In the Laboratory
Determination of Surface Coverage of an Adsorbate on Silica Using FTIR Spectroscopy Jeanne E. Pemberton,* Laurie L. Wood, and Ghanshyam S. Ghoman Department of Chemistry, University of Arizona, Tucson, AZ 85721
Background
Introduction to Quantitative IR Spectroscopy Although IR spectroscopy is most often used for qualitative analysis and structural determination, quantitative analyses by IR spectroscopy are also possible. Owing to a variety of physical and instrumental parameters, quantitative IR measurements have historically been viewed as much less accurate and precise than their counterparts in the ultraviolet (UV) and visible (vis) regions of the spectrum. Many problematic instrumental parameters are eliminated when using modern FTIR spectrometers, however (1). Despite the fact that good quantitative measurements are possible with FTIR, these approaches are seldom introduced into the undergraduate curriculum. This shortcoming is unfortunate for several reasons. First, it prevents students from appreciating the utility of such approaches, especially for problems where other methods are inadequate. Second, without this exposure, students do not develop an appreciation for the magnitudes of quantitative parameters encountered in IR spectroscopy, such as molar absorptivity and path length, relative to those in the UV–vis regions. IR spectroscopy is an absorbance-based measurement, and therefore quantitation is based on Beer’s Law. Absorptivity values in the IR region are considerably smaller than those in the UV–vis regions; thus, IR spectroscopy is generally not as sensitive as UV–vis spectroscopy. Nonetheless, quantitative IR spectroscopy is useful in applications requiring the molecular specificity afforded by a vibrational measurement. The literature contains many interesting applications of the use of quantitative IR spectroscopy to analyze components in solutions, analyze polymer blends, and determine the surface coverage of adsorbed species (1). This report describes an undergraduate laboratory experiment in which the surface coverage of adsorbed bromobenzene on silica is determined. Introduction to Adsorption Adsorption is the general term applied to the interaction between molecules (or atoms) and a surface (2). Adsorption of molecules on surfaces is important in many areas of chemistry, including catalysis, chromatography, chemical sensors, and biocompatibility of materials. In many processes, such as the chromatographic separation of mixtures or the catalytic conversion of one molecule into another, the adsorption of molecules is essential. Thus, the study of adsorption and adsorbate systems has received considerable attention in the chemistry community over the past several decades, but is rarely pursued directly in undergraduate laboratories. *Corresponding author. Email:
[email protected] .
Adsorption of molecules can be further classified according to the strength of the interaction between the adsorbate and the surface of the adsorbent. The term physisorption refers to adsorption systems in which the energy of interaction between the adsorbate and the adsorbent is relatively small. These interactions are based on weak van der Waals interactions such as dispersion interactions, dipole–dipole interactions, and hydrogen bonding. The term chemisorption refers to adsorption systems in which the energy of interaction between the adsorbate and adsorbent is much larger, approaching that of a covalent bond (2). The adsorption system in this study consists of bromobenzene as the adsorbate and silica as the adsorbent. The interactions between bromobenzene and silica are relatively weak dipole–dipole interactions; therefore, this system is a physisorption system.
