**3.2. Molecular adsorption**

The addition of N led to a shift of the valence edge to higher energy, i.e., reduction of semi‐ conductor band gap. The shift is due to the appearance of N-2p orbitals that are strongly hybridized with O-2p orbitals. However, too much N-doping results in the formation of TiN and consequent coloring in the sample, but according to the electronics of the materials, TiN could not work as a sensitizer. Plus, it is known that TiN has no catalytic activity and thus

**Figure 6.** Electronic band structure of TiO2 and TiN. (*Top*) Valence and conduction band electronic states extracted from measured RIXS plane. (*Below*) Calculated Ti, O, and N DOS for TiO2, TiN, and TiO2-xNx, where x amounts to 2%

By confirming RXES measurements with FEFF calculations, we derived an elegant and costeffective strategy for the rational design of novel materials used in the conversion of solar energy into chemical bonds. Together, one is able to map the material electronic structure with

might decrease activity by blocking and/or reducing surface sites.

N-dopant level. (Reproduced from elsewhere [18] with permission)

14 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

Reagents' adsorption is the initial step of any catalytic reaction, and the interaction between reactants and active site is governed by the electronic structure of the catalyst, which deter‐ mines adsorption strength and geometry [2]. Thus, modification of the active site electronic structure alters reactant adsorption parameters. It is therefore crucial to attain information about adsorption parameters, preferentially under relevant conditions. Surface science studies and theoretical calculations provided over the years a platform to acquire information about these parameters under model conditions, and idealized surfaces. However, this valuable information is difficult to correlate with reactions carried out in liquid phase and/or pressure.

Catalytic hydrogenations with molecular H2 are unquestionably the workhorse of catalytic organic synthesis. The synthetic capacity of catalytic hydrogenation is superbly condensed in Rylander's following quotation [22]: *Catalytic hydrogenation is one of the most useful and versatile tools available to the organic chemist. The scope of the reaction is very broad; most functional groups can be made to undergo reduction, frequently in high yield, to any of several products. Multifunctional molecules can often be reduced selectively at any of several functions. A high degree of stereochemical control is possible with considerable predictability, and products free of contaminating reagents are obtained easily*. Most of the above comments apply to heterogeneous catalytic hydrogenations over supported Group VIII metals [23].

Alternatively to molecular hydrogen, hydrogen donors such as isopropanol or formic acid can be applied in transfer hydrogenations but they generate unwanted side products. Several important inventions have been accomplished in the last 150 years, such as the application of highly dispersed metals, e.g., nickel, in the hydrogenation of organic compounds [24], selective semi-hydrogenation of C≡C-bonds in the presence of Pd-Pb/CaCO3 catalysts (Lindlar catalyst) [25], and more recently asymmetric hydrogenations [26], pioneered by Knowles and Noyori for which they received in 2001 the Nobel Prize in Chemistry. Catalytic hydrogenations can be carried out in a variety of ways, namely liquid or gas phase, and in batch-wise or continuous mode.

Hydrogenation reactions involving hydrogen dissociation on the metal surface have high reaction probability on many surface sites: top, bridge, and step sites. For this reason, hydro‐ genation reactions are usually structure-insensitive in respect to hydrogen activation. How‐ ever, the reaction is considered structure-sensitive in respect to the hydrocarbon hydrogenation. Crespo-Quesada et al. [27] reported that semi-hydrogenation of alkynes occurred preferentially on terraces, whereas further hydrogenation to the alkane occurred at the edges. Similarly, Schmidt et al. [28] revealed that in ethyl pyruvate enantioselective hydrogenation the Pt(111)/Pt(100) ratio controlled reaction rate and enantiomeric excess.

Platinum group metals are highly active hydrogenation catalysts, operating at low tempera‐ tures and H2 pressures. However, their scarcity and high cost intensified the research devoted in finding alternative catalysts deprived of noble metals. One should be made aware that fine chemical industries, such as the ones responsible for the production of vitamins and fragrance, afford products of medium added value. Accordingly, the use of noble metals and/or sophis‐ ticated ligands is only justified when the turnover number is higher than 1000 or even 10,000 depending on the cost of the target product [29]. Nonprecious metal catalysts, especially those based on nickel (e.g., Raney nickel [30], Urushibara nickel [31], CENTOPRIME [32] catalysts) have also been developed as economical alternatives, but they are often less active, requiring higher temperature and H2 pressure to attain comparable performance.

