Preparation And Performance Of Hydrophobic Catalysts For Hydrogen-water Isotopic Exchange

Hydrogen isotope exchange reactions between hydrogen and water over a catalyst are important in water detritiation and heavy water production. Hydrophobic catalysts are substances that repel liquid water but allow the transport of gaseous reactants and reaction products to and from catalytic active centers. Given the need to develop these catalysts, we used a statistical thermodynamic method to calculate the equilibrium constants for D/H, T/D, and T/H exchanges. The separation factors for D/H and T/D exchange reactions were measured via static exchange experiments. We analyzed the effect of hydrogen isotope concentration on the separation factor for D/H exchange and studied the ordered structures of hydrophobic catalysts. We successfully prepared foamed and cellular structures of hydrophobic catalysts by adding polymethyl methacrylate (PMMA). Pt surfaces were modified using metal oxides to improve the exchange activities of modified hydrophobic catalysts. The liquid phase catalytic exchange (LPCE) process was designed based on the results of the exchange experiment. The production of deuterium-depleted water and water detritiation by LPCE were also designed. We performed numerical simulations to determine the optimum operating parameters and to predict the exchange performance of the column. The simulation results were verified through the experiments. All the research contents and results are as follows:(1) According to statistical thermodynamic methods and Raoult’s law, the equilibrium constants for D/H, T/D, and T/H exchange reactions between hydrogen and water vapor as well as those for water vapor-liquid water phase transition can be computed. The static exchange equilibria of H2-HDO (1) and D2-DTO (1) were obtained at different temperatures in a closed reactor. Pt/styrene divinylbenzene copolymer (Pt/SDB) was used as a hydrophobic catalyst. The results show that the theoretical equilibrium constants of H2-HDO (1) and D2-DTO (1) exchange reactions were similar to the separation factor at low concentrations of deuterium or tritium. The maximum errors of D/H and T/D were less than8%and2%, respectively. In addition, the effect of low deuterium concentrations on the separation factor could be ignored. The equilibrium constants of vapor phase catalytic exchange, water phase transition, and liquid phase catalytic exchange decreased with increased temperature. Heavy elements in the gas phase were easily transferred to the liquid phase at low exchange temperatures. The results were used to evaluate the exchange capacities.(2) Carbon-supported Pt catalysts were prepared in dispersant ethylene glycol via the impregnation-liquid-reduction method with formaldehyde as the reducing agent. The mean particle size was<2.4nm in the Pt/C catalyst for a Pt-loaded content of<20%. Pt0, Pt2+, and Pt4+coexisted in the catalyst, and about60%of which was Pt0. The Pt (111) crystal surface was easily exposed. The differences in particle sizes of Pt-supporting different carriers were insignificant. The oxidation of the atmosphere greatly influenced the surface chemical states of active metals.(3) Polytetrafluoroethylene (PTFE) was used to bind carbon-supported Pt catalysts and inert carriers as well as provide water resistance for the catalysts. Porous ceramic spheres (diameter,5.5mm; porosity,30%) and a fiber blanket (thickness,0.2mm; porosity,80%) were used as inert carriers for hydrophobic catalysts (0.8%Pt/C/PTFE). Pt/C/PTFE hydrophobic catalysts supported by porous ceramic spheres are strong and easy to fill. On the other hand, Pt/C/PTFE catalysts supported by fiber blankets exhibit a very high binding strength and a variety of loading specifications that are easy to prepare. The overall mass transfer coefficient of hydrogen isotope exchange between hydrogen and water increased with increased active metal loading. The optimal loading ratio was about30%at a low gas-liquid ratio. Hydrogen and water vapor diffused in the catalyst (physical process), and isotope exchange reactions then occurred at Pt and its oxide active sites (chemical process).(4) Hydrophobic catalysts with foamed and cellular structures were successfully prepared by adding PMMA to improve the characteristics of the catalysts. The physical parameters and microstructures of the hydrophobic catalysts were characterized by air permeability, specific surface area, pore volume, and scanning electron microscopy. Deuterium was separated from liquid water via liquid-phase catalytic exchange reactions. The hydrophobic treatment at365℃for15min released significant amounts of gas and resulted in>85%weight loss. Therefore, the modification of hydrophobic catalytic microstructures is satisfactory. The physico-textural characteristics of the modified hydrophobic catalysts were more improved, and their column efficiencies between hydrogen and water increased by20%to25%compared with those of conventional hydrophobic catalysts. The enhanced activities of the modified hydrophobic catalysts are attributed to micro structural factors. PMMA powder decreased the internal diffusion of water vapor and hydrogen inside the catalyst and improved the utilization of active sites (physical process).(5) Transition metals Fe, Ni, and Cr were added to pure Pt to prepare Pt3M/C catalysts via nanocapsule technology. The physical properties of the catalysts were characterized via X-ray photoelectron spectroscopy, transmission electron microscopy, and X-ray diffraction. The activities of hydrophobic Pt3M/C/PTFE catalysts were tested via LPCE experiments between H2and HDO. The results revealed the formation of a solid Pt-Ni alloy. Cr and Fe in oxide forms were observed on the surface of Pt (labeled M oxide@Pt). The activities of the hydrophobic catalysts during H/D isotope exchange followed the order Pt/C/PTFE<Pt3Ni/C/PTFE<Pt3Fe/C/PTFE<Pt3Cr/C/PTFE. The activities of hydrophobic M oxide@Pt catalysts were more improved than those of Pt-alloy hydrophobic catalysts. This result was attributed to the hydrophilic oxides on the surface of platinum. The activation energy of the transportation of hydrogen atom was reduced, the formation of H3O+was accelerated, and the exchange between isotopic hydrogen atom and water molecule proceeded via proton transfer (chemical process).(6) For a gas-liquid ratio of1.53and an exchange temperature of70℃, the theoretical plate height of the hydrophobic catalyst (HETP=34.2cm) was slightly lower than the reported values. Changing the deuterium concentration of the exchange column outlet water (Xb) yielded a nonlinear change in the height of the packing layer (h). The configuration of deuterium-depleted potable water and the detritiation of heavy water provide references for practical applications. The separation factors for liquid-vapor phase transition and catalytic exchange depended on the temperature and concentration throughout the exchange column. The exchange performance of the column can thus be calculated by the reaction equilibrium and the material balance for deuterium. The experimental results were verified by simulations by using a gas-liquid ratio of1and exchange temperatures ranging from20℃to75℃. Increasing the temperature yielded consistency in the calculated results with the experimental values; however, the former were somewhat higher than the latter. The optimum exchange temperature decreased with increasing gas-liquid ratio. The deuterium concentration at the upper column exhibited nonlinearity for all stages. The pressure drop per stage was also determined, which induced insignificant changes in the performance of D/H exchange reaction.