NH3 Oxidation Research Articles

Fine-tuned local coordination environment of Pt single atoms on ceria controls catalytic reactivity

Constructing single atom catalysts with fine-tuned coordination environments can be a promising strategy to achieve satisfactory catalytic performance. Herein, via a simple calcination temperature-control strategy, CeO2 supported Pt single atom catalysts with precisely controlled coordination environments are successfully fabricated. The joint experimental and theoretical analysis reveals that the Pt single atoms on Pt1/CeO2 prepared at 550 °C (Pt/CeO2-550) are mainly located at the edge sites of CeO2 with a Pt–O coordination number of ca. 5, while those prepared at 800 °C (Pt/CeO2-800) are predominantly located at distorted Ce substitution sites on CeO2 terrace with a Pt–O coordination number of ca. 4. Pt/CeO2-550 and Pt/CeO2-800 with different Pt1-CeO2 coordination environments exhibit a reversal of activity trend in CO oxidation and NH3 oxidation due to their different privileges in reactants activation and H2O desorption, suggesting that the catalytic performance of Pt single atom catalysts in different target reactions can be maximized by optimizing their local coordination structures.

Competitive Non-Radical Nucleophilic Attack Pathways for NH3 Oxidation and H2 O Oxidation on Hematite Photoanodes.

The sluggish H2 O oxidation kinetics on photoanodes severely obstructs the overall solar-to-energy efficiency of photoelectrochemical (PEC) cells. Herein, we find a 10 to 55-fold increase of photocurrent by conducting ammonia oxidation reaction (AOR) on hematite (α-Fe2 O3 ) photoanodes under near-neutral pH (9-11) and moderate applied potentials (1.0-1.4 VRHE ) compared to H2 O oxidation. By rate law analysis and operando spectroscopic studies, we confirm the non-radical nucleophilic attack of NH3 molecules on high-valent surface Fe-O species (e.g., FeIV =O) and Fe-N species that produces NOx - and N2 , respectively, which overwhelms the nucleophilic attack of H2 O on surface FeIV =O and contributes to a high Faradaic efficiency of above 80 % for AOR. This work reveals a novel non-radical nucleophilic attack strategy, which is significantly different from the conventional indirect radical-mediated AOR mechanism, for the rational design of high-performance AOR photoelectrocatalysts.

Electrocatalytic Ammonia Oxidation by a Low-Coordinate Copper Complex.

Molecular catalysts for ammonia oxidation to dinitrogen represent enabling components to utilize ammonia as a fuel and/or source of hydrogen. Ammonia oxidation requires not only the breaking of multiple strong N-H bonds but also controlled N-N bond formation. We report a novel β-diketiminato copper complex [iPr2NNF6]CuI-NH3 ([CuI]-NH3 (2)) as a robust electrocatalyst for NH3 oxidation in acetonitrile under homogeneous conditions. Complex 2 operates at a moderate overpotential (η = 700 mV) with a TOFmax = 940 h-1 as determined from CV data in 1.3 M NH3-MeCN solvent. Prolonged (>5 h) controlled potential electrolysis (CPE) reveals the stability and robustness of the catalyst under electrocatalytic conditions. Detailed mechanistic investigations indicate that electrochemical oxidation of [CuI]-NH3 forms {[CuII]-NH3}+ (4), which undergoes deprotonation by excess NH3 to form reactive copper(II)-amide ([CuII]-NH2, 6) unstable toward N-N bond formation to give the dinuclear hydrazine complex [CuI]2(μ-N2H4). Electrochemical studies reveal that the diammine complex [CuI](NH3)2 (7) forms at high ammonia concentration as part of the {[CuII](NH3)2}+/[CuI](NH3)2 redox couple that is electrocatalytically inactive. DFT analysis reveals a much higher thermodynamic barrier for deprotonation of the four-coordinate {[CuII](NH3)2}+ (8) by NH3 to give the copper(II) amide [CuII](NH2)(NH3) (9) (ΔG = 31.7 kcal/mol) as compared to deprotonation of the three-coordinate {[CuII]-NH3}+ by NH3 to provide the reactive three-coordinate parent amide [CuII]-NH2 (ΔG = 18.1 kcal/mol) susceptible to N-N coupling to form [CuI]2(μ-N2H4) (ΔG = -11.8 kcal/mol).

