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

Abstract

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.

1. Introduction

Hydrogen peroxide (H₂O₂) is a colorless, odorless, and slightly acidic liquid used mainly as an oxidant in chemical synthesis [1,2,3]. It is an atom efficient, benign, and eco-friendly oxidant that produces water or oxygen as a degradation product, depending on the catalyst used [4,5]. Commercial H₂O₂ production has a concentration range of 30–70% and the areas of application depend on the concentration of H₂O₂ used. Usually, household, medical/dental, and cosmetic applications need diluted concentrations of Ca. 3–5% H₂O₂. Higher concentrations of up to 70% are needed for synthesis, wastewater treatment, mining, and bleaching applications. The electronics industry needs higher H₂O₂ concentrations (ranging from 70–90%) for cleaning and anti-corrosion purposes. Finally, the concentrated versions of 90–98% are used for military and aerospace purposes [6]. Given its wide range of applications in almost every aspect of human life, the method of H₂O₂ production in the industry is of utmost importance. The demand for H₂O₂ is always increasing with recent processes preferring H₂O₂ as an oxidant. H₂O₂ production can be done using chemical, electrochemical, enzymatic, or photocatalytic means. Of these routes, only the chemical processes are capable of industrial production [3,7] in an economical manner and only these will be discussed in detail in this work.

1.1. Industrial H₂O₂ Manufacture—A Historical Perspective

To discuss the chronological advancements in industrial H₂O₂ production, one has to split the progress into electrochemical and chemical methods. A detailed description of the processes has been summarized in the works of Goor [1,3] and Jones [8] (

Table 1. Historical development in the industrial manufacture of hydrogen peroxide.

Year	Inventor	Catalysis Type	Details
1818	L.J. Thenard	Chemical	Barium peroxide reacts with hydrochloric acid to form barium chloride and H2O2. Yield of 3% H2O2 was 2000 tons/annum (t/a) (Scheme 1a) [9,10]
1853	H. Meidinger	Electrochemical	Electrolysis of sulphuric acid to yield H2O2 [11] (Scheme 1b)
1878	M. Berthelot	Electrochemical	Elucidated the mechanism of sulphuric acid electrolysis. Reported the formation of peroxodisulphuric acid as an intermediate [12] (Scheme 1b)
1901	W. Manchot	Chemical	Autoxidation of hydroquinones and hydrazobenezenes under alkaline conditions in the presence of molecular oxygen to yield H2O2 [13,14,15] (Scheme 1e)
1908	Degussa-Weissenstein Process	Electrochemical	First production plant set up in Wiessenstein, Austria [16,17]
1910	Münchner Process or the Pietzsch-Adolph Process	Electrochemical	Developed by Pietzsch and Adolph at the Elektrochemische Werke, Munich. Used potassium peroxodisulphate instead of sulphuric acid to produce H2O2 [16,17] (Scheme 1c)
1914	Hugo Henkel and Walter Weber	Chemical	First account of direct synthesis of hydrogen peroxide using its constituent gases [18]
1924	Reidl-Löwenstein Process	Electrochemical	Similar to the Pietzsch-Adolph Process; used ammonium peroxodisulphate to produce H2O2 by electrolysis (Scheme 1d). Yield of 100% H2O2 was 35 kt [16,17]
1932	Walton and Filson	Chemical autoxidation	Published their work on the alternate oxidation and reduction of hydrazobenzenes to produce H2O2 [19] (Scheme 1e)
1935	Pfleiderer, Baden Aniline and Soda Factory (BASF)	Chemical autoxidation	Alkaline autoxidation of hydrazobenzenes to form sodium peroxide, later hydrolysed to form H2O2 [20] (Scheme 1f)
1935–1945	Riedl and Pfleiderer	Chemical autoxidation	Anthraquinone autoxidation process (AO) set up in two different cities each with 2000 t capacity [16,21] (Scheme 2a)
1953	E.I. du Pont de Nemors	Chemical autoxidation	Commercial plant setup with based on the Riedl and Pfleiderer process [1,3,8] (Scheme 2a)
1957–1980	Shell process	Chemical autoxidation	Oxidation of 2-propanol to yield H2O2 at a capacity of 15 kt [22] (Scheme 1g)

) and the reaction schemes are depicted in Scheme 1.

1.2. State of the Art in the Industrial Production of H₂O₂

On an industrial scale, the anthraquinone autoxidation (AO) process described by H.J. Riedl and G. Pfleiderer produces H₂O₂ in a cyclic manner (Scheme 2).

Scheme 2a shows the reaction mechanism of the AO process and Scheme 2b depicts the practice of H₂O₂ manufacture in the industry. The hydrogenation chamber is usually a slurry reactor consisting of the alkylated anthraquinone dissolved in a so-called “working solution” along with a catalyst. Hydrogen gas is initially fed through the reactor for hydrogenation to alkyl anthrahydroquinol. After this, the contents of the hydrogenation chamber are degassed in a separate chamber to remove traces of hydrogen. Following this step, the alkyl anthrahydroquinol is transferred to the oxygenation chamber. Here, oxygen or air is used to produce H₂O₂ and alkyl anthrahydroquninone, which is then transferred to a second degassing chamber to remove trace oxygen. The degassed solution is then extracted with water to yield H₂O₂ and the working solution is transferred to the hydrogenation chamber to complete the synthetic cycle. In the mid-1990s, the world capacity of 100% H₂O₂ was approximately 1.5 million tons with an average plant capacity of around 20 kt–40 kt per annum. By 2015, the capacity was 5.5 million tons with a plant capacity of 300 kt per annum [25]. Solvay (30%), Evonik (20%), and Arkema (13%) represented the global contributors of H₂O₂ [26].

