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31.
J Loos PJ Lemstra EME van Kimmenade JW Niemantsverdriet GWH Hhne PC Thüne 《Polymer International》2004,53(7):824-827
The morphology and the thermal behavior of nascent polyethylene (PE) polymerized on flat models for the Phillips CrOx/SiO2 catalyst were investigated using low‐voltage scanning electron microscopy (LVSEM) and differential scanning calorimetry (DSC). Ethylene polymerizations were performed at 25 °C and 70 °C in gas phase. For the applied polymerization conditions, microscopy observations reveal that the formation of the macromolecules at the active catalyst sites, and their subsequent crystallization on the catalyst, result in homogeneous polymer layers having a thickness of the order of micrometers. The surface morphology of the nascent samples consists of spherical entities having a diameter less than one micrometer. Pillar‐like objects form the internal structure of the PE films. Because these entities are only loosely connected, high porosity is observed within the entire film. Thermal analysis of the samples shows that melting temperature and crystallinity of the nascent PE samples polymerized at 25 °C are noticeably higher than after melting and recrystallization. Samples polymerized at 70 °C, however, do not show these features. Copyright © 2004 Society of Chemical Industry 相似文献
32.
J. van de Loosdrecht B. Balzhinimaev J.-A. Dalmon J.W. Niemantsverdriet S.V. Tsybulya A.M. Saib P.J. van Berge J.L. Visagie 《Catalysis Today》2007,123(1-4):293-302
Cobalt catalysts as used in the Fischer-Tropsch synthesis (FTS) are relatively expensive (as compared to iron) and need to have a high metal dispersion and long life to be able to offer a good balance between cost and performance. The oxidation of nano-sized metallic cobalt to cobalt oxide during Fischer-Tropsch synthesis has long been postulated as a major deactivation mechanism. However, to date there is no consistent picture. This paper presents an extensive overview of the literature on this topic of deactivation by means of oxidation for unsupported as well as silica-, alumina- and titania-supported cobalt catalysts. Furthermore, it presents results on the deactivation of an industrial Co/Al2O3 catalyst as obtained by pseudo in situ X-ray diffraction, magnetic measurements and X-ray absorption near-edge spectroscopy. These analyses were performed to study the oxidation state of spent industrial Co/Al2O3 catalyst samples withdrawn from a slurry reactor operating under realistic FTS conditions, and it was concluded that oxidation can be ruled out as a major deactivation mechanism. Finally, these data together with all relevant literature were used to create a common view on the oxidation behaviour of metallic cobalt during FTS. The apparent discrepancies in literature on the oxidation behaviour of cobalt are most likely due to the lack of direct characterisation of the cobalt oxidation state and due to the comparison of catalysts with varying cobalt crystallites sizes, compared at different reactor partial pressures of hydrogen and water (PH2O/PH2). It was shown that the oxidation of cobalt can be prevented by selecting the correct combination of the reactor partial pressures of hydrogen and water (PH2O/PH2) and the cobalt crystallite size. 相似文献
33.
Coulier L. de Beer V.H.J. van Veen J.A.R. Niemantsverdriet J.W. 《Topics in Catalysis》2000,13(1-2):99-108
Model catalysts, consisting of a conducting substrate with a thin SiO2 layer on top of which the active catalytic phase is deposited by spincoating impregnation, were applied to study the formation
of the active CoMoS phase in HDS catalysts. The catalysts thus prepared showed representative activity in the hydrodesulfurization
of thiophene, confirming that these models of HDS catalysts are realistic. Combination of the sulfidation behaviour of Co
and Mo studied by XPS and activity measurements shows that the key in the formation of the CoMoS phase is the retardation
of the sulfidation of Co. Complexing Co to nitrilotriacetic acid complexes retarded the Co sulfidation, resulting in the most
active catalyst. Due to the retardation of Co in these catalysts, the sulfidation of Mo precedes that of Co, thereby creating
the ideal conditions for CoMoS formation. In the CoMo catalyst without NTA the sulfidation of Co is also retarded due to a
Co–Mo interaction. However, the sulfidation of Mo still lags behind that of Co, resulting in less active phase and a lower
activity in thiophene HDS.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
34.
B.L. Mojet L. Coulier J. van Grondelle J.W. Niemantsverdriet R.A. van Santen 《Catalysis Letters》2004,96(1-2):1-4
UV–Raman spectroscopy is demonstrated to be a valuable tool to study molybdenumoxide model catalysts at ambient pressure while visible light Raman yields no information. UV–Raman offers the possibility to characterize MoO x /Al 2O3/Si and MoO x /SiO2/Si model systems with sub-monolayer loading of MoO x even though the surface area of a Si-wafer is much lower than that of a bulk support. Anchoring of MoO x onto Al2O3/Si and SiO2/Si results in similar molybdenum oxide compounds as for bulk catalysts. 相似文献
35.
Pieter L. J. Gunter J. W. Niemantsverdriet Fabio H. Ribeiro Gabor A. Somorjai 《Catalysis Reviews》1997,39(1):77-168
Nanoscale structural information underlies research aimed at fabricating catalysts in a more controlled way. Surface science methods can provide that information, but the complexity of heterogeneous systems in general hinders the application of these methods to their full potential. In the last decades, a solution to this problem has been found in the use of model systems, ranging from well-defined single crystals of the supported phase to films or particles of that phase on flat or spherical model supports. In this paper, we review the literature on the latter model systems, that is, particles on a model support. Attention is payed to both preparation and use of such model systems. 相似文献