Optical approaches in study of nanocatalysis with single-molecule and single-particle resolution

Abstract

Studying the activity of individual nanocatalysts, especially with high spatiotemporal resolution of single-molecule and single-turnover scale, is essential for the understanding of catalytic mechanism and the designing of effective catalysts. Several approaches have been developed to monitor the catalytic reaction on single catalysts. In this review, we summarized the updated progresses of several new spectroscopic and microscopic approaches, including single-molecule fluorescence microscopy, surface-enhanced Raman spectroscopy, surface plasmon resonance microscopy and X-ray microscopy, for the study of single-molecule and single-particle catalysis.

Keywords

nanocatalysis / single-molecule fluorescence / surface-enhanced Raman / localized surface plasmon resonance / X-ray

Introduction

Heterogeneous catalysts made of various materials play a crucial role in modern chemistry, ranging from fine chemical synthesis, energy conversions to pollutant removals [1–3]. Traditional studies of catalysts are focused on ensemble-averaged measurements, assuming that heterogeneous catalysts and catalytic events are spatial and temporal homogeneous. However, due to the complexity of heterogeneous reaction and heterogeneities of individual catalyst particles in both size and morphology, detailed chemical information obtained on heterogeneous catalysts is scarce, leading to a lack of insight understanding of chemical processes. Even on single nanoparticle, active sites come from different types of surface sites such as corner, edge and facet sites, with intrinsically heterogeneities of their nature, distribution and accessibility [4]. In addition to the above mentioned spatial differences, each single catalytic turnover is always transitory due to the morphology changes and surface structural dynamics of catalysts. Therefore, studies of catalytic reactions in real time, and at the single-molecule and single-particle level are needed to address these key challenges [5,6].

Metal nanoparticles (NPs) are important heterogeneous catalysts for many chemical transformations due to their nanometer size-induced high surface-to-volume ration and high chemical potentials [7–9]. The advance in chemical synthesis has allowed the preparation of catalytic nanoparticles, or nanocatalysts, with impressive control of size, shape and composition [10–12]. Even though, studies of nanocatalysts still suffer from lack of sufficient spatiotemporal resolution to gain further direct insights into catalytic mechanism.

Recently, we have witnessed the introduction of some novel spatiotemporal spectroscopy for the characterization of nanocatalysts with unattainable resolution and sensitivity. Figure 1 illustrates the complexity and size of the individual catalyst particle under study (left side), and distinct analysis methods with different spatial resolution and chemical information content (right side). Depending on the chemical information needed and the spatial resolution at which the catalyst material functions, a suitable characterization method [4] can be selected for the evaluation of heterogeneities in space and time for both model catalyst particles (for example large zeolite crystals and metal particles) and real, industrially used catalyst particles (for example catalyst bodies, small zeolite crystals and supported metal nanoparticles).

In this review, we focus on optical approaches in the study of nanocatalysis with single-molecule and single-particle resolution. These spectroscopic and microscopic methods rely on the sensitive detection of optical signals arising from reactants, products, nanocatalysts, or changes of the surroundings evoked by these reactions. Other methods, for instance, electrochemical measurements with ultramicroelectrode [13] and scanning electrochemical microscopy [14], which are capable of measuring catalysis on single particles, but failed to achieve single-reaction resolution for each reaction only resulting in the transfer of one or few electrons, are not covered.

Optical responses from reactants and products

In catalysis, reactants in gas or liquid phase are often converted to desired product molecules on the surface of catalytically active nanoparticles. Thus, a direct means to monitoring catalysis on a single nanoparticle is to detect the optical response changes of reactants or products. This requires ultrasensitive optical resolution because in each catalytic turnover, only one or a few molecules may be consumed or generated on a single nanoparticle.

Single-molecule fluorescence microscopy

This approach builds on the pioneering work in single-enzyme catalysis [15–20]. Roeffaers et al. [21] in 2006, first employed the knowledge obtained from single molecule enzymology and studied the spatial distribution of catalytic activity on a layered double hydroxide crystal. After that, several catalysis research works in studying micro- and nano-scale solid catalysts based on this method were continuing to emerge [22,23]. In this strategy, a fluorogenic substrate is catalytically converted into a strongly fluorescent product, which is then optically detected by total-internal-reflection fluorescence microscopy (TIRFM) [5,24,25].

