| 1. |
- Ahmed, Aseel Bala, et al.
(författare)
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Microwave-enhanced degradation of phenol over Ni-loaded ZnO nanorods catalyst
- 2014
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Ingår i: Applied Catalysis B. - : Elsevier. - 0926-3373 .- 1873-3883. ; 156-157, s. 456-465
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Tidskriftsartikel (refereegranskat)abstract
- Nickel was loaded onto hydrothermally-grown ZnO nanorods on cordierite substrates and tested as catalysts in microwave-enhanced degradation of phenol from its aqueous solution (100 ppm) at 70 degrees C. Effects of metal loadings (1, 10 and 20 mM impregnation solutions) on the degradation of phenol in aqueous solution was investigated. The catalyst performances were monitored based on phenol degradation, product distributions and carbon dioxide (CO2) evolutions. Based on the type of the catalysts, two different mechanistic pathways for the decomposition were observed-through catechol and/or hydroquinone as intermediates. It was found that the 10mM nickel loaded sample catalyzed the degradation through one pathway with hydroquinone as the benzenediol intermediate formed, while the 20 mM nickel impregnated sample catalyzed the reaction through two pathways, producing catechol as well as hydroquinone by products. These differences in reaction pathways were attributed to the variation in the composition of the nickel compounds and surface structures between the two catalysts. Furthermore, the effect of hydrogen peroxide (H2O2) as an oxidant was explored. It was found that although addition of H2O2 led to an increase in the degree of phenol degradation, it also led to enhanced catalyst leaching. There was also an increase in CO2 evolution due to the addition of H2O2. It was observed that 20 mM nickel-loaded sample without the addition of H2O2 exhibited optimum performance in terms of phenol degradation and CO2 evolution with no drawback of catalyst leaching. Catalytic microwave enhanced degradation is an effective means for the removal of dissolved organic compounds from wastewater.
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| 2. |
- Andonova, Stanislava, 1977, et al.
(författare)
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Chemical deactivation by phosphorous under lean hydrothermal conditions over Cu/BEA NH3-SCR catalysts
- 2014
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Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 147, s. 251-263
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Tidskriftsartikel (refereegranskat)abstract
- To obtain a better understanding of the deactivation of SCR catalysts that may be encountered due to the presence of P-containing impurities in diesel exhausts, the effects induced by P over Cu/BEA NH3-SCR catalysts were studied as functions of the temperature of poisoning and P concentration in the feed. Cu/BEA catalysts with different Cu loadings (4 and 1.3 wt% Cu) were exposed to P by controlled evaporation of H3PO4 in the presence of 8% O2 and 5% H2O at 573 and 773 K. The reaction studies were performed by NH3-storage/TPD, NH3/NO oxidation and standard NH3-SCR. In addition, a combination of several characterisation techniques (ICP–AES, BET surface area, pore size distribution, H2-TPR and XPS) was applied to provide useful information regarding the mechanism of P deactivation. Pore condensation of H3PO4 in combination with pore blocking was observed. However, the measured overall deactivation was found to occur mostly by chemical deactivation reducing the number of the active Cu species and hence deteriorating the redox properties of the Cu/BEA catalysts. The process of P accumulation on the surface preferentially occurs on the “over exchanged” Cu active sites with the formation of phosphate species. This is likely the reason for the more severe deactivation of the 4% Cu/BEA compared to 1.3% Cu/BEA. Further, the higher NOx reduction performance at 773 K of the P-poisoned Cu/BEA catalysts was found to originate from the lower selectivity towards NH3 oxidation, which occurs predominately on the “over-exchanged” sites.
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| 3. |
- Andonova, Stanislava, 1977, et al.
