Surface- and Tip-Enhanced Raman Spectroscopy as Operando Probes for Monitoring and Understanding Heterogeneous Catalysis
Surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS) were until recently limited in their applicability to the majority of heterogeneous catalytic reactions. Recent developments begin to resolve the conflicting experimental requirements for SERS and TERS on the one hand, and heterogeneous catalysis on the other hand. This article discusses the development and use of SERS and TERS to study heterogeneous catalytic reactions, and the exciting possibilities that may now be within reach thanks to the latest technical developments. This will be illustrated with showcase examples from photo- and electrocatalysis.
KeywordsHeterogeneous catalysis Raman Scanning probe microscopy Colloidal synthesis Nanostructure Nanotechnology
Surface-enhanced Raman scattering (SERS) is a phenomenon in which the Raman scattering intensity from molecules close to the surface of certain finely divided metals is amplified by several orders of magnitude . As with ordinary Raman spectroscopy, SERS spectra show the molecular-vibration energies based on the frequency shift between the incident and scattered light by matter. The sensitivity enhancement of SERS in comparison to the conventional Raman process has resulted in more widespread applications, especially in surface chemistry where vibrational spectroscopic data reveal how molecules precisely interact with surfaces . The analyte needs to be adsorbed onto the surface of the SERS substrate or very close to it (~10 nm), which can be challenging, however this mechanism has the added value of quenching the fluorescence from adsorbed molecules .
The unexpected enhancement of pyridine Raman signals on Ag electrodes was first reported by Fleischmann et al. in 1974 . Surprisingly strong and potential dependent Raman signals from pyridine adsorbed on an electrochemically roughened Ag electrode have been observed. Subsequent investigations by Albrecht and Creighton  and Jeanmaire and Van Duyne  in 1977, led to the discovery of SERS and its remarkable enhancement effects. It was proposed by Van Duyne and co-workers that the enhancement of the Raman scattering was due to an increase in the electromagnetic field at the roughened surface. Creighten et al. hypothesised that resonance Raman scattering was responsible due to the creation of a charge-transfer absorption band between the adsorbate and the surface. These two ideas, now known as the electromagnetic and chemical enhancement mechanisms, have dominated the mechanistic debate in the SERS community for the past 30 years, and most probably will continue to do so.
The recent revival of interest in SERS as an interesting and valuable approach to probe heterogeneous catalytic reactions has incited a flurry of publications on the subject. Possibly the most interesting reports now deal with the developments leading to the use chemically inert SERS probes [6, 7] as well as to methods for anchoring SERS-active nanoparticles on the external or internal surface of catalyst supports and molecular sieves [8, 9]. They represent exciting new steps on the road towards the wide-ranging use of SERS as an operando probe for monitoring and understanding heterogeneous catalysis.
In this article we discuss the use, recent developments and potential of SERS and TERS in the field of heterogeneous catalysis. Special attention will be focused on some historic developments, recent applications for e.g. reaction monitoring and the technical requirements to be further explored to make this methodology a real asset in the ever-expanding toolbox of in situ spectroscopic methods for heterogeneous catalysis research. The goal of this paper is, however, not to give a comprehensive overview of the entire fast-growing research field. Within this context, we refer the reader to some older and more recent review papers on this topic to put the described developments in the proper context [10, 11, 12].
