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The use of simulations in concert with experiment is a very powerful tool to obtain a better understanding of fundamental phenomena. One such problem of interest is the relationship between the physical and electronic structure of materials and their reactivity. My research will employ experimental and theoretical methods in concert to aid in the rational design of materials, focusing on structure-reactivity relationships at the molecular/atomic level. The general approach involves the construction of well-defined model systems both experimentally and computationally, which allow in-depth investigation of electronic properties and morphology and elucidate their relationship to reactivity.
Model catalysts, consisting of metal particles supported on thin film or single crystal metal oxide surfaces, have been utilized successfully for more than a decade in an effort to understand particle size and support effects in catalysis[1-4]. However, a detailed analysis of size effects is only possible through a monodisperse distribution of clusters. Applying technology from gas phase cluster experiments, Uli Heiz and co-workers have observed fascinating particle size dependent chemistry for the cycloisomerization of acetylene by Pd clusters and the oxidation of CO by gold clusters[5,6]. Similarly, Bruce Gates’ group has made use of metal carbonyl precursors to generate Ir4 clusters and demonstrated remarkable differences between the clusters and bulk iridium catalysts[7]. However, to date, researchers have only scratched the surface of the rich chemistry of such size selected clusters. I will be working with the Cluster Studies Group at Argonne National Lab (http://chemistry.anl.gov/ClusterStudies/index.html) in an effort to examine the activity and electronic structure of bimetallic size selected clusters as a function of their size and composition.
Supporting the experiments at Argonne, my group will employ quantum chemical calculations to gain additional insight into experimental results. Ab initio (from first principles) methods have proven to be a most useful tool in quantum chemistry, especially given the rise of computer power in recent years[8-10]. They allow for the calculation of accurate electronic structures for solid materials and their interactions with adsorbates on an atomic scale. Ab initio quantum chemical computational techniques have been employed to predict the adsorption behavior of simple molecules like CO and oxygen on both metals and metal oxides alike, for both free clusters and extended surfaces[11]. In fact, given an understanding of the rate-limiting step for a given reaction, the design of catalysts can be optimized[12]. By using such techniques, Norskov and co-workers successfully designed a nickel-gold alloy steam reforming catalyst[13]. Due to their complexity, such trends in supported metal clusters regarding cluster size, structure, and the nature of the support have only begun to be explored.
References
[1] M. Baumer, H. J. Freund, Prog Surf Sci, 61 (1999) 127
[2] D. W. Goodman, J Phys Chem, 100 (1996) 13090
[3] C. R. Henry, Surf Sci Rep, 31 (1998) 235
[4] C. T. Campbell, Surf Sci Rep, 27 (1997) 1
[5] U. Heiz, W. D. Schneider, J Phys D Appl Phys, 33 (2000) R85
[6] U. Heiz, E. L. Bullock, J Mater Chem, 14 (2004) 564
[7] Z. Xu, F. S. Xiao, S. K. Purnell, O. Alexeev, S. Kawi, S. E. Deutsch, B. C. Gates, Nature, 372 (1994) 346
[8] W. Koch, M. C. Holthausen, A Chemist's Guide to Density Functional Theory, 2nd (Wiley-VCH, Weinheim, Germany, 2000) vii
[9] E. K. Wilson, C & E News, 82 (2004) 35
[10] R. R. Schaller, IEEE Spectrum, 34 (1997) 52
[11] B. Hammer, J. K. Norskov, Adv Catal, 45 (2000) 71
[12] T. Bligaard, J. K. Norskov, S. Dahl, J. Matthiesen, C. H. Christensen, J. Sehested, J Catal, 224 (2004) 206
[13] F. Besenbacher, I. Chorkendorff, B. S. Clausen, B. Hammer, A. M. Molenbroek, J. K. Norskov, I. Stensgaard, Science, 279 (1998) 1913
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