Selective catalytic oxidation of ammonia over copper-cerium composite catalyst

This work considers the oxidation of ammonia (NH3) by selective catalytic oxidation (SCO) over a copper (Cu)-cerium (Ce) composite catalyst at temperatures between 150 and 400 degrees C. A Cu-Ce composite catalyst was prepared by coprecipitation of copper nitrate and cerium nitrate at various molar concentrations. This study also considers how the concentration of influent NH3 (500-1000 ppm), the space velocity (72,000-110,000 hr(-1)), the relative humidity (12-18%) and the concentration of oxygen (4-20%) affect the operational stability and the capacity for removing NH3. The effects of the O2 and NH3 content of the carrier gas on the catalyst's reaction rate also are considered. The experimental results show that the extent of conversion of NH3 by SCO in the presence of the Cu-Ce composite catalyst was a function of the molar ratio. The NH3 was removed by oxidation in the absence of Cu-Ce composite catalyst, and approximately 99.2% NH3 reduction was achieved during catalytic oxidation over the Cu-Ce (6:4, molar/molar) catalyst at 400 degrees C with an O2 content of 4%. Moreover, the effect of the initial concentration and reaction temperature on the removal of NH3 in the gaseous phase was also monitored at a gas hourly space velocity of less than 92,000 hr(-1).     

Removal of ammonia solutions used in catalytic wet oxidation processes

Ammonia (NH(3)) is an important product used in the chemical industry, and is common place in industrial wastewater. Industrial wastewater containing ammonia is generally either toxic or has concentrations or temperatures such that direct biological treatment is unfeasible. This investigation used aqueous solutions containing more of ammonia for catalytic liquid-phase oxidation in a trickle-bed reactor (TBR) based on Cu/La/Ce composite catalysts, prepared by co-precipitation of Cu(NO(3))(2), La(NO(3))(2), and Ce(NO(3))(3) at 7:2:1 molar concentrations. The experimental results indicated that the ammonia conversion of the wet oxidation in the presence of the Cu/La/Ce composite catalysts was determined by the Cu/La/Ce catalyst. Minimal ammonia was removed from the solution by the wet oxidation in the absence of any catalyst, while approximately 91% ammonia removal was achieved by wet oxidation over the Cu/La/Ce catalyst at 230 degrees C with oxygen partial pressure of 2.0 MPa. Furthermore, the effluent streams were conducted at a liquid hourly space velocity of under 9 h(-1) in the wet catalytic processes, and a reaction pathway was found linking the oxidizing ammonia to nitric oxide, nitrogen and water. The solution contained by-products, including nitrates and nitrites. Nitrite selectivity was minimized and ammonia removal maximized when the feed ammonia solution had a pH of around 12.0.     

Catalytic wet oxidation of ammonia: why is N2 formed preferentially against NO3 -?

The role of catalyst and the reason for the preferential formation of N(2) in the catalytic oxidation reaction of ammonia in water over a Ru (3wt.%)/TiO(2) catalyst were elucidated. It was verified that the catalyst in the reaction had no direct relevance to the selective formation of N(2), but was responsible only for the oxidation of aqueous ammonia, NH(3)(aq), finally giving a molecule of nitrous acid. The preferential production of N(2) was experimentally demonstrated due to the homogeneous aqueous phase reaction of the nitrous acid-dissociated NO(2)(-) with NH(4)(+) ions. Even under the highly oxidizing condition, NO(2)(-) was much more likely to react with NH(4)(+) to form N(2) than being oxidized over the catalyst to NO(3)(-) as long as NH(4)(+) was available in solution.     

