The Transition State in Catalysis: Experiment and Computational Modeling

项目编号：0651182项目负责人：Gellman， Andrew j .机构：卡耐基梅隆大学（Carnegie-Mellon university）研究人员：本项目拟将实验与计算理论相结合，研究催化表面反应的过渡态。几种基本反应的激活势垒Et将使用一组选择性氟化反应物来测量。氟取代基对Et的影响将被用作这些反应过渡态的实验探针。同时，密度泛函理论（DFT）将用于预测相同反应的过渡态结构、电子密度分布和Et。通过计算理论和实验相结合，提出的工作将提供催化过渡态的最准确和良好的基准描述。计算得到的反应物和过渡态之间电子密度分布的差异将与基于取代基效应的预测结果进行比较；用于验证在表面取代基效应解释中使用的一些基本假设；用于探究结构灵敏度等现象的起源；并用于探测金属与氧化物反应中的静电屏蔽效应。这些解决了影响催化活性的几个基本现象。取代基效应方法已经在先前的工作中发展起来，现在正在扩展到探索结构敏感性的起源和表面组成对表面反应动力学的影响。取代基效应将用于探测几种反应：Cu（100）、Cu（111）和Cu（110）上烷基和烷氧基上的ii-氢化物消除，Pt（111）、Pt（100）和Pt（110）上的烷基加氢，Cr O 101 2 2 3上的脱卤反应。红外吸收将用于确定反应物烷基和烷氧基在各种表面上的取向。程序升温反应光谱法将用于测量氟取代反应物中Et对对氢化物的消除、加氢和脱氢。为了定量解释，这些测量结果将直接与相同表面上相同反应物的DFT模拟结果进行比较。使用测量基准理论将为DFT预测的过渡态分析提供前所未有的信心水平。广泛的影响通过对表面反应过渡态的本质以及表面结构和组成对催化反应动力学的影响产生根本性的见解，所提出的努力将产生广泛的影响。因为取代基效应有一个相当简单的物理解释，所以对表面过渡态本质的见解是可以在课堂上展示的。从事这项研究的学生将接触到实验和计算理论。这一点，以及这项工作对基本催化现象的物理洞察力，将有助于在学生中产生对催化现象的兴趣。基于密度泛函理论的更深入的见解将对那些试图在更深层次上理解和模拟催化现象的人感兴趣。拟议的研究产生的结果和概念将通过出版物和通过项目负责人和从事该项目的学生的介绍在科学界广泛传播。此外，专业人员在若干专业学会内都很活跃，并将通过组织有关领域的专题讨论会，为广泛传播成果创造机会。

PROPOSAL NUMBER: 0651182PRINCIPAL INVESTIGATOR: Gellman, Andrew J.INSTITUTION: Carnegie-Mellon UniversityIntellectual MeritThe proposed research effort couples experiment and computational theory to study the transition states to catalytic surface reactions. The activation barriers, Et , to several elementary reactions will be measured using sets of selectively fluorinated reactants. Fluorine substituent effects on the Et will be used as experimental probes of the transition states to those reactions. In parallel, density functional theory (DFT) will be used to predict the transition state structures, electron density distributions and Et for the same reactions. By combining computational theory and experiment, the proposed work will provide the most accurate and well-benchmarked descriptions of catalytic transition states. The computed differences in electron density distributions between reactant and transition state will be compared with predictions based on substituent effects; used to validate some of the basic assumptions used in the interpretation of substituent effects on surfaces; used to probe the origins of phenomena such as structure sensitivity; and used to probe electrostatic screening effects in reactions on metals versus oxides. These address several fundamental phenomena that influence catalytic activity. The substituent effect methodology has been developed in prior work and is now being extended to probe the origins of structure sensitivity and the effects of surface composition of s urface reaction kinetics. Substituent effects will be used to probe several reactions:ii-hydride elimination in alkyl and alkoxy groups on Cu(100), Cu(111) and Cu(110),halkyl hydrogenation on Pt(111), Pt(100), and Pt(110), and dehalogenation reactions on Cr O 101 2 2 3 . Infrared absorption will be used to determine the orientations of the reactant alkyl and alkoxy groups on the various surfaces. Temperature programmed reaction spectroscopy will be used to measure the Et to p-hydride elimination, hydrogenation and dehelogenation in sets of fluorine substituted reactants. For quantitative interpretation, the results of these measurements will be compared directly to the results of DFT simulations of the same reactants on the same surfaces. Using measurements to benchmark theory will provide an unprecedented level of confidence in the analysis of the transition states predicted by DFT.Broad ImpactThe proposed effort will have broad impact by generating a fundamental insight into the nature of transition states for surface reactions and into the influences of surface structure and composition on catalytic reaction kinetics. Because substituent effects have a fairly simple physical interpretation, the insights into the nature of surface transition states are amenable to presentation in the classroom. Students working on this research will be exposed to both experiment and computational theory. This and the fact that the work develops physical insight into fundamental catalytic phenomena will help to generate interest in catalytic phenomena among students. The deeper insights based on the use of density functional theory will be of interest to those trying to understand and model catalytic phenomena at a deeper level. The results and concepts generated by the proposed research will be disseminated widely in the scientific community through publication and through presentations by the PIs and the students working on the project. In addition, the PIs are both active within several professional societies and will generate opportunities for broad dissemination of results through the organization of symposia in relevant areas.