Liquid-phase hydrogenation of N-nitrosodimethylamine over Pd-Ni supported on CeO2-TiO2: The role of oxygen vacancies

Catalytic liquid-phase hydrogenation of N-nitrosodimethylamine (NDMA) over CeO2, TiO2 and CeO2-TiO2 (CexTi, x referred to the molar ratio of Ce to Ti) supported Pd-Ni bimetallic catalysts (Pd-Ni/CexTi) was investigated. Characterization results indicated that Ce-O-Ti solid solutions formed through the incorporation of Ti4+ into the CeO2 lattice, resulting in increased specific surface areas and reduced average crystal sizes of CexTi. Ti doping also promoted the generation of oxygen vacancies (O-v) on the surface of CexTi. For Pd-Ni/CexTi, the amount of oxygen vacancies first increased and then decreased with the increasing of Ti content, and the maximum content of O-v was observed on Pd-Ni/Ce4Ti. Consequently, Pd-Ni/Ce4Ti had the smallest metal particle sizes, due to the strong metal-support interaction formed between Pd-Ni and O-v. Pd-Ni/CexTi exhibited much higher catalytic activities than that of Pd/CeO2 and Pd/TiO2 for NDMA reduction, and the Pd-Ni/Ce4Ti had the highest activity. The catalytic activity of Pd-Ni/CexTi was relevant to the oxygen vacancies on the support, which influenced the metal dispersion and the adsorption of intermediate product of NDMA reduction.