1. |
- Amberntsson, Annika, 1974, et al.
(author)
-
Sulphur deactivation of NOx storage catalysts; Influence exposure conditions and noble metal
- 2003
-
In: Journal of Catalysis. - 0021-9517 .- 1090-2694. ; 217:2, s. 253-263
-
Journal article (peer-reviewed)abstract
- In the present study, barium-based NOx, storage catalysts containing platinum, rhodium, or both noble metals were investigated. The influence of SO2 exposure conditions on the performance of NOx storage catalysts was studied using flow reactor measurements, FTIR, and XPS where the samples were exposed to lean and/or rich SO2-containing gas mixtures, simulating the conditions in a mixed lean application. The main results show that all samples are sensitive to sulfur and that deactivation is faster when SO2 is present in the feed under rich conditions than under lean or continuous SO2 exposure. It was also found that SO2 affects the performance of noble metals strongly and that noble metal deactivation most likely occurs during the rich period of a NOx storage cycle. Additionally, the influence of the noble metals present in the catalysts was investigated with respect to sulfur sensitivity and it was found that a combination of platinum and rhodium seems to be preferable for retaining high performance (high NO oxidation and reduction activity) of the catalyst under SO2 exposure and subsequent regeneration.
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2. |
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3. |
- Andersson, Arne, et al.
(author)
-
Activities of V-Ti-O Catalysts in the Ammoxidation of 3-Picoline
- 1980
-
In: Journal of Catalysis. - : Elsevier BV. - 1090-2694 .- 0021-9517. ; 65:1, s. 9-15
-
Journal article (peer-reviewed)abstract
- Ammoxidation of 3-picoline was studied on reduced V-Ti-O catalysts with V6O13 as the major vanadium oxide. The results showed that the initial activity as a function of the TiO2 content reached a maximum at 50–60 mole% TiO2. It is proposed that there is maximum contact between the vanadium and titanium phases at this composition, which results in a weakening of the (VO)3+ surface bond. The selectivity of formation of nicotinonitrile exhibited a maximum of 83% at 10 mole% TiO2 and minima of 73 and 75% at 0 and 30 mole% TiO2, respectively. At higher TiO2 concentrations the selectivity increased continuously to 83% at 90 mole% TiO2. The variation of the selectivity of formation of nicotinonitrile depends on the View the MathML source ratio in the TiO2 phase. It was also found that the conversion and yields varied with the reaction time, which could be explained by the fact that reduced vanadium oxides were oxidized to V2O5 during the ammoxidation process. This oxidation leads to the formation of active and highly selective boundary surfaces between the TiO2-promoted vanadium oxides V6O13 and V2O5.
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4. |
- Andersson, Arne
(author)
-
Activities of V-Ti-O Catalysts in the Ammoxidation of 3-Picoline. II. Acid-Base Properties and Infrared Spectra
- 1982
-
In: Journal of Catalysis. - : Elsevier BV. - 1090-2694 .- 0021-9517. ; 76:1, s. 144-156
-
Journal article (peer-reviewed)abstract
- The acidity and basicity of prereduced V-Ti-O catalysts were measured by adsorption of NH3 and CO2. It was found that a high activity in the ammoxidation of 3-picoline corresponds to a relatively small amount of acidic sites. A catalyst selective in the formation of nicotinonitrile requires high concentrations of both acidic and basic sites. These results are discussed, and can be correlated if the acidity is considered to be a measure of the hydroxyl group concentration, and the basicity a measure of oxygen vacancies in the vicinity of VO groups. Also, the infrared spectra of the prereduced catalysts were recorded. A new band was found at 995 cm−1. The same band appeared in a homogeneous mixture of V2O5 and V6O13. It is proposed that this band can be assigned to VO bonds in a reduced V2O5phase with a number of disordered vacancies in the lattice, or a nonstoichiometric V6O13 phase. The shift in frequency relative to that of (VO)3+ shows that these bonds are weaker, and probably also more active. The effect of TiO2 is to increase the amount of V4+ in the vanadium oxide lattice. This is caused by dissolution of Ti4+ in the V2O5 melt during the catalyst preparation.
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5. |
- Andersson, Arne, et al.
(author)
-
Catalytic Anisotropy of MoO3 in the Oxidative Ammonolysis of Toluene
- 1988
-
In: Journal of Catalysis. - : Elsevier BV. - 1090-2694 .- 0021-9517. ; 114:2, s. 332-346
-
Journal article (peer-reviewed)abstract
- The oxidative ammonolysis of toluene, i.e., ammoxidation without the presence of molecular oxygen, was studied over a series of samples of MoO3 crystals. The specific surface areas of the various faces were determined from SEM micrographs. Correlations between activities and surface planes were found. For the formation of nitrile the specific activity decreased in the order {001} and {h01} > {100} > {010}. Also, for the formation of carbon oxides the terminations in the [001] direction were found to be especially active. These results are discussed in relation to surface structures and bond strength values of various oxygen species. It is concluded that the presence of both oxygen vacancies and nucleophilic oxygen species is a prerequisite for selective reaction to occur and that electrophilic oxygen species are the source for formation of carbon oxides. The characteristics of the various faces, as they emerge from the results on oxidative ammonolysis of toluene, seem to be of general significance for reactions occurring at the same types of active sites. They are shown to be applicable to results published in the literature on the oxidation of both propene and isobutene.
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6. |
- Andersson, Arne
(author)
-
Structural Dynamics of a V2O5/SnO2 Catalyst in the Ammoxidation of 3-Picoline
- 1981
-
In: Journal of Catalysis. - : Elsevier BV. - 1090-2694 .- 0021-9517. ; 69:2, s. 465-474
-
Journal article (peer-reviewed)abstract
- The influence of the reaction parameters on the conversion of 3-picoline and the yield of nicotinonitrile in the ammoxidation of 3-picoline on a View the MathML source catalyst was studied. The highest yield of nicotinonitrile was 82%. This was obtained at 400 °C, with a space velocity of 4000 h−1 and the following mole ratios: ammonia/3-picoline = 12, air/3-picoline = 210, and water vapour/3-picoline = 40. It was found that the composition of the surface of the catalyst depended strongly on the values of the reaction parameters. The dynamic structure of the catalyst surface was studied by determining the phases present by X-ray diffraction analysis and the average oxidation number of vanadium by titrimetric methods. These results were correlated, showing that catalysts with both V2O5 and V6O13 were especially active and selective. The composition of the SnO2 phase was analyzed and was found to be V0.03Sn0.97O2, and did not depend on the reaction parameters. Infrared investigations showed that the role of SnO2 was to weaken the short VO bonds. This can probably be seen as a consequence of incorporation of Sn4+ in the V2O5 lattice.
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7. |
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8. |
- Jansen, Susan, et al.
