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Single Atoms, On Demand

We chemists spend a lot of time doing things in the solution phase. It makes sense – if you want things to react, getting all the partners dissolved in some medium where they can roam around and contact each other is surely the way to go, most of the time. But it’s also true that we don’t really have a good grasp of what’s happening in solution at the molecular level. Solvation shells, nanoscale aggregates and clusters, effects of interfacial regions: the idealized picture of things floating around is complicated by a lot of real-world effects, which can be very important. They’re particularly so when it comes to dissolution and its opposite, precipitation and crystallization.

One example is the formation of microcrystals. I know that I had a mental picture of these that is probably incorrect. As I imagined it, all sorts of nucleation events happened in solution but the size of the resulting crystals was constrained (perhaps by amount of available solute, or by effects of stirring) so that they just didn’t grow very large. But in many systems, it appears that what happens first is a liquid-liquid phase separation (sound familiar?), what’s called spinodal decomposition into microscopic solute-rich and solute-poor droplets, and the solute-rich ones then crystallize.

This new paper provides another example of liquid-phase weirdness. The authors are looking at a very well-known reaction, reduction of a metal salt to the elemental metal. That sentence hides a lot of complications: for example, what form does this metal arrive in? You can get a mirror deposit on the inside wall of the vessel (as with the famous old Tollens reagent or the other more stable mirror-silvering mixtures), but more often you get some sort of fine black powder. Microscopic examination of that, though, reveals a huge range of particle sizes and morphologies depending on conditions, and it’s just those changes that can make a big difference in (for example) catalyst performance. Finely divided high-surface-area metals (palladium/platinum on carbon, Raney nickel, Rieke metals) are very important in synthetic chemistry, and their properties vary tremendously.

This work provides an ingenious variation. The authors take an aqueous solution of the metal salt (silver nitrate in the first example), freeze it in liquid nitrogen, and then let this ice chunk slowly dissolve in a cold solution of the reducing agent (sodium borohydride). That gives, under the right conditions, essentially an atom-by-atom release of silver ions into the solution, and what you get out is atomically dispersed metal. By electron microscopy, the great bulk of the material is produced as what are apparently single silver atoms, not clusters, aggregates, etc. (there’s also further characterization by EXAFS). I’m surprised that it doesn’t form those on standing, but as I said before, there are a lot of things about the solution phase that my mental models don’t handle well. The authors believe (on the basis of modeling calculations) that the solvation shell around the single metal atoms is a barrier to forming such nanoparticles, and that the release of individual metal atoms is thus key to the whole process. Dimers of silver atoms and the like seem to be just fine in aqueous solution if they can form (or if they’re whittled down to that size); this synthesis just doesn’t let it happen.

The paper demonstrates the same sort of chemistry with a whole list of metals – platinum, palladium, cobalt, nickel, copper, iridium, gold and more. It certainly seems to be a general process, and the resulting atomically dispersed metals would, you’d have to imagine, be very active catalysts in further reactions. What’s more, this entire method would seem to have applications in many other reactions, too, where the slow release of one reactant into an excess of another is important. I’m actually kicking myself not to have thought of something like this – in hindsight it seems like a perfectly simple idea, but try coming up with these in the forward direction!


21 comments on “Single Atoms, On Demand”

  1. ScientistSailor says:

    I stumbled upon this effect years go. I had a reactive reagent (PhSCl) as a solution in benzene. Adding this solution drop-wise to a cold (-78 C) solution of substrate in THF resulted in small frozen drops in the reaction. Since the reagent is bright orange, you could watch each drop dissolve before adding the next drop. Yields were much improved vs. faster addition methods.

    I relayed this technique to another grad student in the lab in front of our prof, and he said “so your contribution to Science is frozen benzene?”

    1. John Wayne says:

      That is a super interesting observation. You implied this, but did adding the same reagent at the same concentration and rate in THF give different results?

      1. ScientistSailor says:

        I made the reagent as a solution in benzene, so I never had the proper control, but slower addition did result in better yield through reduction in formation of a specific by-product. I think the sub-surface addition might have been an alternative, but as the reagent was in benzene, I’m sure the needle would have clogged…

        I made the reagent as a solution in benzene so I could keep it frozen solid, since it was unstable.

    2. milkshake says:

      this is useful trick how to achieve pseudo-high dilution, if local excess of warm PhSeCl ruins the yield. But you could have probably achieve a similar effect by syringe-pumping directly into the reaction mix under the surface of the reaction mix, away from the center vortex, and stirring vigorously. (Of course in a solvent that does not freeze). Maybe it could be even advantageous, as the reagent solution pumped throught the needle immersed in the reaction would have time to cool down to -78C

  2. another guy named Dan says:

    One of the Rules of Engineering “Simple does not imply Easy.”

