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Ammonia (NH 3) dissociation, oxidation, and associated nitric oxide (NO) emission in a vertical cylindrical quartz reactor is investigated to establish the effect of temperature (1000 K–1400 K), initial NH 3 concentration (2%, 4%, 6%, 8%, 10%) and flowrate (250 mL/min, 500 mL/min, 750 mL/min). Ammonia oxidation experiments also examine the effect of equivalence ratio (ɸ = 0.8, 0.9, 1.0, 1.1). The NH 3 and O 2 conversions, N 2 yield, and NO emission are determined by analysing the reactor effluent compositions. Ammonia dissociation of <6% is observed for all conditions tested. Ammonia oxidation is initiated at ∼1100 K, with majority of NH 3 conversion occurring at 1200 K–1300 K before completion at ∼1325 K. NO emission becomes significant at temperatures >1300 K for ɸ ≤ 1.0 and increases with decreasing equivalence ratio. Under fuel-lean conditions, increasing initial NH 3 concentration increases NO emission. Fuel-rich conditions return negligible NO, attributed to the reductive effect of excessive NH 3. • NH 3 dissociation, oxidation and NO emission in a flow reactor are carefully studied. • NH 3 dissociation of <6% is observed under all conditions examined. • NH 3 oxidation initiates at 1100 K, becomes significant at 1200 K, completes at 1300 K. • NO emission is negligible under fuel-rich but significant at fuel-lean conditions. • NO formation precedes NO reduction by (locally) available NH 3. [ABSTRACT FROM AUTHOR]

A novel material (Ag/MgO-Al 2 O 3) for storage of NO x under cold-start conditions and its reduction in the high-temperature stabilized phase is synthesized, and the influence of MgO and Ag loading on its NO x removal performance is investigated. A high MgO loading increases the NO x storage capacity but also decreases the stability of adsorbed NO x. Moreover, it decreases the adsorption rate at early times since NO x spillover is inhibited. The catalysts with the lowest Ag and MgO loading exhibit the highest NO oxidation activity. The amount of NO x storage increases with a decrease in Ag loading and is attributed to enhanced spillover of NO x to the support. Increasing the Ag and MgO content leads to an enhanced NH 3 and C 3 H 6 oxidation activity, which is explained by the presence of Ag metallic clusters. Further, the NH 3 -SCR and C 3 H 6 -SCR activity decrease with Ag and MgO loading. Thus, the catalyst composition can be tuned to obtain the desired activity for various oxidation and reduction reactions. [Display omitted] • Dual-functional NO x adsorption and reduction catalysts synthesized for various compositions. • NO x spillover to support sites inhibited at high MgO loadings but favored at low Ag loadings. • High NO x adsorption capacity but lower stability of nitrates at high MgO loadings. • Undesired NH 3 and C 3 H 6 oxidation with increased MgO content. • High NH 3 -SCR, C 3 H 6 -SCR, and NO oxidation activity for low Ag loadings. [ABSTRACT FROM AUTHOR]

Solid oxide fuel cells (SOFCs) offer a promising technology for clean and efficient power generation. In this study, an SOFC combining in-situ hydrogen generation from ammonia with nitrous oxide reduction to nitrogen was simulated and validated against experimental results for the first time. Ammonia decomposition generates the hydrogen needed for the electroreduction of nitrous oxide, with simultaneous generation of electrical power. Nickel was considered as the anode catalyst since it is highly active for ammonia decomposition. Perovskite catalyst LSCF - lanthanum strontium cobalt ferrite, was used as the cathode where N 2 O, a potent greenhouse gas, is efficiently decomposed into harmless molecular nitrogen and oxygen. The simulation study was performed on ANSYS Fluent, a CFD platform. The developed simulator was revealed to be a powerful tool to understand and optimize the operating conditions of the reactor, when applied to the treatment of the flue gas of a nitric acid plant. Under optimal conditions, the modelled reactor displays current densities >100 mA·cm−2 and power densities of ca. 11 mW·cm−2, where full conversion of N 2 O could be obtained by adjustments of the residence time. Besides nitric acid plants, these findings provide valuable insights for further industries with N 2 O emissions and those involved in the delivery and storage of ammonia, ultimately contributing to the advancement of SOFC as an effective integrated technology for combined electrical generation and flue gas stream treatment. The development of new SOFCs enables the valorisation of potent greenhouse gases such as N 2 O and sustainable power generation. • First-ever report of a Solid Oxide Fuel Cell (SOFC) to eliminate N 2 O emissions while generating power from ammonia. • Successful development and validation of a Computational Fluid Dynamics model for the SOFC. • High current densities (>100 mA⋅cm-2) and power densities (11 mW⋅cm-2) reached at optimal conditions. • Demonstrated the potential for industrial application reducing N 2 O from the Ostwald process tail gas by 99%. [ABSTRACT FROM AUTHOR]

• Coal ash and AAEMs have a significant promoting effect on the NH 3 oxidation. • AAEMs promote the decomposition of N 2 O at 800 °C. • Alkali metals (K and Na) only promote the reduction of NO at 600–800 °C. • AAEMs enhance the adsorption capacity of NH 3 on the active site of the coal ash. • A appropriate O 2 concentration should be selected for the power plant. Ammonia as an eco-friendly and carbon-free energy has been greatly concerned at home and abroad. It has been utilized in the ammonia and coal co-firing process, demonstrating robust market viability. Nonetheless, due to the element N within ammonia causing the production of NO x , the catalytic effect and reaction mechanics of AAEMs in coal ash on the NH 3 oxidation remain elusive. In this study, the catalytic effects of AAEMs (K, Ca, Na and Mg) on NH 3 oxidation were investigated through experiments, and the reaction mechanism was elucidated from a microscopic perspective by density functional theory (DFT). The experimental results show that the addition of AAEMs does enhance the oxidation potential of NH 3 during coal and ammonia co-firing process, and promote the transformation of NH 3 to NO, N 2 O and N 2. However, the promotion effect of different AAEMs is different, consequently influencing the selectivity of NH 3 oxidation pathway. At 600–800 °C, alkaline earth metals (Ca and Mg) facilitate the oxidation of NH 3 while alkali metals (K and Na) have the opposite effect. Mg has the highest promoting effect on the conversion of NH 3 to NO at 600–800 °C, but the catalytic effect of K and Na is more dominant from 800 °C to 900 °C. The transformation from NH 3 to N 2 O remains the same, but the catalytic impact of Ca is mostly pronounced. AAEMs can reduce thermal decomposition activation energy of N 2 O, and the decomposition of N 2 O can be promoted at 800 °C. The DFT results indicate that AAEMs can enhance the adsorption capacity of NH 3 on the active site of the coal ash surface, and reduce the activation energy of NH 3 oxidation, thereby accelerating the transformation of NH 3 to other nitrogen-containing species. This study provides new insight into the evidence for the enhanced catalytic effect of AAEMs on NH 3 oxidation during ammonia and coal co-firing process. [ABSTRACT FROM AUTHOR]