2 A Brief History to SERS of Heterogeneous Catalysis
2.1 1981–1986: First Applications of SERS in the Field of Heterogeneous Catalysis
The first steps towards the use of SERS in the field of heterogeneous catalysis were taken in the 1980s with the study of adsorbates at catalytically relevant interfaces by the Dorain group at Yale University. Initial work was done on Ag powders with a rough surface on the nanoscale, on which surface species such as SO3 2− and SO4 2− were observed and identified [13, 14, 15, 16]. These early forays into kinetic studies utilising the SERS methodology were tentative in their conclusions, as it was already noted early on that the intensities of SERS peaks are dependent on the metal surface roughness as well as on the adsorbate concentration. The SERS-active phase on the Ag powder was formed in situ in the case of the reactions of NO2/N2O4 with Ag. SEM measurements were performed to confirm the theory that Ag microstructures were formed on the surface of the Ag powder. It was confirmed that the formation of these microstructures was due to the surface layer of Ag2O reacting with the initial pulse of the gas to form AgNO3 + Ag + NO. The Ag atoms formed then migrated freely due to the thermal energy released by the reaction, forming Ag microstructures. These first series of studies demonstrated the potential of SERS for studying heterogeneous catalysis, as well as highlighting the challenges of separating the catalytic reaction from the SERS analysis.
2.2 1987–1993: Introduction of SERS in the Field of Electro-catalysis
Following the first reported successes of SERS for heterogeneous catalysis research it could have been expected that the field of ultra-high vacuum (UHV) surface science would take on the challenge of SERS, but this was not really the case. The early recognition that the SERS effect was limited to roughened surfaces of ‘coinage’ metals most probably dampened the interest of the UHV surface-science community. The concurrent emergence of electron energy loss spectroscopy (EELS) and infrared-reflection absorption spectroscopy (IRAS) for UHV-based systems most probably encouraged this indifference, as unlike SERS these vibrational techniques were applicable to the ordered mono-crystalline metal surfaces that were at least in the early days the primary focus of the UHV community. The markedly greater interest shown by the electrochemical community in SERS was probably due in part to the near-exclusive usage (at that time) of polycrystalline metal electrodes, along with the straightforward preparation of SERS-active surfaces by means of controlled-potential oxidation–reduction cycles.
It is important to mention here that the group of Michael Weaver at Purdue University took a new approach to SERS substrates in the late 1980s by coating Au electrodes with thin layers (3–4 monolayer) of transition metals, such as Ru, Rh and Pt . These thin metal layers provided the catalytic surface, whilst the Au electrode beneath enhanced the Raman scattering.
The transition metal thin films were electrodeposited onto an electrochemically roughened gold electrode. The thickness of these transition metal layers can be controlled by varying the potential across the electrode. By utilising transition metals in this way, a much broader range of catalytic reactions were now available for study. The Weaver group investigated catalytic systems ranging from CO and SO2 oxidation to methanol and formic acid oxidation [18, 19, 20, 21].
An early example of these studies is the electro-oxidation of CO over Rh and Ru coated Au electrodes, followed by the observation of SERS bands in the C–O stretching region, which disappeared upon voltage increase at the electrode . Corresponding cyclic voltammetry measurements show CO electro-oxidation occurring at the same voltage at which the SERS bands disappear, confirming the reaction. However, in 1988 it still took 10 min to collect these SERS spectra, preventing practical kinetic measurements.
2.3 1992–1999: Moving Beyond the Field of Electro-catalysis
An important limitation of the transition metal thin films approach developed and used by Weaver and co-workers in the late 1980s and early 1990s was the presence of residual ‘pinhole’ sites that exposed the underlying Au substrate. However, this complication turned out to be much less of an impediment to the utilization of the technique in gas-phase environments, particularly at the elevated temperatures of catalytic relevance, since Au displays near-negligible adsorptive properties under these conditions. It was found that the Pt-, Rh- and Ru- coated Au surfaces exhibited excellent temporal stability when under ambient pressure gas-phase environments, which allowed the exploration of the utility of the SERS approach for probing the adsorption of molecules at such catalytic relevant surfaces .
The advent of CCD technology enabled spectra to be obtained at significantly faster rates (1–2 s). This technical advance also lead to the improved sensitivity in Raman measurements, as demonstrated by McCreery and co-workers detecting adsorbates at the carbon-aqueous interfaces even in the absence of SERS,  and the Tian group obtaining Raman spectra for a diverse range of adsorbates at the transition metal-solution interface in the presence of only mild surface enhancements, more specifically ~30–100 [25, 26].
