Particles rhodium

Figure 4.30. AFM images of rhodium particles deposited by spin-coating impregnation on flat Si02 on a Si(lOO) substrate particle, after reduction in hydrogen. [Adapted from...

The NO + CO reaction is only partially described by the reactions (2)-(7), as there should also be steps to account for the formation of N2O, particularly at lower reaction temperatures. Figure 10.9 shows the rates of CO2, N2O and N2 formation on the (111) surface of rhodium in the form of Arrhenius plots. Comparison with similar measurements on the more open Rh(llO) surface confirms again that the reaction is strongly structure sensitive. As N2O is undesirable, it is important to know under what conditions its formation is minimized. First, the selectivity to N2O, expressed as the ratio given in Eq. (7), decreases drastically at the higher temperatures where the catalyst operates. Secondly, real three-way catalysts contain rhodium particles in the presence of CeO promoters, and these appear to suppress N2O formation [S.H. Oh, J. Catal. 124 (1990) 477]. Finally, N2O undergoes further reaction with CO to give N2 and CO2, which is also catalyzed by rhodium. 

The lower trace in Figure 1 shows the results of heating the tunnel junctions (complete with a lead top electrode) in a high pressure cell with hydrogen. It is seen that the CO reacts with the hydrogen to produce hydrocarbons on the rhodium particles. Studies with isotopes and comparison of mode positions with model compounds identify the dominant hydrocarbon as an ethylidene species (12). The importance of this observation is obviously not that CO and hydrogen react on rhodium to produce hydrocarbons, but that they will do so in a tunneling junction in a way so that the reaction can be observed. The hydrocarbon is seen as it forms from the chemisorbed monolayer of CO (verified by isotopes). As... 

Larpent and coworkers were interested in biphasic liquid-liquid hydrogenation catalysis [61], and studied catalytic systems based on aqueous suspensions of metallic rhodium particles stabilized by highly water-soluble trisulfonated molecules as protective agent. These colloidal rhodium suspensions catalyzed octene hydrogenation in liquid-liquid medium with TOF values up to 78 h-1. Moreover, it has been established that high activity and possible recycling of the catalyst could be achieved by control of the interfacial tension. 

Figure 7.1 Transmission electron micrographs of rhodium particles supported on silica spheres (from Datye and Long [7]). 

In the example of Fig. 7.15 [43j rhodium particles have been deposited by spin coat impregnation of a Si02/Si substrate with an aqueous solution of rhodium trichloride. After drying, the particles were reduced in hydrogen. The images show samples prepared at three different rotation speeds in the spin coating process, but with concentrations adjusted such that each sample contains about the same amount of rhodium atoms. The particles prepared at high rotation speeds are smaller, which... 

Figure 9.3 Rh 3d5/2 binding energy versus particle size for half spherical rhodium particles on an alumina support (data from Huizinga el al. [14], and from Kip et al. [15], see also Fig. 6.18).

Figure 9.7 Transmission electron microscopy of rhodium particles on a model titania support after reduction in H2 at 200 °C (top) and the same catalyst in the SMSI state after reduction at 500 °C (bottom). An amorphous overlayer on the surface of the SMSI catalyst is clearly discerned (from Logan etal. [25]).

The state of a given small particle in a catalyst depends on the composition of the surrounding gas atmosphere, the temperature and the pressure. A rather extreme illustration is provided by the disintegration of rhodium particles in CO. 

Van t Blik et al. [3] exposed a highly dispersed 0.57 wt% RI1/AI2O3 catalyst (H/M=1.7) to CO at room temperature and measured a CO uptake of 1.9 molecules of CO per Rh atom. Binding energies for the Rh 3ds/2 XPS peak increased from 307.5 eV for the reduced catalyst under H2 to 308.7 eV for the catalyst under CO. The latter value equals that of the [Rh+(CO)2Cl]2 complex, in which rhodium occurs as a Rh+ ion. The infrared spectrum of the Rh/Al203 catalyst under CO showed exclusively the gem-dicarbonyl peaks at 2095 and 2023 cm-1. All results point to the presence of rhodium in Rh+(CO)2 entities. However, how can a rhodium particle accommodate so much CO ... 