Introduction to Silica Surfaces ( 3 ) Silica is an amorphous material of the general stoichiometry SiO2. In bulk silica, the Si and O atoms are connected in tetrahedra of SiO 44᎑ units linked through siloxane bonds (Si–O–Si) to form the silica network. The surface of silica exhibits interesting chemistry resulting from truncation of the tetrahedral Si coordination. In ambient environments containing water vapor, the surface reacts with water and becomes covered with silanol (Si–OH) groups, which possess excellent hydrogen bonding characteristics. This silanolcovered silica surface is generally further covered by one to several layers of strongly hydrogen-bonded water. It is with this surface water that adsorbates interact, usually through hydrogen bonding or dipole–dipole interactions. Introduction to the Analysis of Molecules on Surfaces The analysis of molecules on surfaces has been an area of active research in chemistry for several decades. It is important to appreciate the challenge of such an analysis, given the relatively small number of molecules generally adsorbed. A typical monolayer of molecules on a surface is composed of ca. 1014 to 10 15 molecules/cm 2 (i.e., 10 ᎑10 mol/cm2). Many methods have been developed for the analysis of molecules on surfaces (2). IR spectroscopy is particularly attractive for the study of surface species, owing to the molecular specificity associated with the measurement of molecular vibrations (4 ). Additionally, quantitative analysis can be performed with IR spectroscopy, making it further attractive as a surface analysis tool. IR spectroscopy is somewhat limited in sensitivity for surface analysis, however, because the absorptivities of molecular vibrations are several orders of magnitude smaller than those for electronic transitions. Thus, the analysis of surface adsorbates by IR spectroscopy requires
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either a very high surface area solid or some method of enhancing the absorption of IR radiation by the surfaceadsorbed species relative to those in the bulk phase. The analysis to be performed in this experiment is based on the former approach. Overview of Experiment The full experiment can be done in two parts. In Part I the absorptivity α (in cm2/mol) for the bromobenzene vibrational mode at 1580 cm᎑1 (or alternatively, the band at 735 cm ᎑1) is determined using standard transmission experiments. Part II involves determining the surface coverage Γ (in mol/cm 2) of bromobenzene on silica, also in a transmission experiment, using the absorptivity determined in Part I.
Part I: Determination of the Absorptivity of a Bromobenzene Vibrational Band Part I of the experiment has two stages. The overall goal of this part of the experiment is to use Beer’s law to determine absorptivity, α , of the 1580 cm᎑1 (or 735 cm᎑1) band of bromobenzene. To do this, the path length of a liquid IR cell must first be determined. The basis for this determination is the measurement of the interference fringe pattern produced when the transmission of the empty cell is recorded over a range of frequencies. The thickness, b, of the empty cell containing air (whose refractive index is 1.00) can be calculated from the separation in frequency between the interference fringe pattern maxima (or minima) according to the following equation (5): b=
∆m 2 ν1 – ν2
where b = cell thickness in cm (path length) ν1 = frequency in cm᎑1 of first maximum (or minimum) ν2 = frequency in cm᎑1 of last maximum (or minimum) ∆m = number of complete maxima (or minima) in the frequency interval from ν1 to ν2 Once b is known, Beer’s law can be used to determine the value of absorptivity, α, in cm2/mol for bromobenzene. This is accomplished by acquiring spectra on a series of standard solutions of bromobenzene in cyclohexane, plotting the absorbance of a vibrational band of bromobenzene as a function of concentration (in mol/cm3) and determining α (in cm2/mol) from the slope.
Part II: Determination of Bromobenzene Surface Coverage on Silica The sample consists of chromatographic-grade, highsurface-area powdered silica onto which bromobenzene is adsorbed from the vapor phase. Once prepared, this sample is diluted with KBr, and a thin IR-transmissive KBr pellet is prepared for the determination of bromobenzene surface coverage. Although spectra are acquired over the entire frequency range from 400 to 4000 cm᎑1, the intensity of only one vibrational mode is needed for quantitation. For this adsorbate system, several bands are useful for this purpose, the (Cring–Br) mode at ca. 735 cm᎑1 or the ring modes at 672, 684, 1475, or 1580 cm᎑1. Of these, the modes at 672 and 684 cm᎑1 are the weakest in intensity and therefore the least desirable. The mode at 1475 cm᎑1 is very intense, but suffers from interference 254
from a nearby mode of cyclohexane. This interference could compromise the precision and accuracy of Part I. Thus only the bands at 735 and 1580 cm᎑1 remain as possibilities, and either can be used. The 1580 cm᎑1 band has a lower absorptivity than the 735 cm᎑1 band; thus, an analysis based on the 1580 cm᎑1 band may not be quite as sensitive. In addition to spectra from the sample, it is also important to acquire spectra of the background silica, since silica and water adsorbed on the silica surface are strongly absorbing in the IR. Acquisition of spectra from KBr mixtures of silica before exposure to the adsorbate allows identification of these background absorptions. Experimental Details
