Harmless N2 Research Articles

Activity and Mechanism Mapping of Photocatalytic NO2 Conversion on the Anatase TiO2(101) Surface.

NOx emission heavily affects our environment and human health. Photocatalytic denitrification (deNOx) attracted much attention because it is low-cost and nonpolluting, but undesired nitrite and nitrate were produced in reality, instead of harmless N2. Unveiling the active sites and the photocatalytic mechanism is very important to improve the process. Herein, we have employed a combinational scenario to investigate the reaction mechanism of NO2 and H2O on anatase TiO2(101). On the one hand, a polaron-corrected GGA functional (GGA + Lany-Zunger) was applied to improve the description of electronic states in photoassisted processes. On the other hand, a reaction phase diagram (RPD) was established to understand the (quasi) activity trend over both perfect and defective surfaces. It was found that a perfect surface is more active via the Eley-Rideal mechanism without NO2 adsorption, while the activity on defective surfaces is limited by the sluggish recombinative desorption. A photogenerated hole can weaken the OH* adsorption energies and circumvents the scaling relation of the dark reaction, eventually enhancing the deNOx activity significantly. The insights gained from our work indicate that tuning the reactivity by illumination-induced localized charge and diverse reaction pathways are two methods for improving adsorption, dissociation, and desorption processes to go beyond the conventional activity volcano plot limit of dark conditions.

Direct catalytic nitrogen oxide removal using thermal, electrical or solar energy

Considering the significant importance in both ecological and environmental fields, converting nitrogen oxide (NOx, especially NO) into value-added NH3 or harmless N2 lies in the core of research over the past decades. Exploring catalyst for related gas molecular activation and highly efficient reaction systems operated under low temperature or even mild conditions are the key issues. Enormous efforts have been devoted to NO removal by utilizing various driving forces, such as thermal, electrical or solar energy, which shine light on the way to achieve satisfying conversion efficiency. Herein, we will review the state-of-the-art catalysts for NO removal driven by the above-mentioned energies, including a comprehensive introduction and discussion on the pathway and mechanism of each reaction, and the recent achievements of catalysts on each aspect. Particularly, the progress of NO removal by environmentally friendly photocatalysis and electrocatalysis methods will be highlighted. The challenges and opportunities in the future research on the current topic will be discussed as well.

Numerical Simulation of NOx Reduction in a SCR System

The stringent limits of emission standards require advanced emission control technologies to be used in modern on and off highway diesel engines. They include both in-cylinder and aftertreatment measures where the latter now have become almost mandatory. Selective catalytic reduction aftertreatment systems are widely used for nitrogen oxide (NOX) conversion in exhaust gases into harmless N2. To reduce time and costs, at the design stage of SCR systems numerical modelling is applied. Mathematical models and methods providing high prediction accuracy with and acceptable level of computational efforts are required. In this work an approach for complete simulation of SCR systems based on the coupling of commercial CFD software with developed multichannel 1D catalyst model is presented. The first one is used to carefully describe processes occurring upstream in the catalytic converter, particularly, during urea water solution injection and flow mixing. As a result, the distributions of flow parameters at catalyst inlet are derived. They are subsequently imported as boundary conditions into a developed multichannel catalyst model that allows one to take them into account when calculating NOx conversion efficiency. Based on the proposed approach a SCR system was simulated. The effect of non-uniform distributions of NH3 concentration and the gas flow velocity at the catalyst inlet on its performance was investigated. It has been shown that they have a great impact on NOX conversion and should be taken into account during the catalyst modelling.

Nitrogen oxide gas purification using carbon in water as reducing reagent with the aid of microbial fuel cell

Microbial Fuel Cell (MFC) can degrade the organic matter (OM) in wastewater at the anode and transfer electrons to the cathode. In this work, the harmful NOX gas was used as electron acceptor in MFC and converted to harmless N2. The OM in water was indirectly used as a zero-cost reducing agent for NOx removal. More than 80% of NOX was removed continuously by MFC at room temperature. The NOX was directly reduced to N2 at MFC cathode and the cathode activity played a key role on enhancing the NOX removal. The NOX removal efficiency by the cathode of high potential was 1.37 times that by the cathode of low potential. When O2 coexisting with NO as the electron acceptor, not only the NOX removal but also the power output of MFC was improved greatly. The presence of NOX did not decrease the power generation of MFC under the same O2 concentration. The MFCs showed good stability for NOX treatment and power output. Moreover, the possible pathways and advantages of NOX removal by MFC were discussed in detail. These results indicated that the MFC system has the potential to treat wastewater, purify flue gas and recover energy simultaneously.

