Applications of Single-site Photocatalysts to the Design of Unique Surface Functional Materials

The isolated and tetrahedrally coordinated metal oxide (Ti, V, Cr, Mo and W-oxides) moieties can be included in the silica matrixes of silica-based microporous zeolite and mesoporous silica materials and named as “single-site photocatalysts”. Under UV-light irradiation these single-site photocatalysts form the charge transfer excited state, i.e., the excited electron–hole pair state which is located quite near to each other in different from the manner observed on semiconducting materials such as TiO₂, and play a significant role in various photocatalytic reactions. These single-site photocatalysts not only can promote photocatalytic reactions but also can be utilized to synthesis of functional materials. The nano-sized metal catalyst and visible-light sensitive binary oxide photocatalyst can be synthesized on the excited single-site photocatalyst under UV-light irradiation. The transparent mesoporous silica thin film with single-site photocatalyst generates the super-hydrophilic surface. In this review, our recent applications of single-site photocatalysts to synthesis of the surface functional materials have been introduced.     

Atomically Dispersed Metal Sites in MOF‐Based Materials for Electrocatalytic and Photocatalytic Energy Conversion

Metal sites play an essential role in both electrocatalytic and photocatalytic energy conversion. The highly ordered arrangements of the organic linkers and metal nodes as well as the well‐defined pore structures of metal‐organic frameworks (MOFs) make them ideal substrates to support atomically dispersed metal sites (ADMSs) located in their metal nodes, linkers, and pores. Porous carbon materials doped with ADMSs can be derived from these ADMS‐incorporating MOF precursors through controlled treatments. These ADMSs incorporated in pristine MOFs and MOF‐derived carbon materials possess unique advantages over molecular or bulk metal‐based catalysts and bridge the gap between homogeneous and heterogeneous catalysts for energy‐conversion applications. This Review presents recent progress in the design and incorporation of ADMSs in MOFs and MOF‐derived materials for energy‐conversion applications.     

Monitoring surface metal oxide catalytic active sites with Raman spectroscopy

The molecular aspect of the Raman vibrational selection rules allows for the molecular structural and reactivity determinations of metal oxide catalytic active sites in all types of oxide catalyst systems (supported metal oxides, zeolites, layered hydroxides, polyoxometalates (POMs), bulk pure metal oxides, bulk mixed oxides and mixed oxide solid solutions). The molecular structural and reactivity determinations of metal oxide catalytic active sites are greatly facilitated by the use of isotopically labeled molecules. The ability of Raman spectroscopy to (1) operate in all phases (liquid, solid, gas and their mixtures), (2) operate over a very wide temperature (-273 to >1000 °C) and pressure (UHV to ?100 atm) range, and (3) provide molecular level information about metal oxides makes Raman spectroscopy the most informative characterization technique for understanding the molecular structure and surface chemistry of the catalytic active sites present in metal oxide heterogeneous catalysts. The recent use of hyphenated Raman spectroscopy instrumentation (e.g., Raman-IR, Raman-UV-vis, Raman-EPR) and the operando Raman spectroscopy methodology (e.g., Raman-MS and Raman-GC) is allowing for the establishment of direct structure-activity/selectivity relationships that will have a significant impact on catalysis science in this decade. Consequently, this critical review will show the growth in the use of Raman spectroscopy in heterogeneous catalysis research, for metal oxides as well as metals, is poised to continue to exponentially grow in the coming years (173 references).     

