Langmuir 1992,8, 1676-1678
1676
Transmission Infrared Spectroscopy of High Area Solid Surfaces. A Useful Method for Sample Preparation Todd H. Ballinger, Jason C. S. Wong, and John T. Yates, Jr.' Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received January 30, 1992
A method for the preparation of high surface area solids for transmission infrared spectroscopic surface chemistry studies is described. This method consists of mechanically pressing the solid into tiny holes of a support grid, resulting in many tiny self-supported pellets surrounded by the metal grid framework. This allows for maximum efficiency heat transport in the temperature range of 100-1500 K in the sample. Studies of adsorption and desorption phenomena from the pressed powder indicate that diffusion of gases into and out of the pressed sample is rapid. There are no apparent differences in the adsorption and desorptionbehavior of samples produced by this method and by a spray deposition method. Furthermore, contaminant-free samples can be prepared by this method since no solvents are used.
Introduction Transmission infrared spectroscopy is widely used to probe the surface chemistry of high area solids. This method has been effectively employed for studies of oxide powders and of metallic and nonmetallic catalysts supported on oxides for many This paper describes a useful method for the preparation of samples used for this technique and compares the behavior of these samples to those produced by another method. Recently we described a new design for an infrared cell and for sample preparation used in surface chemistry studies of high surface area solid^.^ This cell possesses a number of unique characteristics, the sum of which is not found in other cell designs. These capabilities are (1)ultrahigh vacuum (UHV) operation, (2) optimal gas transport properties to all parts of the solid surface, (3) uniform solid deposition, (4) sample temperature range from 100 to >1500 K, without moving the sample to a furnace region, (5) temperature control to fl K over the entire temperature range, (6) linear temperature programmability a t rates up to 8 K/s, and (7) very rapid sample cooling rates. These useful capabilities are obtained by supporting the solid powder on a flat photoetched tungsten grid, 0.025 mm thick, containing 1000 uniform square openings (0.22 mm X 0.22 mm). Each grid is surrounded by a frame of tungsten foil of 0.025 mm width. The support grid is held rigidly between two nickel clamps. These nickel clamps, positioned in the center of a cubical stainless steel optical cell, are attached to copper feedthroughs which provide contact for programmed electrical heating of the grid and for sample cooling with liquid Nz. Previously, the high surface area solid was deposited onto a tungsten grid by a spray method4using a slurry of the solid in a water/acetone mixture (1:9 ratio). The drawback of depositing the sample by this method is the possibility of contamination from acetone adsorption onto the solid and subsequent decomposition on the surface during the initial sample outgassing at elevated temperatures under vacuum. To avoid such possible contamination of high surface area solids used for transmission infrared studies, we have N
(1) Little, L. H. InfraredSpectraofAdsorbedSpecies;Academic:New York. I9fifi. - -. - - -. (2) Hair, M. L. Infrared Spectroscopy in Surface Chemistry;Dekker:
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New York, 1967;Chapter 5. (3)Bell, A. T.In Vibrational Spectroscopy of Molecules on Surfaces; Yates, J. T., Jr., Madey, T. E., Eds.; Plenum: New York, 1987; Chapter 3. (4) Basu, P.; Ballinger, T. H.; Yates, J. T., Jr.Reu. Sci. Instrum. 1988, 59, 1321.
0743-7463/92/2408-1676$03.00/0
developed an alternative method for pressing the powder into the grid using a hydraulic press. This is similar to pressing a pellet out of the solid, which is the most commonly used method to prepare solids for transmission IR studies. The grid-pressed samples have the very important added feature of excellent heat transport throughout the sample, compared to a massive pressed disk. Pressing solids onto a grid has been previously reported by others5 to prepare solids in a Ta mesh for IR and XPS studies, but detailed studies of this preparation method have not been carried out.
