Showing papers in "Applied Catalysis B-environmental in 1994"

Activity and selectivity of pure manganese oxides in the selective catalytic reduction of nitric oxide with ammonia

Abstract: Manganese oxides of different crystallinity, oxidation state and specific surface area have been used in the selective catalytic reduction (SCR) of nitric oxide with ammonia between 385 and 575 K. MnO2 appears to exhibit the highest activity per unit surface area, followed by Mn5O8, Mn2O3, Mn3O4 and MnO, in that order. This SCR activity correlates with the onset of reduction in temperature-programmed reduction (TPR) experiments, indicating a relation between the SCR process and active surface oxygen. Mn2O3 is preferred in SCR since its selectivity towards nitrogen formation during this process is the highest. In all cases the selectivity decreases with increasing temperature. The oxidation state of the manganese, the crystallinity and the specific surface area are decisive for the performance of the oxides. The specific surface area correlates well with the nitric oxide reduction activity. The nitrous oxide originates from a reaction between nitric oxide and ammonia below 475 K and from oxidation of ammonia at higher temperatures, proven by using 15NH3. Participation of the bulk oxygen of the manganese oxides can be excluded, since TPR reveals that the bulk oxidation state remains unchanged during SCR, except for MnO, which is transformed into Mn3O4 under the applied conditions. In the oxidation of ammonia the degree of oxidation of the nitrogen containing products (N2, N2O, NO) increases with increasing temperature and with increasing oxidation state of the manganese. A reaction model is proposed to account for the observed phenomena.

Mechanism of the selective reduction of nitrogen monoxide on platinum-based catalysts in the presence of excess oxygen

Abstract: A range of alumina-supported platinum catalysts have been prepared and investigated for the selective reduction of nitrogen monoxide in the presence of a large excess of oxygen. Steady-state microreactor experiments have demonstrated that these catalysts are very active and selective for the reduction of nitrogen monoxide by propene at temperatures as low as 200°C. There does not appear to be a simple correlation between the activity for nitrogen monoxide reduction and the platinum surface area. Instead it is found that there is a very good inverse correlation between the maximum nitrogen monoxide reduction activity and the temperature. The most active catalysts for selective nitrogen monoxide reduction are those that generate activity at the lowest temperature. The technique of temporal analysis of products (TAP) has been used to obtain detailed mechanistic data about the selective nitrogen monoxide reduction reaction on an alumina-supported platinum catalyst. Using carbon monoxide, hydrogen or propene as reductant it has been demonstrated that the predominant mechanism for selective nitrogen monoxide reduction involves the decomposition of nitrogen monoxide on reduced platinum metal sites, followed by the regeneration of the active platinum sites by the reductant. In the decomposition step it has been shown that oxygen from nitrogen monoxide is retained on the surface of the platinum and blocks the surface for further adsorption/reaction of nitrogen monoxide; it has been observed that oxidised platinum catalysts are not active for the nitrogen monoxide reduction reaction. Under typical operating conditions, propene is a far more efficient reductant than either carbon monoxide or hydrogen. The greater efficiency of propene as a reductant is explained on the basis of the additional reducing power of the propene molecule, which can react with as many as nine adsorbed oxygen atoms, ensuring that 'patches' of reduced platinum are available for nitrogen monoxide adsorption/reaction. A small additional activity of reduced platinum in the presence of propene, which is not observed when carbon monoxide or hydrogen is used as reductant, has been explained on the basis of a second mechanism involving the carbon-assisted decomposition of nitrogen monoxide at sites on the reduced platinum adjacent to adsorbed carbon-containing moieties, believed to be fragments from adsorbed propene molecules. A model for the selective reduction of nitrogen monoxide on alumina-supported platinum catalysts is presented which is capable of explaining all the results obtained in this work and in the published literature on this subject.

