Jose Antonio Lopez-Sanchez

Solvent-Free Oxidation of Primary Carbon-Hydrogen Bonds in Toluene Using Au-Pd Alloy Nanoparticles

A gold- and palladium-based catalyst can be used to oxidize toluene and form a commercially useful ester. Selective oxidation of primary carbon-hydrogen bonds with oxygen is of crucial importance for the sustainable exploitation of available feedstocks. To date, heterogeneous catalysts have either shown low activity and/or selectivity or have required activated oxygen donors. We report here that supported gold-palladium (Au-Pd) nanoparticles on carbon or TiO2 are active for the oxidation of the primary carbon-hydrogen bonds in toluene and related molecules, giving high selectivities to benzyl benzoate under mild solvent-free conditions. Differences between the catalytic activity of the Au-Pd nanoparticles on carbon and TiO2 supports are rationalized in terms of the particle/support wetting behavior and the availability of exposed corner/edge sites.

Facile removal of stabilizer-ligands from supported gold nanoparticles

Metal nanoparticles that comprise a few hundred to several thousand atoms have many applications in areas such as photonics, sensing, medicine and catalysis. Colloidal methods have proven particularly suitable for producing small nanoparticles with controlled morphologies and excellent catalytic properties. Ligands are necessary to stabilize nanoparticles during synthesis, but once the particles have been deposited on a substrate the presence of the ligands is detrimental for catalytic activity. Previous methods for ligand removal have typically involved thermal and oxidative treatments, which can affect the size or morphology of the particles, in turn altering their catalytic activity. Here, we report a procedure to effectively remove the ligands without affecting particle morphology, which enhances the surface exposure of the nanoparticles and their catalytic activity over a range of reactions. This may lead to developments of nanoparticles prepared by colloidal methods for applications in fields such as environmental protection and energy production.

Direct Catalytic Conversion of Methane to Methanol in an Aqueous Medium by using Copper‐Promoted Fe‐ZSM‐5

Iron copper zeolite (Fe-Cu-ZSM-5) with aqueous hydrogen peroxide is active for the selective oxidation of methane to methanol. Iron is involved in the activation of the carbon–hydrogen bond, while copper allows methanol to form as the major product. The catalyst is stable, re-usable and activates methane giving >90 % methanol selectivity and 10 % conversion in a closed catalytic cycle (see scheme).

Promotion of phenol photodecomposition over TiO2 using Au, Pd, and Au-Pd nanoparticles

Noble metal nanoparticles (Au, Pd, Au-Pd alloys) with a narrow size distribution supported on nanocrystalline TiO(2) (M/TiO(2)) have been synthesized via a sol-immobilization route. The effect of metal identity and size on the photocatalytic performance of M/TiO(2) has been systematically investigated using phenol as a probe molecule. A different phenol degradation pathway was observed when using M/TiO(2) catalysts as compared to pristine TiO(2). We propose a mechanism to illustrate how the noble metal nanoparticles enhance the efficiency of phenol decomposition based on photoreduction of p-benzoquinone under anaerobic conditions. Our results suggest that the metal nanoparticles not only play a role in capturing photogenerated electrons, but are strongly involved in the photocatalytic reaction mechanism. The analysis of the reaction intermediates allows us to conclude that on M/TiO(2) undesired redox reactions that consume photogenerated radicals are effectively suppressed. The analysis of the final products shows that the reusability performance of the catalyst is largely dependent on the pretreatment of the catalyst and the identity of the metal nanoparticle. Interestingly, the as-prepared Pd and Au-Pd decorated TiO(2) materials exhibit excellent long-term photoactivity, in which ~90% of the phenol can be fully decomposed to CO(2) in each cycle.

Selective liquid phase oxidation with supported metal nanoparticles

Over the past twenty years there has been intense interest in the design and understanding of catalysis by gold. More recently, it has been observed that alloying gold with a second metal greatly enhances the catalytic efficacy. These supported nanoparticles offer great potential as catalysts for the synthesis of fine chemicals and they are now at the stage where they can contribute to the sustainable development of chemical processes, in particular selective oxidation. A number of factors have contributed to this increased interest; namely, environmental issues promoting the need for more atom efficient processes and the new advances in the synthesis of nanoparticles as well as the characterisation methods available for their study. New catalytic materials obtained by careful control of the morphology of the metal nanoparticles and their use under solvent-free conditions all contribute to the latest developments. Most importantly their use can obviate the need to use stoichiometric oxidants. In this perspective we demonstrate the recent advances in these materials for selective oxidation reactions.

Selective Oxidation of Glycerol by Highly Active Bimetallic Catalysts at Ambient Temperature under Base-Free Conditions†

Au–Pt alloy nanoparticles deposited on Mg(OH)2 (see STEM-HAADF image) show high activity in the selective oxidation of polyols using molecular oxygen as oxidant at mild and base-free conditions.

