Noble metal catalysts for methane removal

Using a bench-scale rig, the activities of Pt, Pd and Pt+Pd catalysts supported on gamma-Al(2)O(3) and on TiO(2) (anatase) for the complete oxidation of methane (300 ppmv) in air have been measured as a function of temperature; values of T(10), T(50) and T(90) together with the Arrhenius parameters (activation energy and pre-exponential factor) are reported. Pt is less active than Pd when deposited on the surface of the TiO(2), but more active when deposited on gamma-Al(2)O(3), however when combined, the Pt+Pd mixture is more active than either metal individually. The T(10) for Pt+Pd/gamma-Al(2)O(3) was being as low as 228 degrees C. The significance of the Arrhenius parameters, for metal containing catalysts is that they exhibit compensation with increasing activation energy, while securing a more rapid increase in conversion from 0% to 100% when the temperature is increased.     

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.     

Sources of anthropogenic platinum-group elements (PGE): Automotive catalysts versus pge-processing industries

Soil samples from the area of Hanau (Hessen, Germany) were analyzed for anthropogenic platinum-group elements (PGE). The results confirm the existence of two different sources for anthropogenic PGE: 1. automotive catalysts, and 2. PGE-processing plants. Both sources emit qualitatively and quantitatively different PGE spectra and PGE interelemental ratios (especially the Pt/Rh ratio). Elevated PGE values which are due to automotive catalysts are restricted to a narrow-range along roadside soil, whereas those due to PGE-processing plants display a large-area dispersion. The emitted PGE-containing particles in the case of automotive catalysts are subject to transport by wind and water, whereas those from PGE-processing plants are preferably transported by wind. This points to a different aerodynamic particle size. Pt, Pd, and Rh concentrations along motorways are dependent on the amount of traffic and the driving characteristics.     

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.     

Comparison of palladium and platinum in environmental matrices: Palladium pollution by automobile emissions?

A new type of automobile catalyst based on an active Pd/Rh-layer is presently being introduced into the European market. Accordingly, in order to establish baseline information, this work attempts to assess the magnitude of Pd emissions by automobile catalysts to date. Thus, a survey is presented on Pd and Pt levels detected in polluted environmental matrices (grass, soil, dust, sludges) as available from the literature. Additionally, Pd data measured by selective preconcentration/GF-AAS-detection in polluted grass, in a time series of sewage sludge ashes back to 1972, in dust, as well as in urban rain are presented. Since the data basis from the literature is partly inconsistent, the possible impact of different analytical attempts on the data is discussed. The relationship between Pt and Pd concentration from most of the results is between 5 and 10 (Pt/Pd). However, relations down to 0.04 are also reported, thus implying anthropogenic Pd input from additional sources.     

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.     

Surface properties and catalytic performance of La(1-x)Sr(x)FeO(3) perovskite-type oxides for methane combustion

La(1-x)Sr(x)FeO(3) (x=0.0-1.0) perovskites were prepared and tested for the combustion of methane. X-ray diffraction (XRD) patterns revealed the presence of a single perovskite structure for substitutions 0x0.3, however Fe(2)O(3), SrCO(3) and SrFeO(3) phases were observed for substitutions x>0.3. The results of activity test indicate that with La(1-x)Sr(x)FeO(3) as the catalyst, the combustion of methane can take place at low temperatures around 400 degrees C. Partial substitution of La with Sr increases the activity and an optimal substitution fraction (x=0.5) exists in the La(1-x)Sr(x)FeO(3) catalysts. Catalyst activity can be well correlated to the product of the specific surface area and atomic ratio of Fe to La+Sr on the catalyst surface. Experimental results of O(2)-TPD and CH(4)-TPD in the range of 350-500 degrees C indicate that the amount of oxygen desorbed from the La(1-x)Sr(x)FeO(3) catalysts is far larger than that of methane. Therefore, it can be proposed that the catalytic oxidation of CH(4) over these catalysts proceeds with the surface reaction between CH(4) in the gas phase and the adsorbed O(2). Addition of water vapor or CO(2) to the feed inhibited catalyst activity, but the inhibition was reversible and became negligible at high reaction temperature.     

