Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - SINCAT (Single Nanoparticle Catalysis)

Teaser

Small improvements in efficiency and/or selectivity of nanoparticle catalysts, frequently used in the chemical industry, result in billions of Euros of revenue increase. Catalytic nanoparticles are also of central importance for pollution mitigation and in sustainable energy...

Summary

Small improvements in efficiency and/or selectivity of nanoparticle catalysts, frequently used in the chemical industry, result in billions of Euros of revenue increase. Catalytic nanoparticles are also of central importance for pollution mitigation and in sustainable energy technologies such as fuel cells, batteries and hydrogen storage. For these reasons, atomic level engineering of catalytic nanoparticles has an enormous potential impact.
The catalytic performance of nanoparticles is directly controlled by their size, shape and chemical composition. These properties determine which surface sites that are exposed to the reactants and, therefore, activity and selectivity. Accordingly, significant advances in the shape-selected synthesis of nanoparticles have brought about exciting opportunities for capitalizing on such effects. Experimental investigations are, however, traditionally conducted on nanoparticle ensembles. As a result they are plagued by inhomogeneous sample material and averaging effects, which deny access to the understanding of important details related to size, shape, microstructure and composition of nanocatalysts. Conceptually, this problem can be entirely eliminated by experiments on individual nanoparticles. Secondly, under reaction conditions, the gas composition in a macroscopic catalytic reactor is not well defined locally. As a result, the catalyst can take on different oxidation states or experience different reactant mixtures at different positions. In-spite of this, catalyst activity is measured as the average of the production from all catalytic material, assuming that it can be described by a single set of parameters everywhere. This makes it very difficult if not impossible to isolate the importance of different catalyst properties for its performance.
It is therefore the main objective of the SINCAT project to take on this challenge by establishing a nanofluidic reactor that enables the scrutiny of heterogeneous catalytic reactions at the individual catalytic nanoparticle level. The nanoreactor is integrated with local optical plasmonic nanoantenna probes and mass spectrometric readout from a very small number of nanoparticles for the combined in situ and real time analysis of the catalyst surface/oxidation state and of the product molecules. In this way it enables establishing structure-function correlations at operando conditions for multiple individual nanoparticles in parallel, as well as studying mass transport and particle-particle interaction effects in well-controlled nano-confined space mimicking the pores of industrially used support materials.

Work performed

The action has reached the objectives for the reporting period and is following the project plan and the corresponding milestones. In short, highlighting the key results obtained so far, we have developed and built an advanced experimental setup that is comprised of a highly engineered sample holder, through which a nanofabricated nanofluidic reactor chip is connected to a stainless steel gas handling system, a quadrupole mass spectrometer (QMS) for analysis of all gas exiting the nanoreactor, and a power controller for the on-chip heater enabling operation at up to 623 K. This sample holder is then mounted on an optical microscope connected to a spectrometer equipped with an EMCCD camera that facilitates multichannel single particle plasmonic nanospectroscopy from single catalyst particles located inside a nanoreactor. The nanoreactor chip itself is micro- and nanofabricated onto a thermally oxidized silicon wafer and comprised of a microfluidic in-and outlet system that connects to the sample holder towards the high-pressure gas supply side and the low-pressure QMS-side. The successful development of the complex micro-and nanofabrication of these nanoreactor devices is key result of its own. Having the nanoreactor devices and the necessary instrumental infrastructure now at hand it is the third key result of the project that we have been able to show that our nanoreactor approach, in line with our predictions, enables (i) precise positioning of individual catalyst nanoparticles in a controlled nanoscopic volume, (ii) a high level of pressure, temperature and reactant transport control to and from individual catalyst nanoparticles, (iii) analysis of reaction products by mass spectrometry from a very small amount of catalyst, and (iv) spatially resolved real time operando probing of the surface/bulk oxidation state of several tens of single catalyst particles simultaneously by means of plasmonic nanospectroscopy. Finally, as the fourth main result, we have been able to show on the examples of CO oxidation over Cu and Pt nanocatalysts that reactant conversion on a single catalyst nanoparticle creates a gas concentration gradient in a nanofluidic channel, which transiently prohibits the transition of the oxidation state of a downstream particle and shifts the kinetic phase transition point. Furthermore, it shows that particle-specific microstructure dictates the nature of bistable reaction kinetics and the corresponding kinetic phase transitions occurring on the particle surface. In this way, the developed nanoreactors indeed (i) enable the anticipated detailed study of local mass transport effects in nanoconfined space, mimicking the complex mesoporous support arrangements of a real catalyst, and (ii) enable the scrutiny of the interplay between mass-transport, conversion and nanoparticle microstructure, and how it defines catalyst function and activity at the single nanoparticle level.

Final results

Projecting our results obtained so far on the second half of the project it is clear that the platform has significant potential. For example, already now we have strong indications that we will be able to execute on line mass spectrometry on a number of nanoparticles that is small enough (somewhere between 1 and 100) that we can optically track the surface/oxidation state of all of them simultaneously. This constitutes a breakthrough because the common understanding in the field is that at least ca. 10^6 nanoparticles are need for reasonable QMS readout and most other available single particle techniques can only study one nanoparticle at the time. Furthermore, being able to track up to 100 nanoparticles simultaneously but individually by means of light will enable unique insights into the role of the individual in a catalytic reaction and how, for example, communication between individuals via gas phase or support-mediated mass transport and conversion effects dictates the activity and selectivity of the ensemble. Finally, I also expect unique single-particle structure-function correlations to be unraveled by the nanoreactor device when we have fully succeeded with the implementation of transmission electron microscopy readout for the characterization of the nanoparticles localized inside the nanoreactors prior and after a catalysis experiment.