Presence Of Excess Oxygen Research Articles

Selective catalytic reduction of NOxin dielectric barrier discharge plasmas

This article reports observations of significant synergistic effects between dielectric barrier discharge (DBD) plasmas and Cu-ZSM-5 catalysts for C 2 H 4 selective reduction of NO x at 250 °C in the presence of excess oxygen by using a one-stage plasma-over-catalyst (POC) reactor. With a reactant gas mixture of 530 ppm NO, 650 ppm C 2 H 4 , 5.8% O 2 in N 2 , GHSV = 12 000 h -1 and input discharge energy density of 155 J L -1 , the pure catalytic, pure plasma-induced (discharges over fused silica pellets) and plasma-catalytic (in the POC reactor) NO x conversion percentages are 39%, 1.5% and 79%, respectively. A moderate plasma enhancement of NO x reduction by C 2 H 4 was also observed in a two-stage plasma-followed-by-catalyst (PFC) reactor consisting of a discharge stage filled by fused silica pellets and a Cu-ZSM-5 catalyst stage.

Mechanism of acetylene oxidation on the Pt(111) surface using in situ fluorescence yield near-edge spectroscopy

In situ studies of acetylene oxidation on Pt(111) have been performed with both fluorescence yield near-edge spectroscopy (FYNES) and temperature-programmed FYNES (TP-FYNES) for temperatures up to 600 K and flowing oxygen pressures up to 0.009 Torr. Low-temperature spectroscopic (FYNES) results indicate that at 150 K acetylene adsorbs with the C C backbone tilted slightly up from the Pt(111) surface, consistent with the formation of an η 2 - μ 3 -CCH 2 surface intermediate. When acetylene is preadsorbed on the Pt(111) surface, forming the η 2 - μ 3 -CCH 2 intermediate, oxidation occurs in a single step over the 330–420 K temperature range. In the presence of excess oxygen, little effect on the rate of oxidation is seen when the oxygen pressure is increased, as observed previously for oxidation of adsorbed propyne on the Pt(111) surface. Comparison between the intensity of the C H σ ∗ resonance and the intensity in the carbon continuum clearly shows that the adsorbed hydrocarbon intermediate maintains a 1:1 C H stoichiometry throughout oxidation, suggesting that oxydehydrogenation and skeletal oxidation occur simultaneously above 330 K. Identical reaction temperature profiles observed for (1) reaction of coadsorbed acetylene and atomic oxygen, and (2) reaction of acetylene and oxygen from the gas phase signify that in both cases a direct, bimolecular surface reaction mechanism occurs during oxidation. For the catalytic studies, a large temperature hysteresis, which is associated with inhibition of oxygen adsorption by acetylene, is observed during thermal cycling. Detailed isothermal kinetic studies performed in flowing oxygen pressures indicate that the apparent activation energy for oxidation is 20.3 ± 2.0 kcal / mol with a pre-exponential factor of 10 10.3 ± 1.0 s −1 . Taken together, these results clearly indicate that surface properties and reactant properties both play major roles in controlling oxidation reactions over an extended range of reaction conditions.

