Elucidation of intermediates and mechanisms in heterogeneous catalysis using infrared spectroscopy.

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ANNUAL REVIEWS

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Annu. Rev. Phys. Chem. 2014.65:249-273. Downloaded from www.annualreviews.org by George Mason University on 06/12/14. For personal use only.

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Elucidation of Intermediates and Mechanisms in Heterogeneous Catalysis Using Infrared Spectroscopy Aditya Savara1 and Eric Weitz2 1 Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830; email: [email protected] 2 Department of Chemistry and Catalysis Center, Northwestern University, Evanston, Illinois 60208; email: [email protected]

Annu. Rev. Phys. Chem. 2014. 65:249–73

Keywords

The Annual Review of Physical Chemistry is online at physchem.annualreviews.org

deNOx , vibrational spectroscopy, adsorbates, in situ, DRIFTS, IRRAS

This article’s doi: 10.1146/annurev-physchem-040513-103647

Abstract

c 2014 by Annual Reviews. Copyright All rights reserved

Infrared spectroscopy has a long history as a tool for the identification of chemical compounds. More recently, various implementations of infrared spectroscopy have been successfully applied to studies of heterogeneous catalytic reactions with the objective of identifying intermediates and determining catalytic reaction mechanisms. We discuss selective applications of these techniques with a focus on several heterogeneous catalytic reactions, including hydrogenation, deNOx , water-gas shift, and reverse-water-gas shift. The utility of using isotopic substitutions and other techniques in tandem with infrared spectroscopy is discussed. We comment on the modes of implementation and the advantages and disadvantages of the various infrared techniques. We also note future trends and the role of computational calculations in such studies. The infrared techniques considered are transmission Fourier transform infrared spectroscopy, infrared reflection-absorption spectroscopy, polarization-modulation infrared reflection-absorption spectroscopy, sum-frequency generation, diffuse reflectance infrared Fourier transform spectroscopy, attenuated total reflectance, infrared emission spectroscopy, photoacoustic infrared spectroscopy, and surface-enhanced infrared absorption spectroscopy.

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1. INTRODUCTION


Reflectionabsorption infrared spectroscopy (IRRAS): generally performed with an FTIR spectrometer; also known as IRAS or RAIRS

Infrared (IR) spectroscopy has a long history of capturing the fingerprints of chemical compounds, including reaction intermediates. The vibrational frequencies of a molecule are a function of the geometry and bonding of that molecule. For heterogeneous catalysis, identifying chemical intermediates (along with their geometries) and understanding their roles in the overall reaction mechanism are critical to developing a molecular-level understanding of catalytic reactions. Thus, there is a natural marriage between heterogeneous catalysis research and IR spectroscopy. The advantages of using IR spectroscopy to probe reactions on heterogeneous catalysts were first demonstrated over four decades ago (1, 2). Since then, many technological innovations have enabled IR measurements to probe intermediates in a variety of environments and conditions that are particularly relevant to heterogeneous catalysis. These techniques, along with individual examples, have been reviewed elsewhere (3–16). In this review, we focus on a broad overview of uses of IR spectroscopy for identifying reaction intermediates and elucidating reaction mechanisms in heterogeneous catalysis. A secondary theme is that a relatively wide set of techniques has been developed to probe many sets of reaction conditions. Table 1 lists IR spectroscopic techniques and a selective set of advantages and disadvantages for each technique. For most of these techniques, it is assumed that the researcher is using a research-grade Fourier transform infrared (FTIR) spectrometer, and we note when significant further expense may be required to fully utilize the technique. Further details about the different techniques can be found in textbooks and reviews in the literature (3–16). As shown in Table 1, IR spectroscopy can be used to investigate catalytic intermediates on (a) powder samples with complex surfaces and a multiplicity of surface sites or (b) the face of single crystals with well-defined arrays of sites under highly controlled conditions. Additionally, surface intermediates can be inferred indirectly by the observation of (c) gas/liquid intermediates and products that have desorbed from the surface, particularly with the aid of isotopic labeling. A comprehensive understanding of a catalytic mechanism may require studying all three with multiple techniques (including non-IR techniques). Although it is not discussed in detail in this review, valuable kinetic information can be gained from IR studies when either a flow reactor is used or the measurements are obtained in a time-resolved manner. The body of this review is broken into three sections, going from experiments in the most chemically well-defined environments (model catalysis) to the most chemically complex environments (operando catalysis). Finally, we discuss future directions.

2. ULTRAHIGH VACUUM STUDIES OF INTERMEDIATES ON METAL SINGLE CRYSTALS AND ON THIN FILMS ON METAL SINGLE CRYSTALS A major branch of heterogeneous catalysis is focused on fundamental studies of adsorption and reaction occurring under ultrahigh vacuum conditions. These studies typically involve catalysis that occurs on the faces of metal single crystals or on thin films that have been deposited on metal single crystals. The use of metal single crystals and thin films on metal single crystals is prevalent because it allows one (a) to create well-defined surfaces, (b) to use techniques that involve charged particles without charging the sample (e.g., X-ray photoelectron spectroscopy, Auger spectroscopy, low-energy electron diffraction, electron energy loss spectroscopy), and (c) to use scanning tunneling microscopy (17). An additional benefit is that infrared reflectionabsorption spectroscopy (IRRAS) can be used on these types of samples (18). The concepts and methodology for using IRRAS as a technique for detecting surface adsorbates have been 250

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Table 1 List of IR spectroscopic techniques with selected advantages and disadvantagesa IR technique Transmission FTIR/transmission IR

Advantages/disadvantages


IRRAS/IRAS/RAIRS

PM-IRRAS

SFG/RSFG

Is well suited for electronically nonmetallic powders and metals supported on electronically nonmetallic powders Requires a partially transmissive substrate IR absorbance is typically linear with the number of molecules. One source of nonlinearity is when the thickness of the sample is not constant Is not surface sensitive Can be used to probe gas phase products/reactants/intermediates For measurements in which CO2 and water signals from ambient air will cause a problem, the experimental setup must be designed to enable purging/evacuation of the beam path Is commercially available and simple to implement Is compatible with step-scan IR spectroscopy Is compatible with in situ measurements Is compatible with operando measurements for some systems Requires a reflective substrate, or a thin film on top of a reflective substrate, that must be relatively flat. Typically, the reflective substrate is a metal. Metal oxide single crystals have extremely low reflected IR intensities, and thus metal oxides are usually not practical substrates but can be grown as thin films on metals for IRRAS measurements Particles can be studied if deposited directly on the reflective substrate, or with a thin film between the particles and the reflective substrate The adsorbate coverage is often approximately linearly related to the change in reflected intensity (displayed as transmission in most IR spectrometer software). For truly quantitative measurements, IRRAS must be used in conjunction with another technique and a calibration curve Works well in studies on metal single crystals or oxide thin films with well-defined surfaces grown on metals [e.g., Pd(111) and MgO(100)/Mo(100)] Often performed in an ultrahigh vacuum surface science apparatus (which typically adds $100,000–400,000 in costs), requiring specialized knowledge Measurements can also be performed without ultrahigh vacuum equipment by using inert gas flows For measurements in which CO2 and water signals from ambient air will cause a problem, the experimental setup must be designed to enable sufficient gas purging/evacuation of the beam path Is compatible with step-scan IR spectroscopy Is typically suitable for in situ measurements, not typically used for operando measurements Represents a form of IRRAS that is surface sensitive (bulk phase IR absorptions, such as the H2 O and CO2 contributions from ambient air, are almost entirely excluded) Is commercially available Is not compatible with the highest speeds of step-scan IR spectroscopy unless separate measurements are taken with p-polarization and s-polarization in static polarization mode Is compatible with operando measurements in limited situations Is surface sensitive Has low signal intensities and is typically limited to >1,600 cm−1 Does not suffer from blackbody radiation background signals when the sample is at high temperatures (e.g., >500◦ C), unlike IRRAS and transmission IR studies Quantitative analysis is difficult and not always possible Requires one medium that transmits IR light and one that transmits UV/visible light (typically either the solution or the substrate transmits both) Liquid phase studies with metals are possible but not commonly attempted Is best suited for flat surfaces. Limited success has recently been obtained with powders/particles supported on surfaces (Continued )

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Table 1 (Continued) IR technique

