Deposition reduction method

Figure 7. Gold NPs directly deposited on PVC microbeads by deposition-reduction method.

We have recently deposited directly onto activated carbons and polymer microbeads by deposition-reduction method and have found that gold NPs on pol5mers exhibit remarkably high catalytic activity, which may exceed those of Au/activated carbons and Au/ fine Ce02. The type of polymers and their functional groups are critical to the catalytic activity. 

The deposition-reduction (DR) method is based on the weak electrostatic interactions of polymer surfaces with the oppositely charged Au(III) complex ions, leading to the reduction of Au(III) exclusively on the polymer surfaces. Appropriate anionic or cationic Au(III) precursors are chosen based on the zeta potentials of polymer supports (Figure 3.6) [43]. 

Figure 3.6 Schematic representation of deposition-reduction (DR) method (a) positiveiy charged supports and (b) negativeiy charged supports.

Our first attempt of a successive reduction method was utilized to PVP-protected Au/Pd bimetallic nanoparticles [125]. An alcohol reduction of Pd ions in the presence of Au nanoparticles did not provide the bimetallic nanoparticles but the mixtures of distinct Au and Pd monometallic nanoparticles, while an alcohol reduction of Au ions in the presence of Pd nanoparticles can provide AuPd bimetallic nanoparticles. Unexpectedly, these bimetallic nanoparticles did not have a core/shell structure, which was obtained from a simultaneous reduction of the corresponding two metal ions. This difference in the structure may be derived from the redox potentials of Pd and Au ions. When Au ions are added in the solution of enough small Pd nanoparticles, some Pd atoms on the particles reduce the Au ions to Au atoms. The oxidized Pd ions are then reduced again by an alcohol to deposit on the particles. This process may form with the particles a cluster-in-cluster structure, and does not produce Pd-core/ Au-shell bimetallic nanoparticles. On the other hand, the formation of PVP-protected Pd-core/Ni-shell bimetallic nanoparticles proceeded by a successive alcohol reduction [126]. 

Among various methods to synthesize nanometer-sized particles [1-3], the liquid-phase reduction method as the novel synthesis method of metallic nanoparticles is one of the easiest procedures, since nanoparticles can be directly obtained from various precursor compounds soluble in a solvent [4], It has been reported that the synthesis of Ni nanoparticles with a diameter from 5 to lOnm and an amorphous-like structure by using this method and the promotion effect of Zn addition to Ni nanoparticles on the catalytic activity for 1-octene hydrogenation [4]. However, unsupported particles were found rather unstable because of its high surface activity to cause tremendous aggregation [5]. In order to solve this problem, their selective deposition onto support particles, such as metal oxides, has been investigated, and also their catalytic activities have been studied. 

The liquid-phase reduction method was applied to the preparation of the supported catalyst [27]. Virtually, Muramatsu et al. reported the controlled formation of ultrafine Ni particles on hematite particles with different shapes. The Ni particles were selectively deposited on these hematite particles by the liquid-phase reduction with NaBFl4. For the concrete manner, see the following process. Nickel acetylacetonate (Ni(AA)2) and zinc acetylacetonate (Zn(AA)2) were codissolved in 40 ml of 2-propanol with a Zn/Ni ratio of 0-1.0, where the concentration of Ni was 5.0 X lO mol/dm. 0.125 g of Ti02... 

A very early use of anodic alumina as a template involved colonization of the alumina by depositing nanometals in the pores [39]. Somewhat later, Kawai and Ueda templated cobalt and nickel in alumina by electrodeposition [40]. Other metals were deposited by Andersson et al. [41] and Patel et al. [42]. The use of anodic alumina as a template increased after Furneaux et al. developed a convenient voltage-reduction method for detaching the porous anodized alumina from the underlying aluminum [38]. 

Pd on ceria catalysts prepared by various methods were shown to exhibit varying characteristics depending on the preparation methods.625 Specifically, Pd on ceria samples prepared by the deposition-precipitation method are highly active in comparison with the catalyst prepared by the conventional impregnation method. Cationic palladium species are present in the former samples after reduction with hydrogen at 300°C, suggesting that the active species are produced by strong... 

