Supported Co-Re Bimetallic Catalysts with Different Structures as Efficient Catalysts for Hydrogenation of Citral

International audienceBimetallic Co–Re/TiO2 catalysts were developed for efficient citral hydrogenation. Bimetallic catalysts were prepared by co‐impregnation (CI), successive‐impregnation (SI), and surface redox method (SR). The arrangement between the Co and Re species on these systems was fully characterized using several techniques (TEM–energy‐dispersive X‐ray spectroscopy, H2 temperature‐programmed reduction, temperature‐programmed desorption, XRD, CO FTIR spectroscopy, model reaction of cyclohexane dehydrogenation), and their catalytic performances were evaluated for the selective hydrogenation of citral towards unsaturated alcohols. The Re and Co species are completely isolated in the CI sample, presenting a very limited Co–Re interaction. In SI samples, the metals coexist in a Janus‐type structure with a concentration of Re around Co. Decoration/core–shell structures are observed for SR samples resulting from the redox exchange between the metallic surface of the parent Co/TiO2 catalyst and the Re7+ species of the modifier precursor salt. The contact degree between the two metals gradually increases as follows: Isolated structure (CI)<Janus‐type structure(SI)<decoration/core–shell structure (SR). The unchanging structure of all SI samples independent of the Re loading leads to similar electron transfer, and the increase in Re content results in agglomeration of Re, thus decreasing the catalytic activity. Density‐of‐state (DOS) calculations prove that the high valence of Re is a disadvantage for the hydrogenation reaction. For SR samples, the increase of Re loading contributes to the electron transfer from Re to Co that is consistent with a change of structure from decoration to core–shell. The lack of directly accessible Co atoms for SR catalysts with fully coated structure decreases the efficiency of Re reduction. The presence of Co–Re interaction resulting from the close contact between metals plays a dominant role in the hydrogenation of citral. Nevertheless, an excessively high contact degree is unnecessary for citral hydrogenation once Co–Re interaction has formed

Préparation de catalyseurs bimétalliques par réaction d'oxydoréduction de surface (application à l'hydrogénation du citral)

POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Bimetallic Catalysts For Sustainable Chemistry: Surface Redox Reactions For Tuning The Catalytic Surface Composition MINIREVIEW

International audienceThe development of a bimetallic catalyst for a given reaction requires not only the selection of the appropriate metals M1 and M2 but also the control as far as possible of the distribution of the two metals together and at the support surface in the case of supported catalysts. Preparation methods using redox reactions specifically enable the deposition of a second metal M2 at the surface of monometallic M1 nanoparticles, leading in most cases to core-shell nanoparticles with strong metal-metal interactions. Various methods are possible depending on the electrochemical potentials of the species involved: either a direct redox reaction, also named galvanic replacement, or the reduction of an intermediate reducing agent activated at the surface of M1. In this minireview, the fundamental bases of the preparation of bimetallic catalysts by both types of redox reactions and the recent advances in that domain are described

Impact of cerium-based support oxides in catalytic wet air oxidation: Conflicting role of redox and acid-base properties

International audienceHigh surface area ceria is an excellent support of metals (Ru, Pt, etc.) for catalytic wet air oxidation reactions (CWAO). Due to the low solubility of oxygen in water, the rate of transfer of O-2 from the gas phase toward the catalyst has a decisive impact on the whole oxidation process. The role of the ceria support would be to facilitate this oxygen transfer. Redox properties of ceria (generally measured by the oxygen storage capacity, OSC) have a great impact on this step. Attempts were made to modulate the redox properties of ceria by using mixed oxides: CeO2-ZrO2, CeO2-PrO2-ZrO2 or CeO2 -TiO2. In doing so, acid-base properties of the supports are also modified. The role of the redox and acid-base properties on CWAO reactions (acetic acid, aniline and phenol) are reviewed. In most cases, acid-base properties can have a great impact on the catalyst stability (deactivation by carbonates or polymer deposit). Increasing the redox properties may have an adverse effect on the catalytic performances when acid-base properties bias the whole process. In some cases (phenol over CeO2-TiO2 supported catalysts), an adequate Lewis acidity is preferable to a high OSC of the catalyst

Hydrogénation du citral sur cattalyseurs bimétalliques supportés rhodium-germanium, palladium-germanium et palladium-étain