The chemoselective hydrogenation of C=C and C=O-bonds is extensively used in the prepa‐ ration of pharmaceuticals, fragrances, and vitamins precursors. Common substrates are unsaturated ketones, aldehydes, or esters, which have to be hydrogenated selectively either at the C=C or C=O bond depending on their end-use. Palladium and platinum are routinely used to hydrogenate C=C bonds [33]; however, the systems lose their effectiveness when the molecule contains several hydrogenable functionalities. Chemoselective hydrogenation of unsaturated aldehyde, ketones, and esters has been catalyzed over a plethora of 3d transition metal catalysts [34]. For example, Ni and Cu catalysts were found both active and selective in the hydrogenation of C=O instead of C=C [35].

As aforementioned, platinum is routinely used in the chemoselective hydrogenation of C=C and C=O-bonds. Recently, Manyar et al. [36] demonstrated that Pt supported on OMS-2 possesses different affinities in the chemoselective hydrogenation of C=C and C=O-bonds depending on the substrate used. 5 wt% Pt/OMS-2 (Pt average particle diameter 2 nm) supported on cryptomelane manganese oxide octahedral molecular sieve provides high selectivity for the hydrogenation of both ketoisophorone and cinnamaldehyde. However, in the case of ketoisophorone hydrogenation, 98% of the substrate was hydrogenated to (6R)-2,2,6-trimethylcyclohexane-1,4-dione (levodione) at 100% conversion, i.e., the catalyst selective reduced the C=C bond. Whereas in the case of cinnamaldehyde, 80% selectivity for the reduction of the C=O bond forming cinnamyl alcohol at 100% conversion was found using the same catalyst. The observed selectivity in the ketoisophorone hydrogenation contrasts with the expected Pt behavior, which is commonly preferentially to hydrogenate C=O versus C=C bonds. The expected selectivity was observed when the reaction was performed over 5 wt% Pt/Al2O3 under the same reaction conditions. A possible explanation for the differences is different adsorption geometry/strength of the substrates.

The change in the Pt electronic structure following the adsorption of an α, β-unsaturated aldehyde and ketone was followed by in situ HR-XAS in the liquid phase and the adsorption strength calculated with Density Functional Theory (DFT) [37]. Probably one of the most important outcomes of the experiment is that adsorption of a molecule on Pt surface led to an immediate change in XAS single, i.e., Pt electronic structure. The perceptual change relates to molecule coverage, which in this case is below 10% (Fig. 7a). All the molecules produced a Fermi level shift to higher energies, which suggests electronic density donation from Pt to the adsorbed molecule. Larger energy shifts equates to stronger interactions, and consequently higher adsorption energy. Thus, from the Fermi energy level shifts measured by Pt L3-edge HR-XAS, one might interject that cinnamaldehyde has the strongest interaction with Pt followed by H2 and then ketoisophorone (Fig. 7a).

in finding alternative catalysts deprived of noble metals. One should be made aware that fine chemical industries, such as the ones responsible for the production of vitamins and fragrance, afford products of medium added value. Accordingly, the use of noble metals and/or sophis‐ ticated ligands is only justified when the turnover number is higher than 1000 or even 10,000 depending on the cost of the target product [29]. Nonprecious metal catalysts, especially those based on nickel (e.g., Raney nickel [30], Urushibara nickel [31], CENTOPRIME [32] catalysts) have also been developed as economical alternatives, but they are often less active, requiring

The chemoselective hydrogenation of C=C and C=O-bonds is extensively used in the prepa‐ ration of pharmaceuticals, fragrances, and vitamins precursors. Common substrates are unsaturated ketones, aldehydes, or esters, which have to be hydrogenated selectively either at the C=C or C=O bond depending on their end-use. Palladium and platinum are routinely used to hydrogenate C=C bonds [33]; however, the systems lose their effectiveness when the molecule contains several hydrogenable functionalities. Chemoselective hydrogenation of unsaturated aldehyde, ketones, and esters has been catalyzed over a plethora of 3d transition metal catalysts [34]. For example, Ni and Cu catalysts were found both active and selective in