The effects of manganese precursors on Mn-based/TiO2 catalysts for catalytic reduction of NO with NH3

Manganese acetate (MnAc) and manganese nitrate (MnN) were employed as precursors for the preparation of MnAc)/TiO2, Mn (N)/TiO2, Mn(Ac)-Ce/TiO2, and Mn(N)-Ce/TiO2 by impregnation. These complexes were used as catalysts in the low-temperature selective catalytic reduction of NO with NH3. The influence of manganese precursors on catalyst characteristics, the reduction activity, and the stability of the catalysts to poisoning by H2O and SO2 were studied. Experiments showed that Mn(N) produced MnO2 with large grain sizes in Mn(N)/TiO2 catalyst. On the contrary, Mn(Ac) led to highly dispersed and amorphous Mn2O3 in Mn (Ac)/TiO2 catalyst, which had better catalytic activity and stability to SO2 at low temperatures. The doping of cerium reduced the differences in catalytic performance between the catalysts derived from different Mn precursors.     

Superior pillared clay catalysts for selective catalytic reduction of nitrogen oxides for power plant emission control

Fe(3+)-, Cr(3+)-, Cu(2+)-, Mn(2+)-, Co(2+)-, and Ni(2+)-exchanged Al2O3-pillared interlayer clay (PILC) or TiO2-PILC catalysts are investigated for the selective catalytic reduction (SCR) of nitric oxide by ammonia in the presence of excess oxygen. Fe(3+)-exchanged pillared clay is found to be the most active. The catalytic activity of Fe-TiO2-PILC could be further improved by the addition of a small amount of cerium ions or cerium oxide. H2O and SO2 increase both the activity and the product selectivity to N2. The maximum activity on the Ce-Fe-TiO2-PILC is more than 3 times as active as that on a vanadium catalyst. Moreover, compared to the V2O5-WO3/TiO2 catalyst, the Fe-TiO2-PILC catalysts show higher N2/N2O product selectivities and substantially lower activities (by approximately 85%) for SO2 oxidation to SO3 under the same reaction conditions. A 100-hr run in the presence of H2O and SO2 for the CeO2/Fe-TiO2-PILC catalyst showed no decrease in activity.     

Characterization and reactivity of natural manganese ore catalysts in the selective catalytic oxidation of ammonia to nitrogen

Natural manganese ore (NMO) catalysts were characterized and tested in the selective catalytic oxidation of ammonia to nitrogen oxides under dilute conditions. Also, the oxidation of ammonia (NH(3)) was carried out using pure MnO(2), Mn(2)O(3) for comparing with the activity. It is found that the activity of NMO was similar to that of MnO(2) at low temperature below 150 degrees C but above this temperature, the activity of these catalysts showed the difference. In the course of NH(3) oxidation, N(2), NO, N(2)O and H(2)O were produced. But the quantity of NO(2) produced in this experiment was negligible. At temperature below 250 degrees C, selectivity into N(2) from NH(3) oxidation was in the order, NMO > MnO(2) > Mn(2)O(3). This is the reverse of activity of these manganese oxides. Also the characterization of NH(3) oxidation was proposed and supported by the effect of space velocity, inlet O(2) and NH(3) concentration. The increase of space velocity remarkably influenced not only the conversion but also selectivity into N(2). The higher the reaction temperature was, the higher the effect of inlet O(2) and NH(3) concentration on the reaction rate was. By introducing NO during NH(3) oxidation reaction, the possibility of NMO as selective catalytic reduction catalyst at low temperature was studied and showed positive results.     

Photocatalytic oxidation of gaseous DMF using thin film TiO2 photocatalyst

The heterogeneous photocatalytic oxidation of gaseous N,N'-dimethylformamide (DMF) widely used in the manufacture of synthetic leather and synthetic textile was investigated. The experiments were carried out in a plug flow annular photoreactor coated with Degussa P-25 TiO2. The oxidation rate was dependent on DMF concentration, reaction temperature, water vapor, and oxygen content. Photocatalytic deactivation was observed in these reactions. The Levenspiel deactivation kinetic model was used to describe the decay of catalyst activity. Fourier transform infrared (FTIR) was used to characterize the surface and the deactivation mechanism of the photocatalyst. Results revealed that carbonylic acids, aldehydes, amines, carbonate and nitrate were adsorbed on the TiO2 surface during the photocatalytic reaction. The ions, NH4+ and NO3-, causing the deactivation of catalysts were detected on the TiO2 surface. Several treatment processes were applied to find a suitable procedure for the regeneration of catalytic activity. Among these procedures, the best one was found to be the H2O2/UV process.     

