Chapter 16
Infrared Study of Surface Species Under High-Pressure Conditions
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S. D. Worley, J . P. Wey, and W. C. Neely Department of Chemistry, Auburn University, Auburn, A L 36849
The design, construction, and operation of a novel high-pressure infrared -cell reactor is discussed. The reactor functions in the temperature and pressure ranges of 100-600 K and 10 - 10 Torr, respectively. The use of FTIR to monitor surface species during the interactions of N2, H2, D2, O2, and CO with supported Rh catalyst films at ambient temperature and high pressure is discussed. -8
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One of the most important means of spectroscopically studying reactive intermediates on the surfaces of supported catalysts is by use of transmission FTIR. Most of the work reported from these laboratories and elsewhere concerning this topic has referred to reactive systems at subatmospheric pressures. However, industrially important chemical processes are often performed at pressures well above one atmosphere, so it should be desirable to monitor surface species under high-pressure conditions to insure that the mechanistic processes do not change in passing from the low-pressure to highpressure regimes. The two primary limitations which occur in infrared sutdies of catalytic surfaces at high pressure are: (1) interference from small amounts of impurity surface species which have high infrared extinction coefficients and (2) interference from gas-phase reactants or products which have infrared-active vibrational modes. In this work our efforts in addressing these problems for surface species of interest for supported catalyst films will be discussed. Experimental The high-pressure-infrared-cell reactor designed and constructed in these laboratories and used in the work to be described is shown in Figure 1. This cell-reactor has recendy been discussed in detail (7), but briefly it has been fabricated from three 4.62 in stainless steel flanges. The outer two flanges contain water-cooled 13 mm diameter, 2 mm thick CaF2 infrared windows mounted by means of 0.375 in ultra-torr adaptor fittings capable of operation in the 10-8 -10* Torr pressure regime. The inner flange contains a stainless steel block in which a third 25 mm CaF2 window containing the supported catalyst film is mounted. The sample block also contains a U-shaped tunnel drilled about the window for use in sample heating or cooling by means of circulation of
0097-6156/92/0482-0250$06.00/0 © 1992 American Chemical Society In Surface Science of Catalysis; Dwyer, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
16. WORLEY ET AL.
Surface Species Under High-Pressure Conditions
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Figure 1. The high-pressure infrared-cell reactor. A: Outer 4.62 in stainless steel flanges, B: To GC, C: To vacuurn/high-pressure manifold. D: Water cooling copper tubing, E: 13 mm Viton O-ring, F: 13 mm CaF2 window, G: IR beam, H: Sample gas inlet and oudet, 1:1.3 in thermocouple feed-through in centerflange,J: Heating/cooling gas inlet and oudet, K: Cr-Al thermocouple, L: Stainless steel sample holder block, M: 25 mm sample window, N: U-shaped tunnel for heating and cooling gas.
In Surface Science of Catalysis; Dwyer, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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heated or cooled N2 and a chromel-alumel thermocouple for measuring sample temperatures. The cell-reactor is attached to a stainless steel manifold capable of operation in the 10- to 10 Torr pressure regime utilizing a cryopump and 60 L s ion pump. Pressure measurements are made using a 10 Torr MKS Baratron capacitance manometer which establishes the upper pressure limit of the cell reactor. The reactor could be safely operated at least up to a pressure of 16 atm using the current CaF2 windows. The supported catalyst films were prepared by spraying suspensions of RhCl3.3H20, AI2O3, acetone, and distilled water on to the 25 mm sample window held at 80°C so as to achieve a final loading of 2.2% by weight Rh. The solvents rapidly evaporated leaving infrared-transparent films (£fl. 4.4 mg cm-2) which could then be prereduced in the cell-reactor. It has been shown in these laboratories (2) and elsewhere (3) that this technique produces excellent supported catalyst samples for use in transrnission-infrared analyses. Sample gases were purchased from Air Products (H , 99.999%; N , 99.999%; O2, 99.993%) and Matheson (D , 99.5%; CO, 99.99%). Further purification was generally necessary with high-pressure H2, D2, N2, and O2 being passed through a 5% RI1/AI2O3 a catalytic converter heated to 373 K, and then trapped at 77 or 158 K, as appropriate. The CO was trapped at 77 K only. All of the sample films were pretreated by evacuation to 10- Torr at 373 K for 1 h, followed by several reduction/evacuation cycles in 100 Torr H2 at 473 K, and further evacuation at 298 K for 1 h, before exposure to the gases of interest. In some cases preoxidation was effected by exposure to 100 Torr O2 at 298 K for 10 min immediately following reduction treatment All infrared spectra were obtained with an IBM 32 FT spectrometer operated at 2 cmresolution. Generally 500 scans were generated for each spectrum over a period of 7.5 min. All IR data displayed in this work represent difference spectra relative to appropriate reference spectra. 10
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Downloaded by NORTH CAROLINA STATE UNIV on October 12, 2012 | http://pubs.acs.org Publication Date: January 23, 1992 | doi: 10.1021/bk-1992-0482.ch016
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Results and Discussion Dinitrogen. *Wang and Yates (4) have presented a detailed infrared analysis of the N2/Rh/Al203 system for low pressures (
