Thermochemical Conversion of Water and Carbon Dioxide into Synthesis Gas.
A great deal has been written in the energy field about the solid state structural class known as perovskites.
Here is the Wikipedia picture (not bad) of the perovskite structure:
Much of this interest has been directed at the quixotic enterprise of making the so called "renewable" solar industry actually work - it hasn't, it isn't and it won't - research has involved perovskite type structure consisting of lead, a halogen, usually iodine or bromine and an element like cesium.
For example: Imaging the Anomalous Charge Distribution Inside CsPbBr3 Perovskite Quantum Dots Sensitized Solar Cells (Panagrahi et al ACS Nano, ASAP accessed 10/02/17)
Perovskites of this type show high light to electricity efficiency, but like most all solar cells, they suffer from the usual draw backs, low energy to mass ratios which means that their material environmental impact will be enormous. Of course the fact that these cells contain the highly toxic element lead will have no bearing on people declaring them "green," any more that the equally stupid idea of "distributing" for "distributed energy" cadmium in cadmium telluride solar cells prevented these future toxic nightmares from being declared "green." Were they ever to make it to 10 exajoules (out of 570 exajoules used by humanity as a whole) per year - they won't - they would rival dangerous fossil fuels as environmental disasters, simply because of the volume and mass of toxic waste that would be required to be processed.
When I read about perovskite solar cells - and one really has no choice given the cockamamie energy funding pop culture has promoted - I usually want to throw up.
History will not forgive this generation, nor should it.
In more than half a century of nuclear operations in the United States, by contrast to the much more toxic and far less sustainable, successful or safe solar industry, only 75,000 MT of used nuclear fuel - a valuable resource - has accumulated, almost all of it easily contained at the site where it was generated. By contrast any wasteful scheme intended to make the solar industry work - it won't - would require the processing of tens of thousands of toxic materials per day, worse, distributed, where, much of it will be abandoned, making lots of little Flint Michigans (or their cadmium equivalents) all of the world.
Solar waste will never be as easily confined as fission products and actinides are.
My own interest in perovskites goes back a little longer than this recent solar fad. I've been interested in them as oxygen conducting membranes.
Way back in 2011, I spent a few weeks compiling all kinds of literature about perovskite oxygen conducting membranes, and actually built a spreadsheet listing the references I'd reviewed, the elements in the periodic table that they used, and the oxygen flux at reported temperatures...blah...blah...blah.
My interest was motivated by consideration of thermochemical water splitting cycles, of which a great many are known. I was investigating several that would theoretically make a 1:1 stoichiometric mixture of oxygen and carbon dioxide, and I was thinking about separations. (There are much better approaches, by the way, than such separations, but that's another issue.)
Thus my interest was piqued when I came across a paper in the current issue, released today (10/2/17), of the Journal ACS Sustainable Chemistry and Engineering
It's this one:
Oxygen Transport Membrane for Thermochemical Conversion of Water and Carbon Dioxide into Synthesis Gas (Jiang et al ACS Sustainable Chem. Eng. 2017, 5, 8657?8662)
With synthesis gas, one can pretty much make any large scale organic chemical obtained from petroleum, including those utilized in polymers. To the extent these chemicals are obtained from carbon dioxide and water, they represent value added sequestration of carbon dioxide. If the carbon dioxide is removed chemically (or physically) from the atmosphere, or is obtained by the controlled combustion of biomass, this sequestered carbon in theory at least could reverse climate change. (Realistically that is not going to happen. We're going to burn fossil fuels until we simply can't do so any more, all the time prattling on about the grand renewable future that never arrived, is not arriving and will not arrive. I'm speaking "in theory" and not "in practice." )
From the introductory text:
In the past few decades, transforming H2O and CO2 into high energy chemicals by artificial photosynthesis with the aid of solar power is getting more and more attractive, because of its important role in mitigation of energy shortage and global warming.1,2 Synthesis gas, a mixture of CO and H2, is a precursor to liquid hydrocarbon fuels. Synthesis gas can be obtained from splitting of CO2 and H2O using photocatalytic processes,3?7 high-temperature steam/CO2 coelectrolysis,8?11 or solar thermochemical loop processes.12,13 In the photocatalytic process, oxidic materials can decompose H2O and/or CO2 into H2 and/or CO. However, photocatalysis is impeded by its inherently limited energy conversion efficiency associated with band gap excitation.14 By contrast, thermochemical processes operating at elevated temperature can use the solar spectrum for thermal energy and possess fast chemical reaction kinetics. Previous research has demonstrated that the direct thermolysis of H2O and CO2 requires ultrahigh temperatures (>2500 K). To avoid the recombination and the formation of an explosive mixture, the generated gas products have to be separated at such high temperatures.15 To tackle the two issues of (i) ultrahigh temperature and (ii) gas separation at these temperatures, multistep thermochemical cycles - especially two step thermochemical loop cycles using metal oxide redox reactions - have been put forward and widely studied in the past several decades.