NH3 oxidation by NO2 in a jet-stirred reactor: The effect of significant uncertainties in H2NO kinetics

Understanding the kinetics of ammonia (NH3) is becoming increasingly important to a growing variety of applications — ranging from its role as a NOx reduction agent, a key intermediate during combustion of biomass and energetic materials (especially green propellants), and a potential carbon-free energy carrier and storage medium. This wide variety of applications calls for comprehensive NH3 kinetic models that are reliable over wide ranges of temperatures, pressures, and mixtures. Yet, many still consider the present understanding of its kinetics to be incomplete. For example, there are few experimental studies of NH3 oxidation by nitrogen-containing species, which offer the opportunity to probe relatively untested reactions (or combinations thereof) to enable a more comprehensive understanding of NH3 kinetics. To address this gap, we perform jet-stirred reactor experiments of NH3 oxidation by NO2 over an intermediate temperature range (700–1100 K). The mole fractions of NH3, NO2, NO, and O2 are measured through a combination of gas chromatography, chemiluminescence, and infrared absorption. Agreement among different diagnostics (≤4% for NH3 and ≤7% for NO2) and excellent experimental repeatability ensure high confidence in all species measurements. Comparisons of species measurements to model predictions revealed deficiencies in recent kinetic models, particularly for NH3 consumption and NO formation at elevated temperatures (≥900 K). Uncertainty-weighted kinetic analyses point to the importance of reactions that form (NH2 + NO2) and consume (H2NO + NO2, H2NO + OH) H2NO, both of which are uncertain and influential in this system (and many other NH3 oxidation systems). These and other reactions accentuated in the present dataset are also key reactions in NH3/air ignition and N2O formation, both of which remain outstanding challenges for NH3 combustion in engines. Consequently, resolving the modeling deficiencies observed for the present dataset appears especially important to predictive models to enable the use of NH3 as a fuel.

Hydrotalcite-Modified Clinoptilolite as the Catalyst for Selective Catalytic Reduction of NO with Ammonia (NH3-SCR).

A series of clinoptilolite-supported catalysts, modified with hydrotalcite-like phase (HT) by co-precipitation, were prepared and tested in NH3-SCR reactions. It was found that deposition of HT on clinoptilolite increased conversion of NO within 250-450 °C, and that the positive impact on the catalytic activity was independent of HT loading. The promoting effect of clinoptilolite was attributed to Brönsted acid sites present in the zeolite, which facilitated adsorption and accumulation of ammonia during the catalytic process. Concentration of N2O in the post-reaction gas mixture reached its maximum at 300 °C and the by-product was most likely formed as a consequence of NH4NO3 decomposition or side reaction of NH3 oxidation in the high-temperature region. The gradual elimination of nitrous oxide, noticed as the material with the highest concentration of hydrotalcite phase, was attributed to the abundance of oligomeric iron species and the superior textural parameters of the material. UV-Vis experiments performed on the calcined samples indicated that Fe sites of higher nuclearity were generated by thermal decomposition of the hydrotalcite phase during the catalytic reaction. Therefore, calcination of the materials prior to the catalytic tests was not required to obtain satisfactory overall catalytic performance in NO reductions.

Recent progress in electrochemical synthesis of carbon-free hydrogen carrier ammonia and ammonia fuel cells: A review

Ammonia (NH3) is a cornerstone widely used in the modern agriculture and industry, the annual global production gradually increases to almost 200 million tons. Nearly 80% of the produced NH3 is used in the fertilizer industry and is essential for the development of global agriculture and consequently for maintaining population growth. Furthermore, NH3 can power hydrogen (H2) fueled devices, such as H2 fuel cells (FC), to use the interconversion between chemical energy and electric energy of nitrogen (N2) cycle, which can effectively alleviate the intermittent problems of renewable energy. However, the problems faced by NH3 in storage and release still restrict its development. Herein, this review introduces the latest research and development of electrochemical NH3 synthesis and direct NH3 FC, as well as outlines the technical challenges, possible improvement measures and development perspectives. N2 reduction reaction (NRR) and nitrate reduction reaction (NO3−RR) are two potential approaches for electrochemical NH3 synthesis. However, the existing research foundation still faces challenges in achieving high selectivity and efficiency. Direct NH3 FC are easy to transport and are expected to be widely used in mobile energy consuming equipment, but also limited by the lack of highly active and stable NH3 oxidation electrocatalysts. The perspectives of ammonia fuel cells as an alternative green energy are discussed.

