Jack C. Summers

Interaction of cerium oxide with noble metals

Ce oxide interacts with noble metals to affect greatly the metal dispersions and CO oxidation and three-way conversion activities. For fresh Pt catalysts the apparent metal dispersions decrease with increasing Ce content. For fresh Pd catalysts, on the other hand, the metal dispersions are independent of Ce loading. Thermal aging of PtCe · Al2O3 catalysts results in serious losses in apparent Pt dispersion. The apparent dispersion of the aged PtCe · Al2O3 catalysts depends on the gaseous environment (air or H2) in which the catalysts are aged. Infrared spectroscopic investigations of CO chemisorption over Pt and PtCe · Al2O3 catalysts revealed that Ce promotes the oxidation of Pt. Ce did not affect CO chemisorption over the fresh PtCe · Al2O3 catalysts. To determine if bulk compound formation between the noble metals and Ce oxide could occur during thermal aging, mixtures of PtCeO2 and PdCeO2 were heated (600 and 900 °C, 6 hr) in air and H2. None were observed for the Pd system. However, at 900 °C in H2, Pt reacted to form Pt5Ce. The Pt5Ce was identified by X-ray diffraction. Small quantities of Ce oxide (0.6–1.3 wt% Ce) enhance the oxidation of CO over fresh Pt and Pd catalysts. With increasing quantities of Ce the CO conversion activity of the fresh Pt catalysts greatly deteriorates while no such deterioration is observed for Pd. The effect of thermal aging on catalytic activity was studied under oxidizing (air), inert (N2), and reducing (H2) conditions. When aged in air, the activity of Ce-containing Pt and Rh catalysts greatly deteriorated. However, thermal aging in H2 resulted in little or no activity loss under the conditions employed. The three-way activity of an air-aged Rh catalyst could be mostly regenerated upon high-temperature exposure to H2.

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.

The effects of SO2 on the performance of noble metal catalysts in automobile exhaust

Abstract SO 2 inhibits important three-way or stoichiometric catalytic reactions. Of the noble metals studied (Pt, Pd, Rh, Ir), Rh is particularly resistant to SO 2 for NO removal while Pt is strongly inhibited by SO 2 . This is an important reason why Rh is useful for effective NO control. Ir is particularly effective for converting NO under strongly oxidizing conditions in the presence of SO 2 . NH 3 formation is inhibited by SO 2 over the metals studied. Similarly under reducing conditions, propylene conversion is inhibited by SO 2 over the metals studied.

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.

Catalyst impregnation: Reactions of noble metal complexes with alumina

The rates of uptake of noble metal complexes over the surfaces of porous alumina are controlled partially by pore diffusion and partially by the rate of the chemical process between the metal complex and the alumina, resulting in impregnation profiles of different shapes. The rate of removal of metal complexes by alumina from solution is inversely related to the depth to which the noble metal penetrates into the alumina pellet. For those complexes that are rapidly removed from solution, the noble metal profile resembles a sharp shallow band, located at the pellets outer edge. On the other hand, for those complexes that are slowly removed from solution, the noble metal profiles in the pellets are essentially uniform along the radius of the catalyst pellets. Anionic chloride complexes, e.g., MCl6n− and MCl4n−, appear to react with alumina surfaces by a ligand displacement reaction. For a series of (NH4)aMCl6 complexes, the relative reactivities toward γ-alumina are Pd > Rh > Ru ≫ Pt ∼- Ir.

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.

Scavenger and Lead Poisoning of Automotive Oxidation Catalysts

The deactivation of noble metal oxidation catalysts of lead and halide lead scavengers was studied in engine and laboratory experiments. Lead alone or lead plus scavengers produced a persistent poisoning of the catalyst. Lead poisoning effects were increased by increased catalyst temperatures and fuel lead content. Tests with scavengers only, conducted in an engine previously operated on leaded fuel, showed that lead was transported to the catalyst causing lead poisoning even in the absence of lead in the fuel. These experiments showed that the reversible scavenger inhibition effects could be superimposed on the persistent lead poisoning effects.