Littérature scientifique sur le sujet « Urea oxidation reaction »

In this feature article, we discuss the significant role of the pre-oxidation reaction during urea electro-oxidation, and summarize the detailed strategies and recent advances in promoting the pre-oxidation reaction.

Secondary problems, such as the occurrence of side reactions and the accumulation of by-products, are a major challenge in the application of wet denitrification technology through urea solution. We revealed the formation mechanism of urea nitrate and clarified the main and side reaction paths and key intermediates of denitrification. Urea nitrate would be separated from urea absorption solution only when the concentration product of [urea], [H+] and [NO3−] was greater than 0.87~1.22 mol3/L3. The effects of the urea concentration (5–20%) and reaction temperature (30–70 °C) on the denitrification efficiency could be ignored. Improving the oxidation degree of the flue gas promoted the removal of nitrogen oxides. The alkaline condition was beneficial to the dissolution process, while the acidic condition was beneficial to the reaction process. As a whole, the alkaline condition was the preferred process parameter. The research results could guide the optimization of process conditions in theory, improve the operation efficiency of the denitrification reactor and avoid the occurrence of side reactions.

Urea oxidation reaction (UOR) has received a high level of recent interest since electrochemical oxidation of urea can remediate harmful nitrogen compounds in wastewater and accomplish hydrogen fuel production simultaneously. Thus, urea is considered to be potential hydrogen energy source that is inherently safe for fuel cell applications. However, the catalytic reaction suffers from slow kinetics due to six electron transfer in UOR. In this work, β phase NiS is successfully prepared through facile hydrothermal reaction, in which diethanolamine (DEA) was added as chelating agent leading to 3D nanoflower morphology. The crystal structure, surface morphology, and chemical bonding of the β−NiS were characterized by X–ray diffraction (XRD), scanning electron microscope (SEM), and X−ray photoelectron spectroscopy (XPS), respectively. The UOR performance of NiS was evaluated by means of linear sweep voltammetry (LSV), Tafel analysis, electrochemical impedance spectroscopy (EIS), chronoamperometry, and chronopotentiometry in 1 M KOH electrolyte containing 0.33 M urea. Compared to the Ni(OH)2 counterpart, NiS exhibits lower onset potential, increased current responses, faster kinetics of urea oxidation, lower charge transfer resistance, and higher urea diffusion coefficient, leading to the enhanced catalytic performance toward UOR. Moreover, the developed NiS catalyst exhibits superior stability and tolerance towards urea electro−oxidation in 10,000 s test.

The oxidation of formamidine disulfide, FDS, the dimer of thiourea, by aqueous chlorine dioxide has been studied in highly acidic and mildly acidic media. FDS is one of the possible oxidation intermediates formed in the oxidation of thiourea by oxyhalogens to urea and sulfate. The reaction is exceedingly slow, giving urea and sulfate with a stoichiometric ratio of 5 : 14 FDS to chlorine dioxide after an incubation period of up to 72 h and only in highly acidic media which discourages the disproportionation of chlorine dioxide to the oxidatively inert chlorate. Mass spectrometric data suggest that the oxidative pathway proceeds predominantly through the sulfinic acid, proceeding next to the products sulfate and urea, while by-passing the sulfonic acid. Transient formation of the unstable sulfenic acid was also not observed.

Energy shortage and environmental pollution have become the most serious problems faced by human beings in the 21st century. Looking for advanced clean energy technology to achieve sustainable development of the ecological environment has become a hot spot for researchers. Nitrogen-based substances represented by urea are environmental pollutants but ideal energy substances. The efficiency of urea-based energy conversion technology mainly depends on the choice of catalyst. The development of new catalysts for urea oxidation reaction (UOR) has important application value in the field of waste energy conversion and pollution remediation based on UOR. In this work, four metal–organic framework materials (MOFs) were synthesized using ultrasound (NiCo-UMOFs) and hydrothermal (NiCo-MOFs, Ni-MOFs and Co-MOFs) methods to testify the activity toward UOR. Materials prepared using the hydrothermal method mostly form large and unevenly stacked block structures, while material prepared using ultrasound forms a layer-by-layer two-dimensional and thinner structure. Electrochemical characterization shows NiCo-UMOFs has the best electrocatalytic performance with an onset potential of 0.32 V (vs. Ag/AgCl), a Tafel slope of 51 mV dec−1, and a current density of 13 mA cm−2 at 0.5 V in a 1 M KOH electrolyte with 0.7 M urea. A prolonged urea electrolysis test demonstrates that 45.4% of urea is removed after 24 h.

