IR study on the reduction of NO and NO2 by hydrazine monohydrate over Fe-BEA zeolite

The adsorption of NO and NO₂ and their subsequent reduction by hydrazine monohydrate (HDM) over Fe-BEA zeolite were investigated using an FT-IR spectrophotometer equipped with an in-situ cell. Although NO and NO₂ molecules were adsorbed on Fe species in an unaltered state, some of them reacted with oxygen atoms, resulting in the adsorption of NO₂ and NO ₃ ^? , respectively. The reducing species that had originated from HDM on Fe-BEA selectively reduced these molecules to N₂, while a small amount of N₂O was formed in the reduction of NO by HDM. NO and NO₂ were rapidly reduced by HDM through their adsorbed state even at 150 °C, and Fe species were required for their adsorption and for the formation of reducing species from HDM.     

Infrared study of nitrogen monoxide adsorption on palladium ion-exchanged ZSM-5 catalysts

The adsorption of NO at room temperature on a H-ZSM-5 catalyst exchanged with Pd(NH₃) ₄ ²⁺ complex and activated in oxygen at 773 K has been examined by FTIR spectroscopy. After the oxidizing treatment, the Pd tetrammine complex decomposed into Pd(II) ions and/or Pd(II) hydroxyl complexes dispersed in the zeolite channels. The subsequent adsorption of NO at room temperature led to the reduction of Pd(II) to Pd(I) entities, resulting in the formation and adsorption of NO₂ on H-ZSM-5. The Pd(I) entities were shown to adsorb NO and form mononitrosyl complexes dispersed in the zeolite porosity and characterized by a single infrared absorption band at 1881 cm^–1. The Pd(I) mononitrosyl complex was shown to reversibly coordinate water and NO₂ molecules. The resulting nitrosyl complex was characterized by a single NO vibration band at 1836 cm^–1.     

Spectroscopic and catalytic characterization of Co-exchanged mordenites subject to CO/O2 redox treatments

Co-mordenites were prepared by ion exchange and wet impregnation over Na-mordenite and H-mordenite. The prepared solids were calcined and aliquots of these samples were subject to redox treatments first with CO for 2 h at 773 K and then kept 2 h at the same temperature in flowing O₂. A thorough characterization of the solids was carried out by TPR, XPS, CO volumetric adsorption, and FTIR with CO, NO and pyridine adsorption as probe molecules. After the redox treatment, Na based samples showed an important increase in the CO adsorption. TPR and XPS results indicated reduction of the oxides present in the calcined samples and the migration of exchanged cobalt from hidden to more exposed sites was demonstrated by FTIR. The acid catalysts did not change their CO capacity of adsorption; exchanged cobalt ions were mainly in β-type sites and remained in this position after treatment. After the redox treatments, the activity for the selective reduction of NO_x with methane suffered a decrease on both types of mordenites, probably caused by the strong adsorption of NO and the reduction of β-type Co²⁺ to metallic cobalt that would diminish the active sites concentration and also block the channels, thus preventing the access of the reactants.     

Denitrification in aqueous FeEDTA solutions

The biological reduction of nitric oxide (NO) in aqueous solutions of FeEDTA is an important key reaction within the BioDeNOx process, a combined physico‐chemical and biological technique for the removal of NO_x from industrial flue gasses. To explore the reduction of nitrogen oxide analogues, this study investigated the full denitrification pathway in aqueous FeEDTA solutions, ie the reduction of NO₃^?, NO₂^?, NO via N₂O to N₂ in this unusual medium. This was done in batch experiments at 30 °C with 25 mmol dm^?3 FeEDTA solutions (pH 7.2 ± 0.2). Also Ca²⁺ (2 and 10 mmol dm^?3) and Mg²⁺ (2 mmol dm^?3) were added in excess to prevent free, uncomplexed EDTA. Nitrate reduction in aqueous solutions of Fe(III)EDTA is accompanied by the biological reduction of Fe(III) to Fe(II), for which ethanol, methanol and also acetate are suitable electron donors. Fe(II)EDTA can serve as electron donor for the biological reduction of nitrate to nitrite, with the concomitant oxidation of Fe(II)EDTA to Fe(III)EDTA. Moreover, Fe(II)EDTA can also serve as electron donor for the chemical reduction of nitrite to NO, with the concomitant formation of the nitrosyl‐complex Fe(II)EDTA–NO. The reduction of NO in Fe(II)EDTA was found to be catalysed biologically and occurred about three times faster at 55 °C than NO reduction at 30 °C. This study showed that the nitrogen and iron cycles are strongly coupled and that FeEDTA has an electron‐mediating role during the subsequent reduction of nitrate, nitrite, nitric oxide and nitrous oxide to dinitrogen gas. Copyright © 2004 Society of Chemical Industry     

