily to the enhancement of O2 activation capability and, consequently, the improvement in catalytic activity.

PGM-Based Ammonia Oxidation Catalysts for Anion-Exchange Membrane Direct Ammonia Fuel Cells

Low-temperature direct ammonia fuel cells (DAFCs) use carbon-neutral ammonia as a fuel, which has attracted increasing attention recently due to ammonia's low source-to-tank energy cost, easy transport and storage, and wide availability. However, current DAFC technologies are greatly limited by the kinetically sluggish ammonia oxidation reaction (AOR) at the anode. Herein, we report an AOR catalyst, in which ternary PtIrZn nanoparticles with an average size of 2.3 ± 0.2 nm were highly dispersed on a binary composite support comprising cerium oxide (CeO2) and zeolitic imidazolate framework-8 (ZIF-8)-derived carbon (PtIrZn/CeO2-ZIF-8) through a sonochemical-assisted synthesis method. The PtIrZn alloy, with the aid of abundant OHad provided by CeO2 and uniform particle dispersibility contributed by porous ZIF-8 carbon (surface area: ∼600 m2 g−1), has shown highly efficient catalytic activity for the AOR in alkaline media, superior to that of commercial PtIr/C. The rotating disk electrode (RDE) results indicate a lower onset potential (0.35 vs. 0.43 V), relative to the reversible hydrogen electrode at room temperature, and a decreased activation energy (∼36.7 vs. 50.8 kJ mol−1) relative to the PtIr/C catalyst. Notably, the PtIrZn/CeO2-ZIF-8 catalyst was assembled with a high-performance hydroxide anion-exchange membrane to fabricate an alkaline DAFC, reaching a peak power density of 91 mW cm−2. Unlike in aqueous electrolytes, supports play a critical role in improving uniform ionomer distribution and mass transport in the anode. PtIrZn nanoparticles on silicon dioxide (SiO2) integrated with carboxyl-functionalized carbon nanotubes (CNT–COOH) were further studied as the anode in a DAFC. A significantly enhanced peak power density of 314 mW cm−2 was achieved. Density functional theory calculations elucidated that Zn atoms in the PtIr alloy can reduce the theoretical limiting potential of *NH2 dehydrogenation to *NH by ∼0.1 V, which can be attributed to a Zn-modulated upshift of the Pt–Ir d-band that facilitates the N–H bond breakage.

A promising catalyst for catalytic oxidation of chlorobenzene and slipped ammonia in SCR exhaust gas: Investigating the simultaneous removal mechanism

As the emission limits for NOx become further stringent, development of a synergistic control technology to prevent emissions of slipped ammonia (NH3) and chlorinated organics from steel production and waste incineration is both a prospect and a challenge for the environmental issues. In this work, the designed ruthenium-based catalyst (RuNb/ST) showed the excellent chlorobenzene (CB)/NH3 synergistic removal performance and N2/CO2 selectivity above 300 ℃. The activity of NH3 and CB were suppressed at relatively low temperature due to the competitive adsorption of CB and NH3. Whereas, there was a significant improvement in the N2/CO2 selectivity at relatively high temperature. The removal mechanism was studied by experiments, in which the accumulation of chlorine species, the role of NO2 and the oxidation of CB were considerable. In contrast to the carrier, the vast majority of the chloride ions adsorbed onto the active sites after dissociating from CB could be efficiently removed in form of Cl2 via the Deacon Reaction catalyzed by RuO2. CB was predominantly converted into CO2 and H2O with a fraction persisting as organics. Partial of organics were attacked by Cl ions to generate organic chlorine-containing by-products. The presence of NO2 molecule was found to promote the cleavage of the benzene ring within CB, and expedited the carbon migration towards the ultimate product of CO2, resulting in a notable reduction of byproducts.

Tuning surficial atomic configuration of Pt–Ir catalysts for efficient ammonia oxidation and low-temperature direct ammonia fuel cells

Low-temperature direct ammonia fuel cell via ammonia oxidation reaction (AOR) is one of the most attractive ways for ammonia utilization. The Pt–Ir alloyed catalysts have been proven greatly efficient in AOR to overcome the sluggish kinetics and complex multi-electron processes; however, it still remains challenging for further improvement because of the complexity of alloyed types and unclear fundamental understanding. Herein, we systematically fabricated a series of PtxIry/XC-72 catalysts via tuning the Pt/Ir compositions and these catalysts demonstrate significant composition-dependent behaviours in AOR, whereas the Pt favours increasing current densities and the Ir facilitates reducing onset potentials. Importantly, the actual surface Pt/Ir atomic configuration is totally different with the classic view for homogeneous alloys by combining catalytic and characteristic analyses. The Pt prefers to be segregated on the topmost surface. Then the quantified correlations between current density/onset potential and Pt composition in Pt − Ir nanoparticles (NPs) were further established and discussed. The onset potential of AOR is significantly reduced by Ir incorporation while it is insensitive to Ir contents. Accordingly, the well-studied Pt5Ir5/XC-72 catalyst was further employed in an alkaline exchange membrane fuel cell, exhibiting a peak power density (230.0 mW cm−2). Our work provides insights into the effect of Pt/Ir compositions on AOR, with the goal of catalysts engineering for AOR and ammonia fuel cells.