Method of Pressing the Grid The solid powders are pressed into the grid by using pressure transfer dies in a hydraulic press. The dies used are two 1-in. cubes of polished 316 stainless steel. As shown schematically in Figure la, the grid is placed on a piece of weighing paper on the bottom die. Approximately 30100 mg of solid is then evenly distributed over the grid, and a second piece of weighing paper is placed on the top to keep the powder from sticking to the stainless steel die when pressed. A second die is then placed on top, as in Figure 1. The die assembly is placed into the hydraulic press, and 12 000 lb/in.2 pressure is applied, as shown in Figure lb. Upon removal of the grid from the die, visual inspection shows that there is material both on top of the grid and inside each of the holes of the grid, as represented in Figure IC. The top layer of material easily flakes off upon flexing the grid or upon gently scraping with a metal spatula. However, the material which is pressed into the grid holes (see Figure Id) is rigidly held and withstands gentle bending. Thus, before the grid is placed in the clamps for cell assembly, the top material is flexed off of the grid since it would come off during the cell assembly process. After this process, material is left on the grid in a density range from 1.5to 4 mg/cmz,where the geometrical surface area used here is that of the open spaces in the grid. Half of the grid can be filled with solid sample and the other half left blank for transmission infrared observation of gas-phase species. This geometry is achieved by only placing half of the grid in the die. Aspare piece of tungsten grid is placed on the other half of the bottom die to keep the two dies level during pressing. Grids pressed in this manner have a straight, even line of demarcation between the covered and uncovered sections of the support grid. (5)Jin, T.;Zhou,Y.; Mains, G. J.;White, J. M.J. Phys. Chem. 1987, 91, 5931.
1992 American Chemical Society
Preparation of High Surface Area Solids
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Langmuir, Vol. 8, No. 6, 1992 1677
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Figure 1. A side-view schematic diagram of the preparation of a pressed powder sample. The first step (a) shows the die assemblyjust before compreasionin a hydraulic press. The order ofassemblyconsistaofthebottomdye,paper,W grid,solidpowder (or catalyst), paper, and top die. The second step (b) consists of pressing the die assembly in a hydraulic press with 12 OOO lb/in2. This leaves the compressed sample covering the grid and grid holes (c) after it has been removed from the die assembly. Finally, the excesssampleis removed from the grid frames,leaving the sample only in the holes of the grid (d).
Figure 2 shows how the pressed grid looks by scanning electron microscopy (SEM). Figure 2a shows the blank grid before pressing. Figure 2b was taken after high surface area alumina, A1203 (surface area = 101m2/g),was pressed in the grid. This SEM was obtained after the excess material was removed from the grid. As can be seen, little A1203is left on the W grid, and all of the pressed material remains inside the holes of the grid. A close-up SEM taken of an individual grid hole showing the character of the compressed A1203 powder is presented in Figure 2c.
Comparison of Pressed and Sprayed Samples One question which arises is whether the type of sample deposition on the grid makes a difference in the adsorption-desorption behavior of the powder. IR spectroscopy has been used to determine if there is any difference between pressed and sprayed samples. The IR spectra were collected on a Perkin-Elmer 580 B grating spectrometer by signal averaging five times at a resolution of 5.3 cm-1. A first comparisonwas made of the desorption transport behavior through the pores of the A1203 prepared by the two methods. Figure 3 compares the hydroxyl region IR spectra of samples of A1203 deposited by the two methods as a function of dehydroxylation temperature. When the A1203is heated, hydroxyl groups combine to form water and to leave a Lewis acid site, AP+, and an oxide site on the surface.e8 The spectra obtained from the pressed A1203at 200 K increments starting from 400 K are shown in Figure 3A, while those of the sprayed A1203are in Figure 3B. A difference is noted between the two A1203 depositions in the IR intensity of the Al-OH groups. The sprayed A l 2 0 3 sample exhibits higher OH intensity than the pressed A1203 due to the heavier buildup of material on the sprayed grid. However, the effect of temperature on the extent of dehydroxylation is very similar for both methods of sample preparation. Both A1203preparations show a loss of associated hydroxyl groups in the 3600-3200-cm-l region around 600 K (spectra 3b). For the 800 K treatment, all associated OH groups have been eliminated from the surface, as shown in spectra 3c. Furthermore, a t lo00 K almost all of the remaining hydroxyl groups have been removed, as seen in spectra 3d (6) Ballinger, T. H.; Yates, J. T., Jr. Langmuir 1991, 7, 3041. (7) Knbinger, H.; Ratnasamy, P. Catal. Reo. Sci. Eng. 1978,27,31. (8) Peri, J. B.; Hannan, R. B. J . Phys. Chem. 1960,64, 1526.