Advantages/disadvantages

SFG/RSFG

DRIFTS

ATR


IRES

PAS-IR

SEIRA

Requires lasers, laser expertise, and specialized knowledge for interpretation. No commercial units are available for SFG at this time, and the costs for building a system are >$150,000 Is compatible with operando measurements for limited situations Is best suited for electronically nonmetallic powders and for metals supported on electronically nonmetallic powders Quantitative absorbance is possible for a single sample, but comparison between different samples is not routine for nonexperts In situ reactors are commercially available at a low price and are simple to implement Is compatible with operando measurements in various situations Allows in situ/operando liquid phase studies more easily than other techniques listed The detected signals have contributions from the surface region and from the bulk region near the surface at a micrometer scale, making it quasi-surface sensitive. This type of sensitivity enables measurements of surfaces even when the surface is in a relatively absorbing solution Requires an internal reflection element as the substrate or beneath the substrate, which imposes some material constraints Absorbance is generally linear with adsorbate concentration Is commercially available at a low cost and is simple to implement Can be used with both insulating powders and metals The practical spectral range is dependent on the sample temperature Thermal blackbody radiation from the sample and the environment can result in low signal-to-noise ratios No special sample preparation is needed Is applicable to nearly all forms of samples Requires little to no sample preparation Has controllable depth profiling Requires a substantial gas phase to be present, so ultrahigh vacuum applications are not possible Works best with a resonant cavity Is compatible with in situ and operando measurements in appropriate situations Enhancement factors are typically on the scale of 10–100 but can be as great as 105 with tailored structures Can probe systems from ultrahigh vacuum to liquids The sites at which enhancement occurs may not be well defined, which makes obtaining quantitative data difficult (i.e., signals may not be linear with coverage owing to some adsorbates residing near the site of enhancement and other adsorbates residing far from the site of enhancement) Surface selection rules exist but are less simple than for IRRAS and may be ambiguous in practice Is limited to a few systems/substrates Is typically suitable for in situ measurements and is compatible with operando measurements in appropriate situations

a Abbreviations: ATR, attenuated total reflectance; DRIFTS, diffuse reflectance infrared Fourier transform spectroscopy; FTIR, Fourier transform infrared; IRES, infrared emission spectroscopy; IRAS/IRRAS, infrared reflection-absorption spectroscopy; PAS-IR, photoacoustic infrared spectroscopy; PM-IRRAS, polarization-modulation infrared reflection-absorption spectroscopy; RAIRS, reflection-absorption infrared spectroscopy; RSFG, reflectance sum-frequency generation; SEIRA, surface-enhanced infrared absorption spectroscopy; SFG, sum-frequency generation.

reviewed elsewhere (1, 4, 11, 19, 20). The application of polarization modulation to IRRAS (PM-IRRAS) enables discrimination between IR absorption by surface species and IR absorption by bulk reactants and background species (the bulk can be solid, gas, or liquid) (13, 21, 22). Commercial polarization-modulation accessories are now available for some research-grade IR spectrometers. 252

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When IRRAS spectra are obtained for molecules and the moieties produced by molecular or dissociative adsorption, one can use an effect commonly termed the metal surface selection rule to gain information about the geometric orientation of the molecules/intermediates on the surface (19, 23, 24). In the simplest case, no IRRAS absorption is observed for any vibrational mode that is parallel to the surface, as the dipole moments of such modes are effectively cancelled by the formation of image dipoles within the metal substrate that are (essentially) reverse-mirror images of the dynamic dipoles of the molecule (1). The surface selection rule also applies to molecules/intermediates adsorbed on thin films on metals (i.e., films that are tens of monolayers thick; for thicker films, the situation is more complicated). In principle, one can account for the effect of the film thickness in the calculation of IRRAS spectra, if the distance between the adsorbate and the metal substrate is known or can be estimated (25). Additionally, the surface selection rule applies for molecules on metal particles, provided that the particles are nanoscale or larger (26, 27). Detailed analysis of geometries within the context of the surface selection rule is beyond the scope of this review, and we refer readers to prior artful studies (19, 23). As many industrial catalysts are metal based, one can foresee that IRRAS could provide important insights into some industrial reaction mechanisms. One group of industrial catalytic reactions that has been studied with the help of IRRAS is the hydrogenation of alkenes and related compounds on transition metal surfaces. Ethene hydrogenation was one of the first catalytic reactions to be studied both ex situ (after reaction) and in situ (during reaction) using IR spectroscopy. Hydrocarbons were observed on the surface during reaction and after reaction (2). When alkenes adsorb on the surface of a transition metal, the alkene can (a) dissociate, (b) adsorb molecularly in a π-bonded state, or (c) adsorb molecularly in a di-σ-bonded state. Both the π-bonded and di-σbonded states have been observed by IRRAS for ethene on Pd(111) and Pt(111) (28), and the two states can be discriminated based on the positions of the observed vibrational absorptions (28, 29). In the early 1980s, with the use of IRRAS and other vibrational spectroscopies, researchers found that when ethene is adsorbed on Pt(111) and related surfaces, chemisorbed ethylidene and ethylidyne species are formed and virtually completely cover the surface if the surface is exposed to sufficient ethene (30–32). These vibrational spectroscopic observations presented two puzzles for the heterogeneous catalysis community: Was ethylidyne an intermediate in the hydrogenation reaction, and if not, how could hydrogenation occur on an ethylidyne-covered surface? Subsequently, Mohsin et al. (33) performed one of the earliest studies in which IRRAS was used to obtain quantitative data on an adsorbate on a well-defined surface, enabling the extraction of an experimentally determined pre-exponential and activation energy for the conversion of ethene to surface-bound ethylidyne on Pt(111). Using transmission IR spectroscopy, researchers demonstrated that ethylidyne species were formed on metal particles supported on alumina; that is, the same ethylidyne species were detected on surfaces that more closely resembled industrial catalysts (31, 34). By 1994, a detailed analysis of the IRRAS peaks associated with ethylidyne on Pt(111), which proved to be an excellent example of the use of IR symmetry rules and IRRAS surface selection rules, was found to be consistent with low-energy electron diffraction data showing that the ethylidyne moieties likely had a tilt angle of 6◦ relative to the surface normal (23). One year later, a study based on IRRAS experiments on Pt(111), using a mixture of unlabeled and isotopically labeled ethene (C2 H4 and C2 D4 ), found that partially deuterated ethylidyne formed on the surface, indicating the occurrence of hydrogen-deuterium exchange reactions (35). Based on the IRRAS detection of partially deuterated ethylidyne, and other experiments, investigators proposed a mechanism in which ethylidyne facilitates hydrogenation by shuttling hydrogen between the surface and ethene molecules adsorbed on top of the hydrocarbon layer (36). However, various experimental studies and subsequent kinetic modeling (37–41) showed that ethylidyne and other hydrocarbon species formed from alkenes inhibited dissociative hydrogen adsorption, and www.annualreviews.org • IR Detection of Catalytic Intermediates

PM-IRRAS: polarizationmodulation infrared reflection-absorption infrared spectroscopy; also known as PM-IRAS or PM-RAIRS

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Ethylene (g) CH2=CH2

H2 (g)

H (ad)

Ethylene (ad) CH2=CH2 i

b

d

Vinyl (ad) CH2=CH

Ethyl (ad) CH3-CH2

fj

g

k


c Vinylidene (ad) CH2=C

Ethylidene (ad) CH3-CH

Ethane (g) CH3-CH3

Ethylidyne (ad)

Figure 1 Mechanism of ethene hydrogenation on Pt(111). Every hydrocarbon adsorbate shown has been observed by infrared reflection-absorption spectroscopy. Figure reproduced from Aleksandrov et al. (41) with permission of Academic Press via Copyright Clearance Center.

that this was also an important factor in describing the kinetics. Over several decades, IRRAS and other techniques have confirmed the reaction network displayed in Figure 1. Every hydrocarbon adsorbate shown in Figure 1 has been observed by IRRAS, and most of the reactions have been monitored in situ by IRRAS (28, 42–44). Despite the industrial importance of alkene hydrogenation, and the detection of all the relevant surface species involved, it has taken several decades for the mechanism shown to become accepted. The difficulties in forming a comprehensive description of the mechanism and kinetics (42) are that (a) an induction period is observed during which alkylidynes accumulate on the surface, and these species appear to be necessary for sustained hydrogenation to occur, and (b) various studies suggested that the alkylidynes are not an intermediate for alkane formation, yet they appear to exchange hydrogen/deuterium with other hydrocarbons and the surface, complicating attempts to use isotopic labeling to elucidate the mechanism. How can the hydrogenation activity of Pt(111) be rationalized given vibrational spectroscopic evidence that the surface is completely covered by ethylidyne at temperatures of interest (300– 400 K)? Despite clear indications that ethylidyne decreases the hydrogenation rate relative to a clean metal surface (45), one of the roles of the alkylidynes may be to prevent other hydrocarbon species from totally covering the catalyst surface under relevant practical conditions (39, 42, 46). That is, whereas the alkylidynes inhibit hydrogen dissociation, the alkylidynes may also (a) inhibit the adsorption/formation of other hydrocarbon species on the surface that would completely inhibit surface hydrogen formation (significant amounts of aromatic species are not formed, as they would have been detected by IR techniques) and (b) decrease the adsorbate-surface bond strength of relevant intermediates. We note that ethylidyne is a dead end in Figure 1, except for the possibility of an equilibrium with ethylidene (42), yet the ethylidynes may be necessary components for sustained catalytic activity on Pd(111) and Pt(111). Even though Pd(111) and 254

