Research For The Surface Structure Of Pt/C Catalyst And Its Activity To Hydrogen-water Isotopic Exchange Reaction

Some of the research work on hydrogen-water isotopic exchange reaction from the references has been briefly reviewed and its applications to a series of hydrogen isotope separation processes, such as wastewater detritiation, heavy water production, and heavy water detritiation and upgrading etc, have been discussed in detail. Perspective focused on the development of hydrogen-water isotopic exchange process was presented. According to the research status and development, the Pt/C catalyst was selected as the precursor of Pt/C/PTFE ordered bed hydrophobic catalyst for the hydrogen-water isotopic exchange reaction. The dispersion of platinum metal on the carbon support has been thoroughly studied in the manner combined the ab initio calculations and experimental researches. The technique for the preparation of highly dispersed platinum crystallites on carbon has been developed by directly impregnating the surface modified carbon supports with chloroplatinic acid solution.The molecular interaction theory model between platinum clusters and the surface chemical functional groups on carbon carriers has firstly been proposed and adopted to calculate the support effect of the Pt/C catalyst. Typical chemical functional groups on carbon carriers, including carboxyls, vinyls, and cyclic ethers, were taken into account and computated with a basis set of CEP-31G at the B3LYP theoretical level. Computational results have shown that the electronic interactions are so strong that there has occurred the formation of ligand complexes or Ï€ -complex structures between Pt_n (n=1-4) and R-COOH (R=H; C₆H₅-), CH₂=CH₂, and C₄H₈O (cyclic ether). The maximum electronic interaction energy was -671.30 kJ·mol^-1 (-6.957eV) for the HCOOH-Pt₃ complex. The calculation results also indicated that the electronic interaction energy was in the order of R-COOH (R=H; C₆H₅-)>CH₂=CH₂ >C₄H₈O (cyclic ether).The molecular interaction theory model was also adopted the first time to calculate the support effect of the Pt/C catalyst on its catalytic activities to those reactions resulting in hydrogenlysis. Computational results of intermolecular electronic interaction between bonze and PtH molecules suggested that the formation bf Ï€ -complex weakened the Pt-H bond and thus enhanced the catalytic activity ofthe Pt/C catalyst.Thermochemical quantities and equilibrium constants for the isotopic exchange reactions between hydrogen and water vapor were obtained through the ab initio calculation with the basis set of 6-311G** at the B3P86 theoretical level. The results of six different kinds of hydrogen-water isotopic exchange reactions have been presented in this paper. While compared to the experimental values from the reference, the calculated equilibrium constants for HD-H2O isotopic reaction expressed well with existing literature data, resulting in the reasonable deviations less than 6% over the temperature range 283.2K³73.2K at the pressure 0.1 MPa.According to the theoretic study, the surface ligand site coordination technique using chemical functional groups as the ligands has firstly been proposed to produce highly dispersed platinum crystallites in carbon carriers' micropores generated by gasification. Briefly, the procedure consisted of gasifying carbon carriers with a gas flow of 5%O2+N2 in the range of 385°C⁴25°C, and then oxidizing them with concentrated HNO3 under controlled conditions, and finally impregnating them with the aqueous chloroplatinic acid. Characterization of the carbon carriers, before and after surface modification, with IR, XPS, SEM, and temperature-programmed desorption (TPD) analysis suggested that there were many nanopores formed through gasification and surface oxygen functional groups increased by concentrated HNO3 oxidation. Analytic results by XRD and TEM have demonstrated that the dispersion of platinum crystallites on modified carbon carriers was very high with a mean diameter of 7 nm, while the segregation of platinum crystallites occurred obviously for unmodified carbon carriers.The catalytic activities of various Pt/C catalysts to hydrogen-water isotopic exchange reaction have been measured by a method of co-current flow for hydrogen and water vapor and compared with Pt/SBD catalyst. Experimental results for H2-HDO reaction showed that surface modification of carbon supports increased activities of Pt/C catalysts nearly by a factor of two, but all of the Pt/C catalysts had the same reaction order of 0.76 + 0.03 to the reactant of water vapor and the same activation energies, which are much lower than those ones of unsupported platinum catalysts. This suggested that support modification didn't change the reaction mechanism and activity increase was owing to the increased dispersion of platinum.The Pt/C/PTFE ordered bed hydrophobic catalysts were made from Pt/C and Teflon (PTFE) and their activities to H2 (g)-HDO (1) reaction have been investigatedin the manner of counter-current flow. Experimental results showed that the H2 (g) and HDO (1) throughputs were much higher than random bed parking. And while the mole ratio of liquid water to hydrogen gas flow equaled 1.0, their overall gas-phase volume transfer rate (Kya) was higher than 50 [mol (HD) ? s"1 ? m'3] when the hydrogen gas flow was greater than 0.5 m ? s'1.