Removal of nitrogenous compounds by catalytic wet air oxidation. Kinetic study

Aqueous wastes containing organic pollutants can be efficiently treated by wet air oxidation (WAO), i.e. oxidation by molecular oxygen in the liquid phase, under high temperature (200-325 degrees C) and pressure (up to 150 bar). However, organic nitrogen can be relatively resistant to oxidation and can be harmful to the environment. In the course of treatment, organic nitrogen (N-Org) is converted into ammonia (NH(3)), while organic carbon (C-Org) is converted mainly into carbon dioxide (CO(2)). This can be done without catalysts. In the presence of Mn/Ce composite oxides, it is possible to transform ammonia into molecular nitrogen at a temperature close to 260 degrees C. The direct conversion of organic nitrogen into molecular nitrogen also can be achieved using the same catalyst. This paper discusses the results obtained during the treatment of nitrogenous compounds like aniline, nitrophenol, beta-alanine and ammonia. Laboratory investigations were conducted in a stirred batch reactor with Mn/Ce composite oxides as catalysts. Very limited amounts of nitrites and nitrates were observed with amines, but more significant quantities were found with nitro-compounds. The kinetics of oxidation of ammonia, organic compounds, and more particularly aniline, were investigated. The treatment of a real waste (process wastewater) was also investigated. The dependence of the transformation rate on various parameters (amount of catalyst, temperature, etc.) was established. The rates of oxidation are described by first-order kinetic laws with respect to the various nitrogen species (aniline, NH(3)). Several parallel pathways are considered for the transformation of organic nitrogen, amongst which is an interaction with the catalyst surface. The orders with respect to oxygen and catalyst are established.     

Impact of humic substances on the aqueous solubility, uptake and bioaccumulation of platinum, palladium and rhodium in exposure studies with Dreissena polymorpha

Zebra mussels (Dreissena polymorpha) were exposed to different types of water containing PGE salts (PtCl4, PdSO4, RhCl3) to investigate the influence of humic substances on the aqueous solubility, uptake and bioaccumulation of noble metals. The results showed a time dependent decrease of the aqueous PGE concentrations in tank water for all groups. This could mainly be related to non-biological processes. The aqueous solubility of Pd and Rh was higher in humic water compared with non-chlorinated tap water, whereas Pt showed opposing results. Highest metal uptake rates and highest bioaccumulation plateaus were found for Pd, followed by Pt and Rh. Pd uptake and bioaccumulation was significantly hampered by humic substances, whose presence appear to increase Pt uptake and bioaccumulation. No clear trend emerged for Rh. Differences in effects of humic matter among the PGE may be explained by formation of metal complexes with different fractions of humic substances.     

Evaluation of Cu-PPHs as active catalysts for the SCR process to control NOx emissions from heavy duty diesel vehicles

Copper based catalysts supported on mesoporous materials, which were in turn based on a surfactant expanded zirconium phosphate for the formation of silica galleries in the interlayer space, were prepared by the impregnation method. They were then characterised and tested in the selective catalytic reduction of NO with ammonia as active catalysts for the control of the NOx emissions from heavy duty vehicles. Copper catalysts displayed a high catalytic performance, even in the presence of 14% (v/v) of H2O and 100 ppm of SO2. They also displayed improved catalytic behaviour when compared to a CuZSM5 catalyst.     

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.     

