Using Indirect Nanoplasmonic Sensing (INPS) in the Optical Nanocalorimetry of Catalytic Reactions

The heat dissipated from an exothermic catalytic reaction plays a key role in catalytic ignition and light-off in catalytic converters. One example is automobile emission cleaning. This article discusses the measurement of nanoscale local temperature changes using the Indirect Nanoplasmonic Sensing (INPS) platform from Insplorion.

The use of INPS in an optical nanocalorimetry of catalytic reactions over nanoparticle catalysts using realistic pressure and temperature conditions is also described in this article. The exothermic reaction of H₂ + ½O₂→ H₂O on palladium (Pd) nanoparticles is presented as model reaction in this article.

The INPS Technology

INPS investigates changes and processes in/on neighboring functional nanomaterials using the localized surface plasmon resonance (LSPR) excitation in gold sensor nanodisks, by the coupling of the locally enhanced plasmonic near-field to the latter, or by the intrinsic temperature sensitivity of the LSPR (optical calorimetry). Real-time, in situ analysis (time resolution of < 10^-2s) is possible with INPS for quantitative analysis of chemical and physical properties and processes involving metallic and non-metallic thin films and nanostructures.

Experimental Setup and Procedure

Figure 1a shows the experimental setup, where two optical fibers are used for the measurement of optical transmission via a quartz flow-reactor containing the INPS sensor chip. An array of nanofabricated plasmonic gold nanodisks with a height of 30 nm and diameter of 76 nm is present in the INPS sensor chip.

The nanodisks are deposited over a glass substrate and enclosed by a thin SiO₂ layer of approximately 10 nm thickness (Figure 1b). Palladium nanoparticles of a diameter ranging between 18.6–2.2 nm decorate the surface of the sensor chip as shown in Figure 1b.

Figure 1. (a) Schematic depiction of the experimental setup used for this study. (b) TEM images of four different investigated Pd nanoparticle sizes after structural stabilization by catalytic annealing.

The temperature on the sample surface is increased due to the chemical power released during the exothermic reaction of H₂ + O₂ on the palladium nanoparticles. As a result, the spectral peak position (Δλ_max) of the LSPR of the gold sensing nanoparticles experiences a measurable shift due to local heating.

Experimental Results

The measurement of Δλ_max was first performed without catalytic reaction for calibration purposes, as a function of T under inert gas atmosphere. This results in a linear T-dependence and a temperature sensitivity of Δλ_max = 0.0125nm/°C (Figure 2).

Figure 2. Depiction of the linear T-dependence of the INPS LSPR, yielding a temperature sensitivity of Δλ_max = 0.0125nm/°C.

Catalytic light-off traces for palladium nanoparticles of D_mean = 18.6 nm are shown in Figure 3 for three relative concentrations of H₂ and O₂ (α = [H₂] / ([H₂]+[O₂])) in argon carrier gas at 4% total reactant concentration. The heating rate of the reactor and the gas flow rate were 4°C/min and 1000 mL/min, respectively.

Figure 3. Catalytic light-off traces obtained for the hydrogen oxidation on Pd with D_mean = 18.6nm for a = 0.15, 0.25, 0.35 in argon carrier gas at 4% total reactant concentration.

Figure 3 shows Δλ_max values corrected for the spectral shift caused by external heating of the reactor (calibration curve in Figure 2). The curves for α = 0.15, 0.25, and 0.35 would be flat on the abscissa in the absence of power or heating source other than the external heating. The rise above the abscissa is linked to an LSPR peak shift caused by local heating of the gold sensor nanodisks induced by the heat dissipated from the exothermic reaction of H₂ + ½O₂ → H₂O on palladium nanoparticles (ΔH = 250 kJ/mol).

In this way, the temperature rise can be measured locally, which is difficult to perform by other methods. The general form of the curves is a first slow increase of temperature, until the onset of a rapid increase followed by a flattening (transformation from kinetic limitation to mass transport limitation). After reaching the mass transport limited regime, the reaction becomes nonresponsive to further temperature rise.

Due to the lack of formation of a palladium hydride phase at the temperatures considered in this experiment, the potential contribution from hydrogen in solid solution in the metallic palladium to the LSPR signal is absolutely negligible. It can be observed that the light-off is taking place at elevated temperatures as anticipated, and has its origin in the detailed kinetics of the reaction. It is possible to relate the kinetics of the reaction to hydrogen poisoning and associate the reaction activation energy with hydrogen desorption.

Figure 4 shows an Arrhenius analysis of the low-temperature regime (below light-off and the kinetic phase transition) for quantification of these measurements and demonstrates the relationship between local catalyst temperature and measured LSPR shift. The apparent activation energies obtained for three different reactant concentrations are in line with the literature values.

Figure 4. Arrhenius analysis of LSPR temperature traces below light-off (the dotted/dashed lines correspond to Arrhenius-function fi ts to the LSPR data).

Light-off traces for four different palladium particle sizes (Figure 1b) at α = 0.35 are presented in the inset given in Figure 5, showing higher relative activity with reducing particle size. It can be observed that the catalytic activity per surface area for the smallest catalyst particles of D_mean = 2.2 nm is nearly threefold higher than that for the largest particles of D_mean = 18.6 nm.

Figure 5. Light-off traces for four different Pd particle sizes (inset) and a measure of the relative activity for each Pd catalyst particle size.

Conclusion

The results clearly show the ability of Insplorion’s INPS platform to optically measure local catalyst temperatures under realistic (T, p) reaction conditions for calorimetric studies of reactions on nanoparticle catalysts. This provides a novel nanocalorimetric way for catalytic reaction measurements, including size-dependent activity, on small quantities of catalytically active nanoparticles.

About Insplorion

Insplorion develops and sells instruments for researchers with 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.