Abstract: To understand the effect of the doping amount of Cu²⁺ on the structure and reactivity of SnO₂ in NO_x-SCR with NH₃, a series of Sn-Cu-O binary oxide catalysts with different Sn/Cu ratios have been prepared and thoroughly characterized. Using the XRD extrapolation method, the SnO₂ lattice capacity for Cu²⁺ cations is determined at 0.10 g CuO per g of SnO₂, equaling a Sn/Cu molar ratio of 84/16. Therefore, in a tetragonal rutile SnO₂ lattice, only a maximum of 16% of the Sn⁴⁺ cations can be replaced by Cu²⁺ to form a stable solid solution structure. If the Cu content is higher, CuO will form on the catalyst surface, which has a negative effect on the reaction performance. For samples in a pure solid solution phase, the number of surface defects increase with increasing Cu content until it reaches the lattice capacity, as confirmed by Raman spectroscopy. As a result, the amounts of both active oxygen species and acidic sites on the surface, which critically determine the reaction performance, also increase and reach the maximum level for the catalyst with a Cu content close to the lattice capacity. A distinct lattice capacity threshold effect on the structure and reactivity of Sn-Cu binary oxide catalysts has been observed. A Sn-Cu catalyst with the best reaction performance can be obtained by doping the SnO₂ matrix with the lattice capacity amount of Cu²⁺.

Key words: SnO₂-based solid solution, Lattice capacity of Cu²⁺, XRD extrapolation method, NO_x-SCR with NH₃, Threshold effect