(author)
-
EPR Studies of the Coverage Effects in TiO2(B)-Supported Vanadia Catalysts for Toluene Ammoxidation
- 1992
-
In: Journal of Catalysis. - : Elsevier BV. - 1090-2694 .- 0021-9517. ; 138:1, s. 79-89
-
Journal article (peer-reviewed)abstract
- EPR spectroscopy has been employed to determine the nature of vanadium (+4) species in the TiO2(B)-supported vanadia catalysts. IR and EPR studies show that multiple vanadia species are observed as a function of loading. At coverages greater than 3.5 theoretical layers, the EPR signal is reminiscent of bulk V205. At lower coverages, a different magnetic species dominates the resonance and may be attributable to tetrahedral states of vanadium at the oxide interface. The spin concentration and (g) suggest two distinct vanadia phases as a function of coverage. In this study, we have also analyzed catalysts after use in ammoxidation of toluene. These samples show strikingly different features compared with the freshly prepared samples. Changes in both the g-anisotropy and hyperfine coupling were observed relative to the fresh catalysts. Though EPR is not surface sensitive, inference into the nature of the catalytic surface can be made by comparison of catalysts of different coverage and studies of monolayer samples. A study of the TiO2(B)-vanadia interface was made by comparing monolayer catalysts prepared from slightly different technique. Our measurements show strikingly different features in V4+ species as a function of preparation.
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9. |
- Wallin, Mikaela, 1975, et al.
(author)
-
Selective catalytic reduction of NOx with NH3 over H-ZSM-5: Influence of transient ammonia supply
- 2003
-
In: Journal of Catalysis. - 0021-9517 .- 1090-2694. ; 218:2, s. 354-364
-
Journal article (peer-reviewed)abstract
- The effect of ammonia supply on the selective catalytic reduction of NOX over zeolite H-ZSM-5 was investigated using step response experiments between 200 and 500 degrees C. For inlet NO:NO2 ratios > 1, the activity for NOX reduction transiently increased when NH3 was removed from the feed. For NO:NO2 ratios less than or equal to 1, the NOX reduction however decreased. By pulsing NH3 to the feed, the activity for NO reduction was enhanced up to five times compared to continuous supply of ammonia. For NO:NO2 ratios exceeding one, also the selectivity towards N2O formation was lower with transient ammonia supply. Temperature programmed reaction experiments with preadsorbed NH3 showed highest initial NOX reduction activity when ammonia had been adsorbed at 300 or 250 degrees C compared to 200 degrees C. A minimum in NO reduction was observed at 130 degrees C independent of the ammonia adsorption temperature. For NO:NO2 ratios > 1, the results strongly indicate that NO oxidation is the rate determining step in the ammonia selective catalytic reduction (NH3-SCR) reaction over H-ZSM-5.
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10. |
- Agrell, J., et al.
(author)
-
Production of hydrogen from methanol over Cu/ZnO catalysts promoted by ZrO2 and Al2O3
- 2003
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 219:2, s. 389-403
-
Journal article (peer-reviewed)abstract
- Production of H-2 from methanol by steam reforming, partial oxidation, or a combination thereof was studied over Cu/ZnO-based catalysts. The catalysts were characterized by a variety of techniques, including N2O chemisorption, X-ray photoelectron spectroscopy, X-ray diffraction, and temperature-programmed reduction/oxidation. The influence of feed composition, reaction temperature, and catalyst formulation on H-2 production rate, product distribution, and catalyst lifetime was investigated. Distinct differences between the processes were observed with respect to catalyst behavior. ZrO2-containing catalysts, especially Cu/ZnO/ZrO2/Al2O3, exhibit the best performance in the steam reforming reaction. During partial oxidation, however, a binary Cu/ZnO catalyst exhibits the lowest light-off temperature and the lowest level of CO by-product. The redox properties of the catalyst appear to play a key role in determining the pathway for H-2 production. In particular. the extent of methanol and/or H-2 combustion at differential O-2 conversion is strongly dependent on the ease of copper oxidation in the catalyst.
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11. |
- Janin, Emmanuelle, et al.
(author)
-
Adsorption and bonding of 2-butenal on Sn/Pt surface alloys
- 2003
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 215:2, s. 245-253
-
Journal article (peer-reviewed)abstract
- The adsorption of 2-butenal on the Pt(111), Pt(111)Sn-(2 x 2), and Pt(111)Sn-(root3 x root3)R30degrees surfaces has been studied by high-resolution photoelectron spectroscopy and DFT calculations. On Pt(111) 2-butenal adsorbs at a threefold hollow in a eta(3)(C,C,O) configuration. A similar geometry is observed for Pt(111)Sn-(2 x 2), although the oxygen coordinates to tin instead of platinum. As the surface coverage of tin increases, as for Pt(111)Sn-(root3 x root3)R30degrees, the most stable adsorption geometry changes to eta(1)(O), where the carbonyl oxygen once again coordinates to tin. The carbonyl bond thus retains an activated character as Pt/Sn alloys are formed. The alkene/surface interaction is, however, dramatically influenced at increasing surface concentrations of tin.
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12. |
- Paul, Jan, et al.
(author)
-
Cluster calculations of intermediate states in the catalyzed conversion of CH3OH to CH2O on a Cu(100) surface
- 1983
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 84:2, s. 288-296
-
Journal article (peer-reviewed)abstract
- The Hartree-Fock-Slater (HFS) calculations of a CH3OCu5 cluster symbolizing methoxide bound to a Cu(100) surface are presented. Adsorption geometry, molecular orbitals, and symmetry properties of the CH3O group in terms of one-electron energies, charge distributions, and partial densities of states (PDOS) are discussed. A comparison of calculated ionization energies and measured binding energies supports the occupation by methoxide of the hollow adsorption site. The electron distribution of the hydrogen atoms has been analyzed for different geometries and is shown to be a sensitive probe of the oxidation from methoxide (CH3O)/methyl (CH3) to formaldehyde (CH2O)/methylene (CH2).
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13. |
- Thevenin, P. O., et al.
(author)
-
Catalytic combustion of methane over cerium-doped palladium catalysts
- 2003
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 215:1, s. 78-86
-
Journal article (peer-reviewed)abstract
- Various Pd-supported catalysts have been prepared using three different types of alumina as support material: (a) gamma-alumina, (b) Ba-stabilized alumina, and
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14. |
- Wang, M., et al.
(author)
-
Ethylene oligomerization by salen-type zirconium complexes to low-carbon linear alpha-oletins
- 2003
-
In: Journal of Catalysis. - 0021-9517 .- 1090-2694. ; 220:2, s. 392-398
-
Journal article (peer-reviewed)abstract
- A series of zirconium salen-type complexes were prepared by using ZrCl4 and sodium salts of the tetradentate Schiff base ligands. In combination with Et2AlCl, they displayed moderate to high catalytic activities in ethylene oligomerization under a pressure of 1.0-1.8 MPa at 150 degreesC in toluene. The main products are C-4-C-10 olefins with good selectivity to linear alpha-olefins. The turnover frequency (TOF) of catalyst system Zr(salen)Cl-2 (.) THF/Et2AlCl, under a pressure of 1.8 MPa and in an Al/Zr molar ratio of 300, was 4.93 x 10(4) h(-1) with a satisfactory selectivity to C-4-C-10 olefins (89%), and linear alpha-olefins accounted for 95%. The steric and conjugate effects of the ligands on ethylene oligomerization, as well as the effects of cocatalysts, the Al/Zr molar ratios, the reaction temperature, the ethylene pressure, and the lifetime of the catalysts were studied.
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15. |
- Österlund, Lars, 1967-, et al.
