NH3 Oxidation Research Articles

Unveiling the remarkable deNOx performance of MnMoVOx catalysts via dual regulation of the redox and acid sites

Developing NH3-SCR catalysts possessing excellent NO conversion, N2 selectivity, and alkali-tolerance at low-temperatures remains a great challenge. MnOx-based catalysts have attracted attention due to their exceptional low-temperature NH3-SCR activity. However, their strong oxidative capabilities often lead to excessive NH3 oxidation, causing a narrow operating temperature window and low N2 selectivity. In this study, we employed Mo and V to simultaneously fine-tune the acid and redox sites of MnOx, effectively suppressing the excessive NH3 oxidation at medium-high temperatures. The adjusted Mn0.90Mo0.09V0.01Ox catalyst demonstrated excellent low-temperature activity, a significantly broader active temperature window, and robust resistance to alkali metal poisoning. Multiple characterization results indicate that this dual acid-redox sites regulation strategy appropriately weakens the oxidative capacity while notably enhances surface acidity of the catalyst. Furthermore, the combination of in situ DRIFTS and DFT calculations reveal that following the adjustments of Mo and V on MnOx, new Brønsted acid sites are generated. Besides, the regulated catalyst evidently inhibits NO adsorption and nitrate species formation, thus promoting the reaction exclusively through the E-R mechanism, resulting in boosted N2 selectivity. This study also demonstrates that the optimum NH3-SCR performance of catalyst is achieved when oxidation ability and acid sites are harmoniously balanced, rather than following a "stronger is better" trend. In addition, through effective dual-active site regulation, the K-poisoning catalyst retains satisfactory catalytic activity. Thereby, the proposed dual-active sites regulation strategy in this study offers beneficial insights for the development of high-performance denitration catalysts.

NH3 absorption line study and application near 1084.6 cm−1

Ammonia (NH3) holds significant importance as an industrial product, ambient trace pollutant, and a promising zero-carbon energy carrier, underscoring the critical need for accurate quantification in various applications. In this study, a mid-infrared quantum cascade laser (Mid-IR QCL) based tunable diode laser absorption spectroscopy (TDLAS) spectrometer was developed. It was coupled with two reactors, namely a gas cell and a shock tube and operated in three modes: low- and high-frequency (140 Hz and 10 kHz) scan modes and fixed-wavelength mode, tailored to the specific needs of each experimental scenario. The primary objects were NH3 line parameters measurement and absolute NH3 quantification. With the 140 Hz low-frequency scan mode, NH3-Ar pressure broadening coefficients of six transition lines near 1084.6 cm−1 were measured in a gas cell at 295 K and 1–11 mbar for the first time. These coefficients were subsequently employed to quantify the initial NH3 mole fraction in shock tube experiments, considering the substantial adsorption effect of NH3. Using the high-frequency scan mode (10 kHz), spectrally resolved NH3 absorption cross-sections were obtained in a shock tube across a pressure range of 1.46–3.25 bar and a temperature range of 1000–1800 K. This data, acquired for the first time, significantly contributes to the understanding of NH3 behavior in high-temperature environments. Additionally, potential interference from high-temperature water absorption near 1084.6 cm−1 was thoroughly examined. The high-temperature water absorption cross-section was measured over a pressure range of 0.99–3.02 bar and a temperature range of 1322–2905 K, utilizing a near-infrared water laser. Leveraging these comprehensive datasets, the study quantified the NH3 mole fraction during the NH3 oxidation process in the shock tube, employing the fixed-wavelength mode to achieve high time resolution. The quantified NH3 mole fraction was then compared to predictions from eleven recent chemical kinetic mechanisms, demonstrating the utility of the developed spectrometer in validating these mechanisms.

