Supported Rh Particles Research Articles

Study of desorption activation energy on Rh-CO systems

The adsorption of CO on an Rh foil and small Rh particles supported by Al2O3 polycrystalline surfaces was studied using the temperature programmed desorption (TPD). The supported Rh particles were prepared by evaporation, using the principles of electron bombardment. The activation energy of desorption was calculated. The results show the dependence on the CO exposure of the sample as well as the size effect. The desorption and the recombination peaks were measured. The dissociation of CO and the influence of C atoms on the surface are discussed.

Kinetics of CO oxidation by N 2O over Rh(111)

We have studied, for the first time, the reaction of CO with N 2O over a Rh(111) catalyst at pressures between 1 and 20 Torr in an apparatus that couples a moderate pressure reactor with an ultrahigh vacuum analysis chamber. Using 4 Torr of CO and 4 Torr of N20 between 570 and 670 K, we measured an apparent activation energy ( E a) of 40.0 kcal/mol. By varying the reactant pressures ( T = 623 K), we determined that the reaction orders are + 1.1 in N 2O pressure and −1.2 in CO pressure. Both E a and the reaction order are in good agreement with previously reported measurements over alumina supported Rh particles. Kinetic modeling of the data suggests that CO and N 2O compete for a limited number of vacant sites on a surface where O CO ≥ 0.9 ML. Under these conditions, the rate limiting step is N 2O dissociation. We estimate, based on the model, that the barrier for N 2O dissociation ( E diss N 2O is 17.5 kcal/mol. The model also predicts that the N 2O sticking coefficient ( S N 2O ) must be greater than 0.005. Our measurements show that there is excellent agreement between Rh(111) and supported Rh reaction kinetics with regards to both E a and the CO pressure dependence. These experimental results add still another reaction to the growing list of cases where single crystals are excellent models for more practical supported catalysts. Further, our modeling of these results allows us to estimate two previously unknown quantities, S N 2O and E diss N 2O .

Density functional study of carbon monoxide chemisorption on model clusters of rhodium and palladium: a comparative analysis of the site selection

Dimers of rhodium (Rh) and palladium (Pd) catalysts were investigated during a conversion reaction with carbon monoxide (CO). The hydrogenation of CO on supported Rh particles produces a variety of compounds such as hydrocarbons, alcohols, aldehydes, and acids. The chemisorption of CO is different in these two metals, and this report discusses bonding differences that may contribute to this outcome. 78 refs., 4 tabs.

Electron Microscopy study of metal-support interaction in Rh/CeO2 catalysts

In the characterization of supported metal catalysts, it is important to understand the influence of the support on the metal particles since many catalytic properties are controlled by their interaction. Previous studies of supported Rh catalysts have shown that the oxide support plays an Important role in the activity and stability of the catalysts: the morphology and dispersion as well as the activity of the supported Rh particles can be affected significantly. High resolution electron microscopy (HREM), scanning transmission electron microscopy (STEM) and microdiffraction represent powerful techniques for studying the microstructure of supported small metal particles (<50 Å) and their structural relationship with the support.In this study, the techniques of HREM and STEM/microdiffraction have been combined to characterize the structures of the Rh particles in Rh/CeO2 and their structural relationship with the cerium oxide support. The microscopes used were JEM-200CX, JEM-4000EX and VG HB-5 STEM. The following catalyst samples with various treatments have been studied:

Dissociation and oxidation of carbon monoxide over Rh/Al 2O 3 catalysts

The activity of Rh/Al 2O 3 catalysts for CO oxidation was investigated by transient isotopic pulse experiments using a packed-bed reactor. This transient experimental scheme revealed significant CO dissociation activity during CO oxidation over Rh/Al 2O 3 catalysts. Results indicate that the oxidation of CO proceeds via dissociative oxidation by its own oxygen as well as via direct oxidation by gas-phase oxygen on well-dispersed Rh/Al 2O 3 catalysts. The rate of CO dissociation is on the same order of magnitude as the rate of CO oxidation; under steady-state conditions at 300 °C, the rate of CO dissociation is approximately half that of direct oxidation. Differences in CO dissociation activity between single-crystal Rh surfaces and well-dispersed supported Rh particles are explained in terms of the molecular bonding and adsorption characteristics on these two different surfaces. The importance of CO dissociation kinetics in the overall CO oxidation activity of Rh/Al 2O 3 catalysts is further discussed in view of the reaction lightoff behavior.

Hydrogen spillover on alumina—A study by infrared spectroscopy

Infrared spectroscopy has been used to monitor the exchange of D 2 (g) with OH groups chemisorbed on Al 2O 3. It has been shown that near 300 K, the rate of the exchange process is rapid in the presence of supported Rh particles on the Al 2O 3. A qualitative model for hydrogen “spillover” is presented in which dissociative adsorption of dihydrogen by the metal is a key step. It is shown that CO chemisorption on the supported Rh leads to a marked reduction in the “spillover” rate due to site blockage on the Rh. This is consistent with recent studes of behavior of the CO and H coadsorbed on Rh (111).