Mechanism of NO decomposi-tion on perovskite (-like) catalysts

NOx emitting from industrial and mobile exhaust are serious pollutant in air atmosphere, and the removal of them is an urgent task of today in environment-protection field. Although the present three-way-catalyst (TWC) can remove NOx from the mobile exhaust effectively, it will be out of work as lean-burn strategies are used to increase energy efficiency (for example, the diesel engine operated in the lean-burn condition), hence, the technology that can remove NOx in the presence of excess oxygen is desired. In addition, because the capability of noblemetal catalyst for NOx removal is weak at high temperatures (>873 K), it is thus necessary to comprehend the process of NOx decomposition, which would help to solve the problem of NOx removal. For NO decomposition reaction (2NO = N2 + O2), it is generally accepted that N2 was formed through the decomposition of N2O species, i.e. 2NO N2O + Oads and N2Oads N2 + Oads . While the way of oxygen formation is still unclear at present. According to the literatures, there are two proposed ways for oxygen desorption, that is, i) the direct desorption of two vicinal oxygen, Oads + Oads O2; and ii) the indirect desorption with NO as an intermediate, NOads + Oads NO2, NO2 + Oads NO3 ads, NO3 ads NO + O2. Accordingly, there are two ways for NO2 formation, that is, iii) reaction of gaseous NO and O2 occurring in the downstream, O2 + 2NO 2NO2; and iv) reaction of adsorbed O atom and NO occurring on the catalyst surface, Oads + NOads NO2. Recently, Iglesia et al. suggested that the O2 formation over Cu-ZSM-5 mainly occurred through the decomposition of NO3(a) species, but there is still no literature discussing the way of O2 and NO2 formation over perovskite (-like) oxides. In this work, by discussing the possible reaction steps occurring in the process of NO decomposition, we proposed that the way of NO decomposition over perovskite (-like) mixed oxides occurred with NO2 as an intermediate, and NO2 was mainly formed on the catalyst surface in the way of Oads + NOads NO2, while O2 was formed through the dissociation of NO2 species in the way of 2NO2(g) = 2NO(g) + O2(g). The samples, LaSrNiO4 and La0.4Sr0.6Mn0.8Ni0.2O3, were prepared by the conventional citric combustion method. O2-TPD experiment was carried out on a homemade apparatus equipped with a thermal conductivity detector (TCD). The samples (0.2 g) were first treated in O2 at 1073 K for 1 h and cooled to room temperature in the same atmosphere, then swept with pure He (or 0.5% O2/He) at a rate of 11.8 mL/min until the base line on the recorder remained unchanged. Finally, the sample was heated at a rate of 20 K/min in He (or 0.5% O2/He) to record the TPD profile. Steady-state activities of catalysts were evaluated using a single-pass flow micro-reactor made of quartz, with an internal diameter of 6 mm. The reactant gas (1% NO/He + 0% 10% O2/He) was passed through 0.5 g (in the absence of oxygen) or 1.0 g (in the presence of oxygen) catalysts at a rate of 25 or 40 mL/min (in all, to keep W/F = 1.2 g·s·cm ). The gas composition was analyzed before and after the reaction by an on-line gas chromatography, using molecular sieve 5A column for separating NO, N2 and O2. N2O was not analyzed here because it was difficult to form between 773 and 1123 K as reported in ref. [9]. Before the data were obtained, reactions were maintained for a period of ~2 h at each temperature to ensure the steady-state conditions. According to the literatures reported previously, the possible reaction steps occurring in the process of NO decomposition are summarized in Table 1.

Cooperation of reducing species for NO reduction over Ag/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> under oxidizing conditions

The cooperation effect of reducing species for selective reduction of NO over Ag/Al2O3 has been investigated in the presence of excess oxygen. When the combinations of propene or propane and ethanol or methanol were used as reducing agents, NO reduction took place over a wider temperature range, compared with a single hydrocarbon as reducing agent.

Wet-air oxidation of p-nitrophenol

Wet air oxidation rates of p-nitrophenol in the presence of excess oxygen, at different temperatures and oxygen pressures conditions were investigated. The experiments were carried out at between 120 and 200oC and at 1.0-3.0 MPa partial pressures of oxygen, in a glass vessel inserted in a stainless steel reactor. Initial p-nitrophenol concentration was 3.59x10-4 mol/L. Copper sulfate (CuSO4.5H2O) was used as a catalyst. Presence of Cu2+ ions in the reaction medium accelerated oxidation reaction slightly. Oxidation kinetics have also been investigated with respect to p-nitrophenol disappearance and TOC results.