Palladium Catalyzed Reactions: Reductive Cyclization of Nitroarenes, and Oxidative Carbonylation of Aniline Abstract: The thesis is divided into two main chapters: reductive cyclization of nitroarenes, and oxidative carbonylation of aniline. The first chapter involves developing a catalytic system for carbazoles synthesis through reductive cyclization of 2-nitrobiphenyls employing phenyl formate as an in-situ source of CO. Thus, the synthetic chemist can avoid handling pressurized CO lines and perform the reaction in a pressure tube, a cheap and readily available tool for any laboratory. Moreover, the developed protocol can tolerate both air and moisture and can be performed using undried and undistilled commercial DMF. Several carbazoles bearing a wide range of substituents were synthesized in good to excellent yields including some with valuable pharmaceutical or thermo/electrical applications. The reaction could be performed on the grams scale affording carbazole in a very good yield (85%) without the need for chromatographic purification, making our synthetic strategy even more attractive and economically advantageous. The second chapter deals with the palladium/iodide couple which is the most investigated catalytic system for the oxidative carbonylation of amines to give ureas or carbamates. In reinvestigating it, we found that the most prominent role of iodide is to etch the stainless steel of the autoclave employed in most of previous works, releasing in solution small amounts of iron salts. The latter are much better promoters than iodide itself. Iron and iodide have a complex interplay and, depending on relative ratios, can even deactivate each other. The presence of a halide is beneficial, but chloride is better than iodide in this respect. The ideal Fe/Pd ratio is around 10, but even an equimolar amount of iron with respect to palladium (0.02 mol% with respect to aniline, corresponding to 12 ppm Fe with respect to the whole solution) is sufficient to boost the activity of the catalytic system. Such small amount may also come from Fe(CO)5 impurities present in the CO gas when stored in steel tanks. The role of the solvent has also been investigated. It was found that the reason for the better selectivity in some cases is at least in part due to a hydrolysis of the solvent itself, which removes the coproduced water.

Les objectifs de ce travail étaient de dévoiler la dépendance de l'activité et des produits de l'UOR à la structure et à la composition des catalyseurs à base de Ni. Nous avons utilisé la spectroscopie XAS operando pour étudier la structure des catalyseurs et l'état d'oxydation moyen du Ni dans les catalyseurs Ni, NiCo et NiFe pendant leur fonctionnement, et la spectroscopie FTIRS pour détecter les produits et les intermédiaires formés pendant l'UOR. La spectroscopie FTIR operando nous a permis de détecter plusieurs produits d'oxydation de l'urée sur les électrodes Ni, NiFe et NiCo, suggérant que l'UOR suit deux voies parallèles. La voie aboutissant à la formation de carbonate (et donc aussi de N2 qui ne peut pas être détecté par FTIRS) prédomine à des potentiels plus bas, tandis que la formation de nitrite et de cyanate augmente à des potentiels plus élevés. Les mesures XAS operando à l'arête K du Ni nous ont permis de détecter la formation de NiOOH, et la diminution de sa fraction en présence d'urée. Les résultats soutiennent le mécanisme UOR 'EC' (électrochimique - suivi d'une étape chimique) fonctionnant à la fois pour les catalyseurs monométalliques et bimétalliques, le rôle du second composant (Fe, Co) étant principalement lié à son influence sur le potentiel de la transition Ni(OH)2/NiOOH et donc sur le surpotentiel de l'UOR
The objectives of the work were to unveil the dependence of the activity and the products of the UOR on the structure and composition of Ni-based catalysts. We used operando XAS to investigate both the structure of the catalysts and the mean oxidation state of Ni in Ni, NiCo and NiFe catalysts during their operation, and FTIRS to detect products and intermediates formed during the UOR. The operando FTIR spectroscopy allowed us to detect several urea oxidation products on Ni, NiFe and NiCo electrodes, suggesting that the UOR follows two parallel pathways. The pathway resulting in the carbonate formation (and hence also N2 which cannot be detected with FTIRS) predominates at lower potentials, whereas the formation of nitrite and cyanate increases at higher potentials. The operando XAS measurements at Ni K-edge allowed us to detect formation of NiOOH, and decrease of its fraction in the presence of urea. The results support the EC’ (electrochemical – followed by a chemical step) UOR mechanism operating both for monometallic and for bimetallic catalysts, the role of the second component (Fe, Co) mainly related to its influence on the potential of the Ni(OH)2/NiOOH transition and hence the UOR overpotential

