  We have studied the adsorption of NO, and the coadsorption of N and O, on
four physical and hypothetical systems: unstrained and strained Rh(100)
surfaces and monolayers of Rh atoms on strained and unstrained MgO(100)
surfaces. We find that as we go from Rh(100) to Rh/Mg0(100), via the other two
hypothetical systems, the effective coordination progressively decreases, the
$d$-band narrows and its center shifts closer to the Fermi level, and the
strength of adsorption and co-adsorption increases. Both strain and the
presence of the oxide substrate contribute significantly to this. However,
charge-transfer is found to play a negligible role, due to a cancelling out
between donation and back-donation processes. Our results suggest that lowering
effective coordination of Rh catalysts by strain, roughening or the use of
inert substrates might improve reaction rates for the reduction of NO to N$_2$.
