Rhodium dispersion

When the hydrogenation of citral is performed with supported nanoparticles of rhodium metal, for example Rh/Si02 under classical conditions [liquid phase, rhodium dispersion 80% (particles in the range of 1-2nm), citral/Rhs = 200, P(ti2) = 80bar, T = 340 K], the catalytic activity is very high but most of the above products are obtained and the reaction is totally non-selective, even if the major product was citronellal. 

Verna (62) studied the dissociative adsorption of thiophene on platinum, palladium, and rhodium dispersed on alumina. Figure 7 and Table II present the dissociative chemisorption of thiophene to butane on the three metals. The sulfur coverage of platinum is very low compared to the other metals. The sulfur coverage on palladium is about 2.5 times higher than on platinum such a value is similar to the one found by Mathieu and Primet (63). 

H2 chemisorption. Both Rh/R-Ti02 and Rh/A-Ti02 show a decrease in H2 chemisorption when the reduction and evacuation temperature is increased, while at the same time the slope of the chemisorption vs. In t curve decreases. The decrease in H2 chemisorption is of course due to the gradual transition of the Rh particles into the SMSI state. Whatever the explanation for this state, an electronic interaction between metal particles and support or a covering of the metal particles by the support, in this SMSI state the metal particles are unable to adsorb H2. The decreased slope of the H/Rh-ln t curve can be explained in several ways, such as slow H2 chemisorption on Rh because of an activated process, dependence on metal dispersion, or an effect related to the support. The experiments in which H2 chemisorption was started around 200°C proved that the time dependence is indeed due to a slow adsorption at room temperature, but the experiment with Rh/Si(>2 showed that there is no kinetic limitation in the H2 chemisorption on the metal part of the catalyst. In accordance with this conclusion, no effect of rhodium dispersion on the time dependence of the H2 chemisorption was observed for catalysts in the normal state (cf. Figure 1 curves A, B and F). 

Kaspar et al. [61] studied the effect of the rhodium dispersion in rhodium on y-AI2O3 catalysts on the SCR of NO with CO. It was observed that with increasing particle size of rhodium (> 1.2 nm) the reaction rate increases. 

Thus far, only one precursor of each of the precious metals has been discussed in the context of the calcination process. In practice, a number of precursors are available and these can play a major role in determining metal location and dispersion (ref. 47). The effect of precursor on rhodium dispersion on alumina is shown in Table 6 where the absorption of NO is used as a measure, of dispersion. 

Fig. 13. Effect of concentration on rhodium dispersion using (A) [Rh(NH3)3Cl]Cl2 and (B) Rh Cl3 as precursors.

A second difference between the two is the behaviour when the catalysts are fired in air. Claus salt initially decomposes to rhodium metal but in the presence of air is converted to the oxide which sinters rapidly. Thus a worse dispersion of rhodium is observed when Claus salt is fired in air than when it is fired in nitrogen or hydrogen/nitrogen. In the case of rhodium chloride a superior overall rhodium dispersion is achieved and air firing is not so detrimental to dispersion as it is for the ammine complex. These observations can again be explained in terms of the decomposition chemistry of the precursor. Newkirk and McKee (ref. 51) have studied the decomposition of rhodium chloride, both unsupported and supported on alumina,... 

The effect of alumina phase and ageing (8 hrs in air at the specified temperature) on rhodium dispersion (lXRh/A Oj ex [RMNH CljC )... 

The rhodium dispersion becomes progressively worse on the higher temperature and, therefore, lower surface area alumina phases, NO uptake also falls sharply as the ageing temperature of each Rh/A1 0 phase is increased. The lower NO uptake can be explained partially by rhodium sintering (as the oxide) and also by a metal support interaction (Ref. 36). The interaction is less for the high temperature, less reactive alumina phases but even here NO absorption is not measurable after ageing at 850 C. The rhodium/alumina interaction is also observed when temperature programmed reduction (TPR) is performed (Fig. 14(A) and (B). 

Table 2 shows average rhodium dispersions for each of the five weight loaded catalysts. These were calculated from the hydrogen uptakes in Table 1 by assuming each hydrogen atom represented one rhodium surface site and from the definition of dispersion which is the fraction of total rhodium atoms that... 

Calculated average rhodium dispersions as a function of weight loading. 

In recent years, Schmidt and his collaborators published a series of very detailed electron-microscopy studies on palladium, platinum and rhodium dispersed on cerium... 

Although no clear indication about the sample weight and the rhodium dispersion was given by Cunningham et al., a minimum rate of can be estimated for their 2%Rh0x/Ce02 sample at 18°C,... 

Table 9. Particle sizes and catalytic activity of colloidal rhodium dispersions ([Rh] = 0.01 mmol [substrate] = 25 mmol methanol is a solvent)...

Figure 1. The high resolution electron microscopy of rhodium dispersed on silica (from reference [19]).

Ferrandon and Krause investigated the effect of the catalyst support on the performance of rhodium catalysts wash-coated onto cordierite monoliths for autothermal reforming of gasoline [245]. The samples contained 2 wt.% rhodium on gadolinium/ceria, and lanthanum-stabilised alumina [245]. The latter sample showed higher activity and superior selectivity. Only 30 ppm of light hydrocarbons (C > 1) were detected. However, the sample also had higher surface area and rhodium dispersion. Both samples showed stable performance for more than 50-h test duration. 

Larpent, C. Patin, H. (1988) Catalytic hydrogenations in biphasic liquid-liquid systems. 2. Utilization of sulfonated tripod ligands for the stabilization of colloidal rhodium dispersions, J. Mol Catal, 44,191-5. 

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