The Weaver research group built upon their experience in the SERS field to move from electrochemical cells to gas-flow reaction systems. The combination of in situ SERS and in-line mass spectrometry analysis of the gas flow enabled them to investigate a range of chemical reactions. The changing intensities of Raman bands of surface species and simultaneous analysis of the reaction products enabled new physicochemical insight into the reaction kinetics and related mechanisms over transition metal surfaces.
3 Heterogeneous Catalysis and SERS: Developments in the Last Decade
Single-molecule SERS (SM-SERS) was first demonstrated in 1997, garnering new interest in SERS from a much wider audience. Advances in nanotechnology and synthetic methods have also assisted in this new wave of research, leading to several significant advances in the field.
3.1 Revival of SERS in the Field of Electro-Catalysis
Following a 5-year hiatus, a revival of interest in the use of electrochemical SERS around the turn of the millennium led to an expansion of the scientific community involved in the study of SERS of heterogeneous catalysis.
The Weaver group returned to SERS studies of electro-catalysis after several years of focussing on utilising SERS for gas-phase reactions, publishing a couple of new electro-catalytic SERS articles in the midst of more fundamental electrochemical work [43, 44, 45]. The death of Weaver in 2002 clearly left a vacuum in the fields of electrochemistry and SERS that was later on filled by several new research groups. In this respect, it is worthwhile to mention that a special edition of The Journal of Electroanalytical Chemistry was dedicated to the memory of Michael, to which Ertl and co-workers, as well as Koper et al., contributed papers on SERS in the field of electro-catalysis [46, 47].
A new method for coating SERS-active electrodes with a monolayer of small Pt or Pd nanoparticles was developed in 2005 by the Perez group in Alicante [58, 59, 60, 61].The nanoparticles coating of the electrode are small enough (i.e, 4 nm) that the SERS enhancement is maintained, whilst the use of a layer of nanoparticles rather than an ultrathin layer of Pt metal over the electrode gives an improvement in stability. A combined SERS and AFM study was undertaken by Scheijen et al. in 2007, looking at the influence of the Pt nanoparticles deposition procedure over Au electrodes on the electro-oxidation of small organic molecules. These authors concluded that the method of nanoparticles deposition has a strong influence on the size and homogeneity of the nanoparticles, and the resulting catalytic and SERS activity .
Studies of the electrochemical reduction of keto-esters over Pt surfaces have been undertaken by Attard et al. at Cardiff University, using the previously mentioned Tian method of coating Au nanoparticles with Pt. Mechanistic insight into these chemical reactions show the potential of the SERS methodology for measuring increasingly complex catalytic systems [63, 64]. The electrochemical SERS work of Koper and co-workers at Leiden University also utilizes a variety of bare and transition metals-coated electrodes, in combination with which SERS is used to gain a deeper understanding of the systems involved in solar fuels production [47, 62, 65, 66]. Along the same line of research, recent publications of the Bell group at UC Berkeley report on the use of cobalt oxide- and nickel oxide-coated Au electrodes to follow the electrochemical evolution of O2 [67, 68].
3.2 Applications of Anchored SERS Nanoparticles
Moving towards the implementation of the SERS methodology in more industrially relevant catalytic systems, the SERS substrates are required to be physically robust as well as sufficiently Raman signal enhancing. Various methodologies for anchoring SERS nanoparticles to support materials have been reported, which look promising for future use in heterogeneous catalysis. Some highlights on these approaches are discussed below.
In 2004 a core–shell Au–Pt nanoparticle film was fabricated by a self-assembly method on a silicon wafer, and its application as a catalyst and as SERS substrates was investigated . The nanostructured film exhibited high catalytic activity and SERS, demonstrating its potential use in heterogeneous catalysis and as a SERS substrate. In addition, it should be possible to tune the surface properties of the film by controlling the size, composition and surface properties of core–shell nanoparticles. The self-assembly method leads to a highly reproducible substrate, with great potential for sensing applications in catalysis and other fields.