(author)
-
Reactivity of Pt/ceria and Pt/alumina planar model catalysts prepared by colloidal lithography
- 2003
-
In: Journal of Catalysis. - 0021-9517 .- 1090-2694. ; 215:1, s. 94-107
-
Journal article (peer-reviewed)abstract
- We report on the structure, chemical state, and catalytic activity of nanofabricated, planar Pt/ceria and Pt/alumina model catalysts prepared by colloidal lithography. Employing extended etching times, catalyst particles are fabricated that are much smaller than the size of the polystyrene colloidal particle mask. Using CO oxidation as a probe reaction, the influence of various pretreatments has been studied. We find that the H-2/O-2 pretreatment deteriorates the reactivity on Pt/alumina, which is accompanied by blocking of the high-temperature CO adsorption sites as seen by TPD. After prolonged reactions the Pt/ceria particles show pronounced restructuring, indicating a low-temperature, reaction-induced, strong metal-support interaction, where ceria is decorating the Pt particles. In contrast, after an identical reaction history the Pt/alumina particles become facetted. The nanofabrication approach allows us to attribute the structural changes on individual particles to the integrated, macroscopic catalytic response. (C) 2003 Elsevier Science (USA). All rights reserved.
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16. |
- Abdulhamid, Hussam, 1974, et al.
(author)
-
A combined transient in situ FTIR and flow reactor study of NOx storage and reduction over M/BaCO3/Al2O3 (M=Pt, Pd or Rh) catalysts
- 2006
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 244:2, s. 169-182
-
Journal article (peer-reviewed)abstract
- Transient in situ DRIFTS and flow-reactor experiments were performed to study the storage and reduction of NOX over Pt/BaCO3/Al2O3, Pd/BaCO3/Al2O3, and Rh/BaCO3/Al2O3 samples using CO, H-2, C3H6, or C3H8 as the reducing agent. The DRIFTS results show that exposure of the examined samples to NO2 results in the formation of nitrite/nitrate peaks over alumina and barium and that the reduction of stored NOX is influenced by the type of precious metal and reducing agent. Using CO as the reductant results in a lower NOX reduction capacity for Pt/BaCO3/Al2O3 compared with Pd- and Rh-based samples, whereas H-2 shows a significant ability to reduce the stored NOX on all samples examined. In addition, the reduction with CO and C3H6 proceeds via the formation of isocyanate species over both barium and alumina sites. The intensity of barium-isocyanate species for Pt/BaCO3/Al2O3 is significantly lower than the corresponding intensity for the Pd- and Rh-based samples, suggesting that the interaction between barium and Pt is lower than the corresponding interaction in the Pd/BaCO3/Al2O3 and Rh/BaCO3/Al2O3 samples. (c) 2006 Elsevier Inc. All rights reserved.
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17. |
- Abdulhamid, Hussam, 1974, et al.
(author)
-
In situ FTIR study of SO2 interaction with Pt/BaCO3/Al2O3 NOx storage catalysts under lean and rich conditions
- 2006
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 241:1, s. 200-210
-
Journal article (peer-reviewed)abstract
- The interaction of SO2 with Pt/BaCO3/Al2O3 NOx storage catalysts under lean and rich conditions in the absence and presence of water was investigated by means of in situ FTIR spectroscopy at 350 degrees C. For the lean condition (SO2 + O-2), surface and bulk barium sulfates were formed in addition to sulfites and surface sulfates on alumina. Pt does not appear to play an essential role in the formation of these species under lean conditions. In contrast, under reducing conditions (SO2 + H-2), Pt catalysed the reduction of SO2 with H-2 to form reduced sulfur species that accumulate on both Pt and barium sites. A subsequent treatment with NO2 resulted in the reduction of NO2 in the oxidation of these sulfur-containing species to form bulk barium sulfate instead of being stored over barium sites. Introducing water significantly affected sulfur trapping over the catalyst surface. During the lean conditions, the presence of water resulted in almost no surface barium sulfate formation. Meanwhile, the formation of bulk barium sulfate was increased, probably due to enhanced sulfate formation. However, the presence of H2O under reducing conditions resulted in significantly lower sulfur storage on both barium and alumina in comparison with humid lean exposure conditions.
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18. |
- Acke, Filip, 1968, et al.
(author)
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Selective reduction of NO by HNCO over Pt promoted Al2O3
- 1998
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 179:2, s. 528-536
-
Journal article (peer-reviewed)abstract
- The activity of Pt supported on gamma-Al2O3 towards HNCO oxidation and reduction of NO by HNCO under oxygen excess is examined under transient conditions (temperature ramps between 100 and 500 degrees C). Formation of N-2, N2O, NO, and NO2 is observed. Isotope labelled (NO)-N-15 is used to show the scrambling of nitrogens in N-2 ((NN)-N-14-N-15) and N2O ((NNO)-N-14-N-15). Adsorbed species on the surface are characterised by in-situ FTIR spectroscopy in order to obtain information on reaction intermediates. Adsorbed NNx species are discussed as important intermediates in the N-2 and N2O formation. The absence of Pt results in a delay in HNCO ignition and only a small N2O formation is observed.
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19. |
- Amandusson, H., et al.
(author)
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Isotopic study of ethanol dehydrogenation over a palladium membrane
- 2000
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 195:2, s. 376-382
-
Journal article (peer-reviewed)abstract
- The dehydrogenation of ethanol and the subsequent permeation were studied on a Pd membrane in a continuous ethanol supply. Hydrogen could not be extracted as efficiently from ethanol as from methanol. In ethanol, at least four of the six hydrogen atoms were not available for permeation because of methane formation. Hydrogens bonded to a carbon atom in a C-O group were available for permeation, while hydrogen atoms bonded to a carbon atom without oxygen were not. The efficiency of hydrogen permeation from ethanol was 5% compared to that of pure hydrogen, which could be compared to 25% for methanol compared to pure hydrogen. The hydrogen permeation could be enhanced by adding CO to the EtOH + O2 supply. The permeation probability of the hydrogen bonded to the methylene hydrogen increased while the water formation with this hydrogen atom decreased. Acetic acid was formed upstream when oxygen was in excess. The differently bonded hydrogen atoms in an ethanol molecule experienced different reaction pathways. The results did not contradict the models made from surface experiments in ultrahigh vacuum by Davis and Barteau, Holroyd and Bowker, or Bowker et al.
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20. |
- Arnby, Karl, 1974, et al.
(author)
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Investigation of Pt/γ-Al2O3 catalysts with locally high Pt concentrations for oxidation of CO at low temperatures
- 2004
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 221:1, s. 252-261
-
Journal article (peer-reviewed)abstract
- A new method for preparing supported catalysts, in which Pt was distributed in locally high concentrations on the γ-Al2O3 support, was studied. These catalysts were compared with a conventionally prepared Pt/γ-Al2O3 catalyst in which Pt was deposited evenly on the support. The object was to ascertain whether it is possible to prepare catalysts that retain heat released from exothermic reactions to a higher extent and thereby become more low-temperature active than a conventionally prepared catalyst. A significant improvement of the activity was observed for the catalysts prepared with locally high Pt concentrations when CO (1%, 1000 and 100 ppm) was oxidized at a constant O2 concentration (10%). The improved activity is discussed in terms of heat transfer, mass transfer, and structure sensitivity. Differences in heat transfer appear to be the least probable reason for the enhanced activity for the catalysts with locally higher Pt concentrations, whereas structural effects also seem to be an unlikely explanation. Differences in mass transfer seem, however, to be a more likely reason for the improved activity.
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21. |
- Arnby, Karl, 1974, et al.