1.3. Why Is There an Increased Interest in the Direct Synthesis of Hydrogen Peroxide?

Currently, the industrial manufacture of H₂O₂ is based on the work of Riedl and Pfleiderer, also known as the AO process [21] (Scheme 2) [24] using polynuclear hydroquinones [1]. Although this process is capable of meeting the world’s H₂O₂ demand, it does have certain drawbacks such as:

• excessive use of solvents for the process
• a negative environmental impact owing to the production of unwanted waste
• complex and multi-step process
• mass transfer limitations and low efficiency
• transport limitations of reactants between reactors
• organic contamination of H₂O₂ stemming from organic solvents or hydroquinones/hydroquinols during liquid-liquid extraction [27,28].

Considering environmental issues and resource conservation aspects, academia and the industry have set out to design benign and non-polluting processes. The principles of green chemistry helps in achieving this goal. The principles outlined in the 1990s clearly state that it is necessary to design and execute industrial processes that are clean, benign, non-polluting, and safe [29,30,31,32]. Two important terms, atom utilization and E-factor, are important in assessing the greenness of a process. Atom utilization, atom efficiency or atom selectivity (AE) maybe defined as the actual mass of reactants that actually end up in the final desired product, the rest of which is termed as “waste”. AE helps in assessing the amount of wastes generated by a certain process, which in turn, will determine the E-factor. The E-factor is the ratio of the amount of waste produced to the amount of desired product [33,34,35]. Considering the AO process, the amount of solvents and the alkylated hydroquinones used in the process are waste products, as the reaction is not atom efficient [1,3,7,8]. This brings up an important question: why is the AO process still practiced at an industrial level if it is not sustainable? The answer: operating the AO process is economically feasible at a scale of 1 × 10⁵ tons per annum producing high concentrations of H₂O₂, which are diluted prior to use. For a majority of the applications mentioned previously in the introductory section (Section 1), diluted versions of H₂O₂ (typically 3–8%) is required, in small amounts, and on site. To circumvent the issues mentioned above and promote a green H₂O₂ production process, the direct synthesis of H₂O₂ from H₂ and O₂ was researched, but only at the laboratory scale. Theoretically, it is clear that the direct synthesis approach is the simplest way to synthesise H₂O₂; however, issues with respect to practicality limit the industrialization of this process [36,37]. This review will address the challenges and developments in the field of direct synthesis of H₂O₂, focussing on chemical catalytic methods.

2. The Direct Synthesis Approach to H₂O₂ Production Using Chemical Catalysis

In 1914, H. Henkel and W. Weber reported the very first process that was capable of producing H₂O₂ from hydrogen (H₂) and oxygen (O₂). The patent described the reaction of two gaseous mixtures: an oxygen species (free and bound) and hydrogen in a pressurised vessel along with water. An important aspect of the patent was the use of noble metals capable of fixing hydrogen as catalysts; e.g., palladium (Pd), platinum (Pt), nickel (Ni), etc. [18].

2.1. Pros and Cons of the Direct Synthesis Approach to H₂O₂ Synthesis

The direct synthesis approach has not been industrially practiced due to several technological and scientific barriers [38] and

Table 2. Analysis of the advantages/disadvantages of the direct synthesis approach [27,39,40,41,42].

Advantages	Disadvantages
Absence of organic substrates such as anthraquinones or organic solvents	Unselective reactions leading to simultaneous side products other than H2O2, namely water (H2O)
Usage of green solvents like water, methanol, or ethanol	Complex process with mass transfer limitations involving three phases: gas (H2/O2), liquid (reaction medium), and solid (catalyst)
Economical because of fewer downstream operations to produce H2O2	Safety: explosive nature of the H2 and O2 mixture over a wide range of concentrations (4 mol %–94 mol %)
The whole process can be accomplished with a single reactor system	Presence of chloride and/or bromide ions in the reaction medium

lists the advantages and disadvantages of this approach.

Scheme 3 depicts the direct synthesis of H₂O₂. As seen, the reaction produces either water H₂O or H₂O₂ depending on the reaction conditions.

2.2. Mechanism of the Direct Synthesis of H₂O₂

The mechanism of hydrogen peroxide synthesis using H₂ and O₂ is shown in Scheme 4. A possible mechanism of such a synthesis was proposed by Bianchi et al. in 1999 [46], based on the 1980 report published by Zudin et al. [47]. While Zudin et al. used palladium triphenylphosphane in a biphasic system, Bianchi et al. found out that 2,9-dimethyl-4,7-diphenyl-1,10-phenantroline ligand was the best among other ligands tested. Based on these findings, Werner published the proposed mechanism of H₂O₂ synthesis by the reduction of dioxygen [48]. Stahl et al. used a bathocuproine palladium complex in order to catalyse the direct synthesis of H₂O₂ [49]. All three processes utilised acid halides such as hydrochloric acid or hydrogen bromide to facilitate efficient catalysis [46,47,49]. However, the actual mechanism was reported in 2001 by Stahl et al. [49] Scheme 4(i), which was confirmed by Chinta and Lunsford in 2004 using tetrachloropalladate (PdCl₄²⁻) prepared prior to use (Scheme 4(ii)) by replacing nitrogen ligands with chloride [50].

