Surface-mediated processes in heterogenous catalysis
Three-way catalysts contain noble metals such as platinum, palladium and rhodium as the active components (see the figure). One of the recent requirements for catalyst materials is their durability under high temperature conditions. With X-ray Photoelectron Spectroscopy (XPS) it is possible to investigate the chemical composition of the catalyst surfaces, which have an important role in determining the catalytic performance.
The temperature in a close-coupled catalyst converter can rise even above 1000°C, which induces sintering of noble metal clusters and phase transitions of the alumina support. These phenomena, together with irreversible reactions of noble metal and other materials present in the catalyst may lead to a decrease in catalyst conversion capability and increase in pollutant emissions.
The following figure shows Rh 3d photoelectron spectra of a gamma alumina supported Rh catalyst after aging in air for 3 h at temperatures 550°C, 750°C and 950°C followed by reduction in hydrogen. The catalysts were manufactured by Ecocat Oy (former Kemira Metalkat).
The measured spectra were peak-fitted with two components: one for metallic Rh and the other for irreducible Rh oxide. The aging induced effects are clearly seen. The intensity of the Rh 3d peak decreases as a function of aging temperature due to sintering of noble metal particles and structural changes in the non-stabilized gamma alumina. Furthermore, the relative amount of irreducible Rh oxide increases as the aging temperature increases resulting in severe loss of catalytic activity.