(författare)
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Mechanistic investigations of the promoting role of Rh on the NSR performance of NOx storage BaO-based catalysts
- 2013
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Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 132, s. 266-281
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Tidskriftsartikel (refereegranskat)abstract
- To determine the promoting effect of Rh on the overall NOx storage and reduction (NSR) performance, the studies in the current work were directed toward investigating the storage and release ability over Rh NOx storage BaO-based catalysts compared to Pt. In terms of the metal surface dispersion and the ability of the noble metals to release oxygen at lower temperatures, the synthesized catalysts were characterized by means of dynamic CO chemisorption (RT) and N2O dissociation (RI - 773 K). The NOx storage capacity and the thermal stability of the NOx adsorbed species formed on the surface were analyzed via NOx storage tests and temperature programmed desorption (TPD) without and in the presence of CO2 and H2O. In addition, experiments with lean and rich cycling were conducted at 473,573 and 673 K. The results from the N2O dissociation experiments showed the superior ability of Rh/Al and Rh/Ba/Al catalysts compared to Pt toward O-2 release from the catalytic surface at lower temperatures. In this work, we show that the presence of Rh into the BaO/gamma-Al2O3 system has a considerable effect on the spill-over process of NOx to the precious metal, controlling the subsequent desorption of NOx to occur at lower temperatures in comparison with that of the Pt catalysts. It is suggested a mechanism of NOx desorption where the lower temperature of O-2 release from the surface of Rh catalysts could leave a significant number of noble metal sites accessible for adsorption. Thus this could facilitate the rate of spill-over of NOx from the storage site (the surface sites on gamma-Al2O3 and those on BaO) to the noble metal and their desorption at lower temperatures. The limited NOx storage ability of the Rh-based BaO/gamma-Al2O3 catalysts under lean-burn conditions was found to originate from both low NO oxidation activity and NOx reduction activity, while the main limiting factor for the low NSR performance of the Pt-based catalysts was the limited regeneration ability during rich period.
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| 4. |
- Auvray, Xavier, 1986, et al.
(författare)
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Local ammonia storage and ammonia inhibition in a monolithic copper-beta zeolite SCR catalyst
- 2012
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Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 126, s. 144-152
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Tidskriftsartikel (refereegranskat)abstract
- Selective catalytic reduction of NO with NH3 was studied on a Cu-beta zeolite catalyst, with specific focus on the distributed NH3 capacity utilization and inhibition. In addition, several other relevant catalyst parameter distributions were quantified including the SCR zone, or catalyst region where SCR occurs, and NO and NH3 oxidation. We show that the full NH3 capacity (100% coverage) is used within the SCR zone for a range of temperatures. By corollary, unused NH3 capacity exists downstream of the SCR zone. Consequently, the unused capacity relative to the total capacity is indicative of the portion of the catalyst unused for SCR. Dynamic NH3 inhibition distributions, which create local transient conversion inflections, are measured. Dynamic inhibition is observed where the gas phase NH3 and NO concentrations are high, driving rapid NH3 coverage buildup and SCR. Accordingly, we observe dynamic inhibition at low temperatures and in hydrothermally aged states, but predict its existence very near the catalyst front in higher conversion conditions where we did not specifically monitor its impact. While this paper addresses some general distributed SCR performance parameters including Oxidation and SCR zone, our major new contributions are associated with the NH3 capacity saturation within the SCR zone and dynamic inhibition distributions and the associated observations. These new insights are relevant to developing accurate models, designs and control strategies for automotive SCR catalyst applications.
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| 5. |
- Auvray, Xavier, 1986, et al.
