For CO 2 reduction on CeO 2 , the gradient ∂∆χ( ⃗ E)/∂η act ( ⃗ E) = 0.13 also agrees with the linear fit to the experimental data ∂∆χ/∂η act = 0.03 (Fig. ). The true CeO 2 surface will have many arrangements of the electronic defects causing the net electrostatic potential to cancel. Here we have only analysed one such defect complex which has a relatively small dipole moment (⃗ µ ⊥ = 0.14e), accounting for the models overestimation of the electrostatic surface potential. With respect to the surface chemistry, the ambipolar charge transfer reaction remains coupled. As such, there is no stable charged adsorbate state and so a negligible electrostatic surface potential is observed.