Understanding the Catalytic Conversion of Automobile Exhaust Emissions Using Model Catalysts: CO + NO Reaction on Pd(111)

The CO + NO reaction is one of the profoundly important reactions that take place on Pd-based industrial three-way catalysts (TWC). In this review, we discuss results from polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) and conventional IRAS experiments on CO adsorption, NO adsorption and the CO + NO reaction on a Pd(111) model catalyst surface within a wide range of pressures (10^?6–450 Torr) and temperatures (80–650 K). It will be shown that these studies allow for a detailed understanding of the adsorption behavior of these species as well as the nature of the products that are formed during their reaction under realistic catalytic conditions. CO adsorption experiments on Pd(111) at elevated pressures reveal that CO overlayers exhibit similar adsorption structures as found for ultrahigh vacuum (UHV) conditions. On the other hand, in the case of the CO + NO reaction on Pd(111), the pressure dependent formation of isocyanate containing species' was observed. The importance of this observation and its effects on the improvement of the catalytic NO _x abatement is discussed. The kinetics of the CO + NO reaction on Pd(111) were also investigated and the factors affecting its selectivity are addressed.     

Detection of Reduced W n+ Sites on WO3–ZrO2 and Pt/WO3–ZrO2 Catalysts by Infrared Spectroscopy of Adsorbed NO

Adsorption of NO on oxidized WO₃–ZrO₂ catalysts leads to formation of adsorbed N₂O, surface nitrates, NO⁺, and Zr⁴⁺(NO^- ₃)–NO species. When NO is adsorbed on a sample reduced at 523 K, W⁵⁺ –NO (1855 cm^-1) and W⁴⁺(NO)₂ (1785 and 1700 cm^-1) species are formed in low concentration. Reduction at 573 K increases the density of W⁴⁺ sites and this density remains unchanged after reduction up to 673 K. The W⁴⁺(NO)₂ species are stable towards evacuation and in the presence of oxygen; however, they quickly disappear in the simultaneous presence of NO and O₂ as a result of the oxidation of the W⁴⁺ cations to W⁶⁺. The density of the W⁴⁺ sites on a Pt/WO₃–ZrO₂ sample is significant even after reduction at 523 K. This is explained by the promotion effect of platinum on the support reduction. The use of NO as a probe molecule for detection of reduced W ⁿ⁺ sites on tungstated zirconia is discussed.     

Adsorption and reduction of NO2 over activated carbon at low temperature

The reactive adsorption of NO₂ over activated carbon (AC) was investigated at 50 °C. Both the NO₂ adsorption and its reduction to NO were observed during the exposure of AC to NO₂. Temperature programmed desorption (TPD) was then performed to evaluate the nature and thermal stability of the adsorbed species. Adsorption and desorption processes have been proposed based on the nitrogen and oxygen balance data. The micropores in AC act as a nano-reactor for the formation of -C(ONO₂) complexes, which is composed by NO₂ adsorption on existing -C(O) complexes and the disproportionation of adsorbed NO₂. The generated -C(ONO₂) complexes are decomposed to NO and NO₂ in the desorption step. The remaining oxygen complexes can be desorbed as CO and CO₂ to recover the adsorptive and reductive capacity of AC.     