Microbially mediated nitrification plays an important role in the nitrogen (N) cycle, and rates of activity have been shown to change significantly with temperature. Despite this, the substrate affinities of nitrifying bacteria and archaea have not been comprehensively measured and are often assumed to be static in mathematical models of environmental systems. In this study, we measured the oxidation kinetics of ammonia- (NH3) oxidizing archaea (AOA), NH3-oxidizing bacteria (AOB), and two distinct groups of nitrite (NO2–)-oxidizing bacteria (NOB), of the genera Nitrobacter and Nitrospira , by measuring the maximum rates of apparent activity (V max(app)), the apparent half-saturation constant (K m (app)), and the overall catalytic efficiency (V max(app) / K m (app)) over a range of temperatures. Changes in V max(app) and K m (app) with temperature were different between groups, with V max(app) and catalytic efficiency increasing with temperature in AOA, while V max(app) , K m (app), and catalytic efficiency increased in AOB. In Nitrobacter NOB, V max(app) and K m (app) increased, but catalytic efficiency decreased significantly with temperature. Nitrospira NOB were variable, but V max(app) increased while catalytic efficiency and K m (app) remained relatively unchanged. Michaelis–Menten (MM) and Haldane (H) kinetic models of NH3 oxidation and NO2– oxidation based on the collected data correctly predict nitrification potential in some soil incubation experiments, but not others. Despite previous observations of coupled nitrification in many natural systems, our results demonstrate significant differences in response to temperature strategies between the different groups of nitrifiers; and indicate the need to further investigate the response of nitrifiers to environmental changes. [ABSTRACT FROM AUTHOR]

Based on the changes of catalyst properties, a model was constructed to estimate the rate constants of Standard-SCR, NH3 oxidation, and NO oxidation reactions over hydrothermally aged Cu-CHA catalyst. Moreover, the rate constants obtained by the constructed model were applied to the model for estimating NOx conversion over fresh catalyst constructed in previous studies, and the NOx conversion over hydrothermally aged catalyst was estimated. As a result, the calculated NOx conversion over hydrothermally aged Cu-CHA catalyst was in good agreement with the experimental results.

Microbially mediated nitrification plays an important role in the nitrogen (N) cycle, and rates of activity have been shown to change significantly with temperature. Despite this, the substrate affinities of nitrifying bacteria and archaea have not been comprehensively measured and are often assumed to be static in mathematical models of environmental systems. In this study, we measured the oxidation kinetics of ammonia- (NH3) oxidizing archaea (AOA), NH3-oxidizing bacteria (AOB), and two distinct groups of nitrite (NO2–)-oxidizing bacteria (NOB), of the genera Nitrobacter and Nitrospira, by measuring the maximum rates of apparent activity (Vmax(app)), the apparent half-saturation constant (Km(app)), and the overall catalytic efficiency (Vmax(app)/Km(app)) over a range of temperatures. Changes in Vmax(app) and Km(app) with temperature were different between groups, with Vmax(app) and catalytic efficiency increasing with temperature in AOA, while Vmax(app), Km(app), and catalytic efficiency increased in AOB. In Nitrobacter NOB, Vmax(app) and Km(app) increased, but catalytic efficiency decreased significantly with temperature. Nitrospira NOB were variable, but Vmax(app) increased while catalytic efficiency and Km(app) remained relatively unchanged. Michaelis–Menten (MM) and Haldane (H) kinetic models of NH3 oxidation and NO2– oxidation based on the collected data correctly predict nitrification potential in some soil incubation experiments, but not others. Despite previous observations of coupled nitrification in many natural systems, our results demonstrate significant differences in response to temperature strategies between the different groups of nitrifiers; and indicate the need to further investigate the response of nitrifiers to environmental changes.

In this study, several nanometer-thick transition metal overlayers (M) were formed on an Fe–Cr–Al metal (SUS) foil by pulsed cathodic arc-plasma deposition. The thin-film materials prepared, exhibiting a full-coverage M overlayer (M/SUS), were applied as catalysts for NH 3 oxidation in the presence of excess O 2 (300 ppm NH 3 , 8% O 2 , and He balance). Under dry conditions (0% H 2 O in the gas feed), the observed NH 3 oxidation activity decreased in the order of Ir > Pt > Co > Pd > Cu > Rh when the product selectivity was not taken into consideration. The activities of the materials composed of Co and Cu decreased significantly in the presence of 10% H 2 O, while those of the catalysts containing Pt and Ir remained nearly unchanged. Based on the kinetic analyses and X-ray photoelectron spectrometry experiments conducted, it was determined that the contrasting effect of H 2 O on the activity of M/SUS was related to the ease of oxidation of the non-precious metals studied. The resulting hydrophilic oxide surface was more susceptible to the competitive adsorption of H 2 O and thus inhibited the NH 3 reaction than a metallic surface. The negative effect of H 2 O was evident by the decrease in the NH 3 concentration in the gas feed. Notably, the metallic surface of the Pt overlayer was stable even under oxidizing reaction conditions; therefore, the oxidation of NH 3 was less susceptible to the effects of high H 2 O concentrations. [Display omitted] • Various transition metal overlayer catalysts were studied as NH 3 slip catalysts (ASC). • Co and Cu exhibited higher activities than several of the precious metals. • The activities of Co and Cu decreased significantly in the presence of 10% H 2 O. • The oxidation tolerance of the metal is favorable for potential ASC applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • CuO/TiO 2 catalysts are highly active for Hg0 oxidation. • Improvement of NH 3 resistance was observed over CuO/TiO 2 catalysts. • Abundant number of weak acidic sites are presented on CuO/TiO 2 catalysts. • NO 2 formed by NH 3 oxidation could facilitate the Hg0 oxidation. • The effect of NH 3 on Hg0 oxidation differs with temperature raising. In this study, the effects of ammonia (NH 3) were explored for catalytic oxidation of elemental mercury (Hg0) over CeO 2 /TiO 2 , CuO/TiO 2 , Fe 2 O 3 /TiO 2 , and V 2 O 5 -WO 3 /TiO 2 catalysts. In the presence of 50 ppm NH 3 , the results showed that CuO/TiO 2 catalysts could still maintain 98–100 % of Hg0 conversion in the range of 100–400 °C, while the catalytic performance of other catalysts was significantly inhibited by NH 3 in the same temperature range. NH 3 resistance was observed on CuO/TiO 2 catalysts. The catalysts were further characterized by means of XRD, H 2 -TPR, XPS, NH 3 -TPD, BET, and in-situ DRIFTS. For the CuO/TiO 2 catalysts, Cu(II) was the primary Cu species that conducive to the strong reducibility properties which beneficial for the catalytic performance. The mechanism of NH 3 resistance in Hg0 oxidation over CuO/TiO 2 catalysts was proposed. At low-temperature range (lower than about 250 °C), abundant weak acidic sites contributed to the superior Hg0 adsorption capacity. The Hg0 oxidation activity was enhanced correspondingly. At high-temperature range (about 250–400 °C), the Hg0 oxidation performance was mainly affected by the oxidation of NH 3 and the products. The main product of NH 3 oxidation was NO 2 , which could promote Hg0 oxidation activity. [ABSTRACT FROM AUTHOR]