Unraveling the Direct Decomposition of NOx over Keggin Heteropolyacids and Their Deactivation Using a Combination of Gas-IR/MS and In Situ DRIFT Spectroscopy

Keggin heteropolyacids (HPAs) have been known to be efficient NOx absorbers for many years, and to decompose a significant fraction of the NOx species pre-absorbed at 100-150 °C into harmless N2 an...

Biophysical controls on nitrous oxide emissions following rain-induced thawing of frozen soil microcosms by simulated rainfall

In cold temperate humid regions, peak nitrous oxide (N2O) fluxes coincide with spring snowmelt. However, other extreme climatic events in the non-growing season, such as rainfall that causes temporary thawing at the bare soil surface without snow cover, could be responsible for appreciable fluxes of N2O. The objective of this study was to quantify the proportion of ice-trapped N2O and biologically-produced N2O released following a rain-induced thawing event on soils collected in late fall. The lab experiment was done in soil-filled microcosms after the soil surface was frozen at −20 °C for 2 h, which generated a 1–2 cm ice layer at the soil surface, before they were injected with N2, N2O and 15N2O gas at 5 cm depth and then received different amounts of water with customized rainfall simulators (0, 3, 7.5 or 15 mm simulated rainfall h−1) at 4 °C. Gas samples were collected at 2, 6, 12, 24, 48 and 96 h after gas injection. Soil temperature and water content increased faster after rain-induced thawing than after the air-induced thawing event. The release of ice-trapped N2O was not affected by rain-induced thawing because soil pores filled with melted ice-water, which slowed N2O diffusion and reduced N2O to nitrogen gas (N2), based on the production of 15N–N2. However, high simulated rainfall intensity (7.5 and 15 mm h−1) contributed to greater biological N2O emission after 48 h. We conclude that rainfall on a frozen bare soil surface in cold temperate humid regions may cause N loss through harmless N2, rather than the greenhouse gas N2O.

A ClO[rad]-mediated photoelectrochemical filtration system for highly-efficient and complete ammonia conversion

The ability to convert excess ammonia in water into harmless N2 is highly desirable for environmental remediation. We present a chlorine-oxygen radical (ClO)-mediated photoelectrochemical filtration system for highly efficient and complete ammonia removal from water. The customized photochemical device comprised a Ag-functionalized TiO2nanotube array mesh photoanode and a Pd-Cu co-modified nickel foam (Pd-Cu/NF) cathode. Under illumination, holes generated at the anode catalyzed the conversion of H2O and Cl− to HOand Cl, respectively. In turn, these radicals then reacted further, yielding ClO, which selectively decomposed ammonia. The cathode enabled further reduction of anodic byproducts such as NO3− to N2. The complete oxidation of all dissolved ammonia was achieved within 15 min reaction under neutral conditions, where N2 was the dominant product. The impact of key parameters was assessed, which enabled the discovery of optimal reaction conditions and the proposal of the underlying working mechanism. The flow-through configuration demonstrated a 5-fold increase of ammonia oxidation rate compared to the conventional batch reactor. The role of ClO in the oxidation of ammonia was verified with electron paramagnetic resonance and scavenger studies. This study provided greater mechanistic insights into photoelectrochemical filtration technology and demonstrated the potential of future nanotechnology for removing ammonia.

Selective Oxidation of Diethylamine on CuO/ZSM-5 Catalysts: The Role of Cooperative Catalysis of CuO and Surface Acid Sites

It still remains a big challenge that achieving the selective oxidation of hazardous nitrogen-containing volatile organic compounds to the harmless molecular N2. Here we present the synthesis of Cu...

