Controllable synthesis of small-sized Pd nanoparticles on acetic acid-modified halloysite for enhanced toluene...

The particle size of catalyst active components is a key factor for catalytic activity. In this work, functionalized halloysite (Hal) provides a synthetic strategy for the preparation of catalysts loaded with small Pd particles. In this study, halloysite nanotubes were employed as catalyst supports, and the ac-Hal support was synthesized via mild treatment of halloysite nanotubes with acetic acid (ac). The ac-Hal-Pd catalyst was prepared by the coprecipitation method and exhibited excellent catalytic activity for toluene oxidation, with a temperature for 90% toluene conversion (T₉₀) of 179 °C. Transmission electron microscopy (TEM), CO pulse chemisorption, and CO adsorption in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses revealed that the Pd particles on the surface of the ac-Hal-Pd catalyst had a smaller particle size and more uniformly dispersed Pd particles. X-ray photoelectron spectroscopy (XPS) and O₂-temperature-programmed desorption (O₂-TPD) analyses indicate that the reduction in the particle size of Pd particles increases the proportion of Pd⁰ on the catalyst surface and the concentration of adsorbed oxygen, which is also a key factor for the enhancement of catalytic activity. In situ DRIFTS spectra of NH₃ adsorption reveal that ac-Hal-Pd possesses a higher abundance of Lewis acid sites, which is favorable for the catalytic reaction. In situ DRIFTS results suggest that toluene oxidation over ac-Hal-Pd proceeds through the formation of benzyl alcohol/alkoxy species, benzaldehyde, benzoate/benzoic acid species, and maleic anhydride intermediates, which are finally oxidized to CO₂ and H₂O.