In Situ Cutting of Ammonium Perchlorate Particles by Co-Bipy "scalpel" for High Efficiency Thermal Decomposition.

Burning rate of solid propellants can be effectively improved by adding catalysts and using smaller size ammonium perchlorate (AP). Although few reports, the exploration of changing the size of AP primary particles by catalysts is of great significance for improving combustion performance. Here, taking Co-bipy as an example, the potential advantages of such materials as AP decomposition catalysts are reported. Due to the existence of NO3 - combined with oxygen rich environment provided by AP, the structural self-transformation from micronrods to nanoparticles can be quickly realized during the heating process. More importantly, when Co-bipy decomposes, it can play the role of "scalpel" and in situ cut AP particles. Results show that high-temperature decomposition of Co-bipy/AP occurs at 305.8 °C, which is 137.5 °C lower than that of pure AP. Catalytic mechanism is discussed by in situ IR and TG-IR, CoO can effectively increase the content of reactive oxygen species and weaken the N-H bond, realizing the rapid oxidation of NH3 . Eventually, the behavior of Co-bipy cutting AP particles is tested. This interesting catalyst structure self-transformation behavior can not only realize the influence on AP, but also perform a positive function in the combustion process of solid propellants, such as opening the adhesive AP interface.

Poisoning mechanism of alkali metal on Cu–Fe2O3 catalyst for selective catalytic reduction of NOx with NH3

The poisoning mechanism of Na on Cu–Fe2O3 solid solution catalyst was studied by a series of characterization methods, such as X-ray diffraction (XRD), temperature-programmed reduction by hydrogen (H2-TPR), X-ray photoelectron spectroscopy (XPS), Temperature-programmed desorption (TPD) and in-situ Diffuse Reflectance Infrared Fourier Transform spectroscopy (in-situ DRIFT). The characterization results manifest that one part of Na ions can enter into the crystal lattice of Fe2O3, and replace the original Cu2+ in the crystal lattice of Fe2O3. The replacement results in the electrophilicity of Fe3+ species, thereby restraining the generation and adsorption of NO2, nitrate and nitrite species and reducing Lewis acid sites. In addition, the displaced Cu2+ ions form CuO species and lead to excessive oxidation of NH3 to NOx. In addition, Na2O covers the adsorption sites for NH3, NO2, nitrate and nitrite species. According to in-situ DRIFTS results, the reaction still proceeds through both “fast SCR” and Langmuir-Hinshelwood (L-H) reaction mechanism after sodium poisoning, but the reaction rate reduces obviously. This work reveals the influence and poisoning mechanism of alkali metal on the solid solution catalyst, establishes an alkali metal poisoning model, and offers a new way to develop low-temperature SCR catalysts with high alkali resistance.

Mechanism-based kinetic modeling of N2O formation on SCR activity over Cu-SSZ-13 catalysts for diesel vehicle

A three-site SCR model including Z2Cu, ZCuOH and Brönsted acid sites has been developed for accurately describing SCR reactions and N2O formation over Cu-SSZ-13. The N2O formation mechanism proposed by recent works was adopted. Low-temperature N2O formation is attributed to NH4NO3 decomposition on ZCuOH and Brönsted acid sites with different NH4NO3 formation routes. High-temperature N2O formation is derived from NOx-assisted NH3 oxidation on different sites with different routes. NH3 storage, NH3 oxidation, NO oxidation, Standard SCR, Fast SCR, NO2 SCR, NH4NO3 formation and decomposition were included. The NH4NO3 inhibition on low-temperature NH3-SCR reactions and N2O formation due to its deposition on catalyst surface were considered. The model has been validated by different steady-state and transient experiments. The results indicate the model can accurately predict SCR reactions, as well as N2O formation under different conditions. The model can improve NOx and N2O cooperative emission control performance in diesel engine after-treatment system.

