L. Louis Hegedus

Poison-resistant catalysts for the simultaneous control of hydrocarbon, carbon monoxide, and nitrogen oxide emissions

Abstract The paper investigates several features of the operation of noble metal catalysts in automobile exhaust near the stoichiometric air:fuel ratio ( A F ). These features include the extent of intrapellet diffusion limitations as a function of feedstream stoichiometry, the mechanism of poisoning, the effects of noble metal (Pt, Rh, Pd) impregnation profiles on activity and poison resistance, and the effects of A F oscillations on catalyst performance. A pellet configuration, containing an external shell of Pt and internal rings of Rh and Pd, was found to have improved poison resistance and lightoff characteristics. The addition of Ce improved the catalysts initial performance in transient operation.

Effects of platinum and palladium impregnation on the performance and durability of automobile exhaust oxidizing catalysts

Abstract The performance and durability properties of noble metal-alumina oxidation catalysts are strongly influenced by the relative location of the metals along the radius of the porous catalyst pellets. Five Pt- and Pd-containing catalysts were prepared by systematically varying the noble-metal distribution along the radius of the catalyst pellets. The catalysts were poisoned on a dynamometer or sintered in a high-temperature furnace. The results showed sizable improvements in both steady-state and light-off performance when the catalyst had an outer shell of Pt and an inner shell of Pd.

Improving the poison resistance of supported catalysts

Abstract This paper discusses the poison resistance of supported catalysts, in which the poison reacts both with the active component and the catalyst support. For such catalysts, the net rate of the poisoning reaction can be selectively manipulated with respect to the net rate of the main reaction in such a manner that a catalyst with improved poison resistance is obtained. The above statements are illustrated by diffusion-limited lead and phosphorus poisoning experiments using alumina-supported noble metal catalysts. By modifying the pore structure, support surface area, and noble metal impregnation depth of these catalysts, improvements in activity and poison resistance have been attained for automobile exhaust emission-control applications.

Phosphorus accumulation in automotive catalysts

Abstract Steady-state, accelerated phosphorus accumulation experiments were conducted using a multistage integral reactor which was mounted on an engine dynamometer system. Electron microprobe studies showed that the phosphorus accumulation process is controlled by pore diffusion and that the phosphorus tends to form a monolayer over the pore surfaces of the poisoned band in the catalyst pellets. Excellent agreement was found between the experimental results and a progressive shell-type poison accumulation model.

THE SINGLE PELLET DIFFUSION REACTOR: THEORY AND APPLICATIONS

Abstract Most heterogeneous catalysts are either supported on some kind of porous material or the catalyst itself is porous. To satisfy practical constraints such as pressure drop, handling, and separation from products, these catalysts are generally pelletized. Very often the overall chemical reaction rates within these catalyst pellets are determined by a complicated interaction of internal and external transport effects with the intrinsic kinetic rate at the active surface.

Surface reactions of NO, CO, and O2 near the stoichiometric point: I. Pt-alumina☆

Infrared and integral reactor experiments showed that CO chemisorbed over reduced Pt sites appears to act as an inhibitor for the NOCO reaction at rich stoichiometries. A large fraction of the surface is covered by an isocyanate species which does not appear to participate in the reaction. Dissociatively chemisorbed O2 appears to act as an inhibitor for the NOCO reaction at lean stoichiometries. For a Pt-alumina catalyst, the well known operating window around stoichiometry (i.e., the range of stoichiometries where both NO and CO conversions are appreciable) is defined by the range of stoichiometries where the surface isocyanate and the CO chemisorbed on reduced Pt sites have been partially eliminated but the Pt surface has not yet been completely oxidized. Thus, the operating window has been defined in terms of the surface composition.

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.

Geometrie correction factors for the Weisz diffusivity cell

Weisz has proposed a most convenient method of measuring the effective diffusion coefficient of a porous medium in which a spherical pellet of radius a is held in an elastic tube which effectively seals off an equatorial zone between the latitudes ± γ and allows the spherical caps to be exposed to two different concentrations c1 and c2. The effective diffusion coefficient De is related to the observed flux F by De = αwF (πa2)−1[c1 − c2)(2a)]−1 where αw, a correction factor due to the spherical geometry, is a function of γ. A graph of αw is given for 10 ° ≤ γ ≤ 80 °. It agrees well with experimental measurements.

Effects of poisoning and sintering on the pore structure and diffusive behavior of platinum/alumina catalysts in automotive converters

Abstract Fresh, sintered (heat treated) and lead poisoned spherical Pt Al 2 O 3 pellets were investigated by ultrahigh pressure mercury porosimetry measurements and by hydrogen-nitrogen static counterdiffusion. The relationship between the effective diffusivity of these pellets and their pore structure was analyzed using the random pore diffusivity model. Poisoning and sintering were found to have opposing effects on the diffusivity. These effects may counterbalance each other in catalytic converters during the aging process so that it is possible that no net observable change in the diffusivity of H 2 in N 2 is recorded during diffusivity measurements on samples from these converters. These phenomena are interpreted in terms of the changes in the pore structure of the catalysts during poisoning and sintering.

Multicomponent Chromatographic Processes During the Impregnation of Alumina Pellets with Noble Metals

A mathematical model of a competitive, multicomponent diffusion-adsorption process is presented, and applied to the impregnation of porous γ-alumina pellets by a Rh complex, in the presence of HF as a site blocking agent which drives the Rh below the surface of the pellets. The model correctly predicts the measured Rh peak penetration depths and Rh uptakes from solution.