Rhodium supported systems

Zeolitic systems are very active at low temperatures but they also have disadvantages related to their hydrothermal stability and the possibility of inhibition or poisoning by different compounds. These drawbacks drastically limit the industrial applications of these catalysts. Rhodium-supported systems are also active at low temperatures and low N2O concentration, but at high temperatures and in the presence of O2 the noble metal is oxidized. Furthermore, the high cost of Rh may prove to be a limit for industrial applications. 

Recently we reported EXAFS results on bimetallic clusters of iridium and rhodium, supported on silica and on alumina (15). The components of this system both possess the fee structure in Efie metallic state, as do the components of the platinum-iridium system. The nearest neighbor interatomic distances in metallic iridium and rhodium are not very different (2.714A vs. 2.690A). From the results of the EXAFS measurements, we concluded that the interatomic distances corresponding to the various atomic pairs (i.e., iridium-iridium, rhodium-rhodium, and iridium-rhodium) in the clusters supported on either silica or alumina were equal within experimental error. Since the Interatomic distances of the pure metals differ by only 0.024A, the conclusion is not surprising. 

Fahey (16) suggests that intermediate 3 dissociates formaldehyde he finds supportive evidence in the rhodium-based system by observation of minor yields of 1,3-dioxolane, the ethylene glycol trapped acetal of formaldehyde. For reasons to be discussed later, we believe the formation of free formaldehyde is not on the principal reaction pathway. (c) We have also rejected two aspects of the reaction mechanism proposed by Keim, Berger, and Schlupp (15a) (i) the production of formates via alcoholysis of a formyl-cobalt bond, and (ii) the production of ethylene glycol via the cooperation of two cobalt centers. Neither of these proposals accords with the observed kinetic orders and the time invariant ratios of primary products. 

A mechanistic study by Haynes et al. demonstrated that the same basic reaction cycle operates for rhodium-catalysed methanol carbonylation in both homogeneous and supported systems [59]. The catalytically active complex [Rh(CO)2l2] was supported on an ion exchange resin based on poly(4-vinylpyridine-co-styrene-co-divinylbenzene) in which the pendant pyridyl groups had been quaternised by reaction with Mel. Heterogenisation of the Rh(I) complex was achieved by reaction of the quaternised polymer with the dimer, [Rh(CO)2l]2 (Scheme 11). Infrared spectroscopy revealed i (CO) bands for the supported [Rh(CO)2l2] anions at frequencies very similar to those observed in solution spectra. The structure of the supported complex was confirmed by EXAFS measurements, which revealed a square planar geometry comparable to that found in solution and the solid state. The first X-ray crystal structures of salts of [Rh(CO)2l2]" were also reported in this study. 

Efforts to optimize rhodium-based systems for methanol carbonylation led to the development of new supporting ligands containing phosphorus and sulfur donor atoms, both thiolates and thioethers, such as those used in the preparation of complexes (20) and (21). Ligands such as 2-diphenylphosphinothiolate have been shown to give rise to complexes that exhibit higher activities, up to four times faster, for the carbonylation of methanol compared to [Rh(CO)2l2] . ... 

A development in catalyst support systems in which half of the 5-10% rhodium-platinum alloy gauzes were replaced by nonnoble metal supports or by ordinary metal catalysts gave cost economies without adversely affecting operating efficiency [47]. More recently, ammonia oxidation in a two-bed system (Pt gauzes followed by monolithic oxide layers) gave nearly the same ammonia conversion while reducing platinum losses by 50% [48]. 

A surface structure of the type discussed for the rhodium-silica system, where two CO molecules adsorb on one surface metal atom, appears to be possible for some metals existing in certain ranges of crystallite sizes. Guerra and Schulman (112) have, in fact, questioned the existence of this adsorption complex on their rhodium-silica samples, but have suggested a similar type of adsorption mechanism occurring on their rhenium and ruthenium silica supported samples. 

Another classic work in the area of the solid-state NMR of dispersed metal systems is the work of Duncan and Zilm [ 100]. In this work CO was examined on samples of rhodium supported on a silica. CO adsorbs on oxide-supported transition metals in three forms linear, bridge-bonded, and multicarbonyls. As... 

Rh dispersion appears to be a key factor in the activity of zeolite-supported systems. In studies of the effect of Si/Al ratio in Rh/NaX (faujasite-type zeolite), prepared by cation exchange of NaX with [Rh(NH3)5Cl](OH)2, maximum carbonylation activity (ca. 8 mol/(gRh-h) at 200°C) correlates with maximum Rh dispersion in the prepared catalyst (48). Similarly, cation exchange of NaX with [Rh(NH3)5Cl]Cl2 has been reported to give carbonylation rates of ca. 1 mol/(gRh h) at 150°C (49), and NaY exchanged with rhodium salts (50) gives carbonylation activity of 0.4 mol/(gRh-h) at 170°C. These rates are claimed to be higher than those for Rh impregnated alumina, silica-alumina, silica, or titania (49,50). Optimum Rh dispersion corresponded to approximately two Rh atoms per vmit cell (50). 

Recently, rhodium and ruthenium-based carbon-supported sulfide electrocatalysts were synthesized by different established methods and evaluated as ODP cathodic catalysts in a chlorine-saturated hydrochloric acid environment with respect to both economic and industrial considerations [46]. In particular, patented E-TEK methods as well as a non-aqueous method were used to produce binary RhjcSy and Ru Sy in addition, some of the more popular Mo, Co, Rh, and Redoped RuxSy catalysts for acid electrolyte fuel cell ORR applications were also prepared. The roles of both crystallinity and morphology of the electrocatalysts were investigated. Their activity for ORR was compared to state-of-the-art Pt/C and Rh/C systems. The Rh Sy/C, CojcRuyS /C, and Ru Sy/C materials synthesized by the E-TEK methods exhibited appreciable stability and activity for ORR under these conditions. The Ru-based materials showed good depolarizing behavior. Considering that ruthenium is about seven times less expensive than rhodium, these Ru-based electrocatalysts may prove to be a viable low-cost alternative to Rh Sy systems for the ODC HCl electrolysis industry. 

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