The studies discussed above deal with highly dispersed and therefore well-defined rhodium particles with which fundamental questions on particle shape, chemisorption and metal-support interactions can be addressed. Practical rhodium catalysts, for example those used in the three-way catalyst for reduction of NO by CO, have significantly larger particle sizes, however. In fact, large rhodium particles with diameters above 10 nm are much more active for the NO+CO reaction than the particles we discussed here, because of the large ensembles of Rh surface atoms needed for this reaction [28]. Such particles have also been extensively characterized with spectroscopic techniques and electron microscopy we mention in particular the work of Wong and McCabe [29] and Burkhardt and Schmidt [30], These studies deal with the materials science of rhodium catalysts that are closer to the ones used in practice, which is of great interest from an industrial point of view. 

Figure 7 is a differential tunneling spectrum of CO chemisorbed on alumina supported rhodium particles. The identification of the peaks is also shown below and consist of three separate species. These are a gem dicarbonyl Rh (C0)2 a linear carbonyl RhCO and a bridging carbonyl RhxC0. The dicarbonyl is characterized by a peak at 4l3 cm 1 and the linear species by a bending mode at 465 cm-1. 

Figure 7 Differential tunneling spectrum of CO chemisorbed on alumina supported rhodium particles. Peak positions are not corrected for possible shifts due to the top lead electrode. Peak positions vary with rhodium coverage and CO exposure.

Ohtaki, M., Komiyama, M., Hirai, H., and Toshima, N., Effects of polymer support on the substrate selectivity of covalently immobilized ultrafme rhodium particles as a catalyst for olefrn hydrogenation, Macromolecules, 24, 5567, 1991. 

The reduction of acetaldehyde to ethanol could be explained by its chemisorption near rhodium particles and the action of spill over hydrogen. On Rh/La 0, Bell has observed that at low residence times acetaldehyde is the primary product whereas at longer residence times the formation of ethanol becomes the dominant process. They concluded that this pattern is characteristic of the sequential reaction process ... 

Chlorine remains on the surface even upon heating till 527 °C, a temperature at which large rhodium particles are formed. Moreover, dihydrogen reacts with Rh (C0)2/Ti02 at 27 °C to form a monocarbonyl intermediate species, presumably Rh(H)(CO) and to remove partially chlorine from the surface. Further heating at 152 °C under H2 leads to incomplete CO removal, as evidenced by some graphitic carbon and rhodium carbide, both detected on the surface. 

Rhodium-catalyzed silylformylation proceeds smoothly in branched 1-alkynes at 25 °C as shown in Table 3. The stereochemistry at the chiral carbon involved in alkynes is retained intact under the silylformylation conditions. (A)-28, (condensed with mesitylene. 3-Trimethylsilyl-l-propyne 40 reacts similarly to give 41 (Equation (7)). " / //-Butylacetylene does not work as a substrate for the silylformylation because of the bulky tert-huty group on the i/>-carbon. In contrast to /< r/-butylacetylene, trimethylsilylacetylene 42 gives 43 in a fair yield (Equation (8)). ... 

The phenomenon of metal transport via the creation of volatile metal carbonyls is familiar to workers using carbon monoxide as a reactant. It is often found that carbon monoxide is contaminated with iron pentacarbonyl, formed by interactions between carbon monoxide and the walls of a steel container. Thus, it is common practice to place a hot trap between the source of the CO and the reaction vessel. Iron carbonyl decomposes in the hot trap and never reaches the catalyst that it would otherwise contaminate or poison. Transport of a number of transition metals via volatile metal carbonyls is common. For example, Collman et al. (73) found that rhodium from rhodium particles supported on either a polymeric support or on alumina could be volatilized to form rhodium carbonyls in flowing CO. 

Due to the high density of Rh atoms, no species of the form Rh(C0)2 were formed, however. This is expected to be the case on the (111) surface as well. Weak absorptions between 400 and 575 cm"l were seen and are indicative of metal-absorbate stretching and bending vibrations. Inelastic electron tunneling spectroscopic (IETS) measurements on alumina supported rhodium particles (47, 48, 49) add little new structural information... 

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