Predictive Synthesis of Catalytic Metal Nanoparticles

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Predictive Synthesis of Catalytic Metal Nanoparticles using In-Situ. SAXS and Kinetic Modeling ... design of metal colloidal nanoparticles. A novel approach ...
Predictive Synthesis of Catalytic Metal Nanoparticles using In-Situ SAXS and Kinetic Modeling Saeed Mozaffari1, Wenhui Li1, Peiguang Hu1, Coogan Thompson1, Sergei Ivanov2 and Ayman M. Karim1* 1 Virginia Polytechnic Institute and State University, Blacksburg, VA 24060 (United States) 2 Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, NM 87545 (United States) * [email protected] Introduction For structure-sensitive reactions [1], the synthesis of heterogeneous catalysts with specific active sites (nanoparticles of specific size and/or shape) is of extreme importance for controlling the catalyst activity and selectivity. Despite the significant advancements in colloidal synthesis of nanoparticles [2-3], a priori design of nanoparticles of desired sizes and shapes remains a grand challenge and a trial-and-error approach is still often employed. To minimize trial and error and predict the synthesis conditions for desired nanoparticle morphologies, a thorough understanding of the interactions between the metal, ligands and solvent system is required [3]. Furthermore, based on this understanding, developing robust kinetic models that accurately predict size and shape evolution during the synthesis are required, but still lacking. In this work, we will present our recent efforts on integration of thermodynamics, kinetics and advanced in situ characterization measurements to develop a methodology for predictive design of metal colloidal nanoparticles. A novel approach combining microfluidics with in situ X-ray absorption spectroscopy (XAS) and in situ small angle X-ray scattering (SAXS) is used to develop predictive kinetic models for the synthesis of metal (Pd, Pt) colloidal nanoparticles. Our kinetic model captures the nucleation and growth kinetics (measured by SAXS), including the interactions of capping ligands with both the metal precursor and particle's surface, which are often not included in the kinetic models available in the literature. The application of this new kinetic model in predicting the sizes for various synthesis systems (i.e. different types of metals, solvents, and ligands) will be presented. Materials and Methods Different metallic precursors, palladium (Pd) acetate and platinum (Pt) chloride, were used to synthesize phosphine-capped Pd and alkyne-capped Pt nanoparticles. The effects of solvent type (toluene vs. pyridine) as well as concentration of both the precursor and ligand on Pd nanoparticle size were examined. In the case of Pt, the effects of altering the alkyne chain length and temperature on the size were also studied. For our kinetic model input, in situ small angle X-ray scattering (SAXS), was used to obtain the size, size distribution, and number of particles during synthesis. The proposed kinetic model, for the first time, accounts separately for both the nucleation and growth events through simultaneous fitting of number of nanoparticles (nucleation event) and number of atoms in nanoparticles (both nucleation and growth events). Figure 1: Reaction network for the proposed kinetic model

In the model, the reaction rate constants ( k ) and equilibrium rate constants (K) shown in the reaction network (Figure 1) above are used as fitting parameters. However, the early nucleation and growth rates measured from SAXS are used as upper limits for the rate constants. Results and Discussion Figure 2.a (left) shows the excellent agreement between the model fit (lines) and the in situ SAXS results (symbols). The left and right y-axes represent the particle size and number of particles, respectively. The extracted rate constants (and calculated nucleation and growth rates) show that nucleation is slow and continuous, while growth is fast. The slow nucleation with overlapped fast growth was observed under different conditions for different metals, which shows that the synthesis does not follow the classical nucleation and growth mechanism (i.e. LaMer mechanism). Our modeling results further suggest that the capping ligands interaction with both the precursor and nanoparticles’ surface play a crucial role in controlling the rates of nucleation and growth. We will demonstrate that the single most important parameter to determine the final size and polydispersity of the nanoparticles is the ratio of growth to nucleation rates. Additionally, the growth/nucleation rate ratio can be tuned by the solvent type, temperature, precursor and ligand concentrations to synthesize nanoparticles with narrow size distribution ranging from 1-5 nm for Pd and 1-4 nm for Pt. Figure 2.b (right) shows the excellent quantitative agreement between the final particle size predicted by the model (lines) and the experimental results (symbols) for different concentrations of Pd. Other predictions (and experiment results) for the effect of solvent, ligand, temperature and reaction time on the nanoparticles’ size and polydispersity will be presented. Finally, we will show how the kinetic model can be used to predict the synthesis conditions of nanoparticles using weaker-binding ligands that can easily be removed for catalytic applications. Significance The proposed kinetic model is the first model to account for interactions of capping ligands with both precursor and particle's surface. The model can also provide excellent predictions for the nanoparticles size and reaction time that can help predict the synthesis conditions for specific sizes without trial and error.

Figure 2.a

Figure 2.b

References 1. Boudart et al., Adv. Catal., 20 (1969) 153. 2. Watzky et al. Journal of the American Chemical Society, 119 (1997),10382-10400 3. Xia et al., Angewandte Chemie International Edition, 48 (2009) 1-1.