  It is shown that a two-component percolation model can explain an
experimentally observed behavior, namely that a network built up by a mixture
of sintered nanocrystalline semiconducting n- and p-grains can exhibit
selective behavior, i.e. respond with a resistance increase when exposed to a
reducing gas A and with a resistance decrease in response to another reducing
gas B. To this end, a simple model is developed based on realistic assumptions
about the reactions on the grain surfaces. The resistance is calculated by
random walk simulations with nn-, pp- and np-bonds between the grains and the
results are found in very good agreement with the experiments. Contrary to
former assumptions, the np-bonds are crucial to obtain this accordance.