A novel bifunctional Pt/Ce/WZrOx catalyst for efficient selective oxidation of high-concentration NH3

A novel bifunctional catalyst with excellent NH3-SCO and NH3-SCR functions is effective to achieve excellent low-temperature activity and high N2 selectivity simultaneously, especially for removing high-concentration NH3 slipped from NH3-fuel engine exhaust gas. Herein, a small amount of noble metal Pt (0.1 wt%) was introduced in Ce/WZrOx to synthesize a novel bifunctional NH3-SCO catalyst. The experimental results showed that deposition order of Pt and Ce active species supported on WZrOx had a significant influence on NH3-SCO performance. Pt/Ce/WZrOx catalyst prepared through impregnation-precipitation method exhibited a much superior comprehensive NH3 removal performance compared to Pt/Al2O3 catalyst (Pt 0.1 wt%). It achieved an NH3 conversion of 96.4 % and a N2 selectivity of 94.2 % at 310 °C. A series of characterization tests were adopted to compare the physicochemical properties of Pt/Ce/WZrOx and Ce/WZrOx catalysts to reveal the possible functions of Pt and Ce active species in NH3-SCO reactions. The results of H2-TPR implied that there was a strong interaction between the Pt and Ce active species, which greatly enhanced the low-temperature redox property of Pt/Ce/WZrOx catalyst. Besides, electron transfer between Pt0/Ptn+ and Ce3+/Ce4+ cycles facilitated the generation of oxygen vacancies on the Pt/Ce/WZrOx catalyst surface, which played a key role in the reaction for removing high-concentration NH3. In-situ DRIFTS results suggested that NH3-SCO reactions over Pt/Ce/WZrOx catalyst might follow both imide (–NH) and hydrazine (-N2H4) mechanisms.

Experimental and kinetic modeling study on ammonia/polyoxymethylene dimethyl ether 2 oxidation in a jet-stirred reactor

The oxidation of ammonia/polyoxymethylene dimethyl ether 2 (NH3/PODE2) was investigated in a jet-stirred reactor with ammonia addition of 10 %, 25 %, and 50 % in mole fraction. Atmospheric experiments were conducted at an equivalence ratio of 1.0, within the temperature range of 500–1100 K. The initial fuel mole fraction and residence time were fixed at 0.5 % and 2 s, respectively. PODE2 and NH3, along with other intermediates and products such as C1-C2 hydrocarbons and permanent gases, were analyzed using gas chromatography and Fourier transform infrared spectroscopy. The experimental results demonstrate that the addition of NH3 has little impact on the consumption of PODE2, primarily influencing the mole fraction of hydrocarbon intermediates. All three blends exhibited negative temperature coefficient (NTC) behaviors. However, NTC behavior was weakened as the NH3 blending ratios increased due to the competition for O2 at low temperatures. A detailed kinetic model for PODE2/NH3 blends was constructed, incorporating updated rate coefficients from the recent literature and analogy methods. The model also considers the interaction between PODE2 and NH3, which was investigated through reaction pathway analysis and sensitivity analysis. Simulated results show that the mole fraction profiles are reasonably predicted by this model at the test conditions. The interaction between PODE2 and NH3 is facilitated by CH3, NH2 radicals, and NO-NO2 cycle. The rapid reversible reaction between NO and NO2 alters the reaction pathway of radicals in the blends. At low temperatures, PODE2 in blend follows the same oxidation pathway as pure PODE2, while NH3 oxidation is affected by the introduction of additional HO2 and H from PODE2. New OH formation pathways involving NH2, NO, and NO2 occur, while the decomposition of H2O2 is inhibited, thereby affecting the oxidation of PODE2 at intermediate temperatures. Additionally, a notable decrease in the intermediates with carbon–carbon double bonds is observed, due to the consumption of CH3 through the reaction NO2 + CH3 = NO + CH3O. This important reaction also promotes the oxidation of CH3 and the NO-NO2 cycle, resulting in an improvement in system reactivity.