A shock-tube study of NH3 and NH3/H2 oxidation using laser absorption of NH3 and H2O

Ammonia (NH3) is an attractive carbon-free fuel, yet its low reactivity presents many challenges for direct use in combustion applications. These combustion challenges could be resolved by mixing NH3 with more reactive fuels such as hydrogen (H2). To further contribute to NH3 and NH3/H2 kinetics—which arguably still requires much improvement—new experiments were conducted over a wide range of temperatures (1474–2307 K), near-atmospheric pressure, several NH3/O2 mixtures (equivalence ratios varying from 0.56 to 2.07), and near-stoichiometric NH3/H2/O2 mixtures with NH3:H2 ratios of 80:20 and 50:50. During these experiments, laser absorption diagnostics near 10.4 µm and 7.4 µm were simultaneously employed to measure NH3 and H2O time histories, respectively. Characteristic parameters, such as NH3 half-life time and H2O induction delay time, were extracted from the time-history profiles, and these parameters present stringent speciation targets for mechanism validation. After an assessment of most modern kinetics models, three, most accurate, mechanisms were compared against the experimental results. Only one model was able to partially reproduce the pure NH3 experiments, yet none of the models were capable of predicting the NH3/H2 experiments. Reaction pathway analysis showed that NH3 oxidation proceeds via forming NH2 then followed three different routes to form N2. Importantly, the models considered showed different levels of importance for each route. Sensitivity analysis showed that the NH3/H2 experiment is mostly sensitive to NH3+OH⇄NH2+H2O. Interestingly, this reaction showed no sensitivity for the NH3/O2 experiments. Overall, the models exhibited significantly slower reactivity than the NH3/H2 experiments, and the kinetics analysis showed that the start of this reactivity is governed by the levels of H-atoms in the early stages of the experiments. At these early stages of the experiments, propagation and branching reactions in the H2/O2 system are the main contributors to generating H radicals, along with the reaction NH3+H⇄NH2+H2 which proceeds in its reverse direction.

A Survey of Catalytic Materials for Ammonia Electrooxidation to Nitrite and Nitrate.

Studies of the ammonia oxidation reaction (AOR) for the synthesis of nitrite and nitrate (NO2/3 - ) have been limited to a small number of catalytic materials, majorly Pt based. As the demand for nitrate-based products such as fertilisers continues to grow, exploration of alternative catalysts is needed. Herein, 19 metals immobilised as particles on carbon fibre electrodes were tested for their catalytic activity for the ammonia electrooxidation to NO2/3 - under alkaline conditions (0.1 m KOH). Nickel-based electrodes showed the highest overall NO2/3 - yield with a rate of 5.0±1.0 nmol s-1 cm-2 , to which nitrate contributed 62±8 %. Cu was the only catalyst that enabled formation of nitrate, at a rate of 1.0±0.4 nmol s-1 cm-2 , with undetectable amounts of nitrite produced. Previously unexplored in this context, Fe and Ag also showed promise and provided new insights into the mechanisms of the process. Ag-based electrodes showed strong indications of activity towards NH3 oxidation in electrochemical measurements but produced relatively low NO2/3 - yields, suggesting the formation of alternate oxidation products. NO2/3 - production over Fe-based electrodes required the presence of dissolved O2 and was more efficient than with Ni on longer timescales. These results highlight the complexity of the AOR mechanism and provide a broad set of catalytic activity and nitrate versus nitrite yield data, which might guide future development of a practical process for the distributed sustainable production of nitrates and nitrites at low and medium scales.

