(108d) Thermal Stability of Nickel-Silica Core-Shell Nanocatalysts
One of the biggest challenges in the technical application of nanocatalytsts is deactivation due to sintering. While the increasing specific surface area with decreasing particle diameter results in higher catalytic activity, it also increases the thermodynamic driving force for sintering, i.e. the loss of active surface area and hence loss of catalytic activity with time due agglomeration of metal crystallites. To overcome this dilemma, our group has in recent years investigated different approaches towards stabilizing metal nanoparticles via embedding or encapsulation in nanostructured matrices, or via alloying of the active metal phase.
Here, we are presenting the results of a study in which Ni@SiO2 core-shell materials were systematically investigated to identify structure-stability correlations for this type of metal-oxide core-shell materials. The Ni@SiO2 materials were synthesized in a straightforward one-pot reverse-microemulsion templated approach. By varying synthesis parameters, we were able to synthesize not only Ni@SiO2 with carefully controlled metal nanoparticle sizes (~1-7nm), but also to synthesize core-shell nanostructures with a pronounced central cavity (~10 nm). These “nanobubble” materials contain sub-nanometer Ni clusters which decorate the inner wall of the (porous) silica shell. The cavity leaves the surface of the Ni clusters much more accessible than in typical core-shell structures, and is expected to impact the thermal stability of the Ni NPs as it restricts their mobility much less than for the embedded Ni@SiO2 core-shell materials. The materials were characterized by HRTEM, BET, XRD, TPR, and H2-chemisorption, and the thermal stability of the different nanocomposite materials were investigated systematically as function of structure and metal NP size over a temperature range from 500oC to 1000oC.
Silica porosity and pore size distribution decreased significantly with increasing calcination temperature above ~800oC, indicating a softening and restructuring of the silica matrix. More significantly, in typical core-shell structures, Ni sintering occurs within the individual silica particles via a transition from multiple Ni cores to a single, larger Ni core, followed by the diffusion of Ni particles out of the silica particles at ~1000oC. This stability is a function of the ratio of Ni NP size to silica pore diameter, where larger Ni NPs induce significant reconstruction of the silica matrix during diffusion. Surprisingly, the very small Ni nanoclusters in the silica “nanobubbles” show excellent stability up to 800oC, presumably due to stronger interaction with the silica support. This enhanced thermal stability of the “nanobubble” catalysts is confirmed in catalytic CO methanation compared to core-shell catalysts as well as conventional Ni/silica catalyst.