Roles of NH2 reactions in ammonia oxidation at intermediate temperatures: Experiments and chemical kinetic modeling

In recent years, a number of experimental and numerical studies on chemical kinetics of ammonia (NH3) have been performed and NH3 chemical kinetic models have been proposed. However, as NH3 oxidation at intermediate temperatures (below 1400 K) has been investigated by reactor experiments, discrepancies between measurements and model-prediction have been reported. In addition, NH3 oxidation in extremely fuel-rich conditions has not been well studied. This study aims to further explore the chemical kinetics of NH3 oxidation at intermediate temperatures using a micro flow reactor with a controlled temperature profile (MFR). NH3 and H2 in the oxidation of NH3/O2/Ar mixtures at atmospheric pressure were measured using a gas chromatograph and a quadrupole mass spectrometer connected to the MFR. The maximum wall temperatures were varied in the range of 1100–1400 K, and six equivalence ratios (ϕ = 0.5–4.0) were examined. An updated reaction model for NH3 was developed by placing special emphasis on NH2 reactions. The revised model satisfactorily reproduced the experimental and literature data. Reaction analysis revealed that the main channel of the NH2 + HO2 reaction is chain-terminating, as proposed by Klippenstein et al., which was critically important to reproduce the measured profiles of NH3 in the MFR. Furthermore, the contribution of NH2 + NH2 = N2H2 + H2 to H2 production showed large differences among the models because of large uncertainty of its rate constant. Models that used larger rate constants for this reaction overestimated H2 production. Novelty and significance statementThis study addresses unrevealed intermediate temperature reactions in NH3 oxidation where large discrepancies between experiments and model predictions as well as among models have been reported. Species profiles of NH3 and H2 in NH3 oxidation at intermediate temperatures (1100–1400 K) over a wide range of equivalence ratio (0.5–4.0) was obtained using a micro flow reactor with a controlled temperature profile. The model developed in the present study (adopted the rate constants of NH2 reactions in the latest literatures) satisfactorily reproduced the experimental results as well as literature data. Branching reactions of NH2 + + HO2 and NH2 + NH2 are found to be crucial for the NH3 oxidation and H2 production at intermediate temperatures, which would enhance further improvement of the ammonia chemical kinetic model and moreover, help control ammonia combustion.

Electrochemical Synthesis of the Energetic Combustion Catalyst Co(BODN)·9H2O and Its Catalytic Effect on Ammonium Perchlorate Thermal Decomposition.

Safe, efficient, and green synthetic energetic combustion catalysts are of great importance for the application of ammonium perchlorate (AP) in solid propellants. In this study, a novel, simple, efficient, and green electrochemical method for synthesizing energetic combustion catalysts was designed and implemented to successfully synthesize Co(BODN)·9H2O (BODN = [2,2'-bi{1,3,4-oxadiazole}]-5,5'-dinitramide), a novel energetic combustion catalyst. The target products were characterized via single-crystal X-ray diffraction, powder X-ray diffraction, Fourier transform infrared spectroscopy, optical microscopy, scanning electron microscopy, differential scanning calorimetry, and thermogravimetric analysis. Results reveal that Co(BODN)·9H2O crystallizes in the triclinic P1̅ space group and has a density of 1.836 g cm-3. The size of the Co(BODN)·9H2O crystal increases gradually with the increase in the reaction current and the prolongation of the reaction time, respectively. However, the change in reaction current and time does not affect the crystal form. In addition, with the increase in Co(BODN)·9H2O content, the peak temperature of high-temperature decomposition (HTD) and apparent activation energy of AP/Co(BODN)·9H2O gradually decrease, and the heat release during thermal decomposition gradually increases. The HTD peak temperature and apparent activation energy of AP/Co(BODN) 9H2O (10%) decrease by 97.9 °C and 94.2 kJ·mol-1, respectively, compared with those of pure AP, and the heat release during thermal decomposition increases by 1613 J·g-1. Furthermore, compared with those of the propellant containing pure AP, the burning rate and flame temperature of the propellant containing AP/Co(BODN)·9H2O (10%) increase by 8.15 mm s-1 and 458.44 °C, respectively. Real-time Fourier transform infrared spectroscopy reveals that CoO catalyzes the thermal decomposition of AP mainly by promoting electron transfer to accelerate the oxidation of NH3 and the conversion of N2O to NO. In brief, this work provides new insights into synthesizing energetic combustion catalysts. Moreover, Co(BODN)·9H2O synthesized through the electrochemical method exhibits considerable application prospects for improving the thermal and energy performance of AP and the combustion performance of propellants.

Production of Oximes Directly from Sustainable Lignocellulose-Derived Aldehydes and Ammonia over HTS-1 Catalyst.