Instrumentation and Apparatus FTIR spectra were acquired on a Nicolet 510P FTIR spectrometer. A sealed IR cell with KBr windows (SpectraTech, nominally 200 µm thickness) was used for acquisition of the liquid spectra. A KBr pellet die (8 mm diam) made in-house was used to fabricate KBr pellets, although any commercially available pellet die of approximately the same inner diameter would be equally useful. (The critical constraint in pellet dies is that the diameter of the resulting pellet be less than the diameter of the collimated IR beam that impinges on the sample.) Materials and Chemicals IR-grade KBr (Aldrich) and high-purity grade silica gel (Aldrich, 70-230 mesh, 60 Å av pore diam, 500 m 2/g) were stored in an oven at 110 °C when not in use. Bromobenzene and cyclohexane were reagent grade (Fisher) and used as received. Procedure Adsorption of Bromobenzene onto Silica Approximately 100 mg of silica is weighed into a small glass jar. The silica is spread uniformly on the bottom of the jar for maximum exposure to the bromobenzene vapor. (Alternately, a small shallow petri dish could be used.) In the hood, ca. 10 mL of bromobenzene is poured into a large glass jar. Using tongs, the small jar containing the silica is carefully placed into the larger jar and the larger jar is tightly capped. The jar must sit undisturbed for a minimum of 1 hour (longer is preferable), during which time adsorption of the vapor phase bromobenzene onto the silica surface occurs and, it is hoped, reaches equilibrium. Determination of Bromobenzene Absorptivity 1. This experiment utilizes a single-beam FTIR spectrometer, requiring that separate background and sample spectra be acquired. The background spectrum is subtracted from the sample spectrum to get the equivalent double-beam spectrum. This subtraction is done automatically by most spectrometer data systems. 2. The FTIR spectrometer is set at a resolution of 2 cm ᎑1 for 32 scans. 3. A background spectrum is acquired between 400 and 4000 cm᎑1 with nothing in the spectrometer. For the path length determination, this spectrum will be used as the background. 4. A spectrum between 400 and 4000 cm ᎑1 is acquired on the empty liquid cell. The path length can be accu-
Journal of Chemical Education • Vol. 76 No. 2 February 1999 • JChemEd.chem.wisc.edu
In the Laboratory rately determined from the interference fringe pattern observed in this spectrum. 5. Spectra of neat bromobenzene and neat cyclohexane are acquired between 400 and 4000 cm᎑1 for peak identification. 6. Spectra on standard solutions of 1, 2, 3, and 5% (v/v) of bromobenzene in cyclohexane are acquired. These spectra are plotted as absorbance versus frequency.
Determination of Bromobenzene Surface Coverage 1. Approximately 50–60 mg of dry IR-grade KBr is weighed out and ground in an agate mortar for ca. 1 minute. It is important not to grind too long, because the KBr adsorbs water from the atmosphere which can seriously interfere in the analysis. 2. A KBr die is assembled, all of the ground KBr is transferred into the die, and a pellet is pressed. KBr pellets on which spectra are acquired should be translucent and clear of opaque regions or cracks. Fabricating acceptable KBr pellets is a bit of an art, the simple trick being to allot adequate time for fusion of the KBr to occur. This can generally be reproducibly accomplished through a two-step tightening procedure. After tightening the screws the first time, let the pellet sit for several minutes. Then, tighten both screws a second time (this removes the space created in the die by partial fusion of the KBr during first tightening), and let the pellet sit for several more minutes to allow the KBr to continue to fuse. Carefully unscrew one of the screws and examine the resulting KBr pellet. If the pellet is not translucent, replace the screw, tighten both screws again, and press for several more minutes. If, after several attempts at retightening, the pellet is still not acceptable, clean out the die and press a new pellet. With materials such as silica, it is sometimes necessary to allow fusion to occur for 20 to 30 minutes in order to get a usable pellet. Do not rush this step! 3. Change the number of scans acquired to 64. Once an acceptable pellet has been fabricated, acquire a background spectrum of KBr between 400 and 4000 cm᎑1. 4. Approximately 150 mg of IR-grade KBr and 15 mg of silica are weighed out and transferred to an agate mortar. It is important to know the exact weights of the KBr and the silica weighed out in this step! The powders are mixed thoroughly with a spatula and ground lightly with a pestle. This mixing and grinding step must be done relatively quickly to minimize water uptake by these solids from the atmosphere. The entire mixture is reweighed. It is important to know the exact weight of the mixture used in this step! Remove enough of the mixture to make a suitable pellet (50–60 mg) and press a pellet of the mixture as quickly as possible. After the final tightening of the screws, the KBr die should sit for at least 20 minutes before loosening the bolts. During this time, the remaining mixture that was not used is reweighed and the amount of mixture used to make the pellet is determined by difference. Once an acceptable pellet is made, a spectrum between 400 and 4000 cm᎑1 is acquired. 5. Step 4 is repeated using silica onto which bromobenzene has been adsorbed so that the final pellet is ca. 10–15 wt % silica in KBr. After removing the silica with adsorbed bromobenzene from the glass jar, the silica and KBr should be mixed and put into the pellet die as quickly as possible to avoid significant bromobenzene desorption. A pellet
is made from 50–60 mg of this mixture. It is important to know the exact weight of the silica/KBr mixture used to make the pellet. The remaining unmixed silica can be returned to the capped glass jar containing bromobenzene until it is certain that an acceptable spectrum from adsorbed bromobenzene has been obtained.
