NO Oxidation Reaction Research Articles

Oxidation of zeolite acid sites in NO/O2 mixtures and the catalytic properties of the new site in NO oxidation

The NO oxidation reaction kinetics over a number of zeolite structures were investigated at temperatures above 423K using plug-flow microreactors and in situ FTIR spectroscopy. Fast catalytic NO oxidation rates (up to 1000×) are observed over H- and Na-exchanged zeolite frameworks (BEA, MFI, and CHA) at temperatures above 423K with respect to the gas-phase reaction. NO oxidation rates increase with temperature over H- and Na-zeolites, while siliceous zeolites display little activity. Activation energies scale in the order of CHA<MFI<BEA (40.8–55.5kJmol−1), and application of transition state theory shows that the formation of the activated complex is an enthalpically controlled process that benefits from smaller zeolite pore size. In all samples, rates are proportional to NO and O2 concentrations, in contrast to low-temperature catalytic rates Loiland et al. [33], which are second order in NO and first order in O2. This difference indicates that a new reaction mechanism is operative above 423K. In-situ FTIR studies reveal that NO+ coordinated at framework sites in the zeolite pores (SiO−(NO+)Al) plays a direct role in the catalysis, and they also reveal that the NO oxidation rate is proportional to the amount of NO+ present in the sample. Furthermore, it is found that NO+ is in equilibrium with gas-phase NO and that desorption of NO+ (as NO) yields an oxidized acid site (SiOAl). No evidence of NO+ formation is observed over the siliceous zeolite samples. A working model of the reaction mechanism for the high-temperature NO oxidation reaction is proposed for acidic zeolites that is consistent with the observed form of the rate equation and the observed NO+ reaction intermediate.

Si-embedded boron-nitride nanotubes as an efficient and metal-free catalyst for NO oxidation

We study the catalytic capability of Si-embedded boron nitride nanotubes (Si-BNNTs) as a metal-free catalyst for NO oxidation by using density functional theory. Our results reveal that a vacancy defect in BNNT strongly stabilizes the Si adatom and makes it more positively charged. These results confirm the fact that Si-BNNTs are stable enough to be utilized in catalytic NO oxidation reactions. The complete NO oxidation reactions involve the direct oxidization of NO to NO2 by O2 adsorbed on the surface of Si-BNNTs, as in NO+O2→NO2+O. The remaining O atom can further oxidize NO to NO2. The calculated energy barriers range from 0.2 to 2.4kcal/mol, which suggests that the NO oxidation catalyzed by the Si-BNNTs is likely to occur at the room temperature. These values are much less than those processes using noble metal catalysts.

Catalytic performance of CuO/Ce0.8Zr0.2O2 loaded onto SiC-DPF in NOx-assisted combustion of diesel soot

A copper/ceria-zirconia catalyst was incorporated onto a DPF following an environmentally-friendly impregnation procedure. Its catalytic activity was studied for diesel exhaust NO oxidation and soot combustion reactions.

Kinetic modeling of H2-assisted C3H6 selective catalytic reduction of NO over silver alumina catalyst

A global kinetic model was developed for C3H6-SCR consisting of NO oxidation, C3H6 oxidation and C3H6-SCR reactions with and without H2 over Ag-Al2O3. The model is based on a dual role of H2 to remove inhibiting nitrates from the active sites as well as to modify/form new active Ag sites. For model development, a range of temperature programmed reaction (TPR) and transient feed experimental conditions were used. The proposed model was eventually also validated with additional transient experimental data. The kinetic model proposed in this study predicts the experimental data well for a wide-range of feed conditions. Evaluation of mass transfer resistance in the washcoat indicated that during H2-assisted C3H6-SCR, mild internal mass transfer resistance for NO was predicted to be important already at 250°C.