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Pt(111) have the same mechanistic network, the rate-limiting step varies among the surfaces under relevant conditions (41), with a dependence on the temperature and pressures of the reactants. A thorough review of the data, elementary steps, and complications has been published recently (46). The adsorption and hydrogenation of ethene and larger unsaturated hydrocarbons have also been studied on Pt(111), Pd(111), and other metal surfaces using IRRAS and other techniques (46). Our understanding of the factors that govern hydrogenation on metals remains incomplete, but IR studies have been involved in crucial mechanistic investigations and are likely to remain crucial in future mechanistic investigations of hydrogenation. Today, research-grade spectrometers can be purchased with the software required for routine IRRAS and routine PM-IRRAS studies. Some limitations and future possibilities for IRRAS should be noted. As discussed elsewhere (1), IRRAS should be considered semiquantitative in the absence of an external quantitative calibration of the adsorbate surface coverage, primarily because interadsorbate interactions and electronic interactions with the substrate (and through the substrate) may cause IRRAS signals to deviate from a direct proportionality to the number of adsorbates. Typically, the IRRAS signal is linear with the concentration of adsorbates at low coverages, and at high coverages, the intensity is usually within a factor of two of the intensity extrapolated from the very low-coverage limit. IRRAS has generally not yet been able to investigate adsorbates on oxide surfaces unless the oxide surface is grown as a thin film on a metal. IRRAS measurements have been obtained for several specific systems of adsorbates on bulk metal oxides (47, 48). The surface selection rules of IRRAS are different for adsorbates on bulk dielectrics: Vibrational modes of adsorbates can, in principle, be detected regardless of the orientation of the dynamic dipole. However, Weiss and colleagues (49, 50) performed transmission IR measurements on ionic solids (e.g., NaCl) and showed that with transmission IR on such single crystals, one can compare the differences in absorption using s-polarized and p-polarized light sources to obtain geometric information, a capability similar to that provided by the metal surface selection rules of IRRAS. If researchers grow insulator films on top of salt windows with layers thin enough to allow for transmission IR studies, then it is possible that information similar to IRRAS can be obtained for adsorbates on insulator surfaces for cases that are not measurable by IRRAS. Griffith and colleagues (51, 52) have published a technique using an integrating sphere, which has a transmissive medium on top of a metal. Multiple reflections are used to obtain improved signal-to-noise levels, with the enhancement greater for low-intensity peaks than for high-intensity peaks. This is technically an IRRAS technique, but it gives information more similar to transmission techniques and requires a transmissive substrate. An integrating sphere for surface species detection has been described for diffuse reflectance ultraviolet (UV)-visible-near-IR spectroscopy (53), although to our knowledge an integrating sphere has never been applied in this manner for mid-IR surface science studies. Mid-IR integrating sphere accessories are commercially available (54) and could be adapted for studying adsorbates on insulators.

3. INTERMEDIATES ON POWDERS, HIGH VACUUM TO ATMOSPHERE A wide variety of catalytic reactions take place on catalysts that are in powder form, including metal oxides and zeolites, and in situ IR spectroscopy has been employed for the detection of reaction intermediates on such powders. We note that there are several articles in a special issue of Chemical Society Reviews (55) that focus on in situ characterization of heterogeneous catalysis (56–58), which showcase some of the in situ IR techniques that are available. For all the techniques described in this section, if the vibrationally active species is adsorbed on a metal particle, then www.annualreviews.org • IR Detection of Catalytic Intermediates

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the metal surface selection rule described in Section 2 applies, provided that the metal particles are nanoscale or larger (26, 27). Historically, there have been few basic approaches to obtaining IR spectra of powder samples— one challenge being that, in general, powders with dimensions typical for catalytic materials efficiently scatter IR light (59). Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) effectively takes advantage of powders being good scatterers (16). A 1993 review article by Mitchell (60) provides a summary of the history, development, and application of DRIFTS up to that point in time. Pressing pellets from the powder for transmission IR studies is another approach. However, the pellets generally have to be very thin to avoid unacceptable scattering losses and thus are difficult to prepare, handle, and heat evenly while maintaining their physical integrity. An additional approach for transmission IR developed by Yates and colleagues (61) involved spraying a slurry of the desired powder onto a transmissive wire grid that can then be uniformly heated or cooled. This method provides the ability to work with relatively small quantities of powder along with some control of the sample thickness to optimize the desired transmissivity. This approach has been utilized by a number of groups for many studies, sometimes with different methods of sample application, and the original article describing the cell and the technique now has over 165 citations (61). Of course, there are additional challenges to obtaining IR spectra. In our implementation of a wire grid reactor, we typically use ∼10–30 mg of a high–surface area powder that is painted on a tungsten wire grid. For a powder with a surface area of 100 m2 /g, this corresponds to ∼1–3 m2 or ∼2 × 104 cm2 total area. Taking 3 × 1014 molecules/cm2 as a reasonable estimate for an adsorbate saturation coverage would mean that there would be ∼0.6 × 1019 molecules present upon adsorbate saturation. This is normally an ample number of molecules from which to obtain an IR spectrum (62). However, intermediates can be present at a small fraction of the saturation coverage, and whereas zeolites can have surface areas larger than the quoted value, metal oxide powders can have somewhat smaller surface areas, making the detection of intermediates on these materials even more challenging. We also note that in transmission mode, except for wavelengths at which there is a high level of scattering by the solid (59), the integrated absorption of the IR peaks is linear with the quantity of adsorbates, provided that the conditions for Beer’s law are met (14, 59). We now provide an example of an area, deNOx chemistry, in which researchers made extensive use of IR spectroscopy to identify reaction intermediates and to successfully delineate reaction mechanisms. There has been considerable scientific and societal interest in NOx removal from diesel exhaust. Although a diesel engine has a higher efficiency relative to gasoline-powered vehicles, there are higher levels of NOx in diesel exhaust than in gasoline exhaust. The need to find an effective means to remediate NOx emission in diesel exhaust became more critical with recent legislation both in the European Union and in the United States mandating decreases in NOx emissions. IR spectroscopy has played a central role in elucidating the mechanism(s) for catalytic NOx reduction and NOx storage, which has been reviewed elsewhere (63, 64). Below we provide two examples of the use of IR spectroscopy for mechanistic studies of NOx reduction with the detection of intermediates by IR. The first example involves the elucidation of the mechanism for NOx reduction with added ammonia, and the second involves the determination of the operative mechanism(s) for NOx reduction with organic oxygenates as the reductant. There has also been some elegant work on intermediates involved in NOx storage that is beyond the scope of this review (63–66). It has been known for more than a decade that the addition of appropriate amounts of ammonia to a NOx stream, in the presence of a suitable catalyst, will decrease NOx emission (67–72), and this approach has applications to stationary NOx sources (72). It has also been known that the ideal ratio of NOx to ammonia is 1:1:2 (for NO:NO2 :NH3 ). However, the mechanistic rationale for this ratio, the intermediates involved, and the role that NO plays in the mechanism were not well


DRIFTS: diffuse reflectance infrared Fourier transform spectroscopy

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understood. Understanding was greatly aided by studies in which intermediates were detected by IR spectroscopy and the reaction pathways thereby elucidated. Yeom et al. (73) demonstrated that the desired pathway for low-temperature NOx reduction with ammonia proceeds via the formation of ammonium nitrite, which then cleanly and efficiently decomposes to N2 and H2 O at approximately 100◦ C (74) or at even lower temperatures in an acidic environment (75, 76). Within this mechanism, Sachtler and coworkers (77) demonstrated that one of the nitrogen atoms in the N2 molecule formed in the decomposition of ammonium nitrite comes from ammonia and the other comes from the NOx in the feed stream. In competition with the pathway for ammonium nitrite formation is a pathway for ammonium nitrate formation: The formation of the latter is more problematic as ammonium nitrate is stable up to approximately 250◦ C and decomposes to produce N2 O, which is undesirable (76). However, as demonstrated for some catalysts (e.g., BaNa-Y), NO can reduce surface nitrates to surface nitrites and nitric acid to nitrous acid (73, 76), whereas solid acids can facilitate the production of N2 from the reaction of NO with ammonium nitrate (76). All these reduction reactions either directly produce ammonium nitrite, or more labile surface nitrites, or allow for entry into reaction pathways that lead to the production of nitrites rather than nitrates. The mechanism depicted in Figure 2 shows the intermediates that were identified by the use of IR spectroscopy (73). Related and similar mechanistic conclusions have been drawn by Tronconi et al. over other catalysts (78), with interesting 2NO