A recent promising nanomaterial is graphene, and has been recently demonstrated as a support to disperse and stabilize various metal and metal oxide nanoparticles. Huang et al. describe the formation of Au nanoparticle-graphene oxide (Au–GO) and Au nanoparticle-reduced GO (Au-rGO) composites . These composite materials were used to demonstrate SERS of the molecule p-aminothiophenole (pATP) as well as of the catalytic reduction of o-nitro-aniline. The impressive results for both applications due to the electronic interaction of the graphene with the Au nanoparticles, show great promise for the possible combination of SERS and catalysis over graphene substrates, and have already inspired several further investigations into the use of graphene oxide in combination with SERS [73, 74, 75].
In 2012, different shapes of Pt nanoparticles were used to enable the monitoring of a surface reaction on different surface sites. Spherical and octahedral–tetrahedral Pt nanoparticles were used with the (111) surface orientation of the octahedral–tetrahedral nanoparticles facilitating the reduction of acetaldehyde oxime into ethylamine. This reduction process was not observed over spherical nanoparticles, demonstrating the combination of SERS-active metals and the use of preferentially ordered nanoparticles allows SERS studies of structure-sensitive surface reactions. 
3.3 Monitoring Chemical Reactions Directly in Colloidal Suspension
Joseph et al. have also recently used the idea of small catalytic nanoparticles supported on large SERS nanoparticles, following the catalytic reduction of pNTP with NaBH4 over Pd nanoparticles of 2 nm in size supported on 40 nm-sized Au nanoparticles. 
More recent work by the Schlücker group has built upon the principle of small catalytically active nanoparticles supported on larger, SERS active nanoparticles . The 80 nm Au core is encapsulated within a thin silica shell of ~1.5 nm in size, then thiol groups are used to attach 5 nm Au nanoparticles to the surface. These bimetallic particles are used to monitor the Au-catalysed reduction of pNTP in colloidal suspension, whilst the inert silica shell protects the Au metal core surface from direct contact with the chemical species and prevents unwanted photocatalytic side reactions.
Other Au and Ag nanostructures have been synthesised and tested for both SERS and catalytic activity, though not in combination. One such example is the three-dimensional dendritic Au nanostructures developed by Huang et al. . The high surface-to-volume ratio of the dendritic structures led to high catalytic efficiency, and the structure provided multiple sharp corners and edges for SERS hotspots. However, the adaption of such substrates for use in catalytic conditions could be complicated, and is clearly a challenge for future research.
3.4 SERS and Plasmon-Driven Catalysis
For over a decade, pATP has been regarded as a prototypical probe for assessing the nature of SERS measurements . As pATP interacts strongly with Ag and Au, gives strong SERS signals, and has been regarded as a model compound for probing the chemical enhancement of SERS due to its potential and wavelength dependent spectra . However, difficulties in reproducing the SERS spectra of pATP alone led to further investigations, through which two different explanations were derived. Sun and co-workers first hypothesised that the catalytic conversion of pATP to dimercaptoazobenzene (DMAB) accounted for the unexplained SERS spectra in 2009 . This conclusion was supported by further theoretical and experimental work by Sun and co-workers, who then also concluded that DMAB could be produced by 4-nitrobenzenethiol (4-NBT) [81, 84, 85]. It has been found that these plasmon-driven reactions are strongly dependent on substrate, wavelength and time. More significantly, these instances of plasmon-driven catalysis have shown that SERS can be an invasive technique under certain conditions, and that the species measured may not be the original surface species . In contrast, the work of Kim et al. supports the hypothesis that the SERS bands under debate are in fact a result of the chemical enhancement mechanism [87, 88, 89].Within this context it is important to refer to a recent study of Kang et al.  of the Harbin Institute of Technology, who have monitored the plasmon-driven conversion of pNTP into DMAB using a single Ag particle of 2 μm in size and having a roughened surface. This detailed SERS study provided laser wavelength- and power-dependent conversion rates for the reduction of pNTP into DMAB.
