Change in the Pd Oxidation State during Light-Off of Three-Way Catalysis Analyzed by in situ Diffuse Reflectance Spectroscopy

Abstract

The changes in the oxidation state of supported Pd catalysts during light-off of NO−CO−C 3 H 6 −O 2 reactions were evaluated during temperature ramp up and down cycles (10 °C min −1 ) under fuel-rich (air-to-fuel ratio, A/F = 14.1) and fuel-lean (A/F = 15.0) conditions. Real-time analysis of the Pd oxidation state was carried out by acquiring a diffuse reflectance at a wavelength of 450 nm every second. The correlation between the Kubelka−Munk function and the Pd oxidation state was confirmed by Pd 3d X-ray photoelectron spectroscopy. At the onset of the catalytic reaction, the oxidized Pd species (PdO) supported on Al 2 O 3 was converted to metallic Pd to a varying extent, depending on whether the provided gas feed was rich or lean. Under the rich conditions, >70% of Pd was reduced during light-off, which remained unchanged following the completion of the reaction and the subsequent temperature ramp down. Even under lean conditions, >40% of Pd on the surface was temporally reduced to the metallic state at 300 °C, although the thermodynamic estimation revealed that Pd should be stable in the oxide form. The obtained outcomes suggest that the real catalyst surface at the onset of light-off should be in a more reductive environment than that estimated from the equilibrium O 2 concentration in the gas phase. This is because the Pd surface site was predominantly occupied by the adsorbed CO and/or C 3 H 6 . Nevertheless, following the completion of light-off at ≥400 °C, the Pd surface was immediately reoxidized, as the surface concentration of CO/C 3 H 6 became negligible. The Pd reduction during light-off under lean conditions was determined to be considerably less pronounced when using the oxygen storage CeO 2 − ZrO 2 support. Because of its oxygen storage and release abilities, the reductive atmosphere of the Pd surface was mitigated, and the three-phase boundary formed at the interface with CeO 2 −ZrO 2 provided suitable active sites for the CO/C 3 H 6 light-off at the lowest possible temperatures.

Multicomponent 3d Transition-Metal Nanoparticles as Catalysts Free of Pd, Pt, or Rh for Automotive Three-Way Catalytic Converters

“…However, Figure clearly shows that the regeneration of Cu 0 occurs at a much lower temperature of 600 °C. This discrepancy can be rationalized by the fact that the real catalyst surface at the onset of TWC light-off should be in a more reductive environment than that estimated from the equilibrium O 2 concentration in the gas phase, conceivably because the surface is primarily occupied by the chemisorbed CO and/or C 3 H 6 . Furthermore, the higher degree of regeneration of Cu 0 species in the multinary systems may suggest that the regeneration is driven by an entropy-mediated phase stabilization effect.…”

“…This discrepancy can be rationalized by the fact that the real catalyst surface at the onset of TWC light-off should be in a more reductive environment than that estimated from the equilibrium O 2 concentration in the gas phase, conceivably because the surface is primarily occupied by the chemisorbed CO and/ or C 3 H 6 . 57 Furthermore, the higher degree of regeneration of Cu 0 species in the multinary systems may suggest that the regeneration is driven by an entropy-mediated phase stabilization effect. For a more detailed understanding of the structural change of alloy nanoparticles under TWC conditions, high-resolution imaging at the atomic level is needed in a future study.…”

Thermal Deactivation of Pd/CeO2–ZrO2Three-Way Catalysts during Real Engine Aging: Analysis by a Surface plus Peripheral Site Model

“…In the Pd–CZ model, two types of surface-exposed Pd atoms, residing on the surface of a hemisphere (Pd s ) and circular periphery of the Pd/CZ interface (Pd b ), are responsible for TWC. This hypothesis is based on the fact that the interface or the TPB of the metal, support, and gas phase of many catalyst systems comprising metals and CeO 2 -based supports provides active sites for the catalytic reaction owing to the oxygen storage/release functions to/from the support. − , Other reported deep interactions such as decoration, alloying, Pd encapsulation, and surface complex (Pd x CeO 2 ) formation , are not considered here. The decoration of Pd by Ce oxide and Pd–Ce alloying were reported to occur only under a high-temperature reducing atmosphere .…”

“…A key challenge involves reliably predicting and enhancing the long-term durability of the catalyst performance, which is impaired by thermal degradation. The most probable deactivation mode is the sintering of platinum group metals (PGMs) and CeO 2 -based cocatalysts, such as CeO 2 –ZrO 2 (CZ), which reduce the active surface area and therefore the metal–CZ interface. , This interface is particularly important because the three-phase boundary (TPB) of the metal, CZ, and gas phase provides the catalytically active sites. − On these sites, CZ enables the oxygen storage/release in response to an oscillating air-to-fuel ratio (A/F), − promoting the catalytic conversion by PGMs. Oppositely, PGMs increase the rate of oxygen storage/release through spillover to/from CZ. , …”

Role of Ba in an Al2O3‐Supported Pd‐based Catalyst under Practical Three‐Way Catalysis Conditions

“…[24] Alkaline earth and lanthanide metals are often introduced into Al 2 O 3 as structural stabilizers that suppress the γ-to-α phase transformation. [25][26][27] Basic metals, such as La and Ba, are also known to stabilize Pd, which is the most commonly used active metal in modern commercial three-way catalysts [28][29][30][31][32][33] under TWC conditions, resulting in improved Pd dispersion even after catalyst exposure to harsh operating conditions. [34][35][36][37][38][39] In addition to the stabilizing benefits received by the support metals from basic metal additives and the stabilizing supports themselves, these basic metals have been shown to promote catalytic TWC performance.…”