A New Dynamic Approach for N2O Decomposition by Pre‐reduced Rh/CeZrOx Catalysts

AbstractThis study deals with the catalytic decomposition of N2O into harmless N2 and O2, in view of its potential application to decrease the pollution from internal combustion engines. The key of this work was to pre‐reduce a Rh/CeZrOx catalyst under a H2 based atmosphere, in order to enhance the amount of oxygen vacancies in the ceria‐based support. The results demonstrated that both, Rh species and oxygen vacancies of the ceria‐based support catalyze the N2O decomposition reaction. However, the presence of high concentrations of H2O and O2 strongly inhibited the catalytic activity. Nevertheless, small O2 concentrations maintained the performance. Therefore, the pre‐reduced Rh/CeZrOx could be potentially used for the reduction of N2O emitted by internal combustion engines working in stoichiometric conditions. In addition, a short injection of a reducing agent (e. g. H2) allowed to partially recover the initial catalytic activity of the system. All in all, this study proposes a new potential approach to mitigate the overall N2O greenhouse gas emissions.

Pd and Pd,In nanoparticles supported on polymer fibres as catalysts for the nitrate and nitrite reduction in aqueous media

The presence of high concentrations of nitrate and nitrite ions in water sources is an actual environmental problem. Nowadays, there are several technologies for nitrate and nitrite treatment, but most of them involve the concentration of the ions which are later necessary to be decomposed in a second step. The catalytic elimination of nitrates and nitrites from water is a very promising method because the ions are transformed into the harmless N2(g). Two types of synthetic polymeric fibers were proposed as supports of palladium and indium catalytic species with low metallic loading (Pd:In = 1:0.25 wt.%). They were synthesized by electrospinning method with a Pd/In weight ratio 1:0.25. PdInDA catalyst was prepared by dissolving poly (methyl methacrylate) (PMMA) with a mixture of acetone/dimethylformamide, whereas the polymer (PMMA) was dissolved in pure dimethylformamide for the synthesis of PdD and PdInD catalysts. Both bimetallic solids gave rise to good selectivity to N2(g). However, the PdInDA reached middle nitrate conversions (around 50 %) with low ammonia production (under 0.1Nppm). The characterization results suggested that the active sites remained unchanged after the reaction. This proposal involves a green chemical method with low consumption of reactants and producing a small quantity of waste.

Simultaneous removal of NO and SO2 through a new wet recycling oxidation–reduction process utilizing micro-nano bubble gas–liquid dispersion system based on Na2SO3

Wet desulfurization and denitrification technologies are one of the main research directions in the future air pollution control. Compared with the traditional macro-bubbles, the MNBs can spontaneously generate ·OH with high oxidizing ability, and also improve the gas solubility. Na2SO3 is a strong reducing agent and can react with NO, NO2, NO2– and NO3– to convert ionic N to non-toxic and harmless N2. In the study, based on the principle of wet desulfurization and denitrification, a new recycling oxidation–reduction process was firstly proposed for simultaneous removal of NO and SO2 by using the MNBGLS which was generated with a MNBG through recycling Na2SO3 aqueous solution at a given concentration from a column reactor and the simulated flue gas composed of NO, SO2, N2 and/or other gases such as O2 and CO2. The MNBGLS was recycled continuously from the MNBG to the reactor. The effects of various experimental parameters, such as operation time, initial pH and temperature of absorbent, Na2SO3 concentration, NO concentration, SO2 concentration, the presence of O2 and CO2, on the removal of NO and SO2, were investigated. All the investigated parameters affected the removal of NO, whereas only initial solution pH influenced the removal of SO2. This new process can not only achieve high NO and SO2 removal efficiencies but also treat NO2– and NO3– so as to reduce the cost of subsequent wastewater treatment. Moreover, the main removal product, SO42−, can be recycled. Thus it is an economical, environmentally friendly and efficient desulfurization and denitrification process.

