Ruthenium Compound Splits Water

If you work in the energy sector and your focus is on hydrogen, then chances are you spend a lot of time thinking about this reaction:

2H₂ + O₂ --> 2H₂O

Researchers in this field tend to fall into one of two groups. The first group wants to use hydrogen to generate energy, usually in the form of electricity via a fuel cell, and devotes its energies into driving the above reaction as far to the right as possible. The second group wants to use energy to generate hydrogen, usually by electrolysis, possibly using solar photons, and strives to drive the reaction as far to the left as possible. (A third group is concerned with hydrogen storage, using high surface area materials such as MOFs, but that’s a topic for another discussion). Although these the two groups would appear to be diametrically opposed, they have at least one thing in common. In both cases, the efficiencies of the processes are often dependent on the oxygen side of the reaction. During electrolysis, forming the O₂ is the hard part, which explains why most of the advancements in this area are related to the anode. The cobalt phosphate electrode coating announced by MIT last year would be one example. And in fuel cells, it’s the cathode that causes most of the headaches, since it’s more difficult to reduce O₂ then it is to oxidize H₂ (at the anode).

In an attempt to negate the need for electrodes, much work has been devoted to identifying transition metal complexes which might catalyze the photochemical splitting of water in solution. The results have been generally disappointing. In many cases, sacrificial reagents are required, usually to facilitate the formation of O₂, obviously limiting the usefulness of the process. In addition, since the H₂ and O₂ are usually co-generated at the same location, an additional step is required to isolate the H₂. Not good at all.

In a recent article in Science , David Milstein describes some ruthenium chemistry which may have some implications in the solar energy field. When water was added to a ruthenium compound they’ve been working with, a new hydrido-hydroxo complex was formed.



Upon heating, this new complex continues to react with water to produce a dihydroxo ruthenium complex along with free H₂. Irradiating this dihydroxo complex with a halogen lamp causes it to revert back to the original hydrido-hydroxo complex, along with the formation of O₂. Catalytic photochemical splitting of water without the use of sacrificial reagents. Not bad. Even better, since the H₂ and O₂ are produced during different steps, there are no separation issues to be dealt with. This process is a loooong way from being commercially viable, but I enjoy any chemistry where an organometallic compound reacts constructively with water without simply igniting or decomposing into an ugly pile of goo.

Nano-gold Catalysts

Reports of nano-gold and its use as a catalytic material are beginning to pile up. Last week I mentioned nanogold particles in stained glass windows and their ability to oxidize organic pollutants. This week, two more articles showed up, here and here. Hmmm…. seems like we’re beginning to reach a critical mass here. Pretty good for a metal that hasn’t been considered a particularly good catalyst over the years. Perhaps I'll start paying more attention to this area. I’ve read articles about nano-particle based catalysts in the past and have generally been underwhelmed, but nano-gold catalysts may be about to change that view. And that’s because of two words that are often found in these reports.

Room temperature.

Or at least, low temperature.

The obvious reason for using nano-particles is their much larger surface area to mass ratio. Higher surface areas generally mean better catalysts, so the drive to make smaller and smaller catalyst particles has been going on for decades. Back before “nano” became a marketing term, I recall at least one set of researchers making platinum nano-particle catalysts to improve the efficiencies of automotive exhaust catalysts. They succeeded, but unfortunately, at the high operating temperatures (generally above 400^oC) to which these catalysts were subjected, the platinum nano-particles tended to move around and coalesce into much larger particles, losing much of their surface area and eventually not looking all that different from more crudely prepared catalysts. In fact, with these types of catalysts, research is focused less on the catalyst itself and more on the substrate which supports the catalyst -- changing its properties so as to limit the movement of the catalyst particles. This is always a bit of a balancing act since too much interaction between the catalyst and substrate can change the properties of the catalyst in undesirable ways. But if the reactions to be catalyzed take place near room temperature, or at least at lower temperatures, you stand a much better chance of maintaining the catalyst nanoparticles and hence their benefits.

Gold has traditionally been the forgotten metal, with all catalyst discussion centering on its neighbors on the periodic table (Pt, Pd, Rh, Re, Ru, and Ag). That looks like it's about to change. The big question to be answered: Why are nanogold particles so much better than bulk gold? Here are some possibilities to consider.

1. Larger surface areas. Well yes, you would expect some benefit here, but gold has been so unremarkable as a catalyst, I don’t believe a simple increase of surface area would account for this big of an effect. If it were that easy, everyone would just have dumped more gold into their catalyst formulations in order to increase the available surface area.

2. More defect sites. In many cases, catalysis only occurs at specific sites on the catalyst surface, such as the so-called "steps" as shown in the figure below. Perhaps nano-gold particles naturally have more of these locations.

3. More of the appropriate crystal faces. Different lattice faces such as (111) or (100) often have significantly different catalytic rate constants. See figure above. Perhaps nano-gold particles have more of the correct lattice orientations at the defect sites than bulk gold.

4. Different surface oxide properties. One of the reasons metals like Pt and Rh make such good catalysts is their ability to maintain surfaces which are relatively clean of oxygen atoms. Whereas the early transition metals tend to form oxides, the late transition metals favor the metallic state. Heat vanadium or molybdenum in air and you form the oxides. Heat platinum or its salts in air and you wind up with platinum metal. Keeping the surface free of oxygen atoms makes the metal more available for catalyzing the desired reactions. Perhaps smaller gold particles tend to have less oxygen on their surface.

Figure obtained from http://www.pnas.org/content/103/28/10577.full.pdf