In Situ Plasmonic Sensing of Platinum Model Catalyst Sintering on Different Oxide Supports and in O2 and NO2 Atmospheres with Different Concentrations

Effective Optical Properties of Inhomogeneously Distributed Nanoobjects in Strong Field Gradients of Nanoplasmonic Sensors

Accurate and efficient modeling of discontinuous, randomly distributed entities is a computationally challenging task, especially in the presence of large and inhomogeneous electric near-fields of plasmons. Simultaneously, the anisotropy of sensed entities and their overlap with inhomogeneous fields means that typical effective medium approaches may fail at describing their optical properties. Here, we extend the Maxwell Garnett mixing formula to overcome this limitation by introducing a gradient within the effective medium description of inhomogeneous nanoparticle layers. The effective medium layer is divided into slices with a varying volume fraction of the inclusions and, consequently, a spatially varying effective permittivity. This preserves the interplay between an anisotropic particle distribution and an inhomogeneous electric field and enables more accurate predictions than with a single effective layer. We demonstrate the usefulness of the gradient effective medium in FDTD modeling of indirect plasmonic sensing of nanoparticle sintering. First of all, it yields accurate results significantly faster than with explicitly modeled nanoparticles. Moreover, by employing the gradient effective medium approach, we prove that the detected signal is proportional to not only the nanoparticle size but also its size dispersion and potentially shape. This implies that the simple volume fraction parameter is insufficient to properly homogenize these types of nanoparticle layers and that in order to quantify optically the state of the layer more than one independent measurement should be carried out. These findings extend beyond nanoparticle sintering and could be useful in analysis of average signals in both plasmonic and dielectric systems to unveil dynamic changes in exosomes or polymer brushes, phase changes of nanoparticles, or quantifying light absorption in plasmon assisted catalysis.

The physical chemistry and materials science behind sinter-resistant catalysts

This tutorial review highlights recent progress in understanding the physical chemistry and materials science for developing sinter-resistant catalytic systems.

Oxygen-promoted catalyst sintering influences number density, alignment, and wall number of vertically aligned carbon nanotubes

A lack of synthetic control and reproducibility during vertically aligned carbon nanotube (CNT) synthesis has stifled many promising applications of organic nanomaterials. Oxygen-containing species are particularly precarious in that they have both beneficial and deleterious effects and are notoriously difficult to control. Here, we demonstrated diatomic oxygen's ability, independent of water, to tune oxide-supported catalyst thin film dewetting and influence nanoscale (diameter and wall number) and macro-scale (alignment and density) properties for as-grown vertically aligned CNTs. In particular, single- or few-walled CNT forests were achieved at very low oxygen loading, with single-to-multi-walled CNT diameters ranging from 4.8 ± 1.3 nm to 6.4 ± 1.1 nm over 0-800 ppm O₂, and an expected variation in alignment, where both were related to the annealed catalyst morphology. Morphological differences were not the result of subsurface diffusion, but instead occurred via Ostwald ripening under several hundred ppm O₂, and this effect was mitigated by high H₂ concentrations and not due to water vapor (as confirmed in O₂-free water addition experiments), supporting the importance of O₂ specifically. Further characterization of the interface between the Fe catalyst and Al₂O₃ support revealed that either oxygen-deficit metal oxide or oxygen-adsorption on metals could be functional mechanisms for the observed catalyst nanoparticle evolution. Taken as a whole, our results suggest that the impacts of O₂ and H₂ on the catalyst evolution have been underappreciated and underleveraged in CNT synthesis, and these could present a route toward facile manipulation of CNT forest morphology through control of the reactive gaseous atmosphere alone.

Plasmonic Nanospectroscopy of Platinum Catalyst Nanoparticle Sintering in a Mesoporous Alumina Support

In situ plasmonic nanospectroscopy has proven useful to bridge the pressure gap in heterogeneous catalysis. The method has, however, so far been used only for idealized two-dimensional systems without the structural complexity of realistic three-dimensional porous oxides, which generally are used as supports for the catalytically active metal nanoparticles. Here, we report a generic method that addresses this structural gap by demonstrating the possibility to use nanoplasmonic sensing to monitor surface processes in a traditional three-dimensional mesoporous alumina matrix, wet-impregnated with Pt nanoparticles. The capability of the experimental platform is illustrated by measuring sintering kinetics of the Pt nanoparticles inside the mesoporous matrix under oxidizing conditions at atmospheric pressure and at temperatures up to 625 °C. The study thus demonstrates in operando plasmonic nanospectroscopy of realistic, commercial catalyst systems.