(author)
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The effect of platinum distribution in monolithic catalysts on the oxidation of CO and hydrocarbons
- 2005
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 233:1, s. 176-185
-
Journal article (peer-reviewed)abstract
- Monolithic Pt/gamma-Al2O3 catalysts were prepd. by depositing of the platinum phase, either homogeneously or heterogeneously (i.e., local high platinum concn.), in the washcoat. The influence of the platinum distribution on the ignition and extinction processes for oxidn. of CO, propene, and propane, resp., was investigated by both temp.-programmed and oxygen step-response flow-reactor expts. In addn., in situ XANES spectroscopy was used to follow changes in the chem. state of platinum during propane oxidn. For samples with heterogeneous platinum distribution, the results show an improved low-temp. activity for CO oxidn., whereas no clear improvement is obsd. for oxidn. of propene or propane. Comparison of the results for CO and hydrocarbon oxidn. shows that the improved activity cannot be explained by thermal effects. Moreover, calcns. indicate that the enhanced activity for CO oxidn. can be due to mass-transfer phenomena. This could not be confirmed by our expts., however. Instead, the improved activity for CO oxidn. can be due to varying platinum particle size between the samples. Furthermore, the oxidn. of propane is shown to be highly influenced by the oxygen concn., showing an activity max. for gas compositions close to the stoichiometric. High oxygen levels result most probably in a predominantly oxygen-covered surface, which inhibits the reaction and, as supported by the in situ XANES experiments, passivates the surface by forming platinum oxide.
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22. |
- Atakan, Aylin, et al.
(author)
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Time evolution of the CO2 hydrogenation to fuels over Cu-Zr-SBA-15 catalysts
- 2018
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 362, s. 55-64
-
Journal article (peer-reviewed)abstract
- Time evolution of catalytic CO2 hydrogenation to methanol and dimethyl ether (DME) has been investigated in a high-temperature high-pressure reaction chamber where products accumulate over time. The employed catalysts are based on a nano-assembly composed of Cu nanoparticles infiltrated into a Zr doped SiOx mesoporous framework (SBA-15): Cu-Zr-SBA-15. The CO2 conversion was recorded as a function of time by gas chromatography-mass spectrometry (GC-MS) and the molecular activity on the catalyst’s surface was examined by diffuse reflectance in-situ Fourier transform infrared spectroscopy (DRIFTS). The experimental results showed that after 14 days a CO2 conversion of 25% to methanol and DME was reached when a DME selective catalyst was used which was also illustrated by thermodynamic equilibrium calculations. With higher Zr content in the catalyst, greater selectivity for methanol and a total 9.5% conversion to methanol and DME was observed, yielding also CO as an additional product. The time evolution profiles indicated that DME is formed directly from methoxy groups in this reaction system. Both DME and methanol selective systems show the thermodynamically highest possible conversion.
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23. |
- Babucci, Melike, et al.
(author)
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Unraveling the individual influences of supports and ionic liquid coatings on the catalytic properties of supported iridium complexes and iridium clusters
- 2020
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 387, s. 186-195
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Journal article (peer-reviewed)abstract
- Supported iridium complexes were synthesized by the reaction of Ir(CO)(2) (acac) (acac = acetylacetonato) with SiO2, gamma-Al2O3, and MgO. Extended X-ray absorption fine structure (EXAFS) and infrared (IR) spectra demonstrate that the iridium was present as atomically dispersed species anchored to each support. The samples were treated in flowing H-2 at 673 K to form supported iridium clusters. EXAFS spectra and highangle annular dark-field scanning transmission electron microscopy images demonstrate that the average diameter of the iridium clusters on each support was approximately 1.2 nm. The catalysts before and after cluster formation were coated with each of a family of 1,3-dialkylimidazolium ionic liquids (ILs) having varying electron-donor tendencies probed by their nu(C2H) frequencies determined by IR spectroscopy. The coated and uncoated samples were tested as catalysts for partial hydrogenation of 1,3-butadiene in a flow reactor at 333 K, with turnover frequencies determined from differential conversions. The individual influences of the IL coatings and supports on the catalyst performance were found to depend strongly on whether the iridium was site-isolated complexes or present in clusters. The IL coatings as ligands exerted dominant effects on the clusters as catalysts, whereas the supports exerted dominant effects on the isolated iridium atoms. The results indicate how to tune the effects of metal nuclearity, IL coatings, and supports on the electronic environments and catalytic properties of the metals
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24. |
- Becker, Elin, 1981, et al.
(author)
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Methane oxidation over alumina supported platinum investigated by time-resolved in situ XANES spectroscopy
- 2007
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 252:1, s. 11-17
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Journal article (peer-reviewed)abstract
- In situ time-resolved X-ray absorption spectroscopy and mass spectrometry were used to correlate changes in catalyst surface composition with catalytic activity for methane oxidation over alumina supported Pt. Different transient experiments (i.e., pulsing of oxygen or hydrogen to an otherwise constant gas composition) were performed to study the methane oxidation kinetics. Changes in the surface O/Pt ratio were monitored by the introduction of a new analysis method of the white line area corresponding to the Pt L-III-edge XANES spectra. The relevance of the method was confirmed by first-principles calculations demonstrating how hydrogen and oxygen adsorbates modify the electronic structure of Pt. The experimental results show that during the gas-phase transients, the surface O/Pt ratio changes, which in turn affects the methane oxidation rate. Activity maxima are observed for an intermediate surface O/Pt ratio. An oxygen-rich surface seems to hinder the dissociative adsorption of methane, leading to low methane oxidation activity at oxygen excess.
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25. |
- Behravesh, Erfan, et al.
(author)
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Synthesis and characterization of Au nano particles supported catalysts for partial oxidation of ethanol : Influence of solution pH, Au nanoparticle size, support structure and acidity
- 2017
-
In: Journal of Catalysis. - : Elsevier. - 0021-9517 .- 1090-2694. ; 353, s. 223-238
-
Journal article (peer-reviewed)abstract
- Partial oxidation of ethanol to acetaldehyde was carried out over gold catalysts supported on various oxides and zeolites by deposition precipitation. The special focus of this work was on the influence of H-Y zeolite surface charge on Au cluster size and loading linking it to activity and selectivity in ethanol oxidation and comparing with other studied catalysts. The catalysts were characterized by nitrogen physisorption, transmission electron microscopy (TEM), scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDXA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and zeta potential measurements. pH of the solution governed the Au NPs size within the range of 5.8–13.2 nm with less negatively charged surfaces leading to formation of smaller clusters. Au loading on H-Y zeolite with silica to alumina ratio of 80 was increased by raising the pH. In fact, H-Y-12 and H-Beta-25 were selective towards diethyl ether while acetaldehyde was the prevalent product on less acidic H-Y-80. The results demonstrated strong dependency of the catalytic activity on the Au cluster size. Namely turn over frequency (TOF) decreased with an increase in metal size from 6.3 to 9.3 nm on H-Y-80. Selectivity towards acetaldehyde and ethyl acetate did not change significantly on H-Y-80 within 6.3–9.3 nm Au particle size range. On Al2O3 support, however, selectivity towards acetaldehyde increased considerably upon diminishing Au average particle size from 3.7 to 2.1 nm.
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26. |
- Berner, Simon, et al.
