Effect of ceria and zirconia on nanoparticulate rhodium catalysts

Abstract

Rh metal was implemented as a core component in the so-called three-way automotive exhaust catalyst (TWC) due to its excellent properties such as thermal stability, poison resistance and inert behaviour to react with any support materials. These catalysts are often loaded with small amount of promoter elements such as Ce and Zr that enhance their overall catalytic performances and catalyst lifetime. It is therefore desirable to develop new CeOx-based catalyst with both high redox activity and high thermal resistance.
Samples of two surface coverages of rhodium on a non-porous high area alumina been synthesised; at 1.6 Rh and 4 wt % Rh loading. Ceria and zirconia have been deposited on the pre-supported Rh catalysts by the controlled surface reaction between the reduced Rh surface and an organometallic precursors: Zr(acac)4 and Ce(acac)3, with different cerium : zirconium ratios: Ce:Zr;1:0, 1:1, 2:1, 0:1. Ceriated Rh catalysts have been also prepared by a different synthesis method, wherein the ceria was deposited on ?-Al2O3 and subsequently Rh metal was added. A series of Rh catalysts produced has been characterised ex situ by BET surface area measurement, TEM, STEM HAADF, EDX, XPS, and XAFS.
The combination of Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), Mass Spectrometry (MS) and Energy Dispersive X-ray Absorption Fine Structure (EDE) techniques in a synchronous, time-resolved manner has been applied to determine the complementary information about structural and kinetic changes of the chemical system throughout CO exposure and two catalytic processes: CO oxidation and the reduction of NO by CO in the temperature range between 298 K to 573 K.
It has been shown that advanced characterisation techniques, used in the time-resolved, in situ manner were needed in order to understand the behaviour of these complex catalytic systems under operating conditions. The effects of the catalysts’ composition, temperature and pulsed gas flow on catalyst performance have been effectively mapped by the combined DRIFTS/MS/EDE studies. The synthesis procedure has been found to be crucial to achieve the desired interaction between CeOx and Rh particles. No enhancement of ceriated Rh catalysts was observed when ceria was deposited directly on ?-Al2O3 (method I). The addition of each promoter element by the controlled surface modification procedure (method II) exhibits multiple effects on the catalysts performance. The DRIFTS/XAFS studies have confirmed that CeOx and ZrO2 facilitate the protection of Rh particles against extensive oxidation in atmospheres of air, O2 and CO. Larger Rh core particles of ceriated and zirconiated Rh catalysts have been observed when compared with undoped Rh samples. DRIFTS results indicate that by interaction of CO with Rh particles a significantly larger amount of linear CO species was formed on the ceriated Rh surface than in the non-ceriated Rh catalysts case suggesting the presence of predominant metallic Rh phase for that system. Moreover, only ceria doping was found to improve the catalytic activity of promoted Rh catalysts throughout CO oxidation in the temperature ramp mode under different (O2/CO) gas composition.

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