Ammonia synthesis kinetics: Surface chemistry,rate expressions,and kinetic analysis

The results of surface science studies of nitrogen adsorption and desorption on iron single crystals are summarized with respect to ammonia synthesis reaction kinetics. Analytical rate expressions for ammonia synthesis on high surface iron catalysts at industrial reaction conditions are presented and compared to the behavior of microkinetic models based on the surface science results. Microkinetic models based on uniform surfaces are successful in extrapolating results from surface science studies at low surface coverages to describe the performance of iron catalysts at industrial ammonia synthesis reaction conditions. We suggest that the value of rate constant for recombination of nitrogen atoms on the surface is a weak function of surface coverage. Furthermore, modest variations in the heat of N₂ adsorption are sufficient to achieve fractional ammonia orders that are relatively insensitive to the ammonia pressure.     

Promotion in ammonia synthesis: A pressure dependent phenomenon

The origin of the pressure dependence of potassium promotion of iron catalysts has been investigated by measuring the turnover numbers for ammonia synthesis on polycrystalline iron, potassium-doped polycrystalline iron and two triply promoted (K₂O, Al₂O₃, C_aO) industrial ammonia synthesis catalysts at 1, 2 and 31 bar in the temperature range 550–800 K. At 1 and 2 bar the turnover numbers of polycrystalline iron and potassium doped polycrystalline iron are greater than those of the industrial catalysts, suggesting that the promoters are merely decorating the surface of the iron, blocking off part of it for reaction. It is only at 31 bar and at temperatures above 670 K, that the promoting effect of the additions to the industrial catalyst becomes apparent, the turnover numbers of these materials being roughly 20 times greater than that of polycrystalline iron. Potassium doped polycrystalline iron, however, has the same turnover number as those of the industrial catalysts. It is concluded that at temperatures above 670 K in a hydrogen/ nitrogen (3 : 1) mixture and at 31 bar the morphology of the surface of the industrial catalysts is changed with the iron, probably as iron nitride, coating the potassium aluminate, the identity of the turnover numbers of the catalysts and of potassium doped polycrystalline iron showing that potassium plays a key role in this promotion. The surface restructuring of the industrial catalysts does not produce dominantly the (111) surface, the observed turnover numbers being 10³ times lower than that of the (111) surface. The activation energy for ammonia synthesis in the temperature range 670–770 K at 31 bar on these materials is much higher (32–37 kcal mol^–1) than that of iron (111) (19.2 kcal mol^–1) since it incorporates the activation energy for surface reconstruction of iron. The molecular nitrogen desorption spectra from the industrial catalysts exhibit three states: (i) N₂, bonded end on, (ii) N₂, -bonded and (iii) -bonded N₂, vicinal to some form of potassium.     

The behaviour of iron electrode in CO2− saturated neutral electrolyte—II. Radiotracer study and corrosion considerations

The radiotracer method described previously was applied for investigations of adsorption processes occuring in the system: iron electrodeposited electrode and CO₂-saturated (¹⁴C-labelled) neutral electrolyte. In addition to adsorption of ¹⁴C-containing species in the electrodeposited film, both reversible and irreversible adsorption of those species was identified. The irreversible adsorption was interpreted to result from incorporation of ¹⁴C-species (most probably HCO^?₃ ions) to the passive layer formed on the iron electrode. The reversible adsorption was concluded to occur due to the weak interactions of carbonic acid with the oxidized iron surface. Lewis acid and base concept of adsorption was used to account for the results. The role of the reversible adsorption process in the accelerated corrosion of steel due to the presence of carbon dioxide in aqueous solutions is discussed.     

Influence of surface properties and iron addition on the SO2 adsorption capacity of activated carbons

When iron derivatives are added to low ash activated carbons having basic surface characteristics (obtained by suitable oxidation at 800°C with 2% of O₂ in N₂), certain materials are obtained showing high SO₂ sorbent properties from gaseous mixtures having a composition close to that of the flue gases. This behaviour seems to be related to the simultaneous presence of both basic surface sites promoting the initial adsorption of SO₂ and iron promoting the transformation of the adsorbed SO₂ into other, more stable forms. The sorbent properties of these activated carbons are more stable, following consecutive cycles, in the processes of adsorption and desorption of SO₂, than those shown by similar carbons with different surface characteristics.     

