Catalysts highly dispersed

Nappom WT, Laborde H, Leger J-M, Lamy C. 1995. Flectro-oxidation of Ci molecules at Pt-based catalysts highly dispersed into a polymer matrix Effect of the method of preparation. J Electroanal Chem 404 153-159. 

The traditional HDS catalysts are CoMo or NiMo based catalysts. Highly dispersed molybdenum sulphide exhibits a low but stable catalytic activity in a sulfur-containing environment. By adding the promoter elements, cobalt or nickel, in an atomic Co/Mo or NiMo ratio of 0.3 to 0.5 a ten-fold increase in activity could be achieved [1], Further improvements were made by addition of chelating agents, phosphate, and fluorine [2]. 

These requirements can be achieved best with a porous structure, but the realization depends strongly on operating temperature. Since costly precious metals (Pt and Pt alloys) are used as a catalyst, high dispersion of these metals is necessary in order to optimize the surface-to-mass ratio. Starting from metal salts, various chemical routes are usually used to achieve very small metal catalyst particles and thus high dispersion. The metal particle sizes are of the order... 

Supported catalysts are prepared for a large variety of applications such as obtaining bifunctional catalysts, high dispersion of the active phase, better diffusion of gases through the bed, better mechanical resistance to attrition (moving or fluidized-bed reactors), better thermal conductivity, and improved catalytic properties induced by active phase-support interaction, to name but a few of the many potential applications/requirements of oxides as heterogeneous catalysts. 

CatalyticaHy Active Species. The most common catalyticaHy active materials are metals, metal oxides, and metal sulfides. OccasionaHy, these are used in pure form examples are Raney nickel, used for fat hydrogenation, and y-Al O, used for ethanol dehydration. More often the catalyticaHy active component is highly dispersed on the surface of a support and may constitute no more than about 1% of the total catalyst. The main reason for dispersing the catalytic species is the expense. The expensive material must be accessible to reactants, and this requires that most of the catalytic material be present at a surface. This is possible only if the material is dispersed as minute particles, as smaH as 1 nm in diameter and even less. It is not practical to use minute... 

M0S2 is one of the most active hydroprocessing catalysts, but it is expensive, and the economical way to apply it is as highly dispersed material on a support, y-Al202. The activity of the supported catalyst is increased by the presence of promoter ions, Co " or Ni ". The stmctures of the catalysts are fairly well understood the M0S2 is present in layers only a few atoms thick on the support surface, and the promoter ions are present at the edges of the M0S2 layers, where the catalytic sites are located (100,101). 

Successful and reproducible preparation of highly dispersed catalysts crucially depends on the state of the carrier surface and on the concentration and pH of the impregnating solution. It is an art and a science for which several goodbooks and reviews exist.1 5... 

Electrochemical Promotion with Highly Dispersed Catalysts... 

In these laboratory studies the active catalyst phase (Pt) is highly dispersed on an electronically conductive support (C, Au) in contact with the electrolyte. 

MgO-supported model Mo—Pd catalysts have been prepared from the bimetallic cluster [Mo2Pd2 /z3-CO)2(/r-CO)4(PPh3)2() -C2H )2 (Fig. 70) and monometallic precursors. Each supported sample was treated in H2 at various temperatures to form metallic palladium, and characterized by chemisorption of H2, CO, and O2, transmission electron microscopy, TPD of adsorbed CO, and EXAFS. The data showed that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form. In contrast, the sample prepared from the monometallie precursors was characterized by larger palladium particles and by weaker Mo—Pd interactions. ... 

Usually noble metal NPs highly dispersed on metal oxide supports are prepared by impregnation method. Metal oxide supports are suspended in the aqueous solution of nitrates or chlorides of the corresponding noble metals. After immersion for several hours to one day, water solvent is evaporated and dried overnight to obtain precursor (nitrates or chlorides) crystals fixed on the metal oxide support surfaces. Subsequently, the dried precursors are calcined in air to transform into noble metal oxides on the support surfaces. Finally, noble metal oxides are reduced in a stream containing hydrogen. This method is simple and reproducible in preparing supported noble metal catalysts. 

Hi ly dispersed supported bimetallic catalysts with bimetallic contributions have been prepared from molecular cluster precursors containing preformed bimetallic bond [1-2]. For examples, extremely high dispersion Pt-Ru/y-AUOa could be prepared successfully by adsorption of Pt2Ru4(CO)ison alumina [2]. By similar method, Pt-Ru cluster with carbonyl and hydride ligands, Pt3Ru6(CO)2i(p3-H)(p-H)3 (A) was used in this work to adsorb on MgO support. The ligands were expectedly removable from the metal framework at mild conditions without breaking the cluster metal core. 

---