Effect of reductants in N 2O reduction over Fe-MFI catalysts

We investigated the effect of reductants over ion-exchanged Fe-MFI catalysts (Fe-MFI) based on the catalytic performance in N 2O reduction in the presence and absence of an oxygen atmosphere. In the case of N 2O reduction with hydrocarbons (CH 4, C 2H 6, and C 3H 6) in the presence of excess oxygen, the order of N 2O contribution was as follows: CH 4 > C 2H 6 > C 3H 6. This indicates that CH 4 is a more efficient reductant than C 2H 6 and C 3H 6. The TOFs of N 2O decomposition and the N 2O reduction by various reductants (H 2, CO, CH 4) in the absence of oxygen increased with increasing Fe/Al ratio (Fe/Al⩾0.15), wheras the TOFs were lower and constant in the range of Fe/Al⩽0.10. Temperature-programmed reduction with hydrogen (H 2-TPR) showed that the catalysts with a higher Fe/Al ratio were reduced more easily than those with a lower Fe/Al ratio. Temperature-programmed desorption of O 2 (O 2-TPD) showed that oxygen was desorbed at lower temperatures over the catalysts with a higher Fe/Al ratio. As the result of extended X-ray absorption fine structure (EXAFS) analysis, only mononuclear Fe species were observed over Fe(0.10)-MFI after treatment with N 2O or O 2. On the other hand, binuclear Fe species and mononuclear Fe species were observed over Fe(0.40)-MFI after treatment with N 2O or H 2. More reducible Fe species, which gave lower-temperature O 2 desorption, can be due to Fe binuclear species. Since the N 2O reduction with reductants proceeds via a redox mechanism, the reducible binuclear Fe species can exhibit higher activity. Furthermore, CH 4 can be oxidized by N 2O more easily than can H 2 and CO, although it is generally known that the reactivity of methane is very low.

Synthesis and Processing of Barium Hexaaluminogallates

Barium hexaaluminogallate was synthesized by the mixed-oxide method and the coprecipitation method. Barium hexaaluminate and barium hexagallate were found to form barium hexaaluminogallate, Ba(Al,Ga)12O19, at 1400°C. BaCo(Al,Ga)11O19 of magnetoplumbite structure was formed from a mixture of metal oxides at 1200°C for 6 h. Co was successfully introduced to barium hexaaluminogallate, although the synthesis of BaCoAl11O19 is quite difficult via solid-state reaction of oxide powders. Anisotropic BaCo(Al,Ga)11O19 particles crystallized at 1100°C for 2 h through the coprecipitation method using metal nitrates and ammonium carbonate. BaCo(Al,Ga)11O19 supported on a cordierite honeycomb had the ability to reduce NOx with methane over 500°C in the presence of excess oxygen.

Novel nanocrystalline Ga–Al–Zn complex oxide: catalyst for simultaneous treatment of NPAC and lean NO x

Nanocrystalline Ga–Al–Zn complex-oxide (designated here as nano-GAZ) has been successfully synthesized using hydrothermal method. Using TEM, BET, FTIR, DTA–TGA, and XRD, synthesized nano-GAZ particles are shown as high-surface-area nanoparticles comprising typical spinel crystal structures. The nano-GAZ catalyst was tested as a new deNPAC catalyst, using pyridine as the model NPAC compound. The reaction results show that pyridine could be completely oxidized over nano-GAZ catalyst above 400 °C. Furthermore, NO formation was significantly decreased due to the excellent performance of the nano-GAZ catalyst for the selective catalytic reduction of NO x with hydrocarbon in the presence of excess oxygen. An optimal reaction temperature at around 440–500 °C was observed to achieve complete catalytic oxidation of pyridine with the lowest NO x formation. These results demonstrate a promising approach for dual-mitigation of NPAC and NO x pollutants using nano-GAZ catalyst.

Kinetic and Mechanistic Studies of NOXSelective Catalytic Reduction Over In/Al2O3and N2O Decomposition Over Ru/Al2O3

Detailed kinetic studies are presented for two reactions: the nitric oxide (NO) selective catalytic reduction (SCR) by propene over indium/alumina (In/Al2O3) and the nitrous oxide (N2O) reduction over ruthenium/alumina (Ru/Al2O3). Both reactions were studied in the presence of excess oxygen (O2) to simulate the composition of flue gases. Apparent activation energies and apparent orders of reaction were calculated in experiments performed under differential reaction conditions. We used our experimental results to propose the reaction mechanism that leads to nitrogen formation over the two catalysts. The NO reduction proceeds through the initial formation of C X H Y O Z N, a reaction intermediate that reacts with activated nitrogen oxides (NO X ). Nitrous oxide is catalytically decomposed to nitrogen (N2) over Ru/Al2O3.