The mechanism follows these steps: initially, Pd⁰ reduces molecular oxygen in the presence of the nitrogen ligands (Scheme 4(i))/halide ions (Scheme 4(ii)) to form the respective Pd^II complex that contains a peroxo-species. This is replaced by halides yielding H₂O₂. Subsequently, Pd^II is reduced by molecular hydrogen to yield Pd⁰, thereby completing the catalytic cycle.

2.3. Series of Elementary Steps in H₂O₂ Synthesis

Several accounts of the possible elementary steps during the direct synthesis of H₂O₂ have been described in literature by Wilson and Flaherty [40], Plauck et al. [51] and Yi et al. [52], to name a few. Scheme 5 depicts the series of steps that lead to the synthesis of H₂O₂ and H₂O during the direct synthesis approach using Pd catalysts in liquid solvents. In each of the steps in the process, bimolecular reactions exist between the H⁺ and chemically adsorbed intermediates on the catalyst surface. The elementary steps are based on the following assumptions:

• Free energies of H₂ and O₂ adsorption are negligible under saturation conditions
• The adsorption and desorption of the H₂O₂ species is unrestricted

Based on these assumptions, the first step is that the hydrogen adsorbs dissociatively on to the catalyst surface (Step (1) in Scheme 5) yielding H* and is subsequently oxidized (Step (2) in Scheme 5). In Step (3) of Scheme 5, the molecular adsorption of O₂ takes place. This initiates Step (4) in Scheme 5, wherein O₂** undergoes proton-electron transfer under quasi-equilibration conditions to form OOH** (hydroperoxy radical). Alternatively, O₂** cleaves the O-O bond irreversibly to form O*, also known as chemi-absorbed oxygen atoms (Step (7) Scheme 5). The OOH** is then adsorbed, which then reacts further to form either H₂O₂** by proton-electron transfer, as shown in Step (5) of Scheme 5, or dissociates itself (Step (8) of Scheme 5). This finally leads to the release of H₂O₂ (Step (6), Scheme 5) or H₂O (Step (11), Scheme 5) [40,51,52].

2.4. Process Conditions for the Direct Synthesis of H₂O₂ from H₂ and O₂

The direct synthesis of H₂O₂ requires certain operating conditions such as the ratio of H₂ and O₂, reaction medium for the synthesis, the reactor used, additives and/or promoters, a catalyst, and its supporting material [53,54,55]. The following sections explain the influence of each of these parameters on the conversion and yield of the direct synthesis of H₂O₂.

2.4.1. Ratio of the Gaseous Mixture

During direct synthesis, one would expect the reaction of H₂ and O₂ on a catalytic surface to form only H₂O₂ as the product. However, unwanted side reactions also occur that reduce the productivity of this otherwise green process. The first side reaction is the oxidation of H₂ to H₂O instead of H₂O₂ and the second one is the reduction of the H₂O₂ produced to H₂O (Scheme 3). One can influence the selectivity of the process by optimising the parameters mentioned previously, but it is self-explanatory that the amount of H₂ and O₂ in the reaction mixture directly influences the H₂O₂ output. Three combinations of H₂/O₂ are possible for this reaction excess H₂, excess O₂, and stoichiometric amounts. Using excess H₂ would favour the reduction of H₂O₂, while using stoichiometric amounts would increase H₂O₂ concentration during synthesis. However, an excess of oxygen, up to three times compared to hydrogen, would also increase the selectivity and yield of the direct synthesis approach [56]. It is worth mentioning that the flammable and explosive nature of these two gases over a wide concentration range at 25 °C and 0.1 (MPa) (1 atmospheric pressure) is a point of great concern. The flammability limit for H₂ in O₂ is 4% (lower flammability limit) to 94% (upper flammability limit), while the detonation limit is at 15% to 95%, with an increased risk of explosion with increasing pressure [42,54,57]. DuPont faced frequent explosions in their pilot plant by feeding 10% H₂ in O₂ to their process, which led to the discontinuation of the pilot plant studies [27,54].

In order to minimise the explosion risk, it is suitable to perform the reaction at lowered feed rates of H₂ and O₂, diluted with inert gases [54]. The most common diluents are helium (He), argon (Ar), nitrogen (N₂), or carbon dioxide (CO₂) [26]. Most of the recent literature indicates the use of either CO₂ [44,58,59] or N₂ [39,60,61]. The work of Wilson and Flaherty described the use of N₂ and CO₂ as the diluents during synthesis using palladium catalysts (Pd) supported on silica (Si). By doing so, the researchers reported an overall selectivity of 31% towards H₂O₂ using only CO₂ as the diluent. However, the combined effect on the overall selectivity was not reported [40]. Using CO₂ as a diluent is advantageous as it can expand different solvents during the reaction and increase H₂ solubility. Secondly, CO₂ dissolves in water to form carbonic acid (HCO₃⁻), which makes the medium acidic. The acidic condition is helpful as it is the most commonly used storage condition for H₂O₂ [5]. The chemists and engineers in the field have agreed that increasing the solubility of H₂ and O₂ in the reaction medium would also lead to a better adsorption of the gases on to the catalytic surface. This, in turn, would lead to a better yield of H₂O₂ [62]. Selinsek et al. recently reported a process design with two separate tanks containing H₂ and O₂ dissolved in water with a two-fold benefit. First, an explosion is circumvented due to the separate feeding of gases. Second, the H₂ to O₂ ratio in the reaction cell can be easily controlled by varying the flow rate of the pump, ensuring that at any given point in time, the process operates in a safe manner [4]. The recent work of Urban et al. describes the use of an electrochemical sensor system to detect the hydrogen and oxygen amounts present in the system during direct synthesis. Additionally, the sensor is capable of monitoring the H₂O₂ levels in the reactor as well. This sensor is capable of performing under high analyte concentrations and high pressures. The authors also claim that the usage of this novel electrochemical sensor could minimise the risk of explosion due to high accuracy of detection [63].