Mn/TiO2촉매를 이용한 일산화질소의 산화반응 특성 연구

본 연구에서는, <TEX>$Mn/TiO_2$</TEX> 촉매를 이용하여 일산화질소의 산화반응 특성에 따른 연구가 수행되었다. <TEX>$TiO_2$</TEX>의 물리 화학적 특징 및 활성금속인 Mn과 담체인 <TEX>$TiO_2$</TEX>의 interaction에 따라 일산화질소의 산화반응이 서로 다르게 나타남을 관찰하였다. 우수한 NO oxidation 반응을 나타낸 <TEX>$Mn/TiO_2(A)$</TEX>의 경우, Mn의 함량이 10 wt%에서 30 wt%로 증가할수록, 공간속도가 낮아질수록, 반응활성이 증가함을 확인하였다. 이러한 결과를 바탕으로 SCR 전단에 <TEX>$Mn/TiO_2$</TEX> 촉매를 사용함으로써, fast-SCR을 유도하여 SCR반응활성이 증진될 수 있을 것으로 판단된다. In this study, NO oxidation reaction using <TEX>$Mn/TiO_2$</TEX> catalysts was investigated. NO oxidation results revealed that the different trend was observed upon physicochemical properties of <TEX>$TiO_2$</TEX> and interactions between Mn and <TEX>$TiO_2$</TEX> support. <TEX>$Mn/TiO_2(A)$</TEX> catalyst has a superior NO oxidation activity, which increased with decreased space velocity and increased Mn amounts from 10 to 30 wt%. The results indicated that the SCR activity could increase by the fast SCR reaction process using the <TEX>$Mn/TiO_2(A)$</TEX> catalyst located in front of the SCR unit.

Combined experimental and kinetic modeling study of the bi-modal NOx conversion profile on commercial Cu-SAPO-34 catalyst under standard SCR conditions

Bi-modal NOx conversion profile has been consistently reported for selective catalytic reduction (SCR) of NOx with NH3 on certain Cu-exchanged zeolite catalysts under the so-called standard SCR conditions, with two distinct NOx conversion peaks located around 250 and 450°C. However, the nature of this practically important phenomenon remained unexplained. In this work, we have used a combination of experimental studies and kinetic modeling to characterize and de-convolute several side reactions, allowing us to quantitatively reconstruct the overall bi-modal conversion profile. In particular, we have shown that the first peak can be represented as a competition between the standard SCR reaction and parasitic NH3 oxidation. The subsequent increase in the NOx conversion can be attributed to the onset of in-situ NO oxidation to NO2 that enhances NOx reduction via fast SCR reaction route. Specifically, our modeling study indicates that NO oxidation reaction accelerates by orders of magnitude under the standard SCR conditions and becomes the governing step for NO reduction at high temperature.

NO oxidation activity of Ag-doped perovskite catalysts

Ag-doped perovskite catalysts (La1−xAgxMnO3) prepared by the citric acid method were investigated for the catalytic oxidation of NO. Compared to LaMnO3, La1−xAgxMnO3 revealed a superior NO oxidation activity with its maximum activity at x=0.2, confirming the potential of La0.8Ag0.2MnO3 as one of the most promising NO oxidation catalysts. The high reactivity of La0.8Ag0.2MnO3 is mainly due to the high oxygen vacancy concentration on its surface produced by the partial substitution of La3+ with Ag+ in LaMnO3, as determined by the XRD, SEM, XPS, O2-TPD, H2-TPR, (NO+O2)-TPRD and DRIFT studies. The mono- and bi-dentate nitrates formed on the oxygen vacancies of perovskite appear to be the primary reaction intermediates for the NO oxidation reaction over the Ag-doped perovskite catalyst. A plausible reaction pathway of the NO oxidation over La1−xAgxMnO3 has been postulated on the basis of the redox process involving the oxygen vacancy, oxygen, NO, NO2 and NO3.

Understanding ammonia selective catalytic reduction kinetics over Cu/SSZ-13 from motion of the Cu ions

