Se H. Oh

Comparative kinetic studies of COO2 and CONO reactions over single crystal and supported rhodium catalysts

The kinetics of the COO2 and CONO reactions over single crystal Rh(111) and over alumina-supported Rh catalysts have been compared at realistic reactant pressures. For the COO2 reaction, there is excellent agreement between both the specific rates and activation energies measured for the two types of Rh catalysts. The CONO reaction, on the other hand, exhibits substantially different activation energies and specific reaction rates between the single crystal and supported catalysts. This indicates that the kinetics of the CONO reaction, unlike the COO2 reaction kinetics, are sensitive to changes in catalyst surface characteristics. The kinetic data for the COO2 and CONO reactions over Rh(111) and RhAl2O3 were analyzed using mathematical models which account for the individual elementary reaction steps established from surface chemistry studies of the interactions of CO, NO, and O2 with Rh surfaces. The model, when used with the parameter values similar to those reported in the surface chemistry literature, can quantitatively fit the CO oxidation rate data over both the single crystal and supported Rh catalysts. The kinetics of the CO NO reaction over Rh(111) can also be well described by a reaction model using parameter values taken from surface chemistry studies. However, the rate data for the CONO reaction over supported Rh can be rationalized by assuming that the dissociation of molecularly adsorbed NO occurs much more slowly on supported Rh than on Rh(111).

Methane oxidation over alumina-supported noble metal catalysts with and without cerium additives

Laboratory reactor experiments have been conducted to evaluate alumina-supported noble metal catalysts, both in the presence and absence of cerium additives, for their effectiveness in the catalytic oxidation of methane under conditions likely to be encountered in natural-gas vehicle exhaust. Under oxidizing conditions, all of the catalysts promoted the complete oxidation of methane to CO2 and H2O, with the methane oxidation activity ranking given by Pd > Rh > Pt in the absence of Ce and by Rh > Pd ≈ Pt in the presence of Ce. Under reducing conditions, methane oxidation produced substantial amounts of CO and H2 as the principal partial oxidation products. In the absence of Ce, the methane oxidation activity decreases in the order Pd > Rh > Pt, with the tendency to form CO decreasing in the order Rh > Pd > Pt. The activity ranking for methane conversion in reducing feedstreams was not affected by the presence of Ce; however, the addition of Ce to the Pt/Al2O3 and Pd/Al2O3 catalysts almost completely suppressed the formation of the partial oxidation product CO. At a fixed temperature of ≈550°C, the methane conversion over each of the noble metal catalysts goes through a maximum as the feedstream concentration of O2 is varied. The data suggest that O2 inhibits the CH4 oxidation under oxidizing conditions by excluding the more weakly adsorbed species, CH4, from the active sites. Also, methane oxidation experiments in the presence of CO in the feed showed that the methane conversion characteristics of the noble metal catalysts are little affected by the CO.

Effects of cerium addition on CO oxidation kinetics over alumina-supported rhodium catalysts

Abstract The kinetics of CO oxidation in a strongly oxidizing environment (i.e., P O 2 ⪢ P CO ) over a low-loaded Rh/Al 2 O 3 catalyst are not significantly affected by the presence of cerium. Under moderately oxidizing or net-reducing conditions, on the other hand, the addition of sufficient amounts of cerum oxides (≥2 wt% Ce) to the Rh/Al 2 O 3 catalyst was found to cause the following changes in CO oxidation kinetics: suppression of the CO inhibition effect, decreased sensitivity of the reaction rate to gas-phase O 2 concentration, and decreased apparent activation energy. These cerium-induced changes in the kinetics lead to enhancement of CO oxidation activity and can be rationalized on the basis of a mechanism involving CO 2 formation via a reaction between CO adsorbed on Rh and surface oxygen derived from the neighboring ceria particles. The effects of Ce addition on the CO oxidation kinetics were also independent of whether the Ce was deposited before or after the Rh.

