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.

Globally, water disinfection is reliant on chlorination but a route that avoids the formation of chemical residues would be preferred. Hydrogen peroxide, can offer such an alternative, is a broad-spectrum biocide but typically is less effective than traditional approaches to water remediation. Here, we show that the reactive species-including hydroxyl, hydroperoxyl and superoxide radicals-formed over a AuPd catalyst during the synthesis of hydrogen peroxide from hydrogen and air are over 10 7 times more potent than an equivalent amount of pre-formed hydrogen peroxide and over 10 8 times more effective than chlorination under equivalent conditions. The key to bactericidal and virucidal efficacy is the radical flux that forms when hydrogen and oxygen are activated on the catalyst. This approach can form the basis of an alternative method for water disinfection particularly in communities not currently served by traditional means of water remediation or where access to potable water is scarce.

The direct synthesis of hydrogen peroxide (HO) from hydrogen and oxygen has been studied using an Au-Pd/TiO 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 HO 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 HO 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.

The syntheses of five new heteroleptic iridium complexes [Ir(L(1-4))(2)(Diobpy)]PF(6) (where Diobpy = 4,4'-dioctylamido-2,2'-bipyridine) and [Ir(L(3))(2)(bpy)]PF(6) (where L = para-substituted 2,3-diphenylquinoxaline cyclometalating ligands; bpy = 2,2'-bipyridine) are described. The structures of [Ir(L(3))(2)(Diobpy)]PF(6) and [Ir(L(3))(2)(bpy)]PF(6) show that the complexes each adopt a distorted octahedral geometry with the expected trans-N, cis-C arrangement of the cyclometalated ligands. Electrochemical studies confirmed subtle perturbation of the Ir(III/IV) redox couple as a function of ligand variation. Luminescence studies showed the significant contribution of (3)MLCT to the phosphorescent character with predictable and modestly tunable emission wavelengths between 618 and 636 nm. DFT studies provided approximate qualitative descriptions of the HOMO {located over the Ir(5d) center (11-42%) and the phenylquinoxaline ligand (54-87%)} and LUMO {located over the ancillary bipyridine ligands (ca. 93%)} energy levels of the five complexes, confirming significant MLCT character. TD-DFT calculations indicate that UV-vis absorption and subsequent emission has substantial MLCT character, mixed with LLCT. Predicted absorption and emission wavelengths are in good general agreement with the UV-vis and luminescence experiments.

The direct synthesis of hydrogen peroxide (H 2 O 2 ) from molecular H 2 and O 2 offers an attractive, decentralized alternative to production compared to the current means of production, the anthraquinone process. Herein we evaluate the performance of a 0.5%Pd–4.5%Ni/TiO 2 catalyst in batch and flow reactor systems using water as a solvent at ambient temperature. These reaction conditions are considered challenging for the synthesis of high H 2 O 2 concentrations, with the use of sub-ambient temperatures and alcohol co-solvents typical. Catalytic activity was observed to be stable to prolonged use in multiple batch experiments or in a flow system, with selectivities towards H 2 O 2 of 97% and 85%, respectively. This study was carried out in the absence of halide or acid additives that are typically used to inhibit sequential H 2 O 2 degradation reactions showing that this Pd–Ni catalyst has the potential to produce H 2 O 2 selectively. This article is part of a discussion meeting issue ‘Science to enable the circular economy’.