Cu/SSZ-13 catalysts with Si/Al=6 and various Cu/Al ratios are synthesized with solution ion exchange. Catalysts are characterized with surface area/pore volume measurements, Temperature Programmed Reduction (TPR), and Electron Paramagnetic Resonance (EPR) spectroscopy. Catalytic properties are examined using NO oxidation, ammonia oxidation, and standard ammonia selective catalytic reduction (NH3-SCR) reactions. Prior to full dehydration of the zeolite catalysts, hydrated Cu2+ ions are found to be very mobile as judged from EPR. NO oxidation is catalyzed by O-bridged Cu-dimer species that form at relatively high Cu loadings and in the presence of O2. For NH3 oxidation on samples with low to intermediate Cu loadings, transient Cu-dimers are the low-temperature (⩽300°C) active centers, while these dissociate to monomers at 350°C and above and become active centers. For the much more complex standard SCR reaction, transient Cu-dimers are the active sites for reaction temperatures <250°C at very low Cu loadings (Cu/Al⩽0.016). Between ∼250 and 350°C, these Cu-dimers become less stable causing SCR reaction rates to decrease. At temperatures ⩾350°C, Cu2+ monomers that had migrated to faces of 6-membered rings are the active sites. At intermediate Cu loadings, monomeric Cu2+ ions are also active in SCR in the low-temperature regime; these are proposed to be located within CHA cages and next to 8-membered rings, likely in the form of [Cu(OH)]+. At high Cu loadings (i.e., more than one Cu2+ ion in each unit cell), stable Cu-dimers form and these do not dissociate at temperatures above 350°C. These moieties effectively occupy CHA cage space and block pore openings causing decreased efficiency of the catalysts. Also these moieties are highly active in catalyzing the NH3 oxidation reaction thus causing SCR selectivities to decrease above ∼450°C. Finally, our kinetics results strongly support a redox mechanism for standard SCR.

Effect of the calcination temperature on the performance of a CeMoOx catalyst in the selective catalytic reduction of NOx with ammonia

The effect of the calcination temperature on the structure and the catalytic performance of a CeMoOx mixed oxide catalyst in the selective catalytic reduction (SCR) of NOx with NH3 has been investigated. Compared to the catalyst calcined at 400°C, the one calcined at 500°C shows a performance of a slightly lower NOx conversion in the reaction temperature range below 200°C, and a higher NOx conversion above 350°C. However, further increase of the calcination temperature to 600 and 700°C leads to a drop of the NOx conversion and the N2 selectivity in both the low and high reaction temperature ranges. For the samples calcined at 600 and 700°C, Mo significantly enriches on the surface. These samples also favor the formation of N2O, and show a poor redox ability in NO oxidation reaction. The low ratio and low mobility of the surface chemisorbed oxygen correlates to the poor redox ability of the samples calcined at high temperatures. Furthermore, an abrupt drop of the surface area for the sample calcined at 600°C is observed and the destruction of acid sites during the high temperature calcination also recorded.

Theoretical insights into the reaction mechanisms of NO oxidation catalyzed by Cu 2 O(1 1 1)

• Cu 2 O shows superior catalytic activity toward NO oxidation. • Coadsorption of NO and O 2 gives rise to the formation of OONO species, which plays an important role in reducing barrier. • NO reacts with molecular or dissociated oxygen, rather than lattice oxygen of the Cu 2 O(1 1 1) surface. • NO 2 will poison the reaction of NO oxidation if it occurs through Langmuir–Hinshelwood mechanism, and an alternative Eley–Rideal mechanism makes NO 2 desorption possible. The NO oxidation on Cu 2 O(1 1 1) with molecular oxygen, dissociated oxygen, and lattice O, was studied by using periodic density functional theory. Cu 2 O could promote NO oxidation via the more favorable Elay–Rideal mechanism. For NO oxidation with molecular oxygen, path II (NO + O 2 * → O * ONO → NO 2 + O * ; NO + O* → NO 2 * → NO 2 ) was found as the most probable route, in which NO 2 desorption is the reaction rate determining step. The NO oxidation reaction with dissociated oxygen is also possible. In this case, O 2 dissociation occurs after surpassing a barrier of 105 kJ/mol. Thereafter, NO molecule can readily react with oxygen adatoms without barrier or with a moderate-low barrier of 49 kJ/mol. Both of the produced NO 2 molecules will release from the surface. The barrier to be surmounted is 53.3 and 103.2 kJ/mol, respectively. The reaction of NO with lattice O has a high barrier and it is very unlikely. The present results enrich our understanding of the catalytic oxidation of NO by metal-oxide catalysts.

