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Reconstructions in atom probe tomography (APT) are biased by image distortions arising from dynamic changes of the specimen geometry that controls image projection. Despite the strong efforts to build realistic models for understanding and reproducing image artifacts, the current models are too slow or not adapted to be routinely used in image correction approaches. To understand the APT imaging process for real size samples submitted to realistic experimental conditions of electric field and temperature, we propose an alternative simulation tool based on a coarse-grained model of the sample surface. The surface electric field on a meshed surface is calculated by using continuous models describing field evaporation. The dynamic evolution of the sample surface and the image projection are predicted using materials properties. We show that the interplay between temperature and electric field is an important ingredient in predicting the ion projection, in pure metals and in more complex materials. This fast approach accurately reproduces the well-known local magnification and trajectory overlaps effects in the evaporation of small particles. By combining prior knowledge about the sample structure and properties, the model could be used to improve the reconstruction approaches for complex sample geometries. [ABSTRACT FROM AUTHOR]

The enantioselective hydrogenation of N-acetyl dehydrophenylalanine methyl ester (NADPME) to N-acetyl phenylalanine methyl ester is investigated using cinchona-modified Pd/Al2O3 catalysts. The catalyst was prepared using deposition-reduction and was evaluated for the reaction using methanol as solvent with various cinchonine alkaloid/NADPME molar ratios. Enantioselectivity was sensitive to this ratio. For cinchonine at low cinchonine:NADPME molar ratios the S- N-acetyl phenylalanine methyl ester was formed with low enantioselection, and as the cinchonine:NADPME ratio was increased the reaction became less enantioselective. In the extreme the solubility of cinchonine limited the extent of the experimental conditions that could be explored. As expected cinchonidine modified Pd/Al2O3 initially gave R- N-acetyl phenylalanine methyl ester, again with low enantioselection. However, as the cinchonidine:NADPME molar ratio was increased the reaction initially became racemic and then was selective to the formation of S- N-acetyl phenylalanine methyl ester. This unexpected inversion in the sense of enantioselection was observed in a range of solvents. [ABSTRACT FROM AUTHOR]

The enantioselective hydrogenation of buta-2,3-dione to 3-hydroxy-buta-2-one and ethyl pyruvate to ethyl lactate are compared using cinchona-modified Pt/γ-Al2O3 catalysts. The reactions were carried out in a range of solvents and both reactants gave the same linear relationship between enantiomeric excess (e.e.) and the dielectric constant of the solvent. The e.e. for the 3-hydroxy-buta-2-one is lower than that for ethyl pyruvate. For both reactants there is an optimal concentration of the cinchona modifier but the optimal concentration required for the hydrogenation of buta-2,3-dione is approximately an order of magnitude higher than that required for the hydrogenation of ethyl pyruvate. The hydrogenation of buta-2,3-dione in acetic acid as solvent leads to a suppression in the e.e. with an enhancement in rate, whereas the reaction of ethyl pyruvate in acetic acid leads to an enhancement in e.e. The e.e. for 3-hydroxy-buta-2-one formation is independent of conversion in the initial period of the reaction which contrasts markedly with the observed initial induction period for ethyl lactate formation. The results are discussed in terms of the interaction of the reactants with the cinchona alkaloid adsorbed on the Pt surface. [ABSTRACT FROM AUTHOR]

The premodification of a 5 wt% Pt/γ-Al2O3 catalyst with cinchonidine (0.01 and 0.2 g g-1catalyst) is described and discussed. Premodification is carried out by treating the catalyst with a solution of cinchonidine followed by solvent removal. Catalysts premodified in this way give the same ee and initial rate of reaction for the enantioselective hydrogenation of ethyl pyruvate as those using the standard in situ modification procedure. Investigations of different solvents for premodification and reaction (dichloromethane, ethanol) show that it is the solvent used for the reaction that controls the observed enantioselection. Premodified catalysts also display the initial transient behavior typically observed with in situ modified catalysts in which the ee increases with conversion in the early part of the reaction. Premodified catalysts show an enhanced rate of reaction when ethanol is used as the reaction solvent compared with in situ modified catalysts under the same conditions. Premodification using aerobic conditions gives the best results and premodified catalysts can be stored prior to use for up to a week without loss of catalytic performance. [ABSTRACT FROM AUTHOR]