| 1. |
- Andersson, Arne, et al.
(författare)
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Direct Propane Ammoxidation to Acrylonitrile: Kinetics and Nature of the Active Phase
- 1993
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Ingår i: New Frontiers in Catalysis (Studies in Surface Science and Catalysis ). - 0167-2991. ; 75, s. 691-705
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Konferensbidrag (refereegranskat)abstract
- The kinetics of the direct synthesis of acrylonitrile from propane on V-Sb-Al-(W) mixed oxides indicate that acrylonitrile (ACN) forms by two parallel pathways, one directly from propane and the second, which is the prevalent path, through the intermediate formation of propylene (C3=). The limiting factor in the formation of ACN is the relative slowness of the step of allylic oxidation to ACN of the intermediate C3=, and the higher rate of C3= oxidation to carbon oxides as compared to that of ACN to COx. The step of C3= oxidation to ACN is controlled by the surface availability of NH3 which, in turn, depends considerably on the side reaction of NH3 oxidation to N2. The catalytic behavior of different modified V-Sb-(Al)-O systems and their characterization by X-ray diffraction analysis and Raman, Infrared and X-ray Photoelectron spectroscopies indicate that i) a reduction of both V and Sb occurs during the catalytic reaction, ii) the presence of vanadium not stabilized in the rutile-like phase is responsible for the side reaction of NH3 oxidation and lowering of the selectivity, iii) alumina reacts with antimony forming an AlSbO4 rutile phase which could be epitaxially intergrown or in solid solution with the VSbO4/Sb2O4 system, which, in turn, limits the presence of not stabilized (unselective) vanadium species, and iv) antimony oxide supported on alumina is also selective in propane ammoxidation, but forming acetonitrile as the main product. The doping with vanadium of this sample increases slightly the activity, but especially gives rise to the formation of acrylonitrile instead of acetonitrile.
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| 2. |
- Andersson, Arne, et al.
(författare)
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Surface Characterization and Reactivity in Ammoxidation Reactions of Vanadium Antimonate Catalysts
- 1994
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Ingår i: Applied Catalysis A: General. - 0926-860X. ; 113:1, s. 43-57
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Tidskriftsartikel (refereegranskat)abstract
- Unsupported vanadium antimonate catalysts with Sb/V ratios of 1 and 5 and samples with the latter ratio supported on alumina were studied in toluene and propane ammoxidation to benzonitrile and acrylonitrile, respectively, and were characterized by X-ray photoelectron spectroscopy (XPS) analysis before and after catalytic tests. Activity data for toluene ammoxidation suggest that excess antimony with respect to the stoichiometric amount required for forming the VSbO4 rutile phase affects the dispersion of the latter phase giving smaller particles. Vanadium sites are involved both in the activation of toluene and in the insertion of nitrogen in this reaction, whereas antimony does not play a specific role in the reaction mechanism. In propane ammoxidation, on the other hand, due to a higher reaction temperature with respect to toluene (500°C vs. 370°C), free vanadia on the surface of the catalyst has a negative influence on the selectivity because it promotes the conversion of ammonia to nitrogen, decreasing the surface nitrogenous species required for the selective formation of acrylonitrile. Excess antimony is thus necessary for completing the reaction between antimony and vanadium oxides, but antimony also participates in the reaction mechanism. In propane ammoxidation, in fact, XPS data show that both vanadium and antimony sites are reduced. Tentatively, vanadium sites are involved in the activation of propane, while antimony sites insert nitrogen. The differences between the toluene and propane ammoxidation mechanisms are interpreted to be primarily related to the different reaction temperatures.
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| 3. |
- Huuhtanen, Jane, et al.
(författare)
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Catalytic and Spectroscopic Studies of Vanadium Oxide Supported on Group IVb and Vb Metal Oxides for Oxidation of Toluene
- 1993
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Ingår i: Applied Catalysis A: General. - 0926-860X. ; 97:2, s. 197-221
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Tidskriftsartikel (refereegranskat)abstract
- Vanadia was deposited on TiO2, ZrO2, HfO2, Nb2O5, and Ta2O5 supports using impregnation with either an oxalic acid solution of NH4VO3 or a solution of vanadyl acetylacetonate in ethanol. Prepared samples, with a nominal vanadia content in the range 0.5–2 monolayers, were characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman and infrared spectroscopy, and were used in toluene oxidation. XRD did not reveal formation of any vanadia phase. XPS spectra showed deposited vanadium to be present mainly as V5+ on all the supports. A plot of the V:( support metal) ratio determined by XPS showed agglomeration of vanadia on TiO2, ZrO2, HfO2, and Nb2O5 to occur at loadings above 0.5 monolayer, while for Ta2O5 the dispersion seemed independent of the loading. Preparations from vanadyl acetylacetonate gave superior dispersion. Raman spectra showed bands from crystalline V2O5 on all the supports except Nb2O5. Raman and infrared bands from dispersed vanadia were present in the spectra of TiO2 (three species), ZrO2 and HfO2 (both one species) supported catalysts. No bands from vanadium-oxygen vibrational modes were seen in the spectra of Nb2O5 supported samples, but a V-OH band was observed, suggesting an amorphous structure. Some evidence was obtained for formation of amorphous VTa9O25 on Ta205. The activity for toluene oxidation increased with vanadia loading for each support, and the activity varied with respect to support at all loadings in the order TiO2 > ZrO2 > Nb2O5 > HfO2 > Ta2O5. The selectivity for formation of benzaldehyde was the highest using TiO2 and Nb2O5 supports, while for benzoic acid TiO2 was the best support.