2.4.2. Reaction Medium

As mentioned previously, the direct synthesis of H₂O₂ from H₂ and O₂ in the gaseous state without any reaction medium is highly dangerous. This is because the gases form an explosive mixture over a wide range of concentrations [64]. Hence, performing the synthesis of H₂O₂ at lowered temperatures in highly pressurised environments, in an appropriate reaction medium, prevents explosions and produces high yields [27]. Therefore, the choice of the reaction medium is crucial to the success of the process. Most of the existing literature uses water as the reaction solvent, with some exceptions where pure methanol or ethanol is used. Additionally, using water with co-solvents such as methanol or ethanol favours a higher dissolution of H₂ when compared to pure water as a reaction medium [56,65]. In 2001, Hâncu and Beckmann reported the use of CO₂ as a reaction medium for the direct synthesis of H₂O₂ using a CO₂ soluble ligand-supported Pd catalyst. The researchers worked on the assumption that the H₂O₂ solubility in CO₂ is considerably less than the conventional working solutions of the AO process, i.e. organic solvents. Furthermore, the CO₂ used was liquid under the reaction conditions (298 K, 17 MPa) and the presence of a CO₂-phillic catalyst would minimize the contact time of H₂O₂ on Pd, thereby increasing the selectivity of the process (Figure 1) [28,66].

Moreno et al. reported the use of a supercritical CO₂ (scCO₂) with methanol (MeOH) water mixture at an operating temperature range of 283 K–318 K and pressure of 16.7 MPa to synthesize H₂O₂ from H₂ and O₂. N₂ was used as a diluent in this reaction to achieve a yield between 11.6% and 45.9% [42]. Except for these works, almost every direct synthesis of H₂O₂ using chemical catalysis is documented in water, alcohol, or a defined ratio of both. Landon et al. reported that at 31.1 °C, which is the critical temperature of CO₂, more decomposition of H₂O₂ was observed than formation. The researchers suggested that the synthesis was performed at a temperature just below the critical temperature [67]. This could be one of the many reasons as to why the use of CO₂ as a solvent has not been widely researched. Abate et al., on the other hand, used scCO₂-expanded methanol (a solution operating below the triple point in the presence of a solvent to form a two-phase fluid system) as the solvent in the presence of a Pd catalyst supported on mesoporous silica for the direct synthesis of H₂O₂. On doing so, a selectivity of 40% towards H₂O₂ and a productivity of 0.11 mol_H2O2m⁻²·Pd·h⁻¹ at the end of 3 h [65].

2.4.3. Additives/Promoters

Along with the reaction medium, special additives termed “promoters” are often used to stabilize H₂O₂ production and to increase the process yield. The most commonly used promoters are acids or halides. Edwards et al. published the effect of acids and halides on the outcome of the direct synthesis of H₂O₂. In this report, the researchers classified the promoters into two groups:

(i)

oxyacids such as acetic acid, perchloric acid, phosphoric acid (H₃PO₄), nitric acid, and sulphuric acid (H₂SO₄)

(ii)

halide acids such as hydrochloric acid (HCl), hydrobromic acid (HBr), and hydroiodic acid (HI).

The authors reported that the reactions could accept potassium and sodium salts of halide acids as well. The presence of halide ions in the reaction could influence the H₂O₂ conversion in the following order: KF > no halide ions > KCl > KBr and selectivity in the following order: KBr > KCl > no halide ions > KF. The report also claimed that the presence of iodide ions eventually poisoned the catalyst [55].

The next pioneering work in the field was that of Wilson and Flaherty, pointing out that the presence of protons (H⁺) from mineral acids such as HCl, H₂SO₄, H₃PO₄ and HNO₃ aids in reducing molecular oxygen. They also suggested that the corresponding counter ions such as Cl⁻, SO₄²⁻, PO₄²⁻, and NO₃⁻ modify the structure of the metal catalyst. By adding halide groups to the Pd catalyst and performing the synthesis in ethanol at an acidic pH, the selectivity would eventually increase from 60% to 80% [40]. The recent report of Gallina et al. compared the roles of sodium bromide (NaBr), H₃PO₄, and H₂SO₄ on the outcome of the direct synthesis of H₂O₂ by varying the concentrations of NaBr and H₃PO₄. The authors conducted the experiments at 288 K, 2.0 MPa, 3 h reaction time, Pd/C, with a gas mixture percent of H₂/O₂/CO₂ at 4/20/76%. The table (

Table 3. Effect of different promoters on H₂ conversion, H₂O₂ selectivity, H₂O₂ productivity as reported by Gallina et al. [68].

Serial No. (S/N)	Additive Added			Outcome
	NaBr (M *)	H3PO4 (M)	H2SO4 (M)	H2 Conversion (%)	H2O2 Selectivity (%)	H2O2 Productivity (molH2O2·Kg(Pd)−1·h−1)
1	0	0	0	100	0	0
2	0	0.003		100	0	0
3	0.0005	0	0	92	50	740.1
4	0.0005	0.003	0	85	61	891
5	0.0005	0.005	0.025	79	65	830

* mole (mol)/litre (L); molar.