A study on the reaction characteristics of vanadium-impregnated natural manganese oxide in ammonia selective catalytic reduction

This study investigated the effect of adding vanadium (V) to natural manganese oxide (NMO) in ammonia (NH3) selective catalytic reduction (SCR). The addition of V to NMO decreased the catalytic activity at low temperatures by blocking the active site. However, the enhancement of catalytic activity was achieved by controlling NH3 oxidation at high temperatures. From the NH3 temperature programmed desorption and oxygen on/off test, it was confirmed that the amount of Lewis acid site and active lattice oxygen of the catalyst affects the catalytic performance at low temperature.     

Simultaneous absorption of NO and SO2 into hexamminecobalt(II)/iodide solution

An innovative catalyst system has been developed to simultaneously remove NO and SO2 from combustion flue gas. Such catalyst system may be introduced to the scrubbing solution using ammonia solution to accomplish sequential absorption and catalytic oxidation of both NO and SO2 in the same reactor. When the catalyst system is utilized for removing NO and SO2 from the flue gas, Co(NH3)(6)2+ ions act as the catalyst and I- as the co-catalyst. Dissolved oxygen, in equilibrium with the residual oxygen in the flue gas, is the oxidant. The overall removal process is further enhanced by UV irradiation at 365 nm. More than 95% of NO is removed at a feed concentration of 250-900 ppm, and nearly 100% of SO2 is removed at a feed concentration of 800-2500 ppm. The sulfur dioxide co-existing in the flue gas is beneficial to NO absorption into hexamminecobalt(II)/iodide solution. NO and SO2 can be converted to ammonium sulfate and ammonium nitrate that can be used as fertilizer materials. The process described here demonstrates the feasibility of removing SO2 and NO simultaneously only by retrofitting the existing wet ammonia flue-gas-desulfurization (FGD) scrubbers.     

Preparation of the Mn/Co mixed oxide catalysts for low-temperature CO oxidation reaction

The Mn/Co mixed powders with various Mn/Co molar ratios were prepared by the coprecipitation method and used in low-temperature CO oxidation. The physicochemical characteristics of these powders were characterized using the Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), temperature-programmed reduction (TPR), and scanning electron microscopy (SEM) analyses. The results demonstrated that the Mn/Co molar ratio significantly affected both the textural and catalytic properties and the sample with a Mn/Co = 1:1 possessed a BET area of 123.7 m²g⁻¹ with a small mean pore size of 6.44 nm. The catalytic results revealed that the pure cobalt and manganese catalysts possessed the low catalytic activity and the pure Co catalyst is not active at temperatures lower than 140 °C. The highest catalytic activity was observed for the catalyst with a Mn/Co = 1. The obtained results showed that the incorporation of Pd into the Mn/Co catalyst significantly enhanced the catalytic activity for oxidation of carbon monoxide and the highest CO conversion was observed for the catalyst with 1 wt.% Pd and this catalyst exhibited a CO conversion of 100% at 80 ^°C.

     

MnOx／Al-SBA-15催化剂低温NH3选择性催化还原NO的原位红外研究

用原位红外分别进行了MnOx／Al-SBA-15催化剂上NH3和NO＋O2的吸附态和瞬态实验以及NH3＋NO＋02反应的稳态实验。结果表明，催化剂上存在着L酸位和B酸位，NH3吸附在催化剂上形成配位态的NH3和NH4＋，配位态的NH3能脱氢形成-NH2活性中间态。NO＋O2在催化剂上吸附形成硝酸盐类、硝基类和亚硝酸盐类。将NO＋O2通入预吸附NH3的催化剂中时，表面的配位态的NH3、NH4＋和-NH2都会减少直至消失，SCR反应显著。而将NH3通人预吸附NO＋O2的催化剂中时，只有硝基类和亚硝酸盐类减少，硝酸盐类基本不发生变化，SCR反应微弱。NH3＋NO＋O2稳态反应中，催化剂表面稳定存在着NH4＋和硝酸盐类，SCR反应显著。     

