Towards Atomistic Understanding of Catalytic Nanoparticles on Amorphous Supports

Metal nanoparticles (NPs) have attracted considerable attention in heterogeneous catalysis due to their unique nanoscale properties. However, rational design and optimization of supported NP catalysts requires an accurate description of metal-support interactions, which can significantly impact NP stability and catalytic activity. The ability to calculate NP interactions with amorphous supports, which are commonly used in industrial practice, has been inhibited by a lack of accurate atomically-detailed models of amorphous surfaces. \ud We have developed an approach for constructing atomistic models of amorphous silica surfaces, using a combination of classical molecular modeling and density functional theory (DFT) calculations. To experimentally validate our model, we developed an emulsion synthesis procedure yielding mono-disperse 6 nm silica NPs, and a simple approach for high-yield separation of stable NPs from solution. Remarkably, our model accurately reproduces the experimental silanol number and silanol distribution over a wide temperature range, without any adjustable parameters. \ud We next developed a systematic approach for generating distributions of model NP/SiO2 structures using the discrete element method in conjunction with DFT. Using these structures, we studied NP-support interactions between amorphous silica and metal NPs ranging from 0.7 – 1.7 nm in diameter (13 – 147 atoms). Both NP adhesion energetics and charge transfer are local in nature, and depend on the silica hydroxyl density. Because surface hydroxyl content is directly dependent on temperature, our results suggest that both electronic charge and catalyst stability can be tuned via the silica pretreatment temperature, the latter of which we have experimentally validated using in situ X-ray diffraction. Finally, exploiting the local nature of NP-support interactions, we developed a method for predicting NP-support effects of different NP sizes and geometries based on correlations calculated for 13-atom NPs.\ud In this work, we show that an accurate atomistic description of not only amorphous surfaces, but also interactions between NPs and those surfaces can be achieved. Additionally, we developed a similar method for generating model systems of single-atom catalysts on amorphous supports. Insights into the role of catalyst-support interactions on catalyst structure and function gained from these atomistic models may serve to guide the design/optimization of catalytic materials