Infrared cell-reactors

In this paper we will first describe a fast-response infrared reactor system which is capable of operating at high temperatures and pressures. We will discuss the reactor cell, the feed system which allows concentration step changes or cycling, and the modifications necessary for converting a commercial infrared spectrophotometer to a high-speed instrument. This modified infrared spectroscopic reactor system was then used to study the dynamics of CO adsorption and desorption over a Pt-alumina catalyst at 723 K (450°C). The measured step responses were analyzed using a transient model which accounts for the kinetics of CO adsorption and desorption, extra- and intrapellet diffusion resistances, surface accumulation of CO, and the dynamics of the infrared cell. Finally, we will briefly discuss some of the transient response (i.e., step and cycled) characteristics of the catalyst under reaction conditions (i.e.,... 

Extreme interaction regime. The experimental set-up is given in figure 6. The stirred-cell reactor was operated batchwise with respect to the liquid and semi-batchwise with respect to the gas-phase which was also circulated by means of a peristaltic pump over an infrared spectrophotometer for C02 detection. 

Temporal analysis of products (TAP) and flow reactor catalytic measurements were done as reported elsewhere [2,4,8,12]. Fourier-transfomt infrared (FT-1R) studies were carried out using a Perkin Elmer 1750 instrument and a flow reactor infrared cell connected to conventional vacuum... 

Reaction studies were carried out in a specially designed infrared cell which doubled as a flow reactor [10]. Before a given experiment, the catalyst was pressed into the form of a thin wafer, placed into the reactor cell, and reduced at 200-450°C for twenty hours. The catalyst was then run to steady-state in 3.4% CO and 0.8% NO for sixteen hours before any steady-state data were obtained. All data were obtained under differential reactor conditions and analysis of the feed and product gases was accomplished using an automated gas chromatographic system [11],... 

RTD experiments showed that the fixed-bed almost behaves like a plug-flow reactor and the infrared cell like a continuous stirred tank reactor. This fixed-bed is described by the tanks-in-series model, using 9 tanks for the catalyst compartment. The two kinetic models (Equations 1-6) are able to describe the stop-effect experiments at 180 and 200°C, and the considerations made in this work are valid for both temperatures. However, for the sake of clarity, only model discrimination at 180°C will be presented here. In the experimental conditions used here, both models can be simplified the first adsorption step is considered as irreversible, and instantaneous equilibrium is assumed for the second one. With these hypothesis the total number of kinetic parameters is reduced from five (ki, Li, k2, k.2 and ks) to three (ki, K2 and ks), and the models can be expressed as follows ... 

Figure 4.2-7 Layout of the reactor for the synthesis and isolation of CpMn(CO)2(n -H2) from CpMn(CO)3 and H2 in SCCO2. The components are labeled as in Figure 4.2-6 with additional items as follows C, control valve DU, gas dosage unit (NWA) H2, hydrogen cylinder IR, infrared cell P, solid product, CpMn(CO)2(ii -H2) PC, pneumatic compressor (NWA Model CU105) R, variable volume view-cell containing a solution of CpMn(CO)3 in an H2/SCCO2 mixture S, mixer with magnetic stirrer (Kontron M491). (Reproduced with permission from J. A. Banister, P. D. Lee, M. Poliakoff, Organometallics 1995, 14, 3876 American Chemical Society).

Fig. 9. Infrared flow-reactor cell for in-situ measurements of, e.g., activation, reaction and diffusion [149,158]...

The experimental set-up is given in figure 6. A closed reactor-detector system was used to enable detection of small mole fluxes. The stirred cell reactor is 0.10 m in diameter and was filled before each experiment with 120 ml of charged 2.0 M DIPA solution. The gas phase in the system was circulated by means of a flexible tube pump over a flow-through cell in a Perkin Elmer model 257 Infrared Grating Spectrophotometer for CO2 detection. Although spectrophotometers are not exceptionally well-suited for quantitative measurements, we preferred this type of analysis compared to gas chromatography for example because it does not influence the gas phase. 

The experiments were carried out in a stirred cell reactor. The operation was batch-wise with respect to the liquid phase. The stirred cell reactor constructed from resistant glass with interfacial area for mass transfer of 76.93 cm. The internal diameter of the reactor was 10 cm with a total volume of about 1800 cm. The gas and liquid phases were stirred separately using two stirrers. To prevent the formation of vortex, four equidistant baffles were placed inside the reactor. An infrared Rosemount model 880A CO2 analyzer was used to measure the amount of carbon dioxide at the exit of the reactor. 