二氧化钛负载单原子催化剂用于光催化反应的研究

Titania (TiO₂) has been among the most widely investigated and used metal oxides over the past years, as it has various functional applications. Extensive research into TiO₂ and industrial interest in this material have been triggered by its high abundance, excellent corrosion resistance, and low cost. To improve the activity of TiO₂ in heterogeneous catalytic reactions, noble metals are used to accelerate the reactions. However, in the case of nanoparticles supported on TiO₂, the active sites are usually limited to the peripheral sites of the noble metal particles or at the interface between the particle and the support. Thus, highly dispersed single metal atoms are desired for the effective utilization of precious noble metals. The study of oxide-supported isolated atoms, the so-called single-atom catalysts (SACs), was pioneered by Zhang's group. The high dispersion of precious noble metals results helps reduce the cost associated with catalyst preparation. Because of the presence of active centers as single atoms, the deactivation of metal atoms during the reaction, e.g., by coking for large agglomerates, is retarded. The unique coordination environment of the noble metal center provides special sites for the reaction, consequently increasing the selectivity of the reaction, including the enantioselectivity and stereoselectivity. Hence, supported SACs can bridge homogenous and heterogeneous reactions in solution as they provide selective reaction sites and are recyclable. Moreover, owing to the high site homogeneity of the isolated metal atoms, SACs are ideal models for establishing the structure-activity relationships. The present review provides an overview of recent works on the synthesis, characterization, and photocatalytic applications of SACs (Pt₁, Pd₁, Ir₁, Rh₁, Cu₁, Ru₁) supported on TiO₂. The preparation of single atoms on TiO₂ includes the creation of surface defective sites, surface modification, stabilization by high-temperature shockwave treatment, and metal-ligand self-assembly. Conventional characterization methods are categorized as microscopic imaging and spectroscopic methods, such as aberration-corrected scanning transmission electron microscopy (STEM), scanning tunneling microscopy (STM), extended X-ray absorption fine structure analysis (EXAFS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). We attempted to address the critical factors that lead to the stabilization of single-metal atoms on TiO₂, and elucidate the mechanism underlying the photocatalytic hydrogen evolution and CO₂ reduction. Although many fascinating applications of TiO₂-supported SACs in photocatalysis could only be addressed superficially and in a referencing manner, we hope to provide interested readers with guidelines based on the wide literature, and more specifically, to provide a comprehensive overview of TiO₂-supported SACs.     

Photocatalysis on titanium oxide catalysts: Approaches in achieving highly efficient reactions and realizing the use of visible light

The photocatalytic reactivities of various Ti-based photocatalysts have been investigated for various different types of reactions in order to achieve as highly efficient photocatalytic reactivities as possible. The reactivity of powdered TiO2 is dramatically enhanced by the addition of small amounts of Pt which initiates an effective charge separation of the photo-formed electrons and holes. The highly dispersed titanium oxide species prepared and encapsulated within the zeolite cavities as well as into the SiO 2 matrices exhibit high photocatalytic reactivities due to the high reactivities of their charge transfer excited states. With regard to the use of visible light, ion implantation of metal ions such as Cr or V into powdered TiO2 catalysts has been found to modify the electronic state of TiO 2 , resulting in the shift of the absorption band to longer wavelength regions, i.e., into the visible light region. The extent of the shift strongly depends on the type and concentration of the implanted metal. The present study focuses on the preparation of the photocatalysts, a detailed characterization of the active sites and their dynamics, the direct detection of the reaction intermediate species, as well as a clarification of the mechanisms behind the observed photocatalytic reactions at the molecular level. This work significantly contributes to advances in the design of photocatalysts which will be able to operate efficiently and effectively not only under UV irradiation but, most ideally, under visible light.     

Maximizing the Number of Interfacial Sites in Single‐Atom Catalysts for the Highly Selective,Solvent‐Free Oxidation of Primary Alcohols

The solvent‐free selective oxidation of alcohols to aldehydes with molecular oxygen is highly attractive yet challenging. Interfacial sites between a metal and an oxide support are crucial in determining the activity and selectivity of such heterogeneous catalysts. Herein, we demonstrate that the use of supported single‐atom catalysts (SACs) leads to high activity and selectivity in this reaction. The significantly increased number of interfacial sites, resulting from the presence of individually dispersed metal atoms on the support, renders SACs one or two orders of magnitude more active than the corresponding nanoparticle (NP) catalysts. Lattice oxygen atoms activated at interfacial sites were found to be more selective than O₂ activated on metal NPs in oxidizing the alcohol substrate. This work demonstrates for the first time that the number of interfacial sites is maximized in SACs, providing a new avenue for improving catalytic performance by developing appropriate SACs for alcohol oxidation and other reactions occurring at metal–support interfacial sites.     

Determination of the chemical nature of active surface sites present on bulk mixed metal oxide catalysts