Dissertação de Mestrado Integrado em Engenharia Química apresentada à Faculdade de Ciências e Tecnologia
A capacidade dos organismos vivos manterem um equilíbrio harmonioso com a natureza permitiu a sobrevivência e a evolução até aos dias de hoje. Mas este equilíbrio é influenciado por muitos comportamentos que fazem parte da vida quotidiana. A agitação e o ritmo acelerado da atualidade podem ser a origem de doenças, muitas delas irreversíveis. Nos últimos tempos considera-se que as causas de doenças neurodegenerativas como as de Alzheimer, Parkinson e depressão e de muitas outras doenças estão associadas ao estilo de vida atual. As pessoas lidam diariamente com stress no trabalho e no ambiente escolar e familiar. Estas situações são tão habituais que a população em geral já as considera “normais”. Por outro lado, o estilo de vida cada vez mais industrializado, mantém as pessoas em contacto com uma infinidade de fármacos e produtos de cuidado pessoal (PPCPs) que se podem acumular no organismo humano. A exposição contínua a este tipo de situações pode levar a um desequilíbrio da atividade cerebral e causar danos permanentes. Mecanismos fisiológicos diversos, incluindo os de defesa, atuam constantemente tentando restabelecer o equilíbrio. Estes processos podem beneficiar muito da ação de nutrientes específicos. Tendo em conta as considerações anteriores, o objetivo principal deste trabalho consistiu em estudar respostas a stress fisiológico, envolvendo uma hormona de stress ou um conservante de um produto de cuidado pessoal. Para tal utilizaram-se fatias cerebrais (400 µm) de ratos Wistar e efetuaram-se registos de autofluorescência e de fluorescência relacionados com a produção de espécies reativas de oxigénio (ROS) e com o potencial de membrana mitocondrial (Δψm), nas sinapses das fibras musgosas da área CA3 do hipocampo. No primeiro conjunto de experiências utilizou-se a corticosterona, a principal hormona libertada nos ratos em situações de stress, em concentrações de 1 µM, 4 µM e 10 µM. Inicialmente mediram-se sinais de autofluorescência das proteínas mitocondriais NADH e FAD, tendo-se verificado que a oxidação de NADH é mais sensível às variações de corticosterona e que este sinal se aproxima lentamente do valor basal quando a hormona deixa de estar presente. Por outro lado, nestas experiências a corticosterona tem uma ação irreversível na oxidação da proteína FADH2, uma vez que o sinal não recupera quando a hormona de stress é retirada. Fatias incubadas com o indicador fluorescente H2DCFDA, permitiram analisar variações de ROS, que aumentaram entre 1% e 6% para as concentrações estudadas. A ação de 10 µM de corticosterona foi persistente pois o excesso de ROS manteve-se na ausência da hormonaNo estudo feito com o conservante imidazolidinil ureia (IU), libertador de formaldeído, utilizaram-se as seguintes concentrações de IU: 1 µM, 50 µM e 100 µM. Nestas experiências observaram-se aumentos na autofluorescência de FAD, que eram irreversíveis para as concentrações mais elevadas de IU. Em fatias incubadas com o indicador H2DCFDA verificou-se que na presença de 50 µM e 100 µM de IU houve um aumento do sinal de ROS entre 2% a 4%. Foram também detetadas variações no potencial de membrana mitocondrial, em fatias contendo o indicador fluorescente rodamina 123. Os resultados indicam que para todas as concentrações de IU houve um aumento reversível do sinal sendo, para 100 µM, a recuperação lenta.O organismo contém defesas antioxidantes que ajudam a reduzir os efeitos oxidativos provocados por situações como as consideradas anteriormente. Por vezes a sua ação é insuficiente verificando-se um aumento de ROS irreversível, como se observou para as maiores concentrações da hormona de stress corticosterona e do conservante IU. Por isso, a ingestão alimentar pode ser uma fonte muito importante de antioxidantes com uma ação positiva no equilíbrio oxidativo. A importância dada a alimentos com propriedades antioxidantes, como os cogumelos tem sido crescente. Este alimento é rico em compostos bioativos que lhe conferem atividade antioxidante, incluindo compostos fenólicos, fitoquímicos, polissacarídeos, vitaminas, carotenóides e minerais, que proporcionam efeitos benéficos na saúde, para além das propriedades nutricionais. Por estes motivos, realizou-se um terceiro conjunto de estudos focado na caracterização da composição química de três espécies de cogumelos: Lentinula edodes koshin (shiitake), Pleurotus ostreatus e Pleurotus citrinopileatus. Analisaram-se também extratos de fitoquímicos destas espécies, que podem ter um papel importante na redução do stress oxidativo.