(author)
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Activity boost of a biomimetic oxidation catalyst by immobilization onto a gold surface
- 2006
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 244:1, s. 86-91
-
Journal article (peer-reviewed)abstract
- Thiol-functionalized cobalt porphyrins were used as a model system for investigating catalytic activity in homogeneous and heterogeneous oxidation catalysis. Self-assemble monolayers of thiol-functionalized cobalt porphyrins were prepared on a gold surface and served as heterogenous catalysts. These immoblilized molecules prevented the strong inactivation observed for their homogeneous congener. As a result, the turnover number per molecule in heterogeneous catalysis was at least 100 times higher than that of the corresponding homogeneous catalyst. It is atypical for a heterogenized catalyst to outperform its homogeneous congener. The properties of the molecular layers were characterized on the molecular level by means of X-ray photoelectorn spectroscopy (XPS) and scanning tunneling microscopy (STM). The results demonstrate that the performance of these biomimetic catalysts can be dramatically improved if the catalyst arangement can be controlled on the molecular level.
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27. |
- Bjerregaard, Joachim, 1996, et al.
(author)
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Mechanism for SO2 poisoning of Cu-CHA during low temperature NH3-SCR
- 2023
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 417, s. 497-506
-
Journal article (peer-reviewed)abstract
- Density Functional Theory (DFT) calculations are used to investigate low temperature SO2 deactivation of Cu-CHA during ammonia assisted selective catalytic reduction of NO (NH3-SCR). SO2 is found to adsorb on [Cu2IINH34O2]2+ forming a copper sulfate complex. NO and NH3 react over the sulfate complex forming N2, H2O and H2SO4. H2SO4 undergoes an acid-base reaction with NH3 yielding SO4(NH4)2 and HSO4(NH4), where HSO4(NH4) is thermodynamically preferred during typical reaction conditions. The SO2-derived species are bulky and have considerable barriers for inter-cage diffusion. Moreover, the presence of HSO4(NH4) species reduces the probability of having two [CuI(NH3)2]+ complexes in one cage, which is a requirement for O2 activation. The results suggest that the key mechanism for low temperature SO2 deactivation is of physical origin and that the catalyst can be regenerated by exposure to high temperatures where HSO4(NH4) decomposes. The suggested mechanism agrees with experimental observations and provides atomistic understanding of sulfur poisoning of Cu-CHA during NH3-SCR.
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28. |
- Broqvist, Peter, 1975, et al.
(author)
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A DFT Study on CO Oxidation over Co3O4
- 2002
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 210:1, s. 198-206
-
Journal article (peer-reviewed)abstract
- A pilot investigation regarding the CO oxidation mechanism at the Co3O4(110) surface is performed by means of first-principles calculations based on density functional theory. Preferred adsorption of CO is found to occur at a surface Co3+ site. A possible mechanism for CO2(g) formation of the Mars–van Krevelen type is also looked into via the monitoring of two inequivalent oxygen abstraction routes. Experimental observations regarding partial surface deactivation are discussed based on surface intermediates. The oxidative property of the Co3O4 substrate, i.e., Co3++e−→Co2+, is found to be crucial for the O abstraction step. This implies that CoAl2O4 is inactive in CO oxidation, in agreement with experiment.
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29. |
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30. |
- Campbell, Paul S., et al.
(author)
-
Olefin hydrogenation by ruthenium nanoparticles in ionic liquid media : Does size matter?
- 2010
-
In: Journal of Catalysis. - : Elsevier. - 0021-9517 .- 1090-2694. ; 275:1, s. 99-107
-
Journal article (peer-reviewed)abstract
- Tailor-made and size-controlled ruthenium nanoparticles, RuNPs, of three distinct sizes between 1 and 3 nm are generated from the decomposition of (η4-1,5-cyclooctadiene)(η6-1,3,5-cyclooctatriene)ruthenium(0) [Ru(COD)(COT)], under H2 in 1-butyl-3-methylimidazolium bis(trifluoromethanesulphonyl)imide, C1C4ImNTf2, by simply varying experimental conditions. Catalytic hydrogenation of 1,3-cyclohexadiene, CYD, and cyclohexene, CYE, in C1C4ImNTf2, has been used as a probe for the relationship between size and catalytic performance (activity and selectivity) of RuNPs. To allow comparison between different reactions, all catalytic reaction mixtures were diligently prepared in order that the parameters such as substrate/catalyst and substrate/ionic liquid ratio, and therefore, viscosity and mass transport factors remained constant. It was found that the catalytic activity increases with the NP size, while high selectivity is only observed with the smaller NPs. In addition, the studied RuNPs exhibit a high level of recyclability with neither loss of activity nor significant agglomeration.
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31. |
- Cao, Lili, et al.
(author)
-
Putative reaction mechanism of nitrogenase after dissociation of a sulfide ligand
- 2020
-
In: Journal of Catalysis. - : Elsevier BV. - 1090-2694 .- 0021-9517. ; 391, s. 247-259
-
Journal article (peer-reviewed)abstract
- We have investigated the implications of the recent crystallographic findings that the m2-bridging S2B sulfide ligand may reversibly dissociate from the active-site FeMo cluster of nitrogenase. We show with combined quantum mechanical and molecular mechanical (QM/MM) calculations that once S2B has dis- sociated, N2 may bind in that position and can be protonated to two NH3 groups by thermodynamically favourable steps. The substrate forms hydrogen bonds with two protein ligands, Gln-191 and His-195. For all steps, we have studied three possible protonation states of His-195 (protonated on either ND1, NE2 or both). We find that the thermodynamically favoured path involves an end-on NNH2 structure, a mixed side-on/end-on H2NNH structure, a side-on H2NNH2 structure, a bridging NH2 structure and a bridging NH3 structure. In all cases, His-195 seems to be protonated on the NE2 atom. Dissociation of the NH3 pro- duct is often unfavourable and requires either further reduction or protonation of the cluster or rebinding of S2B. In conclusion, our calculations show that dissociation of S2B gives rise to a natural binding and reaction site for nitrogenase, between the Fe2 and Fe6 atoms, which can support an alternating reaction mechanism with favourable energetics.
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32. |
- Carlsson, Per-Anders, 1972, et al.
(author)
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A transient in situ FTIR and XANES study of CO oxidation over Pt/Al2O3 catalysts
- 2004
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 226:2, s. 422-434
-
Journal article (peer-reviewed)abstract
- We report experimental results for the oxidation of CO over supported Pt/Al2O3 catalysts operating in oxygen excess at atmospheric pressure. To study the reaction kinetics under transient conditions we have employed step changes of the O2 concentration by intermittently switching off the O2 supply at various temperatures ranging from 523 to 623 K. Detailed in situ FTIR and XANES data for CO coverage and the chemical state of Pt, respectively, are presented together with the CO conversion, which in both cases was monitored by mass spectrometry. A red-shift of the vibrational frequency of linearly bonded CO which correlates with a blue-shift of the Pt LIII binding energy indicates that the Pt catalyst initially is partially oxidised and gradually reduced when the O2 supply is switched off. Control experiments with a NO2 oxidised Pt/Al2O3 catalyst support these findings. A hysteresis in the catalytic activity due to the different rates whereby Pt is oxidised and reduced as a function of gas-phase composition is observed. The activation energy for the Pt oxide reduction (decomposition) process is estimated to be about 50 kJ/mol. The results further emphasise that the conventional three-step Langmuir-Hinshelwood (LH) scheme used to interpret CO oxidation on Pt surfaces must be complemented by a Pt oxidation and reduction mechanism during transient conditions. Moreover, FTIR data suggest that during the extinction, the partially oxidised platinum surface is reduced by chemisorbed CO which should be explicitly accounted for in the modeling of the reaction mechanism.
