Uranium catalyzed electrolysis of water to produce hydrogen.

[NNadir]

Last edited Wed Mar 7, 2018, 07:14 PM - Edit history (1)

The paper I will discuss in this post is this one:
Uranium-mediated electrocatalytic dihydrogen production from water (Meyer et al Nature volume 530, pages 317–321 (18 February 2016))

First some bitter background:

I don't have much use for the anti-nuke ersatz "climate activist" Joe Romm, who I consider an appalling fool, but despite my general contempt for almost all of his rhetoric, there is one thing about it with which I agree: Hydrogen will never be a useful consumer fuel, useful for powering cars and other dubious artifacts of our modern "screw the planet and the future be damned" culture.

On this, he disagreed with his pal, fellow anti-nuke Moron Amory Lovins, who once promised us Hydrogen Hypercars in Showrooms by 2005 adding to his long list of stupid Ouija board quality prognostications about energy.

The referenced National Geographic puff piece on Lovins was published on October 16, 2001.

At Mauna Loa the weekly average for concentrations of the dangerous fossil fuel waste carbon dioxide in the planetary atmosphere posted on October 14, 2001 was 368.16 ppm.

On October 15, 2017, the posted figure from the same source was 403.97 ppm.

Nevertheless, irrespective of what a fool Amory Lovins is, hydrogen is, and for as long as an industrial society exists, will always be an important captive intermediate for a variety of products, the most important being ammonia, but for many other products as well, including fuels. Hydrogen can be used to reduce ("hydrogenate" ) carbon dioxide or carbon monoxide to make dirty fuels like gasoline (the Fischer-Tropsch process into which the Carter administration put lots of research effort) or clean fuels like dimethyl ether, and less attractively, methanol. This potential for a closed carbon cycle was enthusiastically advanced by the late great Nobel Laureate George Olah in his widely cited 2011 paper Anthropogenic Chemical Carbon Cycle for a Sustainable Future (Olah et al J. Am. Chem. Soc., 2011, 133 (33), pp 12881–12898)

Olah's dead, and despite his noble efforts during his magnificent life, the planet is still dying.

What Lovins, a poorly educated ignoramus who is nevertheless thought by some, including himself, to be a "real stable genius," was too stupid to understand, or simply deliberately avoided since he makes a lot of money "consulting" for huge and very dirty dangerous fossil fuel companies, is that 99% of the hydrogen on this planet is generated by the energy wasting process of reforming dangerous natural gas, and less commonly these days, coal.

Lovins liked to pretend, or at least convince his acolytes, that hydrogen could be industrially made by what he called "soft" technologies - they are actually environmentally egregious nightmares of unsustainable industrial chemistry - the solar driven electrolysis of water.

This is pretty funny, since Lovins, who made his name hyping "energy conservation," while apparently knowing zero about the laws of thermodynamics, never bothered to account for the fact that electrolysis of water is one of the most thermodynamically inefficient processes known for producing hydrogen. About 1% of the hydrogen on the planet is so produced, and of this 1%, almost all of it is produced as a side product in the production of chlorine gas utilized to make bleach, polyvinyl chloride and historically interesting molecules like DDT and CFC's. Until very recently and for most of the period of Lovins' awful career, the main electrode for undertaking these electrolysis efforts was a mercury electrode. Bleach produced still produced this way - and there is some - usually contains small amounts of mercury, making it the third largest contributor to mercury in the environment after coal burning and medical waste.

(By the way, despite all Lovins’ hype about energy conservation, the strategy has failed as badly as the solar and wind industries have failed. In 1973, world energy demand was estimated to be 256 exajoules. As of 2016, world energy consumption is 576 exajoules.

IEA 2017 World Energy Outlook, Table 2.2 page 79 (MTOE converted to exajoules.)
For the 1973 figure see Current Energy Demand; Ethical Energy Demand; Depleted Uranium and the Centuries to Come and references therein)



All the above said, the production of hydrogen via electrolysis also results in the isolation of heavy water which is useful in the production of stable labeled isotopes useful for chemical, biochemical, medical and environmental research. What should be equally important – or would be in a sane world – deuterium is a key component of a potentially extremely mass efficient type (particularly in thorium based cycles) of nuclear reactor, commonly called a CANDU reactor, a result of having been developed in Canada, but otherwise known more generally as a heavy water reactor. The main national nuclear energy program investing in this approach is India’s, although heavy water reactors do still operate in Canada.

Thus there is a role for electrolysis and for improving its efficiency.

This brings me to the paper cited at the outset of this post. The complexity of the electronic structures of the light actinide uranium and the multiple oxidation states suggests - as do other elements with this property of having multiple oxidation states . (This fact, the complexity of the electronic structure of uranium, was the subject of a recent post of mine in this space, Highly sensitive, uranium based UV detectors.)


