Researchers have confirmed that through 'superatom' phenomena it is possible to mimic the properties of precious metals.
Return of the alchemists
With its associations with wizards, spells and magic, alchemy is not a term you expect to come across in a prestigious research journal. Like astrology and divination, the idea of transforming ho-hum materials into wondrous ones at the wave of a wand has long been dismissed by scientists. Yet last week, the US National Academy of Sciences published a paper that not only begins with that taboo word, but even claims to explain how to bring about such transformations.
The implications are certainly startling. According to the authors of the paper, it is possible to mimic certain properties of precious metals as platinum and palladium using combinations of far more mundane materials. And that opens up the prospect of replacing expensive strategic metals in many industrial applications by much cheaper alternatives. The origins of the breakthrough lie in a curious discovery made by chemists around 40 years ago. A number of research teams in Europe and America reported that when carbon atoms were added to atoms of tungsten, the resulting material - tungsten carbide - seemed to possess some of the properties of the far more costly metal platinum. In particular, it mimicked the ability of platinum to act as a catalyst, boosting the efficiency of chemical reactions. Somehow the combination of atoms of two different elements were behaving like a single "superatom" of an entirely different atom.
During the 1980s, more examples of the superatom phenomenon emerged. Researchers at the California Institute of Technology discovered that combinations of aluminium atoms had some of the properties of the far rarer metal ruthenium, used as a catalyst by the chemical industry. In the search for explanations, attention focused on the key players in chemical reactions: electrons. The properties of every atom are dictated by the arrangement of these subatomic particles in so-called "shells" surrounding the central nucleus of their host atoms.
The link between these arrangement and chemical properties is famously demonstrated by the Periodic Table, the block-like diagram that graces the walls of every chemistry lab. Atoms with similar configurations of electrons and thus similar properties, such as alkali metals or inert gases, end up close to each other. The fact that superatoms mimic the behaviour of individual atoms suggests that both have similar electron arrangements. That, in turn, suggests that new types of superatom could be predicted by using the Periodic Table.
Now a team of researchers at Pennsylvania State University led by Prof Welford Castleman has confirmed this, opening the way to a whole new branch of chemistry. The team's discovery stems from its work on a superatom constructed from 13 aluminium atoms, which studies revealed had properties similar to a single atom of iodine. On the Periodic Table, iodine sits in the column of so-called halogen elements, which are just one electron short of becoming inert gases. That suggests adding one more electron to the iodine-like cluster of 13 aluminium atoms would make them chemically inert - which is just what the team found.
Emboldened by this, the team has now used the Periodic Table to predict that a superatom made from atoms of titanium and oxygen should behave like a single atom of nickel. Their reasoning can be understood with basic arithmetic: titanium and oxygen have four and six outermost electrons respectively, while nickel has 10 such electrons. The same idea has led them to predict that zirconium plus oxygen should act like palladium, which - like platinum - is a widely-used but pricey industrial catalyst.
In research published last week in the online early edition of Proceedings of the National Academy of Sciences, the team has now confirmed these predictions - along with the 40-year-old connection between tungsten carbide and platinum. But they have gone further, using spectroscopic studies to show that the root cause really is the similarity in the electron structures of the superatoms and the atoms they mimic.
Now the team is working its way across the big central block of the Periodic Table, consisting of so-called transition metals from scandium - used in aerospace alloys - to gold. Their aim is to discover other superatoms, and to gauge the extent of their similarities to standard atoms. Not surprisingly in view of the commercial implications of success, the Penn State team is not alone in its quest. Researchers at Virginia Commonwealth University recently announced that a cluster of eight caesium atoms plus a vanadium atom mimic the magnetic strength of manganese. The research team has also predicted that superatoms of gold and manganese will be magnetic while not conducting electricity - a combination making them useful in some biomedical applications.
Such discoveries suggest we are witnessing the birth of a whole new branch of chemistry, and one that could not have arrived at a better time - for many critical technologies are crying out for a breakthrough in material science. An example is nuclear fusion power, in which the energy source of the sun and stars is harnessed in giant reactors known as tokamaks. The walls of such reactors are blasted by intense radiation in the form of fast moving neutrons, and the resulting damage and replacement threatens to make nuclear fusion uneconomic. Reactor walls made of superatomic metals may be the answer. The economics of many other technologies, from superconducting power cabling to fuel cells, might also be transformed by the discovery of low-cost superatomic equivalents.
Before it fell into disrepute, alchemy had many distinguished adherents - among them Sir Isaac Newton. Their 21st century successors, armed with a book of spells in the form of the Periodic Table, may soon be achieving feats of magic that will astound us all. Robert Matthews is Visiting Reader in Science at Aston University, Birmingham, England