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33. |
- Chen, Lin, 1990, et al.
(author)
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Activation of oxygen on (NH3–Cu–NH3)+ in NH3-SCR over Cu-CHA
- 2018
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 358, s. 179-186
-
Journal article (peer-reviewed)abstract
- Cu-CHA materials are efficient catalysts for NH 3 –SCR of NO x in oxygen excess. A crucial step in the reaction is oxygen (O 2 ) activation, which still is not well understood. Density functional theory calculations in combination with ab initio thermodynamics and molecular dynamics are here used to study O 2 dissociation on Cu(NH 3 ) 2 + species, which are present under NH 3 –SCR conditions. Direct dissociation of O 2 is found to be facile over a pair of Cu(NH 3 ) 2 + complexes whereas dissociation on a single Cu(NH 3 ) 2 + species is unlikely due to a high activation energy. The presence of NO promotes oxygen dissociation on both single and pairs of Cu(NH 3 ) 2 + complexes. Nitrites and nitrates are easily formed as O 2 dissociates, and NO adsorption over nitrates leads to facile formation of NO 2 . The results stress the importance of ligand-stabilized Cu species in Cu-CHA catalysts for NH 3 –SCR.
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34. |
- Chen, Wenhua, et al.
(author)
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Reduction of NO by C4 hydrocarbons on platinum in the presence of oxygen : influence of sulfur dioxide
- 1997
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 172:1, s. 3-12
-
Journal article (peer-reviewed)abstract
- The reduction of NO, in the presence of i-C4H10or i-C4H8and O2, catalyzed by a platinum foil, was studied in order to understand better the catalytic activity of platinum metal, free of any support or dispersion effects. The reaction products were analyzed by mass spectrometry, and the surface was characterized by X-ray photoelectron spectroscopy at different stages of the reaction. A correlation between the catalytic activity for NO conversion and the presence of adsorbed intermediates is demonstrated. The role of oxygen is interpreted as twofold: formation of active intermediates and deactivation of the surface. The effect of traces of sulfur dioxide in the reacting phase on the reduction of NO by isobutene was investigated. The influence of SO2is very much dependent upon its initial concentration in the gas phase. Low concentrations (<15 ppm) promote the reduction of NO, whereas higher levels poison the reaction by a surface site blocking effect.
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35. |
- Christoffersen, E., et al.
(author)
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Anode materials for low-temperature fuel cells : A density functional theory study
- 2001
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 199:1, s. 123-131
-
Journal article (peer-reviewed)abstract
- Based on density functional calculations, we discuss the effect of alloying Pt with other metals for use as anode catalyst materials in low-temperature fuel cells. We discuss why a few parts per million of CO in the H-2 fuel can poison Pt surfaces and how this problem can be alleviated by alloying, and an extensive data base of the effect of alloying on the reactivity that includes all binary combinations of the transition metals to the right in the periodic table is given. We also discuss the effect of surface segregation and give a calculated data base of segregation energies of binary transition metal alloys. Based on extensive Monte Carlo simulations we show that while the adsorbate-free surface of a Ru0.5Pt0.5 alloy has no Ru in the first layer, the presence of CO can move some Ru to the surface, but all these Ru atoms are covered by CO.
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36. |
- Creaser, Derek, et al.
(author)
-
Transient kinetic analysis of the oxidative dehydrogenation of propane
- 1999
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 182:1, s. 264-269
-
Journal article (peer-reviewed)abstract
- Oxidative dehydrogenation of propane was studied using various transient techniques. Results support a redox mechanism in which propane reduces the catalyst, which is reoxidized by gas-phase oxygen. Only lattice oxygen participates in propene formation. Desorbable oxygen is a major source of poor selectivity, although lattice oxygen also causes total oxidation. Consequently, propene selectivity in the absence of gas-phase O2is superior to co-feed, steady-state selectivity at the same propane conversion. Propene selectivity is further improved by increasing the degree of catalyst reduction.
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37. |
- DAWODY, JAZAER, 1959, et al.
(author)
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Sulfur Deactivation of Pt/SiO2, Pt/BaO/Al 2O3 and BaO/Al2O3 NOx Storage Catalysts: Influence of SO2 Exposure Conditio
- 2005
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 234:1, s. 206-218
-
Journal article (peer-reviewed)abstract
- Flow reactor experiments were performed to study the effect Of SO2 + O-2 and SO2 + H-2 exposures on the NOx storage performance of Pt/BaO/Al2O3 and BaO/Al2O3 catalysts. In addition, Pt/SiO2 samples were used to study the interaction between Pt and SO2 under the two exposure conditions. For BaO/Al2O3 the two SO2 exposure conditions caused similar deactivation of the NO, storage capacity, whereas for Pt/BaO/Al2O3 the decline in the NOx storage capacity was faster during the SO2 + H-2 exposure than for SO2 + O-2 exposure. The presence of Pt enhances the adsorption Of SO2 for both SO2 exposure conditions. Quantitative analysis of sulfur showed that exposure to SO2 + O-2 caused higher accumulation of sulfur in Pt/BaO/Al2O3 samples in comparison with SO2 + H-2 exposure. Thus there is no correlation between the total amount of adsorbed sulfur in the samples and the reduction of the NOx storage capacity, probably because of the adsorption of sulfur on sites not necessarily important for the NOx storage process.
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38. |
- Di, Mengqiao, 1994, et al.
(author)
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Chasing PtO x species in ceria supported platinum during CO oxidation extinction with correlative operando spectroscopic techniques
- 2022
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 409, s. 1-11
-
Journal article (peer-reviewed)abstract
- Industrially relevant, highly dispersed, Pt/ceria and reference Pt/alumina catalysts with narrow Pt particle size distributions have been prepared, characterised ex situ and studied for CO oxidation by operando infrared and X-ray absorption spectroscopy. At high CO conversions, spectator CO ad-species on ionic platinum are observed while the CO oxidation proceeds on Pt particles in a high oxidation state exhibiting significant Pt[sbnd]O coordination. During the protracted catalytic extinction, the CO coverage builds up gradually while the Pt oxidation state and Pt[sbnd]O coordination remain high because of interactions with ceria. The observed CO oxidation at high CO coverage is suggested to involve sites at the platinum-ceria boundary that cannot be CO self-poisoned. This behaviour is in stark contrast to that of Pt/alumina, which shows removal of platinum oxides formed during CO oxidation and the classical drop in catalytic activity caused by rapid CO self-poisoning when reaching a critical temperature.
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39. |
- Dong, Qibing, et al.