4 Challenges Imposed on SERS for Performing Operando Spectroscopy Measurements
The true aim for the SERS methodology in catalysis research is of course to perform operando measurements, leading to new fundamental insights into catalytic intermediates and related transition states. In order to achieve this ambitious goal, further advances must be made to existing SERS probes, enabling the use of SERS under the often extreme conditions encountered inside a catalytic reactor; i.e., at high temperatures and pressures. The most important challenges that are yet to be fully overcome are chemical stability, thermal stability, spectral reproducibility and data analysis. These challenges will now be discussed in more detail below.
4.1 Chemical Stability
One of the primary considerations in the use of SERS for label-free monitoring of catalytic reactions is the exposure of a chemical reaction to an additional transition metal surface in the form of SERS nanoparticles or a planar SERS substrate.
For true operando measurements that are unlimited by the catalyst surface being Au or Ag, SERS surfaces with a high level of chemical stability must be developed, or adapted from existing particles or surfaces. One solution would be to use a SERS surface that could be coated with e.g. Al2O3, however under true reaction conditions it would be preferable to have inert particles that could be distributed within the catalyst material, being preferentially within its shaped form (i.e. catalyst body), without causing any transport limitations or disruption to the gas flow in the catalyst bed.
Previous work in the field of spectroscopy has seen attempts to modify the surface specificity of SERS nanoparticles by various methods. One of the oldest of these approaches is the functionalisation of the nanoparticle surface with alkane thiols, for example, to enable SERS measurements of neutral molecules that would not usually interact with the charged capping of a SERS active nanoparticle . Another method to bring SERS and heterogeneous catalysis together is the use of an ultra-thin shell of other transition metals, such as Pt and Pd . These systems effectively ‘borrow’ the SERS enhancement from the Au core nanoparticles, whilst presenting a catalytically active transition metal outer surface. Until recently, the destabilisation of the nanoparticles upon the removal of the stabilising coating layer has prevented these forms of SERS substrates from application beyond electrochemical systems, though the work of the Schlücker group has begun to address this important topic .
The challenge of the interference of a SERS substrate in a particular system is also a hot topic within e.g. the bio-analysis field, where it has been noted that the exposed metal surface of metal nanoparticles can easily adsorb interfering molecules in the biological environment, leading to variations of the original SERS signals as well as possible biotoxicity . Common solutions include the use of a biomolecule surface-coating, such as bovine serum albumine (BVA), non-toxic polymer coatings, liposome coatings and silica coatings. Such approaches could also inspire the field of heterogeneous catalysis.
4.2 Thermal Stability
Chemical reactions often run well over 300 °C, so it must be possible to use SERS under such conditions. The thermal stability of nanoparticles is a well-known topic in heterogeneous catalysis, with the sintering of nanoparticles at high temperatures being a major field of study. One commonly used solution in heterogeneous catalysis is the synthesis of active nanoparticles within a confined micro- or nanoporous system, which restricts the mobility of the nanoparticles under heating, and so prevents sintering. As discussed above, Silvia et al. utilized a similar concept for their SBA-15 crystals containing Au nanoparticles, though they did not extend their study to look at thermal effects . An investigation into the heating effects on nanoparticle films by Kho et al. confirmed that lateral particle diffusion as a result of heating results in a loss of SERS signal . Other studies focusing on roughened metal substrates have observed some effect of heating on the SERS activity, though the changes are reversible upon cooling [94, 95, 96].
As discussed previously, high-temperature SERS has been achieved with the use of electrochemically-roughened surfaces as SERS substrates. These transition metal surfaces demonstrate a much higher degree of thermal stability than the metal nanoparticles more frequently employed in recent years, though they do not have the extremely high SERS enhancement factors often seen with metal nanoparticles.
