G.U. Kulkarni gave a really nice talk at the conference from which I learnt a few fascinating things about gold nanoparticles. He is co-author of a nice concise review, Size Dependent Chemistry: Properties of Nanocrystals.
Bulk gold surfaces are inert because the dissociative chemisorption energy of oxygen is positive unlike all other metallic elements [Nolan, Accounts Chem. Res. 1998]. However, this energy can be negative for nanoparticles.
Another key property of gold nano-particles is that there is a plasmon collective mode associated with the surface electrons and lies in the visible.
Heterogeneous catalysis with nanoparticles can be enhanced because of the large fraction of atoms on the surface. e.g., for 100 atoms, 50% are on the surface.
Pollution control in cars is achieveed by a catalytic converter which converts CO to CO2 using Pt-Pd, Pt-Rh catalysts.
Problems: expensive materials and bed has to be heated, leading to a search for alternative catalysts.
Au/TiO2 [gold nanoparticles on titania surfaces] is a good catalyst. Hurata (1984) made this revolutionary discovery, which was unexpected because gold was expected to be inert.
25 years later, it still seems anatase TiO2 is the best substrate. A key property is multiple oxidation states of Ti that are possible in titania.
Another key is the strong coupling of delocalised electrons on metal nano-particle surface and oxygen atom on titania surface. [see DFT paper, ref?]
It seems optimum size of gold is about 2-3nm.
Why does it work? Not just high surface area but also electronic structure of gold nano-particle which can undergo a metal-insulator transition as a function of size.
No comments:
Post a Comment