Cu(I)-ZSM-5 zeolites prepared by reaction of H-ZSM-5 with gaseous CuCl: Spectroscopic characterization and reactivity towards carbon monoxide and nitric oxide☆

Abstract: The strongly acidic Bronsted sites of H-ZSM-5 can be quantitatively exchanged with monovalent copper ions by reaction with CuCl at 573 K, as evidenced by the disappearance of the characteristic IR bands of bridged OH groups. Characterization of the Cu-ZSM-5 samples prepared following this route by means of UV-Vis-NIR (diffuse reflectance) and photoluminescence spectroscopies confirms that the protons are substituted by Cu+ ions, which are isolated and located in a few, structurally well defined sites easily accessible to ligand molecules. These Cu+ ions are highly coordinatively unsaturated and can form Cu+ (CO)n (n=1, 2 or 3) carbonylic and Cu+ (NO)n (n=1 or 2) nitrosylic complexes upon dosage of carbon monoxide or nitric oxide at 77 K. The Cu+ (NO)2 dinytrosylic complexes are unstable at room temperature and evolve with formation of nitrous oxide, NO2− and oxidized CuIINO species. This behaviour strongly supports the hypothesis that a redox mechanism is operating in the nitric oxide decomposition reaction leading to nitrogen and oxygen.

Effect of Fenton and photo-Fenton reactions on the degradation and biodegradability of 2 and 4-nitrophenols in water treatment

Abstract: Photo-Fenton, Fenton and biodegradation reactions have been investigated in detail during the degradation of 2 and 4-nitrophenols. Fenton-type reactions accelerated nitrophenols degradation in comparison with direct photolysis using pyrex flasks (λ > 290 nm). The influence of Fe3+, H2O2, light, temperature, reactant concentration and gas atmosphere was systematically studied. Experimental techniques used involved total organic carbon determination (TOC), high pressure liquid chromatography (HPLC), nuclear magnetic resonance (NMR) and spectroscopy (OD). A solution containing 3.6·10−1M of 2-nitrophenol was degraded in ca. 3 h (30°C) in the dark and in ca. 1 h (30°C) under light where continuous photoproduction of the Fenton reagent is achieved. This study shows that the hydrolxylation of the phenol ring is fast as compared to the slower concomitant decrease in DOC in dark or light processes. Using NMR an explanation is proposed in terms of pathways involving direct oxidation of the nitrophenols under study by hydroxy type radicals. Chemical insight is provided why the photo-Fenton degradation observed for 2-nitrophenol proceeds at about half the rate than his homologue 4-nitrophenol. Biodegradability of 2-nitrophenol was monitored before and after photo-Fenton treatment by biochemical oxygen demand (BOD) and dissolved organic carbon (DOC) and indicated the formation of substances which are non-biodegradable during photo-Fenton pretreatment.

Investigation of Pt/Al2O3 and Pd/Al2O3 catalysts for the combustion of methane at low concentrations

Abstract: Pt/Al 2 O 3 and Pd/Al 2 O 3 catalysts have been prepared from chlorine-free precursors and investigated for the combustion of methane under lean, stoichiometric, and rich conditions using dilute mixtures. It has been found that under lean conditions, and at low conversions under stoichiometric or rich conditions, Pd/Al 2 O 3 is a more effective catalyst. However, at higher conversions with stoichiometric or rich mixtures Pt/Al 2 O 3 is a more active catalyst. This change over between Pd/Al 2 O 3 and Pt/Al 2 O 3 is associated with a ‘light-off’ effect observed with Pt/Al 2 O 3 catalysts. Various possible explanations for these effects are discussed. With the Pt/Al 2 O 3 catalysts there is no evidence of a particle size effect, which is in contrast to reports in the literature that the methane combustion reaction is structure sensitive. Reasons for these variations, including the possible inhibition of activity by chlorine used in many catalyst preparations, are discussed. It is concluded that platinum can be a more effective catalyst than palladium for methane combustion under real conditions and that, in consequence, platinum may play an important role in multimetallic catalysts for emission control for natural gas vehicles.

