Thermochemical Carbon Dioxide Splitting With Oxygen Permeable Membranes to Effect Separation.

[NNadir]

The paper I'll discuss in this post is this one: Thermochemical CO2 Splitting Enhanced by In Situ Oxygen Separation through CeO2 and CaTiO3Membranes, Liya Zhu, Heng Pan, Shaocong Chen, and Youjun Lu, Energy & Fuels 2022 36 (19), 12226-12235.

When my son entered his nuclear engineering Ph.D. program, I advised him, as a materials scientist, that if I were to suggest a goal, it would be to develop materials suitable to effect solid to gas heat transfer at temperatures higher than 1400°C. This is the temperature at which cerium dioxide, CeO2, decomposes to give Ce2O3, formally dicerium trioxide. The latter, at a lower temperature, can be reoxidized by carbon dioxide back to CeO2, whereupon the carbon dioxide is reduced to carbon monoxide, a gas that is very useful for making carbon free liquid fuels.

I discussed this topic, carbon dioxide splitting previously in this space, here: Cerium Requirements to Split One Billion Tons of Carbon Dioxide, the Nuclear v Solar Thermal cases.

In that post, now over four years old - using another unit of time, accumulations of carbon dioxide in the planetary atmosphere, 11.20 ppm of carbon dioxide ago; we were at 409.60 ppm when I wrote it - I made the following remark:

Although - probably for funding reasons - this device is purportedly described as "solar," it is actually agnostic for the source of primary energy. It would operate using electricity (which is the source utilized in the paper's description of the experimental operation), dangerous fossil fuels (although this might amount to a material perpetual motion machine), or nuclear energy.

In my calculations, I compared the nuclear and solar cases, because they're illustrative, and nominally carbon free, if one ignores the fact that the world’s largest solar thermal plant – there are actually very, very, very few of them as of 2018 - spends a significant portion of its operating time as a very, very, very expensive and unreliable gas plant.

The paper under discussion qualifies in the same way. It claims the source of heat for this process will be solar energy, which is just stupid, since any plant operating at the required temperatures to be economically viable will also need to be reliable, something solar energy isn't, since as should be obvious but somehow escapes attention, something called "night" exists.

But look, we live in a world where in order to get funding, one has to engage in quasi-religious chanting about so called "renewable energy" which is neither "renewable" nor environmentally acceptable.

I have been a regular reader of Energy and Fuels, a publication of the American Chemical Society, of which I am a proud and long term member, and am now a minor officer. The journal is largely devoted, not entirely, but largely to the development and use of dangerous fossil fuels, fossil fuels that are killing the planet while morons have been carrying on about Three Mile Island for close to half a century, or in climate change time, 81.62 ppm of carbon dioxide concentrations ago. It should be as obvious as the existence of "night" which is the worst disaster, Three Mile Island or climate change, but somehow it's not. As to why I read a journal largely devoted to dangerous fossil fuels, which I abhor, there are useful things in it that might be adopted to clean energy, and in any case, there's the famous cliched maxim: "Know thine enemy."

The journal specifically excludes discussion of the only sustainable form of energy there is, nuclear energy, but also excludes a worthless form of unsustainable energy, wind energy:

Submissions dealing with nuclear, wind and hydraulic power, power grids, or solely with process economics will not be considered.

About the Journal.

So, if one wants to be published in Energy and Fuels, one has to get a little absurd. Nevertheless, thermochemical splitting of water and/or carbon dioxide - in many ways, owing to the existence of the water gas reaction they are equivalent - are worth reading.

Carbon dioxide has a certain value as a working fluid in Brayton cycle devices, and an intriguing concept in carbon dioxide Brayton cycle devices is the Allam cycle, an oxyfuel combustion cycle for which CO2 splitting could be considered a variant with a better heat source than combustion.

Anyway, the paper under discussion addresses a problem about which I've thought quite a bit. Most of the scientific interest in a class of structural compounds known as "perovskites" is currently built around the very bad idea of developing even more toxic solar PV cells, but my own interest in this class of compounds has been built around varieties of these structures to function as oxygen transport membranes.

What is interesting about this paper is not the carbon dioxide splitting itself - cerium oxides are well known to do this - but oxygen transport.

