In situ characterization of interaction of ammonia with carbon surface in oxygen atmosphere

The adsorption and oxidation of ammonia over carbons differing in the chemical structure of surface functional groups have been investigated by FTIR spectroscopy. The reactions of NH₃ with carbons have been studied both in the presence and in the absence of oxygen. As a result of NH₃ chemisorption, in addition to ammonium salts, there are formed surface amide and imide structures. At the higher temperature surface isocyanate species are formed. Thermal stabilities of surface structures, formed as a result of NH₃ chemisorption have been determined by means of FTIR spectroscopy. The activity and selectivity of carbons for the selective catalytic oxidation (SCO) of NH₃ to N₂ with excess O₂ has been shown by microreactor studies at 295-623 K. Carbon catalysts are very active for NH₃ oxidation. Nitrogen is generally the predominant product of ammonia oxidation. The selectivity to N₂, N₂O and NO is determined by the surface oxygen coverage and reaction temperature. The data obtained indicate that the N₂ is formed via selective catalytic reduction (SCR) between NH_x surface species and NO formed from NH₄⁺ oxidation. This implies that ammonia is activated in the form of NH₄⁺ species for both SCR and SCO processes.     

Oxidation of carbon by NOx, with particular reference to NO2 and N2O

The reactions reviewed here concern those between elemental carbon and NO₂, N₂O and NO, sometimes in the presence of oxygen. The section on NO includes only updates to recent reviews. Soots, activated carbons and carbon blacks are more reactive than graphite. The magnitudes of the reaction rates are found to be: NO₂ > N₂O ≈ NO ≈ O₂. The presence of a soluble organic fraction (SOF) in soot is found to influence some reactions, and all three reactions suffer from inhibition by surface products. The mechanisms proposed for the surface adsorbates are summarised. All authors found that two types of active site were present; one forming weak bonds (physisorption), and the other undergoing chemisorption to form groupings such as -C-ONO, -C-ONO₂ or -C-NO₂. The latter decompose to give oxides of carbon, and are sometimes called redox reactions. The adsorbates appear to be the same for all NO_x species. Some elemental nitrogen adsorption takes place, and can involve incorporation into the C skeleton. The attack of NO on carbon proceeds via NO₂, so that catalysts that facilitate this oxidation are effective. Gaseous SO₂ and H₂O assist in the process by forming acids which are good oxidants. The change in activation energy with temperature found experimentally for NO and N₂O may be due to the form of nitrogen on the edge carbon atoms.     

Mechanistic aspects of N2O and N2 formation in NO reduction by NH3 over Ag/Al2O3: The effect of O2 and H2

A mechanistic scheme of N₂O and N₂ formation in the selective catalytic reduction of NO with NH₃ over a Ag/Al₂O₃ catalyst in the presence and absence of H₂ and O₂ was developed by applying a combination of different techniques: transient experiments with isotopic tracers in the temporal analysis of products reactor, HRTEM, in situ UV/vis and in situ FTIR spectroscopy. Based on the results of transient isotopic analysis and in situ IR experiments, it is suggested that N₂ and N₂O are formed via direct or oxygen-induced decomposition of surface NH₂NO species. These intermediates originate from NO and surface NH₂ fragments. The latter NH₂ species are formed upon stripping of hydrogen from ammonia by adsorbed oxygen species, which are produced over reduced silver species from NO, N₂O and O₂. The latter is the dominant supplier of active oxygen species. Lattice oxygen in oxidized AgO_x particles is less active than adsorbed oxygen species particularly below 623 K. The previously reported significant diminishing of N₂O production in the presence of H₂ is ascribed to hydrogen-induced generation of metallic silver sites, which are responsible for N₂O decomposition.     

Selective Catalytic Reduction of NOx over H-ZSM-5 Under Lean Conditions Using Transient NH3 Supply

The selective catalytic reduction (SCR) of NO _x over zeolite H-ZSM-5 with ammonia was investigated using in situ FTIR spectroscopy and flow reactor measurements. The adsorption of ammonia and the reaction between NO _x , O₂ and either pre-adsorbed ammonia or transiently supplied ammonia were investigated for either NO or equimolar amounts of NO and NO₂. With transient ammonia supply the total NO reduction increased and the selectivity to N₂O formation decreased compared to continuous supply. The FTIR experiments revealed that NO _x reacts with ammonia adsorbed on Brønsted acid sites as NH₄ ⁺ ions. These experiments further indicated that adsorbed -NO₂ ^– is formed during the SCR reaction over H-ZSM-5.     