TIRFM employs the unique properties of an induced evanescent wave to selectively illuminate and excite fluorophores in a restricted specimen region immediately adjacent to a glass-water (or glass-buffer) interface. The basic concept of TIRFM is simple, requiring only an excitation light beam traveling at a high incident angle through the solid glass coverslip or plastic tissue culture container, where the cells adhere. Refractive index differences between the glass and water phases regulate how light is refracted or reflected at the interface as a function of incident angle. At a specific critical angle, the beam of light is totally reflected from the glass/water interface, rather than passing through and refracting in accordance with Snell’s Law. The reflection generates a very thin electromagnetic field (usually less than 200 nm) in the aqueous medium, which has an identical frequency to that of the incident light, which enables to detect a single fluorescence molecule [26].

Real-time single-molecule kinetics of single nanoparticlesƒƒChen and coworkers [23,27–30] detected the catalytic activity of individual 6 nm gold (Au) nanoparticles with single turnover precision. The single-molecule turnover of non-fluorescent resazurin to its fluorescent product resorufin by the addition of NH₂OH was monitored as a reporter system (Fig. 2(a)). A continuous flow was applied to accelerate the release of product molecules from the nanoparticles after a short period of stay, leading to a sudden disappearance of the fluorescence emission and an unoccupied nanoparticle for the next chemical reactions. Individual catalytic reactions on a single Au nanoparticle are reported by the fluorescence bursts at a localized spot on the image, corresponding with fluorescence intensity vs. time trajectory (Fig. 2(d)). Based on the stochastic analysis of the on- and off-times of fluorescence bursts measured at different substrate concentrations, both the kinetics of product formation and product dissociation could be revealed. To explain the substrate concentration dependency of the catalytic reaction, two types of surface sites and two products desorption pathways (a direct desorption pathway and a reactant-assisted desorption pathway) have been evoked (Fig. 2(e)). More importantly, individual nanoparticles revealed different selectivity in these two pathways. In a follow-up study by the same group, the relationship of Au nanoparticle size and the catalytic activity was carefully analyzed [29], revealing that the size of nanocatalysts strongly influences its product formation and dissociation rates coupled with surface restructuring. They also applied this strategy to explore the kinetics of electrocatalysis on individual carbon nanotubes [31] as well as single platinum (Pt) nanoparticles [32]. Majima and coworkers [22] also employed this strategy in a photocatalysis system, in which a fluorogenic probe was used to study reactive oxygen species such as singlet oxygen (¹O₂), and hydroxyl radicals (·OH) generated from irradiated TiO₂ photocatalysts. Alivisatos’s group studied the photocatalytic oxidation reaction of Amplex Red to resorufin using Sb-doped TiO₂ nanorods [33].

Super-resolution imaging of single particle nanocatalysisƒƒFor large catalytic particles, where the size of catalytic particles is above 100 nm to even micrometer (μm), their surface structures and distributions of active sites are relatively easy to be imaged by electron microscopes, but difficult for optical microscopes owing to the existence of optical diffraction limit. To solve this problem, a fluorescent spot generated from a catalytic generated fluorescent product could be fitted by a two dimensional Gaussian function, resulting a point spread function (PSF) of single-molecule fluorescence, which allows the imaging of individual product formation event on single nanoparticles with a resolution of tens of nanometers.

A series of pioneer work on super optical resolution imaging of heterogeneous catalysis were also conducted by Chen’s group. They used this method to map catalytic activity distribution and its temporal evolution on pseudo-0D, −1D, and −2D nanocatalysts [34–36]. In these studies, the catalytic formation of resorufin was used in conjunction with Gaussian fitting of PSF to achieve a sub-particle mapping of the reactive centers with a resolution of ca. 40 nm. To take mesoporus silica-coated Au nanorods as an example of pseudo-1D nanocatalysts [34] (Fig. 3(a)), the reactivity of the end sites on the nanorods was found much active center sites and their side facets, revealing a linear gradient from the middle toward the two ends along the side facets of a single Au nanorod (Fig. 3(c)). Furthermore, the reaction activity of the side cites over the center cites, differs for individual Au nanorods and depends on their length. A subsequent study on single Au nanoplates as an example of pseudo-2D nanocatalysts [35], reveals that their corner regions were most catalytic active, followed by edge regions and then flat facets (Fig. 3(d)). These site-specific activity patterns are attributed to an underlying defect density gradient that presumably formed during the seeded growth of the nanocrystals. These results indicate that identifying the active crystal planes is not enough to extrapolate the catalytic activity of metal nanoparticles. Recent investigations by the same group have led to the elucidation of the relationship between the size and catalytic activity by statistically analyzing over 1000 nanoparticles, and their results are able to screen the activity of a large amount of catalyst particles [36].

