Effect of the Support

Apart stabilizing the metal active phase, the support plays a major role in the steam reforming reaction, since it directiy participates to the reaction. The effect of the support in the reaction is the most important for noble metals such as Rh, Pd, and Pt, which are not able to activate water at low temperature [32], whereas other metals such as Ni, Co, Ru and to a certain extent Ir, may be more considered as support insensitive. For noble metals, the support must be not only hydrophilic to fevor the dissociation of water into hydrox])4 groups, but also able to fecilitate their mobility at its surfece. Thus, alumina and ceria are good candidates since they present these two essential properties [33], whereas on Si02, the 

It was proposed by Roh et al. [37] that the presence of partially oxidized Ce sites in Ce,cZri 02 suppresses CH4 formation by acetaldehyde decomposition, thus optimizing the hydrogen yield. In addition, Ce Zri c02 promotes noble metal and transition metals for the water-gas shift reaction (Eq. (24.3)) [38]. Moreover, in the reduced state, CexZii x02 niay reduce water to directly yield hydrogen [39]. Finally, Ce Zri, (02 improves the catalyst stability by (i) limiting the formation of ethylene and (ii) promoting carbon gasification [40]. 

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A.Y. Stakheev, and L.M. Kustov, Effects of the support on the morphology and electronic properties of supported metal clusters modem concepts and progress in 1990s, Appl. Catal. A 188, 3-35 (1999). 

Simple Fe porphyrins whose catalytic behavior in the ORR has been smdied fairly extensively are shown in Fig. 18.9. Literature reports disagree substantially in quantitative characterization of the catalytic behavior overpotential, stability of the catalysts, pH dependence, etc.). It seems plausible that in different studies the same Fe porphyrin possesses different axial hgation, which depends on the electrolyte and possibly specific residues on the electrode surface the thicknesses and morphologies of catalytic films may also differ among studies. AU of these factors may contribute to the variabUity of quantitative characteristics. The effect of the supporting surface on... 

In contrast to a-pinene autoxidation, for which no effect of the support on the reaetion seleetivity was observed, the seleetivity of the eo-oxidation process could be altered by varying the amount of NH2-groups on the support. Thus for Co-POM supported on NH2-MCF and NH2-SBA-I5 (NH2/C0-POM = 30 and 16 mol/mol, respectively), the a-pinene epoxide selectivity attained 94 and 76%, respeetively, at 96% of substrate conversion. 

The complex cation [Mo(T) -Cp)(CO)3] has been grafted successfully on zeolites H- 3 and H-Y to afford 57 by Kiihn and coworkers [190]. The grafting was realized by stirring a solution of the precursor complex [Mo(Ti -Cp)(CO)3Cl] with activated acid zeolites followed by washing to remove physisorbed complexes. The supported cations were characterized by IR spectroscopy (Vco 2026 and 1985 cm ) and the supports by XRD, which indicated minimal effect of the supported complex on the zeolite. The complexes were presumed to be confined to the zeolite by... 

HDL, respectively. By starch block, some of the VLDL exhibit 0 2-mobility, whereas by polyacrylamide gel electrophoresis the pre-j8 band migrates in post-)8 position, a phenomenon due to the sieving effect of the supporting medium. 

In order to stabilize the effect of the support on the activity and selectivity for hydrogen production, several supported platinum catalysts have been prepared and tested [282]. At 225 °C, Pt supported on Ti02 is the most active catalyst. In addition,... 

In the following, we summarize the main features of practical relevance. The position and amplitude of the activity hysteresis depends mainly on partial pressure of O2 and on the nature of the support material. The threshold temperatures of both decomposition and re-formation of PdO increase markedly with O2 partial pressure. Data on the effect of the support material are reported in Table 12.2. 

The effect of the supporting electrolyte on the fpzc of Au electrodes indicates specific affinity of either cations or anions of the dissolved salt to the given surface. For example, the studies performed by Hamelin [34] have shown the effect of the supporting electrolyte on the double-layer capacity - potential curves (see Table 3). 

Tab. 3 The effect of the supporting electrolyte (in aqueous solution) on the potential (versus SHE) of the minimum of the double-layer capacitance [11, 34] of the Au (100) electrode surface...

It is clear that the influence of surface geometry upon catalytic activity is extremely complex and many more studies are required before any definitive relationship between catalytic activity and metal particle size can be established. Such studies will require to take cognisance of such factors as the perturbation of surface structure due to the formation of carbidic residues, as noted by Boudart [289] and by Thomson and Webb [95], and by the modification of catalytic properties on adsorption, as noted by Izumi et al. [296—298] and by Groenewegen and Sachtler [299] in studies of the modification of nickel catalysts for enantioselective hydrogenation. Possible effects of the support, as will be discussed in Sect. 6.3, must also be taken into account. 

We are now in a position to examine, in quantitative detail, the effect of the supporting electrolyte, at least for steady-state homovalent transport in finite planar geometry. The amount of supporting electrolyte present, relative to electroactive electrolyte, is characterized by the support ratio that was defined as... 

The selective hydrogenation of organic compounds which contain a C=C and a C=0 double bonds into unsaturated alcohols is a very challenging subject from an industrial and from a fundamental point of view. Recent studies have shown that some catalytic systems are suitable for a selective hydrogenation in particular bi or trimetallic supported catalysts but no clear conclusion could be drawn concerning the mechanism and the respective effects of the support and the alloying (ref.1). 

These results indicate that alumina acts on CrgOg phase to prevent its clustering and segregation with high coordinate Cr + ions. This dispersive effect of the support provides a suitable environment for the formation on the surface of low coordinate chromium ions (ref. 2). However this effect of alumina tends to depress when the content of chromium exceeds 30 % at 410°C. This result seems to Indicate a saturation of the surface sites of the support which interact with the chromium (surface of alumina covered with a layer of chromium oxide). Then an excess of Cr O deposited leads to its clustering and crystallization. Consequently the coordination of chromium ions changes from tetrahedral (low coordination) to octahedral (high coordination). 

An objection to this model is that it fails to account for the effect of the support, which is known to increase the hydrogenation/hydrogenolysis ratio over that of bulk sulfides. De Beer and Schuit (7) concede that the model may have some merit at higher Co/Mo ratios, but that the major contribution to HDS activity is caused by intercalated systems. 

The above coatings have been applied on various substrates including Ni, steel, Ti and Au. In some cases no effect of the support has been noticed [511,517] (Fig. 27). In other cases, the effect is evident [79, 530], Steel is worse than Ni for thermal coatings [5, 519]. Synergetic effects involving the support are claimed in the case of in situ activation with Co-Mo coatings [79, 528] (Fig. 28). 

It should taken into account that the effects of the supporting electrolyte can also be adverse, for example, reaction mechanisms can be drastically altered (the ions of the supporting electrolyte may complicate the electroactive species through ionic association processes). However, except in specific cases, the advantages indicated make the addition of supporting electrolyte in excess a standard procedure. 

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