The lowest energy structures were further relaxed with QE using the same settings as the slab relaxation until the atomic forces were under 0.01 eV/ Å. The vibration analyses were performed using ASE, and imaginary frequencies for physisorbed species were approximated as 12 cm 1 as discussed in references 29 and 51. The Rh(211) slab and adsorbate DFT calculations used the same settings as the Rh(111) surfaces.