Springer Handbook of Advanced Catalyst Characterization

Inhaltsverzeichnis

1. Frontmatter

2. Vibrational Spectroscopy

   1. Frontmatter

   2. 1. Infrared (IR) Spectroscopy

      Guido Busca

      Abstract

      The application of infrared (IR) spectroscopic methods in the field of heterogeneous catalysis research and development, mainly based on the original work of the author’s group, is summarized. The fundamentals of vibrational spectroscopy are briefly described. The application of infrared spectroscopy in the catalyst skeletal region for structural characterization is considered. The spectra of pure powder-pressed discs of catalytic materials based on oxides or supported with insulating carriers are described. In particular, data arising from the spectra in the OH stretching region (surface and bulk hydroxyl groups) are considered. The absorptions due to the vibrations of surface metal-oxygen bonds are discussed. The use of infrared spectroscopy of surface molecular probes for the characterization of surface acid sites both of the Lewis and the Brønsted type is considered. The application of molecular probes for the infrared characterization of surface basicity and nucleophilicity is also taken into account. Finally, the use of probes for the characterization of the redox state of the surface as well as of the size/shape of metal nanoparticles is also considered. The application of IR spectroscopy for the analysis of reaction products is also briefly discussed.

   3. 2. Case Studies: Infrared (IR) Spectroscopy

      Chiara Negri, Michele Carosso, Eleonora Vottero, Elena Groppo, Silvia Bordiga

      Abstract

      The chapter illustrates the role of FT-IR spectroscopy in clarifying the nature of the surface species formed on a catalyst in reaction conditions and their behavior as a function of the reaction variables (such as the temperature and the reactants’ concentration). In particular, two case studies have been selected, belonging to two different categories of heterogeneous catalysts: a Cu exchanged chabazite zeolite (Cu-CHA), which can be classified as a single-site catalyst; and a Pt/Al₂O₃ catalyst, where the active Pt phase is in the form of nanoparticles and hence is an example of multisite catalyst. Taken together, the two case studies demonstrate the potentials of FT-IR spectroscopy in catalysis, provided that the experiments (either in situ or operando) are properly designed by tuning the reaction conditions.

   4. 3. Reflection Absorption Infrared Spectroscopy

      Ravi Ranjan, Michael Trenary

      Abstract

      Recent case studies are presented that illustrate the capabilities of the technique of reflection absorption infrared spectroscopy (RAIRS) for probing the structure and surface chemistry of model metal catalysts. A key feature of RAIRS is the polarization dependence of the interaction of infrared radiation with surface vibrations. Only the component of polarization parallel to the plane of incidence (p-polarization) interacts with molecules on the metal surface, whereas surface vibrations are invisible to light polarized perpendicular to the plane of incidence (s-polarization). Since both polarizations interact with gas-phase molecules, the polarization dependence of the spectra allows surface vibrations to be distinguished from those of gas-phase species. This is the basis for using RAIRS to study surfaces under ambient pressure conditions, in addition to studies in ultrahigh vacuum. The case studies presented include the use of RAIRS of CO to probe the structure of surfaces, the hydrogenation of acetylene, the activation of CO₂ by H₂O on a ZrO₂ thin-film grown on a Pt₃Zr(0001) surface, and the formation of hydrogen-bonded clusters of methanol on a Pd(111) surface.

   5. 4. Raman Spectroscopy

      Jisue Moon, Meijun Li, Anibal J. Ramirez-Cuesta, Zili Wu

      Abstract

      Raman spectroscopy is one of the mostly utilized optical spectroscopic tools for revealing both the catalyst structure and surface chemistry in heterogeneous catalysis. It has recently seen increasing role in catalysis research, thanks to the development of new Raman instrumentations, reactors, and combination with other techniques, leading to in situ and operando studies with significant temporal and spatial resolutions. This chapter aimed to provide a general overview of the applications of Raman spectroscopy in heterogeneous catalysis. It starts with an introduction to the fundamentals of Raman scattering including theory and pros and cons for catalysis research; followed by a description of the typical setup of a Raman system and recent advances in Raman instrumentations; then a chronology of the applications of Raman spectroscopy for ex situ, in situ, and operando studies of catalysis; elucidations of the advances in improving the temporal and spatial resolution of Raman spectroscopy of catalysis; Raman application case studies related to catalyst synthesis, treatments, and function under reaction conditions; illustrations of the power of multimodal approach including Raman spectroscopy in catalysis research; and ended with a brief summary and a future outlook.

