Gas phase impregnation

An alternative method for producing thin deposits consists in avoiding single-step processes by chemisorbing the precursor and then having it decomposed thermally [1]. Such a method, often named atomic layer deposition or gas phase impregnation-deposition, provides very thin deposits due to the presence of a... 

Two different approaches have been used to graft molybdenum on alumina, namely, either a two-step process involving gas-phase impregnation and further decomposition at high temperature (GPID) or the direct contact of [Mo(CO),5] vapor with the alumina support placed in a hot zone so as to achieve its decomposition. All of the relevant studies point to the existence of a close relationship between the OH group density on the support and the amount of deposited molybdenum as well as the chemical nature of such deposits. Hence, we successively deal with three types of alumina highly, partially and fully dehydroxylated surfaces. 

Figure 9.4 Gas phase impregnation and decomposition of molybdenum hexacarbonyl on a fully hydroxylated alumina surface.

Processes for fabrication of carbon/carbon composites by "gas-phase impregnation" and by "liquid-phase impregnation" were described in the 1975 Symposium (JJ. The vapor impregnation process was first used for fabrication of the special aircraft brakes required for the commercial jet CONCORDE, and is still in use, although most new carbon/carbon parts and even aircraft brakes are now produced by the more economical pitch-impregnation processes. 

Figure 9. (a) TEM micrograph of a supported metallocene/ MAO catalyst particle prepared by gas-phase Impregnation with TMA/H2O. (b) EDX line scan analysis of the metal-locene/MAO-supported silica gel regarding the silicon and aluminum distribution in the volume. Under this condition, the active sites are formed on the outer surface of the particle. The mean particle size shifted from 50 /rm for the silica support to 70 /nm for the catalyst. 

This study relates to a continuous process for the preparation of perfluoroalkyl iodides over nanosized metal catalysts in gas phase. The water-alcohol method provided more dispersed catalysts than the impregnation method. The Cu particles of about 20 nm showed enhanced stability and higher activity than the particles larger than 40 nm. This was correlated with the distribution of copper particle sizes shown by XRD and TEM. Compared with silver and zinc, copper is better active and stable metal. 

The authors concluded that using the gas-phase preparation method leads to better control of variables during preparation and to higher dispersions of the active component over the conventional liquid phase impregnation method. 

In recent years, many academic workers have used impregnation methods as a simple means to deposit metal onto carbon supports. Typically, a solution of a metal salt (or salts) is mixed with the carbon support, the solvent removed by evaporation, and the resulting solid reduced chemically or by gas-phase treatment. Variations on this method include the use of a chemical reducing agenf (e.g., NaBHj) in the slurry phase and the use of sonicationi or microwaves to encourage deposition and reduction. 

Co2(CO)g has been used to obtain encapsulated cobalt clusters in Y-faujasite, which have been used as model catalysts for methane homologation [152]. The gas phase adsorption of Co2(CO)8 under N2 rendered predominately encaged Co4(CO)i2 species whereas Co,s(CO)iis was obtained when the impregnation of Co2(CO)8 was carried out under a CO/H2 atmosphere [152, 155], Samples were oxidized at 80°C, subsequently reduced at 400 °C and then structurally characterized by EXAFS. Clusters of two and three cobalt atoms were formed from encaged Co4(CO)i2 and COis(CO)iis, respectively. Higher methane conversion and selectivity to C2+ products in the CH4 homologation reaction have been obtained for the two atoms-size cluster sample the results were discussed using a DFT model [152]. 

The first fixed-bed application of a supported ionic liquid-phase catalyst was hydroformylation of propylene, with the reactants concentrated in the gas phase (265). The catalyst was a rhodium-sulfoxantphos complex in two ionic liquids on a silica support. The supported ionic liquid phase catalysts were conveniently prepared by impregnation of a silica gel with Rh(acac)(CO) and ligands in a mixture of methanol and ionic liquids, [BMIMJPFg and [BMIM][h-C8Hi70S03], under an argon atmosphere. 

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