Adsorption and reaction of NO on activated carbon in the presence of oxygen and water vapour

A study of the adsorption and reaction of NO in the presence of oxygen and water vapour on an activated carbon obtained from oil palm shells is presented. The study is based on the measurement of breakthrough curves, at temperatures between 100 and 150 °C, and on the subsequent thermal desorption in a fix bed reactor. The concentration of the gas components, NO, O₂ and H₂O, corresponds to a simulation of a flue gas in a coal fired power plant. The experimental results show that the reactions on this system include the simultaneously adsorption, reduction and catalytic oxidation of NO together with the adsorption of created NO₂. During desorption NO₂ reacts to NO through a reductive desorption process. An acceleration of the NO oxidation occurs when the saturation level of the adsorbed NO is reached, resulting in a maximum on the breakthrough curve. Different adsorbed NO species are formed during the process: one thermal unstable NO, and three thermal stable NO species, NO₂, NO and (NO)₂ dimers, respectively.     

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.     

Adsorption and temperature programmed desorption of NO,NO + O2, NO2 and NO + NO2 over CoH-ZSM-5

The NO₂, NO (O₂) adsorption and temperature programmed desorption (TPD) were studied systematically to probe into the selective catalytic reduction of NO by methane (CH₄–SCR) over CoH-ZSM-5 (SiO₂/Al₂O₃ = 25, Co/Al = 0.132–0.312). Adsorption conditions significantly affect the adsorption of NO, NO₂ and NO + O₂. Adsorbed NO species are unstable and desorbed below the reactive temperature 523 K. Increasing adsorption temperature results in the decrease of the adsorbed NO species amount. The amount of –NO_y species formed from NO₂ adsorption increases with the increase of NO₂ concentration in the adsorption process, while decreases significantly with the increase of adsorption temperature. Though NO species are adsorbed weakly on CoH-ZSM-5, competitive adsorption between NO and –NO_y species decreases the amount of adsorbed –NO_y species. Similar desorption profiles of NO₂ were obtained over CoH-ZSM-5 while they were contacted with NO₂ or NO + O₂ followed by TPD. If NO₂ was essential to form adsorbed –NO_y species, the amount of adsorbed –NO_y species for NO + O₂ adsorption should be the least among the adsorptions of NO₂, NO + O₂ and NO + NO₂ because of the lowest NO₂ concentration and highest NO concentration. In fact, the amount of adsorbed –NO_y species is between the other two adsorption processes. These indicate that the formation of adsorbed –NO_y species may not originate from NO₂.     

Detailed kinetic modeling of NOx adsorption and NO oxidation over Cu-ZSM-5

Detailed kinetic modeling was used in combination with flow reactor experiments to investigate the NO_x adsorption/desorption and NO oxidation over Cu-ZSM-5. NO oxidation is likely an important step for selective catalytic reduction (SCR) using urea and hydrocarbons, and thus was investigated separately. First the NO₂ adsorption on Brönstedt acid sites in H-ZSM-5 was modeled using an NO₂ temperature programmed desorption (TPD) experiment. These results, together with the results of the NO₂ TPD and NO oxidation experiments, were used in developing the model for Cu-ZSM-5. A substantial amount of NO₂ was adsorbed on the catalyst. However, the results from a corresponding NO TPD experiment showed that only very small amounts of NO were adsorbed on the catalyst and therefore this step was not included in the model. The model consists of reversible steps for NO₂ and O₂ adsorption, O₂ dissociation, NO oxidation and two steps for nitrate formation. The first nitrate formation step was disproportionation of NO₂ to form NO and nitrates. This step enabled us to describe the NO production during NO₂ adsorption. Further, in the reverse step the NO reacts with the nitrates and decreased their stability. Without this step the nitrates blocked the surface resulting in to low NO oxidation activity. However, we observe that nitrates can be decomposed also without the presence of NO and in the second reversible step were the nitrates decomposed to form NO₂ and oxygen on the copper. These steps enabled us to describe both the TPD and activity measurement results. NO oxidation was observed even at room temperature. Interestingly, the NO₂ decreased when increasing the temperature up to 100 °C and then increased as the temperature increased further. We suggest that this low-temperature NO oxidation occurs with species loosely bound on the surface and that is included in the detailed mechanism. An additional NO₂ TPD at 30 °C was also modeled to describe the loosely bound NO₂ on the surface. The detailed model correctly describes NO₂ storage, NO oxidation and low-temperature NO oxidation.     