Influence of metal particle size and support on the catalytic properties of supported rhodium: COO2 and CONO reactions

The effects of metal particle size and support material on the kinetics of the CO-O{sub 2} and CO-NO reactions over Rh have been investigated. For the CO-O{sub 2} reaction in the CO-inhibition regime, both the specific rate and the apparent activation energy were found to be virtually independent of the Rh particle size, but moderately sensitive to the nature of the support material (with the low-temperature oxidation activity decreasing in the order Rh/{alpha}-Al{sub 2}O{sub 3}.Rh/SiO{sub 2}.Rh/{theta}-Al{sub 2}O{sub 3}). The specific rate of the CO-NO reaction increased drastically with increasing Rh particle size; a 45-fold increase in specific rate was observed as the Rh particle size increased from 10 to 676 {angstrom}. However, support material had only a small effect on both the activity and selectivity of Rh for the CO-NO reaction.

Platinum-rhodium synergism in three-way automotive catalysts

The interactions between Pt and Rh in three-way automotive catalysts were investigated by conducting laboratory reactor experiments with both a PtRh bimetallic catalyst (prepared by stepwise metal impregnation) and a physical mixture of Pt and Rh monometallic catalysts while holding the absolute amount of each of the noble metals constant. Activity measurements in a CONOO2 feedstream have shown that the CO oxidation activity of the bimetallic catalyst is substantially higher than that of the physical mixture over the concentration and temperature ranges characteristic of the converter warmup period. This synergistic enhancement of CO oxidation activity in the bimetallic catalyst can be rationalized by visualizing the catalyst surface structure as a statistical mixture of Pt and Rh sites. It was found that such a desirable surface structure (and the attendant synergistic activity enhancement) cannot be obtained by simultaneous impregnation of the noble metals because this catalyst preparation procedure leads to preferential Rh enrichment on the catalyst surface under the net-oxidizing reaction conditions considered in this study.

Role of NO in inhibiting CO oxidation over alumina-supported rhodium

The inherent rate of the COO2 reaction over alumina-supported Rh is much higher than that of the CONO reaction. Rate measurements in CONOO2 mixtures have shown, however, that the presence of even small amounts of NO in the reactant stream prevents the occurrence of the CO O2 reaction until the extent of the CONO reaction becomes significant, resulting in the simultaneous onset of both the COO2 and CONO reactions near the lightoff temperature for the CONO reaction itself (i.e., in the absence of O2). This indicates that the overall kinetic behavior of RhAl2O3 in CONOO2 mixtures is dominated by the features characteristic of the CONO reaction rather than by those of the COO2 reaction. These kinetic interactions among the reactant species, including the NO inhibition effect on the CO oxidation rate, can be interpreted on the basis of a mechanism involving the blocking of the reactive sites by molecularly adsorbed NO. Similar experiments with a ptal2o3 catalyst have shown that its CO oxidation activity is much less affected by the presence of NO than is the CO oxidation rate over RhAl2O3.

Effects of rhodium addition on methane oxidation behavior of alumina-supported noble metal catalysts

Abstract Laboratory methane oxidation experiments were conducted using a series of catalyst samples prepared by impregnating Rh-free noble metal catalysts (i.e., Pt/Pd, Pt or Pd) with various amounts of rhodium (0.0035, 0.005, and 0.014 wt.-% Rh). The addition of these small amounts of rhodium did not significantly change the temperature required for the onset of the methane oxidation. However, variable-composition experiments conducted at a temperature characteristic of warmed-up catalytic converters (ca. 550°C) reveal that the rhodium addition tends to shift the optimum feedstream stoichiometry (at which a maximum methane conversion occurs) toward more reducing conditions. These Rh-induced effects on methane conversion behavior appear to be independent of how the rhodium is incorporated in the catalyst.

Effects of catalyst particle size on multiple steady states

Carbon monoxide oxidation experiments were carried out over Pt-alumina catalyst particles of various sizes. The width of the conversion-temperature hysteresis loop goes through a maximum as the degree of intrapellet diffusion resistances (i.e., catalyst particle size) is varied. No hysteresis was observed over finely powdered catalysts. The above observations are in agreement with the qualitative predictions of diffusion-reaction theory, and provide further evidence to suggest that the steady-state multiplicities observed here were caused by the interactions between reaction and intrapellet diffusion resistances. The hysteresis loop was found to shift along the temperature axis as the pellet size was varied. The hysteresis occurred at the lowest temperature when catalyst particles of an intermediate size were used. Both larger and smaller particles showed multiplicity at higher temperatures. This observation is also consistent with diffusion-reaction theory.