+O2

2NO2

N2O4

–O2 –H2O

+H2O

BaNa-Y NO+ + NO–3

HNO2 + HNO3

NO–3

NO+

+H+

NO –H+ –H2O

+H2O

HNO3

NO2 NO

+NH3 –NH3

HNO2 + H+

NH4NO3

NO2

NO–2

+H+

NO

+

–H

A+ HNO2

NO2 +NH3 –NH3 NH4NO2

N2 + 2H2O

Figure 2 Mechanism for the conversion of NOx to N2 in the presence of ammonia over BaNa-Y and related catalysts. Most of the species shown have been detected by IR spectroscopy. Figure adapted from Savara et al. (89) with permission of Elsevier BV via Copyright Clearance Center. www.annualreviews.org • IR Detection of Catalytic Intermediates

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differences observed when redox metals are present (79). We note that the disproportionation of N2 O4 (formed from dimerization of two NO2 molecules) to NO+ and NO3 − is an interesting reaction in which the NO+ intermediate is presumably stabilized by interaction with the surrounding ionic environment. Szanyi et al. (80) had previously observed the formation of NO+ and NO3 − and studied and characterized it using isotopically labeled precursors. NO+ has been reported in a number of studies (80, 81). We also note that Grassian and coworkers (82) studied the mechanism of the related reaction of NO2 with both 15 N and normal urea in nanocrystalline NaY zeolite and pointed out potential advantages of the nanomaterial for NOx reduction. The above results also tie into the mechanism for low-temperature NOx reduction using added organic oxygenates as a reductant. There were several early reports of low-temperature reduction of NOx using hydrocarbons as additives over suitable catalysts (83–85). RNCO species were demonstrated as intermediates (86), and Cant and colleagues (87) proposed that HNCO played a critical role as an intermediate when using nitromethane as a reactant with Co-ZSM-5. A provocative study demonstrated that NOx could be removed from NOx streams by adding oxygenates to the streams in the presence of catalysts that contained neither redox active metals nor reducible supports (88). This result indicated that redox active metals were not necessary for the NOx removal process and thus expanded the range of catalysts and additives that could potentially be used for NOx removal. One of the most active oxygenates for NOx reduction over these catalysts is acetaldehyde (90– 92). An important initial study focused on the mechanism for NOx removal with acetaldehyde over a non–transition metal containing zeolite (88). Subsequent studies involved BaNa-Y, which acts functionally as a Ba-Y zeolite (90, 91). Static-cell transmission IR studies led to the conclusion that there are two parallel pathways that lead to NOx removal (90): One involves ionic species (the ionic pathway) and the other radical species (the radical pathway). The ionic pathway has been effectively completely mapped by IR studies and is shown in Figure 3. The reaction of adsorbed acetaldehyde with NO2 leads to the formation of a surface acetate intermediate. Acetate

O

H N

O

CH3NO2

C

O

–

N

H

O NH4NO3

+

–H

O C H3C

CH2NO –2 + CO2

H NO2

HNO3

+H+, –H2O

NO2

+H+

NCO –

N2O + H2O

H2O NH3 + CO2

+ NO2 + H+

NO2

HNO2 HCN + products

O

NH4NO2

N2 + H2O

C O– + NO + H+

H3C

Figure 3 The ionic pathway for NOx reduction over BaNa-Y with acetaldehyde as a reactant. The intermediates detected by IR spectroscopy are designated with red boxes. The dinitromethane anion is designated by a dashed red box because it was identified indirectly through isotopic labeling. The equilibria among the NOx species shown in Figure 2 occur concurrently. 258

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formation is in competition with aldol condensation, but typical concentrations of NO2 present in a NOx stream lead to acetate formation being dominant over aldol condensation. The acetate thus formed then further reacts with NO2 to form nitromethane, which has been directly detected on the BaNa-Y surface with transmission IR spectroscopy (88). Another transmission IR study showed that there was an overlap of intermediates for NOx reduction with acetaldehyde over an Na-Y zeolite with those observed in the study over BaNa-Y (92). Over BaNa-Y, nitromethane forms by the reaction of NO2 with surface acetate (93). Nitromethane has been postulated to be in equilibrium with its ionic form, the aci-anion of nitromethane, and undergoes a further reaction with NO2 to form dinitromethane (94). The dinitromethane molecule is the only intermediate in the ionic pathway that has not been directly detected. The presence of dinitromethane was inferred from the isotopic distribution of products (based on IR data) when starting with isotopically labeled reactants. When 15 NO2 reacts with CH3 14 NO2 , both H15 NCO and H14 NCO are formed in equal amounts, strongly suggesting that the intermediate yielding these products is a symmetric species containing two NO2 moieties (90). The only plausible compound that satisfies these criteria is dinitromethane, a compound that has been reported in the literature (95). Dinitromethane (or its aci-anion) decomposes to give two NCO (NCO− ) moieties, either directly or via an intermediate (90, 94). The NCO moieties add hydrogen and can then hydrolyze to give ammonia (90). The net effect of these reactions can be viewed as leading to the in situ generation of ammonia through hydrolysis of an HNCO species or an HNCO analog, with the subsequent steps in NOx removal being analogous to those operative when ammonia is an additive. Subsequent studies determined that the rate-limiting step in this reaction sequence is the generation of nitromethane (96). Consistent with this conclusion, the addition of nitromethane to an NOx stream led to virtually 100% NOx removal at significantly lower temperatures over Ag-Y than could be achieved starting with acetaldehyde (97). Experimental and theoretical studies also suggested that the stabilization of the ionic intermediates (e.g., nitrate, nitrate, acetate) plays a key role in the catalytic performance of BaNa-Y (89, 98). Figure 3 shows the elucidated mechanism, along with the intermediates that have been detected by IR spectroscopy. To our knowledge, the mechanism for NOx reduction on BaNa-Y using acetaldehyde as a reductant is the longest linear mechanism for heterogeneous catalysis elucidated thus far. The above studies included in situ IR measurements. When identifying intermediates or elucidating mechanisms, there are advantages that result from in situ studies with flow reactors and/or time-resolved static-cell measurements. We note two here: (a) Quantitative measurements of the IR absorption of the intermediate can be correlated with the reaction rate as a function of conditions (99–101), and (b) an isosbestic point can result when the wavelength of the IR absorption (i.e., the peak position) of one species in the reaction overlaps with that of another species that is coupled to the first in a direct A → B process. Owing to space limitations, we do not discuss isosbestic points in detail. However, the observation of an isosbestic point is a strong indicator that two species are directly coupled by the reaction mechanism. An isosbestic point was observed in the interconversion between surface nitrates and surface nitrites on BaNa-Y (89). One valuable approach that was used to help elucidate the mechanism in the above studies is to introduce proposed reaction intermediates directly under reaction conditions. This approach is general and can be used for other systems, as well as in conjunction with isotopic labeling. The observation that the subsequent chemistry was the same as with the starting materials helped to validate the assignment of the proposed intermediates and also to put the subsequent chemistry on a firmer footing. An advantage of this approach for identifying reaction intermediates and elucidating a reaction mechanism is that in a reaction system, intermediates are normally present in a steady state at concentrations that are typically much lower than those of the starting materials. www.annualreviews.org • IR Detection of Catalytic Intermediates

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By independently introducing a proposed intermediate to the catalyst being studied, under reaction conditions, one can control the concentration of the intermediate, and it can be much higher than its normal steady-state value. This approach can make it much easier to obtain larger signals, and thus higher signal-to-noise levels, for species (intermediates) produced by subsequent reactions in the overall mechanism. In fact, it may make it possible to identify intermediates that are simply at too low a concentration under actual reaction conditions for detection. This approach can be particularly important in trying to follow a multistep mechanism such as that shown in Figure 3. For example, in the BaNa-Y and acetaldehyde system, acetate ions were prepared directly from acetic acid to probe subsequent chemistry, and other experiments used nitromethane and HNCO as starting points to investigate their subsequent chemistries. In addition to the above advantages, the introduction of a putative intermediate can decouple the kinetics of a series of sequential reactions. This can make it much easier to determine the rates of individual steps in a complex reaction and also help pinpoint the rate-limiting step(s) in a complex reaction mechanism. As mentioned above, with nitromethane as the added oxygenate, the overall deNOx reaction took place with higher efficiency and at a significantly lower temperature than with acetaldehyde, acetic acid, or ethanol as the added oxygenate (97). This demonstrated that the rate-limiting step with the other additives occurred prior to the reaction of nitromethane. Additional experimental data strongly suggest that the reaction of surface acetate, which is formed with all the aforementioned oxygenates except nitromethane, is the rate-limiting step in the overall deNOx reaction sequence for BaNa-Y under the conditions studied (96). As mentioned above, IR studies have demonstrated that ethanol can be an effective NOx removal additive, over Ag-Y and Ag/γ-Al2 O3 (93). The reaction pathway using ethanol proceeds through acetaldehyde (93, 102). The most efficient catalyst for the process that was studied in Reference 93 was Ag-Y, which efficiently oxidizes ethanol to acetaldehyde at 200◦ C with O2 as the oxidant. The chemistry subsequent to ethanol oxidation is then the same as starting with acetaldehyde. Comparisons of Ag-Y results to observations for the Ag/γ-Al2 O3 system (102) provided additional insights into the operative chemistry. With the latter system, oxidation with NO2 competes with oxidation with O2 at the reaction temperature. At 200◦ C, the dominant oxidant was NO2 , and a product of this oxidation reaction is ethyl nitrite, which can decompose into multiple products, including acetaldehyde (93, 102). But the yield of acetaldehyde is lower than that under comparable conditions with Ag-Y, for which the primary oxidant is O2 (93, 102). In both systems, moieties containing C-N bonds are observed that result from the following pathway: (a) acetaldehyde + NO2 → acetate + NO, (b) acetate + NO2 → nitromethane + CO2 , and (c) nitromethane + NO2 → products containing C-N bonds. Isotopic labeling of both the C and N atoms greatly facilitated the identification of specific C-N-bond-containing species using in situ transmission IR spectroscopy. In both the Ag-Y and Ag/γ-Al2 O3 systems, IR absorptions corresponding to NCO− , CN− , and NC− were observed. R-CN-containing species were also observed, but only on Ag-Y, and only when ethanol, acetaldehyde, or crotonaldehyde was used as a reductant (93). R-CN species were not observed on Ag-Y when acetic acid was used as the starting material. The conclusion reached was that the observed R-CN species are likely formed from a product of an aldol-condensation reaction, and consistent with this conclusion, the amount of R-CNcontaining species decreased when NO2 was in excess compared to ethanol. No R-CN species were seen on Ag/γ-Al2 O3 : It appears that the oxidation of acetaldehyde on Ag/γ-Al2 O3 is fast enough to successfully compete with the aldol-condensation reaction. In another early experiment that relates to the formation of C-N-bond-containing species in a catalytic reaction, Cant et al. (103) investigated the reaction of NO and NH3 in the presence of excess CO over Pt, Pd, or Rh supported on silica. They characterized the formation of HNCO in the gas phase and inferred the formation of surface-bound NCO.


