Enhanced NOx Conversion with Decane Over Ag/Al2O3 and Metal Supported ZSM-5 Composite SCR Catalysts

A combination of Ag/Al₂O₃ and a partial oxidation catalyst M/ZSM-5, M being different metal cations, were evaluated for selective catalytic reduction of NO with decane. Physical mixture of Ag/Al₂O₃ and M/ZSM-5 catalysts showed significant increase in NOx conversion compared to single component Ag/Al₂O₃ catalyst. M/ZSM-5 as a second catalyst component was found to generate more reactive hydrocarbons, such as unsaturated small chain hydrocarbons and oxygenates in situ, and enhance the NOx conversion over Ag/Al₂O₃ HC-SCR catalyst.     

Effect of SO2 on NOx reduction by ethanol over Ag/Al2O3 catalyst

A new Ag/Al₂O₃ catalyst for removing NO_x in diesel engine exhaust gas was developed. The influence of SO₂ on the reduction of lean NO_x by ethanol over the Ag/Al₂O₃ catalyst was evaluated in simulated diesel exhaust and characterized using TPD, XRD, XPS, SEM and BET measurements. The Ag/Al₂O₃ catalyst was highly active for the reduction of NO_x with ethanol in the presence of SO₂ although the reduction of NO_x is suppressed at lower temperatures. The activity for NO_x reduction is high even on the Ag/Al₂O₃ catalyst exposed to a SO₂ (200 ppm)/O₂ (10%)/H₂O (10%) flow for 20 h at 723 K and comparable to that on the fresh Ag/Al₂O₃ catalyst. No crystallized Ag metal and Ag compounds were formed by the SO₂/O₂/H₂O exposure. On the other hand, crystallized Ag₂SO₄ was easily formed when the Ag/Al₂O₃ catalyst was exposed to a SO₂ (200 ppm)/O₂ (10%)/NO (800 ppm)/H₂O (10%) flow for 10 h at 723 K. XRD, SEM and XPS studies showed that the formation of crystallized Ag₂SO₄ results in growing of Ag particles in larger size and lowering the surface content of Ag particles. In addition, the specific surface area of the Ag/Al₂O₃ catalyst decreases from 221 to 193 m²/g. Although the dispersion of Ag particles was decreased by the formation of Ag₂SO₄, the activity for the reduction of lean NO_x was, remarkably, not affected. This suggests that the Ag–alumina sites created by the Ag₂SO₄ formation are still active for the lean catalytic reduction of NO_x. This revised version was published online in July 2006 with corrections to the Cover Date.     

Combination of Ag/Al2O3 and Fe-BEA for High-Activity Catalyst System for H2-Assisted NH3-SCR of NO x for Light-Duty Diesel Car Applications

Low-temperature active Ag/Al₂O₃ and high-temperature active Fe-BEA zeolite were combined and tested for H₂-assisted NH₃-selective catalytic reduction (SCR) of NO _x . The catalysts were either washcoated onto separate monoliths that were placed up- or downstream of each other (dual-brick layout) or washcoated on top of each other in a sandwiched layout (dual-layer). Our results showed that it is highly preferred to have Ag/Al₂O₃ as the upstream or outer layer catalyst. Fe-BEA showed a high NH₃ oxidation giving an NH₃ deficit over the Ag/Al₂O₃. Ag/Al₂O₃ formed NO₂ which enhanced the activity over Fe-BEA through the “fast”-SCR reaction when Fe-BEA was placed downstream or as inner layer. When no H₂, which is needed for the SCR reaction over Ag/Al₂O₃, was added, the dual-layer layout was preferred. The shorter diffusion distance between the layers is a probable explanation.     

