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.
Gold nanocrystals absorbed on metal oxides have exceptional properties in oxidation catalysis, including the oxidation of carbon monoxide at ambient temperatures, but the identification of the active ...catalytic gold species among the many present on real catalysts is challenging. We have used aberration-corrected scanning transmission electron microscopy to analyze several iron oxide-supported catalyst samples, ranging from those with little or no activity to others with high activities. High catalytic activity for carbon monoxide oxidation is correlated with the presence of bilayer clusters that are ~0.5 nanometer in diameter and contain only ~10 gold atoms. The activity of these bilayer clusters is consistent with that demonstrated previously with the use of model catalyst systems.
Hydrogen peroxide (H₂O₂) is an important disinfectant and bleach and is currently manufactured from an indirect process involving sequential hydrogenation/oxidation of anthaquinones. However, a ...direct process in which H₂ and O₂ are reacted would be preferable. Unfortunately, catalysts for the direct synthesis of H₂O₂ are also effective for its subsequent decomposition, and this has limited their development. We show that acid pretreatment of a carbon support for gold-palladium alloy catalysts switches off the decomposition of H₂O₂. This treatment decreases the size of the alloy nanoparticles, and these smaller nanoparticles presumably decorate and inhibit the sites for the decomposition reaction. Hence, when used in the direct synthesis of H₂O₂, the acid-pretreated catalysts give high yields of H₂O₂ with hydrogen selectivities greater than 95%.
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).
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.
Au/C catalysts prepared using aqua regia as a solvent display superior activity in the hydrochlorination reaction of acetylene to vinyl chloride monomer. A synergistic effect between HCl and HNO3 is ...observed, which is linked to the nucleation of Au nanoparticles. Display omitted
► Impregnated Au/C catalyst with aqua regia as a solvent displayed superior activity in the hydrochlorination reaction of acetylene to vinyl chloride monomer. ► A synergistic effect between HCl and HNO3 is present, driving the Au nanoparticles nucleation process over the carbon support. ► The impregnating acid mixture can affect the carbon support enriching the presence of oxygen functional groups. ► The reaction occurs over Au3+ centres at the Au/C interface.
Au/C catalysts are effective materials for the gas phase hydrochlorination of acetylene to vinyl chloride monomer, and to date, the most effective catalyst preparation protocol makes use of impregnation using aqua regia. In the present study, the effect of this solvent is evaluated and discussed in detail by modifying the ratio of HCl and HNO3 and the temperature of the impregnation step. These factors are observed to affect the Au3+/Au0 ratio of the final catalyst, in addition to the modification of the functional groups of the carbon used as support. The results can be rationalised by the oxidation effect of HNO3 on both the gold nanoparticles and the functional groups on the carbon surface, as well as a nucleation effect of HCl towards gold over the carbon support.
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.
Small change with great effect: The addition of a small amount of Pt to a AuPd/CeO2 catalyst can significantly enhance its performance in the direct synthesis of H2O2. The contour diagram shows the productivity of catalysts with different metal ratios (blue/yellow: low/high productivity).
The identity of active species in supported gold catalysts for low temperature carbon monoxide oxidation remains an unsettled debate. With large amounts of experimental evidence supporting theories ...of either gold nanoparticles or sub-nm gold species being active, it was recently proposed that a size-dependent activity hierarchy should exist. Here we study the diverging catalytic behaviours after heat treatment of Au/FeO
materials prepared via co-precipitation and deposition precipitation methods. After ruling out any support effects, the gold particle size distributions in different catalysts are quantitatively studied using aberration corrected scanning transmission electron microscopy (STEM). A counting protocol is developed to reveal the true particle size distribution from HAADF-STEM images, which reliably includes all the gold species present. Correlation of the populations of the various gold species present with catalysis results demonstrate that a size-dependent activity hierarchy must exist in the Au/FeO
catalyst.
Hydrogen peroxide is a widely used chemical but is not very efficient to make in smaller than industrial scale. It is an important commodity chemical used for bleaching, disinfection, and chemical ...manufacture. At present, manufacturers use an indirect process in which anthraquinones are sequentially hydrogenated and oxidized in a manner that hydrogen and oxygen are never mixed. However, this process is only economic at a very large scale producing a concentrated product. For many years, the identification of a direct process has been a research goal because it could operate at the point of need, producing hydrogen peroxide at the required concentration for its applications. Research on this topic has been ongoing for about 100 years. Until the last 10 years, catalyst design was solely directed at using supported palladium nanoparticles. These catalysts require the use of bromide and acid to arrest peroxide decomposition, since palladium is a very active catalyst for hydrogen peroxide hydrogenation. Recently, chemists have shown that supported gold nanoparticles are active when gold is alloyed with palladium because this leads to a significant synergistic enhancement in activity and importantly selectivity. Crucially, bimetallic gold-based catalysts do not require the addition of bromide and acids, but with carbon dioxide as a diluent its solubility in the reaction media acts as an in situ acid promoter, which represents a greener approach for peroxide synthesis. The gold catalysts can operate under intrinsically safe conditions using dilute hydrogen and oxygen, yet these catalysts are so active that they can generate peroxide at commercially significant rates. The major problem associated with the direct synthesis of hydrogen peroxide concerns the selectivity of hydrogen usage, since in the indirect process this factor has been finely tuned over decades of operation. In this Account, we discuss how the gold–palladium bimetallic catalysts have active sites for the synthesis and hydrogenation of hydrogen peroxide that are different, in contrast to monometallic palladium in which synthesis and hydrogenation operate at the same sites. Through treatment of the support with acids prior to the deposition of the gold–palladium bimetallic particles, we can obtain a catalyst that can make hydrogen peroxide at a very high rate without decomposing or hydrogenating the product. This innovation opens up the way to design improved catalysts for the direct synthesis process, and these possibilities are described in this Account.
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