Oxime chemicals are the building blocks of many anticancer drugs and widely used in industry and laboratory. A simple but robust hierarchically porous zeolite (HTS-1) catalyst was prepared by hydrothermal methods and used for the preparation of vanillin oxime from vanillin in NH3 ⋅ H2 O/DIO (v/v 1/10) system. The results of the catalyst characterization showed that the larger pore size and more framework Ti were conducive to promote the transformation of the substrates. The conversion of vanillin and the yield of vanillin oxime were both higher than 99 % under optimized reaction conditions. It was found that the reaction proceeded by oxidation of NH3 to hydroxylamine (NH2 OH), and oximation of hydroxylamine with vanillin to obtain vanillin oxime, where the rate-controlling step was the hydroxylamine formation, and the apparent activation energy was 26.22 kJ/mol. The corresponding oximation products could also be obtained by extending this method to other compounds derived from lignin. Furthermore, the catalytic system was used directly to the conversion of birch biomass to obtain oxime products such as vanillin oxime, syringaldehyde oxime, and furfural oxime etc. This work might give insights into the sustainable production of N-containing high-value products from lignocellulose.

Ignition enhancement and NOx formation of NH3/air mixtures by non-equilibrium plasma discharge

This work computationally studies plasma assisted low temperature NH3/air ignition and NOx formation in a repetitively-pulsed nanosecond discharge at atmospheric pressure by using an experimentally-validated plasma-combustion kinetic model. The results show that plasma discharge significantly enhances low temperature NH3 ignition. Compared with thermal ignition, the ignition delay time is shortened by 2–5 orders of magnitude due to the kinetic enhancement of excited species and radicals. The radicals (NH2, NH, H and O) produced through electron impact dissociation and quenching of electronically excited N2*, O(1D) and N(2D) promote OH production and further accelerate NH3 oxidation. The results show that there exists a non-monotonic dependence of ignition delay time on the reduced electric field strength. The optimum ignition enhancement is achieved at 250 Td at which the production of electronically excited species and radicals is most efficient. The vibrationally excited species produced at lower electric fields (<100 Td) are less effective in enhancing ignition because they only induce gas heating through the fast vibrational-translational relaxation by NH3 and H2O. At a higher electric field, although the efficient production of NH2, NH, H, O and OH by plasma creates new low temperature reaction pathways in enhancing low temperature NH3 ignition, the ignition is inhibited through NH + NO = N2O + H and chain-termination reaction NH2 + HO2 = NH3 + O2. The ignition delay times at different equivalence ratios show that the ignition enhancement by plasma is more effective at fuel-lean conditions due to the faster generation of N2(B), O(1D) and O from air, leading to accelerated NH3 oxidation via O(1D) + NH3→ NH2 + OH, NH3 + O = NH2 + OH and NH2 + O = NH + OH. The sensitivity analysis shows that the reactions involving O and O(1D) production are more effective on NH3 ignition enhancement than the fuel dissociation by electrons. Moreover, the ignition is also enhanced by NOx formation in plasma via reactions NH2 + NO = NNH + OH and NO + HO2 = NO2 + OH. This work advances the understanding of non-equilibrium excitation and NOx formation by plasma discharge on low temperature NH3 ignition.

Copper loading effect on active species formation over Cu/SSZ-13 catalysts for selective catalytic oxidation of high-concentration NH3