Catalytic activity of ethylene oxidation over Au,Ag and Au–Ag catalysts: Support effect

Au, Ag and Au–Ag catalysts on different supports of alumina, titania and ceria were studied for their catalytic activity of ethylene oxidation reactions. An addition of an appropriate amount of Au on Ag/Al₂O₃ catalyst was found to enhance the catalytic activity of the ethylene epoxidation reaction because Au acts as a diluting agent on the Ag surface creating new single silver sites which favor molecular oxygen adsorption. The Ag catalysts on both titania and ceria supports exhibited very poor catalytic activity toward the epoxidation reaction of ethylene, so pure Au catalysts on these two supports were investigated. The Au/TiO₂ catalysts provided the highest selectivity of ethylene oxide with relatively low ethylene conversion whereas, the Au/CeO₂ catalysts was shown to favor the total oxidation reaction over the epoxidation reaction at very low temperatures. In comparisons among the studied catalysts, the bimetallic Au–Ag/Al₂O₃ catalyst is the best candidate for the ethylene epoxidation. The catalytic activity of the gold catalysts was found to depend on the support material and catalyst preparation method which govern the Au particle size and the interaction between the Au particles and the support.     

Promotional effect of Ca on the Pd/Ce–Zr/Al2O3 catalyst for low-temperature catalytic combustion of methane

Promotional effect of Ca on the catalytic property of Pd/Ce–Zr/Al₂O₃ catalyst towards methane combustion is examined. The surface properties and the oxidation/reduction behavior of these catalysts are investigated by BET, TEM, XPS, TPR, TPO and TPSR techniques. Activity tests in methane combustion show that addition of Ca to Pd/Ce–Zr/Al₂O₃ can promote remarkably its low-temperature activity. The thermal stability of the Pd/Ce–Zr/Al₂O₃ catalyst to the exposure at high temperature is also enhanced by Ca loading. XPS and TEM results show that the addition of Ca to Pd/Ce–Zr/Al₂O₃ catalyst generates well-dispersed PdO particles on support. H₂–TPR, O₂–TPO and CH₄/O₂–TPSR experiments show that the addition of Ca improves the reduction/reoxidation properties and thermal stability of the active PdO species, which increases the catalytic activity and thermal stability of the Pd/Ce–Zr/Al₂O₃ catalyst.     

Effect of the pressure on the catalytic oxidation of volatile organic compounds over Ag/Al2O3 catalyst

The catalytic oxidation of volatile organic compounds (VOCs: ethanol, 1-propanol and 2-propanol) over Ag/Al₂O₃ catalyst under low and normal atmospheric pressure conditions has been studied with synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry (PIMS). The partial oxidation intermediates of the VOCs under different pressures were identified by PIMS and their photoionization efficiency (PIE) spectra. Alkene is preferentially formed under the low pressure conditions, while aldehyde and acid are favored under the normal atmospheric pressure conditions. In addition, the low pressure conditions are more suitable for observing the active intermediates, such as ethenol, ketene and propenal. The results indicate that the pressure has a significant effect on the oxidation pathway of VOCs over Ag/Al₂O₃ catalyst.     

The role of silver species on Ag/Al2O3 catalysts for the selective catalytic oxidation of ammonia to nitrogen

The role of Ag species on Ag/Al₂O₃ catalyst for the selective catalytic oxidation (SCO) of NH₃ to N₂ was studied using 10 wt% Ag/Al₂O₃ catalysts prepared with impregnation, incipient wetness impregnation and sol–gel methods. The catalyst characterization was preformed using N₂ adsorption–desorption, UV/Vis, TEM and XRD. O₂-chemisorption and H₂–O₂ titration were measured to confirm the metal dispersion on the catalyst. The Ag species state and Ag particle size have significant influence on the Ag/Al₂O₃ activity and N₂ selectivity of the SCO of NH₃ at low temperature. Ag⁰ is proposed to be an active species on the H₂ pretreated catalyst at low temperature (<140 °C). It is evident that well-dispersed and small particle Ag⁰ enhances catalytic activity at low temperature, whereas large particle Ag⁰ is relate to a high N₂ selectivity. In contrast, Ag⁺ could also be the active species at temperatures above 140 °C.     

