Catalysis: UHV Model Catalysts, High Pressure
- Exposes samples at elevated temperatures to gas mixtures of one atmosphere or below
- Allows heterogeneous catalytic reactions at realistic pressures over model, often single crystal, catalyst materials
- Gas-phase products analyzed by gas chromatography and/or mass spectrometry
- Pre- and post-high pressure reaction surface analysis capabilities include: LEED, AES, TPD, XPS, and ISS
- Performs vibrational spectroscopic measurements during reaction with FTIR
This unique instrument is capable of measuring gas/solid reaction rates under realistic, high-pressure (∼1 atm) conditions using model, low-surface area solid samples. To date, it has been primarily used to measure the kinetics of gas-phase reactions over model heterogeneous catalysts. As with research microreactor (e.g., chemical reaction test stand, RXM-100 catalyst testing and characterization machine) studies of realistic catalyst materials, reaction rates as a function of temperature and varying reagent partial pressures can be measured in this instrument. From such data, a reaction mechanism can be proposed and a phenomenological kinetic model developed. With the use of single crystals as model catalysts, the sensitivity of the catalytic reaction to geometric surface structure can be assessed.
Besides studies of thermal catalyzed reactions, researchers have used this equipment to study the mechanisms of room-temperature photocatalyzed reactions over model semiconductor oxide catalysts such as TiO2. For these reactions, a high-pressure 100-watt mercury arc lamp was interfaced with the high-pressure reaction cell. The instrument is capable of performing a number of surface science spectroscopic measurements of the structure and composition of the catalyst surfaces without exposing the samples to air. These in-situ and ex-situ spectroscopic methods are performed before, during, and after atmospheric pressure kinetic measurements to provide a fairly complete picture of the chemical state of a reactive surface. Notably, they provide information about the concentration of reaction intermediates, the oxidation state(s) of the constituents of the surface, and the structure of the active surface sites.
System Configuration and Operational Overview
This instrument is capable of exposing samples at elevated temperatures to gas mixtures of one atmosphere or below in a reactor situated just below an ultra-high vacuum (UHV) chamber. In particular, heterogeneous catalytic reactions at realistic pressures over model, often single crystal, catalyst materials can be performed in this instrument with the gas-phase products analyzed by gas chromatography and/or mass spectrometry. Pre- and post-high pressure reaction surface analysis with low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and ion-scattering spectroscopy (ISS) is performed in the main UHV chamber. In addition, the capability to perform vibrational spectroscopic measurements during reaction with Fourier transform infrared (FTIR) spectroscopy is available in the atmospheric pressure reactor.
Sample Preparation and Handling
While experiments on realistic metal, oxide (ceramic and glass), and polymer sample materials can be performed using this instrument, it is primarily used to study model (often single crystal) materials. EMSL researchers responsible for this equipment have significant experience with mounting conductive metal, semiconductive, and insulating oxide samples as well as with attaching thermocouples to these types of samples for temperature measurements. Typically, square and (nearly) round samples with areas of about 1 cm2 and thicknesses of 1 to 2 mm or more are used. Pressing oxide powders into a metal mesh material also has been used as a method to study more realistic materials.
All work with this instrument and in the associated laboratory must be performed in compliance with EMSL practices and permits.
Atmospheric Pressure Reactor
The high-pressure reaction cell is constructed from a commercial 2 3/4-inch conflat flange cube. The cell can be pumped either by the main UHV chamber (see details below) or by a 300-L/s turbo pump. The cell can be isolated from the UHV chamber and backfilled with gas mixtures up to a total pressure of about one atmosphere. After high-pressure reactions at temperatures up to 800K to 900K, gas-phase reaction products are analyzed with an HP 5890 Series II gas chromatrograph. A number of gas chromatrograph columns are available; however, users should check to make sure the products of the reactions proposed for study can be properly analyzed. It also is possible to leak the reaction mixture into the main UHV chamber where mass spectrometric detection is available, thus making it possible to perform reactions with mixed isotopes that address more complicated issues of the reaction mechanism. The reactor has capabilities for in-situ FTIR spectroscopy that can be used to identify both adsorbed and gas-phase chemical species present during reaction. The presence or absence of various proposed reaction intermediates can be readily determined from the in-situ spectra.
A custom, long z-motion (19-inch travel) manipulator, with x (±1-inch) and y (±1-inch) motion, and θrotation (±180°), is used to mount samples. The long z travel allows the sample to be moved between the atmospheric pressure reactor and the UHV surface analysis chamber under vacuum without exposing the sample to the ambient temperature. Currently, the manipulator allows for only Type C (tungsten/rhenium, W-5%Re/W-26%Re) thermocouples to be used for sample temperature measurement.
Surface Analysis Chamber
The high-pressure reactor is directly coupled to a UHV chamber that houses a variety of surface analytical probes to include Princeton Scientific reverse-view LEED optics; a Phi single-pass cylindrical mirror analyzer for AES; a UTI 100°C quadrupole mass spectrometer for obtaining temperature-programmed desorption data; and an Omicron EA 125 multichannel energy analyzer. A non-monochromatic Phi Model 04-548 dual (magnesium and aluminum) anode X-ray source and a Kimball Physics Model ILG-2 ion gun are used for XPS and ISS spectroscopies, respectively. The ion gun can be raster scanned as well to enable sputter cleaning of samples. A custom metal evaporator, capable of deposition rates from a monolayer in several minutes to many monolayers per minute, is available. The surface analysis vacuum chamber consists of a custom designed octagonal stainless-steel chamber with several ports for processing and analytical capabilities. This main UHV chamber is pumped by a 500-L/s ion pump and/or a 300 L/s turbo pump. Base pressures are routinely in the low 10-10 torr range. There is also a 220 L/s turbo pump used for differential pumping of various Teflon® seals used in this chamber, as well as for differential pumping of the ion gun. There are no remaining ports available for additional techniques; thus, adding other UHV probes requires removing one or more of the current capabilities.
Individuals may use this instrument for their research independently following the necessary training.
All Related Publications Related Publications
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- Hydrogen Reactivity on Highly-hydroxylated TiO2(110) Surfaces Prepared via Carboxylic Acid Adsorption and Photolysis.
- Visible Light Induced Photodesorption of NO from the α-Cr2O3(0001) Surface.
- Direct Imaging of Site-Specific Photocatalytic Reactions of O2 on TiO2(110).
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