Two series of Cu/SSZ-13 catalysts were synthesized via aqueous solution and solid-state ion exchange using SSZ-13 supports of varying Si/Al ratios. The isolated and multinuclear Cu content of these catalysts were determined by H 2 temperature programmed reduction (H 2 -TPR). Multinuclear Cu in these catalysts, including in situ Cu-dimers formed from ZCuIIOH coupling and permanent CuO clusters, are active species for dry NO oxidation. NH 3 oxidation on these catalysts follows an internal SCR (i-SCR) mechanism, i.e., a portion of NH 3 is first oxidized to NO, then NO is selectively reduced by the remaining NH 3 to N 2. NH 3 oxidation displays distinct kinetic behavior below ∼300 °C and above ∼400 °C. At low temperature the results indicate that NH 3 -solvated mobile Cu-ions are the active centers. CuO clusters, when present, also contribute to the low temperature activity by catalyzing NH 3 oxidation to NO. At high temperature, in situ Cu-dimers and CuO clusters catalyze NH 3 oxidation to NO, and isolated Cu-ions catalyze SCR to realize the cascade turnovers. For both NO and NH 3 oxidation, Cu-dimers balanced by framework charges of close proximity appear to be more active than Cu-dimers balanced by distant framework charges. However, the former Cu-dimers are less stable than the latter and tend to split into monomers in the presence of vicinal Brønsted acid sites. Via density functional theory (DFT) calculations, the i-SCR mechanism for low temperature NH 3 oxidation, i.e., the energetic favorability for the involvement of the NO intermediate, is justified. The DFT results also agree with experimental data that the formation of Cu-dimers from ZCuIIOH dimerization is essential for NH 3 oxidation at high temperature. [Display omitted] • NO and NH 3 oxidation on Cu-SSZ-13 occurs on multinuclear Cu sites, including in situ Cu-dimers and permanent CuO clusters. • NH 3 oxidation follows an internal SCR mechanism, which displays distinct kinetic behavior below 300 °C and above 400 °C. • Solvated CuII(NH 3) 3 OH species oxidize NH 3 at low-temperature and catalyze further reduction of produced NO to N 2 and H 2 O. • Permanent CuO clusters catalyze NH 3 oxidation to NO at low temperatures, clarifying a synergy with isolated Cu-ions. • At high temperature, multinuclear Cu sites sustain NH 3 oxidation, and immobilized & isolated Cu-ions sustain NO reduction. [ABSTRACT FROM AUTHOR]

The strong oxidation capabilities of Mn sites render Mn-based catalysts showing high activities of selective catalytic reduction (SCR) at temperatures < 250℃, but simultaneously lead to the generation of by-product N 2 O with strong greenhouse effect. The activity-N 2 -selectivity trade-off was a prevalent observation in the development of Mn-based SCR catalysts. Here, a polyoxometallic acid enhancement strategy was devised to simultaneously promote the N 2 selectivity and low-temperature SCR (LT-SCR) activity of Mn-based metal oxide catalysts, where the introduction of uniformly dispersed phosphomolybdic acid (HPMo) tuned the oxidation capabilities and enhanced NH 3 adsorption-storage capability of catalysts. The combination of transient reaction, in-situ DRIFTS analysis and DFT calculations revealed that the main origin of N 2 O on the Mn-based catalysts was the deep oxidation of NH 3 , which was inhibited by the interaction between HPMo and Mn sites. Additional NH 3 molecules adsorbed by the abundant acid sites supplied by HPMo further facilitated the SCR reaction. This strategy could guide the design of high-activity and high-selectivity LT-SCR catalysts for industrial denitration. [Display omitted] • The introduction of HPMo tuned the oxidation abilities and enhanced the acidity. • The loading of HPMo can inhibit the oxidation of NO to NO 2 and deep oxidation of NH 3. • Abundant acid sites supplied by HPMo can accelerate the SCR reaction. • The activity-selectivity trade-off can be broken by the modification of HPMo. [ABSTRACT FROM AUTHOR]

• Construction of Er composition (X)-variable Ce 1-X Er X VO 4 (Er X) used as active sites for NH 3 -SCR. • Er 0.5 with the largest numbers of acid sites/defects, desired Lewis acidity, and improved redox character. • Er 0.5 with the greatest catalytic consequences for NH 3 -SCR and NH 3 oxidation at high temperatures. • Er 0.5 minimizing particle sintering and structural dissociation upon hydrothermal aging. • Er 0.5 retaining the best NH 3 -SCR and NH 3 oxidation performance post hydrothermal aging. Catalytic rare earth metal vanadates have shown promise for efficiently converting NO X to N 2 at elevated temperatures (NH 3 -SCR) (e.g. , CeVO 4 , ErVO 4 , and TbVO 4). However, these vanadates have limitations as catalytic sites because of three major issues such as weak hydro-thermal stability, low N 2 selectivity, and limited numbers of major active (Lewis acid) sites. As an efficient way to circumvent these constraints, this study showcases a means of structurally modifying vanadate with additional rare earth metals to generate bimetallic vanadates with variable metal compositions. While selecting Ce and Er as metal constituents, a series of Ce 1-X Er X VO 4 solid solutions were deposited onto WO 3 -promoted TiO 2 supports (WO 3 -TiO 2) to form Er X catalysts, whereas a control simulating a commercial catalyst (V) was also synthesized using WO 3 -TiO 2 for comparison. Bimetallic Ce 1-X Er X VO 4 (X = 0.25, 0.5, and 0.75) showed enhanced redox features, improved the quantities of Lewis/Brönsted acid sites and defects, and increased resistance to hydro-thermal aging relative to their monometallic analogues (X = 0 and 1). The optimal Er composition of Ce 1-X Er X VO 4 studied was found to be X = 0.5. This was because Er 0.5 provided the best redox character, the largest number of active sites with the desired Lewis acid strength, and the greatest hydro-thermal stability among all the Er X and V catalysts studied. This led to the best catalytic consequence of Er 0.5 in the selective NH 3 oxidation and the NH 3 -SCR reactions, both of which should achieve high N 2 productivities at elevated temperatures. In addition, Er 0.5 subjected to hydro-thermal aging also extended its best NH 3 -SCR performance among all aged catalysts studied even to low temperature regime of < 300 °C. This paper remarks the proper combination of rare earth metals used to construct bimetallic vanadates can be adaptable to create high-performance NH 3 -SCR catalysts for use at elevated temperatures. [ABSTRACT FROM AUTHOR]

• Two different N 2 O formation mechanisms over V-based SCR catalyst are proposed. • The decomposition of NH 4 NO 3 is the main way of N 2 O formation at the temperatures below 300 °C. • At high temperatures, NH 3 is oxidized to form NH by V active site and then reacts with NO to produce N 2 O. • The deep oxidation of the adsorbed NH 3 is the rate-determined step for N 2 O formation at high temperatures. Formation of N 2 O during NH 3 -SCR reaction limits the temperature window of SCR catalyst. In the present study, a series of kinetics experiments were carried out over V-based catalyst to study the N 2 O formation characteristics at different temperatures. Transient and temperature-programmed reactions show that the thermal decomposition of solid NH 4 NO 3 formed by the reaction of NH 3 with NO 2 is the main way of N 2 O formation at low temperatures (below 300 °C). At the temperature above 300 °C, N 2 selectivity of NO reduction over V-based catalysts decreases with the increase of the gaseous NH 3 concentration and the V content of the catalysts. DFT calculations were applied to understand the formation mechanism of N 2 O at high temperature. The results show that the NH is found to be formed by NH 3 oxidative dehydrogenation over V Lewis acid site and subsequently reacts with NO to produce N 2 O. In this process, the deep oxidation of the adsorbed NH 3 is the rate-determined step, which can be aggravated by the high temperature and high V content. [ABSTRACT FROM AUTHOR]

Featured Application: Pt/TiO2 catalysts are widely applied in catalysis, especially in oxidation reactions within exhaust gas after treatment. Novel synthetic approaches may significantly improve their catalytic properties. Pulsed laser ablation in liquid offers altered properties of oxide nanopowders such as enhancement of thermostability and increased concentration of defects providing strong interaction with Pt. A set of physicochemical methods, including X-ray photoelectron spectroscopy (XPS), X-ray diffraction, electron microscopy and X-ray absorption spectroscopy, was applied to study Pt/TiO2 catalysts prepared by impregnation using a commercial TiO2-P25 support and a support produced by pulsed laser ablation in liquid (PLA). The Pt/TiO2-PLA catalysts showed increased thermal stability due to the localization of the highly dispersed platinum species at the intercrystalline boundaries of the support particles. In contrast, the Pt/TiO2-P25 catalysts were characterized by uniform distribution of the Pt species over the support. Analysis of Pt4f XP spectra shows that oxidized Pt2+ and Pt4+ species are formed in the Pt/TiO2-P25 catalysts, while the platinum oxidation state in the Pt/TiO2-PLA catalysts is lower due to stronger interaction of the active component with the support due to strong interaction via Pt-O-Ti bonds. The Pt4f XP spectra of the samples after reaction show Pt2+ and metallic platinum, which is the catalytically active species. The study of the catalytic properties in ammonia oxidation showed that, unlike the catalysts prepared with a commercial support, the Pt/TiO2-PLA samples show higher stability during catalysis and significantly higher selectivity to N2 in a wide temperature range of 200–400 °C. [ABSTRACT FROM AUTHOR]