The role of Fe on PtPd oxidation catalyst prepared by simultaneous and sequential incipient wetnesses

The roles and effects of Fe on the catalytic performance and physicochemical properties of a PtPd diesel oxidation catalyst prepared by three different methods were investigated by CO oxidation reaction, X-ray diffraction, temperature-programmed reduction (TPR), temperature-programmed oxidation, and BET surface area. It was found that the roles of Fe depended strongly on the sequential order of Fe introduction during the preparation of the PtPd catalyst. The Fe/PtPd/Al₂O₃ catalyst was prepared by introducing Fe onto the PtPd/Al₂O₃, and the PtPd/Fe/Al₂O₃ catalyst was obtained by loading the PtPd onto the Fe/Al₂O₃. The former had a superior activity. From the TPR results, the catalytic activity of CO oxidation was correlated with the oxygen mobility of the iron oxides. For PtPd/Fe/Al₂O₃, the iron interacted preferentially with the alumina support forming FeAlO₃, which resulted in the stabilization of the support and a reduction in the surface area. The major role of Fe was to promote the enhancement of the catalytic activity of PtPd through an intimate interaction between the PtPd and iron oxides, which had lattice oxygens to generate oxygen with oxidation abilities.     

Catalytic Oxidation of Methanol by Aqueous POM on Al2O3 Supported Catalysts and Electrochemical Performance of POM

Phosphomolybdic acid (H₃PMo₁₂O₄₀, POM) was attempted to be used as the energy‐storage agent in this paper to avoid some problems of the direct methanol fuel cell (DMFC), such as catalyst poisoning and methanol permeation. Catalytic oxidation of methanol by aqueous POM on Al₂O₃ supported catalysts with Pt and Ru active metal was evaluated in the presence of liquid water. The process takes advantage of the high catalytic activities of platinum for methanol oxidation. The effects of temperature, reaction time, and methanol concentration on activity were observed. The catalytic activity of Pt/Al₂O₃ is better than that of Ru/Al₂O₃ for the oxidation of methanol by POM. The methanol conversion rate reached 93.55% on the Pt/Al₂O₃ at 80 °C after reaction for 1 h. The electrochemical experiments indicate that POM shows a larger current density in redox processes on an Au electrode than methanol. The redox process of reduced POM is a reversible multi‐electron transfer process.     

Alumina and Alumina–Baria Supported Cobalt Catalysts for DeNO x : Influence of the Support and Cobalt Content on the Catalytic Performance

Co/Al₂O₃ and Co/Al₂O₃–BaO catalysts with low cobalt loading (0.1, 0.3 and 1 wt%) for the selective catalytic reduction (SCR) of NO _x by C₃H₆ were prepared. The distribution of cobalt species was investigated by UV–vis diffuse reflectance spectroscopy and by H₂-TPR in order to identify the active cobalt species in hydrocarbons (HC)-selective catalytic reduction (SCR). It was found that the nature of cobalt species strongly depends on the cobalt loading as well as on the properties of the support. The barium addition to the alumina slows down solid state diffusion processes, improving the thermal stability of the support and preventing diffusion of cobalt into the bulk. Highly dispersed surface Co²⁺ species over alumina were identified as active sites in the NO-SCR process. Accordingly, a high concentration of surface Co²⁺ sites in Co 1 wt%/Al₂O₃ improves the catalytic performance in NO-SCR, the long term stability as well as the water tolerance. On the contrary, the formation of Co₃O₄ particles in Co 1 wt%/Al₂O₃–BaO promotes the propylene oxidation by oxygen, decreasing the activity and selectivity of the catalyst in NO reduction.     