As an “actinide,” uranium is expected to exhibit a +3 oxidation state, and it does. However the shielding of the 5f orbitals is less effective than it is for the corresponding lanthanides, where the filling of 4f orbitals results in lanthanide chemistry being being dominated by this +3 oxidation state, so much so, that the separation of the lanthanide elements from one another was long problematic.


Because of this ineffective shielding in uranium however, f orbitals are available for chemistry, and this is why, until the Seaborg actinide concept was developed and accepted, uranium was thought to be a cogener of tungsten, rather than a cogener of neodymium, with which it shares only limited chemistry.


Like uranium hexafluoride, a +6 compound, for example, a gaseous compound at moderate temperatures that plays a huge role in isotope separation both for nuclear power and for nuclear weapons, tungsten hexafluoride is a gas, and both tungsten and uranium form, for another example oxocations.

(However for reasons having more to do with quantum chemical formalism than actual chemistry, uranium is -rightly I think - considered an actinide, as is thorium, which effectively exhibits no f related chemistry at all, and in fact, doesn’t really possess a 3+ oxidation state of any significance.)

The availability of multiple oxidation states can be used to reduce water and this brings me (finally!) to a discussion of the paper cited in the opening paragraph of this post.

From the introductory text:

Depleted uranium is a mildly radioactive waste product that is stockpiled worldwide. The chemical reactivity of uranium complexes is well documented, including the stoichiometric activation of small molecules of biological and industrial interest such as H2O, CO2, CO, or N2 (refs 1–11), but catalytic transformations with actinides remain underexplored in comparison to transition-metal catalysis12–14. For reduction of water to H2, complexes of low-valent uranium show the highest potential, but are known to react violently and uncontrollably forming stable bridging oxo or uranyl species15. As a result, only a few oxidations of uranium with water have been reported so far; all stoichiometric2,3,16,17. Catalytic H2 production, however, requires the reductive recovery of the catalyst via a challenging cleavage of the uranium-bound oxygen-containing ligand. Here we report the electrocatalytic water reduction observed with a trisaryloxide U(iii) complex [((Ad,MeArO)3mes)U] (refs 18 and 19)—the first homogeneous uranium catalyst for H2 production from H2O. The catalytic cycle involves rare terminal U(iv)–OH and U(v)=O complexes, which have been isolated, characterized, and proven to be integral parts of the catalytic mechanism. The recognition of uranium compounds as potentially useful catalysts suggests new applications for such light actinides.

Here, from the paper, is the structure of the complex:



The caption:

Figure 2 | Independent synthesis and characterization of the uranium(IV) hydroxo complex [((Ad,MeArO)3mes)U–OH] (2–OH). a, Synthesis of 2–OH with concomitant H2 evolution. b, Molecular structure of the crystallographically characterized complex 2–OH in crystals of C67H84O5U ・ 3(C4H8O), with thermal ellipsoids at 50% probability. All hydrogen atoms except for the hydroxo H were omitted for clarity. c, Infrared vibrational spectra of 2–OH (black) and its isotopomer 2–OD (blue), showing the expected isotopic shift for the O–H stretching vibration ν. The inset is a close-up of the 2–OH spectrum, showing the two OH stretching frequencies at ν = 3,659 cm−1 and ν = 3,630 cm−1.

I very much doubt that this complex - and here I'm referring to the organic ligands and not the final synthesis shown in the graphic - is trivial to synthesize, but then again, it's a catalyst not a reagent, and depending on its stability and turn over rate, it might be viable to make it.

The authors propose the following mechanism for the hydrogen reduction reaction:



The caption:

Figure 3 | Postulated mechanism for the reduction of H2O by the U(iii) complex 1, based on EPR results. The addition of H2O to 1 probably yields a U(iii) aquo species, which forms a fleeting U(v) hydroxo–hydrido intermediate, [((Ad,MeArO)3mes)U(OH)(H)], by intramolecular insertion; this hydroxo–hydrido species then decays to a U(v) oxo species by elimination of H2 (reaction (1)). Subsequently, the U(iv) hydroxo complex 2–OH is formed in a comproportionation reaction between the U(v) oxo and the U(iii) aquo species (reaction (2)). In the net reaction, two U(iii) aquo complexes form two molecules of 2–OH and one equivalent H2.

Their experiments to confirm this mechanism sound like incredible fun:

To elucidate this mechanism, we performed time- and temperature- dependent EPR experiments with a reaction mixture of 1 and H2O in a frozen toluene solution at 7.5 K (Fig. 4). Initially, a spectrum of the neat U(iii) f 3 starting material (10 mM) in toluene was recorded, yielding an almost axial signal with g values centred at 1.56, 1.48, and 1.20 (see Supplementary Information), as expected for [((Ad,MeArO)3mes)U] (1)19. In the following measurement, a mixture of 1 (10 mM) in toluene with a sub-stoichiometric amount of H2O (0.375 equiv.) was prepared. Under these dilute conditions the reaction takes about 2 h at room temperature for completion...