Mars-van Krevelen-like Mechanism of CO Hydrogenation on an Iron Carbide Surface

Computational chemistry is used to explore a mechanism for CO hydrogenation to methane on iron carbides. As CO dissociation is endothermic on carbon terminated Fe₅C₂ (100) cuts, we explore a path starting with the hydrogenation of the surface, which liberates iron 4-fold sites for adsorption and dissociation of CO. The reaction cycle to methane resembles the Mars-van Krevelen mechanism for oxidation reactions.     

Modelling diffusion and adsorption of As species in Fe/GAC adsorbent beds

BACKGROUND: Arsenic decontamination of drinking water by adsorption is a simple and robust operation. When designing packed bed adsorbers for arsenic, the main problems are the slow diffusion kinetics of As in microporous media and the lack of simple equations for predicting the performance of the equipment. Commercial iron‐doped granular activated carbon adsorbents (Fe/GAC) for groundwater arsenic abatement were studied in this work. Basic parameters for arsenate (As^V) adsorption were measured and their performance at larger scale was simulated with an approximate analytical model. RESULTS: In the 0–300 µg_As L^?1 range, the As^V adsorption isotherm on Fe/GAC was found to be approximately linear. Assuming Henry's law for adsorption and homogeneous surface diffusion with constant diffusivity for intrapellet mass transfer, an approximate model for flow and adsorption of arsenate inside packed bed adsorbers was developed, and reduced to an analytic compact solution using the quasi‐lognormal distribution (Q‐LND) approximation. The use of this model with fitted and reported parameters enabled the approximate simulation of industrial adsorbers and home point‐of‐use filters. Results show that industrial adsorbers meet the breakthrough condition with incomplete utilization of the adsorbent unless convenient process configurations are used. In point‐of‐use systems with short residence times intraparticle diffusion would drastically reduce the adsorbent performance. CONCLUSION: Assuming linear adsorption of As^V over Fe/GAC, an analytical approximate solution for flow and adsorption in packed beds can be obtained. The model seems to represent correctly the main features of industrial and home filters, however, more experimental data is necessary for scale‐up purposes. Copyright © 2011 Society of Chemical Industry     

木质素磺酸钠在TiO2/水界面的吸附特性

木质素磺酸钠在固体表面的吸附特性决定了其应用性能,利用红外和紫外分光光度仪,采用剩余质量分数法研究了温度、pH值、无机盐和氢键破坏剂脲对木质素磺酸钠在TiO₂/水界面吸附动力学和等温吸附性能的影响,初步探讨了其在固液界面的吸附作用机理。结果表明,该吸附为单层多点式吸附,随着温度升高和pH值减小,木质素磺酸钠在TiO₂/水界面的吸附速率常数和饱和吸附量均增大,而离子强度的增大和脲的加入却使吸附速率常数减小;木质素磺酸钠在TiO₂/水界面的吸附驱动力为静电、疏水和氢键作用,疏水作用力可显著增加其吸附量。     

In situ surface Raman spectroscopy studies of oxygen adsorbed on electrolytic silver

In situ Raman spectroscopy has been employed to investigate oxygen adsorption on electrolytic silver catalyst under industrial conditions for methanol oxidation to formaldehyde. Both adsorbed atomic and molecular oxygen species are shown to exist on the silver surface in O₂ flow above 870 K. The peroxide species is determined to be a precursor to atomic adsorbed oxygen. In consideration of the industrial process, the molecular mechanism of the partial oxidation of methanol and the adsorption mechanism of oxygen on electrolytic silver surface are discussed.     