Influence of Activation and the HDS Reaction on the Structure of Magnesium Fluoride-Supported Ru Catalysts Derived from Ruthenium Chloride Hydrate

The effect of water and reductants (CO and H₂) on the decomposition of NO _x stored on BaO/Al₂O₃ at 300 °C has been investigated. Water eliminates the initial rapid total uptake of NO₂ but has little effect on the subsequent formation of nitrates that is accompanied by evolution of NO. Water hinders liberation of NO₂ and NO during temperature-programmed decomposition of stored NO _x . Both CO and H₂ lower the temperatures required for decomposition through reduction of NO₂ to NO and N₂ thus restricting NO₂ readsorption. The rate of reduction is lower with H₂ than with CO.     

FTIR Characterization of Fe3+–OH Groups in Fe–H–BEA Zeolite: Interaction with CO and NO

Fe³⁺–OH groups of a Fe–H–BEA sample prepared by conventional ion-exchange method are characterized by an IR band at 3686–3684 cm^?1. They exhibit a weak acidity: upon low-temperature CO adsorption the O–H stretching modes are blue shifted by 100 cm^?1 and the respective carbonyl adducts are observed at 2158 cm^?1. The Fe³⁺–OH groups are reduced at room temperature by NO to form Fe²⁺–NO species and NO⁺ groups in cationic positions. Desorption of pre-adsorbed NO at temperatures above 373 K regenerates the Fe³⁺–OH groups. The relation of the Fe³⁺–OH species to the so-called α-oxygen is discussed.     

In situ DRIFTS study of NO reduction with CH4 over PtCo-ferrierite catalysts

The effect of platinum incorporated into Co-ferrierite catalyst on the selective catalytic reduction of NO_x with CH₄ was studied by means of in situ DRIFTS technique. NO adsorption on Co- and PtCo-ferrierite catalysts gave dinitrosyl and mononitrosyl species on Co²⁺ ions. The adsorption of NO + O₂ on Co-ferrierite yielded NO₂ (NO ₂ ⁺) species and also nitrites and nitrates. Similar species were observed on PtCo-ferrierite, although chemisorbed NO₂ was much more stable since it persisted at reaction temperatures as high as 723 K. The spectra of the pre-reduced bimetallic PtCo-ferrierite catalyst exposed to the CH₄ + NO + O₂ reaction mixture showed bands at 2200–2100 cm^-1, which were similar to results for a Pt-free sample but slightly more intense. In addition, strong bands of nitrate, almost unchanged with temperature, were observed. A very stable Co²⁺–NO₂ intermediate species was developed upon incorporation of Pt into the base Co-ferrierite catalyst.     

Mechanistic study on adsorption and reduction of NO2 over activated carbon fibers

Adsorption and reduction of NO₂ over pitch-based ACFs, both as received and calcined at 1100 °C, were studied in a range of concentrations (NO₂, 250-1000 ppm; O₂, 0-10%) and temperatures (30-70 °C). Repeated adsorption after regeneration at 300 °C, temperature-programmed desorption (TPD) and diffuse reflectance fourier transformation infrared spectroscopy (DRIFTS) were also applied to analyze the adsorbed NO₂ species. Pitch-based ACFs showed rapid NO production and adsorption at 30 °C which stayed at similar conversions until the rapid breakthrough of NO₂. A higher reaction temperature of 70 °C decreased the ratio of NO₂ adsorption to reduction in the stationary state and shortened the breakthrough time. Higher NO₂ concentration increased the rates of both adsorption and reduction to shorten breakthrough time, whereas the presence of oxygen changed the NO₂ profiles by enhancing the NO₂ adsorption rate and decreasing both the rate and the capacity of reduction. It must be noted that 10% O₂ allowed still significant production of NO. The molar O/N ratio evolved from TPD decreased and converged to a constant value according to the NO₂ adsorption time, showing that NO_x species adsorbed on the ACF changed from NO₂ to NO₃ along with the time of NO₂ adsorption. Such a trend was confirmed by DRIFTS spectra of adsorbed NO₂. These results suggest two kinds of NO₂ adsorption sites. Site 1 adsorbs NO₂ molecules strongly, transferring one oxygen to another adsorbed molecule on a similar site to form NO₃ad. Although oxygen in the gas phase oxidized adsorbed NO₂ to some extent, especially in the initial stage, disproportionation is still dominant at 10% O₂. Such disproportionation produces gaseous NO, leaving NO₃ on the surface. Site 2 adsorbs NO₂ weakly. Saturation of both sites terminates the adsorption and reduction and results in the breakthrough of NO₂. Adsorbed NO₃ produces both NO and NO₂ when heated, leaving one or two oxygen atoms on the surface, which are evolved as CO and CO₂ at the same time, restoring a stationary ability for adsorption and reduction of NO₂ through carbon loss.     

