Designing catalytic systems to be fast acting and long-lived, and to form only the desired end products, has been a challenge to industry since the 1920’s. In many of these processes a stream of reactants, often at high temperature, is passed over tiny particles of metal on an oxide support surface.
Gabor Somorjai of Berkeley Lab’s Materials Sciences Division (University of California), has devoted much of his attention to creating ‘high-technology’ catalysts that are stable at high temperatures, resistant to poisoning, and 100 percent selective.
Recently, Somorjai and his team obtained encouraging results using ordered platinum ‘nanocluster’ arrays.
‘We coat the oxidised silicon crystal with a thin film of polymer, then use an electron beam to burn a pattern of holes through the polymer to the substrate,’ said Somorjai. ‘A platinum film is then evaporated onto the polymer, and it fills in the holes; when the polymer is removed, we are left with platinum clusters of uniform size and spacing.’
The result is a wafer of silicon oxide half a square centimetre in area, covered with a billion particles of platinum spaced 100 nanometers apart. In a recent experiment, particles examined with an atomic force microscope and a scanning electron microscope were found to be some 20 nanometers in diameter and about 15 nanometers high.
Initially each metal cluster was polycrystalline, consisting of several crystal domains. Under sufficient load, the tip of an atomic force microscope could break off some of these clusters. But after the wafer was heated, the crystal domains grew together into single crystals of platinum that could no longer be easily broken.
Catalysis tests were run in a reaction chamber designed by Somorjai that allows different reactions at different pressures and temperatures.
Two similar chemicals, cyclohexane and benzene, can be converted into one another. Increased pressure tends to add hydrogen atoms, favouring cyclohexane; increased temperature tends to free hydrogen atoms, favouring benzene.
In both reactions, the nanoparticle array was much more efficient and selective than small foils of pure platinum or wafers of pure silicon. Reactivity remained high even after the platinum surface in each cluster had been greatly reduced.
‘Between each test we cleaned the catalyst, shaving off any organic ‘dirt’ by bombarding the wafer with a beam of ionised neon,’ said Somorjai. ‘At the same time, the beam planed down the platinum clusters, until eventually the surface area was only 40 percent of what it had been.’
The nano array catalyst remained 20 to 30 times more active than the platinum foil. ‘This led us to realise that it wasn’t just the metal that promoted catalysis. The interface between the metal and the oxide was much more important that anyone had previously thought.’
Armed with the knowledge that the metal-oxide interface is crucial to catalysis, and having demonstrated high efficiency and selectivity with single reactions, Somorjai and his team are poised ‘to move the whole field of catalysis into the high-tech mode.’