Majima and coworkers used super-resolution imaging of photocatalytic reduction to identify active sites on single TiO₂ crystals [37] and also found a crystal facet-dependent catalytic activity behavior for the reductive photocatalysis. Monitoring the catalytic turnover on micro-sized anatase crystals revealed that catalytic active sites are mainly reside at the {101} facets. They then extended the super-resolution imaging studies to Au nanoparticle-decorated TiO₂ nanoparticles [38], micrometer-sized crystals [39] and 2D nanocrystals [40]. In a very recent work, this group developed novel fluorogenic molecule probes to visualize photo-induced redox reactions on single TiO₂ and Au-TiO₂ nanoparticles [40]. By chemically modifying the fluorescent probe, the fluorescence quantum efficiency and life time were enhanced. As a result, electron transfer on the surface of the nanocatalysts can be visualized in aqueous environments with significantly improved signal-to-noise ratio. This system was used for super-resolution mapping of reactive sites with an accuracy of approximately 8 nm on Au-TiO₂ nanoparticles (Figs. 3(e) and 3(f)).

This approach was also adopted by Hofkens’ group. Using the method called nanometer accuracy by stochastic chemical reactions (NASCA), Hofkens and coworkers [41] observed single catalytic turnovers on individual ZSM-22 crystals (Fig. 3(g)) and ZSM-5 crystals (Fig. 3(h)) to elucidate the influence of nanoscale features. The reconstructed fluorescence maps clearly revealed the location of the active intergrowth region. They also studied the Ti-MCM-41 catalyzed epoxidation (Fig. 3(i)) of a fluorogenic BODIPY derivative and obtained the turnover mapping on individual crystals [42]. Data analysis over an extended timeframe demonstrates that the product formation was limited to the outer regions (200–300 nm) of the individual catalyst particles.

To take a short summarize, single-molecule fluorescence microcopy allows for the real time, high-throughput observation of catalytic processes on a wide range of catalyst materials under ambient conditions with single-turnover resolution. However, TIRF does not reflect the heterogeneous in catalytic reaction due to the heterogeneities of catalyst itself. Another limitation is that single-molecule fluorescence microscopy requires either products or reactants should be fluorescent and could not be neither enhanced nor quenched by catalysts. Then, the fluorescent products must be dissociated from the catalyst surface after a short period of stay. Thus, the implication of single-molecule fluorescence microcopy in catalytic reaction is quite limited.

Surface-enhanced Raman spectroscopy

Reactants and products in catalytic reactions often exhibit fingerprint vibrational features that are related to their chemical structures, which can be measured by Raman spectroscopy. Because the Raman signals of molecules can be greatly enhanced (~10⁶) in the vicinity of an excited plasmonic material (typically Au or silver (Ag)), surface-enhanced Raman spectroscopy (SERS) [43–47] and tip-enhanced Raman spectroscopy (TERS) are capable of single molecule detection and single nanoparticle catalysis study in real time [48–51].

Kang et al. monitored the plasmon-driven dimerizing reaction of p-nitrothiophenol (pNTP) into p,p′-dimercaptoazobenzene (DMAB) on single Ag particles using SERS [52,53]. The Ag catalyst particle was relatively large (~2 μm diameter), but its surface was roughed with nano-structures, acting as an efficient SERS substrate (Fig. 4(a)). They studied the laser wavelength- and power-dependent conversion rates of the reaction on individual Ag particles and found that 532 nm excitation could accelerate the reaction rate effectively than 633 nm excitation, confirming that the reaction is induced by excitation of surface plasmon on the Ag particle. Moreover, they further found that dimerization of p-aminothiophenol (pATP) into DMAB was actually induced by the efficient energy transfer (plasmonic heating) from surface plasmon resonance to the surface adsorbed pATP, where O₂ (as an electron acceptor) is the necessary oxidant and H₂O (as a deprotonation agent) can dramatically accelerate the reaction (Fig. 4(d)) [54]. Their results demonstrated that surface plasmon assisted catalysis (SPAC) hold great promise for the studying chemical reactions on plasmonic metal nanostructures.