Synthesis of Hexaaluminogallate Catalysts for NOxReduction

BaCo(Al,Ga)11O19with β-alumina structure was found to be an effective catalyst for NOxreduction and successfully synthesized by the coprecipitation method using metal nitrates and ammonium carbonate. Anisotropic BaCo(Al,Ga11O19particles crystallized at 1100 °C for 2 h through the coprecipitation method. BaCo(Al,Ga)11O19supported on a cordierite honeycomb had the NOxremoval activity with methane over 500 °C in the presence of excess oxygen.

Selective catalytic reduction of NO over Cu-Al-MCM-41

Al-MCM-41 and Cu-Al-MCM-41 were prepared by a modified hydrothermal method. The characterization using XRD, NMR, and N 2 adsorption isotherms revealed that the structure of Al-MCM-41 remained unchanged after the complete exchange of protons for copper ions. Cu-Al-MCM-41 catalysts were studied for the selective catalytic reduction (SCR) of NO by C 3H 6 in the presence of excess oxygen. Particular attention was paid to the influence of both copper content and Si/Al ratio on the nature of the active copper sites and, therefore, on the performance of the catalysts. It was observed that the most active catalyst have ∼ 100 % of copper exchange. For the underexchanged catalyst of the same copper content, it was found that the NO reduction activity decreased with decreasing Si/Al ratio. H 2-TPR and NO-TPD experiments showed that copper is mainly in the form of isolated Cu 2+ ions in Cu-Al-MCM-41 catalysts with copper exchange ⩽ 100 % , whereas at higher copper loadings CuO species are also present. Moreover, when the catalyst Si/Al ratio increases copper is more easily reduced. These results are in agreement with the view accepted in the field of Cu-zeolite catalysts that the isolated Cu 2+ ions are involved in the SCR of NO by propylene.

Catalytic oxidation of pyridine on the supported copper catalysts in the presence of excess oxygen

The catalytic oxidation of pyridine pollutant on a series of supported metals/zeolites catalysts in the presence of excess oxygen was studied. All the catalysts were further characterized by means of BET, XRD, H 2-TPR, XPS, and FTIR techniques. The catalyst that could be reduced at lower temperatures had a better oxidation activity. The better NO x control ability could be attributed to the importance of the Lewis acidity of the samples, rather than to that of Brønsted acidity. On supported copper catalysts, Cu(H 2O) 6 2+ ions had higher activity for the NO x control but poorer activity for the pyridine oxidation. The cointeraction of some factors, such as the surface acidity of catalysts, the structure of supports, and the amount of Cu(H 2O) 6 2+ ions on the supports, played a deciding role in the NO x control ability of the catalysts. Comparatively, Cu/beta was the most active for the pyridine oxidation and NO x control, possibly being a potential catalyst for the catalytic removal of pyridine pollutant.

Selective catalytic reduction of nitric oxide by ammonia over Cu-exchanged Cuban natural zeolites

The catalytic selective reduction of NO over Cu-exchanged natural zeolites (mordenite (MP) and clinoptilolite (HC)) from Cuba using NH3 as reducing agent and in the presence of excess oxygen was studied. Cu(II)-exchanged zeolites are very active catalysts, with conversions of NO of ∼95%, a high selectivity to N2 at low temperatures, and exhibiting good water tolerance. The chemical state of the Cu(II) in exchanged zeolites was characterized by H2-TPR and XPS. Cu(II)-exchanged clinoptilolite underwent a severe deactivation in the presence of SO2. However, Cu(II)-exchanged mordenite not only maintained its catalytic activity, but even showed a slight improvement after 20h of reaction in the presence of 100ppm of SO2.

Selective removal of NO2 in the presence of oxygen and NO over Pd/SiO2 catalysts

Catalytic reduction of NO2 in the presence of excess oxygen and NO has been studied over a 0.2% Pd/SiO2 catalyst between 120 and 260°C. CO is effective in selective reduction of NO2 at temperatures below 180°C. Temperature programmed desorption (TPD) and FT-IR studies have been used to investigate the adsorption sites during the reaction conditions. A Langmuir–Hinshelwood mechanism involving oxidised Pd sites is proposed for the reduction of NO2 with CO, where both reactants adsorb competitively on Pd2+ sites. Cl deactivates the catalyst for the reduction of NO2 with CO. This problem can be overcome by using propylene as a co-reductant. Furthermore, addition of propylene widens the temperature window of operation.