As aforementioned, platinum is routinely used in the chemoselective hydrogenation of C=C and C=O-bonds. Recently, Manyar et al. [36] demonstrated that Pt supported on OMS-2 possesses different affinities in the chemoselective hydrogenation of C=C and C=O-bonds depending on the substrate used. 5 wt% Pt/OMS-2 (Pt average particle diameter 2 nm) supported on cryptomelane manganese oxide octahedral molecular sieve provides high selectivity for the hydrogenation of both ketoisophorone and cinnamaldehyde. However, in the case of ketoisophorone hydrogenation, 98% of the substrate was hydrogenated to (6R)-2,2,6-trimethylcyclohexane-1,4-dione (levodione) at 100% conversion, i.e., the catalyst selective reduced the C=C bond. Whereas in the case of cinnamaldehyde, 80% selectivity for the reduction of the C=O bond forming cinnamyl alcohol at 100% conversion was found using the same catalyst. The observed selectivity in the ketoisophorone hydrogenation contrasts with the expected Pt behavior, which is commonly preferentially to hydrogenate C=O versus C=C bonds. The expected selectivity was observed when the reaction was performed over 5 wt% Pt/Al2O3 under the same reaction conditions. A possible explanation for the differences is

The change in the Pt electronic structure following the adsorption of an α, β-unsaturated aldehyde and ketone was followed by in situ HR-XAS in the liquid phase and the adsorption strength calculated with Density Functional Theory (DFT) [37]. Probably one of the most important outcomes of the experiment is that adsorption of a molecule on Pt surface led to an immediate change in XAS single, i.e., Pt electronic structure. The perceptual change relates to molecule coverage, which in this case is below 10% (Fig. 7a). All the molecules produced a Fermi level shift to higher energies, which suggests electronic density donation from Pt to the adsorbed molecule. Larger energy shifts equates to stronger interactions, and consequently higher adsorption energy. Thus, from the Fermi energy level shifts measured by Pt L3-edge HR-XAS, one might interject that cinnamaldehyde has the strongest interaction with Pt

higher temperature and H2 pressure to attain comparable performance.

the hydrogenation of C=O instead of C=C [35].

16 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

different adsorption geometry/strength of the substrates.

followed by H2 and then ketoisophorone (Fig. 7a).

**Figure 7.** (a) Pt L3-edge HR-XAS spectra and 1st derivative (right insert) of Pt/OMS-2 interacting with H2, KIP (ketoiso‐ phorone), and CIN (cinnamaldehyde). The spectra were recorded in situ in 10 mol% methanol in water at 373 K. The lines are from a least squares fitting procedure. (b) Correlation between the experimentally measure shifts in the HR-XAS spectra (Ef ) and the calculated adsorption energies via DFT for the Pt(111) (black) and Pt(211) (red) surfaces. (Re‐ produced from elsewhere [37] with permission)

The resulting shift in the Pt Fermi energy, measured at Pt L3-edge, due to adsorption of molecules was found to be in good agreement with the molecule adsorption energy trends calculated by DFT, which provided valuable insight into the reaction selectivity (Fig. 7b). It should be mentioned that the experiments were carried out at 373 K and 10 bar pressure, i.e., catalyst working conditions. This was possible due to the development of a homemade cell comprised of a stainless steel autoclave reactor with window comprising of a polyether ether ketone (PEEK) insert [38]. The work confirmed that the combination of state-of-the-art spectroscopy (HR-XAS) and theoretical calculations is a powerful and versatile tool to reveal differences in adsorption behavior for reactants in the liquid phase under reaction conditions, with unprecedented resolution and sensitivity.

Carbon monoxide is often used as chemical probe in spectroscopy because it is very sensitive to the electronic structure of materials. Furthermore, CO is a key reagent in several reactions such as Fischer-Tropsch, PROX, and catalytic converter catalysis. RXES can accurately resolve the occupied and unoccupied d-density-of-states (DOS) of Pt after the adsorption of molecules, including CO [39], under reaction conditions. RXES was found capable of differentiating between the possible adsorption geometries of CO adsorbed on Pt, namely atop, bridged, and faced-bridging [40], making it the ideal tool to determine the changes in adsorption geometry caused by an external effect.

RXES measured at Pt L3-edge was used to determine the changes in CO oxidation over platinum sites induced by a magnetic field [41]. CO molecules adsorbed on nonmagnetic Pt nanoparticles supported on a carbon capped Co magnetic nanocore on atop position in the absence of a magnetic field, as expected. Under magnetic field, part of the atop CO changed to the bridged position (Fig. 8), which is a nonreactive state. This result indicates that catalytic activity can be modified by the presence of a magnetic field even if the active site is not magnetic. This opens other potential applications such as molecular displacement on auto-

**Figure 8.** Pt L3M5 Δ-RXES due to the presence of a 50 mT magnetic field on Pt on Co with adsorbed CO (field OFF–field ON). (*Left*) Experimental map differences measured in situ. (*Right*) Calculated map differences. (Reproduced from else‐ where [41] with permission)

poisoning reactions, characterized by poisoning of reagents, products, and/or trace contami‐ nates that cannot be removed or avoided. For example, hydrogen fuel cells are vulnerable to CO poisoning because CO binds strongly to platinum averting hydrogen adsorption, deterring commercial application. By changing adsorption from atop to bridge, CO adsorption strength decreases by ca. 0.1 eV, enabling its removal at lower temperature.