Effect of the addition of Ce to MnOx/Ti catalyst on reduction of N2O in low‐temperature SCR

ABSTRACTThe effect of MnOx/Ti catalyst doped with Ce prepared by sol–gel method at a low temperature on N2O formation during the selective catalytic reduction of NO with NH3 was studied in detail. Results showed that the production of N2O was primarily controlled by two paths: (1) direct reaction of NO and NH3 and (2) oxidation of NH3 with O2. The overoxidation of NH3 was the dominant step in N2O reduction. Addition of Ce to MnOx/Ti catalyst increased the proportion of lattice oxygen on the catalyst surface. Meanwhile, the stability of lattice oxygen, which contributed to the decrease in N2O formation by inhibiting the overoxidation of NH3, on the catalyst surface was the best when the Ce/Ti mole ratio reached 0.1. © 2014 Curtin University of Technology and John Wiley &amp; Sons, Ltd.

Highly dispersed iron species created on alkali-treated zeolite for ammonia SCR

Alkaline treatment using sodium hydroxide was introduced to obtain a hierarchical pore structure in H–ZSM-5 zeolite. Fe-exchanged zeolite catalysts were prepared by impregnation on the original and alkali-treated zeolites, and were evaluated for NOx reduction by NH3, NO oxidation, and NH3 oxidation reactions. The highly dispersed iron species as active sites can be obtained by controlling the pore structure and particle size of zeolite. Therefore, the Fe/ZSM-5 catalyst treated mildly by sodium hydroxide before iron exchange, which contains amounts of highly dispersed Fe species, obtains over 80% NOx conversion at a wide temperature range of 250–500°C.

Effect of preparation method on MnOx-CeO2 catalysts for NO oxidation

The NO oxidation reaction was studied over MnOx-CeO2 catalysts prepared by co-precipitation, impregnation and mechanical mixing method, respectively. It was found that the co-precipitation was the most active and a 60% NO conversion was achieved at 250 °C. X-ray diffraction (XRD), Brumauer-Emmett (BET), H2-temperature programmed reduction (H2-TPR) and oxygen storage capacity (OSC) techniques were employed to characterize the physical and chemical properties of the catalysts. XRD results showed that amorphous MnOx or Mn-O-Ce solid solution existed in co-precipitation and impregnation prepared sample, while crystalline MnOx was found in mechanical mixing catalyst. A larger surface area was observed on co-precipitation prepared catalyst compared to those prepared by impregnation and mechanical mixing. The strong interaction between MnOx and CeO2 enhanced the reducibility of the oxides and increased the amount of Mn4+ and activated oxygen, which are favorable for NO oxidation to NO2.

Catalytic performance of NO oxidation over LaMeO3 (Me = Mn, Fe, Co) perovskite prepared by the sol–gel method

Abstract The catalytic performance of LaMeO 3 (Me = Mn, Fe, Co) perovskite prepared by a sol–gel method was studied. These catalysts were characterized by X-ray diffraction (XRD), N 2 adsorption (BET), H 2 temperature programmed reduction (TPR), NO temperature programmed desorption (TPD) and CO–O 2 pulse. LaCoO 3 exhibited the best activity than that of LaFeO 3 and LaMnO 3 even after hydrothermal ageing. The activity sequence is in accordance with the reducibility of the samples. The activated oxygen species and adsorbed NO play key roles in the NO oxidation reaction.

Influence of Catalyst Composition on NOX Trap Performances

In the present work the influence of the type of noble metals present in NOX storage catalysts was investigated regarding the NO oxidation, NOX storage and NO reduction performances. Monolith samples with combinations of platinum, palladium and rhodium were tested in a flow-reactor. From this study, it was concluded that platinum is necessary for NO oxidation while palladium shows a better ability to oxidize NO to NO2 at high temperature. The presence of Rh on Pt/Rh and Pt/Pd/Rh was found to have a negative effect on NO oxidation reaction. Results suggest also that the storage capacity is partially linked to the oxidation function: Surface area developed by the storage material and the Pt–Ba interaction have an important influence on NOX storage function. The rich step was not efficient to purge completely the trap and leads to accumulation of NOX and so to performances degradation which was assigned to accumulation of NOX after each cycle and storage on of Ba-bulk sites. In presence of Rh, a high oxygen storage capacity leads to high exothermic process between reducing agent and stored oxygen which improves the ability of catalyst to release NOX.