NO Oxidation Kinetics on Iron Zeolites: Influence of Framework Type and Iron Speciation

Zeolites having MFI, FER and *BEA topology were loaded with iron using solid state cation exchange method. The Fe:Al atomic ratio was 1:4. The zeolites were characterized using nitrogen adsorption, FTIR and DR UV–Vis–NIR spectroscopy. The catalytic activity in NO oxidation and the occurrence of NO _x adsorption was determined in a fixed-bed mini reactor using gas mixtures containing oxygen and water in addition to NO and NO₂ and temperatures of 200–350 °C. Under these reaction conditions, the NO _x adsorption capacity of these iron zeolites was negligible. The kinetic data could be fitted with a LHHW rate expression assuming a surface reaction between adsorbed NO and adsorbed O₂. The kinetic analysis revealed the occurrence of strong reaction inhibition by adsorbed NO₂. FER and MFI zeolites were more active than *BEA type zeolite. MFI zeolite is most active but suffers most from NO₂ inhibition of the reaction rate. FTIR and UV–Vis spectra suggest that isolated Fe³⁺ cations and binuclear Fe³⁺ complexes are active NO oxidation sites. Compared to the isolated Fe³⁺ species, the binuclear complexes abundantly present in the MFI zeolite seem to be most sensitive to poisoning by NO₂.     

NO-Assisted N2O Decomposition over ex-Framework FeZSM-5: Mechanistic Aspects

The decomposition of N₂O over an ex-framework FeZSM-5 catalyst is strongly promoted by NO. Activity data show that the promoting effect of NO is catalytic, and that besides NO₂, O₂ is formed much more extensively in the presence, than in the absence of NO. Transient in situ FT-IR/MS measurements indicate that NO is strongly adsorbed on the catalyst surface up to at least 650 K, showing absorption frequencies at 1884 and 1876 cm^–1. A change in gas phase composition from NO to N₂O results in the formation of adsorbed NO₂, identified by a sharp IR band at 1635 cm^–1. Switching back to the original NO gas phase induces a rapid desorption of NO₂, restoring the original NO absorption frequencies. During the IR measurements, bands typical of nitro- or nitrate groups were not observed. Multi-Track (a TAP-like technique) experiments show that the presence of NO or NO₂ on the catalyst surface significantly enhances the rate of oxygen desorption at the time of N₂O exposure to the catalyst. The spectral changes and transient experiments are discussed and catalytic cycles are proposed, to explain the formation of NO₂ and the (enhanced) formation of oxygen. The latter can be either explained by an indirect effect (electronic, steric) of NO adsorbed on sites neighboring the active sites, or by a direct effect involving reaction of adsorbed NO₂ groups with neighboring oxidized sites yielding O₂.     

H2-Induced NO x Adsorption/Desorption over Ag/Al2O3: Transient Experiments and TPD Study

NO adsorption/desorption over 1 wt% Ag/Al₂O₃ was studied by a combination of isothermal transient adsorption/desorption and NO _x temperature-programmed desorption (NO _x -TPD) methods. NO _x -TPD profiles obtained for Ag/Al₂O₃ were identified by comparison with decomposition profiles of “model” AgNO₃/Al₂O₃ and Al(NO₃)₃/Al₂O₃ prepared by impregnation of Al₂O₃ with individual AgNO₃ and Al(NO₃)₃ compounds. The data obtained indicate that H₂-induced NO adsorption leads to the formation of surface Ag and Al-nitrates. Their accumulation on the catalyst surface is accompanied by an intensive NO₂ evolution, which proceeds primarily via reaction of surface nitrates with NO. Thus, NO₂ formation appears to result from an intrinsic stage of the H₂-induced NO _x adsorption process, rather than from the direct oxidation of NO by gaseous oxygen catalyzed by Ag.     

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.     

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.     

Pillared smectite modified with carbon and manganese as catalyst for SCR of NOx with NH3. Part II. Temperature‐programmed studies

Temperature‐programmed desorption (TPD) and surface reaction (TPSR), and additionally FTIR spectroscopy of adsorbed NO molecules were used to characterise surface sites on pillared smectites modified with carbon and manganese. Much higher adsorption of NH₃ than NO was found, but acidic pre‐treatment increased NO sorption to comparable values as well as catalytic performance in SCR of NO_x. In this case formation of strongly bound NO₃ ⁻ species was recognised, which reacted with NH₃ at a temperature 200 °C higher than weaker adsorbed NO. This revised version was published online in July 2006 with corrections to the Cover Date.     

