Adsorption characteristics and surface free energy of AlPO4-5

The micropores and surface characteristics of aluminophosphate-type zeolite, AlPO₄-5, were analyzed by examining the adsorption behavior of water and other adsorbates. Water adsorption on AlPO₄-5 occurred on both structural defects and nonpolar surfaces. Adsorption on structural defects, accompanied by high heats of adsorption, is attributed to adsorption to surface hydroxyls. Water adsorption increased steeply at a certain relative pressure depending on the adsorption temperature, and this was considered attributable to capillary condensation. The contact angle of water on AlPO₄-5 micropore surfaces can be determined quantitatively by applying the Kelvin equation. The surface free energy of AlPO₄-5 calculated on the basis of the contact angle was revealed to be about 120 mJ/m², in agreement with accepted values of the dispersion component of the surface free energy of metal oxides. Adsorption heat values of adsorbates with different polarities indicate that the AlPO₄-5 surface is essentially nonpolar and interacts only with dispersion interaction. In the case of n-hexane the contact angle was assumed to be zero, showing high affinity with the result of enhanced adsorption due to pore filling. Received: 21 May 1998 Accepted: 28 July 1998     

Acetylene-insertion reactions into Pt(II)–H and Pt(II)–SiH3 bonds. An ab initio MO study and analysis based on the vibronic coupling model

Acetylene insertion into Pt(II)–H and Pt(II)SiH₃ bonds of PtH(SiH₃)(PH₃) was investigated using ab initio molecular orbital and M?ller-Plesset perturbation theory methods. The insertion into PtH was predicted to proceed with a smaller activation energy (E _a =12.8 kcal/mol) than that into PtSiH₃ (E _a =20.9 kcal/mol). The reaction energy (ΔE) of the insertion into PtH is 10 kcal/mol smaller than that for the insertion into PtSiH₃, which reflects differences in bond energies between CH and CSi and between PtH and PtSiH₃. A comparison with ethylene insertion revealed that the acetylene insertion occurs more easily, and the latter reaction is more exothermic. A simple vibronic coupling model combined with Toyozawa's interaction mode analysis was used to examine interesting differences in E _a and ΔE between insertions into PtH and PtSiH₃, and between acetylene and ethylene insertions. This analysis suggests that the factors determining E _a are the stiffness of the PtH and PtSiH₃ bonds and the vibronic coupling strength of acetylene and ethylene. Received: 13 August 1998 / Accepted: 2 September 1998 / Published online: 15 February 1999     

Low-temperature epitaxial Ni silicidation: the role of hyperthermal species

We present the results of Ni silicidation on a Si111 surface employing a mass-selected hyperthermal ion beam at 100 eV and discuss the reaction mechanism compared with the conventional Ni silicidation process. It is found that the Ni silicide formation using this technique is different from that achieved by conventional methods such as high-energy Ni-ion implantation or evaporation with thermal species. Namely, the Ni silicide phase formed at 230 degrees C using hyperthermal ions in this study is Ni-rich Ni2Si, in contrast to Si-rich disilicide NiSi2, ordinarily formed when high-energy Ni ions or thermal Ni beams react with Si at elevated temperatures. In addition, this layer is formed epitaxially on Si in spite of a low substrate temperature of 230 degrees C, while a polycrystalline Ni silicide layer is formed with conventional Ni-rich silicidation. This suggests that the reaction mechanism of the silicide formation with hyperthermal Ni particles is different from that using higher- or thermal-energy Ni particles. The atomic rearrangement induced by the thermal spikes most likely plays an important role in the Ni silicidation process employing hyperthermal species.