UC Irvine

Catalytic oxidation of methane to less potent gases has been the current technology for controlling methane emissions from natural gas-powered vehicles. Pd is commonly used as an active metal for methane oxidation, but its amount is desired to be minimized due to its high cost. The stability and activity of Pd species at wide operating temperatures depend on the Pd dispersion and local structure on the CeO2 surface. Here, we adopt flame spray pyrolysis (FSP), a high-temperature synthesis, to stabilize Pd on CeO2. The Pd/CeO2 catalv4yst synthesized in the oxidizing environment generates highly active Pd species, while the reducing environment has less active Pd species for methane oxidation. Decreasing Pd loading on CeO2 can enhance the reaction rate due to better dispersion. Detailed structural characterizations of catalysts synthesized at different loading and synthesis conditions using CO diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments identify the highly active sites as atomically dispersed Pd and the less active sites as the combination of highly dispersed Pdn+, Pd0, and Pd0 cluster. These results indicate that the control of Pd structures is a function of its loading and the FSP synthesis condition. Evaluation of the methane oxidation reaction over these structures reveals that the enhanced methane oxidation activity by the atomically dispersed Pd is due to the facilitation of the conversion from CH2 to carbonates during the methane oxidation reaction. This study demonstrates that FSP can be used to control Pd structures for synthesizing atomically dispersed Pd for enhanced methane oxidation activity.

In addition to controlling the chemical property of catalysts by FSP, the uniformity of catalyst particles can be governed by the release rate of precursors in FSP. However, there is a lack of experimental techniques for the direct measurement of the precursor release, and its quantification through the single droplet combustion (SDC) modeling has been based on the immediate release of the precursor from the droplet. Here, a single droplet combustion model that includes film theory has been developed, and the thickness of the mass boundary layer that limits the release of precursor to the combustion zone is coupled with the droplet temperature for a more accurate prediction of the temporal precursor release during synthesis. The model reveals that the mass boundary layer can influence nanomaterial formation and its coupling with droplet temperature allows a more accurate comparison of precursor release rate. We also developed design rules that can guide experiments on the choice of solvent composition and nozzle operating parameters to improve homogeneity. Quantifying the precursor release through thermodynamic phase relation and diffusion model with two moving boundary conditions is further developed to investigate precursor concentration on catalysts homogeneity and more precise prediction. This new methodology for accurate prediction of precursor release is the first application of film theory for the understanding and design of flame-synthesized homogeneous catalysts.