Structure of Mn-Zr mixed oxides catalysts and their catalytic performance in the gas-phase oxidation of chlorocarbons

The catalytic activity and selectivity of manganese zirconia mixed oxides were evaluated for the oxidation of two common chlorinated pollutants found in waste streams, namely 1,2-dichloroethane (DCE) and trichloroethylene (TCE). Mixed oxides with varying Mn-Zr content were prepared by coprecipitation via nitrates, and subsequent calcination at 600 degrees C for 4 h in air. These catalysts were characterised by means of several techniques such as atomic emission spectrometry, N2 adsorption-desorption, powder X-ray diffraction, temperature-programmed desorption of ammonia, pyridine adsorption followed by diffuse reflectance infrared spectroscopy and temperature-programmed reduction with hydrogen. The active catalytic behaviour of Mn-Zr mixed oxides was ascribed to a substantial surface acidity combined with readily accessible active oxygen species. Hence, the mixed oxide with 40 mol% manganese content was found to be an optimum catalyst for the combustion of both chlorocarbons with a T50 value around 305 and 315 degrees C for DCE and TCE oxidation, respectively. The major oxidation products were carbon dioxide, hydrogen chloride and chlorine. It was observed that the formation of both CO2 and Cl2 was promoted with Mn loading.     

固体超强酸和金属氧化物类催化剂上CH4-SCR还原NOx研究进展

由于还原剂甲烷价廉易得,甲烷选择性催化还原NO_x（简称CH₄-SCR）被认为是最有潜力替代NH₃-SCR的催化还原技术。现有的CH₄-SCR催化剂中,分子筛类催化剂因催化活性高而被广泛研究,但由于其水热稳定性不好,使得非分子筛负载的催化剂成为近年来的研究热点,其中主要包括固体超强酸和氧化物两大类。综述了这两类催化体系在催化活性、反应机理及掺杂改性等方面的研究现状,比较了各种催化剂的优缺点,并对CH₄-SCR的发展前景进行了展望。     

双氧水催化氧化中Cu/γ-Al_2O_3催化剂的稳定性研究

针对废水湿式双氧水催化氧化,采用浸渍法制备Cu催化剂,研究非均相Cu催化剂在常温常压湿式双氧水催化氧化中的稳定性与失活问题。研究表明,催化剂制备条件及催化氧化反应条件对催化剂中Cu2＋溶出均有影响。研究同时表明,催化剂失活与活性组分流失和活性组分被有机中间产物覆盖有关,高温焙烧可对催化剂再生。     

Biological treatment process of air loaded with an ammonia and hydrogen sulfide mixture

The physico-chemical characteristics of granulated sludge lead us to develop its use as a packing material in air biofiltration. Then, the aim of this study is to investigate the potential of unit systems packed with this support in terms of ammonia and hydrogen sulfide emissions treatment. Two laboratory scale pilot biofilters were used. A volumetric load of 680 g H2S m(-3) empty bed day(-1) and 85 g NH3 m(-3) empty bed day(-1) was applied for eight weeks to a unit called BGSn (column packed with granulated sludge and mainly supplied with hydrogen sulfide); a volumetric load of 170 g H2S m(-3) empty bed day(-1) and 340 g NH3 m(-3) empty bed day(-1) was applied for eight weeks to the other called BGNs (column packed with granulated sludge and mainly supplied with ammonia). Ammonia and hydrogen sulfide elimination occur in the biofilters simultaneously. The hydrogen sulphide and ammonia removal efficiencies reached are very high: 100% and 80% for BGSn; 100% and 80% for BGNs respectively. Hydrogen sulfide is oxidized into sulphate and sulfur. The ammonia oxidation products are nitrite and nitrate. The nitrogen error mass balance is high for BGSn (60%) and BGNs (36%). This result could be explained by the denitrification process which would have occurred in anaerobic zones. High percentages of ammonia or hydrogen sulfide are oxidized on the first half of the column. The oxidation of high amounts of hydrogen sulfide would involve some environmental stress on nitrifying bacterial growth and activity.     