Design and Operation of a Novel Impinging Jet Infrared Cell-Recycle Reactor... 

In order to get closer from the catalytic conditions (for example in DeNO reaction exhaust gases are to be treated) investigators developed reactor cells allowing the infrared study of catalysts underflow. The principle of transient technique is then to introduce... 

Although acetone was a major product, it was not observed by infrared spectroscopy. Flowing helium/acetone over the catalyst at room temperature gave a prominent carbonyl band at 1723 cm 1 (not show here). In this study, a DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) cell was placed in front of a fixed reactor DRIFTS only monitored the adsorbed and gaseous species in the front end of the catalyst bed. The absence of acetone s carbonyl IR band in Figure 3 and its presence in the reactor effluent suggest the following possibilities (i) acetone formation from partial oxidation is slower than epoxidation to form PO and/or (ii) acetone is produced from a secondary reaction of PO. 

Features common to all CVD reactors include source evaporators with an associated gas handling system to control input gases and gas-phase precursor concentrations, a reactor cell with a susceptor heated by either radio frequency or infrared radiation, and an exhaust system to remove waste products (which may include a vacuum pump for low-pressure operations). Substrate temperatures can vary from less than 200 °C to temperatures in excess of 1000 °C, depending on the nature of the material layer and precursor used. Schematic diagrams of some simple CVD reactors are shown in Figure 4. 

E. J. Shinouskis built the infrared reactor cell and conducted the infrared reactor experiments. The authors are indebted to J. A. Sell for the modifications of the infrared spectrophotometer, and to Professor A. T. Bell for sharing his experience with infrared reactor construction. The CO conversion data of Figure 3 were obtained by J. E. Carpenter. 

Many of the characterization techniques described in this chapter require ambient or vacuum conditions, which may or may not be translatable to operational conditions. In situ or in opemndo characterization avoids such issues and can provide insight and information under more realistic conditions. Such approaches are becoming more common in X-ray adsorption spectroscopy (XAS) methods ofXANES and EXAFS, in NMR and in transmission electron microscopy where environmental instruments and cells are becoming common. In situ MAS NMR has been used to characterize reaction intermediates, organic deposits, surface complexes and the nature of transition state and reaction pathways. The formation of alkoxy species on zeolites upon adsorption of olefins or alcohols have been observed by C in situ and ex situ NMR [253]. Sensitivity enhancement techniques play an important role in the progress of this area. In operando infrared and RAMAN is becoming more widely used. In situ RAMAN spectroscopy has been used to online monitor synthesis of zeolites in pressurized reactors [254]. Such techniques will become commonplace. 

The parameter can change in a vessel being part of the analytical instrument, for example, an ultraviolet-visible (UV-Vis) spectrophotometric cell [39,41,45,14,47, 48], an infrared (IR) cell [42, 46], or a fluorometer cell [45, 51], or a polarimetric tube [27, 49]. It can change in a reactor vessel where the analytical signal can be read in some way, for example using an optical fiber cell for spectrophotometry [52-54] or a conductometric cell [16,34,40]. Another possibility is to transport the solution from the reaction vessel to the analytical instrument by a peristaltic pump [38]. When altenative ways are not practicable, samples can be taken at suitable time intervals and analyzed apart [29,31,35,39,43,50]. 

Attenuated total reflection (ATR) is sometimes used to measure the infrared spectra of catalysts inside a reactor. The infrared light is coupled into an ATR crystal, which can be either a flat plate (e.g., the wall of a reactor) or a cylindrical rod (surrounded by catalyst particles). The evanescent wave that protrudes outside the crystal when the infrared beam reflects on the inside of its surface is used for the measurement. A review of ATR in catalysis has been published by Biirgi and Baiker [11], and a catalytic cell to apply the method in situ inside a catalyst bed reported by Moser and co-workers [12]. An example of ATR is discussed later in this chapter. 

The catalytic CO oxidation by pure oxygen was selected as a model reaction. The Pt/alumina catalyst In the form of 3.4 mm spherical pellets was used. The CO used In this study was obtained by a thermal decomposition of formic acid In a hot sulphuric acid. The reactor was constructed by three coaxial glass tubes. Through the outer jacket silicon oil was pumped, while air was blown through the inner jacket as a cooling medium. The catalyst was placed in the central part of the tube. The axial temperature profiles were measured by a thermocouple moving axially in a thermowell. Gas analysis was performed by an infrared analyzer or by a thermal conductivity cell. [7]. 

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