Polycrystalline rhodium

Surface science studies of thin films may be very helpful for the understanding of the mechanisms of heterogeneous catalysis on intermetallics. This was true in particular for the AES study of the Ru(0001)-Ce-H2 interface performed by Walker and Lambert (1992) in the context of ammonia synthesis or the growth of cerium films on polycrystalline rhodium (Warren et al. 1993) on top of which carbon monoxide oxidation was performed. 

Table III. Comparison of Polycrystalline Rhodium Foil with a 1% Rh/Al203 Catalyst in the CO—H2 Reaction at Atmospheric Pressure...

Sexton and Somorjai (12) have reported results of Auger and other studies on a polycrystalline rhodium surface exposed to CO, CO2, and H2. They found that after reaction of H2 and CO with the specimen at... 

The 02/ 02 isotopic exchange that occurs during thermal desorption of oxygen from polycrystalline rhodium was investigated by Matsushima. 

Temperature programmed desorption (TPD) or thermal desorption spectroscopy (TDS), as it is also called, can be used on technical catalysts, but is particularly useful in surface science, where one studies the desorption of gases from single crystals and polycrystalline foils into vacuum [2]. Figure 2.9 shows a set of desorption spectra of CO from two rhodium surfaces [14]. Because TDS offers interesting opportunities to interpret desorption in terms of reaction kinetic theories, such as the transition state formalism, we will discuss TDS in somewhat more detail than would be justified from the point of view of practical catalyst characterization alone. 

D. Hasenberg, HCN Synthesis on Polycrystalline Platinum and Rhodium , dissertation, University of Minnesota, 1984. 

The nature of adsorbed hydrogen atoms is not precisely known. Polycrystalline platinum, palladium, and iridium show two major peaks for hydrogen upon potential sweep in the positive potential region (97), indicative of multiple adsorption states. Ruthenium and rhodium exhibit only one adsorbed state at the low-potential, weak adsorption region 102, 103. The surface coverage on the first group of metals varies approximately linearly with potential [Fig. 6 (97.9,5)], in accord with Eq. (24) for a Temkin isotherm. 

The only other observation of chemisorbed hydrogen was made by Pickering and Eckstrom (11) on rhodium films. They found a total of 18 absorption bands between 2193 cm-1 and 1416 cm-1 which they did not attempt to interpret. It is possible that the bands represent adsorption on different crystal faces of a highly polycrystalline material. 

Many studies on carbon monoxide adsorbed on polycrystalline and single crystal Pt, Pd, and Rh electrodes have been carried out during recent years by means of electrochemical methods and IR spectroscopy (EMIRS, SNIFTIRS, IRRAS, etc.), potential-modulated reflectance spectroscopy and other methods.Electrochemical results show that the number of Pt adsorption sites per CO molecule is changing from 2 to 1 with increasing coverage in acidic solution. There is, however, a discussion in the literature about the evaluation of absolute saturation coverage on ordered low-index platinum (and rhodium) electrodes with particular reference to Pt(l 1... 

Both rhodium and iron polycrystalline foils have been used and compared at 6 atm. Again methanation was predominant even at this pressure. Iron was found to be a better methanation catalyst than rhodium, as indicated by Figure 9. The distribution of higher-molecular-weight products from the two metal surfaces are somewhat different as shown in Figure 10. Iron produces hydrocarbon products up to C5 under... 

Figure 10. Comparison of product distributions obtained over initially clean polycrystalline iron and rhodium foils...

Noble metals are eatalytieally very active, and many studies have been earried out on their surfaces, espeeially platinum, palladium, and rhodium. Noble metals have been used as polycrystalline metals or monocrystals, metal blacks, metals supported on graphite, microparticles ineorporated into redox active polymers, ete. The activity of these materials towards the electrocatalytieal hydrogenation depends mainly on the nature of the metal, pH, and supporting electrolyte, and the state of the surface. 

When the fact of hydrogen adsorption on platinum metals was well documented, a search for self-consistent models started, aimed to description of surface hydrogen coverage dependence on electrode potential and temperature. Breiter was the first who studied the temperature dependence of hydrogen adsorption on smooth platinum, rhodium and iridium. He demonstrated a possibility to present experimental dependences typical for polycrystalline platinum by combination of two Fmmkin isotherms (see below in Section IV. 5). 

A series of pubhcations was devoted to the electrocatalytic reduction of nitrate by the Eindhoven group [50-54]. On the basis of these works, a comparative study was performed to determine the reactivity of nitrate ions in 0.1 mol dm concentration on eight different polycrystalline electrodes (platinum, palladium, rhodium, ruthenium, iridium, copper, silver, and gold) in acidic solution using cyclic voltammetry, chronoamperometry, and differential electrochemical mass spectroscopy (OEMS) [50]. 

The HPLP apparatus has been used to study the hydrogenation of carbon monoxide on iron and rhodium polycrystalline specimens as well as on sit e crystals and various rhodium compounds." Postreaction surface analysis revealed the presence of a catalytically active carbonaceous layer on all the samples investigated. In addition, precise control of the rhodium surface oxidation state by oxygen pretreatment in the UHV chamber was found to have a marked effect on the product distribution of the rhodium catalyzed reaction. 

In one of the most relevant papers in this field, Dima et al. [13] studied the electrocatalytic behavior of different polycrystalline metals such as Ru, Rh, Ir, Pd, Pt, Cu, Ag and Au for nitrate (100 mM) reduction in 0.5 M H2SO4. On the basis of the peak current density related to nitrate reduction on cyclic voltammograms, the activities of each electrode were compared. It was determined that rhodium is the most active catalyst among the noble metals for the reduction of nitrate, with the activity decreasing in the order Rh, Ru, Ir, Pt, Pd and Cu, Ag, Au for transition metals. The high electrocatalytic performance of Rh for nitrate reduction was also observed by Brylev et al. [19]. By using Differential Electrochemical Mass Spectrometry (DEMS), a reduction mechanism for nitrate reduction has been determined for transition metals (Fig. 2). 

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