Surface characterization of modified aluminas. Part 6. The poisonous effect of lead

Abstract

The presence of lead in three-way converters (TWC) is considered to be one of the most important causes of catalyst deactivation: its poisoning effect has been ascribed to the irreversible interaction with the catalytically active metal supported on the washcoat, leading to the formation of stable inert alloys. This paper considers the lethal poisonous effect played by lead under certain conditions (e.g., at the high temperatures easily reached by a working catalyst) also on the alumina support (washcoat). The analysis was carried out on pure transition aluminas (A samples) and on Pb-containing alumina systems (APb samples). Structural and morphological studies were carried out by BET, XRD and (high-resolution) TEM, whereas surface features of pure and Pb-containing aluminas were analysed by in-situ FTIR spectroscopy. Data are reported for: (i) the adsorption of CO at room temperature (RT) (reveals strong Lewis acidity) and at 77 K (reveals total Lewis acidity); (ii) the RT adsorption of CO₂ (reveals both Lewis acidity and, in the 1900–1200 cm⁻¹ spectral region, surface basicity). It could be concluded that the presence of Pb on transition alumina supports does not modify appreciably the surface area nor the range of existence of the various crystal phases (i.e., there are no physical effects), but selectively eliminates both strong Lewis acidic properties, and basic properties of the alumina support (surface chemical effects).