Steam effects in three-way catalysis

Abstract: Steam can exert two main effects in three-way catalysis (i) it is a reactant which can oxidize the carbon monoxide by water-gas shift and the hydrocarbons by steam reforming (ii) it is a major component of the exhaust gas which can affect significantly the thermal stability of three-way catalysts The role of water as a co-reactant in oxidation reactions as well as its role in catalyst sintering are successively reviewed Most investigations having been carried out with PtRh catalysts, no definite conclusion can be given as far as palladium catalysts are concerned

Selective catalytic reduction of nitric oxide with ethane and methane on some metal exchanged ZSM-5 zeolites

Abstract: The selective reduction of nitric oxide by methane or ethane, in the presence and in the absence of a large excess of oxygen, has been investigated on Cu/ZSM-5, Co/ZSM-5, Rh/ZSM-5 and Pt/ZSM-5 catalysts over a wide range of temperatures. It has been found that the maximum nitric oxide conversion is higher with ethane than with methane and the temperature of this maximum is lower with ethane. In the absence of oxygen the order of activity is Rh/ZSM-5>Pt/ZSM-5>Co/ZSM-5 > Cu/ZSM-5 with the Cu/ZSM-5 being essentially inactive, while in the presence of oxygen the order is: Rh/ZSM-5>Co/ZSM-5>Cu/ZSM-5 > Pt/ZSM-5 when ethane is used as reductant and: Rh/ZSM-5>Co/ZSM-5 > Cu/ZSM-5>Pt/ZSM-5 when methane is used. The effect of the oxygen content has been investigated for the Co/ZSM-5 catalyst. It has been found that with a small quantity of oxygen the catalytic activity decreases markedly; with higher oxygen content the activity of the catalyst rises again. It appears that two different reaction schemes may be operative, one in the absence of oxygen the other in the presence of oxygen. It is concluded that neither carbonaceous deposits, nor nitrogen dioxide formation in the gas phase are important in the reaction mechanism on metal-containing zeolites. It is proposed that the reaction is essentially a redox process in which decomposition of nitric oxide occurs on reduced metallic or metal ion sites (the relative activity of each of these depending on the choice of metal), leading to the formation of gaseous nitrogen and adsorbed oxygen, followed by the removal of the adsorbed oxygen by the hydrocarbon, thus recreating the active centres.

Propane and propene oxidation over platinum and palladium on alumina: Effects of chloride and water

Abstract: The oxidation of propane and propene was investigated on palladium and platinum supported catalysts. Catalyst intrinsic activities, evaluated by light-off temperatures in slightly oxidizing reactant mixture (5% excess oxygen), show an optimum particle size which maximizes the catalytic activity for a given metal loading. On catalysts prepared from chloride containing precursor salts, chloride poisons the metallic activity whatever the particle size. Moreover, reaction isotherms under varying oxygen levels point out that the effect of chloride is more detrimental under oxidizing conditions. After successive oxidation cycles, this poisoning effect disappears as a consequence of the removal of chloride from the catalyst surface by water produced during propane and propene combustion. On the other hand, addition of relatively large quantities of water (equivalent to the content of the exhaust gas) inhibits the oxidation of hydrocarbon. Poisoning effects of chloride and water are explained by a decreasing active surface for the reactions under consideration.

Intermediacy of organic nitro and nitrite surface species in selective reduction of nitrogen monoxide by propene in the presence of excess oxygen over silica-supported platinum

Abstract: The role of surface species in the selective reduction of nitrogen oxides by propene in the presence of excess oxygen over Pt/SiO 2 has been studied at 393 K mainly with IR spectroscopy. Organic nitro, nitrite and carbonyl species were detected during the reaction. The reactions of those three species with nitrogen dioxide and oxygen took place rapidly, producing N 2 , N 2 O and CO 2 , while the reactivities of those species with nitric oxide and propene were low. Hence, a mechanism is proposed, in which the nitro, nitrite and carbonyl surface species are key reaction intermediates. Similarity in the products between the selective reduction and the oxidation of nitro (nitromethane) or nitrite (n-butylnitrite) compound supported the proposed reaction scheme.

Catalytic combustion of methane over palladium exchanged zeolites

Abstract: Palladium cation exchanged zeolites (ZSM-5, mordenite and ferrierite) were studied as catalysts for methane combustion. Pd-zeolites showed much higher activities than PdO/Al2O3. For comparable palladium loadings, PdO/Al2O3 requires a reaction temperature of ca. 70–80°C higher than Pd-ZSM-5 for conversions between 50–100%. The catalytic activity of Pd-ZSM-5 seems to be related to its reducibility. Temperature-programmed reduction experiments with carbon monoxide showed a lower reduction temperature (ca. 157°C) for Pd-ZSM-5 than for PdO/Al2O3 (225°C). Further, the positioning of the palladium by ion exchange offers a highly dispersed form of PdII supported on the high surface area zeolite.

