Application Note

Nanoparticle Sintering

Introduction the catalyst to be studied, and (iii) is observed (Fig. 2a). Analysis of Δλ preventing the catalyst from directly during the course of the experiment, Catalyst sintering is a major cause of interacting with the Au nanodisks by shows that the LSPR shifts fast in catalyst deactivation, which yearly e.g. alloy formation. the beginning and then more slowly causes billions of dollars of extra cost The Pt model catalyst is formed onto towards the end of the experiment associated with catalyst regeneration the sensor chip by evaporating a 0.5 (Fig. 2b, black curve). In contrast, and renewal. nm thick (nominal thickness) granu- when the same experiment is per- In order to develop more sintering- lar film. This results in individual Pt formed in pure Ar (or in 4% O2 on resistant catalysts, a detailed under- nanoparticles with an average diam- a “blank” sensor without Pt) no standing of the sintering kinetics and eter of = 3.3 nm (+/- 1.1 nm), significant shift during the entire ex- mechanisms is required. It is therefore which mimics the size range of real periment is observed (Fig. 2b, blue of great importance to investigate sin- supported catalysts. curve). These results indicate that tering in situ, in real time and under The change of the localized surface INPS can be used to monitor the realistic catalyst operation conditions plasmon resonance (LSPR) centroid sintering of nanoparticle catalysts (i.e. at high temperatures and pres- wavelength, Δλ, is monitored during (since O2 is a known sintering pro- sures in reactive gas atmospheres). Pt nanoparticle sintering at different moter for Pt). Today there is a lack of available temperatures (< 610°C) in inert (Ar) The sintering of the Pt particles is techniques that allow such studies. or oxidizing (4% O in Ar) environ- confirmed by TEM imaging after Using Pt/SiO as model catalyst, this 2 2 ment at atmospheric pressure. The 5 min and 6 hours in Ar and 4% Application Note illustrates that Indi- remote nature of the INPS approach O2/Ar, respectively (Fig. 2c-f). Slow rect Nanoplasmonic Sensing (INPS) (optical transmission measurement sintering is observed in Ar at 610°C, has the potential to fill this gap and through the INPS chip) makes causing to increase from 3.12 provide novel real time data at low measurements at high temperatures nm to 3.48 nm after 6 hours. cost and with high throughput. and pressures easily possible. All However, already after 5 min in 4%

experiments are performed in an O2 the Pt nanoparticles have sintered Experimental Insplorion X1 gas flow reactor, and more ( = 3.6 nm) than after 6 h in the optical spectra are collected Ar and after 6 h sintering in O , The INPS sensor consists of Au na- 2 and analyzed using the Insplorer® has increased to 8.9 nm. nodisks (average diameter 80 nm, software. To further demonstrate the direct height 20 nm) deposited on a glass correlation between the plasmonic substrate and covered by a 10 nm signal from the INPS sensor and thick SiO spacer layer (Fig. 1a). 2 Results sintering of Pt nanoparticles, experi- The spacer layer serves several func- Exposing the Pt nanoparticles to ments are performed where the tions, including (i) protection of the O , a well-known sintering promoter sintering process is interrupted Au nanodisks from the environment 2 for Pt, causes the LSPR centroid after different exposure times of the and from structural re-shaping, (ii) position, λ, to shift towards shorter Pt particles to 4% O atmosphere providing tailored surface chemistry 2 wavelengths and, simultaneously, a at 550°C (Fig. 3a). TEM imaging is (i.e. catalyst support material) for decrease in the optical absorbance performed after each sintering time

Figure 1: Schematic cross-section of the Indirect Nanoplasmonic Sensing (INPS) chip. The sensing structure consists of an array of gold nanodisks (1) deposited on a conventional glass slide (5)

and covered by a SiO2 spacer layer (2) onto which the Pt model catalyst particles (3) are deposited through thermal evaporation. Sintering of the Pt nanoparticles is sensed through the locally enhanced plasmonic field (4, “nanoantenna”) surrounding the Au nanodisks. Application Note

interval (Fig. 3b-g). Plotting Δλ as a is subsequently used for theoretical contain only a few data points), function of sintering time for the six modeling and curve fitting to the which provides a unique tool for real different experiments (t = 10 min, experimental data. The results indi- time in situ sintering studies. 30 min, 1 h, 3 h, 6 h, and 12 h) cate Ostwald ripening as the domi- (Fig. 3a) shows that the signal is nant sintering mechanism (see Lars- completely reproducible during over- son et al. for details). References lapping sintering intervals for differ- Real Time Indirect Nanoplasmonic ent samples. This demonstrates the in situ Spectroscopy of Catalyst robustness and reproducibility of Conclusions Nanoparticle Sintering, Elin M. Lars- the INPS measurement method. Indirect Nanoplasmonic Sensing son, Julien Millet, Stefan Gustafs- Furthermore, when comparing (INPS) was applied to study sinter- son, Magnus Skoglundh, Vladimir the optical response with particle ing kinetics of a supported catalyst P. Zhdanov, Christoph Langham- size distributions (PSDs) obtained in situ with a temporal resolution in mer, ACS Catalsys, DOI: 10.1021/ from TEM image analysis, an un- the sub-second range under fairly cs200583u (2011). ambiguous correlation is found realistic catalyst operation condi- between the optical response (i.e. the tions. More information about catalyst centroid shift Δλ) of the INPS sensor Due to the superior time resolu- sintering: Mechanisms of Catalyst and the Pt catalyst particle density tion, the INPS sintering kinetics Deactivation, C.H. Bartholomew, during the sintering process. The described above are more detailed Applied Catalysis A: General 212 latter relation is used to calculate than those obtained by other tech- (2001) 17-60. an average particle diameter, which niques (the latter kinetics often

Figure 2 (above): (a) Absorbance spectra obtained at different times during the sinter- ing of the Pt model catalyst in 4% O2/Ar at 610°C. The LSPR peak of the INPS sensor spectrally shifts to shorter wavelengths and, simultaneously, a decrease in the optical absorbance is observed. (b) Graph showing the LSPR peak centroid shift vs. sintering time in 4% O2/Ar (black) and 100% Ar (blue) atmosphere. Clearly, in O2, which is known to promote the sintering of supported Pt catalysts, the centroid shifts fast in the beginning and then more slowly, almost linearly, towards the end. In contrast, in pure Ar only a very small centroid shift is seen. TEM pictures and corresponding PSD histograms of the Pt nanoparticles after 5 min (c) and 6h (d) in pure Ar at 610°C reveal that the particle size is nearly constant with time, indicating almost total ab- sence of sintering. TEM images and PSD histograms after 5 min (e) and 6 h (f) in 4%

O2/Ar at 610°C clearly show significant Pt catalyst particle sintering, in agreement with the INPS measurement.

Figure 3 (right): (a) Real time plasmonic sintering kinetic curves obtained for 6 different samples and sintering times under identical experimental conditions (4%

O2/Ar atmosphere at 550°C) demonstrate reproducibility between experiments and illustrate the robustness of the INPS platform. Corresponding 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 and average particle diam- eters illustrate the significant sintering of the Pt model catalyst, as registered by the INPS sensor.