XAFS and UV Vis

XAFS analysis and UV-vis spectroscopy can be applied to get information about the local structure and state of photocatalytic active phases or species. 

Hsu et al. [55] studied a core-shell Ti02/Fe304 C catalyst during gas-phase photocatalytic degradation of trichloroethylene (TCE) by in situ X-ray absorption near-edge structural (XANES) analysis with a home-made photoreaction cell using a 300 W xenon lamp. 

Despite the importance of solid-state nuclear magnetic resonance (NMR) spectroscopy for the characterization of solid catalysts, in situ studies related to photocatalysts are rare. Mills and O Rourke [59] monitored the selective photooxidation of toluene by in situ NMR using an NMR tube as the photoreactor. A Ti02 precursor paste was prepared by hydrolysis of titanium propoxide and following treatment at 228 °C. The obtained anatase-type titania was mixed with poly(vinyl alcohol). The obtained paste was coated on the walls of the NMR tube, rotated over night and calcined. In parallel, batch experiments were carried out. The reaction mixture containing the catalyst was directly placed into the tube, which was irradiated outside the spectrometer and then inserted into the NMR spectrometer. 

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Using fiber optics, Groothaert et al. (2003a) monitored the decomposition of NO and of N2O on CuZSM-5. A bis(/i-oxo)dicopper species, [Cu2(iu-0)2]2+, which had been previously identified by XAFS and UV-vis analysis (Groothaert et al., 2003b), was proposed to be a key intermediate and 02-releasing species. Although no quantitative correlation was established between decomposition rate and intensity of the absorption band of the copper species, the performance appeared to be related to the intensity of this band. 

This group has further developed multitechnique cells, and designed a cell for SAXS/WAXS and XAFS, and SAXS/WAXS/XAFS and UV-vis spectroscopy (Beale et al., 2006 Grandjean et al., 2005). They reported use of this cell to investigate hydrothermal crystallization processes of inorganic catalysts such as CoAPO-5. The synthesis cell design was simple in concept essentially a mini-autoclave, with a usable volume of 2 mm3, and mica windows, heated by an insulated aluminum block. 

P-26 - Structure of Mo species incorporated into SBA-1 and SBA-3 studied by XAFS and UV-VIS spectroscopies... 

In this section, Ti/Si and Ti/B binary oxide thin films were prepared by the ICB method using multi-ion sources. Characterization studies were carried out by spectroscopic measurements such as XRD, XAFS and UV-Vis, in order to elucidate the local structure of the Ti oxide species of these thin films. The photocatalytic reactivity was also evaluated for significant reactions such as the decomposition of NO to produce N2 and O2 as well as N2O as a minor product under UV light irradiation. Moreover, the photoinduced super-hydrophilic properties were evaluated by measuring the changes in the contact angle of water droplets under UV irradiation and under dark conditions at 298 K. 

Extending the equipment, the authors (Beale et al., 2005) recently added energy dispersive X-ray absorption spectroscopy (XAS). Raman and UV-vis spectra are recorded by illuminating opposite sides of a catalyst bed in a vertical tubular reactor and detecting the scattered and reflected light as described above. XAS is performed in the same horizontal plane but in transmission and with the beam orthogonal to the incident radiation of the other two methods. Example spectra were recorded for samples at 823 K. A combination of UV-vis (fiber optics) and XAFS spectroscopy for investigation of solids has also been described by Jentoft et al. (2004), who reported UV-vis measurements of samples at 773 K. 

TG/DTA, TPR, and complementary techniques for characterizing catalysts in the working state (e.g., XRD Raman, IR, and UV-vis spectroscopies) can provide structural and metal valence information under reaction conditions. However, the capability of TR-XAFS spectroscopy to reveal quantitative phase composition and average metal valence together with the evolution of the local structure of a catalyst under varying (reaction) conditions, combined with a time resolution of 100 ms will continue to be a very powerful tool for kinetics investigations in solid-state chemistry and heterogeneous catalysis. 

Caetano, B.L., Santilli, C.V., Meneau, F., Briois, V., and Pulcinelli, S.H. (2011) In situ and simultaneous UV-vis/SAXS and UV-vis/XAFS time-resolved monitoring of ZnO quantum dots formation and growth. /. Phys. Chem. C, 115, 4404-4412. 

Groothaert et al., using operando UV-vis spectroscopy combined with online GC analysis [176] and operando X-ray absorption fine structure (XAFS) [177], presented the first experimental evidence for the formation of the bis( x-oxo)dicopper core in Cu-ZSM-5 and for its key role of intermediate in the sustained high activity of Cu-ZSM-5 in the direct decomposition of NO into N2 and 02. In particular, monitoring the catalytic conversion of NO and N20 above 673 K, they found that the bis( x-oxo)dicopper core is formed by the O abstraction of the intermediate N20 (Figure 4.14). Subsequently,... 

The three-coordinated unsaturated structure of the Ru complex remained unchanged after 1000 cycles of the stilbene epoxidation as suggested by XPS, DR-UV/Vis and XAFS. The unsaturated Ru complex 5 is active for the epoxidation reaction. Notably, the three-coordinate Ru complex 5 is quite stable under the reaction conditions and also in air despite its unsaturated structure. This remarkable stability and durability made the immobilized catalyst 5 recyclable for the catalytic reactions, maintaining 100% conversion with selectivity higher than 80% [17]. 

Spectroscopy produces spectra which arise as a result of interaction of electromagnetic radiation with matter. The type of interaction (electronic or nuclear transition, molecular vibration or electron loss) depends upon the wavelength of the radiation (Tab. 7.1). The most widely applied techniques are infrared (IR), Mossbauer, ultraviolet-visible (UV-Vis), and in recent years, various forms ofX-ray absorption fine structure (XAFS) spectroscopy which probe the local structure of the elements. Less widely used techniques are Raman spectroscopy. X-ray photoelectron spectroscopy (XPS), secondary ion imaging mass spectroscopy (SIMS), Auger electron spectroscopy (AES), electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopy. 