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DRIFTS has also contributed to our knowledge and understanding of the role of C-N bonds in deNOx processes and related chemistry. DRIFTS accessories are available for standard commercial FTIR spectrometers and are relatively easy to implement for powder samples. Thus, DRIFTS is now a widely used technique for obtaining the IR spectrum of powders and moieties adsorbed on the surface of powders (60). Quantitative comparisons between DRIFTS spectra on different samples have been shown to be possible but are not routine or for casual users (104), although it is straightforward to quantitatively compare adsorbate intensities on a single sample or between different samples from the same preparation by the proportion of the adsorbate band relative to a substrate framework band. A recent article (105) provides a good discussion of the DRIFTS technique, and Reference 60 reviews aspects of its applications. We consider below a few reactions/systems to illustrate the type of information available from DRIFTS studies and the role it has played in the detection of reaction intermediates in selected heterogeneous catalytic reactions. C-N bond formation plays an important role in deNOx chemistry. Tamm et al. (106) used DRIFTS to investigate surface species and used standard FTIR techniques to investigate the gas phase over an Ag/Al2 O3 catalyst in a series of experiments in which they varied the composition of the gas incident on the catalyst to include combinations of propene, oxygen, and nitric oxide. They detected surface CN and NCO on the silver/alumina surface and presented a reaction pathway leading to the formation of these species, followed by hydrolysis to form ammonia, analogous to the hydrolysis step shown in Figure 3. The subsequent chemistry is then presumably the same as what takes place with added ammonia, as described in the previous section. Pietrzyk et al. (107) used transmission IR, electron paramagnetic resonance, and operando DRIFTS to investigate the selective reduction of NO by propene over a Co-BEA zeolite. They reported the presence of multiple intermediates, including cyanide, isocyanide, surface nitrates, and nitrites, as well as a more unusual intermediate: Co-nitrosyls. They noted that all these intermediates are intimately involved in the deNOx mechanism. Interestingly, they presented evidence for three different potential pathways for the formation of dinitrogen from the observed surface intermediates, including oxidation of cyanides by NOx . DRIFTS (and transmission FTIR) has also been used to investigate the chemistry operative in NOx storage systems (108, 109) and other catalytic deNOx systems (110). DRIFTS has also been used to detect reaction intermediates in the water-gas shift and reversewater-gas shift reactions, both of which are extremely important reactions that have been the subject of a myriad of studies by the catalysis community. Meunier and coworkers (111) reported on the active intermediates in the reverse-water-gas shift reaction taking place over a Pt/ceria catalyst. In these studies, the authors used DRIFTS in conjunction with other techniques, specifically mass spectrometry and steady-state isotope transient kinetic analysis. They observed both the production of surface formates and the reduction of CO2 by defects in the ceria, concluding that the mechanism for CO production switches from one in which the reduction of CO2 is dominant (at lower temperature) to one in which surface formate is an important intermediate for CO formation (at higher temperature) (111, 112). This study is another demonstration that the use of multiple techniques to probe reactions can often provide more information than is obtainable with one technique and can provide results that are more solidly grounded. Another reaction in which DRIFTS studies have played a role in increasing our understanding is the selective oxidation of CO with O2 , often in the presence of H2 and/or H2 O. Again, our discussion is by necessity selective. One area of focus has been studies of the oxidation of CO with supported Au catalysts. Kung et al. (113) published a feature article on this topic that provides an excellent summary of the area. Wu et al. (114) reported on the room-temperature oxidation of CO on Au supported on silica using both DRIFTS and quadruple mass spectrometry methods. They found a correlation between the degree of reduction of Au and the activity for CO oxidation. The degree of reduction of Au is deduced by changes in the frequency of adsorbed CO as probed www.annualreviews.org • IR Detection of Catalytic Intermediates

Transmission FTIR: transmission mode Fourier transform infrared spectroscopy; also known as transmission IR spectroscopy

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PAS-IR: photoacoustic infrared spectroscopy


IRES: infrared emission spectroscopy

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by DRIFTS (114), in which the adsorbed CO can be viewed as an intermediate in this oxidation reaction. An interesting aspect of this work is that the vibrational frequency of the surface intermediate (adsorbed CO) is used as a probe of the state of the Au catalyst. Differences have been reported with different supports. In a study of CO oxidation on Au on titania, Kung et al. (115) proposed a hydroxycarbonyl as an intermediate in the oxidation reaction. A study of CO oxidation over Au on ceria reported the involvement of a surface formate and the formation of an Au-CO complex [Au(CO)2 ]+ (116). CO oxidation has also been studied on other catalysts using DRIFTS. For example, Shi et al. (117) studied the selective oxidation of CO on an FeOx/Pt/TiO2 catalyst. They reported that the reaction proceeds via an observed HCOO intermediate in the presence of H2 or water, with the rate-limiting step reported as the oxidative decomposition of HCOO with OH. A subsequent paper proposed further details of the mechanism (118). These IR studies show that even a relatively simple reaction (CO oxidation) can proceed via different mechanisms through different intermediates with the same metal on different substrates—clearly demonstrating that the substrate can affect the reaction mechanism. Photoacoustic infrared spectroscopy (PAS-IR) is another technique that can be used to obtain the vibrational spectrum of adsorbates and the near-surface region of the solid for gas-solid interfaces (3, 119, 120). The sampling depth of this technique is controllable. PAS-IR has been reviewed elsewhere in the context of its potential applications to catalysis and surface science (121, 122). A homemade reactor for in situ PAS-IR was used to detect methoxy species on Na-Y as well as CO on supported Pt particles (123). Presently, there has been relatively little use of PAS-IR for studies of heterogeneous catalysis, although its future in heterogeneous catalysis studies may be promising. Infrared emission spectroscopy (IRES) is another technique that is suitable for studies of surface reactions and intermediates, both on powders and on thin films. An advantage of this technique is that there are effectively no issues involving sample preparation or handling, such as those involved in making thin pellets or in supporting materials on a wire grid for transmission studies. IR radiation is collected from the emitting sample and the molecules on the sample. Sullivan et al. (124) have reviewed the technique and its application to surfaces through 1991. They reported on some of the details of the methodology involved in an application of this technique to a catalytic reaction. They also presented data on the observation of C-N bond formation in the reaction of NO and CO on Pt/γ-Al2 O3 , which they indicate is most likely formed on the support material. Another study reported on the formation of intermediates containing C-N bonds on the reaction of CO with NH3 on MgO (125). Somewhat earlier, van Woerkom & de Groot (126) reviewed the theory involved in the application of this method to heterogeneous catalysts. Although IRES has been applied to studies of surfaces, surprisingly little work has been done with this methodology to probe and identify reaction intermediates. There are, however, studies that report on the observation of chemisorbed moieties. For example, one study reported on CO and pyridine on Pt supported zeolite Y (127), whereas another reported the observation of ruthenium dicarbonyl on the adsorption of CO on Rh/Al2 O3 (128). The IRES technique appears to be promising but underutilized. Accessories are commercially available for in situ transmission IR (3) and in situ DRIFTS measurements (16, 129, 130), including high-temperature versions that go up to 700–1,000 K. It is possible to perform in situ IRES using custom, commercial IRES reactors (which include a detector in the setup) (124) or any modern commercial in situ DRIFTS reactor (131) in conjunction with a commercial IRES detector accessory (132). At present, ex situ measurements can be performed with PAS-IR, transmission IR, and DRIFTS measurements, all in the same accessory (133), but currently these commercial multisampling accessories can be used only for ex situ measurements (and the measurements are not taken simultaneously). In practice, the relative intensity of the peaks associated with different vibrational modes may differ among PAS-IR, transmission IR, and DRIFTS (133). The factors that can lead to changes in the relative intensities of the peaks include Savara

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the following: PAS-IR can sample either the surface and near-surface region or the surface and bulk region (the depth of sampling is qualitatively controllable); DRIFTS peak intensities are modulated by the ability of the powder to scatter light as a function of wavelength and the number of scattering events that occur; for transmission IR experiments through pellets, Beer’s law applies only when the sample is spatially homogeneous (i.e., if some parts of the sample are thicker than other parts of the sample, there will be deviations from Beer’s law that can change the relative intensities of the peaks).