Combining the confocal Raman microscope with high-resolution scanning probe microscopy [e.g., atomic force microscopy (AFM) and scanning transmission microscopy (STM)], usually called TERS, allows for ultrahigh spatially resolving of spectroscopic information even with single-molecule resolution. Using the setup combining a confocal Raman microscope and an AFM, Weckhuysen and coworkers [55] used TERS to investigate a photocatalytic reaction of a self-assembled monolayer of pNTP molecules adsorbed on a gold nanoplate support. The silver-coated AFM tip acted as both the Raman signal enhancer and the catalyst (Fig. 4(e)). They monitored the photocatalytic reduction of pNTP to DMAB at the illuminated Ag tip. The reaction was initiated with a 532 nm laser, whereas a 633 nm laser was used for Raman excitation. By acquiring the Raman spectra of the molecules as a function of 532 nm illumination time (Fig. 4(f)), they found that the catalytic reaction was occurred. Moreover, the complete monolayer coverage of the pNTP reactant was disturbed during catalysis, through either the change in functionality from NO₂ to NH₂ or dimerization. In the meantime, Xu et al. [56] used a high vacuum TERS to investigate the plasmon-driven in situ chemical reaction of 4-nitrobenzenethiol dimerizing to dimercaptoazobenzene, which can be controlled by the plasmon intensity.

Owing to its high chemical structure sensitivity and selectivity, SERS and TERS reveal the potential to offer a general, direct, quantitative, real-time detection method for reactant consumption and product generation on a single catalyst particle, and hold tremendous promise toward the identification of atomic-scale active sites in heterogeneous catalysis. Additionally, Raman scattering is label-free and a non-invasive spectroscopic technique, thus SERS and TERS allow for the investigation of arbitrary substrates. However, these methods lack millisecond-scale temporal resolution and require the use of roughened substrate as Raman enhancer. Single-molecule sensitivity via SERS is possible in both liquid and gaseous environments, but liquid-phase catalysis experiments by TERS are difficult, because reactant/product molecules in the near surface region are hard to exclude from those specifically adsorbed to the catalyst.

Optical responses from catalysts

Heterogeneous catalysis reactions that involve redox always cause changes in the physical and chemical properties of the catalyst particles and the atoms in the catalyst during the catalytic cycle, which can also be measured by optical methods. Therefore, monitoring the performance of catalyst particles in real time becomes another effective approach to study nanocatalysis.

Localized surface plasmon resonance microscopy

Localized surface plasmon resonance (LSPR), is the collective coherent oscillation of conduction electrons in metal nanoparticles especially Au and Ag nanoparticles [57–59]. Owing to their unique plasmonic properties that surprisingly highly sensitive to their size, shape, composition, and charge density as well as local dielectric environment, Au and Ag nanostructures have long been utilized for nanoplasmonic chemical and biologic sensing, counting, tracking and imaging system [60–65]. In addition to high sensitivity, plasmonic nanostructures provide higher intensity, nonblinking, optical stability and easiness to prepare compared with previously reported methods.

Direct localized surface plasmon resonance monitoring of single plasmonic particle nanocatalysisƒƒCatalytic reactions can change the physical and chemical properties of a plasmonic catalyst nanoparticle because of changes of the local environment due to the consumption of reactants and generation of products on the catalyst surfaces. These changes result in the LSPR wavelength shifts in the visible/near-infrared regime. The scattering spectrum of individual nanoparticles can be monitored by dark field microscopy (DFM) [66]. These facilitate the locally probing catalytic reactions in real time by either direct or indirect strategy.

For gold nanoparticles (Au NPs), which play an important role in heterogeneous catalysis, their LSPR spectra can be measured directly to monitor the catalytic reactions occurring on their surfaces. As pioneered by Mulvaney’s groups [67], the electron injection and extraction of a single Au decahedron nanoparticle were monitored during an oxidation of ascorbic acid by dissolved oxygen. The oxidation of ascorbic acid molecules caused electron injection into the Au nanoparticle, resulting in a ~20 nm blue shift of the nanoparticle’s LSPR spectrum in the first 3 min. The subsequent reaction of the Au nanoparticle with O₂ depopulated the accumulated electrons, and concurrently the LSPR spectrum red-shifted back to the initial state (Figs. 5(b) and 5(c)). They calculated the electron injected rate was 4600 electrons per second. The kinetics of atomic deposition onto a single Au nanorod (Fig. 5(d)) was also studied by using this strategy. Recently, Baumberg’s group upgraded DFM with supercontinuum laser source to improve its temporal resolution and real time monitored the growth of nanocrystals [68].

