钯-货币金属合金催化乙炔半氢化反应的理论模拟

Semi-hydrogenation of acetylene to ethylene is currently used to reduce the content of acetylene in ethylene-acetylene mixture to produce polymer-grade ethylene. This reaction embodies the three features of selectivity, i.e., to a specific reaction, to a specific reactant and to a specific product, and thus is an ideal system for investigating selectivity. Studies of selective hydrogenation are of importance and significance because (1) achieving 100% selectivity for the desired product is the focus of catalysis research in 21st century; (2) production of various chemical products and fine chemical products as well as pharmaceuticals involves selective hydrogenation; (3) Pd and Pd-based hydrogenation catalysts possess high dynamic character under relevant reaction conditions. Theoretical simulations of the high dynamic character of the hydrogenation catalysts under reaction conditions are a challenge to theoreticians, and are significant from both practical and theoretical points of view. .Aiming at unraveling the electronic effect and geometric effect of alloy catalysts, and elucidating the promotion effect of promoters and the dynamic behavior of the catalysts under reaction conditions, in the present proposal we will examine the influence of species at the near-surface on the catalytic performance of Pd-M（M=Cu、Ag、Au）alloys which are used as catalysts for the semi-hydrogenation of acetylene in the ethylene-acetylene mixture. Kinetic Monte Carlo method will be employed to explore the evolution of the surface composition and structure of the catalysts under reaction conditions. The reaction mechanism of acetylene hydrogenation on Pd-Ag and Pd surfaces will also be simulated..The results will be useful for better understanding various fundamental concepts of catalysis and furnish valuable information for designing and producing hydrogenation catalysts with nice catalytic performance.

乙炔半氢化为乙烯是降低乙烯-乙炔混合物中乙炔含量的常用方法。该体系涵盖选择性三种含义:即对特定反应、特定产物和特定反应物的选择性，是研究选择性问题的理想体系。一方面，获得目标产物100%的选择性是21世纪催化研究的焦点，许多化工及精细化工产品的生产和制药过程均涉及选择氢化；另一方面，氢化催化剂在反应条件下表现出高度的动态行为，动态行为模拟是理论研究的热点和挑战，故研究选择性氢化具有十分重要的理论和实际意义。.为揭示合金催化的电子效应和几何效应、助剂作用的本质、特别是反应条件下催化剂的动态行为，本申请拟以Pd及Pd-M（M=Cu、Ag、Au）催化乙炔半氢化为对象，系统研究近表面物种对催化行为的影响，运用动力学蒙特卡罗(kMC)方法模拟催化剂近表面组成和结构随反应条件和时间的演化以及半氢化反应机理。研究结果将有助于加深理解众多催化基本问题，也将为研制价廉品优的催化剂提供有价值的信息。

本项目的研究分为两部分：乙炔半氢化生成乙烯反应机理和计算方法及程序发展。在机理研究中我们考察了H和C2H2等系列物种在钯掺杂的M(111)（M = Cu,Ag和Au）、M（111）和PdM(111)表面的吸附；对M(111)和Pd-M(111)表面C2H2->C2H3->C2H4->C2H5->C2H6各步的热力学和动力学进行了计算，研究了乙炔半氢化反应机理。结果表明，次表层钯较表层钯对表面性质的影响大，且随钯含量的增加，影响程度增加。PdCu和PdAg表面形成乙烯基是乙炔氢化为乙烯的速决步；金和银表面氢化能垒低，但主要反应物在表面的吸附能也低。铜表面乙炔和乙烯基的吸附能较大，且能垒相对较低；增加金和银表面与乙炔的作用强度，将有利于乙炔氢化反应的转化率；计算表明Pd-Au表面的氢化能垒较低，活性较高。.我们编写了KMC程序，研究了真空条件下PdCu和PdZn表面偏析、表面组成和结构。Cu会偏析在PdCu表面，偏析主要发生在1-4层，在500K时偏析在~900秒基本完成； PdCu合金表层会形成钯单体、二聚或三聚的钯，它们都有与之键连的次表层钯；我们还研究了450K和650K下热处理2LZn/Pd(111)10分钟的过程，否定了前人提出的低温形成多层表面合金，而高温形成单层PdZn表面合金的观点，完美地解释了有关实验现象。.我们还探索了计算原子相互作用能的新方法-原子的配位相关能方法，并研究了金团簇的演化，结果与实验相符，有望将该方法推广。我们将MC/MD思想与第一性原理DFT方法相结合，编写了MC/MD-DFT程序，研究了PdZn纳米线及单层合金膜的稳定结构、模拟了PdCu合金表面在真空和反应条件下稳定表面的结构和组成，结果均与预期相符，这为模拟动态条件下的催化剂组成和结构打下了坚实的基础。