Controlling Heterogeneous Catalysis With Subsurface Oxygen

Rhodium surfaces play a crucial role in heterogeneous catalysis, driving extensive research on their reactivity. In particular, CO oxidation is of great interest, where different oxygen species at the surface can influence catalytic activity. Under certain conditions, rhodium can also host sub-surface oxygen species, further affecting reaction dynamics. In this work, we combine molecular beam surface scattering, ion imaging, and ultra-high vacuum techniques to investigate the impact of subsurface oxygen on CO oxidation on single-crystal Rh surfaces. When oxidizing CO at the (2 $\times$ 1)-O adlayer without subsurface oxygen, we observe hyperthermal velocity distributions of desorbing CO₂, indicating significant energy release along the translational coordinate directly from the transition state. In contrast, subsurface oxygen induces thermal velocity distributions. DFT calculations indicate the formation of a chemisorption state in presence of subsurface oxygen that is energetically favored and transiently traps product CO₂ long enough for thermalization.