Catalytic decomposition of nitrous oxide on “in situ” generated thermally calcined hydrotalcites

Abstract: Catalytic decomposition of nitrous oxide to nitrogen and oxygen has been carried out on “in situ” generated thermally calcined hydrotalcites of the general formula MAlCO3HT, where M stand for Ni, Co and Cu, in the temperature range 140–310°C at 50 Torr (1 Torr=133.3 Pa) initial pressure of the gas. All the catalysts showed first-order dependence on nitrous oxide without any inhibition by oxygen. Among the catalysts studied, NiAlCO3HT was the most active, followed by the cobalt and copper catalysts. These catalysts are more active in comparison with earlier reported Cu-ZSM-5, Co-ZSM-5 and Rh-ZSM-5 catalysts based on their conversion for the decomposition. The enhanced activity can be attributed to non-stoichiometry and dispersion of the active mixed metal oxides. Prior to the kinetic runs, the catalytic precursors were characterised by X-ray diffraction, thermogravimetry-differential scanning calorimetry measurements, IR, transmission electron microscopy and nitrogen adsorption measurements.

Reduction of sulfur dioxide by carbon monoxide to elemental sulfur over composite oxide catalysts

Abstract: The catalyst activity of fluorite-type oxide, such as ceria and zirconia, for the reduction of sulfur dioxide by carbon monoxide to elemental sulfur can be significantly promoted by active transition metals, such as copper. More than 95% elemental sulfur yield, corresponding to almost complete sulfur dioxide conversion, was obtained over a Cu-Ce-O oxide catalyst with a feed gas of stoichiometric composition ([CO]/[SO 2 ]=2) at temperatures above 450°C. This type of mixed metal oxide catalyst has stable activity and is resistant to water and carbon dioxide poisoning. XPS analysis found copper in the Cu-Ce-O oxide in a reduced oxidation state (Cu 1+ , Cu 0 ). The stable fluorite-type structure, regarded as the backbone structure of the catalyst, existed in both the fresh and the spent catalyst. The high activity resulted from the strong interaction of transition metal and fluorite oxide.

NOx decomposition and reduction by methane over La2O3

Abstract: Nitric oxide reduction by methane was conducted in a quartz microreactor from 773 to 973 K over La2O3, a good methane oxidative coupling catalyst. La2O3 is a better catalyst than MgO and Li/MgO for this reaction because: (1) it has a much higher specific activity, (2) the presence of oxygen enhances the rate of reduction, and (3) it gives a selectivity to nitrogen that is close to 100%. Both Li/MgO and La2O3 catalyst systems have rates that increase continuously with temperature and exhibit no bend over. The apparent activation energy for nitric oxide reduction by methane over La2O3 was 24.4 kcal/mol (26.0 kcal/mol with oxygen present), and the reaction orders in CH4, NO and O2 were 0.26, 0.98 and 0.50, respectively. Unlike Li/MgO, La2O3 is also active for direct nitric oxide decomposition, but the activity is noticeably lower than that for nitric oxide reduction by methane. Both the direct decomposition of nitrous oxide and nitrogen dioxide and their reduction by methane were also studied on La2O3. The rate of direct nitrous oxide decomposition was quite high and essentially unaltered by the presence of methane; in contrast, the rate of nitrogen dioxide decomposition was very low. However, the rate of nitrogen dioxide reduction to nitrogen was greatly enhanced by methane, and the rate became comparable to that for nitric oxide reduction by methane in the presence of 1.0% O2. Methane oxidative coupling over this La2O3 catalyst was verified using oxygen and nitrous oxide as oxidants, whereas little or no coupling occurred when nitric oxide or nitrogen dioxide was used. At this time, the high NOx reductive activity of La2O3 and other coupling catalysts is attributed to the formation of surface methyl radicals.