The ability of living organisms to maintain a harmonious balance with nature has allowed survival and evolution to this day. But this balance is influenced by many behaviors that are part of everyday life. The unrest and the accelerated rhythm of the present day can be the source of diseases, many of them irreversible. In recent times the causes of neurodegenerative diseases such as Alzheimer's, Parkinson's and depression and of many other diseases are considered to be associated with the current lifestyle.People deal with stress at work and in the school and family environment on a daily basis. These situations are so common that the general population already considers them “normal”. On the other hand, the increasingly industrialized lifestyle keeps people in contact with an enormous variety of pharmaceuticals and personal care products (PPCPs) that can accumulate in the human body. Continued exposure to these types of situations can lead to an imbalance in brain activity and cause permanent damage. Various physiological mechanisms, including the defense ones, constantly work to try to restore the balance. These processes can benefit much from the action of specific nutrients. Taking into account the previous considerations, the main objective of this work was to study responses to physiological stress, involving a stress hormone or a preservative of a personal care product. For that, brain slices (400 µm) from Wistar rats were used and autofluorescence and fluorescence records related with the production of reactive oxygen species (ROS) and with the mitochondrial membrane potential (Δψm), were made at the mossy fiber synapses of CA3 hippocampal area.In the first set of experiments, corticosterone, the main hormone released in rats in stressful situations, was used in 1 µM, 4 µM and 10 µM concentrations. Initially, autofluorescence signals from the mitochondrial proteins NADH and FAD were measured and it was found that NADH oxidation is more sensitive to changes in corticosterone and that this signal slowly approaches the baseline when the hormone is not present. On the other hand, in these experiments corticosterone has an irreversible action in the oxidation of the FADH2 protein, since the signal does not recover when the stress hormone is removed. Slices incubated with the fluorescent indicator H2DCFDA allowed the analysis of ROS changes, which increased between 1% and 6% for the studied concentrations. The action of 10 µM of corticosterone was persistent because the excess of ROS remained in the absence of the hormone.In the study carried out with the preservative imidazolidinyl urea (IU), which releases formaldehyde, the following IU concentrations were used: 1 µM, 50 µM and 100 µM. In these experiments, increases in FAD autofluorescence were observed, which were irreversible for the higher concentrations of IU. In slices incubated with the indicator, H2DCFDA, it was found that in the presence of 50 µM and 100 µM IU there was an increase in ROS fluorescence between 2% and 4%. Changes in the mitochondrial membrane potential were also detected in slices containing the fluorescent indicator rhodamine 123. The results indicate that for all IU concentrations there was a reversible signal increase occurring, for 100 µM, a slow recovery.The body contains antioxidant defenses that help reducing the oxidative effects caused by situations like those previously considered. Sometimes their action is insufficient and there is an irreversible increase in ROS, as was observed for the higher concentrations of the stress hormone corticosterone and of the IU preservative. Therefore, food can be a very important source of antioxidants with a positive action in the oxidative balance.The importance given to nourishment with antioxidant properties, such as mushrooms has been growing. This food is rich in bioactive compounds that give it antioxidant activity, including phenolic compounds, phytochemicals, polysaccharides, vitamins, carotenoids and minerals, which provide beneficial effects on health, in addition to the nutritional properties.For these reasons, a third set of studies was carried out focusing on the characterization of the chemical composition of three mushroom species: Lentinula edodes koshin (shiitake), Pleurotus ostreatus and Pleurotus citrinopileatus. Phytochemical extracts from these species, which can play an important role in reducing oxidative stress, were also analyzed.
Outro - Enquadrado no projeto 0340_SYMBIOSIS_3_E, co-financiado pelo FEDER através do Programa INTERREG V A Espanha – Portugal (POCTEP).