CoAl2O4 spinel catalyst for soot combustion with NOx/O2

Mesoporous and nanosized cobalt aluminate spinel with high specific surface area was prepared using microwave assisted glycothermal method and used as soot combustion catalyst in a NO_x + O₂ stream. For comparison, zinc aluminate spinel and alumina supported platinum catalysts were prepared and tested. All samples were characterised using XRD, (HR)TEM, N₂ adsorption–desorption measurements. The CoAl₂O₄ spinel was able to oxidise soot as fast as the reference Pt/Al₂O₃ catalyst. Its catalytic activity can be attributed to a high NO_x chemisorption on the surface of this spinel, which leads to the fast oxidation of NO to NO₂.     

Transformations and destruction of nitrogen oxides—NO, NO2 and N2O—in a pulsed corona discharge reactor

There has been an increasing recent research interest in the removal of NO_x from combustion gases using electrical discharges, especially pulsed corona discharge reactors. The major issues in development of this technology are (a) the energy consumption required to achieve the desired pollutant reduction; and (b) the formation of undesirable byproducts. In this study, the transformations and destruction of nitrogen oxides—NO, NO₂ and N₂O—were investigated in a pulsed corona discharge reactor. Gas mixtures—NO in N₂, N₂O in N₂, NO₂ in N₂ and NO-N₂O-NO₂ in N₂—were allowed to flow through the reactor with initial concentrations, flow rates and energy input as operating variables. The reactor effluent gas stream was analyzed for N₂O, NO, NO₂, by means of an FTIR spectrometer. In some experiments, oxygen was measured using a gas chromatograph.Reaction mechanisms were proposed for the transformations and destruction of the different nitrogen oxides within a unified model structure. The corresponding reaction rates were integrated into a simple reactor model for the pulsed corona discharge reactor. The reactor model brings forth the coupling between reaction rates, electrical discharge parameters, and fluid flow within the reactor. It was recognized that the electron-impact dissociation of the background gas N₂ leads to both ionic and radical product species. In fact, ionic reactions were found responsible for N₂O destruction. Radical reactions were dominant in the transformation and destruction of NO and NO₂. However, decomposition of N₂⁺ ions also leads to indirect production of N radicals; this appears to be a less-power intensive route for NO destruction though longer residence times may be necessary. In addition, the decomposition of N₂⁺ ions limits the N₂O destruction that can be achieved. Comparison with our experimental data, as well as data in the literature, was very encouraging.     

Modeling of NOx conversion during combustion under high CO2 concentration using detailed chemical kinetics

Combustion of fuels under enhanced oxygen atmospheres has been well investigated over the past decades in various types of combustors, varying from diesel engines to coal-fired boilers. Most studies have found significantly lower NO_x emissions during Oxy-coal combustion. In this paper, NO_x combustion chemistry under O₂/CO₂ atmosphere as well as air atmosphere was studied using detailed kinetic model. A suitable reaction mechanism was chosen based on the comparison between the calculation result and the experimental data. The influence of various parameters (temperature, CO₂ concentration) on NO_x conversion was investigated. The chemical effects of high CO₂ concentration on NO formation and destruction process was studied. On the basis of investigations through elementary chemical reactions, it can be concluded that high CO₂ concentration play a pronounced role on NO_x conversion process. Moreover, the dominant reaction steps contribution to production and destruction of NO as well as the most important reactions for NO reduction under different atmospheres were discussed.     

Support modification to improve the sulphur tolerance of Ag/Al2O3 for SCR of NOx with propene under lean-burn conditions

Ag/Al₂O₃ catalysts with 1 wt% SiO₂ or TiO₂ doping in alumina support have been prepared by wet impregnation method and tested for sulphur tolerance during the selective catalytic reduction (SCR) of NO_x using propene under lean conditions. Ag/Al₂O₃ showed 44% NO_x conversion at 623 K, which was drastically reduced to 21% when exposed to 20 ppm SO₂. When Al₂O₃ support in Ag/Al₂O₃ was doped with 1 wt% SiO₂ or TiO₂ the NO_x conversion remained constant in presence of SO₂ showing the improved sulphur tolerance of these catalysts. Subsequent water addition does not induce significant deactivation. On the contrary, a slight promotional effect on the activity of NO conversion to nitrogen is observed after Si and Ti incorporation. FTIR study showed the sulphation of silver and aluminum sites of Ag/Al₂O₃ catalysts resulting in the decrease in the formation of reactive intermediate species such as –NCO, which in turn decreases NO_x conversion to N₂. In the case of Ag/Al₂O₃ doped with SiO₂ or TiO₂, formation of silver sulphate and aluminum sulphate was drastically reduced, which was evident in FTIR resulting in remarkable improvement in the sulphur tolerance of Ag/Al₂O₃ catalyst. These catalysts before and after the reaction have been characterized with various techniques (XRD, BET surface area, transmittance FTIR and pyridine adsorption) for physico-chemical properties.     

