Auのヘテロ接合効果と反応性に関する理論的研究

概要

It is well known that the highly dispersed Au nanoclusters (Au NC) deposited on supports exhibit unique catalytic activities[1]. This is because of a hetero-junction effect derived from the electronic interactions between the clusters and supports. To elucidate this effect as Chemistry by theoretical studies, the representative model catalysts corresponded to the weak hetero-junction effect and the strong hetero-junction effect were taken up as topics.
To elucidate the weak hetero-junction effect, the coordination structures and catalytic activities of the aerobic oxidation of p-hydroxybenzyl alcohols on the polymer- stabilized Au NCs were investigated, using two model catalysts viz. Au38:(PVP36)4 and Au38:(PAA18)8 in water. The former model catalyst represents the Au NCs stabilized by poly(N-vinyl-2-pyrrolidone) [PVP,(C6H9ON)n](abbreviated as Au:PVP) while the latter model catalyst represents Au NCs stabilized by poly(allylamine) [PAA, (C3H5NH3)n] (abbreviated as Au:PAA),and the catalytic activity of Au:PVP is much higher than that of Au:PAA[2].
The results show that the Au38 has 8 electron acceptor cites to adsorb the substrates or the protective polymers. Some electron acceptor cites are not occupied by the protective polymers (abbreviated as open active sites). The results also show that the difference of the number of the open active sites is a good index for predicting the catalytic activities.
Next, the principal reaction route on the aerobic oxidation reaction was analyzed, using modeled ligands as PVP and Au38 cluster. Some structures corresponded to the transition states have imaginary frequencies derived from the bulkiness of the pyrrolidone ring.
Thus, these ligands were replaced with negative point charge of -1, and each system was reoptimized. The results obtained from these models exhibited the same tendency for the reaction pathways. The results are shown as follow. (1) The catalyst oxidized p-HBA into the corresponding aldehyde, and the two elementary reactions competed in the catalytic cycle. One of them was C-H bond dissociation at C1 position of the alcohol, and the other was the hydride elimination from the alcohol by the Au NC. (2) The main reaction pathway was determined based on the difference of the electron withdrawing groups near the reaction site.(3)The produced hydroperoxide exhibits electron withdrawing to Au NC and locally generated cationic Au site to eliminate hydride from the substrate.
To elucidate the strong hetero-junction effect, the selective hydrogenolysis of methyl vinyl carbinol as the modeled substrate over Au-Ni bimetallic catalyst were theoretically investigated including the role of dichloroethane(DCE) solvent and perimeter sites[3].The results show that the chlorinated low coordinate Ni site by DCE eliminated OH- from the substrate as Lewis acid to produce the carbocation intermediate under the neutral solution. The perimeter Ni site adsorbes the intermediate and the active hydrogen species to promote the hydrogenation reaction. The exposed Au site also
promotes the reduction of the oxidized sites on the catalyst, which is interpreted as an important role of the exposed Au sites on the catalyst.

In conclusion, these hetero-junction effects are elucidated by the charge transfer between Au NCs and the substrates or the supports. On the weak hetero-junction effect, the charge transfer from the substrate or the protective polymers promotes the generation of the active oxygen species which promotes the C-H bond dissociation at C1 position of the alcohol. On the other hand, the electron withdrawing groups such as the hydroperoxide generates the cationic Au site to promote the hydride elimination reaction. Thus, these catalytic activities on Au NC are derived from the flexible charge transfer by the adsorbed chemical species.
On the strong hetero-junction effect, the role of Au is interpreted as the electron doner to activate the perimeter Ni sites as the main reaction site and the oxidized Ni sites as Lewis acid. Thus, these electron donation effects generate the unique catalytic activities.
These results are in a good harmony with the property that Au is not easily oxidized. This property is also reflected on its electron withdrawing and the self-reduction promoting properties. For instance, Au NCs accept electron donations from the substrates and the protective polymers while Au NCs directly eliminate hydride from the alcohol by generating the hydroperoxide. On the bimetallic catalyst, Au sites catalytically activate the surrounding Ni sites and the oxidized Ni sites. In other words, it is interpreted that Au compensate the catalytic properties of other metallic clusters by the self-reduction promoting properties.
The obtained results on the hetero-junction effects would contribute to other theoretical studies on various metallic catalysts.