An assessment of the suitable operating conditions for the CeO2/gamma-Al2O3 catalyzed wet air oxidation of phenol

It has been shown that the CeO2/gamma-Al2O3 catalyst is a feasible alternative to CeO2 for the catalytic wet air oxidation (CWAO) of phenol because it remains an effective catalyst and yet is cheaper to prepare. In this study, we found that the optimal cerium content in the CeO2/gamma-Al2O3 catalyst was 20 wt.%, regardless of catalyst loading. Furthermore, at 180 degrees C, with a phenol concentration of 1000 mg l(-1), and an O2 partial pressure of 1.0M Pa or 1.5M Pa, the optimal catalyst loading was 3.0 gl (-1). The efficacy of CWAO of phenol improved with O2 partial pressure, although the effects of O2 pressure were more significant between 0.5 MPa and 1.5 MPa than between 1.5 MPa and 2.0 MPa. After 2 h of reaction, approximately 100% phenol conversion and 80% total organic carbon (TOC) removal was recorded at 180 degrees C, 1000 mg l(-1) of phenol and 3.0 g l(-1) of catalyst. Because these percentages subsequently leveled off, it is suggested that 2 h is a suitable time over which to run the reaction. The efficacy of CWAO of phenol decreased as initial phenol concentration was raised (from 400 to 2500 mg l(-1)), with the exception of phenol conversion after about 2 h, for which 400 mg l(-1) produced the lowest phenol conversion figure. Higher phenol concentrations require both catalyst loading and O2 partial pressure to be increased to maintain high performance. For example, for 2000 mg l(-1) and 2500 mg l(-1) phenol, nearly 100% phenol conversion and 90% TOC removal after 4 h of reaction at 180 degrees C required 4.0 g l(-1) of catalyst and 2.0 MPa.     

Preparation of a titanium dioxide photocatalyst codoped with cerium and iodine and its performance in the degradation of oxalic acid

A novel class of visible light-activated photocatalysts was prepared by codoping TiO(2) with cerium and iodine (Ce-I-TiO(2)). The particles were characterized using the Brunauer-Emmett-Teller method, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and UV-Visible light absorption. Particles of Ce-I-TiO(2) had greater photoabsorption in the 400-800 nm wavelength range than iodine-doped TiO(2) (I-TiO(2)). The effects on the photocatalytic degradation of oxalic acid under visible light or UV-Visible light irradiation were investigated. The photocatalytic activity of Ce-I-TiO(2) calcined at 673 K was significantly higher than that of Ce-I-TiO(2) calcined at 773 K and I-TiO(2) calcined at 673 K in aqueous oxalic acid solution under visible light or UV-Visible light irradiation. Under visible light irradiation, oxalic acid was first adsorbed on the surface of the catalysts rather than reacted with free radicals in the bulk solution, and then oxidized by (·)OH(ads) to CO(2), which was verified by studying the effects of nitrogen purging and scavengers, as well as by gas chromatography/mass spectrometry.     

CeO_x/AC催化剂NH_3选择性催化还原NO

以用不同浓度的HNO3预处理后的椰壳活性炭为载体,负载铈制备SCR催化剂。利用比表面积分析仪（BET）、X射线衍射（XRD）、扫描电镜（SEM）和催化剂活性实验,探讨了不同变量如金属离子的分布、焙烧温度和载体属性对催化活性的影响。结果表明,HNO3处理后,经500℃焙烧金属铈负载量为7%的催化剂表现出优良的催化性能。在90℃时,NO转化率在90%以上,随温度升高,达到接近100%的NO转化率。     