   6. 5. Case Studies: Raman Spectroscopy

      Ragamaye Tigiripalli, Vishal Agarwal, Goutam Deo

      Abstract

      The application of Raman spectroscopy to heterogeneous catalysts has undergone several changes over the years. Based on the information collected during characterization of the catalysts, these changes can be categorized into the three generations of applications. In the first-generation of applications, information from Raman spectroscopy is used to characterize the catalysts when the environment surrounding the catalyst is not well controlled, for example, ambient conditions. This is also referred to as ex situ characterization. The second-generation applications are those where the vibrational information from the catalysts under controlled environments or in situ studies is examined, and the nature of the solid catalyst and gas-solid catalyst interactions are deciphered. Finally, the third-generation applications involve the simultaneous use of in situ Raman spectroscopy and reaction data to directly correlate the structure and reactivity information of the working heterogeneous catalysts. Such studies are referred to as operando Raman spectroscopy. More recently, Raman spectroscopy has been coupled with other characterization techniques to obtain additional information simultaneously, which are then correlated with the reactivity data. Furthermore, computational techniques coupled with operando Raman and other techniques are becoming in vogue for a more comprehensive understanding of the catalytic system. In this chapter, we primarily take up two case studies. In one, we highlight the three generations of application of Raman spectroscopy, and in the other, we show the importance of using density functional theory coupled with Raman spectroscopy to identify the molecular nature of the catalyst. The proper applications of Raman spectroscopy to heterogeneous catalysis is faced with several challenges. However, the future looks bright, and it is expected that we will continue to see a significant increase in research in this exciting area.

   7. 6. Ultraviolet (UV) Raman Spectroscopy

      Peter C. Stair

      Abstract

      Using an ultraviolet laser to excite Raman scattering has the benefit of avoiding fluorescence interference and increasing Raman intensity via resonance enhancement. The high photon energy inherent at ultraviolet wavelengths requires special precautions for sample handling to minimize transformations caused by laser-induced heating and photochemistry. This chapter begins by covering the basic theory of resonance-enhanced Raman spectroscopy and the instrumentation for making measurements with a special focus on sample handling and in situ reaction cells. The remainder of the chapter summarizes studies of catalyst synthesis, catalyst deactivation by coke formation, and catalytic metal oxide speciation. The identification and appearance of resonance-enhanced Raman scattering and how it can be exploited are emphasized.

   8. 7. Surface Enhanced Raman Spectroscopy (SERS)

      Ramón A. Alvarez-Puebla

      Abstract

      SERS is a spectroscopic technique that combines modern laser spectroscopy with the exciting optical properties (localized surface plasmon resonances, LSPRs) of metallic nanostructures that result in strongly increased Raman signals when molecules are attached to nanometer-sized metallic structures. Such enhancement can be over 12 orders of magnitude with respect to the signal obtained by normal Raman scattering. Because the registered signal is essentially a vibrational spectrum and, therefore, it contains all the information of the molecular system studied, SERS spectroscopy is considered as a powerful analytical technique. Since its discovery in the 1970s, SERS has attracted much interest in various fields of physics, chemistry, biology, and medicine.

