The effect of polymer addition on base catalysed glycerol oxidation using gold and gold-palladium bimetallic catalysts

The oxidation of glycerol represents both a viable route to catalytic upgrading of biomass and has become a model reaction for catalytic polyol oxidation. Gold and gold–palladium nanoparticle catalysts prepared by colloidal methods involving polymer additives have been extensively studied. However, the effect of residual polymer at the catalyst surface on reaction pathways has not been decoupled from particle size effects. We show that when using catalysts prepared without polymer stabilisers the addition of either polyvinyl alcohol or polyvinylpyrrolidone to the reaction changes the reaction rate and results in a change in reaction selectivity. We conclude that the polymer additive has a significant effect on the reaction pathway and that these systems should be considered as a metal surface–polymer interface catalytic systems and properties should not be rationalised solely based on nanoparticle size

Plasmonic oxidation of glycerol using AuPd/TiO₂ catalysts

AuPd nanoparticles supported on P25 TiO2 (AuPd/TiO2) were prepared by a facile sol-immobilisation method and investigated for surface plasmon-assisted glycerol oxidation under base-free conditions. The AuPd/TiO2 samples were characterized by UV-vis spectroscopy and transmission electron microscopy. The sol-immobilisation method readily permitted the Au : Pd molar ratio to be changed over a wide range whilst keeping the mean particle size of the AuPd nanoparticles at 3 nm. Visible light irradiation during the reaction has a beneficial effect on the conversion of glycerol with the most marked effect being observed with gold-rich catalysts and the increase of conversion on light irradiation increases linearly with the gold content of the nanoparticles. The reaction selectivity is also affected by the plasmon-assisted oxidation and glycolic acid, not observed during the dark reactions, was observed for all illuminated reactions due to the enhanced activity of these catalysts

Plasmonic oxidation of glycerol using AuPd/TiO₂ catalysts

AuPd nanoparticles supported on P25 TiO2 (AuPd/TiO2) were prepared by a facile sol-immobilisation method and investigated for surface plasmon-assisted glycerol oxidation under base-free conditions. The AuPd/TiO2 samples were characterized by UV-vis spectroscopy and transmission electron microscopy. The sol-immobilisation method readily permitted the Au : Pd molar ratio to be changed over a wide range whilst keeping the mean particle size of the AuPd nanoparticles at 3 nm. Visible light irradiation during the reaction has a beneficial effect on the conversion of glycerol with the most marked effect being observed with gold-rich catalysts and the increase of conversion on light irradiation increases linearly with the gold content of the nanoparticles. The reaction selectivity is also affected by the plasmon-assisted oxidation and glycolic acid, not observed during the dark reactions, was observed for all illuminated reactions due to the enhanced activity of these catalysts.</p

Direct synthesis of hydrogen peroxide in water at ambient temperature

The direct synthesis of hydrogen peroxide (H2O2) from hydrogen and oxygen has been studied using an Au–Pd/TiO2 catalyst. The aim of this study is to understand the balance of synthesis and sequential degradation reactions using an aqueous, stabilizer-free solvent at ambient temperature. The effects of the reaction conditions on the productivity of H2O2 formation and the undesirable hydrogenation and decomposition reactions are investigated. Reaction temperature, solvent composition and reaction time have been studied and indicate that when using water as the solvent the H2O2 decomposition reaction is the predominant degradation pathway, which provides new challenges for catalyst design, which has previously focused on minimizing the subsequent hydrogenation reaction. This is of importance for the application of this catalytic approach for water purification

Identification of single-site gold catalysis in acetylene hydrochlorination

There remains considerable debate over the active form of gold under operating conditions of a recently validated gold catalyst for acetylene hydrochlorination. We have performed an in situ x-ray absorption fine structure study of gold/carbon (Au/C) catalysts under acetylene hydrochlorination reaction conditions and show that highly active catalysts comprise single-site cationic Au entities whose activity correlates with the ratio of Au(I):Au(III) present. We demonstrate that these Au/C catalysts are supported analogs of single-site homogeneous Au catalysts and propose a mechanism, supported by computational modeling, based on a redox couple of Au(I)-Au(III) species. View Full Tex

Acceptorless dehydrogenation of 1-phenylethanol using Pd/TiO2 catalysts prepared by sol immobilisation

Reversible hydrogenation and acceptorless dehydrogenation of organic molecules would enable pre-existing technologies and infrastructure to be leveraged for the storage and transport of hydrogen. We report the synthesis of active Pd/TiO2 catalysts prepared by sol immobilisation for the acceptorless dehydrogenation of 1-phenylethanol to acetophenone with evolution of molecular hydrogen. The catalyst converts 1-phenylethanol with a TOF of 3600 mol1-PE h−1 molPd−1 while maintaining high selectivity (>95%) at over 80% conversion. This represents a 3.5 x increase in catalytic activity with comparable selectivity over commercially available 5% Pd/C catalysts

Gas-Phase Deposition of Gold Nanoclusters to Produce Heterogeneous Glycerol Oxidation Catalysts

Gold nanoparticles prepared by colloidal methods are effective catalysts for selective glycerol oxidation under basic conditions. Large-scale synthesis of catalysts by wet chemical methods leads to large amounts of waste and can result in polymer or salt residues remaining on the catalyst. In contrast, gas-phase cluster deposition (cluster beam deposition) offers a solvent-free method to synthesize controlled nanoparticles/clusters. We show that the deposition of bare gas-phase gold clusters onto carbon powder leads to a catalyst comparable to that prepared by colloidal methods. This shows the feasibility of the synthesis method to produce oxidation catalysts with reduced waste

