Decomposition Products Of Urea Research Articles

Preparation of propylene carbonate by alcoholysis of urea from the perspective of homogeneous reaction: Influence of dissolution and precipitation of metal oxide catalysts

AbstractThe dissolution process of metal oxide catalysts in the alcoholysis of urea by 1, 2‐propylene glycol (PG) to the synthesis of propylene carbonate (PC) was investigated. It was found that the reaction was mainly catalyzed by homogeneous catalysts. The metal oxide reacted with ammonia and isocyanate acid, the decomposition products of urea, to form a complex and dissolved into the reaction system. The dissolved amount of catalyst was positively correlated with the PC yield by using different oxides and salts as catalysts. The Fourier‐transform infrared spectroscopy (FTIR) result of the catalysts reacting with urea showed that the dissolution‐precipitation process of the catalysts was related to the decomposition of urea. The results of catalytic test indicated that both the dissolved amount of catalysts and the structure of the complex affected PC yields. A reaction mechanism was proposed based on the results. From the mechanism, a complex catalyst with at least two ammonia ligands is better for this reaction.

Experimental study and numerical simulation of urea-assisted SAGD in developing exra-heavy oil reservoirs

Laboratory experiments and pilot applications have proved that urea is an efficient agent to enhance heavy oil recovery. Numerous studies have been conducted to investigate the mechanisms of urea co-injection process in steam flooding or cyclic steam stimulation. However, investigations of applying urea in steam assisted gravity drainage (SAGD) process are rarely published. The mechanism of enhanced oil recovery in the urea assisted SAGD process has not been fully understood. Laboratory experiments and numerical simulations were conducted to study the decomposition rate of urea, the effect of urea decomposition products on reservoirs and pipelines, and the distribution of effective decomposition products of urea in the steam chamber at conditions of different concentrations, temperatures, and injection rates. The results show that the solubility of the effective urea decomposition products affects their distribution in the formation because of the huge difference between the solubility in oil and water. In SAGD process, urea cannot be completely decomposed and will produce solid decomposition products, which could block the reservoir pores and production pipelines in certain conditions. Moreover, numerical simulation indicates that the use of low-concentration urea injection in SAGD can effectively enhance oil recovery and oil production rate, while high-concentration urea can reduce the oil recovery and oil production rate. In addition, it was proved that CO2 mainly exists at the edge of the steam chamber and NH3 is distributed in the irreducible water near the well-bore. CO2 is more effective than NH3 for enhancing oil recovery in urea assisted SAGD. Studies indicate that urea co-injection is a feasible method for enhancing oil recovery in the SAGD process.

Corrosion behavior of 15CrMo steel for water-wall tubes in thermal power plants in the presence of urea and its byproducts

Abstract The corrosion behavior of 15CrMo used for water-wall tubes was studied in various urea-containing solution to determine the corrosion problem of water-wall tubes caused by urea in a coal-fired power plant. Urea decomposition tests, together with corrosion experiments, were carried out. The temperature was 320 °C, and the concentrations of urea were 70, 140, 280, 560 and 840 mg/L. Weight loss experiments and surface analysis indicated that the corrosion of 15CrMo steel is mainly manifested as localized corrosion. The corrosion rate of 15CrMo steel increased with the increase of urea concentration, and the maximum value reached 0.686 mm/y (mm per year) when the urea concentration was 840 mg/L. Electrochemical analysis suggested that the corrosion rates of 15CrMo were enhanced substantially by urea decomposition products. The results of UPLC-ESI-MS, infrared spectroscopy and X-ray diffraction showed that urea solution produced corrosive ions NH2COO− during the decomposition process, which caused the corrosion of 15CrMo. Results provided evidence as relevant explanation for the corrosion behavior of 15CrMo in urea solution.