Ce travail est consacré à la préparation de catalyseurs bimétalliques supportés Rh-Ge, Pd-Ge et Pd-Sn pour l hydrogénation sélective du citral en alcools a,b-insaturés. Pour les catalyseurs à base de Rh, l étude porte sur l influence de la nature des sels précurseurs utilisés. L existence d interactions entre les deux métaux des différents systèmes bimétalliques est mise en évidence par diverses techniques de caractérisation (EDX, réaction modèle de déshydrogénation du cyclohexane, RTP sous H2, IRTF de CO adsorbé). La réaction d hydrogénation sélective du citral a montré qu il est possible de modifier profondément les propriétés de ces métaux par les ajouts Ge et Sn, et ainsi former des alcools insaturés. L adsorption de CO suivie par IRTF sur les catalyseurs Rh-Ge/TiO2 révèle la formation d espèces Rh>=1+ avec ajout de Ge, lesquelles permettraient l activation de la fonction carbonyle du citral. De plus, la présence de chlore issu des sels précurseurs semble être favorable à la formation de ces espèces Rh>=1+ sur ces systèmes bimétalliques. Pour le palladium, une bonne sélectivité en alcools insaturés est obtenue lorsqu il y a formation d un alliage de type Pd3M (avec M = Ge ou Sn), formation mise en évidence par différentes caractérisations (RTP et XPS). Cet alliage induit un transfert d électron de Sn vers Pd constaté aussi bien par IRTF que par XPS, qui peut expliquer une adsorption favorisée de la liaison C=O du citral.This work was devoted to the preparation of supported bimetallic Rh-Ge, Pd-Ge and Pd-Sn catalysts for the selective hydrogenation of citral to its corresponding a,b-unsaturated alcohols. In the case of Rh catalysts, the influence of Rh and Ge salt precursors has been studied. The existence of interactions between the two metals for the different bimetallic systems has been highlighted by several characterization methods (EDX, cyclohexane dehydrogenation reaction, TPR under H2, FTIR of adsorbed CO). The reaction of selective hydrogenation of citral showed that the addition of Ge or Sn to Rh and Pd allowed one to strongly modified the catalytic properties of these metals. The adsorption of CO followed by FTIR on Rh-Ge/TiO2 revealed the formation of Rh>=1+ species due to Ge addition. These species would activate the carbonyl group of citral. Moreover, the presence of chlorine resulting from precursor salts seemed favourable to the formation of these Rh>=1+ species. In the case of Pd catalysts, the different characterizations (TPR and XPS) proved the formation of Pd3M alloy (with M = Ge or Sn). This alloy induced an electron transfer from Sn to Pd, evidenced by FTIR and XPS. The formation of these Pd-Snd+ sites could then explain a favoured adsorption of the carbonyl group of citral, which leads to high selectivity in unsaturated alcohol.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

The relationship between the structural properties of bimetallic Pd-Sn/SiO2 catalysts and their performance for selective citral hydrogenation

International audienceThe effect of Sn addition to Pd on the selective liquid-phase hydrogenation of citral to alpha,beta-unsaturated alcohols (VA: nerol and geraniol) was examined. Pd-Sn/SiO2 bimetallic catalysts were prepared by successive impregnation method and were characterized by transmission electronic microscopy (TEM) coupled with energy dispersive X-ray spectroscopy (EDX), temperature-programmed reduction (TPR), Fourier transform infrared (FFIR) spectroscopy of adsorbed CO, and X-ray photoelectron spectroscopy (XPS). Sn addition to Pd/SiO2 catalysts significantly modifies their properties for citral hydrogenation performed at 130 degrees C, under 7 MPa and in isopropanol solvent, inducing a promoting effect on the VA selectivity. This promoting effect is related to the existence of a Pd-Sn interaction highlighted by EDX analysis, TPR under hydrogen and FTIR of adsorbed CO. The latter technique suggested the presence of a geometric effect on catalytic activity. Maximum VA selectivities (>75% at 30% citral conversion) were obtained when an alloy of the Pd3Sn type is formed in the bimetallic particles, as confirmed by TPR and XPS. Moreover, FTIR measurements of the adsorbed CO singleton frequency as well as XPS binding energy shifts strongly imply an electron transfer from Sn to Pd, which is proposed to be responsible for enhanced adsorption of citral C=O bond on the surface of Pd-Sn/SiO2 bimetallic catalysts. To our knowledge, it is the first time that modified Pd catalysts lead to such important UA selectivity values during alpha,beta-unsaturated aldehyde hydrogenation

Influence of the formulation of catalysts deposited on cordierite monoliths for acetic acid oxidation

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Influence of Re–M interactions in Re–M/C bimetallic catalysts prepared by a microwave-assisted thermolytic method on aqueous-phase hydrogenation of succinic acid

International audienceCarbon-supported Re–M (M = Pt and Rh) bimetallic catalysts with controlled size and composition were synthesized by using a microwave-assisted thermolytic method and evaluated in the aqueous phase hydrogenation of succinic acid. The Re–M interaction contributes to the inhibition of aggregation of particles and to the improvement in the catalytic activity for succinic acid hydrogenation through decreasing the activation energy. The Re–M interaction favors the ring opening of γ-butyrolactone, an intermediate product, to 1,4-butanediol instead of the hydrogenation and dehydration to tetrahydrofuran observed over a Re/C catalyst. The kinetic study proves that the Re–M interaction can increase the relative formation rate of 1,4-butanediol more than that of tetrahydrofuran, while the strength of the Re–M interaction has a limited influence on the product selectivity. It was shown that the Re–Rh interaction can reduce the direct hydrogenolysis of succinic acid, but it cannot avoid the hydrogenolysis of 1,4-butanediol, thus limiting the selectivity to this product. According to the kinetic mechanism, ring opening of γ-butyrolactone is favored at low temperature while direct hydrogenation to tetrahydrofuran is favored at high temperature