Experimental Study of the NO Oxidation to NO2 Over Metal Promoted Zeolites Aimed at the Identification of the Standard SCR Rate Determining Step

Steady state and transient kinetic runs devoted to the comparative analysis of NO oxidation and standard SCR reactions over commercial Cu- and Fe-promoted zeolite catalysts are herein presented with the aim to clarify whether NO oxidation to NO2 is the rate determining step (rds) of the standard SCR reaction. It is found that this statement seems questionable in the light both of the herein collected experimental results and of scattered evidence from the literature.

Microkinetic modeling of H2-assisted NO oxidation over Ag–Al2O3

A microkinetic model has been assembled to investigate the mechanism for NO oxidation over a monolith-supported Ag–Al2O3 catalyst both in the presence and absence of H2. The effect of H2 examined in the kinetic model was to reduce self inhibiting surface nitrate species on active silver sites. A reduced factorial design of inlet experimental conditions was used to generate transient experimental data. The kinetic model was developed based on a single channel reactor model which accounted for mass and heat transfer between gas and catalyst washcoat as well as mass transport resistance in the washcoat. In general, the modeling results could reproduce the transient experimental data well with correct levels of outlet concentrations and time scales for transient responses. It was found that the effect of increased NO and NO2 inlet concentration had a negative correlation with the NO oxidation conversion, which in the model was considered related to the formation of nitrate surface species. In addition, the model in agreement with experiments, clearly showed that H2 promoted the NO oxidation mainly at low temperature and this effect tended to decrease at elevated temperatures. When H2 was present in the feed, the kinetic model showed that H2 was consumed rapidly in the front part of the monolith. This was also seen in the experiments where in all cases H2 was entirely consumed. The rapid reaction of H2 along with resulting transport limitations indicated that the H2 promotion of the NO oxidation reaction may have been isolated to only a portion of the catalyst.

The Activities of NO Oxidation over the Pt/γ-Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3 &lt;/sub&gt;Catalyst

Pt catalysts were prepared by incipient wetness impregnation,It was investigated that the effects of the support and the loading of Pt on the activities of NO oxidation over supported Pt catalysts, The investigation indicates that Pt/γ-Al2O3 was better performance than Pt/ZrO2, Pt/CeO2, Pt/TiO2 and Pt/Ce0.4Zr0.6O2 on NO oxidation.The optimal Pt loading over Pt/r-Al2O3 was 1%, the NO conversion efficiency of 54.9% at 300°C, close to the conversion rate of thermodynamic equilibrium of the NO oxidation reaction.

Study of NO oxidation reaction over the Pt cluster supported on γ-Al2O3(111) surface

NO oxidation reaction on the Pt cluster supported on γ-Al2O3(111) surface is investigated using the climbing image nudged elastic band (CI-NEB) calculation based on the density functional theory. This is to verify the possibility of using Pt-based catalyst for NO oxidation reaction which is the rate-limiting step for the NOx oxidation reaction. From investigations of NO adsorption and NO2 desorption on the Pt cluster supported on γ-Al2O3 (111) surface where the dissociated O atoms already formed and lie on the Pt cluster sites, we find the process requires no activation barrier. Furthermore, we also performed the first principles of molecular dynamics simulation to investigate NO oxidation reaction at certain range temperatures (400 K, 600 K and 800 K) for complement of CI-NEB calculation. From the results, we discussed the temperature dependence of NO oxidation reaction.

The effect of partial substitution of Co in LaMnO3 synthesized by sol–gel methods for NO oxidation

LaMn1 − yCoyO3 (y = 0–0.5) catalysts were synthesized by sol–gel methods and characterized by BET and XRD. The catalytic activities were non-linear enhanced by cobalt substitution, and achieved the highest level (76.5%) on LaMn0.9Co0.1O3. The reducibility was investigated by the oxygen storage capacity and H2-TPR measurements. The stability of Mn4+ and activated oxygen species may be the determined factors for NO oxidation reaction, which is attributed to the synergistic effect between cobalt and manganese.