Theoretical studies of palladium on magnesium oxide(100) surface with density functional and transition state theories
by Xu, Lijun, Ph.D., University of Washington, 2006, 126 pages; AAT 3207815Advisor: Campbell, Charles T.
School: University of Washington
School Location: United States -- Washington
Index terms(keywords): Palladium, Magnesium oxides, Metal nanoparticles
Source: DAI-B 67/02, Aug 2006
Source type: Dissertation
Subjects: Chemistry
Publication Number: AAT 3207815
ISBN: 9780542568831
Document URL: [url]http://proquest.umi.com/pqdweb?did=1092106551&sid=5&Fmt=2&clientId=9339&RQT=309&VName=PQD[/url]
ProQuest document ID: 1092106551
Abstract (Document Summary)
Metal nanoparticles supported on oxides are used as catalysts in many industrial processes. Although high activity of metal nanoparticles has been shown under idealized experimental conditions, in practice the nanoparticles tend to sinter (agglomerate into larger particles) at typical reaction temperatures, thus reducing their catalytic activity. Theoretical studies can help elucidate the nature of small metal particles diffusion, growth and agglomeration on oxide surface that can not be easily achieved with experiments only. Among the key issues involving metal/oxide interface dynamics, the adsorption, diffusion, and growth of small metal clusters are most interesting and thus become the focus of this project.
In this work, density functional theory integrated with nudged elastic band and dimer methods has been applied to study the bonding features of small Pd clusters on MgO(100) and explore the diffusion mechanisms involving adatoms and small clusters so that the picture of the early stage of metal nano-particles formation and growth can be drawn with the help of transition state theory and compared with the experimental results to show how well the theoretical model works.
My studies show that small Pd clusters adsorbed on MgO(100) tend to form 3-dimensional structures and most surprisingly, they are quite mobile on the MgO(100) surface. In fact, the tetramer shows higher mobility of Pd tetramers than monomer even at room temperature. Calculations also show that small clusters bind strongly on defect sites. This changes the traditional view of metal island growth on oxides in which the metal adatom is the only mobile species in the growth and ripening process. Analysis of the bonding features of Pd-MgO(100) interface shows significant charge transfer from oxygen in the substrate to Pd adatoms. This will shed light on building an empirical potential to simulate the Pd island growth on a long time scale. Kinetic Monte Carlo simulations have been applied to simulate the island growth over a range of temperatures and the results show that mobility of small clusters is necessary to qualitatively match the experimental observations of island density dependence on temperature.
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