Oxidation of mercury across selective catalytic reduction catalysts in coal-fired power plants

A kinetic model for predicting the amount of mercury (Hg) oxidation across selective catalytic reduction (SCR) systems in coal-fired power plants was developed and tested. The model incorporated the effects of diffusion within the porous SCR catalyst and the competition between ammonia and Hg for active sites on the catalyst. Laboratory data on Hg oxidation in simulated flue gas and slipstream data on Hg oxidation in flue gas from power plants were modeled. The model provided good fits to the data for eight different catalysts, both plate and monolith, across a temperature range of 280-420 degrees C, with space velocities varying from 1900 to 5000 hr(-1). Space velocity, temperature, hydrochloric acid content of the flue gas, ratio of ammonia to nitric oxide, and catalyst design all affected Hg oxidation across the SCR catalyst. The model can be used to predict the impact of coal properties, catalyst design, and operating conditions on Hg oxidation across SCRs.     

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.

     

Factors influencing the preparation of supported iron oxide in fluidized-bed crystallization

Our previous work applied a novel supported iron oxyhydroxide (FeOOH) catalyst to effectively treat benzoic acid by hydrogen peroxide. The FeOOH catalyst was prepared via the oxidation of Fe2+ by H2O2 in the acidic condition using a fluidized-bed crystallization reactor. The major components coated on the surface were identified as amorphous FeOOH and gamma-FeOOH. In terms of the crystallization conditions of FeOOH, some parameters including the operational pH, superficial velocity, specific iron loading, and influent H2O2 concentration were investigated to quantify their effects on the crystallization efficiency. All these parameters were found to significantly influence the crystallization efficiency. Two types of FeOOH catalysts were synthesized: FeOOH I was prepared at pH 3.5, and FeOOH II was formed by aging FeOOH I at pH 13. The percentages of surface amorphous FeOOH reduced from 70% to 30% after aging. The FeOOH II catalyst presented a higher reactivity toward H2O2 but lower stoichiometric efficiency in oxidizing benzoic acid than FeOOH I, similar to the result of the commercial goethite. Therefore, it is concluded that the crystalline property significantly affects the performance of catalytic oxidation.     

Effect of CO on NO oxidation over platinum based catalysts for hybrid fast SCR process

The oxidation characteristics of NO over Pt/TiO2 (anatase, rutile) catalysts have been determined in a fixed bed reactor as a function of O2, CO and SO2 concentrations in the presence of 8% water. The conversion of NO to NO2 increases with increasing O2 concentration up to 12% and it levels off. This saturation effect is more pronounced over rutile-Pt/TiO2 (r-Pt/TiO2) than that of anatase-Pt/TiO2 (a-Pt/TiO2). The presence of CO increases NO oxidation significantly and this enhanced effect is more pronounced on a-Pt/TiO2 than that on r-Pt/TiO2 with increasing CO concentration at lower temperatures. The same effect is also observed on the catalysts with different Pt and tungsten oxide (WO3) loadings. With increasing Pt and WO3 loadings on TiO2 support (Pt-WO3/TiO2), formation of NO2 is high even at lower temperatures. The presence of SO2 significantly suppresses the oxidation of NO over both r-Pt/TiO2 and a-Pt/TiO2 catalysts but it is less pronounced due to low stability of sulfate on a-Pt/TiO2.     

Accumulation of platinum group elements by the marine gastropod Littorina littorea

The accumulation and trophic transfer of the platinum group elements (PGE): Rh, Pd and Pt; have been studied in short-term (5 day) exposures conducted in aquaria containing the marine macroalga, Ulva lactuca, and/or the grazing mollusc, Littorina littorea. Metals added to sea water (to concentrations of 20 μg L⁻¹) were taken up by U. lactuca in the order Rh, Pt > Pd and by L. littorea in the order Pd ≥ Pt ≥ Rh, with greatest metal accumulation in the latter generally occurring in the visceral complex and kidney. When fed contaminated alga, accumulation of Rh and Pd by L. littorea, relative to total available metal, increased by an order of magnitude, while accumulation of Pt was not readily detected. We conclude that the diet is the most important vector for accumulation of Rh and Pd, while accumulation of Pt appears to proceed mainly from the aqueous phase.