CH3OH temperature programmed surface reaction (TPSR) spectroscopy was employed to determine the chemical nature of active surface sites for bulk mixed metal oxide catalysts. The CH3OH-TPSR spectra peak temperature, Tp, for model supported metal oxides and bulk, pure metal oxides was found to be sensitive to the specific surface metal oxide as well as its oxidation state. The catalytic activity of the surface metal oxide sites was found to decrease upon reduction of these sites and the most active surface sites were the fully oxidized surface cations. The surface V5+ sites were found to be more active than the surface Mo6+ sites, which in turn were significantly more active than the surface Nb5+ and Te4+ sites. Furthermore, the reaction products formed also reflected the chemical nature of surface active sites. Surface redox sites are able to liberate oxygen and yield H2CO, while surface acidic sites are not able to liberate oxygen, contain either H+ or oxygen vacancies, and produce CH3OCH3. Surface V5+, Mo6+, and Te4+ sites behave as redox sites, and surface Nb5+ sites are Lewis acid sites. This experimental information was used to determine the chemical nature of the different surface cations in bulk Mo-V-Te-Nb-Ox mixed oxide catalysts (Mo(0.6)V(1.5)Ox, Mo(1.0)V(0.5)Te(0.16)Ox, Mo(1.0)V(0.3)Te(0.16)Nb(0.12)Ox). The bulk Mo(0.6)V(1.5)Ox and Mo(1.0)V(0.5)Te(0.16)Ox mixed oxide catalytic characteristics were dominated by the catalytic properties of the surface V5+ redox sites. The surface enrichment of these bulk mixed oxide by surface V5+ is related to its high mobility, V5+ possesses the lowest Tammann temperature among the different oxide cations, and the lower surface free energy associated with the surface termination of V=O bonds. The quaternary bulk Mo(1.0)V(0.3)Te(0.16)Nb(0.12)Ox mixed oxide possessed both surface redox and acidic sites. The surface redox sites reflect the characteristics of surface V5+ and the surface acidic sites reflect the properties normally associated with supported Mo6+. The major roles of Nb5+ and Te4+ appear to be that of ligand promoters for the more active surface V and Mo sites. These reactivity trends for CH3OH ODH parallel the reactivity trends of propane ODH because of their similar rate-determining step involving cleavage of a C-H bond. This novel CH3OH-TPSR spectroscopic method is a universal method that has also been successfully applied to other bulk mixed metal oxide systems to determine the chemical nature of the active surface sites.     

Maximizing the Number of Interfacial Sites in Single‐Atom Catalysts for the Highly Selective,Solvent‐Free Oxidation of Primary Alcohols

The solvent‐free selective oxidation of alcohols to aldehydes with molecular oxygen is highly attractive yet challenging. Interfacial sites between a metal and an oxide support are crucial in determining the activity and selectivity of such heterogeneous catalysts. Herein, we demonstrate that the use of supported single‐atom catalysts (SACs) leads to high activity and selectivity in this reaction. The significantly increased number of interfacial sites, resulting from the presence of individually dispersed metal atoms on the support, renders SACs one or two orders of magnitude more active than the corresponding nanoparticle (NP) catalysts. Lattice oxygen atoms activated at interfacial sites were found to be more selective than O₂ activated on metal NPs in oxidizing the alcohol substrate. This work demonstrates for the first time that the number of interfacial sites is maximized in SACs, providing a new avenue for improving catalytic performance by developing appropriate SACs for alcohol oxidation and other reactions occurring at metal–support interfacial sites.     

Single-site heterogeneous catalysts

Intellectually, the advantages that flow from the availability of single-site heterogeneous catalysts (SSHC) are many. They facilitate the determination of the kinetics and mechanism of catalytic turnover-both experimentally and computationally-and make accessible the energetics of various intermediates (including short-lived transition states). These facts in turn offer a rational strategic principle for the design of new catalysts and the improvement of existing ones. It is generally possible to prepare soluble molecular fragments that circumscribe the single-site, thus enabling a direct comparison to be made, experimentally, between the catalytic performance of the same active site when functioning as a heterogeneous (continuous solid) as well as a homogeneous (dispersed molecular) catalyst. This approach also makes it possible to modify the immediate atomic environment as well as the central atomic structure of the active site. From the practical standpoint, SSHC exhibit very high selectivities leading to the production of sharply defined molecular products, just as do their homogeneous analogues. Given that mesoporous silicas with very large internal surface areas are ideal supports for SSHC, and that more than a quarter of the elements of the Periodic Table may be grafted as active sites onto such silicas, there is abundant scope for creating new catalytic opportunities.     

A study of the surface region of the Mo-V-Te-O catalysts for propane oxidation to acrylic acid

The bulk mixed Mo-V-Te oxides possess high activity and selectivity in propane oxidation to acrylic acid and represent well-defined model catalysts for studies of the surface molecular structure-activity/selectivity relationships in this selective oxidation reaction. The elemental compositions, metal oxidation states, and catalytic functions of V, Mo, and Te in the surface region of the model Mo-V-Te-O system were examined employing low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS). This study indicated that the surfaces of these catalysts are terminated with a monolayer, which possesses a different elemental composition from that of the bulk. The rates of propane consumption and formation of propylene and acrylic acid depended on the topmost surface V concentration, whereas no dependence of these reaction rates on either the surface Mo or Te concentrations was observed. These findings suggested that the bulk Mo-V-Te-O structure may function as a support for the unique active and selective surface monolayer in propane oxidation to acrylic acid. The results of this study have important practical consequences for the development of improved selective oxidation catalysts by introducing surface metal oxide components to form new surface active V-O-M sites for propane oxidation to acrylic acid.     

