We present dissociative adsorption probabilities of H 2 on Pd͑111͒ computed with the classical trajectory method. We perform both classical ͑C͒ and quasiclassical ͑QC͒ calculations, the latter including, by contrast with the former, the initial zero point energy ͑ZPE͒ of H 2 . We analyze in detail the role played by the ZPE and demonstrate the strong and weak points of both C and QC calculations. We show that ZPE is crucial in accelerating the molecules toward the surface through vibrational softening. However, at low energies, dynamic trapping is quenched in QC calculations by processes of vibration to rotation energy transfer that would be associated with closed channels in a quantum approach. In this study we use a new representation of the H 2 /Pd(111) potential energy surface ͑obtained by interpolation of ab initio data͒ with a significantly better accuracy in the entrance channel region which plays a decisive role in the dissociation dynamics.
Highly concentrated CdS colloids (about 30 wt %) are prepared in acetone through the grafting of 4-fluorophenylthiol at their surface. 19 F NMR measurements show that the controlled oxidation by an aqueous solution of hydrogen peroxide leads to a mixture of dithiol and fluorosulfonate species. Aggregation of particles produces either transparent gels or opaque precipitates, depending on the nature and the concentration of the oxidant. The sol-gel transformation results from an aggregation mechanism, which requires the release of surface groups because of their interaction with water molecules. Considering the formation of mass-fractal clusters, small-angle X-ray scattering measurements show, at least in the first steps, that the growth kinetics is in agreement with the usual reaction limited cluster aggregation mechanism previously observed in the silica system. This can be followed by a saturation effect, enhanced at low water concentration, which is due to the poisoning of the surface by chemical groups produced by oxidation.
We have used a modified Shepard (MS) interpolation method, initially developed for gas phase reactions, to build a potential energy surface (PES) for studying the dissociative chemisorption of H2 on Pt(111). The aim was to study the efficiency and the accuracy of this interpolation method for an activated multidimensional molecule-surface reactive problem. The strategy used is based on previous applications of the MS method to gas phase reactions, but modified to take into account special features of molecule-surface reactions, like the presence of many similar reaction pathways which vary only slightly with surface site. The efficiency of the interpolation method was tested by using an already existing PES to provide the input data required for the construction of the new PES. The construction of the new PES required half as many ab initio data points as the construction of the old PES, and the comparison of the two PESs shows that the method is able to reproduce with good accuracy the most important features of the H2 + Pt(111) interaction potential. Finally, accuracy tests were done by comparing the results of dynamics simulations using the two different PESs. The good agreement obtained for reaction probabilities and probabilities for rotationally and diffractionally inelastic scattering shows clearly that the MS interpolation method can be used efficiently to yield accurate PESs for activated molecule-surface reactions.
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