Orbital-Symmetry Rules for Chemisorption and Catalysis

A new method, based on orbital-symmetry rules, is suggested for predicting the surface sites of most importance in chemisorption and catalysis. The relationship between the orbital symmetry of reactants and products is often successfully used to predict organic reaction paths. Here we compare the symmetry of the highest occupied and lowest unoccupied absorbate molecular orbitals with the symmetry of the substrate wave functions $\phi \left(k_F\right)$ as obtained in band structure calculations. This comparison is quite distinct from previous suggestions involving the matching of charge densities or the consideration of the wave functions of isolated substrate atoms. The effect of Fermi level changes due, e.g., to doping is naturally obtained. Results for ${\mathrm{H}}_2$, ${\mathrm{O}}_2$, ${\mathrm{N}}_2$, ${\mathrm{F}}_2$, H, N, O, and F chemisorption on the graphite basal plane are presented.