Structural Dynamics of Dispersed Titania During Dehydration and Oxidative Dehydrogenation Studied by In Situ UV Raman Spectroscopy

Surface oxygen vacancy engineering on TiO2 (101) via ALD technology for simultaneously enhancing charge separation and transfer

Titanium oxide molecular layers containing extensive SOV content (11.4-16.2%) have been constructed on (101) TiO₂ nanotubes through a precisely controlled atomic layer deposition technique, in which the charge separation efficiency and surface charge transfer efficiency are increased to 28.2% and 89.0%, respectively, about 17 and 2 times those of the initial TiO₂ nanotubes.

Rethinking the molecular structures of WVIOx sites dispersed on titania: distinct mono-oxo configurations at 430 °C and temperature-dependent transformations

The structural properties of the (WO_x)_n phase dispersed on TiO₂ (P25, anatase) at surface densities of 0.5-4.5 W nm^-2 (i.e. up to approximately a monolayer) were explored by using in situ Raman and FTIR spectroscopy, in situ Raman/¹⁸O exchange and Raman spectroscopy in static equilibrium at temperatures of 175-430 °C. Deciphering the temperature and coverage dependence of the spectral features under oxidative dehydration conditions showed that (i) the (WO_x)_n dispersed phase is heterogeneous at 430 °C consisting of two distinct mono-oxo species: Species-I with C_3v-like OW(-O-)₃ configuration (WO mode at 1009-1014 cm^-1) and Species-II with C_4v-like OW(-O-)₄ configuration (WO mode at 1003-1009 cm^-1); (ii) the OW(-O-)₃ site is formed with first order of priority and its formation ceases after the complete consumption of the most basic hydroxyls that are titrated first, hence is abundant at low coverage (<1.5 W nm^-2); (iii) the OW(-O-)₄ site prevails over the OW(-O-)₃ site at medium to high coverage (≥2 W nm^-2) and partially occurs in associated (polymerized) coverages above 2 W nm^-2; (iv) lowering the temperature in the 430 → 250 → 175 °C sequence does not affect the structural and vibrational properties of OW(-O-)₃ but leads to the gradual transformation of the OW(-O-)₄ site to a di-oxo (O)₂W(-O-)₃ site (with a symmetric stretching mode at ∼985 cm^-1) and the partial association of adjacent OW(-O-)₄ units. All temperature-dependent structural/configurational transformations are fully reversible in the 430-175 °C range and are interpreted at the molecular level by a mechanism involving water molecules retained at the surface that act in a reversible temperature-dependent mediative manner resulting in hydroxylation (upon cooling, e.g. to 250 °C) and dehydroxylation (upon heating, e.g. to 430 °C). The Raman spectra obtained for the hydroxyl region confirm the successive hydroxylation/dehydroxylation steps during temperature cycles (e.g. 430 → 250 → 430 °C). One can tune the speciation of the dispersed (WO_x)_n phase under dehydrated conditions by appropriate control of temperature and coverage.

Identification and classification of particle contaminants on photomasks based on individual-particle Raman scattering spectra and SEM images

Raman spectroscopy was used for the detection chemical composition of particle contamination on photomasks. Particle types and sources were identified and classified according to the Raman spectra of individual particles.

New advances in using Raman spectroscopy for the characterization of catalysts and catalytic reactions

Gaining insight into the mode of operation of heterogeneous catalysts is of great scientific and economic interest. Raman spectroscopy has proven its potential as a powerful vibrational spectroscopic technique for a fundamental and molecular-level characterization of catalysts and catalytic reactions. Raman spectra provide important insight into reaction mechanisms by revealing specific information on the catalysts' (defect) structure in the bulk and at the surface, as well as the presence of adsorbates and reaction intermediates. Modern Raman instrumentation based on single-stage spectrometers allows high throughput and versatility in design of in situ/operando cells to study working catalysts. This review highlights major advances in the use of Raman spectroscopy for the characterization of heterogeneous catalysts made during the past decade, including the development of new methods and potential directions of research for applying Raman spectroscopy to working catalysts. The main focus will be on gas-solid catalytic reactions, but (photo)catalytic reactions in the liquid phase will be touched on if it appears appropriate. The discussion begins with the main instrumentation now available for applying vibrational Raman spectroscopy to catalysis research, including in situ/operando cells for studying gas-solid catalytic processes. The focus then moves to the different types of information available from Raman spectra in the bulk and on the surface of solid catalysts, including adsorbates and surface depositions, as well as the use of theoretical calculations to facilitate band assignments and to describe (resonance) Raman effects. This is followed by a presentation of major developments in enhancing the Raman signal of heterogeneous catalysts by use of UV resonance Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS), and shell-isolated nanoparticle surface-enhanced Raman spectroscopy (SHINERS). The application of time-resolved Raman studies to structural and kinetic characterization is then discussed. Finally, recent developments in spatially resolved Raman analysis of catalysts and catalytic processes are presented, including the use of coherent anti-Stokes Raman spectroscopy (CARS) and tip-enhanced Raman spectroscopy (TERS). The review concludes with an outlook on potential future developments and applications of Raman spectroscopy in heterogeneous catalysis.

High Surface Area VOx/TiO2/SBA-15 Model Catalysts for Ammonia SCR Prepared by Atomic Layer Deposition

The mode of operation of titania-supported vanadia (VOx) catalysts for NOx abatement using ammonia selective catalytic reduction (NH3-SCR) is still vigorously debated. We introduce a new high surface area VOx/TiO2/SBA-15 model catalyst system based on mesoporous silica SBA-15 making use of atomic layer deposition (ALD) for controlled synthesis of titania and vanadia multilayers. The bulk and surface structure is characterized by X-ray diffraction (XRD), UV-vis and Raman spectroscopy, as well as X-ray photoelectron spectroscopy (XPS), revealing the presence of dispersed surface VOx species on amorphous TiO2 domains on SBA-15, forming hybrid Si–O–V and Ti–O–V linkages. Temperature-dependent analysis of the ammonia SCR catalytic activity reveals NOx conversion levels of up to ~60%. In situ and operando diffuse reflection IR Fourier transform (DRIFT) spectroscopy shows N–Hstretching modes, representing adsorbed ammonia and -NH2 and -NH intermediate structures on Bronsted and Lewis acid sites. Partial Lewis acid sites with adjacent redox sites are proposed as the active sites and desorption of product molecules as the rate-determining step at low temperature. The high NOx conversion is attributed to the presence of highly dispersed VOx species and the moderate acidity of VOx supported on TiO2/SBA-15.