Catalysis of Organic..

478 Palladium Leachingtemperatures higher than 200 °C. Heat treatment at lower temperatures showed onlyminor changes of the metal leaching (Figure 2).300.0amount of Pd leached [ppm]225.0150.075.00.00 100 200 300 400 500heat treatment temperature [°C]ABCFigure 2 Pd leaching of heat treated catalystsTo identify the cause for this reduced amount of metal leaching after heattreatments the hydrogenation activities and dispersions of all investigated catalystswere determined. As expected, lower activities and dispersions were observed for theheat treated catalysts compared to the non-treated ones (Figure 3).1000.0rel. catalyst activity750.0500.0250.0ABC0.00 100 200 300 400 500heat treatment temperature [°C]Figure 3 Relative hydrogenation activity of heat treated catalysts1000.0rel. catalyst activity750.0500.0250.0ABC0.05.0 10.0 15.0 20.0 25.0Pd dispersion [%}Figure 4 Correlation of relative hydrogenation activity with Pd-dispersionThis detected drop of activity and the increased resistance against palladiumleaching when exposing the catalysts to an increased temperature can be explainedby a sintering of the metal particles at elevated temperatures, which reduces themetal surface. The reduced palladium surface would cause lower hydrogenationactivities and palladium dispersions and a reduced metal leaching (Figure 4). Thegood correlation of the relative hydrogenation activity with the palladium dispersionof the tested catalysts supports this theory.