MOLECULAR DYNAMICS SIMULATIONS OF FILLED AND EMPTY CAGE-LIKE WATER CLUSTERS IN LIQUID WATER AND THEIR SIGNIFICANCE TO GAS HYDRATE FORMATION MECHANISMS

Molecular dynamics simulations are performed to observe the evolutions of 512 and 51262 cage-like water clusters filled with or without a methane molecule immersed in bulk liquid water at 250 K and 230 K. The lifetimes of these clusters are calculated according to their Lindemann index δ (t) using the criteria of δ≥0.07. For both the filled and empty clusters, we find the dynamics of bulk water determines the lifetimes of cage-like water clusters, and that the lifetime of 512 62 cage-like cluster is the same as that of 512 cage-like cluster. Although the methane molecule indeed makes the filled cage-like cluster more stable than the empty one, the empty cage-like cluster still has chance to be long-lived compared with the filled clusters. These observations support the labile cluster hypothesis on the formation mechanisms of gas hydrates.

Understanding the hydroxyl adsorption behavior at Pt electrode surface in high-temperature alkaline solutions

Since the application in fuel cell,the electrochemical adsorption of hydroxyl has received considerable attention in recent years.While most research mainly focus on the room temperature,in this paper,the electrochemical adsorption of hydroxyl in alkaline solution at high temperature was investigated.An unusual oxidation peak was observed at-0.27 V,suggesting new behavior of hydroxyl adsorption occurred.As is known two kinds of cation hydrated clusters exist in alkaline solution,(H_(2)O)_(x-1)M^(+)-H_(2)O-O_(ad)H and(H_(2)O)_xM^(+)-O_(ad)H.For K^(+)and Cs^(+),the cluster shows unstable structure due to the weak interaction between hydrated cation and OH^(-)especially at high temperature.However,For Li^(+),Na^(+)the cluster structure would be stable,as the interaction force between the hydrated cation and OH^(-)is so strong.It was revealed that the unusual oxidation peak has some relationship with the(H_(2)O)_(x-1)M^(+)-H_(2)O-O_(ad)H cluster(K^(+)and Cs^(+))absorbed at Pt electrode surface.When the temperature was raised,(H_(2)O)_(x-1)M^(+)-H_(2)O-and-O_(ad)H was disconnected,then the O_(ad)H absorbed at Pt surface got oxidated.Based on the SEM observation,it was showed the unusual electrochemical oxidation reaction would generate platinum oxides,blocking the reactive sites at Pt electrode surface,thus reducing the electrochemical reactivity of Pt electrode.Accordingly,parameters of alkaline concentration and temperature were systematically studied,it was found that increase temperature or alkaline concentration was in favor of the unusual oxidation reaction.This study provides more understanding of hydroxyl adsorption behavior at Pt electrode surface for the high temperature water solution environment.