Non-thermal plasma-assisted NOx reduction over alkali and alkaline earth ion exchanged Y, FAU zeolites

The catalytic activities of a series of alkali and alkaline earth cation exchanged Y, FAU zeolites were investigated in the non-thermal plasma-assisted NOx reduction reaction using a simulated diesel engine exhaust gas mixture. The catalytic activity of the Y, FAU zeolite showed significant variations with both the nature of the charge compensating cation, and the method of catalyst preparation. Our results show that conventional multiple solution ion exchange is insufficient to prepare the most active catalyst for the given cationic form. The highest NOx conversion level was achieved over a Ba-Y, FAU which was prepared by a multiple ion exchange method, in which each solution ion exchange step was followed by a high temperature calcination. A systematic change in the catalytic activity was observed as a function of the charge density around the charge compensating cation. For both catalyst series (alkali and alkaline earth ion exchanged Y, FAU), the specific activity decreased with increasing electrostatic field around the charge compensating cation. The large difference in the NOx reduction activity at a given e/r ratio. however, may suggest different reaction mechanisms for the two sets of catalysts. Indeed, there is a noticeable difference in the product distribution (selectivity) for the alkali and alkaline earth series of catalysts. Our results also reveal that extreme care must be taken when catalytic activities are compared for seemingly similar materials. We found that two base zeolite materials with identical Si/Al ratios, obtained from the same manufacturer but from different synthesis batches show significantly different catalytic behavior. (C) 2003 Published by Elsevier B.Vclose283

Nonthermal plasma-assisted catalytic NOx reduction over Ba-Y,FAU: the effect of catalyst preparation

The effects of catalyst preparation on the NOx reduction activity of a series of Ba-Y,FAU zeolites were investigated using a simulated exhaust gas mixture. The introduction of Ba2+ ions into Na-Y,FAU results in a large increase in their nonthermal plasma-assisted NOx reduction activity. The NOx reduction activities of Ba-Y,FAU catalysts were found to increase with increasing Ba2+ concentration in the aqueous ion-exchange solutions, which translated into increased Ba2+/Na+ ratios in the resulting materials. Consecutive ion-exchange procedures at a given Ba2+ concentration in the aqueous solution, however, did not improve the NOx reduction activities of Ba-Y,FAU catalysts; i.e., the activity of the four times ion-exchanged material was the same as that of the one that was ion-exchanged only once. The reaction profiles for all of these Ba-Y,FAU catalysts were the same. In contrast, a significant increase in NOx reduction activity was observed when a 773 K calcination step was implemented after each solution ion exchange. The reaction profile was also altered as a result of the ion-exchange/calci nation cycles. Calcination that followed each ion-exchange step seems to further increase the Ba2+/Na+ ratio in the zeolite, and in turn increases the NOx reduction activities of the catalysts prepared this way. Key differences in Na- and Ba-Y,FAU catalysts were found in NO2 adsorption and TPD experiments. The amount of chemisorbed NO2 is about twice as high in Ba-Y,FAU than in Na-Y,FAU, and Ba-Y,FAU holds NOx much stronger than Na-Y,FAU. Published by Elsevier Incclose273

The adsorption of NO2 and the NO+O-2 reaction on Na-Y,FAU: an in situ FTIR investigation

The adsorption of NO and NO2 and the reaction between NO and O(2)were investigated on a Na-Y,FAU zeolite. The interaction between NO and Na-Y is weak and no IR absorption feature is seen upon room temperature adsorption. On the other hand, several NO, species were identified in the adsorption of NOx bonded to Lewis acidic (NO3-, NO2-) and basic sites (NO+ and [NO+][NO2] and [NO+][N2O4]). In the NO + O-2 reaction, N2O3 was formed and adsorbed N2O3 was observed in addition to the species detected upon NO, adsorption. A series of experiments were conducted to unambiguously assign the IR features in the 2000-2120 cm(-1) spectral range. Through reaction and isotopic substitution ((NO)-N-15 and O-18(2)) experiments, these bands were assigned to NO+ adsorbed onto framework O- sites as charge compensating cationsclose525

Adsorption, coadsorption, and reaction of acetaldehyde and NO2 on Na-Y,FAU: An in situ FTIR investigation

The adsorption of acetaldehyde and its coadsorption and reaction with NO, were investigated on a Na-Y,FAU zeolite using in situ FTIR spectroscopy. Acetaldehyde adsorbs strongly over Na-Y and desorbs molecularly at around 400 K with very limited extent of condensation or polymerization. Reaction between CH3CHO and NO, takes place in coadsorption experiments even at 300 K. In the initial step, acetaldehyde is oxidized to acetic acid accompanied by the formation of NO, which can be observed as N2O3 formed via a further reaction between NO and NO2. The key intermediates in the overall NOx reduction in this process are nitromethane and, possibly, nitrosomethane, which form in the next step. Their decomposition and further reaction with adsorbed NOx species lead to the formation of HCN, HNCO, N2O, CO2, and organic nitrile species identified by their characteristic IR vibrational signatures. At 473 K, the reaction between adsorbed CH3CHO and NO, is very fast. The results seem to suggest a mechanism in which N-N bond formation takes place among ionic nitrogen containing species (NO+ and CN- or NCO-). No evidence has been found to suggest the participation of NHx+NOy- type species in the N-N bond formation under the experimental conditions of this study, although their role in the overall N-2 formation process cannot be ruled out under realistic catalytic conditions.close1