Cu/SSZ-13 catalysts not only exhibit superior SCR performance but also possess excellent selective catalytic oxidation (SCO) ability to remove NH3 escaped from engine exhaust gas fueled by NH3. To elucidate the effect of Cu loading on Cu/SSZ-13 catalysts, a series of Cu/SSZ-13 catalysts were prepared through wet impregnation method to remove high-concentration NH3 (5000 ppm). The SCO activity test results showed that when Cu loading increased from 1% to 10%, there was an obvious improvement effect on NH3 conversion performance at low temperature. Cu10/SSZ-13 catalyst reached a high NH3 conversion of 96% at 250 °C, and N2 selectivity was higher than 86% in the whole temperature range. However, when further increasing Cu loading from 10% to 20%, there was no distinctive difference in NH3 conversion and N2 selectivity. The characterization results indicated that there were abundant active species, adsorbed oxygen species, and acid sites on the surface of Cu10/SSZ-13 catalyst. The [Cu(OH)]+ and isolated Cu2+ species in zeolite channels of Cu10/SSZ-13 catalyst rather than CuO species on the surface played a key role in NH3-SCO reactions. Since 20% Cu loading would result in a significant decrease in isolated Cu2+ species, it resulted in both [Cu(OH)]+ species in zeolite channels and superfluous Cu species on zeolite surface being main active species in NH3-SCO reactions. In-situ DRIFTS experiments suggested that NH3-SCO reactions over Cu/SSZ-13 catalysts followed the internal selective catalytic reduction (i-SCR) mechanism.

Insight into NOx formation characteristics of ammonia oxidation in N2 and H2O atmospheres

For the practical combustion of ammonia (NH3) in air, ammonia jet entrains the combustion product inside the furnace, and the reaction zone is diluted with a significant amount of H2O; thus, the impact of H2O on NH3 oxidation cannot be ignored. In this study, NH3 oxidation was conducted under N2 and H2O atmospheres in a jet-stirred reactor. Experimental and numerical investigations were conducted to analyze the impact of various parameters, including the temperature, equivalence ratio, H2O volume fraction, and residence time. The NO formation is enhanced as T increases, while it is inhibited as Φ or XH2O increases. The N2O and NO concentrations are an order of magnitude when T ≤ 1375 K, and the peak N2O concentration occurs at T ≈ 1375 K and Φ = 0.5. The presence of 20 % H2O reduces NO and N2O concentrations by 61.5 % and 29.5 %, respectively, at Φ = 0.5 and T = 1375 K. To reduce NOx formation with an acceptable concentration of unburned NH3, the recommended operation condition of NH3 oxidation is obtained. This study presents a new underlying understanding of the formation of NOx during NH3 oxidation under N2 and H2O atmospheres.

Study on the mechanism of nitrogen conversion in NH3-doped methane premixed flame based on multi-spectral analysis methods

It is essential to obtain the in-situ key radicals and components in the combustion process of NH3 oxidation to reveal the volatile nitrogen conversion mechanism. In this work, multi-spectral analysis methods, including image spectroscopy, Tunable diode laser absorption spectroscopy (TDLAS), and plane laser-induced fluorescence (PLIF) were used to obtain the spatial distribution characteristics of the main excited radicals (C2*/CH*/CN*/OH*), H2O, and NO in NH3-doped methane flames, respectively. Furthermore, the kinetic simulation was combined to obtain the key reactions and main pathways of nitrogen conversion. The experimental results showed that when the concentration of NH3 was low, the NO generated in lean flame was less than that in rich flame, which was more than that in rich flame when the concentration of NH3 was high. NH3 also promoted the formation of H2O in flames. The image spectral intensity of CN* had a significant linear relationship with the CN concentration obtained by simulation, indicating that CN* was mainly generated by CN thermal excitation. The results of sensitivity and path analysis indicated that rich flame contributed to the reaction of N-species with H and that high NH3 concentration promoted the conversion of NH2 to HNO. With increasing NH3 concentration and equivalence ratio, the proportion of CH3 conversion to C2H6 was enhanced, while the proportion of CH3 oxidized decreased. This work reveals the mechanism of volatile nitrogen (NH3) conversion and the influence of equivalence ratio and NH3 concentration on it, which provides theoretical guidance for NOx emission reduction.

Combustion characteristics of premixed ammonia-hydrogen/air flames in a swirl model combustor

Ammonia (NH3)-hydrogen (H2) blends based on the thermal cracking of NH3 are considered as a prominent carbon-free fuel. This study aims to provide the combustion characteristics of premixed NH3-H2(-N2)/air swirl flames under various conditions of fuel-equivalence ratio (ϕ) and H2 content in the fuel blend (xh,f). Results show four representative types of premixed NH3-H2(-N2)/air flames depending on ϕ. Combustion stability limits are extended with increasing xh,f and through the transition from laminar to turbulent combustion condition. With increasing xh,f, NOx emissions are enhanced and their peak values are observed to fuel-leaner condition. The deNOx effect of NH3 is enhanced with increasing xh,f, and different main pathways for the oxidation of NH3 to NO are found depending on ϕ. Flame visualization including the distributions of OH(∗) and NO radicals indicates that OH promotes NO production. No remarkable discrepancies of the combustion characteristics between the premixed NH3-H2/air and NH3-H2-N2/air flames are found.