Evidence for crystal-face-dependent TiO2 photocatalysis from single-molecule imaging and kinetic analysis

According to the concept of active sites, the activity of heterogeneous catalysts correlates with the number of available catalytic sites and the binding affinity of the substrates. Herein, we report a single-molecule, single-particle fluorescence approach to elucidate the inherent photocatalytic activity of exposed surfaces of anatase TiO(2), a promising photocatalyst, using redox-responsive fluorogenic dyes. A single-molecule imaging and kinetic analysis of the fluorescence from the products shows that reaction sites for the effective reduction of the probe molecules are preferentially located on the {101} facets of the crystal rather than the {001} facets with a higher surface energy. This surprising discrepancy can be explained in terms of face-specific electron-trapping probability. In situ observation of the catalytic events occurring at the solid/solution interfaces reveals the hidden role of the crystal facets in chemical reactions and their impact on the efficiency and selectivity of heterogeneous (photo)catalysts.     

Highly Dense Isolated Metal Atom Catalytic Sites: Dynamic Formation and In Situ Observations

Atomically dispersed noble‐metal catalysts with highly dense active sites are promising materials with which to maximise metal efficiency and to enhance catalytic performance; however, their fabrication remains challenging because metal atoms are prone to sintering, especially at a high metal loading. A dynamic process of formation of isolated metal atom catalytic sites on the surface of the support, which was achieved starting from silver nanoparticles by using a thermal surface‐mediated diffusion method, was observed directly by using in situ electron microscopy and in situ synchrotron X‐ray diffraction. A combination of electron microscopy images with X‐ray absorption spectra demonstrated that the silver atoms were anchored on five‐fold oxygen‐terminated cavities on the surface of the support to form highly dense isolated metal active sites, leading to excellent reactivity in catalytic oxidation at low temperature. This work provides a general strategy for designing atomically dispersed noble‐metal catalysts with highly dense active sites.     

均质固体催化剂

最重要的固体催化剂可分成两类:一类通常被假定是多相和多组分的,例如金属和合金催化剂,它们的作用机理还难于阐明,另一类则是单相的,同时,它们的晶体结构明确,分子筛、层柱粘土、杂多酸以及某些复合氧化物等属此。它们的活性中心被假定均匀地分散于整个催化剂体相之中。这一观点是通过寻求与酶催化剂在结构上和催化功能上的类似点而得出的。     

负载型催化剂的表面相互作用及其在大气分子污染物NO和CO消除中的应用基础研究

负载型催化剂具有重要的应用背景,研究表面分散组分与载体之间的相互作用.对于了解催化过程的本质,进而设计开发高效实用的催化剂均有重要的指导意义.本课题组近年来在该领域中进行了一些探索研究,涉及的催化剂以负载型金属氧化物(如CuO/γ-Al_2O_3,CuO/Ce_xZr_(1-x)O_2和CuO/Ti_xSn_(1-x)O_2等)为主,涉及的反应包括CO完全氧化和NO+CO反应.通过改变活性组分的负载量、添加改性剂和改变样品制备条件等制得一系列样品,用多种固体催化剂表征手段考察各组分在催化剂中的作用以及在上述催化反应中的活性变化规律.研究表明:(1)金属氧化物和卤化物等离子化合物在氧化物载体表面的分散容量和分散后的一些性质均可从"嵌入模型"的考虑得到解释;(2)处于不同载体表面的活性物种或同一载体表面不同结构的活性物种,由于其存在状态的差异使得其氧化还原性质和催化性质不同;(3)在相关催化剂体系中进行的CO完全氧化和NO+CO反应的结果显示,通过探索催化剂的"组成-结构-性质"间的关系,有可能为实际催化剂的设计提供理论参考.     

