Insights into the Active Sites for Catalytic Steam Reforming of Methane from Oscillations in the Reaction

A new oscillating catalytic reaction is discovered: steam reforming of CH₄ in the presence of SO₂ over Rh nanoparticles. The reducing products from the reforming reaction convert SO₂ to adsorbed sulfur, which causes deactivation. When activity, and thus the reducing environment, is lost, the H₂O oxidizes adsorbed S back into gaseous SO₂, and activity re-emerges via a transient, S-free, but less active metastable state. Electron microscopy, spectroscopy, and theoretical calculations show that adsorption of S drives a reconstruction of the particle edges, which is reversed when H₂O oxidizes away the sulfur. The reconstructions during S-adsorption alter the particle edges decorated by (211) steps into edges decorated with a (110) step. Our theoretical calculations reveal that the (211) configuration is an order of magnitude more active for reforming and identifies the reconstructed edges as the metastable state. The cycling between the presence and absence of the active (211) step sites thus leads to the discontinuity in activity that results in sustained oscillations. The correlation between activity and the presence of (211) step sites identifies these as the primary active sites for the industrially important catalytic reforming of methane. These results illustrate how heterogeneous catalysts can be extremely sensitive to the detailed configuration of the active site, such as steps with a certain structure. As steps are observed at the edges of the nanoparticles, this also explains the long-standing question of why 3D catalysts with edges can cause oscillations despite being composed of 2D facets that do not cause oscillations at the same conditions.