4. INTERMEDIATES AT SOLID/GAS INTERFACES AT >1 BAR AND AT SOLID/LIQUID INTERFACES

ATR: attenuated total reflectance infrared spectroscopy; also known as ATR-IR spectroscopy SFG: sum-frequency generation

A current trend is to pursue the ability to perform operando spectroscopy, in which measurements are made under conditions that are realistic for a catalytic reaction or industrial process. For the techniques described in Sections 1 and 2, measurements are sometimes untenable under such conditions. (a) If there is a sufficient number of adsorbing reactant or solution molecules in the beam path, they may prevent adequate IR light from reaching the detector, and (b) the IR frequencies of the absorptions of reactant or solution molecules may overlap with the IR frequencies of the absorptions associated with the surface species. In such cases, the IR absorption of the reactant or solution may bury the IR absorption of the surface species, thus making the detection of the IR absorptions associated with surface species very difficult, if not impossible. We discuss two notable classes of IR techniques that can overcome this problem. The first is attenuated total reflectance (ATR) (134–136), and the second is the nonlinear optical technique known as sum-frequency generation (SFG) (22, 137). ATR is an established technique for the detection of intermediates on materials in contact with a solution (or gas) and is relatively easy to implement. In situ commercial ATR-IR accessories are available that can be heated up to ∼200◦ C (138). Recently, the use of a fiber optics probe has been developed for in situ ATR (139). Excellent reviews have been written regarding experimental considerations and the basic theory of ATR (134, 135). In ATR, an IR beam is focused into an internal reflection element that must be largely nonabsorbing in the frequency range of interest and should have a high refractive index. The IR beam is allowed to enter the internal reflection element at an angle such that the beam undergoes internal reflections and evanescent waves propagate outward from the points of reflection. The evanescent waves travel outward from the interface between the internal reflection element and the medium being probed and are attenuated by any IR absorbing species within the region penetrated by the evanescent waves. The extent of the depth of sampling of the external bulk phase is dependent on the angle and wavelength (leading to stronger peaks at lower wave numbers) and is typically of the order of several micrometers. Commercial accessories include software options to account for the wavelength dependence, thus enabling direct comparisons to transmission mode spectra. Powders and films may be deposited on the internal reflection element, enabling heterogeneous catalysis studies to be performed on substrates, provided that the catalytic interface is within the region probed by the evanescent wave. The primary advantages of ATR over transmission IR spectroscopy are that (a) the technique is inherently sensitive to the region near the surface of the internal reflection element and (b) ATR can be performed in the presence of bulk solutions that are too absorbing to perform transmission spectroscopy. As with IRRAS and transmission IR spectroscopy, ATR is commonly used as a difference technique: It is desirable to obtain a background spectrum that can be subtracted from the signal obtained in the presence of the adsorbate being probed. Unlike SFG, the IR absorption associated with the bulk solution is still present, but multivariate analysis (a mathematical technique) coupled with experimental perturbations (e.g., controlled changes in www.annualreviews.org • IR Detection of Catalytic Intermediates

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the feed concentration or temperature) can be used to separate the signals of the desired species from the signals generated by the bulk solution (136). As with IRRAS, the surface selection rules created by image dipoles on metal surfaces will apply when a metal is used for the thin film or the internal reflection element. Several examples of heterogeneous catalytic intermediates detected in situ by ATR at the solid/liquid interface are (a) benzaldimine in the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate on amine-modified silica (135, 140), (b) 2-propoxide in the oxidation of 2-propanol over Pd/Al2 O3 (141), (c) imine species in the hydrogenation of ethanenitrile (136) and butanenitrile (142) over Pt/Al2 O3 (136), and (d ) surface carboxylates in the hydrogenation of 2-methyl-2-pentenoic acid (143) and methylcinnamic acid on Pd/Al2 O3 (144). The intensity of the vibrational features associated with the adsorbate in ATR can be increased using surface-enhanced infrared absorption spectroscopy (SEIRA) (145, 146), in which measurements are performed with a substrate consisting of metal islands, roughened metal surfaces, or solids composed of metal colloids. In all cases of SEIRA, the metal surfaces may be covered by the nonmetallic intermediate layer such as an oxide or polymer, upon which the adsorption/reaction study is to be performed. SEIRA is not limited to ATR: SEIRA effects have been observed in ATR, IRRAS, DRIFTS, and transmission studies (147). The metal surface selection rule described in Section 2 generally applies to adsorbates studied by SEIRA (145, 146). As it relies on the polarizability of the rough surface or islands, it is also in principle possible to perform with oxide nanoparticles. Surface intermediates have been observed in electrochemistry using the technique with an intensity enhancement factor of typically 10–100 (145). In SEIRA, the enhancement is wavelength specific and dependent on the surface structure’s geometry (due to local field effects and polarization), enabling larger enhancements through tailored structures (enhancements of the order of 103 –105 have been realized experimentally) (148, 149). SFG is a surface-sensitive nonlinear optical technique in which photons from an IR source and photons from a UV/visible source are simultaneously used to excite the molecules into a virtual state(s), which then emit UV/visible photons that are detected (137). Only molecules in a noncentrosymmetric environment (e.g., at an interface) are SFG active, and thus SFG probes the vibrations of molecules at interfaces, with chemical specificity provided by the frequency of the IR source. Consequently, this technique selectively probes adsorbates, even with gases and liquids present—provided that both the UV/visible and IR photons can penetrate either bulk phase (the two beams can enter through different bulk phases) to reach the interface. SFG can be used in either a transmission or reflection mode (15). Its application to surface science has been reviewed elsewhere (137, 150–152), and the technique has been compared with both PMIRRAS (13, 15, 22) and Raman spectroscopy (15) regarding advantages and disadvantages for applications in surface studies. Despite the promise of SFG, relatively few groups have applied it to heterogeneous catalysis studies (153). The knowledge and capital required to develop an SFG apparatus, as well as the technique’s experimental limitations, have been discussed previously (15). At present, most intermediates detected by SFG have been in the hydrogenation of hydrocarbons on metals, and these accomplishments have been reviewed elsewhere (150, 154).

5. FUTURE TRENDS: COMPUTATIONAL CALCULATIONS, SPATIAL RESOLUTION, AND PARALLEL SCREENING MEASUREMENTS Until the past decade or two, accurate calculations of vibrational modes of molecules were limited to small gas phase molecules and molecules adsorbed on small clusters of metal atoms. However, because of advances in computing power and algorithms, accurate geometries and energies can now be calculated for small molecules adsorbed on extended surfaces (155). Subsequent to geometry optimization, the calculation of the frequencies of vibrational modes for an adsorbate using electronic 264

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structure calculations, such as density functional theory, requires calculation of the multidimensional potential energy surface for the nuclear motions of the atoms. The computational expense for calculating the vibrational frequencies of normal modes increases nonlinearly with the number of atoms included. Moreover, for an accurate calculation of vibrational frequencies for adsorbates, some atoms from the solid (e.g., the first layer of atoms of the solid, or more) may need to be free to move during geometry optimization and vibrational calculation, further increasing the computational expense. Fortunately, the calculation of vibrational modes using the harmonic approximation is often much cheaper (computationally) than the initial geometry optimization. More computationally expensive vibrational calculations are required for cases that necessitate accounting for anharmonicity to obtain accurate frequencies (e.g., when Fermi resonance occurs). In the past 10–15 years, advances in computing power and algorithms have enabled computational calculations of adsorption energies and vibrational spectra for more complex species on surfaces (156–159). Conventional density functional theory calculations cannot accurately calculate the energy contributions from van der Waals interactions, which are important not only in physisorption, but also in chemisorption. However, recent advances that include additional terms in the energy calculations now enable accurate calculations, even for cases of adsorbates on surfaces in which van der Waals contributions are important (160). Another current trend is the development of instruments with capabilities for spatially resolved IR measurements. Historically, most IR measurements have obtained spectra of the entire surface of the sample, or an arbitrary portion of the solid sample’s surface. Two broad classes of spatially resolved measurements are (a) macroscopic spatial resolution (135, 161, 162), which can be used for parallel screening measurements of multiple samples or multiple conditions simultaneously (in the same measurement) and thus increases the chances of successfully detecting intermediates by monitoring more conditions or samples per measurement (163), and (b) microscopic- to atomicscale spatial resolution (161, 164), which may enable the correlation of intermediates and activity with features on the surface (e.g., steps, edges, and defects). We anticipate that spatial resolution and parallel screening measurements will continue to see increased use in the future, and these trends are also present for non-IR spectroscopies.