This chapter presents an outline of chemical reactions performed while utilizing microwave irradiation. The improvement in economic strategies has taken cognizance and acknowledgment of environment-friendly and cost-effective procedures having the most negligible impact on the environment. Compared to other methods, microwave-assisted methods proved to be more favorable substitutes for conventional laboratory heating systems; many chemical reactions have been accomplished, refining prevailing procedures with practical conclusions than the reactions proceeded under the conventional heating system. Reactions executed via catalysis in an aqueous medium enhanced the eco-friendly procedures, and the reactions executed via catalysis in fluid medium enhanced environmental conventions. In this chapter, we will discuss microwave-assisted catalytic approaches, which have been used for the preparation of various heterocyclic compounds, preparation of peptides, urea and coordination polymers having carboxyl group, and various other chemical reactions. The focus of this work is to highlight the recent advances in the field of Microwave-assisted organic reactions like oxidation, reduction, coupling, functionalization, heterocyclic compounds synthesis, multi-component reactions, and nucleophilic substitutions in water.

Nitric oxide and renin angiotensin system are involved in the pathophysiology and progression of diabetes mellitus chronic complications. To evaluate angiotensin-converting enzyme activity on nitric oxide levels in patients with type 2 diabetes mellitus. Were recruited 20 patients and 20 health volunteers. Blood and urine samples were collected to measure: fasting blood glucose, glycated hemoglobin, plasmatic Na+, K+, urea, creatinine, total cholesterol, triglycerides, thiobarbituric acid reactive substances, nitric oxide levels, angiotensin-converting enzyme activity and microalbuminuria.The mean arterial pressure and body mass index were obtained from the medical records. The results were considered significant when p<0.05. Fasting blood glucose, glycated hemoglobin, body mass index, nitric oxide, urinary thiobarbituric acid reactive substances, angiotensin-converting enzyme activity and microalbuminuria were increased and total cholesterol was reduced in diabetic vs. controls; meanwhile MAP, Na+, triglycerides, urea and creatinine were similar between these two groups. Our study showed that although the angiotensin-converting enzyme was elevated, favored by the high oxidative stress level, demonstrating that there was protection on the cardio-renal axis. Our data suggest that maybe angiotensin-converting enzyme inhibitors acted on AT2, demonstrated by increased nitric oxide and stable blood pressure, revealing how dynamic the renin angiotensin system is and reacts to treatment.

Abstract CO2 is the meeting point of several idealistic criteria for the functionalization of organic substrates: abundance, low cost, nontoxicity, and reusable carboxylation carbon unit. Its potential wide-range utilization is actually limited by its high stability, but motivates the search for general activation processes. One of the best uses of CO2 is the carboxylation of organic halides via Grignard reagents. However, the overall reaction cost implies loss of the halide and oxidation of magnesium (eqn (1)). In contrast, new generation carboxylation processes should involve the direct coupling with hydrocarbons and the use of a more easily reducible metal catalyst.

An increased awareness of global atmospheric carbon levels and heightened efforts to recover industrial emissions prior to their release into the environment has led to the availability of an unprecedented amount of carbon dioxide for industrial utilization. Unfortunately, chemical utilization of carbon dioxide as an industrial feedstock is limited by thermodynamic and kinetic constraints. Toxic carbon monoxide, the main competitor in many processes, is used in industry instead because CO2 is perceived to be less reactive and its efficient catalytic conversion has remained elusive. The major commercial uses of CO2 today are in beverages, fire extinguishers, and refrigerants, where inert physical properties such as oxidative and thermodynamic stability are advantageous. It is this stability that has limited the use of CO2 to only a very few synthetic chemical processes (urea, aspirin, carbonates) despite the enormous availability of this resource. The conversion of CO2 into useful organic compounds will likely rely on the use of metal catalysts to lower energy inputs. Increasingly, the use of supercritical carbon dioxide appears to offer significant advantages in the catalytic activation of CO2 to yield useful products. Liquid or supercritical CO2 (sc CO2) can be used as a reaction medium and can potentially replace conventional organic solvents to serve as an environmentally benign reaction medium (Ikariya and Noyori, 1999; Jessop and Leitner, 1999; Jessop et al., 1995b; Noyori, 1999). A supercritical fluid (SCF) is any substance that has a temperature and pressure higher than their critical values and which has a density close to or higher than its critical density (Jessop and Leitner, 1999; Jessop et al., 1995b). Carbon dioxide has a critical temperature of 31.0 °C and a critical pressure of 71.8 bar. The supercritical region of the phase diagram is the one at temperatures higher than the Tc and pressures higher than the Pc at which the liquid and gas phases become indistinguishable. Below Tc, liquid CO2 can be maintained under relatively modest pressures. Subcritical liquid CO2 behaves like any other nonpolar liquid solvent. Properties such as density are continuous above the Tc and discontinuous below it.

