Hydrogen Feature pic

Manganese (iii) Oxide The Game Changer? Mn2O3 In Hydrogen Production Applications

Already discussed in this series of articles, Mn2O3 already has myriad applications in water treatment/remediation, sensors and in energy storage. Perhaps one of the most promising ongoing and future uses for manganese (iii) oxide is in the production of hydrogen.  

Manganese (iii) oxide, manganese oxide, manganese sesquioxide and Mn2O3 are used interchangeably.

As economies move away from fossil fuels, hydrogen is fast becoming the de facto fuel of the future - but current technologies rely heavily on polluting fossil fuels and the oil and gas industry. In some situations, the hydrogen produced is no better than burning the fossil fuel directly for energy, when the total process is taken into account.

Hydrogen production

The current industrial process for hydrogen production is as follows:

CH4 + H2O → CO + 3H2              (a)
CO + H2O → CO2 + H2                             (b)

Equation a is the reaction of steam with natural gas, this proceeds at between 700 - 1,100 °C and at a pressure of 2 MPa. The mixture of CO and H2 produced is known as synthesis gas. Equation b is the water gas shift reaction, catalysed by an iron catalyst, which also uses high temperature steam. Aside from the high temperatures and pressures (and thus energy) required to sustain these reactions, carbon dioxide is produced and allowed to escape unless captured and used. One mole of methane will produce one mole of carbon dioxide. This process is referred to as ‘grey hydrogen’. ‘Blue hydrogen’ is the same process with carbon capture. Other processes include ‘black’ and ‘brown’ hydrogen, which use the gasification of coal to produce the synthesis gas - with the CO2 emitted into the atmosphere. Crucially, all of these processes use fossil fuels as a starting point and require lots of energy.

It is imperative to develop better methods of hydrogen production if it is to become the de facto fuel of the future. Much research looks into the electrochemical splitting of water (electrolysis), and there are other production methods such as the metal-acid reactions and methane pyrolysis. Manganese (iii) oxide has a part to play in the global hydrogen story: the thermochemical splitting of water:

Mineral Milling Feature 2

Hydrogen Production By Water Splitting

Splitting water into hydrogen and oxygen is quite possibly the ‘holy grail’ of hydrogen production. The bond dissociation energy in a molecule of water - to split the covalent bond between hydrogen and oxygen - is 461.5 kJ mol-1. This relatively high number, whilst not insurmountable by conventional means, would be much more favourable if it were lower. This is where a catalyst comes in. A catalyst is a substance that enables a chemical reaction to proceed at a faster rate or under different conditions than would normally be possible. Furthermore, as the water oxidation reaction is rate limiting, an over voltage (i.e. an excess application of charge, in the electrolysis type methods) is required which therefore leads to inefficiency. Because of the high currents required, other compounds present may also oxidise - which is not acceptable for modern hydrogen production.

Manganese as a catalyst provides a great level of hope in making the production of hydrogen from water much easier. The idea to use manganese for the oxidation of water comes from plants; it is manganese compounds that are responsible for this process within photosystem II inside the plant. Logical, then, that research for the industrial production of hydrogen leans on nature’s precedence. Additionally, as mentioned earlier, the lack of toxicity associated with manganese, its oxides and its other compounds makes it much more attractive to the user.

As early as 1988, oxides such as Mn2O3 had been shown as effective water oxidation catalysts in the presence of ruthenium bipyridine species or sources of cerium(iv)(1). One of the concerns associated with using Mn2O3 alone - and indeed true of other oxides of manganese - is the tendency of the oxide to not form in a layered structure, which is viewed as more active(2). Notwithstanding this, manganese sesquioxide finds use alongside other metal oxides for the thermochemical splitting of water; with the Mn2O3/MnO redox pathway proceeding as follows(3):

Mn2O3 → 2MnO + ½O2                            (c)

2MnO + 2NaOH → 2NaMnO2 + H2                       (d)

2NaMnO2 + H2O → Mn2O3 + 2NaOH                (e)

The step outlined in equation c is endothermic and requires temperatures in excess of 1,560 °C. This does seem outlandish for a solar-assisted reaction, but research shows that it actually comprises two reactions:

Mn2O3 → ⅔Mn3O4 + ⅙O2                                      (d1)

⅔Mn3O4 → 2MnO + ⅓O2                                       (d2)

The reactions d1 and d2 individually may proceed at more modest temperatures. Overall, manganese (iii) oxide is reduced to manganese (ii) oxide, which reacts with sodium hydroxide to produce sodium manganese oxide and hydrogen gas. Sodium manganese oxide may react with water to regenerate the manganese (iii) oxide and sodium hydroxide(4).

It can be said, therefore, that manganese sesquioxide is catalytic in this reaction(5). Research has shown that the reaction may proceed at temperatures as low as 75 °C, but temperatures exceeding 450 °C are required to achieve a respectable rate of conversion - with crucially an excess of sodium hydroxide present. Despite the initial headline-grabbing temperature requirements, researchers suggest that the reaction is kinetically rather than thermodynamically controlled, relying on heat/mass transfer and solid phase transformations. Meaning that the huge temperatures are not necessarily required.