   9. 8. Nanoscale Raman Spectroscopy

      Tanja Deckert-Gaudig, Marie Richard-Lacroix, Volker Deckert

      Abstract

      Heterogeneous metal-catalysis plays a significant role in organic synthesis as it lowers the activation barrier necessary to initiate a chemical reaction. In this field, mainly thermally activated catalysts are established whereby unwanted by-products are often an issue. Photocatalysts can be a promising alternative, especially, if they can be activated with visible light under ambient conditions. To promote the application and development of this rather young class of catalysts, a detailed understanding of their surface structure, processes at the interface, reaction kinetics and mechanisms on the atomic and molecular level is of great importance. Photocatalysts based on noble metal nanoparticles like gold and silver are highly attractive candidates in this field. When irradiated with the appropriate wavelength, resonant light absorption of the nanoparticles can lead to the excitation of localized surface plasmons (LSPs) and the generation of a highly amplified electromagnetic field in their immediate vicinity. This can be exploited to initiate charge-driven reactions with an energy transfer between metal and adsorbate as the so-called “hot electrons” or “hot holes.” (Zhang et al., Acc Chem Res 52:2506–2515, 2019; Wang et al., Appl Mat Today 15:305–314, 2019; Ren et al., RSC Adv 7:31189–31203, 2017; Linic et al., Nat Mater 14:567–576, 2015). Still, there are many fundamental questions unresolved, such as the catalyst-molecule electronic interactions and transfer and the structural (re)-organization of the adsorbates on the catalytic surface. Since the general aspects of plasmon-activated catalysis are discussed in another chapter of this book, in this chapter we focus mainly on the high lateral resolution aspects. The consequences for spectroscopy when using single plasmon particles as analytical probes will be considered specifically in the respective topics. If necessary, specific properties of the probe composition and its influence on the spectra and on the overall results of the reactions are also discussed.

   10. 9. Operando Electrochemical Raman Spectroscopy

       Wolfgang Schuhmann, Denis Öhl, Dulce M. Morales

       Abstract

       Advances in nanoscience and particularly in the fabrication of sophisticated nanostructures have led to substantial progress in the design of catalytic materials. Probing species at an aqueous interface during electrochemical reactions poses a great challenge to experimentalists and has raised considerable attention over the past decades, which is especially intriguing in the field of electrocatalysis. Several approaches for coupling electrochemical measurements to spectroscopic characterization of the electrode itself have been established and such experimental setups now represent state-of-the-art analytical techniques. Alongside with such developments a substantial improvement of already existing technologies for the analysis of nanostructures has been achieved, rendering them suitable to address the aforementioned challenges. A prime example is the case of Raman spectroscopy, which has been substantially developed in recent years toward the study of adsorbed molecules and/or the state of a material during catalytic reactions.

       Boosted by the discovery of the significant signal enhancement upon using nanostructured metals as substrates, Raman spectroscopy has evolved to become an appropriate tool for allowing the detection of Raman-active species in very small concentrations. Moreover, its versatility regarding operating conditions led to the development of operando electrochemical Raman spectroscopy (OERS), namely, Raman spectroscopic investigations of the electrochemical interfaces while electrochemical conversion under a controlled reaction rate is proceeding. Being electrochemists by training, we focus in this contribution on reactions linked to challenges in modern electrochemistry such as water electrolysis and CO₂ conversion, and we highlight recent studies providing a comprehensive overview of developments important to the field. We aim to familiarize the reader with theoretical background of electrochemistry and Raman spectroscopy, and we broadly illustrate the implications of OERS for the understanding of fundamental electrochemical reactions and the elucidation of reaction mechanisms.

   11. 10. Sum Frequency Generation (SFG) Spectroscopy

       Verena Pramhaas, Günther Rupprechter

       Abstract

       Infrared-visible sum frequency generation (SFG) laser spectroscopy is an interface-specific nonlinear optical method that provides vibrational spectra of molecules located at surfaces or interfaces. Provided the interface is accessible to light, SFG allows identifying chemical species, molecular densities, and molecular orientations, with sub-picosecond time resolution. Resulting from its selection rules, SFG is typically not sensitive to bulk or isotropic phases, making the method highly interface-specific. SFG can thus be utilized to investigate buried interfaces in gaseous or liquid environments.