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| 4. |
- Otamiri, JC, et al.
(författare)
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Ammoxidation of Toluene by YBa2Cu3O6 + x and Copper Oxides. Activity and XPS Studies
- 1990
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Ingår i: Applied Catalysis. - 0166-9834. ; 65:1, s. 159-174
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Tidskriftsartikel (refereegranskat)abstract
- The catalytic activities of YBa2Cu3O6+x and copper oxides have been studied in the ammoxidation of toluene. At low oxygen pressures, over YBa2Cu3O6+x selective ammoxidation to benzonitrile occurs, whereas at high pressures, only total combustion of toluene to carbon oxides is observed. X-ray photoelectron spectroscopy (XPS) analysis of YBa2Cu3O6+x catalysts indicates predominance of Cu(I) states under partial ammoxidation conditions, while under total oxidation conditions, Cu(II) states are predominant. Comparison with the catalytic performance of copper oxide catalysts shows that differences do exist. The catalysis by YBa2Cu3O6+x is greatly influenced by the oxygen content of the bulk material despite its surface composition.
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| 5. |
- Otamiri, JC, et al.
(författare)
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Effect of Zn Substitutions in YBa2Cu3O6 + x Phases on Reactivity during the Ammoxidation of Toluene
- 1991
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Ingår i: Journal of the Chemical Society, Faraday Transactions. - : Royal Society of Chemistry (RSC). - 0956-5000 .- 1364-5455. ; 87:8, s. 1265-1271
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Tidskriftsartikel (refereegranskat)abstract
- Zn substitution in Y-Ba-Cu-O phases influences the catalytic ammoxidation of toluene. The activity for formation of both benzonitrile and CO2 at low and high O2 pressures, respectively, decreases strongly with increasing Zn content up to 4%. Above this substitution level, the activity, however, increases slightly. Characterisations of the surface by XPS and of the bulk by XRD analysis, indicate the presence of overlayers on the catalysts. In these overlayers, irrespective of the Zn content of catalysts, Cu1 states predominate at low O2 pressure and Cu11 at high pressure. These states are associated with selective and non-selective properties, respectively. The activity of the catalysts shows a general tendency to increase with surface copper concentrations as found from XPS studies. However, strong deviations from linearity indicate that the activity of the overlayer is greatly influenced by the bulk acting as a support.
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| 6. |
- Andersson, Arne, et al.
(författare)
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Approximate Indexed Lists.
- 1998
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Ingår i: Journal of Algorithms. ; 29:2
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Tidskriftsartikel (refereegranskat)abstract
- Let the position of a list element in a list be the number of elements preceding it plus one. An indexed list supports the following operations on a list: Insert; delete; return the position of an element; and return the element at a certain position. The order in which the elements appear in the list is completely determined by where the insertions take place; we do not require the presence of any keys that induce the ordering.We consider approximate indexed lists, and show that a tiny relaxation in precision of the query operations allows a considerable improvement in time complexity. The new data structure has applications in two other problems; namely, list labeling and subset rank.
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| 7. |
- Andersson, Arne, et al.
(författare)
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Comment on "self-indexed sort"
- 1996
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Ingår i: SIGPLAN notices. - : Association for Computing Machinery (ACM). - 0362-1340 .- 1558-1160. ; 31:8, s. 40-41
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 8. |
- Andersson, Arne, et al.
(författare)
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Fusion trees can be implemented with AC0 instructions.
- 1999
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Ingår i: Theoretical Computer Science. ; 205, s. 337-344
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Tidskriftsartikel (refereegranskat)abstract
- Addressing a problem of Fredman and Willard, we implement fusion trees in deterministic linear space using AC^o instructions only. More precisely, we show that a subset of {0,...,2^(w-1)} of size n can be maintained using linear space under insertion, deletion, predecessor, and successor queries, with O(log n/loglog n) amortized time per operation on a RAM with word size w, where the only computational instructions allowed on the RAM are fuinctions in AC^0. The AC^0 instructions used are not all available on today's computers.
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| 9. |
- Andersson, Arne
(författare)
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General balanced trees
- 1999
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Ingår i: Journal of Algorithms. ; 30, s. 1-28
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Tidskriftsartikel (refereegranskat)abstract
- We show that, in order to achieve efficient maintenance of a balanced binary search tree, no shape restriction other than a logarithmic height is required. The obtained class of trees, general balanced trees, may be maintained at a logarithmic amortized cost with no balance information stored in the nodes. Thus, in the case when amortized bounds are sufficient, there is no need for sophisticated balance criteria.The maintenance algorithms use partial rebuilding. This is important for certain applications, and has previously been used with weight-balanced trees. We show that the amortized cost incurred by general balanced trees is lower than what has been shown for weight-balanced trees.
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| 10. |
- Andersson, Arne, et al.
(författare)
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Implementing Radixsort
- 1998
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Ingår i: ACM Journal of Experimental Algorithmics.. ; 3
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Tidskriftsartikel (refereegranskat)abstract
- We present and evaluate several optimization and implementation techniques for string sorting. In particular, we study a recently published radix sorting algorithm, Forward radixsort, that has a provably good worst-case behavior. Our experimental results indicate that radix sorting is considerably faster (often more than twice as fast) than comparison-based sorting methods. This is true even for small input sequences. We also show that it is possible to implement radix sorting so as to achieve good worst-case running time without sacrificing average-case performance. Our implementations are competitive with the best previously published string sorting programs.
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