Metal-support interaction desorption

Such a difference in terms of product selectivity was attributed to the complete absence of any acidic sites on the carbon nanotubes sur ce and also to the absence of micropores which could induce re-adsorption and consecutive reaction [16]. The presence of micropores could artificially increase the contact time and as a consequence, modify the hydrogenation pathway. The influence of the support nature on the electronic properties of the metallic phase eould also be put forward to explain these results. Depending on the metal-support interaction, the metal particles could exhibit different exposed faces and as a consequence, significantly modify the chemisorption of the reactant on their surface. According to the interaction between the C=C bond and the laces exposed by the palladium particles, the residence time and the desorption of the intermediate could be different and thus, lead to a different selectivity. The presence of palladium aggregates on the activated charcoal as compared to the individual palladium dispersion on the CNTs could be the illustration of this difference in exposed crystalline feces. 

The choice depends on the type of molecules and strength or interaction with the surface sites. Carbon monoxide (CO) and hydrogen (H2) are the most often used but also NO, N2O, ethanol, and methanol. It gives us information about the dispersion of surface active sites, the nature and morphology of the metal sites, as well as metal-support interactions. The desorption profiles may be accompanied by several other profiles of products formed during the desorption. 

Depending on the time, temperature, and Cl-support interaction. Cl can be eliminated or remain adsorbed on the support. In the latter case, calcination or oxidation pretreatments lead to desorption of Cl from the support and to the formation of a stable platinum oxychloride species." Regardless of the detailed mechanism of Cl transport from the support to the metal and vice versa, i.e., surface diffusion versus gas phase transport, all of the results show this dynamic interaction between Cl, Pt, and the support resulting in a specific catalytic activity. 

The surface saturation by sulfur has to be compared to the irreversible adsorbed sulfur introduced by Menon and Prasad (22) and Apesteg-uia et al. (23). The study of H2S adsorption on supported catalysts was carried out by Menon and Prasad (22) and Apesteguia et al., Parera et al., and Barbier et al. and Marecot (23-25). For alumina supports, it was shown (23-25) that chlorine inhibits the adsorption of H2S on the support. Yet this adsorption on pure alumina is wholly reversible at 500°C, as is shown in Fig. 2. On Pt/Al203 at 500°C, only a fraction of the adsorbed sulfur is quickly desorbed in a hydrogen atmosphere. This result enabled the preceding authors (22-25) to develop the notion of reversible and irreversible adsorbed sulfur. The irreversible form, which does not exist on pure alumina, would interact with the metal. The quantity of irreversible sulfur, determined after 30 h of desorption under hydrogen flow at 500°C, does not depend on the sulfiding conditions (Table I). 

Temperature Programmed Desorption (TPD) is a technique widely used for characterizing the thermal evolution of different probe molecules chemisorbed on M/CeO and related catalysts. From these studies, information about the chemical interaction of these molecules with both the dispersed metal phase and the support could be gained. This section will mainly deal with the TPD studies on chemisorbed H and CO, two of the most extensively investigated probe molecules. 

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