Frozen toluene at 7.5K, I'd guess is made by dipping toluene in liquid helium; that my friends has to be fun.

And then...

Hence, the sample was allowed to equilibrate for 5 min at room temperature and then flash-frozen in liquid nitrogen to trap potential intermediate species in a frozen solvent matrix. Indeed, we obtained a convoluted spectrum of at least two species: the U(iii) starting material and another, welldefined rhombic species with simulated g values at 2.73, 1.83, and 1.35, consistent with an intermediate U(v) f 1 species (Fig. 4)

Here's the EPR spectrum:



And its caption:

Figure 4 | X-band EPR spectrum of a frozen 10 mM toluene solution of 1 with a sub-stoichiometric amount of H2O. The EPR data show a convoluted spectrum of two species: the U(iii) starting material and a well-defined rhombic species, tentatively assigned to the fleeting U(v) hydroxo–hydrido species. Experimental conditions are as follows: temperature T = 7.5 K, frequency ν = 8.96286 GHz, power P = 1 mW, modulation width of 1.0 mT. The experimental spectrum (black) and simulation (red) under these conditions are shown. The best fit for the experimental spectrum is a convolution of the signal of 1 in toluene (simulated, green; g values at g1 = 1.56, g2 = 1.48, g3 = 1.20, with line widths of W1 = 21.4 mT, W2 = 30.5 mT, W3 = 14.4 mT; relative weight of 1.0) and the signal of an additional, rhombic transient U(v) species (simulated, blue; g values at g1 = 2.73, g2 = 1.83, g3 = 1.35, with line widths of W1 = 18.9 mT, W2 = 25.5 mT, W3 = 26.5 mT; relative weight of 0.70). The spectra are offset for ease of viewing.

And finally the full cyclic mechanism of the electrolysis, wherein the oxidized uranium is reduced to U(III):



And its caption:

Figure 5 | Postulated electrocatalytic cycle for H2 generation from H2O in the presence of the homogeneous U(iii) catalyst [((Ad,MeArO)3mes)U] (1). Step 1 (top to bottom-right), H2 evolution and formation of [((Ad,MeArO)3mes)U(OH)(THF)] (2–OH) through oxidation of 1 with H2O. Step 2 (bottom-right to bottom-left), electrochemical reduction of 2–OH, forming the transient anion 2–OH−. Step 3 (bottom-left to top), elimination of OH– from 2–OH− to regenerate catalyst 1.

This device is a battery, and like all batteries, it wastes energy, however it wastes less energy than other electrolysis devices.

Regrettably the world has chosen, much to the detriment of the environment to choose to explore so called "renewable energy" to address climate change, surrounding this choice with all kinds of delusional statements designed to obscure the complete and total failure of this choice to address the expanding use of dangerous fossil fuels.

By their very nature, these systems are wasteful, since they necessarily require redundant systems, usually systems involving gas turbines. To the extent that the excess rotational energy of a spinning turbine being shut for a few hours so we can all make excited, if nonsensical, demonstrations of how great solar energy is, can be recovered, a battery is not a bad idea as a brake, as is the case in hybrid cars. At least some of the energy can be recovered and not wasted.

I actually think that this system, the uranium catalyzed electrolysis system might make sense in very limited circumstances, for example in remote systems, such as on space craft powered by RTG's, where the waste heat of the RTG might serve to provide operating temperatures for fuel cells operating on hydrogen.

Large scale energy storage should be a non-starter on environmental grounds but this is not culturally accepted yet, given the general contempt for science and the inexplicable pop enthusiasm for so called "renewable energy."

A better use for depleted uranium in my view, would be to convert it to plutonium and fission it, but that's just my view.

Have a nice evening, and if you're in this Nor'easter, as I am, by all means be safe.

[eppur_se_muova]

1. Man, that is one lipophilic ligand.

If I didn't know better, I'd think they were really trying to minimize hydration of that OH group.

note to self: add comproportionation to vocabulary as synonym for reproportionation.

[NNadir]

2. Note this reaction is carried out in THF, obviously...

...wet THF.

Early in my career I spent a lot of time trying to dry THF; although one can form separable phases with aqueous solutions, THF in a biphasic system will contain a lot of water. One can see this easily by dropping sodium metal into wet THF. In fact, in periods of extreme laziness, when working late in the lab, it's how I used to get rid of excess sodium, since I often had a lot of wet THF laying around. It probably was dangerous to do that, but I was a kid, and what do kids know?

In theory, this catalyst system is a means to dry THF. It may beat the hell out of distilling out over phosphorous pentoxide.

I'm not sure what effect this chemistry might have on the formation of peroxides though. It probably wouldn't be pretty, although the redox system overall might well minimize that problem. I don't know. These scientists didn't blow up, and lived to publish, a good thing, I think.

[Eko]

3. You know, all you have to do is put in your title.

"I hate all energy producing systems except for nuclear" and you would save yourself quite a bit of time.