4.3 Spectral Reproducibility and Interpretation
The lack of measurement reproducibility is a major limitation of SERS measurements that must be taken into consideration in all measurements, and is in no small way also related to the surface chemistry and related stability of SERS substrates. A lack of substrate generality is intrinsic to the SERS mechanism, as one of the major SERS mechanisms—the chemical enhancement mechanism—works via a charge transfer mechanism. Therefore, if an analyte is not chemically compatible with the surface charge of the SERS substrate, signal enhancement is unlikely to occur.
SERS substrates themselves must also be reproducible synthetically for the results to be truly reproducible. Highly-ordered planar substrates are known to give the most regular SERS spectra, however they sacrifice the high spectral intensity seen when SERS nanoparticles form so-called ‘hot-spots’. The use of multivariate data analysis of spectral variance in SERS measurements using colloids is one potential method to untangle some of the variance seen in SERS measurements. The method takes into account the variation of SERS between different batches of colloids, as the synthesis method may not have been reproduced always exactly . This observation also implies that results may differ from one laboratory to another, and even within the same laboratory variations in time may occur as not always the same persons are in charge of synthesizing the SERS substrates or SERS-active nanoparticles.
Combination of techniques, already employed frequently in heterogeneous catalysis research, may shed further light on the interpretation of SERS measurements. For example, an integrated AFM—Raman instrument is able to directly correlate single nanoparticles or clusters of nanoparticles to spectra, and so give further insight into the chemistry occurring on SERS substrates. The integration of AFM with Raman spectroscopy has been recently illustrated by Harvey et al. for the photo-degradation of rhodamine-6G in combination with differently sized and shaped Ag nanoparticles in the presence and absence of air and at elevated temperatures by making use of an in situ reaction chamber .
5 Showcase SERS and TERS Studies from Our Laboratory
Recent SERS and TERS work from our group has focused on two photo-catalytic/plasmon-driven catalytic reactions involving two thiophenols, namely pNTP and pATP. Both photo-reactive molecules have been largely studied in the literature, as discussed before, as their reactivity can be tuned by altering the laser excitation wavelength as well as power. In other words, the reduction processes of pNTP and pATP can be regarded as ideal model reactions for the (further) development of the SERS and TERS spectroscopic toolkit for heterogeneous catalysis research.
5.1 SERS of the Photo-catalytic Reduction of pNTP in a Self-assembled Monolayer
A large discussion point in the literature is the interpretation of the reaction product SERS spectrum during the photo-catalytic reduction of pNTP. There exist mainly two proposals: p-aminothiophenol (pATP) and p,p′-dimercaptoazobisbenzene (DMAB). The challenge in the identification of the reaction product is at least twofold: (1) an extremely low number of molecules that are converted during the photo-catalytic reduction reaction, and (2) the few molecules present or formed are chemically bound to the surface, and therefore non-accessible by other characterization methods with molecular fingerprinting capabilities.
The second approach providing evidence for a dimerization reaction of pNTP was obtained through incorporation of an inert thiol into the SAMs of pNTP, as illustrated in Fig. 11b. Thiophenol was here used as the two-dimensional equivalent of a solvent. It was found that the reaction rate was extremely slowed down by the dilution effect with thiophenol, up to the point that no photo-reduction reaction was observed under the standard reaction conditions applied. As a result, the reduction process of pNTP should be second-order.