Electrochemical characteristics of silver/nickel oxide (Ag/Ni) for direct ammonia oxidation and nitrogen selectivity in paired electrode system

Electrochemical engineering processes are emerging alternatives for ammonia removal owing to high efficiency and low waste product. Ag electrodes were synthesized by electroplating metallic silver nanoparticles (Ag NPs) on different substrates (Ni, Ti, and graphite) for the electrochemical oxidation of ammonia in aqueous solutions. Among tested electrodes, Ag/Ni400 was the best in NH3 oxidation and N2 yield. Behaviors of the two reversible couples, AgO/Ag2O and NiOOH/Ni(OH)2, catalytically mediated the faradaic process were analyzed by voltammetry. The performance of direct electro-oxidation in terms of current efficiency and energy consumption was investigated in an asymmetric cell using Ag/Ni400 anode and copper cathode (Cu/Ni). Parameters of batch constant current electrolysis included initial concentration and current density. The ratio of paired electrodes significantly affected the N2 selectivity. The Ag/Ni400-Cu/Ni electrode system was further applied for the treatment of a real opto-electronic wastewater containing NH3 of around 250 mg-N/L and low concentration of Cl- ion (<50 mg L-1); under an electrode ratio of 2:1, N2 yield reached to>95 % at 10 mA cm−2.

Simultaneous nitrification and denitrification by controlling current density and dissolved oxygen supply in a novel electrically-induced membrane bioreactor

In this work, a simultaneous nitrification and denitrification (SND) process in a novel electrically-induced membrane bioreactor (eMBR) was established to treat nitrogen (N) and carbon wastewater. The impact of applied current density (CD) on the important operational parameters of the eMBR was also explored. Furthermore, the performance of denitrification and N-removal at various applied CDs (0–23 A/m2) were investigated. Results showed that high biological oxidation of NH4+−N and total nitrogen (TN) removals were attained at an intermittent CD of 12.5 A/m2 with a low aeration (<2.5 mg/L DO) provided by the eMBR system. The maximum NH4+−N and TN removal efficiencies of the eMBR were 98.71% and 83.75%, respectively. The average denitrification rate was enhanced by the adequate application of DC fields to the eMBR with an increase of NO3−−N removal efficiency from 8.3% to 90.5% and SND rate from 40.3% to 96.28%. It is possible that autotrophic and heterotrophic denitrification together contributed to the high performance of the eMBR process and the complete removal of the nitrate, thereby achieving average TN effluent concentrations of <5 mg/L. The eMBR also reached constant and substantial chemical oxygen demand (COD) removal efficiencies of 99.45%. Taken together, such technology presented in this work could potentially decrease energy demands and could lead to the further development of an energy self-sustained N-removal process. The eMBR could be used in a new WWTP design or for retrofitting existing treatment plants.

Limitations of Electrochemical Nitrogen Oxidation toward Nitrate.

The electrocatalytic N2 oxidation reaction (NOR) using renewable electricity is a promising alternative to the industrial synthesis of nitrate from NH3 oxidation. However, breaking the triple bond in the nitrogen molecule is one of the most essential challenges in chemistry. In this work, we use density functional theory simulations to investigate the plausible reaction mechanisms of electrocatalytic NOR and its competition with oxygen evolution reaction (OER) at the atomic scale. We focus on the electrochemical conversion of inert N2 to active *NO during NOR. We propose formation of *N2O from *N2 and *O as the rate-determining step (RDS). Following the RDS, a microkinetic model is utilized to study the rate of NOR on metal oxides. Our results demonstrate that a lower activation energy is obtained when a catalyst binds *O weakly. We show that the reaction is extremely challenging but also that design strategies have been suggested to promote electrochemical NOR.

Influence of coke deposition by propene poisoning on Cu-SSZ-13 catalyst for selective catalytic reduction of NOx with NH3

