LOCAL GAS SAMPLING AND SURFACE HYDROGEN DETECTION IN CATALYSIS ON PLANAR SURFACES

A new experimental set-up for in situ studies of heterogeneous catalytic reactions on metal films in the pressure range 10(-8) -2000 mbar is described. The reactions take place on the outermost layer of a metal-insulator-semiconductor structure, whose capacitance-voltage characteristics are extremely sensitive to variations of the hydrogen coverage on the metal surface. It is shown how this device can be used in combination with mass spectrometry and work function measurements. Mass spectrometry is performed with high sensitivity and fast response even at atmospheric pressures, owing to local gas sampling with a capillary leak. Using this technique, reactant pressure gradients close to a catalyst surface can be quantified, allowing true rates of mass transfer limited reactions to be determined. The performance of the system is illustrated by the H2+O2 reaction on Pt in the mbar region. The maximum efficiency of water production is found for a hydrogen/oxygen pressure ratio of 0.1, and the reaction probabilities are S(H)2 = 4 x 10(-3) and S02 = 8 x 10(-4) at this optimum point. The behaviour of the experimental set-up is further demonstrated by the CO + O2 oscillatory reaction on Pd and Pt.