Data Analysis
Definitions Terms used in the data analysis are defined as follows: b = liquid cell path length (in cm) Γ = adsorbate surface coverage (in mol/cm 2) Γ = n/S (1) where n = moles of adsorbate S = total surface area of silica in the (silica + KBr) mixture (in cm2 ) G = total mass of (silica + KBr) mixture in pellet (in g) a = characteristic surface area of silica powder (in cm2/g) wt/wt = mass silica/mass (silica + KBr) mixture (in g/g) α = absorptivity of the adsorbate band (in cm2/mol) A = absorbance (unitless) V = volume of the pellet (in cm3) where V = πr 2d (2) r = radius of pellet (in cm) d = pellet thickness (in cm) C = concentration of adsorbate in pellet (in mol/cm3) where C = n/V (3)
Calculation of Bromobenzene Surface Coverage The determination of the surface coverage of bromobenzene on silica starts with calculation of the liquid cell path length from the interference fringe pattern as described above. A Beer’s law plot (absorbance versus concentration) for the bromobenzene band is created using the absorbance data from the standard solutions and the cell path length is determined from the interference fringe pattern. The absorptivity (in cm2/mol) of this bromobenzene band is determined from the slope of the Beer’s law plot. In the quantitative treatment of the IR data used here, it is assumed that the absorptivities of these bromobenzene bands in the liquid phase are identical to those of adsorbed bromobenzene. Given the relatively weak interaction between bromobenzene and the silica surface, this assumption is probably reasonable. From the wt % of silica in the (silica + KBr) mixture, the total surface area of silica in the pellet, S, is calculated according to S = (wt/wt) a G
(4)
Using the peak-to-baseline absorbance of the bromobenzene band, A, the absorptivity for this band, α, and the pellet radius, r, the number of moles of adsorbed bromobenzene, n, can be calculated. This calculation can be done using Beer’s law according to the following arguments. Assuming that the entire pellet (and therefore, all bromobenzene molecules adsorbed on the silica within the pellet) is sampled by the IR beam (a condition commonly met in today’s FTIR spectrometers in which the diameter of the collimated beam impinging on the sample is typically 1 to 1.5 cm),
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A = αdC = α nd/V
(5)
Cyclohexane
Since V = π r d, 2
a
A = αnd/π r 2d Canceling pellet thickness, d, an unknown (and unknowable, in the context of this experiment) quantity from both numerator and denominator gives the pleasing result (6)
n = A π r 2/ α
(7)
Therefore, Equation 7 indicates that the number of moles of bromobenzene sampled, n, can be accurately determined from the measured absorbance, A, if the absorptivity of the bromobenzene band, α , and the radius of the sampled cylinder, r, are precisely known. Finally, using the total surface area of the silica in the pellet, S, and the number of moles of bromobenzene detected in the measured IR absorbance, n, the surface coverage of bromobenzene on the silica surface (Γ , in mol/cm2) can be calculated according to eq 1.