Highly active Au catalysts can be prepared by an appropriate selection of preparation methods such as co-precipitation (CP), deposition-precipitation (DP), deposition-reduction (DR) and solid grinding (SG) with dimethyl Au(III) acetylacetonate, depending on the kind of support materials and reactions targeted. 

Electrochemical procedures can thus be used in the production of solid catalysts for the reduction of higher valent metal ions, usually present as oxyanions, to a lower valent state, where they are less acidic. Also, the above deposition-precipitation method can be extremely well controlled by electrochemical means. 

Considering now the chemical or physico-chemical interactions between the supported phase and the support, it is clear that a change of reactivity will be observed if the supported phase reacts chemically with the carrier for example, nickel can combine with silica to make some hydroxysilicate compound when the deposition-precipitation method is used for preparation. The reactivity of the hydroxysilicate with hydrogen during activation by reduction is very different (actually lower) compared to that of nickel oxide. But careful analysis of the various examples mentioned in literature shows that quite different situations may exist. 

The WFg H2 System. The reaction WFg + 3H2 W + 6HF is possible from 300 to 800 °C and is also used in thin-film production. There are several disadvantages in comparison to the other reduction methods. The HF formed during the reaction may cause defects, like encroachment or wormholes. The layers show poor adhesion on native Si02, which is always present on Si. Therefore, tungsten is not directly deposited on Si but on a bilayer. One layer provides an ohmic contact with Si, and the other acts as an adhesion promotor for W. 

In the preparation of Ni/Hp catalysts by the deposition-precipitation method (DP), nickel hydrosilicates are formed mainly but not exclusively in the external surface of the Hp zeolite. The strong metal-support interaction induced by the DP preparation method prevents the Ni metal particles from sintering during the activation of the catalysts (calcination and reduction) and a homogeneous distribution of small nickel particles is obtained. The catalyst prepared by DP showed better catalytic activity in the hydrogenation of naphthalene than the catalyst prepared by cationic competitive exchange. 

Precipitation or coprecipitation methods are also often used. Suh et al. [40] analyzed the effect of the oxygen surface functionalities of carbon supports on the properties of Pd/C catalysts prepared by the alkali-assisted precipitation of palladium chloride on carbon supports, followed by liquid-phase reduction of the hydrolyzed salt with a saturated solution of formaldehyde. They observed that the metal dispersion increased with increasing amount of oxygen surface groups. Nitta et al. [41] also used a deposition-precipitation method, with sodium carbonate and cobalt chloride or nitrate, to prepare carbon-supported Co catalysts for the selective hydrogenation of acrolein. 

Kuijpers et al. [5] synthesized Cu/Si02 catalysts using deposition-precipitation method. They proposed that the reaction is structure sensitive and follows reduction-oxidation mechanism. Xue et al. [6] compared Cu-Zn catalysts with Pt/Zr02 and Fe-Cr catalysts. The catalytic activity results of those catalysts at various temperatures are presented in Figure 3.1. They carried out the reaction... 

Recently, Soria et al. [109] also investigated the effect of synthesis method on the WGS activity of Au/Fc203 catalysts. They prepared catalysts by deposition-precipitation method, liquid phase reductive depositions and double impregnation method. Among the various catalysts, the catalyst synthesized by deposition-precipitation method exhibits higher CO conversion. 

There are a number of reports on the synthesis of Au NPs-rGO composites by adopting in situ methods such as chemical reduction, electrochemical deposition, microwave irradiation, and sonochemical reduction [14, 18, 23]. In most of the reduction methods, a GO suspension in water or ethanol is mixed with Au salt, the resulting mixture is then reduced with different reducing agents like sodium borohydride, hydrazine hydrate, glucose, ascorbic acid, sodium citrate, beer solution, oleylamine, ethylene glycol, etc., which results in. simultaneous reduction of the Au ions and GO to form Au NPs-rGO composites. 

---