Naturally occurring clay nanotubes, halloysite (Al₂Si₂O₅(OH)₄·2H₂O), with exterior and interior surfaces, respectively, composed of SiO_x and AlO_x layers, act as an agglomeration-tolerant exhaust catalyst when copper–nickel alloy nanoparticles (Cu–Ni NPs, 2–3 nm) are immobilized at the AlO_x interior. Co-reduction of Cu²⁺ and Ni²⁺ (respectively derived from CuCl₂ and NiCl₂) in the presence of sodium citrate (Na₃C₆H₅O₇·2H₂O) and halloysite yielded the required nanocomposite, Cu–Ni@halloysite. Cu–Ni@halloysite efficiently catalyzes the purification of simulated motor vehicle exhaust comprising nitrogen monoxide (NO) and carbon monoxide (CO) near the activation temperature of Pt-based exhaust catalysts, ≤400 °C, showing its potential as an alternative to Pt-based catalysts. In contrast, a different halloysite nanocomposite with the SiO_x exterior decorated with Cu–Ni NPs, Cu–Ni/halloysite, is poorly active even at >400 °C because of particle agglomeration. The enhanced exhaust-purification activity of Cu–Ni@halloysite can ultimately be attributed to the topology of the material, where the alloy NPs are immobilized at the tubular AlO_x interior and protected from particle agglomeration by the tubular form and SiO_x exterior.