6. SUMMARY IR spectroscopy provides molecular-specific detection of chemical species as a result of its ability to determine the vibrational frequencies of species of interest, which is linked to the geometry and identity of the chemical species. This capability can be applied to study both bulk phases and interfaces, in a variety of environments, and can be used to detect reactants, intermediates, and products in heterogeneous catalysis studies. With these capabilities, IR spectroscopy has been used for detecting intermediates and elucidating reaction mechanisms. Although we have not discussed it above in any detail, IR studies can also be used for quantitative kinetic studies. Many IR studies on heterogeneous catalysis have been aided by the application of other analytical techniques, by the use of isotopic labeling, and also by monitoring the kinetics of the reactions of interest. The types of reactions studied have been diverse and include the hydrogenation of hydrocarbons on metals, the reduction of nitrogen oxides on composite materials, and the oxidation of CO on oxide supported metals. Recent trends include an increased interest in monitoring reactions within the liquid phase, parallel screening measurements, and spatially resolved measurements. The information obtained from IR spectroscopic probes is enhanced when coupled with other analytical techniques or computational calculations. Computational calculations of geometries, energies, and vibrational modes of possible intermediates have grown to be of great benefit for IR studies in heterogeneous catalysis. The identification of intermediates www.annualreviews.org • IR Detection of Catalytic Intermediates

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on surfaces is expected to become more reliant on computational methods (e.g., density functional theory) and other experimental analytical techniques as researchers continually move toward more complex reactions and chemical environments.

SUMMARY POINTS 1. IR techniques enable ex situ, in situ, and operando measurements of surface and gas phase reactants, intermediates, and products in heterogeneous catalytic processes. A large number of IR techniques have been developed to monitor catalytic reactions in a variety of environments (at both gas/solid interfaces and liquid/solid interfaces). Some advantages and disadvantages of the various IR techniques are discussed above. Annu. Rev. Phys. Chem. 2014.65:249-273. Downloaded from www.annualreviews.org by George Mason University on 06/12/14. For personal use only.

2. The bonding in and geometries of reactants, products, and intermediates determine their vibrational frequencies, thus enabling geometric information to be gleaned from IR measurements. 3. IR spectroscopy facilitates the elucidation of reaction mechanisms through a combination of the identification of intermediates, quantitative measurements, time-resolved measurements, isotopic labeling, and the introduction of proposed intermediates. IR spectroscopic techniques have played a role in identifying intermediates and elucidating the reaction mechanisms for the hydrogenation of organic double bonds on metals, deNOx chemistry, carbon monoxide oxidation, and other reactions. 4. Computational calculations of the stabilities and vibrational frequencies of intermediates are expected to be of increased importance in future studies.

FUTURE ISSUES 1. The techniques of PAS-IR and IRES appear to be underutilized at present. Further usage of these techniques for in situ and operando measurements may be of scientific benefit. 2. There is currently a lack of commercial high-temperature reactors for in situ or operando ATR measurements, and the development of such reactors would be beneficial for heterogeneous catalysis research.

DISCLOSURE STATEMENT The authors are not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review.

ACKNOWLEDGMENTS A. Savara thanks Alexandre Tkatchenko and Wei Liu for useful discussions regarding calculations of vibrational modes. Research at Oak Ridge National Laboratory was sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, US Department of Energy. Support of much of the work performed at Northwestern University that is discussed in this publication was supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Science, Office of Science, US Department of Energy under grant DE-FG02-03ER15457. 266

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150. Somorjai GA, Rupprechter G. 1999. Molecular studies of catalytic reactions on crystal surfaces at high pressures and high temperatures by infrared-visible sum frequency generation (SFG) surface vibrational spectroscopy. J. Phys. Chem. B 103:1623–38 151. Somorjai GA, Park JY. 2008. Molecular surface chemistry by metal single crystals and nanoparticles from vacuum to high pressure. Chem. Soc. Rev. 37:2155–62 152. de Aguiar HB, Scheu R, Jena KC, de Beer AGF, Roke S. 2012. Comparison of scattering and reflection SFG: a question of phase-matching. Phys. Chem. Chem. Phys. 14:6826–32 153. Foster AJ, Lobo RF. 2010. Identifying reaction intermediates and catalytic active sites through in situ characterization techniques. Chem. Soc. Rev. 39:4783–93 154. Bratlie K. 2003. High-pressure catalytic reactions of C6 hydrocarbons on platinum single-crystals and nanoparticles. PhD Diss., Univ. Calif., Berkeley 155. Gross A. 2008. Adsorption at nanostructured surfaces from first principles. J. Comput. Theor. Nanosci. 5:894–922 156. Haubrich J, Loffreda D, Delbecq F, Sautet P, Jugnet Y, et al. 2011. Mechanistic and spectroscopic identification of initial reaction intermediates for prenal decomposition on a platinum model catalyst. Phys. Chem. Chem. Phys. 13:6000–9 157. Calaza FC, Xu Y, Mullins DR, Overbury SH. 2012. Oxygen vacancy-assisted coupling and enolization of acetaldehyde on CeO2 (111). J. Am. Chem. Soc. 134:18034–45 158. Vayssilov GN, Mihaylov M, St. Petkov P, Hadjiivanov KI, Neyman KM. 2011. Reassignment of the vibrational spectra of carbonates, formates, and related surface species on ceria: a combined density functional and infrared spectroscopy investigation. J. Phys. Chem. C 115:23435–54 159. Jugnet Y, Bertolini JC, Barbosa LAMM, Sautet P. 2002. Vibrational identification of the surface reaction intermediates for the dehalogenation of trichloroethene on PdCu(110) alloy. Surf. Sci. 505:153–62 160. Liu W, Savara A, Ren XG, Ludwig W, Dostert KH, et al. 2012. Toward low-temperature dehydrogenation catalysis: isophorone adsorbed on Pd(111). J. Phys. Chem. Lett. 3:582–86 161. Stavitski E, Weckhuysen BM. 2010. Infrared and Raman imaging of heterogeneous catalysts. Chem. Soc. Rev. 39:4615–25 162. Snively CM, Oskarsdottir G, Lauterbach J. 2001. Chemically sensitive parallel analysis of combinatorial catalyst libraries. Catal. Today 67:357–68 163. Hendershot RJ, Fanson PT, Snively CM, Lauterbach JA. 2003. High-throughput catalytic science: parallel analysis of transients in catalytic reactions. Angew. Chem. Int. Ed. Engl. 42:1152–55 164. Xu XJG, Rang M, Craig IM, Raschke MB. 2012. Pushing the sample-size limit of infrared vibrational nanospectroscopy: from monolayer toward single molecule sensitivity. J. Phys. Chem. Lett. 3:1836–41

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Contents

Annual Review of Physical Chemistry Volume 65, 2014


A Journey Through Chemical Dynamics William H. Miller p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 1 Chemistry of Atmospheric Nucleation: On the Recent Advances on Precursor Characterization and Atmospheric Cluster Composition in Connection with Atmospheric New Particle Formation M. Kulmala, T. Petäjä, M. Ehn, J. Thornton, M. Sipilä, D.R. Worsnop, and V.-M. Kerminen p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p21 Multidimensional Time-Resolved Spectroscopy of Vibrational Coherence in Biopolyenes Tiago Buckup and Marcus Motzkus p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p39 Phase Separation in Bulk Heterojunctions of Semiconducting Polymers and Fullerenes for Photovoltaics Neil D. Treat and Michael L. Chabinyc p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p59 Nitrogen-Vacancy Centers in Diamond: Nanoscale Sensors for Physics and Biology Romana Schirhagl, Kevin Chang, Michael Loretz, and Christian L. Degen p p p p p p p p p p p p p83 Superresolution Localization Methods Alexander R. Small and Raghuveer Parthasarathy p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 107 The Structure and Dynamics of Molecular Excitons Christopher J. Bardeen p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 127 Advanced Potential Energy Surfaces for Condensed Phase Simulation Omar Demerdash, Eng-Hui Yap, and Teresa Head-Gordon p p p p p p p p p p p p p p p p p p p p p p p p p p p p 149 Ion Mobility Analysis of Molecular Dynamics Thomas Wyttenbach, Nicholas A. Pierson, David E. Clemmer, and Michael T. Bowers p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 175 State-to-State Spectroscopy and Dynamics of Ions and Neutrals by Photoionization and Photoelectron Methods Cheuk-Yiu Ng p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 197 Imaging Fluorescence Fluctuation Spectroscopy: New Tools for Quantitative Bioimaging Nirmalya Bag and Thorsten Wohland p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 225 v

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Elucidation of Intermediates and Mechanisms in Heterogeneous Catalysis Using Infrared Spectroscopy Aditya Savara and Eric Weitz p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 249 Physicochemical Mechanism of Light-Driven DNA Repair by (6-4) Photolyases Shirin Faraji and Andreas Dreuw p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 275 Advances in the Determination of Nucleic Acid Conformational Ensembles Lo¨ıc Salmon, Shan Yang, and Hashim M. Al-Hashimi p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 293 Annu. Rev. Phys. Chem. 2014.65:249-273. Downloaded from www.annualreviews.org by George Mason University on 06/12/14. For personal use only.

