Supported metals vapor phase deposition

In typical surface science experiments as presented previously, oxide-supported metal nanoparticles are deposited onto a clean oxide surface by physical vapor deposition. The precursor in this process is metal atoms in the gas phase, which impinge on the surface, diffuse until they eventually get trapped (either at surface defects or by dimer formation), and then act as nuclei for the growth of larger particles. These processes are well understood for ideal model systems under ultrahigh vacuum (UHV) conditions [56, 57]. In contrast, most realistic supported metal catalyst... 

The preceding paragraphs have introduced the various preparation methods leading eventually to supported metal particles. These methods fall obviously into two categories depending on whether the metal is basically in its zerovalent state (decomposition of metal cluster compound, chemical deposition, ion implantation, and vapor-phase deposition) or in an oxidized state (coprecipitation, impregnation, and ion ex-change)(Table I). 

Semiconductor oxides are also important support materials. Even if a support is inactive in the reaction imder consideration, it can considerably change the reactivity of the catalyst that it supports. As an example, metals such as Ni and Ag are often applied to doped AI2O3 by vapor-phase deposition. The resulting catalyst system behaves like a rectifier in that electrons flow from the support through the catalyst metal to the reac-tauts (Eq. 5-61). Hence in this case acceptor reactions are favored. 

Techniques for the preparation of metal cluster/nanoparticles can be classified into three primary categories condensed phase, gas phase, and vacuum methods. In condensed phase synthesis, metal and semiconductor nanoparticles are prepared by means of chemical synthesis, which is also known as wet chemical preparation. In gas phase synthesis, metal is vaporized, and the vaporized atoms are condensed in the presence or absence of an inert gas. In vacuum methods, the metal of interest is vaporized with high-energy Ar, Kr ions, or laser beams in a vacuum, and thus generated metal vapor is deposited on a support. 

One of the most important target, preparing metal supported catalysts, is to obtain a very high dispersion of the metal on the support (1). In the recent literature, there are few examples of the preparation of highly dispersed catalysts with vapor phase deposition or impregnation of organometallic precursors [2]. 

The chemical structure of iron-phthalocyanine is shown on Figure 1. PcFe is a planar molecule with a central cavity in which the iron atom is located. Metal phthalocyanines may be sublimated and the deposition of PcFe on the carbon support has been done by vapor phase condensation (16). During the condensation, the carbon supports are kept at 235°C, Condensation rates were in the range 2-4 mg PcFe per hour. Samples with PcFe content ranging from 0.5 to 30 % in weight have been prepared with both carbon substrates. 

Model electrodes with a dehned mesoscopic structure can be generated by a variety of means, e.g., electrodeposition, adsorption from colloidal solutions, and vapor deposition and on a variety of substrates. Such electrodes have relatively well-dehned physico-chemical properties that differ signihcantly from those of the bulk phase. The present work analyzes the application of in-situ STM (scanning tunneling microscopy) and ETIR (Eourier Transformed infrared) spectroscopy in determining the mesoscopic structural properties of these electrodes and the potential effect of these properties on the reactivity of the fuel cell model catalysts. Special attention is paid to the structure and catalytic behavior of supported metal clusters, which are seen as model systems for technical electrocatalysts. 

This makes the use of catalysts indispensable. Copper, nickel, zinc based systems are used in the vapor phase, promoted or not in metallic or oxide form, and possibly deposited on a support Tire lower temperature (S 150 Q reaction can be carried out in the liquid phase, in the presence of Raney nickel. Hie thermodynamic equilibrium in this case is shifted in the desired direction by the continuous removal of the hydrogen produced. It is also necessary to extract the acetone from the reaction medium upon its formation, by vaporization for example, owing to the inhibitory action it exerts on the activity of the catalyst... 

In discussing electronic effects and Fig. 2, a number of references have been made to model catalysts formed by deposition of metal from the vapor phase onto an inert support. An attempt is made to simulate the surface of a real catalyst so that it can be better studied by XPS and electron microscopy. In addition, Masson et al. (78, 79) have developed techniques to control and make almost uniform the size of the particles. Rutherford backscattering is used to find the total number of atoms deposited, and the number of nuclei that grows into discrete metal particles is found by electron microscopy. From these measurements the average number of atoms per particle is obtained. Longer deposition times give larger particles. 

A film is deposited in a conventional chemical vapor deposition (CVD) process when the gaseous reactants are presented with a large hot support surface. Supported growth of whiskers occurs also when the gaseous reactants are presented with discrete hot metal catalyst particles located on the surface of a suitable substrate. Unsupported whisker growth occurs when hot metal catalyst particles are freely interspersed with the gaseous reactants in the vapor phase. The most common mechanism for whisker growth is a vapor-liquid-solid transformation, and the most versatile VLS process is a metal particle catalyzed chemical vapor deposition. 

Gas-phase grafting (GG) is characteristic in that gold can be deposited even on the acidic surfaces, such as activated carbon and on Si02 [27]. The vapor of gold acac complex is adsorbed on the support powder probably through the interaction of electron-rich oxygen atoms in acetylacetonate and then calcined in air to decompose it into metallic gold particles. 

An unexpected result was the progressive apparent dechlorination of SiOTiCla. We have verified that this phenomenon was not related to the presence or absence of TiCU either physically adsorbed or in the gas phase. We could also observe the growth of the same IR bands between 1000 and 600 cm using a self-supporting disc. Therefore, the dechlorination of TiCU on silica and the eventual incorporation of Ti as a random mixed metal surface oxide is probably entropy driven. Although the initial chemisorption follows reaction (3) and (4), further dechlorination probably results in the formation of SiCl surface species. The vibrations of this near 7(X) cm would be impossible to detect with a thin film given the low extinction coefficient [15], and in any case, they would be masked by the much stronger SiOTi vibrations. Finally, the results have implications for mixed oxide catalysts which are prepared by chemical vapor deposition. Structural models which are based on the notion that only reactions like those depicted in schemes (3) and (4) occur are probably not valid. 

Several methods for the incorporation of catalysts into microreactors exist, which differ in the phase-contacting principle. The easiest way is to fill in the catalyst and create a packed-bed microreactor. If catalytic bed or catalytic wall microreactors are used, several techniques for catalyst deposition are possible. These techniques are divided into the following parts. For catalysts based on oxide supports, pretreatment of the substrate by anodic or thermal oxidation [93, 94] and chemical treatment is necessary. Subsequently, coating methods based on a Uquid phase such as a suspension, sol-gel [95], hybrid techniques between suspension and sol-gel [96], impregnation and electrochemical deposition methods can be used for catalyst deposition [97], in addition to chemical or physical vapor deposition [98] and flame spray deposition techniques [99]. A further method is the synthesis of zeoUtes on microstructures [100, 101]. Catalysts based on a carbon support can be deposited either on ceramic or on metallic surfaces, whereas carbon supports on metals have been little investigated so far [102]. 

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