Performance of fixed-bed KOH impregnated activated carbon adsorber for NO and NO2 removal in the presence of oxygen

KOH-impregnated activated carbon (K-IAC) was used in this study. This paper contains observation the adsorption behavior of NO and NO₂ with/without oxygen and with different bed depths of adsorbent. The paper also defines surface chemical changes due to NO_x adsorption. By using a simple design of adsorber, the packed amount of adsorbent for NO_x abatement for 6 months on a pilot scale was calculated. When oxygen was present, NO and NO₂ had a great improvement in adsorptivity. Adsorption of NO₂ forms a oxide crystal on the surface of the K-IAC and at the same time produces NO, which acts to bring about increased adsorptivity. The higher the bed of adsorbent was, the more NO was produced and the longer the breakthrough time took. The adsorber was designed in a scale-up condition where NO, NO₂ and O₂ were applied to K-IAC. The adsorbate that consumed the least packed amount was NO₂-air followed by NO₂-N₂, NO-air and NO-N₂. The results of the experiment demonstrated that with regard to adsorption of NO and NO₂ on K-IAC, the presence of oxygen and the bed depth of adsorbent were the biggest variables to adsorptivity.     

Reduction of SO2 on different carbons

The reduction of SO₂ on four carbons (graphite, charcoal, activated carbon and coke) was studied under steady-state conditions and when the kinetics was chemically controlled in a reactor operated under differential conditions. The reaction showed second-order kinetics: first order with respect to carbon and first order with respect to the partial pressure of SO₂. The reactivity of the different carbons, as measured by the second-order rate constants, followed the sequence of decreasing crystallinity: graphite<coke (7.34)<coke (11.73)<charcoal. The difference in reactivity between graphite and charcoal was determined by ΔH^≠, while for cokes it increased with the ash content because of a favorable ΔS^≠. The main reaction products for all carbons were CO₂ and sulfur in the ratio 2:1, considering the sulfur as S₂, which was shown to be formed through the same path. CO, COS, and CS₂ were also detected, and the product distribution depended on the carbon and whether the reaction was diffusion controlled or chemically controlled. Analysis of product ratios strongly suggested that CO, COS and CS₂ were produced from consecutive reactions of the primary products. CO was formed from CO₂ by a slow Boudouard reaction that occurred partially and under conditions of non-equilibrium. Complexed sulfur reacted with CO to form COS and CS₂. There was an interaction between the active site of reduction and the site where sulfur is inserted.     

Sorption–Desorption of NOx from a Lean Gas Mixture on H3PW12O40·6H2O Supported on Carbon Nanotubes

NO _x sorption capacities and efficiencies were measured on a new type of sorbent formed by 12-tungstophosphoric acid (HPW) supported on carbon nanotubes. On such a system, the sorption of both NO and NO₂ was observed but compared with HPW alone, a complementary sorption of NO _x is possible leading to a capacity of 25 mg/g_HPW at 300 °C with an efficiency of 50%. The sorption results from the formation of a [H⁺(NO₂ ^–,NO⁺)] complex on HPW and an additional mode of adsorption by a free-nitrate which was identified by the bands at 2261, 1384 and 1295 cm^–1 using infrared spectroscopy.     

XPS investigation of the reaction of carbon with NO, O2, N2 and H2O plasmas

The interaction of graphite with plasmas of pure gases (O₂, N₂ or H₂O), air or mixtures of gases containing NO has been studied by XPS “in situ” analysis. Depending on the type of plasma, different species of nitrogen, oxygen and carbon have been detected on the surface of graphite. The nitrogen containing species have been attributed to pyridinic, pyrrol, quartenary and oxidized groups adsorbed on the surface. The evolution with the treatment time of the relative intensity of the different nitrogen bands for Ar + NO, N₂ + NO, air or N₂ plasmas has served to propose a model accounting for the reactions of graphite with plasmas of NO containing gases. The model explains why carbon materials (in the form of graphite, soot particles, etc.) can be very effective for the removal of the NO present in exhaust combustion gases excited by a plasma. The analysis of the C1s and O1s photoemission peaks reveals the formation of C/O adsorbed species up to a maximum concentration on the surface of around 10% atomic oxygen. A general evolution is the progressive formation of C/O species where the carbon is sp³ hybridized. This tendency is enhanced when graphite is treated with the plasma of water.     