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where similar intensities of the weak features remain for both methods of A1203 preparation. A second comparison was made to compare the adsorption transport behavior through the pores of the A1203 samples prepared by the two methods. The main concern is whether the macroscopic pores of the material are changed during pressing such that diffusion of a reactant molecule is hindered. To test this, we have monitored the absorbance features of pyridine as it adsorbs onto the surface. On highly dehydroxylated A1203, pyridine adsorption onto the Lewis acid sites results in shifts of the 19b and 8a ring vibrations from the gas-phase values of 1440and 1580cm-1 to frequencies at 1444and 1610cm-l, respectively. To monitor any differences in adsorbate
Ballinger et al.
1678 Langmuir, Vol. 8, No. 6,1992 6. Sprayed
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diffusion kinetics between the two methods of sample preparation, the intensity of the 1444-cm-l band was measured as a function of time after admitting pyridine gas to the cell. The resulting plot in Figure 4 indicates that pyridine adsorption occurs very rapidly for both methods of preparation. In fact, the adsorption appears to be complete for both samples before the first IR scan is collected, in a period of 60 s after pyridine admission, since the initial absorption intensity measured does not change over the 50-min interval used for obtaining a sequence of IR spectra. Although there is an intensity difference between the pressed and sprayed grids due to the different amounts of material deposited on the grid, no gross changes in diffusion kinetics to all surface sites have been observed between the sprayed and pressed Diffusion of pyridine is complete at 300 K in 60 s, or less time. In addition, the saturation absorbance intensity of pyridine per unit mass of A1203 differs little for the two preparation methods. (9) Kline, C.H.,Jr.; Turkevich, J. J. Chem. Phys. 1944, 12, 300.
Figure 5. IR spectra in the 0-H stretching region of a pressed Ti02 sample (A) and a pressed Si02 sample (B) as a function of dehydroxylation temperature. The amount of sample used was 3.96 mg of Ti02/cm2of open grid area and 1.84 mg of SiOz/cm* of open grid area. Both samples were initially heated to 475 K for 36 h to remove adsorbed water from the surface. The spectra were obtained at the indicated dehydroxylationtemperatureafter heating for 30 min: (a) 400 K, (b) 600 K, (c) 800 K, (d) lo00 K.
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Pressed Samples of Other Materials A drawback of the spray preparation method is the limitation to fumed oxides (A1203, Si02, and TiOz), which are the only such materials which have been successfully sprayed onto the grid to produce an adherent deposit. This is apparently related to the ability of these materials to form a well-dispersed slurry in the solvents employed. In contrast, the pressed grid method has successfully been applied to A1203,Si02, TiO2, MgO, and Sr/LazOs. As a further demonstration of this, Figure 5 shows representative IR spectra collected during the dehydroxylation of Si02 and Ti02 between 400 and 1000 K (spectra a-d). Dehydroxylation of SiO2lO-l2 and Ti0213-15 has been previously studied in detail, and the results obtained here using the pressed oxide samples are in agreement with the earlier studies. Summary We have demonstrated a grid pressing method for producing adherent high surface area powdered oxide samples. These samples can be employed for effective transmission IR experiments to study surface chemistry. This method produces pressed oxides in tiny grid openings which are mechanically self-supporting. Good heat transfer between the tungsten grid and the powder permits rapid heating and cooling between 100and 1500K. Rapid adsorbate diffusion to all adsorption sites is observed for pressed A1203 samples. The kinetics of removal of adsorbed OH groups from A1203 do not seem to differ for A1203 samples which have been pressed and A1203 samples produced by a spray deposition technique. Solventderived surface impurities, if present in the spray deposited samples, will not be present in the grid-pressed samples, which do not employ solvents. Acknowledgment. This research was supported by the Northwest College and University Association for Science (Washington State University) under Grant DEFG-06-89ER-75522with the US. Department of Energy. (10) McDonald, R. S. J. Phys. Chem. 1958,62, 1168. (11)Borello, E.;Zecchina, A.; Morterra, C. J. Phys. Chem. 1967, 71, 29.18.
(12) Peri, J. B. J. Phys. Chem. 1966, 70,2937. (13) Jackson, P.; Parfitt, G. D. Trans. Faraday SOC.1971, 67, 2469. 1966,62, 204. (14) Lewis, K. E.;Parfitt, G. D. Trans. Faraday SOC. (15) Primet, M.; Pichat, P.; Mathieu, M. V. J. Phys. Chem. 1971, 75, 1216.