) below describes the observations of Gallina et al. on testing various combinations of the promoters.

The researchers formulated that the absence of promoters (H₃PO₄ or NaBr) led to either (i) a much more prominent water forming reaction than the H₂O₂ forming one or (ii) subsequent reaction of H₂O₂ further to form H₂O. The researchers also concluded that NaBr/H₃PO₄ had a combined effect on the leaching of the Pd catalyst and that there was no fixed NaBr to H₃PO₄ ratio to improve the direct synthesis of H₂O₂ [68]. The authors also suggested that a pH of 2.0 in the reaction medium would favour a better selectivity towards H₂O₂ production from H₂ and O₂ [68].

Another method of acidifying the reaction medium to suit the direct synthesis of H₂O₂ is to use solid acid catalysts (SAC) as reported by the publication of Lewis et al. [57]. The problem of the acidic additives and halide salts being soluble in aqueous medium making the recovery of these a tedious downstream operation was addressed in this work. By using a SAC such as caesium substituted phosphotungstic acid (HPA) in the presence of a Pd or Au-Pd alloyed catalyst, the productivity of H₂O₂ could be increased and the degradation to H₂O could be decreased [69].

2.4.4. Reactor Design

One of the most important parameters to be discussed for the direct synthesis of H₂O₂ is the type of reactor used for the reaction. Until today, slurry reactors, plugged flow reactors, microreactors or trickle bed reactors are often used to synthesise H₂O₂ directly from H₂ and O₂. One of the major requirements when choosing a reactor is that the vessel should withstand high pressure. It is known that the prolonged exposure of H₂O₂ to H₂ in the presence of a Pd catalyst would lead to the formation of water (Scheme 3). One possible way to overcome this phenomenon is to limit the exposure of H₂O₂ to H₂ on the catalytic surface.

Microreactors present a unique way to operate this process in a continuous manner, with defined flow characteristics, large surface area to volume ratio, promising heat and mass transfer rates, and excellent process safety. A microreactor is “a device that contains micro structured features with a sub millimetre dimension, in which chemical reactions are performed in a continuous manner.” The microreactors are constructed from silicon, quartz, glass, metals, polymers, and ceramics, to name a few [70]. The work of Shang and Hessel describes the operational- and reaction-based benefits of using microreactors for the direct synthesis of H₂O₂ in their work [71]. One drawback of using a microreactor for the direct synthesis of H₂O₂ is the incorporation of the metal catalyst within the capillaries of such a reactor [72]. Kanungo et al. described a technique to incorporate an Au-Pd alloy on to the walls of a silica coated capillary microreactor. The innovation in the design lies in the fact that the catalytic particles were formed in situ on the walls of the microreactor by a layer-by-layer self-assembly creating a multi-layer catalyst. By using this approach, the researchers were able to produce 210 mol_H2O2·kg_cat⁻¹·h⁻¹ with a H₂ conversion of 40% and selectivity of 70% (H₂/O₂–1:1, 315 K, 2.0 MPa, 5 h) (Figure 2a) [72]. Paunovic et al. reported a production process using the same catalysts in a microchannel reactor. The researchers were able to obtain five mass percent H₂O₂ solutions at a conversion rate of 15% at 42% selectivity (315 K, 2.0 MPa, 0.05 M H₂SO₄, 9 parts per million (ppm) NaBr, H₂/O₂ ratio 20%) [73]. Voloshin et al. elucidated the mass transfer mechanism that occurs in a microreactor during the direct synthesis of H₂O₂ (Figure 2b). In their work, the researchers claimed that flow of the fluids through the microreactor was slug-flow like. In other words, the liquid flow pattern is interrupted by catalytic particles and the pattern resembles that of a liquid slug being broken down. Using a set of assumptions, the researchers came up with a kinetic model that would explain the behaviour of a packed bed microreactor during direct H₂O₂ synthesis [74]. Hirama et al. used 32 parallel microreactors made up of silica and glass to produce H₂O₂ directly from H₂ and O₂. The authors were able obtain H₂O₂ at 10 mass percent at a productivity of 0.5 kg h^–1. Inoue et al., on the other hand, used four parallel microreactors to produce four mass percent H₂O₂ at 0.042 kg h⁻¹. Ng et al. reported the use of palladium nanoparticles immobilised on to polystyrene based polymer supports in a capillary microreactor, enabling a continuous production of 1.1 mass percent H₂O₂ over 11 days [75]. The reviews of Kolehmainen et al. [56] and Dittmeyer et al. [53] summarize the innovations in the field of catalyst design and reactor engineering in a detailed manner.

Another technique to overcome H₂O₂ decomposition by reactor engineering is the use of a trickle bed reactor or a plugged flow reactor. Almost all works of Biasi et al. uses trickle bed reactors (Figure 3) to improve the selectivity towards H₂O₂ [68,76,77,78,79,80], with the maximum being 80% using Pd on a sulfated ceria (CeS) catalyst.

2.4.5. Influence of the Catalytic Material

A major hindrance in the direct synthesis approach is the process’ low selectivity, as the formation of H₂O₂ is not thermodynamically favoured. Although promoters such as H₂SO₄, H₃PO₄, NaBr, and KBr help enhance the selectivity of the process, one might consider the catalyst and its supporting material to be the most influencing parameter [81]. Furthermore, the presence of noble metals or noble metal alloys as catalysts aid in the hydrogenation and subsequent decomposition of H₂O₂ to water (Scheme 3) [4]. On surveying the recent literature in the direct synthesis of H₂O₂, it is certain that the majority of scientists in the field are dedicated towards developing new, robust and stable catalysts. Existent catalysts and the newly developed versions of noble metal catalysts are characterized based on two criteria: conversion and selectivity. Pd catalysts are almost exclusively used for such reactions either as obtained or alloyed with other metals and/or supports to enhance the selectivity of the process.