## **3.3. Study of catalytic reactions under real working conditions (in situ)**

Determination of metal oxidation states under relevant working conditions is crucial to understanding catalytic behavior. Temperature-programmed reduction (TPR) is often used to determine the temperature at which the catalyst reduces; however, it fails to determine the oxidation state of the catalyst at this specific temperature. That information is normally attained when a spectroscopic measurement is coupled with the TPR. The reduction behavior of NiO [42] and Au2O3 [43] was determined using a novel approach, namely time-resolved RXES or 4D RXES. The experimental concept of such experiment is depicted in Figure 9.

**Figure 9.** Experimental concept for time-resolved RXES measurements. (Reproduced from elsewhere [42] with permis‐ sion)

Figure 10 shows the RXES map before and after reduction of nano-NiO. A shift in the main signal to lower incidence energies after reduction was observed, coherent with the formation of metallic nickel. From the RXES map one can excerpt information about unoccupied states (XAS analysis) and occupied states (XES analysis). In order to determine reaction pathway, the XAS absorption contribution of NiO and Ni metal was plotted as function of time. The transition from oxide to metallic state was found to be very fast, consisting of single-step reduction mechanism.

poisoning reactions, characterized by poisoning of reagents, products, and/or trace contami‐ nates that cannot be removed or avoided. For example, hydrogen fuel cells are vulnerable to CO poisoning because CO binds strongly to platinum averting hydrogen adsorption, deterring commercial application. By changing adsorption from atop to bridge, CO adsorption strength

**Figure 8.** Pt L3M5 Δ-RXES due to the presence of a 50 mT magnetic field on Pt on Co with adsorbed CO (field OFF–field ON). (*Left*) Experimental map differences measured in situ. (*Right*) Calculated map differences. (Reproduced from else‐

Determination of metal oxidation states under relevant working conditions is crucial to understanding catalytic behavior. Temperature-programmed reduction (TPR) is often used to determine the temperature at which the catalyst reduces; however, it fails to determine the oxidation state of the catalyst at this specific temperature. That information is normally attained when a spectroscopic measurement is coupled with the TPR. The reduction behavior of NiO [42] and Au2O3 [43] was determined using a novel approach, namely time-resolved RXES or 4D RXES. The experimental concept of such experiment is depicted in Figure 9.

**Figure 9.** Experimental concept for time-resolved RXES measurements. (Reproduced from elsewhere [42] with permis‐

sion)

decreases by ca. 0.1 eV, enabling its removal at lower temperature.

18 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

where [41] with permission)

**3.3. Study of catalytic reactions under real working conditions (in situ)**

**Figure 10.** RXES map of the initial state (top) and the final state (bottom) of NiO nanoparticle reduction. (Reproduced from elsewhere [42] with permission)

Similar analysis can be performed with XES signals that carry the inherent benefit, when performed in a dispersive-type spectrometer, of high time resolution inaccessible to XAS technique. However, the chemical sensitivity of XES depends on where the analysis is performed. Since this is often unknown a priory, one needs to measure the full RXES map in order to determine the optimal incidence energy to perform XES analysis. The subsequent measurements can be performed only at this range, which decreases drastically the acquisition time, i.e., improved time resolution without losing chemical information. Based on RXES maps, emission analysis above the absorption edge, so-called non-resonant XES or simply XES, is less sensitive to follow the chemical state of atom as compared to resonant XES measured at incoming energies with high discriminating power, in this case at 8325.9 eV (NiO pre-edge) and 8331.9 eV (NiO inflection point) incident beam energy (Fig. 11). This illustrates the importance of knowing at which energy the XES data should be collected.

**Figure 11.** Temporal evolution of XES signals for NiO (top) and Ni0 (bottom) at different incident energies. (Repro‐ duced from elsewhere [42] with permission)

Gold catalysis gained significant popularity after Haruta's discovery that gold nanoparticles are extremely active [44]. Activity of gold is often associated to its metallic phase but some cases consider the involvement of gold in higher oxidations states [45]. The difficulty in detecting gold in oxidations states between 0 and +3 resides in the fact that these intermediate states are unstable and short-lived.