Theoretical Investigation of Hydrogen Adsorption and Dissociation on Iron and Iron Carbide Surfaces Using the ReaxFF Reactive Force Field Method

We have developed a ReaxFF reactive force field to describe hydrogen adsorption and dissociation on iron and iron carbide surfaces relevant for simulation of Fischer?CTropsch (FT) synthesis on iron catalysts. This force field enables large system (>>1000 atoms) simulations of hydrogen related reactions with iron. The ReaxFF force field parameters are trained against a substantial amount of structural and energetic data including the equations of state and heats of formation of iron and iron carbide related materials, as well as hydrogen interaction with iron surfaces and different phases of bulk iron. We have validated the accuracy and applicability of ReaxFF force field by carrying out molecular dynamics simulations of hydrogen adsorption, dissociation and recombination on iron and iron carbide surfaces. The barriers and reaction energies for molecular dissociation on these two types of surfaces have been compared and the effect of subsurface carbon on hydrogen interaction with iron surface is evaluated. We found that existence of carbon atoms at subsurface iron sites tends to increase the hydrogen dissociation energy barrier on the surface, and also makes the corresponding hydrogen dissociative state relatively more stable compared to that on bare iron. These properties of iron carbide will affect the dissociation rate of H₂ and will retain more surface hydride species, thus influencing the dynamics of the FT synthesis process.     

Activated carbon/iron oxide magnetic composites for the adsorption of contaminants in water

In this work the adsorption features of activated carbon and the magnetic properties of iron oxides were combined in a composite to produce magnetic adsorbents. These magnetic particles can be used as adsorbent for a wide range of contaminants in water and can subsequently be removed from the medium by a simple magnetic procedure. Activated carbon/iron oxide magnetic composites were prepared with weight ratios of 2:1, 1.5:1 and 1:1 and characterized by powder XRD, TG, magnetization measurements, chemical analyses, TPR, N₂ adsorption-desorption isotherms, Mössbauer spectroscopy and SEM. The results suggest that the main magnetic phase present is maghemite (γ-Fe₂O₃) with small amounts of magnetite (Fe₃O₄). Magnetization enhancement can be produced by treatment with H₂ at 600 °C to reduce maghemite to magnetite. N₂ adsorption measurements showed that the presence of iron oxides did not significantly affect the surface area or the pore structure of the activated carbon. The adsorption isotherms of volatile organic compounds such as chloroform, phenol, chlorobenzene and drimaren red dye from aqueous solution onto the composites also showed that the presence of iron oxide did not affect the adsorption capacity of the activated carbon.     

Improving the pore structure and adsorption properties of industrial active carbons

The paper describes the possibility of improving the pore structure and adsorption properties of industrial active carbons (AC) by means of their modification with ɛ-caprolactam (CL). Application of ɛ-caprolactam (a product of large-scale organic synthesis) as a modifying substance allows the targeted regulation of the specific surface area and volume of AC mesopores, while the conditions of modification change the chemical composition of the surface of carbon adsorbents and, as a result, their adsorption properties. The process of modification includes three stages. The first is the adsorption of CL from aqueous solution and heating at 300°C in the presence of atmospheric oxygen. The second is carbonization in an argon flow at 900°C, and the third is the vapor-gas activation of the resultant adsorbents at 900°C. Each stage is followed by determining the parameters of the pore structure by N₂ adsorption and the adsorption parameters with respect to benzene, iodine, aqueous solutions of ɛ-caprolactam, and copper sulfate (CuSO₄). When modifying the carbonized samples with an initial ɛ-caprolactam content of 2%, we observe the growth of mesopores over 100% of volume, relative to AG-OV-1 original industrial activated carbon. Depending on the stage of modification, an increase in adsorption is observed, relative to the AG-OV-1 original carbon: benzene adsorption rises by 50%, the adsorption of iodine from aqueous solution rises by 20%, the adsorption of ɛ-caprolactam from aqueous solution grows by more than 30%, and the adsorption of copper(II) ions rises by more than 70%. The procedure for the production of the described modified adsorbent has no rivals abroad; it is protected by a patent and can be used in industry. It is possible to use inexpensive industrial carbons as the original material. The procedure allows the production of carbon adsorbents with predetermined structural and adsorption properties.     