Characterization of TiO2 photocatalysts used in trichloroethene oxidation

Abstract: Kinetic studies show deactivation of TiO2 catalysts during aqueous-phase and gas-phase photooxidation of trichloroethene (TCE). Temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) were used to examine adsorbed species on TiO2 photocatalyst surfaces after reaction, and TPD was used to determine how reactants and products adsorb on the TiO2 surface. Used and deactivated catalysts were analyzed after participating in either aqueous-phase or gas-phase photooxidation of TCE. The XPS spectra showed little difference between the surface composition of fresh TiO2 and that of a deactivated catalyst from the aqueous-phase photoreactor. Chlorine was observed only on catalysts used in the gas-phase photocatalytic decomposition of TCE. Differences due to photoreaction were observed in TPD spectra of water, carbon monoxide, and carbon dioxide. Both the total amount desorbed and the temperature of desorption of carbon monoxide and carbon dioxide were quite different for used and deactivated catalysts from the two photoreactions. Apparently strongly bound species, such as carbonates, accumulated on the surface and formed carbon monoxide upon high-temperature decomposition. Small amounts of chlorinated compounds desorbed from the used and deactivated catalysts following gas-phase photoreaction. Dichloroacetyl chloride (DCAC), a reaction intermediate, can adsorb strongly on TiO2 and readily displaces TCE. Thermally decomposed DCAC reduces the number of available adsorption sites for DCAC and TCE. An interesting low-temperature oxygen desorption peak was observed from catalysts treated with H2O2, which improves catalytic activity. This feature indicates that H2O2 is stable on TiO2 at room temperature and decomposes at 420 K.

Catalytic combustion of diesel soot particles on copper catalysts supported on TiO2. Effect of potassium promoter on the activity

Abstract: We have prepared a TiO2 supported copper catalyst and studied the effect of potassium on its activity in the oxidation of soot particles. The catalysts, with a K/Cu atomic ratio varying between 0 and 2, were calcined at 1073 K. They were characterized by BET surface area measurements, X-ray diffraction and temperature-programmed reduction under hydrogen. The catalytic activity was measured in a microbalance by means of temperature-programmed oxidation in air or argon. The catalytic activity of copper was enhanced by the presence of potassium. This effect was attributed to the formation of mixed KTi oxides which inhibit the sintering of the TiO2 support and thus increases the surface area of the catalyst. Although a redox mechanism can explain the catalytic combustion, no correlation could be established between the reducibility of the different solids and their activity in soot combustion.

Catalytic decomposition of nitrous oxide over Ru-exchanged zeolites

Abstract: We report that ultrastable faujasite-based ruthenium zeolites are highly active catalysts for N2O decomposition at low temperature (120–200°C). The faujasite-based ruthenium catalysts showed activity for the decomposition of N2O per Ru3+ cation equivalent to the ZSM-5 based ruthenium catalysts at much lower temperatures (TOF at 0.05 vol.-% N2O: 5.132 × 10−4 s−1 Ru−1 of Ru-HNaUSY at 200°C versus 5.609 × 10−4 s−1 Ru−1 of Ru-NaZSM-5 at 300°C). The kinetics of decomposition of N2O over a Ru-NaZSM-5 (Ru: 0.99 wt.-%), a Ru-HNaUSY (Ru: 1.45 wt.-%) and a Ru-free, Na-ZSM-5 catalyst were studied over the temperature range from 40 to 700°C using a temperature-programmed micro-reactor system. With partial pressures of N2O and O2 up to 0.5 vol.-% and 5 vol.-%, respectively, the decomposition rate data are represented by: −dN2O/dt=itk(PN2O) (PO2)−0.5 for Ru-HNaUSY, −dN2O/dt=k(PN2O) (PO2)−0.1 for Ru-NaZSM-5, and −dN2O/dt=k(PN2O)−0.2 (PO2)−0.1 for Na-ZSM-5. Oxygen had a stronger inhibition effect on the Ru-HNaUSY catalyst than on Ru-NaZSM-5. The oxygen inhibition effect was more pronounced at low temperature than at high temperature. We propose that the negative effect of oxygen on the rate of N2O decomposition over Ru-HNaUSY is stronger than Ru-NaZSM-5 because at the lower temperatures (<200°C) the desorption of oxygen is a rate-limiting step over the faujasite-based catalyst. The apparent activation energy for N2O decomposition in the absence of oxygen is much lower on Ru-HNaUSY (Ea: 46 kJ mol−1) than on Ru-NaZSM-5 (Ea: 220 kJ mol−1).