In-Situ Observation of Reaction Intermediate in the Selective Catalytic Reduction of N2O with CH4 over Fe Ion-Exchanged BEA Zeolite Catalyst for the Elucidation of Its Reaction Mechanism Using FTIR

The selective catalytic reduction (SCR) of N2O with CH4 in the absence and presence of excess O2 has been studied over ion-exchanged Fe−BEA catalyst by the combination of an activity test with in-situ infrared spectroscopy to understand the nature of the surface species involved in the SCR of N2O with CH4. From the results of flow reaction studies, the Fe−BEA catalyst exhibited high activity in the SCR of N2O with CH4, even in the presence of excess oxygen, which demonstrates that the active Fe species for the SCR are formed on the ion-exchanged Fe−BEA catalyst. In the FTIR spectra of fresh catalysts, the OH band on Fe ion species (Fe−OH) was observed on the Fe−BEA catalyst, and the Fe−OH species can be involved in the SCR of N2O with CH4. Furthermore, the reaction intermediates of methoxy and formate species over the Fe−BEA catalyst were observed during the reaction. We measured the oxidation rates of these surface species with N2O and O2, and found that the methoxy species were oxidized with N2O more ra...

First-principles-based kinetic Monte Carlo simulation of nitric oxide decomposition over Pt and Rh surfaces under lean-burn conditions

First-principles-based kinetic Monte Carlo simulation was used to track the elementary surface transformations involved in the catalytic decomposition of NO over Pt(100) and Rh(100) surfaces under lean-burn operating conditions. Density functional theory (DFT) calculations were carried out to establish the structure and energetics for all reactants, intermediates and products over Pt(100) and Rh(100). Lateral interactions which arise from neighbouring adsorbates were calculated by examining changes in the binding energies as a function of coverage and different coadsorbed configurations. These data were fitted to a bond order conservation (BOC) model which is subsequently used to establish the effects of coverage within the simulation. The intrinsic activation barriers for all the elementary reaction steps in the proposed mechanism of NO reduction over Pt(100) were calculated by using DFT. These values are corrected for coverage effects by using the parametrized BOC model internally within the simulation. This enables a site-explicit kinetic Monte Carlo simulation that can follow the kinetics of NO decomposition over Pt(100) and Rh(100) in the presence of excess oxygen. The simulations are used here to model various experimental protocols including temperature programmed desorption as well as batch catalytic kinetics. The simulation results for the temperature programmed desorption and decomposition of NO over Pt(100) and Rh(100) under vacuum condition were found to be in very good agreement with experimental results. NO decomposition is strongly tied to the temporal number of sites that remain vacant. Experimental results show that Pt is active in the catalytic reaction of NO into N2 and NO2 under lean-burn conditions. The simulated reaction orders for NO and O2 were found to be +0.9 and −0.4 at 723 K, respectively. The simulation also indicates that there is no activity over Rh(100) since the surface becomes poisoned by oxygen.

Mechanism of the selective catalytic reduction of NOx by C2H5OH over Ag/Al2O3

The mechanism of the selective catalytic reduction (SCR) of NOx by C2H5OH over silver catalyst (Ag/Al2O3) was investigated using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Attention was focused on the formation and reactivity of novel enolic species on the Ag/Al2O3 surface. DRIFTS spectra show that the surface enolic species, which was derived from partial oxidation of C2H5OH over Ag/Al2O3 in the presence of excess oxygen, play a crucial role in the formation of isocynate species (NCO) by reaction with NO+O2 or NO3− absorbed on the surface of Ag/Al2O3. 2,3-dihydrofuran was used to an enolic model compound in our DRITFS study, and the results support our assignment. A novel mechanism of the SCR of NOx by C2H5OH is proposed based on the DRIFTS studies. The mechanistic differences between the SCR of NOx by C2H5OH and that by C3H6 are discussed. The high reactivity of the enolic species results in high surface concentration of NCO and high efficiency of NOx reduction using C2H5OH as a reductant. The results of density functional theory (DFT) calculations are in good agreement with the DRIFTS spectra, and support our suggestions about the surface enolic species.

