Promotion of catalytic performance of Pd/Al2O3 for o-xylene oxidation by morphological control

Abstract

Pd/Al2O3 catalysts with different support morphologies (nanoflake, nanorod, nanobeam) were prepared and tested for o-xylene oxidation, and characterized by BET, XRD, TEM, XPS, NMR, CO-TPR-MS, H2-TPR and in-situ DRIFTS. The results indicated that the morphology has a strong effect on the dispersion of Pd species and the oxidation capacity of the catalysts. Compared with nanorod and nanobeam, the nanoflake morphology provides more surface defect sites, penta-coordinated Al3+ and surface OH species. Such sites (defect sites, AlP sites and OH species) enabled the immobilization of Pd to form Pd metal particles of small size, resulting in high Pd dispersion with primary exposure of Pd(100) facets possessing more defect sites for the adsorption. The OH species were also the adsorption sites. Both of them are conducive to the reactivity of adsorbed o-xylene. Moreover, the OH species on Pd/Al2O3 represented activated oxygen, indicating that o-xylene can be oxidized by OH directly. It means that these small Pd particles with more active surface OH species are responsible for the excellent catalytic performance of Pd/Al2O3-NF. Its o-xylene oxidation activity is much better than that of other Pd-based catalyst previously reported. Consequently, morphology control can manipulate Pd particle size and surface OH species to effectively improve the catalytic performance of Pd/Al2O3.