Chemical Composition Of Clusters Research Articles

Effect of modifiers on Fe-Pt/Al2O3 catalysts for alkanes hydrotreatment

Zeolite-containing polyfunctional catalysts Fe-Pt/Al2O3 (КТ-17, КТ-18), modified with additives of molyb-denum, phosphorus and cerium, were synthesized. The developed catalytic systems were studied in treatment of C14 alkane obtaining a gasoline fraction. Reaction products contain C4-C9 iso-alkanes, C10-C14 iso-alkanes, C4-C10 alkanes, aromatic hydrocarbons and C1-C3 alkanes. In addition, 0.2–0.6 % of heavy hydro-carbons was found in reaction products. The yield of C4-C9 iso-alkanes at 380 °C reached 37.9 %. By means of physical and chemical methods it has been found that the zeolite containing catalysts Fe-Pt /Al2O3 modi-fied by various additives are complex systems. Micro-diffraction and Mossbauer spectroscopy methods al-lowed detecting nanosized hetero clusters of Fe-Pt, Fe-Mo, Pt-Mo in catalysts structure. Depending on chem-ical composition of clusters, particle size varies between 20 and 80 Ǻ. KT-18 catalyst demonstrates high ac-tivity in the process of heavy alkanes treatment; sizes of platinum (d = 200 Å) and iron (d = 30–50 Å) parti-cles were determined by electron microscopy. Activity of KT-18 catalyst is higher than that of highly dis-persed KT-17. The main feature of KT catalysts is their polyfunctionality. During alkanes processing simul-taneous and consecutive reactions of hydrocracking, dehydrogenation, isomerization, dehydro-cyclization and hydro-desulfurization take place.

Enhanced positron trapping by Ag nanoclusters at low temperatures: A challenge of positron sensitivity to quantum dots

Microstructure evolution of three Al-Ag alloys with different Ag contents (1 wt. % Ag, 5 wt. % Ag, and 15 wt. % Ag) was studied by positron annihilation spectroscopy during the aging process. In situ measurements of the positron lifetime and Doppler broadening of annihilation radiation indicate the fast formation of Ag-rich clusters during natural aging of the alloys. The formation of Ag-rich clusters was further confirmed by coincidence Doppler broadening measurements. The Ag signal reflected by the Coincidence Doppler broadening spectrum increases with increasing Ag content and is further enhanced after subsequent artificial aging at 140 °C. This might be due to the increase in the size of Ag clusters. The temperature dependence of the Doppler broadening spectra between 10 K and 290 K was measured for the Al-Ag alloys after natural and artificial aging. Detrapping of positrons from Ag clusters with increasing temperature was observed for all the three Al-Ag alloys after natural aging and for the Al-1 wt. % Ag after artificial aging. This indicates that Ag clusters act as shallow positron trapping centers. The thermal detrapping of positrons becomes ambiguous with increasing Ag content in the alloy and is nearly invisible in the artificially aged Al-5 wt. % Ag and Al-15 wt. % Ag. The positron binding energy of the Ag cluster is roughly estimated to be about 18.8 meV and 50 meV in the Al-1 wt. % Ag sample after natural aging and artificial aging at 140 °C, respectively, which suggests that the confinement of positrons in the quantum-dot like state depends on the size or chemical composition of clusters. Theoretical calculations confirm positron trapping by Ag nanoclusters, and the confinement of positrons is enhanced with increasing Ag cluster size.

Formation and reversion of clusters during natural aging and subsequent artificial aging in an Al–Mg–Si alloy

The continuous changes in number density, size distribution and chemical composition of clusters during natural aging (NA), and subsequent artificial aging (AA) at 443K, in an Al–0.62Mg–0.93Si (mass%) alloy have been evaluated using atom probe tomography. The data show almost no change in the average size and Mg/Si ratio (at% ratio) of clusters during NA. The highest hardness and most-dense clusters, following NA, and the largest drop in hardness, during AA for 1.2ks, are observed in the material with the longest NA period. In contrast with the published literature, which suggests that only small clusters revert into solution, the number density of clusters decreased, with no dependence on the cluster size, during AA for 1.2ks, resulting in a slight decrease in the average radius of clusters. Things to be emphasized are that the number density of clusters with the Mg/Si ratio below 0.4 stays pretty much the same during AA, whereas other clusters decrease during the first 1.2ks and then increase again during the prolonged AA for up to 3.6ks. This work is the first to reveal that the typical Si-rich clusters can neither be dissolved nor grow further, although the size is small. In other words, it is believed that the typical Si-rich clusters can lead to the retardation of the hardness increase during AA called as negative effect of NA.