Photocatalytic oxidation of phenol on TiO2 powders. A Fourier transform infrared study

Abstract: In the present paper a detailed investigation of a well known photoreaction, i.e. phenol photodegradation in the presence of TiO2, has been carried out. The Fourier transform infrared (FT-IR) technique has been used to characterize the catalyst surface and to follow the photoprocess by “in situ” measurements in a gas—solid regime, simulating the aqueous liquid—solid regime in which this photodegradation reaction has been usually carried out. The influence on the photoprocess of the surface hydroxylation of the catalyst has been investigated and the presence of some reaction intermediates has been revealed by FT-IR. The experimental results have demonstrated that the chosen gas—solid system in the presence of water vapour is an acceptable simulation of the aqueous liquid—solid regime.

Performance and durability of Pt-MFI zeolite catalyst for selective reduction of nitrogen monoxide in actual diesel engine exhaust

Abstract: Selective catalytic reduction of nitrogen monoxide (NO) by hydrocarbon in an oxidizing atmosphere has been studied over platinum-MFI zeolite (Pt-MFI) in synthesized or actual diesel engine exhaust gases. The activity of Pt-MFI in the synthesized gas, containing 10% water, changed in the early stage of the use, leveled off after 150–200 h, and remained constant for more than 800 h. The Pt-MFI catalyst also showed stable activity at 423–773 K and 10 000–150 000 h − (gas hourly space velocity) in actual engine exhaust with light oil as a fuel. The degree of nitrogen monoxide reduction increased linearly upon addition of ethylene into the exhaust gas.

The unusual hydrothermal stability of Co-ZSM-5

Abstract: We wish to report that cobalt exchanged ZSM-5 has remarkably high hydrothermal stability, especially in comparison to Cu-ZSM-5. Calcination of Co-ZSM-5 in 2% water vapor at 750°C for a period of 99 h has a modest effect upon its activity for nitrous oxide decomposition. Treatment of Cu-ZSM-5 under the same conditions dramatically reduces the performance of the copper exchanged zeolite.

In situ characterization of Cu-ZSM-5 by X-ray absorption spectroscopy: XANES study of the copper oxidation state during selective catalytic reduction of nitric oxide by hydrocarbons

Abstract: Reported here is our recent investigation of the mechanism of the selective nitric oxide reduction by hydrocarbons on Cu-ZSM-5 catalysts in an oxygen-rich gas mixture. We studied the copper oxidation state change during the catalytic reaction using the X-ray Absorption Near Edge Structure (XANES) method. We observe that even under strongly net oxidizing conditions, a significant fraction of the copper ions in ZSM-5 is reduced to Cu I at elevated temperature, when propene is present in the reactant stream. XANES spectra show that the Cu I 1s → 4p transition intensity, which is proportional to cuprous ion concentration, changes with the reaction temperture in a pattern similar to the NO conversion activity. For comparison purposes, we also studied the Cu I concentration change using a gas mixture in which propene was replaced by a stoichiometrically equivalent concentration of methane. Unlike propene, methane provides no NO selective reduction pathway over Cu-ZSM-5. No window of enhanced Cu I concentration was observed using methane as the reductant. Our study indicates that, even in a strongly oxidizing environment, cupric ion can be partially reduced by propene to form Cu I , possibly by way of allylic intermediate, which may be a crucial step for effective NO conversion through a redox mechanism.

