Precursor-Support Interactions

In addition, hydrogen bonding was still observed at 3,583 cm (Fig.l A). IR peaks in VC.O region (Fig.l B) shifted to lower frequency after adsorption due to precursor-support interaction and differed from that of cluster solution implying that precursor was not intact after adsorption on support but still in the form of metal carbonyl species. 

The effect of precursor-support interactions on the surface composition of supported bimetallic clusters has been studied. In contrast to Pt-Ru bimetallic clusters, silica-supported Ru-Rh and Ru-Ir bimetallic clusters showed no surface enrichment in either metal. Metal particle nucleation in the case of the Pt-Ru bimetallic clusters is suggested to occtir by a mechanism in which the relatively mobile Pt phase is deposited atop a Ru core during reduction. On the other hand, Ru and Rh, which exhibit rather similar precursor support interactions, have similar surface mobilities and do not, therefore, nucleate preferentially in a cherry model configuration. The existence of true bimetallic clusters having mixed metal surface sites is verified using the formation of methane as a catalytic probe. An ensemble requirement of four adjacent Ru surface sites is suggested. 

The results of this study suggest that the dynamics of the nucleatlon process are of the utmost Importance In determining the structure and the surface composition of supported bimetallic clusters. Because the surface mobility of the metal phase during pretreatment is strongly influenced by the nature of the precursor-support Interaction, it is useful to consider this Interaction in some detail. 

Metal precursor-support interactions in the case of alumina are quite different. The nature of the H2PtClg-Al203 interaction is still open to question. However, recent "in-situ ultraviolet studies (13-14) suggest the following ... 

The surface dynamics of metal precursor-support interactions dur-... 

The principle of diffusional impregnation is much simpler than when a precursor-support interaction occurs. Two cases may be distinguished ... 

The preparation of alumina supported catalysts by impregnation with a solution containing metallic ions may give rise to precursor-support interactions of different types. Evidence of the formation of certain species can help to understand reaction mechanisms. 

The hydroxyl ions are produced homogeneously throughout the solution, thus obviating the need for strong mixing. Since, in the presence of an active precursor-support interaction, nucleation of the catalyst precursor (metal hydroxide species in the present example) at the surface of the support proceeds at lower concentrations than those needed for precipitation in the bulk of the solution, precipitation exclusively on the support surface can be achieved. This... 

The aim of this work is thus to study the role of active phase precursor-support interactions in deposition-precipitation in an attempt to propose a preparation method avoiding the carbon black activation. The effect of carbon activation on deposition and ion exchange of palladium will be compared. A comparison of deposition-precipitation on two supports showing very different affinity towards noble metal, namely a non activated carbon black and TiOa, will be also provided. 

Electrostatic interactions between the carbon surface and the active-phase precursors have also to be taken into account in the preparation of carbon-supported catalysts. The presence of oxygen functionalities on the carbon surface, which can be produced upon the activation process (for activated carbons) and/or by subsequent oxidation treatments, renders it amphoteric. This implies that it can be more or less charged, positively or negatively, depending on the pH of the surrounding solution. Preparation variables such as the polarity of the solvent, the pH of the solution, the anionic or cationic nature of the metal precursor, and the isoelectric point (lEP) of the carbon support determine the extent of precursor-support interaction and, in this way, the total uptake and dispersion of the active phase in the final catalyst [17,20,37]. Thus, for carbons containing acidic surface groups and, as a consequence, a low isoelectric point, best results in the preparation of supported catalysts are achieved when a cationic precursor is used in basic media. Under these conditions, the acidic complexes (-COOH, -OH) are deprotonated (-COO , -0 ) in such a way that... 

After reviewing the literature we conclude that in all three methods—wet impregnation, incipient-wetness impregnation, and ion adsorption—two major factors govern the final dispersion of the active phase precursor-support interaction and pore structure. These two issues are discussed in the next two subsections. Note that we discuss primarily that literature which adds fundamental insights in the preparation of carbon-supported catalysts. 

Both series of carbons fitted the same curve well, showing a marked decrease in the PZC value as a result of an increase in the oxidation time, along with an increase in the surface acidity of the carbons. Infonnation on the distribution of surface functionalities is a key factor in many carbon applications, such as the manufacturing of carbon-supported catalysis. The chemistry of the support determines the precursor/support interactions and, hence, the nature of the active species formed. Thus, the subject of the distribution of oxygen-containing... 

FTIR spectroscopy is widely used to investigate state of the surface at different stages of catalyst synthesis. When studying the precursor-support interaction, this method allows identification of OH groups involved in chemisorption of the metal complex. Analysis of... 

The reduction temperature of platinum species anchored on SZA support, which characterizes the strength of precursor - support interaction, also has a medium value. 

The amount of metal retained by the solid must be determined by analysis of either the solid or liquid. The extent of metal retained by the support is a function of the precursor-support interaction, which may be chemical or physical (electrostatic) in nature. As WI is defined here, no attention is paid to controlling the impregnation conditions (varying pH, for example) to optimize the interaction. 

Park KC, Jang lY, Wongwiriyapan W, Moiimoto S, Kim YJ, Jung YC, Toya T, Endo M (2010) Carbon-supported Pt-Ru nanoparticles prepared in glyoxylate-reduction system promoting precursor-support interaction. J Mater Chem 20 5345-5354... 

---