3. Electron and Photoelectron Spectroscopy

   1. Frontmatter

   2. 11. Ultraviolet-Visible (UV-Vis) Spectroscopy

      Charlotte Vogt, Caterina Suzanna Wondergem, Bert M. Weckhuysen

      Abstract

      Ultraviolet-Visible (UV-Vis) spectroscopy is a versatile and powerful analytical method, which allows to investigate a wide variety of catalysts in both the liquid-phase and solid-state and their interfaces at elevated temperatures and pressures. In the case of solid catalysts, they can be studied in the form of powders (e.g., in diffuse reflectance mode) and as thin wafers (in transmission mode), and when combined with a microscope even in the form of catalyst bodies (e.g., extrudates) and single crystals. In the past two decades, UV-Vis spectroscopy has been increasingly used under in situ and operando conditions to shed light on/gain insight in the working principles of heterogeneous catalysts, homogeneous catalysts, electrocatalysts, as well as photocatalysts. One of the advantages of this method is that it can simultaneously measure, e.g., the electronic transitions of organic molecules (mainly via their n → π* and π → π* transitions) and transition metal oxides or ions (via their d-d and charge transfer transitions). Unfortunately, absorption bands in the UV-Vis range are often broad and overlapping and hence their interpretations are not always trivial. Advanced theoretical calculations are required to provide a proper foundation of their interpretation, while, e.g., chemometrics can help prevent biased analysis when many (time-resolved) spectra are collected. Finally, UV-Vis spectroscopy is often combined with other analytical methods to provide complementary information. Examples include X-ray absorption spectroscopy and diffraction, next to vibrational spectroscopy (i.e., infrared and Raman) and magnetic resonance (i.e., electron spin resonance and nuclear magnetic resonance) methods. The above-described scientific and instrumental developments will be illustrated by using a selection of showcase examples, covering the different areas of catalysis. The chapter concludes with some main observations as well as some future developments on what might become possible in the not-too-distant future.

   3. 12. Case Studies: Ultraviolet-Visible (UV-Vis) Spectroscopy

      Zixu Yang, Minghui Zhu

      Abstract

      Ultraviolet-visible (UV-Vis) spectroscopy has been demonstrated by scientists to be a powerful tool for exploring the reaction mechanism in heterogeneous catalysis. This chapter aims to review the fundamentals of UV-Vis spectroscopy along with the measurements and data interpretation involved in its application. The advantages of using this spectroscopic technique in investigating the chemical reactions behind a plethora of functional catalysts has been depicted by individual case studies of reactions having great significance in the field of renewable energy and environmental remediation. These reactions include selective catalytic reduction of NO_x by vanadium/copper-based catalysts, dehydrogenation of propane by supported catalysts, electroreduction of CO₂ by molecular catalysts and methanol-to-olefin conversion by zeolite catalysts. The advent of in situ spectroscopy with high spatiotemporal resolution has made it easier to examine the precursor, structure, and oxidation state of catalytic active sites and coke species under various conditions. This chapter concludes by identifying the opportunities for improving the efficiency of UV-Vis spectroscopy method in the rational design of next-generation heterogeneous catalysts.

   4. 13. Fluorescence Microscopy

      Xianwen Mao, Rong Ye, Peng Chen

      Abstract

      Heterogeneous catalysts exhibit intrinsic heterogeneities, both structurally and compositionally. For example, the size and shape of nanoparticle catalysts often show dispersions, and could also change over time during reactions. Therefore, it is important to study heterogeneous catalysts with experimental tools that allow in situ, real-time, spatially resolved characterization of catalytic activities. Single-molecule fluorescence microscopy has recently emerged as a powerful tool with the abovementioned capabilities. In this chapter, we discuss the development and application of single-molecule fluorescence microscopy for characterizations of heterogeneous catalysts at the single-particle to subparticle level, covering topics ranging from the static/dynamic activity heterogeneities of individual catalyst particles and subparticle regions, to the scale-up ability of catalyst screening, and to the catalysis cooperativity between spatially distinct locations.