A Study of the Interaction of NO2 with Carbon Particles

The interaction NO₂ in air (0.5–35 ppm) with carbon particles led to three products: NO gas, and NO₂ ^? and NO₃ ^?, removed from the particles by water extraction. At 4 ppm or below, in dry or humid air, the product distribution, in relative molar amounts, was NO₃ ^? = 2NO₂ ^? = 2NO. At 20 ppm and above, the relative amounts of products depended on the presence of water vapor: in dry air NO = 3NO₃ ^? = 6NO₂ ^?; in humid air NO = NO₂ ^? = 2NO₃ ^?. For carbon slurries in water, [NO₂ ^?] = 6[NO₃ ^?] at an input concentration of NO₂ of 4 ppm. In comparison to carbon, alumina particles and glass beads removed NO₂ ineffectively. These results indicate that NO₂ oxidized the carbon particles while it was reduced to NO. NO₂ adsorbed at oxidized sites on the particles formed a surface species that was analyzed as nitrate. At high enough concentration of NO₂ (20 ppm and above), the interaction of NO and water vapor with the surface nitrate produced NO₂ ^?. In slurries NO, generated from interaction of NO₂ with carbon, reacted with surface nitrate or nitric acid in solution to form the relatively large quantities of nitrite. This work suggests that NO_x reactions with carbon in droplets or on wet surfaces could be important sources for the production of nitrous acid in the environment.     

Nitrate removal from water by quaternized pine bark using choline based ionic liquid analogue

BACKGROUND: The objective of this study was to quaternize pine bark (PB) wood residues using green chemistry and to use the quaternized PB to remove nitrate (NO₃^?) from water. The quaternization process was achieved by reacting the wood residues with an ionic liquid analogue comprised of a choline chloride derivative and urea. Batch adsorption tests were used to delineate the NO₃^? uptake by the modified pine bark (MPB). Fourier Transform Infrared Spectroscopy (FTIR) analysis and Zeta potential measurements were used to characterize the changes at the surface of the PB due to quaternization and NO₃^? uptake. RESULTS: The MPB has a maximum NO₃^? uptake capacity of 2.91 mmol g^?1. The NO₃^? uptake kinetics indicated that diffusion through the boundary layer of the MPB was the rate limiting step. The Langmuir adsorption model provided a better fit for the uptake data than the Freundlich model, indicating monolayer adsorption. The uptake process was found dependent on concentration, pH and ionic strength, and was also spontaneous and exothermic. The desorption–regeneration experimental results indicated a 95% efficiency after five consecutive regeneration cycles. CONCLUSIONS: The quaternization technique was found very effective for developing effective and green anion exchange resins to remove NO₃^? from water. © 2012 Society of Chemical Industry     

Reduction of lean NO2 with diesel soot over metal-exchanged ZSM5, perovskite and γ-alumina catalysts

