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43980-G5
Atomically Resolved Reaction Dynamics Involving Catalytically Active, Surface Supported Clusters

Thomas P. Pearl, North Carolina State University

Opportunities exist for dictating and monitoring the influence of confined geometry, reduced dimensionality, and anisotropic electronic effects on surface reactivity, especially catalytic reactions.  The degree to which an interface can be tuned chemically whether through the substrate or a supported thin film determines the scope for nanostructured media to be synthesized with enhanced reaction selectivity and efficiency.  Over the past two years, funds from the ACS-Petroleum Research Foundation Type G award have been used to achieve a number of different project goals.  The first has been to purchase key equipment, supplies, and materials for positioning experiments to be performed.  An electron beam evaporator for the controlled deposition of various metals on metallic oxides in ultra-high vacuum has been purchased and was delivered in January 2008.  This piece of equipment has been added to our UHV-STM/AFM system which is currently under construction.  Clusters of varying size of key catalytically active metals including Pt and Cu, from isolated adatoms to large, nanometer sized, multilayer facets, can be deposited onto catalytic supports such as graphite (0001) and metallic oxide thin films such as alumina and barium oxide.  Single crystalline alloy samples like NiAl(110) and Ag(111) have likewise been acquired using this award.  Summer support for the PI has been used during year 2 to support project goals as well as expand to other heterogeneous catalytic systems.  Funds from this award have also been used to purchase chemical reagents and liquid cryogen for operating low and variable temperature scanning probe microscopes.

While the majority of our work and funding support for this project has been dedicated to the establishment of capabilities in our group to pursue experiments in catalytic behavior of metallic particles, we have explored further external support from the National Science Foundation and the Department of Energy.  Both single investigator and multi-PI proposals have been submitted over the past year to continue work on selective and tuned catalytic interfaces.  Of particular note is the burgeoning collaboration with a computational group at NCSU led by Marco Buongiorno Nardelli.  We have explored a number of research directions where we can combine density functional theory calculations with our experiments to interrogate reaction energetics and mechanistic pathways.  It our intention to pursue further funding with the ACS-PRF with a New Directions grant to investigate low dimensional materials for selective catalysis, where the role of size and surface defect type and composition can be utilized in the catalytic recycling of carbon dioxide.  

Recently, we have been able to make significant progress in the study of metallic island growth on ferroelectric surfaces.  Specifically we have been studying the growth morphology for Au deposited on to single crystalline, uniformly poled lithium niobate (LiNbO₃).  This ferroelectric has a very high surface charge density (~70 microCoulombs/cm²) and has been studied by our group for selective adsorption through electrostatic anchoring of polarizable molecules.  Our attention has now turned to the study of metal islands and ultra-thin films for the study of surface reactivity, specifically the possibility of controlling surface reactions though electric field driven changes in the local density of states at the interface.  These metal layer experiments are being performed using AFM, LEED, Auger electron spectroscopy, TEM, as well as computational modeling.  In an effort to further capitalize on the unique surface properties of lithium niobate preliminary work was done on characterizing thin metallic layers deposited on the ferroelectric.  Initially gold has been deposited on LN, using e-beam evaporator deposition in UHV, with layer thicknesses ranging from about 10Å to 100Å.  The data for low coverage and higher coverage was compared with and without post annealing effects.  The attached figure highlights one of salient results.  AES proved to be challenging because Au is one of the more difficult elements to resolve with Auger, since we have small coverage in most cases this increased the difficulty.  As a result, work is continuing with microscope to try and get a better handle of the characteristics of the Au/LN (metal/ferroelectric) interface.

The funds from this program have been invaluable to our research group with respect to establishing a new area of work especially in directions we had not initially anticipated.  We have also been able to use this support to begin collaborative work involving high performance simulations and predictions in concert with scanning tunneling microscopy.  We anticipate that we will make even further progress in the study of catalytic activity for thin metal films, metal clusters, and atomic chains that have substantially different electronic and therefore chemical bonding attributes compared to their bulk counterpart.   ADDIN EN.REFLIST

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