From the introduction to the paper:

Establishing a carbon neutral energy economy is becoming a common pursuit of the world nowadays. The conversion of CO2 captured from heavy industry or even from air into value-added chemicals driven by renewable energy serves as an ideal method for both CO2 emission reduction and renewable energy storage. (1,2) The past decades have seen growing research efforts in the field of two-step thermochemical CO2 splitting, with the expectation of using thermal energy from concentrated solar radiation at a high efficiency. (3−5) Usually it is performed with the aid of a metal oxide. The metal oxide is reduced to release oxygen at first, and in the second step, it is oxidized back to the initial state by CO2 to produce CO. CO and O2 are produced separately in two steps. H2O can be split under the same principle, in which case green hydrogen can be produced. The metal oxide is also known as “oxygen carrier” regarding the role it plays in the whole process. The reduction step is highly endothermic, requiring a high temperature (usually >1300 °C). Thus, when it is powered by concentrated solar radiation, solar energy is converted into chemical energy in the fuel gas (CO/H2), which is consequently known as “solar fuel.”

One hears, probably too much about the allegedly "mature" idea of using the heretofore useless (if the goal is to address climate change as opposed to trashing huge land areas of wilderness to make industrial parks to produce insignificant amounts of energy) PV energy scheme to produce electricity which can then be used to make hydrogen or electochemically reduce carbon dioxide.

I like the sly dismissal of this pixilated scheme in this paper:

Solar CO2/H2O splitting could also be realized by the combination of photovoltaic (PV) and electrolysis, in which the technologies involved are more mature. The maximum solar-to-fuel efficiency of PV electrolysis is the product of the efficiencies of PV and electrolysis systems, which is estimated to be 11–16% without considering mismatch between the systems. (6,7)

Thermodynamic inefficiency translates into environmentally disastrous in my opinion.

Later on in the introduction, the authors discuss mechanical Rube Goldberg schemes that have been offered to address the oxygen separation problem:

...Regarding this issue, various innovative designs on the system level for solid–solid heat recovery, including solid heat-transfer medium, (11) moving packed beds, (12) rotating fins/drums, (13,14) were proposed and investigated. However, it still turns out to be a serious technical challenge to handle the solid material, which is highly sensible to atmospheres at such high temperatures with both effectiveness and robustness. (14) An isothermal cycle was thus proposed, in which the oxidation step was suggested to proceed at the same temperature as the reduction step, eliminating the need for repeated heating of the metal oxide. (15,16) This shifts the burden of heat recuperation from the solid phase to the gas phase since the oxidation step is thermodynamically unfavored at high temperatures, and excessive CO2/H2O is required to drive the reaction. (17,18)

The authors then discuss their interest in solving the problem, oxygen transport membranes, discussing previous work:

Recently, researchers went further by introducing the concept of the membrane reactor, in which the oxygen carrier is suggested to be fabricated as a dense membrane, and the reduction and oxidation reactions could proceed simultaneously at two sides, respectively. (19−21) Oxygen transfer between the two reactions is realized by selective oxygen permeation across the membrane. The CO2/H2O splitting reaction on the oxidation side could be shifted to CO/H2 production through in situ separation of oxygen to the reduction side. The principle is not only applicable to oxygen separation but also widely used in the in situ separation of other products, such as hydrogen separation. (22,23) Oxygen permeable membranes have been widely studied for a variety of applications requiring oxygen transfer, including air separation, (24) oxy-fuel combustion, (25) and hydrocarbon reforming. (26,27) In recent years, with the increasing call for the transition to low-carbon economy, they have also shown attractive application prospects in the field of thermochemical CO2 reduction, due to its wide applicability to oxygen-involving redox reactions and obvious technical advantages, such as fast kinetics and system simplicity. (28,29) Fluorite and perovskite oxides are the main alternative materials for oxygen permeable membranes, which, to a large extent, overlap with the materials under investigation for two-step thermochemical cycles. (29,30) This is due to the obvious commonality of these two processes, that both of them are based on the redox reactions of the material and oxygen diffusion inside it. (30,31) However, for the membrane reactor aimed for direct CO2/H2O splitting, the working temperature, which would be as high as 1500 °C, is much higher than that of the applications mentioned above, in which a fuel is usually employed for oxygen removal on the sweeping side. On the one hand, the high working temperature may lead to a fast kinetics since it is expected to be exponentially dependent, but on the other hand, it also poses great challenge to both chemical and physical stability of the membrane material...