Removal of iron from industrial lean methyldiethanolamine solvent by adsorption on sepiolite

Sepiolite clay was used as an adsorbent for the removal of iron ions from industrial methyldiethanolamine (MDEA) solvent. The raw sepiolite clay was modified by different chemical and thermal treatments, and was characterized by FTIR, XRD, SEM-EDX, and BET-specific surface area analysis. Treating sepiolite with nitric acid significantly improved the adsorption capacity of iron ions from lean MDEA. The experimental equilibrium data were represented by Henry, Freundlich, and Langmuir isotherm models. The thermodynamic parameters indicated that the adsorption of iron ions on sepiolite was spontaneous and endothermic process. The kinetic studies showed that the adsorption followed pseudo-second order model.     

Adsorption of oleates of various amines on iron in acidic solution

The adsorption of amine salts of oleic acid has been investigated on iron in 0.5 M H₂SO₄. Measurements made on the differential capacity of the double layer and the kinetic of iron dissolution showed that the adsorption proceeds through the preferable bonding between oleic acid and the metal surface.It has been found for amines with more than two active groups in the molecule that they could play the role of the cross-linking agent, causing the more tight and multilayer structure of the adsorbate. Additionally, such action of amines could result in chemisorptive bonding on the metal surface. In consequence the adsorbed layer acquires better barrier properties and the inhibition becomes more effective.     

Modelling arsenic(III) adsorption from water by sulfate‐modified iron oxide‐coated sand (SMIOCS)

A medium developed by coating BaSO₄ and Fe on quartz sand known as sulfate‐modified iron oxide‐coated sand (SMIOCS) was evaluated for the removal of arsenic(III) from simulated water with an ionic strength of 0.01 M NaNO₃ during batch studies. The medium was characterised for BET surface area, alkali‐resistance, acid‐resistance and the presence of iron and barium on the coated surface. Two simplified kinetic models, ie active available site (AAS) and chemical reaction rate models, were tested to investigate the adsorption mechanisms. The values of rate constants for both the models were found to decrease with increasing As(III) concentrations in the solute. The inverse relationship of rate constants of the reaction rate model with BET surface area showed that As(III) adsorption on SMIOCS was not due to physisorption but to chemisorption. A study of the effect of solute temperature showed that the adsorption of As(III) on SMIOCS media was due to chemisorption. The results of isothermal studies conducted at different pH values showed that adsorption data satisfied both the Langmuir and the Freundlich isotherm models. The adsorption of As(III) on the medium was pH dependent and maximum removal was observed in the pH range of 7–9. © 2002 Society of Chemical Industry     

Low-cost metal oxide activated carbon prepared and modified by microwave heating method for hydrogen storage

Novel microporous activated carbon (MAC) with high surface area and pore volume has been synthesized by microwave heating. Iron oxide nanoparticles were loaded into MAC by using Fe(NO₃)₃·9H₂O followed by microwave irradiation for up to five minutes. The surface modified microporous activated carbon was characterized by BET, XRD, SEM and thermogravimetric examinations. Adsorption data of H₂ on the unmodified and modified MACs were collected with PCT method for a pressure range up to 120 bar at 303 K. Greater hydrogen adsorption was observed on the carbon adsorbents doped with 1.45 wt% of iron oxide nanoparticle loaded due to the joint properties of hydrogen adsorption on the carbon surface and the spill-over of hydrogen molecules into carbon structures.     

Tannin-based adsorbents from green tea for removal of monoaromatic hydrocarbons in water: Preliminary investigations

In this study, we report the adsorption of benzene and toluene from water using rarely reported tannin adsorbents. Tannin gel and tannin powder were synthesized by adding formaldehyde to green tea extract, while iron nanoparticles were synthesized by the addition of FeSO₄?·?7H₂O. The surface morphology of the synthesized adsorbents was determined using SEM and FTIR prior to application to contaminated water. The results show up to 88% removal of benzene and toluene in a batch system after 30?min of reaction time, with a higher rate of removal of toluene compared to benzene. A low pH value of 2 had an adverse effect on the tannin gel, reducing the total adsorption of benzene to approx. 37.5%. On the other hand, iron nanoparticles were least affected by the pH with an adsorption of 62.9% for benzene and 83.3% for toluene.     

