We next investigated the mechanism leading to the high performance of Cu3Al2OF10 by first looking at the effect of thiocyanate anions (SCN -), a well-known poisoning agent specific to metal sites. The addition of 0.1 M KSCN significantly decreased the catalytic current, signifying that the Cu was indeed the active site (Fig. ). We tentatively exclude Al sites as significantly contributing as Al2O3 nanoparticles do not exhibit significant activity for NO3R (Fig. ). Further, removing the F component through an annealing procedure at 650°C for 6h to form the corresponding oxides (reaction (3)) decreases the catalytic current (Fig. ), pointing to the beneficial effects of the fluoride ions within the structure. Likewise, the F-free CuAl2O4 powder does not show exceptional NO3R activity either (Fig. ). Therefore, we believe that the active material consists of small Cu nanoparticles proximal to fluorine ions that enhance their activity, possibly through inductive effects or via the formation of highly active sites at the Cu/oxyfluoride interface.