Methane Oxidation to Methanol in Water

ConspectusMethane represents one of the most abundant carbon sources for fuel or chemical production. However, remote geographical locations and high transportation costs result in a substantial proportion being flared at the source. The selective oxidation of methane to methanol remains a grand challenge for catalytic chemistry due to the large energy barrier for the initial C-H activation and prevention of overoxidation to CO2. Indirect methods such as steam reforming produce CO and H2 chemical building blocks, but they consume large amounts of energy over multistage processes. This makes the development of the low-temperature selective oxidation of methane to methanol highly desirable and explains why it has remained an active area of research over the last 50 years.The thermodynamically favorable oxidation of methane to methanol would ideally use only molecular oxygen. Nature effects this transformation with the enzyme methane monooxygenase (MMO) in aqueous solution at ambient temperature with the addition of 2 equiv of a reducing cofactor. MMO active sites are Fe and Cu oxoclusters, and the incorporation of these metals into zeolitic frameworks can result in biomimetic activity. Most approaches to methane oxidation using metal-doped zeolites use high temperature with oxygen or N2O; however, demonstrations of catalytic cycles without catalyst regeneration cycles are limited. Over the last 10 years, we have developed Fe-Cu-ZSM-5 materials for the selective oxidation of methane to methanol under aqueous conditions at 50 °C using H2O2 as an oxidant (effectively O2 + 2 reducing equiv), which compete with MMO in terms of activity. To date, these materials are among the most active and selective catalysts for methane oxidation under this mild condition, but industrially, H2O2 is an expensive oxidant to use in the production of methanol.This observation of activity under mild conditions led to new approaches to utilize O2 as the oxidant. Supported precious metal nanoparticles have been shown to be active for a range of C-H activation reactions using O2 and H2O2, but the rapid decomposition of H2O2 over metal surfaces limits efficiency. We identified that this decomposition could be minimized by removing the support material and carrying out the reaction with colloidal AuPd nanoparticles. The efficiency of methanol production with H2O2 consumption was increased by 4 orders of magnitude, and crucially it was demonstrated for the first time that molecular O2 could be incorporated into the methanol produced with 91% selectivity. The understanding gained from these two approaches provides valuable insight into possible new routes to selective methane oxidation which will be presented here in the context of our own research in this area.</p

Addressing stability challenges of using bimetallic electrocatalysts: the case of gold?palladium nanoalloys

Bimetallic catalysts are known to often provide enhanced activity compared to pure metals, due to their electronic, geometric and ensemble effects. However, applied catalytic reaction conditions may induce restructuring, metal diffusion and dealloying. This gives rise to a drastic change in surface composition, thus limiting the application of bimetallic catalysts in real systems. Here, we report a study on dealloying using an AuPd bimetallic nanocatalyst (1 : 1 molar ratio) as a model system. The changes in surface composition over time are monitored in situ by cyclic voltammetry, and dissolution is studied in parallel using online inductively coupled plasma mass spectrometry (ICP-MS). It is demonstrated how experimental conditions such as different acidic media (0.1 M HClO4 and H2SO4), different gases (Ar and O-2), upper potential limit and scan rate significantly affect the partial dissolution rates and consequently the surface composition. The understanding of these alterations is crucial for the determination of fundamental catalyst activity, and plays an essential role for real applications, where long-term stability is a key parameter. In particular, the findings can be utilized for the development of catalysts with enhanced activity and/or selectivity

Methane Oxidation to Methanol in Water

Conspectus Methane represents one of the most abundant carbon sources for fuel or chemical production. However, remote geographical locations and high transportation costs result in a substantial proportion being flared at the source. The selective oxidation of methane to methanol remains a grand challenge for catalytic chemistry due to the large energy barrier for the initial C–H activation and prevention of overoxidation to CO2. Indirect methods such as steam reforming produce CO and H2 chemical building blocks, but they consume large amounts of energy over multistage processes. This makes the development of the low-temperature selective oxidation of methane to methanol highly desirable and explains why it has remained an active area of research over the last 50 years. The thermodynamically favorable oxidation of methane to methanol would ideally use only molecular oxygen. Nature effects this transformation with the enzyme methane monooxygenase (MMO) in aqueous solution at ambient temperature with the addition of 2 equiv of a reducing cofactor. MMO active sites are Fe and Cu oxoclusters, and the incorporation of these metals into zeolitic frameworks can result in biomimetic activity. Most approaches to methane oxidation using metal-doped zeolites use high temperature with oxygen or N2O; however, demonstrations of catalytic cycles without catalyst regeneration cycles are limited. Over the last 10 years, we have developed Fe-Cu-ZSM-5 materials for the selective oxidation of methane to methanol under aqueous conditions at 50 °C using H2O2 as an oxidant (effectively O2 + 2 reducing equiv), which compete with MMO in terms of activity. To date, these materials are among the most active and selective catalysts for methane oxidation under this mild condition, but industrially, H2O2 is an expensive oxidant to use in the production of methanol. This observation of activity under mild conditions led to new approaches to utilize O2 as the oxidant. Supported precious metal nanoparticles have been shown to be active for a range of C–H activation reactions using O2 and H2O2, but the rapid decomposition of H2O2 over metal surfaces limits efficiency. We identified that this decomposition could be minimized by removing the support material and carrying out the reaction with colloidal AuPd nanoparticles. The efficiency of methanol production with H2O2 consumption was increased by 4 orders of magnitude, and crucially it was demonstrated for the first time that molecular O2 could be incorporated into the methanol produced with 91% selectivity. The understanding gained from these two approaches provides valuable insight into possible new routes to selective methane oxidation which will be presented here in the context of our own research in this area