Catalytic reduction of NO on copper/MCM-41 studied by in situ EXAFS and XANES

Speciation of copper in the channels of MCM-41 during reduction of NO with CO at 473-773 K was studied by in situ extended X-ray absorption fine structural (EXAFS) and X-ray absorption near edge structural (XANES) spectroscopies in the present work. The component fitted (in situ) XANES spectra of the catalyst showed that about 72% of metallic copper (Cu(0)) in MCM-41 was oxidized to higher oxidation state coppers (Cu(II) (46%) and Cu(I) (26%)) during the NO reduction process (at 473 K). By EXAFS, we also found that in the NO reduction process, oxygen was inserted into the metallic copper matrix and led to a formation of the copper oxide species with a Cu-O bond distance of 1.93 A which was greater than that of the model compound Cu(2)O (typically 1.86 A). At 573-673 K, mainly Cu(II) was found in the channels of MCM-41. Nevertheless, at a higher temperature (e.g., 773 K), about 61% Cu(I), 31% Cu(II), and 8% Cu(O) with averaged Cu-Cu and Cu-O bond distances of 3.04 and 1.88 A, respectively were observed, that might account for the high selectivity-to-decomposition (S/D) ratios for yields of N(2) and CO(2) in the catalytic reduction of NO with CO.     

Fe (III) supported on resin as effective catalyst for the heterogeneous oxidation of phenol in aqueous solution

FeIII supported on resin as an effective catalyst for oxidation was prepared and applied for the degradation of aqueous phenol. Phenol was selected as a model pollutant and the catalytic oxidation was carried out in a batch reactor using hydrogen peroxide as the oxidant. The influent factors on oxidation, such as catalyst dosage, H2O2 concentration, pH, and phenol concentration were examined by considering both phenol conversion and chemical oxygen demand (COD) removal. The FeIII-resin catalyst possesses a high oxidation activity for phenol degradation in aqueous solution. The experimental results of this study show that almost 100% phenol conversion and over 80% COD removal can be achieved with the FeIII-resin catalyst catalytic oxidation system. A series of prepared resin were investigated for improving the oxidation efficiency. It was found that the reaction temperature and initial pH in solution significantly affected both of phenol conversion and COD removal efficiency. The activity of the catalyst significantly decreased at high pH, which was similar to the Fenton-like reaction mechanism. Results in this study indicate that the FeIII-resin catalytic oxidation process is an efficient method for the treatment of phenolic wastewater.     

A predictive mechanism for mercury oxidation on selective catalytic reduction catalysts under coal-derived flue gas

This paper introduces a predictive mechanism for elemental mercury (Hg(o)) oxidation on selective catalytic reduction (SCR) catalysts in coal-fired utility gas cleaning systems, given the ammonia (NH3)/nitric oxide (NO) ratio and concentrations of Hg(o) and HCl at the monolith inlet, the monolith pitch and channel shape, and the SCR temperature and space velocity. A simple premise connects the established mechanism for catalytic NO reduction to the Hg(o) oxidation behavior on SCRs: that hydrochloric acid (HCl) competes for surface sites with NH3 and that Hg(o) contacts these chlorinated sites either from the gas phase or as a weakly adsorbed species. This mechanism explicitly accounts for the inhibition of Hg(o) oxidation by NH3, so that the monolith sustains two chemically distinct regions. In the inlet region, strong NH3 adsorption minimizes the coverage of chlorinated surface sites, so NO reduction inhibits Hg(o) oxidation. But once NH3 has been consumed, the Hg(o) oxidation rate rapidly accelerates, even while the HCl concentration in the gas phase is uniform. Factors that shorten the length of the NO reduction region, such as smaller channel pitches and converting from square to circular channels, and factors that enhance surface chlorination, such as higher inlet HCl concentrations and lower NH3/NO ratios, promote Hg(o) oxidation. This mechanism accurately interprets the reported tendencies for greater extents of Hg(o) oxidation on honeycomb monoliths with smaller channel pitches and hotter temperatures and the tendency for lower extents of Hg(o) oxidation for hotter temperatures on plate monoliths. The mechanism also depicts the inhibition of Hg(o) oxidation by NH3 for NH3/NO ratios from zero to 0.9. Perhaps most important for practical applications, the mechanism reproduces the reported extents of Hg(o) oxidation on a single catalyst for four coals that generated HCl concentrations from 8 to 241 ppm, which covers the entire range encountered in the U.S. utility industry. Similar performance is also demonstrated for full-scale SCRs with diverse coal types and operating conditions.     