Cellular metabolism is divided into catabolism — responsible for converting nutrients into the energy sources and smaller molecules required for the chemical reactions of the body — and anabolism, which is the interconversion and synthesis of the molecules that maintain the body’s structure and function. This chapter examines the control of metabolism and the central metabolic pathways. Such control includes compartmentalization of metabolic processes and the cooperation between the metabolic activities of different organs. Metabolic control is important because metabolism must match the availability of nutrients to the demand for the products of the metabolic processes and both will vary over time. The synthesis of adenosine triphosphate (ATP), with its high-energy phosphate bond, lies at the heart of these central metabolic pathways. Most of the ATP is produced by oxidative phosphorylation in the mitochondria, but glycolysis and the tricarboxylic acid cycle (also known as the citric acid cycle or Krebs cycle) provide additional amounts. Of the nutrients entering the body from the diet, fat, glucose, and amino acids are the main fuels for cellular metabolism. The utilization of lipids, fatty acids, and ketone bodies is important in metabolism in addition to the key role played by glucose. Glucose is the fuel for energy production in glycolysis. It is also manufactured by gluconeogenesis and stored as glycogen by glycogenesis. It is important to know how different organs utilize different fuels and how energy production alters between the fed state and starvation. Amino-acid metabolism and coenzymes in amino acid oxidation are also important although some details, including the urea cycle, have not been covered here. Energy balance and the relationship between food intake and energy expenditure lead to the concept of body mass index (BMI). The BMI offers a quick method of quantifying the nutritional status of a person, and BMI values may be helpful in assessing the risk of, for example, obesity-related diseases such as type II diabetes and coronary heart disease. Cellular metabolism not only contributes to the medical sciences background to clinical reasoning, but there are also a number of identifiable, inborn errors of metabolism. While individually rare (with incidences of approx. 1–25 per 100,000 births), collectively they present a considerable number of new cases each year.

Urea-selective catalytic reduction (SCR) catalysts are regarded as the leading NOx aftertreatment technology to meet the 2010 NOx emission standards for on-highway vehicles running on heavy-duty diesel engines. However, issues such as low NOx conversion at low temperature conditions still exist due to various factors, including incomplete urea thermolysis, inhibition of SCR reactions by hydrocarbons and H2O. We have observed a noticeable reduction in the standard SCR reaction efficiency at low temperature with increasing water content. We observed a similar effect when hydrocarbons are present in the stream. This effect is absent under fast SCR conditions where NO ∼ NO2 in the feed gas. As a first step in understanding the effects of such inhibition on SCR reaction steps, kinetic models that predict the inhibition behavior of H2O and hydrocarbons on NO oxidation are presented in the paper. A one-dimensional SCR model was developed based on conservation of species equations and was coded as a C-language S-function and implemented in Matlab/Simulink environment. NO oxidation and NO2 dissociation kinetics were defined as a function of the respective adsorbate’s storage in the SCR catalyst. The corresponding kinetic models were then validated on temperature ramp tests that showed good match with the test data.

The leading aftertreatment technologies for NOx removal from the exhaust gas of lean burn engines, Diesels in particular, are urea based Selective Catalytic Reduction (SCR), Lean NOx Traps (LNT) and Active Lean NOx Catalysts (ALNC). It is generally believed that the SCR technique has the potential of providing the best NOx conversion efficiency relative to the other techniques. Nonetheless, it is crucial that the high conversion efficiencies be achieved with a minimum slippage of unreacted ammonia as tail pipe emissions. This necessitates a precise control over the urea injection process. The complex behavior of the catalyst substrate with respect to adsorption and desorption of ammonia in conjunction with a lack of “stored ammonia” sensing capabilities makes the control problem challenging. In this paper we present a model-based control design approach using a lumped parameter model of an SCR system that includes the essential dynamics of the plant. The model includes the adsorption, desorption and surface coverage dynamics, along with the NOx reduction and ammonia oxidation dynamics based on the relevant chemical reaction rates.

<div class="section abstract"><div class="htmlview paragraph">To meet the stringent NO_x and particulate emissions requirements of Euro 6 and China 6 standard, Selective Catalyst Reduction (SCR) catalyst integrated with wall flow particulate filter (SCR-DPF) has been found to be an effective solution for the exhaust aftertreatment systems of diesel engines. NO_x is reduced by ammonia generated from urea injection while the filter effectively traps and burns the particulate matter periodically in a process called regeneration. The engine control unit (ECU) effectively manages urea injection quantity, timing and soot burning frequency for the stable functioning of the SCR-DPF without impacting drivability. To control the NO_x reduction and particulate regeneration process, the control unit uses lookup tables generated from extensive hardware testing to get the current soot load and NO_x slip information of SCR-DPF as a function of main exhaust state variables.</div><div class="htmlview paragraph">In the current work, engine dynamometer tests were conducted on a SCR-DPF at different operating conditions covering typical vehicle running conditions. The oxygen assisted and NO₂ assisted soot burning efficiency of the SCR-DPF was measured with and without urea injection at different soot loads. The impact of ammonia on soot burning at different engine operating conditions was studied. Using the test data, a physics based 1-D reaction model was developed with NO_x reduction and soot oxidation reactions. The detailed SCR chemistry includes reactions for ammonia adsorption/desorption, NO oxidation, NH₃ oxidation, standard/fast/slow NO_x reduction and N₂O formation. The soot burning reaction kinetics is described by the oxidation of soot with NO_x. The NO_x reduction and soot regeneration efficiency predictions of the model were validated with test values measured at engine dynamometer conditions under various exhaust flow rate, temperature, and soot load conditions. This 1-D kinetic model can be applied to generate calibration look up tables for the SCR-DPF control system in the vehicle to identify the right soot burning protocol to achieve the target regeneration efficiency. Few of the other areas where the model can be applied are, exhaust aftertreatment (EAT) architectural evaluation, converter sizing, wash coat loading studies, urea injection strategy development and heater element controls optimizations. Compared to the conventional hardware test-based approach, this model-based virtual approach uses less test data thus resulting in faster product development cycle and reduces the testing in engine dynamometer and vehicles.</div></div>