(author)
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Regulating concentration of surface oxygen vacancies in Bi 2 MoO 6 /Bi-MOF for boosting photocatalytic ammonia synthesis
- 2024
-
In: Journal of Catalysis. - 0021-9517 .- 1090-2694. ; 433
-
Journal article (peer-reviewed)abstract
- Surface oxygen vacancies (OVs) engineering has been widely adopted as an effective strategy to enhance photocatalytic performance. At present, photocatalytic systems capable of precisely regulating surface OVs concentrations, which could help illuminate the effects of the surface OVs concentration on N2 fixation activity, are still scarce. Herein, bismuth-based metal organic framework (Bi-MOF) was loaded onto the surface of Bi2MoO6 (BMO) as an operable platform, and the OVs concentration in the Bi-MOF component of BMO/Bi-MOF could be regulated by reduction of bismuth ions therein. Experimental results confirm that optimum construction of OVs in the Bi-MOF promotes the photoelectrons transfer from BMO to Bi-MOF, facilitating the activation of N2 at OVs. Consequently, the optimized catalyst shows superior performance in NH3 production, which reaches 125.78 μmol h−1 g−1, 21.4 higher than that of BMO. This work underline the significance of regulating surface OVs concentration, providing inspiration for the development of efficient OVs-modified photocatalysts.
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40. |
- Du, Dou, et al.
(author)
-
Simulated temperature programmed desorption experiments for calcined nanoceria powders
- 2020
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 384, s. 252-259
-
Journal article (peer-reviewed)abstract
- Density functional theory calculations (DFT), coupled with microkinetic modelling, have been used to simulate Temperature Programmed Desorption (TPD) experiments for calcined ceria nanopowders with the aim to gain insight into the chemistry governing their high redox activity. Our simulations consider two main nanoparticle models. One is a perfect ceria octahedron supercharged with adsorbed oxygen molecules turned into superoxide ions, as has previously been used to explain the enhanced oxygen storage capacity (OSC) in nanoceria. The other model is a variant where we have introduced oxygen vacancies under ridge Ce ions, thereby reducing their coordination numbers to five. The results from our microkinetic modelling suggest that including such five-coordinated Ce adsorption sites results in a TPD spectrum that better matches the experimental counterpart in terms of both peak position and width. In addition, this new structural model allows for the co-existence of Ce3+ ions, superoxide ions and O-2 molecules, as seen in experiments in the literature.
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41. |
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42. |
- Eriksson, Kristofer, et al.
(author)
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Performance of a biomimetic oxidation catalyst immobilized on silica particles
- 2013
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 303, s. 16-21
-
Journal article (peer-reviewed)abstract
- A biomimetic oxidation catalyst, cobalt porphyrin with thiol linkers, was chemically conjugated to silica particles and utilized in the oxidation of hydroquinone to benzoquinone. The cobalt porphyrin/silica particle catalyst was characterized with Inductively Coupled Plasma (ICP) and X-ray Photoelectron Spectroscopy (XPS). The catalytic performance of the cobalt porphyrin molecules was compared to previous results for the same catalyst grafted to a gold surface and on silicon wafers. The measured catalytic activity, after background correction, was 100 times higher than that of its homogeneous counterpart, 10 times higher than that on a silicon wafer, and almost the same as that on a gold surface. The turnover frequency rates after 400 h are still comparable with initial rates reported for homogeneous porphyrins and salophens, whereas the use of particles as support increases the active surface area, which removes the limitations for scale-up associated with the previously used silicon wafers and gold surfaces.
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43. |
- Fashandi, Hossein, et al.
(author)
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Applicability of MOS structures in monitoring catalytic properties, as exemplified for monolayer-iron-oxide-coated porous platinum films
- 2016
-
In: Journal of Catalysis. - : Elsevier. - 0021-9517 .- 1090-2694. ; 344, s. 583-590
-
Journal article (peer-reviewed)abstract
- Metal Oxide Semiconductor (MOS) capacitor devices comprised of monolayer iron oxide-coated as well as non-coated polycrystalline Pt deposited on oxidized silicon carbide substrates have been fabricated and their usefulness as realistic model systems in catalyst studies development was evaluated. The CO oxidation characteristics of both iron oxide- and non-coated Pt catalysts were investigated using mass spectrometry, monitoring the carbon dioxide production rate for different combinations of carbon monoxide (CO) and oxygen concentrations at various temperatures. Additionally, the output capacitance of the MOS model catalysts was recorded for each individual CO oxidation activity. A low-temperature shift in CO oxidation characteristics for the monolayer-coated compared to the non-coated Pt catalysts was observed, similar to that previously reported for monolayer iron oxide grown on single-crystalline Pt substrates. A strong correlation between the output capacitance of the MOS structures and the CO oxidation characteristics was found for both monolayer- and non-coated model catalysts. Furthermore, the devices exhibit retained MOS electrical output and CO oxidation characteristics as well as an unaffected catalyst surface composition, as confirmed by photoelectron spectroscopy, even after 200 h of continuous model catalyst operation. In addition to the implications on practical applicability of monolayer iron oxide coating on widely used polycrystalline Pt films in real-world catalysts and sensors, the findings also point to new possibilities regarding the use of MOS model systems for in situ characterization, high throughput screening, and tailoring of e.g. catalyst- and fuel-cell-electrode materials for specific applications.
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44. |
- Fridell, Erik, 1963, et al.
(author)
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NOx storage in barium-containing catalysts
- 1999
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 183:2, s. 196-209
-
Journal article (peer-reviewed)abstract
- The effect of key parameters on the characteristics of barium oxide-based NOx storage catalysts was systematically investigated. Model Pt/BaO/Al2O3, BaO/Al2O3, Pt–Rh/Al2O3, and Pt–Rh/BaO/Al2O3 catalysts were prepared and evaluated with respect to NOx storage capacity using transient flow reactor studies, temperature-programmed desorption studies (TPD), and in situ Fourier transform infrared (FTIR) absorption spectroscopy. The influence of temperature, storage and regeneration times, NOx source (NO or NO2), oxygen concentration, reducing agent (C3H6, C3H8, CO, or H2), and carbon dioxide concentration onNOx storage capacity was studied. Significant amounts of NOx were found to be stored in the catalysts containing both barium oxide and noble metals. For these catalysts the following observations were made: (1) maximum NOx storage was observed at about 380C;(2) around this temperature no significant differences between NO and NO2 on NOx storage capacity could be observed;(3) a slow increase in stored NOx could be observed with increasing oxygen concentration during the lean phase;(4) significant NOx desorption peaks, mainly of NO, were observed immediately after the switch from lean to rich conditions; and (5) at about 380±C the in situ FTIR spectra show characteristic nitrate peaks in the region 1300–1400 cm¡1 when NOx was stored under lean conditions and isocyanate peaks around 2230 cm-1 when the catalysts were regenerated under rich conditions in the presence of hydrocarbons. The step leading to stored NOx is believed to involve NO2 and the presence of atomic oxygen. During the rich period, the noble metal surfaces are probably reduced, leading to breakthrough peaks when NO desorbs.
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45. |
- Garlisi, Corrado, et al.
(author)
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N-TiO2/Cu-TiO2 double-layer films : Impact of stacking order on photocatalytic properties
- 2017
-
In: Journal of Catalysis. - : ACADEMIC PRESS INC ELSEVIER SCIENCE. - 0021-9517 .- 1090-2694. ; 353, s. 116-122
-
Journal article (peer-reviewed)abstract
- In this work, we report for the first time a unique configuration of N-doped and Cu-doped TiO2 bilayer. The activity of TiO2 was improved by combining Cu- and N-doping in a layered thin-film structure. The impact of the stacking order was studied, pointing out how the best arrangement is by far the one with Cu-TiO2 as the top layer. The results reveal a unique and simple way to enhance the photocatalytic response of TiO2 in the visible domain.