NO reduction by CH4 in the presence of excess O2 over Mn/sulfated zirconia catalysts

Selective catalytic reduction (SCR) of NO with methane in the presence of excess oxygen has been investigated over a series of Mn-loaded sulfated zirconia (SZ) catalysts. It was found that the Mn/SZ with a metal loading of 2–3wt.% exhibited high activity for the NO reduction, and the maximum NO conversion over the Mn/SZ catalyst was higher than that over Mn/HZSM-5. NH3–TPD results of the catalysts showed that the sulfation process of the supports resulted in the generation of strong acid sites, which is essential for the SCR of NO with methane. On the other hand, the N2 adsorption and the H2–TPR of the catalysts demonstrated that the presence of the SO42− species promoted the dispersion of the metal species and made the Mn species less reducible. Such an increased dispersion of metal species suppressed the combustion reaction of CH4 by O2 and increased the selectivity towards NO. The Mn/SZ catalysts prepared by different methods exhibited similar activities in the SCR of NO with methane, indicating the importance of SO42−. The most attractive feature of the Mn/SZ catalysts was that they were more tolerant to water and SO2 poisoning than Mn/HZSM-5 catalysts and exhibited higher reversibility after removal of SO2.

Kinetic Model for the Water Oxidation Method for Treating Wastewater Sludges

AbstractA generalized kinetic model for the hydrothermal oxidation of organic matter in biological wastewater treatment sludge was developed using a simplified first‐order reaction scheme. The model was based on a series of experimental results obtained using a continuous‐flow hydrothermal reactor system used to destroy the organic component of the sludge. Forty‐eight hydrothermal treatment experiments using three sets of inflow conditions were performed. Using excess oxygen, the treatment involved sludge destruction under subcritical (<374d̀C) and supercritical (>374d̀C) water oxidation temperatures. The chemical oxygen demand (COD) was used to measure the organic content and progress of the oxidation reaction. The Arrhenius equation was used to model the reaction rate constant. In the Arrhenius equation, the pre‐exponential factor was fixed and the activation energy was found to vav with temperature. The activation energy increased up to approximately 263d̀C then stabilised at temperatures above 263d̀C. The variation of the activation energy with temperature reflected the complexity of the composition of the organic content of the sludge, which generally consists of proteins, lipids, carbohydrates and fibres. In hydrothermal oxidation, the various organic compounds oxidize at different rates, with the easily oxidized matter being removed first. As such, the activation energy reflected the changing composition of the remaining organic matter with the progress of the oxidation reaction. Above about 263d̀C, the activation energy became virtually independent of temperature. A functional relationship was established between activation energy and average temperature of the reactor. A mathematical model for the destruction of COD in the presence of excess oxygen was set using a kinetic equation having an average pre‐exponential constant and temperature dependent activation energy. With the new equation and known influent COD, efluent COD was simulated for the entire set of experiments and was compared with the actual measured effluent COD.

NO reduction by CH4 in the presence of excess O2 over Pd/sulfated alumina catalysts

Selective catalytic reduction (SCR) of nitric oxide with methane in the presence of excess oxygen has been studied for the first time on a series of Pd loaded sulfated alumina (SA) catalysts. Pd/γ-Al2O3 showed only a low activity for the SCR of NO with methane but high activity for combustion by-reaction. After pretreating the γ-Al2O3 with H2SO4, a remarkably promoting effect on the catalytic activity was achieved. NH3-TPD spectra of different catalysts demonstrated that the sulfation increased the acidity of support which is very important for the reduction of NO with methane over Pd catalysts. The Pd/sulfated alumina catalyst exhibited best catalytic activity when 0.1wt.% Pd was loaded on sulfated alumina (SA) prepared by impregnating the γ-Al2O3 with 2.5M H2SO4. The activity of Pd/SA catalyst can be improved by increasing the surface area of the γ-Al2O3 precursor. Moreover, the Pd/SA catalyst as prepared exhibited high catalytic performance in high space velocity and in the presence of SO2.