Beale AM, van der Eerden AMJ, Kervinen K, Newton MA, Weckhuysen BM. Adding a third dimension to operando spectroscopy a combined UV-Vis, Raman and XAFS setup to study heterogeneous catalysts under working conditions. Chem Commun. 2005 3015. 

Weckhuysen and coworkers (Mesu et al., 2005 Tinnemans et al., 2006) also presented a combination of UV-vis (fiber optics) and XAFS (energy dispersive) spectroscopy for the analysis of homogeneous liquid-phase reactions. Both methods are performed in transmission, with the beams crossing in a cuvette (5-mm path length each) with quartz windows. 

While XAFS spectroscopy is a powerful catalyst characterization method it is clear that XAFS spectroscopy combined with other complementary techniques offers the possibility of providing a more complete understanding of catalyst structure. The designs of cells that allow both XAFS and XRD data to be collected have already been described (above). More recently, the combination of two, or even three, other complementary techniques to XAFS spectroscopy has now been successfully demonstrated. Beale et al. (2005) briefly described a design combining UV-vis... 

FIGURE 38 Photographs and schematic details of the combined UV—vis/Raman/XAFS cell of Tinnemans et al. (2006). Reprinted from (Tinnemans et al., 2006), Copyright 2006, with permission from Elsevier. 

In addition to the structure in the dehydrated state, the structure of supported vanadia catalysts under redox reaction conditions is directly related to the catalytic performance. Vanadia catalysts are usually reduced to some extent during a redox reaction, and the reduced vanadia species have been proposed as the active sites [4, 19-24]. Therefore, information on the valence state and molecular structure of the reduced vanadia catalysts is of great interest. A number of techniques have been applied to investigate the reduction of supported vanadia catalysts, such as temperature programmed reduction (TPR) [25-27], X-ray photoelectron spectroscopy (XPS) [21], electron spin resonance (ESR) [22], UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) [18, 28-32], X-ray absorption fine structure spectroscopy (XAFS) [11] and Raman spectroscopy [5, 26, 33-41]. Most of these techniques give information only on the oxidation state of vanadium species. Although Raman spectroscopy is a powerful tool for characterization of the molecular structure of supported vanadia [4, 29, 42], it has been very difficult to detect reduced supported... 

J. J. Bravo-Suarez, K. K. Bando, J. Lu, M. Haruta, T. Fujitani, S. T. Oyama, Identification of true reaction intermediates in propylene epoxidation on gold/titanosilicate catalysts by in situ UV-vis and XAFS Spectroscopies, J. Phys. Chem. 112 (2008) 1115. 

The present study deals with the distinct characteristics of Ti-oxides incorporated within zeolite frameworks (TS-1, TS-2, Ti-MCM-41, Ti-MCM-48) or on the surface of supports (Ti/FSM-16, TiA ycor glass) and their photocatalytic reactivity including then-selectivity for the NO decomposition reaction as well as for the reduction of CO2 with H2O at the molecular level using various in situ spectroscopic techniques such as photoluminescence, UV-Vis, XAFS (XANES and EXAFS) and ESR spectroscopy. 

In this chapter, the local structures of the transition metal oxides, Ti, V, Mo, and Cr oxide single-site species, incorporated within zeolites or mesoporous sUica framework structures as well as the local structures of transition metal ions such as Ag" " exchanged into zeohte cavities were discussed, based on the results of various in situ spectroscopic investigations such as ESR, UV-Vis, photoluminescence and XAFS (XANES and EXAFS). 

In the case of Ag, probably one of the promoter cations most widely studied, UV-vis, and XAFS techniques were able to indicate that isolated or highly dispersed Ag(I) cations yielded highly active and selective SCR catalysts. This was also a conclusion indirectly extracted by the fact that intermediate loadings of Ag were in fact adequate in order to optimize activity and that addition of Cs also produces further stabilization of Ag(I) cations under reaction conditions. However, the exact nature of the active phase was not revealed until recently when a XAFS (XANES and EXAFS) study was able to provide some further evidence. The XANES spectra of two active catalysts containing 1.5 and 4.5 wt%, and of Ag(I) containing reference samples are shown in Fig. 10.8(a). 

Many combined setups have been developed in the past decades to study catalyst synthesis and reaction processes, many of which employ synchrotron radiation. Perhaps the first example of a successful combination of two techniques is X-ray absorption spectroscopy (XAFS) and diffraction, which was soon followed by the combination of SAXS and WAXS. Other examples in which two techniques have been combined to study systems under reaction include XAFS/FTIR, XRD/Raman, XAFS/UV-Vis, and a number of setups that use non-X-ray based radiation, such as UV-Vis/Raman, FTIR/UV-Vis, NMR/UV-Vis, and EPR/UV-Vis. A number of reports have recently appeared in which the number of combined techniques has been increased to three, including SAXS/WAXS/XAFS, UV-Vis/Raman/XAFS (148), and EPR/UV-Vis/Raman (218), and SAXS/WAXS/XAFS (219). In what follows, we illustrate the power three-in-one in situ spectroscopic methods to unravel chemistry of catalytic solids. 

Propane Dehydrogenation over Supported Molybdenum Catalysts. The combined energy-dispersive (ED)-XAFS, UV-Vis, and Raman represents a powerful device that couples three spectroscopic techniques in one reactor, which probes the same part of a metal oxide catalyst under true reaction conditions and is capable of delivering subsecond time resolution. A scheme of the setup is given in Figure 32. 

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