The deposition of VWOx on the CuCeOy microflower for the selective catalytic reduction of NOx with NH3 at low temperatures

NOx emissions are a major environmental problem, and the selective catalytic reduction (SCR) is the most effective method to convert NOx in flue gas into harmless N2 and H2O. In this work, a new carrier, CuCeOy microflower assembled from a large number of copper-cerium mixed oxide nanosheets, is firstly developed to load vanadium-tungsten mixed oxides (VWOx) for the SCR of NOx with NH3. The resultant optimal VWOx/CuCeOy catalyst exhibits significantly enhanced low-temperature de-NOx performance with the NOx conversion of 60% at 180 °C, over 90% from 240 °C to 390 °C under the gas hourly space velocity (GHSV) of 36,000 h−1. The reason can be mainly attributed the fact that the transfer of electrons among Ce, Cu and V ions is very easy to occur via the following equations Ce3++Cu2+ ↔ Ce4++Cu+, V5+ + Cu+ ↔ V4+ + Cu2+, V4+ + Ce4+ ↔ V5+ + Ce3+, which effectively decreases the apparent activation energy (Ea = 16.59 kJ/mol) of NH3-SCR de-NOx reaction. In addition, the enhanced reducibility and a large number of Brønsted acid sites also contribute the low-temperature de-NOx performance. Both Eley-Rideal and Langmuir-Hinshelwood mechanisms are included in the NH3-SCR de-NOx reaction over the VWOx/CuCeOy catalyst.

Interactions of anaerobic ammonium oxidizers and sulfide-oxidizing bacteria in a substrate-limited model system mimicking the marine environment.

In nature anaerobic ammonium oxidation (anammox) and denitrification processes convert fixed nitrogen to gaseous nitrogen compounds, which are then released to the atmosphere. While anammox bacteria produce N2 from ammonium and nitrite, in the denitrification process nitrate and nitrite are converted to N2 and the greenhouse gas nitrous oxide (N2O). Furthermore, nitrite needed by the anammox bacteria can be supplied by nitrate reduction to nitrite. Consequently, the interplay between nitrogen-transforming microorganisms control the amount of harmless N2 or the greenhouse gas N2O released to the atmosphere. Therefore, it is important to understand the interactions of these microorganisms in the natural environment, where dynamic conditions result in fluctuating substrate concentrations. Here, we studied the interactions between the sulfide-oxidizing denitrifier Sedimenticola selenatireducens and the anammox bacterium Scalindua brodae in a bioreactor mimicking the marine environment by creating sulfide, ammonium and nitrate limitation in distinct operational phases. Through a microbial interaction, Se. selenatireducens reduced nitrate to nitrite, which together with the supplied ammonium was converted to N2 by Sc. Brodae. Using comparative transcriptomics, we determined that Sc. Brodae and Se. selenatireducens had significant responses to ammonium and nitrate limitation, respectively, indicating that the activities of these microorganisms are regulated by different nitrogen compounds.

Self-supported Cu nanosheets derived from CuCl-CuO for highly efficient electrochemical degradation of NO3−

Abstract Degradation of NO3− into harmless N2 is a significant challenge in the field of water treatment. In this work, 3D self-supported hierarchical electrode composed of ultra-thin Cu nanosheets-assembled micro-columns on porous nickel foam (Cu NSs/NF) was prepared by three steps, including electrodeposition of CuCl-Cu, thermal oxidation into CuCl-CuO and electrochemical reduction into Cu NSs. Benefitting from the advantages of large surface area and highly active interface as well as fast electron-transfer path, Cu NSs/NF exhibits the significant positive-shift of potential and higher current density than other Cu-based electrodes in the electrochemical reduction of NO3− (NO3−-ER). Not only that, an extremely high sensitivity of nitrate reduction response up to 11.23 mA mM−1 cm−2 was obtained by chronoamperometric test, which is much higher than those of previously reported works. Moreover, a conversion percentage of 99.7% for NO3−-ER was achieved on Cu NSs/NF cathode in NO3−-ER experiments. Finally, through the cathodic reduction of NO3− coupled with the anodic oxidation of Cl−, and the subsequent chemical reaction of NH4+ and ClO−, the highly efficient degradation of NO3− into N2 with selectivity as high as 97.2% was realized. This work provides a highly efficient Cu NSs/NF electrode and a promising way for degradation of NO3− into N2.