b Absorbance
A = αn/π r 2
1% Bromobenzene/Cyclohexane
3% Bromobenzene/Cyclohexane
c
5% Bromobenzene/Cyclohexane 457
400
735 684 672
600
800
Determination of Surface Coverage of Bromobenzene Adsorbed on Silica A series of experiments were performed to optimize the determination of the surface coverage of bromobenzene adsorbed on silica. The two most important variables in these optimization experiments were the bromobenzene adsorption time and the wt % silica in the KBr pellet. The intensity of the bromobenzene signal in the resulting surface IR spectrum depends on the former and the stability and ease of fabrication
256
1200
1400
1600
1800
2000
Figure 1. FTIR spectra of (a) neat cyclohexane and (b–d) solutions of bromobenzene in cyclohexane: (b) 1% (v/v), (c) 3%, and (d) 5%. Frequencies of important bromobenzene bands are labeled.
3
Absorbance
Determination of the Absorptivity of Bromobenzene The 735 and 1580 cm᎑1 bands of bromobenzene were identified above as suitable for the determination of surface coverage on the basis of two criteria. These bands suffer no interference from either the cyclohexane solvent used for determining absorptivity or from the silica (the spectrum of which is dominated by its adsorbed water). Representative spectra for 1% (v/v), 3%, and 5% bromobenzene in cyclohexane between 400 and 2000 cm᎑1 are shown in Figure 1. From triplicate measurements on each standard, the Beer’s law plots for the 735 and 1580 cm᎑1 bromobenzene bands shown in Figure 2 were generated. The plot for the 1580 cm᎑1 band exhibits excellent linearity over this bromobenzene concentration range. However, the 735 cm᎑1 band behavior exhibits negative deviations at the highest concentration used in this study where the absorbance is in excess of three absorbance units. Only the linear portion of this Beer’s law plot is used for the determination of the 735 cm᎑1 band. From these plots, the absorptivities for the 735 and 1580 cm᎑1 bands were determined to be 3.39 × 10 5 cm2 mol᎑1 and 9.15 × 104 cm2 mol᎑1, respectively.
1580
Wavenumber / cm–1
Typical Results
Determination of Liquid IR Cell Path Length This part of the experiment is routine. A typical interference fringe pattern for a nominally 200- µm path length cell gave a 208-µm observed path length on the basis of the fringe pattern.
1000
d
1475
1069 1000
735 cm–1 2
1580 cm–1
1
0
0
10
20
[Bromobenzene] /
30
(10–5
40
50
mol/cm3)
Figure 2. Beer’s law plots for the 735 and 1580 cm ᎑1 bands of bromobenzene.
of the KBr pellets depends on the latter. To allow sufficient bromobenzene to be adsorbed that adequate signal levels can be observed, bromobenzene adsorption from a vapor-saturated environment must be allowed to proceed for at least one hour; two hours is preferable. Therefore, students must set up the vapor-phase adsorption chamber immediately upon starting the experiment. One other cautionary note should be added. Upon removing the silica with adsorbed bromobenzene from the adsorption vessel, the silica from which the pellet is to be made must be weighed and mixed with the KBr and the pellet pressed at once, to minimize desorption of the weakly adsorbed bromobenzene. In the course of developing this experiment, we also investigated pyridine, which adsorbs more strongly
Journal of Chemical Education • Vol. 76 No. 2 February 1999 • JChemEd.chem.wisc.edu
In the Laboratory
For 14 pellets, the average surface coverage determined for bromobenzene on silica was 2.56 ± 0.55 × 10᎑11 mol/cm2 using both spectral bands. Of course, the accuracy of this value depends on the validity of the assumptions made in the data analysis. Nonetheless, the surface coverages obtained are reasonable based on the surface chemistry of this system. The average surface coverage value roughly corresponds to a fractional surface coverage of ca. 20% if it is assumed that one monolayer corresponds to a close-packed arrangement of bromobenzene molecules oriented parallel to the surface. For such a weakly adsorbed system, this fractional surface coverage is in the ballpark of what one would expect. Thus, the results of this experiment demonstrate quite clearly the mass sensitivity needed for analysis of submonolayer amounts of surface-confined species.
