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Surface segregation processes in Pd−Pt alloys and bimetallic Pd−Pt nanoclusters on alumina and carbon supports (technical catalysts) have been investigated by determining the metal surface composition of these systems by low-energy ion scattering (LEIS). Both Pd-rich (Pd80Pt20) and Pt-rich (Pd20Pt80) systems have been studied. The surface of the Pd−Pt alloys is enriched in Pd after heating in ultrahigh vacuum and thermodynamic equilibrium is reached at about 700 °C. Pd surface segregation is enhanced by heating the alloys in hydrogen or oxygen, and thermodynamic equilibrium is reached already at about 400−500 °C. For Pd−Pt catalysts with low metal dispersions of about 0.3 and 0.8, Pd surface segregation does take place during heating in hydrogen to approximately the same extent as in the Pd−Pt bulk alloys. For Pd−Pt catalysts with a high metal dispersion close to 1, however, surface segregation is completely suppressed during heating in hydrogen and oxygen. We attribute this to the limited supply of Pd atoms from the bulk to the surface of the nanoclusters.

A series of commercial Co-Mo/Al 2 O 3 catalysts, freshly sulfided as well as used in different trickle phase and gas phase reactions, has been characterized by using (scanning) transmission electron microscopy combined with energy dispersive X-ray analysis [(S)TEM-EDX)] and X-ray diffraction (XRD) analysis. While liquid phase sulfided Co-Mo/Al 2 O 3 catalysts contain only MoS 2 single layers, some MoS 2 stacking is observed following H 2 S/H 2 sulfidation. Liquid phase sulfided catalysts as well as catalysts partly sulfided with liquid prior to H 2 S/H 2 sulfidation have higher activities than those sulfided only with H 2 S/H 2 . MoS 2 sintering and Co sulfide segregation are observed in catalysts after commercial use under severe conditions. Compared to what is typically found for Ni-Mo catalysts, metal-rich agglomerates in Co-Mo/Al 2 O 3 catalysts contain more Mo while the Co sulfide crystals are less well developed.

The growing demand for high-quality transportation fuels requires their cost-effective production by hydrodesulfurization of crude oils using heterogeneous catalysts. To study the three-dimensional (3D) structure of such a commercial, sulfided Ni-Mo/γ-Al 2 O 3 catalyst, electron tomography was applied. The MoS 2 particles form an interconnected complex structure within the mesopores of the alumina support. Spatial organization, morphology, and orientation of the MoS 2 particles in the pores were resolved with sufficient accuracy to display the 6-Å-spaced MoS 2 crystal planes. The proximity of the MoS 2 edge planes and more loosely interacting MoS 2 basal planes to the alumina support showed the presence of pores smaller than 3 nm, which was confirmed by physisorption experiments. The actual shape of the MoS 2 particles cannot be described by simple models as derived from studies on model catalysts. Electron tomography is a unique tool to study the actual 3D structure of complex industrial catalysts with sub-nanometer resolution.