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46. |
- Golets, Mikhail, 1987-, et al.
(author)
-
Continuous production of the renewable ρ-cymene from α-pinene
- 2013
-
In: Journal of Catalysis. - : Elsevier. - 0021-9517 .- 1090-2694. ; 307, s. 305-315
-
Journal article (peer-reviewed)abstract
- The aim of this work was to demonstrate the feasibility to produce ρ-cymene, an important commodity chemical, in a continuous, one-pot reaction system from abundant α-pinene, available e.g. as a by-product of pulping industry. The isomerization reactions of α-pinene over bimetallic heterogeneous catalysts, 3 and 5 wt% Pd–Zn (1:1, 1:4, 4:1, 1:0, and 0:1), supported on Al-SBA15 were studied. The principal reaction products were identified as ρ- and m-cymenes, limonene, camphene, and ρ-menthene, respectively. The highest concentration of ρ-cymene reached 77 wt% under the optimized reaction conditions: 300 °C and α-pinene feed of 0.03 mL/min. Two main reaction pathways toward ρ- and m-cymenes were described, and a mechanistic kinetic model, based on a plausible reaction network in line with Langmuir–Hinshelwood approach, was developed. The catalyst characterization revealed the reduction in Pd(II) sites, catalyst coking, and decline of surface area over the course of time. The catalyst recovery and reuse was addressed.
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47. |
- Hanifpour, Fatemeh, et al.
(author)
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Operando quantification of ammonia produced from computationally-derived transition metal nitride electro-catalysts
- 2022
-
In: Journal of Catalysis. - : Elsevier. - 0021-9517 .- 1090-2694. ; 413, s. 956-967
-
Journal article (peer-reviewed)abstract
- Electrochemical reduction of dinitrogen to ammonia is investigated in a micro-reactor flow-cell using thin films of VN, CrN, NbN and ZrN. Chronoamperometry loops are used for ammonia production analysis. Operando ammonia quantification is accomplished in a flow injection analyzer. Results show the effect of presence/absence of N-2(g) within both the electrochemical characterization and ammonia production for ZrN. However, no ammonia is detected from studies on CrN. VN and NbN are inactivated upon reacting their N atoms of the surface top layer(s). Results obtained from ammonia measurements, electrochemical impedance spectroscopy analysis, surface stability checks, and surface characterization using X-ray reflectivity, reveal certain trends indicating catalytic behavior for ZrN. However, the concentration of produced ammonia is below the detection limit of the methods devised to analyze the samples from isotope labeling experiments. The onset of ammonia production on ZrN appears to be in close agreement with that predicted previously by computational studies.
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48. |
- Hedlund, Jonas, et al.
(author)
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Controlling diffusion resistance, selectivity and deactivation of ZSM-5 catalysts by crystal thickness and defects
- 2022
-
In: Journal of Catalysis. - : Elsevier. - 0021-9517 .- 1090-2694. ; 410, s. 320-332
-
Journal article (peer-reviewed)abstract
- A systematic investigation of two sets of defect free and defective ZSM-5 crystals with controlled thickness (T) between 30 and 400 nm and of their performances in methanol conversion was reported for the first time in the present work. The defect free ZSM-5 crystals with a thickness of 35 nm are by far the smallest ever reported and displayed superior activity, stability and selectivity to slower diffusing compounds, which resulted in high yield of e.g. gasoline and the 1,2,4-trimethylbenzene isomer with high octane number, as compared to the other studied catalysts. Almost only products forming in the zeolite pores were detected and consequently, the external surface must be nearly inactive. Strong correlations between T and deactivation rate were observed. Thick crystals deactivated much faster than thin crystals, probably due to formation of carbon species in the zeolite pores, which results in pronounced percolation effects and faster deactivation of the former. At comparable thickness, crystals with defects deactivated much faster than defect free crystals, due to formation of additional small molecular coke species in the former. Strong correlations between T and selectivity were also observed and assigned to control of diffusion resistance by crystal thickness.
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49. |
- Hinz, Andreas, et al.
(author)
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An investigation of the reaction mechanism for the promotion of propane oxidation over Pt/Al2O3 by SO2
- 2001
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 201:2, s. 247-257
-
Journal article (peer-reviewed)abstract
- Total oxidation of propane with oxygen in the absence and the respective presence of SO2 in the feed gas was studied over 1 wt% Pt on gamma-Al2O3 using transient experiments with TAP (temporal analysis of products) and in situ DRIFT spectroscopy. The TAP experiments confirm the promoting role of SO2 on the conversion of propane and, moreover, show an inhibiting effect from SO2 on the conversion of the intermediate products propene and CO. In situ DRIFT spectroscopy reveals the formation of sulphate species on the catalyst surface and indicates the formation of an allylic intermediate in the absence of SO2 only. Interaction between hydrocarbon intermediates and the sulphate species is confirmed by the appearance of thiol fragments in the mass spectra. The transient experiments show that propane in the first step is irreversibly adsorbed at the surface. Analysis of the surface residence times of the products formed at the propane pulse in pump-probe experiments provides information about the reaction pathways. Without S02 in the feed gas, propane reacts consecutively in the sequence propane --> propene -->. ethane --> CO --> CO2. In the presence of SO2 the first step of the main route is the breaking of a C-C bond in propane producing ethane and a C-1 fragment, which then form CO and eventually CO2. It is suggested that the promoting effect by SO2 on the conversion of propane is due to the formation of an acidic site at the Pt/Al2O3/SO42-interface.
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50. |
- Holmberg, Johan, et al.
(author)
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A study of propane ammoxidation on Mo-V-Nb-Te-oxide catalysts diluted with Al2O3, SiO2, and TiO2
- 2006
-
In: Journal of Catalysis. - : Elsevier BV. - 1090-2694 .- 0021-9517. ; 243:2, s. 350-359
-
Journal article (peer-reviewed)abstract
- Mo-V-Nb-Te-oxide catalysts with nominal metal compositions equal to those of M1, M2, and a mixture of the M1 and M2 phases were prepared in neat and sol-diluted forms. The latter catalysts were prepared with 30 wt% of Al2O3, SiO2, or TiO2 sol. All catalysts were characterized by BET, XRD, and TPR and tested for the respective ammoxidation of propane and propene. XRD showed that dilution with SiO2 gave catalysts with the pure M1 and M2 phases. Al2O3 was found to react with Mo and V, giving catalysts with Al-2(MoO4)(3), AlVO4, TeMo5O16, Nb2O5, and, in one case, M2. Dilution of the nominal M1 composition with TiO2 produced Mo5-x(V/Nb/Te)(x)O-14, whereas the M2 and M1/M2 nominal compositions gave the correct type of phases. TPR and XRD reveal that dilution with SiO2 gives improved dispersion of M1 and M2; however, the dilution effect is greater, resulting in no improved activity when expressed per unit surface area of the catalyst. For propane ammoxidation, the activity data show that the neat M1 and M1/M2 catalysts are the best samples. The corresponding SiO2-diluted catalysts are less active and selective. For the ammoxidation of propene, the neat M1 and the SiO2-diluted M1 are the two best catalysts in terms of both activity and selectivity. More selective catalysts with lower activity and some acrolein formation are the neat M2 phase and M2 diluted with TiO2 or SiO2. (c) 2006 Elsevier Inc. All rights reserved.
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