CuSO 4 /TiO 2 catalysts with high catalytic activity and excellent resistant to SO 2 and H 2 O, were thought to be promising catalysts used in Selective catalytic reduction of nitrogen oxides by NH 3. The performance of catalysts is largely affected by calcination temperature. Here, effects of calcination temperature on physicochemical property and catalytic activity of CuSO 4 /TiO 2 catalysts were investigated in depth. Catalyst samples calcined at different temperatures were prepared first and then physicochemical properties of the catalyst were characterized by N 2 adsorption-desorption, X-ray diffraction, thermogravimetric analysis, Raman spectra, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption of NH 3 , temperature-programmed reduction of H 2 and in situ diffuse reflectance infrared Fourier transform spectroscopy. Results revealed that high calcination temperature had three main effects on the catalyst. First, sintering and anatase transform into rutile with increase of calcination temperature, causing a decrement of specific surface area. Second, decomposition of CuSO 4 under higher calcination temperature, resulting in disappears of Brønsted acid sites (S–OH), which had an adverse effect on surface acidity. Third, CuO from the decomposition of CuSO 4 changed surface reducibility of the catalyst and favored the process of NH 3 oxidation to nitrogen oxides (NO x). Thus, catalytic activity of the catalyst calcined under high temperatures (≥600°C) decreased largely. Image 1 [ABSTRACT FROM AUTHOR]

Fine-tuning the dispersion of active metal species on widely used supports is a research hotspot in the catalysis community, which is vital for achieving a balance between the atomic utilization efficiency and the intrinsic activity of active sites. In this work, using bayerite Al(OH) 3 as support directly or after precalcination at 200 or 550 °C, Pt/Al 2 O 3 catalysts with distinct Pt dispersions from single atoms to clusters ( ca . 2 nm) were prepared and evaluated for CO and NH 3 removal. Richer surface hydroxyl groups on AlO x (OH) y support were proved to better facilitate the dispersion of Pt. However, Pt/Al 2 O 3 with relatively lower Pt dispersion could exhibit better activity in CO/NH 3 oxidation reactions. Further reaction mechanism study revealed that the Pt sites on Pt/Al 2 O 3 with lower Pt dispersion could be activated to Pt 0 species much easier under the CO oxidation condition, on which a higher CO adsorption capacity and more efficient O 2 activation were achieved simultaneously. Compared to Pt single atoms, PtO x clusters could also better activate NH 3 into -NH 2 and -HNO species. The higher CO adsorption capacity and the more efficient NH 3 /O 2 activation ability on Pt/Al 2 O 3 with relatively lower Pt dispersion well explained its higher CO/NH 3 oxidation activity. This study emphasizes the importance of avoiding a singular pursuit of single-atom catalyst synthesis and instead focusing on achieving the most effective Pt species on Al 2 O 3 support for targeted reactions. This approach avoids unnecessary limitations and enables a more practical and efficient strategy for Pt catalyst fabrication in emission control applications.

Abstract The laminar flame speeds of CH 4 /NH 3 mixtures during oxy-fuel combustion conditions were measured under variable NH 3 /CH 4 ratios (0.1–0.2), O 2 mole fractions (35%–40%), and CO 2 mole fractions (45%–65%) in a counterflow flame configuration (set at atmospheric pressure and unburnt mixture temperature (T u = 300 K)). These experimental results were compared to the numerical results obtained through three detailed chemical kinetic mechanisms: the Okafor, Mendiara and HUST (Huazhong University of Science and Technology) mechanisms. The comparisons showed that the results obtained through the HUST Mechanism were in good agreement with the experimental results. The experimental results showed that the laminar flame speeds increased linearly with decreasing CO 2 or increasing O 2 concentrations under the conditions considered, while the slopes were irrelevant for the equivalence ratio. Nevertheless, the effects of NH 3 concentration depended on the equivalence ratio: the sensitivity and pathway analyses of NH 3 oxidation revealed that, among the N-containing reactions in the fuel-lean region, NO oxidation and reduction (NO + HO 2 = NO 2 +OH, NH 2 +NO = NNH + OH, NO 2 +H = NO + OH, and CH 3 +NO 2 = CH 3 O + NO) had the largest impact on the laminar flame speeds. In stoichiometric region, the NO reduction pathway (NH 2 +NO = N 2 +H 2 O, NH 2 +NO = NNH + OH, NH + NO = N 2 O + H, and NH + NO = N 2 +OH) greatly contributed to flame propagation. In fuel-rich region, N + NO = N 2 +O and N + OH = NO + H had the biggest impact over laminar flame speeds. Highlights • The S u 0 of CH 4 with high concentration of NH 3 in oxy-fuel combustion were measured. • The S u 0 are linear dependent on O 2 and CO 2 concentrations. • The S u 0 in fuel-lean reduce faster than that in fuel-rich due to the NH 3 addition. • The oxidation and reduction of NO have the most impact on the S u 0 in fuel-lean. • N + NO·N 2 +O and N + OH·NO + H are the most important reactions for S u 0 in fuel-rich. [ABSTRACT FROM AUTHOR]

Abstract The relationship between the formation of N 2 O and NO and the absence and presence of O 2 in NH 3 oxidation process over MnO x /TiO 2 catalysts was studied in this work. Experimental results demonstrated that evident N 2 O and NO formed when only NH 3 with no gaseous O 2 was introduced into the reactor, validating N 2 O originated from NH 3 oxidation by lattice oxygen or surface absorbed oxygen in MnO x /TiO 2 catalysts. In addition, remarkable NO was found during experiments, which provided a new N 2 O formation pathway NH 3 →N→N 2 O or NH 3 →N→NO→N 2 O in NH 3 catalytic oxidation process. Afterwards, different concentrations of O 2 were introduced into the reactor to explore the effect on N 2 O and NO generation. At the same temperature, O 2 moderately promoted the oxidation of NH 3 , but it did not strongly enhance it, implying the weaker oxidation performance of O 2 than the catalyst oxygen in Mn(0.1)/Ti. The NO formation increased monotonically with the temperature, whatever the O 2 concentration. However, the N 2 O concentration did not always coincide with the monotonous variation tendency as NO. With 4% or 8% O 2 , the N 2 O formation suddenly turned down above 350 oC, and the more O 2 led to the greater reduction. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The SCR activity and alkali resistance was tested over a series of vanadia-titania catalysts. • V 4 /Ti catalyst displayed the best performance after doping with potassium. • V4+ species were sensitive to be affected by alkali metals. • Acidity played a critical role in the poisoned catalysts. • Competition of NH 3 oxidation mainly caused the deactivation with V > 4%. Abstract A series of vanadia-titania catalysts with different vanadia loadings for the selective catalytic reduction (SCR) of NO by ammonia was prepared via incipient impregnation method. The alkali resistance and SCR activity at low temperature has been investigated on the prepared catalysts. Increasing the vanadia loading from 1% to 10%, the NO removal efficiency of the catalysts gained a 70% growth at 200 °C. With the increase of the vanadia loading, the high loading vanadia-titania catalysts exhibited higher ratio of V5+/(V4++V5+), which enhanced the oxidation of NH 3 at high temperatures and also improved the standard SCR performance at low temperature. After doping with potassium, V 4 /Ti catalyst (V 2 O 5 = 4 wt%) displayed the best performance over which 80% initial activity was retained from 300 °C to 400 °C. More acid sites and active vanadium species were retained in the high loading vanadia-titania catalysts, which made them exhibit better low temperature alkali resistance. However, when vanadia content was higher than 4%, the competition of NH 3 oxidation severely deactivated the catalytic activity above 250 °C. Additionally, in the presence of SO 2 and H 2 O the SCR performance of the high loading vanadia-titania catalysts were also studied in order to investigate the influence of the actual working conditions. [ABSTRACT FROM AUTHOR]