(författare)
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The effect gas composition during thermal aging on the dispersion and NO oxidation activity over Pt/Al2O3 catalysts
- 2013
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Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 129, s. 517-527
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Tidskriftsartikel (refereegranskat)abstract
- The aging of a model 1 wt.% Pt/Al2O3 catalyst was performed stepwise under different reactive atmosphere to study the evolution of metal dispersion and NO oxidation activity. After each aging step the dispersion was evaluated by CO chemisorptions and the activity of the catalyst for NO oxidation was measured using 500 ppm NO and 8%O-2 diluted in Ar. After a degreening step at 500 degrees C, aging was performed at 600, 700,800 and 900 degrees C. Five wash-coated cordierite monoliths were aged in Ar, 10% O-2, 1% H-2 30 ppm SO2 and 30 ppm SO2 + 10% O-2, respectively. The general trend showed a linear decrease in dispersion when increasing the aging temperature for the lower aging temperatures and for the highest ones the dispersion levels off. When the platinum dispersion decreased the NO oxidation activity increased, due to that the reaction is structure sensitive. H-2 seemed to hinder sintering at low aging temperature. Interestingly, after aging in 10% oxygen at 600 degrees C the NO oxidation activity was significantly higher compared to the Ar aged sample, although the dispersions were similar. Aging in oxygen at higher temperatures resulted in a decrease of dispersion and a slightly decreasing NO oxidation activity. Moreover lower dispersion limit was reached with oxygen aging. Aging in SO2 provoked a severe dispersion drop at low aging temperature meanwhile the activity increased only moderately. However, activity kept increasing with further treatments at higher temperature. The combination of O-2 and SO2 enabled to decrease rapidly the dispersion and to greatly enhance the catalytic NO oxidation activity after the first aging step at only 600 degrees C. The best overall conversion was obtained for the catalyst treated with this mixture after aging at 800 degrees C. A 22-h aging at 250 degrees C in a mixture containing 500 ppm NO, 10% O-2 and 30 ppm SO2 led to a significant decrease of Pt dispersion, which shows the ability of SO2 to promote platinum sintering already 250 degrees C. The low temperature sintering was confirmed with STEM measurements. Several larger particles were observed, but also many small particles remained. Thus the SO2 + O-2 induced low temperature sintering results in a large variation of particle sizes. This treatment resulted in an increase of the maximum NO conversion (after reduction of the sample) from 45% to 76%. The different aging experiments show that it is beneficial to add SO2 during aging and the reason is the increased particle size, but also a clear chemical effect was observed.
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| 6. |
- Baia, L., et al.
(författare)
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TiO2/WO3/Au nanoarchitectures' photocatalytic activity "from degradation intermediates to catalysts' structural peculiarities" Part II: Aerogel based composites - fine details by spectroscopic means
- 2014
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Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373. ; 148, s. 589-600
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Tidskriftsartikel (refereegranskat)abstract
- The "build-up" methodology of a composite photocatalyst is a critical issue regarding the showed photocatalytic performance, including the formation of intermediates. To investigate this issue TiO2/WO3/Au aerogel composites were obtained by sal-gel method and subsequent photoreduction (Au) with UV or visible light. The obtained composites' photocatalytic activity and intermediate formation profiles were evaluated using phenol as a model pollutant. XPS/UPS, XAFS and DRS were used to uncover local coordination, surface chemistry (of the different types of atoms (Ti, W, O and Au) and the band-structure (band-gap, possible electron transitions) of the obtained nanomaterials. The intermediates' evolution profile and structural peculiarities were successfully correlated and it was shown that each minor structural (bulk or surface) change has a significant impact on the photocatalytic activity and intermediate formation dynamics. (c) 2013 Elsevier B.V. All rights reserved.
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| 7. |
- Bajnoczi, Eva G., et al.
(författare)
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The influence of the local structure of Fe(III) on the photocatalytic activity of doped TiO2 photocatalysts-An EXAFS, XPS and Mossbauer spectroscopic study
- 2011
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Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373. ; 103:1-2, s. 232-239
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Tidskriftsartikel (refereegranskat)abstract
- Fe(III)-doped TiO2 based heterogeneous photocatalysts were prepared by the sol-gel technique (S samples) or flame hydrolysis (F samples). In photocatalytic phenol decomposition, the undoped F-sample performed much better, than the undoped S one. However, for the S samples, photocatalytic activity first increased with the increasing Fe(III) concentration, and then passed through a maximum, while Fe(III)-doping in F samples significantly decreased it, even at the smallest dopant level. Since the same dopant caused opposite photocatalytic effects in the two series, their structure was systematically compared to identify the underlying chemical and/or physical reasons. The photocatalysts were first characterized by AAS, DRS, XRD and TEM methods and it has been shown that the differences in the photocatalytic activity cannot be explained by the minor variations in the bulk structural properties of TiO2. Mossbauer and XP spectroscopic measurements performed on representative samples qualitatively proved that the local structure of Fe(III) is different in the two series. To quantify these effects, Fe-K edge X-ray absorption measurements were performed. From the pre-edge and XANES region it was learnt that Fe(III) was present in a distorted octahedral environment in both series, however, the extent of distortion is much more significant within the S than within the F one. Information obtained from the EXAFS region indicated that the structure of Fe2O3 was much more ordered in the F-series then in the S one and vacancies were more abundant in the S than in the F series. Moreover, the geometry around Fe(III) systematically varied within the S-series, which could explain, why photocatalytic activity passed through a maximum with the increasing Fe(III) concentration in these samples. (C) 2011 Elsevier B.V. All rights reserved.