Catalytic destruction of chlorofluorocarbons and toxic chlorinated hydrocarbons

Abstract: The problem of the destruction of the ozone layer by chlorofluorocarbons (CFCs) has led to the proposed phase-out of their production by 1995. A number of chlorinated hydrocarbons which are toxic to man have also been found to destroy the ozone layer. A series of catalysts were tested for their ability to destroy CFC113 and trichloroethylene. CFC113 destruction over alumina-based catalysts led to severe catalyst deactivation due to fluorination of the alumina. However, zirconia-based catalysts were found to have some tolerance against fluorine species. A Pt/ZrO2-(PO4) catalyst was developed which could maintain more than 99% conversion of CFC113 over a 300 hour trial period at 500°C. The major product was CO2 with a yield above 93%. A significant side reaction (selectivity 5%) was observed due to the fluorination of incoming CFC113 feed to form CFC114. A reaction mechanism was hypothesized for CFC113 destruction over zirconia-based catalysts. Trichloroethylene destruction was more easily achieved than CFC113. Over Pt/ZrO2, more than 99.9% conversion was possible at 500°C over a 100 hour trial period. A side reaction associated with the chlorination of the feed to tetrachloroethylene had a selectivity below 0.05% indicating that the reactivity of the fluorine species was far higher than the chlorine species.

Photocatalytic reduction of nitrogen over (Fe, Ru or Os)/TiO2 catalysts

Abstract: Irradiated aqueous suspensions of (Fe, Ru or Os)/TiO 2 catalysts were found to reduce nitrogen to ammonia. The order of catalytic activity was found to be Ru > Fe > Os, while reverse order was found for the stability of the catalysts with respect to ammonia formation. A mechanism involving interaction between H ads and chemisorbed nitrogen is proposed to explain the reactivity and deactivation of these catalysts with the aid of electron paramagnetic resonance results.

Catalytic oxidative decomposition of chlorofluorocarbons (CFCs) in the presence of hydrocarbons

Abstract: The catalytic oxidative decomposition of CFCs containing two carbon atoms was investigated in the presence of hydrocarbons (C 1 –C 4 alkanes) over silica and various acidic metal oxides. It was found that CFC-115 was the most difficult to be decomposed among CFC-113, CFC-114, and CFC-115, and that γ-alumina was the most active catalyst. The CFC-115 conversion over γ-alumina was lower in the presence of methane than in the presence of the other hydrocarbons. In addition, the activities of γ-alumina-supported metal and metal oxide catalysts were investigated for the oxidative decomposition of CFC-115 in the presence of n-butane. Tungsten (VI) oxide and vanadium (V) oxide catalysts exhibited the highest activity.

Optimization of SOx additives of FCC catalysts based on MgO-Al2O3 mixed oxides produced from hydrotalcites

Abstract: A series of magnesium-rich Mg-Al spinels were prepared from hydrotalcites and characterized. When CeO2 was incorporated they showed a very good SOx absorption as FCC SOx additives, but their regeneration capacity was limited. Different transition metal oxides were tried as co-catalysts, and among them CuO, showed excellent properties to catalyze SO2 oxidation as well as catalyst regeneration. The SOx catalyst was stable when steamed at 750°C together with FCC catalysts.

Effects of selective reduction of nitric oxide on zeolite structure

Abstract: The chemical changes that occurred in a Cu-ZSM-5 catalyst during the selective reduction of NO with i-C4H10 in the presence and absence of O2 were catalogued. In the presence of excess O2 complete conversion of the NO to N2 and the hydrocarbon to CO2 and H2O occurred and the Cu2+ concentration estimated from the integrated intensity of the electron paramagnetic resonance (EPR) signal was not significantly changed from its initial value. When the oxygen concentration was lowered below the point of stoichiometry, however, both of these conversions decreased modestly, but when O2 was eliminated from the feed both conversions fell precipitously and the acid catalyzed decomposition products of isobutane appeared in the products instead of CO2 and H2O. These changes were accompanied by corresponding changes in the EPR data. Lowering the O2 below the point of stoichiometry effected a loss of from 30% to 50% of the intensity of the Cu2+ signal. Eliminating O2 reduced the signal by several orders of magnitude. Remarkably, these reduced catalysts could be restored to their initial oxidation states by adding excess O2 into the feed stream, even when there was evidence that Cu0 was present. Dealumination accompanied selective reduction even in excess O2, particularly above 623 K. This was probably caused by steaming of the catalyst by the H2O produced in the reaction.