FeBeta||Mn-Ce/FeBeta catalyst was prepared by coating cordierite monolith with FeBeta zeolite followed by an impregnation of its downstream part by Mn-Ce solution. The resulting dual-zone catalyst demonstrates improved catalytic performance in the selective catalytic reduction of NOx by NH3 (NH3-SCR) as compared to the parent Fe-Beta. The catalyst exhibits 80-90% NOx conversion and 100% selectivity to N2 within wide temperature ranges: 180-500 °C (at GHSV = 18,000 h−1) and 260-500 °C (at GHSV = 54,000 h−1). Moreover, the dual-zone catalyst provides favorable NH3-slip removal efficiency even at NH3/NO = 1.60. [ABSTRACT FROM AUTHOR]

FeBeta||Mn-Ce/FeBeta catalyst was prepared by coating cordierite monolith with FeBeta zeolite followed by an impregnation of its downstream part by Mn-Ce solution. The resulting dual-zone catalyst demonstrates improved catalytic performance in the selective catalytic reduction of NOx by NH3 (NH3-SCR) as compared to the parent Fe-Beta. The catalyst exhibits 80-90% NOx conversion and 100% selectivity to N2 within wide temperature ranges: 180-500 °C (at GHSV = 18,000 h−1) and 260-500 °C (at GHSV = 54,000 h−1). Moreover, the dual-zone catalyst provides favorable NH3-slip removal efficiency even at NH3/NO = 1.60. [ABSTRACT FROM AUTHOR]

A set of physicochemical methods, including X-ray photoelectron spectroscopy (XPS), X-ray diraction, electron microscopy and X-ray absorption spectroscopy, was applied to study Pt/TiO2 catalysts prepared by impregnation using a commercial TiO2-P25 support and a support produced by pulsed laser ablation in liquid (PLA). The Pt/TiO2-PLA catalysts showed increased thermal stability due to the localization of the highly dispersed platinum species at the intercrystalline boundaries of the support particles. In contrast, the Pt/TiO2-P25 catalysts were characterized by uniform distributionof the Pt species over the support. Analysis of Pt4f XP spectra shows that oxidized Pt2+ and Pt4+ species are formed in the Pt/TiO2-P25 catalysts, while the platinum oxidation state in the Pt/TiO2-PLA catalysts is lower due to stronger interaction of the active component with the support due to stronginteraction via Pt-O-Ti bonds. The Pt4f XP spectra of the samples after reaction show Pt2+ and metallic platinum, which is the catalytically active species. The study of the catalytic properties in ammonia oxidation showed that, unlike the catalysts prepared with a commercial support, the Pt/TiO2-PLA samples show higher stability during catalysis and significantly higher selectivity to N2 in a wide temperature range of 200–400 C.

Aiming at the development of an efficient NH 3 oxidation catalyst to eliminate the harmful NH 3 slip from the stationary flue gas denitrification system and diesel exhaust aftertreatment system, a facile ZrO 2 doping strategy was proposed to construct Pt 1 /Ce x Zr 1- x O 2 catalysts with a tunable Pt-CeO 2 interaction strength and Pt-O-Ce coordination environment. According to the results of systematic characterizations, Pt species supported on Ce x Zr 1- x O 2 were mainly in the form of single atoms when x ≥ 0.7, and the strength of the Pt-CeO 2 interaction and the coordination number of Pt-O-Ce bond (CN Pt-O-Ce ) on Pt 1 /Ce x Zr 1- x O 2 showed a volcanic change as a function of the ZrO 2 doping amount. It was proposed that the balance between the reasonable concentration of oxygen defects and limited surface Zr-O x species well accounted for the strongest Pt-CeO 2 interaction and the highest CN Pt-O-Ce on Pt/Ce 0.9 Zr 0.1 O 2 . It was observed that the Pt/Ce 0.9 Zr 0.1 O 2 catalyst exhibited much higher NH 3 oxidation activity than other Pt/Ce x Zr 1- x O 2 catalysts. The mechanism study revealed that the Pt 1 species with the stronger Pt-CeO 2 interaction and higher CN Pt-O-Ce within Pt/Ce 0.9 Zr 0.1 O 2 could better activate NH 3 adsorbed on Lewis acid sites to react with O 2 thus resulting in superior NH 3 oxidation activity. This work provides a new approach for designing highly efficient Pt/CeO 2 based catalysts for low-temperature NH 3 oxidation.

Abstract The reactions of NH 3 and catalyst oxygen in MnO x /TiO 2 catalysts were studied under gaseous O 2 -deficient condition, in order to research N 2 O and NO formation from NH 3 oxidation by catalyst oxygen and also to clarify the authentic role that MnO x /TiO 2 catalysts played in the process. Furthermore, multiple characterization methods were applied to contrastively analyze the physical-chemical properties and the microstructures of the fresh catalyst and the served catalyst, so as to help investigate the possible intrinsic mechanism of N 2 O and NO generation. Experimental results validated that N 2 O and NO originated from NH 3 oxidation by catalyst oxygen obviously, while the characterization results indicated that both the surface absorbed oxygen and the lattice oxygen participated in the process and resulted in the formation of N 2 O and NO. The consumption of catalyst oxygen led to the arising of new small pores inside the catalyst, bringing about the decrease of the average pore diameter and the increase of the total pore volume and the surface area of the catalyst. The NH 3 oxidation process also made partial adjacent catalyst particles gather together, accounting for the aggregation and the non-uniform dispersion of catalyst particles. [ABSTRACT FROM AUTHOR]

Highlights • Incorporated effect of NH 3 oxidation in the LNT-SCR kinetic model. • NH 3 oxidation negatively affects NO x conversion in mixed and dual-layer configurations. • Negative effect of NH 3 oxidation is more prominent at intermediate temperatures. • Decrease in NO x conversion by NH 3 oxidation is higher in mixed configuration. • Activity and selectivity of LNT catalyst for NH 3 oxidation affects NO x conversion. Abstract A global kinetic model for NH 3 oxidation is developed and incorporated into an existing LNT-SCR kinetic model to study its effect on the performance of dual-layer and mixed LNT-SCR configurations. The model predicts a decrease in the NO x conversion and an increase in the NO x storage with the inclusion of NH 3 -oxidation reactions for both the configurations. This is due to the selective oxidation of NH 3 on the LNT catalyst instead of its consumption by the NO x reduction reactions on the SCR catalyst. The decrease in NO x conversion is negligible at low temperatures and is more prominent at intermediate temperatures (∼300 °C). The limited supply of stored NH 3 for the oxidation reactions lowers the negative effect of NH 3 oxidation at high temperatures. The fractional contribution of NH 3 -oxidation reactions towards the consumption of NH 3 is calculated to be higher for the mixed configuration, which is further confirmed by the uniform axial profiles of stored NH 3 in the mixed configuration as compared to the dual-layer configuration. This results in a more prominent decrease in NO x conversion for the mixed configuration as compared to the dual-layer configuration. The formation of NO x at high temperatures by the oxidation of NH 3 in the combined configurations can result in a NO x conversion even lower than the standalone LNT catalyst. A higher NH 3 slip is predicted for the mixed configuration as compared to the dual-layer configuration. It is highlighted that the composition of the LNT catalyst, its activity for NH 3 -oxidation and the selectivity of products formed by NH 3 -oxidation reactions could influence the inferences made by various groups on the comparison of LNT-SCR configurations, with the mixed configuration in particular. [ABSTRACT FROM AUTHOR]