   5. 14. Photoluminescence (PL) Spectroscopy

      Qinghe Li, Masakazu Anpo, Jinmao You, Tingjiang Yan, Xinchen Wang

      Abstract

      This chapter deals with the fundamental of the photoluminescence (PL) spectroscopy and its applications to study the chemical reaction of molecules on solid surfaces and the reactivity of various heterogeneous solid catalysts in relation to their properties in adsorption, catalysis, and photocatalysis. After a short introduction, the basic principles of PL spectroscopy are explained in relation to the definitions of fluorescence and phosphorescence. And, the PL features of the semiconducting catalysts are discussed in relation to the surface band structures. Next, the practical aspects of static and dynamic PL with the spectral parameters including wavelength and spectral shape, lifetime and the Stern-Volmer expression, energy transfer and migration, and ultrafast time-resolved PL spectroscopy are discussed. In Sect. 14.4, which is one of the cores of this chapter, the characterization of the catalytically active sites by applying in situ PL spectroscopy are discussed with various single-sites heterogeneous catalysts such as Ti-oxide, V-oxide, and Mo-oxide single-site containing catalysts, and carbon containing catalysts such as polymeric carbon nitride. In Sect. 14.5, characterization of acidic and basic surface sites is discussed by means of luminescence probe molecules and in situ PL spectroscopy. In Sect. 14.6, in situ PL studies are discussed in relation to the photocatalytic reaction processes on inorganic and organic semiconducting catalysts. Especially, photo-generation of electron and holes, their lifetimes, and reaction dynamics are discussed. In Sect. 14.7, effects of temperature on PL spectra of their intensity and wavelength are discussed. In Sect. 14.8, effect of magnetic fields on PL spectra are discussed. Section 14.9 is the conclusion and outlook of the PL spectroscopy in catalysis and photocatalysis.

   6. 15. Case Studies: Photoluminescence (PL) Spectroscopy

      Lorenzo Mino, Masaya Matsuoka, Gianmario Martra

      Abstract

      The chapter presents two representative case studies, which highlight the potentialities of photoluminescence (PL) spectroscopy for the study of photocatalytic systems at the molecular level in their working state. The first part of the chapter is devoted to the investigation of charge carrier dynamics in semiconductor photocatalysts (TiO₂ and g-C₃N₄) by steady-state and ultrafast time-resolved PL, focusing also on the interaction with the semiconductor surface of key reactants in photocatalytic processes. The second part, on the other hand, describes single-site photocatalytic systems based on transition metal ions dispersed on silica supports, discussing in particular PL studies of NO photo-reduction by CO on Mo⁶⁺/SiO₂ and preferential photo-oxidation of CO by O₂ on Cr⁶⁺/MCM-41.

   7. 16. Near Ambient Pressure (NAP) X-Ray Photoelectron Spectroscopy (XPS)

      Zongyuan Liu, Sanjaya Senanayake, José A. Rodriguez

      Abstract

      Near atmospheric pressure (NAP)-XPS is a technique frequently used for the characterization of catalysts under working conditions and in the study of the surface chemistry associated with catalytic process at solid-gas interfaces. In the last 10 years, NAP-XPS has been used to study the adsorption/desorption of many common molecules (CO, CO₂, NO, H₂O, CH₃OH, CH₄, C₂H₄, C₆H₆, etc.) and a wide set of catalytic reactions (CO oxidation, NO reduction, forward and reverse water-gas shift reaction, CO and CO₂ hydrogenation to methanol or methane, methane dry reforming, methane conversion to methanol, hydrogenation of olefins, desulfurization, etc.). It has provided a fundamental understanding of chemical phenomena associated with the manipulation of C-H, C-O and C-S bonds on model or powder catalysts.

   8. 17. Case Studies: Near Ambient Pressure (NAP) X-Ray Photoelectron Spectroscopy (XPS)

      Franklin Tao, Yu Tang

      Abstract

      Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) is a characterization method of a catalyst in gas phase toward uncovering surface of the catalyst under a reaction or catalytic condition which is inaccessible with high vacuum-based X-ray photoelectron spectroscopy (XPS). It has brought significant value to fundamental understanding of catalytic reactions through establishing a direct correlation between a catalyst surface and its corresponding catalytic performance. It uncovers surface chemistry including constituting elements, composition, oxidation state, chemical environment, and electronic state of a specific type of atoms under a reaction condition or during catalysis and tracks evolution of a catalyst surface driven by a perturbation such as variation of catalysis temperature, increase of reactant pressure, or change of gas composition of reactant mixture under catalytic conditions. With AP-XPS a temperature-dependent evolution of a catalyst surface, a stable intermediate formed at a relatively low temperature, a surface spectator, or a deactivated surface formed at high temperature can be readily identified. By tracking surface chemistry of a catalyst in different reactive environment, reaction-driven restructuring of a catalyst can be observed. Here three cases of AP-XPS studies were discussed toward demonstrating how AP-XPS has been one of the major methods in revealing catalyst surfaces during catalysis.