Hydrogen peroxide (H2O2) is a highly effective, green oxidant that has found application in sectors ranging from the synthesis of fine chemicals and waste stream treatment to the extraction of precious metals and the bleaching of paper pulp and textiles. The growing demand for H2O2 has seen it become one of the 100 most important chemicals in the world. The direct synthesis of H2O2 from H2 and O2 has been a challenge for the scientific community for over 100 years and represents an attractive alternative to the current means of production. Herein we discuss the historical perspective of the direct synthesis process, the recent literature regarding catalyst design and the role of additives as well as the application of H2O2 as an in situ oxidant. We discuss the key problems that remain and conclude that although there has been progress with respect to the selectivity of hydrogen utilisation, there is a need to now concentrate on catalyst activity as the key remaining problem requiring a solution is the concentration of H2O2 that can be achieved especially in flow reactors.

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Recent Advances in the Direct Synthesis of H2O2

Hydrogen peroxide is an important chemical of increasing demand in today’s world Currently, the anthraquinone autoxidation process dominates the industrial production of hydrogen peroxide Herein, hydrogen and oxygen are reacted indirectly in the presence of quinones to yield hydrogen peroxide Owing to the complexity and multi-step nature of the process, it is advantageous to replace the process with an easier and straightforward one The direct synthesis of hydrogen peroxide from its constituent reagents is an effective and clean route to achieve this goal Factors such as water formation due to thermodynamics, explosion risk, and the stability of the hydrogen peroxide produced hinder the applicability of this process at an industrial level Currently, the catalysis for the direct synthesis reaction is palladium based and the research into finding an effective and active catalyst has been ongoing for more than a century now Palladium in its pure form, or alloyed with certain metals, are some of the new generation of catalysts that are extensively researched Additionally, to prevent the decomposition of hydrogen peroxide to water, the process is stabilized by adding certain promoters such as mineral acids and halides A major part of today’s research in this field focusses on the reactor and the mode of operation required for synthesizing hydrogen peroxide The emergence of microreactor technology has helped in setting up this synthesis in a continuous mode, which could possibly replace the anthraquinone process in the near future This review will focus on the recent findings of the scientific community in terms of reaction engineering, catalyst and reactor design in the direct synthesis of hydrogen peroxide

Recent Advances in the Direct Synthesis of Hydrogen Peroxide Using Chemical Catalysis—A Review

The direct synthesis of H2O2 is a dream reaction in the field of selective oxidation and green chemical synthesis. However, the unknown active state of the catalyst and the lack of a defined catalytic mechanism preclude the design and optimization of suitable catalysts and reactor setups. Here direct synthesis of H2O2 over Pd–TiO2 in water was investigated in a continuous flow reactor setup utilizing undiluted oxygen and hydrogen to increase aqueous-phase concentrations safely at ambient temperature and a pressure of 10 bar. In this experiment operando X-ray absorption spectroscopy (XAS) and online flow injection analysis for photometric quantification of H2O2 were combined to build catalyst structure–activity relationships in direct H2O2 synthesis. XAS at the Pd K absorption edge was used to observe the oxidation state and local Pd structure together with H2O2 production for three reactant ratio (H2/O2) regimes: hydrogen rich (>2), hydrogen lean (<0.5), and balanced (0.5–2). During H2O2 production, oxyge...

/pdf/revealing-the-structure-and-mechanism-of-palladium-during-3jdz42ezg7.pdf

Revealing the Structure and Mechanism of Palladium during Direct Synthesis of Hydrogen Peroxide in Continuous Flow Using Operando Spectroscopy

Palladium nanoparticles have been studied extensively as catalysts for the direct synthesis of hydrogen peroxide, where selectivity remains a key challenge. Alloying Pd with other metals and the use of acid and halide promoters are commonly used to increase H2O2 selectivity, however; the sites that can selectively produce H2O2 have not been identified and the role of these additives remains unclear. Here, we report the synthesis of atomically dispersed Pd/C as a model catalyst for H2O2 production without the presence of extended Pd surfaces. We show that these isolated cationic Pd sites can form H2O2 with significantly higher selectivity than metallic Pd nanoparticles in both the reaction of H2 and O2 and the electrochemical oxygen reduction reaction (ORR). This demonstrates that catalysts containing high populations of isolated Pd sites are selective catalysts for this two-electron reduction reaction and that the performance of materials in the direct synthesis reaction and ORR have many similarities.

/pdf/isolated-pd-sites-as-selective-catalysts-for-electrochemical-158iglkgr1.pdf

Isolated Pd Sites as Selective Catalysts for Electrochemical and Direct Hydrogen Peroxide Synthesis

Highly selective hydrogen peroxide (H2O2) synthesis directly from H2 and O2 is a strongly desired reaction for green process. We report a highly efficient palladium-tellurium (Pd-Te/TiO2) catalyst with a selectivity of ~100% toward H2O2 under mild conditions (283 K, 0.1 MPa and a semi-batch continuous flow reactor). The size of Pd particles was remarkably reduced from 2.1 nm to 1.4 nm with the addition of Te; meanwhile, the surface Pd atoms modified by Te could significantly weaken the dissociative activation of O2, leading to the non-dissociative hydrogenation of O2. Density functional theory (DFT) calculations illuminated the critical role of Te in the selective hydrogenation of O2 that the active sites composed of Pd and Te could significantly restrain the side reactions. This work has made a significant progress on the development of high-selectivity catalysts for the direct synthesis of H2O2 at ambient pressure.

Direct and Selective Synthesis of Hydrogen Peroxide over Palladium-Tellurium Catalysts at Ambient Pressure.

The origin of palladium particle size effects in the direct synthesis of H2O2: Is smaller better?

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The origin of palladium particle size effects in the direct synthesis of H2O2: Is smaller better?