NOx adsorption-temperature programmed desorption and surface molecular ions distribution by activated carbon with chemical modification

Activated carbon, the surface of which has been modified with KOH, was used in this study. The study examined adsorption and desorption behaviors and the accompanied surface reaction mechanism as well as the distribution of molecular ions on the surface. The peaks of NO_x desorption behavior may be classified into three bond categories depending on adsorption strength. NO desorption occurs at the earliest stage as chemical adsorption occurs earlier, in a sort of competition, than physical adsorption due to strong basic OH⁻ ion of surface. It was confirmed that the adequate temperature for NO_x desorption was near 390 °C. The potassium that existed on the surface remained without being consumed even with complete desorption of NO_x.     

Pulverized coal combustion in air and in O2/CO2 mixtures with NOx recycle

This paper presents experimental results of a 20 kW vertical combustor equipped with a single pf-burner on pulverised coal combustion in air and O₂/CO₂ mixtures with NO_x recycle. Experimental results on combustion performance and NO_x emissions of seven international bituminous coals in air and in O₂/CO₂ mixtures confirm the previous findings of the authors that the O₂ concentration in the O₂/CO₂ mixture has to be 30% or higher to produce matching temperature profiles to those of coal-air combustion while coal combustion in 30% O₂/70% CO₂ leads to better coal burnout and less NO_x emissions than coal combustion in air. Experimental results with NO_x recycle reveal that the reduction of the recycled NO depends on the combustion media, combustion mode (staging or non-staging) and recycling location. Generally, more NO is reduced with coal combustion in 30% O₂/70% CO₂ than with coal combustion in air. Up to 88 and 92% reductions of the recycled NO can be achieved with coal combustion in air and in 30% O₂/70% CO₂ respectively. More NO is reduced with oxidant staging than without oxidant staging when NO is recycled through the burner. Much more NO is reduced when NO recycled through the burner (from 65 to 92%) than when NO is recycled through the staging tertiary oxidant ports (from 33 to 54%). The concentration of the recycled NO has little influence on the reduction efficiency of the recycled NO with both combustion media—air and 30% O₂/70% CO₂.     

Usefulness of CO2 adsorption at 273 K for the characterization of porous carbons

Characterization of microporous solids (activated carbons and carbon molecular sieves) has been carried out by N₂ (subatmospheric pressures) and CO₂ adsorption (at subatmospheric and high pressures) at 77 and 273 K, respectively. Because the relative fugacity range covered by our CO₂ study is similar to the relative pressure range covered with N₂, a suitable comparison of both adsorptives can be made. The results of such comparison show that both adsorptives give the same micropore size distribution (MPSD) for open porosity activated carbons. This observation confirms that the adsorption mechanism of both adsorptives is similar. However, carbon molecular sieves, with very narrow microporosity, cannot be characterized by N₂ at 77 K, due to the existence of diffusional problems. This is also extensive to many other carbon materials, such as carbon fibers and activated carbons with low degree of activation. As a consequence, in this type of samples, N₂ adsorption at 77 K is useless to determine neither the micropore volumes of the narrowest porosity nor their micropore size distributions (MPSD). In this work, the usefulness of CO₂ for the characterization of carbon molecular sieves and activated carbons with different activation degrees is demonstrated. In addition, examples of applications that cannot be explained from N₂ adsorption but yes by CO₂ are presented. As a result, we strongly encourage the use of CO₂ (i.e. at 273 K) as a complement to N₂ adsorption at 77 K.     

Simultaneous easy CO2 recovery and drastic reduction of SOx and NOx in O2/CO2 coal combustion with heat recirculation

Coal combustion with O₂/CO₂ is promising because of its easy CO₂ recovery, extremely low NO_x emission and high desulfurization efficiency. Based on our own fundamental experimental data combined with a sophisticated data analysis, its characteristics were investigated. It was revealed that the conversion ratio from fuel-N to exhausted NO in O₂/CO₂ pulverized coal combustion was only about one fourth of conventional pulverized coal combustion. To decrease exhausted NO further and realize simultaneous easy CO₂ recovery and drastic reduction of SO_x and NO_x, a new scheme, i.e. O₂/CO₂ coal combustion with heat recirculation, was proposed. It was clarified that in O₂/CO₂ coal combustion, with about 40% of heat recirculation, the same coal combustion intensity as that of coal combustion in air could be realized even at an O₂ concentration of as low as 15%. Thus exhausted NO could be decreased further into only one seventh of conventional coal combustion. Simultaneous easy CO₂ recovery and drastic reduction of SO_x and NO_x could be realized with this new scheme.     

