Raman under reaction conditions

The vibrations of molecular bonds provide insight into bonding and stmcture. This information can be obtained by infrared spectroscopy (IRS), laser Raman spectroscopy, or electron energy loss spectroscopy (EELS). IRS and EELS have provided a wealth of data about the stmcture of catalysts and the bonding of adsorbates. IRS has also been used under reaction conditions to follow the dynamics of adsorbed reactants, intermediates, and products. Raman spectroscopy has provided exciting information about the precursors involved in the synthesis of catalysts and the stmcture of adsorbates present on catalyst and electrode surfaces. 

In the case of selective oxidation catalysis, the use of spectroscopy has provided critical Information about surface and solid state mechanisms. As Is well known( ), some of the most effective catalysts for selective oxidation of olefins are those based on bismuth molybdates. The Industrial significance of these catalysts stems from their unique ability to oxidize propylene and ammonia to acrylonitrile at high selectivity. Several key features of the surface mechanism of this catalytic process have recently been descrlbed(3-A). However, an understanding of the solid state transformations which occur on the catalyst surface or within the catalyst bulk under reaction conditions can only be deduced Indirectly by traditional probe molecule approaches. Direct Insights Into catalyst dynamics require the use of techniques which can probe the solid directly, preferably under reaction conditions. We have, therefore, examined several catalytlcally Important surface and solid state processes of bismuth molybdate based catalysts using multiple spectroscopic techniques Including Raman and Infrared spectroscopies, x-ray and neutron diffraction, and photoelectron spectroscopy. 

Figure 55.4 compares the Raman spectra of the two samples spectra were recorded at 380°C in a 15% O2/N2 stream, on equilibrated catalysts downloaded after reaction. Catalyst VN 1.06 was not oxidized in the air stream, whereas in the case of catalyst PA 1.00 bands typical of a phosphate, ai-VOP04, appeared in the spectrum. These bands were not present in the spectmm of the equilibrated catalyst recorded at room temperature. Indeed, the spectra of the two equilibrated catalysts were quite similar when recorded at room temperature. This result confirms that the surface of catalyst VN 1.06 is less oxidizable than that of catalyst PA 1.00. Therefore, the latter is likely more oxidized than the former one under reaction conditions. A treatment in a more oxidant atmosphere than the reactive n-butane/air feed modifies the surface of catalyst VN 1.06, and leads to the unsteady behavior shown in Figure 55.1. The same treatment did not alter the surface of the equihbrated catalyst P/V 1.00 that was already in an oxidized state under reaction conditions.

A strong point of Raman spectroscopy for research in catalysis is that the technique is highly suitable for in situ studies. The spectra of adsorbed species interfere weakly with signals from the gas phase, enabling studies under reaction conditions to be performed. A second advantage is that typical supports such as silica and alumina are weak Raman scatterers, with the consequence that adsorbed species can be measured at frequencies as low as 50 cm-1. This makes Raman... 

The idea of an one-center template mechanism was initially supported by first-order kinetics in iron. Moreover, intermediates 3a, 43 and 44 ands also their transition states in the catalytic cycle (Scheme 8.18) were proved by computational studies [71]. Moreover, mass spectrometric (ESI) [72] and spectroscopic (EXAFS and Raman) studies indicated complex 45 with two equatorial [3-diketonate ligands to be the catalytically active species in solution (Scheme 8.19) [73]. Actually, 4equiv. of FeCl3-6H20 are needed to generate 1 equiv. of complex 45 under reaction conditions ... 

Finally, we have not discussed cases where Raman spectroscopy can be used to study catalysts indirectly, as for example, by extracting a sample from a reactor and preparing a KBr disc for IR or Raman investigation. Such techniques may be useful in special circumstances (50) but have limited applicability with regard to the direct examination of surfaces under reaction conditions. 

Figure 3 illustrates the concepts of Raman cells that can be used for experiments under reaction conditions. Several commercial cells are suitable for use in combination with Raman microscopy. The rotating sample design was modified by Wachs s group (Figure 3A, Banares et al., 1994) and used to investigate supported oxides during selective alkane oxidation (Banares et al., 2000C Guliants et al., 1995 Sim et al., 1997) and various catalysts...

FIGURE 4 Designs of Raman cells that, for selected reactions, deliver catalyst performance data corresponding to those of an ideal reactor, (A) C.G. Hill [Adapted from Snyder T.P., and Hill C.G.,/ Catal. 132, 536 (1991) Stability of Bismuth Molybdate Catalysts at Elevated-Temperatures in Air and under Reaction Conditions, copyright (1991), with permission from Elsevier (377)], (B) G. Mestl [source, M.A. Banares],... 

FIGURE 13 Raman-GC recorded during propene oxidation on bismuth molybdate with simultaneous activity measurement (A) Raman spectra (Reprinted from J. Catal. 132,536 (1991), Snyder T.P., Hill C.G., Stability of bismuth molbydate catalysts at elevated temperatures in air and under reaction conditions, copyright (1991) with permission from Elsevier) (Snyder and Hill, 1991) (B) simultaneous conversion and selectivity (based on Snyder and Hill, 1991). 

Thus, heteropoly acids may not be stable under reaction conditions (Mestl et al., 2001). This statement is in line with the results of Raman investigations of supported /J-silicomolybdic acid that unambiguously demonstrated its decomposition to surface molybdena during methane oxidation (Banares et al., 1995). It was also shown that silica-supported silicomolybdic acid and silica-supported molybdenum surface oxide species with the same molybdenum loadings performed identically (Banares et al., 1995). Thus, the presence of any heteropoly acid structure during high-temperature oxidation can be ruled out. 