5.2 SHINERS Studies of the Photo-catalytic Reduction of pATP
Harvey and co-workers have recently assessed the performance of SHINERS for investigating the photo-reactivity of the anticipated reaction intermediate in the process of the photo-catalytic reduction of pNTP, i.e., pATP . It is known that pATP, as pNTP, interacts strongly with either Ag and Au, giving strong SERS signals. As mentioned before, analogous to pNTP also pATP is regarded a model compound for probing SERS activity. However, it was noted before that extra Raman bands form in the SERS spectra with an excitation wavelength of 633 nm or below. Difficulties in producing the SERS spectra of pure pATP led to detailed investigations of this effect, from which two potential explanations were derived. The first explanation involves the catalytic conversion of pATP into DMAB, which is then considered to be the molecule responsible for the changes in the SERS spectra of pATP . This line of thinking is supported by further theoretical and experimental work, as discussed in detail above and is regarded as a plasmon-driven reaction, similar pNTP. Another explanation has been put forward by Kim et al., suggesting that the SERS bands under debate are simply a result of charge transfer, and therefore a reversible chemical enhancement [87, 88, 89].
The dominance of the bands at 1064 and 1574 cm−1 in the Raman spectra in Fig. 14a and c are indicative of the presence of pATP, as exposure to 785 nm laser light results only in extremely slow conversion of the starting material into DMAB. The spectra in Fig. 14b and d, however, show clear Raman bands at 1140, 1392 and 1431 cm−1, indicative of the conversion of pATP over both bare gold nanoparticles and Au-based SHINERS. The reason that no complete conversion is observed could be linked to an equilibrium state, and/or also to the fact that only a small fraction of the observed molecules contribute to this signal. The observation of the spectral changes over both bare gold nanoparticles and Au-based SHINERS can be seen as a confirmation that there is a photo-catalytic reduction of pATP into DMAB as charge transfer cannot take place through the oxidic and isolating shell surrounding the Au nanoparticles, as illustrated in Fig. 13b by the HRTEM of the Au-based SHINERS. This observation provides further evidence that the hypothesis of Kim et al. [87, 88, 89] is rather unlikely to occur and therefore we conclude that the origin of the spectral changes is due to the formation of DMAB as the dimerization product of pNTP, as has also been supported by theoretical and experimental work by Sun et al. [81, 84, 85].
5.3 TERS Studies of the Photo-catalytic Reduction of pNTP
6 Future Perspectives
With significant advances being made rapidly throughout the SERS community, the prospect of performing operando SERS and TERS measurements of a catalytic reaction are clearly on the horizon. The introduction and related versatility of SHINERS in the spectroscopic toolkit is an important step forward along the way to introducing the SERS method as a core analysis technique for heterogeneous catalysis research.
The advent of surface-enhanced spatially-offset Raman spectroscopy (SESORS), thus far mainly applied to biological systems, also demonstrates great potential for application in catalysis [106, 107]. Perhaps the most significant challenge in the full applicability of SERS as a standard spectroscopic tool for catalysis research is furthering the work done thus far on the chemical generality or SERS, developing either a single method or a whole toolkit of SERS substrates that enable the detection of any analyte, irrespective of charge or hydrophobicity/hydrophilicity. The first steps have been taken along this promising path, with the initial development of transition metals-coated substrates, and more recently the invention of SHINERS, the first SERS substrate to truly separate heterogeneous catalysis and spectroscopic analysis (reaction monitoring). [19, 108] SHINERS could really become an asset for monitoring the reactants, reaction products, as well as minute amounts of reaction intermediates, in a reactor vessel, as illustrated in Fig. 17b. The showcase example of Heck and co-workers is a clear indication that this indeed may become possible. Moreover, more combinations of SHINERS and reactors and (individual) catalyst particles and catalyst bodies can be envisaged, providing local “Raman antenna” for catalytic performance, including activity, selectivity as well as stability.
This work is supported by the Netherlands Research School Combination-Catalysis (NRSC-C), Netherlands Organization for Scientific Research (CW-NWO) in the frame of a Spinoza and Top research Grant, and an European Research Council (ERC) Advanced Grant (No. 321140). The authors acknowledge Evelien van Schrojentstein en Arjan Mank for their scientific contributions to some of the research described in this article. This article is partially based on Chapter 2 of the PhD thesis of Clare Harvey. Finally, C.E.H. and B.M.W. acknowledge Agnieszka Ruppert for making Fig. 17.
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