Deactivation and coke accumulation during CO2/CH4 reforming over Pt catalysts

摘要

The deactivation of Pt catalysts used in the generation of synthesis gas via CO2/CH4reforming depends strongly on the support and the metal particle size. Methods of physicochemical characterization such as X-ray absorption spectroscopy and hydrogen chemisorption suggest that carbon formation (most likely from methane) rather than sintering is the main cause of catalyst deactivation. The rate of carbon formation decreased in the order Pt/γ-Al2O3Pt/TiO2>Pt/ZrO2. Carbon was formed on the support and on Pt. Using the stability of that carbon toward oxidation it was estimated for Pt/γ-Al2O3that 90% of the carbon was located on the oxidic support. However, even for this catalyst the amount of carbon formed is sufficient to cover only 30% of the total specific surface area of the catalyst. Although carbon can be formed on the metal and the support, evidence is presented that deactivation is caused by carbon formed on the metal and is associated with overgrowth of the catalytically active perimeter between the support and the metal. The reason for the deactivation is the imbalance between the carbon-forming methane dissociation and the oxidation by chemisorbed CO2. Active carbidic carbon seems to be transformed to a less reactive form. Catalysts having larger Pt particles (>1 nm) tend to deactivate more quickly than catalysts with smaller Pt particles.