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| 8. |
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| 9. |
- Bounechada, Djamela, 1984, et al.
(författare)
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Effect of periodic lean/rich switch on methane conversion over a Pd/Rh-based three way catalyst in the exhausts of natural gas vehicles
- 2012
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Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 119-120, s. 91-99
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Tidskriftsartikel (refereegranskat)abstract
- The behavior of a commercial Ce–Zr promoted Pd-Rh/Al2O3 catalyst for the abatement of methane from the exhausts of natural gas vehicles (NGVs) is studied in presence of large amounts of water under both stationary conditions and by periodically switching from lean to rich feed. Under stationary conditions with both stoichiometric (λ = 1.00) and lean (λ = 1.02) feed catalyst deactivation is observed after prolonged exposure to the reaction mixture. Periodic rich pulses in a constant lean feed gas result in the stabilization of catalytic performances.A higher methane conversion than those obtained with stoichiometric and lean feed mixtures is observed under rich conditions, during an experiment carried out by performing lean pulses (λ = 1.02) in a constant rich feed gas (λ = 0.98). The analysis of reactants conversion and products distribution suggests that different chemistries are involved under lean and rich conditions. Only reactions of complete oxidation of H2, CO, CH4 and NO occur under excess of oxygen, whereas under rich conditions NO reduction, CH4 steam reforming and water gas shift also occur.The effect of symmetric oscillation of the exhausts composition around stoichiometry is also addressed by periodically switching from slightly rich to slightly lean composition with different oscillation amplitudes (Δλ = ±0.01, ±0.02 and ±0.03). Higher and more stable methane conversion performances are obtained than those observed under constant λ operations. The presence of a more active PdO/Pd0 state is suggested to explain the enhancement of catalytic performances.
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| 10. |
- Carlsson, Per-Anders, 1972, et al.
(författare)
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Low-temperature oxidation of carbon monoxide and methane over alumina and ceria supported platinum catalysts
- 2011
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Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 101:3-4, s. 669-675
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Tidskriftsartikel (refereegranskat)abstract
- The ignition and extinction processes for total oxidation of CO, CH4 and CO + CH4 mixture in oxygen excess over Pt/Al2O3 and Pt/CeO2 catalysts with the platinum phase distributed either homogeneously or het- erogeneously (i.e., locally high platinum concentration) in the support have been studied by temperature programmed oxidation experiments. Following the preparation methods by Arnby et al. [42], the samples have the same Pt load and dispersion. Generally the catalytic activity follows the order: Pt/CeO2 (hetero- geneous) > Pt/CeO2 (homogeneous) > Pt/Al2 O3 (heterogeneous) > Pt/Al2 O3 (homogeneous) as indicated by lower ignition and/or extinction temperatures. For Pt/Al2O3, the addition of NO2 to the reactant stream increases the rate of oxidation of CO in the pre-ignition regime although the light-off temperature T50 is shifted towards higher temperatures (except for low CO concentrations). In the case of the Pt/CeO2, the CO conversion generally decreases. For CH4 oxidation in the presence of NO2, the conversion increases for Pt/Al2O3 and decreases for Pt/CeO2. The addition of CO2 in the reactant stream has minor influence on CO oxidation over Pt/Al2O3 while for Pt/CeO2, T50 is shifted towards higher temperatures. For the simultaneous oxidation of CO and CH4, a reverse hysteresis for methane oxidation is observed, i.e., the extinction process occurs at higher temperature than the corresponding ignition process. The improved activity for CO oxidation over samples with heterogeneous Pt distribution is likely due to less tendency towards CO self-poisoning through the development of steeper concentration gradients in the Pt con- taining regions in the porous support material. The significant increase of activity for both reactions over ceria-supported Pt is here assigned to highly active sites at the platinum–ceria boundary but also, to some extent, the oxygen storage and release function and dynamics of the transport of oxygen in the Pt/CeO2 system.
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