Li Lu

Palladium-tin catalysts for the direct synthesis of H2O2 with high selectivity

Direct hydrogen peroxide synthesis Hydrogen peroxide is synthesized industrially without direct contact of hydrogen and oxygen in order to achieve high concentrations. For many applications, only dilute aqueous solutions are needed. Freakley et al. report an improvement in the direct synthesis of hydrogen peroxide over using palladium-tin alloys. This catalyst still achieves selectivities of >95%, like palladium-gold alloys, but is cheaper and can suppress reactions that decompose the product. Science, this issue p. 965 Non-noble metal alloys with palladium can suppress decomposition reactions in direct hydrogen peroxide synthesis. The direct synthesis of hydrogen peroxide (H2O2) from H2 and O2 represents a potentially atom-efficient alternative to the current industrial indirect process. We show that the addition of tin to palladium catalysts coupled with an appropriate heat treatment cycle switches off the sequential hydrogenation and decomposition reactions, enabling selectivities of >95% toward H2O2. This effect arises from a tin oxide surface layer that encapsulates small Pd-rich particles while leaving larger Pd-Sn alloy particles exposed. We show that this effect is a general feature for oxide-supported Pd catalysts containing an appropriate second metal oxide component, and we set out the design principles for producing high-selectivity Pd-based catalysts for direct H2O2 production that do not contain gold.

The Direct Synthesis of Hydrogen Peroxide Using Platinum‐Promoted Gold–Palladium Catalysts

The direct synthesis of hydrogen peroxide offers a potentially green route to the production of this important commodity chemical. Early studies showed that Pd is a suitable catalyst, but recent work indicated that the addition of Au enhances the activity and selectivity significantly. The addition of a third metal using impregnation as a facile preparation method was thus investigated. The addition of a small amount of Pt to a CeO2-supported AuPd (weight ratio of 1:1) catalyst significantly enhanced the activity in the direct synthesis of H2O2 and decreased the non-desired over-hydrogenation and decomposition reactions. The addition of Pt to the AuPd nanoparticles influenced the surface composition, thus leading to the marked effects that were observed on the catalytic formation of hydrogen peroxide. In addition, an experimental approach that can help to identify the optimal nominal ternary alloy compositions for this reaction is demonstrated.

Identification of single-site gold catalysis in acetylene hydrochlorination

Gold-on-carbon catalysts are analogs of homogeneous gold catalysts that use a redox couple of Au(I) and Au(III) species. Supported gold ions The mercuric chloride catalyst for acetylene hydrochlorination creates vinyl chloride, an important polymer feedstock. However, a more environmentally friendly catalyst of gold supported on carbon can now replace mercuric chloride. Malta et al. used x-ray spectroscopic studies of the working catalysts and computational modeling to show that the active species are coexisting single-site Au+ and Au3+ cations. These species are analogs of soluble catalysts with single metal atoms that react via a similar redox couple. Science, this issue p. 1399 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.

Atomic-layered Au clusters on α-MoC as catalysts for the low-temperature water-gas shift reaction

Low-temperature CO removal Carbon monoxide deactivates fuel cell catalysts, so it must be removed from H2 generated from hydrocarbons on site. Yao et al. developed a catalyst composed of layered gold clusters on molybdenum carbide (MoC) nanoparticles to convert CO through its reaction with water into H2 and CO2 at temperatures as low as 150°C. Water was activated on MoC to form surface hydroxyl groups, which then reacted with CO adsorbed on the gold clusters. Science, this issue p. 389 Activation of water on α-MoC enables low-temperature reaction with CO adsorbed on gold clusters. The water-gas shift (WGS) reaction (where carbon monoxide plus water yields dihydrogen and carbon dioxide) is an essential process for hydrogen generation and carbon monoxide removal in various energy-related chemical operations. This equilibrium-limited reaction is favored at a low working temperature. Potential application in fuel cells also requires a WGS catalyst to be highly active, stable, and energy-efficient and to match the working temperature of on-site hydrogen generation and consumption units. We synthesized layered gold (Au) clusters on a molybdenum carbide (α-MoC) substrate to create an interfacial catalyst system for the ultralow-temperature WGS reaction. Water was activated over α-MoC at 303 kelvin, whereas carbon monoxide adsorbed on adjacent Au sites was apt to react with surface hydroxyl groups formed from water splitting, leading to a high WGS activity at low temperatures.

Stable amorphous georgeite as a precursor to a high-activity catalyst

Copper and zinc form an important group of hydroxycarbonate minerals that include zincian malachite, aurichalcite, rosasite and the exceptionally rare and unstable—and hence little known and largely ignored—georgeite. The first three of these minerals are widely used as catalyst precursors for the industrially important methanol-synthesis and low-temperature water–gas shift (LTS) reactions, with the choice of precursor phase strongly influencing the activity of the final catalyst. The preferred phase is usually zincian malachite. This is prepared by a co-precipitation method that involves the transient formation of georgeite; with few exceptions it uses sodium carbonate as the carbonate source, but this also introduces sodium ions—a potential catalyst poison. Here we show that supercritical antisolvent (SAS) precipitation using carbon dioxide (refs 13, 14), a process that exploits the high diffusion rates and solvation power of supercritical carbon dioxide to rapidly expand and supersaturate solutions, can be used to prepare copper/zinc hydroxycarbonate precursors with low sodium content. These include stable georgeite, which we find to be a precursor to highly active methanol-synthesis and superior LTS catalysts. Our findings highlight the value of advanced synthesis methods in accessing unusual mineral phases, and show that there is room for exploring improvements to established industrial catalysts.

