Raman spectroscopy under catalytic reaction conditions

In recent years, increasing use has been made of in situ methods in EM—as is true of other techniques of catalyst characterization such as IR, Raman, and NMR spectroscopy, or X-ray diffraction. Although the low mean-free path of electrons prevents EM from being used when model catalysts are exposed to pressures comparable to those prevailing in industrial processes, Gai and Boyes (4) reported early investigations of in situ EM with atomic resolution under controlled reaction conditions to probe the dynamics of catalytic reactions. Direct in situ investigation permits extrapolation to conditions under which practical catalysts operate, as described in Section VIII. 

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... 

Over the past two decades, Raman spectroscopy has been extensively applied during catalytic oxidation reactions by mixed-metal oxides and metals under in situ and operando spectroscopy conditions, which has allowed the direct identification of the catalytic active sites involved in the oxidation reactions. Among the multiple spectroscopic techniques that can provide information about the catalytic active sites under oxidation reaction conditions, Raman spectroscopy is unique because of its ability to directly provide molecular level information that allows discrimination among the different catalytic active sites which may be present in the oxidation catalyst. This chapter provides a snapshot of the types of fundamental information obtainable by Raman spectroscopy, and the different types of catalytic materials and oxidation reactions that have been reported, especially under oxidation reaction conditions. 

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. 

One 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 to be performed under reaction conditions. 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 wavenumbers as low as 50 cm-1. These benefits render Raman spectroscopy a powerful tool for studying catalytically active phases on a support. 

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... 

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]. 

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... 

Czerwosz et al. s findings might be of particular interest to readers familiar with carbon formation on nickel and nickel-coated catalysts that had been exposed to hydrocarbons or carbon monoxide in hydrocarbon synthesis or in so-called re-forming reactions carried out in petroleum refineries. For example, the formation of filamentous carbon on such solids at temperatures in the same range as that used by Czerwosz et al. was reported by McCarthy in 1982 [115]. However, these authors did not analyze the carbon deposits by Raman spectroscopy, nor were they aware of the existence of fullerenes. Their concern was the removal of these carbons by steam or by combustion, because these carbons inactivated the catalyst. It was also unknown to them that these carbons had the lubricating properties that were demonstrated by Lauer and co-workers [60,62]. By using these catalysts under conditions of continuous wear, they could maintain the catalytic effect of the surface. 

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