Byong K. Cho

Chemical modification of catalyst support for enhancement of transient catalytic activity: nitric oxide reduction by carbon monoxide over rhodium

A commercial ceria powder was chemically modified by doping with gadolinia in order to improve its oxygen storage/transport characteristics. The catalytic activity of Rh impregnated on this modified ceria support was measured and compared with those impregnated on conventional ceria or alumina support, using a packed-bed reactor and an isotopic reactant (13CO) for the NO + CO reaction under both cycled- and steady-feed conditions. Results of transient pulse experiments indicated that the oxygen uptakes of both ceria and modified ceria are an order of magnitude greater than that of alumina. This work has demonstrated that the chemical modification of the ceria support can significantly enhance the catalytic activity of Rh for the NO + CO reaction under cycled feedstream conditions at high temperatures above 500°C. This enhancement of catalytic activity of Rh supported on the modified ceria is discussed in light of the oxygen storage and transport characteristics of the modified support.

Kinetics of NO reduction by CO over supported rhodium catalysts: Isotopic cycling experiments

The catalytic activity of RhAl2O3 and RhCeO2 catalysts was investigated for the reduction of NO by CO under feed-composition cycling conditions as well as under steady feed conditions using a packed-bed reactor and an isotopic reactant, 13CO. Results indicate that the formation of N2O is an important intermediate step during the (CO + NO) reaction over RhAl2O3. The enhanced catalytic activity of RhCeO2 compared with RhAl2O3 under cyclic operating conditions is discussed in light of the oxygen storage capacity of ceria support. The effect of asymmetric cycling is examined in view of the nature of the rate-controlling step which changes with temperature. On the basis of our observations an overall reaction scheme is proposed for the (CO + NO) reaction over RhAl2O3 in which N2O is included as a reaction intermediate.

Mechanistic importance of intermediate N2O + CO reaction in overall NO + CO reaction system: I. Kinetic analysis

To investigate the mechanistic importance of the N2O + CO reaction as an intermediate reaction step during the reduction of NO by CO occurring on noble metal exhaust catalysts, we have analyzed theoretically the steady-state kinetics of the NO + CO reaction based on elementary surface processes. Quasilinearization of the nonlinear NO + CO reaction system by identifying a critical kinetic parameter has enabled us to develop a complete set of analytical solutions for the system which includes the intermediate N2O + CO reaction step. The kinetic analysis based on this solution scheme shows a dramatic difference between the rate of the N2O + CO reaction as an intermediate reaction and that as an isolated reaction. Results have revealed that the rate of the N2O + CO reaction as an intermediate reaction in the NO + CO reaction system can be two to three orders of magnitude faster than the isolated N2O + CO reaction, which is known to be very slow compared with the NO + CO reaction. This makes the rate of the intermediate N2O + CO reaction as fast as or even faster than the rate of the NO + CO reaction, suggesting that the former reaction can make a major contribution to the kinetics of the reduction of NO by CO occurring in three-way catalytic converters.

Effect of Aging Atmosphere on Thermal Sintering of Modern Commercial TWCs

The effect of aging atmosphere on the sintering behavior of commercial Pd and Rh catalysts as well as the TWC performance thereof has been investigated under straight oxidizing, reducing and periodic cycling aging conditions, in search of useful guidance in the optimum design of thermally durable TWCs for advanced gasoline engines equipped with the deceleration fuel cutoff technology. Pd and Rh catalysts individually exhibit an opposite trend in the thermal sintering behavior with respect to the aging atmosphere. Under oxidizing conditions, the Pd catalyst becomes more resistant to sintering in higher O2 concentrations, whereas the Rh catalyst reveals the opposite behavior, regardless of the aging temperature. Physicochemical characterizations by using TGA and CO chemisorption have indicated that the state of Pd (PdO vs. Pd0) and Rh (Rh2O3 vs. Rh0) on the catalyst surface formed during the thermal aging plays an important role for determining the trend of thermal sintering; the positive effect of the O2 concentration on Pd sintering is attributable primarily to the formation of PdO, while the main cause for severe deactivation of the Rh catalyst under oxidizing conditions is the diffusion of Rh2O3 into the support along with the agglomeration of Rh particles.

Nitric oxide reduction by ethylene over Pt-ZSM-5 under lean conditions: steady-state activity

Steady-state activity of Pt-ZSM-5 catalysts has been investigated experimentally for the NO + C2H4 + O2 reaction under highly oxidizing conditions, typical of lean-burn gasoline engine exhaust. Effects of temperature, space velocity, feed concentration, Pt loading and water vapor on the catalytic activity have been examined using a packed-bed laboratory reactor. The catalytic activity of Pt-ZSM-5 is discussed in comparison with that of Cu-ZSM-5 and Pt/Al2O3. Results show that Pt-ZSM-5 catalysts are much more active than Cu-ZSM-5 catalysts for lean-NOx reduction at low temperatures, while the kinetic behavior of Pt/Al2O3 is very similar to that of Pt-ZSM-5. Conversion of both NO and C2H4 during the NO + C2H4 + O2 reaction over Pt-ZSM-5 around the reaction lightoff temperature is strongly inhibited by the presence of NO. The NO/C2H4 ratio in the feedstream is an important factor determining the NO reduction activity of the catalyst, and there exists an optimum value of this ratio for a maximum conversion of NO. Based on the steady-state NO conversion data, a correlation between the reactor performance and the feed concentration has been developed, and the feasibility of Pt-based catalysts for lean-NOx reduction is discussed in terms of their activity, selectivity and durability.