Effects of adding lignocellulose-degrading microbial agents and biochar on nitrogen metabolism and microbial community succession during pig manure composting

This study assessed the influence of the additions of lignocellulose-degrading microbial agents and biochar on nitrogen (N) metabolism and microbial community succession during pig manure composting. Four treatments were established: CK (without additives), M (lignocellulose-degrading microbial agents), BC (biochar), and MBC (lignocellulose-degrading microbial agents and biochar). The results revealed that all treatments with additives decreased N loss compared with CK. In particular, the concentrations of total N and NO3−-N were the highest in M, which were 21.87% and 188.67% higher than CK, respectively. Meanwhile, the abundance of denitrifying bacteria Flavobacterium, Enterobacter, and Devosia reduced with additives. The roles of Anseongella (nitrifying bacterium) and Nitrosomonas (ammonia-oxidizing bacterium) in NO3−-N transformation were enhanced in M and BC, respectively. N metabolism pathway prediction indicated that lignocellulose-degrading microbial agents addition could enhance N retention effectively mainly by inhibiting denitrification. The addition of biochar enhanced oxidation of NH4+-N to NO2−-N and N fixation, as well as inhibited denitrification. These results revealed that the addition of lignocellulose-degrading microbial agents individually was more conducive to improve N retention in pig manure compost.

Excellent treatment effect of actual wastewater from a pesticide plant using electro-Fenton process catalyzed by natural pyrite

The experiment used natural pyrite as catalyst for electro-Fenton process to treat actual wastewater containing high concentrations of chlorine salts from a pesticide factory in Guangdong province, China. It revealed that under the optimized conditions of initial pH of 3, electrode spacing of 4 cm, and current density of 40 mA/cm2, excellent treatment effect was achieved using 3 g/L pyrite as catalyst with graphite cathode and Ti/RuO2-IrO2-SnO2 anode after a duration of 3 h. Specifically, the removal efficiencies of chemical oxygen demand (COD), absorbance at 478 nm (I478), ammonia nitrogen (AN) and total nitrogen (TN) reached 99.1 %, 99.8 %, 98.9 % and 46.0 % respectively, and instantaneous current efficiency (ICE) at 3 h and energy consumption (EC) were 48.16 % and 97 kWh/kgCOD. Pyrite exhibited good reusability, mainly due to its proper dissolution capacity to renew surface active sites. It was speculated that the nitrogen removal of pesticide wastewater might mainly owe to the oxidation of NH4+ by chlorine/hypochlorite which were based on anode discharge reaction of contained Cl−. The electro-Fenton process catalyzed by natural pyrite showed good potential for efficiently treating the actual pesticide processing plant wastewater with high contents of chlorine salts.

Theoretical Kinetics Predictions for Reactions on the NH2O Potential Energy Surface.

Recent modeling studies of ammonia oxidation, which are motivated by the prospective role of ammonia as a zero-carbon fuel, have indicated significant discrepancies among the existing literature mechanisms. In this study, high-level theoretical kinetics predictions have been obtained for reactions on the NH2O potential energy surface, including the NH2 + O, HNO + H, and NH + OH reactions. These reactions have previously been highlighted as important reactions in NH3 oxidation with high sensitivity and high uncertainty. The potential energy surface is explored with coupled cluster calculations, including large basis sets and high-level corrections to yield high-accuracy (∼0.2 kcal/mol 2σ uncertainty) estimates of the stationary point energies. Variational transition state theory is used to predict the microcanonical rate constants, which are then incorporated in master equation treatments of the temperature- and pressure-dependent kinetics. For radical-radical channels, the microcanonical rates are obtained from variable reaction coordinate transition state theory implementing directly evaluated multireference electronic energies. The analysis yields predictions for the total rate constants as well as the branching ratios. We find that the NO + H2 channel contributes 10% of the total NH2 + O flux at combustion temperatures, although this channel is not included in modern NH3 oxidation mechanisms. Modeling is used to illustrate the ramifications of these rate predictions on the kinetics of NH3 oxidation and NOx formation. The present results for NH2 + O are important for predicting the chain branching and formation of NO in the oxidation of NH3 and thermal DeNOx.