Performance evaluation of urea injection on the emission reduction of dioxins and furans in a commercial municipal solid waste incinerator

Polychlorinated dibenzo-p-furans and dioxins (PCDD/Fs) that pose a great threat to human health are commonly found during the incineration of municipal solid waste. In this study, industrial urea was injected into a commercial MSW incinerator flue gas to evaluate the suppression performances of PCDD/Fs, and the possible inhibition mechanisms were proposed. The results show that the use of urea dramatically reduced the PCDD/Fs emission concentration from 8.87 to 0.63ng/Nm3, along with a significant decrease in the I-TEQ value (0.26 → 0.047ng I-TEQ/Nm3), below the Chinese national standard of 0.1ng I-TEQ/Nm3. The emission reduction cost of the industrial urea was over 41 % lower than that of using activated carbon. Urea molecule poisoned the metal ions, thus seriously inhibiting the de novo synthesis of PCDD/Fs. Furthermore, the decomposition products of urea reduced the concentration of HCl in the flue gas, thereby reducing the formation of Cl2 and hindering the chlorination reaction, which was identified from the reduction of chlorination degree and lower distribution of high-chlorinated PCDD/Fs. This research provided some practice basis and experience to reduce the emission of PCDD/Fs from municipal solid waste incineration for future commercial promotion and application.

Reducing the Formation of Secondary Pollutants in the Process of NonCatalytic Purification of Flue Gases from Nitrogen Oxides

The possibilities of the technology of non-catalytic reduction of nitrogen oxides (SNCR) to reduce the possible formation of secondary pollutants: ammonia, carbon monoxide and nitric oxide (I) are considered. The conditions for the process of reduction of nitrogen oxides by the thermal decomposition products of urea with a minimum formation of the above pollutants are determined.

Faster Activation and Slower Capacity/Voltage Fading: A Bifunctional Urea Treatment on Lithium‐Rich Cathode Materials

AbstractLi‐rich layered oxides are promising cathode materials for next‐generation Li‐ion batteries because of their extraordinary specific capacity. However, the activation process of the key active component Li2MnO3 in Li‐rich materials is kinetically slow, and the complex phase transformation with electrode/electrolyte side reactions causes fast capacity/voltage fading. Herein, a simple thermal treatment strategy is reported to simultaneously tackle these challenges. The introduction of a urea thermal treatment on Li‐rich material Li1.87Mn0.94Ni0.19O3 leads to oxygen deficiencies and partially reduced Mn ions on the oxide surface for activating the Li‐rich phase. In situ synchrotron study confirms that the urea‐treated cathode shows much faster Li extraction from both Li and transition metal layers with less oxygen evolution upon charging than that of untreated counterparts. Moreover, the decomposition products of urea during thermal treatment subsequently deposit on the surface of cathode material, leading to a unique passivation layer against side reactions between electrode and electrolyte. Soft X‐ray absorption spectroscopy reveals the structural evolution mechanism with a significantly suppressed dissolution of Mn species over cycling measurement. The urea‐treated Li1.87Mn0.94Ni0.19O3 shows accelerated activation kinetics to reach high capacity of 270 mA h g–1 and demonstrates excellent capacity retention of 98.49% over 300 cycles with slower voltage decay.

Preparation of LaTiO2N Using Hydrothermally Synthesized La2Ti2O7 as a Precursor and Urea as a Nitriding Agent

A facile method was successfully developed to prepare perovskite‐type metal oxynitrides, LaTiO2N, from La2Ti2O7 as a precursor in an atmosphere of N2. Urea was employed as a solid‐state nitriding agent, instead of gaseous NH3, to increase the safety of the reaction. The hydrothermally prepared La2Ti2O7 precursor had a nanosheet morphology and contained La(OH)3 as a by‐product. Through the optimization of reaction conditions including heat‐treatment temperature and content of urea, single‐phase LaTiO2N could be obtained. In addition, we revealed that the La(OH)3 present in the La2Ti2O7 precursor played an important role in the nitriding process using urea. La2O2CN2 was generated by the reaction of La(OH)3 with the thermal decomposition products of urea at the medium temperature range. This indicated that the NH3 gas released from the thermal decomposition of urea did not directly cause nitridation of the La2Ti2O7 precursor.