Extremely Efficient Decomposition of Ammonia N to N2 Using ClO• from Reactions of HO• and HOCl Generated in Situ on a Novel Bifacial Photoelectroanode.

The conversion of excess ammonia N into harmless N2 is a primary challenge for wastewater treatment. We present here a method to generate ClO• directionally for quick and efficient decomposition of NH4+ N to N2. ClO• was produced and enhanced by a bifacial anode, a front WO3 photoanode and a rear Sb-SnO2 anode, in which HO• generated on WO3 reacts with HClO generated on Sb-SnO2 to form ClO•. Results show that the ammonia decomposition rate of Sb-SnO2/WO3 is 4.4 times than that of WO3 and 3.3 times than that of Sb-SnO2, with achievement of the removal of NH4+ N on Sb-SnO2/WO3 and WO3 being 99.2 and 58.3% in 90 min, respectively. This enhancement is attributed to the high rate constant of ClO• with NH4+ N, which is 2.8 and 34.8 times than those of Cl• and HO•, respectively. The steady-state concentration of ClO• (2.5 × 10-13 M) is 102 times those of HO• and Cl•, and this is further confirmed by kinetic simulations. In combination with the Pd-Cu/NF cathode to form a denitrification exhaustion system, Sb-SnO2/WO3 shows excellent total nitrogen removal (98.4%), which is more effective than WO3 (47.1%) in 90 min. This study provides new insight on the directed ClO• generation and its application on ammonia wastewater treatment.

Nitrogen conversion during the homogeneous and heterogeneous stages of sludge steam gasification: Synergistic effects of Fenton's reagent and CaO conditioner

Fenton’s reagent (Fe2+ + H2O2) and CaO are the conditioners during sludge dewatering, which also showed abilities in NOx precursors control during drying and pyrolysis process that we have found before. This study further refined the effects of conditioners on homogeneous and heterogeneous processes during sludge steam gasification using a self-designed decoupling reactor. The results show that compared with raw sludge, the NH3 and HCN yields of conditioned sludge decreased by 13.71–17.67 mL/g and 17.77–23.45 mL/g at 873–1073 K respectively. 50.59–98.76% of decrement attributed to the synergistic effect of conditioners on volatiles homogeneous reforming, owing to the less NH3, HCN and tar-N yields in devolatilization product as well as the indirect enhancement of HCN hydrolysis. Fenton’s reagent added alone exhibited a significant effect on volatiles heterogeneous reforming. During the interaction between volatiles and char, iron salts could catalyze the nitrile-N and pyrrole-N in char conversion to stable indoles in tar and inhibited the hydrolysis of amine-N to release NH3. Calcium salts also showed a certain capability on the co-decomposition of quaternary-N in char and indoles/quinolones in tar to harmless N2. Thus, the combination of Fenton’s reagent and CaO can complementarily reduce the emission of NOx precursors at each stage of gasification.

Preparation of a novel Cu-Sn-Bi cathode and performance on nitrate electroreduction.

Cu-Sn-Bi layer coated on Ti substrate was prepared using electrodeposition method and applied as cathode material for electrochemical reduction of nitrate in this research. Linear sweep voltammetry (LSV), chronoamperometry (CA), scanning electron microscope (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD) were used to scrutinize the electrochemical performance and the cathode materials. LSV results illustrated that Cu-Sn-Bi cathode possessed the ability for nitrate reduction. Preparation conditions including deposition time, current density, temperature and the content of Bi were optimized based on NO3 -N removal and byproducts selectivity. Results showed that the cathode with Bi content of 3.18 at.%, and electrodepositing at current density of 6 mA cm-2, 35 °C for 30 min achieved the best performance during the experiment. The increase of Bi content could improve the electrocatalytic activity and stability of the cathode. Compared with other common researched cathodes (Cu and Fe), Cu-Sn-Bi (3.18 at.%) exhibited better performance, i.e. the highest NO3 -N removal of 88.43% and the selectivity of harmless N2 was 77.80%. The kinetic studies showed that the reduction of nitrate on Cu-Sn-Bi followed pseudo-first-order kinetics.