Tuning the Interaction between Platinum Single Atoms and Ceria by Zirconia Doping for Efficient Catalytic Ammonia Oxidation.

Aiming at the development of an efficient NH3 oxidation catalyst to eliminate the harmful NH3 slip from the stationary flue gas denitrification system and diesel exhaust aftertreatment system, a facile ZrO2 doping strategy was proposed to construct Pt1/CexZr1-xO2 catalysts with a tunable Pt-CeO2 interaction strength and Pt-O-Ce coordination environment. According to the results of systematic characterizations, Pt species supported on CexZr1-xO2 were mainly in the form of single atoms when x ≥ 0.7, and the strength of the Pt-CeO2 interaction and the coordination number of Pt-O-Ce bond (CNPt-O-Ce) on Pt1/CexZr1-xO2 showed a volcanic change as a function of the ZrO2 doping amount. It was proposed that the balance between the reasonable concentration of oxygen defects and limited surface Zr-Ox species well accounted for the strongest Pt-CeO2 interaction and the highest CNPt-O-Ce on Pt/Ce0.9Zr0.1O2. It was observed that the Pt/Ce0.9Zr0.1O2 catalyst exhibited much higher NH3 oxidation activity than other Pt/CexZr1-xO2 catalysts. The mechanism study revealed that the Pt1 species with the stronger Pt-CeO2 interaction and higher CNPt-O-Ce within Pt/Ce0.9Zr0.1O2 could better activate NH3 adsorbed on Lewis acid sites to react with O2 thus resulting in superior NH3 oxidation activity. This work provides a new approach for designing highly efficient Pt/CeO2 based catalysts for low-temperature NH3 oxidation.

Ce0.5Zr0.5O2 solid solutions supported Co-Ni catalyst for ammonia oxidative decomposition to hydrogen

A series of CexZr1-xO2 catalysts with different Ce content (x = 1, 0.80, 0.75, 0.50, 0.33) were synthesized by co-precipitation method. The partial doping of Zr in CeO2 framework was observed to enhance the surface area and increase oxygen vacancy of the catalyst, which resulted in promoting of both NH3 oxidation and dehydrogenation in the NH3 oxidative decomposition process. The structural property of the catalysts were characterized by various methods, including XRD, BET, TEM, NH3-TPD, H2-TPR and XPS. It appears that the 5 wt% Co − 5 wt% Ni/Ce0.5Zr0.5O2 (CNCZ-55) catalyst demonstrates excellent activity in the given experimental conditions. The results indicate that when NH3 / O2 = 1:0.25, the conversion rates of both ammonia and oxygen reach 97% and 100%, respectively. Additionally, the hydrogen production achieved a notable efficiency of 63%. Moreover, the stability and repeatability of the CNCZ-55 catalyst have been obtained over 5 cycles and a total experimental duration of 100 h. This suggests that the catalyst exhibits good durability and can consistently perform well in the given reaction conditions. This design concept of improving the performance by optimizing the catalyst carrier may be broadly applicable to catalysts for a wide range of NH3 decomposition and H2 production reaction.

