Gold-support interactions

Metal oxides of variable oxidation state as supports or support modifiers [202] are well known in gold catalysis. In the previous section we have already indicated some metal-support interactions influencing the electronic state of gold nanoparticles as well as the metallic or ionic state of gold. Of the numerous literatures we have to mention Haruta and Date [169], Bond [195], as well as Goodman works [186,203]. Further results can be found on the iron oxide system in recent literatures [162,204]. 

It was not until recently that Chen and Goodman probed the influence of the oxide support material on the intrinsic properties at the metal surface. By covering a titania support with one or two flat atomic layers of gold they eliminated, direct adsorbate-support interactions as well as particle size and shape effects. Their results definitively showed that the electronic properties at the metal surface changed due to charge transfer between the support and the metal. Furthermore, their comparison of one- and two-layer films highlighted the dependence of these effects on the thickness of the metal slab. 

In many catalytic systems, nanoscopic metallic particles are dispersed on ceramic supports and exhibit different stmctures and properties from bulk due to size effect and metal support interaction etc. For very small metal particles, particle size may influence both geometric and electronic structures. For example, gold particles may undergo a metal-semiconductor transition at the size of about 3.5 nm and become active in CO oxidation [10]. Lattice contractions have been observed in metals such as Pt and Pd, when the particle size is smaller than 2-3 nm [11, 12]. Metal support interaction may have drastic effects on the chemisorptive properties of the metal phase [13-15]. Therefore the stmctural features such as particles size and shape, surface stmcture and configuration of metal-substrate interface are of great importance since these features influence the electronic stmctures and hence the catalytic activities. Particle shapes and size distributions of supported metal catalysts were extensively studied by TEM [16-19]. Surface stmctures such as facets and steps were observed by high-resolution surface profile imaging [20-23]. Metal support interaction and other behaviours under various environments were discussed at atomic scale based on the relevant stmctural information accessible by means of TEM [24-29]. 

The use of asymmetric bridging ligands, which permits a selective coordination of the different donor atoms to the metal centers and leads to supported gold-thallium interactions forced by the ligand. 

The now classical methods used for the preparation of supported gold catalysts are hardly capable of giving particles that are both small and bimetallic, when the precursors in solution do not interact strongly with each other. During the subsequent thermal treatment performed to get metal particles, the metals must have enough mobility to migrate on the support, interact with each other, and form bimetallic particles. However, phase separation can be a common problem, especially when the metal ratio falls in the miscibility gap (Section 2.6), or if the intended composition is not thermodynamically stable. 

Since supported gold catalysts prepared by coprecipitation were found to be active for CO oxidation even at temperatures far below room temperature, attempts are increasing to prepare other noble metal catalysts by coprecipitation, deposition-precipitation, and grafting methods, which were used for the preparation of active supported gold catalysts. Although the affinity to CO is markedly different between Pt-group metals and Au supported on selected metal oxides, the contribution of metal-support interactions to the enhancement of low-temperature catalytic activity for CO oxidation appears to be similar, namely, the enhancement of oxygen activation at the perimeter interface. This line of approach may be valid to seek for a new type of catalysts active at lower temperatures for reactions other than CO oxidation [82,83]. 

In order to improve our understanding of metal-support interactions, metal-induced changes in structural properties of typical catalyst supports are investigated. Gold is chosen as metal component, and MgO and TiO in form of anatase are used as support. 

The objectives of this work are to study the influence of gold particles on the properties of typical catalyst supports, namely MgO and TiO. Gold has been chosen because of its relatively low catalytic activity except for oxygen transfer reactions (4). MgO, an insulator, and TiO, a semiconducting material, are widely used as catalyst supports, and for both of them metal-support interactions have been reported in the literature. Our study places main emphasis on the role of gold on thermal stability, phase transformations, solid-phase oxygen exchange activity, and adsorption characteristics of the oxides. 

MgO-supported mono-nuclear gold complex catalysts have been evaluated for ethene (or ethylene) hydrogenation at 353K [464]. The results provided evidence of the stability of the metal complex, complex-support interactions... 

Figure 4.11 Schematic of S-layer stabilized solid supported lipid membranes, (a) S-layer directly recrystallized on gold, with a lipid bilayer on top. (b) Same as (a), with an additional S-layer recrystallized on top of the lipid bilayer, (c) Thiolated SCWPs directly bound to gold and interacting with an S-layer, with a lipid bilayer on top. (d) Same as (c).

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