Adsorption of Copper Ion with Metal Hydroxide from Ammonia Solution

Abstract

The removal of copper(II) from ammonia solution by adsorption on iron (III), aluminum(III), and tin(IV) hydroxide is studied. The effects of experimental parameters such as solution pH and concentration of total ammonia on adsorption are examined both from the change of solution composition and electrical properties of the solid surface. In a moderately high electrolyte concentration, an optimum solution pH is found for the solution composition when the sum of the species fractions of Cu(NH₃)²⁺, Cu(NH₃)₂ ²⁺, and Cu(NH₃)₃ ²⁺ reaches its maximum. The ligand number of aminecopper(II) of the solution is near 2.0 under this optimum condition. The decrease in adsorption in the more basic tetraamminecopper(II) solution is attributed to the competing reaction for copper(II) by the free ammonia in the solution. Adsorption isotherms at various concentrations of total ammonia show decreasing adsorption with high electrolyte concentration as a result of a highly positively charged surface and a reduction of available surface sites. The relative extent of adsorption is discussed for various experimental conditions based on the data and the surface complexation concept.     

Activated carbons with metal containing bentonite binders as adsorbents of hydrogen sulfide

Wood-based activated carbon was ground and mixed with 10% bentonite binders containing either iron, zinc or copper cations adsorbed within the interlayer space and/or on the external surface of bentonite flakes. To better understand the role of transition metals, carbon was also impregnated with iron, zinc and copper salts. The structure of materials after modification was determined using nitrogen adsorption. The modification resulted in a decrease in porosity, especially in micropore volume, as a result of combined mass dilution effect and adsorption/re-adsorption of metals in small pores. Introduction of bentonite binders containing adsorbed metal increased the capacity of carbon for hydrogen sulfide only in the case of material containing copper. Copper also significantly increases the performance of carbon as an H₂S adsorbent when impregnation is applied whereas the effects of other metals used in this study are much less pronounced. It is likely that copper present in the small pores acts as a catalyst for oxygen activation causing hydrogen sulfide oxidation. As a result of this process, elemental sulfur is formed which, when present in small pores, is oxidized to weakly adsorbed SO₂. The SO₂ is removed from the surface when continuous reaction with hydrogen sulfide occurs. Thus, even though binding carbon with spent bentonites after copper adsorption increases the capacity of carbon toward H₂S removal, the formation of SO₂, another undesirable pollutant, does detract somewhat from the procedure.     

Iron Oxide-Coated on Glass Fibers for Arsenic Removal

A highly efficient adsorbent for arsenic removal from water has been prepared by impregnating high surface area iron oxides on glass fibers. Arsenic in water can easily and efficiently be removed by this adsorbent, without the need to pre-oxidize As(III) to As(V). The iron oxides coated on glass fibers (IOCGFs) can remove both arsenic species well below EPA MCL (10 ppb). IOCGFs should have the following four additional advantages: greatly improved contact efficiency; higher adsorption capacity because of high surface area; low cost and easily available adsorbent since the starting reagents (FeCl₃ and NH₃·H₂O) and glass fiber are cheap and readily available; and high adsorption efficiency of As(III) and As(V).     

Impedance investigation of corrosion inhibition of armco iron by thiourea

The inhibitive action of thiourea on the corrosive behaviour of ARMCO iron was investigated in deaerated 0.5 M H₂SO₄ solution, by means of electrochemical impedance spectroscopy. The inhibitor effectiveness increases with concentration, reaches a maximum (at about 1 mm) and then decreases. The adsorptive behaviour of thiourea on the electrode surface up to its peak follows a Frumkin-type isotherm with lateral repulsion, where the molecules are vertically adsorbed on the iron surface via the sulfur atom. Thiourea acts as a mixed inhibitor up to the critical concentration. It decreases the dissolution of iron and the hydrogen evolution reaction by blocking the electrode surface. The free energy of adsorption ΔG ^ad = ?39 kJ mol^?1 and the attraction constant a = ?4.4.