Edwards et al. published the importance of alloying Pd with other metals to increase the efficiency of H₂O₂ production. Their observation stemmed from the highly reactive nature (25 times more active) of an Au-Pd catalyst that was capable of oxidising alcohols better than the corresponding monometallic catalysts. They also reported that when Au-Pd alloys were attached to a titanium dioxide (TiO₂) or alumina (Al₂O₃) support, core-shell structures were formed. With the development of the new catalyst, the researchers still faced the problem of H₂O₂ being reduced to water [82]. The same working group developed a new tin-based Pd alloy to stop the hydrogenation of H₂O₂. With the new alloyed catalyst accompanied by a heat treatment cycle, the hydrogenation reactions were prevented and selectivities of more than 95% were reported [37]. Ntainjua et al. used ruthenium (Ru) alloyed with Au and Pd to perform the direct synthesis of H₂O₂. The authors investigated the Ru-Au, Ru-Pd and Ru-Au-Pd catalyst for H₂O₂ synthesis. The amount of Ru added to the alloy, along with the calcination conditions, had an effect on the catalyst activity and reusability. So far, this report is the only one using an Ru catalyst for the direct synthesis of H₂O₂ [83]. Besides Au, only silver [44,84], tellurium [85], tin [37], and zinc [86] are described as possible metals for alloying with Pd. Xu et al. reported the possible increase of H₂O₂ production by using different metals such as tungsten (W), lead (Pd), molybdenum, etc. and validated their results with density functional theory (DFT). The researchers suggested that all these metals were superior to platinum (Pt) as a promoter. The research was a computational model and experimental evidence to substantiate this fact is needed [87].

Tian et al. reported that by increasing the amount of Pd in the system, the H₂ conversion increases as well, which is self-explanatory. However, the selectivity and productivity increases with decreasing Pd content. From these results, they were able to conclude that having a Pd particle size in the range of 2.5 nm to 1.4 nm would yield a selectivity of approximately 94% with 0.5% Pd loading [88]. Most recently, Howe et al. used microwaves to prepare an Au-Pd alloy supported on TiO₂ for synthesizing H₂O₂ from H₂ and O₂. The authors claim that the catalysts were capable of maintaining their activity for four reaction cycles. Compared to other alloyed Pd particles, these particles have a core-shell structure and can be prepared in 0.25 h [89]. The recent 2017 patent of Desmedt et al. used metallic catalysts supported on sulphate and phosphate to reduce the amount of inorganic acid content in the reaction medium. The inventors varied the metal content between 0.001 mass % and 10 mass % (0.62 to 2 wt % Pd) to obtain conversions ranging between 26.9% and 46% and selectivities between 19.9% and 74% [90].

Table 4. List of operating conditions in literary works involving the direct synthesis of H₂O₂ between 2010 and 2018.

