The effect of sulfidation on the Ni distribution in Ni/USY zeolites

NI/USY zeolite catalysts have been prepared by either an ion- exchange or an impregnation procedure. The modification of support and the effect of sulfidation on the Ni distribution were examined by various techniques (water adsorption, FT i.r. spectroscopy of chemisorbed NO, XPS, Xe-129 n.m.r. and Xe adsorption). in all catalysts the sulfidation of Ni is incomplete and the method of Ni introduction influenced the Ni distribution in Ni/USY zeolites. In unsulfided samples ion- exchange technique led to Ni located essentially in hexagonal prisms, whereas in the other samples prepared by impregnation procedure the Ni concentrates were located at or near the outer zeolite surface. Some nickel redistribution has been observed during catalyst sulfidation. (C) Elsevier Science Inc. 199

The effect of sulfidation on the Ni distribution in Ni/USY zeolites

NI/USY zeolite catalysts have been prepared by either an ion- exchange or an impregnation procedure. The modification of support and the effect of sulfidation on the Ni distribution were examined by various techniques (water adsorption, FT i.r. spectroscopy of chemisorbed NO, XPS, Xe-129 n.m.r. and Xe adsorption). in all catalysts the sulfidation of Ni is incomplete and the method of Ni introduction influenced the Ni distribution in Ni/USY zeolites. In unsulfided samples ion- exchange technique led to Ni located essentially in hexagonal prisms, whereas in the other samples prepared by impregnation procedure the Ni concentrates were located at or near the outer zeolite surface. Some nickel redistribution has been observed during catalyst sulfidation. (C) Elsevier Science Inc. 199

The Vaca Muerta transgression (Upper Jurassic), Neuquén Basin, Argentina: Insights into the evolution and timing of aeolian-marine transitions

Considering the evolution of aeolian to marine transitions for the geological record, either catastrophic or gradual transgressive scenarios showing high or low rates of coastal migration have been proposed. A critical evaluation of modern analogues suggests that a catastrophic transgression shares many characteristics with Holocene transgressions, yet they are caused by different rates of sea-level rise. The present study provides insights into the evolution of aeolian to marine transitions in order to discuss these alternative scenarios of sea-level rise. For this purpose, a sedimentological and ichnological analysis was carried out on ten stratigraphic sections of the Picún Leufú area, Argentina. There, marine deposits of the Vaca Muerta Formation accumulated over the aeolian deposits of the Quebrada del Sapo Formation during the early Tithonian. The sedimentary evolution of the transition can be summarized in: (i) a shutdown of aeolian dune field deposition, generating a planation surface in somewhat elevated areas and reworked megadunes in lowlands; (ii) beach sedimentation caused by episodic marine flooding that contributed to megadune reworking; and (iii) deposition in an embayed marginal-marine setting at the coast, recorded by bay margin bindstone, proximal bay and distal bay sedimentation. This transition indicates very rapid coastline migration and a condensed Transgressive Systems Tract succession throughout the study area. Rates of sea-level rise similar to Holocene ones (millimetres to centimetres per year) may have produced the transition between the Quebrada del Sapo and Vaca Muerta formations. The Late Jurassic represents a non-glacial time, and the global sea-level maximum highstand pre-dated the Vaca Muerta transgressive event. Thus, part of the sea-level rise has to be attributed to tectonic/thermal subsidence and compaction of underlying strata, which may have generated these atypical rapid rates of sea-level rise