Designing a Bifunctional Pt/Cu-SSZ-13 Catalyst for Ammonia-Selective Catalytic Oxidation with Superior Selectivity

Ammonia-selective catalytic oxidation (NH3-SCO) is an effective technology to solve the problem of ammonia leakage from diesel vehicles, while ammonia oxidation catalysts (AOC) still suffer from insufficient low-temperature activity and poor nitrogen selectivity. Herein, we prepared a bifunctional Pt/Cu-SSZ-13 catalyst containing an SCO component (Pt) and a selective catalytic reduction (SCR) component (Cu). This bifunctional catalyst exhibited excellent catalytic performance during NH3 oxidation under practical conditions containing water vapor, achieving >85% N2 selectivity over the whole temperature range. Catalysts were systematically characterized by Brunauer–Emmett–Teller, X-ray diffraction, inductively coupled plasma optical emission spectrometry, X-ray photoelectron spectroscopy, H2 temperature-programmed reduction, NH3 temperature-programmed desorption, high-angle annular dark-field scanning transmission electron microscopy, and X-ray absorption fine structure analysis. The results showed that metallic Pt nanoparticles were mainly present on the exterior surface of the Pt/Cu-SSZ-13 zeolite catalyst, while dispersed Cu2+ cations were present in the interior of the zeolite. Based on operando diffuse reflectance infrared Fourier transform spectroscopy with mass spectrometry experiments, it was confirmed that on Pt/SSZ-13, over-oxidation of NH3 produced large amounts of N2O and NOx at low and high temperatures, respectively, resulting in a poor N2 selectivity. With Cu present, in contrast, the byproduct (NO) would further react with adsorbed NH3 on Cu Lewis acid sites to produce N2 by a tandem SCR reaction, thus contributing to the superior N2 selectivity of the Pt/Cu-SSZ-13 bifunctional catalyst.

High throughput screening of noble Metal-Free High-Entropy alloys catalysts for selective catalytic oxidation of NH3

The high price and scarcity of Pt require the development of alternative low-cost catalysts for selective catalytic oxidation of ammonia (NH3-SCO). This work uses first-principles-based high-throughput calculations to evaluate noble metal-free high-entropy alloys (NMF-HEAs) as potential NH3-SCO candidate catalysts to replace Pt. Firstly, by employing empirical rules to determine the formation probability of single-phase solid solution, 350 NMF-HEAs are found to be single-phase solid solution among the vast configuration space (about 104) consisting of 20 non-noble metals. Among these, NMF-HEAs containing Cu and Zn elements are identified as promising catalysts based on the reactivity and selectivity rules of the NH3-SCO process. Further analysis of elemental composition and distribution on configurations with N-atom adsorption energies reveals that the co-existence of Cu and Zn elements and the symmetric distribution of surface atoms is conducive to tuning N-binding ability in the optimal region. Finally, full reaction paths of NH3-SCO processes on the optimal CuZnNiMoCo HEA are calculated and confirm their activity comparable to that of Pt catalysts and better selectivity. This study gains insight into the effects of the composition and surface configuration of HEAs on the NH3-SCO process and provides a theoretical framework for designing and optimizing efficient catalysts of the NH3-SCO process.

Towards FIB-SEM Based Simulation of Pore-Scale Diffusion in SCR Catalyst Layers

The diffusivity in the upper Cu-Chabazite layer of a dual layer ammonia oxidation catalyst with a lower Pt layer was investigated. In a first step, the pore structure of the upper Cu-Chabazite catalyst layer was determined by Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) slice&view tomography. From the FIB-SEM data the 3D pore structure of the catalyst was reconstructed and diffusion simulations were performed on the reconstructed pore geometry, resulting in an estimated effective diffusivity of Deff/Dgas = 0.31. To validate the FIB-SEM derived estimates of the diffusivity, measurements of CO oxidation on the dual layer catalyst were performed, where the CO was oxidized in the lower Pt-layer while the upper SCR layer served as an inactive diffusion barrier. In this way, the effective diffusivity can be determined from the measured CO conversion. An effective diffusion coefficient of Deff/Dgas = 0.11 was obtained from the CO oxidation measurements, three times lower than the value obtained from the FIB-SEM data, but in line with previous literature data for the effective diffusivity in monolith washcoat layers. Additional NH3 oxidation experiments were performed on the dual layer catalyst. The results were well reproduced by a reactor model applying the effective diffusion coefficient obtained by the CO oxidation experiments. The origin of this apparent inconsistency is currently not understood and requires further investigation.