Influence of Al2O3 support on the activity of Ag/Al2O3 catalysts for SCR of NO with decane

The role of the Al₂O₃ support on the activity of supported Ag catalyst towards the selective catalytic reduction (SCR) of NO with decane is elucidated. A series of Ag/Al₂O₃ catalysts were prepared by impregnation method and characterized by N₂ pore size distribution, XRD, UV–Vis, in-situ FT-IR and acidity measurement by NH₃ and pyridine adsorption. The catalytic activity differences of Ag/Al₂O₃ are correlated with different properties of Al₂O₃ supports and the active Ag species formed. 4wt% Ag supported on sol-gel prepared Al₂O₃ (Ag/Al₂O₃ (SG), showed higher NO _x conversion (65% at 400 °C), compared with the respective catalysts made from commercial Al₂O₃ (Ag/Al₂O₃ (GB), Ag/Al₂O₃ (ALO), (∼26 and 7% at 400 °C). The higher surface area, acidity and pore size distribution in sol–gel prepared Al₂O₃ (SG) results in higher NO and hydrocarbon conversion. Based on the UV–vis characterization, the activity of NO reduction is correlated to the presence of Ag_n^δ+ clusters and acidity of Al₂O₃ support was found to be one of the important parameter in promoting the formation and stabilization of Ag_n^δ+ clusters. Furthermore from pyridine adsorption results, presence of more number of Bronsted acid sites in Ag/Al₂O₃ (SG) is confirmed, which could also contribute to low temperature hydrocarbon activation and improve NO conversion. In situ FT-IR measurements revealed the higher rate of –CN and –NCO intermediate species formation over 4wt% Ag/Al₂O₃ (SG). We conclude that the physico–chemical properties of Al₂O₃ play a crucial role in NO _x conversion over Ag/Al₂O₃ catalysts. Thus, the activity of the Ag/Al₂O₃ catalyst can be tailored by using a proper type of Al₂O₃ support.     

Effective Catalysts for Wet Oxidation Of Formic Acid by Oxygen and Ozone

The catalytic wet-air oxidation and catalytic ozonation of a formic acid solution have been studied at room temperature using more than 20 kinds of catalysts including Pt/C, Pt/Al₂O₃, Pd/Al₂O₃, etc. The most effective catalyst was Pt/Al₂O₃. The apparent activation energies for the catalytic wet-air oxidation and ozonation on Pt/Al₂O₃ were both about 5 kcal/mol. This fact suggests that these reactions on the catalyst were diffusion controlled, and thus, the potential of the catalyst is likely much higher.     

Synthesis, Characterization of Ag/MCM-41 and the Catalytic Performance for Liquid-phase Oxidation of Cyclohexane

Nano-scale silver supported mesoporous molecular sieve Ag/MCM-41 was directly prepared by one-pot synthesis method. The prepared sample was characterized by XRD, TEM, and N₂ sorption. The results showed that the sample of Ag/MCM-41 had no appreciable incorporation of silver into the mesoporous matrix of MCM-41 with good crystallinity, and silver nanoparticles were dispersed inside or outside of the channels in the mesoporous host. The catalytic performance of the sample for the cyclohexane liquid-phase oxidation into cyclohexanone and cyclohexanol by oxygen in the absence of solvents without inducing agents was investigated. The 83.4% selectivity to cyclohexanol and cyclohexanone at 10.7% conversion of cyclohexane was obtained over Ag/MCM-41 catalyst at 428 K for 3 h. The turn over numbers (TONs) of Ag/MCM-41 was up to 2946. The catalytic activity of Ag/MCM-41 was also compared with Ag/TS-1 as well as Ag/Al₂O₃. The results indicated that Ag/MCM-41 showed superior activity to both Ag/TS-1 and Ag/Al₂O₃. A calcined Ag/MCM-41 was found to be an efficient catalyst for the cyclohexane oxidation into cyclohexanol and cyclohexanone using oxygen as oxidant.     

Improvement of thermal stability of NO oxidation Pt/Al2O3 catalyst by addition of Pd

