Alumina metal precursor-support

Metal precursor-support interactions in the case of alumina are quite different. The nature of the H2PtClg-Al203 interaction is still open to question. However, recent "in-situ ultraviolet studies (13-14) suggest the following ... 

Determination of Metal Precursor Mobilities During Pretreatment. Relative precursor mobilities were obtained by premixing the sllica-or alumina-supported metal catalysts with pure silica (Cab-O-Sll, grade M-5, Cabot Corp.) or pure alumina (Alon C, Cabot Corp.) In a 1 2 ratio prior to pretreatment. The catalyst and silica were ground together using a mortar and pestle for at least 0.5 hr. before they were placed in the Pyrex microreactor for pretreatment. 

Palladium can be deposited at low reduction temperatures on alumina or silica supports, either from the gas phase by ALE or from a solution by impregnation methods, using Pd(thd)2 as precursor , as was described for ruthenium in Section lV.B.2.b. During ALE the precursor reacted to yield mainly metallic Pd, whereas on alumina only a small part of the impregnated metal source probably was dissociatively adsorbed yielding Pd(0). Associate adsorption describes the interaction of Pd(thd)2 with the alumina and... 

The preparation of alumina supported catalysts by impregnation with a solution containing metallic ions may give rise to precursor-support interactions of different types. Evidence of the formation of certain species can help to understand reaction mechanisms. 

The XPS technique provides identification of oxidation states of metals in supported catalysts, but the determinations are often inexact and require confirmation by other methods. XPS is especially useful for detecting changes in oxidation states of cluster precursors on various oxide supports. For example, the transformation of Rh4(CO)i2 on silica, alumina, MgO, ZnO, and TiOj to yield different surface species such as a raft of Rh(CO)2(OM) or Rh metal aggregates has been inferred from chemical shifts in XPS data (Fig. 5) 39-41). The XPS technique requires ultrahigh vacuum, and instability of the... 

Supported bimetallic clusters can be prepared simply by contacting a suitable carrier such as silica or alumina with an aqueous solution of salts of the two metals of interest. The material is then dried and contacted with a stream of hydrogen at a temperature high enough to accomplish reduction of the metal precursors to the metallic state. This procedure results in the formation of very small metal crystallites or clusters dispersed on the surface of the carrier. 

Alumina (CK-300 Ketjen y-AbOs, S = 180 m g) supported Sn-Ru catalysts were prepared by co-impregnation or successive impregnation. Various metal precursor salts were used RUCI3XH2O, Ru(acac)3, SnCbx2H20, Ci2H2404Sn... 

Two series of 20-40 mesh y-alumina and silica supported Ni-based catalysts were prepared by the incipient wetness impregnation with nitrate as the metal precursors. The solids were dried overnight in air at 393 K, then calcined at 773 K in air for 6 hrs for complete decomposition of the precursors. For the promoted catalysts, magnesium nitrate and cerous nitrate were added to the support by the same method. Before the reaction, the catalyst was reduced at 973 K in a stream of H2 (99.995%) for at least 2 hrs. 

Pt group metals can activate alcohols and molecular oxygen under close to ambient conditions, producing the corresponding carbonyl or carboxylic acids in high yields. Enhanced selectivity and activity has been obtained by the use of bi- and multimetallic catalysts. " However, as was the case with selective hydrogenation, the optimum catalysts developed to date have involved a combination of supported metals and selectivity promoters. The most commonly used catalysts consist of either Pt or Pd as the active metal combined with Bi or Pb as promoters, commonly on carbon or alumina supports. Other promoters reported include Cd, Co, Cu, Se, ° Ce, Te, Sn, Au and Ru. The catalysts can be prepared by simultaneous deposition and reduction of the metal precursors onto a suitable support. However, more commonly, preparation involves a... 

Thus, the analysis of FITR sp>ectra of alumina provides data on the nature and amount of various surface sites moreover, it allows identification of the sites where active component precursor is anchored during catalyst synthesis, and makes it possible to hypothesize about mechanism and strength of the metal complex-support interaction. 

The support is often coprecipitated simultaneously with the active metals in a single process step. This can be done by mixing in basic precursors of the support, such as sodium sihcate, sodium aluminate, or mixtures thereof, with the base precipitant. These can alternatively be added separately. Also, more acidic compounds like aluminum nitrate can be used as support precursors and, optionally, mixed in with the metal precursor. Compared to preformed supports, the amount of support precursor can be quite low and this is why coprecipitates may contain very high metal levels up to 80-90%, a level not attainable by impregnation. 20-30 mol% of silicate or aluminate relative to the metal is usually sufficient to obtain a high metal surface area after reduction. It may be debated whether the resulting metal silicate, aluminate, or alumina, are real supports or that names like stabilizer or spacer are more appropriate. 

It had been mentioned by Geus (8) that deposition-precipitation by evaporation of solvent is another way to gradually increase the concentration of a metal precursor in solution, but he also reported that this leads to an inhomogeneous distribution of the active material. However, ammonia evaporation of solutions containing ammine or ammine carbonate metal complexes has been reported as a way to gradually decrease the pH of the solution to induce precipitation of hydroxy carbonate compound on alumina support and to prepare Co/AljOj (43, 44), Ni/Al203 (45, 46), CU/AI2O3 catalysts (47). The preparation method was very broadly described in the patent references. It consists of a suspension of alumina... 

Supported platinum catalysts were prepared at room temperature by the adsorption of metal precursors followed by the reduction with sodium tetrahydroborate solution. It was shown that the alumina-supported catalysts so prepared were not only highly active for liquid-phase hydrogenation of cinnamaldehyde but also highly selective for the formation of cinnamyl alcohol at atmospheric pressure of hydrogen and 308 K. The prepared catalysts seemed to be different in the state of dispersion of platinum particles as compared to those prepared by usual hydrogen reduction at 773 K. 

The present work was undertaken to examine this possibility by trying a new method of low-temperature catalyst preparation. The method studied involves the adsorption of metal precursors on supports and the reduction by sodium tetrahydroborate solution for the preparation of supported platinum catalysts. The adsorption and reduction of platinum precursors are carried out at room temperature and the highest temperature during the preparation is 390 K for the removal of solvent. The activities of the catalysts prepared were examined for liquid-phase hydrogenation of cinnamaldehyde under mild conditions. Our attention was directed to not only total activity but also selectivity to cinnamyl alcohol, since it is difficult for platinum to hydrogenate the C=0 bond of this a, -unsaturated aldehyde compared to the C=C bond [2]. We examined the dependence of the catalytic activity and selectivity on preparation variables including metal precursor species, support materials and reduction conditions. In addition, the prepared catalysts were characterized by several techniques to clarify their catalytic features. The activity of the alumina-supported platinum catalyst prepared by the present method was briefly reported in a recent communication [3]. 

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