异相催化还原制备PUREX流程中U/Pu分离试剂U(IV)的反应机理研究

U (IV) is a relative ideal reductant in uranium and plutonium separation stage in the PUREX process. U(IV) is produced by hydrazine reducting U(VI) with Platinum as catalyst, which has advantages of the simple material liquid composition , mild reaction process, short reaction process, and the liquid product can be directly applied. It has been long applied in the nuclear fuel reprocessing plant, but the reaction mechanism is unclear. This project will study the types of primary radicals and the producing way of hydrazine on platinum catalyst. And the purpose is to determine the free radicals reaction of hydrazine on platinum surface and the broken mode of the chemical bond of the hydrazine molecules, and to clarify the types of primary radicals. By investigating the subsequent reaction of the primary radicals and mediums in different concentrations of U (VI), hydrazine, nitric acid etc in reaction system, and examining the generated secondary free radical product,free radical kinds and the concentrations of final products, to determine the reaction chain under different systems, it can get the influence of different concentration of reactants on the reaction mechanism. The article will study reaction heat, dynamics of hydrazine catalytic reduction, and the analog computation is carried out based on the reaction kinetics of the reaction process to determine the reaction mechanism. Thus it can intuitively know the specific reaction mechanism of U (IV) preparation process, understand the characteristics of the reaction, determine the reaction controlling steps and it can be targeted to speed up the reaction rate of this controlling steps, improve the activity and selectivity of the catalyst, which can provide the theory basis and technical support for industrialized production and the optimization of preparation technology of uranium plutonium separation reagent U (IV) in process of spent fuel reprocessing and reducing side reactions.

U(IV)是PUREX流程铀钚分离工艺中较为理想的还原剂。铂催化肼还原U(VI)制备U(IV)的方法具有料液组成简单、反应过程温和、反应流程短、所得产品液能直接应用等优点，但异相催化反应机理尚不清楚，难使反应处于精确可控状态。本项目研究肼在铂催化剂上分解产生初级自由基的种类及途径，以确定肼分子的断键方式，澄清肼在铂上产生初级自由基的反应历程；研究初级自由基与反应体系中U(VI)、肼、硝酸等的后续反应，考察生成的次级自由基产物及最终产物的种类与浓度，确定各反应体系下的反应链，得到不同反应物浓度对反应历程的影响；研究肼催化还原反应的热力学、动力学，并对反应过程的动力学进行模拟计算，进一步确定反应机理；以期直观地认识U(IV)制备的具体反应历程，了解反应特点，确定反应控制步骤，提高催化剂活性及选择性，为铀钚分离试剂U(IV)制备工艺的优化、减少副反应产生、工业化生产提供理论依据及技术支持。

U(IV)是乏燃料后处理PUREX流程铀钚分离工艺中较为理想的还原剂，具备了在核燃料后处理厂中应用的前景。本项目开展铂催化肼还原 U(VI)制备 U(IV)反应类型、历程、过程热动力学及机理等系统研究。采用ESR 法研究肼在铂催化剂上产生的自由基，确定肼分子断键方式为N-N断键与N-H断键共存，N-H断键在随H＋浓度变化过程中起主导作用，N-N断键和N-H断键速率由肼在酸性溶液中的存在形式决定。通过分析硝酸介质下铂催化肼还原U(VI)制备U(IV)过程中的产物，发现产物中没有叠氮酸、氮氧化物及氢气，主要是N2，生成的N2与消耗的肼接近1：1；当存在U（VI）时，生成的NH4+产量较低，当U（VI）反应完全后，NH4+的产率急剧增大；反应温度升高有利于U(VI)还原反应的进行。铂催化肼还原U(VI)制备U(IV)是一个典型的异相催化反应，随温度升高，反应由动力学控制转变为扩散控制，动力学控制过程和扩散控制过程活化能分别为64.65kJ/mol和11.03kJ/mol；U(VI)和肼初始浓度、催化剂固液比为促进反应进行的影响因素，硝酸初始浓度为抑制反应进行的影响因素；反应动力学方程为：-dc(UO22+)/dt)=kc0.5323(UO22+)c0.2074(N2H5+)c-0.2009(H+)，当温度为50℃、催化剂固液比r=0.0667g/mL时，反应的动力学速率常数为0.00199(mol/L)0.4712/min。采用 Gaussion09 软件，对肼催化还原 U(VI)反应的动力学进行模拟计算，得到了UO22+、肼、air状态下UO2(H2O)52+、溶剂化下UO2(H2O)52+、DMPO、DMPO捕捉•OH后等分子的最优几何状态、几何数据、模拟IR光谱和单点能；及中性溶液、质子化肼和添加H3O条件下N-N断键、N-H断键的过渡态分子构型、反应物结构、产物结构、IRC曲线和能垒，对反应物、过渡态、产物结构进行NBO分析，得到了Mayer键级数据。反应机理为：H+在铂原子上发生解吸，肼成功吸附在铂表面的活性位点后，会发生N-N断键或N-H断键，断键产物•H和•N2H3对UO22+进行还原。本项目的研究对乏燃料后处理中铀钚分离试剂U(IV)制备工艺的优化、减少副反应产生及工业化生产具有重要的理论意义，对促进我国核燃料后处理工业的发展具有重要的实际意义。