<div class="section abstract"><div class="htmlview paragraph">The urea-water-solution based selective catalyst reduction (SCR) system is one of the effective devices for reduction of NO_x from diesel engines. In an effort to understand the various levels of oscillation observed in the NO_x measurement downstream of a SCR in which the urea dosage is controlled by a crankshaft-link pump, a zero-dimensional dynamic SCR model is developed in this paper based on conservation of mass. The model contains three states including the concentrations of NO_x and ammonia in the SCR and the surface coverage rate of the catalyst. The temperature-dependent reactions considered in the model include the adsorption, desorption and oxidation of ammonia and the NO_x reduction with the reaction constants provided by the catalyst company. The dynamic SCR model is validated both at steady state and during transient under various engine operating conditions and urea dosing rates. A periodic modulation of the urea dosing rate is adopted to simulate the periodic urea supply resulted from the reciprocating motion of the crankshaft-link pump. The simulation results exhibit similar oscillatory behaviors in the NO_x concentration as observed in the experimental measurement, which is further analyzed and explained based on the nonlinear characteristics between the downstream NO_x and the ammonia dosage. Based on the interpretation of the oscillatory NO_x signal, an algorithm for identification of the cross-sensitivity of the smart NO_x sensor to ammonia is proposed.</div></div>

Motivated by increasingly strict NOx limits, engine manufactures have adopted selective catalytic reduction (SCR) technology to reduce engine-out NOx below mandated levels. In the SCR process, nitrogen oxides (NOx) react with ammonia (NH3) to form nitrogen and water vapor. The reaction is influenced by several variables, including stored ammonia on the catalyst, exhaust gas composition, and catalyst temperature. Currently, measurements from NOx and/or NH3 sensors upstream and downstream of the SCR are used with predictive models to estimate ammonia storage levels on the catalyst and control urea dosing. This study investigated a radio frequency (RF) -based method to directly monitor the ammonia storage state of the SCR catalyst. This approach utilizes the SCR catalyst as a cavity resonator, in which an RF antenna excites electromagnetic waves within the cavity to monitor changes in the catalyst state. A mmonia storage causes changes in the dielectric properties of the catalyst, which directly impacts the RF signal. Changes in the RF signal relative to stored a mmonia (NH3) were evaluated over a wide frequency range as well as temperature and exhaust conditions. The RF response to NH3 storage, desorption, and oxidation on the SCR was observed to be well-correlated with changes in the catalyst state. Calibrated RF measurements demonstrate the ability to monitor the adsorption state of the SCR to within 10 % of the sensor full scale. The results indicate direct measurement of SCR ammonia storage levels, and resulting catalyst feedback control, via RF sensing to have significant potential for optimizing the SCR system to improve NOx conversion and decrease urea consumption.

