In this project funded by the Chemical Structure, Dynamics and Mechanisms Program of the Division of Chemistry, Andrew J. Gellman (Carnegie Mellon University), E. Charles H. Sykes (Tufts University) and David S. Sholl (Georgia Institute of Technology) will collaborate to develop and apply methods for high throughput study of structure sensitive surface chemistry. The core of the experimental program is the preparation, characterization and study of curved single crystal metal surfaces that expose continuous distributions of surface orientations; i.e. regions of the surface that expose different step and kink densities. Spatially resolved experimental tools such as STM, XPS, and LEIS will be used to characterize the local structures of these surfaces and to measure surface reaction kinetics at each point. This effort will resolve the role of step and kink density in several surface reactions. Complementary computational modeling tools will be used to understand the role of surface orientation in surface reaction kinetics. These methods will greatly accelerate the study of structure sensitive surface chemistry. The impact of this work will be development of a fundamental understanding of several catalytically important surface reactions. The broader impact will include outreach to high school students, exposure of undergraduates to research and the development of short videos on surface science and nanoscience for public viewing.
Chemical reactions that occur on metal surfaces are critically important to numerous technologies that impact society. Such catalytic reactions are used to create most of the chemicals, textiles, fuels, etc. that underpin our modern standard of living. For a given catalytic metal, the efficiency of these surface reactions is critically dependent on the atomic structure at the catalyst surface. Understanding the origins of the structure sensitivity is one of the central problems of catalytic surface science and its solution will lead to the development of new catalysts with improved performance over those in use today. Very early work on this problem was awarded the 2007 Nobel Prize in Chemistry. That work relied on the cumbersome preparation and study of numerous metal single crystal surfaces with different surface structures. The work conducted under this grant has developed the basis for vastly accelerating the pace at which catalytic chemistry on different surface structures can be studied. It has prepared curved single crystals called Surface Structure Spread Single Crystals (S4Cs) that expose surfaces with many different crystal structures and then used spatially resolved surface analysis methods to study surface chemistry at each point on the surface. At Carnegie Mellon University the oxidation of a Cu(111)-S4C has been studied with unprecedented resolution, using x-ray photoemission spectroscopy (XPS) to measure in a few days the oxygen uptake on 80 different surface structures, a study that would have taken years using traditional methods. The collaborative study with Tufts University using Scanning Tunneling Microscopy (STM) to image the surface of a Cu(111) S4C prepared at Carnegie Mellon allowed us to understand the origin of the differences in oxidation rates of the many surfaces. That work could not possibly have been done using traditional methods with dozens of different single crystals. In addition to developing methods for accelerated experimental study of structure sensitive surface chemistry, one of the goals of the collaboration was to develop the fundamental basis for understanding of structure sensitivity using modern methods of computational simulation. The adsorption energy of R-3-methylcyclohexanone has been shown experimentally at Carnegie Mellon to vary significantly on Cu surfaces of different atomic structure. These data have been compared to the results of Density Functional Theory (DFT) calculations performed at Georgia Tech. These have shown that a recently developed improvement to DFT that includes dispersion forces results in improved agreement between the results of experiment and the predictions of theory. Furthermore, the simulation predicts the differences in adsorption energies observed between the different surfaces and also provides otherwise unobtainable insight into the bonding and orientations of R-3-methylcyclohexanone on the different Cu surfaces. The combined use of the S4C samples with a variety of measurement methods including XPS and STM and the using of computational methods such as DFT has made it possible to study and solve several problems in surface chemistry that were previously intractable. This advances our understanding of structure sensitive surface chemistry to provide insights that will impact fields such as catalysis, corrosion, adsorption, and others. Work on this grant has contributed to the training of 2 PhD students and 5 MS students in Chemical Engineering at Carnegie Mellon. These students have learned a number of important laboratory research skills including the preparation of S4C samples, their characterization, and the design of experiments to answer important questions in the field of surface chemistry. In addition they have developed skills in writing and in speaking through the preparation and delivery of seminars, papers and reports during the course of the grant period. The work has had additional impact though the publication of research papers and the presentation of results at scientific conferences by the PI and by the students. The PI is also involved in a variety of additional professional service serving as department head in Chemical Engineering at Carnegie Mellon during the course of the grant period, organizing several symposia at national meetings, serving on the Executive Committee of the AVS, as co-chair of the 2015 Gordon Conference on Reactions at Surfaces, and as one of the chairs of the 2015 North American meeting of the Catalysis Society.