Selective oxidation of 5-hydroxymethyl-2-furfural using supported gold–copper nanoparticles

The oxidation of 5-hydroxymethyl-2-furfural was studied under mild reaction conditions using TiO2-supported Au and Au–Cu catalysts synthesized from pre-formed nanoparticles. Bimetallic gold-copper catalysts display superior activity as compared to monometallic gold. Moreover, after reaction, the bimetallic Au–Cu catalysts can be recovered by filtration and reused without significant loss of activity and selectivity whereas gold materials are not stable. STEM-HAADF imagining and XEDS spectra obtained from bimetallic materials show that particles are homogeneous AuCu alloys. No AuCu ordering or segregation effects were noted from these analyses, and the Au:Cu ratio was quite consistent from particle-to-particle irrespective of its absolute size, proving the efficiency of the original method of synthesis utilized. Isolation effects of gold by copper in the alloy nanoparticles is imagined to play a pivotal role in the reaction. The effect of oxygen pressure, metal loading, reaction time, amount of base and temperature were studied in detail and a 99% yield of furandicarboxylic acid was achieved under optimized reaction conditions.

Oxidation of glycerol using gold-palladium alloy-supported nanocrystals

The use of bio-renewable resources for the generation of materials and chemicals continues to attract significant research attention. Glycerol, a by-product from biodiesel manufacture, is a highly functionalised renewable raw material, and in this paper the oxidation of glycerol in the presence of base using supported gold, palladium and gold-palladium alloys is described and discussed. Two supports, TiO(2) and carbon, and two preparation methods, wet impregnation and sol-immobilisation, are compared and contrasted. For the monometallic catalysts prepared by impregnation similar activities are observed for Au and Pd, but the carbon-supported monometallic catalysts are more active than those on TiO(2). Glycerate is the major product and lesser amounts of tartronate, glycolate, oxalate and formate are observed, suggesting a sequential oxidation pathway. Combining the gold and palladium as supported alloy nanocrystals leads to a significant enhancement in catalyst activity and the TiO(2)-supported catalysts are significantly more active for the impregnated catalysts. The use of a sol-immobilisation preparation method as compared to impregnation leads to the highest activity alloy catalysts and the origins of these activity trends are discussed.

Direct Synthesis of Hydrogen Peroxide and Benzyl Alcohol Oxidation Using Au—Pd Catalysts Prepared by Sol Immobilization

We report the preparation of Au-Pd nanocrystalline catalysts supported on activated carbon prepared via a sol-immobilization technique and explore their use for the direct synthesis of hydrogen peroxide and the oxidation of benzyl alcohol. In particular, we examine the synthesis of a systematic set of Au-Pd colloidal nanoparticles having a range of Au/Pd ratios. The catalysts have been structurally characterized using a combination of UV-visible spectroscopy, transmission electron microscopy, STEM HAADF/XEDS, and X-ray photoelectron spectroscopy. The Au-Pd nanoparticles are found in the majority of cases to be homogeneous alloys, although some variation is observed in the AuPd composition at high Pd/Au ratios. The optimum performance for the synthesis of hydrogen peroxide is observed for a catalyst having a Au/Pd 1:2 molar ratio. However, the competing hydrogenation reaction of hydrogen peroxide increases with increasing Pd content, although Pd alone is less effective than when Au is also present. Investigation of the oxidation of benzyl alcohol using these materials also shows that the optimum selective oxidation to the aldehyde occurs for the Au/Pd 1:2 molar ratio catalyst. These measured activity trends are discussed in terms of the structure and composition of the supported Au-Pd nanoparticles.

Au–Pd supported nanocrystals prepared by a sol immobilisation technique as catalysts for selective chemical synthesis

Catalysis by gold and gold-palladium nanoparticles has attracted significant research attention in recent years. These nanocrystalline materials have been found to be highly effective for selective and total oxidation, but in most cases the catalysts are prepared using precipitation or impregnation. We report the preparation of Au-Pd nanocrystalline catalysts supported on carbon prepared via a sol-immobilisation technique and these have been compared with Au-Pd catalysts prepared via impregnation. The catalysts have been evaluated for two selective chemical syntheses, namely, oxidation of benzyl alcohol and the direct synthesis of hydrogen peroxide. The catalysts have been structurally characterised using a combination of scanning transmission electron microscopy and X-ray photoelectron spectroscopy. The catalysts prepared using the sol immobilisation technique show higher activity when compared with catalysts prepared by impregnation as they are more active for both hydrogen peroxide synthesis and hydrogenation, and also for benzyl alcohol oxidation. The method facilitates the use of much lower metal concentrations which is a key feature in catalyst design, particularly for the synthesis of hydrogen peroxide.