Cu/Al2O3 catalysts for soot oxidation: Copper loading effect

Cu/Al₂O₃ catalysts with metal loading from 0.64 to 8.8 wt.% have been prepared and characterized by different techniques: N₂ adsorption at −196 °C (BET surface area), ICP (Cu loading), XRD, selective copper surface oxidation with N₂O (Cu dispersion), TPR-H₂ (redox properties), and XPS (copper surface species). The catalytic activity for soot oxidation has been tested both in air and NO_x/O₂. The activity in air depends on the amount of easily-reduced Cu(II) species, which are reduced around 275 °C under TPR-H₂ conditions. The amount of the most active Cu(II) species increases with the copper loading from Cu_1% to Cu_5% and remains almost constant for higher copper loading. In the presence of NO_x, the first step of the mechanism is NO oxidation to NO₂, and the catalytic activity for this reaction depends on the copper loading. For catalysts with copper loading between Cu_1% and Cu_5%, the catalytic activity for soot oxidation in the presence of NO_x depends on NO₂ formation. For catalysts with higher copper loading this trend is not followed because of the low reactivity of model soot at the temperature of maximum NO₂ production. Regardless the copper loading, all the catalysts improve the selectivity towards CO₂ formation as soot oxidation product both under air and NO_x/O₂.     

Global kinetic modelling of the reaction of soot with O2 and NOx on Fe2O3 catalyst

This study deals with the catalytic reaction of NO_x and soot on Fe₂O₃ to yield N₂ and CO₂ in excess of oxygen. Based on the three types of kinetic experiments, i.e. temperature programmed oxidation (TPO), transient examinations and gradient-free loop reactor experiments, as well as mechanistic studies presented recently a global kinetic model is established. The model includes catalytic effect of the iron oxide on soot/O₂ reaction, whereas it is assumed that NO_x reduction occurs on the soot without direct participation of Fe₂O₃. Furthermore, the model implies global kinetic expressions for the CO_x formation and NO_x reduction. These equations include the evolution of the surface area of soot and the correlation of reactive carbon sites (C_f) with those specifically involved in NO_x reduction (C*). The kinetic model is sequentially developed by accounting for the catalytic and non-catalytic soot/O₂ as well as soot/NO_x/O₂ conversion. Kinetic parameters are taken from the literature and are also determined from a fit to experimental data. Comparison of measured and calculated data shows accurate reproduction of the experiments and the model. Finally, the kinetic model is validated by some simulations.     

Influence of HNO3 oxidation on the structure and adsorptive properties of corncob-based activated carbon

An investigation of the impact of strong oxidation with HNO₃ on the porosity and adsorption characteristics of char and activated carbons, derived from corncobs, is presented. Texture parameters, as obtained from N₂ adsorption at 77 K, showed a considerable decrease in surface area of the activated carbons with enhanced pore widening. The extent of porosity modification was found to depend on the scheme of activation of the precursor, simple carbonization, steam pyrolysis, steam gasification of the char, or chemical activation with H₃PO₄. Surface-chemical changes were detected by FTIR spectroscopy, where absorption bands assigned to carboxyl, carboxylate, carbonyl, and phenolic groups were observed. A SEM study demonstrated the erosive effect of HNO₃, detected by the presence of disintegration of the carbon grains, with the porous structure probably containing very large macropores. As a consequence of the oxidation process, elemental analysis showed high contents of O, H and N, and TG confirmed that the weight loss distribution in the thermogram becomes slower at higher temperatures. The removal of phenol decreased as a result of the formation of oxygen functionalities. Mono-nitrophenols were adsorbed in smaller amounts than phenol, and p-nitrophenol showed a relatively higher uptake than the other two mono-nitrophenols, whereas the uptake of Methylene Blue was improved. Removal of Pb²⁺ from aqueous non-buffered solution was considerably enhanced by chemical oxidation, which may be related to pore widening, increased cation-exchange capacity by oxygen groups, and the promoted hydrophilicity of the carbon surface.     

Investigation of NO Adsorption and NO/O2 Co-adsorption on NOx-Storage-Components by DRIFT-Spectroscopy

NO adsorption and NO/O₂ co-adsorption on CeO₂ at different temperatures was studied by DRIFT-Spectroscopy. The results indicate that this oxide plays an important role in storing NO _x . FTIR studies show that NO adsorption is dominated by the formation of nitrite species. Furthermore, cis- and trans hyponitrite species are detected. Co-adsorption of NO/O₂ leads to the formation of nitrates. The experimental data show that the formation of nitrates is a consecutive reaction: adsorption of NO to form nitrite species (NO₂ ^–), followed by an oxidation to form nitrate species (NO₃ ^–).