Periodic Reporting for period 4 - SINCAT (Single Nanoparticle Catalysis)
Período documentado: 2020-07-01 hasta 2020-12-31
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.
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.
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, as the first one, we have developed and built an advanced experimental setup that is comprised of a custom 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 723 K. This sample holder is then mounted on an optical microscope connected to a spectrometer equipped with an EMCCD camera that facilitates single particle plasmonic nanospectroscopy and nanoimaging from single catalyst particles located inside a nanoreactor.
The successful development of such nanoreactor chips decorated with individual catalyst nanoparticles of different types is the second key result. They are micro- and nanofabricated onto a thermally oxidized silicon wafer and comprised of a microfluidic in-and outlet system.
As the third key result, using this now fully operational platform, we have shown that particle-specific microstructure dictates the nature of bistable reaction kinetics and the corresponding kinetic phase transitions occurring on the single Pt catalyst nanoparticles, as well as that a thin Pt oxide phase that forms at high reaction temperatures constitutes the most active phase at operando conditions.
As the fourth key result, for the CO oxidation reaction on Cu we have how the extreme nanoconfinement of the nanofluidic structure that hosts the single nanoparticles has a dramatic effect on the state of activity of adjacent individuals. We have also shown that our nanoreactor therefore can be regarded as a “model pore” to study the role of mesoporous catalyst support materials in a model system fashion.
As the fifth main result, we have shown how the SINCAT nanoreactor platform can be used to establish single particle structure-activity correlations for catalysis in the liquid phase using fluorescence microscopy, on the example of an Au catalyst and the catalytic decomposition of fluorescein.
As the sixth main result, we have demonstrated that single particle plasmonic nanospectroscopy in combination with electrodynamics simulations can be used to identify the oxidation mechanism of Cu nanoparticles both in the initial oxide shell growth phase and during Kirkendall void formation.
As the seventh main result, we have demonstrated that the nanoreactor concept enables mass spectroscopic activity measurements from a catalyst bed small enough that all individuals in the catalyst bed can be monitored optically with single particle resolution. This means studies down to only 50 individual nanoparticles. For the CO oxidation over Cu catalytic reaction, we have in this way been able to reveal that the oxidation state dynamics during reaction are highly individual for each particle and that they strongly depend on the specific position of the particle in the catalyst bed, as well as on the overall catalyst bed design.
The successful development of such nanoreactor chips decorated with individual catalyst nanoparticles of different types is the second key result. They are micro- and nanofabricated onto a thermally oxidized silicon wafer and comprised of a microfluidic in-and outlet system.
As the third key result, using this now fully operational platform, we have shown that particle-specific microstructure dictates the nature of bistable reaction kinetics and the corresponding kinetic phase transitions occurring on the single Pt catalyst nanoparticles, as well as that a thin Pt oxide phase that forms at high reaction temperatures constitutes the most active phase at operando conditions.
As the fourth key result, for the CO oxidation reaction on Cu we have how the extreme nanoconfinement of the nanofluidic structure that hosts the single nanoparticles has a dramatic effect on the state of activity of adjacent individuals. We have also shown that our nanoreactor therefore can be regarded as a “model pore” to study the role of mesoporous catalyst support materials in a model system fashion.
As the fifth main result, we have shown how the SINCAT nanoreactor platform can be used to establish single particle structure-activity correlations for catalysis in the liquid phase using fluorescence microscopy, on the example of an Au catalyst and the catalytic decomposition of fluorescein.
As the sixth main result, we have demonstrated that single particle plasmonic nanospectroscopy in combination with electrodynamics simulations can be used to identify the oxidation mechanism of Cu nanoparticles both in the initial oxide shell growth phase and during Kirkendall void formation.
As the seventh main result, we have demonstrated that the nanoreactor concept enables mass spectroscopic activity measurements from a catalyst bed small enough that all individuals in the catalyst bed can be monitored optically with single particle resolution. This means studies down to only 50 individual nanoparticles. For the CO oxidation over Cu catalytic reaction, we have in this way been able to reveal that the oxidation state dynamics during reaction are highly individual for each particle and that they strongly depend on the specific position of the particle in the catalyst bed, as well as on the overall catalyst bed design.
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.