The role of water in the methanol-to-olefins (MTO) process over H- has been elucidated by a combined theoretical and experimental approach, encompassing advanced molecular dynamics simulations and in-situ micro-spectroscopy. First principle calculations at the molecular level point out that water competes with methanol and propene for direct access to the Brønsted acid sites. This results in less efficient activation of these molecules, which are crucial for the formation of the hydrocarbon pool. Furthermore, lower intrinsic methanol reactivity towards methoxide formation has been observed. These observations are in line with a longer induction period observed from in-situ UV-Vis micro-spectroscopy experiments. These experiments revealed a slower and more homogeneous discoloration of H-SAPO-34, while insitu confocal fluorescence microscopy confirmed the more homogeneous distribution and larger amount of MTO intermediates when co-feeding water. As such it is show that water induces a more efficient use of the H-SAPO-34 catalyst crystals at the microscopic level. The combined experimental theoretical approach gives a profound insight into the role of water on the catalytic process at the molecular and single particle level.
Surface- and tip-enhanced Raman spectroscopy (SERS and TERS) techniques exhibit highly localized chemical sensitivity, making them ideal for studying chemical reactions, including processes at catalytic surfaces. Catalyst structures, adsorbates, and reaction intermediates can be observed in low quantities at hot spots where electromagnetic fields are the strongest, providing ample opportunities to elucidate reaction mechanisms. Moreover, under ideal measurement conditions, it can even be used to trigger chemical reactions. However, factors such as substrate instability and insufficient signal enhancement still limit the applicability of SERS and TERS in the field of catalysis. By the use of sophisticated colloidal synthesis methods and advanced techniques, such as shell-isolated nanoparticle-enhanced Raman spectroscopy, these challenges could be overcome.
An era of circularity requires robust and flexible catalysts and reactors. We need profound knowledge of catalytic surface reactions on the local scale (i.e., angstrom–nanometer), whereas the reaction conditions, such as reaction temperature and pressure, are set and controlled on the macroscale (i.e., millimeter–meter). Nanosensors operating on all relevant length scales can supply this information in real time during operando working conditions. In this Perspective, we demonstrate the potential of nanoscale sensors, with special emphasis on local molecular sensing with shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) and local temperature sensing with luminescence thermometry, to acquire new insights of the reaction pathways. We also argue that further developments should be focused on local pressure measurements and on expanding the applications of these local sensors in other areas, such as liquid-phase catalysis, electrocatalysis, and photocatalysis. Ideally, a combination of sensors will be applied to monitor catalyst and reactor “health” and serve as feedback to the reactor conditions.
Tip-enhanced Raman spectroscopy (TERS) is a promising technique that enables nondestructive and label-free topographical and chemical imaging at the nanoscale. However, its scope for in situ characterization of catalytic reactions in the liquid phase has remained limited due to the lack of durable and chemically inert plasmonically active TERS probes. Herein, we present novel zirconia-protected TERS probes with 3 orders of magnitude increase in lifetime under ambient conditions compared to unprotected silver-coated probes, together with high stability in liquid media. Employing the plasmon-assisted oxidation of p-aminothiophenol as a model reaction, we demonstrate that the highly robust, durable, and chemically inert zirconia-protected TERS probes can be successfully used for nanoscale spatially resolved characterization of a photocatalytic reaction within an aqueous environment. The reported improved lifetime and stability of probes in a liquid environment extend the potential scope of TERS as a nanoanalytical tool not only to heterogeneous catalysis but also to a range of scientific disciplines in which dynamic solid–liquid interfaces play a defining role.
Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is quickly developing into a powerful characterization tool in heterogeneous catalysis. In this work, we employ Pt catalysts supported on Au@SiO 2 shell-isolated nanoparticles to study hydrogenation reactions. First, we demonstrate the facile preparation of Pt/Au@SiO 2 and its characterization by using adsorption of CO as probe molecule. Next, we use the adsorption and hydrogenation of phenylacetylene as a model reaction for the interaction of triple bonds and aromatic rings with catalytic Pt surfaces. We show that the applicability of SHINERS is not limited to inherently gaseous compounds, thereby expanding the applicability of the technique to more complex systems. Furthermore, by using nonparticipating side groups as labels, we observe the sequential hydrogenation of phenylacetylene into styrene and ultimately ethylbenzene upon reaction with H 2 . Upon the absence of H 2 , the reverse reaction takes place with those molecules still adsorbed onto the catalyst surface, which allowed a more detailed understanding of the reaction mechanism and the assignment of Raman peaks. This strengthens the position of SHINERS as an easily applicable surface sensitive technique that can be used to study a wide variety of chemical reactions in the field of heterogeneous catalysis.
Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) has proven to be a useful characterization tool for heterogeneous catalysis research. The advantage of SHINERS lies in studying surface reactions on solid catalysts, including the detection of reactants, intermediates and products, in real time. However, due to the extremely strong local electric fields, minor amounts of contaminants can already have a big impact on the quality and interpretation of the spectroscopic data obtained. Often, a large part of the organic fingerprint region (1100-1700 cm ) is omitted from SHINER spectra as this is not the main region of interest. However, we show that bands in this region are an important indication of the cleanliness of the substrate. In this work, we propose robust synthesis and measurement protocols to obtain clean SHINERS substrates amenable for catalysis research. By cleaning the substrates with various heat and oxidation treatments, featureless Raman spectra can be obtained. Furthermore, very pure gas feeds are required and must be obtained by flushing the gas lines and the reaction chamber beforehand and installing a filter for further cleaning the gas feed. Controlling the laser power to limit substrate and sample degradation is also a crucial aspect of proper measurement protocols.
Synthesis methods to prepare lower transition metal catalysts and specifically Ni for Shell‐Isolated Nanoparticle‐Enhanced Raman Spectroscopy (SHINERS) are explored. Impregnation, colloidal deposition, and spark ablation have been investigated as suitable synthesis routes to prepare SHINERS‐active Ni/Au@SiO2 catalyst/Shell‐Isolated Nanoparticles (SHINs). Ni precursors are confirmed to be notoriously difficult to reduce and the temperatures required are generally harsh enough to destroy SHINs, rendering SHINERS experiments on Ni infeasible using this approach. For colloidally synthesized Ni nanoparticles deposited on Au@SiO2 SHINs, stabilizing ligands first need to be removed before application is possible in catalysis. The required procedure results in transformation of the metallic Ni core to a fully oxidized metal nanoparticle, again too challenging to reduce at temperatures still compatible with SHINs. Finally, by use of spark ablation we were able to prepare metallic Ni catalysts directly on Au@SiO2 SHINs deposited on a Si wafer. These Ni/Au@SiO2 catalyst/SHINs were subsequently successfully probed with several molecules (i. e. CO and acetylene) of interest for heterogeneous catalysis, and we show that they could be used to study the in situ hydrogenation of acetylene. We observe the interaction of acetylene with the Ni surface. This study further illustrates the true potential of SHINERS by opening the door to studying industrially relevant reactions under in situ or operando reaction conditions.
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