Absorbance
a
1475 735 684 672
400
600
1580
800
1000
1200
1400
Wavenumber /
1600
b 1800
2000
cm–1
Figure 3. FTIR spectra of (a) 9.2 wt % silica–KBr and (b) bromobenzene-adsorbed 9.2 wt % silica–KBr. Frequencies of important bromobenzene bands are labeled.
to silica than bromobenzene, in an attempt to alleviate this desorption problem. However, pyridine is not as spectrally useful as bromobenzene, because its strong absorption bands are in spectral regions where there is significant interference from the silica background. For this reason, we chose to continue with bromobenzene as the adsorbate despite this minor desorption problem. The second important variable in the success of this experiment is the wt % silica in the KBr pellet. Making KBr pellets from silica–KBr mixtures can be a little tricky for students, since more time is required to fuse the KBr around the silica powder than they have used in the past in making pellets of solid organics. In addition, the optical quality and stability of the pellet depend critically on wt % silica. At ca. 15 wt % silica and above, the pellets become more difficult to fuse, powdery, and fragile. On the other hand, the greater the wt% silica in the pellet, the greater the sensitivity of the measurement, since a greater silica surface area (and hence a greater number of adsorbate molecules) is measured. We found that pellets containing 10–15 wt % silica optimized these considerations. Examples of transmission FTIR spectra of silica with and without adsorbed bromobenzene are shown in Figure 3. The spectrum of silica with adsorbed bromobenzene came from a 9.2 wt % KBr pellet of silica, which had been exposed for 3 hours to bromobenzene vapor. This spectrum was obtained on a pellet made from ca. 53.6 mg of the 9.2 wt % silica–KBr mixture pressed for 20 minutes. For this pellet, the total silica surface area was 2.14 × 104 cm2. Using the bromobenzene band at 735 cm᎑1, a surface coverage of 2.56 × 10᎑11 mol/cm2 is calculated. A surface coverage of 2.98 × 10᎑11 mol/cm2 is determined using the 1580 cm᎑1 band. The better signal-tonoise ratio of the 735 cm᎑1 band makes it more desirable for this analysis.
Conclusions An experiment for the quantitative determination of surface coverage of an adsorbate, bromobenzene, on silica using FTIR has been developed. This experiment provides an excellent introduction to several aspects of IR spectroscopy not typically encountered at the undergraduate level, including the use of interference fringe patterns to determine thin-layer cell thickness, the application of Beer’s law to IR data for the determination of absorptivity values of vibrational bands, the surface chemistry of an important adsorbent, silica, and surface analysis based on vibrational spectroscopy. This experiment can be performed in a single four-hour laboratory period. It can be effectively used in either undergraduate instrumental analysis, physical chemistry, or materials chemistry laboratories. One useful variant of this experiment should be noted. The laboratory portion can be shortened if necessary by having students perform only Part II. This strategy would require use of the bromobenzene absorptivities reported here instead of having students experimentally determine these values. Instructors may wish to consider this alternate approach if laboratory time is of major concern. Acknowledgments This work was supported in part by the National Science Foundation (CHE-9504345). We gratefully acknowledge the assistance of Tabitha Sims in the development of this experiment and Michael F. Burke for suggesting bromobenzene as the adsorbate. Literature Cited 1. Griffiths, P. R.; de Haseth, J. A. Fourier Transform Infrared Spectrometry; Wiley: New York, 1986; Chapter 10. 2. Adamson, A. W. Physical Chemistry of Surfaces, 5th edi.; Wiley: New York, 1990; Chapters 4, 7, 8, 11, 15, 16, 17. Somorjai, G. A.; Rupprechter, G. J. Chem. Educ. 1998, 75, 161–176. 3. Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. 4. Bell, A. T. In Vibrational Spectroscopy of Molecules at Surfaces; Yates, J. T.; Madey, T. E., Eds.; Plenum: New York, 1987; Chapter 3. 5. Chia, L.; Ricketts, S. Basic Techniques and Experiments in Infrared and FT-IR Spectroscopy; Perkin Elmer: Norwalk, CT, 1988.
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