HDS Model Catalyst

Several approaches were investigated for the synthesis of the model catalyst, and the procedure resulting in the most homogeneous sample in terms of cluster size and morphology can essentially be divided into two steps Initially, metallic molybdenum is deposited on a single-crystal Au(l 11) substrate in an atmosphere of H2S at 

The clusters are also very similar to each other in size, with an average area of 500 A. This area corresponds to a side length of a triangle of approximately 30 A, which corresponds well to the dimensions of the active clusters in typical HDS catalysts. As a model system for the HDS catalyst, the clusters are therefore a well-characterized reference for further experiments to elucidate details of the structure of the active edges and the reactivity with adsorbed molecules. 

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Although much progress has been made toward understanding the nature and probable catalytic behavior of active sites on CoMo/alumlna catalysts, much obviously remains to be accomplished. Detailed explanation of the acidic character of the reduced metal sites evidently most important In HDS, and presumably In related reactions, must await the Increased understanding which should come from studies of simplified model catalysts using advanced surface science techniques. Further progress of an Immediately useful nature seems possible from additional Infrared study of the variations produced In the exposed metal sites as a result of variations In preparation, pretreatment, and reaction conditions. 

Another SIMS study on model systems concerns molybdenum sulfide catalysts. The removal of sulfur from heavy oil fractions is carried out over molybdenum catalysts promoted with cobalt or nickel, in processes called hydrodesulfurization (HDS) [17]. Catalysts are prepared in the oxidic state but have to be sulfided in a mixture of H2S and H2 in order to be active. SIMS sensitively reveals the conversion of Mo03 into MoSi, in model systems of MoCf supported on a thin layer of Si02 [21]. 

In this paper factors controlling the catalytic activity in the hydrodesulfurization reaction (HDS) are discussed. The SiOa-supported phosphormolybdenum heteropolyacid (HPMo) is used as a model catalyst. Two types of the catalyst deactivation have been shown. The reversible deactivation effect is related with changes in the molybdenum valence, its 0- and 0,S-surrounding and adsorbtion of the S-containing reaction products. The HDS activity is irreversibly changed when the transformation of the catalyst phase composition is carried out ... 

In this paper deactivation of the hydrodesulfurization (HDS) catalysts is examined using the results obtained for the thiophene conversion on the supported phosphormolybdenum heteropolyacid as a model catalyst. 

The study of the HDS activity of HPMo/SiOa as a model catalyst has shown that the reversible deactivation effect is connected with increasing of sulfur iones in Mo5 surrounding, blockage Mos species by S-compounds and deeper reduction of molybdenum. The sample containing only molybdenum sulfide exhibits about two times lower initial and steady thiophene conversion in comparison with partially sulfided HPMo. 

Model 23 R-16 (NiMo) is a new HDS promoted catalyst developed by ICERP to be used in desulphurization units in order to reach the new diesel fuel specifications. The catalyst is based on a new type of alumina obtained by an original preparing method which offers a correct interaction degree between metal and its support. The acidic property and the pore size were improved by the addition of promoteurs such as silica and phosphorus (P2O5). Some of the properties of new 23 R-16 (NiMo) promoted catalyst in comparison with the standard hydrofining catalyst are listed in Table 2. 

F re 2.18 Plot of rates of HD formation, and TOF data, at 425 K, /gainst mean Au nanoparticle size for a series of Au/TiO2(110) model catalysts with the same metal loading. TOF data are referred to Au surface atom at the perimeter of the metal/ support interface. Reproduced from Fujitani et oL with permission from Wiley-VCH. 

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