| dc.description.abstract | The conversion of CO2 into CH4 using H2 can become an important part of the reduction of our carbon
footprint, turning a `waste' molecule into a usable feedstock. This reaction can be catalysed by nickel (Ni)
nanoparticles (NPs). The behaviour of supported nickel catalysts depends on many factors, such as nickel
nanoparticle size and the type of support used[1]. The preparation of monodisperse Ni nanoparticle catalysts
with well-dened particle sizes on various types of supports is dicult using standard catalyst preparation
methods, thereby hampering our understanding and the development of Ni catalysts for CO2 hydrogenation.
A promising approach to address these challenges is the use of colloidal synthesis, where the metal nanoparticles are prepared separately before depositing them on a support[2]. This thesis demonstrates the successful application of colloidal synthesis to prepare Ni nanoparticles supported on several materials.
Control over the size of Ni nanoparticles is typically achieved using phosphorous containing ligands. However,
these ligands are dicult to remove from the Ni NP surface even at high temperatures (500 °C) and phosphorous
often is incorporated in the Ni nanoparticles. In this work we present the synthesis of phosphorous-free Ni
nanoparticles via colloidal synthesis. Using a literature procedure, monodisperse phosphorous-free Ni nanoparticles of 4 and 7 nm were synthesized suspension using a seed-mediated approach by reduction of Ni(acac)2 using a borane tert-butylamine complex[3]. The Ni NPs dispersed in toluene were deposited on a carbon or silica support via sonication-assisted deposition and rapidly dried. 4.2 0.9 nm Ni/SiO2, 4.9 0.8 nm Ni/C,
8.8 2.8 nm Ni/SiO2 and 8.0 1.7 nm Ni/C were successfully obtained in this manner. A mild thermal
treatment was developed in this thesis, comprising a calcination step in O2 at 250 °C and a subsequent reduction
step in H2 at 350 °C. Successful ligand removal on both support types was conrmed by IR spectroscopy
and TGA-MS. Oxidation and reduction behaviour of the Ni NPs was extensively investigated using (in situ)
TEM, TPR and XRD.
Catalytic activity and catalyst stability were tested for Ni/SiO2 and Ni/C during high pressure CO2 hydrogenation
at 30 bar and at temperatures varying from 240 °C to 340 °C, revealing much higher activities
for silica support Ni NPs compared to carbon supported nanoparticles. Signicant sintering was observed for
the Ni NPs supported on SiO2, leading to Ni NPs of up to 100 nm. A comparable colloidal synthesis using
TOP was performed to investigate the potential dierence caused by changing this ligand. This resulted in the
formation of nanoparticles containing phosphorous, which were not active for the reaction. This again illustrates
the importance of developing a synthesis method to created Ni nanoparticles without using phosphorous
containing ligands.To conclude, our methodology can be extended to other supports, and allows detailed investigations of the Ninanoparticle size- and support eects on the catalytic conversion of CO2 to CH4. | |
