Catalysis of Organic..

Pohlmann et al 477this study. In the case of reduced catalysts, the catalysts ERW and URW show arelatively high drop in palladium leaching after four weeks compared to the testperformed the day after the catalyst preparation. In contrast to these results, the dryanalogues of these catalysts (ERD and URD) showed a much smaller decrease inpalladium leaching (Figure 1).Table 2. Relative change of activity and dispersion after four weekscatalyst ERD ERW END ENW URD URW UND UNWrel. activity 581 702 1053 925 395 403 689 568rel. activity 611 697 1074 1010 394 393 695 693after onemonthdispersion 15.8 16.3 19.5 15.6 19.8 18.5 18.1 15.34[%]dispersion– after onemonth [%]16.7 15.3 19.9 19.3 19.9 17.0 17.1 16.5The hydrogenation activity and dispersion of all catalysts remains relativelyconstant during the evaluated period of time. The aging effect thus cannot beexplained by sintering of the palladium crystallites, since this would also reduce theoverall activity and dispersion of the catalyst. TPR experiments of the fresh and agedcatalysts were carried out to investigate the influence of the oxidation state of themetal on the observed aging effect. A comparison of the TPR results of the freshcatalysts with the TPR results after two months shows that the reduced catalysts wereslowly oxidized over time. The total extent of reduction in the TPR experimentdenoted by the amount of H 2 consumed in the TPR to reduce the catalyst increasedover a period of two months for the wet reduced catalysts from 50 ml/g of Pd to120 ml/g of Pd of a 5 % H 2 in Ar mixture for the catalyst ERW and from 32 ml/g ofPd to 156 ml/g of Pd for the catalyst UNW. The corresponding results for the drycatalysts ERD (45 ml/g of Pd after two months) and URD (64 ml/g of Pd after twomonths) did not increase as strongly indicating that the dry catalysts are less prone tooxidation over time. This could also explain the smaller tendency of dry catalysts toshow a reduced amount of leaching when storing the catalyst compared to the wetcatalysts. As reported above the non reduced samples show a higher resistanceagainst metal leaching compared to the reduced samples. These results also indicatethat it is beneficial to use dried catalysts for applications that require a high degree ofreduction of the catalyst, since dry catalysts show a higher resistance againstoxidation during storage.In a second part of this study, the effect of heat treatment under nitrogen of thereduced palladium catalysts A, B and C with an egg-shell type metal distribution onthe metal leaching was investigated. The reduced catalysts were tested for metalleaching after they underwent a heat treatment at temperatures of 100 to 400 °C. Themetal leaching of the investigated catalysts decreased after the heat treatment of