Surface and bulk diffusion of active particles

Many physical-chemical processes on surfaces of solids involve free atoms and radicals as intermediate particles. The latter diffuse along the adsorbent-catalyst surface and govern not only kinetics of catalytic, photocatalytic, or some heterogeneous radiative processes, but also creation of certain substances as a result of the reaction. 

Heterogeneous recombination of active particles and their interaction with molecules of the adlayer are simplest processes of this type. The rates of such reactions as functions of surface coverage by the specified reagents are fully determined by the rate of their surface diffusion towards active centers. In a number of cases, the rate of lateral diffusion is determined not only by the type of diffusing particle, but also (sometimes, predominantly) by the composition and state of the solid substrate surface. Taking into account the role played by the composi- 

As the working temperature of the substrate was increased, the induction period (the delay time) of increased conductivity decreased due to increased rate of lateral diffusion of hydrogen atoms towards the sensor. The activation energy for surface migration of particles along a Si02 substrate estimated from the tilt of the Arrhenius plot was found to be about 20 kj/mol. 

Similar experiments were conducted with deuterium. Comparing the results of these experiments with those obtained with protium, a conclusion was made that the ratio of calculated coefficients of surface diffusion of these particles is close to Vz. In other words, this ratio is a manifestation of a usual isotopic shift for diffusivity given by as one might expect assuming that surface diffusion 

The coefficient of diffusion for one-dimensional motion of particles is given by 

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