Bimetallic modified hydrogenation

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]. 

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. 

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. ... 

Hydrogen evolution Irom the electroreduction of protons at different modified polymer electrodes was first investigated by Tourillon and Gamier, who studied the inclusion of bimetallic Ag-Pt particles into poly-3-methylthiophene(PMeT) and observed their electrocatalytic properties towards the proton reduction reaction [46], They demonstrated the positive effect of the Ag particles (from 15 /ig/cm ) on the reduction current due to an increase of the electrode conduction at low potentials, where PMeT is in its neutral undoped state, and put in evidence a minimum Pt loading (of about 10 tg/cm for a 170 nm thick film) for obtaining an enhanced catalytic activity compared to a platinized Ag-coated Au electrode without a polymeric film. A remarkable stability with time was observed under polarization at a constant potential ( — 0.4 V/SCE) without degradation of the modified electrode. 

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