**Figure 12.** (a) Extracted RXES plane of Au2O from the time-resolved RXES data set compare d to theoretical calcula‐ tions (b). (c) Assessment between the extracted high-energy resolution X-ray absorption (HR-XAS) spectrum and the matching theoretical predictions. The influence of the DOS of the different orbitals to the X-ray absorption spectrum is plotted in the bottom panel. (Reproduced from elsewhere [46] with permission)

We performed Au2O3 TPR coupled with time-resolved RXES to see if one could detected some of these intermediate states. A short-lived Au2O compound was detected for the first time under in situ conditions, permitting a better understanding of the reaction mechanism of Au2O3 reduction [46]. Based on time-resolved RXES data analysis combined with genetic algorithm methodology, we were able to determine the electronic and geometric structure of the unstable Au2O transitional specie (Fig. 12). The data analysis revealed a larger value for the lattice constant of the intermediary Au2O specie as compared to the theoretical predictions. DFT calculations revealed that such structure may indeed be formed, and the expanded lattice constant is justified by the termination of Au2O on the Au2O3 structure. The temporal evolution of the species shows the characteristic behavior of a short-lived intermediate state, in this case Au2O (Fig. 13).

**Figure 11.** Temporal evolution of XES signals for NiO (top) and Ni0

20 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

Gold catalysis gained significant popularity after Haruta's discovery that gold nanoparticles are extremely active [44]. Activity of gold is often associated to its metallic phase but some cases consider the involvement of gold in higher oxidations states [45]. The difficulty in detecting gold in oxidations states between 0 and +3 resides in the fact that these intermediate

**Figure 12.** (a) Extracted RXES plane of Au2O from the time-resolved RXES data set compare d to theoretical calcula‐ tions (b). (c) Assessment between the extracted high-energy resolution X-ray absorption (HR-XAS) spectrum and the matching theoretical predictions. The influence of the DOS of the different orbitals to the X-ray absorption spectrum is

plotted in the bottom panel. (Reproduced from elsewhere [46] with permission)

duced from elsewhere [42] with permission)

states are unstable and short-lived.

(bottom) at different incident energies. (Repro‐

**Figure 13.** Concentration changes of Au2O3, Au2O, and Au(0) during TPR of Au2O3. The data are plotted versus time (bottom scale) and temperature (top scale). (Reproduced from elsewhere [46] with permission)

Excessive amounts of oxidants, in particular H2O2, can injure proteins and lipids resulting in cell death. Oxidative stress has been connected to a multitude of pathophysiological condi‐ tions, such as Alzheimer's and Parkinson's diseases, aging, cancer, genetic damage, and tissue damage in cardiac ischemic/reperfusion injury. In nature, enzymes called catalases conduct the catalytic disproportionation of H2O2 to water and molecular oxygen.

Nitrogen-containing ligand such as imidazoles, is a class of single-site Mn catalase-like complexes. Imidazoles and their derivatives with strong π-donating ability were found to accelerate H2O2 disproportionation. The complexes display elevated activity in organic solvents but exceptionally small reactivity or even none in water under physiological pH values. The activity improved when a base was added as a promoter. We reported a novel manganese (II) complex with a pyridine substitute as a ligand (Fig. 14) [47]. The complex was stable and showed catalase-like activity in neutral aqueous solution.

**Figure 14.** DIAMOND *diagram* showing the coordination environment of Mn(II) in [Mn(2-CH2OHpy)(SO4)(H2O)]n. (Re‐ produced from elsewhere [47] with permission)

The combination of RXES measured at Mn K-edge and theoretical calculations enables us to propose a reaction mechanism (Fig. 15) in which the manganese complex is firstly oxidized due to the loss of a bridge oxygen proton. This is followed by H2O2 coordination and proton abstraction by the sulfate group mediated by water. Release of molecular oxygen and catalyst regeneration entails the involvement of a second complex molecule. This affects catalytic reaction rate because they become diffusion-limited. The projected mechanism was the first effort in trying to understand single-site Mn complexes reactivity, and clearly more work needs to be done to establish and understand Mn-catalase mimics' reaction mechanism. Nonetheless, based on findings two novel Mn complexes were synthetized, displaying catalytic activities several orders of magnitude higher than the parent one [48].