In situ generation of active sites in olefin metathesis

The depth of our understanding in catalysis is governed by the information we have about the number of active sites and their molecular structure. The nature of an active center on the surface of a working heterogeneous catalyst is, however, extremely difficult to identify and precise quantification of active species is generally missing. In metathesis of propene over dispersed molybdenum oxide supported on silica, only 1.5% of all Mo atoms in the catalyst are captured to form the active centers. Here we combine infrared spectroscopy in operando with microcalorimetry and reactivity studies using isotopic labeling to monitor catalyst formation. We show that the active Mo(VI)-alkylidene moieties are generated in situ by surface reaction of grafted molybdenum oxide precursor species with the substrate molecule itself gaining insight into the pathways limiting the number of active centers on the surface of a heterogeneous catalyst. The active site formation involves sequential steps requiring multiple catalyst functions: protonation of propene to surface Mo(VI)-isopropoxide species driven by surface Br?nsted acid sites, subsequent oxidation of isopropoxide to acetone in the adsorbed state owing to the red-ox capability of molybdenum leaving naked Mo(IV) sites after desorption of acetone, and oxidative addition of another propene molecule yielding finally the active Mo(VI)-alkylidene species. This view is quite different from the one-step mechanism, which has been accepted in the community for three decades, however, fully consistent with the empirically recognized importance of acidity, reducibility, and strict dehydration of the catalyst. The knowledge acquired in the present work has been successfully implemented for catalyst improvement. Simple heat treatment after the initial propene adsorption doubled the catalytic activity by accelerating the oxidation and desorption-capturing steps, demonstrating the merit of knowledge-based strategies in heterogeneous catalysis. Molecular structure of active Mo(VI)-alkylidene sites derived from surface molybdena is discussed in the context of similarity to the highly active Schrock-type homogeneous catalysts.     

Grafted metallocalixarenes as single-site surface organometallic catalysts

Metallocalixarenes were grafted onto silica using a surface organometallic approach and shown to be active and selective catalysts for epoxidation of alkenes using organic hydroperoxides. Calixarene-Ti(IV) precursors were anchored at surface densities from 0.1 to near-monolayer coverages (0.025-0.25 calixarene nm(-2)). Several spectroscopic methods independently detected calixarene-Ti(IV) connectivity before and after epoxidation catalysis. Kinetic analyses of cyclohexene epoxidation confirmed that the active sites were anchored on the silica surface and were significantly more active than their homogeneous analogues. The steric bulk and multidentate binding of the calixarenes led to structural stability and to single-site behavior during epoxidation catalysis. Rate constants were independent of surface density for cyclohexene epoxidation with tert-butyl hydroperoxide (11.1 +/- 0.3 M(-2) s(-1)) or cumene hydroperoxide (25 +/- 2 M(-2) s(-1)). The materials and methods reported here allow the assembly of robust surface organometallic structures in which the active sites behave as isolated species, even near saturation monolayer coverages. In turn, this makes possible the rational design and synthesis of a class of heterogeneous oxide catalysts with atomic-scale precision at the active site.     

以一氧化二氮或氧气为氧化剂的丙烯环氧化催化反应

丙烯环氧化制环氧丙烷是催化领域的最关键的挑战之一. 本文对作者等近年来开展的以一氧化二氮为氧化剂的铁催化体系和以氧气为氧化剂的铜催化体系的研究进展进行了综述. 在这两类催化体系中,碱金属离子(特别是K )的修饰作用均是获得较高环氧丙烷选择性的关键. 碱金属离子通过调变催化剂中铁或铜物种的分散度、配位环境和酸碱性等,实现了对反应途径的调控,使反应朝着有利于环氧丙烷生成的方向进行. 活性金属组分(铁或铜)与氧化剂(一氧化二氮或氧气)间的特定的组合对丙烯环氧化反应的发生也起着重要作用. 推测在两类催化体系中,氧化剂均在具有特定结构和价态的铁或铜活性位上活化,产生导致丙烯环氧化反应发生的亲电性活性氧物种.     

High resolution solid state NMR spectroscopy in surface organometallic chemistry: access to molecular understanding of active sites of well-defined heterogeneous catalysts

Recent advances in solid state NMR spectroscopy have made possible the understanding of surface species and active sites of heterogeneous catalysts at a molecular level. This tutorial review describes solid state NMR spectroscopy, what are the possible techniques to obtain high resolution and 2D spectra (structural information), and what are their applications in the context of well-defined heterogeneous catalysts prepared by surface organometallic chemistry.     

UV-VIS-NIR spectroscopy and microscopy of heterogeneous catalysts

This critical review article discusses the characterization of heterogeneous catalysts by UV-VIS-NIR spectroscopy and microscopy with special emphasis on transition metal ion containing catalysts. A review is given of the transitions, that can be observed in the UV-VIS-NIR region and the peculiarities of catalytic solids that have to be taken into account. This is followed by a short discussion of the techniques that have been developed over the years: diffuse reflectance spectroscopy, UV-VIS microscopy, in situ or operando spectroscopy, the combination of UV-VIS spectroscopy with other spectroscopic techniques, with chemometrics and with quantum chemistry. In the third part of this paper four successes of UV-VIS-NIR spectroscopy and microscopy are discussed; (1) coordination of transition metal ions to surface oxygens; (2) quantitative determination of the oxidation states of transition metal ions; (3) characterization of active sites and (4) study of the distribution of transition metal ions and carbocations in catalytic bodies, particles and crystals (104 references).