The catalytic performances of metal-exchanged ZSM5, perovskite and γ-alumina catalysts for the reduction of nitrogen dioxide (NO₂) by diesel soot were investigated. The reaction tests were performed through temperature-programmed reaction (TPR), in which NO₂ and O₂ were passed through a fixed bed of catalyst-soot mixture. On the three types of catalyst, NO₂ was reduced to N₂ by model soot (Printex-U) and most of the soot was converted into CO₂. Pt-, Cu- and Co-exchanged ZSM5 catalysts exhibited reduction activities with conversions of NO₂ into N₂ of about 20%. Among the perovskite catalysts tested, La_0.9K_0.1FeO₃ showed a 32% conversion of NO₂ into N₂. The catalytic activities of the perovskite catalysts were largely influenced by the number and stability of oxygen vacancies. For the γ-alumina catalyst, the peak reduction activity appeared at a relatively high temperature of around 500 °C, but the NO₂ reduction was more effective than the NO reduction, in contrast to the results of the ZSM-5 and perovskite catalysts.     

NO x uptake mechanism on Pt/BaO/Al2O3 catalysts

The NO _x adsorption mechanism on Pt/BaO/Al₂O₃ catalysts was investigated by performing NO _x storage/reduction cycles, NO₂ adsorption and NO + O₂ adsorption on 2%Pt/(x)BaO/Al₂O₃ (x = 2, 8, and 20 wt%) catalysts. NO _x uptake profiles on 2%\Pt/20%BaO/Al₂O₃ at 523 K show complete uptake behavior for almost 5 min, and then the NO _x level starts gradually increasing with time and it reaches 75% of the inlet NO _x concentration after 30 min time-on-stream. Although this catalyst shows fairly high NO _x conversion at 523 K, only ~2.4 wt% out of 20 wt% BaO is converted to Ba(NO₃)₂. Adsorption studies by using NO₂ and NO + O₂ suggest two different NO _x adsorption mechanisms. The NO₂ uptake profile on 2%Pt/20%BaO/Al₂O₃ shows the absence of a complete NO _x uptake period at the beginning of adsorption and the overall NO _x uptake is controlled by the gas–solid equilibrium between NO₂ and BaO/Ba(NO₃)₂ phase. When we use NO + O₂, complete initial NO _x uptake occurs and the time it takes to convert ~4% of BaO to Ba(NO₃)₂ is independent of the NO concentration. These NO _x uptake characteristics suggest that the NO + O₂ reaction on the surface of Pt particles produces NO₂ that is subsequently transferred to the neighboring BaO phase by spill over. At the beginning of the NO _x uptake, this spill-over process is very fast and so it is able to provide complete NO _x storage. However, the NO _x uptake by this mechanism slows down as BaO in the vicinity of Pt particles are converted to Ba(NO₃)₂. The formation of Ba(NO₃)₂ around the Pt particles results in the development of a diffusion barrier for NO₂, and increases the probability of NO₂ desorption and consequently, the beginning of NO _x slip. As NO _x uptake by NO₂ spill-over mechanism slows down due to the diffusion barrier formation, the rate and extent of NO₂ uptake are determined by the diffusion rate of nitrate ions into the BaO bulk, which, in turn, is determined by the gas phase NO₂ concentration.     

Co oxidation on LaCoO3 perovskite

A study of CO oxidation on LaCoO₃ perovskite was performed in an ultrahigh vacuum system by means of adsorption and desorption. All gases were adsorbed at ambient temperature. Two adsorption states (α- and β-) of CO exist. The α-peak at 440 K is attributed to carbonyl species adsorbed on Co³⁺ ions while the β-peak at 663 K likely comes from bidentate carbonate formed by adsorption on lattice oxygens. CO₂ shows a single desorption peak (β-state, 483 K) whose chemical state may be monodentate carbonate. A new CO₂ desorption peak at 590 K can be created by oxidation of CO. O₂ also shows two adsorption states. One desorbs at 600 K, which may reflect adsorption on Co^3- ions. The other apparently incorporates with bulk LaCoO₃ and desorbs above 1000 K. The two adsorption states of CO are oxidized via different mechanisms. The rate determining step in oxidation of a-CO is the surface reaction whereas for that of β-CO, it is desorption of product CO₂.     

