In situ X-ray Scattering and Dynamical Modeling of Pd Catalyst Nanoparticles Formed in Flames
- Juan Wang
- ,
- Sönke Seifert
- ,
- Randall E. Winans
- ,
- Erik Tolmachoff
- ,
- Yuxuan Xin
- ,
- Dongping Chen
- ,
- Hai Wang
- , and
- Scott L. Anderson
Abstract
It has previously been demonstrated that organopalladium precursors can break down under combustion conditions, forming nanoparticles that catalyze ignition. Here, we use in situ small-angle X-ray scattering (SAXS) to probe the formation and growth of palladium nanoparticles in an ethylene flame doped with 28 ppm (mol) of Pd(acetate)2. The particles appear to nucleate in the flame front and are observed by SAXS to grow in size and mass in the high-temperature region of the flame (∼2200 K) with median diameters that evolve from 1.5 to 3.0 nm. Transmission electron microscopy of particles collected on a grid located outside the flame shows that the particles are metallic palladium with sizes comparable to those determined by SAXS. Molecular dynamics simulation of particles of selected sizes indicates that at the flame temperature the particles are molten and the average mass density of the particle material is notably smaller than that of bulk, liquid Pd at the melting point. Both experimental and computational results point to homogeneous nucleation and particle–particle coalescence as mechanisms for particle formation and growth. Aerosol dynamics simulation reproduces the time evolution of the particle size distribution and suggests that a substantial fraction of the particles must be electrically charged during their growth process.
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