Modelling of NOx adsorption over NOx adsorbers

Modelling of the phenomena involved during the adsorption of NO_x on NO_x trap catalysts was developed. The aim of the model is the prediction of the quantity of stocked barium nitrate as well as the emissions of NO and NO₂, as a function of time and temperature. The mechanism of the process is sounded on the adsorption of gas species (NO, NO₂, O₂) on platinum sites, equilibrium reaction between adsorbed species followed by the formation of Ba(NO₃)₂. This formation of barium nitrate is limited by the thermal decomposition reaction which liberates NO in the gas phase. The kinetic constant of decomposition of barium nitrate was determined by temperature programmed thermogravimetry on pure Ba(NO₃)₂, using the method of Freeman and Carroll. Other kinetic constants bound to the mechanism were estimated by fitting the results of the model to experimental results.The mechanism was validated for various values of the molar fraction of O₂, the molar fraction of NO and various values of the NO/NO₂ ratio in the gas entering the reactor. It was also tested with different catalyst compositions (variation of the platinum and BaO concentrations). The importance of oxygen in the process was clearly demonstrated as well as the promoting role of NO₂.     

Regeneration of initial activity of a pitch-based ACF for NO–NH3 reaction at ambient temperature

The reactivity of adsorbed NO (including NO₂) and NH₃ in the presence of 4.0% oxygen in He was examined over a pitch-based ACF calcined at 800°C. Regeneration at 30°C by 4% O₂ in He without NH₃ was found to be optimum for the recovery of the initial activity with complete removal of NO within 3 h, with minimum leaks of adsorbed NO and NH₃. A higher temperature of 40°C for regeneration increased the liberation of adsorbed NO, and NH₃ over ACF was rather slow at a lower temperature of 25°C, slow regeneration being achieved. Oxygen appears necessary to regenerate the ACF through enhancing the reaction of adsorbed NO and NH₃ for the initial activity, which was ascribed to the catalytic activity for NO–NH₃ and adsorption of both NO and NH₃. NH₃ in the gas phase appears to inhibit the regeneration reaction of adsorbed species, by using the leaking amount during the regeneration.     

Removal of nitrogen oxides from air by chemicals-impregnated carbons

Fixation of nitrogen oxides (NO_x) in air onto granular activated carbon impregnated with chemicals was attempted to improve removal efficiency of NO_x by activated carbon adsorption. Nitric oxide (NO) and nitrogen dioxide (NO₂), were tried to remove by a flow test. Fixed-bed adsorption breakthrough curves were obtained when some kinds of carbon were used. The amount adsorbed of NO₂ changed with the amount and kinds of metallic salts impregnated. Chemicals-impregnated carbons were prepared from a commercial activated carbon. Among obtained carbons, the one which showed the highest selectivity for NO_x was chosen, and its performance with the change in humidity was determined. Removal mechanism of NO₂ was estimated, and the carbon impregnated with potassium hydroxide was found to be superior to any other carbon tested. The amount of the adsorbed NO and that produced by the reduction of NO₂ were determined from the breakthrough curves.     

Adsorption of nitrogen dioxide on polycrystalline gold

The adsorption of nitrogen dioxide (NO₂) on a polycrystalline Au surface was studied by temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). Three desorption states due to chemisorbed NO₂ were observed using TPD, with desorption activation energies,E _d , of 11,13, and 17 kcal/mol. The desorption energies reflect the heats of adsorption of NO₂ on the polycrystalline gold surface, since NO₂ adsorption is not an activated process. Desorption of physisorbed NO₂ from N₂O₄ multilayers was also seen at 130–140 K. The sticking probability of NO₂ at 120 K is independent of coverage indicating a strong influence of a precursor state in the adsorption kinetics. Vibrational spectra using HREELS show that chemisorbed NO₂ is molecularly adsorbed on the surface, probably as a Au O,O'-nitrito surface chelate. No evidence for the dissociation of NO₂ on Au was found using AES, TPD, or HREELS, even for large exposures of NO₂ at surface temperatures up to 500 K. Comparison of these results with those for NO₂ adsorption on a Au(111) surface is made. High energy sites, such as steps and kinks, and other crystal faces of Au can chemically bond NO₂ more tightly than occurs on Au(111), but the activation energy for dissociation of NO₂ at all of these sites exceeds 17 kcal/mol, and thus NO₂ adsorption is reversible on Au under low pressure conditions.     

NO2 and N2 sorption in MFI films with varying Si/Al and Na/Al ratios

MFI crystals or films with controlled thicknesses and different Si/Al ratios were grown on seeded cordierite monoliths using a clear synthesis mixture with template or a template-free gel. The materials were analyzed by scanning electron microscopy, X-ray diffraction, inductively coupled plasma-atomic emission spectrometry, X-ray photoelectron spectroscopy, thermogravimetric analysis and sorption experiments using N₂ or NO₂ adsorbates. The films were uniformly distributed over the support surface. As expected, the specific monolayer N₂ adsorption capacity (mol/g_zeolite) was constant and independent of film thickness. The specific molar NO₂ adsorption capacity was significantly lower than the specific molar monolayer N₂ adsorption capacity, indicating that NO₂ is adsorbed at specific sites rather than evenly distributed in a monolayer. A number of NO₂ adsorption sites with varying strengths were observed by TPD experiments. At 30 °C, the amount of adsorbed NO₂ in the MFI films increased with increasing Al and Na content as opposed to the N₂ adsorption capacity, which was independent of these parameters. At 200 °C, the adsorbed amount of NO₂ was lower than at 30 °C and apparently independent on Al concentration in the Na-MFI films. These results indicate that different mechanisms are involved in NO₂ adsorption. NO₂ may adsorb weakly on Na⁺ cations and also react with silanol groups and residual water in the zeolite, the latter two results in more strongly bound species. Upon NO₂ adsorption, formation of NO was observed. This work represents the first systematic study of the effects of Al and Na content on NO₂ adsorption in MFI films.     