Methane combustion over La0.8Sr0.2MnO3+x supported on MAl2O4 (M = Mg, Ni and Co) spinels

Abstract: Catalysts containing 20 wt.-% La0.8Sr0.2MnO3+x supported on spinels (MAl2O4), were prepared by impregnation. Three different supports (MgAl2O4, NiAl2O4 and CoAl2O4) were prepared by coprecipitation. X-ray diffraction line broadening indicated that the mean particle size of the deposited perovskite depended on the spinel used as support. Oxygen evolution measurements carried out in helium in the temperature range from 300 to 1360 K showed that the thermal decomposition proceeded similarly on all three supported catalysts, beginning at about 800 K. The amount of oxygen which evolved was found to depend on the particle size of the perovskite. The smaller the perovskite particles were, the more stable they were toward thermal decomposition. Kinetic tests were performed in a fixed-bed microreactor at 600–1200 K and atmospheric pressure using a reactant mixture with a ratio CH4:O2=1:4. The reaction rate per gram perovskite, measured at 770 K, was more than ten times higher for La0.8Sr0.2MnO3+x/MgAl2O4 than for unsupported La0.8Sr0.2MnO3+x. The high activity of the MgAl2O4-supported perovskite is attributed to the high dispersion of the perovskite on this catalyst, leading to a larger active surface area compared to the unsupported La0.8Sr0.2MnO3+x.

Catalytic oxidation of toxic organics in supercritical water

Abstract: Oxidation of some toxic organic compounds in supercritical water is investigated in the presence of a solid catalyst in an isothermal plug flow fixed-bed reactor. Comparison between catalyzed and non-catalyzed oxidations indicates that the conversions are much higher when the catalyst is present. It is also found that the heterogeneous oxidation route forms less intermediate products during the decomposition of organics such as benzoic acid and 1-methyl-2-pyrrolidone in carbon dioxide.

Influence of the preparation method on the selective reduction of nitric oxide over Cu-ZSM-5. Nature of the active sites

Abstract: Copper ions were introduced in a Na-ZSM-5 zeolite by using exchange, impregnation and precipitation procedures. The catalytic activity of the three solids was measured in the reduction of NO by propane in the presence of oxygen (0 to 10 vol.-%) in the 423–773 K temperature range. At a given temperature the activity for nitrogen formation remarkably increased with the oxygen content in the 0.5–2 vol.-% range, then slightly decreased for higher amounts of oxygen. For an oxygen content exceeding 0.5 vol.-% and regardless of the reaction temperature, the Cu-ZSM-5 sample prepared by exchange was the most active catalyst, the catalytic activity being expressed per gram of introduced copper. The adsorption of carbon monoxide followed by FT-IR spectroscopy suggested that copper was present as isolated Cu+ ions in the three catalysts. The dispersion of cuprous ions was the highest for the sample prepared by the exchange process. The catalytic activity appeared related to a high dispersion of isolated Cu+ ions in the MFI framework.

Effect of water on the kinetics of nitric oxide reduction over a high-surface-area V2O5/TiO2 catalyst

Abstract: The influence of water on the kinetics of nitric oxide reduction with ammonia over a V 2 O 5 /TiO 2 catalyst has been investigated at different water partial pressures (0–300 Pa). An integral reactor operating at space velocity (sv) of 900 000 h −1 was employed. Nitric oxide and ammonia initial partial pressures were 10–200 Pa, oxygen 2700 Pa; the temperature ranged from 250 to 350°C. It has been found that the presence of water in the feed affects the reaction rate, inhibiting it. Two types of kinetic models based on a reaction mechanism involving the formation of a nitrosamidic intermediate compound were developed by assuming the adsorption of water as a reversible or irreversible step. Langmuir, Freundlich and Temkin equations for water adsorption were introduced in the kinetic models. It has been found that a kinetic model based on the irreversible adsorption of water following a Temkin isotherm allows the kinetics of nitric oxide reduction to be described for the whole range of water partial pressures. These results were interpreted according to a poisoning effect of the ammonia adsorbing sites by water.