[Display omitted] • NH 3 and Hg0 can be synergistically oxidized by Cu 3 -ATP under sufficient reaction time despite competition exists. • Cu 3 -ATP shows good sulfur resistance in Hg0 oxidation as HCl presents, but SO 2 partially inhibits NH 3 oxidation. • SO 2 interrupts NH 2 → NO x conversion in iSCR mechanism for NH 3 oxidation. • Surface active Cl for Hg0 oxidation has higher activity when SO 2 is present. NH 3 and Hg0 could be synergistically oxidized by copper modified attapulgite (Cu 3 -ATP) catalyst at the gas hourly space velocity (GHSV) below 5 × 104 h−1 despite competition exist between these two reactions. Its Hg0 oxidation showed good sulfur resistance when HCl was present, but the NH 3 oxidation was partially inhibited after SO 2 was added. In order to explain this phenomenon, the composition and physical characteristics of Cu 3 -ATP before and after sulfurization were characterized by XRF, BET, XRD and FTIR. And H 2 -TPR, NH 3 -TPD, in situ DRIFT and XPS of the catalysts were adopted to reveal the influence of SO 2 on NH 3 /Hg0 oxidation and the reaction mechanisms. The results showed that SO 2 would adsorb and oxidize on the catalyst surface to form very small amount of sulfite and sulfate species, and partially change the composition and structure of Cu 3 -ATP. This reduced the reducibility of Cu 3 -ATP and increased the weak and moderate acid sites on its surface. The presence of SO 2 was beneficial to NH 3 adsorption, but inhibited NH 2 → NO x conversion in iSCR mechanism for NH 3 oxidation. Cl 2 formation were also inhibited by SO 2 , while the generated surface active Cl and O species could ensure high Hg0 oxidation efficiency. [ABSTRACT FROM AUTHOR]

[Display omitted] • Coal ash catalyzes NO reduction by NH 3 , and NH 3 oxidation by O 2. • NH 3 , NO and O 2 can co-adsorb on the coal ash surface. • NH 3 conversion on the coal ash is mainly influenced by O 2. • NH 3 ad-species' main oxidation intermediates are monodentate and bridged nitrates. NH 3 as reducing agent in selective noncatalytic reduction (SNCR) is widely used for the deNO x of coal-fired boiler flue gas, where fly ash significantly influences the conversion of residual NH 3 that does not participate in deNO x. However, few studies have been conducted on the exact nature of this influence. The adsorption and reaction of NH 3 , NO, and O 2 on the fly ash at 50–850 °C were investigated in this study. At 50–350 °C, NH 3 , NO, and O 2 co-adsorbed on the coal ash surface, and the presence of NO and O 2 reduced the physical adsorption of NH 3 on the ash and promoted its chemical adsorption on the ash. At 450–850 °C, coal ash catalyzed NO reduction by NH 3 and NH 3 oxidation by O 2. In the presence of O 2 , the oxidation reaction became dominant and the reduction reaction was inhibited. The mechanism of adsorption and reaction of coal ash showed that NH 3 was adsorbed on the Brönsted and Lewis sites on coal ash and NH 3 at the Lewis site could be dissociated into *NH 2 and *NH. The NO ad-species in coal ash were mainly nitrate and nitrite. *NH 2 reacted with NO to form NH 2 NO, which desorbed as N 2 at 450–850 °C. The main oxidation intermediates for NH 3 ad-species were monodentate nitrate and bridged nitrate, which desorbed as NO at 450–850 °C. The results provide a theoretical basis for adjusting the migration and conversion of residual NH 3 and reducing ammonia slip from power plants. [ABSTRACT FROM AUTHOR]

Montmorillonite intercalated with mixed alumina-iron (III) oxide pillars and promoted with hexamminecobalt (III) chloride was studied in selective catalytic reduction of NO with NH3. Pillaring process extended the interlayer spacing by 0.236 nm and increased specific surface area (SSABET) from 33m2 up to 299m2. Interlayer space width did not change after calcination process, but high temperature could cause decomposition of hexamminecobalt (III) - [Co(NH3)6]3+, possibly to Co2+. Promotion of hexamminecobalt (III) chloride to pillared clay did not significantly change SSABET. The intercalation process influenced positively selective catalytic reduction activity, but very high formation of N2O was observed. Pillared clay, promoted with [Co(NH3)6]3+, showed higher selective catalytic reduction activity and much lower N2O formation than unpromoted, pillared sample. Clay with interlayer cations exchanged for [Co(NH3)6]3+, without pillaring, showed no selective catalytic reduction activity, but still produced high amounts of N2O. [ABSTRACT FROM AUTHOR]

Selective non-catalytic reduction (SNCR) with NH 3 as the reducing agent is widely used for the denitrification of flue gas in coal-fired boilers, where fly ash significantly influences the conversion of the residual NH 3 that does not participate in denitrification. However, there have been few studies on the exact nature of this influence, particularly the adsorption and reaction mechanisms of NH 3 on fly ash. In this study, temperature-programmed desorption (TPD) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were used to study the mechanisms of NH 3 adsorption and reactions over coal ash. In the absence of oxygen, in the temperature range of 50–450 °C, NH 3 was adsorbed on the surface of the coal ash. The adsorption capacity of lignite ash was higher than that of anthracite ash. This difference was attributed to the large specific surface area and surface acidity of the lignite ash. However, between 450–850 °C, coal ash had a catalytic effect on NH 3 decomposition and oxidation. Due to the high surface lattice oxygen content of lignite ash, its catalytic oxidative ability was superior to anthracite ash. Moreover, NH 3 was first adsorbed over Lewis and Brønsted acid sites on the surface of coal ash and later underwent hydrogen abstraction to produce either the NH 2 or the NH intermediate. The intermediates further reacted with the surface lattice oxygen of coal ash to produce NO and N 2 O. These results might be helpful for the management of NH 3 residues from SNCR processes and the utilization of amino reducing agents in coal-fired boilers. [Display omitted] • NH 3 adsorption capacity of lignite ash is better than that of anthracite ash. • Catalytic oxidation capacity of lignite ash is better than that of anthracite ash. • Catalytic oxidation capacity of coal ash depends on surface-active oxygen. • NH 3 adsorbed on Lewis acid sites of coal ash; reacts to form NO and further N 2 O. [ABSTRACT FROM AUTHOR]