These results led to questioning of the presumed similar performance of polymeric and isolated surface species. Raman spectroscopy and UV-vis DRS investigations of zirconia- and alumina-supported vanadia showed that the oxidation state of the surface vanadium oxide species was determined by the propane-to-C>2 ratio in the feed (Garcra-Cortez and Banares, 2002). The combination of two spectroscopic techniques provided more detail about the structural state of the supported species during moderate reduction under reaction conditions (Gao et al., 2002). 

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. 

Raman spectroscopy is one of the most versatile spectroscopies for the characterization of solid catalyst surfaces and of surface species under reaction conditions. Banares and Mestl provide an in-depth description of catalytic reaction cells that allow recording of Raman spectra simultaneously with measurements of catalytic activities and selectivities. The authors discuss the advanced modem equipment and methodologies that permit the detection of Raman spectra at elevated pressures and temperatures (>1270 K) with good time resolution and spatial resolution (Raman microscopy). Measurements can be made during catalyst... 

In addition to the capability of Raman spectroscopy in determining the coordination numbers and bond lengths of molybdate species in bismuth molybdate phases, Raman spectroscopy can also be used as an in situ probe for the bismuth molybdates under reaction conditions. In situ Raman studies have been earned out, for example, on the 6-Bi2Mo209 phase under redox conditions, where insights into the surface mechanism of the... 

A prerequisite for the development indicated above to occur, is a parallel development in instrumentation to facilitate both physical and chemical characterization. TEM and SPM based methods will continue to play a central role in this development, since they possess the required nanometer (and subnanometer) spatial resolution. Optical spectroscopy using reflection adsorption infrared spectroscopy (RAIRS), polarization modulation infrared adsorption reflection spectroscopy (PM-IRRAS), second harmonic generation (SFIG), sum frequency generation (SFG), various in situ X-ray absorption (XAFS) and X-ray diffraction spectroscopies (XRD), and maybe also surface enhanced Raman scattering (SERS), etc., will play an important role when characterizing adsorbates on catalyst surfaces under reaction conditions. Few other methods fulfill the requirements of being able to operate over a wide pressure gap (to several atmospheres) and to be nondestructive. 

Under reaction conditions with the coexistence of ozone and ethanol, the intensities of both these adsorbed species dramatically decreased, indicating that these two species reacted with each other on the catalyst surface. This was also supported by the transient experiment results. When ozone was introduced on a surhice preadsorhed with ethoxide species, the intend of the ethoxide ecies decreased gradually due to the reaction with ozone (gas phase or adsorbed), and that of the peroxide species increased with time due to the removal of ethoxide ecies from the surface. However, if the reaction of ethoxide ecies was mainly due to gas phase ozone, under steady state conditions, the surfrce ould he covered by adsorbed peroxide ecies. The in situ Raman ectra indicated that the reaction of ethoxide species was primarily due to reaction with adsorbed peroxide species because the concentratiorts of both adsorbed ecies decreased dramatically in the presence of both ethanol and ozone. Thus, a Langmuir-ICnshelwood type mechanism appears to be operating ... 

Raman Spectroscopy Used to study the spectral features of solar fuel materials under reaction conditions. 

Raman spectroscopy is a powerful technique for characterization of solids and surfaces, and is well suited for examining oxides and supported oxide catalysts. Over the past few years, our group successfully examined a number of catalytic systems using UV Raman spectroscopy. The use of UV excitation prevents fluorescence from the Raman spectra by exciting the sample at a frequency where fluorescence does not occur. In this study, Raman spectra were obtained from the 1% chromium on alumina catalyst during exposure to propane or propene under reaction conditions. Calcined catalysts were compared to catalysts activated in hydrogen. 

Silver catalysts have been used for the partial oxidation of methanol to formaldehyde this is a very important process in the chemical industry. The role of the silver catalyst and, in particular, the influence of its atomic structure on the catalytic process have been extensively studied with various surface science tools [44—50]. In these investigations, Raman spectroscopy was employed to identify and confirm the role of the oxygen species for the catalytic process. These studies were performed under reaction conditions close to those in industrial processes using Ag(lll) and Ag(llO) samples. Upon extended exposure to oxygen at high temperatures, both samples restructure to (111) planes with a well-defined microstructure and with mesoscopic roughness (on a scale of 1 pm). Therefore, in the course of the oxygen pretreatment, the local nature of the surface of the two samples becomes nearly identical and, hence, their Raman spectra are quite similar [44]. 

Resonance Raman spectroscopy in the spedation of molecular catalysts under reaction conditions... 

Among the multiple spectroscopic techniques that can provide information about the catalytic active sites under reaction conditions (Raman, IR, UV-vis, X-ray absorption (EXAFS (extended X-ray absorption fine structure)/XANES (X-ray absorption near edge structures)), nuclear magnetic resonance (NMR), electron sprin resonance (ESR), etc.), Raman spectroscopy is the technique of choice because of... 

In a relatively short period of time, Raman spectroscopy has emerged as one of the leading catalyst characterization techniques, especially for molecular-level structural information under reaction conditions. It is now possible to obtain real-time in situ Raman analysis of working catalysts with the modem spectrometer systems that have been developed in recent years. The continual development of new and improved Raman spectrometer systems and cell designs will continue to resolve some of the remaining problems in Raman characterization studies of catalysts (fluorescence and photochemical effects). These new capabilities are expected to accelerate the number and types of catalyst issues that are resolvable with Raman spectroscopy. Thus, it is anticipated the exponential growth of Raman spectroscopy of catalysts, shown in Fig. 1, will continue for many more years. 

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