Taking all of this together, the efficiency of the thermal reduction followed by reaction with sodium hydroxide has been calculated to be 74%. Without heat recovery steps, efficiency drops to around 50%. Overall hydrogen production efficiency levels, with other production-side processes taken into account, is between 16 and 22%(6).

Manganese Alongside Other Catalysts

Much of the use cases in the Water splitting are when manganese is used alongside other materials - much of which rely on the above mentioned Mn2O3/MnO redox system mentioned above. When doping cadmium sulfide photocatalysts with manganese oxide, it was found that the presence of manganese ions leads to the formation of trapping sites where electrons or proton pairs can be separated, leading to increased hydrogen yields(7). When combined with cobalt oxide, Mn and Co spinels (formed from manganese sesquioxide) have been shown to be effective at catalysing the hydrogen production via water splitting, in a three step cycle utilising sodium hydroxide(8). Authors claim that the mixed oxide spinel was able to reduce the thermal water splitting temperature range from 1,300 - 1,400 °C to 850 - 1,050 °C. Efficiency is further gained from avoidance of an intermediate forming in the Mn2O3 redox system.

A further example of a mixed oxide solar system for water splitting is the sodium manganese mixed ferrite, operated in a reactor in the 750 - 800 °C regime. Production of 130 - 460 μmol per gram of mixture is reported after just one hour(9). Manganese sesquioxide is used as the source of the manganese. Overall, manganese (iii) oxide finds most uses in water splitting alongside other metal oxides.

water purification process
Beautiful splash of blue water

Improving Pre-Existing Techniques

Whilst it is widely acknowledged that a move away from ‘brown’ hydrogen technologies is essential, moves away from ‘blue’ hydrogen are less discussed. It is imperative that whatever technology or technique is used, it works to the best level possible. In terms of improving ‘blue’ hydrogen production, this means looking at means to reduce the energy requirement in producing the hydrogen from natural gas. Researchers have shown that employing a catalyst - such as manganese sesquioxide -  in a fluidised bed reactor enables the production of hydrogen from natural gas and steam at a much lower energy requirement. The manganese (iii) oxide is supported on silica, where it is reduced, producing hydrogen gas, water, CO2 and moderate quantities of CO(10) - the mixture then proceeds to the water gas shift reaction, producing solely CO2 and H2. Carbon capture technologies may be utilised to reduce carbon dioxide emissions.

Other Methods

Whilst producing hydrogen from natural gas and from water splitting (electrolysis) are the more obvious methods, and naturally to which most attention is given, there are other methods. Hydrogen production via the dehydrogenation of formic acid, as catalysed by a manganese-based catalyst, shows some potential(11), though other metals are used to form the catalyst too. The manganese in such a catalyst may be procured from manganese sesquioxide, as a high-quality source of manganese(iii). Authors cite that most catalysts for this process to date are based on noble metals such as iridium and ruthenium.

In the auto-thermal reforming of ethanol, nickel-alumina catalysts are often used. Research has shown that by adding a source of manganese such as Mn2O3, an ilmenite-type mixed oxide of nickel and manganese, NiMnO3, was formed in situ. This promotion effect greatly improves the reducibility of the Ni-Mn/Al2O3 system. Yields for the unpromoted system were just 1.85 mol H2 per mole ethanol, whereas in the NiMnO3 system, 3.1-3.2 mol H2 were produced - a vast improvement(12).

Formic acid and ethanol are widely available and/or easily produced from waste plant matter.

Water filters. Concept of three glasses on a white blue background. Household filtration system.

Summary

  • Hydrogen is the leading candidate for a sustainable and carbon-free fuel of the future; but current methods of production are poor environmental performers
  • Manganese sesquioxide’s redox system is critical for its success
  • Many applications are based on the idea of making the energy barrier to water splitting lower, manganese (iii) oxide catalysts possess serious potential to make hydrogen production greener when used with other metal oxides - with no residual CO2 production
  • Mn2O3 has been demonstrated as useful in aiding the formation of hydrogen via dehydrogenation of formic acid and ethanol reforming
manganese umber powder in a pot

References

1           A Harriman et al., J. Chem. Soc. Faraday Trans., 1988, 84, 2975

2           M. M. Najafpour et al., Int. J. Hydrogen Energ., 2012, 37, 8753

3           J. A. Botas et al., Int. J. Hydrogen Energ., 2012, 37, 18661

4           T. M. Francis et al., Chem. Eng. Sci., 2010, 65, 3709

5           J Marugán et al., Int. J. Hydrogen Energ., 2014, 39, 5274
6           M. Sturzenegger and P. Nüesch, Energy, 1999, 24, 959
7           L. Guo et al., Int. J. Hydrogen Energ., 2012, 37, 730

8           J Marugán et al., Int. J. Hydrogen Energ., 2017, 42, 13532

9           F. Varsano et al., Int. J. Hydrogen Energ., 2014, 39, 20920

10         Q. Zafar et al., Ind. Eng. Chem. Res., 2005, 44, 3485
11         M. Beller et al., Green Chem., 2020, 22, 913

12         L. Huang et al.Int. J. Hydrogen Energ., 2012, 37, 15908