Rhodium chemisorption

The use of silver fluoroborate as a catalyst or reagent often depends on the precipitation of a silver haUde. Thus the silver ion abstracts a CU from a rhodium chloride complex, ((CgH )2As)2(CO)RhCl, yielding the cationic rhodium fluoroborate [30935-54-7] hydrogenation catalyst (99). The complexing tendency of olefins for AgBF has led to the development of chemisorption methods for ethylene separation (100,101). Copper(I) fluoroborate [14708-11-3] also forms complexes with olefins hydrocarbon separations are effected by similar means (102). 

Although there is a severe paucity of vibrational data for the molecular form of O2 chemisorbed on rhodium surfaces, it is possible to visualize the dinuclear and trinuclear complexes as models for the associative chemisorption of O2 on rhodium. The pq-o values of the complexes Rh2(02)i,2 show little... 

Looking at the trends in dissociation probability across the transition metal series, dissociation is favored towards the left, and associative chemisorption towards the right. This is nicely illustrated for CO on the 4d transition metals in Fig. 6.36, which shows how, for Pd and Ag, molecular adsorption of CO is more stable than adsorption of the dissociation products. Rhodium is a borderline case and to the left of rhodium dissociation is favored. Note that the heat of adsorption of the C and O atoms changes much more steeply across the periodic table than that for the CO molecule. A similar situation occurs with NO, which, however, is more reactive than CO, and hence barriers for dissociation are considerably lower for NO. 

The role of the support on hydrogen chemisorption on supported rhodium catalysts was studied using static and frequency response techniques. In all Instances, several klnetlcally distinct H2 cheml-sorptlve sites were observed. On the basis of the kinetics, at least one site appears to sorb H2 molecularly at temperatures below 150°C, regardless of the support. At higher temperatures, a dissociative mechanism may become dominant. Inducement of the SMSI state In Rh/T102 does not significantly alter Its equilibrium H2 chemisorption. 

These results are consistent with recently reported results by Haller, et al. (10) on the reactions of CO/H2 and NHj over Rh catalysts In which no significant differences were observed between catalysts reduced at low and high temperatures (presumably "normal and SMSI) but In which Rh/S102 was found to behave differently. Thus, there appears to be some correlation between the FRC chemisorption results and the reactivity patterns of supported rhodium catalysts which we would like to believe supports the assertion that the sites at which hydrogen sorbs reversibly are those at which catalytlcally Important reactions occur, and that FRC can monitor the density and relative kinetics of these sites. 

Similar SSIMS and TDS results were obtained for rhodium on tltanla and fiir hydrogen chemisorption on both substrates. In a blank experiment Involving i o metal over layer, temperature programming while following the T1 and TIO SIMS signals (Fig. 4) shows that the tltanla thin film does not begin to change until the temperature reaches about 760 K, well beyond the 615 K where Tl was first noted to Increase on the systems with thin metal overlayers. ... 

Numerous quantum mechanic calculations have been carried out to better understand the bonding of nitrogen oxide on transition metal surfaces. For instance, the group of Sautet et al have reported a comparative density-functional theory (DFT) study of the chemisorption and dissociation of NO molecules on the close-packed (111), the more open (100), and the stepped (511) surfaces of palladium and rhodium to estimate both energetics and kinetics of the reaction pathways [75], The structure sensitivity of the adsorption was found to correlate well with catalytic activity, as estimated from the calculated dissociation rate constants at 300 K. The latter were found to agree with numerous experimental observations, with (111) facets rather inactive towards NO dissociation and stepped surfaces far more active, and to follow the sequence Rh(100) > terraces in Rh(511) > steps in Rh(511) > steps in Pd(511) > Rh(lll) > Pd(100) > terraces in Pd (511) > Pd (111). The effect of the steps on activity was found to be clearly favorable on the Pd(511) surface but unfavorable on the Rh(511) surface, perhaps explaining the difference in activity between the two metals. The influence of... 

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. 

Favorable configurations for CO dissociation are those where optimum overlap exists between the d-orbitals of rhodium and the 271 level of CO the better this overlap is, the lower the energy of the 27t -derived chemisorption... 

The last explanation for methanol formation, which was proposed by Ponec et al., 26), seems to be well supported by experimental and theoretical results. They established a correlation between the gfiethanol activity and the concentration of Pd , most probably Pd. Furthermore, Anikin et al. (27) performed ab initio calculations and found that a positive charge on the palladium effectively stabilizes formyl species. Metals in a non-zero valent state were also proposed by Klier et al. (28) on Cu/ZnO/Al O, by Apai (29) on Cu/Cr O and by Somorjai for rhodium catalyts (30). Recently results were obtained with different rhodium based catalysts which showed the metal was oxidized by an interaction with the support (Rh-0) (on Rh/Al 0 ) by EXAFS ( -32) and by FT-IR ( ) and on Rh/MgO by EXAFS ( ). The oxidation of the rhodium was promoted by the chemisorption of carbon monoxide (, ). ... 

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 ... 

Hayden BE (1997) The structure and reactivity of Ti02 (110) supported palladium and rhodium. In Lambert RM, Pacchioni G (eds) Chemisorption and reactivity on supported clusters and thin films. Kluwer Academic Publishers, Norwell, p 215... 

With the advent of sophisticated experimental techniques for studying surfaces, it is becoming apparent that the structure of chemisorbed species may be very different from our intuitive expectations.10 For example, ethylene (ethene, H2C=CH-2) chemisorbs on platinum, palladium, or rhodium as the ethylidyne radical, CH3—C= (Fig. 6.2). The carbon with no hydrogens is bound symmetrically to a triangle of three metal atoms of a close-packed layer [known as the (111) plane of the metal crystal] the three carbon-metal bonds form angles close to the tetrahedral value that is typical of aliphatic hydrocarbons. The missing H atom is chemisorbed separately. Further H atoms can be provided by chemisorption of H2, and facile reaction of the metal-bound C atom with three chemisorbed H atoms dif-... 

The activities in FT reaction (expressed as turnover rates, Vt of CO transformed to hydrocarbons and oxygenates) of bulk and supported tungsten carbides are compared to that of a rhodium catalyst (3.5 wt%) supported on alumina (Table 18.6). Its dispersion (94%) has been measured by hydrogen chemisorption by assuming unity stoichiometry of adsorbed hydrogen on Rh. 

Thus we may conclude that the pre-adsorption of hydrogen had no effect on CO chemisorption on rhodium while both oxygen and carbon blocked many sites for CO chemisorption and weakened the metal-adsorbate interaction (Vjyj-c decreased, V( q increased,... 

---