Bimetallic catalysts molybdenum

At the initial reaction stage, the activity of oxide Mo-Ni catalysts is higher than that of Mo catalysts by a factor of 5. In three hours, this value decreases to 4 (Fig. la). For the sulfide samples, this effect is less pronounced (Fig. lb). In the next run, the catalyst activity was determined at 400 C, then the temperature was decreased to 300 C. This procedure was repeated for several times. For both oxide and sulfide molybdenum catalysts, activity and the product composition are reproduced with high accuracy in a number of experiments (Fig. 2). At high temperature, the bimetallic catalysts are quickly deactivated during the reaction (and the experimental cycles are not reproducible). 

For both oxide and sulfide molybdenum catalysts, the ratio between butane and butenes is much higher than 1. As temperature rises, this ratio decreases for the oxide forms and increases for the sulfide forms. Polymerization begins at T > 300"C on both catalyst forms. For all bimetallic catalysts, excepting Mo-Ni oxide catalysts, the ratio butane butenes is higher than 1 at 300 C. The amount of resin materials holds with time only for Mo catalysts. For bimetallic catalysts, the conversion of THF into resin materials increases with time by 10-15%. For all catalysts, excepting Mo-Ni sulfide catalysts, the yield of resin con lexes increases with increasing temperature. 

We recently initiated a research programme aimed at providing milestones for the adequate control of the architecture of multimetallic oxidic catalysts for sel tive oxidation reactions. Its main purpose is to carry out a systematic study of t influence exerted by the nature and structure of the metallic precursors on the surface stoichiometry and the dispersion level of the active phase. One of the biggest challenges is however to preserve properly the structure and nuclearity of the selected precursors on the support up to the calcination stage this can be achieved by the judicious choice of the solvent, provided adequate handling and sampling conditions are strictly respe ted to warrant their stabilization. The present report deals with preliminary results obtained in die frame of experimraits on silica-sup rted bimetallic bismuth-molybdenum catalysts. 

A number of bimetallic catalysts have been studied for their activity in ammonia synthesis. A Fe-Mo (1 1) catalyst exhibits a high activity, although it decreased remarkably for a prolonged run when the content of molybdenum is lower than 80%. This catalyst is prepared by calcining the mixtures of metal nitrate and... 

Norskov proposed a more scientific and effective theory, i.e., interpolation in the periodic table, for designing bimetallic catalysts. According to this theory, a reasonable assumption is to combine the most active and less active elements for activation of nitrogen together to construct an active surface to achieve the best performance. The idea was proved by an efficient non-ruthenium catalyst on the basis of cobalt-molybdenum nitride, which can be regarded as the current climax of a systematically but empirically justified development based on the theoretical prediction of the ammonia synthesis catalyst research up to now. 

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. ... 

Bartholomew and co-workers also measured the loss of catalytic activity with time of Ni and Co bimetallics (157, 194), Ni-molybdenum oxide (23, 113), and borided Ni and Co catalysts (161) during methanation in the presence of 10 ppm H2S. Typical activity versus time plots are shown in Figs. 25 and 26. Activity is defined as the ratio of the mass-based rate of methane production at any time t divided by the initial rate. The activitytime curves are generally characteristic of exponential decay some catalysts decay more slowly than others, but all catalysts suffer at least two orders of magnitude loss in activity within a period of 100-150 hr. Accordingly, it does not appear that other metals or metal oxides in conjunction with Ni significantly change the sulfur tolerance defined in terms of steady-state activity of Ni. These materials can, however, influence the rate at which the... 

The fact that butane is not found in the reaction products indicates that THF is not dehydrated (scheme 3) on the study catalysts and oxygen is removed from THF by hydrogenolysis (schemes 1 and 2). On the oxide and sulfide molybdenum catalysts, butane is the main reaction product, that is why hydrogenolysis predominantly follows scheme 1. This is also true for bimetallic sulfide catalysts, if the reaction occurs at high temperature. For the rest catalysts, a part of butenes in the reaction products is rather high. This suggests that hydrogenolysis of the most part of THF follows scheme 2. 

Reaction of a reduced Philipps catalyst with Fischer-type molybdenum or tungsten carbene or carbyne complexes led to very active bimetallic, heterogeneous olefin metathesis catalysts. Surface metal ions might be involved in bonding interactions with the organometallic complex, possibly leading to heterometallic species on inorganic oxides. ... 

The [Mo2RhCp3(CO)5] cluster on Si02 was also used as precursor to a heterogeneous catalyst for the hydroformylation of ethylene and propene. Molybdenum increases the rate of both hydrogenation and hydroformylation of olefins. It was suggested that propanal, the primary product of the hydroformylation, was hydrogenated to propanol on active bimetallic centers. ... 

Heterogeneous, bimetallic metathesis catalysts are formed by reactions of Fischer type carbyne tungsten or molybdenum complexes with the reduced Phillips catalyst, a suface chromium(II) compound on silica (14).(scheme 5). The bimetallic surface compounds can result from 2+1 cycloaddition reactions. Similar reactions are well known by the work of Stone (15). 

Supported bimetallic systems containing molybdenum and a modifier such as Co, Ni... are frequently encountered in the literature, especially as desulfurisation catalysts. On the other hand, little attention has been devoted to Pd-Mo/oxide systems so far, despite their use for the CO + NO reaction by Halasz et al. [1] and several mentions as selective hydrogenation catalysts in the patent literature. 

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