I'm not entirely sanguine about this description of thermochemical cycles, first because many are known thermochemical that do not require temperatures >2500K, (including the most famous thermochemical cycle, the sulfur iodine cycle) and secondly, I find the perfunctory and obligatory reference to solar thermal plants absurd. All of the thermal solar plants ever built on this planet after decades of cheering have been huge commercial and environmental disasters, the most egregious case being the Ivanpah plant in California, which has been more successful on shooting down precooked (or overcooked or even vaporized) birds in flight than in providing meaningful energy. If solar thermal plants were workable, decades of cheering for them would have made them practical and significant. They are neither.
Nevertheless, the paper is interesting; not all "solar thermal" thermochemical cycles are useless simply because it is straight forward to convert them to cleaner energy, that being nuclear energy.
The perovskite oxygen containing membranes are "cobalt free" although they do contain small amounts of praseodymium and cerium, generally the most available (along with neodymium) of the lanthanide elements to which I recently referred in this space while trashing the useless wind industry. Cerium in this case serves at the multivalent element necessary to conduct oxygen gas, along with iron. Their are two perovskites in this paper, utilized as a mixture, a cerium strontium iron version, and a praseodymium strontium iron version.
In order for this system to function efficiently, to remove oxygen from the splitting of carbon dioxide and water, the oxygen must be consumed. In some incarnations of similar systems, this is done by reacting the oxygen with the dangerous fossil fuel methane obtained from dangerous natural gas. And that's what they do here. (There are, of course, better things to do with oxygen, but we'll leave that aside for now.)
In this work, methane was used not only as a sweep gas to consume the permeated oxygen by the POM reaction, but also to produce additional synthesis gas with a H2/CO ratio of 2. Figure S5 presents the influence of temperature on the CH4 conversion, CO selectivity, and yield on the permeate/sweep side. It is shown that both CH4 conversion and CO yield increased with rising temperature. At 930 °C, a CH4 conversion of 62% and a CO selectivity of 99% were achieved, and synthesis gas at a rate of 3.9 mL min?1 cm?2 was obtained.
In any case, the thermochemical cycle can proceed in its entirety at less than 1000 degrees centigrade, and its certainly interesting, if less than entirely practical.
From the conclusion:
In conclusion, for the first time the effective generation of synthesis gas with H2/CO ratio of 2 by the simultaneous decomposition of water and carbon dioxide at the relatively low temperature of <1000 °C was experimentally demonstrated in an oxygen transport membrane reactor. Benefiting from the in situ fast removal of the generated oxygen by the membrane, the effective splitting of CO2 and H2O was achieved at lower temperatures, compared to the usual thermochemical decomposition. A synthesis gas flow rate of 1.3 mL min?1cm?2 on the feed side was obtained at 930 °C at a H2O/CO2 feed ratio of 5. To have a stable and sufficient driving force for oxygen permeation through the membrane, the oxygen partial pressure on the sweep side was effectively reduced using reactive methane as sweep gas. Simultaneously, synthesis gas at a rate of 3.9 mL min?1cm?2 was obtained on the methane side.
In consideration of the disadvantages of the conventional two-step thermochemical route on the requirement of ultrahigh-temperature and discontinuous oxygen transport, the combination of solar energy, catalytic thermolysis, and oxygen transport membrane reactor proposed in this work offers a new perspective and an alternative route to convert water and CO2 into synthesis gas.
Full details can be obtained by accessing the paper in a good science library or with a subscription.
I wish you a pleasant day tomorrow.