Oxidation of methane over palladium catalysts: effect of the support

This work is focused on the deep catalytic oxidation of methane over supported palladium catalysts. The influences of the metal loading, oxidation state of palladium, nature of supports, presence of promoters in the supports (for zirconia-based supports), and thermal stability have been studied experimentally. Catalysts were prepared by incipient wetness of commercially available supports with aqueous solutions of palladium nitrate. For gamma-alumina support, it was observed that the optimal amount of palladium is between 0.5% and 2%, with higher amounts leading to a loss in specific activity. Concerning the oxidation state of the catalyst, it is concluded that for all the supports tested in the present work, a reduction of the catalyst is not needed, yielding the same conversion at steady state catalysts reduced and oxidised. The thermal stability of various supported catalysts were also studied, zirconia supports being the most active. These supports, specially Y-modified zirconia support, do not suffer appreciable deactivation below 500 degrees C.     

Catalytic destruction of dichloromethane using perovskite-type oxide catalysts

Dichloromethane (DCM, also known as methylene chloride [CH2Cl2]) is often present in industrial waste gas and is a valuable chemical product in the chemical industry. This study addresses the oxidation of airstreams that contain CH2Cl2 by catalytic oxidation in a tubular fixed-bed reactor over perovskite-type oxide catalysts. This work also considers how the concentration of influent CH2Cl2 (Co = 500-1000 ppm), the space velocity (GHSV = 5000-48,000 1/hr), the relative humidity (RH = 10-70%) and the concentration of oxygen (O2 = 5-21%) influence the operational stability and capacity for the removal of CH2Cl2. The surface area of lanthanum (La)-cobalt (Co) composite catalyst was the greatest of the five perovskite-type catalysts prepared in various composites of La, strontium, and Co metal oxides. Approximately 99.5% CH2Cl2 reduction was achieved by the catalytic oxidation over LaCoO3-based perovskite catalyst at 600 degrees C. Furthermore, the effect of the initial concentration and reaction temperature on the removal of CH2Cl2 in the gaseous phase was also monitored. This study also provides information that a higher humidity corresponds to a lower conversion. Carbon dioxide and hydrogen chloride were the two main products of the oxidation process at a relative humidity of 70%.     

The Southeastern Aerosol Research and Characterization Study, part 3: continuous measurements of fine particulate matter mass and composition

Deployment of continuous analyzers in the Southeastern Aerosol Research and Characterization Study (SEARCH) network began in 1998 and continues today as new technologies are developed. Measurement of fine particulate matter (PM2.5) mass is performed using a dried, 30 degrees C tapered element oscillating microbalance (TEOM). TEOM measurements are complemented by observations of light scattering by nephelometry. Measurements of major constituents include: (1) SO4(2-) via reduction to SO2; (2) NH4+ and NO3- via respective catalytic oxidation and reduction to NO, (3) black carbon (BC) by optical absorption, (4) total carbon by combustion to CO2, and (5) organic carbon by difference between the latter two measurements. Several illustrative examples of continuous data from the SEARCH network are presented. A distinctive composite annual average diurnal pattern is observed for PM2.5 mass, nitrate, and BC, likely indicating the influence of traffic-related emissions, growth, and break up of the boundary layer and formation of ammonium nitrate. Examination of PM2.5 components indicates the need to better understand the continuous composition of the unmeasured "other" category, because it contributes a significant fraction to total mass during periods of high PM2.5 loading. Selected episodes are presented to illustrate applications of SEARCH data. An SO2 conversion rate of 0.2%/hr is derived from an observation of a plume from a coal-fired power plant during early spring, and the importance of local, rural sources of NH3 to the formation of ammonium nitrate in particulate matter (PM) is demonstrated.