As a carbon-free fuel, NH 3 has been considered as a potential and promising alternative energy carrier in the future transportation system. This study simultaneously measured the temperature and NH 3 -concentration time histories during the oxidation processes of the shock-heated NH 3 /H 2 mixtures by combining laser absorption techniques. The v " = 0, P8 and v " = 1, R21 lines in the fundamental vibrational band of CO were selected for temperature time-history measurements. Nine transition lines in the v 2 fundamental vibrational band of NH 3 near 8.91 µm were selected for NH 3 -concentration time-history measurements. The temperature- and pressure-dependent NH 3 absorption cross-section highly diluted in argon was measured at 1000–2200 K and 0.8–3.9 bar, respectively. The time-dependent temperature and NH 3 concentration were then measured and compared with the predictions based on four mechanisms (Mathieu & Petersen, 2015; Glarborg et al., 2018; Wang et al., 2020; Shrestha et al., 2021). The four mechanisms were substantially slow in predicting the NH 3 -concentration time histories for the stoichiometric NH 3 /H 2 mixtures. The Glarborg et al. mechanism and the Shrestha et al. mechanism showed very good agreements with the experiments for the fuel-rich NH 3 /H 2 mixtures, especially in predicting the NH 3 -concentration time-history profiles in the burnout state. Comprehensive kinetics analyses illustrated that the NH 3 addition evidently changed the mixture reactivity and formation and consumption processes of H and OH radicals. This study updated the rate coefficients of NH 3 + OH = NH 2 + H 2 O in the Glarborg et al. mechanism and the modified mechanism showed improved performances in predicting the NH 3 -concentration profiles. [ABSTRACT FROM AUTHOR]

In the present paper, the kinetics of the NH 3 oxidation on Fe/BEA zeolite catalyst was examined representing a side reaction of the selective catalytic reaction of NO x . The kinetic studies were performed in a gradient-free loop reactor between 723 and 773 K varying the concentration of NH 3 and O 2 . The NH 3 oxidation was described by a global kinetic model implying the adsorption/desorption of NH 3 followed by its reaction with O 2 . The implemented kinetic parameters were suitable for the reproduction and prediction of the experiments over a broad range of NH 3 and O 2 concentrations evidencing the reliability of the model. [ABSTRACT FROM AUTHOR]

[Display omitted] • Pt-V-W supported on TiO 2 catalysts were prepared by co-impregnation method. • Simultaneous oxidations of NH 3 and CO were characterized over Pt-V-W/TiO 2. • Pt-V-W/TiO 2 exhibited improved N 2 selectivity compared to Pt/TiO 2. • Pt-V interaction plays a role in active site of i-SCR and CO-SCR during oxidations. We report a superior N 2 selectivity in the catalytic emission control of NH 3 and CO over PtVW/TiO 2 catalysts synthesized by the co-impregnation method. The PtVW/TiO 2 selective oxidation catalyst (SCO) enabled the following surface reactions over bimetallic Pt-V sites: (1) selective catalytic reduction of internally generated NO x species by NH 3 (i -SCR) and (2) selective catalytic reduction of NO x by the CO (CO-SCR) present in the feed gas. The enhanced N 2 selectivity was characterized in terms of adsorbent behavior and catalyst surface properties as a function of the catalyst formulations. It was found that the strong interaction between the Pt and V metal species increased both the reducibility and the surface acidity of the catalyst, which could be a crucial factor in the enhancement of N 2 selectivity. Surface IR measurement and DFT calculations relates these enhanced catalytic properties to the removal of the generated NO x via reaction with stored imide, amide (–NH x) and isocyanate (–NCO) species on the catalyst surface. These stored species enhance the N 2 selectivity through the i -SCR and CO-SCR mechanisms. A dual-zoned bench-scale monolith reactor has been tested, with a commercial SCR (front) and the developed SCO (rear) in series. This dual-zoned system showed high N 2 selectivity and outstanding CO and NH 3 oxidation. [ABSTRACT FROM AUTHOR]

Abstract: The surface chemistry of NH3 decomposition and oxidation was examined over M n+/Ce n+/Na+/γ-Al2O3 commercial catalysts incorporating Pt, Pd, and Rh in order to clarify the role of these materials in converting NH3 into N2 and NO x within the dense phase of FCC regenerators. All catalysts were active in promoting the decomposition of NH3 into N2 in the 500–700°C temperature range, with the Rh containing catalyst being the most active at 700°C. Oxygen species activated on the surfaces of these materials significantly enhanced the selective conversion of NH3 into N2. When O2 was present in the feed, complete conversion of NH3 – with approximately 80% selectivity to N2 – was observed over the reduced Pt-, Pd-, and Rh-containing materials at temperatures as low as 200, 300, and 350°C, respectively. At temperatures below 350°C, the NH3 oxidation reaction follows an iSCR-type pathway, in which a fraction of NH3 is converted into NO x and subsequently into surface nitrite/nitrate species capable of reacting with the remaining portion of NH3 to form N2. Although the oxidation of NH3 to NO x prevails at 700°C under excess O2 conditions, optimal O2 concentrations can be found under which both complete NH3 conversion and high N2 selectivity are achieved. Under these conditions, the noble metal component of the M n+/Ce n+/Na+/γ-Al2O3 materials promotes the reaction of NH3 with the NO x formed, yielding N2. Excess O2 effectively hinders this reaction pathway at high temperatures. Overall, the results presented herein indicate that the significant increase in NO x emissions, typically observed over conventional Pt-based FCC CO combustion promoters, could be directly attributed to the oxidation of NH3 formed in the reducing zone of the FCC regenerators. [Copyright &y& Elsevier]

Abstract: Selective catalytic reduction with NH3 (NH3-SCR technology), based on V2O5/WO3/TiO2 catalysts, has been previously commercialized for abating NOx emissions from various stationary and mobile lean-burn or diesel engines. However, meeting the uniquely stringent US EPA 2010 regulations for diesel engines required introduction of a new class of SCR catalysts, based on Cu- or Fe-exchanged zeolites. While remarkably active and stable, these new materials proved substantially more difficult than vanadia-based catalysts to operate transiently on the road, due to their much higher NH3 storage. The objective of this work was to develop a concise experimental protocol, elucidating multiple catalytic functions, steady-state and transient, of practical relevance to the mobile SCR applications. This paper provides a comprehensive overview of such functions, using select data from various representative Cu- and Fe-zeolite catalysts. While the bulk of the reported results originated directly from the developed protocol, additional experiments, validating the assumptions or clarifying unexpected experimental observations, are included. [Copyright &y& Elsevier]

Abstract: A series of WO3-promoted zirconia supports were synthesized and calcined between 400 and 800°C. Subsequently vanadium oxide was introduced to obtain 3.5wt% V2O5. The influence of the calcination temperature and potassium-poisoning (K/V=0.2, molar ratio) on the catalytic activity in the selective catalytic reduction (SCR) of NO with NH3 was previously studied. Here, we focus on the influence of these parameters on the catalytic activity for the undesirable oxidation of ammonia, which decrease the N2 selectivity of the SCR process. It is found that potassium doping of the catalysts results in a considerable decrease in the overall ammonia conversion while selectivity to NO increases. [Copyright &y& Elsevier]

Abstract: The mechanism of ammonia oxidation over Pt, Pd, and Rh wires has been investigated in the Temporal Analysis of Products (TAP) reactor at relevant temperatures in industrial ammonia burners. The results of primary and secondary () interactions with isotopically labeled ammonia at 1073 K enable to conclude that the overall reaction pathways to NO, N2O, and N2 are equivalent on the three noble metals. NO is a primary reaction product, while N2 and N2O originate from consecutive NO transformations. The extent of the secondary reactions determines the net NO selectivity. Rhodium is the most active catalyst for the unwanted reduction of NO by NH3, while platinum shows the lowest activity. This explains the superior NO selectivity attained over Pt and, therefore, its industrial application. The TAP-derived selectivity ranking was substantiated by Density Functional Theory calculations on the (100) facets of the noble metals. We proved experimentally that NO selectivity approaching 100% at complete NH3 conversion can be equivalently attained over Pt, Pd, and Rh by increasing the oxygen content in the feed. For a feed of , both N2O and N2 production are suppressed due to the impeded NO dissociation and favored NO desorption at high oxygen coverage. [Copyright &y& Elsevier]