Oxidation of Benzyl Alcohol and Carbon Monoxide Using Gold Nanoparticles Supported on MnO2 Nanowire Microspheres

MnO2 was synthesised as a catalyst support material using a hydrothermal method. This involved reacting MnSO4⋅H2O and (NH4)2S2O8 at 120 °C for a range of crystallisation times, which affords control over the morphology and phase composition of the MnO2 formed. Gold was deposited on these supports using sol-immobilisation, impregnation and deposition precipitation methods, and the resultant materials were used for the oxidation of benzyl alcohol and carbon monoxide. The effect of the support morphology on the dispersion of the gold nanoparticles and the consequent effect on the catalytic performance is described and discussed.

Investigation of the active species in the carbon-supported gold catalyst for acetylene hydrochlorination

The nature of the active species in carbon-supported gold catalysts used for synthesis of vinyl chloride monomer from acetylene hydrochlorination has been investigated using X-ray photoelectron spectroscopy and electron microscopy. Catalysts prepared by impregnation of chloroauric acid dissolved in aqua regia are initially inactive. During the initial reaction they show a pronounced induction period and we have used this opportunity to examine the evolution of the active catalyst as it is transformed during acetylene hydrochlorination. The fresh catalyst comprises a Au(III) surface film which on reaction with acetylene and HCl transforms to a mixture of Au(I)/Au(III). Experiments in which the catalyst is exposed sequentially to HCl and acetylene show that high activity is associated with a catalyst containing significant amounts of cations with both oxidation states and that the Au(I) and Au(III) oxidation cycle is important in the activation of both molecules. These findings are discussed in relation to the nature of the active species.

Biomanufacturing of CdS quantum dots

Nature provides powerful but as-yet largely unharnessed methods for low-cost, green synthesis of inorganic functional materials such as quantum dots. These materials have diverse applications from medicine to renewable energy. Harnessing natures unique ability to achieve cost effective and scalable manufacturing solutions with reduced environmental impact is integral to realizing a future biomanufacturing economy. To address this challenge, a bacterial strain has been engineered to enable biosynthesis of CdS nanocrystals with extrinsic crystallite size control in the quantum confinement range. This strain yields extracellular, water-soluble quantum dots from low-cost precursors at ambient temperatures and pressure. The biomanufacturing approach demonstrated here produces CdS semiconductor nanocrystals with associated size-dependent band gap and photoluminescent properties.

Light alkane oxidation using catalysts prepared by chemical vapour impregnation: tuning alcohol selectivity through catalyst pre-treatment

Fe/ZSM-5(30) catalysts have been prepared by chemical vapour impregnation (CVI) using iron(III) acetylacetonate as the precursor. These materials have been used for the oxidation of methane and ethane using aqueous hydrogen peroxide as oxidant. Heating in air leads to materials that exhibit high catalytic activity and give formic and acetic acid with high selectivity from methane and ethane respectively. Heat treatment of the uncalcined materials under a reducing atmosphere results in partial reduction of iron from the FeIII to FeII oxidation state with the majority of the iron being present as isolated octahedral extra-framework species having oxygen neighbours and showing no evidence of a coordination shell containing Al or Fe, as evidenced from studies using X-ray absorption and UV-Vis spectroscopies. These hydrogen treated catalysts show the same catalytic activity as their analogues formed by heating in air, but in contrast exhibit higher alcohol selectivities for both methane and ethane conversion to oxygenates and are reusable. Our findings for both the oxidation of methane and ethane indicate that the selectivity to the oxidation products, i.e. acids or alcohols, can be controlled by tuning the active site structure and/or oxidation state of the Fe species in Fe/ZSM-5.

Single-enzyme biomineralization of cadmium sulfide nanocrystals with controlled optical properties

Significance Biomineralization is a promising, yet complex, route toward the scalable and green biomanufacturing of functional nanomaterials, involving multiple biomolecules acting in unison to control mineralization and crystal structure. Unraveling the interdependent complexity of biomineralization is a barrier to completely realize this approach. We distill this complexity to a single enzyme that both catalyzes the formation of the reactive precursors required for mineralization and templates nanocrystal growth in solution. This is the first report of a single enzyme capable of providing all of the required functionality for active biomineralization from otherwise unreactive solution. This work provides insight into the mechanism of metal sulfide biomineralization and is an example of the elegance in green functional material synthesis achievable through engineered biomineralization. Nature has evolved several unique biomineralization strategies to direct the synthesis and growth of inorganic materials. These natural systems are complex, involving the interaction of multiple biomolecules to catalyze biomineralization and template growth. Herein we describe the first report to our knowledge of a single enzyme capable of both catalyzing mineralization in otherwise unreactive solution and of templating nanocrystal growth. A recombinant putative cystathionine γ-lyase (smCSE) mineralizes CdS from an aqueous cadmium acetate solution via reactive H2S generation from l-cysteine and controls nanocrystal growth within the quantum confined size range. The role of enzymatic nanocrystal templating is demonstrated by substituting reactive Na2S as the sulfur source. Whereas bulk CdS is formed in the absence of the enzyme or other capping agents, nanocrystal formation is observed when smCSE is present to control the growth. This dual-function, single-enzyme, aerobic, and aqueous route to functional material synthesis demonstrates the powerful potential of engineered functional material biomineralization.