The Role of Ligands in Determining the Exciton Relaxation Dynamics in Semiconductor Quantum Dots Mark D. Peterson, Laura C. Cass, Rachel D. Harris, Kedy Edme, Kimberly Sung, and Emily A. Weiss p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 317 Laboratory-Frame Photoelectron Angular Distributions in Anion Photodetachment: Insight into Electronic Structure and Intermolecular Interactions Andrei Sanov p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 341 Quantum Heat Engines and Refrigerators: Continuous Devices Ronnie Kosloff and Amikam Levy p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 365 Approaches to Single-Nanoparticle Catalysis Justin B. Sambur and Peng Chen p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 395 Ultrafast Carrier Dynamics in Nanostructures for Solar Fuels Jason B. Baxter, Christiaan Richter, and Charles A. Schmuttenmaer p p p p p p p p p p p p p p p p p p 423 Nucleation in Polymers and Soft Matter Xiaofei Xu, Christina L. Ting, Isamu Kusaka, and Zhen-Gang Wang p p p p p p p p p p p p p p p p 449 H- and J-Aggregate Behavior in Polymeric Semiconductors Frank C. Spano and Carlos Silva p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 477 Cold State-Selected Molecular Collisions and Reactions Benjamin K. Stuhl, Matthew T. Hummon, and Jun Ye p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 501 Band Excitation in Scanning Probe Microscopy: Recognition and Functional Imaging S. Jesse, R.K. Vasudevan, L. Collins, E. Strelcov, M.B. Okatan, A. Belianinov, A.P. Baddorf, R. Proksch, and S.V. Kalinin p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 519 Dynamical Outcomes of Quenching: Reflections on a Conical Intersection Julia H. Lehman and Marsha I. Lester p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 537 Bimolecular Recombination in Organic Photovoltaics Girish Lakhwani, Akshay Rao, and Richard H. Friend p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 557 vi

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Mapping Atomic Motions with Ultrabright Electrons: The Chemists’ Gedanken Experiment Enters the Lab Frame R.J. Dwayne Miller p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 583 Optical Spectroscopy Using Gas-Phase Femtosecond Laser Filamentation Johanan Odhner and Robert Levis p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 605 Indexes


Cumulative Index of Contributing Authors, Volumes 61–65 p p p p p p p p p p p p p p p p p p p p p p p p p p p 629 Cumulative Index of Article Titles, Volumes 61–65 p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 632 Errata An online log of corrections to Annual Review of Physical Chemistry articles may be found at http://www.annualreviews.org/errata/physchem

Contents

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Annual Reviews It’s about time. Your time. It’s time well spent.

New From Annual Reviews:

Annual Review of Statistics and Its Application Volume 1 • Online January 2014 • http://statistics.annualreviews.org


Editor: Stephen E. Fienberg, Carnegie Mellon University

Associate Editors: Nancy Reid, University of Toronto Stephen M. Stigler, University of Chicago The Annual Review of Statistics and Its Application aims to inform statisticians and quantitative methodologists, as well as all scientists and users of statistics about major methodological advances and the computational tools that allow for their implementation. It will include developments in the field of statistics, including theoretical statistical underpinnings of new methodology, as well as developments in specific application domains such as biostatistics and bioinformatics, economics, machine learning, psychology, sociology, and aspects of the physical sciences.

Complimentary online access to the first volume will be available until January 2015. table of contents:

• What Is Statistics? Stephen E. Fienberg • A Systematic Statistical Approach to Evaluating Evidence from Observational Studies, David Madigan, Paul E. Stang, Jesse A. Berlin, Martijn Schuemie, J. Marc Overhage, Marc A. Suchard, Bill Dumouchel, Abraham G. Hartzema, Patrick B. Ryan

• High-Dimensional Statistics with a View Toward Applications in Biology, Peter Bühlmann, Markus Kalisch, Lukas Meier • Next-Generation Statistical Genetics: Modeling, Penalization, and Optimization in High-Dimensional Data, Kenneth Lange, Jeanette C. Papp, Janet S. Sinsheimer, Eric M. Sobel

• The Role of Statistics in the Discovery of a Higgs Boson, David A. van Dyk

• Breaking Bad: Two Decades of Life-Course Data Analysis in Criminology, Developmental Psychology, and Beyond, Elena A. Erosheva, Ross L. Matsueda, Donatello Telesca

• Brain Imaging Analysis, F. DuBois Bowman

• Event History Analysis, Niels Keiding

• Statistics and Climate, Peter Guttorp

• Statistical Evaluation of Forensic DNA Profile Evidence, Christopher D. Steele, David J. Balding

• Climate Simulators and Climate Projections, Jonathan Rougier, Michael Goldstein • Probabilistic Forecasting, Tilmann Gneiting, Matthias Katzfuss • Bayesian Computational Tools, Christian P. Robert • Bayesian Computation Via Markov Chain Monte Carlo, Radu V. Craiu, Jeffrey S. Rosenthal • Build, Compute, Critique, Repeat: Data Analysis with Latent Variable Models, David M. Blei • Structured Regularizers for High-Dimensional Problems: Statistical and Computational Issues, Martin J. Wainwright

• Using League Table Rankings in Public Policy Formation: Statistical Issues, Harvey Goldstein • Statistical Ecology, Ruth King • Estimating the Number of Species in Microbial Diversity Studies, John Bunge, Amy Willis, Fiona Walsh • Dynamic Treatment Regimes, Bibhas Chakraborty, Susan A. Murphy • Statistics and Related Topics in Single-Molecule Biophysics, Hong Qian, S.C. Kou • Statistics and Quantitative Risk Management for Banking and Insurance, Paul Embrechts, Marius Hofert

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Understanding the mechanism of catalytic fast pyrolysis by unveiling reactive intermediates in heterogeneous catalysis.

X-ray spectroscopy for chemical and energy sciences: the case of heterogeneous catalysis.

Reactive intermediates in (4)He nanodroplets: infrared laser Stark spectroscopy of dihydroxycarbene.

Identification of different forms of cocaine and substances used in adulteration using near-infrared Raman spectroscopy and infrared absorption spectroscopy.

Herbal tea identification using mid-infrared spectroscopy.

Detection and accumulation of tetrahedral intermediates in elastase catalysis.

Discrimination and content analysis of fritillaria using near infrared spectroscopy.

Structures of aspartic acid-96 in the L and N intermediates of bacteriorhodopsin: analysis by Fourier transform infrared spectroscopy.

Investigating lignin key features in maize lignocelluloses using infrared spectroscopy.

Immunoglobulin G measurement in blood plasma using infrared spectroscopy.

Molecular identification in metabolomics using infrared ion spectroscopy.

Assessment of hyaline cartilage matrix composition using near infrared spectroscopy.

Identifications of household's spores using mid infrared spectroscopy.

Rapid monitoring of grape withering using visible near-infrared spectroscopy.

NMR spectroscopy in the structural elucidation of oligosaccharides and glycosides.

Infrared spectroscopy of DNA.

Infrared and Near-Infrared Spectroscopy of Acetylacetone and Hexafluoroacetylacetone.

Combining Raman and infrared spectroscopy as a powerful tool for the structural elucidation of cyclodextrin-based polymeric hydrogels.

Indirect absorption spectroscopy using quantum cascade lasers: mid-infrared refractometry and photothermal spectroscopy.

Gold π-Complexes as Model Intermediates in Gold Catalysis.

Enhancing the Responsivity of Uncooled Infrared Detectors Using Plasmonics for High-Performance Infrared Spectroscopy.

Fully Automated Lipid Pool Detection Using Near Infrared Spectroscopy.

Surface-enhanced infrared spectroscopy using nanometer-sized gaps.

Detecting concealed information using functional near-infrared spectroscopy.