The increase in the planet's human population put strong pressure on the food supply chain and energy requirements. Photocatalysis is at the forefront of technologies to produce solar fuels. The first significant breakthrough in converting light into chemical energy was published in 1972 by Fujishima and Honda, who reported the electrochemical photolysis of water assisted by a semiconductor under UV-A radiation [49]. However, despite all the scientific advances, it remains a significant challenge to construct a device capable of producing solar fuels, such as hydrogen, at a scale and cost able to compete with fossil fuels. Moreover, developments based on large band gap semiconductors' (>3 eV), such as TiO2, are hampered by the fact that it requires UV-A irradiation to induce charge separation. Nevertheless, TiO2 remains indis‐ putably the best photocatalyst to date [50]. Several efforts are being made to improve sunlight X-Ray Spectroscopy — The Driving Force to Understand and Develop Catalysis http://dx.doi.org/10.5772/61940 23

manganese (II) complex with a pyridine substitute as a ligand (Fig. 14) [47]. The complex was

**Figure 14.** DIAMOND *diagram* showing the coordination environment of Mn(II) in [Mn(2-CH2OHpy)(SO4)(H2O)]n. (Re‐

The combination of RXES measured at Mn K-edge and theoretical calculations enables us to propose a reaction mechanism (Fig. 15) in which the manganese complex is firstly oxidized due to the loss of a bridge oxygen proton. This is followed by H2O2 coordination and proton abstraction by the sulfate group mediated by water. Release of molecular oxygen and catalyst regeneration entails the involvement of a second complex molecule. This affects catalytic reaction rate because they become diffusion-limited. The projected mechanism was the first effort in trying to understand single-site Mn complexes reactivity, and clearly more work needs to be done to establish and understand Mn-catalase mimics' reaction mechanism. Nonetheless, based on findings two novel Mn complexes were synthetized, displaying catalytic activities

The increase in the planet's human population put strong pressure on the food supply chain and energy requirements. Photocatalysis is at the forefront of technologies to produce solar fuels. The first significant breakthrough in converting light into chemical energy was published in 1972 by Fujishima and Honda, who reported the electrochemical photolysis of water assisted by a semiconductor under UV-A radiation [49]. However, despite all the scientific advances, it remains a significant challenge to construct a device capable of producing solar fuels, such as hydrogen, at a scale and cost able to compete with fossil fuels. Moreover, developments based on large band gap semiconductors' (>3 eV), such as TiO2, are hampered by the fact that it requires UV-A irradiation to induce charge separation. Nevertheless, TiO2 remains indis‐ putably the best photocatalyst to date [50]. Several efforts are being made to improve sunlight

stable and showed catalase-like activity in neutral aqueous solution.

22 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

produced from elsewhere [47] with permission)

several orders of magnitude higher than the parent one [48].

**Figure 15.** Proposed reaction mechanism for H2O2 disproportionation over [Mn(2-CH2OHpy)(SO4)(H2O)]n in water. (Reproduced from elsewhere [47] with permission)

absorption, including band gap manipulation by doping with elements, such as N, C, and S [51]. However, these are often counterproductive because they decrease the breadth of reactions that can be photocatalyzed and overall performance.

A more promising strategy to circumvent TiO2 deficiency in absorbing visible light is to use sensitizers capable of harvesting solar light and introducing hot electrons into the TiO2 conduction band (CB). O'Regan and Grätzel [52] developed dye-sensitized solar cells (DSSCs), in which the dye component harvests sunlight and readily injects electrons (τinj < 1 ps) into the TiO2 CB. The success of the dye system depends on fast electron injection and slow back electron transfer. However, organic dyes have low stability, discrete absorption levels (narrowband), and small optical cross sections, thus requiring a high dye coverage, which diminishes the space available for photocatalytic reactions. Besides, holes in oxidized dyes are unreactive, which hinders hydrogen evolution since a sustainable process demands for equal consumption of electrons and holes.

Metallic nanoparticles (NPs) are interesting sensitizer candidates because of their large optical cross sections related to the excitation of localized surface plasmons (LSP). Gold group metals exhibit plasmonic resonances in the visible region, and their absorption can be easily tuned by changing nanoparticles morphology (shape and size), enabling a good match with the solar spectrum. Moreover, their d10 configuration bestows them chemical stability. Recently, the excitation of Au and Ag LSP nanostructures was shown to improve charge transfer from sensitizer to semiconductor [53], increase the photocurrents under solar irradiation [54], and enhance photoinitiated catalytic oxidations [55]. The latest corroborates that the holes in these structures are reactive and can take part of photo-oxidations.

