A new type of gold catalyst

Gold is generally considered to be a rather poor catalyst in heterogeneous reactions, this lack of activity usually being attributed to its chemical stability. For this reason, research into the properties of gold as an industrial catalyst has for long been neglected, but in the light of some recently published work by Professor Bond and his colleagues (1, 2), it seems to be worth while reconsidering the potential applications of gold, especially for the preparation of highly selective catalysts. All metal surfaces might be supposed to have some catalytic activity, and as a rule of thumb the smaller the overall catalytic activity the higher the selectivity of the metal. This purely phenomenological statement is supported by some comparative studies of catalytic properties of the platinum group metals (3), while for gold it can be deduced from studies of gold-palladium alloys (2). In order to get a useful catalyst on the basis of gold, its "natural selectivity" (still speaking from the phenomenological point of view) must be preserved while its catalytic activity is enhanced. With this aim in mind, two useful guidelines for gold catalyst preparation can be deduced from the above-mentioned papers (1, 2), the first employing the effect of transmutation of catalytic properties of finely dispersed supported gold, the second making use of alloys or supported alloys in which the catalytic activity of a very active metal is tempered by gold. Unfortunately, inasmuch as the physical and chemical background of these two effects is still not fully understood, the research chemist is left with the trial-and-error method in looking for useful gold catalysts optimised in respect of a particular chemical reaction. Recently another kind of catalytic behaviour of metal surfaces (4, 5), especially gold, has been investigated using a means to enhance catalytic activity which differs from the classical ones, i.e. a method of enhancing catalytic activity without destroying the high natural selectivity. The findings stemming from these investigations differ from other work essentially in the fact that an electrical current passes through the catalyst during the reaction. As early as 1937, Lennard-Jones (6) supposed that conduction electrons could play a great part in surface reactions on metallic surfaces, namely in energytransferring processes, resulting in the activation of chemisorbed complexes. Increase of the energy carried by the electrons should manifest itself in an increase in the rate of reaction where the particular chemisorbed complexes take part. There is, however, no experimental foundation for this hypothesis, mainly for the reason that the total energy of conduction electrons is determined by the thermal energy, and field energies may be almost neglected, so that energy rise will always be accompanied by a temperature rise of the catalyst metal. The basic idea for overcoming this difficulty is to produce an electric current which carries electrons with excess energy (energies above the Fermi level) by means other than temperature or field excitation. These electrons are called hot electrons, and can quite easily be produced by means of thin-layer devices well known in solid-state electronics. For experiments described in this paper a sandwich structure has been used, composed of a supporting metal base, covered with an insulating layer of alumina 50 to 1500 A thick, in turn covered with a top layer of gold (about 100 A thick) which is the catalytically active surface of the device. Only this top layer is in contact with the reactants. When a small voltage is applied across such a device, the Fermi level is lowered on the positively biased top metal layer and electrons may pass through the insulator into the metal by direct tunnelling or by any other related effect. They will in fact lose energy during the passage through the metallic top layer towards the surface by electron-electron or electronphonon scattering processes, but when this top layer is not too thick a proportion of these electrons reach the surface with an energy greater than the Fermi energy. These hot electrons cannot persist in this excited state and must therefore drop back to the Fermi level, this being their only stable state of thermodynamic equilibrium. The excess energy of those electrons reaching the surface must be transferred to another particle, e.g. a chemisorbed complex, which is in turn activated to a higher energetic state. Hot electrons thus represent a source influencing the energy distribution in the surface population