Patricia Concepción

Transforming Nonselective into Chemoselective Metal Catalysts for the Hydrogenation of Substituted Nitroaromatics

It is generally accepted that good hydrogenation noble and nonnoble metal catalysts such as Pt, Ru, or Ni are not chemoselective for hydrogenation of nitro groups in substituted aromatic molecules. We have found that it is possible to transform nonchemoselective into highly chemoselective metal catalysts by controlling the coordination of metal surface atoms while introducing a cooperative effect between the metal and a properly selected support. Thus, highly chemoselective and general hydrogenation Pt, Ru, and Ni catalysts can be prepared by generating nanosized crystals of the metals on the surface of a TiO 2 support and decorating the exposed (111) and (100) crystal faces by means of a simple catalyst activation procedure. By doing this, it has been possible to change the relative rate for hydrogenating competitive groups present in the molecule by almost 2 orders of magnitude, increasing the chemoselectivity from less than 1% to more than 95%.

Gold–Copper Nanoalloys Supported on TiO2 as Photocatalysts for CO2 Reduction by Water

Commercial P25 modified by Au-Cu alloy nanoparticles as thin film exhibits, for CO2 reduction by water under sun simulated light, a rate of methane production above 2000 μmol (g of photocatalyst)(-1) h(-1). Although evolution of hydrogen is observed and O2 and ethane detected, the selectivity of conduction band electrons for methane formation is almost complete, about 97%. This photocatalytic behavior is completely different from that measured for Au/P25 (hydrogen evolution) and Cu/P25 (lower activity, but similar methane selectivity). Characterization by TEM, XPS, and UV-vis spectroscopy shows that Au and Cu are alloyed in the nanoparticles. FT-IR spectroscopy and chemical analysis have allowed one to detect on the photocatalyst surface the presence of CO2(•-), Cu-CO, and elemental C. Accordingly, a mechanism in which the role of Au is to respond under visible light and Cu binds to CO and directs the reduction pathway is proposed.

Exceptional oxidation activity with size-controlled supported gold clusters of low atomicity

The catalytic activity of gold depends on particle size, with the reactivity increasing as the particle diameter decreases. However, investigations into behaviour in the subnanometre regime (where gold exists as small clusters of a few atoms) began only recently with advances in synthesis and characterization techniques. Here we report an easy method to prepare isolated gold atoms supported on functionalized carbon nanotubes and their performance in the oxidation of thiophenol with O2. We show that single gold atoms are not active, but they aggregate under reaction conditions into gold clusters of low atomicity that exhibit a catalytic activity comparable to that of sulfhydryl oxidase enzymes. When clusters grow into larger nanoparticles, catalyst activity drops to zero. Theoretical calculations show that gold clusters are able to activate thiophenol and O2 simultaneously, and larger nanoparticles are passivated by strongly adsorbed thiolates. The combination of both reactants activation and facile product desorption makes gold clusters excellent catalysts.

Graphene oxide as an acid catalyst for the room temperature ring opening of epoxides

The minute amount of hydrogen sulfate groups introduced into the graphene oxide (GO) obtained by Hummers oxidation of graphite renders this material as a highly efficient, recyclable acid catalyst for the ring opening of epoxides with methanol and other primary alcohols as nucleophile and solvent.

Generation of subnanometric platinum with high stability during transformation of a 2D zeolite into 3D

Single metal atoms and metal clusters have attracted much attention thanks to their advantageous capabilities as heterogeneous catalysts. However, the generation of stable single atoms and clusters on a solid support is still challenging. Herein, we report a new strategy for the generation of single Pt atoms and Pt clusters with exceptionally high thermal stability, formed within purely siliceous MCM-22 during the growth of a two-dimensional zeolite into three dimensions. These subnanometric Pt species are stabilized by MCM-22, even after treatment in air up to 540 °C. Furthermore, these stable Pt species confined within internal framework cavities show size-selective catalysis for the hydrogenation of alkenes. High-temperature oxidation-reduction treatments result in the growth of encapsulated Pt species to small nanoparticles in the approximate size range of 1 to 2 nm. The stability and catalytic activity of encapsulated Pt species is also reflected in the dehydrogenation of propane to propylene.