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.     

Indoor air purification by photocatalyst TiO2 immobilized on an activated carbon filter installed in an air cleaner

The enhancement effect of using TiO₂ immobilized on activated carbon (TiO₂/AC) filter for removing indoor air pollutant at parts-per-billion (ppb) levels has been previously reported. To further evaluate the TiO₂/AC filter for practical application, it was installed in an air cleaner available in the commercial market and tested inside an environmental chamber. Nitrogen oxide (NO) and toluene were selected as target pollutant. Results showed that a higher removal efficiency of NO was achieved using shorter wavelength ultraviolet lamp than longer wavelength ultraviolet lamp. A higher NO removal was achieved using TiO₂/AC filter compared to TiO₂ filter only. The intermediate, NO₂, generated from the photodegradation of NO was also successfully suppressed from exiting the system using TiO₂/AC filter. A 25% higher of nitrogen oxides (NO_x) was achieved using TiO₂/AC filter compared to the TiO₂/AC only. The higher removal efficiency of using TiO₂/AC is owing to the large adsorption capacity provided by the activated carbon. The adsorbed NO is then transferred to the TiO₂ for photodegradation. The difference in toluene removal efficiency using TiO₂/AC filter compared to the TiO₂ filter is even more significant.     

HC-SCR reaction pathways on ion exchanged ZSM-5 catalysts

Metal cation (metal = Cu, In and La) ion exchanged ZSM-5 zeolites as catalysts for the NO selective reduction by propane and propene in excess oxygen. The surface reactions of HC-SCR over catalysts were investigated through in situ DRIFTS method. For C₃H₈-SCR, adsorbed nitrate species (–NO₃) were observed as main reaction intermediates and they could react with gaseous propane to produce N₂, H₂O and CO₂. While for C₃H₆-SCR, adsorbed amine species (–NH₂) were observed as main reaction intermediates and they could react with NO or NO₂ to produce the final products. The different reaction pathways for C₃H₈-SCR and C₃H₆-SCR over catalysts were proposed based on the DRIFTS results and the main factors controlling the activities of catalysts were discussed in details. The competing adsorption between NO–O₂ and HC–O₂ on the Brønsted acid sites of catalysts was responsible for the different reaction pathways in HC-SCR.     

A role of surface nitrite and nitrate complexes in NOx selective catalytic reduction by hydrocarbons under oxygen excess

NO adsorption over three types of catalytic systems, such as cation-exchanged zeolites, transition metal oxides supported on -Al₂O₃, and partially stabilised tetragonal ZrO₂ (PSZ) was studied by using the TPD method. NO forms several surface complexes having different desorption temperatures. TPD results compared with catalytic properties of these systems in the selective reduction of NO _x by propane under oxygen excess showed that strongly bound nitrites and nitrates appeared to be true intermediates in this reaction.     

The Effect of nitric oxide on the photocatalytic oxidation of small hydrocarbons over titania

The catalytic UV photo-oxidation of NO in the absence and presence of ethane, ethene, propane, propene, and n-butane over TiO₂ in the presence of excess oxygen was studied in the temperature range 21–150 °C. It was confirmed in our system that in the absence of hydrocarbon NO was photocatalytically oxidised by oxygen to NO₂ over TiO₂ and was strongly absorbed. Both NO and hydrocarbon could be simultaneously photo-converted with the conversion varying considerably with both NO and hydrocarbon concentration and the nature of the hydrocarbon. In some instances the presence of NO in the feed gas enhanced hydrocarbon oxidation via reactions involving NO₂ that is a powerful oxidant. The extent of this effect depended on the relative strengths of adsorption on TiO₂ of the reactants and products. To reduce surface coverage of hydrocarbon most reactions were run at 150 °C, and it was shown that at this temperature NOx adsorbed on titania could be reduced by photogenerated hydrocarbon surface species to N₂O and N₂ under these conditions. The formation of N₂ was confirmed using ¹⁵NO with helium as carrier gas. By contrast, at room temperature in the presence of propene NO was converted to NO₂.