A Study on Urea-Water Solution Spray-Wall Impingement Process and Solid Deposit Formation in Urea-SCR de-NOx System

Selective catalytic reduction (SCR) has been exhibited as a promising method of NOx abatement from diesel engine emissions. Long-term durability is one of the key requirements for the automotive SCR system. A high NOx conversion, droplet distribution and mixing, and fluid film and solid deposit formation are the major challenges to the successful implementation of the SCR system. The current study is therefore three-fold. Firstly, high-speed images disclose detailed information of the spray impingement on the heated impingement surface. The spray impingement investigation took place in a specially-designed optically-accessible visualization chamber where the Z-type shadowgraph technique was used to capture the high-speed images. Wall temperature has a great influence on the film formation and wall wetting. A higher wall temperature can significantly increase the droplet evaporation, and consequently, wall wetting decreases. The numerical analysis was performed based on the Eulerian-Lagrangian approach using STAR CCM+ CFD code. Secondly, the resultant phenomena due to spray-wall impingement such as fluid film generation and transport, solid deposit formation, and thermal decomposition were recorded using a high-speed camera operating at a low frame rate. Infrared thermal imaging was used to observe the spray cooling effect after impingement. Spray impingement caused local cooling, which led to wall film formation, which introduced urea crystallization. Finally, solid deposits were analyzed and characterized using Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). FTIR analysis revealed that urea decomposition products vary based on the temperature, and undecomposed urea, biuret, cyanuric acid, ammeline, and melamine can be formed at different temperatures. TGA analysis showed that accumulated deposits were hard to remove. Moreover, complete thermal decomposition of deposits is not possible at the regular exhaust temperature, as it requires a comparatively long time span.

Homogeneous hydrolysis of thorium by thermal decomposition of urea

Abstract Thorium was precipitated homogeneously from a thorium nitrate solution by the thermal decomposition products of urea. The kinetics of the hydrolysis were studied at 90 and 100°C by pH measurement during the initial 5 h and the precipitation efficiencies of thorium and radium were measured over a 24 h period. Precipitation of the radium daughters was closely followed with the aim of co-precipitation of radium with thorium. The CO2 formed during urea decomposition dissolved in the solution, forming CO3 2− during the experiment upon reaching a sufficiently high pH level (>7). This allowed radium to co-precipitate partially, thus reducing the activity of the filtrate. After filtration or centrifugation, the precipitate is composed of nanocrystalline thorium dioxide (crystallite size ~10 nm), with weakly bound H2O and CO2.

Effect of electrolyte depletion on characteristics of anodic plasma electrolytic nitrocarburizing

This article considers changes in the composition of aqueous electrolytes containing ammonia chloride and urea during plasma electrolytic nitrocarburizing of low-carbon steels. The main purpose is the identification of the transport mechanism of nitrogen and carbon atoms movement from the urea into treated low-carbon steel. The effect of changes in the composition of the electrolyte on treatment temperature and the structure of the modified layer was also evaluated. Urea decomposition to ammonia and carbon monoxide was confirmed. Electrolyte alkalization during treatment was found due to the action of the urea decomposition products. A hypothesis is proposed to explain the observed rise of the electrolyte surface tension near the vapor phase. This accounts for the observed increase in the sample temperature and in the electric current in the process of carburization. It is found that heating brings about an increase in thickness of the inner diffusion layer, but the thickness of the surface nitrocarburized layer remains almost constant.

Synthesis and up-conversion luminescence properties of Yb3+–Er3+ Co-doped In2O3

In(OH)3:Yb3+/Er3+ particles with different morphologies were obtained by homogeneous precipitation in aqueous solutions through reaction with the thermal decomposition products of urea. The effects of In3+ concentration on the shapes of In(OH)3 particles were investigated. In(OH)3:Yb3+/Er3+ precursors converted to In2O3:Yb3+/Er3+ by an annealing process, which does not change original morphologies. The up-conversion photoluminescence of In2O3:Yb3+/Er3+ exhibits the green and red double emissions under 980nm excitation and the up-conversion mechanism has also been discussed.