Characterization of Fe2+ ions in Fe,H/SSZ-13 zeolites: FTIR spectroscopy of CO and NO probe molecules

The IR spectra of adsorbed CO and NO probe molecules were used to characterize the coordination chemistry of Fe2+ ions in solution ion exchanged Fe,H/SSZ-13 zeolites. The effects of Fe ion exchange levels, as well as the sample pre-treatment conditions, on the adsorption of these probe molecules were investigated. The ion exchange levels (in the range of the study) did not affect significantly the IR spectra of either probe molecule, and the IR features and their intensity ratios were very similar. Experiments with both probe molecules substantiated the presence of two distinct types of Fe2+ ions in cationic positions. We assign these two Fe2+ ions to two distinct cationic positions: Fe2+ in 6R and 8R positions. NO initially adsorbs preferentially onto Fe2+ sites in the 6R position, and then populates sites in the 8R. Fe2+ ions in the 8R positions require the interaction of more than one NO molecule to move them out from their adsorbate-free cationic positions. As soon as they move from their stable positions, they are able to bind to multiple NO molecules, and form mostly tri-nitrosyls. These tri-nitrosyls, however, are only stable in the presence of gas phase NO; under dynamic vacuum they lose one of the NO molecules from their coordination sphere and form stable di-nitrosyls. The adsorption of CO is much weaker on Fe2+ sites than that of NO, and requires cryogenic sample temperatures to initiate CO adsorption. Under the conditions applied in this study, only mono-carbonyl formation was observed. Reduction in H2 at 773 K increased the number of Fe2+ adsorption sites, primarily in the 8R locations. Oxidation by N2O, on the other hand, selectively reduced the adsorption of both CO and NO on the Fe2+ sites in 8R positions. Adsorbed oxygen left behind from the decomposition of N2O at 573 K readily reacted with CO to produce CO2 even at 150 K.clos

Line narrowing in H-1 MAS spectrum of mesoporous silica by removing adsorbed H2O using N-2

The peaks for silanol protons in the high-resolution H-1 NMR spectrum obtained on mesoporous silica materials may be broadened and shifted downfield by hydrogen bonding with adsorbed water molecules. Overlapping of the resonance for hydrogenbonded silanol with the corresponding broad peak due to hydrogen-bonded water may further complicate the spectrum. These complications hamper a quantitative analysis of the spectra for these and similar materials. It is demonstrated in this paper that adsorbed water can be removed by exposing the sample to dry N-2 during magic angle spinning. This results in significant line narrowing for the silanol protons in the H-1 MAS spectrum. The enhanced spectral resolution makes it possible to quantify the various hydroxyl groups in a complex metal-oxide catalyst. Results obtained on tungsten oxide supported on SBA- 15 mesoporous silica materials are reported. Additionally, the proton chemical shift of tungsten hydroxyl is identified for the first time. (C) 2004 Elsevier Inc. All rights reservedclose252

Promotional Effect of CO2 on Desulfation Processes for Pre-Sulfated Pt-BaO/Al2O3 Lean NOx Trap Catalysts

A combination of H-2 TPRX, TR-XRD and XPS analysis has been used to investigate the effects of CO2 on the desulfation of pre-sulfated Pt-BaO/Al2O3 samples. The results demonstrate that the presence of CO2 promotes the removal of sulfur species, especially at temperatures below 500 degrees C, with a corresponding suppression of BaS formation, thus resulting in a lower amount of residual sulfur on the sample after desulfation.close

Oxidation of Ethanol to Acetaldehyde over Na-promoted vanadium oxide catalysts

Sodium-promoted vanadium oxide catalysts supported on MCM-41 and TiO2 (anatase) were investigated for the partial oxidation of ethanol to acetaldehyde. The catalysts were prepared by incipient wetness impregnation with a vanadium oxide content of 6 wt.%. The experimental characterization was performed by X-ray diffraction (XRD), N-2 adsorption, temperature-programmed reduction (TPR), and diffuse reflectance UV-vis. Temperature-programmed oxidation (TPO) was also used to identify carbon deposits on the spent catalysts. The presence of sodium plays a strong role in the dispersion and reducibility of the vanadium species as detected by TPR analysis and optical absorption spectroscopy. While sodium addition increases the dispersion of the VOx species, its presence also decreases their reducibility. Additionally, TPO of the spent catalysts revealed that an increase in the Na loading decreases the carbon deposition during reaction. In the case of the catalysts supported on MCM-41, these modifications were mirrored by a change in the activity and selectivity to acetaldehyde. Additionally, on the VOx/TiO2 catalysts the catalytic activity decreased with increasing sodium content in the catalyst. A model in which sodium affects dispersion, reducibility and also acidity of the supported-vanadia species is proposed to explain all these observations.close1