The selective catalytic reduction (SCR) characteristics of NO and NO2 over V2O5–WO3–MnO2/TiO2 catalyst using ammonia as a reducing agent have been determined in a fixed-bed reactor at 200–400°C. The presence of NO2 enhances the SCR activity at lower temperatures and the optimum ratio of NO2/NO x is found to be 0.5. During the SCR reactions, there are some side reactions occurred such as ammonia oxidation and N2O formation. At higher temperatures, the selective catalytic oxidation of ammonia and the nitrous oxide formation compete with the SCR reactions. The denitrification (DeNO x ) conversion decreases at lower temperatures but it increases at higher temperatures with increasing SO2 concentration. The presence of SO2 in the feeds inhibits N2O formation. [Copyright &y& Elsevier]

Abstract: The effect of oxygen on the activity and selectivity of the NO reduction by NH3 over Pt–Rh (95–5wt.%) alloy gauze at 1023–1073K has been investigated. To this end, the Temporal Analysis of Products (TAP) reactor in combination with isotopic tracers was applied. Single pulse experiments evidenced the rapid activation of NH3 over the catalyst surface covered by adsorbed oxygen. Contrarily, NO requires an essentially reduced surface in order to be dissociated. Adsorbed oxygen species in the O2-pretreated gauze accelerate the reaction of NH3 with NO with respect to the as-received gauze. N2 was the main reaction product and traces of N2O were comparatively formed (N2O/N2 ∼ 10−3). Pulsing of an equimolar O2–15NH3–NO mixture over the Pt–Rh gauze mainly produces 15NO, while the formation of N2 is largely suppressed. The selectivity to 15NO and N2O in the ternary O2–15NH3–NO system diminished upon decreasing the O2/(15NH3 +NO) ratio, in favor of N2. This ratio was qualitatively varied by changing the time delay between O2 and 15NH3–NO in sequential pulse experiments. Our results indicate that the NH3 oxidation by O2 to NO is much faster than the NO reduction by NH3 at similar concentrations of oxygen and nitric oxide. This explains the low production of N2 and N2O in ammonia burners within nitric acid manufacture. [Copyright &y& Elsevier]

Abstract: Factors that direct the selectivity of ammonia oxidation for NO were determined previously (J. Catal. 227 (2004) 90) by the investigation of primary NH3–O2 interactions over pure Pt and Pt–Rh (95–5) alloy gauzes at 973–1173 K in the temporal analysis of products (TAP) reactor. A solid mechanistic understanding of the processes leading to by-products (N2O and N2) requires an analysis of secondary NH3–NO interactions, which we investigated in the TAP reactor with isotopically labeled molecules. Our experiments under transient vacuum conditions indicate that these secondary processes determine the reaction selectivity for N2O and N2 in high-temperature ammonia oxidation over noble metal catalysts. Adsorbed oxygen species initiate the reaction of ammonia with nitric oxide. N2O originates from the coupling of ammonia intermediates (NH x ) and nitric oxide. Different reaction pathways leading to N2 have been identified, including primary (NH3 oxidation) and secondary (NH x and H-assisted NO reduction) processes. The relative contributions of these routes depend on the surface coverage of nitrogen and hydrogen-containing species. A reaction scheme accounting for our experimental observations has been proposed, giving rise to an improved mechanistic description of the complex processes in ammonia burners. [Copyright &y& Elsevier]

The Temporal Analysis of Products (TAP) reactor has been applied to study selectivity-directing factors of the high-temperature NH3 oxidation to NO, N2O, and N2 over commercial knitted Pt and woven Pt–Rh alloy gauzes at 973–1173 K. The unique features of the TAP technique enable investigation of the mechanism of this highly exothermic process under isothermal conditions over catalysts of industrial relevance, and the applicaton of much higher peak pressures as compared to surface science techniques in ultrahigh vacuum. This article focuses on the investigation of primary interactions of NH3 and O2. The overall reaction mechanism was found to be very similar over both Pt and Pt–Rh gauzes. NH3 activation is favored over O2-pretreated gauzes, while the as-received gauzes are virtually inactive for NH3 decomposition to N2. The selectivity to NO primarily depends on the concentration of adsorbed oxygen species. A high ratio of adsorbed O/NHx species favors NO formation, confirming that undesirable secondary reaction paths are minimized at a high O coverage. Besides, our results suggest that the nature of oxygen species influences the distribution of NO and N2 in the product. It is put forward that weakly bounded oxygen species lead to a high NO selectivity, while strongly bounded oxidize NH3 into N2. The interaction of NH3 and NO also contributes to N2 formation, while direct NO decomposition is practically suppressed over the oxidized gauzes. Application of isotopically labeled 15NH3 and high peak pressures were essential for detecting N2O formation during ammonia oxidation. Analysis of secondary interactions of NH3 and NO in the authors'' next project is required to further unravel the origins of reaction by-products like N2O and N2. [Copyright &y& Elsevier]

The oxidation of NH3 over three different types of partially sulphated limestone is investigated experimentally and theoretically. The experiments were made in a laboratory fixed bed reactor at atmospheric pressure at 850°C to simulate conditions of fluidized bed combustion. The catalytic activity of calcined limestone decreases strongly when it is sulphated. The decrease in the rate of NH3 oxidation is mainly caused by the conversion of CaO into less active CaSO4 and by the formation of an impervious layer of CaSO4 around the CaO core.A model based on the grain model has been developed to describe the decrease of activity for NH3 oxidation with increasing conversion of CaO to CaSO4. The changes in structure and in composition of the limestone particle are incorporated in the model. The simulation results are in good agreement with the experimental data.Besides the decrease in activity, the selectivity for the formation of NO decreases during sulphation of the limestone. In experiments, it was shown that the decrease in selectivity is independent of the NO concentration and therefore is not the result of NO reduction by NH3 as proposed in the literature. Further, it was shown that while the formation of NO decreases with increasing CaO conversion the formation of N2 is almost not influenced. From these observations and from simulation results it was concluded that the NH3 oxidation to N2 over CaSO4 is the major cause of the decrease in selectivity. To our knowledge, this is the first validated explanation for the decrease in selectivity for NO during sulphation of calcined limestone. [Copyright &y& Elsevier]

Natural manganese ore (NMO) catalysts were characterized and tested in the selective catalytic oxidation of ammonia to nitrogen oxides under dilute conditions. Also, the oxidation of ammonia (NH3) was carried out using pure MnO2, Mn2O3 for comparing with the activity. It is found that the activity of NMO was similar to that of MnO2 at low temperature below 150 °C but above this temperature, the activity of these catalysts showed the difference. In the course of NH3 oxidation, N2, NO, N2O and H2O were produced. But the quantity of NO2 produced in this experiment was negligible. At temperature below 250 °C, selectivity into N2 from NH3 oxidation was in the order, NMO > MnO2 > Mn2O3. This is the reverse of activity of these manganese oxides. Also the characterization of NH3 oxidation was proposed and supported by the effect of space velocity, inlet O2 and NH3 concentration. The increase of space velocity remarkably influenced not only the conversion but also selectivity into N2. The higher the reaction temperature was, the higher the effect of inlet O2 and NH3 concentration on the reaction rate was. By introducing NO during NH3 oxidation reaction, the possibility of NMO as selective catalytic reduction catalyst at low temperature was studied and showed positive results. [Copyright &y& Elsevier]