Combination of photocatalysis and HC/SCR for improved activity and durability of DeNOx catalysts.

A photocatalytic HC/SCR system has been developed and its high deNOx performance (54.0-98.6% NOx conversion) at low temperatures (150-250 °C) demonstrated by using a representative diesel fuel hydrocarbon (dodecane) as the reductant over a hybrid SCR system of a photocatalytic reactor (PCR) and a dual-bed HC/SCR reactor. The PCR generates highly active oxidants such as O3 and NO2 from O2 and NO in the feed stream, followed by the subsequent formation of highly efficient reductants such as oxygenated hydrocarbon (OHC), NH3, and organo-nitrogen compounds. These reductants are the key components for enhancing the low temperature deNOx performance of the dual-bed HC/SCR system containing Ag/Al2O3 and CuCoY in the front and rear bed of the reactor, respectively. The OHCs are particularly effective for both NOx reduction and NH3 formation over the Ag/Al2O3 catalyst, while NH3 and organo-nitrogen compounds are effective for NOx reduction over the CuCoY catalyst. The hybrid HC/SCR system assisted by photocatalysis has shown an overall deNOx performance comparable to that of the NH3/SCR, demonstrating its potential as a promising alternative to the current urea/SCR and LNT technologies. Superior durability of HC/SCR catalysts against coking by HCs has also been demonstrated by a PCR-assisted regeneration scheme for deactivating catalysts.

Dissociation and oxidation of carbon monoxide over Rh/Al2O3 catalysts

The activity of Rh/Al{sub 2}O{sub 3} catalysts for CO oxidation was investigated by transient isotopic pulse experiments using packed-bed reactor. This transient experimental scheme revealed significant CO dissociation activity during CO oxidation over Rh/Al{sub 2}O{sub 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{sub 2}O{sub 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{degree}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{sub 2}O{sub 3} catalysts is further discussed in view of the reaction lightoff behavior.

Estimation of kinetic parameters for elementary surface processes from integral reactor data: CO oxidation over Pt/Al2O3

A novel method has been developed which can be used for kinetic parameter estimation from laboratory reactor data. It is based on discrete modeling and variational techniques applied to an integral reactor packed with supported catalyst pellets. This new method, similar to the McCabe-Thiele method in distillation column design, has been used to determine kinetic parameters of adsorption, desorption and surface reaction steps for CO oxidation over Pt/ AI2 O3catalysts at 500° C and atmospheric pressure. The results clearly indicate the importance of the coverage dependency of CO desorption activation energy in the steady state kinetics of CO oxidation. The good agreement of the results with those on single crystal Pt surfaces indicates no significant metal-support interactions for the Pt/ Al2O3 system during CO oxidation

Activity normalization for lean-NOx catalysts

Abstract Steady-state activity of two representative lean-NO x catalysts — Pt-ZSM-5 and Cu-ZSM-5 — has been analyzed for the NO + C 2 H 4 + O 2 reaction under highly oxidizing conditions, typical of lean-burn gasoline engine exhaust. Results indicate that the steady-state activity of these catalysts exhibits a first-order homogeneous response with respect to changes in reactant feed concentrations. Identification of this unique kinetic behavior has enabled us to develop an activity normalization scheme by which the activity of a lean-NO x catalyst measured under different experimental conditions can be compared on an equal basis, provided the feed concentration ratios are kept constant. Also discussed is how the normalization scheme can be used for the efficient experimental design and the optimum control of lean-NO x catalysts for maximum catalytic activity along with its limitations.

A combinatorial chemistry method for fast screening of perovskite-based NO oxidation catalyst.

A fast parallel screening method based on combinatorial chemistry (combichem) has been developed and applied in the screening tests of perovskite-based oxide (PBO) catalysts for NO oxidation to hit a promising PBO formulation for the oxidation of NO to NO2. This new method involves three consecutive steps: oxidation of NO to NO2 over a PBO catalyst, adsorption of NOx onto the PBO and K2O/Al2O3, and colorimetric assay of the NOx adsorbed thereon. The combichem experimental data have been used for determining the oxidation activity of NO over PBO catalysts as well as three critical parameters, such as the adsorption efficiency of K2O/Al2O3 for NO2 (α) and NO (β), and the time-average fraction of NO included in the NOx feed stream (ξ). The results demonstrated that the amounts of NO2 produced over PBO catalysts by the combichem method under transient conditions correlate well with those from a conventional packed-bed reactor under steady-state conditions. Among the PBO formulations examined, La0.5Ag0.5MnO3 has been identified as the best chemical formulation for oxidation of NO to NO2 by the present combichem method and also confirmed by the conventional packed-bed reactor tests. The superior efficiency of the combichem method for high-throughput catalyst screening test validated in this study is particularly suitable for saving the time and resources required in developing a new formulation of PBO catalyst whose chemical composition may have an enormous number of possible variations.