FT-IR spectroscopic investigation of the surface reaction of CH4 with NO x species adsorbed on Pd/WO3–ZrO2 catalyst

The interaction of methane at various temperatures with NO _x species formed by room temperature adsorption of NO + O₂ mixture on tungstated zirconia (18.6 wt.% WO₃) and palladium(II)-promoted tungstated zirconia (0.1 wt.% Pd) has been investigated using in situ FT-IR spectroscopy. A mechanism for the reduction of NO over the Pd-promoted tungstated zirconia is proposed, which involves a step consisting of thermal decomposition of the nitromethane to adsorbed NO and formates through the intermediacy of cis-methyl nitrite. The HCOO⁻ formed acts as a reductant of the adsorbed NO producing nitrogen.     

CO and NO adsorption for the IR characterization of Fe2+ cations in ferrierite: An efficient catalyst for NOx SCR with NH3 as studied by operando IR spectroscopy

Iron was introduced by ionic exchange inside the FER structure in order to yield a Fe-FER series with increasing metal loading. Characterization of the Fe²⁺ cations by adsorption of CO at liquid nitrogen temperature followed by infrared spectroscopy allowed to identify three distinct sites for iron. The most abundant iron species are located on easily accessible sites of the FER structure, whereas high metal loading is required to observe more confined Fe²⁺ species. According to the CO adsorption results, the main iron species appears to be coordinatively unsaturated whereas isotopic labelling upon NO adsorption indicates that two distinct iron sites almost give rise to the same mononitrosyl infrared signature. Studying the catalyst upon interaction with NO and O₂ in operando conditions leads to the observation of these mononitrosyl species who behave as reaction intermediates for the NO oxidation into NO₂. All our Fe-FER samples presenting these mononitrosyl complexes are active not only in NO-to-NO₂ reaction but also in the NOx selective catalytic reduction with ammonia. The effects of both NH₃ and SO₂ as adsorption competitor during the low temperature NH₃-SCR are also discussed.     

A novel continuous reactor for catalytic reduction of NOx—fixed bed simulations

A novel dual‐zone fluidized bed reactor was proposed for the continuous adsorption and reduction of NO_x from combustion flue gases. The adsorption and reaction behaviour of such a reactor has been simulated in a fixed bed reactor using Fe/ZSM‐5 catalyst and propylene reductant with model flue gases. Fe/ZSM‐5 exhibited acceptable activity at T = 350°C and GHSV = 5000 h^?1 when O₂ concentration was controlled at levels lower than 1% with a HC to NO molar ratio of about 2:1. XPS and BET surface area measurement revealed the nature of the deactivation of the catalyst. Those performance data demonstrated the feasibility of a continuous dual‐zone fluidized bed reactor for catalytic reduction of NO_x under lean operating conditions.     

NOx Abatement by Catalytic Traps: On the Mechanism of NOx Trapping Under Automotive Conditions

NO _x adsorption was measured with a barium based NO_x storage catalyst at an engine bench equipped with a lean burn gasoline direct injection engine (GDI). In order to study the influence of gas phase NO₂ on the NO_x storage efficiency two different pre-catalysts were used: One with excellent NO oxidation activity to produce a high NO₂ concentration and another pre-catalyst without NO oxidation activity and therefore high NO concentration at the NO _x storage catalyst inlet. Both pre-catalyst had excellent HC and CO conversion efficiency and therefore the CO and HC concentration at the NO _x storage catalyst inlet was practically zero. No lean NO _x reduction was observed. Under that conditions, experiments with NO _x storage catalysts of different length show that a high NO₂ inlet concentration did not enhance the NO _x storage efficiency. Moreover, we observed reduction of NO₂ to NO over the NO_x storage catalyst. However, in presence of a high NO inlet concentration NO₂ formation was observed which may proceed parallel to NO _x storage.