Yi et al. [69] used a similar strategy to quantify electron transfer rates on different facets of single gold nanoparticles during catalytic reactions. The 4-nitrophenol reduction reaction caused electron density changes on high-index facets of elongated tetrahexahedryl Au nanoparticles and low-index facets of Au nanorods (Figs. 5(h) and 5(i)). They observed that the high-index facets were capable of accepting electrons 7 times faster and emitting electrons 2.5 times faster than the low-index facets.

Indirect localized surface plasmon resonance monitoring of single particle nanocatalysisƒƒUnlike Au or Ag, many materials do not support a measurable LSPR signal. To solve this problem, several indirect LSPR sensing strategies have been developed. Larsson et al. [70,71] used arrays of Au nanodisks as an indirect platform to continuously monitor gas-phase reaction at the ensemble level. Recent works by Alivisatos and coworkers [72] developed an Au nanoantenna-based strategy and enable the study chemical properties at a single arbitrary nanoparticle. They used electron beam lithography to position an Au nanostructure at nanometer distances away from a palladium (Pd) nanoparticle. Pd nanoparticles could absorb hydrogen strongly and react with it to form PdH, which changes their electrical properties in a reversible manner; meanwhile, Au nanoparticles are sensitive to dielectric environment and exhibit the best plasmonic scattering features in visible region. Hydrogen gas uptake of Pd nanoparticle changed the dielectric of the single Au nanostructure, causing a shift in its LSPR spectrum (Figs. 6(a) and 6(b)). They suggested that the presence of 33 Torr hydrogen gas induced a 10 nm shift in the maximum LSPR wavelength and the sensitivity was highly dependent on the spatial distance between Pd and Au, as well as the geometry of the Au nanostructures (Fig. 6(c)). This group [73] further extended this indirect strategy to study catalysis on similar hybrid nanostructures (Au@Pd core-shell nanoparticles). They found that the shape, faceting, and Pd shell thickness of Au nanoparticles play essential role in hydrogen uptake trajectories. Using a similar strategy, Song and coworkers [74] studied the decomposition of lactic acid into hydrogen on a single chemically synthesized Au nanostructure capsulated by a platinized CdS shell. The kinetic analysis of the LSPR wavelength shift allowed for calculation of the rate constant, diffusion coefficient, as well as average distance between active sites of Pt/CdS and the Au domains, and showed significant particle-to-particle heterogeneity. Liu et al. [75] investigated the dissociation and uptake of hydrogen on 15-nm Pd films on individual Au nanostructures coated with a thin SiO₂ shell as plasmonic reporters.

Tao and coworkers introduced a plasmonic film-supported nanoparticle for the study of electrocatalysis reaction with a plasmonic-based electrochemical current microscope (P-ECM). Compared with the above mentioned core@shell nanoparticles, they used a plasmonic metal film as the reporter rather than plasmonic particles. The principle of P-ECM is that the Faradaic current is determined by the generation rate of reaction products (or the consumption rate of reactants) in an electrochemical reaction process, which can be imaged by SPR because of the sensitive dependence of SPR signal to the local concentration changes of reactants or products [76]. They first used this strategy to image local electrochemical processes and then extended it to study the electrocatalytic activity of single Pt nanoparticles and low-density arrays of nanoparticles supported on an Au film [77]. This strategy, allowing the measurement of cyclic voltammograms of multiple Pt nanoparticles individually (Figs. 6(d)–6(f)), is not only limited to electrocatalysis, but also other catalytic reactions.

This indirect plasmonic antenna mediated strategy enables LSPR to monitor other catalytic reaction using nonplasmonic nanoparticles. In addition, numerical simulations solving Maxwell equation could be used to predict changes of LSPR signals responding to physical or chemical properties changes of a particle during a catalytic reaction.

There still remain some challenges in monitoring chemical reactions with this nanoplasmonic approach, and this technique is currently not amenable to study nanocatalysis with single turnover resolution. First, Mie’s light scattering theory suggests that the scattering cross-section of a particle with diameter (d) is proportional to d⁶. With a conventional DFM, one can hardly measure the scattering spectrum of nanoparticles with a diameter smaller than 50 nm, which is too big to be a highly active nanocatalyst. The nanoplasmonic catalysts possesses many catalytic active sites, as a result, many reactions occur simultaneously on one catalyst particle, which make it difficult to trace single turnover. While for smaller nanoparticles, specialized instrument with more detecting sensitivity (such as electron-multiplying charge-coupled device, EMCCD) are needed to visualize. Secondly, real-time single-molecule kinetics studies are also limited by the integration time needed to acquire a spectrum (several seconds), considering that almost all of the catalytic reactions are accomplished in less than one second.

