Bimetallic modified hydrogenation catalysts

Activity and selectivity of monometallic Ag catalysts can be controlled by the preparation conditions leading to micro- and meso- to macroporous catalysts which are active and selective in the hydrogenation of crotonaldehyde. In Ag catalysts modified by a second metal, bimetallic sites exhibiting surface polarity and Ag particles in close contact with a partially reduced early transition metal or a rare earth element, or Ag species stabilized and incorporated in these oxides were concluded to be the active species in the working state of these catalysts. Simultaneous introduction of both metals during the sol-gel process under optimized hydrolyzing conditions could further increase the metal-promoter interaction and lead to well-tailored new hydrogenation catalysts. 

The data on differential thermal desorption spectra of the reduction of NiO and CuO proved to be very important. Despite the differences in reduction temperatures of individual oxides (320°C and 270°C), a maximum ee was observed at that reduction when mixture of oxides were reduced at a lower temperature, 220°C, because Ni-Cu catalysts contain two phases a Cu phase and a Cu-Ni-alloy phase. With increase of the copper content, the Ni-Cu phase is enriched in the bulk with Ni and on the surface with Cu. According to Klabunovskii et al. the Ni-Cu (70 30) catalyst is five times more active than the pure Cu catalyst, but after modification of this catalyst with TA, its enantioselectivity in the hydrogenation of EAA was lower than either the modified pure Cu catalyst ee 50%) or pure Ni-TA catalyst ee 33%) under the same conditions. Thus the bimetallic Ni-Cu catalysts revealed S5mergism in catalytic activity but not in enantioselectivity. 

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. 

Bimetallic Pt-Mo catalysts were prepared by successive chemical vapor deposition of Mo(CO)g onto nanometer-size platinum particles dispersed in EMT zeolite [264]. Bimetallic particles of imiform composition in a highly dispersed form were formed. Subsequent decomposition of the Mo precursor at 600 K in hydrogen formed a supported molybdenum carbide phase that was characterized by XPS, TEM and EX AES. Nitridation at 973 K in flowing NH3 led to a nanometer-sized Pt-core covered by a Mo-nitride layer. The coverage of the Pt-clusters by ca. 0.3 nm of carbide or nitride modified greatly the selectivities in n-heptane isomerization and hydrogenolysis. 

Partial hydrogenation of acetylenic compounds bearing a functional group such as a double bond has also been studied in relation to the preparation of important vitamins and fragrances. For example, selective hydrogenation of the triple bond of acetylenic alcohols and the double bond of olefin alcohols (linalol, isophytol) was performed with Pd colloids, as well as with bimetallic nanoparticles Pd/Au, Pd/Pt or Pd/Zn stabilized by a block copolymer (polystyrene-poly-4-vinylpyridine) (Scheme 9.8). The best activity (TOF 49.2 s 1) and selectivity (>99.5%) were obtained in toluene with Pd/Pt bimetallic catalyst due to the influence of the modifying metal [87, 88]. 

In this paper we report the application of bimetallic catalysts which were prepared by consecutive reduction of a submonolayer of bismuth promoter onto the surface of platinum. The technique of modifying metal surfaces at controlled electrode potential with a monolayer or sub-monolayer of foreign metal ("underpotential" deposition) is widely used in electrocatalysis (77,72). Here we apply the theory of underpotential metal deposition without the use of a potentiostat. The catalyst potential during promotion was controlled by proper selection of the reducing agent (hydrogen), pH and metal ion concentration. 

Hydrogen, preadsorbed on noble metals, is commonly used to prepare bimetallic catalysts by redox reaction. This requires the parent metal to chemisorb hydrogen (Pt, Pd, Rh, Ru, etc.) and to introduce a modifier that is reducible by hydrogen (Cu, Re, Ir, Rh, Pd, Pt, Au, etc.). All combinations of these metals have been prepared and characterized. For example, this technique has been used to prepare model Pt-Re reforming catalysts. Also, Pt-Rh and Pd-Rh were preformed to examine the interaction between platinum and rhodium in exhaust gas catalysts [8-10, 15-20, 21]. 

Monometallic Pt (0.4% w/w) and bimetallic Pt-Sn (0.4% 0.49% w/w respectively) catalysts supported on alumina have been modified with alkali metals ( Li, Na, K,Rb Cs, Pt alkali molar ratio of 1 40) have been investigated by TPR, TPD (ammonia and hydrogen), Pt dispersion and TPCO measurements and evaluation of activity for dehydrogenation of n-decane. Activity of alkali promoted mono and bimetallic catalysts are shown in Fig. 6 7 In the case of monometallic catalysts, Pt-Li system exhibits comparable initial actvity while the stability improves significantly. Other alkali elements do not show... 

Iron-ruthenium bimetallic catalysts have also received considerable attention as interesting catalysts in Fischer-Tropsch synthesis [115,116]. It has been reported that the Fe-Ru alloy system results in catalysts that are more stable than monometallic iron catalysts [117], and that the hydrocarbon product distribution in CO hydrogenation can easily be modified when changing the relative proportions of the two metals [118]. 

Activities and selectivities of bimetallic catalysts for hydrogenation of carvone are reported in Table 3. The results show a decrease of the specific activity of bimetallic catalysts compared to that of monometallic ones. Gold addition also modifies the selectivity patterns. The partial hydrogenation of the exo double bond is increased on both large and small particles on these catalysts carvotanacetone is the main product. 

Nevertheless only scare data is available in the recent literature on the application of Group VIII noble metal (M) or rhenium-based mono- and Re-M bimetallic catalysts, in the hydrogenolysis of esters or hydrogenation of acids to alcohols. Recently a few publications, - and patents. have been reported on the transformation of different carbonyl compounds (saturated and unsaturated esters, acids and carboxamides) over rhenium-containing catalysts. In the bimetallic catalysts used for the hydrogenation of carbonyl compounds the rhenium was combined with Pd, or Rh. In the case of catalysts used for the hydrogenation of unsaturated carbonyl compounds the rhenium is usually modified with tin. ... 

Different model reactions were used in order to study the interaction between the modifier and the parent metal. It was observed that an inert additive introduced by a redox reaction generally poisons, more or less, the activity of the parent metal or strongly modifies the selectivity of the reaction, which indicates a deposition of the additive on the parent metal. For example, a decrease in activity for structure insensitive reactions, such as toluene hydrogenation [41] or cyclohexane dehydrogenation [43, 78] proves the existence of bimetallic nanoparticles. Likewise, in the case of the 2,2-dimethylpropane reaction, the modification of both the selectivity and the apparent activation energy, demonstrates an interaction between Pd and Au introduced by direct redox reaction. Conversely, no modification was observed on the catalysts prepared by incipient wetness co-impregnation [75]. 

In general, chiraly modified bimetallic catalysts exhibited low to moderate enantioselectivity in the hydrogenation of EAA. A number of studies were carried out by Klabunovskii s group to investigate new metal catalysts, modified mainly with tartaric acid and amino acids, which were active in enantioselective hydrogenation of ethyl acetoacetate and acetylacetone. 

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