Thermocatalytic CO2 hydrogenation is a scalable route to renewable fuels and chemicals that can also mitigate climate change. Here, we demonstrate how subtle changes in a mixed phase zirconia support induce large changes in catalyst activity and selectivity. Ru/SnxZr1-xO2 catalysts were prepared using co-precipitated Sn-doped ZrO2. Ru/ZrO2 and Ru/Sn0.01Zr0.99O2 both comprised tetragonal (t-) and monoclinic (m-) ZrO2 nanoparticles decorated by RuO2·H2O clusters. Ru/Sn0.2Zr0.8O2 exhibited similar textural properties and metal speciation, however, X-ray diffraction evidenced new crystalline phases wherein Sn atoms substituted for Zr. Operando XRD evidenced minimal perturbation of ZrO2 and Sn-doped ZrO2 phases, despite which 20 atom% Sn switched product selectivity from 99% CH4 for Ru/ZrO2 to 67% CO for Ru/Sn0.2Zr0.8O2, due to increased support CO2 adsorption capacity/strength. Operando infrared spectroscopy identify linear surface CO* as a catalytic spectator with (bi)carbonate a precursor to reactive HCOO*/carboxylate intermediates to methane. Sn0.2Zr0.8O2 promotes CO2/(bi)carbonate/formate decomposition, favouring a reverse water gas shift pathway.