X-ray spectroscopy and microscopy

X-ray absorption spectroscopy (XAS) involves the excitation of core-level electrons of atoms, and the near-edge features and extended fine structures inform its oxidation state and local coordination environment. Therefore, XAS, including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), has been widely used to characterize bulk solids or an ensemble of nanoparticle catalysts [78,79]. One way to measure catalysis on single nanoparticles and in situ nanoscale image of catalysts is performing X-ray absorption spectroscopy and X-ray microscopy with ~10−40 nm spatial resolution using synchrotron sources [80].

A powerful X-ray microscopy method is scanning transmission X-ray microscopy (STXM), which has been developed in recent years to reveal spatial heterogeneities within supported metal catalyst materials at nanoscale. Many in situ STXM studies of working catalysts have since appeared, mainly from the Weckhuysen’s group. Individual working SiO₂-supported Fe-based Fischer-Tropsch synthesis (FTS) catalyst particles have been investigated in great detail by de Smit et al. [81] with soft X-rays (200−2000 eV). Constructed by analyzing the spatially resolved XAS spectra, the authors showed chemical contour maps of the catalyst (Figs. 7(a) and 7(b)), which implied intraparticle heterogeneities, as it was found that the active Fe was heterogeneously distributed within the SiO₂ binder. Furthermore, they proposed that iron carbide formed in the iron-rich region, and the presence of hydrocarbon species in iron-deficient regions suggested a spillover of products from the metal to the support, preventing the blocking of the active sites of the catalyst (Fig. 7(c)). In another STXM experiment, a microbeam X-ray absorption fine structure (μ-XAFS) spectroscopy investigation has been performed by Tada and colleagues [82]. They probed the reforming of methane to produce syngas on an industrially relevant NiO_x/Ce₂Zr₂O_y catalyst material, and revealed the presence of catalytically active and inactive phases within an individual catalyst particle (Figs. 7(d)–7(f)).

Besides X-ray transmission detection modes, transmission X-ray microscopy can also be conducted with other detection modes, such as photoelectron emission and X-ray fluorescence [79]. Advances in X-ray technology have pushed the spatial resolution to 10 nm, even at high temperature and pressure [83], meanwhile, subtle designs of nanoreactor can further allow for high-throughput, 3D chemical/structural imaging in high-pressure gaseous and liquid systems [80]. But the working distance between X-ray focusing optics and the sample is short, which puts a strain on the nanoreactor design. Acquiring STXM images can require tens of minutes, which causes it difficult to track catalytic processes in real time. The long time, high intensity irradiation of X-ray can incur significant damage to the sample (both catalysts and reactants), making it unsuitable for examining a single, small, less than 10 nm catalyst particle.

Conclusions and outlook

In this paper, we reviewed several optical approaches that allow for the study of catalysis on single nanoparticles. Distinguished by advantages and limitations with regard to chemical selectivity, sensitivity, and spatial resolution, these approaches can give complementary and corroborating information. Nonetheless, in the field of nanoscale heterogeneous catalysis, greater highly space- and time-resolved monitoring and imaging of heterogeneities within catalytic processes are still the major challenge. New approaches remain to be developed and explored to investigate an individual catalyst particle with atomic-scale resolution and single-molecule sensitivity under true reaction conditions even high temperatures and pressures.

Some developments in other scientific fields are referable. For example, in the past 10 years we have seen the discovery of several appropriate experimental approaches, particularly developed for life sciences (e.g., stimulated emission depletion (STED) [84], fluorescence photoactivated localization microscopy (FPALM) [85] and stochastic optical reconstruction microscopy (STORM) [86]), that allow determining both axial and lateral positions of individual fluorescent molecules with nanometer accuracy. A similar trend toward increasing 2D and 3D imaging capabilities can be found in the field of X-ray microscopy. X-rays have very short wavelengths and are therefore ideally suited for achieving high spatial resolution. X-ray-based tomography and combining diffraction imaging with STXM are capable of bringing the spatial resolution down to 10 nm [87,88]. It is expected that the introduction of these advanced methods to the field of heterogeneous catalysis, will further increase our understanding of the 3D architecture and heterogeneity of complex catalytic solids as well as their reaction mechanisms.

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