No abstract

Supported palladium nanoparticles with different diameters were synthesized by the water-in-oil microemulsion method using TiO2 as support. The materials were characterized by different physicochemical methods such as X-ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic emission spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and H2 chemisorption. The results confirmed that the microemulsion method permits well-dispersed palladium nanoparticles to be obtained. The size of the nanoparticles was estimated by XPS intensity ratios using models proposed by Davis and by Kerkhof and Moulijn and compared with XRD, TEM, and H2 chemisorption analysis. Good accordance of the two models was found for very small Pd particles (smaller than 3 nm). The Kerkhof−Moulijn model seemed to be very sensitive to the small variation in the particle size distribution. The Davis model seemed to be more adequate to determine the size of small and biggest particles as compared with the Kerkhof−Moulijn model. A good accordance between TEM results and the Davis model was found. The results obtained using the Davis model permitted also understanding of the differences observed between XRD and TEM studies. XPS analysis could be a good and probably more accessible alternative to determine rapidly and with high accuracy nanosize particles of materials, in particular when others physicochemical techniques are not accessible or have a limited resolution.

Carbon nanofibers (CNFs) are a class of graphitic support materials with considerable potential for catalytic conversion of biomass. Earlier, we demonstrated the hydrolytic hydrogenation of cellulose over reshaped nickel particles attached at the tip of CNFs. The aim of this follow-up study was to find a relationship between the acid/metal balance of the Ni/CNFs and their performance in the catalytic conversion of cellulose. After oxidation and incipient wetness impregnation with Ni, the Ni/CNFs were characterized by various analytical methods. To prepare a selective Ni/CNF catalyst, the influences of the nature of oxidation agent, Ni activation, and Ni loading were investigated. Under the applied reaction conditions, the best result, that is, 76 % yield in hexitols with 69 % sorbitol selectivity at 93 % conversion of cellulose, was obtained on a 7.5 wt % Ni/CNF catalyst prepared by chemical vapor deposition of CH(4) on a Ni/γ-Al(2)O(3) catalyst, followed by oxidation in HNO(3) (twice for 1 h at 383 K), incipient wetness impregnation, and reduction at 773 K under H(2). This preparation method leads to a properly balanced Ni/CNF catalyst in terms of Ni dispersion and hydrogenation capacity on the one hand, and the number of acidic surface-oxygen groups responsible for the acid-catalyzed hydrolysis on the other.

No abstract

An ZSM-22 aluminosilicate zeolite was synthesized using the hydrothermal gel method at 150 degrees C. Products obtained after different synthesis times were characterized using various techniques and catalytic testing. Massive formation of ZSM-22 nanocrystals occurs after only a short synthesis time, appearing as isolated rods with a cross section of 12+/-4 nm. Nanorods have aluminum enriched at their external surface. Later in the crystallization process nanorods align and fuse sideways, whereby the external surface is systematically converted into an internal micropore surface. The formation of aluminum bearing micropores by the joining of nanorod surfaces is responsible for the enhanced catalytic activity. For this, the zeolite synthesis of nanoscale crystallites is ineffective for enhancing catalytic activity.

No abstract

No abstract

MoO 3 -based heterogeneous catalysts prepared by dispersion of Mo-oxide species on preformed supports are highly regarded candidates for industrial light olefin metathesis. An original approach for the elaboration of Mo-based catalysts is presented here. Mesoporous ternary Si/Al/Mo mixed oxides are prepared in one step by a nonhydrolytic sol-gel route in nonaqueous medium. Taking advantage of the migration of Mo species during the calcination, effective catalysts with very good textures and highly dispersed surface Mo species are obtained, as shown by XRD, XPS, and TOF-SIMS characterization. The presence of isolated Mo species is evidenced and these species are proposed to be the precursors to the active species in the metathesis of propene. These new materials compare well with catalysts prepared by classical wet impregnation of silica-alumina supports with ammonium heptamolybdate, eventually outperforming them at high Mo loading.