  3. NotHF says:

    I’m also surprised that these don’t seem to aggregate on standing, or especially upon heating. It’d be nice to see some AFM in addition to the STEM just for improved resolution, but the radii and distances observed are too small for dimers, and the XANES/EXAFS data are fairly compelling.

    So in monolayer protected cluster chemistry where I come from, it’s well known empirically that controlling the reduction kinetics of your synthesis dramatically affect your product outcome, this seems like the absolute limit of those strategies, although usually the stoichiometry is reversed so your metal precursor is in excess to your reductant. I’m very tempted to run this in the reverse direction, I bet you’d still get particles. (if I missed that in the manuscript, please let me know).

  4. Barry says:

    How well has Reuben Rieke characterized the finely-divided reagents he makes by reduction of metal salts (in non-aqueous solvents, with heterogeneous reductants)?

  5. Gradschooltips says:

    If your day to day life and mood is affected by malicious gossip that is intended to undercut your standing in the workplace, u dont have to put up with it. It is harrassment. Look it up–it can be litigated. I wish someone would have told me that as a first year. It seam benign at first, but u realize later its simply harrassment.

  6. Gradschooltips says:

    As one example, as I started experiments in my first month of grad school, a postdoc claimed that ” i was not asking questions” and i was then reprimanded by my PI who used invectives. That, my friends, is illegal.

  7. Anonymous says:

    Hydrogen atoms, anyone? Back in the ’70s, a couple of groups were racing to prepare stable “atomic hydrogen.” It was a near tie in ~78, but Silvera, then at Amsterdam, edged out Greytak – Kleppner at MIT. A newer method involves photolysis of H-Cl to generate the spin polarized H atoms.

    “Ordinarily, atomic hydrogen is a very reactive gas, with the atoms eagerly recombining to form molecular hydrogen. However, by polarizing the electronic spins of the atoms, the lifetime and density of the gas are greatly extended to the point where it is essentially stable.”

    I wonder if you can order a tank of spH from Airgas and try some reactions. If you can specify that you want a tank of “spin up H” (or “spin down H”) maybe you can use it for enantioselective hydrogenations in a magnetic field.

    (If you can handle the pressure, you can try metallic hydrogen, too.)

    1. Barry says:

      see “Langmuir torch”

      atomic Hydrogen is old news

      1. Barry says:

        Maybe someone here can answer an old question for me. Would one get a hotter flame passing both the O2 stream and the H2 stream through electric discharges (effectively reacting ozone with atomic hydrogen) rather than just exciting the Hydrogen stream?

      2. Anonymous says:

        I just want to emphasize that STABLE atomic H was a big deal in the ’70s (and onward), cf., reactive atomic H in the Langmuir torch of the ’10s. Here’s a leading ref to the reaction of ozone and atomic H: “The Reactions of Atomic Hydrogen with Ozone and with Oxygen”, McKinley and Garvin, J. Am. Chem. Soc., 1955, 77 (22), pp 5802–5805, DOI: 10.1021/ja01627a007 .

        1. a recovering student says:

          If you are referring to events in the 1970’s, I think you are probably reading the wrong papers. This was back when they thought that cigarettes in primates were a good model system for ectopic pregnancy. Not sciences’ finest decade at all.

  8. Me says:

    Since the comments to this thread have taken a rather surreal tone, i thought I’d add my own point to this:

    this method of obtaining ‘monatomic’ metals is a secret that ‘big pharma’ didn’t want you to know about. Check it out:-

  9. colintd says:

    Whilst the metals aren’t “dissolved” in the classic sense (i.e. you can’t start with the bulk metal and then dissolve by application of water), the paper seems to suggest that you literally get a a “solution” of individual gold/silver/platinum/etc atoms in water as opposed to a colloid.

    If this is the case, is there a practical limit on how concentrated you can make the solution through evaporation? One assumes that at some point the gold atoms will start to clump, but might you instead form an azeotrope?

  10. Thoryke says:

    I will confess that my first thought when I read the title was “So, another proof that homeopathy is really a form of talk-therapy?”

  11. Barry says:

    Is this conceptually different from releasing individual (zero-valent) Nickel atoms from solution in aluminum as Raney did?

    1. Derek Lowe says:

      My understanding is that Raney catalysts, as least as they’re used practically after all the washing, etc., are microporous nickel structures: What happens during the aluminum-eating phase, though, I don’t know. . .

      1. Barry says:

        The activity of Raney Nickel is notoriously sensitive to the washing steps that follow the release of Nickel atoms from the aluminum matrix. I suppose one might run that digestion in the presence of a stabilizer to impede the subsequent aggregation. But carbon monoxide’s the first water-stable such stabilizer that leaps to mind that zero-valent Nickel likes. And Ni(CO)4 is not an interesting way to be mon-atomic.

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