Heterogeneous Catalysis in Supercritical Media: 2. Near-Critical and Supercritical Water

Diffusivity and hydration of hydrazine in liquid and supercritical water through molecular dynamics simulations and split-flow pulse injection experiments

The diffusion properties and hydration structure of hydrazine in an aqueous solution are investigated through molecular dynamics simulations and split-flow pulse injection experiments. The simulations are performed from ambient conditions along the liquid side of the liquid-vapor coexistence curve, up to the critical point, and in the supercritical region at temperatures of 673, 773, 873, and 973 K and at densities ranging from 0.1 to 0.8 g cm(-3). The spatial distributions functions for hydrated water are presented. At ambient conditions, hydrazine is hydrated by 24 water molecules with about 1.6 H-bonds being donated to each nitrogen atom. The hydration number decreases with temperature along the coexistence curve and is seen to increase with system density in the supercritical region. At low density supercritical conditions, hydrazine has no appreciable hydration structure and is surrounded by only 2 water molecules at 873 K and 0.1 g cm(-3). The diffusion coefficients for hydrazine at subcritical state conditions are found to be in agreement with Stokes-Einstein and Wilke-Chang predictions. The diffusion coefficients in the supercritical region are found to correlate more closely with the overall fit to the Dymond equation.

Mn-Ce-Co complex oxide nanoparticles: hydrothermal synthesis and their catalytic subcritical oxidation of 4,4'-Dibromobiphenyl

In situ transformation of 4,4'-Dibromobiphenyl (4,4'-DBB) in water was observed with hydrothermal diamond anvil cell (HDAC) up to 633 K. It shows that 4,4'-DBB dissolves in water to form a homogenous phase at the temperature of 588 K and thus subcritical water oxidation of 4,4'-DBB higher than the temperature can be a homogenous phase. To accelerate the oxidative degradation, some Mn-Ce-Co complex oxide nanoparticles of about 100 nm were prepared by co-precipitation hydrothermal method. The nanoparticles show enough stability and catalytic activity for oxidative degradation of 4,4'-DBB in subcritical water. The catalytic activation increases with some Co doping and as for the complex oxides of Mn(1)Ce(1), Mn(0.9)Ce(1)Co(0.1), Mn(0.5)Ce(1)Co(0.5), Mn(0.1)Ce(1)Co(0.9), and Co(1)Ce(1), the Mn(0.9)Ce(1)Co(0.1) presents the best activation. The main intermediate products of degradation are benzoic acid and phenol. The apparent activation energy (E(a)) is 35.92 with 5% Mn(0.9)Ce(1)Co(0.1) as catalyst and 46.69 kJ/mol with no catalyst about the chemical oxygen demand (COD).