Colloidal nanoparticles as catalysts and catalyst precursors for nitrite hydrogenation

摘要

The most distinguished advantage to use colloidal methods for catalyst preparation is that the size and the shape of nanoparticles can be manipulated easily under good control, which is normally difficult to achieve by using traditional methods, such as impregnation and precipitation. This facilitates studies on structure sensitivity of catalytic reactions. However, the residing stabilizers on the metal surface are normally difficult to remove completely, inducing complex influences on catalytic reactions taking place on the metal surface. Pd catalysts have been found most efficient for nitrite hydrogenation. A high selectivity to N2 of the catalyst is required, because ammonium is also harmful in drinking water. There has been disagreement on the influence of Pd particle size on the catalyst performance, and a study with model catalysts with different Pd particle sizes, which can be prepared with colloidal methods, may answer the open question. In the study for this thesis, a novel method has been developed to remove residing polymer stabilizer (polyvinyl alcohol (PVA)) from Pd nanoparticles (NPs) prepared via colloidal method. It is proposed that chlorine, introduced by HCl, can fully cover the Pd surface in presence of air, suppressing the coverage of the Pd surface by PVA. Furthermore, it is also found that chlorine suppresses the selectivity to ammonium without a significant effect on activity. The choice of polymer stabilizer also significantly influences the catalytic performance of the Pd colloidal catalysts. PVA is found to only blocking Pd active sites; whereas a similar polymer stabilizer, polyvinylpyrrolidone (PVP), also influences the activity per Pd surface atom not covered by PVP, as well as the selectivity to ammonium, besides the blocking effect. Nitrite hydrogenation was studied with Pd catalysts prepared with colloid method and impregnation in a semi-batch operation. Significant increase of the selectivity to ammonium appeared when approaching complete conversion of nitrite, which is caused by ammonium formation when nitrite is completely converted. It is found that nitrogen atoms, covering about 80% of the accessible Pd surface area, convert very slowly to ammonium. Apparently, the Pd surface is largely covered with a relatively unreactive intermediate, i.e. nitrogen atoms.