In this study, Cu-SSZ-13 catalyst was prepared via ion exchange method and then treated with 1000 ppm or 2000 ppm propene (C3H6) poisoning for 20 h. The influence of coke deposition by C3H6 poisoning on Cu-SSZ-13 catalyst for selective catalytic reduction of NOx with NH3 was investigated with standard NH3-SCR conditions based upon the simulated gas experimental platform. The results showed that NOx reduction activity was slightly affected after 1000 ppm C3H6 poisoning while the NOx conversion was remarkably lower than that of fresh catalyst below 400 °C after 2000 ppm C3H6 poisoning. However, the NOx reduction activity was recovered and even mildly higher than the original level beyond 400 °C. The existence of coke deposition was confirmed by TG-DTA analysis. 2000 ppm C3H6 poisoned catalyst had more coke deposition than 1000 ppm C3H6 poisoned catalyst. XRD and BET results showed that the surface area of the catalyst was decreased by blocking the pore channels, especially for 2000 ppm C3H6 poisoned catalyst, but the structure of the catalyst was not collapsed after C3H6 poisoning. XPS and NH3-TPD results illustrated that Cu2+, CuO, Lewis and Brønsted acid sites were more susceptible to be affected under 2000 ppm C3H6 poisoning. NH3 oxidation activity was weakened slightly on poisoned catalysts above 400 °C in NH3-TPO. From the In-situ DRIFTs results, the basic mechanism of inhibition by C3H6 poisoning on Cu-SSZ-13 catalysts was attributed to less available NH3 used in SCR reaction.

Experimental study and kinetic modeling of NH3/CH4 co-oxidation in a jet-stirred reactor

This work reports an experimental and kinetic modeling study of the co-oxidation behavior of NH3/CH4 binary fuel in a jet-stirred reactor (JSR). Experimental tests are conducted by varying the reaction temperature (850–1350 K), equivalence ratio (0.5–2), and NH3 mole fraction in fuel (0–100%) individually at ambient pressure and with N2 being diluent gas. The experimental measurement shows that there exists strong competition between CH4 and NH3 oxidation in fuel-lean condition, where the addition of NH3 tends to suppress CH4 oxidation by postponing the peak CO formation to a higher reaction temperature. However, such change is not observed in stoichiometric and fuel-rich conditions. When gradually increasing the reaction temperature, a dual NO formation behavior can be noticed for both NH3/CH4 mixture and pure NH3 regardless of equivalence ratio. When gradually increasing the equivalence ratio, NO formation becomes higher with the addition of NH3 at 1100 K, while is gradually reduced with the addition of NH3 at 1300 K. Kinetic modeling of the present experimental cases is further performed with different existing reaction mechanisms for NH3/CH4 and NH3. The comparison of NO emission shows that there is still some space for the improvement of the currently existing reaction mechanisms, especially in the low-intermediate temperature range (850–1200 K) where NO is significantly under-predicted by all models for both NH3/CH4 and NH3. Further NO reaction pathway and sensitivity analysis suggest that the reactions associated with C–N interaction, H2NO, NH2, N2H2 and NNH are responsible for the under-prediction of NO in the low-intermediate temperature range, and their reaction parameters should be optimized for obtaining improved performance.

Interrogating nitritation at a molecular level: Understanding the potential influence of Nitrobacter spp.

Water resource recovery facilities (WRRFs) increasingly must maximize nitrogen and phosphorus removal, but concurrently face challenges to reduce their energy usage and environmental footprint. In particular, biological nutrient removal (BNR), which targets removal of phosphorus and nitrogen, exhibits a large energy demand. However, a BNR process achieving partial oxidation of NH3 to NO2 (nitritation) could reduce energy demands, with secondary environmental emission benefits. Research was conducted on bench-scale systems performing nitritation and nitrification to better understand how mixed microbial consortia, cultured on real wastewater, can sustain nitritation. BNR configurations achieved nitrite accumulation ratios of 64–82%, with excellent overall effluent quality. Applying phylogenetic, transcriptomic, and metabolomic methods, coupled with process monitoring, results indicate that partial nitritation may be induced through a combination of: (1) Employing ammonia-based aeration control, with an ammonia setpoint of 2, 3 mgN/L; (2) Maintaining an aerobic period DO of 1.0–2.0 mg/L; and (3) Operating BNR post-anoxically, integrated within enhanced biological phosphorus removal (EBPR). Significant nitritation was achieved despite the presence Nitrobacter spp., but nitrite oxidoreductase must be functionally impaired or structurally incomplete. Overall, this research demonstrated the value of interrogating a mixed microbial consortia at a macro and molecular level to explore unique metabolic responses such as nitritation.

