Activated interiors of clay nanotubes for agglomeration-tolerant automotive exhaust remediation

Naturally occurring clay nanotubes, halloysite (Al2Si2O5(OH)4·2H2O), with exterior and interior surfaces, respectively, composed of SiOx and AlOx layers, act as an agglomeration-tolerant exhaust catalyst when copper–nickel alloy nanoparticles (Cu–Ni NPs, 2–3 nm) are immobilized at the AlOx 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 SiOx 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 AlOx interior and protected from particle agglomeration by the tubular form and SiOx exterior.