Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLID-GAS INTERFACE 181Rates of chemisorption vary over a very wide range and many different experimentalmethods have been evolved to study specific gas-solid interactions. However,before proceeding to a description of the experimental methods, certaincriteria must be defined to ensure that the correct phenomenon will be observed.Before any experiment can begin, it is necessary that all contaminating layers areremoved in order to provide a 'clean' surface which is characteristic of the bulkmaterial (or as near as the degree of unsaturation and lack of symmetry at thesurface will allow). In practice such 'clean' surfaces are quite difficult to obtain andonly in a relatively small number of chemisorption experiments can it be said thatthis criterion has been reasonably satisfied. Secondly, a 'cleaned' surface must bemaintained free of contamination for the duration of the kinetic measurements.This usually requires the use of high vacuum torr) or ultra high vacuum(lo-' torr) techniques, because of the high reactivity of most clean solid surfaces.2.1.1 Preparation of clean surfaces2* 39 l2(a) Chemical techniquesReduction with hydrogen has been applied to metals and semiconductors inorder to remove oxide layers. Purity of the hydrogen is very important andsmall amounts of oxygen or water vapour (0.01 %) prevent successful reduction7.Frankenburg* used a vacuum furnace to clean tungsten powder by reduction attemperatures up to 750" C. The temperature required to reduce the oxide dependson its thermodynamic stability. This temperature is about 20" C for palladiumand platinum, above 300" C for nickel and cobalt and above 400" C for iron.A variant of this method is reduction with atomic hydrogen' produced in thereaction vessel by an electrodeless discharge. This is accomplished by windingthe reaction tube with a few turns of wire connected to a 15 Mc/sec oscillator.A current is then passed through the coil whilst hydrogen is admitted to a sufficientpressure (lo-' torr) to give a luminous discharge. The disadvantages are that theatomic hydrogen might remove contaminants from the walls into the gas phase.Hydrogen will be absorbed by the adsorbent and must be removed by stringentoutgassing. Although surfaces produced in the above manner are active for chemisorption,most of the examples quoted in the literature have not been conclusivelyshown to give a completely clean surface. Cobalt is perhaps an exception whereRudham and Stone' O obtained adsorption results with reduced cobalt very similarto those obtained for evaporated films.Carbon can be removed from tungsten, molybdenum and nickel field emissiontips by annealing in oxygen or hydr~gen'~. Becker et al." have,made a detailedstudy of the removal of carbon from tungsten ribbons and conclude that it can besubstantially removed by heating in oxygen torr) for 24 hours. The efficiencyof carbon removal is dependent on the initial purity of the tungsten and it may benecessary to continue the heating for much longer period^"^.References pp. 270-278