Magnesia support

When the Diels-Alder reaction between butadiene and itself is carried out in the presence of alkah metal hydroxide or carbonate (such as KOH, Na2C02, and K CO on alumina or magnesia supports) dehydrogenation of the product, vinylcyclohexene, to ethylben2ene can occur at the same time (134). The same reaction can take place on simple metal oxides like Zr02, MgO, CaO, SrO, and BaO (135). 

The obvious decrease in the number of electron-acceptor sites with palladium deposition on silica-alumina strongly suggests an interaction between the metal and these sites. Turkevich (28) first demonstrated that palladium behaves like an electron-donor toward tetracyanoethylene we suppose that it can be the same toward an electron-acceptor site of a solid support. In that hypothesis, palladium should have a partial positive charge on the second class of supports. This is actually observed by the adsorption of CO. This adsorbate can be considered as a detector of the electronic state of palladium. The shift toward higher frequencies of the CO band reflects a decrease in the back donation of electrons from palladium to CO. Thus, palladium on silica-alumina or HY is electron-deficient compared with the silica- or magnesia-supported metal. Moreover, the shift of CO vibration frequency is roughly parallel to the increase of activity thus, these two phenomena are connected. We propose that the high activity of palladium on acidic oxides is related to its partial electron deficiency. 

In contrast with the dipersion results, the low values obtained for the ammoniacal preparations show an important precursor effect and a small one on the nature of the support. However, the selectivity values for the formation of the three hydrogenated products reported in Table 2, demonstrate that selectivity depends on the nature of the support. Magnesia support presents the highest selectivity (90 /0 to the carvotanacetone formation. Particle size effects in selectivity were not detected, since, the small changes observed... 

Rh/SiOzand Rh/TiOa catalysts. (Ill) the stereospeclflclty to hydrogen addition in the carvomenthol formation is higher in MgO supported catalyst, (iv) magnesia support effect is found due to the blockage of the metal particles by the MgO. 

Triantafillou, N. D., and Gates, B. C., Magnesia-supported tetrairidium clusters derived from [Ir4(CO)12]./. Phys Chem. 98,8431 (1994). 

All of the hydrotaleite-derived magnesia supports were prepared by first coprecipitating magnesium aluminum hydroxycarbonate in the presence of Mg and Al nitrates, KOH, and K2C03 according to procedures already described (8,9). Hydrotaleites were then decomposed by calcination at 873 K for 12-15 h to yield the binary oxide to be used for a catalyst support. The specific surface area of the hydrotaleites determined by nitrogen adsorption was typically about 220 m2g 1 after calcination. X-ray powder diffraction patterns of the materials were recorded on a Scintag X-ray diffractometer. 

Several methods were used to incorporate Pt metal onto the magnesia support. One method involved the impregnation of aqueous H2PtCl6 onto the dried support until the point of incipient wetness. A second method utilized vapor phase... 

Another test of validity is to check the performance of the model against experimental rate data obtained far from equilibrium. The microkinetic model presented in Table 7.3.1 predicts within a factor of 5 the turnover frequency of ammonia synthesis on magnesia-supported iron particles at 678 K and an ammonia concentration equal to 20 percent of the equilibrium value. This level of agreement is reasonable considering that the catalyst did not contain promoters and that the site density may have been overestimated. The model in Table 7.3.1 also predicts within a factor of 5 the rate of ammonia synthesis over an Fe(lll) single crystal at 20 bar and 748 K at ammonia concentrations less than 1.5 percent of the equilibrium value. 

Some properties of the prepared magnesia supported nickel catalyst samples are summarized in Table 1. 

On the catalyst samples obtained by immersion of magnesia support in solution of highest Ni-concentration, and studied in the propane oxidation, the best selectivity was exhibited on the sample prepared by 2-step immersion and with lowest Ni-loading of 3.19 wt%. On the other two samples, having greater Ni-loading, exhibit lower selectivity at all investigated temperatures. 

The diagrams in Figure 2 also show the effect of the promoter used on the selectivity of CO+H2 formation in propane oxidation by air. The best selectivity at all temperatures was obtained on the Al203-promoted catalyst. The selectivity for the products formation of the main reaction (1) in the presence of investigated promoter decreases in order to Al203>Mg0>Ca0, which is in the correlation with the previously mentioned effect of these promoters on the Ni surface area in magnesia supported nickel catalysts. 

Promoted Ni/MgO catalyst for the propane oxidation by air to produce reducing gas (CO and H2) is obtained by multiple successive impregnation of the low area magnesia support. 

In the presence of the promoter used, the mean Ni crystallite size in the promoted magnesia supported nickel catalyst increases according to the following order Al203> MgO>CaO. 

CO and H2 are formed with greater than 95% selectivity in the propane oxidation at temperatures higher than 800 C on the Al203-promoted catalyst prepared by 6-step impregnation of magnesia support in the 1M Ni"-solution. 

Enhanced steam adsorption can be achieved by the addition of alkali or the use of magnesia support as reflected by a negative reaction order with respeetive to steam (4,15). It is assumed that spill-over of steam from the support to the nickel surface... 

Goodman et al. [170] also report a direct correlation between the increasing concentration of F centers obtained in annealing MgO between 900 and 1300 K, and the catalytic activity of a real Au/MgO catalyst with 4nm gold particles. The validity of the correlation is questionable since the two magnesia supports are different the first is an ultrathin non-hydroxylated MgO fihn supported on Mo(lOO) while the second is a powder, and such an oxide still containing hydroxyl groups after thermal treatment at 900-1300 K. 

Most of the solutions used are aqueous - there are only a few examples of preparations using organic solvents. For instance, magnesia-supported Ni-Mo catalysts can be prepared by nonaqueous impregnation using dimethyl sulfoxide and methanol as solvents [50]. Dimethyl formamide was used as a solvent for the impregnation of phosphomolybdates on y-Al203 [51]. 

New generations of catalysts still at the R D stage combine reducible supports such as (doped) ceria and magnesia supporting noble and non-noble metals [5-19]. 

In summary, the concept of using the interaction between the support and the applied active phases proves to be applicable to zirconia, but not to titania. This evidences clear that a careful selection and characterization of candidate supports is necessary, emphasizing the importance of the possible formation of mixed compounds of the applied components and the support material. The employment of a mixed potassium-iron compound, probably potassium ferrite, KFeOj, to obtain a well-defined supported catalyst able to dehydrogenate and to prevent carbon deposition, was shown to be also applicable to zirconia, yielding a catalyst exhibiting a mote stable activity than titania- or magnesia-supported catalysts. 

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