Geometric and electronic factors determining the differences in reactivity of H-2 on Cu(100) and Cu(111)

We present a comparative density functional theory study of H-2 dissociation over Cu(100) and Cu(111) surfaces. For like reaction geometries we find that the more open (100) surface is the more reactive, as expected from the usual arguments for electronic effects in metal surface reactivity theory. However, when allowing for different reaction geometries, we find that the minimum energy barrier is 0.1 eV larger over the (100) surface compared to over the (111) surface. The larger barrier correlates with a more stretched H-H bond in the transition state and with further separated final hollow sites for atomic H on the (100) surface compared to the (111) surface. Consequently, we assign the minimum energy-barrier ordering to geometrical effects. Finally, using a simple model of the sticking dynamics, the calculated difference in barrier heights over the two facets is discussed in relation to the difference in onset of hydrogen sticking, as measured in molecular beam experiments.