Temperature and Pressure (K and MPa)	Catalyst	Reactor Type	Solvent(s)	Promoters	Conversion and Selectivity (%)	Literature Reference
263 and 2	Palladium-CeS	Trickle bed reactor with Teflon lining	Methanol	None	No data, 80	Biasi et al. [76]
298 and 0.1 (ambient)	Palladium on porous alumina tubing	Membrane reactor	Water	Sulphuric acid (H2SO4) sodium bromide (NaBr), phosphoric acid (H3PO4)	No data, 50	Inoue et al. [91]
283–324 and 4.6–16.7	Palladium on carbon	Stirred slurry batch reactor	Water + scCO2 and methanol + scCO2	H3PO4 and NaBr	No data available	Moreno et al. [42]
298 and 0.1 (ambient)	Pd nanoparticles immobilized on polystyrene based polymer	Capillary microreactor	Methanol	No additive	47 and 0.65	Fei Ng et al. [75]
				H2SO4, KBr	3.9 and 77
301 and 1.01	Insoluble heteropoly acid supported on Pd immobilized on mesostructured foam (MCF) silica	Autoclave reactor	Methanol	H3PO4 and NaBr	85 and 35	Park et al. [92]
298 and 4.5	Metallic Pd deposited on ceramic tubes	Porous tubular membrane reactor	Methanol	H2SO4, NaBr	No data and 83	Pashkova et al. [93]
293 and 4.0	Au-Pd on TiO2 on carbon	Stainless steel autoclave	Methanol/water	No data available	No data available	Pritchard et al. [94]
315 and 2.06	Pd on SiO2.	Microreactor	Water	H2SO4, NaBr	No data and 85	Voloshin et al. [74]
263 and 1.0	Bimetallic Pd-Au catalyst on CeS and sulphated zirconia	Trickle-bed reactor	Methanol	No data available	90	Biasi et al. [77]
293 and 0.65	Pd on SiO2	Teflon coated steel reactor	Methanol	H2SO4	No data and 21	Abate et al. [65]
	Pd on mesoprous silica (SBA-15)				No data and 58
263 & 1.0	Pd-Au CeS	Trickle bed reactor	Methanol	No data available	No data and 50	Biasi et al. [95]
	Pd-Au on ZS				No data and 60
278–313 and 2.0	Pd/C	Batch reactor	Methanol	H2SO4	100 and 35	Biasi et al. [78]
298 and 2.4	Commercial 5% Pd/C	Batch slurry reactor	Methanol	None added	No data available	Gemo et al. [96]
263 and 2.0	Pd on CeS	Packed bed reactor	Methanol	H2SO4	No data and 70	Kilpiö et al. [97]
303 and 5	Pd nanoparticles immobilized on a functionalised resin	Fixed bed reactor	Methanol	None added	No data and 73	Kim et al. [98]
293 and 0.1	Pd on SiO2	Glass stirred tank reactor	Methanol	H2SO4	No data and 60	Menegazzo et al. [99]
275 and 4.0	Ru-Au-Pd catalyst on TiO2 support	Stainless steel autoclave	Methanol/water	None added	No data available	Ntainjua et al. [83]
301 and 1.01	Palladium on zeolite HZSM-5	Autoclave reactor	Methanol	H3PO4	90 and 16	Park et al. [100]
294 and 4.0	AuPd/C	Stainless steel autoclave	Methanol/water	None added	No data available	Piccinini et al. [101]
298 and 2.3	Pd on sulfated zirconia and Pd on alumina	Batch autoclave reactor	Methanol	None added	No data available	Rossi et al. [102]
278–308 and 2.8	Commercial Pd/C	Trickle bed reactor	Water	NaBr	No data and 90	Biasi et al. [79]
298 and 2.4	Commercial Pd/C	Stainless steel batch autoclave	Methanol	None added	No data and 33	Biasi et al. [80]
278 and no data	Commercial Pd/C	Trickle bed reactor	Water	H3PO4 and NaBr	No data available
278 and 1.0	Au-Pd/TiO2	Microreactor	Water/Methanol	None	90 and 25	Freakley et al. [103]
273 and 3.8	Pd on activated carbon cloth (ACC)	Stainless steel autoclave	Methanol	None added	No data and 70	Gudarzi et al. [104]
295 and 3.0	Pd-Au on carbon nanotube (CNT)	Stainless steel autoclave with Teflon coating	Methanol and sulphuric acid	H2SO4	No conversion values and 15–65 depending on H2/O2 flow	Abate et al. [105]
295 and 3.0	Pd on nanocarbon	Stainless steel autoclave with Teflon coating	Methanol	H2SO4	No data and 25	Arrigo et al. [106]
293 and 0.1	Pd nanocubes on silica	Similar to the work of Lee et al. (2011)			10 and 25	Kim et al. [107]
273 and 2.0	Pd on activated carbon cloth (ACC)	Stainless steel microreactor	Methanol	None added	No data and 23	Ratchananusorn et al. [108]
278 and 2.0	Au-Pd on nanostructured TiO2 nanotube support	Stainless steel autoclave	Methanol	HCl	No data available	Torrente-Murciano et al. [109]
275 and 2.0	Pd/C treated with NaBr	Custom made stainless steel batch reactor	Methanol	NaBr	95 and 1	Biasi et al. [58]
293 and 2.0	Pd supported metal organic framework (MOF)	Autoclave reactor	Methanol	None added	No data and 26	Chung et al. [60]
296 and 1.0	Combination of Pd/TiO2 and Au-Pd/TiO2	Microreactor with parallel packed beds (1, 8 and 16)	Water	H2SO4, H3PO4, and NaBr	Varied depending on number of channels and the flow rate	Inoue et al. [110]
303 and 2.0	Supported Au, Pd and Au-Pd	Autoclave reactor	Water	NaBr	15 and 50	Paunovic et al. [111]
			1-pentanol		20 and 80
			Chloroform		18 and 38
			Hexane		20 and 17
			Methyl isobutyl ketone		27 and 10
			1-butanol		28 and 60
			2-butanol		25 and 55
			Isopropanol		35 and 75
			Methanol		45 and 47
			Ethanol		50 and 47
			Dimethyl sulphoxide (DMSO)		25 and 87
			Acetonitrile		32 and 100
			Acetone		35 and 95
			t-butanol		40 and 70
303 and 2.0	Au-Pd catalyst	Autoclaved slurry reactor	Water	H2SO4 and NaBr	5 and no data	Paunovic et al. [112]
313 and 2.0					10 and no data
323 and 2.0					30 and no data
313 and 2.0	Au-Pd colloidal nanoparticles	Microchannel-Silica capillary reactor	Water	H2SO4 and NaBr	80 and 85	Paunovic et al. [73]
298 and 2.6	Commercial Pd/C	Trickle bed reactor	Water	H2SO4, H3PO4, and NaBr	No data available	Abejón et al. [81]
333 and 0.1	Au-Pd/TiO2	Fixed bed reactor	Gas phase synthesis (2% H2/air)	None	No data available	Akram et al. [113]
313 and 9.5	Pd loaded on a sulfonic acid resin	High pressure stirred reactor	Methanol	Three compounds tested: 2-bromo-2-methyl propane, 2-bromopropane, bromobenzene compared to NaBr	No conversion data; selectivity for NaBr was 80 and 75 for the rest	Blanco-Brieva et al. [114]
298 and 0.5	Au-Pd/TiO2	Stainless steel autoclave	Methanol/water	None added	No data available	Crole et al. [36]
275 and 3.7	Pd-Sn	Stainless steel autoclave	Methanol/water	None added	9 and 96	Freakley et al. [37]
275 and 3.0	Pd-Ag (varying amounts)	Teflon coated stainless steel autoclave	Methanol	H2SO4	Depended on alloy ratio	Gu et al. [84]
303 and 2.0	Porous Pd/SiO2	Teflon coated stainless steel autoclave	Methanol	H2SO4	No data and 46	Sierra-Salazar et al. [115]
273–305 and 0.1–3.0	Pd/SiO2	Packed-bed flow reactor	Methanol/water	HCl, H2SO4, and sodium bicarbonate (NaHCO3)	No data and 31	Wilson et al. [40]
288 and 2.0	Commercial Pd/C	Trickle bed reactor	Water	H3PO4, H2SO4, and NaBr	77 and 72	Gallina et al. [68]
278 and 5.0	Pd particles on acidic niobia (Nb)-silica (Si) support	Slurry reactor	Methanol	None added	38 and 78	Gervasini et al. [43]
278 and 10.0			Water		38 and 85
293 and 0.1	Pd/SiO2 (sonochemical approach)	Stirred glass reactor	Ethanol/water	KBr and H3PO4	22 and 85	Han et al. [116]
	Pd/SiO2 (incipient wetness approach)				12 and 85
	Pd/TiO2 (sonochemical approach)				22 and 75
	Pd/TiO2 (incipient wetness approach)				12 and 80
300 and 0.95	Pd/TiO2	Flow reactor comprising of 8, 16 and 32 parallel micro-packed beds	Water	H3PO4, H2SO4, and NaBr	64 and 66 (eight glass beds) 61 and 70 (eight glass + Si beds)	Hirama et al. [117]
315 and 2.0	Au-Pd nanoparticles	Microreactor	Water	H2SO4, NaBr and acetonitrile (MeCN)	20 and 85	Kanungo et al. [72]
273 and 4.0	Pd on a hexadecyl-2-hydroxyethyl-dimethyl ammonium dihydrogen phosphate (HHDMA)	Stirred reactor	Methanol/water	No data available	No data and 80	Lari et al. [61]
275 and 4.0	Au-Pd nanoparticles supported on cesium substituted phosphotungstic acid (HPA)	Stainless steel autoclave	Methanol/water	No data available	69 and 86	Lewis et al. [69]
293 and 0.1	Pd/SiO2	Glass stirred reactor	Ethanol/water	KBr	30 and 27	Seo et al. [45]
283 and 0.1	Pd on hydroxyapatite	Slurry reactor	Ethanol	H2SO4	2 and 94	Tian et al. [88]
283 and 0.1	Pd-Tellerium (Te)/TiO2	Micro triphase reactor	Ethanol	H2SO4	6 and 100	Tian et al. [85]
283 and 0.1	Pd on mesoporous anatase TiO2	Glass triphase reactor	Water	H2SO4	40 and 40	Tu et al. [118]
275 and 3.0	Pd-zinc (Zn) on alumina (Al2O3)	Stainless steel autoclave	Methanol	H2SO4	57 and 78.5	Wang et al. [86]
278 and 3.0	Au-Pd supported on carbon	Stainless steel autoclave	Methanol/water	None added	65 and 60	Yook et al. [119]
275 and 4.0	Au-Pd/TiO2	Stainless steel autoclave	Methanol/water	None added	No data available	Howe et al. [89]
No data and 4.0	Ag-Pd/TiO2	Stainless steel autoclave	Methanol/water	None added	No data available	Khan et al. [44]
275 and 4.0	Au-Pd/mesoporous silica (SBA-15)	Autoclave	Methanol/water	None added	No data available	Rodrigéz-Goméz et al. [5]
298 and 1.1	Pd/TiO2	Teflon coated reaction cell	Water	NaBr	No data available	Selinsek et al. [4]
273–337 and 0.1–3.1	Au-Pd alloy	Plugged flow reactor	Methanol/water	None added	32 and 40	Wilson et al. [39]