The impact of urea on the performance of metal exchanged zeolites for the selective catalytic reduction of NOx: Part I. Pyrolysis and hydrolysis of urea over zeolite catalysts

Urea-SCR over metal exchanged zeolites is one of the leading catalytic technologies to abate NOx emissions in diesel exhaust. Ideally, urea injected into the diesel exhaust upstream of the SCR catalyst decomposes only to the gaseous products CO2 and NH3, where the latter gas can react with NOx emissions to form harmless N2 and H2O. However, solid by-products can be formed as well, and if deposited on the catalyst harm the long-term catalytic performance. In order to identify the impact of various urea decomposition products on the catalytic activity, we studied the pyrolysis and hydrolysis of neat urea and of urea over different zeolites (H-Y, Cu-Y, H-Beta, Na-Beta, and Fe-Beta). The experiments were run in dry and steam-containing N2 between 20 and 750° C by using simultaneous thermogravimetric analysis (TGA), differential thermoanalysis (DTA), and online GC/MS evolved gas analysis. Solid intermediate products at different decomposition temperatures were identified by means of ATR-FTIR and luminescence spectroscopy. As for neat urea, CO2, NH3 and HNCO could be detected as major gaseous products. At 270° C significant amounts of cyanuric acid and ammelide and at 500° C of melem and melon were identified as solid intermediates. Above 625°C, all solid residues decomposed to cyanogen and isocyanic acid. Furthermore, it could be shown clearly that the investigated zeolites significantly accelerate the pyrolysis of urea and cyanuric acid, and the hydrolysis of HNCO, by shifting the decomposition processes to lower temperatures and by inhibiting the formation of solid by-products. In addition, the presence of steam in the feed gas can prevent even further the formation of solid residues and the high temperature adsorption of gaseous products.

Dehydrated halloysite intercalated mechanochemically with urea: Thermal behavior and structural aspects

Urea has been intercalated mechanochemically into dehydrated halloysite and analyzed by X-ray powder diffraction (XRPD), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance ultraviolet/visible spectroscopy (DRUV–VIS), thermal analysis (TGA/DTA), transmission electron microscopy (TEM), and electron paramagnetic resonance (EPR). The basal distance expands from 7.4 to 10.7 Å and the interaction of urea to adjacent layers of halloysite through hydrogen bonds increases the structural order of the matrix. After heat treatment in air at different temperatures, decomposition products begin to appear starting from 100 °C. Although the basal distance remains constant up to 160 °C and collapses to the original value at 200 °C, urea and the decomposition products are still present in the sample. Starting from 125 °C, urea decomposition products reduce halloysite structural Fe 3+ centers to Fe 2+, as indicated by DRUV–VIS and EPR spectroscopy.

Kinetics of Urea Decomposition in the Presence of Transition Metal Ions: Ni2+

The literature on kinetics of the urea decomposition reaction was reviewed for the 333–373 K range of temperature. Possible reactions in the pH range of 5–9 were identified. Kinetic simulations indicated significant accumulation of the cyanate intermediate in the pH‐time–temperature range that was studied. The effects of Ni2+ hydrolysis and complexation with the urea decomposition products were incorporated into the simulations. The kinetic simulation of the rate of Ni2+ removal from the solutions was compared against the experimental data. The experimental results indicated an agglomerative growth mechanism for the precipitation process. Chemical analyses showed that the composition of the precipitate varies with digestion time, in agreement with the predictions of the kinetic simulation.

Effect of microwave radiation on the formation of Ce 2O(CO 3) 2·H 2O in aqueous solution

Reaction between urea and cerium nitrate with/without microwave radiation was investigated. Decomposition of urea occurred in aqueous solution to form NH 4OH when the solution was heated to approximately 80 °C. The decomposition products of urea in aqueous solution reacted with cerium nitrate to form cerium oxycarbonate hydrate powder (Ce 2O(CO 3) 2·H 2O). When the aqueous solution was heated by hot plate to decompose urea, elongated powder of Ce 2O(CO 3) 2·H 2O with an aspect ratio of approximately 3 was obtained. For the reaction in which the microwave radiation was used to heat the solution for only 5 min, the resulting Ce 2O(CO 3) 2·H 2O powder was almost identical to the powder that was formed by hot plate heating. However, when the microwave radiation was used for 15 min, the precipitated powder was spherical in shape. These results indicate that the microwave radiation had an effect on the reaction in the aqueous solution.