A series of well crystallized Ni-Co-Zn-Al LDHs materials has been prepared by the urea hydrolysis method as precursors of mixed oxide catalysts for the Ethanol Steam Reforming (ESR) reaction. The calcination of the layered precursors gives rise to high surface area mixed oxides, mainly a mixture of rock-salt phase (NiO), wurtzite phase (ZnO) and spinel phase. Both precursors and mixed oxides have been throughtfully characterized and the steam reforming of ethanol has been investigated over the calcined catalysts in flow reactor and in-situ FT-IR experiments. The data here reported provide evidence of the good catalytic activity of Co-Zn-Al and Co-Ni-Zn-Al catalysts prepared from hydrotalcite-like LHD precursors for ethanol steam reforming. At 823 K the most active Co/Ni catalyst containains a predominant spinel phase with composition near Zn0.58Ni0.42[Al0.44Co0.56]2O4 and small amounts of NiO and ZnO. On the other side, at 873 K the selectivity to hydrogen increases with cobalt content. In particular, the presence of cobalt increases selectivity to H2 and CO2 and decreases selectivity to methane in the low temperature range 720–870 K. The most selective catalyst is the Ni-free Co-Zn-Al mixed oxide essentially constituted by a single spinel type phase Zn0.55Co0.45[Al0.45Co0.55]2O4. Cobalt catalysts appear consequently to behave better than nickel based catalysts in this temperature range. The key feature for high selectivity to hydrogen is proposed to be associated to a stability of a relatively high oxidation state at the catalyst surface, the most relevant selectivity determining step being constituted by the evolution of surface acetate species. In fact, over oxidized catalyst surface the acetate species evolve producing carbon dioxide and hydrogen while over a more reduced surface they evolve giving rise to methane and COx. Water is supposed to have the main role of allowing surface sites to stay in an unreduced state at least in the temperature range 720–870 K.

Environmental regulations have put stringent requirements on NOx emissions in the transportation industry, essentially requiring the use of exhaust after-treatment on diesel fueled light and heavy-duty vehicles. Urea-Water-Solution (UWS) based Selective Catalytic Reduction (SCR) for NOx is one the most widely adopted methods for achieving these NOx emissions requirements. Improved understanding and optimization of SCR after-treatment systems is therefore vital, and numerical investigations can be employed to facilitate this process. For this purpose, detailed and numerically accurate models are desired for in-cylinder combustion and exhaust after-treatment. The present paper reports on 3-D numerical modeling of the Urea-Water-Solution SCR system using Computational Fluid Dynamics (CFD). The entire process of Urea injection, evaporation, NH3 formation and NOx reduction is numerically investigated. The simulation makes use of a detailed kinetic surface chemistry mechanism to describe the catalytic reactions. A multi-component spray model is applied to account for the urea evaporation and decomposition process. The CFD approach also employs an automatic meshing technique using Adaptive Mesh Refinement (AMR) to refine the mesh in regions of high gradients. The detailed surface chemistry NOx reduction mechanism validated by Olsson et al. (2008) is applied in the SCR region. The simulations are run using both transient and steady-state CFD solvers. While transient simulations are necessary to reveal sufficient details to simulate catalytic oxidation during transient engine processes or under cyclic variations, the steady-state solver offers fast and accurate emission solutions. The simulation results are compared to available experimental data, and good agreement between experimental data and model results is observed.

<div class="section abstract"><div class="htmlview paragraph">A urea-selective catalytic reduction (SCR) system is used for the reduction of NOx emitted from diesel engines. Although this SCR catalyst can reduce NOx over a wide temperature range, improvements in NOx conversion at relatively low temperatures, such as under cold-start or low-load engine conditions, are necessary. A close-coupled SCR (cc-SCR), which was set just after the engine exhaust manifold, was developed to address this issue. The temperature of the SCR catalyst increases rapidly owing to the higher exhaust temperatures, and NOx conversion is then enhanced under cold-start conditions. However, since the diesel oxidation catalyst is not installed before the SCR catalyst, hydrocarbon (HC) emissions pass directly through the SCR catalyst and poison it, leading to lower NOx conversion. Therefore, the mechanism of NOx conversion reduction on HC-poisoned SCR catalysts are required to be studied. In this study, the effects of HC poisoning on the NOx conversion of Cu-CHA catalysts experimentally investigated using propene, n-decane, and 1-methylnaphtalene. In addition, a kinetic model of NH₃-SCR over the HC-poisoned Cu-CHA catalyst was constructed. When 500 ppm propene was passed through the SCR catalyst, the coke was found to be formed on the catalyst, which led the decrease of the NOx conversion (maximum 75% reduction at 210 °C). Conversely, when n-decane or 1-methylnaphthalene was used, no coke was formed at temperatures below 500 °C, and the NOx conversion was unaffected. Even when coke was formed, it decomposed above 350 °C, and the NOx conversion was equivalent to that of a fresh catalyst. Based on the experimental results, a model for NH₃-SCR over an HC-poisoned Cu-CHA catalyst was constructed. The reactor model was the one channel model and one-dimensional mass, momentum, energy and species balances were solved in the channel gas phase, assuming a quasi-steady state. The model reproduced the experimental results reasonably well, including the recovery of the catalyst from poisoning at relatively high temperatures.</div></div>