Investigating Nanoparticle Sintering Using Indirect Nanoplasmonic Sensing (INPS)

Catalyst deactivation is mainly caused by catalyst sintering, and causes billions of dollars of extra expenditure on catalyst regeneration. Gaining insights into the sintering kinetics and mechanisms is essential in the design of more sintering-resistant catalysts.

This can be achieved by performing real-time, in situ sintering analysis under realistic catalyst operation conditions. However, there are very few adequate techniques to perform such studies. This article discusses the ability of Indirect Nanoplasmonic Sensing (INPS) to provide such information in real time, with high throughput but at a low cost. Pt/SiO₂ was used as the model catalyst in this experiment.

Experimental Procedure

The INPS sensor comprises the gold nanodisks of 20 nm height and 80 nm average diameter deposited on a glass substrate and enclosed by a SiO₂ spacer layer of thickness 10 nm
(Figure 1).

Figure 1. Schematic cross-section of the Indirect Nanoplasmonic Sensing (INPS) chip

The functions of the SiO₂ spacer layer are as follows:

• Protecting the gold nanodisks from structural re-shaping and from the environment
• Providing tailored surface chemistry for the catalyst of interest
• Avoiding direct interaction of the catalyst with the gold nanodisks through, for example, alloy formation.

The evaporation of a granular film of 0.5 nm thickness deposits the Pt model catalyst particles onto the INPS sensor chip, yielding individual Pt nanoparticles with 3.3 nm average diameter (+/- 1.1 nm). This size range replicates that of real supported catalysts. This experiment involves monitoring the shift in the localized surface plasmon resonance (LSPR) centroid wavelength (Δλ) during sintering of Pt nanoparticles at various temperatures (< 610°C), at atmospheric pressure, in inert (Ar) or oxidizing (4% O₂ in Ar) environments.

The INPS method’s remote nature (measurement of optical transmission via the INPS chip) allows performing measurements at elevated pressures and temperatures. An Insplorion X1 gas flow reactor is used to perform all experiments and the Insplorer® software is used to analyze the acquired optical spectra.

Experimental Results

O₂ is a renowned sintering promoter for Pt. When the Pt nanoparticles are exposed to O₂, the LSPR centroid position (λ) shifts towards shorter wavelengths, whilst the optical absorbance is decreased, as shown in Figure 2a. The analysis of Δλ during the experiment has revealed that the LSPR shift is rapid in the start of the experiment, and slows down towards the end of the experiment (Figure 2b, black curve).

Conversely, the experiment carried out in pure Ar, or in 4% O₂ on a "blank" sensor in the absence of Pt, shows no drastic shift in LSPR throughout the experiment (Figure 2b, blue curve). These results conclude the applicability of INPS for nanoparticle catalyst sintering, as O₂ is a renowned sintering promoter for Pt.

TEM imaging, after 5 minutes and 6 hours respectively in Ar and in 4% O₂/Ar environments, confirms the Pt nanoparticle sintering (Figures 2c-f). At 610°C, sintering is slow in Ar, and after 6 hours, the average diameter has increased from 3.12 nm to 3.48 nm. Compared to 6 hours sintering in Ar, the sintering of the Pt nanoparticles is much higher in 4% O₂ even after five minutes (average diameter = 3.6 nm). The value of diameter has reached 8.9 nm after six hours of sintering in O₂.

Figure 2. (a) Absorbance spectra obtained at different times during the sintering of the Pt model catalyst in 4% O₂/Ar at 610°C. (b) Graph showing the LSPR peak centroid shift vs. sintering time in 4% O₂/Ar (black) and 100% Ar (blue) atmosphere. TEM pictures and corresponding PSD histograms of the Pt nanoparticles after 5 min (c) and 6h (d) in pure Ar at 610°C. TEM images and PSD histograms after 5 min (e) and 6 hrs (f) in 4% O₂/Ar at 610°C.

Experiments are conducted to show the direct relationship between Pt nanoparticle sintering and the plasmonic signal from the INPS sensor by interrupting the sintering process after different exposure times of the Pt nanoparticles in 4% O₂ atmosphere at 550°C (Figure 3a). TEM imaging is captured after each sintering time interval (Figures 3b-g).

Plotting Δλ as a function of sintering time for the six different experiments (t = 10 min, 30 min, 1 hr, 3 hr, 6 hr, and 12 hr), shown in Figure 3a, reveals the ability to reproduce the signal during overlapping sintering intervals for various samples. These results show the strength and reproducibility of the INPS measurement technique.

Figure 3. (a) Real time plasmonic sintering kinetic curves obtained for 6 different samples and sintering times under identical experimental conditions. TEM micrographs obtained after each sintering time interval, i.e. 10 min (b), 30 min (c), 1 h (d), 3 h (e), 6 h (f) and 12 h (g), with respective PSD histograms.

The comparison of the optical response (i.e. centroid shift Δλ) against the particle size distributions derived from TEM image analysis finds an unambiguous relationship between the Pt catalyst particle density and the optical response of the INPS sensor in the course of the sintering process.

This correlation is applied in the determination of the average particle diameter. Theoretical modeling, and curve fitting to the experimental data, are performed using this average particle diameter value. From the results, Ostwald ripening is the dominant sintering mechanism.

Conclusion

The results clearly demonstrate the applicability of INPS for in situ analysis of sintering kinetics of a supported catalyst with a temporal resolution in the sub-second range at realistic catalyst operation conditions. INPS sintering kinetics are more comprehensive when compared to those with other methods, thanks to the superior time resolution. This yields a powerful tool for real-time, in situ sintering analysis.

About Insplorion

Insplorion develops and sells instruments for researchers with an interest in knowing more about what is happening on their surfaces. Insplorions instruments are used to better understand different areas such as lipid bilayer interactions, dye sorption in thin films and catalytic processes in high temperatures. An instrument system from Insplorion offers its users:

• extreme sensitivity for detection of nanoscale chemical and physical processes in liquid or in air
• real-time, in-situ measurements in temperatures up to 800°C and at pressures up to atmospheric
• high time resolution

The instruments are built around Insplorion’s proprietary optical technology platform Nanoplasmonic Sensing (NPS). The platform is utilizing the optical phenomenon Localized Surface Plasmon Resonance (LSPR).

This information has been sourced, reviewed and adapted from materials provided by Insplorion.

For more information on this source, please visit Insplorion.