Dissociation of N2O on anatase TiO2 (001) surface – The effect of oxygen vacancy and presence of Ag cluster

The increase in concentration of nitrous oxide (N2O) in the atmosphere is one of the major contributors to the greenhouse effect, ozone depletion and climate change. Therefore, it is important to decompose harmful N2O molecule into harmless N2. In the present work, we have studied the decomposition of N2O on anatase TiO2 (001) surface using first principle calculations. The results indicates that the N2O molecule is physisorbed on perfect TiO2 surface without any dissociation, and is dissociated into N2 and oxygen on the reduced TiO2 surface. In addition, it has been found that the interaction between N2O and TiO2 is augmented by the presence of Ag cluster on anatase (001) surface. On the basis of Bader charge analysis and electron density difference plot, it has been found that the excess charge in the reduced anatase TiO2 (001) surface is transferred to the adsorbed N2O molecule, which results the weakening of N–O bond of N2O followed by the decomposition of N2O into N2 and O. Vibrational frequency analysis also performed to confirm the decomposition of N2O molecule. From the pathway for N2O dissociation on reduced TiO2 and Ag/TiO2 surfaces, it has been observed that the dissociation reaction of N2O on TiO2 surface is highly exothermic with activation energy barrier of 0.25eV. The results presented in this work show that the reduced anatase TiO2 with (001) facet is a good catalyst for the decomposition of N2O molecule.

Numerical simulation of copper recovery from converter slags by the utilisation of spent potlining (SPL) from aluminium electrolytic cells

An innovative method was devised to improve copper recovery and operational efficiency of the Cu converter slag-cleaning furnace, by utilising both carbon and fluoride values of an otherwise environmental hazardous material (i.e. spent potlining (SPL)) produced from the aluminium industry. Results based on numerical modelling show that 90% Cu recovery could be achieved by adding as little as 3–4 wt-% of SPL to the molten converter slag. The fluorides and sodium-containing compounds in the SPL reduced the slag viscosity, resulting in a much faster settling rate of matte droplets. In this process, the SPL could be detoxified by destroying the cyanides to form harmless N2 gas, and inertising the fluorides in a much diluted form in the ferro-silicate slag, ensuring a safe disposal to the environment.

Photocatalytic Hydrogen Formation from Ammonia in an Aqueous Solution Over Pt-Enriched TiO2-ZrO2 Photocatalyst.

The aim of this study was to remove ammonia from an aqueous solution by its decomposition to valuable products such as H2 and harmless N2 under UV light. The decomposition of ammonia by photocatalytic process represents an emerging and interesting way of its removal since beside the need of its reduction from the drinking and wastewaters with the respect to its negative impact on human and mammals health, it can lead to generation of hydrogen as an alternative fuel. A laboratory-synthesized Pt/TiO2-ZrO2 photocatalyst was studied and its photocatalytic activity was compared with the activity of commercial TiO2 Evonik P25. The Pt/TiO2-ZrO2 photocatalyst was prepared by combining a sol-gel process controlled within reverse micelles of nonionic surfactant Triton X-114 in cyclohexane, impregnation under vacuum and calcination. Explored photocatalysts were characterized by organic elementary analysis, nitrogen physisorption, XRD, FESEM and UV-Vis spectroscopy. The real platinum content in the Pt/TiO2-ZrO2 photocatalyst was determined by ICP-MS. The photocatalytic decomposition of ammonia was investigated in the time range of 0-12 h. During the first two hours the generation of hydrogen was almost negligible. The generation of hydrogen increased after 4 h of irradiation. Based on time dependences of ammonia decomposition the kinetic rate constants for Pt/TiO2-ZrO2 and TiO2 Evonik P25 photocatalysts were calculated. The ammonia photocatalytic decomposition was described well by the first order kinetic equation. The photocatalytic ammonia decomposition over the platinized TiO2-ZrO2 photocatalyst was proving 2 times higher photocatalytic performance than Evonik P25 (1241 μmol/g(cat) and 665 μmol/g(cat), respectively).