Preparation of Catalytic Filters by the Urea Method and Its Application for Benzene Cracking in H2S-Containing Biomass Gasification Gas

The urea method has been explored to introduce nickel(II) into the pores of ceramic filter substrates to develop a catalytic filter for the combined removal of tars and particles from hot biomass gasification gas. SEM-EDX characterization showed that the above method gave a fairly uniform spatial distribution of nickel throughout the modified filter substrate. It was observed that urea decomposition products such as NH3 and CO2 limited the incorporation of nickel in the filter substrate due to the complexation of NH3 with Ni(II) and loss of impregnation solution by the discharge of CO2. It was demonstrated that the release of CO2 could be suppressed by increasing the starting pH of the impregnation solution. In addition, the urea method was explored to add calcium(II) to the catalyst formulation but it remained unclear if the calcium precursor coprecipitated with the nickel precursor during the urea decomposition or precipitated in the posttreatment of drying. The resulting catalytic filter substrates wer...

Urea as a PCDD/F Inhibitor in Municipal Waste Incineration

ABSTRACT Emissions of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) from municipal waste incineration have been widely studied because of their extensive toxicity, and many efforts have been made to restrict their emissions. Although a number of chemical compounds have been shown in laboratory-scale tests to inhibit the formation of PCDD/Fs, few have been tested in pilot- or full-scale plants. This work evaluates the effect of urea as a PCDD/F inhibitor in a pilot-scale incinerator that uses refuse-derived fuel (RDF). The decomposition of urea under the test conditions was also studied using detailed kinetic modeling. An aqueous solution of urea was injected into the flue gas stream after the furnace at ~730 °C, with varied urea concentrations and flue gas residence times used between the furnace and the sampling point. The results demonstrate that urea can successfully inhibit PCDD/F formation in waste incineration if concentrations and injection points are properly adjusted. The kinetic model showed that urea can be rapidly decomposed under appropriate flue gas conditions, indicating that in addition to the urea molecule itself, decomposition products of urea can also be responsible for the reduction of PCDD/F production during incineration.

Preparation of rare-earth carbonate nanosized particles monolayer by using potentiostatic technique

Gadolinium carbonate and samarium carbonate nanosized particles monolayers were prepared on copper grids coated with carbon film by using potentiostatic technique. The particles were obtained by homogeneous precipitation in aqueous solution by reaction with the thermal decomposition products of urea. The electrocatalytic activity of nanosized particle film was investigated.

尿素法によるイットリア前駆体の水熱合成

Fine particles of monodispersed yttria are desirable for preparing transparent and/or high-performance, -functional ceramics. Various shape of yttria precursors was observed in the precipitates synthesized through homogeneous precipitation from aqueous solution by reacting with each other of thermal decomposition products of urea at 115 °C. Three types precipitates of particle morphologies such as hallow spheres with rough and smooth suefaces, and rodlike shape were synthesized in an aqueous solution containing urea with sulfate ion. All of them crystallized to monophase yttria above 1,100 °C. The calcined particles were hollow basketlike, porous-spherical and rodlike with rough surface. The basketlike hollow spheres were obtained from solution of low [urea]/[yttrium ions] ratio in a short reaction time, and rodlike ones were obtained from solution of high [urea]/[yttrium ions] ratio in a long reaction time. Spherical, rodlike and platelet precipitates were synthesized in an aqueous solution of urea coexisting with nitrate ions, and a monophase of yttria was obtained above 800 °C. These facts indicate that the shape of yttria precursor obtained in the solution coexisting with sulfate or nitrate ions were controlled by the [urea]/[yttrium ions] ratio and reaction time.

Preparation of spherical, monosized Y 2O 3 precursor particles

Spherical, monosized yttria precursor particles were obtained by homogeneous precipitation in aqueous solutions by reaction with the thermal decomposition products of urea. Increasing [Y 3+ above 0.05 M resulted in a deviation from spherical morphology and caused agglomeration of particles. Over the concentration range studied, excess urea did not affect particle morphology, but increased the yield. Increasing aging time appeared to increase particle size as well as to improve yield, as long as the urea was not depleted. The approximate chemical composition of the precipitate was YOHCO 3. The YOHCO 3 particles formed were amorphous to X-rays, and underwent a two-stage thermal decomposition, first forming Y 2O 2CO 3 near 180°C, and then cubic Y 2O 3 above 610°C. Generation of CO 3 2− appeared to be crucial to the formation of the solid phase. Heating the aqueous yttrium solution with trichloroacetic acid (CCl 3COOH) instead of urea as the precipitating agent produced a solid phase, while heating the same solution with formamide (HCONH 2) substituted for urea formed no precipitate.