Inhibiting effect of C3H6 on the performance of Cu-SSZ-13 catalysts for NH3 selective catalytic reduction

Hydrocarbons (HCs) have been found to be a potential source of catalyst poisoning in ammonia selective catalytic reduction (NH3-SCR) of NOx. As one of the predominant constituents of hydrocarbons, propene (C3H6) was selected to investigate its inhibitory effect on Cu-SSZ-13 catalysts utilized in the NH3-SCR reaction. The results of standard SCR tests mixing C3H6 and NH3 showed that the NH3-SCR activity of Cu-SSZ-13 catalysts was inhibited in the range of 150–650 °C with the introduction of C3H6 and the NOx conversion was lowest in the presence of 2000 ppm C3H6. The competitive adsorption and NH3 oxidation activity were extensively studied. It was found that NH3 and C3H6 could both be stored on Brønsted acid sites and Cu sites. The introduction of C3H6 during NH3-TPD experiments caused an increase of weakly adsorbed NH3 on the weak Lewis acid sites and a decrease of adsorbed NH3 on the strong Lewis acid sites and Brønsted acid sites, confirming the existence of the competitive adsorption between NH3 and C3H6. Based on TPO experiments, C3H6 enhanced the NH3 oxidation activities in the temperature range of 250–650 °C, and coke deposition might be generated on the catalysts, causing the decline of NOx conversion during NH3-SCR with C3H6.

Remarkable N2 selectivity enhancement of NH3-SCO reactions over V0.5/Pt0.04/TiO2 catalyst through Cu-Er co-modification.

Pt-V bimetallic catalysts maybe promising substitutes to precious metal catalysts for selective catalytic oxidation (SCO) of NH3. But it remains a major challenge for Pt-V bimetallic catalysts to pursue high NH3 conversion rate and N2 selectivity simultaneously. In this work, both Cu and Er were adopted to modify V0.5/Pt0.04/TiO2 catalyst (denoted as V/PT), and the influences of Cu and Er doping amounts on NH3-SCO performance of V/PT catalysts were investigated systematically. The results indicated that the co-modification of Cu and Er imposed little influence on NH3 conversion efficiency, but significantly boosted N2 selectivity. Compared with other Cu-Er-modified V/PT catalysts, CEV/PT-4 catalyst exhibited outstanding NH3-SCO performance, which attained completely 100% NH3 conversion efficiency and > 90% N2 selectivity in the temperature range of 225-450°C. It was significantly superior to the NH3-SCO performance of most previously reported catalysts. The characterization results indicated that the adequate doping amounts of Cu and Er resulted in an obvious enhancement on redox property and surface acidity of CEV/PT-4 catalyst. It also led to abundant Pt0 and surface chemisorbed oxygen species on catalyst surface, which facilitated the oxidation of NH3 to NOx and enhanced i-SCR reactions. In situ DRIFTS results showed that -NH2 species on the surface of CEV/PT-4 catalyst could actively react with nitrate species to generate N2 and H2O.

Comparing Mn-based oxides filters started by KMnO4 versus K2FeO4 for ammonium and manganese removal: Formation mechanism of active species

A pilot-scale filtration system was adopted to prepare filter media with catalytic activity to remove manganese (Mn2+) and ammonium (NH4+-N). Three different combinations of oxidants (KMnO4 and K2FeO4) and reductants (MnSO4 and FeCl2) were used during the start-up period. Filter R3 started up by KMnO4 and FeCl2 (Mn7+→MnOx) exhibited excellent catalytic property, and the NH4+-N and Mn2+ removal efficiency reached over 80% on the 10th and 35th days, respectively. Filter R1 started up by K2FeO4 and MnSO4 (MnOx←Mn2+) exhibited the worst catalytic property. Filter R2 started up by KMnO4 and MnSO4 (Mn7+→MnOx←Mn2+) were in between. According to Zeta potential results, the Mn-based oxides (MnOx) formed by Mn7+→MnOx performed the highest pHIEP and pHPZC. The higher the pHIEP and pHPZC, the more unfavorable the cation adsorption. However, it was inconsistent with its excellent Mn2+ and NH4+-N removal abilities, implying that catalytic oxidation played a key role. Combined with XRD and XPS analysis, the results showed that the MnOx produced by the reduction of KMnO4 showed early formation of buserite crystals, high degree of amorphous, high content of Mn3+ and lattice oxygen with the higher activity to form defects. The above results showed that MnOx produced by the reduction of KMnO4 was more conducive to the formation of active species for catalytic oxidation of NH4+-N and Mn2+ removal. This study provides new insights on the formation mechanisms of the active MnOx that could catalytic oxidation of NH4+-N and Mn2+.