Selective oxidation of hydrocarbons on V- and/or Co-containing aluminophosphate (MeAPO-5) using molecular oxygen

Abstract MeAPO-5 (Me = V, Co, Mg) and MeVAPO-5 (Me = Co, Mg) were prepared by hydrothermal synthesis and they were characterized by N 2 adsorption, XRD, FTIR of piridine adsorption/desorption, TPR and UV-visible spectroscopy. For comparative purpose, MgO- and Al 2 O 3 -supported vanadium catalysts were also prepared. Their catalytic properties for the oxidative dehydrogenation (ODH) of ethane and the oxidation of cyclohexane were studied. The ODH of ethane was studied, at atmospheric pressure, in the 500–600°C temperature interval, with an ethane/oxygen/helium molar ratio of 4/8/88. The oxidation of cyclohexane was studied at 125°C and an oxygen pressure of 10 kg/cm 2 . Different catalytic behaviour was observed depending on the transition metal incorporated, the catalyst structure and the reaction studied. The importance of the redox and the acid character of the catalysts on their catalytic properties is discussed.

Methanol to olefins: activity and stability of nanosized SAPO-34 molecular sieves and control of selectivity by silicon distribution

Nano-SAPO-34 molecular sieves synthesized in a microwave environment with 20 nm crystal size showed a longer lifetime than SAPO-34 prepared by the conventional hydrothermal method in the reaction of methanol to olefins. It has been found that silicon distribution strongly affects the lifetime and selectivity. Thus, silicon at the border of the silicon islands gives a higher lifetime and lower C2/C3 ratio. This change in activity and selectivity is better explained in terms of different silicon distribution than by preferential diffusion of ethene through the 8MR pores and agrees with transition-state selectivity. The effects of equilibrium of olefins and deactivation by coke were isolated, showing that after full formation of the hydrocarbon pool, selectivity is independent of deactivation by coke.

Synthesis and Stabilization of Subnanometric Gold Oxide Nanoparticles on Multiwalled Carbon Nanotubes and Their Catalytic Activity

Small gold nanoclusters in a very narrow size distribution (1.1 ± 0.5 nm) have been stabilized onto multiwalled carbon nanotubes (MWCNT). Theoretical studies supported by XPS and (16)O(2)/(18)O(2) isotopic exchange experiments have shown that, on small gold nanoparticles (0.9-1.5 nm), dissociation of molecular O(2) and formation of a surface oxide-like layer is energetically favorable and occurs at room temperature, while O(2) recombination and desorption involves a larger activation barrier. CO titration experiments and theoretical studies demonstrate that the reactivity of the oxidized particles toward CO does not only depend on particle size but also on oxygen coverage. The oxidation-reduction process described is reversible, and the oxidized nanoparticles are active in the epoxidation of styrene with air.

Ceria nanoparticles as heterogeneous catalyst for CO2 fixation by ω-aminoalcohols

In contrast to gamma-Al(2)O(3), TiO(2), ZrO(2), MgO and Y(2)O(3), CeO(2) is a reusable catalyst for the reaction of CO(2) with omega-aminoalcohols to form cyclic carbamates; the highest yield (68%) was obtained for the preparation of N-alkyl 1,3-oxazolidin-2-ones from N-alkyl ethanolamines.

The selective oxidative dehydrogenation of propane on vanadium aluminophosphate catalysts

VAPO-5 and V/ ALPO-5 catalysts have been tested for the oxidative dehydrogenation of propane. Depending on the vanadium contents and the preparation procedure, different vanadium species and different catalytic behavior are observed. The catalyst containing V5+ species with a tetrahedral coordination presents the higher yield of propene in the oxidative dehydrogenation of propane. The same yields of CO2 are observed on all vanadium aluminophosphate catalysts, while the higher the yield of propene the lower the yield of CO is.