The present work has been undertaken to tailor Pt/Al₂O₃ catalysts active for NO oxidation even after severe heat treatments in air. For this purpose, the addition of Pd has been attempted, which is less active for this reaction but can effectively suppress thermal sintering of the active metal Pt. Various Pd-modified Pt/Al₂O₃ catalysts were prepared, subjected to heat treatments in air at 800 and 830 °C, and then applied for NO oxidation at 300 °C. The total NO oxidation activity was shown to be significantly enhanced by the addition of Pd, depending on the amount of Pd added. The Pd-modified catalysts are active even after the severe heat treatment at 830 °C for a long time of 60 h. The optimized Pd-modified Pt/Al₂O₃ catalyst can show a maximum activity limited by chemical equilibrium under the conditions used. The bulk structures of supported noble metal particles were examined by XRD and their surface properties by CO chemisorption and EDX-TEM. From these characterization results as well as the reaction ones, the size of individual metal particles, the chemical composition of their surfaces, and the overall TOF value were determined for discussing possible reasons for the improvement of the thermal stability and the enhanced catalytic activity of Pt/Al₂O₃ catalysts by the Pd addition. The Pd-modified Pt/Al₂O₃ catalysts should be a promising one for NO oxidation of practical interest.     

Catalytic oxidation for carbon-black simulating diesel particulate matter over promoted Pt/Al2O3 catalysts

Catalytic oxidation activity of carbon-black (CB) simulating the soot of diesel particulate matters to CO₂ over 3Pt/Al₂O₃, 3Pt5Mn/Al₂O₃ and 3Pt/30Ba–Al₂O₃ catalysts is investigated with model gases of diesel emission. In case of the large amount of CB compared to the amount of catalyst (3/1, w/w) in the mixture sample, insufficient oxygen at the point of sudden increase in the amount of CO₂ is leaded to the partial oxidation using the lattice oxygen of the catalyst. And the peaks of CO₂ after the first peak were attributed to the regional combustion of the CB, which was not in contact with catalyst particles. The fresh 3Pt5Mn was estimated to the oxidation states on the catalyst surface by XPS. For used sample at 700 °C, the BEs of Pt 4d5 was revealed to metallic state Pt(0) (314.4 eV) in a predominant levels compared with Pt(II) (317.3 eV). While BEs of Mn 2p were similar to that obtained from the fresh 3Pt5Mn. It is suggested that Pt is in charge of the roles in CB-oxidation, using the lattice oxygen of the catalyst. Two-stage catalytic system with the strategies of promoting the soot oxidation and NO_x reduction, simultaneously, were composed of the CB oxidation catalyst and the diesel oxidation catalyst. The catalytic oxidation of CB was accelerated by activated oxidants and exothermic reaction resulted from the diesel oxidation catalyst, which lies in upstream of two-stage. But the system with the CB oxidation catalyst sited in the upstream showed the initiation of CB oxidation at a lower temperature than the other case. Two-stage catalytic system composed of 3Pt5Mn with CB in the upstream and DOC in the downstream showed high oxidation activity with 95% consumption rate of CB to the total loaded CB in the range of 100–500 °C during the TPR process.     

Catalytic reduction of N2O with CH4 and C3H6 over Ag–Rh/Al2O3 bimetallic catalyst in the presence of oxygen

A study of nitrous oxide (N₂O) reduction with methane (CH₄) and propene (C₃H₆) in the presence of oxygen (5%) over Ag/Al₂O₃, Rh/Al₂O₃ and Ag–Rh/Al₂O₃ catalysts, with Ag and Rh loadings of 5 wt% and 0.05 wt% respectively, has been performed. From the results, it was observed that the Ag–Rh bimetallic catalyst was the most active for both nitrous oxide removal (more than 95%) and hydrocarbon oxidation. This high activity seems to be connected with a synergistic effect between Ag and Rh. The findings from X‐ray diffraction and X‐ray photoelectron spectroscopy studies showed also, that there were no strong interactions (eg alloying) between Ag and Rh. Copyright © 2005 Society of Chemical Industry     

Dehydrogenation of Propane on Pt or PtSn Catalysts with Al2O3 or SBA-15 Support

Dehydrogenation of propane on Pt or PtSn catalyst over Al₂O₃ or SBA-15 support was investigated. The catalysts were characterized by CO-pulse chemisorption, thermogravimetry, temperature-programmed-reduction of H₂, and diffuse reflectance infrared Fourier transform spectroscopy of absorbed CO. The results show that the platinum species is in oxidation state in the catalyst on Al₂O₃ support, so the catalyst must be reduced in H₂ before dehydrogenation reaction. Addition of Sn improves the Pt dispersion, but the catalyst deactivates rapidly because of the coke formation. The interaction of Pt and Al₂O₃ is strong. On SBA-15 support, the platinum species is completely reduced to Pt⁰ in the calcination process, so the reduction is not needed. Addition of Sn improves the activity and selectivity of the catalyst. The interaction of Pt and SBA-15 is weak, so it is easy for Pt particles to sinter.     