below summarises the reaction conditions and the catalysts for the direct synthesis of H₂O₂ along with the catalyst used.

3. Summary, Conclusions and Future Perspectives

More than a century has passed since the first documented work of Henkel and Weber in 1914 producing H₂O₂ directly from H₂ and O₂ using Pd catalysts, and the direct synthesis has only reached to pilot plant scale of production. This is because the synthesis needs to be operated beyond the explosive range of H₂ and O₂ ratios and the thermodynamic favoring of water formation over hydrogen peroxide formation. The conversion and selectivity of the process depends on several parameters such as the H₂/O₂ ratio, the diluent used in the process, the reaction medium used to prevent explosion, the catalyst and its supporting material, reactor design, and the operating temperature and pressure, to name a few. From recent literature, it is understood that it is an advantage to perform the synthesis at reduced temperatures (263 K to 283 K) and pressures (most commonly 2.0 MPa to 4.0 MPa). To synthesise H₂O₂ selectively from H₂ and O₂, oxygen is to be used in excess to avoid hydrogenation of H₂O₂. With the new developments in microreactor technology, great advancements are being achieved in increasing the selectivity of the process. Moreover, robust and stable catalysts have been the research focus of the scientific community ever since this reaction was reported. The use of Pd in its pure form or as an alloy presents an opportunity towards industrializing this process. Additionally, additives such as NaBr, H₃PO₄, H₂SO₄ etc. aid in stabilising the synthesised H₂O₂. Finally, with the scientific community focusing on green and sustainable processes, ably supported by the advancements in the field of direct synthesis, the first commercial plant producing H₂O₂ using direct synthesis technology is not that far in the future.