Hallett-Tapley et al. [56] defended that the plasmon excitation could drive reactions via thermal, electronic, and/or antenna processes. In the case of photocatalysis, the relevant process is the electronic one, in which electrons and holes are allegedly formed upon plasmon excitation [57]. In theory, plasmonic structures can be used directly in photocatalysts; however, the electron–hole pair is short-lived (few femto-second), making it problematic to drive chemical reactions. Interfacial reactions are relatively slow with kinetics in the milliseconds range. To increase charge separation lifetime, the charges can be confined to spatially separated sites where reactions take place. For example, charge separation lifetime can be dramatically improved by coupling LSP structures with a semiconductor (analogous to DSSCs). Mubeen et al. [58] prepared a proof-of-concept system conglomerating the findings on plasmonic nanostructures, water reduction, and oxidation catalysts to produce simultaneously H2 and O2 under visible light irradiation. This pioneering work demonstrated system's potential but highlighted the need for improvement.

Despite plasmonic nanostructures' potential, until recently there was no spectroscopic evidence that hot carriers are formed after photon absorption. Since this is essential for the utilization of these structures in photocatalysis, we decided to investigate hot carriers forma‐ tion on plasmonic structures during illumination. The creation of electron–hole pairs due to LSP excitation was measured by high-resolution X-ray absorption spectroscopy at the Au L3 edge, which determines Au 5d unoccupied electronic states. LSP excitation led to an upward shift of the ionization energy threshold by ca. 1.0 eV, and an increase of Au d-band hole population, consistent with hot electrons formation, and their promotion to high-energy states [59] (Fig. 16).

To evaluate if the hot electrons possess sufficient energy to be injected into TiO2 CB, we carried out transient broadband mid-IR spectroscopy [19]. Free and trapped electrons in a semicon‐ ductor conduction band produce a broad mid-IR band. Upon excitation, a broad mid-IR band appeared, confirming the presence of electrons in the TiO2 CB. The minimum in transmittance was observed at t=0 ps for both systems, which is the best possible overlap between pump and probe pulses, thus confirming fast injection into the TiO2 and consequently increase in electron lifetime from few fs to 100s of ns. The result corroborates Furube et al. [60] speculated mechanism and more importantly, it indorses that these NPs can drive photocatalytic reactions under solar irradiation.

To evaluate if the hot electrons possess sufficient energy to be injected into TiO2 CB, we carried out transient broadband mid‐IR spectroscopy [19]. Free and trapped electrons in a semiconductor conduction band produce a broad mid‐IR band. Upon excitation, a broad mid‐IR band appeared, confirming the presence of electrons in the TiO2 CB. The minimum in transmittance was observed at t=0 ps for both systems, which is the best possible overlap between pump and probe pulses, thus confirming fast injection into the TiO2 and consequently increase in electron lifetime from few fs to 100s of ns. The result corroborates Furube et al. [60] speculated mechanism and more importantly, it indorses that these NPs

Metallic nanoparticles (NPs) are interesting sensitizer candidates because of their large optical cross sections related to the excitation of localized surface plasmons (LSP). Gold group metals exhibit plasmonic resonances in the visible region, and their absorption can be easily tuned by changing nanoparticles morphology (shape and size), enabling a good match with the solar spectrum. Moreover, their d10 configuration bestows them chemical stability. Recently, the excitation of Au and Ag LSP nanostructures was shown to improve charge transfer from sensitizer to semiconductor [53], increase the photocurrents under solar irradiation [54], and enhance photoinitiated catalytic oxidations [55]. The latest corroborates that the holes in these

Hallett-Tapley et al. [56] defended that the plasmon excitation could drive reactions via thermal, electronic, and/or antenna processes. In the case of photocatalysis, the relevant process is the electronic one, in which electrons and holes are allegedly formed upon plasmon excitation [57]. In theory, plasmonic structures can be used directly in photocatalysts; however, the electron–hole pair is short-lived (few femto-second), making it problematic to drive chemical reactions. Interfacial reactions are relatively slow with kinetics in the milliseconds range. To increase charge separation lifetime, the charges can be confined to spatially separated sites where reactions take place. For example, charge separation lifetime can be dramatically improved by coupling LSP structures with a semiconductor (analogous to DSSCs). Mubeen et al. [58] prepared a proof-of-concept system conglomerating the findings on plasmonic nanostructures, water reduction, and oxidation catalysts to produce simultaneously H2 and O2 under visible light irradiation. This pioneering work demonstrated system's potential but