Role of organic nitro compounds in selective reduction of NOx with ethanol over different supported silver catalysts

The adsorption of organic nitro compounds such as nitromethane and nitroethane on different supported silver catalysts (Ag/Al₂O₃, Ag/TiO₂, Ag/SiO₂) has been studied using infrared spectroscopy. The adsorbed NCO species formation was strongly influenced by the catalyst support and therefore clearly detected on Ag/Al₂O₃ and Ag/TiO₂ catalysts by thermal decomposition of nitromethane and nitroethane at temperatures higher than 150°C. With the Ag/SiO₂ catalyst, very little NCO formation was observed at 350°C. On the other hand, the catalyst support was found to affect the N₂ formation in the selective reduction of NO_x on supported silver catalysts. On the basis of these findings, the role of adsorbed nitromethane, nitroethane and isocyanate species in the selective reduction of NO_x is discussed with respect to the catalyst support effect and the catalytic activity. This revised version was published online in July 2006 with corrections to the Cover Date.     

The catalytic oxidation of ammonia: influence of water and sulfur on selectivity to nitrogen over promoted copper oxide/alumina catalysts

The CuO/Al₂O₃ system is active for ammonia oxidation to nitrogen and water. The principal by-products are nitrous oxide and nitric oxide. Nitrous oxide levels increase with the addition of various metal oxides to the basic copper oxide/alumina system. Addition of sulfur dioxide to the reaction stream sharply reduces the level of ammonia conversion, but has a beneficial effect on selectivity to nitrogen. Added water vapour has a lesser effect on activity but is equally beneficial in terms of selectivity to nitrogen. The CuO/Al₂O₃ is also active for the selective catalytic reduction of nitric oxide by ammonia, but this reaction is not effected by sulfur dioxide addition. A mechanism for ammonia oxidation to nitrogen is proposed wherein part of the ammonia fed to the catalyst is converted into nitric oxide. A pool of monoatomic surface nitrogen species of varying oxidation states is established. N₂ or N₂O are formed depending upon the average oxidation state of this pool. An abundance of labile lattice oxygen species on the catalyst surface leads to overoxidation and to N₂O formation. On the other hand, reduced lability of surface lattice oxygen species favours a lower average oxidation state for the monoatomic surface nitrogen pool and leads to N₂ formation.     

Complete Oxidation of Methane at Low Temperature over Pt Catalysts Supported on High Surface Area SnO2

The catalytic activity of Pt catalysts supported on high surface area tin(IV) oxide in the complete oxidation of CH₄ traces under lean conditions at low temperature was studied in the absence and in the presence of water (10 vol.%) or H₂S (100 vol.ppm). Their catalytic properties were compared to those of Pd/Al₂O₃ and Pt/Al₂O₃. In the absence of H₂S in the feed, Pt/SnO₂appears as a very promising catalyst for CH₄ oxidation, being even significantly more active under wet conditions than the best reference catalyst, Pd/Al₂O₃. Catalysts steamed-aged at 873 K were also studied in order to simulate long term ageing in real lean-burn NGV exhaust conditions. To this respect, Pt/SnO₂ is slightly less resistant than Pd/Al₂O₃. In the presence of H₂S, Pt/SnO₂catalysts are rapidly and almost completely poisoned, comparably to Pd/Al₂O₃and the catalytic activity is hardly restored upon oxidising treatment below 773 K. A synergetic effect between Pt and specific surface SnO₂sites active in CH₄oxidation is proposed to explain the superior catalytic behaviour of Pt/SnO₂.