Oxidation mechanism of ammonia-N/coal-N during ammonia-coal co-combustion

Ammonia-coal co-combustion is a feasible approach to reduce CO2 emissions during thermal power generation, it is necessary to study NO formation mechanism in ammonia-coal co-firing to realize low-carbon and low-nitrogen combustion. The experimental results showed that temperature and ammonia ratio have a significant effect on the NO formation. Under the same ammonia blending amount, the NO production increased first and then decreased with temperature increasing. Theoretical calculations revealed that the formation of NH in NH3→NH→NO is one of the factors restricting ammonia combustion. NH oxidation on the char surface first occurred in the NH/coal/O2 combustion system, and realized the conversion of N to NO, HNO and NO2 through different reaction paths. Combined with the experimental and theoretical calculation results, it was concluded that the reduction of NO by ammonia/char is enhanced at high temperature (>1300 °C), which reduces the conversion of ammonia-N/coal-N to NO.

Effects of Nb-modified CeVO4 to form surface Ce-O-Nb bonds on improving low-temperature NH3-SCR deNOx activity and resistance to SO2 & H2O

A series of novel ternary CeVO4 modified with different Nb contents (CeNbVO) catalysts were synthesized via alkali-assisted hydrothermal method, and applied for selective catalytic reduction (SCR) of NOx with NH3. The as-prepared CeNbVO catalysts show enhanced NH3-SCR performance compared with the CeVO catalyst. Especially, the optimal CeNbVO-2 with appropriate Nb amount displays the superior NH3-SCR activity with above 90 % NO conversion in the range of 210–420 °C with 5 %vol·H2O and excellent resistance to H2O/SO2. Effects of Nb-modified CeVO4 to form surface Ce-O-Nb bonds on improving low-temperature NH3-SCR activity and resistance to SO2 & H2O were investigated by using various characterization methods, including XRD, Raman spectroscopy, N2 physical adsorption, TEM, H2-TPR, NH3-TPD, O2-TPD, NO-TPD, XPS, kinetic tests and in-situ DRIFTS. Different phases can be obtained with the different Nb amount doping, leading to different SCR activity. The pure CeVO4 phase with the short-range ordered Nbn+–O2–-Cen+ bond is formed by adding a small amount of Nb in CeNbVO-2. The superior catalytic performance of CeNbVO-2 can be ascribed to its improved acidity and suitable oxidation ability, which may counteract the negative effects of nonselective NH3 oxidation to N2O. It is found that the formation of Nbn+–O2–-Cen+ bonds can significantly increase the surface active oxygen species and oxygen vacancy of the catalyst. Additionally, the NH3-SCR reaction mechanism is revealed using in-situ DRIFTS and kinetic studies.

Chemical Composition of the Surface and Subsurface of Pt–Pd–Rh–Ru Catalytic Gauzes Used in the NH3 Oxidation with Air at 1133 K

High-temperature oxidation of NH3 on Pt alloy gauzes to NO is widely employed in industry for the production of HNO3, which is used to obtain agricultural fertilizers. Particular attention is paid now to the investigation of the chemical composition of gauzes used in NH3 oxidation. X-ray photoelectron and energy-dispersive X-ray spectroscopies with the depth of analysis ca. 5 nm and ca. 500 nm were applied to investigate the chemical composition of surface and subsurface layers of the new and used in NH3 oxidation with air at T = 1133 K Pt–Pd–Rh–Ru catalytic gauzes (81, 15, 3.5, 0.5 wt.%, respectively). For all the gauzes, adsorption (OHad, COad), graphitic (Cgr) and oxide (Rh2O3) films were found on the surface of the metallic alloy. Under these films, Cab, Nab and Oab atoms absorbed in the subsurface layers were detected on the gauzes used in NH3 oxidation. The obtained data testify to the penetration of Oab and Nab atoms into deeper layers of the alloy during etching with elevation of the catalyst temperature. Oab atoms were accumulated predominantly on dislocations, etch pits and grain boundaries, whereas Nab atoms intercalated mostly into interstitial sites of the alloy lattice.