...Besides, for CeO2, the strong reductive condition required on the reduction side may cause a high oxygen vacancy concentration, which may further lead to the lattice expansion and membrane crack. (36) From this point of view, using a membrane material with a higher oxygen affinity may provide a more effective option for this application. In this work, the performances of membrane reactors made from two distinct oxides, CeO2 and CaTiO3, are investigated and compared, upon which the rate-limiting mechanism of each membrane reactor and the difference on material requirements of the two reaction modes (two-step cycle and membrane reactor), are discussed.

Titanium is one of my favorite elements in the periodic table (although my son says that I refer to way to many elements as "favorite" elements). I had not heard of it as an oxygen transporting material; thus this paper interests me.

A few pictures from the paper:

The introductory cartoon:





The apparatus:





The caption:

Figure 1. Overall structure and local details of the experimental setup: (a) schematics of the whole setup; (b) gas flows in the tubular furnace and the membrane reactor; (c) oxygen diffusion in the membrane; and (d) temperature distribution along the heating zone of the furnace when it is set to be 1500 °C (the position in the furnace is marked as the distance to the bottom end of the heating zone).

Yields and gas behavior:




The caption:

Figure 2. CO and O2 producing profiles and CO/O2 ratios at different temperatures at the CO2 side: (a) alumina tube and (b) CeO2 membrane reactor. The gas configuration is 50CO2/300Ar.

Note that an industrial "Allam like" system would need to do away with the argon diluent.





The caption:

Figure 3. CO production rate of the CeO2 membrane reactors under different temperatures and airflow conditions.

The caption:

Figure 4. CO and O2 producing profiles at different temperatures at the CO2 side of the CaTiO3 membrane reactor. The gas configuration is 50CO2/300Ar.

Figure 5. CO production rate of CaTiO3 membrane reactors under different temperatures and gas configurations. The thinner membrane with a thickness of 0.30 mm was particularly marked.

Some conclusions from the paper:

n this paper, the CO2 splitting characteristics of oxygen permeable membrane reactors made of CeO2 and CaTiO3 were studied. In situ separation of oxygen was realized through the CeO2 and CaTiO3 membranes, and CO production rates much higher than direct CO2 splitting were obtained. Distinct performances of the CeO2 and CaTiO3 membrane reactors under various conditions indicate different rate-limiting regimes, with the former being limited by mass-transfer steps in the gas phase and the latter limited by bulk diffusion. The CO production rate of the CaTiO3 membrane could be significantly improved by decreasing the membrane thickness to ∼0.3 mm. A CO production rate of 12.2 μmol·min–1 was obtained, comparable to the performance to the CeO2 membrane under the same airflow condition (13.7 μmol·min–1). Its significance lies not only in the use of a new material but also showing the applicability of oxides with much higher oxygen affinity as high-temperature oxygen permeable membranes.

An excellent paper if accurate; with heat recovery this type of process could go a long way to providing high efficiency clean energy with a nuclear fuel source.

Have a nice Sunday afternoon.

[Backseat Driver]

1. Sounds similar to human telomeres, except these

"end caps" on DNA strands get shorter with each replication via telomorase--To be a self-sustaining energy source in this analogy, that enzymatic maintenance would require replacement of some sort of osmotic membrane at some point. I read that fruit flies don't use this process but something else. Have these seemingly analogous processes been exploited (researched) in the material world as it does seem human and insect life energy depends on these processes for continued life energy or at least ending??? Anti-aging of bodily organs including neurons maybe equals immortality? (given self-sustaining cellular energy pretty much dependent on H20 balance)
Pardon my "uneducated" reading of your article; I pretty much skipped physics and chemistry in HS and took a basic Life Science college class in which I struggled with chemistry--Amazed myself with the B grade

https://en.wikipedia.org/wiki/Telomerase

[eppur_se_muova]

2. Bookmarking that paper, as if I'll ever have a chance to work in that area. nt

[NNadir]

3. I will certainly never work in this area...

...I did however, send the paper to my son.

He's finishing his first year of graduate school, and from what I understand, his advisor is giving him a lot of freedom to choose a topic for his research.

The lab he's in is engaged in additive manufacture, apparently at the level of nanolayering, and this sort of system might be targeted by such a technology.

He probably doesn't give a rat's ass what I say, and will follow his own scientific interests, but I thought I'd give it a shot at inspiring him in this direction.