Structural Changes in Supported Rhodium in Catalytic Reactions.

The paper to which I'll refer in this post is this one: Rhodium Catalyst Structural Changes during, and Their Impacts on the Kinetics of, CO Oxidation Silvia Marino, Lai Wei, Marina Cortes-Reyes, Yisun Cheng, Paul Laing, Giovanni Cavataio, Christopher Paolucci, and William Epling JACS Au 2023 3 (2), 459-467.

The paper is open to the public, JACS Au is an open source journal.

I'm actually not going to spend a lot of time discussing the content of the paper but, rather discuss the implications with respect rhodium in another setting. Here, nonetheless is the introduction to the paper itself:

My interest is this: Rhodium is an absolutely essential element in automotive catalysts for a reaction which is not the subject of this paper, specifically, the catalytic destruction of NO_x type pollutants, a very serious problem, since the fate of NO_x often passes through the compound N₂O, aka "laughing gas" which is both a greenhouse gas and an ozone depletion agent, no laughing matter. (The main source of N₂O in the environment is not automobiles, however; it's agriculture. However automobiles do not make things better; they add to the problem.)

In recent years I have come to consider air as the working fluid in a certain kind of nuclear reactor, a Brayton type cycle with air as the working fluid with the idea that passing air through such a device might represent a pathway to cleaning it up by radiolytically destroying some onerous pollutants, N₂O among them, but residual CFCs, chlorinated solvents like chloroform and dichloromethane, and HFCs. The idea of air as a working fluid in a Brayton device is not new or unique. Every jet engine in the world is a Brayton cycle device with air as a working fluid.

However, a Brayton device relies on pressure and temperature, conditions under which nitrogen "burns" (endothermically) to form NO_x. In diesel engines, which also compress air and heat it with fuel ignition, a means of addressing this problem is to add urea to the fuel, in what is called the SCR reaction. Rhodium catalysts are inactivated in diesel systems by carbon build up, a lesser problem in Otto type gasoline engines, although certainly not unknown.

Rhodium is a very rare element, and is generally an impurity in other precious metal ores.

It is also, however a fission product and is found in used nuclear fuel, generally in occlusions, alloys of palladium, ruthenium, technetium and rhodium.

It is generally understood that the supply of rhodium in used nuclear fuel exceeds the amount found in natural ores.

Rhodium is a monoisotopic element; its only stable isotope is ¹⁰³Rh; all other isotopes are radioactive. In used nuclear fuel rhodium has a somewhat unique property generally not found in fission products. Although the rhodium is 99% isotopically pure it does contain - the rare aspect - a neutron deficient isotope. Most fission products are neutron rich, they decay by β^- decay to stable isotopes having the same mass number as the fission product itself. In the case of rhodium, however, it contains a nuclear isomer ^102mRh which decays into its ground state isomer ¹⁰²Rh with half-lives of 3.74 years and 207 days respectively by either electron capture or by β⁺ (positron) emission, both processes which release powerful γ rays, in the case of β⁺, annihilation gamma rays at 0.511 MeV. Although the concentration pf the two ¹⁰²Rh nuclear isomers is low, this is serious radiation. In general, it is believed that only rhodium from used nuclear fuel that has aged for about 50 years will be safe to use in automobiles. We have such fuels, but they are limited in supply.

It occurs to me that in closed systems, the radioactivity is not such a big deal; indeed in irradiating air in closed systems, it may be beneficial, again, by destroying serious intractable pollutants, including but not limited to N₂O.

It is known that in the case of automobiles, the catalytic metals are released in nanomolar amounts from the catalysts. Traces of palladium, ruthenium and yes, rhodium, can be detected on roadways. From the paper cited, we can understand why this is so, catalytic particles are in motion. The full paper describes single free atoms detected by means of the techniques utilized.

I do believe the problem can be addressed, and it might be wise to expose air to relatively "fresh" fissiogenic rhodium, rhodium still producing γ rays. γ radiation after all represents the sink for many pollutants in the upper atmosphere, a sink including the natural output, even without the use of fertilizer, of N₂O, which has always been part of the nitrogen cycle on Earth, albeit it is the case that fertilizer has made it problematic. γ radiation also produces the ozone that protects life from UV radiation. As it happens however, owing to the Maxwell-Boltzmann distribution, most anthropogenic pollutants are heavy, and tend to remain close to the surface, limiting the rate at which they can be destroyed by solar radiation. This suggests why exposure of air in the lower atmosphere is a good idea. (Medical x-rays do this, but obviously to a limited extent.)

Still, it's a problem worthy of consideration, the containment of radioactive rhodium in an airflow Brayton device. I'm not sure that the risk is very high for the catalytic use of fissiogenic rhodium before a 50 year cooling period, but again, it is worthy of consideration.

Have a nice day tomorrow.