Despite plasmonic nanostructures' potential, until recently there was no spectroscopic evidence that hot carriers are formed after photon absorption. Since this is essential for the utilization of these structures in photocatalysis, we decided to investigate hot carriers forma‐ tion on plasmonic structures during illumination. The creation of electron–hole pairs due to LSP excitation was measured by high-resolution X-ray absorption spectroscopy at the Au L3 edge, which determines Au 5d unoccupied electronic states. LSP excitation led to an upward shift of the ionization energy threshold by ca. 1.0 eV, and an increase of Au d-band hole population, consistent with hot electrons formation, and their promotion to high-energy states

To evaluate if the hot electrons possess sufficient energy to be injected into TiO2 CB, we carried out transient broadband mid-IR spectroscopy [19]. Free and trapped electrons in a semicon‐ ductor conduction band produce a broad mid-IR band. Upon excitation, a broad mid-IR band appeared, confirming the presence of electrons in the TiO2 CB. The minimum in transmittance was observed at t=0 ps for both systems, which is the best possible overlap between pump and probe pulses, thus confirming fast injection into the TiO2 and consequently increase in electron lifetime from few fs to 100s of ns. The result corroborates Furube et al. [60] speculated mechanism and more importantly, it indorses that these NPs can drive photocatalytic reactions

structures are reactive and can take part of photo-oxidations.

24 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

highlighted the need for improvement.

[59] (Fig. 16).

under solar irradiation.

31 **Figure 16.** HR-XAS experiments portraying variations in the Au DOS induced by continuous wave laser excitation of plasmon resonance at 532 nm, with 100 mW power. (*a*) HR-XAS ground state spectrum of Au nanoparticles (open cir‐ cles black trace), FDMNES calculated spectra of the ground state (black trace Au 5d106s1 ) and excited states (red trace 5d106s0 7p1 and blue trace 5d9 6s1 7p1 ). (*b*) Difference spectra between the excited and ground state: experimental (open circles black trace), and calculated assuming 2% excitation Au 5d106s1 7p0 → Au 5d106s0 7p1 (red trace); Au 5d106s1 7p0 → Au 5d9 6s0 7p1 (dashed blue trace). (Reproduced from elsewhere [59] with permission)

We hope that all the arguments mentioned so far demonstrate the capabilities of HR-XAS in understanding catalytic reactivity. However, HR-XAS suffers from an important limitation, namely its time resolution, which is at best of the order of tenths of seconds for ideal samples with high metal concentration. The limitation starts from the fact that HR-XAS measurements entail scanning of the incoming energy, which is restricted to the speed at which the mono‐ chromator can be moved. This makes it problematic to monitor a catalytic process in real time. Ideally, the experiments should be carried out in continuous mode in which data collection is carried out synchronously and uninterruptedly, enabling the identification of metastable regimes and intermediate species. HEROS can provide element-specific information about the unoccupied density of states [61], and due to the scanning free arrangement of von Hamos spectrometer, HEROS spectra can be recorded on a shot-to-shot basis with extraordinary time resolution (only depending on sample concentration and photon flux), while upholding spectral energy resolution [62]. Moreover, HEROS spectra are not perturbed by the selfabsorption process [63]. This makes HEROS a commanding tool to identify and quantify the catalyst electronic changes during reaction and as it happens.

**Figure 17.** (a) Temporal evolution of the HEROS spectra during CO/O2 switches at 300 °C on 1.3 wt% Pt/Al2O3. (b) Temporal evolution of the HEROS signals at 9426 and 9427 eV (whiteline region) during CO/O2 switches. (Reproduced from elsewhere [64] with permission)

We performed an in situ time-resolved HEROS study with subsecond resolution providing insight into the oxidation and reduction steps of a Pt catalyst during CO oxidation with 500 ms resolution [64]. Figure 17 displays a gentle oxidation step, comprised of two distinguishing stages, namely dissociative adsorption of oxygen followed by partial oxidation of Pt subsur‐ face. By comparing the experimental spectra with theoretical calculations, we found that the intermediate chemisorbed O on Pt is adsorbed on atop position, insinuating CO surface poisoning or surface reconstruction. This indicates HEROS ability to perform chemical speciation with subsecond time resolution, which opens exciting opportunities to follow catalysis in real time. Since the HEROS spectra are collected in a single shot, the time resolution can be further improved since it depends only on the number of incoming photons and element concentration, which makes it particularly suited for experiments at the XFELs. Recently, it was shown that experiments with 100 ms resolution could be achieved at the synchrotron [65].
