The effect of a nickel promoter on the reducibility of a commercial Cu/ZnO/Al₂O₃ catalyst for CO oxidation

CO oxidation was studied for a commercial Cu/ZnO/Al₂O₃ catalyst with and without a nickel promoter. The addition of a small amount of nickel can effectively enhance the activity of the Cu/ZnO/Al₂O₃ catalysts. A strong interaction between Ni²⁺ ions and Al₂O₃ is the main factor causing an increase in the catalytic activity of Cu/ZnO/Al₂O₃. Owing to the formation of CuAl₂O₄, complete reduction of the Cu/ZnO/Al₂O₃ catalyst is difficult, even if pretreated by reduction at 573 K for 5 h. As Ni²⁺ ions complex with Al₂O₃ in the copper catalyst and form NiAl₂O₄, they weaken the interaction between Cu²⁺ and Al₂O₃, leading to enrichment of the catalyst surface in Cu²⁺ ions. This Ni addition could induce the aggregation of Cu atoms on the surface into larger Cu particles, possibly leading to a lower Cu surface area or easy reduction to Cu⁰ by H₂.